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Measuring wall shear stress in bend pipe

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Old   July 16, 2014, 12:56
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Nara Shikamaru
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Thanks Amir for the formulation. Having taken solid mech courses I was also skeptical about the traction vector that was being calculated. I see that you are not active anymore in this forum, but big thanks for the helpful posts.

I am was working on generating the traction on the interface between two incompressible immiscible fluids (VOF formulation). I am almost done with that. The only part I have doubt about is the stress tensor term being used in this formulation -
Should the viscous stress tensor be used for calculating the Traction vector? This is what I would do if I was doing solids

\tau_{ij}=\mu (\frac{\partial u_i}{\partial x_j}+\frac{\partial u_j}{\partial x_i})

or, The total stress tensor including the pressure term should be used?

\sigma_{ij}=-p\delta_{ij}+\mu (\frac{\partial u_i}{\partial x_j}+\frac{\partial u_j}{\partial x_i})

Last edited by shikamaru; July 16, 2014 at 19:21. Reason: [Typo in equation]
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Old   July 16, 2014, 15:51
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Quote:
Originally Posted by shikamaru View Post
Thanks Amir for the formulation. Having taken solid mech courses I was also skeptical about the traction vector that was being calculated. I see that you are not active anymore in this forum, but big thanks for the helpful posts.

I am was working on generating the traction on the interface between two incompressible immiscible fluids (VOF formulation). I am almost done with that. The only part I have doubt about is the stress tensor term being used in this formulation -
Should the viscous stress tensor be used for calculating the Traction vector? This is what I would do if I was doing solids

\tau_{ij}=\mu (\frac{\partial u_i}{\partial x_j}+\frac{\partial u_i}{\partial x_i})

or, The total stress tensor including the pressure term should be used?

\sigma_{ij}=-p\delta_{ij}+\mu (\frac{\partial u_i}{\partial x_j}+\frac{\partial u_i}{\partial x_i})
Hi,

Reasonably, the total traction force would be originated from the total stress tensor consisted of both pressure and viscous terms; however, it has a typo and needs to be modified like this:

\sigma_{ij}=-p\delta_{ij}+\mu (\frac{\partial u_i}{\partial x_j}+\frac{\partial u_j}{\partial x_i})

Bests,
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Old   July 16, 2014, 19:21
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Thanks for catching the typo, fixed.

Last edited by shikamaru; July 16, 2014 at 23:55.
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Old   July 17, 2014, 09:16
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Since pressure always acts normal to the surface, if anyone only wants the shear stress the pressure term is not needed. However, in my case I am calculating both normal and shear stress on a curved surface. So, I need the pressure term in my calculation.

Please correct me if I am wrong.
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Old   July 17, 2014, 09:28
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Quote:
Originally Posted by shikamaru View Post
Since pressure always acts normal to the surface, if anyone only wants the shear stress the pressure term is not needed. However, in my case I am calculating both normal and shear stress on a curved surface. So, I need the pressure term in my calculation.

Please correct me if I am wrong.
Well, this was a concept we were talking about all the time. The viscous term includes both normal and in-plane components, but the pressure term only contains normal component. So if anyone would be in need of shear stress, the pressure term must be definitely omitted and the viscous term should be modified....

Bests,
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Old   August 9, 2014, 09:07
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I wrote a post processing utility myWallShearStress according to this thread.

And thanks @Amir a lot.

My utility is general, suitable for incompressible and compressible flows using RAS or LES turbulence models.
Attached Files
File Type: gz myWallShearStress.tar.gz (2.2 KB, 19 views)
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Old   August 17, 2014, 10:56
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Anastasios Stampoultzoglou
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Hi all,

I am a new user on openfoam . I read your posts and it was very useful. I would like to ask you which method the utility "wallShearStress" use (for example log law method or momentum method or Reynolds stress method, etc. )

Thank you very much
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Old   September 9, 2014, 11:34
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Quote:
Originally Posted by epi_c View Post
I wrote a post processing utility myWallShearStress according to this thread.

And thanks @Amir a lot.

My utility is general, suitable for incompressible and compressible flows using RAS or LES turbulence models.
Thanks for the utility! That being said, the units for the compressible stress tensor are wrong, as they should be \frac{kg}{ms^2} and not \frac{m^2}{s^2}. The results are not affected by this, though.

Regards
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Old   September 13, 2014, 05:25
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Greetings to all!

This has become a rather long and somewhat confusing thread, but unfortunately I will only be able to try and clarify Tasos' question:
Quote:
Originally Posted by Tasos View Post
I read your posts and it was very useful. I would like to ask you which method the utility "wallShearStress" use (for example log law method or momentum method or Reynolds stress method, etc. )
The answer is given in some of the previous posts, from which I'll quote the most important details I could quickly spot:
Quote:
Originally Posted by ngj View Post
BTW: In stead of making the stress tensor yourself, you could benefit from the turbulence model by calling devReff(). The only thing to remember is that this tensor is also non-tangential with the wall, so you have to perform the same projection (as you already do) onto the boundary face.

The upside of using the turbulence model is that you do not have to implement turbulence model specific shear stress methods, because the turbulence model itself tells, how the shear stress is defined.
And:
Quote:
Originally Posted by Nucleophobe View Post
Edit:
I just tried this for my case, and I get the same result doing either of the following:
tau = (wallShearStress) * rho
tau = (wallGradU) * mu

Is that expected? Was the wallShearStress utility fixed? I am solving a laminar incompressible flow with a density rho close to that of water.
@Tasos: you'll have to draw your own specific conclusions, but I'd say from the quotes above, the utility basis closely resembles the "Reynolds stress method" you've considered, namely "devReff()": https://github.com/OpenFOAM/OpenFOAM...inar/laminar.H
Quote:
Code:
 //- Return the effective sub-grid turbulence stress tensor
// including the laminar stress
virtual tmp<volSymmTensorField> devReff() const;
In addition, I'll quote from another post of mine on another thread:
Quote:
Originally Posted by wyldckat View Post
My guess is that you did not take into account the fact that the wallShearStress utility will give you the results in the same units of the pressure field: http://www.cfd-online.com/Wiki/Wall_shear_stress
Which for incompressible flow, in OpenFOAM, it usually is in "kinematic pressure", namely in "m2/s2".

In post #46 of the other thread you pointed out, you'll find this comment:
Quote:
Originally Posted by Nucleophobe View Post
Edit:
I just tried this for my case, and I get the same result doing either of the following:
tau = (wallShearStress) * rho
tau = (wallGradU) * mu
This means that the value are seeing in OpenFOAM does not yet take into account the density of the fluid. You can use the Calculator filter in ParaView to multiply the "wallShearStress" field by the density value you have for your fluid.
I hope this answers your question and hopefully clear up some details asked above.

Best regards,
Bruno
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Old   September 13, 2014, 08:55
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Mr Bruno thank you very very much, your help is priceless.
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