The simulation of unsteady cavitating flows with external perturbations

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In this study, the unsteady cavitating flows arising from the abrupt or gradual changes in the freestream flow speed and emergence of cavitator are simulated. The flows are governed by the one-fluid compressible Euler equations with cavitation phenomenon modelled by the cut-off cavitation formulation. The two-dimensional (2D) axisymmetric cavitating flows around a cylinder with a blunt or sharp-edged head are resolved to investigate the cavity evolution associated with the freestream flow speed variation and emergence of cavitator, respectively. The numerical study indicates that the cavity over the cylinder may collapse with an abrupt and severe change of the freestream flow speed. A very strong pressure wave can be generated following the cavity collapse. However, if the freestream flow speed is gradually increased, the original cavity may be sustained without complete collapse. For a given amplitude of speed increase, the longer response time allowed for the change corresponds to a slower and smoother deformation of the cavity. On the other hand, the larger amplitude of speed variation leads to a more significant deformation during the cavity evolution for a given response time. Separately, an emerging cavitator is introduced to the cylinder head to control the cavity development for a given flow speed. It is found that the introduced cavitator can amplify the original cavity. The emerging cavitator may change a partial cavity to a supercavity and therefore potentially reduces the friction drag on the object.