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Aspects of the numerical simulation for the flow in penstocks

The flow in a full-scale penstock has been computed. Characteristic of this flow is the very high device Reynolds-number, which ranges from up to 100 millions. The results show that for the numerical simulations special care has to be applied to mesh, boundary conditions, turbulence models and numerics. A good understanding of the models and assumptions involved in the CFD is necessary, especially for the wall function, which is used in order to reduce the wall resolution in the mesh. Recent research results show that the parameters of the law of the wall at high Reynolds numbers change compared to the established values at typical turbulent conditions (device Reynolds-Number up to 1 million). Standard procedures can therefore lead to an incorrect computation of the wall shear stress, thus strongly affecting the results (pressure drop, velocity profile, secondary flow). Mesh quality has been identified as a key parameter for this kind of flows. The very low viscosity present in the flow can lead to instabilities, which can be of physical or numerical nature. Both affect the convergence of steady state simulations, thus making a transient approach necessary. In the paper numerical and physical aspects for this kind of simulations and flows are included. The numerical data is then evaluated in order to assess the impact of flow distortion on ultrasonic measurements, with the goal to improve the accuracy of the measured integral predictions.