Cross-code verification of non-neutral ABL and single wind turbine wake modelling in LES

Abstract

Large Eddy Simulations (LES) of atmospheric boundary layer (ABL) flow with the actuator disc (AD) for turbine modelling is a widely used method of simulating wind farm flows. Hence, it is important to understand the requirements for achieving a good comparison between ABL flow and turbine wakes between different research group setups, despite unavoidable differences which include LES numerical framework, sub-grid scale (SGS) model, and turbine modelling, and at grid resolutions achievable in large wind farm simulations. In this work, conventionally neutral (CNBL), stable (SBL) and convective (CBL) boundary layers, and single turbine wakes under these different conditions, are compared between the EPFL pseudo-spectral code WIRE LES and the DTU finite-volume code EllipSys3D. ABL profiles largely agree well with hub height velocity magnitudes agreeing to 0.7%, 3.8% and 0.5% for the CNBL, SBL and CBL respectively. The scale-dependent SGS model of WIRE LES results in reduced grid dependency, while EllipSys3D required higher grid resolution in the SBL. Wake flows show improved wake recovery and greater added turbulence intensity with increasing grid resolution, and good agreement is achieved with a radius R to cell size ratio of R/(dxdydz)1/3 ≥ 6.5. Trends in wake flow with different stability conditions, such as the influence of inflow turbulence intensity or shear, are well replicated between codes. Likewise, wake deficit and added TI profiles, and distributions of turbine power and thrust also agree well. Mean power output predictions match to 4.3%, 7.2% and 3.8% in the CNBL, SBL and CBL respectively between the two codes. Overall, these results demonstrate that good agreement is possible with aligned turbine data and sufficient grid resolution.