422
P. Liu
wave changes the pressure distribution on the surface of the model and
destroys the normal flow around it, which is completely different from the
flow around the prototype flow field. After the ventilation wall is adopted,
there are both real wall and “air wall” on the ventilation wall. Some shock
waves generated by the model meet the real wall and generate reflected
shock waves, while waves generated by the other part meet the “air wall”
and generate reflected expansion waves. These separately reflected shock
waves and reflected expansion waves meet at a certain distance from the
cave wall and cancel each other out. As long as the ventilation rate of
the wall is appropriate, the interference of the wall reflection wave can
be minimized. In particular, when the geometric parameters of the hole
are properly selected, the “no reflection” degree can be achieved within
a considerable Mach number range. (3) To reduce or eliminate the cave
wall interference in subsonic velocity. The cave wall interference is very
serious in subsonic velocity in transonic wind tunnel. The effect of wall
interference in closed and open sections is the opposite. Therefore, the
use of ventilation wall, as long as the open-close ratio of ventilation wall
selection is appropriate, can reduce or nearly eliminate wall interference.
2. Typical wind tunnel
The world’s first practical transonic wind tunnel was developed in 1947.
At present, there are two development trends of transonic wind tunnel:
one is to further increase the Reynolds number of transonic experiment;
the other is to further reduce the interference of transonic tunnel wall.
With the development of aerospace industry, the Reynolds number of
modern large aircraft flying at transonic speed is as high as 6 × 10 7 while
the experimental Reynolds number of general transonic wind tunnel can
only reach 1/10 of the flight Reynolds number, and the maximum is
not more than 1/6. The difference between the experimental Reynolds
number and the flight Reynolds number is so large that many experimental results are unreliable. For example, because of the low Reynolds
number in the experiment, the location and pressure distribution of shock
waves on the wing surface in the wind tunnel experiment are far different
from the flight situation. Due to the low experimental Reynolds number,
it is difficult to carry out the experiment of large angle of attack and high
maneuverability of aircraft, and to carry out the experimental research
of advanced airfoils such as peak airfoils and supercritical airfoils. If the
experimental Reynolds number is increased to more than or equal to 4 ×
10 7 (the characteristic length is the average aerodynamic string), it is generally considered that the change in aerodynamic coefficient with Reynolds
number is no longer significant. This experimental Reynolds number is
P. Liu
wave changes the pressure distribution on the surface of the model and
destroys the normal flow around it, which is completely different from the
flow around the prototype flow field. After the ventilation wall is adopted,
there are both real wall and “air wall” on the ventilation wall. Some shock
waves generated by the model meet the real wall and generate reflected
shock waves, while waves generated by the other part meet the “air wall”
and generate reflected expansion waves. These separately reflected shock
waves and reflected expansion waves meet at a certain distance from the
cave wall and cancel each other out. As long as the ventilation rate of
the wall is appropriate, the interference of the wall reflection wave can
be minimized. In particular, when the geometric parameters of the hole
are properly selected, the “no reflection” degree can be achieved within
a considerable Mach number range. (3) To reduce or eliminate the cave
wall interference in subsonic velocity. The cave wall interference is very
serious in subsonic velocity in transonic wind tunnel. The effect of wall
interference in closed and open sections is the opposite. Therefore, the
use of ventilation wall, as long as the open-close ratio of ventilation wall
selection is appropriate, can reduce or nearly eliminate wall interference.
2. Typical wind tunnel
The world’s first practical transonic wind tunnel was developed in 1947.
At present, there are two development trends of transonic wind tunnel:
one is to further increase the Reynolds number of transonic experiment;
the other is to further reduce the interference of transonic tunnel wall.
With the development of aerospace industry, the Reynolds number of
modern large aircraft flying at transonic speed is as high as 6 × 10 7 while
the experimental Reynolds number of general transonic wind tunnel can
only reach 1/10 of the flight Reynolds number, and the maximum is
not more than 1/6. The difference between the experimental Reynolds
number and the flight Reynolds number is so large that many experimental results are unreliable. For example, because of the low Reynolds
number in the experiment, the location and pressure distribution of shock
waves on the wing surface in the wind tunnel experiment are far different
from the flight situation. Due to the low experimental Reynolds number,
it is difficult to carry out the experiment of large angle of attack and high
maneuverability of aircraft, and to carry out the experimental research
of advanced airfoils such as peak airfoils and supercritical airfoils. If the
experimental Reynolds number is increased to more than or equal to 4 ×
10 7 (the characteristic length is the average aerodynamic string), it is generally considered that the change in aerodynamic coefficient with Reynolds
number is no longer significant. This experimental Reynolds number is