Physics of Fluids publishes in traditional areas of fluid dynamics as well as in novel and emerging areas of the field including dynamics of gases, liquids, and complex or multiphase liquids. Physics of Fluids is published with the cooperation of the APS Division of Fluid Dynamics.
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We propose a novel strategy for designing chaotic micromixers using curved channels confined between two flat planes. The location of the separatrix between the Dean vortices, induced by centrifugal forces, is dependent on the location of the maxima of axial velocity. An asymmetry in the axial velocity profile can change the location of the separatrix. This is achieved physically by introducing slip alternatingly at the top and bottom walls. This leads to streamline crossing and Lagrangian chaos. An approximate analytical solution of the velocity field is obtained using perturbation theory. This is used to find the Lagrangian trajectories of fluid particles. Poincare sections taken at periodic locations in the axial direction are used to study the extent of chaos. We study two microchannel designs, called circlet and serpentine, in which the Dean vortices in adjacent half cells are corotating and counterrotating, respectively. The extent of mixing, at low Re and low slip length, is shown to be greater in the serpentine case. Wide channels are observed to have much better mixing than tall channels; an important observation not made for separatrix flows till now. Eulerian indicators are used to gauge the extent of mixing, with varying slip length, and it is shown that an optimum slip length exists which maximizes the mixing in a particular geometry. Once the parameter space of relatively high mixing is identified, detailed variance computations are carried out to identify the detailed features.

Computational results are presented which covalidate published experimental observations of steady streaks, deliberately generated by the steady wake of a wire placed upstream of a flat plate with a prescribed leading edge. The largest streak occurs when the wake is generated from a wire placed upstream of the wind tunnel contraction. Normal vorticity passing through the contraction leads to the creation of streamwise vorticity in the testsection via tilting and stretching. The computational results allow the original experiment to be reinterpreted as a receptivity experiment that demonstrates the boundary layer is more receptive to steady streamwise vorticity than normal vorticity. It also suggests an interesting mechanism for the generation of Klebanoff streaks in wind tunnels. The effect of shifting the attachment point at the leading edge on receptivity is also demonstrated. The streak growth is compared to the Optimal streak often used in computational studies. The modal growth of the streak generated by freestream normal vorticity is found to have a streamwise location of peak energy close to the Optimal streak for wavelengths larger than the leading edge thickness. However, the location of the peak energy for the streamwise vorticity streak varies substantially with wavelength. Differences in wallnormal profiles are also noted.

We use confocal microscopy to directly visualize the simultaneous flow of both a wetting and a nonwetting fluid through a model threedimensional (3D) porous medium. We find that, for small flow rates, both fluids flow through unchanging, distinct, connected 3D pathways; in stark contrast, at sufficiently large flow rates, the nonwetting fluid is broken up into discrete ganglia. By performing experiments over a range of flow rates, using fluids of different viscosities, and with porous media having different geometries, we show that this transition can be characterized by a state diagram that depends on the capillary numbers of both fluids, suggesting that it is controlled by the competition between the viscous forces exerted on the flowing oil and the capillary forces at the pore scale. Our results thus help elucidate the diverse range of behaviors that arise in twophase flow through a 3D porous medium.

Sedimentation behaviours of an ellipsoidal particle in narrow and infinitely long tubes are studied by a multirelaxationtime lattice Boltzmann method (LBM). In the present study, both circular and square tubes with 12/13 ⩽ D/A ⩽ 2.5 are considered with the Galileo number (Ga) up to 150, where D and A are the width of the tube and the length of major axis of the ellipsoid, respectively. Besides three modes of motion mentioned in the literature, two novel modes are found for the narrow tubes in the higher Ga regime: the spiral mode and the vertically inclined mode. Near a transitional regime, in terms of average settling velocity, it is found that a lighter ellipsoid may settle faster than a heavier one. The relevant mechanism is revealed. The behaviour of sedimentation inside the square tubes is similar to that in the circular tubes. One significant difference is that the translation and rotation of ellipsoid are finally constrained to a diagonal plane in the square tubes. The other difference is that the anomalous rolling mode occurs in the square tubes. In this mode, the ellipsoid rotates as if it is contacting and rolling up one corner of the square tube when it settles down. Two critical factors that induce this mode are identified: the geometry of the tube and the inertia of the ellipsoid.

We investigate bubble dispersion in turbulent TaylorCouette flow. The aim of this study is to describe the main mechanisms yielding preferential bubble accumulation in nearwall structures of the flow. We first proceed to direct numerical simulation of TaylorCouette flows for three different geometrical configurations (three radius ratios η = R 1/R 2: η = 0.5, η = 0.72, and η = 0.91 with the outer cylinder at rest) and Reynolds numbers corresponding to turbulent regime ranging from 3000 to 8000. The statistics of the flow are discussed using two different averaging procedures that permit to characterize the mean azimuthal velocity, the Taylor vortices contribution and the smallscale turbulent fluctuations. The simulations are compared and validated with experimental and numerical data from literature. The second part of this study is devoted to bubble dispersion. Bubble accumulation is analyzed by comparing the dispersion obtained with the full turbulent flow field to bubble dispersion occurring at lower Reynolds numbers in previous works. Several patterns of preferential accumulation of bubbles have been observed depending on bubble size and the effect of gravity. For the smaller size considered, bubbles disperse homogeneously throughout the gap, while for the larger size they accumulate along the inner wall for the large gap width (η = 0.5). Varying the intensity of buoyancy yields complex evolution of the bubble spatial distribution. For low gravity effect, bubble entrapment is strong leading to accumulation along the inner wall in outflow regions (streaks of low wall shear stress). When buoyancy effect dominates on vortex trapping, bubbles rise through the vortices, while spiral patterns stretched along the inner cylinder are clearly identified. Force balance is analyzed to identify dominating forces leading to this accumulation and accumulation patterns are compared with previous experiments.