New interfacial fluid thickness approach in aero-optics with applications to compressive turbulence

A general approach in aero-optics is proposed based on the physical thickness of refractive fluid interfaces. In turbulent flows between dissimilar-index-of-refraction or optically different gas streams, particularly at large Reynolds numbers, the interfacial fluid thickness is highly variable. The role of this interfacial fluid thickness in aero-optical interactions is examined by directly relating the optical path length (OPL) to the interfacial fluid thickness (IFT) variations. This is done by expressing the OPL as an integral of the IFT variations along the beam propagation path. The proposed IFT approach is demonstrated on refractive-field measurements in large Reynolds number (Re similar to 10(6)) high-compressibility (M(c) similar to 1) shear layers between optically different gases. Highly irregular networks of isolated high-gradient interfaces are observed at various transverse locations in the flow, that is, both in the interior and near the outer boundaries. The observation that the high-gradient interfaces are spatially isolated and the OPL interpretation in terms of the IFT variations are utilized to propose and demonstrate a new modeling approach for compressible flow where the high-gradient interfaces are the dominant elements needed to capture the large-scale optical distortions. The present approach suggests a new point of view for relating aero-optical distortions to the flow structure in terms of the IFT variations.