The generation of hexahedral meshes for assembly geometry: survey and progress

The finite element method is being used today to model component assemblies in a wide variety of application areas, including structural mechanics, fluid simulations, and others. Generating hexahedral meshes for these assemblies usually requires the use of geometry decomposition, with different meshing algorithms applied to different regions. While the primary motivation for this approach remains the lack of an automatic, reliable all‐hexahedral meshing algorithm, requirements in mesh quality and mesh configuration for typical analyses are also factors. For these reasons, this approach is also sometimes required when producing other types of unstructured meshes. This paper will review progress to date in automating many parts of the hex meshing process, which has halved the time to produce all‐hex meshes for large assemblies. Particular issues which have been exposed due to this progress will also be discussed, along with their applicability to the general unstructured meshing problem. Published in 2001 by John Wiley & Sons, Ltd.     

Three-dimensional stress and progressive failure analysis of ultra thick laminates and experimental validation

Test methods and analysis capabilities for fibre reinforced composites are generally limited to thin laminates. However, extending the range of application of composite materials to thick laminates is essential for a multitude of possible composite structures. This paper presents an adapted three-point bending test for a new quasi isotropic stacking sequence for non crimped fabrics for the application in ultra thick laminates (UTL). In addition, numerical simulation capabilities for thick laminates using a multiscale analysis are shown. The three-point bending test setup is developed to examine the failure behaviour of 30–60 mm thick coupons.     

Theory of second harmonic thermal-wave generation: One-dimensional geometry

The analysis of the thermal-wave second harmonic generation induced by the time-modulated heating of the material with temperature-dependent heat capacity C(T) and thermal conductivity k(T) is presented. The developed theory describes nonmonotonic behavior of the second harmonic amplitude in a semiinfinite medium. An enhanced spatial resolution of nonlinear photothermal imaging in materials with dominant role of the k(T) temperature dependence (i.e., for is predicted.On leave from International Laser Center, Moscow State University, 119899 Moscow, Russia.On leave from Jenoptik GmbH, Jena, Germany.     

Two-dimensional representation of machining geometry and tool path generation for ball-end milling of sculptured surfaces

This article applies a two-dimensional representation of the machining geometry relevant to tool path generation for the three-axis ball-end milling of sculptured surfaces. A two-dimensional geometric model detecting the machined strip is suggested as the concept for the ‘effective cutting profile’ which fits well into the three-dimensional machining geometry. The model is the same as the intersection of the cutter with the plane perpendicular to the tangent direction of the cutter location curve and incident with the cutter location point. In order to achieve the specified machining accuracy, an iterative approach is needed. The paper also presents a new iterative method to generate tool paths with a constant scallop height. It is based on the proposed model which resorts to a two-dimensional representation of the three-dimensional machining geometry. The proposed method reduces significantly the computing time to generate tool paths. Implementations and illustrated examples are discussed.     

Multiscale analysis of stiffness and fracture of nanoparticle-reinforced composites using micromechanics and global–local finite element models

A combined micromechanics analysis and global–local finite element method is proposed to study the interaction of particles and matrix at the nano-scale near a crack tip. An analytical model is used to obtain the effective elastic modulus of nanoparticle-reinforced composites, then a global–local multi-scale finite element model with effective homogeneous material properties is used to study the fracture of a compact tension sample. For SiO₂ particle-reinforced epoxy composites with various volume fractions, the simulation results for effective elastic modulus, fracture toughness, and critical strain energy release rate show good agreement with previously published experimental data. It is demonstrated that the proposed parametric multi-scale model can be used to efficiently study the toughness mechanisms at both the macro and nano-scale.     

An efficient dense and stable particular elements generation method based on geometry

In this paper, a new discrete elements generation method based on geometry is proposed to fill geometric domains with particles (disks or spheres). By generating particles each one with a random radius or with a radius calculated from the iteration to ensure no overlaps exist between particles and identifying unstable particles and changing them to stable ones, a dense and stable packing can be created. A partitioning particle radius interval method and a particle stability inspection and improvement method are introduced to guarantee the algorithm's success and the stability of the particles. Some packings were created to evaluate the performances of the new method. The results showed that the algorithm was very efficient and was able to create isotropic packings of low porosities and large coordinate numbers. The partitioning particle radius interval method improved the generation efficiency significantly and increased the packing densities. Through the comparisons with several existing methods proposed recently, the method proposed in this work is found to be more efficient and can fill geometric domains with the lowest porosities. In addition, the stability of the particles is guaranteed and no complex triangular or tetrahedral mesh is required in particle generation, thereby making the new method simpler. Copyright © 2016 John Wiley & Sons, Ltd.     

液化天然气冷能利用发电技术浅析

阐述了国内外LNG利用的形势,论述了LNG作为燃料的优势.通过分析LNG冷量利用原理及对LNG进行的分析,得出回收LNG冷量发电不仅有效利用能源,而且减少机械制冷造成的大量电能消耗,具有可观的经济效益和社会效益.对国内外利用LNG冷量发电方式进行总结分类,具体给出一些典型流程图和成功的应用范例.     

Robust generation of high‐quality unstructured meshes on realistic biomedical geometry

In this paper, we propose efficient and robust unstructured mesh generation methods based on computed tomography (CT) and magnetic resonance imaging (MRI) data, in order to obtain a patient‐specific geometry for high‐fidelity numerical simulations. Surface extraction from medical images is carried out mainly using open source libraries, including the Insight Segmentation and Registration Toolkit and the Visualization Toolkit, into the form of facet surface representation. To create high‐quality surface meshes, we propose two approaches. One is a direct advancing front method, and the other is a modified decimation method. The former emphasizes the controllability of local mesh density, and the latter enables semi‐automated mesh generation from low‐quality discrete surfaces. An advancing‐front‐based volume meshing method is employed. Our approaches are demonstrated with high‐fidelity tetrahedral meshes around medical geometries extracted from CT/MRI data. Copyright © 2005 John Wiley & Sons, Ltd.     

A Multiscale Progressive Failure Modeling Methodology for Composites That Includes Fiber Strength Stochastics

A multiscale modeling methodology was developed for continuous fiber composites that incorporates a statistical distribution of fiber strengths into coupled multiscale micromechanics/ finite element (FE) analyses. A modified twoparameter Weibull cumulative distribution function, which accounts for the effect of fiber length on the probability of failure, was used to characterize the statistical distribution of fiber strengths. A parametric study using the NASA Micromechanics Analysis Code with the Generalized Method of Cells (MAC/GMC) was performed to assess the effect of variable fiber strengths on local composite failure within a repeating unit cell (RUC) and subsequent global failure. The NASA code FEAMAC and the ABAQUS finite element solver were used to analyze the progressive failure of a unidirectional SCS-6/ TIMETAL 21S metal matrix composite tensile dogbone specimen at 650°C. Multiscale progressive failure analyses were performed to quantify the effect of spatially varying fiber strengths on the RUCaveraged and global stress-strain responses and failure. The ultimate composite strengths and distribution of failure locations (predominately within the gage section) reasonably matched the experimentally observed failure behavior. The predicted composite failure behavior suggests that use of macroscale models that exploit global geometric symmetries are inappropriate for cases where the actual distribution of local fiber strengths displays no such symmetries. This issue has not received much attention in the literature. Moreover, the model discretization at a specific length scale can have a profound effect on the computational costs associated with multiscale simulations.     

Woven fabric permeability: From textile deformation to fluid flow mesoscale simulations

A two-step methodology is proposed in order to estimate from numerical simulations the permeability of deformed woven fabrics. Firstly, the shear deformation of a glass plain weave until the shear locking is studied from a mesoscale analysis achieved with a representative volume element (RVE) of the periodic plain weave. Simulations have been carried out within the scope of large transformations, accounting for yarn–yarn contacts, and assuming that yarns behave as hypoelastic materials with transverse isotropy. From the simulated deformed solid RVE, a complementary periodic fluid RVE is then built and the slow flow of an incompressible Newtonian fluid within it is investigated. This allows to compute, in a second step, the permeability of the deformed plain weave. The role of the shear deformation on the permeability of multi-layers or single layer preforms is discussed.     

Automatic generation of random distribution of fibers in long-fiber-reinforced composites and mesomechanical simulation

An algorithm for the automatic generation of 2D representative volume element (RVE) of unidirectional long-fiber-reinforced composites (LFRCs) is presented in this paper. Both high fiber volume fraction and random fiber distribution are considered in the RVE. Two procedures which are named as global crisscrossing and local disturbing are included in this algorithm. Based on the model generated, mesomechanical analysis is carried out by using general finite element method (FEM) software ABAQUS. Firstly, the effect of the randomness of fiber distribution on the transverse modulus is investigated. Secondly, user subroutine to redefine field variables at a material point (USDFLD) in ABAQUS is used to simulate the damage behavior. A series of computational experiments are performed to evaluate the influence of mesh size on the ultimate load of the composites. The obtained results prove that the algorithm is capable of capturing the random distribution nature of these materials and the RVE produced could be used for predicting the damage onset and propagation of LFRCs.     

Mesh generation for aerospace applications

In recent years there has been much research activity in the field of compressible flow simulation for aerodynamic applications. In the 1970’s and 1980’s the advances in the numerical solution of the Full Potential and Euler equations made, in principle, the inviscid flow simulation around complex aerodynamic shapes possible. At this stage much attention was focused on methods capable of generating meshes on which such calculations could be performed. In this paper an overview is presented of some techniques which have been developed to generate meshes for aerospace applications. Structured mesh generation techniques are discussed and their application to complicated shapes utilising the multiblock approach is highlighted. Unstructured mesh generation methods are also discussed with particular emphasis given to the Delaunay triangulation method. Finally, the advantages and disadvantages of the structured and unstructured approaches are discussed and new work is presented which attempts to utilise both these approaches in an efficient and flexible manner. An erratum to this article is available at .     

New materials technologies for hydrogen generation

Abstract

A thermochemical cycle is described that demonstrates the use of biomass or lignite-derived synthesis gas to reduce New Zealand ironsand, which is subsequently steam-oxidised to generate pure hydrogen. The redox reactions for this cycle have been thermodynamically modelled, giving an indication of the temperature ranges required to achieve water splitting and the potential sensitivity of the process to partial pressures of CO₂. A fixed bed reactor showed reproducible oxide reduction behaviour over 10 redox cycles but only achieved limited conversion to 64% residual Fe₃O₄ (18%Fe). Fluidised bed reactor studies carried over a comparable timeframe using CO/N₂ mixtures demonstrated a high degree of reduction to 76%Fe.     

Numerical simulation of cascaded third harmonic generation for both phase matched and mismatched schemes

Numerical solutions to the nonlinear coupling-wave equations of second harmonic (SH) and third harmonic (TH) generators are investigated for both phase matched and phase mismatched configurations. For phase mismatched TH generation, several kinds of schemes (the phase mismatch either in second harmonic generation (SHG) or sum-frequency generation (SFG) process) are considered and analyzed. The physical nature corresponding to the different ratios of the coupling coefficients is discussed.     

Pulsed laser energy through fiberoptics for generation of ultrasound

Laser pulses are an effective, noncontacting technique for generating ultrasound in materials. However, for this approach to be practical, a versatile and safe method of delivering the laser pulses must be developed that eliminates exposed beams steered by mirrors and focused by lenses. Investigations by several researchers using fiberoptic delivery systems indicate that fiberoptics may be a viable method for the delivery of laser energy to generate acoustic energy. The main problem experienced with the fiberoptic delivery systems has been the inability to deliver high-energy, short-duration pulses via a fiber for thousands of pulses with no fiber damage and with constant energy output. This paper presents a technique for laser generation of sound using fiberoptics that continuously delivers sustained 20 ns pulses at a pulsing rate of 30 Hz from a doubled, Q-switched Nd:YAG laser operating at 532 nm with output energy from the fiber-optic system up to 26 mJ/pulse. The delivery system is used to excite ultrasound in a molten weld pool as part of a research effort to develop a noncontacting sensing system for real-time weld inspection.     

A three-stage graphics processing unit-based finite element analyses matrix generation strategy for unstructured meshes

With the development of parallel computing architectures, larger and more complex finite element analyses (FEA) are being performed with higher accuracy and smaller execution times. Graphics processing units (GPUs) are one of the major contributors of this computational breakthrough. This work presents a three-stage GPU-based FEA matrix generation strategy with the key idea of decoupling the computation of global matrix indices and values by use of a novel data structure referred to as the neighbor matrix. The first stage computes the neighbor matrix on the GPU based on the unstructured mesh. Using this neighbor matrix, the indices and values of the global matrix are computed separately in the second and third stages. The neighbor matrix is computed for three different element types. Two versions for performing numerical integration and assembly in the same or separate kernels are implemented and simulations are run for different mesh sizes having up to three million degrees of freedom on a single GPU. Comparison with GPU-based parallel implementation from the literature reveals speedup ranging from 4× to 6× for the proposed workload division strategy. Furthermore, the same kernel implementation is found to outperform the separate kernel implementation by 70% to 150% for different element types.     

Two effective approaches for hydroelectric generation scheduling

Abstract

This paper presents and compares two systematic and effective approaches for the hydroelectric generation scheduling problem (HSP). The objective of the HSP is to determine the best substitution of hydro energy for thermal energy in electric generation so that the fuel cost is minimized while meeting the system load and constraints. In the first approach, HSP is formulated as an optimal control problem, for which a multiplier method‐based differential dynamic programming algorithm developed by Chen et al. [4] is adopted as the solution algorithm. The second approach exploits the network structure of the water balance relation and takes linear approximation of the first formulation to formulate HSP as a minimum cost, linear network flow problem. Bertsekas and Tsengs’ RELAX code is applied to solve such a large‐scale network optimization problem. Numerical results indicate that both approaches generate results close to TPC's empirical schedules, are efficient in computation and suggest predictive operation strategies. We address the differences between the two approaches in solution optimality and computational efficiency and provide necessary information for tradeoffs in selecting a suitable approach.     

An efficient method for analyzing second harmonic generation with the consideration of pump depletion

We propose a simple method to analyze second harmonic generation (SHG) in general quasi-phase-matching media with the consideration of pump depletion. It allows us to conveniently predict precise result from undepleted pump approximation solution. Based on this method, multi-wavelength generation with arbitrary target efficiency and wavelength spacing is engineered. The effect of pump depletion on the performance of constructed multi-wavelength generator is analyzed. We demonstrate that it is efficient to design nonlinear device for multi-wavelength SHG by proposed method.     

Dual-band windows (DBW) transparent for fundamental and second harmonic generation frequencies

Using the known “looking glass” transformation property (z → 2π − z, y → 2π − y) of the optical phase thickness z and y of matching layers of two-layer anti-reflection coating, together with the fact that optical characteristics of any film do not change after addition of a half-wavelength layer, we designed dual-band anti-reflection coatings transparent at any preset wavelengths λ₁ and λ₂. On the basis of this result common fractional anti-reflection coatings for second and higher harmonics generation using dispersionless coating materials are developed. Explicit analytical relationships between refractive indices of the layers and substrate are deduced. Since for second harmonic generation the dispersion of materials may be a factor we show how to compensate the known dispersion of the coating materials by special choice of dispersion of a suitable substrate.