Fluids

Fluids

Minute Germs of liquid inside vapor

    • zakmt293
      Subscriber

      Greetings


      Respected experts I hope you are fine. I working on simulation of single bubble growth in a super heated domain. The case that I am considering is an axi-symmetric case in which a small germ of vapor is already present. The figure is given as below with all the boundary co


      VOF interface tracking method is used and UDF is utilized for adding mass source terms at interface. Phase change model of lee is implemented for adding mass source terms. While acceleration due to gravity is in positive direction of x-axis. I am using interfacial anti diffusion to keep the interface sharp to avoid interface diffusion. PISO is used for coupling while 2nd order upwind schemes are used for energy and momentum solution.Geo reconstruction is used to solve VOF. 


      As the bubble grow there appear very small germs of liquid near the origin. It seems that from the axis samall germs of liquid move 



      It seems that liquid is coming from the contact between the vapor with both the walls. How can I avoid it and what could be the possible cause for it. Also the interface does not remain sharp some times. Can the experts recommend me any way so that the interface remain sharp.


      Thank You 

    • Rob
      Ansys Employee

      Use an axis condition in place of symmetry (may not make any difference) and if you display contours don't select anything from the surface list (this avoids the mesh imprint you have). 


      Replot with node values off to see if the result is the same.  A blurry interface can mean poor convergence: also how big is the bubble relative to the domain. 

    • zakmt293
      Subscriber

      Greetings


      Thank You for your answer. I will show here every detail related to problem so that we can sort it out. Let first start with residuals. The  convergence criterion to 1e-8 for energy and 1e-4 for all other governing equations. 



       


      Now I will show the the contours of vapor with nodal values off 



       


      These are contours of User defined memory of mass transfer which is implemented through lee model at interface. The interface near the wall on axis is very thick. And from this thick region small particle moves into vapor.Though in other regions the interface remain sharp                 



      Now lets come to the size of bubble. The domain has height along negative of x-axis as 10mm and in horizontal direction it is 6mm. The radius now is round about 1mm 



      I am willing to share any other about my problem. I would be thankful for a repsonese from you side. How can I solve this problem I would be very thankful for your comments.


      Thank You

    • Rob
      Ansys Employee

      Check for convergence on each step. How are you doing the phase change?

    • zakmt293
      Subscriber

      Greetings


      I have been waiting for you. At each step the solution get converged. When the normalized residuals of energy equations are below 1e-8 and and for other equation it is below 1e-4. Now I am having 30 iterations in each time step and the solutions get converged at each time step.


      I am using phase change model of lee. It loops through each cell and finds cells having both liquid and vapor. As soon as the cell is retrieved mass is transferred. Frequency of evaporation is set now on a value of 100. Please let me know if there is anything needed. Please note that a udf is used for phase change implementation.

    • Rob
      Ansys Employee

      OK. That makes sense: I'm sure someone else had a similar case on here recently.   


      Think about the UDF logic, and how the VOF model works.  If you have a cell that's tagged for phase change you will have a cell with some vapour & some liquid.  Depending on the flow you could get a "neck" of liquid, so you have a few nearly adjacent cells with some liquid & gas. If this happens you can very quickly get a smudged region. 

    • Rob
      Ansys Employee

      Have a look in the documentation to see how the VOF model works and how the free surface is tracked. 

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