# how to set two bubbles using VOF method

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 January 7, 2008, 07:41 how to set two bubbles using VOF method #1 xujjun Guest   Posts: n/a I want to simulate the bubble using VOF method in CFX10.0,i have simulated the single bubble,but I want to know how to set two bubbles in initial boundary condition,,any help will be appreciated. thanks in advance xjj

 January 7, 2008, 18:15 Re: how to set two bubbles using VOF method #2 Glenn Horrocks Guest   Posts: n/a Hi, You can use the CEL expressions to define regions of fluid and gas. Either use a step function or a 3D interpolation function (cloud of points). Glenn Horrocks

 January 8, 2008, 08:51 Re: how to set two bubbles using VOF method #3 pankaj Guest   Posts: n/a Hi xujjun! can u pls help me to formulate a steady state problem of multiphase for a single bubble in cfx?

 January 8, 2008, 22:54 Re: how to set two bubbles using VOF method #4 HK Guest   Posts: n/a Hello, 1) Did you check the steady state terminal settling velocity compared to some theoretical calculation? 2) Did you monitor the recirculation of air inside the bubble? HK

 January 9, 2008, 07:08 Re: how to set two bubbles using VOF method #5 xujjun Guest   Posts: n/a Thanks,Glenn Horrocks,how to set the initial condition of the two bubble,i think that the two different regions on the bubbles need to set, i do not know that whether the subdomain region is used or not. Hi,pankaj, I simulated single bubble using transient method, I am sorry i do not know how to simulate single using steady method.

 January 9, 2008, 07:16 Re: how to set two bubbles using VOF method #6 xujjun Guest   Posts: n/a Hi,HK, i will compare the terminal settling velocity with some theoretical calculation, but there are few numerical results because of the too many time of the completing a case.

 January 14, 2008, 06:55 Re: how to set two bubbles using VOF method #7 Andreas Guest   Posts: n/a You do not need subdomains for this. Use e.g. the sum of step functions as initial condition for the volume fraction of one phase: step(((2. [mm])^2-x^2-y^2-z^2)/1. [m^2])+ step(((2. [mm])^2-x^2-(y-6. [mm])^2-z^2)/1. [m^2]) Then, two spherical bubbles with radius 2 mm are at x=y=z=0 and x=z=0, y=6 mm. Perhaps a piecewise linear step funtion (or a tanh function) is better than this discontinuous step...

 January 14, 2008, 07:23 Re: how to set two bubbles using VOF method #8 xujjun Guest   Posts: n/a thanks Andreas, i will try using your method. recently, i simulated a single bubble of quiescent water in a closed tank using vof model, however, i found the volume fraction of bubble is decreasing with increasing time, the interface of bubble is blurry,even to disappear, my mesh is very good and the number of the meshes in a bubble is about 20, i think the mesh is very enough, i do not know how to do ?

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