Aero-Optical Turbulent Boundary Layer/Shear Layer Experiment On The KC-135 Aircraft Revisited

Abstract

This paper examines the aero-optical effects associated with propagating a laser beam through both an aircraft turbulent boundary layer and artificially generated shear layers. The data present comparisons of observed optical performances with those inferred from aerodynamic measurements of unsteady density and correlation lengths within the same random flow fields. Using optical instrumentation with tens of microseconds temporal resolution through a finite aperture, optical performance degradation was determined and contrasted with the infinite-aperture, time-averaged aerodynamic measurement. In addition, the optical data were artificially clipped to compare to theoretical scaling calculations. Optical instrumentation consisted of a custom Q-switched Nd:YAG double-pulsed laser and a holographic camera that recorded the random flow field in a double-pass, double-pulse mode. Aero-dynamic parameters were measured using hot film anemometer probes and a five-hole pressure probe. Each technique is described with its associated theo-retical basis for comparison. The effects of finite aperture and spatial and temporal frequencies of the random flow are considered. The results presented represent five flights flown at altitudes from 1.8 km to 10.7 km and at Mach numbers from 0.32 to 0.79. Single-pass phase deviations for the boundary layer were from 0.06 to 0.17 waves (at X = 0.53 ;Am) with piston and tilt components removed. Measured phase deviations for the artificially induced shear flows were from 0.10 to 0.279 waves (at X = 0.53 /um) with piston and tilt components removed. However, when low order aberrations through coma were removed, the remaining deviations were only 0.09 to 0.18 waves. This resulted in a 33 to 250% increase in the Strehl ratio at the 14 cm optical aperture. It was further shown that the low order aberrations corresponded to the longer wavelengths in the random flow, and these waves propagated with a longer characteristic time than the higher order aberrations (200 to 1000 /As versus 50 to 100