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Old   March 9, 2011, 06:40
Default outflow boundary conditions
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Hi everybody,

I am using the KIVA 3V code to run simulations and I am experiencing some problems with the outflow boundary condition (BC) when I use ICEM-CFD to generate the mesh.

I created a cylindrical mesh with ICEM-CFD to be read in by KIVA. The situation is very interesting now since:

1. when I set the outer region of the cylinder as wall, the code works fine and I get nice results.
2. when I set the outer region as outflow, the code fails because the velocity vectors field shows that all the vectors (3-D) are oriented in one direction.

I reckon the problem might be linked to the mesh configuration due to I have a cylindrical mesh and I don't have axis.
Has anyone experienced a problem like this?

I am thinking of perhaps I should create my own "outflow BC" and forget about what KIVA does in the code. But, how could I program an outflow BC for a Control Volume Method code?

any help would be most appreciated,

thanks a lot
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Old   March 10, 2011, 00:03
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Raashid Baig
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Hi Dain,
Can you post a picture of your model and where you have defines boundary conditions ? I had a similar problem while working with a water pump simulation in CFX. There were 2 solutions to this increase the length of the farfield (making outflow boundary farther away from geometry) or make change the boundary condition to opening.
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Old   March 10, 2011, 05:01
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Hi Raashid,

Thanks a lot for your replay.
These are two pictures of the situation.
Picture 1 is the mesh in which inflow bc is set on the bottom plane, while top and outer region of the domain were set as outflow.
Picture 2 is a plane right in the middle of the domain (Y=0). As you can see on it, all the flow field within the domain (3-D) is oriented in that direction.
It does not happen when wall is set on the outer region, picture 3.

Thanks

daniel
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File Type: jpg pic2.jpg (97.4 KB, 22 views)
File Type: jpg pic3.jpg (96.3 KB, 22 views)
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Old   March 11, 2011, 06:54
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Raashid Baig
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Hi,
I am wondering why is the cylindrical domain set at outflow. Shouldn't it be set at wall ? It will be help full if you can put a picture of your model along with the computational domain.
What my suggestion was that you can set your outflow surface further away (i.e. make a larger cylindrical computational domain).
this is just a suggestion hope this will be useful.
Raashid
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Old   May 12, 2011, 14:32
Question Problem with outflow bc
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Vijay Paul
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I have similar problem with KIVA 3V. I followed the instructions as per the documentation but the code never seems to be affected by pamb (which is what the exit pressure should be when the outflow is subsonic). Pressure at outflow is set to 1.0 (CGS units) and I am not sure why it is set to that value. As a result, velocity jumps to unphysically high values near the outflow boundary.

I would be grateful if some one can post the itape17 (or iprep) and itape5 for a problem with outflow boundary. Thanks.
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