Analyzing the vibration characteristics of an underwater rotating propeller is crucial for predicting the structural damage of the propeller and preventing the failure of an entire propulsion system. The laser Doppler vibrometer (LDV) measurement technology is a good alternative to conventional contact sensors for the analysis of underwater structures. In this paper, a tracking LDV technique for the vibration measurement of propeller blades that rotate underwater is presented. The underwater vibration measurement of a rotating propeller is affected by the environmental conditions of the water, cavitation, and turbidity. The purpose of this study is to investigate the effects of underwater environmental conditions on the vibration measurement of a rotating propeller with the tracking LDV measurement technique. The vibration response measurements were performed in a water environment, and cavitation bubble and turbidity effects were simulated. To improve the quality of the measurement signal, the PCA-based feature extraction technique was used to remove abnormal signals including noise and irrelevant components and determine the natural frequency of the marine propeller. The final results show that the tracking LDV method with PCA-based feature extraction is suitable for in-situ health monitoring of rotating objects in real underwater environments.

Investigation of underwater environmental effects in rotating propeller blade tracking laser vibrometric measurement

Taking significant merits of high spatial resolution of image sensing, the vibration mode shape of a cantilever beam (CB) is obtained from the free vibration process for local crack identification and structural health monitoring. This vision-based vibration measurement uses a cost-effective rolling shutter camera and a computationally efficient singular value decomposition (SVD). It shows that vibration modes of a continuous structure such as a CB are mainly determined by spatial resolution and less rely on temporal resolution. Moreover, images recorded by the camera maintain the critical information about mode profiles though distortions are introduced into the image. Decomposing the images by SVD allows the vibration modes to be extracted with good degree of accuracy. Both simulation study and experimental verification shows that this novel approach allows different sizes of cracks and locations to be identified.

Vision-based vibration mode shape extraction for identifying cracks in a cantilever beam

The design of lifting blade shapes is a key engineering application, especially in domains such as those of marine propellers, hydrofoils, and tidal energy converters. In particular, the excitation frequency must be different from that of the structure to avoid resonance. The natural frequency in the cases where the fluid–structure interaction (FSI) is considerably different if considering the coupling added mass (AM) of the water. In this study, vibration experiments were performed using a real propeller in air and water. The modal parameters, natural frequencies, and mode shapes were determined. Validations were performed using 3D solid and acoustic elements in a direct coupling finite element format. The modal results and AM ratios were in agreement with the experimental results. Convenient application and high efficiency are basic requirements for an engineering application. Therefore, an empirical formula was established for the first-order FSI natural frequency to enable rapid estimation, thereby satisfying this requirement.

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Fluid–Structure Interaction Vibration Experiments and Numerical Verification of a Real Marine Propeller

Based on the homodyne detection, a compact and cost-effective all-fiber laser Doppler vibrometer (LDV) with high resolution is presented. For the signal processing, the discrimination algorithm combined with the nonorthogonal correction is applied. The algorithm corrects the quadrature imbalance and other nonlinearity. In the calibration experiment, with the glass pasted on a piezoceramic transducer (PZT), the velocity resolution of 62 nm/s at 4 kHz and displacement resolution of 2.468 pm are achieved. For the LDV-based acousto-optic communication, the minimum detectable sound pressure level (SPL) reached 0.12 Pa under the hydrostatic air-water surface. The results demonstrate that the designed homodyne LDV has a low system background noise and can offer high precision in the vibration measurement.

https://www.mdpi.com/1424-8220/20/20/5801/pdf

Development of a High-Resolution All-Fiber Homodyne Laser Doppler Vibrometer.

Discussion de l'application du controle des vibrations considere comme une partie des programmes d'entretien, et description de plusieurs methodes incluant en particulier les techniques de controle continu ou periodique. Importance du choix des transducteurs de vibration en fonction du type specifique de machine. Necessite d'une interpretation par specialiste des donnees de vibration afin d'identifier les defauts de fonctionnement dont plusieurs exemples sont donnes

Dong-Ho Kim

Papers

Underwater vibration analysis method for rotating propeller blades using laser Doppler vibrometer