Drag and lift forces on microscopic bubbles entrained by a vortex

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Abstract
The forces that act on bubbles as they are entrained by a vortex are measured using particle image velocimetry. Triple exposure images are used to measure the velocity and acceleration of the fluid and the bubbles simultaneously. Distinction between phases is achieved by using fluorescent particles as liquid flow tracers. The buoyancy, pressure, and inertia forces are computed from the data, while the drag and the lift forces are determined from a force balance on each bubble. It is found that in the present range of bubble diameters, 500 μm<d<800 μm, and Reynolds numbers, 20<Re<80, the drag on a bubble is similar to that on a solid body. Vorticity does not have a significant effect on the drag coefficient. The lift coefficients are significantly higher than currently available analytical and numerical estimates. The coefficients are independent of the Reynolds number and are proportional to the fourth root of the local vorticity. Estimates of the Bassett force show that it can be neglected in the present experiment. Computed bubble trajectories, based on the measured lift and drag coefficients, compare well with experimental observations.

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