Due to the coexistence of compressibility,viscosity,and threedimensional effects,laminar flow is difficult to maintain for high-speed boundary layer on complex geometries.The unstable disturbance waves in the boundary...Due to the coexistence of compressibility,viscosity,and threedimensional effects,laminar flow is difficult to maintain for high-speed boundary layer on complex geometries.The unstable disturbance waves in the boundary layer are excited and rapidly increase during the receptivity process,so sufficiently large Reynolds stress causes the basic flow velocity profile to change,and the formation of turbulence is inevitable.展开更多
The diffuse-interface immersed boundary method(IBM)possesses excellent capabilities for simulating flows around complex geometries and moving boundaries.In this method,the flow field is solved on a fixed Cartesian mes...The diffuse-interface immersed boundary method(IBM)possesses excellent capabilities for simulating flows around complex geometries and moving boundaries.In this method,the flow field is solved on a fixed Cartesian mesh,while the solid boundary is discretized into a series of Lagrangian points immersed in the flow field.The boundary condition is implemented by introducing a force term into the momentum equation,and the interaction between the immersed boundary and the fluid domain is achieved via an interpolation process.Over the past decades,the diffuse-interface IBM has gained popularity and spawned many variants,effectively handling a wide range of flow problems from isothermal to thermal flows,from laminar to turbulent flows,and from complex geometries to fluidstructure interaction scenarios.This paper first outlines the basic principles of the diffuse-interface IBM,then highlights recent advancements achieved by the authors’research group,and finally shows the method’s excellent numerical performance and wide applicability through several case studies involving complex moving boundary problems.展开更多
The traditional topology optimization method of continuum structure generally uses quadrilateral elements as the basic mesh.This approach often leads to jagged boundary issues,which are traditionally addressed through...The traditional topology optimization method of continuum structure generally uses quadrilateral elements as the basic mesh.This approach often leads to jagged boundary issues,which are traditionally addressed through post-processing,potentially altering the mechanical properties of the optimized structure.A topology optimization method of Movable Morphable Smooth Boundary(MMSB)is proposed based on the idea of mesh adaptation to solve the problem of jagged boundaries and the influence of post-processing.Based on the ICM method,the rational fraction function is introduced as the filtering function,and a topology optimization model with the minimum weight as the objective and the displacement as the constraint is established.A triangular mesh is utilized as the base mesh in this method.The mesh is re-divided in the optimization process based on the contour line,and a smooth boundary parallel to the contour line is obtained.Numerical examples demonstrate that the MMSB method effectively resolves the jagged boundary issues,leading to enhanced structural performance.展开更多
Vorticity is locally generated on a boundary at a rate measured by the boundary vorticity flux(BVF),which can be further decomposed into the sum of the orbital rotation and the generalized spin(specifically,the sum of...Vorticity is locally generated on a boundary at a rate measured by the boundary vorticity flux(BVF),which can be further decomposed into the sum of the orbital rotation and the generalized spin(specifically,the sum of shear and streaming vorticity)under the field description.For an incompressible viscous flow interacting with a stationary wall,the full expressions of the boundary fluxes of the orbital rotation and the spin are derived,for the first time,to elucidate their boundary creation mechanisms.Then,these new findings are successfully extended to the study of the boundary enstrophy dynamics,as well as the Lyman vorticity dynamics as an alternative interpretation to the boundary vorticity dynamics.Interestingly,it is found that the boundary coupling of the longitudinal and transverse processes is only embodied in the boundary spin flux,which is definitely not responsible for the generation of the boundary orbital-rotation flux.In addition,the boundary fluxes of enstrophy are directly associated with the boundary source of the second principal invariant of the velocity gradient tensor(VGT)and the two quadratic forms representing the spin-geometry interaction.The present exposition provides a new perspective and an additional dimension for understanding the vorticity dynamics on boundaries,which could be valuable in clarifying the formation mechanisms of near-wall coherent structures and flow noise at the fundamental level.展开更多
Cowl-induced incident Shock Wave/Boundary Layer Interactions (SWBLI) under the influence of gradual expansion waves are frequently observed in supersonic inlets. However, the analysis and prediction of interaction len...Cowl-induced incident Shock Wave/Boundary Layer Interactions (SWBLI) under the influence of gradual expansion waves are frequently observed in supersonic inlets. However, the analysis and prediction of interaction lengths have not been sufficiently investigated. First, this study presents a theoretical scaling analysis and validates it through wind tunnel experiments. It conducts detailed control volume analysis of mass conservation, considering the differences between inviscid and viscous cases. Then, three models for analysing interaction length under gradual expansion waves are derived. Related experiments using schlieren photography are conducted to validate the models in a Mach 2.73 flow. The interaction scales are captured at various relative distances between the shock impingement location and the expansion regions with wedge angles ranging from 12° to 15° and expansion angles of 9°, 12°, and 15°. Three trend lines are plotted based on different expansion angles to depict the relationship between normalised interaction length and normalised interaction strength metric. In addition, the relationship between the coefficients of the trend line and the expansion angles is introduced to predict the interaction length influenced by gradual expansion waves. Finally, the estimation of normalised interaction length is derived for various coefficients within a unified form.展开更多
The Beijing 325 m meteorological tower stands as a pivotal research platform for exploring atmospheric boundary layer physics and atmospheric chemistry.With a legacy spanning 45 years,the tower has played a crucial ro...The Beijing 325 m meteorological tower stands as a pivotal research platform for exploring atmospheric boundary layer physics and atmospheric chemistry.With a legacy spanning 45 years,the tower has played a crucial role in unraveling the complexities of urban air pollution,atmospheric processes,and climate change in Beijing,China.This review paper provides a comprehensive overview of the measurements on the tower over the past two decades.Through long-term comprehensive observations,researchers have elucidated the intricate relationships between anthropogenic emissions,meteorological dynamics,and atmospheric composition,shedding light on the drivers of air pollution and its impacts on public health.The vertical measurements on the tower also enable detailed investigations into boundary layer dynamics,turbulent mixing,and pollutant dispersion,providing invaluable data for validating chemical transport models.Key findings from the tower’s research include the identification of positive feedback mechanisms between aerosols and the boundary layer,the characterization of pollutant sources and transport pathways,the determination of fluxes of gaseous and particulate species,and the assessment of the effectiveness of pollution control measures.Additionally,isotopic measurements have provided new insights into the sources and formation processes of particulate matter and reactive nitrogen species.Finally,the paper outlines future directions for tower-based research,emphasizing the need for long-term comprehensive measurements,the development of innovative tower platforms,and integration of emerging technologies.展开更多
In order to better describe the phenomenon of biological invasion,this paper introduces a free boundary model of biological invasion.Firstly,the right free boundary is added to the equation with logistic terms.Secondl...In order to better describe the phenomenon of biological invasion,this paper introduces a free boundary model of biological invasion.Firstly,the right free boundary is added to the equation with logistic terms.Secondly,the existence and uniqueness of local solutions are proved by the Sobolev embedding theorem and the comparison principle.Finally,according to the relevant research data and contents of red fire ants,the diffusion area and nest number of red fire ants were simulated without external disturbance.This paper mainly simulates the early diffusion process of red fire ants.In the early diffusion stage,red fire ants grow slowly and then spread over a large area after reaching a certain number.展开更多
Asymmetric tilt boundaries on conventional twin boundaries(TBs)are significant for understanding the role of twins on coordinating plastic deformation in many metallic alloys.However,the formation modes of many asymme...Asymmetric tilt boundaries on conventional twin boundaries(TBs)are significant for understanding the role of twins on coordinating plastic deformation in many metallic alloys.However,the formation modes of many asymmetric tilt boundaries are hard to be accounted for based on traditional theoretical models,and the corresponding solute segregation is complex.Herein,atomic structures of a specific asymmetric boundary on{1012}TBs were reveled using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM),molecular dynamics(MD)and density functional theory(DFT)simulations.Reaction between<a60>M dislocations and the{1012}TB can generate a~61°/25°asymmetric tilt boundary.The segregation of Gd and Zn atoms is closely related to the aggregateddislocations and the interfacial interstices of the asymmetric tilt boundary,which is energetically favorable in reducing the total system energy.展开更多
The combination of dual-main-phase(DMP)(Nd,Ce)-Fe-B magnets and grain boundary diffusion process(GBDP)is currently a research topic for obtaining high-cost performance materials in rare earth permanent magnet fields.T...The combination of dual-main-phase(DMP)(Nd,Ce)-Fe-B magnets and grain boundary diffusion process(GBDP)is currently a research topic for obtaining high-cost performance materials in rare earth permanent magnet fields.The novel structural features of GBDP(Nd,Ce)-Fe-B magnets give a version of different domain reversal processes from those of non-diffused magnets.In this work,the in-situ magnetic domain evolution of the DMP magnets was observed at elevated temperatures,and the temperature demagnetization and coercivity mechanism of the GBDP dual-main-phase(Nd,Ce)-Fe-B magnets are discussed.The results show that the shell composition of different types of grains in DMP magnets is similar,while the magnetic microstructure results indicate the Ce-rich grains tend to demagnetize first.Dy-rich shell with a high anisotropic field caused by GBDP leads to an increase in the nucleation field,which enhances the coercivity.It is found that much more grains exhibit single domain characteristics in the remanent state for GBDP dual-main-phase(Nd,Ce)-Fe-B magnets.In addition,the grains that undergo demagnetization first are Ce-rich or Nd-rich grains,which is different from that of non-diffused magnets.These results were not found in previous studies but can be intuitively characterized from the perspective of magnetic domains in this work,providing a new perspective and understanding of the performance improvement of magnetic materials.展开更多
The grain boundary diffusion process(GBDP)has proven to be an effective method for enhancing the coercivity of sintered Nd-Fe-B magnets.However,the limited diffusion depth and thicker shell struc-ture have impeded the...The grain boundary diffusion process(GBDP)has proven to be an effective method for enhancing the coercivity of sintered Nd-Fe-B magnets.However,the limited diffusion depth and thicker shell struc-ture have impeded the further development of magnetic properties.Currently,the primary debates re-garding the mechanism of GBDP with Tb revolve around the dissolution-solidification mechanism and the atomic substitution mechanism.To clarify this mechanism,the microstructure evolution of sintered Nd-Fe-B magnets during the heating process of GBDP has been systematically studied by quenching at different tem peratures.In this study,it was found that the formation of TbFe_(2) phase is related to the dis-solution of _(2)Fe_(14)B grains during GBDP with Tb.The theory of mixing heat and phase separation further confirms that the Nd_(2)Fe_(14)B phase dissolves to form a mixed phase of Nd and TbFe_(2),which then solidifies into the(Nd,Tb)_(2)Fe_(14)B phase.Based on the discovery of the TbFe_(2) phase,the dissolution-solidification mechanism is considered the primary mechanism for GBDP.This is supported by the elemental content of the two typical core-shell structures observed.展开更多
Pb(Zr,Ti)O_(3)-Pb(Zn_(1/3)Nb_(2/3))O_(3) (PZT-PZN) based ceramics, as important piezoelectric materials, have a wide range of applications in fields such as sensors and actuators, thus the optimization of their piezoe...Pb(Zr,Ti)O_(3)-Pb(Zn_(1/3)Nb_(2/3))O_(3) (PZT-PZN) based ceramics, as important piezoelectric materials, have a wide range of applications in fields such as sensors and actuators, thus the optimization of their piezoelectric properties has been a hot research topic. This study investigated the effects of phase boundary engineering and domain engineering on (1-x)[0.8Pb(Zr_(0.5)Ti_(0.5))O_(3)-0.2Pb(Zn_(1/3)Nb_(2/3))O_(3)]-xBi(Zn_(0.5)Ti_(0.5))O_(3) ((1-x)(0.8PZT-0.2PZN)- xBZT) ceramic to obtain excellent piezoelectric properties. The crystal phase structure and microstructure of ceramic samples were characterized. The results showed that all samples had a pure perovskite structure, and the addition of BZT gradually increased the grain size. The addition of BZT caused a phase transition in ceramic samples from the morphotropic phase boundary (MPB) towards the tetragonal phase region, which is crucial for optimizing piezoelectric properties. By adjusting content of BZT and precisely controlling position of the phase boundary, the piezoelectric performance can be optimized. Domain structure is one of the key factors affecting piezoelectric performance. By using domain engineering techniques to optimize grain size and domain size, piezoelectric properties of ceramic samples have been significantly improved. Specifically, excellent piezoelectric properties (piezoelectric constant d_(33)=320 pC/N, electromechanical coupling factor kp=0.44) were obtained simultaneously for x=0.08. Based on experimental results and theoretical analysis, influence mechanisms of phase boundary engineering and domain engineering on piezoelectric properties were explored. The study shows that addition of BZT not only promotes grain growth, but also optimizes the domain structure, enabling the polarization reversal process easier, thereby improving piezoelectric properties. These research results not only provide new ideas for the design of high-performance piezoelectric ceramics, but also lay a theoretical foundation for development of related electronic devices.展开更多
Numerical simulations were conducted on a 10-blade Sevik rotor ingesting wake downstream of two turbulence-generating grids.These simulations were based on implicit large-eddy simulation(ILES)and the boundary data imm...Numerical simulations were conducted on a 10-blade Sevik rotor ingesting wake downstream of two turbulence-generating grids.These simulations were based on implicit large-eddy simulation(ILES)and the boundary data immersion method(BDIM)for compressible flows,which were solved using a fully self-programmed Fortran code.Results show that the predicted thrust spectrum aligns closely with the experimental measurements.In addition,it captures the thrust dipole directivity of the noise around the rotating propeller due to random pressure pulsations on the blades,as well as the flow structures simultaneously.Furthermore,the differences in the statistical characteristics,flow structures,and low-frequency broadband thrust spectra due to different turbulence levels were investigated.This analysis indicates that the interaction between the upstream,which is characterized by a lower turbulence level and a higher turbulent length of scale,and the rotating propeller results in a lower amplitude in force spectra and a slight increase in the scale of tip vortices.展开更多
When immersed in sand and dust environment,aero-engine blades are exposed to harsh erosion which may lead to failure if erosion is severe.Using Physical Vapor Deposition(PVD)to prepare hard ceramic coatings can greatl...When immersed in sand and dust environment,aero-engine blades are exposed to harsh erosion which may lead to failure if erosion is severe.Using Physical Vapor Deposition(PVD)to prepare hard ceramic coatings can greatly enhance the operational capabilities of aero-engine.However,due to the“line-of-sight”processing characteristic of PVD process,uneven coating deposition rates occur when preparing coatings on obstructed areas such as blisks.Quantitative research on such phenomena is few,and it is even rarer in the study of aero-engine coatings.Based on the analyses and considerations of the geometric shape of blade surfaces and the influence of both deposition and re-sputtering effect,an ideal model is established to analyze the deposition rate variation along blocked region in complex self-shadowing boundaries.The relative deposition rates at various locations on the blade surface within the inter-blade gaps are quantitatively calculated and experimentally validated.Furthermore,differences in erosion resistance of the coatings are tested.The conclusions are drawn as follows:the geometric configuration of the obstructed shape and resputtering phenomenon significantly influence the deposition rates within the inner wall of blade gaps.Taking the structural configuration as an example,in a 25 mm×60 mm×15 mm gap,the coating thickness can vary more than 252%from the thickest to the thinnest location.The deposition rates of various locations are proportional to the solid angle of incident ion in more obstructed regions,and the re-sputtering is more prominent in open regions.Obstructive boundaries directly affect the erosion resistance at various locations within the gaps,with erosion failure time decreasing by 40%in heavily blocked region compared to open region.展开更多
Hypersonic boundary-layer receptivity to freestream entropy and vorticity waves is investigated using direct numerical simula-tions for a Mach 6 flow over a 5.08 mm nose radius cone.Two frequencies of 33 kHz and 150 k...Hypersonic boundary-layer receptivity to freestream entropy and vorticity waves is investigated using direct numerical simula-tions for a Mach 6 flow over a 5.08 mm nose radius cone.Two frequencies of 33 kHz and 150 kHz are considered to be rep-resentative of the first and second instability modes,respectively.For the first mode,wall pressure fluctuations for both entropy and vorticity wave cases exhibit a strong modulation yet without a growing trend,indicating that the first mode is not generated despite its instability predicted by linear stability theory.The potential reason for this is the absence of postshock slow acoustic waves capable of synchronizing with the first mode.By contrast,for the second mode,a typical three-stage boundary-layer response is observed,consistent with that to slow acoustic waves studied previously.Furthermore,the postshock disturbances outside the boundary layer can be decomposed into the entropy(density/temperature fluctuations)and vorticity components(ve-locity fluctuations),and the latter is shown to play a leading role in generating the second mode,even for the case with entropy waves where the density/temperature fluctuations dominate the postshock regions.展开更多
This study was used oceanographic database in the Sea of Okhotsk between the period from 1929 to 2020(131286 stations).The paper used gas hydrate dissociation parameters for the“pure methane-seawater”system obtained...This study was used oceanographic database in the Sea of Okhotsk between the period from 1929 to 2020(131286 stations).The paper used gas hydrate dissociation parameters for the“pure methane-seawater”system obtained in the study by Dickens GR and Quinby-Hunt MS.The results have elucidated the spatiotemporal variability of distribution of such parameters at the upper boundary of the gas hydrate stability zone(GHSZ)as water temperature,salinity,and top depth of the stability zone.As the study has shown(based on average long-term spatial distributions),the minimum temperature and depth values of the GHSZ upper boundary in the Sea of Okhotsk occur off the western and southwestern parts of the water area.The maximum temperature and depth values of the GHSZ upper boundary are typical of the southeastern sea area and over the Kamchatka Peninsula slope.This study has also identified an area,where there are no thermobaric conditions for the emergence and stable existence of methane hydrates in the water column.The results presented agree well with the materials of observations conducted during expeditions and the previous data of predictive simulations for the Sea of Okhotsk.展开更多
Grain boundary diffusion technology is pivotal in the preparation of high-performance NdFeB magnets.This study investigates the factors that affect the efficiency of grain boundary diffusion,starting from the properti...Grain boundary diffusion technology is pivotal in the preparation of high-performance NdFeB magnets.This study investigates the factors that affect the efficiency of grain boundary diffusion,starting from the properties of the diffusion matrix.Through the adjustment of the sintering process,we effectively prepared magnets with varied densities that serve as the matrix for grain boundary diffusion with TbH,diffusion.The mobility characteristics of the Nd-rich phase during the densification stage are leveraged to ensure a more extensive distribution of heavy rare earth elements within the magnets.According to the experimental results,the increase in coercivity of low-density magnets after diffusion is significantly greater than that of relatively high-density magnets.The coercivity values measured are 805.32 kA/m for low-density magnets and 470.3 kA/m for high-density magnets.Additionally,grain boundary diffusion notably enhances the density of initial low-density magnets,addressing the issue of low density during the sintering stage.Before the diffusion treatment,the Nd-rich phases primarily concentrate at the triangular grain boundaries,resulting in an increased number of cavity defects in the magnets.These cavity defects contain atoms in a higher energy state,making them more prone to transition.Consequently,the diffusion activation energy at the void defects is lower than the intracrystalline diffusion activation energy,accelerating atom diffusion.The presence of larger cavities also provides more space for atom migration,thereby promoting the diffusion process.After the diffusion treatment,the proportion of bulk Nd-rich phases significantly decreases,and they infiltrate between the grains to fill the cavity defects,forming continuous fine grain boundaries.Based on these observations,the study aims to explore how to utilize this information to develop an efficient technique for grain boundary diffusion.展开更多
Drains play an important role in seepage control in geotechnical engineering.The enormous number and one-dimensional(1D)geometry of drainage holes make their nature difficult to be accurately modeled in groundwater fl...Drains play an important role in seepage control in geotechnical engineering.The enormous number and one-dimensional(1D)geometry of drainage holes make their nature difficult to be accurately modeled in groundwater flow simulation.It has been well understood that drains function by presenting discharge boundaries,which can be characterized by water head,no-flux,unilateral or mixed water head-unilateral boundary condition.It has been found after years of practices that the flow simulation may become erroneous if the transitions among the drain boundary conditions are not properly considered.For this,a rigorous algorithm is proposed in this study to detect the onset of transitions among the water head,noflux and mixed water head-unilateral boundary conditions for downwards-drilled drainage holes,which theoretically completes the description of drain boundary conditions.After verification against a numerical example,the proposed algorithm is applied to numerical modeling of groundwater flow through a gravity dam foundation.The simulation shows that for hundreds of downwards-drilled drainage holes used to be prescribed with water head boundary condition,56%and 2%of them are transitioned to mixed water head-unilateral and no-flux boundary conditions,respectively.The phreatic surface around the drains will be overestimated by 25e33 m without the use of the mixed boundary condition.For the first time,this study underscores the importance of the mixed water head-unilateral boundary condition and the proposed transition algorithm in drain modeling,which may become more essential for simulation of transient flow because of groundwater dynamics.展开更多
The influence of Impedance Boundary Condition (IBC) on transonic compressors is investigated. A systematic input–output analytical framework is developed, which treats the nonlinearities as unknown forcing terms. The...The influence of Impedance Boundary Condition (IBC) on transonic compressors is investigated. A systematic input–output analytical framework is developed, which treats the nonlinearities as unknown forcing terms. The framework is validated through the experiments of rotating inlet distortion within a low-speed compressor. The input–output method is subsequently applied to transonic compressors, including NASA Rotor37 and Stage35, wherein impedance optimization is studied along with the exploration of its fundamental mechanisms. The IBC is employed to model the effect of Casing Treatment (CT). The optimal complex impedance values are determined through predicted results and tested across a range of circumferential modes and forcing frequencies. The IBC significantly reduces the energy and Reynolds stress gain, notably at the first-order circumferential mode and within the Rotor Rotating Frequency (RRF) range. Output modes reveal that transonic compressors with fine-tuned impedance values exhibit a more confined perturbation distribution and redistribute the perturbations compared to the uncontrolled case. Additionally, the roles of resistance and reactance are elucidated through input–output analysis, and resistance determines the energy transfer direction between flow and pressure waves and modulates the amplitude, whereas reactance modifies the phase relationships and attenuates the perturbations.展开更多
Second period elements(B,C,N,and O)usually appear at the grain boundary(GB)and strongly affect the mechanical performance in austenitic stainless steels.Therefore,it is significant to investigate the effect of solute ...Second period elements(B,C,N,and O)usually appear at the grain boundary(GB)and strongly affect the mechanical performance in austenitic stainless steels.Therefore,it is significant to investigate the effect of solute elements(B,C,N,and O)on the GB.The first-principles calculation based on the density function theory was applied to explore the effect of B,C,N,and O onγ-FeΣ5(210)[001]GB.The GB energy,the segregation energy,the Voronoi volume,and the theoretical tensile test were calculated to investigate the segregation behavior and the strengthening effect.The structural change and electronic evolution were also investigated by bond change,charge density distribution,and density of states.The results show that B is favored to segregate at the capped trigonal prism(CTP)position with a large void and has a strengthening effect on the GB strength,while O and N are preferred to locate at the octahedral(OCT)site and have an embrittling effect on GB cohesion.C can segregate at both the CTP site and the OCT location with little energy difference.As C segregates at the OCT site,it is beneficial for GB strength.However,it is detrimental at the CTP position.It can be seen that the influence of solutes is closely related to the element type and segregated position.展开更多
As a rare earth solute element in Mg alloys,Y has the beneficial effects of increasing both the strength and the ductility as well as weakening the crystallographic texture.To achieve a more fundamental understanding ...As a rare earth solute element in Mg alloys,Y has the beneficial effects of increasing both the strength and the ductility as well as weakening the crystallographic texture.To achieve a more fundamental understanding on how Y addition affects the microstructural evolution and mechanical properties,the Y segregation behavior at grain boundaries was investigated in Mg-1wt.%Y and Mg-7wt.%Y alloys at different conditions.The segregation intensity and its dependence on the grain boundary misorientation angle were experimentally characterized and computationally predicted.Strong segregation at grain boundaries was observed in both low and high Y-containing alloys.Y segregation was found to remain in alloy Mg-7Y after high-temperature annealing heat treatment at 540℃.No direct correlation between the Y segregation intensity and the grain boundary misorientation angle could be established based on either the experimental characterization or the atomistic simulation with a spectral model.We thus conclude that grain boundary segregation of Y is independent of grain boundary misorientation angle.展开更多
文摘Due to the coexistence of compressibility,viscosity,and threedimensional effects,laminar flow is difficult to maintain for high-speed boundary layer on complex geometries.The unstable disturbance waves in the boundary layer are excited and rapidly increase during the receptivity process,so sufficiently large Reynolds stress causes the basic flow velocity profile to change,and the formation of turbulence is inevitable.
基金partially supported by the National Natural Science Foundation of China(Nos.92271103,12202191)。
文摘The diffuse-interface immersed boundary method(IBM)possesses excellent capabilities for simulating flows around complex geometries and moving boundaries.In this method,the flow field is solved on a fixed Cartesian mesh,while the solid boundary is discretized into a series of Lagrangian points immersed in the flow field.The boundary condition is implemented by introducing a force term into the momentum equation,and the interaction between the immersed boundary and the fluid domain is achieved via an interpolation process.Over the past decades,the diffuse-interface IBM has gained popularity and spawned many variants,effectively handling a wide range of flow problems from isothermal to thermal flows,from laminar to turbulent flows,and from complex geometries to fluidstructure interaction scenarios.This paper first outlines the basic principles of the diffuse-interface IBM,then highlights recent advancements achieved by the authors’research group,and finally shows the method’s excellent numerical performance and wide applicability through several case studies involving complex moving boundary problems.
基金supported by the National Natural Science Foundation of China(Grant 12472113).
文摘The traditional topology optimization method of continuum structure generally uses quadrilateral elements as the basic mesh.This approach often leads to jagged boundary issues,which are traditionally addressed through post-processing,potentially altering the mechanical properties of the optimized structure.A topology optimization method of Movable Morphable Smooth Boundary(MMSB)is proposed based on the idea of mesh adaptation to solve the problem of jagged boundaries and the influence of post-processing.Based on the ICM method,the rational fraction function is introduced as the filtering function,and a topology optimization model with the minimum weight as the objective and the displacement as the constraint is established.A triangular mesh is utilized as the base mesh in this method.The mesh is re-divided in the optimization process based on the contour line,and a smooth boundary parallel to the contour line is obtained.Numerical examples demonstrate that the MMSB method effectively resolves the jagged boundary issues,leading to enhanced structural performance.
基金Project supported by the National Natural Science Foundation of China(No.12402262)。
文摘Vorticity is locally generated on a boundary at a rate measured by the boundary vorticity flux(BVF),which can be further decomposed into the sum of the orbital rotation and the generalized spin(specifically,the sum of shear and streaming vorticity)under the field description.For an incompressible viscous flow interacting with a stationary wall,the full expressions of the boundary fluxes of the orbital rotation and the spin are derived,for the first time,to elucidate their boundary creation mechanisms.Then,these new findings are successfully extended to the study of the boundary enstrophy dynamics,as well as the Lyman vorticity dynamics as an alternative interpretation to the boundary vorticity dynamics.Interestingly,it is found that the boundary coupling of the longitudinal and transverse processes is only embodied in the boundary spin flux,which is definitely not responsible for the generation of the boundary orbital-rotation flux.In addition,the boundary fluxes of enstrophy are directly associated with the boundary source of the second principal invariant of the velocity gradient tensor(VGT)and the two quadratic forms representing the spin-geometry interaction.The present exposition provides a new perspective and an additional dimension for understanding the vorticity dynamics on boundaries,which could be valuable in clarifying the formation mechanisms of near-wall coherent structures and flow noise at the fundamental level.
基金co-supported by the National Natural Science Foundation of China (No. 12172175)the National Science and Technology Major Project, China (No. J2019-II0014-0035)the Science Center for Gas Turbine Project, China (Nos. P2022-C-II-002-001, P2022-A-II-002-001)
文摘Cowl-induced incident Shock Wave/Boundary Layer Interactions (SWBLI) under the influence of gradual expansion waves are frequently observed in supersonic inlets. However, the analysis and prediction of interaction lengths have not been sufficiently investigated. First, this study presents a theoretical scaling analysis and validates it through wind tunnel experiments. It conducts detailed control volume analysis of mass conservation, considering the differences between inviscid and viscous cases. Then, three models for analysing interaction length under gradual expansion waves are derived. Related experiments using schlieren photography are conducted to validate the models in a Mach 2.73 flow. The interaction scales are captured at various relative distances between the shock impingement location and the expansion regions with wedge angles ranging from 12° to 15° and expansion angles of 9°, 12°, and 15°. Three trend lines are plotted based on different expansion angles to depict the relationship between normalised interaction length and normalised interaction strength metric. In addition, the relationship between the coefficients of the trend line and the expansion angles is introduced to predict the interaction length influenced by gradual expansion waves. Finally, the estimation of normalised interaction length is derived for various coefficients within a unified form.
基金supported by the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB0760200)the National Natural Science Foundation of China(Grant Nos.42330605 and 42377101).
文摘The Beijing 325 m meteorological tower stands as a pivotal research platform for exploring atmospheric boundary layer physics and atmospheric chemistry.With a legacy spanning 45 years,the tower has played a crucial role in unraveling the complexities of urban air pollution,atmospheric processes,and climate change in Beijing,China.This review paper provides a comprehensive overview of the measurements on the tower over the past two decades.Through long-term comprehensive observations,researchers have elucidated the intricate relationships between anthropogenic emissions,meteorological dynamics,and atmospheric composition,shedding light on the drivers of air pollution and its impacts on public health.The vertical measurements on the tower also enable detailed investigations into boundary layer dynamics,turbulent mixing,and pollutant dispersion,providing invaluable data for validating chemical transport models.Key findings from the tower’s research include the identification of positive feedback mechanisms between aerosols and the boundary layer,the characterization of pollutant sources and transport pathways,the determination of fluxes of gaseous and particulate species,and the assessment of the effectiveness of pollution control measures.Additionally,isotopic measurements have provided new insights into the sources and formation processes of particulate matter and reactive nitrogen species.Finally,the paper outlines future directions for tower-based research,emphasizing the need for long-term comprehensive measurements,the development of innovative tower platforms,and integration of emerging technologies.
基金Supported by National Natural Science Foundation of China(12101482)Postdoctoral Science Foundation of China(2022M722604)+2 种基金General Project of Science and Technology of Shaanxi Province(2023-YBSF-372)The Natural Science Foundation of Shaan Xi Province(2023-JCQN-0016)Shannxi Mathmatical Basic Science Research Project(23JSQ042)。
文摘In order to better describe the phenomenon of biological invasion,this paper introduces a free boundary model of biological invasion.Firstly,the right free boundary is added to the equation with logistic terms.Secondly,the existence and uniqueness of local solutions are proved by the Sobolev embedding theorem and the comparison principle.Finally,according to the relevant research data and contents of red fire ants,the diffusion area and nest number of red fire ants were simulated without external disturbance.This paper mainly simulates the early diffusion process of red fire ants.In the early diffusion stage,red fire ants grow slowly and then spread over a large area after reaching a certain number.
基金supported by the Scientific and Technological Developing Scheme of Jilin Province under grants no.YDZJ202301ZYTS538the Chinese Academy of Sciences Youth Innovation Promotion Association under grants number 2023234+3 种基金the National Natural Science Foundation of China under grants number U21A20323the Scientific and Technological Developing Scheme of Jilin Province under grants no.SKL202302038the Major Scientific and Technological Projects of Hebei Province under grants No.23291001Zthe Scientific and Technology Project of Hanjiang District.
文摘Asymmetric tilt boundaries on conventional twin boundaries(TBs)are significant for understanding the role of twins on coordinating plastic deformation in many metallic alloys.However,the formation modes of many asymmetric tilt boundaries are hard to be accounted for based on traditional theoretical models,and the corresponding solute segregation is complex.Herein,atomic structures of a specific asymmetric boundary on{1012}TBs were reveled using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM),molecular dynamics(MD)and density functional theory(DFT)simulations.Reaction between<a60>M dislocations and the{1012}TB can generate a~61°/25°asymmetric tilt boundary.The segregation of Gd and Zn atoms is closely related to the aggregateddislocations and the interfacial interstices of the asymmetric tilt boundary,which is energetically favorable in reducing the total system energy.
基金supported by the National Key Research and Development Program of China(Nos.2021YFB3503003,2021YFB3503100,and 2022YFB3505401).
文摘The combination of dual-main-phase(DMP)(Nd,Ce)-Fe-B magnets and grain boundary diffusion process(GBDP)is currently a research topic for obtaining high-cost performance materials in rare earth permanent magnet fields.The novel structural features of GBDP(Nd,Ce)-Fe-B magnets give a version of different domain reversal processes from those of non-diffused magnets.In this work,the in-situ magnetic domain evolution of the DMP magnets was observed at elevated temperatures,and the temperature demagnetization and coercivity mechanism of the GBDP dual-main-phase(Nd,Ce)-Fe-B magnets are discussed.The results show that the shell composition of different types of grains in DMP magnets is similar,while the magnetic microstructure results indicate the Ce-rich grains tend to demagnetize first.Dy-rich shell with a high anisotropic field caused by GBDP leads to an increase in the nucleation field,which enhances the coercivity.It is found that much more grains exhibit single domain characteristics in the remanent state for GBDP dual-main-phase(Nd,Ce)-Fe-B magnets.In addition,the grains that undergo demagnetization first are Ce-rich or Nd-rich grains,which is different from that of non-diffused magnets.These results were not found in previous studies but can be intuitively characterized from the perspective of magnetic domains in this work,providing a new perspective and understanding of the performance improvement of magnetic materials.
基金supported by the National Key Research and Development Program of China(2022YFB3505503)the National Natural Science Foundation of China(52201230)+2 种基金the Key R&D Program of Shandong Province(2022CXGC020307)the China Postdoctoral Science Foundation(2022M71204)the Beijing NOVA Program(Z211100002121092).
文摘The grain boundary diffusion process(GBDP)has proven to be an effective method for enhancing the coercivity of sintered Nd-Fe-B magnets.However,the limited diffusion depth and thicker shell struc-ture have impeded the further development of magnetic properties.Currently,the primary debates re-garding the mechanism of GBDP with Tb revolve around the dissolution-solidification mechanism and the atomic substitution mechanism.To clarify this mechanism,the microstructure evolution of sintered Nd-Fe-B magnets during the heating process of GBDP has been systematically studied by quenching at different tem peratures.In this study,it was found that the formation of TbFe_(2) phase is related to the dis-solution of _(2)Fe_(14)B grains during GBDP with Tb.The theory of mixing heat and phase separation further confirms that the Nd_(2)Fe_(14)B phase dissolves to form a mixed phase of Nd and TbFe_(2),which then solidifies into the(Nd,Tb)_(2)Fe_(14)B phase.Based on the discovery of the TbFe_(2) phase,the dissolution-solidification mechanism is considered the primary mechanism for GBDP.This is supported by the elemental content of the two typical core-shell structures observed.
基金National Natural Science Foundation of China (52202139, 52072178)。
文摘Pb(Zr,Ti)O_(3)-Pb(Zn_(1/3)Nb_(2/3))O_(3) (PZT-PZN) based ceramics, as important piezoelectric materials, have a wide range of applications in fields such as sensors and actuators, thus the optimization of their piezoelectric properties has been a hot research topic. This study investigated the effects of phase boundary engineering and domain engineering on (1-x)[0.8Pb(Zr_(0.5)Ti_(0.5))O_(3)-0.2Pb(Zn_(1/3)Nb_(2/3))O_(3)]-xBi(Zn_(0.5)Ti_(0.5))O_(3) ((1-x)(0.8PZT-0.2PZN)- xBZT) ceramic to obtain excellent piezoelectric properties. The crystal phase structure and microstructure of ceramic samples were characterized. The results showed that all samples had a pure perovskite structure, and the addition of BZT gradually increased the grain size. The addition of BZT caused a phase transition in ceramic samples from the morphotropic phase boundary (MPB) towards the tetragonal phase region, which is crucial for optimizing piezoelectric properties. By adjusting content of BZT and precisely controlling position of the phase boundary, the piezoelectric performance can be optimized. Domain structure is one of the key factors affecting piezoelectric performance. By using domain engineering techniques to optimize grain size and domain size, piezoelectric properties of ceramic samples have been significantly improved. Specifically, excellent piezoelectric properties (piezoelectric constant d_(33)=320 pC/N, electromechanical coupling factor kp=0.44) were obtained simultaneously for x=0.08. Based on experimental results and theoretical analysis, influence mechanisms of phase boundary engineering and domain engineering on piezoelectric properties were explored. The study shows that addition of BZT not only promotes grain growth, but also optimizes the domain structure, enabling the polarization reversal process easier, thereby improving piezoelectric properties. These research results not only provide new ideas for the design of high-performance piezoelectric ceramics, but also lay a theoretical foundation for development of related electronic devices.
基金Supported by the National Key R&D Program of China(2022YFB3303500).
文摘Numerical simulations were conducted on a 10-blade Sevik rotor ingesting wake downstream of two turbulence-generating grids.These simulations were based on implicit large-eddy simulation(ILES)and the boundary data immersion method(BDIM)for compressible flows,which were solved using a fully self-programmed Fortran code.Results show that the predicted thrust spectrum aligns closely with the experimental measurements.In addition,it captures the thrust dipole directivity of the noise around the rotating propeller due to random pressure pulsations on the blades,as well as the flow structures simultaneously.Furthermore,the differences in the statistical characteristics,flow structures,and low-frequency broadband thrust spectra due to different turbulence levels were investigated.This analysis indicates that the interaction between the upstream,which is characterized by a lower turbulence level and a higher turbulent length of scale,and the rotating propeller results in a lower amplitude in force spectra and a slight increase in the scale of tip vortices.
基金financially supported by the Shaanxi Provincial Science and Technology Innovation Team,China(No.2024RS-CXTD-26)。
文摘When immersed in sand and dust environment,aero-engine blades are exposed to harsh erosion which may lead to failure if erosion is severe.Using Physical Vapor Deposition(PVD)to prepare hard ceramic coatings can greatly enhance the operational capabilities of aero-engine.However,due to the“line-of-sight”processing characteristic of PVD process,uneven coating deposition rates occur when preparing coatings on obstructed areas such as blisks.Quantitative research on such phenomena is few,and it is even rarer in the study of aero-engine coatings.Based on the analyses and considerations of the geometric shape of blade surfaces and the influence of both deposition and re-sputtering effect,an ideal model is established to analyze the deposition rate variation along blocked region in complex self-shadowing boundaries.The relative deposition rates at various locations on the blade surface within the inter-blade gaps are quantitatively calculated and experimentally validated.Furthermore,differences in erosion resistance of the coatings are tested.The conclusions are drawn as follows:the geometric configuration of the obstructed shape and resputtering phenomenon significantly influence the deposition rates within the inner wall of blade gaps.Taking the structural configuration as an example,in a 25 mm×60 mm×15 mm gap,the coating thickness can vary more than 252%from the thickest to the thinnest location.The deposition rates of various locations are proportional to the solid angle of incident ion in more obstructed regions,and the re-sputtering is more prominent in open regions.Obstructive boundaries directly affect the erosion resistance at various locations within the gaps,with erosion failure time decreasing by 40%in heavily blocked region compared to open region.
基金supported by the National Natural Science Foundation of China(GrantNo.12072231).
文摘Hypersonic boundary-layer receptivity to freestream entropy and vorticity waves is investigated using direct numerical simula-tions for a Mach 6 flow over a 5.08 mm nose radius cone.Two frequencies of 33 kHz and 150 kHz are considered to be rep-resentative of the first and second instability modes,respectively.For the first mode,wall pressure fluctuations for both entropy and vorticity wave cases exhibit a strong modulation yet without a growing trend,indicating that the first mode is not generated despite its instability predicted by linear stability theory.The potential reason for this is the absence of postshock slow acoustic waves capable of synchronizing with the first mode.By contrast,for the second mode,a typical three-stage boundary-layer response is observed,consistent with that to slow acoustic waves studied previously.Furthermore,the postshock disturbances outside the boundary layer can be decomposed into the entropy(density/temperature fluctuations)and vorticity components(ve-locity fluctuations),and the latter is shown to play a leading role in generating the second mode,even for the case with entropy waves where the density/temperature fluctuations dominate the postshock regions.
基金the POI FEB RAS,entitled“Study of the Structure and Dynamics of World’s Oceans Waters in Conditions of Current Climate Change”(reg.no.124022100079-4).
文摘This study was used oceanographic database in the Sea of Okhotsk between the period from 1929 to 2020(131286 stations).The paper used gas hydrate dissociation parameters for the“pure methane-seawater”system obtained in the study by Dickens GR and Quinby-Hunt MS.The results have elucidated the spatiotemporal variability of distribution of such parameters at the upper boundary of the gas hydrate stability zone(GHSZ)as water temperature,salinity,and top depth of the stability zone.As the study has shown(based on average long-term spatial distributions),the minimum temperature and depth values of the GHSZ upper boundary in the Sea of Okhotsk occur off the western and southwestern parts of the water area.The maximum temperature and depth values of the GHSZ upper boundary are typical of the southeastern sea area and over the Kamchatka Peninsula slope.This study has also identified an area,where there are no thermobaric conditions for the emergence and stable existence of methane hydrates in the water column.The results presented agree well with the materials of observations conducted during expeditions and the previous data of predictive simulations for the Sea of Okhotsk.
基金Project supported by the National Natural Science Foundation of China(52361033)National Key Research and Development Program(2022YFB3505400)+3 种基金Ministry of Industry and Information Technology Heavy Rare Earth Special Use of Sintered NdFeB Project(TC220H06J)Academic and Technical Leaders in Major Disciplines in Jiangxi Province(2022BCJ23007)Jiangxi Province Science and Technology Cooperation Key Project(20212BDH80007)Jiangxi Graduate Student Innovation Special Fund Project(YC2023-B213)。
文摘Grain boundary diffusion technology is pivotal in the preparation of high-performance NdFeB magnets.This study investigates the factors that affect the efficiency of grain boundary diffusion,starting from the properties of the diffusion matrix.Through the adjustment of the sintering process,we effectively prepared magnets with varied densities that serve as the matrix for grain boundary diffusion with TbH,diffusion.The mobility characteristics of the Nd-rich phase during the densification stage are leveraged to ensure a more extensive distribution of heavy rare earth elements within the magnets.According to the experimental results,the increase in coercivity of low-density magnets after diffusion is significantly greater than that of relatively high-density magnets.The coercivity values measured are 805.32 kA/m for low-density magnets and 470.3 kA/m for high-density magnets.Additionally,grain boundary diffusion notably enhances the density of initial low-density magnets,addressing the issue of low density during the sintering stage.Before the diffusion treatment,the Nd-rich phases primarily concentrate at the triangular grain boundaries,resulting in an increased number of cavity defects in the magnets.These cavity defects contain atoms in a higher energy state,making them more prone to transition.Consequently,the diffusion activation energy at the void defects is lower than the intracrystalline diffusion activation energy,accelerating atom diffusion.The presence of larger cavities also provides more space for atom migration,thereby promoting the diffusion process.After the diffusion treatment,the proportion of bulk Nd-rich phases significantly decreases,and they infiltrate between the grains to fill the cavity defects,forming continuous fine grain boundaries.Based on these observations,the study aims to explore how to utilize this information to develop an efficient technique for grain boundary diffusion.
基金Financial support from the National Natural Science Foundation of China(Grant Nos.51925906 and U2340228)the Natural Science Foundation of Hubei Province(Grant No.2022CFA028)is acknowledged.
文摘Drains play an important role in seepage control in geotechnical engineering.The enormous number and one-dimensional(1D)geometry of drainage holes make their nature difficult to be accurately modeled in groundwater flow simulation.It has been well understood that drains function by presenting discharge boundaries,which can be characterized by water head,no-flux,unilateral or mixed water head-unilateral boundary condition.It has been found after years of practices that the flow simulation may become erroneous if the transitions among the drain boundary conditions are not properly considered.For this,a rigorous algorithm is proposed in this study to detect the onset of transitions among the water head,noflux and mixed water head-unilateral boundary conditions for downwards-drilled drainage holes,which theoretically completes the description of drain boundary conditions.After verification against a numerical example,the proposed algorithm is applied to numerical modeling of groundwater flow through a gravity dam foundation.The simulation shows that for hundreds of downwards-drilled drainage holes used to be prescribed with water head boundary condition,56%and 2%of them are transitioned to mixed water head-unilateral and no-flux boundary conditions,respectively.The phreatic surface around the drains will be overestimated by 25e33 m without the use of the mixed boundary condition.For the first time,this study underscores the importance of the mixed water head-unilateral boundary condition and the proposed transition algorithm in drain modeling,which may become more essential for simulation of transient flow because of groundwater dynamics.
基金co-supported by the National Natural Science Foundation of China(Nos.52325602,52306036 and 52306035)the National Science and Technology Major Project of China(No.Y2022-II-0003-0006 and Y2022-II-0002-0005)+1 种基金the project funded by China Postdoctoral Science Foundation(No.2022M720346)supported by the Key Laboratory of Pre-Research Management Centre of China(No.6142702200101).
文摘The influence of Impedance Boundary Condition (IBC) on transonic compressors is investigated. A systematic input–output analytical framework is developed, which treats the nonlinearities as unknown forcing terms. The framework is validated through the experiments of rotating inlet distortion within a low-speed compressor. The input–output method is subsequently applied to transonic compressors, including NASA Rotor37 and Stage35, wherein impedance optimization is studied along with the exploration of its fundamental mechanisms. The IBC is employed to model the effect of Casing Treatment (CT). The optimal complex impedance values are determined through predicted results and tested across a range of circumferential modes and forcing frequencies. The IBC significantly reduces the energy and Reynolds stress gain, notably at the first-order circumferential mode and within the Rotor Rotating Frequency (RRF) range. Output modes reveal that transonic compressors with fine-tuned impedance values exhibit a more confined perturbation distribution and redistribute the perturbations compared to the uncontrolled case. Additionally, the roles of resistance and reactance are elucidated through input–output analysis, and resistance determines the energy transfer direction between flow and pressure waves and modulates the amplitude, whereas reactance modifies the phase relationships and attenuates the perturbations.
基金supported by National Key R&D Program of China(No.2022YFB3705202)National Natural Science Foundation of China(Nos.51831008,52171049 and 52104330).
文摘Second period elements(B,C,N,and O)usually appear at the grain boundary(GB)and strongly affect the mechanical performance in austenitic stainless steels.Therefore,it is significant to investigate the effect of solute elements(B,C,N,and O)on the GB.The first-principles calculation based on the density function theory was applied to explore the effect of B,C,N,and O onγ-FeΣ5(210)[001]GB.The GB energy,the segregation energy,the Voronoi volume,and the theoretical tensile test were calculated to investigate the segregation behavior and the strengthening effect.The structural change and electronic evolution were also investigated by bond change,charge density distribution,and density of states.The results show that B is favored to segregate at the capped trigonal prism(CTP)position with a large void and has a strengthening effect on the GB strength,while O and N are preferred to locate at the octahedral(OCT)site and have an embrittling effect on GB cohesion.C can segregate at both the CTP site and the OCT location with little energy difference.As C segregates at the OCT site,it is beneficial for GB strength.However,it is detrimental at the CTP position.It can be seen that the influence of solutes is closely related to the element type and segregated position.
基金supported by PRISMS(PRedictive Integrated Structural Materials Science)center which is located at University of Michigan and funded by U.S.Department of Energy,Office of Basic Energy Science,Division of Materials Science and Engineering(Grant award number DE-SC0008637)support from Michigan Center for Materials Characterization(MC2)at University of Michigan and Canmet MATERIALS,Natural Resources Canada+1 种基金the Extreme Science and Engineering Discovery Environment(XSEDE)Stampede2 at the TACC through allocation TG-MSS160003the National Energy Research Scientific Computing Center(NERSC),a U.S.Department of Energy Office of Science User Facility operated under Contract No.DE-AC02-05CH11231。
文摘As a rare earth solute element in Mg alloys,Y has the beneficial effects of increasing both the strength and the ductility as well as weakening the crystallographic texture.To achieve a more fundamental understanding on how Y addition affects the microstructural evolution and mechanical properties,the Y segregation behavior at grain boundaries was investigated in Mg-1wt.%Y and Mg-7wt.%Y alloys at different conditions.The segregation intensity and its dependence on the grain boundary misorientation angle were experimentally characterized and computationally predicted.Strong segregation at grain boundaries was observed in both low and high Y-containing alloys.Y segregation was found to remain in alloy Mg-7Y after high-temperature annealing heat treatment at 540℃.No direct correlation between the Y segregation intensity and the grain boundary misorientation angle could be established based on either the experimental characterization or the atomistic simulation with a spectral model.We thus conclude that grain boundary segregation of Y is independent of grain boundary misorientation angle.
