Bubbly Shock Propagation as a Cause of Sheet to Cloud Transition of Partial Cavitation and Stationary Cavitation Bubbles Forming on a Delta Wing Vortex.

Abstract

a) Bubbly Shock Propagation as a Cause of Sheet to Cloud Transition of Partial Cavitation: Upon changes in the flow conditions, partial cavitation in separated flows can transition from stable partial cavities to periodically growing and collapsing cavities accompanied by shedding of vapour clouds. Understanding of the mechanisms of transition from stable to shedding cavitation, a key contributor of surface erosion of propeller blades, and its relationship with underlying flow properties is crucial for effective design. In the present study, cavitation dynamics on a backward facing wedge in a re-circulating water tunnel is studied using time resolved X-ray densitometry. Different regimes of cavitation dynamics and point of transition from stable to shedding cavities are identified. Based on the findings, the study then focuses on the mechanisms of transition from partial stable cavities to periodic shedding of vapour clouds. From the experiments, presence of re-entrant flow and propagating condensation shock fronts are identified as mechanisms that are responsible for the shedding of vapour clouds for transitory cavities. For periodically shedding cavities, condensation shock induced shedding is found to be dominant mechanism. Upon further investigation of mechanism dynamics by pressure measurements the observed condensation shock dynamics is shown to be broadly consistent with simplified analysis using conservation laws. Cavity growth rate is found to be key flow process that dictates the type of the mechanism observed. b) Stationary Cavitation Bubbles Incepting on a Delta Wing Vortex: Vortex cavitation is usually the first observed type of cavitation in marine propellers and other lifting surfaces and a strong source of acoustic noise. The relationship between the single phase vortex properties and the observed bubble size, bubble dynamics, and acoustics is yet to be completely understood. A delta wing vortex with rapidly varying vortex properties and experiencing vortex breakdown, unlike a rolled up tip vortex, provides unique conditions to study vortex cavitation. The present study aims to relate the observed cavitation bubble dynamics and acoustics of vortex cavitation on a delta wing in a re-circulating water channel with the properties of the underlying vortical flow.