Using TMP_EXTERIOR and TMP_EXTERIOR_RAMP

[mcgratta]

Discussed in #16261

^{Originally posted by uporersith May 12, 2026}
The TMP_EXTERIOR and TMP_EXTERIOR_RAMP feature is quirky, so I am posting this here to document my findings. (using the sample case pressure_boundary.fds from the validation folder)

If I set TMP_EXTERIOR=20, FDS appears to ignore the TMP_EXTERIOR_RAMP and just set the OPEN boundary temperature to 20 for all time. For example:
&VENT MB = 'XMAX' SURF_ID = 'OPEN', DYNAMIC_PRESSURE=2.4, PRESSURE_RAMP='wind_ramp', TMP_EXTERIOR=20.0, TMP_EXTERIOR_RAMP='temp_ramp' /

If I set the TMP_EXTERIOR=25.0, then FDS will utilize the ramp, however If i set the F > 1, the behavior is unexpected.
&RAMP ID='temp_ramp', T=0., F= 1.5 / &RAMP ID='temp_ramp', T=10., F= 2.0 / &RAMP ID='temp_ramp', T=15., F= 2.0 /
I expect the temperature coming in to be 37.5 and go up to 50 degC, but it tops out at 30 deg C.

If I set the TMP_EXTERIOR=50, and use a ramp with F <= 1 then FDS behaves as expected:
&RAMP ID='temp_ramp', T=0., F= 0.5 / &RAMP ID='temp_ramp', T=10., F= 1.0 / &RAMP ID='temp_ramp', T=15., F= 1.0 /
Also FDS does not like it if you set TMP_EXTERIOR=1. and set the TMP_EXTERIOR_RAMP F=[actual temperature]

I kept getting numerical instability. maybe the way it applies the Ramp to 1degC trips up the solver...

In conclusion, set the TMP_EXTERIOR to some large value, and use the F <=1 to set the time history.

[mcgratta]

I need a simple example input file demonstrating the problem. Let's start with just one example.

[uporersith]

This case I have set TMP_EXTERIOR to 20, and it ignores the RAMP completely.

pressure_boundary.txt

[uporersith]

This case TMP_EXTERIOR=50.

It follows the ramp, but I am expecting 25, 50, 75, and instead I am getting 35, 50, 65.
I added a DEVC output to see the temps better.

pressure_boundary1.txt

[mcgratta]

The ambient temperature is 20 C and the exterior temperature is 20 C. You are ramping from one temperature to the same one.

[uporersith]

The ambient temperature is 20 C and the exterior temperature is 20 C. You are ramping from one temperature to the same one.

I was expecting to be able to change the exterior temperature as a function of time via the TMP_EXTERIOR_RAMP. The manual suggests I should be able to change the time history of the EXTERIOR temperature this way.

If given experimental data of pressure and temperature entering a domain over time, I want to be able to model that in FDS.

[mcgratta]

You can change it. You need to set TMP_EXTERIOR to something other than the ambient. Then the ramp tells you where you are between the two values.

[uporersith]

AH okay, It was not clear to me how the TMP_EXTERIOR_RAMP functioned. Section 7.4.2 - Special Vents does not mention that TMP_EXTERIOR_RAMP ramps between T_AMB and TMP_EXTERIOR. It is not what the section would lead you to believe.

I think it would make it more clear if instead of referencing section 11.1, reference equation 11.2 which is applicable to how TMP_EXTERIOR_RAMP works, and that you need to set TMPA appropriately.

Knowing that it is going between TMPA=20 and TMP_EXTERIOR=50, the second case values of 35 and 65 make sense.

[mcgratta]

I changed the link.