Response of the wind-turbine wake to a turbulent atmospheric boundary-layer flow

The wake characteristics of a wind turbine for different regimes occurring throughout the diurnal cycle are investigated systematically by means of large-eddy simulation with the geophysical flow solver EULAG. A methodology to maintain the turbulence of the background flow in wind-turbine simulations with open streamwise boundaries, without the necessity of the permanent import of turbulence data, was developed. These requirements are fulfilled by applying the turbulent fluctuations of the spectral energy distribution of a neutral boundary layer in the wind-turbine simulations. Further, idealized diurnal cycle simulations over homogeneous and heterogeneous surface were performed. Under homogeneous conditions, the diurnal cycle significantly impacts the low-level wind shear and the atmospheric turbulence. A strong vertical wind shear and veering with height occur in the nocturnal stable boundary layer and in the morning boundary layer, whereas the atmospheric turbulence is much larger in the convective boundary layer and in the evening boundary layer. The increased shear under heterogeneous conditions change these characteristics, counteracting the formation of the night-time Ekman spiral. Synchronized turbulent inflow data from the diurnal cycle simulations drive the wind-turbine simulations. The resulting wake is strongly influenced by the stability of the atmosphere and has an impact on the efficiency of the wind turbine. The flow in the wake recovers more rapidly under convective conditions during the day, than under stable conditions at night. The wake characteristics of the transitional periods are influenced by the flow regime prior to the transition. To alleviate the computational extremely expensive diurnal cycle simulations, the turbulence preserving method was extended to a parameterization, which includes a stratification related weighting and suitable background wind profiles, both resulting from the idealized diurnal cycle precursor simulation over homogeneous surface. The following parameterization wind-turbine simulations are in good agreement with the synchronized diurnal cycle wind-turbine simulations over homogeneous surface and reduce the computational costs by a factor of O(102). Therefore, they result in a simple, numerically efficient, and computationally fast parameterization of turbulent wind-turbine flows for large-eddy simulations of different thermal stratifications.