40
2 Wind Tunnels and Other Aerodynamic Test Facilities
– The six components balance measures the forces exerted by the support beam on
the model.
– The seven measurements from the accelerometers and the three measurements
from the gyro meters allow the reconstitution of the matrix of the model inertial
forces.
– The displacement system gives the positions, attitudes and velocities of the model.
The wire suspension technique offers the possibility of carrying out complex tests
close to free flight conditions; this new test method enables the study of the synthesis
of control surface loads to identify aerodynamic forces that are usually difficult to
access.
Magnetic levitation: to avoid interferences from a solid support, systems to position the model in the test section by magnetic fields were developed in the 50s. This
principle based on magnetic levitation consists of placing a ferromagnetic model
in an electromagnetic field generated within the working section. By varying the
magnetic field intensity on each side of the test section or any direction the model
can be oriented in the desired position or attitude. In the magnetic levitation system
developed by ONERA, the position of the model in space is detected by light beams
reflected on photodiodes. Any displacement of the model causes a current change in
the cells incorporated in a feedback circuit, which induces a change in the current
in the coils and thus the magnetic field intensity so as to bring the model back to
its nominal position (see Fig. 2.10). The set-up developed at ONERA was very reliable, even at high Mach numbers. However, the technique had serious drawbacks,
including the shape of the model that could not be too complex, the complexity of the
installation and the limited measurement on the model which could be transmitted.
These limitations would not have been an issue today. It is to be hoped that this
ingenious solution will regain renewed interest in view of the remarkable progress
made in the field of electromagnetics, electro-optics, data acquisition and processing
techniques (see Chaps. 9–11).
2.5.3 Freestream Vortical and Acoustic Perturbations
Earlier issues due to low Reynolds number limitation and unwanted laminar to turbulent transition during wind tunnel testing were discussed. However, wind tunnel
study of stability and transition of laminar flows is even more complicated due to
freestream perturbations generated in the tunnel as opposed to the quiet environment
at cruise and other flight conditions where they are assumed to be very low as the air
is stationary. The perturbations in the tunnels are in the form of vortical structures
which are generated upstream and/or acoustic waves which can propagate in all directions. These perturbations can either trigger the growth of instability modes in the
flow through the process of receptivity or accelerate the transition process especially
in the presence of high freestream turbulence. Due to these effects the wind tunnel
results cannot be extrapolated to flight conditions even for similar Reynolds number
2 Wind Tunnels and Other Aerodynamic Test Facilities
– The six components balance measures the forces exerted by the support beam on
the model.
– The seven measurements from the accelerometers and the three measurements
from the gyro meters allow the reconstitution of the matrix of the model inertial
forces.
– The displacement system gives the positions, attitudes and velocities of the model.
The wire suspension technique offers the possibility of carrying out complex tests
close to free flight conditions; this new test method enables the study of the synthesis
of control surface loads to identify aerodynamic forces that are usually difficult to
access.
Magnetic levitation: to avoid interferences from a solid support, systems to position the model in the test section by magnetic fields were developed in the 50s. This
principle based on magnetic levitation consists of placing a ferromagnetic model
in an electromagnetic field generated within the working section. By varying the
magnetic field intensity on each side of the test section or any direction the model
can be oriented in the desired position or attitude. In the magnetic levitation system
developed by ONERA, the position of the model in space is detected by light beams
reflected on photodiodes. Any displacement of the model causes a current change in
the cells incorporated in a feedback circuit, which induces a change in the current
in the coils and thus the magnetic field intensity so as to bring the model back to
its nominal position (see Fig. 2.10). The set-up developed at ONERA was very reliable, even at high Mach numbers. However, the technique had serious drawbacks,
including the shape of the model that could not be too complex, the complexity of the
installation and the limited measurement on the model which could be transmitted.
These limitations would not have been an issue today. It is to be hoped that this
ingenious solution will regain renewed interest in view of the remarkable progress
made in the field of electromagnetics, electro-optics, data acquisition and processing
techniques (see Chaps. 9–11).
2.5.3 Freestream Vortical and Acoustic Perturbations
Earlier issues due to low Reynolds number limitation and unwanted laminar to turbulent transition during wind tunnel testing were discussed. However, wind tunnel
study of stability and transition of laminar flows is even more complicated due to
freestream perturbations generated in the tunnel as opposed to the quiet environment
at cruise and other flight conditions where they are assumed to be very low as the air
is stationary. The perturbations in the tunnels are in the form of vortical structures
which are generated upstream and/or acoustic waves which can propagate in all directions. These perturbations can either trigger the growth of instability modes in the
flow through the process of receptivity or accelerate the transition process especially
in the presence of high freestream turbulence. Due to these effects the wind tunnel
results cannot be extrapolated to flight conditions even for similar Reynolds number