Phenomenon of generalized synchronization in klystron chaos generators with delayed feedback: Experiment and numerical simulation

The system of unidirectionally coupled klystron chaos generators with delayed feedback has been experimentally and theoretically investigated. A new technique for diagnostics of generalized chaotic synchronization is proposed, which is based on the spectral analysis of a signal from a response klystron generator. Comparison of the obtained experimental and theoretical results is performed.     

Chaos in fractional-order generalized Lorenz system and its synchronization circuit simulation

The chaotic behaviours of a fractional-order generalized Lorenz system and its synchronization are studied in this paper. A new electronic circuit unit to realize fractional-order operator is proposed. According to the circuit unit, an electronic circuit is designed to realize a 3.8-order generalized Lorenz chaotic system. Furthermore, synchronization between two fractional-order systems is achieved by utilizing a single-variable feedback method. Circuit experiment simulation results verify the effectiveness of the proposed scheme.     

Two-dimensional numerical simulation on galloping detonation in a narrow channel

The numerical simulations of the two-dimensional galloping detonation performed by using two-dimensional full Navier–Stokes simulations with a detailed chemistry model are presented. The detonation in a narrow channel with d = 5 mm, which is approximately twice the half-reaction length of hydrogen, shows a feature of galloping detonation with two initiations during its propagation under the laminar flow assumption. The distance between these two initiations is approximately 1300 mm, which causes the induction time behind the leading shock wave. As the channel width increases, the galloping feature diminishes. The detonation propagates approximately 4% lower than D_CJ for d = 10 and 15 mm. By increasing the channel width, the strength of the detonation increases, as shown in the maximum pressure histories. The effects of turbulence behind the detonation show that the galloping feature disappears, although its propagation velocity becomes 0.9 D_CJ. The strength of the detonation becomes significantly weak compared with the detonation propagating in the wide channel widths, and this feature is similar to the laminar assumption. The trend of the velocity deficits in the NS simulations agrees fairly well with the trend of the modified ZND calculations with η = 0.25.     

一类节点结构互异的复杂网络的混沌同步

提出了一种实现节点结构互异的复杂网络的混沌同步方法.以异结构混沌系统作为节点构造复杂网络,基于Lyapunov稳定性定理确定了复杂网络中连接节点的耦合函数的形式.以Rssler系统、Coullet系统以及Lorenz系统作为网络节点构成的复杂网络为例,仿真模拟发现,整个复杂网络存在稳定的混沌同步现象.此方法不但可以实现任意混沌系统作为节点的网络混沌同步,而且网络节点数对整个复杂网络同步的稳定性也无影响,因而,具有一定的普适性. 关键词： 混沌同步 复杂网络 异结构 Lyapunov稳定性定理     

Chaos synchronization and communication of mutual coupling lasers ring based on incoherent injection

A chaos secure communication system of mutual coupling lasers ring based on incoherent optical injection is proposed, in which fine tuning of optical frequency is not required compared with other schemes based on coherent optical injection. Therefore the secure communication scheme is attractive for experimental investigation. The dynamics of semiconductor lasers in the coupling ring are examined. Numerical investigations indicate that zero lag synchronization can be achieved under equal coupling time and strength of mutual coupling. Furthermore, by chaos shift keying (CSK), secure communication is simulated with a random bit stream of 1.0 Gbit/s. The results confirm the possibility of applying incoherent schemes of mutual coupling lasers ring to realize chaotic secure communication.     

Chaos synchronization of the Chua system with a fractional order

Chaos synchronization of two identical Chua systems with the same fractional order is studied by utilizing the Pecora–Carroll (PC) method, the active–passive decomposition (PAD) method, the one-way coupling method and the bidirectional coupling one. The sufficient conditions for achieving synchronization between these two systems are derived via the Laplace transformation theory. Numerical simulations show the effectiveness of the theoretical analyses.     

Space-time response of a cloud communication channel to an optical signal

An optical signal,an impulse in space and step in time,is incident on a plane,parallel fixed,homogeneous cloud. The incident signal is modelled as a stream of photons, each of them executing a random walk. A Monte Carlo simulation technique is applied to obtain the space-time response of the channel when an optical receiver with a specified field of view is located at a specified distance below the cloud. The response is obtained as angular, spatial and temporal dispersions. Various simulation steps are described. For thin clouds two variance reduction techniques, i.e. Russian roulette and forced collision, are used. It is shown that the bottom of the cloud tends to diffuse uniformly and the 3 dB exit beam width saturates. The rise time in this channel is found to be of the order of several nanoseconds.On leave from Electrical Engineering Department, Indian Institute of Technology, New Delhi, 110029, India.     

Chaos synchronization in injection-locked semiconductor lasers with optical feedback

Based on the rate equations, we have investigated three types of chaos synchronizations in injection-locked semiconductor lasers with optical feedback. Numerical simulation shows that the synchronization can be realized by the symmetric or asymmetric laser systems. Also, the influence of parameter mismatches on chaos synchronization is investigated, and the results imply that these two lasers can achieve good synchronization, with smaller tolerance of parameter mismatch existing.     

Chaos synchronization in semiconductor lasers with polarization-rotated optical feedback

Chaotic oscillations of the transverse magnetic (TM) mode, which is not a common lasing mode, are excited by using polarization-rotated optical feedback from the transverse electric (TE) mode in a semiconductor laser. In our previous paper, we found that the dynamics were strongly dependent on their RF components under the condition of moderate optical feedback from the TE mode to the TM mode and that they were divided into three RF regions; low-pass filtered signals with a lower frequency than the laser relaxation oscillation frequency, intermediate RF components including the relaxation oscillation frequency, and high-pass filtered signals with a higher frequency higher than the relaxation oscillation frequency. Depending on the frequency bands, the laser outputs showed different correlations. In the present study, using such schemes, the polarization-rotated beam from a transmitter laser (i.e., the rotated TE-mode beam of a transmitter laser) is injected into a receiver laser. We experimentally observe chaos synchronization in accordance with the dynamics of RF components on the transmitter laser side. We also perform numerical calculations using a model and obtain good agreement between the theoretical and experimental results.     

Time-domain simulation of channel crosstalk and inter-modulation distortion in gain-clamped semiconductor optical amplifiers

A time-domain model is implemented for gain-clamped semiconductor optical amplifiers (GC-SOAs) based on a combination of the separated traveling-wave equations and effective Bloch equations. The key feature of this model lies in its capability of handling the lasing-signal, signal-signal, and signal-noise interactions over a broad wavelength band. Therefore, various nonlinear phenomena such as the cross-gain saturation (XGS) and nondegenerate four-wave mixing (ND-FWM) can readily be captured. After being implemented and validated, this model is applied to the simulation of GC-SOA dynamic behaviors such as the channel crosstalk and intermodulation distortion (IMD). Simulation results show that the third-order IMD can be effectively suppressed by a gain-clamping lasing mode in GC-SOAs in comparison with that in conventional SOAs. The channel crosstalk can also be suppressed to some extent in GC-SOAs, but not as effectively. Other than a homogeneous reduction, the gain-clamping in GC-SOAs does not change the dependence of the channel crosstalk and IMD on the input signal power and channel spacing. It is also shown that the channel crosstalk, unlike the IMD, cannot be efficiently reduced by enlarging the channel spacing even in GC-SOAs.     

Chaos synchronization and communication of the polarization modes for two unidirectionally coupled vertical-cavity surface-emitting lasers

We theoretically investigate the synchronization performance of the polarization modes of two unidirectionally coupled vertical-cavity surface-emitting lasers (VCSELs), where the master VCSEL subjects to polarization-preserved optical feedback and the slave VCSEL subjects to polarization-preserved optical injection. We demonstrate that high synchronization can be achieved between corresponding polarization components of the master laser and the slave laser. We also analyze the influence of the internal mismatched parameters on the synchronization performance. Furthermore, this system is used to dual-channel communication and the different transmitted digital information is respectively recovered at each polarization branch of the slaver laser successfully with the encryption scheme of chaos masking (CM). The results open an opportunity for multichannel chaotic communication by utilizing different polarization components in single-mode or multi-transverse-mode VCSELs.     

喷射式空调系统的实验与数值模拟研究

利用二维轴对称,真实气体模型对喷射式制冷空调系统的喷射器进行CFD计算。搭建喷射式空调实验系统,进行喷射式空调实验,验证CFD模型的可行性。利用验证的CFD模型,进行实验工况以外的数值计算,得到了喷射器在不同发生条件、蒸发条件和冷凝条件下的性能变化。在喷射器结构确定,其它条件不变时,发生温度tg必须要在一定范围内,71℃≤tg≤95℃,喷射器才能正常运行。在计算条件下,蒸发温度te越高,喷射器的性能越好。冷凝温度tc要在一定范围内:20℃≤tc≤40℃,喷射器才能正常工作。当冷凝压力低于临界压力时,喷射器的性能表现出恒能力特性。     

Phase transitions and dynamic entropy in small two-dimensional systems: Experiment and numerical simulation

The results of experimental and numerical analysis are presented for phase transitions in strongly nonequilibrium small systems of strongly interacting Brownian particles. The dynamic entropy method is applied to analysis of the state of these systems. Experiments are carried out with kinetic heating of the structures of micron-size particles in a laboratory rf discharge plasma. Three phase states of these small systems are observed: crystalline, liquid, and transient. The mechanism of phase transitions in cluster structures of strongly interacting particles is described.     

电光双稳态系统的混沌控制与同步

根据长延时状态下电光双稳系统的特点,提出了实现其混沌控制与同步的具体方案.数值模拟的结果表明：适当选取驱动强度及驱动系统的状态,不仅可以实现对响应系统不同周期状态的稳定控制,还可以实现驱动系统与响应系统间的广义混沌同步.以最大李雅普诺夫指数为标准,给出了实现混沌同步的参数范围.     

Sprott-B和Sprott-C系统之间的耦合混沌同步

Sprott-B和 Sprott-C是拓扑等价的异结构混沌系统. 利用系统变量线性耦合方法实现二系统之间的混沌同步；根据Lyapunov稳定性理论，确定了二系统达到同步时耦合系数的阈值. 设计了实现二系统耦合同步的实验电路，实验结果证明了理论分析的正确性. 基于Sprott-B和 Sprott-C异结构系统混沌同步，提出了一种新的具有更好保密性能的混沌保密通讯方法. 关键词： 线性耦合 混沌同步 异结构系统 拓扑等价     

Chaos synchronization in networks of coupled maps with time-varying topologies

Complexity of dynamical networks can arise not only from the complexity of the topological structure but also from the time evolution of the topology. In this paper, we study the synchronous motion of coupled maps in time-varying complex networks both analytically and numerically. The temporal variation is rather general and formalized as being driven by a metric dynamical system. Four network models are discussed in detail in which the interconnections between vertices vary through time randomly. These models are: 1) i.i.d. sequences of random graphs with fixed wiring probability, 2) groups of graphs with random switches between the individual graphs, 3) graphs with temporary random failures of nodes, and 4) the meet-for-dinner model where the vertices are randomly grouped. We show that the temporal variation and randomness of the connection topology can enhance synchronizability in many cases; however, there are also instances where they reduce synchronizability. In analytical terms, the Hajnal diameter of the coupling matrix sequence is presented as a measure for the synchronizability of the graph topology. In topological terms, the decisive criterion for synchronization of coupled chaotic maps is that the union of the time-varying graphs contains a spanning tree.     

Direct numerical simulation of viscoelastic-fluid-based nanofluid turbulent channel flow with heat transfer

Our previous experimental studies have confirmed that viscoelastic-fluid-based nanofluid(VFBN) prepared by suspending nanoparticles in a viscoelastic base fluid(VBF, behaves drag reduction at turbulent flow state) can reduce turbulent flow resistance as compared with water and enhance heat transfer as compared with VBF. Direct numerical simulation(DNS) is performed in this study to explore the mechanisms of heat transfer enhancement(HTE) and flow drag reduction(DR) for the VFBN turbulent flow. The Giesekus model is used as the constitutive equation for VFBN. Our previously proposed thermal dispersion model is adopted to take into account the thermal dispersion effects of nanoparticles in the VFBN turbulent flow. The DNS results show similar behaviors for flow resistance and heat transfer to those obtained in our previous experiments. Detailed analyses are conducted for the turbulent velocity, temperature, and conformation fields obtained by DNSs for different fluid cases, and for the friction factor with viscous, turbulent, and elastic contributions and heat transfer rate with conductive, turbulent and thermal dispersion contributions of nanoparticles, respectively. The mechanisms of HTE and DR of VFBN turbulent flows are then discussed. Based on analogy theory, the ratios of Chilton–Colburn factor to friction factor for different fluid flow cases are investigated, which from another aspect show the significant enhancement in heat transfer performance for some cases of water-based nanofluid and VFBN turbulent flows.     

Direct numerical simulation of elastic turbulence and its mixing-enhancement effect in a straight channel flow

In this paper,we present a direct numerical simulation(DNS) of elastic turbulence of viscoelastic fluid at vanishingly low Reynolds number(Re = 1) in a three-dimensional straight channel flow for the first time,using the Giesekus constitutive model for the fluid.In order to generate and maintain the turbulent fluid motion in the straight channel,a sinusoidal force term is added to the momentum equation,and then the elastic turbulence is numerically realized with an initialized chaotic velocity field and a stretched conformation field.Statistical and structural characteristics of the elastic turbulence therein are analyzed based on the detailed information obtained from the DNS.The fluid mixing enhancement effect of elastic turbulence is also demonstrated for the potential applications of this phenomenon.     

Chaos synchronization in RCL-shunted Josephson junctions via a common chaos driving

We study chaos synchronization in two resistive-capacitive-inductive-shunted (RCL-shunted) Josephson junctions (RCLSJJs) by using a common chaos driving. The numerical simulations confirm that the synchronization of two RCLSJJs can be achieved with a suitable driving intensity when the maximum condition Lyapunov exponent (MCLE) is negative.