Modelling open channel flow with wave at the interface

Member Posts: 5

Hi,

I want to simulate open channel flows where flow velocity and waves on interface co-exist.

My 2D geometry is shown here:

I am using VOF model, clicked open channel flow & open channel wave BC options.

I am using pressure outlet at free surface and pressure outlet (Please refer the image) :

I am using velocity inlet in the inlet, where water is supposed to flow from along with wave at the interface. I have clicked open channel wave BC & Segregated velocity inputs, where I have provided velocity for the incoming water. Primary phase has no velocity. Please refer the image below:

However, the flow velocity does not get simulated correctly. I am able simulate waves at the free surface in the absence of water flow velocity. However, along with flow velocity, the simulation is not giving results.

Kindly suggest where am I making mistakes.

Regards,

Ayan

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• UKPosts: 8,855Forum Coordinator

The surface "Free surface" needs to be away from the water free surface and set as a pressure boundary but not part of the open channel. Open channel is for where the waves enter and leave the domain.

• Posts: 59Member

Dear sir,

I did what you suggest. The free surface in my image is at z=0.5 m and water free surface is at z=0.2 m. Please check pressure outlet boundary condition.

• UKPosts: 8,855Forum Coordinator

What is pressure outlet set as? You need more spare height and if the domain is only a few metres long you may want to consider how big the waves should be and what the model was designed for.

• Posts: 59Member

The pressure outlet is se as pressure outlet. The domain dimension: length, L=5 m, water depth, d=0.2 m, domain height, H=0.5 m. Velocity of water, v=0.2 m/s.

• UKPosts: 8,855Forum Coordinator

I'd make the height enough that the waves aren't anywhere near the top of the domain. With a shallow depth you'll need to choose the wave conditions carefully, you also need more mesh in the domain to track the free surface.

• Posts: 59Member

Thanks.

Few points:

1. I provided a wave of zero amplitude to check if I get water flow velocity contour properly. I thought upon getting that, I would introduce waves of non-zero amplitude for future simulations. But the velocity contour came to be weird -- my main issue is with simulating the water flow.
2. If my way of producing open channel flow (without wave) is wrong, please suggest the better way.

Regards,

Ayan

• UKPosts: 8,855Forum Coordinator

Did you patch in the water? We do use the model without waves regularly for various applications.

• Posts: 59Member

Hi,

I did try patching the water. Even with this my velocity came to be unreal.

For example, I used only open channel option (leaving open channel wave BC) :

I used water density as compressible:

I did patch the domain of water.

Boundary conditions:

Inlet- velocity inlet,

outlet & top of the domain- pressure outlet

bottom- wall

The solution I got after 24 seconds is:

• Posts: 931Member

You can ignore the compresability of water ... I am not sure is your inlet 0.5 m height or 0.2 m height (Whole domain height, or just water height)

Second with the dimension you mention and the mesh on the first post, you will not get the free surface. You need fine mesh around the interface ...

Finally, if you simulated 24 seconds between your comments ... you are using a super super fast HPC or you are using very large time step. And most probably you don't do enough iterations to converge

• Posts: 59Member

Hi,

My domain height is 0.5 m, water height is 0.2 m. My entire inlet is 0.5 m.

Is my method is correct?

Regards,

AKB

• Posts: 931Member

I think you should have the inlet velocity inlet of a height of 0.2 m, not 0.5 m. If you keep it 0.5 m, you will have flow rate more than you want. Also. it might be source of instability of the free surface.

• Posts: 59Member

Hi,

I tried to do so. In that case, the problem is what condition should I use for top 0.3 m? If I use pressure inlet for top 0.3 m, then it takes secondary phase by default. That's the problem.

• Posts: 931Member

You are right, It should take the secondary phase. But you should check the open channel wave BC to be able to put the free surface at the inlet

• Posts: 59Member

But if it takes secondary phase in the top 0.3 m, where secondary phase (water) does not exist, won't it give wrong results?

• Posts: 931Member

No, it will take air

• Posts: 59Member

Thanks. I will try and inform again.

Regards,

Ayan

• Posts: 59Member

One point:

I am using velocity inlet in the water inlet. I have clicked open channel wave BC & Segregated velocity inputs, where I have provided velocity for the incoming water. Primary phase has no velocity. Please refer the image in the first message. Is it okay ?

• Posts: 59Member

Hi,

If I use height of 0.2 m to be the inlet, then the problem is what condition should I use for top 0.3 m ? If I take pressure inlet, it takes secondary phase by default.

That's the problem.

• UKPosts: 8,855Forum Coordinator

If you pick "waves" then the velocity boundary gives

So you specify inlet velocity for both phases. Note, you need sufficient resolution to do this. If you don't have waves we'd tend to use a velocity inlet for the water & pressure for air that's above it.

• Posts: 59Member

Hi,

I have followed all the suggestions.

My domain height is 0.5 m, water height is 0.2 m. I have two inlets- one water inlet , and another is air inlet. Water flows in the channel and air remains stationary. I am using velocity inlet for the water & pressure inlet for air that's above it. I am using open channel wave BC option along with segregated velocity input (water velocity -0.15 m/s, air vel- 0 m/s).

My resolution is high, time step is 0.001 (initially) and 0.01 later on.

But velocity contour is not acceptable.

Kindly suggest.

Regards,

Ayan