Numerical simulation and control of self- propelled swimming of two- and three-dimensional biomimetic fish school in a viscous flow are investigated. With a parallel computational fluid dynamics package for the two- a...Numerical simulation and control of self- propelled swimming of two- and three-dimensional biomimetic fish school in a viscous flow are investigated. With a parallel computational fluid dynamics package for the two- and three-dimensional moving boundary problem, which combines the adaptive multi-grid finite volume method and the methods of immersed boundary and volume of fluid, it is found that due to the interactions of vortices in the wakes, without proper control, a fish school swim with a given flap- ping rule can not keep the fixed shape of a queue. In order to understand the secret of fish swimming, a new feedback con- trol strategy of fish motion is proposed for the first time, i,e., the locomotion speed is adjusted by the flapping frequency of the caudal, and the direction of swimming is controlled by the swinging of the head of a fish. Results show that with this feedback control strategy, a fish school can keep the good order of a queue in cruising, turning or swimming around circles. This new control strategy, which separates the speed control and direction control, is important in the construction of biomimetic robot fish, with which it greatly simplifies the control devices of a biomimetic robot fish.展开更多
Ionic Liquid Electrospray Thrusters(ILETs)are well suited for micro-nano satellite applications due to their small size,low power consumption,and high specific impulse.However,the limited thrust of a single-emitter IL...Ionic Liquid Electrospray Thrusters(ILETs)are well suited for micro-nano satellite applications due to their small size,low power consumption,and high specific impulse.However,the limited thrust of a single-emitter ILET restricts its use in space missions.To optimize the performance of ILETs and make them suitable for a wider range of space missions,we designed a Circular-emitter ILET(CILET)to convert a one-dimensional(point)emission into a twodimensional(line)emission.The CILET can self-organize multiple Taylor cones simultaneously.The cones were photographed and the axial emission currents were measured under different voltage and pressure difference conditions with a CILET experimental system.The emission can be divided into two stable states and one unstable state based on the flow and current characteristics.The current in Stable state Ⅰ increases non-linearly with the voltage,while that in Stable state Ⅱ is nearly linear with respect to the voltage.The number of cones increases with the voltage in stable states,while the cones become short and crowded under high-voltage conditions.The variation law of the number of cones can be explained with the self-organization theory.The variation in the current exhibits a good correlation with the number of cones.This study demonstrates the feasibility of circular emitters and experimentally indicates that the emission current is improved by approximately two orders of magnitude compared to that of a single capillary.展开更多
To enhance the anti-corrosion performance of TC4 alloy across a wide temperature range for modern aircrafts operating in increasingly harsh environments, the (TiB+TiC) hybrid reinforced TC4 composites were prepared by...To enhance the anti-corrosion performance of TC4 alloy across a wide temperature range for modern aircrafts operating in increasingly harsh environments, the (TiB+TiC) hybrid reinforced TC4 composites were prepared by laser melting deposition (LMD) via the in-situ reaction between B_(4)C reinforcement and molten TC4 alloy. The effect of B_(4)C content (0, 0.5, 1.5, wt%) on the microstructure and room/high-temperature corrosion behaviour of the composites was investigated. Microstructural analysis revealed that the microstructure of the composites was significantly influenced by the B_(4)C content. The composite containing 0.5 wt% B_(4)C exhibited an optimal microstructure characterized by refined grains, equiaxed α-Ti transformed from lath-shaped α-Ti, well-distributed (TiB+TiC) phases with a proper amount and reduced pore/dislocation defects. This composite also demonstrated the best corrosion resistance at both room temperature (25 ℃) and high temperature (800 ℃), which was primarily attributed to its comprehensive advantages including a favorable microstructure, a uniform dispersion of thermally stable (TiB+TiC) phases and a stable passivation film.展开更多
The influence of the squeeze film between the tube and the support structure on flow-induced vibrations is a critical factor in tube bundles subjected to two-phase cross-flow.This aspect can significantly alter the th...The influence of the squeeze film between the tube and the support structure on flow-induced vibrations is a critical factor in tube bundles subjected to two-phase cross-flow.This aspect can significantly alter the threshold for fluidelastic instability and affect heat transfer efficiency.This paper presents a mathematical model incorporating the squeeze film force between the tube and the support structure.We aim to clarify the mechanisms underlying fluidelastic instability in tube bundle systems exposed to two-phase flow.Using a self-developed computer program,we performed numerical calculations to examine the influence of the squeeze film on the threshold of fluidelastic instability in the tube bundle system.Furthermore,we analyzed how the thickness and length of the squeeze film affect both the underlying mechanisms and the critical velocity of fluidelastic instability.展开更多
The crack initiation and early propagation are of great significance to the overall fatigue life of material.In order to investigate the anisotropic fracture behavior of laser metal deposited Ti-6Al-4V alloy(LMD Ti64)...The crack initiation and early propagation are of great significance to the overall fatigue life of material.In order to investigate the anisotropic fracture behavior of laser metal deposited Ti-6Al-4V alloy(LMD Ti64)during the early stage,the fourpoint bending fatigue test was carried out on specimens of three different directions,as well as the forged specimens.The results indicate the anisotropic crack initiation and early propagation of LMD Ti64.The direction perpendicular to the deposition direction exhibits a better fatigue resistance than the other two.The crack initiation position and propagation path are dominated by the microstructure in the vicinity of U-notch.LMD Ti64 has a typical small crack effect,and the early crack propagation velocities in three directions are similar.Affected by the slip system of LMD Ti64,secondary cracks frequently occur,which are often found to have an angle of 60°to the main crack.The electron backscatter diffraction analysis indicates that LMD Ti64 has preferred orientations,i.e.,strong 0001//Z texture and 001//Z texture.Their crystallographic orientation will change as the direction of columnarβgrains turns over,resulting in the fatigue anisotropy of LMD Ti64 in crack initiation and early crack propagation process.展开更多
To detect space gravitational waves in the extremely low-frequency band,the telescope and optic-al platform require high stability and reliability.However,the cantilevered design presents challenges,espe-cially in the...To detect space gravitational waves in the extremely low-frequency band,the telescope and optic-al platform require high stability and reliability.However,the cantilevered design presents challenges,espe-cially in the glass-metal hetero-bonding process.This study focuses on the analysis and experimental re-search of the bonding layer in the integrated structure.By optimizing the structural configuration and select-ing suitable bonding processes,the reliability of the telescope system is enhanced.The research indicates that using J-133 adhesive achieves the best performance,with a bonding layer thickness of 0.30 mm and a metal substrate surface roughness of Ra 0.8.These findings significantly enhance the reliability of the optical sys-tem while minimizing potential risks.展开更多
Revealing the combined influence of interfacial damage and nonlinear factors on the forced vibration is significant for the stability design of fluid-conveying pipes, which are usually assembled in aircraft. The nonli...Revealing the combined influence of interfacial damage and nonlinear factors on the forced vibration is significant for the stability design of fluid-conveying pipes, which are usually assembled in aircraft. The nonlinear forced resonance of fluid-conveying layered pipes with a weak interface and a movable boundary under the external excitation is studied. The pipe is simply supported at both ends, with one end subject to a viscoelastic boundary constraint described by KelvinVoigt model. The weak interface in the pipe is considered in the refined displacement field of the layered pipe employing the interfacial cohesive law. The governing equations are derived by Hamilton's variational principle. Geometric nonlinearities including nonlinear curvature, longitudinal inertia nonlinearity and nonlinear constraint force are comprehensively considered during the theoretical derivation. Amplitude-frequency bifurcation diagrams are obtained utilizing a perturbation-Incremental Harmonic Balance Method(IHBM). Results show that interfacial damage and viscoelastic constraints from boundary and foundation have an important influence on the linear and nonlinear dynamic behavior of the system.展开更多
How multi-unmanned aerial vehicles(UAVs)carrying a payload pass an obstacle-dense environment is practically important.Up to now,there have been few results on safe motion planning for the multi-UAVs cooperative trans...How multi-unmanned aerial vehicles(UAVs)carrying a payload pass an obstacle-dense environment is practically important.Up to now,there have been few results on safe motion planning for the multi-UAVs cooperative transportation system(CTS)to pass through such an environment.The prob-lem is challenging because it is difficult to analyze and explicitly take into account the swing motion of the payload in planning.In this paper,a modeling method of virtual tube is proposed by fus-ing the advantages of the existing modeling algorithm for regu-lar virtual tube and the expansion environment method.The pro-posed method can not only generate a safe and smooth tube for UAVs,but also ensure the payload stays away from the dense obstacles.Simulation results show the effectiveness of the method and the safety of the planned tube.展开更多
This work attempts to optimize Graphene nanoplatelets(GPLs)distribution in the face sheet of sandwich plates to pursue the minimum thermal deflection and transverse shear stresses at interfaces.Thus,an Improved Legend...This work attempts to optimize Graphene nanoplatelets(GPLs)distribution in the face sheet of sandwich plates to pursue the minimum thermal deflection and transverse shear stresses at interfaces.Thus,an Improved Legendre Higher-order plate Theory combined with Isogeometric Analysis(ILHT-IGA)is,first,proposed to accurately predict thermomechanical behaviors of GPLs-reinforced sandwich plates,which can ensure the reliability of the optimized results.Then,an accelerated multi-objective optimization approach is proposed to optimize thermomechanical behaviors.The trained machine learning algorithm based on ILHT-IGA is employed as a surrogate model to accelerate the optimization process.Finally,X-shaped GPLs distribution can provide the maximum stiffness to resist thermal expansion.However,X-shaped GPLs distribution on face sheets will result in large difference of stiffnesses at adjacent surfaces of face sheets and core layer.Thus,transverse shear stresses at interfaces are obviously increased.To avoid a sudden increase of transverse shear stresses at interfaces,an alternative optimized GPLs distribution has been obtained,where GPLs gradually increase toward the upper and lower surfaces of face sheets and suddenly decrease near the surface of face sheets.Such distributions can effectively enhance the stiffness of sandwich plates to resist thermal expansion behaviors and decrease transverse shear stresses at interfaces.展开更多
The harmonic balance method(HBM)has been widely applied to get the periodic solution of nonlinear systems,however,its convergence rate as well as computation efficiency is dramatically degraded when the system is high...The harmonic balance method(HBM)has been widely applied to get the periodic solution of nonlinear systems,however,its convergence rate as well as computation efficiency is dramatically degraded when the system is highly non-smooth,e.g.,discontinuous.In order to accelerate the convergence,an enriched HBM is developed in this paper where the non-smooth Bernoulli bases are additionally introduced to enrich the conventional Fourier bases.The basic idea behind is that the convergence rate of the HB solution,as a truncated Fourier series,can be improved if the smoothness of the solution becomes finer.Along this line,using non-smooth Bernoulli bases can compensate the highly non-smooth part of the solution and then,the smoothness of the residual part for Fourier approximation is improved so as to achieve accelerated convergence.Numerical examples are conducted on systems with non-smooth restoring and/or external forces.The results confirm that the proposed enriched HBM indeed increases the convergence rate and the increase becomes more significant if more non-smooth bases are used.展开更多
A vibro-impact system is a hot topic in the study on nonlinear dynamics due to its generality and importance in engineering.In general,the alternating frequency-time harmonic balance(AFT-HB)method can be used to solve...A vibro-impact system is a hot topic in the study on nonlinear dynamics due to its generality and importance in engineering.In general,the alternating frequency-time harmonic balance(AFT-HB)method can be used to solve elastic collision.However,since the system is non-smooth,the required Fourier/harmonic truncation order is high in order to achieve the theoretical convergence rate,resulting in expensive computational cost.Furthermore,for rigid body collision,the periodic response of the system cannot be solved with the AFT-HB method due to the discontinuous velocity of the system.In order to accelerate the convergence and solve highly non-smooth systems,an enriched harmonic balance(HB)method is proposed,which is derived from the AFT-HB method in the framework of event-driven Gauss quadrature.The basic idea is to augment the Fourier bases by introducing a non-smooth Bernoulli base such that the non-smooth Bernoulli base compensates for the non-smooth part of the solution and the smooth part of the solution is approximated by the Fourier bases,thus achieving accelerated convergence.Based on the enriched HB method,gear pair systems with gear backlash and oscillator systems with rigid impact are solved,and the dynamic response characteristics are analyzed in this work.Then,based on the Floquet theory,the event-driven monodromy matrix method for non-smooth systems is used to analyze the stability and bifurcation of the periodic solutions.The numerical example shows that the results obtained from the enriched HB method are consistent with those from the Runge-Kutta method,which proves that the presented method is an effective method for analyzing the dynamic response characteristic of the vibro-impact system.展开更多
This paper,the first-part of a two part series of surveys on Incremental Nonlinear Dynamic Inversion(INDI),provides an overview of the evolution and developments in INDI.Written in a tutorial style,it presents differe...This paper,the first-part of a two part series of surveys on Incremental Nonlinear Dynamic Inversion(INDI),provides an overview of the evolution and developments in INDI.Written in a tutorial style,it presents different basic INDI variants and their specifics,such as modelbased INDI,sensor-based INDI,and hybrid INDI.Furthermore,it sets these different approaches in context with each other.Later developments of INDI explicitly consider actuator dynamics.Those concepts are summarized and discussed in detail.Subsequently,studies that relate INDI to other control methods are summarized.Finally,an overview of various applications of INDI is given,covering different types of control loops and various types of vehicles and plants.This paper seeks to set these developments into context with each other.The purpose of this paper is twofold.INDI is already well-known in the domain of flight control but less so in other fields.Therefore,the paper is written in a comprehensive tutorial style to provide easy access to readers unfamiliar with the topic.On the other hand,the paper can serve as a reference for readers familiar with the topic.展开更多
The wing of a butterfly consists of partially overlapping forewing and hindwing,and forewing sweeping can dynamically change the shape of the whole wing.In this work,the effect of forewing sweeping on aerodynamic perf...The wing of a butterfly consists of partially overlapping forewing and hindwing,and forewing sweeping can dynamically change the shape of the whole wing.In this work,the effect of forewing sweeping on aerodynamic performance of a butterfly like model is studied using a solver based on immersed boundary method and adaptive mesh.For aerodynamic performance,adding a"forward-backward-forward"sweeping motion to the forewing makes it more suitable for fast cruising flight,and compared to the situation without forewing sweeping,the drag is reduced by 46%and the lift to drag ratio is increased by 45%.On the contrary,adding a"backward-for ward-backward"sweeping motion to the forewing increases lift and makes it more suitable for climb flight.For downstroke and middle to late upstroke,the forewing sweeping affects the Leading-Edge Vortex(LEV)through two factors:sweeping velocity and forward sweeping angle,and their effects are coupled.A large forward sweeping velocity can enhance the strength of LEV,while a large forward sweeping angle can weaken it.For early upstroke,the forewing sweeping can affect the wake capture mechanism,sweeping backward can enhance it while sweeping forward can weaken it.The findings in this work provide insight into the design of butterfly like Micro AirVehicles(MAVs).展开更多
The nozzle is a critical component responsible for generating most of the net thrust in a scramjet engine.The quality of its design directly affects the performance of the entire propulsion system.However,most turbule...The nozzle is a critical component responsible for generating most of the net thrust in a scramjet engine.The quality of its design directly affects the performance of the entire propulsion system.However,most turbulence models struggle to make accurate predictions for subsonic and supersonic flows in nozzles.In this study,we explored a novel model,the algebraic stress model k-kL-ARSM+J,to enhance the accuracy of turbulence numerical simulations.This new model was used to conduct numerical simulations of the design and off-design performance of a 3D supersonic asymmetric truncated nozzle designed in our laboratory,with the aim of providing a realistic pattern of changes.The research indicates that,compared to linear eddy viscosity turbulence models such as k-kL and shear stress transport(SST),the k-kL-ARSM+J algebraic stress model shows better accuracy in predicting the performance of supersonic nozzles.Its predictions were identical to the experimental values,enabling precise calculations of the nozzle.The performance trends of the nozzle are as follows:as the inlet Mach number increases,both thrust and pitching moment increase,but the rate of increase slows down.Lift peaks near the design Mach number and then rapidly decreases.With increasing inlet pressure,the nozzle thrust,lift,and pitching moment all show linear growth.As the flight altitude rises,the internal flow field within the nozzle remains relatively consistent due to the same supersonic nozzle inlet flow conditions.However,external to the nozzle,the change in external flow pressure results in the nozzle exit transitioning from over-expanded to under-expanded,leading to a shear layer behind the nozzle that initially converges towards the nozzle center and then diverges.展开更多
The lack of systematic and scientific top-level arrangement in the field of civil aircraft flight test leads to the problems of long duration and high cost.Based on the flight test activity,mathematical models of flig...The lack of systematic and scientific top-level arrangement in the field of civil aircraft flight test leads to the problems of long duration and high cost.Based on the flight test activity,mathematical models of flight test duration and cost are established to set up the framework of flight test process.The top-level arrangement for flight test is optimized by multi-objective algorithm to reduce the duration and cost of flight test.In order to verify the necessity and validity of the mathematical models and the optimization algorithm of top-level arrangement,real flight test data is used to make an example calculation.Results show that the multi-objective optimization results of the top-level flight arrangement are better than the initial arrangement data,which can shorten the duration,reduce the cost,and improve the efficiency of flight test.展开更多
The aerial deployment method enables Unmanned Aerial Vehicles(UAVs)to be directly positioned at the required altitude for their mission.This method typically employs folding technology to improve loading efficiency,wi...The aerial deployment method enables Unmanned Aerial Vehicles(UAVs)to be directly positioned at the required altitude for their mission.This method typically employs folding technology to improve loading efficiency,with applications such as the gravity-only aerial deployment of high-aspect-ratio solar-powered UAVs,and aerial takeoff of fixed-wing drones in Mars research.However,the significant morphological changes during deployment are accompanied by strong nonlinear dynamic aerodynamic forces,which result in multiple degrees of freedom and an unstable character.This hinders the description and analysis of unknown dynamic behaviors,further leading to difficulties in the design of deployment strategies and flight control.To address this issue,this paper proposes an analysis method for dynamic behaviors during aerial deployment based on the Variational Autoencoder(VAE).Focusing on the gravity-only deployment problem of highaspect-ratio foldable-wing UAVs,the method encodes the multi-degree-of-freedom unstable motion signals into a low-dimensional feature space through a data-driven approach.By clustering in the feature space,this paper identifies and studies several dynamic behaviors during aerial deployment.The research presented in this paper offers a new method and perspective for feature extraction and analysis of complex and difficult-to-describe extreme flight dynamics,guiding the research on aerial deployment drones design and control strategies.展开更多
To enhance the adhesion of ceramic coatings in turbine blade Thermal Barrier Coatings(TBCs)systems,Laser Surface Texturing(LST)was employed to create microstructures on the metal bond coat.The bonding conditions and f...To enhance the adhesion of ceramic coatings in turbine blade Thermal Barrier Coatings(TBCs)systems,Laser Surface Texturing(LST)was employed to create microstructures on the metal bond coat.The bonding conditions and failure mechanisms of the ceramic coatings within these microstructures were thoroughly investigated.Femtosecond laser technology was used to fabricate three types of high-quality microstructure grooves:linear,sine wave,and grid patterns.These grooves exhibit uniform morphology,well-defined edges,and smooth inner walls.After ceramic coating deposition,columnar crystal structures grew perpendicularly along the groove walls,completely filling the microstructures and forming an arched support structure that significantly enhances mechanical interlocking and adhesion.Among the different microstructures,grid patterns demonstrated the best adhesion performance.In scratch tests,grid-patterned microstructures exhibited only localized small block spalling under high load conditions,avoiding large-scale delamination.This superior performance is attributed to the ability of grid pattern to effectively distribute stress in multiple directions and prevent crack propagation.By reducing stress concentration and enhancing mechanical interlocking points,grid-patterned microstructures also showed excellent resistance to spallation during thermal cycling,markedly improving the thermal resistance and adhesion of coating.展开更多
The Distributed Propulsion Wing(DPW)presents prominent advantages in terms of energy conservation during flight,but the intense integration of propulsive internal flow with aerodynamic external flow brings significant...The Distributed Propulsion Wing(DPW)presents prominent advantages in terms of energy conservation during flight,but the intense integration of propulsive internal flow with aerodynamic external flow brings significant design challenges.To tackle this issue,this paper undertakes a comprehensive investigation of the aero-propulsive coupling performance of the DPW under both hovering and cruising conditions,and subsequently proposes a multi-level collaboration optimization design method based on the decomposition principle.Specifically,the complex 3D surfaces of DPW are systematically dissociated into simple 2D curves with inherent relationships for design.The decomposition is achieved based on the analysis results of the aero-propulsive coupling characteristics.And a DPW design case is conducted and subsequently analyzed in order to further validate the effectiveness and feasibility of the proposed design method.It is shown that a 115.75%drag reduction of DPW can be achieved at cruise under a specified thrust level.Furthermore,the DPW exhibits inherent characteristics of consistent lift-to-drag ratio with the thrust-drag balance constraint,regardless of variations in incoming flow velocity or total thrust.展开更多
This paper proposed a new libration decoupling analytical speed function(LD-ASF)in lieu of the classic analytical speed function to control the climber's speed along a partial space elevator to improve libration s...This paper proposed a new libration decoupling analytical speed function(LD-ASF)in lieu of the classic analytical speed function to control the climber's speed along a partial space elevator to improve libration stability in cargo transportation.The LD-ASF is further optimized for payload transportation efficiency by a novel coordinate game theory to balance competing control objectives among payload transport speed,stable end body's libration,and overall control input via model predictive control.The transfer period is divided into several sections to reduce computational burden.The validity and efficacy of the proposed LD-ASF and coordinate game-based model predictive control are demonstrated by computer simulation.Numerical results reveal that the optimized LD-ASF results in higher transportation speed,stable end body's libration,lower thrust fuel consumption,and more flexible optimization space than the classic analytical speed function.展开更多
The icing characteristics of supercooled large droplet(SLD)impacting carbon fiber-reinforced composites(CFRCs)remain poorly understood,hindering the enhancement of ice protection capabilities and the certification of ...The icing characteristics of supercooled large droplet(SLD)impacting carbon fiber-reinforced composites(CFRCs)remain poorly understood,hindering the enhancement of ice protection capabilities and the certification of ice-accreted composite aircraft.The paper systematically investigates the effects of the supercooling degree,the surface temperature,and the impact velocity on the ice accretion behavior of SLDs impacting carbon fiber-reinforced epoxy composite surfaces.To address the ice-prone nature of CFRCs,nanoparticle-modified anti-icing coatings are developed,and the icing characteristics of SLD-impacted modified carbon fiber-reinforced epoxy composite surfaces are analyzed.Results demonstrate that surface-modified carbon fiber-reinforced epoxy composite exhibits significantly delayed ice formation.Under conditions of droplet temperature(−15℃)and surface temperature(−18℃),the icing time of hydrophobic-modified CFRCs was delayed by over 1100 ms,representing a 5.4-fold improvement compared to the unmodified carbon fiber-reinforced epoxy composite.展开更多
基金supported by the National Natural Science Foundation of China(10172095 and 10672183)
文摘Numerical simulation and control of self- propelled swimming of two- and three-dimensional biomimetic fish school in a viscous flow are investigated. With a parallel computational fluid dynamics package for the two- and three-dimensional moving boundary problem, which combines the adaptive multi-grid finite volume method and the methods of immersed boundary and volume of fluid, it is found that due to the interactions of vortices in the wakes, without proper control, a fish school swim with a given flap- ping rule can not keep the fixed shape of a queue. In order to understand the secret of fish swimming, a new feedback con- trol strategy of fish motion is proposed for the first time, i,e., the locomotion speed is adjusted by the flapping frequency of the caudal, and the direction of swimming is controlled by the swinging of the head of a fish. Results show that with this feedback control strategy, a fish school can keep the good order of a queue in cruising, turning or swimming around circles. This new control strategy, which separates the speed control and direction control, is important in the construction of biomimetic robot fish, with which it greatly simplifies the control devices of a biomimetic robot fish.
基金co-supported by the National Key R&D Program of China(No.2020YFC2201001)the Shenzhen Science and Technology Program,China(No.20210623091808026)。
文摘Ionic Liquid Electrospray Thrusters(ILETs)are well suited for micro-nano satellite applications due to their small size,low power consumption,and high specific impulse.However,the limited thrust of a single-emitter ILET restricts its use in space missions.To optimize the performance of ILETs and make them suitable for a wider range of space missions,we designed a Circular-emitter ILET(CILET)to convert a one-dimensional(point)emission into a twodimensional(line)emission.The CILET can self-organize multiple Taylor cones simultaneously.The cones were photographed and the axial emission currents were measured under different voltage and pressure difference conditions with a CILET experimental system.The emission can be divided into two stable states and one unstable state based on the flow and current characteristics.The current in Stable state Ⅰ increases non-linearly with the voltage,while that in Stable state Ⅱ is nearly linear with respect to the voltage.The number of cones increases with the voltage in stable states,while the cones become short and crowded under high-voltage conditions.The variation law of the number of cones can be explained with the self-organization theory.The variation in the current exhibits a good correlation with the number of cones.This study demonstrates the feasibility of circular emitters and experimentally indicates that the emission current is improved by approximately two orders of magnitude compared to that of a single capillary.
基金supported by the Tianjin Municipal Natural Science Foundation(No.23JCYBJC00040)the National Nat-ural Science Foundation of China(No.52175369)the Tian-jin Research Innovation Project for Postgraduate Students(No.2022SKY134).
文摘To enhance the anti-corrosion performance of TC4 alloy across a wide temperature range for modern aircrafts operating in increasingly harsh environments, the (TiB+TiC) hybrid reinforced TC4 composites were prepared by laser melting deposition (LMD) via the in-situ reaction between B_(4)C reinforcement and molten TC4 alloy. The effect of B_(4)C content (0, 0.5, 1.5, wt%) on the microstructure and room/high-temperature corrosion behaviour of the composites was investigated. Microstructural analysis revealed that the microstructure of the composites was significantly influenced by the B_(4)C content. The composite containing 0.5 wt% B_(4)C exhibited an optimal microstructure characterized by refined grains, equiaxed α-Ti transformed from lath-shaped α-Ti, well-distributed (TiB+TiC) phases with a proper amount and reduced pore/dislocation defects. This composite also demonstrated the best corrosion resistance at both room temperature (25 ℃) and high temperature (800 ℃), which was primarily attributed to its comprehensive advantages including a favorable microstructure, a uniform dispersion of thermally stable (TiB+TiC) phases and a stable passivation film.
基金financially supported by the National Natural Science Foundation of China(Grant No.12072336).
文摘The influence of the squeeze film between the tube and the support structure on flow-induced vibrations is a critical factor in tube bundles subjected to two-phase cross-flow.This aspect can significantly alter the threshold for fluidelastic instability and affect heat transfer efficiency.This paper presents a mathematical model incorporating the squeeze film force between the tube and the support structure.We aim to clarify the mechanisms underlying fluidelastic instability in tube bundle systems exposed to two-phase flow.Using a self-developed computer program,we performed numerical calculations to examine the influence of the squeeze film on the threshold of fluidelastic instability in the tube bundle system.Furthermore,we analyzed how the thickness and length of the squeeze film affect both the underlying mechanisms and the critical velocity of fluidelastic instability.
基金National Natural Science Foundation of China(12172292,12072287)。
文摘The crack initiation and early propagation are of great significance to the overall fatigue life of material.In order to investigate the anisotropic fracture behavior of laser metal deposited Ti-6Al-4V alloy(LMD Ti64)during the early stage,the fourpoint bending fatigue test was carried out on specimens of three different directions,as well as the forged specimens.The results indicate the anisotropic crack initiation and early propagation of LMD Ti64.The direction perpendicular to the deposition direction exhibits a better fatigue resistance than the other two.The crack initiation position and propagation path are dominated by the microstructure in the vicinity of U-notch.LMD Ti64 has a typical small crack effect,and the early crack propagation velocities in three directions are similar.Affected by the slip system of LMD Ti64,secondary cracks frequently occur,which are often found to have an angle of 60°to the main crack.The electron backscatter diffraction analysis indicates that LMD Ti64 has preferred orientations,i.e.,strong 0001//Z texture and 001//Z texture.Their crystallographic orientation will change as the direction of columnarβgrains turns over,resulting in the fatigue anisotropy of LMD Ti64 in crack initiation and early crack propagation process.
文摘To detect space gravitational waves in the extremely low-frequency band,the telescope and optic-al platform require high stability and reliability.However,the cantilevered design presents challenges,espe-cially in the glass-metal hetero-bonding process.This study focuses on the analysis and experimental re-search of the bonding layer in the integrated structure.By optimizing the structural configuration and select-ing suitable bonding processes,the reliability of the telescope system is enhanced.The research indicates that using J-133 adhesive achieves the best performance,with a bonding layer thickness of 0.30 mm and a metal substrate surface roughness of Ra 0.8.These findings significantly enhance the reliability of the optical sys-tem while minimizing potential risks.
文摘Revealing the combined influence of interfacial damage and nonlinear factors on the forced vibration is significant for the stability design of fluid-conveying pipes, which are usually assembled in aircraft. The nonlinear forced resonance of fluid-conveying layered pipes with a weak interface and a movable boundary under the external excitation is studied. The pipe is simply supported at both ends, with one end subject to a viscoelastic boundary constraint described by KelvinVoigt model. The weak interface in the pipe is considered in the refined displacement field of the layered pipe employing the interfacial cohesive law. The governing equations are derived by Hamilton's variational principle. Geometric nonlinearities including nonlinear curvature, longitudinal inertia nonlinearity and nonlinear constraint force are comprehensively considered during the theoretical derivation. Amplitude-frequency bifurcation diagrams are obtained utilizing a perturbation-Incremental Harmonic Balance Method(IHBM). Results show that interfacial damage and viscoelastic constraints from boundary and foundation have an important influence on the linear and nonlinear dynamic behavior of the system.
基金supported by the National Natural Science Foundation of China(6237338661973327).
文摘How multi-unmanned aerial vehicles(UAVs)carrying a payload pass an obstacle-dense environment is practically important.Up to now,there have been few results on safe motion planning for the multi-UAVs cooperative transportation system(CTS)to pass through such an environment.The prob-lem is challenging because it is difficult to analyze and explicitly take into account the swing motion of the payload in planning.In this paper,a modeling method of virtual tube is proposed by fus-ing the advantages of the existing modeling algorithm for regu-lar virtual tube and the expansion environment method.The pro-posed method can not only generate a safe and smooth tube for UAVs,but also ensure the payload stays away from the dense obstacles.Simulation results show the effectiveness of the method and the safety of the planned tube.
基金supported by the National Natural Sciences Foundation of China(No.12172295)。
文摘This work attempts to optimize Graphene nanoplatelets(GPLs)distribution in the face sheet of sandwich plates to pursue the minimum thermal deflection and transverse shear stresses at interfaces.Thus,an Improved Legendre Higher-order plate Theory combined with Isogeometric Analysis(ILHT-IGA)is,first,proposed to accurately predict thermomechanical behaviors of GPLs-reinforced sandwich plates,which can ensure the reliability of the optimized results.Then,an accelerated multi-objective optimization approach is proposed to optimize thermomechanical behaviors.The trained machine learning algorithm based on ILHT-IGA is employed as a surrogate model to accelerate the optimization process.Finally,X-shaped GPLs distribution can provide the maximum stiffness to resist thermal expansion.However,X-shaped GPLs distribution on face sheets will result in large difference of stiffnesses at adjacent surfaces of face sheets and core layer.Thus,transverse shear stresses at interfaces are obviously increased.To avoid a sudden increase of transverse shear stresses at interfaces,an alternative optimized GPLs distribution has been obtained,where GPLs gradually increase toward the upper and lower surfaces of face sheets and suddenly decrease near the surface of face sheets.Such distributions can effectively enhance the stiffness of sandwich plates to resist thermal expansion behaviors and decrease transverse shear stresses at interfaces.
基金supported by the National Natural Science Foundation of China (Grant No. 12372028)the National Key Research and Development Program of China (Grant No. 2020YFC2201101)the Guangdong Basic and Applied Basic Research Foundation (Grant No.2022A1515011809)。
文摘The harmonic balance method(HBM)has been widely applied to get the periodic solution of nonlinear systems,however,its convergence rate as well as computation efficiency is dramatically degraded when the system is highly non-smooth,e.g.,discontinuous.In order to accelerate the convergence,an enriched HBM is developed in this paper where the non-smooth Bernoulli bases are additionally introduced to enrich the conventional Fourier bases.The basic idea behind is that the convergence rate of the HB solution,as a truncated Fourier series,can be improved if the smoothness of the solution becomes finer.Along this line,using non-smooth Bernoulli bases can compensate the highly non-smooth part of the solution and then,the smoothness of the residual part for Fourier approximation is improved so as to achieve accelerated convergence.Numerical examples are conducted on systems with non-smooth restoring and/or external forces.The results confirm that the proposed enriched HBM indeed increases the convergence rate and the increase becomes more significant if more non-smooth bases are used.
基金Project supported by the National Natural Science Foundation of China(No.12372028)the Guangdong Basic and Applied Basic Research Foundation(No.2022A1515011809)。
文摘A vibro-impact system is a hot topic in the study on nonlinear dynamics due to its generality and importance in engineering.In general,the alternating frequency-time harmonic balance(AFT-HB)method can be used to solve elastic collision.However,since the system is non-smooth,the required Fourier/harmonic truncation order is high in order to achieve the theoretical convergence rate,resulting in expensive computational cost.Furthermore,for rigid body collision,the periodic response of the system cannot be solved with the AFT-HB method due to the discontinuous velocity of the system.In order to accelerate the convergence and solve highly non-smooth systems,an enriched harmonic balance(HB)method is proposed,which is derived from the AFT-HB method in the framework of event-driven Gauss quadrature.The basic idea is to augment the Fourier bases by introducing a non-smooth Bernoulli base such that the non-smooth Bernoulli base compensates for the non-smooth part of the solution and the smooth part of the solution is approximated by the Fourier bases,thus achieving accelerated convergence.Based on the enriched HB method,gear pair systems with gear backlash and oscillator systems with rigid impact are solved,and the dynamic response characteristics are analyzed in this work.Then,based on the Floquet theory,the event-driven monodromy matrix method for non-smooth systems is used to analyze the stability and bifurcation of the periodic solutions.The numerical example shows that the results obtained from the enriched HB method are consistent with those from the Runge-Kutta method,which proves that the presented method is an effective method for analyzing the dynamic response characteristic of the vibro-impact system.
文摘This paper,the first-part of a two part series of surveys on Incremental Nonlinear Dynamic Inversion(INDI),provides an overview of the evolution and developments in INDI.Written in a tutorial style,it presents different basic INDI variants and their specifics,such as modelbased INDI,sensor-based INDI,and hybrid INDI.Furthermore,it sets these different approaches in context with each other.Later developments of INDI explicitly consider actuator dynamics.Those concepts are summarized and discussed in detail.Subsequently,studies that relate INDI to other control methods are summarized.Finally,an overview of various applications of INDI is given,covering different types of control loops and various types of vehicles and plants.This paper seeks to set these developments into context with each other.The purpose of this paper is twofold.INDI is already well-known in the domain of flight control but less so in other fields.Therefore,the paper is written in a comprehensive tutorial style to provide easy access to readers unfamiliar with the topic.On the other hand,the paper can serve as a reference for readers familiar with the topic.
基金supported by the National Natural Science Foundation of China(No.52175277)The computations were conducted in Computing Center in Xi'an.The authors thankfully acknowledge these institutions.
文摘The wing of a butterfly consists of partially overlapping forewing and hindwing,and forewing sweeping can dynamically change the shape of the whole wing.In this work,the effect of forewing sweeping on aerodynamic performance of a butterfly like model is studied using a solver based on immersed boundary method and adaptive mesh.For aerodynamic performance,adding a"forward-backward-forward"sweeping motion to the forewing makes it more suitable for fast cruising flight,and compared to the situation without forewing sweeping,the drag is reduced by 46%and the lift to drag ratio is increased by 45%.On the contrary,adding a"backward-for ward-backward"sweeping motion to the forewing increases lift and makes it more suitable for climb flight.For downstroke and middle to late upstroke,the forewing sweeping affects the Leading-Edge Vortex(LEV)through two factors:sweeping velocity and forward sweeping angle,and their effects are coupled.A large forward sweeping velocity can enhance the strength of LEV,while a large forward sweeping angle can weaken it.For early upstroke,the forewing sweeping can affect the wake capture mechanism,sweeping backward can enhance it while sweeping forward can weaken it.The findings in this work provide insight into the design of butterfly like Micro AirVehicles(MAVs).
基金supported by the Zhejiang Provincial Key Research and Development Program of China(No.2020C01020).
文摘The nozzle is a critical component responsible for generating most of the net thrust in a scramjet engine.The quality of its design directly affects the performance of the entire propulsion system.However,most turbulence models struggle to make accurate predictions for subsonic and supersonic flows in nozzles.In this study,we explored a novel model,the algebraic stress model k-kL-ARSM+J,to enhance the accuracy of turbulence numerical simulations.This new model was used to conduct numerical simulations of the design and off-design performance of a 3D supersonic asymmetric truncated nozzle designed in our laboratory,with the aim of providing a realistic pattern of changes.The research indicates that,compared to linear eddy viscosity turbulence models such as k-kL and shear stress transport(SST),the k-kL-ARSM+J algebraic stress model shows better accuracy in predicting the performance of supersonic nozzles.Its predictions were identical to the experimental values,enabling precise calculations of the nozzle.The performance trends of the nozzle are as follows:as the inlet Mach number increases,both thrust and pitching moment increase,but the rate of increase slows down.Lift peaks near the design Mach number and then rapidly decreases.With increasing inlet pressure,the nozzle thrust,lift,and pitching moment all show linear growth.As the flight altitude rises,the internal flow field within the nozzle remains relatively consistent due to the same supersonic nozzle inlet flow conditions.However,external to the nozzle,the change in external flow pressure results in the nozzle exit transitioning from over-expanded to under-expanded,leading to a shear layer behind the nozzle that initially converges towards the nozzle center and then diverges.
基金supported by the National Natural Science Foundation of China(62073267,61903305)the Fundamental Research Funds for the Central Universities(HXGJXM202214).
文摘The lack of systematic and scientific top-level arrangement in the field of civil aircraft flight test leads to the problems of long duration and high cost.Based on the flight test activity,mathematical models of flight test duration and cost are established to set up the framework of flight test process.The top-level arrangement for flight test is optimized by multi-objective algorithm to reduce the duration and cost of flight test.In order to verify the necessity and validity of the mathematical models and the optimization algorithm of top-level arrangement,real flight test data is used to make an example calculation.Results show that the multi-objective optimization results of the top-level flight arrangement are better than the initial arrangement data,which can shorten the duration,reduce the cost,and improve the efficiency of flight test.
基金co-supported by the Natural Science Basic Research Program of Shaanxi,China(No.2023-JC-QN-0043)the ND Basic Research Funds,China(No.G2022WD).
文摘The aerial deployment method enables Unmanned Aerial Vehicles(UAVs)to be directly positioned at the required altitude for their mission.This method typically employs folding technology to improve loading efficiency,with applications such as the gravity-only aerial deployment of high-aspect-ratio solar-powered UAVs,and aerial takeoff of fixed-wing drones in Mars research.However,the significant morphological changes during deployment are accompanied by strong nonlinear dynamic aerodynamic forces,which result in multiple degrees of freedom and an unstable character.This hinders the description and analysis of unknown dynamic behaviors,further leading to difficulties in the design of deployment strategies and flight control.To address this issue,this paper proposes an analysis method for dynamic behaviors during aerial deployment based on the Variational Autoencoder(VAE).Focusing on the gravity-only deployment problem of highaspect-ratio foldable-wing UAVs,the method encodes the multi-degree-of-freedom unstable motion signals into a low-dimensional feature space through a data-driven approach.By clustering in the feature space,this paper identifies and studies several dynamic behaviors during aerial deployment.The research presented in this paper offers a new method and perspective for feature extraction and analysis of complex and difficult-to-describe extreme flight dynamics,guiding the research on aerial deployment drones design and control strategies.
基金supported by the National Science and Technology Major Project,China(No.J2019-VII-0013-0153)the Sichuan Science and Technology Program,China(Nos.2021ZDZX0001 and 2021ZDZX0002)。
文摘To enhance the adhesion of ceramic coatings in turbine blade Thermal Barrier Coatings(TBCs)systems,Laser Surface Texturing(LST)was employed to create microstructures on the metal bond coat.The bonding conditions and failure mechanisms of the ceramic coatings within these microstructures were thoroughly investigated.Femtosecond laser technology was used to fabricate three types of high-quality microstructure grooves:linear,sine wave,and grid patterns.These grooves exhibit uniform morphology,well-defined edges,and smooth inner walls.After ceramic coating deposition,columnar crystal structures grew perpendicularly along the groove walls,completely filling the microstructures and forming an arched support structure that significantly enhances mechanical interlocking and adhesion.Among the different microstructures,grid patterns demonstrated the best adhesion performance.In scratch tests,grid-patterned microstructures exhibited only localized small block spalling under high load conditions,avoiding large-scale delamination.This superior performance is attributed to the ability of grid pattern to effectively distribute stress in multiple directions and prevent crack propagation.By reducing stress concentration and enhancing mechanical interlocking points,grid-patterned microstructures also showed excellent resistance to spallation during thermal cycling,markedly improving the thermal resistance and adhesion of coating.
基金co-supported by the Equipment Advance Research Project of China(No.50911040803)the National Defense Pre-research Foundation of China(No.2021-JCJQJJ-0805)the Aeronautical Science Foundation of China(No.2024Z006053001)。
文摘The Distributed Propulsion Wing(DPW)presents prominent advantages in terms of energy conservation during flight,but the intense integration of propulsive internal flow with aerodynamic external flow brings significant design challenges.To tackle this issue,this paper undertakes a comprehensive investigation of the aero-propulsive coupling performance of the DPW under both hovering and cruising conditions,and subsequently proposes a multi-level collaboration optimization design method based on the decomposition principle.Specifically,the complex 3D surfaces of DPW are systematically dissociated into simple 2D curves with inherent relationships for design.The decomposition is achieved based on the analysis results of the aero-propulsive coupling characteristics.And a DPW design case is conducted and subsequently analyzed in order to further validate the effectiveness and feasibility of the proposed design method.It is shown that a 115.75%drag reduction of DPW can be achieved at cruise under a specified thrust level.Furthermore,the DPW exhibits inherent characteristics of consistent lift-to-drag ratio with the thrust-drag balance constraint,regardless of variations in incoming flow velocity or total thrust.
基金funded by the National Natural Science Foundation of China(12102487)Basic and Applied Basic Research Foundation of Guangdong Province,China(2023A1515012339)+1 种基金Shenzhen Science and Technology Program(ZDSYS20210623091808026)the Discovery Grant(RGPIN-2024-06290)of the Natural Sciences and Engineering Research Council of Canada。
文摘This paper proposed a new libration decoupling analytical speed function(LD-ASF)in lieu of the classic analytical speed function to control the climber's speed along a partial space elevator to improve libration stability in cargo transportation.The LD-ASF is further optimized for payload transportation efficiency by a novel coordinate game theory to balance competing control objectives among payload transport speed,stable end body's libration,and overall control input via model predictive control.The transfer period is divided into several sections to reduce computational burden.The validity and efficacy of the proposed LD-ASF and coordinate game-based model predictive control are demonstrated by computer simulation.Numerical results reveal that the optimized LD-ASF results in higher transportation speed,stable end body's libration,lower thrust fuel consumption,and more flexible optimization space than the classic analytical speed function.
基金supported by the National Key Laboratory of Advanced Composite Materials(No.KZ42191814)。
文摘The icing characteristics of supercooled large droplet(SLD)impacting carbon fiber-reinforced composites(CFRCs)remain poorly understood,hindering the enhancement of ice protection capabilities and the certification of ice-accreted composite aircraft.The paper systematically investigates the effects of the supercooling degree,the surface temperature,and the impact velocity on the ice accretion behavior of SLDs impacting carbon fiber-reinforced epoxy composite surfaces.To address the ice-prone nature of CFRCs,nanoparticle-modified anti-icing coatings are developed,and the icing characteristics of SLD-impacted modified carbon fiber-reinforced epoxy composite surfaces are analyzed.Results demonstrate that surface-modified carbon fiber-reinforced epoxy composite exhibits significantly delayed ice formation.Under conditions of droplet temperature(−15℃)and surface temperature(−18℃),the icing time of hydrophobic-modified CFRCs was delayed by over 1100 ms,representing a 5.4-fold improvement compared to the unmodified carbon fiber-reinforced epoxy composite.
