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How to decide the boundary condition for free surface?

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Old   February 5, 2011, 22:29
Default How to decide the boundary condition for free surface?
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Hi

I know that at the bottom we can use the wall function to determine the BC for turbulent kinetic energy k and dissipation e.
So how about the free surface?
Does the wall function still apply to free surface or we can use something else?

Thank you in advance
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Old   February 6, 2011, 03:10
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Mohammad Reza Hadian
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at the free surface the velocity vector normal to the free surface is zero and the zero gradient is applied for tangential velocity vector.
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Old   February 6, 2011, 19:10
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Thank you for your explainnation.
I know how to set velocity at the free surface.
I mean, at present i used k-e model to simulate the turbulence.
I want to know how to set k and e at the free surface.
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Old   February 7, 2011, 01:50
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you can use zero gradient for k normal to free surface and some researchers apply a damping factor to it too.
for \epsilon, Celik and Rodi (1988) suggested \epsilon=k^{3/2} /(0.43*h) at free surface. but Nezu and Nakagawa (1993) mentioned \epsilon=k^{3/2} /(0.18*h).
h in both equations is water depth.
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Old   February 7, 2011, 02:16
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I have got it
Thank you again
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Old   February 16, 2011, 07:48
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Hi...

I am a bit confused about how to specify boundary condition for the top surface. I have seen many ppl set the surface as symmetry. I am wondering when such boundary condition can be applied and when do we need to use other types of boundary conditions?

Specifically, if we are modeling the flow crossing a weir, what type of boundary condition should I specify for the top surface (both in front and after the weir)...

Thanks!
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