[chc]

I'm encountering an issue when trying to utilize a fixed pressure boundary condition instead of a zeroGrad where the value would float. I'm attempting to simulate a supersonic aircraft inlet and what happens when I choke the flow.

For these cases, I have a geometry with mainly walls, 1 inlet, and two outlets. One of the outlets belongs to the downstream portion within the aircraft inlet. I'm using an expanded duct to use a constant back pressure as one of the BCs, from this article on page 40 of the pdf: https://apps.dtic.mil/sti/tr/pdf/ADA291365.pdf

With initial cases, I'm allowing the outlet in question to be zeroGrad to let the pressure float to determine what values I should look to test around. I get to uniform values around 700kPa.
However, when I try to use a uniform pressure boundary condition on that outlet, I notice an exponential rise in the pressure that grows without bound. I have not ran the simulations beyond .25s so I'm unsure about its stability in the long term but regardless, this is problematic, since I'm using the same pressure value as was seen in the zeroGrad test.

I am under the impression that I should be achieving similar results by setting the uniform pressure BC as with the zeroGrad case.

I have pictures and plots within my case on github in the README portion which helps visualize it.
https://github.com/CChe36/0_22/tree/main
Can anyone provide insight as to why this may be happening and what I may be missing? Thank you.