Mesh Scheme Research Articles

Simulation study on ultrasonic tomography for grouted reinforced concrete by finite element

A finite element reconstruction algorithm for ultrasound tomography based on the Helmholtz equation in frequency domain is presented to monitor the grouting defects in reinforced concrete structures. In this algorithm, a hybrid regularizations-based iterative Newton method is implemented to provide stable inverse solutions. Furthermore, a dual mesh scheme and an adjoint method are adopted to reduce the computation cost and improve the efficiency of reconstruction. Simultaneous reconstruction of both acoustic velocity and attenuation coefficient for a reinforced concrete model is achieved with multiple frequency data. The algorithm is evaluated with numerical simulation under various practical scenarios including varied transmission/receiving modes, different noise levels, different source/detector numbers, and different contrast levels between the heterogeneity and background region. Results obtained suggest that the algorithm is insensitive to noise, and the reconstructions are quantitatively accurate in terms of the location, size and acoustic properties of the target over a range of contrast levels.

Elastoplastic Finite Element analysis of masonry shear walls

Masonry is the most important construction material in Turkey. It has been used for public and residential buildings in the past several thousand years. A great number of well-preserved old masonry structures still exist proving that this form of construction can successfully resist loads and environmental impact. Traditionally, most major buildings were solid walled structures with the walls bearing directly on the ground. Engineers work hard to convert the highly indeterminate, ambiguous and nonlinear behavior of historic masonry construction into something which can be understood with mathematical certainty. Therefore, practical and accurate structural analysis techniques are needed for the preserve the historical monuments as a huge cultural heritage. This paper is focused on Nonlinear Finite Element (NLFE) modeling of masonry shear walls at a macro-level taking the geometric arrangement of constituents. In this study, 3D elasto-plastic Finite Element (FE) analysis for the masonry walls that subjected to the combinations of vertical and lateral loads, are determined to find a practical method. An original meshing procedure is introduced to consider the orthotropy along the two natural directions of the masonry while the material is still assumed to be isotropic. The paper further examines parameter studies carried out to show that the relation suggested for cohesion values of mortar joint masonry can also be adopted for the masonry walls with dry joints employing compressive stresses on the top surface of the wall despite using its compressive strength. The accuracy of the proposed approach is verified by simulating a series of experiments reported in the literature. Those papers include shear tests on masonry walls with both dry and mortar joints by Raijmakers and Vermeltfoort, Oliveira and Roca. Comparisons between the predicted and measured failure loads of the walls confirm that it is possible to reproduce the fundamental features of masonry shear walls with the proposed meshing scheme. Finally, the proposed approach is shown to fit quite well the experimental load-deformation plots of masonry walls with both dry- and mortar joint under shear-compression fracture.

Polar Embedding for Aurora Image Retrieval.

Exploring the multimedia techniques to assist scientists for their research is an interesting and meaningful topic. In this paper, we focus on the large-scale aurora image retrieval by leveraging the bag-of-visual words (BoVW) framework. To refine the unsuitable representation and improve the retrieval performance, the BoVW model is modified by embedding the polar information. The superiority of the proposed polar embedding method lies in two aspects. On the one hand, the polar meshing scheme is conducted to determine the interest points, which is more suitable for images captured by circular fisheye lens. Especially for the aurora image, the extracted polar scale-invariant feature transform (polar-SIFT) feature can also reflect the geomagnetic longitude and latitude, and thus facilitates the further data analysis. On the other hand, a binary polar deep local binary pattern (polar-DLBP) descriptor is proposed to enhance the discriminative power of visual words. Together with the 64-bit polar-SIFT code obtained via Hamming embedding, the multifeature index is performed to reduce the impact of false positive matches. Extensive experiments are conducted on the large-scale aurora image data set. The experimental result indicates that the proposed method improves the retrieval accuracy significantly with acceptable efficiency and memory cost. In addition, the effectiveness of the polar-SIFT scheme and polar-DLBP integration are separately demonstrated.

Partially Averaged Navier Stokes simulation of turbulent heat transfer from a square cylinder

A variable resolution modeling approach, namely the Partially-Averaged Navier Stokes (PANS) approach, is used to study heat transfer in a separated turbulent flow field. The simulations are performed to assess the applicability and effectiveness of the PANS approach. Two types of grids are compared: a structured grid with full hexahedral elements with wall functions model (SM-WF) and an unstructured grid with hybrid tetrahedral-prismatic elements with wall resolve model (UM-WR). A heated square cylinder in cross air stream at Re=2.2×104 is considered and the simulations are performed using a finite volume based opensource software, OpenFOAM. It is observed that the PANS approach, with both the meshing strategies adopted, is able to predict the unsteady flow behavior around the cylinder and in the wake. But, only the wall resolve case reproduces the thermal and the Nusselt number behaviors around the cylinder as compared to experimental results. Further the wall resolve model is used to study the heat transfer phenomena at different faces of the cylinder over a complete vortex shedding cycle using the phase-averaged analysis of the shedding phenomenon. The turbulent heat fluxes are also studied to understand the effect of turbulence on convection.

Computational fluid dynamics simulation of gas-liquid two phases flow in 320 m3 air-blowing mechanical flotation cell using different turbulence models

According to the recently developed single-trough floating machine with the world’s largest volume (inflatable mechanical agitation flotation machine with volume of 320 m3) in China, the gas-fluid two-phase flow in flotation cell was simulated using computational fluid dynamics method. It is shown that hexahedral mesh scheme is more suitable for the complex structure of the flotation cell than tetrahedral mesh scheme, and a mesh quality ranging from 0.7 to 1.0 is obtained. Comparative studies of the standard k-e, k-ω and realizable k-e turbulence models were carried out. It is indicated that the standard k-e turbulence model could give a result relatively close to the practice and the liquid phase flow field is well characterized. In addition, two obvious recirculation zones are formed in the mixing zones, and the pressure on the rotor and stator is well characterized. Furthermore, the simulation results using improved standard k-e turbulence model show that surface tension coefficient of 0.072, drag model of Grace and coefficient of 4, and lift coefficient of 0.001 can be achieved. The research results suggest that gas-fluid two-phase flow in large flotation cell can be well simulated using computational fluid dynamics method.

Application of FEA to image-based models of electrical trees with uniform conductivity

X-ray computed tomography and serial block-face SEM have provided detailed threedimensional reconstructions of electrical trees for the first time. The application of finite element analysis (FEA) to the analysis of electrical fields in an epoxy block containing a tree is considered. Illustrations are provided by way of a number of case studies. It is shown that the limitations of FEA do not arise from the discrete nature of the meshing: rather uncertainties are more concerned with material properties in high fields on the micrometer scale, the limitations imposed by the pixel size of the imaging technique, and the discrete nature of the image reconstruction technique. For a dynamic model of tree growth space charge dynamics on the same physical scale need also to be modelled. A meshing strategy is used, calibrated against the charge simulation method, to ensure accurate but manageable computations in critical parts of a tree such as branch tips. Examples of field values are given using geometric constructs and low-field material characteristics as illustrative values. The field variation around a conducting tree structure, including the maximum field direction as a branch starts to bifurcate, is determined as an example. These yield values in excess of those expected if space charge movement was considered, but consistent with analytical calculations.

High performance ocean energy harvesting turbine design–A new casing treatment scheme

Delaying a stall improves the performance of any turbomachinery system. TC (tip clearance), which is used in a bi-directional flow Wells turbine of an ocean wave energy device, changes the flow pattern on the turbine blade suction surface, while changing or modifying the TC zone can help obtaining a delayed stall. In the present work, a new tip grooving scheme is introduced and the performance is compared for different tip groove depths and TCs of a Wells turbine. The performance is defined in terms of wider operating range or stall delay, power production and efficiency. The problem was solved by a numerical analysis technique. A multi-block meshing scheme was employed to generate structured and hexahedral elements in the computational domain and the flow was solved in ANSYS CFX® v14.5 by solving Reynolds-averaged Navier Stokes equations. It was found that the grooves improve the turbine operating range and power production as compared to those of the turbine without a groove. The groove depth of 3% of the chord length produced highest power and widest operating range. Using the circumferential groove, 26% increase in turbine power output for a particular operating point is achieved.

Investigation of meshing strategies and turbulence models of computational fluid dynamics simulations of vertical axis wind turbines

Making good use of vertical axis wind turbines (VAWTs) is an attractive and potential way to deal with the energy and environmental issues due to the unique superiorities of it. Computational Fluid Dynamics (CFD) technology is a useful tool for the design process of VAWTs. Various turbulence models have been developed and available for turbulent flow simulations. Currently, there have been few researchers studying on meshing strategies and turbulence model selections of VAWT simulations. In this paper, 2D unsteady models under 4 meshing strategies and 6 turbulence models were established and simulated to investigate the effect of the above two aspects on numerical simulations of VAWTs. The numerical results were compared with the experimental data of Oler et al. (“Dynamic stall regulation of the Darrieus Turbine,” SAND Report No. 83–7029, Sandia National Laboratories, Albuquerque, 1983, pp. 67–96) and the analytical solution of Deglaire et al. (Eur. J. Mech., B: Fluids 28(4), 506–520 (2009)). The results reveal that a mesh of 213 656 grids is sufficient to meet the requirements of grid independence with the help of boundary layer and size function techniques. Besides, the realizable k-ε model enables the closest CFD simulation of the experimental data and shows better prediction performance than the analytical model of Deglaire et al. and other turbulence models.

Large-Eddy Simulation of Turbulent Barotropic Flows in Spectral Space on a Sphere

Abstract Numerical simulations of atmospheric circulation models are limited by their finite spatial resolution, so large-eddy simulation (LES) is a preferred approach to study these models. In LES, a low-pass filter is applied to the flow field to separate the large- and small-scale motions. In implicitly filtered LES, the computational mesh and discretization schemes are considered to be the low-pass filter, while in the explicitly filtered LES approach, the filtering procedure is separated from the grid and discretization operators and allows for better control of the numerical errors. The aim of this paper is to study and compare implicitly filtered and explicitly filtered LES of atmospheric circulation models in spectral space. To achieve this goal, the results of implicitly filtered and explicitly filtered LES of a barotropic atmosphere circulation model on a sphere in spectral space are presented and compared with the results obtained from direct numerical simulation (DNS). Different numerical experiments are performed to investigate the efficiency of explicit filtering over implicit filtering under different dissipation terms and rotation rates. The study shows that explicit filtering increases the accuracy of the computations and improves the results, particularly where the location of coherent structures is concerned, a topic of particular importance in LES of atmospheric flows for climate and weather applications.

On Linear Spaces of Polyhedral Meshes.

Polyhedral meshes (PM)-meshes having planar faces-have enjoyed a rise in popularity in recent years due to their importance in architectural and industrial design. However, they are also notoriously difficult to generate and manipulate. Previous methods start with a smooth surface and then apply elaborate meshing schemes to create polyhedral meshes approximating the surface. In this paper, we describe a reverse approach: given the topology of a mesh, we explore the space of possible planar meshes having that topology. Our approach is based on a complete characterization of the maximal linear spaces of polyhedral meshes contained in the curved manifold of polyhedral meshes with a given topology. We show that these linear spaces can be described as nullspaces of differential operators, much like harmonic functions are nullspaces of the Laplacian operator. An analysis of this operator provides tools for global and local design of a polyhedral mesh, which fully expose the geometric possibilities and limitations of the given topology.

An Eulerian multi-material scheme for elastic–plastic impact and penetration problems involving large material deformations

An Eulerian multi-material scheme is developed for studying elastic–plastic impact and penetration problems involving large material deformations and shock waves. It consists of a Lagrangian plus remap (onto the original mesh) strategy at each time-step. The stress components of each material in a mixed cell are computed independently by assuming a common strain-rate to all materials. Similarly, the energy of each material in a mixed cell is updated independently and the mean pressure is determined using volume weighted average. This simplified approach eliminates commonly used iteration based pressure relaxation procedures and equation of state (EOS) closure models to find an equilibrium pressure in a mixed cell. A similar procedure is used to find equivalent stress in a mixed cell. The volume fraction of each material in a mixed cell is updated using a volume-of-fluid (VOF) interface tracking scheme. A new predictor–corrector (PC) scheme is developed to integrate the governing equations of the proposed multi-material model. The remapping of the stress components of the individual materials is achieved by a combination of a second-order monotonic upwind scheme and the VOF method. It is demonstrated that, despite using a ‘simple’ volume weighted scheme for calculating the total mean stress in mixed cells, the proposed scheme yields reasonable agreement with known analytical and experimental results of various impact and penetration problems.

A novel approach based on ALE and delamination fracture mechanics for multilayered composite beams

A novel approach able to predict debonding or fracture phenomena in multilayered composite beams is proposed. The structural model is based on the first-order shear deformable laminated beam theory and moving mesh strategy developed in the framework of Arbitrary Lagrangian–Eulerian (ALE) formulation. The former is utilized to evaluate fracture parameters by using a multilayer approach, in which a low number of interface elements are introduced along the thickness, whereas the latter is utilized to reproduce crack tip motion due to the crack extension produced by moving boundaries. The model is able to avoid computational complexities introduced by an explicit crack representation in bi-dimensional structures, in which typically high computational efforts are expected for handling moving boundaries. To this aim, a moving mesh strategy is proposed for the first time in the context of beam modeling based on a multilayered configuration. Such an approach, essentially based on ALE formulation, is able to reproduce interfacial crack paths by using a low number of computational elements. The numerical method is proposed in the framework of the finite element formulation for a quasi-static or dynamic evolution of the crack tip front. In order to investigate the accuracy and to validate the proposed methodology, comparisons with experimental data and existing formulations available from the literature are developed. Moreover, a parametric study in the framework of dynamic fracture is developed to investigate the capability of the proposed model to reproduce more complex loading cases.

Moving meshes to solve the time-dependent neutron diffusion equation in hexagonal geometry

To simulate the behaviour of a nuclear power reactor it is necessary to be able to integrate the time-dependent neutron diffusion equation inside the reactor core. Here the spatial discretization of this equation is done using a finite element method that permits h–p refinements for different geometries. This means that the accuracy of the solution can be improved refining the spatial mesh (h-refinement) and also increasing the degree of the polynomial expansions used in the finite element method (p-refinement). Transients involving the movement of the control rod banks have the problem known as the rod-cusping effect. Previous studies have usually approached the problem using a fixed mesh scheme defining averaged material properties. The present work proposes the use of a moving mesh scheme that uses spatial meshes that change with the movement of the control rods avoiding the necessity of using equivalent material cross sections for the partially inserted cells. The performance of the moving mesh scheme is tested studying one-dimensional and three-dimensional benchmark problems.

Simulating radiation damage accumulation in α-Fe: A spatially resolved stochastic cluster dynamics approach

Abstract Neutron irradiation of high purity α -Fe to 10 - 1 DPA is simulated using spatially resolved stochastic cluster dynamics (SRSCD). In doing so, a novel scheme is developed that addresses the following challenges to simulations of defect accumulation in bulk materials: how to introduce displacement cascades in a numerically efficient manner while maintaining the spatial resolution necessary for accurate defect evolution within cascades; how to account for interactions between defects already present in the material and cascades during the thermal spike phase; and how to choose appropriate cascade energies that reflect damage accumulation due to high-energy neutrons. The spatial resolution necessary for displacement cascade implantation is maintained via an accelerated adaptive meshing scheme. Direct cascade mixing with point defects present in the material is studied and shown to be necessary for defect saturation above 10 - 3 DPA. Defect population evolution is shown to only weakly depend on the cascade energy used in this study, allowing the use of a single cascade energy for simulating high energy neutron irradiation. Using this scheme, traps for mobile one-dimensional diffusing SIA loops are shown to strongly affect the final defect microstructure, and simulated defect accumulation results are compared to experimental results over the range of DPA studied here. The computational efficiency and accuracy of this technique make it a good candidate for inclusion in multi-scale models of radiation-induced material behavior changes.

Interactive three-dimensional boundary element stress analysis of components in aircraft structures

Computer aided design of mechanical components is an iterative process that often involves multiple stress analysis runs; this can be time consuming and expensive. Significant efficiency improvements can be made by increasing interactivity at the conceptual design stage. One approach is through real-time re-analysis of models with continuously updating geometry. Thus each run can benefit from an existing mesh and governing boundary element matrix that are similar to the updated geometry.For small problems, amenable to real-time analysis, re-integration accounts for the majority of the re-analysis time. This paper assesses how efficiency can be achieved during re-integration through both algorithmic and hardware based methods. For models with fewer than 10,000 degrees of freedom, the proposed algorithm performs up to five times faster than a standard integration scheme. An additional six times speed is achieved on eight cores over the serial implementation. By combining this work with previously addressed meshing and solution schemes, real-time re-analysis becomes a reality for small three-dimensional problems. Significant acceleration of larger systems is also achieved.This work demonstrates the viability of application in the aerospace industry where rapid validation of a range of similar models is an essential tool for optimising aircraft designs.

Adaptive Tetrahedral Mesh Generation of 3D Heterogeneous Objects

ABSTRACTIn this paper, a step-wise tetrahedral mesh generation method is introduced to solve the adaptive meshing problems of 3D heterogeneous objects, in which both the material heterogeneity and the geometric complexity have to be taken into account. In light of that, the proposed meshing strategy embraces three steps: initial mesh generation, material-oriented refinement, and geometry-oriented refinement. In the material-oriented refinement, a global refinement algorithm based on optimal Delaunay triangulation (ODT) is designed to control the mesh adaptation in terms of the material distribution. Compared to the refinement strategies based on local modifications, the global refinement algorithm takes full advantages of the material heterogeneity information and distributes the material composition variation over elements as equally as possible. As a result, the number of elements is reduced to a great extent regarding a given material threshold. Implementations show that the proposed approach guarantee...

Debonding detection of FRP strengthened concrete beams by using impedance measurements and an ensemble PSO adaptive spectral model

An impedance-based midspan debonding identification method for RC beams strengthened with FRP strips is presented in this paper using piezoelectric ceramic (PZT) sensor–actuators. To reach this purpose, firstly, a two-dimensional electromechanical impedance model is proposed to predict the electrical admittance of the PZT transducer bonded to the FRP strips of an RC beam. Considering the impedance is measured in high frequencies, a spectral element model of the bonded-PZT–FRP strengthened beam is developed. This model, in conjunction with experimental measurements of PZT transducers, is used to present an updating methodology to quantitatively detect interfacial debonding of these kinds of structures. To improve the performance and accuracy of the detection algorithm in a challenging problem such as ours, the structural health monitoring approach is solved with an ensemble process based on particle of swarm. An adaptive mesh scheme has also been developed to increase the reliability in locating the area in which debonding initiates. Predictions carried out with experimental results have showed the effectiveness and potential of the proposed method to detect prematurely at its earliest stages a critical failure mode such as that due to midspan debonding of the FRP strip.

RICH: OPEN-SOURCE HYDRODYNAMIC SIMULATION ON A MOVING VORONOI MESH

We present here RICH, a state of the art 2D hydrodynamic code based on Godunov's method, on an unstructured moving mesh (the acronym stands for Racah Institute Computational Hydrodynamics). This code is largely based on the code AREPO. It differs from AREPO in the interpolation and time advancement scheme as well as a novel parallelization scheme based on Voronoi tessellation. Using our code we study the pros and cons of a moving mesh (in comparison to a static mesh). We also compare its accuracy to other codes. Specifically, we show that our implementation of external sources and time advancement scheme is more accurate and robust than AREPO's, when the mesh is allowed to move. We performed a parameter study of the cell rounding mechanism (Llyod iterations) and it effects. We find that in most cases a moving mesh gives better results than a static mesh, but it is not universally true. In the case where matter moves in one way, and a sound wave is traveling in the other way (such that relative to the grid the wave is not moving) a static mesh gives better results than a moving mesh. Moreover, we show that Voronoi based moving mesh schemes suffer from an error, that is resolution independent, due to inconsistencies between the flux calculation and change in the area of a cell. Our code is publicly available as open source and designed in an object oriented, user friendly way that facilitates incorporation of new algorithms and physical processes.

Transient heat transfer between a turbulent lean partially premixed flame in limit cycle oscillation and the walls of a can type combustor

In this paper transient fluid-structure thermal analyses of the Limousine test rig have been conducted while the combustor was exposed to saturated amplitude limit cycle combustion oscillations (LCO). The heat transfer between hot combustion gases and the liner wall cooled by convection will affect thermo-acoustic instabilities, and therefore the relevance of prediction of the transient heat transfer rate in gas turbine combustors is explored. The commercial CFD code ANSYS CFX is used to analyze the problem. Fluid and solid regions are resolved simultaneously in a monolithical approach with application of a finite volume approach. Since the spatial scales of the solid temperature profiles are different in case of steady state and transient oscillatory heat transfer, special care has to be taken in the meshing strategy. It is shown that for the transient oscillatory heat transfer in to the solid in LCO operation, the mesh distribution and size of the grid in the solid part of the domain will play a very important role in determining the magnitude for the heat flow in the solid and the gas pressure fluctuations, and the grid resolution needs to be adapted to the thermal penetration depth. Moreover, compared to the calculations of only the fluid domain with adiabatic/isothermal boundary wall conditions, the results demonstrated that application of the Conjugated Heat Transfer (CHT) model leads to significant accuracy improvements in the prediction of the characteristics of the combustion instability.

Hierarchical template-based quadrilateral mesh generation

In the context of any domain decomposition meshing strategy, this paper describes a quadrilateral mesh generation algorithm ideally suited for subdomains in which transition of mesh refinement is required. The algorithm is based on an automatic hierarchical region decomposition in which, in the last level, it is possible to generate quadrilateral elements with a conventional mapping strategy. In two dimensions, a subdomain is usually a triangle or a rectangle. In this algorithm, a subdomain with two boundary curves may also be allowed. Direct use of mapping algorithms imposes restrictions on the number of boundary curve segments of a subdomain to be meshed. The proposed hierarchical template scheme eliminates these restrictions, requiring only an even number of boundary segments. Other algorithms in the literature present similar characteristics. However, the implementation of the hierarchical decomposition and its templates presented here is quite simple compared to other approaches. Six high-level templates are considered for a subdomain, depending on the number of boundary curves and the number of segments on each curve. Some examples demonstrate that this simple idea may result in structured meshes of good quality. We also show that the quadrilateral elements can be used as input to generate hexahedral elements. Three-dimensional sweeping examples, which use the proposed meshing scheme in the source and target surfaces, are also shown.