Unexpected High Negative Pressure

[Fraser Munnoch]

Hello all,

I am having problems with high negative pressures within my simulation. I am using FDS 6.7.9, but have also tested the model in 6.8.0, with the same results.

The model consists of a stairwell, lobby, corridor and fire compartment. When the door between the lobby and stairwell is open, i am getting over 200Pa pressure difference across the door when the extract in the lobby is running at 3m3/s.

I have run test models of just the stairwell as well as the stairwell and lobby. The pressure problem does not arise in these test models. To construct the test models, I used the 'problem' model and removed the fire compartment and corridor. I did not make any changes to the lobby or stairwell.

There are pressure sensors within the stairwell and these corroborate the slices.

I have attached images of the problem and the comparison with the test model. I have also attached my input file.

Any help would be greatly appreciated.

[Kevin McGrattan]

I will run the case.

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[Kevin McGrattan]

This case has been transferred here

https://github.com/firemodels/fds/issues/12641

When I ran the case with the latest code, it failed after 4 time steps with an array over-shoot error. We're working the case, but you might want to do the following:

1. Do not specify SURF_ID='HVAC' for VENTs that use localized leakage. Provide a SURF_ID similar to the one you use for the entire door.

2. Do not apply localized leak VENTs to OBSTructions that can disappear. Put the vents on the floor instead.

[Fraser Munnoch]

Hi Kevin,

I have updated my model with the comments above about the HVAC surfaces and not putting the vents on obstructions that have controls. Unfortunately, these have not fixed the pressure problem on my end.

However, i have changed some of the mesh dimensions and made the FC, lobby and bottom of the stairwell all in the same mesh, and so far, it looks like this might fix the problem. I am not sure why altering the mesh may have fixed the problem.

I have attached my latest revision.

[Kevin McGrattan]

There appears to be a problem where the merging of pressure zones is not working right. This might involve communication among the meshes, which is why your solution may have solved this particular problem. I'll continue to work the issue.