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  • 1. Alegre, Cesar
    et al.
    McNally, Donal
    Choi, Kwing-So
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    The effect or arterial flow elasticity on the flow through a stenosis2016In: Proceedings of the International Conference on Bionic Engineering 2016, 2016Conference paper (Refereed)
  • 2. Alegre-Martínez, C.
    et al.
    Choi, K. -S
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    McNally, D.
    On the axial distribution of plaque stress: Influence of stenosis severity, lipid core stiffness, lipid core length and fibrous cap stiffness2019In: Medical Engineering and Physics, Vol. 68, p. 76-84Article in journal (Refereed)
    Abstract [en]

    Numerical simulations of blood flow through a partially-blocked axisymmetric artery are performed to investigate the stress distributions in the plaque. We show that the combined effect of stenosis severity and the stiffness of the lipid core can drastically change the axial stress distribution, strongly affecting the potential sites of plaque rupture. The core stiffness is also an important factor when assessing plaque vulnerability, where a mild stenosis with a lipid-filled core presents higher stress levels than a severe stenosis with a calcified plaque. A shorter lipid core gives rise to an increase in the stress levels. However, the fibrous cap stiffness does not influence the stress distributions for the range of values considered in this work. Based on these mechanical analyses, we identify potential sites of rupture in the axial direction for each case: the midpoints of the upstream and downstream regions of the stenosis (for severe, lipid-filled plaques), the ends of the lipid core (for short cores), and the middle of the stenosis (for mild stenoses with positive remodelling of the arterial wall). 

  • 3.
    De Vita, Francesco
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Rosti, Marco E.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Izbassarov, Daulet
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Duffo, L.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Hormozi, S.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Elastoviscoplastic flows in porous media2018In: Journal of Non-Newtonian Fluid Mechanics, ISSN 0377-0257, E-ISSN 1873-2631, Vol. 258, p. 10-21Article in journal (Refereed)
    Abstract [en]

    We investigate the elastoviscoplastic flow through porous media by numerical simulations. We solve the Navier–Stokes equations combined with the elastoviscoplastic model proposed by Saramito for the stress tensor evolution [1]. In this model, the material behaves as a viscoelastic solid when unyielded, and as a viscoelastic Oldroyd-B fluid for stresses higher than the yield stress. The porous media is made of a symmetric array of cylinders, and we solve the flow in one periodic cell. We find that the solution is time-dependent even at low Reynolds numbers as we observe oscillations in time of the unyielded region especially at high Bingham numbers. The volume of the unyielded region slightly decreases with the Reynolds number and strongly increases with the Bingham number; up to 70% of the total volume is unyielded for the highest Bingham numbers considered here. The flow is mainly shear dominated in the yielded region, while shear and elongational flow are equally distributed in the unyielded region. We compute the relation between the pressure drop and the flow rate in the porous medium and present an empirical closure as function of the Bingham and Reynolds numbers. The apparent permeability, normalized with the case of Newtonian fluids, is shown to be greater than 1 at low Bingham numbers, corresponding to lower pressure drops due to the flow elasticity, and smaller than 1 for high Bingham numbers, indicating larger dissipation in the flow owing to the presence of the yielded regions. Finally we investigate the effect of the Weissenberg number on the distribution of the unyielded regions and on the pressure gradient.

  • 4.
    Ge, Zhouyang
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Loiseau, Jean Christophe
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. Arts et Métiers ParisTech, France.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    An efficient mass-preserving interface-correction level set/ghost fluid method for droplet suspensions under depletion forces2018In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 353, p. 435-459Article in journal (Refereed)
    Abstract [en]

    Aiming for the simulation of colloidal droplets in microfluidic devices, we present here a numerical method for two-fluid systems subject to surface tension and depletion forces among the suspended droplets. The algorithm is based on an efficient solver for the incompressible two-phase Navier–Stokes equations, and uses a mass-conserving level set method to capture the fluid interface. The four novel ingredients proposed here are, firstly, an interface-correction level set (ICLS) method; global mass conservation is achieved by performing an additional advection near the interface, with a correction velocity obtained by locally solving an algebraic equation, which is easy to implement in both 2D and 3D. Secondly, we report a second-order accurate geometric estimation of the curvature at the interface and, thirdly, the combination of the ghost fluid method with the fast pressure-correction approach enabling an accurate and fast computation even for large density contrasts. Finally, we derive a hydrodynamic model for the interaction forces induced by depletion of surfactant micelles and combine it with a multiple level set approach to study short-range interactions among droplets in the presence of attracting forces.

  • 5.
    Ge, Zhouyang
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH Mech, INTERFACE Ctr, SE-10044 Stockholm, Sweden..
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH Mech, INTERFACE Ctr, SE-10044 Stockholm, Sweden..
    Flow-assisted droplet assembly in a 3D microfluidic channel2019In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 15, no 16, p. 3451-3460Article in journal (Refereed)
    Abstract [en]

    Self-assembly of soft matter, such as droplets or colloids, has become a promising scheme to engineer novel materials, model living matter, and explore non-equilibrium statistical mechanics. In this article, we present detailed numerical simulations of few non-Brownian droplets in various flow conditions, specifically, focusing on their self-assembly within a short distance in a three-dimensional (3D) microfluidic channel, cf. [Shen et al., Adv. Sci., 2016, 3(6), 1600012]. Contrary to quasi two-dimensional (q2D) systems, where dipolar interaction is the key mechanism for droplet rearrangement, droplets in 3D confinement produce much less disturbance to the underlying flow, thus experiencing weaker dipolar interactions. Using confined simple shear and Poiseuille flows as reference flows, we show that the droplet dynamics is mostly affected by the shear-induced cross-stream migration, which favors chain structures if the droplets are under an attractive depletion force. For more compact clusters, such as three droplets in a triangular shape, our results suggest that an inhomogeneous cross-sectional inflow profile is further required. Overall, the accelerated self-assembly of a small-size droplet cluster results from the combined effects of strong depletion forces, confinement-mediated shear alignments, and fine-tuned inflow conditions. The deterministic nature of the flow-assisted self-assembly implies the possibility of large throughputs, though calibration of all different effects to directly produce large droplet crystals is generally difficult.

  • 6. Ghosh, Sukhendu
    et al.
    Ranganathan, Usha
    Govindarajan, Rama
    Tammisola, Outi
    The University of Nottingham, UK.
    Inviscid instability of two-fluid free surface flow down an incline2015In: Meccanica (Milano. Print), ISSN 0025-6455, E-ISSN 1572-9648, p. 1-18Article in journal (Refereed)
    Abstract [en]

    The inviscid temporal stability analysis of two-fluid parallel shear flow with a free surface, down an incline, is studied. The velocity profiles are chosen as piecewise-linear with two limbs. The analysis reveals the existence of unstable inviscid modes, arising due to wave interaction between the free surface and the shear-jump interface. Surface tension decreases the maximum growth rate of the dominant disturbance. Interestingly, in some limits, surface tension destabilises extremely short waves in this flow. This can happen because of the interaction with the shear-jump interface. This flow may be compared with a corresponding viscous two-fluid flow. Though viscosity modifies the stability properties of the flow system both qualitatively and quantitatively, there is qualitative agreement between the viscous and inviscid stability analysis when the less viscous fluid is closer to the free surface.

  • 7. Imagawa, K.
    et al.
    Bellani, Gabriele
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Higuchi, H.
    Hayase, T.
    Measurement-Integrated simulations and Kalman filter applied to a co-flowing jet2010In: 5th Flow Control Conference, 2010, p. 2010-4420-Conference paper (Refereed)
    Abstract [en]

    This paper deals with the experimental evaluation of a flow analysis system based on the integration between an under-resolved Navier-Stokes simulation and experimental measurements with the mechanism of feedback (referred to as Measurement-Integrated simulation), applied to the case of a planar turbulent co-flowing jet. The experiments are performed with inner-to-outer-jet velocity ratio around 2 and the Reynolds number based on the inner-jet heights about 10000. The measurement system is a high-speed PIV, which provides time-resolved data of the flow-field, on a field of view which extends to 20 jet heights downstream the jet outlet. The experimental data can thus be used both for providing the feedback data for the simulations and for validation of the MI-simulations over a wide region. The effect of reduced data-rate and spatial extent of the feedback (i.e. measurements are not available at each simulation time-step or discretization point) was investigated. At first simulations were run with full information in order to obtain an upper limit of the MI-simulations performance. The results show the potential of this methodology of reproducing first and second order statistics of the turbulent flow with good accuracy. Then, to deal with the reduced data different feedback strategies were tested. It was found that for small data-rate reduction the results are basically equivalent to the case of full-information feedback but as the feedback data-rate is reduced further the error increases and tend to be localized in regions of high turbulent activity. Moreover, it is found that the spatial distribution of the error looks qualitatively different for different feedback strategies. Feedback gain distributions calculated by optimal control theory are presented and proposed as a mean to make it possible to perform MI-simulations based on localized measurements only. So far, we have not been able to low error between measurements and simulations by using these gain distributions.

  • 8.
    Izbassarov, Daulet
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Rosti, Marco E.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Niazi Ardekani, Mehdi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Sarabian, Mohammad
    Hormozi, Sarah
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Computational modeling of multiphase viscoelastic and elastoviscoplastic flows2018In: International Journal for Numerical Methods in Fluids, ISSN 0271-2091, E-ISSN 1097-0363, Vol. 88, no 12, p. 521-543Article in journal (Refereed)
  • 9.
    Juniper, Matthew
    et al.
    Department of Engineering, Cambridge University, Cambridge.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    The local and global stability of confined planar wakes at intermediate Reynolds number2011In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 686, p. 218-238Article in journal (Refereed)
    Abstract [en]

    At high Reynolds numbers, wake flows become more globally unstable whenconfined within a duct or between two flat plates. At Reynolds numbers around100, however, global analyses suggest that such flows become more stable whenconfined, while local analyses suggest that they become more unstable. Theaim of this paper is to resolve this apparent contradiction. In this theoreticaland numerical study, we combine global and local stability analyses of planarwake flows at Re = 100 to determine the effect of confinement. We find thatconfinement acts in three ways: it modifies the length of the recirculation zoneif one exists, it brings the boundary layers closer to the shear layers, and itcan make the flow more locally absolutely unstable. Depending on the flowparameters, these effects work with or against each other to destabilize orstabilize the flow. In wake flows at Re = 100 with free slip boundaries, flowsare most globally unstable when the outer flows are 50% wider than the halfwidthof the inner flow because the first and third effects work together. Inwake flows at Re = 100 with no slip boundaries, confinement has little overalleffect when the flows are weakly confined because the first two effects workagainst the third. Confinement has a strong stabilizing effect, however, whenthe flows are strongly confined because all three effects work together. Bycombining local and global analyses, we have been able to isolate these threeeffects and resolve the apparent contradictions in previous work.1.

  • 10.
    Magri, Luca
    et al.
    Shinshu University, Nagano, Japan.
    See, Y.-C.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI).
    Ihme, Mattias
    Juniper, Matthew
    Department of Engineering, Cambridge University, Cambridge.
    Multiple-scale thermoacoustic stability analysis of a coaxial jet combustor2016In: Proceedings of the Combustion Institute, ISSN 1540-7489, E-ISSN 1873-2704Article in journal (Refereed)
    Abstract [en]

    In this paper, asymptotic multiple-scale methods are used to formulate a mathematically consistent set of thermo-acoustic equations in the low-Mach number limit for linear stability analysis. The resulting sets of nonlinear equations for hydrodynamics and acoustics are two-way coupled. The coupling strength depends on which multiple scales are used. The double-time-double-space (2T-2S), double-time-single-space (2T-1S) and single-time-double-space (1T-2S) limits are revisited, derived and linearized. It is shown that only the 1T-2S limit produces a two-way coupled linearized system. Therefore this limit is adopted and implemented in a finite-element solver. The methodology is applied to a coaxial jet combustor. By using an adjoint method and introducing the intrinsic sensitivity, (i) the interaction between the acoustic and hydrodynamic subsystems is calculated and (ii) the role of the global acceleration term, which is the coupling term from the acoustics to the hydrodynamics, is analyzed. For the confined coaxial jet diffusion flame studied here, (i) the growth rate of the thermo-acoustic oscillations is found to be more sensitive to small changes in the hydrodynamic field around the flame and (ii) increasing the global acceleration term is found to be stabilizing in agreement with the Rayleigh Criterion.

  • 11.
    Nicholson, J. M. P.
    et al.
    Univ Nottingham, Dept Mech Mat & Mfg Engn, Nottingham NG7 2RD, England..
    Power, H.
    Univ Nottingham, Dept Mech Mat & Mfg Engn, Nottingham NG7 2RD, England..
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hibberd, S.
    Univ Nottingham, Sch Math Sci, Nottingham NG7 2RD, England..
    Kay, E. D.
    Norton Straw Consultants, Darley Abbey Mills, Steam Engine House, Derby DE22 1DZ, England..
    Fluid dynamics of the slip boundary condition for isothermal rimming flow with moderate inertial effects2019In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 31, no 3, article id 033602Article in journal (Refereed)
    Abstract [en]

    Motivated by evaluating coating oil films within bearing chambers in an aero-engine application, an analysis is presented for the fluid dynamics relevant in their dual capacity as both the coolant and lubricant in highly sheared flows that may approach microscale thickness. An extended model is developed for isothermal rimming flow driven by substantial surface shear within a stationary cylinder. In particular, a partial slip condition replaces the no-slip condition at the wall whilst retaining inertial effects relevant to an intrinsic high speed operation. A depth-averaged formulation is presented that includes appropriate inertial effects at leading-order within a thin film approximation that encompasses a more general model of assessing the impact of surface slip. Non-dimensional mass and momentum equations are integrated across the film depth yielding a one dimensional problem with the a priori assumption of local velocity profiles. The film flow solutions for rimming flow with wall slip are modeled to a higher order than classical lubrication theory. We investigate the impact of wall slip on the transition from pooling to uniform films. Numerical solutions of film profiles are provided for the progressively increased Reynolds number, within a moderate inertia regime, offering evaluation into the effect of film slippage on the dynamics of rimming flow. We find that slip allows non-unique solution regions and existence of multiple possible steady state solutions evaluated in transforming from smooth to pooling film solutions. Additionally, boundary slip is shown to enhance the development of recirculation regions within the film which are detrimental to bearing chamber flows. 

  • 12.
    Rosti, Marco E.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Izbassarov, Daulet
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Hormozi, Sarah
    Ohio Univ, Dept Mech Engn, Athens, OH 45701 USA..
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Turbulent channel flow of an elastoviscoplastic fluid2018In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 853, p. 488-514Article in journal (Refereed)
    Abstract [en]

    We present numerical simulations of laminar and turbulent channel flow of an elastoviscoplastic fluid. The non-Newtonian flow is simulated by solving the full incompressible Navier-Stokes equations coupled with the evolution equation for the elastoviscoplastic stress tensor. The laminar simulations are carried out for a wide range of Reynolds numbers, Bingham numbers and ratios of the fluid and total viscosity, while the turbulent flow simulations are performed at a fixed bulk Reynolds number equal to 2800 and weak elasticity. We show that in the laminar flow regime the friction factor increases monotonically with the Bingham number (yield stress) and decreases with the viscosity ratio, while in the turbulent regime the friction factor is almost independent of the viscosity ratio and decreases with the Bingham number, until the flow eventually returns to a fully laminar condition for large enough yield stresses. Three main regimes are found in the turbulent case, depending on the Bingham number: for low values, the friction Reynolds number and the turbulent flow statistics only slightly differ from those of a Newtonian fluid; for intermediate values of the Bingham number, the fluctuations increase and the inertial equilibrium range is lost. Finally, for higher values the flow completely laminarizes. These different behaviours are associated with a progressive increases of the volume where the fluid is not yielded, growing from the centreline towards the walls as the Bingham number increases. The unyielded region interacts with the near-wall structures, forming preferentially above the high-speed streaks. In particular, the near-wall streaks and the associated quasi-streamwise vortices are strongly enhanced in an highly elastoviscoplastic fluid and the flow becomes more correlated in the streamwise direction.

  • 13. Samuelsson, John
    et al.
    Tammisola, Outi
    Juniper, Matthew
    Department of Engineering, Cambridge University, Cambridge.
    Breaking axi-symmetry in stenotic flow lowers the critical transition Reynolds number2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27Article in journal (Refereed)
    Abstract [en]

    Flow through a sinuous stenosis with varying degrees of non-axisymmetric shape variations and at Reynolds numberranging from 250 to 750 is investigated using direct numerical simulation (DNS) and global linear stability analysis. At low Reynolds numbers (Re < 390), the flow is always steady and symmetric for an axisymmetric geometry. Two steady state solutions are obtained when the Reynolds number is increased: a symmetric steady state and an eccentric, non-axisymmetric steady state. Either one can be obtained in the DNS depending on the initial condition. A linear global stability analysis around the symmetric and non-axisymmetric steady state reveals that both flows are linearly stable for the same Reynolds number, showing that the first bifurcation from symmetry to antisymmetry is subcritical. When the Reynolds number is increased further, the symmetric state becomes linearly unstable to an eigenmode, which drives the flow towards the non-axisymmetric state. The symmetric state remains steady up to Re = 713, while the non-axisymmetric state displays regimes of periodic oscillations for Re ≥ 417 and intermittency for Re ≳ 525. Further, an offset of the stenosis throat is introduced through the eccentricity parameter E. When eccentricity is increased from zero to only 0.3% of the pipe diameter, the bifurcation Reynolds number decreases by more than 50%, showing that it is highly sensitive to non-axisymmetric shape variations. Based on the resulting bifurcation map and its dependency on E, we resolve the discrepancies between previous experimental and computational studies. We also present excellent agreement between our numerical results and previous experimental results.

  • 14.
    Söderberg, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. STFI-Packforsk, Sweden.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    The fundamental mechanism behind headbox jet break-up2008In: TAPPI Press - Paper Conference and Trade Show, PaperCon '08, 2008, p. 3564-3596Conference paper (Refereed)
    Abstract [en]

    It has previously been shown that MD streaks are created in the headbox jet, which is closely connected to the appearance of waves on the jet surface. The fundamental mechanism behind this break-up is presented. This has been achieved by implementing state-of-the-art methods for determining the characteristics and evolution of hydrody-namic instabilities. The methodology also allows the headbox slice to be designed in order to minimise jet break-up. This possibility has been evaluated in pilot-scale.

  • 15.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Global stability of X-junction flow. Part 2: Sensitivity and control2012In: Proceedings of the Global Flow Instability and Control Symposium 2012, 2012Conference paper (Refereed)
  • 16.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Linear stability of plane wakes and liquid jets: global and local approach2009Licentiate thesis, comprehensive summary (Other academic)
  • 17.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Numerical stability studies of one-phase and immiscible two-phase jets and wakes2011Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The initial linear instability growth of two-dimensional plane wakes and jetsis investigated, by temporal two-dimensional global modes, and local spatialstability analysis. Comparisons are also made to experiments, direct numericalsimulations, and methods designed for weakly-non-parallel flows. The studiesproceed through three different flow setups with increasing complexity.The first flow analysed is a convectively unstable liquid sheet surroundedby a stagnant or co-flowing gas. The experimentally measured growth rates arefound to be in excellent agreement with spatial stability calculations, if the airboundary layer is taken into account, and not otherwise. The stabilizing effectof moderate air co-flow is quantified in the numerical study, and the governingparameters found to be the speed difference between water and air, and theshear from air at the water surface (inversely proportional to the air boundarylayer thickness).The second flow case is a one-phase confined wake, i.e. a wake in a channel.The effect of confinement (wall distance) on the global stability of wakes isanalysed by linear global modes, and compared to the results from DNS andweakly-non-parallel theory. At Re = 100, confinement is globally stabilizing,mostly due to a faster development towards a parabolic profile for confinedflows. The stabilizing effect of confinement almost disappears at Re ≈ 400.However, when the structural sensitivity of the wakes is analysed by an adjointbasedapproach, fundamental differences are seen in the global wavemakers ofconfined and unconfined wakes at Re ≈ 400.The third and most complex flow case is immiscible two-fluid wakes andjets. A parallel multi-domain spectral code is developed, where the kinematicand dynamic conditions on the interface are imposed as coupling conditions. Itis shown that intermediate values of surface tension can destabilize stable wakesand jets. In addition, surface tension has a considerable influence on the globaloscillation frequency and spatial shape of the global mode for unstable wakes.The character of the mode is gradually changed from a wake instability to aglobal shear layer instability. Both symmetric and antisymmetric modes areencountered for both wakes and jets, depending on the strength of the surfacetension (value of the Weber number) and the flow case.iii

  • 18.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Optimal wavy surface to suppress vortex shedding using second-order sensitivity to shape changes2016In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, article id 10.1016/j.euromechflu.2016.12.006Article in journal (Refereed)
    Abstract [en]

    A method to find optimal 2nd-order perturbations is presented, and applied to find the optimal spanwise-wavy surface for the suppression of cylinder wake instability. As shown in recent studies (Hwang et al., 2013, Tammisola et al., 2014, Del Guercio et al., 2014), 2nd-order perturbations are required to capture the stabilizing effect of spanwise waviness, which is ignored by standard adjoint-based sensitivity analyses. Here, previous methods are extended so that (i) 2nd-order sensitivity is formulated for base flow changes satisfying the linearised Navier–Stokes, and (ii) the resulting method is applicable to a 2D global instability problem. This makes it possible to formulate the 2nd-order sensitivity to shape modifications. This formulation is used to find the optimal shape to suppress the a cylinder wake instability. The optimal shape is then perturbed by random distributions in full 3D stability analysis to confirm that it is a local optimal at the given amplitude and wavelength. At Re=100, surface waviness of maximum height 1% of the cylinder diameter is sufficient to stabilize the flow. The optimal surface creates streaks passively by extracting energy from the base flow derivatives and altering the tangential velocity component at the wall. This paper extends previous techniques to a fully two-dimensional method to find boundary perturbations which optimize the 2nd-order drift. The method should be applicable to generic flow instability problems, and to different types of control, such as boundary forcing, shape modulation or suction.

  • 19.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Oscillatory sensitivity patterns for global modes in wakes2012In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 701, p. 251-277Article in journal (Refereed)
    Abstract [en]

    Globally unstable wakes with co-flow at intermediate Reynolds numbers are studied, to quantify important spatial regions for the development and control of the global instability. One region of high structural sensitivity is found close to the inlet for all wakes, in agreement with previous findings for cylinder wakes. A second, elongated region of high structural sensitivity is seen downstream of the first one for unconfined wakes at Re = 400. When base-flow modifications are considered, a spatially oscillating sensitivity pattern is found inside the downstream high-structural-sensitivity region. This implies that the same change in the base flow can either destabilize or stabilize the flow, depending on the exact position where it is applied. It is shown that the sensitivity pattern remains unchanged for different choices of streamwise boundary conditions and numerical resolution. The actual base-flow is modified in selected configurations, and the linear global modes recomputed. It is confirmed that the linear global eigenvalues move according to the predicted sensitivity pattern for small-amplitude base-flow modifications, for which the theory applies. We also look at the implications of a small control cylinder for the flow. Only the upstream high-sensitivity region proves to be robust in terms of control, but one should be careful not to disturb the flow in the downstream high-sensitivity region, in order to achieve control. The findings can have direct implications for the numerical resolution requirements for wakes at higher Reynolds numbers. Furthermore, they provide one more possible explanation for why confined wakes have a more narrow frequency spectrum than unconfined wakes.

  • 20.
    Tammisola, Outi
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. Univ Nottingham, England.
    Juniper, M. P.
    Coherent structures in a swirl injector at Re=4800 by nonlinear simulations and linear global modes2016In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 792, p. 620-657Article in journal (Refereed)
    Abstract [en]

    The large-scale coherent motions in a realistic swirl fuel-injector geometry are analysed by direct numerical simulations (DNS), proper orthogonal decomposition (POD), and linear global modes. The aim is to identify the origin of instability in this turbulent flow in a complex internal geometry. The flow field in the nonlinear simulation is highly turbulent, but with a distinguishable coherent structure: the precessing vortex core (a spiralling mode). The most energetic POD mode pair is identified as the precessing vortex core. By analysing the fast Fourier transform (FFT) of the time coefficients of the POD modes, we conclude that the first four POD modes contain the coherent fluctuations. The remaining POD modes (incoherent fluctuations) are used to form a turbulent viscosity field, using the Newtonian eddy model. The turbulence sets in from convective shear layer instabilities even before the nonlinear flow reaches the other end of the domain, indicating that equilibrium solutions of the Navier-Stokes are never observed. Linear global modes are computed around the mean flow from DNS, applying the turbulent viscosity extracted from POD modes. A slightly stable discrete m = 1 eigenmode is found, well separated from the continuous spectrum, in very good agreement with the POD mode shape and frequency. The structural sensitivity of the precessing vortex core is located upstream of the central recirculation zone, identifying it as a spiral vortex breakdown instability in the nozzle. Furthermore, the structural sensitivity indicates that the dominant instability mechanism is the Kelvin-Helmholtz instability at the inflection point forming near vortex breakdown. Adjoint modes are strong in the shear layer along the whole extent of the nozzle, showing that the optimal initial condition for the global mode is localized in the shear layer. We analyse the qualitative influence of turbulent dissipation in the stability problem (eddy viscosity) on the eigenmodes by comparing them to eigenmodes computed without eddy viscosity. The results show that the eddy viscosity improves the complex frequency and shape of global modes around the fuel-injector mean flow, while a qualitative wavemaker position can be obtained with or without turbulent dissipation, in agreement with previous studies. This study shows how sensitivity analysis can identify which parts of the flow in a complex geometry need to be altered in order to change its hydrodynamic stability characteristics.

  • 21.
    Tammisola, Outi
    et al.
    University of Nottingham, Coates Building, United Kingdom.
    Juniper, Matthew
    Department of Engineering, Cambridge University, Cambridge.
    Adjoint sensitivity analysis of hydrodynamic stability in a gas turbine fuel injector2015In: Proceedings of the ASME Turbo Expo 2015, ASME Press, 2015Conference paper (Refereed)
    Abstract [en]

    Hydrodynamic oscillations in gas turbine fuel injectors help to mix the fuel and air but can also contribute to thermoacoustic instability. Small changes to some parts of a fuel injector greatly affect the frequency and amplitude of these oscillations. These regions can be identified efficiently with adjoint-based sensitivity analysis. This is a linear technique that identifies the region of the flow that causes the oscillation, the regions of the flow that are most sensitive to external forcing, and the regions of the flow that, when altered, have most influence on the oscillation. In this paper, we extend this to the flow from a gas turbine’s single stream radial swirler, which has been extensively studied experimentally (GT2008-50278) [8].

    The swirling annular flow enters the combustion chamber and expands to the chamber walls, forming a conical recirculation zone along the centreline and an annular recirculation zone in the upstream corner. In this study, the steady base flow and the stability analysis are calculated at Re 200–3800 based on the mean flow velocity and inlet diameter. The velocity field is similar to that found from experiments and LES, and the local stability results are close to those at higher Re (GT2012-68253) [11].

    All the analyses (experiments, LES, uRANS, local stability, and the global stability in this paper) show that a helical motion develops around the central recirculation zone. This develops into a precessing vortex core. The adjoint-based sensitivity analysis reveals that the frequency and growth rate of the oscillation is dictated by conditions just upstream of the central recirculation zone (the wavemaker region). It also reveals that this oscillation is very receptive to forcing at the sharp edges of the injector. In practical situations, this forcing could arise from an impinging acoustic wave, showing that these edges could be influential in the feedback mechanism that causes thermoacoustic instability.

    The analysis also shows how the growth rate and frequency of the oscillation change with either small shape changes of the nozzle, or additional suction or blowing at the walls of the injector. It reveals that the oscillations originate in a very localized region at the entry to the combustion chamber, which lies near the separation point at the outer inlet, and extends to the outlet of the inner pipe. Any scheme designed to control the frequency and amplitude of the oscillation only needs to change the flow in this localized region.

  • 22.
    Tammisola, Outi
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Loiseau, Jean-Christophe
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Effect of viscosity ratio on the self-sustained instabilities in planar immiscible jets2017In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 2, no 3, article id 033903Article in journal (Refereed)
    Abstract [en]

    Previous studies have shown that intermediate magnitude of surface tension has a counterintuitive destabilizing effect on two-phase planar jets. In the present study, the transition process in confined two-dimensional jets of two fluids with varying viscosity ratio is investigated using direct numerical simulations (DNSs). The outer fluid coflow velocity is 17% of that of the central jet. Neutral curves for the appearance of persistent oscillations are found by recording the norm of the velocity residuals in DNS for over 1000 nondimensional time units or until the signal has reached a constant level in a logarithmic scale, either a converged steady state or a "statistically steady" oscillatory state. Oscillatory final states are found for all viscosity ratios ranging from 10-1 to 10. For uniform viscosity (m = 1), the first bifurcation is through a surface-tension-driven global instability. On the other hand, for low viscosity of the outer fluid, there is a mode competition between a steady asymmetric Coanda-type attachment mode and the surface-tension-induced mode. At moderate surface tension, the first bifurcation is through the Coanda-type attachment, which eventually triggers time-dependent convective bursts. At high surface tension, the first bifurcation is through the surface-tension-dominated mode. For high viscosity of the outer fluid, persistent oscillations appear due to a strong convective instability, although it is shown that absolute instability may be possible at even higher viscosity ratios. Finally, we show that the jet is still convectively and absolutely unstable far from the inlet when the shear profile is nearly constant. Comparing this situation to a parallel Couette flow (without inflection points), we show that in both flows, a hidden interfacial mode brought out by surface tension becomes temporally and absolutely unstable in an intermediate Weber and Reynolds regime. By an energy analysis of the Couette flow case, we show that surface tension, although dissipative, can induce a velocity field near the interface that extracts energy from the flow through a viscous mechanism. This study highlights the rich dynamics of immiscible planar uniform-density jets, where different self-sustained and convective mechanisms compete and the nature of the instability depends on the exact parameter values.

  • 23.
    Tammisola, Outi
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Loiseau, Jean-Christophe
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    The role of viscosity stratification on the global stability of two-phase jets2016In: Proceedings of the International Conference on Multiphase Flow, 2016Conference paper (Refereed)
  • 24.
    Tammisola, Outi
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Hellstrom, Georg
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Lagerstedt, Torgny
    Spin coating of Blu-Ray disks: modeling, experiments, limitations, and manipulation2010In: JCT Research, ISSN 1547-0091, E-ISSN 2168-8028, Vol. 7, no 3, p. 315-323Article in journal (Refereed)
    Abstract [en]

    A major challenge in the production of Blu-Ray Disks (BDs) is making the cover layer over the information area. The layer has to be both thick enough to protect the information and even enough for the information to be read through it optically. Furthermore, it is preferred not to cover the hole in the center of the disk. Spin coating is a candidate method for the production of these layers in a rapid reproduction process. When dispensing is performed off-center (in order not to cover the hole), a new complication appears, namely the formation of a slope toward the inner rim of the liquid film. Here, fundamental limitations for achieving even films in this system and ways to overcome the difficulties by manipulation of the process are studied. A mathematical model for this particular case of spin coating is obtained and validated by comparison with experiments made in industrial equipment aiming at producing BDs. The model agrees well with the experimental data. The model is then used to show that cover layers that fulfil the Blu-Ray specification are very difficult to produce with the spin-coating technique. Manipulation by inline curing and surface shear is added to the model and the results show that it is considerably easier to meet the BD specification when utilizing the manipulation.

  • 25.
    Tammisola, Outi
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wehrfritz, Armin
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, Daniel
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Global linear and nonlinear stability of viscous confined plane wakes with co-flow2011In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 675, p. 397-434Article in journal (Refereed)
    Abstract [en]

    The global stability of confined wakes is studied numerically, using two-dimensionallinear global modes and nonlinear direct numerical simulations (DNS).The wake inflow velocity is varied between different amounts of co-flow (basebleed), while the density and viscosity are assumed to be constant everywherein the flow domain. In accordance with previous studies, we find that thefrequencies of both the most unstable linear and the saturated nonlinear globalmode increase with confinement. Here, we also find that for wake Reynoldsnumber Re = 100, the confinement is stabilising. It decreases both the growthrate of the linear and the saturation amplitude of the nonlinear modes. Weconclude that the dampening effect is connected to the streamwise developmentof the base flow, and for higher Reynolds numbers this effect decreases, sincethe flow becomes more parallel. The linear analysis reveals that the criticalwake velocities below which the flow becomes unstable are almost identicalfor unconfined and confined wakes at Re ≈ 400. Also, the present resultsare compared with literature data for an inviscid parallel wake due to thesimilarity of inflow profile. The confined wake is found to be more stable thanits inviscid counterpart, while the unconfined wake is more unstable than theinviscid wake. The main reason to both can be explained by the base flowdevelopment. A detailed comparison of the linear and nonlinear results revealsthat the most unstable linear global mode gives an excellent prediction of theinitial nonlinear behaviour and therefore the stability boundary, in all cases.However, the nonlinear saturated state is quite different in particular for higherReynolds numbers. For Re = 100, the saturated frequency also differs less than5% from the linear frequency, and trends regarding confinement observed in thelinear analysis are confirmed.141

  • 26.
    Tammisola, Outi
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Global linear stability of confined wakes with co-flowManuscript (Other academic)
    Abstract [en]

    The global stability of confined wakes is studied numerically, using 2D global modes. The wake inlet velocity is varied, and the main focus lies in wakes with co-flow (base bleed). The density and viscosity are assumed to be constant in and outside the wake. We find that for wake Reynolds number 100, the confinement is stabilising in general. The frequency of the unstable mode increases, and wavelength decreases with confinement. By comparing with an artificial, more slowly developing wake, we conclude that the dampening effect is due to the mean flow development. For higher Reynolds numbers this effect decreases. The critical inlet velocities for which the wake becomes unstable are almost identical for unconfined and confined wakes at $Re=400$. Also, the present results are compared with results for an inviscid parallel wake found in literature, since the inlet profile is very similar to the inviscid profile. The confined wake is found to be more stable than its inviscid counterpart, while the unconfined wake is more unstable than the inviscid wake. The reason to both can be explained by the mean flow development.

     

     

     

  • 27.
    Tammisola, Outi
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics.
    On the global stability of a plane liquid jet surrounded by gas: problem formulation and preliminary resultsManuscript (Other academic)
    Abstract [en]

    The global stability of a liquid sheet in gas is studied. The global 3D stability problem for a 2D base flow is formulated, including surface tension of the interface, and the viscosity and density of both phases. The implementational requirements are clarified, and met by using a parallel code for eigenvalue computations based on the mathematical software libraries PARPACK and ScaLAPACK. Preliminary eigenvalue spectra and eigenmodes are presented for the case of water jet surrounded by air.

     

  • 28.
    Tammisola, Outi
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Surface tension induced global destabilization of plane jets and wakesManuscript (preprint) (Other academic)
    Abstract [en]

    The effect of surface tension on global stability of confined co-flow jets andwakes at a moderate Reynolds number is studied. The flow cases under studyare globally stable without surface tension. It is found that surface tensioncan cause the flow to be unstable if the inlet shear is strong enough. For evenstronger surface tension, the flow is re-stabilized. As long as there is no changeof the most unstable mode, increasing surface tension seems to decrease the oscillationfrequency and increase the wavelength of the mode. The critical shear(minimum shear at which an instability is found) is found to occur for antisymmetricdisturbances for the wakes and symmetric disturbances for the jets.However, at stronger shear, the opposite symmetry might be the most unstableone, in particular for wakes at high surface tension. The results show strongeffects of surface tension that should be possible to reproduce experimentallyas well as numerically.

  • 29.
    Tammisola, Outi
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Sasaki, Atsushi
    Matsubara, Masaharu
    Global stability of a plane liquid jet surrounded by gas2010In: SEVENTH IUTAM SYMPOSIUM ON LAMINAR-TURBULENT TRANSITION / [ed] Schlatter P; Henningson DS, 2010, Vol. 18, p. 403-408Conference paper (Refereed)
    Abstract [en]

    The global stability of a liquid sheet in gas is studied. The global 3D stability problem for a 2D base flow is formulated, including surface tension of the interface, and the viscosity and density of both phases. The implementational requirements are clarified, and met by using a parallel code for eigenvalue computations based on the mathematical software libraries PARPACK and ScaLAPACK. Preliminary eigenvalue spectra and eigenmodes are presented for the case of a water jet surrounded by air.

  • 30.
    Tammisola, Outi
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Söderberg, L. Daniel
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Effect of surface tension on global modes of confined wake flows2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 1, p. 014108-Article in journal (Refereed)
    Abstract [en]

    Many wake flows are susceptible to self-sustained oscillations, such as the well-known von Karman vortex street behind a cylinder that makes a rope beat against a flagpole at a distinct frequency on a windy day. One appropriate method to study these global instabilities numerically is to look at the growth rates of the linear temporal global modes. If all growth rates for all modes are negative for a certain flow field then a self-sustained oscillation should not occur. On the other hand, if one growth rate for one mode is slightly positive, the oscillation will approximately obtain the frequency and shape of this global mode. In our study, we first introduce surface tension between two fluids to the wake-flow problem. Then we investigate its effects on the global linear instability of a spatially developing wake with two co-flowing immiscible fluids. The inlet profile consists of two uniform layers, which makes the problem easily parametrizable. The fluids are assumed to have the same density and viscosity, with the result that the interface position becomes dynamically important solely through the action of surface tension. Two wakes with different parameter values and surface tension are studied in detail. The results show that surface tension has a strong influence on the oscillation frequency, growth rate, and shape of the global mode(s). Finally, we make an attempt to confirm and explain the surface-tension effect based on a local stability analysis of the same flow field in the streamwise position of maximum reverse flow.

  • 31.
    Tammisola, Outi
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Söderberg, L. Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Surface tension-induced global instability of planar jets and wakes2012In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 713, p. 632-658Article in journal (Refereed)
    Abstract [en]

    The effect of surface tension on global stability of co-flow jets and wakes at a moderate Reynolds number is studied. The linear temporal two-dimensional global modes are computed without approximations. All but one of the flow cases under study are globally stable without surface tension. It is found that surface tension can cause the flow to be globally unstable if the inlet shear (or, equivalently, the inlet velocity ratio) is strong enough. For even stronger surface tension, the flow is restabilized. As long as there is no change of the most unstable mode, increasing surface tension decreases the oscillation frequency. Short waves appear in the high-shear region close to the nozzle, and their wavelength increases with increasing surface tension. The critical shear (the weakest inlet shear at which a global instability is found) gives rise to antisymmetric disturbances for the wakes and symmetric disturbances for the jets. However, at stronger shear, the opposite symmetry can be the most unstable one, in particular for wakes at high surface tension. The results show strong effects of surface tension that should be possible to reproduce experimentally as well as numerically.

  • 32.
    Tammisola, Outi
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Sasaki, Atsushi
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Mechanics.
    Matsubara, Masaharu
    Söderberg, L. Daniel
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Mechanics.
    Stabilizing effect of surrounding gas flow on a plane liquid sheet2011In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 672, p. 5-32Article in journal (Refereed)
    Abstract [en]

    The stability of a plane liquid sheet is studied experimentally and theoretically, with an emphasis on the effect of the surrounding gas. Co-blowing with a gas velocity of the same order of magnitude as the liquid velocity is studied, in order to quantify its effect on the stability of the sheet. Experimental results are obtained for a water sheet in air at Reynolds number Re-t = 3000 and Weber number We = 300, based on the half-thickness of the sheet at the inlet, water mean velocity at the inlet, the surface tension between water and air and water density and viscosity. The sheet is excited with different frequencies at the inlet and the growth of the waves in the streamwise direction is measured. The growth rate curves of the disturbances for all air flow velocities under study are found to be within 20% of the values obtained from a local spatial stability analysis, where water and air viscosities are taken into account, while previous results from literature assuming inviscid air overpredict the most unstable wavelength with a factor 3 and the growth rate with a factor 2. The effect of the air flow on the stability of the sheet is scrutinized numerically and it is concluded that the predicted disturbance growth scales with (i) the absolute velocity difference between water and air (inviscid effect) and (ii) the square root of the shear from air on the water surface (viscous effect).

  • 33.
    Tammisola, Outi
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Sasaki, Atsushi
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Matsubara, Masaharu
    Stabilisation of a plane liquid sheet by gas flow: experiments and theoryManuscript (Other academic)
    Abstract [en]

    The stability of a plane liquid jet is studied experimentally and theoretically. The main emphasis is on the effect of the surrounding air. Co-blowing with a gas velocity of the same order of magnitude as the liquid velocity is studied. Experimental results are obtained and the growth rates of the disturbances are found to be within 20\% of the values obtained from local spatial stability analysis. The effect of different parameters of the gas flow on the stability of the jet is scrutinized and it is concluded that the predicted disturbance growth is mostly sensitive to the shear from the air on the liquid surface and the velocity difference between the water and the air.

    The overall disturbance growth and wavelength scales with the difference between the gas and liquid velocity.

     

     

1 - 33 of 33
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