Articles | Volume 2, issue 2
Wind Energ. Sci., 2, 533–545, 2017
Wind Energ. Sci., 2, 533–545, 2017

Research article 20 Nov 2017

Research article | 20 Nov 2017

Impacts of the low-level jet's negative wind shear on the wind turbine

Walter Gutierrez1, Arquimedes Ruiz-Columbie2, Murat Tutkun3,4, and Luciano Castillo5 Walter Gutierrez et al.
  • 1Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas 79409, USA
  • 2National Wind Institute, Texas Tech University, Lubbock, Texas 79409, USA
  • 3Institute for Energy Technology (IFE), Kjeller, Norway
  • 4Department of Mathematics, University of Oslo, Oslo, Norway
  • 5School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, USA

Abstract. Nocturnal low-level jets (LLJs) are defined as relative maxima in the vertical profile of the horizontal wind speed at the top of the stable boundary layer. Such peaks constitute major power resources for wind turbines. However, a wind speed maximum implies a transition from positive wind shears below the peak to negative ones above. The effect that such a transition has on wind turbines has not been thoroughly studied.

This research study employed a methodical approach to the study of negative wind shear's impacts on wind turbines. Up to now, the presence of negative shears inside the turbine's rotor in relation to the presence of positive shears has been largely ignored. A parameter has been proposed to quantify that presence in future studies of LLJ–wind-turbine interactions. Simulations were performed using the NREL aeroelastic simulator FAST code. Rather than using synthetic profiles to generate the wind data, all simulations were based on real data captured at the high frequency of 50 Hz, which allowed us to perform the analysis of a turbine's impacts with real-life, small scales of wind motions.

It was found that the presence of negative wind shears at the height of the turbine's rotor appeared to exert a positive impact on reducing the motions of the nacelle and the tower in every direction, with oscillations reaching a minimum when negative shears covered the turbine swept area completely. Only the tower wobbling in the spanwise direction was amplified by the negative shears; however, this occurred at the tower's slower velocities and accelerations. The forces and moments were also reduced by the negative shears. The aforementioned impacts were less beneficial in the rotating parts, such as the blades and the shafts. Finally, the variance in power production was also reduced. These findings can be very important for the next generation of wind turbines as they reach deeper into LLJ's typical heights.

The study demonstrated that the presence of negative shears is significant in reducing the loading on wind turbines. A major conclusion of this study is that the wind turbines of the future should probably be designed with the aim of reaching the top of the nightly boundary layer more often and therefore the altitudes where negative shears are more frequent. Doing so will help to reduce the positive shear's associated damage and to capture the significant LLJ energy.

Short summary
Low-level jets (LLJ) are fast flows in the low atmosphere, usually seen at night, with a wind speed peak between 100–1000 m above the ground. More wind energy can be captured if an LLJ is present. The positive wind shear below the peak augments the damage to wind turbines. However, our results show that the negative shears above decrease the mechanical loading. Therefore, reaching negative shears more often reduces the LLJs' adverse impacts and makes it more feasible to harness their power.