Study of Flow Field and Pressure Distribution on a Rotor Blade of HAWT in Yawed Flow Conditions

Abstract This paper describes the flow field and the blade pressure distribution of a horizontal axis wind turbine in various yawed flow conditions. These measurements were carried out with 2.4m-diameter rotor with pressure sensors and a 2-dimensional laser Doppler velocimeter for each azimuth angle in a wind tunnel. The results show that aerodynamic forces of the blade based on the pressure measurements change according to the local angle of attack during rotation. Therefore the wake of the yawed rotor becomes asymmetric for the rotor axis. Furthermore, the relations between aerodynamic forces and azimuth angles change according to tip speed ratio. By the experimental analysis, the flow field and the aerodynamic forces for each azimuth angle in yawed flow condition were clarified. Keywords : Wind Turbine, Rotor Aerodynamics, Fluid Machinery, Yaw, Flow Field, Pressure Distribution 1. Introduction The wind energy is the most reliable renewable energy and the installed capacity of wind turbines in 2008 was the largest in all generation plants [1]. However to reduce the energy from fossil fuel the production of electricity by wind turbines is still not enough. There are many ways to increase the amount of energy production, which are upsizing of the rotor, finding the most suitable layout of the wind turbine and improving of the capacity factor, and so on. These ways are much affected by the aerodynamic efficiency of wind turbine rotor. Therefore, the efficiency improvement of wind turbine rotor is necessary to increase the amount of energy production. The commercial wind turbines are operated in natural wind where both direction and speed are unsteady. The nacelle direction is changed at some intervals to follow the mean wind directions. It means the wind turbine may be operated under yawed flow conditions. The flow field around yawed rotor is very complicated. There are some numerical studies related to the flow field around yawed rotor [2], and the experimental studies are also done for the averaged flow field around yawed rotor [3],[4]. However the experimental data are not sufficient for verification in the instantaneous pressure distribution on the blade. The detailed measurements of local and instantaneous pressure distribution will give deep understanding of the yawed flow properties and be used for the verification of the numerical studies. In this study, the flow field and the instantaneous aerodynamic forces for each azimuth angle were clarified by reproducing yawed flow conditions in a wind tunnel and measuring the flow field around the wind turbine and the pressure distribution on the blad e.