Heat transfer via thermal radiation is a common occurrence in industry; be it in flue gas, boilers, reactors, or in supersonic combustor such as in a scramjet propulsion.
Numerous studies have been done regarding the phenomena interacting with thermal radiation in different types of turbulent flows.

For compressible flows the effect of thermal radiation on the turbulent field via Turbulent Radiation Interaction (TRI) has been research to a lesser degree than for incompressible flows. Therefore, a more in-depth study on the impact of optical thickness in compressible flows should help create a better understanding regarding this.
This study consists of an investigation into the effect of thermal radiation in a non-reacting supersonic channel flow where the optical thickness of the fluid is changed.
The fluid dynamics is simulated using a Direct Numerical Simulation (DNS) code for compressible turbulence and for the thermal radiation a grey-gas Finite Volume Method is used. A fictitious fluid is used with two different Planck numbers (Pl = 0.1, 0.01). Furthermore, for cases with Pl = 0.01 the constant absorption coefficient is varied between κ = 1, 5, 10.

The effects of thermal radiation on the temperature and density fields are discussed. Changing the optical thickness via the absorption coefficient shows a strong change in the behaviour of the fluctuating fields.
Compressibility is shown to be affected by thermal radiations, where a stronger thermal radiation characterized by a high optical thickness and low Pl number show characteristics of an incompressible flow while being supersonic.

A model to determine the fluctuating incident radiation developed for high optical thickness incompressible flows is applied to this study as-is to investigate if the model is suited. It is shown that the assumptions made for the model, especially regarding thermal structures size, do not hold for compressible fluids and a different approach is needed.