Solution ProcessA key aspect in this work is not only validation of the flow solver for utility scale turbines, but the development of a robust and easy to use modeling approach. To facilitate this requirement, we focus on a process that exploits 3-d CAD modeling, automated unstructured meshing, automated solution set-up, and automated post processing. The work process is illustrated below:

Validation of High Fidelity CFD Modeling Approach for Utility Scale Wind Turbines

Dave Corson - Altair Engineering, Inc. Paul Lees - PAX Streamline

Copyright © 2011 Altair Engineering, Inc. All trademarks are property of their respective owners.

AbstractA methodology is presented to use a commercially available finite element based flow solver, AcuSolve™, to study utility scale wind turbine aerodynamics employing advanced, high fidelity modeling techniques. The wind turbine studied here is based upon the NREL 5 MW offshore design. This machine uses a horizontal axis, 3-bladed rotor with a diameter of 126 meters. Due to the large size of this rotor, the simulations present challenges to CFD practitioners to develop accurate, efficient, and robust modeling approaches. Using the techniques described in this document, steady state Reynolds Averaged Navier Stokes (RANS) and fully transient Detached Eddy Simulation (DES) results were computed for a range of wind speeds and rotor RPMs. Rotor thrust, torque, and power were resolved and compare favorably to accepted results obtained from researchers at laboratories and academic institutions. Flow structures were also identified and compared for different wind speeds using the commercial post processing package, FieldView.

Modeling MethodologySimulations were constructed using the process described in Figure 1. A thorough mesh sensitivity study was performed to ensure grid independent results for the simulations. The final mesh for the full rotor contained approximately 13 millions nodes and 55 million elements. The following images illustrate the shape of the blade and a representative unstructured mesh.

Post Processing Using FieldView

AcuSolve Flow Solution

Unstructured Mesh Generator

3-d Solid Model of Turbine Geometry

Customizable Graphical User Interface

CAD based meshing ensures accurate representation of complex airfoils

Unstructured meshing technology operates directly on the underlying CAD model

Python scripting used to automate the set-up of the meshing controls, boundary conditions, and solver parameters

FVX scripting enables automated batch processing of CFD results

Figure 1: Flow chart illustrating the simulation process

Figure 2: Blade geometry for the 5 MW rotor models.

Figure 3: Unstructured mesh used for wind turbine simulations. Note the structured edge meshing on the leading and trailing edges as well as the anisotropic surface triangles. These techniques provide an efficient method of resolving the pressure field on the surface without the need to use structured meshing

Two operating points were selected for simulation. The lower power case corresponds to a 9 m/s wind speed and 10.3 RPM rotational speed. The higher power case entails a wind speed of 11 m/s and a rotational speed of 11.9 RPM. Steady RANS simulations using the Spalart-Allmaras turbulence model were performed for both conditions, while a full sliding mesh DES simulation was performed only for the lower power case. All simulations were performed using a 64 core AMD Opteron cluster with an Infiniband message passing network. Steady state simulations of the full rotor model required approximately 10 hours of compute time on the cluster to reach a steady state solution.

ResultsThe steady RANS solution provides detailed information about the performance of the rotor. The local pressure field on the high and low pressure side of the blade for the lower power case is shown in Figure 4.

Figure 4: Surface pressure distribution on the wind turbine blade.

The CFD solution is also successful at capturing the detailed flow structures in the wake of the turbine. Adequately capturing these features requires highly accurate numerical methods to propagate the wake downstream without the need to use excessive compute resources.

The power and thrust for each of the simulations was computed and compared to the results published by Riso1.

Figure 5: Flow structures in the wake of the 5 MW rotor models. The image on the left depicts iso-surfaces of the Q-criterion colored by velocity magnitude. This clearly illustrates the root and tip vortices as well as the trailing edge vortex sheet. The image on the right is a cut plane showing contours of vorticity in the wake of the rotor.

The AcuSolve results compare well to the Riso simulations, indicating that the unstructured meshing/finite element solution methodology provides accurate results for this application. Additionally, the DES approach is found to provide similar results as the steady RANS simulations. For this application, the additional compute cost of the DES approach is not warranted if integrated quantities such as power and thrust are the only items of interest. However, this also implies that the DES approach produces accurate results and can be used for inherently transient applications such as acoustic and fluid-structure interaction simulations.

ConclusionsAn unstructured grid based CFD modeling methodology has been developed and successfully used to simulate the flow around a utility scale wind turbine rotor. The total power and thrust predicted by the simulations compare favorably with results obtained by other research groups. To facilitate the ease of performing the full rotor simulations, all gridding was performed using fully automated unstructured mesh generation techniques, and post processing was performed using automated batch processing.

The wealth of insight provided by CFD simulations gives designers and engineers the opportunity to rapidly investigate advanced design concepts and establish the improvements in efficiency, reliability, and cost effectiveness that are required to propel wind power technology into the future. This validation effort represents an important step in achieving these improvements and can easily be extended to encompass more complex physics such as sheared wind conditions, gust events, and fluid structure interaction.

References1. UPWIND, Aerodynamics and aero-elasticity. Rotor aerodynamics

in atmospheric shear flows. Niels N. Sorensen. Wind Energy Department, Risoe National Laboratory, www.risoe.dtu.dk/rispubl/art/2007_140_paper.pdf.

Figure 6: Power and thrust comparisons between AcuSolve and Riso simulations.