help for VOF
 

hi everyone:

I am dealing with free surface flow now. And I use VOF method. When I read the paper, named 'volume of fluid (VOF) method for the dynamics of free boundaried' by C.W. Hirt and B.D. Nichols, I have a question about the determination of acceptor and donor cell. It said that the acceptor cell is used when the surface is convected mostly normal to itself; otherwise, the donor cell value is used, page 213. Did it mean if surface is vertical and flow is horizontal we use acceptor cell? And for the Fig.5 (d), left cell surface is vertical and right one is horizontal. Why there is another part between those two? I assumed that the surface is not connected to each other for piecewise constant surface scheme. Your help will be greatly appreciated!!

Kai