CFD Online Discussion Forums - Measuring wall shear stress in bend pipe

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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?

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

Quote:

Originally Posted by shikamaru (Post 501794)

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,

Thanks for catching the typo, fixed.

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.

Quote:

Originally Posted by shikamaru (Post 501979)

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,

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.

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

Quote:

Originally Posted by epi_c (Post 505165)

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

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 (Post 506342)

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 (Post 378727)

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 (Post 418210)

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 (Post 509408)

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 (Post 418210)

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

Mr Bruno thank you very very much, your help is priceless. :D

Hello Amir
could you tell me why
n=-mesh.Sf().boundaryField()[patchi] / mesh.magSf().boundaryField()[patchi]
there is a minus "-" here

And I also check the code in Openfoam for the forces calculation,there also calculate the viscous force which is the also wall shear stress
but in the utility ,the code is:
sfb[patchI]&devrhoReffb[patch] do you think there are the same?

Best regards!