Turbulent boundary layer flow over rigid and flexible isolated roughness elements

Turbulent boundary layer response to rigid and flexible isolated roughness elements was experimentally investigated using planar particle image velocimetry (PIV) and volumetric particle tracking velocimetry (PTV). The flow measurements were made at a Reynolds number of approximately 60,000, based on the boundary layer thickness and freestream velocity. The effects of flexibility were studied for roughness elements in the form of thin flat plates with aspect ratios h/w = 0.5, 1, 2, and 4, where w is the width of the plates (in the spanwise direction) and h is their height (in the wall-normal direction). The roughness elements occupied approximately 15% of the incoming turbulent boundary layer thickness and were subjected to the same flow conditions to identify the effects of flexibility. The planar PIV measurements highlight the effects of flexibility on the mean flow, boundary layer perturbation, Reynolds shear stress, and turbulent kinetic energy in the central streamwise-wall-normal plane, while the volumetric PTV measurements highlight the effects of the flexibility on the coherent vortical structures in the roughness element wake. The results also highlight the sensitivity of the flow over rigid and flexible thin plates to their aspect ratio.