Effect of roughness on the spatio-temporal characteristics of inner-outer interactions in a turbulent boundary layer

Turbulent boundary layers over smooth and rough walls are investigated using high frame-rate PIV in a refractive-index-matched (RIM) flow facility to better understand the structure of inner-outer modulating interactions. The RIM enables well resolved investigation of the near-wall buffer and roughness sub- layers to identify the effect of large and very large scales on the same. A hexagonally-packed hemispherical roughness pattern is investigated, together with baseline measurements over a smooth wall. Amplitude and frequency modulation coefficients are computed for each case, and preliminary results have shown strong correlation coefficients over roughness compared to the smooth-wall baseline. The wall-normal velocity fluctuations in the roughness sub-layer are stronger compared to that in the smooth-wall near-wall region, and exhibit frequency modulation. The on-going work aims to investigate the effect of roughness geometry on this interaction structure using conditional averaging methods.