Wanning Minimum Density

[Oluwafemi Salami]

Hello Pls,

How can I resolve this instability?

[fde]

Have you checked User Guide  4.2 Common Error Statements >  Numerical Instability Errors: I

[Ballal Patil]

Could you share your file?

Usually I've got this error only when there is no ambient flow and gravity is also disabled. For some reason FDS seems to struggle with pressure based flows.

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[Randy McDermott]

See section 19.5.2 Density in the user guide.  There you will see that you can control the min and max density that FDS permits.  Within the applicability of the low-Mach assumption and the usual caveats about velocities errors at mesh and solid boundaries (which need to be tight for sealed compartments), FDS usually only has issues with pressure-based flows if the user specifies an unphysical condition, like a mass imbalance.

If I think I will be close to the minimum density limit, I will add

&CLIP MINIMUM_DENSITY=0.01/ 

If everything else is set correctly, this should get you over that hump, but possibly at the cost of a smaller time step.

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[Oluwafemi Salami]

Hello, Please see the file. Thanks 

[dr_jfloyd]

&SURF ID='Fire',
      COLOR='RED',
      HRRPUA=2.9E+4,
      TAU_Q=-1.0,
      TMP_FRONT=300.0/

&VENT ID='Vent fire', SURF_ID='Fire', XB=101.0,103.0,51.0,53.0,0.0,0.0, SPREAD_RATE=0.0, XYZ=102.0,52.0,0.0,COLOR='BLUE'/

(103.0 m - 101.0 m) * (53.0 m - 51.0 m) * 29,000 kW/m2 = 116,000 kW

In 1 s you are trying to go from a 0 kW fire to a 116 MW fire. This is not a fire. This is an explosion.

[Oluwafemi Salami]

Hello Scholars, 

I have corrected the fire size and area and it is still having that "Mining density " problem. I suspect it is from the ventilation duct. Because when I tried the code without the ducts and HVAC, it did work. Please help me look at this. Thanks 

[Kevin McGrattan]

I will run the case. I suggest that you run the case without a fire, just to test the HVAC parameters.

[Kevin]

What version of FDS are you running?

[Kevin McGrattan]

I am running the case with a nightly test version and it has surpassed 50 s.

[Oluwafemi Salami]

I am using fds 7.7

[Oluwafemi Salami]

Pls, Did you run it without the HVAC?

On Monday, October 16, 2023 at 11:00:33 AM UTC-5 Kevin McGrattan wrote:

[Kevin McGrattan]

There is no fds 7.7

Show me what is written at the top of the .out file, like this

 Fire Dynamics Simulator

 Current Date     : October 16, 2023  10:04:34
 Revision         : FDS-6.8.0-635-g2e1bf02-master
 Revision Date    : Mon Sep 18 10:20:24 2023 -0400
 Compiler         : ifort version 2021.7.1
 Compilation Date : Sep 18, 2023 15:48:02

 MPI Enabled;    Number of MPI Processes:       8
 OpenMP Disabled

 MPI version: 3.1
 MPI library version: Intel(R) MPI Library 2021.7 for Linux* OS


 Job TITLE        : Base_Model
 Job ID string    : model3

[Oluwafemi Salami]

Sorry I meant  6.7.7

[Oluwafemi Salami]

[Kevin McGrattan]

Time to upgrade

[Oluwafemi Salami]

I did have the modification.  I still got some numerical instability errors but now the problem has changed to " Warning: Maximum Density "

On Monday, October 16, 2023 at 3:11:34 PM UTC-5 Kevin McGrattan wrote:

Time to upgrade

[Kevin McGrattan]

Why do you embed your corridor within a mesh of unused cells? I do not understand what you are trying to do. My simulation of your input file also failed with a density error which tells me that there is something fundamentally wrong with your set up. I suggest you simplify the mesh and try the HVAC without the fire.

[Oluwafemi Salami]

Hello Kevin,

Thanks for the help. I have simplified the model. I am trying to model fires in the development face of an underground mining environment. The "corridor ' is the main drift of the mine and we want to see how the smoke spread from the development face to the main drift. 

However. this model still has a numerical instability problem. I have attached the fds file. I guess there is something I am not doing well about the ventilation and HVAC. Please help me look at this model. Thank you 

[Kevin McGrattan]

Did you test the HVAC system without a fire like I told you? What were the results?

[Oluwafemi Salami]

Yes. Running the model without a fire. Numerical instability at less than 1 s

[sssq ss]

may be you could plus this to your file：
&MISC CHECK_HT=TRUE./
&PRES VELOCITY_TOLERANCE=0.01,MAX_PRESSURE_ITERATIONS=100/