fds leak

[yILCCY]

I design a case which is natural gas leak in a Limited space. A wind field is set on the surface where the leakage port is located, in the same direction as the leakage, with a wind speed of 1 m/s. Run in 10s. But why does the gas flow downwards? Ideally, methane is less dense than air and should diffuse upwards. The follow is the code and results. Thanks for answering!

[rmcdermo]

With a velocity of 33 m/s you have a momentum driven scenario. This will set up complex flow patterns in the sealed domain. Buoyancy will not be the driving factor in the transport.