Optical refraction from high Mach number turbulent boundary layer structures

An experimental and numerical study was conducted to investigate the grazing incidence refraction of optical rays due to interaction with the discrete structures present in a high Mach number turbulent boundary layer. This study was motivated by the development of the Radiatively Driven Hypersonic Wind Tunnel for which high energy laser light must be coupled into supersonic air with minimum wall heating. Data are presented to illustrate the instantaneous structure of the turbulent boundary layer over a flat plate in a Mach 8 flow, with and without normal sonic injection of helium through a transverse linear slot located just downstream of the boundary layer trip. The striking ability of helium injection to reduce the turbulent mixing is illustrated and discussed. A numerical model for propagating optical rays through an arbitrary density field was employed to examine the interaction of rays with the turbulent structures. Representative flow field images were analyzed using a bimodal density model with a density gradient at the turbulent structure interface in order to evaluate the first order sensitivity of the ray refraction to outer layer behavior. Ray tracing Graduate Student, Mechanical & Aerospace Engrg., Senior Member, AIAA t Graduate Student, Mechanical & Aerospace Engrg., Member, AIAA * Professor, Mechanical & Aerospace Engrg., Ohio State University, Member, AIAA § Professor, Mechanical & Aerospace Engrg., Associate Fellow, AIAA This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. results are presented to illustrate these effects and implications regarding ray penetration to the wall and total reflection are discussed. Preliminary analyses revealed very weak refraction for densities corresponding to the experimental test conditions. However, strong refraction and scattering were observed for the high density conditions expected in a high pressure hypersonic wind tunnel. Helium injection, which smoothes the boundary layer interface, reduces the size of outer layer structures, and increases the density change from free stream to wall, may reduce ray penetration to the wall.