Evaluation of adaptive, unstructured CFD calculations of the flow around the DU91 airfoil

In this paper we investigate the application of an adaptive meshing algorithm in Computational Fluid Dynamics (CFD) for the computation of the flow around a wind turbine airfoil. We use the DU91 wind turbine airfoil, which is a 25% thick airfoil, found typically at the mid-span section of a wind turbine blade. Wind turbine applications typically involve unsteady flows due to changes in the angle of attack and to unsteady flow separation at high angles of attack. In order to obtain reasonably accurate results for all these conditions one should use a mesh which is refined in many regions, which is not computationally efficient. Our solution is to apply an automated mesh adaptation technique. In this paper we test an adaptive refinement strategy developed for unstructured hexahedral meshes by NUMECA Int. for steady flow conditions. Local flow sensors for pressure, velocity and density determine the regions in the mesh to be refined or coarsened. This way the mesh is refined only in those regions necessary for high accuracy, retaining computational efficiency. A verification and validation study is performed on a fixed mesh for three angles of attack: -3.121°, 1.00° and 8.21°. These results are compared with experimental data and with the results obtained with the adapted meshes for two of these angles of attack. From these computations it can be concluded that for a given computational time, adapted meshes result in solutions closer to the experimental data compared to the results obtained with non-adapted meshes.