Alternative Procedures for the Simulation of Thunderstorm Downbursts and Associated Wind Turbine Loads

Abstract

This study is focused on the simulation of thunderstorm downburst wind fields defined separately from non-turbulent and turbulent parts and of associated loads on a 5-MW wind turbine. The non-turbulent part is simulated using computational fluid dynamics (CFD) tools as well as an alternative semi-empirical model; the turbulent part is simulated as additive stochastic processes using standard turbulence power spectral density functions and coherence functions. A generalized CFD based model—referred to as the Paused Downburst model—is developed to simulate a non-turbulent downburst wind field that is suitable for engineering applications. The CFD-based model also serves as an alternative to an available semi-empirical model that has been developed based on recorded downburst events. These models include as main parameters, the maximum radial velocity as well as the elevation above ground and the radial distance to the maximum radial velocity. Other storm parameters—such as the storm intensity, the storm translation velocity, and the direction of the storm path—are all added to both models in order to simulate the evolution of a downburst. A wind turbine response simulation during a thunderstorm downburst also takes into account ambient (environmental) winds and the storm touchdown point relative to the turbine. Turbine loads are generated using aeroelastic response simulation of a model of the selected utility-scale 5-MW turbine. Using a representative downburst scenario, turbine loads are compared based on wind fields generated using the two approaches. Based on the similarity in response predictions with the two models, it is concluded that the CFD-based Paused Downburst model can serve as an alternative to the semi-empirical one in evaluating thunderstorm downburst scenarios and their effects on wind farms.