Optical Characterization of a Simulated Weakly Compressible Shear Layer: Unforced and Forced

Abstract

This paper uses a discrete-vortex code to examine a shear layer’s response to forcing at its origin and to develop a relationship between a shear layer’s optical characteristics and the commonly used characteristic growth length, vorticity thickness. The code and its thermodynamic overlay have been used in previous studies to predict the optically-aberrating characteristics of relatively-high-Mach-number, subsonic shear layers that can be classified as weakly compressible. A weighted average natural frequency is introduced and used to characterize the unforced shear layer in terms of an optical characteristic length referred to as optical coherence length. It is shown that optical coherence length is related to vorticity thickness by a factor of approximately 3.18. The study also shows that the use of singlefrequency forcing produces a regularized shear layer for distances preceding the point where the unforced shear layer’s natural frequency matches the forcing frequency. In the case of the forced shear layer, a greater thickness is produced closer to its point of origin until collapsing * Graduate Research Assistant, Aerospace and Mechanical Engineering Department, Fitzpatrick Hall, Notre Dame, IN 46556, DEPS Fellow, AIAA Student Member. ** Research Assistant Professor, Aerospace and Mechanical Engineering Department, Hessert Center, Notre Dame, IN 46556, AIAA Member. § Professor, Aerospace and Mechanical Engineering Department, Hessert Center, Notre Dame, IN 46556, AIAA Fellow.