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Jan 2011

Volume 23, Issue 1, Articles (01xxxx)

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Phys. Fluids 23, 011702 (2011); http://dx.doi.org/10.1063/1.3541844 (4 pages)

Hongjie Zhong, Shiyi Chen, and Cunbiao Lee
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back to top Laminar Flows

Electromagnetically driven oscillatory shallow layer flow

Aldo Figueroa, Sergio Cuevas, and Eduardo Ramos

Phys. Fluids 23, 013601 (2011); http://dx.doi.org/10.1063/1.3531729 (10 pages)

Online Publication Date: 6 January 2011

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We report experimental observations of the laminar flow in a thin horizontal layer of electrolyte, generated by a time-periodic Lorentz force produced by an alternate, unidirectional electric current and the field of a small permanent magnet. The force drives a periodically oscillating dipolar vortex which displays some spatial and temporal symmetries. The attention is focused on the motion of the oscillatory layer in vertical planes perpendicular to both the bottom wall and the injected current. For different frequencies of the injected current, velocity fields were obtained using particle image velocimetry in the zone of more intense magnetic field as well as close to the edges of the magnet where the inhomogeneity of the field is more pronounced. Velocity profiles as functions of the normal coordinate are determined in characteristic points at different phases and oscillation frequencies. Experimental results are compared with a simple analytical solution and a full three-dimensional numerical simulation that reproduces satisfactorily the experimental observations. Under the explored conditions and available experimental resolution, results indicate that except in the zone above of the lateral edges of the magnet, no recirculating flows appear and vertical velocity components are negligible.
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47.65.-d Magnetohydrodynamics and electrohydrodynamics
47.15.-x Laminar flows
47.32.-y Vortex dynamics; rotating fluids
47.80.Jk Flow visualization and imaging
47.11.-j Computational methods in fluid dynamics

Structure of trailing vortices: Comparison between particle image velocimetry measurements and theoretical models

C. del Pino, L. Parras, M. Felli, and R. Fernandez-Feria

Phys. Fluids 23, 013602 (2011); http://dx.doi.org/10.1063/1.3537791 (12 pages) | Cited 2 times

Online Publication Date: 11 January 2011

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The velocity field of the trailing vortex behind a wing at different angles of attack has been measured through the stereo particle image velocimetry technique in a water tunnel for Reynolds numbers between 20 000 and 40 000, and for several distances to the wing tip. After filtering out the vortex meandering, the radial profiles of the axial and the azimuthal velocity components and of the radial profiles of the vorticity were compared to the theoretical models for trailing vortices by [ G. K. Batchelor, J. Fluid Mech. 20, 645 (1964) ] and by [ D. W. Moore and P. G. Saffman, Proc. R. Soc. London, Ser. A 333, 491 (1973) ], whose main features are conveniently summarized. We take into account the downstream evolution of these profiles from just a fraction of the wing chord to more than ten chords. The radial profiles of the vorticity and the azimuthal velocity are shown to fit quite well to Moore and Saffman’s trailing vortex model, while Batchelor’s model does not fit so well, especially in the tails of the profiles. At the downstream distances considered, the radial profiles of the axial velocity do not adjust so well to Moore and Saffman’s model as the azimuthal velocity profiles do, but the disagreement with Batchelor’s model is quite manifested, especially at the axis. Thus, the details of the flow structure are in better agreement with the predictions of Moore and Saffman’s model. The downstream evolution of several key features of the measured velocity profiles is also in agreement with the predictions of Moore and Saffman’s model, within the dispersion of the experimental data, but up to the largest axial distance considered in this work we cannot decide if they follow the asymptotic behavior predicted by this model.
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47.32.cd Vortex stability and breakdown
47.80.Jk Flow visualization and imaging
47.80.Cb Velocity measurements
47.27.nb Boundary layer turbulence
47.27.wb Turbulent wakes
47.85.Np Fluidics

Convection-dominated dispersion in channels with fractal cross-section

Alessandra Adrover

Phys. Fluids 23, 013603 (2011); http://dx.doi.org/10.1063/1.3526759 (10 pages)

Online Publication Date: 14 January 2011

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We focus on the characterization of dispersion processes in microchannels with fractal boundaries (and translational symmetry in the longitudinal direction) in the presence of laminar axial velocity field. This article extends the theory of laminar dispersion in finite-length channel flows at high Peclet numbers by analyzing the role of the fractal cross-section in the convection-dominated transport regime. In this regime, the properties of the dispersion boundary layer and the values of the scaling exponents controlling the dependence of the moment hierarchy on the Peclet number are determined by the local near-wall behavior of the axial velocity. Specifically, different scaling laws in the behavior of the moment hierarchy occur, depending whether the cross-sectional boundary is smooth or nonsmooth (e.g., presenting corner points or cusps). The limit case of a fractal boundary is analyzed in detail. Analytical and numerical results are presented for two fractal cross-sections (the classical Koch curve and the Koch snowflake) in the Stokes regime.
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47.15.Rq Laminar flows in cavities, channels, ducts, and conduits
47.53.+n Fractals in fluid dynamics
47.60.Dx Flows in ducts and channels
47.15.G- Low-Reynolds-number (creeping) flows
47.15.Cb Laminar boundary layers

Measures of mixing quality in open flows with chaotic advection

E. Gouillart, O. Dauchot, and J.-L. Thiffeault

Phys. Fluids 23, 013604 (2011); http://dx.doi.org/10.1063/1.3506817 (11 pages) | Cited 3 times

Online Publication Date: 25 January 2011

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We address the evaluation of mixing efficiency in experiments of chaotic mixing inside an open-flow channel. Since the open flow continuously brings new fluid into the limited mixing region, it is difficult to define relevant mixing indices as fluid particles experience typically very different stretching and mixing histories. The repeated stretching and folding of a spot of dye leads to a persistent pattern. We propose that the normalized standard deviation of this characteristic pattern is a good measure of the mixing quality of the flow. We discuss the link between this measure and mixing of continuously injected dye, and investigate it using an idealized map.
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47.60.Dx Flows in ducts and channels
47.52.+j Chaos in fluid dynamics
64.75.Ef Mixing
44.27.+g Forced convection
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