Fluids

Fluids

Multiphase VOF – Flow in a channel

    • harshabharadwaj
      Subscriber

      Hello,


      I am currently working on simulating flow through a channel. I have been facing some issues with the simulation. With transient, I have played around with different values of Max number of iterations, time steps and time step size and the solution does not converge.


      When it converges in some time steps, it does not stop and goes ahead to complete the maximum number of iterations. What does this mean? Shouldn't the computation stop once it reaches convergence?


      And with steady, the solution does not converge at all.
      Not only does the nature of the flow make any sense there is also a small gap between the moving substrate (bottom) and the flow channel that I can’t seem to eliminate. I can give more details if necessary. Someone please help me out with this.



      The axes in the final image are changed. But it still reflects what I get with the original setup itself.

    • DrAmine
      Ansys Employee

      Include your settings and models used in the screenshots or add a better but concise description.

    • harshabharadwaj
      Subscriber

      Thank you for the reply sir. For some reason it did not let me attach all the information that I needed to initially. So I has to shorten it. Below is the detailed description as what I am doing. 


      I have modeled this as a multiphase flow with air and liquid. I am trying to simulate the flow from a 3d printer nozzle. I am doing a pressure based steady simulation ( I have tried transient as well). I use Implicit VOF with the default Volume Fraction Cutoff. I have not considered the surface tension. 


      I am using a UDF to set the BC and to initialize the volume fraction. I have edited one of the pictures above to reflect the correct BC. The liquid comes in from the top (-ve z axis). Once it leaves the cylindrical space, it moves onto the moving plate at the bottom, where air is present.


      I am currently using a green-gauss node based gradient, PRESTO! pressure, QUICK momentum and Modified HRIC Volume Fraction. 


      I am having trouble getting convergence. And since it is Implicit I dont think I can set the Courant Number. In addition as seen in the last image attached above, there is a gap between the moving substrate and the flow channel which I cannot seem to account for.


      Different numbers for iterations, time step size give me different kinds of flows. I do not know which is correct. Please guide me with this. I really appreciate the help. Let me know if anymore information or screenshots are needed.

    • DrAmine
      Ansys Employee

      You can use adaptive time stepping and set CFL number to something like 5.

    • harshabharadwaj
      Subscriber

      Hello,


      I try doing the same. But it still does not converge. However, I noticed one thing. Decreasing the viscosity significantly helps me reach convergence. Since I am using a molten plastic as the liquid ( viscosity = 1000 Pa-s), it does not seem to reach at all. 


       


      Do you have any suggestions for this?. Thanks for the help. And also, I should be doing Steady low right instead of Transient?. When do I use steady and when do I use transient?

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