[brainok]

Originally Posted by ghorrocks

There should be no need to do as you say. Have you seen my post #15? That explains how to do it.

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https://drive.google.com/file/d/0B-Q...it?usp=sharing

Many Thanks
HyoSuk

[ghorrocks]

You seem to be defining 2 points in 3D space. WSS exists as a variable on the wall surface. So you need a line object on the wall surface, not in 3D space. You can generate this from the intersection of a plane with the wall surface (or many other means).

[singer1812]

Glenn, regarding your method. Wouldnt it have been easier to create an additional variable WSS*Normal.

Plot that on the surface.

Put your line on the surface.

Pull out the additional variable values that lie on the line?

[ghorrocks]

Yes, that probably would be easier - thanks for the tip.

[brainok]

Originally Posted by ghorrocks

You seem to be defining 2 points in 3D space. WSS exists as a variable on the wall surface. So you need a line object on the wall surface, not in 3D space. You can generate this from the intersection of a plane with the wall surface (or many other means).

Thanks for giving me insights.
I am a doctor so I have little knowledge about fluid dynamics.
I want to measure wall shear stress on the stenotic artery.
According to your reply, I have to draw line on the wall not to 3D model.
Does It mean that I have to draw line on the mesh before simulation?

And, dear Endmund, what is WSS*Normal? How can I use in CFX?
Thanks

[ghorrocks]

No, you just need to generate a line object which lies on the wall boundary. There are lots of ways of doing this:
* you could draw a line in 3D space and project it onto the surface
* you could get the intersection of a surface and the wall surface
* you could extract a contour of some description on the wall surface
* many others...

All these methods generate a line object which lies on the wall. Now you have a line object on the wall you should be able to extract wall variables (like wall shear stress) along this line.

Normal is a variable defined at surfaces, where it gives the surface normal at that point. Look it up the CFX Reference manual for further information.

[brainok]

I will do according to your comments.
I appreciate your kindness.
Thanks
HyoSuk

[pjohhn]

Hi mrkmrk,

I tried to implement the tangential wall shear stress in CFD post along the curved surface as you mentioned i.e.
tangential wall shear stress vector= T-((T.N)N)

I created three Tx,Ty,Tz expressions with Normal X, Normal Y and Normal Z and then created a tangential wall shear stress vector in variable list.

However, I didn't get the negative tangential wall shear stress value.

Can you please suggest how did you solve the same, as I am also intersted in the negative co-efficient of friction for the separation zone?

[mrkmrk]

Hi,

I calculated it on a curve and not a surface,
how did you create Tx?(type your expression)

[pjohhn]

I calculated Tx with consideration of Normal X. I think what I did was mistake since we get the vector of shear stress from the CFD post. I used
Tx = Tx - (Tx.N)N) and it is wrong.

I think when we apply tangential wall shear stress vector= T-((T.N)N), it will be same as the wall shear stress vector obtained from the CFD post or not??

Actually I also want along the curve..I have the pipe bend and over the symmetry plane boundary I want to have the co efficient of friction.

Can you pls. tell me how did you calculated along the curve?? By considering the dot product of TX, Ty, Tz and the unit tangent vector pointing towards the flow direction?

[mrkmrk]

I think in the CFD post "Wall Shear" variable represents wall shear stress vector that is always tangential to your surface but when you want its contour on a surface CFD post calculates only the magnitude of this vector that is always positive. If you want negative values of tangential wall shear stress, you should calculate it on a line/curve. For calculating tangential wall shear stress on a curve you should export this curve wall shear X, Y, Z data and then process them with a mathematical software (like Maple or Matlab) by the method that Glenn said in post#15.

[pjohhn]

Thanks for your suggestion...The problem is solved now...:-)

[bongbang]

Originally Posted by ghorrocks

No, you just need to generate a line object which lies on the wall boundary. There are lots of ways of doing this:
* you could draw a line in 3D space and project it onto the surface

How do you do that projection? I've looked and looked in vain in the documentation and elsewhere. (I actually want to project a point, but figure it's the same process.)

[Nils_de]

Dear all,

I want to calculate wall shear stress in a steady state condition for laminar flow. How did you calculate it?

Thanks in advance

[guxin7005]

Originally Posted by Nils_de

Dear all,

I want to calculate wall shear stress in a steady state condition for laminar flow. How did you calculate it?

Thanks in advance

If you open in Fluent, you can easily find it in post-processing, Countours/Wall Fluxes

[Nils_de]

Originally Posted by guxin7005

If you open in Fluent, you can easily find it in post-processing, Countours/Wall Fluxes

I am using simpleFoam in OpenFOAM. I am noly able to get velocity contours but not wall shear stress.

[srinivas09]

how to create a intersection for two different parts in one plane