CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > ANSYS Meshing & Geometry

Mesh size and equivalent sand grain rougthness

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

Reply
 
LinkBack Thread Tools Display Modes
Old   January 27, 2011, 05:52
Exclamation Mesh size and equivalent sand grain rougthness
  #1
New Member
 
Join Date: Mar 2009
Posts: 19
Rep Power: 8
kordou is on a distinguished road
Hi!

I am trying to model a windtunnel experiment and i have to apply a wall roughness on the ground equal to 1.53m.

In cfx for rough wall you need to specify the equivalent sand grain rougthness which is about 30*1.53. (according to this http://www.mi.uni-hamburg.de/fileadm...5-2007-www.pdf).

In the cfx manual there is the a notation about the cell size of the first cell. it says that the center of the first cell must not be smaller than the sand grain roughness. So for a wind tunnel of hight 200m (full scale) i have to have a fisrst cell with size 22.95m which is extremely hight.

any recomentation about how create this mesh?

P.S. the geometry is an empty simple rectangular box
P.S2. the ground material on the wind tunnel have roughness 0.0045m (full scale) of 0.0002 m (wind tunnel scale)

Thank you in advanced
kordou is offline   Reply With Quote

Old   January 29, 2011, 21:11
Default
  #2
Senior Member
 
PSYMN's Avatar
 
Simon Pereira
Join Date: Mar 2009
Location: Ann Arbor, MI
Posts: 2,662
Blog Entries: 1
Rep Power: 35
PSYMN has a spectacular aura aboutPSYMN has a spectacular aura about
What mesher? What kind of mesh?

You need boundary layers, but there are many ways to get them.
PSYMN is offline   Reply With Quote

Old   February 14, 2011, 06:28
Default
  #3
New Member
 
Join Date: Mar 2009
Posts: 19
Rep Power: 8
kordou is on a distinguished road
I was not thinking about mesher. I was thinking as a general problem. Let's say ICEM with Hexahedral mesh

as for the boundary layer can you please tell me how can I get them?
kordou is offline   Reply With Quote

Old   February 14, 2011, 11:01
Default
  #4
Senior Member
 
PSYMN's Avatar
 
Simon Pereira
Join Date: Mar 2009
Location: Ann Arbor, MI
Posts: 2,662
Blog Entries: 1
Rep Power: 35
PSYMN has a spectacular aura aboutPSYMN has a spectacular aura about
In ICEM CFD, you would start with a single block, and create an OGRID. You would put an OGRID Face at the inlet and outlet so your inflation would be parallel to the side walls, but pass thru the inlet and outlet.

Then you would set the Ogrid edge distribution with the Edge params so that your spacing 1 = what ever first cell height you wanted and your growth ratio was what you wanted. You could also set the growth law to geometric1 or exponential1 or what ever you wanted.

Beyond that starts to get specific again to your problem, but if necessary, you can adjust the total thickness of your OGrid, have different initial heights on different edges, etc.
PSYMN is offline   Reply With Quote

Old   February 15, 2011, 07:16
Default
  #5
New Member
 
Rob Connell
Join Date: Feb 2011
Location: Christchurch, New Zealand
Posts: 1
Rep Power: 0
RobC56 is on a distinguished road
The value you quote is what is expected. See Blocken et al. Atmospheric Environment 41 (2007) 238 - 252 for full discussion about this how to do ABL's in CFD.

Regards,
__________________
RobC56
RobC56 is offline   Reply With Quote

Reply

Tags
cfx, icem, mesh, roughness

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 00:54.