Numerical studies of the hydrodynamic damping of a vibrating hydrofoil in torsional mode

The vortex-induced vibration may lead to a premature failure of hydraulic mechanical systems, especially under the resonance condition in the torsional mode. To predict the structural fatigue life, a careful consideration of the dynamic response to the hydraulic excitations is essential in the design phase. This study focuses on the numerical investigation of the relationship between the flow velocity, the added mass and the hydrodynamic damping, particularly, with respect to a Donaldson-type hydrofoil, vibrating in the first torsional mode. A two-way fluid-structure interaction (FSI) method is used to predict above two parameters. The flow velocity is in the range of 0 m/s–20m/s. To evaluate the hydrodynamic damping ratio, an identification method is proposed, based on a modified version of the logarithmic decay method. The relative deviations of the simulated natural frequencies and hydrodynamic damping ratios as compared with the experimental data for the first torsional modes, are within 8.1% and 16.6%, respectively. The analysis results show that the added mass coefficient for the first torsional mode is in the range of 1.59–1.86, and is around 44% of that for the first bending mode. The trends of the boundary layer thickness and the wake width against the reduced velocity are found to be opposite to that of the hydrodynamic damping ratio. The theoretical equation for predicting the hydrodynamic damping ratio is modified, which is shown to be more reliable due to its consideration of the velocity independent hydrodynamic damping phase.