Interaction and ordering of bubbles levitated in vortical flow (original) (raw)
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Effect of bubble’s arrangement on the viscous torque in bubbly Taylor-Couette flow
Physics of Fluids, 2015
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The effect of bubbles on the evolution of vortical flows near a wall are studied by direct numerical simulations, using a finite volume/front tracking technique that accounts fully for the effect of fluid inertia, viscosity, bubble deformability, and surface tension. Two problems have been studied. In one, the interaction of bubbles with a welldefined vortical flow, consisting of a parabolic velocity profile and a pair of counter-rotating straight vortex filaments near a wall, parallel to the flow direction, is followed. For a wide range of injection sites and bubble sizes, as well as for different number of bubbles, the motion of the bubbles into the vortex core leads to a cancellation of the original vorticity with secondary wall vorticity, resulting in a small transient reduction of the wall shear. In the other study, bubbles are injected near the wall in a turbulent channel flow. The evolution of the bubbles and the modification of the flow is followed as the bubbles migrate away from the wall.
The role of coherent structures in bubble transport by turbulent shear flows
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Using Auton's force law for the unsteady motion of a spherical bubble in inhomogeneous unsteady flow, two key dimensionless groups are deduced which determine whether isolated vortices or shear-layer vortices can trap bubbles. These groups represent the ratio of inertial to buoyancy forces as a relaxation parameter 17 = AU2/2gx and a trapping parameter r = AUI VT where AUis the velocity difference across the vortex or the shear layer, x is streamwise distance measured from the effective origin of the mixing layer and VT is the terminal slip speed of the bubble or particle. It is shown here that whilst buoyancy and drag forces can lead to bubbles moving in closed orbits in the vortex flows (either free or forced), only inertial forces result in convergent trajectories. Bubbles converge on the downflow side of the vortex at a location that depends on the inertial and lift forces. It is important to note that the latter have been omitted from many earlier studies.
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