Phys. Fluids (1994-Present) - Physics of Fluids

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Experimental visualization of Lagrangian coherent structures in aperiodic flows

A. Chrisohoides and F. Sotiropoulos

Phys. Fluids 15, L25 (2003); http://dx.doi.org/10.1063/1.1540111 (4 pages) | Cited 6 times

Online Publication Date: 22 January 2003

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A technique is introduced for extracting the coherence time scale of Lagrangian coherent structures (LCS) in aperiodic flows from experimental, light intensity time series. The technique employs digital photography to record the transport of passive tracers in a chaotic instantaneous flow. Coherent eddies are detected by time-averaging the instantaneous light intensity field on finite-size temporal windows. The optimal size of the time-average window (the coherence time scale of the LCS) is extracted from the light intensity field using statistical scaling arguments based on the central limit theorem. The technique is applied to visualize LCS in the vicinity of a surface-piercing rectangular block mounted at one corner of a rectangular open channel. © 2003 American Institute of Physics.

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47.52.+j	Chaos in fluid dynamics
47.80.-v	Instrumentation and measurement methods in fluid dynamics
47.60.-i	Flow phenomena in quasi-one-dimensional systems

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Inapplicability of the dynamic Clark model to the large eddy simulation of incompressible turbulent channel flows

Hiromichi Kobayashi and Yutaka Shimomura

Phys. Fluids 15, L29 (2003); http://dx.doi.org/10.1063/1.1553756 (4 pages) | Cited 5 times

Online Publication Date: 5 February 2003

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The inapplicability of the dynamic Clark model to the large eddy simulation of incompressible turbulent channel flows is proved both analytically and numerically. The reason is neither a negative subgrid-scale eddy viscosity nor an incorrect near-wall scaling, but a negative effective viscosity in the viscous sublayer for the tensor diffusivity term that models the sum of the Leonard and the cross terms. © 2003 American Institute of Physics.

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47.27.-i	Turbulent flows
47.60.-i	Flow phenomena in quasi-one-dimensional systems
47.11.-j	Computational methods in fluid dynamics

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Statistical theory of compressible turbulence based on mass-weighted averaging, with an emphasis on a cause of countergradient diffusion

Akira Yoshizawa

Phys. Fluids 15, 585 (2003); http://dx.doi.org/10.1063/1.1536977 (12 pages) | Cited 6 times

Online Publication Date: 17 January 2003

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Statistical theory of inhomogeneous compressible turbulence is formulated on the basis of mass-weighted averaging. The velocity and the concentration of matters are resolved into the mass-weighted means and the fluctuations around them, whereas the density and the pressure are resolved into the ensemble means and the fluctuations around them. The former fluctuations multiplied by the density are normalized by the mean density, leading to new variables characterizing velocity and concentration fluctuations. In the use of those variables, turbulent fluxes may be examined in the presence of spatially varying mean velocity and scalar, as in incompressible flow. With the aid of the theoretical results about the fluxes, a cause of countergradient diffusion is discussed in the context of a premixed turbulent flame. © 2003 American Institute of Physics.

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47.27.-i	Turbulent flows
47.40.-x	Compressible flows; shock waves

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Chemical fronts in Hele-Shaw cells: Linear stability analysis based on the three-dimensional Stokes equations

Rainer Demuth and Eckart Meiburg

Phys. Fluids 15, 597 (2003); http://dx.doi.org/10.1063/1.1536972 (6 pages) | Cited 11 times

Online Publication Date: 17 January 2003

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We present linear stability results based on the three-dimensional Stokes equations for chemically reacting, propagating fronts giving rise to an unstable density stratification in a Hele-Shaw cell. The results are compared with the experiments in M. Böckmann and S. C. Müller [Phys. Rev. Lett. 85, 2506 (2000)], as well as with a corresponding linear stability analysis based on the Darcy equations that was performed in A. De Wit [Phys. Rev. Lett. 87, 054502 (2001)]. The reason for the good agreement between these earlier Darcy data and the experimentally observed growth rates is found in the relatively low experimental value of the Rayleigh number, Ra = 79, for which the flow is approximately of Poiseuille type. Already for Ra values as low as 300, we observe a discrepancy between the stability results based on the Darcy and Stokes equations, respectively, with the Darcy results overpredicting both the most amplified wavenumber, as well as the corresponding growth rate, by about a factor of two. This indicates that three-dimensional effects quickly gain importance as Ra increases, so that the stability analysis needs to be based on the full, three-dimensional Stokes equations. The stability results based on the Stokes equations furthermore demonstrate the stabilizing influences of both an increasing interfacial thickness, as well as increasing frontal propagation velocities, confirming the earlier Darcy-based findings by De Wit. An argument in terms of vorticity is forwarded to explain the latter effect. A more rapidly advancing front deposits vorticity over a wider layer of fluid particles, so that the concentrated regions of vorticity needed for rapid instability growth cannot form. Somewhat surprisingly, however, slowly propagating fronts are seen to be more unstable than nonreacting fronts of equivalent thickness, as the chemical reaction leads to the formation of more compact perturbations in the interfacial region. © 2003 American Institute of Physics.

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47.70.Fw	Chemically reactive flows
47.20.-k	Flow instabilities
47.55.Hd	Stratified flows
47.56.+r	Flows through porous media

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Peter B. V. Johansson, William K. George, and Michael J. Gourlay

Phys. Fluids 15, 603 (2003); http://dx.doi.org/10.1063/1.1536976 (15 pages) | Cited 12 times

Online Publication Date: 22 January 2003

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Equilibrium similarity considerations are applied to the axisymmetric turbulent wake, without the arbitrary assumptions of earlier theoretical studies. Two solutions for the turbulent flow are found: one for infinite local Reynolds number which grows spatially as x^1/3; and another for small local Reynolds number which grows as x^1/2. Both solutions can be dependent on the upstream conditions. Also, the local Reynolds number diminishes with increasing downstream distance, so that even when the initial Reynolds number is large, the flow evolves downstream from one state to the other. Most of the available experimental data are at too low an initial Reynolds number and/or are measured too near the wake generator to provide evidence for the x^1/3 solution. New results, however, from a laboratory experiment on a disk wake and direct numerical simulations (DNS) are in excellent agreement with this solution, once the flow has had large enough downstream distance to evolve. Beyond this the ratio of turbulence intensity to centerline velocity deficit is constant, until the flow unlocks itself from this behavior when the local Reynolds number goes below about 500 and the viscous terms become important. When this happens the turbulence intensity ratio falls slowly until the x^1/2 region is reached. No experimental data are available far enough downstream to provide unambiguous evidence for the x^1/2 solution. The prediction that the flow should evolve into such a state, however, is confirmed by recent DNS results which reach the x^1/2 solution at about 200 000 momentum thicknesses downstream. After this the turbulence intensity ratio is again constant, until box-size affects the calculation and the energy decays exponentially. © 2003 American Institute of Physics.

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47.27.wb	Turbulent wakes
47.27.tb	Turbulent diffusion
47.50.-d	Non-Newtonian fluid flows
47.11.-j	Computational methods in fluid dynamics

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Vortex streets generated by a moving momentum source in a stratified fluid

S. I. Voropayev and S. A. Smirnov

Phys. Fluids 15, 618 (2003); http://dx.doi.org/10.1063/1.1539475 (7 pages) | Cited 3 times

Online Publication Date: 22 January 2003

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The aim of this paper is to present the results of experiments conducted in a stratified fluid with a momentum source modeled by a moving jet in the asymptotic case when the size of the source is negligibly small but the resulting force (momentum flux), which acts on the fluid, remains finite. In this case the forcing may be considered as a “point” force moving in the fluid and the problem has no obvious characteristic length scale, compared to the case of a bluff body. The latter, towed horizontally in the stratified fluid, frequently leaves behind a highly organized vortex street, the characteristics of which strongly depend on the body diameter. It is shown that a similar type of the vortex streets may be generated by a “point” momentum source and conditions when this occurs are found. Experimental data on the characteristics of the vortex streets are presented and compared with the towed body case. © 2003 American Institute of Physics.

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47.32.C-	Vortex dynamics
47.55.Hd	Stratified flows
47.27.wg	Turbulent jets

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The statistics of power injected in a closed turbulent flow: Constant torque forcing versus constant velocity forcing

Jean Hugues Titon and Olivier Cadot

Phys. Fluids 15, 625 (2003); http://dx.doi.org/10.1063/1.1539856 (16 pages) | Cited 14 times

Online Publication Date: 22 January 2003

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A confined turbulent flow forced by two counter-rotating stirrers is investigated. Two processes of energy injection are studied. In the first one, called the Ω mode, the forcing devices are driven at constant angular velocity. In the second, called the Γ mode, the forcing devices are driven with a constant torque. For each forcing mode, the power injected and the wall pressure fluctuations are simultaneously measured in the range of Reynolds numbers from 20 000 to 500 000. For each mode, the probability density functions of the injected power are approximately Gaussian with slight reversed skewness between both modes. These asymmetries are due to an excess of low dissipation events compared to high dissipation events. For each mode, the skewness varies as the logarithm of the Reynolds number. On the other hand, the fluctuation rate of the injected power does not depend on the Reynolds number whatever the forcing modes, however, their magnitudes are drastically different. A value of only 5.9% is obtained for the Γ mode against 10.9% for the Ω mode. The local properties of the turbulence at smaller scales, deduced from either hot film anemometry or wall pressure measurements, are identical for both forcing modes. The mean dissipated power remains unchanged as well. It is then deduced that the injected power fluctuations are caused by the turbulence feedback rather than the turbulence production. The difference in the fluctuation rates is a consequence of the natural control of the turbulence feedback that occurs for the Γ mode. © 2003 American Institute of Physics.

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47.80.-v	Instrumentation and measurement methods in fluid dynamics
47.27.-i	Turbulent flows

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The effect of electric fields on the rupture of thin viscous films by van der Waals forces

K. Savettaseranee, D. T. Papageorgiou, P. G. Petropoulos, and B. S. Tilley

Phys. Fluids 15, 641 (2003); http://dx.doi.org/10.1063/1.1538250 (12 pages) | Cited 16 times

Online Publication Date: 23 January 2003

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We examine the stability of a thin two-dimensional incompressible liquid film when an electric field is applied in a direction parallel to the initially flat bounding fluid interfaces, and study the competition between surface tension, van der Waals, viscous, and electrically induced forces. The film is assumed to be sufficiently thin, and the surface tension and electrically induced forces are large enough that gravity can be ignored to the leading order. We analyze the nonlinear stability of the flow by deriving and numerically solving a set of nonlinear evolution equations for the local film thickness and the horizontal velocity. We find that the electric field forces enhance the stability of the flow and can remove rupture. If rupture occurs then the form of the singularity, to leading order, is that found in the absence of an electric field. © 2003 American Institute of Physics.

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68.15.+e	Liquid thin films
34.20.Gj	Intermolecular and atom-molecule potentials and forces
66.20.-d	Viscosity of liquids; diffusive momentum transport
68.03.Cd	Surface tension and related phenomena
47.20.-k	Flow instabilities

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Experimental friction factor of a liquid flow in microtubes

D. Brutin and L. Tadrist

Phys. Fluids 15, 653 (2003); http://dx.doi.org/10.1063/1.1538612 (9 pages) | Cited 28 times

Online Publication Date: 23 January 2003

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The friction factor of a water laminar flow through fused silica microtubes with diameters ranging from 50 to 530 μm is investigated experimentally. The existing works in the literature are analyzed. They show a range of experimental results which agree or not with the conventional theory. Pressure drops and flowing masses are measured to determine the flow characteristics using a transient method. Several experiments are developed for two fluids (distilled and tap water) and tube compositions. The experimental results are discussed. The disparity from the classical theory is found to be due to the fluid’s ionic composition. The experimental results for tap water and classical fused silica surfaces indicate a disparity from the conventional theory. A friction factor increase with small uncertainties (<5%) is observed for decreasing diameters. © 2003 American Institute of Physics.

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47.60.-i	Flow phenomena in quasi-one-dimensional systems
47.15.-x	Laminar flows
46.55.+d	Tribology and mechanical contacts
47.85.Np	Fluidics

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The velocity distribution function in direct simulation Monte Carlo method with an application to extended thermodynamics

Luca Marino

Phys. Fluids 15, 662 (2003); http://dx.doi.org/10.1063/1.1536971 (6 pages) | Cited 2 times

Online Publication Date: 23 January 2003

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A numerical experiment is carried out to prove the difference between the value of the kinetic temperature and that of the thermodynamic temperature in a gas in the presence of molecular transport. As a reference situation the velocity distribution function is evaluated between two concentric cylinders at different wall temperatures. A direct simulation Monte Carlo method (DSMC) is adopted for various Knudsen numbers Kn and comparisons are made with existing data. The results prove quantitatively that the difference between the two differently defined temperatures increases with Kn and with the temperature gradient, as predicted by the theory for systems which are almost in thermodynamical equilibrium. The present article does not aim to validate DSMC, but rather to illustrate how the method can be used to address fundamental issues in gas dynamics. © 2003 American Institute of Physics.

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47.11.-j	Computational methods in fluid dynamics
47.45.-n	Rarefied gas dynamics

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Three-dimensional linear stability analysis of the flow in a liquid spherical droplet driven by an alternating magnetic field

V. Shatrov, J. Priede, and G. Gerbeth

Phys. Fluids 15, 668 (2003); http://dx.doi.org/10.1063/1.1535410 (11 pages) | Cited 8 times

Online Publication Date: 30 January 2003

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The paper presents a numerical stability analysis of the flow driven by an alternating (ac) magnetic field in an electromagnetically levitated liquid metal droplet. The basic axisymmetric flow is found to become unstable at Reynolds numbers in the order of 100. The critical Reynolds number Re_c and the corresponding most unstable azimuthal wave number m are found for several configurations of the magnetic field depending on the skin-depth δ. For a uniform external ac magnetic field the azimuthal wave number of the most unstable mode is m = 3. An additional steady (dc) magnetic field imposed along the axis of symmetry increases the stability of the flow. © 2003 American Institute of Physics.

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47.55.D-	Drops and bubbles
47.65.-d	Magnetohydrodynamics and electrohydrodynamics
52.55.Tn	Ideal and resistive MHD modes; kinetic modes
47.20.-k	Flow instabilities
52.65.Kj	Magnetohydrodynamic and fluid equation

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Thin front propagation in steady and unsteady cellular flows

M. Cencini, A. Torcini, D. Vergni, and A. Vulpiani

Phys. Fluids 15, 679 (2003); http://dx.doi.org/10.1063/1.1541668 (10 pages) | Cited 14 times

Online Publication Date: 30 January 2003

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Front propagation in two-dimensional steady and unsteady cellular flows is investigated in the limit of very fast reaction and sharp front, i.e., in the geometrical optics limit. For the steady flow, a simplified model allows for an analytical prediction of the front speed v_f dependence on the stirring intensity U, which is in good agreement with numerical estimates. In particular, at large U, the behavior v_f ∼ U/log(U) is predicted. By adding small scales to the velocity field we found that their main effect is to renormalize the flow intensity. In the unsteady (time-periodic) flow, we found that the front speed locks to the flow frequency and that, despite the chaotic nature of the Lagrangian dynamics, the front evolution is chaotic only for a transient. Asymptotically the front evolves periodically and chaos manifests only in its spatially wrinkled structure. © 2003 American Institute of Physics.

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47.70.Fw	Chemically reactive flows
47.20.-k	Flow instabilities
47.52.+j	Chaos in fluid dynamics

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Vapor flows condensing at incidence onto a plane condensed phase in the presence of a noncondensable gas. I. Subsonic condensation

Satoshi Taguchi, Kazuo Aoki, and Shigeru Takata

Phys. Fluids 15, 689 (2003); http://dx.doi.org/10.1063/1.1539476 (17 pages) | Cited 12 times

Online Publication Date: 30 January 2003

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A steady flow of a vapor in a half space condensing onto a plane condensed phase of the vapor at incidence is considered in the case where another gas that neither evaporates nor condenses (the noncondensable gas) is present near the condensed phase. The behavior of the vapor and noncondensable gas is investigated on the basis of kinetic theory under the assumption that the molecules of the noncondensable gas are mechanically identical with those of the vapor. In particular, the relation, among the parameters of the vapor at infinity (the pressure, temperature, and flow velocity of the vapor), those related to the condensed phase (the temperature of the condensed phase and the corresponding saturation pressure of the vapor), and the amount of the noncondensable gas, that admits a steady solution is obtained numerically by the use of a model Boltzmann equation proposed by Garzó et al. [Phys. Fluids A 1, 380 (1989)]. The present analysis is the continuation of an earlier work by Sone et al. [Transp. Theory Stat. Phys. 21, 297 (1992)], where the case in which the vapor flow is condensing perpendicularly onto the condensed phase is investigated exclusively. The case with subsonic condensation is discussed in the present paper (the case with supersonic condensation is left to the subsequent paper). © 2003 American Institute of Physics.

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47.55.Kf	Particle-laden flows
47.40.Dc	General subsonic flows

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Wall-pressure-array measurements beneath a separating/reattaching flow region

Laura M. Hudy, Ahmed M. Naguib, and William M. Humphreys

Phys. Fluids 15, 706 (2003); http://dx.doi.org/10.1063/1.1540633 (12 pages) | Cited 20 times

Online Publication Date: 30 January 2003

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A database of wall-pressure-array measurements was compiled for studying the space–time character of the surface-pressure field within a separating/reattaching flow region. The experimental setup consisted of a long splitter plate located within the wake of a fence and instrumented with an array of flush-mounted microphones. Data were acquired for a Reynolds number of 7900, based on the fence height above the splitter plate. Two distinctive regions, defined based on their location relative to the position of the mean reattachment point (x_r) of the shear layer, emerged from this investigation. Upstream, from the fence to 0.25x_r, the surface-pressure signature was dominated by large time scale disturbances and an upstream convection velocity of 0.21U_∞. Beyond 0.25x_r, turbulent structures with smaller time scales and a downstream convection velocity of 0.57U_∞ generated most of the pressure fluctuations. Interestingly, the low-frequency wall-pressure signature typically associated with the flapping of the separated shear layer was found to be composed of standing and downstream/upstream propagating wave components. The latter seemed to originate from a point near the middle of the reattachment zone, suggesting the existence of an absolute instability of the recirculation bubble, which may be the cause of the flapping of the shear layer. © 2003 American Institute of Physics.

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47.55.D-	Drops and bubbles
47.27.wb	Turbulent wakes

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Short-wave, localized disturbances in jets, with applications to flows on a beta plane with topography

E. S. Benilov

Phys. Fluids 15, 718 (2003); http://dx.doi.org/10.1063/1.1540100 (12 pages) | Cited 3 times

Online Publication Date: 30 January 2003

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We examine the stability of jets over topography on the so-called barotropic beta plane (which models oceanic currents in mid-latitudes). Attention is focused on disturbances with a large wave number, for which an asymptotic solution of the normal-mode eigenvalue problem is presented. It is demonstrated that short-wave modes, if they exist, are localized in narrow strips “centered” at local extrema of the velocity profile U(y) (y is the transverse variable). It is further shown that an extremum, say y = y_l, can support a short-wave mode only if the vorticity gradient and potential vorticity (PV) gradient at y_l are of opposite signs. If they are, the leading-order solution of the eigenvalue problem describes a mode with a phase speed slightly larger than U(y_l), if y_l is a maximum; or slightly smaller than U(y_l), if y_l is a minimum. In other words, the mode does not have critical levels in the vicinity of y_l, although it may have them elsewhere, at a distant point. If that indeed happens, an additional condition is required to guarantee the existence of the mode: namely, the PV gradient at y_l and that at the critical level must have opposite signs. If they do, the mode exists and is weakly unstable (the phase speed has a small imaginary part). Thus, a change of sign of the PV gradient does not necessarily destabilize the flow; and in order to guarantee instability, the PV gradient should have opposite signs at the “important” points, i.e., the localization point and critical level. The asymptotic results are tested against the numerical solution of the exact normal-mode eigenvalue problem. The former and the latter are in good agreement, and not only for large wave numbers, but also for moderate ones. It is also demonstrated that our approach can be applied to other cases of jets in fluids and plasmas. © 2003 American Institute of Physics.

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47.27.wg	Turbulent jets
47.20.-k	Flow instabilities

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The undular hydraulic jump in turbulent open channel flow at large Reynolds numbers

Wolfgang Grillhofer and Wilhelm Schneider

Phys. Fluids 15, 730 (2003); http://dx.doi.org/10.1063/1.1538249 (6 pages) | Cited 4 times

Online Publication Date: 3 February 2003

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Plane flow over a bottom of constant slope is considered in the double limit of very large Reynolds numbers, i.e., Re_τ→∞, and Froude numbers approaching the critical value, i.e., Fr = 1+ math

ε with ε→0. Fully developed turbulent flow far upstream is assumed. Owing to the large Reynolds number the inclination angle of the bottom, α, is small. The undular jump is analyzed for a parameter regime that is characterized by values of α that are of the same order of magnitude as ε², i.e., α/ε² = O(1). The first-order perturbation equations contain unknown functions that are determined from a solvability condition of the second-order equations. Without making use of a turbulence model or empirical parameters, the following equation is obtained for the shape of the free surface: (d³H₁/dX³)+(H₁−1)(dH₁/dX)−βH₁ = 0, with H₁→0 as X→−∞. H₁(X) denotes the first-order perturbation of the surface elevation as a function of the nondimensional longitudinal coordinate X, and the parameter β = ⅓αε^−3/2 characterizes the slow changes of amplitudes and wavelengths, respectively. Numerical solutions of this ordinary differential equation are compared with experimental data. © 2003 American Institute of Physics.

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47.27.nb	Boundary layer turbulence
47.35.-i	Hydrodynamic waves
47.60.-i	Flow phenomena in quasi-one-dimensional systems
47.11.-j	Computational methods in fluid dynamics
02.30.Hq	Ordinary differential equations

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Force on a spinning sphere moving in a rarefied gas

Karl I. Borg, Lars H. Söderholm, and Hanno Essén

Phys. Fluids 15, 736 (2003); http://dx.doi.org/10.1063/1.1541026 (6 pages) | Cited 11 times

Online Publication Date: 3 February 2003

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The force acting on a spinning sphere moving in a rarefied gas is calculated. It is found to have three contributions with different directions. The transversal contribution is of opposite direction compared to the so-called Magnus force normally exerted on a sphere by a dense gas. It is given by F = −α_τξ⅔πR³mnω×v, where α_τ is the accommodation coefficient of tangential momentum, R is the radius of the sphere, m is the mass of a gas molecule, n is the number density of the surrounding gas, ω is the angular velocity, and v is the velocity of the center of the sphere relative to the gas. The dimensionless factor ξ is close to unity, but depends on ω and κ, the heat conductivity of the body. © 2003 American Institute of Physics.

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47.45.-n	Rarefied gas dynamics
47.55.Kf	Particle-laden flows
47.27.T-	Turbulent transport processes

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Free-surface evolution due to an impulsive bottom sink at uniform depth

Kjetil B. Haugen and Peder A. Tyvand

Phys. Fluids 15, 742 (2003); http://dx.doi.org/10.1063/1.1542888 (10 pages) | Cited 4 times

Online Publication Date: 3 February 2003

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The free-surface evolution due to an impulsively started point sink at the bottom of a uniform horizontal layer of inviscid and incompressible fluid is investigated analytically. A third-order small-time expansion of the full nonlinear problem is performed. The dip formation above the sink depends on the Froude number of the sink. An initially critical Froude number 0.1376 is found for the sign change of the third-order elevation above the point sink. A physically critical Froude number 0.109 is identified for the threshold for the dip to be swallowed into the point sink. The same problem is solved in two dimensions, where a uniform line sink is turned on impulsively at the bottom. © 2003 American Institute of Physics.

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47.60.-i

Flow phenomena in quasi-one-dimensional systems

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Vorticity production and turbulent cooling of “hot channels” in gases: Three dimensions versus two dimensions

Yair Kurzweil, Eli Livne, and Baruch Meerson

Phys. Fluids 15, 752 (2003); http://dx.doi.org/10.1063/1.1539477 (11 pages) | Cited 3 times

Online Publication Date: 3 February 2003

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Hot channels (HCs), created in a gas by a rapid energy release in the quasi-cylindric geometry, cool anomalously fast by turbulent flow. Picone and Boris [Phys. Fluids 26, 365 (1983)] suggested that turbulent mixing results from the vorticity generation by the baroclinic mechanism during the early, shock-wave dominated stage of the dynamics. This scenario was confirmed, with important modifications, in a recent series of two-dimensional (2D) hydrodynamic simulations. This work reports three-dimensional (3D) hydrodynamic simulations of the HC evolution, and compares the results with those of 2D simulations. Assuming a small perturbation of the cylindric shape of the energy release region, we followed a typical HC up to 200 acoustic times. The simulations capture well the phenomenology of the HC cooling. The details of vorticity production, that results in a fast mixing of the cold ambient gas into the HC, are clearly identified. The cooling process can be interpreted as turbulent diffusion. The empiric diffusion coefficient and cooling time agree with experiment. The late-time morphology of the HC and the empiric turbulent diffusion coefficient are dimension-dependent, the 3D cooling being faster than 2D cooling. © 2003 American Institute of Physics.

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47.27.tb	Turbulent diffusion
47.32.C-	Vortex dynamics
47.60.-i	Flow phenomena in quasi-one-dimensional systems
47.55.Kf	Particle-laden flows
47.11.-j	Computational methods in fluid dynamics
47.40.Nm	Shock wave interactions and shock effects

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Mechanisms of particle deposition in a fully developed turbulent open channel flow

Chidambaram Narayanan, Djamel Lakehal, Lorenzo Botto, and Alfredo Soldati

Phys. Fluids 15, 763 (2003); http://dx.doi.org/10.1063/1.1545473 (13 pages) | Cited 14 times

Online Publication Date: 3 February 2003

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Particle dispersion and deposition in the region near the wall of a turbulent open channel is studied using direct numerical simulation of the flow, combined with Lagrangian particle tracking under conditions of one-way coupling. Particles with response times of 5 and 15, normalized using the wall friction velocity and the fluid kinematic viscosity, are considered. The simulations were performed until the particle phase reached a statistically stationary state before calculating relevant statistics. For both response times, particles are seen to accumulate strongly very close to the wall in the form of streamwise oriented streaks. Deposited particles were divided into two distinct populations; those with large wall-normal deposition velocities and small near-wall residence times referred to as the free-flight population, and particles depositing with negligible wall-normal velocities and large near-wall residence times (more than 1000 wall time units), referred to as the diffusional deposition population. Diffusional deposition (deposition induced by the small residual turbulent fluctuations near the wall) is found to be the dominant mechanism of deposition for both particle response times. The free-flight mechanism is shown to gain in importance only for τp+ = 15 particles. For τp+ = 5 particles only 10% deposit because of free flight, whereas the fraction is around 40% for τp+ = 15 particles. This result runs counter to the widely held opinion that free flight is the dominant mechanism of deposition in wall-bounded flows and clearly quantifies the relative importance of the two mechanisms. A simple relationship between the particle wall-normal velocity on deposition and the residence time for free-flight particles is presented. Particle deposition locations over the period of the entire simulation reveal that, while diffusional deposition occurs mostly along streamwise oriented lines below the near-wall particle accumulation patterns, free-flight particles deposit more evenly over the wall. © 2003 American Institute of Physics.

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47.55.Kf	Particle-laden flows
47.27.-i	Turbulent flows
47.60.-i	Flow phenomena in quasi-one-dimensional systems
47.11.-j	Computational methods in fluid dynamics

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Three-dimensional numerical simulation of Marangoni flow instabilities in floating zones laterally heated by an equatorial ring

M. Lappa

Phys. Fluids 15, 776 (2003); http://dx.doi.org/10.1063/1.1543147 (14 pages) | Cited 2 times

Online Publication Date: 4 February 2003

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Instability of Marangoni convection in floating zones (full zone configuration) of a low Prandtl number fluid under microgravity conditions is investigated by parallel supercalculus and direct three-dimensional and time-dependent simulation of the problem. A parametric analysis (still absent in literature) of the influence of the aspect ratio of the liquid column on the features of the three-dimensional bifurcation of Marangoni flow is carried out. A novel distribution is introduced for the surface heat flux corresponding to the radiative flux generated by a ring heater positioned around the equatorial plane of the full zone at a distance h from the free interface. Axisymmetric computations are used to obtain the steady basic state, then the three-dimensional Navier–Stokes equations are solved to investigate the evolution of azimuthal disturbances. These disturbances always exhibit antisymmetric behavior with respect to the equatorial plane. The mirror symmetry with respect to this plane is broken. Strong interaction occurs in fact between the toroidal convection rolls located in the upper part and lower part of the liquid column. This leads for some values of the aspect ratio to a heretofore unseen “apparent” doubling or quadrupling of the azimuthal wave number of the azimuthal velocity distribution in the midplane. The present analysis points out that the instability of the half zone flow is not relevant for the full zone configuration. © 2003 American Institute of Physics.

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47.27.T-	Turbulent transport processes
47.20.Ky	Nonlinearity, bifurcation, and symmetry breaking

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Convection-induced enhancement of mass transfer through an interface separating two immiscible liquids in a two-layer horizontal annulus

A. Yu. Gelfgat, A. L. Yarin, and P. Z. Bar-Yoseph

Phys. Fluids 15, 790 (2003); http://dx.doi.org/10.1063/1.1545081 (11 pages) | Cited 5 times

Online Publication Date: 4 February 2003

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Two-fluid natural-convection flow in the horizontal cylindrical annulus and its effect on mass transfer through the liquid-liquid interface of two immiscible fluids are studied numerically. The liquids are stratified by gravity, with the denser one occupying the lower part of the annulus. The convective motion is driven by heating of the inner or outer cylindrical boundary. It is shown that the mass transfer of a passive scalar (say, a protein) through the interface can be significantly enhanced by the convective flow. Varying the radii ratio from 0.1 to 0.5, it is found that the mass transfer is more intensive in annuli with smaller radii ratio. No significant difference in the mass transfer rates was found between the heating of either inner or outer cylinder. A possibility of further mass transfer enhancement using more complicated temperature distribution on the boundaries is demonstrated. The problem is related to the search for novel bioseparator devices. © 2003 American Institute of Physics.

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05.60.-k	Transport processes
64.75.-g	Phase equilibria
47.27.T-	Turbulent transport processes
47.55.Hd	Stratified flows
87.14.E-	Proteins
68.08.-p	Liquid-solid interfaces

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Dispersion of particle pairs and decay of scalar fields in isotropic turbulence

David J. Thomson

Phys. Fluids 15, 801 (2003); http://dx.doi.org/10.1063/1.1540634 (13 pages) | Cited 7 times

Online Publication Date: 5 February 2003

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The decay of homogeneous scalar fields in isotropic turbulence is addressed by considering the dispersion of particle pairs. The evolution of the variance is studied by considering the dispersion backwards in time from the measurement time to the source time. The decay is shown to depend on the scalar field’s low wave number spectral exponent and a critical exponent of 8/3 is derived. This critical value separates cases where the dominant contribution to the variance at large time comes from the initial large scales from cases where the dominant contribution comes from the initial variance-containing scales. This confirms the results obtained for the particular case of the Kraichnan white noise model by Eyink and Xin and shows that the value of the critical exponent is not an artifact of the white noise model. The evolution of the scalar spectrum and correlation function has also been considered and results have been derived for the backtransfer of variance to low wave numbers, the permanence of large (scalar) eddies, the asymptotic convergence for different initial conditions and the self-similarity of the evolution. In particular it is shown that the form of the backtransfer of variance to low wave numbers previously found in models (i.e., the proportionality of the backtransfer to the fourth power of the wave number) is a direct consequence of the finite variance of particle separations and can be derived without modelling assumptions. Finally the particular case of the “mandoline” source geometry often used in wind tunnel studies is addressed.

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47.27.Gs

Isotropic turbulence; homogeneous turbulence

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Vortex shedding in subcritical conditions

E. Buffoni

Phys. Fluids 15, 814 (2003); http://dx.doi.org/10.1063/1.1543943 (3 pages) | Cited 2 times

Online Publication Date: 5 February 2003

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An expressly developed apparatus has been applied to the study of the phenomenon of vortex shedding at Reynolds numbers inferior to the critical value. During previous research it was discovered that vortex shedding could be triggered under subcritical conditions by imparting low-amplitude transversal vibrations at specific frequencies to a cylinder positioned in the flow. © 2003 American Institute of Physics.

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47.32.C-

Vortex dynamics

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Effect of a “slip” splitter plate on vortex shedding from a cylinder

S. Mittal

Phys. Fluids 15, 817 (2003); http://dx.doi.org/10.1063/1.1540632 (4 pages) | Cited 8 times

Online Publication Date: 5 February 2003

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The effect of placing a “slip” splitter plate in the wake of a circular cylinder along the line of symmetry is studied. Such a hypothetical plate allows slip of velocity along itself but prevents any flow normal to it. Unlike the conventional splitter plate the slip plate does not have a wake of its own. The objective of the present study is to increase our understanding of vortex shedding by tracking down that part of the wake that needs to be constrained to suppress it completely. Computations for various configurations of the plate for the Re=100 flow are carried out using a finite element formulation. It is found that the shortest length of the plate required to suppress vortex shedding is two cylinder diameters, approximately, and needs to be located in the latter part of the wake bubble of the basic unperturbed flow. In this region the vertical component of velocity in each of the two standing vortices is towards the centerline. The upstream edge of the optimal plate is located very close to the region in the wake bubble where the vertical velocity component changes direction. As compared to the downstream edge, the location of the upstream edge of the plate has a much more significant effect on the unsteadiness of the flow. This study establishes that the latter part of the wake bubble of the basic unperturbed solution plays an important role in vortex shedding. © 2003 American Institute of Physics.

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