力学学报

EXPERIMENTAL STUDY OF HYPERSONIC OVERFLOW COOLING

Yuan Chaokai, Li Jinping, Chen Hong, Jiang Zonglin, Yu Hongru

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.

2018, 50(1): 1-8. doi: 10.6052/0459-1879-17-289

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NUMERICAL STUDY OF RESPONSE PERFORMANCE OF VORTEX-INDUCED VIBRATION ON A FLEXIBLE CYLINDER WITH PINNED-PINNED BOUNDARY CONDITION

Gao Yun, Zou Li, Zong Zhi

Nowadays, the study methods of VIV on long flexible cylinders contain experimental method, computational fluid dynamic method and semi-empirical method. Due to the expensive cost of experimental method and the long computational time of computational fluid dynamics method, a semi-empirical method study based on a wake oscillator model has been conducted to determine the response performance of vortex-induced vibration (VIV) on a long flexible cylinder with pinned-pinned ends under linearly sheared currents. The coupled model of the structural oscillator of a flexible cylinder and wake oscillator is established, and then the model is discretized and solved based on a standard central finite difference method of the second order. The VIV response characteristics including vibration wavelength, vibration frequency, vibration displacement and frequency varied with time at different shear parameters are compared. The numerical results show that the VIV response is composed of standing wave response and travelling wave response. For a cylinder with small dimensionless bending stiffness, the standing wave response is dominated near the ends while the travelling wave response is dominated near the middle part. However, for a cylinder with large dimensionless bending stiffness, the standing wave response is dominated along the entire cylinder span. As the sheared current increases, the vibration wavelength and displacement decrease, however, the vibration frequency and frequency width increase.

2018, 50(1): 9-20. doi: 10.6052/0459-1879-17-340

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Numerical simulation of vortex-induced vibration of a flexible cylinder exposed to shear flow at different shear rates

Chunning Ji, Yang Hua, Dong Xu, Guoyuan Xing, Weilin Chen

In this paper, the immersed boundary method was used to simulate the vortex-induced vibration of a slender flexible cylinder exposed to linear shear flows. The vibration of the cylinder was simulated by using a three-dimensional cable model pinned at both ends. The cylinder has a mass ratio of 6 and an aspect ratio of 50. The normalized top tension is 496. The incoming flow has different linear velocity profiles with the shear rates ranging from 0 to 0.024. The maximum Reynold number is 250. It was found that the transverse vibration shows a standing wave pattern while the streamwise vibration shows a combined traveling-standing wave pattern. With the increase of the shear rate, the distribution of Power Spectrum Density (PSD) of vibration responses shows a multi-frequency mode, and the vibration energy shifts to low frequency bands. The mean drag coefficient varies in the spanwise direction while the root-mean-square (RMS) values of fluctuating drag and lift coefficients show a two-peak pattern. The distribution of the fluid-solid energy transferring coefficient indicates that the vibration-exciting region coincides with the high-velocity region while the vibration-damping region matches the low-velocity region. For the cases with low shear rates, the vortex-shedding behind the cylinder shows the interwoven pattern. However, for the cases with large shear rates, the vortex-shedding displays the oblique pattern. Due to the spanwise variation of the vortex-shedding frequency, the vortex-splitting occurs in the near-wake, leading to the vortex cells with different vortex-shedding frequencies.

2018, 50(1): 21-31. doi: 10.6052/0459-1879-17-222

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The seismic wave input method for soil-structure dynamic interaction analysis based on the substructure of artificial boundaries

Liu Jingbo, Tan Hui, Bao Xin, Wang Dongyang, Li Shutao

：Numerical simulation is an important approach to conduct soil-structure dynamic interaction analysis, while the realization of the seismic wave input determines the accuracy of the simulation. Wave method is one of the most commonly used methods for seismic wave input, which converts the input wave into the equivalent loads on the artificial boundaries. Comparing with other methods, the wave method has high precision, but the implementation is relatively complicated. Based on another form of equivalent input seismic loads in the finite element model, a new seismic wave input method was proposed. In the new method, by imposing the free-field displacements on the nodes of the substructure composed of the elements containing the artificial boundary nodes, the equivalent input seismic loads are obtained through dynamic analysis of the substructure. Afterwards, the equivalent input seismic loads are applied to the nodes of the artificial boundaries to complete the seismic wave input and the seismic analysis of the soil-structure dynamic interaction model are performed. In comparison with the original wave method, the new method avoids the complex processes of calculating the free field stress on the artificial boundaries and the additional forces caused by the artificial boundaries, and determining the directions of the loads on different artificial boundaries. Therefore, it is simple to calculate the equivalent input seismic loads and easy to implement the seismic wave input process in the new method. The validity of the new method is verified by the numerical examples of the seismic analysis of the elastic half-space and layered half-space under vertical and oblique incident seismic waves.

2018, 50(1): 32-43. doi: 10.6052/0459-1879-17-336

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NONLINEAR MECHANICS MODEL FOR JOINT INTERFACE OF ASSEMBLED STRUCTURE

Wang Dong, Xu Chao, Hu Jie, Wan Qiang, Chen Hongyong

The existence of complex multi-scale, multi-physics and nonlinear behaviors on joint interfaces is mainly response for complex dynamics of assembled structures. Modeling of mechanical joint interfaces is also a challenging scientific problem, due to the complexity of interface behaviors and difficulties of direct experimental observation. Firstly, the multi-scale physics of contact surface is considered. The namely smooth surface is assumed as a rough surface covered with asperities with random height distribution. The micro-scale stick-slip physics of asperity contact is analyzed to conduct the relationship between the tangential load and deformation. The statistical theory of GW model is used to yield the formulation of total contact load of rough surface and verified by a comparison with published experimental results. Then, an improved Iwan constitutive model is proposed to describe the nonlinear behaviors of joint interface. The nonlinear behaviors calculated by the finite element analysis are used to identify the parameters of proposed Iwan model, and verified by a comparison with the results of finite element analysis. The results show that the total contact load of rough surface predicted by the proposed multi-scale model agrees very well with the experimental results at lower normal load. The nonlinear behaviors predicted by the proposed Iwan model also agree very well with finite element analysis.

2018, 50(1): 44-57. doi: 10.6052/0459-1879-17-125

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STABILITY OF PLASTIC FLOW OF METALLIC CHIPS

Ma Wei

This work deals with the experimental and analytical investigations of the formation mechanisms and the plastic flow stability of chips in the orthogonal cutting processes. In the cutting experiments, four kinds of metal are chosen as modelling materials. In the high-speed cutting process of each testing metal, we observed the transformation of chip morphology from continuous to serrated and determined the critical cutting speed which depends on the material properties of workpiece and the cutting conditions. Based on the experimental results, a two-dimensional orthogonal cutting model is proposed for analyzing the two-dimensional effects of chip flow and a corresponding basic theoretical framework is established under the plane strain loading condition. By introducing a group of scaling quantities that related to the cutting condition parameters, a system of dimensionless governing equations is obtained by normalization, a main dimensionless controlling parameter is determined in terms of experimental conditions and numerical simulation results, an instability criterion is established by the linear perturbation analysis under plane strain loading conditions, and the asymptotic flow fields on the velocity and stress in the extended chip formation zone are obtained by approximate analysis of chip flow. The theoretical results showed that, provided the cutting speed is sufficient high, the plastic flow of continuous chip will be unstable. This instability behavior of chip would be the non-localization unstable flow of the continuous chip caused by plane loadings and differs from the shear-localized instability in the serrated chip. The dimensionless controlling parameter, called as the modified Reynold number, did play a leading role since it better describes the plastic instability behavior of continuous chip flow and the shear-localized instability behavior of the serrated chip in the orthogonal cutting process of metals.

2018, 50(1): 58-67. doi: 10.6052/0459-1879-17-270

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REANALYSIS OF THE CONTACT MECHANICS FOR ROUGH SURFACES

Sun Jianjun, Ji Zhengbo, Ma Chenbo

To overcome the scale dependence of the contact model based on statistical parameters and the shortcoming that the initial profile of existing fractal contact models depends on contact area or sampling length, a new fractal contact model for rough surface was established based on fractal dimension $D$, fractal roughness $G$ and the base size $l$ of the largest asperity. The asperity deformation mechanism and the relationship between the true contact area and contact load were explored. The variation law of porosity of the contact interface and real contact area under different surface topographies and normal forces were discussed. The maximum deformation $\delta $ that characterizes the deformation to compact the profile pores for different morphology interfaces was also given. The results show that the asperity deformation initiates from elastic deformation to elastoplastic deformation, and then transforms into full plastic deformation with the increase of average contact pressure $p_{\rm m}$. The initial porosity $\phi_{0}$ of the contact interface increases with increasing $D$ and then the maximum deformation also increases. The porosity $\phi $ decreases with the increase of contact pressure $p_{c}$, and rapidly decreases with increasing $D$ and decreasing $G$ until it becomes zero. The effect of the increase of $G$ on the increase of the real contact area can be ignored when $D$ is small. However, when $D$ is large, an obvious increase of the real contact area can be found as $G$ increase. The research could provide a theoretical basis for the lubrication and sealing design of friction pairs.

2018, 50(1): 68-77. doi: 10.6052/0459-1879-17-272

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Implicit numerical integration of an elasto-plastic constitutive model for structured clays

Dajiang Geng, Peijun Guo, Shunhua Zhou

：Compared with the general constitutive models, the highly nonlinear elasto-plastic constitutive models for structured clays are more complex, which leads to the problems of Jacobian matrix singularity and nonconvergence more easily when the implicit algorithm of Newton-CPPM is used for the numerical implementation. To solve the problems, two implicit algorithms are proposed in this paper. Considering the Newton-CPPM implicit algorithm is a local convergence algorithm, the homotopy continuation algorithm of global convergence is introduced to improve the iterative initial value of the Newton-CPPM algorithm, so the method can be called as homotopy-Newton-CPPM algorithm. Considering that the calculation of every iteration for the Newton-CPPM implicit algorithm is too large, a two-stage iterative algorithm based on the idea of the fully explicit algorithm is presented. The consistency parameter is calculated in the first-stage, taking the consistency parameter as a known quantity and the algorithm similar to the explicit algorithm is used to solve the values of state variables in the second-stage. Then, taking the SANICLAY model that including destructuration as an example, from the two aspects of the composition of the elasto-plastic constitutive model and the characteristics of the algorithm, the reasons for Jacobian matrix singularity and nonconvergence are analyzed. The convergence, accuracy and cost of four algorithms, including the explicit algorithm, traditional implicit algorithm and two kinds of improved implicit algorithms, are compared with reference to the numerical simulations of single element tests. Finally, the homotopy-Newton-CPPM algorithm and the traditional implicit algorithm are applied to the multi-element calculation of subgrade bearing capacity. The results show that the homotopy-Newton-CPPM algorithm can effectively improve convergence and avoid singularity of Jacobian matrix compared with the traditional implicit algorithm.

2018, 50(1): 78-86. doi: 10.6052/0459-1879-16-340

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DYNAMIC MODELLING AND SIMULATION OF ORBIT AND ATTITUDE COUPLING PROBLEMS FOR STRUCTURE COMBINED OF SPATIAL RIGID RODS AND SPRING

Yin Tingting, Deng Zichen, Hu Weipeng, Li Qingjun, Cao Shanshan

For the strong coupling dynamic problems of the sail tower solar power satellite in orbit, a simplified model combined of spatial rigid rods and spring that describes the coupling dynamic behaviours of orbit and attitude is established. The coupling dynamic effects of the simplified model are analyzed by the symplectic geometry method and the numerical results can be verified indirectly by the energy conservation of the system. Firstly, based on the variational principle, by introducing the symplectic variables the Lagrange equation describing the dynamic behaviour of the simplified model combined by spatial rigid rods and spring is expressed in the form of the Hamilton system, and the associated canonical governing equations of the simplified model are established. And then, the influence of the Earth non-shape perturbation on the orbit, attitude coupling dynamic motion is simulated by the symplectic Runge-Kutta method and the energy deviation of the simplified model is also analyzed by the symplectic Runge-Kutta method. According to the numerical results, it can be concluded that with the increase of the initial attitude angle velocity, the disturbance of the orbital radius increases and the coupling dynamics between orbit and attitude increases. The effect of zonal harmonic term is higher than that of the tesseral harmonic term at least about two orders of magnitude. And the symplectic Runge-Kutta method proposed could reproduce the dynamic properties of the sail tower solar power satellite associated with the Earth non-shape perturbation rapidly and preserve the energy well with excellent long-time numerical stability, which will give a new approach to obtain the real-time dynamic response of the ultra-large spatial structure for the real-time feedback controller design.

2018, 50(1): 87-98. doi: 10.6052/0459-1879-17-337

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The research of time delay vibration control with time - varying parameters

Li Shuai, Zhou Jilei, Ren Chuanbo, Shao Sujuan

The time delay dynamic vibration absorber has good vibration control effect on harmonic excitation, but it has no obvious vibration control effect on the random excitation, which is shown that the vibration control effect of the time-delay dynamic absorber is almost the same as that of the passive vibration absorber. In view of the above problems, this paper proposed a new method of time varying time delay parameter vibration control. On the basis of the original time delay vibration control method, firstly, the time delay gain is changed from fixed value form to time function form, then, the time-varying time delay control parameters are obtained by time varying optimization, and they are applied to the vibration control process in a certain time cycle. By this method, the vibration control performance of the time-delay dynamic absorber is further improved. Finally, this paper takes the Two-degree-of-freedom damping system with time-delay feedback control as the target system, the vibration response of the primary system as the simulation object, using the Precise integration method to solve the dynamic equations with time varying time delay parameters, and obtained the time domain simulation results of the vibration of the primary system under influence of harmonic excitation and random excitation respectively. The results show that, compared with the fixed parameter time delay dynamic vibration absorber , when the primary system under the control of time-varying parameters time delay vibration absorber, its vibration displacement, vibration speed and vibration acceleration are all greatly reduced, whether it is subjected to harmonic excitation or random excitation. The vibration control performance of the time-delay dynamic absorber has been greatly improved.

2018, 50(1): 99-108. doi: 10.6052/0459-1879-17-207

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Stable variable mass mechanical systems constructed by using a gradient system with negative-definite matrix

Li Yan-Min, Zhang Ting-Ting, Mei Feng-Xiang

With the development of science and technology, it is more and more important to study the dynamics of variable mass system such as jet aircraft and rocket, and it is always hoped that the solutions of the variable mass system are stable or asymptotically stable. It is difficult to study the stability by using Lyapunov direct methods because of the difficulty of constructing Lyapunov functions directly from the differential equations of the mechanical system. This paper presents an indirect method for studying stability, that is, gradient system method. This method can not only reveal the internal structure of dynamic system, but also help to explore the dynamic behavior such as the stability, asymptotic and bifurcation. The function V of the gradient system is usually taken as a Lyapunov function, so the gradient system is more suitable to be studied with the Lyapunov function. The equations of motion for the holonomic mechanical system with variable mass are listed, and all generalized accelerations are obtained in the case of non-singular system. A class of gradient system with negative-definite matrix is proposed, and the stability of the solutions of the gradient system is studied. This kind of gradient system and variable mass mechanical system are combined, then the conditions under which the solutions of the mechanical systems with variable mass can be stable or asymptotically stable are given. Further the mechanical system with variable mass whose solution is stable or asymptotically stable is constructed by using the gradient system with non-symmetrical negative-definite matrix. Through specific examples, it is studied that the solutions of the single degree of freedom motion of a variable mass system are stable or asymptotically stable under some conditions of the laws of mass change, particle separation velocity and force. The method is also suitable for the study of other constrained mechanical systems.

2018, 50(1): 109-113. doi: 10.6052/0459-1879-17-283

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PROBABILISTIC MODELLING OF DYNAMIC RESPONSE OF UNDERWATER VEHICLE STRUCTURE VIA MAXIMUM ENTROPY METHOD

Zhou Chunxiao, Wang Ruiqiong, Nie Zhaokun, Li Gang, Zeng Yan

Hydrodynamic loads of underwater vehicle structure have a high degree of randomness in time and space, which leads to random responses of structure such as maximum internal forces, time and position they taking place, etc. Dynamic response of underwater vehicle will also be nonlinear due to different tension and compression stiffness of its connecting or separated structures. To analyse the influence of nonlinearity of connection on statistics of response of underwater vehicle based on limited sample data of hydrodynamic external load, probabilistic modelling of dynamic response of underwater vehicle is completed by using maximum entropy method in which statistical moments of response samples are obtained from a simplified dynamic model of the structure. Probability density functions of maximum internal forces, time and position they taking place, are proposed separately to predict the most dangerous cases of the underwater vehicle. Accuracy of results proposed here is verified through Monte Carlo simulation. Effects of nonlinear parameters of connections on dynamic response are studied and discussed. Final conclusions are as follows: Nonlinearity of connection leads to axial force response of structures under transverse loads; peak of probabilistic density function of maximum axial force of the structure increases with nonlinear degree of connections which leads to significant effect of propagation of uncertainty; internal force response of structure with nonlinear connections under normal random loads is non-normal; position of maximum internal force response of underwater vehicle structure is influenced by degree of nonlinearity of connections. These findings can provide technical support for structural optimization.

2018, 50(1): 114-123. doi: 10.6052/0459-1879-17-022

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A THEORETICAL MICROFLUIDIC FLOW MODEL FOR THE CELL CULTURE CHAMBER UNDER THE PRESSURE GRADIENT AND ELECTRIC FIELD DRIVEN LOADS

Wang Zhaowei, Wu Xiaogang, Chen Kuijun, Xue Yanan, Wang Ningning, Zhao Teng, Yu Weilun, Wang Yanqin, Chen Weiyi

Fluid shear stress (FSS) induced in the microfluidic bioreactor under the driven loads of applied external physical fields (Fluid pressure gradient or Electric filed), which make the cells regulate its genes expression and promote differentiation and growth, so does in the natural tissue microtubule system. It is difficult to experimental quantifications the fluid flowing behavior in the cell culture chamber. The theoretical modeling is thought to be an effective way. In this paper, a theoretical model for microfluidic flow in the rectangle cell culture chamber is developed to link the applied external physical fields (pressure gradient and electrical field) to intraluminal fluid velocity (FV), FSS and fluid flow rate (FFR). The results predict the model solutions are nearly match the others experiment results. Specifically, the solutions under pressure-electricity synergic driven are the superposition of the driven solutions of each pressure gradient and the electric field. FSS, FV and FFR amplitudes in chamber are proportional to the amplitudes of applied external physical fields, but decrease and change little as the frequencies of pressure gradient and electric field grows, respectively. The higher chamber height is, the larger FSS and FFR amplitudes generalized in the pressure gradient driven model, while not obvious change in the electric field driven model. Besides, the results are not influenced when the culture medium temperature varies at the physiological level. At the generalized cell response level, electric field driven model can provides the larger FSS amplitude, while the pressure driven flow model is good at inducing the larger FFR amplitude. The combined pressure-electricity synergic driven model can be used as the theoretical basis to design the experimental cell microfluidic bioreactor system, meanwhile, provides the references to research the mechanism of cell growth and differentiation under the stimulus of （shear）stress and electricity.

2018, 50(1): 124-137. doi: 10.6052/0459-1879-17-317

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FLUID-STRUCTURE INTERACTION ANALYSIS OF LOCAL STRESSES IN ATHEROSCLEROTIC PLAUQE AND THE INTERVENTION OF ENHANCED EXTERNAL COUNTERPULSATION TREATMENT

Du Jianhang, Wang Liang, Wu Guifu, Zheng Zhensheng, Dai Gang, Feng Mingzhe

It is generally agreed that biomechanical stresses play important roles in advanced atherosclerotic plaque progression and rupture. This paper aims to perform a pilot study to access the influences of blood perfusion, blood pressure, histology and material properties of plaques on the levels of blood flow stress (FSS) and plaque structural stress (PSS), and meanwhile to access the intervention of enhanced external counterpulsation (EECP), a kind of clinical non-invasive assisted circulation therapy. A method combining in vivo measurements performed in a porcine model and 3D fluid-structure interaction (FSI) numerical simulation was adopted. Results showed that, when the stenotic degree was fixed to 50%, FSS level of the plaque depended mainly on blood perfusion, while PWS level was mainly determined by both blood pressure and fibrous cap (FC) length. Only when FC was thin enough, plaque material properties had significant influence on PSS. A thinnest FC together with a softest lipid pool led to the peak critical plaque wall stress (3D CPWS) of 257.72 kPa (normal physiological state) and 300.20 kPa (EECP state). Note that changes in FC length or lipid pool material property only induced the variation of 3D CPWS significantly, we suggested that 3D CPWS was a stress-based factor that might play a much more important role during plaque progression than max WSS (MWSS) or global max plaque stress (GMPWS). Moreover, EECP treatment significantly increased the levels of both FSS and PWS, whether it would intervene the progression and remodeling of advanced plaque, and should be brought the attention in its clinical applications might need more detailed evaluations.

2018, 50(1): 138-146. doi: 10.6052/0459-1879-17-150

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A NUMERICAL WELL TEST MODEL FOR MULTI-FRACTURED HORIZONTAL WELLS BASED ON DISCRETE-FRACTURE MODEL AND ITS APPLICATION

Wan Yizhao, Liu Yuewu, Wu Nengyou, Hu Gaowei

Horizontal well fracturing technology has become a key technology in the development of low permeability oil and gas reservoir, shale gas and tight gas and other unconventional reservoirs. This paper developed a numerical well test model for multi-fractured horizontal wells based on discrete-fracture model which simplifies the fractures as lines. The finite element method was applied to solve the mathematical model and get the type curves and pressure profiles. The log-log type curve of bottom hole pressure can be divided into seven stages: wellbore storage, fractures linear flow, fractures-formation bilinear flow, fractures interacting flow, formation linear flow, radial flow, and boundary domain flow. Among them, the fractures-formation bilinear flow and fractures interacting flow are the two typical features. The effect of fractures’ numbers, distance of fractures, asymmetry of fractures, fractures with unequal length, and some fracturing failure on the transient pressure and derivative responses were also studied. The analysis results showed that the number of fractures and the spacing between fractures have the greatest influence on the type curves. More fractures, fractures with larger distance, symmetrical fractures with equal length were propitious to reduce the flow resistance of bottom hole and increase production capacity. The multi-fractured well test model proposed in this paper was applied in the interpretation of build-up tests of multi-fractured horizontal wells. The results of a field case in Sichuan Basin shown that the model matched with the tested data very well. The developed numerical well test model can be used to obtain the parameters of reservoirs and fractures, and to provide technical support for the design and evaluation of hydraulic fracturing.

2018, 50(1): 147-156. doi: 10.6052/0459-1879-17-319

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INFLUENCE OF MACROSCOPIC TOPOGRAPHY ORIENTATIONS OF WHEELS ON ADHESION COEFFICIENT OF HIGH SPEED WHEEL/RAIL UNDER WATER LUBRICATION

Jiang Huazhen, Wang Baoan, Li Zhengyang, Cai Baochun, Yang Bing, Ren Zhiyuan

Abstract Though tyre and asphalt are low elastic modulus materials, water lubrication resulting from hydrodynamic action would present even the speed is low. For high elastic modulus of materials, such as wheel and rail, water lubrication would present when the speed is over 200 km/h, causing potential unsafety for train operations. Increasing surface roughness will improve the wheel/rail adhesion coefficient. However it is shown that the topography orientation also has great effect on adhesion coefficient when the value of surface roughness is nearly the same under mixed lubrication. In this paper, a numerical analysis based on unified Reynolds equation was adopted. The behavior of three patterns of roughness orientations on wheels, i.e. longitudinal, transverse and rhombus, with high speed up to 500km/h under mixed lubrication were analyzed. The simulation results were compared with the results of the average flow model and the existing experimental results. It is concluded that the adhesion coefficients of wheel/rail decreased with speed increasing, while the adhesion coefficient of rhombus pattern is greater than that of the transverse, and transverse pattern is greater than that of the longitudinal. The adhesion coefficient is mainly depended on the ratio of asperity contact pressure to the total pressure. When the ellipticity $k<1$ in wheel/rail point contact, the lateral flow effect could not be neglected, the results of average flow model will result in error.

2018, 50(1): 157-166. doi: 10.6052/0459-1879-17-129

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CFD SIMULATION ON THE PENETRATION OF FFP INTO UNIFORM CLAY

Liu Jun, Zhang Yuqin

The cone penetration test (CPT) has been widely used to measure the soil undrained shear strength. On the basis of CPT, the free fall penetrometer (FFP) is developed to improve the test efficiency, which penetrates into soil by its kinetic energy gained from free fall in the water/air column and potential energy. However, the soil-FFP interaction is rather complex, which refers to the shear strain rate effect and drag force. Therefore, it is necessary to analyze the forces acting on the FFP accurately to improve its practicability and the accuracy of soil strength measurement. The FFP penetration procedure in uniform soils was simulated in the present study by using the commercial software ANSYS CFX 17.0, which is based on the computational fluid dynamics (CFD) approach. The dynamic mesh approach was applied to simulate the moving boundary. The thin layer element method was proposed to simulate the FFP-soil interaction. In the CFD simulation, the soil was modeled as non-Newtonian fluid and the shear strain rate effect was considered. Different FFP velocities, soil strengths and densities, interface frictional coefficients and shear strain rate parameters were considered to investigate their effects on the bearing and sleeve resistances of FFP. The fitted formulas of the cone bearing capacity factor, the strain rate parameters and drag coefficients for the cone and sleeve were established based on the present numerical results. In addition, the process to estimate the undrained shear strength of clayed soils was put forward, which may be beneficial for analyzing the recorded data from FFP.

2018, 50(1): 167-176. doi: 10.6052/0459-1879-17-284

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REVIEW OF THE SECOND NATIONAL SYMPOSIUM ON EXPLOSION AND IMPACT DYNAMICS FOR YOUNG SCHOLARS

Zheng Zhijun, Zhan Shige, Dai Lanhong

The Second National Symposium on Explosion and Impact Dynamics for Young Scholars was briefly introduced and all of the scientific reports presented at this symposium were reviewed. The scientific reports include three invited talks, 14 thematic invitations and 24 topic invitations, which were divided into five research topics, i.e. Detonation and Explosion Dynamics, Structural Dynamics and Multi-scale High Performance Computing, Material Dynamics and Experimental Testing Techniques, Dynamic Mechanical Behavior of Composite Structures, and Energy Absorption Characteristics and Optimization Design of Lightweight Structures.

2018, 50(1): 177-187.

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