Abstract
Knowledge about the structure and development of wakes behind wind turbines is important for power optimization of wind power farms. The high turbulence levels in the wakes give rise to undesired unsteady loadings on the downstream turbines, which in the long run might cause fatigue damages. In the present study, the near wake behind a small-scale model wind turbine was investigated experimentally in a wind tunnel. The study consists of measurements with particle image velocimetry using two different inlet conditions: a freely developing boundary layer, causing an almost uniform inflow across the rotor disc, and an inflow with strong shear across the rotor disc, in order to model the atmospheric boundary layer. The results show a faster recovery of the wake in the case with shear inflow, caused by the higher turbulence levels and enhanced mixing of momentum. The increased inlet turbulence levels in this case also resulted in a faster breakdown of the tip vortices as well as different distributions of the streamwise and vertical components of the turbulence intensity in the wake. An analysis comparing vortex statistics for the two cases also showed the presence of strong tip vortices in the case with lower inlet turbulence, while the case with higher inlet turbulence developed a different distribution of vortices in the wake.
Highlights
The turbulent wake behind a horizontal axis wind turbine is mainly characterized by a helical system of tip vortices, a mean velocity deficit and a turbulence distribution
An analysis comparing vortex statistics for the two cases showed the presence of strong tip vortices in the case with lower inlet turbulence, while the case with higher inlet turbulence developed a different distribution of vortices in the wake
Velocity measurements were performed in the wake behind a small-scale model wind turbine placed in a wind tunnel
Summary
The turbulent wake behind a horizontal axis wind turbine is mainly characterized by a helical system of tip vortices, a mean velocity deficit and a turbulence distribution. M. Khan et al 106 cause a reduced power output and increased loadings for the downwind turbines in wind power farms. Knowledge about the development of wind turbine wakes is important for optimization of wind power farms, both in terms of power output and lifetime of the farm. Most of the research is focused on the far wakes, where the subsequent turbines are placed. This is a simpler region to construct a general model for, since it is less dependent on the specific blade aerodynamics of the turbine. The near wake constitutes the boundary conditions for the far wake, and the specific structure and dynamics of the near wake is important in the strive for increased knowledge of turbine wakes
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