Experimental investigation on the influence of liner non-uniformities on prevailing modes

From the in-flight measured circumferential modal spectra of the Rolls-Royce Tay 650 engine mounted on the Fokker 100, it was found that the sound field propagating upstream in the inlet is strongly modulated by intercostal hard-walled strips in the lined area, the non-cylindrical geometry of the duct, and the non-axisjnmnetric flow velocity distribution. To study the effect of the modulation of the acoustic field by the hard-walled strips separately, an experimental program in the NLR spinning mode synthesizer was carried out. In the first place, the effect of scattering of modes of low circumferential order in the absence of flow was studied using an array of loudspeakers as noise source. Sound was generated in the frequency range from 400 Hz to 3000 Hz. The target circumferential mode numbers ranged from 0 to 3. Modal scattering caused by simulated hard-walled strips was studied by measuring the incident and transmitted acoustic energy flux is not much influenced by the scattering of the incident modes, whereas the m-mode spectra of the transmitted field clearly show modulation effects caused by the hard-walled strips and the modulation increases with increasing mode number.