Learning Forum

[Jinhong Wang]

Hi, I am trying to run a supersonic compressor cascade (ARL-SL19) case from the literature. All the literature suggested using pressure far-field inlet B.C and I did get the expected results using this. 

However, if I swap to pressure inlet condition (specifying both total and supersonic static pressure), fluent converged to a result with subsonic upstream flows. I looked into the literature and did not manage to find any supersonic inlet cases with a pressure-inlet B.C. I am wondering if it is the nature of the pressure-inlet B.C. for the supersonic inlet because it seems my specified supersonic inlet static pressure gets neglected after iterations. My current understanding is that there is a difference between the characteristic (pressure far-field) B.C. and pressure B.C. and the flexibility of characteristic B.C. will work better for my cascade case, but I would really appreciate it if you could explain it a bit further.

If it helps, I also tried the same case in CFX but the supersonic inlet option gives overflow and an exceptionally high Mach number was reported just at the inlet.

Moreover, the next step I am trying to do is to implement real gas calculation and it seems pressure far-field B.C. does not work for real flows. I am wondering if you have any recommendations. Thank you very much.

[Federico]

How are you initializing the solution? If you start with the fluid at rest and the solution tries to "accelerate" the flow to the inlet BC, you might get choked flow with subsonic regime upstream before the flow can become supersonic.

[Jinhong Wang]

Sorry for not replying properly. As the last comment, I have initialised everything from inlet condition (supersonic)

[Federico]

Make sure the Gaute Total pressure and Supersonic/Initial Gauge pressure are defined relative to your operating conditions, and look at your contours after initialization to confirm that the flow is initialized as you expect. The Supersonice/Initial Gauge pressure will be ignored if the flow is subsonic.

[Jinhong Wang]

I am pretty sure that my initialisation condition is supersonic by checking both the total and static values after initialisation.

[Jinhong Wang]

I am initialising everything from the inlet (i.e. supersonic regime)

[Shiva K.]

I also had the same issue when I was simulating ARL-s19, the same airfoil you are using. Try defining the periodic interfaces properly. I will advice to go with P-far field inlet.