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Old   August 27, 2012, 17:22
Question boundary condition
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Hello everybody!

I am practically a complete noob in openFoam and actually I am staying in front of the following task:

I have to create a fluid stream on a plate and analyze the boundary layer. All in all it seems to be a not very complicated task, but the difficulty is, that I have to put the inlet of the stream in a specific distance to the edge of the semi-infinity plate.

This means, that i can't use the fixed velocity-value boundary condition for the inlet and somehow have to define a special velocity function u instead, which depends on the distance to the plate (y). I have to set the function u(y) for the velocity as an inlet boundary condition, so to speak.

I have attached a simple drawing of this task to post, to make it more clear.

My problem is, that I don't have a clou on how I can manage to set this special inlet boundary condition in openFoam?
I searched the internet for a while now, but I didn't find a good or at least an easy-to-understand solution for my problem.

I would be glad if you could help me with my task at this point!
greetings and thanks a lot!
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Old   August 28, 2012, 02:26
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Try 'groovybc' in google. It allows you to supply functions as boundary condition, just what you want.
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Old   August 28, 2012, 11:25
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thank you for this hint, Bernhard, but actually I didn't really find a tutorial on how to use this tool for my task.

Could you please describe this tool and how the syntax works? Or is there any documentation for groovybc? I couldn't find one..
What do I actually have to include in the u-file to use this for a function u(y) and is groovybc also implemented in openFoam v2.1.0?

Or is there maybe also a possiblity to use a table of different velocities in accordance to the height over the plate for the inlet boundary?
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Old   August 28, 2012, 11:30
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GroovyBC in now part of swak4Foam, which you should download and install seperately. More info can be found here: http://openfoamwiki.net/index.php/Contrib/swak4Foam . Examples are included in the download, or you can read non-updated page on groovyBC http://openfoamwiki.net/index.php/Contrib_groovyBC
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Old   August 28, 2012, 13:25
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Ok, I have read through the ReadMe of swak4Foam so far and I think the way better solution for my problem might be to simply use a table with given values (from the Blasius-Theory) for the velocities at the inlet in accordance to the y coordinate.

Is there a simple way to do this?
I have found something called setfields, but I am not really sure if this is the right one.

In which form is it possible to implement the table with the values and link it to the boundary for the velocity?
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Old   August 28, 2012, 19:29
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Hello again and thanks for your help so far,

I actually managed to make a velocity-inlet variation by a value table based on the Blasius-values. (instead of a function)

I used timeVaryingMappedFixedValue for the inlet boundary for this purpose.
My project is in the attachment of this post.

Now, I just have some trouble with the boundary layer on the plate: the thickness of the boundary layer simply doesn't increase after the inlet but just stays constant.

Do you know why this happens or what I have made wrong?
I would be glad for any help!
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Old   August 29, 2012, 18:42
Default boundary layer thickness
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Quote:
Originally Posted by suflaki View Post
Hello again and thanks for your help so far,

I actually managed to make a velocity-inlet variation by a value table based on the Blasius-values. (instead of a function)

I used timeVaryingMappedFixedValue for the inlet boundary for this purpose.
My project is in the attachment of this post.

Now, I just have some trouble with the boundary layer on the plate: the thickness of the boundary layer simply doesn't increase after the inlet but just stays constant.

Do you know why this happens or what I have made wrong?
I would be glad for any help!
Hi Suflaki,

i didn't read your project but this problem comes some time when cell size is greater than boundary layer thickness, so you must take care when specifies cells number in blockMeshDict file for y dimension.

regards,
Ahmed
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Old   September 24, 2012, 18:07
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Quote:
Originally Posted by suflaki View Post
Hello everybody!

I am practically a complete noob in openFoam and actually I am staying in front of the following task:

I have to create a fluid stream on a plate and analyze the boundary layer. All in all it seems to be a not very complicated task, but the difficulty is, that I have to put the inlet of the stream in a specific distance to the edge of the semi-infinity plate.

This means, that i can't use the fixed velocity-value boundary condition for the inlet and somehow have to define a special velocity function u instead, which depends on the distance to the plate (y). I have to set the function u(y) for the velocity as an inlet boundary condition, so to speak.

I have attached a simple drawing of this task to post, to make it more clear.


My problem is, that I don't have a clou on how I can manage to set this special inlet boundary condition in openFoam?
I searched the internet for a while now, but I didn't find a good or at least an easy-to-understand solution for my problem.

I would be glad if you could help me with my task at this point!
greetings and thanks a lot!
Hi,

i think there is a new type of BC called polynomial in the release note of last openFOAM release.
generally, you can make your mesh of two hexagons each one has a different velocity in its inlet.

good luck.
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