Effects of wind turbine wakes on bird gliding aerodynamic performance.

Wind power, as a critical source of renewable energy to combat global warming, is growing rapidly worldwide. A great concern has arisen over the killing of a great number of birds by wind turbines. There has been much research on this issue based on observational and statistical data. However, it remains unknown how the wind turbine wakes affect the bird flight from a fluid dynamic and mechanical perspective. The wind turbine wakes calculated by the large eddy simulation are used as boundary input to simulate the flying of bird. A number of 62 bird flight simulations are run to calculate the lift-to-drag ratio, and then fitted to get its distribution over the whole wake area. Results reveal bird flights are affected most near the upper blade tip, with a maximum reduction in lift-to-drag ratio of about 30%. Conversely, turbine wakes below bottom tip level assist bird flight, suggesting a potential low-altitude route through wind farms. Wake superposition exacerbates adverse aerodynamic effects, and a staggered layout strategy can reduce the negative impact. The findings will benefit the conservation of birds and assist the sustainable development of wind farms.