Structure of Turbulent Boundary Layers and Surface Pressure Fluctuations on a Patch of Large Roughness Elements
Rusche, Max Thomas
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Measurements were made in a zero pressure gradient turbulent boundary layer over two roughness patches containing hemispherical and cubical elements. The elements were 3 mm in height and spaced 16.5 mm apart in an array containing 7 streamwise rows and 6 spanwise columns for a total of 42 elements per patch. The boundary layer thickness was approximately 60 mm, so the ratio of element height to that thickness was a large amount at k= = 1=20. A three velocity component laser Doppler velocimeter measured instantaneous velocities. Mean flow and turbulence statistics were calculated as well velocity energy spectra. Surface pressure fluctuations were measured using a two-microphone subtraction method. The results show that hemispherical elements produce larger turbulence quantities in their wakes compared to the cubes. This is due to the hemispheres having a frontal area nearly 60% larger than that of the cubes. The turbulence levels behind the hemispheres is a maximum behind the first streamwise row of elements, and decreases afterwards. The cubical elements maintain a nearly constant amount of turbulence in their wake, signifying little interaction between cubical elements. Surface pressure fluctuations vary little in the streamwise direction of the patches. The hemispherical elements produce a larger sound pressure level behind them than the cube elements do. Velocity spectra results show large normal stress energy for regions at and below the element height. The energy for locations high in the boundary layer increases as the flow moves downstream. Coherency plots show that there is a large correlation between the turbulent structure and production of shear stress at the roughness height. Any measurements taken at or below the roughness height are highly correlated under 10 kHz, while locations higher in the boundary layer are correlated under 2 kHz.
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