Aero-Optical Assessments of Hypersonic Flowfields

Abstract

A potential future use of hypersonic platforms is for responsive Intelligence, Surveillance, and Reconnaissance (ISR). It is common for these types of missions to employ Electro-Optical/Infrared (EO/IR) sensors and Radio Frequency (RF) sensors to collect information. If an optical signal were to travel through a hypersonic flowfield, the type of high-speed flow analysis required to perform reliable assessments of sensor performance is unclear. In order to provide this information, the hypersonic flow phenomena of thermochemistry, high speed turbulence, and high enthalpy turbulence are studied.

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 flowfield, an aero-optical analysis that considers thermochemical nonequilibrium is necessary to assist in sensor design. In the present thesis, we assess the optical properties of a nonequilibrium flowfield. The optical distortions are quantified using optical path length (OPL) and optical path difference (OPD). Optical distortion is also assessed based on flowfield modeling performed using a perfect gas assumption, and these distortions are provided for comparison. The primary contributions of nonequilibrium flow properties on optical distortion are identified. For all Mach numbers, OPLs are higher when the effects of dissociation and vibrational relaxation are included. Oxygen dissociation is the dominant nonequilibrium flow phenomenon affecting the optical distortion. As Mach number increases, atomic nitrogen begins to have an additional influence on optical aberrations.

Additional distortion can arise in the hypersonic regime from high speed turbulence within the boundary layer. An initial study is conducted on a Mach 6 flow over a flat plate. This simulation is performed using air as a calorically perfect gas with varying levels of turbulence modeling and resolution. The cases are run using a laminar flow assumption (no turbulence), the Menter SST closure model for the Reynolds Averaged Navier-Stokes equations (RANS), and Implicit Large Eddy Simulation (ILES). It was found that the OPDs produced by the highest fidelity ILES approach can be as much as two orders of magnitude larger than solutions produced by the Menter SST model and laminar flow. While RANS was able to predict the mean flow quantities, it is unable to correctly predict the optical distortion.

The effects of thermochemical nonequilibrium on turbulent flow, and consequently optical distortion, are also presented in this dissertation. In the present study, numerical simulations are utilized to perform implicit large eddy simulations of flows over an adiabatic flat plate. The simulations are run with and without thermochemistry models to determine the effects of thermochemical nonequilibrium on optical distortion. Accounting for thermochemical nonequilibrium predicts less variation in OPD across the sensor aperture. The RMS average of OPD is significantly smaller for the real gas simulation when compared to a perfect gas. These differences in OPD occur because nonequilibrium energy exchanges act to damp out turbulent fluctuations, as illustrated with the Taylor-Green Vortex problem in this thesis. It is therefore necessary to include these physical flow effects in optical assessments to obtain an accurate description of the aero-optic distortions.