Numerical Investigations of the Unsteady Flow in the Stuttgart Swirl Generator with OpenFOAM
Examensarbete för masterexamen
As a consequence of the current change in the electric energy supply structure and present available electric energy storage capabilities, hydro power plants are tending to be operated on a wide spread of off-design conditions. The operation of turbines not running at their best operating point could lead to a physical phenomenon called helical vortex or vortex rope. Although the helical vortex has been investigated for several decades already, it is still far from being completely understood. Moreover, it could have undesirable impacts on the hydraulic system of the power plant, namely, an increase of the risk of fatigue failure, the development of resonance vibration and a decrease of the efficiency of the power plant. In order to analyze the vortex rope and to assess its effects on the hydraulic system, it is therefore in the interest of both science and industry to simulate the physical phenomenon of the helical vortex accurately. In response to this, the Institute of Fluid Mechanics and Hydraulic Machinery at the University of Stuttgart, Germany, constructed a Swirl Generator to investigate a swirling flow, which is similar to the downstream of the turbine runners. With the help of eight non-rotating blades and a conical diffuser, this test rig is capable of generating a helical vortex. In this work, cases for the simulation of the unsteady flow in the Stuttgart Swirl Generator with OpenFOAM are presented. To do so, this work firstly explains the background theory of the helical vortex phenomenon and the method of Computational Fluid Dynamics. Furthermore, the underlying test rig, geometry and mesh, discretisation schemes, algorithm for interequation coupling, solvers for the systems of linear algebraic equations and boundary and initial conditions are elucidated. As the flow in the Swirl Generator is characterized by aReynolds number in a range of approximately 1.1*105 to 1.8*105 and unsteady flow features caused by thehelical vortex, the treatment of turbulence plays a vital role for an appropriate setup of the cases. Here, common Reynolds-Averaged Navier-Stokes (RANS) turbulence models are assumed to be limited in their capability of dealing with unsteady flows. The method of Large Eddy Simulation (LES) on the other hand, would be capable of addressing these drawbacks, but are still unfavorable in terms of their demands on the computational effort. This work therefore, investigates hybrid strategies, which combine the advantages of both the RANS and LES approach. For this, cases based on two standard high-Reynolds number RANS models: k-ε and k-ω SST, and three hybrid RANS-LES models:k-ω SST SAS, Spalart-Allmaras DDES and Spalart-Allmaras IDDES are set up. In order to assess the results of the simulations, the present work shows a general evaluation of the results and compares experimental and simulated measurement data.
Strömningsmekanik , Energi , Hållbar utveckling , Fluid mechanics , Energy , Sustainable Development