On synthetic jet actuation for aerodynamic load control

A major goal for the wind energy industry is the reduction of the cost of energy. This drives the design towards increasingly larger wind turbines. The technology of smart rotor control is expected to allow wind turbines to increase even further in size due to its potential to change the local aerodynamic characteristics of the blades such that fatigue inducing variations in blade loads are alleviated. In the present research, synthetic jets have been investigated as a potential option for smart rotor control. Synthetic jets are generated by repeated ingestion and subsequent ejection of air, into and out of a cavity below the surface of the blade, respectively, through holes or slits in the surface of this blade. A multi-purpose computational method has been developed that solves the unsteady Reynolds-averaged Navier-Stokes equations for unsteady compressible viscous flow, together with the equation(s) of an eddy-viscosity turbulence model. Time-dependent inflow/outflow boundary conditions have been included that enable the simulation of flows with synthetic jets. A parameter study has been performed of synthetic jet actuation through long spanwise slits close to the trailing edge of a non-rotating airfoil. For low actuation frequencies, computational results have been compared to to experimentally obtained surface pressure measurements. The computational method predicts well the effect of low-frequency actuation, but the effect is overpredicted in comparison with the experimental results, probably due to three-dimensional flow effects that are not represented in the two-dimensional flow configuration considered in the numerical simulations. Only synthetic jet actuation at high actuation frequencies is a feasible option for smart rotor control. At these frequencies, vortices are generated that cause a displacement of the main flow along the airfoil. This displacement influences the direction of the flow at the trailing edge, i.e. the circulation around the airfoil, which is associated with a change of the lift and drag coefficients. A combination of the studied parameters has been found that yields a performance close to that needed for smart rotor control. However, the associated power consumption appears to be prohibitive. Further study is needed to find a less power intensive combination of the involved parameters.