Towards a Configurable Laser-Plasma Flow Actuator: Development of Experimental Diagnostic Method, Validation Through Exploration of Parameter Space, & Local Flowfield Control via Phase Modulation

Recently, there has been renewed interest in the use of laser-generated plasma for various flow control applications, ranging from enhanced mixing in internal combustion engines, turbulent mixing in supersonic ramjets, and even subsonic and supersonic external flow control. As a high-intensity laser pulse is focused into a small region, the laser can ionize the gas, creating a suspended plasma spark. The sudden change in thermodynamic state caused by the laser-induced breakdown of gases results in a complex flowfield consisting of a blast wave (pressure gradient) and a variable-density region (density gradient) at high temperature. The interaction between the pressure and density gradients result in regions of highly rotational flow, lasting for up to milliseconds. The current state of research aims to control the spatiotemporal development of the flowfield. However, due to the large variation in timescales and the three-dimensional nature of the flowfield, experiments in a laboratory setting have shown difficulties in capturing the important quantitative flowfield dynamics. Additionally, numerical investigations, which have been successful in reproducing key features of the flowfield, are dependent on simplifying assumptions of the flowfield that unfortunately limit the scope of investigations aimed at controlling the spatiotemporal development of the flowfield. In this research, a novel technology is developed that demonstrates a high degree of spatiotemporal flowfield control. Theoretical formulations are developed that link the electromagnetic properties of the laser pulse to the development of the plasma spark and the resulting fluid dynamics. An experimental diagnostic method is developed to recover quantitative velocity information from a time-resolved series of flow visualization images. The experimental methods are then used to collect data from two key experiments. The first experiment investigates the well-researched parameter space, consisting of 50 data points. The results of the first experiment validate the experimental methods and reveal key scaling behaviors that are consistent for a wide range of parameters. The second experiment investigates a novel method to control the spatiotemporal development of the flowfield by modulating the phase distribution of the laser pulse. It has been observed that each phase distribution produces unique flowfield features that persist well into the millisecond time range, showing strong potential for use in flow control applications.近年来，激光生成等离子体（laser-generated plasma）在各类流动控制场景中的应用重新受到关注，其应用场景涵盖内燃机内强化混合、超音速冲压发动机内湍流混合，乃至亚音速与超音速外流控制。当高强度激光脉冲聚焦至微小区域时，激光可使气体电离，形成悬浮等离子体火花。激光诱导气体击穿所引发的热力学状态突变，会形成包含激波（压力梯度）与高温变密度区域（密度梯度）的复杂流场（flowfield）。压力与密度梯度的相互作用会产生高度旋转的流场区域，其持续时长可达毫秒量级。当前研究的核心目标是实现该流场的时空演化调控。然而，由于流场的时间尺度跨度较大且具有三维特性，实验室环境下的实验难以捕捉到关键的定量流场动力学特征。此外，虽已成功复现流场关键特征的数值研究，需基于流场的简化假设，而这类假设恰恰限制了流场时空演化调控相关研究的拓展范围。本研究开发了一种可实现高精度流场时空调控的新技术。研究建立了将激光脉冲电磁特性与等离子体火花演化及后续流体动力学过程相关联的理论公式。本研究开发了一种实验诊断方法，可从时间分辨的流动可视化图像序列中提取定量速度信息。随后利用该实验方法开展了两项关键实验并采集数据。第一项实验针对已被充分研究的参数空间展开，共包含50组数据点。该实验结果验证了实验方法的有效性，并揭示了适用于宽参数范围的关键尺度律。第二项实验探索了一种通过调制激光脉冲相位分布来实现流场时空演化调控的新方法。研究发现，不同的相位分布会产生独特的流场特征，且这些特征可维持至毫秒级时长，展现出在流动控制场景中的巨大应用潜力。