CFD Online Discussion Forums (https://www.cfd-online.com/Forums/)
-   OpenFOAM Running, Solving & CFD (https://www.cfd-online.com/Forums/openfoam-solving/)
-   -   pressure in incompressible solvers e.g. simpleFoam (https://www.cfd-online.com/Forums/openfoam-solving/79411-pressure-incompressible-solvers-e-g-simplefoam.html)

 chrizzl August 23, 2010 08:45

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 simpleFoam-solver.

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

 chrizzl August 24, 2010 03:23

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

 Bjw August 24, 2010 05:41

Changing the scale and/or the unit of pressure should not change the physics of your problem.

 kjetil August 24, 2010 06:02

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

 chrizzl August 24, 2010 06:07

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 density-normalization to your simpleFoam-setup 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.

 sven82 August 24, 2010 08:08

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 ;)

 Bjw August 24, 2010 08:41

Christian, sorry that I misunderstood your Question.

Quote:
 If you set your pressure difference, which is known from an experiment without density-normalization to your simpleFoam-setup 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?
Yes, absolutely. You must divide the pressure (Unit Pa) by density first when using an incompressible solver.

 chrizzl August 24, 2010 11:19

Quote:
 Originally Posted by Bjw (Post 272506) Yes, absolutely. You must divide the pressure (Unit Pa) by density first when using an incompressible solver.
Thanks, that's exactly what I was looking for.

 prasa April 8, 2016 03:47

Quote:
 Originally Posted by chrizzl (Post 272379) Dear All, I have a general question about the setup of the pressure in an incompressible solver like the simpleFoam-solver. 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

Hi,

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.

 Jan Brunner June 9, 2016 11:01

Quote:
 Originally Posted by prasa (Post 594031) units [pressure/density] = m2/s3
Hi Prasa

Thanks for wanting to clarify this but the unit should be m2/s2, right? ;)

All the best,
Jan

 prasa June 9, 2016 12:30

Quote:
 Originally Posted by Jan Brunner (Post 604134) Hi Prasa Thanks for wanting to clarify this but the unit should be m2/s2, right? ;) All the best, Jan
Hi Jan,

Yes it should be m2/s2.

Regards,
Prasa.

 Manojmech March 27, 2017 09:42

For Hydrogen ??

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

 piu58 March 27, 2017 11:49

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.

 Manojmech March 28, 2017 05:49

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 1e-06
field U tolerance 1e-06
field "(k|epsilon|omega|f|v2)" tolerance 1e-06

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 = 2.18237e-05, 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 = 5.37614e-06, No Iterations 2
smoothSolver: Solving for Uy, Initial residual = 0.108497, Final residual = 8.93021e-05, 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 = 1.23498e-05, No Iterations 2
smoothSolver: Solving for Uy, Initial residual = 0.133453, Final residual = 1.14443e-05, No Iterations 2
smoothSolver: Solving for Uz, Initial residual = 0.123063, Final residual = 8.81908e-06, 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 = 6.19314e-05, 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 = 6.72522e-05, No Iterations 3
smoothSolver: Solving for Uz, Initial residual = 0.101372, Final residual = 2.39837e-05, 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 = 1.21663e-05, No Iterations 3
smoothSolver: Solving for Uz, Initial residual = 0.127022, Final residual = 1.99985e-05, 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
{
}

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

 All times are GMT -4. The time now is 19:32.