Aero-optical analysis of compressible flow over an open cavity
P. E. Cassady, Stanley Francis Birch, P. John Terry · AIAA Journal · 1989
The optical rays that form the image of an object viewed through an open port in a high-altitude transonic aircraft are refracted by the unsteady density variations in a compressible turbulent shear layer. A time-dependent, two-dimensional Navier-Stokes computer code has been used to model the flowfield including the reattachment process. Optical ray tracing and image formation codes using fast Fourier transforms have been developed to generate both intensity profiles and intensity contours at the focal plane. Values of centerline intensity loss, steady and unsteady beam steering, and image distortion were extracted from the results of calculations made over a range of Mach numbers between 0.6 and 0.8 at altitudes of 40,000-50,000 ft. Two-dimensional predictions of image steering agree with those generated by a less detailed self-similar model. Image distortion was found to be almost entirely made up of wavefront tilt without appreciable higher-order distortion. Higher-order image distortion is shown to depend on the details of coherent structures in the shear layer that are presently very difficult to predict theoretically.