Diffusive interface in VOF model
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I am trying to simulate droplet flow in a small 2D slit. For this I have introduced a small inlet into the channel. Flow of primary phase,water(red,see image) is on both sides of the channel; the dispersed phase toluene(blue is coming through the inlet.
I have used these settings: second order schemes for all including time, PISO and implicit scheme with 1e8 cutoff. Mesh is in mm (0.15x0.25 mm inlet) with 10 micron quad mapped cells. I start with first order scheme and then after first iteration revert to second order. My flow rates are of 4mm/s, time step is 1e7 , flow is purely laminar and Courant number is not a problem. The continuity criterion are all 10^8 with only vfphase criterion 10^6. I have worked with finite elements and I have known the overdiffusive effects of second order terms in hyperbolic flows. I am new to finite volume but I doubt such large diffusion can be caused by it laminar flows. I am struggling to understand this diffusive spreading of the interface. From looking at the mass conservation given in FLUENT manual it seems that this spreading can occur only if there is a source term which is set to zero by default. In this case the mass in and mass out should cancel and nothing should move out. I am baffled by this spreading. Any suggestions will be useful. Thanks. 
the diffusive behaviour that you are getting is due to the application of the secondorder upwind (I assume) scheme for discretizing the volume fraction transport equation
due to the discrete nature of the interface, special interpolation schemes are required in order to capture a sharp boundary between the phases when using a implict time formulation, it is recommended that you use HRIC or Compressive schemes in a explicit time calculation, the Georeconstruct method is most commonly applied hope it helps Quote:

Thanks. The simulation is now running well with explicit Geo reconstruct.

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