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2015 Vol. 41, No. 6

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2015 Year 41 Volume 6 Issue E-journal
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Strategies for reconciling tradeoff between conductivity and swelling in alkaline polymer electrolytes membrane
XIANG Yan, SI Jiangju
2015, 41(6): 961-968. doi: 10.13700/j.bh.1001-5965.2015.0174
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Alkaline polymer electrolytes membrane fuel cells (APEMFC) have been investigated as an alternative to proton-exchange membrane fuel cells (PEMFC) because of their compatibility with nonprecious-metal catalyst, favorability toward fuel oxidation, together with the lower cost, where the charge carrier is OH- rather than H+. However, the performance of APEMFC, especially the conductivity, has thus far lagged that of PEMFC because of the intrinsic lower mobility of OH- than that of H+. The improvement of ion-exchange capacity (IEC) by increasing the grafting degree (GD) of cationic functional groups can, to some extent, solve this issue; however, a high IEC is always accompanied by excessive water uptake, swelling, and backbone degradation. Balancing the ionic conductivity and the dimensional stability in APEs has been a formidable scientific challenge. Here, we reviewed the research progress of the strategies for reconciling the tradeoff between conductivity and dimensional stability. These strategies include physical stratigies, such as blending and filling pores to restain the swelling, chemical cross-linking, enrichment of quaternary ammonium cation groups in the side chains and constructing efficient ionic channels by hydrophilic/hydrophobic phase segregation morphological structure like Nafion® membranes to enhance the mobility of OH-. The strategies mentioned above can all realize high ion conductivity and low water uptake and swelling at the same time to some extent.
3D thermal deformation measurement of superalloy honeycomb panels in time-varying thermal radiation environment
PAN Bing, JIANG Tianyun, WU Dafang
2015, 41(6): 969-975. doi: 10.13700/j.bh.1001-5965.2014.0391
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Superalloy honeycomb panels with the advantages of light weight, high strength and excellent heat-shielding properties have been widely used in the field of aeronautics and astronautics. Deformation measurement of superalloy honeycomb panels due to transient thermal loading is essential for the design of heat-shielding structures. Firstly, a self-developed infrared radiation transient aerodynamic heating simulation system was used to simulate conditions similar to transient aerodynamics service conditions and a novel active imaging three-dimensional digital image correlation (3D-DIC) method was used to measure 3D thermal deformation of a superalloy honeycomb panel sample with a size of 210 mm×210 mm at different times in time-varying thermal radiation environment. Secondly, to ensure the reliability of measurement by using 3D-DIC, a new technique for making large-area high-temperature speckle pattern on a test sample was proposed. The high-temperature speckle pattern stayed stable throughout the experiment and could be used as an effective carrier of thermal deformation. Finally, the largest warping displacement was also calculated by Hoff's equivalent stiffness theory. Study results indicate that in-plane thermal expansion is homogeneous when the panel is heated one-side by radiation heating, while the out-plane displacements show evident axisymmetric warp deformation with the largest warping displacement of approximate 1.6 mm at a temperature of 900℃. The experimental results agree well with theoretical predictions made by Hoff's equivalent stiffness theory.
Discrete element simulation of mechanical characteristics of sands under principal stress axes rotation
TONG Zhaoxia, WANG Bo, ZUO Zhaokun, YAO Yangping
2015, 41(6): 976-981. doi: 10.13700/j.bh.1001-5965.2014.0434
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The mechanical behavior of sands and other granular materials subjected to rotation of principal stress axes is significantly dependent on the micro fabric properties and their evolution. The discrete element simulation for sands under cyclic rotation of principal stress axes was validated by comparing the numerical simulation and laboratory test results on the development of two normal strains, the relationship between shear stress and shear strain, the evolution of the volumetric strain, and the effects of initial void ratio and stress ratio. Four variables characterized by the spatial distribution features of particle major axis orientation and inter-particle contact normal directions were used to describe the fabric properties of granular materials. The evolution of the four fabric variables in a single cycle was studied for granular materials subjected to cyclic rotation of principal stress axes. The effects of initial void ratio and stress ratio on the evolution of fabric variables were also investigated in detail.
Head bone tissue extraction algorithm based on CTA image
CAO Chunhong, AI Liang, XU Guangxing
2015, 41(6): 982-988. doi: 10.13700/j.bh.1001-5965.2014.0502
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Vascular tissue and bone tissue cannot be clearly separated based solely on grayscale information in images of computed tomography angiography (CTA). The algorithm based on the growth of three-dimensional (3D) region improved bone tissue outside the bone contour extraction and the extraction algorithm based on improved Snake model combined with the characteristics of CTA grayscale images were proposed. Combining the knowledge of probability theory to improve the accuracy of determining condition of the region growing, fast skeletal regional seed extraction method of 3D region growing was proposed. It made it possible to obtain more accurate bone tissue area. After the Snake model was selected and some improvements were made to the model, energy image information items were increased, so that the model can better solve the current problems. Finally, the experimental results were given and compared with results from the traditional algorithm. It is confirmed the proposed segmentation of bone tissue extraction algorithm works well.
Pedal simulator and braking feel evaluation in brake by wire system
JI Fenzhu, ZHOU Xiaoxu, ZHU Wenbo
2015, 41(6): 989-994. doi: 10.13700/j.bh.1001-5965.2014.0067
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The pedal characteristic from experiments for target, a pedal simulator with elastic components and hydraulic unit as the basic simulation unit was designed, which based on the analysis on the pedal characteristics in traditional hydraulic braking system. The "breaking feeling" evaluation index was analyzed. The simulation model of pedal simulator was established by using AMESim and Matlab/Simulink. The pedal characteristic was simulated. The simulation results show that, the pedal reaction force increases with the increase of the pedal stroke, the pedal characteristics could be able to change following the target pedal characteristics; the different pedal characteristics could be obtained by changing the stiffness of elastic component and the pressure of hydraulic cylinder; the "breaking feeling" index was evaluated on three different simulators, and the maximum braking feel index value is 92.4 points, which means a "good" brake feeling. The pedal characteristics could adapt to different vehicle or meet needs of different drivers by adjusting the parameters of simulator.
New control technology for visible light reflection characteristics of space target
HUANG Chen, WANG Jianjun, XUE Li, GAO Xin, SONG Ping
2015, 41(6): 995-999. doi: 10.13700/j.bh.1001-5965.2014.0422
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For geosynchronous (GEO) orbit space object's detection and recognition is mainly dependent on analysis, identification and judgment of reflected sunlight photometric curve. Based on analysis of visible light reflection characteristics calculation model of space target surface diffuse reflection properties, a study on different shape reflector light reflection characteristics was carried out. Moreover, a different shape reflector method was proposed to control space target light reflection characteristics. Taking an example of a GEO satellite, simulate and analyze the effects of cone, sphere and cylinder as reflection control body on satellite reflection characteristics by using Monte Carlo non-sequential light tracing method. The results show that the method can change the satellite original visible light reflection curve, reduce the probability of being detected and recognized, and improve the invisibility of space object.
Ultrasonic phased array inspection of L-shaped components based on dynamic aperture focusing
XU Na, HE Fangcheng, ZHOU Zhenggan
2015, 41(6): 1000-1006. doi: 10.13700/j.bh.1001-5965.2014.0519
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When L-shaped component is detected by ultrasonic phased array inspection technique based on total focusing method, wave propagation path in the specimen became more complex to increase the computational difficulty of propagation time due to the complicated geometries of component. To solve this problem, a dynamic aperture focusing method, adjusting focusing program according to actual samples dynamically, was proposed on the basis of the total focusing method using full matrix data. Then, the dynamic aperture focusing program was designed for the surface characteristics of L-shaped components. A calculation method of wave propagation time was posed up based on the Snell's principle. Finally, a couple of full matrix data was simulated using the finite-difference time domain method to validate the dynamic aperture focusing method. The inspection experiment was carried out to image the defect in customized L-shaped metal specimen, yielding experiment results that are in good quantitative agreement with the true values. The results show that it is feasible and effective for dynamic aperture focusing method to test the L-shaped components.
K-π interaction in finite volume and the K* resonance
ZHOU Dan, CUI Erliang, CHEN Huaxing, GENG Lisheng, SHEN Chengping
2015, 41(6): 1007-1011. doi: 10.13700/j.bh.1001-5965.2014.0411
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Firstly, the K-π interaction in P-wave in a finite volume using the chiral unitary approach was studied, to find an efficient strategy to obtain K-π phase shifts and thus the K* meson properties from energy levels could be obtained in lattice calculations. This method has been applied to provide K-π shifts in infinite space very successfully. Secondly, the energy levels, which are functions of the cubic box size and the pion mass, were evaluated, and the K-π phase shifts were evaluated based on the physical properties of K* meson. Finally, in order to compare with the lattice quantum chromodynamics (QCD) results, non-physical pion masses were adopted to calculate energy levels of the K* meson. Comparison shows that results from approach mentioned above agree well with those from lattice QCD: using pion mass 266 MeV, we obtain two energy levels, 924.0 MeV and 1483.0 MeV respectively, which accord well with the lattice QCD results, 915.6 MeV and 1522.3 MeV respectively.
Comprehensive evaluation model of multidimensional visual coding on display interface
XIAO Xu, WANYAN Xiaoru, ZHUANG Damin
2015, 41(6): 1012-1018. doi: 10.13700/j.bh.1001-5965.2014.0428
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To predict and evaluate the effects of multidimensional visual coding on pilot's information identification, a new comprehensive evaluation mathematical model of multidimensional visual coding on display interface, which adopted meta-analyses and integrated performance values (IPVs) for the unitary quantification of multidimensional visual coding, and combined the weight coefficient determined by G1 method with fuzzy weighted average operator, was proposed based on a large number of existing research about single-dimensional visual coding. In order to verify the validity of the model, interfaces of Boeing and Airbus were comprehensively evaluated, based on which optimized interface was designed. 22 subjects performed an instrument supervision task under different display interface. Subjective evaluation, performance measure and physiological assessment, including eye-movement tracking and electrocardiogram (ECG) time domain index, were introduced to record the experimental data. The integrated experimental results reveal that theoretical predictions calculated by the model is significantly correlated with the practical experimental results, and the optimized interface provides performance advantages, especially for more difficult tasks. This comprehensive evaluation model of multidimensional visual coding on display interface will provide a reference for the optimized design of aircraft cockpit display interface.
Swing motion control of lower extremity exoskeleton based on admittance method
LIU Difei, TANG Zhiyong, PEI Zhongcai
2015, 41(6): 1019-1025. doi: 10.13700/j.bh.1001-5965.2014.0462
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To solve the problem of the pilot motion intent's identification and tracking during the swing motion of lower extremity exoskeleton, a control algorithm based on admittance method was proposed at first. This control method used the admittance characteristics between the force and velocity of moving object. And the interaction force between pilot and lower extremity exoskeleton was converted into the desired motion trajectories via the reasonable design of admittance parameters. Then the traditional control was employed to track these trajectories accurately. Finally the coordinated movement of pilot and exoskeleton was achieved. The human-machine system model including interaction force information was established and the simulations were conducted according to the system model. The results show that the interaction force between pilot and exoskeleton is reduced by 85% compared with the unpowered exoskeleton under the normal swing frequency, and the accurate tracking to pilot's motion trajectories is implemented successfully with the error ranged from -0.3° to 0.3°.
Robust trajectory tracking for quadrotor aircraft based on ADRC attitude decoupling control
YANG Liben, ZHANG Weiguo, HUANG Degang
2015, 41(6): 1026-1033. doi: 10.13700/j.bh.1001-5965.2014.0392
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An attitude decoupling algorithm based on active disturbance rejection control (ADRC) was designed for underactuated quadrotor aircraft. The algorithm can overcome some shortcomings of traditional control method for underactuated quadrotor, such as strong coupling between system states, weak anti-interference ability and high sensitivity of tracking performance to modeling errors, etc. The state coupling was tracked and estimated by extended state observer (ESO), system interference can be estimated by ESO at the same time. The interference of the system includes internal and external disturbances. The nonlinear multiple-input multiple-output (MIMO) system was transformed into linear single-input single-output (SISO) system by dynamic feedback linearization. Then using the nonlinear feedback control law to achieve high quality control of the attitude system, and study the robust trajectory tracking problem of the aircraft based on the attitude decoupling control algorithm. The simulation results show that the above attitude control algorithm can improve the robustness of the trajectory tracking system. The algorithm does not rely on the accurate system model, reduces the difficulty of practical application, and has strong anti-interference capability and practical application value as well.
Optimization of carrier-based aircraft repairable spares inventory systems under limited maintenance ability
ZHANG Shuai, LI Junliang, LYU Weimin, TENG Kenan
2015, 41(6): 1034-1041. doi: 10.13700/j.bh.1001-5965.2014.0393
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The repair capability of carrier-based aircraft is constrained by finite repair channels. To make Palm's theorem and Metric theory applicable to the optimal allocation of carrier-base aircraft repairable spares, the modified accommodative models of repair and supply channels under conditions of dedicated channels, general channels and hybrid channels were researched and proposed by making the sum of actual repair time and waiting time under finite repair channels equal to the repair time under infinite repair channels based on the queuing theory. According to the modified accommodative models, the mean and variance models of two indenture spares in repair and supply channels were found. Combined with the application instance, the impact of spares demand rate and the amount of repair channels on the repair time were analyzed. The application effect of the stockpiles programs under the finite repair channels model and infinite repair channels model were compared and analyzed. The applicable conditions of each model were given. And the results show that the modified accommodative models are effective.
Topology optimization for periodic thermal conductive material using SIMP method
JIA Jiao, CHENG Wei, LONG Kai
2015, 41(6): 1042-1048. doi: 10.13700/j.bh.1001-5965.2014.0401
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In order to obtain periodic material microstructure under macroscopic thermal conduction condition, the optimal topological model of periodic structure was built by solid isotropic material with penalization (SIMP) method. The volume fraction was referred as constraint and minimized thermal compliance was taken as optimization objective in this model. To satisfy the periodic constraint, the designable domain was divided into a certain number of identical unit cells and the thermal compliance was reallocated. The filtered variable implicitly as a solution of a partial differential equation (PDE) was applied to eliminate the checkerboard patterns and mesh-dependence problems efficiently. The optimal topological configurations were analyzed and compared with different numbers of unit cells and different load cases. The numerical results indicate that proposed periodic model is valid in design of periodic material microstructure with macroscopic steady state thermal conduction condition. Microstructure configurations are different when number of unit cells changes and it reflects the influences of size effect to periodic material design. With an increasing number of unit cells, the optimal results gradually converge to the results using homogenization method.
Conduction mechanism analysis of modified polyimide composite
LEI Weiqun, WU Jiang, PENG Ping, YANG Pei, ZHENG Xiaoquan
2015, 41(6): 1049-1054. doi: 10.13700/j.bh.1001-5965.2014.0427
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Firstly, semi-conductive granules in different diameters from nanometer to micron were used to modify polyimide resin that strengthened by glass fabric. Four kinds of polyimide resin with non-alkalis glass fibers were produced, including the original, micro-filler, nano-filler, and micro/nano-filler modified samples; besides, one kind of polyimide resin with glass fabric that contained small amount of alkalis was prepared as well. The dielectric properties and non-linear conductivity property of all the samples were tested. As a result, it is discovered that the size of filler has little influence on materials' dielectric properties. However, fillers in nanometer decrease a bit of the volume conductivity of the materials, while those in micrometer not only make it increase obviously but also change it into non-linear conductivity. Furthermore, comparing with samples strengthened by non-alkalis glass fabric, the threshold electric field of non-linear conductivity of the polyimide resin strengthened by the glass fabric with small amount of alkalis has a distinct decrease.
Effects of distributed propulsion crucial variables on aerodynamic performance of blended wing body aircraft
YAN Wanfang, WU Jianghao, ZHANG Yanlai
2015, 41(6): 1055-1065. doi: 10.13700/j.bh.1001-5965.2014.0390
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Based on a 350-passenger blended wing body (BWB) integrated aircraft with a distributed propulsion system, effects of crucial design variables of distributed propulsion on aerodynamic performances of the aircraft are studied by a computational fluid dynamics method. It is shown that the effect of the mass flow rate (MFR) on aerodynamic performance in cruising becomes more obvious when the propulsion system moving forward along the aircraft and the aerodynamic efficiency increases with the increase of MFR as well. With the intake location of the propulsion system moving backward along the aircraft, the lift-to-drag ratio increases, but the non-uniform intake flow also increases which decreases the efficiency of the propulsion system. It needs a trade-off to determine the intake location. The aerodynamic efficiency of the aircraft can also be increased by a suitable intake height of the propulsion system meanwhile maintain a uniform intake flow. The lift can be enhanced by up to 20% with a larger MFR in taking off, compared with the aircraft without distributed propulsion system.
Progressive damage analysis of open-hole composite laminates under compression load
ZHOU Rui, GUAN Zhidong, LI Xing, ZHUO Yue
2015, 41(6): 1066-1072. doi: 10.13700/j.bh.1001-5965.2014.0395
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Firstly, compressional experiments were conducted on open-hole composite laminates. The progressive damage during the loading process and the final damage mode of the specimen were detected with a digital microscope, scanning electron miroscope and an X-scan equipment. Damage initiation and delamination between layups of 45° and 90° were observed during the experiments. Secondly, the failure modes of composite laminates are divided into intra-laminar failure and inter-laminar failure. A multiscale model based on the micro-mechanics of failure MMF3 theory and the interface cohesive element method for the damage analysis of open-hole compression laminates was developed. At last, the initiation and propagation of the damage and the failure modes of the laminates were predicted with the model. Numerical results such as the damage initiation position of fiber and matrix, the delamination propagation process and the final failure modes are in good agreement with experimental results, which indicates that the model is applicable to analysis of progressive damage of open-hole composite laminates under compression load.
Numerical simulation of lateral control in supersonic cross jet flow
LI Yachao, YAN Chao, ZHANG Xiang, MENG Jun
2015, 41(6): 1073-1079. doi: 10.13700/j.bh.1001-5965.2014.0398
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To investigate the effect of angle of attack and jet position on the lateral control force in the cross jet flow, the paper firstly simulated the supersonic cross jet flow by the numerical methods. The calculated longitudinal wall pressure distribution agreed well with the experimental results. Jet force amplification factor in longitudinal direction and center of interaction force were used to evaluate the actual performance of lateral control force in jet interaction. Moreover, normal interaction force accumulation coefficient was introduced to analyze how four characteristic areas on the body surface affect the lateral control force in jet interaction. The numerical results indicate that the lateral control force in jet interaction, which is equivalent to a normal force, together with a nose-down moment around center of jet, are not equal to the jet force in design. The wake zone of low pressure is a determinant factor affecting the actual performance of lateral control force. As the angle of attack increases, the strength of the interaction pressure in the wake zone decreases, which will enhance the lateral control force. With the jet location moving backward, the scope of the wake zone diminishes, which will improve the actual performance of lateral control force.
Fault case information extraction method research based on ontology
KE Qianyun, LI Qing, SUN Yong
2015, 41(6): 1080-1086. doi: 10.13700/j.bh.1001-5965.2014.0419
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To solve the accumulation and reusing problems of fault case knowledge that are described as unstructured and unnormalized information in the current maintenance support activities of aircraft, research on the knowledge representation and information extraction method of aircraft fault case was carried out. Firstly,ontology model of aircraft fault case knowledge was established according to the particularity of aircraft fault domain and the actual demand of knowledge sharing and reusing. Then with Chinese segmentation tools and general architecture for text engineering (GATE) frame, semantic annotation and rule based information extraction technology of aircraft fault case documents were studied. Finally, the hidden knowledge was discovered by using apache Jena inference engine, and knowledge base was expanded by the new knowledge found in the process of information extraction. Moreover, the prototype system for information extraction was developed and was used to extract structured fault case information from different types of documents, the information was then stored and managed by using database. This method was proved feasible to improve the reusability of fault case knowledge.
Video avatar-based remote video collaboration
LIU Siqi, YU Chun, SHI Yuanchun
2015, 41(6): 1087-1094. doi: 10.13700/j.bh.1001-5965.2014.0532
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Most existing remote video communication systems are confined to how to accurately deliver adequate information in real time, but lose sight of communicators' interaction demands. Meanwhile, traditional 2D-based video communications cannot make full use of people's 3D information, which shows great potential for video communication to achieve immersive, natural and efficient interactions with 3D technique. To enhance the immersion sense and expand interaction modes in video collaboration, the design goal for ‘immersive' video communication systems was proposed and a novel, avatar-based remote video collaboration system was designed and implemented. Specifically, using Creative Senz3D depth camera, the proposed system extracted people's foreground images through background segmentation as their video avatars, and located the avatars together in a common virtual scenario. Natural and immersive interactions among people and between people and virtual scenes were also well designed, which expanded modes of interaction and collaboration in video communication. Finally, a user study was conducted and the result indicates that the proposed video avatar-based remote video cooperation mode can effectively enhance people's sense of immersion in telecommunication.
Evaluation method for radar anti-stealth performance based on evidence fusion
SHI Junpeng, HU Guoping, WANG Xin
2015, 41(6): 1095-1101. doi: 10.13700/j.bh.1001-5965.2014.0388
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Aiming at the phenomenon of complicated process and imperfect algorithm in the evaluation of radar anti-stealth performance, a new method for the evaluation based on improved evidence theory was presented. Firstly, to overcome the shortcomings of strong subjectivity and weak correlation, the credibility assignment function was corrected in real time according to the relationship among credibility factor, entropy weight and supported coefficient, which is based on the objective analysis of entropy theory and Euclidean distance. Secondly, combined with the multi-hierarchy indexes of radar anti-stealth performance, the rank order of whole radars anti-stealth performance on utility interval was attained. At last, an objective and accurate assessment results were obtained. The analysis of theory and simulation proved that, in contrast to other fusion algorithm, the improved fusion algorithm had better ability of assessment and better robustness.
Design of constant current source to drive analog coil of space magnetic environment
ZHANG Pengfei, QI Bojin, ZHENG Minxin, ZHANG Wei
2015, 41(6): 1102-1107. doi: 10.13700/j.bh.1001-5965.2014.0403
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To reach the requirements that the space environment simulation coil's magnetic induction intensity can continuously adjust from 0 to 20 Gs, and the magnetic field stability can be better than 1%, main circuit topology structure using a voltage source in series with a current source, combined with the voltage double closed loop control and the current closed loop negative feedback control methods, achieved a stable current output, reduced the power consumption of the metal-oxide-semiconductor field-effect transistor (MOSFET), and improved the efficiency of the constant current source. The test results show that the constant current source output current ranges from 0 to 10 A, Helmholtz coil's center magnetic induction intensity can reach the design requirements of 20 Gs, the current stability is better than 0.1%, and the magnetic field stability is better than 1%.
Aerodynamic and overall parameters analysis of buoyancy-lifting hybrid airship
MI Panpan, MENG Junhui, LYU Mingyun
2015, 41(6): 1108-1116. doi: 10.13700/j.bh.1001-5965.2014.0404
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In order to balance the aerodynamic performance and buoyancy-lifting characteristics of airship, to make full use of their advantages and to strengthen the controllability, a hybrid airship configuration which has combined envelopes that are based on high lift NACA airfoil was proposed. With computational fluid dynamics (CFD) and FLUENT computing modeling, the lift-drag characteristics, longitudinal static stability and buoyancy-lifting property under different velocities and attack angles of the airship were simulated, furthermore, its overall performance was evaluated and lastly it was compared with conventional airship. The results show that the airship configuration with combined envelopes has good aerodynamic performance and efficiency. It can produce three times aerodynamic lift than that of conventional airship under the same condition. When the magnitude of cruise velocity is greater than 26 m/s, hybrid airship begins to have a higher total lift efficiency and better buoyancy-lift property. Besides, the hybrid airship's size is smaller than conventional airship under the same design load, which contributes to increasing payloads. This can provide a reference for low-attitude large hybrid airship's research.
Effect of swirl cup's primary swirl number on ignition performance
LIU Guigui, LIN Yuzhen, HU Haosheng, WANG Xiaofeng, ZHANG Chi
2015, 41(6): 1117-1121. doi: 10.13700/j.bh.1001-5965.2014.0408
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All swirl cups have the same primary inclined multi hole and secondary radial straight blade and a simplex pressure atomizer. The effects of primary swirl number of airblast atomizer on small combustion ignition performance were examined. The ignition experiments for swirl cups with three different primary swirl numbers were conducted under atmospheric pressure and temperature in a rectangular reflux combustion chamber, and the pressure drop across the flame tube was between 0.5% and 5% of inlet total pressure. The experimental results show that the primary swirl number significantly influence the ignition performance. Under the same pressure drop, within a certain range, the increase of primary swirl number affects the downstream flow field distribution, which is beneficial for fire nuclear spreading along the upstream of recirculation region and forming a steady flame in the swirl cup exit. So the swirl cup with strong primary vortex has better ignition performance.
Human action recognition based on locality-constrained linear coding
BAI Chen, SUN Junhua
2015, 41(6): 1122-1127. doi: 10.13700/j.bh.1001-5965.2014.0414
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Large intra-class variations of action features lead to low classification accuracy of action recognition, on the other hand, current algorithms exist drawbacks in computational complexity and extension of recognizable action classes. A method based on locality-constrained linear coding (LLC) for action recognition from depth images was proposed. In order to reduce the intra-class variations and increase classification accuracy, joints' positions, velocities and acceleration features were concatenated to form local action features, then LLC was used to calculate sparse representations of local action features. Analytical solution of LLC ensures computational speed of our method is up to 760 frames per second. Dictionary is composed by sub-dictionaries learned by K-means from features of each class separately, so global optimization is avoided during extending recognizable action classes. Moreover, to avoid classifier to be over-fitting, a dimensionality reduction method based on labels of dictionary items was proposed. The proposed method was evaluated on MSR-Action3D dataset captured by depth cameras. The experimental results show that the proposed approach achieves classification accuracy of 85.7%.
Adaptive backstepping control of a nonlinear aeroelastic system
LIU Songdan, LI Daochun, XIANG Jinwu
2015, 41(6): 1128-1134. doi: 10.13700/j.bh.1001-5965.2014.0423
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For a two-dimensional airfoil with leading-edge and trailing-edge control surfaces, the nonlinear aeroelastic equations under the supposition of quasi-steady aerodynamic forces were established and were described in state space form. The control variables of the leading and trailing edges were coupled resulting that the backstepping control method could not be used directly. To solve the problem, two equivalent control laws were newly defined. Supposing that the system has parametric uncertainty in the cubic nonlinearity in pitch, an adaptive control law was designed based on Lyapunov stability theory. In order to verify the validation of the control law, the dynamic equations were solved numerically by using Runge-Kutta method. The simulation results show that the open-loop aeroelastic system is unstable with limit cycle oscillation, while the close-loop system reaches to stable as a result of the adaptive control law. With double control surfaces, the flutter critical velocity is improved after the control design. Taking the limits of the control surface deflection in reality into account, the invalidation problems of the single control surface are discussed. Just considering the effectiveness of the single control surface, the system using the trailing edge control surface is better than that of using the leading edge control surface.
Modeling and analysis of gyroscope's random drift based on GP+GA method
LYU Lin, QUAN Wei
2015, 41(6): 1135-1140. doi: 10.13700/j.bh.1001-5965.2014.0440
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Gyroscope is the key component in an inertial navigation system (INS). It depends on the precision of the INS. In order to improve the gyro's operational accuracy and compensate the random error caused by the nonlinear and weak-stability characteristic of gyro's random drift, the nonlinear random drift error model based on genetic programming (GP)+genetic algorithm (GA) was established taking the time series of gyro's random drift as study object. Firstly, the time series of gyro's random drift were got through analyzing and preprocessing the measured data of gyro. Then the model from the data based on the GP was established and the nonlinear mathematic relationship between the current time and the former times was obtained. Finally, GA was used to optimize the parameters of the mathematic relationship in order to get the more accurate model. The experiment result indicates that compared with classical auto regressive (AR) model, this model can effectively reflect the characteristics of gyro's random drift. The square error of the gyro's random drift has decreased by 73.72% and the effects have increased by 4.72% compared with classical AR model. The establishing model method effectively compensates the gyro's random drift and improved the stability of the system.
Angle of repose and contact-force distribution in granular pile under variable g
CHEN Hui, ZHAO Xianqiong, LIU Yilun
2015, 41(6): 1141-1146. doi: 10.13700/j.bh.1001-5965.2014.0468
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Abstract:
Taking the discontinuity and random character of granular system into account, a kinematic model of particles was established by three-dimensional discrete element method in order to investigate the relationship between angle of repose and gravity. Accumulation process of granular pile was simulated using the model under variable gravity. The angle of repose and probabilistic distribution of contact forces were obtained in the model. Results show that the force-chain formed from contact forces within particle pile has the characteristics of structure of irregular mesh. Contact forces within particle pile are approximately log-normal distributed and there are about 65% of contacts carrying a force lower than the mean. Moreover, there are about 70% of contacts whose friction force are fully mobilized and the ratio of tangential force to normal force among the rest of contacts is uniformly distributed. The distributions of contact force of granular piles in different gravity field have similar properties. The contact forces, which are normalized with respect to particle's gravity, of piles with variable gravity have nearly the same distribution. The angle of repose is not affected by the gravity, though granular pile has a randomness property of microscopic structure.
Learning based intelligent image retargeting technique
LI Ke, YANG Yizhen, YAN Bo
2015, 41(6): 1147-1154. doi: 10.13700/j.bh.1001-5965.2014.0473
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Abstract:
There has been a wide range of image retargeting (IR) approaches, in order to solve the problem of adapting images to different display resolutions. A fast image retargeting method was proposed, which was based on image columns. Firstly, the method would calculate a saliency map of the original image. Secondly, a group of scaling factors were generated for image pixel fusion, which was used to get the result image of the target image size. Each image column corresponded to its scaling factor. For different images, an adaptive upper bound was obtained by machine learning, for scaling factor assignment. This upper bound was set to limit the column width and can reduce image distortion. The experiment results prove that this adaptive upper bound results in a better performance. Moreover, this method has a low complexity, thus it calculates fast, as it is based on image columns instead of pixels.

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