We present an assumed enhanced strain finite element framework for the simulation of tensile fracturing processes in transversely isotropic rocks.Fractures along the weak bedding planes and through the anisotropic roc...We present an assumed enhanced strain finite element framework for the simulation of tensile fracturing processes in transversely isotropic rocks.Fractures along the weak bedding planes and through the anisotropic rock matrix are treated with distinct enrichment,and a recently proposed dualmechanism tensile failure criterion for transversely isotropic rocks is adopted to determine crack initiation for the two failure modes.The cohesive crack model is adopted to characterize the response of embedded cracks.As for the numerical implementation of the proposed framework,both algorithms for the update of local history variables at Gauss points and of the global finite element system are derived.Four boundary-value problem simulations are carried out with the proposed framework,including uniaxial tension tests of Argillite,pre-notched square loaded in tension,three-point bending tests on Longmaxi shale,and simulations of tensile cracks induced by a strip load around a tunnel in transversely isotropic rocks.Simulation results reveal that the proposed framework can properly capture the tensile strength anisotropy and the anisotropic evolution of tensile cracks in transversely isotropic rocks.展开更多
This paper presents a finite element framework for imposing frictional contact conditions on embedded fracture faces,implemented by the constant-strain assumed enhanced strain(AES)method,where penalty method is used t...This paper presents a finite element framework for imposing frictional contact conditions on embedded fracture faces,implemented by the constant-strain assumed enhanced strain(AES)method,where penalty method is used to impose both non-penetration constraint and Coulomb’s law of friction.The proposed constant-strain AES method for modeling embedded frictional contact can be cast into an integration algorithm similar to those used in the classical plasticity theory,where displacement jump is calculated from the local traction equilibrium at Gauss point,so the method does not introduce any additional global degrees of freedom.Moreover,constant-strain elements are often desirable in practice because they can be easily created automatically for large-scale engineering applications with complicated geometries.As encountered in other enriched finite element methods for frictional contact,the problem of normal contact pressure oscillations is also observed in the constant-strain AES method.Therefore,we developed a strain-smoothing procedure to effectively mitigate the oscillations.We investigated and verified the proposed AES framework through several numerical examples,and illustrated the capability of this method in solving challenging nonlinear frictional contact problems.展开更多
In this study, the stress based finite element method is coupled with the boundary element method in two different ways. In the first one, the ordinary distribution matrix is used for coupling. In the second one, the ...In this study, the stress based finite element method is coupled with the boundary element method in two different ways. In the first one, the ordinary distribution matrix is used for coupling. In the second one, the stress traction equilibrium is used at the interface line of both regions as a new coupling process. This new coupling procedure is presented without a distribution matrix. Several case studies are solved for the validation of the developed coupling procedure. The results of case studies are compared with the distribution matrix coupling, displacement based finite element method, assumed stress finite element method, boundary element method, ANSYS and analytical results whenever possible. It is shown that the coupling of the stress traction equilibrium with assumed stress finite elements gives as accurate results as those by the distribution matrix coupling.展开更多
The "cascade static lens （CSL） gauge" has a high sensitivity（S） because the emitted electrons repeat the go and back oscillation before they are received by the electrodes. (S=18.6 Pa<sup>-1</su...The "cascade static lens （CSL） gauge" has a high sensitivity（S） because the emitted electrons repeat the go and back oscillation before they are received by the electrodes. (S=18.6 Pa<sup>-1</sup> (2480 Torr<sup>-1</sup> in a展开更多
By combining femtoscopic interferometry with an optical deblurring algorithm,we present a novel method to image the source in heavy-ion collisions(HICs),while simultaneously determining the interaction strength betwee...By combining femtoscopic interferometry with an optical deblurring algorithm,we present a novel method to image the source in heavy-ion collisions(HICs),while simultaneously determining the interaction strength between particle pairs.The spatial distribution of the emission source has been reconstructed for protons(p)and antiprotons(^(-)p)from the respective pp and^(-)p^(-)p correlation functions in Au+Au collisions at√S_(NN)=200 GeV.Within experimental uncertainties,protons and antiprotons share the same freeze-out distribution showing higher density in the center compared to the widely assumed Gaussian shape.The results evidence the matter–antimatter symmetry in coordinate space at the freeze-out moment before the nucleons are fully randomized in the collisions.展开更多
A hybrid LES (Large Eddy Simulation)/assumed sub-grid PDF (Probability Density Function) closure model has been devel-oped for supersonic turbulent combustion. Scalar transport equations for all species in a given che...A hybrid LES (Large Eddy Simulation)/assumed sub-grid PDF (Probability Density Function) closure model has been devel-oped for supersonic turbulent combustion. Scalar transport equations for all species in a given chemical kinetic mechanism were solved, which are necessary in the supersonic combustion where the non-equilibrium chemistry is essentially involved. The clipped Gaussian PDF of temperature and multivariate ? PDF of composition were used to close the sub-grid chemical sources that appear in the conservation equations. The sub-grid variances of temperature and composition were constructed based on scale similarity approach. A semi-implicit approach based on the PDF model was proposed to tackle the resulting numerical stiffness associated with finite rate chemistry. The model was applied to simulate a supersonic, coaxial H2-air burner, where both the mean and rms (root mean square) results were compared with the experimental data. In general, good agree-ments were achieved, which indicated that the present sub-grid PDF method could work well in simulating supersonic turbu-lent combustion. Moreover, the calculation showed that the sub-grid fluctuations of temperature and major species in the combustion region were of the order of 10%-20% of their rms, while the sub-grid fluctuation of hydroxyl might be as high as 40%-50% of its rms.展开更多
A fuzzy adaptive control method is proposed for a flexible robot manipulator. Due to the structure characteristics of the flexible manipulator, the vibration modes must be controlled to realize the high-precision tip ...A fuzzy adaptive control method is proposed for a flexible robot manipulator. Due to the structure characteristics of the flexible manipulator, the vibration modes must be controlled to realize the high-precision tip position. The Lagrangian principle is utilized to model the dynamic function of the single-degree flexible manipulator incorporating the assumed modes method. Simulation results of the fuzzy adaptive control method in the location control and the trajectory tracking with different tip disturbances are presented and compared with the results of the classic PD control. It shows that the controller can obtain the stable and robust performance.展开更多
The 2024 UN Climate Change Conference(COP29)will convene from 11 to 22 November in Baku,Azerbaijan.This year’s conference assumes special significance due to conflicts in some regions,geopolitical tensions and height...The 2024 UN Climate Change Conference(COP29)will convene from 11 to 22 November in Baku,Azerbaijan.This year’s conference assumes special significance due to conflicts in some regions,geopolitical tensions and heightened uncertainties in the world.World leaders are expected to report on the progress in implementing their climate commitments.展开更多
Ⅰ.Each of the statements below is followed by four alternative answers.Choose the one that wouldbest complete the statement.1.Polysemy is a common feature peculiar to ___.A.English only B.Chinese only C.all natural l...Ⅰ.Each of the statements below is followed by four alternative answers.Choose the one that wouldbest complete the statement.1.Polysemy is a common feature peculiar to ___.A.English only B.Chinese only C.all natural languages D.some natural languages2.From the ____ point of view,polysemy is assumed to be the result of growth and development of thesemantic structure of one and same word.A.linguistic B.diachronic C.synchronic D.展开更多
A nonlinear dynamic model of a simple nonholonomic system comprising a self-rotating cantilever beam subjected to a unilateral locked or unlocked constraint is established by employing the general Hamilton＇s Variatio...A nonlinear dynamic model of a simple nonholonomic system comprising a self-rotating cantilever beam subjected to a unilateral locked or unlocked constraint is established by employing the general Hamilton＇s Variational Principle. The critical values, at which the trivial equilibrium loses its stability or the unilateral constraint is activated or a saddle-node bifurcation occurs, and the equilibria are investigated by approximately analytical and numerical methods. The results indicate that both the buckled equilibria and the bifurcation mode of the beam are different depending on whether the distance of the clearance of unilateral constraint equals zero or not and whether the unilateral constraint is locked or not. The unidirectional snap-through phenomenon （i.e. catastrophe phenomenon） is destined to occur in the system no matter whether the constraint is lockable or not. The saddle-node bifurcation can occur only on the condition that the unilateral constraint is lockable and its clearance is nonzero. The results obtained by two methods are consistent.展开更多
In the traditional method of earthquake loss estimation, all the social wealthes are classified according to their structural type and occupational use. Inventory data is collected and the total loss is estimated from...In the traditional method of earthquake loss estimation, all the social wealthes are classified according to their structural type and occupational use. Inventory data is collected and the total loss is estimated from each facility class separately. For many regions of the world, however, the vast amount of data required by this method is difficult or impossible to obtain. The traditional method is also unable to estimate quickly the loss from an unexpected disaster earthquake. It is difficult to give the necessary risk information to help the government to rescue and relief the earthquake disaster. This paper proposes a simple estimation method of earthquake loss based on macroscopic economical index of Gross Domestic Product (GDP) and population distribution data. A preliminary nonlinear relation among hazard loss, seismic intensity and social wealth was developed by means of some earthquake disaster records during 1980～1995. This method was applied to analyze several assumed earthquakes. The preliminary analysis results show that the new method is effective and reasonable for quick assessment of earthquake loss.展开更多
Reactive oxygen species(ROS)and kynurenines:Kynurenines represent a relatively heterogenous group of tryptophan metabolites(Figure 1 A).The amino acid tryptophan is metabolized in the humans by the kynurenine or serot...Reactive oxygen species(ROS)and kynurenines:Kynurenines represent a relatively heterogenous group of tryptophan metabolites(Figure 1 A).The amino acid tryptophan is metabolized in the humans by the kynurenine or serotonin pathway.For a long time,the kynurenine pathway was assumed primarily to constitute the source for nicotinamide-adenine dinucleotide phosphate,one of the most utilized redox active enzyme cofactors.展开更多
Infertility is a major health issue,affecting approximately 15%of couples of child-bearing age.Although nearly half of idiopathic infertility cases are assumed to have a genetic basis,the underlying causes remain larg...Infertility is a major health issue,affecting approximately 15%of couples of child-bearing age.Although nearly half of idiopathic infertility cases are assumed to have a genetic basis,the underlying causes remain largely unknown in most infertile men.展开更多
Tlie rigid-flexible coupling dynamic modeling theory and the discretization methods of a rotating flexible rectangular thin plate are investigated in this paper.Based on the continuum mechanics,the rigid-flexible coup...Tlie rigid-flexible coupling dynamic modeling theory and the discretization methods of a rotating flexible rectangular thin plate are investigated in this paper.Based on the continuum mechanics,the rigid-flexible coupling dynamic model is established for the flexible rectangular thin plate undergoing large overall rotation,and the coupling term of the deformation which is caused by transverse deformation is considered.Assumed mode method(AMM),spline finite point method(SFPM)and Beizer finite point method(BFPM)are used to describe the deformation of the flexible rectangular plate,and then the dynamic equations of a rotating flexible rectangular thin plate undergoing overall motion are derived by Lagrange^equation of the second kind.The dynamics of a cantilever plate undergoing large overall rotation is simulated via using different dynamic models,and the simulation results of the first order approximation model are compared with those of the traditional zero-order approximation model.It is shown that the first order approximation model with the dynamic stiffening terms can correctly describe the dynamic behavior of the system undergoing large overall rotation,while the zero-order approximation model cannot get the correct results.And AMM.SFPM.BFPM can well describe the deformation of a rotating flexible rectangular plate.展开更多
A set of basic deformation modes for hybrid stress finite elements are directly derived from the element displacement field. Subsequently, by employing the so-called united orthogonal conditions, a new orthogonalizati...A set of basic deformation modes for hybrid stress finite elements are directly derived from the element displacement field. Subsequently, by employing the so-called united orthogonal conditions, a new orthogonalization method is proposed. The result- ing orthogonal basic deformation modes exhibit simple and clear physical meanings. In addition, they do not involve any material parameters, and thus can be efficiently used to examine the element performance and serve as a unified tool to assess different hybrid elements. Thereafter, a convenient approach for the identification of spurious zero-energy modes is presented using the positive definiteness property of a flexibility matrix. More- over, based on the orthogonality relationship between the given initial stress modes and the orthogonal basic deformation modes, an alternative method of assumed stress modes to formulate a hybrid element free of spurious modes is discussed. It is found that the orthogonality of the basic deformation modes is the sufficient and necessary condition for the suppression of spurious zero-energy modes. Numerical examples of 2D 4-node quadrilateral elements and 3D 8-node hexahedral elements are illustrated in detail to demonstrate the efficiency of the proposed orthogonal basic deformation mode method.展开更多
The objective of this investigation is to examine the correctness and efficiency of the choice of boundary conditions when using assumed mode approach to simulate flexible multi-body systems. The displacement field du...The objective of this investigation is to examine the correctness and efficiency of the choice of boundary conditions when using assumed mode approach to simulate flexible multi-body systems. The displacement field due to deformation is approximated by the Rayleigh-Ritz assumed modes in floating frame of reference （FFR） formulation. The deformations obtained by the absolute nodal coordinate （ANC） formulation which are transformed by two sets of reference coordinates are introduced as a criterion to verify the accuracy of the simulation results by using the FFR formulation. The relationship between the deformations obtained from different boundary conditions is revealed. Nu- merical simulation examples demonstrate that the assumed modes with cantilevered-free, simply-supported and free- free boundary conditions without inclusion of rigid body modes are suitable for simulation of flexible multi-body system with large over all motion, and the same physical deformation can be obtained using those mode functions, differ only by a coordinate transformation. It is also shown that when using mode shapes with statically indeterminate boundary conditions, significant error may occur. Furthermore, the slider crank mechanism with rigid crank is accurate enough for investigating boundary condition problem of flexible multi-body system, which cost significant less simulating time.展开更多
Flame stabilization is the key to extending scramjets to hypersonic speeds;accordingly,this topic has attracted much attention in theoretical research and engineering design.This study performed large eddy simulations...Flame stabilization is the key to extending scramjets to hypersonic speeds;accordingly,this topic has attracted much attention in theoretical research and engineering design.This study performed large eddy simulations(LESs)of lifted hydrogen jet combustion in a stepped-wall combustor,focusing on the flame stabilization mechanisms,especially for the autoignition effect.An assumed probability density function(PDF)approach was used to close the subgrid chemical reaction source.The reliability of the solver was confirmed by comparing the LES results with experimental data and published simulated results.The hydrogen jet and the incoming stream were first mixed by entraining large-scale vortices in the shear layer,and stable combustion in the near-wall region was achieved downstream of the flame induction region.The autoignition cascade is a transition of fuel-rich flame to stoichiometric ratio flame that plays a role in forming the flame base,which subsequently causes downstream flame stabilization.Three cases with different jet total temperatures are compared,and the results show that the increase in the total temperature reduces the lift-off distance of the flame.In the highest total temperature case,an excessively large scalar dissipation rate inhibits the autoignition cascade,resulting in a fuel-rich low-temperature flame.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.52038005 and 52201326)the fellowship of China Postdoctoral Science Foundation(Grant No.2022M721883)Tsinghua University Initiative Scientific Research Program.
文摘We present an assumed enhanced strain finite element framework for the simulation of tensile fracturing processes in transversely isotropic rocks.Fractures along the weak bedding planes and through the anisotropic rock matrix are treated with distinct enrichment,and a recently proposed dualmechanism tensile failure criterion for transversely isotropic rocks is adopted to determine crack initiation for the two failure modes.The cohesive crack model is adopted to characterize the response of embedded cracks.As for the numerical implementation of the proposed framework,both algorithms for the update of local history variables at Gauss points and of the global finite element system are derived.Four boundary-value problem simulations are carried out with the proposed framework,including uniaxial tension tests of Argillite,pre-notched square loaded in tension,three-point bending tests on Longmaxi shale,and simulations of tensile cracks induced by a strip load around a tunnel in transversely isotropic rocks.Simulation results reveal that the proposed framework can properly capture the tensile strength anisotropy and the anisotropic evolution of tensile cracks in transversely isotropic rocks.
基金supported by the Fundamental Research Funds for the Central Universities (Grant No.2021FZZX001-14)and ZJU-ZCCC Institute of Collaborative Innovation (Grant No.ZDJG2021005).
文摘This paper presents a finite element framework for imposing frictional contact conditions on embedded fracture faces,implemented by the constant-strain assumed enhanced strain(AES)method,where penalty method is used to impose both non-penetration constraint and Coulomb’s law of friction.The proposed constant-strain AES method for modeling embedded frictional contact can be cast into an integration algorithm similar to those used in the classical plasticity theory,where displacement jump is calculated from the local traction equilibrium at Gauss point,so the method does not introduce any additional global degrees of freedom.Moreover,constant-strain elements are often desirable in practice because they can be easily created automatically for large-scale engineering applications with complicated geometries.As encountered in other enriched finite element methods for frictional contact,the problem of normal contact pressure oscillations is also observed in the constant-strain AES method.Therefore,we developed a strain-smoothing procedure to effectively mitigate the oscillations.We investigated and verified the proposed AES framework through several numerical examples,and illustrated the capability of this method in solving challenging nonlinear frictional contact problems.
文摘In this study, the stress based finite element method is coupled with the boundary element method in two different ways. In the first one, the ordinary distribution matrix is used for coupling. In the second one, the stress traction equilibrium is used at the interface line of both regions as a new coupling process. This new coupling procedure is presented without a distribution matrix. Several case studies are solved for the validation of the developed coupling procedure. The results of case studies are compared with the distribution matrix coupling, displacement based finite element method, assumed stress finite element method, boundary element method, ANSYS and analytical results whenever possible. It is shown that the coupling of the stress traction equilibrium with assumed stress finite elements gives as accurate results as those by the distribution matrix coupling.
文摘The "cascade static lens （CSL） gauge" has a high sensitivity（S） because the emitted electrons repeat the go and back oscillation before they are received by the electrodes. (S=18.6 Pa<sup>-1</sup> (2480 Torr<sup>-1</sup> in a
基金supported by the National Natural Science Foundation of China(Grant Nos.12335008 and 12205160)the support of the Center for High Performance Computing and Initiative Scientific Research Program in Tsinghua University。
文摘By combining femtoscopic interferometry with an optical deblurring algorithm,we present a novel method to image the source in heavy-ion collisions(HICs),while simultaneously determining the interaction strength between particle pairs.The spatial distribution of the emission source has been reconstructed for protons(p)and antiprotons(^(-)p)from the respective pp and^(-)p^(-)p correlation functions in Au+Au collisions at√S_(NN)=200 GeV.Within experimental uncertainties,protons and antiprotons share the same freeze-out distribution showing higher density in the center compared to the widely assumed Gaussian shape.The results evidence the matter–antimatter symmetry in coordinate space at the freeze-out moment before the nucleons are fully randomized in the collisions.
基金supported by the National Natural Science Foundation of China (Grant Nos. 50906098 and 91016028)
文摘A hybrid LES (Large Eddy Simulation)/assumed sub-grid PDF (Probability Density Function) closure model has been devel-oped for supersonic turbulent combustion. Scalar transport equations for all species in a given chemical kinetic mechanism were solved, which are necessary in the supersonic combustion where the non-equilibrium chemistry is essentially involved. The clipped Gaussian PDF of temperature and multivariate ? PDF of composition were used to close the sub-grid chemical sources that appear in the conservation equations. The sub-grid variances of temperature and composition were constructed based on scale similarity approach. A semi-implicit approach based on the PDF model was proposed to tackle the resulting numerical stiffness associated with finite rate chemistry. The model was applied to simulate a supersonic, coaxial H2-air burner, where both the mean and rms (root mean square) results were compared with the experimental data. In general, good agree-ments were achieved, which indicated that the present sub-grid PDF method could work well in simulating supersonic turbu-lent combustion. Moreover, the calculation showed that the sub-grid fluctuations of temperature and major species in the combustion region were of the order of 10%-20% of their rms, while the sub-grid fluctuation of hydroxyl might be as high as 40%-50% of its rms.
文摘A fuzzy adaptive control method is proposed for a flexible robot manipulator. Due to the structure characteristics of the flexible manipulator, the vibration modes must be controlled to realize the high-precision tip position. The Lagrangian principle is utilized to model the dynamic function of the single-degree flexible manipulator incorporating the assumed modes method. Simulation results of the fuzzy adaptive control method in the location control and the trajectory tracking with different tip disturbances are presented and compared with the results of the classic PD control. It shows that the controller can obtain the stable and robust performance.
文摘The 2024 UN Climate Change Conference(COP29)will convene from 11 to 22 November in Baku,Azerbaijan.This year’s conference assumes special significance due to conflicts in some regions,geopolitical tensions and heightened uncertainties in the world.World leaders are expected to report on the progress in implementing their climate commitments.
文摘Ⅰ.Each of the statements below is followed by four alternative answers.Choose the one that wouldbest complete the statement.1.Polysemy is a common feature peculiar to ___.A.English only B.Chinese only C.all natural languages D.some natural languages2.From the ____ point of view,polysemy is assumed to be the result of growth and development of thesemantic structure of one and same word.A.linguistic B.diachronic C.synchronic D.
基金The project supported by the National Natural Science Foundation of China （10272002） and the Doctoral Program from the Ministry of Education of China （20020001032） The English text was polished by Yunming Chen.
文摘A nonlinear dynamic model of a simple nonholonomic system comprising a self-rotating cantilever beam subjected to a unilateral locked or unlocked constraint is established by employing the general Hamilton＇s Variational Principle. The critical values, at which the trivial equilibrium loses its stability or the unilateral constraint is activated or a saddle-node bifurcation occurs, and the equilibria are investigated by approximately analytical and numerical methods. The results indicate that both the buckled equilibria and the bifurcation mode of the beam are different depending on whether the distance of the clearance of unilateral constraint equals zero or not and whether the unilateral constraint is locked or not. The unidirectional snap-through phenomenon （i.e. catastrophe phenomenon） is destined to occur in the system no matter whether the constraint is lockable or not. The saddle-node bifurcation can occur only on the condition that the unilateral constraint is lockable and its clearance is nonzero. The results obtained by two methods are consistent.
文摘In the traditional method of earthquake loss estimation, all the social wealthes are classified according to their structural type and occupational use. Inventory data is collected and the total loss is estimated from each facility class separately. For many regions of the world, however, the vast amount of data required by this method is difficult or impossible to obtain. The traditional method is also unable to estimate quickly the loss from an unexpected disaster earthquake. It is difficult to give the necessary risk information to help the government to rescue and relief the earthquake disaster. This paper proposes a simple estimation method of earthquake loss based on macroscopic economical index of Gross Domestic Product (GDP) and population distribution data. A preliminary nonlinear relation among hazard loss, seismic intensity and social wealth was developed by means of some earthquake disaster records during 1980～1995. This method was applied to analyze several assumed earthquakes. The preliminary analysis results show that the new method is effective and reasonable for quick assessment of earthquake loss.
基金funded by Austrian Science Fund(FWF),grant No.P 24630-B21(to VC)。
文摘Reactive oxygen species(ROS)and kynurenines:Kynurenines represent a relatively heterogenous group of tryptophan metabolites(Figure 1 A).The amino acid tryptophan is metabolized in the humans by the kynurenine or serotonin pathway.For a long time,the kynurenine pathway was assumed primarily to constitute the source for nicotinamide-adenine dinucleotide phosphate,one of the most utilized redox active enzyme cofactors.
基金supported by the Fundamental Research Funds for the Central Universities(WK2070080005)。
文摘Infertility is a major health issue,affecting approximately 15%of couples of child-bearing age.Although nearly half of idiopathic infertility cases are assumed to have a genetic basis,the underlying causes remain largely unknown in most infertile men.
基金the National Natural Science Foundation of China(Nos.11502098 and 11772158)the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province(No.15KJB130003)+1 种基金the Doctoral Scientific Research Foundation of Jiangsu University of Science and Technology(No.120140003)the Fundamental Research Funds for Central Universities of China(No.30917011103)。
文摘Tlie rigid-flexible coupling dynamic modeling theory and the discretization methods of a rotating flexible rectangular thin plate are investigated in this paper.Based on the continuum mechanics,the rigid-flexible coupling dynamic model is established for the flexible rectangular thin plate undergoing large overall rotation,and the coupling term of the deformation which is caused by transverse deformation is considered.Assumed mode method(AMM),spline finite point method(SFPM)and Beizer finite point method(BFPM)are used to describe the deformation of the flexible rectangular plate,and then the dynamic equations of a rotating flexible rectangular thin plate undergoing overall motion are derived by Lagrange^equation of the second kind.The dynamics of a cantilever plate undergoing large overall rotation is simulated via using different dynamic models,and the simulation results of the first order approximation model are compared with those of the traditional zero-order approximation model.It is shown that the first order approximation model with the dynamic stiffening terms can correctly describe the dynamic behavior of the system undergoing large overall rotation,while the zero-order approximation model cannot get the correct results.And AMM.SFPM.BFPM can well describe the deformation of a rotating flexible rectangular plate.
基金Project supported by the National Natural Science Foundation of China(No.10972188)the Fundamental Research Funds for the Central Universities of China(No.2010121073)the Scientific Program of Fujian Province of China(No.2007F3096)
文摘A set of basic deformation modes for hybrid stress finite elements are directly derived from the element displacement field. Subsequently, by employing the so-called united orthogonal conditions, a new orthogonalization method is proposed. The result- ing orthogonal basic deformation modes exhibit simple and clear physical meanings. In addition, they do not involve any material parameters, and thus can be efficiently used to examine the element performance and serve as a unified tool to assess different hybrid elements. Thereafter, a convenient approach for the identification of spurious zero-energy modes is presented using the positive definiteness property of a flexibility matrix. More- over, based on the orthogonality relationship between the given initial stress modes and the orthogonal basic deformation modes, an alternative method of assumed stress modes to formulate a hybrid element free of spurious modes is discussed. It is found that the orthogonality of the basic deformation modes is the sufficient and necessary condition for the suppression of spurious zero-energy modes. Numerical examples of 2D 4-node quadrilateral elements and 3D 8-node hexahedral elements are illustrated in detail to demonstrate the efficiency of the proposed orthogonal basic deformation mode method.
基金supported by the National Natural Science Foundation of China (10872126)the Research Fund of the Doctoral Program of Higher Education of China (20100073110007)
文摘The objective of this investigation is to examine the correctness and efficiency of the choice of boundary conditions when using assumed mode approach to simulate flexible multi-body systems. The displacement field due to deformation is approximated by the Rayleigh-Ritz assumed modes in floating frame of reference （FFR） formulation. The deformations obtained by the absolute nodal coordinate （ANC） formulation which are transformed by two sets of reference coordinates are introduced as a criterion to verify the accuracy of the simulation results by using the FFR formulation. The relationship between the deformations obtained from different boundary conditions is revealed. Nu- merical simulation examples demonstrate that the assumed modes with cantilevered-free, simply-supported and free- free boundary conditions without inclusion of rigid body modes are suitable for simulation of flexible multi-body system with large over all motion, and the same physical deformation can be obtained using those mode functions, differ only by a coordinate transformation. It is also shown that when using mode shapes with statically indeterminate boundary conditions, significant error may occur. Furthermore, the slider crank mechanism with rigid crank is accurate enough for investigating boundary condition problem of flexible multi-body system, which cost significant less simulating time.
基金National Natural Science Foundation of China(Nos.91741205 and 11522222)。
文摘Flame stabilization is the key to extending scramjets to hypersonic speeds;accordingly,this topic has attracted much attention in theoretical research and engineering design.This study performed large eddy simulations(LESs)of lifted hydrogen jet combustion in a stepped-wall combustor,focusing on the flame stabilization mechanisms,especially for the autoignition effect.An assumed probability density function(PDF)approach was used to close the subgrid chemical reaction source.The reliability of the solver was confirmed by comparing the LES results with experimental data and published simulated results.The hydrogen jet and the incoming stream were first mixed by entraining large-scale vortices in the shear layer,and stable combustion in the near-wall region was achieved downstream of the flame induction region.The autoignition cascade is a transition of fuel-rich flame to stoichiometric ratio flame that plays a role in forming the flame base,which subsequently causes downstream flame stabilization.Three cases with different jet total temperatures are compared,and the results show that the increase in the total temperature reduces the lift-off distance of the flame.In the highest total temperature case,an excessively large scalar dissipation rate inhibits the autoignition cascade,resulting in a fuel-rich low-temperature flame.
