| PorscheGTIII |
November 1, 2013 01:08 |
Frozen Rotor Simulation Trouble with a Hydrokinetic Propeller Turbine
Greetings all!
I come to you very frustrated at the moment. I'm simulating a propeller-type hydrokinetic turbine in a large channel as shown bellow.
http://i66.photobucket.com/albums/h2...ATION_GEOM.png
I'm using the frozen rotor approach to solve this problem using simpleFoam with the MRF option and the k-omegaSST turbulence model. I create an unstructured tetrahedral and prism mesh using ANSYS Meshing and then import the mesh to OpenFOAM format using fluent3DMeshToFoam. Once the file structure is set up, I check the mesh quality with checkMesh, decompose the mesh for a parallel run using the scotch method, and renumber the mesh to reduce the bandwidth. The simulations are initialized with potentialFoam for both the velocity and pressure fields.
For a very coarse mesh (~2 million cells), the simulation behaves very well. Values for torque and thrust (drag) on the turbine converge very nicely on firm values and the residuals drop very well as shown bellow.
http://i66.photobucket.com/albums/h2...i/linear-1.png
However, when I refine the mesh (~7.5 million cells), the simulation does not behave as nicely. The residual for pressure only drops about 2 orders of magnitude and values for torque and thrust are not even resolved to one decimal place. (See below)
http://i66.photobucket.com/albums/h2...iii/linear.png
So I thought maybe because I was capturing more flow features was the reason the residuals were hanging, so I created a 47 million cell mesh and thought the residuals would improve. Nope, the behaved much of the same as the 7.5 million cell mesh.
This is the results from my checkMesh runs between the 2 and 7.5 million cell mesh:
2 Million Cell Mesh Stats
Code:
/*---------------------------------------------------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.2.2 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
Build : 2.2.2-9240f8b967db
Exec : checkMesh -case testCase
Date : Nov 01 2013
Time : 01:15:37
Host : "Ranka"
PID : 26751
Case : ./testCase
nProcs : 1
sigFpe : Enabling floating point exception trapping (FOAM_SIGFPE).
fileModificationChecking : Monitoring run-time modified files using timeStampMaster
allowSystemOperations : Disallowing user-supplied system call operations
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
Create time
Create polyMesh for time = 0
Time = 0
Mesh stats
points: 146558
faces: 1280820
internal faces: 1258192
cells: 605624
faces per cell: 4.19239
boundary patches: 5
point zones: 0
face zones: 3
cell zones: 2
Overall number of cells of each type:
hexahedra: 0
prisms: 116496
wedges: 0
pyramids: 20
tet wedges: 0
tetrahedra: 489108
polyhedra: 0
Checking topology...
Boundary definition OK.
Cell to face addressing 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 2364 1749 ok (non-closed singly connected)
outlet 2400 1767 ok (non-closed singly connected)
river_bed 7592 3913 ok (non-closed singly connected)
free_surface 5144 3315 ok (non-closed singly connected)
turbine 5128 2566 ok (closed singly connected)
Checking geometry...
Overall domain bounding box (-3.048 -1.8288 -3.048) (3.048 1.2192 3.048)
Mesh (non-empty, non-wedge) directions (1 1 1)
Mesh (non-empty) directions (1 1 1)
Boundary openness (6.72913e-17 -2.14284e-16 -2.24694e-17) OK.
Max cell openness = 9.18905e-16 OK.
Max aspect ratio = 26.5342 OK.
Minimum face area = 2.56528e-07. Maximum face area = 0.0276275. Face area magnitudes OK.
Min volume = 7.93254e-10. Max volume = 0.00137809. Total volume = 88.9365. Cell volumes OK.
Mesh non-orthogonality Max: 82.8675 average: 14.8611
*Number of severely non-orthogonal (> 70 degrees) faces: 34.
Non-orthogonality check OK.
<<Writing 34 non-orthogonal faces to set nonOrthoFaces
Face pyramids OK.
Max skewness = 2.38013 OK.
Coupled point location match (average 0) OK.
Mesh OK.
End
7.5 Million Cell Mesh Stats
Code:
/*---------------------------------------------------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.2.2 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
Build : 2.2.2-9240f8b967db
Exec : checkMesh -case 7357266_Cells
Date : Oct 28 2013
Time : 14:19:03
Host : "Ranka"
PID : 19244
Case : ./7357266_Cells
nProcs : 1
sigFpe : Enabling floating point exception trapping (FOAM_SIGFPE).
fileModificationChecking : Monitoring run-time modified files using timeStampMaster
allowSystemOperations : Disallowing user-supplied system call operations
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
Create time
Create polyMesh for time = 0
Time = 0
Mesh stats
points: 1771385
faces: 15543560
internal faces: 15383084
cells: 7357266
faces per cell: 4.20355
boundary patches: 5
point zones: 0
face zones: 3
cell zones: 2
Overall number of cells of each type:
hexahedra: 0
prisms: 1497580
wedges: 0
pyramids: 0
tet wedges: 0
tetrahedra: 5859686
polyhedra: 0
Checking topology...
Boundary definition OK.
Cell to face addressing 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 15540 9823 ok (non-closed singly connected)
outlet 15958 10032 ok (non-closed singly connected)
free_surface 35872 20655 ok (non-closed singly connected)
river_bed 56652 28637 ok (non-closed singly connected)
turbine 36454 18229 ok (closed singly connected)
Checking geometry...
Overall domain bounding box (-3.048 -1.8288 -3.048) (3.048 1.2192 3.048)
Mesh (non-empty, non-wedge) directions (1 1 1)
Mesh (non-empty) directions (1 1 1)
Boundary openness (3.19358e-16 -5.6173e-15 4.75543e-16) OK.
Max cell openness = 1.7247e-15 OK.
Max aspect ratio = 31.2746 OK.
Minimum face area = 1.48502e-07. Maximum face area = 0.0035493. Face area magnitudes OK.
Min volume = 3.97334e-11. Max volume = 6.47112e-05. Total volume = 88.9538. Cell volumes OK.
Mesh non-orthogonality Max: 80.6111 average: 14.8982
*Number of severely non-orthogonal (> 70 degrees) faces: 165.
Non-orthogonality check OK.
<<Writing 165 non-orthogonal faces to set nonOrthoFaces
Face pyramids OK.
Max skewness = 2.48881 OK.
Coupled point location match (average 0) OK.
Mesh OK.
End
System Files:
https://dl.dropboxusercontent.com/u/806642/controlDict
https://dl.dropboxusercontent.com/u/806642/fvSchemes
https://dl.dropboxusercontent.com/u/806642/fvSolution
BC's
https://dl.dropboxusercontent.com/u/806642/U
https://dl.dropboxusercontent.com/u/806642/k
https://dl.dropboxusercontent.com/u/806642/nut
https://dl.dropboxusercontent.com/u/806642/omega
https://dl.dropboxusercontent.com/u/806642/p
I'm a little frustrated right now. Is my issue a mesh issue? Or is there something that is flawed about my simulation?
Thanks for your help so far! |
