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

Can I turn an element "off" in CFX?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   May 15, 2008, 16:49
Default Can I turn an element "off" in CFX?
  #1
Joshua
Guest
 
Posts: n/a
Hi,

If I specify a continuity source in CFX with 0 kg/s total mass inflow and 0 m/s momentum inflow, does this effectively turn the element where the source is located "off"? By that I mean is the momentum forced to be zero at those locations? If not, can anyone suggest an effective way to force zero velocity at a particular element? Thanks!

- Joshua
  Reply With Quote

Old   May 15, 2008, 20:02
Default Re: Can I turn an element "off" in CFX?
  #2
andy2o
Guest
 
Posts: n/a
"If I specify a continuity source in CFX with 0 kg/s total mass inflow and 0 m/s momentum inflow, does this effectively turn the element where the source is located "off"?"

No - the source is just what you add to the domain. So adding nothing (0 kg/s, 0 momentum) is just like not having a source at all....

For a subdomain, you can use the 'stiff spring' approach to fix velocity values. Look up the word 'Dirichlet' in the CFX help search system - it should come up with a section describing general momentum sources of the form S = -C*(v-v_target). You choose C to be very large, then this source term dominates all other terms at that node, so the solver effectively ends up solving -C*(v-v_target) = 0 in that sub-domain node, and hence you end up with v = v_target in that subdomain. This technique has worked well for me.

I've never seen a way of dealing with an individual element though... I hope others will advise you more.

Good luck. Andy

  Reply With Quote

Old   May 15, 2008, 21:14
Default Re: Can I turn an element "off" in CFX?
  #3
Joshua
Guest
 
Posts: n/a
Thanks for the reply. I just finished verifying your reply regarding the continuity sources. I've already confirmed with CFX help that momentum sources cannot be specified at individual elements. Thanks again.

-- Joshua
  Reply With Quote

Old   May 25, 2008, 22:57
Default Re: Can I turn an element "off" in CFX?
  #4
CycLone
Guest
 
Posts: n/a
Hi Joshua,

What it is it you are trying to accomplish by turning off an element? If you explain your goal, rather than a step you have determined, you may find there is another way altogether.

-CycLone
  Reply With Quote

Old   May 26, 2008, 08:14
Default Re: Can I turn an element "off" in CFX?
  #5
Joshua
Guest
 
Posts: n/a
Hi,

I'm trying to simulate surface roughness in a DNS study of transition over a flat plate affected by surface roughness of a given statistical height, spacing, etc. By turning "off" a few nodes, they become a blockage similar to a roughness element. I've also looked at specifying a source and a source/sink pair to simulate the roughness element. Any other suggestions?

-- Joshua
  Reply With Quote

Old   May 26, 2008, 12:00
Default Re: Can I turn an element "off" in CFX?
  #6
CycLone
Guest
 
Posts: n/a
Hi Joshua,

You should model the roughness in the geometry by creating a pattern and meshing it appropriately.

Note that just turning off an element wouldn't be enough in this case, as you would need more than a single element around the roughness to resolve how the fluid reacts to the roughness. If you intend to do Direct Numerical Simulation, this is the only way, otherwise you are introducing some form of modeling.

Of course if you aren't doing DNS, then the wall roughness option in the turbulence model can do the trick.

-CycLone
  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
discretizer - gmshToFoam Andyjoe Open Source Meshers: Gmsh, Netgen, CGNS, ... 13 March 14, 2012 05:35
ATTENTION! Reliability problems in CFX 5.7 Joseph CFX 14 April 20, 2010 15:45
mesh file for flow over a circular cylinder Ardalan Main CFD Forum 6 April 17, 2010 23:40
Importing solutions in CFX. Alphonso CFX 1 August 1, 2008 14:01
CFX 10's solutions differ from CFX 5.7's Atit Koonsrisuk CFX 4 July 26, 2006 11:59


All times are GMT -4. The time now is 18:33.