# Pipe with sudden enlargement

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 January 22, 2008, 07:17 Hi, I'm a new OpenFoam user. #1 New Member   Marco Zecchi Join Date: Mar 2009 Location: Italy Posts: 8 Rep Power: 10 Sponsored Links Hi, I'm a new OpenFoam user. My first openFoam experience is the study of turbolent flow in a circular pipe with a sudden enlargement. I'm interested about steady state, and so I choose simpleFoam with standard k-epsilon mode. The geometry is axi-symmetric, so for the mesh I use the blockMesh and makeAxialMesh utilities. About the boundary conditions I set:  inlet BC for the pipe inlet, so fixed value for U (1m/s), k and epsilon. About k and epsilon I extimated the values following the Programmer's Guide, page 58.  outlet BC for pipe outlet: fixed value (0) for pressure; (but in fvSolution I change the pRefValue to 101.325 = 101325/1000)  wall BC for pipe surface: fixed value (0 0 0) for U (no-slip condition);  wedge BC for the front and back planes. In the transportProperties I set 1e-6 for nu value (the fluid is water). In this conditions the Reynold's number is 10000. I use upwind in the fvScheme to ensure boundedness: divSchemes { default none; div(phi,U) Gauss upwind; div(phi,k) Gauss upwind; div(phi,epsilon) Gauss upwind; div(phi,R) Gauss upwind; div(R) Gauss linear; div(phi,nuTilda) Gauss upwind; div((nuEff*dev(grad(U).T()))) Gauss linear; } Everything seems to work fine, but when I try to compute the head loss coefficient xi I don't get the correct value: the areas ratio is A0/A2 = 0.2; hence, according with theory, I should find 0.64 = (1 - A0/A2)2. In order to calculate xi I use the mean velocity in the smaller pipe (U0) and the pressure difference between section where streamlines return parallels to pipe axis (p2) and section immediately after the enlargement (p1): csi = 2(p2-p1)/U02 my result is 0.28... Have you got an idea about which is my mistake? Thank you! Marco

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