Distributed Roughness and Transient Growth in a Flat Plate Boundary Layer

Very few experiments or numerical simulations have deliberately studied how distributed surface roughness affects boundary layer transition through the transient-growth mechanism. In this experimental work, transient growth initiated by randomly distributed, streamwise-extended surface roughness is experimentally studied in the Klebanoff--Saric Wind Tunnel at Texas A\&M University. Two distributed roughness surfaces with the same surface topography, but different amplitudes, were manufactured and mounted flush with the wall in a flat-plate boundary layer. Detailed measurements of the roughness wake were made using hotwire anemometry at three sub-critical Reynolds numbers. Transient growth was observed at multiple spanwise wavelengths. Multiple wavelengths underwent robust growth but only weak growth was observed near the so-called ``optimal'' wavelength. Comparing cases with different Reynolds numbers and roughness heights reveals how the steady velocity disturbance scales and gives futher insight into the receptivity mechanisms of distributed surface roughness.