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

Laminar model the same as DNS?

Register Blogs Community New Posts Updated Threads Search

Like Tree1Likes
  • 1 Post By Kerrin

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   February 20, 2003, 15:59
Default Laminar model the same as DNS?
  #1
Kerrin
Guest
 
Posts: n/a
Please forgive me for asking a question which may be obvious to many of you, but:

Does a DNS simulation solve exactly the same equations as those used in laminar flow simulations? I have been trying to figure this out from the textbook but couldn't quite pin it down.

Thanks,

Kerrin
bhanuday.sharma likes this.
  Reply With Quote

Old   February 22, 2003, 00:05
Default Re: Laminar model the same as DNS?
  #2
versi
Guest
 
Posts: n/a
I did not do DNS. But I figure out the equations are the same, except that DNS use huge number of grids to resolve turbulent scales, and the solutions are not always steady state, yet you can make certain kind of statistics and average in time to obtain meaningful results.
  Reply With Quote

Old   February 22, 2003, 08:15
Default Re: Laminar model the same as DNS?
  #3
gorka
Guest
 
Posts: n/a
I've never performed any DNS simulation but as far I know the solved equations are EXACTLY the same: Navier-Stokes equations and continuity equation (+ energy equation if needed).

NO extra term is added, neither any physical modelling assumed. (In comparison with Reynolds Averaging approach)

Hope this helps

Gorka
  Reply With Quote

Old   February 22, 2003, 10:10
Default Re: Laminar model the same as DNS?
  #4
Holidays
Guest
 
Posts: n/a
The main problem with DNS is that the number of nodes that is necessary is roughly proportional to the turbulent Re at the power of 3 (turbulent Re ~1% of Re) per turbulent recirculation length unit. Hence the cost in computer power, time (the time step needs be small on top of the grid size issue) and storage! Otherwise the equation are even, in principle, simpler to solve than the RANS.
  Reply With Quote

Old   February 23, 2003, 15:59
Default Re: Laminar model the same as DNS?
  #5
Kerrin
Guest
 
Posts: n/a
Thanks for your replies. I had a suspician that the equations were the same but was not sure.

If I was solving on a single workstation (ie limited computing power), I imagine that a normal RANS simulation would produce more accurate results than a DNS simulation (with far too few mesh points). I guess what I am asking is: Is there any point using a DNS simulation (with too few mesh points) to check the results of a turbulence model? Or is that just a waste of time?

The reason I ask is that another CFD user said they were doing just this to check whether their RANS solutions were correct. This sounded wrong to me.

Any thoughts?

Kerrin
  Reply With Quote

Old   February 24, 2003, 04:51
Default Re: Laminar model the same as DNS?
  #6
Tom
Guest
 
Posts: n/a
For a good DNS you need to solve the full three-dimensional (!) and time dependent Navier-Stokes equations (no terms neglected!). The time step has to be really small (CFL number around one) because you need to resolve all the rapid fluctuations in time. In general turbulent kinetic energy will be produced in the flow and this has to be taken away by dissipation otherwise it will accumulate. So need to resolve the small dissipative scales. In many practical applications the Reynolds number is so high (and the dissipative scales so small) that it is impossible to perform a full DNS, especially with a slow commercial CFD code. So I guess that 'they' are doing a very poorly resolved DNS and that the dissipative scales are absolutely not resolved. A good code would just blow up because of the accumulation of energy. That they can do a simulation means that the computations are somehow stabilized with a lot of numerical viscosity. You can say that they are doing some kind of large eddy simulation poorly defined and probably poorly resolved large eddy simulation (LES). The sub-grid model is the numerical viscosity added by the numerical scheme. I guess that you can use these kind of computation in the best case just to get a very global idea about the flow patterns but absolutely not for any kind of quantitave check of the RANS results.
  Reply With Quote

Old   February 24, 2003, 16:12
Default Re: Laminar model the same as DNS?
  #7
Kerrin
Guest
 
Posts: n/a
Hi Tom,

Thanks for helping to clarify this for me. I had suspicians along the line of what you had said but wanted to check it out with someone else.

Kerrin
  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
Can I use turbulent model to solve a laminar flow? nikhil FLUENT 5 February 1, 2011 11:42
Low Reynolds k-epsilon model YJZ ANSYS 1 August 20, 2010 14:57
DNS always returns to laminar initial solution kat Main CFD Forum 10 September 9, 2007 10:20
Laminar flow in species transport model? Neha Rao FLUENT 7 February 25, 2006 03:23
Premixed combustion -Laminar finite rate model Shankar FLUENT 4 March 17, 2004 16:32


All times are GMT -4. The time now is 14:39.