Surface damage induced by a cavitation bubble in a flow through a gap

We study cavitation damage on a metal surface induced by a single bubble that is laser-seeded in a narrow, water-filled gap with background flow of adjustable velocity. The bubble dynamics and collapse shock wave emission is observed using high-speed imaging from two perspectives of the bubble. Surface damage is inspected ex-situ using 3D surface profilometry. First, in stagnant liquid the bubble dynamics and resulting surface damage are studied for different bubble-to-wall distances. Then, the effect of the flow on the bubble dynamics and the surface damage is studied for increasing flow velocities. Different damage patterns are found that can be matched to the different bubble dynamics. In stagnant liquid at small enough stand-off distances, the surface damage appears as a single deep hole with its volume increasing for decreasing initial bubble-to-wall distance. In a moderate background flow of a few meters per second, the surface damage occurs as an indentation cloud, i.e., multiple smaller indentations are scattered over a large area. The indentation cloud is shifted downstream from the nucleation site due to the translation of the bubble with the flow. High-speed imaging at sub-picosecond exposure times reveals the influence of the background flow on the final stage of the bubble collapse and the shock wave self-focusing mechanism. • A background flow deforms laser-induced cavitation bubbles at solid surfaces and alters the bubble dynamics. • The surface damage crucially depends on the nucleation distance and flow velocity. • When bubbles collapse in a stagnant liquid in a narrow gap, a ring collapse with a weakened self-focusing mechanism is visible. • A background flow in the narrow gap results in the non-coordinated bubble collapse further reducing the energy self-focusing.