Aero-optical characteristics induced by shock wave in hypersonic flow field

Abstract

Hypersonic flow field around flight vehicle is complicated and induce various problem. Especially, flight vehicle with optical system experience aero-optical aberration induced by hypersonic flow field. Aero-optics is a field which studies interaction between flow field and electromagnetic wave propagation near flight vehicle. Turbulence, shock wave and boundary layer at near flow field of flight vehicle induce index of refraction variation, distorting electromagnetic wave propagation characteristics. Aero-optical phenomena decrease optical performance and limits accuracy of optical system loaded on flight vehicle. Research have been conducted to find out the relation between flow field characteristics and optical characteristics. Accordingly, various devices have been developed to measure and analyze aero-optical characteristics. Recently, an optical sensor called Shack-Hartmann sensor was developed and performance was compared with other optical sensor. In this thesis, hypersonic flow field is demonstrated through shock tunnel and aero-optics experiment is conducted. Shock tunnel performance is validated by analyzing pressure distribution inside driven tube and measuring test time inside test section. Conical nozzle and contoured nozzle is installed at the shock tunnel to generate Mach 7 flow. Nozzle flow characteristics is studied through stagnation pressure measurement with pitot rake and flow visualization of wedge model with shadowgraph or schlieren technique. Conical nozzle has 63% decrease of stagnation pressure during nozzle flow expansion and exit flow is Mach 6.8. Contoured nozzle has little loss of stagnation pressure and exit flow is Mach 6.9. After shock tunnel performance and nozzle performance is validated, aero-optical characteristics is measured using Shack-Hartmann sensor. Shack-Hartmann sensor performance is studied and validated with experiments on optical table. Wedged window and hot wire is used as a density disturbance. Wedged window tilts the laser to angle which is same as refractive angle calculated through refraction law. Hot wire experiment result reveals increase of optical aberration as power consumption of hot wire increase. Aero-optical characteristics induced by shock wave in hypersonic flow field is studied. Wedge model is installed inside shock tunnel test section and Shack-Hartmann sensor measured phase and intensity, which is converted to point spread function (PSF). From PSF, aero-optical parameter such as bore sight error (BSE), Strehl ratio is calculated. In the results, average BSE and tilt increased as stagnation pressure increased. Average BSE is 80 ~ 160μrad, and Strehl ratio is 0.65 ~ 0.835. Shock wave in hypersonic flow increase BSE, tilt and decrease Strehl ratio.