6 Wind and Water Tunnel Equipment
397
under the action of an adverse pressure gradient. If the expansion
angle of the diffuser is larger, the friction loss can be reduced and the
diffuser loss can be increased. On the contrary, when the expansion
angle decreases, the friction loss will increase and the diffuser loss will
decrease. A large number of experiments have proved that the optimal
expansion angle of the three-dimensional circular section is 5–6°
(3) The corner and the deflector are to solve the separation problem
caused by the airflow turning in the loop wind tunnel. A loop wind
tunnel usually has four right-angle corners behind the airflow. The
airflow from the test section passes through the first, second, third,
and fourth corners at one time and bends 360° in total. When the air
flows through the corner, it is easy to generate separation and vortex,
forming the pulsation and non-uniform region in the flow. This is
because, when the airflow enters the corner, the streamline bends and
centrifugal inertial force occurs. The flow rate decreases and the pressure increases along the direction away from the center of curvature.
The velocity increases and the pressure decreases along the direction of
the center of curvature. So, if the flow rate on the inside of the corner
increases and the pressure decreases, there is a contraction effect. If the
flow rate on the outside decreases and the pressure increases, there is a
diffuser effect. That leads to the pressure on the outside of the corner
higher than the pressure on the inside. After the airflow flows through
the corner, there is a diffuser effect inside the corner and a contraction effect outside. The diffusion results in the separation of the inner
and outer walls of the airflow and the formation of many small-scale
vortexes. This effect changes with the distance between the inside and
outside. For this reason, the wind tunnel corners are equipped with
deflectors with the function equivalent to dividing a large corner into
several small corners. For each small corner, the distance between the
inside and outside is significantly reduced, so the flow separation and
vortex are significantly weakened. The function of the flow deflector
is to reduce the generation of separation when the air flows through
the corner and reduce the intensity of the secondary flow vortex, so as
to reduce the energy loss of the airflow and to improve the flow field
performance after the air flows through the corner. For example, the
performance of the flow field behind the second corner is improved to
reduce the wind tunnel energy loss. For example, the flow field performance behind the fourth corner is also improved to improve the flow
field quality in the test section. The shape of the cross section of the
deflector is a circular arc, arc plus straight line, and wing profile.
397
under the action of an adverse pressure gradient. If the expansion
angle of the diffuser is larger, the friction loss can be reduced and the
diffuser loss can be increased. On the contrary, when the expansion
angle decreases, the friction loss will increase and the diffuser loss will
decrease. A large number of experiments have proved that the optimal
expansion angle of the three-dimensional circular section is 5–6°
(3) The corner and the deflector are to solve the separation problem
caused by the airflow turning in the loop wind tunnel. A loop wind
tunnel usually has four right-angle corners behind the airflow. The
airflow from the test section passes through the first, second, third,
and fourth corners at one time and bends 360° in total. When the air
flows through the corner, it is easy to generate separation and vortex,
forming the pulsation and non-uniform region in the flow. This is
because, when the airflow enters the corner, the streamline bends and
centrifugal inertial force occurs. The flow rate decreases and the pressure increases along the direction away from the center of curvature.
The velocity increases and the pressure decreases along the direction of
the center of curvature. So, if the flow rate on the inside of the corner
increases and the pressure decreases, there is a contraction effect. If the
flow rate on the outside decreases and the pressure increases, there is a
diffuser effect. That leads to the pressure on the outside of the corner
higher than the pressure on the inside. After the airflow flows through
the corner, there is a diffuser effect inside the corner and a contraction effect outside. The diffusion results in the separation of the inner
and outer walls of the airflow and the formation of many small-scale
vortexes. This effect changes with the distance between the inside and
outside. For this reason, the wind tunnel corners are equipped with
deflectors with the function equivalent to dividing a large corner into
several small corners. For each small corner, the distance between the
inside and outside is significantly reduced, so the flow separation and
vortex are significantly weakened. The function of the flow deflector
is to reduce the generation of separation when the air flows through
the corner and reduce the intensity of the secondary flow vortex, so as
to reduce the energy loss of the airflow and to improve the flow field
performance after the air flows through the corner. For example, the
performance of the flow field behind the second corner is improved to
reduce the wind tunnel energy loss. For example, the flow field performance behind the fourth corner is also improved to improve the flow
field quality in the test section. The shape of the cross section of the
deflector is a circular arc, arc plus straight line, and wing profile.