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Apr 2009

Volume 21, Issue 4, Articles (04xxxx)

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Phys. Fluids 21, 045106 (2009); http://dx.doi.org/10.1063/1.3112686 (22 pages)

Felix Keiderling, Leonhard Kleiser, and Christophe Bogey
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back to top Laminar Flows

Secondary flow behavior in a double bifurcation

Fong Yew Leong, Kenneth A. Smith, and Chi-Hwa Wang

Phys. Fluids 21, 043601 (2009); http://dx.doi.org/10.1063/1.3100211 (11 pages) | Cited 3 times

Online Publication Date: 6 April 2009

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Secondary flows in the form of multivortex structures can occur in bifurcation models as the result of upstream influence. Results from numerical modeling of steady inspiratory flows indicate that, for the case of a symmetric planar double bifurcation, four counter-rotating vortices develop in each of the grand daughter branches. In this paper, experimental visualization and verification is provided by particle image velocimetry measurements on a modified single bifurcation model. A splitter plate was positioned in the mother tube so that secondary vorticity was introduced into the fluid core. The axial velocity profile before the bifurcation junction resembles the M-shaped velocity profile commonly observed in bifurcated tube flows. The result of this manipulation is the development of a physically observable four-vortex configuration in the cross sections of the daughter branches, thus demonstrating the strong influence of upstream secondary vorticity. Through numerical visualization of vortex lines, it is shown that secondary vorticity is amplified by the extension of vortex lines due to secondary flow within the daughter tube. Order-of-magnitude arguments have been applied to the vorticity transport equation; and key dimensionless parameters have been obtained, accounting for curvature effects. Results indicate that the secondary vorticity goes through a maximum with increasing downstream distance, as a result of the interplay between vortex stretching and viscous effects.
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47.20.Ky Nonlinearity, bifurcation, and symmetry breaking
47.32.-y Vortex dynamics; rotating fluids
47.80.Jk Flow visualization and imaging

Characterization of linear plasma synthetic jet actuators in an initially quiescent medium

Arvind Santhanakrishnan, Daniel A. Reasor, Jr., and Raymond P. LeBeau, Jr.

Phys. Fluids 21, 043602 (2009); http://dx.doi.org/10.1063/1.3097004 (18 pages) | Cited 5 times

Online Publication Date: 22 April 2009

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The plasma synthetic jet actuator (PSJA) is a geometrical variant of the aerodynamic plasma actuator that can be used to produce zero-mass flux jets similar to those created by mechanical devices. This jet can be either three-dimensional using annular electrode arrays (annular PSJA) or nearly two dimensional using two rectangular-strip exposed electrodes and one embedded electrode (linear PSJA). Unsteady pulsing of the PSJA at time scales decoupled to the ac input frequency results in a flow field dominated by counter-rotating vortical structures similar to conventional synthetic jets, and the peak velocity and momentum of the jet is found to be affected by a combination of the pulsing frequency and input power. This paper investigates the fluid dynamic characteristics of linear plasma synthetic jet actuators in an initially quiescent medium. Two-dimensional particle image velocimetry measurements on the actuator are used to validate a previously developed numerical model wherein the plasma behavior is introduced into the Navier–Stokes equations as an electrohydrodynamic force term calculated from Maxwell’s equations and solved for the fluid momentum. The numerical model was implemented in an incompressible, unstructured grid code. The results of the simulations are observed to reproduce some aspects of the qualitative and quantitative experimental behavior of the jet for steady and pulsed modes of actuator operation. The self-similarity behavior of plasma synthetic jets are examined and compared to mechanically driven continuous and synthetic jets.
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52.75.-d Plasma devices
52.30.-q Plasma dynamics and flow

Pressure-driven miscible two-fluid channel flow with density gradients

K. C. Sahu, H. Ding, P. Valluri, and O. K. Matar

Phys. Fluids 21, 043603 (2009); http://dx.doi.org/10.1063/1.3122779 (10 pages) | Cited 12 times

Online Publication Date: 24 April 2009

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We study the effect of buoyancy on pressure-driven flow of two miscible fluids in inclined channels via direct numerical simulations. The flow dynamics are governed by the continuity and Navier–Stokes equations, without the Boussinesq approximation, coupled to a convective-diffusion equation for the concentration of the more viscous fluid through a concentration-dependent viscosity and density. The effect of varying the density ratio, Froude number, and channel inclination on the flow dynamics is examined, for moderate Reynolds numbers. We present results showing the spatiotemporal evolution of the flow together with an integral measure of mixing.
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47.60.Dx Flows in ducts and channels
47.55.P- Buoyancy-driven flows; convection
47.51.+a Mixing
47.11.-j Computational methods in fluid dynamics
66.10.C- Diffusion and thermal diffusion
66.20.-d Viscosity of liquids; diffusive momentum transport
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