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:
Surface Area [in^2]
Normal Force [grf]
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?