Experimental study on the effect of throat length in the dynamics of internal unsteady cavitating flow

Abstract

Cloud cavitation, both in external and internal flow fields, has been an active field of research because of its different harmful effects, such as noise, vibration, and material damage, in several applications. In the present work, the same is studied experimentally using venturi geometries. Venturi geometry was selected because of its diverse applications. The two venturi geometries chosen are nearly identical in all respect except the throat length. The influence of throat length is examined in this study because previously, these two venturi geometries (with and without throat) produced contradictory results in terms of the underlying mechanisms of cavity shedding, namely, re-entrant jets and condensation shocks observed at different cavitation numbers. Different diagnostic strategies were adopted to characterize cavitation events, viz., sound pressure level, dynamic pressure fluctuations, and high-speed imaging. High-speed images were studied to obtain mean cavity length. Proper orthogonal decomposition along with wavelet analysis was also employed. From these analyses, it was shown that for the venturi with 23 mm throat length, the condensation shock is followed by the re-entrant jet as cavitation number is reduced, while reverse is seen for venturi with zero throat length. Simulations of unsteady, non-cavitating, turbulent flow through these venturis show that this difference in the order of predominance of the two mechanisms can be explained by the product of cavity thickness (approximated by boundary layer height) and average pressure gradient value.