Electromagnetic backscattering from one-dimensional drifting fractal sea surface Ⅰ:Wave–current coupled model

To study the electromagnetic backscattering from a one-dimensional drifting fractal sea surface,a fractal sea surface wave–current model is derived,based on the mechanism of wave–current interactions.The numerical results show the effect of the ocean current on the wave.Wave amplitude decreases,wavelength and kurtosis of wave height increase,spectrum intensity decreases and shifts towards lower frequencies when the current occurs parallel to the direction of the ocean wave.By comparison,wave amplitude increases,wavelength and kurtosis of wave height decrease,spectrum intensity increases and shifts towards higher frequencies if the current is in the opposite direction to the direction of ocean wave.The wave–current interaction effect of the ocean current is much stronger than that of the nonlinear wave–wave interaction.The kurtosis of the nonlinear fractal ocean surface is larger than that of linear fractal ocean surface.The effect of the current on skewness of the probability distribution function is negligible.Therefore,the ocean wave spectrum is notably changed by the surface current and the change should be detectable in the electromagnetic backscattering signal.     

Effective dielectric constant model of electromagnetic backscattering from stratified air–sea surface film–sea water medium

Studies of surface film medium on the sea surface are carried out in this paper for developing the technology to automatically detect and classify sea surface films, and an effective dielectric constant model of electromagnetic backscattering from a stratified air–ocean interface. Numerical results of the new model show the characteristics of effective dielectric constants for the air–sea surface film–sea water medium as follows. The effective dielectric constants decrease with increasing relative dielectric constants of the sea surface films. The effective dielectric constants decrease in horizontal polarization(abbr. HH polarization) and increase in VV vertical polarization(abbr. VV polarization) with increasing radar incident angle. Effective dielectric constants vary with relative sea surface film thickness as a cosinusoidal function of sea surface film thickness. Effective dielectric constant of VV polarization is larger than that of HH polarization. Two potential applications are found with our model, i.e., the retrieval of dielectric constants from the sea surface film, and the film thickness retrieval with our model. Our model has a highly significant influence on improving the technology related to the remote sensing of sea surface films.     

Electromagnetic backscattering from freak in （1＋1）-dimensional deep-water

To study the electromagnetic （EM） backscatter characteristics of freak waves at moderate incidence angles, we establish an EM backscattering model for freak waves in （1＋1）-dimensional deep water. The nonlinear interaction between freak waves and Bragg short waves is considered to be the basic hydrodynamic spectra modulation mechanism in the model. Numerical results suggest that the EM backscattering intensities of freak waves are less than those from the background sea surface at moderate incidence angles. The normalised radar cross sections （NRCSs） from freak waves are highly polarisation dependent, even at low incidence angles, which is different from the situation for normal sea waves; moreover, the NRCS of freak waves is more polarisation dependent than the background sea surface. NRCS discrepancies between freak waves and the background sea surface with using horizontal transmitting horizomtal （HH） polarisation are larger than those using vertical transmitting vertical （VV） polarisation, at moderate incident angles. NRCS discrepancies between freak waves and background sea surface decreases with the increase of incidence angle, in both HH and VV polarisation radars. As an application, in the synthetic-aperture radar （SAR） imaging of freak waves, we suggest that freak waves should have extremely low backscatter NRCSs for the freak wave facet with the strongest slope. Compared with the background sea surface, the freak waves should be darker in HH polarisation echo images than in VV echo images, in SAR images. Freak waves can be more easily detected from the background sea surface in HH polarisation images than in VV polarisation images. The possibility of detection of freak waves at low incidence angles is much higher than at high incidence angles.     

Electromagnetic scattering from a PEC object above a dielectric rough sea surface by a hybrid PO–PO method

In this paper, the electromagnetic scattering from a perfectly electric conducting object above a dielectric sea surface is investigated by an efficient hybrid method combining physical optics (PO) with physical optics (PO). Both the scatterings of an object and the underlying sea surface are calculated by the PO method and the mutual couplings between them are solved by the equivalence principle and multi-path scattering strategy. In numerical simulations, the monostatic and bistatic normalized radar cross-section of the composite model is computed by the proposed method and is compared with that by the conventional method of moments. The results show the hybrid PO–PO method has a good accuracy, presence of PO and can greatly reduce the computational time and memory requirement.     

Scattering of decimetric acoustic waves from the sea’s surface

The influence of the sea’s surface slopes caused by wind waves whose wavelength exceeds that of the Bragg components on the backscattering of acoustic radiation is discussed. The analysis is performed for long waves whose wavelengths are greater than 1.5 m. The study is based on the data of full-scale measurements of the sea’s surface slopes. Expressions relating the backscattering coefficient to the statistical characteristics of the sea’s surface slopes caused by long waves are presented.     

Subcooled pool boiling heat transfer in fractal nanofluids:A novel analytical model

A novel analytical model to determine the heat flux of subcooled pool boiling in fractal nanofluids is developed. The model considers the fractal character of nanofluids in terms of the fractal dimension of nanoparticles and the fractal dimen- sion of active cavities on the heated surfaces; it also takes into account the effect of the Brownian motion of nanoparticles, which has no empirical constant but has parameters with physical meanings. The proposed model is expressed as a function of the subcooling of fluids and the wall superheat. The fractal analytical model is verified by a reasonable agreement with the experimental data and the results obtained from existing models.     

A hybrid IEM–PO method for composite scattering from a PEC object above a dielectric sea surface with large windspeed: HH polarization

In the paper, for HH polarization the composite scattering from 2-D perfectly electric conducting (PEC) object situated above 1-D dielectric sea surface with large windspeed is investigated by an efficient hybrid method combining integral equation method with physical optics (IEM–PO). In the hybrid method, IEM is applied to study the direct scattering of underlying sea surface, whereas PO is employed to deal with the direct scattering of object. The mutual couplings between sea surface and object are also solved by PO method based on the equivalence principle and multi-path scattering strategy. In numerical simulations, the bistatic normalized radar cross-section (NRCS) of composite model are calculated by the proposed method and are validated by numerical experiments for different incident angles and windspeeds. The results show the hybrid IEM-PO method has a good accuracy and can greatly reduce the computational time and memory requirement.     

FAB-MS spectra from Teflon® surface: Peak pattern development with increasing bombarding dose

The potential of FAB-MS for characterization of Teflon® surface is investigated and the character of the spectra compared with SIMS results from several different experiments. The change in the spectral peak pattern with increasing atom bombarding dose is examined in light of the radiation damage accumulation in the surface layer of the polymer and surface morphology development at higher atom bombarding doses. The correlation of the fragment ion yield with surface morphology shows that the overall change of the bombarding geometry plays an important role in the decrease of signal intensity with time. An irregular development of the relative ion fragment yields with increasing bombarding dose indicates that processes in the surface region are more complicated than a simple radiation damage accumulation of the polymer fragments created by a polymer chain scissoring mechanism.     

Growth of ZnO nanorod arrays by sol–gel method: transition from two-dimensional film to one-dimensional nanostructure

ZnO nanorod arrays were prepared by a sol–gel method in the present work. The effects of doping concentration and annealing time on the morphologies of ZnO:Al (ZAO) layers were investigated to clearly explore the growth process of ZnO nanorods by designing gradient structures and adjusting the annealing time. The results show that the doping level in the films is a key factor for the formation of nanorods and they cannot form at a low doping level. Out-of-plane anisotropic grain growth instead of the traditional in-plane coarsening process is observed with increasing annealing time. The growth model of nanorods is proposed in terms of the surface diffusion and Ehrlich–Schwoebel barrier (ES barrier) theory.     

(2 + 1) dimensional Hărăgus-Courcelle-Il’ichev model for the liquid surface waves in the presence of sea ice or surface tension: Bäcklund transformation,exact solutions and possibly observable effects

The wave propagation on an ocean or water surface in the presence of sea ice or surface tension is of current importance. In this paper, we investigate the (2 + 1) dimensional 6th-order model proposed recently by Hrgus-Courcelle and Ilichev for such wave propagation. Firstly, we correct some errors in the original derivations of this model. With computerized symbolic computation and truncated Painlevé expansion, we then obtain an auto-Bäcklund transformation and types of the solitonic and other exact analytic solutions to the model, with the solitary waves as a special case, able to be dealt with the powerful Wu method. Based on the results, we later propose some possibly observable effects for the future experiments, and in the end, provide a possible way to explain the regular structure of the open-sea ice break-up observations.Received: 21 July 2004, Published online: 23 December 2004PACS: 47.11. + j Computational methods in fluid dynamics - 05.45.Yvi Solitons - 47.35. + i Hydrodynamic waves - 02.70.Wz Symbolic computation (computer algebra)This revised version was published online in January 2005 with corrections to the sequence of the second authors affiliations.     

Electromagnetic backscattering from one-dimensional drifting fractal sea surface I: Wave-current coupled model

To study the electromagnetic backscattering from a one-dimensional drifting fractal sea surface, a fractal sea surface wave-current model is derived, based on the mechanism of wave-current interactions. The numerical results show the effect of the ocean current on the wave. Wave amplitude decreases, wavelength and kurtosis of wave height increase, spectrum intensity decreases and shifts towards lower frequencies when the current occurs parallel to the direction of the ocean wave. By comparison, wave amplitude increases, wavelength and kurtosis of wave height decrease, spectrum intensity increases and shifts towards higher frequencies if the current is in the opposite direction to the direction of ocean wave. The wave-current interaction effect of the ocean current is much stronger than that of the nonlinear wave-wave interaction. The kurtosis of the nonlinear fractal ocean surface is larger than that of linear fractal ocean surface. The effect of the current on skewness of the probability distribution function is negligible. Therefore, the ocean wave spectrum is notably changed by the surface current and the change should be detectable in the electromagnetic backscattering signal.     

Numerical study of electromagnetic scattering from one-dimensional nonlinear fractal sea surface

In recent years, linear fractal sea surface models have been developed for the sea surface in order to establish an electromagnetic backscattering model. Unfortunately, the sea surface is always nonlinear, particularly at high sea states. We present a nonlinear fractal sea surface model and derive an electromagnetic backscattering model. Using this model, we numerically calculate the normalized radar cross section (NRCS) of a nonlinear sea surface. Comparing the averaged NRCS between linear and nonlinear fractal models, we show that the NRCS of a linear fractal sea surface underestimates the NRCS of the real sea surface, especially for sea states with high fractal dimensions, and for dominant ocean surface gravity waves that are either very short or extremely long.     

改进的二维分形模型在海面电磁散射中的应用

提出了一种改进的二维分形海面模型，其表面谱函数在空间波数小于基波波数及大于基波波数时分别满足正幂率关系和负幂率关系.通过比较可以发现在不同风速时，改进模型的空间自相关函数及表面轮廓谱和有关文献结果有较好的吻合.在满足Kirchhoff近似条件下推导了改进分形模型的散射系数及散射强度系数的计算公式并进行了数值计算，比较了改进模型和经典模型的后向散射强度系数角分布并详细讨论了它们随入射频率、海上风速和风向的变化. 关键词： 改进分形模型 粗糙海面 电磁散射 Kirchhoff近似     

双尺度动态分形粗糙海面的电磁散射及多普勒谱研究

利用更为符合海面实际散射场计算的双尺度法分析并计算了一维动态分形粗糙海面的后向散射截面，同时与微扰法有关结果进行了比较.研究了动态分形海面后向散射信号的多普勒谱分形特征，分析了不同极化状态下后向散射截面和多普勒谱谱宽随入射角及分维的变化规律，最后详细讨论了多普勒谱中心频率与海面Bragg谐振频率间的关系. 关键词： 双尺度模型 分形海面 电磁散射 多普勒谱     

Polarimetric scattering from a two-dimensional improved sea fractal surface

This paper studies the influence of wind parameters and fractal dimension from an improved two-dimensional sea fractal surface on the polarimetric scattering by using facet integration.A two-dimensional improved sea surface simulated is discretized into three matrices of sea surface facets including a height matrix and two slope matrices on orthogonal directions.Based on the Kirchhoff approximation,the polarimetric scattered field is derived in the Cartesian coordinate system by integration of three matrices mentioned above.Finally,the fully polarised radar cross section is numerically simulated and the dependence of the polarimetric scattering on the sea fractal surface,such as the wind speed,the wind direction,as well as the fractal dimension,is discussed in detail.     

Study of backscattering from a complex scatterer at 3mm wave band

The backscattering characteristics of complex scatterer has been studied theoretically at 3 mm wavelength for the first time. RCS of the practial complex scatterer was calculated with GTD and the physical optics. The constrict model was also measured. The experimental results are in agreement with the theoretical calculations.     

Electromagnetic similitude and inconsistency of laser scattering for scale model

In this paper the electromagnetic similitudes for lossy and dispersive system are discussed. If both the geometric size of an object and incident wavelength are scaling-down simultaneously by analogy with microwave band, the inconsistencies exist for laser scattering in visible and near infrared bands. Another method of measurements and simulation for laser scattering in the geometric model is analyzed. That is, the geometric size of the object is only scaled down but the incident wavelength, material and configuration are keeping to not change. Although this way does not satisfy the principles of the electrodynamic similitude, it can overcome the electromagnetic inconsistencies for laser scattering. It is possible that the LRCS of the full-scale tested object are evaluated by those of the corresponding scaling-down model at identical frequency to establish theoretical models and data basis for LRCS of objects.     

RCS测试中的低散射载体设计及仿真分析

在雷达散射截面（RCS）测试中,低散射载体的设计结果取决于被测目标的RCS量级。为进行细节性隐身设计,获得台阶与缝隙排除载体干扰后的RCS水平,提出一种在台阶与缝隙RCS测试中的低散射载体设计方法。采用多层快速多极子算法,对低散射载体相邻曲面连接方式、台阶与缝隙在载体上的位置、前缘尖削度、倒圆半径对散射特性的影响进行了仿真分析。仿真结果表明,控制载体表面曲率的变化、加大前缘尖削度、减小拼接曲面倒圆半径能有效降低载体前向散射水平,由于载体曲面一侧的散射水平低于平面一侧散射水平,台阶与缝隙特征应位于曲面上。对RCS测试中低散射载体设计具有指导意义。     

热防护层覆盖弹体目标雷达散射截面的修正的等效电流近似法和图形计算电磁学法分析

将修正的等效电流近似法与图形计算电磁学法相结合引入到热防护层覆盖弹体目标的电磁散射问题的研究中.应用修正的等效电流近似法对介质和有耗表面进行散射计算,结合图形计算电磁学法,借助于计算机显示技术,将三维目标图形在计算机屏幕上投影,由图形加速卡完成遮挡和消隐工作,利用图形计算电磁学的积分公式,把三维空间的计算转化为二维空间的计算,大大降低了计算时间和复杂度.计算结果表明:当入射波频率较低时,热防护层的厚度不会影响弹体雷达截面值的大小,当频率升高,随着热防护层厚度的增加,弹体雷达截面值不断减小,说明热防护涂层为有耗介质,介电常数的虚部越大其消耗能量的能力越强,弹体雷达截面变化越明显;当热防护层中存在孔隙,热防护层厚度一定,孔隙率越大,雷达截面值越大,孔隙率为零时,雷达截面值最小.当孔隙率相同,热防护层越薄,其雷达截面值越大;当弹体在高空中出现脱粘现象,对弹体的雷达截面值影响不大.