On the road to silencing future aircraft engines

Air traffic significantly contributes to the overall environmental burden on airports vicinities, and noise certainly represents the pollution that has the highest impact on people. Although modern fleets are less noisy than decades ago, the race to higher thrust-efficiency and lower fuel consumption unavoidably yields new noise issues, which are more and more challenging to tackle.

The trade-off between air and noise pollution is probably the most important challenge for the aircraft industry in the coming years.

More specifically, modern aircraft engines tend to have much larger fans, thus producing much lower frequencies, while the nacelle (the ”body” surrounding the engine) becomes much shorter and thinner, which limits the available space for acoustic treatments. To date, it is literally impossible to lower those new noise emissions with conventional acoustic material, and technological breakthroughs are urgently required to limit noise impact of future aircrafts.

In our recent paper (Billon et al, Applied Acoustics 191, 108655, 2022), we present a new active technology aiming at providing such low-frequency noise reduction while fitting in a very limited space, comparable to existing acoustic treatments. The so-called ”active acoustic liner” designates a surface made of compact loudspeakers (and the associated electronics, including control microphones), which membranes are controlled so as to present an optimal sound absorption frequency-wise. That way, the active acoustic liner is able to significantly reduce the transmitted sound energy, with tunable frequency ranges of operation. The concept has been optimized through numerical simulations, and a working prototype has been developed and assessed in a flow-duct facility with an external noise source, allowing testing the noise reduction performance under different flow conditions.

Experimental setup picture of the Ca?man wind tunnel tests

Active cells mounted in the Ca?man wind tunnel : a) inside view (without wiremesh), b) outside view

The Ca?man wind tunnel is an experimental test bench composed by a straight duct with a square 66x66 mm2 section. The guided termination reproduces an anechoic condition. Some liners of longitudinal length below 320 mm can be tested with a maximum speed flow of Mach 0.4 (137 m/s). Acoustic quantities are estimated with a four microphones technique in the plane wave regime.

Figure 3a shows the inside view (without wiremesh) of the cells installed in the Ca¨?man wind tunnel. Figure 3b shows the outside view with the electronic cards, the wires for the electrical supply and the wires for the communication between computer and cells. A rigid panel is installed on the opposite side of the active liner and a wiremesh is glued in front of the cells to protect them from the flow.

Figure 4: Effect of the variation of the parameters of the control law (mu1 = 0.4, mu2 = [0.2, 0.4, 1, 2]) and target resistance (Rat = 0.5.r0c0) : a) Absorption coefficient, b) Insertion loss

Figures 4 shows the performances of the control when the parameters of the control law are varied. The mu1 parameter of the local law is 0.4 and mu2 is changed and takes values : 0.2, 0.4, 1, 2. The target resistance is equal to 0.5 times the characteristic impedance of the air. The control law is implemented by pressure-based, current-driven digital architecture for impedance control. Frequencies where the control is efficient depending on the parameters mu1 and mu2 of the control law. The adaptability and the stability of the system between 300 and 1500 Hz with the tested parameters of local control law have been validated. Different indicators are computed (maximum and average insertion loss and absorption coefficient on the frequency band of interest) to compare the configurations.

Acknowledgements: The SALUTE project has received funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 821093. This publication reflects only the author’s view and the JU is not responsible for any use that may be made of the information it contains.

Link to the SALUTE public website : https://salute-h2020.epfl.ch/