导弹多喷流干扰特性数值计算分析

给出一种直接力/气动力复合控制的空空导弹计算模型,应用CFD软件对其侧向多喷流干扰流场进行了数值模拟;选择四种代表情况,重点研究分析了不同侧多喷流对导弹气动特性的影响。研究结果表明,喷流干扰使得升力系数和俯仰力矩系数减小,并产生较大的偏航干扰力矩,同时喷流的间距也影响俯仰力矩系数和偏航力矩系数。所得结论具有一定的参考价值。     

合成双射流控制翼型分离流动的数值研究

合成双射流激励器是合成射流技术发展的最新成果,所形成的射流具有更高能量、流动更稳定的特点。采用数值模拟的方法,对比研究了合成射流与合成双射流对翼型分离流动的改善效果。结果表明:合成射流可以将翼型失速攻角提高2°、最大升力系数增加18%,合成双射流可以将翼型失速攻角提高4°、最大升力系数增加35%,证明了合成双射流具有更好的分离流动控制效果。另外着重分析了合成双射流工作频率和动量系数对控制效果的影响,发现当激励器工作频率为流场特征频率的1和2倍时,对翼型气动特性的改善效果最好,同时控制效果会随动量系数的增加而增大。     

Kelvin–Helmholtz instability in a twisting solar polar coronal hole jet observed by SDO/AIA

We investigate the conditions under which the fluting ($m=2$), $m=3$, and $m=12$ magnetohydrodynamic (MHD) modes in a uniformly twisted flux tube moving along its axis become unstable in order to model the Kelvin–Helmholtz (KH) instability in a twisting solar coronal hole jet near the northern pole of the Sun. We employed the dispersion relations of MHD modes derived from the linearized MHD equations. We assumed real wavenumbers and complex angular wave frequencies, namely complex wave phse velocities. The dispersion relations were solved numerically at fixed input parameters (taken from observational data) and varying degrees of torsion of the internal magnetic field. It is shown that the stability of the modes depends upon five parameters: the density contrast between the flux tube and its environment, the ratio of the external and internal axial magnetic fields, the twist of the magnetic field lines inside the tube, the ratio of transverse and axial jet’s velocities, and the value of the Alfvén Mach number (the ratio of the tube axial velocity to Alfvén speed inside the flux tube). Using a twisting jet of 2010 August 21 by SDO/AIA and other observations of coronal jets we set the parameters of our theoretical model and have obtained that in a twisted magnetic flux tube of radius of $9.8$ Mm, at a density contrast of $0.474$ and fixed Alfvén Mach number of $?0.76$, for the three MHD modes there exist instability windows whose width crucially depends upon the internal magnetic field twist. It is found that for the considered modes an azimuthal magnetic field of $1.3$–$1.4$ G (computed at the tube boundary) makes the width of the instability windows equal to zero, that is, it suppress the KH instability onset. On the other hand, the times for developing KH instability of the $m=12$ MHD mode at instability wavelengths between 15 and 12 Mm turn out to be in the range of $1.9$–$4.7$ min that is in agreement with the growth rates estimated from the temporal evolution of the observed unstable jet’s blobs in their initial stage.     

Metal Enrichment Processes

There are many processes that can transport gas from the galaxies to their environment and enrich the environment in this way with metals. These metal enrichment processes have a large influence on the evolution of both the galaxies and their environment. Various processes can contribute to the gas transfer: ram-pressure stripping, galactic winds, AGN outflows, galaxy-galaxy interactions and others. We review their observational evidence, corresponding simulations, their efficiencies, and their time scales as far as they are known to date. It seems that all processes can contribute to the enrichment. There is not a single process that always dominates the enrichment, because the efficiencies of the processes vary strongly with galaxy and environmental properties.     

不同喷压比下欠膨胀射流对特征频率激励的响应

为了揭示欠膨胀激励射流的流动机理,以及考察不同喷压比下射流对相同激励的流动响应,采用大涡模拟方法,对喷压比NPR=5.60和9.34的欠膨胀定常射流和激励射流进行了三维数值计算。激励频率为定常射流中固有的轴对称频率=14.569kHz,激励形式为在射流喷管入口处施以正弦压力扰动。结果表明,特征频率激励影响射流的声场特征,缩小射流核心区的范围,减少射流近场的激波胞格数目,并影响射流气体与环境空气的混合。同时,激励射流的特征频率转变为激励频率及其高阶倍频,激励射流的主不稳定模态均为轴对称模态。其中,NPR=9.34的欠膨胀射流的主不稳定模态和外加压力扰动的形式相一致,射流与外加激励发生了更加剧烈的流动耦合和响应。这使得在NPR=9.34时,射流核心区长度减小得更多,压力脉动的振幅更大,激励对射流混合的增强作用更加明显。     

阵列射流-扰流柱复合冷却结构换热和压降特性数值研究

为研究阵列射流-扰流柱耦合换热结构对热端部件综合冷却效果的影响,采用数值模拟方法研究了阵列射流-扰流柱复合冷却结构的流动换热特性,重点关注扰流柱相对于射流孔的布置方式(顺排和叉排)、扰流柱直径d_p相对于射流孔的直径d_j之比(d_p/d_j=0.5,1,2)的影响。为体现阵列射流-扰流柱复合冷却结构的导热-对流强耦合传热过程特征,引入了靶板加热侧当量对流换热系数的概念,并分别采用冲击靶面对流换热系数和靶板加热侧当量对流换热系数进行了综合性能的评价分析。研究结果表明,采用绕流柱顺排和叉排方式的冲击靶面对流换热相较于光滑靶板分别增加约30%和10%,而扰流柱相对于射流孔呈顺排方式的对流换热效果要优于叉排方式,同时,顺排方式的压力损失系数却低于叉排方式。至于扰流柱相对于射流孔的直径比的影响,基于冲击靶面和靶板加热侧对流换热系数的综合性能评价存在显著的差异。     

互击式气-气喷注器混合场影响因素流动显示

虽然针对互击式气-液喷注器开展了大量研究,但对互击式气-气喷注器的了解甚少。为了深入理解互击式气-气喷注器气气混合过程的特点,利用粒子图像测速(PIV)系统对其开展了流动显示实验研究。实验结果表明:对于O-F-O互击式气-气喷注器,气流撞击角度和氧/氢动量比直接影响着气流撞击作用的大小,控制着气流混合过程;对于喷孔间距保持一定时,撞击角越大,气流间的相互作用越强,气流混和效果越好;氧/氢动量比越大,气流间撞击力越强,混合距离越短,对中心气流的挤压作用越厉害,其流场分布三维特征越明显;可以通过组合高的氧压降、低的氢压降和大撞击角来设计混合效果较好的互击式气-气喷注器。     

Radio-faint BL Lac objects and their impact on the radio/gamma-ray connection

Radio and gamma-ray emissions in Active Galactic Nuclei (AGNs) are both related to the presence of relativistic particles in jets. With the advent of the Fermi Large Area Telescope (LAT), and thanks to its large sensitivity up to several GeV, many observational results are changing our understanding of these phenomena. BL Lac objects, which made up only a fraction of the known extragalactic gamma-ray source population before Fermi, have now become the most abundant class. However, since they are relatively weak radio sources, most of them are poorly known as far as their parsec scale structure and multi-wavelength properties are concerned. For this reason, we have selected a complete sample of 42 low redshift BL Lacs (independently of their gamma-ray properties) to study with a multi-wavelength (radio, optical, X-ray, gamma-ray) approach. Here, we present results and images of sources in the sample (most of which have never been observed before), using new VLBA observations at 8 and 15 GHz. Beyond this sample of BL Lacs, the population of gamma-ray AGNs has also dramatically enlarged in the Fermi era, permitting us to discuss the presence of a correlation between radio and gamma-ray properties with improved statistical significance. We explore the radio-gamma relation with several hundreds sources and using both simultaneous and archival radio data, thus tackling the impact of time variability.     

高速列车通风设计方法的研究

 由于高速列车车厢高度密闭的特性,高速列车的通风设计必须满足旅客的安全性和舒适性.而传统的依赖自由射流的经验公式确定车厢内温度场和速度场的方法因无法考虑送排风气流和室内障碍物的影响,通风设计很大程度上依赖于模型实验.本研究将送排风气流与车厢型状及座椅作为一体来考虑、通过数值模车厢内温度场和速度场来保证满足舒适空调要求的同时,还可方便地确定出送风速度和送风温差.该方法不仅适用于高速列车的通风设计,还能适用于一般载人列车及飞机客舱的通风设计.     

液体射流撞击液膜振荡行为的实验研究

液体射流撞击过程是液体火箭发动机常见的高效雾化方式。为了进一步探究液体射流撞击雾化过程,采用高速摄像仪和图像处理技术对两股液体射流撞击液膜的振荡行为进行了实验研究,考察了射流韦伯数(21≤We≤1356)和撞击角(2θ=60°,90°,120°)对液膜振荡的影响。结果表明,随着射流韦伯数增大(We250),撞击液膜会从竖直稳定模式转变为振荡模式,促进液膜破裂和雾化。增大撞击角能加剧液膜振荡和增加雾化角。2θ=60°和90°的液膜振荡区间为250We400,2θ=120°的液膜振荡区间为We250,无量纲振幅的时均极大值约为5。结果揭示了随着液体射流速度的增大,射流不稳定性的发展会引起撞击点的动量不平衡,进而形成液膜振荡。此外,液膜向下传播时与空气界面间的Kelvin-Helmholtz不稳定性也促进液膜振荡。