The Effects of Upstream Wall Roughness on Separated and Reattached Flows Over a Forward-Facing Step

Separating and reattaching turbulent flows induced by a forward-facing step submerged in thick oncoming turbulent boundary layers developed over smooth and rough walls were investigated using time-resolved particle image velocimetry. Both smooth and fully rough upstream bottom wall conditions were examined and the resultant oncoming boundary layer thickness were 4.3 and 6.7 times the step height, respectively. The Reynolds number based on the step height and free-stream velocity was 7800. The mean velocities, Reynolds stresses analyzed in both Cartesian and curvilinear coordinate systems, eddy viscosity, correlation coefficient and third order moments are discussed. The results indicate that, due to the enhanced turbulence intensity and shear rate in the fully rough case, distinct elevated regions of vertical and shear Reynolds stresses are consistent upstream of the leading edge of the step while the magnitude of the Reynolds stresses are consistently higher than observed in the smooth case. The correlation coefficient, eddy viscosity and third order moments also show distinct elevated regions upstream of the leading edge of the step in the fully rough case. Above the step, distinct elevated regions of the Reynolds stresses, eddy viscosity and correlation coefficient are observed in both cases with the peak values at a vertical location corresponding to the maximum elevation of the mean separating streamline.