Generation Of Computational Grids Research Articles

Modelling and optimization of shallow underground thermal energy storage

Shallow geothermal reservoirs are excellent candidates for low-enthalpy energy storage, and can serve as heat batteries providing constant discharge of base heat, as well as rapid discharge of heat in periods of high demand. Recharging can be done by pumping down hot water, heated using excess heat from e.g. waste incinerators. In addition to having a very low carbon footprint, such systems also require limited surface infrastructure, and can easily be placed near or under the end-user, such as below residential buildings. The geological setting is typically complex, with horizons, faults, and intertwined patterns of natural fractures, and the nearwell region is often hydraulically fractured to enhance inter-well communication. Therefore, in order to fully utilize the potential of shallow geothermal heat storage, numerical simulations are imperative. In this work, we show how to practically model such systems, including generation of computational grids with a large number of wells and fractures, numerical discretizations with discrete fractures, and complex storage strategies with multiple wells working together under common group targets. We also discuss how adjoint-based methods can be used to tune model parameters (e.g. well injectivities, rock properties, and hydraulic fracture conductivities) so that the model fits observed data, and to find well controls (e.g. rates and temperatures) that optimize storage operations. The methodology is demonstrated using one artificial and two real underground thermal energy storage projects currently under development, and we highlight important challenges and our suggested solutions related to each of them. Thematic collection: This article is part of the Earth as a thermal battery: future directions in subsurface thermal energy storage systems collection available at: https://www.lyellcollection.org/topic/collections/thermal-energy

Computer-aided determination of the effective diffusion coefficient of a gas diffusion layer for polymer electrolyte fuel cells

Accurate measurement of the gas diffusivity in a gas diffusion layer (GDL) for polymer electrolyte fuel cells (PEFCs) is extremely important in order to improve the performance of PEFCs and design fuel cell stacks. In the present study, in order to improve the accuracy in the determination of the gas diffusivity when uisng the simple and inexpensive method newly proposed by Yoshimune (2022), which uses an infrared absorption carbon dioxide sensor, three-dimensional (3D) simulations reproducing the experiments are performed. The results reveal that the gas transport resistance of the measurement apparatus is not small (approximately seven times higher than that of a commercial carbon paper (TGP-H060, Toray)), and the 3D simulation, which considers the resistance, reduces the error in the effective diffusion coefficient of the GDL by 25%. The effective diffusion coefficient for a new sample can be determined using the simulation results already obtained, when the apparatus is unchanged. In addition, a new one-dimensional (1D) model extended from the simple 1D model presented in Yoshimune (2022) is proposed. Since the extended model incorporates the gas transport resistance of the apparatus and dispenses with the generation of computational grids and the use of an expensive computer needed in conducting 3D simulations, this model enables us to obtain the accurate effective diffusion coefficient of a GDL with ease even when using a measurement apparatus with different sizes.

Towards automatic reconstruction of 3D city models tailored for urban flow simulations

In the computational fluid dynamics simulation workflow, the geometry preparation step is often regarded as a tedious, time-consuming task. Many practitioners consider it one of the main bottlenecks in the simulation process. The more complex the geometry, the longer the necessary work, meaning this issue is amplified for urban flow simulations that cover large areas with complex building geometries. To address the issue of geometry preparation, we propose a workflow for automatically reconstructing simulation-ready 3D city models. The workflow combines 2D geographical datasets (e.g., cadastral data, topographic datasets) and aerial point cloud-based elevation data to reconstruct terrain, buildings, and imprint surface layers like water, low vegetation, and roads. Imprinted surface layers serve as different roughness surfaces for modeling the atmospheric boundary layer. Furthermore, the workflow is capable of automatically defining the influence region and domain size according to best practice guidelines. The resulting geometry aims to be error-free: without gaps, self-intersections, and non-manifold edges. The workflow was implemented into an open-source framework using modern, robust, and state-of-the-art libraries with the intent to be used for further developments. Our approach limits the geometry generation step to the order of hours (including input data retrieval and preparation), producing geometries that can be directly used for computational grid generation without additional preparation. The reconstruction done by the algorithm can last from a few seconds to a few minutes, depending on the size of the input data. We obtained and prepared the input data for our verification study in about 2 hours, while the reconstruction process lasted 1 minute. The unstructured computational meshes we created in an automatic mesh generator show satisfactory quality indicators and the subsequent numerical simulation exhibits good convergence behavior with the grid convergence index of observed variables less than 5%.

A proposed cigarette emissions topography protocol reflecting smokers' natural environment use behavior.

The FTC, in 2008, rescinded its 1966 guidance regarding use of the Cambridge Filter Method, noting the yields from the method are relatively poor indicators of tar, nicotine, and carbon monoxide exposure. This article proposes a set of puffing conditions for cigarette emissions testing, with the goal of developing product-specific emissions characterizations which can subsequently be used to realistically model the yield of particulate matter and constituents to the mouth of a smoker, while accounting for the actual puffing behavior of the smoker. Synthesis of data was conducted on data collected from a prior one-week observation of 26 adult cigarette smokers, using their usual brand cigarette in each smokers' natural environment including the puff flow rate, duration, volume and time of day of each puff taken were recorded with a cigarette topography monitor. Data was analyzed to determine the empirical joint probability function and cumulative distribution function of mean puff flow rate and puff duration. The joint CDF was used to define an emissions topography protocol using concepts common to computational grid generation. Analysis of 8,250 cigarette puffs indicated the middle 95% of mean puff flow rates varied between 15 and 121 [mL/s] while the middle 95% of puff duration varied from 0.55 to 3.42 [s]. Thirteen conditions of varying mean puff flow rate and puff duration are proposed for a comprehensive cigarette emissions topography protocol. The proposed protocol addresses inadequacies associated with common machine-puffing profiles used for generating cigarette emissions.

有翼大気圏再突入機の空力特性予測に適したプリズム格子生成法

This study proposes a computational grid generation method to generate prismatic grids suitable for shock adaptation around a winged re-entry vehicle. The grid generation method proposed here is based on an existing method (Takanashi, S., Computers & Fluids, 19 (1991), pp. 393--399), which uses the theory of electric force lines to determine the computational grid lines. In order to control the distortion of the prismatic grids, the proposed method introduces three modifications to the existing method: 1) the grid generation process is divided into two stages, 2) the height of the prism layers is adjusted by considering the curvature of geometries, and 3) the cell width is optimized using the spring analogy. Both the proposed and existing methods are applied to generation of computational grids for a winged re-entry vehicle, HYFLEX. It is found that the existing method gives a computational grid containing cells not suitable for flow computations. On the other hand, the proposed method successfully gives a computational grid with which the flow computation is stably performed. By applying an adaptive refinement of prismatic layers to the detached shock wave, a smooth flow field is obtained behind the sharply resolved shock wave.

Numerical investigation of stall characteristics for winglet blade of a horizontal axis wind turbine

Wind energy is one of the renewable energy resources which is clean and sustainable energy and the wind turbine is used for harnessing energy from the wind. The blades are the key components of a wind turbine to convert wind energy into rotational energy. Recently, wingtip devices are used in the blades of horizontal axis wind turbine (HAWT), which decreases the vortex and drag, while increases the lift and thereby improve the performance of the turbine. In the present study, a winglet is used at the tip of an NREL phase VI wind turbine blade. Solidworks, Pointwise, and Ansys-Fluent are used for geometric modeling, computational grid generation, and CFD simulation, respectively. The computational result obtained using SST k-ω turbulence modeling is well validated with the experimental data of NREL at 5 and 7 m/s of wind speeds. Numerical investigation of stall characteristics is carried out for wingleted blade at higher turbulence intensity (21% and 25%) and angle of attack (00 to 300 at 50 intervals) at 7 m/s wind speed. The result found that wingletd blade delay stall to 150 for both the cases of turbulence intensity. Increasing the turbulence intensity increases the lift coefficient at stall angle but drag coefficient also increases and thus a lower aerodynamic performance (CL/CD ratio = 13) is obtained. Wingleted blade improves the performance as the intensity of vortices is smaller compared to baseline blade

The Effects of Grid Accuracy on Flow Simulations: A Numerical Assessment

High-quality, accurate grid generation is a critical challenge in the computational simulation of fluid flows around complex geometries. In particular, the accuracy of the grids is an effective factor in order to achieve a successful numerical simulation. In the current study, we present a series of systematic numerical simulations for fluid flows around a NACA 0012 airfoil using different computational grid generation techniques, including the standard second-order, fourth-order compact, and Theodorsen transformation approaches, to assess the effects of grid accuracy on the flow solutions. The flow solvers are based on the second- and fourth-order schemes for spatial discretizations and Beam-Warming linearization method for time advancement. The obtained grids, as well as the metrics and the corresponding numerical flow solution for each grid generation technique, are compared and studied in detail. It is demonstrated that the quality and orthogonality of the grids is improved by using the fourth-order compact scheme. Moreover, the numerical assessment showed that the accuracy and the quality of the grids directly influence the numerical flow solutions. Finally, the higher-order accurate flow solvers are found to be more sensitive to the accuracy of the generated grid.

A spectral analysis approach for the a priori generation of computational grids in the 2-D hydrodynamic-based runoff simulations at a basin scale

The increasing availability of high-resolution terrain data fostered the development of two-dimensional hydrodynamic-based surface runoff models at basin scale, for flood hazard assessment and mapping purposes. The generation of the computational domain is one of the most important preliminary steps for practical applications of this kind of modeling. Several studies can be found in the technical literature showing the influence exerted by the computational grid type used on the results in terms of water depth, flow velocity and discharge. Several approaches were proposed for the mesh generation even though, in practice, it is subject to the modeller choice and experience. In any case, the reason why a specific mesh is used is rarely discussed and its impact on the model results in terms of outflow hydrographs, runoff volumes and flooded areas extent is seldom analysed. In this work a rigorous approach for an a priori design of a computational grid is proposed. The a priori term suggests that the proposed procedure aims at generating a computational mesh, close to the optimal one, in a time-saving process which is not based on trial meshes and trial simulations. Namely, the process is based on the application of the two-dimensional Fourier analysis to a high-resolution digital elevation model in order to compute the power spectrum and finally to identify the significant waves lengths and the characteristic scales. The latter ones are located in the catchment and used for the generation of a computational grid characterized by different elements size. The simulations of surface runoff at a basin scale have shown that the application of the proposed method for the a priori construction of the grid leads to the generation of a computational mesh which reduces the calculation time by approximately 87%, if compared to a fine grid taken as reference, with negligible errors on outflow hydrographs and flooded areas extent.

Criteria of computational grid generation for turbulence models taking into account laminar-turbulent transition

Criteria of structured and unstructured mesh generation for turbulence models taking into account laminar-turbulent transition were developed using flat plate boundary layer flow. It is shown the first near wall cell should be placed in the viscous sublayer in the transition region and stretching factor should not exceed 1.4. For the unstructured meshes all the boundary layer should be covered by prismatic layers. The results of the prediction of the flow around a NLF(1)-0414F airfoil show that the use of relatively coarse meshes provide acceptable accuracy of the solution.

Computational Grid Generation for the Design of Free-Form Shells with Complex Boundary Conditions

AbstractFree-form grid structures have been widely used in various public buildings, and many are bounded by complex curves including internal voids. Modern computational design software enables th...

A Study of Multi-Objective Aerodynamic Optimization Design for Variable Camber Airfoils and High Lift Devices

Aiming at the synthetical optimization of the aerodynamic performance between the low-speed condition of two-dimensional high lift devices during take-off and landing phase and the high-speed condition of variable camber airfoil during cruise phase, an aerodynamic optimization design method for high lift device based on Kriging based surrogate model and multi-objective genetic algorithm has been developed. With the application of Adaptive Dropped Hinge Flap mechanism, the low-speed take-off and landing performance and high-speed cruise performance of the aircraft is improved by coupling deflection of the flap and spoiler. The position of flap hinge, deflection angle of spoiler and deflection angle of flap are taken as design variables; The Navier-Stokes equations are used to predict the aerodynamic forces of initial samples; The Kriging based surrogate model is employed to establish the algebraic relation between design variables and aerodynamic forces at take off, landing and cruise, obtaining four efficient prediction models for aerodynamic forces; Multi-objective optimization design with multi-objective genetic algorithm is conducted on the basis of surrogate models. The automatic generation of computational grid is achieved by the mesh deformation method based on RBF (Radial Basis Function) when the design variables change. On the basis of efficient global multi-objective optimization design platform, the synthetical optimization of high-speed and low-speed aerodynamic performance is conducted; The multi-objective solution set of the Pareto frontier is verified and analyzed, and the optimal solution with well matched high and low speed performance is selected.

Strong-form approach to elasticity: Hybrid finite difference-meshless collocation method (FDMCM)

• FDMCM combines the finite difference and the strong form meshless method. • FDMCM scheme uses a uniform Cartesian grid embedded in complex geometry. • The method is suited for problems with geometries known imperfectly from images. • FDMCM method combines the advantages of both FD and meshless methods. We propose a numerical method that combines the finite difference (FD) and strong form (collocation) meshless method (MM) for solving linear elasticity equations. We call this new method FDMCM. The FDMCM scheme uses a uniform Cartesian grid embedded in complex geometries and applies both methods to calculate spatial derivatives. The spatial domain is represented by a set of nodes categorized as (i) boundary and near boundary nodes, and (ii) interior nodes. For boundary and near boundary nodes, where the finite difference stencil cannot be defined, the Discretization Corrected Particle Strength Exchange (DC PSE) scheme is used for derivative evaluation, while for interior nodes standard second order finite differences are used. FDMCM method combines the advantages of both FD and DC PSE methods. It supports a fast and simple generation of grids and provides convergence rates comparable to weak formulations. We demonstrate the appropriateness and robustness of the proposed scheme through various benchmark problems in 2D and 3D. Numerical results show good accuracy and h -convergence properties. The ease of computational grid generation makes the method particularly suited for problems where geometries are very complicated and known only imperfectly from images, frequently occurring in e.g. geomechanics and patient-specific biomechanics, where the proposed FDMCM method, after its extension to non-linear regime, appears to be a promising alternative to the traditional weak form-based numerical schemes used in the field.

Algebraic generation of single domain computational grid for twin screw machines. Part I. Implementation

Special attention is required for generation of computational grids in highly deforming working chambers of twin screw machines for 3D CFD calculations. Two approaches for customised grid generation are practically available. The first is an algebraic grid generation and the second is a differential decomposition method. This paper reports on new developments in the algebraic approach that has the advantages associated with both algebraic and differential methods. Two control functions are introduced for regularisation of the initial algebraic distribution. One is based on an analytical control function in transformed coordinate system while the other uses background blocking structure in order to guide the initial algebraic distribution towards a single computational mesh. This paper presents implementation and grid characteristics of these new functions. Developed grids have been tested and results from flow calculations on a dry air compressor have been validated in part II of the paper [29].It was possible to achieve two distinct characteristics desirable in a twin screw rotor domain mesh. Firstly, it is possible to independently control grid refinement in the interlobe region thereby providing better accuracy in representation of the leakage gaps. Secondly and most importantly, it is possible now to eliminate the non-conformal interface between the two rotor domains thereby producing a single domain structured grid for the rotors, while still maintaining the fully hexahedral cell topology. An improvement in the global orthogonality of the cells was achieved. Despite of a decrement in the Face warp quality, aspect ratio of cells retained similar scale.

A stabilized Nitsche‐type extended embedding mesh approach for 3D low‐ and high‐Reynolds‐number flows

SummaryThis paper presents a stabilized extended finite element method (XFEM) based fluid formulation to embed arbitrary fluid patches into a fixed background fluid mesh. The new approach is highly beneficial when it comes to computational grid generation for complex domains, as it allows locally increased resolutions independent from size and structure of the background mesh. Motivating applications for such a domain decomposition technique are complex fluid‐structure interaction problems, where an additional boundary layer mesh is used to accurately capture the flow around the structure. The objective of this work is to provide an accurate and robust XFEM‐based coupling for low‐ as well as high‐Reynolds‐number flows. Our formulation is built from the following essential ingredients: Coupling conditions on the embedded interface are imposed weakly using Nitsche's method supported by extra terms to guarantee mass conservation and to control the convective mass transport across the interface for transient viscous‐dominated and convection‐dominated flows. Residual‐based fluid stabilizations in the interior of the fluid subdomains and accompanying face‐oriented fluid and ghost‐penalty stabilizations in the interface zone stabilize the formulation in the entire fluid domain. A detailed numerical study of our stabilized embedded fluid formulation, including an investigation of variants of Nitsche's method for viscous flows, shows optimal error convergence for viscous‐dominated and convection‐dominated flow problems independent of the interface position. Challenging two‐dimensional and three‐dimensional numerical examples highlight the robustness of our approach in all flow regimes: benchmark computations for laminar flow around a cylinder, a turbulent driven cavity flow at Re = 10000 and the flow interacting with a three‐dimensional flexible wall. Copyright © 2016 John Wiley & Sons, Ltd.

Development of a regional coastal management decision-aided system. Part A: Establishment of an automatic numerical simulation program

An automatic numerical simulation program is established for coastal waters. The program is developed based on an idea of nested model. The whole process of engineering data acquisition, computational domain drafting, computational grid generation, initial and open boundary condition abstraction, parameter settings, program running and simulation results extraction is automatically carried out. The program structure and the implementation are described in detail. Based on the requirements of computational stability and efficiency, one model bank and two rule sets are established. Different from existing hydrodynamic programs, this program achieves a thorough automatic process, which can be set up and then automatically run without much user intervention. Being compiled into an EXE file, it can be easily transplanted into MGIS and other similar information system.

Evaluation of endplate impact on tip loaded propeller performance using a RANSE solver

Systematic variations of the endplate geometry for two endplate propellers are made in order to assess the impact of the blade tip shape on propeller performance. Several types of modifications are analyzed including variations in plate width, contraction angle, plate sweep and flap angle. The effect of endplate geometry modifications in the radial distribution of circulation is illustrated. The magnitude of the circulation is derived from the analysis of the tangential velocities behind the propeller. The contribution of pressure and friction components to the total endplate forces is also shown. Comparisons are made at full scale. A special procedure for the generation of the computational grids is implemented in order to minimize computational errors in the comparison of the alternative geometries. RANS code FINFLO is used in the evaluation.

Numerical calculation of the transient behaviour of two pure cross-flow heat exchangers coupled by a circulating flow stream

The transient thermal behaviour of a heat shifting system consisting of two pure cross-flow heat exchangers coupled by a circulating flow stream is studied theoretically. A suitable mathematical description of the system is based on the energy balance equation for general flow processes yielding a system of coupled hyperbolic partial differential equations in two dimensions. System responses to perturbations of inlet temperatures and mass flow rates are numerically calculated with an explicit finite difference method. A criterion for the generation of computational grids minimising effects of numerical dispersion and dissipation is applied to the system of coupled pure cross-flow heat exchangers which has not been considered up to now. Due to its internal circulation the coupled system shows a different behaviour compared to single cross-flow heat exchangers like inverse response and oscillatory behaviour to non-oscillating input signals.

Comparison of different spatial grids for numerical schemes of geophysical fluid dynamics

The generation of computational grids is an important component contributing to the efficiency of numerical schemes of atmosphere/ocean dynamics. In this study the problem of construction of the most uniform grids based on conformal mappings of spherical domains is considered. Stereographic, cylindrical and conic grids for computational rectangles are developed and their uniformity is compared. Numerical experiments with two schemes approximating shallow water equations are performed in order to assess the practical efficiency of the constructed grids and to compare the numerical results with analytical evaluations.

Efficient nonlinear solvers for Laplace–Beltrami smoothing of three-dimensional unstructured grids

The Laplace–Beltrami system of nonlinear, elliptic, partial differential equations has utility in the generation of computational grids on complex and highly curved geometry. Discretization of this system using the finite-element method accommodates unstructured grids, but generates a large, sparse, ill-conditioned system of nonlinear discrete equations. The use of the Laplace–Beltrami approach, particularly in large-scale applications, has been limited by the scalability and efficiency of solvers. This paper addresses this limitation by developing two nonlinear solvers based on the Jacobian-Free Newton–Krylov (JFNK) methodology. A key feature of these methods is that the Jacobian is not formed explicitly for use by the underlying linear solver. Iterative linear solvers such as the Generalized Minimal RESidual (GMRES) method do not technically require the stand-alone Jacobian; instead its action on a vector is approximated through two nonlinear function evaluations. The preconditioning required by GMRES is also discussed. Two different preconditioners are developed, both of which employ existing Algebraic Multigrid (AMG) methods. Further, the most efficient preconditioner, overall, for the problems considered is based on a Picard linearization. Numerical examples demonstrate that these solvers are significantly faster than a standard Newton–Krylov approach; a speedup factor of approximately 26 was obtained for the Picard preconditioner on the largest grids studied here. In addition, these JFNK solvers exhibit good algorithmic scaling with increasing grid size.

Multi-Scale Visual Analysis of Trauma Injury

We develop a multi-scale high-fidelity biomechanical and physiologically based modeling tools for trauma (ballistic/impact and blast) injury to brain, lung and spinal cord for resuscitation, treatment planning and design of personnel protection. Several approaches have been used to study blast and ballistic/impact injuries. Dummy containing pressure sensors and synthetic phantoms of human organs have been used to study bomb blast and car crashes. Large animals like pigs also have been equipped with pressure sensors exposed to blast waves. But these methods do not provide anatomically and physiologically, full optimization of body protection design and require animal sacrifice. Anatomy and medical image-based high-fidelity computational modeling can be used to analyze injury mechanisms and to optimize the design of body protection. This paper presents novel approach of coupled computational fluid dynamics and computational structures dynamics to simulate fluid (air, cerebrospinal fluid)–solid (cranium, brain tissue) interaction during ballistic/blast impact. We propose a trauma injury simulation pipeline concept staring from anatomy and medical image-based high-fidelity 3D geometric modeling, extraction of tissue morphology, generation of computational grids, multi-scale biomechanical and physiological simulations, and data visualization.