Wavelet-based multiresolution analysis of cavitation-induced loading instabilities under phase effect in a waterjet pump

Abstract

As observed experimentally and numerically, the blade cavitation in a waterjet pump presents multi-temporal-spatial scales under the inlet guide vanes (IGVs) wake perturbations. Meanwhile, the phase effects on blade cavitation are specifically identified and confirmed to contribute to loadings and force spectrum instabilities. In light of this, the state-of-the-art multiresolution-proper orthogonal decomposition (MRPOD) and dynamic mode decomposition (MRDMD) are employed to separate the cavitating flow scales regarding to tonal- and broadband-spectrum (TS and BS respectively) to perform a space-time-frequency analysis. The results show that the surface cavitation on the adjacent blades globally evolves in a distinct way due to the phase difference between IGVs and rotor blades. As resolved by MRPOD, the TS oscillations are mainly arising around where cavitation dynamically develops, which is majorly attributable to the rotor-stator interference (RSI). The BS oscillations are in general emerging closing to hub regimes wherein usually deviate from the design operating point and perform more flow instabilities. Comparatively, the MRDMD identifies a frequency-jittering spectrum that slightly oscillates around the IGVs passing frequency, which is physically triggered by the cavitation-enhanced irregular loading impingements. The MRDMD relevant TS modes perform similar tendencies as MRPOD but alternatively oscillate with time around the leading edge due to the initial phase difference. The studies on pump cavitation dynamics accounting phase effects via multiresolution analysis (MRA) aim to gain a fundamental understanding of the cavitation-induced vibration to facilitate its control.