Nonequilibrium Effects on Aero-Optics in Hypersonic Flows

Hypersonic flows present unique challenges due to the complex interplay of fluid dynamics, chemical reactions, and optical phenomena. When an optical signal traverses a hypersonic flow field, a comprehensive understanding of the impact of nonequilibrium effects on aero-optics becomes crucial. This understanding is necessary to accurately assess optical distortions, sensor performance, and signal loss across the flow. Hypersonic flows exhibit high complexity, encompassing nonequilibrium, reactive flows (involving chemistry), energy transfer between molecular energy modes, turbulence, boundary layers, unsteadiness, and shocks. These factors can directly or indirectly affect and degrade optical signals as they propagate through the flow medium. While previous research in aero-optics has predominantly focused on low-speed applications with perfect gas assumptions, this study investigates the influence of real gas effects, particularly nonequilibrium effects. The study specifically examines three objectives: the impact of chemical reactions on the polarizability constant, the optical properties of a typical hypersonic flow (including the index of refraction and the dielectric constant of the medium), and the effects of turbulence on the flow. This research reveals significant findings. Firstly, it demonstrates that polarizability cannot be assumed to be solely a function of the signal’s frequency. Instead, it is also influenced by the particle’s state, characterized by its energy modes, which are closely related to the temperature. Secondly, the study highlights the strong correlation between the index of refraction and the dielectric constant of the medium with the flow’s density. Lastly, the investigation establishes that turbulence has a discernible impact on the optical properties of the flow. In summary, this work underscores the importance of considering nonequilibrium effects in aero-optics within hypersonic flows. It sheds light on the complexities involved and provides valuable insights into the impact of chemical reactions, density variations, and turbulence on optical signals. These findings contribute to advancing our understanding and pave the way for improved modeling and optimization of aero-optical systems in hypersonic applications.