Suppression of Transition Behind a Discrete Roughness Element Using a Downstream Element
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abstract
Discrete roughness elements in laminar boundary layers trigger breakdown to turbulence when the associated roughness Reynolds number, Rekk, exceeds a critical value. Recent work by Suryanarayanan et al. (2017a) has demonstrated that for the case of a 45-inclined rectangular roughness at modest supercritical Rekk, transition can be prevented by placing a second oppositely inclined rectangular roughness element in the wake of the upstream element. Extensive hotwire measurements downstream of the single and twoelement roughness configurations are made to determine how the addition of the second element suppresses transition. The data show that although the second element does not dramatically reduce the maximum velocity disturbance, disturbance velocity gradients are reduced and, as a result, unsteady velocity fluctuations do not grow as quickly. Then, as the wake recovers, the unsteady fluctuations decay without forming a turbulent wedge. Two-dimensional eigenmode calculations are made using measured steady flow data and these corroborate the measured fluctuation data.
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Proceeding of Tenth International Symposium on Turbulence and Shear Flow Phenomena