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

why first grid point is at log layer in ke model

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   May 12, 2007, 01:41
Default why first grid point is at log layer in ke model
  #1
student
Guest
 
Posts: n/a
sir, why we cannot go upto the wall when applying std ke model..what will happen if we apply.. how low Re ke model eliminates this defect. ie when applying Low Re ke model we can go up the wall. how? plz expalin thanks
  Reply With Quote

Old   May 12, 2007, 08:30
Default Re: why first grid point is at log layer in ke mod
  #2
Anantha
Guest
 
Posts: n/a
Hello,

In the k-e model, when you write the modeled k and e equations, you have few constants with the production and dissipation terms of the epsilon equation. The values are calculated based on some experiments and are valid only in the log law region. This is one reason. More important is, when you see some terms (consider production term) in the modeled 'k' equation and exact 'k' equation, the behavior will not be same as we go close to the wall in your usual k-e equation. In fact there is one order of magnitude difference. To solve this usually damping functions are used such that even the modeled terms behave as same as the exact terms. The models with these damping functions are referred as Low Re Number models.

Regards Anantha
  Reply With Quote

Old   May 12, 2007, 09:20
Default Re: why first grid point is at log layer in ke mod
  #3
student
Guest
 
Posts: n/a
Helo Anantha..thank you very much for your brief expalnation. but the models with damping functions termes as Low Re model..what Low Re exactly represents. whether this model is applied only low turbulent reynolds no..
  Reply With Quote

Old   May 12, 2007, 09:22
Default Re: why first grid point is at log layer in ke mod
  #4
student
Guest
 
Posts: n/a
slight correction in previous question. sori for that..

Helo Anantha..thank you very much for your brief expalnation. but why models with damping functions termed as Low Re model..what Low Re exactly represents. whether this model is applied only low turbulent reynolds no..

  Reply With Quote

Old   May 12, 2007, 09:26
Default Re: why first grid point is at log layer in ke mod
  #5
Anantha
Guest
 
Posts: n/a
Hello,

The Low Re number does not mean that it is applicable only for the flows with very low over all Re. It refers to the local Re. As you go close to the wall, due to no slip condition, velocity goes to zero rapidly. As I mentioned earlier the damping functions take care of the behavior of each terms in the transport equations. But far away from the wall when Re is high, the effect of damping functions won't be there (damping functions are designed so). Thus it means that these models are good in the entire domain and is not restricted to low turb Re areas. But the difficulty is you need to resolve your grid with a y+ close to 1.

Regards Anantha
  Reply With Quote

Old   May 13, 2007, 00:38
Default Re: why first grid point is at log layer in ke mod
  #6
student
Guest
 
Posts: n/a
thankyou very much Anantha. can i have your mail id.. if i have further doubts i can clarify with you.

  Reply With Quote

Old   May 13, 2007, 13:25
Default Re: why first grid point is at log layer in ke mod
  #7
Anantha
Guest
 
Posts: n/a
Hello, The above email link will do I suppose... you are welcome to send me emails and what ever help I can offer, i will definitely do. All the best.
  Reply With Quote

Reply


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
Water subcooled boiling Attesz CFX 7 January 5, 2013 04:32
Grid independence and prismatic layer, a doubt franco-bosi Main CFD Forum 0 December 15, 2010 12:53
udf to set vel. = 0 at a grid point or a cell Ralf Schmidt FLUENT 0 September 22, 2006 10:45
UDF to define source term at every grid point Ralf Schmidt FLUENT 0 September 21, 2006 12:32
errors Fahad Main CFD Forum 0 March 23, 2004 14:20


All times are GMT -4. The time now is 06:27.