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April 22, 2010, 07:06 
Forces in compressible internal flow

#1 
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sasha
Join Date: Feb 2010
Posts: 10
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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 

April 24, 2010, 17:58 

#2 
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sasha
Join Date: Feb 2010
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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 14:52. 

April 26, 2010, 03:54 

#3 
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sasha
Join Date: Feb 2010
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i tried refine the mesh  the same result.
what i should to do 

April 26, 2010, 10:13 

#4 
Member

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 lowRe turbulence model that can solve the boundary layer. Hope it helps, CX 

April 26, 2010, 10:44 

#5 
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sasha
Join Date: Feb 2010
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Hi Carlos,
thank you for your answer, i use ke model, and not used wall function 1.where i can specify them? and what the rule? 2.which lowRe 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. 

April 26, 2010, 10:59 

#6 
Member

I never used a turbulence model in Foam, my experience is with FLUENT.
>>1.where i can specify them? and what the rule? All highRe RANS turbulence models include wallfunctions 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 lowRe model you can suggest, and what's Re limitation for this model. If you want to use a lowRe you need a y+<=1. I could suggest SpalartAllmaras, kw or kw 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. 

April 26, 2010, 11:21 

#7 
New Member
sasha
Join Date: Feb 2010
Posts: 10
Rep Power: 8 
hi,
>>All highRe RANS turbulence models include wallfunctions 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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