# Simulation CFD

#### Why does the water level rise?

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03/07/2018 9:34 AM

Hi! ;) Why does the water level rise? Where is the mistake? Example file.

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#### Re: Why does the water level rise?

03/07/2018 10:21 AM

Hi @svetashov,

Thanks for posting!

As a starting point I would take a look at this guide for setting up free surface simulations. You should use two volume one to represent the initial height of the water and one to represent the initial air space. Apply water to both then assign an initial condition specifying the height if the water using the bottom volume.

Once that is done you need to use an appropriate time step. 1 second is far too large to accurately capture the physical flow behavior. I would first try with 0.01 seconds.

Hope that helps.

All the best,

David

David Short
Technical Support Specialist, Simulation
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#### Re: Why does the water level rise?

03/12/2018 3:11 AM

Hi! thx! ;) 1) sent, check whether it is correct?, attached the file. 2) Did I build the mesh correctly?

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#### Re: Why does the water level rise?

03/12/2018 10:14 AM

Hi @svetashov,

Set up looks pretty good except for the mesh. At the moment you have mesh size of 0.5 m this means that the free surface will look very chunky (imagine a wave made out of lego). You need to refine the mesh to give smooth representation of the free surface. This will require a lot of mesh so you will have to find a balance between accurate, time and your computing capacity. Remember that 1 million elements requires roughly 2 GB of RAM so you must setup your simulation with a mesh that can be realistically handled by your machine.

I would split your model in half and use a symmetry plane. You will then save half the total element count allowing for further refinement. Royce does the same in this webinar.

Also maybe just start with just one P=0 BC at the outlet. It is good practice to avoid such BCs touching one another. If the top side BC turns out to be required you can add it later. For now keep it simple and get something running.

Hope that helps,

David

David Short
Technical Support Specialist, Simulation
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