As fundamental vibration data for the fan on steady driving, accelerations at the duct root were measured with an
acceleration pick-up (PCB-352C22). One example of frequency components of the measured acceleration is shown in Fig. 4.
The figure shows the frequency components of 26 Hz (¼f0), which corresponds to the number of revolutions of the fan,
and of 105 Hz, which is four times f0, i.e., N f0: N¼4 is the number of blades (Felli et al., 2008). In addition, the component
of four times N f0, 420 Hz, can also be seen.