High-fidelity CFD simulations for the wake characteristics of the NTNU BT1 wind turbine

Abstract

In this work, we present the results of the wake characteristics of a model-scale horizontal-axis wind turbine (HAWT) obtained from high-fidelity CFD simulations. A thorough verification and validation (V&V) study is performed to quantify the numerical uncertainties in the CFD results. The RANS equations with the k−ω SST turbulence model are adopted in the present study. The geometry of the HAWT is fully resolved including the blades, hub, nacelle, and tower. The Moving-Grid-Formulation (MVG) approach with a sliding interface technique is leveraged to handle the relative motion between the rotating hub and turbine blades and the stationary tower and nacelle. First, to evaluate the spatial and temporal discretization uncertainties in the predicted wake characteristics, a simulation matrix consisting of three systematically refined grids coupled with three different time increments is established. In all the simulations, a tip speed ratio (TSR) of 6 and an inlet velocity of 10 m/s are specified. The numerical uncertainties for the predicted velocity and turbulent kinetic energy at different downstream locations are then obtained by applying a systematic verification procedure to the CFD results. Second, a validation study is carried out by comparing the CFD results against the experimental data. It is shown that the CFD predictions are in good agreement with the measurements. In addition, the details of the wind turbine wake are visualized and discussed. It is found that the tower wake is skewed by the rotating turbine blades. As the result, the wake profiles at downstream locations are asymmetric. Further, the tower wake is carried upwards by the rotating blade wakes, and thus the location of the asymmetry peak at different downstream distances is changing. We therefore confirm that the asymmetry of the wake profile is physical and it is not a measurement error in the experiment as suspected by other researchers in the previous studies. Finally, the interaction between the rotor wake and the tower wake is visualized and discussed. It is observed that due to the presence of the tower, the wake below the hub height is highly unsteady compared to the wake above the hub height. The results show that the methodologies adopted in this study are capable of accurately capturing the wake characteristics of the NTNU BT1 wind turbine.