Learning Forum

[Emma CFD]

hello!

I want to apply the wall shear stress BC on my 3d tube wall, the problem is in BC panel I have to specify shear stress in x, y, and z-directions and I have a cylindrical coordinates system (3d tube). So how to apply this condition in my case study?

any guidelines please I would be grateful !! 

[Rob]

Are you expecting swirl in the pipe? If not then you can ignore two components. If yes, then you can use an expression: you may want to refresh your trigonometry skills. 

[Emma CFD]

Thank you, sir, for the reply!

actually I have an elbow (horizontal pipe and a curved pipe so the swirling flow will be in the curved pipe), the objective of my study is to compare the no-slip condition and the wall shear stress condition.

should I use a UDF for that? Do you think that I have to transform the coordinate system from cartesian to cylindrical one?

[Rob]

Does the shear vary in each direction? 

[Emma CFD]

I think it will be significant in the radial direction than the other directions 

[Rob]

I assume you mean circumferential/tangential over radial?  I think you're going to need a UDF giving the values based on position, there should be an example in the documentation. 

[Ritz Ri]

The simplest way to apply a wall shear stress boundary condition in a cylindrical coordinate system is to convert the wall shear stress into components in the radial and axial directions. The radial component should be zero since shear stress has no effect on the radial direction in a cylindrical coordinate system. The axial component can be calculated from the wall shear stress using the equation:

Axial Shear Stress = Wall Shear Stress x cos(angle between wall normal and axial direction).

Once the axial shear stress is determined, it can be applied to the boundary condition in the axial direction.