1 Reply Latest reply on Jan 27, 2015 1:38 AM by Jared Conway

# Force calculation does not agree with plotted variables

I have a simulation of a solid object resting on a perforated drum, with a vacuum pressure behind, as pictured: Please focus only on the highlighted face and ignore the rest.

As you can see there are only a few places where there is solid-fluid contact, namely at the holes (which are 5mm in diameter.) The pressure is approximately uniform, a -25 inches of water. Only 12 of the holes have vacuum pressure. Neglecting friction, one would expect the force on this solid to be = 12 x (25 inches H2O) x pi() x (2.5mm)^2. This equation is equal to approximately 150 gram-force (http://www.wolframalpha.com/input/?i=3*4*25+in+wc*pi*%282.5mm%29%5E2+to+gram-force).

When I do a surface parameter on this face I get this result from Flow Sim:

 Integral Parameter Value X-component Y-component Z-component Surface Area [in^2] Normal Force [grf] 1745.461 -1202.013 -1265.621 -3.873e-004 0.3972 Force [grf] 1745.461 -1202.013 -1265.621 -3.872e-004 0.3972

As you can see the force and normal force are identical, there is no shear force. The error here is not small, it is a factor of 10 off from what I would expect. Does anybody know why this would be? Did I miss something?

• ###### Re: Force calculation does not agree with plotted variables

check the swx kb and help

you have to be really careful selecting open faces when plotting forces because it uses the reference pressure

you may need to adjust it to get good results

I would also just set expectations using something a little simpler to make sure your methodology is good and your expectations are reasonable