• Proceedings of the Computational Structural Engineering Institute Conference
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• 1992.04a
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• pp.58-64
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• 1992

It is usual that underground structures are constructed within multi-layered medium. In this paper, an efficient numerical model ling of multi-layered structural systems is studied using coupled analysis of finite elements and boundary elements. The finite elements are applied to the area in which the material nonlinearity is dominated, and the boundary elements are applied to the far field area where the nonlinearity is relatively weak. In the boundary element model 1 ins of the multi-layered medium, fundamental solutions are restricted. Thus, methods which can utilize existing Kelvin and Melan solution are sought for the interior multi-layered domain problem and semi infinite domain problem. Interior domain problem which has piecewise homogeneous layers is analyzed using boundary elements with Kelvin solution; by discretizing each homogeneous subregion and applying compatibility and equilibrium conditions between interfaces. Semi-infinite domain problem is analyzed using boundary elements with Melan solution, by superposing unit stiffness matrices which are obtained for each layer by enemy method. Each methodology is verified by comparing its results which the results from the finite element analysis and it is concluded that coupled analysis using boundary elements and finite elements can be reasonable and efficient if the superposition technique is applied for the multi-layered semi-infinite domain problems.

• Nuclear Engineering and Technology
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• v.44 no.8
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• pp.871-888
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• 2012

This paper aims at presenting the state of the art, the recent progress, and the perspective for the future, in the modelling of two-phase flow in the horizontal legs of a PWR. All phenomena relevant for safety analysis are listed first. The selection of the modelling approach for system codes is then discussed, including the number of fluids or fields, the space and time resolution, and the use of flow regime maps. The classical two-fluid six-equation one-pressure model as it is implemented in the CATHARE code is then presented and its properties are described. It is shown that the axial effects of gravity forces may be correctly taken into account even in the case of change of the cross section area or of the pipe orientation. It is also shown that it can predict both fluvial and torrential flow with a possible hydraulic jump. Since phase stratification plays a dominant role, the Kelvin-Helmholtz instability and the stability of bubbly flow regime are discussed. A transition criterion based on a stability analysis of shallow water waves may be used to predict the Kelvin-Helmholtz instability. Recent experimental data obtained in the METERO test facility are analysed to model the transition from a bubbly to stratified flow regime. Finally, perspectives for further improvement of the modelling are drawn including dynamic modelling of turbulence and interfacial area and multi-field models.

• Smart Structures and Systems
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• v.26 no.6
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• pp.809-817
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• 2020

In cells, the microtubules are surrounded by viscoelastic medium. Microtubules, though very small in size, perform a vital role in transportation of protein and in maintaining the cell shape. During performing these functions waves propagate and this propagation of waves has been investigated using nonlocal elastic theory. But the effect of surrounding medium was not taken into account. To fill this gap, this study considers the viscoelastic medium along with nonlocal elastic theory. The analytical formulas of the velocity of waves, and the results reveal that the presence of medium reduces the velocity. The axisymmetric and nonaxisymmetric waves are separately discussed. Furthermore, the results are compared with the results gained from the studies of free microtubules. The presence of medium around microtubules results in the increase of the flexural rigidity causing a significant decrease in radial wave velocity as compared to axial and circumferential wave velocities. The effect of viscoelastic medium is more obvious on radial wave velocity, to a lesser extent on torsional wave velocity and least on longitudinal wave velocity.

• Composites Research
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• v.13 no.4
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• pp.67-74
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• 2000

The general solution of the anti-plane shear problem for the curved interfacial crack between viscoelastic foam and composites was investigated with the complex variable displacement function. Kelvin-Maxwell three parameter model is used to present viscoelasticity and the Laplace transform was applied to treat the viscoelastic characteristics of foam in the analysis. The stress intensity factor near the interfacial crack tip was predicted by considering both anisotropic and viscoelastic properties of two different materials. The results showed that the stress intensity factor increased with increasing the curvature of the curved interfacial crack and it also increased and eventually converged to a specific value with increasing time. The stress intensity factor increased with increasing the ratio of stiffness coefficients between foam and composites and the effect of fiber orientation on the stress intensity factor decreased with increasing the ratio of stiffness coefficients between foam and composites.

• Advances in aircraft and spacecraft science
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• v.10 no.3
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• pp.257-279
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• 2023

This manuscript is dedicated to deriving the closed form solutions of free vibration of viscoelastic nanobeam embedded in an elastic medium using nonlocal differential Eringen elasticity theory that not considered before. The kinematic displacements of Euler-Bernoulli and Timoshenko theories are developed to consider the thin nanobeam structure (i.e., zero shear strain/stress) and moderated thick nanobeam (with constant shear strain/stress). To consider the internal damping viscoelastic effect of the structure, Kelvin/Voigt constitutive relation is proposed. The perforation geometry is intended by uniform symmetric squared holes arranged array with equal space. The partial differential equations of motion and boundary conditions of viscoelastic perforated nonlocal nanobeam with elastic foundation are derived by Hamilton principle. Closed form solutions of damped and natural frequencies are evaluated explicitly and verified with prestigious studies. Parametric studies are performed to signify the impact of elastic foundation parameters, viscoelastic coefficients, nanoscale, supporting boundary conditions, and perforation geometry on the dynamic behavior. The closed form solutions can be implemented in the analysis of viscoelastic NEMS/MEMS with perforations and embedded in elastic medium.

• Computers and Concrete
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• v.15 no.4
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• pp.687-707
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• 2015

A finite element computer code for short-term analysis of steel-concrete composite structures is extended to study long-term effects under service loads, in the present work. Long-term effects are important in engineering design because they influence stress and strain distribution of the structural system and therefore contribute to the increment of deflections in these structures. For creep analysis, a rheological model based on a Kelvin chain, with elements placed in series, was employed. The parameters of the Kelvin chain were obtained using Dirichlet series. Creep and shrinkage models, proposed by the CEB FIP 90, were used. The shear-lag phenomenon that takes place at the concrete slab is usually neglected or not properly taken into account in the formulation of beam-column finite elements. Therefore, in this work, a three-dimensional numerical model based on the assemblage of shell finite elements for representing the steel beam and the concrete slab is used. Stud shear connectors are represented for special beam-column elements to simulate the partial interaction at the slab-beam interface. The two-dimensional representation of the concrete slab permits to capture the non-uniform shear stress distribution in the horizontal plane of the slab due to shear-lag phenomenon. The model is validated with experimental results of two full-scale continuous composite beams previously studied by other authors. Results are given in terms of displacements, bending moments and cracking patterns in order to shown the influence of long-term effects in the structural response and also the potentiality of the present numerical code.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.23 no.3
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• pp.193-204
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• 2011

To develop more robust oil boom which is vulnerable to various failure mode under severe weather condition, highly accurate wave model is developed using Spatially filtered Navier-Stokes Eq., LDS (Lagrangian Dynamic Smagorinsky model) for residual stresses, SPH (Smoothed Particle Hydrodynamics). To clarify the hydraulic characteristics of floating type oil boom, we numerically simulate the behavior of oil spill around oil boom under very energetic progressive waves. At the first stage, we firmly anchored the oil boom, and then, allowed the excursion of the oil boom. It turns out that oil boom with skirt of enough length (longer than 30% of depth) effectively confines the oil spill even against very energetic waves. We can also observe obliquely descending vertical eddies between y = 1~2 m as horizontal vortices shedding at the interface of oil spill and water are diffused toward the bottom, which is believed to be the birth, growing and break-down of Kelvin-Helmholz wave.

• Journal of the Korean earth science society
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• v.28 no.7
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• pp.857-870
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• 2007

The structure and evolution of a thunderstorm outflow in two dimensions with no environmental wind are investigated using a cloud-resolving model with explicit liquid-ice phase microphysical processes (ARPS: Advanced Regional Prediction System). The turbulence structure of the outflow is explicitly resolved with a high-resolution grid size of 50m. The simulated single-cell storm and its associated Kelvin-Helmholtz (KH) billows are found to have the lift stages of development maturity, and decay. The secondary pulsation and splitting of convective cells resulted from interactions between cloud dynamics and microphysics are observed. The cooled downdrafts caused by the evaporation of rain and hail in the relatively dry lower atmosphere result in thunderstorm cold-air outflow. The outflow head propagates with almost constant speed. The KH billows formed by the KH instability cause turbulence mixing from the top of the outflow and control the structure of the outflow. Ihe KH billows are initiated at the outflow head, and pow and decay as moving rearward relative to the gust front. The numerical simulation results of the ratio of the horizontal wavelength of the fastest growing perturbation to the critical shear-layer depth and the ratio of the horizontal wavelength of the billow to its maximum amplitude are matched well with the results of other studies.

• Advances in nano research
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• v.6 no.1
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• pp.39-55
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• 2018

Forced vibration analysis of a cracked functionally graded microbeam is investigated by using modified couple stress theory with damping effect. Mechanical properties of the functionally graded beam change vary along the thickness direction. The crack is modelled with a rotational spring. The Kelvin-Voigt model is considered in the damping effect. In solution of the dynamic problem, finite element method is used within Timoshenko beam theory in the time domain. Influences of the geometry and material parameters on forced vibration responses of cracked functionally graded microbeams are presented.

• Journal of ILASS-Korea
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• v.12 no.2
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• pp.101-107
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• 2007

The purpose of this study is to analyze spray and evaporation characteristics of DME fuel at the high pressure and temperature. For the numerical analysis of dimethyl ether(DME) fuel spray characteristics, hybrid breakup model was applied to the DME spray and its breakup process. In order to obtain experimental results for comparison with the predicted ones, the visualization of the spray evolution process was executed by using a Nd:YAG laser. Also, the numerical investigation was conducted by the two hybrid models for primary and secondary breakup of the DME spray. The primary breakup model was used the Kelvin-Helmholtz(KH) breakup model. In the secondary breakup process, Rayleigh-Taylor(RT) and Drop Deformation Breakup(DDB) model was applied. The results of this study provide the macroscopic characteristics of the spray such as spray tip penetration and cone angle, and prediction accuracy of the two hybrid model.