Assessment of Hypersonic Flow Physics on Aero-Optics

In a hypersonic environment, the high kinetic energy of the oncoming flow causes the molecules in the flow to be thermally excited, leading to dissociation. In such a flow field, an aero-optical analysis that considers thermochemical nonequilibrium may be necessary to assist in sensor design. The present study assesses the optical properties of a nonequilibrium, real gas flow field around a hemisphere cylinder. The simulations are conducted at freestream Mach numbers of 11, 13, and 15. The optical distortions are quantified using optical path length and optical path difference. Optical distortion is also predicted using a perfect gas assumption, and these distortions are provided for comparison. The primary contributions of nonequilibrium parameters on optical distortion are identified. For all Mach numbers, optical path lengths are higher when the effects of dissociation and vibrational relaxation are included. The computational results show that oxygen dissociation is the dominant nonequilibrium flow phenomena affecting the optical distortion. However, as the freestream Mach number increases, atomic nitrogen begins to have an additional influence on optical aberrations.