4 Replies Latest reply on Jan 14, 2011 2:00 PM by Bill McEachern

    Internal Flows - Does Flow Simulation consider a vessel empty initially ?

    Tom Sawyer

      Hello,

       

      I am currently doing a project which involves internal flows in a vessel.

       

      I want to know if Flow Simulation considers the vessel initially empty or if it assumes the vessel is already completely filled with the flowing fluid when the solver begins the calculations.

       

      I'm new to Flow Simulation and up to now i have not come across any piece of information that can help me on that.

       

      Your help or suggestions are very much appreciated.

       

      Thanks in advance

       

      Tom

        • Re: Internal Flows - Does Flow Simulation consider a vessel empty initially ?
          Chris Michalski

          Tom -

           

          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).

            • Re: Internal Flows - Does Flow Simulation consider a vessel empty initially ?
              Tom Sawyer

              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.

                • Re: Internal Flows - Does Flow Simulation consider a vessel empty initially ?
                  Chris Michalski

                  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.