Towards a general description of the cavitation threshold in acoustic systems

Abstract

Traditionally, the occurrence of cavitation has been related to the ratio between flow velocity and pressure gradient in the case of hydrodynamic cavitation, or some combination of vapor pressure and surface tension. However, both formulations present a large discrepancy with experimental data for cases in which cavitation is induced by acoustic waves. The present study aims to identify a more suitable cavitation threshold for such cases. The methodology adopted in this work consists of a combination of visualization with high-speed cameras and direct measurements using a hydrophone. The data collected confirmed that the vapor pressure is not a proper indicator of cavitation occurrence for an acoustic system characterized by high frequencies. The main reason behind the inability of vapor pressure to predict incipient cavitation in acoustic systems is that they evolve very quickly toward strong gradients in pressure, and the quasi-static assumptions used by traditional models are not valid. Instead, the system evolves towards a metastable state [Brennen, 2013], where the liquid exhibits an elastic behavior and can withstand negative pressures. A new cavitation number accounting for the tensile strength of the liquid was defined. An acoustic analogy is also proposed for the description, with the same framework, of an impulsive cavitation phenomenon.