[pe_sta]

Hello all,

I cannot stop being impressed by the high ratio of versatility over complexity that the REEF3D modelling system displays. However, as a new user I still have some difficulties when the level of customization increases...

I'm trying to simulate the wave field that reaches a specific beach using realistic geography/bathymetry, with SFLOW.

The goal is to use this as the control simulation and then make additional tests by placing breakwaters at specific places, which will hopefully showcase an attenuation of wave energy reaching the coast.

The problem I'm facing is that although things start well, at some point an instability starts developing in the form of a strong depression in eta (see figure - contours show bathymetry with 1m increments). This instability grows and, although it does not lead to a blow-up, eventually it takes over most of the domain. This is a strong sign that something is wrong with my setup. The same happens when I place breakwaters in the grid (as solids).

The domain is about 400m x 700m in size, with a maximum depth of ~20m, and forced by 2nd order Stokes wave generation. I'm using the latest version of REEF3D. I include the control and ctrl files for the run shown in the figure, however I have tried numerous other configurations so far by combining a multitude of parameter changes.
Things I've tried include:

1. Designating faces C12, C13 as either symmetry planes (3), numerical beaches (7), or walls (21).
2. Changing the resolution and crop of the grid.
3. Changing between geodata for topo and solid (G 9)
4. Different convection schemes for momentum (A211)
5. Diffusion for velocities on/off (A212)
6. Roughness on/off (A 218)
7. Different pressure schemes (A 220)
8. Hydrostatic pressure in shallow areas (A 242)
9. Wetting/drying on/off
10. Different turbulence models (A 260)

I have the impression that the reflected wave at the beach is creating this instability, but couldn't find a way to adjust the reflection (or increase the dumping coeff). I understand that adjusting the numerical beach parameters doesn't have to do with the actual beach slope due to topography.

Could anyone please help with hints/suggestions?

Best,
Stamatis

PS: I have run the above configuration successfully, with good results, using the FNPF solver, but could not make the solid breakwaters work and this is why I turned to the SFLOW solver.

[harenaobsidet]

Hello Stamatios,

When you tried running the simulation with FNPF, did you check if the solid was high enough to pierce the water surface?
I just tried the FNPF tutorial, you asked about in a previous thread, with an added solid and that seemed to be working fine. The z-dimension of the solid needed to be a lot larger than the defined domain to account for the geo data.

[Felix_Sp]

Hello

I have two Ideas for SFLOW. First, have you tried a uniform grid? I think SFLOW in general does not support a non-uniform grid yet, as in your setup.

Second. I had a number of problems with wetting and drying for some time (cell gets dry due to a low water depth and generates a small shock wave). Did you try changing A244 or A245 (default for wetting and drying)? Also, I think a non-uniform grid is not accurate for a combination with A245, as the grid size for the wetting and drying is wrong.

Cheers

[pe_sta]

Originally Posted by harenaobsidet

Hello Stamatios,

When you tried running the simulation with FNPF, did you check if the solid was high enough to pierce the water surface?
I just tried the FNPF tutorial, you asked about in a previous thread, with an added solid and that seemed to be working fine. The z-dimension of the solid needed to be a lot larger than the defined domain to account for the geo data.

Thank you Alexander,

I am trying two cases; one with the breakwater slightly submerged (1m bellow the surface) and one with the breakwater piercing the surface.
The surface field in both cases doesn't seem to be affected by the breakwater (in the second case I even made the breakwater a lot taller than the still water level).
I am using S 84 - "hexahedron object" to create the solid. How did you create it?
Apart from the solid, are you changing anything else to the tutorial files?

Best,
Stamatis

[harenaobsidet]

Hello Stamatis,

I only added a S 10 object. The object was higher than the highest point of the geo-data and large compared to the cell size.
However, I only checked the Fifsf value of the FSF output.

Tutorial.jpg

The upper box is a S 84 and the lower one is S 10.
Does this look more like what you want it to be?

[pe_sta]

Originally Posted by harenaobsidet

I only added a S 10 object. The object was higher than the highest point of the geo-data and large compared to the cell size.
However, I only checked the Fifsf value of the FSF output.

The upper box is a S 84 and the lower one is S 10.
Does this look more like what you want it to be?

Yes, I was expecting something similar to your figure. I also made the object much taller than the geo-data highest point, but I'm not getting similar results.
I will check again my input and try what you suggest, i.e. make the object relatively larger compared to the cell size (by decreasing the cell size, as the object's size must remain the same).

One last thing I would like to ask is, can the FNPF solver handle objects that are submerged (e.g. just bellow the surface, but enough to induce wave braking)? I would expect the potential flow solver to be able to handle these situations.

Thanks again,
Stamatis

[pe_sta]

Originally Posted by Felix_Sp

Hello

I have two Ideas for SFLOW. First, have you tried a uniform grid? I think SFLOW in general does not support a non-uniform grid yet, as in your setup.

Second. I had a number of problems with wetting and drying for some time (cell gets dry due to a low water depth and generates a small shock wave). Did you try changing A244 or A245 (default for wetting and drying)? Also, I think a non-uniform grid is not accurate for a combination with A245, as the grid size for the wetting and drying is wrong.

Cheers

Hello Felix,

thanks a lot for your response and suggestions.

I don't understand exactly what you mean by "uniform grid". Does it mean having a square cell? I have been using ocean models with highly irregular or curvilinear grids throughout the years, so the grid I'm using here with a rectangular cell size of ~2.5m x 2.9m seams pretty uniform to me (everything is similar everywhere, all cells are the same size and I think I'm not using any stretching factor in the horizontal direction, i.e. B101/102/103 are off). Am I missing something?


As I mentioned, I tried turning off the wetting/drying by turning A243 off, but this resulted in an early blowup.
I will try again by making the grid more uniform, by removing B2 and trying to make all the cells square. I will also try to change A244 and 245 instead of A243 and let you know how it went.

Cheers,
Stamatis


P.S.:
I understand your hypothesis regarding the creation of small shock-waves, and indeed I find it very plausible. However, there must be an additional existent instability, as, the creation of shock-waves alone does not explain how these instabilities grow so large and take over the entire domain.