2 Aerodynamics
153
noise. The pure numerical method uses the numerical method to
calculate the propagation, so the calculation model allows obstacles
between the sound source and the observation point, while the mixed
method does not. However, due to the magnitude difference between
the sound field pressure and the flow field pressure, this method is
very demanding for calculation. On the one hand, it needs a grid
with dense near-field and far-field, on the other hand, it needs a
low dissipation and low dispersion format. Therefore, the main difficulty of this method is the large amounts of calculation of far-field
aerodynamic noise.
(4) Method of combining CFD with “acoustic analogy”
The combination of CFD and “acoustic analogy” is also known as a
hybrid method, which is the most commonly used method to solve
aerodynamic noise. In this method, the unsteady RANS, LES/DES,
and other methods are used to calculate the sound source, or the
RANS is added to the disturbance factor to simulate the sound source,
and then the LEE (linearized Euler equation) method or FW-H,
Kirchhoff, and other integration methods are used to calculate the
propagation or radiation of the sound wave. The hybrid method is
usually divided into two parts. First, the source information (flow field
characteristics) is calculated, and then the sound propagation or farfield radiation is calculated according to the needs. At present, the
main aerodynamic noise calculation method is the hybrid method
because of the influence of grid, the accuracy of discrete format, the
amount of calculation, and the time of calculation.
2.9 Stall Characteristics of Low-Speed Airfoil
and Wing
1. Stall characteristics of airfoil
As the angle of attack of airfoil increases, the lift coefficient of airfoil
will appear maximum and then decrease. This is the result of the separation of the flow around the airfoil. Stall characteristics of airfoil refer
to the aerodynamic performance near the maximum lift coefficient. The
phenomenon of airfoil separation is closely related to the flow and pressure
distribution on the leeward side of the airfoil.
At a certain angle of attack, when the low-speed airflow bypasses the
airfoil, it can be seen from the pressure distribution and velocity change in
the upper wing surface that the airflow on the upper wing surfacestarts to
153
noise. The pure numerical method uses the numerical method to
calculate the propagation, so the calculation model allows obstacles
between the sound source and the observation point, while the mixed
method does not. However, due to the magnitude difference between
the sound field pressure and the flow field pressure, this method is
very demanding for calculation. On the one hand, it needs a grid
with dense near-field and far-field, on the other hand, it needs a
low dissipation and low dispersion format. Therefore, the main difficulty of this method is the large amounts of calculation of far-field
aerodynamic noise.
(4) Method of combining CFD with “acoustic analogy”
The combination of CFD and “acoustic analogy” is also known as a
hybrid method, which is the most commonly used method to solve
aerodynamic noise. In this method, the unsteady RANS, LES/DES,
and other methods are used to calculate the sound source, or the
RANS is added to the disturbance factor to simulate the sound source,
and then the LEE (linearized Euler equation) method or FW-H,
Kirchhoff, and other integration methods are used to calculate the
propagation or radiation of the sound wave. The hybrid method is
usually divided into two parts. First, the source information (flow field
characteristics) is calculated, and then the sound propagation or farfield radiation is calculated according to the needs. At present, the
main aerodynamic noise calculation method is the hybrid method
because of the influence of grid, the accuracy of discrete format, the
amount of calculation, and the time of calculation.
2.9 Stall Characteristics of Low-Speed Airfoil
and Wing
1. Stall characteristics of airfoil
As the angle of attack of airfoil increases, the lift coefficient of airfoil
will appear maximum and then decrease. This is the result of the separation of the flow around the airfoil. Stall characteristics of airfoil refer
to the aerodynamic performance near the maximum lift coefficient. The
phenomenon of airfoil separation is closely related to the flow and pressure
distribution on the leeward side of the airfoil.
At a certain angle of attack, when the low-speed airflow bypasses the
airfoil, it can be seen from the pressure distribution and velocity change in
the upper wing surface that the airflow on the upper wing surfacestarts to