With the increasing complexity of industrial automation,planetary gearboxes play a vital role in largescale equipment transmission systems,directly impacting operational efficiency and safety.Traditional maintenance s...With the increasing complexity of industrial automation,planetary gearboxes play a vital role in largescale equipment transmission systems,directly impacting operational efficiency and safety.Traditional maintenance strategies often struggle to accurately predict the degradation process of equipment,leading to excessive maintenance costs or potential failure risks.However,existing prediction methods based on statistical models are difficult to adapt to nonlinear degradation processes.To address these challenges,this study proposes a novel condition-based maintenance framework for planetary gearboxes.A comprehensive full-lifecycle degradation experiment was conducted to collect raw vibration signals,which were then processed using a temporal convolutional network autoencoder with multi-scale perception capability to extract deep temporal degradation features,enabling the collaborative extraction of longperiod meshing frequencies and short-term impact features from the vibration signals.Kernel principal component analysis was employed to fuse and normalize these features,enhancing the characterization of degradation progression.A nonlinear Wiener process was used to model the degradation trajectory,with a threshold decay function introduced to dynamically adjust maintenance strategies,and model parameters optimized through maximum likelihood estimation.Meanwhile,the maintenance strategy was optimized to minimize costs per unit time,determining the optimal maintenance timing and preventive maintenance threshold.The comprehensive indicator of degradation trends extracted by this method reaches 0.756,which is 41.2%higher than that of traditional time-domain features;the dynamic threshold strategy reduces the maintenance cost per unit time to 55.56,which is 8.9%better than that of the static threshold optimization.Experimental results demonstrate significant reductions in maintenance costs while enhancing system reliability and safety.This study realizes the organic integration of deep learning and reliability theory in the maintenance of planetary gearboxes,provides an interpretable solution for the predictive maintenance of complex mechanical systems,and promotes the development of condition-based maintenance strategies for planetary gearboxes.展开更多
The electrospray thruster supplied by ionic liquid is a promising micro-propulsion thruster with small size and precise thrust, which can emit both cations and anions to achieve self-neutralization. In order to furthe...The electrospray thruster supplied by ionic liquid is a promising micro-propulsion thruster with small size and precise thrust, which can emit both cations and anions to achieve self-neutralization. In order to further investigate the effect of ion solvation energy on the evaporation of cations and anions from ionic liquid under the action of a uniform electric field, this paper establishes a transient Electrohydrodynamic (EHD) model for free ionic liquid droplets undergoing ion evaporation. The dynamic processes of droplet deformation and ion evaporation are simulated. And the study further focuses on the influence of different ion solvation energies for cations on the droplet morphology and the ion evaporation characteristics at the positively charged end and negatively charged end of the droplet. The results indicate that, when the ion solvation energy for cations is higher than that of anions, it will cause the ion evaporation at the positively charged end of the droplet to lag behind the ion evaporation at the negatively charged end. And the higher the ion solvation energy for the cations, the longer the evaporation lag time at the positively charged end of the droplet, which will lead to a higher peak of surface charge density that can be reached, resulting in a larger evaporation current and sharper droplet stretching deformation. Additionally, the peak surface charge density of the positively charged end of the droplet is linearly related to the ion solvation energy for cations, while the peak surface charge density of the negatively charged end remains almost unchanged and is not significantly affected by the ion solvation energy for cations.展开更多
Coastal wetlands are crucial for the‘blue carbon sink’,significantly contributing to regulating climate change.This study util-ized 160 soil samples,35 remote sensing features,and 5 geo-climatic data to accurately e...Coastal wetlands are crucial for the‘blue carbon sink’,significantly contributing to regulating climate change.This study util-ized 160 soil samples,35 remote sensing features,and 5 geo-climatic data to accurately estimate the soil organic carbon stocks(SOCS)in the coastal wetlands of Tianjin and Hebei,China.To reduce data redundancy,simplify model complexity,and improve model inter-pretability,Pearson correlation analysis(PsCA),Boruta,and recursive feature elimination(RFE)were employed to optimize features.Combined with the optimized features,the soil organic carbon density(SOCD)prediction model was constructed by using multivariate adaptive regression splines(MARS),extreme gradient boosting(XGBoost),and random forest(RF)algorithms and applied to predict the spatial distribution of SOCD and estimate the SOCS of different wetland types in 2020.The results show that:1)different feature combinations have a significant influence on the model performance.Better prediction performance was attained by building a model using RFE-based feature combinations.RF has the best prediction accuracy(R^(2)=0.587,RMSE=0.798 kg/m^(2),MAE=0.660 kg/m^(2)).2)Optical features are more important than radar and geo-climatic features in the MARS,XGBoost,and RF algorithms.3)The size of SOCS is related to SOCD and the area of each wetland type,aquaculture pond has the highest SOCS,followed by marsh,salt pan,mud-flat,and sand shore.展开更多
Polyvinylidene fluoride/lead zirconate titanate(PVDF/PZT)composite films have been prepared by direct ink writing and the effect of PZT content on crystallization behavior and electrical properties of film were system...Polyvinylidene fluoride/lead zirconate titanate(PVDF/PZT)composite films have been prepared by direct ink writing and the effect of PZT content on crystallization behavior and electrical properties of film were systematically investigated.The composite films were characterized by scanning electron microscope(SEM),X-ray diffractometer(XRD),Flourier transform infrared spectroscope(FTIR)and differential scanning calorimeter(DSC).The results show that,surface modified PZT powder(PZT@PDA)is successfully coated by polydopamine(PDA),resulting in a large number of polar groups that interact with the-CF_(2)-groups in PVDF,inducing the generation of polarβphase due to hydrogen bonding formed in the interaction.Theβphase content in composite film increases with increasing PZT@PDA content,up to 28.09%as with 5 wt.%PZT@PDA.PZT@PDA plays a role of nucleating agent to promote the generation of polar phases in the film and also acts as an impurity hindering the growth of nuclei to reduce crystallinity.Moreover,the presence of PZT@PDA in interfaces provides more sites for the occurrence of interfacial polarization and thus improving the electrical properties of films.The composite film with 5 wt.%PZT@PDA possesses the highest dielectric constant(8.61)and residual polarization value(0.6803μC/cm^(2)).展开更多
A Discrete Boltzmann Method(DBM)with a Maxwell-type boundary condition is constructed to investigate the influence of rarefaction on laminar Shock Wave/Boundary Layer Interaction(SWBLI).Due to the complexity of compre...A Discrete Boltzmann Method(DBM)with a Maxwell-type boundary condition is constructed to investigate the influence of rarefaction on laminar Shock Wave/Boundary Layer Interaction(SWBLI).Due to the complexity of compressible flow,a Knudsen number vector Kn,whose components include the local Knudsen numbers such as Kn_(ρ)and Kn_(U),is introduced to characterize the local structures,where Kn_(ρ)and Kn_(U)are Knudsen numbers defined in terms of the density and velocity interfaces,respectively.Since first focusing on the steady state of SWBLI,the DBM considers up to the second-order Kn_(ρ)(rarefaction/non-equilibrium)effects.The model is validated using Mach number 2 SWBLI and the necessity of using DBM with sufficient physical accuracy is confirmed by the shock collision problem.Key findings include the following:the leading-edge shock wave increases the local density Knudsen number Kn_(ρ)and eventually leads to the failure of linear constitutive relations in the Navier-Stokes(N-S)model and surely also in the lower-order DBM;the non-equilibrium effect differences in regions behind the leading-edge shock wave are primarily correlated with Kn_(ρ),while in the separation region are primarily correlated with Kn_(U);the non-equilibrium quantities D_(2)and D_(4,2),as well as the viscous entropy production rate S_(NOMF)can be used to identify the separation zone.The findings clarify various effects and main mechanisms in different regions associated with SWBLI,which are concealed in N-S model.展开更多
Cellular structures,distinguished by their porous characteristics,are frequently adopted in designs aimed at impact isolation,owing to their lightweight attributes and exceptional ability to absorb energy during impac...Cellular structures,distinguished by their porous characteristics,are frequently adopted in designs aimed at impact isolation,owing to their lightweight attributes and exceptional ability to absorb energy during impact events.Lattice structures often rely on plastic deformation to absorb energy.However,in applications such as sports protection and robotic grasping,there exists a requirement for a reusable structure designed to isolate impacts,which can be effectively achieved by three-dimensional flexible lattice structures.In this work,a theoretical calculation method for soft lattice structures is proposed,and in light of this method,a three-dimensional soft lattice structure aimed at isolating impacts has been carefully designed.The predictive theory for the quasistatic mechanical properties,including stiffness and buckling strength for three-dimensional soft lattice structures is described.On the basis of the quasizero stiffness characteristics inherent in body-centered cubic,octahedral,and regular diamond structures,a soft impact isolation structure is designed.The soft structure,fabricated with thermoplastic polyurethane material,demonstrated a peak impact isolation efficiency of 83%,despite possessing a thickness of 24 mm described.The work provides a novel design methodology for three-dimensional soft lattice structures and supports the development of reusable impact isolation structures for applications such as reconfigurable robots and space capture missions.展开更多
High-resolution remote sensing images(HRSIs)are now an essential data source for gathering surface information due to advancements in remote sensing data capture technologies.However,their significant scale changes an...High-resolution remote sensing images(HRSIs)are now an essential data source for gathering surface information due to advancements in remote sensing data capture technologies.However,their significant scale changes and wealth of spatial details pose challenges for semantic segmentation.While convolutional neural networks(CNNs)excel at capturing local features,they are limited in modeling long-range dependencies.Conversely,transformers utilize multihead self-attention to integrate global context effectively,but this approach often incurs a high computational cost.This paper proposes a global-local multiscale context network(GLMCNet)to extract both global and local multiscale contextual information from HRSIs.A detail-enhanced filtering module(DEFM)is proposed at the end of the encoder to refine the encoder outputs further,thereby enhancing the key details extracted by the encoder and effectively suppressing redundant information.In addition,a global-local multiscale transformer block(GLMTB)is proposed in the decoding stage to enable the modeling of rich multiscale global and local information.We also design a stair fusion mechanism to transmit deep semantic information from deep to shallow layers progressively.Finally,we propose the semantic awareness enhancement module(SAEM),which further enhances the representation of multiscale semantic features through spatial attention and covariance channel attention.Extensive ablation analyses and comparative experiments were conducted to evaluate the performance of the proposed method.Specifically,our method achieved a mean Intersection over Union(mIoU)of 86.89%on the ISPRS Potsdam dataset and 84.34%on the ISPRS Vaihingen dataset,outperforming existing models such as ABCNet and BANet.展开更多
Endocrine disrupting chemicals(EDCs) in the secondary effluent discharged from wastewater treatment plants are of great concern when water reuse is intended. Ozonation and ultrafiltration(UF) are powerful technologies...Endocrine disrupting chemicals(EDCs) in the secondary effluent discharged from wastewater treatment plants are of great concern when water reuse is intended. Ozonation and ultrafiltration(UF) are powerful technologies reported to eliminate EDCs. Due to the importance of effluent organic matters(EfOMs) in secondary effluent, the effects of three kinds of EfOM on the treatment of five EDCs using ozonation and UF were investigated. The three kinds of EfOM studied were humic acid sodium salt(NaAH), bovine serum albumin(BSA)and sodium alginate(NaAg); and the five EDCs were estrone, 17β-estradiol, estriol, 17α-ethynyl estradiol and bisphenol A. The results showed that EfOM accelerated the decay rate of ozone and inhibited the degradation efficiency of EDCs by ozonation in the order NaAH > BSA > NaAg.The ultraviolet absorbance at 280 nm(UVA_(280)) has potential for use as a surrogate indicator to assess EDC removal via ozonation without conducting difficult EDC analyses. When the decline in UVA_(280) exceeded 18%, the five EDCs had been completely removed. The UF behavior of NaAH,BSA and NaAg was found to follow the cake filtration law. The fouling potential of EfOM followed the order NaAg > NaAH > BSA; while EfOM on the membrane surface enhanced EDC removal in the order NaAH > BSA > NaAg. The mean retention rate of the membrane was increased by 24%, 10% and 8%, respectively. The properties of EDCs and EfOM cakes both influenced the EDC removal rates due to adsorption, size exclusion and charge attraction.展开更多
The recently developed discrete Boltzmann method(DBM), which is based on a set of uniform linear evolution equations and has high parallel efficiency, is employed to investigate the dynamic nonequilibrium process of K...The recently developed discrete Boltzmann method(DBM), which is based on a set of uniform linear evolution equations and has high parallel efficiency, is employed to investigate the dynamic nonequilibrium process of Kelvin-Helmholtz instability(KHI). It is found that, the relaxation time always strengthens the global nonequilibrium(GNE), entropy of mixing, and free enthalpy of mixing. Specifically, as a combined effect of physical gradients and nonequilibrium area, the GNE intensity first increases but decreases during the whole life-cycle of KHI. The growth rate of entropy of mixing shows firstly reducing, then increasing, and finally decreasing trends during the KHI process. The trend of the free enthalpy of mixing is opposite to that of the entropy of mixing. Detailed explanations are:(i) Initially,binary diffusion smooths quickly the sharp gradient in the mole fraction, which results in a steeply decreasing mixing rate.(ii) Afterwards, the mixing process is significantly promoted by the increasing length of material interface in the evolution of the KHI.(iii) As physical gradients are smoothed due to the binary diffusion and dissipation, the mixing rate reduces and approaches zero in the final stage. Moreover, with the increasing Atwood number, the global strength of viscous stresses on the heavy(light) medium reduces(increases), because the heavy(light) medium has a relatively small(large) velocity change. Furthermore, for a smaller Atwood number, the peaks of nonequilibrium manifestations emerge earlier, the entropy of mixing and free enthalpy of mixing change faster, because the KHI initiates a higher growth rate.展开更多
Single thermal cycle simulation tests were carried out for X80 high strength steel pipes from three steel mills by a Gleeble 3500HS thermal simulation test machine,and coincidence degree of the thermal simulation curv...Single thermal cycle simulation tests were carried out for X80 high strength steel pipes from three steel mills by a Gleeble 3500HS thermal simulation test machine,and coincidence degree of the thermal simulation curve with the set curve under heat inputs of 6–30 kJ/cm was observed;The relationship between different heat inputs and microstructure,impact toughness and hardness of steel pipe CGHAZ(coarse grain heat affected zone)was studied by metallographic examination,impact test and hardness test.The results show that with the increase of heat input,original austenite grain size increases gradually,the lath bainite ratio decreases and the granular bainite ratio increases.The impact toughness of C steel pipe is lower than those of A and B steel pipe,and the impact toughness of CGHAZ from the three steel pipes show different trends:for A steel pipe CGHAZ,impact toughness increases first and then decreases,with the highest value of 270–320 J under 20–25 kJ/cm;for B steel pipe CGHAZ,impact toughness decreases slightly;for C steel pipe CGHAZ,impact toughness increases,with the highest value of 260–300 J under 25 kJ/cm.As the heat input increases,the hardness of three X80 steel pipes CGHAZ shows a decreasing trhighend,and C steel pipe has the largest decreasing range.展开更多
Creep age forming techniques have been widely used in aerospace industries. In this study, we investigated the effect of aging temperature(143 °C-163 °C) on the creep behavior of Al-Li-S4 aluminum alloy and ...Creep age forming techniques have been widely used in aerospace industries. In this study, we investigated the effect of aging temperature(143 °C-163 °C) on the creep behavior of Al-Li-S4 aluminum alloy and their mechanical properties at room temperature. The mechanical properties were tested by tensile testing, and the microstructural evolution at different aging temperatures was examined by transmission electron microscopy. Results show that the creep strains and the room-temperature mechanical properties after creep aging increase with the aging temperature. As the aging temperature increases, the creep strain increases from 0.018% at 143 °C to 0.058% at 153 °C, and then to 0.094% at 163 °C. Within 25 h aging, the number of creep steps increases and the duration time of the same steps is shortened with the growth of aging temperatures. Therefore, the increase in aging temperatures accelerates the progress of the entire creep. Two main strengthening precipitates θ′(Al2 Cu) and T1(Al2 Cu Li) phases were characterized. This work indicates that the creep strain and mechanical properties of Al-Li-S4 alloys can be improved by controlling aging temperatures.展开更多
The effect of cold rolling reduction(50%-90%)on the grain structures of solutionized 1445 Al-Li alloy sheet at525-575 ℃ was investigated through electron backscatter diffraction(EBSD).Although the solutionization tem...The effect of cold rolling reduction(50%-90%)on the grain structures of solutionized 1445 Al-Li alloy sheet at525-575 ℃ was investigated through electron backscatter diffraction(EBSD).Although the solutionization temperature is elevated to 575 ℃,the sheet is not completely recrystallized.The main recrystallization model is subgrain coalescence and growth,and the non-recrystallization is due to the formed nano-sized Al3(Sc,Zr)dispersoids,which pin the grain boundaries,subgrain boundaries and dislocations.With increasing the cold rolling reduction,the fraction and size of the recrystallized grains in the sheet solutionized at525 ℃ are decreased,but the fraction of the subgrains is increased,leading to a decrease in the fraction of the deformed structures.Meanwhile,the number fraction of high-angle boundaries(HABs)is increased.Due to the decreased fraction of the deformed structures and increased fraction of the HABs,the T8-aged 1445 Al-Li alloy sheet displays a decrease trend in the strength and heterogeneity with increasing the cold rolling reduction.At higher solutionization temperature of 575 ℃,the fraction of the recrystallized grains and their size are obviously increased.展开更多
We present an improved lattice Boltzmann(LB) model for thermal liquid-vapor system.In the new model,the Windowed Fast Fourier Transform(WFFT) and its inverse are used to calculate both the convection term and the exte...We present an improved lattice Boltzmann(LB) model for thermal liquid-vapor system.In the new model,the Windowed Fast Fourier Transform(WFFT) and its inverse are used to calculate both the convection term and the external force term of the LB equation.By adopting the WFFT scheme,Gibbs oscillations can be damped effectively in unsmooth regions while high resolution feature of the spectral method can be retained in smooth regions.As a result,spatial discretization errors are dramatically decreased,conservation of the total energy is much better preserved,and the spurious velocities near the liquid-vapor interface are significantly reduced.The high resolution,together with the low complexity of the WFFT approach,endows the proposed method with considerable potential for studying a wide class of problems in the field of multiphase flows.展开更多
2024 aluminum alloys were consolidated by using spark plasma sintering(SPS) method,and then heat treated by solid solution treatment(SST) and aging treatment(AT) procedures.The average grain size of bulk samples sinte...2024 aluminum alloys were consolidated by using spark plasma sintering(SPS) method,and then heat treated by solid solution treatment(SST) and aging treatment(AT) procedures.The average grain size of bulk samples sintered with 5,20 and 50 μm powders was 3.72,4.73 and 8.11 μm,respectively.The difference between the average grain size and original powder size was attributed to the recrystallization during short sintering process.The number of the inclusion phases in these samples decreased after SST and increased after subsequent AT.Besides,it was observed that intergranular corrosion(IGC) cracks initiated from stable pits due to the electrochemical inhomogeneity between the intermetallic particles(IMPs) and the aluminum matrix(176.02,110.83 and 164.80 mV for as-SPS,as-SST and as-AT samples,respectively).Besides,the cracks would propagate along the grain boundaries(GBs) and bypass the IMPs at GBs during propagation.It was revealed that the sample after SST presented the best IGC resistance,and this was ascribed to the reduce of IMPs,both in size and number.展开更多
The tracking of orientation and angular velocity is a primary attitude control task for an on-orbit spacecraft.The problem for a rigid spacecraft tracking a desired angular velocity profile is addressed using an adapt...The tracking of orientation and angular velocity is a primary attitude control task for an on-orbit spacecraft.The problem for a rigid spacecraft tracking a desired angular velocity profile is addressed using an adaptive feedback control.An angular velocity feedback tracking algorithm is firstly developed based on the precisely known attitude dynamics of the spacecraft,and the global tracking of the control algorithm is proved based on the Lyapunov analysis.An adaptation mechanism is then designed to deal with the dynamic uncertainties of the spacecraft.Such an adaptation mechanism enables the controller to track any desired angular velocity trajectories even in the presence of uncertain inertia parameters,although it does not guarantee the inertia tensor being precisely identified.To verify the effectiveness of the proposed adaptive control policy,computer simulations on dynamic equations of a spacecraft are conducted and their results are discussed.展开更多
Potassium ion capacitors(PICs)are regarded as promising large-scale aqueous energy storage systems.However,due to the poor K^(+)transport kinetics and the structural instability of the cathode materials,the key issues...Potassium ion capacitors(PICs)are regarded as promising large-scale aqueous energy storage systems.However,due to the poor K^(+)transport kinetics and the structural instability of the cathode materials,the key issues of limited energy density and poor cyclic stability are obstacles to the in-depth growth of PICs.Herein,a novel O-doped perovskite fluoride is demonstrated via an in-situ electrochemical oxidation strategy as the cathode for PICs,introducing additional defects that improve the capacitance and facilitate the reaction kinetics of the electrode.During the electrochemical oxidation process,it is discovered that the perovskite fluoride crystal tends to transform into disordered O-doped KMnF 3(K_(x)MnF_(y)O_(z)),realizing a structural reconstruction at the electrode material/electrolyte interface.The First-principles calculations based on density functional theory(DFT)are performed to confirm that the improved electrical conduc-tivity and low ionic adsorption energy may be ascribed to the substitution of oxygen for fluorine.The obtained K_(1.14)MnF_(1.17)O_(1.26) cathode achieves a high specific capacitance of 694 F g^(-1) at 1 A g^(-1),as well as high capacitance retention of 91.3%after 10,000 charge/discharge cycles in mild K_(2)SO_(4) electrolyte.This study provides an effective strategy to improve the capacitive performance of perovskite fluoride cathode materials in electrochemical energy storage.展开更多
During the supersonic re-entry of multi-nozzle heavy rockets into the atmosphere,the basic flow state becomes increasingly complex due to the coupling effect between the retropropulsion plumes and the freestream.A num...During the supersonic re-entry of multi-nozzle heavy rockets into the atmosphere,the basic flow state becomes increasingly complex due to the coupling effect between the retropropulsion plumes and the freestream.A numerical method using the hybrid Reynolds-Averaged Navier-Stokes and Large Eddy Simulation(RES)method and discrete coordinate method is developed to accurately estimate the thermal environment.In addition,finite rate chemical kinetics is used to calculate the afterburning reactions.The numerical results agree well with wind tunnel data,which confirms the validity and accuracy of the numerical method.Computations are conducted for the heavy carrier rocket re-entry from 53.1 km to 39.5 km altitude with 180°angle of attack by using three different Supersonic Retro-Propulsion(SRP)modes.The numerical results reveal that these three SRP flow fields are all Short Penetration Models(SPM).As the re-entry altitudes decrease,both the plume-plume interaction and the plume-freestream interaction become weaker.The highest temperatures in the plume shear layers of the three SRP modes increase by 8.36%,7.33%and 6.92%respectively after considering afterburning reactions,and all occur at a reentry altitude of 39.5 km.As the rocket re-enters the atmosphere,the maximum heat flux on the rocket base plate of three SRP modes stabilizes at 290,170 and 200 kW/m^(2) respectively,but the maximum heat flux on the side wall increases significantly.When the altitude declines to 39.5 km,the extreme heat flux of the three modes increase by 84.16%,49.45%and 62.97%respectively compared to that at 53.1 km.展开更多
The libration control problem of space tether system(STS)for post-capture of payload is studied.The process of payload capture will cause tether swing and deviation from the nominal position,resulting in the failure o...The libration control problem of space tether system(STS)for post-capture of payload is studied.The process of payload capture will cause tether swing and deviation from the nominal position,resulting in the failure of capture mission.Due to unknown inertial parameters after capturing the payload,an adaptive optimal control based on policy iteration is developed to stabilize the uncertain dynamic system in the post-capture phase.By introducing integral reinforcement learning(IRL)scheme,the algebraic Riccati equation(ARE)can be online solved without known dynamics.To avoid computational burden from iteration equations,the online implementation of policy iteration algorithm is provided by the least-squares solution method.Finally,the effectiveness of the algorithm is validated by numerical simulations.展开更多
The placement optimization of piezoelectric actuators and active vibration control of a membrane structure are studied in this paper. The classical linear quadratic regulator con- trollers are designed to suppress the...The placement optimization of piezoelectric actuators and active vibration control of a membrane structure are studied in this paper. The classical linear quadratic regulator con- trollers are designed to suppress the unwanted vibration. Simulation results indicate that the optimal locations of piezoelectric actuators are affected deeply by the additional mass and stiff- ness of actuators, the computational efficiency of particle swarm optimizer is higher than that of genetic algorithm for this particular problem, and the control performance of optimally placed actuators is better than that of non-optimally placed actuators.展开更多
Fine zirconium diboride （ZrB2） powders with high purity were successfully prepared by combustion synthesis through magnesiothermic reduction process in Mg-B2O3-ZrO2 system. The reaction mechanism was investigated by...Fine zirconium diboride （ZrB2） powders with high purity were successfully prepared by combustion synthesis through magnesiothermic reduction process in Mg-B2O3-ZrO2 system. The reaction mechanism was investigated by differential thermal analysis and quenching experiment. The results show that the whole magnesio-thermic reduction process includes three stages： first, molten B2O3 and Mg formed above the temperature of 650 ℃, and glassy B2O3 and solid ZrO2 particles were coated on the surface of the molten Mg; thus, the hollow balls can be formed when the molten Mg was exuded under capillary function. Second, ZrO2 particles reacted with molten Mg to form Zr and MgO with dissolution-precip-itation mechanism, which released a large amount of heat to induce the diffusion reaction between B203 and Mg to form B and MgO. Last, Zr reacted with B to form ZrB2 grains. The preparation of ZrB2 by self-propagating syn-thesis in Mg-B2O3-ZrO2 system is a solid-liquid-liquid reaction.展开更多
基金funded by scientific research projects under Grant JY2024B011.
文摘With the increasing complexity of industrial automation,planetary gearboxes play a vital role in largescale equipment transmission systems,directly impacting operational efficiency and safety.Traditional maintenance strategies often struggle to accurately predict the degradation process of equipment,leading to excessive maintenance costs or potential failure risks.However,existing prediction methods based on statistical models are difficult to adapt to nonlinear degradation processes.To address these challenges,this study proposes a novel condition-based maintenance framework for planetary gearboxes.A comprehensive full-lifecycle degradation experiment was conducted to collect raw vibration signals,which were then processed using a temporal convolutional network autoencoder with multi-scale perception capability to extract deep temporal degradation features,enabling the collaborative extraction of longperiod meshing frequencies and short-term impact features from the vibration signals.Kernel principal component analysis was employed to fuse and normalize these features,enhancing the characterization of degradation progression.A nonlinear Wiener process was used to model the degradation trajectory,with a threshold decay function introduced to dynamically adjust maintenance strategies,and model parameters optimized through maximum likelihood estimation.Meanwhile,the maintenance strategy was optimized to minimize costs per unit time,determining the optimal maintenance timing and preventive maintenance threshold.The comprehensive indicator of degradation trends extracted by this method reaches 0.756,which is 41.2%higher than that of traditional time-domain features;the dynamic threshold strategy reduces the maintenance cost per unit time to 55.56,which is 8.9%better than that of the static threshold optimization.Experimental results demonstrate significant reductions in maintenance costs while enhancing system reliability and safety.This study realizes the organic integration of deep learning and reliability theory in the maintenance of planetary gearboxes,provides an interpretable solution for the predictive maintenance of complex mechanical systems,and promotes the development of condition-based maintenance strategies for planetary gearboxes.
基金supported by the National Key R&D Program of China(No.2020YFC2201100)the National Natural Science Foundation of China(Nos.12175032,12102082,12275044,12402327,12405290 and 12211530449)+4 种基金the Joint Program of the Science and Technology Program of Liaoning,China(No.2023JH2/101700285)the Fundamental Research Funds for the Central Universities of China(Nos.DUT22RC(3)078,DUT23RC(3)040 and DUT24ZD106)the S&T Program of Hebei,China(No.246Z2301G)the S&T Innovation Program of Hebei,China(Nos.SJMYF2022X18 and SJMYF2022X06)the Beijing Engineering Research Center of Efficient and Green Aerospace Propulsion Technology and Advanced Space Propulsion Laboratory of BICE,China(No.LabASP-2023-07).
文摘The electrospray thruster supplied by ionic liquid is a promising micro-propulsion thruster with small size and precise thrust, which can emit both cations and anions to achieve self-neutralization. In order to further investigate the effect of ion solvation energy on the evaporation of cations and anions from ionic liquid under the action of a uniform electric field, this paper establishes a transient Electrohydrodynamic (EHD) model for free ionic liquid droplets undergoing ion evaporation. The dynamic processes of droplet deformation and ion evaporation are simulated. And the study further focuses on the influence of different ion solvation energies for cations on the droplet morphology and the ion evaporation characteristics at the positively charged end and negatively charged end of the droplet. The results indicate that, when the ion solvation energy for cations is higher than that of anions, it will cause the ion evaporation at the positively charged end of the droplet to lag behind the ion evaporation at the negatively charged end. And the higher the ion solvation energy for the cations, the longer the evaporation lag time at the positively charged end of the droplet, which will lead to a higher peak of surface charge density that can be reached, resulting in a larger evaporation current and sharper droplet stretching deformation. Additionally, the peak surface charge density of the positively charged end of the droplet is linearly related to the ion solvation energy for cations, while the peak surface charge density of the negatively charged end remains almost unchanged and is not significantly affected by the ion solvation energy for cations.
基金Under the auspices of National Natural Science Foundation of China(No.42101393,41901375,52274166)Hebei Natural Science Foundation(No.D2022209005,D2023209008)Central Guided Local Science and Technology Development Fund Project of Hebei Province(No.236Z3305G,246Z4201G)Key Research and Development Program of Science and Technology Plan of Tangshan,China(No.22150221J)。
文摘Coastal wetlands are crucial for the‘blue carbon sink’,significantly contributing to regulating climate change.This study util-ized 160 soil samples,35 remote sensing features,and 5 geo-climatic data to accurately estimate the soil organic carbon stocks(SOCS)in the coastal wetlands of Tianjin and Hebei,China.To reduce data redundancy,simplify model complexity,and improve model inter-pretability,Pearson correlation analysis(PsCA),Boruta,and recursive feature elimination(RFE)were employed to optimize features.Combined with the optimized features,the soil organic carbon density(SOCD)prediction model was constructed by using multivariate adaptive regression splines(MARS),extreme gradient boosting(XGBoost),and random forest(RF)algorithms and applied to predict the spatial distribution of SOCD and estimate the SOCS of different wetland types in 2020.The results show that:1)different feature combinations have a significant influence on the model performance.Better prediction performance was attained by building a model using RFE-based feature combinations.RF has the best prediction accuracy(R^(2)=0.587,RMSE=0.798 kg/m^(2),MAE=0.660 kg/m^(2)).2)Optical features are more important than radar and geo-climatic features in the MARS,XGBoost,and RF algorithms.3)The size of SOCS is related to SOCD and the area of each wetland type,aquaculture pond has the highest SOCS,followed by marsh,salt pan,mud-flat,and sand shore.
基金Project(22020JJ4729)supported by the Natural Science Foundation of Hunan Province,China。
文摘Polyvinylidene fluoride/lead zirconate titanate(PVDF/PZT)composite films have been prepared by direct ink writing and the effect of PZT content on crystallization behavior and electrical properties of film were systematically investigated.The composite films were characterized by scanning electron microscope(SEM),X-ray diffractometer(XRD),Flourier transform infrared spectroscope(FTIR)and differential scanning calorimeter(DSC).The results show that,surface modified PZT powder(PZT@PDA)is successfully coated by polydopamine(PDA),resulting in a large number of polar groups that interact with the-CF_(2)-groups in PVDF,inducing the generation of polarβphase due to hydrogen bonding formed in the interaction.Theβphase content in composite film increases with increasing PZT@PDA content,up to 28.09%as with 5 wt.%PZT@PDA.PZT@PDA plays a role of nucleating agent to promote the generation of polar phases in the film and also acts as an impurity hindering the growth of nuclei to reduce crystallinity.Moreover,the presence of PZT@PDA in interfaces provides more sites for the occurrence of interfacial polarization and thus improving the electrical properties of films.The composite film with 5 wt.%PZT@PDA possesses the highest dielectric constant(8.61)and residual polarization value(0.6803μC/cm^(2)).
基金support from the National Key R&D Program of China(No.2020YFC2201100)the Foundation of National Key Laboratory of Shock Wave and Detonation Physics,China(No.JCKYS2023212003)+1 种基金the National Natural Science Foundation of China(No.12172061)the Opening Project of State Key Laboratory of Explosion Science and Safety Protection(Beijing Institute of Technology)(No.KFJJ25-02M).
文摘A Discrete Boltzmann Method(DBM)with a Maxwell-type boundary condition is constructed to investigate the influence of rarefaction on laminar Shock Wave/Boundary Layer Interaction(SWBLI).Due to the complexity of compressible flow,a Knudsen number vector Kn,whose components include the local Knudsen numbers such as Kn_(ρ)and Kn_(U),is introduced to characterize the local structures,where Kn_(ρ)and Kn_(U)are Knudsen numbers defined in terms of the density and velocity interfaces,respectively.Since first focusing on the steady state of SWBLI,the DBM considers up to the second-order Kn_(ρ)(rarefaction/non-equilibrium)effects.The model is validated using Mach number 2 SWBLI and the necessity of using DBM with sufficient physical accuracy is confirmed by the shock collision problem.Key findings include the following:the leading-edge shock wave increases the local density Knudsen number Kn_(ρ)and eventually leads to the failure of linear constitutive relations in the Navier-Stokes(N-S)model and surely also in the lower-order DBM;the non-equilibrium effect differences in regions behind the leading-edge shock wave are primarily correlated with Kn_(ρ),while in the separation region are primarily correlated with Kn_(U);the non-equilibrium quantities D_(2)and D_(4,2),as well as the viscous entropy production rate S_(NOMF)can be used to identify the separation zone.The findings clarify various effects and main mechanisms in different regions associated with SWBLI,which are concealed in N-S model.
文摘Cellular structures,distinguished by their porous characteristics,are frequently adopted in designs aimed at impact isolation,owing to their lightweight attributes and exceptional ability to absorb energy during impact events.Lattice structures often rely on plastic deformation to absorb energy.However,in applications such as sports protection and robotic grasping,there exists a requirement for a reusable structure designed to isolate impacts,which can be effectively achieved by three-dimensional flexible lattice structures.In this work,a theoretical calculation method for soft lattice structures is proposed,and in light of this method,a three-dimensional soft lattice structure aimed at isolating impacts has been carefully designed.The predictive theory for the quasistatic mechanical properties,including stiffness and buckling strength for three-dimensional soft lattice structures is described.On the basis of the quasizero stiffness characteristics inherent in body-centered cubic,octahedral,and regular diamond structures,a soft impact isolation structure is designed.The soft structure,fabricated with thermoplastic polyurethane material,demonstrated a peak impact isolation efficiency of 83%,despite possessing a thickness of 24 mm described.The work provides a novel design methodology for three-dimensional soft lattice structures and supports the development of reusable impact isolation structures for applications such as reconfigurable robots and space capture missions.
基金provided by the Science Research Project of Hebei Education Department under grant No.BJK2024115.
文摘High-resolution remote sensing images(HRSIs)are now an essential data source for gathering surface information due to advancements in remote sensing data capture technologies.However,their significant scale changes and wealth of spatial details pose challenges for semantic segmentation.While convolutional neural networks(CNNs)excel at capturing local features,they are limited in modeling long-range dependencies.Conversely,transformers utilize multihead self-attention to integrate global context effectively,but this approach often incurs a high computational cost.This paper proposes a global-local multiscale context network(GLMCNet)to extract both global and local multiscale contextual information from HRSIs.A detail-enhanced filtering module(DEFM)is proposed at the end of the encoder to refine the encoder outputs further,thereby enhancing the key details extracted by the encoder and effectively suppressing redundant information.In addition,a global-local multiscale transformer block(GLMTB)is proposed in the decoding stage to enable the modeling of rich multiscale global and local information.We also design a stair fusion mechanism to transmit deep semantic information from deep to shallow layers progressively.Finally,we propose the semantic awareness enhancement module(SAEM),which further enhances the representation of multiscale semantic features through spatial attention and covariance channel attention.Extensive ablation analyses and comparative experiments were conducted to evaluate the performance of the proposed method.Specifically,our method achieved a mean Intersection over Union(mIoU)of 86.89%on the ISPRS Potsdam dataset and 84.34%on the ISPRS Vaihingen dataset,outperforming existing models such as ABCNet and BANet.
基金supported by the National Natural Science Foundation of China (No.51808008)the Natural Science Foundation of Hebei Province (No.E2017409004)the Youth Foundation of Hebei Province Education Department (No.QN2016224)
文摘Endocrine disrupting chemicals(EDCs) in the secondary effluent discharged from wastewater treatment plants are of great concern when water reuse is intended. Ozonation and ultrafiltration(UF) are powerful technologies reported to eliminate EDCs. Due to the importance of effluent organic matters(EfOMs) in secondary effluent, the effects of three kinds of EfOM on the treatment of five EDCs using ozonation and UF were investigated. The three kinds of EfOM studied were humic acid sodium salt(NaAH), bovine serum albumin(BSA)and sodium alginate(NaAg); and the five EDCs were estrone, 17β-estradiol, estriol, 17α-ethynyl estradiol and bisphenol A. The results showed that EfOM accelerated the decay rate of ozone and inhibited the degradation efficiency of EDCs by ozonation in the order NaAH > BSA > NaAg.The ultraviolet absorbance at 280 nm(UVA_(280)) has potential for use as a surrogate indicator to assess EDC removal via ozonation without conducting difficult EDC analyses. When the decline in UVA_(280) exceeded 18%, the five EDCs had been completely removed. The UF behavior of NaAH,BSA and NaAg was found to follow the cake filtration law. The fouling potential of EfOM followed the order NaAg > NaAH > BSA; while EfOM on the membrane surface enhanced EDC removal in the order NaAH > BSA > NaAg. The mean retention rate of the membrane was increased by 24%, 10% and 8%, respectively. The properties of EDCs and EfOM cakes both influenced the EDC removal rates due to adsorption, size exclusion and charge attraction.
基金Supported by the Natural Science Foundation of China under Grant Nos.91441120,51806116,11875001,and 11602162the China Postdoctoral Science Foundation under Grant No.2017M620757+2 种基金the Center for Combustion Energy at Tsinghua Universitythe Natural Science Foundation of Hebei Province under Grant Nos.A2017409014,ZD2017001,and A201500111the UK Engineering and Physical Sciences Research Council under Project UK Consortium on Mesoscale Engineering Sciences(UKCOMES)under Grant Nos.EP/L00030X/1 and EP/R029598/1
文摘The recently developed discrete Boltzmann method(DBM), which is based on a set of uniform linear evolution equations and has high parallel efficiency, is employed to investigate the dynamic nonequilibrium process of Kelvin-Helmholtz instability(KHI). It is found that, the relaxation time always strengthens the global nonequilibrium(GNE), entropy of mixing, and free enthalpy of mixing. Specifically, as a combined effect of physical gradients and nonequilibrium area, the GNE intensity first increases but decreases during the whole life-cycle of KHI. The growth rate of entropy of mixing shows firstly reducing, then increasing, and finally decreasing trends during the KHI process. The trend of the free enthalpy of mixing is opposite to that of the entropy of mixing. Detailed explanations are:(i) Initially,binary diffusion smooths quickly the sharp gradient in the mole fraction, which results in a steeply decreasing mixing rate.(ii) Afterwards, the mixing process is significantly promoted by the increasing length of material interface in the evolution of the KHI.(iii) As physical gradients are smoothed due to the binary diffusion and dissipation, the mixing rate reduces and approaches zero in the final stage. Moreover, with the increasing Atwood number, the global strength of viscous stresses on the heavy(light) medium reduces(increases), because the heavy(light) medium has a relatively small(large) velocity change. Furthermore, for a smaller Atwood number, the peaks of nonequilibrium manifestations emerge earlier, the entropy of mixing and free enthalpy of mixing change faster, because the KHI initiates a higher growth rate.
基金supported by Science and Technology Research Project of Universities of Hebei Province(No.QN201.221)。
文摘Single thermal cycle simulation tests were carried out for X80 high strength steel pipes from three steel mills by a Gleeble 3500HS thermal simulation test machine,and coincidence degree of the thermal simulation curve with the set curve under heat inputs of 6–30 kJ/cm was observed;The relationship between different heat inputs and microstructure,impact toughness and hardness of steel pipe CGHAZ(coarse grain heat affected zone)was studied by metallographic examination,impact test and hardness test.The results show that with the increase of heat input,original austenite grain size increases gradually,the lath bainite ratio decreases and the granular bainite ratio increases.The impact toughness of C steel pipe is lower than those of A and B steel pipe,and the impact toughness of CGHAZ from the three steel pipes show different trends:for A steel pipe CGHAZ,impact toughness increases first and then decreases,with the highest value of 270–320 J under 20–25 kJ/cm;for B steel pipe CGHAZ,impact toughness decreases slightly;for C steel pipe CGHAZ,impact toughness increases,with the highest value of 260–300 J under 25 kJ/cm.As the heat input increases,the hardness of three X80 steel pipes CGHAZ shows a decreasing trhighend,and C steel pipe has the largest decreasing range.
基金Project(2017YFB0306300)supported by National key R&D Program of ChinaProject(zzyikt2015-05)supported by the Project of State Key Laboratory of High Performance Complex Manufacture,China。
文摘Creep age forming techniques have been widely used in aerospace industries. In this study, we investigated the effect of aging temperature(143 °C-163 °C) on the creep behavior of Al-Li-S4 aluminum alloy and their mechanical properties at room temperature. The mechanical properties were tested by tensile testing, and the microstructural evolution at different aging temperatures was examined by transmission electron microscopy. Results show that the creep strains and the room-temperature mechanical properties after creep aging increase with the aging temperature. As the aging temperature increases, the creep strain increases from 0.018% at 143 °C to 0.058% at 153 °C, and then to 0.094% at 163 °C. Within 25 h aging, the number of creep steps increases and the duration time of the same steps is shortened with the growth of aging temperatures. Therefore, the increase in aging temperatures accelerates the progress of the entire creep. Two main strengthening precipitates θ′(Al2 Cu) and T1(Al2 Cu Li) phases were characterized. This work indicates that the creep strain and mechanical properties of Al-Li-S4 alloys can be improved by controlling aging temperatures.
文摘The effect of cold rolling reduction(50%-90%)on the grain structures of solutionized 1445 Al-Li alloy sheet at525-575 ℃ was investigated through electron backscatter diffraction(EBSD).Although the solutionization temperature is elevated to 575 ℃,the sheet is not completely recrystallized.The main recrystallization model is subgrain coalescence and growth,and the non-recrystallization is due to the formed nano-sized Al3(Sc,Zr)dispersoids,which pin the grain boundaries,subgrain boundaries and dislocations.With increasing the cold rolling reduction,the fraction and size of the recrystallized grains in the sheet solutionized at525 ℃ are decreased,but the fraction of the subgrains is increased,leading to a decrease in the fraction of the deformed structures.Meanwhile,the number fraction of high-angle boundaries(HABs)is increased.Due to the decreased fraction of the deformed structures and increased fraction of the HABs,the T8-aged 1445 Al-Li alloy sheet displays a decrease trend in the strength and heterogeneity with increasing the cold rolling reduction.At higher solutionization temperature of 575 ℃,the fraction of the recrystallized grains and their size are obviously increased.
基金Supported by the Science Foundations of Laboratory of Computational Physics and China Academy of Engineering Physics under Grant Nos.2009A0102005 and 2011A0201002National Natural Science Foundation of China under Grant Nos.11075021,91130020,11074300, and 10874242+2 种基金Fundamental research funds for the central university under Grant No.2010YS03Technology Support Program of LangFang under Grant Nos.2010011030,201101118/21/23/24Teaching and Research Foundation of NCIAE under Grant Nos.JY-2011-027-Y and JY-2011-028-Y
文摘We present an improved lattice Boltzmann(LB) model for thermal liquid-vapor system.In the new model,the Windowed Fast Fourier Transform(WFFT) and its inverse are used to calculate both the convection term and the external force term of the LB equation.By adopting the WFFT scheme,Gibbs oscillations can be damped effectively in unsmooth regions while high resolution feature of the spectral method can be retained in smooth regions.As a result,spatial discretization errors are dramatically decreased,conservation of the total energy is much better preserved,and the spurious velocities near the liquid-vapor interface are significantly reduced.The high resolution,together with the low complexity of the WFFT approach,endows the proposed method with considerable potential for studying a wide class of problems in the field of multiphase flows.
基金financially supported by the National Nature Science Foundations of China (Nos.51271012 and 51671013)。
文摘2024 aluminum alloys were consolidated by using spark plasma sintering(SPS) method,and then heat treated by solid solution treatment(SST) and aging treatment(AT) procedures.The average grain size of bulk samples sintered with 5,20 and 50 μm powders was 3.72,4.73 and 8.11 μm,respectively.The difference between the average grain size and original powder size was attributed to the recrystallization during short sintering process.The number of the inclusion phases in these samples decreased after SST and increased after subsequent AT.Besides,it was observed that intergranular corrosion(IGC) cracks initiated from stable pits due to the electrochemical inhomogeneity between the intermetallic particles(IMPs) and the aluminum matrix(176.02,110.83 and 164.80 mV for as-SPS,as-SST and as-AT samples,respectively).Besides,the cracks would propagate along the grain boundaries(GBs) and bypass the IMPs at GBs during propagation.It was revealed that the sample after SST presented the best IGC resistance,and this was ascribed to the reduce of IMPs,both in size and number.
基金Supported by the Innovation Fund of Shanghai Aerospace Science and Technology(SAST 201308)
文摘The tracking of orientation and angular velocity is a primary attitude control task for an on-orbit spacecraft.The problem for a rigid spacecraft tracking a desired angular velocity profile is addressed using an adaptive feedback control.An angular velocity feedback tracking algorithm is firstly developed based on the precisely known attitude dynamics of the spacecraft,and the global tracking of the control algorithm is proved based on the Lyapunov analysis.An adaptation mechanism is then designed to deal with the dynamic uncertainties of the spacecraft.Such an adaptation mechanism enables the controller to track any desired angular velocity trajectories even in the presence of uncertain inertia parameters,although it does not guarantee the inertia tensor being precisely identified.To verify the effectiveness of the proposed adaptive control policy,computer simulations on dynamic equations of a spacecraft are conducted and their results are discussed.
基金the financial support from Liaoning Sci-ence and Technology Development Foundation Guided by Cen-tral Government(No.2021JH6/10500139)the Fundamental Research Funds for the Central Universities(No.N2205003)+2 种基金the financial support from the National Natural Science Foundation of China(No.52003007)Nat-ural Science Foundation of Hebei Province(No.E2019409063)Langfang top-notch talent(No.LFBJ202004).
文摘Potassium ion capacitors(PICs)are regarded as promising large-scale aqueous energy storage systems.However,due to the poor K^(+)transport kinetics and the structural instability of the cathode materials,the key issues of limited energy density and poor cyclic stability are obstacles to the in-depth growth of PICs.Herein,a novel O-doped perovskite fluoride is demonstrated via an in-situ electrochemical oxidation strategy as the cathode for PICs,introducing additional defects that improve the capacitance and facilitate the reaction kinetics of the electrode.During the electrochemical oxidation process,it is discovered that the perovskite fluoride crystal tends to transform into disordered O-doped KMnF 3(K_(x)MnF_(y)O_(z)),realizing a structural reconstruction at the electrode material/electrolyte interface.The First-principles calculations based on density functional theory(DFT)are performed to confirm that the improved electrical conduc-tivity and low ionic adsorption energy may be ascribed to the substitution of oxygen for fluorine.The obtained K_(1.14)MnF_(1.17)O_(1.26) cathode achieves a high specific capacitance of 694 F g^(-1) at 1 A g^(-1),as well as high capacitance retention of 91.3%after 10,000 charge/discharge cycles in mild K_(2)SO_(4) electrolyte.This study provides an effective strategy to improve the capacitive performance of perovskite fluoride cathode materials in electrochemical energy storage.
基金co-supported by the National Level Project,China and Shanghai Municipal Major Science and Technology Project,China.In addition,the authors gratefully acknowledge the guidance on English writing and numerical methods of Professor Junfeng ZHANG from the Faculty of Engineering at Laurentian University.
文摘During the supersonic re-entry of multi-nozzle heavy rockets into the atmosphere,the basic flow state becomes increasingly complex due to the coupling effect between the retropropulsion plumes and the freestream.A numerical method using the hybrid Reynolds-Averaged Navier-Stokes and Large Eddy Simulation(RES)method and discrete coordinate method is developed to accurately estimate the thermal environment.In addition,finite rate chemical kinetics is used to calculate the afterburning reactions.The numerical results agree well with wind tunnel data,which confirms the validity and accuracy of the numerical method.Computations are conducted for the heavy carrier rocket re-entry from 53.1 km to 39.5 km altitude with 180°angle of attack by using three different Supersonic Retro-Propulsion(SRP)modes.The numerical results reveal that these three SRP flow fields are all Short Penetration Models(SPM).As the re-entry altitudes decrease,both the plume-plume interaction and the plume-freestream interaction become weaker.The highest temperatures in the plume shear layers of the three SRP modes increase by 8.36%,7.33%and 6.92%respectively after considering afterburning reactions,and all occur at a reentry altitude of 39.5 km.As the rocket re-enters the atmosphere,the maximum heat flux on the rocket base plate of three SRP modes stabilizes at 290,170 and 200 kW/m^(2) respectively,but the maximum heat flux on the side wall increases significantly.When the altitude declines to 39.5 km,the extreme heat flux of the three modes increase by 84.16%,49.45%and 62.97%respectively compared to that at 53.1 km.
基金supported by the National Natural Science Foundation of China(No.62111530051)the Fundamental Research Funds for the Central Universities(No.3102017JC06002)the Shaanxi Science and Technology Program,China(No.2017KW-ZD-04).
文摘The libration control problem of space tether system(STS)for post-capture of payload is studied.The process of payload capture will cause tether swing and deviation from the nominal position,resulting in the failure of capture mission.Due to unknown inertial parameters after capturing the payload,an adaptive optimal control based on policy iteration is developed to stabilize the uncertain dynamic system in the post-capture phase.By introducing integral reinforcement learning(IRL)scheme,the algebraic Riccati equation(ARE)can be online solved without known dynamics.To avoid computational burden from iteration equations,the online implementation of policy iteration algorithm is provided by the least-squares solution method.Finally,the effectiveness of the algorithm is validated by numerical simulations.
基金supported by the Natural Science Foundation of China(Grant No.11772187)the Natural Science Foundation of Shanghai(Grant No.16ZR1436200)
文摘The placement optimization of piezoelectric actuators and active vibration control of a membrane structure are studied in this paper. The classical linear quadratic regulator con- trollers are designed to suppress the unwanted vibration. Simulation results indicate that the optimal locations of piezoelectric actuators are affected deeply by the additional mass and stiff- ness of actuators, the computational efficiency of particle swarm optimizer is higher than that of genetic algorithm for this particular problem, and the control performance of optimally placed actuators is better than that of non-optimally placed actuators.
基金financially supported by the National Natural Science Foundation of China(No.91016014)
文摘Fine zirconium diboride （ZrB2） powders with high purity were successfully prepared by combustion synthesis through magnesiothermic reduction process in Mg-B2O3-ZrO2 system. The reaction mechanism was investigated by differential thermal analysis and quenching experiment. The results show that the whole magnesio-thermic reduction process includes three stages： first, molten B2O3 and Mg formed above the temperature of 650 ℃, and glassy B2O3 and solid ZrO2 particles were coated on the surface of the molten Mg; thus, the hollow balls can be formed when the molten Mg was exuded under capillary function. Second, ZrO2 particles reacted with molten Mg to form Zr and MgO with dissolution-precip-itation mechanism, which released a large amount of heat to induce the diffusion reaction between B203 and Mg to form B and MgO. Last, Zr reacted with B to form ZrB2 grains. The preparation of ZrB2 by self-propagating syn-thesis in Mg-B2O3-ZrO2 system is a solid-liquid-liquid reaction.
