2 Aerodynamics
151
obtained. This equation shows that the sound field produced by the interaction between the moving object and the fluid is composed of quadrupole
source, dipole source, and monopole source. The FW-H equation is the
most effective tool to solve the aerodynamic noise radiation problem of
fans, propellers, and compressor rotors.
At the same time, Lighthill’s theory has achieved great success, but has
also led to some new problems. Because the sound field and the flow field
are essentially unified, their governing equations are all N-S equations.
Theoretically, the flow field solution and the sound field solution can be
obtained directly from the equations. However, the Lighthill sound simulation theory divides the solution of the flow field and the sound field into
two steps, which cannot explain such basic problems as how the sound
field and the flow field interact, how the sound energy is generated and
transferred in the fluid, etc. Therefore, after Lighthill, the development
of aeroacoustics continues in accordance with Lighthill ‘s thought on the
one hand, and on the other hand, the connotation and research scope of
aeroacoustics continue to expand. In 1964, Powell Alan put forward the
theory of vortex sound, which shows that the generation of sound wave
is closely related to the interaction between vortices, potential flow, and
vortices in the fluid, and the formation and transformation of sound wave
energy are also completed through these nonlinear interactions.
3. Calculation method of aerodynamic noise
Aerodynamic noise is mainly related to aerodynamics and acoustics, so
the calculation of aerodynamic noise can be divided into two parts: the
generation of aerodynamic noise in the flow field and the propagation
in the sound field. The two parts can be calculated at the same time
or separately. Farassat (1975), the aeroacoustics expert, summed up the
calculation methods of aerodynamic noise as follows: pure theoretical
method; semiempirical method; pure numerical method; CFD combined
with “acoustic analogy”. It is specified below.
(1) Pure theoretical method
The pure theory method is to obtain the analytical results of the flow
field and sound field using the mathematical theory tools directly.
This method is usually suitable for the basic research of the relevant
simplified model, which is an important basis for the development
of other methods, and also a standard tool to verify the correctness
of other methods. The model solved by this method is as simple as
possible on the premise of catching some physical phenomena before
obtaining the analytical solution. For example, Ffowcs Williams and
Hall (1970) solved the aerodynamic noise at the trailing edge of a
151
obtained. This equation shows that the sound field produced by the interaction between the moving object and the fluid is composed of quadrupole
source, dipole source, and monopole source. The FW-H equation is the
most effective tool to solve the aerodynamic noise radiation problem of
fans, propellers, and compressor rotors.
At the same time, Lighthill’s theory has achieved great success, but has
also led to some new problems. Because the sound field and the flow field
are essentially unified, their governing equations are all N-S equations.
Theoretically, the flow field solution and the sound field solution can be
obtained directly from the equations. However, the Lighthill sound simulation theory divides the solution of the flow field and the sound field into
two steps, which cannot explain such basic problems as how the sound
field and the flow field interact, how the sound energy is generated and
transferred in the fluid, etc. Therefore, after Lighthill, the development
of aeroacoustics continues in accordance with Lighthill ‘s thought on the
one hand, and on the other hand, the connotation and research scope of
aeroacoustics continue to expand. In 1964, Powell Alan put forward the
theory of vortex sound, which shows that the generation of sound wave
is closely related to the interaction between vortices, potential flow, and
vortices in the fluid, and the formation and transformation of sound wave
energy are also completed through these nonlinear interactions.
3. Calculation method of aerodynamic noise
Aerodynamic noise is mainly related to aerodynamics and acoustics, so
the calculation of aerodynamic noise can be divided into two parts: the
generation of aerodynamic noise in the flow field and the propagation
in the sound field. The two parts can be calculated at the same time
or separately. Farassat (1975), the aeroacoustics expert, summed up the
calculation methods of aerodynamic noise as follows: pure theoretical
method; semiempirical method; pure numerical method; CFD combined
with “acoustic analogy”. It is specified below.
(1) Pure theoretical method
The pure theory method is to obtain the analytical results of the flow
field and sound field using the mathematical theory tools directly.
This method is usually suitable for the basic research of the relevant
simplified model, which is an important basis for the development
of other methods, and also a standard tool to verify the correctness
of other methods. The model solved by this method is as simple as
possible on the premise of catching some physical phenomena before
obtaining the analytical solution. For example, Ffowcs Williams and
Hall (1970) solved the aerodynamic noise at the trailing edge of a