  Yaw control has proven to be promising in alleviating the wake effects that
plague the efficiency of wind farms. In this work, the actuator line modeling
(ALM) method is adopted to simulate the flows over two tandem turbines
distanced by $3 - 7$ rotor diameters, with the yaw angle of the upstream rotor
varying from $\gamma_1=0^{\circ}$ to $50^{\circ}$. The aim is to provide a
comprehensive aerodynamic characterization of a simple wind farm under yaw
misalignment. With increasing yaw angle, the power generated by the downstream
rotor increases, compensating the power loss in the upstream rotor, and
resulting in higher total power of the two turbines than that without yaw
control. The maximum power output is achieved as the upstream wake of the yawed
rotor is redirected away from the downstream rotor plane. Behind the downstream
rotor, the secondary steering phenomenon is observed, where the wake is also
redirected from the centerline. The use of the actuator line model also reveal
unsteady aerodynamic characteristics that can not be captured by lower-fidelity
models. For the upstream rotor, the yaw misalignment results in time-varying
change in the local angle of attack on the blade, giving rise to unsteady
loading. The downstream rotor is partially submerged in the deflected wake
incurred by the yawed upstream rotor. As the blade revolves into and out of the
wake deficit, the blade experiences cyclic loading, leading to even stronger
fluctuations in the aerodynamic loads than the upstream rotor. These analysis
provides a comprehensive understanding of the yaw control effects on the two
tandem rotors from the perspectives of aerodynamic performance, wake profiles,
and unsteady characteristics. The insights gained from the study can aid the
design of collective yaw control strategies of wind farms, and lay the
foundation for assessing the fatigue damage associated with yaw misalignment.

Fluid Dynamics