The peridynamics(PD),as a promising nonlocal continuum mechanics theory,shines in solving discontinuous problems.Up to now,various numerical methods,such as the peridynamic mesh-free particlemethod(PD-MPM),peridynamic...The peridynamics(PD),as a promising nonlocal continuum mechanics theory,shines in solving discontinuous problems.Up to now,various numerical methods,such as the peridynamic mesh-free particlemethod(PD-MPM),peridynamic finite element method(PD-FEM),and peridynamic boundary element method(PD-BEM),have been proposed.PD-BEM,in particular,outperforms other methods by eliminating spurious boundary softening,efficiently handling infinite problems,and ensuring high computational accuracy.However,the existing PD-BEM is constructed exclusively for bond-based peridynamics(BBPD)with fixed Poisson’s ratio,limiting its applicability to crack propagation problems and scenarios involving infinite or semi-infinite problems.In this paper,we address these limitations by introducing the boundary element method(BEM)for ordinary state-based peridynamics(OSPD-BEM).Additionally,we present a crack propagationmodel embeddedwithin the framework ofOSPD-BEM to simulate crack propagations.To validate the effectiveness of OSPD-BEM,we conduct four numerical examples:deformation under uniaxial loading,crack initiation in a double-notched specimen,wedge-splitting test,and threepoint bending test.The results demonstrate the accuracy and efficiency of OSPD-BEM,highlighting its capability to successfully eliminate spurious boundary softening phenomena under varying Poisson’s ratios.Moreover,OSPDBEMsignificantly reduces computational time and exhibits greater consistencywith experimental results compared to PD-MPM.展开更多
In this letter,the effect of slip boundary on the origin of subcritical transition in two-dimensional chan-nel flows is studied numerically and theoretically.It is shown that both the positive and the negative slip le...In this letter,the effect of slip boundary on the origin of subcritical transition in two-dimensional chan-nel flows is studied numerically and theoretically.It is shown that both the positive and the negative slip lengths will increase the critical Reynolds number of localized wave packet and hence postpone the transition.By applying a variable transformation and expanding the variables about a small slip length,it is illustrated that the slip boundary effect only exists in the second and higher order modulations of the no-slip solution,and hence explains the power law found in simulations,i.e.the relative increment of the critical Reynolds number due to the slip boundary is proportional to the square of the slip length.展开更多
By direct numerical simulations of the plane Couette flow (PCF) in a large computational domain, it is shown that an isolated turbulent band decays monotonically at low Reynolds numbers but experiences transient growt...By direct numerical simulations of the plane Couette flow (PCF) in a large computational domain, it is shown that an isolated turbulent band decays monotonically at low Reynolds numbers but experiences transient growth before the eventual relaminarization at moderate Reynolds numbers. The lower bound Reynolds number of the transient-growth regime is determined as 286. The width, length, and tilt angle of the iso- lated band structure are defined based on the disturbance kinetic energy in the mid-plane, and the geometric characteristics of the band can be described with a tilted rectangle. It is illustrated that before its eventual fragmentation, the isolated turbulent band decays in a style of longitudinal contraction, where the center, width, and tilt angle of the band keep almost constant but the band length contracts with a statistically constant velocity.展开更多
Lobes and clefts are characteristic structures at the front of sand storms.In this paper,their original formation mechanism and geometric features are studied experimentally and theoretically.A rotatable lock-exchange...Lobes and clefts are characteristic structures at the front of sand storms.In this paper,their original formation mechanism and geometric features are studied experimentally and theoretically.A rotatable lock-exchange tank is utilized to avoid the strong local disturbances existing in the conventional horizontal apparatus,and the original lobe size is selected as the dominant spanwise wavelength by a statistical method instead of the mean lobe size used in the literature.It is shown that at the initial formation stage,the measured lobe sizes for different tank geometries and density differences(or Atwood numbers)have a 1/3 scaling law with the Grashof number(Gr)as predicted by a Rayleigh-Taylor(RT)model,especially at moderate Gr range,and substantially depend on the diffusion effect represented by the Schmidt number.Furthermore,using turbulent Schmidt number and eddy viscosity for atmosphere in the RT model predicts large scale dominant lobes,whose sizes agree qualitatively with those observed in a real sand storm,revealing that the underlying control mechanism for these lobe and cleft structures is intrinsically related to the RT instability.展开更多
The effective properties of composite materials have been predicted by various micromechanical schemes.For composite materials of constituents which are described by the classical governing equations of the local form...The effective properties of composite materials have been predicted by various micromechanical schemes.For composite materials of constituents which are described by the classical governing equations of the local form,the conventional micromechanical schemes usually give effective properties of the local form.However,it is recognized that under general loading conditions,spatiotemporal nonlocal constitutive equations may better depict the macroscopic behavior of these materials.In this paper,we derive the thermo-elastic dynamic effective governing equations of a fibre-reinforced composite in a coupled spatiotemporal integral form.These coupled equations reduce to the spatial nonlocal peridynamic formulation when the microstructural inertial effects are neglected.For static deformation and steady-state heat conduction,we show that the integral formulation is superior at capturing the variations of the average displacement and temperature in regions of high gradients than the conventional micromechanical schemes.The approach can be applied to analogous multi-field coupled problems of composites.展开更多
The instabilities of a pendent viscous thin film underneath two corrugated ceilings are studied numerically and theoretically in comparison with the case of a flat wall. With the same initial interface perturbations, ...The instabilities of a pendent viscous thin film underneath two corrugated ceilings are studied numerically and theoretically in comparison with the case of a flat wall. With the same initial interface perturbations, it is shown numerically that both the supercritical instability and the subcritical instability can be retarded by the in-phase corrugated ceilings. The lubrication approximation is used to explain the retardation effect of the corrugated ceiling on the supercritical instability of the pendant film, where the linear growth rate is revealed to be power three of the initial film thickness.展开更多
3D printing or additive manufacturing (AM) has revolutionized the way of manufacturing by designing complex structures in a customized feature which cannot be realized by traditional processing methods. Incoming mater...3D printing or additive manufacturing (AM) has revolutionized the way of manufacturing by designing complex structures in a customized feature which cannot be realized by traditional processing methods. Incoming materials are trying to adopt 3D printing techniques which directly fabricate sophisticated entities with multifunctionality like mechanical, electrical, thermal and magnetic properties etc. For the realization of advanced materials, 3D printing techniques are emerging from single material to composite materials manufacturing by simply introducing the nano- and micro-reinforcements with the matrix. In this review, we provide an outline of 3D printing graphene-based composites according to various AM techniques including fused deposition modeling (FDM), direct ink writing (DIW), stereolithography (SLA) and selective laser sintering (SLS). First a brief introduction of various AM techniques is given to get a basic understanding of the principles of 3D printing, and then the fabrication process, structural characteristics and applications of different 3D printing techniques for graphene-based composites are summarized. In addition, some effective simulation and characterization methods are also included. We hope that this review would clarify the potential of AM techniques for composite materials and can open new prospects for designing of novel materials.展开更多
Flexible transparent conductive films are indispensable for nowadays wearable electronic devices with various applications.However,existing solutions such as ITO and metal mesh were limited by their poor intrinsic str...Flexible transparent conductive films are indispensable for nowadays wearable electronic devices with various applications.However,existing solutions such as ITO and metal mesh were limited by their poor intrinsic stretching ability.In this work,we designed and fabricated silver nanowires(AgNWs)on graphene hybrid films for enhanced mechanical and electrical performance.In situ TEM characterizations show that,beside conductive paths,silver nanowire network,can also contribute to the toughening mechanisms of the hybrid films.Furthermore,bending and electrical tests were applied to examine the corresponding flexible electronics’perfor-mance.Finally,we showed that the fabrication of our AgNW/graphene hybrid films could be scaled up for large film applications and extended to other 1D/2D hybrid systems.展开更多
基金supported by the National Key R&D Program of China(2020YFA0710500).
文摘The peridynamics(PD),as a promising nonlocal continuum mechanics theory,shines in solving discontinuous problems.Up to now,various numerical methods,such as the peridynamic mesh-free particlemethod(PD-MPM),peridynamic finite element method(PD-FEM),and peridynamic boundary element method(PD-BEM),have been proposed.PD-BEM,in particular,outperforms other methods by eliminating spurious boundary softening,efficiently handling infinite problems,and ensuring high computational accuracy.However,the existing PD-BEM is constructed exclusively for bond-based peridynamics(BBPD)with fixed Poisson’s ratio,limiting its applicability to crack propagation problems and scenarios involving infinite or semi-infinite problems.In this paper,we address these limitations by introducing the boundary element method(BEM)for ordinary state-based peridynamics(OSPD-BEM).Additionally,we present a crack propagationmodel embeddedwithin the framework ofOSPD-BEM to simulate crack propagations.To validate the effectiveness of OSPD-BEM,we conduct four numerical examples:deformation under uniaxial loading,crack initiation in a double-notched specimen,wedge-splitting test,and threepoint bending test.The results demonstrate the accuracy and efficiency of OSPD-BEM,highlighting its capability to successfully eliminate spurious boundary softening phenomena under varying Poisson’s ratios.Moreover,OSPDBEMsignificantly reduces computational time and exhibits greater consistencywith experimental results compared to PD-MPM.
基金The present research is financially supported by the National Natural Science Foundation of China(Grants No.91752203).
文摘In this letter,the effect of slip boundary on the origin of subcritical transition in two-dimensional chan-nel flows is studied numerically and theoretically.It is shown that both the positive and the negative slip lengths will increase the critical Reynolds number of localized wave packet and hence postpone the transition.By applying a variable transformation and expanding the variables about a small slip length,it is illustrated that the slip boundary effect only exists in the second and higher order modulations of the no-slip solution,and hence explains the power law found in simulations,i.e.the relative increment of the critical Reynolds number due to the slip boundary is proportional to the square of the slip length.
基金Project supported by the National Natural Science Foundation of China(Nos.91752203,11490553,and 11602148)
文摘By direct numerical simulations of the plane Couette flow (PCF) in a large computational domain, it is shown that an isolated turbulent band decays monotonically at low Reynolds numbers but experiences transient growth before the eventual relaminarization at moderate Reynolds numbers. The lower bound Reynolds number of the transient-growth regime is determined as 286. The width, length, and tilt angle of the iso- lated band structure are defined based on the disturbance kinetic energy in the mid-plane, and the geometric characteristics of the band can be described with a tilted rectangle. It is illustrated that before its eventual fragmentation, the isolated turbulent band decays in a style of longitudinal contraction, where the center, width, and tilt angle of the band keep almost constant but the band length contracts with a statistically constant velocity.
基金support by the National Natural Science Foundation of China(Grants No.91752203,11490553)is acknowledged.
文摘Lobes and clefts are characteristic structures at the front of sand storms.In this paper,their original formation mechanism and geometric features are studied experimentally and theoretically.A rotatable lock-exchange tank is utilized to avoid the strong local disturbances existing in the conventional horizontal apparatus,and the original lobe size is selected as the dominant spanwise wavelength by a statistical method instead of the mean lobe size used in the literature.It is shown that at the initial formation stage,the measured lobe sizes for different tank geometries and density differences(or Atwood numbers)have a 1/3 scaling law with the Grashof number(Gr)as predicted by a Rayleigh-Taylor(RT)model,especially at moderate Gr range,and substantially depend on the diffusion effect represented by the Schmidt number.Furthermore,using turbulent Schmidt number and eddy viscosity for atmosphere in the RT model predicts large scale dominant lobes,whose sizes agree qualitatively with those observed in a real sand storm,revealing that the underlying control mechanism for these lobe and cleft structures is intrinsically related to the RT instability.
文摘The effective properties of composite materials have been predicted by various micromechanical schemes.For composite materials of constituents which are described by the classical governing equations of the local form,the conventional micromechanical schemes usually give effective properties of the local form.However,it is recognized that under general loading conditions,spatiotemporal nonlocal constitutive equations may better depict the macroscopic behavior of these materials.In this paper,we derive the thermo-elastic dynamic effective governing equations of a fibre-reinforced composite in a coupled spatiotemporal integral form.These coupled equations reduce to the spatial nonlocal peridynamic formulation when the microstructural inertial effects are neglected.For static deformation and steady-state heat conduction,we show that the integral formulation is superior at capturing the variations of the average displacement and temperature in regions of high gradients than the conventional micromechanical schemes.The approach can be applied to analogous multi-field coupled problems of composites.
基金Project supported by the National Natural Science Foundation of China(Nos.91752203,11490553,and 11521091)
文摘The instabilities of a pendent viscous thin film underneath two corrugated ceilings are studied numerically and theoretically in comparison with the case of a flat wall. With the same initial interface perturbations, it is shown numerically that both the supercritical instability and the subcritical instability can be retarded by the in-phase corrugated ceilings. The lubrication approximation is used to explain the retardation effect of the corrugated ceiling on the supercritical instability of the pendant film, where the linear growth rate is revealed to be power three of the initial film thickness.
基金supported by NSFC (Grant No.11672002)NSAF (Grant No. U1730103)
文摘3D printing or additive manufacturing (AM) has revolutionized the way of manufacturing by designing complex structures in a customized feature which cannot be realized by traditional processing methods. Incoming materials are trying to adopt 3D printing techniques which directly fabricate sophisticated entities with multifunctionality like mechanical, electrical, thermal and magnetic properties etc. For the realization of advanced materials, 3D printing techniques are emerging from single material to composite materials manufacturing by simply introducing the nano- and micro-reinforcements with the matrix. In this review, we provide an outline of 3D printing graphene-based composites according to various AM techniques including fused deposition modeling (FDM), direct ink writing (DIW), stereolithography (SLA) and selective laser sintering (SLS). First a brief introduction of various AM techniques is given to get a basic understanding of the principles of 3D printing, and then the fabrication process, structural characteristics and applications of different 3D printing techniques for graphene-based composites are summarized. In addition, some effective simulation and characterization methods are also included. We hope that this review would clarify the potential of AM techniques for composite materials and can open new prospects for designing of novel materials.
基金This work was supported by the City University of Hong Kong[ARG 9667194]Natural Science Foundation of Shaanxi Province[2020JQ-295]+3 种基金Key Research and Development Program of Shaanxi[2020GY-252]Young and Middle-aged Academic and Technical Leaders Reserve Talents Program of Yunnan Province[2017HB060]National Key Laboratory of Science and Technology on Vacuum Technology and Physics[HTKJ2019KL510007]National Natural Science Foundation of China(NSFC)[11922215,21761016,61904141].
文摘Flexible transparent conductive films are indispensable for nowadays wearable electronic devices with various applications.However,existing solutions such as ITO and metal mesh were limited by their poor intrinsic stretching ability.In this work,we designed and fabricated silver nanowires(AgNWs)on graphene hybrid films for enhanced mechanical and electrical performance.In situ TEM characterizations show that,beside conductive paths,silver nanowire network,can also contribute to the toughening mechanisms of the hybrid films.Furthermore,bending and electrical tests were applied to examine the corresponding flexible electronics’perfor-mance.Finally,we showed that the fabrication of our AgNW/graphene hybrid films could be scaled up for large film applications and extended to other 1D/2D hybrid systems.
