Effect of Surface Roughness on the Flow Structures of a Normally Impinging Jet

Impinging jets are widely utilized in different applications including propulsion systems, thermal management, and industrial spray processes due to their high convective transport capabilities. However, the resulting flow is highly complex, governed by interactions between jet momentum, stand-off distance, and surface topology. In this study, the flow characteristics of a round jet impinging normally on smooth and rough impermeable surfaces were experimentally investigated using planar Particle Image Velocimetry (PIV). Surface roughness was introduced by affixing sandpaper of grit sizes 240, 400, and 1000 to the impingement plane. Velocity distributions in the free-jet region and flow structures in the impingement zone were analyzed at jet-to-surface spacing ratios of H/D=12 and 6 across Reynolds numbers ranging from 2,500 to 10,500. The flow field is classically divided into three regions: the free-jet, impingement, and wall-jet zones. Results reveal that surface roughness influences vortex dynamics near the stagnation region and alters shear layer development. These findings provide new insights into how surface texture affects impinging jet behavior, with implications for optimizing heat and momentum transfer in engineering systems.