Flow induced vibrations at stay vanes: Experience at site and CFD simulation of von Kármán vortex shedding

Experience in some existing hydro power plants demonstrates that vibrations due to Karman vortex shedding may cause fatigue cracking in the stay vanes of large turbines. This paper describes the approach of Ontario Power Generation (OPG) to fully understand the fluid dynamics affecting the stay vanes in one of their northern power plants, and the use of a finite element analysis (FEM) of the vane to understand the stresses, deflections and natural frequencies involved. Special emphasis is given to the CFD analysis of von Karman vortex shedding. In a first step a steady state analysis is carried out. This CFD analysis provides the velocity distribution at the trailing edge of the stay vanes which can be used to confirm the von Karman vortex shedding frequency. In a second step an advanced time dependent CFD analysis of vortex shedding is presented for a basic blade profile and two different geometries of the trailing edge including the dove-tail modification. The CFD results are discussed in comparison to basic test data with respect to three dimensional effects as well as the influence of the computational grid and turbulence modelling. Based on the elementary unsteady CFD analysis, some first CFD work is presented for the case of the OPG power plant. These results show qualitatively good results but leave open some questions about the quantitative accuracy, such as the prediction of three dimensional flow patterns and the pressure amplitude.