Energetic structural materials(ESMs)are a new type of structural materials with bearing and damage characteristics.In this work the microstructure,mechanical properties and energy release characteristics of multi-elem...Energetic structural materials(ESMs)are a new type of structural materials with bearing and damage characteristics.In this work the microstructure,mechanical properties and energy release characteristics of multi-element Ti-Zr-Ta alloys with good casting performance were studied.The microstructure of the Ti_(x)ZrTa alloys gradually change from BCC+HCP to single BCC structure with the increase of Ti.While the Ti_(2)Zr_(y)Ta alloys was still uniform and single BCC structure with the increase of Zr.The evolution of microstructure and composition then greatly affect the mechanical properties and energy-release characteristics of Ti-Zr-Ta alloys.The synergistic effect of dual phase structure increases the fracture strain of Ti_(x)ZrTa(x=0.2,0.5)with the Ti content decreases,while the fracture strain of Ti_(x)ZrTa(x=2.0,3.0,4.0)gradually increase with the Ti content increases caused by the annihilation of the obstacles for dislocation movement.And as Zr content increases,the fracture strain of Ti_(2)Zr_(y)Ta alloys decrease,then the oxidation reaction rate and fragmentation degree gradually increase.The higher oxidation rate and the lager exposed oxidation area jointly leads the higher releasing energy efficiency of Ti_(x)ZrTa alloys with low Ti content and Ti_(2)Zr_(y)Ta alloys with high Zr content.展开更多
In this paper,three kinds of materials including graphite,titanium(Ti)and molybdenum(Mo)are used as anodes to figure out the influence factors of anode material on the characteristics of the intense electron beam diod...In this paper,three kinds of materials including graphite,titanium(Ti)and molybdenum(Mo)are used as anodes to figure out the influence factors of anode material on the characteristics of the intense electron beam diode.The results show that the characteristics of diode are mainly determined by the cathode plasma motion under a 15 mm diode gap,in which the typical electron beam parameters are 280 kV,3.5 kA.When the diode gap is reduced to 5 mm,the voltage of the electron beam reduces to about 200 kV,and its current increases to more than 8.2 kA.It is calculated that the surface temperatures of Ti and Mo anodes are higher than their melting points.The diode plasma luminescence images show that Ti and Mo anodes produce plasmas soon after the bombardment of electron beams.Ti and Mo lines are respectively found in the plasma composition of Ti and Mo anode diodes.Surface melting traces are also observed on Ti and Mo anodes by comparing the micromorphologies before and after bombardment of the electron beam.These results suggest that the time of anode plasma generation is closely related to the anode material.Compared with graphite,metal Ti and Mo anodes are more likely to produce large amounts of plasma due to their more significant temperature rise effect.According to the moment that anode plasma begins to generate,the average expansion velocities of cathode and anode plasma are estimated by fitting the improved space-charge limited flow model.This reveals that generation and motion of the anode plasma significantly affect the characteristics of intense electron beam diode.展开更多
The radiofrequency(RF) inductive cathode has great prospects in space missions with long mission cycles, large speed increments, and rapid response requirements as the main electron source and neutralizer in Hall thru...The radiofrequency(RF) inductive cathode has great prospects in space missions with long mission cycles, large speed increments, and rapid response requirements as the main electron source and neutralizer in Hall thrusters and ion thrusters. This paper proposes a comprehensive multi-physics RF inductive cathode model in which the RF electromagnetic field, electrostatic field for extracting electrons, flow field, plasma transport and electrochemical reaction process are all accounted for. Each physical field mentioned above can form a closed partial differential equation. The two-dimensional finite element code COMSOL is used to solve the multi-physics model. With this model, the formation process of the anode spot is exhibited and demonstrates the non-bipolar flow theory in practice. The simulation results demonstrate that the current jump in the RF inductive cathode is caused by the anode spot. Furthermore, the influences of preset discharge parameters such as RF power, bias voltage and actuating gas flow as well as structural parameters like the coil structure, discharge chamber size and ion collector area, emission hole size, distance between the anode target and the emission hole etc on the cathode performance are investigated, and some important optimal parameters are proposed.展开更多
Steel-tube-confined concrete(STCC) targets are provided with excellent anti-penetration performance over semi-infinite concrete(SIC) targets since the steel tube imposes passive restraint on the in-filled concrete dur...Steel-tube-confined concrete(STCC) targets are provided with excellent anti-penetration performance over semi-infinite concrete(SIC) targets since the steel tube imposes passive restraint on the in-filled concrete during the penetration process. Grid STCC system with square steel tubes is a potential solution to protective structures. In this paper, experiments of 9-cell grid STCC targets penetrated by 12.7 mm Armor Piercing Projectile(APP) were performed. The influence of side length and thickness of steel tube,steel ratio and impact velocity on anti-penetration performance were taken into account. Additionally,single-cell square STCC targets were also designed and tested for comparison with the 9-cell grid STCC targets. Damage modes and parameters of the tested targets were measured and discussed. Moreover,the stiffness of radial confinement of grid STCC targets is achieved according to the elastic solution of infinite cylindrical shell in Winkler medium. Furthermore, the penetration resistance and depth of penetration(DOP) for grid STCC targets are obtained on the basis of the dynamic finite spherical cavityexpansion(FSCE) models including radial confinement effect. It is shown that the 9-cell grid STCC targets with optimal dimension match of thickness and side length of steel tube can reduce the DOP by about17 % and 23 % in comparison with the SIC targets and single-cell square STCC targets, respectively, due to both the confinement of square steel tube to concrete in the impacted cell and the additional confinement of the surrounding cells to the impacted cell;the penetration resistance and DOP of the grid and cellular STCC targets with similar steel ratio is close, and thus the grid STCC targets with simpler manufacturing process and excellent in-plane expandability are preferred in engineering practice;moreover, the predicted results of DOP model based on the FSCE models agree well with the tested results with the maximum disparity less than 12 % and the proposed model is more applicable to the grid and cellular STCC targets with high radial confinement.展开更多
It is important to calculate the reachable domain(RD)of the manned lunar mission to evaluate whether a lunar landing site could be reached by the spacecraft. In this paper, the RD of free return orbits is quickly eval...It is important to calculate the reachable domain(RD)of the manned lunar mission to evaluate whether a lunar landing site could be reached by the spacecraft. In this paper, the RD of free return orbits is quickly evaluated and calculated via the classification and regression neural networks. An efficient databasegeneration method is developed for obtaining eight types of free return orbits and then the RD is defined by the orbit’s inclination and right ascension of ascending node(RAAN) at the perilune. A classify neural network and a regression network are trained respectively. The former is built for classifying the type of the RD, and the latter is built for calculating the inclination and RAAN of the RD. The simulation results show that two neural networks are well trained. The classification model has an accuracy of more than 99% and the mean square error of the regression model is less than 0.01°on the test set. Moreover, a serial strategy is proposed to combine the two surrogate models and a recognition tool is built to evaluate whether a lunar site could be reached. The proposed deep learning method shows the superiority in computation efficiency compared with the traditional double two-body model.展开更多
Unsteady wake from upstream components of landing gear impinging on downstream components could be a strong noise source.The use of a plane jet is proposed to reduce this flow-induced noise.Tandem rods with different ...Unsteady wake from upstream components of landing gear impinging on downstream components could be a strong noise source.The use of a plane jet is proposed to reduce this flow-induced noise.Tandem rods with different gap widths were utilized as the test body.Both acoustic and aerodynamic tests were conducted in order to validate this technique.Acoustic test results proved that overall noise emission from tandem rods could be lowered and tonal noise could be removed with use of the plane jet.However,when the plane jet was turned on,in some frequency range it could be the subsequent main contributor instead of tandem rods to total noise emission whilst in some frequency range rods could still be the main contributor.Moreover,aerodynamic tests fundamentally studied explanations for the noise reduction.Specifically,not only impinging speed to rods but speed and turbulence level to the top edge of the rear rod could be diminished by the upstream plane jet.Consequently,the vortex shedding induced by the rear rod was reduced,which was confirmed by the speed,Reynolds stress as well as the velocity fluctuation spectral measured in its wake.This study confirmed the potential use of a plane jet towards landing gear noise reduction.展开更多
The Si3N4-BN composites have been prepared via die pressing and precursor infiltration and pyrolysis route using borazine as precursor, and the effect of sintering additives on properties of the composites has been in...The Si3N4-BN composites have been prepared via die pressing and precursor infiltration and pyrolysis route using borazine as precursor, and the effect of sintering additives on properties of the composites has been investigated. After sintering additives are adopted, the α to β phase transition of Si3N4 and the mechanical properties of the composites at both room temperature and high temperature are all increased with small extent. When using Y2O3+Al2O3 as additives, the phase transition of Si3N4 and the mechanical properties of the composites have better results. The β-Si3N4 content is 17.47%. The fl exural strength, elastic modulus and fracture toughness of the composites are 188.74 MPa, 84.34 GPa and 2.96 MPa?m1/2, respectively. After exposed at 1 000 ℃ in the air for 15 min, the fl exural strength of the composites is 154.62 MPa with a residual ratio of 81.92%. The elongated β-Si3N4 grains appear in all composites with different sintering additives. Relatively more rod like β-Si3N4 grains can be observed in composites with Y2O3+Al2O3 as additives, making it to possess better mechanical properties.展开更多
A nonlinear controller for disturbances rejection and collision avoidance is proposed for spacecraft formation flying.The formation flying is described by a nonlinear model with the J_2 perturbation and atmospheric dr...A nonlinear controller for disturbances rejection and collision avoidance is proposed for spacecraft formation flying.The formation flying is described by a nonlinear model with the J_2 perturbation and atmospheric drag. Based on the theory of the state-dependent Riccati equation(SDRE), a finite time nonlinear control law is developed for the nonlinear dynamics involved in formation flying. Then, a compensative internal mode(IM) control law is added to eliminate disturbances.These two control laws compose a finite time nonlinear tracking controller with disturbances rejection. Moreover, taking safety requirements into account, the repulsive control law is incorporated in the composite controller to perform collision avoidance manoeuvres. A numerical simulation is presented to demonstrate the effectiveness of the proposed method.Compared to the conventional control method, the proposed method provides better performance in the presence of the obstacles and external disturbances.展开更多
In this study, we focused on the effect of the underwater explosion parameters of multi-point array explosion. The shock wave and bubble parameters of aggregate charge, two charges, and four charges were measured thro...In this study, we focused on the effect of the underwater explosion parameters of multi-point array explosion. The shock wave and bubble parameters of aggregate charge, two charges, and four charges were measured through an underwater explosion test, and their influence on the explosion power field of charge quantity and array distance was analyzed. Results show that the multi-shock wave collision of array explosion can be approximated to a linear superposition, and the interaction of delayed shock wave can be deemed as the increase of the shock wave baseline. Shock wave focusing and delayed superposition increase the shock wave peak pressure. Compared with the aggregate charge, the greater the number of array explosion points is, the higher the impulse and the gain of the bubble peak pressure are. At the same array distance, the smaller the charge quantity is, the higher the bubble impulse will be. At the same charge quantity, the smaller the array distance is, the higher the bubble impulse will be. The bubble period decreases gradually with the increase of the charge quantity, but the test orientation has little effect on the bubble period.展开更多
A novel method for estimating the space range of battery-powered vertical take-off and landing(VTOL) aircraft is presented. The method is based on flight parameter optimization and numerical iteration. Subsystem model...A novel method for estimating the space range of battery-powered vertical take-off and landing(VTOL) aircraft is presented. The method is based on flight parameter optimization and numerical iteration. Subsystem models including required thrust, required power and battery discharge models are presented. The problem to be optimized is formulated, and then case study simulation is conducted using the established method for quantitative analysis. Simulation results show that the space range of battery-powered VTOL aircraft in a vertical plane is an oblate curve, which appears horizontally long but vertically short, and the peak point is not located on the vertical climb path. The method and results are confirmed by parameter analysis and validations.展开更多
Experimental studies which focus on flow visualization and the velocity field of a supersonic laminar/turbulent flow over a compression ramp were carried out in a Mach 3.0 wind tunnel. Fine flow structures and velocit...Experimental studies which focus on flow visualization and the velocity field of a supersonic laminar/turbulent flow over a compression ramp were carried out in a Mach 3.0 wind tunnel. Fine flow structures and velocity field structures were obtained via NPLS(nanoparticle-tracer planar laser scattering) and PIV(particle image velocimetry) techniques, timeaveraged flow structures were researched, and spatiotemporal evolutions of transient flow structures were analyzed. The flow visualization results indicated that when the ramp angles were 25°, a typical separation occurred in the laminar flow,some typical flow structures such as shock induced by the boundary layer, separation shock, reversed flow and reattachment shock were visible clearly. While a certain extent separation occurred in turbulent flow, the separation region was much smaller. When the ramp angles were 28°, laminar flow separated further, and the separation region expanded evidently,flow structures in the separation region were complex. While a typical separation occurred in turbulent flow, reversed flow structures were significant, flow structures in the separation region were relatively simple. The experimental results of velocity field were corresponding to flow visualization, and the velocity field structures of both compression ramp flows agreed with the flow structures well. There were three layered structures in the U component velocity, and the V component velocity appeared like an oblique "v". Some differences between these two compression ramp flows can be observed in the velocity profiles of the shear layer and the shearing intensity.展开更多
Battery powered vertical takeoff and landing(VTOL) aircraft attracts more and more interests from public, while limited hover endurance hinders many prospective applications. Based on the weight models of battery, mot...Battery powered vertical takeoff and landing(VTOL) aircraft attracts more and more interests from public, while limited hover endurance hinders many prospective applications. Based on the weight models of battery, motor and electronic speed controller, the power consumption model of propeller and the constant power discharge model of battery, an efficient method to estimate the hover endurance of battery powered VTOL aircraft was presented. In order to understand the mechanism of performance improvement, the impacts of propulsion system parameters on hover endurance were analyzed by simulations, including the motor power density, the battery capacity, specific energy and Peukert coefficient. Ground experiment platform was established and validation experiments were carried out, the results of which showed a well agreement with the simulations. The estimation method and the analysis results could be used for optimization design and hover performance evaluation of battery powered VTOL aircraft.展开更多
Space electromagnetic docking technology,free of propellant and plume contamination,offers continuous,reversible and synchronous controllability,which is widely applied in the future routine on-orbit servicing mission...Space electromagnetic docking technology,free of propellant and plume contamination,offers continuous,reversible and synchronous controllability,which is widely applied in the future routine on-orbit servicing missions.Due to the inherent nonlinearities,couplings and uncertainties of an electromagnetic force model,the dynamics and control problems of them are difficult.A new modeling approach for relative motion dynamics with intersatellite force is proposed.To resolve these control problems better,a novel nonlinear control method for soft space electromagnetic docking is proposed,which combines merits of artificial potential function method,Lyapunov theory and extended state observer.In addition,the angular momentum management problem of space electromagnetic docking and approaches of handling it by exploiting the Earth's magnetic torque are investigated.Finally,nonlinear simulation results demonstrate the feasibility of the dynamic model and the novel nonlinear control method.展开更多
On the basis of considering electrochemical reactions and collision relations in detail, a direct numerical simulation model of a helicon plasma discharge with three-dimensional two-fluid equations was employed to stu...On the basis of considering electrochemical reactions and collision relations in detail, a direct numerical simulation model of a helicon plasma discharge with three-dimensional two-fluid equations was employed to study the characteristics of the temporal evolution of particle density and electron temperature. With the assumption of weak ionization, the Maxwell equations coupled with the plasma parameters were directly solved in the whole computational domain. All of the partial differential equations were solved by the finite element solver in COMSOL Multiphysics^(TM) with a fully coupled method. In this work, the numerical cases were calculated with an Ar working medium and a Shoji-type antenna. The numerical results indicate that there exist two distinct modes of temporal evolution of the electron and ground atom density, which can be explained by the ion pumping effect. The evolution of the electron temperature is controlled by two schemes: electromagnetic wave heating and particle collision cooling. The high RF power results in a high peak electron temperature while the high gas pressure leads to a low steady temperature. In addition, an OES experiment using nine Ar I lines was conducted using a modified CR model to verify the validity of the results by simulation, showing that the trends of temporal evolution of electron density and temperature are well consistent with the numerically simulated ones.展开更多
The physical process of a single-stage planar-pulsed-inductive accelerator is investigated.Measurements include the waveforms of circuit current,capacitor voltage,plasma radiation intensity,and temporal plasma structu...The physical process of a single-stage planar-pulsed-inductive accelerator is investigated.Measurements include the waveforms of circuit current,capacitor voltage,plasma radiation intensity,and temporal plasma structure photos captured by a high-speed camera.Experiments are conducted under static ambient fill condition using argon as propellant.Varied values of capacitor voltage and gas pressure are compared.Further discussions quantify the EM interaction between circuit and plasma,as well as their energy deposition and current sheet acceleration.Based on the results of experiments,physical mechanisms of the initial ionization phase and the following acceleration phase are analyzed theoretically.展开更多
Precise control of the discharge in space and time is of great significance for better applications of discharge plasma.Here,we used a femtosecond laser filament to trigger and guide a highvoltage DC pulse discharge t...Precise control of the discharge in space and time is of great significance for better applications of discharge plasma.Here,we used a femtosecond laser filament to trigger and guide a highvoltage DC pulse discharge to achieve spatiotemporal control of the discharge plasma.In space,the discharge plasma is distributed strictly along the channel generated by the femtosecond laser filament.The breakdown voltage threshold is reduced,and the discharge length is extended.In time,the electrical parameters such as the electrode voltage and the electrode gap affect discharge delay time and jitter.By optimizing the parameters,we can achieve sub-nanosecond jitter of the discharge.Based on the spatiotemporal control of the discharge,we applied filamenttriggered discharge for one-dimensional composition measurements of the gas flow field.Besides,the technique shows great potential in studying the spatiotemporal evolution of discharge plasma.展开更多
Ablative pulsed plasma thrusters(APPTs)are considered as an attractive propulsion option for station-keeping and drag makeup purposes for mass-and power-limited satellites.In order to understand the physical mechanism...Ablative pulsed plasma thrusters(APPTs)are considered as an attractive propulsion option for station-keeping and drag makeup purposes for mass-and power-limited satellites.In order to understand the physical mechanism of APPTs,high-speed camera and optical emission spectroscopy are utilized to investigate the plasma characteristics including the spatial distribution and composition between the electrodes.The plume images and spectra at different times and positions are experimentally recorded,and the spatial distribution,composition,and trajectory of plasmas can be concluded through analyzing them.With the increase of the distance from the ablation surface,two clusters of plasmas near the anode and cathode meet downstream,and the species and density of plasmas tend to be uniform.展开更多
The development of China’s space industry puts forward urgent requirements for high-performance debris shielding materials and high energy density rechargeable battery.In this review,the recent progress on debris shi...The development of China’s space industry puts forward urgent requirements for high-performance debris shielding materials and high energy density rechargeable battery.In this review,the recent progress on debris shielding fibrous materials and high energy density Li-S battery are particularly summarized.According to the experimental results,basalt fibers and silicon carbide fibers were chosen as the effective filling shielding materials.The geometric structure of fabrics was also investigated.For the novel shielding materials,high-strength and flexible silicon carbide micro-nano fibrous membranes were designed and fabricated.The obtained membranes with excellent mechanical properties portend the potential applications in debris protection structure.Furthermore,the high specific energy lithium sulfur batteries have made remarkable progress in fundamental research and application research in recent years.In order to solve the key problems of polysulfides shuttle and slow redox kinetics in lithium sulfur battery,a series of transition metal compound@hollow carbon-based material as sulfur host with dual functions of catalysis and adsorption towards polysulfides were designed and constructed.The obtained Li-S pouch cells with high areal sulfur loading of 6.9 mg·cm^(-2)yield exceptional high practical energy density of 382 W·h·kg^(-1)under lean electrolyte of 3.5μL·mg^(-1),demonstrating the great potential of realistic high-energy Li-S batteries.展开更多
More than 300 electrostatic solitary waves(ESWs)with a large perpendicular component which is a bi-polar waveform structure are observed in the boundary layer within the magnetic reconnection diffusion region in the n...More than 300 electrostatic solitary waves(ESWs)with a large perpendicular component which is a bi-polar waveform structure are observed in the boundary layer within the magnetic reconnection diffusion region in the near-Earth magnetotail.Such ESWs are called two-dimensional ESWs.A Singe-reconnection-based-statistical study of two-dimensional ESWs shows that:(1)ESWs can be continuously observed in the plasma sheet boundary layer(PSBL)associated with the magnetic reconnection diffusion region,and their amplitude ranges are mainly from several tens to hundreds ofμV/m;(2)both one-dimension-like ESWs(very small magnitude on E⊥)and two-dimension-like ESWs(large magnitude on E⊥,which are even comparable to that in the E||)are observed within a small time interval;(3)within the observation time spans,more than 61%of ESWs are regarded as two-dimensional ESWs for the I_(2)D>20%.We discuss the bi-polar structure in E⊥.The observation of ESWs with a large bi-polar structure in the perpendicular electric field gives evidence that the unique waveform differs from previous understanding from observations and simulations which suggests that it should be a uni-polar waveform structure in the E⊥of ESWs.展开更多
The flexible inflatable film structure has many advantages such as small folding volume,light weight,portability,simple transportation,rapid deployment,low cost,and high reliability,and it is gradually becoming widely...The flexible inflatable film structure has many advantages such as small folding volume,light weight,portability,simple transportation,rapid deployment,low cost,and high reliability,and it is gradually becoming widely used in spacecraft,which greatly reduces the overall weight and volume of the spacecraft and decreases the cost of spacecraft launch and operation on the track,and has become a research hotspot at home and abroad. However,the load-bearing characteristics of the flexible inflatable film structure are quite different from those of the general hard solid structures,whose bearing characteristics in the space environment have attracted much attention of scholars. In this paper,a typical flexible inflatable film biaxial curved structure-wing was taken as an example to study the bearing stress characteristics of flexible inflatable film aerospace structure under the condition of space temperature and pressure.Meanwhile,based on the method of thin film theory and flexible film structure static balance,an analysis model was constructed for the flexible inflatable film of biaxial bearing stress. The finite element ABAQUS software was used to simulate the calculation,and the load-bearing stress characteristics and deformation characteristics of the flexible inflatable film structure were studied under certain inflation pressure conditions. Besides, the load and stress characteristics of the flexible film were obtained under different conditions of the internal pressure of 1 KPa,1.3 KPa,and 1.6 KPa for the flexible inflatable film structure-wing. The results show that in the space vacuum environment,the bearing characteristics of the flexible film are closely related to various factors such as internal inflation pressure,film material properties,and film surface structure. As the internal inflation pressure increases,the load stress carried by the flexible film increases,too. Meanwhile,there is an obvious stress concentration and warpage deformation at the local joint of the flexible film,the load-bearing stress change is small around the region with a gentle curvature of the flexible inflatable film,and the flexible film is subject to biaxial stress load. However,at the region with large curvature,the uniaxial load bearing stress is significantly different from the load bearing stress in the other direction. But the deformation tends to change with a large curvature,and the biaxial load-bearing stress and deformation change law of the flexible inflatable film structure are also significantly different. Therefore,the results obtained in this paper could provide an effective theoretical reference for the flexible inflatable mechanical deformation analysis and structural optimization design of the flexible inflatable film structure(wing)on spacecrafts.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.52171166,11972372 and U20A20231)supported by Sinoma Institute of Materials Research(Guang Zhou)Co.,Ltd。
文摘Energetic structural materials(ESMs)are a new type of structural materials with bearing and damage characteristics.In this work the microstructure,mechanical properties and energy release characteristics of multi-element Ti-Zr-Ta alloys with good casting performance were studied.The microstructure of the Ti_(x)ZrTa alloys gradually change from BCC+HCP to single BCC structure with the increase of Ti.While the Ti_(2)Zr_(y)Ta alloys was still uniform and single BCC structure with the increase of Zr.The evolution of microstructure and composition then greatly affect the mechanical properties and energy-release characteristics of Ti-Zr-Ta alloys.The synergistic effect of dual phase structure increases the fracture strain of Ti_(x)ZrTa(x=0.2,0.5)with the Ti content decreases,while the fracture strain of Ti_(x)ZrTa(x=2.0,3.0,4.0)gradually increase with the Ti content increases caused by the annihilation of the obstacles for dislocation movement.And as Zr content increases,the fracture strain of Ti_(2)Zr_(y)Ta alloys decrease,then the oxidation reaction rate and fragmentation degree gradually increase.The higher oxidation rate and the lager exposed oxidation area jointly leads the higher releasing energy efficiency of Ti_(x)ZrTa alloys with low Ti content and Ti_(2)Zr_(y)Ta alloys with high Zr content.
文摘In this paper,three kinds of materials including graphite,titanium(Ti)and molybdenum(Mo)are used as anodes to figure out the influence factors of anode material on the characteristics of the intense electron beam diode.The results show that the characteristics of diode are mainly determined by the cathode plasma motion under a 15 mm diode gap,in which the typical electron beam parameters are 280 kV,3.5 kA.When the diode gap is reduced to 5 mm,the voltage of the electron beam reduces to about 200 kV,and its current increases to more than 8.2 kA.It is calculated that the surface temperatures of Ti and Mo anodes are higher than their melting points.The diode plasma luminescence images show that Ti and Mo anodes produce plasmas soon after the bombardment of electron beams.Ti and Mo lines are respectively found in the plasma composition of Ti and Mo anode diodes.Surface melting traces are also observed on Ti and Mo anodes by comparing the micromorphologies before and after bombardment of the electron beam.These results suggest that the time of anode plasma generation is closely related to the anode material.Compared with graphite,metal Ti and Mo anodes are more likely to produce large amounts of plasma due to their more significant temperature rise effect.According to the moment that anode plasma begins to generate,the average expansion velocities of cathode and anode plasma are estimated by fitting the improved space-charge limited flow model.This reveals that generation and motion of the anode plasma significantly affect the characteristics of intense electron beam diode.
文摘The radiofrequency(RF) inductive cathode has great prospects in space missions with long mission cycles, large speed increments, and rapid response requirements as the main electron source and neutralizer in Hall thrusters and ion thrusters. This paper proposes a comprehensive multi-physics RF inductive cathode model in which the RF electromagnetic field, electrostatic field for extracting electrons, flow field, plasma transport and electrochemical reaction process are all accounted for. Each physical field mentioned above can form a closed partial differential equation. The two-dimensional finite element code COMSOL is used to solve the multi-physics model. With this model, the formation process of the anode spot is exhibited and demonstrates the non-bipolar flow theory in practice. The simulation results demonstrate that the current jump in the RF inductive cathode is caused by the anode spot. Furthermore, the influences of preset discharge parameters such as RF power, bias voltage and actuating gas flow as well as structural parameters like the coil structure, discharge chamber size and ion collector area, emission hole size, distance between the anode target and the emission hole etc on the cathode performance are investigated, and some important optimal parameters are proposed.
基金the projects supported by the Natural Science Foundation of Hunan Province, China (No. 2018JJ2470 and 2021JJ30776)。
文摘Steel-tube-confined concrete(STCC) targets are provided with excellent anti-penetration performance over semi-infinite concrete(SIC) targets since the steel tube imposes passive restraint on the in-filled concrete during the penetration process. Grid STCC system with square steel tubes is a potential solution to protective structures. In this paper, experiments of 9-cell grid STCC targets penetrated by 12.7 mm Armor Piercing Projectile(APP) were performed. The influence of side length and thickness of steel tube,steel ratio and impact velocity on anti-penetration performance were taken into account. Additionally,single-cell square STCC targets were also designed and tested for comparison with the 9-cell grid STCC targets. Damage modes and parameters of the tested targets were measured and discussed. Moreover,the stiffness of radial confinement of grid STCC targets is achieved according to the elastic solution of infinite cylindrical shell in Winkler medium. Furthermore, the penetration resistance and depth of penetration(DOP) for grid STCC targets are obtained on the basis of the dynamic finite spherical cavityexpansion(FSCE) models including radial confinement effect. It is shown that the 9-cell grid STCC targets with optimal dimension match of thickness and side length of steel tube can reduce the DOP by about17 % and 23 % in comparison with the SIC targets and single-cell square STCC targets, respectively, due to both the confinement of square steel tube to concrete in the impacted cell and the additional confinement of the surrounding cells to the impacted cell;the penetration resistance and DOP of the grid and cellular STCC targets with similar steel ratio is close, and thus the grid STCC targets with simpler manufacturing process and excellent in-plane expandability are preferred in engineering practice;moreover, the predicted results of DOP model based on the FSCE models agree well with the tested results with the maximum disparity less than 12 % and the proposed model is more applicable to the grid and cellular STCC targets with high radial confinement.
基金supported by the National Natural Science Foundation of China (12072365)the Natural Science Foundation of Hunan Province of China (2020JJ4657)。
文摘It is important to calculate the reachable domain(RD)of the manned lunar mission to evaluate whether a lunar landing site could be reached by the spacecraft. In this paper, the RD of free return orbits is quickly evaluated and calculated via the classification and regression neural networks. An efficient databasegeneration method is developed for obtaining eight types of free return orbits and then the RD is defined by the orbit’s inclination and right ascension of ascending node(RAAN) at the perilune. A classify neural network and a regression network are trained respectively. The former is built for classifying the type of the RD, and the latter is built for calculating the inclination and RAAN of the RD. The simulation results show that two neural networks are well trained. The classification model has an accuracy of more than 99% and the mean square error of the regression model is less than 0.01°on the test set. Moreover, a serial strategy is proposed to combine the two surrogate models and a recognition tool is built to evaluate whether a lunar site could be reached. The proposed deep learning method shows the superiority in computation efficiency compared with the traditional double two-body model.
基金Project partially supported by the European Union FP7 Clean Sky Joint Technology Initiative“ALLEGRA”(Grant No.308225)
文摘Unsteady wake from upstream components of landing gear impinging on downstream components could be a strong noise source.The use of a plane jet is proposed to reduce this flow-induced noise.Tandem rods with different gap widths were utilized as the test body.Both acoustic and aerodynamic tests were conducted in order to validate this technique.Acoustic test results proved that overall noise emission from tandem rods could be lowered and tonal noise could be removed with use of the plane jet.However,when the plane jet was turned on,in some frequency range it could be the subsequent main contributor instead of tandem rods to total noise emission whilst in some frequency range rods could still be the main contributor.Moreover,aerodynamic tests fundamentally studied explanations for the noise reduction.Specifically,not only impinging speed to rods but speed and turbulence level to the top edge of the rear rod could be diminished by the upstream plane jet.Consequently,the vortex shedding induced by the rear rod was reduced,which was confirmed by the speed,Reynolds stress as well as the velocity fluctuation spectral measured in its wake.This study confirmed the potential use of a plane jet towards landing gear noise reduction.
基金Founded by the National Natural Science Foundation of China(Nos.90916019 and 50902150)Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province and Aid Program for Innovative Group of National University of Defense Technology
文摘The Si3N4-BN composites have been prepared via die pressing and precursor infiltration and pyrolysis route using borazine as precursor, and the effect of sintering additives on properties of the composites has been investigated. After sintering additives are adopted, the α to β phase transition of Si3N4 and the mechanical properties of the composites at both room temperature and high temperature are all increased with small extent. When using Y2O3+Al2O3 as additives, the phase transition of Si3N4 and the mechanical properties of the composites have better results. The β-Si3N4 content is 17.47%. The fl exural strength, elastic modulus and fracture toughness of the composites are 188.74 MPa, 84.34 GPa and 2.96 MPa?m1/2, respectively. After exposed at 1 000 ℃ in the air for 15 min, the fl exural strength of the composites is 154.62 MPa with a residual ratio of 81.92%. The elongated β-Si3N4 grains appear in all composites with different sintering additives. Relatively more rod like β-Si3N4 grains can be observed in composites with Y2O3+Al2O3 as additives, making it to possess better mechanical properties.
基金supported by the National Natural Science Foundation of China(Grant No.11404404)
文摘A nonlinear controller for disturbances rejection and collision avoidance is proposed for spacecraft formation flying.The formation flying is described by a nonlinear model with the J_2 perturbation and atmospheric drag. Based on the theory of the state-dependent Riccati equation(SDRE), a finite time nonlinear control law is developed for the nonlinear dynamics involved in formation flying. Then, a compensative internal mode(IM) control law is added to eliminate disturbances.These two control laws compose a finite time nonlinear tracking controller with disturbances rejection. Moreover, taking safety requirements into account, the repulsive control law is incorporated in the composite controller to perform collision avoidance manoeuvres. A numerical simulation is presented to demonstrate the effectiveness of the proposed method.Compared to the conventional control method, the proposed method provides better performance in the presence of the obstacles and external disturbances.
文摘In this study, we focused on the effect of the underwater explosion parameters of multi-point array explosion. The shock wave and bubble parameters of aggregate charge, two charges, and four charges were measured through an underwater explosion test, and their influence on the explosion power field of charge quantity and array distance was analyzed. Results show that the multi-shock wave collision of array explosion can be approximated to a linear superposition, and the interaction of delayed shock wave can be deemed as the increase of the shock wave baseline. Shock wave focusing and delayed superposition increase the shock wave peak pressure. Compared with the aggregate charge, the greater the number of array explosion points is, the higher the impulse and the gain of the bubble peak pressure are. At the same array distance, the smaller the charge quantity is, the higher the bubble impulse will be. At the same charge quantity, the smaller the array distance is, the higher the bubble impulse will be. The bubble period decreases gradually with the increase of the charge quantity, but the test orientation has little effect on the bubble period.
文摘A novel method for estimating the space range of battery-powered vertical take-off and landing(VTOL) aircraft is presented. The method is based on flight parameter optimization and numerical iteration. Subsystem models including required thrust, required power and battery discharge models are presented. The problem to be optimized is formulated, and then case study simulation is conducted using the established method for quantitative analysis. Simulation results show that the space range of battery-powered VTOL aircraft in a vertical plane is an oblate curve, which appears horizontally long but vertically short, and the peak point is not located on the vertical climb path. The method and results are confirmed by parameter analysis and validations.
基金supported by the National Natural Science Foundation of China(Grant Nos.11172326 and 11302256)
文摘Experimental studies which focus on flow visualization and the velocity field of a supersonic laminar/turbulent flow over a compression ramp were carried out in a Mach 3.0 wind tunnel. Fine flow structures and velocity field structures were obtained via NPLS(nanoparticle-tracer planar laser scattering) and PIV(particle image velocimetry) techniques, timeaveraged flow structures were researched, and spatiotemporal evolutions of transient flow structures were analyzed. The flow visualization results indicated that when the ramp angles were 25°, a typical separation occurred in the laminar flow,some typical flow structures such as shock induced by the boundary layer, separation shock, reversed flow and reattachment shock were visible clearly. While a certain extent separation occurred in turbulent flow, the separation region was much smaller. When the ramp angles were 28°, laminar flow separated further, and the separation region expanded evidently,flow structures in the separation region were complex. While a typical separation occurred in turbulent flow, reversed flow structures were significant, flow structures in the separation region were relatively simple. The experimental results of velocity field were corresponding to flow visualization, and the velocity field structures of both compression ramp flows agreed with the flow structures well. There were three layered structures in the U component velocity, and the V component velocity appeared like an oblique "v". Some differences between these two compression ramp flows can be observed in the velocity profiles of the shear layer and the shearing intensity.
文摘Battery powered vertical takeoff and landing(VTOL) aircraft attracts more and more interests from public, while limited hover endurance hinders many prospective applications. Based on the weight models of battery, motor and electronic speed controller, the power consumption model of propeller and the constant power discharge model of battery, an efficient method to estimate the hover endurance of battery powered VTOL aircraft was presented. In order to understand the mechanism of performance improvement, the impacts of propulsion system parameters on hover endurance were analyzed by simulations, including the motor power density, the battery capacity, specific energy and Peukert coefficient. Ground experiment platform was established and validation experiments were carried out, the results of which showed a well agreement with the simulations. The estimation method and the analysis results could be used for optimization design and hover performance evaluation of battery powered VTOL aircraft.
基金supported by the National Natural Science Foundation of China(11172322)
文摘Space electromagnetic docking technology,free of propellant and plume contamination,offers continuous,reversible and synchronous controllability,which is widely applied in the future routine on-orbit servicing missions.Due to the inherent nonlinearities,couplings and uncertainties of an electromagnetic force model,the dynamics and control problems of them are difficult.A new modeling approach for relative motion dynamics with intersatellite force is proposed.To resolve these control problems better,a novel nonlinear control method for soft space electromagnetic docking is proposed,which combines merits of artificial potential function method,Lyapunov theory and extended state observer.In addition,the angular momentum management problem of space electromagnetic docking and approaches of handling it by exploiting the Earth's magnetic torque are investigated.Finally,nonlinear simulation results demonstrate the feasibility of the dynamic model and the novel nonlinear control method.
基金funding from National Natural Science Foundation of China under grant agreement No. 11305265 (research on the acceleration mechanism of an electric double layer in a helicon plasma with a divergent magnetic field)
文摘On the basis of considering electrochemical reactions and collision relations in detail, a direct numerical simulation model of a helicon plasma discharge with three-dimensional two-fluid equations was employed to study the characteristics of the temporal evolution of particle density and electron temperature. With the assumption of weak ionization, the Maxwell equations coupled with the plasma parameters were directly solved in the whole computational domain. All of the partial differential equations were solved by the finite element solver in COMSOL Multiphysics^(TM) with a fully coupled method. In this work, the numerical cases were calculated with an Ar working medium and a Shoji-type antenna. The numerical results indicate that there exist two distinct modes of temporal evolution of the electron and ground atom density, which can be explained by the ion pumping effect. The evolution of the electron temperature is controlled by two schemes: electromagnetic wave heating and particle collision cooling. The high RF power results in a high peak electron temperature while the high gas pressure leads to a low steady temperature. In addition, an OES experiment using nine Ar I lines was conducted using a modified CR model to verify the validity of the results by simulation, showing that the trends of temporal evolution of electron density and temperature are well consistent with the numerically simulated ones.
基金Project supported by the Hunan Provincial Natural Science Foundation of China(Grant No.2018JJ3592).
文摘The physical process of a single-stage planar-pulsed-inductive accelerator is investigated.Measurements include the waveforms of circuit current,capacitor voltage,plasma radiation intensity,and temporal plasma structure photos captured by a high-speed camera.Experiments are conducted under static ambient fill condition using argon as propellant.Varied values of capacitor voltage and gas pressure are compared.Further discussions quantify the EM interaction between circuit and plasma,as well as their energy deposition and current sheet acceleration.Based on the results of experiments,physical mechanisms of the initial ionization phase and the following acceleration phase are analyzed theoretically.
基金funded by National Natural Science Foundation of China(NSFC)(Nos.51806149,91741205)。
文摘Precise control of the discharge in space and time is of great significance for better applications of discharge plasma.Here,we used a femtosecond laser filament to trigger and guide a highvoltage DC pulse discharge to achieve spatiotemporal control of the discharge plasma.In space,the discharge plasma is distributed strictly along the channel generated by the femtosecond laser filament.The breakdown voltage threshold is reduced,and the discharge length is extended.In time,the electrical parameters such as the electrode voltage and the electrode gap affect discharge delay time and jitter.By optimizing the parameters,we can achieve sub-nanosecond jitter of the discharge.Based on the spatiotemporal control of the discharge,we applied filamenttriggered discharge for one-dimensional composition measurements of the gas flow field.Besides,the technique shows great potential in studying the spatiotemporal evolution of discharge plasma.
基金National Natural Science Foundation of China(No.11772354)for the financial assistance provided for this work。
文摘Ablative pulsed plasma thrusters(APPTs)are considered as an attractive propulsion option for station-keeping and drag makeup purposes for mass-and power-limited satellites.In order to understand the physical mechanism of APPTs,high-speed camera and optical emission spectroscopy are utilized to investigate the plasma characteristics including the spatial distribution and composition between the electrodes.The plume images and spectra at different times and positions are experimentally recorded,and the spatial distribution,composition,and trajectory of plasmas can be concluded through analyzing them.With the increase of the distance from the ablation surface,two clusters of plasmas near the anode and cathode meet downstream,and the species and density of plasmas tend to be uniform.
基金Supported by the National Natural Science Foundation of China(52002400)Young Elite Scientists Sponsorship Program by CAST(YESS20200093)。
文摘The development of China’s space industry puts forward urgent requirements for high-performance debris shielding materials and high energy density rechargeable battery.In this review,the recent progress on debris shielding fibrous materials and high energy density Li-S battery are particularly summarized.According to the experimental results,basalt fibers and silicon carbide fibers were chosen as the effective filling shielding materials.The geometric structure of fabrics was also investigated.For the novel shielding materials,high-strength and flexible silicon carbide micro-nano fibrous membranes were designed and fabricated.The obtained membranes with excellent mechanical properties portend the potential applications in debris protection structure.Furthermore,the high specific energy lithium sulfur batteries have made remarkable progress in fundamental research and application research in recent years.In order to solve the key problems of polysulfides shuttle and slow redox kinetics in lithium sulfur battery,a series of transition metal compound@hollow carbon-based material as sulfur host with dual functions of catalysis and adsorption towards polysulfides were designed and constructed.The obtained Li-S pouch cells with high areal sulfur loading of 6.9 mg·cm^(-2)yield exceptional high practical energy density of 382 W·h·kg^(-1)under lean electrolyte of 3.5μL·mg^(-1),demonstrating the great potential of realistic high-energy Li-S batteries.
基金Supported by the Post-Doctoral Research Program of China(42956)the National Basic Research Program of Chinathe National Natural Science Foundation of China under Grant No 40890163.
文摘More than 300 electrostatic solitary waves(ESWs)with a large perpendicular component which is a bi-polar waveform structure are observed in the boundary layer within the magnetic reconnection diffusion region in the near-Earth magnetotail.Such ESWs are called two-dimensional ESWs.A Singe-reconnection-based-statistical study of two-dimensional ESWs shows that:(1)ESWs can be continuously observed in the plasma sheet boundary layer(PSBL)associated with the magnetic reconnection diffusion region,and their amplitude ranges are mainly from several tens to hundreds ofμV/m;(2)both one-dimension-like ESWs(very small magnitude on E⊥)and two-dimension-like ESWs(large magnitude on E⊥,which are even comparable to that in the E||)are observed within a small time interval;(3)within the observation time spans,more than 61%of ESWs are regarded as two-dimensional ESWs for the I_(2)D>20%.We discuss the bi-polar structure in E⊥.The observation of ESWs with a large bi-polar structure in the perpendicular electric field gives evidence that the unique waveform differs from previous understanding from observations and simulations which suggests that it should be a uni-polar waveform structure in the E⊥of ESWs.
基金Hunan Province Science and Technology Plan Project Funding(2018JJ3591)the National Natural Science Foundation of China(11802334)
文摘The flexible inflatable film structure has many advantages such as small folding volume,light weight,portability,simple transportation,rapid deployment,low cost,and high reliability,and it is gradually becoming widely used in spacecraft,which greatly reduces the overall weight and volume of the spacecraft and decreases the cost of spacecraft launch and operation on the track,and has become a research hotspot at home and abroad. However,the load-bearing characteristics of the flexible inflatable film structure are quite different from those of the general hard solid structures,whose bearing characteristics in the space environment have attracted much attention of scholars. In this paper,a typical flexible inflatable film biaxial curved structure-wing was taken as an example to study the bearing stress characteristics of flexible inflatable film aerospace structure under the condition of space temperature and pressure.Meanwhile,based on the method of thin film theory and flexible film structure static balance,an analysis model was constructed for the flexible inflatable film of biaxial bearing stress. The finite element ABAQUS software was used to simulate the calculation,and the load-bearing stress characteristics and deformation characteristics of the flexible inflatable film structure were studied under certain inflation pressure conditions. Besides, the load and stress characteristics of the flexible film were obtained under different conditions of the internal pressure of 1 KPa,1.3 KPa,and 1.6 KPa for the flexible inflatable film structure-wing. The results show that in the space vacuum environment,the bearing characteristics of the flexible film are closely related to various factors such as internal inflation pressure,film material properties,and film surface structure. As the internal inflation pressure increases,the load stress carried by the flexible film increases,too. Meanwhile,there is an obvious stress concentration and warpage deformation at the local joint of the flexible film,the load-bearing stress change is small around the region with a gentle curvature of the flexible inflatable film,and the flexible film is subject to biaxial stress load. However,at the region with large curvature,the uniaxial load bearing stress is significantly different from the load bearing stress in the other direction. But the deformation tends to change with a large curvature,and the biaxial load-bearing stress and deformation change law of the flexible inflatable film structure are also significantly different. Therefore,the results obtained in this paper could provide an effective theoretical reference for the flexible inflatable mechanical deformation analysis and structural optimization design of the flexible inflatable film structure(wing)on spacecrafts.
