[avi@lpsc]

We are trying to model Venting/draining of liquid from a tank using VOF model. Liquid occupies 35% portion of tank and rest of the tank volume is filled with helium gas. VOF model is employed to capture interface tracking without heat and mass transfer(Coupled Level Set formulation). Interface movement has been captured and liquid has been seen to drain/vent out of the tank. An adiabatic expansion process happens for helium gas, as the gas volume is increased on venting of liquid from this tank. In spite of liquid draining out of the tank and increase of gas volume(adiabatic expansion), the pressure of helium gas has not been seen to decrease. Temperature of helium gas is not dropping as such from its initial(starting) value. A simple analysis of adiabatic expansion can show that expansion of gas due increase in volume by 50% can decrease the helium gas pressure by half from its initial value. This decrease in pressure and hence decrease in temperature has not been seen in our VOF simulation. What should be the possible reason for this deviation from thermodynamic understanding. Please assess this problem and suggest what changes should be incorporated in the VOF based CFD model to match results with adiabatic expansion process.