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P. Liu
Reynolds number, the size of the test section, and the size of the
flow velocity reach a certain value. In addition, the airflow in the test
section should be stable, the velocity and direction should be evenly
distributed in space, and the turbulence, noise intensity, and static
pressure gradient of the incoming flow should be low. These characteristics of the flow in the test section are called flow field quality.
The flow field quality index has been stipulated in China. As seen in
the military standard, in the model area of the test section, the flow
direction |α| and |β| shall not be greater than 0.5 and the absolute value of the average flow deviation angle shall not be greater than
0.1. Along the axial static pressure gradient in the center line of the
test section, the length of the model area L shall be within the range
of L ·
dc p
dx
≤ 0.005 ≤ 0.1% (Cp is the static pressure strength
coefficient) and the turbulence degree in the center of the model area
shall not be greater than 0.1%. The cross-sectional shapes of the lowspeed wind tunnel test section are rectangular, square, round, oval,
and octagonal. Most of the existing wind tunnels are rectangular with
angle cutting. The length of the closed section is generally 1.5−2.5
times of the equivalent diameter of the cross-sectional area, and the
length of the open section is generally 1−1.5 times. Generally, the
wind tunnel has a diffusion angle along the axial direction (along the
direction of incoming flow), or the area of each section is gradually
reduced along the axial cutting angle, so that the cross-sectional area
is gradually expanded along the axial direction. The absolute value of
the negative static pressure gradient caused by axial thickening of the
wall boundary layer is then reduced, so as to meet the requirements
of flow field quality.
(2) Diffuser section includes the first and second diffusion sections. In a
low-speed wind tunnel, the diffuser section is a section of the pipe
expanding along the direction of the airflow, so it is also called the
diffusion section. Its function is to slow down the airflow in the test
section and change the kinetic energy into pressure energy, so as to
reduce the energy loss of the airflow in the wind tunnel and reduce
the power required for the operation of the wind tunnel. The total
power lost by the airflow in the pipe is generally proportional to the
cubic velocity. The diffuser decelerates the flow from the test section
and reduces the power loss of the whole wind tunnel. However, the
diffuser can also cause the energy loss of the flow, including friction
loss and diffuser loss. The so-called diffusion loss refers to the loss
caused by the airflow avoiding the thickening of the boundary layer
P. Liu
Reynolds number, the size of the test section, and the size of the
flow velocity reach a certain value. In addition, the airflow in the test
section should be stable, the velocity and direction should be evenly
distributed in space, and the turbulence, noise intensity, and static
pressure gradient of the incoming flow should be low. These characteristics of the flow in the test section are called flow field quality.
The flow field quality index has been stipulated in China. As seen in
the military standard, in the model area of the test section, the flow
direction |α| and |β| shall not be greater than 0.5 and the absolute value of the average flow deviation angle shall not be greater than
0.1. Along the axial static pressure gradient in the center line of the
test section, the length of the model area L shall be within the range
of L ·
dc p
dx
≤ 0.005 ≤ 0.1% (Cp is the static pressure strength
coefficient) and the turbulence degree in the center of the model area
shall not be greater than 0.1%. The cross-sectional shapes of the lowspeed wind tunnel test section are rectangular, square, round, oval,
and octagonal. Most of the existing wind tunnels are rectangular with
angle cutting. The length of the closed section is generally 1.5−2.5
times of the equivalent diameter of the cross-sectional area, and the
length of the open section is generally 1−1.5 times. Generally, the
wind tunnel has a diffusion angle along the axial direction (along the
direction of incoming flow), or the area of each section is gradually
reduced along the axial cutting angle, so that the cross-sectional area
is gradually expanded along the axial direction. The absolute value of
the negative static pressure gradient caused by axial thickening of the
wall boundary layer is then reduced, so as to meet the requirements
of flow field quality.
(2) Diffuser section includes the first and second diffusion sections. In a
low-speed wind tunnel, the diffuser section is a section of the pipe
expanding along the direction of the airflow, so it is also called the
diffusion section. Its function is to slow down the airflow in the test
section and change the kinetic energy into pressure energy, so as to
reduce the energy loss of the airflow in the wind tunnel and reduce
the power required for the operation of the wind tunnel. The total
power lost by the airflow in the pipe is generally proportional to the
cubic velocity. The diffuser decelerates the flow from the test section
and reduces the power loss of the whole wind tunnel. However, the
diffuser can also cause the energy loss of the flow, including friction
loss and diffuser loss. The so-called diffusion loss refers to the loss
caused by the airflow avoiding the thickening of the boundary layer