# How do I model a sub-critical flow with FLOW-3D?

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 November 8, 2012, 17:31 How do I model a sub-critical flow with FLOW-3D? #1 New Member   abbas bakhtiari Join Date: Nov 2012 Posts: 4 Rep Power: 7 Hi dear I want to model a sub-critical flowin FLOW-3D. When the height of the lower boundary is greater than of the upper boundary the water will enter the downstream boundary of the computational region. As a result of my assumptions is destroyed. Please help me

November 10, 2012, 15:24
#2
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max
Join Date: Oct 2011
Posts: 10
Rep Power: 8
Quote:
 Originally Posted by abbaspwut Hi dear I want to model a sub-critical flowin FLOW-3D. When the height of the lower boundary is greater than of the upper boundary the water will enter the downstream boundary of the computational region. As a result of my assumptions is destroyed. Please help me
I don't see why the water enters the downstream when height of the downstream boundary is greater than upper boundary?

 November 11, 2012, 05:40 #3 New Member   abbas bakhtiari Join Date: Nov 2012 Posts: 4 Rep Power: 7 Ask better questions: Why height of downstream is not more than height of upstream? if y2>y1 then flow-3d enter water of downstream boundary condition. y2=height of downstream y1=height of upstream i want y2>y1 and flow direction is upstream --> downstream but after Run flow direction is downstream --> upstream Last edited by abbaspwut; November 11, 2012 at 06:37.

November 11, 2012, 05:44
#4
New Member

abbas bakhtiari
Join Date: Nov 2012
Posts: 4
Rep Power: 7
Quote:
 Originally Posted by abbaspwut Hi dear I want to model a sub-critical flowin FLOW-3D. When the height of the lower boundary is greater than of the upper boundary the water will enter the downstream boundary of the computational region. As a result of my assumptions is destroyed. Please help me
Why height of downstream is not more than height of upstream?
if y2>y1 then flow-3d enter water of downstream boundary condition.
y2=height of downstream
y1=height of upstream
i want y2>y1 and flow direction is upstream --> downstream but after Run flow direction is downstream --> upstream
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Last edited by abbaspwut; November 11, 2012 at 06:37.

 November 27, 2012, 12:37 #5 Senior Member   Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 10 FLOW-3D is a transient solver. You are using assumptions of steady-state gradually-varied flow. The only way to model this kind of situation is to include the control structure that causes the downstream free surface elevation to be higher than the upstream elevation. That is, you must model the contraction, critical transition structure, or bed elevation change explicitly, instead of assuming that by setting h2 > h1 you can force FLOW-3D to assume that the flow is steady/gradually-varied.

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