Reference density

[mattbilson76]

Is it possible to set a reference density for buoyancy forces? My understanding is that you can set Tinf on the MISC line, which then would compute rho_0 (or rho_inf) via the value of Pinf and equation of state, but not set a reference density in a direct way.

[drjfloyd]

Just via ambient T and P.

[mattbilson76]

Thanks. Just looked at the user guide, and I think if I set Tinf to the reference level I want, and then I can control the "ambient" conditions via the initial conditions, wall temperature and TMP_EXTERIOR on OPEN boundaries.

[mattbilson76]

I am testing some analysis where I adjust TMPA, as I want to manipulate the reference density for buoyancy. Maybe this will have no impact, but I want to run some specific cases and see. When I adjust TMPA, I notice in the *.out file that things like Cp peg their lower temperature to TMPA. Maybe it affects some combustion parameters too. Question is - I want to increase TMPA, say to 100 deg C, but then I will set the initial temperature and temperature on openings, walls, etc. to 20 deg C. This would then make the reference density be pegged to 100 deg C, but is this change possibly going to perturb things elsewhere? For instance, if I set TMPA to 150 deg C, I do not get any burning - could that be that I have "muted" all buoyancy forces since temperature never gets large enough to mix things, or is something else going in in the code perhaps.

[mcgratta]

Do you have a simple file where combustion does not occur with a high TMPA?

[mattbilson76]

Do you have a simple file where combustion does not occur with a high TMPA?

This case attached is not zero HRR, but it is a higher TMPA and is trending the same way (should be 5 MW, but the combustion is very uneven with time compared with cases having lower TMPA)

SimpleRoom.fds.txt

[mcgratta]

The temperatures might be the same, but the densities are different, and "heavy" air pours down from the hole into the room.

[mattbilson76]

Yes, that is the sort of thing I would expect to see as the reference density is changed. It seems that the flow can be driven one way or another with reference density choice. The sensible choice here is the ambient temperature, but in some situations, this might drive too much buoyant flow, hence the experimentation here with adjusting TMPA.

[mcgratta]

Keep in mind also that FDS automatically stratifies the atmosphere unless you say otherwise. Pressure and density decrease with height, but temperature does not unless you specify a profile or lapse rate.