Effect of Surface Roughness on Cloud Cavitataion

Abstract

Cloud cavitation in hydraulic systems produces severe noise and erosion through collective bubble collapse and shock formation, yet the influence of wall surface roughness on cloud dynamics remains incompletely understood despite its practical importance in marine propulsion, turbomachinery, and biomedical devices. This study investigates the effect of controlled surface roughness on cloud cavitation dynamics in a converging-diverging Venturi test section with throat dimensions 50 × 5 mm. Four roughness levels (Ra = 5, 20, 55, 80 μm) were produced by laser etching on the diverging wall, spanning hydraulically smooth (k+ = 4.68), transitionally rough (k+ = 18.71 and 51.45), and fully rough (k+ = 74.84) regimes at a Reynolds number of 215492. High-speed videography combined with optical flow algorithm (RAFT) enabled quantitative extraction of cavity length, shedding frequency, and re-entrant jet characteristics. Results demonstrate that increasing surface roughness systematically reduces mean cavity length and increases shedding frequency. Re-entrant jets become progressively thinner and shorter as roughness increases, while microscale grooves on roughness elements act as nucleation sites that trap bubbles and, at higher roughness levels, disrupt the boundary layer and alter vorticity production. These findings confirm that roughness modifies cloud cavitation through coupled mechanisms such as, enhanced nucleation at element-scale features and altered boundary-layer dynamics that weaken large-scale re-entrant jet formation.