# multiphaseEulerFoam high Courant number

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 April 19, 2016, 11:41 multiphaseEulerFoam high Courant number #1 New Member   Join Date: Sep 2015 Location: Germany Posts: 11 Rep Power: 3 Hello Everybody, I am using multiphaseEulerFoam (OF 2.2.2) to simulate a gas-liquid flow through a vertical pipe. My simulation works fine but there are very sudden velocity spikes without any obvious cause (and consequently high Courant number and small time step). However the simulation is not diverging afterwards and these spikes happen at different moment for different runs. I am looking at convergence to a quasi steady state: these velocity spikes seems to be just transient events which damp down, but they do slow the process of course... I already tried to reduce maxCo (now I have: maxAlphaCo = 0.5, nAlphaSubCycles = 5, maxCo = 1, maxDeltaT = 0.0005), different numerical schemes and linear solver. The case is running fine with interFoam but gives unsatisfactory results, while multiphaseEulerFoam (with interface compression) predicts good agreement in terms of pressure drop and liquid holdup (if the spikes are not considered in their calculation). Code: ```Courant Number mean: 0.03354866023 max: 0.5000822782 Interface Courant Number mean: 0.03291234457 max: 0.3301922196 deltaT = 3.279629714e-05 Time = 4.057664 smoothSolver: Solving for omega, Initial residual = 0.0004233724736, Final residual = 6.483884521e-09, No Iterations 6 smoothSolver: Solving for k, Initial residual = 0.0008657965129, Final residual = 8.034028879e-09, No Iterations 9 MULES: Solving for alphawater water volume fraction, min, max = 0.1935806296 -1.206282135e-06 1.000053919 MULES: Solving for alphaair air volume fraction, min, max = 0.8064353529 -5.78019834e-12 1.000795038 Phase-sum volume fraction, min, max = 1.000015982 0.9682864229 1.047777234 MULES: Solving for alphawater water volume fraction, min, max = 0.1935810693 -9.551659934e-07 1.000053951 MULES: Solving for alphaair air volume fraction, min, max = 0.8064349129 -5.779948852e-12 1.000795241 Phase-sum volume fraction, min, max = 1.000015982 0.9682452577 1.047778703 MULES: Solving for alphawater water volume fraction, min, max = 0.193581509 -7.133944747e-07 1.000053984 MULES: Solving for alphaair air volume fraction, min, max = 0.8064344729 -5.779697411e-12 1.000795445 Phase-sum volume fraction, min, max = 1.000015982 0.9682044639 1.04778016 MULES: Solving for alphawater water volume fraction, min, max = 0.1935819488 -4.803904539e-07 1.000054016 MULES: Solving for alphaair air volume fraction, min, max = 0.8064340329 -5.779445978e-12 1.00079565 Phase-sum volume fraction, min, max = 1.000015982 0.9681640235 1.047781602 alphawater phase volume in zone AB = 995.5642595 ml alphaair phase volume in zone AB = 4147.374824 ml alphawater phase volume in zone pipe30D = 574.3235992 ml alphaair phase volume in zone pipe30D = 2511.472611 ml GAMG: Solving for p, Initial residual = 0.006124217476, Final residual = 3.692759527e-05, No Iterations 4 GAMG: Solving for p, Initial residual = 0.0002267828647, Final residual = 2.12083789e-06, No Iterations 11 time step continuity errors : sum local = 1.436653067e-08, global = -1.385048028e-11, cumulative = -2.031026982e-05 GAMG: Solving for p, Initial residual = 0.0003193666751, Final residual = 3.041920407e-06, No Iterations 9 GAMG: Solving for p, Initial residual = 4.551235006e-05, Final residual = 4.080258665e-07, No Iterations 11 time step continuity errors : sum local = 2.764300915e-09, global = -1.142671761e-10, cumulative = -2.031038409e-05 GAMG: Solving for p, Initial residual = 0.0004827085861, Final residual = 4.492423075e-06, No Iterations 36 GAMG: Solving for p, Initial residual = 5.059787016e-05, Final residual = 9.289342781e-07, No Iterations 16 time step continuity errors : sum local = 6.295698326e-09, global = 6.69707674e-12, cumulative = -2.031037739e-05 ExecutionTime = 349381.87 s Courant Number mean: 0.07088353367 max: 19054.01219 Interface Courant Number mean: 0.03293067193 max: 13.63270585 deltaT = 8.606139431e-10 --> FOAM Warning : From function Time::operator++() in file db/Time/Time.C at line 1039 Increased the timePrecision from 7 to 10 to distinguish between timeNames at time 4.057663995 Time = 4.057663995 smoothSolver: Solving for omega, Initial residual = 1.190823172e-08, Final residual = 3.336175019e-14, No Iterations 1 smoothSolver: Solving for k, Initial residual = 2.334359121e-08, Final residual = 1.532873766e-13, No Iterations 1 --> FOAM Warning : From function Time::operator++() in file db/Time/Time.C at line 1039 Increased the timePrecision from 10 to 11 to distinguish between timeNames at time 4.057663994 MULES: Solving for alphawater water volume fraction, min, max = 0.1935819488 -3.638118437e-07 1.000054016 MULES: Solving for alphaair air volume fraction, min, max = 0.8064340329 -1.805508425e-12 1.00079565 Phase-sum volume fraction, min, max = 1.000015982 0.9681640234 1.047781603 MULES: Solving for alphawater water volume fraction, min, max = 0.1935819488 -2.473435533e-07 1.000054016 MULES: Solving for alphaair air volume fraction, min, max = 0.8064340328 -1.51373208e-16 1.00079565 Phase-sum volume fraction, min, max = 1.000015982 0.9681640232 1.047781603 MULES: Solving for alphawater water volume fraction, min, max = 0.1935819488 -1.309847101e-07 1.000054016 MULES: Solving for alphaair air volume fraction, min, max = 0.8064340328 -1.510822602e-16 1.00079565 Phase-sum volume fraction, min, max = 1.000015982 0.9681640231 1.047781603 MULES: Solving for alphawater water volume fraction, min, max = 0.1935819488 -1.473444722e-08 1.000054015 MULES: Solving for alphaair air volume fraction, min, max = 0.8064340328 -1.510822596e-16 1.000795649 Phase-sum volume fraction, min, max = 1.000015982 0.9681640229 1.047781603 alphawater phase volume in zone AB = 995.5642597 ml alphaair phase volume in zone AB = 4147.374824 ml alphawater phase volume in zone pipe30D = 574.323599 ml alphaair phase volume in zone pipe30D = 2511.472611 ml GAMG: Solving for p, Initial residual = 0.9990622067, Final residual = 0.009030592757, No Iterations 7 GAMG: Solving for p, Initial residual = 0.01058062846, Final residual = 9.957216274e-05, No Iterations 31 time step continuity errors : sum local = 9.470303504e-13, global = 7.257069631e-17, cumulative = -2.031037739e-05 GAMG: Solving for p, Initial residual = 0.07303887886, Final residual = 0.000665584782, No Iterations 14 GAMG: Solving for p, Initial residual = 0.001270285605, Final residual = 1.229286986e-05, No Iterations 37 time step continuity errors : sum local = 1.023783463e-13, global = -4.891270899e-16, cumulative = -2.031037739e-05 GAMG: Solving for p, Initial residual = 0.0004048321254, Final residual = 3.79214576e-06, No Iterations 14 GAMG: Solving for p, Initial residual = 0.0001298401762, Final residual = 9.118084172e-07, No Iterations 15 time step continuity errors : sum local = 7.590509253e-15, global = -3.318214066e-16, cumulative = -2.031037739e-05 ExecutionTime = 349386.51 s Courant Number mean: 1.057986249e-06 max: 0.08949085779 Interface Courant Number mean: 8.6273426e-07 max: 8.337475345e-05 deltaT = 1.032736731e-09 Time = 4.0576639961``` Attached are an image of the mesh (inlet: annular flow), checkMesh output, initial residual, fvSchemes and fvSolution. Any help is appreciated. Thanks, Francesco linear.png Inlet.jpg fvSolution.txt fvSchemes.txt checkMesh.txt

 November 7, 2016, 12:12 #2 New Member   Mohsen Join Date: Nov 2014 Posts: 7 Rep Power: 4 Hi Francesco, I am using this solver too. I have kind of the same issue. I get those spikes too. After trying many things I ended up increasing the nCorrectors to 5. Also I am using a very small time step, 2.5e-5, and the Co is about 0.1 If I double the time step or for Co number larger than 0.2 the simulation will be unstable. Another problem that I have is that the simulations blow up after some many time steps and if I restart it form the latest time it continues without any problem. Did you find any solution? Thanks, Mohsen

 November 9, 2016, 12:32 #3 New Member   Join Date: Sep 2015 Location: Germany Posts: 11 Rep Power: 3 Hi Mohsen, In my case I reduced the maximum time step to 5e-6 which gives a maximum Co below 0.05 during the run. In this way the simulation was stable, however the increased computational effort did not allow me to study the problem further. Are you sure that if you restart the simulation, it continues till you reach a steady state and it's not blowing up later? If you are looking for a steady state I would give a try also to the PIMPLE algorithm, setting nOuterCorrectors higher than 1. Kind regards, Francesco

 November 9, 2016, 17:30 #4 New Member   Mohsen Join Date: Nov 2014 Posts: 7 Rep Power: 4 Hi Frank, Maybe I need to reword what I typed. If I restart a simulation it continues for a few minutes (let's say 30 min) and then blows up. Then I can start from the time that it blow up and again it continues for a few minutes (another 30 mins) and again blows up! I was not able to find any sources of problem. At the time that simulation blows up, yo can find some cells, nodes, with very high velocity. I tried using PIMPLE and also using very low relaxation factors but it didn't help.

 November 16, 2016, 20:16 #5 New Member   Mohsen Join Date: Nov 2014 Posts: 7 Rep Power: 4 I finally could solve the problem by increasing the number of non orthogonal corrector. Set the nNonOrthogonalCorrectors to a number greater than 1. It should help alot. Cheers Frenk_T likes this.

 November 24, 2016, 04:23 #6 Member   Ping Chang Join Date: Feb 2016 Location: Perth Posts: 88 Rep Power: 3 Hi Francesco, I think you can find the answer from this post. http://www.cfd-online.com/Forums/blo...hm-part-i.html Regards, Ping Frenk_T likes this.

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