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March 26, 2010, 07:56 

#21 
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Claus Meister
Join Date: Aug 2009
Location: Wiesbaden, Germany
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Hey Foamers,
does anybody know a book or paper where the formula in wallShearStress.C is mathematically derived? I need something which I can cite it in my scientific work (I must guarantee that it work). Furthermore, does anybody know a justifaction that I can use this formular even for twophase flows, especally, in VOF? Cheers in advanced! 

April 15, 2010, 03:46 

#22 
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Claus Meister
Join Date: Aug 2009
Location: Wiesbaden, Germany
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Hey folks!
I have used interFoam with an kemodel. I modified wallShearStress.C that the material property directory is reading correctly. Now I need to know if the formular is still valid. cheers 

April 16, 2010, 06:53 

#23 
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Anders Bųje
Join Date: Apr 2010
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Hi
I am trying to implement calculations of the shear stress in an interFoam case, simulating injection moulding. I have been looking at the threads on this forum trying to figure out how to do this. In this thread it seems that something similar is done, but being new to the world of OpenFOAM I am looking for some more basic instructions. Where do I implement the c++ code? Do I have to create new files or modify existing ones? If someone has done shear stress implementation, I would much appreciate some help. For instance if you have a case where this is done I could take a look at, not nessesarily interFOAM. The same goes for the wallShearStress utility. Best Regards Anders 

June 8, 2010, 17:20 

#24 
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Daniele
Join Date: Feb 2010
Posts: 134
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Hi
In Ras turbolent model, if i use wallShearStress tool, the result is normalised? Best regards 

June 9, 2010, 02:08 

#25 
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Claus Meister
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What you mean by "normelised"? If you mean that the calculated vectors have the length one, then No !. The vectors are actually tensor of rank 1, i.e. the vectors points in the direction to where forces acts and the magnitude of them are force acting.
Cheers 

June 9, 2010, 07:56 

#26 
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Daniele
Join Date: Feb 2010
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No, i would know if is there rho or not in the result
Best regards 

June 25, 2010, 07:00 

#27 
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Daniele
Join Date: Feb 2010
Posts: 134
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Hi
How can I calulate the wallShearStress runtime? Thanks 

June 25, 2010, 09:04 

#28 
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Daniele
Join Date: Feb 2010
Posts: 134
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Can I use sampleDict to sample wallShearStress on bottom? I do:
1)blockMesh 2)simpleFoam 3)wallShearStress 4)sample latestTime but in my set directory output file of whall shear stress is 0 everywhere... 

September 16, 2010, 16:22 

#29 
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Peter
Join Date: Aug 2010
Posts: 16
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Hi@all,
this week, I simulated a channel flow using rhoSimpleFoam with kOmegaSSTModell. To validate the results, I need to calculate the friction coefficient cf(Tau_wall / (0.5*roh_0*U_0^2). There are now 2 possibilities to get Tau_wall. First by taking the wall shear stress computed by the WallShearStress postprocessing tool. (Simply taking the magnitude of the wallshearstresses as Tau_wall). The second approach is to use the wallGradU tool, and compute the cf as described above by Santos. Surprisingly, the results were quite different (diffference of more than 10 Percent). Is anybody able to explain the value differences? My guess is that the shear stresses calculated by the wallshearstress tool are not exact. Is that true? Thanks, Peter 

February 27, 2011, 19:45 

#30  
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Jie
Join Date: Jan 2010
Location: Australia
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Quote:
I have tried both laminar flow and turbulent flow wss as you suggested for the same simulation. How come the wss calculated by the two methods vary so much from each other? Do you need to multiple rho*nu for the turbulent case? Thanks 

April 11, 2011, 12:17 

#31 
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Sergio
Join Date: Apr 2011
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Hello Guys
I am also very new to OpenFoam and CFD, and I was wondering how can I calculate the Viscous Stresses (say in a given point of the fluid). In addition, note that instead of RAS, I am using a LES Turbulence model. Thanks for your help Sergio 

June 23, 2011, 09:33 

#32  
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David GISEN
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Location: Germany
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There are so much unanswered questions in here, so I try to answer two of them:
At least for incompressible cases, rho is excluded, so you have to multiply the results from wallShearStress by your density. Quote:
Code:
surfaces ( type patch; patchName bottom; ) Hope that helps someone! 

January 14, 2012, 08:55 

#33 
New Member
alex
Join Date: Jun 2009
Posts: 17
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Hello
So if we run the wallShearStress utility we must multiply the values by the rho? For instance in my case i'm simulating water flow in a tube with spacers and after running the wallShearStress it gives me as in the figure. The value of maximum magnitude of "displayed" wall shear stress is near 0.00043 (in unknown units...). If I multiply by the density of water (near 996 kg/m³) its gives 4.3 (in unkown units). Is this new units Pascal? Thanks, Duarte Silva Last edited by oort; January 14, 2012 at 10:25. 

January 14, 2012, 10:35 

#34 
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Claus Meister
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Yes, according to the wallShearStress.C code you must multiply by rho to obtain pascal. But to be sure, open the dictionaries wallShearStress in the time directories by using a editor and take a look at the dimension line. Here you can get the dimension of the data. Just do an dimension analysis and you will see that the data have to be multiplied by the density.


August 28, 2012, 08:12 

#35  
New Member
Elise
Join Date: Jan 2012
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Quote:
What does the quoted code means; mesh.Sf/mag(mesh.Sf) ; what does this term do? : gives a unit vector normal to the boundary element & (turbulence>R()() ; is this the lookup of the wall shear stress from the Reynolds stress? : Yes, Reynolds stress Last edited by Elise; August 29, 2012 at 02:28. Reason: solved 

May 10, 2013, 08:54 

#36 
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Yosmcer Mocktai
Join Date: Apr 2013
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I know this post is old, but it is the best one I found related to my problem.
I have a wall that is parallel to the X axis, flow is laminar. I compute the wallShearStress, and have a result. I use paraFoam to look at this with Paraview. I plot over line the wallShearsStress by placing the line on the boundary. I see that I have a value for the magnitude of the wallShearStress, and three others values, one for each direction. As my face is parallel to the X axis, I expect to have a signed value in the X direction, a value approximating zero in the two other direction, and the absolute value of the X's direction value. But I also have a Y direction value, so a normal compound to the face. And this value is not negligible (the z value is negligible, but no flow in this direction). The magnitude is the vectorial sum of the X and Y values (without the sign). [EDIT] I think this is because the shears stress must equilibrate at edges. So there is a normal compound corresponding to the tangential one of the perpendicular face, and a continuity of this across the face. Image from : http://www.codecogs.com/reference/en...ear_stress.php So, considering the AB face, there is a normal compound to this face at the points A and B, but opposite, so there must be a continuous field across the face, and the resulting shear stress in not totally tangential to the face. Please, correct me if there is/are mistake(s). [\EDIT] Last edited by Yosmcer; May 22, 2013 at 15:18. 

May 22, 2013, 08:11 

#37  
Member
Join Date: Nov 2012
Location: Liverpool, UK
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Hi Yosmcer,
I am using interFoam, however the wallShearStress utility is intended for single phase transport models. I modified it to include "incompressible/incompressibleTwoPhaseMixture/twoPhaseMixture.H" and etc., and I've modified the option file according to that of interFoam. The compiling is ok, but when I run it, error message pops out. Quote:
Could you shed some light on my problem please? Many thanks in advance. Sophie 

May 22, 2013, 10:15 

#38 
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Pablo Higuera
Join Date: Jan 2011
Location: Singapore
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Hi Sophie,
alpha1 is not in the object registry in this case. You have to do the same as with U, read it from the time folder. Best, Pablo 

May 22, 2013, 10:40 

#39 
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Location: Liverpool, UK
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Hi Pablo,
Thank you so much! I've got it solved. Best, Sophie 

December 9, 2013, 13:45 

#40  
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Camille Bilger
Join Date: Jul 2013
Posts: 42
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Quote:
I have the exact same issue and unfortunately did not understand how to do this as I am myself very new to C++ and OpenFOAM. I would like to calculate dU/dy for my fuel injection with interFoam and then the shear stresses. I would appreciate your knowledge on this, if you did figure it out. Thank you very much. 

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