Electromagnetic Fluid Discharge Characteristics Of Typical Aircraft Protection Structures Exposed To Lightning Strike

Although the advanced avionics system and lighter composite materials are adopted in modern aircraft design,the whole aircraft system becomes more sensitive to lightning strike.Some clear requirements have been put forward in aircraft design manual for the lightning protection design.Therefore,the purpose of lightning protection research is to improve the reliability and safety of aircraft structure under extreme lightning electromagnetic environment.It should be noted that lightning protection of typical aircraft structures,especially aircraft skin and radome,is an important problem to be solved.The influence of plasma discharge channel is ignored in previous thermal-electric coupling model of composite lightning.It is considered that the resistance-thermal effect in carbon fiber reinforced composite under lightning strike is the critical mechanism leading to structural damage.While this numerical model provides a reasonable explanation to ablation damage in composite effectively,but it is unable to study the forming process of plasma channel above the segmented guide strip and lightning protection mechanism.Consequently,it is necessary to propose a new full coupling numerical method to comprehensively describe characteristics the complex interaction between lightning discharge channel and composite structure.In this thesis,lightning direct effects of composite structures are studied considering electromagnetic fluid characteristics of lightning discharge.The main work of the paper is as follows:1.In order to study the simulated lightning arc in laboratory,a two-dimensional axisymmetric model of DC free-burning arc is established based on the assumptions of local thermodynamic equilibrium,single-phase flow and local neutral Newtonian flow.The magneto hydro dynamic(MHD)equations coupled with the hydrodynamic equations and the Maxwell equations are solved.By comparing the results of argon arc simulation and experiment,the validity of this MHD model is evaluated.A three-dimensional lightning arc model is established to study the plasma characteristics of different lightning current based on lightning standard environment waveform and MHD theory.2.Lightning damage of composite laminates is studied.A high temperature ablation model coupled with lightning arc is established and arc attachment characteristics above composite surface are analyzed.The results indicate that arc attachment characteristics are related to the conductivity distribution of composite surface.The arc spreads in a direction perpendicular to composite fibers under the local electric field.Arc extension causes lightning damage perpendicular to fiber direction and lightning damage in fiber and thickness directions is owing to lightning current conduction.3.Lightning protection of composite laminates sprayed with aluminum on the surface is studied under lightning strike.Lightning tests and numerical simulations of lightning damage are carried out for the unprotected laminates and composite laminates with different protection methods.Lightning protection effects of different protection methods are compared under the standard waveform current.The results show that ablation damage area and depth of test piece increase with the increase of action integral under the applied impact current.Aluminum layer has a good lightning protection effect under the same impact current.The fully sprayed aluminum and the partially sprayed aluminum have both good lightning protection effects.4.Based on magnetohydro dynamic theory,transient Electro-Magneto-thermal multiphysics coupling model is established to analyze protection mechnism of the segmented diverter strip.Lightning breakdown characteristics of the segmented diverter strip applied for radome protection are studied under high voltage.The effects of voltage rise rate,gap width between metal segments and metal segment geometry are investigated on breakdown voltage of the segmented diverter strip.Breakdown voltage increases with the rate of voltage rise and the width of segment gap.The segmented diverter strips with square-shaped metal segments installed at an angle of 45 ° and round-shaped metal segments have the same breakdown voltage,but the square-shaped metal segments have less metal coverage.5.A high current breakdown model of the segmented diverter strip is established to predict the evolution of plasma arc above the strip from a series of small arcs to a plasma channel.According to temperature distribution,conductivity,electric field and current density in the model,formation mechanism of plasma channel is revealed.A series of small arcs above the segmented diverter strip are driven by local strong electric field to form a plasma channel.The simplified model of segmented diverter strip based on breakdown electric field is proposed.According to this model,the arrangement rule of the strips is consistent with the test results in the literature.