Study of turbulent separations induced by a forward-facing step using particle tracking velocimetry

Turbulent separations induced by a forward-facing step (FFS) submerged in a thick turbulent boundary layer (TBL) were experimentally studied using a time-resolved particle tracking velocimetry. The Reynolds number based on the free-stream velocity and step height was 13200. The oncoming TBL was developed over a cube-roughened wall, and its thickness was 6.5 times the step height. Consequently, the FFS was exposed to excessively strong mean shear, turbulence intensity and large-scale motion (LSM) in the oncoming TBL. The complete set of Reynolds stresses and triple velocity correlations are, for the first time, reported for turbulent flows over FFS. The Reynolds stresses in the streamwise-vertical plane exhibit dual local peaks: one upstream of FFS and the other over FFS, whereas the spanwise Reynolds normal stress peak immediately upstream of the leading edge. The local peaks of Reynolds stresses upstream of FFS signify the interaction of oncoming LSM with FFS. On the frontal surface of FFS, a pair of counter-rotating vortices are dominant. Particles approaching the step are deflected towards either positive or negative spanwise directions before passing over the step in helical trajectories. This mechanism induces strong ejection events immediately upstream of the leading edge.