Mesh Domain Research Articles

Numerical Simulation of Micromixing of Particles and Fluids with Galloping Cylinder

Micromixers are significant segments inside miniaturized scale biomedical frameworks. Numerical investigation of the effects of galloping cylinder characteristics inside a microchannel Newtonian, incompressible fluid in nonstationary condition is performed. Governing equations of the system include the continuity equation, and Navier–Stokes equations are solved within a moving mesh domain. The symmetry of laminar entering the channel is broken by the self-sustained motion of the cylinder. A parameter study on the amplitude and frequency of passive moving cylinder on the mixing of tiny particles in the fluid is performed. The results show a significant increase to the index of mixing uses of the galloping body in biomedical frameworks in the course of micro-electromechanical systems (MEMS) devices.

Nonlinear Finite Volume Method for 3D Discrete Fracture-Matrix Simulations

Summary We present a lower dimensional discrete fracture-matrix (DFM) model for general nonorthogonal meshes populated by anisotropic permeability tensors in 3D spatial dimension. The discrete fractures are represented as 2D planes embedded in a 3D matrix domain and serve as internal boundaries for conforming meshing of the entire computational domain. The nonlinear finite volume method (FVM) is used to derive flux for both matrix-matrix connections and fracture-fracture connections to account for permeability anisotropy in the matrix and inside the fracture planes, whereas the linear two-point flux approximation (TPFA) is used to couple the matrix and fracture together. The nonlinear method proceeds by first constructing two one-sided fluxes for a connection, and then a unique flux is obtained by a convex combination of the two one-sided fluxes. Construction of one-sided fluxes requires introducing the so-called harmonic averaging points as auxiliary points. While the nonlinear FVM can be applied to derive the flux for matrix-matrix connections in a straightforward way, difficulties arise for fracture-fracture connections because of the presence of fracture intersections. Therefore, to construct the one-sided fluxes for fracture-fracture connections, we first present a novel generalization of the concept of harmonic averaging point so that auxiliary points can be calculated at fracture intersections. Unique nonlinear fluxes are then derived for fracture-fracture connections and fracture intersections. Results of the numerical examples demonstrate that the linear TPFA coupling of matrix and fracture seems to be adequate even for relatively strong anisotropy on a non-K-orthogonal grid, and the new DFM model can accurately capture the permeability anisotropy effect inside the fracture planes as well as the permeability anisotropy in the matrix domain compared with the equidimensional models in which the fractures are gridded explicitly. Finally, the DFM model is applied successfully to deal with complex fracture networks embedded in a heterogeneous matrix domain or fracture network with challenging geometric features.

Coupling FEM with a Multiple-Subdomain Trefftz Method

We consider 2D electromagnetic scattering at bounded objects consisting of different, possibly inhomogeneous materials. We propose and compare three approaches to couple the finite element method (FEM) in a meshed domain encompassing material inhomogeneities and the multiple multipole program (MMP) in the unbounded complement. MMP is a Trefftz method, as it employs trial spaces composed of exact solutions of the homogeneous problem. Each of these global basis functions is anchored at a point that, if singular, is placed outside the respective domain of approximation. In the MMP domain we assume that material parameters are piecewise constant, which induces a partition: one unbounded subdomain and other bounded, but possibly very large, subdomains, each requiring its own Trefftz trial space. Coupling approaches arise from seeking stationary points of Lagrangian functionals that both enforce the variational form of the equations in the FEM domain and match the different trial functions across subdomain interfaces. Hence, on top of the transmission conditions connecting the FEM and MMP domains, one also has to impose transmission conditions between the MMP subdomains. Specifically, we consider the following coupling approaches: We compare these approaches in a series of numerical experiments with different geometries and material parameters, including examples that exhibit triple-point singularities.

An ALE-FE method for two-phase flows with dynamic boundaries

The present work aims at developing a new flexible computational framework to simulate macro and microscale two-phase flows with dynamic boundaries. Such a technique is extremely useful for periodic and very large domains which requires exhaustive computational resources, consequently reducing the required numerical domain. In this article an interface tracking Finite Element (FE) method is used to solve the equations governing the motion of two immiscible incompressible fluids in the Arbitrary Lagrangian–Eulerian framework (ALE). The equations are written in axisymmetric coordinates, however the proposed moving boundary technique can be easily extended to 3-dimensional flows and other methods using the ALE framework such as the finite volume method. The two-phase interface separating the fluids is a subset of the domain mesh, therefore a layer of zero thickness is achieved assuring sharp transition of properties among phases. At the scale of interest, surface tension plays an important role and is thus considered in the flow equations. Several validations and results are presented for gravity dominated problem, including the sessile drop test and rising of spherical and Taylor bubbles, as well as the divergent and sinusoidal channels, showing accuracy for modeling two-phase flows in large and periodic domains.

Accurate, grid-robust and versatile combined-field discretization for the electromagnetic scattering analysis of perfectly conducting targets

Recent implementations of the Electric-Field Integral Equation (EFIE) for the electromagnetic scattering analysis of perfectly conducting targets rely on the electric current expansion with the monopolar-RWG basis functions, discontinuous across mesh edges, and the field testing over volumetric subdomains attached to the surface boundary triangulation. As compared to the standard RWG-based EFIE-approaches, normally continuous across edges, these schemes exhibit enhanced versatility, allowing the analysis of geometrically non-conformal meshes, and improved accuracy, especially for subwavelength sharp-edged conductors. In this paper, we present a monopolar-RWG discretization by the Method of Moments (MoM) of the Combined-Field Integral Equation (CFIE) resulting from the addition of a volumetrically tested discretization of the EFIE and the Galerkin tested MFIE-implementation. We show for sharp-edged conductors the degree of improved accuracy in the computed RCS and the convergence properties in the iterative search of the solution. More importantly, as we show in the paper, these implementations become in practice advantageous because of their robustness to flaws in the grid generation or their agility in handling complex meshes arising from the interconnection of independently meshed domains. The hybrid RWG/monopolar-RWG discretization of the CFIE defines the RWG discretization over geometrically conformal and smoothly varying mesh regions and inserts the monopolar-RWG expansion strictly at sharp edges, for improved accuracy purposes, or over boundary lines between partitioning mesh domains, for the sake of enhanced versatility. These hybrid schemes offer similar accuracy as their fully monopolar-RWG counterparts but with fewer unknowns and allow naturally non-conformal mesh transitions without inserting additional inter-domain continuity conditions or new artificial currents.

An Intelligent CFD-Based Optimization System for Fluid Machinery: Automotive Electronic Pump Case Application

Improving the efficiency of fluid machinery is an eternal topic, and the development of computational fluid dynamics (CFD) technology provides an opportunity to achieve optimal design in limited time. A multi-objective design process based on CFD and an intelligent optimization method is proposed in this study to improve the energy transfer efficiency, using the application of an automotive electronic pump as an example. Firstly, the three-dimensional CFD analysis of the prototype is carried out to understand the flow loss mechanism inside the pump and establish the numerical prediction model of pump performance. Secondly, an automatic optimization platform including fluid domain modeling, meshing, solving, post-processing, and design of experiment (DOE) is built based on three-dimensional parametric design method. Then, orthogonal experimental design and the multi-island genetic algorithm (MIGA) are utilized to drive the platform for improving the efficiency of the pump at three operating flowrates. Finally, the optimal impeller geometries are obtained within the limited 375 h and manufactured into a prototype for verification test. The results show that the highest efficiency of the pump increased by 4.2%, which verify the effectiveness of the proposed method. Overall, the flow field has been improved significantly after optimization, which is the fundamental reason for performance improvement.

A robust Delaunay-AFT based parallel method for the generation of large-scale fully constrained meshes

Making full use of a sequential Delaunay-AFT mesher, a parallel method for the generation of large-scale tetrahedral meshes on distributed-memory machines is developed. To generate meshes with the required and the preserved properties, a Delaunay-AFT based domain decomposition (DD) technique is employed. Starting from the Delaunay triangulation (DT) covering the problem domain, this technique creates a layer of elements dividing the domain into several zones. The initially coarsely meshed domain is partitioned into DTs of subdomains which can be meshed in parallel. When the size of a subdomain is smaller than a user-specified threshold, it will be meshed with the standard Delaunay-AFT mesher. A two-level DD strategy is designed to improve the parallel efficiency of this algorithm. A dynamic load balancing scheme is also implemented using the Message Passing Interface (MPI). Out-of-core meshing is introduced to accommodate excessive large meshes that cannot be handled by the available memory of the computer (RAM). Numerical tests are performed for various complex geometries with thousands of surface patches. Ultra-large-scale meshes with more than ten billion tetrahedral elements have been created. Moreover, the meshes generated with different numbers of DD operations are nearly identical in quality: showing the consistency and the stability of the automatic decomposition algorithm.

Numerical Modelling of Boundary Layers and Far Field Acoustic Propagation in Thermoviscous Fluid

To model linear acoustics in a thermoviscous fluid in open domain and time-harmonic regime, a Finite Element formulation in a bounded meshed domain is combined with the integral representation of the field for the propagative solution. The integrals are non-singular and involve the only Finite Element node values for temperature variation and particle velocity variables. To overcome the non-uniqueness of solutions at fictitious resonant frequencies, a Burton-Miller combination of integral representation is used. This formulation is suitable to compute acoustic radiation, scattering and diffraction by objects or mutual interaction between transducers. Two-dimensional computational experiments are presented in an infinite, open domain (exterior), showing that the model can be achieved in meshing only a thin domain surrounding the physical boundaries of a device.

On the automatic and a priori design of unstructured mesh resolution for coastal ocean circulation models

This study investigates the design of unstructured mesh resolution and its impact on the modeling of barotropic tides along the United States East Coast and Gulf Coast (ECGC). A discrete representation of a computational ocean domain (mesh design) is necessary due to finite computational resources and an incomplete knowledge of the physical system (e.g., shoreline and seabed topography). The selection of mesh resolution impacts both the numerical truncation error and the approximation of the system’s physical domain. To increase confidence in the design of high-resolution coastal ocean meshes and to quantify the efficacy of current mesh design practices, an automated mesh generation approach is applied to objectively control resolution placement based on a priori information such as shoreline geometry and seabed topographic features. The simulated harmonic tidal elevations for each mesh design are compared to that of a reference solution, computed on a 10.8 million vertex mesh of the ECGC region with a minimum shoreline resolution of 50 m. Our key findings indicate that existing mesh designs that use uniform resolution along the shoreline and slowly varying resolution sizes on the continental shelf inefficiently discretize the computational domain. Instead, a targeted approach that places fine resolution in narrow geometric features, along steep topographic gradients, and along pronounced submerged estuarine channels, while aggressively relaxing resolution elsewhere, leads to a mesh with an order of magnitude fewer vertices than the reference solution with comparable accuracy (within 3% harmonic elevation amplitudes in 99% of the domain).

Design and analysis of finite volume methods for elliptic equations with oblique derivatives; application to Earth gravity field modelling

We develop and analyse finite volume methods for the Poisson problem with boundary conditions involving oblique derivatives. We design a generic framework, for finite volume discretisations of such models, in which internal fluxes are not assumed to have a specific form, but only to satisfy some (usual) coercivity and consistency properties. The oblique boundary conditions are split into a normal component, which directly appears in the flux balance on control volumes touching the domain boundary, and a tangential component which is managed as an advection term on the boundary. This advection term is discretised using a finite volume method based on a centred discretisation (to ensure optimal rates of convergence) and stabilised using a vanishing boundary viscosity. A convergence analysis, based on the 3rd Strang Lemma [9], is conducted in this generic finite volume framework, and yields the expected O(h) optimal convergence rate in discrete energy norm.We then describe a specific choice of numerical fluxes, based on a generalised hexahedral meshing of the computational domain. These fluxes are a corrected version of fluxes originally introduced in [29]. We identify mesh regularity parameters that ensure that these fluxes satisfy the required coercivity and consistency properties. The theoretical rates of convergence are illustrated by an extensive set of 3D numerical tests, including some conducted with two variants of the proposed scheme. A test involving real-world data measuring the disturbing potential in Earth gravity modelling over Slovakia is also presented.

Elastohydrostatic simulation of thrust bearing Compensated with orifice restrictor operation with non-Newtonian lubricant

In the present work influence of elastohydrostaic lubrication on orifice compensated bearing has been shown. The thrust pad has been modeled by using three-dimensional mesh and lubrication domain has been modeled by using two dimensional mesh. The analysis of Reynold equation is coupled with the analysis of solution of equilibrium equation of thrust pad domain. The Present results shows that elasticity of thrust pad results a significant decrement in the fluid film Elasticity and it results an increase in fluid film stiffness. In the present case Circular, Elliptical, square and rectangular shape of recess has been taken from the analysis.

Parametric Study for Calculating Surface Deflection Using the Method of Images

The solution of the elastic field for inclusions in an elastic half space has been applied widely in many contact analyses and engineering designs. A popular method to solve the half-space inclusion problem resorts to the method of images, where the solution is decomposed into 3 components consisting of the full space inclusion, mirrored inclusion, and the surface traction cancellation. In the process of cancelling the redundant surface tractions determined from a full space inclusion problem, the computation domain is supposed to be limited in a finite size and there is inevitably truncation error. It has not been quantitatively investigated how the truncation error will influence the accuracy of the numerical computations based on the method of images. This work studies the deflection of the boundary surface of a half-space containing an Eshelby inclusion. Errors due to mesh refinement and domain truncation are quantitatively analyzed. Parametric studies are performed for a systematic examination of surface redundant tractions and their influences.

Improving accuracy and efficiency of CFD predictions of propeller open water performance

This research proposes mesh and domain optimization strategies for a popular Computational Fluid Dynamics (CFD) technique to estimate the open water propulsive characteristics of fixed pitch propellers accurately and time-efficiently based on examining the effect of various mesh and computation domain parameters. It used a Reynolds-Averaged Navier-Stokes (RANS) solver to predict the propulsive performance of a fixed pitch propeller with varied meshing, simulation domain and setup parameters. The optimized mesh and domain size parameters were selected using Design of Experiments (DoE) methods enabling simulations in a limited memory and in a timely manner without compromising the accuracy of results. The predicted thrust and torque for the propeller were compared to the corresponding measurements for determining the prediction accuracy. The authors found that the optimized meshing and setup arrangements reduced the propeller opens simulation time by at least a factor of six as compared to the generally popular CFD parameter setup. In addition, the accuracy of propulsive characteristics was improved by up to 50% as compared to published simulation results. The methodologies presented in this paper can be similarly applied to other simulations such as calm water ship resistance, ship propulsion etc. to systematically derive the optimized meshing arrangement for simulations with minimal simulation time and maximum accuracy. This investigation was carried out using a commercial CFD package; however, the findings can be applied to any RANS solver.

Sediment transport problems by the particle finite element method (PFEM)

This paper presents a particle finite element model for the numerical simulation of sediment transport problems. Fluid motion and sediment motion are coupled in a two-way manner, through boundary shear stress field and through flow domain mesh deformation. Solution of equations for fluid and sediment flows is performed by an explicit particle Lagrangian procedure (PFEM-2), including a fractional step strategy for the fluid flow equations. Previous particle finite element techniques concentrate on the simulation of erosion by removing elements. Here, transport problem includes erosion, saltation, deposition, and avalanche of grains. Numerical tests explore the efficiency of the methodology, either quantitatively or qualitatively.

Numerical simulation of seepage problem in porous media

In the paper, a mesh-free method called smoothed particle hydrodynamics (SPH) is presented to deal with seepage problem in porous media. In the SPH method, the computational domain is discredited by means of some nodes, and there is no need for computational domain meshing. Therefore, it can be said that it is a truly mesh-free approach. The method has been applied to analyze seepage problem in earth dams and foundations. The results were compared with ones obtained by analyzing with the finite element-based software, Geostudio (SEEP/W). There was a good agreement between results. Moreover, the SPH method is efficient and capable of seepage analysis specifically for the problems with complex geometry.

Nondestructive detection of underlying moldy lesions of apple using frequency domain diffuse optical tomography

Nondestructive tomographic identification of underlying moldy lesions of apple is demonstrated experimentally, for the first time, by using frequency domain diffuse optical tomography (FD-DOT). The custom applicator-probe had 18 optodes distributed on a spheroid apple-size imaging domain. The 12 source channels were time-multiplexed to a laser source modulated at 110 MHz, and the 6 detector channels were individually conditioned for parallel heterodyning at 10 kHz. The iterative image reconstruction was performed without priors in an open-source finite-element-method (FEM) package with custom meshing of the imaging domain. Cylindrical moldy lesions in apple at controlled center depths ranging from 10 mm to 35 mm were established using 10 mm diameter cylinder inclusions harvested from mildew flesh, and freeform lesions in apple were developed by injecting colonies of Alternaria alternate. The moldy lesions not deeper than 20 mm from the peel were resolvable on the absorption images. The moldy lesions were unremarkable on the reduced scattering images. Contrast-detail-analysis is also performed in simulation to assess the condition of identifying a moldy-core lesion by FD-DOT.

Efficient preprocessing of complex geometries for CFD simulations

ABSTRACTPreprocessing remains one the main bottlenecks in the computational fluid Dynamics simulations of flows involving complex geometries as advances in the algorithm development, turbulence modelling and parallel computing are made. To this end, two approaches are presented here to efficiently deal with complex geometries in order to reduce the preprocessing time and manual effort. First, a hybrid blocking approach, combining the medial axis-based method with level set iso-surface is presented to aid the block topology generation for subsequent structured meshing of complex 3D external flow domains. Secondly, a hierarchical geometry handling approach is demonstrated which makes use of the lower order modelling, overset meshes and zonal blocking for efficient preprocessing. Typical external aerodynamics cases have been showcased to describe how such techniques can be used to address modern challenges in the preparation of complex geometries for flow simulations.

Issues on characterization of cement paste microstructures from [formula omitted]-CT and virtual experiment framework for evaluating mechanical properties

There are issues on the microstructure characterization of cement paste obtained from μ-CT due to resolution limits, and evaluation of properties through virtual experiments. A phase separation procedure between the solid and pore phases, which can be used for pure cement paste microstructures, is proposed. The problems of underestimation of microstructural characteristics such as porosity in virtual specimens from μ-CT, as compared to real specimens, are addressed. Reflecting such underestimation, the process of input modeling parameter determination for virtual experiments on mechanical property evaluation using the phase field fracture model is elaborated. Through virtual tests, the effects of domain size and mesh resolution on the evaluated properties are investigated, and the correlation between the microstructural characterization parameters and mechanical properties is reconfirmed. It is shown that the virtual experiment framework proposed in this study can be used as a loading tool to supplement time and effort consuming real experiments for evaluating the mechanical properties of cement paste at the micro-scale.

Volumetric quantitative optical coherence elastography with an iterative inversion method.

It is widely accepted that accurate mechanical properties of three-dimensional soft tissues and cellular samples are not available on the microscale. Current methods based on optical coherence elastography can measure displacements at the necessary resolution, and over the volumes required for this task. However, in converting this data to maps of elastic properties, they often impose assumptions regarding homogeneity in stress or elastic properties that are violated in most realistic scenarios. Here, we introduce novel, rigorous, and computationally efficient inverse problem techniques that do not make these assumptions, to realize quantitative volumetric elasticity imaging on the microscale. Specifically, we iteratively solve the three-dimensional elasticity inverse problem using displacement maps obtained from compression optical coherence elastography. This is made computationally feasible with adaptive mesh refinement and domain decomposition methods. By employing a transparent, compliant surface layer with known shear modulus as a reference for the measurement, absolute shear modulus values are produced within a millimeter-scale sample volume. We demonstrate the method on phantoms, on a breast cancer sample ex vivo, and on human skin in vivo. Quantitative elastography on this length scale will find wide application in cell biology, tissue engineering and medicine.

Effects of Multiple Semiconducting Screens on Line Parameters and Wave Properties of Underground Cable

Along with the geometric and electromagnetic properties, incorporation of multiple semiconducting screens in an underground (UG) power cable influences the primary line parameters as well as wave properties of the cable. The impacts of multiple semiconducting screens in UG cable structure compared to the cable with single and without semiconducting screen have been reported in this paper. The effective impedance of conductor-semiconductor assembly in cable structure is determined based on the electromagnetic theory of tubular conductor. Further, the complete impedance matrix of the UG cable with multiple semiconducting screens is determined in both mesh and phase domain and reported in this paper. The expressions of the impedances of UG cable with multiple semiconducting screens as obtained using the electromagnetic approach also have been reproduced using the conventional loop current method to justify the robustness and accuracy of the adopted approach. Comparative analyses of various line parameters and wave properties between UG cables with multiple, single and without semiconducting screens have been carried out by varying the frequency as well as semiconducting screen properties. Such analyses indicate that the inclusion of multiple semiconducting screens can lead to the improvement in wave properties of UG cable in high frequency zone.