Cavitation occurs whenever the pressure drops to the cavita- tion pressure pcav , and the pressure is constant and equal to the cavitation pressure p ¼ pcav in the cavitated region Sc . For simplicity, the cavitation pressure is assumed to be equal to zero, pcav ¼ 0. At the same time, the density of the cavitating fluid is lower than the (constant) density ϱ0 of the intact fluid. This can be compactly written in the form of the following complementarity conditions,