I have no idea what I'm doing wrong. Please see picture below; we modeled part of a tank that is going to basically be at atmospheric. The tank is to be 12 ft tall with a vapor space cushion of 8" since it will be heated.
Per the picture, something in the way we're applying the non-uniform water pressure force is yielding results in the opposite direction than expected with respect to displacement of the wall surface. I applied the Hydrodynamic force to both the wall and horizontal W-beams as shown. In the last 5 minutes, I've gone back and made the fluid density a negative 0.0349 lbf / in^3 - this did result in changing the direction of the hydro force, but is this correct. I also had gravity and thermal both at 1 with a Temp=125 F.
Any thoughts on my setup and results?
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Have you displayed the pressure arrows in the Results environment -- just to make sure that the results do agree with the direction of the load. ("Results Options > View > Loads and Constraints".)
My only thought applies if you are using plate elements. It looks like you are using brick (or tet) elements, but I can't tell for sure from the image. If you are using plate elements, the problem could be that you have not specified which side of the plate is the "top" and which is the "bottom". Pressures are applied to the "bottom" surface. You use the Element Definition to enter a coordinate in space; the side of each element that faces this coordinate (the Element Normal coordinate) is the bottom of the plate element.
16.9 years experience with Simulation Mechanical
As AstroJohn indicated you may have the direction of the pressures not selected correctly. If you are running your simulation with brick elements I would suggest changing to plate elements. I've had very good success using the plate/shell elements with a setting of Thin for the Element Formulation found under Element Definition -> Advanced tab. The results are generally similar to calculations and lab tests for thin plate deflection. If you do not feel comfortable using the plate/shell elements I would suggest you double check the deflection results using a hand calc. Please keep in mind that if the deflection is greater than 1/2 the material thickness then many formulas may not apply.