Inner-outer interactions in a turbulent boundary layer overlying complex roughness

Hot-wire measurements were performed in turbulent flow overlying a smooth and a rough wall (the latter a multiscale topography) with the intent of investigating the details of inner-outer interactions. One-point measurements with a traversing probe were made to investigate the distribution of streamwise turbulent kinetic energy and large scale-small scale interactions at different regions of the smooth- and rough-wall cases. In addition, 2-probe simultaneous measurements were conducted and enabled investigation of inner-outer interactions wherein the large scales were always sampled in the outer layer. After identifying different regions across the boundary layer in the timedelayed correlation maps of the amplitude modulation coefficient, roughness-induced changes to the near-wall behavior were investigated. It was observed that the amplitude modulation influence on the near-wall small scales by the large scales in logarithmic region appears augmented in the rough-wall case compared to smooth-wall flow. Further, among the different spanwise positions investigated over the complex roughness (previously found to induce persistent secondary flows owing to spanwise heterogeneity of the topography), the low-momentum pathway region tended to have a 'stronger' interaction compared to the high-momentum pathway position.