124
5 Supersonic Wind Tunnels
The air supply and circulation is provided by a centrifugal compressor with three
stages, driven by a motor of 450 kW. The cooling is through a heat exchanger operated
at similar power to cool down the air to a temperature close to atmospheric. The
settling section is equipped with 7 fine meshes to reduce the freestream turbulence
prior to the beginning of the contraction. The tests sections are fixed to concrete blocks
resting on vibration isolation or damping material, the various parts being connected
to the rest of the wind tunnel by flexible joints in order to avoid the transmission
of mechanical vibrations within the measurement zones. The air is desiccated by a
molecular sieve dryer. The dust is removed by paper filter banks that extract 99% of
particles larger than 1 μm. The pressure level in the circuit is regulated by a system
maintaining a constant pressure with an accuracy of ±13 Pa, with a response time
of a few seconds ensuring a very good aerodynamic stability.
The main devices for damping disturbances: In high velocity flow, the disturbances to be eliminated are mainly perturbations of temperature, pressure and
freestream velocity or turbulence due to the mechanically power system. The temperature disturbances are produced by the compressor, where at the outlet it could
reach about 200 °C. As mentioned above, the air is brought back to near atmospheric
conditions by means of a large heat exchanger, which ensures unstratified flow in the
ducts and in pressure gradient zones.
The pressure disturbances produced by the compressor are related to the rotation of
the rotor which introduces a fluctuation of the fluid causing it to jolt at the outlet. These
pulsations are eliminated by a Helmholtz filter tuned to the resonance frequency of
the rotation speed of the compressor (similar to the anti-pogo device used on some
rockets) and by organ pipes for the harmonics corresponding to the number of blades.
Downstream of these devices, the discrete frequencies related to the mechanical
system are no longer discernible on the spectra of the fluctuations. On the return part
of the circuit, cavities prevent the noise produced mechanically from propagating
towards the test sections from downstream of the nozzles.
The freestream turbulence intensity are reduced or damped by conventional means
applied in the subsonic tunnel: careful design of corners with vanes to avoid the
formation of longitudinal vortices; divergent portions with a small opening angle,
possibly equipped with grids, so as to avoid separation; and quiet rooms as indicated
above. The grid of the heat exchanger stages helps to destroy larger eddies. The very
strict dust extraction from the air allows measurements by hot wire at a lower risk of
damage. In the absence of natural contamination of the air, measurements by LDV
or PIV require a seeding, the presence of the filter making it possible to control the
particle size distribution. This seeding is recollected with a device downstream of
the measurement zones.
Figure 5.12 shows a study of the reflection of a shock wave in the test section of
the S7 wind tunnel. The shock wave is generated by a flat plate with sharp leading
edge placed above the study wall.
Characteristics of the flow: The flows generated in the nozzles are largely free
of upstream disturbances, the detected fluctuations being essentially the turbulence
radiated by the boundary layers. For example, in a Mach 2.3 test section, with the
boundary layer tripped by roughness on the side walls upstream of the throat and for
5 Supersonic Wind Tunnels
The air supply and circulation is provided by a centrifugal compressor with three
stages, driven by a motor of 450 kW. The cooling is through a heat exchanger operated
at similar power to cool down the air to a temperature close to atmospheric. The
settling section is equipped with 7 fine meshes to reduce the freestream turbulence
prior to the beginning of the contraction. The tests sections are fixed to concrete blocks
resting on vibration isolation or damping material, the various parts being connected
to the rest of the wind tunnel by flexible joints in order to avoid the transmission
of mechanical vibrations within the measurement zones. The air is desiccated by a
molecular sieve dryer. The dust is removed by paper filter banks that extract 99% of
particles larger than 1 μm. The pressure level in the circuit is regulated by a system
maintaining a constant pressure with an accuracy of ±13 Pa, with a response time
of a few seconds ensuring a very good aerodynamic stability.
The main devices for damping disturbances: In high velocity flow, the disturbances to be eliminated are mainly perturbations of temperature, pressure and
freestream velocity or turbulence due to the mechanically power system. The temperature disturbances are produced by the compressor, where at the outlet it could
reach about 200 °C. As mentioned above, the air is brought back to near atmospheric
conditions by means of a large heat exchanger, which ensures unstratified flow in the
ducts and in pressure gradient zones.
The pressure disturbances produced by the compressor are related to the rotation of
the rotor which introduces a fluctuation of the fluid causing it to jolt at the outlet. These
pulsations are eliminated by a Helmholtz filter tuned to the resonance frequency of
the rotation speed of the compressor (similar to the anti-pogo device used on some
rockets) and by organ pipes for the harmonics corresponding to the number of blades.
Downstream of these devices, the discrete frequencies related to the mechanical
system are no longer discernible on the spectra of the fluctuations. On the return part
of the circuit, cavities prevent the noise produced mechanically from propagating
towards the test sections from downstream of the nozzles.
The freestream turbulence intensity are reduced or damped by conventional means
applied in the subsonic tunnel: careful design of corners with vanes to avoid the
formation of longitudinal vortices; divergent portions with a small opening angle,
possibly equipped with grids, so as to avoid separation; and quiet rooms as indicated
above. The grid of the heat exchanger stages helps to destroy larger eddies. The very
strict dust extraction from the air allows measurements by hot wire at a lower risk of
damage. In the absence of natural contamination of the air, measurements by LDV
or PIV require a seeding, the presence of the filter making it possible to control the
particle size distribution. This seeding is recollected with a device downstream of
the measurement zones.
Figure 5.12 shows a study of the reflection of a shock wave in the test section of
the S7 wind tunnel. The shock wave is generated by a flat plate with sharp leading
edge placed above the study wall.
Characteristics of the flow: The flows generated in the nozzles are largely free
of upstream disturbances, the detected fluctuations being essentially the turbulence
radiated by the boundary layers. For example, in a Mach 2.3 test section, with the
boundary layer tripped by roughness on the side walls upstream of the throat and for