Time - Resolved CFD Simulation of a Turbulent Atmospheric Boundary Layer Interacting with a Wind Turbine
The present article describes results of an unsteady CFD simulation of the generic 5MW NREL turbine under unsteady atmospheric inflow conditions. Thereby, the blade loads are evaluated and the wake development is investigated.
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Abstract. The present article describes results of an unsteady CFD simulation of the generic 5MW NREL turbine under unsteady atmospheric inflow conditions. Thereby, the blade loads are evaluated and the wake development is investigated.
1 Introduction Wind turbines in operation are exposed to the turbulent atmospheric boundary layer. The boundary layer is characterized by large scale eddies, shear flow and velocity variation over time and height. These characteristics strongly affect the blade loads as the relative angle of attack changes permanently. The change in angle of attack causes unsteady nonlinear blade load response in the linear region of originally steady aerodynamic polars which should be considered for reduced frequencies higher than 0.04 [1]. Further the large scale turbulence interacts with the turbine wake and the wake begins to meander [2]. Commonly used for wind turbine simulations in an atmospheric boundary layer is the Actuator Line Method in combination with a LES [3] [4] which uses two dimensional airfoil data in combination with 3d models to determine the three dimensional turbine flow. However, due to the fact of modeling when using the Actuator Line Method only CFD methods calculating directly the unsteady pressure distribution can consider unsteady three dimensional and nonlinear effects physically correct. For this purpose the Institute of Aerodynamics and Gas Dynamics (IAG) performs CFD simulations of fully meshed wind turbines in an unsteady atmospheric boundary layer. The results should be used to better predict turbine loads and improve the understanding of the near wake flow and the interaction of turbines with atmospheric K. Meister · Th. Lutz · E. Kr¨amer Institute of Aerodynamics & Gas Dynamics (IAG), University of Stuttgart, Pfaffenwaldring 21 70569 Stuttgart e-mail: {meister,lutz,kraemer}@iag.uni-stuttgart.de M. H¨olling et al. (eds.), Wind Energy – Impact of Turbulence, Research Topics in Wind Energy 2, c Springer-Verlag Berlin Heidelberg 2014 DOI: 10.1007/978-3-642-54696-9_28,
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K. Meister, Th. Lutz, and E. Kr¨amer
turbulence. In doing so this article describes the numerical model and simulation results of the generic NREL 5MW turbine that has been used as reference turbine in the European UpWind and is improved in the ongoing OFFWINDTECH project [5].
2 Description of the Numerical Model The simulations are hybrid RANS / LES simulations (DES) and are performed by using the CFD solver FLOWer, which is provided by the German Aerospace Center (DLR) and was successfully used for wind turbine simulations during the MexNext project [6]. DES means, that the blade boundary layer is calculated in RANS mode with statistical turbulence modeling. The atmospheric boundary layer is simulated in LES mode with temporal and spatial resolution of the relevant large scale turbulent structures, except a thin wall layer where the used DES 97 method switches to RANS. In comparison to prior simulations [7] the time step is reduced to 0.04 s (≈ 2.9 ◦ ), the distance from the inflow to
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