Synergistic mechanism of cavitation cloud cleaning: An experimental study on surface contaminant removal

Ultrasonic cavitation has emerged as an environmentally sustainable technique for the removal of marine biofouling and surface contaminants from submerged structures. However, cleaning performance cannot be adequately characterized by acoustic input alone because the collective transport and accumulation of cavitation bubbles modify the effective cavitation activity delivered to the target surface. In this study, systematic experiments were conducted to investigate the coupled effects of cavitation micro-jets and acoustic streaming on contaminant removal under different ultrasonic operating conditions. To quantify this collective bubble-transport response, a dimensionless Bubble Migration Number, N b ∗ , was introduced together with the dimensionless stand-off distance, d ∗ . The experimental results demonstrate that the decontamination efficiency is jointly influenced by the Bubble Migration Number and the ultrasonic stand-off distance. At a fixed stand-off distance, increasing N b ∗ initially enhances contaminant removal owing to intensified cavitation micro-jet impingement. Beyond a critical threshold, however, excessive bubble accumulation is associated with a shielding-dominated regime and consequently deteriorates the cleaning performance. Based on these observations, a physics-informed semi-empirical scaling framework was developed to quantitatively describe the nonlinear evolution of the decontamination efficiency, η ∗ = C ( d ∗ ) α ( N b ∗ ) β e − γ N b ∗ . The proposed framework captures the combined effects of stand-off distance, bubble transport enhancement, and shielding-related attenuation. The model reproduces the experimental trends and yields an optimal Bubble Migration Number of 8.53 as a point estimate under the present experimental conditions. The proposed dimensionless framework provides a quantitative basis for identifying favorable ultrasonic operating regimes and for the future optimization of underwater ultrasonic cleaning systems.