Vibration response of a waterjet propulsion pump under low-gas-fraction microbubble inflow

This study experimentally investigates the vibration attenuation effect of low-inlet-gas-volume-fraction microbubble inflow in a waterjet propulsion pump. Hydraulic performance and triaxial vibration responses at the inlet, impeller, and outlet were measured under IGVF values of 0–0.5%. The vibration signals were analyzed using root-mean-square values, fast Fourier transform, empirical mode decomposition, and the Hilbert–Huang transform. The results show that microbubble inflow has only a limited influence on the head coefficient and efficiency, while effectively reducing vibration responses at all measuring locations. The impeller region exhibits the highest vibration intensity and the strongest sensitivity to gas entrainment. Frequency-domain analysis indicates that the main spectral peak locations remain nearly unchanged, while the amplitudes of several prominent peaks and the broadband vibration response decrease under microbubble inflow. The EMD and Hilbert marginal energy analyses show that the first two IMF components account for most of the summed IMF energy and that microbubble inflow reduces the vibration signal energy across a broad frequency range. Additional pressure-pulsation measurements and pressure–vibration coherence analysis reveal frequency-dependent changes in selected hydraulic excitation components and their linear relationship with vibration. These results demonstrate vibration attenuation under the tested low-IGVF conditions and characterize the accompanying changes in pressure pulsations.