Natural frequency extraction - "huge" generalized mass

[ebrehe]

Hi all,

I am interested in natural frequencies of a steel structure, which is in contact with water.

I am expecting sideways and vertical deflection (or vibration or mode shapes whatever you want to call it), and I could get this through Abaqus when there is no contact, i.e. empty machine.

However, when I couple water with the steel, the first mode shape which is vertical deflection of the part happens at zero frequency. Any suggestion regarding the reason of this would be a great help!

The contact between water and steel is made through *tie interaction. (There is no point of using non-linear contact in a linear pertubation analysis)

All the warnings in the .dat file are not relevant with frequency analysis. However as you can see in the attachment, the generalized mass is rather big and I suspect that it might be the problem.

And only the first mode shape occurs at zero frequency, and then the higher mode shapes are OK.

Thank you very much for your help!

 

[ebrehe]

For future reference here is how I solved this problem:

I have added a non-reflecting circular acoustic impedence to model, by this way Abaqus gave the correct results. But beware apparantly "planar" type acoustic impedence has no absorbing qualities so they also don't work. It should be "circular".