Assessing the Structural Impact of Low Level Jets over Wind Turbines

Low Level Jets (LLJs) are defined as regions of relatively strong winds in the lower part of the atmosphere. They are a common feature over the Great Plains in the United States. It has been reported that 75 % of LLJs in the Great Plains occur at night and with seasonal patterns, affecting significantly the wind energy production. Present results have corroborated some of the LLJ's known characteristics. LLJs develop due to the formation of stable stratification in the lower atmosphere. This paper is focused on the determination of the static/dynamic impact that real LLJs produced in West Texas have over wind turbines. High-frequency (50Hz) observational data from the 200-m tower data (Reese, Texas) have been input as inflow conditions into the NREL FAST code in order to evaluate the structural impacts of LLJ's on a typical wind turbine. Due to the higher levels of wind speed, the potential for power increase proportional to the cube of the velocity. It has been observed that during an LLJ event the level of turbulence intensities and TKE are significantly much lower than those during unstable conditions; as a result, cyclical aerodynamic loads on turbine blades are different. Low-frequency oscillations prevail in stable conditions with formation of LLJ, as opposed to high-frequency oscillations more prevalent in unstable conditions. The turbulent kinetic energy is lower in LLJ but the energy concentrates in particular frequencies that can stress the turbine. From the point of view of the wind turbine loads/stresses, we have detected frequencies that can be correlated with those from the incoming wind.