Spatiotemporal modal dynamics and nonlinear characteristics of pressure fluctuations in a centrifugal pump under off-design operation

Abstract

Off-design operation reorganizes centrifugal-pump pressure pulsations through interaction between the rotating impeller discharge and the stationary volute. To clarify how the non-uniform impeller-outlet pressure field is concentrated and modulated in the volute, unsteady computational fluid dynamics was performed at 0.8Qd, 1.0Qd, and 1.2Qd. Fast Fourier transform, proper orthogonal decomposition (POD), variational mode decomposition, sample entropy, permutation entropy, and Lempel–Ziv complexity were combined to characterize the frequency content, spatial organization, temporal scales, and complexity of the pressure response. The 1.2Qd condition produced the strongest pulsations. The impeller response was dominated by the shaft frequency fr = 29.17 Hz and its low-order harmonics, whereas the volute response was governed by the blade passing frequency fimp = 175 Hz and its harmonics. The first four POD modes captured 91% and 90% of the fluctuating-pressure energy in the impeller and volute, respectively. The impeller modes were spatially distributed and phase shifted, whereas the volute modes were localized near the tongue and impeller outlet. The dominant impeller intrinsic mode functions retained 96.2% and 96.5% of the decomposed energy and remained close to the shaft-frequency scale; the dominant volute components exceeded 85% and clustered near fimp and 2fimp. Complexity analysis revealed an ordered low-frequency backbone with low-energy high-frequency complexity in the impeller, but stronger complexity across the blade passing frequency, its harmonics, and higher-frequency scales in the volute. The volute tongue concentrates and modulates the non-uniform impeller-outlet pressure field in a region-dependent multiscale response.