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-   -   Can anyone give me some hint on how to make traction free boundary condition? (http://www.cfd-online.com/Forums/openfoam/87623-can-anyone-give-me-some-hint-how-make-traction-free-boundary-condition.html)

 poplar April 25, 2011 06:31

Can anyone give me some hint on how to make traction free boundary condition?

Traction free boundary condition is often described at open boundary with setting the pressure p to be zero or known. The 3 unknown velocity components at open boundary may be calculated from three equations:f.e., du/dx=0, du/dy+dv/dx=0 and dw/dx+du/dz=0 at a vertical b.c. normal to x. Then tangential (shear) stress and normal stress are both zero. For a rising jet or plume, fluid will be entrained in from side boundary. Traction free boundary condition may give better entrainment than Slip BC(shear stress is zero, no entrainment) or zeroGradient(normal stress is zero, unstable). PressureInletOutletVelocity BC may permit the entrainment and give stable results but not very good.

I can implement the Traction free boundary under FDM. but I do not how to implement it under OpenFOAM. I have read many b.c. source files of OpenFOAM, thay all seem to be different from traction free boundary.

I wonder why it is not a built in b.c. in CFD softwares including OpenFOAM?

 prasanthnitt September 20, 2012 07:00

Traction free boundary condition

Hi,

Could you please explain how to implement the traction free boundary condition in FDM. Take the case of a Jet, at the bottom wall dirichlet velocity bc so the pressure bc is Neumann. Lateral boundary if it is traction free then Pressure is zero. What should be done at the outflow boundary? I m facing some issues while trying to implement traction free bc. My poisson solver is not converging at the very first time-step.

If you can share some example code that would be great.

Cheers
Prasanth

 sloti May 16, 2013 03:02

Traction free boundary condition

Hey there,
I know your post was already a while ago, but none the less I am curious whether you found a solution for your problem?
Implementing a traction free bc condition might solve my current problems, too.
I am trying to use a moving boundary to "suck in" air from outside the geometry. right now I use zeroGradient for the velocity and given pressure but it is not working correctl. Also with inletOutlet conditions I do not get good results.