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Phonon Transport of Rough Si/Ge Superlattice Nanotubes
Get PDFNanostructuring of thermoelectric materials bears promise for manipulating physical parameters to improve the energy conversion efficiency of thermoelectrics. In this paper the thermal transport in Si/Ge superlattice nanotubes is investigated by performing nonequilibrium molecular dynamics simulations aiming at realizing low thermal conductivity by surface roughening. Our calculations revealed that the thermal conductivity of Si/Ge superlattice nanotubes depends nonmonotonically on periodic length and increases as the wall thickness increases. However, the thermal conductivity is not sensitive to the inner diameters due to the strong surface scattering at thin wall thickness. In addition, introducing roughness onto the superlattice nanotubes surface can destroy the phonon tunneling in superlattice nanotubes, which results in thermal conductivity even surpassing amorphous limit. Furthermore, a nonmonotonic dependence of the thermal conductivity of rough Si/Ge superlattice nanotubes with thicker wall thickness is found, while for thinner wall the thermal conductivity of rough Si/Ge superlattice nanotubes decreases monotonically with surface roughness increasing. Our results provide guidance for designing high performance thermoelectrics using superlattice nanotube
Effects of the Velocity and the Nature of the Inert Gas on the Stainless Steel Laser Cut Quality
Get PDFThe effects of inert assisting gas nature and velocity on laser cut quality are investigated. A pure fusion cutting process just above melting point is considered, where the molten steel velocity is given as a function of the two acting forces represented by the pressure gradient and the frictional forces applied by the laminar gas flow. In the case of nitrogen assisting gas, the stainless steel melt film exhibits a visible separation point. The point where the melt flow is separated out from the solid wall depends strongly on the gas velocity. It is pushed down the cut surface when the gas velocity is increased. Furthermore, we have investgated the use of different inert gases (nitrogen, argon and helium) to blow the molten material out of the kerf, and it was noted that the argon and the nitrogen gases evacuate more easily the molten metallic film, compared to the helium gas from their cooling rates point of view. It is concluded that the two first gases are more efficient in laser cutting process of metals. We have studied a 4 mm stainless steel plate thickness without taking into account the transverse movement of the treated workpiece, the numerical solution is obtained by the volume of fluid (VOF) and solidification/melting models, implemented by Fluent CFD software
Convective Film Condensation in an Inclined Channel with Porous Layer
Get PDFThe present work is a numerical study of laminar film condensation from vapor-gas mixtures in an inclined channel with an insulated upper wall and an isothermal lower wall coated with a thin porous material. A two-dimensional model is developed using a set of complete boundary layer equations for the liquid film and the steam-air mixture while the Darcy-Brinkman-Forchheimer approach is used for the porous material. The governing equations are discretized with an implicit finite difference scheme. The resulting systems of algebraic equations are numerically solved using Gauss and Thomas algorithms. The numerical results enable to determine the velocity, temperature and vapor concentration profiles in the steam-air mixture, the liquid film and the porous substrate. The axial evolution of the condensate flow rate and the wall heat flux are also presented and analyzed for different operating conditions. It is found that the inclination angle, the inlet values of relative humidity and the Reynolds number exert an influence on the condensation process much more significant than that coming from a change in the porous layer properties
Effect of Double Stratification on Free Convection in a Power-Law Fluid Saturated Porous Medium
Get PDFFree convection and related heat and mass transfer along a vertical plate embedded in a power-law fluid saturated Darcy porous medium with thermal and solutal stratification effects is studied. The governing partial differential equations are transformed into ordinary differential equations using similarity transformations and then solved numerically by means of a shooting method. The variations of non-dimensional velocity, temperature and concentration are presented graphically for various values of the power-law index, and of the thermal and solutal stratification parameters. In addition, the heat and mass transfer rates are tabulated for different values of the governing nondimensional numbers
Influence of Scale Specific Features on the Progressive Damage of Woven Ceramic Matrix Composites (CMCs)
Get PDFIt is well known that failure of a material is a locally driven event. In the case of ceramic matrix composites (CMCs), significant variations in the microstructure of the composite exist and their significance on both deformation and life response need to be assessed. Examples of these variations include changes in the fiber tow shape, tow shifting/nesting and voids within and between tows. In the present work, the influence of many of these scale specific architectural features of woven ceramic composite are examined stochastically at both the macroscale (woven repeating unit cell (RUC)) and structural scale (idealized using multiple RUCs). The recently developed MultiScale Generalized Method of Cells (MSGMC) methodology is used to determine the overall deformation response, proportional elastic limit (first matrix cracking), and failure under tensile loading conditions and associated probability distribution functions. Prior results showed that the most critical architectural parameter to account for is weave void shape and content with other parameters being less in severity. Current results show that statistically only the post-elastic limit region (secondary hardening modulus and ultimate tensile strength) is impacted by local uncertainties both at the macro and structural level
Correspondence Relations for Fracture Parameters of Interface Corners in Anisotropic Viscoelastic Materials
Get PDFThe problems of the interface corners between two dissimilar anisotropic viscoelastic materials are studied in this paper. Through the use of the well-known correspondence principle between linear elasticity and linear viscoelasticity, fracture parameters in the Laplace domain can be obtained from the path-independent H-integral for the corresponding problems of anisotropic linear elastic materials. Further application of the correspondence relations for fracture parameters proposed in our recent study then leads us the solutions of fracture parameters in the time domain. To show the applicability and accuracy of the proposed method, several different kinds of numerical examples are presented such as a centered interface crack, free edges between two dissimilar materials, and the interface corners appeared within the electronic packages. The fracture parameters calculated in this study include the orders of stress singularity and the stress intensity factors of opening mode, shearing mode and tearing mode. The proposed method allows the orders of stress singularity be real or complex, repeated or distinct, and the fracture mode be pure mode or mixed mode
Numerical Evaluation of Variation in ‘Characteristic Distance’ due to Fracture Specimen Thickness and Temperature
Get PDFThe present numerical study is an attempt to understand the dependency of characteristic distance on the fracture specimen thickness and temperature. The presented work will be useful to establish the characteristic distance prediction methodology using three dimensional FEM model. Based on the methods proposed for the numerical prediction of characteristic distance, it comes out that it depends on fracture specimen thickness and finally it converges after a specified thickness of fracture specimen. In Armco iron material, characteristic distance varies in temperature ranges where dynamic strain ageing phenomenon is observed, initially decrease and then increases again
Deletion of the TPM1 and MDM20 Genes Affect the Mechanical and Structural Properties of Yeast Cells
Get PDFMany diseases including cancer are associated with a disorganised cytoskeleton. The process of characterising how cytoskeletal disorganisation affects the mechanical properties of cells offers the potential to develop new drugs and treatment regimes that may exploit mechanical weakness in cells and tissues. This work investigated the role of actin associated proteins, namely tropomyosin 1 (tpm1p) and mitochondrial distribution and morphology protein 20 (mdm20p), on the mechanical and morphological properties of yeast cells. For the first time it was shown that deletion of both the TPM1 and MDM20 genes resulted in a decrease in Young’s modulus when compared to the wild-type cells. The deletion strains appeared to have aberrant cell walls when compared to the wild-type strain and also appeared to have lost the characteristic elliptical morphology that is normally exhibited by yeast. Deletion of the TPM1 gene resulted in a significant increase in mean conjugate cell diameter when compared to the wild-type cells, however deletion of the MDM20 gene did not have any significant effect upon the mean conjugate diameter of the yeast cells
Low and Intermediate Re Solution of Lid Driven Cavity Problem by Local Radial Basis Function Collocation Method
Get PDFThis paper explores the application of Local Radial Basis Function Collocation Method (LRBFCM) [Šarler and Vertnik (2006)] for solution of Newtonian incompressible 2D fluid flow for a lid driven cavity problem [Ghia, Ghia, and Shin (1982)] in primitive variables. The involved velocity and pressure fields are represented on overlapping five-noded sub-domains through collocation by using Radial Basis Functions (RBF). The required first and second derivatives of the fields are calculated from the respective derivatives of the RBF’s. The momentum equation is solved through explicit time stepping. The method is alternatively structured with multiquadrics and inverse multiquadrics RBF’s. In addition, two different approaches are used for pressure velocity coupling (Fractional Step Method (FSM) [Chorin (1968)] and Artificial Compressibility Method (ACM) [Chorin (1967)] with Characteristic Based Split (CBS) [Zienkiewicz and Codina (1995); Zienkiewicz, Morgan, Sai, Codina and Vasquez (1995)]). The method is tested for several low and intermediate Reynolds numbers (100, 400, 1000 and 3200) and node arrangements (41x41, 81x81, 101x101, 129x129). The original contribution of the paper represents extension of the LRBFCM to Reynolds number beyond 400 and assessment of the method for two different types of RBFs and two different types of pressure-velocity couplings. The obtained numerical results, in terms of mid-plane velocities, are in a good agreement with the data calculated in several reference publications and by commercial code. Both RBF’s used give approximately the same results. Both pressure-velocity coupling methods give approximately the same results, however the FSM turns out to be slightly more efficient. The advantages of the method are simplicity, accuracy and straightforward applicability in non-uniform node arrangements
Thermal-Mechanical and Thermodynamic Properties of Graphene Sheets using a Modified Nosé-Hoover Thermostat
Get PDFThe investigation assesses the thermal-mechanical and thermodynamic properties of various graphene sheets using a modified Nosé-Hoover (NH) thermostat method incorporated with molecular dynamics (MD) simulation. The investigation begins with an exploration of their thermal-mechanical properties at atmospheric pressure, including Young’s modulus, shear modulus, Poisson’s ratio, specific heats and linear and volumetric coefficients of thermal expansion (CTE). Two definitions of the line change ratio (ΔL/L) are utilized to determine the linear CTE of graphene sheets, and the calculations are compared with each other and data in the literature. To estimate the volumetric CTE values, the Connolly surface method is applied to predict the volume of the deformed graphene sheets in the free relaxation state and under temperature loading. Their specific heats are also determined by estimating the ratio of the amount of heat energy per unit mass that is required to raise the temperature by one degree. Finally, the dependences of the size and temperature on the thermal-mechanical and thermodynamic properties are examined. The calculations are validated by comparison with the results obtained from the existing thermostats and with the literature experimental and theoretical data The results indicate that the presently calculated thermal-mechanical and thermodynamic properties of graphene sheets are very similar to the published experimental and theoretical results. The graphene sheets tend to have a negative linear CTE at temperatures below 300 K. Additionally, the calculated linear CTE of graphene sheets depends strongly on the line-change-ratio assumptions. The modified NH thermostat is the only one of five thermostats that can accurately reproduce the Debye T3-dependent specific heat at temperatures below the Debye temperature