Mesh Geometry Research Articles

One-step Monte Carlo global homogenization based on RMC code

Abstract Due to the limitation of the computers, the conventional homogenization method is based on many assumptions and approximations, and some tough problems such as energy spectrum and boundary condition are faced. To deal with those problems, the Monte Carlo global homogenization is adopted. The Reactor Monte Carlo code RMC is used to study the global homogenization method, and the one-step global homogenization method is proposed. The superimposed mesh geometry is also used to divide the physical models, leading to better geometric flexibility. A set of multigroup homogenization cross sections is online generated for each mesh under the real neutron energy spectrum and boundary condition, the cross sections are adjusted by the superhomogenization method, and no leakage correction is required. During the process of superhomogenization, the author-developed reactor core program NLSP3 is used for global calculation, so the global flux distribution and equivalent homogenization cross sections could be solved simultaneously. Meanwhile, the calculated homogenization cross section could accurately reconstruct the non-homogenization flux distribution and could also be used for fine calculation. This one-step global homogenization method was tested by a PWR assembly and a small reactor model, and the results show the validity.

Prediction of sand erosion in CFD with dynamically deforming pipe geometry and implementing proper treatment of turbulence dispersion in particle tracking

Predicting more accurate sand erosion rate in oil and gas production is important for maintaining safe and reliable operations while maximizing output efficiency. Computational fluid dynamic (CFD) is a powerful tool for erosion prediction and provides detailed erosion pattern in complex geometry. To improve accuracy of erosion prediction, first part of this paper proposes an algorithm to include the effect of dynamically changing pipe geometry in CFD based erosion modelling and second part of this paper provides guidelines for proper treatment of turbulence dispersion in CFD based erosion modelling.Sand erosion is a time-dependent phenomenon where most severe erosion scars are localized and occurs under disturbed flow conditions. These localized high erosion rates cause substantial change in pipe geometry, creating cavities which modify the flow field, the particle trajectories and eventually the erosion pattern. This work has illustrated that the erosion prediction can be influenced by including the effect of dynamically changing pipe geometry in CFD modelling. An algorithm was developed to couple erosion rate calculations with a Moving-Deforming-Mesh, which dynamically deforms the solid wall surface geometry and computational mesh based on the local erosion rate at regular time intervals. This effect will be significant for geometries involving sharp corners and/or regions of recirculation zones.Sand erosion largely depends on particle impact location, impact angle and impact velocity - which is the commutative effect of particle trajectory leading to the bounding surface. The particle trajectory is determined based on mean and the turbulence of the flow field. In steady state CFD simulations with Reynolds Averaged Naiver Stokes (RANS) turbulence models, turbulent dispersion is obtained from mean turbulence quantities using a stochastic random walk model, which estimates the instantaneous characteristics of a turbulent eddy. However, stochastic random walk model parameters will have significant effects on turbulence dispersion of the particles and hence the erosion prediction. Specific guidelines for these model parameters can improve the accuracy and consistency in CFD based erosion prediction.

Analysis of the bias induced by voxel and unstructured mesh Monte Carlo models for the MCNP6 code in orthovoltage applications

ABSTRACTThe use of Monte Carlo methods for the set up of Treatment Planning Systems (TPS) in radiotherapy applications is a current standard. The most advanced modeling techniques aim at directly link the output of CT scans to a patient-specific model build up instead of standard phantoms and look-up tables. This link represents a critical step since even the most accurate segmentation and organ volume definition must be translated into a suitable input for the Monte Carlo code. During that step, the segmented volume is usually mapped on a regular geometrical lattice (voxels). A more sophisticated option appears to be a volume description based on an Unstructured Mesh (UM) geometry typical of current finite element codes. In this paper, we compared the two approaches analyzing the bias induced by the different choices. Starting from anonymous patient DICOM files coming from a CT scan, suitable segmentation and volume definition have been carried out and voxel and UM-based equivalent models for the Monte Carlo code MCNP6 have been built. Some computational phantoms, covering some significant portion of the human body (head and lower limb), have been used as a benchmark of the dose distribution obtained from X-ray sources commonly used in orthovoltage applications. The specific clinical application has been chosen because, despite being used on a growing number of patients and with multi-Gy doses, treatment planning is still based on poorly characterized dose mapping systems such as look-up tables or low-resolution computational phantoms for MC codes. Also, experimental measurements on phantom slabs irradiated by an X-ray source were carried out preliminarily to validate the simulated radiation source. As shown in the results, the UM computational phantoms (built through the Simpleware SCAN-IP™ tool) can reduce the bias induced by the regularity of the classical cubic voxel geometry, give a more accurate description of volumes and complex surfaces and, thanks to an optimized discretization of the volumes, are also able to reduce the computational work. For the same UM models, various comparisons with different voxel sizes have been produced to investigate the dose distributions and evaluate the voxelization effects. The comparison shows a convergence pattern of the voxel model to the UM one. It has been possible to verify that the most significant bias occurs where the dose gradient is higher, along the beam borders and at tissue interfaces. The simulations showed that the UM can be really effective and reliable to compute the dose distributions within computational anthropomorphic phantoms obtained from the patient’s CT scans.

Use of 3D mesh geometries and additive manufacturing in neutron beam experiments

PGAI-NT, a neutron-based element composition and structure analysis method, is well applicable to the non-destructive characterization of valuable artefacts, paleontological, bulk geological samples or to industrial reverse engineering. To set up the measurement geometry and scanning positions for items with unconstrained shapes, sizes, and matrices, accurate knowledge of the object’s geometry is a must. We applied portable structured-light 3D optical scanning or segmented neutron/X-ray tomography data to produce 3D meshes of the complex samples. Subsequently, 3D printing was used to fabricate detailed replicas of museum objects, as well as their gentle ad hoc sample holders, or produce custom parts of the Budapest PGAA instrument.

Efficient Design Method for Applying Vortex Generators in Turbomachinery

Secondary flow limits the aerodynamic loading level of turbomachinery. Vortex generators (VGs) offer the potential to attenuate secondary flow when implemented at the endwall of the blade passage. Customary design usually relies on computational fluid dynamics (CFD); however, VG geometry modeling and mesh generation are challenging. This paper presents an efficient method for designing the optimal VG layout. In this approach, first, a mathematical model (BAYC) is introduced to replace the actual VGs; hence, simulation can be carried out without detailed VG gridding. Second, an optimization procedure with response surface methods is employed to determine the optimal VG layout. To illustrate the proposed method, compressor cascades with one and three VGs are used as the test cases. The results demonstrate that the optimal VG layout may effectively weaken the secondary flow and can decrease the aerodynamic loss by 15–25% in almost all incidence angle ranges, particularly at positive incidence angles. Flow mechanism analysis indicates that VGs can enhance the boundary layer kinetic energy, thereby elevating the capability to withstand adverse pressure gradients.

Benchmarking Open-Source Static 3D Mesh Codecs for Immersive Media Interactive Live Streaming

This paper provides a systematic understanding of the requirements of live 3D mesh coding, targeting (tele-) immersive media streaming applications. We thoroughly benchmark in rate-distortion and runtime performance terms, four static 3D mesh coding solutions that are openly available. Apart from mesh geometry and connectivity, our analysis includes experiments for compressing vertex normals and attributes, something scarcely found in the literature. In addition, we provide a theoretical model of the tele-immersion pipeline that calculates its expected frame rate, as well as lower and upper bounds for its end-to-end latency. In order to obtain these measures, the theoretical model takes into account the compression performance of the codecs and some indicative network conditions. Based on the results we obtained through our codec benchmarking, we used our theoretical model to calculate and provide concrete measures for these tele-immersion pipeline’s metrics and discuss on the optimal codec choice depending on the network setup. This offers deep insight into the available solutions and paves the way for future research.

Optimisation of Pelton turbine jet deflector using CFD analysis

The high head Pelton turbines operate with extreme conditions. In case to need a sudden stop of operation, one of the important parts is jet deflector, which can be in push-out, or cut-of options. The paper presents the optimisation procedure of Pelton Turbine push-out jet deflector shape using numerical simulation of three-dimensional turbulent flow in order to minimize the forces, stresses, deformations and torque of jet deflector servomotor. The calculations are performed using the CFD package Ansys CFX. The steps for flow modelling, 3-D geometry definition, mesh generation, pre-processing and post-processing, as well as the optimised results obtained from the numerical simulation of the flow in the jet-deflector are presented. The work corresponds with the actual approach of methods development for flow simulation and calculations of Pelton Turbines. The kinematic and dynamic flow parameters are calculated based on CFD simulations. The results of the calculations represents reliable tool in the procedure of development and construction of Pelton Turbines parts.

Hydrodynamic Simulation of Wave Energy Converter Using Particle-Based Computational Fluid Dynamics

Wave energy from the ocean is currently a very popular renewable energy, and its development has primarily focused on the shape of the wave energy converter (WEC) used to efficiently convert wave energy into electrical energy. However, the free surface ocean wave problem is very complex and the parameters affecting WEC behavior are difficult to understand. In this paper, based on the Lattice-Boltzmann method, we present particle-based CFD simulation results for the pivoted-type WEC that exhibits both vertical and horizontal motions. In this method, the computation domain need not be a mesh and complex geometry is not a limiting factor. Using a free-surface turbulence model, we simulated the fluid–structure interaction. We detail our simulation results, which show good agreement with those in the literature.

The effect of local inertia around the crack-tip in dynamic fracture of soft materials

Phase-field or gradient-damage approaches offer elegant ways to model cracks. Material stiffness decreases in the cracked region with the evolution of the phase-field or damage variable. This variable and, consequently, the decreased stiffness are spatially diffused, which essentially means the loss of the internal links and the bearing capacity of the material in a finite region. Considering the loss of material stiffness without the loss of inertial mass seems to be an incomplete idea when dynamic fracture is considered. Loss of the inertial mass in the damaged material region may have significant effect on the dynamic failure processes. In the present work, dynamic fracture is analyzed using a theory, which takes into account the local loss of both material stiffness and inertia. Numerical formulation for brittle fracture at large deformations is based on the Cosserat point method, which allows suppressing the hourglass type deformation modes in simulations. Based on the developed algorithms, the effect of the material inertia around a crack tip is studied. Two different problems with single and multiple cracks are considered. Results suggest that in dynamic fracture the localized loss of mass plays an important role at the crack tip. It is found, particularly, that the loss of inertia leads to lower stresses at the crack tip and, because of that, to narrower cracks as compared to the case in which no inertia loss is considered. It is also found that the regularized problem formulation provides global convergence in energy under the mesh refinement. At the same time, the local crack pattern might still depend on the geometry of the unstructured mesh.

Verification of FEM modelling of composite materials based on the results of static strength test

The article presents the verification of FEM modelling of composite materials based on the results of static strength test. The aim of the work was to examine whether the applied modelling of composite materials is correct and verify it with finite element method (FEM). The composite structure of the PW-6U glider was used as a model. In the program the numerical model (geometry and finite element mesh) of the glider’s wing was created. The wing is made of glass fabrics and a spar with flanges with a glass roving. The composite structure of the wing, including composition, layout and thickness of laminate layers, fiber arrangement was exactly modelled in the program and then subjected to loads. Having the measurements from the static strength tests of the glider, the numerical results were compared with the experimental results. Thanks to the applied modelling, the obtained numerical results were satisfactory and very close to the experimental results from the structural static tests of the glider. Therefore, it can be concluded that the conducted verification of FEM modelling of composite materials is correct. Nowadays application of composite materials is increasingly expanding. Therefore, the modelling of composites becomes a significant issue. FEM modeling allows verification of the structure. At the stage of modelling modifications can be implemented and thus time and costs associated with subsequent changes in the production process may be saved. This is a very good solution which already at the design stage of the structure allows examination of its strength.

Influences of the mesh in the CAE simulation for plastic injection molding

Abstract Although computer-aided engineering (CAE) software has been used for many years in the plastic industry, identifying the most appropriate mesh geometry and density remains a challenge. It can affect the accuracy of the simulation, the time and the costs. The evaluation of the most suitable mesh is not easy because the difficulties to obtain the real the values of the pressure and temperature inside the mold. The current work investigates this issue. A mold was manufactured and sensors were installed in its interior. CAE simulations using different mesh geometries and densities were evaluated against the experimental data. The results showed that the computational time was mostly influenced by the mesh geometry. The use of 2D mesh and lower density can lead to a faster and more precise simulation of pressure inside the mold and 3D mesh with lower density can provide a faster and precise simulation of the temperature.

Theoretical study of electromagnetic and optical properties of periodic conductive networks based on Voronoi diagrams

Abstract This paper is a preliminary report on a theoretical study of transparent shielding materials with potentially very high shielding efficiency based on quasi-periodic conductive networks. Optical transmittance as a function of beam incident angle and mesh geometry is modelled in the geometrical approximation by exact ray-tracing and compared to the analytical solution for ideal hexagonal meshes. An empirical model for rapid calculation of shielding efficiency vs frequency and mesh geometrical parameters is suggested based on numerical simulation results.

Two Triangular Membrane Elements Based on Strain

The strain formulation approach improves accuracy and removes complications, such as shear parasitic errors and sensitivity to mesh distortions. For analyzing plane stress and strain problems, two new strain-based triangular elements are proposed. Both compatibility and equilibrium conditions are imposed to these elements. Contrary to the quadrilateral shape, triangular element facilitates proper meshing of various geometries. To formulate these elements, the linear strain field is assumed. The first element is a five-node triangular element, in which each node has two degrees of freedom. In the second one, which is a four-node element, drilling degrees of freedom are added to improve applicability of the element for bending problems. Various numerical examples and patch tests verify high accuracy and efficiency of the suggested elements in comparison with the other existing plane elements.

Laboratory study of ballast material reinforced by flat geogrid under the dynamic load

Ballast material suffers from continuous degradation under cyclic load. This leads to rail track settlement and necessitates its constant maintenance. It is serious problem that costs Russia millions of dollars every year. Easily accessible plastic geogrid was proposed to reinforce ballast and to prevent its rapid degradation. However, the optimal parameters of geogrid (its mesh size, geometry and number of layers) remains an open question. In current work effects of number of geogrid layers and geogrid mesh size on ballast settlement are studied by laboratory dynamic load tests.

Gear mesh geometry effect on performance improvement for external gear pumps

The parameters of the involute and cycloidal gearing which influence on fluid volume at the tooth space are analysed. It is established that to a greater extent on the flow rate of the pump with involute gearing is influenced by the radial clearance coefficient and the profile shift coefficient. The radii ratio of the auxiliary and pitch circles, as well as the radial clearance coefficient, mainly affect the flow rate of the pump with cycloidal gearing. Since the gear module is determined for reasons of pump flow rate, the teeth has a significant safety factor for contact stress and especially bending stress. This allows a wide variation of the gearing parameters in greater limits than in transmissions. The dependences of the fluid volume at the tooth space in the cycloidal gearing on the radial clearance coefficient, on the radii ratio of the auxiliary and pitch circles, on the fillet radius coefficient, is found. The boundaries of the variation of the gearing parameters were established taking into account the bending strength of the teeth. The analysis is carried out using the finite element method. Recommendations for the use of cycloidal gearing in external gear pumps have been developed.

Construction of polyhedral finite element meshes based upon marching cube algorithm

Hexahedral meshes that exhibit the superiority in terms of solution accuracy and convergence rate are preferred to other types of meshes in the finite element analysis. However, the construction of the hexahedral meshes for complex geometries is still considered troublesome due to their poor geometric adaptability. This paper presents an efficient grid-based scheme to automatically generate polyhedral meshes including the hexahedral elements, and thus to provide hexahedral-dominant meshes for three-dimensional geometry with complex shapes. On the basis of the marching cube algorithm with a background grid composed of a regular arrangement of cubes, surface topologies for the background cubes are defined to represent the three-dimensional boundaries of a given domain. Then, in order to generate a three-dimensional finite element mesh, the surface topologies of the marching cube algorithm are systematically expanded to polyhedral volume topologies. Meanwhile, a topology ambiguity problem inherent in the marching cube algorithm is effectively resolved to generate an appropriate polyhedral mesh even for an arbitrary complex geometry. Several examples including biostructure modeling demonstrate that the proposed mesh generation scheme can easily discretize complex three-dimensional domains with hexahedral-dominant meshes, which are composed of the polyhedral elements near the domain boundaries and the hexahedral elements that come from the background cubes inside the domains. Furthermore, to show the applicability and effectiveness of polyhedral meshes in the finite element analysis, some structural analyses are performed using the smoothed finite element method that can be straightforwardly adapted to polyhedral elements of arbitrary shape.

Effects of misalignment and crowning on contact characteristics of crown gear coupling

Although crown gear couplings are widely used in rotating machinery, very little is known about their contact behavior and load distribution characteristics. In this study, the manufacturing methods are presented for crown gear coupling. Complete geometrical mathematical models from tools to crown gear coupling are proposed based on the theories of differential geometry and gear mesh. Then, a high-fidelity finite element model verified by tooth contact analysis under light load is employed to investigate load distribution along the crown gear coupling interfaces. The effects of meshing position, torque, and angular misalignment are investigated on load distributions along with crowning amount depending on the displacement circle radius. Finally, it is observed that when the contact position is 0.2 times the width of the tooth, the radius of the displacement circle is the optimal result for the performance of the crown gear coupling.

Wave isogeometric analysis method for calculating dispersive properties of guided waves in rotating damped cylinders

The reliable guided wave inspection techniques depend on the accurate understanding of dispersive properties. The classic finite element method has been proven to be very practical for modeling wave propagation in arbitrary waveguides. However, when it comes to modeling on complex geometries, it still has a major drawback: the geometric discrete errors and the high consumption of resources to improve accuracy. Recently, Isogeometric analysis has been found advantageous over the classic finite element method on geometry representation and mesh generation. Here, a wave isogeometric analysis (WIGA) method is proposed for the computation of guided wave characteristics in rotating damped cylinders, based on the Floquet’s principle. According to the linear incremental theory and the principle of virtual power, elastodynamic wave equation is established, with regard to the WIGA. To ensure its effectiveness, the convergence and accuracy of the proposed method are considered compared with that of the wave finite element method. The size of unit cell and the solvable frequency range are discussed in detail. Furthermore, dispersive behaviors are computed considering the effects of damping and rotation. The results demonstrate that flexural wave modes are very sensitive to the rotation effect. Particularly, in phase velocity spectra the characteristic frequency at the amplitude peak equals to the rotational angular speed, where wave propagating and attenuating both vanish.

Response Site Analyses of 3D Homogeneous Soil Models

The seismic excitation at the surface can be determined through Site Response Analyses (SRA) as to account for the specific soil properties of the site. However, the obtained results are largely affected by the model choice and setting, and by the depth of the considered soil layer. This paper proposes a refined 3D analytical approach, by the application of OPENSEES platform. A preliminary analysis has been performed to check the model adequacy as regards the mesh geometry and the boundary conditions. After the model setting, a SRA has been performed on various soil profiles, differing for the shear velocity and representing the different soil classes as proposed by the Eurocode 8 (EC8). Three levels of seismic hazard have been considered. The seismic input at the bedrock has been represented consequently, through as much ensembles of seven ground motions each, spectrum-compatible to the elastic spectra provided by EC8 for the soil-type A (bedrock). Special attention has been paid to the role of the considered soil depth on the evaluation of the surface seismic input. Different values of depth have been considered for each soil type and seismic intensity, in order to check its effect on the obtained results.

Parallel computation of aeroacoustics of industrially relevant complex-geometry aeroengine jets

Jet noise is still a distinct noise component when a commercial aircraft is taking off. A parallel high-fidelity simulation framework for industrial jet noise prediction is presented in this paper. This framework includes complex geometry meshing and Ffowcs Williams-Hawkings (FW-H) surface placement during preprocessing, a parallel hybrid RANS-LES flow solver coupled with an FW-H acoustic solver in the simulation and mean and unsteady data processing after the simulation. The use of this framework is demonstrated through two jet noise prediction cases: in-flight heated jets and installed ultra-high bypass-ratio (UHBPR) engines. These simulations can provide more insight than experimental tests into jet flow physics for engineering model improvement. Additional advantages are also shown in the cost and turn-around time. Thus there is great potential for high-fidelity jet noise simulations to partly replace rig tests for industrial use in the future.