Abstract
The control of flow separation on aerodynamic surfaces remains a fundamental goal for future air transportation. On airplane wings and control surfaces, the effects of flow separation include decreased lift, increased drag, and enhanced flow unsteadiness and noise, all of which are detrimental to flight performance, fuel consumption, and environmental emissions. Many types of actuators have been designed in the past to counter the negative effects of flow separation, from passive vortex generators to active methods like synthetic jets, plasma actuators, or sweeping jets. At the Chair of Aerodynamics at TU Berlin, significant success has been achieved through the use of pulsed jet actuators (PJA) which operate by ejecting a given amount of fluid at a specified frequency through a slit-shape slot on the test surface, thereby increasing entrainment and momentum in a separating boundary layer and thus delaying flow separation. Earlier PJAs were implemented using fast-switching solenoid valves to regulate the jet amplitude and frequency. In recent years, the mechanical valves have been replaced by fluidic oscillators (FO) in an attempt to generate the desired control authority without any moving parts, thus paving the way for future industrial applications. In the present article, we present in-depth flow and design analysis which affect the operation of such FO-based PJAs. We start by reviewing current knowledge on the mechanism of flow separation control with PJAs before embarking on a detailed analysis of single-stage FO-based PJAs. In particular, we show that there is a fundamental regime where the oscillation frequency is mainly driven by the feedback loop length. Additionally, there are higher-order regimes where the oscillation frequency is significantly increased. The parameters that influence the oscillation in the different regimes are discussed and a strategy to incorporate this new knowledge into the design of future actuators is proposed.
Highlights
The objective of flow separation control is to enhance the effectiveness and efficiency of aerodynamic systems that are naturally limited by boundary-layer separation
The design of an fluidic oscillators (FO)-pulsed jet actuator (PJA) starts with the choice of exit velocity and switching frequency since these parameters are relevant for the flow physics of separation control
The internal dynamics and the parameters influencing the frequency of pulsed jet actuators based on fluidic oscillators were investigated
Summary
The objective of flow separation control is to enhance the effectiveness and efficiency of aerodynamic systems that are naturally limited by boundary-layer separation. The present paper is concerned with another type of unsteady fluidic actuator, the pulsed jet actuator (PJA), which operates by injecting a given amount of fluid at a specified frequency through a slit-shape slot. Such actuators were successfully used in several wind-tunnel experiments [9,10] and flight tests [11] for the open and closed-loop control of flow separation.
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