Interaction diagrams for carbon nanotubes under combined shortening-twisting

A theoretical investigation on the strength and stiffness of carbon nanotubes (CNTs) under combined shortening and twisting strains is presented. CNTs with similar length-to-diameter aspect ratios, L/D, but different atomic structures (zig-zag, armchair and chiral) have been selected. Molecular dynamics (MD) simulations have been performed to study the critical buckling behaviour and the pre-critical and post-critical stiffness of CNTs under combined shortening-twisting conditions. The main results are presented in the form of interaction diagrams between the critical strain and the critical angle of twist per unit of length. An interaction equation is proposed and validated by comparison with the MD results. If shortening is more dominant than twisting, the strain energy at the onset of buckling drops considerably with the increase of the twisting-shortening rate. If twisting is more influential than shortening, the energy at the onset of buckling decreases very slowly with the twisting-shortening rate. We also found an interaction factor of 1.5 for CNTs under combined shortening-twisting, which is much lower than the value 2.0 commonly adopted for circular tubes at macro-scale. We conclude that CNTs are much more sensitive to buckling under shortening-twisting interaction than macro-scale tubes.     

Hierarchical theories for the free vibration analysis of functionally graded beams

This work addresses a free vibration analysis of functionally graded beams via several axiomatic refined theories. The material properties of the beam are assumed to vary continuously on the cross-section according to a power law distribution in terms of the volume fraction of the material constituents. Young’s modulus, Poisson’s ratio and density can vary along one or two dimensions all together or independently. The three-dimensional kinematic field is derived in a compact form as a generic N-order polynomial approximation. The governing differential equations and the boundary conditions are derived by variationally imposing the equilibrium via the Principle of Virtual Displacements. They are written in terms of a fundamental nucleo that does not depend upon the approximation order. A Navier-type, closed form solution is adopted. Higher-order displacements-based theories that account for non-classical effects are formulated. Classical beam models, such as Euler–Bernoulli’s and Timoshenko’s, are obtained as particular cases. Bending, torsion and axial modes are investigated. Slender as well as short beams are considered. Numerical results highlight the effect of different material distributions on natural frequencies and mode shapes and the accuracy of the proposed models.     

Analysing and modelling the 3D shear damage behaviour of hybrid yarn textile-reinforced thermoplastic composites

The presented work focuses on the examination of the 3D shear damage behaviour and its phenomenological failure process of a thermoplastic composite made of E-glass/polypropylene hybrid yarn with a woven reinforcement. Experimental shear characterisation is performed by means of the Iosipescu testing approach for both in-plane and through-thickness directions. A procedure for the manufacturing of through-thickness shear specimens is presented in this study. The characterisation of the chronological failure process and deformation analysis is supported by high speed camera system and Digital Image Correlation. Based on the experimental observations, material modelling strategies are derived and performed within the finite element environment Ls-Dyna.     

Buckling analysis and design of anisogrid composite lattice conical shells

Composite lattice anisogrid shells have now become a popular choice in many aerospace applications. Their use in various structural components, such as rocket interstages, payload adapters for spacecraft launchers, fuselage components for aerial vehicles, and parts of the deployable space antennas requires the development of more advanced finite-element models and analysis techniques capable of predicting buckling behaviour of these structures under variety of loadings. A specialised finite-element model generation procedure (design modeller) is developed and applied to the buckling analysis of the composite anisogrid conical shells treated as three-dimensional frames composed of the curvilinear ribs made of unidirectional composite material. Featuring a dedicated control procedure for positioning the beam elements, the design modeller enables a close approximation of the original twisted geometry of the curvilinear ribs. The parametric finite-element buckling analyses of the anisogrid conical shells subjected to axial compression, transverse bending, pure bending, and torsion showed the robustness and potential of the modelling approach. It was demonstrated that the buckling resistance can be significantly enhanced by either increasing the stiffness of a few hoop ribs located in the close proximity to the section with the larger diameter, or by introducing the additional hoop ribs in the same part of the conical shell. The effectiveness of the design analyses is demonstrated using particular examples. It has been shown that the resultant optimised designs can produce up to 22% mass savings in comparison with the non-optimised lattice shells.     

Numerical modelling of oxidized microstructure and degraded properties of 2D C/SiC composites in air oxidizing environments below 800 °C

In this paper, a numerical model which incorporates the modelling of oxidized microstructure and computing of degraded elastic moduli is presented for simulation of the oxidation behaviors of 2D C/SiC composites exposed to air oxidizing environments below 800 °C. Regarding the multi-scale characteristics of 2D C/SiC composite, the microstructure modelling is carried out on microscopic and macroscopic scale, respectively to compute the degraded elastic properties in terms of time duration, temperature and pressure, whose influences upon the oxidation microstructure morphology and degraded properties of 2D C/SiC composites are also investigated. It is shown that the simulation models well the microstructure morphology after oxidation and numerical results are found to be in good agreement with experimental data.     

Simulation of energy saving in Iranian buildings using integrative modelling for insulation

According to one survey on energy consumption in Iran, commercial and building sector consume more energy than any other economic sectors. For example, about 38% of total energy that consumed in year 2001 has been used for space heating. Insulation in external walls of buildings has an important role to reduce the environmental effects on indoor space condition. Therefore, always using insulation is an alternative to avoid from the energy loss. In this paper, the effects of the using of a proper insulation on the energy saving in Iranian buildings are studied. For this purpose, an integrative modelling is used for simulation of the energy consumption in buildings. It is shown that energy consumption per square meter of buildings can be reduced up to 35.2% when insulation is used for external walls.     

System performance of a deep borehole heat exchanger

Deep borehole heat exchanger (BHE) systems, installed in abandoned boreholes, have been operative in Switzerland for several years now. The operational conditions of the 2302 m deep BHE plant at Weggis have been monitored continuously since 1994. In the first operational phase, lasting from October 1994 to May 1996, the plant was severely underused, as shown by the high production temperatures (40 °C). This behaviour was investigated by a numerical model accounting for the heat transport in the rock matrix and along the different tubing systems, with special emphasis on the heat transfer in a multi-layer insulated central pipe. Lacking detailed logging data or undisturbed temperature profiles, an axis-symmetrical model had to be used, assuming uniform rock parameters. Sensitivity studies highlighted the effect of varying flow rate or operation/recovery cycle lengths and helped to develop a strategy that allowed us to make an accurate calculation of the long-term Weggis production history. The initial model assumptions, based on this detailed treatment of the tubing system, could explain the operational data. By means of slight model variations that account only for the minor effects of metallic sleeves, the long-term production temperature history of the Weggis plant could be accurately fitted. These findings were confirmed by a detailed analysis of the May 1996 data. Due to the low degree of utilization, only numerical sensitivity analyses were able to highlight the potential of the deep BHE plant at Weggis. The results indicate that the low utilisation of 40 kW during the first operational phase could be increased to over 200 kW. The specific yield of deep systems is much higher than in conventional shallow BHE systems. Our simulation procedure proves that the heat transfer in a deep BHE system is well understood.     

Study of Real—Time Simulation for Two—Phase Flow Network

To meet the need of real-time simulation for two-phase flow network in power plants, this paper presents a mathematical model based on basic principles and a numerical solution method. This model can be used to describe nonhomogeneous two-phase flow networks. The algorithm presented in this paper makes use of the sparsity, the symmetry and the diagonal dominancy of the coefficient matrix to save storage space, reduce computational time and meet the requirements of real-time simulations.     

A case study to prepare for the utilization of aerosol forecasts in solar energy industries

Precise aerosol information is indispensable in providing accurate clear sky irradiance forecasts, which is a very important aspect in solar facility management as well as in solar and conventional power load prediction. In order to demonstrate the need of detailed aerosol information, direct irradiance derived from Aerosol Robotic Network (AERONET) ground based measurements of aerosol optical depth (AOD) was compared in a case study over Europe to irradiance calculated using a standard aerosol scenario. The analysis shows an underestimation of measurement-derived direct irradiance by the scenario-derived direct irradiance for locations in Northern Europe and an overestimation for the Mediterranean region.Forecasted AOD of the European Dispersion and Deposition Model (EURAD) system was validated against ground based AERONET clear sky AOD measurements for the same test period of February 15th to 22nd, 2004. For the time period analyzed, the modelled AOD forecasts of the EURAD system slightly underestimate ground based AERONET measurements. To quantify the effects of varying AOD forecast quality in their impact on the application in solar energy industry, measured and forecasted AOD were used to calculate and compare direct, diffuse, and global irradiance. All other influencing variables (mainly clouds and water vapour) are assumed to be modelled and measured correctly for this analysis which is dedicated to the specific error introduced by aerosol forecasting. The underestimated AOD results in a mean overestimation of direct irradiance of +28 W/m² (+12%), whereas diffuse irradiance is generally underestimated (−19 W/m² or −14%). Mean global irradiance values where direct and diffuse irradiance errors compensate each other are very well represented (on average +9 W/m² or +2%).     

“FRAC ” — A simulation code for forced fluid flow and transport in fractured, porous rock

The 3D Finite Element Program FRACTure was developed with the specific aim of studying the coupling of interactive mechanisms in geoscience and in particular those relevant to the long term behaviour of a Hot Dry Rock reservoir. The flexible modular structure facilitates the addition of further processes and elements to the existing library and the handling of linear and non-linear constitutive laws and the calculation of their interactions. The Hot Dry Rock applications involve essentially forced fluid flow of cool fluid injected into a hot fractured rock matrix. A study of the relevant processes required finite element solutions for the hydraulic, thermal and elastic fields and especially their interactions. Particular attention has been paid to modelling the perturbations arising from poro-elastic and thermo-elastic effects in the rock matrix and of a non-linear, stress dependent joint closure law. The fracture network is designed in a way to represent nature as realistically as possible: fracture flow takes place along planes or in pipes (fracture intersections), porous matrix flow in 3D bodies. In devising the coupling scheme careful thought has been given to the different time constants for each physical process. Apart of simulating the coupled Hot Dry Rock behaviour, the code has been successfully practised in a variety of applications in geoscience: analysis of geoelectric measurements, radon transport, solute tracer tests, simulations of heat probe operations and hydrogeologic system modelling.