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

mixing length

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

Reply
 
LinkBack Thread Tools Display Modes
Old   July 24, 2002, 04:44
Default mixing length
  #1
mukkarum
Guest
 
Posts: n/a
I got the sentence from w. Rodi's book TURBULENCE MODELS AND THEIR APPLICATION IN HYDRAULICS that " eddy viscosity proportional to a velocity characterizing the fluctuation motion and to a typical lenth of this motion which prandlt called mixing length" i want to know what is the meaning of velocity characterizing and typical length and mixing length.
  Reply With Quote

Old   July 24, 2002, 05:53
Default Re: mixing length
  #2
h.jordi
Guest
 
Posts: n/a
unit of viscosity are m2/s. this therefore translates to being proportional to a length, m * a velocity, m/s with added constants.

therefore to quantify the additional stresses caused by the tubulent motion (reynold stresses) we must choose an appropriate viscosty and as seen above this will be found from choosing an apprapriate length and velocity scale for the turbulent motion.

the velocity scale is often chosen as the sqrt(of turbulnt energy) so a tranport equation can be solved for KEt.

the length sclae is more difficult to choose as high reynolds nuber turbulence has a range of scales present. we therefore usually choose the integral scale which is the size of the largest eddies. (the justification of choosing the largest sclaes is based on the idea of an energy cascade where the small scale motions is determined by the energy passed to them from the largest scales).

the term mixing length originates from prandtls orginal model, where a velocity gradient dudy, over a small distance l, is said to cause fluctuations of order l*dudy. the fluctuatons cause momentum transfer and serve to mix the flow.

hope this helps.
  Reply With Quote

Old   July 24, 2002, 23:07
Default Re: mixing length
  #3
mukkarum
Guest
 
Posts: n/a
What you mean by dudy and energy cascade
  Reply With Quote

Old   July 25, 2002, 04:41
Default Re: mixing length
  #4
h.jordi
Guest
 
Posts: n/a
you should look in a turbulence book at the ideas of kolmogorov and the -5/3 law - this will explain the energy cascade. it is basically the idea that large turbulent scales will pass energy to smaller scales and so on, unitl viscosity dissipates the smallest scales. (one of the best books is by tennekes and lumley).

by dudy i meant the velocity gradient, i.e. change in velocity / change in distance.
  Reply With Quote

Old   July 25, 2002, 05:16
Default Re: mixing length
  #5
mukkarum
Guest
 
Posts: n/a
i will contect you after studyind books within 10 days may i have ur email address thanks for help
  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
How to define the turbulence intensity and mixing length at an outlet (for k-eps)? david39 OpenFOAM Running, Solving & CFD 6 February 2, 2011 04:01
Turbulence mixing length value eva Main CFD Forum 1 February 27, 2009 15:19
Find the mixing length using FLUENT (Heeeelp!!!) Park.C.Y FLUENT 0 May 8, 2004 09:00
Prandtl mixing length vs Isotropic assumption Mohammad Kermani Main CFD Forum 1 March 26, 2000 15:47
Prandtl Mixing length Vs. Isotropic turbulent Mohammad Kermani Main CFD Forum 0 March 24, 2000 00:06


All times are GMT -4. The time now is 02:21.