Over the last 20 years wind energy has undergone a significant growth. The installation of new wind farms is increasing globally with an average rate (over the last five years) of 10 percent every year. Current commercial wind turbines (WTs) have diameters that range between 90 to 150 meters and their total height exceeds 200 meters. Manufacturer's ambition points towards even bigger turbines with rotor diameters that within the next decade will reach sizes of about 250m. The above development will consequently lead to larger areas occupied by future wind farms. On the other hand, helicopters often execute low altitude flights following trajectories that cross wind farm areas. Furthermore, helicopters are commonly used as transport mean for maintenance staff transportation missions to distant offshore wind farms (as for example in the North Sea where many wind farms have been deployed over the last years). It is therefore foreseen that in the coming years the possibility for a helicopter to fly in interaction with a wind turbine wake will increase. Lack of previous experience or evidence renders safety checks necessary. In this respect of particular significance is to know whether safe passage through or in strong interaction with the wake of an operating wind turbine is possible or in such occasion the turbine should be shutdown. In the present paper, the aerodynamic interaction of a helicopter main rotor with a wind turbine wake is analyzed on the basis of free wake vortex analysis. Helicopter forward flight crossings of wind turbine wakes are simulated using different wake flow models.
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