Numerical Simulation of Wake Vortices Generated by an A330-200 Aircraft in the Nearfield Phase

In order to overcome the typical limitation of earlier studies,where the simulation of aircraft wake vortices was essentially based on the half-model of symmetrical rectangular wings,in the present analysis the entire aircraft(a typical A330-200 aircraft)geometry is taken into account.Conditions corresponding to the nearfield phase(takeoff and landing)are considered assuming a typical attitude angle of 7°and different crosswind intensities,i.e.,0,2 and 5 m/s.The simulation results show that the aircraft wake vortices form a structurally eudipleural four-vortex system due to the existence of the sweepback angle.The vortex pair at the outer side is induced by the pressure difference between the upper and lower surfaces of the wings.The wingtip vortex is split at the wing by the winglet into two smaller streams of vortices,which are subsequently merged 5 m behind the wingtip.Compared with the movement trend of wake vortices in the absence of crosswind,the aircraft wake vortices move as a whole downstream due to the crosswind to be specific,the 2 m/s crosswind can accelerate the dissipation of wake vortices and is favorable for the reduction of the aircraft wake separation.The 5 m/s crosswind results in significantly increased vorticity of two vortex systems:the wingtip vortex downstream the crosswind and the wing root vortex upstream the crosswind due to the energy input from the crosswind.However,the crosswind at a higher speed can accelerate the deviation of wake vortices,and facilitate the reduction in wake separation of the aircraft taking off and landing on a single-runway airport.

Nearfield acoustic holography in a moving medium based on particle velocity input using nonsingular propagator

Nearfield acoustic holography in a moving medium is a technique which is typically suitable for sound sources identification in a flow.In the process of sound field reconstruction,sound pressure is usually used as the input,but it may contain considerable background noise due to the interactions between microphones and flow moving at a high velocity.To avoid this problem,particle velocity is an alternative input,which can be obtained by using laser Doppler velocimetry in a non-intrusive way.However,there is a singular problem in the conventional propagator relating the particle velocity to the pressure,and it could lead to significant errors or even false results.In view of this,in this paper,nonsingular propagators are deduced to realize accurate reconstruction in both cases that the hologram is parallel to and perpendicular to the flow direction.The advantages of the proposed method are analyzed,and simulations are conducted to verify the validation.The results show that the method can overcome the singular problem effectively,and the reconstruction errors are at a low level for different flow velocities,frequencies,and signal-to-noise ratios.

各向异性电离层中沿地磁场取向有限长柱体目标的近场散射

为了研究甚低频(very low frequency,VLF)电磁波对电离层中目标的近场散射机制,提出了一种VLF电磁波在该环境中对有限长金属和介质柱体近场散射的计算方法。首先,通过定义散射系数和交叉散射系数,分析各向异性电离层中电型波和磁型波激励下的散射矩阵;然后,根据积分方程法和矩量法,计算得到各向异性电离层中有限长金属和介质柱体的近区散射场和表面电流半解析表达式;最后,通过各向异性电离层中VLF波的特征波分析,给出柱体近区散射场中寻常波(O波)和非寻常波(E波)分量空间分布情况。计算结果表明:电离层中VLF波在金属和介质目标表面激励了不同的电流,并在不同的电离层环境参数、柱体尺寸和入射角度下产生相应的近场散射和耦合散射;通过对比O波和E波表面电流分布和散射分量,进一步揭示了VLF波与暴露在均匀等离子体环境中的柱体目标的散射机理。本文的计算方法和结果可为电离层中散射通信和目标探测提供理论依据。

Patch nearfield acoustic holography combined with sound field separation technique applied to a non-free field

The conventional nearfield acoustic holography(NAH) is usually based on the assumption of free-field conditions, and it also requires that the measurement aperture should be larger than the actual source. This paper is to focus on the problem that neither of the above-mentioned requirements can be met, and to examine the feasibility of reconstructing the sound field radiated by partial source, based on double-layer pressure measurements made in a non-free field by using patch NAH combined with sound field separation technique. And also, the sensitivity of the reconstructed result to the measurement error is analyzed in detail. Two experiments involving two speakers in an exterior space and one speaker inside a car cabin are presented. The experimental results demonstrate that the patch NAH based on single-layer pressure measurement cannot obtain a satisfied result due to the influences of disturbing sources and reflections, while the patch NAH based on double-layer pressure measurements can successfully remove these influences and reconstruct the patch sound field effectively.

Enhanced optical absorption in semiconductor nano- particles enabled by nearfield dielectric scattering

The optical absorption of semiconducting AgBr nanocubes is significantly increased by up to 5 times in the measured spectral range when they are bonded to the surface of dielectric SiO2 nanospheres through electrostatic interaction. The absorption enhancement factor depends on the wavelength and the size of the SiO2 nanoparticles （NPs）. Finite-difference time-domain calculations provide the nearfield intensity mapping of a heterostructure that is composed of a AgBr nanocube in close contact with a SiO2 nanosphere. The electric-field distributions indicate the field enhancement near the SiO2/AgBr interface due to light scattering and absorption enhancement in the AgBr nanocube, implying that the enhanced scattering nearfield increases the absorption cross section of the AgBr nanocube. The absorption cross-section spectra calculated using Mie theory agree with the experimental observations. This discovery sheds light on the utilization of dielectric spherical particles to increase the absorption in semiconductor NPs, thus improving the light-harvesting efficiency for solar-energy conversion.

The method eliminating nearfield effect for measurement sound absorption coefficient

The numerical simulation is used in this work to study the nearfield radiation pressure of a piston projector, the smooth effect on the pressure field by using a 'big' receiver and the influence of the nearfield and in the transition region on measurement of sound absorption coefficient. The result indicates that the big error would appear when using usual absorption coefficient measurement method. According to the reason causing the error, the method eliminating nearfield effect (MENE) is proposed and some digital results are given for a / λ= 3, 9 and b = 0, 0.5a, a (a and b are the transducer and receiver radii respectively). The calculation results show that the method not only widens measurement range in nearfield, but also gives accurate results.

Landweber iterative regularization for nearfield acoustic holography

On the basis of the distributed source boundary point method (DSBPM)-based nearfield acoustic holography (NAH), Landweber iterative regularization method is proposed to stabilize the NAH reconstruction process, control the influence of measurement errors on the reconstructed results and ensure the validity of the reconstructed results. And a new method, the auxiliary surface method, is pro- posed to determine the optimal iterative number for optimizing the regularization effect. Here, the optimal number is determined by minimizing the relative error between the calculated pressure on the auxiliary surface corresponding to each iterative number and the measured pressure. An experiment on a speaker is investigated to demonstrate the high sensitivity of the reconstructed results to measurement errors and to validate the chosen method of the optimal iterative number and the Landweber iterative regularization method for controlling the influence of measurement errors on the reconstructed results.

Resolution enhancement of nearfield acoustic holography by interpolation using band-limited signal restoration method

A new method based on interpolation using band-limited signal restoration method was proposed for enhancing the resolution of the nearfield acoustic holography. According to the band-limited property of the pressure on the hologram surface, a band-limited signal restoration method named Papoulis-Gerchberg algorithm was used to realize the interpolation of acoustic pressure. Therefore acoustic pressure data on the hologram surface were increased, the sampling interval was reduced, the information on evanescent waves which was lost because of the large sampling interval was partially recovered, and the resolution of nearfield acoustic holography image was improved. The experimental result shows that the method can enhance the resolution of the nearfield acoustic holography image efficiently.

压缩感知等效源法对板件近场重构精度改进的研究

为改进目前板件压缩感知等效源法近场声全息中存在的稀疏基稀疏度不足、适用范围小等问题,提出一种优化等效源面范围的方法。使用板件振动模态与声辐射模态对板件倏逝波分布区域进行联合研究。利用板件振动模态得到高波数倏逝波分布情况,结合声辐射模态得到等效源向量在波数域分布情况,提出减小等效源范围从而达到过滤高波数倏逝波,减小重建误差的效果。数值模拟实验结果表明,该方法增加了等效源向量的稀疏度,对重建精度有大幅度的改善作用,扩大了压缩感知等效源法的使用范围。

基于格林函数法研究分数阶Bessel-Gaussian光束近场传输

为揭示FBG(fractional-order Bessel-Gaussian)光束在近场传输时的光强和相位演化情况,基于光传播的独立性和叠加性原理,通过引入虚光源点,并结合格林函数法,严格推导了FBG光束的非傍轴积分表达式,并进一步利用非傍轴下的各阶修正解,精确计算了FBG光束轴上的光强分布.计算结果表明:对于分数阶Bessel-Gaussian光束的远场传输,利用经典的傍轴近似理论可以得到准确的计算结果,但在近场传输时,傍轴近似理论的计算结果误差很大,只有利用非傍轴修正解才能精确研究FBG光束的近场光学传输.研究结果为将FBG光束更好地应用于近场光学奠定必要的理论基础.

Extension of sound field reconstruction based on element radiation superposition method in a sparsity framework

Nearfield acoustic holography(NAH)is a powerful tool for realizing source identification and sound field reconstruction.The wave superposition(WS)-based NAH is appropriate for the spatially extended sources and does not require the complex numerical integrals.Equivalent source method(ESM),as a classical WS approach,is widely used due to its simplicity and efficiency.In the ESM,a virtual source surface is introduced,on which the virtual point sources are taken as the assumed sources,and an optimal retreat distance needs to be considered.A newly proposed WS-based approach,the element radiation superposition method(ERSM),uses piston surface source as the assumed source with no need to choose a virtual source surface.To satisfy the application conditions of piston pressure formula,the sizes of pistons are assumed to be as small as possible,which results in a large number of pistons and sampling points.In this paper,transfer matrix modes(TMMs),which are composed of the singular vectors of the vibro-acoustic transfer matrix,are used as the sparse basis of piston normal velocities.Then,the compressive ERSM based on TMMs is proposed.Compared with the conventional ERSM,the proposed method maintains a good pressure reconstruction when the number of sampling points and pistons are both reduced.Besides,the proposed method is compared with the compressive ESM in a mathematical sense.Both simulations and experiments for a rectangular plate demonstrate the advantage of the proposed method over the existing methods.

基于源偶极子的扫描近场光学显微镜的数值模拟

红外和太赫兹范围内的扫描近场光学显微镜能够突破衍射极限,实现更小的空间分辨率,在纳米尺度结构的光学特性分析检测方面具有重要应用。为了进一步理解探针与样品的相互作用,对探针近场的分析和数值描述是必不可少的。基于真实的探针形状的解析模型,结合数值模拟的开发了源偶极子模型(SDM),能够直接获得近场检测信息,提高计算效率。基于模拟结果,解释了天线效应、尖端半径影响和电荷量的影响,并将SDM模型结果与全波仿真(FWS)结果进行了比较。为进一步理解近场光学显微镜中的针尖-样品结提供了新的视角。

超临界燃油喷射近场结构与流量特性研究

组合发动机中燃油常处于超临界状态喷射入燃烧室中,其喷射特性决定着燃烧效率与稳定性。通过数值模拟系统地研究了滞止压力(2.5~3.5MPa),滞止温度(640~740K),环境压力(0.1~2.1MPa)及真实气体效应对超临界燃油喷射近场结构与流量的影响规律。研究结果表明:当环境压力小于声速临界压力时,射流近场产生膨胀扇区,膨胀扇区径向边界上存在低于环境温度的环形低温区;膨胀扇区的长度与直径均随滞止压力或滞止温度的升高而增大,随环境压力的降低而显著增大,且会出现由中度欠膨胀向极度欠膨胀的转变;燃油质量流量随滞止压力的升高而增大,随滞止温度的升高而减小,是否受环境压力影响取决于燃油流动是否处于壅塞状态;当滞止温度低于临界温度时,喷嘴内部会出现冷凝现象,流量明显提升;相较于理想气体模型,真实气体模型得到的燃油出口质量流量较高,射流核心区长度较小。

非均匀流体介质内部散射声场重建的逐层计算方法

入射声波激励下非均匀流体介质内部散射声场的重建方法对超声层析成像具有重要意义。以往采用矩量法求解,但该方法全域离散形成的复数满秩矩阵规模随着分辨率与计算精度的提高急剧增大,对算力具有很高的要求,一定程度上限制了其在实际中的应用。为克服上述缺陷,以逐层离散、逐层计算为核心思想,以声散射基本公式与近场声全息理论为基础,推导出逐层计算非均匀流体介质内部散射声场的理论公式并给出对应的几何离散模型。为验证该方法的可行性,以矩量法为参照,对同样的介质模型进行介质内部声场重构仿真。结果表明,逐层算法不仅可以有效地重建非均匀流体介质内部散射声场,且大幅度减小了求解规模。

基于近场扫描的屏蔽罩内辐射源重构方法

提出的屏蔽罩内辐射源重构方法,采用仅包含幅值信息的近场扫描技术,克服了精确测量相位信息的困难。首先,扫描屏蔽罩不存在时自由空间中真实未知电磁干扰(EMI)源的幅值近场云图,应用动态差分进化(DDE)算法获得等效偶极子模型。然后,将等效偶极子模型连同屏蔽罩模型带入全波仿真,用以表征屏蔽罩中的未知EMI源。经仿真和测量实例验证,存在屏蔽罩情况下,受扰微带线耦合电压的测量值和仿真预测值之间的最大偏差小于6 dB,等效偶极子模型可以用来表征屏蔽罩中的未知EMI源。最后,结合工程应用,建议屏蔽罩内受扰微带线摆放应恰当选择倾角并分别考虑非谐振频率和谐振频率两种情况时与EMI源的耦合关系。

2000年姚安地震的震源参数

根据近场小孔径观测台网记录的余震序列资料 ,研究了 2 0 0 0年 1月 15日云南姚安MS6 .4地震序列的地震物理过程 .用地震标定律关系估算主震的地震矩M0 =1.5 8× 10 1 8N·m ,矩震级MW =6 .0 ,平均位错 D =0 .6 3m ,断层长度L =16 .6km ,断层宽度W =5 .6km .余震序列的高精度定位结果和能量分布走向 ,很好地证实了主震的断层破裂走向为N5 0°W ,震区马尾菁断裂为主震发震构造 ,断层错动性质以右旋走滑为主 .用横波记录资料及波谱分析方法估算出余震的震源参数 :地震矩范围为 10 1 0 ～ 10 1 6 N·m ,震源破裂半径a为 80～ 5 0 0m ,地震应力降范围为 0 .0 1～ 9.5MPa .较大应力降 (Δσ >1.0MPa)沿主断层线性排列 ,大应力降 (Δσ >2 .0MPa)与ML≥ 3.0级地震相关 .余震能量释放和高应力降的地震多发生在 6 .0～ 11km的深度范围 ,说明在这一深度范围内最大程度地集中了地壳中的应力 .

高功率底发射VCSELs的制作与特性研究

研究制作了大面积底发射氧化限制面发射激光器,并分析了器件特性。通过增加有源区面积,改进制作工艺,采用Al2O3作钝化膜和多层复合HfO2作增透膜等方法,提高了激光器输出功率。分析了最大输出功率与有源区直径和注入电流之间的依赖关系。结果表明:有源区直径分别为500μm和600μm的单管,室温下均达到连续输出功率1.95W,这也是目前国际上所实现的单管室温连续输出最高功率;实验所得最大输出功率与有源区直径和注入电流之间的依赖关系与理论计算所得结果一致。并特别讨论了直径200μm的器件的近场和远场光强分布,获得单横模工作。

PVDF计在水中爆炸近场压力测试中的应用

为得到炸药水中爆炸冲击波在一定近场范围内的压力剖面曲线和峰值压力随传播距离的衰减规律,采用在炸药起爆轴线上布置4个PVDF计测量冲击波压力以及在相同实验情况下对炸药轴线上冲击波轨迹进行高速扫描的方法,研究了PVDF计测试近场压力的可行性。结果表明,PVDF计可以用于水中爆炸近场冲击波压力的测量,在量程允许范围内,能得到较准确的冲击波压力峰值;在冲击波强度小于4GPa的情况下,得到了冲击波的压力衰减历程,获得近场压力剖面曲线,对评估舰船抗爆性能具有重要意义。

高功率激光装置光束精密调控性能研究进展

光束精密调控是惯性约束聚变(ICF)研究对激光驱动器的基本需求,它是一项装置层面的系统工程。主要介绍了中国工程物理研究院激光聚变研究中心近年来在靶面光强控制、脉冲波形控制、光束近场控制以及在新型光束探索方面所取得的重要进展。

基于波数域外推方法的近场声全息

在基于快速傅里叶变换(FFT)法的近场声全息技术中,为了减小算法误差对结果的影响,要求测量面(全息孔径)至少为声源面积的2倍。由于要求的全息孔径较大,该技术测量工作异常繁琐以及在大尺寸结构上应用困难。分析基于FFT法的近场声全息技术中算法误差产生的机理,并引入声压外推过程予以克服,得到基于波数域外推方法的近场声全息。该方法能有效抑制重建过程中算法误差的影响,不再须要靠增大全息孔径来减小误差,因此在小全息孔径条件下仍具有较高精度。仿真和试验结果表明,该方法在小全息孔径条件下取得了良好效果,显著减小了对全息孔径大小的要求,从而方便了工程运用。