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Understanding ventilation strategy of a supercavity is important fordesigning high-speed underwater vehicles wherein an artificial gas pocket iscreated behind a flow separation device for drag reduction. Our studyinvestigates the effect of flow unsteadiness on the ventilation requirements toform (CQf) and collapse (CQc) a supercavity. Imposing flow unsteadiness on theincoming flow has shown an increment in higher CQf at low free stream velocityand lower CQf at high free stream velocity. High-speed imaging revealsdistinctly different behaviors in the recirculation region for low and highfreestream velocity under unsteady flows. At low free stream velocities, therecirculation region formed downstream of a cavitator shifted vertically withflow unsteadiness, resulting in lower bubble collision and coalescenceprobability, which is critical for the supercavity formation process. Therecirculation region negligibly changed with flow unsteadiness at high freestream velocity and less ventilation is required to form a supercavity comparedto that of the steady incoming flow. Such a difference is attributed to theincreased transverse Reynolds stress that aids bubble collision in a confinedspace of the recirculation region. CQc is found to heavily rely on the verticalcomponent of the flow unsteadiness and the free stream velocity. Interfacialinstability located upper rear of the supercavity develops noticeably with flowunsteadiness and additional bubbles formed by the distorted interface shed fromthe supercavity, resulting in an increased CQc. Further analysis on thequantification of such additional bubble leakage rate indicates that thedevelopment and amplitude of the interfacial instability accounts for thevariation of CQc under a wide range of flow unsteadiness. Our study providessome insights on the design of a ventilation strategy for supercavitatingvehicles in practice.

Insights into ventilation hysteresis shift due to flow unsteadiness in ventilated supercavitation