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

stochastic model

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

Reply
 
LinkBack Thread Tools Display Modes
Old   June 28, 2001, 10:27
Default stochastic model
  #1
Alex
Guest
 
Posts: n/a
Hi everybody, does anybody know how fluent tracks a particle using the stochastic model?what I mean is,what equations are used to track the particle? force balance? how do we know if the particle passed one eddy and is moving on to the next one. if we're using the force balance equ., what is the calculated velocity?instantenous velocity?I'm very much confused. I looked at lots of literatures but I got more confused?!!! thanks in advance alex
  Reply With Quote

Old   June 28, 2001, 13:47
Default Re: stochastic model
  #2
Lanre
Guest
 
Posts: n/a
A momentum balance on a particle sets up the equation of motion for the particles solved by FLUENT, i.e., mass of particle x acceleration of particle = SUM (forces acting on particle).

The drag force term contains the relative velocity (U_f-U_p) where U_f is mean fluid velocity. However, to model influence of turbulence on the particle U_f is decomposed into u_bar + u'. The instantaneous velocity u' is computed from the local tke in the cell => turbulence intensity, and a randomly generated number (since u' can fluctuate about u_bar). The eddy lifetime, or lifetime of exposure to u', is based on a characteristic time constant (or variable).
  Reply With Quote

Old   June 28, 2001, 14:14
Default Re: stochastic model
  #3
Alex
Guest
 
Posts: n/a
thanx Lanre, I understand it much better now.in fluent's mannual I found these explanations : "The instantaneous characteristics of the gas are computed by adding random fluctuations to the mean flow solution from the k-e simulation. Thus, U = Ubar+Cu' where, u'=(2/3k)^1/2 equation 1 where c is random between 1 and -1." do you think there is any mistakes there?I guess it's Ubar that can be calculated from equ. 1.also, you mentioned u' can be calculated from turbulent intensity and randomly generated number.which one is correct?!!! alex

  Reply With Quote

Old   June 30, 2001, 08:47
Default Re: stochastic model
  #4
Alain
Guest
 
Posts: n/a
Ubar come from the velocity field calculated by time averaged NS equation.

u' is an instataneaous fluctuation which is evaluated from turbulent quantities (basicly eddies lenght scale and turbulent kinetic energy).

Try to have a look at "multiphase flows with droplets and particles", C. CROWE and al., CRC press,1998

Best regards

Alain
  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
Centrifugal Pump and Turbulence Model Michiel CFX 12 January 25, 2010 04:20
Problems bout CFD model of biomass gasification, Downdraft gasifier wanglong FLUENT 2 November 26, 2009 00:27
Reynolds Stress model in CFX vs Fluent Tim CFX 1 October 7, 2009 06:19
help for different between les model (subgrid-scale model) liuyuxuan FLUENT 1 October 2, 2009 15:25
Grid resolution for full-scale and down scaled model gravis Main CFD Forum 0 October 2, 2009 10:27


All times are GMT -4. The time now is 05:03.