On the numerical prediction of added damping and added mass of vibrating disc-like structures in heavy fluids

Abstract

Disk-like structures are common in hydraulic turbomachinery. These structures are prone to near-resonant vibrations. Accurately determining their modal parameters, however, is not a trivial task. Due to the submersion in a heavy, viscous fluid, the inertia and damping of the structure are significantly influenced by the fluid added mass and damping. As a step towards accurate vibration prediction, the added damping and mass of a vibrating, water-submerged disc with a variable axial distance from a rigid wall is numerically investigated. First, a computation approach of the added mass and added damping is derived based on the vibration-induced fluid reaction force. Using this approach, requirements on numerical flow simulation for accurate added damping prediction and related uncertainties are identified by validation against experimental results. Next, the numerical flow field is analyzed, exposing the vibration-induced fluid phenomena. Additionally, we propose a methodology to study the mechanisms of the added mass and added damping effects and their transfer from fluid to structure. As a result, a numerical configuration that accurately predicts the added mass and added damping in both trend and magnitude for a set of vibration modes, vibration amplitudes, and axial gap sizes is presented for the disc system. Moreover, the cause of the nonlinear behavior of the added damping force is revealed. We demonstrate that the phase lead of the fluid reaction force with respect to the structural oscillations increases with rising vibration amplitude, implying that both the added mass and added damping depend on the vibration level of the system.