Numerical simulation of the hydrodynamic damping of a vibrating hydrofoil

Abstract

The periodic loads from Rotor-Stator interaction is believed to be the main fatigue contributor in High Head Francis turbines. The calculation of the structural response, and thus fatigue, is heavily reliant on the proper hydrodynamic damping characteristics of the water - structure system. The relationship between the water velocity and the hydrodynamic damping is also of great interest. To investigate this, the hydrodynamic damping characteristics of a submerged hydrofoil is simulated in ANSYS CFX. A one-way coupling is implemented, where the blade is forced to vibrate with the first bending mode at the natural frequency, while the hydrodynamic work is calculated over a vibrational period. The velocity of the flow over the hydrofoil is varied in the range v = 2.5 - 45 m/s. Two distinct damping regimes are observed depending on whether the vortex shedding frequency is below or above the lock-in region. The hydrodynamic damping is approximately constant before, and linearly increasing after this region. Experimental data from the Norwegian University of Science and Technology is available for validation, and shows the same trends. The sensitivity with respect to maximum vibrational amplitude is tested, and shows that the hydrodynamic damping is independent of the amplitude as long as the deflections are small.