Minimum density - Clipped in Mesh 1 error

Muneeb Khalid

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Mar 13, 2024, 6:33:41 AMMar 13

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Hi all,

I am running a model and I got this error half way through my simulation. The model previously ran perfectly some years ago. Now it is giving this error to me when i made few changes. Any insights on this would be highly appreciated.

Capture.PNG

Randy McDermott

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Mar 13, 2024, 6:49:23 AMMar 13

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Please post your input file. There are many possible reasons for a case to go unstable.

On Wed, Mar 13, 2024 at 6:33 AM Muneeb Khalid <khalid.m...@gmail.com> wrote:

Hi all,

I am running a model and I got this error half way through my simulation. The model previously ran perfectly some years ago. Now it is giving this error to me when i made few changes. Any insights on this would be highly appreciated.

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Muneeb Khalid

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Mar 13, 2024, 6:56:38 AMMar 13

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File is attached. I am running it on FDS 6.7.9.

Model_1_1 - Copy.fds

Randy McDermott

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Mar 13, 2024, 7:07:36 AMMar 13

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Not that this should cause an instability, but you do not need to specify CO as a lumped component. And you should not use a special FORMULA for propane. Just specify FUEL='PROPANE' and then FDS will use the correct propane thermochemical properties.

Do you have sealed domains with pressure zones? This is likely the cause of an instability. The systems have to be perfectly mass balanced; make sure there is a path to an OPEN boundary. And lastly, you might try using the ULMAT pressure solver, as this eliminates mass errors at zone boundaries.

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Muneeb Khalid

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Mar 13, 2024, 7:12:39 AMMar 13

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Thanks for your reply. All is understandable but I just dont know how to implement the last point about ULMAT pressure solver

Randy McDermott

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Mar 13, 2024, 7:22:54 AMMar 13

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Per the user guide...

&PRES SOLVER='ULMAT'/

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dr_jfloyd

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Mar 13, 2024, 8:22:24 AMMar 13

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You have 12 extraction vents at the tops of the two major wings of the building that each remove 20 m3/s. You have no makeup air, no lekaage (not that typical leakage could reasonably keep up with 240 m3/s of flow), and I don't see any openings in the exterior envelope of the building to the outside. If you have a sealed volume, and you remove air from it, you are going to reduce the density.

Muneeb Khalid

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Mar 13, 2024, 9:17:14 AMMar 13

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yes, i understood my mistake now. Although, to be clear, only 6 fans activate, remaining 6 do not activate for this simulation. But I have added inlets now to the model to make up for the air being extracted

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