CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Transiet Animation - vortex shedding

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

Reply
 
LinkBack Thread Tools Display Modes
Old   May 4, 2010, 22:46
Default Transiet Animation - vortex shedding
  #1
Member
 
Derwin Parkin
Join Date: Feb 2010
Posts: 31
Rep Power: 7
derz is on a distinguished road
Hi guys,
I'm trying to get on top of transient simulations in CFX. I've decided to try and model flow past a cylinder - I want to get CFX to show me some vortex shedding. So I'm using the same parameters as this guy used: here is his video. Inlet turbulence is 5%.

He seems to get some nice vortex shedding.

Here is my video...

I'm using the exact same parameters as him - same dimensions too. Pretty much everything he displays at the start of the video, I used. The time step is 0.025 seconds and 15 seconds in total - that's 600 timesteps. also I know I should probably be using a laminar model due to low Re, but I used k-omega because the youtube guy did too. With the laminar I don't get any shedding either...

Seeming as he is also using CFX, I can't understand why I'm not getting any vortex shedding. In fact, my animation shows no changes in the wake at all.

I ran this as a 2D simulation. However I've also tried 3D with similar results. I'm getting CFX to output results after each timestep - then when the animation is finished, I use the keyframe animation movie maker to make the movie of the changing velocity contours.

The only thing that does not match this youtube's guys simulation is the mesh - could my mesh be the reason I'm not getting any shedding? The mesh isn't very fine in the region behind the cylinder - but around the cylinder I've used some pretty fine inflation (here is an image).

Just wondering if anyone had any ideas where I'm going wrong?

Thanks,

Derwin
derz is offline   Reply With Quote

Old   May 5, 2010, 04:04
Default
  #2
Senior Member
 
Matthias Voß
Join Date: Mar 2009
Location: Berlin, Germany
Posts: 446
Rep Power: 11
mvoss is on a distinguished road
hey,

did you even look at the resolution of th ICEM mesh?
Your mesh is way too coarse. Do some refinement in the vortex regions.

neewbie
mvoss is offline   Reply With Quote

Old   May 5, 2010, 04:55
Default
  #3
Member
 
Derwin Parkin
Join Date: Feb 2010
Posts: 31
Rep Power: 7
derz is on a distinguished road
hi...yeah i noticed he used ICEM mesh. I have workbench ansys 12.0. i don't think i have ICEM mesh. i tried making it less coarse but still same result.

i would like to give my mesh the same type of shape as the one ICEM gives, but not sure how to in the CFX Workbench environment. It only gives me patch conforming/patch independent options.

thanks for your response...i'll focus on improving the mesh for now.
derz is offline   Reply With Quote

Old   May 5, 2010, 06:40
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,831
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Some issues I can see...

1) Make the top and bottom walls art of the inlet boundary.
2) Your mesh is probably too coarse.
3) Your mesh is probably low quality (can tell by the jaggedness of your contours).
4) A hex mesh carefully fitted to the cylinder will help a lot.
5) And dump the turbulence model, it just damps stuff out. Run it laminar.
ghorrocks is offline   Reply With Quote

Old   May 5, 2010, 07:02
Default
  #5
Member
 
Derwin Parkin
Join Date: Feb 2010
Posts: 31
Rep Power: 7
derz is on a distinguished road
Thanks ghorrocks. Yes I have started to run it in laminar. It seems my mesh is the issue - I will attempt to make it more like the meshes I've seen in many papers I've read - like this one but with a cylinder (sorry for the large picture).
derz is offline   Reply With Quote

Old   May 10, 2010, 09:21
Default
  #6
Member
 
Derwin Parkin
Join Date: Feb 2010
Posts: 31
Rep Power: 7
derz is on a distinguished road
following up - i've improved my mesh a great deal. and its made a huge difference. before it took like 15 seconds until i saw a little bit of movement in the wake...now its moving noticeably after 4 secs or so. thanks for everyone's help...
derz 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
K-Epsilon for Vortex Shedding Sham FLUENT 30 July 10, 2015 21:13
Vortex shedding, FSI-analysis, turbulence numerics tallknuseren CFX 3 May 10, 2010 04:31
Vortex shedding? rbel038 CFX 4 April 27, 2008 19:57
Vortex shedding behind cylider in cross flow Muthu FLUENT 0 March 6, 2006 11:29
basic vortex shedding john Main CFD Forum 4 November 6, 2000 14:23


All times are GMT -4. The time now is 03:15.