One problem affecting the use of high speed endo-atmospheric intercept vehicles is turbulent flow aero-optical distortion caused by seeker window cooling jets. Work is underway at NEAR to determine a method for predicting and reducing this distortion. Current studies have been focused on windows cooled by external jets, but the technology is applicable to other configurations, such as internally cooled windows.

The NEAR prediction method uses Large Eddy Simulation (LES) as the model of turbulence. In LES, the structures and motions of the dominant scales of turbulence are resolved explicitly in the three-dimensional, time dependent solutions of the compressible Navier-Stokes equations. This approach is very different from the Reynolds-averaged CFD methods typically employed in this field. LES accurately predicts, with no empirical input, many characteristics of turbulence in a mixing layer; the reduced growth rate at elevated Mach numbers, the turbulent stresses, and the dominant structure of turbulence. LES has proved an accurate and economical means of assessing aero-optical distortion and turbulence control concepts.

It is possible to achieve a significant reduction in turbulent density fluctuations by curving the shear layer in the streamwise direction. In practice this curvature is produced by a window whose outer surface is convex in the axial direction. For a case involving moderate curvature, the peak shear stress and density fluctuations are reduced by 85% compared to an otherwise similar flat shear case. Predictions of optical intensity versus angular refraction are made using a simple ray tracing method applied to the computed density field. The figures below show the difference in optical quality of flat and curved mixing layers. A visible image of a point source is given as a contour plot of the intensity versus refraction angles in both the streamwise and spanwise directions. The results show that turbulent aero-optical distortion is significantly reduced by the introduction of curvature.

Commercial Application: The principal commercial applications of this work are in the development of interceptor missiles' guidance systems which operate near optical or infrared wavelengths.

Reference:

Childs, R. E., "Prediction and Control of Turbulent Aero-Optical Distortion Using Large Eddy Simulation." AIAA Paper 93-2670.