[W.N.]

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.

[Opaque]

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 ?

[W.N.]

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

[W.N.]

Originally Posted by Opaque

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.

[W.N.]

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

[ghorrocks]

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

[Shomaz ul Haq]

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.

[yaseen wsu]

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)

[W.N.]

Originally Posted by yaseen wsu

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?

[yaseen wsu]

Originally Posted by W.N.

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