Precise knowledge of the actual wind tunnel model geometry or position of moving model parts under wind load is a prerequisite for accurately determining aircraft performance. DNW offers its customers the application of an optical technique, based on three-dimensional stereoscopic point tracking of non-intrusive, high contrast markers on wind tunnel model components. Smart processing algorithms allow precise determination of geometric model properties such as wing bending and twisting (mode shapes) or propeller/rotor blade position in real time.
System description
DNW’s stereoscopic point tracking system utilizes image analysis techniques to determine the three-dimensional geometric properties of a wind tunnel model during testing. Images are acquired from two or more high-resolution cameras monitoring the wind tunnel model equipped with non-intrusive markers from different angles simultaneously.
The system is sufficiently flexible to handle virtually any distribution / lay-out and number of markers. Markers can be either passive by applying fluorescent spray paint, or active by controlled flush-mounted light sources. The marker locations obtained from image analysis software serve as input for software, determining (online) geometric properties of the model such as its precise attitude, position, wing bending and wing twisting.
Capabilities
The system is permanently installed in the Large Low-speed Facility LLF of DNW but also available in other facilities. Capabilities include, but are not limited to:
Model attitude and position determination, including tracking of rigid bodies
Model deformation such as wing bending and twisting
Phase-locked measurements, since the system is triggerable, which is useful for helicopter and propeller blade tracking
Accuracy
The spatial resolution of the system depends on camera type, model length scale and marker configuration. Pixel resolutions are achievable of up to 1/10th of a pixel. Coordinate resolutions are dependent on camera chip size and model size (order of magnitude: 1/(10 * R), where R is the amount of pixels per imaged meter). For a typical full span wind tunnel model in the LLF or HST, the coordinate detection accuracy is approximately 0.5 ‰ of the chord length for wing displacement and 0.1 degree for wing twist (torsion).
Data presentation
Wing deformation is represented by modelling a 3D finite element surface geometry (triangular wire–frame in space without discontinuities) based on individual marker grid points.
Data, that include statistical error estimates based on error propagation algorithms, are provided in real time (up to 500 Hz) and in ASCII file format.