# Outlet/opening boundary condition

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 November 21, 2012, 06:37 Outlet/opening boundary condition #1 New Member   Ed Musgrove Join Date: Nov 2012 Posts: 2 Rep Power: 0 Sponsored Links Hi I am currently trying to model 2D free surface flow through a sump. I am having issues with my outlet boundary as I want to set it so there is no defined pressure or velocity. If I do set a pressure condition then fluid wants to flow back into the system due to the head loss through the sump. If I resolve this using an open boundary it is not realistic. I have looked at the 2d flow over bump tutorial but that sets a down stream water level which I don't want to do. Is there any outlet boundary condition or expression I can use which does not need to have a pressure or velocity set? Thank you

 November 21, 2012, 10:04 #2 Senior Member   Bruno Join Date: Mar 2009 Location: Brazil Posts: 279 Rep Power: 14 You can set the pressure level as an explicit result of the water level calculated by the solver. For a 2D case with a rectangular outlet boundary condition and gravity in +Y, this might do it: Code: ```LIBRARY: CEL: EXPRESSIONS: DenRef = 1.185 [kg m^-3] DenWater = 997 [kg m^-3] DenH = (DenWater - DenRef) areaWater = areaInt(Water.Volume Fraction)@outflow bottomYposition = 0 [m] domainWidth = 0.01 [m] aveWaterHeight = bottomYposition + areaWater / domainWidth HidPressure = DenH * g * (aveWaterHeight - y) * step( (aveWaterHeight - y)/1[m] ) END END END``` where 'HidPressure' is the value you set at the outflow region. It will probably make convergence harder, since you're no longer tying the outlet condition. Cheers

 November 22, 2012, 10:51 #3 New Member   Ed Musgrove Join Date: Nov 2012 Posts: 2 Rep Power: 0 Thanks that has worked for me. I have also run the test with a long channel downstream of the sump. Both methods give roughly the same results. Thanks again Ed brunoc likes this.

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