Aeroacoustic damping performance studies on off-axial double-layer in-duct orifices at low Mach and Helmholtz number

Both experimental and theoretical studies are conducted in this work to evaluate aeroacoustics damping performances of off-axial double-layer in-duct orifices at low Mach (≤0.0041) and Helmholtz number. The damping performance is characterized by using power absorption coefficient α. It denotes the fraction of incident sound energy being absorbed The effects of 1) mean duct flow (behaving like a bias flow) Mach number M_a, 2) the porosities σ₁ and σ₂ of the front- and back-plates (with respect to the flow direction), 3) the off-axial distance d_v (normalized with the plate radius) of the orifice relative to the plate's centre point are experimentally examined first. Varying d_v1 and d_v2 is found to have an impact on the noise damping performance, especially at a higher Ma. Power absorption coefficient α is shown to be increased by 15%, when d_v1 = 30% and d_v2 = 0 in comparison with that when d_v1 = d_v2 = 0. In addition, the aeroacoustics damping performances of the double-layer in-duct orifices are compared with that of a single-layer one. The double-layer perforated plates with a smaller diameter orifice (D_1,2 ≤ 6 mm) are found to involve with a larger α than that of single-layer orifice plates over a broader Helmholtz number range. Finally, a simplified 1D theoretical model is derived to simulate the experimental measurements. It is based on Rayleigh conductivity. It is found that a qualitatively good agreement is achieved for single-layer in-duct orifice between the theoretical predictions and the experimentally measured ones. As far as double-layer in-duct orifices are concerned, there is some discrepancy between the theoretical and experimental results.