基于有限元/边界元耦合方法的管道进口声传播及声辐射模型

发展了一种基于有限元和边界元耦合方法的管道进口声传播及声辐射计算模型.该模型将整个声场分为内部有限域和外部无界域,分别用有限元和边界元方法求解控制方程,在两者之间的界面上使用具有物理意义的声阻抗参数进行匹配,并通过一种快速迭代方法实现全声场求解.这种迭代方法可以保证有限元刚度矩阵等带宽以及对称的特性不被破坏,有助于提高计算效率.该模型先得到了Levine-Schwinger标准解的检验,进而在无流动情况下对于简化的航发短舱进口管道模型进行了噪声辐射现象的数值模拟,最后基于计算结果分析了声衬对远场声辐射的影响.      

一种基于有限元/边界元耦合方法的管道进口声传播及声辐射模型

本文发展了一种基于有限元和边界元耦合方法的管道进口声传播及声辐射计算模型。该模型将整个声场分为内部有限域和外部无界域,分别用有限元和边界元方法求解控制方程,在二者之间的界面上使用具有物理意义的声阻抗参数进行匹配,并通过一种快速迭代方法实现全声场求解。这种迭代方法可以保证有限元刚度矩阵等带宽以及对称的特性不被破坏,有助于提高计算效率。该模型先得到了Levine and Schwinger标准解的检验,进而在无流动情况下对于简化的航发短舱进口管道模型进行了噪声辐射现象的数值模拟,最后基于计算结果分析了声衬对远场声辐射的影响。     

Light scattering in planetary atmospheres

This paper reviews scattering theory required for analysis of light reflected by planetary atmospheres. Section 1 defines the radiative quantities which are observed. Section 2 demonstrates the dependence of single-scattered radiation on the physical properties of the scatterers. Section 3 describes several methods to compute the effects of multiple scattering on the reflected light.     

Type-III radio bursts and their interpretation

The observations of type-III solar radio bursts are briefly reviewed to set requirements on a model for their interpretation. The most important of these requirements is that the source must be an electron stream which is in a state of continuous quasilinear relaxation and which initially must have a nearly monotonically decreasing velocity distribution. The problem of constructing a model is broken into three parts: (1) The plasma wave source which depends on the interaction of the electron stream with electron plasma waves. (2) The radiation source which depends on the interaction of plasma waves and transverse electromagnetic waves or in a magnetized plasma the ordinary and extraordinary modes of magnetoionic theory. (3) The propagation of radiation between the source and the observer which depends on the transmission of radiation through a scattering refracting absorbing magnetized plasma.Progress on a model for the plasma wave source is reviewed and it is concluded that no existing models are adequate. The equations which would lead to an adequate model are written down, but not solved. These include, in addition to collisional damping, Landau damping both by the exciting stream and the background plasma, and spontaneous and induced processes for a three-dimensional distribution of plasma waves. Possible limitations to a quasilinear approach such as pile-up of plasma waves and nonlinear effects are considered. Processes which affect the gross structure of the source such as electron trajectories in coronal streamers and electron scattering by inhomogeneities are reviewed.Progress on the radiation source is considered both in the absence and presence of a magnetic field. At high frequencies (e.g., 80 MHz) observations of radiation near the fundamental and second harmonic of the plasma frequency allow a unique determination of source size and the energy density in plasma waves within the uncertainties of geometry by source ray tracing. This determination is extremely critical because the fundamental must be amplified and thus production of the fundamental is effectively a much more highly nonlinear process than production of the second harmonic. At low frequencies (e.g., 500 kHz) the second harmonic is shown to be dominant because amplification of the fundamental becomes an inefficient process.Calculations of scattering of radiation in a random medium are reviewed. It is concluded that these are adequate at high and low frequencies, but have not been carried out properly at intermediate frequencies where amplification of the fundamental may still be present. It is shown in particular that when scattering is taken into account at high frequencies all observations can be explained by isotropic emission near the second harmonic. At low frequencies the nature of the scatterers is determined by source occultations unlike the case at high frequencies where these are free parameters. This fact allows the possibility of determining true source sizes at low frequencies by subtracting out the contribution due to scattering. A mechanism for producing the possibly observed linear or highly elliptical polarization of type-III bursts, which must be imposed far from the source due to Faraday rotation, is reviewed.Finally, the questions of what remains to be done and what we can hope to obtain upon completion of this work are briefly considered.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

旋转体源声辐射的近场分析

详细讨论了旋转运动声源的近声场频域解。所得近场解析式表现为远场解基础上的近场修正。范例计算显示出这种修正的特点, 并表明源的旋转半径和轴向偏移对声场影响的性质。风扇实验初步证明了文中分析。      

超声速混合层中边频扰动的气动声场

为研究超声速流动下混合层声辐射机理,提高对超声速混合层气动噪声的认识,利用抛物化稳定性方程(PSE)考察一对频率接近失稳扰动的非线性演化,分析近场差频扰动的演化特征,并结合Wu积分理论计算远场声辐射特性。结果表明,超声速混合层中频谱拓宽与差频扰动有关,差频扰动的产生,拓宽了远场马赫波辐射范围,增大了远场马赫波辐射强度。对于快模态扰动,差频扰动频率越小,其增长能力越强,远场马赫波辐射区域越宽;对于慢模态扰动,差频扰动频率大小对其增长能力影响不明显,远场马赫波辐射范围和强度变化不大。      

Prediction of noise radiation from basic configurations of landing gears by means of computational aeroacoustics

Noise radiation from aircraft during the takeoff and landing has become a major issue for inhabitants living in the vicinity of airports and thus for regulation authorities and aircraft developers. However the numerical simulation of aeroacoustic noise, especially for complex geometries like a landing gear, remains one of the most difficult challenges in aeroacoustics. The present study, aiming at predicting noise radiation from basic geometries as well as the noise radiation of a simplified landing gear, employs a hybrid approach that combines a CFD simulation with the decoupled computational aeroacoustics (CAA) simulation. Flow-induced noise is assumed to originate from turbulence. Reynolds-averaged Navier–Stokes equations with different closure approaches can be employed to gain the required turbulent quantities. Subsequently, quantities as the mean flow velocities, pressure, density, turbulent kinetic energy and dissipation rate of the CFD simulation are the starting point for the generation of the transient acoustic sources by the stochastic noise generation and radiation (SNGR) method. It is assumed that the acoustic phenomena do not provide feedback to the mean flow field and turbulence and thus a recalculation of the flow field is not required. Since the propagation of sound is insignificantly influenced by turbulent and viscous effects, it can be described by the Euler equations in the near field. The CAA simulation is extended with a Ffowcs Williams Hawkings (FWH) module that calculates the noise levels in the far field upon integrating the surface source terms on a porous FWH surface within the CAA domain. The results of the simulations are compared with experimental data, obtained by measurements in an acoustic wind tunnel.     

Estimating Target Lengths Shorter Than the Radar Pulse Width

This paper discusses the performance and sensitivity of a maximum-likelihood length estimation algorithm which was originally developed to estimate the separation between unresolved point scatterers. It shows that when the two-scatterer target model is valid, the algorithm reliably estimates lengths shorter than the Rayleigh limit provided that the strength of no third interior scatterer (or its equivalent created by a few very closely spaced scatterers) is greater than the geometric mean of the strengths of the fore and aft scatterers.     

多喷管射流气动声学特性的数值研究

采用大涡模拟结合动力Smagorinsky亚格子模型对四喷管超声速高温射流进行了三维非稳态数值模拟。基于非稳态计算的结果,利用FW-H面积分方法对四喷管射流远场的声学特性进行计算,并将结果与单喷管得到的结果进行对比。结果表明,多喷管射流形成了复杂的流场,其产生的气动声场并不仅仅是单喷管简单的叠加,由于四股射流之间的相互影响使得声场也产生了新的特点,不但表现在总声压级数值上,还对改变了射流噪声辐射的指向性,最大声压级的位置由原来的50°左右变化到30°。     

Robust autofocus algorithm for ISAR imaging of moving targets

A robust autofocus approach, referred to as AUTOCLEAN (AUTOfocus via CLEAN), is proposed for the motion compensation in ISAR (inverse synthetic aperture radar) imaging of moving targets. It is a parametric algorithm based on a very flexible data model which takes into account arbitrary range migration and arbitrary phase errors across the synthetic aperture that may be induced by unwanted radial motion of the target as well as propagation or system instability. AUTOCLEAN can be classified as a multiple scatterer algorithm (MSA), but it differs considerably from other existing MSAs in several aspects: (1) Dominant scatterers are selected automatically in the 2D image domain; (2) scatterers may not be well isolated or very dominant; (3) phase and RCS information from each selected scatterer are combined in an optimal way; (4) the troublesome phase unwrapping step is avoided. AUTOCLEAN is computationally efficient and involves only a sequence of FFTs. Another good feature associated with AUTOCLEAN is that its performance can be progressively improved by assuming a larger number of dominant scatterers for the target. Numerical and experimental results have shown that AUTOCLEAN is a very robust autofocus tool for ISAR imaging     

S弯二元喷管红外辐射特性实验

基于轴对称排气系统设计了一套S弯二元喷管的实验模型,在实验工况下测量了喷管实验件的部分流场参数和远场红外辐射特性,对S弯二元喷管的红外辐射特性进行了分析和研究,并与基准轴对称喷管的红外辐射实验结果进行了对比。结果表明:上方探测平面是S弯二元喷管的主要辐射方向,其最大辐射角度为15°,在该方向角上固体壁面辐射的贡献最大。下方和侧向探测平面各方向角的红外辐射强度明显小于上方探测平面,主要是由于S弯二元喷管遮挡了大部分的内部高温壁面。与基准轴对称喷管对比,S弯二元喷管有着明显的红外抑制作用,在尾部方向S弯二元喷管的积分辐射强度相比轴对称喷管降低了81%,最大辐射强度相比轴对称喷管降低了47%,燃气辐射降低50%。      

非线性扰动方程研究平面混合层发声机制

在流动过程中由脉动引起的噪声问题在自然界和工业界中广泛存在,研究流动过程中不稳定波的发声机制对于理解、预测并最终控制气动噪声有着重要意义。本文以二维 Blasius 相似性解作为基本流场,以超声速混合层作为研究对象,采用非线性扰动方程(NLDE)研究超声速混合层不稳定波的近场动力学特性与远场声辐射之间的内在联系。针对不同类型的对流马赫数,分别研究对流马赫数 Mc =0.5(<1)和 Mc =1.2(≥1)两种情况下的扰动发声机制,结果表明：当对流马赫数 Mc <1时,流场中的发声机制主要由大尺度结构的涡条发声,且基本波与亚谐波之间的非线性作用能增强辐射强度;当对流马赫数 Mc ≥1时,根据扰动相速度是相对于上层还是下层自由流速度为超声速,可以进一步分为快慢两种模态,分别对快慢两种模态以及其相互作用模态进行了数值模拟研究,计算得到流场中的发声机制是以马赫波形式辐射向远场,即马赫波辐射,其辐角的计算结果与理论值相符。     

劈绕射系数的一种通用表达式及应用

 在讨论矩形波导辐射场、飞行器翼边电磁散射场时,要用到二维金属尖劈的绕射系数表达式。许多学者对尖劈绕射场进行了详细讨论,给出了明确结果,其中级数形式的严格解收敛速度较慢,简化公式是当源置于无限远处取得的,对于尖劈近区场源问题没有触及,文献[2]给出的简化解析式在应用上有一定的困难。因此我们重新进行了推导,获得一种新的表达式,适合于近、远场源问题的数值计算。矩形波导棱边的绕射是普通劈尖的一个特例。     

距离扩展目标检测中的 强散射点自适应估计方法

高分辨雷达通过积累多个距离单元上的目标回波,达到提高距离扩展目标检测性能的目的。由于目标散射中心的稀疏分布,常规的能量积累检测方法存在陷落损失,而基于目标强散射点的积累检测方法可有效克服因陷落损失造成的检测性能下降问题。利用最大类间方差法对目标强散射点进行自适应估计,并通过仿真验证了该方法改善检测性能的有效性。     

静电电磁脉冲辐射场诱发针-球电极结构放电试验

针对当前国内外诱发静电放电（ESD）研究较少的实际情况，为了解自然环境下静电放电辐射场诱发放电的基本特性，进一步分析诱发放电的特征规律，建立了大气条件下诱发放电试验系统。主要采取理论分析、试验验证的方式，对静电放电辐射场诱发针-球电极结构放电的基本规律进行摸索，初步得出了诱发放电的基本规律。研究结果表明：在外界条件一定的情况下，随着辐射场强的不断增大，放电重复频率不断增大；在辐射场强一定的情况下，随着高压源电压的不断增大，诱发放电重复频率不断增大；在外界环境条件一定的情况下，能够确定该条件下的诱发放电唯一阈值。这些试验规律的取得，为真空条件下的诱发放电特性研究提供了试验方法和理论依据，对开展航天器运行环境中的诱发放电研究和防护方法设计具有重要意义。     

Statistical estimation of navigation errors

An algorithm for estimating navigation system accuracy without the use of so-called ground truth data is presented. The estimates are expressed in terms of system covariance matrices and relative bias vectors; if one of the systems is a dead-reckoning (DR) or inertial system, the algorithm also estimates drift rate parameters. A summary of tests with US Navy ships backs up the results of Monte Carlo testing. The algorithm should also have applications outside the field of navigation     

Raster Scan Track of a Radar Scatterer Ensemble

A method is discussed for tracking the radar cross-section centroid of an ensemble of scatterers, using a raster scan observation technique. Estimation of the mean-squared dimensions of the ensemble is also considered. A maximum permissible beam separation is defined for the scan. Expressions for the bias and variance of the centroid estimates are given. Advantages of the approach are that individual scatterers need not be resolvable, and that the number of beams is independent of the number of scatterers.     

High-Resolution Imaging of Tropospheric Radio Scatterers

A two-dimensional matched filter technique is described for processing ing the signal from a pulsed coherent troposcatter link to obtain high crosspath resolution "maps" showing the relative positions of individual scatterers. For a typical tropolink (station displacement 200 km; midpath beam halfwidth 2 km; wavelength 0.3 meters) crosspath resolution is about 4.5 meters under perfect conditions. Vertical resolution is inversely proportional to scatterer height, but even for wide bandwidth signals is never better than hundreds of meters. Slight velocity mismatches between the signal and filter degrade crosspath resolution considerably so that the scatterers' velocity must be known accurately a priori. Crosspath resolution is relatively insensitive to mismatch in signal and filter record lengths. Random fluctuations of the velocity of a scatterer seriously degrade crosspath resolution. For the typical tropolink parameters and with a 10 m/s scatterer crosspath velocity and a 0.2 m/s standard deviation ion fluctuation the crosspath resolution degrades by an order of magnitude. It is concluded that the matched filter imaging scheme will produce images of individual scatterers if specific atmospheric conditions ions exist. The best results are obtained if the received signal comes from a single scattering layer at a known height, with scatterers moving at a known constant horizontal velocity, with no along-path or vertical components. Such a layer would be observed during night time or early morning when convective and mixing processes are at a minimum.     

Jupiter's radiation belts

The recent close encounters of Pioneer-10 (December 1973) and Pioneer-11 (December 1974) with the planet Jupiter provided the first in situ observations of zenomagnetically trapped particle radiation. Such observations represented a major advance in planetary research. Prior estimates of radiation intensities (particle fluxes) at Jupiter had necessarily relied (in the case of electrons) upon inferences from Jovian decimetric radio emission observed at the Earth and (in the case of protons) upon postulates for the numerical scaling from terrestrial proton intensities. The Pioneer-10 and Pioneer-11 observations have stimulated continuing theoretical efforts to understand the reported findings and to extrapolate from them to other planets and other epochs. While the analysis of trapped-radiation data from the Pioneer spacecraft is far from being completed, a consensus has developed with respect to the physical mechanisms that must be considered. The observed radiation belts seem to be populated by radial diffusion from an external source. The diffusion coefficient seems to be that derived from fluctuations in the polarization electric field produced by neutral winds in the Jovian ionosphere, which is coupled to the magnetosphere by equipotential B-field lines. Radiation-belt electrons lose energy and change their equatorial pitch angles by virtue of synchrotron emission. Radiation-belt ions and electrons both may be subject to pitch-angle diffusion caused by waves that the respective particle anisotropies have created through plasma instabilities. Finally, radiation-belt ions and electrons seem to experience absorption by the inner Jovian satellites (moons) in a manner that may depend upon the species and energy of the incident radiation-belt particle. It is not yet known whether satellite-associated clouds of sodium and sulfur contribute substantially to the inferred particle absorption. Also still open is the question of whether the satellites provide a substantial source of radiation-belt particles. Moreover, there remains doubt concerning the configuration of the outer Jovian magnetosphere and the influence of this configuration on the zenomagnetic trapping of energetic charged particles.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

Rank one phase error estimation for range-Doppler imaging

A range-Doppler image is corrupted when each pulse in the corresponding range-time array contains a single time-dependent phase error. A previously devised algorithm, the rank one phase estimation (ROPE) technique developed by F. Dyson, et al. (1992), estimates the phase errors, which are then removed so that range-Doppler imaging can proceed in the usual manner. The iterative algorithm works homogeneously across the two-dimensional range-time array (no bright scatterers are necessary for focusing). The estimation technique is model based, but image recovery is successful for many images that do not fit the model. Extensions of both theory and application of the previously devised algorithm are presented. Theoretical insights pertaining to convergence, initialization, optimality, and recovery for images that do not fit the model are presented. Several examples, drawn from simulations, postmission corrupted field data, and field data that are intrinsically corrupted, are also contained herein