AIM:To study the relationships between amplitude of low-frequency fluctuations(ALFF)changes and clinical ophthalmic parameters in patients with primary open angle glaucoma(POAG)and analyze the diagnostic value of ALFF...AIM:To study the relationships between amplitude of low-frequency fluctuations(ALFF)changes and clinical ophthalmic parameters in patients with primary open angle glaucoma(POAG)and analyze the diagnostic value of ALFF.METHODS:Twenty-four POAG patients and 24 healthy controls(HCs)underwent resting-state functional magnetic resonance imaging(rs-fMRI).Nonparametric rank-sum tests were used to compare the ALFF values in the slow-4 and slow-5 bands,and Spearman or Pearson correlation analysis was used to assess the correlation between ALFF changes and clinical ophthalmic parameters in POAG patients.Receiver operating characteristic(ROC)curves were used to evaluate the diagnostic performance of the ALFF.RESULTS:There were 16 males in POAG patients(median age 48y)and 12 males in HCs(median age 39y).Compared with HCs,POAG patients presented increased or decreased ALFF values in different brain regions,and similar changes were observed in mild POAG patients.The ALFF values were correlated with retinal nerve fiber layer(RNFL)thickness,inner limiting membrane-retinal pigment epithelium thickness changes and the degree of visual field defects.Analysis of the diagnostic value of the ALFF via ROC curves revealed that the right medial frontal gyrus[area under the curve(AUC)=0.9063]and superior frontal gyrus(AUC=0.9097)had better diagnostic value than did the optic disc area(AUC=0.8019),visual field index(VFI%,AUC=0.8988)and macular parameters.CONCLUSION:POAG patients present altered cortical function that is significantly correlated with the optic nerve and retinal thickness and had good diagnostic value,which may reflect the underlying neuropathological mechanism of POAG.展开更多
In view of the limitations of a Rn-Gn model in the low frequency range and the defects of an En-In model in common use now, this paper builds a complete En-In model according to the theory of random harmonic. The para...In view of the limitations of a Rn-Gn model in the low frequency range and the defects of an En-In model in common use now, this paper builds a complete En-In model according to the theory of random harmonic. The parameters for the low-noise design such as the equivalent input noisy voltage Ens, the optimum source impedance Zsopt and the minimum noise figure Fmin can be calculated accurately by using this En-In model because it considers the coherence between the noise sources fully. Moreover, this paper points out that it will cause the maximum 30% miscalculation when neglecting the effects of the correlation coefficient 7. Using the series-series circuits as an example, this paper discusses the methods for the En-In noise analysis of electronic circuits preliminarily and demonstrates its correctness through the comparison between the simulated and measured results of the minimum noise figure Fmin of a single current series negative feedback circuit.展开更多
With the aid of ANSYS software, the effect of different mould external part materials on magnetic flux density in the aluminum melt and magnetic field interaction of four coils applied with same currents were investig...With the aid of ANSYS software, the effect of different mould external part materials on magnetic flux density in the aluminum melt and magnetic field interaction of four coils applied with same currents were investigated. Calculating results showed that magnetic flux density in the aluminum melt was greatly improved and the magnetic field interaction among different coils was decreased when external part of mould is made of soft magnetic material. Based on the finding, a four-strand low-frequency electromagnetic casting 6063 aluminum alloy experiment was carried out in the laboratory . The experiment showed that the surface of the billet was smooth and had no exudations and cold shuts, the as-cast microstructures were fine, uniform, equiaxed, net-globular or globular under low-frequency electromagnetic field. The microstructure becomes finer with increased current value.展开更多
In order to obtain a lower frequency band gap,this paper proposes a novel locally resonant meta-beam incorporating a softening nonlinear factor.An improved camroller structure is designed in this meta-beam to achieve ...In order to obtain a lower frequency band gap,this paper proposes a novel locally resonant meta-beam incorporating a softening nonlinear factor.An improved camroller structure is designed in this meta-beam to achieve the softening nonlinear stiffness of the local oscillators.Firstly,based on Hamilton's principle and the Galerkin method,the control equations for the coupled system are established.The theoretical band gap boundary is then derived with the modal analysis method.The theoretical results reveal that the band gap of the meta-beam shifts towards lower frequencies due to the presence of a softening nonlinear factor,distinguishing it from both linear metamaterials and those with hardening nonlinear characteristics.Then,the vibration attenuation characteristics of a finite size meta-beam are investigated through numerical calculation,and are verified by the theoretical results.Furthermore,parameter studies indicate that the reasonable design of the local oscillator parameters based on lightweight principles helps to achieve further broadband and efficient vibration reduction in the low-frequency region.Finally,a prototype of the meta-beam is fabricated and assembled,and the formations of the low-frequency band gap and the amplitude-induced band gap phenomenon are verified through experiments.展开更多
Low-frequency vibroseis acquisition has become a routine operation in land seismic surveys,given the advantages of low-frequency signals in characterizing geological structures and enhancing the imaging of deep explor...Low-frequency vibroseis acquisition has become a routine operation in land seismic surveys,given the advantages of low-frequency signals in characterizing geological structures and enhancing the imaging of deep exploration targets.The two key points of low-frequency sweep design techniques include controlling the distortion and improving the output energy during the low-frequency stage.However,the vibrators are limited by the maximum fl ow provided by the hydraulic systems at the low-frequency stage,causing difficulty in satisfying exploration energy requirements.Initially,a theoretical analysis of the low-frequency acquisition performance of vibrators is conducted.A theoretical maximum output force below 10 Hz is obtained by guiding through theoretical formulas and combining actual vibrator parameters.Then,the signal is optimized according to the surface characteristics of the operation area.Finally,detailed application quality control and operational procedures are established.The new low-frequency sweep design method has overcome the maximum flow limitations of the hydraulic system,increased the low-frequency energy,and achieved broadband acquisition.The designed signal has been tested and applied on various types of ground surfaces in the Middle East desert region,yielding good performance.The proposed low-frequency sweep design method holds considerable value for the application of conventional vibroseis in low-frequency acquisition.展开更多
Controlling low-frequency noise presents a significant challenge for traditional sound absorption materials,such as foams and fibrous substances.Recently developed acoustic absorption metamaterials,which rely on local...Controlling low-frequency noise presents a significant challenge for traditional sound absorption materials,such as foams and fibrous substances.Recently developed acoustic absorption metamaterials,which rely on local resonance can effectively balance the volume occupation and low-frequency absorption performance.However,these materials often exhibit a very narrow and fixed absorption band.Inspired by Helmholtz resonators and bistable structures,we propose bistable reconfigurable acoustic metamaterials(BRAMs)that offer multiband low-frequency absorption.These BRAMs are fabricated using shape-memory polylactic acid(SM-PLA)via four-dimension(4D)printing technology.Consequently,the geometry and absorption performance of the BRAMs can be adjusted by applying thermal stimuli(at 55℃)to switch between two stable states.The BRAMs demonstrate excellent low-frequency absorption with multiband characteristics,achieving an absorption coefficient of 0.981 at 136 Hz and 0.998 at 230 Hz for stable state I,and coefficients of 0.984 at 156 Hz and 0.961 at 542 Hz for stable state II.It was found that the BRAMs with different inclined plate angles had linear recovery stages,and the recovery speeds range from 0.75 mm/s to 1.1 mm/s.By combining a rational structural design and 4D printing,the reported reconfigurable acoustic metamaterials will inspire further studies on the design of dynamic and broadband absorption devices.展开更多
Microgrid stability analysis is a critical issue especially due to the inverters’low-inertia nature.The voltage and current control loops influences on stability are researched frequently most of which focus on mediu...Microgrid stability analysis is a critical issue especially due to the inverters’low-inertia nature.The voltage and current control loops influences on stability are researched frequently most of which focus on medium and high-frequency characteristic.Although the complete state-space model aims at low-frequency characteristic,it is too complicated and the calculation amount is huge with the scale of the microgrid increasing.One available reduced-order model of an inverter is simple,but it is suitable for only single inverter without network dynamic in microgrid.To fill in these gaps,a novel modeling method is proposed in this paper to investigate the low-frequency instability phenomenon and describe the whole DG connected system including network.In consideration of the high penetration level of induction motor(IM)loads and constant power(CP)loads in practical applications,the low-frequency mathematical model of IM and CP loads on the basis of static load is also built in this paper.Simulation and experimental results verify the effectiveness of the proposed model.展开更多
Metamaterials can control and manipulate acoustic/elastic waves on a subwavelength scale using cavities or additional components.However,the large cavity and weak stiffness components of traditional metamaterials may ...Metamaterials can control and manipulate acoustic/elastic waves on a subwavelength scale using cavities or additional components.However,the large cavity and weak stiffness components of traditional metamaterials may cause a conflict between vibroacoustic reduction and load-bearing capacity,and thus limit their application.Here,we propose a lightweight multifunctional metamaterial that can simultaneously achieve low-frequency sound insulation,broadband vibration reduction,and excellent load-bearing performance,named as vibroacoustic isolation and bearing metamaterial(VIBM).The advent of additive manufacturing technology provides a convenient and reliable method for the fabrication of VIBM samples.The results show that the compressive strength of the VIBM is as high as 9.71 MPa,which is nearly 87.81%higher than that of the conventional grid structure(CGS)under the same volume fraction.Moreover,the vibration and sound transmission are significantly reduced over a low and wide frequency range,which agrees well with the experimental data,and the reduction degree is obviously larger than that obtained by the CGS.The design strategy can effectively realize the key components of metamaterials and improve their application scenarios.展开更多
Low-frequency structural vibrations caused by poor rigidity are one of the main obstacles limiting the machining efficiency of robotic milling.Existing vibration suppression strategies primarily focus on passive vibra...Low-frequency structural vibrations caused by poor rigidity are one of the main obstacles limiting the machining efficiency of robotic milling.Existing vibration suppression strategies primarily focus on passive vibration absorption at the robotic end and feedback control at the joint motor.Although these strategies have a certain vibration suppression effect,the limitations of robotic flexibility and the extremely limited applicable speed range remain to be overcome.In this study,a Magnetorheological Joint Damper(MRJD)is developed.The joint-mounted feature ensures machining flexibility of the robot,and the millisecond response time of the Magnetorheological Fluid(MRF)ensures a large effective spindle speed range.More importantly,the evolution law of the damping performance of MRJD was revealed based on a low-frequency chatter mechanism,which guarantees the application of MRJD in robotic milling machining.To analyze the influence of the robotic joint angle on the suppression effect of the MRJD,the joint braking coefficient and end braking coefficient were proposed.Parallel coordinate plots were used to visualize the joint range with the optimal vibration suppression effect.Finally,a combination of different postures and cutting parameters was used to verify the vibration suppression effect and feasibility of the joint angle optimization.The experimental results show that the MRJD,which directly improves the joint vibration resistance,can effectively suppress the low-frequency vibration of robotic milling under a variety of cutting conditions.展开更多
Analysis is done of five-year low-pass filtered data by a five-layer low-order global spectral model, indicating that although any non-seasonal external forcing is not considered in the model atmosphere,monthly-scale ...Analysis is done of five-year low-pass filtered data by a five-layer low-order global spectral model, indicating that although any non-seasonal external forcing is not considered in the model atmosphere,monthly-scale anomaly takes place which is of remarkable seasonality and interannual variability.Analysis also shows that for the same seasonal external forcing the model atmosphere can exhibit two climatic states,similar in the departure pattern but opposite in sign, indicating that the anomaly is but the manifestation of the adverse states, which supports the theory of multi-equilibria proposed by Charney and Devore(1979) once again.Finally, the source for the low-frequency oscillation of the global atmosphere is found to be the convective heat source / sink inside the tropical atmosphere as discussed before in our study.Therefore, the key approach to the exploration of atmospheric steady low-frequency oscillation and the associated climatic effect lies in the examination of the distribution of convective heat sources / sinks and the variation in the tropical atmosphere.展开更多
When tracking a unmanned aerial vehicle(UAV)in complex backgrounds,environmen-tal noise and clutter often obscure it.Traditional radar target tracking algorithms face multiple lim-itations when tracking a UAV,includin...When tracking a unmanned aerial vehicle(UAV)in complex backgrounds,environmen-tal noise and clutter often obscure it.Traditional radar target tracking algorithms face multiple lim-itations when tracking a UAV,including high vulnerability to target occlusion and shape variations,as well as pronounced false alarms and missed detections in low signal-to-noise ratio(SNR)envi-ronments.To address these issues,this paper proposes a UAV detection and tracking algorithm based on a low-frequency communication network.The accuracy and effectiveness of the algorithm are validated through simulation experiments using field-measured point cloud data.Additionally,the key parameters of the algorithm are optimized through a process of selection and comparison,thereby improving the algorithm's precision.The experimental results show that the improved algo-rithm can significantly enhance the detection and tracking performance of the UAV under high clutter density conditions,effectively reduce the false alarm rate and markedly improve overall tracking performance metrics.展开更多
Objective:To study the clinical efficacy of applying the Meridian Flow Low-Frequency Therapy Device combined with Chinese herbal enema for patients with acute and chronic pelvic inflammatory disease(PID).Methods:Sixty...Objective:To study the clinical efficacy of applying the Meridian Flow Low-Frequency Therapy Device combined with Chinese herbal enema for patients with acute and chronic pelvic inflammatory disease(PID).Methods:Sixty-two patients with acute and chronic PID admitted from January 2024 to October 2025 were selected and randomly divided into a standard group(n=31)and an experimental group(n=31).The standard group received conventional medication+Chinese herbal enema treatment.The experimental group received the Meridian Flow Low-Frequency Therapy Device in addition to the standard group’s treatment.The clinical efficacy,changes in inflammatory markers,and pelvic improvement were compared between the two groups.Results:The excellent-good rate of treatment in the experimental group was higher than that in the standard group(p<0.05).The levels of various inflammatory factors in the experimental group were lower than those in the standard group(all p<0.05).The improvement in pelvic mass diameter and pelvic effusion depth in the experimental group was superior to that in the standard group(both p<0.05).Conclusion:The Meridian Flow Low-Frequency Therapy Device combined with Chinese herbal enema has a definite curative effect in treating pelvic inflammatory disease.It can effectively alleviate clinical symptoms,reduce inflammatory response,and promote the improvement of the pelvic environment.展开更多
A barotropic model containing large-scale topography and zonal mean flow is established to discuss the effects of large-scale topography on the low-frequency waves. The results show that what affects low-frequency wav...A barotropic model containing large-scale topography and zonal mean flow is established to discuss the effects of large-scale topography on the low-frequency waves. The results show that what affects low-frequency waves mostly is maximal height of topography and topographic slope. The former makes frequency of topographic Rossby waves decrease, the latter makes Rossby waves instable. Moreover, when topographic slope is appropriate, it can also make Rossby waves turn into low-frequency waves.展开更多
A non-contact low-frequency(LF)method of diagnosing the plasma surrounding a scaled model in a shock tube is proposed.This method utilizes the phase shift occurring after the transmission of an LF alternating magnetic...A non-contact low-frequency(LF)method of diagnosing the plasma surrounding a scaled model in a shock tube is proposed.This method utilizes the phase shift occurring after the transmission of an LF alternating magnetic field through the plasma to directly measure the ratio of the plasma loop average electron density to collision frequency.An equivalent circuit model is used to analyze the relationship of the phase shift of the magnetic field component of LF electromagnetic waves with the plasma electron density and collision frequency.The applicable range of the LF method on a given plasma scale is analyzed.The upper diagnostic limit for the ratio of the electron density(unit:m^(-3))to collision frequency(unit:Hz)exceeds 1×10^(11),enabling an electron density to exceed 1×10^(20)m^(-3)and a collision frequency to be less than 1 GHz.In this work,the feasibility of using the LF phase shift to implement the plasma diagnosis is also assessed.Diagnosis experiments on shock tube equipment are conducted by using both the electrostatic probe method and LF method.By comparing the diagnostic results of the two methods,the inversion results are relatively consistent with each other,thereby preliminarily verifying the feasibility of the LF method.The ratio of the electron density to the collision frequency has a relatively uniform distribution during the plasma stabilization.The LF diagnostic path is a loop around the model,which is suitable for diagnosing the plasma that surrounds the model.Finally,the causes of diagnostic discrepancy between the two methods are analyzed.The proposed method provides a new avenue for diagnosing high-density enveloping plasma.展开更多
Model evaluation using benchmark datasets is an important method to measure the capability of large language models(LLMs)in specific domains,and it is mainly used to assess the knowledge and reasoning abilities of LLM...Model evaluation using benchmark datasets is an important method to measure the capability of large language models(LLMs)in specific domains,and it is mainly used to assess the knowledge and reasoning abilities of LLMs.Therefore,in order to better assess the capability of LLMs in the agricultural domain,Agri-Eval was proposed as a benchmark for assessing the knowledge and reasoning ability of LLMs in agriculture.The assessment dataset used in Agri-Eval covered seven major disciplines in the agricultural domain:crop science,horticulture,plant protection,animal husbandry,forest science,aquaculture science,and grass science,and contained a total of 2283 questions.Among domestic general-purpose LLMs,DeepSeek R1 performed best with an accuracy rate of 75.49%.In the realm of international general-purpose LLMs,Gemini 2.0 pro exp 0205 standed out as the top performer,achieving an accuracy rate of 74.28%.As an LLMs in agriculture vertical,Shennong V2.0 outperformed all the LLMs in China,and the answer accuracy rate of agricultural knowledge exceeded that of all the existing general-purpose LLMs.The launch of Agri-Eval helped the LLM developers to comprehensively evaluate the model's capability in the field of agriculture through a variety of tasks and tests to promote the development of the LLMs in the field of agriculture.展开更多
In this paper,we establish and study a single-species logistic model with impulsive age-selective harvesting.First,we prove the ultimate boundedness of the solutions of the system.Then,we obtain conditions for the asy...In this paper,we establish and study a single-species logistic model with impulsive age-selective harvesting.First,we prove the ultimate boundedness of the solutions of the system.Then,we obtain conditions for the asymptotic stability of the trivial solution and the positive periodic solution.Finally,numerical simulations are presented to validate our results.Our results show that age-selective harvesting is more conducive to sustainable population survival than non-age-selective harvesting.展开更多
In recent years,there has been an increasing need for climate information across diverse sectors of society.This demand has arisen from the necessity to adapt to and mitigate the impacts of climate variability and cha...In recent years,there has been an increasing need for climate information across diverse sectors of society.This demand has arisen from the necessity to adapt to and mitigate the impacts of climate variability and change.Likewise,this period has seen a significant increase in our understanding of the physical processes and mechanisms that drive precipitation and its variability across different regions of Africa.By leveraging a large volume of climate model outputs,numerous studies have investigated the model representation of African precipitation as well as underlying physical processes.These studies have assessed whether the physical processes are well depicted and whether the models are fit for informing mitigation and adaptation strategies.This paper provides a review of the progress in precipitation simulation overAfrica in state-of-the-science climate models and discusses the major issues and challenges that remain.展开更多
Utilizing finite element analysis,the ballistic protection provided by a combination of perforated D-shaped and base armor plates,collectively referred to as radiator armor,is evaluated.ANSYS Explicit Dynamics is empl...Utilizing finite element analysis,the ballistic protection provided by a combination of perforated D-shaped and base armor plates,collectively referred to as radiator armor,is evaluated.ANSYS Explicit Dynamics is employed to simulate the ballistic impact of 7.62 mm armor-piercing projectiles on Aluminum AA5083-H116 and Steel Secure 500 armors,focusing on the evaluation of material deformation and penetration resistance at varying impact points.While the D-shaped armor plate is penetrated by the armor-piercing projectiles,the combination of the perforated D-shaped and base armor plates successfully halts penetration.A numerical model based on the finite element method is developed using software such as SolidWorks and ANSYS to analyze the interaction between radiator armor and bullet.The perforated design of radiator armor is to maintain airflow for radiator function,with hole sizes smaller than the bullet core diameter to protect radiator assemblies.Predictions are made regarding the brittle fracture resulting from the projectile core′s bending due to asymmetric impact,and the resulting fragments failed to penetrate the perforated base armor plate.Craters are formed on the surface of the perforated D-shaped armor plate due to the impact of projectile fragments.The numerical model accurately predicts hole growth and projectile penetration upon impact with the armor,demonstrating effective protection of the radiator assemblies by the radiator armor.展开更多
Large-scale topography, such as a seamount, substantially impacts low-frequency sound propagation in an ocean waveguide, limiting the application of low-frequency acoustic detecting techniques. A three-dimensional(3D)...Large-scale topography, such as a seamount, substantially impacts low-frequency sound propagation in an ocean waveguide, limiting the application of low-frequency acoustic detecting techniques. A three-dimensional(3D) coupledmode model is developed to calculate the acoustic field in an ocean waveguide with seamount topography and analyze the3D effect. In this model, a correction is introduced in the bottom boundary, theoretically making the acoustic field satisfy the energy conservation. Furthermore, a large azimuth angle calculation range is obtained by using the operator theory and higher-order Pade approximation. Additionally, the model has advantages related to the coupling mode and parabolic equation theory. The couplings corresponding to the effects of range-dependent environment are fully considered, and the numerical implementation is kept feasible. After verifying the accuracy and reliability of the model, low-frequency sound propagation characteristics in the seamount environment are analyzed. The results indicate lateral variability in bathymetry can lead to out-of-plane effects such as the horizontal refraction phenomenon, while the coupling effect tends to restore the abnormal sound field and produces acoustic field diffraction behind the seamount. This model effectively considers the effects of the horizontal refraction and coupling, which are proportional to the scale of the seamount.展开更多
The National Geophysical Data Center(NGDC)of the United States has collected aeromagnetic data for input into a series of geomagnetic models to improve model resolution;however,in the Tibetan Plateau region,ground-bas...The National Geophysical Data Center(NGDC)of the United States has collected aeromagnetic data for input into a series of geomagnetic models to improve model resolution;however,in the Tibetan Plateau region,ground-based observations remain insufficient to clearly reflect the characteristics of the region’s lithospheric magnetism.In this study,we evaluate the lithospheric magnetism of the Tibetan Plateau by using a 3D surface spline model based on observations from>200 newly constructed repeat stations(portable stations)to determine the spatial distribution of plateau geomagnetism,as well as its correlation with the tectonic features of the region.We analyze the relationships between M≥5 earthquakes and lithospheric magnetic field variations on the Tibetan Plateau and identify regions susceptible to strong earthquakes.We compare the geomagnetic results with those from an enhanced magnetic model(EMM2015)developed by the NGDC and provide insights into improving lithospheric magnetic field calculations in the Tibetan Plateau region.Further research reveals that these magnetic anomalies exhibit distinct differences from the magnetic-seismic correlation mechanisms observed in other tectonic settings;here,they are governed primarily by the combined effects of compressional magnetism,thermal magnetism,and deep thermal stress.This study provides new evidence of geomagnetic anomalies on the Tibetan Plateau,interprets them physically,and demonstrates their potential for identifying seismic hazard zones on the Plateau.展开更多
基金Supported by National Natural Science Foundation of China(No.82260203).
文摘AIM:To study the relationships between amplitude of low-frequency fluctuations(ALFF)changes and clinical ophthalmic parameters in patients with primary open angle glaucoma(POAG)and analyze the diagnostic value of ALFF.METHODS:Twenty-four POAG patients and 24 healthy controls(HCs)underwent resting-state functional magnetic resonance imaging(rs-fMRI).Nonparametric rank-sum tests were used to compare the ALFF values in the slow-4 and slow-5 bands,and Spearman or Pearson correlation analysis was used to assess the correlation between ALFF changes and clinical ophthalmic parameters in POAG patients.Receiver operating characteristic(ROC)curves were used to evaluate the diagnostic performance of the ALFF.RESULTS:There were 16 males in POAG patients(median age 48y)and 12 males in HCs(median age 39y).Compared with HCs,POAG patients presented increased or decreased ALFF values in different brain regions,and similar changes were observed in mild POAG patients.The ALFF values were correlated with retinal nerve fiber layer(RNFL)thickness,inner limiting membrane-retinal pigment epithelium thickness changes and the degree of visual field defects.Analysis of the diagnostic value of the ALFF via ROC curves revealed that the right medial frontal gyrus[area under the curve(AUC)=0.9063]and superior frontal gyrus(AUC=0.9097)had better diagnostic value than did the optic disc area(AUC=0.8019),visual field index(VFI%,AUC=0.8988)and macular parameters.CONCLUSION:POAG patients present altered cortical function that is significantly correlated with the optic nerve and retinal thickness and had good diagnostic value,which may reflect the underlying neuropathological mechanism of POAG.
文摘In view of the limitations of a Rn-Gn model in the low frequency range and the defects of an En-In model in common use now, this paper builds a complete En-In model according to the theory of random harmonic. The parameters for the low-noise design such as the equivalent input noisy voltage Ens, the optimum source impedance Zsopt and the minimum noise figure Fmin can be calculated accurately by using this En-In model because it considers the coherence between the noise sources fully. Moreover, this paper points out that it will cause the maximum 30% miscalculation when neglecting the effects of the correlation coefficient 7. Using the series-series circuits as an example, this paper discusses the methods for the En-In noise analysis of electronic circuits preliminarily and demonstrates its correctness through the comparison between the simulated and measured results of the minimum noise figure Fmin of a single current series negative feedback circuit.
文摘With the aid of ANSYS software, the effect of different mould external part materials on magnetic flux density in the aluminum melt and magnetic field interaction of four coils applied with same currents were investigated. Calculating results showed that magnetic flux density in the aluminum melt was greatly improved and the magnetic field interaction among different coils was decreased when external part of mould is made of soft magnetic material. Based on the finding, a four-strand low-frequency electromagnetic casting 6063 aluminum alloy experiment was carried out in the laboratory . The experiment showed that the surface of the billet was smooth and had no exudations and cold shuts, the as-cast microstructures were fine, uniform, equiaxed, net-globular or globular under low-frequency electromagnetic field. The microstructure becomes finer with increased current value.
基金supported by the National Natural Science Foundation of China(Nos.12172014,U224126412332001)。
文摘In order to obtain a lower frequency band gap,this paper proposes a novel locally resonant meta-beam incorporating a softening nonlinear factor.An improved camroller structure is designed in this meta-beam to achieve the softening nonlinear stiffness of the local oscillators.Firstly,based on Hamilton's principle and the Galerkin method,the control equations for the coupled system are established.The theoretical band gap boundary is then derived with the modal analysis method.The theoretical results reveal that the band gap of the meta-beam shifts towards lower frequencies due to the presence of a softening nonlinear factor,distinguishing it from both linear metamaterials and those with hardening nonlinear characteristics.Then,the vibration attenuation characteristics of a finite size meta-beam are investigated through numerical calculation,and are verified by the theoretical results.Furthermore,parameter studies indicate that the reasonable design of the local oscillator parameters based on lightweight principles helps to achieve further broadband and efficient vibration reduction in the low-frequency region.Finally,a prototype of the meta-beam is fabricated and assembled,and the formations of the low-frequency band gap and the amplitude-induced band gap phenomenon are verified through experiments.
基金The authors would like to express their sincere appreciation to the research project of CNPC Geophysical Key Lab(2022DQ0604-4)National Natural Science Foundation of China(Grant No.42074141).
文摘Low-frequency vibroseis acquisition has become a routine operation in land seismic surveys,given the advantages of low-frequency signals in characterizing geological structures and enhancing the imaging of deep exploration targets.The two key points of low-frequency sweep design techniques include controlling the distortion and improving the output energy during the low-frequency stage.However,the vibrators are limited by the maximum fl ow provided by the hydraulic systems at the low-frequency stage,causing difficulty in satisfying exploration energy requirements.Initially,a theoretical analysis of the low-frequency acquisition performance of vibrators is conducted.A theoretical maximum output force below 10 Hz is obtained by guiding through theoretical formulas and combining actual vibrator parameters.Then,the signal is optimized according to the surface characteristics of the operation area.Finally,detailed application quality control and operational procedures are established.The new low-frequency sweep design method has overcome the maximum flow limitations of the hydraulic system,increased the low-frequency energy,and achieved broadband acquisition.The designed signal has been tested and applied on various types of ground surfaces in the Middle East desert region,yielding good performance.The proposed low-frequency sweep design method holds considerable value for the application of conventional vibroseis in low-frequency acquisition.
基金financially supported by National Key Research and Development Program of China(Grant No.2023YFB4604800)National Natural Science Foundation of China(Grant No.52275331)financial support from the Hong Kong Scholars Program(Grant No.XJ2022014).
文摘Controlling low-frequency noise presents a significant challenge for traditional sound absorption materials,such as foams and fibrous substances.Recently developed acoustic absorption metamaterials,which rely on local resonance can effectively balance the volume occupation and low-frequency absorption performance.However,these materials often exhibit a very narrow and fixed absorption band.Inspired by Helmholtz resonators and bistable structures,we propose bistable reconfigurable acoustic metamaterials(BRAMs)that offer multiband low-frequency absorption.These BRAMs are fabricated using shape-memory polylactic acid(SM-PLA)via four-dimension(4D)printing technology.Consequently,the geometry and absorption performance of the BRAMs can be adjusted by applying thermal stimuli(at 55℃)to switch between two stable states.The BRAMs demonstrate excellent low-frequency absorption with multiband characteristics,achieving an absorption coefficient of 0.981 at 136 Hz and 0.998 at 230 Hz for stable state I,and coefficients of 0.984 at 156 Hz and 0.961 at 542 Hz for stable state II.It was found that the BRAMs with different inclined plate angles had linear recovery stages,and the recovery speeds range from 0.75 mm/s to 1.1 mm/s.By combining a rational structural design and 4D printing,the reported reconfigurable acoustic metamaterials will inspire further studies on the design of dynamic and broadband absorption devices.
基金This work was supported by the National key research and development plan 2016YFB0900300National Natural Science Foundation of China under Grant51677162Natural Science Foundation of Hebei Province E2017203337。
文摘Microgrid stability analysis is a critical issue especially due to the inverters’low-inertia nature.The voltage and current control loops influences on stability are researched frequently most of which focus on medium and high-frequency characteristic.Although the complete state-space model aims at low-frequency characteristic,it is too complicated and the calculation amount is huge with the scale of the microgrid increasing.One available reduced-order model of an inverter is simple,but it is suitable for only single inverter without network dynamic in microgrid.To fill in these gaps,a novel modeling method is proposed in this paper to investigate the low-frequency instability phenomenon and describe the whole DG connected system including network.In consideration of the high penetration level of induction motor(IM)loads and constant power(CP)loads in practical applications,the low-frequency mathematical model of IM and CP loads on the basis of static load is also built in this paper.Simulation and experimental results verify the effectiveness of the proposed model.
基金Project supported by the National Natural Science Foundation of China(Nos.11991032 and 52241103)the Hunan Province Graduate Research Innovation Project of China(No.KY0409052440)。
文摘Metamaterials can control and manipulate acoustic/elastic waves on a subwavelength scale using cavities or additional components.However,the large cavity and weak stiffness components of traditional metamaterials may cause a conflict between vibroacoustic reduction and load-bearing capacity,and thus limit their application.Here,we propose a lightweight multifunctional metamaterial that can simultaneously achieve low-frequency sound insulation,broadband vibration reduction,and excellent load-bearing performance,named as vibroacoustic isolation and bearing metamaterial(VIBM).The advent of additive manufacturing technology provides a convenient and reliable method for the fabrication of VIBM samples.The results show that the compressive strength of the VIBM is as high as 9.71 MPa,which is nearly 87.81%higher than that of the conventional grid structure(CGS)under the same volume fraction.Moreover,the vibration and sound transmission are significantly reduced over a low and wide frequency range,which agrees well with the experimental data,and the reduction degree is obviously larger than that obtained by the CGS.The design strategy can effectively realize the key components of metamaterials and improve their application scenarios.
基金supported by the National Natural Science Foundation of China(No.U20A20294)the National Natural Science Foundation of China(No.52322511)the National Natural Science Foundation of China(No.52188102).
文摘Low-frequency structural vibrations caused by poor rigidity are one of the main obstacles limiting the machining efficiency of robotic milling.Existing vibration suppression strategies primarily focus on passive vibration absorption at the robotic end and feedback control at the joint motor.Although these strategies have a certain vibration suppression effect,the limitations of robotic flexibility and the extremely limited applicable speed range remain to be overcome.In this study,a Magnetorheological Joint Damper(MRJD)is developed.The joint-mounted feature ensures machining flexibility of the robot,and the millisecond response time of the Magnetorheological Fluid(MRF)ensures a large effective spindle speed range.More importantly,the evolution law of the damping performance of MRJD was revealed based on a low-frequency chatter mechanism,which guarantees the application of MRJD in robotic milling machining.To analyze the influence of the robotic joint angle on the suppression effect of the MRJD,the joint braking coefficient and end braking coefficient were proposed.Parallel coordinate plots were used to visualize the joint range with the optimal vibration suppression effect.Finally,a combination of different postures and cutting parameters was used to verify the vibration suppression effect and feasibility of the joint angle optimization.The experimental results show that the MRJD,which directly improves the joint vibration resistance,can effectively suppress the low-frequency vibration of robotic milling under a variety of cutting conditions.
文摘Analysis is done of five-year low-pass filtered data by a five-layer low-order global spectral model, indicating that although any non-seasonal external forcing is not considered in the model atmosphere,monthly-scale anomaly takes place which is of remarkable seasonality and interannual variability.Analysis also shows that for the same seasonal external forcing the model atmosphere can exhibit two climatic states,similar in the departure pattern but opposite in sign, indicating that the anomaly is but the manifestation of the adverse states, which supports the theory of multi-equilibria proposed by Charney and Devore(1979) once again.Finally, the source for the low-frequency oscillation of the global atmosphere is found to be the convective heat source / sink inside the tropical atmosphere as discussed before in our study.Therefore, the key approach to the exploration of atmospheric steady low-frequency oscillation and the associated climatic effect lies in the examination of the distribution of convective heat sources / sinks and the variation in the tropical atmosphere.
基金supported in part by National Natural Science Founda-tion of China(No.62372284)in part by Shanghai Nat-ural Science Foundation(No.24ZR1421800).
文摘When tracking a unmanned aerial vehicle(UAV)in complex backgrounds,environmen-tal noise and clutter often obscure it.Traditional radar target tracking algorithms face multiple lim-itations when tracking a UAV,including high vulnerability to target occlusion and shape variations,as well as pronounced false alarms and missed detections in low signal-to-noise ratio(SNR)envi-ronments.To address these issues,this paper proposes a UAV detection and tracking algorithm based on a low-frequency communication network.The accuracy and effectiveness of the algorithm are validated through simulation experiments using field-measured point cloud data.Additionally,the key parameters of the algorithm are optimized through a process of selection and comparison,thereby improving the algorithm's precision.The experimental results show that the improved algo-rithm can significantly enhance the detection and tracking performance of the UAV under high clutter density conditions,effectively reduce the false alarm rate and markedly improve overall tracking performance metrics.
文摘Objective:To study the clinical efficacy of applying the Meridian Flow Low-Frequency Therapy Device combined with Chinese herbal enema for patients with acute and chronic pelvic inflammatory disease(PID).Methods:Sixty-two patients with acute and chronic PID admitted from January 2024 to October 2025 were selected and randomly divided into a standard group(n=31)and an experimental group(n=31).The standard group received conventional medication+Chinese herbal enema treatment.The experimental group received the Meridian Flow Low-Frequency Therapy Device in addition to the standard group’s treatment.The clinical efficacy,changes in inflammatory markers,and pelvic improvement were compared between the two groups.Results:The excellent-good rate of treatment in the experimental group was higher than that in the standard group(p<0.05).The levels of various inflammatory factors in the experimental group were lower than those in the standard group(all p<0.05).The improvement in pelvic mass diameter and pelvic effusion depth in the experimental group was superior to that in the standard group(both p<0.05).Conclusion:The Meridian Flow Low-Frequency Therapy Device combined with Chinese herbal enema has a definite curative effect in treating pelvic inflammatory disease.It can effectively alleviate clinical symptoms,reduce inflammatory response,and promote the improvement of the pelvic environment.
文摘A barotropic model containing large-scale topography and zonal mean flow is established to discuss the effects of large-scale topography on the low-frequency waves. The results show that what affects low-frequency waves mostly is maximal height of topography and topographic slope. The former makes frequency of topographic Rossby waves decrease, the latter makes Rossby waves instable. Moreover, when topographic slope is appropriate, it can also make Rossby waves turn into low-frequency waves.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.52107162 and 12202479)the Science and Technology Projects of Shaanxi Province,China(Grant Nos.2022CGBX-12 and 2022KXJ-57)the Science and Technology Projects of Xi’an City,China(Grant Nos.23KGDW0023-2022 and 23GXFW0011)。
文摘A non-contact low-frequency(LF)method of diagnosing the plasma surrounding a scaled model in a shock tube is proposed.This method utilizes the phase shift occurring after the transmission of an LF alternating magnetic field through the plasma to directly measure the ratio of the plasma loop average electron density to collision frequency.An equivalent circuit model is used to analyze the relationship of the phase shift of the magnetic field component of LF electromagnetic waves with the plasma electron density and collision frequency.The applicable range of the LF method on a given plasma scale is analyzed.The upper diagnostic limit for the ratio of the electron density(unit:m^(-3))to collision frequency(unit:Hz)exceeds 1×10^(11),enabling an electron density to exceed 1×10^(20)m^(-3)and a collision frequency to be less than 1 GHz.In this work,the feasibility of using the LF phase shift to implement the plasma diagnosis is also assessed.Diagnosis experiments on shock tube equipment are conducted by using both the electrostatic probe method and LF method.By comparing the diagnostic results of the two methods,the inversion results are relatively consistent with each other,thereby preliminarily verifying the feasibility of the LF method.The ratio of the electron density to the collision frequency has a relatively uniform distribution during the plasma stabilization.The LF diagnostic path is a loop around the model,which is suitable for diagnosing the plasma that surrounds the model.Finally,the causes of diagnostic discrepancy between the two methods are analyzed.The proposed method provides a new avenue for diagnosing high-density enveloping plasma.
文摘Model evaluation using benchmark datasets is an important method to measure the capability of large language models(LLMs)in specific domains,and it is mainly used to assess the knowledge and reasoning abilities of LLMs.Therefore,in order to better assess the capability of LLMs in the agricultural domain,Agri-Eval was proposed as a benchmark for assessing the knowledge and reasoning ability of LLMs in agriculture.The assessment dataset used in Agri-Eval covered seven major disciplines in the agricultural domain:crop science,horticulture,plant protection,animal husbandry,forest science,aquaculture science,and grass science,and contained a total of 2283 questions.Among domestic general-purpose LLMs,DeepSeek R1 performed best with an accuracy rate of 75.49%.In the realm of international general-purpose LLMs,Gemini 2.0 pro exp 0205 standed out as the top performer,achieving an accuracy rate of 74.28%.As an LLMs in agriculture vertical,Shennong V2.0 outperformed all the LLMs in China,and the answer accuracy rate of agricultural knowledge exceeded that of all the existing general-purpose LLMs.The launch of Agri-Eval helped the LLM developers to comprehensively evaluate the model's capability in the field of agriculture through a variety of tasks and tests to promote the development of the LLMs in the field of agriculture.
基金Supported by the National Natural Science Foundation of China(12261018)Universities Key Laboratory of Mathematical Modeling and Data Mining in Guizhou Province(2023013)。
文摘In this paper,we establish and study a single-species logistic model with impulsive age-selective harvesting.First,we prove the ultimate boundedness of the solutions of the system.Then,we obtain conditions for the asymptotic stability of the trivial solution and the positive periodic solution.Finally,numerical simulations are presented to validate our results.Our results show that age-selective harvesting is more conducive to sustainable population survival than non-age-selective harvesting.
基金the World Climate Research Programme(WCRP),Climate Variability and Predictability(CLIVAR),and Global Energy and Water Exchanges(GEWEX)for facilitating the coordination of African monsoon researchsupport from the Center for Earth System Modeling,Analysis,and Data at the Pennsylvania State Universitythe support of the Office of Science of the U.S.Department of Energy Biological and Environmental Research as part of the Regional&Global Model Analysis(RGMA)program area。
文摘In recent years,there has been an increasing need for climate information across diverse sectors of society.This demand has arisen from the necessity to adapt to and mitigate the impacts of climate variability and change.Likewise,this period has seen a significant increase in our understanding of the physical processes and mechanisms that drive precipitation and its variability across different regions of Africa.By leveraging a large volume of climate model outputs,numerous studies have investigated the model representation of African precipitation as well as underlying physical processes.These studies have assessed whether the physical processes are well depicted and whether the models are fit for informing mitigation and adaptation strategies.This paper provides a review of the progress in precipitation simulation overAfrica in state-of-the-science climate models and discusses the major issues and challenges that remain.
文摘Utilizing finite element analysis,the ballistic protection provided by a combination of perforated D-shaped and base armor plates,collectively referred to as radiator armor,is evaluated.ANSYS Explicit Dynamics is employed to simulate the ballistic impact of 7.62 mm armor-piercing projectiles on Aluminum AA5083-H116 and Steel Secure 500 armors,focusing on the evaluation of material deformation and penetration resistance at varying impact points.While the D-shaped armor plate is penetrated by the armor-piercing projectiles,the combination of the perforated D-shaped and base armor plates successfully halts penetration.A numerical model based on the finite element method is developed using software such as SolidWorks and ANSYS to analyze the interaction between radiator armor and bullet.The perforated design of radiator armor is to maintain airflow for radiator function,with hole sizes smaller than the bullet core diameter to protect radiator assemblies.Predictions are made regarding the brittle fracture resulting from the projectile core′s bending due to asymmetric impact,and the resulting fragments failed to penetrate the perforated base armor plate.Craters are formed on the surface of the perforated D-shaped armor plate due to the impact of projectile fragments.The numerical model accurately predicts hole growth and projectile penetration upon impact with the armor,demonstrating effective protection of the radiator assemblies by the radiator armor.
基金Project supported by the National Natural Science Foundation of China(Grant No.11804360)the IACAS Frontier Exploration Project(Grant No.QYTS202103)the Key Laboratory Foundation of Acoustic Science and Technology(Grant No.2021-JCJQ-LB-066-08)。
文摘Large-scale topography, such as a seamount, substantially impacts low-frequency sound propagation in an ocean waveguide, limiting the application of low-frequency acoustic detecting techniques. A three-dimensional(3D) coupledmode model is developed to calculate the acoustic field in an ocean waveguide with seamount topography and analyze the3D effect. In this model, a correction is introduced in the bottom boundary, theoretically making the acoustic field satisfy the energy conservation. Furthermore, a large azimuth angle calculation range is obtained by using the operator theory and higher-order Pade approximation. Additionally, the model has advantages related to the coupling mode and parabolic equation theory. The couplings corresponding to the effects of range-dependent environment are fully considered, and the numerical implementation is kept feasible. After verifying the accuracy and reliability of the model, low-frequency sound propagation characteristics in the seamount environment are analyzed. The results indicate lateral variability in bathymetry can lead to out-of-plane effects such as the horizontal refraction phenomenon, while the coupling effect tends to restore the abnormal sound field and produces acoustic field diffraction behind the seamount. This model effectively considers the effects of the horizontal refraction and coupling, which are proportional to the scale of the seamount.
基金supported by the CAS Pioneer Hundred Talents Program and Second Tibetan Plateau Scientific Expedition Research Program(2019QZKK0708)as well as the Basic Research Program of Qinghai Province:Lithospheric Geomagnetic Field of the Qinghai‒Tibet Plateau and the Relationship with Strong Earthquakes(2021-ZJ-969Q).
文摘The National Geophysical Data Center(NGDC)of the United States has collected aeromagnetic data for input into a series of geomagnetic models to improve model resolution;however,in the Tibetan Plateau region,ground-based observations remain insufficient to clearly reflect the characteristics of the region’s lithospheric magnetism.In this study,we evaluate the lithospheric magnetism of the Tibetan Plateau by using a 3D surface spline model based on observations from>200 newly constructed repeat stations(portable stations)to determine the spatial distribution of plateau geomagnetism,as well as its correlation with the tectonic features of the region.We analyze the relationships between M≥5 earthquakes and lithospheric magnetic field variations on the Tibetan Plateau and identify regions susceptible to strong earthquakes.We compare the geomagnetic results with those from an enhanced magnetic model(EMM2015)developed by the NGDC and provide insights into improving lithospheric magnetic field calculations in the Tibetan Plateau region.Further research reveals that these magnetic anomalies exhibit distinct differences from the magnetic-seismic correlation mechanisms observed in other tectonic settings;here,they are governed primarily by the combined effects of compressional magnetism,thermal magnetism,and deep thermal stress.This study provides new evidence of geomagnetic anomalies on the Tibetan Plateau,interprets them physically,and demonstrates their potential for identifying seismic hazard zones on the Plateau.
