Learning Forum

[2712]

Hi
I simulate 3d seawater in the case of dispersion thermal analysis of the power plant cooling water using ansys fluent, the geometry and namely boundary conditions that I modelled is shown in the figure below. In modelling physical models used k-epsilon and active energy. the material used is only one phase, namely water. Temperature normal seawater set 30? and temperature discharge outlet 37?. The boundary conditions are discharged outlets, ambient inlet, outlet, ocean, seabed and seashore. The discharge outlet is the discharge area of the cooling water power plant modelled as an inlet velocity with a velocity of 1.71 m / s and a temperature value of 37?, the ambient inlet boundary condition is modelled as an inlet velocity at 0.4 m / s and a temperature value of 30?. Outlet modelled as outflow. Seabed and seashore as a wall.


For ocean boundary conditions I model as symmetry. what I want to ask, in my case is it okay if I model ocean as symmetry? Can anyone explain that? Or do I need to use other boundary conditions?

[Amine Ben Hadj Ali]

Symmetry or free slip wall is commonly used as alternative to model a "real" free surface: air over sea or river. If you expect that the free surface is not deforming and not wavy than as first approximation it should be enough. If you mean with ocean boundary something else please add more details.

[2712]

thankyou for your responde, 

you explaination is about free surface: air over sea or river. sorry maybe missunderstanding but the ocean boundary conditions that I mean are areas on the high seas not the sea level . what your opinion about this?

 

[Amine Ben Hadj Ali]

Add more details please. If it is the top boundary then it is for me a free surface as it is not facing anything (only air).

[2712]

 

The ocean boundary I mean is the area that I marked yellow

thankyou

[Amine Ben Hadj Ali]

And what is this boundary in reality?

[2712]

it is sea water

[Amine Ben Hadj Ali]

I still do not really understand the model: So the water height is along that sea water / ocean Boundary? In which direction does gravity point to?

[2712]

sorry for my unclear explanation, the water height is along that ocean boundary. the gravity point to in y direction

maybe the picture below explains in more detail

 

best regrads, thankyou

[Amine Ben Hadj Ali]

Yes use symmetry or free surface wall. Do not use Outflow boundary rather pressure outlet and provide hydrostatic profile there.

[2712]

So is it allowed if I set ocean as a symmetry boundary condition?? I need more explanation about that, can you explain more detail about why it's possible to use symmetry boundary conditions in my model? because I want to make sure whether it's true if I set the ocean boundary conditions as symmetry. 

 

best regards, thank you

[Rob]

If you don't expect any flow from the ocean into your domain it can be set as symmetry. Basically, you need to decide what is/can flow where and set the boundaries to suit. Modelling the free surface (water-air boundary) as symmetry is common as it saves the need for using a multiphase model. 

[Amine Ben Hadj Ali]

That is your simulation and model task. You should be aware about inflow and outflows into your domain.

I agree what all what rwoolhou said. Symmetry or free slip means that the normal gradient to the boundary are zero and the vfelocity normal is zero. There is no flow penetration into the boundary.

[2712]

thank you for the best answer, in my model task I have decided that normal velocity into ocean boundary is not dominant so I set zero normal velocity and there is no incoming flow from ocean to the domain. so it is suitable if i set as symmetry?

[SALMAN]

I want to perform steady-state flow simulation in 2D open channel using VOF model. Can I use the SYMMETRY boundary condition at the top wall of the open channel? The image of the model is attached. I am a beginner so somebody please kindly help me out. I have used pressure inlet and pressure outlet at the top wall of the open channel but nothing worked. 

 

[Karthik Remella]

Hello
Yes, you can treat it as a symmetry condition as long as the normal gradient at the boundary is zero. Please have a look at both Rob’s and Amine’s answers. If you expect the waves to interact at this boundary, then modeling it as a pressure outlet makes more sense. It depends on what you expect would happen at this boundary.

Hope this helps.

Best Karthik

[SALMAN]

Thank u very much for your guidance. 

While performing experiments under uniform flow conditions, I observed a small increment (bulge) in the flow depth above the cavity (see figure) but the depth of flow remains uniform in an open channel at upstream and downstream of the cavity. 

Actually water is flowing horizontally in an open channel from inlet then it goes into the cavity and when the cavity gets filled with water than water will start flowing towards the outlet without wave formation.

Can I use symmetry bc for the top surface of channel?

 

[Amine Ben Hadj Ali]

But put it far wary from the region of interest. You can model it as symmetry. Actually I usually natural pressure opening but again as far as possible to avoid affecting the flow through flow reversal. Symmetry can affect if put very near through artificial reflection.