Effects of roughness and adverse pressure gradient on the turbulence structure

The manuscript presents an experimental study of turbulent flows over smooth and rough walls in a channel that consists of an upstream parallel section to produce a fully developed channel flow and a diverging section to produce an adverse pressure gradient (APG) flow. The roughness element consists of two-dimensional transverse square ribs of nominal height, k = 3 mm that were spaced to produce pitch-to-height ratio of p/k = 2 and 8, corresponding to d-type and k-type rough walls. A particle image velocimetry system was used to conduct detailed velocity measurements over the smooth and rough walls in both the parallel and diverging sections. The mean defect velocity and Reynolds stresses as well as Galilean decomposition, quadrant decomposition, two-point velocity correlation and linear stochastic estimation were used to document the salient effects of APG and wall roughness on the flows. The results indicated that APG significantly augments turbulence level compared to the flows in the parallel section. The flow fields were populated with vortex cores that formed hairpin vortex packets. The packets were inclined at shallow angles relative to the wall as revealed by the two-point velocity correlation. The quadrant analysis revealed that the ejections, sweeps, and inward and outward interaction motions associated with the structures were influenced by roughness and APG. The existence of structures was further supported by linear stochastic estimates conditioned on the prograde vortices.