Models and methods for bird strike load predictions
Typical bird shapes used in numerical bird strike analyses include a number of primitive geometries, such as the right circular cylinder, hemi-spherical cylinder, sphere and ellipsoid. There are also some simplifying assumptions made regarding the modeling of the bird material. Yet, the open literature on the subject includes no comparative study to systematically investigate the effect of the projectile shape or bird model material on the accuracy of impact loads analyses. This research filled this gap by showing how various primitive projectile shapes, a more complex bird-like shape, and several homogeneous and heterogeneous material models affect the calculated impact loads. Comparisons to actual bird impact test data were provided, wherever possible, to validate these results. The bird strike event is characterized by three important phases: shock compression, shock decay, and the establishment of a steady state condition. In addition to reproducing the entire pressure-time history, a rational numerical simulation of bird strike analysis should accurately replicate both the Hugoniot shock pressure and the steady state stagnation pressure. Since the bird is generally represented as a soft body material to model its hydrodynamic behavior, an accurate representation of the bird’s equation of state is critical to predicting correct impact loads. In this research, the study of various homogeneous and heterogeneous bird materials depended on the use of accurate equation of state models for each material. Initially, two equations of state – representing the shock compression phase and the steady state compression phase – were studied. This approach follows that of previous research; but, the full 3-D numerical simulation requires a single equation of state to represent the entire impact event. Therefore, an equation of state that combined these two models was derived. Its validity was confirmed by comparing the analytical results from this equation of state with the results from experimental tests. In addition, the effect of porosity in combination with this equation of state was investigated. Additionally, the effects of oblique impact and target flexibility on the bird impact loads were investigated using the traditional, homogeneous bird torso model. Both of these factors acted to lower the Hugoniot shock pressures. The exploration of multi-material bird models in this work began with the study of two different materials, randomly distributed throughout a simple, hemispherical-ended cylindrical bird model. This was followed by introducing high and low density regions in this bird torso model to include the effects of bones and lungs, respectively. Finally, this work studied a more complete, geometrically accurate bird model that included physical shapes and material properties to represent the head, neck, bone structure, lungs, and wings, in addition to the torso. The effect of these multi-material bird models on both the Hugoniot pressure and the steady state pressure is presented in this work.
Thesis (Ph.D.)--Wichita State University, College of Engineering, Dept. of Aerospace Engineering