Experimental Investigation of Image Distortion in Hypersonic Flows

The goal of this work is to qualitatively and quantitatively investigate how imaging is impaired through the flow field around a body in a laminar low enthalpy hypersonic flow for application to hypersonic EO/IR sensor analysis. In particular, we would like to quantify image distortion due to aero-optic effects such as boresight error and distortion for remote sensing from a vehicle in hypersonic flight. For this purpose, a Mach 6 wind tunnel at Wright Patterson Air Force Base is intended to be used for the experimental work, thus restricting the flow conditions to lower enthalpies, likely without significant dissociation or vibrational or electronic excitation. In terms of spectral characteristics, the broadest range of interest might be from 0.2 to 5 ƒÝm, and the proposed work will focus on the visible and possibly near-infrared wavelength range between 400 and 1000 nm. The wavelength range of measurement will depend on the following: transmissivity of the window material, sensitivity of cameras (in Year 1 will be in 400 ¡V 800 nm), and chromatic aberration compensation of the lens systems. In Year 2, we will strive to increase the wavelength range of measurements to the IR using wave front measurements in conjunction with a laser system and further replace the current wind tunnel windows with the suitable window materials. The original problem is the observation of the outside world from the inside of a body through the flow field at hypersonic speed. In case a wind tunnel is used, that means that observation optics will have to be mounted inside a model which is exposed to the high speed flow. A defined pattern outside the flow would be observed through these optics with and without a flow around the model. Deformation of the image patterns will give important information about image distortion. Scale modeling techniques would then be needed to scale the effects to imaging objects from a bigger vehicle in a larger distance. Building such a model and designing optics of sufficiently high quality which fit into the model and are robust enough to survive the test in good working conditions is a technological challenge and may require significant financial support. The design of such a system would require relatively precise knowledge of the anticipated effects and the conditions to be investigated. To build such predictive knowledge, it is proposed here to start by experimentally investigating the reverse path and observe patterns on a model in the flow from outside of the wind tunnel. Thus, the optical setup can be adapted to the observed effects and an optimized setup can be developed from lessons learned from the initial testing. A comparison of data before, during, and after the testing will be investigated with particular respect to optical distortion patterns. Analysis procedures will be developed in the early phase of the contract using the imaging setup in the University of Kentucky Lab with commercially available targets as shown in Fig. 2. After characterizing the imaging quality of the optical setup itself (without a body in the optical path), bodies (such as optical plates, wedges, etc.) will be introduced in the optical path to create defined optical distortions. These results will be used to interpret the aero-optical effects to be observed later when the experiments are conducted in the wind tunnel. Different experimental approaches can be distinguished in terms of object illumination, passive or active signal generation as well as model shape and material choice. In all cases, the information about distortion processes will be derived from the difference between measurements with and without flow, typically before, during, and after application of a hypersonic flow to a model. Although shapes of interest to represent the real-world situation are probably spherically blunted cones or even more complex geometries, it will likely be useful to provide flat surfaces for the investigation of surface patterns as discussed later. The lessons learned from imaging surface patterns in the first phase will be used to define the second phase of the project. The Year 2 activities will involve wave front deformation measurements with the same wedge models used in the first phase will include some combination of additional measurements with various model shapes and/or imaging of external structures from inside a model in the flow. Scale model correlations to real flight based on the wind tunnel experiments simulations will be investigated.