# Shear Stress

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October 10, 2013, 16:04
Shear Stress
#1
New Member

Marcus Fernandes Araujo Filho
Join Date: Sep 2013
Posts: 10
Rep Power: 5
Dear all,

I´ve beeing try to evaluate the shear stress of a channel but i´ve found some issues. I´ve attached a picture of the results to help you guys to understand.

the problem is that it just doesn´t look right. The changes are very sudden and the values seem off chart. Does any have one have an idea of what might be happening?

My first thought is the my mesh is too course. Is there a "rule of thumb" of how dense should my mesh be in order to correctly estimate the shear stress? The mesh is around 0,5m in a 10m wide channel.

Thank you all.
Attached Images
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 October 17, 2013, 10:57 #2 Senior Member   Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 9 Yes, Marcus, the shear stress output is mesh-size-dependent. Try halving your mesh size until the shear stress output doesn't vary by more than 3% between runs: you can call that the 'converged' mesh size. Also, please consider upgrading to v10.1 if you haven't already: the shear stress calculations are much better in v10.0 and v10.1 than in earlier versions. - Jeff

 October 22, 2013, 08:03 #3 New Member   Marcus Fernandes Araujo Filho Join Date: Sep 2013 Posts: 10 Rep Power: 5 Thank you Jeff! One more question. Is there a "rule of thumb" of how many mesh cells must I have in a certain depth in order to have some first acceptable results? Marcus

 October 22, 2013, 10:39 #4 Senior Member   Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 9 Yes. The rules of thumb are: 1) at least 3 to 4 cells across the narrowest flow width (usually a depth, but might be a gate) in order to get a model that works. 2) at least 10 to 12 cells across the narrowest flow width to get a model that's accurate. The best approach is still to start with rule #1 above and successively refine the mesh two or three more times (total of three or four meshes) and demonstrate that the difference between the finest mesh results and the next-closest mesh results are so small that the model can be considered 'converged'. I know that's not always possible, so the rules of thumb above can be a shortcut. Marcus.filho likes this.

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