Assessment of RANS turbulence closure models for predicting airflow in neutral ABL over hilly terrain

Abstract

AbstractImplementing wind farms in heights of a hilly terrain where wind speed is expected to be large may be viewed as a means to increase wind energy production without occupying fertile lands. Micro sitting of a wind farm in these conditions can gain dramatically from CFD simulation of fluid flow in the ABL above complex topography. However, this issue still poses tough challenges regarding the turbulence model to be used and the way to operate the near wall treatment in the presence eventually of separation. In this work, prediction capacity of RANS turbulence models was studied for a typical hill under the assumption of steady state and incompressible airflow regime in neutral ABL. Two models were analyzed by using COMSOL Multiphysics software packages. These included standard , and shear-stress transport . The most up-to-date procedures dedicated to near wall treatment were applied along with refined closer coefficients adjusted for the particular case of ABL. Considering wind tunnel test data, performance of the previous models was discussed in terms of converging mesh, computational time, reattachment point position and propensity of the model to retrieve the right level of turbulence flow in conditions of neutral stratifications. Then, a numerical simulation of the turbulent airflow over two slopes shapes of the symmetry hill by the validation of the experimental data has been then carried out. Both turbulence models agree well with air-velocity tested windward of the hills H3 and H5. Therefore, it was found that the standard model performs very well at the different positions of the low slope hill, and at the summit of a steep hill, but it over-predicts wind speed close to the wall, which requires an improvement of the near-wall treatment. However, the model in neutral case of the ABL was given consistent simulation results with experimental data for prediction of the flow separation and recirculation region at the leeward side of a steep hill, whereas standard model under the neutral condition and the model by using standard coefficients were failed to predict accurately detailed characteristics of recirculation region process.