106
P. Liu
(3) Periodic shedding stage of trailing edge vortex
With the increase in airfoil velocity, the energy of vortex motion in
the separation bubble increases continuously, which makes the kinetic
energy accumulation in the bubble unable to self consume, so the
separation bubble opens and forms vortex shedding (as shown in
Fig. 2.36). At the same time, the centrifugal inertia force around the
trailing edge is increased and the negative pressure near the trailing
edge is increased due to the increase in air velocity, which causes the
separation point of the trailing edge area of the upper wing surface to
move to the trailing edge (flow from high pressure to low pressure).
At this time, the boundary layer of the flow around the upper wing
begins to form, which is relatively slower than the lower wing, thus
increasing the speed of the outflow area of the upper wing boundary
layer, and the boundary layer flow around the near area of the airfoil
surface begins to play a role. The attached vorticity is not zero, and the
lift begins to be greater than zero. With the increase in airfoil velocity,
the vortices generated from the trailing edge are constantly shed out,
and they are dumped downstream with the flow, and the flow velocity
around the trailing edge is increasing. With the increase in centrifugal
inertia force at the trailing edge, the negative pressure near the trailing
edge increases continuously, which leads to the separation point of the
trailing edge of the upper wing moving further to the rear edge. At
this time, the flow boundary layer around the upper wing develops
continuously, and the flow boundary layer around the lower wing
begins to form and develop. The velocity difference in the outflow
region of the upper and lower wing boundary layer further increases,
the attached vorticity around the airfoil continues to increase, and the
lift also increases.
(4) Stable equilibrium stage of boundary layer
The periodic shedding of the trailing edge separation vortices is
repeated until the airfoil reaches a uniform speed and no longer accelerates. At this time, the separation point of the upper wing surface
Fig. 2.36 Period of falling edge separation vortex periodically falling off (acceleration)
P. Liu
(3) Periodic shedding stage of trailing edge vortex
With the increase in airfoil velocity, the energy of vortex motion in
the separation bubble increases continuously, which makes the kinetic
energy accumulation in the bubble unable to self consume, so the
separation bubble opens and forms vortex shedding (as shown in
Fig. 2.36). At the same time, the centrifugal inertia force around the
trailing edge is increased and the negative pressure near the trailing
edge is increased due to the increase in air velocity, which causes the
separation point of the trailing edge area of the upper wing surface to
move to the trailing edge (flow from high pressure to low pressure).
At this time, the boundary layer of the flow around the upper wing
begins to form, which is relatively slower than the lower wing, thus
increasing the speed of the outflow area of the upper wing boundary
layer, and the boundary layer flow around the near area of the airfoil
surface begins to play a role. The attached vorticity is not zero, and the
lift begins to be greater than zero. With the increase in airfoil velocity,
the vortices generated from the trailing edge are constantly shed out,
and they are dumped downstream with the flow, and the flow velocity
around the trailing edge is increasing. With the increase in centrifugal
inertia force at the trailing edge, the negative pressure near the trailing
edge increases continuously, which leads to the separation point of the
trailing edge of the upper wing moving further to the rear edge. At
this time, the flow boundary layer around the upper wing develops
continuously, and the flow boundary layer around the lower wing
begins to form and develop. The velocity difference in the outflow
region of the upper and lower wing boundary layer further increases,
the attached vorticity around the airfoil continues to increase, and the
lift also increases.
(4) Stable equilibrium stage of boundary layer
The periodic shedding of the trailing edge separation vortices is
repeated until the airfoil reaches a uniform speed and no longer accelerates. At this time, the separation point of the upper wing surface
Fig. 2.36 Period of falling edge separation vortex periodically falling off (acceleration)