Impact of the Diurnal Cycle of the Atmospheric Boundary Layer on Wind-Turbine Wakes: A Numerical Modelling Study

Abstract

The wake characteristics of a wind turbine for different regimes occurring throughout the diurnal cycle are investigated systematically by means of large-eddy simulation. Idealised diurnal cycle simulations of the atmospheric boundary layer are performed with the geophysical flow solver EULAG over homogeneous and heterogeneous terrain. Under homogeneous conditions, the diurnal cycle significantly impacts the low-level wind shear and 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 changes these wind characteristics, counteracting the formation of the night-time Ekman spiral. The convective, stable, evening, and morning regimes of the atmospheric boundary layer over a homogeneous surface as well as the convective and stable regimes over a heterogeneous surface are used to study the flow in a wind-turbine wake. Synchronized turbulent inflow data from the idealized atmospheric boundary-layer simulations with periodic horizontal boundary conditions are applied to the wind-turbine simulations with open streamwise boundary conditions. The resulting wake is strongly influenced by the stability of the atmosphere. In both cases, the flow in the wake recovers more rapidly under convective conditions during the day than under stable conditions at night. The simulated wakes produced for the nighttime situation completely differ between heterogeneous and homogeneous surface conditions. The wake characteristics of the transitional periods are influenced

Wind-turbine wakes responding to stably stratified flow over complex terrain

“…1(d)). For a more detailed discussion see [6]. The wind-turbine wakes over homogeneous surface corresponding to the MBL and the EBL are comparable to the SBL, with a slightly longer (shorter) wake extension and larger (smaller) velocity deficit values for the MBL (EBL).…”

“…All simulations presented in this paper are performed with open streamwise and periodic spanwise boundary conditions and are conducted with synchronized turbulent inflow data of a diurnal cycle-driven boundary-layer flow over homogeneous surface on 512 × 512 × 64 grid points with a resolution of 5 m. This precursor simulation was initialised with a geostrophic wind of 10 m s −1 in streamwise direction and zero for the lateral and vertical wind components and evolved in response to the sensible heat flux provided at the horizontally homogeneous surface over 30 h [6]. Figure 1(a) illustrates the temporal evolution of the streamwise wind component u e in the course of the day at three heights of 50 m, 100 m, and 150 m, representing bottom tip, hub height, and top tip of a common wind turbine.…”

“…In this paper, the state of the diurnal cycle situation after 24 h is considered as stable boundary layer (SBL) case, after 29 h as morning boundary layer (MBL) case, and after 18 h as evening boundary layer (EBL) case, following [6].…”

“…Its intensity is strongly affected by the thermal stratification and the vertical shear of the horizontal wind. Generally, a more rapid wake recovery is observed under convective conditions during the day compared to night-time periods [3], [4], [5], [6]. Transitional periods in the evening and the morning follow the respective flow regime prior to the transition [6].…”

“…The individual impacts of complex terrain [1], [2] and of the ABL [3], [4], [5], [6] on windturbine wakes have been investigated in several studies within the last years. The deformation of the wind while passing over hilly terrain increases the turbulence and advects it leewards into possible wind-turbine wakes [1], [2].…”