Viscous model for compressible flows through micro nozzle
 

Hi guys, I am currently working on a micro nozzle design for nano satellites. I have a doubt regarding the viscous model which will suit best to my compressible flow inside my convergent nozzle. There can be rarefied flow, flow separation and adverse pressure gradient at the throat. I have used K-omega SST model and k-epsilon realizable model as they are said to be working good in flow separation and low Reynolds number problems but I am not sure about my results.

I have gone through some papers also there they are mainly using DSMC schemes which is not available in ANSYS Fluent and it is also computationally intensive. Can you guys suggest me which viscous model or numerical method should i choose to get the approximate best solution:).