
[Sponsors] 
November 2, 2010, 23:51 
uniform mass fraction throughout domain

#1 
New Member
ranjith
Join Date: Dec 2009
Posts: 19
Rep Power: 7 
Hi,
I am simulating the mixing of two fluids with different viscosities. I am doing a transient analysis in which the domain is initially filled with fluid A and then gradually fluid B enters the domain and mixing takes place. The problem is that when I look at the results in post, the mass fraction is uniform through out the domain. There is no variation with respect to space. But there is variation with respect to time. I am expecting a variation with respect to both time and space. any ideas? ranjith 

November 3, 2010, 02:29 

#2 
Member
Ali Torbaty
Join Date: Jul 2009
Location: Sydney, Australia
Posts: 71
Rep Power: 8 
assuming you have fluids with different densities, using isoVolume with density variable you should be able to track fluids' mixing.


November 4, 2010, 01:16 

#3 
New Member
ranjith
Join Date: Dec 2009
Posts: 19
Rep Power: 7 
Thanks ali for ur response.....but I am still not able to visualize the mixing. The properties are apparently calculated in bulk for the whole domain. I getting a uniform mass fraction and uniform density throughout the domain..


November 4, 2010, 01:42 

#4 
Member
Ali Torbaty
Join Date: Jul 2009
Location: Sydney, Australia
Posts: 71
Rep Power: 8 
if your fluids get mixing (like brine and water) and model supposed to have a range of densities, I suggest you to follow the Tutorial about Reacting Flow in Mixing Tube, your case might be simpler but you have to follow the same method to create a multicomponent fluid which describes the mixture.


November 4, 2010, 04:38 

#5 
New Member
ranjith
Join Date: Dec 2009
Posts: 19
Rep Power: 7 
I did create a multicomponent fluid, in fact my fluid is a variable composition mixture of components A and B to form a mixture C. A has a mass fraction 1 initially in the domain and in the fluid(C) entering the domain B has a mass fraction 1. I then do a transient simulation with total time equal to the time taken by the fluid to traverse the length of the domain


November 4, 2010, 06:25 

#6 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,662
Rep Power: 84 
If you set the initial mass fraction as 1 and inject stuff with a mass fraction of 1 into the domain then nothing will happen. As Ali suggests, have a look at the tutorial examples for how to set up multicomponent models. I think there is a CO2/air tutorial which will also be of relevance for you.


November 4, 2010, 06:26 

#7 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,662
Rep Power: 84 
If the initial mass fraction is 1 then you have to inject stuff with a mass fraction of 0 (or anything other than 1) to get anything to happen.


November 5, 2010, 23:55 

#8 
New Member
ranjith
Join Date: Dec 2009
Posts: 19
Rep Power: 7 
Thanks Glen for ur response, but u have got me wrong. Initially the mass fraction is 1 for component A(0 for component B). The fluid entering the domain has mass fraction 1 for component B(0 for component A)....and I have taken total time equal to the time taken by the fluid to traverse through the domain.


November 6, 2010, 07:17 

#9 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,662
Rep Power: 84 
If you have set the diffusivity too high it will rapidly diffuse over the entire domain. Are you sure the diffusivity is correct?


November 7, 2010, 06:15 

#10 
Member
Ali Torbaty
Join Date: Jul 2009
Location: Sydney, Australia
Posts: 71
Rep Power: 8 
just for check, you should use the Multicomponent C material over your boundaries and then define the subcomponents fraction. you should not use the components A and B directly over the boundaries.


November 10, 2010, 06:50 

#11 
New Member
ranjith
Join Date: Dec 2009
Posts: 19
Rep Power: 7 
@glen I have checked with different diffusivity values but I am getting the same results...
@ali at inlet I have defined multicomponent fluid with mass fractions. my definition is correct 

November 10, 2010, 17:33 

#12 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,662
Rep Power: 84 
Your diffusivity could be out by orders of magnitude. Small changes will then have no effect. Try a diffusivity of 1e20 and see if that makes a difference.


November 11, 2010, 00:39 

#13 
New Member
ranjith
Join Date: Dec 2009
Posts: 19
Rep Power: 7 
Thankyou Glenn!! you are absolutely right. I am getting the mixing pattern now... Thank you very much


Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Free jet simulation  msarkar  OpenFOAM Running, Solving & CFD  33  January 7, 2015 23:10 
Please Help.....Insufficient Catalogue Size  Paresh Jain  CFX  17  March 11, 2014 12:40 
Domain Reference Pressure and mass flow inlet boundary  AdidaKK  CFX  12  February 11, 2010 21:28 
I need UDF help.  S.Whitney  FLUENT  0  October 15, 2007 11:29 
Mass fraction of Species  Chris  FLUENT  0  November 20, 2000 14:09 