Beijing Jiaotong University
Recent publications
An in-fiber Mach-Zehnder interferometer is proposed for the discrimination of strain and temperature. The sensor is based on two cascaded standard single mode fibers using three peanut tapers fabricated by simple splicing. The cascaded structure excites more frequency components, which induce four sets of interference dips in the transmission spectrum. One set of the spectrum dips have different sensitivities to temperature and strain from those of the other three. The sensor can discriminate strain and temperature by monitoring the wavelength shifts of two spectrum dips. Repeated experiments are taken both for strain and temperature increasing and decreasing scenarios. Experimental results show that Dip 1 has an average strain sensitivity of −0.911 pm/µε and an average temperature sensitivity of 49.98 pm/°C. The strain sensitivity for Dip 2 is negligible and its average temperature sensitivity is 60.52 pm/°C The strain and temperature resolutions are ±3.82 µε and ±0.33 °C.
This paper deals with the type synthesis of reconfigurable single-loop mechanisms (RSLMs) using screw theory. A novel approach is proposed to synthesize RSLMs by inserting two or four revolute joints to a planar rhombus 4R mechanism. Using the proposed approach, four types of wrench systems of RSLMs are obtained. The reconfiguration analysis of an RSLM with a constraint force system is then carried out using the kinematics close-loop equation and screw theory method. By locking the R joints, it can be found that the mechanism can realize a 1-DOF planar 4R mode, a 1-DOF Bricard mode, a 2-DOF serial mode, a 1-DOF coaxial mode, a 1-DOF Bennett mode, and a 2-DOF spatial 8R mode. Furthermore, four transition configurations among different modes are identified. To verify motion modes of the mechanisms, serval 3D printed models are also presented. This work provides a framework for further study of type synthesis reconfigurable mechanisms from overconstrained single-loop mechanisms.
Aiming at the construction method and theoretical analysis of deployable mechanism, the construction method of loop-construction mechanism based on quadrilateral mechanisms and the network mechanism are proposed. Firstly, the construction method of the loop-construction mechanism is given, and the constraint conditions of the mechanism are derived. Two types of loop-construction mechanisms based on the Bricard mechanisms are obtained, namely the plane-symmetric and the generalized trihedral loop-construction mechanisms. Then, the constraint conditions and kinematic paths of the two loop-construction mechanisms are analyzed respectively. Thirdly, a special threefold-symmetric loop-construction mechanism is obtained, and based on the regular hexagonal shape characteristics of the mechanism, the construction method of network mechanism and the motion characteristics are given. Finally, four prototypes are fabricated to verify the correctness of the analytical results and the feasibility of the construction method.
High-speed railway axles are suffered from extremely complex variable amplitude (VA) loads during operation. However, in the design codes and operation maintenance of railway axles, most of them are based on the equivalent constant amplitude (CA) load spectrum to perform the fatigue resistance design by using the traditional nominal stress method, and carrying out the routine inspection based on running experiments and data. In this paper, an innovative time-domain stepwise fatigue assessment (TSFA) approach has been introduced to include the influence of VA loads on the accumulated damage and fatigue crack propagation. Based on this procedure, the fatigue load spectrum from the multi-body system (MBS) dynamics simulation was integrated into the fatigue damage and remaining lifetime assessment of the axles. The calculation indicates that the safe lifetime of the powered and non-powered axles under the time-domain loading is considerably larger than the design value, which shows that the axle could safely operate until they are retired. However, for an original crack with the morphology ratio of 0.6 propagating to 45.0 mm, the lifetime of the powered and non-powered axle is sharply reduced to 1.32, and 2.07 years, respectively. Compared with the classical nominal stress method, the newly-developed TSFA procedure could produce a more conservative but practical result.
The onboard direct current (DC) system in electric aircraft could face a severe fault current, which is tens of times the nominal current in a short-circuit fault. It is critical to limit the fault current and clear the fault within a few milliseconds to prevent any damage to the DC system. A protection method using a resistive superconducting fault current limiter (SFCL) with a solid-state DC circuit breaker (SSCB) to manage the DC short-circuit fault is proposed and experimentally verified. A bifilar SFCL coil prototype with two types of connection to achieve low and high inductance is designed and tested, which reduces the fault current considerably from 2000 A to below 1000 A. The performance when integrating the low and high inductance SFCL with a solid-state DC circuit breaker are investigated. It is found that when integrating the SFCL with the SSCB, a high voltage is induced across the high inductance SFCL during current interruption tests. In terms of reliability and durability, the low inductance SFCL is preferred to integrate with the SSCB. The experimental results show that the low inductance SFCL can be an effective solution to protect the DC system from severe fault currents and then SSCB can rapidly and reliably interrupt the fault current at 1000 A. © 2017 Elsevier Inc. All rights reserved.
A numerical investigation on laminar nanofluid flow and convective heat transfer at the entry region of microtubes subjected to constant wall temperature and constant heat flux boundary conditions has been carried out employing a multiphase Eulerian–Lagrangian method. The impacts of the Peclet number (175 ≤ Pe ≤ 3500), nanoparticle volume fraction (0.1 ≤ φ ≤ 1.0%), and nanoparticle diameter (40 ≤ dp ≤ 130 nm) on thermal characteristics of Al2O3-water nanofluid flow through a microtube are analyzed in detail. The results indicate that the influence of the Reynolds number on apparent friction factor and the impact of the axial heat conduction on the Nusselt number for the nanofluids have to take into account in the entry region. Compared with the influences of particle concentration and particle size, the entrance effect dominates pressure drop and thermal performance of the nanofluids near the entrance of the channel. As the dimensionless axial distance increases, the entrance effect is weakened, and the influences of particle concentration and particle size are gradually reflected in flow and heat transfer results. Besides, the particle effects on thermal performance of the nanofluids are earlier and higher than that on flow resistance. Performance evaluation demonstrates that the nanofluids in the entrance region have not only heat transfer enhancement but also a good economy. When Pe = 175, the PEC of Al2O3-water nanofluids with particle concentrations of 0.1, 0.2, 0.5, and 1% increased by 104.0, 103.1, 113.8, and 128.5% under constant heat flux condition, respectively, at the dimensionless axial distance x* = 0.01 compared with deionized water; the PEC improved by 74.6, 77.2, 85.6, and 102.5% under constant wall temperature condition compared with deionized water, respectively.
The slip flow and heat transfer problems inside a micro- or nano-channel are active research topics. Understanding the velocity and temperature distributions in these channels is essential in both theory and applications. In this paper, the analytical solutions of the fully developed slip flow and heat transfer under the H1 boundary condition in elliptical microchannels are obtained with the velocity slip and temperature jump. The separation of variables method is employed to solve the momentum and energy equations, and the coefficient of the velocity profile is calculated using the binomial series twice. Based on the result of the velocity profile, the analytical solution of the temperature profile is also derived. A new model is deduced to predict the Poiseuille number in elliptical microchannels. The close-form solution of the Nusselt number is first obtained and can serve as a benchmark for other approaches.
An experimental investigation was performed to study combustion characteristics of ADN based liquid propellant droplet with resistive ignition method. The experiment was conducted in Ar atmosphere and different oxidizing gas atmospheres under an on-load voltage range of 190–230 V; the oxidizing gases included N2O, NO and O2. The influence of the atmosphere, on-load voltage, droplet volume on the ignition energy, ignition delay time, burning duration, and droplet equivalent diameter development during ignition and combustion was studied. ADN based liquid propellant showed feasibility to combustion by the resistive ignition method in the inert gas atmosphere. The ignition energy accounted for more than 80 % of the total energy during the ignition and combustion process. With the increase of the droplet volume, the value of the ignition delay time and burning duration increased. The atmosphere had a significant influence on the burning duration and droplet diameter, but had little influence on the ignition delay time. Under our experiment conditions, the N2O and O2 atmospheres improved the combustion process, while, the NO atmosphere inhibited the oxidation of methanol.
Performance degradation and operational safety are vital issues for lithium-ion batteries. After summarizing each battery failure accident, it is found that local defects caused by the inconsistency within the battery are one of the critical reasons for battery failure. To study the influence of the internal inconsistency of the battery on its safety performance, a 3D distributed circuit and electrochemical coupling model that can reflect the internal structure of the battery are established. Moreover, the model’s solid-phase potential and liquid-phase lithium ion concentration are corrected to improve the simulation accuracy. The RMSE of the model is less than 10 mV under 1.0 C constant current charging condition and dynamic discharge condition. Based on the proposed distributed model, this paper investigates the influence of the cell size, tab position, and other structural parameters on the internal inconsistency of the cell. And the phenomenon of “current reversal” and the possible local lithium deposition away from the tab position is discovered. The model’s correctness and efficacy in simulating internal inconsistencies are further illustrated by reproducing the local lithium deposition of a homemade LFP cell and a commercial NCM cell by controlling the charging current.
Rail train operation energy consumption mainly focuses on train traction energy consumption. Reducing train traction energy consumption in rail transit operation is significant to developing a green and low-carbon economy and reducing operation costs. The rail train operation energy-saving optimization framework is developed considering the utilization of regenerative braking energy. Firstly, three objectives of punctual arrival, fixed-point parking and minimum energy consumption are provided by train operation strategy analysis. Secondly, the improved brute-force search is developed to solve train operation energy-saving multi-objective problems. The running time, speed, distance, power, and energy consumption of operation intervals are calculated. Finally, Guangzhou Metro Line 7 is taken as an example to verify the effectiveness of the developed optimization model. The results show that the improved brute-force search method effectively finds a more energy-saving turning point under constant interval operation time and has a better energy-saving effect than two other heuristic algorithms.
Development of biocompatible hydrogel adhesives with robust tissue adhesion to realize instant hemorrhage control and injury sealing, especially for emergency rescue and tissue repair, is still challenging. Herein, we report a potent hydrogel adhesive by free radical polymerization of N-acryloyl aspartic acid (AASP) in a facile and straightforward way. Through delicate adjustment of steric hindrance, the synergistic effect between interface interactions and cohesion energy can be achieved in PAASP hydrogel verified by X-ray photoelectron spectroscopy (XPS) analysis and simulation calculation compared to poly (N-acryloyl glutamic acid) (PAGLU) and poly (N-acryloyl amidomalonic acid) (PAAMI) hydrogels. The adhesion strength of the PAASP hydrogel could reach 120 kPa to firmly seal the broken organs to withstand the external force with persistent stability under physiological conditions, and rapid hemostasis in different hemorrhage models on mice is achieved using PAASP hydrogel as physical barrier. Furthermore, the paper-based Fe³⁺ transfer printing method is applied to construct PAASP-based Janus hydrogel patch with both adhesive and non-adhesive surfaces, by which simultaneous wound healing and postoperative anti-adhesion can be realized in gastric perforation model on mice. This advanced hydrogel may show vast potential as bio-adhesives for emergency rescue and tissue/organ repair.
Underdeveloped infrastructure and inconvenient water and heating in winter are important factors restricting the improvement of living conditions and quality of life in cold rural areas in China. Developing energy-saving, sustainable, and autonomous communities for self-sufficiency in suitable areas is an effective way to overcome the above difficulties and improve the quality of life of farmers, which is significant to developing economy and saving land and energy. Based on the self-sufficiency technology, the paper puts forward the concept of Autonomous Community for Self-sufficiency and implementation strategy. Contraposing the regional climate feature, community environment and facility, utilization of energy and resources in Beijing countryside, it analyzes the technical feasibility and application of rural autonomous community for self-sufficiency from three aspects of neighborhood, cluster and community under co-construction and sharing. The energy-saving principle, structure and construction method, frontier development and application effectiveness of technologies of autonomous community are discussed afterwards. It gives the conclusion and suggestion on the technological choice and management mechanism at the end.
Failure of soil slopes is often associated with instability of soils. Instability refers to a behavior in which large plastic strains are generated rapidly when a soil element sustains a given load or stress. Currently, the research related to instability of soils is primarily conducted at saturated conditions through undrained triaxial tests on loose saturated soils (e.g., Lade, J Geotech Eng 118:51–72, 1992; Leong et al., Geotech Test J 23:178–192, 2000; Yang, Geotechnique 52:757–760, 2002) and drained constant shear tests on saturated medium and dense sands (Chu et al., Can Geotech J 40:873–885, 2003). However, many natural soil deposits encountered in engineering practice are often unsaturated. During rainfall infiltration, a reduction in soil suction causes a decrease in the shear strength, which leads to the development of plastic strains and ultimately to the instability of the soil. This process can be idealized as a wetting path along which the shear stress and net mean stress keep constant, but the suction decreases over time. However, the instability behavior of unsaturated granular soils along the wetting path has seldom been investigated.
Seepage in unsaturated media is a common phenomenon in geotechnical and geoenvironmental engineering, such as rainfall infiltration into slopes (Ng and Shi, Comput Geotech 22:1–28, 1998; Wu et al., Comput Geotech 117, 2020a), water flow through capillary barrier covers (Chen et al., Comput Geotech 118, 2020; Zhan et al., Sci Total Environ 718, 2020), and contamination migration in unsaturated soils (Bai et al., Int J Heat Mass Transf 153, 2020; Wei et al., Migration and transformation of chromium in unsaturated soil during groundwater table fluctuations induced by rainfall. J Hazard Mater, 126,229, 2021.).
In this chapter, the hydraulic properties of the five design soils listed in Table 2.2 are investigated. The SWCCs were measured in the laboratory using the newly developed low suction SWCC device (Fig. 4.2) and three types’ commercial available devices (i.e., Fredlund SWCC device, Model 1600 pressure plate extractor, and WP4 dewpoint potentiometer device). The soil hydraulic conductivity functions (SHCFs) were measured using the wetting front advancing column tests as described in Chap. 4.
Colluvial soils are widely distributed on natural terrains. Shallow-seated failures with sliding depths varying between 0.5 and 3.0 m are the main failure mode in colluvial soil deposits during rainstorms. The corresponding confining stresses are in a low range between 5 and 25 kPa. It is pertinent to study the behavior of loose, coarse, widely-graded colluvial soils under very low confining pressures to provide better understanding of shallow-seated failures in coarse-grained soils.
The microporosity structure of soil provides important information on the shear strength, compressibility, hydraulic conductivity, and soil–water characteristics of the soil. The soil microporosity structure changes with stress state, transfer of water and air, temperature fluctuations, long-term gravimetric actions, and weathering. To investigate the formation of a microporosity structure during compaction, the evolution of microporosity structure after saturation, and the variation of microporosity structure during wetting–drying, scanning electron microscopy (SEM) was used to characterize the soil surface structure directly, and mercury intrusion porosimetry (MIP) was used to quantify the soil pore-size distribution (PSD).
Quite a few studies on individualized predictions based on neuroimage made use of the similarities among the samples, and it is of vital importance to estimate these similarities. Conventionally, the similarity or distance between two samples can be evaluated based on Euclidean distance, with neuroimage-derived features implicitly approached as points located on a linear manifold. In most cases, however, it is more appropriate to assume points to be sampled from a nonlinear manifold, and the similarities among samples can accordingly be evaluated based on the locally linear assumption. In this study, we aim to perform individualized predictions based on similarities among neuroimage-derived features, with the similarities evaluated with a novel manifold-based unsupervised metric learning framework, which bridges the gap between deep metric learning and manifold learning. We evaluate the performance of the proposed framework based on functional connectivity extracted from resting-state fMRI (rfMRI) data, with the supervised and semi-supervised predictions of individuals’ age and sex used as test cases. The results demonstrate the effectiveness of the proposed framework, in both supervised and semi-supervised settings.
By giving previously unknown a pair of orthogonal orthomorphisms of cyclic groups of order 18t+9 for any positive integer t, we complete the existence spectrum of a pair of orthogonal orthomorphisms of cyclic groups. As a corollary, we complete the existence spectrum of a difference matrix with four rows over any finite abelian group.
Let G be a connected simple graph with vertex set V(G). Let Ω be a subset with cardinality at least two of V(G). A path containing all vertices of Ω is said to be an Ω-path of G. Two Ω-paths T1 and T2 of G are internally disjointif V(T1)∩V(T2)=Ω and E(T1)∩E(T2)=∅. For an integer ℓ with 2≤ℓ, the ℓ-path-connectivity πℓ(G) is defined as πℓ(G)=min{πG(Ω)|Ω⊆V(G) and |Ω|=ℓ}, where πG(Ω) represents the maximum number of internally disjoint Ω-paths. In this paper, we completely determine 3-path-connectivity of the k-ary n-cube Qnk. By deeply exploring the structural proprieties of Qnk, we show that π3(Qnk)=⌊6n−14⌋ with n≥1 and k≥3.
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9,625 members
Zhiqun He
  • Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology
Limin Jia
  • State Key Lab on Rail Traffic Control and Safety
Xuezhong Zhou
  • School of Computer and Information Technology
Zhibao Cheng
  • School of Civil Engineering
Beijing, China