Signal Processing, Wavelets and high-speed Image Interpretation of Bird Impact
Bird impact on aircraft has been well documented and has been of interest for researchers in aircraft design. The process, though of very short duration, is complex in nature. A bird, which can be treated as a soft body behaves more like a fluid at high-speeds. When aircraft components become targets of bird strikes, the impact can have consequences for safety, and hence the study of the phenomena has engineering implications. In this paper, strain signals from a specially instrumented stiff fixture, high-speed imaging and wavelets are used to describe the nature of the phenomenon. Gelatin-based artificial birds were impacted on the fixture fired through an air gun at two different velocities. high-speed imaging showed different behaviours with a rebound at low-velocity (~50 m/s) and a flow behaviour at high-velocity (~100 m/s). High sampling data acquisition was used to measure the dynamic strain exerted on the fixture during bird impact. Time histories of strain signals obtained in the raw form were processed to get a Fourier spectrum and continuous wavelet transform to gain more information about different frequency patterns and the temporal distribution of the frequencies, when such impacts occurred. The frequency content for low-velocity and high-velocity impacts is characterised. It can be noted that the behaviour as described by earlier researchers was seen here as well at higher velocities, though at lower velocities, the bird behaved more like a solid. Many aircraft have approach speeds that are about 60 m/s rather than 100-200 m/s, making it important to study behaviour at lower velocities as well. The short time interval events identified in the signals provide insight into the nature of the loads on targets. This information can aid in tuning simulation models of the birds which use Lagrangian, Eulerian and smooth particle hydrodynamic models.
Defence Science Journal, 2011, 61(1), pp.62-71, DOI:http://dx.doi.org/10.14429/dsj.61.21
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