question about small imaginary part for eigenfrequency

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Hello all,

I am running a study for acoustic in a hard wall cylindrical tank. The problem can be easily solved by separation of variables. Theoretically, the eigenfrequencies are all real. But why the eigenfrequencies solved in comsol included imaginary part, such as 1847.9832949527051+8.535829728636379E-12i. All imaginary parts are very small which are compared with real parts. How and why does this imaginary part exist? I am really confused about this part.

Regards.

Margaret


1 Reply Last Post 6 Aug 2020, 10:05 GMT-4
Henrik Sönnerlind COMSOL Employee

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Posted: 2 months ago 6 Aug 2020, 10:05 GMT-4

Hi,

This is just numerical noise due to roundoff errors. The eigenvalue solver in COMSOL is the same for damped (in which case the eigenfrequencies should be complex) and undamped problems.

You can often reduce the noise by setting a reasonable value in the Search for eigenfrequencies around text field.

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Henrik Sönnerlind
COMSOL
Hi, This is just numerical noise due to roundoff errors. The eigenvalue solver in COMSOL is the same for damped (in which case the eigenfrequencies should be complex) and undamped problems. You can often reduce the noise by setting a reasonable value in the *Search for eigenfrequencies around* text field.

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