Numerical investigations of the dynamics of the full load vortex rope in a Francis turbine.

The draft tube vortex rope occurring in Francis Turbines at full load is a potential cause of severe system instabilities, limiting the operating range of concerned hydro power plants. Studies focusing on physical mechanisms [1], methods for stability assessment based on numerical flow simulation [2, 3], studies on simplified configurations [4] and detailed numerical simulations [5] have been published. The pulsations of the rope volume being the core element of the system instability, the size and shape and dynamic evolution of the vortex rope predicted by numerical methods are essential in the validation of these methods. More experimental results [6, 7] of the study case underlying former numerical simulations [3], allow a review of these cases with improved boundary conditions, based on measurements instead of assumptions. On the other hand, the completed experimental database provides more validation cases to better understand the shortcomings of the numerical simulation techniques and identify the potential for improvement of predication capabilities. Advances in computational resources and optimization of the numerical parameters have allowed completing the range of comparisons of numerical versus experimental results close to the onset of instability. Results are compared in terms of the globally unsteady behavior as well as local pressure and velocity measurements at different locations in the draft tube.