Verification and validation of CFD simulations of the NTNU BT1 wind turbine

Results obtained from CFD simulations are sensitive to many numerical settings, such as grid spacing, time increment, turbulence models, etc. Therefore, rigorous quantification of the numerical uncertainties is necessary to obtain reliable solutions from CFD simulations. In the present study, the performance of the NTNU Blind Test 1 wind turbine is analyzed in the CFD simulations by using the CFD code ReFRESCO, and a thorough verification and validation (V&V) study is performed for the numerical predictions. To begin with, the methodologies leveraged in the current study, including the CFD approach and the adopted V&V procedure, are described in detail. Next, the two experiments adopted in the simulations, i.e. the experiment for the S826 airfoil and the NTNU BT1 experiment, are presented. Then, As a benchmark case, CFD simulations for a wing section of the NREL S826 airfoil under the Reynolds number (Re) of 1.0 x 105 at three different Angles of Attack (AoA) are performed. The simulation results are compared against the experimental data directly, and good agreement between the prediction and the measurement is achieved. Further, to quantify the spatial and temporal discretization uncertainties in the simulations targeting the performance of the BT1 wind turbine, a simulation matrix is established by using four systematically refined computational grids and four different time increments. In all the computational grids, the wind turbine geometry is fully resolved, including the blades, hub, nacelle, and tower. An inlet velocity of 10 m/s and a tip speed ratio (TSR) of 6 are used for the verification study. Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations with the k - w SST turbulence model are performed. The Moving-Grid-Formulation (MVG) approach with a sliding interface technique is leveraged to handle the rotating turbine blades. By applying a modern verification procedure to the numerical predictions, the spatial and temporal numerical uncertainties of the predicted thrust (CT) and power (CP) coefficients are determined. Finally, simulations are performed for a range of TSRs, and a validation study is carried out by comparing the CFD results to the experimental data. It is observed that the validation for CP is achieved in the entire range of TSR, while failed for CT at high TSRs.