Simple Pipe Flow
 

Hello - I am trying to solve steady, laminar flow in a cylindrical pipe due to pressure gradient (Hagen-Poiseuille Flow). I have been using the simpleFoam solver and setting the RASModel to laminar. I have been able to mesh a pipe with a 0.701m diameter and 1m length. I wanted to have an inlet pressure of 1.002 m^2/s^2 and outlet pressure of 0 (atmosphere). I think I am going wrong with my p and U but I am not sure. That or I need to make my pipe longer?

For p I have:
dimensions [0 2 -2 0 0 0 0];

internalField uniform 0;

boundaryField
{
inlet
{
type fixedValue;
value uniform 1.002;
}

outlet
{
type fixedValue;
value uniform 0;
}

walls
{
type zeroGradient;
}

}

For U:
dimensions [0 1 -1 0 0 0 0];

internalField uniform (0 0 0);

boundaryField
{
inlet
{
type pressureInletVelocity;
value uniform (0 0 0);
}

outlet
{
type zeroGradient;
}

walls
{
type fixedValue;
value uniform (0 0 0);
}

}

My solution is coming out really strange. Any help is appreciated!
Betsy

Hi Betsy,

your boundary conditions look OK, the problem must be located somewhere else.
Useful information to give further help would be:
- complete case (with mesh preferred, otherwise with an image of the mesh)
- checkMesh output
- image of your strange solution
- image the residuals

Best regards

Martin

- I've attached the files including mesh.

- checkMesh output:

Create time

Create polyMesh for time = 0

Time = 0

Mesh stats
points: 2541
faces: 6500
internal faces: 5500
cells: 2000
boundary patches: 3
point zones: 0
face zones: 0
cell zones: 0

Overall number of cells of each type:
hexahedra: 2000
prisms: 0
wedges: 0
pyramids: 0
tet wedges: 0
tetrahedra: 0
polyhedra: 0

Checking topology...
Boundary definition OK.
Point usage OK.
Upper triangular ordering OK.
Face vertices OK.
Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces ...
Patch Faces Points Surface topology
inlet 100 121 ok (non-closed singly connected)
outlet 100 121 ok (non-closed singly connected)
walls 800 840 ok (non-closed singly connected)

Checking geometry...
Overall domain bounding box (-0.01036 0 -0.01036) (0.06036 0.1 0.06036)
Mesh (non-empty, non-wedge) directions (1 1 1)
Mesh (non-empty) directions (1 1 1)
Boundary openness (1.52156e-16 4.04658e-17 -2.00692e-16) OK.
Max cell openness = 2.04154e-16 OK.
Max aspect ratio = 3.74725 OK.
Minumum face area = 1.04378e-05. Maximum face area = 5.71473e-05. Face area magnitudes OK.
Min volume = 5.21888e-08. Max volume = 2.85736e-07. Total volume = 0.000391154. Cell volumes OK.
Mesh non-orthogonality Max: 61.528 average: 18.9707
Non-orthogonality check OK.
Face pyramids OK.
Max skewness = 0.978201 OK.

Mesh OK.

End

-I've attached images of the pipe inlet and outlet. It looks strange to me because I was expecting to see more of a bullseye with higher velocities in the center and zero velocities on the pipe walls.

-Residuals:
I am not sure if this is what you mean but here is part of my output:

Time = 500

smoothSolver: Solving for Ux, Initial residual = 0.110048, Final residual = 0.00234335, No Iterations 4
smoothSolver: Solving for Uy, Initial residual = 0.0500392, Final residual = 0.00197118, No Iterations 4
smoothSolver: Solving for Uz, Initial residual = 0.100236, Final residual = 0.0024594, No Iterations 4
GAMG: Solving for p, Initial residual = 0.312405, Final residual = 0.025525, No Iterations 1
time step continuity errors : sum local = 2.90786e+07, global = -1.72903e+07, cumulative = -2.9503e+08
ExecutionTime = 5.64 s ClockTime = 6 s

Thank you so much for your help, I really appreciate it.
Betsy

Hi Betsy,

here is a modified case (modifications are done mainly in the fvScheme file). Convergence is not that nice, you'll see if you let it run for 50000 iterations... However results look better...

The mesh topology is not optimal for a pipe, although it should do in your case. Have a look at image of post #6 of this thread: http://www.cfd-online.com/Forums/ope...arge-mesh.html

Martin

Hi Betsy,

You wrote that the pipe diameter is 0.7 m and the length is 1 m. But if you look in checkMesh the diameter is 0.07 m and the length 0.1 m.

jean