Experimental Investigation on Damping Characteristics of a Vibrating Hydrofoil in Low-Order Modes

doi:10.3901/JME.2022.05.108

Abstract

Abstract: Added mass and damping effects are induced by the fluid structure interaction（FSI） between submerged structure of hydraulic machines and the environmental fluid, which have significant influences on the vibration characteristics of submerged structures. The damping characteristic parameters of the first four-order modes of a vibrating hydrofoil in air and water are measured,based on experimental tests. By inserting a piezoelectric patch（PZTp） on the leading edge of the hydrofoil to apply excitation, a laser Doppler vibrometer（LDV） and another PZTp（on the trailing edge） are used to simultaneously obtain vibration response signals and calibrate each other, and then a modal parameter testing system is constructed. A single mode shape measurement method is proposed by taking the signal obtained by the PZTp as the reference signal and synchronizing the multi-point response signals obtained by the LDV. Five kinds of commonly used damping ratio identification methods including logarithmic decay method, Hilbert transform method, resonance amplification method, half power bandwidth method and circle fitting method are analyzed. The results show that the accuracies and uncertainties of the five damping ratio identification methods are at the same level. After comprehensive comparisons, the logarithmic decay method is recommended for the free vibration response signal, and the half power bandwidth method is recommended for the forced vibration response signal. At the same vibration mode in air and water, the damping ratios of different measurement positions of the hydrofoil are nearly the same with the maximum deviation of 5.8%. The natural frequency reduction rates of hydrofoil in water range from 6.49%-26.73% as compare to air condition, and no difference in the mode shapes of test modes is observed. The damping characteristics of the vibrating hydrofoil in water are depended on the specific modes.Specifically, the damping ratios of the first bending, first torsional and first bending-torsion combined modes in water are all higher（13.09%, 8.61% and 14.34%, respectively） than those in air. However, the damping ratio of the second bending mode in water is 16.25% lower than that in air.

Key words: hydraulic machinery, hydrofoil, damping identification method, damping ratio, mode shape

CLC Number:

TV136

CENG Yong-shun, QI Xin, YAO Zhi-feng, WANG Jing-zhu, WANG Yi-wei, WANG Fu-jun. Experimental Investigation on Damping Characteristics of a Vibrating Hydrofoil in Low-Order Modes[J]. Journal of Mechanical Engineering, 2022, 58(5): 108-118.

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