Although wind turbines have been well studied from a blade aerodynamics perspective, the interactions among these massive structures and the atmospheric turbulent boundary layer (ATBL) are still not understood in detail. It is important to understand such interactions in order to maximize the energy that can be extracted from the available wind resource.
Consequently, in order to improve the understanding of the vertical transport of momentum and kinetic energy across a boundary layer flow with wind turbines, wind-tunnel experiments were performed to include a scaled down wind farm of 3x5. Particle-image-velocity measurements in a volume surrounding a target wind turbine are used to compute mean velocity and turbulence properties averaged on horizontal planes. The impact of vertical transport of kinetic energy due to turbulence and mean flow correlations is quantified. It is found that the fluxes of kinetic energy associated with the Reynolds shear stresses are of the same order of magnitude as the power extracted by the wind turbines, highlighting the importance of vertical transport of turbulence in the boundary layer and thus in wind farms.
The concept of coherent transfers of energy is employed here as means to uncover the scales responsible for the entrainment of mean kinetic energy into the array. The major contributions to the MKE entrainment are achieved by large-scale motions associated with sums of the Reynolds shear stress, (idiosyncratic) modes (see figure 1). Thus, the sum of the first 9 modes yield 54% of the total energy entrainment, with scales given by L ~ 13D associated with this sum. From these results, it is clear that scales of the order of the total wind farm size are those, which are critical in determining how much power can be extracted from the atmospheric boundary layer.
Luciano Castillo is the Don Kay and Clay Cash Foundation Engineering Chair in Wind Energy and Professor in the mechanical Engineering Department at Texas Tech University. He has a PhD in Mechanical Engineering from the State University of New York at Buffalo. His research interests include:
Modeling & Experimental Wind Energy Array
Single−Blade Aerodynamics for Turbine Blades
Multi−Scale & Asymptotic Methods in Turbulent Boundary Layers
Experimental, Theoretical, & Numerical Fluid Mechanics
Forced Convection Heat Transfer