In the area closing to the boundary, nevertheless, its flow pattern is more complicated and subjected to the boundary condition; the theory is not applicable to this situation any more, and the dominant frequency is the blade frequency.
From the frequency graph of monitors S1~S6 in Figure 10, it is figured out that the dominant frequency of monitors close to blade is 275 Hz and that in the center region is doubling blade frequency. This can be explained by the Kubota theory as analyzed in Section 2.2.