About selecting Turbulence model in VOF model
Hi I am confused about selecting the type of flow as laminar or turbulence when I want solve two phase flow problem within VOF model should I estimate Reynolds number based on the mixture velocity (the sum of superficial velocities of phases) or estimate Reynolds for each phase separately based on the physical velocity of each phase? So which one is the correct one in determining the type of flow? knowing that my case is stratified flow?

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I do not foresee any issuse as to why you cannot calculate Reynolds number separately for each phase. But I definitely would not use the mixture velocity (that leads to a completely wrong interpretation of Reynolds number) if the flow is stratified. Hopefully you end up with a case where both phases are both clearly laminar or both turbulent, since you can't choose separate models for each reason. 
Hi Lucky thanks for the answer. In my case the oil volume fraction is 0.39 , oil superficial velocity is 0.03 m/s and its density is 828 kg/m^3 , viscosity is 0.3 N.m/s and water superficial velocity is 0.15 m/s , density 1000 kg/m^3 and viscosity 0.001 N.m/s the pipe diameter is 0.026 m and length 12 m. When I estimate Reynolds for oil phase it be just 4.3 while Reynolds for water phase is 4756 so is my case should be simulated as laminar or turbulent?
Mariam 
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Your units on viscosity are wrong. If it is a wellmixed flow you use the proper weighting scheme to calculate a single Reynolds number. If it is straified flow you actually have 3 Reynolds numbers, 1 for the oil in a pipe, 1 for the water in a pipe, and 1 for the interface between the oil and water. If you still have 1 laminar and 1 turbulent like you mentioned then there is difficulty now, as there is both a laminar and turbulent region. Unfortunately there are no models that can handle this well (and selecting both laminar and turbulent is not an option obviously). 
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Your units on viscosity are wrong. If it is a wellmixed flow you use the proper weighting scheme to calculate a single Reynolds number. If it is straified flow you actually have 3 Reynolds numbers, 1 for the oil in a pipe, 1 for the water in a pipe, and 1 for the interface between the oil and water. If you still have 1 laminar and 1 turbulent like you mentioned then there is difficulty now, as there is both a laminar and turbulent region. Unfortunately there are no models that can handle this well (and selecting both laminar and turbulent is not an option obviously). 
Yes its a stratified flow case. The 0.39 is the oilholdup at inlet and the oil level is 10.8 mm. The oil at the lower part of the pipe and water above it. I am trying to get wavy interface depending at the inlet velocities which i took it from a paper (I failed to attached it here coz its 1MB type me your email if you want see it) . The VOF model equations mentioned at the manual reference to the average velocities and properties for mixture hence I think selecting the type of flow is depend on the Reynolds number which it defined as :
Rem=(U(mix)*D*density(mix))/viscosity (mix) where density(mix)=density(oil)*vof(oil)+density(water)* vof(water) viscosity(mix)=viscosity(oil)*vof(oil)+viscosity(w ater)*vof(water) U(mix) = U(oil)+U(water) I am right? Yes i type the viscosity units wrong its in Pa.s 
I should've asked this from the start but: What are your inlet conditions? Is it turbulent inflow? If so then obviously it will not suddenly become laminar again. Your fluid at the inlet must come from some source, and is that source laminar or turbulent? Is this developing or fullydeveloped? If it is developing, where did the flow come from and was that flow already turbulent? These are important questions that will greatly assist deciding if the nature of the flow.
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Unfortunately, since a single set of equations is used for both zones, Fluent cannot solve a case where one fluid is laminar and the other is turbulent. You can try your luck with a transitional model. But to choose either laminar or turbulent is wrong, at least in my opinion. 
Dear Lucky many thanks to the information you post. What is the transitional model ? is it another type of VOF model and can it solve stratified flow case? what it's capabilities?

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