# Is there a way to export a simulation runs' cell data for the viscous forces vectors?

It does not look like I am able to analyze the viscous forces from a Sim run in Paraview.

The only results available to me in a run, that includes a ‘Forces and Moments’ result, are these:

While there is ‘p’ pressure cell data, that ‘p’ represents ONLY the ‘pressure force’ on the nodes and cells.

Where is the viscous cell data from which these ‘OpenFoam Solver Log’ viscous results were totaled from? :

``````forces forces1 output:
sum of forces:
pressure : (0.899477815465 0.000250002893751 0.00434935196386)
viscous  : (0.000435184212691 8.80383335398e-06 0.0015251541872)
porous   : (0 0 0)
sum of moments:
pressure : (9.05475938616e-06 -0.0246267281685 -3.38115630888e-05)
viscous  : (1.73593995622e-06 -9.99376338653e-05 -2.6845190272e-07)
porous   : (0 0 0)``````

@jousefm , who can we call to this topic that may have an answer

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Hi Dale,

Sorry for the late response, quite busy at the moment. Does this thread help you? - Drag Coefficient result?

If not, please let me know! I have answered this question from another person in the past I guess but cannot find the thread now Just a small side note here: In CFD we have mainly two types of forces, pressure force and viscous force, the pressure force is calculated as the pressure normals integrated over the given area and the viscous force is calculated as the shear stress times the area.

Hope that makes sense and I could help a bit! @1318980, in case you want to add something here.

Best,

Jousef

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No, that does not help, the question is about the fact that viscous results do not seem to export when results are downloaded…

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My answer implies that you would need to use the calculator/filter in order to achieve these results. Tagged Darren in case he knows - maybe the @cfd_squad can also add their comments & thoughts here.

Cheers!

Jousef

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The calculator/filter only works on data that resides in the case.foam structure, I can not find the data in the structure… see dropdown list on first image that shows all the data available to view in ParaView…

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Would have to check that Dale, might take a bit though

Cheers!

Jousef

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Hi @DaleKramer, @jousefm, I think I did write a post before that describes how you can calculate the viscous forces. In its essence, you can add wall fluxes as a field result in the simulation, then you can use this similarly to normal pressures to calculate the viscous force.

Best,
Darren

That was the idea but I thought Dale was missing some output in his case.foam to work with - could not find your post so far but will keep digging

Cheers and thanks for the follow up!

Jousef

@1318980

Darren,

I will add ‘wall fluxes - wall shear stress’ to my standard ‘results’ that I use.

Using ‘Advanced Searching’, and other than this topic, I can not find any posts you have ever made which have ‘wall fluxes’ in them. I will try to use same approach in ParaView for calculating lift and drag forces from ‘p’…

Thanks,
Dale

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Darren,

I am trying to integrate the wallShearStress_Z*Normals_Z to get viscous lift and similar for drag but the results don’t make sense (viscous lift ~ same magnitude as viscous drag and viscous drag is negative )…

Do you see what I am doing wrong?

Thanks,
Dale

Not sure here, I think wall shear stress is vectorised already, so you just need to do integrate varaible on wall shear vector to get the viscous forces in x,y,z but would need to look into it to be sure.

Best,
Darren

Do you know what units the Normals are?

You might want to try the same thing I am trying here, verify that the OpenFoam results for CL, CD and CM truly include viscous forces AND that ParaView integrates the results to the same number as OpenFoam.

So far with pressure forces only ParaView gives close but not exactly matching values

EDIT: Also, that ~6/1 range of -ve pressures to +ve pressures is in line for experimental data at 10aoa so, something is fishy in Denmark