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Forces in compressible internal flow

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Old   April 22, 2010, 08:06
Default Forces in compressible internal flow
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sasha
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Hi all,

I tried calculate forces that act on cylinder inside a pipe. when i used simplefoam ,i get very good result. but when i switch to rhoporoussimplefoam (with zero porosity - i use it as a template for steady state compressible flow - air) i get wrong direction and component of force greater x150 .
for finding force i used :

functions
(
forces
{
type forces;
functionObjectLibs ("libforces.so"); //Lib to load
patches (buhna); // change to your patch name
rhoInf 1.24; //Reference density for fluid
CofR (0 0 0); //Origin for moment calculations
}
);

Is there anyone who can explain how do it right.
thanks in advance
Sasha
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Old   April 24, 2010, 18:58
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sasha
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hi all,

anyone have expirience
with force Utility for use it for Compressible Flow Fields ?
i tried look at forum but nothing.

please help, becouse i have very strange result ,and i don't understand where is problem
thank you

p.s.
i tried also with rhoSimpleFoam, and with rhoTurbFoam and i get similar result as in rhoporoussimplefoam.I work in O.F. 1.5 and geometry from SALOME. If there some indication that problem in mesh? i read that yplus in turbulent flow must be 30<y+<100, in my case average of y+ on cylinder is 1 (flow is turbulent, RE>20000).
wait for your suggestion .
thank in advance
Sasha

Last edited by zabar; April 25, 2010 at 15:52.
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Old   April 26, 2010, 04:54
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sasha
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i tried refine the mesh - the same result.
what i should to do
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Old   April 26, 2010, 11:13
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Carlos Xisto
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I' sorry,

But I never need to calculate forces.

What is the turbulence model that you are using? Do you use wall functions?

Because if you do the y+ around 30 and 100 should work fine, but if you don't use wall functions you need a low-Re turbulence model that can solve the boundary layer.

Hope it helps,

CX
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Old   April 26, 2010, 11:44
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sasha
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Hi Carlos,
thank you for your answer,
i use k-e model, and not used wall function
1.where i can specify them? and what the rule?
2.which low-Re model you can suggest, and what's Re limitation for this model.
3.i read that y+ must be between 30 and 100 for fully turbulent flow- is right?
4.if your suggestion valid both for compressible and incompressible flow ?

thank you very much
Sasha.
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Old   April 26, 2010, 11:59
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Carlos Xisto
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I never used a turbulence model in Foam, my experience is with FLUENT.

>>1.where i can specify them? and what the rule?

All high-Re RANS turbulence models include wall-functions because it is inappropriate to use them without. The rule is that for use wall functions you need a valid y+ between 30 and 100 (300 in Fluent).

>>2.which low-Re model you can suggest, and what's Re limitation for this model.

If you want to use a low-Re you need a y+<=1. I could suggest Spalart-Allmaras, k-w or k-w SST.

>>3.i read that y+ must be between 30 and 100 for fully turbulent flow- is right?

Yes I think you are right, but If you want such y+ you need to coarse your mesh, in FLUENT is mandatory to use wall functions I think that in foam is also a need.
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Old   April 26, 2010, 12:21
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sasha
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hi,

>>All high-Re RANS turbulence models include wall-functions because it is inappropriate to use them without.

i found them in RASproperties file

i will try another RAS model,maybe it will help
thank you for your help.
sasha
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