Effects of ambient pressure on the frequency characteristics of self-excited cavitating waterjet

Abstract

The effects of ambient pressure on the self-excited cavitating waterjet(SECJ)’s frequency characteristics were experimentally studied under different operating conditions and nozzle resonant chamber geometries. The frequency characteristics were obtained by analyzing the waterjet’s dynamic pressure fluctuation and noise. Experimental results indicate that the fundamental self-excited frequency and its harmonics increase as ambient pressure rises from 0 to 5.5 MPa (corresponding 0 - 500 m water depth) under constant pressure drop ΔP=25MPa, which results in a higher-order resonance mode of the jet. The dimensionless parameters, including the Strouhal number Sd and cavitation number σ, were introduced to uncover ambient pressure’s influence mechanism. Dimensional analysis results indicate that the ambient pressure alters the self-excited frequency and modulates the frequency characteristics by changing the Strouhal number. The relationship between SdN and σ can be fitted as a power function SdN=Naσb (N is harmonic order, a and b are constant-coefficient). Based on this law and the mechanism of SECJ, the ambient pressure’s effects on the frequency characteristics can be predicted, which may be used to guide the optimization of SECJ system under a high ambient environment.