Output frequency and time step selection

Altair Forum User

Hi,

 

My question is about integration time step and load output frequency. In case of dynamic stress, how to make sure that the time step is small enough to capture the dynamic effect on the load? (Kind of like a Courant condition).

 

Also when use of Component Mode Synthesis method, where only chosen elastic deformation modes are selected to represent the deformation of a flexible solid, is there a link between highest frequency mode chosen and time step settings?

 

Best,

Jean

Praful

H Jean - To ensure that the time step is adequate, a convergence study is needed. This is done by solving multiple iteration with gradually reducing time step until there is no significant changes in the results. If the forces dont change abruptly, you can achieve convergence  with a relatively higher time step. If the force is very transient, it would need lower step. One effecient way is to break the simulation into multiple simulate and wherever there is higher transient, you use lower steps.

 

Coming to the CMS method, generally higher the frequency, lower time step is needed if that frequency effects are desired. By default, frequencies above 1000 hz are critically damped so that their influence is avoided. So the flexbody is solved efficiently even with a larger time step. This works well for most of the case.

 

This default can be changed if those higher frequencies are of importance. The time step in this case needed should be atleast 1/20th of the inverse of the frequency.

 

Regards

Praful