How to choose the number of timesteps (time discretization) for vibroacoustic simulation?

MANATEE software automatically checks the quality of the discretization input by the user during the simulation. In case of low discretization quality, some warnings are given in MANATEE console.

The number of time steps is given per rotor revolution, so it is based on the mechanical period of the rotor, which includes slip effect in induction machines. This total number of time steps per mechanical period is defined in the simulation script by

Input.Simu.Nt_tot= 8*1000;

MANATEE automatically identifies applicable time and space symmetries when Input.Simu.is_symmetry= 1. If a symmetry of order 8 is identified in time domain, it is the advised to use a multiple of 8 for Nt_tot.

The number of time steps must be chosen to catch the highest exciting frequency, and to include the maximum acoustic frequency of interest. According to Shannon theorem if the maximum vibration and noise frequency to be studied is Input.Simu.freq_max_spec=6400; Hz the sampling frequency must be at least 2freq_max_spec (12800 Hz in this case).

The maximum frequency of interest depends on the machine topology and on the application - for instance, large electrical machines used in hydroelectric generators have low natural frequencies and low exciting forces due to low rotational speed, therefore main electromagnetically-excited noise and vibration may occur below 1200 Hz ; on the contrary, in small electric motors with high switching frequency noise and vibration issues might occur up to 10 000 Hz, so a smaller simulation timestep is necessary.

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