THE INFLUENCE OF IMPOSED STRAIN RATE AND CIRCULATION ON BUBBLE AND CLOUD DYNAMICS

Within the presented work the effects of rotation and strain on the bubble dynamics inside a cavitating cloud is analyzed and discussed. To follow this task, the mixture of cavitation bubbles and liquid is treated as a continuous medium. With the Helmholtz vortex theorem in mind a torus is considered to be preferable cloud geometry. In fact analyzing cloud cavitation horse show cloud structures are dominant and motivate the torus shape we focus on. The flow inside the cloud is treated quasi one-dimensional, the flow outside is modeled by a potential flow. The excitation of the cloud is carried out dynamical by a pressure distribution at infinity and/or kinematical by imposing a circulation and/or a temporal strain distribution. The research was performed in two steps: In a first step the influence of the time scales and void fraction on bubble growth, cloud breathing, cloud collapse delay, and work done on the cloud are considered. Based on a simplification of the governing equations the effects are divided in those of homogeneous, all bubbles in the cloud are equal sized in each time step, and heterogeneous bubble interaction. In a second step the influences of circulation and strain on these quantities have been analyzed. It was found that the circulation enhances the bubble growth in the center of the cloud especially for low void fractions. As consequences the cloud breathing (expansion of the cloud) and the work done on the cloud increased in comparison to the circulation free case, the collapses are delayed; the shocks moving to the center are attenuated. These effects depending on the level of circulation and therefor the density reduction in the cloud core region. The circulation, if the excitation time is high enough, strengths the acoustic load of the environment, represented by the far field pressure, and the maximum compression of the bubbles in the center. An imposed strain results in a density-specific vorticity production and therefor works like an increased circulation but independent of interaction and excitation time if the cloud related strain rate is constant. To bring it to a point: Strain and circulation increase the damage potential.