Blade Element Momentum Study of Rotor Aerodynamic Performance and Loading for Active and Passive Microjet Systems

This study investigates the performance of microjets for load reduction on the NREL-5 MW wind turbine and identifies optimal system parameters. Microjets provide blowing normal to the blade surface and can rapidly increase or decrease lift on a blade section, enabling a wind turbine to respond to local, short-term changes in wind condition. As wind turbine rotors become larger, control methods that act on a single blade or blade section are increasingly necessary to reduce critical fatigue and extreme loads. However, microjets require power to operate, and thus, it is crucial that the fatigue reduction justifies any energy input to the system. To examine the potential for fatigue reduction of a range of potential microjet system configurations, a blade element momentum (BEM) code and a flow energy solver were used to estimate the energy input and the change in primary fatigue metrics. A parametric analysis was conducted to identify the optimal spanwise position and length of the microjets over a range of air mass flow rates. Both active and passive air supply methods were considered. A passive microjet system applied to the NREL 5-MW rotor produced a 3.7% reduction in the maximum flapwise root bending moment (FRBM). The reduction in the peak bending moment increased to 6.0% with a 5 kPa blower that consumes approximately 0.1% of the turbine output power. The most effective configurations placed microjets between the blade midspan to three-quarters span. Load reduction was achieved for both active and passive modes of air supply to the microjet system.