90
P. Liu
When the airfoil starts at the beginning, because the viscous boundary
layer has not been formed on the airfoil surface, the velocity circulation
around the airfoil is zero, and the rear stagnation point is not at the trailing
edge, but at some point on the upper airfoil surface, the air will flow around
the rear edge to the upper airfoil surface. With the development of time, the
boundary layer on the wing surface is formed. When the air on the lower
wing surface flows around the rear edge, it will form a large velocity and a
low pressure. There is a large counter pressure gradient from the rear edge
point to the rear stagnation point, which causes the boundary layer to separate, resulting in a counter-clockwise circulation, which is called the starting
vortex (as shown in Fig. 2.17). The starting vortices are separated from the
trailing edge of the airfoil and flow downstream with the airflow, and the
closed fluid line also moves with the airflow, but it always surrounds the
airfoil and the starting vortices. According to the law of vorticity retention,
there must be a counter-clockwise velocity circulation around the airfoil, so
that the total circulation around the closed fluid line is zero. This way, the
position of the rear stagnation point of the airfoil moves backward. As long
as the rear stagnation point has not moved to the trailing edge point, there
is a continuous anticlockwise vortex shedding from the trailing edge of the
airfoil, so the circulation around the airfoil continues to increase until the
airflow leaves smoothly from the trailing edge point (the rear stagnation point
moves to the trailing edge, as shown in Fig. 2.18), forming the final attachment vortex and starting vortex (as shown in Fig. 2.19). The starting vortices
are far behind, and the attached vortices are superimposed on the airfoil and
move at a constant speed with the airfoil, which has an important impact on
the aerodynamic force of the airfoil (as shown in Fig. 2.20).
The earliest wings were made to imitate kites, with a piece of cloth on the
skeleton, basically a flat plate. In practice, it is found that the bending plate is
better than the flat plate, and can be used in a larger range of angle of attack.
In 1903, the Wright brothers developed a thin airfoil with positive curvature.
Fig. 2.17 Unbalanced airfoil around the boundary layer during start-up
P. Liu
When the airfoil starts at the beginning, because the viscous boundary
layer has not been formed on the airfoil surface, the velocity circulation
around the airfoil is zero, and the rear stagnation point is not at the trailing
edge, but at some point on the upper airfoil surface, the air will flow around
the rear edge to the upper airfoil surface. With the development of time, the
boundary layer on the wing surface is formed. When the air on the lower
wing surface flows around the rear edge, it will form a large velocity and a
low pressure. There is a large counter pressure gradient from the rear edge
point to the rear stagnation point, which causes the boundary layer to separate, resulting in a counter-clockwise circulation, which is called the starting
vortex (as shown in Fig. 2.17). The starting vortices are separated from the
trailing edge of the airfoil and flow downstream with the airflow, and the
closed fluid line also moves with the airflow, but it always surrounds the
airfoil and the starting vortices. According to the law of vorticity retention,
there must be a counter-clockwise velocity circulation around the airfoil, so
that the total circulation around the closed fluid line is zero. This way, the
position of the rear stagnation point of the airfoil moves backward. As long
as the rear stagnation point has not moved to the trailing edge point, there
is a continuous anticlockwise vortex shedding from the trailing edge of the
airfoil, so the circulation around the airfoil continues to increase until the
airflow leaves smoothly from the trailing edge point (the rear stagnation point
moves to the trailing edge, as shown in Fig. 2.18), forming the final attachment vortex and starting vortex (as shown in Fig. 2.19). The starting vortices
are far behind, and the attached vortices are superimposed on the airfoil and
move at a constant speed with the airfoil, which has an important impact on
the aerodynamic force of the airfoil (as shown in Fig. 2.20).
The earliest wings were made to imitate kites, with a piece of cloth on the
skeleton, basically a flat plate. In practice, it is found that the bending plate is
better than the flat plate, and can be used in a larger range of angle of attack.
In 1903, the Wright brothers developed a thin airfoil with positive curvature.
Fig. 2.17 Unbalanced airfoil around the boundary layer during start-up