Simulating Cavitation and Hydraulic Flip Inside Hydroentangling Nozzles

Hydroentangling owes its success to the peculiar properties of coherent water jets. For hydroentangling to be feasible at higher pressures, it is extremely important that water jets maintain their collimation for an appreciable distance downstream of the nozzle. How ever, water-jet breakup accelerates at high pressures. Recent studies have shown that cavitation severely affects the integrity of high-pressure water jets. Investigating cavita tion experimentally is not trivial. Computational fluid dynamics simulations offer appro priate tools as a first step. This paper discusses the results of an unsteady-state simulation, which shows the inception and time-evolution of a cavitation cloud inside a hydroentan gling nozzle. Under certain conditions, the cavity cloud extends to the nozzle outlet, resulting in the so-called hydraulic flip. Once hydraulic flip occurs, cavitation suddenly vanishes because the downstream air moves upward into the nozzle and fills the cavity. This air envelops the water flow inside the nozzle, which results in the depletion of cavitation-induced instabilities from the jet surface and elongates the jet breakup length. Moreover, our simulations reveal the approximate time scales of cavity growth through the nozzle. This information is highly relevant for experimental visualization of nozzle cavitation. The discharge and velocity coefficient obtained from the simulation are in a good agreement with published experimental data.