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  • 1.
    Alghalibi, Dhiya
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. College of Engineering, University of Kufa, Al Najaf, Iraq.
    Rosti, Marco E.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Inertial migration of a deformable particle in pipe flow2019Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, nr 10, artikel-id 104201Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We perform fully Eulerian numerical simulations of an initially spherical hyperelastic particle suspended in a Newtonian pressure-driven flow in a cylindrical straight pipe. We study the full particle migration and deformation for different Reynolds numbers and for various levels of particle elasticity, to disentangle the interplay of inertia and elasticity on the particle focusing. We observe that the particle deforms and undergoes a lateral displacement while traveling downstream through the pipe, finally focusing at the pipe centerline. We note that the migration dynamics and the final equilibrium position are almost independent of the Reynolds number, while they strongly depend on the particle elasticity; in particular, the migration is faster as the elasticity increases (i.e., the particle is more deformable), with the particle reaching the final equilibrium position at the centerline in shorter times. Our simulations show that the results are not affected by the particle initial conditions, position, and velocity. Finally, we explain the particle migration by computing the total force acting on the particle and its different components, viscous and elastic.

  • 2.
    Alizad Banaei, Arash
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Rahmani, Mona
    Univ British Columbia, Dept Math, Vancouver, BC V6T 1Z2, Canada..
    Martinez, D. Mark
    Univ British Columbia, Dept Chem & Biol Engn, Vancouver, BC V6T 1Z3, Canada..
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Inertial settling of flexible fiber suspensions2020Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 5, nr 2, artikel-id 024301Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the inertial settling of suspensions of flexible and rigid fibers using an immersed boundary method. The fibers considered are inextensible and slender, with an aspect ratio of 20. For a single Galileo number of Ga = 160, we examine a range of dimensionless bending rigidities 0.1 < gamma < 20 and fiber concentrations 0.5 < nL(3) < 25, with n being the fiber number density and L the fiber length, that spans dilute and semidilute regimes. The settling fibers form streamers, regions where the fibers are packed and settle faster than the average settling velocity of the suspension, for nL(3) > 10. In the low-concentration regions outside the streamers, the fibers either go upward or have low settling velocities. Flexible fibers exhibit higher packing inside the streamers and smaller streamers compared to the streamers formed by the rigid fibers. Due to this higher packing, the flexible fibers settle faster compared to the rigid fibers. The formation of the streamers counterbalances the hindering of the settling velocity at higher concentrations. At higher nL(3), however, the maximum local concentration of fibers relative to a uniform distribution diminishes for both flexible and rigid fibers as the mobility of the fibers becomes limited due to the presence of other fibers in their vicinity. Due to this limited mobility, the deformation of the fibers and their settling orientation become insensitive to nL(3) for nL(3) > 7. In both the dilute and semidilute regimes, flexible fibers are more aligned with the direction perpendicular to gravity compared to rigid fibers.

  • 3.
    Arratia, Cristobal
    et al.
    Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.;Ecole Polytech Fed Lausanne, Lab Fluid Mech & Instabil, CH-1015 Lausanne, Switzerland.;Nordita SU.
    Mowlavi, Saviz
    Ecole Polytech Fed Lausanne, Lab Fluid Mech & Instabil, CH-1015 Lausanne, Switzerland.;MIT, Dept Mech Engn, Cambridge, MA 02139 USA..
    Gallaire, Francois
    Ecole Polytech Fed Lausanne, Lab Fluid Mech & Instabil, CH-1015 Lausanne, Switzerland..
    Absolute/convective secondary instabilities and the role of confinement in free shear layers2018Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 3, nr 5, artikel-id 053901Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the linear spatiotemporal stability of an infinite row of equal point vortices under symmetric confinement between parallel walls. These rows of vortices serve to model the secondary instability leading to the merging of consecutive (Kelvin-Helmholtz) vortices in free shear layers, allowing us to study how confinement limits the growth of shear layers through vortex pairings. Using a geometric construction akin to a Legendre transform on the dispersion relation, we compute the growth rate of the instability in different reference frames as a function of the frame velocity with respect to the vortices. This approach is verified and complemented with numerical computations of the linear impulse response, fully characterizing the absolute/convective nature of the instability. Similar to results by Healey on the primary instability of parallel tanh profiles [J. Fluid Mech. 623, 241 (2009)], we observe a range of confinement in which absolute instability is promoted. For a parallel shear layer with prescribed confinement and mixing length, the threshold for absolute/convective instability of the secondary pairing instability depends on the separation distance between consecutive vortices, which is physically determined by the wavelength selected by the previous (primary or pairing) instability. In the presence of counterflow and moderate to weak confinement, small (large) wavelength of the vortex row leads to absolute (convective) instability. While absolute secondary instabilities in spatially developing flows have been previously related to an abrupt transition to a complex behavior, this secondary pairing instability regenerates the flow with an increased wavelength, eventually leading to a convectively unstable row of vortices. We argue that since the primary instability remains active for large wavelengths, a spatially developing shear layer can directly saturate on the wavelength of such a convectively unstable row, by-passing the smaller wavelengths of absolute secondary instability. This provides a wavelength selection mechanism, according to which the distance between consecutive vortices should be sufficiently large in comparison with the channel width in order for the row of vortices to persist. We argue that the proposed wavelength selection criteria can serve as a guideline for experimentally obtaining plane shear layers with counterflow, which has remained an experimental challenge.

  • 4.
    Atzori, Marco
    et al.
    Johannes Kepler Univ Linz, Dept Particulate Flow Modelling, A-4040 Linz, Austria..
    Torres, Pablo
    Univ Politecn Valencia, Inst Univ Matemat Pura & Aplicada, Valencia 46022, Spain..
    Vidal, Alvaro
    Parallel Works, Chicago, IL 60654 USA..
    Le Clainche, Soledad
    Univ Politecn Madrid, Sch Aerosp Engn, Madrid 28040, Spain..
    Hoyas, Sergio
    Univ Politecn Valencia, Inst Univ Matemat Pura & Aplicada, Valencia 46022, Spain..
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    High-resolution simulations of a turbulent boundary layer impacting two obstacles in tandem2023Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 8, nr 6, artikel-id 063801Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    High-fidelity large-eddy simulations of the flow around two rectangular obstacles are carried out at a Reynolds number of 10 000 based on the freestream velocity and the obstacle height. The incoming flow is a developed turbulent boundary layer. Mean-velocity components, turbulence fluctuations, and the terms of the turbulent-kinetic-energy budget are analyzed for three flow regimes: skimming flow, wake interference, and isolated roughness. Three regions are identified where the flow undergoes the most significant changes: the first obstacle's wake, the region in front of the second obstacle, and the region around the second obstacle. In the skimming-flow case, turbulence activity in the cavity between the obstacles is limited and mainly occurs in a small region in front of the second obstacle. In the wake-interference case, there is a strong interaction between the freestream flow that penetrates the cavity and the wake of the first obstacle. This interaction results in more intense turbulent fluctuations between the obstacles. In the isolated-roughness case, the wake of the first obstacle is in good agreement with that of an isolated obstacle. Separation bubbles with strong turbulent fluctuations appear around the second obstacle.

  • 5.
    Atzori, Marco
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Stroh, Alexander
    Karlsruhe Techonol, Inst Fluid Mech, D-76131 Karlsruhe, Germany..
    Gatti, Davide
    Karlsruhe Techonol, Inst Fluid Mech, D-76131 Karlsruhe, Germany..
    Frohnapfel, Bettina
    Karlsruhe Techonol, Inst Fluid Mech, D-76131 Karlsruhe, Germany..
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Uniform blowing and suction applied to nonuniform adverse-pressure-gradient wing boundary layers2021Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 6, nr 11, artikel-id 113904Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A detailed analysis of the effects of uniform blowing, uniform suction, and body-force damping on the turbulent boundary layer developing around a NACA4412 airfoil at moderate Reynolds number is presented. The flow over the suction and the pressure sides of the airfoil is subjected to a nonuniform adverse pressure gradient and a moderate favorable pressure gradient, respectively. We find that the changes in total skin friction due to blowing and suction are not very sensitive to different pressure-gradient conditions or the Reynolds number. However, when blowing and suction are applied to an adverse-pressure-gradient (APG) boundary layer, their impact on properties such as the boundary-layer thickness, the intensity of the wall-normal convection, and turbulent fluctuations are more pronounced. We employ the Fukagata-Iwamoto-Kasagi decomposition [K. Fukagata et al., Phys. Fluids 14, 73 (2002)] and spectral analysis to study the interaction between intense adverse pressure gradient and these control strategies. We find that the control modifies skin-friction contributions differently in adverse-pressure-gradient and zero-pressure-gradient boundary layers. In particular, the control strategies modify considerably both the streamwisedevelopment and the pressure-gradient contributions, which have high magnitude when a strong adverse pressure gradient is present. Blowing and suction also impact the convection of structures in the wall-normal direction. Overall, our results suggest that it is not possible to simply separate pressure-gradient and control effects, a fact to take into account in future studies on control design in practical applications.

  • 6.
    Banaei, Arash Alizad
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik.
    Shahmardi, Armin
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik.
    Numerical study of suspensions of nucleated capsules at finite inertia2021Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 6, nr 4, artikel-id 044301Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the rheology of suspensions of capsules with a rigid nucleus at negligible and finite flow inertia by means of numerical simulations. The capsule membrane is modeled as a thin Neo-Hookean hyperelastic material and the nucleus as a rigid particle with radius equal to half the radius of the undeformed spherical capsules. The fluid and solid motion are coupled with an immersed boundary method, validated for both the deformable membrane and the rigid nucleus. We examine the effect of the Reynolds number, capillary number, and volume fraction on the macroscopic properties of the suspensions, comparing with the case of capsules without nuclei. To explain the rheological measurables, we examine the mean capsule deformation, the mean orientation with respect to the flow direction, and the stress budget. The results indicate that the relative viscosity decreases with the capillary number, i.e., increasing deformability, and increases with inertia. The presence of a nucleus always reduces the membrane deformation. Capsules align more in the flow direction at higher capillary numbers and at higher volume fractions, where we also see a significant portion of them oriented with their longer deformed axis in the spanwise direction. When increasing inertia, the alignment with the flow decreases while more capsules orient in the spanwise direction. The first normal stress difference increases with the capillary number and it is always less for the nucleated capsules. Finally, the relative viscosity and the first normal stress difference increase with the capsule volume fraction, an effect more pronounced for the first normal stress difference.

  • 7.
    Borthakur, Manash Pratim
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik.
    Nath, Binita
    CNR, Inst Complex Syst, I-00185 Rome, Italy..
    Biswas, Gautam
    Indian Inst Technol Kanpur, Dept Mech Engn, Kanpur 208016, Uttar Pradesh, India..
    Dynamics of a compound droplet under the combined influence of electric field and shear flow2021Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 6, nr 2, artikel-id 023603Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a numerical investigation on the dynamics of a compound droplet under the combined influence of an applied electric field and shear flow. The paper is carried out by solving the electro-hydrodynamic equations in a two-dimensional framework, and the interface is captured using a volume-of-fluid approach. Both perfect dielectric as well as leaky dielectric fluids are considered. For the case of dielectric fluids, the deformation of both the inner and outer interfaces can be modulated by either variation of the permittivity contrast between the fluids or the applied field strength. The nature of the polarization forces acting at both the interfaces can be either compressive or tensile depending on the magnitude of the permittivity ratio. The investigations for leaky dielectric fluids reveal that the ratio of electrical permittivity and conductivity between the two phases plays a critical role in deciding the magnitude of deformation and orientation of the compound droplet. The variation of charge accumulated at the interfaces modifies the behavior of the Coulombic forces thereby fundamentally altering the droplet deformation and orientation characteristics. Furthermore, it is demonstrated that the electric field can be suitably applied to engender breakup of the compound droplets.

  • 8.
    Brandenburg, Axel
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Kahniashvili, Tina
    Carnegie Mellon Univ, McWilliams Ctr Cosmol, 5000 Forbes Ave, Pittsburgh, PA 15213 USA.;Carnegie Mellon Univ, Dept Phys, 5000 Forbes Ave, Pittsburgh, PA 15213 USA.;Laurentian Univ, Dept Phys, Ramsey Lake Rd, Sudbury, ON P3E 2C, Canada.;Ilia State Univ, Abastumani Astrophys Observ, 3-5 Cholokashvili St, Tbilisi 0194, Rep of Georgia..
    Mandal, Sayan
    Carnegie Mellon Univ, McWilliams Ctr Cosmol, 5000 Forbes Ave, Pittsburgh, PA 15213 USA.;Carnegie Mellon Univ, Dept Phys, 5000 Forbes Ave, Pittsburgh, PA 15213 USA..
    Pol, Alberto Roper
    Univ Colorado, Lab Atmospher & Space Phys, Boulder, CO 80303 USA.;Univ Colorado, Dept Aerosp Engn Sci, Boulder, CO 80303 USA..
    Tevzadze, Alexander G.
    Carnegie Mellon Univ, McWilliams Ctr Cosmol, 5000 Forbes Ave, Pittsburgh, PA 15213 USA.;Carnegie Mellon Univ, Dept Phys, 5000 Forbes Ave, Pittsburgh, PA 15213 USA.;Ilia State Univ, Abastumani Astrophys Observ, 3-5 Cholokashvili St, Tbilisi 0194, Rep of Georgia.;Ivane Javakhishvili Tbilisi State Univ, Fac Exact & Nat Sci, 3 Chavchavadze Ave, Tbilisi 0179, Rep of Georgia..
    Vachaspati, Tanmay
    Arizona State Univ, Dept Phys, Tempe, AZ 85287 USA..
    Dynamo effect in decaying helical turbulence2019Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, nr 2, artikel-id 024608Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We show that in decaying hydromagnetic turbulence with initial kinetic helicity, a weak magnetic field eventually becomes fully helical. The sign of magnetic helicity is opposite to that of the kinetic helicity-regardless of whether the initial magnetic field was helical. The magnetic field undergoes inverse cascading with the magnetic energy decaying approximately like t(-1/2). This is even slower than in the fully helical case, where it decays like t(-2/3). In this parameter range, the product of magnetic energy and correlation length raised to a certain power slightly larger than unity is approximately constant. This scaling of magnetic energy persists over long timescales. At very late times and for domain sizes large enough to accommodate the growing spatial scales, we expect a crossover to the t(-2/3) decay law that is commonly observed for fully helical magnetic fields. Regardless of the presence or absence of initial kinetic helicity, the magnetic field experiences exponential growth during the first few turnover times, which is suggestive of small-scale dynamo action. Our results have applications to a wide range of experimental dynamos and astrophysical time-dependent plasmas, including primordial turbulence in the early universe.

  • 9.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Influence of spanwise rotation and scalar boundary conditions on passive scalar transport in turbulent channel flow2019Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, nr 1, artikel-id 014602Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Direct numerical simulations of passive scalar transport in turbulent channel flow subject to spanwise rotation are carried out with two different boundary conditions for the scalar. In the first case the scalar transport is driven by an assigned scalar difference at the walls and in the second case by a constant mean streamwise scalar gradient. The Reynolds number Re = U(b)h/nu is fixed at 14 000 and the rotation number Ro = 2 Omega h/U-b is varied from 0 to 0.75, where U-b is the mean bulk velocity, h half the channel gap width, and Omega the rotation rate. This work is a continuation of Brethouwer [J. Fluid Mech. 844, 297 ( 2018)] to further study the influence of rotation and also the influence of scalar boundary conditions on scalar transport in channel flow. Mean scalar profiles and other scalar statistics differ in the two cases with different boundary conditions but are similar in the near-wall region in terms of local wall units. The conclusion of Brethouwer that the Reynolds analogy for scalar-momentum transfer does not apply to rotating channel flow is independent of scalar boundary conditions. Rotation influences the turbulent scalar flux differently than the Reynolds shear stress and strongly reduces the turbulent Prandtl number on the unstable channel side, irrespective of the scalar boundary conditions. Scalar structures are larger than the turbulence structures in rotating channel flow, in contrast to nonrotating channel flow where these are similar.

  • 10.
    Canton, Jacopo
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Chin, Cheng
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Reynolds number dependence of large-scale friction control in turbulent channel flow2016Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 1, nr 8, artikel-id 081501Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The present work investigates the effectiveness of the control strategy introduced by Schoppa and Hussain [Phys. Fluids 10, 1049 (1998)] as a function of Reynolds number (Re). The skin-friction drag reduction method proposed by these authors, consisting of streamwise-invariant, counter-rotating vortices, was analyzed by Canton et al. [Flow, Turbul. Combust. 97, 811 (2016)] in turbulent channel flows for friction Reynolds numbers (Re t) corresponding to the value of the original study (i.e., 104) and 180. For these Re, a slightly modified version of the method proved to be successful and was capable of providing a drag reduction of up to 18%. The present study analyzes the Reynolds number dependence of this drag-reducing strategy by performing two sets of direct numerical simulations (DNS) for Re-tau = 360 and 550. A detailed analysis of the method as a function of the control parameters (amplitude and wavelength) and Re confirms, on the one hand, the effectiveness of the large-scale vortices at low Re and, on the other hand, the decreasing and finally vanishing effectiveness of this method for higher Re. In particular, no drag reduction can be achieved for Re t = 550 for any combination of the parameters controlling the vortices. For low Reynolds numbers, the large-scale vortices are able to affect the near-wall cycle and alter the wall-shear-stress distribution to cause an overall drag reduction effect, in accordance with most control strategies. For higher Re, instead, the present method fails to penetrate the near-wall region and cannot induce the spanwise velocity variation observed in other more established control strategies, which focus on the near-wall cycle. Despite the negative outcome, the present results demonstrate the shortcomings of the control strategy and show that future focus should be on methods that directly target the near-wall region or other suitable alternatives.

  • 11. Chan, C. , I
    et al.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Chin, R. C.
    Univ Adelaide, Sch Mech Engn, Adelaide, SA 5005, Australia..
    Large-scale and small-scale contribution to the skin friction reduction in a modified turbulent boundary layer by a large-eddy break-up device2022Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 7, nr 3, artikel-id 034601Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The role of streamwise length scales (lambda x) in turbulent skin friction generation is investigated using a direct numerical simulation data set of an incompressible zero pressure gradient turbulent boundary layer and the spectral analysis based on the FukagataL73 (2002)]. The total skin friction generation associated with motions scaled with local boundary layer thickness delta of lambda x 3 delta and lambda x 3 delta) contribute to a significant portion of turbulent skin friction. However, it is found that the large-scale ejection and sweep events with streamwise length scales at lambda x 3 delta are equally important. The turbulent skin friction reduction associated with the modification of largeand small-scale quadrant events is studied, using well-resolved simulation data sets of a large-eddy break-up (LEBU) device in a turbulent boundary layer. The results reveal that LEBUs modify both the large- and small-scale ejection and sweep events, yielding a substantial turbulent skin friction reduction.

  • 12.
    Chicchiero, Claudio
    et al.
    Univ Pisa, Dipartimento Ingn Aerosp, I-56122 Pisa, Italy..
    Segalini, Antonio
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Camarri, Simone
    Univ Pisa, Dipartimento Ingn Aerosp, I-56122 Pisa, Italy..
    Triple-deck analysis of the steady flow over a rotating disk with surface roughness2021Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 6, nr 1, artikel-id 014103Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The effect of surface roughness on the steady laminar flow induced by a rotating disk submerged by fluid otherwise at rest is investigated here theoretically and numerically. A theory is proposed where a triple-deck analysis is applied leading to a fast evaluation of the steady-flow modification due to the rough surface. The theory assumes that the roughness is much smaller than the boundary-layer height and is characterized by a significantly longer length scale (slender roughness). Only the leading-order correction is developed here, corresponding to a velocity-field correction that is linear with the roughness height. The proposed theory neglects some curvature terms (here partially accounted by means of a stretching of the radial coordinate and of a scaling of the dependent variables). Numerical simulations performed with different roughness geometries (axisymmetric roughness, radial grooves, and localized bumps) have been used to validate the theory. Results indicate that the proposed theory leads to a good quantification of the flow modifications due to surface roughness at a very low computational cost. A demonstration of the capabilities of the theory is finally proposed where the statistical effects on the flow due to a random (but statistically known) roughness distributed on the surface of a rotating disk are characterized.

  • 13.
    Chin, R. C.
    et al.
    Univ Adelaide, Sch Mech Engn, Adelaide, SA 5005, Australia..
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Cardesa, J. , I
    Noorani, Azad
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Chong, M. S.
    Univ Melbourne, Dept Mech Engn, Melbourne, Vic 3010, Australia..
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Backflow events under the effect of secondary flow of Prandtl's first kind2020Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 5, nr 7, artikel-id 074606Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A study of the backflow events in the flow through a toroidal pipe at friction Reynolds number Re-tau approximate to 650 is performed and compared with the results in a straight turbulent pipe flow at Re-tau approximate to 500. The statistics and topological properties of the backflow events are analysed and discussed. Conditionally averaged flow fields in the vicinity of the backflow event are obtained, and the results for the torus show a similar streamwise wall-shear stress topology which varies considerably for the azimuthal wall-shear stress when compared to the pipe flow. In the region around the backflow events, critical points are observed. The comparison between the toroidal pipe and its straight counterpart also shows fewer backflow events and critical points in the torus. This is attributed to the secondary flow of Prandtl's first kind present in the toroidal pipe, which is responsible for the convection of momentum from the inner to the outer bend through the core of the pipe, and back from outer bend to the inner bend along the azimuthal direction. These results indicate that backflow events and critical points are genuine features of wall-bounded turbulence, and are not artefacts of specific boundary or inflow conditions in simulations and/or measurement uncertainties in experiments.

  • 14.
    Deshpande, Rahul
    et al.
    Department of Mechanical Engineering, University of Melbourne, Parkville, Victoria 3010, Australia.
    Van Den Bogaard, Aron
    Department of Mechanical Engineering, University of Melbourne, Parkville, Victoria 3010, Australia; Physics of Fluids Group, University of Twente, P.O. Box 217, 7500AE Enschede, Netherlands.
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Lindić, Luka
    Department of Mechanical Engineering, University of Melbourne, Parkville, Victoria 3010, Australia.
    Marusic, Ivan
    Department of Mechanical Engineering, University of Melbourne, Parkville, Victoria 3010, Australia.
    Reynolds-number effects on the outer region of adverse-pressure-gradient turbulent boundary layers2023Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 8, nr 12, artikel-id 124604Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the Reynolds-number effects on the outer region of moderate adverse-pressure-gradient (APG) turbulent boundary layers (TBLs) and find that their small-scale (viscous) energy reduces with increasing friction Reynolds number (Reτ). The trend is based on analyzing APG TBL data across 600≲Reτ≲7000 and contrasts with the negligible variation in small viscous-scaled energy noted for canonical wall flows. The data sets considered include those from a well-resolved numerical simulation [Pozuelo, J. Fluid Mech. 939, A34 (2022)0022-112010.1017/jfm.2022.221], which provides access to an APG TBL maintained at near-equilibrium conditions across 1000≲Reτ≲ 2000, with a well-defined flow history, and a new high-Reτ (∼7000) experimental study from the large Melbourne wind tunnel, with its long test section modified to permit development of an APG TBL from a "canonical"upstream condition. The decrease in small-scale energy with Reτ is revealed via decomposing the streamwise normal stresses into small- and large-scale contributions, based on a sharp spectral cutoff. The origin for this trend is traced back to the production of turbulent kinetic energy in an APG TBL, the small-scale contribution to which is also found to decrease with Reτ in the outer region. The conclusion is reaffirmed by investigating attenuation of streamwise normal stresses due to changing spatial resolutions of the numerical grid or hotwire sensors, which reduces with increasing Reτ and is found to be negligible at Reτ∼7000 in this study. The results emphasize that new scaling arguments and spatial-resolution corrections should be tested rigorously across a broad Reτ range, particularly for pressure gradient TBLs.

  • 15.
    Dogan, Eda
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Hearst, R. Jason
    Univ Southampton, Engn & Environm, Southampton SO17 1BJ, Hants, England.;Norwegian Univ Sci & Technol, Dept Energy & Proc Engn, NO-7491 Trondheim, Norway..
    Hanson, Ronald E.
    Univ Southampton, Engn & Environm, Southampton SO17 1BJ, Hants, England.;York Univ, Dept Mech Engn, Toronto, ON M3J 1P3, Canada..
    Ganapathisubramani, Bharathram
    Univ Southampton, Engn & Environm, Southampton SO17 1BJ, Hants, England..
    Spatial characteristics of a zero-pressure-gradient turbulent boundary layer in the presence of free-stream turbulence2019Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, nr 8, artikel-id 084601Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Particle image velocimetry (PIV) measurements are performed to examine the structural organization inside a turbulent boundary layer under the influence of free-stream turbulence (FST). In particular, streamwise-wall-normal plane PIV measurements are presented for two cases at two different turbulent intensity levels (about 13% and 8%). The free-stream turbulence is generated using an active grid in a wind tunnel. The statistical information of the flow regarding the wall-normal velocity and Reynolds shear stress are presented. The effect of increasing the turbulence level in the free stream for these flows has been found to have similarities with increasing Reynolds number for high-Reynolds-number canonical flows. Quadrant analysis is performed to determine the contributions of different Reynolds-stress-producing events. In this regard, the distribution of momentum transport events shows some similarity with channel flows, which can be justified by comparison of similar intermittency characteristics of both flows. In addition, the coherent structures found inside the boundary layer have inclined features that are consistent with the previous studies for canonical flows. The fact that the external disturbance, such as FST in this study, does not alter the organization of the structures inside the boundary layer supports the growing evidence for a universal structure for wall-bounded flows.

  • 16.
    Fan, Yitong
    et al.
    Shanghai Jiao Tong Univ, Sch Aeronaut & Astronaut, Shanghai 200240, Peoples R China..
    Li, Weipeng
    Shanghai Jiao Tong Univ, Sch Aeronaut & Astronaut, Shanghai 200240, Peoples R China..
    Atzori, Marco
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Pozuelo, Ramon
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Decomposition of the mean friction drag in adverse-pressure-gradient turbulent boundary layers2020Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 5, nr 11, artikel-id 114608Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this study, we exploit the Renard-Deck identity [J. Fluid Mech. 790, 339 (2016)] to decompose the mean friction drag in adverse-pressure-gradient turbulent boundary layers (APG-TBLs) into three components, associated with viscous dissipation, turbulence kinetic energy production, and spatial growth of the flow, respectively. We consider adverse-pressure-gradient turbulent boundary layers developing on flat plates and airfoils, with friction Reynolds numbers in the range 200 < Re-tau < 2000, and with Rotta-Clauser pressure-gradient parameters (beta) ranging from 0 to 50. The effects of Reynolds number, adverse pressure gradient, and the pressure-gradient history on the contributing components are individually investigated, and special attention is paid to the comparisons with zero-pressure-gradient turbulent boundary layers (ZPG-TBLs). Our results indicate that the inner peaks of the dissipation and production terms are located at y(+) approximate to 6 and y(+) approximate to 16.5, respectively, and their outer peaks scale with the 99% boundary-layer thickness (delta(99)), i.e., y/delta(99) approximate to 0.7 and 0.53, respectively. These results are independent of the friction Reynolds number, the magnitude of beta, and its development history. Moreover, the spatial-growth component is negative in the investigated APG-TBLs, and its magnitude increases with beta.

  • 17.
    Feneuil, Blandine
    et al.
    Univ Oslo, Dept Math, Oslo, Norway.;SINTEF Ind Petr, Trondheim, Norway..
    Iqbal, Kazi Tassawar
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik, Tillämpad strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Jensen, Atle
    Univ Oslo, Dept Math, Oslo, Norway..
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. NTNU, Dept Energy & Proc Engn, Trondheim, Norway..
    Tammisola, Outi
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Carlson, Andreas
    Univ Oslo, Dept Math, Oslo, Norway..
    Experimental and numerical investigation of bubble migration in shear flow: Deformability-driven chaining and repulsion2023Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 8, nr 6, artikel-id 063602Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the interaction-induced migration of bubbles in shear flow and observe that bubbles suspended in elastoviscoplastic emulsions organize into chains aligned in the flow direction, similarly to particles in viscoelastic fluids. To investigate the driving mechanism, we perform experiments and simulations on bubble pairs, using suspending fluids with different rheological properties. First, we notice that, for all fluids, the interaction type depends on the relative position of the bubbles. If they are aligned in the vorticity direction, then they repel, if not, then they attract each other. The simulations show a similar behavior in Newtonian fluids as in viscoelastic and elastoviscoplastic fluids, as long as the capillary number is sufficiently large. This shows that the interaction-related migration of the bubbles is strongly affected by the bubble deformation. We suggest that the cause of migration is the interaction between the heterogeneous pressure fields around the deformed bubbles, due to capillary pressure.

  • 18.
    Forooghi, Pourya
    et al.
    Karlsruhe Inst Technol, Inst Fluid Mech, D-76131 Karlsruhe, Germany..
    Stroh, Alexander
    Karlsruhe Inst Technol, Inst Fluid Mech, D-76131 Karlsruhe, Germany..
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Frohnapfel, Bettina
    Karlsruhe Inst Technol, Inst Fluid Mech, D-76131 Karlsruhe, Germany..
    Direct numerical simulation of flow over dissimilar, randomly distributed roughness elements: A systematic study on the effect of surface morphology on turbulence2018Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 3, nr 4, artikel-id 044605Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Direct numerical simulations are used to investigate turbulent flow in rough channels, in which topographical parameters of the rough wall are systematically varied at a fixed friction Reynolds number of 500, based on a mean channel half-height h and friction velocity. The utilized roughness generation approach allows independent variation of moments of the surface height probability distribution function [thus root-mean-square (rms) surface height, skewness, and kurtosis], surface mean slope, and standard deviation of the roughness peak sizes. Particular attention is paid to the effect of the parameter Delta defined as the normalized height difference between the highest and lowest roughness peaks. This parameter is used to understand the trends of the investigated flow variables with departure from the idealized case where all roughness elements have the same height (Delta = 0). All calculations are done in the fully rough regime and for surfaces with high slope (effective slope equal to 0.6-0.9). The rms roughness height is fixed for all cases at 0.045h and the skewness and kurtosis of the surface height probability density function vary in the ranges -0.33 to 0.67 and 1.9 to 2.6, respectively. The goal of the paper is twofold: first, to investigate the possible effect of topographical parameters on the mean turbulent flow, Reynolds, and dispersive stresses particularly in the vicinity of the roughness crest, and second, to investigate the possibility of using the wall-normal turbulence intensity as a physical parameter for parametrization of the flow. Such a possibility, already suggested for regular roughness in the literature, is here extended to irregular roughness.

  • 19.
    Fouxon, Itzhak
    et al.
    Technion, Dept Chem Engn, IL-32000 Haifa, Israel.;Yonsei Univ, Dept Computat Sci & Engn, Seoul 120749, South Korea..
    Rubinstein, Boris
    Stowers Inst Med Res, 1000 E 50th St, Kansas City, MO 64110 USA..
    Ge, Zhouyang
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Leshansky, Alexander
    Technion, Dept Chem Engn, IL-32000 Haifa, Israel..
    Theory of hydrodynamic interaction of two spheres in wall-bounded shear flow2020Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 5, nr 5, artikel-id 054101Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The seminal Batchelor-Green's (BG) theory on the hydrodynamic interaction of two spherical particles of radii a suspended in a viscous shear flow assumes unbounded fluid. In the present paper we study how a rigid plane wall modifies this interaction. Using an integral equation for the surface traction we derive the expression for the particles' relative velocity as a sum of the BG's velocity and the term due to the presence of a wall at finite distance, z(0). Our calculation is not the perturbation theory of the BG solution, so the contribution due to the wall is not necessarily small. We indeed demonstrate that the presence of the wall is a singular perturbation, i.e., its effect cannot be neglected even at large distances. The distance at which the wall significantly alters the particles interaction scales as z(0)(3/5). The phase portrait of the particles' relative motion is different from the BG theory, where there are two singly connected regions of open and closed trajectories both of infinite volume. For finite z(0), besides the BG's domains of open and closed trajectories, there is a domain of closed (dancing) and open (swapping) trajectories that do not materialize in an unbounded shear flow. The width of this region grows as 1/z(0) at smaller separations from the wall. Along the swapping trajectories, which have been previously observed numerically, the incoming particle is turning back after the encounter with the reference particle, rather than passing it by, as the BG theory anticipates. The region of dancing trajectories has infinite volume and is separated from a BG-type domain of closed trajectories that becomes compact due to presence of the wall. We found a one-parameter family of equilibrium states that were previously overlooked, whereas the pair of spheres flows as a whole without changing its configuration. These states are marginally stable and their perturbation yields a two-parameter family of the dancing trajectories, whereas the test particle is orbiting around a fixed point in a frame comoving with the reference particle. We suggest that the phase portrait obtained at z(0) >> a is topologically stable and can be extended down to rather small z(0) of several particle diameters. We confirm this hypothesis by direct numerical simulations of the Navier-Stokes equations with z(0) = 5a. Qualitatively the distant wall is the third body that changes the global topology of the phase portrait of two-particle interaction.

  • 20.
    Ge, Zhouyang
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Holmgren, Hanna
    Uppsala Univ, Dept Informat Technol, Div Sci Comp, Box 337, S-75105 Uppsala, Sweden..
    Kronbichler, Martin
    Tech Univ Munich, Inst Computat Mech, Boltzmannstr 15, D-85748 Garching, Germany..
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Kreiss, Gunilla
    Uppsala Univ, Dept Informat Technol, Div Sci Comp, Box 337, S-75105 Uppsala, Sweden..
    Effective slip over partially filled microcavities and its possible failure2018Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 3, nr 5, artikel-id 054201Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Motivated by the emerging applications of liquid-infused surfaces (LIS), we study the drag reduction and robustness of transverse flows over two-dimensional microcavities partially filled with an oily lubricant. Using separate simulations at different scales, characteristic contact line velocities at the fluid-solid intersection are first extracted from nanoscale phase field simulations and then applied to micronscale two-phase flows, thus introducing a multiscale numerical framework to model the interface displacement and deformation within the cavities. As we explore the various effects of the lubncant-toouter-fluid viscosity ratio A2/A0 th(mu)over tilde( )c(mu)over tilde(1), thary number Ca, the static contact angle A> and t theta(s), filling fraction of the cavity <5, we f delta d that the effective slip is most sensitive to the parameter S. The effects of A2/A1 an(mu)over tilde( )A(mu)over tilde(a )re ge theta(s)erally intertwined but weakened if <5 < 1. delta M 1er, for an initial filling fraction S = 0.94 delta our results show that the effective slip is nearly independent of the capillary number when it is small. Further increasing Ca to about O.OIA1/A20.01(mu)over tilde(1)/(mu)over tilde(2)ntify a possible failure mode, associated with lubricants draining from the LIS, for A2/A1 A (mu)over tilde(2)1(mu)over tilde(1)V less than or similar to y viscous lubricants (e.g., A2/A1 > (mu)over tilde()),(mu)over tilde(h)owever, are immune to such failure due to their generally larger contact line velocity.

  • 21.
    Ge, Zhouyang
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. Univ British Columbia, Dept Mech Engn, Vancouver, BC V6T 1Z4, Canada..
    Martone, Raffaella
    Univ Campania Luigi Vanvitelli, Dept Engn, Real Casa Annunziata, Via Roma 29, I-81031 Aversa, Italy..
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Minale, Mario
    Univ Campania Luigi Vanvitelli, Dept Engn, Real Casa Annunziata, Via Roma 29, I-81031 Aversa, Italy..
    Irreversibility and rate dependence in sheared adhesive suspensions2021Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 6, nr 10, artikel-id L101301Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recent experiments report that slowly sheared noncolloidal particle suspensions unexpectedly exhibit rate(omega)-dependent complex viscosities in oscillatory shear, despite a constant relative viscosity in steady shear. Using a minimal hydrodynamic model, we show that van der Waals attraction gives rise to this behavior. At volume fractions phi = 20-50%, the complex viscosities in both experiments and simulations display power-law reductions in shear, with a phi-dependent exponent maximum at phi = 40%, resulting from the interplay between hydrodynamic, collision, and adhesive interactions. Furthermore, this rate dependence is accompanied by diverging particle diffusivities and pronounced cluster formations after repeated oscillations.  Previous studies established that suspensions transition from reversible absorbing states to irreversible diffusing states when the oscillation amplitude exceeds a ϕ-dependent critical value γc0,ϕ. Here, we show that a second transition to irreversibility occurs below an ω-dependent critical amplitude, γc0,ω≤γc0,ϕ, in the presence of weak attractions.

  • 22.
    Gowda, V. Krishne
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Rydefalk, Cecilia
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Söderberg, Daniel
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Fiber- och polymerteknologi, Fiberteknologi. KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Centra, Wallenberg Wood Science Center.
    Lundell, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Centra, Wallenberg Wood Science Center.
    Formation of colloidal threads in geometrically varying flow-focusing channels2021Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 6, nr 11, artikel-id 114001Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Threads of colloidal dispersions can be formed in microfluidic channel systems and are often used for analytical purposes or to assemble macroscopic structures from colloidal particles. Here, we report a combined experimental and numerical study of thread formation in channel systems with varying geometry. In the reference flow-focusing configuration, the sheath flows impinge the core flow orthogonally while in four other channel configurations, the sheath flows impinge the core flow at different confluence angles, which are both positive and negative with respect to the reference sheath direction. Tomographic measurements of the thread development are made using optical coherence tomography (OCT) and are compared to numerically simulated 3D data. The numerical simulations performed with an immiscible fluid solver show good agreement with the experiments in terms of 3D thread shapes, wetted region morphologies, and velocity fields provided an ultralow interfacial tension is applied between the low viscosity (solvent) sheath flows and the high viscosity (dispersion) core flow. Such an ultralow interfacial tension is motivated by the so-called Korteweg stresses induced as a result of the concentration gradient between two miscible fluids in nonequilibrium state. These stresses mimic the effect of interfacial tension, and are often modeled as an effective interfacial tension (EIT), an approach chosen in the present work as well. The value of interfacial tension applied in the simulations was determined through an optimization procedure and compares well with a value deduced from a scaling analysis utilizing the downstream development of experimentally determined thread shape. The experimental and numerical results show that for channel configurations with modest deviations from orthogonal sheath flows, the effect on the thread is similar regardless of whether the sheath flows are co- or counterflowing the core flow. In fact, for these cases, the effect of co- and counterflowing sheath flows can be reproduced with orthogonal sheath flows, if the sheath channel width is increased. However, for channel configurations with larger deviations from orthogonal sheath flows, the effects on the thread are direction dependent. The one-to-one comparison and analysis of numerical and experimental results bring useful insights to understand the behavior of miscible systems involving high-viscosity contrast fluids. These key results provide the foundation to tune the flow-focusing for specific applications, for example in tailoring the assembly of nanostruc-tured materials.

  • 23.
    Hu, Ruifeng
    et al.
    Lanzhou Univ, Ctr Particle Laden Turbulence, Key Lab Mech Disaster & Environm Western China, Minist Educ, Lanzhou 730000, Peoples R China.;Lanzhou Univ, Coll Civil Engn & Mech, Lanzhou 730000, Peoples R China..
    Dong, Siwei
    State Key Lab Aerodynam, Mianyang 621000, Peoples R China..
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    General attached eddies: Scaling laws and cascade self-similarity2023Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 8, nr 4, artikel-id 044603Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The attached-eddy model (AEM) is one of the most successful coherent-structure-based phenomenological models in wall turbulence. In the classical AEM, the probability density of eddy pe is assumed to follow an inverse law with the eddy size he, i.e., pe oc h-1 e , to satisfy a constant Reynolds-shear-stress distribution in the inertial layer of canonical wall-bounded turbulent flows. In this paper, we first extend the AEM to general attached eddies with pe oc h-" e , where " is an arbitrary positive real number. Scaling laws for velocity covariance (Reynolds stress) by general attached eddies are derived. Preliminary evidence for the validity of the model is provided from adverse-pressure-gradient turbulent boundary layer and turbulent wing boundary layer flows. Second, considering that the eddy cascade self-similarity is manifested by generalized power laws for probability density pe, population density Me, area coverage Ce, and volume fraction Ve of eddies, i.e., pe oc h-"e, Me oc h-ae , Ce oc h-?e , and Ve oc h-zeta e, we directly connect the exponents with the fractal dimension De of the general attached eddies in a simple and clear way. The present paper highlights that the scaling laws of velocity covariance in the inertial layer of wall-bounded turbulent flows can be directly linked to the characteristics of the cascade self-similarity of the general attached eddies. We believe that the scaling laws derived here and the generalized power-law relationships are useful for a deeper understanding of the connection between coherent structures and turbulence statistics.

  • 24.
    Izbassarov, Daulet
    et al.
    Aalto Univ, Dept Mech Engn, FI-00076 Aalto, Finland..
    Ahmed, Zaheer
    Mehran Univ Engn & Technol, Dept Mech Engn, SZAB Campus, Khairpur Mirs 66020, Sindh, Pakistan..
    Costa, Pedro
    Univ Iceland, Fac Ind Engn Mech Engn & Comp Sci, Hjardarhagi 2-6, IS-107 Reykjavik, Iceland..
    Vuorinen, Ville
    Aalto Univ, Dept Mech Engn, FI-00076 Aalto, Finland..
    Tammisola, Outi
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Muradoglu, Metin
    Koc Univ, Dept Mech Engn, TR-34450 Istanbul, Turkey..
    Polymer drag reduction in surfactant-contaminated turbulent bubbly channel flows2021Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 6, nr 10, artikel-id 104302Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Polymer additives are commonly utilized to manipulate bubbly flows in various applications. Here we investigate the effects of clean and contaminated bubbles driven upward (upflow) in Newtonian and viscoelastic turbulent channel flows. Interface-resolved direct numerical simulations are performed to examine sole and combined effects of soluble surfactant and viscoelasticity using an efficient three-dimensional finite-difference-front-tracking method. The incompressible flow equations are solved fully coupled with the FENE-P viscoelastic model and the equations governing interfacial and bulk surfactant concentrations. The latter coupling is accomplished by a nonlinear equation of state that relates the surface tension to the surfactant concentration. For Newtonian turbulent bubbly flows, the effects of Triton X-100 and 1-pentanol surfactant are examined. It is observed that the sorption kinetics highly affect the dynamics of bubbly flow. A minute amount of Triton X-100 is found to be sufficient to prevent the formation of bubble clusters restoring the single-phase behavior while even two orders of magnitude more 1-pentanol surfactant is not adequate to prevent the formation of layers. For viscoelastic turbulent flows, it is found that the viscoelasticity promotes formation of the bubble wall-layers and thus the polymer drag reduction is completely lost for the surfactant-free bubbly flows, while the addition of small amount of surfactant (Triton X-100) in this system restores the polymer drag reduction resulting in 25% drag reduction for the Wi = 4 case.

  • 25.
    Izbassarov, Daulet
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Centra, SeRC - Swedish e-Science Research Centre. Aalto Univ, Dept Mech Engn, FI-00076 Espoo, Finland..
    Tammisola, Outi
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Dynamics of an elastoviscoplastic droplet in a Newtonian medium under shear flow2020Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 5, nr 11, artikel-id 113301Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The dynamics of a single elastoviscoplastic drop immersed in plane shear flow of a Newtonian fluid is studied by three-dimensional direct numerical simulations using a finite-difference and level-set method combined with the Saramito model for the elastoviscoplastic fluid. This model gives rise to a yield stress behavior, where the unyielded state of the material is described as a Kelvin-Voigt viscoelastic solid and the yielded state as a viscoelastic Oldroyd-B fluid. Yielding of an initially solid drop of Carbopol is simulated under successively increasing shear rates. We proceed to examine the roles of nondimensional parameters on the yielding process; in particular, the Bingham number, the capillary number, the Weissenberg number, and the ratio of solvent and total drop viscosity are varied. We find that all of these parameters, and not only the Bingham number, have a significant influence on the drop dynamics. Numerical simulations predict that the volume of the unyielded region inside the droplet increases with the Bingham number and the Weissenberg number, while it decreases with the capillary number at low Weissenberg and Bingham numbers. A new regime map is obtained for the prediction of the yielded, unyielded, and partly yielded modes as a function of the Bingham and Weissenberg numbers. The drop deformation is studied and explained by examining the stresses in the vicinity of the drop interface. The deformation has a complex dependence on the Bingham andWeissenberg numbers. At low Bingham numbers, the droplet deformation shows a nonmonotonic behavior with an increasing drop viscoelasticity. In contrast, at moderate and high Bingham numbers, droplet deformation always increases with drop viscoelasticity. Moreover, it is found that the deformation increases with the capillary number and with the solvent to total drop viscosity ratio. A simple ordinary differential equation model is developed to explain the various behaviours observed numerically. The presented results are in contrast with the heuristic idea that viscoelasticity in the dispersed phase always inhibits deformation.

  • 26.
    Johansson, Petter
    et al.
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Biofysik. KTH, Centra, Science for Life Laboratory, SciLifeLab.
    Hess, Berk
    KTH, Centra, Science for Life Laboratory, SciLifeLab. KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Biofysik. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Electrowetting diminishes contact line friction in molecular wetting2020Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 5, nr 6, artikel-id 064203Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We use large-scale molecular dynamics to study the dynamics at the three-phase contact line in electrowetting of water and electrolytes on no-slip substrates. Under the applied electrostatic potential the line friction at the contact line is diminished. The effect is consistent for droplets of different sizes as well as for both pure water and electrolyte solution droplets. We analyze the electric field at the contact line to show how it assists ions and dipolar molecules to advance the contact line. Without an electric field, the interaction between a substrate and a liquid has a very short range, mostly affecting the bottom, immobilized layer of liquid molecules which leads to high friction since mobile molecules are not pulled towards the surface. In electrowetting, the electric field attracts charged and polar molecules over a longer range, which diminishes the friction.

  • 27.
    Johansson, Petter
    et al.
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Biofysik.
    Hess, Berk
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Biofysik.
    Molecular origin of contact line friction in dynamic wetting2018Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 3, nr 7, artikel-id 074201Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A hydrophilic liquid, such as water, forms hydrogen bonds with a hydrophilic substrate. The strength and locality of the hydrogen bonding interactions prohibit slip of the liquid over the substrate. The question then arises how the contact line can advance during wetting. Using large-scale molecular dynamics simulations we show that the contact line advances by single molecules moving ahead of the contact line through two distinct processes: either moving over or displacing other liquid molecules. In both processes friction occurs at the molecular scale. We measure the energy dissipation at the contact line and show that it is of the same magnitude as the dissipation in the bulk of a droplet. The friction increases significantly as the contact angle decreases, which suggests suggests thermal activation plays a role. We provide a simple model that is consistent with the observations.

  • 28.
    Kamble, Chetna
    et al.
    Texas A&M Univ, Dept Ocean Engn, College Stn, TX 77843 USA..
    Girimaji, Sharath
    Texas A&M Univ, Dept Ocean Engn, College Stn, TX 77843 USA..
    Razi, Pooyan
    Gen Motors, Warren, MI 48092 USA..
    Tazraei, Pedram
    Convergent Sci, Madison, WI 53719 USA..
    Wallin, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik.
    Closure modeling in near-wall region of steep resolution variation for partially averaged Navier-Stokes simulations2022Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 7, nr 4, artikel-id 044608Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We seek to develop a closure model to enable scale-resolving simulation (SRS) of a turbulent flow to optimally switchover from a Reynolds-averaged Navier-Stokes (RANS) calculation at the wall to a specified degree of resolution in the wake or free-stream region. The closure model is derived by (i) using the physical principle that the total energy of resolved and unresolved scales should be conserved in the switchover region and (ii) establishing consistency with equilibrium boundary layer scaling of the partially resolved field. The model development is performed in the context of a partially averaged Navier-Stokes (PANS) scale-resolving method by quantifying and modeling the commutation terms resulting from varying resolutions in the wall-normal direction. The resulting wall-modeled PANS (WM-PANS) is used to compute the turbulent channel flow in the Re,. range 180 - 8000. The influence of the RANS-SRS switchover location on the computed flow field is examined. It is then demonstrated that the mean flow is reproduced with reasonable accuracy at modest computational effort without discernible log-layer mismatch even at the highest Reynolds number considered. While the Reynolds stresses are also recovered accurately over most of the flow domain, a noticeable computational transition from RANS to unsteady SRS flow behavior is observed and the underlying physics is examined. Irrespective of the location of computational transition, the unsteady features of the flow away from the wall are well captured. It is demonstrated that the proposed closure is able to inject the appropriate amount of resolved turbulence without the need for artificially generated synthetic turbulence. Overall, WM-PANS presents an accurate and computationally viable option for scale-resolving computations of near-wall high-Reynolds number flows.

  • 29.
    Kato, Kentaro
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Lingwood, Rebecca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Department of Mechanical and Aerospace Engineering, Brunel University London, Uxbridge, UB8 3PH, United Kingdom.
    Boundary-layer transition over a rotating broad cone2019Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, nr 7, artikel-id 071902Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The route to turbulence in the boundary layer on a rotating broad cone is investigated using hot-wire anemometry measuring the azimuthal velocity. The stationary fundamental mode is triggered by 24 deterministic small roughness elements distributed evenly at a specific distance from the cone apex. The stationary vortices, having a wave number of 24, correspond to the fundamental mode and these are initially the dominant disturbance-energy carrying structures. This mode is found to saturate and is followed by rapid growth of the nonstationary primary mode as well as the stationary and nonstationary first harmonics, leading to transition to turbulence. The amplitudes of these are plotted in a way to highlight the continued growth after saturation of the fundamental stationary mode.

  • 30.
    Kato, Kentaro
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Kawata, T.
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Lingwood, Rebecca
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Investigation of the structures in the unstable rotating-cone boundary layer2019Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, nr 5, artikel-id 053903Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This work reports on the unstable region and the transition process of the boundary-layer flow induced by a rotating cone with a half apex angle of 60 degrees using the probability density function (PDF) contour map of the azimuthal velocity fluctuation, which was first used by Imayama et al. [Phys. Fluids 24, 031701 (2012)] for the similar boundary-layer flow induced by a rotating disk. The PDF shows that the transition behavior of the rotating-cone flow is similar to that on the rotating disk. The effects of roughness elements on the cone surface have been examined. For the cone with roughnesses, we reconstructed the most probable vortex structure within the boundary layer from the hot-wire anemometry time signals. The results show that the PDF clearly describes the overturning process of the high-momentum upwelling of the spiral vortices, which due to vortex meandering cannot be detected in the phase-averaged velocity field reconstructed from the point measurements. At a late stage of the overturning process, our hot-wire measurements captured high-frequency oscillations, which may be related to secondary instability.

  • 31.
    Kern, J. Simon
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik, Turbulent simulations laboratory.
    Lupi, Valerio
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik, Turbulent simulations laboratory.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik, Turbulent simulations laboratory.
    Floquet stability analysis of pulsatile flow in toroidal pipes2024Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 9, nr 4, artikel-id 043906Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The linear temporal stability of the fully developed pulsatile flow in a torus with high curvature is investigated using Floquet theory. The baseflow is computed via a Newton-Raphson iteration in frequency space to obtain basic states at supercritical Reynolds numbers in the steady case for two curvatures, δ=0.1 and 0.3, exhibiting structurally different linear instabilities for the steady flow. The addition of a pulsatile component is found to be overall stabilizing over a wide range of pulsation amplitudes, in particular for high values of curvature. The pulsatile flows are found to be at most transiently stable with large intracyclic growth rate variations even at small pulsation amplitudes. While these growth rates are likely insufficient to trigger an abrupt transition at the parameters in this work, the trends indicate that this is indeed likely for higher pulsation amplitudes, similar to pulsatile flow in straight pipes. At the edge of the considered parameter range, subharmonic eigenvalue orbits in the local spectrum of the time-periodic operator, recently found in pulsating channel flow, have been confirmed also for pulsatile flow in toroidal pipes, underlining the generality of this phenomenon.

  • 32.
    Khan, Monsurul
    et al.
    Purdue Univ, Dept Mech Engn, Indiana, PA 47905 USA..
    More, Rishabh V.
    Purdue Univ, Dept Mech Engn, Indiana, PA 47905 USA.;MIT, Dept Mech Engn, Cambridge, MA 02139 USA..
    Banaei, Arash Alizad
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik. KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Ardekani, Arezoo M.
    Purdue Univ, Dept Mech Engn, Indiana, PA 47905 USA..
    Rheology of concentrated fiber suspensions with a load-dependent friction coefficient2023Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 8, nr 4, artikel-id 044301Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate the effects of fiber aspect ratio, roughness, flexibility, and volume fraction on the rheology of concentrated suspensions in a steady shear flow using direct numerical simulations. We model the fibers as inextensible continuous flexible slender bodies with the Euler-Bernoulli beam equation governing their dynamics suspended in an incompressible Newtonian fluid. The fiber dynamics and fluid flow coupling is achieved using the immersed boundary method. In addition, the fiber surface roughness might lead to interfiber contacts, resulting in normal and tangential forces between the fibers, which follow Coulomb's law of friction. The surface roughness is modeled as hemispherical pro-trusions on the fiber surfaces. Their deformation results in a normal load-dependent friction coefficient. Our simulations accurately predict the experimentally observed shear thinning in fiber suspensions. Furthermore, we find that the suspension viscosity eta increases with increasing the volume fraction, roughness, fiber rigidity, and aspect ratio. The increase in eta is the macroscopic manifestation of a similar increase in the microscopic contact contribution to the total stress with these parameters. In addition, we observe positive and negative first N1 and second N2 normal stress differences, respectively, with |N2| < |N1|, in agreement with previous experiments. Last, we propose a modified Maron-Pierce law to quantify the reduction in the jamming volume fraction by increasing the fiber aspect ratio and roughness. Our results and analysis establish the use of fiber surface tribology to tune the suspension flow behavior.

  • 33.
    Khan, Monsurul
    et al.
    Department of Mechanical Engineering, Purdue University, Indiana 47905, USA.
    More, Rishabh V.
    Department of Mechanical Engineering, Purdue University, Indiana 47905, USA.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Ardekani, Arezoo M.
    Department of Mechanical Engineering, Purdue University, Indiana 47905, USA.
    Rheology of dense fiber suspensions: Origin of yield stress, shear thinning, and normal stress differences2023Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 8, nr 6, artikel-id 064306Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We explain the origins of yield stress, shear thinning, and normal stress differences in rigid fiber suspensions. We investigate the interplay between the hydrodynamic, noncontact attractive and repulsive, and interfiber contact interactions. The shear-thinning viscosity and finite yield stress obtained from the Immerse Boundary Method simulations are in quantitative agreement with experiential results from the literature. In this study, we show that attractive interactions result in yield stress and shear thinning rheology in the suspensions of rigid fibers. This is an important finding, given the ongoing discussion regarding the origin of the yield stress for suspensions of fibers. The ability of the proposed model to quantitatively predict the rheology is not limited to only shear thinning and yield stress but also extends to normal stresses.

  • 34.
    Kozul, Melissa
    et al.
    NTNU, Dept Energy & Proc Engn, N-7491 Trondheim, Norway..
    Costa, Pedro S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre. Univ Iceland, Fac Ind Engn Mech Engn & Comp Sci, IS-107 Reykjavik, Iceland.
    Dawson, James R.
    NTNU, Dept Energy & Proc Engn, N-7491 Trondheim, Norway..
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. NTNU, Dept Energy & Proc Engn, N-7491 Trondheim, Norway.
    Aerodynamically driven rupture of a liquid film by turbulent shear flow2020Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 5, nr 12, artikel-id 124302Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The rupture of a liquid film due to coflowing turbulent shear flows in the gas phase is studied using a volume-of-fluid method. To simulate this multiphase problem, we use a simplified numerical setup where the liquid film is "sandwiched" between two fully developed boundary layers from a turbulent channel simulation. The film deforms and eventually ruptures within the shear zone created by the coflows. This efficient setup allows systematic variation of physical parameters to gauge their role in the aerodynamically driven deformation and rupture of a liquid film under fully developed sheared turbulence. This work presents a detailed study of the developing pressure field over the deforming film and related aerodynamic effects, as previously suggested by other authors, in particular the role of the inviscid lift and drag forces. A cumulative lift force is introduced to capture the effect of the alternating pressure minima and maxima forming over the film, which amplify and eventually rupture the film. A velocity scale derived from the lift-induced drag force reflects the state of the turbulent boundary layer over the film and collapses the temporal development of this cumulative lift force as well as the amplitude of film deformation with some success for the different film thicknesses and Reynolds numbers.

  • 35. Kurzthaler, Christina
    et al.
    Zhu, Lailai
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Pahlavan, Amir A.
    Stone, Howard A.
    Particle motion nearby rough surfaces2020Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 5, nr 8, artikel-id 082101Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the hydrodynamic coupling between particles and solid, rough boundaries characterized by random surface textures. Using the Lorentz reciprocal theorem, we derive analytical expressions for the grand mobility tensor of a spherical particle and find that roughness-induced velocities vary nonmonotonically with the characteristic wavelength of the surface. In contrast to sedimentation near a planar wall, our theory predicts continuous particle translation transverse and perpendicular to the applied force. Most prominently, this motion manifests itself in a variance of particle displacements that grows quadratically in time along the direction of the force. This increase is rationalized by surface roughness generating particle sedimentation closer to or farther from the surface, which entails a significant variability of settling velocities.

  • 36.
    Kvick, Mathias
    et al.
    Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia, Canada.
    Martinez, D. Mark
    Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia, Canada.
    Hewitt, Duncan R.
    Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 0WA, United Kingdom.
    Balmforth, Neil J.
    Department of Mathematics, University of British Columbia, Vancouver, British Columbia, Canada.
    Imbibition with swelling: Capillary rise in thin deformable porous media2017Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 2, nr 7Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The imbibition of a liquid into a thin deformable porous substrate driven by capillary suction is considered. The substrate is initially dry and has uniform porosity and thickness. Two-phase flow theory is used to describe how the liquid flows through the pore space behind the wetting front when out-of-plane deformation of the solid matrix is considered. Neglecting gravity and evaporation, standard shallow-layer scalings are used to construct a reduced model of the dynamics. The model predicts convergence to a self-similar behavior in all regions except near the wetting front, where a boundary layer arises whose structure narrows with the advance of the front. Over time, the rise height approaches the similarity scaling of t1/2, as in the classical Washburn or BCLW law. The results are compared with a series of laboratory experiments using cellulose paper sheets, which provide qualitative agreement.

  • 37.
    Lacagnina, Giovanni
    et al.
    University of Southampton.
    Chaitanya, Paruchuri
    University of Southampton.
    Berk, Tim
    University of Southampton.
    Kim, Jung-Hoon
    University of Nottingham.
    Joseph, Phillip
    University of Southampton.
    Ganapathisubramani, Bharathram
    University of Southampton.
    Hasheminejad, Seyed Mohammad
    Brunel University London.
    Chong, Tze Pei
    Brunel University London.
    Stalnov, Oksana
    Technion - Israel Institute of Technology.
    Choi, Kwing-So
    University of Nottingham.
    Mechanisms of airfoil noise near stall conditions2019Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, nr 12Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The focus of this paper is on investigating the noise produced by an airfoil at high angles of attack over a range of Reynolds number Re≈ 2× 10^5 – 4× 10^5. The objective is not modeling this source of noise but rather understanding the mechanisms of generation for surface pressure fluctuations, due to a separated boundary layer, that are then scattered by the trailing edge. To this aim, we use simultaneous noise and surface pressure measurement in addition to velocimetric measurements by means of hot wire anemometry and time-resolved particle image velocimetry. Three possible mechanisms for the so-called “separation-stall noise” have been identified in addition to a clear link between far-field noise, surface pressure, and velocity fields in the noise generation.

  • 38.
    Lacis, Ugis
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll.
    Olivieri, Stefano
    Mazzino, Andrea
    Bagheri, Shervin
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll.
    Passive control of a falling sphere by elliptic-shaped appendages2017Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 2, artikel-id 033901Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The majority of investigations characterizing the motion of single or multiple particles in fluid flows consider canonical body shapes, such as spheres, cylinders, discs, etc. However, protrusions on bodies – being either as surface imperfections or appendages that serve a function – are ubiquitous in both nature and applications. In this work, we characterize how the dynamics of a sphere with an axis-symmetric wake is modified in the presence of thin three-dimensional elliptic-shaped protrusions. By investigating a wide range of three-dimensional appendages with different aspect ratios and lengths, we clearly show that the sphere with an appendage may robustly undergo an inverted-pendulum-like (IPL) instability. This means that the position of the appendage placed behind the sphere and aligned with the free-stream direction is unstable, in a similar way that an inverted pendulum is unstable under gravity. Due to this instability, non-trivial forces are generated on the body, leading to turn and drift, if the body is free to fall under gravity. Moreover, we identify the aspect ratio and length of the appendage that induces the largest side force on the sphere, and therefore also the largest drift for a freely falling body. Finally, we explain the physical mechanisms behind these observations in the context of the IPL instability, i.e., the balance between surface area of the appendage exposed to reversed flow in the wake and the surface area of the appendage exposed to fast free-stream flow.

  • 39.
    Leskovec, Martin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Centra, Wallenberg Wood Science Center.
    Innings, Fredrik
    Lund Univ, Dept Food Technol Engn & Nutr, SE-22100 Lund, Sweden.;Tetra Pak Proc Syst AB, SE-22355 Lund, Sweden..
    Pipe flow with large particles and their impact on the transition to turbulence2020Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 5, nr 11, artikel-id 112301Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The classical transition from laminar to turbulent flow is affected if solid particles are added. The transition behavior is a function of particle size d and solid volume fraction phi and the flow undergoes a smooth transition, as opposed to intermittent, if phi exceeds a certain threshold. In this work we show that, for particle-laden pipe flows with large particle-to-pipe diameter ratios d/D, the phi threshold for altering the transition is much lower than previously reported for smaller particles. Magnetic resonance velocimetry reveals that particles introduce turbulent-like fluid velocity fluctuations in laminar flow. Factors that might control the limits between "classical" and "smooth" transition in the state space spanned by d/D and phi are discussed based on scaling analyses.

  • 40.
    Liu, Ying
    et al.
    Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA..
    Rallabandi, Bhargav
    Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA.;Univ Calif Riverside, Dept Mech Engn, Riverside, CA 92521 USA..
    Zhu, Lailai
    KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA..
    Gupta, Ankur
    Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA..
    Stone, Howard A.
    Princeton Univ, Dept Mech & Aerosp Engn, Princeton, NJ 08544 USA..
    Pattern formation in oil-in-water emulsions exposed to a salt gradient2019Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, nr 8, artikel-id 084307Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Flow instabilities can occur in a fluid system with two components that have significantly different diffusivities and that have opposite effects on the fluid density, as is the scenario in traditional double-diffusive convection. Here, we experimentally show that an oil-in-water emulsion exposed to salt concentration gradients generates a flowerlike pattern driven by vertical and azimuthal instabilities. We also report numerical and analytical studies to elaborate on the mechanism, the instability criteria, and the most unstable modes that determine the details of the observed patterns. We find that the instability is driven by buoyancy and stems from the differential transport between the dissolved salt and the suspended oil droplets, which have opposing effects on the density of the medium. Consequently, we identify a criterion for the development of the instability that involves the relative densities and concentrations of the salt and oil droplets. We also argue that the typical wave number of the pattern formed scales with the Peclet number of the salt, which here is equivalent to the Rayleigh number since the flow is driven by buoyancy. We find good agreement of these predictions with both experiments and numerical simulations.

  • 41.
    Maffioli, Andrea
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Vertical spectra of stratified turbulence at large horizontal scales2017Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 2, nr 10, artikel-id 104802Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Stably stratified turbulence is investigated with the aim of increasing our limited understanding of the vertical structure of this type of turbulent flow. For strongly stratified turbulence there is a theoretical prediction that the energy spectra in the vertical direction of gravity are very steep, possessing the well-known form E-h(k(v)) alpha N(2)kv(-3) , where N is the Brunt-Vaisala frequency and kv is the vertical wave number, but supporting evidence from experiments and numerical simulations is lacking. We conduct direct numerical simulation (DNS) with uniform background stratification and forcing at large scales. In order to consider the large anisotropic scales only, the vertical energy spectra are decomposed into large-scale vertical spectra E-large(k(v)) and small-scale vertical spectra E-small(k(v)) using a horizontal demarcation scale. We find that this approach gives results that are in close agreement with E-large(k(v)) alpha N(2)k(v)(-3) for the DNS runs performed. This result holds approximately over the wave-number range k(b) <= k(v) <= k(oz), where kb is the buoyancy wave number and koz is the Ozmidov wave number, in agreement with theory. Similarly, large-scale vertical spectra of potential energy are found to be E-p,E-large(k(v)) alpha N(2)k(v)(-3) , over a narrower range of wave numbers. The evidence supports the existence of a scale-by-scale balance between inertia and buoyancy occurring in strongly stratified turbulence at large horizontal scales. Finally, the current results are put in the context of ocean turbulence by making a comparison with measurements of vertical shear spectra made in the ocean interior.

  • 42.
    Marchioli, C.
    et al.
    Univ Udine, Dept Engn & Architecture, I-33100 Udine, Italy.;CISM, Dept Fluid Mech, I-33100 Udine, Italy..
    Bhatia, H.
    Univ Udine, Dept Engn & Architecture, I-33100 Udine, Italy..
    Sardina, G.
    Chalmers Univ Technol, Dept Mech & Maritime Sci, Div Fluid Dynam, S-41258 Gothenburg, Sweden..
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Soldati, A.
    Univ Udine, Dept Engn & Architecture, I-33100 Udine, Italy.;TU Wien, Inst Fluid Mech & Heat Transfer, A-1040 Vienna, Austria..
    Role of large-scale advection and small-scale turbulence on vertical migration of gyrotactic swimmers2019Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, nr 12, artikel-id 124304Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this work, we use direct-numerical-simulation-based Eulerian-Lagrangian simulations to investigate the dynamics of small gyrotactic swimmers in free-surface turbulence. We consider open-channel flow turbulence in which bottom-heavy swimmers are dispersed. Swimmers are characterized by different vertical stability, so that some realign to swim upward with a characteristic time smaller than the Kolmogorov timescale, while others possess a reorientation time longer than the Kolmogorov timescale. We cover one order of magnitude in the flow Reynolds number and two orders of magnitude in the stability number, which is a measure of bottom heaviness. We observe that large-scale advection dominates vertical motion when the stability number, scaled on the local Kolmogorov timescale of the flow, is larger than unity: This condition is associated to enhanced migration toward the surface, particularly at low Reynolds number, when swimmers can rise through surface renewal motions that originate directly from the bottom-boundary turbulent bursts. Conversely, small-scale effects become more important when the Kolmogorov-based stability number is below unity: Under this condition, migration toward the surface is hindered, particularly at high Reynolds, when bottom-boundary bursts are less effective in bringing bulk fluid to the surface. In an effort to provide scaling arguments to improve predictions of models for motile microorganisms in turbulent water bodies, we demonstrate that a Kolmogorov-based stability number around unity represents a threshold beyond which swimmer capability to reach the free surface and form clusters saturates.

  • 43.
    Massaro, Daniele
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Lupi, Valerio
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Peplinski, Adam
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. Institute of Fluid Mechanics (LSTM), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen 91058, Germany.
    Global stability of 180-bend pipe flow with mesh adaptivity2023Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 8, nr 11, artikel-id 113903Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The global stability of the flow in a spatially developing 180∘-bend pipe with curvature δ=R/Rc=1/3 is investigated by performing direct numerical simulations to understand the underlying transitional mechanism. A unique application of the adaptive mesh refinement technique is used during the stability analysis for minimizing the interpolation and quadrature errors. Independent meshes are created for the direct and adjoint solutions, as well as for the base flow extracted via selective frequency damping. The spectrum of the linearized Navier-Stokes operator reveals a pair of complex conjugate eigenvalues, with frequency f≈0.233. Therefore, the transition is attributed to a Hopf bifurcation that takes place at Reb,cr=2528. A structural sensitivity analysis is performed by extracting the wavemaker. We identify the primary source of instability located on the outer wall, θ≈15 downstream of the bend inlet. This region corresponds to the separation bubble on the outer wall. We thus conclude that the instability is caused by the strong shear resulting from the backflow, similar to the 90-bend pipe flow. We believe that understanding the stability mechanism and characterizing the base flow in bent pipes is crucial for studying various biological flows, like blood vessels. Hence, this paper aims to close the knowledge gap between a 90-bend and toroidal pipes by investigating the transition nature in a 18-bend pipe flow.

  • 44.
    Massaro, Daniele
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teknisk mekanik, Strömningsmekanik och Teknisk Akustik. Department of Aerospace Sciences and Technologies, Politecnico di Milano, via La Masa 34 20156 Milano, Italy.
    Martinelli, Fulvio
    Laboratoire d'Hydrodynamique (LadHyX), CNRS-École Polytecnique, F-91128 Palaiseau, France.
    Schmid, Peter
    Department of Mechanical Engineering, PSE Division, KAUST, 23955 Thuwal, Saudi Arabia.
    Quadrio, Maurizio
    Department of Aerospace Sciences and Technologies, Politecnico di Milano, via La Masa 34 20156 Milano, Italy.
    Linear stability of Poiseuille flow over a steady spanwise Stokes layer2023Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 8, nr 10, artikel-id 103902Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The temporal linear stability of plane Poiseuille flow modified by spanwise forcing applied at the walls is considered. The forcing consists of a stationary streamwise distribution of spanwise velocity that generates a steady transversal Stokes layer, known to reduce skin-friction drag in a turbulent flow with little energetic cost. A large numerical study is carried out, where the effects of both the physical and the discretization parameters are thoroughly explored, for three representative subcritical values of the Reynolds number Re. Results show that the spanwise Stokes layer significantly affects the linear stability of the system. For example, at Re=2000 the wall forcing is found to more than double the negative real part of the least-stable eigenvalue, and to decrease by nearly a factor of 4 the maximum transient growth of perturbation energy. These observations are Re dependent and further improve at higher Re. Comments on the physical implications of the obtained results are provided, suggesting that spanwise forcing might be effective to obtain at the same time a delayed transition to turbulence and a reduced turbulent friction.

  • 45. Meibohm, J.
    et al.
    Candelier, F.
    Rosén, Tomas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Einarsson, J.
    Lundell, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Mehlig, B.
    Angular velocity of a sphere in a simple shear at small Reynolds number2016Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 1, nr 8, artikel-id 084203Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We analyze the angular velocity of a small neutrally buoyant spheroid log rolling in a simple shear. When the effect of fluid inertia is negligible the angular velocity. equals half the fluid vorticity. We compute by singular perturbation theory how weak fluid inertia reduces the angular velocity in an unbounded shear, and how this reduction depends upon the shape of the spheroid (on its aspect ratio). In addition we determine the angular velocity by direct numerical simulations. The results are in excellent agreement with the theory at small but not too small values of the shear Reynolds number Res, for all aspect ratios considered. For the special case of a sphere we find omega/s = -1/2 + 0.0540 Re-s(3/2) where s is the shear rate. The O( Re-s(3/2)) correction differs from that derived by Lin et al. who obtained a numerical coefficient roughly three times larger.

  • 46.
    Meibohm, Jan
    et al.
    Gothenburg Univ, Dept Phys, SE-41296 Gothenburg, Sweden..
    Pandey, Vikash
    Tata Inst Fundamental Res, TIFR Ctr Interdisciplinary Sci, Hyderabad 500046, India..
    Bhatnagar, Akshay
    NORDITA SU.
    Gustavsson, Kristian
    Gothenburg Univ, Dept Phys, SE-41296 Gothenburg, Sweden..
    Mitra, Dhrubaditya
    NORDITA SU, ;Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Perlekar, Prasad
    Tata Inst Fundamental Res, TIFR Ctr Interdisciplinary Sci, Hyderabad 500046, India..
    Mehlig, Bernhard
    Gothenburg Univ, Dept Phys, SE-41296 Gothenburg, Sweden..
    Paths to caustic formation in turbulent aerosols2021Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 6, nr 6, artikel-id L062302Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The dynamics of small, yet heavy, identical particles in turbulence exhibits singularities, called caustics, that lead to large fluctuations in the spatial particle-number density, and in collision velocities. For large particle inertia, the fluid velocity at the particle position is essentially a white-noise signal and caustic formation is analogous to Kramers escape. Here we show that caustic formation at small particle inertia is different. Caustics tend to form in the vicinity of particle trajectories that experience a specific history of fluid-velocity gradients, characterized by low vorticity and a violent strain exceeding a large threshold. We develop a theory that explains our findings in terms of an optimal path to caustic formation that is approached in the small inertia limit.

  • 47.
    Mitra, Dhrubaditya
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.
    Perlekar, Prasad
    Tata Inst Fundamental Res, Ctr Interdisciplinary Sci, Hyderabad 500107, Andhra Pradesh, India..
    Topology of two-dimensional turbulent flows of dust and gas2018Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 3, nr 4, artikel-id 044303Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We perform direct numerical simulations (DNS) of passive heavy inertial particles (dust) in homogeneous and isotropic two-dimensional turbulent flows (gas) for a range of Stokes number, St < 1. We solve for the particles using both a Lagrangian and an Eulerian approach (with a shock-capturing scheme). In the latter, the particles are described by a dust-density field and a dust-velocity field. We find the following: the dust-density field in our Eulerian simulations has the same correlation dimension d(2) as obtained from the clustering of particles in the Lagrangian simulations for St < 1; the cumulative probability distribution function of the dust density coarse grained over a scale r, in the inertial range, has a left tail with a power-law falloff indicating the presence of voids; the energy spectrum of the dust velocity has a power-law range with an exponent that is the same as the gas-velocity spectrum except at very high Fourier modes; the compressibility of the dust-velocity field is proportional to St(2). We quantify the topological properties of the dust velocity and the gas velocity through their gradient matrices, called A and B, respectively. Our DNS confirms that the statistics of topological properties of B are the same in Eulerian and Lagrangian frames only if the Eulerian data are weighed by the dust density. We use this correspondence to study the statistics of topological properties of A in the Lagrangian frame from our Eulerian simulations by calculating density-weighted probability distribution functions. We further find that in the Lagrangian frame, the mean value of the trace of A is negative and its magnitude increases with St approximately as exp(-C/St) with a constant C approximate to 0.1. The statistical distribution of different topological structures that appear in the dust flow is different in Eulerian and Lagrangian (density-weighted Eulerian) cases, particularly for St close to unity. In both of these cases, for small St the topological structures have close to zero divergence and are either vortical (elliptic) or strain dominated (hyperbolic, saddle). As St increases, the contribution to negative divergence comes mostly from saddles and the contribution to positive divergence comes from both vortices and saddles. Compared to the Eulerian case, the Lagrangian (density-weighted Eulerian) case has less outward spirals and more converging saddles. Inward spirals are the least probable topological structures in both cases.

  • 48.
    Montecchia, Matteo
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Brethouwer, Gert
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Johansson, Arne V.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Wallin, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Taking large-eddy simulation of wall-bounded flows to higher Reynolds numbers by use of anisotropy-resolving subgrid models2017Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 2, artikel-id 034601Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Properly resolved large-eddy simulations of wall-bounded high Reynolds number flows using standard subgrid-scale (SGS) models requires high spatial and temporal resolution. We have shown that a more elaborate SGS model taking into account the SGS Reynolds stress anisotropies can relax the requirement for the number of grid points by at least an order of magnitude for the same accuracy. This was shown by applying the recently developed explicit algebraic subgrid-scale model (EAM) to fully developed high Reynolds number channel flows with friction Reynolds numbers of 550, 2000, and 5200. The near-wall region is fully resolved, i.e., no explicit wall modeling or wall functions are applied. A dynamic procedure adjusts the model at the wall for both low and high Reynolds numbers. The resolution is reduced, from the typically recommended 50 and 15 wall units in the stream-and spanwise directions respectively, by up to a factor of 5 in each direction. It was shown by comparison with direct numerical simulations that the EAM is much less sensitive to reduced resolution than the dynamic Smagorinsky model. Skin friction coefficients, mean flow profiles, and Reynolds stresses are better predicted by the EAM for a given resolution. Even the notorious overprediction of the streamwise fluctuation intensity typically seen in poorly resolved LES is significantly reduced whenEAMis used on coarse grids. The improved prediction is due to the capability of the EAM to capture the SGS anisotropy, which becomes significant close to the wall.

  • 49.
    Nambiar, Sankalp
    et al.
    Nordita, SU.
    Wettlaufer, John
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Yale Univ, New Haven, CT 06520 USA..
    Hydrodynamics of slender swimmers near deformable interfaces2022Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 7, nr 5, artikel-id 054001Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the coupled hydrodynamics between a motile slender microswimmer and a deformable interface that separates two Newtonian fluid regions. From the disturbance field generated by the swimming motion, we quantitatively characterize the interface deformation and the manner in which the coupling modifies the microswimmer translation itself. We treat the role of the swimmer type (pushers and pullers), size and model an interface that can deform due to both surface tension and bending elasticity. Our analysis reveals a strong dependence of the hydrodynamics on the swimmer orientation and position. Given the viscosities of the two fluid media, the interface properties and the swimmer type, a swimmer can either migrate toward or away from the interface depending on its configurations. When the swimmer is oriented parallel to the interface, a pusher-type swimmer is repelled from the interface at short times if it is swimming in the more viscous fluid. At long times, however, pushers are always attracted to the interface, and pullers are always repelled from it. However, swimmers oriented orthogonal to the interface exhibit a migration pattern opposite to the parallel swimmers. In consequence, a host of complex migration trajectories emerge for swimmers arbitrarily oriented to the interface. We find that confining a swimmer between a rigid boundary and a deformable interface results in regimes of attraction toward both surfaces depending on the swimmer location in the channel, irrespective viscosity ratio. The differing migration patterns are most prominent in a region of order the swimmer size from the interface, where the slender swimmer model yields a better approximation to the coupled hydrodynamics.

  • 50.
    Nambiar, Sankalp
    et al.
    Nordita, SU; Present address: Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India; snambiar@iitd.ac.in.
    Wettlaufer, John
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Departments of Earth & Planetary Sciences, Mathematics and Physics, Yale University, New Haven, Connecticut 06520-8109, USA.
    Stochastic reorientations and the hydrodynamics of microswimmers near deformable interfaces2024Ingår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 9, nr 2, artikel-id 023102Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the hydrodynamic interaction between a microswimmer and a deformable interface when the swimmer can stochastically reorient itself. We consider a force- and torque-free swimmer, modeled as a slender body, that can execute random orientation tumbles or active Brownian rotations in the plane of the deformable interface. When the swimmer is in the more viscous fluid, our analysis shows that both tumbles and Brownian rotations acting on timescales comparable to that of interface deformations can lead to a pusher-type swimmer rotating away from the interface, while enhancing its attraction towards the interface. In turn, the intrinsic orientational stochasticity of the microswimmer favors a stronger migration of pushers towards the interface at short times, but migration away from the interface in the long-time limit. However, irrespective of the viscosity ratio of the two fluid medium, the tendency of a pusher to align parallel to the interface is suppressed; the results for puller-type swimmers are the opposite. Our study has potential consequences for the residence time of swimming microorganisms near deformable boundaries.

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