Mixing chamber - 2 inlets same medium , 1 outlet

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Sidemountyucatan
Posts: 20
Joined: Wed Apr 17, 2019 2:08 pm

Mixing chamber - 2 inlets same medium , 1 outlet

Post by Sidemountyucatan » Mon Jun 24, 2019 10:32 am

Dear all,

I tried to solve another example for get myself more familiar with the CFD topic. It is a quite simple structure of
a pipe with 2 inlets (round shaped) and 1 outlet. The medium is the same for both inlets. I did meshing with cfmesh
and surface refinement at the inlets and outlet.
I tried a steady and a transient run.

Both did not work with certain boundary condition at the outlet.

Error message was:
21.1: OpenFOAM fatal error: Continuity error cannot be removed by adjusting the outflow.
21.2: Simulation exited with error


It has something to do with the boundary condition "outflow" because with "pressure" as boundary condition at the outlet it did
at least start to run eventhough residuals are somewhere in the dimension of 10^-1 to 10^-2 which is quite too high I guess.

I have the following questions:
(1) What is the issue with the "outflow" boundary ?
(2) How can I bring down the residuals ? Finer mesh in whole 3D model ? Mesh refinement )? Other values for k and w ?


Thx a lot !
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oliveroxtoby
Posts: 281
Joined: Fri Dec 23, 2016 9:43 am
Location: South Africa

Re: Mixing chamber - 2 inlets same medium , 1 outlet

Post by oliveroxtoby » Thu Jun 27, 2019 7:52 pm

Sidemountyucatan wrote:
Mon Jun 24, 2019 10:32 am
Dear all,

I tried to solve another example for get myself more familiar with the CFD topic. It is a quite simple structure of
a pipe with 2 inlets (round shaped) and 1 outlet. The medium is the same for both inlets. I did meshing with cfmesh
and surface refinement at the inlets and outlet.
I tried a steady and a transient run.

Both did not work with certain boundary condition at the outlet.

Error message was:
21.1: OpenFOAM fatal error: Continuity error cannot be removed by adjusting the outflow.
21.2: Simulation exited with error


It has something to do with the boundary condition "outflow" because with "pressure" as boundary condition at the outlet it did
at least start to run eventhough residuals are somewhere in the dimension of 10^-1 to 10^-2 which is quite too high I guess.

I have the following questions:
(1) What is the issue with the "outflow" boundary ?
(2) How can I bring down the residuals ? Finer mesh in whole 3D model ? Mesh refinement )? Other values for k and w ?


Thx a lot !
The outflow boundary extrapolates all quantities and is only useful in rare conditions (this is why there is a warning to use with care). In most cases you need to specify some quantity at an outlet for it to be well posed.

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