Measurement of Aero-Optic Distortion in Transonic and Hypersonic, Turbulent Boundary Layers with Gas Injection

Abstract

Here, we summarize the results of our investigation of means for controlling high-speed flows using helium injection, with particular emphasis on the wavefront distortion of light transmitted through the boundary layer. Quantitative estimates of wavefront distortion were obtained, with and without helium injection, and new diagnostic techniques were developed, including a MHz rate Hartmann wavefront sensor. In particular, we demonstrated the ability to capture real-time wavefront data in two dimensions using an ultra-fast Shack-Hartmann sensor in hypersonic and transonic turbulent boundary layers. These measurements have high temporal resolution (2 μ μ μ μs), while providing full dimensional information on the planar field. Such measurements are vital for making accurate and complete estimates of aero-optic distortion due to boundary layer turbulence in air-borne laser applications, and to assess quantitatively the effectiveness of control methods. I. Introduction he index-of-refraction variations that degrade the performance of on-board optical systems for tracking and imaging targets are primarily due to the presence of turbulent free shear layers and turbulent boundary layers, and density discontinuities across shock waves. In directed energy and communication link applications, density fluctuations in the boundary layer cause image break-up and rapidly varying motions, reducing both image and target resolution, and diminishing the power and total energy on target. For tracking applications another important issue is the variation of “bore-sight” error with missile angle-of-attack. Bore-sight error is the difference between the apparent location of the target and its actual location relative to the missile reference frame. As the missile angle-of-attack varies, the apparent location of the target changes because the light rays are bent as they pass through shock waves and through the boundary layer before entering the window. As the name suggests, aero-optics is the study of the interaction between light and air. More specifically, it is the study and measurement of the distortion of light due to propagation through an index of refraction field which is thin with respect to the imaging (or projecting) aperture. In contrast, atmospheric propagation involves distortion distances much greater than the aperture, and has been studied at some length in the context of ground based astronomical telescopes. For airborne laser systems, this is called the far field problem. Astronomers are now able to measure and compensate for these distortions in real time, effectively ‘de-twinkling’ the stars. Aero-optics is concerned with the near field problem, where the distortions are caused by the vehicle flow field itself, and is by comparison at a nascent stage of development.