Influence of a Supersonic Free Jet in an Over-Expanded State on Laser Transmission

When an aircraft flies at supersonic speeds in the atmosphere, light rays often experience distortion, jitter, and divergence as they pass through the flow field, which significantly negatively impacts the transmission of optical signals. This study designs a supersonic (Mach number approximately 2.43) free jet generation device with a nozzle exit height of 1 mm to investigate the effects of the flow field on laser beams under different total pressures and positions of the over-expanded state in real atmospheric conditions. In this experiment, a high-speed camera and a schlieren system are employed to measure the laser spot distribution and the schlieren images of the jet. The experimental results indicate that in the laminar region of the free jet, as the distance from the nozzle exit increases, the jitter of the laser spot becomes increasingly pronounced, characterized by a gradual increase in the centroid deviation of the spot. In the transition region between the laminar and turbulent segments, the centroid deviation reaches a maximum. In the turbulent segment of the free jet, as the distance from the nozzle exit increases, the centroid deviation gradually decreases and stabilizes. Therefore, to avoid significant centroid deviation and reduce the jitter effect of the spot when the laser traverses the free jet flow field, it is essential for the laser to pass through the laminar segment of the free jet for effective transmission.