Using Large-eddy Simulations to Examine the Variability of Wind Speed and Direction in the Hurricane Boundary Layer

Tropical cyclones subject wind turbines to environmental conditions that may significantly affect their loading, durability, and operation. The strongest class of wind turbines outlined in the International Electrotechnical Commission standards (i.e., IEC Class I) can withstand reference wind speeds of 50 m s-1 with gusts up to 70 m s-1. These wind speed specifications correspond to a Category 3 hurricane on the Saffir-Simpson hurricane wind scale. However, the standards do not encompass Category 4 and 5 hurricanes that occur in the Gulf of Mexico and south of the Carolinas. In addition, the standards were not specifically designed for the unique qualities of the hurricane boundary layer (HBL) (i.e., frequent gusts (Figure 1), swift directionality changes, and shear across the rotor layer). Because of the paucity of observational data in these conditions, we use the Cloud Model Version I (CM1) large-eddy simulation model to simulate high spatial- (< 10 m) and temporal- (< 0.1 s) resolution data across a theoretical turbine in the HBL. This unique dataset is used to answer the following questions: how often do severe gusts (> 70 m s-1) occur, how much time occurs between these critical gusts, is a new gust factor required to represent the HBL, and lastly, how quickly does wind direction change across the rotor diameter? The answers to these questions will help wind turbine load simulators better estimate the loads that turbines would experience in the extreme conditions of the HBL and assess whether current yaw technology is adequate for the rate of directionality changes in a major hurricane.