**Investigations on the Unsteady Aerodynamic Characteristics of a Horizontal-Axis Wind Turbine during Dynamic Yaw Processes**

#### **Xiaodong Wang 1,\*, Zhaoliang Ye 1,2, Shun Kang 1 and Hui Hu 3**


Received: 11 June 2019; Accepted: 7 August 2019; Published: 14 August 2019

**Abstract:** Wind turbines inevitably experience yawed flows, resulting in fluctuations of the angle of attack (AOA) of airfoils, which can considerably impact the aerodynamic characteristics of the turbine blades. In this paper, a horizontal-axis wind turbine (HAWT) was modeled using a structured grid with multiple blocks. Then, the aerodynamic characteristics of the wind turbine were investigated under static and dynamic yawed conditions using the Unsteady Reynolds Averaged Navier-Stokes (URANS) method. In addition, start-stop yawing rotations at two di fferent velocities were studied. The results sugges<sup>t</sup> that AOA fluctuation under yawing conditions is caused by two separate e ffects: blade advancing & retreating and upwind & downwind yawing. At a positive yaw angle, the blade advancing & retreating e ffect causes a maximum AOA at an azimuth angle of 0◦. Moreover, the e ffect is more dominant in inboard airfoils compared to outboard airfoils. The upwind & downwind yawing effect occurs when the wind turbine experiences dynamic yawing motion. The e ffect increases the AOA when the blade is yawing upwind and vice versa. The phenomena become more dominant with the increase of yawing rate. The torque of the blade in the forward yawing condition is much higher than in backward yawing, owing to the reversal of the yaw velocity.

**Keywords:** HAWT; aerodynamic characteristics; dynamic yawing process; near wake; start-stop yaw velocity
