# How to specify BCs to solve a Poiseuille flow

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 February 11, 2008, 15:01 Hello, I was trying to solv #1 Senior Member   Kārlis Repsons Join Date: Mar 2009 Location: Latvia Posts: 111 Rep Power: 9 Hello, I was trying to solve a parabolic flow in 2D square-like domain. * In blockMeshDict and boundary file I specified just "patch" for inlet and outlet, * In /0/U I set zeroGradient on in/out, (0 0 0) on walls + empty * In /0/p I set fixedValue on in/out, zeroGradient on walls + empty. But.... solution obviously diverge. Aren't those BCs sufficent to describe a problem? I think, I don't understand how to set BCs in OpenFOAM, for example - why should I use physicalType? Maybe you can advice something useful at least for that poiseuille flow?

 February 11, 2008, 15:55 Oh, I see a parabolic velocity #2 Senior Member   Kārlis Repsons Join Date: Mar 2009 Location: Latvia Posts: 111 Rep Power: 9 Oh, I see a parabolic velocity distribution now and it feels so good! The problem was - I forgot to take into account a fact, that for icoFoam pressure must be divided by density. So I tried to solve for 1000 times greater pressure! Maybe I'll see somewhere else why physicalType should be specified...

 February 11, 2008, 17:14 I checked solution more carefu #3 Senior Member   Kārlis Repsons Join Date: Mar 2009 Location: Latvia Posts: 111 Rep Power: 9 I checked solution more carefully and .. trouble doesn't stop - I have almost exactly 2x greater U values comparing with analytical! So in SI: nu = 1e-4, geometry: dx=0.01, dy=0.1, dz=0.001, flow is in y direction, delta p = 0.016 (divided by ro) According to wikipedia, Umax = 0.01. But solution gives 0.02!! How come?

 February 11, 2008, 18:12 Hi Karlis The Poiseuille-fl #4 Senior Member   Niels Gjoel Jacobsen Join Date: Mar 2009 Location: Deltares, Delft, The Netherlands Posts: 1,702 Rep Power: 27 Hi Karlis The Poiseuille-flow I could find on wiki (please add link in the future), was for a circular pipe flow. Your setup is for a 2D case, i.e. Poiseuille flow between two flat plates. The ratio between u_max in these two cases is exactly a factor of 2. / Niels __________________ Please note that I do not use the Friend-feature, so do not be offended, if I do not accept a request.

 February 12, 2008, 04:07 Thank you, Niels! I was think #5 Senior Member   Kārlis Repsons Join Date: Mar 2009 Location: Latvia Posts: 111 Rep Power: 9 Thank you, Niels! I was thinking in a quick way - take a plane of cylinder and calculate for it.. Actually different problem is solved in such way.

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