Influence of non-rigid surfaces on the dynamic response of a submerged and confined disk

Abstract

Determining the dynamic response of submerged and confined disk-like structures is of interest in the hydraulic turbines field. Disks present similar dynamic characteristics than hydraulic turbine runners, especially in the mode-shapes associated with each natural frequency. The dynamic response of submerged structures is heavily affected by the added mass and damping as well as the proximity of solid boundaries. In all studies the solid boundaries are commonly considered completely rigid, however in real cases this is not always true. This fact occurs in some hydraulic turbines where the casing is not completely stiff and it may have some dynamic response in the frequency range of the runner, which may affect its dynamic behavior. To determine the influence of the casing stiffness on a vibrating disk submerged in water an experimental investigation has been carried out. A test rig consisting of a disk attached to a shaft inside a cylindrical tank closed with two different covers with different stiffness has been built up. The disk was impacted using a hammer and both disk and covers responses were measured with different accelerometers. Pressure sensors mounted on the covers have also been used. Natural frequencies of the disk have been obtained, compared and discussed for different disk to cover distances.