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

Vortex Mehtod : Internal flow

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   June 22, 2005, 00:28
Default Vortex Mehtod : Internal flow
  #1
MC Ryu
Guest
 
Posts: n/a
Dear ,

I'm making a code for predicting lid-driven cavity flow using the conventional Lagrangian vortex method.

In my results, many particles go outside the box domain, especially near the upper two corners. I think the reason is that there are singular behaviours there. But except the moving wall vorticity and secondary vortex near two lower corners, overall flow patterns and the centerline velocity profiles are nealry same with Ghia's resutls after converging.

I think the wrong prediction of the moving wall vorticity profile is due to the outside particles. Am I right ?

And please let me know how can I reduce or eliminates the particles going outside ?

Thank you,

cf) dt = 0.01, Blob size = 1/64 for Re = 100
  Reply With Quote

Old   June 22, 2005, 01:10
Default Re: Vortex Mehtod : Internal flow
  #2
Adrin Gharakhani
Guest
 
Posts: n/a
Are you using random walk for diffusion? This would explain so many particles outside the box. How are you dealing with the velocity boundary conditions and diffusion from the wall? A good/correct implementation would not lead to so many particles going out of the domain. And, anyway, particles that move outside should be removed from the computation, as keeping them will lead to incorrect flow field.

Adrin Gharakhani
  Reply With Quote

Old   June 22, 2005, 03:14
Default Re: Vortex Mehtod : Internal flow
  #3
MC Ryu
Guest
 
Posts: n/a
Thank you for your kindness. I use PSE scheme for diffusion. And for no-slip condition at the wall, diffusion process is iterated. So, strengths of the particels are modified.

"Particles that move outside should be removed from the computation" means that outside particels must be eliminated from the computation even though they have large enough strength or they should be forced to have zero strength ?

Thank you.
  Reply With Quote

Old   June 22, 2005, 10:42
Default 2) Vortex Mehtod : Internal flow
  #4
MC Ryu
Guest
 
Posts: n/a
Thank you,

I cured some mistakes in my program, so particles don't go outside. But, it is seen some wrong vectors in the region of the right bottom corner, very close to the corner. In the position direction of the velocity vector is north-west, so in the corner region streamline contours are slightly different from Ghia's solution. Please give me some advices.

  Reply With Quote

Old   June 22, 2005, 20:04
Default Re: Vortex Mehtod : Internal flow
  #5
Adrin Gharakhani
Guest
 
Posts: n/a
Removing means removing particles from the computations. Setting the strengths to zero has essentially the same effect, but adds to your cost. A zero strength is equivalent to the case where no particles exist - of course when using PSE you will see some nearby particles gaining strength eventually, due to diffusion.

There is nothing wrong with removing the particles. What happens is that the loss of vorticity gets compensated for in the next timestep when you apply the no-slip BC. Essentially, you'll have a vortex sheet distribution at the walls with strength that accommodate the loss of particles. Nevertheless, if you are losing particles, then you may still have errors in implementation and/or your timestep size may be too large. A few particles here and there is not a problem, but an accurate solution should not lead to more losses.

Adrin Gharakhani
  Reply With Quote

Old   June 22, 2005, 20:15
Default Re: 2) Vortex Mehtod : Internal flow
  #6
Adrin Gharakhani
Guest
 
Posts: n/a
Well, since in one day you found problems that significantly improved your errors, I wonder if you will find other errors should you spend some more time!!

As for the vector going "in the wrong direction", I'm not so sure, because I don't have the vector field in front of me to make sense. However, barring you have additional errors (quite likely), I don't see a problem with the direction of the vector you described. You're supposed to have a very small secondary vortex there - it just makes physical sense - and the vector direction you mention, sort of, agrees with that. Try higher and lower resolution runs (_after_ you're sure you have no bugs) and see what happens. I won't be surprised one bit that you can capture small scales better than a finite-difference solution!

Adrin Gharakhani
  Reply With Quote

Old   June 22, 2005, 23:07
Default Thank you !!! Mr. Gharakhani
  #7
MC Ryu
Guest
 
Posts: n/a
I'm very appreciated in your kind answers. I'll try to do following your comments.

Thank you again.
  Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Advice on turbulence model for complex internal flow JPBodner Main CFD Forum 1 September 3, 2011 04:07
Modeling internal and external flow for a nozzle smschnob Main CFD Forum 1 November 11, 2010 18:12
Can 'shock waves' occur in viscous fluid flows? diaw Main CFD Forum 104 February 16, 2006 06:44
Need help with BC's for a internal flow problem ravi FLUENT 2 March 16, 2005 01:45
Vortex flow fields & Nature Oliver Main CFD Forum 4 September 13, 2000 18:47


All times are GMT -4. The time now is 10:03.