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  • 51. Hiwatashi, Kazuaki
    et al.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Tillmark, Nils
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Nagata, M.
    Experimental observations of instabilities in rotating plane Couette flow2007In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, no 4Article in journal (Refereed)
    Abstract [en]

    The transition from the two-dimensional (2D) longitudinal roll cell state to 3D flows in the rotating plane Couette system, predicted by the theoretical investigation [M. Nagata, J. Fluid Mech. 358, 357 (1998)], is examined experimentally. The streamwise and spanwise wave numbers of observed steady 3D flows seem to agree with those predicted by the theory when the rotation rate is relatively large. However, we observe unsteady 3D states in the region where the theory predicts stable steady 3D flows when the rotation rate is small.

  • 52. Hoepffner, Jerome
    et al.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Stochastic approach to the receptivity problem applied to bypass transition in boundary layers2008In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 20, no 2Article in journal (Refereed)
    Abstract [en]

    To study the flow behavior in the presence of external disturbances of chaotic nature, a stochastic approach is pursued. In particular, transition to turbulence in boundary layers exposed to high levels of free-stream turbulence is considered. The late stages of this transition scenario, characterized by the growth and breakdown of streamwise-elongated streaks, are examined by considering the linear evolution of perturbations to a base flow consisting of the Blasius profile and the streaks. A stochastic initial condition is considered where the free-stream perturbations are described by the correlations of isotropic homogeneous turbulence. The spatial correlation of the excited flow at later times can be computed by the numerical solution of a Lyapunov equation. It is shown that free-stream turbulence has the necessary features to excite secondary energy growth, thus playing a central role in the transition to turbulence. The method proposed here can be used to examine the receptivity of other flows to external noise whose statistical properties are known or can be modeled.

  • 53. Hogberg, M.
    et al.
    Bewley, T. R.
    Henningson, Dan S.
    KTH, Superseded Departments, Mechanics.
    Relaminarization of Re-tau=100 turbulence using gain scheduling and linear state-feedback control2003In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 15, no 11, p. 3572-3575Article in journal (Refereed)
    Abstract [en]

    The first successful application of linear full-state feedback optimal control theory to consistently relaminarize turbulent channel flow at Re-tau=100 with full state information and gain scheduling is reported. The actuation is zero-net mass-flux blowing and suction on the channel walls. Two key issues central to the success of this strategy are: (a) the choice of the mean-flow profile about which the equations are linearized for the computation of the linear feedback gains, and (b) the choice of an objective function which targets the control effort on the flow perturbations of interest. A range of mean-flow profiles between the laminar and fully turbulent profiles and a weighted energy measure which targets flow perturbations in the near-wall region were found to provide effective feedback gains. A gain-scheduling strategy to tune the feedback gains to the nonstationary mean-flow profile is introduced, resulting in consistent relaminarization of the turbulent flow in all realizations tested.

  • 54. Hogberg, M.
    et al.
    Chevalier, M.
    Henningson, Dan S.
    KTH, Superseded Departments, Mechanics.
    Linear compensator control of a pointsource induced perturbation in a Falkner-Skan-Cooke boundary layer2003In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 15, no 8, p. 2449-2452Article in journal (Refereed)
    Abstract [en]

    We focus on the problem of controlling pointsource induced perturbations on an infinite swept wing using linear control theory. Based on wall measurements in a spatial simulation of localized disturbances in Falkner-Skan-Cooke boundary layers, an extended Kalman filter is used to estimate the full three-dimensional wave packet. The estimated field is in turn used to calculate a feedback control which changes the growth of the disturbance into decay. This is the first time that optimal control and estimation concepts are successfully applied to construct a dynamic output feedback compensator which is used to control disturbances in spatially developing boundary layers.

  • 55.
    Holm, Richard
    et al.
    KTH, Superseded Departments, Mechanics.
    Storey, S.
    Martinez, Mark
    Söderberg, L. Daniel
    KTH, Superseded Departments, Mechanics.
    Visualization of streaming-like structures during settling of dilute and semi-dilute rigid fibre suspensions2004In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666Article in journal (Other academic)
  • 56.
    Ilak, Milos
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Chevalier, Mattias
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan Stefan
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Stability of a jet in crossflow2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 9, p. 091113-Article in journal (Refereed)
    Abstract [en]

    [No abstract available]

  • 57.
    Imayama, Shintaro
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lingwood, Rebecca J.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    A new way to describe the transition characteristics of a rotating-disk boundary-layer flow2012In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 24, no 3, p. 031701-Article in journal (Refereed)
    Abstract [en]

    A new method of graphically representing the transition stages of a rotating-disk flow is presented. The probability density function contour map of the fluctuating azimuthal disturbance velocity is used to show the characteristics of the boundary-layer flow over the rotating disk as a function of Reynolds numbers. Compared with the variation of the disturbance amplitude (rms) or spectral distribution, this map more clearly shows the changing flow characteristics through the laminar, transitional, and turbulent regions. This method may also be useful to characterize the different stages in the transition process not only for the rotating-disk flow but also for other flows.

  • 58. Jacob, Boris
    et al.
    Casciola, Carlo Massimo
    Talamelli, Alessandro
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Scaling of mixed structure functions in turbulent boundary layers2008In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 20, no 4Article in journal (Refereed)
    Abstract [en]

    We address the issue of the scaling of the anisotropic components of the hierarchy of correlation tensors in the logarithmic region of a turbulent boundary layer over a flat plate, at Re-theta approximate to 15 000. We isolate the anisotropic observables by means of decomposition tools based on the SO(3) symmetry group of rotations. By employing a dataset made of velocity signals detected by two X probes, we demonstrate that the behavior of the anisotropic fluctuations throughout the boundary layer may be understood in terms of the superposition of two distinct regimes. The transition is controlled by the magnitude of the mean shear and occurs in correspondence with the shear scale. Below the shear scale, an isotropy-recovering behavior occurs, which is characterized by a set of universal exponents which roughly match dimensional predictions based on Lumley's argument [J. L. Lumley, Phys. Fluids 8, 1056 (1965)]. Above the shear scale, the competition between energy production and transfer mechanisms gives rise to a completely different scenario with strong alterations of the observed scaling laws. This aspect has significant implications for the correct parametrization of the anisotropy behavior in the near wall region since, approaching the wall, an increasingly larger fraction of the scaling interval tends to conform to the shear-dominated power laws.

  • 59. Khaleque, Tania S.
    et al.
    Fowler, A. C.
    Howell, P. D.
    Vynnycky, Mikael
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Casting of Metals.
    Numerical studies of thermal convection with temperature-and pressure-dependent viscosity at extreme viscosity contrasts2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 7, article id 076603Article in journal (Refereed)
    Abstract [en]

    Motivated by convection of planetary mantles, we consider a mathematical model for Rayleigh-Benard convection in a basally heated layer of a fluid whose viscosity depends strongly on temperature and pressure, defined in an Arrhenius form. The model is solved numerically for extremely large viscosity variations across a unit aspect ratio cell, and steady solutions for temperature, isotherms, and streamlines are obtained. To improve the efficiency of numerical computation, we introduce a modified viscosity law with a low temperature cutoff. We demonstrate that this simplification results in markedly improved numerical convergence without compromising accuracy. Continued numerical experiments suggest that narrow cells are preferred at extreme viscosity contrasts, and this conclusion is supported by a linear stability analysis.

  • 60.
    Kjellander, Malte
    et al.
    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.
    Energy concentration by spherical converging shocks generated in a shock tube2012In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 24, no 12, p. 126103-Article in journal (Refereed)
    Abstract [en]

    Spherical converging shock waves are produced in a conventional shock tube with a circular cross-section. Initially, plane shocks are transformed into the shape of a spherical cap by means of a smoothly convergent cross-section. The wall shape in the transformation section is designed to gradually change the form of the shock wave until it approaches a spherical shape. Thereafter, the shock enters a conical section where it converges towards the apex of the cone. Numerical calculations with the axisymmetric Euler equations show that the spherical form is only slightly dependent on the initial Mach number of the plane shock within the range 1.5 < MS < 5.5, and is preserved to a close vicinity of the focal point. The test gas is heated to very high temperatures as a result of shock convergence and emits a bright light pulse at the tip of the test section. The light radiation is collected by optical fibers mounted at the tip of the convergence chamber and investigated by photometric and spectroscopic measurements. Experiments are performed with argon and nitrogen and with different initial Mach numbers. The radiation of the shock-heated argon closely resembles blackbody radiation. Fits to the experimental data result in apparent blackbody temperatures in argon of up to ~27 000 K, some 250 ns after the focusing instant. The initial Mach number in these spectrometric runs is MS = 3.9, indicating an efficient amplification of the shock wave strength.

  • 61.
    Kjellander, Malte
    et al.
    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.
    Experimental determination of self-similarity constant for converging cylindrical shocks2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 11, p. 116103-Article in journal (Refereed)
    Abstract [en]

    Guderley's self-similarity solution r = r(0)(1 - t/t(0)) a for strong converging cylindrical shocks is investigated experimentally for three different gases with adiabatic exponents gamma = 1.13; 1.40; and 1.66 and various values of the initial Mach number. Corresponding values of the similarity exponent a which determines the strength of shock convergence are obtained for each gas thus giving the variation of alpha with gamma. Schlieren imaging with multiple exposure technique is used to track the propagation of a single shock front during convergence. The present experimental results are compared with previous experimental, numerical, and theoretical investigations.

  • 62.
    Kjellander, Malte
    et al.
    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.
    Thermal radiation from a converging shock implosion2010In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 22, no 4, p. 046102-Article in journal (Refereed)
    Abstract [en]

    High energy concentration in gas is produced experimentally by focusing cylindrical shock waves in a specially constructed shock tube. The energy concentration is manifested by the formation of a hot gas core emitting light at the center of a test chamber at the instant of shock focus. Experimental and numerical investigations show that the shape of the shock wave close to the center of convergence has a large influence on the energy concentration level. Circular shocks are unstable and the resulting light emission varies greatly from run to run. Symmetry and stability of the converging shock are achieved by wing-shaped flow dividers mounted radially in the test chamber, forming the shock into a more stable polygonal shape. Photometric and spectroscopic analysis of the implosion light flash from a polygonal shock wave in argon is performed. A series of 60 ns time-resolved spectra spread over the 8 mu s light flash shows the emission variation over the flash duration. Blackbody fits of the spectroscopic data give a maximum measured gas temperature of 5800 K in the beginning of the light flash. Line emissions originating in transitions in neutral argon atoms from energy levels of up to 14.7 eV were also detected.

  • 63.
    Kjellander, Melte
    et al.
    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.
    Shock dynamics of strong imploding cylindrical and spherical shock waves with real gas effects2010In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 22, no 11, p. 116102-Article in journal (Refereed)
    Abstract [en]

    Strong cylindrical and spherical shock implosion in a monatomic gas is considered A simple solution is obtained by Whitham's geometrical shock dynamics approach modified to account for the real gas effects The real gas effects are introduced by jump relations over the shock and include several levels of ionization, Coulomb interaction as well as internal energy of the excited electrons It is shown that ionization has a major effect on temperature and density behind the converging shock as well as on the shock acceleration The temperature and acceleration being substantially reduced and density substantially increased as compared to the ideal nonionizing case The ionization effect on the pressure behind the converging shock is less pronounced It is also shown that for the considered test case of initial Mach number M-0=8 the gas becomes completely ionized behind the spherical shock at approximately 1% of the initial radius from the focal point and its speed being decreased by a factor of 1 8 as compared to the ideal case

  • 64.
    Klinkenberg, Joy
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    de Lange, H. C.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Modal and non-modal stability of particle-laden channel flow2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 6, p. 064110-Article in journal (Refereed)
    Abstract [en]

    Modal and non-modal linear stability analysis of channel flow with a dilute particle suspension is presented where particles are assumed to be solid, spherical, and heavy. The two-way coupling between particle and fluid flow is therefore modeled by the Stokes drag only. The results are presented as function of the particle relaxation time and mass fraction. First, we consider exponentially growing perturbations and extend previous findings showing the potential for a significant increase of the critical Reynolds number. The largest stabilization is observed when the ratio between the particle relaxation time and the oscillation period of the wave is of order one. By examining the energy budget, we show that this stabilization is due to the increase of the dissipation caused by the Stokes drag. The observed stabilization has led to the hypothesis that dusty flows can be more stable. However, transition to turbulence is most often subcritical in canonical shear flows where non-modal growth mechanisms are responsible for the initial growth of external disturbances. The non-modal analysis of the particle-laden flow, presented here for the first time, reveals that the transient energy growth is, surprisingly, increased by the presence of particles, in proportion to the particle mass fraction. The generation of streamwise streaks via the lift-up mechanism is still the dominant disturbance-growth mechanism in the particle laden flow; the length scales of the most dangerous disturbances are unaffected, while the initial disturbance growth can be delayed. These results are explained in terms of a dimensionless parameter relating the particle relaxation time to the time scale of the instability. The presence of a dilute solid phase therefore may not always work as a flow-control strategy for maintaining the flow as laminar. Despite the stabilizing effect on modal instabilities, non-modal mechanisms are still strong in internal flows seeded with heavy particles. Our results indicate that the initial stages of transition in dilute suspensions of small particles are similar to the stages in a single phase flow.

  • 65.
    Kurian, Thomas
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Boundary layer receptivity to free-stream turbulence and surface roughness over a swept flat plate2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 3, p. 034107-Article in journal (Refereed)
    Abstract [en]

    An experimental study of the receptivity of disturbances and their subsequent development into a three-dimensional boundary layer has been carried out. The three-dimensional boundary layer was set up using a flat plate with a swept leading edge and a pressure gradient using a displacement body at the ceiling of the test section. Low level free-stream turbulence was generated with five different screens and was shown to generate traveling crossflow modes for all but the lowest turbulence level, i.e., for Tu > 0.2%, where instead a stationary crossflow disturbance dominated. Stationary crossflow disturbances were triggered by small cylindrical roughness elements arranged in an array. For high enough roughness Reynolds number (Re-k) stationary disturbances growing exponentially were seen and their amplitude seems to scale with Re-k(2.3).

  • 66.
    Lenaers, Peter
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Li, Qiang
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brethouwer, Geert
    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.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Rare backflow and extreme wall-normal velocity fluctuations in near-wall turbulence2012In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 24, no 3, p. 035110-Article in journal (Refereed)
    Abstract [en]

    Rare negative streamwise velocities and extreme wall-normal velocity fluctuations near the wall are investigated for turbulent channel flow at a series of Reynolds numbers based on friction velocity up to Re-tau = 1000. Probability density functions of the wall-shear stress and velocity components are presented as well as joint probability density functions of the velocity components and the pressure. Backflow occurs more often (0.06% at the wall at Re-tau = 1000) and further away (up to y(+) = 8.5) from the wall for increasing Reynolds number. The regions of backflow are circular with an average diameter, based on ensemble averages, of approximately 20 viscous units independent of Reynolds number. A strong oblique vortex outside the viscous sublayer is found to cause this backflow. Extreme wall-normal velocity events occur also more often for increasing Reynolds number. These extreme fluctuations cause high flatness values near the wall (F(v) = 43 at Re-tau = 1000). Positive and negative velocity spikes appear in pairs, located on the two edges of a strong streamwise vortex as documented by Xu et al. [Phys. Fluids 8, 1938 (1996)] for Re-tau = 180. The spikes are elliptical and orientated in streamwise direction with a typical length of 25 and a typical width of 7.5 viscous units at y(+) approximate to 1. The negative spike occurs in a high-speed streak indicating a sweeping motion, while the positive spike is located in between a high and low-speed streak. The joint probability density functions of negative streamwise and extreme wall-normal velocity events show that these events are largely uncorrelated. The majority of both type of events can be found lying underneath a large-scale structure in the outer region with positive sign, which can be understood by considering the more intense velocity fluctuations due to amplitude modulation of the inner layer by the outer layer. Simulations performed at different resolutions give only minor differences. Results from experiments and recent turbulent boundary layer simulations show similar results indicating that these rare events are universal for wall-bounded flows. In order to detect these rare events in experiments, measurement techniques have to be specifically tuned.

  • 67. Levenstam, M.
    et al.
    Amberg, Gustav
    KTH, Superseded Departments, Mechanics.
    Winkler, C.
    Instabilities of thermocapillary convection in a half-zone at intermediate Prandtl numbers2001In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 13, no 4, p. 807-816Article in journal (Refereed)
    Abstract [en]

    The stability of thermocapillary convection inside a cylindrical liquid bridge is studied using both a direct numerical simulation of the three-dimensional problem and linear stability analysis of the axisymmetric basic state. Previously this has been studied extensively for low and high Prandtl numbers. However, the intermediate range of Prandtl numbers between approximately 0.07 and 0.8 which joins the low and high ranges is quite complicated and has not been studied to the same extent. One striking feature is that the axisymmetric base state is much more stable in this intermediate range than at high or low Prandtl numbers. We identify four different oscillatory modes in this range, which have different qualitative features. Direct numerical simulations have been carried out for representative parameter values, and show that the bifurcations are supercritical.

  • 68.
    Levin, Ori
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Davidsson, Niklas
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Transition thresholds in the asymptotic suction boundary layer2005In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 17, no 12Article in journal (Refereed)
    Abstract [en]

    Energy thresholds for transition to turbulence in an asymptotic suction boundary layer is calculated by means of temporal direct numerical simulations. The temporal assumption limits the analysis to periodic disturbances with horizontal wave numbers determined by the computational box size. Three well known transition scenarios are investigated: oblique transition, the growth and breakdown of streaks triggered by streamwise vortices, and the development of random noise. Linear disturbance simulations and stability diagnostics are also performed for a base flow consisting of the suction boundary layer and a streak. The scenarios are found to trigger transition by similar mechanisms as obtained for other flows. Transition at the lowest initial energy is provided by the oblique wave scenario for the considered Reynolds numbers 500, 800, and 1200. The Reynolds number dependence on the energy thresholds are determined for each scenario. The threshold scales like Re-2.6 for oblique transition and like Re-2.1 for transition initiated by streamwise vortices and random noise, indicating that oblique transition has the lowest energy threshold also for larger Reynolds numbers.

  • 69. Li., J.
    et al.
    Scheffel, Jan
    Boundary Larmor Radius Effect on Electrostatic Waves1991In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. B3, p. 3013-Article in journal (Refereed)
    Abstract [en]

    The linearized Vlasov–Poisson equations, which combine to an integrodifferential equation for the perturbed electric potential, are used to investigate the effect of finite plasma size on the stability of electrostatic waves in a homogeneous plasma slab. The distortion of the gyromotion of the particles at the plasma boundary influences wave stability, a phenomenon termed the boundary Larmor radius (BLR) effect. The integrodifferential equation, treated as an eigenvalue problem, is discretized into a matrix dispersion equation by use of the Galerkin method and is then solved numerically. It is found that the ion Bernstein wave,which is undamped in an infinite homogeneous plasma, now becomes damped with a maximum damping rate of 0.35 ωci at rG/L (ion Larmor radius over wall distance)≊0.15. In general, the damping is less pronounced at shorter perpendicular wavelengths. It implies a necessity to take into account the BLR effect in the kinetic stability studies for sufficiently large ion Larmor radius in comparison to the characteristic dimension.

  • 70. Li, J.
    et al.
    Scheffel, Jan
    Kinetic High-beta Equilibria and Stability in a Plasma Slab1991In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. B3, p. 2506-Article in journal (Refereed)
    Abstract [en]

    Self‐consistent, high‐beta kinetic equilibria are derived from the Vlasov equation for a plasma slab in a nonzero magnetic field with a density gradient and a gravitational field. It is shown that a magnetic field inhomogeneity and an electric field arise as necessary consequences of the high‐beta state. Hence the corresponding equilibrium forces should not be neglected in calculations of individual particle motion. Explicit expressions are derived for equilibrium quantities, useful, e.g., for large Larmor radius (LLR) studies of the Rayleigh–Taylor instability.

  • 71.
    Liberman, M. A.
    et al.
    Department of Physics, Uppsala University.
    Ivanov, M. F.
    Department of Physics, Uppsala University.
    Peil, O. E.
    Department of Physics, Uppsala University.
    Valiev, Damir
    Department of Physics, Uppsala University.
    Eriksson, L.-E.
    Volvo Aero Corporation, Combustor Division, Trollhättan .
    Self-acceleration and fractal structure of outward freely propagating flames2004In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, no 7, p. 2476-2482Article in journal (Refereed)
    Abstract [en]

    Flame acceleration associated with development of the Landau-Darrieus hydrodynamic instability is studied by means of direct numerical simulation of the Navier-Stokes equations including chemical kinetics in the form of the Arrhenius law. The fractal excess for radially expanding flames in cylindrical geometry is evaluated. Two-dimensional (2-D) simulation of radially expanding flames in cylindrical geometry displays a radial growth with 1.25 power law temporal behavior after some transient time. It is shown that the fractal excess for 2-D geometry obtained in the numerical simulation is in good agreement with theoretical predictions. The difference in fractal dimension between 2-D cylidrical and three-dimensional spherical radially expanding flames is outlined. Extrapolation of the obtained results for the case of spherical expanding flames gives a radial growth power law that is consistent with temporal behavior obtained in the survey of experimental data.

  • 72.
    Lindborg, Erik
    et al.
    KTH, Superseded Departments, Mechanics.
    Alvelius, K.
    The kinetic energy spectrum of the two-dimensional enstrophy turbulence cascade2000In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 12, no 5, p. 945-947Article in journal (Refereed)
    Abstract [en]

    A direct numerical simulation of forced two-dimensional turbulence with hyperviscosity is performed at resolution 4096(2). A stage is reached at which the flux of enstrophy from large to small scales is approximately constant in time. The cubic and quintic relations for the third-order velocity structure function derived by Lindborg [J. Fluid Mech. 388, 259 (1999)] are verified. The calculated kinetic energy spectrum in the constant enstrophy flux range has the form E(k)=K epsilon(omega)(2/3)k(-3), where epsilon(omega) is the enstrophy dissipation. This is in accordance with the prediction of Kraichnan [Phys. Fluids 10, 1417 (1970)] and Batchelor [Phys. Fluids 12, II233 (1969)]. The logarithmic correction, suggested by Kraichnan [J. Fluid Mech. 47, 525 (1970)], is not present in the calculated spectrum.

  • 73.
    Lindborg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mohanan, Ashwin Vishnu
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    A two-dimensional toy model for geophysical turbulence2017In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 29, no 11, article id 111114Article in journal (Refereed)
    Abstract [en]

    A toy model for large scale geophysical turbulence is constructed by making two modifications of the shallow water model. Unlike the shallow water model, the toy model has a quadratic expression for total energy, which is the sum of Available Potential Energy (APE) and Kinetic Energy (KE). More importantly, in contrast to the shallow water model, the toy model does not produce any shocks. Three numerical simulations with different forcing are presented and compared with the simulation of a full General Circulation Model (GCM). The energy which is injected cascades in a similar way as in the GCM. First, some of the energy is converted from APE to KE at large scales. The wave field then undergoes a forward energy cascade displaying shallow spectra, close to k−5/3, for both APE and KE, while the vortical field either displays a k−3-spectrum or a more shallow spectrum, close to k−5/3, depending on the forcing. In a simulation with medium forcing wave number, some of the energy which is converted from APE to KE undergoes an inverse energy cascade which is produced by nonlinear interactions only involving the rotational component of the velocity field. The inverse energy cascade builds up a vortical field at larger scales than the forcing scale. At these scales, coherent vortices emerge with a strong dominance of anticyclonic vortices. The relevance of the simulation results to the dynamics of the atmosphere is discussed as in possible continuations of the investigation.

  • 74.
    Lindborg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vallgren, Andreas
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Testing Batchelor's similarity hypotheses for decaying two-dimensional turbulence2010In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 22, no 9, p. 091704-Article in journal (Refereed)
    Abstract [en]

    We carry out three high resolution direct numerical simulations of the two-dimensional Navier-Stokes equation to test Batchelor's similarity hypotheses of an equilibrium spectral range and an inertial subrange where the enstrophy wave number spectrum has the form Phi(k)=C chi(2/3)k(-1), where chi is the mean enstrophy dissipation rate and C is a constant. We use very different initial conditions in the three simulations and find that Batchelor's hypotheses are well satisfied in each simulation. However, there is a small but significant difference between the equilibrium range spectrum of one of the simulations as compared to the spectra of the other two. We suggest that the difference is linked to the stronger degree of large scale variation of the enstrophy dissipation which is observed in this simulation as compared to the other two.

  • 75.
    Lindgren, Björn
    et al.
    KTH, Superseded Departments, Mechanics.
    Johansson, Arne
    KTH, Superseded Departments, Mechanics.
    Tsuji, Yoshiyuki
    Universality of probability density distributions in the overlap region in high Reynolds number turbulent boundary layers2004In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, no 7, p. 2587-2591Article in journal (Refereed)
    Abstract [en]

    The probability density functions (PDFs) of the streamwise mean velocity in high Reynolds number turbulent boundary layers have been studied. The hypothesis of self-similar, normalized with the root mean square velocity, PDFs has been tested using the KTH database of high Reynolds number zero pressure-gradient turbulent boundary layer flow. The self-similarity was tested using the Kullback-Leibler divergence measure and it was found that the region of self-similar PDFs extends from about 160 viscous wall units to about 0.3 boundary layer thicknesses (in delta(95)). This region is somewhat larger than the universal overlap region. The PDF shape in the universal overlap region is close to Gaussian allowing for a Gram-Charlier expansion approximation of the measured PDFs. A remarkable collapse was found for 57 normalized PDF distributions for different positions within the universal overlap region and Reynolds numbers based on the momentum-loss thickness between 4300 and 12 600, strongly indicating a high degree of flow universality within the universal overlap region. Within the range studied, the Gram-Charlier expansion coefficients (related to the PDF moments) show no Reynolds number trend further supporting the self-similarity hypothesis.

  • 76.
    Liverts, Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Ram, O.
    Sadot, O.
    Apazidis, Nicholas
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Ben-Dor, G.
    Mitigation of exploding-wire-generated blast-waves by aqueous foam2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 7, article id 076103Article in journal (Refereed)
    Abstract [en]

    In this work, we implement an exploding wire technique to generate small-scale cylindrical blast waves in aqueous foam. The exploding wire system offers an easy to operate and effective tool for studying blast-wave/foam interaction related phenomena in real explosion scenarios. The mitigation of blast waves as a function of the thickness of the foam barrier is discussed and quantified. A fluid mixture pseudo-gas based numerical approach with the aid of the point explosion theory is used to separate the mitigation mechanisms into the near-and the far-field related groups and to analyze the contribution of each group to the overall losses of the blast wave energy.

  • 77.
    Lundell, Fredrik
    KTH, Superseded Departments, Mechanics.
    Streak oscillations of finite length: Disturbance evolution and growth2004In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, no 8, p. 3227-3230Article in journal (Refereed)
    Abstract [en]

    Secondary instabilities of low-velocity streaks are studied in a plane channel flow. The secondary instability is forced by a sinusoidal signal consisting of N periods, where N ranges from 0.5 to 30. The disturbance distribution of the secondary instability is found to be similar to previous measurements on streaks forced by infinitely long wave trains. The amplitude growth of the localized secondary instabilities is close to exponential for N>4. However, for N<4 it is found that the exponential growth factor varies considerably both in the streamwise direction and with N with high values at upstream positions. It is proposed that these effects are due to transient growth of the localized secondary instabilities.

  • 78.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    The effect of particle inertia on triaxial ellipsoids in creeping shear: From drift toward chaos to a single periodic solution2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 1, p. 011704-Article in journal (Refereed)
    Abstract [en]

    The motion of inertial, triaxial ellipsoids in creeping shear flow is explained for a wide range of aspect ratios and Stokes numbers (St, quantifying particle inertia). Particle inertia induces a drift toward rotation around the shortest axis, with this axis aligned with the vorticity axis of the flow. For aspect ratio combinations in a certain region, this periodic state is unstable for low St and the particle moves in a chaotic manner. At higher St, the instability is stabilized and one single final periodic motion is well defined also for (in the limit of St=0) unstable aspect ratios.

  • 79.
    Lundell, Fredrik
    et al.
    KTH, Superseded Departments, Mechanics.
    Alfredsson, Henrik
    KTH, Superseded Departments, Mechanics.
    Streamwise scaling of streaks in laminar boundary layers subjected to free-stream turbulence2004In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, no 5, p. 1814-1817Article in journal (Refereed)
    Abstract [en]

    A flat plate laminar boundary layer subjected to free-stream turbulence, generated by grids upstream of the leading edge, has been studied. Correlations between a wall wire and a hot wire in the boundary layer have been obtained. The hot wire has been traversed to 328 positions for each of the five wall-wire positions studied. The length, height, and width of the correlation distributions is seen to increase in the downstream direction being self-similar in coordinates scaled with the boundary-layer length scale. The propagation speeds of the structures are obtained from the data and its variation in the streamwise and wall-normal directions is found to agree with what is obtained from a simple kinematic model.

  • 80.
    Lögdberg, Ola
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Angele, Kristian
    Vattenfall Research and Development AB.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    On the scaling of turbulent separating boundary layers2008In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 20, no 7, p. 075104-1-075104-4Article in journal (Refereed)
    Abstract [en]

    This study focuses on the mean velocity distribution of turbulent boundary layers near, at, and after separation. The proper mean velocity scaling of the outer region in strong adverse pressure gradients and separated turbulent boundary layers is still under debate and over the years various different velocity scales have been proposed. Here the scaling proposed by Zagarola and Smits [J. Fluid Mech. 373, 33 (1998)] is applied to data from three different separated flows. In all cases the mean velocity defect profiles are self-similar in the region between separation and the position of maximum mean reverse flow. Downstream of the reverse flow maximum, the profiles change, but they are still self-similar within that region. It was also found that the mean velocity defect profiles of all three pressure gradients show similarity in the region between separation and the position of maximum mean reverse flow.

  • 81.
    MacKenzie, Jordan
    et al.
    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 Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Swerin, Agne
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Surface and Corrosion Science. RISE Research Institutes of Sweden.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Turbulent stress measurements of fibre suspensions in a straight pipe2018In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 30, no 2, article id 025104Article in journal (Refereed)
    Abstract [en]

    The focus of the present work is an experimental study of the behaviour of semi-dilute, opaque fibre suspensions in fully developed cylindrical pipe flows. Measurements of the normal and turbulent shear stress components and the mean flow were acquired using phase-contrast magnetic resonance velocimetry. Two fibre types, namely, pulp fibre and nylon fibre, were considered in this work and are known to differ in elastic modulus. In total, three different mass concentrations and seven Reynolds numbers were tested to investigate the effects of fibre interactions during the transition from the plug flow to fully turbulent flow. It was found that in fully turbulent flows of nylon fibres, the normal, < u(z)u(z)>(+), and shear, < u(z)u(z)>(+) (note that <.> is the temporal average, u is the fluctuating velocity, z is the axial or streamwise component, and r is the radial direction), turbulent stresses increased with Reynolds number regardless of the crowding number (a concentration measure). For pulp fibre, the turbulent stresses increased with Reynolds number when a fibre plug was present in the flow and were spatially similar in magnitude when no fibre plug was present. Pressure spectra revealed that the stiff, nylon fibre reduced the energy in the inertial-subrange with an increasing Reynolds and crowding number, whereas the less stiff pulp fibre effectively cuts the energy cascade prematurely when the network was fully dispersed.

  • 82.
    Mariani, Raffaello
    et al.
    University of Manchester.
    Kontis, Konstantinos
    University of Manchester.
    Experimental Studies on Coaxial Vortex Loops2010In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 22, no 12, article id 126102Article in journal (Refereed)
    Abstract [en]

    An experimental study has been conducted on the formation and propagation of coaxial vortex loops using a shock tube facility. The study aimed at evaluating the flow characteristics of pairs of corotating vortex rings that generate the leapfrogging phenomenon. The driver and driven gas of the shock tube were air. Three driver pressures were used (4, 8, and 12 bars) with the driven gas being at ambient conditions. The Mach numbers of the shock wave generated inside the shock tube were 1.34, 1.54, and 1.66, respectively. The sudden expansion present at the diaphragm location effectively decreased the Mach number value of the traveling shock wave. Results showed that apair of vortex rings staggered with respect to time and with the same direction rotation lead to leapfrogging. Results also indicated that the number of leapfrogging occurrences is related to the Reynolds number of the vortex ring pairs with a decrease in leapfrogs at higher Reynolds numbers.

  • 83. Marin, O.
    et al.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Obabko, A. V.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Characterization of the secondary flow in hexagonal ducts2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 12, article id 125101Article in journal (Refereed)
    Abstract [en]

    In this work we report the results of DNSs and LESs of the turbulent flow through hexagonal ducts at friction Reynolds numbers based on centerplane wall shear and duct half-height Re-tau,Re- c similar or equal to 180, 360, and 550. The evolution of the Fanning friction factor f with Re is in very good agreement with experimental measurements. A significant disagreement between the DNS and previous RANS simulations was found in the prediction of the in-plane velocity, and is explained through the inability of the RANS model to properly reproduce the secondary flow present in the hexagon. The kinetic energy of the secondary flow integrated over the cross-sectional area < K >(yz) decreases with Re in the hexagon, whereas it remains constant with Re in square ducts at comparable Reynolds numbers. Close connection between the values of Reynolds stress (uw) over bar on the horizontal wall close to the corner and the interaction of bursting events between the horizontal and inclined walls is found. This interaction leads to the formation of the secondary flow, and is less frequent in the hexagon as Re increases due to the 120 degrees aperture of its vertex, whereas in the square duct the 90 degrees corner leads to the same level of interaction with increasing Re. Analysis of turbulence statistics at the centerplane and the azimuthal variance of the mean flow and the fluctuations shows a close connection between hexagonal ducts and pipe flows, since the hexagon exhibits near-axisymmetric conditions up to a distance of around 0.15D(H) measured from its center. Spanwise distributions of wall-shear stress show that in square ducts the 90 degrees corner sets the location of a high-speed streak at a distance z(nu)(+) similar or equal to 50 from it, whereas in hexagons the 120 degrees aperture leads to a shorter distance of z(nu)(+) similar or equal to 38. At these locations the root mean square of the wall-shear stresses exhibits an inflection point, which further shows the connections between the near-wall structures and the large-scale motions in the outer flow. Published by AIP Publishing.

  • 84.
    Marstorp, Linus
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    A stochastic subgrid model with application to turbulent flow and scalar mixing2007In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, no 3, p. 035107-Article in journal (Refereed)
    Abstract [en]

    A new computationally cheap stochastic Smagorinsky model which allows for backscatter of subgrid scale energy is proposed. The new model is applied in the large eddy simulation of decaying isotropic turbulence, rotating homogeneous shear flow and turbulent channel flow at Re-tau=360. The results of the simulations are compared to direct numerical simulation data. The inclusion of stochastic backscatter has no significant influence on the development of the kinetic energy in homogeneous flows, but it improves the prediction of the fluctuation magnitudes as well as the anisotropy of the fluctuations in turbulent channel flow compared to the standard Smagorinsky model with wall damping of C-S. Moreover, the stochastic model improves the description of the energy transfer by reducing its length scale and increasing its variance. Some improvements were also found in isotropic turbulence where the stochastic contribution improved the shape of the enstrophy spectrum at the smallest resolved scales and reduced the time scale of the smallest resolved scales in better agreement with earlier observations.

  • 85.
    Mittal, Nitesh
    PSL Res Univ, CNRS UMR8231, ESPCI ParisTech, Lab Colloides & Mat Divises,Inst Chem Biol & Inno, F-75231 Paris, France.
    Dynamics of step-emulsification: From a single to a collection of emulsion droplet generators2014In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 26, article id 082109Article in journal (Refereed)
    Abstract [en]

    Microfluidics has proven to be an efficient tool for making fine and calibrated emulsion droplets. The parallelization of drop makers is required for producing large amounts. Here, we investigate the generation of emulsion drops along a series of shallowmicrochannels emerging in a deep one, in other words the step-emulsification process. The dynamics of a single drop maker is first characterized as a function of interfacial tension and viscosities of both phases. The characteristic time scale of drop formation, namely, the necking time that finally sets drop size, is shown to be principally governed by the outer phase viscosity to interfacial tension ratio with a minor correction linked to the viscosity ratio. The step emulsification process experiences a transition of fragmentation regime where both the necking time and drop size suddenly raise. This transition, that corresponds to a critical period of drop formation and thus defines a maximum production rate of small droplets, is observed to be ruled by the viscosity ratio of the two phases. When drops are produced along an array of microchannels with a cross flow of the continuous phase, a configuration comparable to a one-dimensional membrane having rectangular pores, a drop boundary layer develops along the drop generators. In the small drop regime, the local dynamics of drop formation is shown to be independent on the emulsion cross flow. Moreover, we note that the development of the drop boundary layer is even beneficial to homogenize drop size along the production line. On the other hand, in the large drop regime, drop collision can trigger fragmentation and thus lead to size polydispersity.

  • 86.
    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. 

  • 87.
    Nicolai, Claudia
    et al.
    Sapienza University of Rome.
    Jacob, Boris Francesco
    CNR-INSEAN.
    Piva, Renzo
    Sapienza University of Rome.
    On the spatial distribution of small heavy particles in homogeneous shear turbulence2013In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 25, no 8, article id 083301Article in journal (Refereed)
    Abstract [en]

    We report on a novel experiment aimed at investigating the effects induced by a large-scale velocity gradient on the turbulent transport of small heavy particles. To this purpose, a homogeneous shear flow at Reλ = 540 and shear parameter S* = 4.5 is set-up and laden with glass spheres whose size d is comparable with the Kolmogorov lengthscale η of the flow (d/η ≈ 1). The particle Stokes number is approximately 0.3. The analysis of the instantaneous particle fields by means of Voronoï diagrams confirms the occurrence of intense turbulent clustering at small scales, as observed in homogeneous isotropic flows. It also indicates that the anisotropy of the velocity fluctuations induces a preferential orientation of the particle clusters. In order to characterize the fine-scale features of the dispersed phase, spatial correlations of the particle field are employed in conjunction with statistical tools recently developed for anisotropic turbulence.The scale-by-scale analysis of the particle field clarifies that isotropy of the particle distribution is tendentially recovered at small separations, even though the signatures of the mean shear persist down to smaller scales as compared to the fluid velocity field.

  • 88.
    Noorani, Azad
    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.
    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.
    Evidence of sublaminar drag naturally occurring in a curved pipe2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 3, article id 035105Article in journal (Refereed)
    Abstract [en]

    Steady and unsteady flows in a mildly curved pipe for a wide range of Reynolds numbers are examined with direct numerical simulation. It is shown that in a range of Reynolds numbers in the vicinity of Re-b approximate to 3400, based on bulk velocity and pipe diameter, a marginally turbulent flow is established in which the friction drag naturally reduces below the laminar solution at the same Reynolds number. The obtained values for friction drag for the laminar and turbulent (sublaminar) flows turn out to be in excellent agreement with experimental measurements in the literature. Our results are also in agreement with Fukagata et al. ["On the lower bound of net driving power in controlled duct flows," Phys. D 238, 1082 (2009)], as the lower bound of net power required to drive the flow, i.e., the pressure drop of the Stokes solution, is still lower than our marginally turbulent flow. A large-scale traveling structure that is thought to be responsible for that behaviour is identified in the instantaneous field. This mode could also be extracted using proper orthogonal decomposition. The effect of this mode is to redistribute the mean flow in the circular cross section which leads to lower gradients at the wall compared to the laminar flow.

  • 89.
    Noorani, Azad
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Aspect ratio effect on particle transport in turbulent duct flows2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 11, article id 115103Article in journal (Refereed)
    Abstract [en]

    The dynamics of dilute micron-sized spherical inertial particles in turbulent duct flows is studied by means of direct numerical simulations of the carrier phase turbulence with one-way coupled Lagrangian particles. The geometries are a square and a rectangular duct with width-to-height aspect ratio AR of 3 operating at Re-tau,Re-c = 360 (based on the centerplane friction velocity and duct half-height). The present study is designed to determine the effect of turbulence-driven secondary motion on the particle dynamics. Our results show that a weak cross-flow secondary motion significantly changes the cross-sectional map of the particle concentration, mean velocity, and fluctuations. As the geometry of the duct is widened from AR = 1 to 3, the secondary vortex on the horizontal wall significantly expands in the spanwise direction, and although the kinetic energy of the secondary flow increases close to the corner, it decays towards the duct centreplane in the AR = 3 case so as the turbulent carrier phase approaches the behavior in spanwise-periodic channel flows, a fact that significantly affects the particle statistics. In the square duct the particle concentration in the viscous sublayer is maximum at the duct centreplane, whereas the maximum is found closer to the corner, at a distance of |z/h| approximate to 1.25 from the centreplane, in the AR = 3 case. Interestingly the centreplane concentration in the rectangular duct is around 3 times lower than that in the square duct. Moreover, a second peak in the accumulation distribution is found right at the corners for both ducts. At this location the concentration increases with particle inertia. The secondary motion changes also the cross-stream map of the particle velocities significantly in comparison to the fluid flow statistics. These directly affect the particle velocity fluctuations such that multiple peaks appear near the duct walls for the particle streamwise and wall-normal velocity fluctuations.

  • 90.
    Nooranidoost, M.
    et al.
    Koc Univ, Dept Mech Engn, TR-34450 Istanbul, Turkey.;Univ Cent Florida, Dept Mech & Aerosp Engn, Orlando, FL 32816 USA..
    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. Koc Univ, Dept Mech Engn, TR-34450 Istanbul, Turkey..
    Tasoglu, S.
    Univ Connecticut, Connecticut Inst Brain & Cognit Sci, Inst Collaborat Hlth Intervent & Policy, Dept Biomed Engn,Inst Mat Sci,Dept Mech Engn, Storrs, CT 06269 USA..
    Muradoglu, M.
    Koc Univ, Dept Mech Engn, TR-34450 Istanbul, Turkey..
    A computational study of droplet-based bioprinting: Effects of viscoelasticity2019In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 31, no 8, article id 081901Article in journal (Refereed)
    Abstract [en]

    Despite significant progress, cell viability continues to be a central issue in droplet-based bioprinting applications. Common bioinks exhibit viscoelastic behavior owing to the presence of long-chain molecules in their mixture. We computationally study effects of viscoelasticity of bioinks on cell viability during deposition of cell-loaded droplets on a substrate using a compound droplet model. The inner droplet, which represents the cell, and the encapsulating droplet are modeled as viscoelastic liquids with different material properties, while the ambient fluid is Newtonian. The model proposed by Takamatsu and Rubinsky ["Viability of deformed cells," Cryobiology 39(3), 243-251 (1999)] is used to relate cell deformation to cell viability. We demonstrate that adding viscoelasticity to the encapsulating droplet fluid can significantly enhance the cell viability, suggesting that viscoelastic properties of bioinks can be tailored to achieve high cell viability in droplet-based bioprinting systems. The effects of the cell viscoelasticity are also examined, and it is shown that the Newtonian cell models may significantly overpredict the cell viability. Published under license by AIP Publishing.

  • 91. Olivieri, S.
    et al.
    Picano, Francesco
    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.
    Sardina, Gaetano
    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.
    Iudicone, D.
    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.
    The effect of the Basset history force on particle clustering in homogeneous and isotropic turbulence2014In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 26, no 4, p. 041704-Article in journal (Refereed)
    Abstract [en]

    We study the effect of the Basset history force on the dynamics of small particles transported in homogeneous and isotropic turbulence and show that this term, often neglected in previous numerical studies, reduces the small-scale clustering typical of inertial particles. The contribution of this force to the total particle acceleration is, on average, responsible for about 10% of the total acceleration and particularly relevant during rare strong events. At moderate density ratios, i.e., sand or metal powder in water, its presence alters the balance of forces determining the particle acceleration.

  • 92.
    Pak, Shun
    et al.
    Dept. of Mechanical and Aerospace Engineering University of California, San Diego, USA.
    Zhu, Lailai
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. 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.
    Micropropulsion and microrheology in complex fluids via symmetry breaking2012In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 24, no 10, p. 103102-Article in journal (Refereed)
    Abstract [en]

    Many biological fluids have polymeric microstructures and display non-Newtonian rheology. We take advantage of such nonlinear fluid behavior and combine it with geometrical symmetry-breaking to design a novel small-scale propeller able to move only in complex fluids. Its propulsion characteristics are explored numerically in an Oldroyd-B fluid for finite Deborah numbers while the small Deborah number limit is investigated analytically using a second-order fluid model. We then derive expressions relating the propulsion speed to the rheological properties of the complex fluid, allowing thus to infer the normal stress coefficients in the fluid from the locomotion of the propeller. Our simple mechanism can therefore be used either as a non-Newtonian micro-propeller or as a micro-rheometer.

  • 93. Pandit, R.
    et al.
    Banerjee, D.
    Bhatnagar, Akshay
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholms University, Sweden.
    Brachet, M.
    Gupta, A.
    Mitra, Dhrubaditya
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholms University, Sweden.
    Pal, N.
    Perlekar, P.
    Ray, S. S.
    Shukla, V.
    Vincenzi, D.
    An overview of the statistical properties of two-dimensional turbulence in fluids with particles, conducting fluids, fluids with polymer additives, binary-fluid mixtures, and superfluids2017In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 29, no 11, article id 111112Article in journal (Refereed)
    Abstract [en]

    We present an overview of the statistical properties of turbulence in two-dimensional (2D) fluids. After a brief recapitulation of well-known results for statistically homogeneous and isotropic 2D fluid turbulence, we give an overview of recent progress in this field for such 2D turbulence in conducting fluids, fluids with polymer additives, binary-fluid mixtures, and superfluids; we also discuss the statistical properties of particles advected by 2D turbulent fluids.

  • 94.
    Pouransari, Zeinab
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Biferale, L.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Statistical analysis of the velocity and scalar fields in reacting turbulent wall-jets2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 2, p. 025102-Article in journal (Refereed)
    Abstract [en]

    The concept of local isotropy in a chemically reacting turbulent wall-jet flow is addressed using direct numerical simulation (DNS) data. Different DNS databases with isothermal and exothermic reactions are examined. The chemical reaction and heat release effects on the turbulent velocity, passive scalar, and reactive species fields are studied using their probability density functions (PDFs) and higher order moments for velocities and scalar fields, as well as their gradients. With the aid of the anisotropy invariant maps for the Reynolds stress tensor, the heat release effects on the anisotropy level at different wall-normal locations are evaluated and found to be most accentuated in the near-wall region. It is observed that the small-scale anisotropies are persistent both in the near-wall region and inside the jet flame. Two exothermic cases with different Damkohler numbers are examined and the comparison revealed that the Damkohler number effects are most dominant in the near-wall region, where the wall cooling effects are influential. In addition, with the aid of PDFs conditioned on the mixture fraction, the significance of the reactive scalar characteristics in the reaction zone is illustrated. We argue that the combined effects of strong intermittency and strong persistency of anisotropy at the small scales in the entire domain can affect mixing and ultimately the combustion characteristics of the reacting flow.

  • 95.
    Pouransari, Zeinab
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne J.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Direct numerical simulation of an isothermal reacting turbulent wall-jet2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 8, p. 085104-Article in journal (Refereed)
    Abstract [en]

    In the present investigation, Direct Numerical Simulation (DNS) is used to study a binary irreversible and isothermal reaction in a plane turbulent wall-jet. The flow is compressible and a single-step global reaction between an oxidizer and a fuel species is solved. The inlet based Reynolds, Schmidt, and Mach numbers of the wall-jet are Re = 2000, Sc = 0.72, and M = 0.5, respectively, and a constant coflow velocity is applied above the jet. At the inlet, fuel and oxidizer enter the domain separately in a non-premixed manner. The turbulent structures of the velocity field show the common streaky patterns near the wall, while a somewhat patchy or spotty pattern is observed for the scalars and the reaction rate fluctuations in the near-wall region. The reaction mainly occurs in the upper shear layer in thin highly convoluted reaction zones, but it also takes place close to the wall. Analysis of turbulence and reaction statistics confirms the observations in the instantaneous snapshots, regarding the intermittent character of the reaction rate near the wall. A detailed study of the probability density functions of the reacting scalars and comparison to that of the passive scalar throughout the domain reveals the significance of the reaction influence as well as the wall effects on the scalar distributions. The higher order moments of both the velocities and the scalar concentrations are analyzed and show a satisfactory agreement with experiments. The simulations show that the reaction can both enhance and reduce the dissipation of fuel scalar, since there are two competing effects; on the one hand, the reaction causes sharper scalar gradients and thus a higher dissipation rate, on the other hand, the reaction consumes the fuel scalar thereby reducing the scalar dissipation.

  • 96. Pralits, J. O.
    et al.
    Hanifi, Ardeshir
    Optimization of steady suction for disturbance control on infinite swept wings2003In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 15, no 9, p. 2756-2772Article in journal (Refereed)
    Abstract [en]

    We present a theory for computing the optimal steady suction distribution to suppress convectively unstable disturbances in growing boundary layers on infinite swept wings. This work includes optimization based on minimizing the disturbance kinetic energy and the integral of the shape factor. Further, a suction distribution in a continuous control domain is compared to an approach using a number of discrete pressure chambers. In the latter case, the internal static pressures of these chambers are optimized. Optimality systems are derived using Lagrange multipliers. The corresponding optimality conditions are evaluated using the adjoint of the parabolized stability equations and the adjoint of the boundary layer equations. Results are presented for an airfoil designed for medium range commercial aircraft. We show that an optimal suction distribution based on a minimization of the integral of the shape factor is not always successful in the sense of delaying laminar-turbulent transition. It is also demonstrated that including different types of disturbances, e.g., Tollmien-Schlichting and cross-flow types, in the analysis may be crucial.

  • 97.
    Rasam, Amin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Johansson, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    A stochastic extension of the explicit algebraic subgrid-scales models2014In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 26, no 5, p. 055113-Article in journal (Refereed)
    Abstract [en]

    The explicit algebraic subgrid-scale (SGS) stress model (EASM) of Marstorp et al. ["Explicit algebraic subgrid stress models with application to rotating channel flow," J. Fluid Mech. 639, 403-432 (2009)] and explicit algebraic SGS scalar flux model (EASFM) of Rasam et al. ["An explicit algebraic model for the subgrid-scale passive scalar flux,"J. Fluid Mech. 721, 541-577 (2013)] are extended with stochastic terms based on the Langevin equation formalism for the subgrid-scales by Marstorp et al. ["A stochastic subgrid model with application to turbulent flow and scalar mixing," Phys. Fluids 19, 035107 (2007)]. The EASM and EASFM are nonlinear mixed and tensor eddy-diffusivity models, which improve large eddy simulation (LES) predictions of the mean flow, Reynolds stresses, and scalar fluxes of wall-bounded flows compared to isotropic eddy-viscosity and eddy-diffusivity SGS models, especially at coarse resolutions. The purpose of the stochastic extension of the explicit algebraic SGS models is to further improve the characteristics of the kinetic energy and scalar variance SGS dissipation, which are key quantities that govern the small-scale mixing and dispersion dynamics. LES of turbulent channel flow with passive scalar transport shows that the stochastic terms enhance SGS dissipation statistics such as length scale, variance, and probability density functions and introduce a significant amount of backscatter of energy from the subgrid to the resolved scales without causing numerical stability problems. The improvements in the SGS dissipation predictions in turn enhances the predicted resolved statistics such as the mean scalar, scalar fluxes, Reynolds stresses, and correlation lengths. Moreover, the nonalignment between the SGS stress and resolved strain-rate tensors predicted by the EASM with stochastic extension is in much closer agreement with direct numerical simulation data.

  • 98. Ren, Jie
    et al.
    Fu, Song
    Hanifi, Ardeshir
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Swedish Def Res Agcy FOI, Stockholm, Sweden.
    Stabilization of the hypersonic boundary layer by finite-amplitude streaks2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 2, article id 024110Article in journal (Refereed)
    Abstract [en]

    Stabilization of two-dimensional disturbances in hypersonic boundary layer flows by finite-amplitude streaks is investigated using nonlinear parabolized stability equations. The boundary-layer flows at Mach numbers 4.5 and 6.0 are studied in which both first and second modes are supported. The streaks considered here are driven either by the so-called optimal perturbations (Klebanoff-type) or the centrifugal instability (Gortler-type). When the streak amplitude is in an appropriate range, i.e., large enough to modulate the laminar boundary layer but low enough to not trigger secondary instability, both first and second modes can effectively be suppressed. (C) 2016 AIP Publishing LLC.

  • 99.
    Samanta, Arghya
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Department of Chemical Engineering, Indian Institute of ScienceBangalore, Karnataka, India .
    Effect of surfactants on the instability of a two-layer film flow down an inclined plane2014In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 26, no 9, p. 094105-Article in journal (Refereed)
    Abstract [en]

    The effect of insoluble surfactants on the instability of a two-layer film flow down an inclined plane is investigated based on the Orr-Sommerfeld boundary value problem. The study, focusing on Stokes flow [P. Gao and X.-Y. Lu, "Effect of surfactants on the inertialess instability of a two-layer film flow," J. Fluid Mech. 591, 495-507 (2007)], is further extended by including the inertial effect. The surface mode is recognized along with the interface mode. The initial growth rate corresponding to the interface mode accelerates at sufficiently long-wave regime in the presence of surface surfactant. However, the maximum growth rate corresponding to both interface and surface modes decelerates in the presence of surface surfactant when the upper layer is more viscous than the lower layer. On the other hand, when the upper layer is less viscous than the lower layer, a new interfacial instability develops due to the inertial effect and becomes weaker in the presence of interfacial surfactant. In the limit of negligible surface and interfacial tensions, respectively, two successive peaks of temporal growth rate appear in the long-wave and short-wave regimes when the interface mode is analyzed. However, in the case of the surface mode, only the long-wave peak appears.

  • 100. 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.

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