Canadian machinery vibration association 1 OPERATIONAL MODAL ANALYSIS OF A STRUCTURE SUBJECTED TO A TURBULENT FLOW

The mechanical systems are subjected to various excitations during their operation. According to the type of excitation, the vibratory response differs too. Generally the excitations such as the repetitive shocks and random excitations excite the resonance frequencies of the system. In the special case of the hydraulic turbines, the blades operating under water are subjected to turbulent flows and mechanical constraints generating vibrations may be amplified when the resonance occurs. This fact may harm the state of the turbine blades subjected to the rupture due to the appearance of cracks and the slackening of the welding. This is reflected on the safety operation of the turbine, and affects notably its energetic efficiency. One can easily understand the importance of the knowledge of the structural resonance frequencies of the machine in order to specify a safety range of operating conditions. It is well known that the resonance frequencies of underwater structures are not the same when these structures vibrate in air because of the effects of the added mass and added damping. The first effect is the result of the presence of water itself or fluid in general, the second is caused by the fluid flows. However, if the identification of the parameters of the structures in air is well controlled nowadays, the identification of th