Experimental investigation of turbulent flow over surfaces of rigid and flexible roughness

This study presents a comparative experiment to investigate turbulent channel flows over surfaces of rigid and flexible roughness. Using digital inline holographic particle tracking velocimetry in a refractive-index-matched flow channel, the turbulent flow and roughness deformation are measured simultaneously. The mean velocity profiles of the two cases match closely, but the flow over flexible roughness shows a substantial reduction of Reynolds stresses with a corresponding decrease of coherence in roughness-induced flow structures. For flexible roughness, the streamwise roughness deformation exhibits higher energy levels at lower frequencies where the fluid velocity spectra peak, while the spanwise deformation shows a more prominent peak at the natural frequency of the roughness elements. Combining the measurements of turbulent flow and roughness kinematics, we show a close match of the strain energy of the flexible roughness with the deficit of turbulent kinetic energy (TKE) due to roughness compliance, and a connection between large roughness deformation and elevated levels of Reynolds stresses. Overall, these analyses demonstrate the anisotropic process of turbulence-roughness interaction, in which the TKE is dampened and primarily converted into streamwise deformation of roughness, and is partially returned to the smaller scale turbulence through spanwise oscillation of roughness at its natural frequency.