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.