Experimental studies on sound absorption coefficients of perforated pipes with bias-grazing flows at low Mach and Strouhal number

Abstract

This work considers the sound absorption characteristics of acoustic absorbers through which acoustical energy is absorbed partially by a number of circular orifices perforated on a cylindrical pipe (i.e. a perforated liner) in the presence of a mean flow. Sound absorption coefficient Δa is used to characterize the noise damping performance. It describes the fraction of incident acoustical energy being absorbed. To protect such liner in gas turbines and aero-engines, it is subjected to a range of joined grazing-bias flows. The bias flow passes through each orifice to create a cooling flow. Another stream of either cold/hot air flow grazes the same edge side of each orifice. The grazing and bias flow velocities are characterized by Mach number Mg and Mb respectively. Experimental measurements of Δa are conducted on 7 perforated pipes with different porosity δp but implemented in a single-layer configuration, over Strouhal number St ranged from 0.27 to 1.1. It is found that decreasing the grazing flow Mg leads to the liner's damping performance being improved over the interested St range. However, it is shown that when the bias flow Mb is increased, Δa is enhanced. The porosity δp is found to affect Δa and the optimum δp corresponding to max⁡{Δa} is found to be 1.84%. Finally, further comparison study is conducted on single- and double-layer perforated liners. In general, the present work shed lights on aeroacoustics damping characteristics of single-layer liners in the presence of both bias and grazing flow over the low Mach and St ranges.