Numerical Analysis of Interaction Between Single-Pulse Laser-Induced Plasma and Bow Shock in a Supersonic Flow

The interaction of laser-induced plasma and bow shock over a blunt body is inves- tigated numerically in an M∞ =6.5 supersonic flow. A ray-tracing method is used for simulating the process of laser focusing. The gas located at the focused zone is ionized and broken down and transformed into plasma. In a supersonic flow the plasma moves downstream and begins to interact with the bow shock when it approaches the surface of the blunt body. The parameters of flowfield and blunt body surface are changed due to the interaction. By analyzing phenomena occurring in the complex unsteady flowfield during the interaction in detail, we can better under- stand the change of pressure on the blunt body surface and the mechanism of drag reduction by laser energy deposition. The results show that the bow shock is changed into an oblique shock due to the interaction of the laser-induced low-density zone with the bow shock, so the wave drag of the blunt body is reduced.

Formation of the bow shock indentation: MHD simulation results

Simulation results from a global magnetohydrodynamic(MHD)model are used to examine whether the bow shock has an indentation and characterize its formation conditions,as well as its physical mechanism.The bow shock is identified by an increase in plasma density of the solar wind,and the indentation of the bow shock is determined by the shock flaring angle.It is shown that when the interplanetary magnetic field(IMF)is southward and the Alfvén Mach number(Mα)of solar wind is high(>5),the bow shock indentation can be clearly determined.The reason is that the outflow region of magnetic reconnection(MR)that occurs in the low latitude area under southward IMF blocks the original flow in the magnetosheath around the magnetopause,forming a high-speed zone and a low-speed zone that are upstream and downstream of each other.This structure hinders the surrounding flow in the magnetosheath,and the bow shock behind the structure widens and forms an indentation.When Mαis low,the magnetosheath is thicker and the disturbing effect of the MR outflow region is less obvious.Under northward IMF,MR occurs at high latitudes,and the outflow region formed by reconnection does not block the flow inside the magnetosheath,thus the indentation is harder to form.The study of the conditions and formation process of the bow shock indentation will help to improve the accuracy of bow shock models.

A Review of Density Holes Upstream of Earth＇s Bow Shock

Larmor size transient structures with density depletions as large as 99%of ambient solar wind density levels occur commonly upstream of Earth’s collisionless bow shock.These "density holes" have a mean duration of 17.9±10.4s but holes as short as 4 s have been observed.The average fractional density depletion（δn/n） inside the holes is 0.68±0.14.The density of the upstream edge moving in the sunward direction can be enhanced by five or more times the solar wind density. Particle distributions show the steepened edge can behave like a shock,and measured local field geometries and Mach number support this view.Similarly shaped magnetic holes accompany the density holes indicating strong coupling between fields and particles.Current densities as large as 150nA·m-2 are observed at the leading compressed edge.The waves are elliptically polarized and rotating in the sense of ions（left hand） in the plasma frame.The waves appear to grow and steepen as the density holes convect with the solar wind toward the Earth.The transient nature of density holes suggests that the temporal features could represent the different stages of nonlinear evolutionary processes that produce a shock-like structure.The density holes are only observed with upstream particles,suggesting that back-streaming particles interacting with the solar wind are important. The significance of these observations is still being investigated.

Rapid and Asymmetric Response of the Earth＇s Bow Shock： Multipoint Observations

We analyze observations of three bow shock crossings which occurred during 2007.using upstream data from STEREO A/B.ACE and WIND,combined with multi-point THEMIS and Cluster data,and TC-1 data located near noon.During the crossing of 7 May 2007.we find that following a rapid reduction in solar wind ram pressure and subsequent pressure pulse seen by ACE and WIND upstream,the bow shock responds asymmetrically from dawn to dusk.Cluster data on the dawn-side suggest the bow shock is significantly flared and responds rapidly to the pulse arrival,while TC-1 at noon,and THEMIS on the dusk-side,are well matched to the model bow shock,but show a delayed response.The crossings observed on 21 May and 2 June show contrasting response matching the model boundary for northward Interplanetary Magnetic Field(IMF).The IMF and solar wind plasma data suggest that,the bow shock crossing at dawn-dusk side and subsolar point were mainly caused by large and smaller scale features of the solar wind ram pressure rise rather than the influence of IMF.

Shape and position of Earth's bow shock near-lunar orbit based on ARTEMIS data

Earth's bow shock is the result of interaction between the supersonic solar wind and Earth's magnetopause. However, data limitations mean the model of the shape and position of the bow shock are based largely on near-Earth satellite data. The model of the bow shock in the distant magnetotail and other factors that affect the bow shock, such as the interplanetary magnetic field(IMF) B_y, remain unclear. Here, based on the bow shock crossings of ARTEMIS from January 2011 to January 2015, new coefficients of the tail-flaring angle a of the Chao model(one of the most accurate models currently available) were obtained by fitting data from the middle-distance magnetotail(near-lunar orbit, geocentric distance -20R_E>X>-50R_E). In addition, the effects of the IMF B_y on the flaring angle a were analyzed. Our results showed that:(1) the new fitting coefficients of the Chao model in the middle-distance magnetotail are more consistent with the observed results;(2) the tail-flaring angle a of the bow shock increases as the absolute value of the IMF B_y increases. Moreover, positive IMF B_y has a greater effect than negative IMF B_y on flaring angle. These results provide a reference for bow shock modeling that includes the IMF B_y.

Dipole tilt controls bow shock location and flaring angle

The dipole tilt angle has beenfound to affect Earth's bow shock.This work presents a quantitative relationship between the dipole tilt angle and the bow shock location and flaring angle.We collected a large data set of bow shock crossings from four different satellites(IMP 8,Geotail,Magion 4,and Cluster),including some recent crossings obtained during 2012-2013.The results from a statistical analysis demonstrate that:(1)the subsolar standoff distance increases but the flaring angle decreases with increasing dipole tilt angle;(2)when the dipole tilt angle changes sign from negative to positive,the dayside bow shock moves toward Earth and the shift can be as much as 2.29 R_E,during which the flaring angle increases;and(3)the shape of bow shock in the northern and southern hemispheres differs.For the northern hemisphere bow shock,with increasing positive/negative dipole tilt angle,the flaring angle increases/decreases.While for the southern hemisphere,the trend is the opposite;with increasing positive/negative dipole tilt angle,the flaring angle decreases/increases.These results are helpful for future bow shock modeling that needs to include the effects of dipole tilt angle.

Effects of the interplanetary magnetic field clock angle on the shape of bow shock

Using the global magnetohydrodynamics(MHD) simulation model, we investigated the effects of the interplanetary magnetic field(IMF) clock angle on the shape of bow shock, including its rotational asymmetry and subsolar point. For general northward IMF( z component Bz > 0), the rotational symmetry of the bow shock is broken by the effects of fast magnetosonic Mach number(Mms), and the cross-sectional line of the bow shock is an ellipse with the semi-major axis along the direction perpendicular to the IMF. The ratio or D-value between semi-major and semi-minor axis can be used to illustrate the extent of asymmetry of the bow shock. On the basis of the multiple parameters fitting, we obtain the changing relationship of both semi-axes with the clock angle and the distance away from the Earth. For general southward IMF(Bz < 0), the cross sectional line of the bow shock is highly asymmetrical under the multiple effects of magnetopause and Mms. The effects of IMF clock angle on subsolar point depend mainly on the changing subsolar point of magnetopause as an obstacle. The distance of subsolar point of bow shock from the Earth increases with the increasing IMF clock angle for Bz > 0, and decreases with the increasing IMF clock angle for Bz < 0.

Characteristics of the double layer associated with terrestrial bow shock by THEMIS observation

This study presents observation and detailed analysis on the double layers （DLs） in the ramp and the foreshock contacting with the foot of the terrestrial bow shock by THEMIS on September 14, 2008 under enhanced dynamic pressure in the solar wind. The results reveal that： （1） The time duration of the double layers is nus 10-40 mV/m. （2） On assuming a propagation speed at the ion mainly 3-8 ms, and their max parallel electric field is miacoustic speed （vs）, their spatial scale is estimated to be 0.3-1.15 km （about 75-200 2D）. （3） The net potential drop of DLs is estimated to be 5-32 V. （4） The DLs in the ramp and the foreshock contacting to the foot of the bow shock is current-carrying as a result of development and evolution of nonlinear phase of instability in the self-consistent current-carrying plasma. The DLs may play an important role in strong turbulence in the foreshock contacting with the foot of the bow shock.

On the vorticity behind 3-D detached bow shock wave

For the general case of a spatial isoenergetic flow of ideal gas,Helmholtz’s theorems are generalized and the speed with which vortex tubes move is found,keeping the intensity.It is shown that along the streamline without stagnation point,vorticity either is equal to zero everywhere,or it is non zero at all.The pattern of vortex lines behind the three-dimensional detached bow shock wave is specified.

Comparison between magnetic coplanarity and MVA methods in determining the normal of Venusian bow shock

With the measurements of magnetic field of Venus Express (VEX), magnetic coplanarity and minimum variance analysis (MVA) methods are analyzed and their validity is tested to determine the normal of Venusian bow shocks. It is found that MVA method is the better than magnetic coplanarity, and 95% shock crossings can be accurately determined by the method. However, the occurrence of the shock normal which is not determined accurately by magnetic coplanarity increases with the decrease of the solar zenith angle (SZA). At the same time, compared with quasi-parallel shocks, there is more occurrence of the shock normal which cannot be determined accurately by magnetic coplanarity for quasi-perpendicular shocks.

Role of on-board discharge in shock wave drag reduction and plasma cloaking

In the present paper, a physical model is proposed for reducing the problem of the drag reduction of an attached bow shock around the nose of a high-speed vehicle with on-board discharge, to the problem of a balance between the magnetic pressure and gas pressure of plane shock of a partially ionized gas consisting of the environmental gas around the nose of the vehicle and the on-board discharge-produced plasma. The relation between the shock strength and the discharge-induced magnetic pressure is studied by means of a set of one-fluid, hydromagnetic equations reformed for the present purpose, where the discharge-induced magnetic field consists of the electron current （produced by the discharge）-induced magnetic field and the partially ionized gas flow-induced one. A formula for the relation between the above parameters is derived. It shows that the discharge-induced magnetic pressure can minimize the shock strength, successfully explaining the two recent experimental observations on attached bow shock mitigation and elimination in a supersonic flow during on-board discharge [Phys. Plasmas 9 （2002） 721 and Phys. Plasmas 7 （2000） 1345]. In addition, the formula implies that the shock elimination leaves room for a layer of higher-density plasma rampart moving around the nose of the vehicle, being favourable to the plasma radar cloaking of the vehicle. The reason for it is expounded.

A case study of large-amplitude ULF waves in the Martian foreshock

Foreshock ultralow frequency (ULF) waves constitute a significant physical phenomenon in the plasma environment of terrestrial planets. The occurrence of these waves, associated with backstreaming particles reflected and accelerated at the bow shock, implies specific conditions and properties of the shock and its foreshock. Using magnetic field and ion measurements from MAVEN, we report a clear event of ULF waves in the Martian foreshock. The interplanetary magnetic field connected to the Martian bow shock, forming a shock angle of ~51°. Indicating that this was a fast mode wave is the fact that ion density varied in phase with perturbations of the wave field. The peak frequency of the waves was about 0.040 Hz in the spacecraft frame, much lower than the local proton gyrofrequency (~0.088 Hz). The ULF waves had a propagation angle approximately 34° from ambient magnetic field and were accompanied by the whistler mode. The ULF waves displayed left-hand elliptical polarization with respect to the interplanetary magnetic field in the spacecraft frame. All these properties fit very well with foreshock waves excited by interactions between solar wind and backstreaming ions through right-hand beam instability.

The Mars orbiter magnetometer of Tianwen-1:in-flight performance and first science results

The Mars Orbiter MAGnetometer(MOMAG)is a scientific instrument onboard the orbiter of China’s first mission for Mars—Tianwen-1.Since November 13,2021,it has been recording magnetic field data from the solar wind to the magnetic pile-up region surrounding Mars.Here we present its in-flight performance and first science results,based on its first one and one-half months’data.Comparing these early MOMAG observations to the magnetic field data in the solar wind from NASA’s Mars Atmosphere and Volatile EvolutioN(MAVEN)mission,we report that the MOMAG magnetic field data are at the same level in magnitude,and describe the same magnetic structures with similar variations in three components.We recognize 158 clear bow shock(BS)crossings in these MOMAG data;their locations match well statistically with the modeled average BS.We also identify and compare five pairs of datasets collected when Tianwen-1’s orbiter and the MAVEN probe made simultaneous BS crossings.These BS crossings confirm the global shape of modeled BS,as well as the south-north asymmetry of the Martian BS.Two cases presented in this paper suggest that the BS is probably more dynamic at flank than near the nose.So far,MOMAG performs well,and provides accurate magnetic field vectors.MOMAG is continuously scanning the magnetic field surrounding Mars.Data from MOMAG’s measurements complement data from MAVEN and will undoubt edly advance our understanding of the plasma environment of Mars.

EFP冲击起爆带盖板装药的可行性分析

聚能装药形成的金属射流和 EFP是战争中对付坦克、装甲车辆等目标的技术手段 ,研究 EFP对炸药的引爆机理对于评估弹药受 EFP侵彻时的安全性具有非常重要的意义。通过计算 EFP侵彻带盖板装药时在盖板中产生的脱体冲击波压力 ,以及该压力在盖板与炸药界面处产生反射和透射作用后进入炸药内部的透射波压力强度 ,分析了 EFP破甲后冲击起爆炸药的可行性。

膛口初始流场对火药燃气流场影响的数值研究

枪炮发射时,身管内被弹丸压缩的空气柱喷出膛口后形成初始流场,它对后续火药燃气流场的发展及弹丸的运动产生很大影响,甚至会降低射击精度。因而,研究初始流场对火药燃气流场的影响机理对于武器设计具有重要的意义。基于ALE方程的有限体积方法,利用AUSM+格式和分区结构化贴体网格对含有复杂形状弹丸的膛口流场进行了数值模拟。根据数值结果绘制的计算阴影图与实验阴影照片符合较好。分别计算了有、无初始流场两种条件下的流场发展过程,详细比较讨论了它们的流场结构与参数特征。结果表明,初始流场的存在是火药燃气流场形成冠状冲击波的必要条件。同时,含有初始流场条件下的近膛口区域最大滞止压力相对于无初始流场条件增加了2倍以上。

跨声速轴流压气机动叶弯和掠效应的应用

对一单级跨声速轴流压气机的动叶分别进行了前掠和正弯研究,然后针对前掠和正弯叶片各自所存在的问题分别作了改进。对掠动叶中部的叶型做了二维改型以降低中部的激波强度和分离损失;对弯动叶不同截面叶型的安装角进行调整以消除正弯对动叶出口气流角的影响。最终应用前掠和正弯动叶的压气机的效率得到明显的提高。在对正弯和前掠动叶研究的基础之上,对动叶进行了前掠和正弯相联合的设计以综合利用弯和掠的有利作用。同时对弯掠动叶中部截面的叶型进行了二维改型以降低中部分离损失,并且对不同叶高截面的安装角进行调整以消除对出口气流角的影响。最终应用弯掠动叶的压气机效率和失速裕度都得到显著提高。

激光等离子体点源减阻技术中入射能量对气动阻力的影响

数值模拟了来流马赫数为5时的圆球绕流冷流场,以及来流与头部驻点附近注入激光能量而产生的等离子体相互耦合形成的流场的演化过程。计算结果表明:施加局部能量点源能够改变原有的弓形激波结构,使其变为斜激波,从而减小气动阻力;气动阻力随入射激光能量的增大而逐渐减小,当入射激光能量为1.1 J时,气动阻力减小的百分比高达40%。

地球弓激波的旋转非对称性

通过对太阳风-磁层-电离层系统的全球MHD模拟,研究地球弓激波相对日地连线的旋转非对称性.模拟限于太阳风速度沿日地连线、地球磁偶极矩和行星际磁场(IMF)与日地连线垂直的简单情况.模拟结果表明,即便对于IMF强度为零的情况,弓激波相对日地连线也不具备旋转对称性质:终端面(晨昏子午面)及其向阳侧的弓激波截线的东西宽度大于南北宽度(约9%～11%),终端面尾侧的弓激波截线东西宽度小于南北宽度(约8%).在存在IMF的情况下,弓激波的位形同时受到磁层顶的形状和快磁声波速度各向异性的影响.磁层顶向外扩张并沿IMF方向拉伸,且其扩张和拉伸程度随IMF由北转南而增强.在磁鞘中,垂直于磁场方向的快磁声波速度高于平行方向.因此,磁层顶拉伸方向与快磁声波速度最大方向垂直,它们对弓激波位置的效应恰好相反;弓激波的最终形状取决于何种效应占据主导地位.对于终端面尾侧,快磁声波速度的各向异性起主导作用,弓激波截线沿IMF垂直方向的宽度大于平行方向.对于终端面及其向阳侧,弓激波截线的形状与IMF取向有关:在准北向或晨昏向IMF情况下,弓激波截线沿IMF垂直方向的宽度仍大于平行方向;在准南向IMF情况下,弓激波截线沿IMF垂直方向的宽度小于平行方向的.鉴于弓激波形状同IMF取向之间的密切关系,我们提议以IMF为基准方向,提取弓激波截线的平行半宽度R_b_∥垂直半宽度R_b_⊥作为尺度参数.这些尺度参数和通常引入的弓激波截线的东西半宽度y_b和南北半宽度z_b相比,更为合理地表征了弓激波的几何性质.模拟结果表明,在终端面上,y_b/z_b和R_b_∥/R_b_⊥在IMF各向同性取向下的统计平均值均低于1,与观测得到的结论一致.

带减阻杆高超声速飞行器外形气动特性研究

采用高超声速风洞测力试验方法测量钝头飞行器头部减阻杆的高超声速气动特性,研究减阻杆的气动减阻原理,分析了多组不同构型减阻杆的减阻效果。结果表明,减阻杆显著减少了钝头飞行器高超声速的阻力,最大的减阻率达到60%之多;减阻效果与减阻杆构型和迎角状态密切相关;减阻杆会诱发稳定性、'热斑'以及非定常脉动等不利问题。

1.5级跨音速压气机内部流场数值分析

本文以正在建设的1.5级跨音压气机试验台所采用的压气机为研究对象,利用NUMECA软件计算了该压气机在不同转速下的特性曲线,并在设计转速下,对其内部流场进行了分析研究。研究表明:激波位置和强度随压气机运行工况的变化而变化;在近失速工况和堵塞工况时,压气机内部流动出现分离;在设计工况时,压气机内部流动状况良好。上述结论,可以为试验台建设中,探头位置的确定和激光测窗位置的选取等提供指导和帮助。此外,该算例作为一盲解,将来同实验结果对比后,可以用来检验和校准CFD程序,为国内F级燃机设计平台的建设打下良好的基础。