Separation of hydrodynamic perturbations in acoustic liner insertion loss measurements at a fan rig

Abstract

The scope of this study is the separation of the hydrodynamic pressure perturbations beneath a turbulent boundary layer in acoustic measurements in annular ow ducts. The hydrodynamic pressure perturbations superpose with the acoustic perturbations and cannot be distinguished by a microphone. In ow ducts with acoustical treatment, the magnitude of the hydrodynamic signal fraction can exceed that of the broadband acoustic fraction and therefore bias insertion loss measurements. In order to improve the measurement quality approaches for the separation of the hydrodynamic and acoustic pressure perturbations were studied. The investigations were carried out for an axial array of 60 microphones mounted in the bypass duct of the AneCom Universal Fan Facility for Acoustics (UFFA). In the paper is shown that the dierent constituents could be separated by means of an axial wave number decomposition. In a subsequent step ltered power spectral densities were reconstructed from the decomposed data, which represented solely the downstream propagating acoustic modes. Thus the liner insertion loss could be determined with signicantly improved accuracy. In an accompanied study the suitability of semi-empirical models to predict the spectrum of hydrodynamic pressure uctuations was examined. For this purpose a parametric study was conducted at a specic DLR test rig. Outcome was that using the dynamic pressure as pressure scale and the boundary layer thickness together with the ow speed as time scale an existing model can be adapted to the UFFA fan rig data. Thus, by subtraction of the of the unwanted hydrodynamic contributions the quality of liner insertion loss measurements can be improved also in cases when not enough microphones are available for wave number decomposition.