44
2 Wind Tunnels and Other Aerodynamic Test Facilities
turbulence structures and the overall turbulence intensity in the test section. In a
descent wind tunnel, the freestream turbulence of the flow should be below 0.1%,
whereas in the ambient atmosphere where the air is at rest it is 10 times lower. As
mentioned above, these higher values in the wind tunnel can be an issue for the
study of the stability of flows and laminar to turbulent transition in most facilities
(see Sect. 5.4).
– The contraction section convergent part, accelerates the flow at the entrance of
the test section. In supersonic tunnels, the contraction section is followed by a
convergent-divergent nozzle that accelerates the flow to the supersonic regime
(see Chap. 5). The contraction ratio is defined as the ratio between the inlet and
the outlet of the contraction section. This section is an important component of
the wind tunnel and the wall curvature must be carefully design to ensure a good
homogeneity of velocity throughout the test section. It also helps in reducing the
size of the freestream turbulent scale entering the test section. A contraction ratio
close to 10 is recommended for a tunnel with a decent flow quality.
– The test section is where the model is mounted and as we know there are two types:
the guided test section with walls and the open test section constituting of a free
jet emerging from the contraction section and then followed by the diffuser. The
open test section has the advantage of providing direct access to the model, which
facilitates observation of the flow field and measurements by optical methods. Its
disadvantage is the shear layer instability and the turbulent mixing at the boundary
of the jet which is a strong source of unsteadiness and acoustic disturbances. The
guided test section does not allow direct access to the model, this can hinder
observation of the flow around and on the model (for visualisations and optical
measurements). It has the advantage of a better flow quality due to the absence of
the shear layer. The section of the guided test section is often slightly divergent
so as to maintain a constant velocity by compensating for the thickening of the
boundary layers on the walls. The initial tendency was to use a circular shape as
test section, for structural reasons and better quality of the flow by avoiding corner
vortices. The curved wall, on the other hand, has the disadvantage of complicating
optical measurements. For this reason, and also to facilitate installation of the
models, rectangular test sections tend to impose themselves.
– The diffuser consists of a divergent duct widening towards the fan or the exhaust
in the open circuit tunnel. To reduce the size of the installation, it is advantageous
to adopt a diffuser which is as short as possible, so a high angle of divergence.
However, too great a divergence causes a rapid increase in pressure; therefore a risk
of separation of the boundary layer on the diffuser wall, resulting in a significant
efficiency loss and noise. The design rule that was imposed very early is not to
exceed a divergence angle of 6°–7°.
– The motor and fan unit restores the pressure level of the flow before ejecting it
into the atmosphere (Eiffel type tunnels) or reintroducing it into the return circuit
(closed loop tunnels). For high Mach number wind tunnels, which require a very
high pressure ratio, the air flow is often produced by the discharge of air stored at
high pressure. These installations are called blow down facilities, the useful test
time being limited in particular by the storage capacity.
2 Wind Tunnels and Other Aerodynamic Test Facilities
turbulence structures and the overall turbulence intensity in the test section. In a
descent wind tunnel, the freestream turbulence of the flow should be below 0.1%,
whereas in the ambient atmosphere where the air is at rest it is 10 times lower. As
mentioned above, these higher values in the wind tunnel can be an issue for the
study of the stability of flows and laminar to turbulent transition in most facilities
(see Sect. 5.4).
– The contraction section convergent part, accelerates the flow at the entrance of
the test section. In supersonic tunnels, the contraction section is followed by a
convergent-divergent nozzle that accelerates the flow to the supersonic regime
(see Chap. 5). The contraction ratio is defined as the ratio between the inlet and
the outlet of the contraction section. This section is an important component of
the wind tunnel and the wall curvature must be carefully design to ensure a good
homogeneity of velocity throughout the test section. It also helps in reducing the
size of the freestream turbulent scale entering the test section. A contraction ratio
close to 10 is recommended for a tunnel with a decent flow quality.
– The test section is where the model is mounted and as we know there are two types:
the guided test section with walls and the open test section constituting of a free
jet emerging from the contraction section and then followed by the diffuser. The
open test section has the advantage of providing direct access to the model, which
facilitates observation of the flow field and measurements by optical methods. Its
disadvantage is the shear layer instability and the turbulent mixing at the boundary
of the jet which is a strong source of unsteadiness and acoustic disturbances. The
guided test section does not allow direct access to the model, this can hinder
observation of the flow around and on the model (for visualisations and optical
measurements). It has the advantage of a better flow quality due to the absence of
the shear layer. The section of the guided test section is often slightly divergent
so as to maintain a constant velocity by compensating for the thickening of the
boundary layers on the walls. The initial tendency was to use a circular shape as
test section, for structural reasons and better quality of the flow by avoiding corner
vortices. The curved wall, on the other hand, has the disadvantage of complicating
optical measurements. For this reason, and also to facilitate installation of the
models, rectangular test sections tend to impose themselves.
– The diffuser consists of a divergent duct widening towards the fan or the exhaust
in the open circuit tunnel. To reduce the size of the installation, it is advantageous
to adopt a diffuser which is as short as possible, so a high angle of divergence.
However, too great a divergence causes a rapid increase in pressure; therefore a risk
of separation of the boundary layer on the diffuser wall, resulting in a significant
efficiency loss and noise. The design rule that was imposed very early is not to
exceed a divergence angle of 6°–7°.
– The motor and fan unit restores the pressure level of the flow before ejecting it
into the atmosphere (Eiffel type tunnels) or reintroducing it into the return circuit
(closed loop tunnels). For high Mach number wind tunnels, which require a very
high pressure ratio, the air flow is often produced by the discharge of air stored at
high pressure. These installations are called blow down facilities, the useful test
time being limited in particular by the storage capacity.