5.2 Analysis of unsteady pressure field
Unsteady simulations were carried out for the fan operating at the
nominal point considering the transient-rotor-stator interface and a
time step of\(\ 41.66\ \mu s\), which is adequate for capturing the
effect of the relative motion. The static pressure fluctuations for one
rotor revolution are invariant with the reference frame and were
recorded at monitor positions at 10 %, 50 % and 90 % of span of
IGV/rotor interface and at the rotor/outlet interface as seen from Fig.
14. Pressure fluctuation and spectrums shown by Figs. 15(a-c) depict 11
lobs corresponding to the blades count, where the first peak corresponds
to the fundamental blade passing frequency (BPF)\(f_{s}=m\frac{\mathrm{\Omega}}{2\pi}N_{r}=1\frac{6000}{60}\frac{1}{2\pi}=1100\ \text{Hz}\).
The harmonics of the pressure signal corresponding to multiples of the
fundamental frequency are embedded in the broad band of the frequency
domain are suitably predicted. The spectrum is mainly connected with the
fundamental harmonic (1100 Hz) and its multiples, and it would be
logical to have essentially this principal one in case of a single row.
The main peaks at the frequency of 1100 Hz and 2200 Hz are also
predicted according to Table 4 and 5. Another peak of lower amplitudes
related to IGV wake occurs at frequencies multiple of 1900 Hz. The
pressure fluctuation related to the potential effect related to blade
movement is propagating both upstream and downstream in the form of
pressure wave. One could observe a great variation of pressure
fluctuations between the points located at interface IGV/rotor and at
rotor /outlet, which the amplitudes tend to decrease downstream of rotor
blades (Fig. 16). Near the leading edge of rotor blades BPF is always
the dominant structure while moving away the potential mode of
interaction is dumped either downstream or upstream. Figure 16
illustrates the existence of other modes of different amplitudes
superimposed related to wakes interaction from IGV and rotor as well as
the secondary flows and tip vortices. A comparison between the harmonics
indicates high peaks at 90 % of span followed by those obtained near
hub, whereas the moderate amplitudes are seen at mid-span. Besides, the
most disturbed region is characterized by the highest amplitude as it is
influenced by the vortex structures near the blade tip and the secondary
flow near the nub.