CFD Online URL
[Sponsors]
Home > Forums > CFX

Schmidt Number setting in Buoyance model

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

Like Tree1Likes
  • 1 Post By ghorrocks

Reply
 
LinkBack Thread Tools Display Modes
Old   March 18, 2012, 04:48
Default Schmidt Number setting in Buoyance model
  #1
New Member
 
Joe
Join Date: Aug 2011
Posts: 14
Rep Power: 5
qsx4881 is on a distinguished road
Dear all,
I am simulating thermal stratification using k-e model with full buoyancy. But I am not sure how to set the buoyancy turbulence term in the Fluid models/Buoyancy Turbulence. The default setting is none, while other choices are production, production and dissipation, see attachment.

I have run some cases with the default setting and production and dissipation (Turbulent Schmidt Number=1.0 and Dissipation Coefficient=1.0), but the results really show big differences. For example, the maximum temperature difference through cross-sections is bigger while use production and dissipation. It seems that the Schmidt number and dissipation coefficient make the thermal stratification more seriously, see attachment. Besides, the Schmidt number also make the simulation difficult to converge,

So my question is what the influence of Schmidt number in buoyancy simulation? Does it relate to convection and buoyance? Which result is reasonable? Should I select production and dissipation?

Thanks in advance! Look forward to your replay.
Attached Images
File Type: gif 12.gif (38.4 KB, 12 views)
File Type: jpg 2.jpg (50.4 KB, 18 views)

Last edited by qsx4881; March 18, 2012 at 06:10.
qsx4881 is offline   Reply With Quote

Old   March 18, 2012, 07:20
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 9,863
Rep Power: 78
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
I do not know those parameters in detail as they are of no importance to most simulations. After a quick read of the CFX theory manual they introduce new terms in to the production and dissipation equations to deal with buoyant effects.

If your simulation has found that they are important (and your results suggest they are) then you better do some reading of the CFX theory manual and the references quoted in it. You will have to understand these parameters to know how to apply them properly.

My suggestion is you probably want to activate this option and leave the Schmidt numbers at the default values. I would also compare results to the SST model which is a good baseline turbulence model. And hopefully you have some experimental results to compare to so you know if you are going in the right direction. If not you need a good benchmark case to compare against. And don't forget to do a sensitivity analysis on mesh, time step size and convergence tolerance so you know you are not kidding yourself.
happy likes this.
ghorrocks is offline   Reply With Quote

Old   March 18, 2012, 13:05
Default
  #3
New Member
 
Joe
Join Date: Aug 2011
Posts: 14
Rep Power: 5
qsx4881 is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
I do not know those parameters in detail as they are of no importance to most simulations. After a quick read of the CFX theory manual they introduce new terms in to the production and dissipation equations to deal with buoyant effects.

If your simulation has found that they are important (and your results suggest they are) then you better do some reading of the CFX theory manual and the references quoted in it. You will have to understand these parameters to know how to apply them properly.

My suggestion is you probably want to activate this option and leave the Schmidt numbers at the default values. I would also compare results to the SST model which is a good baseline turbulence model. And hopefully you have some experimental results to compare to so you know if you are going in the right direction. If not you need a good benchmark case to compare against. And don't forget to do a sensitivity analysis on mesh, time step size and convergence tolerance so you know you are not kidding yourself.
Glenn,thank you for your earnest answer.

Just as you said, I want to active the option. I have read the theory manual, it doesn't help. In related papers, some active the option while others not.Professor Abdus Salam give a lector named "The.State.of.Turbulent.Mixing.and.Future.Directio ns.for.Research" http://v.163.com/movie/2010/7/I/A/M6...M6G40T8IA.html refers to Schmidt number, but I haven't understand. It really confused me.

Your suggestion almost include all possibilities, I am going to try. I have refined the mesh and tried little timestep (0.1s, now 0.5s). It seems doesn't matter. We are preparing the experiment but it need some time. So now I will find a benchmark to verify the option.

Thanks again.
qsx4881 is offline   Reply With Quote

Old   March 18, 2012, 18:57
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 9,863
Rep Power: 78
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Schmidt number is a key concept in turbulence modelling and there are several Schmidt numbers in the model. I recommend you get a good turbulence modelling textbook (eg "Turbulence Modelling for CFD" by Wilcox) and you get some background in this area.
ghorrocks is offline   Reply With Quote

Reply

Tags
schmidt number.buoyance

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
How to modify the Schmidt number in CFX for additional variables model? rockzh CFX 11 February 10, 2012 21:36
Dispersion Prandtl Number in Mixture Mutliphase Model ajayanthi42 Main CFD Forum 0 October 25, 2010 19:59
Schmidt number col16 CFX 4 March 4, 2008 18:39
CCL for changing the Schmidt number Andrew CFX 2 July 3, 2006 19:42
a low Re number model for impinging jets mahesh prakash Main CFD Forum 14 September 3, 1999 17:40


All times are GMT -4. The time now is 21:30.