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978-3-8439-4772-5, Reihe Ingenieurwissenschaften
Numerical Study of Transition, Flow Separation and Wake Phenomena of Wind Turbines with Enhanced Hybrid RANS/LES Methods
257 Seiten, Dissertation Universität Stuttgart (2021), Hardcover, A5
Computational Fluid Dynamics (CFD) is becoming increasingly important in the aerodynamic design and assessment of wind turbines and their components. The state-of-the-art Reynolds-averaged Navier-Stokes (RANS) approach is established as reasonably accurate prediction tool for aerodynamic performance and loads, for basic operational conditions, where the flow around the rotor blades is attached.
However, the accuracy of RANS turbulence models is known to decrease significantly, at the borders of turbine operation, where flow separation can occur. An emerging approach to calculate detached flows are scale-resolved methods. Among them, the hybridization of RANS and Large-Eddy Simulation (LES) methods has become very popular, in view of industrial applications. The enhancement and application of hybrid RANS/LES methods for flows around wind turbines, as well as the study of stall and wake phenomena are the central topics of this thesis.
For this purpose, a novel shielding function for the interface definition between RANS and LES is developed, which prevents grid-induced detachment. Moreover, model extensions are implemented to improve the unnaturally slow development of turbulence after switching to LES mode. As a further component, different methods for transition prediction, which are based on transport equations, are integrated to the hybrid RANS/LES procedure. All methods are successfully validated with canonical test cases and aerodynamically relevant problems.
As a representative rotor application, the flow field of the "Mexico" experimental turbine is simulated with the developed methodology, and compared in detail with measured data. The highly resolved simulations provide valuable insights into the physics of separation phenomena and wake development.