# Pressure correction at pressure outlet (multiphase)

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 July 30, 2013, 03:06 Pressure correction at pressure outlet (multiphase) #1 New Member   Nara Shikamaru Join Date: Apr 2012 Posts: 22 Rep Power: 14 I am stuck with this problem for sometime now. Any help is appreciated. There is two phases in the domain (air and water – modeling with VOF) . I am trying to maintain a constant level of water in the domain. Top surface is open to atm (0 atm pressure boundary with operating pressure set to 1atm). There is a inlet on the right wall (50mm) with 0.006 m/s velocity through which water comes into the domain. The bottom is a pressure outlet where I am doing a pressure correction based on burnolli’s equation (same as FLUENTS built in target mass flow for single phase). Gravity is turned on. However, when I use the udf I wrote the mass flow rate oscillates between ±50% of the target mass flow and decreases very very slowly with time. I thought something was wrong with the udf so I used my udf with 0.05 under relaxation factor (same as fluent) and applied the same problem for single flow. The result was same as what I get when I use targer mass flow in FLUENT. It converges very slowly to the target mass flow. I tried with the constant pressure boundary condition and found that even without any target mass flow option the mass flow rate at the outlet oscillates. I don’t know why this happens. Can anyone explain this behavior? Does anyone know why the target mass flow is not available for Multiphase flows or is it even possible to maintain a constant level of liquid for a transient case? Any suggestions is welcome.

 April 6, 2014, 02:31 suggestion #2 New Member   Bitte56 Join Date: Mar 2013 Location: India Posts: 15 Rep Power: 13 By seeing towards the fig, it looks the whole bootom surface is available for water to go out. is that so.? in that case, since gravity has been applied water will suddenly move out physically. Try doing transient simulation and observe mass flowrate value at outlet bounary. I guess it should decrease very rapidly within few iterations. do reply what you found. thanks and regards Rohit S Adhav There is two phases in the domain (air and water – modeling with VOF) . I am trying to maintain a constant level of water in the domain. Top surface is open to atm (0 atm pressure boundary with operating pressure set to 1atm). There is a inlet on the right wall (50mm) with 0.006 m/s velocity through which water comes into the domain. The bottom is a pressure outlet where I am doing a pressure correction based on burnolli’s equation (same as FLUENTS built in target mass flow for single phase). Gravity is turned on. However, when I use the udf I wrote the mass flow rate oscillates between ±50% of the target mass flow and decreases very very slowly with time. I thought something was wrong with the udf so I used my udf with 0.05 under relaxation factor (same as fluent) and applied the same problem for single flow. The result was same as what I get when I use targer mass flow in FLUENT. It converges very slowly to the target mass flow. I tried with the constant pressure boundary condition and found that even without any target mass flow option the mass flow rate at the outlet oscillates. I don’t know why this happens. Can anyone explain this behavior? Does anyone know why the target mass flow is not available for Multiphase flows or is it even possible to maintain a constant level of liquid for a transient case? Any suggestions is welcome.[/QUOTE]

 Tags multiphase, pressure, vof