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

2D vs 3D

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

Like Tree2Likes

Reply
 
LinkBack Thread Tools Display Modes
Old   January 11, 2012, 14:17
Default
  #21
Senior Member
 
Martin Hegedus
Join Date: Feb 2011
Posts: 467
Rep Power: 9
Martin Hegedus is on a distinguished road
Quote:
Originally Posted by lakeat View Post
Thank you guys,

Hybrid would be definitely better than (U)RANS, but it is still too costly for large Re. Many hybrid still require a low y_1st_plus be well set, around unity or similar.

Low-Re turb model is very promising, but I am not sure which one is more stable. I am testing.

Yes, the 3D simulation I mentioned is LES, I will never do 3D URANS, I don;t think it worth the effort.

And btw, in 3DURANS, the vortices do break in the wake a little bit, but 2D rings persist for a quite long distance, this is what I've seen from many published papers.

These days, I am so interested in 2D URANS' performance, is that I am doing some project, where we have thousands of cases to run, or we have body motion dynamics mesh etc., all of these demand the single run to be as quickly as possible.
Since you are dealing with a separation bubble at the top and bottom of your square, which is driven by the sharp leading edge, I'm not really sure I know how y+=1 translates into a real grid spacing and how dependent the solution is on it. You may find that the required grid spacing for low and high Re number, at least in regards to your pressure forces, is similar. However, your viscous drag will be off. But, viscous forces are probably a smaller percentage of your overall loads and you can tweak it during post processing to get better results.

If your separation is smooth body separation (i.e. a circle), then the pressure and viscous solution is dependent on initial grid spacing.

Yes, as flow moves much further away from the body, I do expect the URANS flow to become unstable as eddy viscosity diminishes and the grid (in general) becomes coarser.
mali likes this.
Martin Hegedus is offline   Reply With Quote

Old   September 1, 2014, 04:18
Default
  #22
Member
 
Albert Tong
Join Date: Dec 2010
Location: Perth, WA, Australia
Posts: 76
Blog Entries: 1
Rep Power: 6
tfuwa is on a distinguished road
Quote:
Originally Posted by lakeat View Post
Yet see how many papers were published with 2D-LES...


I am more interested in the lift coefficient RMS values than in the Cd.

But I got impression that ALL URANS would give sine wave time history, and low RMS(Cl) values.

Is there any better solution within the scope of 2D (2D-mesh) simulations? That what I am looking hard for.
Hi All,

I have a simple question that why 2-D les is not physically right?
__________________
Kind regards,

Albert
tfuwa is offline   Reply With Quote

Old   September 1, 2014, 05:22
Default
  #23
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,654
Rep Power: 23
FMDenaro will become famous soon enough
Quote:
Originally Posted by tfuwa View Post
Hi All,

I have a simple question that why 2-D les is not physically right?

the key is to understand what you mean for 2d turbulence... 2d turbulence is a real model for large scale flows, such as atmospheric or oceanic turbulence becose you have a 2d plane of large scale and the third dimension (the vertical direction) is much smaller compared to them. For such flows you can perform 2d LES computation.

On the other hand, if you have a 3d problem where all the characteristic scales in the three dimensions are comparable each other, then it makes no sense to perform an LES computation in 2d
FMDenaro is offline   Reply With Quote

Old   September 10, 2014, 01:40
Default
  #24
Member
 
Albert Tong
Join Date: Dec 2010
Location: Perth, WA, Australia
Posts: 76
Blog Entries: 1
Rep Power: 6
tfuwa is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
the key is to understand what you mean for 2d turbulence... 2d turbulence is a real model for large scale flows, such as atmospheric or oceanic turbulence becose you have a 2d plane of large scale and the third dimension (the vertical direction) is much smaller compared to them. For such flows you can perform 2d LES computation.

On the other hand, if you have a 3d problem where all the characteristic scales in the three dimensions are comparable each other, then it makes no sense to perform an LES computation in 2d
Hi Filippo,

Thank you for your promote reply. That is to say 2-D LES is similar to 2-D RAS? I read through this thread and some people seem to suggest 2-D LES is not mathematically right.

I am simulating pipeline sitting on seabed, and the pipeline is very long compared to its diameter. Is 2-D LES appropriate for such application, or 2-D RAS is more appropriate?

Many thanks.
__________________
Kind regards,

Albert
tfuwa is offline   Reply With Quote

Old   September 10, 2014, 03:21
Default
  #25
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,654
Rep Power: 23
FMDenaro will become famous soon enough
No, that's not a mathematical lack, LES can be formally performed even in 1D...
The key is in the physics of the flow problem.... as happen in geophysical flows, when you have turbulent structures extending several order of magnitude more in two dimensions compared to the third dimension, a 2D LES can be a reasonable approximate model.

Pipe and channel flows are typically flow problems with 3D turbulent structures and you have to use 3D LES.

RANS is used in 2D as it implies a statistical averaging, that's very different idea from LES
FMDenaro 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



All times are GMT -4. The time now is 18:07.