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multiphase openchannel flow

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Old   September 15, 2019, 02:02
Default multiphase openchannel flow
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Mohsin Raza
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Hi,
I am simulating an open channel flow placing a cylinder in the center of the channel. I have done many test runs, but the problem I am facing is, the flow depth is under predicted, throughout the channel, specially downstream of the cylinder.
I have at the inlet flow rate, reynolds no, slope of the channel and manings coefficient etc.
Attached is a zip file of the initial condition.
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File Type: zip New WinRAR ZIP archive.zip (4.5 KB, 2 views)

Last edited by moraza; September 19, 2019 at 03:51. Reason: wanted to add the details
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Old   September 16, 2019, 04:17
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Please share the full case if you want some help.
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Old   September 16, 2019, 08:55
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Attached is the full case. due to size constraints I have removed the polymesh folder. I have checked the results, depth is under predicted. I don,t know the reason is but I think this ,ay be due to outlet boundary conditions.
Thank you for the quick reply
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Last edited by moraza; September 17, 2019 at 12:11.
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Old   September 20, 2019, 06:05
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We would also need the mesh. So pack it up and share via Gdrive/dropbox/onedrive etc. if possible.
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Old   September 20, 2019, 10:40
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Hi Mohsin,

At the outlet, if you use zeroGradient for U, like you did, and you have a subcritical flow in the inlet, there is a tendency o having a critical flow in the outlet. See for instance the book o Chaudrhy "Open-Channel Flow" page 8 and chapter 3 in the 2nd editon.
You may control the water level at the outlet using a fixedValue condition for U at the outlet. This condition, together with the imposed flow rate at the inlet, will give you an imposed water level at the outlet, because you have Q=V.A, if Q and V are imposed then you are also imposing the area A, that is related to the water level.

P.S. (To Linnemann) There is no need to post the mesh. Just run blockMesh

Best Regards,
Paulo
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Old   September 25, 2019, 05:01
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Quote:
Originally Posted by linnemann View Post
We would also need the mesh. So pack it up and share via Gdrive/dropbox/onedrive etc. if possible.
Running "blockmesh" command in the terminal will generate mesh.
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Old   September 25, 2019, 05:14
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Hi Paulo,
Thank you for the reply.
You are right.

One minor problem, If i have to use the fixedvalue bc at the outlet, I have to assign a value(e.g, uniform 1 etc). Right? As for my problem there isn,t any value that I can assign.
One more question. I have a depth value at the inlet, How can I impose that value at the inlet?
From depth and flow rate velocity in the horizontal direction can be calculated using manings equation, Can I use the velocity value(only) as boundary condition at the inlet, or do I have to use depth or flow rate in combination with velocity?
I am also not sure about the p-rgh boundary conditions to be used.
Thank you
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Mohsin

Last edited by moraza; September 30, 2019 at 06:59. Reason: wanted to add further details
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Old   September 30, 2019, 03:38
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Originally Posted by vatavuk View Post
Hi Mohsin,

At the outlet, if you use zeroGradient for U, like you did, and you have a subcritical flow in the inlet, there is a tendency o having a critical flow in the outlet. See for instance the book o Chaudrhy "Open-Channel Flow" page 8 and chapter 3 in the 2nd editon.
You may control the water level at the outlet using a fixedValue condition for U at the outlet. This condition, together with the imposed flow rate at the inlet, will give you an imposed water level at the outlet, because you have Q=V.A, if Q and V are imposed then you are also imposing the area A, that is related to the water level.

P.S. (To Linnemann) There is no need to post the mesh. Just run blockMesh

Best Regards,
Paulo
Well, I tried using fixedvalue at the outlet, but I am not getting the desired results.
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Old   October 7, 2019, 12:11
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Hi Mohsin,

To assign a water level to the inlet you may divide the inlet surface in two regions, inletw, for instance, will be the water inlet and inleta is the air inlet or a zero velocity surface like a wall. You may check a similar setup in the spillway tutorial at https://www.hpc.ntnu.no/display/hpc/...llway+Tutorial. This tutorial is a bit old but the mesh generation still works with minor adjustments.

About the outlet condition, you may use the velocity to control the water level. As you decrease the inlet velocity, the water level increases because you have a constant flow rate at the inlet. But caution, if you decrease the outlet velocity too much, the channel will flood and water will scape through the atmosphere surface.

Best regards,
Paulo
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