# Air inlet boundary condition for free surface in open channels

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 February 15, 2017, 06:21 Air inlet boundary condition for free surface in open channels #1 New Member   mohammad.ramezani Join Date: Dec 2015 Posts: 12 Rep Power: 9 hi I want to know that for the Air inlet boundary condition, above the water surface, which of the followings must be determine? 1. A velocity similar to the water velocity? 2. The real air velocity above the water surface? 3. A portion of the water velocity at water inlet?(for example 1/100) 4. A linear velocity profile that at water surface has a velocity equal with water and reaches to zero at higher levels? 5. Or there is no need to define an air inlet, and choosing homogeneous model is sufficient for simulation? Thanks a lot

 February 15, 2017, 17:58 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,321 Rep Power: 138 Please post an image of what you are trying to do showing the flows you want to model and the boundary conditions you intend to use.

February 17, 2017, 09:11
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Join Date: Dec 2015
Posts: 12
Rep Power: 9
this is a siphon spillway,in which, in the upstream the reservoir level rises by increasing the discharge.
at the upstream:
1)water inlet height= 0<h<0.35m
2)a wall=0.35<h<0.39
3)air inlet height=0.39<h<0.45m
(water height of the reservoir would not increase than 0.39m)
the down stream is 0.25m drown and the outlet is opening
Attached Images
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 February 19, 2017, 18:54 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,321 Rep Power: 138 Your questions are in the reverse order to what you should consider them. First: Homogenoeus or inhomogeneous is chosen depending on whether the free surface is distinct and clear or whether bubbles, foam or other features you do not intend to directly model are present. Once you have made that choice you need to look at the boundary conditions.

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