Experimental/Numerical Study of Ducted-Fan Noise: Effect of Duct Inlet Shape

The paper presents an investigation of flow and noise produced by the generic fan of an environmental control system installed in a circular duct, with a focus on the evaluation of the effect of mean flow distortions and elevated turbulence levels upstream of the fan, which are here created by replacing a baseline smooth bell-mouth duct inlet by inlets with a rectangular-to-circular transition of the duct cross section and with a T junction formed by two circular pipes. The study includes both experiments, which are aimed primarily at supplying data for a validation of simulation approaches and numerical simulations based on a hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation approach. An acoustic modal decomposition of the measured and computed unsteady pressure fields is then carried out, which allows extracting "pure" (with filtered out hydrodynamic/turbulent fluctuations) acoustic modes propagating upstream and downstream under reflection-free conditions. A comparison with the experiment is performed for both the "raw" pressure spectra on the duct walls and the extracted individual acoustic modes. It is shown that, in terms of acoustics, a replacement of the clean inlet by the rectangular/circular one is almost neutral, whereas the T-junction inlet causes a strong noise penalty.