This large eddy simulation shows air going through a dual-rotor wind turbine. The three lines at the front are the blades of the main rotor; the secondary rotor is embedded in the ring near the center. (Anupam Sharma)
Large wind turbine blades disturb the wind, creating a wake behind them and reducing the energy harvest of any downwind turbines. A turbine sitting in the slipstream of another can lose 8 to 40 percent of its energy production, depending on conditions. By adding a smaller, secondary rotor mounted mounted in front of the big rotor, the two sets of blades are separated by the nacelle that houses the generating machinery on top of the tower.

The extra blades can increase a wind farm’s energy harvest by 18 percent. Researchers are using advanced computer simulations, including high-fidelity computational fluid dynamics analysis and large eddy simulations, to find the best aerodynamic design for a dual-rotor turbine. Where, for example, should the second rotor be located? How big should it be? What kind of airfoil should it have? Should it rotate in the same direction as the main rotor or in the opposite direction?