Large-scale refractive turbulent interfaces and aero-optical interactions in high Reynolds number compressible separated shear layers

Abstract

Refractive turbulent interfaces and their role in large-scale aero-optical interactions are examined in high Reynolds number compressible separated shear layers. A new variable pressure turbulent flow facility has been developed which enables experiments for direct imaging of refractive turbulent fields in purely gaseous flows. In the present experiments, the freestream fluid consists of air molecularly premixed with acetone vapor and the ambient gas is air. Direct imaging of the refractive index field is conducted using laser-induced fluorescence and digital photography techniques. A pulsed ultraviolet laser sheet is utilized to excite the acetone vapor resulting in fluorescence in the visible spectrum which is recorded with a high-resolution digital camera system. High-resolution two-dimensional spatial images of the refractive index field are recorded at a Reynolds number of Re ∼ 6 × 10⁶, based on the visual thickness, with a convective Mach number of Mc ∼ 0.4 and a test section pressure of p ∼ 3 atm. Quantitative visualization of the refractive interfacial thickness field shows that the large-scale high-gradient interfaces are highly irregular and present both in the flow interior and at the outer boundaries of the compressible separated shear layer. These outer and internal interfaces separate large-scale low-gradient refractive index field regions. While these interfaces occupy spatially a relatively small fraction of the refractive index field, they are found to generate the primary contributions to the large-scale aero-optical interactions because of the large magnitudes of the refractive index gradients across these interfaces. This observation suggests that the high-gradient refractive interfaces, both in the flow interior and at the outer boundaries, determine the large-scale aero-optical interactions. Significant robustness of the aero-optical interactions to reduction in refractive field information is found, by retaining only high-gradient interfaces, which provides a key ingredient for physical descriptions, modeling, and optimization in aero-optics.

Acknowledgements

The present work is part of a research program on turbulence and the dynamics of flows. We are grateful for the support of the Air Force Office of Scientific Research and the Defense University Research Instrumentation Program. Insightful discussions with Col. D. Kyrazis and Professors D. Dunn-Rankin, S. Elghobashi, R. Hugo, E. Jumper, J. LaRue, and R. Truman are acknowledged. We are also grateful to the referees for their helpful comments.