Comparison of Computational and Experimental Unsteady Cavitation on a Pitching Foil

Abstract

Comparisons are made between a nonlinear method for predicting unsteady sheet cavitation and available experimental data for a pitching foil for the purposes of verifying the calculations and to further analyze the flow. A dynamical approach is employed in which the form of the instantaneous cavity surface is modeled as a semiellipse. The cavity length (major axis), thickness (semiminor axis), and position are determined such that the nonlinear cavity-surface boundary conditions are satisfied approximately. The pressure on the instantaneous cavity surface is prescribed using an unsteady thick-foil potential-flow method based on Green’s second identity. The computational method yields best results in predicting the cavity dynamics, but underpredicts the cavity length. For fixed cavitation number, mean foil angle, and pitch amplitude, the cavity dynamics, such as maximum cavity size and cavity surface behavior, are shown to depend on the ratio of the cavity natural frequency for the foil fixed at the maximum pitch amplitude to the foil reduced frequency. For a certain value of this ratio, the cavitation response is shown to be a minimum. The experimental results confirm the computational trends up to the point that experimental data were obtained.