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June 21, 2022, 03:33 |
mass imbalance / div(U) vs div(phi)
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#1 |
Senior Member
Join Date: Dec 2019
Location: Cologne, Germany
Posts: 355
Rep Power: 8 |
hi all,
i searched up the forum about this issue, but could not find a satisfactory explanation. i would like to know what the mass imbalance is in every iteration step for a steady state calculation with simpleFoam. 1.normally, for an incompressible, steady state case the continuity equation would be: div(U)=0. from the computational side this equation should be div(U)=r, meaning there should be a residual r in every iteration. 2. Integral of div(U) over a control volume is equal to integral over the bounding surface of this control volume when Gauss theorem is applied: Integral(div(U))dV=Integral(U_f*vector_n)dS, further this Integral can be discretized for a polyhedra -> SumOverAllFaces(U_f*vector_n*faceArea). This sum should be 0 ideally, but will give a residual r for every iteration step, which could be viewed as the mass imbalance for each cell. i want to compute and output exactly this value. so the question is, what do i need to do to compute this mass imbalance. i know that there is the postprocessing utility "div". i did execute div(U) and div(phi) for icofoam. both values differ from each other (in this thread Meaning of "fvc::div(phi)" , post #20, it is claimed that both should give the same value). the dimension for both is 1/second. i did calculate div(U) values with excel for this case manually, and got the same values which are shown in paraview. i hope anybody can tell me how to compute the mass imbalance correctly. |
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