6 Wind and Water Tunnel Equipment
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boundary layer of cave wall. When the Mach number of the incoming
flow is approximately 0.8, the Mach number reaches 1 in the minimum
section. Although the incoming flow is still in the subsonic velocity range,
no matter how to improve the pressure ratio of the wind tunnel, the Mach
number of the incoming flow cannot be further increased due to the limitation of the minimum section flow. In other words, there is no transonic
flow in the solid wall section. The ventilation wall solves this problem. The
ventilation wall is adopted in the test section and the air in the laboratory
is removed appropriately. A part of the air in front of the model flows into
the laboratory, and the part that could not pass due to the limitation of
the minimum section could be discharged through the laboratory. In this
way, the Mach number of the flow before the minimum section can exceed
0.8, thus establishing a transonic flow in the test section. As long as the
indoor pressure is kept appropriately low, equal to the Mach number at the
time of wind tunnel operation and the corresponding static pressure under
the total pressure, then, once the airflow enters the test section, some air
will pass through the ventilation wall and enter the chamber. This flow
that passes through the ventilation wall continues until the pressure in
the test section is equal to the pressure in the chamber. Finally, a uniform
transonic flow with the Mach number in line with the requirements of
wind tunnel operation is obtained. Thus, when the airflow flows downstream along the test section, using the ventilation wall to continuously
reduce the airflow flow through the test section can play the same role
as the geometric nozzle. In the process of the wind tunnel, how to keep
the indoor air out? One method is pumping, in which a vacuum pump
is used to remove air from the chamber, and the air extracted should flow
back into the wind tunnel at the appropriate location for a recirculating
continuous wind tunnel. The other method is the mainstream ejector
type. Import cross-sectional area is slightly greater than the test section
diffuser outlet cross-sectional area, forming the gap from the chamber to
the diffuser. Under the influence of the mainstream of the ejector, the air
in the chamber is drawn out through a slit into the diffuser. The size of
the pressure in the chamber is related to the size of the aperture. The size
of the aperture of imports can be adjusted by adjusting the diffuser wall
plate. (2) To reduce or eliminate the interference of the reflected wave
of the tunnel wall. When the Mach number of the test section is close
to 1 or slightly greater than 1, the reflected shock wave generated from
the model with a large wave angle meets the real wall and returns to the
model. If the shock wave meets the free air boundary (“air wall”), it generates a reflection expansion wave that hits the model back. The reflected
421
boundary layer of cave wall. When the Mach number of the incoming
flow is approximately 0.8, the Mach number reaches 1 in the minimum
section. Although the incoming flow is still in the subsonic velocity range,
no matter how to improve the pressure ratio of the wind tunnel, the Mach
number of the incoming flow cannot be further increased due to the limitation of the minimum section flow. In other words, there is no transonic
flow in the solid wall section. The ventilation wall solves this problem. The
ventilation wall is adopted in the test section and the air in the laboratory
is removed appropriately. A part of the air in front of the model flows into
the laboratory, and the part that could not pass due to the limitation of
the minimum section could be discharged through the laboratory. In this
way, the Mach number of the flow before the minimum section can exceed
0.8, thus establishing a transonic flow in the test section. As long as the
indoor pressure is kept appropriately low, equal to the Mach number at the
time of wind tunnel operation and the corresponding static pressure under
the total pressure, then, once the airflow enters the test section, some air
will pass through the ventilation wall and enter the chamber. This flow
that passes through the ventilation wall continues until the pressure in
the test section is equal to the pressure in the chamber. Finally, a uniform
transonic flow with the Mach number in line with the requirements of
wind tunnel operation is obtained. Thus, when the airflow flows downstream along the test section, using the ventilation wall to continuously
reduce the airflow flow through the test section can play the same role
as the geometric nozzle. In the process of the wind tunnel, how to keep
the indoor air out? One method is pumping, in which a vacuum pump
is used to remove air from the chamber, and the air extracted should flow
back into the wind tunnel at the appropriate location for a recirculating
continuous wind tunnel. The other method is the mainstream ejector
type. Import cross-sectional area is slightly greater than the test section
diffuser outlet cross-sectional area, forming the gap from the chamber to
the diffuser. Under the influence of the mainstream of the ejector, the air
in the chamber is drawn out through a slit into the diffuser. The size of
the pressure in the chamber is related to the size of the aperture. The size
of the aperture of imports can be adjusted by adjusting the diffuser wall
plate. (2) To reduce or eliminate the interference of the reflected wave
of the tunnel wall. When the Mach number of the test section is close
to 1 or slightly greater than 1, the reflected shock wave generated from
the model with a large wave angle meets the real wall and returns to the
model. If the shock wave meets the free air boundary (“air wall”), it generates a reflection expansion wave that hits the model back. The reflected