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- - **Pipe with sudden enlargement **
(*https://www.cfd-online.com/Forums/openfoam-solving/59207-pipe-sudden-enlargement.html*)

Hi, I'm a new OpenFoam user.
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 A<sub>0</sub>/A<sub>2</sub> = 0.2; hence, according with theory, I should find 0.64 = (1 - A<sub>0</sub>/A<sub>2</sub>)<sup>2</sup>. In order to calculate xi I use the mean velocity in the smaller pipe (U<sub>0</sub>) and the pressure difference between section where streamlines return parallels to pipe axis (p<sub>2</sub>) and section immediately after the enlargement (p<sub>1</sub>): csi = 2(p<sub>2</sub>-p<sub>1</sub>)/U<sub>0</sub><sup>2</sup> my result is 0.28... Have you got an idea about which is my mistake? Thank you! Marco |

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