waveNGauges for extract surface elevation affected forces?
 

Dear waves2Foamers:

I compiled waves2foam in OpenFoam 2.3.1. and I did some forced osccilations in the numerical wave tank using waveDyMFoam.

I found that when I included waveNGauges for extract surface elevation, the time series for forces on the cylinder were completely wrong. However, when I removed this file, the force signals were correct. Maybe it was due to something wrong for probe function in dynamic mesh?

Best regards
Hao

Hallo Hao,

This sounds very weird and I do not understand it.

You said that it could have something to do with the probes-function, but are you using probes or forces to extract the force on your structure? 'forces' should provide you with the pressure force on the surface of the structure, whereas probes is using point locations on a moving mesh, but if the probes are static and the structure is moving, it might be sub-obtimal. I had to make modifications to the surfaceElevation utility in order to obtain correct values on moving meshes, so there might be a clash in the code; not that I can see, where it should be coming from.

Furthermore, what do you mean by 'completely wrong'? Could you show an example of the time series. Furthermore, does it matter whether your force/probe is before or after the surface sampling?

Kind regards,

Niels

Hi Niels:

I do not have my case at hand. I will rerun both of them and upload some example figure later on.

The basic set up is that, the case is 2D, in the left and right hand there are two relaxation zones to absorb the radiated waves. The horizontal cylinder is in the middle of the tank in the free surface, half immersed.

I use 'force' function object to in integrate the pressure and skin friction to get the total force on the cylinder over time. Meanwhile I hope that I can use waveGaugesNProbe to measure the radiated wave height at run time. However, when I use both function objects, the force signal are completely wrong. There are almost no variations over time but it should have a sinasoidal signal as I expect.

I said there might be something wrong with probe function on moving mesh because I saw it somewhere that probe function object does not work properly for moving mesh? And is waveGaugesNProbe based on the probe
function? So I just guess that.

I will also have a look at the waveGaugesNProbe on moving mesh and see if it works properly.

Best regards
Hao

I run the two cases, one with surface elevation and one without surface elevation, they have same mesh, fvSchemes and fcSolutions. The one with surface elevation has almost no variation in forces signals, the other one has obvious variation as I expect. I run them in parallel with 8 cores in my desktop.

I also test them in foam extend 3.1, basically there is no problem for force signal no matter with or without surface elevation. However I met serious problem in parallel execution, as I post here:

http://www.cfd-online.com/Forums/ope...ecutation.html

But no problem with parallel execution in OpenFOAM 2.3.1.

Best regards
Hao

Good morning Hao,

To me, it sounds as if there are problems with the sampling utilities on 2.3 (and perhaps even above?). Could you please perform the following couple of tests:

1. Test whether the problem persists on static meshes, e.g. make a dambreak onto your cylinder with and without the surfaceElevation utility.

2. Perform your test case with line sampling instead of surfaceElevation. surfaceElevation is derived from the sampling utility in version 1.6 (if I remember correctly), so there could be some discrepancies.

If the problem still persists for '2', it suggests that there is a problem with multiple sampling in 2.3+ and that it is not related to surfaceElevation itself.

Good luck,

Niels

Hi Niels:

I've tested the cases you suggested.

For 1 and 2, No problem for both.

So it indicates that the problem lies in the surface elevation utility in dynamic mesh in version 2.3.1 (or maybe above.)

Best regards
Hao

Hallo Hao,

Thank you for the testing. It does indeed seem that the problem lies with the dynamic meshes on recent versions of the official release.

To be honest, this is beyond the scope that I 'support' in waves2Foam, so the time horizon for any bug-fix will be far into the future. You are very welcome to discuss, how you could resolve this bug yourselves. I would probably start at the sampling utility and merge the necessary bits of code into a version of the sampling utility for version 2.3.1. I would also appreciate a lot, if you could subsequently share this modification with other waves2Foam users directly in the svn-repository.

Kind regards,

Niels