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Scapin, N., Costa, P. & Brandt, L. (2020). A volume-of-fluid method for interface-resolved simulations of phase-changing two-fluid flows. Journal of Computational Physics, 407, Article ID 109251.
Open this publication in new window or tab >>A volume-of-fluid method for interface-resolved simulations of phase-changing two-fluid flows
2020 (English)In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 407, article id 109251Article in journal (Refereed) Published
Abstract [en]

We present a numerical method for interface-resolved simulations of evaporating two-fluid flows based on the volume-of-fluid (VoF) method. The method has been implemented in an efficient EFT-based two-fluid Navier-Stokes solver, using an algebraic VoF method for the interface representation, and extended with the transport equations of thermal energy and vaporized liquid mass for the single-component evaporating liquid in an inert gas. The conservation of vaporizing liquid and computation of the interfacial mass flux are performed with the aid of a reconstructed signed-distance field, which enables the use of well-established methods for phase change solvers based on level-set methods. The interface velocity is computed with a novel approach that ensures accurate mass conservation, by constructing a divergence-free extension of the liquid velocity field onto the entire domain. The resulting approach does not depend on the type of interface reconstruction (i.e. can be employed in both algebraic and geometrical VoF methods). We extensively verified and validated the overall method against several benchmark cases, and demonstrated its excellent mass conservation and good overall performance for simulating evaporating two-fluid flows in two and three dimensions.

Place, publisher, year, edition, pages
ACADEMIC PRESS INC ELSEVIER SCIENCE, 2020
Keywords
Interface-resolved direct numerical simulations, Volume-of-fluid method, Phase change
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-271763 (URN)10.1016/j.jcp.2020.109251 (DOI)000519535500014 ()2-s2.0-85078608275 (Scopus ID)
Note

QC 20200406

Available from: 2020-04-06 Created: 2020-04-06 Last updated: 2020-04-06Bibliographically approved
Le Clainche, S., Izbassarov, D., Rosti, M. E., Brandt, L. & Tammisola, O. (2020). Coherent structures in the turbulent channel flow of an elastoviscoplastic fluid. Journal of Fluid Mechanics, 888, Article ID A5.
Open this publication in new window or tab >>Coherent structures in the turbulent channel flow of an elastoviscoplastic fluid
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2020 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 888, article id A5Article in journal (Refereed) Published
Abstract [en]

In this numerical and theoretical work, we study the turbulent channel flow of Newtonian and elastoviscoplastic fluids. The coherent structures in these flows are identified by means of higher order dynamic mode decomposition (HODMD), applied to a set of data non-equidistant in time, to reveal the role of the near-wall streaks and their breakdown, and the interplay between turbulent dynamics and non-Newtonian effects. HODMD identifies six different high-amplitude modes, which either describe the yielded flow or the yielded-unyielded flow interaction. The structure of the low- and high-frequency modes suggests that the interaction between high- and low-speed streamwise velocity structures is one of the mechanisms triggering the streak breakdown, dominant in Newtonian turbulence where we observe shorter near-wall streaks and a more chaotic dynamics. As the influence of elasticity and plasticity increases, the flow becomes more correlated in the streamwise direction, with long streaks disrupted for short times by localised perturbations, reflected in reduced drag. Finally, we present streamwise-periodic dynamic mode decomposition modes as a viable tool to describe the highly complex turbulent flows, and identify simple well-organised groups of travelling waves.

Place, publisher, year, edition, pages
Cambridge University Press, 2020
Keywords
nonlinear instability, viscoelasticity, turbulent boundary layers
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-268813 (URN)10.1017/jfm.2020.31 (DOI)000511269300001 ()2-s2.0-85079267695 (Scopus ID)
Note

QC 20200221

Available from: 2020-02-21 Created: 2020-02-21 Last updated: 2020-02-21Bibliographically approved
Picano, F., Tammisola, O. & Brandt, L. (2020). Editorial. Meccanica (Milano. Print), 55(2), 295-297
Open this publication in new window or tab >>Editorial
2020 (English)In: Meccanica (Milano. Print), ISSN 0025-6455, E-ISSN 1572-9648, Vol. 55, no 2, p. 295-297Article in journal, Editorial material (Other academic) Published
Place, publisher, year, edition, pages
SPRINGER, 2020
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-269045 (URN)10.1007/s11012-019-01112-1 (DOI)000512775600001 ()2-s2.0-85077563686 (Scopus ID)
Note

QC 20200311

Available from: 2020-03-11 Created: 2020-03-11 Last updated: 2020-03-11
Zade, S., Shamu, T. J., Lundell, F. & Brandt, L. (2020). Finite-size spherical particles in a square duct flow of an elastoviscoplastic fluid: an experimental study. Journal of Fluid Mechanics, 883, Article ID A6.
Open this publication in new window or tab >>Finite-size spherical particles in a square duct flow of an elastoviscoplastic fluid: an experimental study
2020 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 883, article id A6Article in journal (Refereed) Published
Abstract [en]

The present experimental study addresses the flow of a yield stress fluid with some elasticity (Carbopol gel) in a square duct. The behaviour of two fluids with lower and higher yield stress is investigated in terms of the friction factor and flow velocities at multiple Reynolds numbers Re* is an element of (1, 200) and, hence, Bingham numbers Bi is an element of (0.01, 0.35). Taking advantage of the symmetry planes in a square duct, we reconstruct the entire 3-component velocity field from two-dimensional particle image velocimetry (PIV). A secondary flow consisting of eight vortices is observed to recirculate the fluid from the core towards the wall centre and from the corners back to the core. The extent and intensity of these vortices grows with increasing Re* or, alternately, as the plug size decreases. The second objective of this study is to explore the change in flow in the presence of particles. To this end, almost neutrally buoyant finite-size spherical particles with a duct height, 2H, to particle diameter, d(p), ratio of 12 are used at two volume fractions phi = 5 and 10 %. Particle tracking velocimetry is used to measure the velocity of these refractive-index-matched spheres in the clear Carbopol gel, and PIV to extract the fluid velocity. Additionally, simple shadowgraphy is also used to qualitatively visualise the development of the particle distribution along the streamwise direction. The particle distribution pattern changes from being concentrated at the four corners, at low flow rates, to being focussed along a diffused ring between the centre and the corners, at high flow rates. The presence of particles induces streamwise and wall-normal velocity fluctuations in the fluid phase; however, the primary Reynolds shear stress is still very small compared to turbulent flows. The size of the plug in the particle-laden cases appears to be smaller than the corresponding single-phase cases. Similar to Newtonian fluids, the friction factor increases due to the presence of particles, almost independently of the suspending fluid matrix. Interestingly, predictions based on an increased effective suspension viscosity agrees quite well with the experimental friction factor for the concentrations used in this study.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2020
Keywords
particle/fluid flow, suspensions
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-267152 (URN)10.1017/jfm.2019.868 (DOI)000508121500006 ()
Note

QC 20200217

Available from: 2020-02-17 Created: 2020-02-17 Last updated: 2020-02-17Bibliographically approved
Rosti, M. E., Olivieri, S., Banaei, A. A., Brandt, L. & Mazzino, A. (2020). Flowing fibers as a proxy of turbulence statistics. Meccanica (Milano. Print), 55, 357-370
Open this publication in new window or tab >>Flowing fibers as a proxy of turbulence statistics
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2020 (English)In: Meccanica (Milano. Print), ISSN 0025-6455, E-ISSN 1572-9648, Vol. 55, p. 357-370Article in journal (Refereed) Published
Abstract [en]

The flapping states of a flexible fiber fully coupled to a three-dimensional turbulent flow are investigated via state-of-the-art numerical methods. Two distinct flapping regimes are predicted by the phenomenological theory recently proposed by Rosti et al. (Phys. Rev. Lett. 121:044501, 2018) the under-damped regime, where the elasticity strongly affects the fiber dynamics, and the over-damped regime, where the elastic effects are strongly inhibited. In both cases we can identify a critical value of the bending rigidity of the fiber by a resonance condition, which further provides a distinction between different flapping behaviors, especially in the under-damped case. We validate the theory by means of direct numerical simulations and find that, both for the over-damped regime and for the under-damped one, fibers are effectively slaved to the turbulent fluctuations and can therefore be used as a proxy to measure various two-point statistics of turbulence. Finally, we show that this holds true also in the case of a passive fiber, without any feedback force on the fluid.

Place, publisher, year, edition, pages
Springer Netherlands, 2020
Keywords
Dispersed flows, Fiber, Multiphase flows, Turbulence, Multiphase flow, Numerical methods, Dispersed flow, Flapping behavior, Phenomenological theory, Resonance condition, Three-dimensional turbulent flow, Turbulence statistics, Turbulent fluctuation, Two point statistics, Fibers
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-263285 (URN)10.1007/s11012-019-00997-2 (DOI)000512775600006 ()2-s2.0-85068170932 (Scopus ID)
Note

QC 20191105

Available from: 2019-11-05 Created: 2019-11-05 Last updated: 2020-03-11Bibliographically approved
Alizad Banaei, A., Rahmani, M., Martinez, D. M. & Brandt, L. (2020). Inertial settling of flexible fiber suspensions. Physical Review Fluids, 5(2), Article ID 024301.
Open this publication in new window or tab >>Inertial settling of flexible fiber suspensions
2020 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 5, no 2, article id 024301Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
American Physical Society, 2020
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-268814 (URN)10.1103/PhysRevFluids.5.024301 (DOI)000511202700003 ()
Note

QC 20200221

Available from: 2020-02-21 Created: 2020-02-21 Last updated: 2020-02-21Bibliographically approved
Costa, P., Brandt, L. & Picano, F. (2020). Interface-resolved simulations of small inertial particles in turbulent channel flow. Journal of Fluid Mechanics, 883, Article ID A54.
Open this publication in new window or tab >>Interface-resolved simulations of small inertial particles in turbulent channel flow
2020 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 883, article id A54Article in journal (Refereed) Published
Abstract [en]

We present a direct comparison between interface-resolved and one-way-coupled point-particle direct numerical simulations (DNS) of gravity-free turbulent channel flow laden with small inertial particles, with high particle-to-fluid density ratio and diameter of approximately three viscous units. The most dilute flow considered, solid volume fraction O(10(-5)), shows the particle feedback on the flow to be negligible, whereas differences with respect to the unladen case, notably a drag increase of approximately 10 %, are found for a volume fraction O(10(-4)). This is attributed to a dense layer of particles at the wall, caused by turbophoresis, flowing with large particle-to-fluid apparent slip velocity. The most dilute case is therefore taken as the benchmark for assessing the validity of a widely used point-particle model, where the particle dynamics results only from inertial and nonlinear drag forces. In the bulk of the channel, the first- and second-order moments of the particle velocity from the point-particle DNS agree well with those from the interface-resolved DNS. Close to the wall, however, most of the statistics show major qualitative differences. We show that this difference originates from the strong shear-induced lift force acting on the particles in the near-wall region. This mechanism is well captured by the lift force model due to Saffman (J. Fluid Mech., vol. 22 (2), 1965, pp. 385-400), while other widely used, more elaborate, approaches aiming at extending the lift model for a wider range of particle Reynolds numbers can actually underpredict the magnitude of the near-wall particle velocity fluctuations for the cases analysed here.

Place, publisher, year, edition, pages
Cambridge University Press, 2020
Keywords
multiphase flow, particle, fluid flows
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-265513 (URN)10.1017/jfm.2019.918 (DOI)000499724600001 ()2-s2.0-85075801940 (Scopus ID)
Note

QC 20191213

Available from: 2019-12-13 Created: 2019-12-13 Last updated: 2020-01-08Bibliographically approved
Banaei, A. A., Rosti, M. E. & Brandt, L. (2020). Numerical study of filament suspensions at finite inertia. Journal of Fluid Mechanics, 882, Article ID A5.
Open this publication in new window or tab >>Numerical study of filament suspensions at finite inertia
2020 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 882, article id A5Article in journal (Refereed) Published
Abstract [en]

We present a numerical study on the rheology of semi-dilute and concentrated filament suspensions of different bending stiffness and Reynolds number, with the immersed boundary method used to couple the fluid and solid. The filaments are considered as one-dimensional inextensible slender bodies with fixed aspect ratio, obeying the Euler-Bernoulli beam equation. To understand the global suspension behaviour we relate it to the filament microstructure, deformation and elastic energy and examine the stress budget to quantify the effect of the elastic contribution. At fixed volume fraction, the viscosity of the suspension reduces when decreasing the bending rigidity and grows when increasing the Reynolds number. The change in the relative viscosity is stronger at finite inertia, although still in the laminar flow regime, as considered here. Moreover, we find the first normal stress difference to be positive as in polymeric fluids, and to increase with the Reynolds number; its value has a peak for an intermediate value of the filament bending stiffness. The peak value is found to be proportional to the Reynolds number, moving towards more rigid suspensions at larger inertia. Moreover, the viscosity increases when increasing the filament volume fraction, and the rate of increase of the filament stress with the bending rigidity is stronger at higher Reynolds numbers and reduces with the volume fraction. We show that this behaviour is associated with the formation of a more ordered structure in the flow, where filaments tend to be more aligned and move as a compact aggregate, thus reducing the filament-filament interactions despite their volume fraction increases.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2020
Keywords
suspensions
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-266939 (URN)10.1017/jfm.2019.794 (DOI)000506238300005 ()
Note

QC 20200203

Available from: 2020-02-03 Created: 2020-02-03 Last updated: 2020-02-03Bibliographically approved
Rosti, M. E., Pramanik, S., Brandt, L. & Mitra, D. (2020). The breakdown of Darcy's law in a soft porous material. Soft Matter, 16(4), 939-944
Open this publication in new window or tab >>The breakdown of Darcy's law in a soft porous material
2020 (English)In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 16, no 4, p. 939-944Article in journal (Refereed) Published
Abstract [en]

We perform direct numerical simulations of the flow through a model of deformable porous medium. Our model is a two-dimensional hexagonal lattice, with defects, of soft elastic cylindrical pillars, with elastic shear modulus G, immersed in a liquid. We use a two-phase approach: the liquid phase is a viscous fluid and the solid phase is modeled as an incompressible viscoelastic material, whose complete nonlinear structural response is considered. We observe that the Darcy flux (q) is a nonlinear function - steeper than linear - of the pressure-difference (Delta P) across the medium. Furthermore, the flux is larger for a softer medium (smaller G). We construct a theory of this super-linear behavior by modelling the channels between the solid cylinders as elastic channels whose walls are made of material with a linear constitutive relation but can undergo large deformation. Our theory further predicts that the flow permeability is an universal function of Delta P/G, which is confirmed by the present simulations.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2020
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-268803 (URN)10.1039/c9sm01678c (DOI)000510894800006 ()31845717 (PubMedID)
Note

QC 20200225

Available from: 2020-02-25 Created: 2020-02-25 Last updated: 2020-02-25Bibliographically approved
Brockmann, P., Kazerooni, H. T., Brandt, L. & Hussong, J. (2020). Utilizing the ball lens effect for astigmatism particle tracking velocimetry. Experiments in Fluids, 61(2)
Open this publication in new window or tab >>Utilizing the ball lens effect for astigmatism particle tracking velocimetry
2020 (English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 61, no 2Article in journal (Refereed) Published
Abstract [en]

In the present study, a simple method is developed to apply astigmatism particle tracking velocimetry (APTV) to transparent particles utilizing backlight illumination. Here, a particle acts as ball lens and bundles the light to a focal point, which is used to determine the particle's out-of-plane position. Due to the distance between focal point and particle, additional features have to be considered in ball lens astigmatism particle tracking velocimetry (BLAPTV) compared to conventional APTV. We describe required calibration steps and perform parameter studies to show how the autocorrelation coefficient and the light exposure affect the accuracy of the method. It is found that the accuracy and robustness of the Euclidean calibration approach as also used in conventional APTV (Cierpka et al. in Meas Sci Technol 22(1):015401, 2010a) can be increased if an additional calibration curve for the light intensity of the particle's focal point is considered. In addition, we study the influence of the particle diameter and the refractive index jump between liquid and particles on the calibration curves and the accuracy. In this way, particles of the same size, but different material, can be distinguished by their calibration curve. Furthermore, an approach is presented to account for shape changes of the calibration curve along the depth of the measurement volume. Overall, BLAPTV provides high out-of-plane particle reconstruction accuracies with respect to the particle diameter. In test cases, position uncertainties down to 1.8% of the particle diameter are achieved for particles of dp=124 mu m. The measurement technique is validated for a laminar flow in a straight rectangular channel with a cross-sectional area of 2.3x30 mm2. Uncertainties of 0.75% for the in-plane and 2.29% for out-of-plane velocity with respect to the maximum streamwise velocity are achieved.Graphic abstract [Figure not available: see fulltext.]

Place, publisher, year, edition, pages
SPRINGER, 2020
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-270921 (URN)10.1007/s00348-020-2900-5 (DOI)000514649900007 ()2-s2.0-85079521446 (Scopus ID)
Note

QC 20200319

Available from: 2020-03-19 Created: 2020-03-19 Last updated: 2020-03-19Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-4346-4732

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