pressure in incompressible solvers e.g. simpleFoam
Dear All,
I have a general question about the setup of the pressure in an incompressible solver like the simpleFoamsolver. As we all know in simpleFoam the "strange" pressure units are a result of the partition by the density. This is reasonable, because the density will not change in an incompressible flow. Hence the only property of the fluid which can be changed by the user is the kinematic viscosity. In my opinion in a pressure driven flow (pressure difference between inlet and outlet are known from an experiment) you have to normalise the fixed pressure values at inlet and outlet by the density. In common commercial flow solvers you have to define a reference density. That is not possible in simpleFoam and why I wrote down my thoughts in this forum. It would be gald if someone could help me in this open discussion. Regards, Christian 
Another reason for posting this massage is, that I have seen many users who don't care about the normalization of the pressure. I just want to make it clear (for me) in this forum.
There are experimental data like: Inlet: 1,013,250 Pa Outlet: 101,325 Pa > Delta p: 911,925 Pa With a density of air round about 1,2 kg/m³ you've got: Inlet: 844,375 m²/s² Outlet: 84,437.5 m²/s² > Delta p: 759,937.5 m²/s² Which setup is the correct one? In my opinion it's the second one;) Thanks, Christian 
Changing the scale and/or the unit of pressure should not change the physics of your problem.

Well, you could just have a look at the units atop in the pfile. From there it should be very clear if your normalization is correct.

Thank you for your reply.
I know, that the physics are the same, but the first setup only make sense in a compressible simulation, the second one in an incompressible simulation. If you set your pressure difference, which is known from an experiment without densitynormalization to your simpleFoamsetup you will get a setup, which is different to the experiment (factor in this case 1.2). > The setup does not mirror your experiment. Is this correct? From your reply I guess, that my thoughts are correct. Thanks a lot. 
Hi everyone,
I'm with Cristian, was your experimental data's initialization with a pressure difference, you must dived the pressure through the density ("for a normalization"). After this the results should look like the experimental data's. A other opportunity is the work with a post processor. best regards Sven ;) 
Christian, sorry that I misunderstood your Question.
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Hi, I was reading this thread these days. And I found this will help to future readers. Like to add a comment. In simplefoam continuity equation doesn't include the density because incompressible fluid. But in momentum equation density is a parameter(constant). Therefore pressure has introduced as pressure/density. units [pressure/density] = m2/s3 cheers. 
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Thanks for wanting to clarify this but the unit should be m2/s2, right? ;) All the best, Jan 
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Yes it should be m2/s2. Regards, Prasa. 
For Hydrogen ??
Sorry for reopening this thread.
I would like to know if the fluid is hydrogen. Density of Hydrogen is very less compared to Air. its around 0.0899 kg/m3 and Inlet pressure shoots up and I am getting divergence in simpleFoam. Can anyone clarify this ?? Regards, Manoj 
Dear Manoj,
you get stabilisation problems not for the density but for the much larger Renolds number which results from it. It is well known that larger Re numbers may arise destabilisation. But can be solved, of course. It depends on the solver you use, which arrangements are possible. in simpleFoam you may chose a lesser value for relaxationFactors in fvSolution. 
Hello Uwe Pilz,
Thank you for your response. I tried reducing the relaxationFactors as low as 0.1 for U but did not work :(. My simulation residuals are as below : Create mesh for time = 0 SIMPLE: convergence criteria field p tolerance 1e06 field U tolerance 1e06 field "(kepsilonomegafv2)" tolerance 1e06 Reading field p Reading field U Reading/calculating face flux field phi Selecting incompressible transport model Newtonian Selecting turbulence model type laminar No MRF models present No finite volume options present Starting time loop Time = 1 smoothSolver: Solving for Ux, Initial residual = 1, Final residual = 0.00189998, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1, Final residual = 0.00172237, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 1, Final residual = 0.0025047, No Iterations 1 GAMG: Solving for p, Initial residual = 1, Final residual = 0.0285601, No Iterations 1 time step continuity errors : sum local = 2736.75, global = 639.791, cumulative = 639.791 ExecutionTime = 4.78 s ClockTime = 5 s Time = 2 smoothSolver: Solving for Ux, Initial residual = 0.0690268, Final residual = 0.000134747, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.0667625, Final residual = 0.000110036, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.100766, Final residual = 0.000232851, No Iterations 1 GAMG: Solving for p, Initial residual = 0.893178, Final residual = 0.0296773, No Iterations 2 time step continuity errors : sum local = 664.798, global = 147.554, cumulative = 492.237 ExecutionTime = 7.27 s ClockTime = 8 s Time = 3 smoothSolver: Solving for Ux, Initial residual = 0.539464, Final residual = 0.000930136, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.559662, Final residual = 0.00101096, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.460855, Final residual = 0.000848781, No Iterations 1 GAMG: Solving for p, Initial residual = 0.562958, Final residual = 0.0145996, No Iterations 2 time step continuity errors : sum local = 816.835, global = 396.305, cumulative = 888.542 ExecutionTime = 9.68 s ClockTime = 10 s Time = 4 smoothSolver: Solving for Ux, Initial residual = 0.181828, Final residual = 0.000369129, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.191699, Final residual = 0.000363309, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.163456, Final residual = 0.000388016, No Iterations 1 GAMG: Solving for p, Initial residual = 0.445896, Final residual = 0.0413308, No Iterations 1 time step continuity errors : sum local = 1300.99, global = 453.554, cumulative = 1342.1 ExecutionTime = 11.96 s ClockTime = 12 s Time = 5 smoothSolver: Solving for Ux, Initial residual = 0.157346, Final residual = 0.000294152, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.178279, Final residual = 0.000372299, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.12979, Final residual = 0.000339263, No Iterations 1 GAMG: Solving for p, Initial residual = 0.333383, Final residual = 0.0327735, No Iterations 2 time step continuity errors : sum local = 865.563, global = 95.1892, cumulative = 1437.29 ExecutionTime = 14.3 s ClockTime = 15 s Time = 6 smoothSolver: Solving for Ux, Initial residual = 0.118667, Final residual = 0.000266071, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.14578, Final residual = 0.000287402, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.164746, Final residual = 0.000527067, No Iterations 1 GAMG: Solving for p, Initial residual = 0.340376, Final residual = 0.0297977, No Iterations 1 time step continuity errors : sum local = 933.368, global = 248.937, cumulative = 1188.35 ExecutionTime = 16.52 s ClockTime = 17 s Time = 7 smoothSolver: Solving for Ux, Initial residual = 0.0951829, Final residual = 0.000200016, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.138162, Final residual = 0.000303547, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.138874, Final residual = 0.00046242, No Iterations 1 GAMG: Solving for p, Initial residual = 0.194145, Final residual = 0.014261, No Iterations 2 time step continuity errors : sum local = 493.546, global = 191.549, cumulative = 996.799 ExecutionTime = 18.97 s ClockTime = 19 s Time = 8 smoothSolver: Solving for Ux, Initial residual = 0.108677, Final residual = 0.000179489, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.225636, Final residual = 0.000503802, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.11593, Final residual = 0.000352237, No Iterations 1 GAMG: Solving for p, Initial residual = 0.256212, Final residual = 0.0223614, No Iterations 1 time step continuity errors : sum local = 794.502, global = 202.588, cumulative = 794.211 ExecutionTime = 21.27 s ClockTime = 22 s Time = 9 smoothSolver: Solving for Ux, Initial residual = 0.0982557, Final residual = 0.000304908, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.185531, Final residual = 0.000533666, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.0994782, Final residual = 0.00034999, No Iterations 1 GAMG: Solving for p, Initial residual = 0.293328, Final residual = 0.0275425, No Iterations 1 time step continuity errors : sum local = 1043.34, global = 151.985, cumulative = 642.226 ExecutionTime = 23.59 s ClockTime = 24 s Time = 10 smoothSolver: Solving for Ux, Initial residual = 0.0937519, Final residual = 0.000187036, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.0959, Final residual = 0.000166799, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.0952078, Final residual = 0.000233081, No Iterations 1 GAMG: Solving for p, Initial residual = 0.368527, Final residual = 0.0356445, No Iterations 1 time step continuity errors : sum local = 1467.9, global = 92.6168, cumulative = 549.61 ExecutionTime = 27.46 s ClockTime = 28 s Time = 11 smoothSolver: Solving for Ux, Initial residual = 0.164868, Final residual = 0.000163476, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.222946, Final residual = 0.000334008, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.12722, Final residual = 0.000234201, No Iterations 1 GAMG: Solving for p, Initial residual = 0.477769, Final residual = 0.0298537, No Iterations 1 time step continuity errors : sum local = 1505.12, global = 19.7979, cumulative = 529.812 ExecutionTime = 29.72 s ClockTime = 30 s Time = 12 smoothSolver: Solving for Ux, Initial residual = 0.130392, Final residual = 0.000355599, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.321622, Final residual = 0.000903749, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.12949, Final residual = 0.00030575, No Iterations 1 GAMG: Solving for p, Initial residual = 0.703616, Final residual = 0.0332269, No Iterations 1 time step continuity errors : sum local = 2847.31, global = 90.554, cumulative = 439.258 ExecutionTime = 31.96 s ClockTime = 32 s Time = 13 smoothSolver: Solving for Ux, Initial residual = 0.0826217, Final residual = 0.000238347, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.130508, Final residual = 0.000542229, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.18985, Final residual = 0.000381059, No Iterations 1 GAMG: Solving for p, Initial residual = 0.833578, Final residual = 0.0424293, No Iterations 1 time step continuity errors : sum local = 6137.85, global = 204.729, cumulative = 234.529 ExecutionTime = 34.2 s ClockTime = 35 s Time = 14 smoothSolver: Solving for Ux, Initial residual = 0.130854, Final residual = 0.000467332, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.104228, Final residual = 0.000302874, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.119158, Final residual = 0.000711339, No Iterations 1 GAMG: Solving for p, Initial residual = 0.883089, Final residual = 0.0555208, No Iterations 1 time step continuity errors : sum local = 14200.1, global = 11.3717, cumulative = 223.157 ExecutionTime = 36.42 s ClockTime = 37 s Time = 15 smoothSolver: Solving for Ux, Initial residual = 0.103926, Final residual = 0.000248405, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.233039, Final residual = 0.000832855, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.115342, Final residual = 0.000299915, No Iterations 1 GAMG: Solving for p, Initial residual = 0.893818, Final residual = 0.0572619, No Iterations 1 time step continuity errors : sum local = 18571, global = 876.462, cumulative = 653.305 ExecutionTime = 38.64 s ClockTime = 39 s Time = 16 smoothSolver: Solving for Ux, Initial residual = 0.186822, Final residual = 0.0013917, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.206877, Final residual = 0.000716568, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.129085, Final residual = 2.18237e05, No Iterations 2 GAMG: Solving for p, Initial residual = 0.929582, Final residual = 0.0374351, No Iterations 2 time step continuity errors : sum local = 37929.5, global = 19801.9, cumulative = 19148.6 ExecutionTime = 41.11 s ClockTime = 42 s Time = 17 smoothSolver: Solving for Ux, Initial residual = 0.0989814, Final residual = 0.000175765, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.136444, Final residual = 0.00032175, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.0903033, Final residual = 0.000223593, No Iterations 1 GAMG: Solving for p, Initial residual = 0.893951, Final residual = 0.0713946, No Iterations 1 time step continuity errors : sum local = 89091.8, global = 5091.64, cumulative = 24240.2 ExecutionTime = 43.34 s ClockTime = 44 s Time = 18 smoothSolver: Solving for Ux, Initial residual = 0.16088, Final residual = 0.000557163, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.232857, Final residual = 0.000855333, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.0866188, Final residual = 0.000243466, No Iterations 1 GAMG: Solving for p, Initial residual = 0.802139, Final residual = 0.0397683, No Iterations 1 time step continuity errors : sum local = 150507, global = 9986.03, cumulative = 34226.2 ExecutionTime = 45.56 s ClockTime = 46 s Time = 19 smoothSolver: Solving for Ux, Initial residual = 0.0714521, Final residual = 0.000127453, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.0880485, Final residual = 0.000378587, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.0896318, Final residual = 0.00025341, No Iterations 1 GAMG: Solving for p, Initial residual = 0.936714, Final residual = 0.069373, No Iterations 1 time step continuity errors : sum local = 580022, global = 21106.1, cumulative = 55332.3 ExecutionTime = 47.79 s ClockTime = 48 s Time = 20 smoothSolver: Solving for Ux, Initial residual = 0.104275, Final residual = 5.37614e06, No Iterations 2 smoothSolver: Solving for Uy, Initial residual = 0.108497, Final residual = 8.93021e05, No Iterations 2 smoothSolver: Solving for Uz, Initial residual = 0.0775309, Final residual = 0.000112957, No Iterations 2 GAMG: Solving for p, Initial residual = 0.991097, Final residual = 0.0726207, No Iterations 1 time step continuity errors : sum local = 1.70661e+08, global = 369551, cumulative = 314219 ExecutionTime = 51.79 s ClockTime = 52 s Time = 21 smoothSolver: Solving for Ux, Initial residual = 0.130851, Final residual = 1.23498e05, No Iterations 2 smoothSolver: Solving for Uy, Initial residual = 0.133453, Final residual = 1.14443e05, No Iterations 2 smoothSolver: Solving for Uz, Initial residual = 0.123063, Final residual = 8.81908e06, No Iterations 2 GAMG: Solving for p, Initial residual = 0.980152, Final residual = 0.0740928, No Iterations 5 time step continuity errors : sum local = 5.06379e+09, global = 1.63071e+08, cumulative = 1.62757e+08 ExecutionTime = 54.92 s ClockTime = 55 s Time = 22 smoothSolver: Solving for Ux, Initial residual = 0.304109, Final residual = 0.000529039, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 0.0874946, Final residual = 0.000185472, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 0.114907, Final residual = 6.19314e05, No Iterations 1 GAMG: Solving for p, Initial residual = 0.948518, Final residual = 0.0931426, No Iterations 1 time step continuity errors : sum local = 1.11399e+10, global = 1.46106e+08, cumulative = 3.08863e+08 ExecutionTime = 57.19 s ClockTime = 58 s Time = 23 smoothSolver: Solving for Ux, Initial residual = 0.118614, Final residual = 0.0010459, No Iterations 2 smoothSolver: Solving for Uy, Initial residual = 0.129134, Final residual = 6.72522e05, No Iterations 3 smoothSolver: Solving for Uz, Initial residual = 0.101372, Final residual = 2.39837e05, No Iterations 3 GAMG: Solving for p, Initial residual = 0.423058, Final residual = 0.0259104, No Iterations 1 time step continuity errors : sum local = 2.47431e+14, global = 5.41145e+12, cumulative = 5.41176e+12 ExecutionTime = 59.73 s ClockTime = 60 s Time = 24 smoothSolver: Solving for Ux, Initial residual = 0.120392, Final residual = 0.000117413, No Iterations 3 smoothSolver: Solving for Uy, Initial residual = 0.131364, Final residual = 1.21663e05, No Iterations 3 smoothSolver: Solving for Uz, Initial residual = 0.127022, Final residual = 1.99985e05, No Iterations 3 GAMG: Solving for p, Initial residual = 0.924117, Final residual = 0.0600726, No Iterations 1 time step continuity errors : sum local = 5.14431e+19, global = 2.03359e+17, cumulative = 2.03353e+17 ExecutionTime = 62.41 s ClockTime = 63 s I stopped at this stage Because I know this will diverge. Would you like to look at my BC. May be something wrong in that may be ? :( Physics is Inlet at 1.8 bar and outlet at 1.5 bar. Flow in a micro channel with Hydrogen as fluid. My p file FoamFile { version 2.0; format ascii; class volScalarField; object p; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // dimensions [0 2 2 0 0 0 0]; internalField uniform 1112347.05; boundaryField { WALL { type zeroGradient; } INLET { type fixedValue; value uniform 2002224.694; } OUTLET { type fixedValue; value uniform 1668520.57; } } My U file \**/ FoamFile { version 2.0; format ascii; class volVectorField; object U; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // dimensions [0 1 1 0 0 0 0]; internalField uniform (0 0 0); boundaryField { INLET { type pressureInletVelocity; value uniform (0 0 0); } WALL { type noSlip; } OUTLET { type inletOutlet; inletValue uniform (0.0001 0 0); value uniform (0.0001 0 0); } } Regards, Manoj 
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