Magnetic Reconnection in Extreme Astrophysical Environments

Magnetic reconnection is a fundamental plasma physics process in which ideal-MHD??s frozen-in constraints are broken and the magnetic field topology is dramatically re-arranged, which often leads to a violent release of the free magnetic energy. Most of the magnetic reconnection research done to date has been motivated by the applications to systems such as the solar corona, Earth??s magnetosphere, and magnetic confinement devices for thermonuclear fusion. These environments have relatively low energy densities and the plasma is adequately described as a mixture of equal numbers of electrons and ions and where the dissipated magnetic energy always stays with the plasma. In contrast, in this paper I would like to introduce a different, new direction of research??reconnection in high energy density radiative plasmas, in which photons play as important a role as electrons and ions; in particular, in which radiation pressure and radiative cooling become dominant factors in the pressure and energy balance. This research is motivated in part by rapid theoretical and experimental advances in High Energy Density Physics, and in part by several important problems in modern high-energy astrophysics. I first discuss some astrophysical examples of high-energy-density reconnection and then identify the key physical processes that distinguish them from traditional reconnection. Among the most important of these processes are: special-relativistic effects; radiative effects (radiative cooling, radiation pressure, and radiative resistivity); and, at the most extreme end??QED effects, including pair creation. The most notable among the astrophysical applications are situations involving magnetar-strength fields (10¹⁴?C10¹⁵ G, exceeding the quantum critical field B _??4×10¹³ G). The most important examples are giant flares in soft gamma repeaters (SGRs) and magnetic models of the central engines and relativistic jets of Gamma Ray Bursts (GRBs). The magnetic energy density in these environments is so high that, when it is suddenly released, the plasma is heated to ultra-relativistic temperatures. As a result, electron-positron pairs are created in copious quantities, dressing the reconnection layer in an optically thick pair coat, thereby trapping the photons. The plasma pressure inside the layer is then dominated by the combined radiation and pair pressure. At the same time, the timescale for radiation diffusion across the layer may, under some conditions, still be shorter than the global (along the layer) Alfvén transit time, and hence radiative cooling starts to dominate the thermodynamics of the problem. The reconnection problem then becomes essentially a radiative transfer problem. In addition, the high pair density makes the reconnection layer highly collisional, independent of the upstream plasma density, and hence radiative resistive MHD applies. The presence of all these processes calls for a substantial revision of our traditional physical picture of reconnection when applied to these environments and thus opens a new frontier in reconnection research.     

Parametric analyses for synthetic jet control on separation and stall over rotor airfoil

Numerical simulations are performed to investigate the effects of synthetic jet control on separation and stall over rotor airfoils. The preconditioned and unsteady Reynolds-averaged Navier–Stokes equations coupled with a k x shear stream transport turbulence model are employed to accomplish the flowfield simulation of rotor airfoils under jet control. Additionally,a velocity boundary condition modeled by a sinusoidal function is developed to fulfill the perturbation effect of periodic jets. The validity of the present CFD procedure is evaluated by the simulated results of an isolated synthetic jet and the jet control case for airfoil NACA0015. Then, parametric analyses are conducted specifically for an OA213 rotor airfoil to investigate the effects of jet parameters(forcing frequency, jet location and momentum coefficient, jet direction, and distribution of jet arrays) on the control effect of the aerodynamic characteristics of a rotor airfoil. Preliminary results indicate that the efficiency of jet control can be improved with specific frequencies(the best lift-drag ratio at F+= 2.0) and jet angles(40 or 75) when the jets are located near the separation point of the rotor airfoil. Furthermore, as a result of a suitable combination of jet arrays, the lift coefficient of the airfoil can be improved by nearly 100%, and the corresponding drag coefficient decreased by26.5% in comparison with the single point control case.     

短距冲击流动下的流型及换热

本文阐述了短距冲击情况下各种流型及相应的各种局部换热特性。对于二维缝射流 ,冲击流流型及局部换热特性显著地受射流紊度的发展程度、靶面对射流的反影响以及高射流速度下压缩性等因素的影响 ,靶面上局部换热的多峰分布取决于这些因素的发展。而对于圆射流排 ,除上述因素外 ,尚有射流三维性质的影响 ,在短冲击距下流动的三维性导致驻点线两侧局部换热有比二维缝射流更陡峭得多的下降速度。     

多喷口喷流对侧向喷流流场影响的风洞试验研究

侧向喷流控制研究一个很重要的目的在于了解、掌握喷流与来流的干扰,寻找提高喷流控制效率的方法,不同截面多喷流同时工作便是其中一种。多喷流同时作用时,下游喷流会受到上游喷流的影响,与直接来流干扰现象不同,控制效率不同。针对这种情况,φ1m高超声速风洞从测压租测力两方面进行了多喷口喷流对侧向喷流控制影响的风洞试验研究。试验采用锥柱模型,喷管均位于同一母线上,喷管数目为单喷和三喷。结果表明：上游喷流的低压区会影响下游喷流,当喷流数目增加时,喷流与来流的干扰与多个单喷的叠加完全不同。     

射流孔交错偏置排布下的楔形凹表面对流换热数值研究

通过三维数值模拟的方法研究了射流孔交错偏置排布下的楔形凹表面对流换热,在射流孔偏置间距比（L/d）为0~2.5、射流孔距凹腔前缘间距比（H/d）为6~12、射流冲击雷诺数（Re）为10000~28000的研究参数下,研究结果表明射流孔交错偏置排布在凹腔中诱导复杂的涡流,对应于射流冲击驻点区的局部表面传热系数得到增强;相对于直线排布的射流孔,小射流孔交错偏距比能够使得凹腔前缘附近沿弦向的展向平均努塞尔数相对射流孔直线排布方式有所改善,随着射流孔交错偏距比增加,沿弦向的展向平均努塞尔数最大值所对应的弦向位置逐渐向凹腔前缘下游迁移.为了在保持凹腔前缘对流换热能力不受到显著削弱的前提下,改善凹腔前缘附近的射流冲击对流换热能力,射流孔交错偏距比宜选择在1倍射流孔直径左右.      

Numerical investigation of transcritical liquid film cooling in a methane / oxygen rocket engine

Transcritical film cooling was investigated by numerical study in a methane cooled methane/oxygen rocket engine.The respective time-averaged Navier-Stokes equations have been solved for the compressible steady three-dimensional(3-D) flow.The flow field computations were performed using the semi-implicit method for pressure linked equation(SIMPLE) algorithm on several blocks of nonuniform collocated grid.The calculation was conducted over a pressure range of 202 650.0 Pa to 1.2×107 Pa and a temperature range of 120.0 K to 3 568.0 K.Twenty-nine different cases were simulated to calculate the impact of different factors.The results show that mass flow rate,length,diameter,number and diffused or convergence of film jet channel,injection angle and jet array arrangements have great impact on transcritical film cooling effectiveness.Furthermore,shape of the jet holes and jet and crossflow turbulence also affect the wall temperature distribution.Two rows of film arranged in different axial angles and staggered arrangement were proposed as new liquid film arrangement.Different radial angles have impact on the film cooling effectiveness in two row-jets cooled cases.The case of in-line and staggered arrangement are almost the same in the region before the second row of jets,but a staggered arrangement has a higher film cooling effectiveness from the second row of jets.     

周期性湍流射流冲击下的传热与流场特征

实验研究了周期性变化的湍流射流冲击平板时的传热和流场特性,对实验现象和射流冲击强化传热机理进行了探索和分析。利用一个特殊的质量流量控制装置产生周期性的冲击射流,采用典型的正弦和矩形变化的射流,在不同频率下进行实验。研究表明,两种周期性射流冲击传热性能有很大差异,矩形冲击射流的传热性能优于正弦冲击射流,提高频率有助于射流冲击传热的强化。用粒子图像测速(PIV)技术对流场进行锁相实验测量,研究表明,在较高频率下,阶跃变化的矩形射流冲击时整个变化周期内平板表面处于强烈的冲击和涡流不断的产生和传播状态,从而能有效地强化对流换热过程。平缓连续变化的正弦射流的锁相平均速度场低于矩形射流,卷吸和冲击作用、涡流的形成和传播过程均没有矩形射流冲击下那么强烈,从而导致换热强化效果相对较弱。     

合成射流激励器实现共轴射流掺混控制

对多排列合成射流激励器控制共轴射流掺混流场进行了详细的二维非定常数值模拟。结果表明,激励器的采用可以增强共轴射流的掺混效果;相邻合成射流产生涡对在孔口处经过相互耦合,形成了更强的旋涡对,并以一种新生合成射流作用周围流场;两排激励器较单排激励器相比,前者控制掺混能力更强,这主要是由于相邻合成射流比单个合成射流有着更显著作用力的缘故。     

横向射流影响缓燃向爆震转捩过程的试验研究

为探究横向射流对缓燃转爆震过程的影响,在高为20 mm,深为6 mm,总长为810 mm的矩形通道内进行了横向射流加速起爆的单爆试验。采用甲烷和氧气的预混气为反应物,点火源为点火能量为50 mJ的弱火花塞点火。垂直于主流的横向喷射孔径为1 mm,喷射介质为与反应物同浓度的混气。试验主要研究射流延迟时间、射流位置、射流数量和射流分布形式对起爆特性的影响。结果显示,射流各喷射位置下均存在相应的最佳射流延迟喷射时间使得预混气的起爆时间最短。射流位置分别位于距点火区90、270 mm处的平行双射流和90、180 mm处的交错射流均具有最短的起爆时间,约为1 ms。     

Grid-Adaptive Computations of Magnetized Jets

We present grid-adaptive numerical simulations of magnetized plasma jets, modeled by means of the compressible magnetohydrodynamic equations. The Adaptive Mesh Refinement strategy makes it possible to investigate long-term jet dynamics where both large-scale and small-scale effects are at play. We extend recent findings for uniformly magnetized, periodic shear layers to planar and fully 3D extended jet segments. The jet lengths cover multiple, typically 10, axial wavelengths of the fastest growing Kelvin–Helmholtz (KH) like modes. The dominant linear MHD instabilities of the jet flows are quantified by means of MHD spectroscopic analysis. In cases characterized by sonic Mach numbers about unity and large plasma beta values, both single and double shear layers (planar jets) manifest self-organizing trends to large scales, e.g. by continuous pairing/merging between co-rotating vortices, simultaneously with the introduction of small-scale features by magnetic reconnection events. The vortices form as a result of KH unstable shear-flow layers, and their coalescence arises from the growth of subharmonic modes at multiple wavelengths of the fastest growing KH instability. In extended two-dimensional jet segments, we investigate how varying jet width alters this coalescence process occurring at both edges, e.g. by introducing Batchelor-like coupling between counter-rotating vortices formed at opposing weakly magnetized, close shear layers. Finally, periodic segments of supersonic magnetized jets are simulated in two- and three-dimensional cases, which are characterized by violent shock-dominated transients.     

喷流对飞机尾流涡影响的试验研究

飞机产生的尾流涡,特别是大尺度的翼尖涡,对尾随其后的飞行器是非常有害的,本文旨在探索利用飞机发动机产生的喷流加速尾流涡消亡的方法。试验采用简化的飞机模型(有尾翼),建立了包含一对翼尖涡及一对反向旋转的尾翼涡(通过以负迎角安装尾翼得到)的4涡尾流系统。在无外来扰动的情况下,不同的尾翼设置下得到的尾翼涡对翼尖涡的作用效果不同,有的能导致翼尖涡提前消亡,有的则不能。考察了不同强度的喷流对不同4涡尾流系统的影响,且作为对比,对无尾翼(2涡系统)及无喷流下的各种情况也分别作了观测。试验在拖曳水槽中进行,运用体视粒子图像测速(SPIV)技术,观测了与模型拖曳方向垂直的、从机翼后缘到下游约45翼展间均布的一系列切面。结果表明:当喷流直接作用于涡时,其效果主要取决于两者之间的初始距离及相对强度;而当喷流作用于整个4涡尾流系统时,其引入的扰动对不同的系统均能起到一定程度的改善作用,这种作用的关键在于利用喷流优化对翼尖涡进行扰动的机制,而不仅仅取决于喷流的强度。     

亚声速旋拧射流噪声中的温度效应

采用大涡模拟（LES）方法模拟亚声速旋拧射流,着重考察温度效应对旋拧射流近场流动演化过程、湍流脉动空间发展和远场噪声的影响。线性稳定性分析表明,旋拧射流中提高射流中心温度会增加剪切层的扰动增长率;数值结果显示,加热会促进剪切层中大尺度结构的产生及相互作用,促使流动更快进入湍流状态,并缩短射流势核区的长度。在初始层流发展阶段,加热会提高中心线上的流向速度脉动峰值,但是对剪切层中的流向速度脉动峰值几乎没有影响;在湍流发展阶段,提高射流中心温度会提高流向速度脉动衰减率,并降低脉动幅值。此外,在非等温射流中,密度脉动幅值要远高于等温射流。在30°方位角附近,等温射流的总声压级幅值最高,冷射流的噪声幅值最低。方位角大于50°时,加热使总声压级降低,且随着方位角幅值的增大,降低越明显;而冷却则会提高总的声压级幅值。     

CONTROL OF TWO DIMENSIONAL JETS USING MINIATURE ZERO MASS FLUX JETS

Zero mass flux jets, synthesized by acoustic actuators, have been used for the purpose of jet mixing enhancement and jet vectoring. Zero mass flux jets composed of entirely entrained fluid allow momentum transfer into the embedding flow. In the present experiments, miniature-scale high aspect ratio actuator jets are placed along the long sides and near the exit plane of a primary two-dimensional jet. In different modes, the primary jet can be vectored either towards or away from the actuator jets and the jet mixing is enhanced. The disturbance of the excitation frequency is developed while the unstable frequency of the primary jet is completely suppressed.     

Numerical investigation on flow mechanism in a supersonic fluidic oscillator

A type of supersonic fluidic oscillator is proposed and its ability to generate pulsating supersonic jet is proved in this paper. Unsteady two-dimensional numerical simulations reveal that the fluid transforms from subsonic to supersonic condition in the mixing chamber of oscillator after the supplied flow pressure increases from 1.1 × 10⁵ Pa to 5.0 × 10⁵ Pa. When the supersonic flow is formed inside the oscillator, the wall-attached flow represents expansion wave and compression wave alternately. The oscillating frequency will saturate to a certain value with the increase of supplied pressure. Examination of the internal fluid dynamics indicates that the flow direction inside the FeedBack Channel (FBC) is related to the change of the local pressure at the inlet and the outlet of the feedback channel. The vortices produced in the mixing chamber present different distribution characteristics with the change of the fluid’s direction in the FBC. The sweeping jet is divided into two jets with varying flow rate over time by the splitter. In the end of two channels, two jets are accelerated above sound speed by convergent-divergent nozzle. Therefore, pulsating supersonic jets are produced at two outlets for this type of fluidic oscillator.     

合成射流控制主流矢量的发展过程

合成射流激励器射流矢量控制具有对吸气式发动机进气道出流和发动机喷管流动进行矢量控制的潜力。通过对四种不同出口构型合成射流激励器控制主流矢量偏转的数值模拟和分析,对合成射流控制主流矢量的发展过程进行了研究。结果表明:合成射流控制主流矢量的发展过程可分为三个不同阶段,第一阶段,在主流通道内,激励器工作引起的主流通道压强梯度形成的侧向力致偏主流,在出口处主流矢量偏转Ⅰ;第二阶段,在主流出口附近,合成射流对主流的卷吸作用和引射作用,致偏主流Ⅱ;第三阶段,在出口下游,合成射流与主流自由剪切层发生相互耦合作用,致偏Ⅲ。因此,主射流最终偏转角度是其在经历三个不同阶段时受到的合成射流致偏作用之和,即=Ⅰ+Ⅱ+Ⅲ。     

双微射流作动器定向控制主流数值模拟

采用改进的Beam-Warming近似因式分解法求解二维、粘性、非定常、可压全N-S方程,对双微射流作动器定向控制主流的合成流场进行数值模拟。计算结果揭示了主流在双微射流作用下会比在单微射流作用下产生更大的偏转,并探讨了其产生偏转的原因。分析了主流的偏转程度与相邻作动器相位差之间的关系,即相邻作动器之间的相位差必须在一定的范围内,主流才能发生更大的偏转。     

侧向多喷流流场数值研究

为研究侧向喷流对高超声速拦截弹喷流直接力控制性能的影响，发展了在有限体积离散方法框架下的三维非定常N—S方程，采用LU—SSOR隐式时间推进格式和残值项空间迎风型格式求解技术，对大气层内高超声速拦截弹多个姿态控制喷流发动机同时工作时的干扰流场进行了数值求解。计算给出了自由流与侧喷流、侧喷流与侧喷流之间复杂干扰流场结构图(等压云图、等马赫云图)以及喷流附近的压强分布，其结果与单一侧向喷流流场情况进行了比较和分析。结果显示，喷流前的压强场基本不受后喷流的影响，处于前喷流尾流区后的喷流的干扰效率将受到较大影响。     

声激励喷流流动的实验研究

本文就喷管上游加入“声激振”对喷流流动特性的影响进行了机理性实验研究,得出在不同声激振参数和喷流工况条件下的喷流参数变化规律及相应的流动显示图象。实验结果表明,一定的声激振能量可引起喷流核心区长度缩短,掺混宽度增加,并对外界空气增大引射掺混。      

射流间的声屏蔽对啸叫的作用

用纹影流动显示、声学特性测量和理论分析相结合方法，研究欠膨胀超声速双射流布局对声学特性的影响。欠膨胀超声速双射流的啸叫基频模式与单自由射流相比，在模式切换对应压比上不同，但基本规律相同。但双射流的三维空间流动特性，使不同空间方位上测得的声学特性不同，射流间对彼此处于螺旋啸叫C模式的声辐射有显著的屏蔽作用，在上、下游靠近主轴的方位角处屏蔽效果最大。利用双射流的声屏蔽特性，可达到降噪目的。     

零质量自耦合射流控制喷流矢量实验

以压电陶瓷振为激振源的自耦合射流激发器,在发射孔产生一系列平均质量流率为零的涡环,对垂直布置的主流产生复杂的相互作用,使主流实现矢量偏转。研究了自耦合射流激发器自身机理和自耦合射流使主流实现矢量偏转的实验,在实验条件下,自耦合射流使20m s流速的主流产生了最大17°的矢量偏转。