Numerical prediction of the fluid damping of a standing disc with a variable axial distance from a rigid wall

Abstract Near-resonant flow-induced vibrations are a significant cause of fatigue damage in hydraulic impellers. The amplitude of these vibrations is limited mainly by the fluid damping, making its quantification paramount for the safety of hydraulic turbines. To study the fluid damping for low-specific-speed radial impellers, a submerged disc in still water with a variable vertical distance from a rigid wall is investigated. This setup serves as an abstraction of the axial gap between impeller hub and head cover, and of the modal behavior of impellers for the case of diametrical nodal lines. The fluid damping is computed numerically for the 3-nodal-diameters mode using the imposed modal motion approach for various axial gap sizes and vibration amplitudes. The results are contrasted with the experimental damping measured with the same setup. First, a simulation configuration that accurately replicates the experimentally measured damping in value and tendency is presented. The factors that would normally impede a valid numerical prediction are exposed and solutions are suggested. Second, the valid simulation results are thoroughly studied, and the flow field is visualized to gain insight into the physical mechanisms that cause the fluid mass and damping. As a result, the interaction between fluid and structure in this system is understood.