Visualization and measurements of shock waves in cavitating flow

Martin Petkovšek,Marko Hočevar,Matevž Dular

doi:10.1016/j.expthermflusci.2020.110215

Abstract

Upon cavitation cloud collapse an omnidirectional shock wave is emitted. It then travels through the flow field, causing a cascade of events resulting in erosion, noise, vibration and the cavitation shedding process.Despite that the accumulated data points evidently to the presence of the shock waves, the direct measurements hardly exist - and even then, they are very expensive and time consuming to perform.In the present paper, the possibility of detecting shock waves inside cavitating flow is shown.The methodology bases on using two conventional high speed cameras. With the first one cavitating flow from a distance is observed, determining the position of the wave, while the second camera with a microscopic lens enables a close-up view to determine the number and size change of air bubbles as a shock wave passed them. By calibration and reference measurements the amplitude of the shock waves was determined.This relatively simple approach enabled the first observation of shockwaves which occur at the cavitation cloud collapse (downstream of the attached cavity). Several examples of shock wave dynamics are shown and how they influence the general cavitation cloud behaviour. Shock wave front velocities and local pressure waves caused by cloud collapse were estimated from visualization, reaching values to more than 700 m/s and over 5 MPa respectively.

Highlights

Cavitation is characterized as inception, growth, and collapse of vapour-gas bubbles, due to the local change in the pressure
Developed hydrodynamic cavitation with periodic shedding of vaporous structures (Fig. 1) is one of the several cavitation types which can form in the low-pressure region of the flow tract
The flow separates in downstream and upstream part - the latter enters the attached cavity and upon losing the momentum, causes its separation - forming a detached cavitation cloud, which carries a significant amount of potential energy

Summary

Introduction

Cavitation is characterized as inception, growth, and collapse of vapour-gas bubbles, due to the local change in the pressure. In many cases, it is an unavoidable and undesirable phenomenon in hydraulic machinery, as it causes vibration, noise, deterioration of efficiency and even erosion of the elements of the flow tract. Developed hydrodynamic cavitation with periodic shedding of vaporous structures (Fig. 1) is one of the several cavitation types which can form in the low-pressure region of the flow tract. It is acknowledged, that two mechanisms govern the shedding process:. As the cloud is carried downstream, it enters a higher pressure region, due to which it collapses and emits shock waves in the order of MPa [3]

Methods

Results

Conclusion