Influences of the effective slope of rough surface on the quasi-streamwise vortex and Reynolds shear stress under favorable-adverse pressure gradient

In this research, direct numerical simulations were performed on the transitional flow over a flat plate subject to a pressure gradient and equipped with a three-dimensional irregular rough surface. The roughness configurations were developed based on varying ratios of spanwise to streamwise effective slopes (ESz/ESx), and were positioned upstream as well as in the proximity of the separation point induced by the adverse pressure gradient. The investigation primarily centered on the influence of the adverse pressure gradient and the rough surface's effective slope on the downstream quasi-streamwise vortices and the sweep and ejection events within the lift-up mechanism. Findings indicate that a rough surface aligned with the streamwise direction fosters more pronounced and intact hairpin vortices and promotes the development of spanwise secondary flows. When located near the separation point, the adverse pressure gradient emerges as a predominant factor shaping the downstream vortex structure, leading to vortex cores mixing and an increase in wall-normal vortices within the reverse flow region. The distribution of sweep and ejection events downstream of the roughness is largely consistent, whereas spanwise-aligned rough surface significantly mitigates these events. In the intense regions of these events, ejection events surpass sweep events in strength, while maintaining streamwise continuity. For the energy-rich segments of the most active coherent structures in the low-frequency band, the ejection event is markedly attenuated along the streamwise direction under the adverse pressure gradient, with its continuity disrupted, whereas the sweep event remains largely unaffected, with the evolutionary degrees of both events being similar.