Noise control in turbomachines has always been a popular topic in industrial R&D and academic research sectors. As one of the typical acoustic noise dampers, Helmholtz resonators have been widely implemented in automobile muffler systems due to their simple structure and good sound absorption characteristics over a low frequency range. However, such Helmholtz resonators generally have a narrow effective frequency bandwidth. Thus optimizing and broadening the resonators’ noise damping capacity is needed. This could be achieved by modifying its resonant cavity structure and/or neck shape. For gas turbine engines with both grazing and bias flows present, perforated plates are widely applied as a sound absorber. The perforated plates could be implemented in either single- or double-layer configuration in practice. In addition, the perforated orifices could be in different shapes and sizes, corresponding to different porosities. Maximizing the acoustic noise damping performance of the perforated plates is also critical and needed.

In this work, a modified Helmholtz resonator with a rigid baffle implemented in its cavity is designed and evaluated to enhance the performances of the transmission loss and to enlarge its effective frequency range. A 2D linearized Navier-Stokes model is developed to discuss the effects of 1) the width Lp of the rigid baffle, 2) the installation position/height Hg, 3) its installation configurations (e.g. implemented to the left or right sidewall), 4) the grazing (i.e. tangential) mean flow Mach number and 5) the shape of resonator neck on noise damping effect. The simulation results show that the maximum transmission loss and the resonant frequencies cannot be projected by applying the theoretical equation ω2=c2S/VLeff , when the rigid baffle is installed in 2 different positions, especially, as the grazing Mu is greater than 0.07. Apart from this, as the cavity width Dr is more than two times that of rigid baffle Lp e.g. Lp/Dr ≤0.5, there is an optimum the grazing mean flow Mu which correspond to the transmission loss peak. As the rigid width increases to Lp/Dr=0.75, there is an additional transmission loss peak at approximately 400 Hz. The structure and sound interaction causes the generation of the 12 dB transmission loss peak at 400 Hz. Finally, changing the conventional neck shape into an arc one brings about the dominant resonant frequency being rose by approximately 20% and the secondary transmission loss peak being increased by 2-5 dB.

Besides the study on the modified Helmholtz resonator, single- and double-layer perforated plates are also considered in this work. Acoustic damping performances of double-layer perforated plates in a flow duct are investigated at different Helmholtz number (He) condition

to analyse the impacts on 1) Mach number, 2) the porosities of the double-layer perforated plates, and 3) the axial length between the two layers. The damping of orifice is represented by noise absorption coefficient α denoting the part of incident noise being damped. In order to investigate the effects, a new model and experiments are created and applied. When 𝑀𝑎 equals zero, αmax is found to be smaller than that 𝑀𝑎 takes other values by experiments. In addition, when 𝑀𝑎 is rose to and above 0.037 and porosity is less than 9%, the partial minimum of noise absorption of the double-layer perforated plates are more discrete and smaller. Furthermore, when σ1 =9% and σ2 =2.25% or σ1 =2.25% and σ2 =9%, the acoustic damping performance of the two plates is quite different in respect of the partial maximum αmax. Besides, αmin and αmax increase by 10% when the value of Lc/Ld changes from 3.5% to 11.7%. Finally, it is shown that the double-layer perforated plates appear a larger absorption coefficient than single-layer plate in a wide He range.

In summary, systematic studies on Helmholtz resonators and perforated plates are conducted to maximize their noise damping performance. Both theoretical, numerical simulations and experimental tests are conducted. The present findings are helpful on designing effective aeroacoustics noise dampers such as Helmholtz resonator and perforated plates for propulsion and power generation applications.