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

Time resolved tomographic PIV is used to study flow structures in the outer region of a rough-wall turbulent boundary layer, focusing on imprints of the roughness on the outer layer. Measurements are performed in a transparent channel installed in the JHU optically index matched facility. The roughness consists of pyramids with height, k$=$0.46 mm, and wavelength, $\lambda =$3.2 mm, satisfying h/k$=$55 (h$=$25.4 mm is the channel half-height), k$^{+}=$64 and Re$=$40000. The TPIV setup consists of four high-speed cameras operating at 3 kHz, which view the sample volume through acrylic prisms. The flow field is illuminated by an Nd:YLF laser. Following enhancement, calibration, and reconstruction, 64$^{3}$ voxels interrogation volumes with 0.75 overlap provide 3D velocity fields with spacing of 0.588$^{3}$ mm$^{3}$. Formation and transport of near-wall 3D U-shaped vortex structures, with base in front of the pyramids, and quasi-streamwise legs extending between pyramid crest lines are evident from the data. Extended streamwise regions of high wall-normal vorticity appear ``latched'' to the roughness elements close to the wall, but are transported downstream at higher elevations. Also evident are traveling streamwise low velocity streaks, which cover many roughness elements.