# Boundary condition problem in Ansys fluent

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Rana Nasser posted this 17 January 2018

Good morning every body, I'm a beginner in Ansys and I have some problems in simulating the water reserved upstream and down stream a gate of a regulator (irrigation structure) to study their hydrodynamic effect on  the structural elements.  The first attached file contains a 1 meter strip of the model which consists of a concrete wall subjected to water on 2 sides with different water levels. I thought that I must add another fluid domain to represent the air above the water at each side to make sure that all the boundary conditions are defined "I'm not sure if this is worthy or not!".The second one is the same strip without the air domain, in this file I tried to simplify the model, but it didn't work too.  Now I think I have a problem in defining boundary conditions in fluent, so I'm looking for your help!

Attached Files

peteroznewman posted this 17 January 2018

Hello Rana,

I am a beginner with CFD, so wait for confirmation from a CFD expert member, but from what I have read, you can use a Multiphase VOF model for your retaining wall fsi 02 Workbench project.

When you do this, you create a domain large enough for both water and air to exist, and Fluent computes which cells contain 100% air, 100% water or a fraction of each at the free surface, which can change with time.

Raef Kobeissi has an example of a transient structural with a water free surface.

Note to readers with the Student license: 2-way FSI such as the above example, requires System Coupling, which is not included in  the Student license.

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raul.raghav posted this 18 January 2018

So looking at your workbench files, the approach with both air and water sounds the most appropriate for your case. I looked into the "Meshing" tab of the Fluent module and notices that your mesh is very coarse for this case you are trying to investigate. I meshed the geometry so you can test your case. Remember, you would have to refine the mesh iteratively in order to check grid independence (once everything starts working properly). And i took the liberty to change the named selections in the mesh to give you a direction. For the boundary conditions, refer the following links:

Modeling open channel flow

open channel flow

Rahul

Attached Files

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vganore posted this 19 January 2018

Hi Rana,

I am not sure why FSI model is required to solve this problem. If fluid is stationary and your aim to find out the effect of hydrostatic force on the structure then I could go ahead with hydrostatic pressure boundary condition in static structural instead of FSI.

If this makes sense then test case 42 (VMMCH042) in Workbench verification manual could help you.

Vishal Ganore, ansys.com/student

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raul.raghav posted this 19 January 2018

Since you mention that water is reserved upstream and downstream of the regulator gate, what Vishal mentioned would be the best approach. However, since a regulator plays a role in controlling or regulating the flow, you would expect fluctuations in the flowrate and the selection of the FSI would be appropriate at that point. But this totally depends on the focus of your investigation and what you are trying to achieve at the end. Good luck!

Rahul

peteroznewman posted this 19 January 2018

@Vishal, Rana found me through my YouTube videos and in an email she said,

I'm working on my master thesis which study the effect of  the dynamic load (produced from the passing trains) on an irrigation structure "regulator".

which is why the FSI is required. In a two-way FSI model, Rana has a transient load in a Transient Structural system linked to the Fluent system through a Coupling system.

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Rana Nasser posted this 4 weeks ago

First of all, thanks very much to all of you for giving me a hand to solve my problem. Then, as peter said my main issue is studying the effect of the  dynamic load that produced from the trains passing close to the regulator. I thought that as the soil and the structural elements will vibrate during the trains motion the water will vibrate also and during its vibration it will hit the gates of the regulator and this will add another load on the structure. Do you think I'm too conservative? If anyone see reasonable simplification please share it with us!

The files attached above are 1 meter strip of the regulator, they are just trials to model the whole system on a smaller geometry. I will attach the main model geometry file to let you have a complete imagination of the model.

Rana Nasser posted this 4 weeks ago

So looking at your workbench files, the approach with both air and water sounds the most appropriate for your case. I looked into the "Meshing" tab of the Fluent module and notices that your mesh is very coarse for this case you are trying to investigate. I meshed the geometry so you can test your case. Remember, you would have to refine the mesh iteratively in order to check grid independence (once everything starts working properly). And i took the liberty to change the named selections in the mesh to give you a direction. For the boundary conditions, refer the following links:

Modeling open channel flow

open channel f

Thank you Raul for you attention and your time, could you please send me the edited file again but in ansys 15 version?!

raul.raghav posted this 4 weeks ago

Rana, attached is the workbench archive file in Ansys ver.15.0.

Rahul

Attached Files