Prediction of vibrations induced by underground railway traffic in Beijing

This paper examines the problem of subway induced vibrations on line 4 of Beijing metro, which is currently under construction and is planned to pass in close proximity of the Physics Laboratory of Beijing University. The laboratory has a lot of equipment that is very sensitive to traffic induced vibrations and future operation of metro line 4 is a matter of concern. Hence, it is important to study the influence of subway induced vibrations inside the laboratory and to propose a viable solution to mitigate the vibrations. In this paper, the tunnel north of Chengfulu station is modelled using a coupled periodic FE-BE model and the free-field response due to moving trains is predicted. In addition, vibration measurements have been performed on the site of the Physics Laboratory to estimate the existing vibration levels due to road traffic. The predicted and measured vibrations are superimposed to assess the vibrations due to the combined effect of road and railway traffic in the vicinity of the Physics Laboratory. Apart from the numerical investigations, vibration measurements have also been performed on a similar site at line 1 of Beijing metro to substantiate the estimated results on metro line 4. Finally, it is studied how the vibrations can be controlled using a floating slab track, which is widely used as an effective measure of vibration isolation in tunnels. The efficiency of a 7.9 Hz floating slab track as a vibration countermeasure is assessed in this paper. This study demonstrates the applicability of the numerical model for the relevant assessment of subway induced vibrations and its use to study the performance of different track structures in the tunnel.     

An accurate and robust numerical method for micromagnetics simulations

We propose a new robust, accurate, and fast numerical method for solving the Landau–Lifshitz equation which describes the relaxation process of the magnetization distribution in ferromagnetic material. The proposed numerical method is second-order accurate in both space and time. The approach uses the nonlinear multigrid method for handling the nonlinearities at each time step. We perform numerical experiments to show the efficiency and accuracy of the new algorithm on two- and three-dimensional space. The numerical results show excellent agreements with exact analytical solutions, the second-order accuracy in both space and time, and the energy conservation or dissipation property.     

无源测向跟踪的数值预测方法

本文提出了一类基于最小二乘估计的数值算法，用于解决无源被动测向系统预置跟踪的实际问题，通过建立观测器及目标运动模型，分析了算法理论上的合理性，计算机模拟实验结果示出了预测方法的良好性能，并就其对机动目标的适应能力进行了比较。     

A new numerical method for nonlocal electrostatics in biomolecular simulations

The electrostatic behavior of biomolecules solved in water can be described by an elliptic system of partial differential equations for the potential. In previous studies, this system has been solved by the Boundary Element Method (BEM).     

A new method for predicting nonlinear structural vibrations induced by ground impact loading

Based on nonlinear theory and field measurements, a hybrid method for modeling the time history of structural vibrations resulting from impact loading in the vicinity of a structure is presented in this paper. The characteristics of the medium present between the impact source and a measured location inside of a structure are depicted by a nonlinear system that can be modeled by a Volterra functional series. The nonlinear system can be identified by the inputs and the corresponding output signals, which can be obtained by performing in situ experiments. Then, the predicted signal induced by a known impact loading at the measured location can be calculated using the identified Volterra functional series. Moreover, the structural vibration under a known impulse train can be controlled by adjusting the delay period according to the predicted results. In addition, the method has been verified by applying it in two practical applications: the cases of impact loading induced by either the impact of a hammer or blasting. The results show that the method features convenient application, high precision and extensive applicability for various types of impact loadings.     

The stochastic method for numerical simulations:: Higher order corrections

We derive discrete versions of stochastic differential equations governing the evolution of some random variable x(t) to arbitrary order in Δt, giving explicit formulae to second order. These are tested in the static case by examples where x takes values in the groups U(1) and SU(2).     

A frequency domain numerical method for airfoil broadband self-noise prediction

This paper describes a numerical approach, based in the frequency domain, for predicting the broadband self-noise radiation due to an airfoil situated in a smooth mean flow. Noise is generated by the interaction between the boundary layer turbulence on the airfoil surface and the airfoil trailing edge. Thin airfoil theory is used to deduce the unsteady blade loading. In this paper, the important difference with much of the previous work dealing with trailing edge noise is that the integration of the surface sources for computation of the radiated sound field is evaluated on the actual airfoil surface rather than in the mean-chord plane. The assumption of flat plate geometry in the calculation of radiation is therefore avoided. Moreover, the solution is valid in both near and far fields and reduces to the analytic solution due to Amiet when the airfoil collapses to a flat plate with large span, and the measurement point is taken to the far field.Predictions of the airfoil broadband self-noise radiation presented here are shown to be in reasonable agreement with the predictions obtained using the Brooks approach, which are based on a comprehensive database of experimental data. Also investigated in this paper is the effect on the broadband noise prediction of relaxing the ‘frozen-gust’ assumption, whereby the turbulence at each frequency comprises a continuous spectrum of streamwise wavenumber components. It is shown that making the frozen gust assumption yields an under-prediction of the noise spectrum by approximately 2dB compared with that obtained when this assumption is relaxed, with the largest occurring at high frequencies.This paper concludes with a comparison of the broadband noise directivity for a flat-plat, a NACA 0012 and a NACA 0024 airfoil at non-zero angle of attack. Differences of up to 20 dB are predicted, with the largest difference occurring at a radiation angle of zero degrees relative to the airfoil mean centre line.     

The experimental validation of a numerical model for the prediction of the vibrations in the free field produced by road traffic

The objective of the present paper is the experimental validation of a numerical model for the prediction of traffic-induced vibrations. The vibrations in the free field, generated by the passage of a vehicle on an uneven road, are predicted in two stages. First, the equations of motion of the vehicle are solved to determine the dynamic axle loads. Next, these axle loads are applied to the road and the free field vibrations are computed. An elaborate measurement campaign has been set up to validate this model. The response of a Volvo FL6 truck and the response in the free field have been measured simultaneously during the passage of the truck over an artificial road unevenness. The parameters related to the vehicle, the road and the soil have been determined experimentally. A comparison of the predicted and the measured response demonstrates the predictive qualities of the numerical model. Furthermore, the results provide a clear insight in the influence of the vehicle speed on the vehicle's and the free field response.     

Transfer matrix method for internal noise prediction of an arbitrary shell-cavity structure induced by temporal-spatial random surface excitation

Combining structural finite element method(FEM),acoustic finite element and boundary element methods,a model of elastic shell vibration of an arbitrary shell-cavity structure coupled with internal and external sound fields is built.In addition,the transfer matrices from the excitation force to vibration of the shell and internal sound field are calculated.As the fluctuating pressure of turbulent boundary layer(TBL) is a temporal-spatial random surface excitation,the overall shape function matrix is introduced,and then the relationship between power spectral density matrix of the generalized nodal force of the elastic shell and power spectral density of the temporal-spatial random surface excitation is derived.Utilizing the vibro-acoustic coupled transfer matrix,relationships between the power spectral densities of vibration of the elastic shell/internal sound field and the power spectral density matrix of the generalized nodal force are obtained.Thus,the calculation method of vibration and internal sound field of an arbitrary shell-cavity structure induced by temporal-spatial random surface excitation is established.A typical vibro-acoustic coupled model of a rectangular cavity with acoustic media internally and externally,and with elastic rectangular plate on one side,is taken as example.The vibration of the elastic shell and power spectral density of the internal sound field are calculated and compared with the analytical method.The two results generally agree with the analytical one,with deviations of about 1 dB and 2 dB,respectively.The transfer matrix method has good adaptability which is not restricted by the shell-cavity structure and the shape of the inner region.     

传递矩阵法预报时空随机激励下任意薄壳腔体内部噪声

利用结构有限元结合声有限元及边界元方法,建立了任意薄壳腔体弹性壳板振动与内外声场的耦合模型,并计算了激励力与壳板振动和内部声场之间的传递矩阵;湍流边界层脉动压力具有时空随机面激励特性,引入整体形状函数矩阵,进一步推导弹性壳板广义节点力功率谱密度函数矩阵与随机面分布激励力功率谱密度函数的关系,再利用声振耦合传递矩阵,得到弹性壳板振动和内部声场功率谱密度函数与广义节点力功率谱密度函数矩阵的关系,形成随机分布激励下任意薄壳腔体结构振动及内部声场的计算方法。以典型的内外均有声介质且一面为弹性矩形板的矩形腔声振耦合模型为例,计算了弹性壳板振动和内部声场功率谱密度函数,并与解析方法进行了比较,两者基本吻合,偏差分别为1 dB和2 dB左右。传递矩阵法不受腔体结构及其内部区域形状的制约,具有良好的适用性。     

A numerical model for the 3-D non-linear vibrations of an N-string

We present a non-linear numerical model describing the 3-D vibrations of a planar network of N sections of string which are connected together at one common mobile extremity. We call such a network N-string. For small-amplitude vibrations perpendicular to the N-string equilibrium plane, the numerical results coincide with the already known analytical solutions of the linear model. This non-linear model makes it possible to describe small- or large-amplitude 3-D vibrations of any kind of N-string subjected to an initial plucking. The equations of motion are also presented in a dimensionless form and a vast dimensionless physical parameter space is identified. The numerical model can be extended to more complex networks of strings.     

Exploring an opinion network for taste prediction: An empirical study

We develop a simple statistical method to find affinity relations in a large opinion network which is represented by a very sparse matrix. These relations allow us to predict missing matrix elements. We test our method on the Eachmovie data of thousands of movies and viewers. We found that significant prediction precision can be achieved and it is rather stable. There is an intrinsic limit to further improve the prediction precision by collecting more data, implying perfect prediction can never obtain via statistical means.     

Two-lane traffic simulations with a blockage induced by an accident car

Based on the two-lane traffic model proposed by Chowdhury et al., a highway traffic model with a blockage induced by an accident car is proposed, in which both symmetric lane changing rules and asymmetric lane changing rules are adopted. The fundamental diagrams and spatial-temporal profiles are presented after the numerical simulation and the jam transition is studied. It is shown that the accident car not only causes a local jam behind the accident car, but also causes vehicles to cluster in the bypass lane. The asymmetric lane changing rules are more advantageous in reducing the local jam than the symmetric lane changing rules when the accident car is in the right lane, and the symmetric lane changing rules are superior when the accident car is in the left lane. Furthermore the curves of lane-changing frequency against the total density are given. It is found that the vehicles will change lane more frequently when traffic is inhomogeneous with different types of vehicle or with an accident car.     

基于最大Lyapunov指数预测的EMD端点延拓

将最大Lyapunov指数预测引入到经验模式分解（EMD）的端点延拓中,提出用最大Lyapunov指数预测后的数据进行EMD分解的方法.该方法充分考虑了信号的内在特性,使端点处的延拓更加合理,从而使得三次样条曲线在端点处不会发生大的摆动.通过线性、非线性和实测信号的分析表明该方法能够有效地抑制端点效应. 关键词： EMD 非线性 Lyapunov指数 端点效应     

Verification of numerical simulation method for entropy generation of radiation heat transfer in semitransparent medium

The numerical simulation method of radiative entropy generation in participating media presented by Caldas and Semiao [Entropy generation through radiative transfer in participating media: analysis and numerical computation. JQSRT 2005;96:423-37] is extended to analyze the radiative entropy generation in the enclosures filled with semitransparent media. A discrete ordinates method is used to solve radiative transfer equation and radiative entropy generation. Two different examples are employed to verify the numerical simulation method of radiative entropy generation in the enclosure. Numerical results of dimensionless radiative entropy generation of enclosure are identical to that of entire thermodynamics analysis for the enclosure system. This numerical simulation method can be used in the entropy generation analysis of high-temperature systems such as boilers and furnaces, in which radiation is the dominant mode of heat transfer.     

New proposal for numerical simulations of theta-vacuum-like systems

We propose a new approach to perform numerical simulations of theta-vacuum-like systems, test it in two analytically solvable models, and apply it to CP3. The main new ingredient in our approach is the method used to compute the probability distribution function of the topological charge at theta=0. We do not get unphysical phase transitions (flattening behavior of the free energy density) and reproduce the exact analytical results for the order parameter in the whole theta range within a few percent.     

Two-dimensional numerical simulations on the performance of an annular jet pump

Two-dimensional numerical simulations were carried out to investigate the effects of the shape of mixing chamber on the performance of annular jet pumps for several jet flow rates. We confined the computational domain to a jet, a mixing chamber, and the outlet of the pump. Several computations have been carried out to seek the effects of reducing angles on the suction performance and the efficiency of the jet pump. Some numerical results regarding suction flow rates, head ratios, the efficiencies of the pump, flow fields and pressure distributions are shown and discussed.