  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.
