Wind shear effect on water level

wouterremmerie · April 25, 2016, 08:12

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 · July 12, 2019, 07:40

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 · August 12, 2019, 15:40

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 · December 21, 2020, 00:52

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 · December 28, 2020, 17:01

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?

April 25, 2016, 08:12	Wind shear effect on water level	#1
wouterremmerie Member Wouter Join Date: Aug 2013 Posts: 41 Rep Power: 13	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.

August 12, 2019, 15:40	Approach to modeling the water level	#3
wouterremmerie Member Wouter Join Date: Aug 2013 Posts: 41 Rep Power: 13	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!

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July 12, 2019, 07:40		#2
Lupocci New Member Lupo Ci Join Date: Aug 2010 Posts: 18 Rep Power: 16	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?

December 21, 2020, 00:52		#4
Thaw Tar Member Thaw Tar Join Date: Apr 2013 Location: Yangon, Myanmar Posts: 35 Rep Power: 13	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.

December 28, 2020, 17:01		#5
wouterremmerie Member Wouter Join Date: Aug 2013 Posts: 41 Rep Power: 13	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?