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.