Ducted-fan unmanned aerial vehicle(UAV)can realize take-off,landing and hovering,which has a simple and reliable structure and high safety coefficient.In view of the restricted take-off and landing environment,redundancy of folding wing mechanism and low aerodynamic efficiency of conventional water-air amphibious UAVs,a new dual ducted fan water-air amphibious trans-media UAV is designed.The fuselage is optimized with reference to underwater unmanned underwater vehicle(UUV)and UAV,and the tilting dual ducted fan power system is used.Based on the improved blade element momentum theory,the theoretical formula for calculating the lift of small ducted fan is derived.The structural vibration pattern of UAV as a whole under the mechanical physical field is analyzed,and the aerodynamic performance of the whole vehicle is verified by flow field finite element simulation.The analyzed results show that the calculated results of the improved small ducted fan lift calculation formula are basically consistent with the wind tunnel test results,with the maximum error between the two results is 4.5%,and the designed small ducted fan can provide 20% of additional lift with high aerodynamic efficiency.Through the modal analysis,it is found that the UAV fuselage structure is more stable,and the vibration pattern is reasonable.The wind tunnel test and the test flight verification of the ducted fan water-air amphibious UAV in different media are completed,which proves the feasibility of the UAV design scheme.
A SBE_YOLOv8s small target detection algorithm based on multi-scale attention mechanism is proposed for the UAV aerial images with high density,small target size and complex background.First,a feature extraction module EF_C2f(EMA-Faster Block_C2f) based on the multi-scale attention mechanism is designed to replace the C2f module in the YOLOv8 network to improve the network’s ability to extract small target features.And then a P1 detection layer is added to the feature fusion network,and a cross-scale feature fusion structure BPAN(Bi-Path Aggregation Network) is designed to fuse the small target feature information.Finally,a tiny target detection head is added,and SIoU Loss is used as the bounding-box loss function to improve the detection accuracy of small targets and the convergence speed of the network.The proposed algorithm is validated on the public dataset VisDrone2019.Compared with YOLOv8s algorithm,the proposed algorithm improves the detection accuracy by 6.9% and mAP50 by 9.1%,and reduces the amount of parameters of the model is by 46.4%,and the detection speed is 28 fps.The experimental results show that the proposed algorithm has a certain degree of utility in the field of small target detection.
The near-field blast wave characteristics(0.06m/kg1/3<Z<1m/kg1/3)of typical charge structures(sphere and cylinder)are investigated for supporting the protection of sympathetic detonation through near-field explosion experiment and numerical simulation.The load of near-field blast wave of cylindrical charge is experimentaly investigated,and a numerical caculation model of near-field explosion is established,The near-field blast wave structures and spatial distributions for peak parameters of spherical and cylindrical charges are analyzed.The results show that the near-field explosion loads are affected by the detonation products,and the reflected overpressure histories exhibit multi-peak and jagged form,which are due to the effects of multiple reflection waves and the complex flow of detonation products after the Mach region.The inverse of pressure and density gradient between detonation products and shock wave interface leads to the Rayleigh-Taylor instability effect.The generated interfacial microjets of detonation products promote the complexity of multiple reflection wave structure,resulting in large uncertainty of measured results of near-field reflected loads.The near-field incident overpressure curve shows a typical double-peak structure,which comes from the air shock wave and detonation products.Due to the structural effects of charge,the incident wave loads of cylindrical charge distribute non-uniformly along the axial and radial directions.The spatial distribution of incident parameters drastically varies when the azimuth angle is between 30°-60° because of the bridging wave effects.The modified predictive model can capture the incident peak overpressure and impulse in the range of 0.06m/kg1/3<Z<1m/kg1/3 for cylindrical charge(L/D=0.8)with center initiation under arbitrary azimuth angle,and the relative deviations among predicted results and numerical results of incident peak overpressure and impulse are less than 20%.
To investigate the detonation characteristics of composite charges containing the reactive materials and explosives,a composite charge containing Al/PTFE/TiH2 ternary reactive materials and RDX explosives is prepared.The influences of TiH2 powder content on detonation performance and afterburning effect are systematically studied by the air blast experiment and the colorimetric temperature measurement technology.The experimental results show that the shock wave parameters,explosion fireball duration and maximum average temperature of the composite charges increase first and then decrease with the increase in TiH2 powder content,reaching the maximum values when the content of TiH2 powders is 10%.Compared with the composite charge without TiH2 powders,the shock wave peak pressure,explosion fireball duration and maximum average temperature of composite charges containing 10% of TiH2 powders are increased by 21.6%,105.9% and 7.1%,respectively.Furthermore,the effects of TiH2 and Ti powders on the explosion energy release characteristics of composite charges are compared.It is found that the enhancement effects of TiH2 powders on the explosion shock wave parameters and fireball temperature field of the composite charge are superior to those of Ti powders due to the participation of free hydrogen in the afterburning reaction.The research results can provide valuable references for the application design of Al/PTFE/TiH2 ternary reactive materials for composite charges as well as the weapon damage assessment.
Counter-terrorism personnel need to address both stabbing and shooting threats when dealing with urban violent terrorist attack.Single-function protective gear is insufficient to ensure life safety.A multifunctional protective effect can be achieved by combining the bulletproof and stab-resistant materials based on their protective mechanisms.Currently,the development of dual-protection suits based on composite materials primarily relies on trial and error in experiments,resulting in low design efficiency and high costs.To address this issue,a theoretical design and optimization method is proposed for fiber-mixed bulletproof and stab-proof structures based on energy absorption criteria.Ballistic experiment and dynamic puncture test are conducted using single protective materials,namely resin-coated aramid fabric and unidirectional ultra-high molecular weight polyethylene(PE)fabric.The ballistic and puncture kinetic energy absorptions of single material are calculated.Based on the law of energy conservation,a combined bulletproof and stab-proof structure with a given areal density of 8500g/m2 is designed,and the range of layers for bulletproof and stab-proof materials is determined,After validation through ballistic and dynamic puncture experiments,the theoretical calculations demonstrate sufficient safety margins,The combination of 11 layers of resin-coated aramid fabric and 18 layers of unidirectional PE fabric meets the requirements of GA68-2019 Police Stab-resistant Clothing and police level III bulletproof standards while being the lightest with an areal density of 8390g/m2.The bulletproof and stab-proof structure is optimized to achieve the lightest combination structure with an areal density of 8370g/m2,consisting of 9 layers of resin-coated aramid fabric and 21 layers of unidirectional PE fabric.These research findings can provide reference for the engineering design and innovative application of dual-protection suits.
The shock wave protection ability is the main performance index of individual soldier helmet,which is of great significance to improve solider battlefield survivability.To improve the protective effect of the individual soldier helmet against shock wave,the structure and arrangement of the liner of advanced combat helmet (ACH) are explored,and a design of ring-shaped outer edge liner is proposed,and its protective effect is evaluated.The protection performances of helmets with different structures and sizes of outer edge liner are tested in shock tube and real explosion field.The waveform and peak values of shock wave on the forehead,top and rear of head model are compared and analyzed.The experimental results show that,after wearing the helmet,the shock wave mainly enters through the gap between the helmet and head in diffraction form,and the accumulation and superposition of shock waves in the gap may increase the pressure.In addition,the local high pressure region which is far from the incoming wave is easy to form because of the underwash effect.The annular composite liner composited with polyurea-polyurethane foam-sponge can effectively prevent the diffraction wave from entering the inside of the helmet and reduce the superposition effect of diffraction wave.The attenuation rate of shock wave overpressure at each measuring point can reach more than 40% after wearing the helmet with annular composite liner.The smaller the gap formed during the shock wave action is under the condition of the incident shock wave (weak shock wave),the better the protective effect is.When the width of polyurea-polyurethane foam reaches 15 mm,the outer edge liner and the head model have the best fit degree and the protection effect is optimal.The experimental results show that the overpressure attenuation rates at the measuring points on forehead,top,and rear of head can reach 92.26%,88.82%,and 87.19%,respectively.
In order to control the energy distribution of underwater explosion of explosive for the directional striking,the distribution characteristics of the explosion pressure field of a typical explosive-air column charge structure are studied.The pressure and impulse distribution of underwater explosions of explosives with different sizes of air domains are explored through experiment and numerical simulation.It is found that the air domain has a directional enhancement effect on the underwater explosions.The simulated results show that the air domain would cause the explosive products to impact the air-water boundary,resulting in an enhancement of peak pressure in the direction of the air domain in the near field water.With the change in bubble morphology,a vase-shaped bubble isolating energy is formed,leading to an enhancement of impulse behind the air domain.The above enhancement effects gradually disappear with the increase in distance,and the influence range of the impulse enhancement effect is wider.The research results indicate that the presence of air domain can significantly enhance the destructive effect within the near field in that direction.
The formation mechanism of coated reactive explosively formed projectiles (EFP) and the influence laws of liner structure on the formation behavior of EFP are studied. A Lagrange-Euler numerical simulation model of coated EFP is established, which reveals that the formation process of coated reactive EFP mainly includes axial double-liner impacting phase, radial coat closing phase and metal precursor penetrator stretching phase. In the axial double-liner impacting phase, the velocities of two liners rise in turns with axial kinetic energy transfer. In the radial coat closing phase, the copper liner folds forward to the axis and its tail completely coats the reactive liner. It is mentioned that a metal precursor penetrator is formed on the edge of copper liner. After that, the metal precursor penetrator is stretched and even fractured over time. Further, the influences of the shape parameters of copper and reactive liners on the coating formation are studied. With the decrease in the copper liner edge thickness from 2.0mm to 0.5mm, the closing time decreases from 57.9μs to 23.9μs, meanwhile the tip velocity increases from 1 851m/s to 2370m/s, and the penetrator length increase from 76mm to 110.5mm. As the curvature radius of copper liner decreases from 60mm to 40mm, the closing time of coating decreases from 42.1μs to 28.1μs, meanwhile the tip velocity increases from 1789m/s to 2242m/s, and the penetrator length increases from 66mm to 100mm. With the decrease in the reactive liner thickness from 6mm to 2mm, the closing time decreases from 52.0μs to 32.1μs, meanwhile the reactive liner mass decreases from 6.47g to 2.37g. With the decrease in the reactive liner diameter from 32mm to 16mm, the closing time decreases from 34.4μs to 30.8μs, meanwhile the reactive liner mass decreases from 6.42 g to 1.61g. The results can provide guidance and reference for the design of coated reactive EFP shaped charge.
The pressing ring is an indispensable part of an explosively formed projectile (EFP) charge structure for fastening the liner on the charge.A typical EFP charge structure with a copper spherical segment liner is adopted to study the influence of pressing ring on the EFP formed by the liner during explosion.The Influence of pressing ring on EFP forming properties is modeled and simulated using 3D dynamic finite element software, including Lagrange algorithm, arbitrary Lagrangian-Eeulerian (ALE) algorithm, smoothed particle hydrodynamics (SPH) method, and finite element method and smooth particle hydrodynamics (FEM-SPH) adaptive coupling algorithm.The EFP’s velocity and shape characteristics calculated by each algorithm are compared with the EFP images captured by pulse X-ray photography, and the FEM-SPH algorithm is used to obtain the high precision simulation results of EFP formation.On this basis, for the basic EFP charge structure with mass ratio of pressing ring to liner MR/ML≤0.2, the influences of the pressing ring’s rectangular and non-rectangular parameters (axial and radial thicknesses, and sectional shape) and the materials of pressing ring on the EFP’s initial velocity, mass conversion ratio, aspect ratio, and aerodynamic characteristics (compactness and windward area) parameters are simulated and calculated.Results show that the influences of axial and radial thicknesses and material parameters on the initial velocity of EFP is within 3%.The mass conversion ratio of EFP gradually decreases (up to 12.6%), the aspect ratio of EFP shows a gradually decreasing trend (with maximum reduction of 19.2%), and the compactness of of EFP with pressing ring is increased by 32.6% compared with the case without a ring.The upwind area shows a decreasing trend, indicating that the increase of ring weight is beneficial to the formation of EFP and thus decreases its upwind resistance.The results of this study provide guidance for the optimal design of EFP charge structure.
The large volume charge for fuel-air explosive weapons is a main developing trend.However,it is very difficult for a single cloud detonation device to spray large amounts of fuel.The mainstream solution is multiple cloud collaborative detonation.Therefore,how to design the distribution of multiple clouds to maximize the damage power of synergistic detonation overpressure is a technical bottleneck that needs to be resolved.The superposition effect of shock waves of double clouds collaborative detonation and the influence of distribution structural parameters on the detonation overpressure field are analyzed through numerical simulation.On this basis,the maximum damage gain of synergistic detonation is quantitatively investigated,and an optimal distribution scheme of multiple clouds detonation and a theoretical prediction model of damage area are given.The results show that there exists an optimal distance between two cloud regions,which maximizes the damage area of synergistic detonation.The dimensionless optimal distance between two cloud regions is related to the concentration of fuel.Based on the optimal distance between two cloud regions,the optimal radius of distribution field in multiple clouds with equidistant circumferential distribution is designed,and the maximum damage area of multiple cloud collaborative detonation under this optimal distribution is proportional to the 2/3 power of fuel mass and the 1/3 power of the number of cloud regions.
In order to explore the rules of damage of explosives with different energy structures to ships, the damage characteristics of underwater explosion of explosives with different energy structures to ship multi-cabin structures are studied.The energy output structure of underwater explosions of four types of explosives, namely TNT, PBXC19, H6, and PBXN111, is simulated using arbitrary Lagrange-Euler (ALE) algorithm, and the proportions of shock wave energy and bubble energy for different explosives are obtained.Subsequently, the damage mechanisms of explosives with different energy structures on multi-cabin structures are analyzed, and the damage characteristics of explosives with different energy structures on multi cabin structures are obtained.The research results show that, under the same charge,the larger the shock wave energy of the explosive is,the more severe the damage in the shock wave stage is,and the larger the bubble energy is,the more severe the damage in the bubble stage is.PBXC19 explosive of which shock wave energy is 2.28 times more than that of TNT increases the rupture diameter of the side shell by 30.2% during the shock wave stage compared to TNT When bubbles break on the free surface,the explosive H6 of which bubble energy is 1.44 times more than that of TNT increases the rupture diameter of the side shell by 9.9% during the bubble stage compared to TNT.Increasing the shock wave energy and bubble energy of explosives significantly increase the damage effect of shock waves and bubble loads on ship multi cabin structures.
The influence of sandwich structural composites with non-metallic core layer on the explosion propagation law of methane-air mixture is studied.The explosion experiments of the materials with different core layers,such as polyvinylidene fluoride(PVDF)foam board,hyriopsis cumingii shell and super absorbent polymer(SAP)colloid,are conducted on a self-developed experimental explosion pipe network platform.The influence rules of the sandwich structural composites with non-metallic cores on the explosion overpressure,flame propagation velocity,flame temperature and quenching parameter are analyzed.The research indicates that the sandwich structura; composites with non-metallic cores have good inhibition effects on the explosion propagation of methane-air mixture.Specifically,the composite material with PVDF foam board core layer shows the best inhibition effect on the flame temperature.The composite material with SAP colloid core layer shows the best inhibition effect on the explosion overpressure and flame propagation velocity,and its back-end quenching parameter is 9.20MPa·℃.Accordingly,the composite material with SAP colloid core layer has excellent explosion suppression properties.
The electrostatic safety boundary between coated energetic explosive particles in the production process is studied.A theoretical model of electrostatic interaction between charged coated energetic explosive particles is established by using the re-expansion method.The boundaries of charge density and electric field intensity threshold for electrostatic discharge of coated energetic explosive particle are defined based on the modified Paschen’s law. The influences of key particle structure and electrical properties on electrostatic agglomeration and discharge are discussed.The results show that the polarization effect of the coated medium enhances the electric field in the gap and attractive force,resulting in a like-charge attraction phenomenon of charged particles and exacerbating the discharge risk.Under the same charge,the attractive force between the coated active metal conductor particles is much higher than that between coated single-component explosive dielectric particles.When the surface charge density of coated active metal conductor particle is ±5.0μC/m2,the electric field exceeds the air breakdown threshold.For the coated dielectric particles,the electric field surpasses the breakdown threshold only when the surface charge density reaches ±50μC/m2.Therefore,the coated active metal conductor particles possess less electrostatic safety than that of coated single-component dielectric particles,and the greater risk of electrostatic agglomeration and electrostatic discharge.
Nitro gradiently distributed propellant is a kind of environmentally friendly smokeless propellant.The controllable construction of its structure is a complex physicochemical process,which is the key to determine the combustion performance of propellant.In order to study the kinetics of reaction process of nitro gradiently distributed propellant during preparation,the cylindrical granular single-base propellant sample was used as the research object, which density was tested by the liquid static weighing method.The influence rules of denitration reaction temperature and reaction liquid concentration on the denitration rate were investigated.The control steps of the reaction process were studied by using the Avrami model,and a reaction kinetics equation was established.The experimental results showed that the bigger the degree of denitration of the cylindrical granular single-base propellant sample was,the smaller its density was.The denitration reaction process of cylindrical granular single-base propellant was consistent with the Avrami model of n=0.816,the apparent activation energy was 49.223kJ/mol,the apparent reaction order of the reaction liquid concentration was 1.028,and the denitration reaction process was controlled by the mixture of diffusion and chemical reaction.Therefore,the chemical reaction effect or diffusion effect can be strengthened to improve the denitration rate of the cylindrical granular single-base propellant.This study provides a theoretical basis for the establishment of productive process conditions for nitro gradiently distributed propellant.
In order to address the issue of frequency sweeping jamming that a frequency modulated continuous wave (FMCW) fuze is susceptible to,a method for suppressing the frequency sweeping jamming based on matching transform order in fractional Fourier domain is designed.This method is based on the characteristics that the target echo beat signal of FMCW fuze is superimposed by multiple single-tone signals,and the jamming beat signal is linear frequency modulation signal.The position of jamming signal is obtained using the aggregation of linear frequency modulation signals at a specific order in the fractional Fourier domain through one-dimensional constant false alarm rate(1D-CFAR)detection.This method eliminates the impact of jamming on fuze by reconstructing the amplitude and phase of signal at the interfered location.Moreover,a fast search algorithm for frequency modulation,utilizing the iterative golden section search(GSS)method,is developed for the optimization of fractional order of jamming signals.The effectiveness of the proposed method was verified through the simulated and measured data.The results indicate that the proposed method can effectively suppress the frequency sweeping jamming when the signal to interference plus noise ratio(SINR)is below-20dB.
Icing on aircraft wings and engine air intakes leads to frequent flight accidents.In response to the problem of icing on wings and engine air intakes caused when an aircraft flies in a cold climate,a reinforcing fabric for electrically heated composites is developed for anti-icing and de-icing of aircraft.The electrically heated fabric,which is consisted of Ni-Cr alloy wire as an electric heating element,glass fiber plain fabric as a base fabric and polyester wire as the surface and bottom lines,is designed and prepared based on the tailored fiber placement (TFP) technology.The influences of TFP process parameters,such as stitch length,thread tension,and wiring speed,on the transaxial and zonal tensile properties of electrically heated fabric are investigated by tensile experiment,so as to select the optimal process parameters.Three kinds of electrically heated fabrics with different electrically heating element arrangement densities are prepared based on the optimal process parameters,and the electrothermal properties of the three kinds of electrically heated fabrics under different voltages are evaluated.The test results of tensile and electrothermal properties show that the retention rates of transaxial and zonal tensile strengths of Ni-Cr alloy wire electrically heated fabrics prepared based on TFP technology could reach up to 98.34% and 95.71%,respectively.The temperatures of three kinds of electrically heated fabrics could reach the highest balance temperature in 35 seconds.When the applied voltage is constant,The bigger the arrangement densities of the resistance wires are,the higher the highest balance temperature of the fabric is.The results can provide a reinforced fabric scheme for electrically heated composites for aircraft anti-icing and de-icing.
The effect of pulse triggering on the internal ballistics and transient flow field characteristics of solid rocket motor (SRM) is studied.An internal ballistic model of pulse triggering,a erosion combustion model and a dynamic flow field simulation model are established by numerical calculation method.The transient flow field of solid rocket motor in the process of pulse triggering is simulated.The results show that,the pressure prediction error is less than 5% compared with the experimental result.The pulse triggering increases the velocity of transverse gas flow in the combustion chamber,and the propellant near the pulse end is burning with erosion,which contributes 44% to the pressure rise.When the pulse is triggered,the closer the propellant is to the pulse inlet,the more aggressive the erosive burning is,and the maximum erosion ratio can reach 7.32.The simulated results show that the higher the pulse charge is,the greater the peak pressure is,the higher the pressure rise rate is,and the higher the attenuation rate is after the pulse is terminated.
To enhance the target detection efficiency of distributed multiple input and multiple output (MIMO) radar systems in low-altitude penetration combat scenarios,a cooperative game power allocation (CGPA) method is introduced.A distributed MIMO radar signal model used in a low-altitude multipath environment is established based on the support information with potential errors,and a detection model is derived based on the Neyman-Pearson criterion.The signal to interference plus noise ratio (SINR) is used as the utility function for the optimization model by incorporating the Max-Min criterion.On this basis,a weighted method is employed to streamline the computation of the Shapley value pertaining to coalition interests,therefore resulting in a cooperative resource allocation framework that meets both Pareto optimality and fairness criteria.The fluctuations and fading of received signal amplitude stemming from the multipath effect can be significantly mitigated by meticulously managing transmission power resources.Furthermore,in addition to enhancing the stability of the received signal,the abundant scattering features in the multipath environment are mined and harnessed,thereby effectively boosting the detection efficiency of the radar system.Simulated results validate the superior performance of distributed MIMO radar system for detecting the low-altitude multipath target.The proposed power allocation method significantly enhances the detection capabilities of the system while maintaining excellent real-time performance.
For the needs of target detection and precision guidance of image guided missile,the ground target intercept probability of image guided missile when it is transferred to terminal guidance based on a rectangular focal plane detector is investigated.The analytical solution of irregular quadrilateral detection field(IQDF) and the simplified rectangular detection field(SRDF) model for ground detection are derived from the altitude,attitude and detector parameters of missile.The main error sources affecting the target intercept probability are identified.The target intercept probability is decomposed into the missile state probability andthe probability of a target falling into the ground field-of-view in the current state by using the conditional probability idea,and the finite element segmentation algorithm is utilized to derive the probability of the target falling into the ground field of view(FOV),so as to establish a model of target intercept probability in the case of multi-source error.The effects of the main error sources on the probability of the target falling into the ground FOV in IQDF and SRDF are analyzed by simulation,and the derived target intercept probability is compared with the Monte Carlo simulation results in the case of multi-source errors,which verifies the effectiveness of the target intercept probability calculation method.
The dynamic response characteristics of human ear subjected to high-level sound intensity shock wave are studied.A series of experiments are designed and carried out by a physical headform comprising a realistic ear canal with/without hearing protectors under shock wave from an individual rocket launching.On the basis of the characteristics analysis of shock wave,a human ear test device model is designed as the research object.A test method of protective performance against shock waves for hearing protection devices(HPDs) is developed.The shock wave overpressures in the ears of gunner and assistant gunner are measured and analyzed.The measured data are analyzed by the wavelet transform method.The two kings of evaluation method for the safety limit of shock wave are compared.The results indicate that the high-intensity sound shock wave has instability,peak overpressure,short duration and multiple pulses during launching.It also has a wide spectrum with complex components and typical characteristics of low frequency and high sound pressure.The HPDs significantly reduce the effect of shock wave on the human ear,resulting in a prolonged action time and a significantly decreased peak overpressure.The peak overpressure and sound pressure level of shock wave on the gunner right ear could be attenuated by 95.7% and 14.8%,respectively.The calculated results about allowable exposure times per day from the evaluation modelin the modified GJB 2A-1996 and the AHAAH model in MIL-STD-1474E is roughly equivalent.The refined penetration formula in GJB 2A-1996 is feasible and can predict the permitted daily exposures times.This study can provide theoretical basis and practical guidance for test system design optimization of the individual weapons.
Time series matching technology is widely used in vehicle handling consistency evaluation.An evaluation method of vehicle throttle control action consistency based on segmented dynamic time warping (DTW) is proposed for the motion consistency evaluation of a coach vehicle in the process of cooperative driving.On the basis of the inconsistent number of sample data points in the cooperative control test of coach vehicle and the slope constraint of dynamic bending path, the traditional DTW rectangular search area is transformed into a parallelogram search area to reduce the area of the search area by changing the slope of search path, thus greatly reduce the calculation amount.Four groups of typical throttle action curves are selected to carry out 50 rounds of iterative experiments for verification, and the DTW distance matrix between the action curves of real vehicles A and B is calculated by segmented DTW method.A minimum deviation leveling method is used to combine the cluster objects for the actions of vehicles A and B, so as to complete the consistency evaluation of throttle action data.The experimental results show that the average matching accuracy of the improved DTW algorithm in each throttle action can reach 89.2%, which is about 3.2% higher than that of the single DTW algorithm, and the average matching time is about 92.45s, which is about 12.6% lower, thus verifying the feasibility and superiority of the segmented DTW algorithm in the consistency evaluation of throttle action.
With the widespread application of high-switching-frequency SiC power devices in the field of high-speed permanent magnet synchronous motor(PMSM)drives,the prominent dead-time effect leads to an increase in the contents of 5th and 7th high-frequency harmonic currents in high-speed domains.Traditional harmonic suppression strategies based on quasi-proportional resonant controllers exhibit significant resonant frequency deviations,which increase with the rise of the fundamental frequency,leading to suboptimal current harmonic suppression.Addressing the issue above,this paper proposes a current harmonic suppression strategy for high-speed PMSM based on zero-deviation quasi-proportional resonant control.By analyzing the frequency deviations of quasi-proportional resonant control under different discretization methods,a discrete transformation expression with zero frequency deviation is constructed,and the basis for setting the control parameters is provided,effectively suppressing the resonant frequency deviation.Both the simulated and experimental results demonstrate that the proposed control strategy achieves zero-deviation resonant frequency control,effectively suppressing the current harmonics of PMSM.
The high-speed electromechanical transmission(EMT) system for vehicle is characterized by a highly compact structure and high-speed components,leading to significant electromechanical coupling effects and susceptibility to resonance.This paper analyzes the electromechanical coupling inherent vibration characteristics of a series EMT system.An inherent vibration model of the system is derived,its natural frequencies and mode shapes are studied,and six typical vibration modes of the system are summarized based on the characteristics of each mode shape.The influences of high-speed operating conditions and electromechanical coupling effects on the system’s natural frequencies and mode shapes are investigated.The resonance speeds of the system within a wide operating speed range are studied using Campbell diagrams and modal energy method.The results indicate that the electromechanical coupling effects alter the low-frequency vibration characteristics of the system,and the high-speed operating conditions can significantly change the certain natural frequencies of the system.Both factors should be fully considered in dynamics modeling.The research of resonance speed provides a reference for the dynamic regulation and optimization of system.
As the power density of diesel engines increases,the main bearing assembly structure may suffer from reliability problems such as the increased deformation of main bearing and the decreased strength of key components.The strength,stiffness,contact strength and lightweight performance of main bearing assembly structure are taken as the optimization objectives,and a mathematical model for the collaborative optimization design of multi-structural characteristics is constructed.Aiming at the poor computational efficiency of the traditional non-dominated sorting genetic algorithm II(NSGA-II) in solving the complex engineering problems with small population size and restricted evolutionary generations,an improved elephant herding optimization(EHO) algorithm is proposed for solving the reliability matching mathematical model for the main bearing assembly structure based on the Pareto optimization theory by introducing the adaptive strategy and fixed sized candidate set random testing algorithm.Furthermore,the performance of the improved EHO algorithm and the reliability matching design scheme are experimentally verified.The results show that the improved EHO algorithm has stronger solving ability and higher computational efficiency with small population size and restricted evolutionary generations.After optimization,the first principal stresses in the stress-concentrated areas of engine block and main bearing cover are decreased by 18.67% and 11.06%,respectively,with a mass fluctuation of only 0.6%; and the average out-of-roundness in each examined section is decreased by 14.39%; and the maximum contact pressure on the contact surface between the engine block and the main bearing cover is decreased by 18.92%.
High-entropy alloys are applied in extreme environments,such as high-speed collision and explosive impact,due to their excellent comprehensive mechanical properties.To study the deformation,damage,and failure behaviors of high-entropy alloys under dynamic loading,HfZrTiTaAl-based refractory high-entropy alloys are designed and prepared.Quasi static compression experiment and split Hopkinson bar (SHPB) experiment are conducted on the high-entropy alloys,and the Johnson-Cook constitutive model parameters containing damage are obtained through numerical simulation.The damage evolution process and failure of material under dynamic loading are simulated.The results indicate that the refractory high-entropy alloy exhibits good plasticity under quasi-static compression conditions.Within the strain rate range of 0.001s-1-3500s-1,the HfZrTiTaAl-based high-entropy alloy exhibits a strain rate effect,with a yield strength increasing from 1140MPa to 1568MPa.Numerical simulation shows that the damage of specimen is mainly concentrated on the cross-section at 45° angle to the loading direction under high strain rate loading,and the damage degree of the elements in the middle of the specimen is greater than that of the elements on both sides of the specimen.As the loading strain rate increases,the percentage of elements with a damage degree greater than 0.8 in the total specimen elements gradually increases.
The penetration into steel target is a typical process of large plastic deformation,and the construction of constitutive and damage models for the target material is a crucial prerequisite for achieving finite element simulations.In this study,the static and dynamic tests under different stress states and strain rates,and the multilayer target penetration experiments,are conducted on AISI 4340 steel.To facilitate the finite element simulation of steel target penetration,a Swift-Voce hybrid plasticity model is introduced to describe the stress-strain response of material in the large plastic deformation stage,specifically after necking.Additionally,an extended Hosford-Coulomb fracture initiation model,considering stress triaxiality,Lode angle parameters,strain rate,and temperature,is incorporated.In the parameter identification process,the parameters of the plasticity model are first identified based on experimental data and finite element simulations.The parameters of fracture initiation model are identified using an error function and the linear programming algorithm of the simplex method.Finally,the finite element models for quasi-static tension,dynamic tension,and penetration tests are established to inversely verify the parameters of the plasticity and fracture initiation models.The research results show that AISI 4340 steel exhibits the noticeable strain rate strengthening and thermal softening effects.And the newly developed constitutive and damage models are capable of accurately predicting the deformation and fracture behavior of steel target material.The simulated results are good agreement with the experimental data.
The hysteresis loop is a stress-strain data loop continuously monitored during low-cycle plastic loading,which contains rich accumulated information on material fatigue damage.However,due to the difficulties in hysteresis loop data collection,feature extraction and analysis,there are few relevant research reports.In order to deeply evaluate the low-cycle torsional fatigue properties of 42CrMo high-strength steel,a fatigue test analysis method based on hysteresis loop data is proposed.A torsional fatigue testing machine is used to obtain the hysteretic loop data of the material,and a feature extraction algorithm is designed to obtain the rules of evolution of cyclic stress,shear modulus,residual stress and plastic strain with the loading cycle as well as the cyclic softening characteristics of the material.Through experimental design and analysis,it is confirmed that the 42CrMo material does not meet the Masing characteristics,and as the strain amplitude increases,the non-Masing characteristics become more significant.The low-cycle torsional fatigue test data is used to analyze the cyclic stress-strain relationship characteristics and strain-life relationship characteristics of the material,and calculate shear modulus G,cyclic strength coefficient K',cyclic strain hardening inde xn',fatigue strength index b,fatigue strength coefficient τ'f,fatigue ductility index c,and fatigue ductility coefficient γ'f,etc.The proposed method makes full use of a large amount of loading hysteresis loop data in the torsional fatigue test to extract the hysteresis loop characteristics of each cycle,and uses rich analysis data to describe the fatigue properties of the material.
The nitrogen loss and the reduction in mechanical properties of high nitrogen steel may happen during additive manufacturing.The conventional pulsed gas metal arc(P-GMA)and P-GMA superimposed with ultrasonic frequency pulse current at the peak stage of pulse current are used to conduct the arc additive manufacturing experiments of high-nitrogen steel,and the single-pass multi-layer high nitrogen steel straight wall bodies under different processing parameters are prepared.The influence of ultrasonic frequency pulse current superposition on the solidification mode,microstructure evolution and mechanical properties of arc additive manufactured high-nitrogen steel parts were studied.Due to the nitrogen loss of arc additive manufactured high-nitrogen steel parts,the solidification mode of metal molten pool of high-nitrogen steel changes from A to FA in different arc additive manufacturing modes.Compared with the conventional P-GMA,the high-frequency ultrasonic effect generated by ultrasonic frequency pulse current can increase the diffusion of nitrogen elements,promote the austenite phase transformation,and limit the growth of ferrite dendrites.After comparative analysis,the impact of ultrasonic frequency pulse current on the Y-axis direction of ferrite dendrites is greater than that on the Z-axis direction,the impact of it on the mechanical properties in the Y-axis direction is also greater than that in the Z-axis direction.When the frequency is 60kHz,the tensile strength in the Y-axis direction is increased by 9.9%,and the yield strength is increased by 15.9%.
To address the scale contradiction problems of centimeter-level thickness and meter-level low-frequency detection wavelength of underwater vehicle covering layer,a secondary emission array of giant magnetostrictive transducer is developed and the surface configuration of emission unit is optimized.The active echo control effect of large underwater samples is simulated and evaluated based on the active echo control method of phased array.The sound pressure levels and cloud maps of three positions in the spatial sound field are verified by comparing the numerically simulated results of acoustic finite element model with the test data in a large pressure silencing tank,and the sound field consistency of six frequency points in the low band range of 300-2000Hz with different array layouts is studied.The results show that the higher the frequency is,the worse the consistency of planar sound field is.The consistency of planar sound field formed by the array with square disk is better than that of the disk array.The optimal array layout can form a better planar sound field at 10 m away from it,and the optimal configuration of the emission unit can further improve the consistency of sound field under the active echo control,so as to achieve better control effect.
The sub-axis of traditional compound axis stabilized sighting system has a small motion range,which is easy to reach saturation integration and causes the failure of system.As to the problem above,a self-returning-zero compound axis stabilized sighting system is proposed.On the basis of analyzing the reasons why the sub-axis of traditional compound axis stabilized sighting system is prone to integral saturation and leads to system failure,a self-returning-zero compound axis stabilized sighting system of which the main axis moves along with the sub axis according to the strength function is proposed.The feasibility and advantages of the proposed stabilized sighting system to avoid the integral saturation and enhance the anti-interference characteristics are visually demonstrated,theoretically analyzed,and simulated with mathematical simulation software,and a test platform is built for experimental verification.The test shows that,in the low frequency band,the self-returning-zero compound axis stabilized sighting system can not only guarantee the vibration rejection ability of sub axis,but also greatly enhance the vibration rejection ability of main axis,so that the main axis error is maintained at a small value,so as to avoid the limit saturation of sub axis.This shows the feasibility of the self-returning-zero compound axis stabilized sighting system and its advantages in avoiding the limit saturation of sub axis and maintaining stability of the system.
The evaluation of military equipment maintainability in operational test is characterized by insufficient field test data,various failure modes,different repair states and maintenance tasks of equipment.The fusion of prior information related to the system and its failure modes and field test data can expand the source of information and provide new ideas for the evaluation of equipment maintainability.To this end,the probability of each failure mode combination in the test is updated using discrete Bayes theorem and fault data.The distribution parameters of maintenance time of each failure mode are estimated under Bayes framework.On this basis,the evaluated results of test system maintainability are obtained,and the evaluation of test system maintainability under the condition of battlefield repair is considered.The application of the proposed method is illustrated with an example.It is verified that the proposed method can be used to more accurately estimate the maintenance time density function and system maintainability compared with the existing methods.Result shows that in the context of operational test,taking the test data as a multi-failure mode is conducive to more deeply understand the mechanism for the distribution of maintenance time and grasp the specific logic for the maintenance assessment in more detail,thus obtaining more reasonable assessment results.