CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Running, Solving & CFD

k-omega SST flow over a cylinder

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree6Likes

Reply
 
LinkBack Thread Tools Display Modes
Old   June 8, 2013, 14:21
Default
  #21
MDB
New Member
 
Manuel Díaz Brito
Join Date: Jun 2013
Posts: 16
Rep Power: 4
MDB is on a distinguished road
Well, that was my first thought and they seem to be OK. On the other hand the residuals worry me, as you can see in the attached figure they are pretty high, but I'm not able to spot where errors might come from.
Attached Images
File Type: png residuals.png (19.8 KB, 43 views)
JR22 likes this.
MDB is offline   Reply With Quote

Old   June 9, 2013, 01:25
Default getting used to crappy k-Omega SST residuals
  #22
Senior Member
 
JR22's Avatar
 
Jose Rey
Join Date: Oct 2012
Posts: 122
Rep Power: 8
JR22 will become famous soon enough
I am getting used to seeing k-omega SST runs with decrepit residuals. Even when you are careful about your y+ (keeping it below 5 or even below 1 at all times) as you have been. Just run the motorbike tutorial and you will see residuals that make yours look amazing (regardless of how long you let it run). After lots of tweaking your mesh (mine come from snappyHexMesh), it is possible to improve residuals on a case by case. In the case of snappyHexMesh, sometimes less means more. When I ask snappyHexMesh to do lots of iterations, it sometimes endsup worsening the mesh.

Quote:
Originally Posted by MDB View Post
Well, that was my first thought and they seem to be OK. On the other hand the residuals worry me, as you can see in the attached figure they are pretty high, but I'm not able to spot where errors might come from.
JR22 is offline   Reply With Quote

Old   June 14, 2013, 15:48
Default further issues
  #23
MDB
New Member
 
Manuel Díaz Brito
Join Date: Jun 2013
Posts: 16
Rep Power: 4
MDB is on a distinguished road
Thanks Frédéric,

I found out that for 2D Aref has to be the spanwise cell thickness, and modifying it I get proper values of c_d (for 2D-SRANS...). This was using omegaWallFunction and y1+ in the range of 4-42. When I tried with a finer mesh (aiming for y1+ about unity) the case crashes due to floating point exception before 700 iterations as seen in the attached picture.

The output of checkMesh is as follows:
Code:
/*---------------------------------------------------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.1.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
Build  : 2.1.1-221db2718bbb
Exec   : checkMesh
Date   : Jun 14 2013
Time   : 20:43:29
Host   : "lyceum2"
PID    : 16785
Case   : /home/mdb1g12/OpenFOAM/mdb1g12-2.1.1/run/sst7
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:           101320
    internal points:  0
    faces:            201230
    internal faces:   99910
    cells:            50190
    boundary patches: 4
    point zones:      0
    face zones:       0
    cell zones:       0
Overall number of cells of each type:
    hexahedra:     50190
    prisms:        0
    wedges:        0
    pyramids:      0
    tet wedges:    0
    tetrahedra:    0
    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                  
    frontAndBack        100380   101320   ok (non-closed singly connected)  
    inlet               347      696      ok (non-closed singly connected)  
    outlet              231      464      ok (non-closed singly connected)  
    wall                362      724      ok (non-closed singly connected)  
Checking geometry...
    Overall domain bounding box (-20.4992 -20.5 0) (20.5 20.5 0.00015)
    Mesh (non-empty, non-wedge) directions (1 1 0)
    Mesh (non-empty) directions (1 1 0)
    All edges aligned with or perpendicular to non-empty directions.
    Boundary openness (-5.16794e-22 -1.3413e-24 2.9497e-15) OK.
    Max cell openness = 2.61712e-14 OK.
    Max aspect ratio = 867.882 OK.
    Minumum face area = 1.5e-09. Maximum face area = 7.5.  Face area magnitudes OK.
    Min volume = 1.30171e-11. Max volume = 0.001125.  Total volume = 0.224971.  Cell volumes OK.
    Mesh non-orthogonality Max: 62.0435 average: 6.24136
    Non-orthogonality check OK.
    Face pyramids OK.
    Max skewness = 0.572015 OK.
    Coupled point location match (average 0) OK.
Mesh OK.
End
What are your thoughts about this? It worries me that such a simple 2D-steady case seems to be this complicated to run properly, provided that I pretend to be using hybrid turbulence models in unsteady 3D configuration asap...

Thanks in advance
Attached Images
File Type: jpg crash.jpg (36.6 KB, 22 views)
MDB is offline   Reply With Quote

Old   June 14, 2013, 16:10
Default
  #24
Senior Member
 
HECKMANN Frédéric
Join Date: Jul 2010
Posts: 236
Rep Power: 7
fredo490 is on a distinguished road
Just to remind me, what is your Reynolds number?
fredo490 is offline   Reply With Quote

Old   June 14, 2013, 16:35
Default
  #25
MDB
New Member
 
Manuel Díaz Brito
Join Date: Jun 2013
Posts: 16
Rep Power: 4
MDB is on a distinguished road
I'm setting up the case at Re = 3e+6
MDB is offline   Reply With Quote

Old   June 15, 2013, 00:38
Default
  #26
Senior Member
 
HECKMANN Frédéric
Join Date: Jul 2010
Posts: 236
Rep Power: 7
fredo490 is on a distinguished road
Simulating a cylinder is not as easy as it might look... An airfoil is much easier to converge. How does your data look like (flow field) at 500 iterations for example? Is there any un physical properties?

You might have to try with a simple first order (upwind) at e beginning and when you get a good general flow field, you can go to second order. It is hard to convergence directly to second order a case as complicated as a flow behind a cylinder (the recirculation area is not easy to find)
fredo490 is offline   Reply With Quote

Old   August 11, 2013, 17:15
Default
  #27
New Member
 
Rafael Coelho
Join Date: Jun 2012
Location: Portsmouth
Posts: 22
Rep Power: 5
Rafael_Coelho is on a distinguished road
Dear All,

I sent the last two days trying to simulate this tricky problem. I had some discussions about it on:

Drag coefficient too high at flow around a cyclinder

As Fredo well said simulate the flow around a cylinder is harder than it seems. I found on my backups some data from a similar wind tunnel experiment I did back in 2008. We where measuring the turbulence on the cylinder wake and drag wasn't the main purpose. The Reynolds was 2*10⁴ and I have two results for Cd=0.925 and on another experiment Cd=1.019. According to Schlitching Cd=1.2. Those fellows (http://www.disasterzone.net/projects...ent.pdf‎) also had some problems and found Cd=1.06 and Cd=0.96.

After 29 attempts I am getting Cd around 1.1. I will upload the complete case to dropbox and post the link here.
Attached Images
File Type: jpg movie.0008.jpg (34.2 KB, 41 views)
Rafael_Coelho is offline   Reply With Quote

Old   August 11, 2013, 17:32
Default
  #28
New Member
 
Rafael Coelho
Join Date: Jun 2012
Location: Portsmouth
Posts: 22
Rep Power: 5
Rafael_Coelho is on a distinguished road
I am using a 3D domain and snappyHexMesh in order to make "scaleable" and "swapable" in case of running a different body . The bottom plane is treated as a wall (as it is on motorbike tutorial). The same case can run with pimpleFoam renaming the FvSolutions and FvScheme files.
I would like to compare some Ux results on a plane 40mm behind the cylinder. I have the analytical and experimental data of it and it will be good to compare. Does anyone kno a easy way to get this data?

Thanks!
Attached Files
File Type: gz cylinder30mm_simpleFoam.tar.gz (36.4 KB, 67 views)
Rafael_Coelho is offline   Reply With Quote

Old   August 13, 2013, 14:54
Default
  #29
New Member
 
Rafael Coelho
Join Date: Jun 2012
Location: Portsmouth
Posts: 22
Rep Power: 5
Rafael_Coelho is on a distinguished road
Just ran it using pimpleFoam and got Cd=1.175

http://youtu.be/2LsQC_LJ8uo
Rafael_Coelho is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
benchmark: flow over a circular cylinder goodegg Main CFD Forum 12 January 22, 2013 12:47
k-omega SST simulation of turbulent flow around a circular cylinder DanM OpenFOAM Running, Solving & CFD 16 December 12, 2012 14:39
Flow induced vibration of a mobile cylinder Hooman Main CFD Forum 0 December 31, 2010 09:48
Simulation Flow Around cylinder 3D Jwolf CFX 19 November 25, 2009 15:21
Flow over a cylinder Anna Main CFD Forum 9 March 24, 2006 15:32


All times are GMT -4. The time now is 15:32.