
[Sponsors] 
October 28, 2010, 21:13 
difference between isotropic and homogeneous in turbulence

#1 
New Member
Join Date: Feb 2010
Posts: 12
Rep Power: 8 
Some literature wrote that 'One might argue that no real turbulent flow is isotropic or even homogeneous in the large scales.'
It sounds that homogeneous is stricter than isotropic! SO who can give me any hints about the difference between isotropic and homogeneous? Any comments will be appreciated. 

October 29, 2010, 01:10 

#2 
Member
ganesh
Join Date: Mar 2009
Posts: 40
Rep Power: 9 
Dear Dut_thinker,
Isotropic turbulence demands that there is no mean shear, rotation or buoyancy effects in the flow as this can lead to anisotropy. Homogeneous turbulence is indicative of the fact that there are no mean flow gradients. In a more simpler sense, homogeneity deals with invariance in translation, isotropy deals with invariance in rotation. Take a look at MIT OCW notes on turbulence if you wish for more information. Regards, Ganesh 

October 29, 2010, 13:26 

#3 
Senior Member
Join Date: Apr 2009
Posts: 129
Rep Power: 9 

October 29, 2010, 13:48 

#4 
Senior Member
Join Date: Feb 2010
Posts: 145
Rep Power: 9 
Thanks fw! This link is very helpful! I'm typing the information below in case the book disappears.
In homogeneous turbulence, the fluctuating velocity field u(x,t) is statistically homogeneous. It is consistent with this definition for the mean velocity gradients d(Ui)dxj to be nonzero but uniform. A good approximation to homogeneous turbulence can be achieved in windtunnel experiments and homogeneous turbulence is the simplest class of flows to study using DNS. A statistically homoegeous field U(x,t) is, by definition, statistically invariant under translations (i.e. shifts in the origin of the coordinate system). If the field is also statistically invariant under rotations and reflections of the coordinate system, then it is (statistically) isotropic. Reference: Turbulent Flows by S. B. Pope 

October 30, 2010, 23:34 

#5 
New Member
Join Date: Oct 2010
Posts: 7
Rep Power: 7 
An isotropic turbulent flow as other members have already replied, is basically a turbulent flow where the time averaged turbulent quantities ( like u r.m.s.) have the same value at each and every location.
A homogeneous flow on the other hand is one where the turbulent quantities at any given location are the same in all the directions. (ex. u r.m.s = v r.m.s.) example a concrete wall may be isotropic if the concentration of steel and cement is the same at all locations in the wall. But it is not homogeneous because at any point, there may be steel in one direction and cement in the other direction. Of course it is actually impossible to get either homogeneous or isotropic turbulence in a wind tunnel. But you can get close to it and ppl want to get close to it cause theoretically a homogeneous isotropic turbulence is very attractive. And ya it is much more impossible to get actual 100% isortopic turbulence cause turbulent flows are by definition dissipative. So the turbulent quantities ( like u r.m.s.) will decay into heat energy with time. Unless you the right amount of energy at the right time it will not be isotropic. 

January 2, 2013, 00:11 
Mistake maybe?!

#6 
New Member
Join Date: Jan 2013
Posts: 1
Rep Power: 0 
simulationman, Is definition of homogeneous and isotropic in your answer switched maybe?


January 2, 2013, 12:10 

#7 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,374
Rep Power: 30 
just to make an example, often the turbulence in channel flow is simulated, the test is composed by two parallel plates that are replicated by periodic boundary conditions. The flow is substained by a forcing pressure gradient.
This flow is simultaneously homogeneous along the two directions of periodicity and inhomogeneous in the direction normal to walls 

November 8, 2015, 00:44 
Update?

#8 
New Member
Nick
Join Date: Oct 2013
Posts: 2
Rep Power: 0 
Homogeneous turbulence: invariance in translation
Isotropic turbulence: invariance in rotation 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
axisymmetric homogeneous isotropic turbulence  RAJANI  Main CFD Forum  1  April 24, 2006 06:44 
the homogeneous isotrotic compressible turbulence.  loong  Main CFD Forum  0  March 30, 2006 00:18 
Help needed on homogeneous isotropic turbulence an  Guoping Xia  Main CFD Forum  0  March 12, 2006 22:54 
ISOTROPIC HOMOGENEOUS TURBULENCE  Valdemir  Main CFD Forum  2  September 1, 2003 23:04 
isotropic homogeneous turbulence  tingguang  Main CFD Forum  0  August 1, 2002 17:08 