Aero-optical wavefronts and scale-local characterization in large-Reynolds-number compressible turbulence

A new technique is proposed that enables the scale-local characterization of aero-optical wavefronts. Because optical wavefronts degraded by turbulent flows are physically highly anisotropic and exhibit distortions over a wide range of scales, a method is needed to examine the wavefront structure at varying degrees of anisotropy and as a function of scale. We introduce an aero-optical-wavefront-anisotropy (AWA) parameter as the ratio of scaling factors for the wavefront distortions and for the wavefront transverse extent. This AWA parameter, combined with box counting, enables a scale-local anisotropic examination of the wavefronts. We demonstrate this technique on wavefronts derived from experiments on large-Reynolds-number (Re similar to 10(6)) high-compressibility (M-c similar to 1) turbulent shear layers between dissimilar-index-of-refraction gases. Variation of the AWA parameter and scale-local examination of the distortions reveal the presence of anisotropic self-similarity, or self-affinity, at small scales spanning nearly a decade. This finding shows the utility of the technique to detect scaling in large-Reynolds-number flow experiments from the aero-optical behavior. The present finding and technique provide key ingredients to extrapolate the small-scale properties of compressible-flow aero-optical wavefronts to higher Reynolds numbers and are also useful for modeling and for computational aero-optics.