Examination of ventilated cavities in the wake of a two-dimensional bluff body

TL;DR

This study experimentally investigates ventilated cavities behind a 2D bluff body, identifying different cavity closures and their governing conditions, revealing the influence of flow parameters on cavity geometry, gas ejection, and hysteresis effects.

Contribution

It provides a systematic regime map of cavity closures based on flow velocity and ventilation, and analyzes gas ejection mechanisms and hysteresis in cavity formation and maintenance.

Findings

Four stable cavity closure types identified.

Cavity closure depends on Froude number and ventilation coefficient.

Ventilation hysteresis affects gas flux requirements.

Abstract

Ventilated cavities in the wake of a two-dimensional bluff body are studied experimentally via time-resolved X-ray densitometry. With a systematic variation of flow velocity and gas injection rate, expressed as Froude number ($Fr$) and ventilation coefficient ($C_{qs}$), four stable cavities with different closures are identified. A regime map governed by $Fr$ and $C_{qs}$ is constructed to estimate flow conditions associated with each cavity closure type. Each closure exhibits a different gas ejection mechanism, which in turn dictates the cavity geometry and the pressure inside the cavity. Three-dimensional cavity closure is seen to exist for the supercavities at low $Fr$. However, closure is nominally two-dimensional for supercavities at higher $Fr$. At low $C_{qs}$, cavity closure is seen to be wake-dominated, while supercavities are seen to have wave-type closure at higher $C_{qs}$, irrespective of $Fr$. With the measured gas fraction, a simple gas balance analysis is performed to quantify the gas ejection rate at the transitional cavity closure during its formation. For a range of $Fr$, the transitional cavity closure is seen to be characterised by liquid re-entrant flow, whose intensity depends on the flow inertia, dictating the gas ejection rates. Two different ventilation strategies were employed to systematically investigate the formation and maintenance gas fluxes. The interaction of wake and gas injection is suspected to dominate the cavity formation process and not the maintenance, resulting in ventilation hysteresis. Consequently, the ventilation gas flux required to maintain the supercavity is significantly less than the gas flux required to form the supercavity.