LES of Wind Fields and Wind Turbine Load Estimation

Abstract

Stochastic simulation of wind fields based on Fourier methods, commonly used in wind turbine loads computations, is unable to account for contrasting stability conditions in the atmospheric boundary layer (ABL). Flow fields in the stable boundary layer (SBL), for instance, exhibit enhanced wind shear and veering wind direction profiles that can greatly influence loads on utility-scale wind turbines. To investigate these influences, we use large-eddy simulation (LES) to generate inflow wind fields and to estimate loads on a 5-MW wind turbine model. First, we carry out a parameter study to relate large-scale environmental conditions (referred to as external parameters) to wind field characteristics at turbine scale (referred to as internal parameters). Next, we investigate how the various internal parameters influence turbine load statistics. Finally, we attempt to relate the large-scale environmental conditions (i.e., the external parameters) to the wind turbine load statistics. Several external parameters that define the initial conditions for the LES computations, including atmospheric and surface conditions, are considered. Among these, the geostrophic wind speed and surface cooling rate have the greatest influence on flow field characteristics and, thus, on wind turbine loads. Higher geostrophic winds lead to increased wind speed mean and standard deviation values at hub height; geostrophic wind speed is, on the other hand, negatively correlated with the level of wind shear. Increased surface cooling rates lead to steeper shear profiles and appear to significantly increase fatigue damage associated when out-of-plane bending moments are studied. Overall, our studies suggest that LES may be used effectively to model wind fields in the SBL, to study characteristics of wind fields for various atmospheric stability conditions, and to assess turbine loads for conditions not typically examined in design standards.