How to decide the boundary condition for free surface?

dut_thinker · February 5, 2011, 21:29

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

hadian · February 6, 2011, 02:10

at the free surface the velocity vector normal to the free surface is zero and the zero gradient is applied for tangential velocity vector.

dut_thinker · February 6, 2011, 18:10

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.

hadian · February 7, 2011, 00:50

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.

dut_thinker · February 7, 2011, 01:16

I have got it
Thank you again

newcomer · February 16, 2011, 06:48

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!

February 5, 2011, 21:29	How to decide the boundary condition for free surface?	#1
dut_thinker New Member Join Date: Feb 2010 Posts: 12 Rep Power: 16	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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February 6, 2011, 02:10		#2
hadian Member Mohammad Reza Hadian Join Date: Mar 2009 Location: Yazd, Iran Posts: 52 Rep Power: 17	at the free surface the velocity vector normal to the free surface is zero and the zero gradient is applied for tangential velocity vector.

February 6, 2011, 18:10		#3
dut_thinker New Member Join Date: Feb 2010 Posts: 12 Rep Power: 16	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.

February 7, 2011, 00:50		#4
hadian Member Mohammad Reza Hadian Join Date: Mar 2009 Location: Yazd, Iran Posts: 52 Rep Power: 17	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 $\epsilon=k^{3/2} /(0.43h)$ at free surface. but Nezu and Nakagawa (1993) mentioned $\epsilon=k^{3/2} /(0.18h)$ . h in both equations is water depth.

February 7, 2011, 01:16		#5
dut_thinker New Member Join Date: Feb 2010 Posts: 12 Rep Power: 16	I have got it Thank you again

February 16, 2011, 06:48		#6
newcomer New Member Gillian Join Date: Sep 2010 Posts: 15 Rep Power: 15	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!