Time-resolved PIV measurement of influence of upstream roughness on separated and reattached turbulent flows over a forward-facing step

This study reports an experimental investigation of the effects of upstream roughness on a low Reynolds number turbulent boundary layer over a forward-facing step. Two types of upstream roughness were investigated, including a transitionally rough 16-grit sandpaper (ks+ ≈ 69) and fully rough staggered cubes (ks+ ≈ 500). A two-dimensional two-component time-resolved particle image velocimetry (2D-2C TR-PIV) method was used to measure the time-averaged mean velocities, Reynolds stresses, temporal auto-correlations and frequency spectra of the flow field to quantify the influence of upstream roughness on the downstream evolution of the turbulence over the step. The results indicate that upstream roughness decreased the vortex shedding frequency. Roughness also decreased the reattachment length by enhancing the streamwise turbulence intensity level, reducing the magnitude of backflow and suppressing the vortex shedding frequency in comparison to the smooth wall. In the recirculation region, upstream roughness reduced the mean streamwise velocity only in the outer layer. The Reynolds stresses remained relatively unchanged by the sandpaper roughness but were significantly modified by the cube roughness. Downstream of the leading edge, the staggered cubes increased the streamwise Reynolds stress both near the wall and outside the shear layer but decreased the wall-normal Reynolds stress and Reynolds shear stress within the shear layer. These modifications are inversely proportional to distance in the recirculation region. The life times of the streamwise and wall-normal velocity fluctuations increase with streamwise distance and are much longer in the redevelopment region than in the recirculation region. Quadrant decomposition and joint probability density functions of the velocity fluctuations were also measured to characterize upstream roughness effects on the downstream evolution of the dominant motions producing the Reynolds shear stress.