# High number iteration of pressure equation

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 July 11, 2014, 09:12 High number iteration of pressure equation #1 New Member   Join Date: Nov 2012 Posts: 27 Rep Power: 7 Hallo everyone, I am doing DNS of heated pipe flow (one phase) with OF 2.2.2. The solver is buoyantpimplefoam. The pipe diameter is 2mm and length of 30D. I use Inflow Generator as inlet U BC (zeroGradient for p_rgh), convective BC (fixedValue for p_rgh) as outlet U BC. On the wall is fixedValue for U and fixedFluxPressure for p_rgh. Mesh for pipe is O type. Total mesh number is 16 Mio. (y1 plus 0.15 (y1 under 1e6m)), time step is about 2.5e-5. The simulations is running but I observed that solving implicit pressure equation p_rgh withg PCG solver costs extremly high iterations (about 3000). Even with GAMG, the number of iteration reduce, but the simulation time is not shorter. Can anyone tell me the reason? Thanks. Code: ```Courant Number mean: 0.139046 max: 0.382011 Time = 0.18235 diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0 PIMPLE: iteration 1 DILUPBiCG: Solving for Ux, Initial residual = 0.00172453, Final residual = 1.09164e-05, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 0.0120354, Final residual = 7.28735e-05, No Iterations 1 DILUPBiCG: Solving for Uz, Initial residual = 0.0120533, Final residual = 7.37222e-05, No Iterations 1 DILUPBiCG: Solving for h, Initial residual = 0.002042, Final residual = 1.96612e-05, No Iterations 1 DICPCG: Solving for p_rgh, Initial residual = 0.277668, Final residual = 0.0276332, No Iterations 15 diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0 time step continuity errors : sum local = 1.98488e-06, global = 5.99754e-08, cumulative = 0.000204476 DICPCG: Solving for p_rgh, Initial residual = 0.159447, Final residual = 0.0159405, No Iterations 2306 diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0 time step continuity errors : sum local = 1.15001e-06, global = 3.65048e-08, cumulative = 0.000204512 PIMPLE: iteration 2 DILUPBiCG: Solving for Ux, Initial residual = 0.000840124, Final residual = 5.48588e-07, No Iterations 2 DILUPBiCG: Solving for Uy, Initial residual = 0.00545121, Final residual = 1.47463e-07, No Iterations 3 DILUPBiCG: Solving for Uz, Initial residual = 0.00546338, Final residual = 1.82867e-07, No Iterations 3 DILUPBiCG: Solving for h, Initial residual = 2.04831e-05, Final residual = 6.68786e-07, No Iterations 1 DICPCG: Solving for p_rgh, Initial residual = 0.187843, Final residual = 0.0180666, No Iterations 28 diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0 time step continuity errors : sum local = 1.87581e-06, global = -3.96611e-08, cumulative = 0.000204472 DICPCG: Solving for p_rgh, Initial residual = 0.165657, Final residual = 9.80264e-07, No Iterations 3181 diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0 time step continuity errors : sum local = 3.85402e-10, global = -9.35153e-14, cumulative = 0.000204472 ExecutionTime = 48955.7 s ClockTime = 49428 s```

 July 11, 2014, 16:24 #2 New Member   Join Date: Nov 2012 Posts: 27 Rep Power: 7 I also wanna to mention that, in my DNS pipe flow without heat transfer (Re5400, d=2mm,time step=2.5e-5s,L=5D,cyclic bc, pimpleFoam), the number of pressure iteration is still high,about 300 to 500 iterations.

 October 8, 2014, 03:47 #3 Member   Sebastian W. Join Date: Nov 2012 Location: Saxony, Germany Posts: 43 Rep Power: 7 What does checkMesh say about your mesh? Also, an O type mesh for a pipe is a really bad idea since you get cells with really small angles in the center of the pipe. For any circular geometry it is a standard to use a OH grid or butterfly topology. Hope this helps! Sebastian

 June 8, 2016, 12:12 #4 New Member   Lennart Join Date: Feb 2016 Posts: 22 Rep Power: 3 Although this is an old thread, I'd still be interested in an answer.

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