非均匀网格下的QUICK格式应用于强分离湍流计算

本文从湍流模型和数值格式两方面出发,研究了对绕方形柱体的强分离低速湍流的数值模拟.比较了标准k-ε两方程模型和J-B模型,将QUICK格式推广应用于非均匀网格,采用LU分解的强隐式求解技术(LU-SIP)保证了求解的稳定性和效率.计算表明 J-B模型具有明显的优越性,与QUICK格式联合应用可十分显著地改善计算结果.     

Sajben扩压器复杂流动中湍流模型的性能评估

用8个常用的湍流模型对Sajben扩压器中跨声速流动进行了数值模拟,评估了Spalart-Allmaras, 标准k-ε, RNG (re-normalization group) k-ε,realizable k-ε,标准k-ω,SST(shear stress transport) k-ω,v2-f,Reynolds stress共8个湍流模型对激波/湍流边界层相互作用的模拟预测能力.通过与实验数据比较发现:SST k-ω模型和v2-f模型比其他模型模拟的更准确,其中SST k-ω模型比v2-f更能准确地预测壁面压力,然而对于分离点、再附点以及分离区长度v2-f比SST k-ω预测得更准确.      

液体煤油超声速燃烧数值分析的湍流模型

利用计算流体力学软件对煤油在所设计的双模态燃烧室内的喷雾燃烧进行了数值模拟。采用k-ε三种形式、k-ω两种形式和雷诺应力湍流模型,计算了在飞行马赫数为5,煤油与空气的当量比为0.551时的情况。通过将数值模拟结果与实验数据进行对比,证明这些湍流模型都可用于煤油超声速燃烧的数值计算。并且,k-εRNG模型和k-ωSST模型的计算结果与实验数据最为接近,可以认为这两个模型在模拟煤油超声速燃烧流场中是非常有效的;而雷诺应力模型效果较差。     

基于气体射流的气液两相流动减阻特性

为了明确射流表面减阻特性,基于气液两相流动原理,建立具有射流功能的气液两相流动减阻计算模型,采用数值计算方法,通过RNG k-ε湍流模型对射流表面不同射流方向角、以及不同气体射流速度情况下的流场进行数值模拟及实验验证,分析气体射流对壁面黏性阻力、减阻率及流场的影响,研究气体射流的气液两相的减阻特性。结果表明:气体射流表面能够减小壁面黏性阻力,具有较好的减阻效果;四种射流方向角下,射流表面减阻率均在气体射流速度为5m/s时达到极大值,并且当射流方向角在xoz平面为-30°时,减阻率最大,为7.18%;数值模拟的减阻率效果略高于实验结果;气体射流的气液两相流动对壁面边界层进行了有效控制,减小了壁面的黏性剪应力和雷诺应力,导致壁面的黏性阻力减小,使得射流表面具有减阻性能。     

应用修正的k-ε模型研究超声速H2/Air燃烧

为了研究超声速燃烧中流体可压缩性的影响,对标准k-ε湍流模型进行可压缩性修正(包括结构可压缩性修正和膨胀可压缩性修正两部分)。分别应用标准k-ε模型、修正的k-ε模型和雷诺应力模型(RSM),考虑氢气/空气详细化学反应机理(GR I-M ech 2.11机理,10组分,28基元反应),数值模拟有壁面限制的超声速混合层冷态及热态流场。结果表明:壁面和燃烧对湍流影响都很大;修正模型对冷态以及燃烧场的预测结果优于其它两个;修正模型预测的混合层厚度更薄,燃烧区域更窄,与实验结果吻合地更好。     

湍流模型对湍流射流CFD模拟的影响

横流中的湍流射流涡旋结构复杂,湍流模型对其模拟结果影响很大。采用S-A,Realizable k-ε,k-ω和SST k-ω四种湍流模型对吹风比为0.5和1.5工况下的横流中射流进行了数值模拟研究。与实验值比较验证了数值模拟结果的可信性。分析了旋转张量和剪切张量对湍流生成的影响。发现四种湍流模型对下游速度分布的预测均较为准确,在小吹风比下S-A模型性能最好;在大吹风比下k-ω模型较为准确。     

涡轮叶片前缘气膜冷却数值模拟

对具有双排气膜孔的涡轮叶片前缘进行了气膜冷却数值模拟,在吹风比M为0.5,1.0的情况下得到了相邻两排孔的流场和气膜冷却效率的分布规律,并将计算结果与实验进行了对比分析.结果表明:①在不同的吹风比下,两排孔的冷气射流运动非常复杂,冷却效率沿流向的分布都是非线性的;在M为1.0时,25°的小角度喷射气膜冷却效果较好.②利用两层k-ε湍流模型比传统的k-ε湍流模型数值预测的精确度高,可较好地模拟叶片前缘的气膜冷却特性.      

轴承腔油气两相流数值模拟的试验验证

为探索一种适于模拟轴承腔油气两相流流动特性的数值计算模型,建立了一套简化结构的轴承腔试验装置,通过试验研究获得了不同工况对轴承腔通风口出气比例和回油口滑油温升的影响规律.应用Fluent软件,采用Mixture多相流模型和Eulerian多相流模型分别搭配标准k-ε湍流模型和RNG(renormalization group) k-ε湍流模型对选定试验工况进行数值模拟.对比结果表明:当转速为2200r/min时,使用Mixture多相流模型搭配RNG k-ε湍流模型计算的滑油温升与试验值的相对误差仅为2.72%,体现了该计算模型在高转速时的精确性,并在此基础上获得了油气两相流速度场、压力场和温度场的分布特征.      

两种k-ε模型用于扩压器内流数值模拟的比较

同时采用标准k-ε及非线性k-ε模型在附体坐标系下求解定常不可压N-S方程组计算扩压器内湍流流场, 得到了扩压器的压力恢复系数和速度分布, 与实验结果对比发现非线性k-ε模型预测的结果更吻合实验值, 且能测出临界扩压角下的局部分离流。文中着重研究了非对称扩压器, 并给出了用两种模型计算的湍流量。      

应用不同湍流模型计算叶栅流场的结果与比较

采用有限体积TVD-LW格式求解N-S方程,使用了B-L模型、高雷诺数k-ε模型和低雷诺数k-ε模型等多种湍流模型。并且用具体算例说明本文所用程序的可靠性,比较了3种的湍流模型的特点。B-L模型最大的优点是简单、使用方便。k-ε方程适应性强,分为低雷诺数k-ε方程和高雷诺数k-ε方程两种。高雷诺数k-ε方程比低雷诺数k-ε方程精度略低,但所需计算时间和存储量都比后者小,网格也可以粗些。k-ε模型对粘性层、分离区有比较高的分辨能力,高雷诺数k-ε模型比较适于一般数值计算。      

The First two Years of Image

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) is the first satellite mission that is dedicated to imaging the Earth's magnetosphere. Using advanced multispectral imaging techniques along with omnidirectional radio sounding, IMAGE has provided the first glimpses into the global structure and behavior of plasmas in the inner magnetosphere. Scientific results from the two-year prime mission include the confirmation of the theory of plasmaspheric tails and the discovery of several new and unpredicted features of the plasmasphere. Neutral-atom imaging has shown how the ring current develops during magnetic storms and how ionospheric ions are injected into the ring current during substorms. The first global imaging of proton auroras has allowed the identification of the ionospheric footprint of the polar cusp and its response to changes in the interplanetary magnetic field. Detached subauroral proton arcs have been found to appear in the afternoon sector following south-north and east-west rotations of the IMF. Low-energy neutral atom imaging has shown global-scale ionospheric outflow to be an immediate response to solar-wind pressure pulses. Such imaging has also provided the first measurements of solar wind and interstellar neutral atoms from inside the magnetosphere. Radio sounding has revealed the internal structure of the plasmasphere and identified plasma cavities as the source of kilometric continuum radiation. These and numerous other scientific results now set the stage for the extended mission of IMAGE in which the imaging perspective will change markedly owing to orbital evolution while the magnetospheric environment undergoes a transition from solar maximum toward solar minimum.     

Technological evaluation of two AFIS systems

This paper provides a technological evaluation of two automatic fingerprint identification systems (AFIS) used in forensic applications. Both are installed and working in Spanish police premises. The first is a Printrak AFIS 2000 system with a database of more than 450,000 fingerprints, while the second is a NEC AFIS 21 SAID NT-LEXS Release 2.4.4 with a database of more than 15 million fingerprints. Our experiments reveal that, although both systems can manage inkless fingerprints, the latest one offers better experimental results.     

A comparison of two adaptive detection schemes

Two schemes for adaptive detection are compared: Kelly's generalized likelihood ratio test (GLRT) and the mean level adaptive detector (MLAD). Detection performance, P_D, is predicted for the two schemes under the assumptions that the input noises are zero-mean complex Gaussian random variables that are temporally independent but spatially correlated; and the amplitude of the desired signal is Rayleigh distributed. P_D is computed as a function of the false alarm probability, the number of input channels, the number of independent samples per channel, and the matched filtered output signal-to-noise (S/N) power ratio. In this analysis the GLRT is shown to have better detection performance than the MLAD. The difference in detection performance increases as one uses fewer input samples. However, the required number of samples necessary to have only a 3 dB detection loss for both detection schemes is approximately the same. This is significant since for the present, the MLAD is considerably less complex to implement than the GLRT     

两种镁合金的化学处理

对AZ91D和AM50合金进行化学处理，用SEM和电化学方法来分析化学转化膜的表面形貌及在腐蚀性电解质中的稳定性。结果表明，铬酸盐处理时，AM50合金较AZ91D合金的成膜反应容易进行，成膜均匀，在3．5％NaCI水溶液中耐蚀性能好；而磷酸盐处理时，结果恰恰相反，AZ91D合金转化膜的性能优于AM50合金的膜。     

Monopulse DOA estimation of two unresolved Rayleigh targets

This paper provides for new approaches to the processing of unresolved measurements as two direction-of-arrival (DOA) measurements for tracking closely spaced targets rather than the conventional single DOA measurement of the centroid. The measurements of the two-closely spaced targets are merged when the target echoes are not resolved in angle, range, or radial velocity (i.e., Doppler processing). The conditional Cramer Rao lower bound (CRLB) is developed for the DOA estimation of two unresolved Rayleigh targets using a standard monopulse radar. Then the modified CRLB is used to give insight into the boresight pointing for monopulse DOA estimation of two unresolved targets. Monopulse processing is considered for DOA estimation of two unresolved Rayleigh targets with known or estimated relative radar cross section (RCS). The performance of the DOA estimator is studied via Monte Carlo simulations and compared with the modified CRLB     

两种跨声速气动弹性问题分析研究

在非结构运动网格基础上,采用中心有限体积法进行空间离散和双时间方法进行时间推进求解非定常欧拉方程.通过与气动力方程的联立求解,在时域内用四步龙格-库塔方法求解结构运动方程.分析和研究了二维嗡鸣和三维机翼颤振这两种跨声速非线性气动弹性问题.二维嗡鸣问题的研究考虑了翼面-舵面系统的缝隙间网格运动、缝隙对嗡鸣的影响和扰流片对嗡鸣的抑制.耦合多自由度Lagrange结构运动方程数值模拟了三维机翼的颤振问题.通过跨声速标模算例AGARD445.6机翼的颤振计算,计算的颤振临界速度与实验值有5%左右的误差,验证本方法的正确性.由于本方法是在对外形具有良好普适性的非结构动网格基础上完成的,具有良好的工程实践价值.     

Bayesian tracking of two possibly unresolved maneuvering targets

The paper studies the problem of maintaining tracks of two targets that may maneuver in and out formation flight, whereas the sensor and measurement extraction chain produces false and possibly unresolved or missing measurements. If the possibility of unresolved measurements is not modelled then it is quite likely that either the two tracks coalesce or that one of the two tracks diverges on false measurements. In literature a robust measurement resolution model has been incorporated within an interacting multiple model/multiple hypothesis tracking (IMM/MHT) track maintenance setting. A straightforward incorporation of the same model within an IMM and probabilistic data association (PDA)-like hypothesis merging approach suffers from track coalescence. In order to improve this situation, the paper develops a track-coalescence avoiding hypotheses merging version for the two target problem considered. Through Monte Carlo simulations, the novel filters are compared with applying hypotheses merging approaches that ignore the possibility of unresolved measurements or track-coalescence.