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foamToVTK will have a different values?! And the fvScheme questions. 

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November 18, 2014, 21:48 
foamToVTK will have a different values?! And the fvScheme questions.

#1 
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Hello,everybody! Have a nice day!
QUESTION 1: I am confused with the foamToVTK utility... When I run the case /root/OpenFOAM/root2.3.0/run/tutorials/incompressible/pisoFoam/les/pitzDaily, i got the different values with the paraFoam(load all of the time values:0 , 0.001,0.002 etc.) but when i use the foamToVTK utility and post process it with the paraview utility, i got the almost the different values, why? (1)VTK(paraFoam) value (2)different time (paraFoam) value. I think the foamToVTK value is more "credible" since the inlet velocity is 10m/s... QUESTION 2: The previous case is my test case , i want to know the different divSchemes' dissipation.(I use the linear, limitedLinearV 1, linearUpwindV cellMDLimited, LUST grad(U) as the div(phi,U) fvScheme, the almost have the same value: the maximun value is around 16 m/s.) But when i use Gauss linear as div(phi,U) fvScheme to calculate my case(attachment 3 :the flow jet inlet velocity is 9.5m/s.) the velocity seems unstable.. Any suggestions about my schemes, may i use other fvScheme such as LUST grad(U),limitedLinearV 1 or somthing else? my mesh check as follows: Checking topology... Boundary definition OK. Cell to face addressing OK. Point usage OK. Upper triangular ordering OK. Face vertices OK. Number of regions: 1 (OK). Checking patch topology for multiply connected surfaces... Patch Faces Points Surface topology wall 2400 2480 ok (nonclosed singly connected) out_let 5440 5481 ok (nonclosed singly connected) out_pressure 20000 20080 ok (nonclosed singly connected) outer_wall 320 400 ok (nonclosed singly connected) inner_wall 640 720 ok (nonclosed singly connected) co_flow 3200 3280 ok (nonclosed singly connected) air_coal 1040 1081 ok (nonclosed singly connected) ch4 240 320 ok (nonclosed singly connected) Checking geometry... Overall domain bounding box (0.03 0.04 0.04) (0.25 0.04 0.04) Mesh (nonempty, nonwedge) directions (1 1 1) Mesh (nonempty) directions (1 1 1) Boundary openness (6.205103531e16 1.178824882e15 1.5696863e15) OK. Max cell openness = 3.329184873e16 OK. Max aspect ratio = 44.4510692 OK. Minimum face area = 1.373471837e08. Maximum face area = 8.494790386e06. Face area magnitudes OK. Min volume = 3.323882784e12. Max volume = 1.412579007e08. Total volume = 0.001256192872. Cell volumes OK. Mesh nonorthogonality Max: 31.4382777 average: 2.385880292 Nonorthogonality check OK. Face pyramids OK. Max skewness = 0.4751537125 OK. Coupled point location match (average 0) OK. Mesh OK. And divSchemes as follows: divSchemes { default none; //default none; div(phi,U) Gauss linear;//upwind;//LUST grad(U);//limitedLinearV 1;//better than the linearUpwind scheme.//Gauss linearUpwindV cellMDLimited Gauss linear 1; div(phid,p) Gauss limitedLinear 1; div(phi,k) Gauss limitedLinear 1; div(phi,Yi_h) Gauss multivariateSelection { O2 limitedLinear01 1; CH4 limitedLinear01 1; N2 limitedLinear01 1; H2O limitedLinear01 1; CO2 limitedLinear01 1; h limitedLinear 1; }; div((muEff*dev2(T(grad(U))))) Gauss linear; div(phi,omega) Gauss limitedLinear 1; div(phi,K) Gauss linear;//limitedLinear 1; div(U) Gauss linear; } ddtSchemes { default backward;//Euler; } gradSchemes { default Gauss linear; // default cellMDLimited Gauss linear 0.5; // grad(U) cellMDLimited Gauss linear 0.5 } laplacianSchemes { default Gauss linear limited 1;//corrected;//orthogonal; } snGradSchemes { default limited 1;//orthogonal;//corrected;//orthogonal; } fluxRequired { default no; p ; } And my maxCo is 0.25, and the URF is: fields { // ".*" 0.7; "(p)" 0.3; } equations { ".*" 0.7; } And pimple settings: nCorrectors 3;//PISO correction times while SIMPLE always 1; nOuterCorrectors 3;//in order to increase the stability you will need to add more corrector steps nNonOrthogonalCorrectors 1; Any suggestions will be helpful,thank you in advance... my best wishes, wen 

November 19, 2014, 04:37 

#2 
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Philipp
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Wen, I am not sure I get you right... is this LES? I guess. Anyway, one general hint: Your mesh is good enough (orthogonal quality) to run without any "nNonOrthogonalCorrectors". You can set it to zero.
The velocity becomes unstable when switching from upwind to central differencing, because you remove all the additional dissipation that comes from the upwind scheme. I think this is pretty regular behavior. The question is how to get rid of it Can you post some of the log output, when the velocity is unstable?
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November 19, 2014, 05:06 

#3 
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Dear,Philipp,
(1) yes,it is LES; (2)To stablize the calculation, i set nOuterCorrectors 3; and use small maxco which i currently use is 0.25. Any idea to get rid of the unstable situation? (3)Sorry, i did not save the log output, but i post the result after i change the settings every 0.0005s.I found that there is no precise time that the velocity become unstable. Thank you again! best wishes, wen 

November 19, 2014, 05:19 

#4 
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Philipp
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What do you mean by "velocity becomes unstable" anyway?
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November 19, 2014, 05:23 

#5 
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I mean that the mean inlet velocity is 9.5m/s, when i use upwind scheme the maximum velocity will be 12m/s around,but when i changed to the linear scheme, the maximum velocity will rise up to 26m/s as you see. Thank you for your reply!
best wishes, wen 

November 19, 2014, 05:36 

#6 
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What do you think i change the scheme to Gauss limitedLinearV 1; or you think the velocity is reasonable? (sorry, i can not judge whether the velocity is ok.. ) or please give me some hints how to let the velocity become more "stable"..
thank you! best, wen 

November 19, 2014, 05:37 

#7 
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Philipp
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I don't want to just guess something without knowing that everything else is alright. So without the log file I would rather say nothing.
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November 19, 2014, 06:05 

#8  
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Quote:
Quote:
sorry for that. Now i changed to calculate the combustion use the same slover. And i also changed the scheme to Gauss limitedLinearV 1; Now this is the log file for a while..Thank you very much. wen 

November 19, 2014, 06:18 

#9 
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Philipp
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To me, the log output looks pretty nice. I see two pictures, one in post #1 and one in post #3, they look different. Which one are we dealing with here?
Maybe you will get better help (from other people) if you name your thread better and put it in the correct sub forum (OpenFOAM Running, Solving & CFD, or even in the main cfd forum). This seems to be an LES question. I don't see anything suspicious here. Every equation converges. Of course you will get much lower velocity fluctuations if you set convective scheme to upwind. You damp everything with numerical diffusion. When switching to less dissipative schemes, you remove the damping. Picture number 3 in post #1 looks strange, because the fluid seems to accelerate in the inlet pipe. But I don't know your boundary conditions, so this also might be physical.
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November 19, 2014, 06:53 

#10 
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Thank you for your help, your points are precious.
The post #3 picture is the one i changed the scheme not for a while, and the post #1 is the one have flow for some time(0.02s). Thank you for your suggestions, very nice of you. regards, wen 

November 19, 2014, 08:30 

#11 
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Tobias Holzmann
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Hello together,
as far as I get to the thread I want to mention a few things at the beginning:
foamToVTK and paraView You should know that paraview interpolate the values from the cell centres to the points and you ALWAYS will get interpolation errors. If you just want to integrate the mass flow (phi) through one inlet patch you will be amazed how bad the calculated value is. The best way to do things like that is to provide a patch (internal or boundary) and use openfoam calculations like:
Numerical schemes Its not a big deal to know that "Gauss linear" is very unstable. There are a lot of papers and publications. Additionally you can check out that page: http://www.holzmanncfd.de/index.php...ericalschemes There should be a video for linear (but is at the moment not working). I will check this the next hours. I will not repeat all things about numerical schemes because you wlil find a lot of stuff in this forum, papers, publications and world wide web. But as some hints:
Your combustion case I am not sure about your model because I only used the flamelet model for combustion and this model is total different to the other once. But with my experience I think the temperature you have: Code:
T gas min/max = 300, 3444.701528 Additionally your relaxation is wrong: Code:
fields { "(p)" 0.3; } equations { ".*" 0.7; } Code:
fields { p 0.3; pFinal 1; } equations { ".*" 0.7; ".*Final" 1; }
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Best regards, Tobias Holzmann Some interesting OpenFOAM tutorials, publications and videos on www.Holzmanncfd.de OpenFOAM Beginners should check out the new wiki on wiki.openfoam.com A list of some active OpenFOAM contributers can be found »here« A book about the basics of »Mathematics, Numerics, Derivations and OpenFOAM« can be found on www.Holzmanncfd.de 

November 19, 2014, 09:18 

#12  
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Thanks very much for your time,Tobias Holzmann. Your reply is so clearly and helpful, today i'm the happiest person in the word!
(1) I am really confused with the different fvSchemes and i read almost all of the forum about the schemes. Now i know that the upwind scheme i could use to get the initial value, and then change to the higher order schemes. Which one should i choose? Now maybe i already have the answer. (i have test the different scheme :TVD (limited) and QUICK and linear and linearUpwind using the case .../les/pitzDily, they almost have the same result. this report also compare the different schemes and concluded that the linearUpwind almost the best. LARGEEDDY SIMULATION OF AIRFOIL FLOW USING OPENFOAM) (3)Yes, i already notice that the temperature is too high!!! But when i use upwind scheme, it do not go to so high. I want know that the whether scheme have influence on the scalar, so afternoon i use the combustionFoam to calculate the simulation. (i will calculate the coal combustion in the future, and coal will be injected to the domain later.) (4) i am not sure whether the relation form is wrong. I read a post and say that when we use this form: Quote:
the last final step will use the relax factor, and if we use this form: Quote:
Thank you again! very nice of you! I also want to use your flamelet model lately. Very interesting! (this model could be used in LES?) Now i use the infintelyFast model that openfoam has built in. Also i have compile the EBU model succeed, if you are interested in it, i could share with you! best regards, wen Quote:


November 19, 2014, 09:31 

#13 
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Tobias Holzmann
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Hi,
you are welcomed. But for the future some hints (typeset)
Can you please post some link. I never saw that before. Also if you check out all tutorials: Code:
cd $FOAM_TUTORIALS grep rni e "(p)" * Code:
"p.*" 0.3; For more information about the PIMPLE look here: http://openfoamwiki.net/index.php/ThePimpleAlgorithm http://www.cfdonline.com/Forums/blo...hmparti.html
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Best regards, Tobias Holzmann Some interesting OpenFOAM tutorials, publications and videos on www.Holzmanncfd.de OpenFOAM Beginners should check out the new wiki on wiki.openfoam.com A list of some active OpenFOAM contributers can be found »here« A book about the basics of »Mathematics, Numerics, Derivations and OpenFOAM« can be found on www.Holzmanncfd.de 

November 19, 2014, 09:45 

#14 
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Sorry, tobi. I am really new to this forum and do not know much about the typeset. Next time i will remember your suggestions and you will see the clearly questions, h h.
Regarding the relaxtion form i think you are right, i will change that. Very nice of you and i should know much from you in the future not only combustion but also anything else, such as type, English etc. PS:this link is very useful:http://www.holzmanncfd.de/index.php...s/gausslinear Thank you again!!! Best regards, wen 

September 18, 2015, 12:03 

#15 
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shashank moghe
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Hello Wen Xu,
I am trying to simulate a free jet flow in OpenFOAM using the interFoam solver. Can I know what is the length of the jet inlet and the length of the cylinder that you shoot into? Also, what is the mesh resolution? I am using the RANS komega SST model. And I am having difficulties having the jet to show the fanning that other commercial codes have already validated. I am pretty sure I have the BCs set appropriately. Any help will be highly appreciated. 

September 18, 2015, 20:55 

#16 
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Dear moghe,
I do not think it is very important to set the jet inlet to generate the turbulence for RANS. You know in RANS framework, the time average value is used instead of instantaneous values as used in LES. You will see in LES the inlet jet or turbulence generation technique is often used but you usually can not find them in RANS, because that's meaningless.. Also, the mesh resolution is not also a typical requirement in RANS as that in LES. Because you just want the "time average values". Best, Wen 

September 19, 2015, 00:42 

#17 
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shashank moghe
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Wen, I understand all your points. I just wanted to know what your case configuration is. I am using a min cell dimension of 0.1 mm, an inlet value of 35 m/s, water jet issuing into stagnant air. I am having difficulty getting the jet to fan out. I have ~ 50D length downstream of the jet. The jet just passes through the air without showing any breakup.
Hence I wanted to know about your case setup. Thanks. 

September 19, 2015, 02:27 

#18 
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Yes, in fact I have answered your questions. It is not easy to see "eddies" in RANS simulation, because it is "time averaged values" (the eddies can only be found in instantaneous simulation, such as LES and DNS. ) Do you ever found any eddy in RANS simulation? If it is, please tell me (counter plot).
Figure 1 and 2 attached in #1 have the same configuration as that in pisoFoam/les/pitzDaily The configuration as show in figure 3 attached in #1 is 10D*40D with the minimum mesh of 0.01D. I strongly recommend you to compare the simulation results with LES and RANS for the same configuration. I also recommend you to test this in pisoFoam/les/pitzDaily and pimpleFoam or icoFom/ras/pitzDaily at the tutorial. Best regards, Wen 

September 21, 2015, 13:39 

#19 
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shashank moghe
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I understand. I will post my results when I am done comparing RANS and LES. What is the max cell size I can use, if I want to run my case with LES? Unfortunately, due to processor limit (16), I cannot go D/100 on the entire mesh domain as you did.
Any paper/material directing me towards this would be great! 

September 21, 2015, 20:49 

#20 
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Sorry, I do not have specific material to describe the mesh resolution of LES simulation. You can google it.
But we typically do test the mesh resolution of the LES simulation, which called "grid independence test". That means you implement different mesh resolution to simulate the case, if the simulation results have not much difference, then we can say the simulation results has no relation with the grid numbers, in other word, the grid has achieved its dependence. Best, wen 

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