  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.
