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Turbulence at steady free surface

    • kevin.anny
      Subscriber

      As we know, vof model are often used for free surface features simulation such as distortion and borken. While, sometimes the interface are relatively stable, that means the free surface shape nearly doesn't change with time. Take open channel flow as an example. When the gas shearing effects is weak and the flow is steady, the vof model is not necessary.

      In ANSYS Fluent, we can get a model shown in the figure. Vof model is not used. If we use RANS k-epsilon to simulation the weak natural convection, then the turbulent viscosity is zero both at free surface and crucible. Because the default boundary type of crucible and free surface is WALL, Fluent treat WALL type boundary with zero value of k and epsilon transportation equation. That means turbulence shouldn't happen at WALL.

      While how could I considering the turbulence at free surface? Is there any methods to modify the boundary condition of k and epsion transportation equation?

    • Rob
      Ansys Employee

      If you want to mimic the free surface with a meshed boundary it's common to use symmetry, or a slip wall. The latter may be better here if you also want to see heat transfer. 

      • kevin.anny
        Subscriber

        Dear Rob, thank you for your reply. I did use a slip wall, as shown in the figure. While we can see the turbulent kinetic energy is zero at free surface. That means zero value boundary conditon is used for k and epsilon transportation equation. The turbulence viscosity is also zero at free surface. But free surface is not wall bounded, turbulence should happen there.

        So I'm thinking could I modify boundary condition for k and epsilon transportation equation, like change zero value to zero flux for k and epsion at free surface?

    • Rob
      Ansys Employee

      I've never felt the need to do that, but I'd also replot with node values off before drawing too many conclusions. 

      • kevin.anny
        Subscriber

        Here you can see the report with node values off. At free surface, the kinetic energy is nearly zero.

    • Rob
      Ansys Employee

      You can rewrite wall functions, but it'll be a lot of work: there should be something in the documentation.  Looking at a physical effect, how much will the turbulence effect the other phase? Eddies will be damped at a free surface. There's a reason we added some damping functions into the multiphase models! 

      • kevin.anny
        Subscriber

        The wall function is not used in specified shear bondary, as shown in the user guide. Also, i want to modify only the free surface rather not crucible wall. If i rewrite wall function, then the crucible wall will change too.

        Do you have any other suggestions?

    • Rob
      Ansys Employee

      You may be able to link the function to certain facets/walls, but you'd need to check the UDF manual. 

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