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Research on momentum transfer in simple railgun

Defence technology（防务技术） 2019年第5期15卷 735-740页

作者： Yingtao Xu Bo Tang Baoming Li The National Key Laboratory of Transient Physics Nanjing University of Science and Technology

In this paper,the momentum conservation equation in the longitudinal direction for simple railgun was ***,a three-dimensional model was established and the finite element method was utilized to simulate the *** on the simulation,some results such as the surface force on the electromagnetic field,the electromagnetic force on the conductor in the longitudinal direction were ***,the surface force density on different longitudinal section of railgun was analyzed and discussed in *** results showed that the momentum of the railgun system was conserved when the surface force of the electromagnetic field was ***,large amount of longitudinal force was transmitted to the breech by the electromagnetic field in the form of surface *** exact position and distribution of recoil were related to the current input device.

关键词： Railgun Momentum conservation Recoil Surface force

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Transverse Particle Trapping Using Finite Bessel Beams Based on Acoustic Metamaterials

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Physical Review Applied 2023年第3期19卷 034032-034032页

作者： Xudong Fan Yifan Zhu Zihao Su Ning Li Xiaolong Huang Yang Kang Can Li Chunsheng Weng Hui Zhang Weiwei Kan Badreddine Assouar National Key Laboratory of Transient Physics Nanjing University of Science and Technology Nanjing 210094 China Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments School of Mechanical Engineering Southeast University Nanjing 211189 China School of Science MIIT Key Laboratory of Semiconductor Microstructure and Quantum Sensing Nanjing University of Science and Technology Nanjing 210094 China Université de Lorraine CNRS Institut Jean Lamour F-54000 Nancy France

We investigate the transverse trapping force acting on a small particle on or off the central axis of zero-order finite Bessel beams. Combining the simulated fields with the Gorkov force potential, the transverse trapping behaviors for small objects are analyzed, and the reversal of the trapping behaviors is recovered when varying the paraxiality parameter of the Bessel beam. The results prove the possibility of using axisymmetric Bessel beams to trap both dense and stiff particles as well as light and soft ones. The particles can be trapped at the maximum central pressure in the main lobe and also the maximum pressure of the other transverse locations. A lossy acoustic metastructure with the ability to decouple the phase and amplitude modulations is used to generate the desired sound field, which is used to trap a foam ball as an example. Our research opens a promising avenue to the design and development of simplified acoustic tweezers.

关键词： Acoustic metamaterials Acoustic wave phenomena Acoustics

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Discrete Boltzmann modeling of Rayleigh-Taylor instability: Effects of interfacial tension, viscosity, and heat conductivity

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Physical Review E 2022年第1期106卷 015102-015102页

作者： Jie Chen Aiguo Xu Dawei Chen Yudong Zhang Zhihua Chen Laboratory of Computational Physics Institute of Applied Physics and Computational Mathematics P.O. Box 8009-26 Beijing 100088 China Key Laboratory of Transient Physics Nanjing University of Science and Technology Nanjing 210094 China State Key Laboratory of Explosion Science and Technology Beijing Institute of Technology Beijing 100081 China HEDPS Center for Applied Physics and Technology and College of Engineering Peking University Beijing 100871 China School of Mechanics and Safety Engineering Zhengzhou University Zhengzhou 450001 China

The two-dimensional Rayleigh-Taylor instability (RTI) in compressible flow with intermolecular interactions is probed via the discrete Boltzmann method. The effects of interfacial tension, viscosity, and heat conduction are investigated. It is found that the influences of interfacial tension on the perturbation amplitude, bubble velocity, and two kinds of entropy production rates all show differences at different stages of RTI evolution. It inhibits the RTI evolution at the bubble acceleration stage, while at the asymptotic velocity stage, it first promotes and then inhibits the RTI evolution. Viscosity and heat conduction inhibit the RTI evolution. Viscosity shows a suppressive effect on the entropy generation rate related to heat flow at the early stage but a first promotive and then suppressive effect on the entropy generation rate related to heat flow at a later stage. Heat conduction shows a promotive effect on the entropy generation rate related to heat flow at an early stage. Still, it offers a first promotive and then suppressive effect on the entropy generation rate related to heat flow at a later stage. By introducing the morphological boundary length, we find that the stage of exponential growth of the interface length with time corresponds to the bubble acceleration stage. The first maximum point of the interface length change rate and the first maximum point of the change rate of the entropy generation rate related to viscous stress can be used as a criterion for RTI to enter the asymptotic velocity stage.

关键词： Complex fluids Entropy production Flow instability Interface & surface thermodynamics Nonequilibrium & irreversible thermodynamics Rayleigh-Taylor instability Thermodynamics of mixing

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Kinetics of Rayleigh-Taylor instability in van der Waals fluid: the influence of compressibility

arXiv

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arXiv 2024年

作者： Chen, Jie Xu, Aiguo Zhang, Yudong Chen, Dawei Chen, Zhihua National Key Laboratory of Transient Physics Nanjing University of Science and Technology Nanjing210094 China National Key Laboratory of Computational Physics Institute of Applied Physics and Computational Mathematics P. O. Box 8009-26 Beijing100088 China State Key Laboratory of Explosion Science and Safety Protection Beijing Institute of Technology Beijing100081 China HEDPS Center for Applied Physics and Technology College of Engineering Peking University Beijing100871 China National Key Laboratory of Shock Wave and Detonation Physics Mianyang621999 China School of Mechanics and Safety Engineering Zhengzhou University Zhengzhou450001 China

Early studies on Rayleigh-Taylor instability (RTI) primarily relied on the Navier-Stokes (NS) model. As research progresses, it becomes increasingly evident that the kinetic information that the NS model failed to capture is of great value for identifying and even controlling the RTI process;simultaneously, the lack of analysis techniques for complex physical fields results in a significant waste of data information. In addition, early RTI studies mainly focused on the incompressible case and the weakly compressible case. In the case of strong compressibility, the density of the fluid from the upper layer (originally heavy fluid) may become smaller than that of the surround- ing (originally light) fluid, thus invalidating the early method of distinguishing light and heavy fluids based on density. In this paper, tracer particles are incorporated into a single-fluid discrete Boltzmann method (DBM) model that considers the van der Waals potential. By using tracer par- ticles to label the matter-particle sources, a careful study of the matter-mixing and energy-mixing processes of the RTI evolution is realized in the single-fluid framework. The effects of compress- ibility on the evolution of RTI are examined mainly through the analysis of bubble and spike velocities, the ratio of area occupied by heavy fluid, and various entropy generation rates of the system. It is demonstrated that: (i) compressibility has a suppressive effect on the spike velocity, and this suppressive impact diminishes as the Atwood number (At) increases. The influence of compressibility on bubble velocity shows a staged behavior with increasing At. (ii) The impact of compressibility on the entropy production rate associated with the heat flow ( SNOEF ) is related to the stages of RTI evolution. Moreover, this staged impact of compressibility on SNOEF varies with At. Compressibility exhibits an inhibitory effect on the entropy production rate associated with viscous stresses ( SNOMF). (iii) By incor

关键词： Compressibility

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Ets-Net: Edge Enhanced Two-Stream Network for Medical Image Segmentation

SSRN

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SSRN 2023年

作者： Zhu, Shiyu Han, Junyang Liu, Hongzhi Xie, Shipeng Chen, Yang National Key Laboratory of Transient of Physics Nanjing University of Science and Technology Nanjing China School of Computer Science and Engineering Southeast University Nanjing China College of Telecommunications Information Engineering Nanjing University of Posts and Telecommunications Nanjing China

Over the past several years, deep learning methods were widely applied to some medical image processing task such as medical image segmentation and achieved adequate performance. However, universal and effective methods are still needed to segment the details such as the edge of object in some medical image segmentation task. And the medical images nowadays tend to contain richer detail information with the development of the medical imaging technology, which makes the above challenge more obvious. To this end, we propose an edge enhanced two-streams network (ETS-Net) to improve the segmentation near the edge of the object. The proposed network contains a regular stream aiming at getting the contextual information and an edge detail stream aiming at capturing more detail features near the edge. Moreover a multi-scale supplement module (MSSM) is applied to the regular stream to obtain more contextual information from different scales and a context enhance fusion module (CEFM) is used to fuse the features from the two streams. The proposed method is thoroughly validated in three different datasets, and the experimental results demonstrate that the proposed method generally outperforms other state-of-the-art algorithms for various metrics. © 2023, The Authors. All rights reserved.

关键词： Image segmentation

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Numerical Research on the Effect of Propeller Rotation Direction on the Propulsion Performance and Load Difference of Inner and Outer Propeller in a Four-Screw Ship

Numerical Research on the Effect of Propeller Rotation Direc...

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OCEANS

作者： Wenxuan Ma Xin Chang Yu Lu Wenquan Wang Naval Architecture and Ocean Engineering College Dalian Maritime University Dalian China School of Naval Architecture and Ocean Engineering Guangzhou Maritime University Guangzhou China National Key Laboratory of Transient Physics Nanjing University of Science and Technology Nanjing China

ISBN: (数字)9798350362077

ISBN: (纸本)9798350362084

In the actual sailing of the four-screw ship, due to the asymmetrically distributed wake, the inner propeller and outer propeller show obvious load difference. The load difference will affect the machine-propeller matching and reduce the service life of the main engine. This paper proposes a method to reduce the load difference by changing the rotation direction of the inner and outer propellers. The RANS method is used in this paper to simulate the self-propulsion of a four-screw ship with different rotational arrangements. By analyzing the propeller load variation, the propeller and wake flow interrelationships are illustrated. The peak value of the main propeller blade is related to the circumferential velocity distribution. When the inner and outer propellers are rotating inward at the same time, the efficiency of the inner propeller is increased and the torque difference between inner and outer propellers is reduced to within 1%, the lateral loads are reduced by 15% to 40%.

关键词： Time-frequency analysis Torque Propellers Blades Oceans Propulsion Load management

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On the Cavity Flow of a Cylinder Exits Water Obliquely

SSRN

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SSRN 2022年

作者： Guo, Zeqing Zhao, Yue Zhang, Xingyu Lyu, Xujian National Key Laboratory of Transient Physics Nanjing University of Science and Technology Nanjing210094 China School of Energy and Power Engineering Nanjing University of Science and Technology Nanjing210094 China

A CFD method based on VOF multiphase flow interface-capturing technology and overset mesh technology is adopted to numerically calculate the interaction of vapor-liquid-air multiphase flow when a high-speed cylinder exits water obliquely. Based on the verification of the calculation model by mesh convergence and water entry experiment, the cavitation evolution and flow distribution characteristics of the high-speed cylinder during oblique water exit is analyzed. Moreover, the effects of water exit angles on the surface pressure distribution and drag characteristics of cylinder are studied. Calculation results show that the fluid at the end of the cavity impacts the shoulder of the cylinder during oblique water exit, resulting in the pressure difference between the upper and lower surfaces of the cylinder, which may finally leads to motion deviation. With the decreasing water-exit angle, the negative peak of the drag coefficient decreases while the peak of the surface pressure coefficient increases. © 2022, The Authors. All rights reserved.

关键词： Computational fluid dynamics

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Analysis on the Influence of Breech Voltage on the Pulsed Discharge Characteristics of Capacitive Pulsed Power Supply in EML 2

Analysis on the Influence of Breech Voltage on the Pulsed Di...

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2nd Spring International Conference on Defence technology, ICDT 2020

作者： Li, Zhenxiao Hao, Shuangpeng Ma, Fuqiang Li, Baoming National Key Laboratory of Transient Physics Nanjing University of Science and Technology Nanjing210094 China

Corresponding to the different gradient of the ideal working current, the movement process of the armature in the railgun bore can be divided into rising phase, stable phase and decreasing phase. Based on the load characteristic and the lumped-element equivalent circuit of the railgun, the characteristics of the railgun's breech voltage in above three phases were analysed. Under the condition that the size of the launching system is close to the same, the change ranges of the breech voltage of the augmented railgun is much larger than that of the simple railgun, and the breech voltage of the augmented railgun is a very high forward voltage in the rising phase. The influence of the breech voltage on the discharge characteristics of the capacitive pulsed power supply was studied. In the rising phase, the high breech voltage of the augmented railgun has a significant adverse effect on the pulsed discharge characteristics of the capacitive pulsed power supply whose pulsed forming net consists of a large number of pulsed forming units with small energy storage and high resonance frequency. As a result, part of the stored energy of the capacitive pulsed power supply cannot be used effectively, which leads to the decrease of the working efficiency of the launching system. Using the launching system with a 13-MJ capacitive pulsed power supply and a medium calibre augmented railgun, the launching simulation and test were completed, and the analysis was verified. The research has important enlightenment for improving the match between the capacitive pulsed power supply and the railgun. © Published under licence by IOP Publishing Ltd.

关键词： Rail guns

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Modeling and numerical simulation on launch dynamics of integrated launch package in electromagnetic railgun 2

Modeling and numerical simulation on launch dynamics of inte...

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2nd Spring International Conference on Defence technology, ICDT 2020

作者： Lin, Qinghua Li, Baoming National Key Laboratory of Transient Physics Nanjing University of Science and Technology Nanjing210094 China

This paper presents a numerical approach to modeling launch dynamics of integrated launch package in railguns. This approach was based on the three-dimensional transient finite element solutions of the multi-physical field equations. Numerical simulations were conducted for a railgun system composed of the capacitor-based pulse forming network, advanced containment launcher, and integrated launch package. Some issues about induced eddy currents, stress distributions, and projectile lateral motions were discussed. The results show that the in-bore motion of the projectile is sensitive to the launch package configuration and launcher stiffness. Moreover, the armature under a heavy current is prone to serious deformation and then leads to launch failure. The launch dynamics model provides a method to predict the in-bore processes and evaluate the in-bore survivability of the integrated launch package in a railgun. © Published under licence by IOP Publishing Ltd.

关键词： Rail guns

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Numerical study of the turbulent channel flow under space-dependent electromagnetic force control at different Reynolds numbers

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Applied Mathematics and Mechanics(English Edition) 2019年第4期40卷 435-448页

作者： Daiwen JIANG Hui ZHANG Baochun FAN Zijie ZHAO Jian LI Mingyue GUI Science and Technology on Transient Physics Laboratory Nanjing University of Science and Technology Department of Mathematics Imperial College London

In this paper, the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with the direct numerical simulation(DNS) methods for different Reynolds numbers. A formulation is derived to express the relation between the drag and the Reynolds shear stress. With the application of optimal electromagnetic force, the in-depth relations among characteristic structures in the flow field, mean Reynolds shear stress, and the effect of drag reduction for different Reynolds numbers are discussed. The results indicate that the maximum drag reductions can be obtained with an optimal combination of parameters for each case of different Reynolds numbers. The regular quasi-streamwise vortex structures, which appear in the flow field, have the same period with that of the electromagnetic *** structures suppress the random velocity fluctuations, which leads to the absolute value of mean Reynolds shear stress decreasing and the distribution of that moving away from the wall. Moreover, the wave number of optimal electromagnetic force increases,and the scale of the regular quasi-streamwise vortex structures decreases as the Reynolds number increases. Therefore, the rate of drag reduction decreases with the increase in the Reynolds number since the scale of the regular quasi-streamwise vortex structures decreases.

关键词： flow control drag reduction electromagnetic force turbulent channel flow

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