One-layered Model Research Articles

Saturated Ground Vibration Analysis Based on a Three-Dimensional Coupled Train-Track-Soil Interaction Model

A novel three-dimensional (3D) coupled train-track-soil interaction model is developed based on the multi-body simulation (MBS) principle and finite element modeling (FEM) theory using LS-DYNA. The novel model is capable of determining the highspeed effects of trains on track and foundation. The soils in this model are treated as saturated media. The wheel-rail dynamic interactions under the track irregularity are developed based on the Hertz contact theory. This model was validated by comparing its numerical results with experimental results obtained from field measurements and a good agreement was established. The one-layered saturated soil model is firstly developed to investigate the vibration responses of pore water pressures, effective and total stresses, and displacements of soils under different train speeds and soil moduli. The multi-layered soils with and without piles are then developed to highlight the influences of multi-layered soils and piles on the ground vibration responses. The effects of water on the train-track dynamic interactions are also presented. The original insight from this study provides a new and better understanding into saturated ground vibration responses in high-speed railway systems using slab tracks in practice. This insight will help track engineers to inspect, maintain, and improve soil conditions effectively, resulting in a seamless railway operation.

A study on two-layered (K.L-Newtonian) model of blood flow in an artery with six types of mild stenoses

A mathematical model of the steady flow of blood in arteries with six different types of mild stenoses (symmetric, triangular, trapezoidal, elliptical, bell-shaped and composite) is analyzed. The model comprises a core (suspension of cells) surrounded by a layer of plasma near the wall. The rheology of blood in the core region is characterized by a K-L (non-Newtonian) fluid model and the peripheral plasma layer as a Newtonian fluid. An attempt has been made to obtain analytical expressions for the velocity of blood flow in the stenotic arterial region, radius of the plug-core region, the velocity of the fluid in the plug-flow region, flow flux, shear stress at wall and resistance to flow, using the suitable boundary conditions. The computational results were presented graphically for several values of the parameters involved in the present analysis, and also several comparisons have been made with the existing models to endorse the applicability of the present model. The study revealed that the velocity of blood decreases by increasing values of the yield stress of the blood and K-L fluid parameters and an opposite nature is observed for wall shear stress and resistive impedance to flow. It is found that the shape of the plug flow region is purely dependent on the shape of the stenosis present in the cardiovascular system. It is also recovered that the wall shear stress and flow resistance are significantly reduced for the two-layered model (K.L-Newtonian) than those of the one-layered model (K-L) irrespective of the type of geometries of stenosis. Also, the axial variation of wall shear stress and resistive impedance are lowest for the bell-shaped stenosis than those of the other stenoses considered in the present study, and they found to be highest for the trapezoidal stenosis. A comparison of the velocity profiles obtained from the present study with the experimental data is made, and a good agreement between them is found. Further, the relevant physiological applications of the present mathematical model have been analyzed.

Numerical modeling of a prototype cardiac assist device by implementing fluid-structure interaction

The purpose of this study is to simulate the blood flow in a 2D axisymmetric model for the aorta assisted with a type of cardiac assist device in which the balloon part is surrounding the ascending aorta and can pump the blood flow by its inflation/deflation. The blood flow and von Mises stress in the assisted aorta are investigated by fluid–structure interaction analysis (FSI). Taking into consideration three materials and seven thicknesses for the balloon in one layered and three-layered aorta models, the influences of each of these states on the mentioned parameters have been studied. The numerical simulation demonstrates that by using this cardiac assist device, the maximum blood flow velocity increases up to 92% at the time of maximum pressure on the balloon and this configuration augments the maximum von Mises stress. The magnitude of the maximum von Mises stress and the maximum blood velocity reduce by thickening the balloon from 0.8 mm to 2 mm up to 8.9% and 9.95%, respectively. The maximum pressure at the outlet of the aorta increases from 9.77 kPa to 10.4 kPa at the time of applying maximum pressure on the balloon. With the one-layered aorta model, the maximum von Mises stress occurs at the innermost layer of the aorta's wall on each line across the wall and the balloon. Using the three-layered aorta model, the location of the maximum stress is shifted to the juncture of intima and media layers.

Microstructural quantification of collagen fiber orientations and its integration in constitutive modeling of the porcine carotid artery

BackgroundMechanical characteristics of vascular tissue may play a role in different arterial pathologies, which, amongst others, requires robust constitutive descriptions to capture the vessel wall’s anisotropic and non-linear properties.Specifically, the complex 3D network of collagen and its interaction with other structural elements has a dominating effect of arterial properties at higher stress levels.The aim of this study is to collect quantitative collagen organization as well as mechanical properties to facilitate structural constitutive models for the porcine carotid artery.This helps the understanding of the mechanics of swine carotid arteries, being a standard in clinical hypothesis testing, in endovascular preclinical trials for example. MethodPorcine common carotid arteries (n=10) were harvested and used to (i) characterize the collagen fiber organization with polarized light microscopy, and (ii) the biaxial mechanical properties by inflation testing.The collagen organization was quantified by the Bingham orientation density function (ODF), which in turn was integrated in a structural constitutive model of the vessel wall.A one-layered and thick-walled model was used to estimate mechanical constitutive parameters by least-square fitting the recorded in vitro inflation test results.Finally, uniaxial data published elsewhere were used to validate the mean collagen organization described by the Bingham ODF. ResultsThick collagen fibers, i.e.the most mechanically relevant structure, in the common carotid artery are dispersed around the circumferential direction.In addition, almost all samples showed two distinct families of collagen fibers at different elevation, but not azimuthal, angles.Collagen fiber organization could be accurately represented by the Bingham ODF (κ1,2,3=[13.5,0.0,25.2] and κ1,2,3=[14.7,0.0,26.6]; average error of about 5%), and their integration into a structural constitutive model captured the inflation characteristics of individual carotid artery samples.Specifically, only four mechanical parameters were required to reasonably (average error from 14% to 38%) cover the experimental data over a wide range of axial and circumferential stretches.However, it was critical to account for fibrilar links between thick collagen fibers.Finally, the mean Bingham ODF provide also good approximation to uniaxial experimental data. ConclusionsThe applied structural constitutive model, based on individually measured collagen orientation densities, was able to capture the biaxial properties of the common carotid artery. Since the model required coupling amongst thick collagen fibers, the collagen fiber orientations measured from polarized light microscopy, alone, seem to be insufficient structural information. Alternatively, a larger dispersion of collagen fiber orientations, that is likely to arise from analyzing larger wall sections, could have had a similar effect, i.e. could have avoided coupling amongst thick collagen fibers. Statement of SignificanceThe applied structural constitutive model, based on individually measured collagen orientation densities, was able to capture the biaxial and uniaxial properties of the common carotid artery. Since the model required coupling amongst thick collagen fibers, an effective orientation density that accounts for cross-links between the main collagen fibers has been porposed. The model provides a good approximation to the experimental data.

Dynamic neural cortical ensembles model

One-layered model of cortical neurons as a set of overlapping ensembles, each with a structure similar to Hopfield network, is proposed. Ensemble equilibrium equation is solved and formulas for connections weights calculation for given set of attractors are obtained. Concept of dynamic attractors that consists of consequent recalling of stored patterns with moving activity through the network is introduced. Role of dynamic attractors in long-term memory is discussed and mechanism for memory recovery after destruction of some neurons is proposed. Results of experiments on associative memory recovery after partial removal of neurons are shown.

Mapping shallow soil moisture profiles at the field scale using full-waveform inversion of ground penetrating radar data

Full-waveform inversions were applied to retrieve surface, two-layered and continuous soil moisture profiles from ground penetrating radar (GPR) data acquired in an 11-ha agricultural field situated in the loess belt area in central Belgium. The radar system consisted of a vector network analyzer combined with an off-ground horn antenna operating in the frequency range 200–2000 MHz. The GPR system was computer controlled and synchronized with a differential GPS for real-time data acquisition. Several inversion strategies were also tested using numerical experiments, which in particular demonstrated the potentiality to reconstruct simplified two-layered configurations from more complex, continuous dielectric profiles as prevalent in the environment. The surface soil moisture map obtained assuming a one-layered model showed a global moisture pattern mainly explained by the topography while local moisture patterns indicated a line effect. Two-layered and profile inversions provided consistent estimates with respect to each other and field observations, showing significant moisture increases with depth. However, some discrepancies were observed between the measured and modeled GPR data in the higher frequency ranges, mainly due to surface roughness effects which were not accounted for. The proposed GPR method and inversion strategies showed great promise for high-resolution, real-time mapping of soil moisture at the field scale.

A two-layered suspension blood flow through an overlapping stenosis

The problem of blood flow through an overlapping constriction in arteries has been investigated. To account for the hematocrit and peripheral layer, blood has been represented by a two-layered macroscopic model consisting of a core region of suspension of all the erythrocytes assumed to be a particle–fluid suspension (i.e., a suspension of erythrocytes in plasma) and a peripheral layer of plasma (Newtonian fluid). The expression for the flow characteristics, namely, the impedance, the wall shear stress, the shear stress at the stenosis throats and at critical height of the stenosis has been derived. The impedance increases with the hematocrit as well as with the stenosis size and assumes lower magnitude in two-layered model than its corresponding value in one-layered model for any given hematocrit. The shear stress at the two stenosis throats is same and assumes considerably higher value than its corresponding magnitude at the critical height. With respect to any parameter, the shear stresses at the throats and at the critical height possess the characteristics similar to that of impedance.

Two-layered quasi-3D finite element model of the oesophagus

Analysis of oesophageal mechanoreceptor-dependent responses requires knowledge about the distribution of stresses and strains in the layers of the organ. A two-layered and a one-layered quasi-3D finite element model of the rat oesophagus were used for simulation. An exponential pseudo-strain energy density function was used as the constitutive equation in each model. Stress and strain distributions at the distension pressures 0.25 and 1.0 kPa were studied. The stress and strain distributions depended on the wall geometry. In the one-layered model, the stress ranged from −0.24 to 0.38 kPa at a pressure of 0.25 kPa and from −0.67 to 2.57 kPa at a pressure of 1.0 kPa. The stress in the two-layered model at the pressure of 0.25 and 1.0 kPa varied from −0.52 to 0.64 kPa and from −1.38 to 3.84 kPa. In the two-layered model, the stress was discontinuous at the interface between the muscle layer and the mucosa–submucosa layer. The maximum stress jump was 1.67 kPa at the pressure of 1.0 kPa. The present study provides a numerical simulation tool for characterising the mechanical behaviour of a multi-layered, complex geometry organ.

Buckling instabilities of one-layered growing tissues.

Growth and folding in one-layered model tissue sheets are studied in a stochastic, lattice-free single cell model which considers the discrete cellular structure of the tissue, and in a coarse grained analytical approach. The polarity of the one-layered tissue is considered by a bending term. Cell division gives rise to a locally increasing metric. An exponential and a power-law growth regime are identified. In both regimes, folding occurs as soon as the bending contribution becomes too small to compensate the destabilizing effect of the cell proliferation. The potential biological relevance is discussed.

Prediction of sampling depth and photon pathlength in laser Doppler flowmetry

Monte Carlo simulation of photon migration in tissue was used to assess the sampling depth, measuring depth and photon pathlength in laser Doppler flowmetry. The median sampling depth and photon pathlength in skin, liver and brain tissue were calculated for different probe geometries. The shallowest median sampling depth found was 68 microns for a 120 microns diameter single fibre probe applied to a one-layered skin tissue model. By using separate transmitting and receiving fibres, the median sampling depth, which amounted to 146 microns for a 250 microns fibre centre separation, can be successively increased to 233 microns when the fibres' centres are separated by 700 microns. Total photon pathlength and thereby the number of multiple Doppler shifts increase with fibre separation, thus favouring the choice of a probe with a small fibre separation when linearity is more important than a large sampling depth. Owing mainly to differences in the tissue g-value and scattering coefficient, the median sampling depth is shallower for liver and deeper for brain, in comparison with skin tissue. For skin tissue, the influence on the sampling depth of a homogeneously distributed blood volume was found to be limited to about 1 per cent per percentage increase in tissue blood content, and may, therefore, be disregarded in most practical situations. Simulations show that the median measuring depth is strongly dependent on the perfusion profile.

Rayleigh wave propagation in a solid with a cold-worked surface layer

The propagation of the Rayleigh surface wave is experimentally studied along the top surface of used railroad rail under conditions where ultrasonic pulses have carrier frequencies ranging from 0.4 to 3.0 MHz and approximately 10 µs duration. The generation of the first higher (M21 or Sezawa) mode as well as the fundamental (M11) mode and their dispersion properties are observed. These phenomena are attributable to the presence of the cold-worked surface layer caused by the wheel passage. It is shown that a theoretical model of a single layer overlying a half space, whose elastic constants are determined by a destructive method, yields results which agree with the dispersion curves obtained experimentally. On the basis of this one-layered model, an inversion method to estimate the layer thickness and its elastic constants is discussed.

Love wave propagation in a solid with a cold-worked surface layer

In this paper, experiments show Love wave generation along the top surface of used railroad rail, where the shear wave velocity has been slightly reduced by the cold-working of wheel passage for years. The rf pulses used in the experiments have about 10 µs duration and a relatively narrow frequency spectrum. The group velocity of the Love wave is found to have a strong dependence on the carrier frequency over the tested range of 0.45–3.1 MHz. Application of the seismological one-layered model to the experimental measurements yields an NDE technique for the elastic properties and the thickness of the cold-worked surface layer. The results are interpreted on the basis of a destructive observation by micro-Vickers hardness testing.