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Author Topic: Direction of internal fans changed when FDS5 --> FDS6  (Read 4527 times)


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Direction of internal fans changed when FDS5 --> FDS6
« on: January 06, 2014, 12:24:32 pm »

Dear all,

I often use internal fans in car park smoke exhaust simulations. I had a case that caused me quite some troubles in FDS5 due to repeated numerical instabilities. In FDS6 however the simulations remain stable, so that's a good step forward!

But I had a minor problem when making the switch: all the internal fans changed flow direction: they all started to blow backwards.

To investigate the problem, I started a pyrosim 2012 session and a pyrosim 2013 session; in both I imported the FDS5 fan_test sample file that comes with both installations. I ran both models and indeed: fan directions are switched!

Anyone out there who encountered the same?
Is there an easy to understand reason why this happens?

I switched velocity value signs on my car park fans, and launched the simulation again.  I hope to got the same results as in the Pyrosim 2012 model...

happy modeling,

Jon Albrecht

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Re: Direction of internal fans changed when FDS5 --> FDS6
« Reply #1 on: January 07, 2014, 09:22:12 am »


Sorry for the confusion. I took a look at the sample file you mentioned, and thought it may be pertinent to explain a few things. The FDS5 sample file, fan_test, uses multiple variables that no longer exist in FDS6 (and PyroSim 2013). The SURF variable, POROUS, used to identify jet fan surfaces, no longer exists. To simulate a jet fan in 2013, see our examples on using the new HVAC features.

The duct path variables have also been removed. This means that there in the fan_test example, there is nothing describing the directionality of the vents, except for the +/- signs on the VEL variable.

According to the FDS User's Guide, "If VEL is negative, the flow is directed into the computational domain, i.e., a supply vent. If VEL is positive, the flow is drawn out of the domain, i.e., an exhaust vent.

Hopefully that helps. If you have any more specific questions, please let us know.