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Aeroacoustics


Aeroacoustics

Aeroacoustics is a division of physics, which studies the problems of aerodynamic noise generation, noise propagation in the moving medium, noise interaction with unsteady flow, as well as the methods of acoustic and aerodynamic noise reduction. Aeroacoustics is closely related to the divisions of continuum dynamics, such as aerodynamics, hydrodynamic stability, vortex dynamics, and turbulence theory.

Aeroacoustic noises can be divided into two groups: the noise caused by mixing of the air particles (for example, the jet noise) and the noise that appears in the flow over solid bodies (for example, in the flow over an aircraft, propeller etc.). Formation of vortices and their fast motion in nonuniform flow field around solid bodies as well as in the gas outflow into a steady-state or moving medium is the main cause of aerodynamic noise generation. Unsteady flow components in the boundary layers near streamlined bodies or in the free layers such as the jet mixing zone lead to continuous vortex generation and increase the flow turbulence. Due to the medium compressibility, part of the flow energy is radiated in the form of acoustic waves. Thermal processes that occur during combustion, as well as in the heated gas flows, play a significant role in the aeroacoustic noise generation.

Characteristics of the turbulent flow noise are calculated based on the nonuniform convective wave equation under certain assumptions. For example, M. J. Lighthill considered small flow velocities, which allowed him to determine velocity of the sound emitted by the entire jet and its separate segments, as well as spectrum and directionality characteristics of the turbulent jet noise. Turbulent subsonic jet creates almost continuous wide-band noise. In case of the jet outflow from the nozzle, high-frequency noise is emitted near the nozzle, and low-frequency noise is emitted farther away from the nozzle, with maximum sound emission being observed at an angle of approximately 30° to the jet axis.

The spectrum of noise from propellers and turbines contains both wide-band background noise caused by the flow over the blades and harmonic components, which frequency is proportional to the product of the number of blades by the propeller revolution. This noise is calculated using Gutin approach, according to which the blades’ effect on the medium is replaced by the action of elementary forces distributed along the blade.

Noise propagation in the channel with impedance walls is of great interest in aeroacoustics. This is due to the necessity to create aeroacoustic noise suppressors. Choosing the wall impedance allows to efficiently reduce the noise within the chosen frequency range.

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