Development of a Downwind Wind Turbine: Encouraging Faster Wake Recovery Through Coned Rotors

A downwind turbine configuration offers the opportunity for the use of lighter, larger, and more flexible blades by relaxing blade-tower clearance constraints. This design can reduce costs and increase farm power in flow-aligned rows while expanding rotor-swept area for turbines. However, the tower shadow effect raises concerns about periodic blade loading, increased bending moments, and reduced efficiency. Scaled wind tunnel experiments provide insight into how upwind and downwind configurations with 0°- 20° coning angles differ in power production and wake behavior. This study examines 20 cm-diameter model turbines subjected to a 3 m/s freestream velocity in the Portland State University wind tunnel, directly comparing upwind and downwind orientations. Power output was measured using a hub-mounted generator, and stereo particle image velocimetry (PIV) was used to characterize mean velocity fields and turbulent Reynolds stresses in the wake. Four 20 cm × 20 cm streamwise-vertical planes were blended to create a detailed map of the wake structure at streamwise positions from x/D = 0.5–2.5 rotor diameters (D) downstream. Two vertically combined planes were also used in the spanwise- vertical plane capturing the wake at 1D, 2D, and 3D downstream of the turbine. Results demonstrate that turbine geometry and orientation significantly affect wake development, turbulence distribution, and power efficiency, while the tower shadow effect has negligible impact on power. By comparing wake dynamics and power production between upwind and downwind turbines coning was found to partially mitigate the tower shadow effects influence on turbulent energy production and the downward deflection caused by the tower-blade interaction. An increase in turbulence intensity and mixing was also observed, impacting wake recovery in the near wake by promoting faster mixing and vortex breakdown at the tips and along the centerline. Coning was found to improve wake recovery, but at the cost of increased turbulence intensity and power capacity reduction. These results inform the feasibility, performance trade-offs, and potential advantages of adopting downwind turbines for utility-scale wind energy applications.