This study utilizes a visualization nozzle and spray experimental platform to experimentally investigate the flow focusing/blurring nozzle.It is found that the working mode of the nozzle transitions from flow focusing...This study utilizes a visualization nozzle and spray experimental platform to experimentally investigate the flow focusing/blurring nozzle.It is found that the working mode of the nozzle transitions from flow focusing to flow transition and eventually to flow blurring as the gas flow rate increases or the tube hole distance decreases.Conversely,an increase in liquid flow rate only facilitates the transition from flow focusing to flow transition.Changes in the gas/liquid flow rate or tube hole distance influence the gas shear effect and the gas inertial impact effect inside the nozzle,which in turn alters the working mode.An increase in gas flow rate results in a shift of the droplet size distribution towards smaller particle sizes in the flow blurring mode,whereas an increase in liquid flow rate produces the opposite effect.Notably,the impact of the gas flow rate on these changes is more pronounced than that of the liquid flow rate.展开更多
Reducing the defocus blur that arises from the finite aperture size and short exposure time is an essential problem in computational photography.It is very challenging because the blur kernel is spatially varying and ...Reducing the defocus blur that arises from the finite aperture size and short exposure time is an essential problem in computational photography.It is very challenging because the blur kernel is spatially varying and difficult to estimate by traditional methods.Due to its great breakthrough in low-level tasks,convolutional neural networks(CNNs)have been introdu-ced to the defocus deblurring problem and achieved significant progress.However,previous methods apply the same learned kernel for different regions of the defocus blurred images,thus it is difficult to handle nonuniform blurred images.To this end,this study designs a novel blur-aware multi-branch network(Ba-MBNet),in which different regions are treated differentially.In particular,we estimate the blur amounts of different regions by the internal geometric constraint of the dual-pixel(DP)data,which measures the defocus disparity between the left and right views.Based on the assumption that different image regions with different blur amounts have different deblurring difficulties,we leverage different networks with different capacities to treat different image regions.Moreover,we introduce a meta-learning defocus mask generation algorithm to assign each pixel to a proper branch.In this way,we can expect to maintain the information of the clear regions well while recovering the missing details of the blurred regions.Both quantitative and qualitative experiments demonstrate that our BaMBNet outperforms the state-of-the-art(SOTA)methods.For the dual-pixel defocus deblurring(DPD)-blur dataset,the proposed BaMBNet achieves 1.20 dB gain over the previous SOTA method in term of peak signal-to-noise ratio(PSNR)and reduces learnable parameters by 85%.The details of the code and dataset are available at https://github.com/junjun-jiang/BaMBNet.展开更多
Efficient elastic wave focusing is crucial in materials and physical engineering.Elastic coding metasurfaces,which are innovative planar artificial structures,show great potential for use in the field of wave focusing...Efficient elastic wave focusing is crucial in materials and physical engineering.Elastic coding metasurfaces,which are innovative planar artificial structures,show great potential for use in the field of wave focusing.However,elastic coding lenses(ECLs)still suffer from low focusing performance,thickness comparable to wavelength,and frequency sensitivity.Here,we consider both the structural and material properties of the coding unit,thus realizing further compression of the thickness of the ECL.We chose the simplest ECL,which consists of only two encoding units.The coding unit 0 is a straight structure constructed using a carbon fiber reinforced composite material,and the coding unit 1 is a zigzag structure constructed using an aluminum material,and the thickness of the ECL constructed using them is only 1/8 of the wavelength.Based on the theoretical design,the arrangement of coding units is further optimized using genetic algorithms,which significantly improves the focusing performance of the lens at different focus and frequencies.This study provides a more effective way to control vibration and noise in advanced structures.展开更多
Dynamically tunable terahertz(THz)beam focusing plays a critical role in emerging applications including reconfigurable imaging,localized spectral analysis,and micro-machining.Conventional methods,however,frequently e...Dynamically tunable terahertz(THz)beam focusing plays a critical role in emerging applications including reconfigurable imaging,localized spectral analysis,and micro-machining.Conventional methods,however,frequently employ complex wavefront modulators and external control algorithms,resulting in increased system footprint and limited tuning efficiency.In this work,we present an all-silicon mechanically rotatable cascaded metasurface capable of dynamic THz beam focusing.By independently adjusting the relative rotation angles between the two metasurface layers,real-time repositioning of the focal spot is achieved for orthogonal circular polarization channels.The proposed design facilitates polarization-division multiplexing without requiring external algorithms or active materials while preserving high focusing efficiency and beam quality across a predefined focal plane.Numerical simulations reveal a quasi-linear shift of the focal position with the rotation angle,with stable focusing efficiency and full-width at half-maximum observed in both polarization channels.This strategy offers an efficient and reliable approach to dynamic wavefront control for compact,reconfigurable THz imaging,sensing,and communication systems.展开更多
Adjustable or programmable metamaterials offer versatile functions,while the complex multi-dimensional regulation increases workload,and hinders their applications in practical scenarios.To address these challenges,we...Adjustable or programmable metamaterials offer versatile functions,while the complex multi-dimensional regulation increases workload,and hinders their applications in practical scenarios.To address these challenges,we present a mechanically programmable acoustic metamaterial for real-time focal tuning via one-dimensional phase-gradient modulation in this paper.The device integrates a phase gradient structure with concave cavity channels and an x-shaped telescopic mechanical framework,enabling dynamic adjustment of inter-unit spacing(1 mm-3 mm)through a microcontroller-driven motor.By modulating the spacing between adjacent channels,the phase gradient is precisely controlled,allowing continuous focal shift from 50 mm to 300 mm along the x-axis at 7500 Hz.Broadband focusing is also discussed in the range6800 Hz-8100 Hz,with transmission coefficients exceeding 0.5,ensuring high efficiency and robust performance.Experimental results align closely with simulations,validating the design's effectiveness and adaptability.Unlike conventional programmable metamaterials requiring multi-dimensional parameter optimization,this approach simplifies real-time control through single-axis mechanical adjustment,significantly reducing operational complexity.Due to the advantages of broadband focusing,simple control mode,real-time monitoring,and so on,the device may have extensive applications in the fields of acoustic imaging,nondestructive testing,ultrasound medical treatment,etc.展开更多
Hall thrusters with large height-radius ratio,owing to their unique advantages in compactness,lightweight,and high performance,have progressively emerged as a preferred choice for diverse space propulsion applications...Hall thrusters with large height-radius ratio,owing to their unique advantages in compactness,lightweight,and high performance,have progressively emerged as a preferred choice for diverse space propulsion applications in the future.However,the amplification of the annular effect in structures with a large height-radius ratio poses a practical problem of plume over-focusing,which seriously restricts the further improvement of Hall thruster performance and the extension of its life.In this study,the formation mechanism of over-focused plume is deeply investigated,and it is ascertained that an intensified radial electric field directed towards the inner wall within the channel serves as a key contributing factor.This phenomenon is fundamentally attributed to structural characteristics of large height-radius ratio that induce pronounced inward inclination of field lines within strong magnetic field zone.Based on this,the design concept of focused magnetic field is proposed,wherein straight magnetic field lines are established within the strong magnetic field zone to generate a quasi-axial accelerating electric field.Simultaneously,the symmetrical magnetic field inside the channel ensures ionization concentration near the channel center,thereby achieving optimal matching between the ionization zone and accelerating field.Experimental results demonstrate that employing a focused magnetic field significantly reduces the divergence half-angle of the plume and yields an excellently barrel-shaped focusing plume morphology in HEP-1350PM.Consequently,the total efficiency of the thruster surpasses 60%,while erosion belt on the inner wall is shortened by nearly 50%.These advancements effectively enhance thruster performance and prolong its operational lifespan.This study can not only resolve practical problems associated with plume over-focusing,but also provide a fundamental guiding principle for magnetic field design of Hall thrusters.展开更多
Background For static scenes with multiple depth layers,existing defocused image deblurring methods have the problems of edge-ringing artifacts or insufficient deblurring owing to inaccurate estimation of the blur amo...Background For static scenes with multiple depth layers,existing defocused image deblurring methods have the problems of edge-ringing artifacts or insufficient deblurring owing to inaccurate estimation of the blur amount,and prior knowledge in nonblind deconvolution is not strong,which leads to image detail recovery challenges.Methods To this end,this study proposes a blur map estimation method for defocused images based on the gradient difference of the boundary neighborhood,which uses the gradient difference of the boundary neighborhood to accurately obtain the amount of blurring,thereby preventing boundary ringing artifacts.The obtained blur map is then used for blur detection to determine whether the image needs to be deblurred,thereby improving the efficiency of deblurring without manual intervention and judgment.Finally,a nonblind deconvolution algorithm was designed to achieve image deblurring based on the blur amount selection strategy and sparse prior.Results Experimental results showed that our method improves PSNR(Peak Signal-to-Noise Ratio)and SSIM(Structural Similarity Index)by an average of 4.6%and 7.3%,respectively,compared to existing methods.Conclusions Experimental results showed that the proposed method outperforms existing methods.Compared to existing methods,our method can better solve the problems of boundary ringing artifacts and detail information preservation in defocused image deblurring.展开更多
Although previous studies have demonstrated that transcranial focused ultrasound stimulation protects the ischemic brain,clear criteria for the stimulation time window and intensity are lacking.Electrical impedance to...Although previous studies have demonstrated that transcranial focused ultrasound stimulation protects the ischemic brain,clear criteria for the stimulation time window and intensity are lacking.Electrical impedance tomography enables real-time monitoring of changes in cerebral blood perfusion within the ischemic brain,but investigating the feasibility of using this method to assess post-stroke rehabilitation in vivo remains critical.In this study,ischemic stroke was induced in rats through middle cerebral artery occlusion surgery.Transcranial focused ultrasound stimulation was used to treat the rat model of ischemia,and electrical impedance tomography was used to measure impedance during both the acute stage of ischemia and the rehabilitation stage following the stimulation.Electrical impedance tomography results indicated that cerebral impedance increased after the onset of ischemia and decreased following transcranial focused ultrasound stimulation.Furthermore,the stimulation promoted motor function recovery,reduced cerebral infarction volume in the rat model of ischemic stroke,and induced the expression of brain-derived neurotrophic factor in the ischemic brain.Our results also revealed a significant correlation between the impedance of the ischemic brain post-intervention and improvements in behavioral scores and infarct volume.This study shows that daily administration of transcranial focused ultrasound stimulation for 20 minutes to the ischemic hemisphere 24 hours after cerebral ischemia enhanced motor recovery in a rat model of ischemia.Additionally,our findings indicate that electrical impedance tomography can serve as a valuable tool for quantitatively evaluating rehabilitation after ischemic stroke in vivo.These findings suggest the feasibility of using impedance data collected via electrical impedance tomography to clinically assess the effects of rehabilitatory interventions for patients with ischemic stroke.展开更多
Computational simulations on structurally different detonation generator are carried out to study the phenomena,the mechanism and the gas dynamics characteristics of flame implosion and shock wave focusing.Two-dimensi...Computational simulations on structurally different detonation generator are carried out to study the phenomena,the mechanism and the gas dynamics characteristics of flame implosion and shock wave focusing.Two-dimensional axisymmetric and unsteady Navier-Stokes equations are numerically solved and detailed chemical reaction kinetics of hydrogen/air mixture is used.The simulation results show that the laminar flame generated by low energy spark in the jet flame burner is accelerated under the narrow channel,the jet flame impinging on the axis strengthens shock wave and the shock wave enhances flame acceleration.Under the function of multiple shock waves and flame,a number of hot spots appear between the wave and the surface.The spots enlarge rapidly,thus forming an over-drive detonation with high pressure,and then declining to stable detonation.Through calculation and analysis,the length of detonation initiation and stable detonation are obtained,thus providing the useful information for further experimental investigations.展开更多
A numerical simulation of the toroidal shock wave focusing in a co-axial cylindrical shock tube is inves- tigated by using discontinuous Galerkin （DG） finite element method to solve the axisymmetric Euler equations....A numerical simulation of the toroidal shock wave focusing in a co-axial cylindrical shock tube is inves- tigated by using discontinuous Galerkin （DG） finite element method to solve the axisymmetric Euler equations. For validating the numerical method, the shock-tube problem with exact solution is computed, and the computed results agree well with the exact cases. Then, several cases with higher incident Mach numbers varying from 2.0 to 5.0 are simulated. Simulation results show that complicated flow-field structures of toroidal shock wave diffraction, reflection, and focusing in a co-axial cylindrical shock tube can be obtained at different incident Mach numbers and the numerical solutions appear steep gradients near the focusing point, which illustrates the DG method has higher accuracy and better resolution near the discontinuous point. Moreover, the focusing peak pres- sure with different grid scales is compared.展开更多
An experimental scheme for the generation of directional focusing waves in a wave basin is established in this paper. The effects of the directional range, frequency width and center frequency on the wave focusing are...An experimental scheme for the generation of directional focusing waves in a wave basin is established in this paper. The effects of the directional range, frequency width and center frequency on the wave focusing are studied. The distribution of maximum amplitude and the evolution of time series and spectra during wave packet propagation and the variation of water surface parameters are extensively investigated. The results reveal that the characteristics of focusing waves are significantly influenced by wave directionality and that the breaking criteria for directional waves are distinctly different from those for unidirectional waves.展开更多
The degraded parameters recognition is very important for the restoration of blurred images. There are two common types of blurs for most camera systems. One is the defocus blur due to the optical system's defocus...The degraded parameters recognition is very important for the restoration of blurred images. There are two common types of blurs for most camera systems. One is the defocus blur due to the optical system's defocus phenomenon and the other is the motion blur due to the relative movement between the objectives and the camera. Compared with the recognition for the blurred image with only one blur model, the parameter estimation for the picture combining defocus and motion blur models is a more complicated mission. A method was proposed for computer to estimate the parameters of defocus blur and motion blur in cepstrum area simultaneously. According to characters of both blur models in the frequency domain, an adjustment approach was suggested in the frequency area and then convert to the cepstrum field to increase the accuracy of measurement.展开更多
This paper presents a method for the automatic adjustment of the laser defocusing amount in micro-laser-induced breakdown spectroscopy. A microscopic optical imaging system consisting of a CCD camera and a 20× ob...This paper presents a method for the automatic adjustment of the laser defocusing amount in micro-laser-induced breakdown spectroscopy. A microscopic optical imaging system consisting of a CCD camera and a 20× objective lens was adopted to realize the method. The real-time auto-focusing of the system was achieved by detecting the effective pixels of the light spot generated by the laser pointer. The focusing accuracy of the method could achieve 3 μm. The element concentrations of Mn and Ni in low-alloy steels were analyzed at a crater diameter of about 35 μm using the presented method. After using the presented method, the determination coefficients of Mn and Ni both exceeded 0.997, with the root-mean-square errors being 0.0133 and 0.0395, respectively. Scanning analysis was performed on the inclined plane and the curved surface by means of focusing control and non-focusing control. Ten characteristic spectral lines of Fe were selected as the analysis lines. With the focusing control, the average relative standard deviations obtained on the inclined plane and curved surface were both less than 5%, and much less than the values without focusing control, 14.6% and 40.39%.展开更多
Four focusing models for generation of freak waves are presented. An extreme wave focusing model is presented on the basis of the enhanced High-Order Spectral （HOS） method and the importance of the nonlinear wave-wa...Four focusing models for generation of freak waves are presented. An extreme wave focusing model is presented on the basis of the enhanced High-Order Spectral （HOS） method and the importance of the nonlinear wave-wave interaction is evaluated by comparison of the calculated results with experimental and theoretical data. Based on the modification of the Longuet-Higgins model, four wave models for generation of freak waves （a. extreme wave model ＋ random wave model; b. extreme wave model ＋ regular wave model; e. phase interval modulation wave focusing model; d. number modulation wave focusing model with the same phase） are proposed. By use of different energy distribution techniques in the four models, freak wave events are obtained with different Hmax/Hs in finite space and time.展开更多
Two-dimensional focusing waves are generated and investigated by numerical method. The numerical model is developed by introducing the wave maker boundary on the high-order spectral （HOS） method proposed by Dommermu...Two-dimensional focusing waves are generated and investigated by numerical method. The numerical model is developed by introducing the wave maker boundary on the high-order spectral （HOS） method proposed by Dommermuth and Yue in 1987 and verified by theoretical and experimental data. Some cases of focusing waves considering different parameters such as assumed focusing amplitudes, frequency bandwidth, central frequency and frequency spectrum are generated. Characteristics of the focusing wave including surface elevations, the maximum crest, shift of focusing points and frequency spectra are discussed. The results show that the focusing wave characteristics are strongly affected by focusing amplitudes, frequency bandwidth, central frequency and frequency spectrum.展开更多
The flow focusing nozzle is a new type of nozzle that performs effective atomization of the discrete phase by means of high-speed motion of the continuous phase.The flow pattern and its morphological changes have a si...The flow focusing nozzle is a new type of nozzle that performs effective atomization of the discrete phase by means of high-speed motion of the continuous phase.The flow pattern and its morphological changes have a significant effect on the atomization, but the influence of different parameters on the morphological change of the flow pattern remains unclear.The flow focusing pattern and morphological changes in the two-phase flow inside the nozzle were simulated numerically, based on the volume of fluid method.The results demonstrate that the ratio of the nozzle-to-capillary distance and capillary diameter, the gas–liquid velocity ratio, and capillary diameter have significant effects on the flow pattern.When the ratio of the nozzle-to-capillary distance H and capillary diameter D increases, or the capillary diameter D increases, the flow pattern tends to transform into a laminar form; however, when the gas–liquid velocity ratio V increases, the flow pattern tends to transform into a turbulence form.Furthermore, we define the cone-shaped expansion rate, cone-shaped focusing rate,and cone angle in order to study the morphological changes in the cone shape inside the nozzle.The results indicate that the morphological change of the cone shape and flow pattern transformation is interrelated.When the cone shape tends to be unstable, the flow pattern changes towards flow blurring, whereas, a stable cone indicates that the flow tends to exhibit a droplet pattern.展开更多
According to intensity distribution of the synchrotron radiation source focused by a toroidal mirror at the Beijing synchrotron radiation biological macromolecule station, theoretical modeling of the Beijing synchrotr...According to intensity distribution of the synchrotron radiation source focused by a toroidal mirror at the Beijing synchrotron radiation biological macromolecule station, theoretical modeling of the Beijing synchrotron radiation source is developed for capillary optics. Using this theoretical modeling, the influences of the configuration curve of the polycapillary X-ray lens on transmission efficiency and working distance are analyzed. The experimental results of the transmission efficiency and working distance at the biological macromolecule station are in good agreement with the theoretical results.展开更多
We extend Bespalov-Talanov (B-T) theory on small-scale self-focusing (SSSF) to include vector effect of a very narrow intense laser beam with application of the vector self-focusing model. The gain spectrum for pertur...We extend Bespalov-Talanov (B-T) theory on small-scale self-focusing (SSSF) to include vector effect of a very narrow intense laser beam with application of the vector self-focusing model. The gain spectrum for perturbations is obtained by using the standard linear instability analysis. It is shown that the influence on SSSF of vector effect is closely related to the beam width. For a very narrow beam, the role played by vector effect becomes significant, it reduces the fastest growing frequency and the maximum growth rate, and shortens the frequency range for perturbation growing, and thus deviates the gain spectrum from that of B-T theory.展开更多
We present a new model of an electron gun for generating subrelativistic femtosecond （fs） electron pulses. The basic idea is to utilize a dc acceleration stage combined with a time focusing region, the time focusing...We present a new model of an electron gun for generating subrelativistic femtosecond （fs） electron pulses. The basic idea is to utilize a dc acceleration stage combined with a time focusing region, the time focusing electrode generates an electron energy chirp for bunching at the target. Without considering the space charge effects, simulations of the electron gun were carried out under the conditions of different dc voltages and various slopes of the voltage added on the time focusing electrode. Tracing and simulating large numbers of photoelectrons through Monte-Carlo and finite difference methods, the electron pulses with 1 ps can be compressed to 55 fs, which will allow significant advances in the field of ultrafast diagnosis.展开更多
基金the National Natural Science Foundation of China(52276026).
文摘This study utilizes a visualization nozzle and spray experimental platform to experimentally investigate the flow focusing/blurring nozzle.It is found that the working mode of the nozzle transitions from flow focusing to flow transition and eventually to flow blurring as the gas flow rate increases or the tube hole distance decreases.Conversely,an increase in liquid flow rate only facilitates the transition from flow focusing to flow transition.Changes in the gas/liquid flow rate or tube hole distance influence the gas shear effect and the gas inertial impact effect inside the nozzle,which in turn alters the working mode.An increase in gas flow rate results in a shift of the droplet size distribution towards smaller particle sizes in the flow blurring mode,whereas an increase in liquid flow rate produces the opposite effect.Notably,the impact of the gas flow rate on these changes is more pronounced than that of the liquid flow rate.
基金supported by the National Natural Science Foundation of China (61971165, 61922027, 61773295)in part by the Fundamental Research Funds for the Central Universities (FRFCU5710050119)+1 种基金the Natural Science Foundation of Heilongjiang Province(YQ2020F004)the Chinese Association for Artificial Intelligence(CAAI)-Huawei Mind Spore Open Fund
文摘Reducing the defocus blur that arises from the finite aperture size and short exposure time is an essential problem in computational photography.It is very challenging because the blur kernel is spatially varying and difficult to estimate by traditional methods.Due to its great breakthrough in low-level tasks,convolutional neural networks(CNNs)have been introdu-ced to the defocus deblurring problem and achieved significant progress.However,previous methods apply the same learned kernel for different regions of the defocus blurred images,thus it is difficult to handle nonuniform blurred images.To this end,this study designs a novel blur-aware multi-branch network(Ba-MBNet),in which different regions are treated differentially.In particular,we estimate the blur amounts of different regions by the internal geometric constraint of the dual-pixel(DP)data,which measures the defocus disparity between the left and right views.Based on the assumption that different image regions with different blur amounts have different deblurring difficulties,we leverage different networks with different capacities to treat different image regions.Moreover,we introduce a meta-learning defocus mask generation algorithm to assign each pixel to a proper branch.In this way,we can expect to maintain the information of the clear regions well while recovering the missing details of the blurred regions.Both quantitative and qualitative experiments demonstrate that our BaMBNet outperforms the state-of-the-art(SOTA)methods.For the dual-pixel defocus deblurring(DPD)-blur dataset,the proposed BaMBNet achieves 1.20 dB gain over the previous SOTA method in term of peak signal-to-noise ratio(PSNR)and reduces learnable parameters by 85%.The details of the code and dataset are available at https://github.com/junjun-jiang/BaMBNet.
基金Project supported by the National Natural Science Foundation of China(Grant No.12404531)the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province,China(Grant No.23KJB140011)。
文摘Efficient elastic wave focusing is crucial in materials and physical engineering.Elastic coding metasurfaces,which are innovative planar artificial structures,show great potential for use in the field of wave focusing.However,elastic coding lenses(ECLs)still suffer from low focusing performance,thickness comparable to wavelength,and frequency sensitivity.Here,we consider both the structural and material properties of the coding unit,thus realizing further compression of the thickness of the ECL.We chose the simplest ECL,which consists of only two encoding units.The coding unit 0 is a straight structure constructed using a carbon fiber reinforced composite material,and the coding unit 1 is a zigzag structure constructed using an aluminum material,and the thickness of the ECL constructed using them is only 1/8 of the wavelength.Based on the theoretical design,the arrangement of coding units is further optimized using genetic algorithms,which significantly improves the focusing performance of the lens at different focus and frequencies.This study provides a more effective way to control vibration and noise in advanced structures.
基金supported by the National Natural Science Foundation of China(Grants U22A2008,12404484,12464016,and 62405219)the Double First Class Joint Special Key Project of Yunnan Science and Technology Department and Yunnan University(Grant 202401BF070001-012)Sichuan Provincial Science and Technology Support Program(Grant 25QNJJ2419).
文摘Dynamically tunable terahertz(THz)beam focusing plays a critical role in emerging applications including reconfigurable imaging,localized spectral analysis,and micro-machining.Conventional methods,however,frequently employ complex wavefront modulators and external control algorithms,resulting in increased system footprint and limited tuning efficiency.In this work,we present an all-silicon mechanically rotatable cascaded metasurface capable of dynamic THz beam focusing.By independently adjusting the relative rotation angles between the two metasurface layers,real-time repositioning of the focal spot is achieved for orthogonal circular polarization channels.The proposed design facilitates polarization-division multiplexing without requiring external algorithms or active materials while preserving high focusing efficiency and beam quality across a predefined focal plane.Numerical simulations reveal a quasi-linear shift of the focal position with the rotation angle,with stable focusing efficiency and full-width at half-maximum observed in both polarization channels.This strategy offers an efficient and reliable approach to dynamic wavefront control for compact,reconfigurable THz imaging,sensing,and communication systems.
基金supported by the National Natural Science Foundation of China(Grant No.12374416)。
文摘Adjustable or programmable metamaterials offer versatile functions,while the complex multi-dimensional regulation increases workload,and hinders their applications in practical scenarios.To address these challenges,we present a mechanically programmable acoustic metamaterial for real-time focal tuning via one-dimensional phase-gradient modulation in this paper.The device integrates a phase gradient structure with concave cavity channels and an x-shaped telescopic mechanical framework,enabling dynamic adjustment of inter-unit spacing(1 mm-3 mm)through a microcontroller-driven motor.By modulating the spacing between adjacent channels,the phase gradient is precisely controlled,allowing continuous focal shift from 50 mm to 300 mm along the x-axis at 7500 Hz.Broadband focusing is also discussed in the range6800 Hz-8100 Hz,with transmission coefficients exceeding 0.5,ensuring high efficiency and robust performance.Experimental results align closely with simulations,validating the design's effectiveness and adaptability.Unlike conventional programmable metamaterials requiring multi-dimensional parameter optimization,this approach simplifies real-time control through single-axis mechanical adjustment,significantly reducing operational complexity.Due to the advantages of broadband focusing,simple control mode,real-time monitoring,and so on,the device may have extensive applications in the fields of acoustic imaging,nondestructive testing,ultrasound medical treatment,etc.
基金financial support from the National Key R&D Program of China(No.2022YFE0204100)the National Natural Science Foundation of China(Nos.U23B20152 and 52402479)。
文摘Hall thrusters with large height-radius ratio,owing to their unique advantages in compactness,lightweight,and high performance,have progressively emerged as a preferred choice for diverse space propulsion applications in the future.However,the amplification of the annular effect in structures with a large height-radius ratio poses a practical problem of plume over-focusing,which seriously restricts the further improvement of Hall thruster performance and the extension of its life.In this study,the formation mechanism of over-focused plume is deeply investigated,and it is ascertained that an intensified radial electric field directed towards the inner wall within the channel serves as a key contributing factor.This phenomenon is fundamentally attributed to structural characteristics of large height-radius ratio that induce pronounced inward inclination of field lines within strong magnetic field zone.Based on this,the design concept of focused magnetic field is proposed,wherein straight magnetic field lines are established within the strong magnetic field zone to generate a quasi-axial accelerating electric field.Simultaneously,the symmetrical magnetic field inside the channel ensures ionization concentration near the channel center,thereby achieving optimal matching between the ionization zone and accelerating field.Experimental results demonstrate that employing a focused magnetic field significantly reduces the divergence half-angle of the plume and yields an excellently barrel-shaped focusing plume morphology in HEP-1350PM.Consequently,the total efficiency of the thruster surpasses 60%,while erosion belt on the inner wall is shortened by nearly 50%.These advancements effectively enhance thruster performance and prolong its operational lifespan.This study can not only resolve practical problems associated with plume over-focusing,but also provide a fundamental guiding principle for magnetic field design of Hall thrusters.
基金Supported by the National Natural Science Foundation of China (62172190)the“Double Creation”Plan of Jiangsu Province (JSSCRC2021532)the“Taihu Talent-Innovative Leading Talent”Plan of Wuxi City (Certificate Date:202110)。
文摘Background For static scenes with multiple depth layers,existing defocused image deblurring methods have the problems of edge-ringing artifacts or insufficient deblurring owing to inaccurate estimation of the blur amount,and prior knowledge in nonblind deconvolution is not strong,which leads to image detail recovery challenges.Methods To this end,this study proposes a blur map estimation method for defocused images based on the gradient difference of the boundary neighborhood,which uses the gradient difference of the boundary neighborhood to accurately obtain the amount of blurring,thereby preventing boundary ringing artifacts.The obtained blur map is then used for blur detection to determine whether the image needs to be deblurred,thereby improving the efficiency of deblurring without manual intervention and judgment.Finally,a nonblind deconvolution algorithm was designed to achieve image deblurring based on the blur amount selection strategy and sparse prior.Results Experimental results showed that our method improves PSNR(Peak Signal-to-Noise Ratio)and SSIM(Structural Similarity Index)by an average of 4.6%and 7.3%,respectively,compared to existing methods.Conclusions Experimental results showed that the proposed method outperforms existing methods.Compared to existing methods,our method can better solve the problems of boundary ringing artifacts and detail information preservation in defocused image deblurring.
基金supported by the Fundamental Research Funds for the Central Universities,Nos.G2021KY05107,G2021KY05101the National Natural Science Foundation of China,Nos.32071316,32211530049+1 种基金the Natural Science Foundation of Shaanxi Province,No.2022-JM482the Education and Teaching Reform Funds for the Central Universities,No.23GZ230102(all to LL and HH).
文摘Although previous studies have demonstrated that transcranial focused ultrasound stimulation protects the ischemic brain,clear criteria for the stimulation time window and intensity are lacking.Electrical impedance tomography enables real-time monitoring of changes in cerebral blood perfusion within the ischemic brain,but investigating the feasibility of using this method to assess post-stroke rehabilitation in vivo remains critical.In this study,ischemic stroke was induced in rats through middle cerebral artery occlusion surgery.Transcranial focused ultrasound stimulation was used to treat the rat model of ischemia,and electrical impedance tomography was used to measure impedance during both the acute stage of ischemia and the rehabilitation stage following the stimulation.Electrical impedance tomography results indicated that cerebral impedance increased after the onset of ischemia and decreased following transcranial focused ultrasound stimulation.Furthermore,the stimulation promoted motor function recovery,reduced cerebral infarction volume in the rat model of ischemic stroke,and induced the expression of brain-derived neurotrophic factor in the ischemic brain.Our results also revealed a significant correlation between the impedance of the ischemic brain post-intervention and improvements in behavioral scores and infarct volume.This study shows that daily administration of transcranial focused ultrasound stimulation for 20 minutes to the ischemic hemisphere 24 hours after cerebral ischemia enhanced motor recovery in a rat model of ischemia.Additionally,our findings indicate that electrical impedance tomography can serve as a valuable tool for quantitatively evaluating rehabilitation after ischemic stroke in vivo.These findings suggest the feasibility of using impedance data collected via electrical impedance tomography to clinically assess the effects of rehabilitatory interventions for patients with ischemic stroke.
文摘Computational simulations on structurally different detonation generator are carried out to study the phenomena,the mechanism and the gas dynamics characteristics of flame implosion and shock wave focusing.Two-dimensional axisymmetric and unsteady Navier-Stokes equations are numerically solved and detailed chemical reaction kinetics of hydrogen/air mixture is used.The simulation results show that the laminar flame generated by low energy spark in the jet flame burner is accelerated under the narrow channel,the jet flame impinging on the axis strengthens shock wave and the shock wave enhances flame acceleration.Under the function of multiple shock waves and flame,a number of hot spots appear between the wave and the surface.The spots enlarge rapidly,thus forming an over-drive detonation with high pressure,and then declining to stable detonation.Through calculation and analysis,the length of detonation initiation and stable detonation are obtained,thus providing the useful information for further experimental investigations.
基金Supported by the National Natural Science Foundation of China(50976072,51106099,10902070)the Leading Academic Discipline Project of Shanghai Municipal Education Commission(J50501)the Science Foundation for the Excellent Youth Scholar of Higher Education of Shanghai(slg09003)~~
文摘A numerical simulation of the toroidal shock wave focusing in a co-axial cylindrical shock tube is inves- tigated by using discontinuous Galerkin （DG） finite element method to solve the axisymmetric Euler equations. For validating the numerical method, the shock-tube problem with exact solution is computed, and the computed results agree well with the exact cases. Then, several cases with higher incident Mach numbers varying from 2.0 to 5.0 are simulated. Simulation results show that complicated flow-field structures of toroidal shock wave diffraction, reflection, and focusing in a co-axial cylindrical shock tube can be obtained at different incident Mach numbers and the numerical solutions appear steep gradients near the focusing point, which illustrates the DG method has higher accuracy and better resolution near the discontinuous point. Moreover, the focusing peak pres- sure with different grid scales is compared.
基金This research was partially supported by the National Natural Science Foundation of China (Grant No. 50379002),the Korea Research Council of Public Science and Technology (Principal R&D Program) and Korea Ministry of Science and Technology (International Collaboration Research Program)
文摘An experimental scheme for the generation of directional focusing waves in a wave basin is established in this paper. The effects of the directional range, frequency width and center frequency on the wave focusing are studied. The distribution of maximum amplitude and the evolution of time series and spectra during wave packet propagation and the variation of water surface parameters are extensively investigated. The results reveal that the characteristics of focusing waves are significantly influenced by wave directionality and that the breaking criteria for directional waves are distinctly different from those for unidirectional waves.
基金The National Natural Science Foundation of China (No 30570485)
文摘The degraded parameters recognition is very important for the restoration of blurred images. There are two common types of blurs for most camera systems. One is the defocus blur due to the optical system's defocus phenomenon and the other is the motion blur due to the relative movement between the objectives and the camera. Compared with the recognition for the blurred image with only one blur model, the parameter estimation for the picture combining defocus and motion blur models is a more complicated mission. A method was proposed for computer to estimate the parameters of defocus blur and motion blur in cepstrum area simultaneously. According to characters of both blur models in the frequency domain, an adjustment approach was suggested in the frequency area and then convert to the cepstrum field to increase the accuracy of measurement.
基金supported by the National Key Research and Development Program of China (Grant No. 2017YFF0106202)National Natural Science Foundation of China (Grant No. 61473279)+1 种基金the Key Research Program of Frontier Sciences, CAS (Grant No. QYZDJ-SSW-JSC037)the Youth Innovation Promotion Association, CAS
文摘This paper presents a method for the automatic adjustment of the laser defocusing amount in micro-laser-induced breakdown spectroscopy. A microscopic optical imaging system consisting of a CCD camera and a 20× objective lens was adopted to realize the method. The real-time auto-focusing of the system was achieved by detecting the effective pixels of the light spot generated by the laser pointer. The focusing accuracy of the method could achieve 3 μm. The element concentrations of Mn and Ni in low-alloy steels were analyzed at a crater diameter of about 35 μm using the presented method. After using the presented method, the determination coefficients of Mn and Ni both exceeded 0.997, with the root-mean-square errors being 0.0133 and 0.0395, respectively. Scanning analysis was performed on the inclined plane and the curved surface by means of focusing control and non-focusing control. Ten characteristic spectral lines of Fe were selected as the analysis lines. With the focusing control, the average relative standard deviations obtained on the inclined plane and curved surface were both less than 5%, and much less than the values without focusing control, 14.6% and 40.39%.
基金supported by the National Natural Science Foundation of China (Grant No.50779004)
文摘Four focusing models for generation of freak waves are presented. An extreme wave focusing model is presented on the basis of the enhanced High-Order Spectral （HOS） method and the importance of the nonlinear wave-wave interaction is evaluated by comparison of the calculated results with experimental and theoretical data. Based on the modification of the Longuet-Higgins model, four wave models for generation of freak waves （a. extreme wave model ＋ random wave model; b. extreme wave model ＋ regular wave model; e. phase interval modulation wave focusing model; d. number modulation wave focusing model with the same phase） are proposed. By use of different energy distribution techniques in the four models, freak wave events are obtained with different Hmax/Hs in finite space and time.
基金This research was respectively supported by the National Natural Science Foundation of China(Grant No.50379002)Programfor New Century Excellent Talents in University(NCET-05-0282)the Korea ResearchCouncil of Public Science and Technology(Principal R&D Program)
文摘Two-dimensional focusing waves are generated and investigated by numerical method. The numerical model is developed by introducing the wave maker boundary on the high-order spectral （HOS） method proposed by Dommermuth and Yue in 1987 and verified by theoretical and experimental data. Some cases of focusing waves considering different parameters such as assumed focusing amplitudes, frequency bandwidth, central frequency and frequency spectrum are generated. Characteristics of the focusing wave including surface elevations, the maximum crest, shift of focusing points and frequency spectra are discussed. The results show that the focusing wave characteristics are strongly affected by focusing amplitudes, frequency bandwidth, central frequency and frequency spectrum.
基金Supported by the National Natural Science Foundation of China(51776016,51606006)Beijing Natural Science Foundation(3172025,3182030)+2 种基金National Key Research and Development Program(2017YFB0103401)National Engineering Laboratory for Mobile Source Emission Control Technology(NELMS2017A10)the Talents Foundation of Beijing Jiaotong University(2018RC017)
文摘The flow focusing nozzle is a new type of nozzle that performs effective atomization of the discrete phase by means of high-speed motion of the continuous phase.The flow pattern and its morphological changes have a significant effect on the atomization, but the influence of different parameters on the morphological change of the flow pattern remains unclear.The flow focusing pattern and morphological changes in the two-phase flow inside the nozzle were simulated numerically, based on the volume of fluid method.The results demonstrate that the ratio of the nozzle-to-capillary distance and capillary diameter, the gas–liquid velocity ratio, and capillary diameter have significant effects on the flow pattern.When the ratio of the nozzle-to-capillary distance H and capillary diameter D increases, or the capillary diameter D increases, the flow pattern tends to transform into a laminar form; however, when the gas–liquid velocity ratio V increases, the flow pattern tends to transform into a turbulence form.Furthermore, we define the cone-shaped expansion rate, cone-shaped focusing rate,and cone angle in order to study the morphological changes in the cone shape inside the nozzle.The results indicate that the morphological change of the cone shape and flow pattern transformation is interrelated.When the cone shape tends to be unstable, the flow pattern changes towards flow blurring, whereas, a stable cone indicates that the flow tends to exhibit a droplet pattern.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 11179010 and 11075017 )the Natural Science Foundation of Beijing,China (Grant No. 1102019)the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20100003120010)
文摘According to intensity distribution of the synchrotron radiation source focused by a toroidal mirror at the Beijing synchrotron radiation biological macromolecule station, theoretical modeling of the Beijing synchrotron radiation source is developed for capillary optics. Using this theoretical modeling, the influences of the configuration curve of the polycapillary X-ray lens on transmission efficiency and working distance are analyzed. The experimental results of the transmission efficiency and working distance at the biological macromolecule station are in good agreement with the theoretical results.
基金Supported by the High Technology Research and Development Program of China(Grant No.863-416-5).
文摘We extend Bespalov-Talanov (B-T) theory on small-scale self-focusing (SSSF) to include vector effect of a very narrow intense laser beam with application of the vector self-focusing model. The gain spectrum for perturbations is obtained by using the standard linear instability analysis. It is shown that the influence on SSSF of vector effect is closely related to the beam width. For a very narrow beam, the role played by vector effect becomes significant, it reduces the fastest growing frequency and the maximum growth rate, and shortens the frequency range for perturbation growing, and thus deviates the gain spectrum from that of B-T theory.
基金supported the Young Scientists Fund of the National Natural Science Foundation of China(Grant No.60901036)the National Natural Science Foundation of China(Grant No.60777027)the Innovation Laboratory Fund of the Chinese Academy of Sciences(Grant No.CXJJ-10-S13)
文摘We present a new model of an electron gun for generating subrelativistic femtosecond （fs） electron pulses. The basic idea is to utilize a dc acceleration stage combined with a time focusing region, the time focusing electrode generates an electron energy chirp for bunching at the target. Without considering the space charge effects, simulations of the electron gun were carried out under the conditions of different dc voltages and various slopes of the voltage added on the time focusing electrode. Tracing and simulating large numbers of photoelectrons through Monte-Carlo and finite difference methods, the electron pulses with 1 ps can be compressed to 55 fs, which will allow significant advances in the field of ultrafast diagnosis.
