Active yaw angle control is one of the promising methods for wind farm power optimization, it is implemented by intentionally misaligning the upwind turbine to the incoming wind direction, as a result, the wake is deflected away from the downwind turbine, and the latter turbine can extract more energy from the un-waked flow. For an application of such an operational control, it is important to have a good knowledge about the wake characteristics by a yawed wind turbine. However, studies on the symmetry to yaw direction have not sufficiently covered until now. Hence, in this paper, numerous large eddy simulations are performed, to investigate the wake properties of wind turbines with same yaw angle magnitude but opposite directions, and to further explore the effects of asymmetry in wake steering on wind farm performance. We find that the wake shape and wake deflection magnitude are asymmetrical for positive and negative yaw angles. What's more, clockwise and counter-clockwise upstream turbine yaw rotations do not produce equal benefits at the downstream wind turbines. More importantly, it is observed that rows of turbines compound the effects of the asymmetry in wake steering, the power gain in the positive yaw case greatly exceeds the negative one. According to the detailed analysis of the yawed turbine wake in oppositely signed yaw angles, we encourage to incorporate the effects of asymmetry in wake steering into the empirical engineering wake models used for designing wind farm controllers.
With the rapid growth of requirement for renewable energy, a growing number of wind farms has been installed worldwide. However, for a large-scale wind plant, the reduced wind speed and increased level of turbulence intensity, arising from the wake interference, can greatly affect the efficiency of power production (Wu et al, 2015). In the past, wake steering through yaw misalignment (Bastankhah et al, 2015; Jiménez et al, 2009) has been proven to be a possible and promising way to increase the overall energy yield of the wind farm, in which, the upstream turbine intentionally misaligns its yaw angle to deflect the wake such that avoiding the downstream turbine. As a consequence, the power output of the downstream wind turbine increases and compensates the power loss of the upstream turbine in a global view.