Fluids

Fluids

Why and when need to turn on “operating density”

    • cs437
      Subscriber

      Hi,


       


      I am dealing with a mixing tank problem with multiphases VOF. I am wondering whether I need to tick "operating density" when doing the simulation. If that, should I set it to the lighter phase, air. Some comments suggested using operating density is better for multiphase flows. I would like to know how difference with/without operating density is in terms of results.


      Best regards,

    • DrAmine
      Ansys Employee
      Best to trst on single phase run straight vertixal duct with oressure inlet and pressure outlet with same temperature outside and heated wall. Here the value you provide will dramatically affect the flow. The reason is Fluent does a variable changes as it solves for motion pressure and hydrostatic head due to reference density is excluded from pressure calculation. More information in the help.
    • cs437
      Subscriber

      Hi,


       


      Yes, I have read "Help" and the operating density seems to be associated with a natural convection. For my case, water is incompressible fluid while air is compressible with isothermal in a mixing tank; however, the flow is not natural-convection flows (it is created by external force). Therefore, there is no need to specify operating density, right? Am I understand correctly?


       


      Best regards,

    • DrAmine
      Ansys Employee
      Yes there is a need to enhance stability and abstract hydrostatic pressure. I would for multiphase provide a density of zero and provide the full pressure at boundaries. For stratified flows you can provide the one if the lighter phase. At pressure boundary you need then to provide the profile excluding that pressure due to phase say g ×height×(density-reference)
    • cs437
      Subscriber

      Hi, 


       


      Thank you. How different multi-phases you mentioned and stratified flow since the latter is also multi-phases. Is operating density zero for only compressible fluid? For operating reference pressure, I put at the top boundary of primary phase. 


      Best regards,

    • DrAmine
      Ansys Employee
      If you put zero umyou need to provide the pressure profile at boundary including hydrostatic head.
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