Non-uniform velocity at outlet when using mass flow rate boundary condition

Julian121 · June 9, 2019, 09:52

I am applying total pressure and total temperature boundary conditions at inlet to solve a problem. At outlet, I am using mass flow rate boundary condition.

Although I have applied the mass flow rate at the outlet, the velocity at the outlet is not uniform.

At the inlet, the velocity is uniform.

Is there a problem at the outlet?

ghorrocks · June 9, 2019, 20:43

Your inlet graph shows the velocity is 16m/s or above. It does not say it is uniform.

The mass flow outlet condition has a number of different settings, but by default it does not apply uniform velocity across the outlet. Have a look at your mass flow outlet setup and change the option if you want it to be uniform. But I suggest this is not a problem, if the flow wants to be uneven then it probably is uneven and you should let it do that.

karachun · June 10, 2019, 03:14

Maybe this link give some clarity.
https://en.wikipedia.org/wiki/Entrance_length
It will be strange if flow remain uniform in channel IRL.

Julian121 · June 10, 2019, 03:14

In a transient simulation at any instant, should the mass flow at inlet be equal to mass flow at outlet to satisfy the conservation of mass?

Opaque · June 10, 2019, 07:39

Depends:
1 - Are you working with an incompressible fluid? YES
2 - Are you working with a compressible fluid? NO

June 9, 2019, 20:43		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Your inlet graph shows the velocity is 16m/s or above. It does not say it is uniform. The mass flow outlet condition has a number of different settings, but by default it does not apply uniform velocity across the outlet. Have a look at your mass flow outlet setup and change the option if you want it to be uniform. But I suggest this is not a problem, if the flow wants to be uneven then it probably is uneven and you should let it do that. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

June 10, 2019, 03:14		#3
karachun Senior Member Join Date: Nov 2015 Posts: 244 Rep Power: 11	Maybe this link give some clarity. https://en.wikipedia.org/wiki/Entrance_length It will be strange if flow remain uniform in channel IRL. Julian121 likes this.

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June 10, 2019, 03:14		#4
Julian121 Senior Member Join Date: Aug 2012 Posts: 268 Rep Power: 14	In a transient simulation at any instant, should the mass flow at inlet be equal to mass flow at outlet to satisfy the conservation of mass?

June 10, 2019, 07:39		#5
Opaque Senior Member Join Date: Jun 2009 Posts: 1,804 Rep Power: 32	Depends: 1 - Are you working with an incompressible fluid? YES 2 - Are you working with a compressible fluid? NO