6 Wind and Water Tunnel Equipment
423
close to the flying Reynolds number, known as a high-Reynolds number
wind tunnel. The methods to increase the experimental Reynolds number
are as follows: increasing the size of the wind tunnel; high density gas
being used as the working medium of the wind tunnel; increasing the total
pressure of airflow in the wind tunnel; reducing the total temperature of
the airflow. Increasing the size of a wind tunnel, like a low-speed wind
tunnel, is not realistic for a transonic wind tunnel because it is too expensive to build. It is theoretically feasible to change the working medium
of the wind tunnel to increase the experimental Reynolds number, but
there is no gas with the same specific heat capacity as air, higher density
than air, and cheaper. Since 1966, pressure wind tunnel has been used to
increase the experimental Reynolds number, which has achieved certain
results. However, it has caused an increase in dynamic pressure, resulting
in the strength of the model and the support. Meanwhile, the driving
power of the wind tunnel has increased significantly, which has limited
the further improvement of the Reynolds number. Since the 1970s, a
new type of wind tunnel, low-temperature wind tunnel, has shown more
and more advantages. A low-temperature wind tunnel is a wind tunnel
where the temperature of the working medium is less than 173 K. As
the temperature decreases, the viscosity coefficient μ and sound velocity a
decrease, the density ρ increases, and the Reynolds number Re increases.
With lower temperature, while the density increases, the sound velocity
decreases, and wind speed decreases under the same Mach number, thus
dynamic pressure can basically remain unchanged. The driving power of
the wind tunnel will drop slightly, to avoid the general pressure due to the
increase of Reynolds number in the wind tunnel model too much load
and driving power too large. Compared with the conventional ambient
temperature wind tunnel, by reducing the total temperature, the Reynolds
number can be increased by as much as six times with the wind tunnel size
unchanged and the gas flow pressure basically unchanged. If the total pressure of the airflow is appropriately increased while the total temperature of
the airflow is reduced, that is, the combination of low-temperature wind
tunnel and pressure wind tunnel, the experimental Reynolds number will
be increased even more. For example, in transonic wind tunnel at NASA
Langley Experiment Center National, the world’s largest high-Reynolds
number transonic wind tunnel was formal operating in 1983, the
transverse dimension of the test section is 2.5 m × 2.5 m. The Mach
number is within the testing range 0.2−1.2. The total pressure can be
up to 9 × 10 5 Pa. The airflow temperature can be reduced to 100 K by
using the method of liquid nitrogen injection. When the Mach number
423
close to the flying Reynolds number, known as a high-Reynolds number
wind tunnel. The methods to increase the experimental Reynolds number
are as follows: increasing the size of the wind tunnel; high density gas
being used as the working medium of the wind tunnel; increasing the total
pressure of airflow in the wind tunnel; reducing the total temperature of
the airflow. Increasing the size of a wind tunnel, like a low-speed wind
tunnel, is not realistic for a transonic wind tunnel because it is too expensive to build. It is theoretically feasible to change the working medium
of the wind tunnel to increase the experimental Reynolds number, but
there is no gas with the same specific heat capacity as air, higher density
than air, and cheaper. Since 1966, pressure wind tunnel has been used to
increase the experimental Reynolds number, which has achieved certain
results. However, it has caused an increase in dynamic pressure, resulting
in the strength of the model and the support. Meanwhile, the driving
power of the wind tunnel has increased significantly, which has limited
the further improvement of the Reynolds number. Since the 1970s, a
new type of wind tunnel, low-temperature wind tunnel, has shown more
and more advantages. A low-temperature wind tunnel is a wind tunnel
where the temperature of the working medium is less than 173 K. As
the temperature decreases, the viscosity coefficient μ and sound velocity a
decrease, the density ρ increases, and the Reynolds number Re increases.
With lower temperature, while the density increases, the sound velocity
decreases, and wind speed decreases under the same Mach number, thus
dynamic pressure can basically remain unchanged. The driving power of
the wind tunnel will drop slightly, to avoid the general pressure due to the
increase of Reynolds number in the wind tunnel model too much load
and driving power too large. Compared with the conventional ambient
temperature wind tunnel, by reducing the total temperature, the Reynolds
number can be increased by as much as six times with the wind tunnel size
unchanged and the gas flow pressure basically unchanged. If the total pressure of the airflow is appropriately increased while the total temperature of
the airflow is reduced, that is, the combination of low-temperature wind
tunnel and pressure wind tunnel, the experimental Reynolds number will
be increased even more. For example, in transonic wind tunnel at NASA
Langley Experiment Center National, the world’s largest high-Reynolds
number transonic wind tunnel was formal operating in 1983, the
transverse dimension of the test section is 2.5 m × 2.5 m. The Mach
number is within the testing range 0.2−1.2. The total pressure can be
up to 9 × 10 5 Pa. The airflow temperature can be reduced to 100 K by
using the method of liquid nitrogen injection. When the Mach number