Investigation of hypersonic flows through a cavity with sweepback angle in near space using the DSMC method

Near space has been paid more and more attentionin recent years due to its military application value.However,flow characteristics of some fundamental configurations(e.g.,the cavity)in near space have rarely been investigated due to rarefied gas effects,which make the numerical simulation methods based on continuous flow hypothesis lose validity.In this work,the direct simulation Monte Carlo(DSMC),one of the most successful particle simulation methods in treating rarefied gas dynamics,is employed to explore flow characteristics of a hypersonic cavity with sweepback angle in near space by considering a variety of cases,such as the cavity at a wide range of altitudes 20-60 km,the cavity at freestream Mach numbers of 6-20,and the cavity with a sweepback angle of 30°-90°.By analyzing the simulation results,flow characteristics are obtained and meanwhile some interesting phenomena are also found.The primary recirculation region,which occupies the most area of the cavity,causes pressure and temperature stratification due to rotational motion of fluid inside it,whereas the pressure and temperature in the secondary recirculation region,which is a small vortex and locates at the lower left corner of the cavity,change slightly due to low-speed movement of fluid inside it.With the increase of altitude,both the primary and secondary recirculation regions contract greatly and it causes them to separate.A notable finding is that rotation direction of the secondary recirculation region would be reversed at a higher altitude.The overall effect of increasing the Mach number is that the velocity,pressure,and temperature within the cavity increase uniformly.The maximum pressure nearby the trailing edge of the cavity decreases rapidly as the sweepback angle increases,whereas the influence of sweepback angle on velocity distribution and maximum temperature within the cavity is slight.

Adaptive Sampling for Near Space Hypersonic Gliding Target Tracking

For modern phased array radar systems,the adaptive control of the target revisiting time is important for efficient radar resource allocation,especially in maneuvering target tracking applications.This paper presents a novel interactive multiple model(IMM)algorithm optimized for tracking maneuvering near space hypersonic gliding vehicles(NSHGV)with a fast adaptive sam-pling control logic.The algorithm utilizes the model probabilities to dynamically adjust the revisit time corresponding to NSHGV maneuvers,thus achieving a balance between tracking accuracy and resource consumption.Simulation results on typical NSHGV targets show that the proposed algo-rithm improves tracking accuracy and resource allocation efficiency compared to other conventional multiple model algorithms.

Aerodynamic map for soft and hard hypersonic level flight in near space

In this note,we design a velocity-altitude map forhypersonic level flight in near space of altitude 20-100 km.This map displays aerodynamic-related parameters associ-ated with near space level flight,schematically or quantita-tively.Various physical conditions for the near-space levelflight are then characterized,including laminar or turbulentflow,rarefaction or continuous flow,aerodynamic heating,aswell as conditions for sustaining level flight with and withoutorbital effect.This map allows one to identify conditions tohave soft flight or hard flight,and this identification wouldbe helpful for making correct planning on detailed studies ofaerodynamics or making initial design of near space vehicles.

Hypersonic sliding target tracking in near space

To improve the tracking accuracy of hypersonic sliding target in near space,the influence of target hypersonic movement on radar detection and tracking is analyzed,and an IMM tracking algorithm is proposed based on radial velocity compensating and cancellation processing of high dynamic biases under the earth centered earth fixed(ECEF) coordinate.Based on the analysis of effect of target hypersonic movement,a measurement model is constructed to reduce the filter divergence which is caused by the model mismatch.The high dynamic biases due to the target hypersonic movement are approximately compensated through radial velocity estimation to achieve the hypersonic target tracking at low systematic biases in near space.The high dynamic biases are further eliminated by the cancellation processing of different radars,in which the track association problem can be solved when the dynamic biases are low.An IMM algorithm based on constant acceleration(CA),constant turning(CT) and Singer models is used to achieve the hypersonic sliding target tracking in near space.Simulation results show that the target tracking in near space can be achieved more effectively by using the proposed algorithm.

Fault tolerant control for near space vehicle:a survey and some new results

A review on fault-tolerant control(FTC) for near space vehicle(NSV) is presented.First,the concept of near space is introduced,the background of NSV is emphasized,and the model characteristics of NSV in faulty case are investigated.Then,a comparison of different existing approaches is briefly carried out,and achievements on the current research in this field are also presented in the view of the practical application.Furthermore,several existing advanced FTC results for nonlinear flight control systems are given.Finally,the recent literature of NSV are presented to provide an overall view of future developments in this area.

Adaptive sliding mode backstepping control for near space vehicles considering engine faults

A fault tolerant control methodology based adaptive sliding mode(ASM) backstepping is proposed for near space vehicle(NSV) attitude control system under engine faults. The proposed scheme combined adaptive backstepping with the sliding mode control strategy could guarantee the system's stability and track desired signals under external disturbances and engine faults. Firstly, attitude mode description and the engine faulty model are given. Secondly, a nominal control law is designed.Thirdly, a sliding mode observer is given later in order to estimate both the information of engine faults and external disturbances. An adaptive sliding mode technology based on the previous nominal control law is developed via updating faulty parameters. Finally,analyze the system's fault-tolerant performance and reliability through experiment simulation, which verifies the proposed design of fault-tolerant control can tolerate engine faults, as well as the strong robustness for external disturbance.

Prescribed performance neural control to guarantee tracking quality for near space kinetic kill vehicle

A prescribed performance neural controller to guarantee tracking quality is addressed for the near space kinetic kill vehicle (NSKKV) to meet the state constraints caused by side window detection. Different from the traditional prescribed performance control in which the shape of the performance function is constant, this paper exploits new performance functions which can change the shape of their function according to different symbols of initial errors and can ensure the error convergence with a small overshoot. The neural backstepping control and the minimal learning parameters (MLP) technology are employed for exploring a prescribed performance controller (PPC) that provides robust tracking attitude reference trajectories. The highlight is that the transient performance of tracking errors is satisfactory and the computational load of neural approximation is low. The pseudo rate (PSR) modulator is used to shape the continuous control command to pulse or on-off signals to meet the requirements of the thruster. Numerical simulations show that the proposed method can achieve state constraints, pseudo-linear operation and high accuracy.

Switching disturbance rejection attitude control of near space vehicles with variable structure

A switching disturbance rejection attitude control law is proposed for a near space vehicle(NSV) with variable structure.The multiple flight modes, system uncertainties and disturbances of the NSV are taken into account based on switched nonlinear systems. Compared with traditional backstepping design methods,the proposed method utilizes the added integrals of attitude angle and angular rate tracking errors to further decrease the tracking errors. Moreover, to reduce the computation complexity, a rapid convergent differentiator is employed to obtain the derivative of the virtual control command. Finally, for disturbance rejection, based on the idea from the extended state observer(ESO), two disturbance observers are designed by using non-smooth functions to estimate the disturbances in the switched nonlinear systems. All signals of the closed-loop system are proven to be uniformly ultimately bounded under the Lyapunov function framework. Simulation results demonstrate the effectiveness of the proposed control scheme.

Preliminary Study on the Distribution of Meridional Wind in Near Space

Based on high-altitude wind data in about one year,the seasonal changes and vertical profiles of meridional wind in near space were studied,and the reasons for the increase of meridional wind in spring and winter were analyzed.Meanwhile,the seasonal distribution of atmospheric circulation in the stratosphere was studied,and the seasonal changes and mechanism of wind conversion from the east to the west were revealed.The results show that the speed of meridional wind in near space was the highest in spring,followed by autumn and winter,while it was the lowest in summer;the increase of meridional wind in spring and winter was related to weather systems in the troposphere,such as the development of strong warm ridge over Xinjiang and cold trough system,and the system resulted in the increase of meridional wind.

Current status and prospects of terminal guidance laws for intercepting hypersonic vehicles in near space:a review

The unique performance advantages of hypersonic vehicles represent a critical challenge for existing defense systems.To facilitate defensive operations against hypersonic vehicles in near space,this paper systematically discusses both the advantages of these vehicles and the difficulties in intercepting them.Focusing on the state-of-the-art terminal guidance laws for intercepting hypersonic vehicles in near space,we examine research progress in the area of single-and multi-interceptor cooperative guidance laws and summarize their advantages and disadvantages.We also highlight future research directions for developing an effective terminal guidance law for multi-interceptor cooperative interception of hypersonic vehicles,based on four aspects:the information domain,space domain,physical domain,and effect-cost ratio.The findings provide a reference for further research into near-space interceptor terminal guidance technologies.

Near-Vector Spaces Constructed over Zp, for p a Prime

The purpose of this paper is to construct near-vector spaces using a result by Van der Walt, with Zp for p a prime, as the underlying near-field. There are two notions of near-vector spaces, we focus on those studied by André [1]. These near-vector spaces have recently proven to be very useful in finite linear games. We will discuss the construction and properties, give examples of these near-vector spaces and give its application in finite linear games.

DSC-backstepping based robust adaptive LS-SVM control for near space vehicle’s reentry attitude

Purpose–The purpose of this paper is to propose a robust control scheme for near space vehicle’s(NSV’s)reentry attitude tracking problem under aerodynamic parameter variations and external disturbances.Design/methodology/approach-The robust control scheme is composed of dynamic surface control(DSC)and least squares support vector machines(LS-SVM).DSC is used to design a nonlinear controller for HSV;then,to increase the robustness and improve the control performance of the controller.LS-SVM is presented to estimate the lumped uncertainties,including aerodynamic parameter variations and external disturbances.The stability analysis shows that all closed-loop signals are bounded,with output tracking error and estimate error of LS-SVM weights exponentially converging to small compacts.Findings-Simulation results demonstrate that the proposed method is effective,leading to promising performance.Originality/value-First,a robust control scheme composed of DSC and adaptive LS-SVM is proposed for NSV’s reentry attitude tracking problem under aerodynamic parameter variations and external disturbances;second,the proposed method can achieve more favorable tracking performances than conventional dynamic surface control because of employing LS-SVM to estimate aerodynamic parameter variations and external disturbances.

Near space airship conceptual design and optimization

Near space airships have gained widespread attention owing to the unique characteristics of long endurance at high altitudes;because of this characteristic,such airships have great potential for use in the fields of communication relay,early warning,earth observation etc.Near space airships can substitute near-earth geostationary satellites owing to the advantages of high load and long endurance at altitude exceeding 20 km.Buoyancy against gravity is generated by the airship hull,which is completely filled with helium,while the near space airship is equipped with an electrical propulsion system powered by solar energy.Unlike conventional aircraft,nearspace airships are a new type of vehicles with less design experience;coupled multi-disciplinary problems related to aerodynamics,structure,energy,control,etc.exist.With the aim of finding a solution for opti-mal near-space airship design,this study proposed a design using a multidisciplinary design optimization method.

Attitude tracking control for variable structure near space vehicles based on switched nonlinear systems

An adaptive robust attitude tracking control law based on switched nonlinear systems is presented for a variable structure near space vehicle (VSNSV) in the presence of uncertainties and disturbances. The adaptive fuzzy systems are employed for approximating unknown functions in the flight dynamic model and their parameters are updated online. To improve the flight robust performance, robust controllers with adaptive gains are designed to compensate for the approximation errors and thus they have less design conservation. Moreover, a systematic procedure is developed for the synthesis of adaptive fuzzy dynamic surface control (DSC) approach. According to the common Lyapunov function theory, it is proved that all signals of the closed-loop system are uniformly ultimately bounded by the continuous controller. The simulation results demonstrate the effectiveness and robustness of the proposed control scheme.

Adaptive functional link network control of near-space vehicles with dynamical uncertainties

The control law design for a near-space hypersonic vehicle(NHV) is highly challenging due to its inherent nonlinearity,plant uncertainties and sensitivity to disturbances.This paper presents a novel functional link network(FLN) control method for an NHV with dynamical thrust and parameter uncertainties.The approach devises a new partially-feedback-functional-link-network(PFFLN) adaptive law and combines it with the nonlinear generalized predictive control(NGPC) algorithm.The PFFLN is employed for approximating uncertainties in flight.Its weights are online tuned based on Lyapunov stability theorem for the first time.The learning process does not need any offline training phase.Additionally,a robust controller with an adaptive gain is designed to offset the approximation error.Finally,simulation results show a satisfactory performance for the NHV attitude tracking,and also illustrate the controller's robustness.

Shock wave standoff distance of near space hypersonic vehicles

The shock wave standoff distances of near space hypersonic vehicles,which execute missions mainly at the altitude of 25 km to55 km,are vital in aerothermodynamic analysis.The implicit finite volume schemes are derived from axisymmetric Navier-Stokes equations for chemical equilibrium flow,and programmed in FORTRAN.Taking a sphere cone for example,the effects of Mach numbers(from 22 to 36) on the shock wave standoff distance and the average density behind the shock are simulated at different altitudes from 25 km to 55 km.The numerical results illustrate that the turning point of the standoff distance is corresponding to that of the average density with the variation of Mach numbers.Based on the numerical results,we propose a formula for shock wave standoff distance,which is the function of the radius of the blunt body,the Mach number and the altitude in the atmosphere.Compared with previous correlations,the new formula can overcome the drawbacks of larger relative errors and complex calculations of the average density.

Real scalar field scattering in the nearly extremal Schwarzschild de Sitter space

Reasonable approximations are introduced to investigate the real scalar field scattering in the nearly extremal Schwarzschild-de Sitter (SdS) space.The approximations naturally lead to the invertible x(r) and the global replacement of the true potential by a Pshl-Teller one.Meanwhile,the Schrdinger-like wave equation is transformed into a solvable form.Our numerical solutions to the wave equation show that the wave is characteristically similar to the harmonic under the tortoise coordinate x,while the wave piles up near the two horizons and the wavelength tends to its maximum as the potential approaches to the peak under the radial coordinate r.

Turbulence in the near-Venusian space: Venus Express observations

With Venus Express magnetic field measurements at 32 Hz from 2006 to 2012, we investigate statistically the magnetic fluctuations in the near-Venusian space. The global spatial distribution of their spectral scaling features is presented in MHD and kinetic regimes. It can be observed that turbulence is a common phenomenon in the solar wind in both regimes. The solar wind MHD turbulence is modified at the Venusian bow shock;MHD turbulence is absent in the Venusian magnetosheath but present at the magnetosheath boundary layer. Pre-existing kinetic turbulence from the far upstream solar wind is modified in the near solar wind region, while kinetic turbulence can be extensively observed throughout the Venusian magnetosheath and in some regions of the induced magnetosphere. Our results reveal that, in the near-Venusian space, energy cascade can be developed at the boundary between magnetosheath and wake, and the turbulence-related dissipation of magnetic energy occurs extensively in the magnetosheath and the induced magnetosphere.

全球空间安全治理视域下临近空间飞行监管研究

临近空间作为全球一体化的重要连接点,是实现长期驻空侦察、全球精准打击的军事战略高点。受空天划界论争的影响,临近空间飞行的监管规则仍依附于国内立法,在国际层面尚未实现有效规制和协调。在临近空间法律地位不清、监管主体不明、监管客体不定、监管内容复杂、飞行活动存有争议的情况下,现有国际航空航天法律制度已无法满足调整此类新型利益关系的实践需要。构建针对性的监管制度不仅是行业有序发展的基础,也是确保地面国国防安全和临近空间资源有序开发利用的重要保障。在国内立法层面,中国可参鉴《临近空间管制公约(草案)》的相关条款,积极推进临近空间融入航空法或航天法的制度设计,或制定单行的临近空间法,以国内立法引领国际立法;在国际立法层面,应以国际民用航空组织为主导,通过修订《国际民用航空公约》(又称《芝加哥公约》)相关附件或增设临近空间飞行的新附件标准,推进国际统一规则的构建。

宽孔距爆破模型试验及近场动力学计算研究

针对宽孔距爆破破岩技术中炮孔密集系数合理取值的问题,在总结国内外宽孔距爆破破岩机理的基础上,开展了炮孔密集系数为1.7、2.5、3.5、4.5的有机玻璃板室内爆破试验,并基于常规态型近场动力学原理,开发了爆破计算模块,对宽孔距爆破破岩中裂纹扩展过程、介质位移响应规律进行了研究。结果表明:临空面一侧的径向裂纹受到临空面反射拉伸应力波的作用,表现出向临空面拐角分叉的特征,并逐渐向临空面扩展,最后形成主裂纹组成漏斗形状。在最小抵抗线固定不变条件下,随着炮孔密集系数的增加,应力降低区逐渐增大,同排炮孔间的径向贯通裂隙消失,过大的炮孔密集系数也进一步造成前后排孔间贯通裂纹消失,最后导致单孔爆破的情况发生。位移计算结果进一步证实了上述规律,并且当炮孔密集系数较小时,前排炮孔留下的岩埂刚好被后排炮孔爆破破碎,随着炮孔密集系数的增大,后排孔爆破对前排炮孔所留下的岩埂破碎作用效果逐渐降低。建议实际工程中根据岩体力学性质、地质条件以及爆破目的适当调整密集系数来降低炸药单耗与大块率。