Learning Forum

[kashfia.mahin]

I have done a simulation in the transient analysis where the time step size is 0.0002 sec and step end time is 0.1 sec and 500 steps have been created. Now I want to convert it to the frequency domain. nCan someone explain to me the theoretical way to find the maximum Frequency?n

[peteroznewman]

nHave you done a Modal analysis? That should always be run to learn what natural frequencies are in the structure whenever a transient analysis will be done. Note that the default number of modes requested is only 6 but you should ask for 20 modes. Create Deformation plots for each of these nodes. Those can be created with a few clicks, not 20 individual plot requests. Look at the animation of each of these mode shapes. Decide on the highest mode shape of interest you would want to see in the transient simulation. Call that highest frequency f. The time step size needed to see that frequency in the transient simulation is t = 1/20f seconds. Running that calculation in the other direction, the highest frequency you can see in your transient simulation with a 0.0002 second time step is 250 Hz.nWhat is the load in your transient analysis? You should compute the FFT of that load to see what frequencies are in the load. What is the peak frequency? The time step should be no larger than 1/20f of that frequency.nWhich mode from the modal analysis is closest to the peak frequency in the FFT of the load? That mode is likely to be excited by the transient load.nIs the load periodic? If so, you could compute the steady-state response of the structure to that load using a Harmonic Response analysis.n