Measurements of Distributed Roughness Receptivity
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2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. Receptivity of laminar boundary layers to distributed 3D roughness is not well under- stood. One key question is whether distributed roughness undergoes a linear or nonlinear receptivity process. This question relates to the concept of roughness shielding," the possibility that only the largest roughness elements have an impact on the flow and this impact is reduced by the surrounding roughness. Experiments presented here explore this phenomenon by placing variable-height cylindrical roughness elements in a field of similarly sized distributed roughness and measuring the steady and unsteady velocity perturbations that result at different heights of the adjustable roughness. A clear shielding effect is found simply for the distributed roughness. That roughness fails to produce transition even when its roughness peaks substantially exceed a critical roughness height predicted by an isolated-roughness transition correlation. When the cylindrical roughness elements are introduced, these have minimal impact on the flow until they approximately equal the peak heights of the distributed roughness. Transition only occurs once the cylinder- roughness heights substantially exceeds the maximum roughness heights in the distributed field. Overall, the results show clear, quantitative evidence of the shielding phenomenon and provide direction for continued study of distributed-roughness receptivity.
