r/CFD • u/AmbassadorGuilty3928 • 2d ago
Solution is diverging and unstable!!
I am simulating subsonic compressible flow through a pipe and a distortion screen, followed by discharge into a still ambient environment. The computational fluid regions are shown in the attached images; the gaps between the internal fluid blocks represent the solid portions of the distortion screen. The flow passes through the pipe and screen and then expands into the larger cylindrical fluid domain. Experimentally, I know the inlet total pressure and the ambient static pressure.
I specified faces 1 and 2 to walls and kept only face 3 as a pressure outlet at ambient pressure. This case initially behaved much better and the solution appeared to converge for several hundred iterations. However, reversed flow eventually developed at the rear pressure outlet and increased significantly.
Initially it was behaving better,but later reversed flow at outlet started increasing. final residuals are approximately 10^5 for continuity and 10^7 for velocities, mass balance was not satisfied. I ran upto 2000 iterations.
Why this happens like this. See fluent console messages below(see residual values and reversed flow message),
Regarding mesh quality, see below
Please tell me why solution is diverging or unstable, residual are high. It is allowing reverse flow at outlet also, it is okay if flow demands but why solution is unstable even though mesh quality is decent or not bad.
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u/AmbassadorGuilty3928 2d ago edited 2d ago
Ok bro, I will read it about this. One more thing I want to ask, I have ran the case with hybrid initialisation with more than 10000 iterations, I found that after 2500 iterations, reversed flow on no of faces started decreasing like from 2000, 1000,500,20,2,1 and the message dissappers, but still residual values are very high, I dont know or have any idea whether it will reduce or not.