Numerical and Experimental Investigation of the Structural Behavior during Aircraft Emergency Landing on Water

Kurzfassung

Although occurring infrequent, the emergency landing of aircraft on water constitutes a crucial facet within aviation safety and, hence, it engages aircraft manufacturers within design and certification processes. Currently employed methods to analyze ditching comprise experimental testing, comparison with already ditching-certified aircraft designs, and semi-analytical as well as uncoupled numerical simulations. Since these means comprise several drawbacks and limitations, there is the motivation to employ advanced, coupled numerical methods to enhance the analysis capabilities of the structural behavior under ditching loads. Moreover, there is no fundamental understanding of the occurring hydrodynamic phenomena, the detailed fluid-structure interaction and the dynamic structural response in ditching. The subject is of particular interest as aforementioned methods do not consider effects due to structural deformations, which limits their validity. It is claimed by the author that structural deformations significantly affect the hydrodynamic loads acting during a ditching, as they modify the boundary conditions the fluid is facing; therefore, they should be taken into account for an accurate assessment of the structural behavior through coupled simulation approaches. In this context, the present thesis investigates the structural behavior under representative impact conditions by means of an evaluation of experimental data as well as numerical simulations. In particular, it is investigated how and to what extent structural deformations affect the hydrodynamic loading during water impact at high horizontal velocity. For that purpose, first the state of the art of methods for ditching analysis is reviewed with particular focus on advanced numerical simulation approaches. A more detailed insight is given into the coupled approach of Smoothed Particle Hydrodynamics (SPH) and Finite Element (FE) method, as it is adopted in this thesis. Comprehensive experimental data of novel, unique guided ditching experiments conducted by CNR-INSEAN during the research project SMAES are evaluated. Based thereon fundamental knowledge about determining factors and key physical effects involved in the hydrodynamic loading and the corresponding structural response under ditching conditions is established. Starting with a characterization of the hydrodynamics, the structural behavior, and their interaction, the effects of impact conditions combination of pitch angle and horizontal impact velocity), panel curvature, and structural deformations on hydrodynamic loads, resulting forces, and local strains are described. Most interestingly, hydrodynamic loads are found to increase considerably as soon as structural deformations occur, which supports the above thesis. The assessment of the results of this evaluation establishes the basis for the subsequent development and validation of the numerical model. Based on these findings, a numerical simulation model of the guided ditching experiments is developed adopting the coupled SPH-FE approach. Particular focus is put on enhancements of the fluid modeling, which previously did not permit such simulations at high forward velocity. Due to the application of state-of-the-art numerical techniques, the developed simulation model is robust and efficient, which for the first time enables profound numerical analyses. Finally, results of comprehensive parameter studies are presented. These permit validating the developed SPH-FE simulation model based on extensive comparison with experimental data for a broad range of test cases. The regarded parameters are horizontal impact velocity, pitch angle, lateral panel curvature, and panel type (combination of material and thickness). Panels made of aluminum and composite materials with thicknesses between 15 and 0.8~mm are considered. Subsequently, the validated simulation model is employed to explore and to assess in detail the structural response of highly deformable structures under hydrodynamic loading. The mechanisms that were experimentally identified to affect the hydrodynamic loading of deformable structures are further investigated in order to assess their importance. Consequently, the findings of this numerical investigation corroborate the thesis that structural deformations should be taken into account in the analysis of the structural behavior through coupled simulation approaches. Overall, the attained knowledge in this thesis contributes to a deeper comprehension of the behavior of aeronautical structures under hydrodynamic loading.