Learning Forum

[murali]

I had conducted the transient analysis of a beam with a mass at the end. The damping ratio was given as 0.01 for the fundamental frequency of 12.38 Hz (in damping controls). But when I compared the time response of the beam with that in matlab, it seemed to have a huge difference. I had previously conducted the modal analysis of the same system and the natural frequency value had matched. It seems that system in ansys has somehow been more damped than required. What might be the reason? 

[Saurabh Patil]

Hi

 

Based on the context provided, there could be several reasons for the discrepancy you're observing between your Ansys and Matlab results. One possible reason could be the difference in the damping models used in Ansys and Matlab. In Ansys, the damping ratio you specified is used in the modal superposition method for transient analysis. This method transforms the governing equation of motion into an alternate form using modal coordinates and a linear combination of mode shapes. It is more efficient than the full solution method as it converts the coupled equation of motion into modal form. Another possible reason could be related to the 'Missing Mass' method in Ansys. If you have not extracted or included enough modes in your analysis, the 'Missing Mass' effect tries to include the effect of these modes in an additional pseudo-static mode shape. This could potentially lead to a more damped system in Ansys compared to Matlab.

[murali]

 

 

Hi Saurabh,

Thanks for the inputs. Meanwhile I was trying to solve it by changing various analysis settings.

Originally, I did this as a two step analysis, in the first step the initial displacement was given and in thze second step the diaplacement was deactivated. When I did the analysis first, I had turned on the ‘Auto time stepping’ and ‘Large deflection’ ON for the second step. But when I turned both of them off I got a much better result (given below). I was curious about what do these two settings do to the problem that the result varies drastically??

 

 

 

[Erik Kostson]

 

Hi

See this course for an explanation on what the large deflections/goemetric nonlinear analysis is:

/courses/index.php/courses/large-deformation/#:~:text=Solids%20behave%20differently%20when%20they,nonlinearity%20known%20as%20geometric%20nonlinearity.

/courses/index.php/courses/structural-nonlinearity/

 

 

All the best

Erik

 

[murali]

Thanks Erik. The course really helped to bring things into perspective.