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Nina February 27, 2008 13:46

problem with static pressure
Hi, I've a problem with the static pressure.My geometry consist in a pipe, with a diameter of 2,5mm and a lenght of 29mm, inside to another pipe, with a diameter of 4mm, that after 20mm to the end of the first pipe presents a bifurcation.I have imposed as a BC, at the input of the pipe, a velocity equal to 0.8m/s and at the end of the two branches of bifurcation a gauge pressure of 0 Pa.I have used k-epsilo model. I have obtained that the static pressure at the end of first pipe is negative. So I wonder: -How can be the static pressure negative and then become zero? -Why the pressure drop is different from what I expect from theoretical calculations? Where I have wrong? Please help me!!! Thanks a lot


jesse February 28, 2008 06:22

Re: problem with static pressure it really a turbulence flow here? 2.didnt quite understand your problem specification,air/water flows between the pipes, or individually in the pipes? pressure outlet is qualified,but what happens in the end? flow gemixed or just jet separately into air?actually, no matter what, these two pipe can share a commmen Boundary condition,that is,pressure outlet. 3.pressure drop is of course reasonable,the flow experinces accerelation.

Nina February 28, 2008 08:28

Re: problem with static pressure
I have only PFC (density 1780 kg/m^3) in my pipes. I have a Reynold number of 2966 so, in the reality I'm in a transitional flow. PFC flows only in the smaller pipe until the latter does not end and then results in the biggest. From here part the bifurcation in two tubes at the end of which were set two boundary conditions of pressure outlet (gauge pressure 0 Pa for both). I know that pressure drop exist but in my simulation is different...ten times bigger than the theoretical calculations! Do not understand why. May be wrong the choice of model K-epsilon? Thanks a lot


jesse February 28, 2008 14:08

Re: problem with static pressure
it certainly has nothing to do with the model, Re about 3000 is actually very critical,flow transfer from laminar to turbulent,so turb Model should be qualified.

large pressure drop has its physical reasoning,you gotta find out where is it from, most likely from the aspect of fluid mechanics.

PS.since the flow is pretty slow(incompressible),you wont have to set the gauge pressure at outlet,take it as default,that is quantitively one atmosphere pressure,althought in the interface of BC it appears as 0,which is in respect to one atmosphere.

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