Aeroacoustics damping performances of double-layer perforated pipes at low grazing-bias flow Mach number

Abstract

The present work studies the aeroacoustics damping characteristics of double-layer perforated pipes (DPP) at low grazing-bias flow Mach number. The noise damping mechanism involved with thermos-viscous and vortex shedding, through which acoustical energy is somehow absorbed by a few tiny circle-shaped orifices perforated on the cylindrical pipes. Here, Δb (sound absorption coefficient) is used as a noise damping measure. It characterizes the fraction of incident sound energy being attenuated. DPP are associated with a bias flow passing through each orifice and a grazing flow. It is another flow stream grazing the inner surface of the inner perforated pipe. The grazing flow velocity is denoted by Mach number Mg. Since the DPP have different porosities δi and δo, we use Mb to denote the bias flow Mach number at the outlet of the cooling flow pump. Experimental evaluations on Δa are performed on five double-layer perforated pipes over the Strouhal number range of 0.25 ≤ St ≤ 1.2. It is shown that increasing the grazing flow Mg gives rise to deteriorated overall acoustic damping performances over the measured St range. However, it is found that acoustic damping is improved, i.e., Δa is increase with the implementation configuration of δi > δo.