Fluid-Optic Interactions II
Abstract
This report describes fluid-optic interaction research at the University of Notre Dame. When a laser beam propagates through a variable-index-of-refraction, turbulent fluid, its optical wavefront becomes aberrated, reducing associated optical system performance. For flight above 0.6 Mach, "compressibility effects" alone become important in aberrating wavefronts. Scaling optical wavefront data collected in ground-test facilities to realistic flight conditions requires an understanding of the compressibility mechanism that produces wavefront distortions This mechanism was studied using an analytical/numerical model. Shear-layer flows of low convective Mach number can be considered weakly-compressible, which allowed their velocity fields to be approximated using a discrete vortex method. From these "known" velocity fields, the concomitant pressure and density fields were determined by iteratively solving the unsteady Euler equations. The results showed that unsteady pressure fluctuations present in shear layers had a dominant effect on the resulting density/Index fields; this effect has historically been neglected. The computed index fields produced simulated schlieren images which closely resembled experimental schlierens. Optical wavefronts computed from the simulation reasonably matched the behavior of large-scale aberrations measured in a transonic wind tunnel at AEDC. The compressibility model was used to suggest and test relations for scaling optical wavefronts from one flow condition to another. A simple density ratio successfully scaled distortion variations with altitude when shear layer Mach numbers were held constant; an additional temperature ratio was required if the velocity difference was kept the same. The spatial and temporal frequencies of optical distortions produced by dissimilar-index, incompressible, mixing flows were found to logically scale, provided the beam's diameter was larger than the largest flow structures.
Document Details
- Document Type
- Technical Report
- Publication Date
- Mar 22, 2000
- Accession Number
- ADA376028
Entities
People
- Eric J. Jumper
Organizations
- University of Notre Dame