Dave Corson - Altair Engineering, Inc.                                                                                    ...
Two operating points were selected for simulation. The lower power        The power and thrust for each of the simulations...
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Validation of High Fidelity CFD Modeling Approach for Utility Scale Wind Turbines

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

  1. 1. Dave Corson - Altair Engineering, Inc. Paul Lees - PAX StreamlineValidation of High Fidelity CFD Modeling Approach forUtility Scale Wind TurbinesAbstractA methodology is presented to use a commercially available finite element based flow solver, AcuSolve™, to study utility scalewind turbine aerodynamics employing advanced, high fidelity modeling techniques. The wind turbine studied here is basedupon the NREL 5 MW offshore design. This machine uses a horizontal axis, 3-bladed rotor with a diameter of 126 meters. Dueto the large size of this rotor, the simulations present challenges to CFD practitioners to develop accurate, efficient, and robustmodeling 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. Rotorthrust, torque, and power were resolved and compare favorably to accepted results obtained from researchers at laboratoriesand academic institutions. Flow structures were also identified and compared for different wind speeds using the commercialpost processing package, FieldView.Solution Process Modeling MethodologyA key aspect in this work is not only validation of the flow solver for Simulations were constructed using the process described in Figure 1.utility scale turbines, but the development of a robust and easy to A thorough mesh sensitivity study was performed to ensure griduse modeling approach. To facilitate this requirement, we focus on independent results for the simulations. The final mesh for the fulla process that exploits 3-d CAD modeling, automated unstructured rotor contained approximately 13 millions nodes and 55 millionmeshing, automated solution set-up, and automated post processing. elements. The following images illustrate the shape of the blade andThe work process is illustrated below: a representative unstructured mesh. 3-d Solid Model of Turbine Geometry CAD based meshing ensures accurate representation of complex airfoils Customizable Graphical User Interface Figure 2: Blade geometry for the 5 MW rotor models. Python scripting used to automate the set-up of the meshing controls, boundary conditions, and solver parameters Unstructured Mesh Generator Unstructured meshing technology operates directly on the underlying CAD model AcuSolve Flow Solution FVX scripting enables automated batch processing of CFD results Post Processing Using FieldView Figure 3: Unstructured mesh used for wind turbine simulations. Note the structured edge meshing on the Figure 1: Flow chart illustrating the simulation process 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 meshingCopyright © 2011 Altair Engineering, Inc. All trademarks are property of their respective owners.
  2. 2. Two operating points were selected for simulation. The lower power The power and thrust for each of the simulations was computed andcase corresponds to a 9 m/s wind speed and 10.3 RPM rotational compared to the results published by Riso1.speed. The higher power case entails a wind speed of 11 m/s anda rotational speed of 11.9 RPM. Steady RANS simulations usingthe Spalart-Allmaras turbulence model were performed for bothconditions, while a full sliding mesh DES simulation was performedonly for the lower power case. All simulations were performedusing a 64 core AMD Opteron cluster with an Infiniband messagepassing network. Steady state simulations of the full rotor modelrequired approximately 10 hours of compute time on the cluster toreach a steady state solution. Figure 6: Power and thrust comparisons between AcuSolve and Riso simulations.Results The AcuSolve results compare well to the Riso simulations, indicatingThe steady RANS solution provides detailed information about the that the unstructured meshing/finite element solution methodologyperformance of the rotor. The local pressure field on the high and low provides accurate results for this application. Additionally, the DESpressure side of the blade for the lower power case is shown approach is found to provide similar results as the steady RANSin Figure 4. 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. Conclusions An unstructured grid based CFD modeling methodology has been developed and successfully used to simulate the flow around a utility Figure 4: Surface pressure distribution on the wind turbine blade. scale wind turbine rotor. The total power and thrust predicted by the simulations compare favorably with results obtained by otherThe CFD solution is also successful at capturing the detailed research groups. To facilitate the ease of performing the full rotorflow structures in the wake of the turbine. Adequately capturing simulations, all gridding was performed using fully automatedthese features requires highly accurate numerical methods to unstructured mesh generation techniques, and post processing waspropagate the wake downstream without the need to use excessive performed using automated batch processing.compute resources. 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. Figure 5: Flow structures in the wake of the 5 MW rotor models. References The image on the left depicts iso-surfaces of the Q-criterion 1. UPWIND, Aerodynamics and aero-elasticity. Rotor aerodynamics colored by velocity magnitude. This clearly illustrates the root and tip vortices as well as the trailing edge vortex sheet. in atmospheric shear flows. Niels N. Sorensen. Wind Energy The image on the right is a cut plane showing contours of Department, Risoe National Laboratory, vorticity in the wake of the rotor. www.risoe.dtu.dk/rispubl/art/2007_140_paper.pdf.

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