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Fourth International Symposium on Turbulence and Shear Flow Phenomena
June, 27-29, 2005, Marriot Hotel, Williamsburg, Virginia, USA

DOI: 10.1615/TSFP4

FUNDAMENTAL STUDIES OF ACTIVE DIMPLES

pages 283-288
DOI: 10.1615/TSFP4.480
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ABSTRACT

The effect of local time-dependent dimple forcing of laminar channel and boundary-layer flow is investigated. Simulations of channel flow with a linearized boundary condition (no separation) for the dimple show how it generates stream-wise vorticity and that, as the frequency of forcing increases, progressively more of the vorticity flux is generated by the local surface acceleration as opposed to the pressure-gradient mechanisms operating in the case of a static dimple. Flow visualization in a laminar boundary layer with small-amplitude dimple forcing over a range of frequencies shows that, at quite low frequencies, an inner horseshoe vortex is generated by surface pressure gradients, producing common flow away from the surface along the streamwise horizontal centre-line. At higher frequencies, an increase in viscous blockage leads to the formation of an outer horseshoe vortex, in addition to the inner one to which it rotates in the opposite sense. There are therefore two regions of common flow towards the surface, each one outboard of, and immediately adjacent to, the streamwise legs of the inner horseshoe. The region of common flow away from the surface within the inner horseshoe persists. At the two highest frequencies, the leading edge of the outer horseshoe has more or less broken up into an array of jet-like structures, while the legs of the inner horseshoe vortex appear to merge such that a stretched vortex ring is now formed. A schematic of this model is presented.

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