Unsteady Cavitation in Separating and Re-attaching Shear Flows

Abstract

The unsteady cloud cavitation is a significant source of unwanted noise, erosion and performance deterioration in hydrodynamic applications. Despite previous experimental and numerical studies on sheet to cloud transition in partial cavities the role of underlying flow features is yet to be fully explored. Previous studies have reported that the transition from sheet to cloud cavitation is caused either by re-entrant liquid flow or by bubbly shockwaves. The generation of liquid re-entrant flow is governed by kinematics of the incompressible liquid flow, while occurrence of bubbly shockwaves is related to the compressibility of the vapor-liquid mixture. To model and predict cavitating flows with bubbly shockwaves, an understanding of the barotropic relation and hence the speed of sound is required. The first part of the thesis addresses the validity of homogenous mixture models of speed of sound (‘frozen’ and ‘equilibrium’) under two extreme thermodynamic assumptions, in predicting occurrence of shockwaves in cavitating flow behind a backward facing step. The second part of the thesis explores the relative importance of either re-entrant flow or shockwaves to cause cloud shedding on a NACA0015 hydrofoil and the relation to mixture compressibility. High speed cinematography, x-ray densitometry and pressure measurements (static and unsteady) are primary tools of investigation.

The backward facing step provides a canonical flow configuration for examining developed cavitation in separating and re-attaching shear layers. Three types of cavitation regimes at different inlet cavitation numbers are designated as type A, B and C. Type A cavitation occurs in the streamwise vortices and mean void fraction values are low. Periodic self-sustained shedding occurs for type B cavities where bubbly shockwaves appear. Type C cavities are similar to the type B cavities but occur at a lower shedding rate. The shockspeed estimated from 1D Rankine Hugoniot jump relation aligns well with shockspeed measured using X-T diagrams of void fraction values. The homogeneous frozen model of speed of sound predicts subsonic to supersonic transition in Mach number (based on flow velocity at step) that aligns well with the experimentally observed onset of bubbly shockwaves when transitioning from type A to B cavitites. The homogenous equilibrium model on the other hand overpredicts the Mach number by two orders of magnitude.

Partial cavity shedding dynamics on a NACA0015 hydrofoil are studied and the underlying flow features are visualized. Based on these flow features the cavities are classified as type(i) to (iv) in the increasing order of cavity length. Type(i) oscillatory cavities have shed vapor cloud from the aft of the cavity. Both re-entrant liquid flow and shockwaves cause cavity pinch-off from leading edge for type(ii) cavities with no preference in dominance of either flow front. Shockwaves dominate as a shedding mechanism for type(iii) and (iv) cavities, while re-entrant flow can also cause pinch-off in a few cycles. Unlike type(iii) cavities, the trailing edge cavitation in type(iv) cavities actively interacts with the main cavity leading to alternate lobe cavitation similar to near wake cavitation in the wake of a bluff body. The relative importance of re-entrant flows and shockwaves is quantified using event-probability plot. The effect of compressibility is assessed by estimating Mach number based on inlet flow velocity and frozen model of soundspeed. When Mach number exceeds unity the probability of shockwaves to cause pinch-off increases to 0.6.