Predefined boundary condition 'OPEN'

[SXFire12]

Dear all,

I run two distinct simulations with the same domain characteristics, using open boundary conditions for XMAX, XMIN, YMAX, YMIN, and ZMAX.

In Case 1, the smoke in ZMAX vanishes (as I expected) at the top, whereas in Case 2, it is reflected into the domain, creating a cavity. Both were run in the FDS v6.10.1.

I tried to define the boundary condition in the domain with DB as well, but the problem persists.

Attached the codes and the smoke evolutions for both cases.

[FDS Code.zip](https://github.com/user-attachments/files/24409536/FDS.Code.zip)

[rmcdermo]

Please attach the two different input files (use .txt extension). Thanks. (The zip file is not linking for some reason.)

[SXFire12]

case 1.txt
case 2.txt
Please kindly have a look at it.

Thank you.

[mcgratta]

I see several differences between the two files that are not directly related to the upper boundary condition. It is difficult to diagnose the problem because each of the differences is a potential cause. Can you simplify the cases so that they are identical as much as possible.

[SXFire12]

Thanks, Kevin. I followed your instructions, isolated some of the code, and re-ran. It turned out that the RAMP for wind speed is the cause. I tried with a different RAMP, and it worked as expected. I changed the wind speed in the line &WIND SPEED with the original RAMP, but it didn't work.
case1.txt

[mcgratta]

OK, I'll take a look at the case.

[mcgratta]

I ran case1 and I see that it appears that the plume is trapped in a box. In a way, it is. The way you have specified the WIND is more appropriate for a case where the domain is large and the boundaries are far from the fire. In a sense, you are specifying the "background" wind which exists in the absence of a fire. Here, however, your fire is relatively close to the boundary, and its entrainment velocity is comparable to the wind velocity. I suggest in this case that you use a PRESSURE_GRADIENT_FORCE. Read more in the User's Guide, but in brief, a PGF of , say 0.05 Pa/m will generate a wind of a few meters per second. This force acts over the entire domain, not just the boundary. However, you have less control over the exact value of the wind velocity. You need to experiment with a range of values.

[SXFire12]

Thanks, Kevin, I got your idea. I will try to explore the PGF