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

BRIEF COMMUNICATIONS

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Analysis of the flow pattern modifications in a bubbly Couette-Taylor flow

A. Mehel, C. Gabillet, and H. Djeridi

Phys. Fluids 19, 118101 (2007); http://dx.doi.org/10.1063/1.2786584 (4 pages) | Cited 1 time

Online Publication Date: 7 November 2007

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The aim of this Brief Communication is to discuss the bubble effect on the Couette-Taylor flow patterns in the transition from laminar to turbulent flow, especially in the weakly turbulent regime. It is shown that bubble location and local void fractions both in the vortices cores and in the near wall regions directly influence the axial wavelength. Bubbles trapped in the vortices tend to increase the vorticity and reduce the axial diffusivity. Bubbles near the wall contribute to “shear induced” turbulence depending on the void fraction gradient near the wall and the bubble size.

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47.55.D-	Drops and bubbles
47.15.Fe	Stability of laminar flows
47.27.Cn	Transition to turbulence
47.32.-y	Vortex dynamics; rotating fluids

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Saffman-Taylor instability of shear thinning fluids

Ph. Tordjeman

Phys. Fluids 19, 118102 (2007); http://dx.doi.org/10.1063/1.2795213 (4 pages) | Cited 1 time

Online Publication Date: 7 November 2007

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A theoretical study of the Saffman-Taylor instability criterion in a Hele-Shaw cell when an inviscid fluid pushes a shear thinning fluid, is presented. The shear thinning fluid model consists of three independent fluids, two of which are Newtonian while the viscosity of the third is linearly dependent of the shear rate. The corresponding Darcy’s law is computed, and then the dispersion relation is obtained in a linear perturbative analysis. We show how the dispersion relation characteristic of the Saffman-Taylor instability is controlled by a “shear thinning characteristic number” that takes into account the rheological behavior of the shear thinning fluid in the Hele-Shaw cell. The model is discussed compared with the experimental pattern formation.

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47.20.Ft	Instability of shear flows (e.g., Kelvin-Helmholtz)
47.50.Gj	Instabilities
47.56.+r	Flows through porous media
47.50.Cd	Modeling
47.54.-r	Pattern selection; pattern formation
47.57.Qk	Rheological aspects

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Experimental study of small-amplitude lateral vibrations of an axisymmetric liquid bridge

C. Ferrera and J. M. Montanero

Phys. Fluids 19, 118103 (2007); http://dx.doi.org/10.1063/1.2804282 (4 pages) | Cited 4 times

Online Publication Date: 8 November 2007

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Optical imaging is used to measure the interface deformation due to small-amplitude lateral vibrations of an axisymmetric liquid bridge with a precision of the order of micrometers. Crucial aspects of the experimental procedure are identified. Comparison with the predictions of the Navier–Stokes equations for zero capillary number shows good agreement, even for oscillation amplitudes much smaller than the pixel size. A possible application of the experimental procedure is to measure the surface tension for zero Bond number.

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68.03.Cd

Surface tension and related phenomena

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K. A. M. Moinuddin, M. B. Jones, P. N. Joubert, and M. S. Chong

Phys. Fluids 19, 118104 (2007); http://dx.doi.org/10.1063/1.2803346 (4 pages)

Online Publication Date: 9 November 2007

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Turbulent boundary layer development, with zero pressure gradient, over an orthogonal external corner was investigated both experimentally and numerically. The results suggest that within the experimental and numerical limitations of the study that a similarity solution for the mean streamwise velocity profile exists along an external corner bisector. However, to confirm the self-similar solution, further independent measurement of the wall shear stress would be required. For the numerical simulation the Reynolds stress model was used. The experimental and numerical results were all consistent with a logarithmic self-similar solution. However the constants appearing in the self-similar solution were found to differ between the experimental and numerical results and reasons for these discrepancies are discussed. The results also provide scope for benchmarking of commercial software packages against a generic three-dimensional turbulent boundary layer that occurs in many engineering applications.

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47.20.Ib	Instability of boundary layers; separation
47.27.N-	Wall-bounded shear flow turbulence
47.11.-j	Computational methods in fluid dynamics

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Young’s law and the effects of interfacial energy on the pressure at the solid-fluid interface

Ivan Lunati

Phys. Fluids 19, 118105 (2007); http://dx.doi.org/10.1063/1.2800040 (4 pages) | Cited 7 times

Online Publication Date: 12 November 2007

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In a recent paper [ R. Finn, Phys. Fluids 18, 047102 (2006) ] the attention has been drawn on the tension component perpendicular to the solid surface that results from Young’s law. Considering the problem of a solid sphere floating in zero gravity, it has been argued that this component yields a spurious net vertical force acting on the sphere, which remains unbalanced. Therefore, the validity of Young’s law has been questioned in favor of an energy argument. The scope of this Brief Communication is to restate the equivalence between the energy and the force descriptions of the problem. It is shown that there is no contradiction in Young’s law if one accounts for the different stress acting on the solid surface in the two fluids. The net force resulting from the latter balances the force resultant associated with the tension component perpendicular to the solid.

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68.03.Cd

Surface tension and related phenomena

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Small-scale isotropy and universality of axisymmetric jets

F. Picano and C. M. Casciola

Phys. Fluids 19, 118106 (2007); http://dx.doi.org/10.1063/1.2804955 (4 pages) | Cited 8 times

Online Publication Date: 12 November 2007

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This paper addresses the long-standing debate about whether the asymptotic jet can be universal. It is argued that the ansatz of a universal spreading rate is inconsistent with more accurate and recent measurements where the scatter in the opening angle is found to exceed the experimental accuracy, implying a true lack of universality in these basic quantities. A universal scaling theory may still be proposed by including the spreading rate in the similarity transformations. We will show that this approach leads to a conflict with the presumed recovery of isotropy in the dissipative scales of the flow. The issue is addressed here by using direct numerical simulations of free-jets to finally show that a complete universality is also incompatible with the experimental data available in the literature.

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47.27.Gs	Isotropic turbulence; homogeneous turbulence
47.27.wg	Turbulent jets

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A Eulerian model for large-eddy simulation of concentration of particles with small Stokes numbers

Babak Shotorban and S. Balachandar

Phys. Fluids 19, 118107 (2007); http://dx.doi.org/10.1063/1.2804956 (4 pages) | Cited 5 times

Online Publication Date: 19 November 2007

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In order to conduct the large-eddy simulation (LES) of particle-laden turbulent flows through a two-fluid approach, a model is proposed for the concentration of particles using an equilibrium assumption in which the Eulerian velocity of particles can be expressed in terms of the velocity and acceleration of the fluid phase as well as the gravitational acceleration through an asymptotic series expansion. This assumption is valid only for small Stokes numbers. The assessment of the model is carried out in forced isotropic turbulence. The LES model results well compared against results obtained by direct numerical simulation.

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47.27.Gs	Isotropic turbulence; homogeneous turbulence
47.55.Kf	Particle-laden flows
47.11.-j	Computational methods in fluid dynamics
47.27.ek	Direct numerical simulations
47.27.ep	Large-eddy simulations

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Analysis of vortex core in steady turbulent flow

Eduard Amromin

Phys. Fluids 19, 118108 (2007); http://dx.doi.org/10.1063/1.2813045 (3 pages) | Cited 2 times

Online Publication Date: 21 November 2007

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Profiles of velocity and pressure for the vortex core in turbulent flow were obtained by solving Reynolds equation for the circumferential component of the fluid momentum. The viscous core radius is defined as a function of viscosity coefficient, vortex intensity, and a Reynolds stress component. The obtained velocity profiles are in much better agreement with known experimental data than are the Rankin vortex profiles.

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47.32.-y	Vortex dynamics; rotating fluids
47.27.-i	Turbulent flows
47.10.-g	General theory in fluid dynamics

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Experimental investigation of a flow driven by a combination of a rotating and a traveling magnetic field

A. Cramer, J. Pal, and G. Gerbeth

Phys. Fluids 19, 118109 (2007); http://dx.doi.org/10.1063/1.2801407 (4 pages) | Cited 7 times

Online Publication Date: 21 November 2007

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Velocity measurements in a liquid metal flow were performed in order to study the combined action of a rotating (RMF) and a traveling magnetic field (TMF). In a cylindrical container, a RMF alone, as well as a TMF alone, cause axisymmetric volume forces, which drive corresponding axisymmetric base flows. The combination of both fields gives rise to an inherent three-dimensional constituent of the electromagnetic force distribution. In the case of equal RMF and TMF frequencies, this combination drives a three-dimensional flow consisting of a single large-scale helical motion, which implies an intense mixing of the melt. The necessary admixture of a TMF to a given RMF to cause a qualitative change of the flow structure is shown to be two orders of magnitude smaller in the case of equal field frequencies compared to the case of differing frequencies.

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47.32.Ef	Rotating and swirling flows
47.65.-d	Magnetohydrodynamics and electrohydrodynamics

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On a three-dimensional implementation of the baker’s transformation

Philippe Carrière

Phys. Fluids 19, 118110 (2007); http://dx.doi.org/10.1063/1.2804959 (4 pages) | Cited 2 times

Online Publication Date: 27 November 2007

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A three-dimensional, steady flow configuration intended to mimic the baker’s map is studied by means of numerical simulation. The Poincaré sections computed from a finite element solution of the velocity field show that the behavior is dominated by chaotic advection. The value obtained for the Lyapunov exponent is very close to the theoretical value of ln 2 predicted by the baker’s map.

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