Closed-loop active flow control systems: Actuators

Closed-loop active flow control (CLAFC), the capability to estimate, efficiently alter and maintain a flow state, relies on the control authority of available actuators as a primary enabling technology. The requirements from the actuation systems are outlined and a critical review of available actuation technology is offered. Since the relevance of a given actuator depends on the application, separation control is considered over a wide range of operational conditions. Unsteady zero-net-mass-flux (ZNMF) periodic excitation was proven to be significantly more effective than steady blowing and simpler to apply than steady suction for the control of boundary layer separation. Furthermore, it can utilize flow instability as efficiency magnifier. When generated by Piezo-fluidic actuators, it has a bandwidth that is suitable for a wide range of feedback control applications. However, the current state-of-the-art ZNMF actuators lack, for certain applications, sufficient control authority. Therefore, effective methods for coupling the excitation to the most unstable modes of the flow should preferably be sought after and utilized. A new robust and simple actuator concept that combines steady suction and pulsed blowing is presented. It can generate wide band-width near sonic oscillations. Its performance was modeled and validated in several scales. The valve design allows highly efficient operation, not nullifying the favorable effects of future CLAFC schemes. Three-dimensional (3D) excitation modes should be explored, as the flow naturally becomes 3D even if the baseline flow and the excitation are nominally two-dimensional (2D). To be for industrial applications, overall system efficiency should always be assessed, not only the improvement in aerodynamic performance. Three performance based criteria for comparing different actuation concepts are presented and discussed. The first criterion evaluates the actuator based on its force or linear momentum generation capability as it operates in still fluid, while considering its weight, volume and power consumption. The second criterion is simply the actuator peak velocity Mach number relative to the free-stream Mach number, where it is rare to find any benefit from actuator with Mach ratio smaller than 0.1 and/or momentum coefficient smaller than 0.01%. The third, application dependent, criterion is the Aerodynamic figure of merit, an energy efficiency criterion, based on the improvement of the controlled performance (e.g., lift to drag ratio) of a certain application, when the power consumption (and also the weight) of the actuation system are taken into account.