力学学报

2013, 45(2): 149-150. doi: 10.6052/0459-1879-2013-2-20130200

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STUDY OF NON-SUBMERGED GROIN TURBULENCE FLOW IN A SHALLOW OPEN CHANNEL BYLES

Bai Jing, Fang Hongwei, He Guojian

Turbulent flow past series of groins in a shallow open channel is studied by large eddy simulation (LES) in this paper. A direct-forcing immersed boundary method is proposed to approximate complex boundaries around groins with round heads. Time-averaged velocity and turbulence intensities at the water surface obtained by a PIV experiment are used to validate the LES model, and the numerical results from LES agree well with the PIV measurements. Moreover a comparing case for groin flow is set up to investigate the effect of the groin aspect ratio L/D on flow characteristics, involving flow patterns, turbulent intensities, vorticity distribution and vortex dynamics. The distance D between two groins is smaller when the groin aspect ratio L/D is bigger and L is constsant. And this a ects the strength of turbulence intensities and vorticity in the mixing layer and flow patterns in the groin field. After producing around the groin heads, the shape of the vortex group varies as the vortices being transported downstream by the flow.

2013, 45(2): 151-157. doi: 10.6052/0459-1879-12-309

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MULTIPLE TIME SCALES OF AEOLIAN AND FLUVIAL PROCESSES AND DEPTH-AVERAGED INTEGRAL MODELLING

Li Zhijing, Cao Zhixian, Hu Peng, Gareth Pender

A depth-averaged numerical model is introduced for aeolian sediment transport modeling. Model validity has been assessed using experimental data, and the results show that the major features of aeolian sediment transport can be well captured by the proposed model. Furthermore, based on the integral model, the recent theoretical work on the multiple time scales of fluvial processes is extended to aeolian processes, the corresponding time scales concerning the adaptation of sediment transport to equilibrium state are purposely defined and quantitatively analyzed. The results provide a theoretical justification for aeolian saltation that it can be adapted to equilibrium state very rapidly, while it is found that a much longer time and space is needed for aeolian suspension to adapt to equilibrium.

2013, 45(2): 158-163. doi: 10.6052/0459-1879-12-311

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NUMERICAL STUDY ON THE GROWTH PROCESS OF SECONDARY AEROSOL IN THE FOG

Ding Jue, Wang Qingtao, Liu Yi, Ying Mengkan

The collision and coagulation between aerosols are the dynamical base for migration and growth of aerosols in the fog. Based on the particle population balance equation and multi-Monte Carlo method, a numerical study on growth process of liquid secondary aerosol was carried out. The influence of flow turbulence and brown mechanisms on secondary aerosol collision and coagulation was analyzed for the initial monodispersal and exponential distribution. The computed results show that collision and coagulation between aerosols lead to the aerosol number decreased gradually, and the average volume of aerosol increased gradually. For the initial scale of free molecular area and monodispersal aerosols, when brown movement time is up to 600 s, the average volume of aerosol is increased to about 202.7 times of the initial, but its number is decreased to 0.006 of the initial. Considering the turbulent effect and brown motion, during the short period (100 s), secondary aerosol particle size is increased to 163 times of the initial, and its number is decreased to 0.025 times of the initial. The above phenomena illustrate that flow turbulence can cause strong transport and accumulation of aerosols, which lead to raise probability of aerosol collision and coagulation, and to increase aerosol scale gradually.

2013, 45(2): 164-170. doi: 10.6052/0459-1879-12-310

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GAS-WATER TWO-PHASE DIFFUSION MODEL FOR UNSATURATED LAYERED COVER SYSTEMS

Guan Chi, Xie Haijian, Lou Zhanghua

A one-dimensional diffusion model of gas is developed to investigate landfill gas transport in unsaturated layered cover systems. Coupled processes of gas diffusion in the unsaturated cover and transient water transport in the media were considered. The mathematical model was solved by using the finite element analysis software COMSOL Multiphysics 4.3. The results obtained by the proposed model agree well with those obtained by the experimental methods. It is shown that the variation of water content has great influence on landfill gas transport in a 1-meter-thickness unsaturated landfill cover soil. For the 1-meter-thcikness unsaturated cover soil, the relative gas concentrations for the case assuming coupled transport was approximately 8 times smaller than those assuming steady transport at the time range of 20 d to 100 d. It is indicated that the water content is of great importance when predicting the unsteady gas migration in unsaturated landfill cover systems.

2013, 45(2): 171-176. doi: 10.6052/0459-1879-12-313

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NUMERICAL SIMULATION OF WIND-BLOWN SAND MOVEMENT BASED ON SPH

Lu Bo, Maimtimin·Geni, Jin Afang, Xu Yujuan

The smooth particle hydrodynamics (SPH) method is used to analyze the mutual coupling motion characteristics between sands and the airflow, and the SPH numerical methods of sand flow is proposed and simulated in this study. Firstly, the SPH modeling method and basic theory about the sand flow is proposed and the SPH simulation platform of the flow of sand is established. Secondly, the SPH model and boundary conditions were used to simulate the movement of sand in the wind movement, the motion curve and characteristics of the sand are studied completely. Finally, by contrasting with the related research analysis, the effectiveness of the SPH method completed is verified. Allowing for the variability of the flow field, the way of loading (windy) and unloading (blowing-out) are applied in the SPH calculation model of sand flow, the motion curve and characteristics of the sand have been observed and comparatively studied. It will be helpful to further study of the constant dynamic nonlinear behavior of the sand flow by SPH theoretical basis and numerical analysis.

2013, 45(2): 177-182. doi: 10.6052/0459-1879-12-312

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TRPIV MEASUREMENT OF DRAG-REDUCTION IN THE TURBULENT BOUNDARY LAYER OVER RIBLETS PLATE

Li Shan, Yang Shaoqiong, Jiang Nan

Time series of velocity vector field and statistics in the turbulent boundary layer(TBL) over the riblets surface and smooth one were measured utilizing the time-resolved particle image velocimetry (TRPIV); Several characteristic parameters in the TBL, such as the mean velocity profile, the Reynolds shear stress and the turbulent intensity, etc. were compared at the same free-stream velocity(0.19m/s) for the different surface plates. We firstly detected the coherent structures using multi-scale spatial locally-averaged structure function of the streamwise velocity component at different scales. Then, we utilized the conditional sampling and phase-average method to extract the spatial topologies of physical quantities, such as the velocity fluctuation and the spanwise vorticity, etc. based on the ejection or sweep events of coherent structures in the TBL. Results reveal that a drag-reduction of nearly 10.73 percent was acquired over the riblets surface when we compared the skin friction coe cient of such two Acrylic Plexiglas plates at the same free-stream velocity. In addition, that the streamwise turbulent intensity and the Reynolds shear stress of the TBL over riblets surface are both smaller than the ones over smooth surface at the same wall-normal position indicates riblets surface weaken the flow turbulence reducing the momentum exchange and energy loss in the flow. Lastly, we compared several statistic characteristics, mentioned above, based on the ejection and sweep events of coherent structures to clarify the effect of the drag-reduction over the riblets surface with triangle cross-section. More importantly, we find that the drag-reduction achieved by means of riblets surface suppressing the bursting events of coherent structures.

2013, 45(2): 183-192. doi: 10.6052/0459-1879-12-262

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MULTI-OBJECTIVE OPTIMIZATION AND AERODYNAMIC PERFORMANCE ANALYSIS OF THE UPPER SURFACE FOR HYPERSONIC VEHICLES

Gao Taiyuan, Cui Kai, Hu Shouchao, Wang Xiuping

To aim at analyzing the variation of the aerodynamic performance as well as the volume of hypersonic vehicles caused by the modification of the upper surface, a two-dimensional multi-objective optimization study is carried out by considering the design condition of flight Mach number 6.5, flight altitude 27 km, and 4? flight angle of attack. The CFD-embedded pareto genetic algorithm is used as the optimization driver. On the basis of 2D optimization results, several typical 3D configurations are generated, and a primary relationship between the aerodynamic performance and the volume is obtained by numerical simulation. The results show that the lift-to-drag ratio is approximately linear inverse proportion to the volume for both two-dimensional and three-dimensional configurations, though there are significant di erences between the 2D and 3D aerodynamic coe cient values. Moreover, the lift-to-drag ratio can only gain a little increment (about 0.36%) by adjusting the symmetrical profile shape of the upper surface when the volume is a constant, while the volume has a relatively large adjustable range (about 1.93%) under the condition of fixing the lift-to-drag ratio. Besides, the numerical results also demonstrate that the adjustment range of the pitch moment of the vehicle is about 5% by modifying the shape of the upper surface when the lift-to-drag ratio and the volume are all fixed simultaneously.

2013, 45(2): 193-201. doi: 10.6052/0459-1879-12-227

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VISCOUS FLOWFIELD BASED ON DISCONTINUOUS BOUNDARY ELEMENT METHOD AND VORTEX METHOD

Ding Jinghu, Ye Jihong

The two-dimensional, three-dimensional viscosity and incompressible flow fields are simulated bases on a combination application of discontinuous boundary element method and vortex method in our present study. Discrete vortex elements are used to analogue the vorticity generation, accumulation and transport mechanisms of the unsteady separated flow fields. And it decomposes the computing domain into an interior domain of vortex blobs and a thin numerical boundary layer of vortex sheets. The convection and stretch of the vortical field is imitated by Lagrangian vortex method, and the random walk method is adopted to describe the diffusion process of the vortical field. Additionally, vortex element's vortical velocity is calculated by generalized Biot-Savart law, while discontinuous boundary element method is used to compute potential velocity. To avoid the discontinuous of normal velocity, all nodes of discontinuous boundary element are selected at smooth boundary. Since a large scale boundary element equation set with a nonsymmetrical coefficient matrix should be solved, the present study import a pre-conditioning the generalized minimum residual (GMRES) iterative algorithm, which takes full advantage of the boundary element method. Moreover, regularization algorithm that applies at interior points close to the boundary, which the nearly singular surface integrals are transformed into a series of line integrals along the contour of the element, help to eliminate the unacceptable results of potential velocity and velocity gradient in potential calculation. The accuracy of present method is verified in both examples of two-dimension and three-dimension flow field calculation, as well as the significant increased simulation precision and efficiency.

2013, 45(2): 202-213. doi: 10.6052/0459-1879-12-171

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TOPOLOGY OPTIMIZATION FOR GEOMETRIC STABILITY OF STRUCTURES WITH COMPENSATION DISPLACEMENTS

Su Wenzheng, Zhang Yongcun, Liu Shutian

The special parts of structures are always required to be geometric stable on output displacements under different loadings in many engineering cases. Introduction of the adjustable compensation displacements in the special region of structures is the effective technique. In the scope of the linearly elasticity and small deformation, the purpose of this paper is to study the topology optimization of structures with the geometric stability in the given region through the lowest compensation cost. The objective of the present optimization model is to obtain the maximum sti ness of structures with the stable ideal displacement responses of structural given region. There are two types of design variables in the model which are the structural topology variables and the compensation displacement variables. These two types of variables are coupled through the two-level search method. The adjoint method is performed to compute the sensitivities of the objective function with respect to the two types of variables respectively. The results show that the present optimization formulation can realize the objective of shape control of structures through the low controlling cost.

2013, 45(2): 214-222. doi: 10.6052/0459-1879-12-295

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THERMAL SHOCK PROTECTION BASED ON MICROTUBE AND MICROPOST ARRAY SURFACE

Zhang Wenlong, Zhou Ping, Wu Chengwei

To prevent primary pump shaft and the piping system from crack caused by the thermal shock of cold water at the nuclear power facilities, a new method for thermal shock protection is proposed. The proposed thermal shock protection technique is based on bionic surface with microstructure covered with a layer of water film. Due to the low thermal diffusivity of the water film,the thermal stress at the surface caused by the thermal shock is reduced and the probability of the thermal fatigue failure of the structure will be decreased. In the present paper,using COMSOL multifield coupling analysis software,the temperature field and thermal stress distribution of microstructure at the solid surface are analyzed combining finite elements method and infinite elements method. Effects of thermal shock time, the transition corner between microstructure and bulk material, the thickness of viscous boundary layer and the boundary temperature di erence on the surface thermal stress are studied. Based on the simulation results,it is found that the surface thermal stress is remarkably reduced contributed to the surface microtubes and microposts array structure, and there is an optimal shape of transition structure between microstructure layer and bulk material for minimizing thermal stress.

2013, 45(2): 223-228. doi: 10.6052/0459-1879-12-175

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SOUND ABSORBING PROPERTY OF POROUS METAL MATERIAL AT HIGH TEMPERATURE AND HIGH SOUND PRESSURE LEVEL

Zhou Han, Wu Jiuhui, Hu Zhiping

On the basis of the statistical turbulence theory of the porous metal material, an analysis model which can be used under the high temperature and high sound pressure level (SPL) conditions is obtained by considering the influence of temperature and pressure on the acoustical parameters. The relationship between the acoustical pressure amplitude and wire diameter, porosity and other physical parameters at various temperatures and SPL are investigated. It is shown in the results that the acoustic pressure amplitude increases with the increase of either the temperature or the sound pressure level. The theoretical results are in good agreement with existing experimental results. The work in this paper provides a theoretical basis for the vibration and noise reduction design of porous metal materials at high temperature and high acoustic pressure level.

2013, 45(2): 229-235. doi: 10.6052/0459-1879-12-161

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A NEW METHOD OF IDENTIFYING MAIN FAILURE MODE ABOUT TRUSS STRUCTURE

Chen Weidong, Li Jiancao, Yu Yanchun, Yang Wenmiao, Wang wei, Yan Han

On analyzing two kinds of the existing identification methods of main failure mode, the problem that the reserved candidate failure elements are not all the necessary elements of failure modes has been studied. According to the basic theory of structural mechanics, the effects of failure elements on residual structure internal force has been analyzed, and the relationships have been deduced; According to the relationships, the linear correlation between failure modes which have the same basic failure elements has been proved; and based on the probability theory, it can be concluded that the main failure modes contain only basic failure elements, thus that the necessary elements of main failure modes are exactly the basic elements has been proved. Based on structure geometry analysis theory, failure modes of structural system have been analyzed, and the basic elements range of structural system failure has been discussed. And then using the idea of gradually building structure, the divided method of minimum existence range of the basic elements has been set up. A new method of identifying main failure mode about trusswork has been put forward combined with the branch bounding method. According to examples analysis, it can prove that this method is rational and has high-efficiency, and simultaneously it can ensure no omission of any main failure modes.

2013, 45(2): 236-244. doi: 10.6052/0459-1879-12-248

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A NEW LINEARIZATION METHOD OF MOHR-COULOMB YIELD SURFACE FOR LOWER BOUND PROBLEMS

Li Chunguang, Zhu Yufei, Liu Feng, Deng Qin, Zheng Hong

Compared with the limit equilibrium method, the lower limit analysis has a more rigorous mechanics foundation, and the safety factor acquired by the lower limit analysis is more conservative and valuable. Although many scholars have done many useful researches on it, however, the classical linearization method cannot solve the general problem of the anisotropy of strength. In this paper, spatial discretization is implemented, and the yield criterions on the discrete directions are built. Finally, pseudo cohesion is introduced to keep the property of lower bound analysis. The examples show that the result can converge to exact solution stably from below. Proposed method not only enriches the lower bound theory based FEM and linear programming, also lays a solid foundation for anisotropic problems.

2013, 45(2): 245-250. doi: 10.6052/0459-1879-12-187

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A HIGH-ORDER RIGID-FLEXIBLE COUPLING MODEL OF A ROTATING FLEXIBLE BEAM UNDER LARGE DEFORMATION

Chen Sijia, Zhang Dingguo, Hong Jiazhen

The dynamic modeling theory of a flexible beam which is rotating in a plane is further studied, and the highorder coupling dynamic model is investigated here. Both the transversal deformation and the longitudinal deformation of the flexible beam are considered. And the non-linear coupling deformation term, also known as the longitudinal shortening term caused by transversal deformation, is considered here. The high-order terms related to the non-linear coupling term are retained, which are ignored in the first-order approximation coupling modeling. Thus, we can get the rigid-flexible coupling dynamic equations of the system. The high-order coupling model can not only be used in small deformation case, but also in large deformation case. Then simulations, compared to the absolute nodal coordinate formulation and the first-order coupling model, are given to prove the validity of the high-order coupling model. And the results show that the high-order coupling model can make up for the deficiency of the first-order approximation coupling model in the large deformation situation.

2013, 45(2): 251-256. doi: 10.6052/0459-1879-12-221

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STOCHASTIC P-BIFURCATION OF TRI-STABLE VAN DER POL-DUFFING OSCILLATOR

Hao Ying, Wu Zhiqiang

This paper aims to investigate the stochastic P-bifurcations in the tri-stable Van der Pol-Duffing oscillator with additive and multiplicative Gauss noise. By using the stochastic averaging method, the stationary probability density function of amplitude is obtained. Then based on the singularity theory of the deterministic system, the explicit parameter conditions for P-bifurcation are deduced, and eleven types of qualitatively different probability density curves are founded. Finally the effects of three coe cients, one for linear damping and two for random excitation strength, are discussed. The results are verified by Monte-Carlo numerical simulations. The method used here is also suitable for other systems' P-bifurcation analysis.

2013, 45(2): 257-264. doi: 10.6052/0459-1879-12-169

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COLLINEAR LIBRATION POINTS WITH CONTINUOUS LOW THRUST

Hou Xiyun, Liu Lin

Collinear libration points of the restricted three-body problem with constant low thrust are studied. Varying the direction of the low thrust, the positions of the collinear libration points vary correspondingly. The set of the point L1 or L2 forms the shape of an elliptical “sphere” in space while the set of the point L3 forms the shape of a “banana”. The triangular libration points are also studied to show how they are connected with the collinear libration point L3. Concentrating on the points L1 and L2, linear solutions of the motions around them are obtained. Due to the asymmetry introduced by the low thrust, the linear solution has a form different from that of the unperturbed case. High order analytical solutions are also constructed. These solutions are compared with numerically integrated orbits. The comparison shows good agreement with each other. At last, periodic families around these two points are studied. These families include the planar and the vertical Lyapunov families, the halo family and another special periodic family. Bifurcation phenomena of these families are described.

2013, 45(2): 265-273. doi: 10.6052/0459-1879-12-191

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EFFECTS OF 2-TUNNEL AND 3-TUNNEL FIXATION ON BIOMECHANICS OF TIBIOFEMORAL JOINT

Huang Rongying, Guo Yunfei, Zhang Gaolong, Zheng Hongguang, Wei Xiaodong

Based on MR images of normal human knee at four flexion angles(0?, 25?, 60?, 80?), we built 3D finite element models of the normal tibiofemoral joint and two medial meniscus transplantation models (2-tunnel fixation and 3-tunnel fixation). Two kinds of loads (a single compressive load, the combined loads of compressive load and internal rotation torque) were applied to models in the finite element simulation, and the stress and displacement distribution were extracted from all models. We compared the von-Mises equivalent stress of articular cartilage and meniscus and the displacement of meniscus in transplantation models with those in the normal model, respectively. We concluded that the 3-tunnel fixation was superior to 2-tunnel fixation, and 3-tunnel fixation can restore the tibiofemoral joint function better.

2013, 45(2): 274-282. doi: 10.6052/0459-1879-12-261

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NUMERICAL MODELING FOR FLUID-STRUCTURE INTERACTION UNDER BLAST AND IMPACT LOADING RESPONSE

Guo Pan, Liu Jun, Wu Wenhua

In this paper, a framework for numerical modeling of fluid and structure interaction (FSI) under blast shock wave has been developed and implement by loose coupling method on the basis of the equivalence of displacement condition. The nonlinear displacement of solid field is simulated by Discontinuous Galerkin Finite Element Method (DGFEM) which is based on the Lagrange method. The flow field is computed by using the cell-centered finite-volume scheme on the assumption of the improved spring analogy. Compared to the analytical results of one-dimensional spring piston problem, the present loose coupling method shows the good performance in computational accuracy in simulating fluid-structure interaction problem. Furthermore, the program framework exhibits the good abilities in simulating the coupling interacting behavior of the reflection and superposition of flow field due to the complex solid deformations.

2013, 45(2): 283-287. doi: 10.6052/0459-1879-12-118

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PREDICTIONS OF EFFECTIVE OUT-PLANE SHEAR MODULUS AND SIZE EFFECT OF HEXAGONAL HONEYCOMB

Zhang Weihong, Duan Wendong, Xu Yingjie, Zhu Jihong

In this paper, a cylinder torsion model and a torsion energy method are proposed to predict equivalent outplane shear modulus of hexagon cellular materials. An analytical expression for its size effect is constructed in terms of volume fraction(ν), number of cells in circumference(n), radius of the hollow cylinder(r) and number parameter of cell layers(m). Comparisons are made among the results of torsion energy method, finite element numerical simulation and G-A microstructure mechanical method. The size effect is revealed and proved theoretically. Numerical results show that when the cell size trends to be infinitely small with regard to the size of the structure, predicted results approach those obtained by mesoscopic mechanics method. Due to the cyclic symmetry of periodic cellular materials, it is shown that the computing efficiency can be greatly increased by means of the substructure model.

2013, 45(2): 288-292. doi: 10.6052/0459-1879-12-073

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A NEW SPATIAL THIN-WALLED BEAM ELEMENT INCLUDING TRANSVERSE AND TORSIONAL SHEAR DEFORMATION

Wang Xiaofeng, Yang Qingshan

Based on the Timoshenko and Benscoter's theory, a new spatial thin-walled beam element with an arbitrary open or closed cross section is proposed in this paper, accounting for the influences of shear deformation, flexural and torsional coupling and warping shear stress. With introduction of an interior node to the element, three-node interpolation functions are adopted for bending angles and warping angle to consider shear deformation and warping shear stress, and to avoid shear locking simultaneously. Through a kinematic description of the cross section of a deformed thin-walled beam under loads, the flexural-torsional coupling is included in the displacement and strain equations. In order to verify its accuracy and convergence, some numerical examples are analyzed and their results obtained from the present element are compared with theoretical solutions and numerical solutions of the commercial finite element software and other literatures. Comparisons indicate that the present element is free of shear locking and more accurate than those beam elements presented in other documents.

2013, 45(2): 293-296. doi: 10.6052/0459-1879-12-218

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DYNAMIC ANALYSIS OF BOUNDED DOMAINS BY SBFE AND THE IMPROVED CONTINUED-FRACTION EXPANSION

Chen Denghong, Du Chengbin

By using the continued-fraction solution and introducing auxiliary variables, the dynamic sti ness and mass matrices of bounded domains are expressed in high-order matrices. The formulation is based on an improved continuedfraction solution of the scaled boundary finite element equation in dynamic sti ness. This continued-fraction solution converges over the whole frequency range with increasing order of expansion. Compared to the original approach, it leads to numerically more robust for large-scale systems and arbitrarily high orders of expansion. The eigenvalue problem of a regular octagon is considered. Transient analysis of a bounded rectangular plane is also presented. The results of two numerical examples demonstrate that the proposed method is superior to the original approach and it is suitable for dynamic analysis of large-scale systems.

2013, 45(2): 297-301. doi: 10.6052/0459-1879-12-198

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NUMERICAL SIMULATION STUDY ON DYNAMIC RESPONSE UNDER RUDDER CONTROL

Chen Jianqiang, Chen Qi, Yuan Xianxu, Xie Yufei

The vehicle would have a dynamic response process in aerodynamics under a rudder deflecting manipulation. However, this dynamic process is often ignored in current research about the characteristics of rudder manipulation and only the static characteristics such as the rudder's inversed effect and control efficiency are taken into account. It is focused in this paper the dynamic response process about a supersonic maneuvering aircraft under the rudder manipulation. A dynamic mesh technique and unsteady numerical methods are developed to simulate the relative motion between the rudder and the aircraft body and the dynamic responses. Supersonic flow about a square cross-section missile and the flow about a forced pitching airfoil are simulated to verify the numerical methods. Then the rudder manipulation processes of the square cross-section missile under the three generic rudder deflection types (step-type, pulse-type and sinusoid-type) are simulated, and dynamic response of the missile's pitching motion under the rudder control is obtained. The results indicate that a fast rudder control process can even cause instability in the pitching motion of the missile.

2013, 45(2): 302-306. doi: 10.6052/0459-1879-12-222

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