Interpretation Problem
 

Hi,
I have the following problem:
In a free surface simulation with water and air, both substances mix to an extent that no exact phase boundary can be drawn. Why?

A plane on which the volume fraction is applied shows percentages from 0% to 100% water. How should I treat e.g. a 10% part of water? Is it a liquid phase or does the water reside dissolved in the air?

Hi,

Assuming you are talking about free surface modelling where there is not meant to be any mixing of gas and liquid phases:

The volume fraction diffuses because of numerical diffusion. To minimise this diffusion use the recommended compressive differencing scheme for the volume fraction, make sure your simulation is adequately converged, your timesteps are small enough and you mesh fine enough. You should also be using the homogenous model. If there are foamy regions then you should consider the inhomogenous model.

The interface is the 0.5 volume fraction surface.

Glenn Horrocks