Use Of Fine Mesh Research Articles

Finite element modelling of plain and reinforced concrete specimens with the Kotsovos and Pavlovic material model, smeared crack approach and fine meshes

Modelling of concrete through 3 D constitutive material models is a challenging subject due to the numerous nonlinearities that occur during the monotonic and cyclic analysis of reinforced concrete structures. Additionally, the ultimate limit state modelling of plain concrete can lead to numerical instabilities given the lack of steel rebars that usually provide with the required tensile strength inducing numerical stability that is required during the nonlinear solution procedure. One of the commonly used 3 D concrete material models is that of the Kotsovos and Pavlovic, which until recently it was believed that when integrated with the smeared crack approach, it can only be used in combination with relatively larger in size finite elements. The objective of this study is to investigate into this misconception by developing different numerical models that foresee the use of fine meshes to simulate plain concrete and reinforced concrete specimens. For the needs of this research work, additional experiments were performed on cylindrical high strength concrete specimens that were used for additional validation purposes, whereas results on a reinforced concrete beam found in the international literature were used as well. A discussion on the numerical findings will be presented herein by comparing the different experimental data with the numerically predicted mechanical response of the under study concrete material model.

CARMA—Cellular Automata with Refined Mesh Adaptation—The Easy Way of Generation of Structural Topologies

This paper is focused on the development of a Cellular Automata algorithm with the refined mesh adaptation technique and the implementation of this algorithm in topology optimization problems. Traditionally, a Cellular Automaton is created based on regular discretization of the design domain into a lattice of cells, the states of which are updated by applying simple local rules. It is expected that during the topology optimization process the local rules responsible for the evaluation of cell states can drive the solution to solid/void resulting structures. In the proposed approach, the finite elements are equivalent to cells of an automaton and the states of cells are represented by design variables. While optimizing engineering structural elements, the important issue is to obtain well-defined solutions: in particular, topologies with smooth boundaries. The quality of the structural topology boundaries depends on the resolution level of mesh discretization: the greater the number of elements in the mesh, the better the representation of the optimized structure. However, the use of fine meshes implies a high computational cost. We propose, therefore, an adaptive way to refine the mesh. This allowed us to reduce the number of design variables without losing the accuracy of results and without an excessive increase in the number of elements caused by use of a fine mesh for a whole structure. In particular, it is not necessary to cover void regions with a very fine mesh. The implementation of a fine grid is expected mainly in the so-called grey regions where it has to be decided whether a cell becomes solid or void. The benefit of the proposed approach, besides the possibility of obtaining high-resolution, sharply resolved fine optimal topologies with a relatively low computational cost, is also that the checkerboard effect, mesh dependency, and the so-called grey areas can be eliminated without using any additional filtering. Moreover, the algorithm presented is versatile, which allows its easy combination with any structural analysis solver built on the finite element method.

Neutronic Simulation of Fuel Assembly Vibrations in a Nuclear Reactor

The mechanical vibrations of core internals such as fuel assemblies (FAs) cause oscillations in the neutron flux that require in some circumstances nuclear power plants to operate at a reduced power level. This work simulates and analyzes the changes of the neutron flux throughout a nuclear core due to the oscillation of a single FA without considering thermal-hydraulic feedback. The amplitude of the FA vibration is bounded to a few millimeters, and this implies the use of fine meshes and accurate numerical solvers due to the different scales of the problem. The results of the simulations show a main oscillation of the neutron flux with the same frequency as the FA vibration along with other harmonics at multiples of the vibration frequency much smaller in amplitude. Also, this work compares time domain analysis and frequency domain analysis of the mechanical vibrations. Numerical results show a close match between these two approaches for the fundamental frequency.

Novel in situ predator exclusion method reveals the relative effects of macro and mesopredators on sessile invertebrates in the field

Predation is considered an important structuring process in ecology; however, the effect size attributed to predation can vary across manipulative experiments. Complex interactions between predators of different sizes and trophic levels can confound observations in field-based experimental studies. Excluding large (macro) predators has demonstrated their potential to affect prey assemblage dynamics and structure. However, successful manipulations of smaller (meso) predators are limited due to the difficulty of excluding them in field studies while avoiding procedural artefacts. Here, we aim to manipulate both macro and mesopredators through the combination of exclusion cages (for macropredators) and a chemical deterrent (for mesopredators), avoiding the use of fine mesh and associated experimental artefacts. Using a novel chemical deterrent technique, we successfully manipulated predatory flatworm abundance in sessile hard substrate assemblages with no significant artefacts. Combined with the use of large cages to exclude fish, we tested the individual and interactive effects of macro and mesopredators on target prey species within sessile hard substrate assemblages. Flatworms reduced live barnacle abundances in the absence of fish, although this effect was small and only evident in the later successional stages. The effect of flatworms on barnacles was reduced in the presence of fish, resulting in more live barnacles persisting under the multiple predator scenario. Here, predatory interactions between different trophic levels were antagonistic, leading to reduced predation pressure on the common prey item. Considering mesopredator predation in predator exclusion experiments is important for correctly attributing effect sizes of larger bodied predators; failing to do so may confound experimental interpretations.

Numerical study on dynamic behavior of slope models including weak layers from deformation to failure using material point method

This paper describes numerical studies on the dynamic behavior of experimental slope models, including various inclined weak layers. These studies were conducted by simulation of shaking table tests using the material point method (MPM), which allows seamless treatment of various considerations, from elastic behavior to discontinuous collapse behavior of slopes, on the basis of an elasto-plastic constitutive law. The simulation results showed that the failure modes, progressive deformation and downward sliding of the numerical slope models, which were similar to that observed in the shaking table tests, can be obtained using the numerical method. In addition, sensitivity analyses of the numerical models used in this study were performed to determine the effects of mesh size, number of particles per cell (PPC) and damping constants on the simulation results. The results of these analyses indicated the use of fine meshes and high damping constants seems to produce sensible results and reduce numerical noises, respectively.

Sharp Boundary Electrocardiac Simulations

We present a sharp boundary electrocardiac simulation model based on the finite volume embedded boundary method for the solution of voltage dynamics in irregular domains with anisotropy and a high degree of anatomical detail. This method is second order accurate uniformly up to boundaries and is able to resolve small features without the use of fine meshes. This capability is necessary to enable the repeated simulations required for future verification and validation and uncertainty quantification studies of defibrillation, where fine-scale heterogeneities, such as those formed by small blood vessels, play an important role and require resolution.

Mechanical characterisation of additively manufactured material having lattice microstructure

Many natural and engineered structures possess cellular and porous architecture. This paper is focused on the mechanical characterisation of additively manufactured lattice structures. The lattice consists of a stack of polylactic acid (PLA) filaments in a woodpile arrangement fabricated using a fused deposition modelling 3D printer. Some of the most promising applications of this 3D lattice material of this type include scaffolds for tissue engineering and the core for sandwich panels. While there is a significant body of work concerning the manufacture of such lattice materials, attempts to understand their mechanical properties are very limited. This paper brings together manufacturing with the need to understand the structure-property relationship for this class of materials. In order to understand the elastic response of the PLA-based lattice structures obtained from the fused deposition modelling process, single filaments manufactured using the same process were experimentally characterised first. The single PLA filaments were manufactured under different temperatures. These filaments were then characterised by using tensile testing. The stress-strain curves are presented. The variability of the measured results is discussed. The measured properties are then taken as input to a finite element model of the lattice material. This model uses simple one-dimensional elements in conjunction with a novel method achieving computational economy which precludes the use of fine meshes. Using this novel model, the apparent elastic modulus of lattice along the filaments has been obtained and is presented in this paper.

The MARS Simulation of the ATLAS Main Steam Line Break Experiment

A main steam line break (MSLB) test at the ATLAS facility was simulated using the best-estimate thermal-hydraulic system code, MARS-KS. This has been performed as an activity at the third domestic standard problem for code benchmark (DSP-03) that has been organized by Korea Atomic Energy Research Institute (KAERI). The results of the MSLB experiment and the MARS input data prepared for the previous DSP-02 using the ATLAS facility were provided to participants. The preliminary MSLB simulation using the base input data, however, showed unphysical results in the primary-to-secondary heat transfer. To resolve the problems, some improvements were implemented in the MARS input modelling. These include the use of fine meshes for the bottom region of the steam generator secondary side and proper thermal-hydraulics calculation options. Other input model improvements in the heat loss and the flow restrictor models were also made and the results were investigated in detail. From the results of simulations, the limitations and further improvement areas of the MARS code were identified.

Compressible large eddy simulations of wall-bounded turbulent flows using a semi-implicit numerical scheme for low Mach number aeroacoustics

Large eddy simulations (LES) of low-speed, wall-bounded turbulent flows were conducted by numerically integrating the compressible Navier–Stokes equations in a generalized curvilinear coordinate system. An efficient numerical scheme based on a third-order additive semi-implicit Runge–Kutta method for time advancement and a sixth-order accurate, compact finite-difference scheme for spatial discretization were used. The convective terms in the wall-normal direction were treated implicitly to remove the time-step limitation associated with the use of fine meshes in the near-wall region for high Reynolds number viscous flows. The dynamic Smagorinsky subgrid-scale eddy viscosity model was used to close the filtered equations. Generalized characteristic-based non-reflecting boundary conditions were used together with grid stretching and enhanced damping in the exit zone. The accuracy and efficiency of the numerical scheme was assessed by simple acoustic model problems and by comparing LES predictions for fully developed turbulent channel flow and turbulent separated flow in an asymmetric diffuser to previous direct numerical simulation (DNS) and experimental data, respectively. LES predictions for both flows were in reasonable agreement with the DNS and experimental mean velocity and turbulence statistics. The findings suggest that the numerical approach employed here offers comparable accuracy to similar recent studies at approximately one-third of the computational cost and may provide both an accurate and efficient way to conduct computational aeroacoustics studies for low Mach number, confined turbulent flows.

Electromagnetic Modelling of Fine Features in Photonic Applications

Straightforward application of numerical modelling approaches to Photonic Band-Gap structures demands the use of fine meshes, notably finer that the size of a single scattering element, in order that high accuracy is achieved. However simulations employing sufficiently fine meshing to correctly represent the geometry of the scatterers lead to very long computational run-times and huge memory consumption. Such direct numerical approaches to PBG characterisation are only suited to the analysis of single unit cells or for benchmarking. In practice, the computational overheads significantly increase when one deals with an array of cells or when modelling the global behaviour of a large number of devices integrated on a single substrate. Therefore in order to model the macroscopic response of PBG structures the possibility of employing meshes of much larger size than the individual scattering elements has been explored. A suitable approach, initially developed for Electromagnetic Compatibility predictions, has been modified to permit modelling of Photonic Crystal Waveguides. The method provides second order accuracy and leaves a designer with the flexibility to discretise the entire problem space with an arbitrary number of scatterers per mesh cell. Results are first presented for small clusters of scatterers and subsequently for complete photonic structures.

Heat transfer in an asymmetrically heated duct (2nd report, Turbulent flow in a rectangular duct)

The objective of this article is to study theoretically and experimentally the effects nonuniform beating on turbulent heat transfer characteristics for flow in a horizontal rectangular duct ; a vertical side wall was uniformly heated, and the other wall were insulated. In our theoretical approach, the zero-equation model for turbulent eddy viscosity was employed. The effects of mesh size of finite difference on the calculation results were examined, and some refined compensation for wall temperatures and wall shear stresses by no use of fine mesh were proposed to reduce the calculation time. The heat transfer coefficients in thermally developing region for a nonuniformly heated duct obtained from numerical solutions are larger than the one for uniformly heated case. The buoyancy effects on heat transfer were evaluated. However, it was seen that the secondary flow due to buoyancy force was hardly expected to enhance heat transfer in a turbulent duct flow. Experiments were performed to measure the velocity and temperature profiles in a turbulent duct flow with a nonuniform heated wall. The experimental results were in good agreement with the theoretical ones.