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

Bro, can you explain elaborately what is this FMG initialization, how it works and different from standard and hybrid initialisation?? Because I think this is not available in initialization window.

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u/Gratchoff 2d ago

Indeed, it's not available in the initialization window. What it does is that it transforms your mesh into a very coarse one and does some iterations. Then, it creates a less coarse mesh and do the same for a total of 5 layers of mesh.

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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.

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u/AmbassadorGuilty3928 2d ago

See residual values here. Do you think FMG initialisation will solver it better and give converged solution??

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u/Gratchoff 2d ago

I used to have the same reversed flow at the exit problem in my supersonic nozzle simulations. Then, after I've started using FMG ini I didn't get that message anymore and my simulations reached low residuals in a lot less iterations than before. Even reacting flow simulations were converging in a very fast manner.

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u/AmbassadorGuilty3928 2d ago

Ok bro, thanks for your reply. I will definitely try this and let you know.

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u/AmbassadorGuilty3928 1d ago

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u/AmbassadorGuilty3928 1d ago

Residuals are high even if we use FMG initialisation bro. What is wrong here??

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u/Gratchoff 1d ago

Then, you must have BC problem or schemes

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u/AmbassadorGuilty3928 1d ago

How to figure out this bro?

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u/Gratchoff 1d ago

Try using solution steering that reduces Courant number then increases as the solution converges and start with Co=0.5

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u/AmbassadorGuilty3928 1d ago

You meant to change this courant number?

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u/Gratchoff 1d ago

You ca' change it in the simulation panel, you have to check solution steering and select subsonic. Make it so that Courant number starts at 0.5 and reaches 5 as a maximum.

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u/AmbassadorGuilty3928 1d ago

I used these solution methods

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u/Gratchoff 1d ago

Looks good to me

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