Onset of Ultrasonic Cavitation in Tap Water

Abstract

The conditions influencing the onset of acoustically-induced cavitation in tap water have been investigated, with special attention to the role of air-filled nuclei. Cavitation was induced by exciting the water, contained in a spherical flask, at the resonant frequency of a radial mode near 25 kc/sec. Air-filled nuclei were detected by observing the reverberation decay of sound in the sphere at frequencies from 150 to 700 kc/sec, the presence of undissolved air cavities causing an increase in the decay rate. The results indicate that tap water standing undisturbed for several hours contains no air nuclei with radii larger than 5 × 10−4 cm, even when the water is supersaturated with dissolved air. The smaller nuclei (not detectable by reverberation decay) seem to become progressively smaller, for the measured threshold sound pressure at the onset of cavitation increases several-fold as the water stands undisturbed for several days. The observed influence of the air content on the threshold peak sound pressure Pt is given by Pt≃A1 − A2Pe where Ps, is the equilibrium pressure of the dissolved air, and the constants have the values A1=612 atmos, A2=4, at 22°C and one atmosphere static pressure. The variation of the threshold pressure with varying static pressure P0 depends on the direction of the pressure change; in the increasing direction, dPt/dP0 is larger than it is in the decreasing direction.