The low frequency electromagnetic casting(LFEC)was used to prevent hot cracking during the solidification process of GH4742 superalloy ingot.The effects of LFEC on the solidification macrostructure of the ingot were i...The low frequency electromagnetic casting(LFEC)was used to prevent hot cracking during the solidification process of GH4742 superalloy ingot.The effects of LFEC on the solidification macrostructure of the ingot were investigated through experiments and simulation.The results show that the average grain size decreases after application of LFEC.At the same time,the fraction of equiaxed grains increases compared with the ingots that without LFEC.In addition,the average grain size decreases and the fraction of equiaxed grains increases with increasing the current frequency.When the current frequency increases from 5 Hz to 20 Hz,the average grain size decreases from 5.39 mm to 4.74 mm,and the fraction of equiaxed grains increases from 41.21%to 55.24%.The distribution of Lorentz force,melt flow field and temperature field in the melt was simulated using COMSOL Multiphysics software.It is found that the Lorentz force increases and the forced convection is enhanced with increasing the current frequency,thus the melt flow velocity and heat transfer in the melt are promoted.It can facilitate the heterogenous nucleation in the melt,resulting in grain refinement,and further preventing hot cracking of large size ingots.展开更多
An easily stackable multi-layer quasi-zero-stiffness(ML-QZS)meta-structure is proposed to achieve highly efficient vibration isolation performance at low frequency.First,the distributed shape optimization method is us...An easily stackable multi-layer quasi-zero-stiffness(ML-QZS)meta-structure is proposed to achieve highly efficient vibration isolation performance at low frequency.First,the distributed shape optimization method is used to design the unit cel,i.e.,the single-layer QZS(SL-QZS)meta-structure.Second,the stiffness feature of the unit cell is investigated and verified through static experiments.Third,the unit cells are stacked one by one along the direction of vibration isolation,and thus the ML-QZS meta-structure is constructed.Fourth,the dynamic modeling of the ML-QZS vibration isolation metastructure is conducted,and the dynamic responses are obtained from the equations of motion,and verified by finite element(FE)simulations.Finally,a prototype of the ML-QZS vibration isolation meta-structure is fabricated by additive manufacturing,and the vibration isolation performance is evaluated experimentally.The results show that the vibration isolation performance substantially enhances when the number of unit cells increases.More importantly,the ML-QZS meta-structure can be easily extended in the direction of vibration isolation when the unit cells are properly stacked.Hence,the ML-FQZS vibration isolation meta-structure should be a fascinating solution for highly efficient vibration isolation performance at low frequency.展开更多
Power electronic-interfaced renewable energy sources(RES)exhibit lower inertia compared to traditional synchronous generators.The large-scale integration of RES has led to a significant reduction in system inertia,pos...Power electronic-interfaced renewable energy sources(RES)exhibit lower inertia compared to traditional synchronous generators.The large-scale integration of RES has led to a significant reduction in system inertia,posing significant challenges for maintaining frequency stability in future power systems.This issue has garnered considerable attention in recent years.However,the existing research has not yet achieved a comprehensive understanding of system inertia and frequency stability in the context of low-inertia systems.To this end,this paper provides a comprehensive review of the definition,modeling,analysis,evaluation,and control for frequency stability.It commences with an exploration of inertia and frequency characteristics in low-inertia systems,followed by a novel definition of frequency stability.A summary of frequency stability modeling,analysis,and evaluation methods is then provided,along with their respective applicability in various scenarios.Additionally,the two critical factors of frequency control—energy sources at the system level and control strategies at the device level—are examined.Finally,an outlook on future research in low-inertia power systems is discussed.展开更多
The very low frequency(VLF)regime below 30 MHz in the electromagnetic spectrum has presently been drawing global attention in radio astronomical research due to its potentially significant science outcomes exploring m...The very low frequency(VLF)regime below 30 MHz in the electromagnetic spectrum has presently been drawing global attention in radio astronomical research due to its potentially significant science outcomes exploring many unknown extragalactic sources,transients,and so on.However,the nontransparency of the Earth’s ionosphere,ionospheric distortion and artificial radio frequency interference(RFI)have made it difficult to detect the VLF celestial radio emission with ground-based instruments.A straightforward solution to overcome these problems is a space-based VLF radio telescope,just like the VLF radio instruments onboard the Chang’E-4 spacecraft.But building such a space telescope would be inevitably costly and technically challenging.The alternative approach would be then a ground-based VLF radio telescope.Particularly,in the period of post 2020 when the solar and terrestrial ionospheric activities are expected to be in a’calm’state,it will provide us a good chance to perform VLF ground-based radio observations.Anticipating such an opportunity,we built an agile VLF radio spectrum explorer co-located with the currently operational Mingantu Spectra Radio Heliograph(MUSER).The instrument includes four antennas operating in the VLF frequency range 1-70 MHz.Along with them,we employ an eight-channel analog and digital receivers to amplify,digitize and process the radio signals received by the antennas.We present in the paper this VLF radio spectrum explorer and the instrument will be useful for celestial studies of VLF radio emissions.展开更多
For the partial discharge test of electrical equipment with large capacitance,the use of lowfrequency voltage instead of power frequency voltage can effectively reduce the capacity requirements of test power supply.Ho...For the partial discharge test of electrical equipment with large capacitance,the use of lowfrequency voltage instead of power frequency voltage can effectively reduce the capacity requirements of test power supply.However,the validity of PD test under low frequency voltage needs to be evaluated.In order to investigate the influence of voltage frequency on corona discharge in the air,the discharge test of the tip-plate electrode under the frequency from 50 to0.1 Hz is carried out based on the impulse current method.The results show that some of the main features of corona under low frequency do not change.The magnitude of discharge in a positive half cycle is obviously larger than that in a negative cycle.The magnitude of discharge and interval in positive cycle are random,while that in negative cycle are regular.With the decrease of frequency,the inception voltage increases.The variation trend of maximum and average magnitude and repetition rate of the discharge in positive and negative half cycle with the variation of voltage frequency and magnitude is demonstrated,with discussion and interpretation from the aspects of space charge transportation,effective discharge time and transition of discharge modes.There is an obvious difference in the phase resolved pattern of partial discharge and characteristic parameters of discharge patterns between power and low frequency.The experimental results can be the reference for mode identification of partial discharge under low frequency tests.The trend of the measured parameters with the variation of frequency provides more information about the insulation defect than traditional measurements under a single frequency（usually 50 Hz）.Also it helps to understand the mechanism of corona discharge with an explanation of the characteristics under different frequencies.展开更多
A wide passband frequency selective surface(FSS)is proposed using a five-layer stacked structure.The proposed structure applies four layers of dielectric plates and five layers of metal patches to provide a passband a...A wide passband frequency selective surface(FSS)is proposed using a five-layer stacked structure.The proposed structure applies four layers of dielectric plates and five layers of metal patches to provide a passband and exhibits more stable frequency responses and lower insertion loss under wide-angle oblique incidence compared with the typical three-layer metal-dielectric structure.According to the simulation results,the proposed FSS can achieve a passband range of 1.7-2.7 GHz with an insertion loss of less than 0.5 d B and a relative bandwidth of 44.1%,and it can preserve stable transmission characteristics with the incident angle ranging from 0°to 45°.展开更多
Ocean vector acoustic measurement is feasible affected by the hydrodynamic interference caused by the flow fluctuations and structural vibrations, especially in the very-low-frequency monitoring. Hence, a novel horizo...Ocean vector acoustic measurement is feasible affected by the hydrodynamic interference caused by the flow fluctuations and structural vibrations, especially in the very-low-frequency monitoring. Hence, a novel horizontal floating platform including a horizontal floating cable, vertical mooring cable and floating main body is proposed and described in this paper. It has the advantages of good maneuverability along with the current and multi-stage vibration isolation. The main application of this platform is to measure the ocean ambient noise coming from the wave fluctuation and the deterministic acoustic signals such as aquatic organisms, underwater targets and sailing vehicles. The influence of the current fluctuation on the attitude angle and flow induced vibration of cables and main body are analyzed with some previous sea test data. Moreover, the comparison between the vertical type platform used before and the horizontal type platform is also discussed. It is concluded that there is obvious relevance between the attitude angle and ocean current variation. Meanwhile, the abnormal influence on the main body is caused by the vibration transmission from the fluctuation of cables. There will be the influence on the accuracy of the acoustic measurement above 100 Hz, and the inherent vibration characteristic of the main body is the primary reason.展开更多
The effects of low frequency electromagnetic field on the macro-physical fields in the semi-continuous casting process of aluminum alloys and the microstructure and crack in the billets were studied and analyzed by th...The effects of low frequency electromagnetic field on the macro-physical fields in the semi-continuous casting process of aluminum alloys and the microstructure and crack in the billets were studied and analyzed by the numerical and experimental methods.Comparison of the results for the macro-physical fields in the low frequency electromagnetic casting（LFEC） process with the conventional DC casting process indicates the following characters due to the application of electromagnetic field：an entirely changed direction and remarkably increased velocity of melt flow;a uniform distribution and a decreased gradient of temperature;elevated isothermal lines;a reduced sump depth;decreased stress and plastic deformation.Further,the microstructure of the billets is refined remarkably and the crack in the billets is eliminated in LFEC process because of modification of the macro-physical fields induced by the application of low frequency electromagnetic field.展开更多
The objective of the study was to explore the effect of acute low temperature stress on VO2 and Vf of Silurus meridionalis and Pelteobag vachelli after 10 minutes cold water bath with different temperature.The investi...The objective of the study was to explore the effect of acute low temperature stress on VO2 and Vf of Silurus meridionalis and Pelteobag vachelli after 10 minutes cold water bath with different temperature.The investigation was operated under the temperature of 24 ℃.It was found that the VO2 and Vf of Silurus meridionalis after 6 and 0 ℃ stress showed a decrease-increase-decrease trend while other groups showed a rapid increase then slowly recovery trend.The VO2 and Vf of Pelteobag vachelli after 0 ℃stress showed a decrease-increase-decrease trend while other groups showed an increase then slowly recovery process.It was suggested that Pelteobag vachelli was more adaptive to acute cold stress,but it cost more energy adapting to cold stress compared to Silurus meridionalis.展开更多
The wideband CMOS voltage-controlled oscillator（VCO）with low phase noise and low power consumption is presented for a DRM/DAB（digital radio mondiale and digital audio broadcasting）frequency synthesizer.In order to...The wideband CMOS voltage-controlled oscillator（VCO）with low phase noise and low power consumption is presented for a DRM/DAB（digital radio mondiale and digital audio broadcasting）frequency synthesizer.In order to obtain a wide band and a large tuning range,a parallel switched capacitor bank is added in the LC tank.The proposed VCO is implemented in SMIC 0.18-μm RF CMOS technology and the chip area is 750 μm×560 μm,including the test buffer circuit and the pads.Measured results show that the tuning range is 44.6%;i.e.,the frequency turning range is from 2.27 to 3.57 GHz.The measured phase noise is-122.22 dBc/Hz at a 1 MHz offset from the carrier.The maximum power consumption of the core part is 6.16 mW at a 1.8 V power supply.展开更多
A new frequency compensation technique for low-power, area-efficient multistage amplifiers is introduced. Using nested active capacitors, our scheme achieves better bandwidth-to-power and slew-rate-to-power performanc...A new frequency compensation technique for low-power, area-efficient multistage amplifiers is introduced. Using nested active capacitors, our scheme achieves better bandwidth-to-power and slew-rate-to-power performances than previous works. Implemented in standard 0.35μm CMOS technology, our three-stage amplifier achieves 105dB DC gain, 3.3M GBW,68 phase margin, and 2.56V/μs average slew rate under a 150pF capacitive load. All of these are realized with only 40μW power consumption under a 2V power supply,with very small compensation capacitors.展开更多
The factors affecting the reflection and transmission coefficient of the ionosphere have been analyzed.These factors include wave frequency,incident angle,geomagnetic inclination,electron density and collision frequen...The factors affecting the reflection and transmission coefficient of the ionosphere have been analyzed.These factors include wave frequency,incident angle,geomagnetic inclination,electron density and collision frequency in the ionosphere.The ionosphere refractive index is also analyzed.The ionosphere above 70 km is considered to be homogeneous and anisotropic,and the reflection and transmission coefficient matrix is calculated using matrix method.Simultaneously the Booker quartic equation is solved to get the refractive index in the ionosphere.The results show that when the wave frequency is higher,it is easier to penetrate into the ionosphere from its bottom boundary and the propagation attenuation in the ionosphere is smaller.TE（traverse electric） wave and TM（traverse magnetic） wave can both penetrate into the ionosphere with a small incident angle,while TE wave can hardly transmit into the ionosphere when the incident angle is large.The transmission coefficient decreases as the geomagnetic inclination increases.TE and TM wave cannot penetrate into the ionosphere at magnetic equator.When the electron collision frequency is higher,it is easier for VLF wave to penetrate into the ionosphere and the attenuation of ordinary wave is weaker,which may be caused by the energy transportation between the waves and the particles.The ordinary（O） wave experiences severer attenuation than extraordinary（X） wave,and X wave is a penetration mode whereas O wave is a non-penetration mode in the ionosphere.All the results indicate that VLF wave with higher frequency is easier to penetrate into the ionosphere and to be recorded by the satellites at high latitude.It is hard for ULF and the lower frequency VLF wave to transmit into the ionosphere directly for the severe reflection and attenuation.It may transmit into the ionosphere with a small incident angle due to the nonlinear effect,for example,the interaction between the waves and the particles or cross modulation,and then propagate along the whistle duct with small attenuation.This work may be a preliminary theoretical exploration for the future calculation on the response of ground based VLF artificial transmitter in the ionosphere and further study on the seismic ionosphere coupling model.展开更多
The mid—Holocene climate about 6000 years ago was simulated by using the atmospheric general circulation model. The orbital parameters for 6 ka BP (before present) were prescribed and other forcing factors were set i...The mid—Holocene climate about 6000 years ago was simulated by using the atmospheric general circulation model. The orbital parameters for 6 ka BP (before present) were prescribed and other forcing factors were set in the modern conditions. Results show that the large—scale climate change in the African—Asian monsoon areas during the summer—time is strongly compared to the present climate, while the changes in other seasons and regions are generally weak. The results also revealed the change of the low frequency oscillation in the atmosphere. Key words Seasonal climate - Low frequency oscillation - Mid—Holocene This research was jointly supported by the National Natural Science Foundation of China (NSFC) key project “ Variation of Paleo—environment over East Asia and Its Dynamic Relation with the Global Change” under contract 49894170 and the NSFC project “ Coupling of the AGCM with an Biome Model and the Simulation on the Mid-Holocene Climate” under contract 49975018.展开更多
Low-fi＇equency vertical vibration isolation systems play important roles in precision measurements to reduce seismic and environmental vibration noise. Several types of active vibration isolation systems have been de...Low-fi＇equency vertical vibration isolation systems play important roles in precision measurements to reduce seismic and environmental vibration noise. Several types of active vibration isolation systems have been developed. However, few researches focus on how to optimize the test mass install position in order to improve the vibration transmissibility. An active low-frequency vertical vibration isolation system based on an earlier instrument, the Super Spring, is designed and implemented. The system, which is simple and compact, consists of two stages： a parallelogram-shaped linkage to ensure vertical motion, and a simple spring-mass system. The theoretical analysis of the vibration isolation system is presented, including terms erroneously ignored before. By carefully choosing the mechanical parameters according to the above analysis and using feedback control, the resonance frequency of the system is reduced from 2.3 to 0.03 Hz, a reduction by a factor of more than 75. The vibration isolation system is installed as an inertial reference in an absolute gravimeter, where it improved the scatter of the absolute gravity values by a factor of 5. The experimental results verifies the improved performance of the isolation system, making it particularly suitable for precision experiments. The improved vertical vibration isolation system can be used as a prototype for designing high-performance active vertical isolation systems. An improved theoretical model of this active vibration isolation system with beam-pivot configuration is proposed, providing fundamental guidelines for vibration isolator design and assembling.展开更多
Assessing the fatigue life of mooring systems is important for deep water structures. In this paper, a comprehensive fatigue analysis is conducted on the mooring lines applied in a semi-submersible platform with speci...Assessing the fatigue life of mooring systems is important for deep water structures. In this paper, a comprehensive fatigue analysis is conducted on the mooring lines applied in a semi-submersible platform with special focus on the low frequency(LF) fatigue damage. Several influential factors, including water depth, wave spectral parameters, and riser system, are considered. Numerical simulation of a semi-submersible platform with the mooring/riser system is executed under different conditions, and the fatigue damage of mooring lines is assessed by using the time domain analysis method as a benchmark. The effects of these factors on the mooring line tension and the fatigue damage are investigated and discussed in detail. Research results indicate that the LF fatigue damage only accounts for a very small portion of the total damage, although the LF components dominate the global motion response and the mooring line tension of the semi-submersible platform. However, it is demonstrated that the LF fatigue damage is clearly affected by the influential factors. The increase in water depth and spectral peak periods, and the existence of risers can weaken the contribution of the LF components to the mooring line fatigue damage, while the fatigue damage due to the LF components increases with the increase of significant wave height.展开更多
Elastic wave on seafloor caused by low frequency noise radiated from ship is called ship seismic wave which can be used to identify ship target. In order to analyze the wave components and the propagating properties o...Elastic wave on seafloor caused by low frequency noise radiated from ship is called ship seismic wave which can be used to identify ship target. In order to analyze the wave components and the propagating properties of ship seismic wave, the numerical calculation of synthetic seismograms on seafloor aroused by a low frequency point sound source is carried out using a wave number integration technique combined with inverse Fourier transform. According to the numerical example of hard seafloor, the time series of seismic wave on seafloor are mostly composed of interface waves and normal mode waves. Each normal mode wave has a well defined low cut-off frequency, while the interface wave doesn't have. The frequency dispersion of normal mode wave is obvious when frequency is lower than 100Hz, while the interface wave is dispersive only in the infra-sound frequency range. The time series of seismic wave is dominated by the interface wave when the source frequency is less than the minimal cut-off frequency of normal mode wave.展开更多
The influences of the low frequency electromagnetic field on the horizontal direct chill casting process were investigated experimentally. Ingots of 2024 aluminum alloy with a cross size of 40 mm× 200 mm were pro...The influences of the low frequency electromagnetic field on the horizontal direct chill casting process were investigated experimentally. Ingots of 2024 aluminum alloy with a cross size of 40 mm× 200 mm were produced by the conventional horizontal chill casting process and low frequency electromagnetic horizontal chill casting processre- spectively. The as-cast structures and the mechanical property of the ingots were examined. The results showed that the low frequency electromagnetic field could sub- stantially refine the microstructures and pronouncedly reduce the macrosegregation in the horizontal direct chill casting process. Moreover, the surface quality of the ingot was prominently improved by the low frequency electromagnetic field. The fracture strength and elongation percentage of the ingot was increased with the low frequency electromagnetic field.展开更多
We present a design method for calculating and optimizing sound absorption coefficient of multi-layered porous fibrous metals （PFM） in the low frequency range. PFM is simplified as an equivalent idealized sheet with...We present a design method for calculating and optimizing sound absorption coefficient of multi-layered porous fibrous metals （PFM） in the low frequency range. PFM is simplified as an equivalent idealized sheet with all metallic fibers aligned in one direction and distributed in periodic hexagonal patterns. We use a phenomenological model in the literature to investigate the effects of pore geometrical parameters （fiber diameter and gap） on sound absorption performance. The sound absorption coefficient of multi- layered PFMs is calculated using impedance translation theorem, To demonstrate the validity of the present model, we compare the predicted results with the experimental data. With the average sound absorption （low frequency range） as the objective function and the fiber gaps as the design variables, an optimization method for multi-layered fibrous metals is proposed. A new fibrous layout with given porosity of multi-layered fibrous metals is suggested to achieve optimal low frequency sound absorption. The sound absorption coefficient of the optimal multi-layered fibrous metal is higher than the single- layered fibrous metal, and a significant effect of the fibrous material on sound absorption is found due to the surface Dorosity of the multi-layered fibrous.展开更多
Vibration energy harvesting is to transform the ambient mechanical energy to electricity. How to reduce the resonance frequency and improve the conversion efficiency is very important. In this paper, a layer-separated...Vibration energy harvesting is to transform the ambient mechanical energy to electricity. How to reduce the resonance frequency and improve the conversion efficiency is very important. In this paper, a layer-separated piezoelectric cantilever beam is proposed for the vibration energy harvester(VEH) for low-frequency and wide-bandwidth operation, which can transform the mechanical impact energy to electric energy. First,the electromechanical coupling equation is obtained by the Euler-Bernoulli beam theory.Based on the average method, the approximate analytical solution is derived and the voltage response is obtained. Furthermore, the physical prototype is fabricated, and the vibration experiment is conducted to validate the theoretical principle. The experimental results show that the maximum power of 0.445 μW of the layer-separated VEH is about3.11 times higher than that of the non-impact harvester when the excitation acceleration is 0.2 g. The operating frequency bandwidth can be widened by increasing the stiffness of the fundamental layer and decreasing the gap distance of the system. But the increasing of operating frequency bandwidth comes at the cost of reducing peak voltage. The theoretical simulation and the experimental results demonstrate good agreement which indicates that the proposed impact-driving VEH device has advantages for low-frequency and wide-bandwidth. The high performance provides great prospect to scavenge the vibration energy in environment.展开更多
By use of the filter analysis technique, the Complex Empirical Othogonal Function (CEOF) method and the ECMWF/WMO 2.5°×2.5°grid data of the geopotential heights during the summer months in 1988, an inte...By use of the filter analysis technique, the Complex Empirical Othogonal Function (CEOF) method and the ECMWF/WMO 2.5°×2.5°grid data of the geopotential heights during the summer months in 1988, an interseasonal process that the western Pacific subtropical high (WPSH) was anomalously far to the north in the first and second ten days of July is studied. It has been found that in the western Pacific subtropical region in the first and second ten days of July,it is the continuous assembly of low frequency geopotential waves (LFGWs) that leads to the abnormality of WPSH. This abnormality emerges with the enhancement of wave assembling and ceases while the wave assembling situation disappears. The structure of the low frequency assembling waves corresponds to the structure of subtropical high in its abnormal period. The effect of the assembling waves on the abnormality of subtropical high can be considered as the accumulation of disturbance energy carried by the low frequency waves from different directions in the western Pacific region.展开更多
基金financially supported by the Major Projects in Aviation Engines and Gas Turbines (Grant No.2019-VI-0020-0136)the National Key Research and Development Program of China (Grant Nos.2022YFB3705101&2022YFB3705102)+1 种基金the National Natural Science Foundation of China (Grant No.U1708253)the Fundamental Research Funds for the Central Universities,China (Grant No.N2302005)。
文摘The low frequency electromagnetic casting(LFEC)was used to prevent hot cracking during the solidification process of GH4742 superalloy ingot.The effects of LFEC on the solidification macrostructure of the ingot were investigated through experiments and simulation.The results show that the average grain size decreases after application of LFEC.At the same time,the fraction of equiaxed grains increases compared with the ingots that without LFEC.In addition,the average grain size decreases and the fraction of equiaxed grains increases with increasing the current frequency.When the current frequency increases from 5 Hz to 20 Hz,the average grain size decreases from 5.39 mm to 4.74 mm,and the fraction of equiaxed grains increases from 41.21%to 55.24%.The distribution of Lorentz force,melt flow field and temperature field in the melt was simulated using COMSOL Multiphysics software.It is found that the Lorentz force increases and the forced convection is enhanced with increasing the current frequency,thus the melt flow velocity and heat transfer in the melt are promoted.It can facilitate the heterogenous nucleation in the melt,resulting in grain refinement,and further preventing hot cracking of large size ingots.
基金supported by the National Natural Science Foundation of China(Nos.12122206 and 12272129)the Natural Science Foundation of Hunan Province of China(No.2024JJ4004)the Zhejiang Provincial Natural Science Foundation of China(No.LQ24A020006)。
文摘An easily stackable multi-layer quasi-zero-stiffness(ML-QZS)meta-structure is proposed to achieve highly efficient vibration isolation performance at low frequency.First,the distributed shape optimization method is used to design the unit cel,i.e.,the single-layer QZS(SL-QZS)meta-structure.Second,the stiffness feature of the unit cell is investigated and verified through static experiments.Third,the unit cells are stacked one by one along the direction of vibration isolation,and thus the ML-QZS meta-structure is constructed.Fourth,the dynamic modeling of the ML-QZS vibration isolation metastructure is conducted,and the dynamic responses are obtained from the equations of motion,and verified by finite element(FE)simulations.Finally,a prototype of the ML-QZS vibration isolation meta-structure is fabricated by additive manufacturing,and the vibration isolation performance is evaluated experimentally.The results show that the vibration isolation performance substantially enhances when the number of unit cells increases.More importantly,the ML-QZS meta-structure can be easily extended in the direction of vibration isolation when the unit cells are properly stacked.Hence,the ML-FQZS vibration isolation meta-structure should be a fascinating solution for highly efficient vibration isolation performance at low frequency.
基金supported by the National Natural Science Foundation of China(U2166601)。
文摘Power electronic-interfaced renewable energy sources(RES)exhibit lower inertia compared to traditional synchronous generators.The large-scale integration of RES has led to a significant reduction in system inertia,posing significant challenges for maintaining frequency stability in future power systems.This issue has garnered considerable attention in recent years.However,the existing research has not yet achieved a comprehensive understanding of system inertia and frequency stability in the context of low-inertia systems.To this end,this paper provides a comprehensive review of the definition,modeling,analysis,evaluation,and control for frequency stability.It commences with an exploration of inertia and frequency characteristics in low-inertia systems,followed by a novel definition of frequency stability.A summary of frequency stability modeling,analysis,and evaluation methods is then provided,along with their respective applicability in various scenarios.Additionally,the two critical factors of frequency control—energy sources at the system level and control strategies at the device level—are examined.Finally,an outlook on future research in low-inertia power systems is discussed.
基金funded by the National Natural Science Foundation of China(NSFC,Nos.11573043,11790305,11433006)National Key R&D 278 Program of China(2018YFA0404602)+1 种基金the CE4 mission of the Chinese Lunar Exploration Program:the Netherlands-China Low Frequency Explorer(NCLE)Chinese Academy of Sciences(CAS)Strategic Priority Research Program(XDA15020200)。
文摘The very low frequency(VLF)regime below 30 MHz in the electromagnetic spectrum has presently been drawing global attention in radio astronomical research due to its potentially significant science outcomes exploring many unknown extragalactic sources,transients,and so on.However,the nontransparency of the Earth’s ionosphere,ionospheric distortion and artificial radio frequency interference(RFI)have made it difficult to detect the VLF celestial radio emission with ground-based instruments.A straightforward solution to overcome these problems is a space-based VLF radio telescope,just like the VLF radio instruments onboard the Chang’E-4 spacecraft.But building such a space telescope would be inevitably costly and technically challenging.The alternative approach would be then a ground-based VLF radio telescope.Particularly,in the period of post 2020 when the solar and terrestrial ionospheric activities are expected to be in a’calm’state,it will provide us a good chance to perform VLF ground-based radio observations.Anticipating such an opportunity,we built an agile VLF radio spectrum explorer co-located with the currently operational Mingantu Spectra Radio Heliograph(MUSER).The instrument includes four antennas operating in the VLF frequency range 1-70 MHz.Along with them,we employ an eight-channel analog and digital receivers to amplify,digitize and process the radio signals received by the antennas.We present in the paper this VLF radio spectrum explorer and the instrument will be useful for celestial studies of VLF radio emissions.
基金supported by the National Key R&D Program of China(2017YFB0902704)the Science and Technology Project of SGCC(GY71-15-048)
文摘For the partial discharge test of electrical equipment with large capacitance,the use of lowfrequency voltage instead of power frequency voltage can effectively reduce the capacity requirements of test power supply.However,the validity of PD test under low frequency voltage needs to be evaluated.In order to investigate the influence of voltage frequency on corona discharge in the air,the discharge test of the tip-plate electrode under the frequency from 50 to0.1 Hz is carried out based on the impulse current method.The results show that some of the main features of corona under low frequency do not change.The magnitude of discharge in a positive half cycle is obviously larger than that in a negative cycle.The magnitude of discharge and interval in positive cycle are random,while that in negative cycle are regular.With the decrease of frequency,the inception voltage increases.The variation trend of maximum and average magnitude and repetition rate of the discharge in positive and negative half cycle with the variation of voltage frequency and magnitude is demonstrated,with discussion and interpretation from the aspects of space charge transportation,effective discharge time and transition of discharge modes.There is an obvious difference in the phase resolved pattern of partial discharge and characteristic parameters of discharge patterns between power and low frequency.The experimental results can be the reference for mode identification of partial discharge under low frequency tests.The trend of the measured parameters with the variation of frequency provides more information about the insulation defect than traditional measurements under a single frequency（usually 50 Hz）.Also it helps to understand the mechanism of corona discharge with an explanation of the characteristics under different frequencies.
基金supported by ZTE Industry-University-Institute Cooperation Funds under Grant No.IA20220800001。
文摘A wide passband frequency selective surface(FSS)is proposed using a five-layer stacked structure.The proposed structure applies four layers of dielectric plates and five layers of metal patches to provide a passband and exhibits more stable frequency responses and lower insertion loss under wide-angle oblique incidence compared with the typical three-layer metal-dielectric structure.According to the simulation results,the proposed FSS can achieve a passband range of 1.7-2.7 GHz with an insertion loss of less than 0.5 d B and a relative bandwidth of 44.1%,and it can preserve stable transmission characteristics with the incident angle ranging from 0°to 45°.
基金financially supported by the National Natural Science Foundation of China(Grant No.61801275)Major Science and Technology Innovation This work is financially supported by the Stable Supporting Fund of Acoustic Science and Technology Laboratory(Grant No.SSJSWDZC2018014)+3 种基金the Major Science and Technology Innovation Project of Shandong Province(Grant No.2018YFJH0707)the Key Research and Development Plan(International Cooperation)Project of Shandong Province(Grant No.2018JHZ002)the International Science and Technology Cooperation Project of Shandong Academy of Sciences(Grant No.2019GHZD01)the provincial Key Research and Development Program of Shandong(Military and Civilian Integration)(Grant No.2016JMRH0541)
文摘Ocean vector acoustic measurement is feasible affected by the hydrodynamic interference caused by the flow fluctuations and structural vibrations, especially in the very-low-frequency monitoring. Hence, a novel horizontal floating platform including a horizontal floating cable, vertical mooring cable and floating main body is proposed and described in this paper. It has the advantages of good maneuverability along with the current and multi-stage vibration isolation. The main application of this platform is to measure the ocean ambient noise coming from the wave fluctuation and the deterministic acoustic signals such as aquatic organisms, underwater targets and sailing vehicles. The influence of the current fluctuation on the attitude angle and flow induced vibration of cables and main body are analyzed with some previous sea test data. Moreover, the comparison between the vertical type platform used before and the horizontal type platform is also discussed. It is concluded that there is obvious relevance between the attitude angle and ocean current variation. Meanwhile, the abnormal influence on the main body is caused by the vibration transmission from the fluctuation of cables. There will be the influence on the accuracy of the acoustic measurement above 100 Hz, and the inherent vibration characteristic of the main body is the primary reason.
基金Project(2005CB623707)supported by National Basic Research Project of China
文摘The effects of low frequency electromagnetic field on the macro-physical fields in the semi-continuous casting process of aluminum alloys and the microstructure and crack in the billets were studied and analyzed by the numerical and experimental methods.Comparison of the results for the macro-physical fields in the low frequency electromagnetic casting（LFEC） process with the conventional DC casting process indicates the following characters due to the application of electromagnetic field：an entirely changed direction and remarkably increased velocity of melt flow;a uniform distribution and a decreased gradient of temperature;elevated isothermal lines;a reduced sump depth;decreased stress and plastic deformation.Further,the microstructure of the billets is refined remarkably and the crack in the billets is eliminated in LFEC process because of modification of the macro-physical fields induced by the application of low frequency electromagnetic field.
基金Supported by the National Natural Science Foundation of China(30371121)the Natural Science Foundation of Chongqing the Educa-tion Committee Foundation of Chongqing~~
文摘The objective of the study was to explore the effect of acute low temperature stress on VO2 and Vf of Silurus meridionalis and Pelteobag vachelli after 10 minutes cold water bath with different temperature.The investigation was operated under the temperature of 24 ℃.It was found that the VO2 and Vf of Silurus meridionalis after 6 and 0 ℃ stress showed a decrease-increase-decrease trend while other groups showed a rapid increase then slowly recovery trend.The VO2 and Vf of Pelteobag vachelli after 0 ℃stress showed a decrease-increase-decrease trend while other groups showed an increase then slowly recovery process.It was suggested that Pelteobag vachelli was more adaptive to acute cold stress,but it cost more energy adapting to cold stress compared to Silurus meridionalis.
文摘The wideband CMOS voltage-controlled oscillator（VCO）with low phase noise and low power consumption is presented for a DRM/DAB（digital radio mondiale and digital audio broadcasting）frequency synthesizer.In order to obtain a wide band and a large tuning range,a parallel switched capacitor bank is added in the LC tank.The proposed VCO is implemented in SMIC 0.18-μm RF CMOS technology and the chip area is 750 μm×560 μm,including the test buffer circuit and the pads.Measured results show that the tuning range is 44.6%;i.e.,the frequency turning range is from 2.27 to 3.57 GHz.The measured phase noise is-122.22 dBc/Hz at a 1 MHz offset from the carrier.The maximum power consumption of the core part is 6.16 mW at a 1.8 V power supply.
文摘A new frequency compensation technique for low-power, area-efficient multistage amplifiers is introduced. Using nested active capacitors, our scheme achieves better bandwidth-to-power and slew-rate-to-power performances than previous works. Implemented in standard 0.35μm CMOS technology, our three-stage amplifier achieves 105dB DC gain, 3.3M GBW,68 phase margin, and 2.56V/μs average slew rate under a 150pF capacitive load. All of these are realized with only 40μW power consumption under a 2V power supply,with very small compensation capacitors.
基金supported by Chinese National Science and Technology Support Pro-gram (2008BAC35B01)Basic Research Project from Institute of Earthquake Science,China Earthquake Ad-ministration (02092408)
文摘The factors affecting the reflection and transmission coefficient of the ionosphere have been analyzed.These factors include wave frequency,incident angle,geomagnetic inclination,electron density and collision frequency in the ionosphere.The ionosphere refractive index is also analyzed.The ionosphere above 70 km is considered to be homogeneous and anisotropic,and the reflection and transmission coefficient matrix is calculated using matrix method.Simultaneously the Booker quartic equation is solved to get the refractive index in the ionosphere.The results show that when the wave frequency is higher,it is easier to penetrate into the ionosphere from its bottom boundary and the propagation attenuation in the ionosphere is smaller.TE（traverse electric） wave and TM（traverse magnetic） wave can both penetrate into the ionosphere with a small incident angle,while TE wave can hardly transmit into the ionosphere when the incident angle is large.The transmission coefficient decreases as the geomagnetic inclination increases.TE and TM wave cannot penetrate into the ionosphere at magnetic equator.When the electron collision frequency is higher,it is easier for VLF wave to penetrate into the ionosphere and the attenuation of ordinary wave is weaker,which may be caused by the energy transportation between the waves and the particles.The ordinary（O） wave experiences severer attenuation than extraordinary（X） wave,and X wave is a penetration mode whereas O wave is a non-penetration mode in the ionosphere.All the results indicate that VLF wave with higher frequency is easier to penetrate into the ionosphere and to be recorded by the satellites at high latitude.It is hard for ULF and the lower frequency VLF wave to transmit into the ionosphere directly for the severe reflection and attenuation.It may transmit into the ionosphere with a small incident angle due to the nonlinear effect,for example,the interaction between the waves and the particles or cross modulation,and then propagate along the whistle duct with small attenuation.This work may be a preliminary theoretical exploration for the future calculation on the response of ground based VLF artificial transmitter in the ionosphere and further study on the seismic ionosphere coupling model.
文摘The mid—Holocene climate about 6000 years ago was simulated by using the atmospheric general circulation model. The orbital parameters for 6 ka BP (before present) were prescribed and other forcing factors were set in the modern conditions. Results show that the large—scale climate change in the African—Asian monsoon areas during the summer—time is strongly compared to the present climate, while the changes in other seasons and regions are generally weak. The results also revealed the change of the low frequency oscillation in the atmosphere. Key words Seasonal climate - Low frequency oscillation - Mid—Holocene This research was jointly supported by the National Natural Science Foundation of China (NSFC) key project “ Variation of Paleo—environment over East Asia and Its Dynamic Relation with the Global Change” under contract 49894170 and the NSFC project “ Coupling of the AGCM with an Biome Model and the Simulation on the Mid-Holocene Climate” under contract 49975018.
基金Supported by Tsinghua University’s Scientific Research Initiative Program,China(Grant No.2010THZ05)
文摘Low-fi＇equency vertical vibration isolation systems play important roles in precision measurements to reduce seismic and environmental vibration noise. Several types of active vibration isolation systems have been developed. However, few researches focus on how to optimize the test mass install position in order to improve the vibration transmissibility. An active low-frequency vertical vibration isolation system based on an earlier instrument, the Super Spring, is designed and implemented. The system, which is simple and compact, consists of two stages： a parallelogram-shaped linkage to ensure vertical motion, and a simple spring-mass system. The theoretical analysis of the vibration isolation system is presented, including terms erroneously ignored before. By carefully choosing the mechanical parameters according to the above analysis and using feedback control, the resonance frequency of the system is reduced from 2.3 to 0.03 Hz, a reduction by a factor of more than 75. The vibration isolation system is installed as an inertial reference in an absolute gravimeter, where it improved the scatter of the absolute gravity values by a factor of 5. The experimental results verifies the improved performance of the isolation system, making it particularly suitable for precision experiments. The improved vertical vibration isolation system can be used as a prototype for designing high-performance active vertical isolation systems. An improved theoretical model of this active vibration isolation system with beam-pivot configuration is proposed, providing fundamental guidelines for vibration isolator design and assembling.
基金financial support by the National Basic Research Program of China (Grant No. 2011CB013704)the major program of the National Natural Science Foundation of China (Grant No. 51490675)+1 种基金the Shandong Provincial Science & Technology Development Project (Grant No. 2013GHY11503)the Taishan Scholars Program of Shandong Province
文摘Assessing the fatigue life of mooring systems is important for deep water structures. In this paper, a comprehensive fatigue analysis is conducted on the mooring lines applied in a semi-submersible platform with special focus on the low frequency(LF) fatigue damage. Several influential factors, including water depth, wave spectral parameters, and riser system, are considered. Numerical simulation of a semi-submersible platform with the mooring/riser system is executed under different conditions, and the fatigue damage of mooring lines is assessed by using the time domain analysis method as a benchmark. The effects of these factors on the mooring line tension and the fatigue damage are investigated and discussed in detail. Research results indicate that the LF fatigue damage only accounts for a very small portion of the total damage, although the LF components dominate the global motion response and the mooring line tension of the semi-submersible platform. However, it is demonstrated that the LF fatigue damage is clearly affected by the influential factors. The increase in water depth and spectral peak periods, and the existence of risers can weaken the contribution of the LF components to the mooring line fatigue damage, while the fatigue damage due to the LF components increases with the increase of significant wave height.
基金Sponsored by National Nature Science Foundation of China ( 51179195)National Defense Foundation of China ( 513030203-02)
文摘Elastic wave on seafloor caused by low frequency noise radiated from ship is called ship seismic wave which can be used to identify ship target. In order to analyze the wave components and the propagating properties of ship seismic wave, the numerical calculation of synthetic seismograms on seafloor aroused by a low frequency point sound source is carried out using a wave number integration technique combined with inverse Fourier transform. According to the numerical example of hard seafloor, the time series of seismic wave on seafloor are mostly composed of interface waves and normal mode waves. Each normal mode wave has a well defined low cut-off frequency, while the interface wave doesn't have. The frequency dispersion of normal mode wave is obvious when frequency is lower than 100Hz, while the interface wave is dispersive only in the infra-sound frequency range. The time series of seismic wave is dominated by the interface wave when the source frequency is less than the minimal cut-off frequency of normal mode wave.
文摘The influences of the low frequency electromagnetic field on the horizontal direct chill casting process were investigated experimentally. Ingots of 2024 aluminum alloy with a cross size of 40 mm× 200 mm were produced by the conventional horizontal chill casting process and low frequency electromagnetic horizontal chill casting processre- spectively. The as-cast structures and the mechanical property of the ingots were examined. The results showed that the low frequency electromagnetic field could sub- stantially refine the microstructures and pronouncedly reduce the macrosegregation in the horizontal direct chill casting process. Moreover, the surface quality of the ingot was prominently improved by the low frequency electromagnetic field. The fracture strength and elongation percentage of the ingot was increased with the low frequency electromagnetic field.
基金the support of the National Basic Research Program(973 Program)of China(Grant No.2011CB610304)the National Natural Science Foundation of China(Grant Nos.11332004 and 11402046)+2 种基金China Postdoctoral Science Foundation(No.2015M571296)the 111 Project(B14013)the CATIC Industrial Production Projects(Grant No.CXY2013DLLG32)
文摘We present a design method for calculating and optimizing sound absorption coefficient of multi-layered porous fibrous metals （PFM） in the low frequency range. PFM is simplified as an equivalent idealized sheet with all metallic fibers aligned in one direction and distributed in periodic hexagonal patterns. We use a phenomenological model in the literature to investigate the effects of pore geometrical parameters （fiber diameter and gap） on sound absorption performance. The sound absorption coefficient of multi- layered PFMs is calculated using impedance translation theorem, To demonstrate the validity of the present model, we compare the predicted results with the experimental data. With the average sound absorption （low frequency range） as the objective function and the fiber gaps as the design variables, an optimization method for multi-layered fibrous metals is proposed. A new fibrous layout with given porosity of multi-layered fibrous metals is suggested to achieve optimal low frequency sound absorption. The sound absorption coefficient of the optimal multi-layered fibrous metal is higher than the single- layered fibrous metal, and a significant effect of the fibrous material on sound absorption is found due to the surface Dorosity of the multi-layered fibrous.
基金Project supported by the National Natural Science Foundation of China(Nos.11672008,11702188,and 1832002)
文摘Vibration energy harvesting is to transform the ambient mechanical energy to electricity. How to reduce the resonance frequency and improve the conversion efficiency is very important. In this paper, a layer-separated piezoelectric cantilever beam is proposed for the vibration energy harvester(VEH) for low-frequency and wide-bandwidth operation, which can transform the mechanical impact energy to electric energy. First,the electromechanical coupling equation is obtained by the Euler-Bernoulli beam theory.Based on the average method, the approximate analytical solution is derived and the voltage response is obtained. Furthermore, the physical prototype is fabricated, and the vibration experiment is conducted to validate the theoretical principle. The experimental results show that the maximum power of 0.445 μW of the layer-separated VEH is about3.11 times higher than that of the non-impact harvester when the excitation acceleration is 0.2 g. The operating frequency bandwidth can be widened by increasing the stiffness of the fundamental layer and decreasing the gap distance of the system. But the increasing of operating frequency bandwidth comes at the cost of reducing peak voltage. The theoretical simulation and the experimental results demonstrate good agreement which indicates that the proposed impact-driving VEH device has advantages for low-frequency and wide-bandwidth. The high performance provides great prospect to scavenge the vibration energy in environment.
文摘By use of the filter analysis technique, the Complex Empirical Othogonal Function (CEOF) method and the ECMWF/WMO 2.5°×2.5°grid data of the geopotential heights during the summer months in 1988, an interseasonal process that the western Pacific subtropical high (WPSH) was anomalously far to the north in the first and second ten days of July is studied. It has been found that in the western Pacific subtropical region in the first and second ten days of July,it is the continuous assembly of low frequency geopotential waves (LFGWs) that leads to the abnormality of WPSH. This abnormality emerges with the enhancement of wave assembling and ceases while the wave assembling situation disappears. The structure of the low frequency assembling waves corresponds to the structure of subtropical high in its abnormal period. The effect of the assembling waves on the abnormality of subtropical high can be considered as the accumulation of disturbance energy carried by the low frequency waves from different directions in the western Pacific region.
