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

将流体脉动注入横流边界层是防止或延缓流体从固体表面分离的一种普遍方法。尽管几十年来一直在机翼模型上进行研究，但这种方法简化机械复杂的高升力配置的潜力尚未在飞机工业中使用。从空气动力学的角度来看，这主要是由于产生脉冲射流的装置的质量流量消耗过大。这个项目的首要目标是建立一种新的方法来选择分离控制参数，从而在保留控制权威的同时显著减少质量流量消耗。在第一个资助期(2019-2022年)，讨论了脉冲射流的基本性质，揭示了与所研究方法的作用原理有关的特征涡旋结构和基本机制。作为一个重要的发现，在一般的装置中，已经确定了一个流动固有的时间尺度，该时间尺度支配着脉冲射流的动量添加之后流动分离的再现。这产生了显著的质量流量节省，因为分离连续脉冲的时间延迟可以被优化。实验结果还证实了我们最初的假设，因为驱动占空比的降低确实会导致系统效率的提高。在第二个资助期内，这些结果将被转移到与空气动力学社区更相关的流动结构中，同时解决在此类应用中出现的实际问题。具体地说，带有简单铰链襟翼的有限跨度机翼模型将配备一组脉冲喷注致动器，以对抗襟翼上的边界层分离。首先，我们将评估在第一个资助期建立的方法是否可以应用于这一与应用程序更相关的设置。然后，将考虑周期激励引起的动态载荷的发生，并评估相移强迫信号作为潜在的应对措施。最后，将调查相邻执行器出口之间的间距，这是减少质量流量消耗的另一种方法。本项目所获得的结果有望在未来的分离控制应用中允许更明智地选择操作参数。

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.