Compressiblity (in rooms's air simulation) + Data Validation

[Saint]

Hello again,

1
for simulation of a hot plate in a room, for measuring the natural convection, radiation and back reflection of different surfaces:

Should I introduce the compressibility to my simulation ? or because the system is at low speed, it is not needed?
I know air is compressible, but one simulation video has confused me about it, in the video, they mentioned that they are introducing the compressibility due to the high air speed (high mach of their airplane)...therefore I am a little confused.

If yes, is the "thermal" compressibility the correct choice ?


2
Very important question, after simulation, how can I validate my data?
I can't easily take a thermometer or measure the radiation of surfaces, there are way too many noise and disruptive signals to the system !?
or maybe it is the only true way to validate my own data, with experimental results... should I try to confine the experiment environment or:

maybe I can validate it with different programs, or going to the source, or maybe different models (maybe different viscosity models?)
Or maybe there is a genius way to spot possible errors or flaws of my simulation ?

[kevinarden]

flows are usually treated as being incompressible when the Mach number (the ratio of the speed of the flow to the speed of sound) is smaller than 0.3 (since the density change due to velocity is about 5% in that case)

[raback]

Yes, "thermal" would be the correct approach. It uses the ideal gas law to compute the density when temperature is known but pressure will not affect the density. -Peter

[Saint]

Thank you so much Mr. Arden and Mr. Raback,

Although I am a little confused, but I think this is a good sum:
"Air, is usually considered as incompressible in low speeds, but if I want to apply a compressibility model, then the "thermal" model is the correct choice"

?

But how about the second question, data validation !
It is a scary topic to me, and yet very important. I have seen some presentations from Ansys, and even they have shown different results for the same simulation! How can I know which data is correct ?

especially, if some other student/scientist comes and presents a different data ! How can I fight back and defend my results ? or validate his/her findings...

[Saint]

Just remembered another question:

Assuming, my understanding in last post is correct:
"Air, is usually considered as incompressible in low speeds, but if I want to apply a compressibility model, then the "thermal" model is the correct choice"

Should I use the compressibility model of "None" or "Incompressible" ?
I don't know how software will react, in the two scenarios above!

There are other similar positions as well, like emissivity of air:
What is the difference if I leave it empty or put 0 ?

(I believe, when I put 0, it will naturally nullify the equation, but maybe there are other effects as well )

[kevinarden]

Validation is generally done by comparison to a known or accepted solution, either a closed form mathematical solution, or comparison to another code that has been validated.
Verification is comparison to test data.
Validation is 2+2=4, and if I run the simulation I get 4 it is validation that the software is doing the calculation correctly.
Verification is 2 stones on the table and I add two more stones, there is 4 stones on the table. If the software determines 4 stones it is verified.

[Saint]

Thanks,

So, to verify my simulation, I need to compare it with experimental data.

But as for validation, it returns to the code and the mathematical process... I don't know if it is expected from me or is it something very advanced and only expected in programming level?

[kevinarden]

Validation is done at the programming level, the test cases are here.
https://github.com/ElmerCSC/elmerfem/tr ... /fem/tests

However since each users case is usually unique, there is some level of validation expected at the user level.
An example validation case,
viewtopic.php?t=7005

[Saint]

Thank you again !

Just remembered another question:

Assuming, my understanding in last post is correct:
"Air, is usually considered as incompressible in low speeds, but if I want to apply a compressibility model, then the "thermal" model is the correct choice"

Should I use the compressibility model of "None" or "Incompressible" ?
I don't know how software will react, in the two scenarios above!

There are other similar positions as well, like emissivity of air:
What is the difference if I leave it empty or put 0 ?

(I believe, when I put 0, it will naturally nullify the equation, but maybe there are other effects as well )

[raback]

Do not put "emissivity" for air. It does not make any sense. Air cannot have emissivity since it cannot have a surface. The radiating material should have emissivity defined. The direction of the normal vector will be set from the material that has emissivity defined outwards. For this reason emissivity for air leads to problems. Also, emissivity=0 is not a good idea because it is not physical. The equation for Gebhart factor will be impossible to solve then. All materials radiate. -Peter