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Author Topic: Numerical instability  (Read 2145 times)

Torbjørn Haugen Knutson

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Numerical instability
« on: April 07, 2015, 10:57:09 am »

Hello again.

I'm having a problem with "numerical instability". I thought it had something to do with too much turbulence in the model's air-/smokeflow, but whatever I do it still stops after about 80-90 seconds (out of 300 s)

My efforts to fix it:
- Split meshes (till a total of 4 meshes)
- Increase vent sizes (air out) for HVAC
- Incresae vent size (air out) for smoke hatchet
- Increase vent size (air in) for HVAC/smoke hatchet
- Decrease flow rate for smoke hatchet
- Change location of vent (air in), further away from smoke hatchet
- Decrease number of fires (burners) from 2x350kW to 1x700kW
- Remove smoke hatchet completely

If someone have some other suggestions, I would be most grateful.

Kind regards


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Re: Numerical instability
« Reply #1 on: April 15, 2015, 09:28:02 am »

I started to run the simulation and the quick answer for why your problem is crashing is that the pressure is continually declining. See the attached image (this is just a contour plot, it would be better to add a device measuring pressure to get a time history plot).

Unfortunately, I do not have time right now to try to debug your model, but somehow you are withdrawing more air than is being added to the model. Usually a building should have at least one vent open to the atmosphere to act as a supply. If you have a closed system, you need to make sure your total flow in and out of the building is zero.

Daniel Swenson
Thunderhead Engineering


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Re: Numerical instability
« Reply #2 on: April 20, 2015, 03:58:07 am »

Did you specify a natural outlet (an opening), you will need a pressure relief if you have a heat source.