Discussion Forum

this subject have been investigated in great details by the applied mathematician community over the Years. there is a number of possibilities and no perfect solution.
So you need to learn what exist out there and decide what is best for your problem.
Google "transparent boundary condition" as a starter... but again this is a large subject. Typically you cannot find universally transparent BC but partially transparet that will remove some waves but reflect some others... so you need to decide what is best for your problem either from theory or trying different options and compare...

as an example you can choose to be perfectly transparent for normally incident wave but get large reflection for grazing waves.. or the opposite or a blend..

JF

Hi Jean Francois,
I have already read some papers about the topic.
I saw that that, usually, such a BC is implemented as a sort of generalization of the Sommerfeld radiation condition.
In my case I would like to simulate an object which vibrates in a free fluid.
My computational domain is a sphere and the analysis is of eigenfrequency type. I think it is better for my case to have a boundary which is transparent to normally incident waves.
I have already tried some different BCs (all of them are implemented like a mixed-type condition, i.e. Neumann + Dirichlet), but results are always unphysical.
I'm wondering if there exists a way in Comsol to write your own PML (or also an "infinite" element) to mimic a domain with an infinite extension.
Any further suggestion?
Thanks in advance.

Alessandro

I have not done that on a long time.
but out of my memory,

If these BC are properly written and applied [ that is your modelling is correct for the problem you want to solve] the result should not be unphysical.
One way to test it is to get a larger domain [ possibly on symmetric case so that you can use symmmetry to control the number of DOF if memory or cpu time is an issue and compare the result.
after that make sure the bc you use is really the one you want to use [ these things can be tricky sometime]
JF

a radiation boundary condition that is mathematically exact exists and works well; it derives from a boundary element method and has been applied to many problem ,see attached files :

_ application of bem on elastic wave(baw),can work on stokes infinite losses and infinite element for porous flow


the secret here is to use a green function ......

Thanks for the update .. seems that science progress even when you dont monitor it :-) [ I worked on these problems in the late 80's early 90's...]
JF

I wont spent more time than that but reading the wejung_dong paper..

it works in spherically symmetry [ R is the only space variable] so that solutions have only "normal" components [ if you think of the solution as a pseudo differential integral in spherical coordinates " a la Hormander" [ a generalized green function] and this paper in fact is just an application of the well known result that you can write exact transparent condition in One direction of propagation but not in all of them.

My guess is that when you break the symmetry and angular variable teta and phi shows up you lose the "exacteness" of the BC..and then have to find the best compromise....
but this belong to my past so i really cannot say or do more than that o this subject ...

JF

Thanks a lot to Jean Francois and Louvet.
I will read very carefully the pdf in attachment and if I will manage to solve my problem I will post the BC I have employed.
Thanks a lot again!

Alessandro

hi guys ,

i am doing close to that model a buoy loaded. the buoy motion created waves. i was wndring could I use the same mthod to abzorbe the waves?
Ricci : i am new in comsol i want to model this in frequancy domain is posible ? and wht kind of mode i should pick


thanks

Hi,

I am a new user in COMSOL and trying to estimate the radiation force on a sphere using acoustic module. I wonder if the radiation boundary condition on the boundary of sphere makes it a compressible sphere as nothing gets reflected back !
Please help me out !

Kind regards
Puja
Southampton University