Computational Fluid Dynamics

With a full CFD simulation usually a Reynolds Averaged Navier Stokes (RANS) calculation for blade boundary layer inflow is meant which solves the flow equations in the full three dimensional space surrounding the turbine. These methods do not require the input of aerodynamic coefficients and the full three-dimensional shape is to be prescribed. Turbulence is solved using a turbulence model, which relates turbulent velocity fluctuations to gradients in the flow. This involves closure relations that are tuned depending on the flow topology of interest and the numerical stability required. The relatively crude modeling of turbulence and the overly dissipative nature of these turbulence models sometimes favor the application of even more advanced methods like LES (Large Eddy Simulation) or DES (Detached Eddy Simulation; RANS for boundary layer flows, LES for the remainder) type models for resolving wake flow. Although these are the highest fidelity models available, they have shown to not always predict the onset of airfoil separation accurately. However, as was shown in IEA Task 29 [Schepers and Boorsma, 2012], for more complex load cases involving yawed flow or separated flow these methods clearly outperform lower fidelity models.

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