ACOUSTIC RESONNANCE IN CAVITATION FREE AND CAVITATING FLOWS

At part load operation of Francis turbine, the swirl in the draft tube leads to flow instability known as vortex breakdown. This flow instability can interact with the rest of the hydraulic circuit through axially propagating plane-waves. Moreover, at low cavitation index , the gaseous rope is suspected to modify locally the propagation velocity. Acoustic models have been commonly used to tackle this problematic. In order to validate the parameters of those models, an experiment with equivalent phenomenology has been setup. The experiment is designed so that the flow characteristic are similar to draft tube surge, but with strong simplification in order to facilitate the study of the flow instability and its interaction with the acoustic field. The hydraulic circuit consists in a square pipe connecting two constant pressure reservoirs. The excitation mechanism is obtained by the shedding of vortices in the wake of a bluff body placed at ¾ of the pipe length. The excitation frequency can be adjusted through the flow velocity. To examine the influence of vapor cavity formed in the wake of the obstacle, the mean pressure inside the pipe is also adjustable. Without cavitation, the analyses of the pressure field along the circuit highlights the acoustic modes shapes and Eigen frequencies of the system. It is also demonstrated that the amplitudes along the circuit are strongly increased as the excitation frequency matches the Eigen frequency of the system. In a second step, the relation of the cavitation index with the Eigen mode shapes and frequencies has been systematically analyzed. A direct influence of the vapor cavity on the local propagation velocity is shown. Finally, at particular cavitation index, important amplification of fluctuations has been noticed. From the author’s point of view, it is a consequence of the vapor volume unsteadiness.