The synthesis and decomposition of forces do not apply to dynamics in COMSOL?

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Hi everyone.

I have built three 2D plane strain models in COMSOL, each model was applied dynamic concentrated forces (1 MHz, last 10 microsecond), which are equivalent, at point A as shown in the figure named 'resultant decomposed force', and the results were the displacement of Rayleigh wave in the far field (at point B) on the free surface.

Except for the forces are different, everything else for the three models are the same, the results should be the same since the forces are equivalent. However, the results are not the same, results of force 2 and 3 are the same, results of force 1 is different.

The results for force 1, 2, 3 can be found in the attached pictures. I have no idea why the results are not the same. Does the synthesis and decomposition of forces not apply to dynamics?

Would you please help me? Thank you a lot.



2 Replies Last Post Aug 26, 2019, 2:32 PM EDT
Henrik Sönnerlind COMSOL Employee

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Posted: 3 weeks ago Aug 26, 2019, 10:42 AM EDT

Hi,

In principle this should be equivalent. So assuming that you did the decomposition correctly, my first guess would be that the time step or tolerance is too relaxed so that all three solutions are wrong but in different ways.

BTW: Did you use local coordinate systems for the force input, or just a manual tranformation?

Regards,
Henrik

Hi, In principle this should be equivalent. So assuming that you did the decomposition correctly, my first guess would be that the time step or tolerance is too relaxed so that all three solutions are wrong but in different ways. BTW: Did you use local coordinate systems for the force input, or just a manual tranformation? Regards, Henrik

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Posted: 3 weeks ago Aug 26, 2019, 2:32 PM EDT

Hi Henrik,

Thank you for your answer, I solved the problem just now, I used a rotated coordinate system to load the inclined force, but the sign of the angle I put in was wrong, I would like the original frame to rotate 30 degrees clockwise, so it should be a negative angle, but I filled in a positive angle. Once I correct this, the results are correct. Thank you again.

Hi Henrik, Thank you for your answer, I solved the problem just now, I used a rotated coordinate system to load the inclined force, but the sign of the angle I put in was wrong, I would like the original frame to rotate 30 degrees clockwise, so it should be a negative angle, but I filled in a positive angle. Once I correct this, the results are correct. Thank you again.

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