Meshing and CFD Simulations of Wind Turbines

The first requirement for a CFD simulation is the discretization of the fluid domain. Generating a high-quality grid is no trivial matter. It is an even harder task if the geometry to be discretized presents a complicated shape such as a wind turbine blade.

At Fraunhofer IWES, we introduced two fully automated structured mesh generation tools for CFD simulations of wind turbines, namely bladeBlockMesher and windTurbineMesher. These tools were designed and developed to reduce the complexity of the mesh generation process and, moreover, to enable users to perform efficient CFD simulations of a complete wind turbine rotor. Please find hereafter a few images to illustrate these methods. Furthermore, a complete mesh of the NREL phase VI turbine can be found on the Fraunhofer Gritlab wepage.

Rectangular domain of turbine with tower created with windTurbineMesher

Half-spherical domain for simulation of different inflow directions

Surface-grid of blades close to the hub (created with bladeBlockMesher)

Refinement of the turbine wake (created with windTurbineMesher)

At Fraunhofer IWES, different approaches to simulate wind turbine rotors are used: from high fidelity blade-resolved CFD simulations to medium fidelity methods such as the actuator line. The latter are mainly used to investigate the aerodynamic loads, flow behavior, and wake characteristics of wind turbine rotors or for validation of BEM (Blade Element Momentum) simulations.

The blade-resolved CFD simulations can include the tower and nacelle geometry. Different methods can be used for solving the flow equations, these range from computationally cheap RANS (Reynolds-Averaged Navier-Stokes) to eddy-resolved ones such as DES (Detached-Eddy Simulation), DDES (Delayed Detached-Eddy Simulation), and IDDES (Improved Delayed Detached-Eddy Simulation). An example case of the steady simulation of the NREL phase VI turbine can be found on Fraunhofer Gitlab webpage.

 

DDES of a model research turbine

DDES of the NREL 5MW turbine in downwind configuration with lattice tower subjected to turbulent inflow

In the case of complex load cases such as standstill situations or extreme yaw cases, wind turbine design tools based on BEM clearly operate outside of their validated range.

Complex flow situations involving very large angles of attack or significant vortex shedding therefore require higher-fidelity aerodynamic models such as CFD. At the same time, it is possible to observe a strongly non-linear interaction between the aerodynamics and the flexible blade structures.

In order to investigate such cases, Fraunhofer IWES developed a fluid-structure coupled CFD solver, which can be used to simulate the aero-elastic performance of complete rotors or single blades under extreme conditions. Coupling OpenFOAM’s flow solvers with a non-linear beam model based on the geometrically exact beam theory, the high-fidelity solver can also compute complex blade designs involving cross-sectional material couplings as bend-twist coupling.

Deformation of the NREL 5MW blade under standstill conditions

The actuator line method can be used when the characteristics of the wake are studied, and details of the blade aerodynamics are less important. This method offers a relatively fast solution even when the wakes of a complete wind farm are investigated. For the actuator line method, we use Large Eddy Simulations (LES).

Comparison of a 5MW turbine under turbulent inflow using LES with actuator line and blade-resolved simulations: