How to decide the boundary condition for free surface?

dut_thinker · February 5, 2011, 22:29

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, 03:10

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, 19:10

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, 01:50

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, 02:16

I have got it
Thank you again

newcomer · February 16, 2011, 07:48

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, 22:29	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

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Wind turbine simulation	Saturn	CFX	60	July 17, 2024 06:45
Surface with double boundary condition	Marcopolopeich	CFX	2	January 16, 2011 11:41
RPM in Wind Turbine	Pankaj	CFX	9	November 23, 2009 05:05
boundary condition at free surface	vineet	FLUENT	2	April 14, 2002 07:13
CFX4.3 -build analysis form	Chie Min	CFX	5	July 13, 2001 00:19

February 6, 2011, 03:10		#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, 19:10		#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, 01:50		#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, 02:16		#5
dut_thinker New Member Join Date: Feb 2010 Posts: 12 Rep Power: 16	I have got it Thank you again

February 16, 2011, 07:48		#6
newcomer New Member Gillian Join Date: Sep 2010 Posts: 15 Rep Power: 16	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!