Experimental and frequency-domain study of acoustic damping of single-layer perforated plates

Perforated plates/liners have been widely applied in aeroengines and gas turbines to dissipate unwanted noise. In this work, numerical simulations and experimental study of acoustic damping performance of single-layer perforated plates are conducted. For this, a frequency-domain numerical model is developed via solving 3-dimensional Helmholtz equations. The model validation is performed first by evaluating and comparing the calculated acoustic damping performances of three perforated plates with the experimental results available in literature. These plates are perforated with a number of circle-shaped orifices, which have different porosities σ. Each plate backed by a resonant cavity is placed at the end of a rectangular-shaped pipe with a loudspeaker implemented on the other end. To simulate real engines, a mean flow (also known as bias flow) is applied to pass through the perforated orifices. The effects of 1) the mean flow Mach number Ma, 2) the resonance parameter Q, 3) the porosity σ and 4) the plate thickness T are studied one at a time. Acoustic damping performance of these plates is characterized by using sound absorption coefficient α and specific acoustic impedance z. It is found that sound absorption coefficient α is increased first and then decreased with increased Ma. The real part of z characterizing acoustic resistance/damping is linearly increased, as Ma is increased. However, the imaginary part of z characterizing acoustic reactance remains almost unchanged. It is also found that the presence of the bias flow greatly increases the maximum sound absorption coefficient αmax by approximately 33% in comparison with that of without mean flow. The plate thickness T is shown to shift the frequency corresponding to the maximum sound absorption coefficient. Comparing the present simulation results with the experimental and analytical ones available in the literature reveals that good agreement is obtained at lower frequency range. However, as frequency is increased, the model predicts that the bias flow leads to the local αmax reduced by 15%. The plate with a larger thickness is associated with another local αmax, which is not observed for a lower thickness plate. Finally, experimental measurements of the acoustic damping of 2 single-layer in-duct perforated plates are performed. The frequency-domain model is then applied to simulate the experiment. Good agreement between our experimental and numerical results is obtained.