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9 Characterisation of Flow Properties at the Surface
9.6 Measurement of Deformations of the Models
One of the aims of testing in large wind tunnels is to validate performance precalculated by CFD simulation. The differences sometimes observed can be explained
by various causes, among which are the interactions with the supports of the model
and the effects due to the confinement of the wind tunnel walls (see Sect. 2.5). Another
important cause is the deformation of the models under the influence of aerodynamic
forces, in particular that of the wings. In case of considerable loading, the twist of a
wing can reach 1° which is non-negligible. The comparison with simulation is then
flawed due to the disparity with the deformed geometry during the experiment.
The method for measuring the actual deformed shape of the model under aerodynamic loads (MDM for Model Deformation Measurement) is based on stereoimaging. As shown in Fig. 9.31a, the principle is to use two cameras that are focussed
on the wing model.
The wing is equipped with fixed markers consisting of white “stickers” of negligible thickness, with a black disk at their centre (see Fig. 9.31b). By capturing two
images using the stereo-vision it is possible to calculate the position of the markers in
space and thus to measure the deformation of the model during the test. The cameras
must be calibrated with a known pattern, to correct for the optical errors and the
position of the cameras relative to the model.
There is also a technique for “stiffening” the model that can be applied in some
wind tunnels. This technique consists in carrying out two consecutive tests, under
the same Mach number conditions, but with different stagnation pressures. The measurements of the corresponding aerodynamic forces can then be linearly extrapolated
to a zero-stagnation pressure for which the model is theoretically not deformed.
(a) A two-camera set-up stereo imaging
(b) View showing the markers
Fig. 9.31 Setup for measuring the deformation of a model subjected to aerodynamic loading
(© ONERA)
9 Characterisation of Flow Properties at the Surface
9.6 Measurement of Deformations of the Models
One of the aims of testing in large wind tunnels is to validate performance precalculated by CFD simulation. The differences sometimes observed can be explained
by various causes, among which are the interactions with the supports of the model
and the effects due to the confinement of the wind tunnel walls (see Sect. 2.5). Another
important cause is the deformation of the models under the influence of aerodynamic
forces, in particular that of the wings. In case of considerable loading, the twist of a
wing can reach 1° which is non-negligible. The comparison with simulation is then
flawed due to the disparity with the deformed geometry during the experiment.
The method for measuring the actual deformed shape of the model under aerodynamic loads (MDM for Model Deformation Measurement) is based on stereoimaging. As shown in Fig. 9.31a, the principle is to use two cameras that are focussed
on the wing model.
The wing is equipped with fixed markers consisting of white “stickers” of negligible thickness, with a black disk at their centre (see Fig. 9.31b). By capturing two
images using the stereo-vision it is possible to calculate the position of the markers in
space and thus to measure the deformation of the model during the test. The cameras
must be calibrated with a known pattern, to correct for the optical errors and the
position of the cameras relative to the model.
There is also a technique for “stiffening” the model that can be applied in some
wind tunnels. This technique consists in carrying out two consecutive tests, under
the same Mach number conditions, but with different stagnation pressures. The measurements of the corresponding aerodynamic forces can then be linearly extrapolated
to a zero-stagnation pressure for which the model is theoretically not deformed.
(a) A two-camera set-up stereo imaging
(b) View showing the markers
Fig. 9.31 Setup for measuring the deformation of a model subjected to aerodynamic loading
(© ONERA)