"bit" phenomenon in Vortex Induced Vibration?
 

hi, all

i'm not sure that the "bit" phenomenon in Vortex Induced Vibration means that there are two more frequencies or amplitudes of structural responses?

i found both the two in my simulation of flow over a circular cylinder, and they didnot show at the same time(that is in the same case).

please lend me a hand , and thanx in advance sincerely.