RANS Simulation of two co-axially positioned HAWT under different thermal stratification conditions

This paper proposes numerical method for aerodynamic performance predictions of Horizontal Axis Wind Turbines (HAWTs) immersed on Atmospheric Boundary Layer (ABL) flows under unstable, stable and neutral conditions. The flow field has been described using the three-dimensional Reynolds Averaged Navier Stokes (RANS) equations and the k-ϵ turbulence model with modified constants corresponding to ABL flows is considered. Based on Monin-Obukhov similarity, the ABL profiles under neutral and stratified conditions have been implemented into OpenFOAM where the flow over the fetch has been calculated through an in-house solver (ABLSolversimpleFoam) developed for steady-state turbulent flows. The immersed HAWT has been modeled using the actuator disk approach coupled to the Blade Element theory for the loads estimation. The obtained results have indicated that the velocity defect in the wake of a wind turbine is more significant in unstable than in stable thermal stability conditions. Consequently, significant variation of the wind turbine performances have been noticed; a power output drop of 37.64% and 13.67% being recorded for the second rotor placed in the wake of another and the first one respectively.