Efficiency enhancement of pulsed-jet actuators through duty-cycle optimization

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The pulsatile injection of fluid into a cross-flowing boundary layer is a widespread method to prevent or delay its separation from a solid surface. Despite being investigated on wing models for decades, the potential of this approach to simplify mechanically complex high-lift configurations is yet unused in the aircraft industry. From an aerodynamics perspective, this is mainly due to the excessive mass-flow consumption of the devices generating pulsed jets. The overriding objective in this project is to establish a novel approach for choosing separation-control parameters that lead to a significant reduction of the mass flow consumption while retaining the control authority. During the first funding period (2019-2022), fundamental properties of pulsed jets have been addressed, shedding some light on characteristic vortical structures und the underlying mechanisms associated with the function principle of the studied method. As a major finding, a flow-inherent time scale governing the recurrence of flow separation subsequent to the momentum addition by means of pulsed jets has been identified in a generic setup. This gave rise to significant mass flow savings as the time delay separating successive pulses could be optimized. The experimental findings have also corroborated our initial hypothesis inasmuch as a reduction the actuation duty cycle indeed leads to systematic efficiency gains.During the second funding period, these results will be transferred to a flow configuration that is more relevant to the aerodynamics community while addressing practical issues that arise in such applications. Specifically, a finite-span wing model with a simple-hinged flap will be equipped with an array of pulsed-jet actuators to counter boundary layer separation on the flap. First, we will estimate whether the method established during the first funding period can be applied to this more application-relevant setup. Then, the occurrence of dynamic loads due to the period actuation will be considered and phase-shifted forcing signals will be assessed as a potential counter-measure. Finally, the spacing between neighbouring actuator outlets will be investigated, representing a further means to reduce the mass flow consumption.The findings obtained in this project are expected to allow for a more informed choice of operating parameters in future separation control applications.