Hydrodynamic Damping Identification from an Impulse Response of a Vibrating Blade

In the present study, the identification of hydrodynamic damping is investigated in the case of 2D blade placed in the test section of the EPFL High Speed Cavitation Tunnel. A non intrusive technique, based on a spark generated bubble, is used to generate a wide band mechanical excitation. An underwater electric discharge at the bottom of the test section creates a fast growing and collapsing bubble which generates strong shock waves in the test section. The response of the blade to this excitation impulse is monitored with the help of a digital Laser Doppler vibrometer. Assuming a Single Degree of Freedom (SDOF) system, the hydrodynamic damping is estimated in the time domain by fitting an exponentially damped function on the impulse response envelope. Tests are performed for flow velocities ranging from rest up to 15 m/s. A linear relationship is found between the hydrodynamic damping and the reduced flow velocity for the first bending mode, while surprisingly a constant damping factor is experienced for the first torsion mode.