Time Resolved Tomographic PIV Measurements of Rough-Wall Turbulent Channel Flow

Rough-wall turbulent channel flow is investigated via time-resolved tomographic particle image velocimetry (TPIV). To facilitate 3D measurements in the bulk flow as well as very close to the rough surface the optical refractive index of the transparent acrylic channel wall is matched with that of the liquid, a concentrated solution of sodium iodide in water (62%-64% by weight). The roughness consists of staggered pyramidal elements with h/k = 55 (half channel to roughness heights ratio) and k+ = 64, which satisfies the \u93well-characterized\u94 rough wall boundary layer conditions. The Reynolds number based on half channel height is 40000. Our research is aimed at studying mechanisms involved with generation of coherent structures as large-scale outer layer turbulence and the mean flow interact with the roughness elements, and their subsequent propagation away from the wall. Data is acquired using four high speed cameras at 3000 frames per second, the sample volume size is 40.6×44.1×17.4 mm3, and the vector spacing after 3D cross-correlation with 75% overlap is 0.588×0.588×0.588 mm3. Consistent with recent observations based on holographic PIV measurements, the TPIV data confirms the formation of U-shaped vortices that wrap around the low speed regions above peaks of the pyramids. Interactions among quasi-streamwise \u93legs\u94 of vortices generated by neighboring pyramids generate powerful ejection events that lift these structures away from the wall