[Dmitrjs]

Hi everyone,

First of all, Niels, thank you very much for the tool and your effort in helping everyone to solve their problems.
My problem is the following:
I am using waveFlume tutorial to simulate waves propagating in a wave tank. I had to change the length of the tank and the depth so that it matches the required dimensions. I need to simulate a wave with a period of 0.913 seconds and a wave height of 0.033 metres. The depth of the wave tank is now 1 metre and the length is 17.965.
The problem I experience is that the wave's amplitude decreases with the length of the tank (x-coordinate) and it is also out of the phase when I compare it to the analytical solution of the Stokes First Wave theory (this is what I am using). The following graphs demonstrate it (surface elevation at x=0, x=4, x=7.46 metres).

x_0.jpg

x_4.jpg

x_746.jpeg

I am also attaching my case files:

https://www.dropbox.com/s/glv29onc0k2ehof/My_case.zip

What I tried to do was refining the mesh significantly, but it made absolutely no difference. I also tried to use Stokes Second Wave Theory, but it gave exactly the same results.
I also went through this thread and found a discussion of the similar problem to mine. It was posted by JanL (Jan Lohrmann), post #57, page 3.

Originally Posted by JanL

Hi Niels,


I'm very interested in your tool and have been testing it recently. I'd like to use it for seakeeping-analysis and hence I'm interested in Stokes-theories at first. I calculated different cases, compared them with the analytical solutions and unfortunately got some strange results. I specifically analysed the results at x=2.5 (middle of inlet-relaxation-zone), x=5 (end of inlet-relaxation-zone) and x=9 (middle of free computational domain). Here is what I have done:

1. I have been solving the standard tutorial of waveFlume for StokesFirst. The standardised amplitude changes by the distance from the inlet-relaxation-zone quite dramatically (see figure 1).
2. Since that case is designed for shallow water, I modified the domain to a depth of 2m (which I also changed in waveProperties.org) to have a deepwater case. Now the amplitude generally decreases by the distance from the inlet-relaxation-zone and the wave-length increases, so that the waves get out-of-phase from the analytical solution (see figure 2).
3. Afterwards I tried StokesSecond in the standard waveFlume case which gives almost perfect results (see figure 3)!
4. Running the same case for deepwater (2m) again produces results with lower amplitudes and longer wave-lengths (see figure 4).

Have you, or anybody else experienced similar results? I already tested different settings like refining the mesh or the time-step or calculating under turbulent conditions which had different minor effects on the solution.
Have you any ideas what could have gone wrong?


Regards


Jan

It seems like he had some problems with using the inappropriate amplitude with the deep water case which was not valid for Stokes First Theory. After he changed the wave height to 0.02 (with the depth of 2) it worked out for him. In my case I am having a wave height of 0.033 with the depth of 1 and it should be valid for Airy Stokes Theory.

I would really appreciate if someone would take a look at my problem or maybe someone has had it already? Thank you in advance!

[ngj]

Good morning

@Dimitrij: When you are having less than 2 cells over the wave height, I am not surprised that you are having dissipation of wave energy.

Kind regards,

Niels

[Dmitrjs]

Hi Niels,
Thank you very much for your quick reply to my post (post #822). I refined the mesh to get around 13 cells over the wavelength, but still I get the same results and the amplitude decreases with time. The graph from this simulation is shown below which compares the solution at 2 metres and 7.46 metres:

0.913_13_cells.jpeg

I am also attaching my case files:
https://www.dropbox.com/sh/6mhys9pci...iNmfXNaXho7yoa

Could there be any other reasons why I am getting such results? What else can I try to resolve this problem?

Thank you in advance,
Dmitrijs

[ngj]

Hi Dimitrij,

Try more points per wave length, 13 are way too little. If the problem continues, could you please try another version of OF. The VOF scheme was change from 2.2 to 2.3, so there might have been introduced some additional diffusion.

Kind regards,

Niels

[Dmitrjs]

Hi Niels,
Thank you again for your replies! Can I just ask what would be the recommended number of points per both wave length and wave height to get reasonable results?
Thanks,
Dmitrijs

[Dmitrjs]

Originally Posted by Dmitrjs

Hi Niels,
Thank you again for your replies! Can I just ask what would be the recommended number of points per both wave length and wave height to get reasonable results?
Thanks,
Dmitrijs

I addition to this question, may I ask is it possible to control the reflection from the relaxation zone (e.g by specifying some damping coefficient)? Would that mean making changes to the source code?

Thanks,
Dmitrijs