When an aircraft takes off, the noise level from the jet stream of an aircraft engine can reach values from 110 to 130 dB, which can cause painful sensations for people and animals in the vicinity. To reduce noise, comprehensive measures are used, including banning the construction of residential buildings near airports, developing new take-off and landing methods, and changes in engine design.
The main source of noise is the engines themselves. Engineers are working to develop sound-absorbing materials for the internal engine ducts and passenger cabin trim. On the other hand, a large part of the noise reduction task is related to the sound from the jet of gases beating out of the engine nozzle. One of the most effective methods of noise reduction is to reduce the velocity of the gases by increasing the diameter of the fan.
However, this approach can lead to an increase in size and weight, as well as deterioration of the aerodynamic characteristics of the aircraft. Now they are looking for another way – to change the design of engine parts, for example, nozzles. Scientists at Perm National Research Polytechnic University (PNIPU) have found out which of the innovations is the most effective.
A nozzle is necessary to create a jet that provides movement and acceleration of the aircraft. Usually the nozzle has the shape of a truncated cone. Using different mechanisms to reduce jet noise can result in increased engine power loss. It is important to find the optimal balance to effectively reduce noise with minimal loss of thrust. Chevron nozzles, which have triangular teeth along the edge to improve mixing between the jet and the surrounding air, are now widely used. This results in a reduction in low-frequency noise of the gas jet, but it may increase at high frequencies.
Another type of nozzle is corrugated with wave-like folds on the shear. Recent research has shown that increasing the number of corrugations (more than 4-6 pieces) can increase the nozzle noise reduction efficiency. This creates the potential for improved noise reduction at both low and high frequencies. PNIPU researchers are working to optimise the design of corrugated nozzles for more effective noise reduction and improved overall performance.
PNIPU has conducted a series of studies to optimise the design of corrugated nozzles and reduce jet noise. By 3D printing thermoplastic nozzles with different numbers and sizes of lobes, they compared the results with the use of standard conical nozzles. The analysis showed that increasing the number of lobes to 12 and the amplitude of the corrugations significantly reduced the noise level and extended the noise reduction frequency range. Additional calculations have confirmed that nozzles with improved design, while maintaining noise reduction efficiency, have minimal thrust loss, which makes them comparable to chevron nozzles.
The use of nozzles with 12 lobes allows to reduce the noise level up to 2-3 dB in the frequency range from 20 to 20 000 Hz. A slight deterioration of thrust characteristics is possible, but these changes are extremely small, not exceeding 0.4-0.6 per cent. These results are considered acceptable and further research of jet noise reduction methods is planned to create safer and more environmentally friendly turbojet nozzles.
