when you setup a flow simulation and define the initial conditions (fluid, temperature, pressure) it fills in all of the fluid cells with this information.
If you were to define a given volume as at zero absolute pressure then it would assume it empty, otherwise the initial conditions define the cell default state.
Just like if you define a BC as having a velocity, it initially sets all other fluid cells to zero velocity (again, unless you specify a non-zero initial condition).
thanks for your quick reply Chris
it all makes sense to me now but I have one question about the BC though:
If the fluid exits to the atmosphere, do i specify the environment pressure at the outlet lid as that of the atmospheric pressure ? (the height difference between the inlet and outlet of the fluid is about 13m )
Because sometimes i get negative pressure warnings and sometimes the solver runs fine (this is when i try different fluids)
Any idea about that?
Thanks in advance again.
I've never had negative pressure warnings - I will get warnings about a vortex crossing an opening (some flow in, some flow out at the same BC).
Then again almost all of my simulations are gases, not liquids so the difference of 13m wouldn't really have any influence on properties.
With a liquid column I would say you can specify the outlet as atmospheric pressure and as long as you have gravity enabled it should take into account the headloss as well as frictional losses.
How do the warnings pop up? For vortex warnings it simply says something like "Inlet/outlet = 0.015" or something to give you an indication of how severe the peculiarity is. Sometimes they resolve themselves before the simulation comes to a solution, other times even when it completes it still shows warnings.
You can get negative pressure warnings - they are not big deal as long as they go away as the solution progresses. Probably indicates that the mesh is a bit coarse in the area where it occurs. Ensure you have everything in terms of absolute pressures. Flow sim is not designed to use gauge pressures (as it use the equation of state). If they don't go away you can turn on the cavitation model if your liquid is supported. The pressure potential switch should also be set - probably is.