bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical study on boundary-layer receptivity with localized wall blowing/suction
Authors: Lu Chang-Gen  , Shen Lu-Yu  ,   School of Marine Science, Nanjing University of Information Science and Technology, Nanjing 210044, China;,   College of Atmospheric Science, Nanjing University of Information Science and Technology, Nanjing 210044, China
Published: December 2020

Most of previous studies focused on the boundary-layer receptivity to the convected disturbances in the free stream interacting with localized wall roughness. Whereas the research on the boundary-layer receptivity induced by localized blowing or localized suction is relatively few. In this paper, we investigate two-dimensional boundary-layer receptivity induced by localized blowing/suction within free-stream turbulence through using direct numerical simulation and fast Fourier transformation. High-order compact finite difference schemes in the y-direction, fast Fourier transformation in the x-direction, and a Runge-Kutta scheme in time domain are used to solve the Navier-Stokes equations. The numerical results show that Tollmien-Schlichting (T-S) wave packets are excited by the free-stream turbulence interacting with localized blowing in the two-dimensional boundary layer, which are superposed by a group of stable, neutral and unstable T-S waves. The dispersion relations, growth rates, amplitude distributions and phase distributions of the excited waves accord well with theoretical solutions of the linear stability theory, thus confirming the existence of the boundary-layer receptivity. And the frequencies of the instability waves are between the upper and lower branches of the neutral stability curves. According to the evolutions of the wave packets, the positions of peaks and valleys are tracked over time to calculate the propagation speed by taking the average. The propagation speeds of the wave packets are approximately one-third of the free-stream velocity, which are in accordance with Dietz's measurements. The propagation speeds of wave packets are also close to the phase speeds of the most unstable waves for the numerical results. The relations of the receptivity response to the forcing amplitude, the blowing intensity, and the blowing width are found to be linear, when the forcing amplitude and the blowing intensity are less than 1% free-steam velocity amplitude and 0.01, respectively. And the maximum amplitudes of the T-S waves can be excited while the blowing length is equal to the resonant wavelength /(TS-FS), where TS is the wave-number of the T-S wave, and FS is the wave-number of the forcing disturbance. These results are similar to those given by Dietz [Dietz A J 1999 J. Fluid Mech. 378 291]. Additionally, T-S waves with the same dispersion relations but opposite phases are generated by localized blowing and localized suction respectively, and the amplitudes of the T-S waves excited by localized blowing are nearly 15% greater than those by localized suction under the same condition. According to this theory, an optimal design of localized suction device is able to enhance or delay the laminar-turbulent transition for turbulent control.
Numerical analysis on air entrapment during droplet impacting on a wetted surface
Authors: Li Da-Shu  , Qiu Xing-Qi  , Zheng Zhi-Wei  ,   College of Chemical Engineering, China University of Petroleum, Qingdao 266555, China
Published: December 2020

A numerical model is developed using the coupled level set and volume of fluid method including heat transfer and contact resistance to simulate air entrapment during a droplet impacting on a wetted surface. The dynamic characteristics of the phase interface are analysed. The mechanisms of deformation of the phase interface and formation of entrapped air are explored. The effects of impacting velocity and thickness of liquid film on characteristics of entrapped air are studied. The mechanism of heat transfer is also obtained in this article. The obtained results are as follows. The pressure difference between liquid and gas before the droplet impacting is the main factor determining the deformation of phases interface and the formation of air entrapment. The larger the impacting velocity, the larger the pressure inside the compressed air film is. When the droplet contacts the liquid film, the velocities of the droplet and liquid film increase to their maximum values, and at the impacting axis, they are approximately the same, nearly half the impacting velocity. The velocity distributions of phase interface of the droplet and liquid film are nearly the same in the area of impacting center. The impacting velocity has important effects on the dimensionless arc from bottom to breaking point and the dimensionless diameter of the air. The dimensionless arc and dimensionless diameter decrease with increasing impacting velocity. The dimensionless deforming heights of the droplet and liquid film are closely related to Stokes number: the larger the Stokes number, the larger the dimensionless deforming heights are, and they can be expressed as a power function with Stokes number. The initial thickness of liquid film also affects dimensionless deforming heights of the droplet and liquid film and dimensionless diameter of the entrapped air: the larger the dimensionless thickness of the liquid film, the larger the dimensionless deforming heights are, and the dimensionless diameter decreases with increasing dimensionless thickness of the liquid film. At the very initial stage of the impact, the entrapped air is important for surface heat flux distribution. The entrapped air presents contraction, breakup and detachment. The surface heat flux distribution changes closely with evolution of the entrapped air and tends to be uniform. The effect of the entrapped air on the surface heat flux distribution decreases gradually.
Polarized properties of typical surface types over China based on the multi-angular polarized remote sensing measurements
Authors: Xiang Kun-Sheng  , Cheng Tian-Hai  , Gu Xing-Fa  , Guo Hong  , Chen Hao  , Wang Ying  , Wei Xi  , Bao Fang-Wen  ,   State Key Laboratory of Remote Sensing, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing 100101, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

The surface polarized reflectance is able to fully reflect the physical characteristics of surface, such as vegetation classification, plant biomass estimation, leaf angle distribution and surface water content. The bidirectional polarization distribution function is a useful tool for quantitatively describing the surface polarized reflectance. The multi-angle polarized remote sensing reveals a significant advantage in capturing the radiation and polarization information. The polarization and directionality of the earth reflectance (POLDER) instrument is the only sensor which has provided a long-term trend of polarized measurements. Its combination of multi-spectral, polarization and multi-angle observations has considerable capability for retrieving ocean, land, cloud and aerosol properties. Until now, data obtained from POLDER has been widely used to study various surface bidirectional polarized reflectance models, especially Nadal model. However, parameters of Nadal model reveal a low accuracy in China region. In this study, the parameters of Nadal model suitable for China region are obtained and analyzed based on the POLDER-2 polarized reflectance data. Based on the modified parameters of Nadal model, polarized reflectance under different surface types is further analyzed. Our results show that the polarized reflectance retrieved from modified parameters of Nadal model reveals better correlation with the POLDER-2 products than the polarized reflectance from Nadal official parameters under different surface types. The polarization properties of three typical surfaces (forest, grassland and desert) are further investigated and reveal that 1) different surface polarized reflectances decrease with the increase of the scattering angle, and the polarized reflectance of the same object decreases as the normalized difference vegetation index increases; 2) significant discrepancies exist between the polarized reflectances of different surfaces, the polarized reflectance of forest is the lowest in the three surface types, then that of grass is the second lowest, and desert reveal the largest value (about twice that of forest), 3) the discrepancies of polarized reflectance between different surfaces have an increasing trend as satellite view zenith angle increases. This study will provide a priori knowledge for the detection of surface polarization properties and aerosol parameters based on multi-angle polarization remote sensing data, and also establish a good foundation for the quantitative applications of GF-5 satellite multi-angle polarization imager to be launched soon in China.
Structural phase transition, electronic structures and optical properties of ZnTe
Authors: Hu Yong-Jin  , Wu Yun-Pei  , Liu Guo-Ying  , Luo Shi-Jun  , He Kai-Hua  ,   School of Science, Hubei University of Automotive Technology, Shiyan 442002, China;,   School of Mathematics and Physics, China University of Geosciences, Wuhan 430074, China
Published: December 2020

The equations of state and phase transition of ZnTe in zinc blende (ZB) and cinnabar (CB) structures under high pressure are investigated by the projected augmented wave method in the scheme of density functional theory. The primitive cell volumes, electronic structures and optical properties are also predicted before and after phase transition. The variations of the calculated total energy with volume, for the structures of ZB and CB, yield the information about the static equation of state and phase stability. The results show that the ZB phase of ZnTe has lower energy, and is more stable than its CB phase. The pressure-induced transition occurs along the common tangent line connecting the tangential points on the two enthalpy-volume curves. The calculations show that the phase transition pressure is 8.6 GPa from the ZB structure to the CB structure. The value is also compatible with those of other available theoretical and experimental results. Just before the ZB phase is transferred to the CB phase at about 8.6 GPa, the volume is reduced by 13.0% relative to the former volume at the ambient pressure condition. The calculated critical volumes and volume compressibilities by using two methods agree well with other results in the literature. The lattice parameters and equations of state of the two structures are also obtained. Metallization case of other similar materials such as ZnS caused by high pressure does not occur here. The CB phase has the behavior of indirect band gap with 0.98 eV along the symmetry of GK. After phase transition, the distributions of density of states of Zn and Te atoms of the CB structure shift towards lower energy, especially in the conduction band bottom, and the band gap decreases. Energy level overlapping is more obvious in the CB structure, and orbital hybridizations still exist, that is the reason why it is the stable phase under high pressure condition. Stronger orbital hybridization helps the transitions between Te 5p and Zn 3d electrons. The main peak of imaginary part of dielectric constant is enhanced apparently with abnormal red shift, while other two peaks disappear at the same time. Macroscopic dielectric constant of ZB structure decreases as pressure increases. For CB structure, the macroscopic dielectric constant with 13.60 eV is not affected by pressure. The results provide a theoretical basis for the polarization research of ZnTe material in static electric field under high pressure.
Discrete iterative-map modeling and dynamical analysis of digital voltage-mode controlled buck converter with dual-edge modulation
Authors: Liu Xiao-Tian  , Zhou Guo-Hua  , Li Zhen-Hua  , Chen Xing  ,   School of Electrical Engineering, Southwest Jiaotong University, Chengdu 610031, China
Published: December 2020

The operation principle of digital voltage-mode controlled buck converter with dual-edge modulation is analyzed in this paper. Based on the state equation of buck converter and six possible evolutions in one switching cycle, the discrete iterative-map model of digital voltage-mode controlled buck converter with dual-edge modulation is established. Ignoring the quantization error of analog-digital converter and on the basis of its discrete iterative-map model, the nonlinear dynamical behavior of digital voltage-mode controlled buck converter with dual-edge modulation is investigated in detail. Taking the input voltage and the load resistance as bifurcation parameters, the output voltage bifurcation diagram and the inductor current bifurcation diagram are plotted. Through analyzing the bifurcation diagrams, it is indicated that there are two kinds of similar but different Hopf bifurcation phenomena. By use of Poincar section, time-domain simulation waveforms and phase portraits, two different Hopf bifurcations and low-frequency oscillation phenomena are compared and studied. Observing the inductor current and capacitor voltage waveforms respectively, it is obviously found that their oscillation frequencies and amplitudes are different, the shapes of two Poincar$ sections and phase portraits are also different. In order to verify the correctness of the simulation and theoretical analysis, the eigenvalues of Jacobian matrix of the discrete iterative map model are introduced and solved in two kinds of stable evolutions. Through analyzing variation of eigenvalues of Jacobi matrix with input voltage, the existence and difference of two kinds of Hopf bifurcation phenomena are proved theoretically. Moreover, it is observed in this paper that the odd period-doubling bifurcation phenomenon exists in digital voltage-mode controlled buck converter with dual-edge modulation for the first time, where the operation state of the buck converter turns from period-one into period-three. Its authenticity is verified by using the time-domain simulation waveforms and phase portraits. In order to approach to the actual circuit, the equivalent series resistances of capacitor and inductor are considered. The actual circuit is simulated by using the software Psim. A comparison shows that there are little differences between the theoretical simulation and the actual circuit simulation. So the theoretical simulation can be used to analyze the performances of the actual circuit. The research results in this paper have guiding significance and practical value for designing the digital voltage-mode controlled buck converter with dual-edge modulation.
Progress of terahertz metamaterials
Authors: Bao Di  , Shen Xiao-Peng  , Cui Tie-Jun  ,   State Key Laboratory of Millimetre Waves, School of Information Science and Engineering, Southeast University, Nanjing 210096, China;,   Synergetic Innovation Center of Wireless Communication Technology, Southeast University, Nanjing 210096, China;,   Department of Physics, China University of Mining and Technology, Xuzhou 221116, China;,   Cooperative Innovation Centre of Terahertz Science, University of Electronic Science and Technology of China, Chengdu 610054, China
Published: December 2020

In the past decades, terahertz metamaterials have attracted considerable attention due to the capability of realizing essential terahertz functional devices and potential applications in sensing, imaging, spectroscopy and monitoring. In this review, we first present a brief introduction to the theory and development of terahertz metamaterials, and then focus on some terahertz devices including both triple-band and broadband metamaterial absorbers, the spoof surface plasmon polaritons (SPP) waveguides, the SPP bend, the SPP beam splitter, and the SPP ring resonator. The metamaterial absorbers are fabricated and measured in THz band, while the SPP devices are verified through numerical simulations. All the designs are easy to fabricate and favorable for practical applications.
Research progress on avalanche multiplication GaAs photoconductive terahertz emitter
Authors: Shi Wei  , Yan Zhi-Jin  ,   Applied Physics Department, Xi'an University of Technology, Xi'an 710048, China
Published: December 2020

GaAs photoconductive switch illuminated by a femto-second laser has been widely used in a terabertz (THz) time domain spectroscopy system as a THz wave emission antenna. Now, all of the GaAs photoconductive switches are used in linear mode. However, when the GaAs photoconductive switch operates in an avalanche multiplication mode, the power capacity of output ultrafast electric pulse is much higher than that in a linear mode. So far, nobody has proposed the idea of generating THz waves by using the GaAs photoconductive switches in the avalanche multiplication mode. In this paper, we report the feasibility and research progress of using the GaAs photoconductive switches in the avalanche multiplication mode as the THz sources. By theoretical analysis and experimental research, some results are obtained experimentally as follows. 1) The GaAs photoconductive antenna can operate in an avalanche multiplication mode when illuminated by a femto-second laser pulse with an energy on the order of nJ. 2) The maintaining time of the avalanche multiplication mode, i.e, lock-on period, can be reduced by the quenching mode of photo-activated charge domain. These results lay the foundation for generating the high intensity THz emission by the GaAs photoconductive antenna with the avalanche multiplication mechanism.
High precision and fast method for absolute distance measurement based on resampling technique used in FM continuous wave laser ranging
Authors: Meng Xiang-Song  , Zhang Fu-Min  , Qu Xing-Hua  ,   State Key Laboratory of Precision Measurement Technology and Instrument, Tianjin University, Tianjin 300072, China
Published: December 2020

Frequency modulated continuous wave (FMCW) laser ranging is one of the most interesting techniques for precision distance metrology. It is a promising candidate for absolute distance measurement at large standoff distances (10 to 100 m) with high precision and accuracy, and no cooperation target is needed during the measuring process. How to improve the measurement resolution in practice has been the research focus of the FMCW laser ranging in recent years.FMCW laser ranging system uses the method which may convert the measurement of flight time to the frequency measurement, while the ranging resolution can be determined by the tuning range of the optical frequency sweep in theory. The main impact-factor that reduces the resolution is the tuning nonlinearity of the laser source, which may cause an amount of error points within the sampling signal. So a dual-interferometric FMCW laser ranging system is adopted in this paper. Compared to the traditional Michelson scheme, an assistant interferometer is added. The assistant interferometer has an all-fiber optical Mach-Zehnder configuration, and the delay distance is at least 2 times longer than OPD (optical path difference) of the main interferometer. Because it provides the reference length, the length of the fiber must remain unchanged. The interference signal is obtained on the photodetector. At the time points of every peak and bottom of the auxiliary interferometer signal, the beating signal from the main interferometer is re-sampled. The original signal is not the equal time intervals, while the re-sampled signal is the equal optical frequency intervals. Based on the property of the re-sampled signal, a method by splicing the re-sampled signal to optimize the signal processing is proposed, by which the tuning range of the laser source limitation can be broken and high precision can be easily obtained. Also, a simple high-speed measuring method is proposed.Based on all the above principles, the two-fiber optical frequency-modulated continuous wave laser ranging system is designed. The delay fiber in the FMCW laser ranging system is 40.8 m long, and the tuning speed and tuning range of the laser source are set to 10 nm/s and 40 nm respectively. Experiments show that the optimization method can effectively improve the measurement resolution and measuring efficiency; in the 26 measuring ranges, 50 m resolution can be easily obtained and the error is less than 100 m.
Study on in-situ real-time measurement for thin film growth of organic semiconductors
Authors: Xu Jia-Jia  , Hu Chun-Guang  , Chen Xue-Jiao  , Zhang Lei  , Fu Xing  , Hu Xiao-Tang  ,   State Key Laboratory of Precision Measuring Technology and Instrument, Tianjin University, Tianjin 300072, China;,   School of Precision Instruments and Opto-Electronics Engineering, Tianjin University, Tianjin 300072, China;,   School of Control Science and Technology, Shandong University, Jinan 250061, China
Published: December 2020

We propose an approach for in-situ real-time measuring the optical and electric properties of a thin film in parallel during the process of growth. The method is developed based on two techniques: differential reflectance spectroscopy (DRS) and field effect transistor (FET) structure based electrical characteristics testing method. In order to demonstrate the performance of the method, FETs with a bottom-gate structure are manufactured and the pentacene organic thin film is deposited by vacuum thermal evaporation as a transport layer on the top of the transistor, i.e. the insulator substrate of SiO2. The optical and electrical properties of the organic thin film are in-situ investigated during its growth. As obtained from the optical spectra, the DRS signal moves up and down along the wavelength. Its fluctuation amplitude increases quickly and is very sensitive to the variation of the thickness of the top most film since the shutter of the molecular evaporation source is open. A good agreement between the experimental data and the computational results with a four-layer structure model of Si/SiO2/pentacene/air suggests that the DRS signal here is mainly due to the interference that exists in the multilayer interfaces. In addition, there are two characteristic peaks at 629 nm (1.97 eV) and 673 nm (1.84 eV) appearing occurs clearly in the DRS spectra at the initial stage of the growth. It means that the pentacene layer forms a thin film phase structure. Furthermore, the growth rate is evaluated to be 0.23 nm/min. When the effective thickness of the pentacene layer reaches 28 nm, calculated from the growth rate and the measured time, the conductivity of the organic FET becomes noticeable. It implies that an electrical conducting layer is already formed. After that, the thickness of the conducting layer continuously increases, while the current between the drain and the source increases slowly and turns to be saturated. After a 15-hour film growth, the sample has a threshold voltage of -20 V and the charge carrier mobility is 3.1×10-3 cm2/(V· s). These data confirm that the sample is an FET although its electronic properties are not good enough. These results show that the proposed approach is a useful measurement tool to build the relationships among the data of the optical spectrum, the electrical property, and the structure of the thin films. Hence, it is valuable for both the explanation of the growth mechanism of the thin film in research and the optimization of its preparation process in industry.
Theoretical study on electron-impact excitation cross section and polarization for 5s2S1/2 → 5p2P3/2 of Cd
Authors: Jiao Fei  , Jiang Jun  , Xie Lu-You  , Zhang Deng-Hong  , Dong Chen-Zhong  , Chen Zhan-Bin  ,   Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China
Published: December 2020

The electron-impact excitation (EIE) cross-sections of 5s2S1/2 → 5p2P3/2 of Cd+ have been calculated by using the relativistic distorted-wave (RDW) method which we have developed recently. In order to discuss the electronic correlation effects, four different models are used to describe the target wave functions, namely Model A, 8S, 5SD and 6SD. Model A is a single configuration model, it includes 5s1/2, 5p1/2 and 5p3/2 only. The 8S is a valence-valence correlation model, it considers the impacts of 6s, 7s, 8s and 6p, 7p, 8p orbitals on wave function of 5s1/2, 5p3/2, respectively. The 5SD is a core-valence correlation model, it includes all the virtual single and double excitations from the 4s, 4p, 4d, 5s and 5p shells into the unoccupied 4f, 5p, 5d, and 5f shells. The 6SD is also a core-valence correlation model, it includes all the virtual single and double excitations from the 4s, 4p, 4d, 5s and 5p shells into the unoccupied 4f, 5p, 5d, 5f, 6s, 6p and 6d shells. The oscillator strength calculated by Model A is 0.72, which is in disagreement with the experiment 0.55 measured by Xu et al., while the results of 6SD 0.57 is in agreement with these of the experiment very well. For the EIE cross-sections, the core-valence correlation is very important. The results of Model A and 8S are larger than the experimental results obtained by Gomonai et al., while the results of 5SD and 6SD is obviously smaller than the results of Model A. In low energy range (2P3/2 → 5s2S1/2 emission lines after excitation are very small, while the linear polarization of Model A, 5SD and 6SD are in consistent with each other and also in good agreement with other theories. But for high energies, the theoretical results have big difference from the experimental results obtained by Goto et al.
A design of high-gain end-fire antenna based on split-ring resonator structures
Authors: Liu Hong-Xi  , Gao Jun  , Cao Xiang-Yu  , Liu Yan-Fang  , Zhang Di  , Li Si-Jia  ,   Information and Navigation College of Air Force Engineering University, Xi'an 710077, China;,   Inner Mongolia Ultra High Voltage Power Supply Bureau, Hohhot 010080, China
Published: December 2020

Vivaldi antennas have wide applications in practice due to the ultra-wide band properties; however, their gain and directivity are relatively low. In this paper, a new method is presented to improve the gain and directivity of Vivaldi antennas in a broad band using split-ring resonator (SRR). Based on the peculiar electromagnetic properties of SRR, a novel high-gain SRR-Vivaldi end-fire antenna in C and X bands is designed and fabricated. The size of the antenna is only 0.33λ ×0.33λ ×0.013λ, a significant miniaturization. Equivalent analysis method has been adopted to study the resonance characteristic of an SRR structure. By adding the SRR structures the singular metamaterials in the front of the Vivaldi antenna have an exponential taper slot, and the SRR structures can play a role as a director which has the ability to enhance the antenna's directivity so that the surface currents will focus on the end-fire direction. The SRR structures have been analyzed, designed, and fabricated, which can be embedded into the original Vivaldi antenna smoothly and compactly. As a result, the gain of the SRR-Vivaldi antenna are enhanced effectively, while the size and bandwidth of the original antenna can be kept, with the reflection coeffcient less than -10 dB from 4 to 13.6 GHz after using SRR. The novel Vivaldi antenna based on the SRR has good features of high gain, high directivity, low return loss and low cross-polarization. Compared to the original Vivaldi antenna, the simulation and measured results demonstrate that the gain of the novel SRR-Vivaldi antenna in C band has been increased by an average value of 75.44% and the half-power beam width has been decreased by 20 degrees in xoy and xoz planes. Meanwhile, the gain has been increased by an average value of 24.46% in X band and the half-power beam width has been decreased by 25 degrees in xoz plane. Testing result of the fabricated antenna demonstrates the reliability of the design. A good agreement between simulations and measurements is obtained. The design owns the merits of low cost, simple design and ease in fabrication and conformation, thus provides a new idea for end-fire antenna gain and directivity improvement. The new antenna has great potentials in applications.
A demodulation method of high-speed fiber Bragg grating based on dispersion-compensating fiber
Authors: Li Zheng-Ying  , Sun Wen-Feng  , Li Zi-Mo  , Wang Hong-Hai  ,   National Engineering Laboratory for Fiber Optic Sensing Technology, Wuhan University of Technology, Wuhan 430070, China;,   Key Laboratory of Optical Fiber Sensing Technology and Information Processing, Ministry of Education, Wuhan University of Technology, Wuhan 430070, China
Published: December 2020

Fiber Bragg grating sensing is one of the most attractive researches in the field of optical fiber sensing. It has made considerable progress due to its advantages in high multiplexing, high precision, small size, light weight, good corrosion resistance and immunity to electromagnetic interference. However, the traditional fiber Bragg grating demodulation technology can hardly achieve high-speed demodulation of multiplexing gratings, which seriously limits its extensive application. A novel high-speed fiber Bragg grating demodulation method is proposed and demonstrated in this paper. Large dispersion will be generated when light going through the long-distance dispersion compensation fiber. Based on the dispersion effect of dispersion compensation fiber, a light beam of different wavelength will generate different time delay, and the wavelength shift of the fiber Bragg grating sensor is then transformed into time domain, and ultimately the fiber Bragg grating wavelength demodulation can be realized by measuring the delay of grating reflective light pulse. The reflective light pulse train of all the cascade fiber Bragg grating sensors can be obtained only through one pulse of light source. This method can be applied in all-fiber structure without wavelength scanning so that it can promote the demodulation speed greatly and can be applied to the demodulation of quasi-distributed fiber Bragg grating sensor network. Disturbing influence of dispersion compensating fibers can be eliminated by introducing the reference grating, and the demodulation process is immune to light intensity disturbance. A test system is set up to demodulate a quasi-distributed sensor network which is made up of three fiber Bragg grating sensors. Results show that the linearity of the demodulated wavelength is good and the demodulation speed can be up to 1 MHz. The demodulation linearity is about 0.9969, and the error is about 27.8 pm after 10 times average. The novel demodulation method proposed in this paper has been tested through theoretical analysis and experimental demonstration, its feasibility to realize high-speed demodulation of fiber grating has been proved, but significant improvements still can be made in the demodulation system. The next step of research work will focus on how to realize decoupling between the location information and the wavelength, to avoid the influence of temperature disturbance on wavelength demodulation, so as to further improve the wavelength resolution and demodulation accuracy.
Coupled oscillation of bubbles in a spherical bubble cluster
Authors: Wang Cheng-Hui  , Mo Run-Yang  , Hu Jing  , Chen Shi  ,   Shaanxi Key Laboratory of Ultrasonics, Shaanxi Normal University, Xi'an 710062, China
Published: December 2020

The pressure wave emitted by a pulsating bubble affects the motions of other bubbles, so in an acoustic field bubbles are in a state of coupled oscillation. In this paper, a cluster with cavitation bubbles inside is considered, and a mathematical model is developed to describe the dynamics of the bubbles of the same radius inside a spherical cluster when the effects of coupled oscillation are included. Based on this new model, the nonlinear acoustic response of cavitation bubbles is analyzed numerically. Comparison of our model with those in the literature, shows that bubbles are suppressed heavily. Because of the coupled oscillations of bubbles, the motions of a bubble are affected by more constraints in the system, which cause the decrease of natural frequency of the bubbles. The nonlinear acoustical response of bubbles is improved by the coupled oscillation in a bubble cluster. With the rise in number density of the cluster, the suppression of bubble oscillation is enhanced. For a cluster of 1 mm radius, when the bubble number is below 500, the change of bubble number may cause a sharp decrease of maximum radial displacement of the bubbles. In cavitation region, there are bubble clusters and large-sized bubble, and the moving large bubble can absorb small bubbles from the surface of bubble cluster, so the bubble numbers inside a cluster varies with time, which may change the acoustic response of coupled oscillating bubbles. The increase of the liquid static pressure can suppress the oscillation of bubbles too, and there is a sensitive region (1-2 atm) that affects remarkably the acoustical response of bubbles. Driving ultrasound can affect the motion of bubble greatly. The range of cavitation bubble size is narrowed when the wave frequency increases. The bubbles whose initial radii are close to 5 m are easy to be activated by ultrasound under given acoustic conditions, i.e. sizes of bubble cluster, surrounding liquid and inner gas. The cluster oscillation of bubbles may suppress the motion of individual bubbles, and weaken the cavition effects caused by individual bubbles. However, the collapse time of the bubbles may be delayed, and the cavitation region may become larger than that for a single bubble. As a result, cavitation effects are amplified in the cluster region.
A gradient representation of holonomic system in the event space
Authors: Wu Hui-Bin  , Mei Feng-Xiang  ,   School of Mathematics, Beijing Institute of Technology, Beijing 100081, China;,   School of Aerospace Engineering, Beijing Institute of Technology, Beijing 100081, China
Published: December 2020

The dynamics research in the event space has important geometric and mechanical meanings, and great progress has been made in this field. A gradient system is a kind of important systems in differential equations and dynamical systems, and is receiving more and more attention. In this paper, a gradient representation and a fractional gradient representation of a holonomic system in the event space are studied. First, the differential equations of motion for the system are established and expressed in the first order form. Second, we have obtained the condition under which the system can be considered as a gradient system and also the condition under which the system can be considered as a fractional gradient system. When a constrained mechanical system is transformed into a gradient system or a fractional gradient system, one can use the properties of the gradient system or the fractional gradient system to study the integration and the stability of a constrained mechanical system. Finally, two examples are given to illustrate the application of the results. The event space is known as more extensive than the configuration space, therefore, the result in the configuration space is a special case of this paper.
Horizontal jet characteristics of an underwater solid rocket motor at the beginning of working
Authors: Tang Yun-Long  , Li Shi-Peng  , Liu Zhu  , Sui Xin  , Wang Ning-Fei  ,   School of Aerospace Engineering, Beijing Institute of Technology, Beijing 100081, China
Published: December 2020

Compared with the conventional solid rocket motor which works in atmosphere, the underwater solid rocket motor has different performance characteristics, because the buoyancy of water cannot be neglected, while in the atmosphere only gravity is present. In order to get a deep understanding of the mechanism of thrust oscillation characteristics and of the jet structure of underwater solid rocket motor, a 3-D numerical simulation using the volume of fluid (VOF) model is given in this paper to investigate the horizontal jet structure and the oscillating flow field induced by the gravity and buoyance. The principle of momentum is carried out to analyze the oscillating flow field in the initial working period. Result of analysis indicates that the behavior of rising gas bubbles is remarkably influenced by gravity and buoyancy. Compared to the conventional solid rocket motor in the atmosphere environment, the phenomenon shows that the rising gas bubbles are more obvious in the water. It is concluded that thrust-gravity and buoyancy coupling can generate bigger roll moment in the vertical direction than the conventional solid rocket motor. Numerical calculations in conjunction with experiment in the references proves that the model contains gravity, buoyancy and the VOF model can be used to describe the jet structure and thrust oscillation of underwater solid rocket motor; gravity and buoyancy cannot be ignored in the research of underwater solid rocket motor performance characteristics.
Experimental research on laser-produced Gd target plasma source for 6.7 nm lithography
Authors: Dou Yin-Ping  , Xie Zhuo  , Song Xiao-Lin  , Tian Yong  , Lin Jing-Quan  ,   School of Science, Changchun University of Science and Technology, Changchun 130022, China
Published: December 2020

Extreme ultraviolet (EUV) lithography at λ =6.7 nm is a challenging subject for next generation semiconductor lithography beyond 13.5 nm. The availability of strong radiation at the operating wavelength and low-debris of the plasma source are the two most important aspects for the development of laser-produced Gd plasma source at 6.7 nm. In this paper, experimental research on the extreme ultraviolet source based on the laser-produced Gd plasma is performed. Strong radiation near 6.7 nm from the source has been obtained, which is attributed to the n=4-n=4 transitions in Gd ions that overlap to yield an intense unresolved transition array (UTA). Dependence of spectral variation near the strong emission region of Gd plasma on the incident laser power density and detection angles is given. It is found that the intensity of EUV radiation around 6.7 nm is increased with increasing laser power density, and the emission peak around 7.1 nm increases faster than that of emission peak around 6.7 nm after the laser intensity reaching 6.4×1011 W/cm2, which is ascribed to the unique spectroscopic behavior of Gd ions. In addition, the energy of the ion debris from laser-produced Gd plasma source as well as the angular distribution of the ion yield off the target normal are measured with Faraday cup. Results show that the ion energy corresponding to the peak position of Gd ion energy distribution is about 2.6 keV at 10° off the target normal, and the yield of Gd ions decreases with the increase of the angle from the target normal. Furthermore, the stopping ability of an ambient magnetic field for ion debris from laser Gd plasma source is evaluated, and the result shows that the energetic Gd ion can be effectively mitigated by applying a 0.9 T magnetic field.
Relaxation of granular elasticity
Authors: Sun Qi-Cheng  , Liu Chuan-Qi  , Gordon G D Zhou  ,   State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China;,   Institute of Mountain Hazards and Environment, Chinese Academy of Sciences & Ministry of Water Conservancy, Chengdu 610041, China
Published: December 2020

In granular materials, particles constitute a complex force chains network through contact with each other, and elastic energies are stored due to deformation of particles. This elastic behavior is macroscopic manifestation of inter-particle contacts. Elastic constants or elastic moduli are of fundamental importance for granular material. Due to the hyper-static property of inter-particle forces, the bulk elastic energy stored in the contacts is metastable in the viewpoint of energy landscape, i.e. a high energy state may approaches a more stable state (i.e. relatively lower state) under the action of external perturbations or internal stress, resulting in the elastic modulus reduction. This process is the so-called elasticity relaxation. It may be more obvious in granular materials.The time-dependent behavior of granular materials, especially the creep, has been studied in experiments and numerical simulations, while the stress relaxation has few reported investigations. Stress relaxation is defined as the process in vohich the initial strain is maintained and the stress decays with the time. From energetic viewpoint, elastic energy is stored in the deformation of particles. The granular system is in a metastable state when confined in a state easy to break the balance. Generally speaking, the shape and grading of particles, volume fraction, surface friction properties, initial structure features, ageing time, loading strain rate will all play important roles in stress relaxation.In this work, it is believed that the elastic relaxation is the only mechanism to describe the stress relaxation, and the mechanism of it is analyzed from the viewpoint of the potential energy surface. Stress relaxation is calculated by means of the so-called two-granular temperature theory (TGT) we developed previously (Sun Q et al. 2015 Sci. Rep. 5 9652). The stress decays fast at the beginning, then decreases gradually slowly to a stable value. The logarithmic fit is first proposed to describe the stress decay in the compressed system. Calculated results of stress relaxation match well with the measured results in a recently published paper (Miksic A, Alava M J 2013 Phys. Rev. E 88 032207). Both elastic energy and granular temperature may be reduced with increasing time. It is found that the initial value of the granular temperature has a great influence on the stress relaxation, and at present its effect is input by trial and error. It would be a major problem how to determine the initial value of the granular temperature. Moreover, the relaxation coefficient of elastic stress is basically chosen as a function of granular temperature which is described by the Arrhenius equation that need be further investigated.
The multi-scale theoretical models of Young's modulus of doped monocrystalline silicon nano-film
Authors: Lv Huan-Ling  , Wang Jing  ,   College of Physical Science and Technology, XinJiang University, Urumqi 830046, China
Published: December 2020

The study of physical properties of silicon nano-materials is very important for its application in semiconductor technology. Doping is beneficial to improving the physical properties of silicon nano-materials, it can improve the application value as well. Young's modulus of the crystal in the direction of [100] of the doped silicon nano-film is studied by an analytical model, which is based on the semi-continuum approach. In the model, the strain energy is obtained from the Keating strain energy model. The relationship between the Young's modulus and film thickness are also discussed. Results show that the Young's modulus decreases with the increase of the thickness of the silicon film, especially with the small size; the variation tendency of the Young's modulus of doped silicon films is similar to the pure silicon film. And the Young's modulus decreases as the doping concentration decreases for different doping position. Neither the doping concentration nor the doping position, it is the thickness that shows the most important effect on the Young's modulus of the doped silicon nano-film. Findings in this paper may serve as a reference for similar study, and can offer a totally new idea of the doped monocrystalline silicon materials as well.
Research of coupling behavior based on series-parallel flux-controlled memristor
Authors: Wang Yan  , Yang Jiu  , Wang Li-Dan  , Duan Shu-Kai  ,   School of Electronic and Information Engineering, Southwest University, Chongqing 400715, China
Published: December 2020

Memristor is a nanoscale element with low power consumption and high integration, having great potential in applications. A single memristor has rich electrical properties, and its series-parallel circuit exhibits more abundant dynamic behaviors. However, memristors' coupled effects cannot be ignored in high-density integrated environment. Therefore, this paper first deduces the mathematical model of coupled memristor in detail based on the coupled flux controlled memristors. Second, considering the different polarity connection and coupling strength, we discuss the coupled condition of two flux-controlled memristors in series and parallel connections. Then the detailed theoretical analysis is illustrated, and the variation of memristance in terms of voltage, time and flux as well as the relations between voltage and currents are examined via numerical simulations to further explore the influence of coupled effects on the memristive system. At the same time, a graphical user interface of series-parallel coupled circuit based on Matlab is designed. Through this interface, we can adjust the initial value of memristor and coupling coefficient, select different connection modes, obtain corresponding connection diagram and output waveform which intuitively show the dynamic behavior of different parameters directly and provide experimental reference for further study of the circuit design. Furthermore, this paper shows the influence of initial value on the normal working range of memristors in the presence of coupling. From the table 1 it can be easily obtained that when the memristors are connected in the same direction, the range of memristance without coupling is greater than that with coupling. And the situation is opposite when the memristors are connected in different directions. Finally, the hysteresis curve with different coupling coefficients and the change of memristance with time are shown via building the Pspice simulator of coupled memristors, so the coupling effects of memristor is confirmed by circuit simulations. Experimental results reflect that the coupling with the same polarity enhances the change of resistance, and the coupling with different polarity with slow down it. Such dynamical properties can be well utilized in memristive networks and provide a strong theoretical basis for the comprehensive consideration of the design of memristive system.