Gas entrainment behaviors in the formation and collapse of a ventilated supercavity

Abstract

The present work reports some interesting gas entrainment behaviors in the formation and collapse of a ventilated supercavity under steady and unsteady flow conditions. Our experiments show that the gas entrainment required to establish a supercavity are much greater than the minimum gas entrainment required to sustain it, and these gas entrainment values depend on Froude (Fr) number, cavitator size and the flow unsteadiness. Specifically, the measurements of the formation gas entrainment coefficients under different Fr numbers indicate that it does not monotonically increase with Fr but displays increasing and decreasing trends in different regimes of Fr. On the other hand, the collapse air entrainment coefficient initially decreases with Fr and then approaches to a constant. Similar trends of formation and collapse gas entrainment coefficient are observed for different cavitator sizes. Moreover, the introduction of unsteady gusts causes a slight monotonic increase in the formation and collapse gas entrainment

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

“…For instance, Karn et al [5], explored the ventilation hysteresis phenomenon in great detail and established that the ventilation demands to form and to sustain a supercavity may be significantly different, the latter being much smaller than the former. In a followup work, Karn et al [6] investigated the ventilation demands of the supercavity under various flow settings and provided a detailed explanation of the cavity formation and collapse processes, relating each with bubble coalescence proficiency and pressure balance near the closure. Their research has been conducted for a backward-facing model (BFM) with only a disk type cavitator.…”

“…Their detailed analysis on the momentum balance between the air injection and the estimated re-entrant jet at the closure further supported that the re-entrant jet governs the cavity collapse process. However, although it has been reported that the ventilation demand depends crucially on the flow unsteadiness [6], which may significantly alter the operation of the supercavitating object [3,12], the investigations on ventilation demand and ventilation hysteresis to date have been limited to the steady flow conditions only. Therefore, to connect the lab-scale experiments with the practical situations of underwater vehicles encountering surface waves, experimental investigations exploring the role of different cavitator shapes and mounting strut effects in unsteady flows is needed, not only to understand general cavity behaviors, but to investigate the underlying physics with an express emphasis on the ventilation demand and ventilation hysteresis.…”

“…Though mostly limited to the general cavity behaviors or the ventilation demands of the BFM supercavity, a few recent studies investigated ventilated supercavitation under unsteady flows by using a gust generator that consists of flapping hydrofoils [6,[12][13][14]. Such a setup was deployed to simulate unsteady incoming flows by controlling either the angle of attack ( ) or the flapping frequency ( ) of the hydrofoils.…”

“…They observed transitions across these states with a change in either or and further proposed a stability criterion for these state transitions. Karn et al [6] studied the formation and collapse ventilation demand trends with respect to the change in and for BFM and observed that all these demands increase with higher flow unsteadiness. They noted that such flows impose a vertical perturbation to the individual bubble movements that lead to the increased formation ventilation demand.…”

Interaction between Natural and Ventilated Cavitation around a Base Ventilated Hydrofoil

“…Air cavities surrounded by the vapor sheet could promote vapor growth, resulting in an increase in the cavity shedding frequency. Karn et al (2016) proposed that operation of a supercavitating underwater vehicle is driven by an interplay between the natural and ventilated supercavitation. Effect of these two distinct modes of supercavitation on each other during supercavity formation has been systematically discussed in their recent work (Karn and Chawdhary (2018).…”