# Numerical Study on the Yaw Control for Two Wind Turbines under Different Spacings
## 5 Conclusions  
Complex wake effects between turbines in a wind farm will significantly affect the output power, blade load, etc. In this study, under the same inflow condition, the wake superposition of two turbines under different yaw and spacing conditions are numerically simulated. The rules of angular speed of the rotors, instantaneous power, and average power with the changes of yaw angle and turbine spacing are analyzed, and the corresponding physical mechanisms are revealed through the wake velocity, turbulence intensity and vortices structures of the wind farm. Finally, the ABL inflow is used, and it is compared with the uniform inflow to explore the effect of yaw control strategy. 1. With the yaw-based control of the upstream turbine, the wake of the upstream turbine will be shifted, making the downstream turbine be partially affected by the wake of the upstream turbine, and then the total power of two turbines will increase. When the downstream turbine works in the asymmetric wake, the instantaneous output power fluctuates with a larger amplitude over time. When the spacing is small, the fluctuation of the downstream turbine's output power increases as the yaw angle of the upstream turbine increases. When the spacing becomes large, the increasing trend of fluctuation of the output power occurs in the yaw angle range from \(0^{\circ}\) to \(20^{\circ}\). 2. When two turbines have small spacing, a larger yaw angle is needed to obtain greater total power. When the spacing between two turbines is large enough, the increase in yaw angle from \(20^{\circ}\) to \(30^{\circ}\) will not significantly increase the total power of the wind farm. The power increase in the downstream turbine due to yaw control cannot offset the power loss of the upstream turbine. In this case, the power output fluctuation of the downstream turbine also increases. When the yaw angle is fixed, the total power increases nonlinearly, and the growth rate decreases with the increase in spacing. In general, the effect of yaw control is greater than that of spacing. Under appropriate spacing and yaw angle conditions, the wind field can obtain the best output power. 3. Both velocity deficit and turbulence increase in the wind farm are induced by the upstream turbine. When the upstream turbine has yaw angles, the peak position of its wake velocity profile deviates obviously, which is reset behind the downstream turbine. The larger the spacing between two turbines, the larger the inflow wind speed  
Figure 19: The average output power of the turbines in the ABL wind field compared with uniform inflow under 7D conditions. can be obtained by the downstream turbine. With the increase in yaw angle, the wake velocity deficit behind the downstream turbine becomes more asymmetrical. In some cases, the maximum wake velocity deficit area moves downstream of the downstream turbine. This is due to the sophisticated interference of wake fields of two turbines. 4. With the increase in yaw angle, the interaction of vortices becomes more complex, and the asymmetric distribution of vortices intensifies. These vortices interactions make the flow field near the downstream turbine unstable. Thus, the wake velocity deficit and the wind turbine performance are regulated by the asymmetric vortex structures. Therefore, we need to study these situations thoroughly in the actual project. 5. The background turbulence has little effect on the power of the upstream turbine. The background turbulence increases the inflow velocity of the downstream turbine by reducing the wake effect of the upstream turbine. When the yaw angle of the upstream turbine is small, the effect on the downstream turbine is very significant. The larger the background turbulence intensity, the smaller the yaw angle of the upstream turbine is required to obtain a good comprehensive optimization effect.