This chapter contains information about the aero-optics model, which allows you to assess the impact of fluid flow on the functioning of an optical device. For example, you may be simulating the flow of air around an aircraft fitted with a turret, which contains a laser system used for communications, imaging, or energy-based weapons. When the flow around the optical aperture has complicated compressible turbulent flow features (from the boundary and shear layers, wakes, shocks, and so on), the fluctuations in density result in a corresponding fluctuation in the refractive index of the fluid. This is referred to as "aero-optical distortion", and it affects the performance of the optical device in terms of jitter, defocus, and effective range. The aero-optics model in Fluent allows you to define optical beams from various surfaces that are directed through the flow features of interest; ray-tracing calculations are performed for the faces of each designated surface using multi-domain architecture (through a one-way coupling), so that optical phase distortion data for each beam is computed concurrently with the flow simulation and is then available for postprocessing.