Issue with Numerical Instability and Open Vents Placement in PyroSim Model

[Darwishjahan]

Hi all,

I'm working on a model where I’ve already matched the supply and exhaust volume flow rates, as suggested by some experts earlier, to ensure proper airflow. However, I'm still encountering numerical instability. My mesh size is 0.2, which is smaller than the maximum mesh cell size recommended by the D-star calculator from Thunderhead Engineering. I am doing this for a model similar to basement car parking.

Additionally, I tried changing the supply surface to "inert" and placed open vents on each supply surface to control the air intake, following the guidance from the PyroSim User Manual (Chapter 1, Point #5). Despite this, I’m receiving the error: "ERROR: OPEN, MIRROR, OR PERIODIC VENT must be an exterior boundary."

Has anyone experienced a similar issue or can provide guidance on how to resolve this?

Thanks in advance!

[Vojta-Salek]

Please provide your input file and check if you have any open VENT away from the exterior boundary as the error message suggests.

[Er9y714]

Can you share your input file?

[Darwishjahan]

Hi, Thanks for the response. I am still in the learning phase and trying to understand the correct setup.

While going through the manual, I came across the recommendation that the supply and extraction should be balanced, and that it’s advisable to either set the supply or extraction fans to an open or ambient state in a closed volume to ensure that pressure differences are matched appropriately. This led me to make the supply surface open and the extraction surface set to an exhaust with a defined volume flow.

I now understand that the OPEN Surface can only be placed on the mesh's exterior boundary. However, I would like to clarify further: Is it possible to define an extraction flow rate at the extract surface while leaving the supply as ambient/open, as I intended? Could you please advise on how best to set this up within the constraints?

Additionally, I downloaded a small basement parking CAD file and did set up the model. when meshing throughout the volume it has 1.4 million mesh cells approximately. I have an i7 processor with 8 logical processors and base speed 3.60GHz. After Generating the mesh volume, I did split it using the partition mesh option to 8 meshes and ran FDS parallel. It shows me around 250+ hours to complete 1200s. Does it seem to be approximately right or too long for a this kind of a volume.

Looking forward to your guidance!

Note: I am unable to share the file since I am traveling and do not have the PC with me.

Best regards,
Darwish Jahan

[Vojta-Salek]

I am not sure if I understand your question(s). Yes, you can have open VENT as a supply and a different VENT with specified flow rate somewhere else in your domain to induce a flow.

The estimate of computational time (250+ hours) sounds reasonable, your domain is relatively large. However, you stated you are in a learning phase. Therefore, considering the size of your domain and computational costs, I would recommend starting with a smaller domain to get in touch with the behavior of all your model features. You can try very simple and much smaller geometries to get a feel for what the various vents are doing and how to specify them. I would also recommend to think about your cell size which should correspond to the goals of your simulation (which are unknown as well as unit of your cell size of 0.2).

[Darwishjahan]

Yes exactly. My question is how to have an open vent the interior a confined volume. For example I have a closed room with extract defined as 5m3/sec and supply be given from a vent opening as open, could make a similar small model and share me?

Yes, exactly. I want to understand how to create an open vent to ventilate the interior of a confined space. For example, if I have a closed room with an exhaust airflow rate of 5 m³/sec defined on a vent, and the supply air comes through an open vent, could you help me model this scenario on a smaller scale and share the model?

[drjfloyd]

SURF_ID='OPEN' can only be used on the exterior boundary of a mesh. Either you need to define your MESH(es) such that your supply vent is on the exterior boundary or you can use the HVAC model.

[Vojta-Salek]

@Darwishjahan I guess we did not understand each other. My answer supposed your supply open VENT is at domain boundary as you specified that you are familiar with that limitation.

You already have a clear answer from Dr. Floyd. If it is not possible for you to geometrically put your supply at exterior boundary, go for HVAC. I will just add that there are many simple examples provided for various HVAC utilization. You can study HVAC in User's Guide and ask if something particular is unclear there.

[Darwishjahan]

Thank you very much Dr. Floyd and Mr. Vojta, the issue has been resolved. As suggested, I made HVAC vent with ambient end point in the interior and it helped. Hence resolved. Thanking once again for the information.