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

LES with rough surface

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

Reply
 
LinkBack Thread Tools Display Modes
Old   October 2, 2013, 11:33
Default LES with rough surface
  #1
Member
 
Join Date: Jun 2011
Posts: 35
Rep Power: 6
mikeP is on a distinguished road
Dear Foamers,

How do you define surface roughness in LES? I read that OpenFOAM allows one to use RANS wall functions also in LES? How good is it, and also how should the mesh be then? 30 < y+ < 100?

or should I use a constant shear stress on the wall?
mikeP is offline   Reply With Quote

Old   November 4, 2013, 17:35
Default
  #2
Member
 
Vivek Kumar
Join Date: Mar 2009
Location: Switzerland
Posts: 34
Blog Entries: 1
Rep Power: 8
vivekcfd is on a distinguished road
does not make much sense to use roughness wall function for LES. Even wall function approach for standard smooth pipe LES is not straight forward and questionable. How can one think of being doing LES using rough wall functions.
vivekcfd is offline   Reply With Quote

Old   April 22, 2014, 15:30
Default
  #3
New Member
 
Ashvin Chaudhari
Join Date: Aug 2011
Location: Finland
Posts: 20
Rep Power: 5
ashvinc9 is on a distinguished road
Hi,

In my opinion, one can use the wall-function for LES too. It will reduce near wall mesh-resolution ans thus the CPU time. For the smooth surfaces and for low Re (or Re_Tau) of course it may not seen useful. But how about for rough surfaces, e.g. atmospheric boundary layers flows? The surface roughness (z_0 or K_s are implemented only using the wall functions). For the rough surface, I would recommend "nutURoughWallFunction" available in OF-2.2 which is meant for RANS simulation (based on Ks: Sand grain-roughness) but works perfectly in LES too. I have tested several times this function.


-- Ashvin
ashvinc9 is offline   Reply With Quote

Old   April 23, 2014, 03:06
Default
  #4
Member
 
Vivek Kumar
Join Date: Mar 2009
Location: Switzerland
Posts: 34
Blog Entries: 1
Rep Power: 8
vivekcfd is on a distinguished road
Wall functions follow log-law of the wall. It is not valid for LES for two reasons:

1) mostly the flow is not attached or accompany separation zones where no wall functions is valid. Moreover in most applications, flow is not at all fully developed.

2) Secondly, the log-law is applicable to the mean flow field and not to the instantaneous flow velocity field. In other words, the instantaneous wall shear-stress and flow profile can be well away from the reality.


However, the good thing is that wall function provides the possibility to compute high-Re flows. People use Werner-Wengel wall functions for LES simulations which is not exactly same as the RANS wall-functions

If you are getting good results in your case then it signifies that in your applications:

1. the role of wall is not that important

2. The results which you are comparing or predicting may be unaffected by the flow anisotropy near the wall.

For very accurate simulations (which indeed anyway make sense for LES), I would not use wall functions. In case one is interested in turbulence fluctuations, in my opinion it is cheaper or may be better to employ IDDES using SST or other RANS model. However drawback 1) of wall-functions still can not be overcome. On the other hand wall-functions may still give you good results for some cases. Since the definition of good and bad is relative and depends very much on the type of application and the precision requirement from the simulation.

I hope this helps you.
vivekcfd 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
Problem with Gmsh nishant_hull Open Source Meshers: Gmsh, Netgen, CGNS, ... 18 April 22, 2015 08:43
killed "snappyHexMesh" parkh32 OpenFOAM Pre-Processing 2 April 8, 2012 17:12
[ICEM] Automatic mesh generation script surface intersection problem stuart23 ANSYS Meshing & Geometry 0 May 13, 2011 01:10
Recover surface from surface mesh in IcemCFD Jerry Tanner CFX 0 August 20, 2008 12:48
CFX 4.4 New free surface option Viatcheslav Anissimov CFX 0 April 3, 2002 06:27


All times are GMT -4. The time now is 11:24.