The time sequence of longitudinal velocity component at different vertical locations in turbulent boundary layer was finely measured in a wind tunnel. The concept of coarse_grained velocity structure functions, which ...The time sequence of longitudinal velocity component at different vertical locations in turbulent boundary layer was finely measured in a wind tunnel. The concept of coarse_grained velocity structure functions, which describes the relative motions of straining and compressing for multi_scale eddy structures in turbulent flows, was put forward based on the theory of locally multi_scale average. Based on the consistency between coarse_grained velocity structure function and Harr wavelet transformation,detecting method was presented, by which the coherent structures and their intermittency was identified by multi_scale flatness factor calculated by locally average structure function. Phase_averaged evolution course for multi_scale coherent eddy structures in wall turbulence were extracted by this conditional sampling to educe scheme. The dynamics course of multi_scale coherent eddy structures and their effects on statistics of turbulent flows were studied.展开更多
The high order compact d if ference method is developed for solving the perturbation equations based on Navi er Stokes equations, and is used in studying complex evolution processes from w all negative pulse to the ...The high order compact d if ference method is developed for solving the perturbation equations based on Navi er Stokes equations, and is used in studying complex evolution processes from w all negative pulse to the turbulent coherent structure in the channel flow. Th is method contains three dimensional coupling difference scheme with high accur acy and high resolution, and the high order time splitting methods. Compared with the general spectral method, the method can be used to research turbule nt coherent structure under more general boundary conditions and in flow domains . In this paper, the generation and evolution of the turbulent coherent structur es ind uced by wall pulse in the channel flow are simulated, and the basic characterist ics and rules of the turbulent coherent structure are shown. Computational r esults indicate that a wall negative pulse is more convenient than the resonant three wave model.展开更多
The development of metallic mineral resources generates a significant amount of solid waste,such as tailings and waste rock.Cemented tailings and waste-rock backfill(CTWB)is an effective method for managing and dispos...The development of metallic mineral resources generates a significant amount of solid waste,such as tailings and waste rock.Cemented tailings and waste-rock backfill(CTWB)is an effective method for managing and disposing of this mining waste.This study employs a macro-meso-micro testing method to investigate the effects of the waste rock grading index(WGI)and loading rate(LR)on the uniaxial compressive strength(UCS),pore structure,and micromorphology of CTWB materials.Pore structures were analyzed using scanning electron microscopy(SEM)and mercury intrusion porosimetry(MIP).The particles(pores)and cracks analysis system(PCAS)software was used to quantitatively characterize the multi-scale micropores in the SEM images.The key findings indicate that the macroscopic results(UCS)of CTWB materials correspond to the microscopic results(pore structure and micromorphology).Changes in porosity largely depend on the conditions of waste rock grading index and loading rate.The inclusion of waste rock initially increases and then decreases the UCS,while porosity first decreases and then increases,with a critical waste rock grading index of 0.6.As the loading rate increases,UCS initially rises and then falls,while porosity gradually increases.Based on MIP and SEM results,at waste rock grading index 0.6,the most probable pore diameters,total pore area(TPA),pore number(PN),maximum pore area(MPA),and area probability distribution index(APDI)are minimized,while average pore form factor(APF)and fractal dimension of pore porosity distribution(FDPD)are maximized,indicating the most compact pore structure.At a loading rate of 12.0 mm/min,the most probable pore diameters,TPA,PN,MPA,APF,and APDI reach their maximum values,while FDPD reaches its minimum value.Finally,the mechanism of CTWB materials during compression is analyzed,based on the quantitative results of UCS and porosity.The research findings play a crucial role in ensuring the successful application of CTWB materials in deep metal mines.展开更多
Improving the volumetric energy density of supercapacitors is essential for practical applications,which highly relies on the dense storage of ions in carbon-based electrodes.The functional units of carbon-based elect...Improving the volumetric energy density of supercapacitors is essential for practical applications,which highly relies on the dense storage of ions in carbon-based electrodes.The functional units of carbon-based electrode exhibit multi-scale structural characteristics including macroscopic electrode morphologies,mesoscopic microcrystals and pores,and microscopic defects and dopants in the carbon basal plane.Therefore,the ordered combination of multi-scale structures of carbon electrode is crucial for achieving dense energy storage and high volumetric performance by leveraging the functions of various scale structu re.Considering that previous reviews have focused more on the discussion of specific scale structu re of carbon electrodes,this review takes a multi-scale perspective in which recent progresses regarding the structureperformance relationship,underlying mechanism and directional design of carbon-based multi-scale structures including carbon morphology,pore structure,carbon basal plane micro-environment and electrode technology on dense energy storage and volumetric property of supercapacitors are systematically discussed.We analyzed in detail the effects of the morphology,pore,and micro-environment of carbon electrode materials on ion dense storage,summarized the specific effects of different scale structures on volumetric property and recent research progress,and proposed the mutual influence and trade-off relationship between various scale structures.In addition,the challenges and outlooks for improving the dense storage and volumetric performance of carbon-based supercapacitors are analyzed,which can provide feasible technical reference and guidance for the design and manufacture of dense carbon-based electrode materials.展开更多
Advanced chemical engineering for simultaneous modulation of nanomaterial morphology, defects, interfaces, and structure to enhance electromagnetic and microwave absorption (MA) performance. However, accurately distin...Advanced chemical engineering for simultaneous modulation of nanomaterial morphology, defects, interfaces, and structure to enhance electromagnetic and microwave absorption (MA) performance. However, accurately distinguishing the MA contributions of different scale factors and tuning the optimal combined effects remains a formidable challenge. This study employs a synergistic approach combining template protection etching and vacuum annealing to construct a controlled system of micrometer-sized cavities and amorphous carbon matrices in metal-organic framework (MOF) derivatives. The results demonstrate that the spatial effects introduced by the hollow structure enhance dielectric loss but significantly weaken impedance matching. By increasing the proportion of amorphous carbon, the balance between electromagnetic loss and impedance matching can be effectively maintained. Importantly, in a suitable graphitization environment, the presence of oxygen vacancies in amorphous carbon can induce significant polarization to compensate for the reduced conductivity loss due to the absence of sp2 carbon. Through the synergistic effects of morphology and composition, the samples exhibit a broader absorption bandwidth (6.28 GHz) and stronger reflection loss (−61.64 dB) compared to the original MOF. In conclusion, this study aims to elucidate the multiscale impacts of macroscopic micro-nano structure and microscopic defect engineering, providing valuable insights for future research in this field.展开更多
By integrating topology optimization and lattice-based optimization,a novel multi-scale design method is proposed to create solid-lattice hybrid structures and thus to improve the mechanical performance as well as red...By integrating topology optimization and lattice-based optimization,a novel multi-scale design method is proposed to create solid-lattice hybrid structures and thus to improve the mechanical performance as well as reduce the structural weight.To achieve this purpose,a two-step procedure is developed to design and optimize the innovative structures.Initially,the classical topology optimization is utilized to find the optimal material layout and primary load carrying paths.Afterwards,the solid-lattice hybrid structures are reconstructed using the finite element mesh based modeling method.And lattice-based optimization is performed to obtain the optimal crosssection area of the lattice structures.Finally,two typical aerospace structures are optimized to demonstrate the effectiveness of the proposed optimization framework.The numerical results are quite encouraging since the solid-lattice hybrid structures obtained by the presented approach show remarkably improved performance when compared with traditional designs.展开更多
A large eddy simulation (LES) is performed for two subsonic jets with a Reynolds number of , which have different core temperatures, i.e., the cold and hot jet. The far-field overall sound pressure levels (OASPL) and ...A large eddy simulation (LES) is performed for two subsonic jets with a Reynolds number of , which have different core temperatures, i.e., the cold and hot jet. The far-field overall sound pressure levels (OASPL) and noise spectra are well validated against previous experimental results. It is found that the OASPL is raised by heating at shallow angles. The most energetic coherent structures are extracted with specified frequencies using the filter based on the frequency domain variant of the snapshot method of proper orthogonal decomposition (POD). The modes have high coherence of near-field pressure for both jets, while the coherence of modes is enhanced greatly by heating. Based on the coherent structures, spatial wavepackets are educed and the characteristics of growth, saturation and decay are analyzed and compared between the two jets in detail. The results show that heating would enhance the linear growth rate for high frequency components, and nonlinear growth rates for low frequency components in general, which are responsible for higher OASPL in the hot jet. The far-field sound generated by wavepackets is computed using the Kirchhoff extrapolation, which matches well with that of LES at shallow angles. This indicates that the wavepackets associated with coherent structures are dominant sound sources in forced transitional turbulent jets. Additionally, the present POD method is proven to be a robust tool to extract the salient features of the wavepackets in turbulent flows.展开更多
Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify t...Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify the interaction of the riblet surfaces with the coherent, energy-bearing eddy structures in the near-wall region. Results provide statistical evidence that the averaged organization of the streamwise vortices in the drag-reducing case is lifted above the riblet tip, while in the drag-increasing case the streamwise vortices are embedded further into the riblet cove. In the spanwise direction, the cores of the streamwise vortices over the riblet surfaces are shown to be closer to each other than those for flow over the smooth wall, and wider riblet spacing leads to more reduction on their spanwise distances. In the cases with riblets the streamwise vortices have longer streamwise lengths, but their inclination angles do not change much.展开更多
The problem of how the coherent structures in the wall region of a turbulent boundary layer could be excited by the disturbances from the outer region was investigated by using direct numerical simulation (DNS) method...The problem of how the coherent structures in the wall region of a turbulent boundary layer could be excited by the disturbances from the outer region was investigated by using direct numerical simulation (DNS) method. The results show that velocity disturbances at the upper boundary of the wall region could excite coherent structures in the wall region, thus offering a more comprehensive model for individual coherent structures.展开更多
When using a miniature single sensor boundary layer probe, the time sequences of the stream-wise velocity in the turbulent boundary layer （TBL） are measured by using a hot wire anemometer. Beneath the fully develope...When using a miniature single sensor boundary layer probe, the time sequences of the stream-wise velocity in the turbulent boundary layer （TBL） are measured by using a hot wire anemometer. Beneath the fully developed TBL, the wall pressure fluctuations are attained by a microphone mechanism with high spatial resolution. Analysis on the statistic and spectrum properties of velocity and wall pressure reveals the relationship between the wall pressure fluctuation and the energy-containing structure in the buffer layer of the TBL. Wavelet transform shows the multi-scale natures of coherent structures contained in both signals of velocity and pressure. The most intermittent wall pressure scale is associated with the coherent structure in the buffer layer. Meanwhile the most energetic scale of velocity fluctuation at y＋ = 14 provides a specific frequency f9 ≈ 147 Hz for wall actuating control with Ret = 996.展开更多
A numerical simulation is performed to find out a key vortical structure in the laminar-turbulent transition. A low-speed streak is generated inside a laminar boundary layer using an isolated cuboid roughness, aimed a...A numerical simulation is performed to find out a key vortical structure in the laminar-turbulent transition. A low-speed streak is generated inside a laminar boundary layer using an isolated cuboid roughness, aimed at providing an environment unstable to outer disturbances. Then, a short duration jet is issued into the boundary layer. When the jet velocity is low, some vortices appear in the boundary layer, but the transition of the boundary layer does not take place.However, when the jet velocity exceeds a certain threshold, two vortices newly appear above the elongated legs of a V-shaped vortex and only one of them is stretched and survives. After that,vortices are generated one after another around the survived one. By comparing the decayed and the survived vortices, it is found that the difference in their heights is the key characteristic which leads to the transition.展开更多
The influence of drag-reducing superhydrophobic(SHPo)surface on turbulent boundary layer(TBL)is investigated.A large area of the SHPo surface(about 10δ99 in the streamwise and 5δ99 in the spanwise)is fabricated to f...The influence of drag-reducing superhydrophobic(SHPo)surface on turbulent boundary layer(TBL)is investigated.A large area of the SHPo surface(about 10δ99 in the streamwise and 5δ99 in the spanwise)is fabricated to fully evolve the coherent structures in the TBL.A comparative experiment is carried out by time-resolved particle image velocimetry on a smooth surface and the SHPo surface at Re_(τ)=528.Velocity profiles with high spatial resolution are obtained by the single-pixel resolution ensemble correlation method.The reduction of the streamwise velocity gradient is observed in the near-wall region of y<0.05δ99 on the SHPo surface.By comparing the turbulence statistics,it is discovered that the Reynolds shear stress is reduced by 15.7%,and the turbulent kinetic energy is reduced by 12.3%on the SHPo surface.The coherent structures are investigated by the snapshot proper orthogonal decomposition(POD)and conditional average method.The intensity of Q_(2)/Q_(4)events on the SHPo surface has declined by 16.1%and 12.7%,respectively.The number of clockwise spanwise vortices is substantially reduced by 47%.Through spatial two-point correlation analysis,the streamwise and wall-normal direction scales of the coherent structures on the SHPo surface are suppressed.展开更多
This paper deals with the concurrent multi-scale optimization design of frame structure composed of glass or carbon fiber reinforced polymer laminates. In the composite frame structure, the fiber winding angle at the ...This paper deals with the concurrent multi-scale optimization design of frame structure composed of glass or carbon fiber reinforced polymer laminates. In the composite frame structure, the fiber winding angle at the micro-material scale and the geometrical parameter of components of the frame in the macro-structural scale are introduced as the independent variables on the two geometrical scales. Considering manufacturing requirements, discrete fiber winding angles are specified for the micro design variable. The improved Heaviside penalization discrete material optimization interpolation scheme has been applied to achieve the discrete optimization design of the fiber winding angle. An optimization model based on the minimum structural compliance and the specified fiber material volume constraint has been established. The sensitivity information about the two geometrical scales design variables are also deduced considering the characteristics of discrete fiber winding angles. The optimization results of the fiber winding angle or the macro structural topology on the two single geometrical scales, together with the concurrent two-scale optimization, is separately studied and compared in the paper. Numerical examples in the paper show that the concurrent multi-scale optimization can further explore the coupling effect between the macro-structure and micro-material of the composite to achieve an ultralight design of the composite frame structure. The novel two geometrical scales optimization model provides a new opportunity for the design of composite structure in aerospace and other industries.展开更多
In order to better understand the fatigue mechanisms of steel structures working under high temperature, a multi-scale fatigue damage model at high temperature is developed. In the developed model, the macroscopic fat...In order to better understand the fatigue mechanisms of steel structures working under high temperature, a multi-scale fatigue damage model at high temperature is developed. In the developed model, the macroscopic fatigue damage of metallic materials due to the collective behavior of micro-cracks is quantified by using the generalized self-consistent method. The influence of temperature on fatigue damage of steel structures is quantified by using the previous creep damage model. In addition, the fatigue damage at room temperature and creep damage is coupled in the multi-scale fatigue damage model. The validity of the developed multi-scale damage model is verified by comparing the predicted damage evolution curve with the experimental data. It shows that the developed model is effectiveness. Finally, the fatigue analysis on steel crane runway girders (CRGs) of industrial steel melt shop is performed based on the developed model.展开更多
The short-range property of interactions between scales in incompressible turbulent flow was examined. Some formulae for the short-range eddy stress were given. A concept of resonant-range interactions between extreme...The short-range property of interactions between scales in incompressible turbulent flow was examined. Some formulae for the short-range eddy stress were given. A concept of resonant-range interactions between extremely contiguous scales was introduced and some formulae for the resonant-range eddy stress were also derived. Multi-scale equations for the incompressible turbulent flows were proposed. Key words turbulence - incompressible flow - interactions between scales - multi-scale equations MSC 2000 76F70展开更多
In present study, the subgrid scale （SGS） stress and dissipation for multiscale formulation of large eddy simulation are analyzed using the data of turbulent channel flow at Ret = 180 obtained by direct numerical si...In present study, the subgrid scale （SGS） stress and dissipation for multiscale formulation of large eddy simulation are analyzed using the data of turbulent channel flow at Ret = 180 obtained by direct numerical simulation. It is found that the small scale SGS stress is much smaller than the large scale SGS stress for all the stress components. The dominant contributor to large scale SGS stress is the cross stress between small scale and subgrid scale motions, while the cross stress between large scale and subgrid scale motions make major contributions to small scale SGS stress. The energy transfer from resolved large scales to subgrid scales is mainly caused by SGS Reynolds stress, while that between resolved small scales and subgrid scales are mainly due to the cross stress. The multiscale formulation of SGS models are evaluated a priori, and it is found that the small- small model is superior to other variants in terms of SGS dissipation.展开更多
In this paper, an experimental investigation on the flow structures in a turbulent bounda- ry layer employing a special laser light sheet-Hydrogen bubble flow visualization technique is described. It is observed that ...In this paper, an experimental investigation on the flow structures in a turbulent bounda- ry layer employing a special laser light sheet-Hydrogen bubble flow visualization technique is described. It is observed that the high/low speed streaks are directly related to the hairpin or horseshoe-like vortices. This observation can give a better understanding of the physical mechanism in the turbulent boundary layer.展开更多
Using the idea of general resonant triad of the hydrodynamic stability, the theoretical models for coherent structures in the wall region of a turbulent boundary layer is proposed. The interaction between coherent str...Using the idea of general resonant triad of the hydrodynamic stability, the theoretical models for coherent structures in the wall region of a turbulent boundary layer is proposed. The interaction between coherent structures in the wall region of a turbulent boundary layer is studied by combining the compact finite differences of high numerical accuracy and the Fourier spectral hybrid method for solving the three dimensional Navier Stokes equations. In this method, the third order mixed explicit implicit scheme is employed for the time integration. The fifth order upwind compact finite difference schemes for the nonlinear convection terms in the physical space, and the sixth order center compact schemes for the derivatives in spectral space are descried, respectively. The fourth order compact schemes satisfied by the velocities and pressure in spectral space is derived. As an application, the method is implemented to the wall region of a turbulent boundary to study the interaction between coherent structures. It is found that the numerical results are satisfactory.展开更多
By a suitable manipulation of hydrogen bubble generation,some new results were obtained: (1)The long-streaks arc generated along the interfaces between low and high-speed streaks.The long-streaks are generally stretch...By a suitable manipulation of hydrogen bubble generation,some new results were obtained: (1)The long-streaks arc generated along the interfaces between low and high-speed streaks.The long-streaks are generally stretching and are moving faster than its neighboring high-speed streaks.The hydrogen bubbles in long-streaks have longer life.(2)The stream-wise vortices are also generated along the interfaces.展开更多
In lhis paper experimental research is carried out for the existence of larqe andsmall scale .structures in turbulent boundary layer and the interactions between these structures. Based on the experimental results ,a ...In lhis paper experimental research is carried out for the existence of larqe andsmall scale .structures in turbulent boundary layer and the interactions between these structures. Based on the experimental results ,a new cethod is suggested io (iescribethe coherent strucures in which the interactions between between the coherent and small-scale slructures are considered ,Using this method a new pattern recognition method is and rough wall conditions. The results show that the suggested method is reasonable .展开更多
文摘The time sequence of longitudinal velocity component at different vertical locations in turbulent boundary layer was finely measured in a wind tunnel. The concept of coarse_grained velocity structure functions, which describes the relative motions of straining and compressing for multi_scale eddy structures in turbulent flows, was put forward based on the theory of locally multi_scale average. Based on the consistency between coarse_grained velocity structure function and Harr wavelet transformation,detecting method was presented, by which the coherent structures and their intermittency was identified by multi_scale flatness factor calculated by locally average structure function. Phase_averaged evolution course for multi_scale coherent eddy structures in wall turbulence were extracted by this conditional sampling to educe scheme. The dynamics course of multi_scale coherent eddy structures and their effects on statistics of turbulent flows were studied.
文摘The high order compact d if ference method is developed for solving the perturbation equations based on Navi er Stokes equations, and is used in studying complex evolution processes from w all negative pulse to the turbulent coherent structure in the channel flow. Th is method contains three dimensional coupling difference scheme with high accur acy and high resolution, and the high order time splitting methods. Compared with the general spectral method, the method can be used to research turbule nt coherent structure under more general boundary conditions and in flow domains . In this paper, the generation and evolution of the turbulent coherent structur es ind uced by wall pulse in the channel flow are simulated, and the basic characterist ics and rules of the turbulent coherent structure are shown. Computational r esults indicate that a wall negative pulse is more convenient than the resonant three wave model.
基金Project(2022YFC2904103)supported by the National Key Research and Development Program of ChinaProjects(52374112,52274108)supported by the National Natural Science Foundation of China+1 种基金Projects(BX20220036,BX20230041)supported by the Postdoctoral Innovation Talents Support Program,ChinaProject(2232080)supported by the Beijing Natural Science Foundation,China。
文摘The development of metallic mineral resources generates a significant amount of solid waste,such as tailings and waste rock.Cemented tailings and waste-rock backfill(CTWB)is an effective method for managing and disposing of this mining waste.This study employs a macro-meso-micro testing method to investigate the effects of the waste rock grading index(WGI)and loading rate(LR)on the uniaxial compressive strength(UCS),pore structure,and micromorphology of CTWB materials.Pore structures were analyzed using scanning electron microscopy(SEM)and mercury intrusion porosimetry(MIP).The particles(pores)and cracks analysis system(PCAS)software was used to quantitatively characterize the multi-scale micropores in the SEM images.The key findings indicate that the macroscopic results(UCS)of CTWB materials correspond to the microscopic results(pore structure and micromorphology).Changes in porosity largely depend on the conditions of waste rock grading index and loading rate.The inclusion of waste rock initially increases and then decreases the UCS,while porosity first decreases and then increases,with a critical waste rock grading index of 0.6.As the loading rate increases,UCS initially rises and then falls,while porosity gradually increases.Based on MIP and SEM results,at waste rock grading index 0.6,the most probable pore diameters,total pore area(TPA),pore number(PN),maximum pore area(MPA),and area probability distribution index(APDI)are minimized,while average pore form factor(APF)and fractal dimension of pore porosity distribution(FDPD)are maximized,indicating the most compact pore structure.At a loading rate of 12.0 mm/min,the most probable pore diameters,TPA,PN,MPA,APF,and APDI reach their maximum values,while FDPD reaches its minimum value.Finally,the mechanism of CTWB materials during compression is analyzed,based on the quantitative results of UCS and porosity.The research findings play a crucial role in ensuring the successful application of CTWB materials in deep metal mines.
基金funded by the Joint Fund for Regional Innovation and Development of National Natural Science Foundation of China(U21A20143)the National Science Fund for Excellent Young Scholars(52322607)the Excellent Youth Foundation of Heilongjiang Scientific Committee(YQ2022E028)。
文摘Improving the volumetric energy density of supercapacitors is essential for practical applications,which highly relies on the dense storage of ions in carbon-based electrodes.The functional units of carbon-based electrode exhibit multi-scale structural characteristics including macroscopic electrode morphologies,mesoscopic microcrystals and pores,and microscopic defects and dopants in the carbon basal plane.Therefore,the ordered combination of multi-scale structures of carbon electrode is crucial for achieving dense energy storage and high volumetric performance by leveraging the functions of various scale structu re.Considering that previous reviews have focused more on the discussion of specific scale structu re of carbon electrodes,this review takes a multi-scale perspective in which recent progresses regarding the structureperformance relationship,underlying mechanism and directional design of carbon-based multi-scale structures including carbon morphology,pore structure,carbon basal plane micro-environment and electrode technology on dense energy storage and volumetric property of supercapacitors are systematically discussed.We analyzed in detail the effects of the morphology,pore,and micro-environment of carbon electrode materials on ion dense storage,summarized the specific effects of different scale structures on volumetric property and recent research progress,and proposed the mutual influence and trade-off relationship between various scale structures.In addition,the challenges and outlooks for improving the dense storage and volumetric performance of carbon-based supercapacitors are analyzed,which can provide feasible technical reference and guidance for the design and manufacture of dense carbon-based electrode materials.
基金supported by the National Natural Science Foundation of China(52172091,52172295)Defense Industrial Technology Development Program(JCKY2023605C002)+4 种基金Frontier Leading Technology Basic Research Major Project of Jiangsu Province(SBK2023050110)the National Key Laboratory on Electromagnetic Environmental Effects and Electro-optical Engineering(NO.61422062301)the Opening Project of Science and Technology on Reliability Physics and Application Technology of Electronic Component Laboratory(ZHD202305)the Opening Project of Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology(ASMA202303)the Postgraduate Research&Practice Innovation Program of Jiangsu Province(KYCX23_0371).
文摘Advanced chemical engineering for simultaneous modulation of nanomaterial morphology, defects, interfaces, and structure to enhance electromagnetic and microwave absorption (MA) performance. However, accurately distinguishing the MA contributions of different scale factors and tuning the optimal combined effects remains a formidable challenge. This study employs a synergistic approach combining template protection etching and vacuum annealing to construct a controlled system of micrometer-sized cavities and amorphous carbon matrices in metal-organic framework (MOF) derivatives. The results demonstrate that the spatial effects introduced by the hollow structure enhance dielectric loss but significantly weaken impedance matching. By increasing the proportion of amorphous carbon, the balance between electromagnetic loss and impedance matching can be effectively maintained. Importantly, in a suitable graphitization environment, the presence of oxygen vacancies in amorphous carbon can induce significant polarization to compensate for the reduced conductivity loss due to the absence of sp2 carbon. Through the synergistic effects of morphology and composition, the samples exhibit a broader absorption bandwidth (6.28 GHz) and stronger reflection loss (−61.64 dB) compared to the original MOF. In conclusion, this study aims to elucidate the multiscale impacts of macroscopic micro-nano structure and microscopic defect engineering, providing valuable insights for future research in this field.
基金supported by National Key Research and Development Program(No.2017YFB1102800)Key Project of NSFC(Nos.51790171 and 51761145111)NSFC for Excellent Young Scholars(No.11722219)。
文摘By integrating topology optimization and lattice-based optimization,a novel multi-scale design method is proposed to create solid-lattice hybrid structures and thus to improve the mechanical performance as well as reduce the structural weight.To achieve this purpose,a two-step procedure is developed to design and optimize the innovative structures.Initially,the classical topology optimization is utilized to find the optimal material layout and primary load carrying paths.Afterwards,the solid-lattice hybrid structures are reconstructed using the finite element mesh based modeling method.And lattice-based optimization is performed to obtain the optimal crosssection area of the lattice structures.Finally,two typical aerospace structures are optimized to demonstrate the effectiveness of the proposed optimization framework.The numerical results are quite encouraging since the solid-lattice hybrid structures obtained by the presented approach show remarkably improved performance when compared with traditional designs.
基金supported by the National Natural Science Foundation of China (Grants 11232011, 11402262, 11572314, 11621202)the Fundamental Research Funds for the Central Universities
文摘A large eddy simulation (LES) is performed for two subsonic jets with a Reynolds number of , which have different core temperatures, i.e., the cold and hot jet. The far-field overall sound pressure levels (OASPL) and noise spectra are well validated against previous experimental results. It is found that the OASPL is raised by heating at shallow angles. The most energetic coherent structures are extracted with specified frequencies using the filter based on the frequency domain variant of the snapshot method of proper orthogonal decomposition (POD). The modes have high coherence of near-field pressure for both jets, while the coherence of modes is enhanced greatly by heating. Based on the coherent structures, spatial wavepackets are educed and the characteristics of growth, saturation and decay are analyzed and compared between the two jets in detail. The results show that heating would enhance the linear growth rate for high frequency components, and nonlinear growth rates for low frequency components in general, which are responsible for higher OASPL in the hot jet. The far-field sound generated by wavepackets is computed using the Kirchhoff extrapolation, which matches well with that of LES at shallow angles. This indicates that the wavepackets associated with coherent structures are dominant sound sources in forced transitional turbulent jets. Additionally, the present POD method is proven to be a robust tool to extract the salient features of the wavepackets in turbulent flows.
基金the National Basic Research Program of China (973 program)(Grants 2014CB744802 and 2014CB744804)the National Natural Science Foundation of China (Grants 11772194 and 91441205).
文摘Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify the interaction of the riblet surfaces with the coherent, energy-bearing eddy structures in the near-wall region. Results provide statistical evidence that the averaged organization of the streamwise vortices in the drag-reducing case is lifted above the riblet tip, while in the drag-increasing case the streamwise vortices are embedded further into the riblet cove. In the spanwise direction, the cores of the streamwise vortices over the riblet surfaces are shown to be closer to each other than those for flow over the smooth wall, and wider riblet spacing leads to more reduction on their spanwise distances. In the cases with riblets the streamwise vortices have longer streamwise lengths, but their inclination angles do not change much.
文摘The problem of how the coherent structures in the wall region of a turbulent boundary layer could be excited by the disturbances from the outer region was investigated by using direct numerical simulation (DNS) method. The results show that velocity disturbances at the upper boundary of the wall region could excite coherent structures in the wall region, thus offering a more comprehensive model for individual coherent structures.
基金Project supported by the National Basic Research Program of China(Grant Nos.2012CB720101 and 2012CB720103)the National Natural Science Foundation of China(Grant Nos.11272233,11332006,and 11411130150)
文摘When using a miniature single sensor boundary layer probe, the time sequences of the stream-wise velocity in the turbulent boundary layer （TBL） are measured by using a hot wire anemometer. Beneath the fully developed TBL, the wall pressure fluctuations are attained by a microphone mechanism with high spatial resolution. Analysis on the statistic and spectrum properties of velocity and wall pressure reveals the relationship between the wall pressure fluctuation and the energy-containing structure in the buffer layer of the TBL. Wavelet transform shows the multi-scale natures of coherent structures contained in both signals of velocity and pressure. The most intermittent wall pressure scale is associated with the coherent structure in the buffer layer. Meanwhile the most energetic scale of velocity fluctuation at y＋ = 14 provides a specific frequency f9 ≈ 147 Hz for wall actuating control with Ret = 996.
文摘A numerical simulation is performed to find out a key vortical structure in the laminar-turbulent transition. A low-speed streak is generated inside a laminar boundary layer using an isolated cuboid roughness, aimed at providing an environment unstable to outer disturbances. Then, a short duration jet is issued into the boundary layer. When the jet velocity is low, some vortices appear in the boundary layer, but the transition of the boundary layer does not take place.However, when the jet velocity exceeds a certain threshold, two vortices newly appear above the elongated legs of a V-shaped vortex and only one of them is stretched and survives. After that,vortices are generated one after another around the survived one. By comparing the decayed and the survived vortices, it is found that the difference in their heights is the key characteristic which leads to the transition.
基金the National Natural Science Foundation of China(Grant Nos.11732010,11972251,11872272,11902218,and 12172242)the Ministry of Industry and Information Technology(Grant No.[2019]360).
文摘The influence of drag-reducing superhydrophobic(SHPo)surface on turbulent boundary layer(TBL)is investigated.A large area of the SHPo surface(about 10δ99 in the streamwise and 5δ99 in the spanwise)is fabricated to fully evolve the coherent structures in the TBL.A comparative experiment is carried out by time-resolved particle image velocimetry on a smooth surface and the SHPo surface at Re_(τ)=528.Velocity profiles with high spatial resolution are obtained by the single-pixel resolution ensemble correlation method.The reduction of the streamwise velocity gradient is observed in the near-wall region of y<0.05δ99 on the SHPo surface.By comparing the turbulence statistics,it is discovered that the Reynolds shear stress is reduced by 15.7%,and the turbulent kinetic energy is reduced by 12.3%on the SHPo surface.The coherent structures are investigated by the snapshot proper orthogonal decomposition(POD)and conditional average method.The intensity of Q_(2)/Q_(4)events on the SHPo surface has declined by 16.1%and 12.7%,respectively.The number of clockwise spanwise vortices is substantially reduced by 47%.Through spatial two-point correlation analysis,the streamwise and wall-normal direction scales of the coherent structures on the SHPo surface are suppressed.
基金financial support for this research was provided by the Program (Grants 11372060, 91216201) of the National Natural Science Foundation of ChinaProgram (LJQ2015026 ) for Excellent Talents at Colleges and Universities in Liaoning Province+3 种基金the Major National Science and Technology Project (2011ZX02403-002)111 project (B14013)Fundamental Research Funds for the Central Universities (DUT14LK30)the China Scholarship Fund
文摘This paper deals with the concurrent multi-scale optimization design of frame structure composed of glass or carbon fiber reinforced polymer laminates. In the composite frame structure, the fiber winding angle at the micro-material scale and the geometrical parameter of components of the frame in the macro-structural scale are introduced as the independent variables on the two geometrical scales. Considering manufacturing requirements, discrete fiber winding angles are specified for the micro design variable. The improved Heaviside penalization discrete material optimization interpolation scheme has been applied to achieve the discrete optimization design of the fiber winding angle. An optimization model based on the minimum structural compliance and the specified fiber material volume constraint has been established. The sensitivity information about the two geometrical scales design variables are also deduced considering the characteristics of discrete fiber winding angles. The optimization results of the fiber winding angle or the macro structural topology on the two single geometrical scales, together with the concurrent two-scale optimization, is separately studied and compared in the paper. Numerical examples in the paper show that the concurrent multi-scale optimization can further explore the coupling effect between the macro-structure and micro-material of the composite to achieve an ultralight design of the composite frame structure. The novel two geometrical scales optimization model provides a new opportunity for the design of composite structure in aerospace and other industries.
文摘In order to better understand the fatigue mechanisms of steel structures working under high temperature, a multi-scale fatigue damage model at high temperature is developed. In the developed model, the macroscopic fatigue damage of metallic materials due to the collective behavior of micro-cracks is quantified by using the generalized self-consistent method. The influence of temperature on fatigue damage of steel structures is quantified by using the previous creep damage model. In addition, the fatigue damage at room temperature and creep damage is coupled in the multi-scale fatigue damage model. The validity of the developed multi-scale damage model is verified by comparing the predicted damage evolution curve with the experimental data. It shows that the developed model is effectiveness. Finally, the fatigue analysis on steel crane runway girders (CRGs) of industrial steel melt shop is performed based on the developed model.
文摘The short-range property of interactions between scales in incompressible turbulent flow was examined. Some formulae for the short-range eddy stress were given. A concept of resonant-range interactions between extremely contiguous scales was introduced and some formulae for the resonant-range eddy stress were also derived. Multi-scale equations for the incompressible turbulent flows were proposed. Key words turbulence - incompressible flow - interactions between scales - multi-scale equations MSC 2000 76F70
基金supported by the National Natural Science Foundation of China(10472053 and 10772098)
文摘In present study, the subgrid scale （SGS） stress and dissipation for multiscale formulation of large eddy simulation are analyzed using the data of turbulent channel flow at Ret = 180 obtained by direct numerical simulation. It is found that the small scale SGS stress is much smaller than the large scale SGS stress for all the stress components. The dominant contributor to large scale SGS stress is the cross stress between small scale and subgrid scale motions, while the cross stress between large scale and subgrid scale motions make major contributions to small scale SGS stress. The energy transfer from resolved large scales to subgrid scales is mainly caused by SGS Reynolds stress, while that between resolved small scales and subgrid scales are mainly due to the cross stress. The multiscale formulation of SGS models are evaluated a priori, and it is found that the small- small model is superior to other variants in terms of SGS dissipation.
文摘In this paper, an experimental investigation on the flow structures in a turbulent bounda- ry layer employing a special laser light sheet-Hydrogen bubble flow visualization technique is described. It is observed that the high/low speed streaks are directly related to the hairpin or horseshoe-like vortices. This observation can give a better understanding of the physical mechanism in the turbulent boundary layer.
文摘Using the idea of general resonant triad of the hydrodynamic stability, the theoretical models for coherent structures in the wall region of a turbulent boundary layer is proposed. The interaction between coherent structures in the wall region of a turbulent boundary layer is studied by combining the compact finite differences of high numerical accuracy and the Fourier spectral hybrid method for solving the three dimensional Navier Stokes equations. In this method, the third order mixed explicit implicit scheme is employed for the time integration. The fifth order upwind compact finite difference schemes for the nonlinear convection terms in the physical space, and the sixth order center compact schemes for the derivatives in spectral space are descried, respectively. The fourth order compact schemes satisfied by the velocities and pressure in spectral space is derived. As an application, the method is implemented to the wall region of a turbulent boundary to study the interaction between coherent structures. It is found that the numerical results are satisfactory.
基金The project is supported by the National Natural Science Foundation of China.
文摘By a suitable manipulation of hydrogen bubble generation,some new results were obtained: (1)The long-streaks arc generated along the interfaces between low and high-speed streaks.The long-streaks are generally stretching and are moving faster than its neighboring high-speed streaks.The hydrogen bubbles in long-streaks have longer life.(2)The stream-wise vortices are also generated along the interfaces.
文摘In lhis paper experimental research is carried out for the existence of larqe andsmall scale .structures in turbulent boundary layer and the interactions between these structures. Based on the experimental results ,a new cethod is suggested io (iescribethe coherent strucures in which the interactions between between the coherent and small-scale slructures are considered ,Using this method a new pattern recognition method is and rough wall conditions. The results show that the suggested method is reasonable .
