about VOF model

rey

Hi,

I am modeling two immiscible liquids (oil-water) which flow in different velocities in slightly upward inclined pipe. The pipe is 1 meter in length and 14mm in diameter.

My interest is following: 1) Location of interface. 2) Velocity profile.

I did following for a startup.

Code: FLUENT 6 Problem: 3-D Model: VOF Laminar flow. Steady-state with implicit scheme.

Discretization

Pressure: body force weighted

Pressure velocity coupling: SIMPLE

Momentum: 2nd order upwind

Volume fraction: 2nd order upwind

Under-relaxation factor: 0.2

Operation condition: gravity added.

Boundary conditions: Two velocity inlet boundary conditions at one end of the pipe, one outflow boundary condition at the other end of the pipe. No slip condition on the pipe wall.

Grid: some 140,000 cells

The size of the cell is 0.3x0.3x5 mm.

After running the FLUENT, the solution was converged but there is a numerical dispersion in the interface and there is a big discrepancy between FLUENT result and the result in literature (velocity profile and holdup).

Following is my questions:

1) Is my VOF setting OK?

2) How can I get less dispersed interface between the phases (I increased the mesh number, but again I got dispersed interface)?

3) In order to read the location of the interface, I set the level of the contour to 2, and then read the interface location with the mouse or I write the volume fraction of both phase with their special coordinates to the file then I draw the curves and then take the number where both of the volume fraction curves intersect. Is there any good way to read the interface location correctly?

4) In the FLUENT manual (VOF), it says that in cases where large velocity differences exit between the phases, the accuracy of the velocities computed near the interface can be adversely affected. My other question is that what is the upper limit of these differences which course the VOF model works not well?

Thanks for your attention.

Be well,

Rey