# On the pimpleFoam solution algorithm

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 February 1, 2011, 06:56 On the pimpleFoam solution algorithm #1 Senior Member   Vesselin Krastev Join Date: Jan 2010 Location: University of Tor Vergata, Rome Posts: 368 Rep Power: 12 Hi all, I'm trying to go deeper into the solution algorithm implemented in the pimpleFoam transient solver, especially for what concerns URANS incompressible applications. This is what I have understood about the solver till now (please, correct me if I'm wrong) 1) At the beginning of the time step, the solver applies the PISO algorithm for pressure-velocity coupling (with a given number of PISO loops, tipically 2) and then solves the transport equations for the turbulent quantities 2) If the outer loops parameter (nOuterCorrectors) is set > 1, the solver turns back and performs an additional integration (formally identical to the procedure described in point (1) ), but this time using as initial values the final values (eventually under-relaxed) obtained after the previous time integration. 3) Point 2 is repeted a number of times depending on the nOuterCorrectors value First question: if all of those statements are correct, this means that with nOuterCorrectors set to 1 the whole procedure reduces to the classical PISO algorithm, is it right? Second question: using the outer loops (and under-relaxation) allows to increase a lot the Courant number without loosing stability of the solution. However, what is the dependency between the number of the outer loops and the max Co allowed? I mean, using only one additional loop (thus performing two complete integrations per time step) is enough for pushing the max Co above 2 (maybe 4-5)? Third question: what about the time accuracy? Of course the time resolution is far less critical in the URANS approach than in the LES one, but how will be affected the solution by a very large time step (assuming that the outer correction still preserves from instability)? Any opinion and/or suggestion will be really appreciated Thank you all V.

 February 1, 2011, 09:25 #2 Senior Member   Vesselin Krastev Join Date: Jan 2010 Location: University of Tor Vergata, Rome Posts: 368 Rep Power: 12 I've shifted the tread here http://www.cfd-online.com/Forums/ope...tml#post293083 V.

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