Help!!!Cfx modeling Volume Fraction Keeps Changing

Night · January 4, 2014, 09:24

Hi all,

For cfx simulation.

I'm trying to set up a model which is a cup of fuel and air in a rotating frame. I chose my materials to be air@25c and c8h18l(all has constant density). And I set the volume fraction in the domain initialization to be air:0.6, c8h18l:0.4.

I only used no slip wall(bottom and side) and 0atm total pressure(reference pressure 1 atm)opening(top) as boundary conditions so I expect to see the volume fraction to be constant all the time. But in the results I was super surprised to find the volume fraction changes with time(I used the transient state simulation). I went back to cfx-pre and added a monitor point volumeAve(Fuel.vf)@Tank and ran the solver again and got the same result. The fuel's volume function starts from around .25 instead of .4 and it drops steeply to .03 and then gradually rises to about .28 and stays around that value till the end.

My question is why??????? is the volume fraction changing????????Why doesn't it start from the initial value I set it to be and stay there?????? Help I'm so confused right now

ghorrocks · January 12, 2014, 06:10

It could be:

a) Your physics is wrong - you have selected physics which allows the volume fraction to decrease
b) Your physics is correct but your numerics is inaccurate - then weird things can happen as the numerics is generating rubbish.

Can you post an image of what you are modelling and your CCL?

January 4, 2014, 09:24	Help!!!Cfx modeling Volume Fraction Keeps Changing	#1
Night New Member Join Date: Jan 2014 Posts: 2 Rep Power: 0	Hi all, For cfx simulation. I'm trying to set up a model which is a cup of fuel and air in a rotating frame. I chose my materials to be air@25c and c8h18l(all has constant density). And I set the volume fraction in the domain initialization to be air:0.6, c8h18l:0.4. I only used no slip wall(bottom and side) and 0atm total pressure(reference pressure 1 atm)opening(top) as boundary conditions so I expect to see the volume fraction to be constant all the time. But in the results I was super surprised to find the volume fraction changes with time(I used the transient state simulation). I went back to cfx-pre and added a monitor point volumeAve(Fuel.vf)@Tank and ran the solver again and got the same result. The fuel's volume function starts from around .25 instead of .4 and it drops steeply to .03 and then gradually rises to about .28 and stays around that value till the end. My question is why??????? is the volume fraction changing????????Why doesn't it start from the initial value I set it to be and stay there?????? Help I'm so confused right now

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January 12, 2014, 06:10		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	It could be: a) Your physics is wrong - you have selected physics which allows the volume fraction to decrease b) Your physics is correct but your numerics is inaccurate - then weird things can happen as the numerics is generating rubbish. Can you post an image of what you are modelling and your CCL?