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simple 3D pipe flow diverges with icoFoam

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Old   June 23, 2011, 09:24
Default simple 3D pipe flow diverges with icoFoam
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Rol
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Hello!

I am trying to get grips with OpenFoam. I set up a really EASY case. It is some kind of straight tube. The simulation should be easy, but after 6 iterations, icoFoam diverges. I use the solver settings from the elbow testcase.
Possible problems i thought on:
mesh: it is really fine, much too fine (just to be sure)
time step: is also set really small (dx=2/40 m, U=1m/s -> dt=0.05 should be sufficient, i set it to 0.01)
boundary conditions: inlet with Velocity set to 1 m/s; p at outlet set to 0 Pa. The rest is wall. I checked BC several times, and i find no mistake

I include the test case.

the case is ready to

blockMesh

and

icoFoam

you will see, it diverges after 6 time steps.

any hint is really appreciated!

Roland
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Old   June 23, 2011, 13:33
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I think my test case was not as simple as i thought. The geometry of the tube leads to secondary flows. however, i found out convergence with upwind divScheme is much better.
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Old   June 24, 2011, 09:55
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Quote:
Originally Posted by Rol View Post
I think my test case was not as simple as i thought. The geometry of the tube leads to secondary flows. however, i found out convergence with upwind divScheme is much better.

I think the length of your tube in y direction is very small and you can not impose zeroGradient at the outlet I suggest you, to increase your length ten times of square root of section area
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Old   June 24, 2011, 12:03
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mechy, thanks, although i checked the solver conditions several times i payed less attention on the geometry itself. It makes sense that the short tube interferes with the zeroGradient boundary condition. Thanks!
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Old   June 25, 2011, 04:28
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Originally Posted by Rol View Post
mechy, thanks, although i checked the solver conditions several times i payed less attention on the geometry itself. It makes sense that the short tube interferes with the zeroGradient boundary condition. Thanks!

is your problem solved with increasing length of tube ?
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Old   June 27, 2011, 17:02
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yes, increasing the length solves the problem. Also i set the time step very low. maybe setting pressure boundary condition instead of velocity at inlet would also help convergence. but it works for me. thanks
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