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

TL;DR

This study investigates how flow unsteadiness affects the ventilation requirements for supercavity formation and collapse, revealing velocity-dependent behaviors and the role of flow-induced instabilities in underwater vehicle design.

Contribution

It provides new insights into the influence of flow unsteadiness on supercavity ventilation, highlighting the velocity-dependent effects and the importance of interfacial instabilities.

Findings

Flow unsteadiness increases CQf at low velocities and decreases it at high velocities.

Vertical flow unsteadiness shifts the recirculation region, affecting bubble collision.

Interfacial instability influences CQc and bubble leakage rates.

Abstract

Understanding ventilation strategy of a supercavity is important for designing high-speed underwater vehicles wherein an artificial gas pocket is created behind a flow separation device for drag reduction. Our study investigates the effect of flow unsteadiness on the ventilation requirements to form (CQf) and collapse (CQc) a supercavity. Imposing flow unsteadiness on the incoming flow has shown an increment in higher CQf at low free stream velocity and lower CQf at high free stream velocity. High-speed imaging reveals distinctly different behaviors in the recirculation region for low and high freestream velocity under unsteady flows. At low free stream velocities, the recirculation region formed downstream of a cavitator shifted vertically with flow unsteadiness, resulting in lower bubble collision and coalescence probability, which is critical for the supercavity formation process. The recirculation region negligibly changed with flow unsteadiness at high free stream velocity and less ventilation is required to form a supercavity compared to that of the steady incoming flow. Such a difference is attributed to the increased transverse Reynolds stress that aids bubble collision in a confined space of the recirculation region. CQc is found to heavily rely on the vertical component of the flow unsteadiness and the free stream velocity. Interfacial instability located upper rear of the supercavity develops noticeably with flow unsteadiness and additional bubbles formed by the distorted interface shed from the supercavity, resulting in an increased CQc. Further analysis on the quantification of such additional bubble leakage rate indicates that the development and amplitude of the interfacial instability accounts for the variation of CQc under a wide range of flow unsteadiness. Our study provides some insights on the design of a ventilation strategy for supercavitating vehicles in practice.