Excessive vibrations of railway vehicles induced by dynamic impact loadings have a significant impact on train operating safety and stability; however, due to the complexity and diversity of railway lines and service environment, they are extremely difficult to eliminate. A comprehensive overview of recent studies on the impact vibration behavior of railway vehicles was given in this paper. First, the sources of impact excitations were categorized in terms of wheel-rail contact irregularity, aerodynamic loads, and longitudinal impulses by train traction/braking. Then the main research approaches of vehicle impact vibration were briefly introduced in theoretical, experimental, and simulation aspects. Also, the impact vibration response characteristics of railway vehicles were categorized and examined in detail to various impact excitation sources. Finally, some attempts of using the railway vehicle vibration to detect track defects and the possible mitigation measures were outlined.
An artificial intelligence (AI) open-loop control system is developed to manipulate a turbulent boundary layer (TBL) over a flat plate, with a view to reducing friction drag. The system comprises six synthetic jets, two wall-wire sensors, and genetic algorithm for the unsupervised learning of optimal control law. Each of the synthetic jets through rectangular streamwise slits can be independently controlled in terms of its exit velocity, frequency and actuation phase. Experiments are conducted at a momentum-thickness-based Reynolds number Reθ of 1450. The local drag reduction downstream of the synthetic jets may reach 48% under conventional open-loop control. This local drag reduction rises to 60%, with an extended effective drag reduction area, under the AI control that finds optimized non-uniform forcing. The results point to the significant potential of AI in the control of a TBL given distributed actuation.
Aiming at the problem of unstable buffering process of electromagnetic buffer (EMB) under intensive impact load, a three-segment electromagnetic buffer is proposed. The inner tube and air-gap of EMB are divided into three segments. The finite element analysis and impact test results show that the resultant resistance force (RRF) curve has two hump-shaped peaks, which is the reason for the unstable buffering process. In order to stabilize the buffering process, a multi-objective optimization design method of EMB based on Nash game theory is proposed. Firstly, the optimization model is established by taking the two peaks of the RRF curve and the maximum buffer displacement as the optimization objectives. Secondly, the multi-objective optimization model is transformed into a game model by sensitivity analysis and fuzzy clustering. Then, a Nash equilibrium solution strategy of EMB Nash game model based on symmetric elitist information exchange is proposed, which integrates gene expression programming (GEP) surrogate model and genetic algorithm (GA) as an optimization solver. Finally, the Nash equilibrium of the game model is obtained. The results show that the smoothness of the RRF curve has been significantly improved, which proves the effectiveness of the game strategy.
A shape-memory double network hydrogel consists of two polymer networks: a chemically crosslinked primary network that is responsible for the permanent shape and a physically crosslinked secondary network that is used to fix the temporary shapes. The formation/melting transition of the secondary network serves as an effective mechanism for the double network hydrogel's shape-memory effect. When the crosslinks in the secondary network are dissociated by applying an external stimulus, only the primary network is left to support the load. When the secondary network is re-formed by removing the stimulus, both the primary and secondary networks support the load. In the past, models have been developed for the constitutive behaviors of double network hydrogels, but the model of shape-memory double network hydrogels is still lacking. This work aims to build a constitutive model for the polyacrylamide-gelatin double network shape-memory hydrogel developed in our previous work. The model is first calibrated by experimental data of the double network shape-memory hydrogel under uniaxial loading and then employed to predict the shape-fixing performance of the hydrogel. The model is also implemented into a three-dimension finite element code and utilized to simulate the shape-memory behavior of the double network hydrogel with inhomogeneous deformations related to applications.
Graphic abstract
A shape-memory double network hydrogel consists of a chemically crosslinked primary network and a physically crosslinked secondary network. The formation/melting transition of the secondary network serves as an effective mechanism for the shape-memory effect of the double network hydrogel. This work built a constitutive model for the polyacrylamide-and-gelatin double network shape-memory hydrogel. The model was first calibrated by experimental data and then employed to predict the shape-fixing performance of the hydrogel. The model was also implemented into a three-dimension finite element code and utilized to simulate the shape-memory behavior of double network hydrogel in complex geometries.
Understanding working principles and thermodynamics behind phase separations, which have significant influences on condensed molecular structures and their performances, can inspire to design and fabricate anomalously and desirably mechanoresponsive hydrogels. However, a combination of techniques from physicochemistry and mechanics has yet been established for the phase separation in hydrogels. In this study, a thermodynamic model is firstly formulated to describe solvent-aided phase and microphase separations in the hydrogels, which present significantly improved mechanoresponsive strengths. Flory–Huggins theory and interfacial energy equation have further been applied to model the thermodynamics of concentration-dependent and temperature-dependent phase separations. An intricately detailed phase map has finally been formulated to explore the working principle. The thermodynamic methodology of phase separations, combined with the constitutive stress–strain relationships, has a great potential to explore the working mechanisms in mechanoresponsive hydrogels.
The Boltzmann-Bhatnagar-Gross-Krook(BGK)model is investigated for its validity regarding the collision term approximation through relaxation evaluation. The evaluation is based on theoretical analysis and numerical comparison between the BGK and direct simulation Monte Carlo(DSMC) results for three specifically designed relaxation problems. In these problems, one or half component of the velocity distribution is characterized by another Maxwellian distribution with a different temperature. It is analyzed that the relaxation time in the BGK model is unequal to the molecular mean collision time. Relaxation of component distribution fails to involve enough contribution from other component distributions, which makes the BGK model unable to capture details of velocity distribution, especially when discontinuity exists in distribution. The BGK model,however, predicts satisfactory results including fluxes during relaxation when the temperature difference is small. Particularly, the model-induced error in the BGK model increases with the temperature difference, thus the model is more reliable for low-speed rarefied flows than for hypersonic flows. 相似文献
A new numerical technique named interval finite difference method is proposed for the steady-state temperature field prediction with uncertainties in both physical parameters and boundary conditions. Interval variables are used to quantitatively describe the uncertain parameters with limited information. Based on different Taylor and Neumann series, two kinds of parameter perturbation methods are presented to approximately yield the ranges of the uncertain temperature field. By comparing the results with traditional Monte Carlo simulation, a numerical example is given to demonstrate the feasibility and effectiveness of the proposed method for solving steady-state heat conduction problem with uncertain-but-bounded parameters. 相似文献
In this work, an enhanced treatment of the solid boundaries is proposed for smoothed particle hydrodynamics with implicit time integration scheme (Implicit SPH). Three types of virtual particles, i.e., boundary particles, image particles and mirror particles, are used to impose boundary conditions. Boundary particles are fixed on the solid boundary, and each boundary particle is associated with two fixed image particles inside the fluid domain and two fixed mirror particles outside the fluid domain. The image particles take the flow properties through fluid particles with moving least squares (MLS) interpolation and the properties of mirror particles can be obtained by the corresponding image particles. A repulsive force is also applied for boundary particles to prevent fluid particles from unphysical penetra- tion through solid boundaries. The new boundary treatment method has been validated with five numerical examples. All the numerical results show that Implicit SPH with this new boundary-treatment method can obtain accurate results for non-Newtonian fluids as well as Newtonian fluids, and this method is suitable for complex solid boundaries and can be easily extended to 3D problems. 相似文献
The effect of mechanical properties of erythrocytes on the near-wall motion of platelets was numerically studied with the immersed boundary method. Cells were modeled as viscous-fluid-filled capsules surrounded by hyper-elastic membranes with negligible thickness. The numerical results show that with the increase of hematocrit, the near-wall approaching of platelets is enhanced, with which platelets exhibit larger deformation and orientation angle of its near-wall tank-treading motion, and the lateral force pushing platelets to the wall is increased with larger fluctuation amplitude. Meanwhile the near-wall approaching is reduced by increasing the stiffness of erythrocytes. 相似文献
The present study focuses on the analysis of free vibrations of axisymmetric functionally graded hollow spheres. The material is assumed to be graded in radial di- rection with a simple power law. Matrix Frrbenious method of extended power series is employed to derive the analytical solutions for displacement, temperature, and stresses. The dispersion relations for the existence of various types of pos- sible modes of vibrations in the considered hollow sphere are derived in a compact form. In order to explore the character- istics of vibrations, the secular equations are further solved by using fixed point iteration numerical technique with the help of MATLAB software. The numerical results have been presented graphically for polymethyl methecrylate materials in respect of natural frequencies, frequency shift, inverse quality factor, displacement, temperature change, and radial stress. 相似文献
The transient response of a central crack in an orthotropic strip under the in-plane shear impact loading is studied by using
the dual integral equation method proposed by Copson and Sih. The general formula for the shear stress intensity factor
near the crack tip is derived. Numerical results of
with
in various cases are obtained by solving the second kind Fredholm integral equation and by performing the inverse Laplace
transform. 相似文献
This study describes a multidimensional 3D/lumped parameter(LP) model which contains appropriate inflow/outflow boundary conditions in order to model the entire human arterial trees. A new extensive LP model of the entire arterial network(48 arteries) was developed including the effect of vessel diameter tapering and the parameterization of resistance, conductor and inductor variables. A computer aided-design(CAD) algorithm was proposed to effciently handle the coupling of two or more 3D models with the LP model, and substantially lessen the coupling processing time. Realistic boundary conditions and Navier–Stokes equations in healthy and stenosed models of carotid artery bifurcation(CAB) were used to investigate the unsteady Newtonian blood flow velocity distribution in the internal carotid artery(ICA). The present simulation results agree well with previous experimental and numerical studies. The outcomes of a pure LP model and those of the coupled 3D healthy model were found to be nearly the same in both cases. Concerning the various analyzed 3D zones, the stenosis growth in the ICA was not found as a crucial factor in determining the absorbing boundary conditions.This paper demonstrates the advantages of coupling local and systemic models to comprehend physiological diseases of the cardiovascular system. 相似文献
A particle nonlinear two-scale turbulence model is proposed for simulating the anisotropic turbulent two-phase flow. The particle kinetic energy equation
for two-scale fluctuation, particle energy transfer rate equation for large-scale fluctuation, and particle turbulent kinetic
energy dissipation rate equation for small-scale fluctuation are derived and closed. This model is used to simulate gas–particle
flows in a sudden-expansion chamber. The simulation is compared with the experiment and with those obtained by using another
two kinds of tow-phase turbulence model, such as the single-scale two-phase turbulence model and the particle two-scale second-order moment (USM) two-phase turbulence model. It is shown that
the present model gives simulation in much better agreement with the experiment than the single-scale two-phase turbulence model does and is almost as good as the particle two-scale USM turbulence model.
The project supported by China Postdoctoral Science Foundation (2004036239). 相似文献
An exact-designed mesh shape with favorable surface accuracy is of practical significance to the performance of large cable-network antenna reflectors. In this study, a novel design approach that could guide the generation of exact spatial parabolic mesh configurations of such reflector was proposed. By incorporating the traditional force density method with the standard finite element method, this proposed approach had taken the deformation effects of flexible ring truss supports into consideration, and searched for the desired mesh shapes that can satisfy the requirement that all the free nodes are exactly located on the objective paraboloid. Compared with the conventional design method,a remarkable improvement of surface accuracy in the obtained mesh shapes had been demonstrated by numerical examples. The present work would provide a helpful technical reference for the mesh shape design of such cable-network antenna reflector in engineering practice. 相似文献
The upper limit of the solid scatters density ns (x), a key parameter for the simulation of flows in porous media with a gray Lattice Boltzmann Method, is studied by an analytical way for the infiltration Poiseuille flow between two infinite parallel plates. Analyses of three different gray Lattice Boltzmann schemes, separately proposed by Gao and Sharma et al., Dardis and McCloskey, and Thorne and Sukop, indicate that the effective domain of Gao and Sharma's scheme is restricted to ns 〈 1/2√3≈0.289, Dardis and McCloskey's scheme is restricted to ns 〈 (√57-1)/28≈0.234, and that there is no extra restriction on ns(x) with Thorne and Sukop's scheme. These results are obtained for the dimensionless relaxation time τ= 1. The above analytical results are verified by our numerical simulations. The use of a gray LBM is further illustrated by simulating the flow at the interface of a porous medium. Simulation results yield velocity profiles which agree very well with Brinkman's prediction. 相似文献
An analogue experiment is proposed to simulate flame flickering comprising a free ascending column fed on its side with a light gas (helium) emerging from a vertical slot in ambient air. The convective motion of the helium jet is considered to represent the motion of burnt gases of buoyant jet flames. The helium jet is accelerated by buoyancy effects and the flow field is similar to that of burnt gases observed for real buoyant flames. The vertical velocity profile of the steady helium jet is measured at different vertical distances. The unsteady helium jet is also studied by measuring the instability frequency as a function of ambient pressure at different injection flow rates, and by analyzing the tomography images of the helium jet. The instability morphology is the same as that observed on real buoyant flames. We conclude that this type of instability can be approximately characterized by the maximum vertical velocityumax, and the distance betweenumax in the helium ascending column andu = o in the ambient air. For this type of instability the local vorticity is proportional to
which can be influenced by gravity and ambient pressure. Theoretical prediction of the instability frequency as a function of gravity and ambient pressure has been obtained, and is in good agreement with the experimental results.List of symbols
C1,C2
constants
-
F
instability frequency
-
Fc
critical frequency
-
Fm
the most amplified frequency
-
F (K, )
function defined in (11)
-
g
gravitational acceleration
-
g
reduced gravity acceleration g(0-*)/*
-
k
real wave number of the disturbance
-
K
reduced wave numberK=2k
-
Kc
reduced wave number of the critical instability mode
-
Km
nondimensional wavenumber of the most amplified mode
-
L
vertical characteristic length (in x direction)
-
P
ambient pressure
-
u
local vertical buoyant velocity (inx direction)
-
umax
local maximum vertical velocity
-
v
local velocity component iny direction (horizontal)
-
V0
injection velocity of helium (iny direction)
-
x
vertical distance measured from the leading edge of boundary layer
-
y
horizontal distance measured from the exit plane of the vertical slot
-
Z(K, )
function defined in equation (11)
Greek symbols
distance betweenumax in the helium ascending column andu = o in the ambient air
-
-
wavelength of instability
- c
critical wavelength
- m
the most amplified wavelength
- *
helium density at slot exit
- 0
ambient air density
- *
helium dynamic viscosity at slot exit
-
v*
helium kinematic viscosity at slot exit
-
complex number presented in disturbanceei(kx+t)
- i
imaginary part of , representing the amplification rate of disturbance
- r
real part of , where (r/k) represents the group velocity
-
reduced complex number of , defined
相似文献
The segregation of bismuth atoms on the [101] tilt copper grain boundaries Σ3 (
) 70.53°, Σ33 (
) 58.99°, Σ11 (
) 50.48° and Σ9 (
) 38.94° has been studied by pseudo-molecular dynamics using the empiricalN-body potentials. The relationship between bismuth segregation and grain boundary structure has been discussed in detail.
The subject supported by the Chinese Academy of Sciences and National Natural Science Foundation of China 相似文献