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- - **Questions about the simple algo**
(*https://www.cfd-online.com/Forums/openfoam-solving/59537-questions-about-simple-algo.html*)

Hi,
As I have to make a litHi,
As I have to make a little report about the way Foam use to solve the Navier-Stokes, I have some questions to make it clear: 1/ In the SIMPLE algo, why phi is calculated before the correction of U? If phi is calulated after the correction of U, isn't better? 2/i/ I understand that the coefficient aP and aN depend on U. BUT how are they guessed a the FIRST iteration in the SIMPLE algo???? I can not imagine the A matrix actually! ii/ And the most important: what " /aP" means??? I mean HOW CAN WE DIVIDE by aP which is not a scalar (if I'm not wrong). What does it mean? 3/ To confirm: the previous U is just needed in the relaxation of UEqn, isn't it? Any clues or advice, or attempts of answer will be welcome. (and Hrv's reply will be more than appreciated) Lam |

OK, OK.
- At the stage wherOK, OK.
- At the stage where you're looking phi contains only a part of the interpolated velocity dotted with the face area vector. Have a look at phi -= pEqn.flux(); - this is where the rest of the flux comes from and it's after the pressure solution. All is well - aP is the central coefficient of the momentum matrix, packed up as a field: one value for each cell. That would be a scalar if your discretisation is easy; once you get more clever (than the current implementation), aP will be a 3x3 tensor and division will be a multiplication with the matrix inverse. Advanced, though - for easy stuff have a look at my Thesis - FIRST SIMPLE iteration: well, you guess U as well, right? That is called the initial guess. - to confirm: no. You also have it in UEqn.H() - read the forum again, I've explained this. Hrv - |

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