Assessment of continuum breakdown for high-speed chemically reacting wake flows

The Generalized Chapman Enskog (GCE) breakdown parameters for a two temperature model have been used to assess continuum breakdown for a Mach 24 reacting flow past a circular cylinder. These rigorously derived parameters indicate the regions in the flow field where a solution based on the continuum approximation would no longer accurately reproduce the governing physics. The analysis performed in this work indicates the presence of breakdown regions at the cylinder shock and near the cylinder surface, in the front part of the cylinder. Breakdown regions were also found to extend in the cylinder wake up to the aft cylinder surface. The breakdown parameter indicated that the strong velocity gradients set up in wake region are responsible for distorting the velocity distribution function leading to continuum breakdown. It is also observed that within the recirculation zone adjacent to the cylinder wall, the steep gradients in the temperature are responsible for causing continuum breakdown. A comparison between the rigorously obtained Generalized Chapman Enskog breakdown parameters and the state of the art Gradient-Length-Local Knudsen number reveals that former predict larger breakdown regions, especially in the cylinder wake region.