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-   -   UDF for slip boundary condition (http://www.cfd-online.com/Forums/fluent-udf/110407-udf-slip-boundary-condition.html)

 rasoulb December 10, 2012 14:56

UDF for slip boundary condition

Hi all

I have written a UDF for slip boundary condition (=*du_dy) by C_U_G(c,t)[1] Macro.
1- I test my UDF for 2 & 3D steady laminar channel flow, it's work good with low slip length (0.0001-0.005) but for larger slip length not converged. Can anyone guide me for this convergence problem?

2. I need to use velocity gradient of previous time step for unsteady simulation. what macros I should use?

thanks

 flotus1 December 10, 2012 15:13

Do you know that a boundary condition like this is already implemented in Fluent?

 rasoulb December 10, 2012 15:29

Thanks

 flotus1 December 10, 2012 17:42

Last time i read about it was in the version 13 manual, but i think it can also be found in earlier versions.
I am out of office right now, so all I can do is recommend a google search with 'fluent high knudsen boundary'.
I think you will figure it out by yourself, otherwise feel free ask again.

 rasoulb December 11, 2012 03:46

slip velocity based on knudsen number is appropriate for gases and Not applicable for liquids. please say another method.

thanks

 flotus1 December 11, 2012 05:07

This holds true for a microscopic point of view.

But if you just want a boundary condition in the form (http://www.cfd-online.com/Forums/vbL...f9527e29-1.gif=http://www.cfd-online.com/Forums/vbL...c40486a6-1.gif*du_dy), the high Knudsen number boundary condition in fluent is the right choice, no matter what type of fluid you are using.

 rasoulb December 13, 2012 12:56

high Knudsen number boundary condition is for Low-Pressure Gas Systems and available only when the Laminar model is selected in the Viscous Model panel (based on explanation expressed in fluent 6.3 help). but my Model is LES and pressure is high in my case.

 flotus1 December 14, 2012 05:53

"high" pressure is not the problem in your case.
The boundary treatment can be used at any pressure level. The term "low pressure" comes from one of the applications of the model in low-pressure systems. But the model is appropriate at arbitrary pressure levels, whenever the Knudsen number is high.
I am currently studying flows at normal pressure levels with a BC like this.

But I see now that this BC is not an option since your Model is LES.
Perhaps it is possible to activate the BC with a LES model with a text command. This would be a question for the fluent support.

 m zahid November 13, 2014 05:03

hi

hi, i need a UDF for slip boundary condition at the bottom of the domain or at the ground (wall), case is just like a flow over a building. if anybody have sample UDF please share this, z is a vertical axis of my domain. thanks

mziqureshi@hotmail.com

regards,

 pakk November 13, 2014 05:12

Quote:
 Originally Posted by m zahid (Post 518922) hi, i need a UDF for slip boundary condition at the bottom of the domain or at the ground (wall), case is just like a flow over a building. if anybody have sample UDF please share this, z is a vertical axis of my domain. thanks mziqureshi@hotmail.com regards,
Please read the messages in this thread more carefully. If you would have done that, you would not have asked for a UDF.

 m zahid November 13, 2014 09:42

hi, thanks pakk, here rasoulb use slip length instead of dynamic viscosity, as given in the link
http://www.cfd-online.com/Wiki/Wall_shear_stress

do u know the relationship between slip length and dynamic viscosity.

thanks

 pakk November 13, 2014 10:13

First question you should ask yourself: What is the equation for slip that you want to implement?

Second question you should ask yourself: What is the equation for slip that Fluent has implemented? (This is written in the Help, look it up.)

Third question: How can you choose parameters such that the Fluent implementation is the same as what you want?

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