# Compare 2D axi-symmetric and 3D. Is OpenFOAM calculate the velocity wrong?

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June 11, 2010, 06:02
Compare 2D axi-symmetric and 3D. Is OpenFOAM calculate the velocity wrong?
#1
Member

Join Date: Jun 2010
Posts: 33
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Hi,

I simulate a swirl flow. A big cylinder (in which I took a displacer) following a small cylinder.
I try 2 dimensional axi-symmetric and 3 dimensional. All settings are equal. I used pisoFoam, Rankine vortex at inlet

I think OpenFOAM have problems with calculating the velocity in a 2D case. Enclose two picture.

U_x at the inlet is 10 m/s.
The continuity equation says: u1*A1=u2*A2

In my case, the velocity in the small passage in the middle should be 16 m/s.

u2 = u1 * (r1²/(r1²-rd²)) (With rd, I mean the radius of the displacer)
3D its right!

But in the 2D case the velocity is to high. Does OpenFOAM calculate the area ratio wrong?
If i would calculate like u2 = u1 *(r1/(r1-rd)). I get the velocity in the 2D axi-symmetric case.

The second problem is, that the flow at the axis makes no sense. Here I think OpenFOAM have problems with radial exchange in a 2D axi-symmetric case.

Have you any idea why the velocity is such different between these two cases?
Could it be, that I have forgot something to set in the 2d axi-symmetric case, or is there an additional setting to control something like area ration etc.

Regards
Ralph
Attached Images
 2D.jpg (12.6 KB, 255 views) 3D.jpg (15.0 KB, 230 views)

June 14, 2010, 12:09
2d case/wedge, is radial area ratio respected?
#2
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Johannes Kneer
Join Date: Mar 2009
Location: Germany, Karlsruhe
Posts: 13
Rep Power: 9
Hi Ralph,

nice to meet you here

If I understand you correctly, your problem with the 2d wedge case is as follows: the inlet is at a smaller radius than the outlet and has a smaller area as the outlet. Yet the results for U from OF look like they were the same area?

Quote:
 Originally Posted by RalphS Have you any idea why the velocity is such different between these two cases? Could it be, that I have forgot something to set in the 2d axi-symmetric case, or is there an additional setting to control something like area ration etc.
Could you post your polyMesh/boundary file of the 2d case, or maybe even the complete 2d case?

cheers,
Johannes

 June 14, 2010, 12:39 #3 Senior Member   Laurence R. McGlashan Join Date: Mar 2009 Posts: 370 Rep Power: 15 Based on what the continuity equation gives you, it sounds to me as if you're not simulating a wedge, but an infinite plane. Are you following this: http://openfoamwiki.net/index.php/Ma...s/AxiSymmetric __________________ Laurence R. McGlashan :: Website

 June 14, 2010, 13:08 #4 New Member   ernest Join Date: Jun 2010 Posts: 21 Rep Power: 7 Hi Johannes...I am a new user to OpenFOAM and I am trying to simulate flow in a cyclone. I imported the mesh from a program caedium which created the .ofm. I get the following errors when i try to run please help thank you. (Ia m using simplefoam). I have also enclosed the offending file (ie RASproperties). thanks! Errors below: /*---------------------------------------------------------------------------*\ | ========= | | | \\ / F ield | OpenFOAM: The Open Source CFD Toolbox | | \\ / O peration | Version: 1.5 | | \\ / A nd | Web: http://www.OpenFOAM.org | | \\/ M anipulation | | \*---------------------------------------------------------------------------*/ Exec : c:\cfd\OpenFOAM-1.5\bin\simpleFoam Date : Jun 14 2010 Time : 20:07:31 Host : hostname PID : 3416 Case : C:\Users\user\Documents\PhD\CFD\Openfoam\My-tutorials\cyclone\cyclone-flow nProcs : 1 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // Create time Create mesh for time = 0 Reading field p Reading field U Reading/calculating face flux field phi Selecting incompressible transport model Newtonian Selecting RAS turbulence model kEpsilon Programme terminated with errors: exit code 1, status 0. Error messages: keyword kEpsilonCoeffs is undefined in dictionary "C:\Users\user\Documents\PhD\CFD\Openfoam\My-tutorials\cyclone\cyclone-flow\constant\RASProperties" file: C:\Users\user\Documents\PhD\CFD\Openfoam\My-tutorials\cyclone\cyclone-flow\constant\RASProperties from line 14 to line 15. From function dictionary::subDict(const word& keyword) in file C:\tmp\OpenFOAM-1.5\src\OpenFOAM\db\dictionary\dictionary.C at line 288. FOAM exiting Offending file below: /*--------------------------------*- C++ -*----------------------------------*/ // OpenFOAM 1.6.x // Generated by Symscape's Caedium on: 14 Jun 2010 16:20:41, www.symscape.com FoamFile { version 2.0; format binary; class dictionary; object RASProperties; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // RASModel kEpsilon; turbulence on;

 November 12, 2010, 12:17 Split wedge patch... #5 New Member   Valerio Novaresio Join Date: Mar 2009 Location: Polonghera, Cuneo, Italy Posts: 27 Rep Power: 9 Hi RalphS, You probably have defined a single wedge patch (called for example front&Back) in wich you have put every wedge surfaces. Try to generate two different patchs for every "wedge side" (for example called front and back). This two patch of course will be "specular". Now when you run your simulations the continuity equation will give you the "naturals" expected results. :-) Regards! __________________ ...The best way to acquire new knowledge is to share it...

 November 13, 2010, 08:00 #6 Member   Join Date: Jun 2010 Posts: 33 Rep Power: 8 Close the thread please. Why you answered a question from june?

 November 13, 2010, 21:51 #7 New Member   Valerio Novaresio Join Date: Mar 2009 Location: Polonghera, Cuneo, Italy Posts: 27 Rep Power: 9 I have answered becouse the thread original question wasn't solved (or the solution was not clearly described). I had the same problem some times ago and I hoped that my answer could help someone who encounter the same problems with wedge patchs. krene, hfsf and Rokerda like this. __________________ ...The best way to acquire new knowledge is to share it...

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