# how to calculate forces on the wall with free surface?

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 July 4, 2010, 14:58 how to calculate forces on the wall with free surface? #1 Senior Member   Pei-Ying Hsieh Join Date: Mar 2009 Posts: 314 Rep Power: 11 Sponsored Links Dear CFD experts: For a case where liquid drains toward to bottom, I would like to calculate forces on the wall. The major forces I can think of are: shear stress and forces due to surface tension (and/or wall contact angle?). The shear stress is viscosity * (dU/dn). How about the force due to surface tension and/or wall contact angle? How to calculate them? It will be appreciated if someone can point to me a direction. phsieh2005

 July 12, 2010, 20:29 #2 New Member   Rod Join Date: Jun 2010 Posts: 3 Rep Power: 9 Hmmm . . . what do you mean wall contact angle? Do you mean fluid hitting the wall at an oblique angle?? - -this would be resolved by determining pressure at the wall. Only in extreme circumstances would surface tension have a significant effect on wall forces.

 July 13, 2010, 08:11 #3 Senior Member   Pei-Ying Hsieh Join Date: Mar 2009 Posts: 314 Rep Power: 11 Hi, Rod, Thanks a lot for the reply. When a liquid drop sits stationary on a flat surface, the angle between the liquid/air interface with the wall is called wall contact angle. This contact angle depends on the liquid (basically surface tension) and the wall material. Consider draining of liquid inside a test tube. The liquid level goes down during draining. I would like to calculate the force due to surface tension on the wall (this will be only at the location where the interface intersect with the wall) and the force due to the contact angle (will also meaningful at the location where interface intersect with the wall). The shear stress is easy. Of course, the interface moves downward as liquid is being drained. phsieh2005

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