Learning Forum

[drdmsthesis]

Hi,

In the case of applying a hydrostatic load one the lower half and a pressure load on the upper half of a tank in a static analysis, which case is correct in Ansys? 

a. Step 1 hydrostatic load on, step 2 hydrostatic load and pressure load on

b. Step 1 hydrostatic load on, step 2 pressure load on, hydrostatic load deactivated 

In case b, is the step 1 behaving like a pre-stressed step? The idea is to avoid the ramping effect and superimposition of gravity loads. 

Thank you. 

 

[peteroznewman]

I show a Hydrostatic load on the lower half.

If the tank walls are exposed to atmospheric pressure inside and out, then there is no net force on the walls and the stress from the hydrostatic pressure is correct.

If the tank is closed and can be pressurized, then all the interior walls would get the same pressure and that would be in addition to the hydrostatic pressure.  This will increase the stress.

So you can have a 2 step solution and have just the hydrostatic pressure for step 1 to show the unpressurized tank stress. You would have a pressure load, but just set the pressure to 0 in step 1. Then in step 2 you can use a nonzero pressure and see how much the stress increases.

[drdmsthesis]

Thank you Peter, it is indeed a closed tank. So I assume Step 1 = Initialization with Hydrostatic load, Step 2: Internal Pressure, Hydrostatic load deactivated. Interestingly, I get different stress profiles with the two cases. Maybe Ansys need to include some more data on the use of the hydrostatic load with other loading. 

[peteroznewman]

No, don’t deactivate the hydrostatic pressure unless you want to see the stress in the tank when it has no liquid, but is pressurized. 

Just leave the hydrostatic pressure active in step 2 and also have a separate pressure load on all the tank wall faces, including the faces below the liquid free surface.

The air pressure is pushing on the free surface and adding a constant pressure offset to the liquid, which transfers it to the wall.

[drdmsthesis]

Thank you Peter, I think my error was in not including the surfaces that are submerged below the liquid (lower half of tank) as acted upon by the internal pressure. Are there benchmarks for the hydrostatic loads in Ansys? I could not find any.