Experimental study of coatings' effect on cavitating flow

Influence of surface coatings on cavitating flow around hydrofoils is studied experimentally. Hydrofoils are painted with epoxy coating and fluorocarbon coating, respectively. High speed visualization system is used to observe the cavitation flow patterns in different stage. The results show three main phenomena. Firstly, in stage of incipient cavitation, obvious cavities occur in form of bubbles dispersal along spanwise direction on the suction side of hydrofoil coated by epoxy when σ=1.82, while no cavitation occurs around fluorocarbon hydrofoil. So the fluorocarbon coating is helpful in restrict on the occurrence of cavitation. The incipient cavitation number for fluorocarbon hydrofoil is 1.50. Secondly, in stage of attached sheet cavitation, cavitating flow around fluorocarbon hydrofoil develops into sheet cavitation when σsmall than 1.63, which is earlier than that of epoxy foil. The cavitating flow consists of clusters of dispersive bubbles and cavities become larger and move along the surface of foil. The bubbles collapse in high pressure region and keep moving downstream in form of many horseshoe vortexes. With same σ, the cavity length along surface of the epoxy hydrofoil is relatively larger than fluorocarbon hydrofoil. But cavity length around epoxy hydrofoil and fluorocarbon hydrofoil are equal almost with decreasing of σ. Restricting effect of epoxy coating on cavitation becomes stronger gradually in sheet cavitation stage. Thirdly, the cavitating flow develops into cloudy cavitation with periodical shedding of cloudy bubbles when σ=0.87. Both period of cavitating flow and non-dimensional cavity area for epoxy hydrofoil are smaller than that of fluorocarbon hydrofoil. Restriction on unsteady development of cavitation generated by epoxy coating is stronger than fluorocarbon coating.