离子液体电喷推力器羽流中和特性研究

电推进装置的带电羽流会对航天器产生多种危害,为了降低离子液体电喷推力器带电羽流对航天器产生的潜在安全风险,需要对其进行羽流中和。根据离子液体电喷推力器的羽流自中和特性,提出了时间变化法和空间变化法,并对其进行了实验验证。用时间变化法分析了单推力器的脉冲电压的频率、幅值和占空比对羽流中和特性的影响。以20Hz作为实验的条件,单推力器样机实现羽流中和的正负电压幅值比为0.95~0.98,在发射电压幅值分别为±2.2,±2.6,±3.0kV情况下,羽流中和时正电压占空比分别为35.5%,38.1%,40.4%。空间变化法中使用双推力器模块分析了电压幅值对羽流中和特性的影响。实验结果表明,两个推力器模块羽流中和时的正负电压幅值比为0.95~0.98,与时间变化法的实验结果基本一致,证明了两个推力器协同工作情况下羽流中和的可行性。

A Real-time Modeling of Photovoltaic Array

This paper mainly aims at the modeling problem of the photovoltaic (PV) array with a 30 kW PV grid-connected generation system. An iterative method for the time-varying parameters is proposed to model a plant of PV array. The relationship of PV cell and PV array is obtained and the solution for PV array model is unique. The PV grid-connected generation system is used to demonstrate the effectiveness of the proposed method by comparing the calculated values with the actual output of the system.

Operation safety risk analysis method of hydropower project considering time-dependent effect

In order to consider the time-dependent characteristic of risk factors of hydropower project,the method of stochastic process simulating structure resistance and load effect is adopted.On the basis of analyzing the structure characteristics and mode of operation,the operation safety risk rate assessment model of hydropower project is established on the comprehensive application of the improved analytic hierarchy process,the time-dependent reliability theory and the risk rate threshold.A scheme to demonstrate the time-dependent risk rate assessment method for an example of the earth-rock dam is particularly implemented by the proposed approach.The example shows that operation safety risk rate is closely related to both the service period and design standard;considering the effect of time-dependent,the risk rate increases with time and the intersection of them reflects the technical service life of structures.It could provide scientific basis for the operation safety and risk decision of the hydropower project by predicting the trend of risk rate via this model.

Spectrum Allocation for Cognitive Radio Networks with Non-Deterministic Bandwidth of Spectrum Hole

The spectrum allocation for cognitive radio networks(CRNs) has received considerable studies under the assumption that the bandwidth of spectrum holes is static. However, in practice, the bandwidth of spectrum holes is time-varied due to primary user/secondary user(PU/SU) activity and mobility, which result in non-determinacy. This paper studies the spectrum allocation for CRNs with non-deterministic bandwidth of spectrum holes. We present a novel probability density function(PDF) through order statistics as well as its simplified form to describe the statistical properties of spectrum holes, with which a statistical spectrum allocation model based on stochastic multiple knapsack problem(MKP) is formulated for spectrum allocation with non-deterministic bandwidth of spectrum holes. To reduce the computational complexity, we transform this stochastic programming problem into a constant MKP through exploiting the properties of cumulative distribution function(CDF), which can be solved via MTHG algorithm by using auxiliary variables. Simulation results illustrate that the proposed statistical spectrum allocation algorithm can achieve better performance compared with the existing algorithms when the bandwidth of spectrum holes is time-varied.

A Research on Particle-Based Parallel Methods for Fluid Animation

In this paper, we present an acceleration strategy for Smoothed Particle Hydrodynamics （SPH） on multi-GPU platform. For single-GPU, we first use a neighborhood search algorithm of compacting cell index combined with spatial domain characteristics For multi-GPU, we focus on the changing patterns of SPH＇s computational time. Simple dynamic load balancing algorithm works well because the computational time of each time step changes slowly compared to previous time step. By further optimizing dynamic load balancing algorithm and the communication strategy among GPUs, a nearly linear speedup is achieved in different scenarios with a scale of millions particles. The quality and efficiency of our methods are demonstrated using multiple scenes with different particle numbers.

Anti-unwinding attitude maneuver control of spacecraft considering bounded disturbance and input saturation

The attitude maneuver control of spacecraft under bounded disturbance and input saturation is investigated in this paper.Using the Lyapunov stability theory,an anti-saturation sliding mode control algorithm is given by designing a time-varying sliding mode to solve the input saturated attitude control problem without disturbance.Then combining the merit in anti-saturation of time-varying sliding mode controller with the merit in anti-disturbance of time-invariant sliding mode controller,a switching strategy between the two controllers is designed to solve the attitude maneuver control problem under input saturation and bounded disturbance.However,double value of quaternions makes the attitude slewing unwinding.To solve the problem,an attitude error vector is designed.Based on the error vector,a modified sliding mode control algorithm is proposed which simultaneously has the ability of anti-saturation,anti-disturbance and anti-unwinding.Simulation results show that the algorithm in this paper is feasible and effective.

A modified highly precise direct integration method for a class of linear time-varying systems

It is always a bottleneck to design an effective algorithm for linear time-varying systems in engineering applications.For a class of systems,whose coefficients matrix is based on time-varying polynomial,a modified highly precise direct integration(VHPD-T method)was presented.Through introducing new variables and expanding dimensions,the system can be transformed into a timeinvariant system,in which the transfer matrix can be computed for once and used forever with a highly precise direct integration method.The method attains higher precision than the common methods(e.g.RK4 and power series)and high efficiency in computation.Some numerical examples demonstrate the validity and efficiency of the method proposed.

Rashba Effect in Presence of Time-Dependent Interaction

Rashba effect in presence of a time-dependent interaction has been considered.Then time-evolution of such a system has been studied by using Lewis–Riesenfeld dynamical invariant and unitary transformation method.So appropriate dynamical invariant and unitary transformation according the considered system have been constructed as well as some special cases have come into this article which are common in physics.

Temporal behavior of low-amplitude two-photon screening-photovoltaic grey spatial solitons

The time-dependent formation of one-dimensional two-photon screening-photovoltaic(PV) grey spatial solitons under low-amplitude conditions is presented theoretically.The time-dependent propagation equation of two-photon screeningphotovoltaic solitons is obtained by the numerical method.The results indicate that as the time evolves,the intensity width of grey screening-photovoltaic spatial solitons decreases monotonously to a minimum value towards the steady state.The higher the ratio of soliton peak intensity to the dark irradiation intensity,the narrower the width of grey solitons within the propagation time.

The Sino-US Relations and Alliance Structures in Northeast Asia

This paper addresses the issue of security alliances in Northeast Asia--their origins and current status. It observes some fundamental changes of such alliances in the region with the change of time, and concludes that the present alliances are still hedging against uncertainty. In particular, it elaborates the legitimacy of alliance due to collective defense, rather than cooperative aggressiveness that erodes the credibility of alliance and leads to distrust of nations. The paper raises the Taiwan Question and DPRK issue as key examples to analyze how China and the US have entered a strategically distrustful relationship and how these two issues have been intertwined. It also discusses how to de-link them and build a more stable and secure relationship between China and the US through forward-looking cooperation.

The effect of diffusion loss on the time-varying giant Panda population

It has been certificated that corridors can help giant pandas to keep their habitat from fragmenting. However there are still losses during the process of moving along corridors. In this study, a mathematical model with Allee effect is carried out to describe the diffusion of giant pandas between n patches. Some criteria are obtained to keep the system persisting. It is proved that the system has a unique positive w-periodic solution which is globally asymptotically stable. The ecological meanings of these findings are discussed following the results. And some numerical simulations in the Qinling Mountain giant panda nature reservation area are also presented in the end.