Stability and Drag Reduction in a Boundary Layer with Microbubbles.
PENNSYLVANIA STATE UNIV UNIVERSITY PARK APPLIED RESEARCH LAB
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When microbubbles are injected in a turbulent boundary layer, they promote a reduction of the local skin friction and a sharp mean velocity gradient between the two microbubble free regions in the boundary layer, i.e., the outer edge and the near wall region. In connection with the apparent stability of the bubbles in the boundary layer, the stability of a hollow vortex sheet in an infinite expanse of fluid as well as in the presence of a wall are studied. We concluded that the simple hollow vortex sheet is unconditionally unstable while a stability range was found for a hollow vortex sheet perturbed in the presence of a wall. In all cases, however, the computations suggest that the hollow vortex radius is invariably a destabilizing factor. Regarding the second aspect of the problem, a heuristic, but physically sound explanation of the mechanisms of the drag reduction is proposed. Based on these ideas and using a phenomenological approach, the flat plate boundary layer is numerically analyzed.
- Fluid Mechanics