Shamit Bakshi

An experimental investigation of cavitating structures in the near-wake region of a cylinder is presented. From high-speed imaging of this subcritical flow (Reynolds number of 64,000), it is found that inception of cavities occurs in the shear layer. At the developed cavitation condition, the cavities in the separated zone and the free shear layer merge. A distinct spanwise variation in cavitation activity is observed. The non-dimensionalized correlation length at inception varies from close to a non-cavitating value of about 3.5 to about 1 at developed cavitation. The non-dimensionalized length of formation, characterized by crossover of the free shear layer and the wake axis, increases from 1 to 1.8 as the cavitation number is reduced from 85% to 50% of the inception value. A frequency analysis of the cavity dynamics indicates that although the vortex shedding frequency is dominant in the shear layer, there are peaks corresponding to other frequencies in other flow regions. The presence of a sharp peak at 125 Hz, corresponding to a Strouhal number of 0.2, along with a range of frequencies, is also verified independently through measurement of fluctuating pressure at the cylinder surface.

Cavitation structures inside orifice of high pressure fuel injector nozzles have a major influence on internal flow and quality of spray. The present work provides a detailed understanding of the effect of injection pressure on cavitation and its shedding in the form of cloud cavities inside a cylindrical orifice. Commercial computational fluid dynamics (CFD) software, ANSYS Fluent with Reboud’s correction on the eddy viscosity term of k-ω SST turbulence model is used for the numerical investigations. Comparison of the results with literature on experimental studies shows that the employed modification of turbulent viscosity term by a user defined function helps in prediction of reentrant jet induced cavity shedding. Compressible, multi-phase simulations revealed non-cavitating, periodic cloud shedding, super cavitating and hydraulic flip regimes. Fourier transformation is performed on the time series data of fluctuating vapor fraction to predict the frequency of shedding of cavities for various cavitation numbers. It is shown that with a reduction in cavitation number, length of cavity increases and shedding frequency decreases.