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-   -   Open Channel Boundary Conditions Issues (http://www.cfd-online.com/Forums/fluent/105422-open-channel-boundary-conditions-issues.html)

 Env July 30, 2012 17:34

Open Channel Boundary Conditions Issues

I have modeled an open channel using FLUENT 6.3 (3D). I have encounter many issues in the boundary conditions defining.

I specified the inlet by "Pressure Inlet" BC. As I used the "Open Channel" option on the "multi-phase" tab of the Pressure Inlet BC panel, I entered the following parameter to define the BC: Free Surface Level, Bottom Level, and Velocity magnitude as well as k and epsilon.

My problem is that after solving the field, the inlet velocity differs significantly from the value I have entered in the Pressure Inlet BC condition. I have searched all the FLUENT documentation to find equations of open channel BCs, but nothing specific for open channel are there. Does anyone know how the FLUENT have used the velocity magnitude I have entered? and why the velocity magnitude in the inflow face after calculation is not the same as the velocity magnitude I have entered?

I have checked the "Mass Flow Inlet" BC for open channel which requires the same parameters as the "Pressure Inlet" without velocity magnitude! That means "Mass Flow Inlet" may define the BC using only Free Surface Level and Bottom Level. How is it possible? Is the "Pressure Inlet" BC over defined?

I appreciate if any one help me understand how do boundary conditions for open channel work.

Multi-phase Model: Implicit VOF
Turbulence Model: k-e (RNG)
Outlet BC: Pressure outlet
Top BC: Symmetry

Env

 rsaha August 31, 2012 14:19

Hi,

I am facing similar problem. The velocity magnitude specified in the pressure inlet boundary condition does not matches with the simulation result. I have tried using the velocity inlet boundary condition. But the solution does not converge. Please let me know if you have found details about the open channel boundary conditions in VOF.

Quote:
 I have checked the "Mass Flow Inlet" BC for open channel which requires the same parameters as the "Pressure Inlet" without velocity magnitude! That means "Mass Flow Inlet" may define the BC using only Free Surface Level and Bottom Level. How is it possible?
>> Actually you specify the mass flow rate of both phases by individually selecting that phase in the boundary condition setting.

 Env September 1, 2012 23:02

Hi rasha,
Thank you for your reply. I have asked sense guys in my university to post my question on Ansys Support portal. However, if you can do that please post on the link below.
https://www1.ansys.com/customer/default.asp
If I found any solution, I'll let you know. If you found anything, please let me know, too.
Thanks,
Env

Quote:
 Originally Posted by rsaha (Post 379761) Hi, But the solution does not converge. Please let me know if you have found details about the open channel boundary conditions in VOF. >> Actually you specify the mass flow rate of both phases by individually selecting that phase in the boundary condition setting.

 siamakghh2000 October 21, 2012 14:36

I've tested a way which I think works. let's first remember that if flow is driven due to gravity, then the velocity at the pressure inlet must be unique. That one velocity magnitude is nothing but the true magnitude which may be measured in the lab. So, what you do is you put the actual velocity there, otherwise you should approach that velocity in an iterative manner without the lab measurement. you can start with zero magnitude for velocity in the inlet and solve(now V is not zero anymore), next, calculate the velocity by dividing the flow flux(flux in the pressure inlet) by flow area again at the entrance. Now you have a velocity, go back to BC and put it there and iterate and so on. But usually doing so for one time works good.

 Tanjina June 27, 2016 12:19

Magnitude difference in velocity in BC and simulated data

Hello Env and Rsaha,

I am facing the same problem. The provided velocity at BC is not matching with simulated velocity. Did you get any solution for that problem?

Regards,
Tanjina

 Env June 27, 2016 19:41

Quote:
 Originally Posted by Tanjina (Post 606799) Hello Env and Rsaha, I am facing the same problem. The provided velocity at BC is not matching with simulated velocity. Did you get any solution for that problem? Regards, Tanjina
As I understood, the pressure inlet BC means that FLUENT keeps the total pressure at the BC constant. Thus, the pressure (water surface) and velocity may change as it solves the flow field, but the total pressure (summation of velocity head and static head) will always be constant.

For my case I ignored the changes of velocity at the BC, since the interest region was far from the BC.

I hope it helps you.

 Tanjina June 27, 2016 22:15

Thanks Env.

For my case, I am defining free surface level ( I guess which means constant water level), so there is no chance to change the pressure. Then why I am getting different velocity? :( :( it changes from 1.54m/s to 0.0003 m/s !

 Env June 28, 2016 13:14

Quote:
 Originally Posted by Tanjina (Post 606856) Thanks Env. For my case, I am defining free surface level ( I guess which means constant water level), so there is no chance to change the pressure. Then why I am getting different velocity? :( :( it changes from 1.54m/s to 0.0003 m/s !
Specifying the free surface level does not mean that the solver will preserve the water surface level. It will preserve the total pressure which comes from your specified values for free surface level and velocity. So it might slightly change the flow velocity or surface level at BC during computation to make is consistent with the flow field, but will keep it balanced so the total pressure is always constant.

However, you have significant changes in flow velocity. It means something else is wrong. It could be mesh issue, solution controls and equations, or may be your specified values of velocity and water surface is not consistent with your domain. Check your outlet BC, gravity direction etc.

 Tanjina June 28, 2016 16:05

Thanks Env. I will look into this by changing these variables.

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