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Not equal flow mass,open channel

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Old   April 19, 2017, 07:42
Default Not equal flow mass,open channel
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I'm trying to model a free surface,multi phase (water,air) open channel flow with gate across the channel section. The problem is when calculating flow mass across any channel plane isn't equal to inlet mass flow ,especially the region after the gate given small value of mass flow . I don't know the reason for not equal flow mass.
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Old   April 19, 2017, 10:02
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Have you looked at monitors for transient imbalance in the ANSYS CFX Solver Manager ?

The mass flow across different planes will only be equal to the inlet mass flow once the solution has become stationary; otherwise, you will see the transient imbalance. Has your solution converged to its steady state condition ?
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Old   April 19, 2017, 12:53
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thanks to you for reply . The solution converged to RMS 0.0001 in 83 iteration. Mass -water imbalance 37.8976٪‏ , Mass-air imbalance-14.5323% . Then I tried to do more iterations and after1000 iteration more. water imbalance 0.1952%and air imbalance 0.0842% . Is that the the residual imbalance that you mean. Note that, the flow in the region after the gate not steady at all .it is a turbulent flow region. ATTACH]55448[/ATTACH]IMG_1244.JPGIMG_1245.JPG
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Old   April 20, 2017, 12:30
Default Not equal flow mass,open channel
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Quote:
Originally Posted by Opaque View Post
Have you looked at monitors for transient imbalance in the ANSYS CFX Solver Manager ?



The mass flow across different planes will only be equal to the inlet mass flow once the solution has become stationary; otherwise, you will see the transient imbalance. Has your solution converged to its steady state condition ?


I don't know how to reach for steady state condition?
I want to know in what the difference in mass flow was consumed?
As I know the flow discharge doesn't change with the time.
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Old   June 10, 2017, 14:00
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This is my results after several attempts to achieve stability
Inlet mass flow =~ outlet mass flow, but the planes between them still have different mass flow IMG_1338.JPGIMG_1339.JPGIMG_1340.JPGIMG_1341.JPG
Can anyone help me?I will be very grateful to him
Thank you in advance
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Old   June 11, 2017, 05:53
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The periodic oscillations in the residuals suggests you might have a transient flow condition which you are trying to model with a steady state flow. This is discussed in the FAQ: https://www.cfd-online.com/Wiki/Ansy...gence_criteria
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Old   June 12, 2017, 11:10
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I agree with Glenn. You should just run a transient simulation to check whether you model is steady state or transient. Do not worry about the Re number shown in output by solver.
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Old   July 12, 2017, 16:08
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in my view, this does not due to transient effect, flow under sluice gate is steady state, but I think the problem due to in appropriate outlet boundary condition. (strong jump occur)
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Old   July 22, 2017, 15:15
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Quote:
Originally Posted by yaseen wsu View Post
in my view, this does not due to transient effect, flow under sluice gate is steady state, but I think the problem due to in appropriate outlet boundary condition. (strong jump occur)


I used pressure as outlet boundary condition and entered water level, which outlet condition that you suggest?
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Old   July 23, 2017, 02:25
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Quote:
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I used pressure as outlet boundary condition and entered water level, which outlet condition that you suggest?
that is good, static pressure at outlet, but the water level that you measured from experiment should be at the location when the flow become fully develop.
it means the boundary must be far enough from the jump location
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