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  • 101.
    Sattarzadeh, Sohrab S.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Experimental investigation on the steady and unsteady disturbances in a flat plate boundary layer2014In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 26, no 12, p. 124103-Article in journal (Refereed)
    Abstract [en]

    Recent experiments have shown that miniature vortex generators (MVGs) are coveted devices to stabilize unsteady disturbances in flat plate boundary layers and to delay the onset of turbulence by modulating the base flow in the spanwise direction. The spanwise modulation is a result from the non-modal transient growth of steady and spanwise periodic streamwise vortices being generated by the MVGs. The present experimental investigation aims at studying the transient growth of non-modal disturbances induced by a spanwise periodic array of MVGs and its stabilizing effect on non-linear unsteady disturbances in the boundary layer originating from planar Tollmien-Schlichting (TS) waves. Measurements consist of cross-stream planes at different downstream locations in the boundary layer and a spatio-temporal analysis of different modes of the disturbances is carried out. In the streaky boundary layer generated by the MVGs the fundamental spanwise mode, with the same wavelength as the MVG pairs in the array, and its first harmonic, both undergo transient growth whereas the higher harmonics decay immediately downstream of the array. In the unstable region formed in the wake of the MVG blades, i.e., just downstream of the array, a wide range of spanwise modes contributes to an initial growth in the energy of unsteady disturbances. Similar behavior is observed upstream of branch II position of the neutral stability curve where the unsteady disturbances undergo a second energy growth in the unstable region. It is shown that the spatial gradients of the base flow in the wall-normal and spanwise directions are contributing to the amplification and attenuation of the TS wave disturbances, respectively, in the streaky boundary layer.

  • 102.
    Schlatter, Philipp
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    de lange, H. C.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On streak breakdown in bypass transition2008In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 20, no 10, p. 101505-Article in journal (Refereed)
    Abstract [en]

    Recent theoretical, numerical, and experimental investigations performed at the Department of Mechanics, KTH Stockholm, and the Department of Mechanical Engineering, Eindhoven University of Technology, are reviewed, and new material is presented to clarify the role of the boundary-layer streaks and their instability with respect to turbulent breakdown in bypass transition in a boundary layer subject to free-stream turbulence. The importance of the streak secondary-instability process for the generation of turbulent spots is clearly shown. The secondary instability manifests itself as a growing wave packet located on the low-speed streak, increasing in amplitude as it is dispersing in the streamwise direction. In particular, qualitative and quantitative data pertaining to temporal sinuous secondary instability of a steady streak, impulse responses both on a parallel and a spatially developing streak, a model problem of bypass transition, and full simulations and experiments of bypass transition itself are collected and compared. In all the flow cases considered, similar characteristics in terms of not only growth rates, group velocity, and wavelengths but also three-dimensional visualizations of the streak breakdown have been found. The wavelength of the instability is about an order of magnitude larger than the local boundary-layer displacement thickness delta*, the group velocity about 0.8 of the free-stream velocity U(infinity), and the growth rate on the order of a few percent of U(infinity)/delta*. The characteristic structures at the breakdown are quasistreamwise vortices, located on the flanks of the low-speed region arranged in a staggered pattern.

  • 103.
    Schlatter, Philipp
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Quantifying the interaction between large and small scales in wall-bounded turbulent flows: A note of caution2010In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 22, no 5, p. 051704-Article in journal (Refereed)
    Abstract [en]

    Turbulent flow close to solid walls is dominated by an ensemble of fluctuations of large and small spatial scales. Recent work by Mathis [J. Fluid Mech. 628, 311 (2009); Phys. Fluids 21, 111703 (2009)] introduced and used a decoupling procedure based on the Hilbert transformation applied to the filtered small-scale component of the fluctuating streamwise velocity. This method is employed as a robust tool to quantify a dominant amplitude modulation of the small scales by the large scales found in the outer part of the boundary layer. In the present study, however, we demonstrate by means of experimental and synthetic signals that the correlation coefficient used to quantify the amplitude modulation is related to the skewness of the original signal, and hence, for the Reynolds numbers considered here, may not be an independent tool to unambiguously detect or quantify the effect of large-scale amplitude modulation of the small scales.

  • 104.
    Schlatter, Philipp
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Li, Qiang
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Turbulent boundary layers up to Re-theta=2500 studied through simulation and experiment2009In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Physics of Fluids, Vol. 21, no 5, p. 051702-Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations (DNSs) and experiments of a spatially developing zero-pressure-gradient turbulent boundary layer are presented up to Reynolds number Re-theta=2500, based on momentum thickness theta and free-stream velocity. For the first time direct comparisons of DNS and experiments of turbulent boundary layers at the same (computationally high and experimentally low) Re-theta are given, showing excellent agreement in skin friction, mean velocity, and turbulent fluctuations. These results allow for a substantial reduction of the uncertainty of boundary-layer data, and cross validate the numerical setup and experimental technique. The additional insight into the flow provided by DNS clearly shows large-scale turbulent structures, which scale in outer units growing with Re-theta, spanning the whole boundary-layer height.

  • 105.
    Schrader, Lars-Uve
    et al.
    Univ Ottawa, Ottawa, Canada.
    Mavriplis, Catherine
    Univ Ottawa, Ottawa, Canada.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Spatial linear disturbances in a plane wall jet2012In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 24, no 5, p. 054104-Article in journal (Refereed)
    Abstract [en]

    A two-dimensional direct numerical simulation study of the linear instability in a laminar plane wall jet is presented. The evolution of the wall jet disturbances is in reasonable agreement with predictions by spatial linear stability theory only with regard to the wavelength and the amplitude shape of the disturbance, whereas significant differences in the linear growth rate are noticed. As a consequence, the "stable island" on the instability map based on linear stability theory turns out to be connected with the outer stable region in the simulations, thus taking the form of a "stable peninsula". The failure of the theory is attributed to the rapid streamwise spread and decay of the wall jet, which is incompatible with the assumption of parallel flow. We also assess the maximum possible transient linear amplification of two-dimensional disturbances in the plane wall jet, using the concept of optimal initial disturbances. The transient energy growth relies on the Orr mechanism, and the upper bound of the disturbance energy increases linearly in time for the present flow configuration. The optimal disturbances exhibit local maxima near the edge of the jet and close to the wall, where sites of effective receptivity are hence expected. We find that the outer region of the plane wall jet is more receptive to time periodic forcing than the inner region.

  • 106.
    Segalini, Antonio
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Talamelli, A.
    Experimental analysis of dominant instabilities in coaxial jets2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 2, p. 024103-Article in journal (Refereed)
    Abstract [en]

    An experimental analysis of the dominant instabilities in the near field of two coaxial jets is presented and discussed. Different inner/outer jet velocity pairs (U-i, U-o) have been tested in order to investigate the effect of both velocity ratio r(u) = U-o/U-i and Reynolds number. Three main instabilities have emerged depending on the velocity ratio. For r(u) < 0.75, the coaxial jets dynamic is driven by the inner shear layer. For r(u) > 1.6, the outer shear layer dominates the near field vortex dynamics while for r(u) nearly unitary, the vortex shedding behind the separating wall imposes its own dynamics. A new scaling relationship is proposed to improve the estimation of the shedding frequency with respect to the one found in literature.

  • 107.
    Sembian, Sundarapandian
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Liverts, Michael
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Apazidis, Nicholas
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Attenuation of strong external blast by foam barriers2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 9, article id 096105Article in journal (Refereed)
    Abstract [en]

    The mitigation of externally generated strong blast waves by an aqueous foam barrier of varying configurations within fixed distance between the explosion origin and the object to be protected is investigated and quantified both experimentally and numerically. The blast waves of shock Mach number 4.8 at 190 mm from the explosion plane are generated using exploding wire technique. The initially cylindrical blast waves are transformed into a plane blast wave in a specially constructed test unit in which the experiments are performed. The shock waves emanating from the foam barrier are captured using shadowgraph technique. A simple numerical model treating the foam by a pseudo-gas approach is used in interpreting and reconstructing the experimental results. The additional contribution of the impedance mismatch factor is analysed with the aid of numerical simulation and exploited for achieving greater blast wave pressure reduction.

  • 108.
    Sembian, Sundarapandian
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Liverts, Michael
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Tillmark, Nils
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Apazidis, Nicholas
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Plane shock wave interaction with a cylindrical water column2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 5, article id 056102Article in journal (Refereed)
    Abstract [en]

    A complex system of waves propagating inside a water column due to the impact of plane shock wave is investigated both experimentally and numerically. Flow features, such as, focusing of expansion waves generating large negative pressure, nucleation of cavitation bubbles, and a re-circulation zone are observed and discussed qualitatively and quantitatively. Experiments are conducted on a 22 mm diametrical water column hit by shock waves with Mach numbers 1.75 and 2.4 in a newly constructed exploding wire facility. A new technique to create a properly shaped, repeatable, large diameter water column with straight walls is presented. Qualitative features of the flow are captured using the shadowgraph technique. With the aid of numerical simulations the wave motions inside the column are analyzed; the spatial location of the expansion wave focusing point and the corresponding negative peak pressures is estimated.

  • 109. Shiomi, J.
    et al.
    Amberg, Gustav
    KTH, Superseded Departments, Mechanics.
    Active control of a global thermocapillary instability2002In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 14, no 9, p. 3039-3045Article in journal (Refereed)
    Abstract [en]

    Active control was applied to oscillatory thermocapillary flow in an open cylindrical container filled with Silicone oil. Thermocapillary convection was driven by imposing a radial temperature gradient on a flat free surface. This is an extension of the previous work by Shiomi who applied proportional feedback control by locally heating the surface at a single position using the local temperature signal at a different position fed back through a simple algorithm. Although significant attenuation of the oscillation was detected, an uncertainty remained if global stabilization was achieved. In the present paper, two sensor/heater pairs were installed to achieve the global suppression of the oscillation. Successful global stabilization of the oscillation was achieved in a range of Marangoni number, with the best performance in the weakly nonlinear regime. Using a reliable temperature measurement method, quantitative analysis is carried out to quantify the performance of the control. The optimal values of gain and relative position of sensor/heater pairs were identified.

  • 110. Shiomi, J.
    et al.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
    Numerical investigation of feedback control of thermocapillary instability2005In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 17, no 5Article in journal (Refereed)
    Abstract [en]

    Control of oscillatory thermocapillary convection in an annular geometry with a horizontal free surface is investigated by means of a numerical simulation. The objective is to suppress oscillations using a feedback opposition control. The temperature is measured at certain positions on the interface, this signal is amplified and used to apply local heating on the free surface. Many features of the controlled system observed in previous experiments could be reproduced by the simulation. The numerical simulation allows us to clarify the picture of the spatial structures of the controlled oscillation, which was not accessible in the experiments. In addition to what was found in the previous experiments, the present simulations also permit us to investigate the importance of the positioning of sensors and heaters, and the influence of the properties of the heaters.

  • 111.
    Sinibaldi, Giorgia
    et al.
    Sapienza University of Rome.
    Lacagnina, Giovanni
    Sapienza University of Rome.
    Marino, Luca
    Sapienza University of Rome.
    Romano, Giovanni Paolo
    Sapienza University of Rome.
    Aeroacoustics and aerodynamics of impinging supersonic jets: Analysis of the screech tones2013In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 25, no 8, article id 086104Article in journal (Refereed)
    Abstract [en]

    The interaction between acoustics and aerodynamics of a supersonic jet is an actual fundamental topic which has been a matter of discussion in the last decades. The present paper is devoted to the experimental analysis of free and impinging jets with particular attention on the effect of an impinging surface on screech tones. The acoustics is studied using free-field microphones, while Particle Image Velocimetry is used to investigate the velocity field. The analysis of acquired data allowed to verify and explain the coupling between acoustic discrete tones and mean and fluctuating flow velocities.

  • 112.
    Sjögren, Torbjörn
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Johansson, Arne, V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Development and calibration of algebraic nonlinear models for terms in the Reynolds stress transport equations2000In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, no 12, p. 1554-1572Article in journal (Refereed)
    Abstract [en]

    A simple and straightforward method is presented for the derivation and calibration of algebraic nonlinear models for terms in Reynolds stressturbulence closures. The method extensively utilizes data from direct numerical simulations to allow an investigation of the model performance over the entire Reynolds stressanisotropy-invariant map. The model constants are determined from the condition of minimizing the mean square error over the invariant map, in order to give good model behavior for as wide a class as possible of flow situations. A low Reynolds number closure is proposed based on the most general form for closing the Reynolds stresstransport equations in terms of Reynolds stresses and total dissipation rate. It is shown that forcing the closure to satisfy realizability in a strict sense leads to a good model behavior even for the complicated flow situation near a wall, without any use of ad-hoc wall damping functions in the closure. The model behavior in homogeneous turbulent flow is analyzed by formulating equations for invariant measures, yielding several quite general results for the behavior of the present and other existing models. A new approach to the modeling effects of rotation in the context of Reynolds stress closures is presented and tested for some different homogeneous flows subjected to rotation.

  • 113. Skote, M.
    et al.
    Haritonidis, J. H.
    Henningson, Dan S.
    KTH, Superseded Departments, Mechanics.
    Varicose instabilities in turbulent boundary layers2002In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 14, no 7, p. 2309-2323Article in journal (Refereed)
    Abstract [en]

    An investigation of a model of turbulence generation in the wall region of a turbulent boundary layer is made through direct numerical simulations. The model is based on the varicose instability of a streak. First, a laminar boundary layer disturbed by a continuous blowing through a slot is simulated in order to reproduce and further investigate the results reported from the experiments of Acarlar and Smith [J. Fluid Mech. 175, 43 (1987)]. An isolated streak with an inflectional profile is generated that becomes unstable, resulting in a train of horseshoe vortices. The frequency of the vortex generation is equal to the experimental results. Comparison of the instability characteristics to those predicted through an Orr-Sommerfeld analysis are in good agreement. Second, a direct numerical simulation of a turbulent boundary layer is performed to point out the similarities between the horseshoe vortices in a turbulent and a laminar boundary layer. The characteristics of streaks and the vortical structures surrounding them in a turbulent boundary layer compare well with the model streak. The results of the present study show that one mechanism for the generation of horseshoe vortices in turbulent boundary layers is related to a normal inflectional instability of the streaks.

  • 114. Stolz, S.
    et al.
    Schlatter, Philipp
    Kleiser, L.
    High-pass filtered eddy-viscosity models for large-eddy simulations of transitional and turbulent flow2005In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 17, no 6Article in journal (Refereed)
    Abstract [en]

    Classical fixed-coefficient eddy-viscosity models for large-eddy simulations (LES), e. g., the Smagorinsky [Mon. Weather Rev. 93, 99 (1963)] and the structure-function model [Metais and Lesieur, J. Fluid Mech. 239, 157 (1992)], have deficiencies with accurately describing laminar flow regions and the viscous sublayer of near-wall turbulence. This is mainly due to unphysically large model contributions in such flow regions and is one main reason for the difficulty with correctly predicting laminar-turbulent transition using such models. For transitional flows, subgrid-scale (SGS) models must be able to properly deal with at least the two limits of the laminar and the turbulent fluid state. The dynamic Smagorinsky model [Germano et al., Phys. Fluids A 3, 1760 (1991)] alleviates this problem by dynamically computing a small, but not necessarily vanishing, value for the model coefficient in laminar regions and in the vicinity of the wall. In this contribution we analyze high-pass filtered (HPF) eddy-viscosity models which were proposed recently and independently by Vreman [Phys. Fluids 15, L61 (2003)] and Stolz et al. [Direct and Large-Eddy Simulation V (Kluwer, Dordrecht, 2004), pp. 81-88]. We investigate high-pass filtering suitable for such HPF eddy-viscosity models, e.g., the Smagorinsky or the (filtered) structure-function model. Furthermore, we suggest suitable high-pass filters and examine the influence of different high-pass filters on the results of LES of transitional and turbulent incompressible channel flow. Except for the filter shape, the cutoff wavenumber of the high-pass filter is the only parameter besides the eddy-viscosity model coefficient, and its influence can be minimized by a proposed adaptation of the model coefficient. We find that high-pass filtering employed to the computational quantities prior to the calculation of the eddy-viscosity and strain rate in the SGS model significantly improves the quality of the prediction of transitional and turbulent flows without using any ad-hoc adaptation or dynamic procedure. Of particular importance is that the sensitivity of the results to the model coefficient is considerably reduced by the high-pass filtering. Furthermore, the proposed high-pass filters enable the computation of the structure function in the filtered or HPF structure-function models in all spatial directions also for inhomogeneous flows and on non-equidistant grids, removing the arbitrariness of a special treatment of selected (e.g., wall-normal) directions. Simulation results are presented for incompressible turbulent channel flow at Reynolds numbers Re-tau (based on the channel half-width and the friction velocity) of 180 and 590 and for forced laminar-turbulent transition in a plane channel, demonstrating the effectiveness of the proposed approach. The results are compared to data of direct numerical simulations and to data obtained using the dynamic Smagorinsky model with different test filters.

  • 115. Stroh, A.
    et al.
    Frohnapfel, B.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hasegawa, Y.
    A comparison of opposition control in turbulent boundary layer and turbulent channel flow2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 7, article id 075101Article in journal (Refereed)
    Abstract [en]

    A comparison between classical opposition control applied in the configuration of a fully developed turbulent channel flow and applied locally in a spatially developing turbulent boundary layer is presented. It is found that the control scheme yields similar drag reduction rates if compared at the same friction Reynolds numbers. However, a detailed analysis of the dynamical contributions to the skin friction coefficient reveals significant differences in the mechanism behind the drag reduction. While drag reduction in turbulent channel flow is entirely based on the attenuation of the Reynolds shear stress, the modification of the spatial flow development is essential for the turbulent boundary layer in terms of achievable drag reduction. It is shown that drag reduction due to this spatial development contribution becomes more pronounced with increasing Reynolds number (up to Re-tau = 660, based on friction velocity and boundary layer thickness) and even exceeds drag reduction due to attenuation of the Reynolds shear stress. In terms of an overall energy balance, it is found that opposition control is less efficient in the turbulent boundary layer due to the inherently larger fluctuation intensities in the near wall region.

  • 116.
    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.

  • 117.
    Trip, R.
    et al.
    Laboratory for Aero and Hydrodynamics (3ME-P and E), Delft University of Technology.
    Kuik, D. J.
    Laboratory for Aero and Hydrodynamics (3ME-P and E), Delft University of Technology.
    Westerweel, J.
    Laboratory for Aero and Hydrodynamics (3ME-P and E), Delft University of Technology.
    Poelma, C.
    Laboratory for Aero and Hydrodynamics (3ME-P and E), Delft University of Technology.
    An experimental study of transitional pulsatile pipe flow2012In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 24, no 1, p. 014103-Article in journal (Refereed)
    Abstract [en]

    The transitional regime of a sinusoidal pulsatile flow in a straight, rigid pipe is investigated using particle image velocimetry. The main aim is to investigate how the critical Reynolds number is affected by different pulsatile conditions, expressed as the Womersley number and the oscillatory Reynolds number. The transition occurs in the region of Re = 2250-3000 and is characterized by an increasing number of isolated turbulence structures. Based on velocity fields and flow visualizations, these structures can be identified as puffs, similar to those observed in steady flow transition. Measurements at different Womersley numbers yield similar transition behavior, indicating that pulsatile effects do not play a role in the regime that is investigated. Variations of the oscillatory Reynolds number also appear to have little effect, so that the transition here seems to be determined only by the mean Reynolds number. For larger mean Reynolds numbers, a second regime is observed: here, the flow remains turbulent throughout the cycle. The turbulence intensity varies during the cycle, but has a phase shift with respect to the mean flow component. This is caused by a growth of kinetic energy during the decelerating part and a decay during the accelerating part of the cycle. Flow visualization experiments reveal that the flow develops localized turbulence at several random axial positions. The structures quickly grow to fill the entire pipe in the decelerating phase and (partially) decay during the accelerating phase.

  • 118.
    Trip, Renzo
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bluff body boundary-layer modification and its effect on the near-wake topology2017In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 29, no 9, article id 095105Article in journal (Refereed)
  • 119.
    Trip, Renzo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Boundary layer modification by means of wall suction and the effect on the wake behind a rectangular forebody2014In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 26, no 12, p. 125105-Article in journal (Refereed)
    Abstract [en]

    The wake characteristics of a two-dimensional rectangular forebody with a smooth leading edge and a blunt trailing edge are investigated. Wall suction is applied along the forebody in order to modify the developing boundary layer. An initially laminar boundary layer subject to suction yields an asymptotic suction boundary layer at the trailing edge of the body, whereas a high enough suction coefficient relaminarizes an initially turbulent boundary layer. The critical suction velocity required to achieve this significant modification of the boundary layer properties is typically in the order of 1% of the free-stream velocity, where the critical suction coefficient depends on the Reynolds number. We show that a thinner boundary layer induces a higher vortex shedding frequency and a lower base pressure. Furthermore, the boundary layer state, laminar or turbulent, has a significant influence on the wake. For example, the Strouhal number based on the effective body thickness is being reduced by 25% from laminar to turbulent inlet conditions.

  • 120.
    Törnblom, Olle
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    A Reynolds stress closure description of separation control with vortex generators in a plane asymmetric diffuser2007In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, no 11, p. 115108-Article in journal (Refereed)
    Abstract [en]

    A way to model the effects of streamwise vortices in a turbulent flow with one homogeneous direction is presented. The Reynolds averaged Navier-Stokes equations are solved with a differential Reynolds stress turbulence model. Assuming that the vortices can be approximated with the Lamb-Oseen model, wall-normal Reynolds stress distributions are calculated, corresponding to the spanwise variances of the estimated velocity distribution downstream of the vortex generators. The Reynolds stress contributions that are due to the vortex generators are added to the Reynolds stresses from the turbulence model so as to mimic the increased mixing due to the vortex generators. Volume forces are applied also in the mean momentum equations to account for the drag of the vortex generators. The model is tested and compared with experimental data from a plane asymmetric diffuser flow which is separating without vortex generators. The results indicate that the model is able to mimic the major features of vortex generator flow control and that the flow case in question is susceptible to separation control. The model results show that the pressure recovery of the diffuser could be increased by almost 10% by applying vortex generators and that, if keeping the shape of the vortex generators fixed, their optimal position is close to the diffuser inlet. Computations also indicated that the time to re-establish the separation zone when the control suddenly is turned off is substantially longer than the time it takes to remove the separation after the control is turned on again. Some work on adapting a differential Reynolds stress turbulence model was necessary in order to make it capable of realistic predictions of the asymmetric diffuser flow in which the vortex generator model is tested. However, the main focus of the article is on the modelling of vortex generator effects.

  • 121.
    van Wyk, Stevin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bulusu, Kartik V.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Plesniak, Michael W.
    Non-Newtonian perspectives on pulsatile blood-analog flows in a 180 degrees curved artery model2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 7, article id 071901Article in journal (Refereed)
    Abstract [en]

    Complex, unsteady fluid flow phenomena in the arteries arise due to the pulsations of the heart that intermittently pumps the blood to the extremities of the body. The many different flow waveform variations observed throughout the arterial network are a result of this process and a function of the vessel properties. Large scale secondary flow structures are generated throughout the aortic arch and larger branches of the arteries. An experimental 180. curved artery test section with physiological inflow conditions was used to validate the computational methods implemented in this study. Good agreement of the secondary flow structures is obtained between experimental and numerical studies of a Newtonian blood-analog fluid under steady-state and pulsatile, carotid artery flow rate waveforms. Multiple vortical structures, some of opposite rotational sense to Dean vortices, similar to Lyne-type vortices, were observed to form during the systolic portion of the pulse. Computational tools were used to assess the effect of blood-analog fluid rheology ( i.e., Newtonian versus non-Newtonian). It is demonstrated that non-Newtonian, blood-analog fluid rheology results in shear layer instabilities that alter the formation of vortical structures during the systolic deceleration and onwards during diastole. Additional vortices not observed in the Newtonian cases appear at the inside and outside of the bend at various times during the pulsation. The influence of blood-analog shear-thinning viscosity decreases mean pressure losses in contrast to the Newtonian blood analog fluid.

  • 122.
    Vinuesa, Ricardo
    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.
    Bobke, Alexandra
    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.
    Örlü, Ramis
    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.
    Schlatter, Philipp
    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.
    On determining characteristic length scales in pressure-gradient turbulent boundary layers2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 5, article id 055101Article in journal (Refereed)
    Abstract [en]

    In the present work, we analyze three commonly used methods to determine the edge of pressure gradient turbulent boundary layers: two based on composite profiles, the one by Chauhan et al. ["Criteria for assessing experiments in zero pressure gradient boundary layers," Fluid Dyn. Res. 41, 021404 (2009)] and the one by Nickels ["Inner scaling for wall-bounded flows subject to large pressure gradients," J. Fluid Mech. 521, 217-239 (2004)], and the other one based on the condition of vanishing mean velocity gradient. Additionally, a new method is introduced based on the diagnostic plot concept by Alfredsson et al. ["A new scaling for the streamwise turbulence intensity in wall-bounded turbulent flows and what it tells us about the 'outer' peak," Phys. Fluids 23, 041702 (2011)]. The boundary layers developing over the suction and pressure sides of a NACA4412 wing section, extracted from a direct numerical simulation at chord Reynolds number Re-c = 400 000, are used as the test case, besides other numerical and experimental data from favorable, zero, and adverse pressure-gradient flat-plate turbulent boundary layers. We find that all the methods produce robust results with mild or moderate pressure gradients, although the composite-profile techniques require data preparation, including initial estimations of fitting parameters and data truncation. Stronger pressure gradients (with a Rotta-Clauser pressure-gradient parameter beta larger than around 7) lead to inconsistent results in all the techniques except the diagnostic plot. This method also has the advantage of providing an objective way of defining the point where the mean streamwise velocity is 99% of the edge velocity and shows consistent results in a wide range of pressure gradient conditions, as well as flow histories. Collapse of intermittency factors obtained from a wide range of pressure-gradient and Re conditions on the wing further highlights the robustness of the diagnostic plot method to determine the boundary layer thickness (equivalent to delta(99)) and the edge velocity in pressure gradient turbulent boundary layers.

  • 123. Vinuesa, Ricardo
    et al.
    Bobke, Alexandra
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Mechanics.
    Örlü, Ramis
    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. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    On determining characteristic length scales in pressure-gradient turbulent boundary layers2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28Article in journal (Refereed)
    Abstract [en]

    In the present work we analyze three commonly used methods to determine the edge of pressure gradient turbulent boundary layers: two based on composite profiles, the one by Chauhan et al. (Fluid Dyn. Res. 41:021401, 2009) and the one by Nickels (J. Fluid Mech. 521:217–239, 2004), and the other onebased on the condition of vanishing mean velocity gradient. Additionally, a new method is introduced based on the diagnostic plot concept by Alfredsson et al. (Phys. Fluids 23:041702, 2011). The boundary layers developing over the suction and pressure sides of a NACA4412 wing section, extracted from a directnumerical simulation at chord Reynolds number Rec = 400, 000, is used as the test case, besides other numerical and experimental data from favorable, zero and adverse pressure-gradient flat-plate turbulent boundary layers. We find that all the methods produce robust results with mild or moderate pressure gradients, although the composite-profile techniques require data preparation, including initial estimations of fitting parameters and data truncation. Stronger pressure gradients (with a Rotta–Clauser pressure-gradient parameter β larger than around 7) lead to inconsistent results in all the techniques except the diagnosticplot. This method also has the advantage of providing an objective way of defining the point where the mean streamwise velocity is 99% of the edge velocity, and shows consistent results in a wide range of pressure gradient conditions, as well as flow histories. Collapse of intermittency factors obtained from a wide range of pressure-gradient and Re conditions on the wing further highlightsthe robustness of the diagnostic plot method to determine the boundary layert hickness (equivalent to δ99 ) and the edge velocity in pressure gradient turbulent boundary layers.

  • 124.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics. Department of Mechanical, Materials and Aerospace Engineering (MMAE), Illinois Institute of Technology (IIT), Chicago, IL, United States.
    Hites, Michael
    Wark, Candace
    Nagib, Hassan
    Documentation of the role of large-scale structures in the bursting process in turbulent boundary layers2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 10Article in journal (Refereed)
    Abstract [en]

    The scaling of the bursting frequency with Reynolds number is investigated experimentally by means of hot-wire measurements in turbulent boundary layers over the range 1580 < ReΘ < 23 700. Bursting events are detected by means of the u-level technique, and the effect of filtering and the choice of the threshold are also assessed. We find that the inner scaling produces excellent collapse of the high-pass filtered data in the buffer and log regions, regardless of the cutofffrequency and the threshold. This is due to the fact that the high-pass filter attenuates the low frequency component of the streamwise velocity signal, which is associated with large-scale structures in the flow. On the other hand, the bursting frequency scales with outer variables when the time-series is low-pass filtered with very low cutofffrequencies, highlighting the connection between bursting phenomena and the outer flow.

  • 125.
    Wang, Yuli
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
    Do-Quang, Minh
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
    Viscoelastic droplet dynamics in a y-shaped capillary channel2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 3, p. 033103-Article in journal (Refereed)
    Abstract [en]

    Non-Newtonian droplet dynamics commonly exist in microfluidic systems. We report simulations of viscoelastic (VE) droplets traveling in a two dimensional capillary bifurcation channel. A numerical system combining phase field method, VE rheology, and Stokes flow equations is built. As a generic microfluidic two-phase problem, we study how a non-Newtonian droplet that approaches a channel bifurcation will behave. We identify conditions for when a droplet will either split into two or be directed into one of the branches. In particular, we study the importance of the non-Newtonian properties. Our results reveal two different non-Newtonian mechanisms that can enhance splitting and non-splitting of droplets with respect to Newtonian droplets, depending on the size of droplet and capillary number.

  • 126.
    Wei, Liang
    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.
    Elsinga, Gerrit E.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    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.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Universality and scaling phenomenology of small-scale turbulence in wall-bounded flows2014In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 26, no 3, p. 035107-Article in journal (Refereed)
    Abstract [en]

    The Reynolds number scaling of flow topology in the eigenframe of the strain-rate tensor is investigated for wall-bounded flows, which is motivated by earlier works showing that such topologies appear to be qualitatively universal across turbulent flows. The databases used in the current study are from direct numerical simulations (DNS) of fully developed turbulent channel flow (TCF) up to friction Reynolds number Re-tau approximate to 1500, and a spatially developing, zero-pressure-gradient turbulent boundary layer (TBL) up to Re-theta approximate to 4300 (Re-tau approximate to 1400). It is found that for TCF and TBL at different Reynolds numbers, the averaged flow patterns in the local strain-rate eigenframe appear the same consisting of a pair of co-rotating vortices embedded in a finite-size shear layer. It is found that the core of the shear layer associated with the intense vorticity region scales on the Kolmogorov length scale, while the overall height of the shear layer and the distance between the vortices scale well with the Taylor micro scale. Moreover, the Taylor micro scale collapses the height of the shear layer in the direction of the vorticity stretching. The outer region of the averaged flow patterns approximately scales with the macro scale, which indicates that the flow patterns outside of the shear layer mainly are determined by large scales. The strength of the shear layer in terms of the peak tangential velocity appears to scale with a mixture of the Kolmogorov velocity and root-mean-square of the streamwise velocity scaling. A quantitative universality in the reported shear layers is observed across both wall-bounded flows for locations above the buffer region.

  • 127.
    Weinkauf, Tino
    et al.
    Zuse Institute Berlin, Germany.
    Hege, H. -C
    Noack, B. R.
    Schlegel, M.
    Dillmann, A.
    Coherent Structures in a Transitional Flow around a Backward-Facing Step2003In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 15, no 9Article in journal (Refereed)
  • 128. Widlund, O.
    et al.
    Zahrai, S.
    Bark, Fritz H.
    KTH, Superseded Departments, Mechanics.
    Structure information in rapid distortion analysis and one-point modeling of axisymmetric magnetohydrodynamic turbulence2000In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 12, no 10, p. 2609-2620Article in journal (Refereed)
    Abstract [en]

    It has recently been suggested that dimensionality information, as carried by the Reynolds dimensionality tensor, should be included in an extended Reynolds stress closure for modeling of magnetohydrodynamic (MHD) turbulence at low magnetic Reynolds numbers. This would enable more accurate modeling of the Joule dissipation, and capture the length-scale anisotropies and tendencies towards two-dimensionality characteristic of MHD turbulence. In the present work, an evolution equation for the Reynolds dimensionality tensor is derived, based on the spectral formulation of the Navier-Stokes equations. Most of the terms in the equation require modeling. Rapid distortion theory (RDT) is applied to study the behavior of the different magnetic terms of the dimensionality and Reynolds stress tensor equations; a variety of different anisotropy states could be examined by letting magnetic forcing act on a number of initial spectral energy distributions obtained from axisymmetric strain. The properties and limitations of linear or bilinear invariant tensor models for the magnetic terms are evaluated. In the limit of large interaction numbers (where Joule dissipation dominates), the resulting model equations for the energy decay have analytic solutions. By choosing one model constant appropriately, these are made consistent with the asymptotic energy decay K similar to t(-1/2) predicted earlier by Moffatt. The long-term objective of these efforts is the development of an effective second-moment closure for engineering applications.

  • 129. Wikstrom, P. M.
    et al.
    Wallin, Stefan
    Johansson, Arne V.
    Derivation and investigation of a new explicit algebraic model for the passive scalar flux2000In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 12, no 3, p. 688-702Article in journal (Refereed)
    Abstract [en]

    An algebraic relation for the scalar flux, in terms of mean flow quantities, is formed by applying an equilibrium condition in the transport equations for the normalized scalar flux. This modeling approach is analogous to explicit algebraic Reynolds stress modeling (EARSM) for the Reynolds stress anisotropies. The assumption of negligible advection and diffusion of the normalized passive scalar flux gives, in general, an implicit, nonlinear set of algebraic equations. A method to solve this implicit relation in a fully explicit form is proposed, where the nonlinearity in the scalar-production-to-dissipation ratio is considered and solved. The nonlinearity, in the algebraic equations for the normalized scalar fluxes, may be eliminated directly by using a nonlinear term in the model of the pressure scalar-gradient correlation and the destruction and thus results in a much simpler model for both two-and three-dimensional mean flows. The performance of the present model is investigated in three different flow situations. These are homogeneous shear flow with an imposed mean scalar gradient, turbulent channel flow, and the flow field downstream a heated cylinder. The direct numerical simulation (DNS) data are used to analyze the passive scalar flux in the homogeneous shear and channel flow cases and experimental data are used in the case of the heated cylinder wake. Sets of parameter values giving very good predictions in all three cases are found.

  • 130.
    Winroth, Marcus
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Mechanics.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    On shock structures in dynamic exhaust valve flows2019In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 31, no 2, article id 026107Article in journal (Refereed)
    Abstract [en]

    The gas dynamics of the flow past an exhaust valve has been investigated using Schlieren photography. An experimental setup was designed and constructed, which allowed optical access to the valve head and seat region as well as to the exhaust port. The setup was constructed so that the shock structures of a steady flow, with a static valve, could be compared to the structures found in experiments with a more realistic dynamically discharging cylinder, with a moving valve. The steady flow experiments were carried out at a valve lift to a port diameter ratio of 0.155 with cylinder pressures up to 325 kPa. The dynamic valve experiments were performed with an initial cylinder pressure of 300 kPa and at an equivalent engine speed of 1350 rpm. The steady flow experiments belonged to one of the two flow regimes, depending on the cylinder pressure: regime I, a wall-bounded supersonic jet (for low cylinder pressures) or regime II, a fully expanded supersonic nozzle-flow (for high cylinder pressures). By comparing the images from the dynamic valve experiment to those of the steady flow experiments, it was shown that the flow in the dynamic experiments exhibits more similarities with regime I. However, large differences in the shock structures between the steady flow in regime I and the dynamic valve flow remain. This indicates that experiments using a steady flow and a fixed valve lift do not encompass the essential physics found in real engine flows and should be avoided. 

  • 131. Xiong, Xiangming
    et al.
    Tao, Jianjun
    Chen, Shiyi
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Turbulent bands in plane-Poiseuille flow at moderate Reynolds numbers2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 4, article id 041702Article in journal (Refereed)
    Abstract [en]

    In this letter, we show via numerical simulations that the typical flow structures appearing in transitional channel flows at moderate Reynolds numbers are not spots but isolated turbulent bands, which have much longer lifetimes than the spots. Localized perturbations can evolve into isolated turbulent bands by continuously growing obliquely when the Reynolds number is larger than 660. However, interactions with other bands and local perturbations cause band breaking and decay. The competition between the band extension and breaking does not lead to a sustained turbulence until Re becomes larger than about 1000. Above this critical value, the bands split, providing an effective mechanism for turbulence spreading.

  • 132. Yoshioka, S
    et al.
    Fransson, Jens H.M.
    KTH, Superseded Departments, Mechanics.
    Alfredsson, P. Henrik
    KTH, Superseded Departments, Mechanics.
    Free stream turbulence induced disturbances in boundary layers with wall suction2004In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, no 10, p. 3530-3539Article in journal (Refereed)
    Abstract [en]

    An experimental investigation of free stream turbulence (FST) induced disturbances in asymptotic suction boundary layers (ASBL) has been performed. In the present study four different suction rates are used and the highest is 0.40% of the free stream velocity, together with three different FST levels (Tu=1.6, 2.0, and 2.3%). A turbulence generating grid of the active type is used and offers the possibility to vary the Tu-level while the scales of the turbulence remain almost constant. It is known that FST induces elongated disturbances consisting of high and low velocity regions, usually denoted streaky structures, into the boundary layer. The experiments show that wall suction suppresses the disturbance growth and may significantly delay or inhibit the break-down to turbulence. Two-point correlation measurements in the spanwise direction show that the averaged streak spacing decreases with increasing FST-level, whereas the spanwise scale in the ASBL is more or less constant if scaled with the free stream velocity and viscosity. This is in contrast to what is observed in a Blasius boundary layer where streaks develop and adapt their spanwise scale close to the boundary layer thickness.

  • 133.
    Zhu, Lailai
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lauga, Eric
    Dept. of Mechanical and Aerospace Engineering, University of California, San Diego, USA.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Self-propulsion in viscoelastic fluids: pushers vs. pullers2012In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 24, no 5, p. 051902-Article in journal (Refereed)
    Abstract [en]

    We use numerical simulations to address locomotion at zero Reynolds number in viscoelastic (Giesekus) fluids. The swimmers are assumed to be spherical, to self-propel using tangential surface deformation, and the computations are implemented using a finite element method. The emphasis of the study is on the change of the swimming kinematics, energetics, and flow disturbance from Newtonian to viscoelastic, and on the distinction between pusher and puller swimmers. In all cases, the viscoelastic swimming speed is below the Newtonian one, with a minimum obtained for intermediate values of the Weissenberg number, We. An analysis of the flow field places the origin of this swimming degradation in non-Newtonian elongational stresses. The power required for swimming is also systematically below the Newtonian power, and always a decreasing function of We. A detail energetic balance of the swimming problem points at the polymeric part of the stress as the primary We-decreasing energetic contribution, while the contributions of the work done by the swimmer from the solvent remain essentially We-independent. In addition, we observe negative values of the polymeric power density in some flow regions, indicating positive elastic work by the polymers on the fluid. The hydrodynamic efficiency, defined as the ratio of the useful to total rate of work, is always above the Newtonian case, with a maximum relative value obtained at intermediate Weissenberg numbers. Finally, the presence of polymeric stresses leads to an increase of the rate of decay of the flow velocity in the fluid, and a decrease of the magnitude of the stresslet governing the magnitude of the effective bulk stress in the fluid.

  • 134.
    Zhu, LaiLai
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Rabault, Jean
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Ecole Polytech, F-91128 Palaiseau, France.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    The dynamics of a capsule in a wall-bounded oscillating shear flow2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 7, article id 071902Article in journal (Refereed)
    Abstract [en]

    The motion of an initially spherical capsule in a wall-bounded oscillating shear flow is investigated via an accelerated boundary integral implementation. The neo-Hookean model is used as the constitutive law of the capsule membrane. The maximum wall-normal migration is observed when the oscillation period of the imposed shear is of the order of the relaxation time of the elastic membrane; hence, the optimal capillary number scales with the inverse of the oscillation frequency and the ratio agrees well with the theoretical prediction in the limit of high-frequency oscillation. The migration velocity decreases monotonically with the frequency of the applied shear and the capsule-wall distance. We report a significant correlation between the capsule lateral migration and the normal stress difference induced in the flow. The periodic variation of the capsule deformation is roughly in phase with that of the migration velocity and normal stress difference, with twice the frequency of the imposed shear. The maximum deformation increases linearly with the membrane elasticity before reaching a plateau at higher capillary numbers when the deformation is limited by the time over which shear is applied in the same direction and not by the membrane deformability. The maximum membrane deformation scales as the distance to the wall to the power 1/3 as observed for capsules and droplets in near-wall steady shear flows.

  • 135.
    Åkervik, Espen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Hoepffner, Jérôme
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Marxen, Olaf
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Steady solutions of the Navier-Stokes equations by selective frequency damping2006In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 18, no 6, p. 068102-Article in journal (Refereed)
    Abstract [en]

    A new method, enabling the computation of steady solutions of the Navier-Stokes equations in globally unstable configurations, is presented. We show that it is possible to reach a steady state by damping the unstable (temporal) frequencies. This is achieved by adding a dissipative relaxation term proportional to the high-frequency content of the velocity fluctuations. Results are presented for cavity-driven boundary-layer separation and a separation bubble induced by an external pressure gradient.

  • 136.
    Örlü, Ramis
    et al.
    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.
    On the fluctuating wall-shear stress in zero pressure-gradient turbulent boundary layer flows2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 2, p. 021704-Article in journal (Refereed)
    Abstract [en]

    Recent direct numerical simulation (DNS) results relating to the behavior of the fluctuating wall-shear stress tau(+)(w,rms) in turbulent boundary layer flows are discussed. This new compilation is motivated by a recent article [Wu and Moin, "Transitional and turbulent boundary layer with heat transfer," Phys. Fluids 22, 085105 (2010)], which indicates a need for clarification of the value of tau(+)(w,rms). It is, however, shown here, based on other recent DNS data, that most results, both in boundary layer and channel geometry, yield tau(+)(w,rms)approximate to 0.4 plus a small increase with Reynolds number coming from the growing influence of the outer spectral peak. The observed discrepancy in experimental data is mainly attributed to spatial resolution effects, as originally described by Alfredsson et al. [" The fluctuating wall-shear stress and the velocity field in the viscous sublayer, "Phys. Fluids 31, 1026 (1988)].

  • 137.
    Österlund, Jens, M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Johansson, Arne, V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Nagib, Hassan, M.
    Illinois insitute of Technology.
    Comment on "A note on the intermediate region in turbulent boundary layers" [Phys. Fluids 12, 2159 (2000)]2000In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 12, p. 2360-2363Article in journal (Refereed)
  • 138.
    Österlund, Jens, M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Johansson, Arne, V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Nagib, Hassan, M.
    Massachusetts Institute of Technology.
    Hites, Michael, H.
    A note on the overlap region in turbulent boundary layers2000In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, no 12, p. 1-4Article in journal (Refereed)
    Abstract [en]

    Two independent experimental investigations of the behavior of turbulent boundary layers with increasing Reynolds number were recently completed. The experiments were performed in two facilities, the Minimum Turbulence Level (MTL) wind tunnel at Royal Institute of Technology (KTH) and the National Diagnostic Facility (NDF) wind tunnel at Illinois Institute of Technology (IIT). Both experiments utilized oil-film interferometry to obtain an independent measure of the wall-shear stress. A collaborative study by the principals of the two experiments, aimed at understanding the characteristics of the overlap region between the inner and outer parts of the boundary layer, has just been completed. The results are summarized here, utilizing the profiles of the mean velocity, for Reynolds numbers based on the momentum thickness ranging from 2500 to 27 000. Contrary to the conclusions of some earlier publications, careful analysis of the data reveals no significant Reynolds number dependence for the parameters describing the overlap region using the classical logarithmic relation. However, the data analysis demonstrates that the viscous influence extends within the buffer region to y+≈200, compared to the previously assumed limit of y+≈50.Therefore, the lowest Reθ value where a significant logarithmic overlap region exists is about 6000. This probably explains why a Reynolds number dependence had been found from the data analysis of many previous experiments. The parameters of the logarithmic overlap region are found to be constant and are estimated to be κ=0.38, B=4.1 and B1=3.6 (δ=δ95).

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