Evaluation criteria and performance comparison of actuators for bluff-body flow control

Active flow control (AFC) relies on actuators' control authority as a primary enabling technology for flow manipulation. The requirements from the actuation systems are revisited and tools for critical evaluation of actuators are offered. The application in mind is boundary layer separation control. Effective yet robust methods for coupling the excitation to unstable flow modes should be experimented. The recent introduction of the SaOB (steady Suction and Oscillatory Blowing) actuator concept to AFC and the use of similar no-moving parts oscillators have the potential to be both effective at higher speeds and overall efficient. To be accepted by industry, system efficiency should always be assessed, not only the improvement in aerodynamic performance. A few performance based criteria for comparing different actuation concepts are presented and discussed. The first criterion is the actuator conversion efficiency, from input energy (electric or fluidic) to mechanical energy. The second criterion evaluates the actuator based on its force or momentum generation capability as it operates in still fluid, while considering its weight, volume and power consumption. An additional, 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, wind turbine power production or drag reduction efficiency) of a certain application, when the power consumption (and possibly also the weight) of the actuation system are taken into account. Relevant recent separation control data are presented, compared and discussed. It was found that Piezo fluidic actuation is effective at low speeds while SaOB actuation is preferred at higher speeds. Mechanical or electromagnetic actuators are less effective mainly due to excessive weight while DBD Plasma actuators are orders of magnitude less efficient (0.1%). A minimal actuation efficiency requirement based on CFD results is also outlined.