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Old   July 22, 2021, 10:45
Default Generate Focused Wave
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Rouxiao Chen
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Dear Sir,

I want to generate focused wave, and I involved B81, B82, B85, B86, B89,B92, B93 in ctrl.txt, but the error that missing input parameter in ctrl.txt appeared. The case can be succussfully done when I removed B81 but the wave maker did not produce focused wave. How should I do for generating focused wave? The B92 is used to generate theoritical focused wave rather than numerical focused wave, right?

Best wishes,
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Last edited by Freya; July 25, 2021 at 10:30.
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Old   July 26, 2021, 04:14
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Arun Kamath
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Hi Freya,
B 92 is used the select the type of wave theory you want to use as the basis for the focused wave generation. Eg. B 92 41 will use the 1st order focused wave theory

Further
B 81 is used to provide the x- y- and time locations for the wave focusing
B 82 the type of focus amplitude
B 85 the spectrum
B 86 the number of components
B 87 the range of angular frequencies involved

Since you want to run a 2D case, the focus location along the y-axis is 0.0

Changing B 81 to 0.0 0.0 19.0 will generate a focused wave at x=0, y=0 at t=19 s in your case.
You receive a missing input parameter as B 81 expects three input values and you provide only two.

Hope that helps
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Old   July 26, 2021, 18:03
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Hi, Dr. Kamath

I have modified my file according to your suggestions. I set focusing location at x=0, y=0, and the focusing time at t=0. However, I got strange focused wave when I plot water surface profile at the focusing location. The maximum amplitude is much smaller than I input by B93 which is 0.05. Furthermore, the output wave frequency is higher than theoretical solution based on same input properties.
Actually, I don't know how to design the focusing time on REEF:CFD because initial time always start from 0 rather than negative time. Is there any principle to design the focusing time or the focusing time is randomly set?
The profile has noises, is there any line to smooth it or the finer mesh should be used to smooth it?

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Old   July 27, 2021, 04:39
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I would advice not to focus at x=0 y=0 t=0.
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Old   July 27, 2021, 08:40
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Hi, Dr. Kamath,

I got very small wave amplitude, even the biggest wave amplitude is almost zero, but I input this parameter at 0.05 in B93. I don't know what went wrong. Did I input a wrong line?

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Old   July 27, 2021, 09:25
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I am not entirely sure what you mean.
The first input to B 93 is the wave height or in your case the focused wave height.

Set the focus location somewhere towards the middle of the wave tank so that the components have sufficient distance to propagate and focus. Also provide some adequate time for this to occur. Pick the times and locations from an experiment, eg. Ning et al. (doi.org/10.1016/j.oceaneng.2009.07.011) or Bihs et al. (doi.org/10.1115/1.4036206)
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Old   July 27, 2021, 09:27
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Also wrt your grid, while REEF3D supports it, I am not really a fan of using -ve coordinates, so wont go into details there.
But most important- your focus location for the central grid (I hope you have changed it after choosing a new focus location) and the focus parameter- 0.05 will barely stretch the grid. Use a ratio closer to 1.
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Old   July 29, 2021, 18:42
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Hi,

I followed your suggestions and input wave height 0.1 into B93, but I still got very small water surface elevations and failed to got focused wave shown as below image. The water surface elevation before 35s is smaller than that after 35s.

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Old   July 30, 2021, 09:59
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Hi Rouxiao Chen,

right now your water depth for the wave theory is 0. Either set B 94 to 1.25 or change the reference frame of your setup, so that the bed is at z=0. The same for the x-coordinate: best to start the tank at 0, as this is the origin for X_f, the focusing location.
Also, you need to turn on iowave by specifying B 90 1.

Another thing: for the given wave, the mesh is overly fine. You will obtain good result with much coarser grids.
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Old   August 8, 2021, 11:37
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Hi,

I sucessfully generated focused wave, but the water surface elevation doesn't has good agreement with theoretical solution. I simulated a very nonlinear case which the wave height is 0.3m and the ouput maximum crest should much higher than 2nd-order theoretical solution. I've search the largest eta over all x location and plot water surface elevation over time at that location, but the peak is smaller than the theoretical solution(shown as below figure). Does this result make sense for REEF3D:CFD?
Since 2nd-order input results in spend much more time to simulation than linear input does and the results will not change much, I used B92 41 for first estimation. Also, refine mesh is a weakly improvement in this case.
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Old   August 18, 2021, 05:26
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Hi Freya,

Good to see your progress. However, I'd like to suggest some changes in your control files for possible improvement of the simulations.

1) Since you are running a 2D simulation, it is good to keep the y-dimension of your numerical wave tank one cell size in control.txt: B 10 0 30 0.0 0.025 0 1.55, since your cell size is 0.025 m.

2) The vertical refinement is better kept uniform around the free surface, the mismatch of the focused crest can be a result of low resolution above water level. Meanwhile, try different stretching factors, for example: B 126 42 1.05 1.5 40 1.25 1.0 4 1.55 1.5, between 1.05 and 1.45 I used a uniform stretching with a factor of 1.0.

3) F 42 command in ctrl.txt should cover the height of the numerical wave tank, in your case should be 1.55 instead of 1.0. F 42 > F60 in most cases: the height of NWT is higher than water level.

4) I see that you didn't have wave gauges for time series, I suppose you used other post-processing tools to extract the time series from VTU files, which is subject to smoothing. I recommend to use REEF3D wave gauge command P 51 15.0 0.0125 in ctrl.txt for obtaining time series.

5) If the result is still not satisfactory after the above changes, you can try to use B 98 3 Dirichlet boundary condition for the wave generation, it might help to preserve the nonlinearity of your input wave.

Try the suggestions and let us know how it works out, I hope they help, good luck!

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