Sir/Madam,

I am using Fluent for my proj and i have a problem regarding boundary conditions which i am unable to sort it out.The physical situation of the problem is as follows.

I am trying to model 2 phase laminar incompressible film flow over an inclined flat plate using the VOF model.The 2 fluids are water and air.Water is flowing in the form of a film over which air is flowing .The relative velocity between the fluids are small so that laminar regime is ensured.At the inlet ,there is discrete regions for air and water flow since the initial film thickness of water film is known.My problem is about specifying the bcs for this situation.At the inlet ,i have given VELOCITY INLET bc.At the top of the computational domain,i have given PRESSURE INLET bc.I dont know what bc should i give at the outlet as Fluent is asking for volume fraction for the secondary phase(water) as the input which is the unknown if i give PRESSURE OUTLET bc.So,pls send me the bcs which must be used for this situation.

With regards, Praveen.

Dear Praveen, I did not understand completely how you set your inlet BC. Do you have one inlet (velocity inlet) for both air and water, or is your water inlet the velocity inlet and the air inlet a pressure inlet? What do you mean by "at the top of the computational domain"? Is there a wall? I am also working with the VOF-Model and a pressure-outlet. I calculated the volume fraction of air at the pressure outlet with: massflowrate(inflow)=massflowrate(outflow) and the ideal gas law.

With regards, Nicola

You should split the single velocity inlet into two inlets on which you specify the voulme fractions for the liquid and air.

On the pressure outlet, FLUENT asks for the volume fractions. But the volume fractions get used on when there's a backflow. It sounds like your situation is likely not to involve any backflow through the pressure outlet. So, you leave it to the default value.