Performance recovery of a thick turbulent airfoil using a distributed closed-loop flow control system

This paper describes an experimental study aimed at controlling the performance of a thick airfoil, typical to the root section of a wind turbine blade. The main purpose is recovering decreased performance due to degraded surface quality, leading to decreased lift and increased drag. Since wind turbines are designed to operate for decades, the blades’ surface quality degradation due to environmental effects is unavoidable. This process promotes early transition to turbulent flow, leading to premature boundary layer separation in the post-transitional regime. In addition, non-uniform and unsteady wind speeds cause dynamic loads on the blade and on the overall turbine structure. Controlling unsteady and non-uniform loads by changing the blades’ (or its cross-section) performance will allow building larger, lighter and more durable to aging wind turbines. Active flow control (AFC) is a possible remedy to boundary layer separation, including rough surface effects. Currently, three arrays of synthetic jet actuators are controlled based on state estimation provided by feedback from hot-film and pressure sensors. The unsteady pressure sensors’ data are used to estimate the lift while the unsteady and un-calibrated hot-films data are used to determine the flow separation location and define the relative magnitude of actuation imparted by each of the three actuator rows. The aerodynamic results demonstrate that the “clean” turbine blade performance, with lift-based controller, is recovered by the closed-loop active flow control system at Reynolds numbers around half a million and excitation at Strouhal numbers larger than 10. The total closed-loop AFC system energy efficiency was measured and shown to increase by up to 60 % compared to the airfoil with degraded surface quality. The current results indicate the potential of a closed-loop AFC system to provide significant increase in the net energy harvesting capability of a wind turbine blade with degraded surface quality over a wide range of incidence angles and Reynolds numbers.