Prediction of vibration amplitudes on hydraulic profiles under von Karman vortex excitation

Abstract

The shedding of von Karman vortices on the trailing edges of turbine hydraulic profiles such as stay vanes, guide vanes and runner blades is a well-known mechanism of excitation for high frequency structural vibrations. Most commonly problems occur on stay vanes because the flow across the trailing edges is relatively uniform, and structural damping is low. Our analysis of the excitation mechanism under von Karman vortex shedding shows that in the vicinity of resonance the hydrodynamic damping becomes a non-linear function of amplitude potentially resulting in unacceptable limit amplitudes. Past studies have focused on frequency prediction, resonance avoidance and special trailing edge shapes to reduce dynamic forcing. On modern low head turbines resonance cannot always be avoided without undue hydraulic performance penalties. In our paper we demonstrate the applicability of a fluid structure interaction based approach to predict vibration amplitudes under von Karman vortex shedding. This type of prediction can be used to ensure that permissible amplitudes are not exceeded. We also present a strategy to achieve low vibration amplitudes.