Thermal analysis - low heat input gives wrong results

Following on from a previous post, I have now got the analysis running. However when I use the correct value for the heat input, I get incorrect results.

[code:1]LOADING=AFFE_CHAR_THER(MODELE=MODEL,
FLUX_REP=_F(GROUP_MA='heat_in',
FLUN=37500,),
ECHANGE=_F(GROUP_MA=('bot_air','top_air',),
COEF_H=5.6,
TEMP_EXT=20.0,),
ECHANGE_PAROI=(_F(GROUP_MA_1='mnt_cont',
GROUP_MA_2='bot_cont',
COEF_H=3000.0,),
_F(GROUP_MA_1='bot_mate',
GROUP_MA_2='top_mate',
COEF_H=3000.0,),),);

TEMP=THER_LINEAIRE(MODELE=MODEL,
CHAM_MATER=MATFIELD,
EXCIT=_F(CHARGE=LOADING,),);
[/code:1]

I have a volume 'mount', of which 'heat_in' is a face. mount contacts 'bottom', with faces 'mnt_cont' and 'bot_cont'. bottom contacts 'top', with faces 'bot_mate' and 'top_mate'. Heat loss is through surfaces 'bot_air', 'top_air' to an ambient temperature of 20*C. The air contact surfaces are both of the order of 100x the heat input area.

If I increase my heat input by a factor of 10 then I get sensible results, a temperature distribution ranging between 20.5*C and 37*C. However if I use the correct heat input value 37500W/m^2, my resulting temperature distribution cannot be right, since it covers the range 2.7*C to 4.5*C. Since ambient is 20*C this is clearly wrong.

Can anyone suggest a change to the aster commands in order to correct this?


Thanks,
Joe<br /><br />Post edited by: Joe Dowsett, at: 2010/12/13 12:10

I've just done a test, and this problem is not to do with low heat input.

I doubled again the heat input (so now 20x correct input). Everything is linear so I expected that the temp distribution that used to range between 20.5 and 37 degrees would now range between 21 and 54 degrees (ie, double the temperature relative to ambient, 20 degrees). This didn't happen, instead it has doubled the actual temperatures, the distribution now ranging between 40.5 and 74 degrees.

Since heat loss only occurs through the

[code:1]ECHANGE=_F(GROUP_MA=('bot_air','top_air',),
COEF_H=5.6,
TEMP_EXT=20.0,),
[/code:1]

command, why is it not relative to 20 degrees?

So, it turns out to be a meshing problem.

The only way I was able to get the contact areas to have the same number of nodes in the mesh was to fuse the three parts and then partition it.

This doesn't work.

If I
- fuse mount and bottom
- cut mount from this fuse
- build a compound of mount and the resulting cut
then mesh, I get an equal number of nodes on the contacting faces (allowing code_aster to run), and I get meaningful results.

My problem is now a salome one - the geometry of top and bottom seem to be too complicated, if I fuse top and bottom and then try to cut one from the other, Salome crashes, i sit forever with the 'busy' mouse icon.

Is there a better way to ensure that contacting surfaces have the same number of mesh nodes??