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

long period oscillations in simpleFoam results

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

Reply
 
LinkBack Thread Tools Display Modes
Old   March 5, 2014, 13:52
Default long period oscillations in simpleFoam results
  #1
New Member
 
Jim Conger
Join Date: Jan 2014
Location: California, USA
Posts: 18
Rep Power: 3
james.conger is on a distinguished road
I have been using simpleFoam to simulate 2D turbulent air flow over sails. This works well for cases where the air flow is roughly parallel to the airfoil (sail) shape. The solutions typically converge in under 2000 iterations.

In cases where the sail is fully stalled (sailing downwind) the solutions oscillate over a period of ~800 iterations. The oscillations are periodic and continue indefinitely. This appears to roughly mimic the real-world situation where vortex shedding creates oscillations when sailing downwind. In other words, I suspect that the long period oscillations are a reflection of reality to some extent.

For my purposes I am interested in the average behavior. I would be interested in what approaches other people have taken in dealing with this type of meta-stable behavior.

A movie of the velocity profile over time and a plot of the oscillations is available at:

https://sites.google.com/site/sailcf...-visualization
james.conger is offline   Reply With Quote

Old   March 6, 2014, 19:47
Default
  #2
Senior Member
 
Joachim Herb
Join Date: Sep 2010
Posts: 294
Rep Power: 9
jherb is on a distinguished road
A computational expensive solution would be to do transient simulations (with pimpleFoam). The simulation should converge for every time step. Then you can average over the converged solutions of the time steps.

Perhaps also the fixedMeanValue boundary condition for pressure does help:
Pressure outlet boundary condition
Pressure outlet boundary condition
jherb is offline   Reply With Quote

Old   March 7, 2014, 12:39
Default Good idea
  #3
New Member
 
Jim Conger
Join Date: Jan 2014
Location: California, USA
Posts: 18
Rep Power: 3
james.conger is on a distinguished road
I had overlooked that mean value boundary condition. Certainly worth a test.

Many thanks!
james.conger is offline   Reply With Quote

Old   August 22, 2014, 19:00
Default 3D simulations avoid the problem
  #4
New Member
 
Jim Conger
Join Date: Jan 2014
Location: California, USA
Posts: 18
Rep Power: 3
james.conger is on a distinguished road
Surprisingly enough, it turns out that 3D simulations under the same conditions usually converge quickly without showing the oscillations. The 3D simpleFoam results show a large eddy behind the stalled sails (downwind simulations) with a complex flow pattern. This Reynolds averaged result is nice to work with and so far correlates with real data taken from the test boat.

I would be interested if other people have seen cases which are easier to get to converge in 3D vs 2D.

Partial summary of 3D results at:

https://sites.google.com/site/sailcf...3d-close-reach
Attached Images
File Type: jpg broad reach streamlines from aft.jpg (48.4 KB, 6 views)
james.conger is offline   Reply With Quote

Old   August 22, 2014, 19:39
Default
  #5
Member
 
joegi
Join Date: Nov 2009
Location: genoa
Posts: 67
Rep Power: 7
joegi.geo is on a distinguished road
Hi, I will give you my two cents. That behavior is simply an indication of a highly unsteady flow, so the steady solver have problems getting to a converge solution.


In 3d you get to a converge solution because of the diffusion due to the coarse mesh. I bet that your 2d mesh is very fine so that one resolves better the spatial scales (it captures the unsteadiness). If you use a coarser mesh in 2d you will get to a converge solution as well. Also, if you use a very fine mesh in 3d you will see that you wont get a converge solution.



You can try to reduce the relaxation factors, maybe you are using the standard values (p=0.3, U=0.7), try to reduce U (an the turbulence variables) to 0.5.


Maybe after getting the steady solution, it can be a good idea to change to an unsteady solver and let the simulation run for a few seconds.




jg
joegi.geo is offline   Reply With Quote

Old   August 23, 2014, 10:50
Default Good thoughts
  #6
New Member
 
Jim Conger
Join Date: Jan 2014
Location: California, USA
Posts: 18
Rep Power: 3
james.conger is on a distinguished road
Thanks for your suggestions. You are right that on average there was a finer resolution on the 2D mesh. Also, I made the 3D mesh much more uniform, which might have contributed to the solution.

Note that we have taken a movie of a line of streamers held in the eddy region of the real boat and it certainly demonstrates that things are chaotic (as expected.)

I will have to go to a non steady state solution in the future to deal with wave motion, so I can experiment with that idea too.

Thanks again.
james.conger is offline   Reply With Quote

Reply

Tags
instability, oscillations, sail, sailboat, simplefoam

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
Laminar simpleFoam and inviscid simpleFoam herenger OpenFOAM Running, Solving & CFD 7 July 11, 2013 06:27
Different Results from Fluent 5.5 and Fluent 6.0 Rajeev Kumar Singh FLUENT 6 December 19, 2010 12:33
Oscillations in Results and Residuals Freeman FLUENT 27 December 18, 2010 14:09
Different results with icoFoam and simpleFoam..why??? nileshjrane OpenFOAM Running, Solving & CFD 11 September 13, 2010 15:08
Unstable flow simpleFoam 2nd order Valle OpenFOAM 0 August 26, 2009 08:12


All times are GMT -4. The time now is 02:36.