A general description of the cavitation threshold in acoustic systems

Traditionally, the cavitation threshold is defined by some combination of vapor pressure and surface tension. However, these formulations present a large discrepancy with experimental data for cases where cavitation is induced by acoustic waves. The present study aims to identify a more suitable cavitation threshold for such cases. The method adopted in this work consists of a combination of visualizations with a high-speed camera and direct measurements using a hydrophone. The data collected confirmed that 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 toward a metastable state [Brennen, Cavitation and Bubble Dynamics, 2013], where the liquid exhibits an elastic behavior and can withstand negative pressures. A new cavitation number is defined to properly describe the phenomenology of acoustic cavitation. However, to provide a complete description of the cavitation threshold, future experiments, in a wide range of acoustic frequencies, are still necessary. An acoustic analogy is also proposed for the description, with the same framework, of an impulsive cavitation phenomenon.