KC-135 aero-optical turbulent boundary-layer/shear-layer experiments

The aerooptics effects associated with the propagation of a laser beam through the turbulent boundary layer of an aircraft, as well as artificially generated shear layers, are examined through comparisons of observed optical performance levels with those inferred from aerodynamic measurements of unsteady density and correlation lengths within the same random flows. Results are presented from 13 flights conducted at 0.3-11.3 km altitudes and Mach numbers from 0.25 to 0.85. Single-pass phase variance for the boundary layer varied from 0.08 to 0.18 waves at 0.63 microns, for aircraft dynamic pressures ranging from 0.04 to 0.19 standard atm. These values are associated with density correlation lengths of 1-3.5 cm for the 30 cm thick boundary layer disturbances. Aircraft skins are found to act as adiabatic walls. The collected data allow scaling with wavelengths and aperture as well as aircraft parameters, such as boundary layer thickness and dynamic pressure.