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Wei K.,Peking University | Chen H.,Beijing Institute of TechnologyBeijing | Pei Y.,Peking University | Fang D.,Peking University | Fang D.,Beijing Institute of TechnologyBeijing
Journal of the Mechanics and Physics of Solids | Year: 2016

The unexpected thermal distortions and failures in engineering raise the big concern about thermal expansion controlling. Thus, design of tailorable coefficient of thermal expansion (CTE) is urgently needed for the materials used in large temperature variation circumstance. Here, inspired by multi-fold rotational symmetry in crystallography, we have devised six kinds of periodic planar lattices, which incorporate tailorable CTE and high specific biaxial stiffness. Fabrication process, which overcame shortcomings of welding or adhesion connection, was developed for the dual-material planar lattices. The analytical predictions agreed well with the CTE measurements. It is shown that the planar lattices fabricated from positive CTE constituents, can give large positive, near zero and even negative CTEs. Furthermore, a generalized stationary node method was proposed for aperiodic lattices and even arbitrary structures with desirable thermal expansion. As an example, aperiodic quasicrystal lattices were designed and exhibited zero thermal expansion property. The proposed method for the lattices of lightweight, robust stiffness, strength and tailorable thermal expansion is useful in the engineering applications. © 2015 Elsevier Ltd.

Lou T.,Zhengzhou University of Light Industry | Zhao L.,Beijing Institute of TechnologyBeijing
Acta Astronautica | Year: 2016

A robust integrated navigation algorithm based on a special robust desensitized extended Kalman filtering with analytical gain (ADEKF) during the Mars atmospheric entry is proposed. The robust ADEKF is realized by minimizing a new function penalized by a trace weighted norm of the state error sensitivities and giving a closed-form gain matrix. The uncertainties of the Mars atmospheric density and the lift-to-drag ratio are modeled. Sensitivity matrices are defined to character the parameter uncertainties, and corresponding perturbation matrices are introduced to describe the navigation errors with respect to the parameter uncertainties. The numerical simulation results show that the robust integrated navigation algorithm based on the robust ADEKF effectively reduces the negative effects of the two parameter uncertainties and has good consistency during the Mars entry. © 2015 IAA.

Wang Y.,Beijing Institute of TechnologyBeijing | Wang W.,Beijing Institute of TechnologyBeijing | Zhang S.,Beijing Institute of TechnologyBeijing | Zhao Z.,Beijing Institute of TechnologyBeijing
Tribology International | Year: 2015

Abstract This paper proposes a dynamic model to investigate skidding in angular contact ball-bearings with considering the interaction between balls and raceways, cage and lubricant. The differential equations governing the motions of bearing elements are established and solved using a fourth-order Runge-Kutta algorithm; traction forces between balls and raceways are evaluated based on elastohydrodynamic lubrication theory. The results show that the applied axial load significantly influences the behavior of skidding due to the changes of internal load, orbital and rotation speeds of ball under different operating conditions; appropriate axial load can be determined to avoid severe skidding. © 2015 Elsevier Ltd.

Fang X.-Y.,Yanshan University | Yu X.-X.,Yanshan University | Zheng H.-M.,Yanshan University | Jin H.-B.,Beijing Institute of TechnologyBeijing | And 2 more authors.
Physics Letters, Section A: General, Atomic and Solid State Physics | Year: 2015

Abstract We established a calculation model of the conductivity of multilayer graphene based on Boltzmann transport equation and 2D electron gas theory. Numerical simulations show that the conductivities of few-layer graphene and graphene nanosheets are reduced when thickness is increased. The reduction rate decreases for micron-range thicknesses and remains constant thereafter. Moreover, the conductivity increases with the increase in temperature, in which the increase rate declines as temperature increases. Higher thickness exhibits a more obvious temperature effect on conductivity. Such effect also increases with the increase in temperature. © 2015 Elsevier B.V.

Cao M.-S.,Beijing Institute of TechnologyBeijing | Wang X.-X.,Beijing Institute of TechnologyBeijing | Cao W.-Q.,Beijing Institute of TechnologyBeijing | Cao W.-Q.,Minzu University of China | Yuan J.,Minzu University of China
Journal of Materials Chemistry C | Year: 2015

Ultrathin graphene, a wonder material, exhibits great promise in various fields with its unique electronic structure and excellent physical, chemical, electrochemical, thermal and mechanical properties. Graphene presents great progress in electromagnetic interference (EMI) shielding. Herein, we review the advance in graphene-based EMI shielding materials. Towards graphene composites, we intensively evaluate EMI shielding efficiency and meaningfully describe the mechanism, such as polarization, hopping conduction and interface scattering. Moreover, we highlight an important direction for enhancing EMI shielding, the architectures, including alignment, paper, film and foam. Following that, the problems are summarized and the prospect is also highlighted for significant applications of ultrathin graphene in the field of EMI shielding. This journal is © The Royal Society of Chemistry.

Zhao Y.,Beijing Institute of TechnologyBeijing | Wang G.,Beijing Institute of TechnologyBeijing | Huang B.,Beijing Institute of TechnologyBeijing | Wu Q.,Beijing Institute of TechnologyBeijing
International Journal of Heat and Mass Transfer | Year: 2016

Lagrangian investigations of vortex dynamics, including Lagrangian Coherent Structures (LCS) and particle trajectory, are conducted to highlight the mechanisms of cloud cavitating flows around a Clark-Y hydrofoil. Numerical simulations are performed using a transport equation-based cavitation model and the large eddy simulation (LES) approach. Good agreements are observed between numerical predictions and experimental measurements, including time-averaged turbulence statistics, velocity, vorticity profiles and the periods of unsteady shedding process of the vortex structures near the trailing edge. Besides, present numerical predictions are capable of capturing the unsteadiness of cloud cavitation, including the initiation, growth toward the trailing edge and subsequent shedding of cavities. Based on the Lagrangian analysis of vortex dynamics in non-cavitating flows, two LCSs, namely LE-LCS and TE-LCS, are defined. In cloud cavitating flows, distributions of the two LCSs in different cavitation developing stages illustrate different behaviors of vortex structures. (a) In the attached sheet cavity growing stage, the LE-LCS extends to the trailing edge, which implies the expansion of the attached sheet cavity, and the TE-LCS rolls up and extends downstream, which implies the detachment of cloud cavity. In addition, particle tracers indicate that the Leading edge vortex (LEV) is enhanced by the attached sheet cavity, and there is no direct interaction between attached cavity's expansion and cloud cavity's shedding. (b) In the re-entrant jet developing stage, the LE-LCS and TE-LCS connect together near the middle of the hydrofoil, which implies that two vortex structures mix together inside of the stable attached cavity. Particle tracers clearly show the re-entrant jet flow and the unsteadiness of the vortex structures inside of the stable attached cavity. Furthermore, trapped particles tracers indicate the semi-Vortex Street in the wake, which is induced by the resistance effect from the stable cavity on the partial shedding of the LEV. (c) In the cloud cavity shedding stage, no connection between the LE-LCS and TE-LCS can be observed near the middle part of the hydrofoil, which implies the break-up of vortex structures inside the attached cavity. Meanwhile, particle tracers show the breakup of vortex structure inside the attached cavity, as well as the shedding process of the rear part, which is enhanced by the detached cloud cavity. © 2015 Published by Elsevier Ltd.

Chen D.,Beijing Institute of TechnologyBeijing | Zhong Y.,Beijing Institute of TechnologyBeijing
Biophysical Journal | Year: 2015

Abstract Normal left-right patterning in vertebrates depends on the rotational movement of nodal cilia. In order to produce this ciliary motion, the activity of axonemal dyneins must be tightly regulated in a temporal and spatial manner; the specific activation pattern of the dynein motors in the nodal cilia has not been reported. Contemporary imaging techniques cannot directly assess dynein activity in a living cilium. In this study, we establish a three-dimensional model to mimic the ciliary ultrastructure and assume that the activation of dynein proteins is related to the interdoublet distance. By employing finite-element analysis and grid deformation techniques, we simulate the mechanical function of dyneins by pairs of point loads, investigate the time-variant interdoublet distance, and simulate the dynein-triggered ciliary motion. The computational results indicate that, to produce the rotational movement of nodal cilia, the dynein activity is transferred clockwise (looking from the tip) between the nine doublet microtubules, and along each microtubule, the dynein activation should occur faster at the basal region and slower when it is close to the ciliary tip. Moreover, the time cost by all the dyneins along one microtubule to be activated can be used to deduce the dynein activation pattern; it implies that, as an alternative method, measuring this time can indirectly reveal the dynein activity. The proposed protein-structure model can simulate the ciliary motion triggered by various dynein activation patterns explicitly and may contribute to furthering the studies on axonemal dynein activity. © 2015 Biophysical Society.

Yu Q.,Beijing Institute of TechnologyBeijing | Cao C.,Beijing Institute of TechnologyBeijing
Nanoscience and Nanotechnology Letters | Year: 2015

As an alternative to conventional solid-state photovoltaic, regenerative photoelectrochemical (PEC) cells based on semiconductor/liquid junctions have attracted more and more attentions and attained competitive photoconversion efficiencies. However, most of the efficiency increases were achieved by various methods that can only increase the photocurrents, but not improve the photovoltage limits. Here, we show voltage-added regenerative PEC cell by combining n-Si photoanode with p-Si photocathode. By using CH3CN as the solvent, LiClO4 as the supporting electrolyte, and decamethylferrocenium/decamethylferrocene (Me10Fc+/0 ) as the redox couple, the n-p PEC cell can achieve an open-circuit photovoltage of 0.627 V, exceeding 0.480 V and 0.213 V for comprising n-PEC and p-PEC respectively. In addition, the PEC performances of single-crystal planar Si photoelectrodes and nanowire-array Si photoelectrodes prepared by electroless etching were compared. The wire-array electrodes showed an enhancement in photocurrent, but a slight decrease in phtovoltage. The high open-circuit voltage (>0.6 V) is also the highest ever reported voltage for Si-based PEC solar cells. This is paving the way for the development of a new generation of highly efficient PEC cells with high photovoltge and photocurrent. Copyright © 2015 American Scientific Publishers.

Su L.,Beijing Institute of TechnologyBeijing | Li X.,Beijing Institute of TechnologyBeijing | He X.,Beijing Institute of TechnologyBeijing | Liu F.,Beijing Institute of TechnologyBeijing
Energy Conversion and Management | Year: 2015

In order to optimize the fuel/air mixture formation and decrease pollutant emissions of direct injection (DI) diesel engines, a new concept of forced swirl combustion system (FSCS) included double swirl combustion system (DSCS) and lateral swirl combustion system (LSCS) was proposed and implemented by a unique design of the geometric shape of the combustion chamber. Related numerical research on combustion and emission characteristics of this new system was conducted and acceptable numerical simulation results about its feasibility were drawn initially. To make a better understanding of the mechanism of fuel/air mixture formation in FSCS, visualization of diffusive flame was conducted in a constant volume vessel. The flame images were captured by a high speed camera and the image results indicated that, compared with the traditional omega combustion system (OMECS), both of the flame spread space and spread area of FSCS were increased after the spray impingement. Then, fuel economy and emission performance of FSCS were tested in a single cylinder engine to evaluate its application in diesel combustion system. Test results shown that the LSCS could achieve a better fuel consumption and less soot emission property than DSCS. Both of the two combustion systems (LSCS and DSCS) have the promising benefit in fuel consumption and soot emission compared to OMECS. It is suggested that the fuel/air mixing and engine performance could be promoted in FSCS owing to the introduction of swirling combustion. © 2015 Elsevier Ltd.

Cao W.-Q.,Beijing Institute of TechnologyBeijing | Cao W.-Q.,Minzu University of China | Wang X.-X.,Beijing Institute of TechnologyBeijing | Yuan J.,Minzu University of China | And 2 more authors.
Journal of Materials Chemistry C | Year: 2015

Ultrathin, lightweight graphene composites exhibit high-efficiency microwave absorption at elevated temperatures as well as thermal-stability permittivity. The minimum reflection loss reaches -42 dB and the widest bandwidth covers the entire X-band (-10 dB). More significantly, the composites possess one high-efficiency absorption belt with a value ≤-15 dB, as well as two 'islands' of reflection loss of ≤-17 dB and -30 dB. These excellent properties arise from the synergistic effect of polarization and conductivity. Our finding demonstrates that ultrathin graphene is a promising microwave absorber for microwave attenuation devices, information security and electromagnetic pollution defense. © The Royal Society of Chemistry.

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