Replies: 16 comments 14 replies
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I'll take a look. |
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If I delete the HVAC lines the odd pressure behavior still exists. I think this is a result of having a large negative dynamic pressure at one of the open boundaries that draws a vacuum in its mesh until doors open to the structure. |
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I replaced the green vent with a |
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Could it be that a large cross section, short length, low resistance |
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The LOSS is low for a louver. A louvered opening in a wall is going act like an orifice in a wall with a discharge coefficient (C_d) of ~0.6 to ~0.7. Loss (K) = 1/C_d^2 or K ~2.04 to ~2.78. |
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The loss on the louvre belongs to only the louvre element (excluding inlet and outlet losses, since they are calculated by CFD)(~90% opening rate). The reason of using DUCT instead of a HOLE is DUCT gives me the opportunity to adjust the LOSS exactly the value I enter. On the other hand, this project started with 50% opening rate with higher loss. Later it changed to 90% opening rate with low loss value. That is why I encountered this setup. All in all, is this issue linked to low loss value? Shall I check if it changes with higher loss value? |
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HVAC_MASS_TRANSPORT does not change how the duct pressure solution is done. It just adds transport delays. Since your case is all ambient temperature, I would not expect to see significant differences with HVAC_MASS_TRANSPORT. What duct length crashed with 200 nodes? You are imposing a 100 Pa pressure drop on the interior side of the louver. With a louver loss of 0.25, the steady velocity is ~26 m/s. That will wind up being ~600 air changes per hour inside the rooms. This may be too challenging of a condition for this geometry and HVAC. What is that 100 Pa pressure meant to represent? Is it realistic that such a pressure drop would be maintained once a path is opened up to the louvers? |
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Thank you very much for the update. We have continued on the project with a workaround. Meanwhile, I will continue to follow up this thread. Please let me know if you would like me to try any setup. |
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I'll work the issue too. One thing to try is to apply leakage to the walls, and I would avoid the use of |
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If I set the |
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You may be doing this already, but if not, try adding lines like this to your input file
This creates a leak in the wall near the louvered vent. This one is quite large, like 0.3 m by 0.3 m. You can experiment with different sizes. A longer term fix to this problem would be to couple the louvered vent with the overall pressure solver for the entire domain. |
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In your larger model, does it make sense to include leakage? Unless the facility is specially designed, it is going to leak. These leaks can help to dampen pressure oscillations, both real and numerical. |
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I think I found a possible issue with how I deal with connected zones in HVAC; however, for this case that issue would not have any impact. I did; however, discover another possible issue. If I run this case with one MPI process things look OK. I took your case, removed the DUMP line to just get all the output and added a DEVC for the inside node pressure. &DEVC QUANTITY='NODE PRESSURE',NODE_ID='Node01',ID='N1'/ With one process (n=1, blue solid line) the oscillations dampen out quickly. With 8 (n=8 orange dotted line) the oscillations stay. |
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Did you use the same input file for both cases, just changing the number of MPI processes? |
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If so, can you post the exact input file you used. |
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I am creating an issue for the case where the results are different depending on the number of MPI processes. I'll keep the discussion open in case there are other aspects of this problem that fall outside that narrow issue. |
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Hi,
I am posting this to discussions instead of issues because a similar issue was discussed before (#11951) and the behaviour was deemed normal. So this time I would like to share an another setup (DUCT with HVAC) and ask your opinion. Is this normal, if yes, how can I prevent it? Thank you.
I have observed a pressure issue in a complex simulation I am working on. In short, a room is connected to outside via a HVAC duct. When all doors are closed, the pressure in the room with duct does not drop to 0 Pa as I would expect.
I have tried to simplify the case as much as possible. I have also tightened the tolerances to the level I cannot apply to the complete project file. Yet, the issue is still present.
Although it is not an option for large project file, I have tried running this small case with ULMAT as well. It was significantly slower and fluctuation was still there.
(Similar issue #11951) Based on the previous posts/issues, I am aware that closing a door suddenly is expected to create such fluctuations but I have not yet found an alternative door opening/closing method. Furthermore, the average of the pressure in the room when all doors are closed is positive and not reducing. With a duct of 1.5 m2 area, I would expect there is not sufficient air supply to balance this pressure fluctuation but it appears not the case.
I have tried running this small case with ULMAT as well. It was significantly slower and fluctuation was still there. (This solver is not an option to use in complex project file)
Similar issue #11951
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