# Open Channel Boundary Conditions Issues

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 July 30, 2012, 17:34 Open Channel Boundary Conditions Issues #1 New Member   Join Date: Jul 2012 Posts: 5 Rep Power: 13 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. Other information about the model: Multi-phase Model: Implicit VOF Turbulence Model: k-e (RNG) Outlet BC: Pressure outlet Top BC: Symmetry Many thanks in advance, Env

August 31, 2012, 14:19
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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.

September 1, 2012, 23:02
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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 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.

 October 21, 2012, 14:36 #4 New Member   Siamak Gharahjeh Join Date: Aug 2012 Posts: 27 Rep Power: 13 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.

 June 27, 2016, 12:19 Magnitude difference in velocity in BC and simulated data #5 Senior Member   Tanjina Afrin Join Date: May 2013 Location: South Carolina Posts: 169 Rep Power: 12 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

June 27, 2016, 19:41
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Quote:
 Originally Posted by Tanjina 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.

 June 27, 2016, 22:15 #7 Senior Member   Tanjina Afrin Join Date: May 2013 Location: South Carolina Posts: 169 Rep Power: 12 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 !

June 28, 2016, 13:14
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Quote:
 Originally Posted by Tanjina 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.

 June 28, 2016, 16:05 #9 Senior Member   Tanjina Afrin Join Date: May 2013 Location: South Carolina Posts: 169 Rep Power: 12 Thanks Env. I will look into this by changing these variables.

 Tags boundaries condition, fluent, multi phase, open channel, vof model

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