4 Replies Latest reply on Oct 9, 2008 5:42 PM by Loic Ancian

    flow simulation / time dependent

    Loic Ancian
      Hi,

      I would like to simulate the heating of my product. There is a fan (fluid = air) and a heat source (10kw). Some cavities in my model are filled with water at T°=5°C and their temperature is supposed to increase...

      I could run a stationary analysis. But when I activate the "time dependent" option, my calculation takes a lot of time (10 mn for 1s of physical time).

      Can anyone tell me what numerical method is bein g used here? My goals are useless when I'm in time dependent method, right?
      Is it an explicit method? if yes can I change the time step? Any autostepping?

      Thanks in advance
      Loïc
        • flow simulation / time dependent
          Bill McEachern
          The goals are not used for convergence in a transient analysis but I wouldn't call them useless. You can manually control the time step. Just look under convergence control options. Without knowing more of the specifics various options are available such as flow freezing but that would be dependent on whether the convection cells in the "water" cavities is considered important as opposed tomodeling hte water as a "solid" only considering conduction. You can always develop a model for the water so that you could model it as a solid (as far as heating it up goes. Natural convection and high turbulence are typically not compatible with flow freezing.

          Flow freezing is there to aid problems where the convective processes have short times to stabalize and the diffsuive process take a lot longer. When flow freezing is enabled the advection terms are held constant during the freezing interations.

          With manual time stepping you would need to have fairly short time steps in the beginning so that the flow develops properly and then you could increase them once that occurs. However if you have natural convection in the water cells and it is deemed important then short cuts might be difficult to come by.
          • flow simulation / time dependent
            Loic Ancian
            Hello,
            In my case, the convection cells within water are not considered, therefore I used a solid "water" instead.
            The "flow freezing" option is also a nice feature that works well.

            I could compute my case with a manual time step of 1s from 0 to 1000s, with the warning that the automatic defined one would be something around 0.0001 s. What is the risk of taking such a larger step in my case?

            Thks
            Loïc
            • flow simulation / time dependent
              Loic Ancian
              Ok, thanks for your help !
              but because this is an explicit time integration, I thought that -like in some other softwares- I could specify a small time step for the beginning and then floworks would "auto-step" automatically and increase the time step to speed up the computation... (I don't know if my explanation is clear... )

              Because specifying a too large time step would introduce an error...and next step will even exagerate this divergence...

              Loïc