Thunderstorm downbursts have been reported to cause damage or failure to wind turbine arrays. We extend a large-eddy simulation model used in previous work to generate downburst-related inflow fields with a view toward defining correlated wind fields that all turbines in an array would experience together during a downburst. We are also interested in establishing what role contrasting atmospheric stability conditions can play on the structural demands on the turbines. This interest is because the evening transition period, when thunderstorms are most common, is also when there is generally acknowledged time-varying stability in the atmospheric boundary layer. Our results reveal that the structure of a downburst’s ring vortices and dissipation of its outflow play important roles in the separate inflow fields for turbines located at different parts of the array; these effects vary with stability. Interacting with the ambient winds, the outflow of a downburst is found to have greater impacts in an “average” sense on structural loads for turbines farther from the touchdown center in the stable cases. Worst-case analyses show that the largest extreme loads, although somewhat dependent on the specific structural load variable considered, depend on the location of the turbine and on the prevailing atmospheric stability. The results of our calculations show the highest simulated foreaft tower bending moment to be 85.4 MN-m, which occurs at a unit sited in the array farther from touchdown center of the downburst initiated in a stable boundary layer.

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The late afternoon hours in the diurnal cycle precede the development of the nocturnal stable boundary layer. This “evening transition” (ET) period is often when energy demand peaks. This period also corresponds to the time of day that is a precursor to late-afternoon downbursts, a subject of separate interest. To capture physical characteristics of wind fields in the atmospheric boundary layer (ABL) during this ET period, particularly the interplay of shear and turbulence, stochastic simulation approaches, although more tractable, are not suitable. Large-eddy simulation (LES), on the other hand, may be used to generate high-resolution ABL turbulent flow fields. We present a suite of idealized LES four-dimensional flow fields that define a database representing different combinations of large-scale atmospheric conditions (characterized by associated geostrophic winds) and surface boundary conditions (characterized by surface heat fluxes). Our objective is to evaluate the performance of wind turbines during the ET period. Accordingly, we conduct a statistical analysis of turbine-scale wind field variables. We then employ the database of these LES-based inflow wind fields in aeroelastic simulations of a 5-MW wind turbine. We discuss how turbine loads change as the ET period evolves. We also discuss maximum and fatigue loads on the rotor and tower resulting from different ABL conditions. Results of this study suggest that, during the ET period, the prevailing geostrophic wind speed affects the mean and variance of longitudinal winds greatly and thus has significant influence on all loads except the yaw moment which is less sensitive to uniform and symmetric incoming flow. On the other hand, surface heat flux levels affect vertical turbulence and wind shear more and, as a result, only affect maximum blade flapwise bending and tower fore-aft bending loads.

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Severe winds produced by thunderstorm downbursts pose a serious risk to the structural integrity of wind turbines. However, guidelines for wind turbine design (such as the International Electrotechnical Commission Standard, IEC 61400-1) do not describe the key physical characteristics of such events realistically. In this study, a large-eddy simulation model is employed to generate several idealized downburst events during contrasting atmospheric stability conditions that range from convective through neutral to stable. Wind and turbulence fields generated from this dataset are then used as inflow for a 5-MW land-based wind turbine model; associated turbine loads are estimated and compared for the different inflow conditions. We first discuss time-varying characteristics of the turbine-scale flow fields during the downbursts; next, we investigate the relationship between the velocity time series and turbine loads as well as the influence and effectiveness of turbine control systems (for blade pitch and nacelle yaw). Finally, a statistical analysis is conducted to assess the distinct influences of the contrasting stability regimes on extreme and fatigue loads on the wind turbine.@en

The evening transition represents an interesting period lasting a few hours before the nocturnal stable boundary layer develops. It is also often a period of high energy demand. Large-eddy simulation is an efficient technique for the simulation of turbulent inflow fields that can be used in the computation of wind turbine loads. Unlike traditional methods (e.g., stochastic simulation), LES can be used to generate high-resolution turbulent flow fields in the ABL (atmospheric boundary layer) and to capture variations in key parameters related to wind turbulence and shear during the evening transition period. We conduct a number of idealized LES simulations to establish a database of four-dimensional flow fields with different combinations of atmospheric conditions (e.g., geostrophic wind) and surface boundary conditions (e.g., surface heat flux rate). A statistical analysis of turbine- scale variables (e.g., hub-height mean wind speed and standard deviation of wind speed in longitudinal direction, wind speed shear, etc.) reveals the high interrelations with each other. Further investigation shows that the surface heat flux rate plays an important role in the statistics of wind shear. Finally, we apply the database of inflow fields to the estimation of loads on a 5-MW wind turbine model. In particular, we study the variation in turbine load statistics during the evolution of the flow fields during the evening transition.@en

The late afternoon hours in the diurnal cycle precede the development of the nocturnal stable boundary layer. This so-called evening transition (ET) period is often when energy demands peak. To capture physical characteristics of the wind fields in the atmospheric boundary layer (ABL)-particularly the interplay of shear and turbulence-during this evening transition period, stochastic simulation approaches though inadequate are commonly used to assess wind turbine loads because they are more tractable. Large-eddy simulation (LES), on the other hand, may be used to generate high-resolution ABL turbulent flow fields. We present a suite of idealized LES 4-dimensional flow fields that define a database representing different combinations of large-scale atmospheric conditions (characterized by, for instance, geostrophic wind) and surface boundary conditions (characterized by surface heat flux). Our ultimate objective is to evaluate the performance of wind turbines during the ET period. Accordingly, we conduct a statistical analysis of turbine-scale variables (including hub-height mean wind speed, standard deviation of wind speed in the streamwise direction, and wind speed shear) and discuss patterns of inter-relationships among them. Surface heat fluxes are seen to greatly influence wind shear. Finally, we apply the database of LES-based inflow wind fields in aeroelastic simulation of a 5-MW wind turbine model. We discuss how turbine loads change as the wind field evolves during the evening transition. We also discuss extreme and fatigue loads on the rotor and tower that result from different ABL conditions.@en