A HIGH-SPEED PHOTOGRAPHIC STUDY OF CAVITATION DAMAGE

Cylindrical cavities, viewed through the side as they collapsed onto solid surfaces, were studied using high-speed streak and framing photography. The cavities were collapsed asymmetrically using shock waves of varying amplitude so that the rear surface formed a high-speed jet which crossed the cavity and interacted with the target surface. Schlieren optics were used to visualise waves in the fluid and in the target. Two features of the collapsing bubble affected the damage to the target surface. The first was the impact of the high-speed liquid jet on either the rear wall of the cavity or the target itself. The second was the production of a strong compression wave on the rebound of the bubble after it reached minimum volume. Damage to the targets related to their material properties. Metals, with low compressive but higher tensile strengths, plastically deformed beneath the penetrating jet to form a pit. Brittle materials, with high compressive but low tensile strengths, deformed by cracking. The position of the cavity relative to the surface had a major effect upon the geometry of the damage. With the cavity close to the target, the penetrating jet dominated the damage leaving single pits. With the cavity at some distance, the rebound wave was more important than the jet giving rise to a circular damage mark. This mechanism can be used to re-interpret previous experimental observations [Y. Tomita and A. Shima, J. Fluid Mech. 119, 535(1986)]. (C) 1995 American Institute of Physics,