力学学报

DRAG REDUCTION CONTROL BASED ON ACTIVE WALL DEFORMATION

Ge Mingwei, Xu Chunxiao, Huang Weixi, Cui Guixiang

Direct numerical simulation was performed to turbulent channel flow with active wall deformation under opposition control. It is shown that when the maximum wall displacement is limited within 5 wall units, the pressure drag is negligible and the friction drag can be reduced by 7.6%. Under the control, the turbulence intensity and Reynolds shear stress are suppressed obviously, and the logarithmic region in mean velocity profile is moved upward. Influenced by the vertical motion of the wall, streaky structures are weakened in strength and enlarged in scale. The streamwise vortices moved further away from the wall with weakened strength, and their tilting and twisting angles are both reduced. The wall appears in the shape of streamwise elongated streaky structures with the mean spacing of 90 wall units.

2012, (4): 653-663. doi: 10.6052/0459-1879-11-198

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NUMERICAL STUDY ON THE RM INSTABILITY OF A HEAVY-GAS CYLINDER INTERACTED WITH RESHOCK

Wang Xiansheng, Si Ting, Luo Xisheng, Yang Jiming

The evolution of the Richtmyer-Meshkov (RM) instability of a two-dimensional gas cylinder interacted with incident planar shock and reshock is investigated numerically. The procedure VAS2D based on the finite volume method combined with an adaptive mesh refinement is used to effectively capture the wave propagation and interface deformation. The incident planar shock Mach number is 1.2, and the gas cylinder is filled with sulfur hexafluoride (SF₆) surrounded by air, and the shock tube end wall is regarded as a solid wall. In the simulations, the distances from the gas cylinder to the end wall are changed to obtain the influence of the reshock on the gas cylinder shape, the interface scale and circulation variation trend of the evolving interface. It is found that the evolution of the reshock inhomogeneity and the variation of the circulation with time are dependent on the development of the distorted gas cylinder when the reshock arrives. The generation and distribution of the vorticity during the interaction of the reshock with the evolving gas cylinder are different from the case without reshock, which indicates the physical mechanism of the interface evolution under different conditions.

2012, (4): 664-672. doi: 10.6052/0459-1879-11-245

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DIRECT NUMERICAL SIMULATION OF DECAYING COMPRESSIBLE ISOTROPIC TURBULENC

Li Hu, Zhang Shuhai

Statistical characteristics of turbulent flow and its compressibility effect within decaying isotropic turbulence have been studied using direct numerical simulation through fifth-order WENO scheme. The research reveals that turbulent kinetic energy mainly comes from solenoidal velocity field and small scale motions are sensitive to the compressibility. As the compressibility increases, turbulent kinetic energy transport between the large and the small turbulent motions becomes more rapid. Furthermore, the increasing initial turbulent Mach number leads to a fast rise of fraction of dissipation rate due to dilatation. Through scaling law analysis, it is shown that transverse velocity structure function (TVSF) still satisfies extended self-similarity (ESS), so does longitudinal velocity structure function (LVSF) at low order; while if the order of LVSF is high (p ≥ 5), the ESS will not hold true. For weakly compressible turbulence, the transverse turbulent velocity fluctuations are more intermittent than the longitudinal fluctuations as same as that in the incompressible turbulence. In contrast, longitudinal fluctuations are more intermittent than transverse fluctuations for strongly compressible turbulence.

2012, (4): 673-686. doi: 10.6052/0459-1879-11-353

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SPATIAL INSTABILITY ANALYSIS OF AN ANNULAR SWIRLING VISCOUS LIQUID JET

Yan Kai, Ning Zhi, Lü Ming

The effect of liquid viscosity on an annular swirling viscous liquid jet with different swirl strength values is studied with linear instability analysis. The spatial mode instability analysis for para-sinuous mode and para-varicose mode is carried out. An analytical form of dispersion relation is derived, which governs the disturbance growth for an annular swirling viscous liquid sheet with a solid vortex swirl velocity profile subjected to three-dimensional disturbances. The results show that, with higher swirl strength, the increase of viscosity hinders the breakup and makes the domain frequency and domain wave number smaller. However, at low swirl strength, the increase of viscosity speeds up the breakup, makes the domain frequency in the para-sinuous mode first decrease and then increase and makes the domain frequency in the para-varicose mode increase. Furthermore, at low swirl strength, the domain wave number in the para-sinuous mode first decreases and then increases, the domain wave number in the para-varicose mode increases with an increase in liquid viscosity.

2012, (4): 687-693. doi: 10.6052/0459-1879-11-197

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DETERMINATION OF ENTRAINMENT RATE FOR SUPERCAVITATION VEHICLE

Yang Wugang, Wen Kaige, Li Sancai, Zhang Yuwen

The effect of liquid viscosity on an annular swirling viscous liquid jet with different swirl strength values is studied with linear instability analysis. The spatial mode instability analysis for para-sinuous mode and para-varicose mode is carried out. An analytical form of dispersion relation is derived, which governs the disturbance growth for an annular swirling viscous liquid sheet with a solid vortex swirl velocity profile subjected to three-dimensional disturbances. The results show that, with higher swirl strength, the increase of viscosity hinders the breakup and makes the domain frequency and domain wave number smaller. However, at low swirl strength, the increase of viscosity speeds up the breakup, makes the domain frequency in the para-sinuous mode first decrease and then increase and makes the domain frequency in the para-varicose mode increase. Furthermore, at low swirl strength, the domain wave number in the para-sinuous mode first decreases and then increases, the domain wave number in the para-varicose mode increases with an increase in liquid viscosity.

2012, (4): 694-700. doi: 10.6052/0459-1879-11-320

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A THEORETICAL INVESTIGATION ON THE MAXIMUM PRESSURE OF THE UNSTEADY CAVITY CLOSURE POSITION

Chen Weiqi, Wang Baoshou, Yi Shuqun, Zheng Jun, Chu Xuesen

The cavity closes on the body surface with a re-entry jet at its rear when the body moves with a high speed and induces a cavity in the fluid. The re-entry jet at the closure position of the cavity causes the overpressure on the body. The closure position moves with a considerable speed when the cavity is in the gravity field or when the body velocity or the pressure in the cavity varies. The unsymmetrical distribution of the moving closure position with overpressure arouses significant effect on the motion stability of the body. As far as we know, no theoretical researches about overpressure on the moving closure position owing to the unsteady of the cavity have been reported in literature. In this paper, based on the theory of the potential flow, the position of maximum pressure in closure region of unsteady cavity in gravity field is investigated, and the theoretical formula of maximum pressure is presented, which reveals the relation between the maximum pressure and the developing speed of the cavity. Two special fluid characters of maximum pressure are proved. Finally, a verification experiment is designed and performed for measuring the maximum pressure at the closure position, and the comparison is finally performed between the results of the theoretical formulation and the experiment, which confirms the theoretical approach in this study.

2012, (4): 701-708. doi: 10.6052/0459-1879-11-261

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STUDY ON OPTIMIZATION OF TRAFFIC FLOW VELOCITY-DENSITY MODELS FOR URBAN FREEWAY

Wu Zheng, Guo Mingmin, Xu Qian

Traffic flow models are important means to describe various complex traffic procedures and to establish intelligence traffic systems of urban freeway. Generally the formation of a traffic flow model requires massive observation as well as in-depth analysis of real road traffic. Empirical data are indispensable to identify the parameters of the corresponding model. It is drawn in this paper that 210920 "car velocity-headway distance" data pairs are obtained from the long time video recordings of urban freeway in four Chinese cities. Three traffic flow data-samples are made up of parts of these measuring data and are applied to an optimization study of several manifold well-known traffic flow velocity-density models. Some qualitative and quantitative results of the optimal free velocity value in the models have been obtained. The superiorities of modified Kerner-Konhauser equilibrium function and Payne's one are found in simulating the measuring data under the traffic condition of be free and be crowd, respectively, while Greenshields model (i.e. 1D pipe fluid model when m=3) can give the satisfying results for the measuring data when the traffic condition is between free and crowd.

2012, (4): 709-717. doi: 10.6052/0459-1879-11-377

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STUDY ON THE IMPACT OF SETTINGS OF BUS STOPS WITHOUT BUS BAY ON TRAFFIC FLOW

Kang Sanjun, Xue Yu

In this paper, the impact of the arrangement of bus stations without bus bay along the same section of the highway on traffic flow is studied by using the cellular automata traffic model on two-lane. Simulated results show the impact of the distributed bus stops on traffic capacity is significantly reduced via comparing with the centralized ones. The gap of the location of bus stops, the length of the platforms, stopping time and the ratio of bus in mixing traffic have a major impact on the urban road capacity.

2012, (4): 718-726. doi: 10.6052/0459-1879-11-212

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STUDY ON RESPONSES OF WIND-INDUCED INTERNAL PRESSURE FOR BUILDING WITH INTERNAL PARTITIONING AND OPENINGS WHEN CONSIDERING ROOF FLEXIBILITY

Yu Xianfeng, Quan Yong, Gu Ming

The equations governing wind-induced internal pressure responses for a two-compartment building when the roof responds in dynamic and quasi-static manner are derived based on some reasonable assumptions. The gain functions, power spectra and RMS of internal pressures coefficients in both compartments calculated from the dynamic and quasi-static models are analyzed by the numerical example. The results show that the dynamic model must be used to calculate the response of internal pressure when the roof is flexible, otherwise, the simplified quasi-static model can be applied; with increased natural frequency of the roof, all the three resonance frequencies of the "internal pressure-roof" system increase, and the peak internal pressure responses at the first two frequencies also increase, while it decreases at the third resonance frequency. In addition, the RMS internal pressure coefficients obtained from both models also increase, but their difference decreases.

2012, (4): 727-734. doi: 10.6052/0459-1879-11-314

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THE ROBUST OPTIMIZATION DESIGN BASED ON MOMENT ESTIMATION METHOD

Song Shufang, Lü Zhenzhou

Robust design is a widely used method for design under uncertainty to seek an optimal solution where the design objective is insensitive to the variations of input variables while the design feasibility under the variations is maintained. In this paper, the first four statistical moments, i.e., mean, variance, skewness and kurtosis, are used to measure the robustness of the objective function and probabilistic constraints. With the weighting factor method, a new robust design problem is modeled as a single objective optimization problem on the basic of first four statistical moments. We present moment estimation method as a rigorous method for statistical moment calculation and sensitivity computation with arbitrary input distributions and examine its effectiveness when applied to robust design. Three robust design examples of truss structures are provided to check the feasibility and effectiveness of robust design using moment estimation method and the proposed sensitivity analysis. \vskip 1mm oindent \bf

2012, (4): 735-744. doi: 10.6052/0459-1879-11-256

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MECHANISM OF HIGH FREQUENCY INSTABILITY CAUSED BY TRANSMITTING BOUNDARY AND METHOD OF ITS ELIMINATION——SH WAVE

Xie Zhinan, Liao Zhenpeng

To solve near-field wave problems by finite element method, absorbing boundary conditions were needed to simulate the unbound field robustly and efficiently. Taking numerical simulation of wave motion in SH waveguide as an example, high-frequency instability caused by transmitting boundary were discussed. Directly from discrete model, mechanism of instability was found by analyzing the matching relationship of motion equations of interior nodes and nodes on absorbing boundary. Results showed that instability was caused by high frequency wave, which was allowed to enter continuously from the absorbing boundary into the computational region without excitation. Thus an approach of eliminating such instability was proposed by adjusting the motion equations of interior nodes to change the matching relationship. Theoretical and numerical results showed that the present approach was efficient.

2012, (4): 745-752. doi: 10.6052/0459-1879-11-312

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DYNAMIC CHARACTERISTICS OF A SPHERICAL CAVITY IN NEARLY SATURATED VISCOELASTIC SOIL

Gao Huaxi, Wen Minjie

The steady-state dynamic responses of a spherical cavity in nearly saturated fractional derivative viscoelastic soil are investigated subject to the internal water pressure in the frequency domain. By a stress coefficient depending on the porosity of soil, the values of internal water pressure in medium and in pore water are determined, respectively. The soil skeleton and the lining are treated as a viscoelastic medium with fractional derivative constitutive relations and a porous flexible material, respectively. Based on Boit's theory, the analytical solutions for the displacement, stress and pore water pressure of the partial sealed spherical cavity in the nearly saturated soil subject to the internal water pressure are obtained by the displacement potential functions. The dynamic responses of the spherical cavity are analyzed for different physical and geometry parameters. It is shown that the dynamic behaviors of the soil are described reasonably by the fractional derivative constitutive model. The saturation degree has great influences on the stress and pore water pressure, while it has a smaller impact on the displacement.

2012, (4): 753-761. doi: 10.6052/0459-1879-11-303

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SUPER-HARMONIC RESONANCE OF FRACTIONAL-ORDER DUFFING OSCILLATOR

Shen Yongjun, Yang Shaopu, Xing Haijun

The super-harmonic resonance of Duffing oscillator with fractional-order derivative is studied, and the first-order approximate solution is obtained by averaging method. The definitions of equivalent linear damping coefficient and equivalent linear stiffness for super-harmonic resonance are established, and the effects of the fractional-order parameters on the equivalent linear damping coefficient and equivalent linear stiffness are also analyzed. The amplitude-frequency equation for steady-state solution associated with the stability condition is also presented, and the comparisons of the fractional-order and the traditional integer-order Duffing oscillator are fulfilled. At last the numerical simulation is used to analyze the effects of the parameters in fractional-order derivative on the amplitude-frequency curves.

2012, (4): 762-768. doi: 10.6052/0459-1879-11-378

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SYMPLECTIC RANDOM VIBRATION ANALYSIS FOR COUPLED VEHICLE-TRACK SYSTEMS WITH PARAMETER UNCERTAINTIES

Zhao Yan, Xiang Pan, Zhang Youwei, Lin Jiahao

A new random vibration-based assessment method for coupled vehicle-track system with uncertain parameters subjected to random track irregularity is proposed in this paper. The vehicle system is simplified as a spring-mass-damper system model by using physical coordinates, and the uncertainties in the primary suspension and secondary suspension for the body, bogies and wheels are modeled by Gaussian random variables. The track is treated as a Bernoulli-Euler beam connected to sleepers and the ballast and is regarded as an infinite periodic structure. The state equation for a typical sub-structure of the track is established in the Hamiltonian system. The dynamic equations of the coupled vehicle-track system under the mixed physical coordinates and symplectic dual coordinates are established based on the wheel-rail coupling relations. The control equation with respect to the uncertain parameters is derived by using the Hermitian orthogonal polynomials for dynamic analysis of the coupled systems. By using the periodic features of the track, the "curse of dimensionality" of the control equation is effectively reduced. The wheel-rail contact forces due to the track irregularity are assumed to be fully coherent stationary random processes. An assessment of the random vibration with respect to the uncertain parameters is established for the coupled vehicle-track system by using the pseudo-excitation method (PEM). The proposed method is compared with the Monte Carlo simulations, and it is found that good agreements are achieved even for cases with strong uncertainties in system parameters.

2012, (4): 769-778. doi: 10.6052/0459-1879-11-361

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COMPUTATION OF NEURONAL ENERGY BASED ON INFORMATION CODING

Wang Rubin, Zhang Zhikang

By re-examining the energy model for neuronal activities, we show the inadequacy in the current understanding of the energy consumption associated with the activity of a neuron. Specifically, we show computationally that a neuron first absorbs energy and then consumes energy during firing action, and this result cannot be produced from any current models of neurons or biological neural networks. Based on this finding, we provide an explanation for the observation that when neurons are excited in the brain, blood flow increases significantly while the incremental consumption of oxygen is very small. We can also explain why external stimulation and perceptual emergence are synchronized.

2012, (4): 779-786. doi: 10.6052/0459-1879-11-340

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SUPPRESSION OF VORTEX SHEDDING FROM AN OSCILLATING CIRCULAR CYLINDER BY A SPLITTER PLATE

Wang Sai, Shao Chuanping

A splitter plate was used to suppress vortex shedding from a cylinder oscillating in stream-wise direction. The splitter plate was set on the centerline of the cylinder wake, and the control parameters were the length of the splitter plate L/D and the gap between the central position of cylinder oscillation and the leading edge of the plate G/D, where D is the diameter of cylinder. The value ranges of L/D and G/D in the test were 1.0～ 4.0 and 0.8～ 4.4, respectively, and the ranges of Reynolds number and the reduced frequency of cylinder oscillation were Re=V_∞D/v=1.01×10⁴～1.69×10⁴ and f_eD/V_∞=0～0.03, respectively. Results of smoke wire visualization and hot-wire measurement in wind tunnel show that vortex shedding from both sides of oscillating circular cylinder can be suppressed when the splitter plate is placed in a certain region called the effective zone. The effective zone area increases with the increase of length of splitter plate and decreases with the increases of Reynolds number and the reduced frequency of cylinder oscillation.

2012, (4): 787-791. doi: 10.6052/0459-1879-11-341

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AN IMPLICIT ALGORITHM FOR DISCONTINUOUS GALERKIN METHOD BASED ON NEWTON/GAUSS-SEIDEL ITERATIONS

Liu Wei, Zhang Laiping, He Xin, He Lixin, Zhang Hanxin

An efficient implicit algorithm was developed for high-order discontinuous Galerkin (DGM) based on Newton/Gauss-Seidel iteration approach. The second-order to the forth order DGMs based on Taylor basis functions were employed to carry out the spatial discretization. Newton iteration scheme was used to solve the nonlinear system, and the linear system was solved with one-step Gauss-Seidel iteration. In addition, the effects of several parameters in the implicit scheme, such as the CFL number, the Newton sub-iteration steps, and the update frequency of mass-matrix, have been investigated for two-dimensional Euler equations. Two typical cases, including subsonic flows over a bump and a NACA0012 airfoil, were simulated, and compared with the traditional explicit Runge-Kutta scheme. The numerical results demonstrate that the present implicit scheme can accelerate the convergence history evidently.

2012, (4): 792-796. doi: 10.6052/0459-1879-11-352

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ADVANCED EQUIVALENT PLANE METHOD FOR STRUCTURAL SYSTEM RELIABILITY

Chen Weidong, Li Jiancao

The theory of multiple correlations was adopted in structural system reliability analysis for the first time and the recursive formula of equivalent function for parallel system was deduced, and multiple correlation-equivalent plane method was presented. The method solves the problem that equivalent principles of equivalent plane method don't include the correlation information, and overcomes the lack of great error of equivalent plane method used in parallel system reliability. The result proves that the method has high accuracy and is better to traditional structure system analysis method, and is suitable for large structural system reliability analysis.

2012, (4): 797-801. doi: 10.6052/0459-1879-11-328

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DYNAMIC ANALYSIS OF MULTIBODY SYSTEMS WITH PROBABILISTIC PARAMETERS

Zhao Kuan, Chen Jianjun, Yan Bin, Ma Hongbo

Dynamic analysis of multibody systems with probabilistic parameters was presented. Dynamic modeling of multibody systems was obtained by Lagrange's method. The probabilistic differential algebraic equations were transformed into pure probabilistic differential equations by generalized coordinate partitioning method. The Newmark step-by-step integration method was used to calculate the results. Using the method of random factor method, the numerical characteristics of the system response were derived, and the results were expressed in statistic view. As an illustrating example, dynamic modeling of a rotating bar and sliding block system considering the probabilistic of load, geometric and physical parameters was presented. Compared with the result of Monte-Carlo numerical simulation method, the accuracy and efficiency of the method are verified. The results illustrate that the probabilistic parameters affect the dynamic response of the multibody system and the dynamic modeling with probabilistic parameters can objectively reflect the dynamic behavior of the objective systems.

2012, (4): 802-806. doi: 10.6052/0459-1879-11-342

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2012 Vol. 44, No. 4

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