Multicomponent and Unsteady Velocity Measurements of Transient Disturbances
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Two experiments are conducted to investigate transient growth and bypass transition in boundary layers with moderate- to high-amplitude roughness arrays. Unsteady and multicomponent velocity measurements are obtained in a flat plate boundary layer downstream of a spanwise array of cylindrical roughness elements with roughness Reynolds numbers of Rek = 202, 264 and 334. Measurements of the unsteady streamwise velocities in the wake indicate that growth of a 720 Hz fluctuation leads rapidly to bypass transition at Re k = 334. The Strouhal number of this fluctuation, St = 0.17, is in good agreement with previous experiments. The locations of maximum fluctuation intensity suggest that the fluctuations result from a Kelvin-Helmholz-type instability of the spanwise-varying wake. Additionally, steady spanwise velocities are measured in an effort to supply the initial conditions for the downstream transient growth of the streamwise velocity disturbance. Time-averaged streamwise and spanwise velocity components are measured using a combination of multiple slanted and straight hotwire sensors. The multicomponent measurements reveal the expected horseshoe vortex structure that is symmetric around the roughness centerline.
