Determination the ability of military vehicles to override vegetation

Military operations usually include movement over existing roads and also through natural terrain. Wooded terrain is one of the most challenging environments which affect vehicle mobility. The ability of a vehicle to cross a forest area depends on the possibility of determining if the vehicle is able to manoeuvre between tree stems or can override individual trees. Overriding tree obstacles can be more effective if a vehicle needs a shorter time to cross some tree stems rather than manoeuvring around them. Vehicle movement to cross a forest stand depends on vegetation factors as the stem diameter, stem spacing, and also on tree root parameters, which determine the mechanical tree stability, and a vehicle’s ability to override the trees. Also, the technical parameters (width, length, turning radius, weight, traction force) of the selected military vehicle are important to classify the cross-country movement options. This study describes both the theoretical predictions of the movement of vehicles in forest stands and summarizes the results of one of the most extensive testing of vehicles’ ability to cross individual trees.     

Investigation of the soil thrust interference effect for tracked unmanned ground vehicles (UGVs) using model track tests

The traction force of a tracked unmanned ground vehicle (UGV) depends on the soil thrust generated by the shearing action on the soil-track interface. In the development of soil thrust, because the continuous-track system consists of a number of single-track systems connected to each other, interference occurs between the adjacent single-track systems through the surrounding soil. Thus, the total soil thrust of the continuous-track system is not equal to the sum of the soil thrust of each single-track system, and the interference effect needs to be carefully considered. In this study, model track tests were conducted on model single-, double-, and triple-track systems according to relative density of soil and shape ratio (i.e., the length of the track plate to grouser depth). The test results indicated that the interference effect reduced soil thrust due to the overlapping shear zones between adjoining single-track systems. The loss of soil thrust increased as the relative density of the soil increased and the shape ratio decreased. Based on these findings, a soil thrust multiplier that can be utilized to assess the soil thrust of a continuous-track system was developed.     

基于扩展有限元法的裂尖场精度研究

扩展有限元方法基于单元分解的基本思想,通过引入位移加强函数来表征裂纹的不连续性和裂尖的奇异性。在裂尖加强单元与常规单元之间有一层混合单元,当对裂尖特定区域进行加强时,混合单元个数相应增加,混合单元个数与计算精度存在一定联系。本文提出一种正方形裂尖加强区域的选择方式,可得到较单个加强和圆形加强精度更高、更稳定的计算结果。对于不同长度的裂纹,表征裂尖场奇异性所需的裂尖加强范围存在较大差异,以正方形裂尖加强方式进行计算,得到了不同裂纹长度下最优的加强尺寸。     

An efficient method for the prediction of the motion of individual bubbles

We present a method for the simulation of two-phase flows which can be applied to problems characterised by the presence of up to several hundreds of gas bubbles. The bubble model is kept simple, requiring only six parameters to describe the shape of a single bubble. The model is coupled to a conventional time discrete finite-volume scheme for the solution of the Navier–Stokes equations by the density field which is calculated on basis of the information on the positions and the shapes of the bubbles before each time step. The motion of the bubbles is in turn calculated from an analysis of the computed flow field. Systematical errors due to simplifications are eliminated by the introduction of correction factors. For a selection of fluid dynamical problems, the results of simulations using the method are compared to experimental data. Good quantitative agreement could be found.     

An efficient very large eddy simulation model for simulation of turbulent flow

Among the various hybrid methodologies, Speziale's very large eddy simulation (VLES) is one that was proposed very early. It is a unified simulation approach that can change seamlessly from Reynolds Averaged Navier–Stokes (RANS) to direct numerical simulation (DNS) depending on the numerical resolution. The present study proposes a new improved variant of the original VLES model. The advantages are achieved in two ways: (i) RANS simulation can be recovered near the wall which is similar to the detached eddy simulation concept; (ii) a LES subgrid scale model can be reached by the introduction of a third length scale, that is, the integral turbulence length scale. Thus, the new model can provide a proper LES mode between the RANS and DNS limits. This new methodology is implemented in the standard k ? ? model. Applications are conducted for the turbulent channel flow at Reynolds number of Re_τ = 395, periodic hill flow at Re = 10,595, and turbulent flow past a square cylinder at Re = 22,000. In comparison with the available experimental data, DNS or LES, the new VLES model produces better predictions than the original VLES model. Furthermore, it is demonstrated that the new method is quite efficient in resolving the large flow structures and can give satisfactory predictions on a coarse mesh. Copyright © 2012 John Wiley & Sons, Ltd.     

An efficient method for solving the mixed direct-inverse problem of the transonic rotational flow in plane casades

The method in [1] has been extended to the case of rotational flow in this paper. A new method for dealing with the shock wave is presented. This method has the advantages of both the shock-fitting and the shock capturing methods. The direct problem and the mixed direct-inverse problem of the rotational flow in a transonic plane cascade at both design and off design conditions are solved, and the results show that the present method has rapid convergence rate and high accuracy even for the flow with moderately strong shocks. The calculations have been carried out on the DPS-8 computer, and for the direct problem, only 50–80 iterations are needed, and 50–80 seconds of CPU time are required.     

Data-driven optimization study of the multi-relaxation-time lattice Boltzmann method for solid-liquid phase change

Sharp phase interfaces and accurate temperature distributions are important criteria in the simulation of solid-liquid phase changes. The multi-relaxation-time lattice Boltzmann method(MRT-LBM) shows great numerical performance during simulation;however, the value method of the relaxation parameters needs to be specified. Therefore,in this study, a random forest(RF) model is used to discriminate the importance of different relaxation parameters to the convergence, and a support vector machine(SV...     

The coupling method for torsion problem of the square cross-section bar with cracks

By coupling natural boundary element method (NBEM) with FEM based on domain decomposition, the torsion problem of the square cross-sections bar with cracks have been studied, the stresses of the nodes of the cross-sections and the stress intensity factors have been calculated, and some distribution pictures of the stresses have been drawn. During computing, the effect of the relaxed factors to the convergence speed of the iterative method has been discussed. The results of the computation have confirmed the advantages of the NBEM and its coupling with the FEM. Foundation item: the State Key Laboratory of Science and Engineering Computation Biography: ZHAO Hui-ming (1971-)     

A wavelet-based variational multiscale method for the LES of incompressible flows in a high-order DG-FEM framework

This work focuses upon the development of a wavelet-based variant of the variational multiscale method (VMS) for accurate and efficient large eddy simulation (LES) called wavelet-based VMS-LES (WMS-LES). This approach has been incorporated within the framework of a high-order incompressible flow solver based upon the pressure-stabilized discontinuous Galerkin finite element method (DG-FEM). The VMS approach is designed to produce an a priori scale separation of the governing equations, in a manner which makes no assumptions on either the boundary conditions or the mesh uniformity. Using second-generation wavelets (SGWs) elementwise for scale separation ensures, on one hand, the preservation of the computational compactness of the DG-FEM scheme and, on the other hand, the ability to achieve scale separation in wavenumber space. The optimal space-frequency localization property of the SGW provides an improvement over the commonly used Legendre polynomials. The suitability of the elementwise SGW scale-separation operation as a tool for error indication has been demonstrated in an h-adaptive computation of the reentrant corner test case. Finally, the DG-FEM solver and the WMS-LES method have been assessed through simulations upon the three-dimensional Taylor-Green vortex test case. Our results indicate that the WMS-LES approach exhibits a distinct improvement over the monolevel LES approach. This effect is not produced by a change in the magnitude of the subgrid dissipation but rather by the redistribution of the subgrid dissipation in wavenumber space.     

An indirect shooting method based on the POD/DEIM technique for distributed optimal control of the wave equation

This paper presents a fast numerical method, based on the indirect shooting method and Proper Orthogonal Decomposition (POD) technique, for solving distributed optimal control of the wave equation. To solve this problem, we consider the first‐order optimality conditions and then by using finite element spatial discretization and shooting strategy, the solution of the optimality conditions is reduced to the solution of a series of initial value problems (IVPs). Generally, these IVPs are high‐order and thus their solution is time‐consuming. To overcome this drawback, we present a POD indirect shooting method, which uses the POD technique to approximate IVPs with smaller ones and faster run times. Moreover, in the presence of the nonlinear term, to reduce the order of the nonlinear calculations, a discrete empirical interpolation method (DEIM) is applied and a POD/DEIM indirect shooting method is developed. We investigate the performance and accuracy of the proposed methods by means of 4 numerical experiments. We show that the presented POD and POD/DEIM indirect shooting methods dramatically reduce the CPU time compared to the full indirect shooting method, whereas there is no significant difference between the accuracy of the reduced and full indirect shooting methods.     

A computationally efficient solution of the wave equation for the transient response of infinite reservoirs

In this paper, the transient response of an infinite reservoir is analyzed using the dual-reciprocity boundary element method. A vertical and an inclined-face rigid dam are analyzed under a transient loading. Sharan-type boundary-condition transmission is implemented in the formulation. The results are compared with the exact solution and those obtained by using the finite element method. It is seen that the application of the dual-reciprocity boundary element method is simpler and the results are in very good agreement with the exact solution and those obtained by using the finite element method.     

An efficient second‐order accurate cut‐cell method for solving the variable coefficient Poisson equation with jump conditions on irregular domains

A numerical method is presented for solving the variable coefficient Poisson equation on a two‐dimensional domain in the presence of irregular interfaces across which both the variable coefficients and the solution itself may be discontinuous. The approach involves using piecewise cubic splines to represent the irregular interface, and applying this representation to calculate the volume and area of each cut cell. The fluxes across the cut‐cell faces and the interface faces are evaluated using a second‐order accurate scheme. The deferred correction approach is used, resulting in a computational stencil for the discretized Poisson equation on an irregular (complex) domain that is identical to that obtained on a regular (simple) domain. In consequence, a highly efficient multigrid solver based on the additive correction multigrid (ACM) method can be applied to solve the current discretized equation system. Several test cases (for which exact solutions to the variable coefficient Poisson equation with and without jump conditions are known) have been used to evaluate the new methodology for discretization on an irregular domain. The numerical solutions show that the new algorithm is second‐order accurate as claimed, even in the presence of jump conditions across an interface. Copyright © 2006 John Wiley & Sons, Ltd.     

Perturbation initial parameter method for solving the geometrical nonlinear problem of axisymmetrical shells

In the previous paper[7], the author presented a System of First-Order Differential Equations for the problem of axisynrm’trically loaded shells of revolution with small elastic. strains and arbitrarily large axial deflections, and a Method of Variable-Characteristic Nondimensionization with a Scale of Load Parameter. On this basis, by taking the weighted root-mean-square deviation of angular deflection from linearity as perturbation parameter, this paper pressents a perturbation system of nondimensional differential equations for the problem, thus transforms the geometrical nonlinear problem into several linear problems. This paper calculates these linear problems by means of the initial parameter method of numerical integration. The numerical results agree quite well with the experiments[4].     

Application of the quadrilateral area coordinate method： a new element for laminated composite plate bending problems

Recently, some new quadrilateral finite elements were successfully developed by the Quadrilateral Area Coordinate (QAC) method. Compared with those traditional models using isoparametric coordinates, these new models are less sensitive to mesh distortion. In this paper, a new displacement-based, 4-node 20-DOF (5-DOF per node) quadrilateral bending element based on the first-order shear deformation theory for analysis of arbitrary laminated composite plates is presented. Its bending part is based on the element AC-MQ4, a recent-developed high-performance Mindlin-Reissner plate element formulated by QAC method and the generalized conforming condition method; and its in-plane displacement fields are interpolated by bilinear shape functions in isoparametric coordinates. Furthermore, the hybrid post-processing procedure, which was firstly proposed by the authors, is employed again to improve the stress solutions, especially for the transverse shear stresses. The resulting element, denoted as AC-MQ4-LC, exhibits excellent performance in all linear static and dynamic numerical examples. It demonstrates again that the QAC method, the generalized conforming condition method, and the hybrid post-processing procedure are efficient tools for developing simple, effective and reliable finite element models. The project is supported by the National Natural Science Foundation of China (10502028), the Special Foundation for the Authors of the Nationwide (China) Excellent Doctoral Dissertation (200242), and the Science Research Foundation of China Agricultural University (2004016).     

A procedure of the method of reverberation ray matrix for the buckling analysis of a thin multi-span plate

A procedure of the method of reverberation ray matrix(MRRM) is developed to perform the buckling analysis of thin multi-span rectangular plates having internal line supports or stiffeners. A computation algorithm for the reverberation ray matrix in the MRRM is derived to determine the buckling loading. Specifically, the analytical solutions are presented for the buckling of the structure having two opposite simply-supported or clamped-supported edges with spans, while the constraint condition of...     

基于平稳过滤有色噪声地震动模型的MTMD性能评价

研究了由许多刚度和阻尼保持为常量且频率呈线性分布的TMDs形成的MTMD的地震特性。基于虚拟激励法和平稳过滤有色噪声地震动模型，建立了结构－MTMD系统的传递函数，进而导出了设置MTMD时结构的动力放大系数（DMF）明确表达式。于是MTMD的优化准则可定义为Min.Min.Max.DMF。通过最优搜寻可得MTMD的最优频率间隔、平均阻尼比、调谐频率比和相应的无量纲控制有效性指标。目前的论文通过考虑结构受控频率与地震加速度卓越频率比的不同取值和不同场地类型，研究地震卓越频率和场地类型对MTMD最优参数及有效性的影响。同时与基于Kanai-Tajimi平稳过滤白噪声地震动模型和不考虑地震加速度卓越频率(ωg=∝）两种情况的结果进行了比较。     

Development of a three-dimensional solver based on the local domain-free discretization and immersed boundary method and its application for incompressible flow problems

Recently, the author and two other coauthors have proposed a two-dimensional hybrid local domain-free discretization and immersed boundary method (LDFD-IBM), which can be used to solve the flow problem with complex geometries. In this paper, the LDFD-IBM is extended to solve a three-dimensional unsteady incompressible flow with the complex computational domain. The technical issues related to the implementation of the LDFD-IBM in three-dimensional problems are discussed in detail, particularly for the discretization of Navier-Stokes equations, mesh strategies for a three-dimensional flow, and the fast algorithm on the identification of the status of mesh nodes (ie, to identify if the mesh node is located in the solid domain, in the fluid domain, or near the immersed boundary). Numerical tests show that the LDFD-IBM can accurately solve three-dimensional incompressible problems with ease.     

钢纤维混凝土受拉性能实验方法研究

在分析国内外钢纤维混凝土受拉性能实验研究存在困难的基础上,提出了新的实验方法——钢纤维混凝土环状试件,在内油压作用下的受拉性能实验研究。通过实验装置的合理设计及正确的实验步骤,成功地测出了钢纤维混凝土环状受拉应力－ 应变全过程曲线,为深入研究钢纤维混凝土受拉性能提供参考依据。     

大理岩动态劈裂拉伸的SHPB实验研究

利用直径100 mm的Hopkinson压杆和薄圆形铝片作为波形整形器,用不同弹速径向冲击平台巴西圆盘试样以研究大理岩的动态拉伸强度。分析了试样的应变率、破坏时间、破坏模式,以及破坏过程中的载荷-应变关系,得到了关于大理岩在高应变率下拉伸强度及弹性模量的一些结论。考虑了试样的尺寸大小及两个平台附近应力的时间不均匀性与空间不均匀性对实验结果的影响。同时,利用有限元法对平台巴西圆盘试样的动态应力分布进行了数值模拟,验证了动态劈裂拉伸实验的有效性。