Study Of Horizontal-axis Wind Turbine Wake Characteristics

With the continued rapid development of the world’s wind power in 2015, a new record has been seen in new wind installations, adding 63’690 MW.As wind turbines clustered in wind farms, wind turbine wake has been a intense topic of research once again. Limited land resources may result in aerodynamic interactions when one wind turbine of them operates in the wake of other turbines. This would reduce power output because of the velocity deficit introduced by the upstream wind turbine and increase the turbulence intensity in the wake, consequently increasing the dynamic loads and shortening the lifespan of the wind turbine. Hence, the research of wind turbine wake is significant to the power extraction, the physical analysis of the flow around, the calculation of the loads and aerodynamic performance, as well as the reasonable arrangement of wind turbines in wind farm.A 2-bladed HAWT(horizontal-axis wind turbine) is simulated at various tip speed ratios by solving N-S(Navier–Stokes) equations, which employs a cell-centered finite-volume method, to investigate the wake characteristics by analyzing the three-dimensional vortex structure, the tip vortex trajectories, the velocity distribution, as well as the velocity deficit and recovery, the wake boundary and turbulent intensity. The conclusions are as follows:1)the tip vortices lead a negative partial torque at the blade tip, which shed not exactly from the blade tip but from the blade span of 96.5% 99% radius of the rotor. The pitch of the tip vortex varies inversely with λ. The thrust coefficient decreases with the increase of λ, as well as the larger radial expansion near the tip and the larger radial contraction near the root. The velocity in near wake varies periodically with the azimuth angle. As the increase of λ, the axial velocity deficits become larger, but the peak value of tangential velocity fluctuation caused by the vortex sheets pass is smaller, with the longer effect time.2)The obvious velocity deficits appear as the turbine has extracted momentum from the incoming airflow and produced a wake when the airflow passes through the wind turbine instantly, as well as the rapidly increase of the turbulence intensity, whose peak is located at the tip vortex shedding point.At y/d=1, the distinct W-shaped velocity profiles and the inverted W-shaped turbulence intensity profiles appear. As the increase of λ, the axial velocity deficits become larger, so as the turbulence intensity near the tip.There is a slight increase of the velocity deficits and the turbulence intensity at y/d=2, where the axial velocity deficits get to be the largest, and the wake expand again. The velocity deficits decrease and the turbulence intensity corresponding to the hub and tips decay gradually due to momentum recovery with the distance to the rotor plane increasing before y/d=8, while the profiles of the velocity deficits in the central zone is opposite. At y/d=8, the axial velocity profiles are transformed from W-shape into V-shape due to momentum recovery. After this cross-section, y﹥8d, the velocity deficits in the wake have been reduced up to the outlet at y/d=18, and the wake boundaries keep the small linear expansion. At the same time, the turbulence intensity in the central zone is beginning to increase, opposite to the decay of the global. And the turbulence intensity tends to be uniform in the whole section. However, the wake effects are still quite noticeable for the large velocity deficits and turbulent intensity in the far wake at a distance of 18 d.