| Radar detection and infrared detection are the main ways to detect aircrafts,while the engine exhaust system,as a typical large cavity,is not only the major radiation source of aircraft in the 3?m-5?m wave band,but also the main scattering source.Therefore,it is an important work on how to reduce the infrared and radar signal intensity of the exhaust system.Under this circumstance,a research was conducted on the infrared(IR)radiation and electromagnetic scattering of an aircraft as well as its exhaust system.In the IR radiation research,a numerical method was developed,and a further research was conducted on the aerosol suppression technique and S shaped nozzle suppression technology.Then based on UAV with inlet and exhaust system,the influence was studied on how solar radiation,earth radiation,and sky radiation affect UVA's infrared radiation characteristics.In the study of electromagnetic scattering,a calculation method was studied on cavities' electromagnetic scattering,and then the numerical accuracy was verified by a darkroom experiment.Finally,a research was conducted on geometry design,media coating,and other Radar Cross Section(RCS)reduction technologies.The main research contents and results are as follows:(1)In the aspect of the infrared radiation numerical method.A topology method,which is based on the unstructured grids “surface numbering”,was proposed to solve the problem of the data inconsistency and accuracy loss for IR calculation and CFD calculation.The method can avoid transforming the CFD result and calculation mesh to 3D topologically,which could bring the accuracy loss.Besides,the method is also suitable for periodic boundary conditions of infrared radiation.Then a study was carried out on line-by-line calculation method and Malkmus statistical narrow band model,Combing Malkmus statistical narrow band model and HITEMP database,a gas database was built for researching 3?m-5?m and 8?m-14?m gas radiation.Due to low computing efficiency of using narrow-band model to study atmospheric attenuation,a CG approximation Malkmus model was combined with HITRAN database.It reduced the computational time to build background radiation,applying to the background model radiation calculation.For gas transforming into particles medium by using aerosol suppression technology,a particle medium scattering model was built based on the optical parameters of spherical particles,sparse particle and scattering statistical probability model.Combining this model with Reverse Monte Carlo method,a software was developed to calculate the particle medium infrared radiation,which provided a powerful tool to study aerosol suppression methods.Solar radiation model,ground radiation model and sky radiation model was built for considering environment reflectance radiation,then combing those radiation models with Bidirectional Reflectance Distribution Function(BRDF),the wing body surface environment aircraft infrared radiation simulation was finally achieved,which provided support to study the impacts of the target and the environment.(2)In aspect of aerosol inhibition measurements.In order to study the inhibitory effect of SiO2,the axis-symmetry exhaust system with particles was studied.The results show that particle injection can effectively inhibit the IR radiation of the exhaust system and change the constitution of the IR radiation under large detection angle.By analyzing the influences of the particle's injection flow,injection speed,injection angle,diameter,outflow Mach number and other parameters on the characteristics of the exhaust system IR radiation,another conclusion was found that discrete particles injecting too much into the exhaust system could enhance infrared radiation at large angle.With the existence of outflow,the inhibitory effect of different particle injection speed and angle was not much different.When the outflow Mach number is 0.4,the inhibition efficiency under large detection angle was better than the same case with the outflow Mach number of 0.6 or 0.8.(3)In aspect of geometric block suppression technology.In order to find out how geometric block technology influences the IR radiation of exhaust system,the IR radiation of double S exhaust system,single S exhaust system and axisymmetric exhaust system were compared,the results showed that double S exhaust system could reduce the objective of maximum infrared radiation intensity by 95%.The effects of different aspect ratios,different first corner positions,different areas of the first bend,different export miter angles and different outlet shapes influences on the performance and IR radiation characteristics of the double S exhaust system were researched,and the influences laws of the geometric parameter changes on the double S exhaust system was found.(4)In aspect of background and objective infrared radiation research.Combining theoretical analysis with numerical simulation methods,a research was carried out on the aerodynamic performance,the internal and external flow influence of clean flying wing UAV model and flying wing UAV with inlet and exhaust system.A further research was carried out on the flying wing UAV with single S inlet and single S exhaust system,UAV with single S inlet and double S-shaped exhaust system.The results show that flying wing UAV with single S inlet and double S-shaped exhaust system has lower IR signals.Then based on UAV with single S intake and double S exhaust system,a research was conducted in UAV skin reflective background radiation.The results show that solar radiation was the main reflected radiation of aircraft skin within 3?m-5?m wave band,while terrestrial radiation and sky radiation were the main reflected radiation within 8?m-14?m wave band.(5)In aspect of electromagnetic scattering calculation methods research and RCS experiment.The Iterative physical optics(IPO)and impedance boundary condition were combined to develop large cavity dielectric surface coated calculation software,and integrated with infrared radiation calculation software.The calculation methods were certificated on the simplified axis-symmetrical exhaust system model,and the results verify that the IPO calculation software could deal with the exhaust system.In order to minimize the influence of exhaust system wall to RCS results based on large cavity RCS testing requirements,two shells adapting to the weak scattering background noise requirements were built and used in axisymmetric exhaust system test and double S exhaust system test.Using stepped frequency-testing system on axisymmetric exhaust system model and double S exhaust system model under 5 different wave bands in darkroom,it is found out that the double S structure could effectively reduce the RCS of the exhaust system.(6)In aspect of exhaust system RCS dispersion technology.The electromagnetic scattering characteristics of simplified axisymmetric exhaust system,single S exhaust system and double S exhaust system were analyzed in X band,the results show that double S structure reduces the RCS of exhaust system by 74.4%.Geometric parameters of aspect ratios,area of the first bend and first band position influencing electromagnetic scattering characteristics of double S exhaust system were analyzed,and the change laws between geometric parameters and double S exhaust system were obtained.Research on different coating parts of double S exhaust system was also carried out,and the results show that absorbing wave material coating could reduce the RCS of exhaust system,and coating absorbing material on the surface around the outlet of exhaust system has good proportion between the efficiency and cost. |
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