Abstract
When determining the acoustic characteristics of a single-layer sample of a honeycomb liner by experiments in interferometers with normal wave incidence, a significant effect of incomplete lateral cells of the sample on acoustic characteristics was revealed. The experiments were carried out on two interferometers with channel diameters of 30 and 50 mm in the frequency range 500-3500 Hz at sound pressure levels of 120-155 dB. Based on the results of the experiments, an assessment was made of the effect on the acoustic characteristics of the percentage of perforation, which is the same both for one honeycomb cell and for the entire SAS sample. Based on the results of the research, recommendations were made to improve the accuracy of obtaining the acoustic characteristics of samples of honeycomb liner when testing them in interferometers of normal wave incidence.
Highlights
In connection with the tightening of international standards for aircraft noise on the ground, it is necessary to constantly improve the means of reducing the noise of aircraft engines and to include in consideration an increasing number of possible factors affecting the efficiency of such means.As you know, in modern turbofan engines, due to the high degree of bypass, the dominant source is the fan [1, 2], and the main means of reducing the fan noise are resonant liner
The first of these drawbacks can be the reason that semi-empirical models weakly "catch" the influence of small deviations of the geometric parameters of the liner, due to the manufacturing technology, on the acoustic characteristics, which can be very strong, as field tests of liner samples show
In the results of numerical simulation and experimental data, there is a good agreement between the values of the resonance frequency and characteristics in general. This is important for confirming the operability of the applied numerical modeling technique in predicting the acoustic characteristics of the liner used for facing the channels of an aircraft engine
Summary
In connection with the tightening of international standards for aircraft noise on the ground, it is necessary to constantly improve the means of reducing the noise of aircraft engines and to include in consideration an increasing number of possible factors affecting the efficiency of such means. Liner design is based on semi-empirical models [3,4,5,6,7,8,9,10], the main disadvantages of which include: 1) simplified mathematical formulations, which do not fully take into account physical effects (compressibility, viscosity, thermal conductivity, vortex formation), which are important from the point of view of an accurate description of SAS operation processes at high sound pressure levels typical of aircraft engine channels; 2) relatively large scatter of semiempirical coefficients used in the models (for example, the attached length of the resonator throat varies from 0.785 to 0.85 of the diameter of the perforation hole in different works [11,12,13], and in the presence of a tangential flow it varies even more [14]), which noticeably affects the accuracy of the description of the liner impedance; 3) the acoustic characteristics of only 1layer liner are predicted relatively acceptable The first of these drawbacks can be the reason that semi-empirical models weakly "catch" the influence of small deviations of the geometric parameters of the liner, due to the manufacturing technology, on the acoustic characteristics, which can be very strong, as field tests of liner samples show. This work continues the study of the influence on the acoustic characteristics of the features of the geometry of liner samples of different diameters, manufactured for testing in interferometers of normal incidence
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