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Theoretical Simulation of AlN Nanobelts and Nanorings
Get PDFAn extension of our previously reported periodic cluster model (J. Phys. Chem. C 2008, 112, 6667-6676 ) to nanorings and nanobelts is presented. This new scheme allows for accurately calculating reasonably large nanostructures while preserving a very small number of optimization parameters. The model has been applied to a number of AlN semiconducting structures using ab initio pair potentials. Attention has been paid to the variation of the B1-B4 phase transition pressure as the the size of the structures is varied
Comparison and a Possible Source of Disagreement between Experimental and Numerical Results in a Czochralski Model
Get PDFExperimental and numerical observations of oscillatory instability of melt flow in a Czochralski model are compared, and a disagreement observed at small crystal dummy rotation rates is addressed. To exclude uncertainties connected with flow along the free surface, the latter is covered by a no-slip thermally insulating ring. Experiments reveal an appearance of oscillations at temperature differences smaller than the numerically predicted critical ones. At the same time, a steep increase of the oscillations amplitude is observed just beyond the computed threshold values. By increasing the dummy rotation gradually, we are able to qualitatively confirm the numerically predicted flow destabilization. A good quantitative comparison is reached only with a rather strong rotation of the crystal dummy. Focusing on the disagreement in the non-rotating case, we argue that the experimentally observed instability is triggered by an external excitation that results from low-amplitude temperature oscillations in thermostatic baths. This argument is supported by a numerical simulation of the parametrically excited model
Identification of Parameters of a Nonlinear Material Model Considering the Effects of Viscoelasticity and Damage
Get PDFThis work deals with mechanical properties of a rubber material that is used in modern tram wheels as a damping element. Nonlinear static response as well as strain softening and hysteresis are captured in the material model that is selected. Method of identification of the model's parameters is developed. The identification method relies on successive minimizations with respect to different sets of parameters. Tests in tension, compression and simple shear are performed. Parameters of the material model are identified based on the tension and compression data, while the shear data are used for validation only
On Multiscale Modeling Using the Generalized Method of Cells: Preserving Energy Dissipation across Disparate Length Scales
Get PDFA mesh objective crack band model was implemented within the generalized method of cells micromechanics theory. This model was linked to a macroscale finite element model to predict post-peak strain softening in composite materials. Although a mesh objective theory was implemented at the microscale, it does not preclude pathological mesh dependence at the macroscale. To ensure mesh objectivity at both scales, the energy density and the energy release rate must be preserved identically across the two scales. This requires a consistent characteristic length or localization limiter. The effects of scaling (or not scaling) the dimensions of the microscale repeating unit cell (RUC), according to the macroscale element size, in a multiscale analysis was investigated using a finite-notched, modified, double cantilever beam specimen. Additionally, the ramifications of the macroscale element shape, compared to the RUC, was studied
Theoretical Modeling of the Radiative Properties and Effective Thermal Conductivity of the Opacified Silica Aerogel
Get PDFIn this paper, we investigate the radiative properties and the effective thermal conductivity (ETC) of the opacified silica aerogel by theoretical method. The radiative properties of the opacified silica aerogel are obtained by the modified Mie Scattering Theory that is used for particle scattering in absorbing medium. The modified gamma distribution is used to take account of the non-uniformity of the particle size. The solid thermal conductivity of the composite material is obtained by considering the scale effect of the particles. Based on these calculated thermophysical properties the coupled heat conduction and radiation through the evacuated opacified aerogel are solved by the finite volume method. And the radiation flux is computed by the P-1 approximation combined with the gray-band model. Results show that, the calculated thermophysical properties of the TiO2-doped silica aerogel are close to the experimental data. The optimal mean radius for the largest radiation extinction of the SiC particles is about 1μm. The presented data of optimal doping amount of the SiC particles at different temperature conditions for the evacuated silica aerogel is very useful for thermal insulation material design
Problems of Micromorphic Elastic Bodies Approached by Lagrange Identity Method
Get PDFTaking advantage of the flexibility of Lagrange’s identity, we prove the uniqueness theorem and some continuous dependence theorems without recourse to any energy conservation law, or to any boundedness assumptions on the constitutive coefficients. Also, we avoid the use of positive definiteness assumptions on the constitutive coefficients, even if these results are related to the difficult mixed problem in elasticity of micromorphic bodies
Taguching the Atmospheric Plasma Spraying Process: Influence of Processing Factors on Droplet Impact Properties Obtained on Dense ZrO<sub>2</sub> and H<sub>2</sub>Ar75% Plasma Gas
No full textIn this paper a study of the atmospheric plasma spraying process was conducted. The Jets&Poudres code was used to solve the partial differential equations for the conservation of mass, momentum and energy involved in the problem together with the K-e turbulent model. The Taguchi technique was used to study the influence of processing factors on droplet impact properties obtained on dense zirconia (ZrO2) under H2Ar75% plasma gas that allow optimal functioning condition. The test of the operating parameters for the studied ranges showed that the "thermal power" factor plays a key role on the state of sprayed powder. It was found also that the carrier gas flow rate and the powder size do not influence the dispersion of the impact behavior of droplets/particles. The validation of the chosen levels combination was conducted on the Jets&Poudres code and showed good agreement with the results predicted by the Taguchi technique
Comparison of EHD-Driven Instability of Thick and Thin Liquid Films by a Transverse Electric Field
Get PDFThis study aims to explore the effect of liquid film thickness on the electrohydrodynamic-driven instability of the interface separating two horizontal immiscible liquid layers. The fluids are confined between two electrodes and the light and less conducting liquid is overlaid on the heavy and more conducting one. Direct Numerical Simulations (DNSs) are performed using a front tracking/finite difference scheme in conjunction with Taylor-Melcher leaky dielectric model. For the range of physical parameters used here, it is shown that for a moderately thick lower liquid layer, the interface instability leads to formation of several liquid columns and as a result of competition between these columns eventually a big column is formed. On the other hand, for a thin lower layer the lower electrode strongly influences the growth of the instability, leading to a short and a longer column that are connected together by a thin liquid film. When the film becomes too thick, more columns are formed, but the fluid system does not reach a steady state because the liquid columns grow so rapidly that they hit the top electrode. The flow structure is examined and the variation of the steady state kinetic energy of the system with the film thickness and the applied electric voltage is explored
The Post-Buckling Behavior of A Tubular String in An Inclined Wellbore
No full textA down-hole tubular string in an inclined wellbore, under variable axial and torsional loading, may simultaneously undergo a sinusoidal as well as helical buckling, at different sections. In this paper, the buckling equation for a tubular string, in an inclined wellbore, subjected to axial and torsional loading, is established by an equilibrium method. The analytical solutions for the buckling equations, for sinusoidal and helical configurations of buckled tubular string, are obtained by Galerkin and nonlinear scaling methods. Methods for computing the contact forces between the buckled tubular string and wellbore, are developed. The analytical solutions are in good accordance with the numerical results, for the nonlinear buckling equation. The critical loads for sinusoidal as well as helical buckling of a down-hole tubular string are determined, using the constraint condition under which the contact force is nonnegative. Thus, the post-buckling behavior of a tubular string, with different configurations, in an inclined wellbore, is determined by the presented analytical method
On the Multigrid Method Based on Finite Difference Approximate Inverses
No full textDuring the last decades, multigrid methods have been extensively used in order to solve large scale linear systems derived from the discretization of partial differential equations using the finite difference method. Approximate Inverses in conjunction with Richardon’s iterative method could be used as smoothers in the multigrid method. Thus, a new class of smoothers based on approximate inverses could be derived. Effectiveness of explicit approximate inverses relies in the fact that they are close approximants to the inverse of the coefficient matrix and are fast to compute in parallel. Furthermore, the class of finite difference approximate inverses proposed in conjunction with the explicit preconditioned Richardson method present improved results against the classic smoothers such as Jacobi and Gauss – Seidel method. Moreover, a dynamic relaxation scheme is proposed based on the Dynamic Over / Under Relaxation (DOUR) algorithm. Furthermore, results for the multigrid preconditioned Bi-CGSTAB based on approximate inverse smoothing and a dynamic relaxation technique are presented for a class of model problems