FFLD boundary condition Answered
Hello, I am trying to figure out how the FFLD boundary condition works. I want to simulate different scattering objects using the same incoming wave. I split my test example up into two simulations. One runs on a 400x400 grid and propagates a plane wave in y direction (basically the 11 lines example from the onscale website). Physical coordinates are 0 - 0.1 m in both directions.
In a second simulation I want to simulate only the upper half of it: grid 400x200, physical y coordinates from 0.05 - 0.1 m, x as before. On the ymin side I put an FFLD boundary condition.
I write an FFLD-file in the first simulation with nodes i=1...400 j=jstart to jend (around 200) and depending on the values I get an error message that the boundary at y=0.05 is not included in the FFLD data or for some values it works.
j=200 corresponds to 0.05 m as set with the geom ycrd cmd, which is the location of the ymin boundary in the 2nd model.
jstart = jend = 200 -> Error
jstart =199 jend=201 -> Error
jstart = 199 jend =202 works
jstart = 200 jend =202 works
jstart = 200 jend =201 -> Error
In conclusion, storing from 200 - 202 seems to be the minimum that has to be stored.
The only documentation of how the nodes for the FFLD OUT should be selected is that they "should properly encompass the boundary", without further definition of "properly". It also seems to be necessary to calculate stresses with CALC STRS. It would be nice if you could provide some documentation of this feature and what is the minimum to be stored because I would like to better understand under which conditions it works.
Thanks.
Georg
2 comments
Hi Georg,
I am aware there is not much content on FFLD models. We will put some more information on our Help Center so it is clear how to use it. For now I have a word document with some information. In this document, it explains that the FFLD OUT region must span at least 2 elements in the grid to ensure stresses can be calculated correctly.
I hope this helps.
Best Regards,
Chloe
Hi Allison,
thanks for the quick answer. This explains the minimum selection of nodes that worked in my example.
Best regards,
Georg
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