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phsieh2005 August 29, 2005 21:31

VOF and strong surface tension effect
 
Hi, CFD experts,

When applying VOF, what are the differences between strong surface tension effect and weak surface tension effect? That is:

is cell quality more critical for strong surface tension effect? is there any special treatment needed for strong surface tension effect flow? .... etc.

This is for micro-fluidics flows.

Thanks!

phsieh2005

Hrvoje Jasak August 30, 2005 06:56

Re: VOF and strong surface tension effect
 
When surface tension is weak (or geometry is large), you more or less have a single fluid -like problem with a jump in density and viscosity + some additional complications regarding the indicator (VOF) variable (I call it gamma), where you have to preserve the sharpness of the interface.

For cases where surface tension dominates, you get a different mode of coupling: the curvature of the interface (so, second derivative of the gamma equation) produces massive sources in the momentum equation - surface tension + the momentum solution prescribes the motion of the interface which changes the curvature. For small droplets, say 0.1-0.5 mm, this term may totally dominate the momentum equation, being several orders of magnitude larger.

So, in the first case, you are dominated by the pressure-velocity coupling problem, while in the second you have massive velocity-gamma-pressure (you need pressure for continuity) coupling which is much nastier because of the curvature. If the curvature or the coupling is messed up, you will see "parasitic velocities" around the interface which either mess up the solution pretty badly or make it blow up.

If you want to see an example of what I am talking about, try setting up a case with an air bubble in water with full density and viscosity ratio and a small diameter - this should tell you a lot about the VOF code you're using.

Enjoy,

Hrv


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