Experimental Investigation of Image Distortion in a Mach 6 Hypersonic Flow

OF THESIS EXPERIMENTAL INVESTIGATION OF IMAGE DISTORTION IN A MACH 6 HYPERSONIC FLOW The image distortion that is inherently present when imaging through a flow field at hypersonic speeds was investigated. The original problem involves observation of the outside world from the inside an aircraft moving at hypersonic speeds. For this work, a Mach 6 hypersonic wind tunnel at Wright Patterson Air Force Base (WPAFB) was used and optical patterns were imaged with and without flow field characteristics. Two test campaigns were scheduled to conduct experiments that would provide answers to the proposed problem of the effect on observable aberrations through flow fields. During the first test campaign, October 2017, optical patterns were laser etched on anodized aluminum inserts that would couple to a 15°-degree wedge probe that had been operated with the Mach 6 tunnel previously. During this test phase, lessons learned were extremely acknowledged for preparing for the second campaign in February-March 2019. A primary effect observed was due to tunnel vibrations that created apparent optical distortion by “smearing” the optical patterns over the acquisition time of the camera. During the second test campaign there were 2 primary test models that would be mounted in the tunnel for optical analysis. Newly manufactured steel plates were coupled to the already investigated 15°-degree wedge probe for verification of what was observed previously. Also, a 7° half angle cone was manufactured as a replica of a cone that was already in operation at WPAFB. Characterization of optical distortion was done by using a quantity known as a Strehl Ratio. The Strehl Ratio is defined as the ratio of the peak intensity of a point source from an aberrated image, which has been affected due to distortion, to the corresponding point source from a diffraction limited system. Line Distribution Functions (LDFs) were identified to expand the definition from a ratio of maximums to a ratio of the shapes of the line widths. Measured vibrational influences were extracted in both the axial and vertical directions of flow to account for any artificial distortion mechanisms. These lines in both directions created our optical patterns simultaneously giving information of vibrational influences in either direction as well as the measured distortion over the test targets. Lastly, there was an attempt to relate the experimental findings to real world applications. Considerations from the first test campaign using the wedge probe are presented for this using what is known from the General Image Quality Equation (GIQE). This was developed as an analytical solution for determining image quality parameters within the National Imagery Interpretability Rating Scale (NIIRS).