Volume 20, Issue 10, October 2008

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 boundarylayer streaks and their instability with respect to turbulentbreakdown in bypass transition in a boundary layer subject to freestream turbulence. The importance of the streak secondaryinstability process for the generation of turbulent spots is clearly shown. The secondary instability manifests itself as a growing wave packet located on the lowspeed 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 threedimensional visualizations of the streak breakdown have been found. The wavelength of the instability is about an order of magnitude larger than the local boundarylayer displacement thickness , the group velocity about 0.8 of the freestream velocity , and the growth rate on the order of a few percent of . The characteristic structures at the breakdown are quasistreamwise vortices, located on the flanks of the lowspeed region arranged in a staggered pattern.
 ARTICLES

 Interfacial Flows

Dewetting of nanometer thin films under an electric field
View Description Hide DescriptionThe dewetting of a nanoscale water film under the action of an electric field is studied with molecular dynamics simulation. Results show that the onset of film rupture is induced by a spontaneous instability mechanism. After the rupture, the rim of the film recedes with a dynamic contact angle. The transient streamlines at a typical moment show that the liquid molecule near the rim moves almost vertically upwards, driven by the repulsive force from the solid surface. The oscillatory behavior of the density profile, resulting from the interaction between attractive and repulsive potentials, is observed near the solid surface. The analyses of the dewetting process demonstrate that the applied electric field will increase the wettability of graphite walls, thus suppressing the rupture, reducing the dynamic contact angle, and raising the liquid density adjacent to both the solid and liquidvacuumsurfaces. Owing to the polarity of water, the positive voltage produces stronger influences than the negative one.

Capillary rise of a liquid between two vertical plates making a small angle
View Description Hide DescriptionThe penetration of a wettingliquid in the narrow gap between two vertical plates making a small angle is analyzed in the framework of the lubrication approximation. At the beginning of the process, the liquid rises independently at different distances from the line of intersection of the plates except in a small region around this line where the effect of the gravity is negligible. The maximum height of the liquid initially increases as the cubic root of time and is attained at a point that reaches the line of intersection only after a certain time. At later times, the motion of the liquid is confined to a thin layer around the line of intersection whose height increases as the cubic root of time and whose thickness decreases as the inverse of the cubic root of time. The evolution of the liquid surface is computed numerically and compared with the results of a simple experiment.

Instabilities and Taylor dispersion in isothermal binary thin fluid films
View Description Hide DescriptionExperiments with glycerolwater thin films flowing down an inclined plane reveal a localized instability that is primarily three dimensional. These transient structures, referred to as “dimples,” appear initially as nearly isotropic depressions on the interface. A linear stability analysis of a binary mixture model in which barodiffusive effects dominate over thermophoresis (i.e., the Soret effect) reveals unstable modes when the components of the mixture have different bulk densities and surface tensions. This instability occurs when Fickian diffusion and Taylor dispersion effects are small, and is driven by solutalcapillary stresses arising from gradients in concentration of one component, across the depth of the film. Qualitative comparison between the experiments and the linear stability results over a wide range of parameters is presented.
 Viscous and NonNewtonian Flows

Unsteady contact melting of a rectangular crosssection material on a flat plate
View Description Hide DescriptionThe work in this paper concerns a mathematical model of the contact melting process of a rectangular material in contact with a hot plate. The problem is described by a coupled system of heat equations in the solid and melt layer, fluid flow in the melt, a Stefan condition at the melt interface, and a force balance between the weight of the solid and the fluid pressure. Since the melt layer remains thin throughout the process, we use the lubrication approximation to the fluid equations and assume that the heat flow in the fluid is dominated by conduction across the thin film. In the solid we employ a heat balance integral method. Results show that the film height has initial and final rapid increases, whereas for intermediate times the height slowly increases. The quasisteady state of previous models is never attained: This is shown to be an effect of neglecting the change in mass and conduction in the solid. The previously observed initial infinite velocity of the melt is shown to be a result of the perfect thermal contact assumption. For a waterice system the melting rate is shown to be approximately linear, this allows us to reduce the problem to solving a single first order differential equation for the liquid layer thickness. The main analysis is carried out in two dimensions, but we briefly highlight the extension to three dimensions. The method is verified by comparison with previously published experimental results on the melting of octadecane.

On the appearance of Moffatt eddies in viscous cavity flow as the aspect ratio varies
View Description Hide DescriptionTwodimensional Stokes flow in a rectangular driven cavity is studied. Flow in very tall or shallow cavities has many counterrotating eddies lying along the cavity centerline. This structure is investigated by constructing asymptotic approximations to the flow based on the assumptions and , where is the cavity’s aspect ratio. We show that the number of eddies increases as tends to infinity or zero and derive asymptotic formulas for the values of the aspect ratio at which the streamline topology bifurcates and a new eddy appears. We concentrate particularly on flow driven by translation of the top and bottom cavity walls with equal and opposite velocities. For this benchmark problem our asymptotics are able to connect existing computations (performed in the approximate range of ) with Moffatt’s 1964 theory for an infinite channel (i.e., or ). We show that tall cavities have sequences of eddies which match the infinite channel flow, as observed in the previous computations. In contrast, shallow cavities have only half the number of eddies and a more complicated streamline topology. In both cases our asymptotic approximations give analytic formulas for the number of eddies and the shape of the streamlines. Other flows, corresponding to different driving at the boundaries, are also discussed and can be treated by the asymptotic methods we derive.
 Particulate, Multiphase, and Granular Flows

Convection in deep vertically shaken particle beds. I. General features
View Description Hide DescriptionIt has long been known that shaking a granular bed can produce circulating convection. This is the first in a series of papers that explores convection in beds a hundred particles or more deep. In such deep beds, the pressures are high enough that the particles remain in contact with their neighbors throughout much of the vibrational period. As such, they interact elastically and convection becomes dependent on the elastic properties of the bed. Changing the particle stiffness can dramatically alter the convection, and for very soft or very hard particles, eliminate it completely. In this paper, the effect of stiffness, bed geometry, and frictional properties on the global convection properties are assessed.

Convection in deep vertically shaken particle beds. II. The relationship between convection and internal wave propagation
View Description Hide DescriptionThe convective motion in deep vertically shaken beds is not continuous but occurs only during a brief portion of a cycle that roughly repeats over three periods of vibration. The convection is coordinated by a series of waves that propagate through the bed, a compression wave formed as the flask’s bottom pushes upward against the bottom of the bed, and two expansion waves: a Type 1 expansion wave that is the reflection of the compression wave and a Type 2 expansion wave that forms as the flask’s bottom moves away from the bottom of the bed. Convection only is observed after Type 1 expansion wave has passed the convective zone, relaxing the stresses in the bed and leaving the particles free to move. However the convective motion is confined to the region above Type 2 wave and convection disappears as a Type 2 wave passes.

Convection in deep vertically shaken particle beds. III. Convection mechanisms
View Description Hide DescriptionConvection in a deep vertically vibrated twodimensional cell of granular material occurs in the form of counterrotating cells that move material from the walls to the center of the channel and back again. At least for deep beds, where for much of the cycle, particles are in long duration contact with their neighbors, convection only appears for a short potion of every third vibrational period. That period is delimited by the interaction of three types of internal waves, a compression wave, and two types of expansion waves. Four mechanisms are identified that drive the four basic motions of convection: (1) particles move upward at the center as the result of compression wave, (2) downward at the wall as a combined effect of frictional holdback by the walls and the downward pull of gravity, (3) from the center to the walls along the free surface due to the heaping of the bed generated by the compression wave, and (4) toward the center in the interior of the box to form the bottom of convection rolls due to the relaxation of compressive stresses caused by an expansion wave.Convection only occurs when the conditions are right for all four mechanisms to be active simultaneously.

LatticeBoltzmann simulation of finite Reynolds number buoyancydriven bubbly flows in periodic and wallbounded domains
View Description Hide DescriptionA latticeBoltzmann method is used to probe the structure and average properties of suspensions of monodisperse, spherical, noncoalescing bubbles rising due to buoyancy with Reynolds numbers based on the bubble terminal velocities of 5.4 and 20. Unbounded suspensions subject to periodic boundary conditions exhibit a microstructure with a strong tendency toward horizontal alignment of bubble pairs even at volume fractions of as high as 0.2. This microstructure leads to a mean rise velocity whose dependence on the bubble volume fraction is not well fitted by a standard powerlaw function. Simulations with bounding vertical walls exhibit a deficit of bubbles near each wall and a peak of volume fraction approximately one bubble diameter from the wall. We attribute this structure to the effects of a repulsive wallinduced force and a lift force associated with the liquid flow driven by the variation in the buoyancy force with horizontal position. Weaker peaks of bubble volume fraction extend into the bulk of the suspension and these peaks are separated by a distance equal to the peak in the pair distribution function for bubble pairs in an unbounded fluid. This suggests that the layering is a result of hydrodynamic bubblebubble interactions.

Detailed characteristics of dropladen mixing layers: Large eddy simulation predictions compared to direct numerical simulation
View Description Hide DescriptionResults are compared from direct numerical simulation (DNS) and large eddy simulation(LES) of a temporal mixing layer laden with evaporating drops to assess the ability of LES to reproduce detailed characteristics of DNS. The LES used computational drops, each of which represented eight physical drops, and a reduced flow field resolution using a grid spacing four times larger than that of the DNS. The LES also used models for the filtered source terms, which express the coupling of the drops with the flow, and for the unresolved subgridscale (SGS) fluxes of species mass, momentum, and enthalpy. The LESs were conducted using one of three different SGSflux models: dynamiccoefficient gradient (GRD), dynamiccoefficient Smagorinsky (SMD), and constantcoefficient scale similarity (SSC). The comparison of the LES with the filteredandcoarsened (FC) DNS considered detailed aspects of the flow that are of interest in ignition or full combustion. All LESs captured the largestscale vortex, the global amount of vapor emanating from the drops, and the overall size distribution of the drops. All LESs tended to underpredict the global amount of irreversible entropy production (dissipation). The SMD model was found unable to capture either the global or local vorticity variation and had minimal smallscale activity in dynamic and thermodynamic variables compared to the FCDNS. The SMD model was also deficient in predicting the spatial distribution of drops and of the dissipation. In contrast, the GRD and SSC models did mimic the smallscale activity of the FCDNS and the spatial distribution of drops and of the dissipation. Therefore, the GRD and SSC models are recommended, while the SMD model seems inappropriate for combustion or other problems where the local activity must be predicted.

Experimental study of oscillatory motion of particles and bubbles with applications to Coriolis flow meters
View Description Hide DescriptionThe present experimental study is designed to measure the motion of a spherical particle in a noninertial reference frame when the environment oscillates horizontally at a prescribed frequency and amplitude. Measurements are compared with theoretical equations of motion, for example, Basset [A Treatise on Hydrodynamics (Deighton Hall, London, 1888), Vol. 2], over wide ranges of density ratio , inverse Stokes number , and amplitude ratio , the three most critical nondimensional parameters. The experimental configuration consists of a bubble or solid sphere rising or falling in a bubble column while vibration occurs in the horizontal direction. Motion is measured with a high speed video camera and contemporary image and signal processing techniques are used to evaluate the data. The setup closely resembles multiphase flow in a Coriolisflow meter, a device which measures mass flow rate and density by oscillating two tubes at resonance. Accurate predictions of the motion of the sphere may lead to estimates of measurement errors due to entrained gas or solid particles. Excellent agreement for amplitude and phase shift is found between theory and experiment over the full range of testing, which is defined by small oscillatory Reynolds numbers, finite Strouhal numbers , widely varying density ratios , inverse Stokes numbers , and amplitude ratios .

Inertial migration of spherical particles in circular Poiseuille flow at moderately high Reynolds numbers
View Description Hide DescriptionThe inertial migration of spherical particles in a circular Poiseuille flow is numerically investigated for the tube Reynolds number up to 2200. The periodic boundary condition is imposed in the streamwise direction. The equilibrium positions, the migration velocity, and the angular velocity of a single particle in a tube cell are examined at different Reynolds numbers, particletube size ratios, and tube lengths. Inner equilibrium positions are observed as the Reynolds number exceeds a critical value, in qualitatively agreement with the previous experimental observations [J.P. Matas, J. F. Morris, and E. Guazzelli, J. Fluid Mech.515, 171 (2004)]. Our results indicate that the hydrodynamic interactions between the particles in different periodic cells have significant effects on the migration of the particles at the tube length being even as large as 6.7 particle diameters and they tend to stabilize the particles at the outer Segré–Silberberg equilibrium positions and to suppress the emergence of the inner equilibrium positions. A mirrorsymmetric travelingwavelike structure is observed when the particle Reynolds number is large enough. A pair of counterrotating streamwise vortices exists at both upstream and downstream of the particle but with different rotating directions. The fluids in the half of the pipe without the particle flow more slowly and most fluids in the other half with the particle move faster with respect to the parabolic profile. The intensity of the structure is influenced by the local particle Reynolds number, the particle motion, and the tube length. In addition, the migration of multiple particles in a periodic tube cell is examined. We attribute the disparity in the critical particle Reynolds number for the occurrence of the inner particle annulus for the experiments and our simulations to the effect of the tube length or the periodic boundary condition in our numerical model.
 Laminar Flows

Hydrodynamic interactions in dissipative particle dynamics
View Description Hide DescriptionDissipative particle dynamics (DPD) has recently attracted great interest due to its potential to simulate the dynamics of colloidal particles in fluidic devices. In this work, we explore the validity of DPD to reproduce the hydrodynamic interaction between a suspended particle and confining solid walls. We first show that a relatively large Schmidt number of the DPD fluid can be obtained by increasing the ratio between the strength of the dissipative force and the kinetic energy of the particles. We then measure the mobility and diffusion coefficient of the colloidal particles and show good agreement with the predicted results. We then focus on the particlesolid interactions and measure the force on a colloidal particle moving both parallel and perpendicular to two parallel walls. In both cases we found good agreement with the theoretical predictions based on Stokes flows for separations as small as onetenth of the particle radius.

Patterns in surface driven flows
View Description Hide DescriptionFlow in a rectangular basin driven by a surface force is considered. The problem is motivated by flow in geophysical bodies of water driven by wind at the water surface. Results are obtained via numerical computations of the Navier–Stokes equations assuming constant density. The numerical integration is achieved with a splitting method, with Crank–Nicolson for the linear terms, and Adams–Bashforth for the nonlinear terms. Spatial derivatives are treated with finite differences. The forcing has a sinusoidal variation across the top with a sequence of length scales. The results show a symmetric steady stable flow for small Reynolds numbers. As the Reynolds number is increased, the system experiences either a subcritical or supercritical pitchfork bifurcation to an asymmetric steady stable flow, or a local Hopf bifurcation, depending on the aspect ratio of the container and the length scale of the forcing. The asymmetric flow is cellular for forcing length scales commensurate with the depth. For smaller forcing length scales, the asymmetric flow has a basinfilling character at the bottom portion of the basin.

Flow past an oscillating circular cylinder in a channel with an upstream splitter plate
View Description Hide DescriptionA transversely oscillating circular cylinder confined in a channel has the potential to promote mixing and heat transfer at moderate Reynolds numberflows. In the present study, simulation results for flow past a circular cylinder subjected to forced crossflow oscillations in a straight channel with an upstream splitter plate are presented in a wide range of cylinder oscillation frequencies, including the subharmonic, superharmonic, and primary lockin regimes. Simulations are performed at , with cylinder oscillation amplitude of 0.4 diameters and a blockage ratio of 1/3. A spectral element algorithm based on the arbitrary Lagrangian Eulerian formulation is utilized. The numerical method exhibits spectral accuracy and allows large mesh deformation in the computational domain without mesh refinements. The main objective of this study is systematic investigations of the cylinder oscillation on the vortex shedding mechanism, downstream vortex patterns, and forces exerted on the cylinder.
 Instability and Transition

On linear stability of Rayleigh–Bénard Poiseuille flow of viscoplastic fluids
View Description Hide DescriptionThe present paper deals with the onset of the twodimensional Rayleigh–Bénard convection for a plane channel flow of viscoplasticfluid. The influence of the yield stress on the instability and stability conditions characterized by the Rayleigh numbers denoted, respectively, and is investigated in the framework of linear analysis using modal and energetic approaches. The results show that the yield stress, represented by the Bingham number , delays the onset of convection. For low values of the Reynolds number Re, the critical conditions and tend to be equal and the difference increases with increasing Re, highlighting the nonnormality of the linear operator. For and large , it is shown that the critical Rayleigh number increases as and the critical wave number evolves according to .

Boundarylayer theory for Taconis oscillations in a heliumfilled tube
View Description Hide DescriptionThis paper develops a onedimensional formulation to simulate Taconis oscillations in a heliumfilled, quarterwavelength tube in cryogenics within a framework of the boundarylayer theory. Dividing an acoustic field in the tube into a boundary layer on the wall and a mainflow region outside of it, the fluiddynamical equations are averaged over the whole cross section of the tube, from which the equations averaged over the mainflow region are derived by using the boundarylayer solutions. Nonlinear theory is employed for the mainflow region, whereas the boundary layer is assumed to be described by the linear theory. Resultant equations for the mainflow region are posed in the form of integrodifferential equations due to memory effects by the boundary layer. An initial and boundaryvalue problem is solved numerically for the evolution of a small disturbance. It is demonstrated that for temperature distribution of a smooth, step function, a transient behavior leading to emergence of selfexcited Taconis oscillations can be simulated numerically. Although the firstorder theory in the boundarylayer thickness has been regarded as being incapable of describing Taconis oscillations, it turns out to be applicable to a case with plausible temperature gradient.

Rayleigh–Taylor instability in cylindrical geometry with compressible fluids
View Description Hide DescriptionA linear stability analysis of the Rayleigh–Taylor instability(RTI) between two ideal inviscid immiscible compressible fluids in cylindrical geometry is performed. Threedimensional (3D) cylindrical as well as twodimensional (2D) axisymmetric and circular unperturbed interfaces are considered and compared to the Cartesian cases with planar interface. Focuses are on the effects of compressibility, geometry, and differences between the convergent (gravity acting inward) and divergent (gravity acting outward) cases on the early instability growth. Compressibility can be characterized by two independent parameters—a static Mach number based on the isothermal sound speed and the ratio of specific heats. For a steady initial unperturbed state, these have opposite influence, stabilization and destabilization, on the instability growth, similar to the Cartesian case [D. Livescu, Phys. Fluids16, 118 (2004)]. The instability is found to grow faster in the 3D cylindrical than in the Cartesian case in the convergent configuration but slower in the divergent configuration. In general, the direction of gravity has a profound influence in the cylindrical cases but marginal for planar interface. For the 3D cylindrical case, instability grows faster in the convergent than in the divergent arrangement. Similar results are obtained for the 2D axisymmetric case. However, as the flow transitions from the 3D cylindrical to the 2D circular case, the results above can be qualitatively different depending on the Atwood number, interface radius, and compressibility parameters. Thus, 2D circular calculations of RTI growth do not seem to be a good model for the fully 3D cylindrical case.

Global endwall effects on centrifugally stable flows
View Description Hide DescriptionWe investigate the stability of a fluid confined between two cylinders that rotate at same constant angular speed. In the case of infinite cylinders, or endwalls rotating with the cylinders, the flow is in solidbody rotation and hence linearly stable for any rotation speed. However, when the endwalls are stationary, a largescale circulation is driven by radially inward boundary layer flow on the endwalls. For sufficiently high angular speeds, this circulation becomes unstable to azimuthal waves. As the lengthtogap aspect ratio of the system is increased, a wealth of instabilities is revealed. It is particularly interesting that for all these instabilities the associated energy is localized in the equatorial region, as far from the endwalls as possible. This shows that care must be taken when assuming localized endwall effects in simplified models.

Instability of plane Poiseuille flow in a fluidporous system
View Description Hide DescriptionThe instability of Poiseuille flow in a fluidporous system is investigated. The system consists of a fluid layer overlying porous media and is subjected to a horizontal plane Poiseuille flow. We use Brinkman’s model instead of Darcy’s law to describe the porous layer. The eigenvalue problem is solved by means of a Chebyshev collocation method. We study the influence of the depth ratio and the Darcy number on the instability of the system. We compare systematically the instability of Brinkman’s model with the results of Darcy’s model. Our results show that no satisfactory agreement between Brinkman’s model and Darcy’s model is obtained for the instability of a fluidporous system. We also examine the instability of Darcy’s model. A particular comparison with early work is made. We find that a multivalued region may present in the plane, which was neglected in previous work. Here is the dimensionless wavenumber and Re is the Reynolds number.