Scientists in Perm have discovered a method to reduce airplane noise.

The study has been published in the journal Russian Aeronautics and conducted with the support of the Russian Science Foundation.

At high speeds, the jet stream from the aircraft engine causes turbulence in the air, resulting in noise. This noise can also arise from interactions with the surrounding environment or obstacles such as parts of the aircraft. In modern aerospace engineering, limiting harmful noise is achieved by increasing the diameter of the engine, which reduces the exhaust velocity and, consequently, the noise.

The aircraft engine can be mounted on the airplane in two ways. Most commonly, it is placed under the wing of the aircraft. In this case, the increase in diameter reduces the distance between the engine and the wing, which amplifies low-frequency noise. The other mounting method is above the wing. This arrangement blocks the engine and reduces the noise from its jet stream. However, if the engine is positioned too closely, the noise from the interaction between the air stream and the wing will increase.

Therefore, aerospace engineering employs special devices known as noise-reducing nozzles. These are channels in a conical or cylindrical shape that manage the acceleration and direction of gas flows exiting the engine. Common types include chevron nozzles with triangular serrations at the edges. While they reduce low-frequency noise, they can increase high-frequency noise. Another type is the corrugated nozzle, which features ribbing in the form of petals. With a limited number of petals (5-6), the effect of the corrugated nozzle is similar to that of the chevron: a reduction in harmful noise at low frequencies and an increase at high frequencies.

Researchers from Perm Polytechnic University conducted experiments showing that the corrugated nozzle with an increased number of petals performs better against both high and low-frequency noise.

“We examined conical, chevron, and corrugated nozzles of the same size with twelve chevron petals. The experiments were conducted in a chamber equipped with a special setup that allows the study of noise from a single-stream jet at flow speeds up to 0.65 M. The nozzles were manufactured using a 3D printer. The flow was generated by two sequentially connected fans with a power of 45 kW, while six microphones measured the outgoing noise,” comments Oleg Kustov, leading researcher at the Noise Generation Mechanisms and Modal Analysis Laboratory of the Acoustic Research Center at PNIPU, PhD in Technical Sciences.

“The experiments showed that the chevron nozzle reduces low-frequency noise at the peak of the emission spectrum by about two decibels, but at frequencies above 6,000 Hz, there was unwanted high-frequency noise—up to 1.3 dB at a frequency of 12,000 Hz. Meanwhile, the corrugated nozzle with twelve petals reduced noise by 2.5 dB across the entire frequency range studied,” explains Igor Kramtsov, associate professor at the Department of Rocket and Space Technology and Energy Systems at PNIPU, PhD in Technical Sciences.

The addition of a closely positioned plate, simulating an aircraft wing, yielded similar results. Compared to the conical nozzle, the chevron increased low-frequency noise, while the corrugated one, on the contrary, reduced it. The researchers note that the practical use of such a nozzle for real aircraft engines is feasible when the jet noise dominates over other sources of noise from the aircraft at certification points. Nonetheless, comprehensive verification of acoustic and aerodynamic characteristics is required for each specific configuration.

The research conducted by scientists at Perm Polytechnic University aids in enhancing aircraft engine designs to improve the acoustic characteristics of airplanes and mitigate the negative impact of aviation noise on human health. The implementation of new design solutions in nozzle engineering will improve the quality of life for people living near airports.