# Dirichlet and Nuemann

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 April 28, 2012, 14:26 Dirichlet and Nuemann #1 New Member   cc Join Date: Mar 2012 Posts: 8 Rep Power: 7 Hi everyone, I want to simulate turbulence over a flat plate. Now I have met a problem about the boundary conditions. I would like to apply Dirichlet Boundary conditions, velocity in the direction y and Nuemann Boundary conditions, stress, for one boundary at the same time. The most important is " at the same time for one boundary" Could you please tell me how can I do this in Fluent? Thank you so much!

April 29, 2012, 11:27
#2
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Lucky Tran
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Quote:
 Originally Posted by Zacc Hi everyone, I want to simulate turbulence over a flat plate. Now I have met a problem about the boundary conditions. I would like to apply Dirichlet Boundary conditions, velocity in the direction y and Nuemann Boundary conditions, stress, for one boundary at the same time. The most important is " at the same time for one boundary" Could you please tell me how can I do this in Fluent? Thank you so much!
Which direction is the y direction? Is that plate-normal or plate-parallel?

The plate-normal velocity is 0 by kinematic constraints, flow can't flow through the plate.
The plate-parallel velocity at the wall is also 0 by no-slip conditions, here is where wall functions jump in.

April 29, 2012, 13:28
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Quote:
 Originally Posted by LuckyTran Which direction is the y direction? Is that plate-normal or plate-parallel? The plate-normal velocity is 0 by kinematic constraints, flow can't flow through the plate. The plate-parallel velocity at the wall is also 0 by no-slip conditions, here is where wall functions jump in.
Thank you! Actually I didn't make it clear. The boundary conditions are applied for the farfield where there is a suction in the direction y (normal to the plate)

April 29, 2012, 16:27
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Lucky Tran
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Quote:
 Originally Posted by Zacc Thank you! Actually I didn't make it clear. The boundary conditions are applied for the farfield where there is a suction in the direction y (normal to the plate)
I'm having a hard time imagining this boundary. Can you clarify?

What is the extent of this boundary? And you intend to specify the velocity across this boundary + shear? Since flow is in y, you want to specify velocity in (x,z) and shear in (x,z). Is the velocity uniform? What about shear?
I am also unclear on another point: Once the velocity distribution is specified, the shear stress is automatically known. How can you specify it further?

Providing a sketch would help.

April 29, 2012, 23:36
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Quote:
 Originally Posted by LuckyTran I'm having a hard time imagining this boundary. Can you clarify? What is the extent of this boundary? And you intend to specify the velocity across this boundary + shear? Since flow is in y, you want to specify velocity in (x,z) and shear in (x,z). Is the velocity uniform? What about shear? I am also unclear on another point: Once the velocity distribution is specified, the shear stress is automatically known. How can you specify it further? Providing a sketch would help.
Thank you! First, what I want to do is to simulate the turbulent separation bubble. So I have to do my suction-Velocity in the direction y which is normal to the flow as showed in the image. But we have met a problem of rotation. So the second thing is to simulate the situation irrotational by applying the shear in the farfield. Because with the condition of irrotation, we have du/dy=dv/dx, knowing that v is the velocity of suction.
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