Study on Unsteady Flow around a HAWT Rotor by Panel Method (Calculation of Yawed Inflow Effects and Evaluation of Angle of Attack in the Three-Dimensional Flow Field)

The precise prediction of the performance of a wind turbine largely depends on the accurate knowledge of the flow around the rotor as it is subject to the induced velocity caused by the spiral vortex wake and the unsteady aerodynamic loads caused by the rapid change of wind direction. In this paper the flow around a horizontal axis wind turbine (HAWT) rotor in a yawed flow condition is analyzed by a panel method with a free wake model and the cyclic fluctuations of the aerodynamic load due to the asymmetric inflow condition are obtained. To include the viscous effects in the calculated results, the local angle of attack is investigated and is found to be defined well by the angle of incidence at a quarter chord point in the non-yawed condition. In the yawed condition the effects of highly skewed vortex wake on angle of attack are also discussed.