2 Aerodynamics
145
small, the transmission of sound wave satisfies the classical wave equation,
which is a linear wave. If the disturbance is large, the linear wave equation
is not satisfied, and the wave dispersion and shock wave will appear.
The experiment shows that the propagation speed of sound wave is
constant in the same medium and normal temperature and pressure. For
example, the propagation speed of sound wave in air is 340 m/s; in water
is 1345 m/s; and in solid is 4000–8000 m/s. If dissipation is not taken
into account, the waveform of sound wave will remain unchanged in
the process of propagation. The sound wave heard by the human ear is
called an audible sound wave, which is between 20 Hz and 20 kHz. The
wave less than 20 Hz is called infrasound wave, and the wave greater than
20 kHz is called ultrasonic wave.
The sound pressure level is defined as
L p = 10 lg
p 2
p 2
0
= 20 lg
p
p 0
(unit : dB)
Among them, p 0 = 2×10 −5 Pa, which is the reference sound pressure.
The range of sound pressure that the human ear can bear is as follows: the
lowest sound pressure p 0 = 2×10 −5 Pa, the highest sound pressure p m =
20 Pa, which are called hearing threshold and pain threshold, respectively,
with a difference of one million times, between 20 and 120 dB.
Aeroacoustics is a new interdisciplinary subject based on aerodynamics
and classical acoustics. It studies the mechanism and control of the
sound production of gas itself and the interaction between gas and solid
boundary. It has successfully applied the classical acoustics, especially the
physical concepts, basic rules, and skills of dealing with problems similar
to the acoustics of moving media. The noise radiated by the disturbance
caused by the gas flow or the interaction between the object and the gas is
called aerodynamic noise. The main excitation mechanism of aerodynamic
noise is the propagation of internal stress and pressure disturbance in the
medium caused by the relative movement of solid and gas and the irregular
movement of flow itself. The sound source of aerodynamic noise is generated by moving vortex, or free gas movement, or the interaction between
solid and gas. In addition, the heat source can also generate aerodynamic noise. The sound sources of aerodynamic noise can be divided into
monopole (pulsating volume excitation), dipole (oscillating force excitation), and quadrupole (turbulence excitation) according to their different
sound generation mechanism. Monopole sound source is the sound source
formed when the mass or heat inflow into the medium is not uniform, and
145
small, the transmission of sound wave satisfies the classical wave equation,
which is a linear wave. If the disturbance is large, the linear wave equation
is not satisfied, and the wave dispersion and shock wave will appear.
The experiment shows that the propagation speed of sound wave is
constant in the same medium and normal temperature and pressure. For
example, the propagation speed of sound wave in air is 340 m/s; in water
is 1345 m/s; and in solid is 4000–8000 m/s. If dissipation is not taken
into account, the waveform of sound wave will remain unchanged in
the process of propagation. The sound wave heard by the human ear is
called an audible sound wave, which is between 20 Hz and 20 kHz. The
wave less than 20 Hz is called infrasound wave, and the wave greater than
20 kHz is called ultrasonic wave.
The sound pressure level is defined as
L p = 10 lg
p 2
p 2
0
= 20 lg
p
p 0
(unit : dB)
Among them, p 0 = 2×10 −5 Pa, which is the reference sound pressure.
The range of sound pressure that the human ear can bear is as follows: the
lowest sound pressure p 0 = 2×10 −5 Pa, the highest sound pressure p m =
20 Pa, which are called hearing threshold and pain threshold, respectively,
with a difference of one million times, between 20 and 120 dB.
Aeroacoustics is a new interdisciplinary subject based on aerodynamics
and classical acoustics. It studies the mechanism and control of the
sound production of gas itself and the interaction between gas and solid
boundary. It has successfully applied the classical acoustics, especially the
physical concepts, basic rules, and skills of dealing with problems similar
to the acoustics of moving media. The noise radiated by the disturbance
caused by the gas flow or the interaction between the object and the gas is
called aerodynamic noise. The main excitation mechanism of aerodynamic
noise is the propagation of internal stress and pressure disturbance in the
medium caused by the relative movement of solid and gas and the irregular
movement of flow itself. The sound source of aerodynamic noise is generated by moving vortex, or free gas movement, or the interaction between
solid and gas. In addition, the heat source can also generate aerodynamic noise. The sound sources of aerodynamic noise can be divided into
monopole (pulsating volume excitation), dipole (oscillating force excitation), and quadrupole (turbulence excitation) according to their different
sound generation mechanism. Monopole sound source is the sound source
formed when the mass or heat inflow into the medium is not uniform, and