# uniform mass fraction throughout domain

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 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 1e-20 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

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