An experimental and numerical study on cavitating shear flows.

The dynamics of incipient and developed vortex cavitation was examined. The present work is divided into two parts: the first part deals with cavitation inception, that was studied numerically, and the second part deals with developed cavitation, that was studied experimentally. Incipient cavitation was studied by using a Direct Numerical Simulations (DNS) method for multi-phase flows developed by Unverdi and Tryggvason (1992). The interaction between non-cavitating and cavitating bubbles and the vortex flow was simulated. Incipient cavitation is difficult to observe experimentally because the minimum length scales of the cavitating flow are very small (the size of cavitation nuclei are often on the order of 10 mum) and because the nuclei capture process is stochastic and occurs at different locations in a turbulent flow. The numerical study analyzed the capture time of the bubbles, the bubble trajectory as it gets entrained by the vortex, the effect of the bubble and vortex core diameter ratio, the effect of the cavitation number, and bubble Weber number. The DNS results were compared with a simple one-way coupled particle-tracking (PT) model, using the equation of Johnson and Hsieh (1966) and the Raleigh-Plesset equation (Plesset, 1948). In the PT model different drag and lift coefficients were used in order to determine which model agrees well with the DNS results. It was determined that the drag coefficient given by Haberman and Morton (1953) gave a trajectory that was closer to the trajectory computed with DNS than the drag coefficient by Clift et al. (1978). A lift coefficient that is between the values given by Saffman (1965) and Dandy and Dwyer (1990) would give better comparison with the DNS results. Developed cavitation in a shear layer was studied experimentally in order to determine the effect that the growth and collapse of cavitation have on the dynamics of shear flows. Planar Particle Imaging Velocimetry (PIV) was used to measure the velocity field, the vorticity, strain rates and Reynolds stresses of the flow downstream of the cavitating and non-cavitating shear layer, and the flow pressures and void fraction were also measured. Cavitating shear flows with different cavitation numbers were compared to the non-cavitating shear flow. It was determined that the cavitating flows have higher levels of vorticity, strain rate, and Reynolds stresses in the core of the shear layer, but the differences were not significant. The average flow field was not significantly altered both in terms of average velocities and mean pressure drop across the test section. Downstream of the shear layer, the development of cavitation reduced somewhat the growth of the shear layer (by a factor of about 5%). The reduction of the shear layer thickness is consistent with the observations of Brown and Roshko (1974), Hermanson and Dimotakis (1989), and Belahadji et al. (1995).Ph.D.Applied SciencesMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132806/2/9990911.pd