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August 19, 2010, 14:35 
solver for subsonic compressible turbulent flow in OF 1.7

#1 
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Nilesh Rane
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Hi all,
I want to solve a susonic, turbulent compressible flow in OF 1.7. Which solver should i use??? Please help..its argent
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August 20, 2010, 02:13 

#2 
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Alberto Passalacqua
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Depending upon the case (steady/unsteady), take a look at:
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August 20, 2010, 03:17 

#3 
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Nilesh Rane
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Thank you Alberto for the reply.
My doubt it can i use these solvers for the flow which has Mach numbers upto 1..??? Are the pressure based solvers able to handle higher subsonic mach numbers???
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August 20, 2010, 03:24 

#4 
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Alberto Passalacqua
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You asked about subsonic flows. Not highly supersonic. :)
There are other solvers able to do that (see rhoCentralFoam and the various "sonic" solvers). Best,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

August 20, 2010, 08:01 

#5 
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Nilesh Rane
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With all due respect Sir, i said "highly subsonic" means mach number arnd 0.8 or 0.9. Sorry if my statement was ambiguous.
Thanks for your quick replies. One more question, my friend suggested me that "rhoturbFoam" is the best choice for my problem but its not there in OF 1.7.. So what is the difference between "rhoturbForm" and say "rhosonicForm"??
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August 20, 2010, 10:31 

#6  
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Alberto Passalacqua
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Quote:
Quote:
rhoSonicFoam is a densitybased inviscid solver. About your original question, yes, in theory pressurebased solver should work with those values of Mach number (See for example Peric's papers and book on the topic). Best,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

August 20, 2010, 14:20 

#7 
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Nilesh Rane
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Thank you very much Sir.
Your lucid answer really helped me a lot. This lack of documentation thing really makes it a difficult task for a newbie like me to get going with OF.
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August 20, 2010, 15:04 

#8 
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Alberto Passalacqua
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You are welcome. If you need further help, please don't hesitate to ask. The forum is a often a quick way to get answers.
Best,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

August 22, 2010, 07:58 

#9 
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Nilesh Rane
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Hello Alberto Sir,
As you have suggested, i am using rhopisoFoam for my problem. Brief description of my problem: Its cold flow simulation of mixing in a secondary combustor of a rocket motor. In this air and fuel streams are getting mixed together. I am planning to use RNGkepsilon model for this using rhoPisoFoam. BCs: Air inlet: Mach 0.3, stgT = 840K, staticT = 825K, Vin = 172m/s, massflow = 5.19kg/s, k and eps can be calculated using formulae Fuel Inlet : Mach = 1, staticT = 650 >>Vin 244m/s, massflow = 0.2581kg/s, k and eps can be calculated using formulae Wall: Adiabetic Outlet: there is a chocked nozzle at the end of combustor which i am not simulating, but from chocked condition the pressure at the inlet of nozzle can be calculated as 7.2 bar. I saw the tut for rhoPisoFoam. It uses SST kw model..but the "0" directory contains files for alphat, p, t, u, k, eps, omega, R. I got why we need u,p,t,k,omega. but why other files are present? I know what eps is, alphat is probably diffusivity but i dont know what is R. Also i am confused that, out of the known BC parameters which should be specified??? Also i found out some of the BCs as: Code:
movingWall { type compressible::kqRWallFunction; value uniform 0.00325; } Can you give me any direction???
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August 22, 2010, 14:10 

#10  
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Alberto Passalacqua
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Just Alberto Quote:
alphat is the turbulent thermal diffusivity, and it is used essentially in wallfunctions. Quote:
Best,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

August 22, 2010, 15:05 

#11 
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Nilesh Rane
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Alberto,
Thanks for the reply. What i am confused about is, do i really need to specify the values for omega, mut, alphat and R??? if yes how should i obtain them based on the BCs i have??? I know the formula for calculating omega. but not for the others... A thought: Is it like, the variables we need to depend are based on the turbulence model we use?? like for keps we need k, eps...for kw we need k and omega, for RSM mode we would need k, and R..and alphat in case of heat transfer is needed to be calculated????
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August 22, 2010, 15:16 

#12  
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Alberto Passalacqua
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Quote:
You should have mut and alphat, which are specified using wallfunctions at walls, and are calculated elsewhere. Take a look at this tutorial /OpenFOAM1.7.x/tutorials/compressible/rhoPimpleFoam/angledDuct/0/ It is for rhoPimpleFoam, however the only difference is in the iterative procedure. There you can see how alphat and mut are set up. Quote:
Best,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

August 22, 2010, 15:40 

#13 
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Nilesh Rane
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Thanks..that makes the things more clear now.
Actually i have worked on a keps research code for long time, and there such parameters as alpha or mut would be calculated internally, so i never had to specify them externally. So it didnt occur to me quickly.. But thank you very much for the quick replies. I have one more doubt if i may trouble some more.. I need to have info on mixing, which means air/fuel mixture fraction distribution which obviously not gonna be there in rhoPisoFoam. So how to get this data?? One approach i could think of is to define a scaler which will have value 1 for fuel and 0 for air and thus can give the mixture fraction straightaway. But i think i will need to solve transport equation for that. Is there any better way?? If not how to do this scalar thing???
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August 22, 2010, 15:45 

#14  
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Alberto Passalacqua
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Quote:
Quote:
Best,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

August 22, 2010, 15:59 

#15  
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Nilesh Rane
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Quote:
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August 24, 2010, 06:47 

#16  
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Nilesh Rane
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Hello Alberto,
I am facing this error for rhoPisoFoam. Do you know what it means??? Quote:
Thank you.
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August 24, 2010, 10:08 

#17  
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Alberto Passalacqua
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Quote:
It means that there is some problem in your setup. Compare with a tutorial case, and start changing it step by step to adapt it to your case. Best,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

August 31, 2010, 05:43 

#18 
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Nilesh Rane
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Hi,
I somehow managed to make the solver run. But now i am facing one more problem. The Co number keeps increasing to large numbers. I cannot make the mesh more coarser and the Co problem makes the time step so small that the simulation will take thousand years to complete.. (Thank god i have i7 processor) Now i am using the default solver settings of rhoPisoFoam from the tut of OF1.7. I want to know how can i tackle this Co problem without changing mesh and timestep, but modifying say relaxation factors and scheme only??? For this which scheme can be better or which UR factors can be good. (This is part of experience which i dont have much. so asking here). Any suggestions??? PFA my case files.
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August 31, 2010, 06:55 

#19 
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Christian Lucas
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Hi,
set "adjustTimeStep" to yes and perhaps start with smaller delta T Regards, Christian 

August 31, 2010, 07:06 

#20 
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Nilesh Rane
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Hi chris,
I have tried that. It does not help. My friend just told me that rhoPisoFoam does not solve 'rho' equation and thats why probably is problematic. He suggested me to use 'rhoSonicFoam' and see what happens.
Also i read somewhere on forum that rhopimpleFoam is stable for Co>1 as well. is that right?? Does that mean its an implicit solver??? BTW i have one more doubt, how would i know which solver is implicit and which is explicit?? Where to look for this ans and what to look for?? Thank you for the replies..
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