EM Computation Of Inhomogeneous Plasma And Its Application

In air and space flight conditions,signal transmission is mainly carried out by radio waves.In this communication environment,the non-uniform dispersive media exist sometimes,such as satellite plume and vehicle sheath,etc.Due to the special properties of those substances,they will cause some propagation behaviors different from that in conventional media such as air,which will inevitably affect the communication quality and even threaten entity security.In order to prevent such problems,it is necessary to fully grasp the propagation behaviors of radio waves in these media.It is in this research background that,based on the radio frequency and different algorithms,this thesis constructs a full-band electromagnetic calculation scheme for non-uniform dispersive media,which is then applied to the analysis of engineering problems.In the calculation scheme,the low frequency problems are treated with the time-domain finite-difference method,and the high frequency problems are solved by using the ray tracing and physical optics.Overall,the work of the thesis can be mainly summarized as the following:1.Aiming at the difficulty of the high complexity of the electromagnetic analysis in high-dimensional problems,a method of dimension reduction is proposed.The basic idea of the method is to transform the calculation of high-dimensional problems into the low-dimensional by some mathamatical means according to the geometric and physical characteristics of the target,so as to reduce the computational complexity.In the work of this thesis,the application scenarios of the method are given in many cases,including surface segmentation,ray tracing modeling,engineering data processing and modeling,and complex function integral,etc.Corresponding to this technique,an dimension elevation method is also proposed for the modeling of low-dimensional model to its high-dimensional case.In addition,the calculation acceleration of ray tracing is also discussed in this thesis,based on the dimension reduction calculation method.2.Using numerical analysis and geometric optics,an engineering electromagnetic algorithm for tackling non-uniform media is presented and applied to electromagnetic transmission problems in the vehicle sheath and satellite plume.To establish the high-dimensional electromagnetic model,the dimension reduction method and data interpolation are applied to transform the high-dimensional model processing into low-dimensional problem,extremely reducing the computational complexity.Then,according to the principle of radio wave propagation in plasma,some numerical methods such as Runge-Kutta method and neighboring point query are used to determine the ray equation of electromagnetic waves through the plasma sheath and plume;and the study is mainly focused on the electromagnetic transmission in the vehicle sheath.According to different conditions such as vheicle shape,flight height and speed,the influence of the plusma sheath on the behavior of electriomagnetic waves is analyzed.Studies show that,due to the introduction of the ray tracing method,the propagation behavior of electriomagnetic waves in the sheath is presented from different angles;at the same time,some special propagation phenomena such as the directionality of electromagnetic transmission and the waveguide behavior of the sheath are observed.Combined with the physical properties of the vehicle sheath,proper explainations are given to the results obtained.3.The difficulty of the electromagnetic simulation on non-uniform media lies in finding effective and versatile modeling methods.Focusing on this issue,the thesis carries out a research in-depth,including:(1)An image-based electromagnetic modeling method is proposed.This method uses the relationship between the optical image and the corresponding physical quantity to restore the data in the photo by digital image technology;then according to the rotational symmetry of the target,its three-dimensional model can be created by using the dimension elevation method.Simulation results verify the effectiveness of the method.Finally,the method is applied to the modeling of the satellite plume,and its influence on the transmission behaviors of radio waves is discussed.(2)A high-precision layered modeling method is proposed.This method applies the transformation domain idea to determine the layering scheme,and to builds the layered model;the control equation of modeling accuracy is derived theoretically,which is free from the difficulty that the layering method often used is limited by the uncontrollable modeling precision.The numerical results prove the correctness of the method.In addition,the electromagnetic modeling method based on the interpolation of discrete data is also discussed.4.SO-FDTD method is introduced into the electromagnetic calculation of the hypersonic vehicle,in which the propagation and scattering characteristics of the electriomagnetic waves at the frequency range of 0 ~ 10 GHz is studied.Firstly,in order to better simulate the propagation environment of radio waves,the commercial software COMSOL is used to simulate the external flow field of the hypersonic vehicle,covering both blunt and pointed models;and the antenna and radome models are established in the vehicle as well.Then,SO-FDTD and some numerical methods are applied to analyze influences of the factors such as flight height and speed on the electromagnetic transmission and scattering characteristics of the vehicle;and the investigation on the influence of the vehicle structure on the characteristics is firstly launched in this thesis.Results show that the electriomagnetic waves exhibit rich propagation and scattering behaviors due to the complex impact of the plasma sheath and the vehicle structures;for example,the wave propagation shows characteristics of band movement,tail diffraction and energy resonance absorption;and the scattering characteristics exhibit that the shape,edge and antenna structure of the vehicle contribute significantly to the scattering behavior.5.For the scattering problems of non-uniform electrically large objects,on the one hand,conventional algorithms such as time-domain finite-difference method and finite element method can be hardly used;on the other hand,the application of traditional physical optics has failed.In order to obtain the scattering characteristics of targets in this case,the Stratton-Chu integral formula over a closed surface is transformed into the integral over a infinite plane by using Gauss' s theorem and dimension reduction method.Then,some mathematical methods such as the engineering approximation and vector analysis are applied to convert the infinite plane-integral form of the formula into the integral over a finite plane;and two new far-field calculation formulas are further developed.Subsequently,their consistency problem is discussed,and the third form of the formulas is derived.The results obtained are characterized by the simple calculation and clear physical meaning.On this basis,the time-domain extrapolation of the electromagnetic fields is further studied,resulting in the derivation of the corresponding calculation formula whose properties are briefly discussed later.Finally,numerical examples verify the correctness of the results.