Learning Forum

[Fady Elshazly]

Hello, 

 

I am trying to read the response at different points in LSDYNA. My d3plot is saved at dt of 0.01 sec and the analysis dt is 1*10^-5. When I read the displacement at a node and double differentiate it to get acceleration, the acceleration is logical but different if I read the acceleration directly at this point. What would cause this? I am thinking of dt as when I differentiate I use the 0.01 sec while the direct acceleration is calculated using the analysis dt. Is this hypothesis correct? Or what would be the reason? 

Thanks 

[Jim Day]

This can mostly be explained by the fact your output interval is larger than the time step. Differentiation is especially sensitive to output resolution. You can prove that to yourself by outputting data at 0.005 s (or smaller) and comparing the doubly-differentiated displacement time history to that from your current run.