inflow into closed container

ehooi · June 10, 2011, 04:36

Hi,
Is it possible to model, say a cylindrical domain where one one end, there is an inlet with a diameter smaller than the cylinder. At this end, the velocity inlet boundary condition is prescribed. The rest of the wall is assumed to be stationary no slip wall.

In this case, since we have an inlet flow into a closed domain, can the conservation of mass equation still be satisfied? I tried solving this in FLUENT and the solution converges. I am wary of its solution because intuition tells me that conservation of mass is not satisfied since there is no outflow to balance the inflow. Is the solution of FLUENT correct?

Thanks.

Sincerely,
EH

Ford Prefect · June 10, 2011, 06:40

Incompressible?

ehooi · June 10, 2011, 07:57

Yes, it is solved with the k-e model.

Ford Prefect · June 10, 2011, 08:57

Quote:

Originally Posted by ehooi

Yes, it is solved with the k-e model.

I assume that you mean "Yes and it is solved with the k-e model.", hoping this is not a troll attempt.

Anyways, to your main question: No it should not work in this case, and I assume that your solution has not reached convergence and that you have terminated it before it blew up.

Cheers!

ehooi · June 10, 2011, 09:01

Quote:

Originally Posted by Ford Prefect

I assume that you mean "Yes and it is solved with the k-e model.", hoping this is not a troll attempt.

Anyways, to your main question: No it should not work in this case, and I assume that your solution has not reached convergence and that you have terminated it before it blew up.

Cheers!

Well, I am solving a transient problem and my continuity criteria was set at 1e-6. After about 60s, the residual for continuity, velocities, k an e are less than 1e-6. When I plotted the contour for residual for mass imbalance, it is of the order of 10-9.

Ford Prefect · June 10, 2011, 09:49

If you calculate the mass flux across all boundaries what do you get?

ehooi · June 10, 2011, 10:11

I assumed you meant mass flow rate? Well, the mass flow rate at all the walls are zero except for the inlet where a value of 0.015kg/s was computed.

I guess it means there is a 0.015kg/s flow into the domain...but how is it that convergent is satisfied? This is puzzling. By the way, the mass flow rate at the interior calculated from fluent is -0.18kg/s...does this imply that mass vanishes somehow at the interior?

Ford Prefect · June 10, 2011, 12:18

Ok, I did a quick test with Fluent and I get the same results as you. However when I try the same in COMSOL and PHOENICS the solution diverges as expected. I have no good explanation as to why this works in Fluent. I will think more on the matter during the weekend. A cold beer is a good catalyst for "problems" like this

Cheers!

ehooi · June 13, 2011, 02:25

Quote:

Originally Posted by Ford Prefect

Ok, I did a quick test with Fluent and I get the same results as you. However when I try the same in COMSOL and PHOENICS the solution diverges as expected. I have no good explanation as to why this works in Fluent. I will think more on the matter during the weekend. A cold beer is a good catalyst for "problems" like this

Cheers!

Hi,
If you happen to check the mass flow rate on the boundary, there is a positive flow rate into the container. However, if you calculate the mass flow rate at the interior as well, then there is a sink, which is somehow greater than the net mass flow rate at the boundary (at least in my model). Really puzzling...

Ford Prefect · June 13, 2011, 06:03

Quote:

Originally Posted by ehooi

Hi,
If you happen to check the mass flow rate on the boundary, there is a positive flow rate into the container. However, if you calculate the mass flow rate at the interior as well, then there is a sink, which is somehow greater than the net mass flow rate at the boundary (at least in my model). Really puzzling...

Yes it is, however as I see it the net flow rate is unaffected by the value in the interior. Strange.

June 10, 2011, 04:36	inflow into closed container	#1
ehooi Member EH Join Date: Dec 2010 Posts: 61 Rep Power: 16	Hi, Is it possible to model, say a cylindrical domain where one one end, there is an inlet with a diameter smaller than the cylinder. At this end, the velocity inlet boundary condition is prescribed. The rest of the wall is assumed to be stationary no slip wall. In this case, since we have an inlet flow into a closed domain, can the conservation of mass equation still be satisfied? I tried solving this in FLUENT and the solution converges. I am wary of its solution because intuition tells me that conservation of mass is not satisfied since there is no outflow to balance the inflow. Is the solution of FLUENT correct? Thanks. Sincerely, EH

June 10, 2011, 06:40		#2
Ford Prefect Senior Member Join Date: Mar 2009 Posts: 157 Rep Power: 17	Incompressible? __________________ "Trying is the first step to failure." - Homer Simpson

June 10, 2011, 09:49		#6
Ford Prefect Senior Member Join Date: Mar 2009 Posts: 157 Rep Power: 17	If you calculate the mass flux across all boundaries what do you get? __________________ "Trying is the first step to failure." - Homer Simpson

June 10, 2011, 12:18		#8
Ford Prefect Senior Member Join Date: Mar 2009 Posts: 157 Rep Power: 17	Ok, I did a quick test with Fluent and I get the same results as you. However when I try the same in COMSOL and PHOENICS the solution diverges as expected. I have no good explanation as to why this works in Fluent. I will think more on the matter during the weekend. A cold beer is a good catalyst for "problems" like this Cheers! __________________ "Trying is the first step to failure." - Homer Simpson

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June 10, 2011, 07:57		#3
ehooi Member EH Join Date: Dec 2010 Posts: 61 Rep Power: 16	Yes, it is solved with the k-e model.

June 10, 2011, 10:11		#7
ehooi Member EH Join Date: Dec 2010 Posts: 61 Rep Power: 16	I assumed you meant mass flow rate? Well, the mass flow rate at all the walls are zero except for the inlet where a value of 0.015kg/s was computed. I guess it means there is a 0.015kg/s flow into the domain...but how is it that convergent is satisfied? This is puzzling. By the way, the mass flow rate at the interior calculated from fluent is -0.18kg/s...does this imply that mass vanishes somehow at the interior?