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Old   September 11, 2013, 01:19
Unhappy Boundary Condition Problem
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Shu Shu Chang
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Hi ANSYS users,

I am a new FLUENT user, this is my first post in CFD online, maybe this is a easy question.

there is a channel have a inlet and a outlet, the inlet velocity is a sine function depend on time, and the outlet just outflow.

As you know, the sine wave is positive at 0 to PI, negative at PI to 2*PI.
When time at PI to 2*PI, the velocity become the outlet velocity but reverse the direction, and the inlet become outflow.

Easy to say, the inlet and outlet boundary condition exchange.


I have a problem now. How to set the boundary condition as the example in fluent? Should I use UDFs?

BTW, I had tried using "Dynamic mesh -> event", but appear the error message "the BCs are same type", maybe it's a wrong way.

Please give me some suggest.

Thanks
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Old   September 11, 2013, 07:51
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Andrew Walmsley
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With regards to the transient boundary condition, yes you will to use UDFs. There are quite a few examples around online for transient boundary conditions. Alas, none for what I need.

I can't help with the other issue, sorry I don't have experience with dynamic meshing as of this time.
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Old   September 11, 2013, 21:54
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Maybe you can use velocity inlet and velocity outlet, or velocity inlet and pressure outlet.
Is the flow fluid compressible or not? The compressibility will affect this periodic flow process.
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Old   September 12, 2013, 01:32
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Shu Shu Chang
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Thank quantities, AWalmsley11 reply.

Maybe I had found the way to solve this problem.

I let FLUENT calculate the time at 0 to PI, when time equal to PI, FLUENT calculate completed.

And I can change the boundary condition between inlet and outlet, and don't initialized solution, let FLUENT keep calculating time at PI to 2*PI.

This way can work, but I don't know it will cause what bad effect.
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Old   September 12, 2013, 09:38
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If it's done that way it will not have the sine function, it will be constant for that time. Think of it as it will appear as a digital signal rather than analogue in graphical terms.
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