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 Select ON THE UNIFIED PHASE-FIELD THEORY FOR DAMAGE AND FAILURE IN SOLIDS AND STRUCTURES: THEORETICAL AND NUMERICAL ASPECTS 1) Wu Jianying Chinese Journal of Theoretical and Applied Mechanics    2021, 53 (2): 301-329.   DOI: 10.6052/0459-1879-20-295 Abstract （540）   HTML （6）    PDF（pc） （6488KB）（444）       Save Cracking-induced damage and fracture are the most commonly encountered failure modes of engineering materials and structures. In order to prevent such failure, it is a prerequisite in structural designs to understand how cracks nucleate, propagate, branch, coalesces and even fragmentation, etc., in solids, and more importantly, to quantify their adverse effects to the loss of integrity and even catastrophic collapse of structures. Aiming to provide a feasible approach in the modeling of damage and quasi-brittle failure in solids, this work presents systematically the theoretical and numerical aspects of the unified phase-field theory proposed recently by the author, with applications to a couple of representative benchmark problems. Being a variational approach for regularized cracks, this theory incorporates intrinsically the strength-based nucleation and energy-based propagation criteria, as well as the energy minimization-oriented path following criterion, in a standalone framework. Not only several popular phase-field models for brittle fracture can be recovered as particular examples, but also a novel model——the phase-field regularized cohesive zone model (or shortly, PF-CZM)——that applies to both brittle fracture and quasi-brittle failure, emerges naturally. This model can be numerically implemented in context of the coupled finite element method. In order to solve efficiently the discretized governing equations, several numerical algorithms are discussed, with the monolithic BFGS quasi-newton method being the most efficient one. Representative two- and three-dimensional numerical examples reveal that the PF-CZM is capable of reproducing complex fracture configurations in both brittle and quasi-brittle solids under quasi-static, dynamic and multi-physical environments. Remarkably, in all cases objective numerical predictions are achieved independent of the incorporated length scale and mesh discretization. Therefore, the PF-CZM can be used as a numerically predictive approach for the modeling of damage and failure in engineering structures. Finally, some research topics deserving further studies are suggested.
 Select ARTIFICIAL NEURAL NETWORK-BASED SUBGRID-SCALE MODELS FOR LARGE-EDDY SIMULATION OF TURBULENCE 1) Xie Chenyu, Yuan Zelong, Wang Jianchun, Wan Minping, Chen Shiyi Chinese Journal of Theoretical and Applied Mechanics    2021, 53 (1): 1-16.   DOI: 10.6052/0459-1879-20-420 Abstract （965）   HTML （42）    PDF（pc） （15651KB）（1436）       Save Large eddy simulation (LES) is an important method to investigate different types of complex turbulent flows, which has been widely applied to the turbulent flows in aerospace, combustion, acoustics, atmospheric boundary layer, etc. Large eddy simulation effectively solves the large-scale motions of turbulence and models the effects of small-scale dynamics on the large-scale structures by using subgrid-scale (SGS) models. Traditional SGS models only use the single-point information based on some simple forms of analytical functions to approximate the SGS terms. Thus, traditional models exhibit quite large relative errors in the a priori study, and have excessive dissipations in the a posteriori study. Recently, machine learning approaches have been widely used to develop turbulence models, including the Reynolds-averaged Navier-Stokes (RANS) models and LES models. In this paper, we review the recent developments of artificial neural network (ANN) methods for SGS models in LES of turbulence. We discuss three different ANN-based SGS models, including artificial neural network mixed model (ANNMM), spatial artificial neural network (SANN) model and deconvolutional artificial neural network (DANN) model. Due to the strong data interpolation capability of artificial neural networks, the new SGS models exhibit improved accuracy in both a priori study and a posteriori study. In the a priori study, the new SGS models can predict the SGS stress much more accurately than the traditional SGS models: the correlation coefficients predicted by new SGS models can be made larger than 99%. In the a posteriori study, the new SGS models can give better predictions on turbulence statistics and instantaneous flow structures, as compared to a variety of traditional SGS models including the implicit LES (ILES), dynamic Smagorinsky model (DSM), and dynamic mixed model (DMM). It is shown that artificial neural network-based methods have strong potentials for the developments of advanced SGS models in the LES of complex turbulence.
 Select EXPERIMENTAL STUDY ON PROPERTIES OF TURBULENT/NON-TURBULENT INTERFACE OVER RIBLETS SURFACES AT LOW REYNOLDS NUMBERS 1) Li Sicheng, Wu Di, Cui Guangyao, Wang Jinjun Chinese Journal of Theoretical and Applied Mechanics    2020, 52 (6): 1632-1644.   DOI: 10.6052/0459-1879-20-211 Abstract （188）   HTML （18）    PDF（pc） （2896KB）（214）       Save The turbulent/non-turbulent(T/NT) interface usually refers to the thin layer that separates turbulent flow from irrotational flow. The research of T/NT interface is of great importance to deepen the understanding of matter, momentum and energy transfer between the turbulent and irrotational flow. The geometrical and dynamical properties of the T/NT interface at different Reynolds numbers over smooth and riblets surfaces with zero-pressure-gradient at different Reynolds numbers are experimentally investigated using two-dimensional time-resolved particle image velocimetry (2D-TRPIV). The Reynolds number range is about $Re_{\tau } \approx$ 400 $\sim$ 1000 in the present experiment. The T/NT interface is detected with the turbulent kinetic energy criterion. The probability density function of the interface position, fractal characteristics and the conditional averaged velocity and vorticity near the interface are analyzed. It is shown that the mean heights of the interfaces are around 0.8 $\sim$ 0.9$\delta_{99}$ under different Reynolds numbers for both smooth and riblets surfaces. The probability density function of the interface position over drag reduction riblets surface is kept the same with the smooth surface, whose distribution agrees well with normal distribution. While for the drag increment riblets surface, the distribution of interface position deviates from the normal distribution and presents positive skewness. The fractal dimension of the interface and the velocity jump across the interface will gradually increase with the Reynolds number under the present experiment condition. Moreover, the dimensionless conditional averaged vorticities have similar distributions in the vicinity of the interface over both smooth and riblets surfaces when $Re_{\tau }$ is less than 1000, correspondingly, the maximum gradient of dimensionless conditional average streamwise velocity is approximately constant.
 Select STATE OF THE ART OF CONSTITUTIVE RELATIONS OF HYPERELASTIC MATERIALS 1) Peng Xiangfeng, Li Luxian Chinese Journal of Theoretical and Applied Mechanics    2020, 52 (5): 1221-1234.   DOI: 10.6052/0459-1879-20-189 Abstract （391）   HTML （34）    PDF（pc） （305KB）（495）       Save Hyperelastic materials are commonly used in practical engineering with the prominent feature that a very large deformation may be produced under a force but the initial state can be completely recovered when the force is removed. Hyperelastic materials are typically nonlinear elastic ones, whose behaviors are in general characterized by their strain energy functions. For several decades, a lot of mathematical models and physical models have been proposed to study their constitutive relations through constructing the form of energy functions. However, a complete constitutive relation suitable for varied deformation modes and the entire deformation range is still the significant issue to expect in this field. This paper summarizes and analyzes the latest research status of constitutive relations of hyperelastic materials from three perspectives: (1) volume change modes including incompressible and compressible ones; (2) deformation modes such as uniaxial tension, shearing, biaxial tension and combined stretch and shear; (3) the entire range of deformation including small deformation, moderate deformation and large deformation. The latest progresses indicate that, in order to comprehensively describe experimental data of a given hyperelastic material and to apply it in practical problems, it is necessary to establish a complete constitutive relationship of compressible hyperelastic materials, which is suitable for varied deformation modes and the entire range of deformation. The authors suggest an implementation procedure for establishing the complete constitutive relationship of an actual hyperelastic material and an approach to construct the strain energy function of a compressible material.
 Select CONSTITUTIVE MODELS AND HYPERGRAVITY PHYSICAL SIMULATION OF SOILS$^{\bf 1)}$ Chen Yunmin,Ma Pengcheng,Tang Yao Chinese Journal of Theoretical and Applied Mechanics    2020, 52 (4): 901-915.   DOI: 10.6052/0459-1879-20-059 Abstract （493）   HTML （43）    PDF（pc） （13172KB）（511）       Save Numerical simulation and physical simulation are two main methods to analyze the settlement and stability of soil mass. As the mathematical equations of the soil stress-strain relationships, constitutive models are the foundations of numerical simulation. Soil is a type of granular materials, leading to three essential characteristics of it including compressive hardening, shear dilatancy and friction. They are the main characteristics differing soils from metals and should be considered in constitutive model of soils. Traditional soil mechanics, which are widely applied in engineering at present, analyze the deformation and strength of soils separately by elastic theory and limit equilibrium theory based on rigid plasticity, respectively. However, the accuracy of their calculation results is generally difficult to satisfy the requirement of quantitative analysis because the essential characteristics of soils cannot be fully reflected. Cam-clay model is the first elasto-plastic constitutive model that can fully reflect the essential characteristics of soils. It unifies the deformation and strength of soils and can well describe the stress-strain relationships of normal consolidated clays; thus, Cam-clay model is regarded as the beginning of modern soil mechanics. Through introducing a unique unified hardening parameter, unified hardening model further develops the Cam-clay model and enlarges the application scope to over-consolidated clays. The authors believe that the challenge of constitutive model research in the future is how to consider the phase change of soil skeleton and the multi-field coupling in soils, to solve significant geotechnical problems in the field of energy, traffic, environment and hydraulic engineering, which cannot be analyzed quantitatively by current models. Due to the effects of scale compression and time compression, hypergravity physical simulation can overcome the disadvantage that stress level in small-scale model is lower than the prototype level in normal gravity physical simulation. Especially, hypergravity physical simulation is very appropriate to the problems of large scale and long duration. Compared with numerical simulation, hypergravity physical simulation has the advantages of being able to test the rationality of soil constitutive models and reveal the unknown features that cannot be described by current models. Finally, an engineering case of large-diameter steel pipe pile analyzed by combining numerical simulation and hypergravity physical simulation was presented.
 Select PROGRESS IN CONSTITUTIVE BEHAVIOR OF ALUMINUM FOAM 1) Jia Ran, Zhao Guiping Chinese Journal of Theoretical and Applied Mechanics    2020, 52 (3): 603-622.   DOI: 10.6052/0459-1879-20-020 Abstract （563）   HTML （54）    PDF（pc） （17934KB）（783）       Save As a typical representation in various metallic foams, the aluminum foam is a kind of new material that integrates both structural properties and functional properties. With the improvement of production technology and the development of national economy, the application of the aluminum foam is becoming deeper and wider in the fields of aerospace engineering, transportation, construction engineering, machinery manufacturing and so forth. The complex load conditions in engineering application put forward higher requirements for the accuracy and practicability of the aluminum foam constitutive models. A great deal of experimental research and finite element numerical analysis on aluminum foam have been carried out by scholars both at home and abroad. With the proposal, verification and modification of various research constitutive models and numerical constitutive models, the understanding of researchers on the mechanical properties of aluminum foam becomes deeper and deeper. First of all, the development of experimental research and finite element numerical analysis on the mechanical properties of aluminum foam is briefly summarized in this paper; after that, the research progress and the present situation of constitutive model of aluminum foam both at home and abroad are emphatically reviewed; at last, aiming at the existing problems, the development trend of foam aluminum constitutive model is discussed and prospected. The important research directions in the existing aluminum foam constitutive model system are as follows: supplementing the characteristic parameters that are required for the characterization of constitutive model; introducing the anisotropic material assumption or the transversely isotropic material assumptions into the model building system; clarifying the weight of hydrostatic compression response and uniaxial compression response in material hardening; establishing hardening models which can truly and accurately reflect the hardening process of aluminum foam, such as the kinematic hardening model; introducing the research results of strain rate effect into the constitutive model and so forth.
 Select DOES STRUCTURE DETERMINE PROPERTY IN AMORPHOUS SOLIDS? 1) Wang Yunjiang, Wei Dan, Han Dong, Yang Jie, Jiang Mingqiang, Dai Lanhong Chinese Journal of Theoretical and Applied Mechanics    2020, 52 (2): 303-317.   DOI: 10.6052/0459-1879-19-368 Abstract （555）   HTML （37）    PDF（pc） （4835KB）（414）    PDF（mobile） （4835KB）（68）    Save The mechanical properties and plastic deformation mechanisms of crystalline solids are mainly determined by their structural defects, e.g., the motion of the versatile dislocations. However, how structures determine properties in non-crystalline solids remains as a major unsolved issue in both solid mechanics, materials sciences, as well as condensed matter physics. Structure determines property is the traditional paradigm of materials science. Following this rule, there are vast experimental characterizations, theoretical studies, and computer simulations appeared in the literature, trying to establish a one-to-one correspondence between a specific structural feature with a unique dynamic property in the general amorphous solids. However, up to date, people gain very little understanding of the structure-property relationships in amorphous solids, not to mention whether there exists any hidden rule behind the structure-property relationships. For this purpose, we focus on the unique features of deformations mechanisms in amorphous solids as well as their microstructure characteristics. Thorough proper samplings of the activation energies of the excitation of these structural parameters by an advanced molecular dynamics technique, we are trying to quantitatively assess the validity of simple short-range structures and medium- to long-range structures in determination of their properties. This is done by examination of the possible correlation between parameters of structures with their activation energies, which implies the level of difficulty in activation of the events. By this we find that the hidden governing rule of structure-property relationship in amorphous solids involves a critical role of spatial autocorrelation length of the specific structural parameter. Constraint is more relevant than geometry itself. If only one structural descriptor presents spatial autocorrelation length up to sub nanometer level, it might effectively predict the mechanical property of amorphous solids; otherwise, the short-range local structures lacking such correlation length fails to predict property. Furthermore, we present a general metric to assess the utilities of structures in determining functions of the amorphous solids, which can be served as a screening rule to seeking for effective structures in amorphous solids.
 Select THE BOUNCE DYNAMICS OF A RISING SINGLE BUBBLE NEAR A VERTICAL WALL 1) Sun Jiao, Zhou Wei, Cai Runze, Chen Wenyi Chinese Journal of Theoretical and Applied Mechanics    2020, 52 (1): 1-11.   DOI: 10.6052/0459-1879-19-228 Abstract （487）   HTML （37）    PDF（pc） （17585KB）（261）       Save By using high speed photography technology combined with shadow method, the motion of a single rising bubble near a vertical wall in stationary water is experimentally studied. The effects of bubble size and the initial dimensionless distance between the nozzle and the wall ($S^{\ast})$ on the rising motion characteristics of bubbles were compared. The wall effect, bubble dynamic mechanism and energy variation rule before and after the collision between bubbles and the walls are analyzed. The results show that for the Reynolds number $Re \approx 580\sim 1100$, and the initial dimensionless distance between the nozzle and the wall $S^{\ast } < 2\sim 3$, the bubbles collide with the wall surface and the bubble trajectory changes from three-dimensional spiral under unconstrained conditions to two-dimensional zigzag periodic motion. However, when $S^{\ast } > 2\sim 3$, the wall effect weakens, and the movement characteristics of the bubble with wall constraint tends to be consistent with that without constraint. Before and after the bubble collides with the wall, the wall effect causes the peak value of transverse velocity to drop to 70% of the original peak value, and vertical velocity drop to 50%. Before the bubble collides with the wall, the vertical velocity variation rule can be predicted by the distance between the bubble center and the wall ($x/R)$ and the modified Stokes number correlation formula. In the process of collision between the rising bubble and the wall surface, the deformation energy of the bubble surface is transmitted to the transverse kinetic energy of the bubble in one direction, so that the deforming bubbles can maintain a relatively constant bouncing motion. The prediction model of the average resistance coefficient of bubbles in the repeated bouncing with the wall surface is proposed, which can describe the dimensionless parameters of Reynolds number, Weber number and Eo number reflected by the experimental data.
 Select ADVANCES IN DYNAMIC MODELING AND OPTIMIZATION OF FLEXIBLE MULTIBODY SYSTEMS 1) Sun Jialiang,Tian Qiang,Hu Haiyan Chinese Journal of Theoretical and Applied Mechanics    2019, 51 (6): 1565-1586.   DOI: 10.6052/0459-1879-19-212 Abstract （772）   HTML （57）    PDF（pc） （7422KB）（739）       Save Flexible multibody system is a kind of mechanical system composed of many flexible components and kinematic pairs, such as flexible robot arms, helicopter rotors, deployable antennas of a satellite, and solar sail spacecraft. Flexible multibody systems serve as useful models in aerospace engineering, vehicle engineering, mechanical engineering, weapon engineering and so on. Recently, with the development of the engineering technology, new challenges have arisen to establish an accurate dynamic model of a flexible multibody system, as well as for the dynamic optimization design of such a flexible multibody system, especially of a flexible multibody system with variable-length components. As a matter of fact, when the component gets more and more flexible, the interactions between the component and the flexible multibody system cannot be disregarded when performing optimization design. The component-based structural optimization, hence, should be extended to the flexible multibody system-based structural optimization. In this review, the research background and significance of the dynamic optimization of flexible multibody systems are firstly surveyed. Three methods for investigating flexible multibody dynamics including flexible multibody systems with variable-length components are briefly outlined, i.e., floating frame of reference formulation (FFRF), geometrically exact formulation (GEF), and absolute nodal coordinate formulation (ANCF). Afterwards, the recent advances are systematically reviewed in the dynamic response optimization, the dynamic characteristics optimization, and the dynamic sensitivity analysis of flexible multibody systems, as well as the structural optimization, i.e., size optimization, shape optimization, and topology optimization of the flexible components in a flexible multibody system. Finally, several open problems are addressed for future studies.
 Select PATHOGENETIC MECHANISMS OF GLAUCOMA------RESEARCH PROCESS ON THE DEFORMATION OF LAMINA CRIBROSA 1) Zhang Ting, Li Long, Song Fan Chinese Journal of Theoretical and Applied Mechanics    2019, 51 (5): 1273-1284.   DOI: 10.6052/0459-1879-18-321 Abstract （718）   HTML （80）    PDF（pc） （21642KB）（450）       Save Glaucoma is the first cause of irreversible blinding eye disease in the world. Glaucomatous optic nerve damage is directly associated with the intraocular pressure, and tight control of intraocular pressure is still the only therapeutic approach available for the treatment of glaucoma, while the pathogenesis of glaucoma remains unknown. It has now been confirmed that the primary site of glaucoma is the lamina cribrosa: the pressure difference between the intraocular pressure and intracranial pressure respectively exerted on the anterior and posterior surfaces of lamina cribrosa can cause the change in the structure and morphology of lamina cribrosa, then the deformation of lamina cribrosa squeezes the optic nerves passing through the lamina cribrosa to make their damages; and finally, the damages produce irreversible visual loss. As a result, the pathogenesis of glaucoma is essentially associated with the mechanical properties of lamina cribrosa and mechanical environment surrounding the lamina cribrosa. Since lamina cribrosa was identified as the primary site of glaucomatous optic nerve damage, it has become the hot spot of glaucomatous optic nerve damage research. As an effective method, we can study the deformation of lamina cribrosa under the effect of intraocular pressure and intracranial pressure by developing mechanical model of lamina cribrosa, and analyze the effect of the deformation of lamina cribrosa on the optic nerve damage. This method has helped us to reveal the mechanism of glaucomatous optic nerve damage and the pathogenesis of glaucoma to some extent. This review will introduce the research progress and the present existing problems on the deformation of lamina cribrosa during glaucoma from the related experimental, theoretical, computational and clinical aspects.
 Select PROGRESS IN MOLECULAR DYNAMICS SIMULATIONS OF SURFACTANT SOLUTION FOR TURBULENT DRAG REDUCTION 1) Wei Jinjia,Liu Fei,Liu Dongjie Chinese Journal of Theoretical and Applied Mechanics    2019, 51 (4): 971-990.   DOI: 10.6052/0459-1879-18-372 Abstract （601）   HTML （42）    PDF（pc） （61824KB）（312）       Save Surfactant additives for turbulent drag reduction have been widely used in energy power and chemical industry. The addition of a small amount of surfactant additives in the pipeline fluid can greatly reduce the flow friction resistance and save energy. In recent years, the research on the mechanism of surfactant drag reduction is also a hot scientific topic. This paper not only summarized our work on the rheology of surfactant solution, surfactant drag reduction, the correlation with surfactant drag reduction and heat transfer, Brownian dynamics simulations in the latest years, but also concerning some works based on the coarse grained molecular dynamics (CGMD) simulations in the past three years, which will be elaborated in detail. The CGMD simulation is developed these years and now widely used in chemistry, biology and many other aspects. Our CGMD simulation work includes three parts, which are the rheology properties and its microstructures of the surfactant solution, the mechanism of turbulent drag reduction by surfactant additives, the analysis of turbulent drag reduction failure phenomenon on the pipeline transportation system. Through reviewing the progress in our CGMD simulation work, we believe that the CGMD simulation method can reasonably explain the rheological behavior of surfactant solutions, and the relationship between the rheology and the surfactant micelle structure can be well studied by using the coarse grained model. The breakage and the recombination behaviors of surfactant micelles can be evaluated from a multidimensional system including the extensional energy, the breakage energy, the maximum reasonable stretching distance, coalescence energy, zeta potential, or hydrophobic driving effect. Besides, the "viscoelasticity theory" can be proved from a molecular scale. Last but not least, the mechanism of turbulent drag reduction failure phenomenon can also be analyzed by CGMD simulation by simulating different failure reasons. At last, we summarize the CGMD simulation work on surfactant in recent years and then the direction of the future work about CGMD simulation work on surfactant is predicted.
 Select A REVIEW ON THE DEFORMATION AND FAILURE OF SEALS IN OILFIELD 1) Zhengjin Wang, Tiejun Wang Chinese Journal of Theoretical and Applied Mechanics    2019, 51 (3): 635-655.   DOI: 10.6052/0459-1879-19-061 Abstract （1013）   HTML （51）    PDF（pc） （45847KB）（549）       Save Oil and gas will still be the major energy resource of human in a long period. Sealing is one of the most important operations in oil and gas exploitation, which plays an essential role in horizontal drilling and hydraulic fracturing, booming the exploitation of unconventional oil and gas like shell gas. The quality of sealing is critical to the performance and safety of wells. The deformation and failure of seals may lead to severe environmental pollution, losses of life and property. Therefore, it is important to study the mechanical behavior of seals in oilfields systematically and deeply for safety assessment as well as better sealing of wells. Cement and swellable elastomers are the most widely used sealing materials in oilfield. Well cementing has been developed for more than 100 years. While swellable elastomeric packer is relatively new. In this review, we present the recent advances in the studies of the deformation and failure of cement sheath and swellable packers. The first part focuses on the cement sheath, including stress evolution during the solidification process and production stage, failure modes, and failure criteria. In the second part, emphasis is placed on the work principle, basic theory, test method, failure modes and instability of swellable packers.
 Select RESEARCH PROGRESS OF PLASMA/MHD FLOW CONTROL IN INLET Yiwen Li, Yutian Wang, Lei Pang, Lianghua Xiao, Zhiwen Ding, Pengzhen Duan Chinese Journal of Theoretical and Applied Mechanics    2019, 51 (2): 311-321.   DOI: 10.6052/0459-1879-18-290 Abstract （690）   HTML （39）    PDF（pc） （11997KB）（315）       Save In order to realize wide-speed-range flight of high-speed vehicle, it is of great importance to maintain the performance of inlet at off-design. Compared with traditional passive control methods, plasma and magnetohydrodynaimic(MHD) flow control are novel active flow control methods, and they have attracted extensive attention worldwide, as a result of some advantages, such as simple structure, fast response and feedback control based on actual flight condition, etc. In this paper, the main applications of plasma and MHD in hyper/supersonic inlet and dynamics models are introduced. When the inlets are in supercritical state, the shockwaves can be push back to cowl as a result of the virtual surface produced by plasma and MHD, which is based on the effect of thermal chocking. This technology is expected to applied on the hypersonic missile if only short-time flow control is required. The plasma and MHD actuators can be mounted flush on the wall, so that its requirement for thermal protection is less than that of roughness at hypersonic flight condition. The applications of high-frequency plasma and MHD actuation to produce disturbances in boundary layer have been validated through supersonic wind tunnel experiment, and the physical mechanism can be interpreted from the point of stability theory. The innovative developments of plasma source technology and the way of actuation, as well as coupled model of plasma and fluid dynamics and efficient algorithms are required in future, which can provide guidance for engineering application.
 Select ADVANCES IN DYNAMICS AND VIBRATION CONTROL OF LARGE-SCALE FLEXIBLE SPACECRAFT Cao Dengqing, Bai Kunchao, Ding Hu, Zhou Xubin, Pan Zhongwen, Chen Liqun, Zhan Shige Chinese Journal of Theoretical and Applied Mechanics    2019, 51 (1): 1-13.   DOI: 10.6052/0459-1879-18-054 Abstract （1199）   HTML （65）    PDF（pc） （23857KB）（910）       Save With the gradual implementation of the major projects in aerospace engineering, the spacecraft design is heading towards the direction of ultra-high speed, large scale and multi-function, and its launching and operating environment is likely to worsen. The problems on vibration and active and/or passive control in spacecraft launching process, dynamic modeling and response analysis of large flexible spacecraft in orbit, and the hybrid control of structural vibration and attitude maneuver of the spacecraft are getting more and more complicated and difficult to deal with. The enlarged scale and increased flexibility of the spacecraft structure (suchas the large aperture antenna and solar panels) present a challenge to the ground test and semi-physical simulation. The dynamics and control problems involved in the large-scale flexible spacecraft such as the whole-spacecraft vibration isolation in the spacecraft lunching process, the dynamic modeling and vibration response analysis of large-scale flexible structures, and the coupling dynamics and hybrid control of structural vibration and attitude and or orbital maneuver of the large-scale flexible spacecraft are presented. The key scientific issues seriously in the fields of spacecraft dynamics and control could then be extracted as follows: the dynamic modeling and order reduction of multi-rigid flexible body systems (including the dynamic modeling of the flexible structure with large deformation, the collaborate simulations with multi-solvers, model reductions, the analytical approach for the dynamic modeling of composite structures, etc.), the construction of state space model of complicated structures and its controllability investigation (including the theoretical and experiment methods of the state space model formulation, the observability and controllability of the control system for complex structures, etc.), and the design of hybrid control law of structural vibration and attitude maneuver for the large-scale flexible spacecraft (concerning the robust hybrid control of attitude maneuver and structural vibration, the collaborative control of actuating mechanism and piezoelectric actuator, etc.
 Select SHOCK TUNNEL THEORY AND METHODS FOR DUPLICATING HYPERSONIC FLIGHT CONDITIONS 1) Jiang Zonglin, Li Jinping, Hu Zongmin, Liu Yunfeng, Yu Hongru Chinese Journal of Theoretical and Applied Mechanics    2018, 50 (6): 1283-1291.   DOI: 10.6052/0459-1879-18-238 Abstract （991）   HTML （27）    PDF（pc） （591KB）（559）       Save This paper reviews the scientific research philosophy and discipline layout of Prof. Yung-huai Kuo in the field of thermo-chemically reacting flows occurring in hypersonic flights, and summarizes the research progress in shock tunnel theories and methods for duplicating hypersonic flight conditions. The work has been achieved from 50 years effort dedicated by the High Temperature Gas Dynamics Research Team founded by Prof. Kuo. Rapid generation and rapid application of high temperature gas are an ideal method for wind tunnel operation, and a shock tunnel is such an experimental facility. The fundamental theory and governing equations for shock tunnel are presented first, and these demonstrate the unique advantages of shock tunnel technology for the ground-based testing of hypersonic vehicles. Then the feasibility, basic equations and key problems in the shock tunnel technology for duplicating required hypersonic flight conditions are discussed. Aiming at solving the key problems, a theory is proposed for the detonation-driven shock tunnel from the technical development of detonation driver and its engineering application/verification. Finally, the tailored condition for the detonation-driven shock tunnel is introduced, and lays the foundation for the operation of shock tunnels with long test time. This condition is one of the most difficult problems encountered in developing high-enthalpy shock tunnels. The problem has been investigated for decades, but not solved perfectly. With the proposed theory and methods, several high-enthalpy tunnels are developed for covering the full flight envelope of hypersonic vehicles and its applications show that the theory proposed here is successful and important for aerodynamic and kinetic study in the hypersonic research field.The Team's research work on the hypersonic ground testing facilities has realized the strategic goal of Prof. Kuo's discipline planning, and a world leading research platform was established for exploring hypersonic thermo-chemically reacting flows.
 Select ON THE DEGREES OF FREEDOM OF A MECHANICAL SYSTEM 1) Hu Haiyan Chinese Journal of Theoretical and Applied Mechanics    2018, 50 (5): 1135-1144.   DOI: 10.6052/0459-1879-18-219 Abstract （1595）   HTML （20）    PDF（pc） （1970KB）（706）       Save The definition of degrees of freedom of a mechanical system originated from the number of independent coordinates to describe the system configuration. The definition turned to be the number of independent variations of generalized coordinates after the studies on non-hololomic constraints in the development of analytic mechanics. The paper points out that the above definition of degrees of freedom has some flaws for the mechanical system with non-holonomic constraints and may impose excessive limits on the system dynamics. The paper, hence, studies the accessible state manifold of a mechanical system with non-holonomic constraints in the state space and shows that the dimensions of the accessible state manifold is equal to the number of minimal unknown variables to describe the system dynamics, governed by a set of ordinary differential equations of the first order, such as the Gibbs-Appell equations together with the relation of generalized velocities and psudo-velocities. Then, the paper defines the degrees of freedom of a mechanical system as a half of the dimensions of the accessible state manifold. Afterwards, the paper demonstrates how to understand the concept of a half degree of freedom of a mechanical system with a single non-holonomic constraint via two case studies, that is, the vibration system having a viscoelastic mounting and the sleigh system moving on an inclined plane, presenting the relation between a half degree of freedom and the two neighboring integer degrees of freedom. Furthermore, the paper gives two examples of mechanical systems, each of which has two non-holonomic constraints and results in the reduction of a single degree of freedom, and addresses the dimensions of tangent and cotangent bundles of the two systems.
 Select ANALYSIS OF DSMC REACTION MODELS FOR HIGH TEMPERATURE GAS SIMULATION 1) Yang Chao, Sun Quanhua Chinese Journal of Theoretical and Applied Mechanics    2018, 50 (4): 722-733.   DOI: 10.6052/0459-1879-18-056 Abstract （829）   HTML （9）    PDF（pc） （7207KB）（563）       Save The non-equilibrium phenomenon of thermochemical coupling has been a difficult problem in high temperature aerothermal dynamics, and hinders to analyze phenomena such as cell structure of detonation wave and ignition speed of low temperature combustion. In this paper, typical chemical reaction models (TCE, VFD, QK models) employed in the direct simulation Monte Carlo (DSMC) simulation are analyzed using two examples (namely, N $2$ dissociation at high temperature, and chain displacement reaction in H $2$ ? O $2$ combustion) from microscopic reaction probability, vibrational state specific reaction rates, total reaction rate under thermal nonequilibrium condition, and post-collision redistribution of internal energy. It is found that the probability distribution of vibrational energy of reacted molecules deviates from the equilibrium Boltzmann distribution for both the high temperature dissociation reaction having high activation energy and the chain displacement reaction having low activation energy. The VFD model with strong vibrational favored contribution can predict well the high temperature dissociation reaction, whereas the TCE model (a special case of VFD model) and QK model are better for the chain displacement reaction. Besides, the post-collision redistribution of internal energy should follow the principle of detailed balance, as small deviations may cause inequality between the translational and vibrational energy under final equilibrium state. The DSMC simulation results also show that the vibrational favor of chemical reactions has an obvious effect on the thermochemical coupling process. Particularly, because molecules having high vibrational energy are more easily to have chemical reactions, the decrease of the average vibrational energy of the gas will affect the subsequent chemical reactions.
 Select WALL-MODEL FOR LARGE-EDDY SIMULATION AND ITS APPLICATIONS Wu Ting, Shi Beiji, Wang Shizhao, Zhang Xing, He Guowei Chinese Journal of Theoretical and Applied Mechanics    2018, 50 (3): 453-466.   DOI: 10.6052/0459-1879-18-071 Abstract （1267）   HTML （18）    PDF（pc） （9520KB）（827）       Save Large-eddy simulation (LES) is an important method to investigate unsteady turbulent flows. The cost of the wall-resolved LES is comparable to that of direct numerical simulation, which prevents the applications of the LES to wall-bounded turbulences at high Reynolds numbers. The grid length would be of the order of the viscous length to resolve the near-wall flow structures, which causes the prohibitive computational cost of the wall-resolved LES. Wall-models circumvent the flow details near the wall to avoid resolving all the flow structures near the wall, which significantly reduce the computation cost and have been successfully combined with the LES for turbulent flows. We discuss the basic idea of wall-models for LES and review the wall-stress models with implementation details. The construction and characteristics of the equilibrium models and the two-layer models are discussed in detail. The limitations of the wall-stress models and their improvements to account for the non-equilibrium effects are also discussed. We review the state of the art of the wall shear stress models and provide a hierarchical diagram for the current models. Finally, we present the applications of the Werner-Wengle model to the LES of flows over periodic hills.
 Select DIRECT NUMERICAL SIMULATION OF HYPERSONIC SHOCK WAVE AND TURBULENT BOUNDARY LAYER INTERACTIONS 1) Tong Fulin, Li Xin, Yu Changping, Li Xinliang Chinese Journal of Theoretical and Applied Mechanics    2018, 50 (2): 197-208.   DOI: 10.6052/0459-1879-17-239 Abstract （1208）   HTML （8）    PDF（pc） （14581KB）（809）       Save The peak of local thermal load might be severe due to the interactions of hypersonic shock wave and turbulent boundary layer. It has significant effect on the aerodynamic performance and flight safety of vehicle. Most previous studies on the interaction in hypersonic condition were based on the Reynolds-averaged methods, the corresponding direct numerical simulation is relatively scarce. The direct numerical analysis of hypersonic shock wave and turbulent boundary layer interaction are helpful to the understanding of the relevant mechanisms and the improvement of existing turbulent modes and sub-grid stress models. Numerical analysis of hypersonic shock wave and turbulent boundary layer interactions in a 34° compression ramp are carried out by means of direct numerical simulation for a free-stream Mach number $M ∞ = 6.0$ . Based on the Reynolds stress anisotropy tensor, the evolution of turbulent boundary layer along the compression ramp is analyzed. The compressibility effects on turbulent kinetic energy and its transport mechanism are studied through item by item analysis of transport equation. Using dynamic mode decomposition method, the characteristic of unsteadiness in the interaction region is investigated. It is found that along the flow developing downstream, the turbulent state in the near wall region is gradually turned into two-component turbulence from two-component axisymmetric state. The turbulence in outer region approaches the isotropic state from axisymmetric expansion. The results exhibit that there exist significant compressibility effects in the interaction region. The pressure-dilation correlation in turbulent kinetic energy budgets is enhanced significantly. However, it has little effect on the dilatational dissipation. The low-frequency oscillation in hypersonic compression ramp is characterized by the breathing motion of separation bubble. According to the spatial structure of low frequency dynamic modes, the unsteadiness is strongly associated with the separated shear layer.
 Select EXPERIMENTAL STUDY OF HYPERSONIC OVERFLOW COOLING Yuan Chaokai, Li Jinping, Chen Hong, Jiang Zonglin, Yu Hongru Chinese Journal of Theoretical and Applied Mechanics    2018, 50 (1): 1-8.   DOI: 10.6052/0459-1879-17-289 Abstract （919）   HTML （6）    PDF（pc） （10064KB）（583）       Save Hypersonic overflow cooling is a new type of aircraft thermal protection method. The basic idea is that the overflow hole is arranged in the high heat flux area, and the coolant poured out in an overflow way.The liquid film spreads through the aircraft surface friction forming a thermal buffer layer to reduce the surface heat flux. Now, the overflow cooling technology is still in the exploratory stage, and a large number of experimental verification and mechanism research work need to do. In this paper, wind tunnel experiment platform for overflow cooling was build, adopting the heat flux measurement, liquid film thickness measurement and liquid film motion observation technology. The feasibility of applying overflow cooling to hypersonic thermal protection was verified, and the characteristics of liquid film flow under hypersonic flow field were preliminary analyzed. Reserch results show that: (1) In hypersonic flow field, the liquid film can be formed on the vehicle surface , and effectively isolate the external high temperature air to reduce the surface heat flux; (2) On wedge surface, the leading velocity of the liquid film gradually decelerate. Increase coolant flow rate, the liquid film thickness change is not obvious, but the leading velocity of liquid film will increase; (3) Surface waves exist in liquid film, and evolve in time and space direction, which leads to slight perturbation of liquid film thickness; (4) There is a lateral expansion phenomenon in the liquid film layer, that is, the width of the liquid film is greater than that of the overflow hole. The reason is that the liquid film layer don’t match the flow field boundary condition, and there is pressure gradient, forcing the coolant to flow to low pressure area, thus broadening the liquid film layer.
 Select LARGE EDDY SIMULATION ON THE EVOLUTION AND THE FAST-TIME PREDICTION OF AIRCRAFT WAKE VORTICES Lin Mengda, Cui Guixiang, Zhang Zhaoshun, Xu Chunxiao, Huang Weixi Chinese Journal of Theoretical and Applied Mechani    2017, 49 (6): 1185-1200.   DOI: 10.6052/0459-1879-17-198 Abstract （1074）   HTML （10）    PDF（pc） （18448KB）（812）       Save As the commercial air transport increasing in China, the problem of flight delays is increasingly serious. Wake separation (the minimon separation between the leading and following aircraft to avoid wake encounter) limits the capacity of airports. Aimed at this problem, this paper study the evolution of wake vortex with large eddy simulation (LES). The self-adaptive grid method is applied to the LES of wake vortex to improve the computation efficiency and a lift-drag model is applied to the wake vortex generation process to simulate the roll up phase. Based on the LES wake vortex evolution database, a fast-time wake separation prediction system is established. Given the real time ambient wind field and the aircraft parameters, the prediction system can output the suggested wake separation. The results show that under the average wind condition in Beijing Capital International Airport in 2014, the current wake separation can be reduced by 7%~50% with the established system, thus the airport capacity can be considerably improved.
 Select SIZE EFFECT OF THE INTERFACE ENERGY DENSITY IN BI-NANO-SCALED-METALLIC PLATES Wang Shuai, Yao Yin, Yang Yazheng, Chen Shaohua Chinese Journal of Theoretical and Applied Mechani    2017, 49 (5): 978-984.   DOI: 10.6052/0459-1879-17-142 Abstract （912）   HTML （4）    PDF（pc） （35255KB）（562）       Save The interface free energy density is an important quantity characterizing the mechanical property of interface in nanocomposite systems. In this paper, molecular dynamics simulation method is adopted to investigate the interface energy density of different FCC metallic bi-nano-scaled plates. The morphology of the interface crystal structure and the interface effect on the atomic potential are analyzed. It is found that interface atoms have periodically wrinkled rarefied or serried configurations, and the potential energy of interface atoms is also periodically distributed. The potential energy of atoms near the interface is obviously different from that of atoms inside the nano-plates. Both the Lagrange interface energy and the Eulerian one increase with the increase of the thickness of the bi-material, which approach the interface energy of a bulk bi-material finally.
 Select REVIEW OF BANDGAP CHARACTERISTICS AND ACOUSTO-OPTICAL COUPLING IN PHOXONIC CRYSTALS Ma Tianxue, Su Xiaoxing, Dong Haowen, Wang Yuesheng, Zhang Chuanzeng Chinese Journal of Theoretical and Applied Mechani    2017, 49 (4): 743-757.   DOI: 10.6052/0459-1879-17-130 Abstract （1029）   HTML （12）    PDF（pc） （28794KB）（1293）       Save Phoxonic crystals are periodic structures which possess photonic and phononic bandgaps simultaneously. Phoxonic crystals can be applied as systematic platforms for manipulating electromagnetic and elastic waves simultaneously, and can be utilized in various fields such as optical, acoustic and acouto-optical devices, and cavity optomechanics. This paper firstly introduces the basic concepts of phoxonic crystals, including the constituting materials, their classifications according to spatial periodicity, the numerical calculation methods of band structures. We elaborate the characteristics of phoxonic dual bandgaps for different systems, and the topology optimization method applied in optimizing the bandgap width of phoxonic dual bandgaps. The field of cavity optomechanics, as well as the quasistatic method and optomechanical coupling coefficient method for evaluating the acousto-optical coupling strength are introduced. The acousto-optical coupling phenomena in various phoxonic crystal structures are summarized. Then this paper introduces the research works related to phoxonic crystal waveguides and sensors. Finally, we outline the prospects of phoxonic crystals based on state of the art, including the enhancements of acousto-optical interaction in phoxonic crystal cavities,the investigations of three-dimensional phoxonic crystals, the designs of different phoxonic metamaterials, the phoxonic crystal device designs and related applications, and so on.
 Select PROBLEMS AND CHALLENGES OF MECHANICS IN SHALE GAS EFFICIENT EXPLOITATION Liu Zhanli, Zhuang Zhuo, Meng Qingguo, Zhan Shige, Huang Keh-Chih Chinese Journal of Theoretical and Applied Mechani    2017, 49 (3): 507-516.   DOI: 10.6052/0459-1879-16-399 Abstract （1216）   HTML （16）    PDF（pc） （229KB）（2194）       Save Shale gas is unconventional natural gas stored in shale in free or absorbed forms and sometimes in free fluid phase. The exploitation of shale gas has become a promising field of green energy development in China. Although great success has been achieved in shale gas revolution in North America with the technique of hydraulic fracturing, there is only 5%～15% of the stored oil and gas could be exploited, which is still a puzzle for petroleum engineers. Compared with the North America, China's shale gas reservoirs are deep burial, the geologic construction conditions are complicated and natural quality is low, therefore, efficient exploitation is facing more difficulties and challenges. In recent years, aiming at the national major energy strategy and the frontier of technological development, China's academia and industry have carried out the preliminary study on some of the key scientific and technical issues. Around the new issues encountered in the shale gas extraction in Sichuan and Chongqing areas in recent three years, this paper introduces and summarizes the key mechanics problems and challenges that the high efficient shale gas extraction is facing, mainly includes the multifield coupling safe and high quality drilling mechanics, hydraulic fracturing and multi-scale fracture network formation mechanism and multi-scale seepage and desorption mechanism of shale gas, to solve the challenges in deep exploitation below 3500 meters in China, such as geologic sedimentation, different fracture development, increasing overburden pressure, the change of horizontal stress, etc. The deep shale gas exploitation is not only to adapt to the national energy demand, but also has scientific and engineering significance. To realize the efficient exploitation of shale oil and gas, it needs the interdisciplinary collaboration of mechanical engineering, petroleum engineering, geophysics, chemical engineering and environmental engineering to carry out basic theoretical research, physical simulation, numerical simulation and field experiment. It has been recognized that interdisciplinary research is the bridge and the key to breakthrough the technology bottleneck and realize the efficient exploitation of shale gas. It is necessary of the deep collaboration between mechanics, petroleum engineering, earth science and other disciplines to promote the development of shale gas and other unconventional oil and gas resources.
 Select ESSENTIAL ISSUES AND THEIR RESEARCH PROGRESS IN TUNNEL AND UNDERGROUND ENGINEERING Zhang Dingli Chinese Journal of Theoretical and Applied Mechani    2017, 49 (1): 3-21.   DOI: 10.6052/0459-1879-16-348 Abstract （1311）   HTML （9）    PDF（pc） （25170KB）（1594）       Save As three essential issues in tunnel and underground engineering contain stability of surrounding rocks, interact relation between support and surrounding rock as well as structural dynamic response of a support system are also the key problems in subject study.All around these issues, this paper emphatically analyzes the surrounding rock mechanical characteristics and load effect, and establishes mechanical model of internal and external surrounding rock.Based on the study of stability of structure layer in model above, analytic formula of ground reaction curve and computing method of surrounding rock load effect are given.According to the analysis of interact relation between support and surrounding rock, this interactional dynamic process is divided into four stages:free ground deforming, advance supporting, preliminary supporting, secondary supporting, thus description method of the dynamic process is raised.On idea of generalized load and special load, basic functions of support: "mobilizing" and "assisting" are proposed, then their implementations are clarified, which are"mobilizing" surrounding rock to bear the load reinforcement by applying surrounding rock reinforcement, advance reinforcement and bolt support, "assisting"surrounding rock to bear the load by using the supporting structure.Aiming at the complex tunnel support system, the concept of dynamic optimization of multi-objective and staged synergistic effect is put forward, which can realize the coordination of various supporting structures in terms of time and space to improve the reliability.In view of the safety characteristics of extremely unstable complex surrounding rock, the safety accident mechanism model of three patterns is established.A new concept of safety classification is put forward based on the characteristics of engineering response, and a gradation index system and classification method are established.On account of the underwater tunnel and water-enriched surrounding rock, three patterns of water inrush mechanism model are established, and the theory and method of safety control based on deformation control of surrounding rock are put forward.At last, the hotspot and core problems of the discipline development are analyzed and prospected.
 Select REVIEW OF RESEARCH PROGRESSES OF THE QUANTIFYING JOINT ROUGHNESS COEFFICIENT Chen Shijiang, Zhu Wancheng, Wang Chuangye, Zhang Fei Chinese Journal of Theoretical and Applied Mechani    2017, 49 (2): 239-256.   DOI: 10.6052/0459-1879-16-255 Abstract （994）   HTML （1）    PDF（pc） （6641KB）（1715）       Save The joint roughness coefficient (JRC) method was suggested by International Society for Rock Mechanics to estimate joint roughness in 1978. Although this method is widely used in engineering practices, there is a shortcoming of subjectivity and experience relied on investigator. So, the research works about quantifying joint roughness were developed by authors in the world. In this article, firstly, we introduced the research progress about quantitative characterization on both joint profiles and discontinuities roughness, and summarized the results of relationship between the parameters and JRC. Secondly, we evaluated the essential properties and applicability of each parameter, pointed out the problems about the parameters in obtained process including sampling interval influence, deciding triangulation of an elementary surface and determining the weight of each parameter in the comprehensive parameters method. At the same time, we gave the author's ideas of solving the problems. In addition, we discussed anisotropy and size effect of surface roughness in detail, on which are focused by researchers. Finally, we predict that fractal dimension remain a method to describe surface roughness and 3D printing technology can be helpful to research anisotropy and size effect of rock discontinuities roughness.
 Select FRACTIONAL DERIVATIVE MODELING OF FREQUENCY-DEPENDENT DISSIPATIVE MECHANISM FORWAVE PROPAGATION IN COMPLEX MEDIA Cai Wei, Chen Wen Chinese Journal of Theoretical and Applied Mechani    2016, 48 (6): 1265-1280.   DOI: 10.6052/0459-1879-16-186 Abstract （993）   HTML （5）    PDF（pc） （4444KB）（1474）       Save The existing experimental data indicate that the attenuations of acoustic waves propagating in complex media always exhibit a non-integer power-law dependence on frequency. Such phenomenon is di cult to be characterized by traditional damping wave equation or approximate thermo-viscous wave equation, which can only describe the frequency independent or frequency-squared dependent attenuation, respectively. With the dynamic development and wide applications of fractional calculus, wave equations with fractional derivative terms have been successfully applied to depicting the frequency dependent attenuation. Based on the research achievements of our group, this paper aims at presenting a review of the various fractional derivative wave equations, discussing the corresponding mechanical constitutive relationships and statistical interpretation, and laying the foundation for the in-depth study in the future. The time-and space-fractional derivative wave equations for soft matters are introduced, which can be classified into two groups:the constitutive models and the phenomenological models. The connections and di erences between such models are also discussed. Then, the successful applications of fractional derivative in modeling wave propagation in porous media are also summarized. The statistical interpretation for the power-law dependent exponent covering[0, 2] is presented via linking the space-fractional diffusion equation with Lévy stable distribution. Finally, the key problems in such area for future explorations are highlighted.
 Select DEVELOPMENT STATUS, TREND AND THE PROBLEMS OF MECHANICS OF TIDAL CURRENT ENERGY Zhang Li, Li Zhichuan Chinese Journal of Theoretical and Applied Mechani    2016, 48 (5): 1019-1032.   DOI: 10.6052/0459-1879-15-299 Abstract （1443）   HTML （21）    PDF（pc） （25548KB）（1015）       Save As energy and climate change are currently two of the most important issues in society, many governments in the world are paying more attentions to renewable energy. Tidal current energy is a kind of ocean renewable energy; its resources are rich and its development prospects are very good. Since the 21st century, the development of tidal current energy has made great progress worldwide; a variety of tidal current energy generating devices has been in stage of demonstration project or stage of pre-commercialization. In this aspect, people are faced with the key mechanical problems of how to use all kinds of conversion devices to improve the conversion efficiency of tidal current energy. The situation of tidal current energy resources in China, the latest status of development and utilization at home and abroad, key technologies for tidal current energy utilization and utilization prospect of tidal current energy have been reviewed. Finally, the suggestion of technological development has been given combining with the present situation in China. The main contents of this paper are as follows:First, it is about the generation, characteristics and distribution of tidal current energy. The generation of tidal current, the characteristics of tidal current energy and the distribution of tidal current energy resources are mainly introduced in this part. Second, it's the development status of the technologies of tidal current energy. The process of the technologies for tidal current energy utilization and the development status in China and abroad are introduced. Third, the key technologies for tidal current energy utilization are described, including the capture and conversion of energy, transmission systems and turbines, array layout of tidal current energy conversion, construction of offshore power grid, support structures, etc. Fourth, it is the prospect of development and utilization of tidal current energy. The research focus of tidal current energy technologies at home and abroad are pointed out. Finally, the suggestions and prospects for the development and utilization of tidal current energy in China are given.
 Select A CRITICAL REVIEW ON CRITERIA OF FRACTURE MECHANICS Ji Xing Chinese Journal of Theoretical and Applied Mechani    2016, 48 (4): 741-753.   DOI: 10.6052/0459-1879-16-069 Abstract （1653）   HTML （12）    PDF（pc） （1396KB）（2605）       Save In this paper, the K criterion for linear fracture mechanics, the G criterion for interfacial fracture mechanics, and the J criterion for elastoplastic fracture mechanics has been briefly reviewed, from the preliminary works of Inglis and Griffith to the foundation contributions of Irwin and Rice. Recently, on the basis of the G criterion for interfacial fracture mechanics, the K criterion for interfacial fracture mechanics was derived. This shows that the criteria of fracture mechanics need to be further improved. Thereupon, some debatable issues in the criteria of fracture mechanics are raised. Then, the strain discontinuity at the crack tip is assumed as the source of the stress singularity at a crack tip, and the physical meaning of the stress intensity factor is discussed. At the end, it is concluded that an improved and reliable analysis for the elastoplastic stress field near a crack tip is the key for the establishment of a criterion of elastoplastic fracture mechanics, and an improved and reliable analysis for the elastoplastic stress field near a crack tip depends on the determination of the magnitude of the stress at the crack tip, which caused the strain discontinuity at the crack tip.
 Select NUMERICAL STUDY ABOUT AERODYNAMIC CHARACTERISTICS AND FLOW FIELD STRUCTURES FOR A SKIN OF AIRFOIL WITH ACTIVE OSCILLATION AT LOWREYNOLDS NUMBER Liu Qiang, Liu Zhou, Bai Peng, Li Feng Chinese Journal of Theoretical and Applied Mechani    2016, 48 (2): 269-277.   DOI: 10.6052/0459-1879-15-188 Abstract （1011）   HTML （1）    PDF（pc） （10494KB）（1574）       Save For the poor aerodynamic performance of airfoil at low Reynolds number, the paper targets the active oscillation for flexible skin of airfoil in order to improve its aerodynamic characteristics and flow field structures.Roe method with preconditioning technique was used to solve the unsteady compressible N-S equations and simulate the flow for NACA4415 airfoil at low Reynolds number.The aerodynamic force characteristics and laminar flow separation structures were compared by time-average and unsteady methods when the skin is static or oscillating.Preliminary studies indicate when the flexible skin actively oscillates with appropriate amplitude and frequency, the time-average lift and drag characteristics increase significantly.The separation bubble structures transform from trailing-edge laminar separation bubble to classic long laminar separation bubble.The separation position moves downstream and the separation region reduces.On this basis, the paper studied the alterations of the unsteady flow structures and pressure coefficient distributions more detailed during one period with the two states of the skin.The flow field structures and the pressure distributions keep steady in the front part of separation region at a static skin.The flow approximates steady separation and the unsteady flow phenomena like Karman vortex streets only appear at the trailing edge.Otherwise, for the oscillating skin, the unsteady vortexes generate near the separation position, subsequently move, and then shed along the airfoil surface.The flow presents unsteady separation and displays a wide range of pressure oscillation.Owing to the oscillating skin, the fluid movements are closer to the wall.The large scale laminar flow separation phenomenon is suppressed apparently.