[wouterremmerie]

Hi all!

I'm working on a case (can't share too much, so I'll keep it abstract enough) where I want to analyse the effect of the wind on the water leven in a basin.

Basin could be for example 50x50m, with just 100mm of water in it (with the basin 200mm deep). Then, the wind blows parallel to the surface, "dragging" the water along, causing a pile-up at the far end of the basin.

At least that is what happens in reality, now I want to model it.

My approach was the following:
- use interFoam
- use blockMesh to generate a mesh which has very low thickness cells, because I'm mainly interested in what happens vertically (I want to see small changes in water level) and I don't expect much changes in the horizontal plane. --> cells with for example 1000x1000x10mm size (with 10mm being the height).
- the floor is not entirely flat, so these cells have a slight angle;
- build a "wind tunnel" around the setup, using symmetry at the sides and the top
- wind speed given at the inlet, open patch at the outlet.

First results show that there is indeed interaction between wind and water, but when plotting the water surface (iso surface for alpha = 0.5), the water level actually goes down towards the end of the basin. also, there is quite some difference between the 0.01 and 0.99 alpha levels (quite a lot of mixing between water and air?). The 0.01 level actually does rise towards the end of the basin. But which one should I believe?

Do I need more detail in the mesh?
Do I need to post-process differently (different alpha values)?
Do I need to use a different solver?
...


Many thanks for any inputs!

Regards,

Wouter.

[Lupocci]

Hi I would like to do the same thing. Did you have any success? i run at test case with air at 5 m/s and a quiescent sea underneath and water does not move. How can I include the shear stress at the VOF interface?

[wouterremmerie]

Dear Alberto,

it's been quite a while but yes, in the end I managed to get it to work, luckily!
Still can't share anything, as it was for a customer under confidentiality.

But it worked by creating cells that were very thin in the vertical direction, to capture enough vertical detail, but still long & wide enough to limit total cell count.

Ended up having cells with an aspect ratio of around 100. It was difficult to get it all stable, had to gradually make them more "square" towards the ends of the basin. So that meant playing a lot with blockMesh (did the entire mesh with BlockMesh).

Hope this helps you at least a bit!
Keep trying, it'll work, it's possible!

[Thaw Tar]

Same problem here. Blowing air at 3.3 m/s and the free-surface is not moving

Tried very thin cells for free-surface but still not working.

[wouterremmerie]

Hi Thaw,

it's been about 6 years by now, I don't remember how I got it working in the end, apart from the very thin cells...perhaps you need A LOT of iterations to start seeing the effect?

[aquaris21]

Hi Wouter,

i know it's been such a long time, but in case you still remembered... Why did you chose the VOF method over Euler-Euler (as in twoPhaseEulerFoam for example)? I have a similar problem and I'm encouraged to use the second one by my boss (because you have more control over each phase and how they interact, to correctly simulate the wind dragging the water) but I'm getting better results with interFoam (VOF), maybe you have some insights on this?

Thank you very much.

[wouterremmerie]

Ah, this has indeed been years!!

I was not even aware of the Euler-Euler method (it may have existed already back then, but I don't know). So I never made any comparison, and mainly went by what I had seen from the dam break tutorial...

So I have to disappoint you, the choice was not based on any intelligent comparison

I did work on another project afterwards, analyzing the wind loads on water levels on a roof under wind shear loads (piling up the water on one side) - worked pretty well, but I didn't have any data to validate against.

And then I put all my time in AirShaper, so no more multiphase (at least for the time being ).

Good luck!!

[aquaris21]

Thank you very much for your reply, I didn't think you were still receiving these notifications.

It's quite helpful though; it goes to show that you CAN model the wind dragging the water, contrary to what I've been told.

I may just develop both methods in parallel and in the end see which one's better.

The problem is all the computational resources that OpenFOAM is taking with still "only" 400.000 cells... But we'll see.

Cheers!

[wouterremmerie]

I'm glad it was helpful!

One thing I did do, was the following:
the roof was very large (football field size) and the water level deltas very low (mm to cm). So I worked with very thin, high aspect ratio cells across most of the surface. Only towards the edges of the roof I improved the aspect ratio a bit to avoid crashes (as the flow is more 3D there).

This allowed me to have mm resolution on the water level, while still analyzing a large surface area.

Hope this makes sense and is helpful!