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In the visible light communication (VLC) for parallel vehicles, the light emitted by the headlamps cannot reach the photodiode (PD), which is the receiver installed on another vehicle. To solve this problem, the mirror array-based intelligent reflecting surface- (IRS-) aided VLC system is designed for parallel vehicles, and the system performance with different number of mirrors and transmission distance is analyzed. The maximum distance between adjacent IRSs is calculated via the exhaustive method, and the nearest neighbor iterative search (NNIS) algorithm is proposed for IRS node selection. Numerical results show that the SNR increases with the number of mirrors increasing. If the number of mirrors is 6 × 6 in the IRS, the maximum distance between adjacent IRSs is 37 meters. When the interval between adjacent IRSs is 32 meters, the required BER can be satisfied with merely three IRSs working together according to the NNIS algorithm.

The silica aerogel monoliths (SAMs) with three-dimensional (3D) porous structures showing unique physical and chemical properties promise a new era of technologies. However, the critical challenge remains in these materials due to their poor mechanical properties in large-scale fabrication. Several attempts have been adopted to enhance the mechanical properties of SAMs to overcome this challenge and explore their applications in various communities. In this review, we introduce the general synthetic methods of SAMs and highlight the use of mechanical reinforcement strategies to improve their properties. Furthermore, the applications of SAMs in killer applications have been introduced, including heat and acoustic insulation, catalysis, and electronic devices. Critically, perspectives on the challenges and opportunities of SAMs are highlighted as potential targets for commercialization.

As an emerging type of network architecture, SDN is widespread used and security issues have also received more and more attention. Fingerprint attacks represent one of the most significant threats to network security. Attackers obtain key fingerprint information of the target network, which lays the foundation for subsequent more threatening attacks. Currently, research on domestic and international SDN fingerprint attacks focuses on how to attack, and less research is being done on how to defend against fingerprinting attacks. This paper proposes a mechanism for defending fingerprint attacks that combines dynamic disturbance and information entropy detection. This mechanism adopts the principle of fingerprint attack, combined with a moving average algorithm, Bloom Filter, and packet delay tool, to confuse opponents by disturbing a small number of packets, simultaneously, combined with the information entropy detection to make real-time processing feedback to the network. The experimental results show that this mechanism works effectively to defend SDNs against fingerprint attacks without affecting the normal network communication.

Thin-walled structure is widely used in automobile, aviation and other industrial fields due to its lightweight, high energy absorption efficiency and low cost. Four thin-walled hybrid multi-cell structures with circular and square sections are proposed in this paper. The energy absorption characteristics and crushing deformation of the hybrid structures are investigated by experimental testing and numerical analysis. Meanwhile, the theoretical models of mean crushing force and specific energy absorption of the hybrid structures are developed by simplified super folding element (SSFE) theory. It is found from the sensitivity analysis of parameters that the dimension of the external tube (D) and the wall thickness (T) of the structure have significant effects on energy absorption. Furthermore, the multi-objective optimization including multiple surrogate models is performed to obtain optimal crashworthiness of the hybrid multi-cell structures. The results show that multiple surrogate models are more favorable and accurate for the crashworthiness design, and the hybrid multi-cell structure with the outer circle and inner square section (CS2) has the best crashworthiness performance. Finally, the multi-objective optimization solutions are analyzed and chosen by the normal boundary intersection (NBI) method to carry out the crashworthiness comparison with the typical multi-cell structures of the same mass, and the hybrid structure (CS2) outperforms multi-cell tubes with the single circular or square section.

The combustion effect of natural gas-air mixed combustion is optimised through an effective and rational design of the burner structure, thereby reducing the emission of atmospheric pollutants and achieving energy saving and emission reduction. This paper starts from the geometric model of the burner and uses Computational Fluid Dynamics (CFD) technology to numerically simulate the combustion distribution in the burner, which results in the combustion distribution of the burner after optimising the structure. The guiding effect of the blower guide vanes on the airflow is weak, and the back pressure effect of the guide vanes mounting body is significant, which reduces the primary air volume and prolongs the combustion history in the central region of the flame; The temperature field is evenly distributed and the combustion is well distributed; CH4 and CO in the furnace chamber are basically burned out and the NO concentration in the furnace chamber is about 800 ppm, which basically meets the emission standard and the combustion effect is good. This paper has certain guiding significance for the study of burner structure.

The fast neutron reactor, one of the promising generation-IV nuclear reactors, particularly requires its coolant to have a strong capacity of transferring heat allied with a weak ability of moderating neutrons, and thus a new scheme of applying the dispersed flow as the coolant is presented in this study. Such a flow was generated by injecting subcooled droplets into the overheated steam with the help of a nozzle, simply referred to as the nozzle-atomized dispersed flow. The obtained heat transfer coefficients maintain high values, vary from 40 to 90 kW/m2/K and are greatly improved as high as 50 times compared to those of the traditional annular-mist flow. The result implies that the nozzle-atomized dispersed flow, especially in the saturated boiling region, meets the heat transfer requirements of the coolant for a fast neutron reactor. Through meticulous analyses, appropriately higher inlet pressure, increased wall heat flux, larger inlet quality, boosted mass flow rate and shorter mix chamber length were found to either reinforce the heat transfer process or shorten the boiling regions along the tube. The standard database on hydrodynamic and heat transfer characteristics of the nozzle-atomized dispersed flow under violent conditions at the pressure of ∼7 MPa and the mass flow rate of ∼250 kg/m2/s was established and laid foundations for further engineering design.

Thin-walled columns have been widely used in the protective structure due to their excellent energy absorption capacity and lightweight property. The paper proposes a new design method of the sandwich column composed of a double circular column and a core column to enhance the structural crushing performance under the axial crushing load. The quasi-static crushing test is conducted to investigate the collapse process of the AA6061-O aluminum alloy sandwich column. The good agreement between the experimental test and numerical analysis validates the accuracy of the finite element model of the sandwich column. Then, the numerical investigations of the sandwich column and corresponding members are performed under the dynamic impacting load. The results show that the interaction effect of the sandwich column and corresponding members can significantly improve the energy absorption of the sandwich column. Furthermore, the effects of the geometrical characteristics of the sandwich column are further investigated on the crashworthiness. The results show that the peak height A and number N of the peak of the core column have an important influence on the energy absorption and distribution of the collapse mode. Moreover, the sandwich column with A = 4 mm and N = 14 has the greatest energy absorption capacity. In addition, the wall thickness distribution of the sandwich column plays an important role in energy absorption during the crushing process. Therefore, the results of this paper provide a new method to design the thin-walled structures with high energy absorption.

Cyclone combustion chamber has the advantages of compact structure, short flame length, good combustion stability, high combustion efficiency and low emissions of pollutants, etc. As one of the key components of the cyclone combustion chamber, the cyclone has a direct impact on the working performance of the combustion chamber. In this paper, we propose a slotted swirler and describe its structure in detail. We simulate the cold state flow field and the hot state temperature field in the combustion chamber of a cyclone with open seam using the CFD method, taking into account the effect of open seam on the air flow field and temperature distribution after the cyclone at different angles. The simulation shows that the temperature distribution in the slotted swirl combustor exhibited a center-focused flame of higher temperature, compared with the non-slotted swirler, the slotted swirler exhibits certain advantages in terms of drag reduction, combustion stability and combustion efficiency, which has a certain guiding effect on the improvement of the working performance of the combustion chamber.

The paper proposes a carbon fiber tube reinforced polyurethane foam material (CPM). Mechanical behavior of the CPM is investigated by quasi static crushing experiment. The results show that the carbon fiber tube reinforced design can obviously improve the energy absorption and load carrying capacity of the polyurethane (PU) foam. Parameter studies show that the density of the PU foam, the number and the diameter of the thin carbon fiber tube have significant effect on mechanical performance of the CPM. Moreover, the CPM filled thin walled columns (aluminum column or carbon fiber column) are further developed and their crushing behaviors are investigated under axial crushing conditions. The results show that the CPM filled design can effectively improve the crashworthiness of the aluminum column or the carbon fiber column. The interaction behavior between the CPM and the thin-walled column presents a significant contribution to energy absorption of the CPM filled columns. Moreover, the CPM filled carbon fiber column has greater potential to improve the crashworthiness than the CPM filled aluminum column. The research findings of this paper provide a new method for designing the lightweight protective structure.

A four-stroke free-piston engine with internal and external irreversibilities of finite combustion rate of the fuel, heat transfer and friction is investigated in this paper. Under the condition of the fixed fuel consumption per cycle, the optimal piston motion trajectory for maximizing the net work output of different cases is derived by applying optimal control theory. Considering the path constraints and boundary conditions of this free-piston Miller cycle, a Gauss pseudospectral method (GPM) is presented for solving optimal motion trajectory. The results show that the optimal piston trajectory improves the efficiency by more than 10% and also suitably reduces the heat transfer losses compared to the conventional piston motion. By optimizing the piston motion around the top dead center (TDC), the in-cylinder gas pressure and temperature have a remarkable improvement while the heat transfer losses have a suitable reduction. The effects of other parameters, such as combustion duration and the frictional coefficient on the piston trajectory, are also investigated. It is shown that optimizing the piston motion trajectory is a good approach for the free-piston engine to improve the efficiency and it can provide guidelines for the optimal control of the practical free-piston engine.

The performance of an irreversible air standard Miller cycle in a four-stroke free-piston engine is analyzed using finite-time thermodynamic. In the model, the relation between the internal irreversibility described by using the compression and expansion efficiencies, the specific heat of the working substance depending on its temperature, the heat transfer loss as a percentage of fuel's energy and the friction loss computed according to the mean velocity of the piston is considered. Moreover, the influences of the excess air coefficient, initial temperature, compression ratio and another compression ratio corresponding to expansion level on the Miller cycle are analyzed by detailed numerical examples. The results show that the efficiency increases with the decrease of specific heat of working substance. The heat transfer loss and friction loss have negative effects on the performance. Comparison of the Miller and Otto cycles shows that the Miller cycle has a higher efficiency through extra expansion work. The conclusions of this investigation are of importance to provide guidelines for the performance evaluation and improvement of practical Miller engines.

Since the era of big data is coming, the first important problem is how to enhance the speed of database query. For the query optimization of distributed database, the speed of query depends on the data transfer and order of join. The cost model minimizing communication cost is the emphasis of research. Parallel Genetic Algorithm-Max-Min Ant System was proposed to seek a best query execution plan, which combines faster convergence of Genetic Algorithm, globally search ability of Max-Min Ant System and parallel property of both them. The experiment results show that the proposed algorithm is effective for query processing of multi-join, and plays important role in improving the performance of distributed database.

Auto-reclosing time and the success rate of the single-phase auto-reclosing operation are decided in the extinguishing time of the secondary arc. The extinction characteristic of the secondary arc is the key to guarantee the success of the auto-reclosing operating. At present, the only effective way to achieve the extinction characteristic of secondary arc in UHV transmission lines is often to have a simulation test in the laboratory. To overcome the problem, in this paper the equivalence of proof with the UHV transmission line secondary arc, the laboratory simulation test arc form theoretic analysis and laboratory test was provided. Depended on South of Huaihe River-West of Shanghai UHV transmission and distribution Project in China, the compensation-free, normal compensation and compensation cases simulation test in the laboratory, and secondary arc self-extinction characteristics on UHV transmission lines were investigated. The results can provide a scientific basis to determine the reclosing time of the circuit breaker, which can be used as reference of secondary arc self-extinction characteristics for simulation research.

It is very important to extract all four coupling-of-modes (COM) parameters of the electrode cells for the simulation and optimal design of a low loss surface acoustic wave (SAW) filter. A new approach for fast and full extraction of the COM parameters for a multielectrode-type grating is proposed. The field distribution of the wave under the periodic shorted grating is calculated by the periodic Green's function method. The phase of the reflection is determined from the positions of the standing wave node. The transduction coefficient and its phase are determined by the charge distribution at low frequency. The COM parameters of the commonly used electrode width controlled (EWC) single phase unidirectional transducer (SPUDT) is computed. It shows that this is a simple and direct way to extract all the COM parameters for SPUDT and, accordingly, is a powerful tool for the optimization of the filter structure.

Disconnector switching in gas insulated switchgear (GIS) generates very fast transient overvoltage (VFTO). VFTO may damage the insulation of GIS equipment. This paper studies the VFTO of a 1000kV GIS substation. The simulation models are obtained according to the configuration of GIS. Operation modes of disconnector were summarized. The characteristics of VFTO were obtained. During the system commissioning test of the substation, VFTO were measured during disconnector switching. Comparison was made between the test and simulation results. The simulation results were in agreement with the test results. The accuracy of the simulation model is verified. Simulation and test study shows that it is safe to do switching operation of disconnector in this GIS substation without damping resistor.

Reducing the information transmission delay in the Vehicular Ad Hoc Networks(VANETs) can improve the timeliness of receiving information and avoid the happening of the accident. As a potential solution, cooperative transmission, which may effectively reduce the transmission delay by exploiting spatial diversity, has attracted lots of attentions recently. In this paper, we modeled the information transmission delay minimization of the VANETs in which cooperative transmission is allowed, and proposed a gradient projection algorithm based on the model. The RSU can schedule the vehicle transmission behavior according to the algorithm, so as to solve the problem of the information transmission delay minimization in the VANETs where multipath routings(MRTDM) are considered. Finally, our simulation results show that the algorithm not only has good convergence, but also can solve the MRTDM in VANETs.

In this brief, we present a novel approach to optical character recognition that utilizes ring-projection-wavelet-fractal signatures (RP-WFS). In particular, the proposed approach reduces the dimensionality of a 2-D pattern by way of a ring-projection method and, thereafter, performs Daubechies' wavelet transformation on the derived 1-D pattern to generate a set of wavelet transformation subpatterns, namely, curves that are nonself-intersecting. Further, from the resulting nonself-intersecting curves, the divider dimensions are readily computed. These divider dimensions constitute a new feature vector for the original 2-D pattern, defined over the curves' fractal dimensions. We have conducted several experiments in which a set of printed alphanumeric symbols of varying fonts and orientation were classified, based on the formulation of our new feature vector. The results obtained from these experiments have consistently shown the character recognition approach with the proposed feature vector can yield an excellent classification rate of 100%.

The Fukushima Daiichi nuclear accident was a series of equipment failures, nuclear meltdowns, and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the Tōhoku earthquake and tsunami on 11 March 2011. According to the cause and process of Fukushima severe accident, several possible scenarios for CPR1000 nuclear power plant were analyzed in this paper, under the assumed situation similar to Fukushima severe accident, with the severe accident analysis code MAAP4. According to the analysis results, several weak points of CPR1000 power plant under situation similar to Fukushima accident were found. The electric power and cooling ability for CPR1000 power plant appear to be most important factors under such accident. Then, several temporary cooling strategies for CPR1000 power plant were suggested, including ASG water supply strategy, temporary injection strategy for primary loop, temporary injection strategy for secondary loop, which would improve the safety of CPR1000 power plant under the situation similar to Fukushima accident. At the last, assessments of effectiveness for these strategies were performed, and the results were compared with analysis without these strategies. The comparisons showed that correctly actions of these strategies would effectively prevent the accident process of CPR1000 power plant under situation similar to Fukushima accident.Copyright © 2013 by ASME