[Ford Prefect]

Originally Posted by MachZero

I was wondering if anyone can shed some light on explicit vs implicit VOF methods in fluent. My understanding is that explicit is simply an interface tracking scheme. It "sees" the interface at time t, obtains the momentum at that point, then extrapolates to get a continuous interface at time t+1. My understanding of the implicit formulation is that it actually calculates the interface location, therefore inverting a matrix with the volume fraction equation. This method is significantly more stable because of it, and much larger timesteps can be used. In theory, since the interface tracking is done by calculation and not tracking, it should be more accurate, no?

Where this stems from:
Whenever you talk to one of fluent's service engineers and ask anything about the methods (whether it be "how do I interpret this huge color band, or what is most appropriate for this particular problem) they always say that the method that gives you the clearest and cleanest interface is the explicit with geo-reconstruct. While this makes perfect sense after reading through the guide... is this actually more accurate or just "prettier". Sure the interface looks better, but is the solution any better? Or is it actually less accurate.

Any insight and experince would be much appreciated!

The original method was explicit. You can find it here.

I don't understand your statement about accuracy. Advecting a volume fraction more than one cell per time-step makes no sense and the solution you obtain will not be time accurate. Most free surface flows are not steady. I also believe that you will suffer from diffusion of the interphase if you use an implicit scheme for the VOF equation.

So, yes the solution is most likely better with the explicit scheme.