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1
Said, Rajab. "Unstructured parallel grid generation." Thesis, Swansea University, 2003. https://cronfa.swan.ac.uk/Record/cronfa42637.
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The ultimate goal of this study is to develop a 'tool' by which large-scale unstructured grids for realistic engineering problems can be generated efficiently on any parallel computer platform. The adopted strategy is based upon a geometrical partitioning concept, where the computational domain is sub-divided into a number of sub-domains which are then gridded independently in parallel. This study focuses on three-dimensional applications only, and it implements a Delaunay triangulation based generator to generate the sub-domain grids. Two different approaches have been investigated, where the variations between them are limited to (i) the domain decomposition and (ii) the inter-domain boundary gridding algorithms only. In order to carry out the domain decomposition task, the first approach requires an initial tetrahedral grid to be constructed, whilst the second approach operates directly on the boundary triangular grid. Hence, this thesis will refer to the first approach as 'indirect decomposition method' and to the second as 'direct decomposition method'. Work presented in this thesis also concerns the development of a framework in which all different sub-algorithms are integrated in combination with a specially designed parallel processing technique, termed as Dynamic Parallel Processing (DPP). The framework adopts the Message Passing Library (MPL) programming model and implements a Single Program Multiple Data (SPMD) structure with a Manager/Workers mechanism. The DPP provides great flexibility and efficiency in exploiting the available computing resources. The framework has proved to be a very effective tool for generating large-scale grids. Grids of realistic engineering problems and to the order of 115 million elements, generated using one processor on an SGI Challenge machine with 512 MBytes of shared memory, will be presented.
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Burgess, David A. "Parallel computing for unstructured mesh algorithms." Thesis, University of Oxford, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318758.
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Bailey, R. H. "Unstructured grid methods and moving boundary problems." Thesis, Swansea University, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.636006.
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The work presented in this thesis is concerned with the modelling of moving boundary problems, with particular reference to the solution of the problem of the release of a store from beneath an aircraft wing. Chapter 2 presents a two-dimensional unstructured mesh generation procedure for generating grids of three-noded triangular elements about any number of arbitrarily shaped geometries and within an arbitrarily shaped domain. The method combines a Quadtree point generation procedure with a Delaunay triangulation algorithm. The method is used to generate the grids for the moving boundary algorithm. Chapter 3, a moving boundary flow solution algorithm and the corresponding data control structure are presented. The flow solver uses the explicit timestepping procedure of Lohner et al. A multiple grid or grid embedding procedure is used to model the motion of a body relative to another or other stationary bodies. A minor grid encloses the moving body and is allowed to move under a prescribed motion over the grid enclosing the stationary bodies spanning the domain. A number of steady state problems are analysed and a simple store release case is examined. Chapter 4 presents an implicit finite element scheme for the solution of the flow problems using an unstructured computational grid. The algorithm is based upon the centred finite difference scheme of Lerat et al. The governing equations are solved using a Generalized Minimal Residual method, which is related to the Conjugate Gradient method. A number of steady state flow solutions are presented. Finally, the implicit algorithm is incorporated into the moving boundary data structure of Chapter 3 and results for the new scheme are presented.
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Haselbacher, Andreas C. "A grid-transparent numerical method for compressible viscous flows on mixed unstructured grids." Thesis, Loughborough University, 1999. https://dspace.lboro.ac.uk/2134/7257.
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The goal of the present work is the development of a numerical method for compressible viscous flows on mixed unstructured grids. The discretisation is based on a vertex-centred finite-volume method. The concept of grid transparency is developed as a framework for the discretisation on mixed unstructured grids. A grid-transparent method does not require information on the cell types. For this reason, the numerical method developed in the present work can be applied to triangular, quadrilateral, and mixed grids without modification. The inviscid fluxes are discretised using the approximate Riemann solver of Roe. A limited linear-reconstruction method leads to monotonic capturing of shock waves and second-order accuracy in smooth regions of the flow. The discretisation of the viscous fluxes on triangular and quadrilateral grids is first studied by reference to Laplace's equation. A variety of schemes are evaluated against several criteria. The chosen discretisation is then extended to the viscous fluxes in the Navier-Stokes equations. A careful study of the various terms allows a form to be developed which may be regarded as a thin-shear-layer approximation. In contrast to previous implementations, however, the present approximation does not require knowledge of normal and tangential coordinate directions near solid surfaces. The effects of turbulence are modelled through the eddy-viscosity hypothesis and the one-equation model of Spalart and Allmaras. The discrete equations are marched to the steady-state solution by an explicit Runge-Kutta method with local time-stepping. The turbulence-model equation is solved by a point-implicit method. To accelerate the convergence rate, an agglomeration multigrid method is employed. In contrast to previous implementations, the governing equations are entirely rediscretised on the coarse grid levels. The solution method is applied to various inviscid, laminar, and turbulent flows. The performance of the multigrid method is compared for triangular and quadrilateral grids. Care is taken to assess numerical errors through grid-refinement studies or comparisons with analytical solutions or experimental data. The main contributions of the present work are the careful development of a solution method for compressible viscous flows on mixed unstructured grids and the comparison of the impact of triangular, quadrilateral, and mixed grids on convergence rates and solution quality.
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Tasri, Adek. "Accuracy of nominally 2nd order unstructured grid, CFD codes." Thesis, University of Newcastle Upon Tyne, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.420042.
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Sekhar, Susheel Kumar. "Viscous hypersonic flow physics predictions using unstructured Cartesian grid techniques." Diss., Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/45857.
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Aerothermodynamics is an integral component in the design and implementation of hypersonic transport systems. Accurate estimates of the aerodynamic forces and heat transfer rates are critical in trajectory analysis and for payload weight considerations. The present work seeks to investigate the ability of an unstructured Cartesian grid framework in modeling hypersonic viscous flows. The effectiveness of modeling viscous phenomena in hypersonic flows using the immersed boundary ghost cell methodology of this solver is analyzed.
The capacity of this framework to predict the surface physics in a hypersonic non-reacting environment is investigated. High velocity argon gas flows past a 2-D cylinder are simulated for a set of freestream conditions (Reynolds numbers), and impact of the grid cell sizes on the quality of the solution is evaluated. Additionally, the formulation is verified over a series of hypersonic Mach numbers for the flow past a hemisphere, and compared to experimental results and empirical estimates.
Next, a test case that involves flow separation and the interaction between a hypersonic shock wave and a boundary layer, and a separation bubble is investigated using various adaptive mesh refinement strategies. The immersed boundary ghost cell approach is tested with two temperature clipping strategies, and their impact on the overall solution accuracy and smoothness of the surface property predictions are compared.
Finally, species diffusion terms in the conservation equations, and collision cross-section based transport coefficients are installed, and hypersonic flows in thermochemical nonequilibrium environments are studied, and comparisons of the off-surface flow properties and the surface physics predictions are evaluated. First, a 2-D cylinder in a hypersonic reacting air flow is tested with an adiabatic wall boundary condition. Next, the same geometry is tested to evaluate the viscous chemistry prediction capability of the solver with an isothermal wall boundary condition, and to identify the strengths and weaknesses of the immersed boundary ghost cell methodology in computing convective heating rates in such an environment.
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Lyra, P. R. M. "Unstructured grid adaptive algorithms for fluid dynamics and heat conduction." Thesis, Swansea University, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.637967.
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This work is concerned with the development of reliable and versatile computational tools for the numerical simulation of two-dimensional heat conduction and incompressible and compressible laminar fluid flow problems. Issues related to adaptive techniques, discretisation methodologies (upwind or centred type) and the design of high-resolution shock-capturing schemes are investigated in this thesis. Three distinct research works have been pursued here. In the first work, attention is focused on the construction of an adaptive finite element procedure with mesh refinement, by mesh enrichment, in time and space, and with automatic time stepping for the heat conduction problem in a stationary medium. The Galerkin finite element method and the Euler-backward time marching scheme are used as the basis to obtain the steady-state and transient approximate solutions. Particular emphasis concentrates on the design of the adaptive strategy and the combined influence of time and spatial adaptation. The second task is concerned with the derivation of adaptive remeshing strategies for both steady and unsteady solution of the incompressible Navier-Stokes equations in primitive variables. A Petrov-Galerkin formulation, which automatically introduces streamline upwinding and allows equal order interpolation for all variables, combined with either an explicit or implicit time integration represents the general discretisation methodologies adopted. The adaptive redefinition of the mesh, the error estimate and specific features, such as the presence of singularities on the solution and accumulation of interpolation errors inherent to a transient remeshing, are carefully analysed with some remedies proposed to deal with such difficulties. In the final part of the thesis, the most relevant mathematical-physical properties of the first order hyperbolic model equations are discussed. The utilisation of upstream or centred discretisation and several ways to produce high-resolution schemes to deal with this class of problems are described and compared for one-dimensional test cases. With regard to upwind discretisation techniques, the most popular flux difference splitting, flux vector splitting and some recently proposed hybrid splitting methodologies are considered.
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Zagaris, George. "Parallel Unstructured Grid Generation for Complex Real-World Aerodynamic Simulations." W&M ScholarWorks, 2008. https://scholarworks.wm.edu/etd/1539626877.
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Zhao, Qiuying. "Towards Improvement of Numerical Accuracy for Unstructured Grid Flow Solver." University of Toledo / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1353107603.
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Liu, Zhuo. "Development of Large-Scale Unstructured Grid Storm Surge and Sub-Grid Inundation Models for Coastal Applications." W&M ScholarWorks, 2018. https://scholarworks.wm.edu/etd/1550153651.
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Storm surge and inundation induced by hurricanes and nor'easters pose a profound threat to coastal communities and ecosystems. These storm events with powerful winds, heavy precipitation, and strong wind waves can lead to major flooding for cities along U.S. Coasts. Recent examples of Hurricane Irene (2011) in North Carolina and Virginia and Hurricane Sandy (2012) in New York City not only demonstrated the immense destructive power by the storms, but also revealed the obvious, crucial need for improved forecasting of storm tide and inundation. in part I, a large-scale unstructured-grid 3-D barotropic storm tide model SCHISM (Semi-implicit Cross-scale Hydroscience Integrated System Model) is developed with open ocean boundary aligning along the 60-degree West longitude to catch most Atlantic hurricanes that may make landfall along U.S. East and Gulf Coasts. The model, driven by high-resolution NAM (North America Mesoscale) and ECMWF (European Centre for Medium-Range Weather Forecasts) atmospheric fields, was coupled with Wind Wave Model (WWMIII) to account for wave effects, and used to simulate storm surge in 3-D barotropic mode rather than the traditional 2-D vertical average mode. For Hurricane Sandy, the fully coupled wave-current interaction 3-D model using ECMWF atmospheric forcing performs the best. The storm tide results match well with observation at all nine NOAA tidal gauges along the East Coast. The maximum total water level in New York City, is accurately simulated with absolute error of amplitude less than 8 cm, and timing difference within 10 minutes. The scenarios of "2-D" versus "3-D" and "with" versus "without" wind wave model were compared and discussed in details. Overall, the wave contribution amounts to 5-10% of surge elevation during the event. Also, the large-scale model with similar setup is applied to hindcasting storm tide during Hurricane Irene and the results are excellent when compared with observed water level along Southeast Coast and inside Chesapeake Bay. in part II, a high-resolution sub-grid inundation model ELCIRC-sub (Eulerian-Lagrangian CIRCulation) was developed from the original finite-volume-based ELCIRC model. It utilized the sub-grid method for imbedding high-resolution topography/bathymetry data into the traditional model grid and delivering the inundation simulation on the street level scale. The ELCIRC-sub contains an efficient non-linear solver to increase the accuracy and was executed in the MPI (Message Passing Interface) parallel computing platform to vastly enlarge the water shed coverage, and to expand the numbers of sub-grids allowed. The ELCIRC-sub is first validated with a wetting/drying analytic solution and then applied in New York City for Hurricane Sandy (2012). Temporal comparisons with NOAA and USGS water level gauges showed excellent performance with an average error on the order of 10 cm. It accurately captured the highest surge (during Hurricane Sandy) at Kings Point on both maximum surge height and the explosive surge profile. Spatial comparisons of the modeled peak water level at 80 locations around New York City showed an average error less than 13 cm. The modeled maximum modeled inundation extent also matched well with 80% of the FEMA flooding map. in terms of robustness and efficiency for practical application, ELCIRC-sub surpasses the prototype model UnTRIM2.
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Taghizadeh, Manzari M. "An unstructured grid finite element algorithm for compressible turbulent flow computations." Thesis, Swansea University, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.639151.
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This work describes the design and implementation of an algorithm for solving the compressible turbulent flow equations, in two-dimensional space, using two-equation turbulence models. The basic governing equations of the fluid flow are presented and the transport equations for the turbulence kinetic energy and dissipation energy are given for both the k-epsilon and the k-omega models. Some basic physical concepts of turbulence are explained and several different versions of both the k-epsilon and the k-omega models are considered. The principle of upwind discretisation is discussed and some physical and mathematical aspects of the Euler equations are presented. Roe's flux-difference splitting scheme and the ideas behind a MUSCL higher-order extension are introduced. A brief discussion is also included on the concept of limiting. The finite element formulation employed in solving the governing equations is presented and some numerical issues regarding boundary conditions, time integration and robustness are discussed. Several test cases are solved and the effects of the limiters, mesh resolution, shock-boundary layer and shock-shock interactions and turbulence models are studied. Finally, some conclusions are drawn and a few guidelines for future research are presented.
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Zheng, P. "Development of a fully coupled, unstructured grid, coastal morphodynamic model system." Thesis, University of Liverpool, 2017. http://livrepository.liverpool.ac.uk/3020596/.
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A new fully coupled, unstructured grid, three-dimensional coastal morphodynamic model system is developed in this research. Based on two original independent models, i.e. the original unstructured-grid version of the third generation spectral wave model Simulating WAves Nearshore (UnSWAN) and the original Finite Volume Coastal Ocean Model (FVCOM), the development of this model system is achieved by accomplishing the following procedures: Coupling UnSWAN with FVCOM to enable the full representation of the wave-current interaction in the nearshore region, by building a new wave-current coupling scheme based on the vortex-force (VF) approach to represent the wave-current interaction and developing a new coupling module to facilitate the communication between UnSWAN and FVCOM in the parallel computing and realise the model coupling procedure. A GLS turbulence model is also modified to better reproduce wave-breaking generated turbulence, together with a roller transport model to account for the effects of surface wave roller. An alternative wave model based on Mellor et al. is also implemented in the present model system. The original advection-diffusion (AD) module is modified for the representation of particle suspension and subsequent transport under the combined flows. In this module, the contribution of wave-induced stokes drift to particle transport is included which is absent in the original FVCOM model. A new bed load transport module based on the SANTOSS formulae is built to represent various processes within the oscillatory boundary layer. Based on the semi-unsteady "half-cycle" concept, this SANTOSS formulae distinguish the sediment transports during the positive “crest” and the negative “trough” half-cycles and have the advantages over the traditional steady ’equilibrium’ transport formula that many wave-induced unsteadiness effects are included, including the wave asymmetry, sediment grain size effects and etc. Finally, the wave, circulation, suspended sediment and bed-load transport modules are integrated into the fully coupled, three-dimensional coastal morphodynamic model system, in which a sediment continuity (Exner) equation is also included to resolve the morphology evolution.
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Kim, Kyusup. "Three-dimensional hybrid grid generator and unstructured flow solver for compressors and turbines." Diss., Texas A&M University, 2003. http://hdl.handle.net/1969.1/1437.
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A numerical method for the simulation of compressible turbulent flows is presented. This method includes a novel hybrid grid generation for airfoil cascades and an unstructured mesh flow solver. The mesh tool incorporates a mapping technique and a grid smoothing method. The mapping technique is used to build an initial volume mesh and the grid smoothing method is used to improve the quality of the initial mesh. The grid smoothing is based on the optimization of mesh-quality parameters. The further improvement of the smoothed mesh is achieved by an edge-swapping and node-insertion technique. The unstructured flow solver is developed for a hybrid grid. This flow solver uses a rotational frame of reference. The convective and viscous fluxes are numerically solved by an upwind scheme and an averaged nodal gradient. A higher-order spatial accuracy is achieved by a piece-wise linear reconstruction. An explicit multi-stage method is employed for integration in time. The Menters k −τ model is implemented to simulate the turbulence effects. The flow solver is validated against the analytical and experimental results. A parametric study is performed for a high speed centrifugal compressor.
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Globisch, G., and S. V. Nepomnyaschikh. "The hierarchical preconditioning having unstructured grids." Universitätsbibliothek Chemnitz, 1998. http://nbn-resolving.de/urn:nbn:de:bsz:ch1-199801398.
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In this paper we present two hierarchically preconditioned methods for the fast solution of mesh equations that approximate 2D-elliptic boundary value problems on unstructured quasi uniform triangulations. Based on the fictitious space approach the original problem can be embedded into an auxiliary one, where both the hierarchical grid information and the preconditioner by decomposing functions on it are well defined. We implemented the corresponding Yserentant preconditioned conjugate gradient method as well as the BPX-preconditioned cg-iteration having optimal computational costs. Several numerical examples demonstrate the efficiency of the artificially constructed hierarchical methods which can be of importance in the industrial engineering, where often only the nodal coordinates and the element connectivity of the underlying (fine) discretization are available.
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Parrish, Michael H. "A selective approach to conformal refinement of unstructured hexahedral meshes /." Diss., CLICK HERE for online access, 2007. http://contentdm.lib.byu.edu/ETD/image/etd1985.pdf.
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Tysell, Lars. "Hybrid Grid Generation for Viscous Flow Computations Around Complex Geometries." Doctoral thesis, KTH, Mekanik, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-11934.
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A set of algorithms building a program package for the generation of twoandthree-dimensional unstructured/hybrid grids around complex geometrieshas been developed. The unstructured part of the grid generator is based on the advancing frontalgorithm. Tetrahedra of variable size, as well as directionally stretched tetrahedracan be generated by specification of a proper background grid, initiallygenerated by a Delaunay algorithm. A marching layer prismatic grid generation algorithm has been developedfor the generation of grids for viscous flows. The algorithm is able to handleregions of narrow gaps, as well as concave regions. The body surface is describedby a triangular unstructured surface grid. The subsequent grid layers in theprismatic grid are marched away from the body by an algebraic procedurecombined with an optimization procedure, resulting in a semi-structured gridof prismatic cells. Adaptive computations using remeshing have been done with use of a gradientsensor. Several key-variables can be monitored simultaneously. The sensorindicates that only the key-variables with the largest gradients give a substantialcontribution to the sensor. The sensor gives directionally stretched grids. An algorithm for the surface definition of curved surfaces using a biharmonicequation has been developed. This representation of the surface canbe used both for projection of the new surface nodes in h-refinement, and theinitial generation of the surface grid. For unsteady flows an algorithm has been developed for the deformationof hybrid grids, based on the solution of the biharmonic equation for the deformationfield. The main advantage of the grid deformation algorithm is that itcan handle large deformations. It also produces a smooth deformation distributionfor cells which are very skewed or stretched. This is necessary in orderto handle the very thin cells in the prismatic layers. The algorithms have been applied to complex three-dimensional geometries,and the influence of the grid quality on the accuracy for a finite volumeflow solver has been studied for some simpler generic geometries.QC 20100812
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Kumar, Amitesh. "Hole patching in 3D unstructured surface mesh." Birmingham, Ala. : University of Alabama at Birmingham, 2007. http://www.mhsl.uab.edu/dt/2007m/kumar.pdf.
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Longo, Joel Joseph. "Unsteady Turbomachinery Flow Simulation With Unstructured Grids Using ANSYS Fluent." The Ohio State University, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=osu1376875053.
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Hu, Guanghui. "Numerical simulations of the steady Euler equations on unstructured grids." HKBU Institutional Repository, 2009. http://repository.hkbu.edu.hk/etd_ra/1106.
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Panguluri, Sri S. "IMPLEMENTATION AND VALIDATION OF THE HYBRID TURBULENCE MODELS IN AN UNSTRUCTURED GRID CODE." UKnowledge, 2007. http://uknowledge.uky.edu/gradschool_theses/435.
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Since its introduction in 1997, the use of Detached Eddy Simulation (DES) and similar hybrid turbulence techniques has become increasingly popular in the field of CFD. However, with increased use some of the limitations of the DES model have become apparent. One of these is the dependence of DES on grid construction, particularly regarding the point of transition between the Reynolds-Averaged Navier-Stokes and Large Eddy Simulation models. An additional issue that arises with unstructured grids is the definition of the grid spacing in the implementation of a DES length scale. To lay the ground work to study these effects the Spalart-Allmaras one-equation turbulence model, SA based DES hybrid turbulence model, and the Scale Adaptive Simulation hybrid turbulence model are implemented in an unstructured grid CFD code, UNCLE. The implemented SA based DES model is validated for flow over a three-dimensional circular cylinder for three different turbulent Reynolds numbers. Validation included studying the pressure, skin friction coefficient, centerline velocity distributions averaged in time and space. Tools to output the mean velocity profiles and Reynolds stresses were developed. A grid generation code was written to generate a two/three dimensional circular cylinder grid to simulate flow over the cylinder in UNCLE. The models implemented and validated, and the additional tools mentioned will be used in the future.
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Edwards, William Vincent. "Towards a level set reinitialisation method for unstructured grids." Thesis, University of Edinburgh, 2012. http://hdl.handle.net/1842/6257.
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Interface tracking methods for segregated flows such as breaking ocean waves are an important tool in marine engineering. With the development in marine renewable devices increasing and a multitude of other marine flow problems that benefit from the possibility of simulation on computer, the need for accurate free surface solvers capable of solving wave simulations has never been greater. An important component of successfully simulating segregated flow of any type is accurately tracking the position of the separating interface between fluids. It is desirable to represent the interface as a sharp, smooth, continuous entity in simulations. Popular Eulerian interface tracking methods appropriate for segregated flows such as the Marker and Cell Method (MAC) and the Volume of Fluid (VOF) were considered. However these methods have drawbacks with smearing of the interface and high computational costs in 3D simulations being among the most prevalent. This PhD project uses a level set method to implicitly represent an interface. The level set method is a signed distance function capable of both sharp and smooth representations of a free surface. It was found, over time, that the level set function ceases to represent a signed distance due to interaction of local velocity fields. This affects the accuracy to which the level set can represent a fluid interface, leading to mass loss. An advection solver, the Cubic Interpolated Polynomial (CIP) method, is presented and tested for its ability to transport a level set interface around a numerical domain in 2D. An advection problem of the level set function demonstrates the mass loss that can befall the method. To combat this, a process known as reinitialisation can be used to re-distance the level set function between time-steps, maintaining better accuracy. The goal of this PhD project is to present a new numerical gradient approximation that allows for the extension of the reinitialisation method to unstructured numerical grids. A particular focus is the Cartesian cut cell grid method. It allows geometric boundaries of arbitrary complexity to be cut from a regular Cartesian grid, allowing for flexible high quality grid generation with low computational cost. A reinitialisation routine using 1st order gradient approximation is implemented and demonstrated with 1D and 2D test problems. An additional area-conserving constraint is introduced to improve accuracy further. From the results, 1st order gradient approximation is shown to be inadequate for improving the accuracy of the level set method. To obtain higher accuracy and the potential for use on unstructured grids a novel gradient approximation based on a slope limited least squares method, suitable for level set reinitialisation, is developed. The new gradient scheme shows a significant improvement in accuracy when compared with level set reinitialisation methods using a lower order gradient approximation on a structured grid. A short study is conducted to find the optimal parameters for running 2D level set interface tracking and the new reinitialisation method. The details of the steps required to implement the current method on a Cartesian cut cell grid are discussed. Finally, suggestions for future work using the methods demonstrated in the thesis are presented.
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Majhool, Ahmed Abed Al-Kadhem. "Advanced spray and combustion modelling." Thesis, University of Manchester, 2011. https://www.research.manchester.ac.uk/portal/en/theses/advanced-spray-and-combustion-modelling(eb3ef22a-53d0-4e70-aa9d-bec37775d451).html.
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The thesis presents work across three different subjects of investigations into the modelling of spray development and its interaction with non-reactive and reactive flow. The first part of this research is aimed to create a new and robust family of convective scheme to capture the interface between the dispersed and the carrier phases without the need to build up the interface boundary. The selection of Weighted Average Flux (WAF) scheme is due to this scheme being designed to deal with random flux scheme which is second-order accurate in space and time. The convective flux in each cell face utilizes the WAF scheme blended with Switching Technique for Advection and Capturing of Surfaces (STACS) scheme for high resolution flux limiters. However in the next step, the high resolution scheme is blended with the scheme to provide the sharpness and boundedness of the interface by using switching strategy. The proposed scheme is tested on capturing the spray edges in modelling hollow cone type sprays without need to reconstruct two-phase interface. A test comparison between TVD scheme and WAF scheme using the same flux limiter on convective flow on hollow cone spray is presented. Results show that the WAF scheme gives better prediction than the TVD scheme. The only way to check the accuracy of the presented models are evaluations according to physical droplets behaviour and its interaction with air. In the second part, due to the effect of evaporation the temperature profile in the released fuel vapour has been proposed. The underlying equation utilizes transported vapour mass fraction. It can be used along with the solution of heat transfer inside a sphere. After applying boundary conditions, the equation can provide a solution of existing conditions at liquid-gas interface undergoing evaporation and it is put in a form similar to well-known one-third rule equation. The resulting equation is quadratic type that gives an accurate prediction for the thermo-physical properties due to the non-linear relation between measured properties and temperature. Comparisons are made with one-third rule where both equations are implemented in simulating hollow cone spray under evaporation conditions. The results show the presumed equation performs better than one-third rule in all comparisons. The third part of this research is about a conceptual model for turbulent spray combustion for two combustion regimes that has been proposed and tested for n-heptane solid cone spray type injected into a high-pressure, high-temperature open reactor by comparing to the available experimental data and to results obtained using two well known combustion models named the Combined Combustion Model (CCM) and the unsteady two-dimensional conditional moment closure (CMC) model. A single-zone intermittent beta-two equation turbulent model is suggested to characterise the Lumped zone. This model can handle both unburned and burned zones. Intermittency theory is used to account for the spatially non-uniform distribution of viscous dissipation. The model suggests that the Lumped zone can be identified by using the concept of Tennekes and Kuo-Corrsion of isotropic turbulence that suggests that dissipative eddies are most probably formed as vortex tubes with a diameter of the order of Kolmogorov length scale and a space of the order of Taylor length scale. Due to the complexity of mixture motion in the combustion chamber, there exist coherent turbulent small scale structures containing highly dissipative vortices. The small size eddies play an important role in extinguishing a diffusion spray flame and have an effect on the combustion reaction at molecular scale because small scales turbulence increase heat transfer due to the dissipation. A common hypothesis in constructing part of the model is if the Kolmogorov length scale is larger than the turbulent flame thickness. The Lumped strategy benefits from capturing small reactive scales information provided by numerics to improve the modelling and understand the exact implementation of the underlying chemical hypothesis. The Lumped rate is estimated from the ratio of the turbulent diffusion to reaction flame thickness. Three different initial gas temperature test cases are implemented in simulations. Lumped spray combustion model shows a very good agreement with available experimental data concerning auto-ignition delay points.
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Jawahar, P. "A High-Resolution Procedure For Euler And Navier-Stokes Computations On Unstructured Grids." Thesis, Indian Institute of Science, 2000. http://hdl.handle.net/2005/226.
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A finite-volume procedure, comprising a gradient-reconstruction technique and a multidimensional limiter, has been proposed for upwind algorithms on unstructured grids. The high-resolution strategy, with its inherent dependence on a wide computational stencil, does not suffer from a catastrophic loss of accuracy on a grid with poor connectivity as reported recently is the case with many unstructured-grid limiting procedures. The continuously-differentiable limiter is shown to be effective for strong discontinuities, even on a grid which is composed of highly-distorted triangles, without adversely affecting convergence to steady state. Numerical experiments involving transient computations of two-dimensional scalar convection to steady-state solutions of Euler and Navier-Stokes equations demonstrate the capabilities of the new procedure.
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McDonald, Cameron L. "Automatic, Unstructured Mesh Generation for 2D Shelf Based Tidal Models." Diss., CLICK HERE for online access, 2006. http://contentdm.lib.byu.edu/ETD/image/etd1550.pdf.
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Spirkin, Anton M. "A three-dimensional particle-in-cell methodology on unstructured Voronoi grids with applications to plasma microdevices." Link to electronic dissertation, 2006. http://www.wpi.edu/Pubs/ETD/Available/etd-050506-145257/.
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Fryer, Yvonne Delia. "A control volume unstructured grid approach to the solution of the elastic stress-strain equations." Thesis, University of Greenwich, 1993. http://gala.gre.ac.uk/6172/.
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The aim of the research is to develop and apply algorithms for the solution of elastic stress-strain equations based upon a control volume approach on unstructured meshes. The purpose is to integrate the solution of solid mechanics with that of fluid flow and heat transfer within the context of solidification problems in the casting of metals. The conventional way of solving stress-strain problems is to use a finite element approach which yields a set of linear equations relating loads to displacements. This approach works very well and can also deal well with temperature loading, but becomes problematic when flow and change of phase are included in a transient context. A major set of projects are under way to develop an integrated suite of algorithms to model the flow-cooling-solidification-residual stress development process. This project concerns the component associated with the development of stress-strain distributions under temperature and other loads. A control volume formulation solution procedure for the elastic stress-strain equations in two-dimensions has been developed that solves directly for displacements. The formulation works for mixtures of quadrilateral and triangular elements in an unstructured mesh. The control volume finite element code has been tested on a range of problems, such as a cantilever loaded at one end, a beam with a thermal gradient applied and a multi-material mixed element non-regular shape with a load applied. The results for these test cases have been compared to those obtained by standard finite element codes and analytical solutions where available. Besides the plane stress and plane strain options, the model has been extended to include axisymmetric problems. Two examples are used to test the validity of the algorithm for axisymmetric problems. The results compare very well against other numerical results and analytic solutions for the three special two-dimensional cases. Another problem considered at an inteiface is that of friction between two solids. This non-linear boundary condition has been included in the model. An example of this is Silica in a mould being pressed, results for the stress-strain code are compared to previous results. The control volume stress-strain code has been integrated with the solidification heat transfer code. Problems of simple castings have been considered. When a liquid solidifies it may deform away from the mould, so possible air gap formation at the mould/metal inteiface has been included in the model. Prediction of hole formation in solidification, in the form of volumetric shrinkage and porosity, has been included into the coupled heat transfer stress code via a simple model. Examples show encouraging results when compared to experimental porosity results.
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Blades, Eric Lindsay. "A sliding interface method for unsteady unstructured parallel flow simulations." Diss., Mississippi State : Mississippi State University, 2004. http://library.msstate.edu/etd/show.asp?etd=etd-10142004-165050.
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Shen, Tao. "Development of a Storm Surge Model using a High-Resolution Unstructured Grid Over a Large Domain." W&M ScholarWorks, 2009. http://www.vims.edu/library/Theses/Shen09.pdf.
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Teng, Yi-Cheng. "Developing an Unstructured Grid, Coupled Storm Surge, Wind Wave and Inundation Model for Super-regional Applications." W&M ScholarWorks, 2012. https://scholarworks.wm.edu/etd/1539616874.
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During extreme weather conditions such as hurricanes and nor'easters, both the currents and wind waves generated by the atmospheric forces are important. Although they may act and dominate on different temporal and spatial scales, their interactions and combined effects are without doubt significant. In this dissertation, a major effort has been made to couple an unstructured grid circulation model SELFE (semi-implicit, Eulerian-Lagrangian finite element model) and the WWM II (Wind Wave model II). Moreover, this new coupled model system can be executed in a parallel computational environment. After the coupled model was successfully built, the model was verified with ideal test cases, either through comparisons with analytic solutions or with laboratory experiments. It was further validated by field-measured data during two hurricane events. The SELFE-WWM II model framework described above was used to participate in a SURA testbed project that was recently funded by the NOAA IOOS program. The purpose was to improve the storm surge and inundation modeling skill throughout the Gulf of Mexico as well as along the U.S. East Coast. The coupled tide, surge, and wind wave models in two and three dimensions were tested and compared systematically. Two well-known cases were investigated in detail. One was the event of Hurricane Ike of 2008 in the Gulf of Mexico and the other was the April Nor'easter of 2007 in the Gulf of Maine. For the Gulf of Mexico study, the key scientific issue is the origin of the forerunner. It has long been recognized that the forerunner plays an important role in generating large hurricane-induced storm surge in the Gulf of Mexico. The forerunner is a phenomenon whereby water level throughout the vast coastal region was elevated days before the hurricane makes landfall. The forerunner can contribute significantly to the total water level that results subsequently during the primary surge when the hurricane makes landfall. The 2008 Hurricane Ike, which devastated the Galveston Bay along the Texas Coast, is a good example: 1.4 m out of 4.5 m maximum surge was contributed by the forerunner in the Gulf of Mexico. The consensus from initial results of multiple models indicates that the forerunner occurred as a result of Ekman set-up along the broad Louisiana-Texas (LATEX) shelf by the shore-parallel wind field. By contrast, the primary surge was dominated by the low pressure and the maximum wind along a path perpendicular to the shore as the hurricane made landfall. It was found that the cross-shore Ekman set-up is highly sensitive to the bottom boundary layer (BBL) dynamics, especially to the drag coefficient. Given the fact that the Gulf of Mexico is known to be rich in fluid mud, and near-bed flows generally are very weak under fair-weather conditions, one plausible hypothesis is that, during the stormy condition, the suspended sediment-induced density stratification is likely to be ubiquitously present at the bottom boundary layer. A sediment-transport model and wave-current bottom boundary layer sub-model including the sediment-induced stratification effect were coupled to the unstructured grid circulation and wind wave model (SELFE-WWM II) for simulating the forerunner during Hurricane Ike. The model results demonstrate that the bottom boundary layer dynamics have a significant effect on the velocity veering as well as the Ekman set-up across the shelf. In the Gulf of Maine study, the high-resolution coupled SELFE-WWM II model was applied in the Scituate Harbor, a small, shallow coastal embayment, south of Boston. The key issue for the study was the recurring inundation related to the role played by wind waves during nor'easter events. With limited observation data in the Scituate, the model result from SELFE was compared with that from FVCOM. The major findings are summarized as follows: (1) wind waves generated by the nor'easter can profoundly affect the coastal current by increasing the magnitude and altering its direction, (2) while the mean water level inside the Harbor stays the same, the total transport across the harbor mouth increases when wind waves are included, and (3) the total inundation area, primarily in the northern and southern basins within the Harbor, does increase when wind waves are included. There is a question as to why the inclusion of the wind waves did not cause the mean water level to change inside the Harbor while the inundation area was increased. The plausible explanation is that this lack of impact could be that the Stokes transport was small and the increase of water level by the wave set-up was compensated by the expansion of the inundation area in the shallow region.
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Wang, Jingyu. "Development and Application of a Local Correlation-Based Transition Model in an Unstructured Grid CFD Solver." University of Toledo / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1408444919.
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Shin, Sangmook. "Reynolds-Averaged Navier-Stokes Computation of Tip Clearance Flow in a Compressor Cascade Using an Unstructured Grid." Diss., Virginia Tech, 2001. http://hdl.handle.net/10919/28947.
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A three-dimensional unstructured incompressible RANS code has been developed using artificial compressibility and Spalart-Allmaras eddy viscosity model. A node-based finite volume method is used in which all flow variables are defined at the vertices of tetrahedrons in an unstructured grid. The inviscid fluxes are computed by using the Roe's flux difference splitting method, and higher order accuracy is attained by data reconstruction based on Taylor series expansion. Gauss theorem is used to formulate necessary gradients. For time integration, an implicit scheme based on linearized Euler backward method is used.
A tetrahedral unstructured grid generation code has been also developed and applied to the tip clearance flow in a highly staggered cascade. Surface grids are first generated in the flow passage and blade tip by using several triangulation methods including Delaunay triangulation, advancing front method and advancing layer method. Then the whole computational domain including tip gap region is filled with prisms using the surface grids. Each prism is divided into three tetrahedrons. To accomplish this division in a consistent manner, connectivity pattern is assigned to each triangle in the surface grids. A new algorithm is devised to assign the connectivity pattern without reference to the particular method of triangulation. This technique offers great flexibility in surface grid generation.
The code has been validated by comparisons with available computational and experimental results for several test cases: invisicd flow around NACA section, laminar and turbulent flow over a flat plate, turbulent flow through double-circular arc cascade and laminar flow through a square duct with 90° bend. For the laminar flat plate case, the velocity profile and skin friction coefficient are in excellent agreement with Blasius solution. For the turbulent flat plate case, velocity profiles are in full agreement with the law of the wall up to Reynolds number of 1.0E8, however, the skin friction coefficient is under-predicted by about 10% in comparison with empirical formula. Blade loading for the two-dimensional circular arc cascade is also compared with experiments. The results obtained with the experimental inflow angle (51.5° ) show some discrepancies at the trailing edge and severely under-predict the suction peak at the leading edge. These discrepancies are completely remedied if the inflow angle is increased to 53.5° . The code is also capable of predicting the secondary flow in the square duct with 90° bend, and the velocity profiles are in good agreement with measurements and published Navier-Stokes computations.
Finally the code is applied to a linear cascade that has GE rotor B section with tip clearance and a high stagger angle of 56.9° . The overall structure of the tip clearance flow is well predicted. Loss of loading due to tip leakage flow and reloading due to tip leakage vortex are presented. On the end wall, separation line of the tip leakage vortex and reattachment line of passage vortex are identified. The location of the tip leakage vortex in the passage agrees very well with oil flow visualization. Separation bubble on the blade tip is also predicted. Mean streamwise velocity contours and cross sectional velocity vectors are compared with experimental results in the near wake, and good agreements are observed. It is concluded that Spalart-Allmaras turbulence model is adequate for this type of flow field except at locations where the tip leakage vortex of one blade interacts with the wake of a following blade. This situation may prevail for blades with longer span and/or in the far wake. Prediction of such an interaction presents a challenge to RANS computations.
The effects of blade span on the flow structure have been also investigated. Two cascades with blades of aspect ratios of 0.5 and 1.0 are considered. By comparing pressure distributions on the blade, it is shown that the aspect ratio has strong effects on loading distribution on the blade although the tip gap height is very small (0.016 chord). Grid convergence study has been carried out with three different grids for pressure distributions and limiting streamlines on the end wall.Ph. D.
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Mahmutyazicioglu, Emel. "Development Of An Octree Based Grid Coarsening And Multigrid Flow Solution." Phd thesis, METU, 2010. http://etd.lib.metu.edu.tr/upload/12612549/index.pdf.
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The multigrid technique is one of the most effective techniques to achieve the reduction of the CPU cost for flow solvers. The multigrid strategy uses the multilevel grids which are the coarsening subsets of fine grid. An explicit solver rapidly reduces the high frequency errors on the computational grids. Since high frequency errors on coarse grids correspond to low frequency errors on fine grids, cycling through the coarse grid levels rapidly reduces the errors ranging from high-to-low frequency. The aim of this study is, therefore, to accelerate SENSE3D solver developed by TUBITAK-SAGE by implementating multigrid concept.
In this work, a novel grid coarsening method suitable for cell-centered hybrid/unstructured grids is developed to provide the cells with high aspect ratio. This new grid coarsening technique relies on the agglomeration of cells based on their distribution on octree data structure. Then, the multigrid strategy is implemented to the baseline flow solver. During this implementation, the flux calculation along the face loops is modified without changing cell-centered scheme.
The performance of the coarsening algorithm is investigated for all grid types in two and three dimension. The grid coarsening algorithm produces well defined, nested, body fitted coarser grids with aspect ratios of one and the coarse grids have similar characteristics of Cartesian grids. Then, the multigrid flow solutions are obtained at inviscid, laminar and turbulent flows. It is shown that, the convergence accelerations are up to 14 times for inviscid flows and in a range of 4 to 110 fold for turbulent flow solutions.
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Wang, Mianzhi. "Numerical Analysis of Transient Teflon Ablation with a Domain Decomposition Finite Volume Implicit Method on Unstructured Grids." Digital WPI, 2012. https://digitalcommons.wpi.edu/etd-theses/284.
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This work investigates numerically the process of Teflon ablation using a finite-volume discretization, implicit time integration and a domain decomposition method in three-dimensions. The interest in Teflon stems from its use in Pulsed Plasma Thrusters and in thermal protection systems for reentry vehicles. The ablation of Teflon is a complex process that involves phase transition, a receding external boundary where the heat flux is applied, an interface between a crystalline and amorphous (gel) phase and a depolymerization reaction which happens on and beneath the ablating surface. The mathematical model used in this work is based on a two-phase model that accounts for the amorphous and crystalline phases as well as the depolymerization of Teflon in the form of an Arrhenius reaction equation. The model accounts also for temperature-dependent material properties, for unsteady heat inputs and boundary conditions in 3D. The model is implemented in 3D domains of arbitrary geometry with a finite volume discretization on unstructured grids. The numerical solution of the transient reaction-diffusion equation coupled with the Arrhenius-based ablation model advances in time using implicit Crank-Nicolson scheme. For each time step the implicit time advancing is decomposed into multiple sub-problems by a domain decomposition method. Each of the sub-problems is solved in parallel by Newton-Krylov non-linear solver. After each implicit time-advancing step, the rate of ablation and the fraction of depolymerized material are updated explicitly with the Arrhenius-based ablation model. After the computation, the surface of ablation front and the melting surface are recovered from the scalar field of fraction of depolymerized material and the fraction of melted material by post-processing. The code is verified against analytical solutions for the heat diffusion problem and the Stefan problem. The code is validated against experimental data of Teflon ablation. The verification and validation demonstrates the ability of the numerical method in simulating three dimensional ablation of Teflon.
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Aybay, Orhan. "Implementation Of The Spalart-allmaras Turbulence Model To A Two-dimensional Unstructured Navier-stokes Solver." Master's thesis, METU, 2005. http://etd.lib.metu.edu.tr/upload/12605725/index.pdf.
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An unstructured explicit, Reynolds averaged Navier-Stokes solver is developed to operate on inviscid flows, laminar flows and turbulent flows and one equation Spalart-Allmaras turbulence modeling is implemented to the solver. A finite volume formulation, which is cell-center based, is used for numerical discretization of Navier-Stokes equations in conservative form. This formulation is combined with one-step, explicit time marching upwind numerical scheme that is the first order accurate in space. Turbulent viscosity is calculated by using one equation Spalart-Allmaras turbulence transport equation. In order to increase the convergence of the solver local time stepping technique is applied.
Eight test cases are used to validate the developed solver,for inviscid flows, laminar flows and turbulent flows. All flow regimes are tested on NACA-0012 airfoil. The results of NACA-0012 are compared with the numerical and experimental data.
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Kashefi, Ali. "A Finite-Element Coarse-GridProjection Method for Incompressible Flows." Thesis, Virginia Tech, 2017. http://hdl.handle.net/10919/79948.
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Coarse grid projection (CGP) methodology is a novel multigrid method for systems involving decoupled nonlinear evolution equations and linear elliptic Poisson equations. The nonlinear equations are solved on a fine grid and the linear equations are solved on a corresponding coarsened grid. Mapping operators execute data transfer between the grids. The CGP framework is constructed upon spatial and temporal discretization schemes. This framework has been established for finite volume/difference discretizations as well as explicit time integration methods. In this article we present for the first time a version of CGP for finite element discretizations, which uses a semi-implicit time integration scheme. The mapping functions correspond to the finite-element shape functions. With the novel data structure introduced, the mapping computational cost becomes insignificant. We apply CGP to pressure correction schemes used for the incompressible Navier Stokes flow computations. This version is validated on standard test cases with realistic boundary conditions using unstructured triangular meshes. We also pioneer investigations of the effects of CGP on the accuracy of the pressure field. It is found that although CGP reduces the pressure field accuracy, it preserves the accuracy of the pressure gradient and thus the velocity field, while achieving speedup factors ranging from approximately 2 to 30. Exploring the influence of boundary conditions on CGP, the minimum speedup occurs for velocity Dirichlet boundary conditions, while the maximum speedup occurs for open boundary conditions. We discuss the CGP method as a guide for partial mesh refinement of incompressible flow computations and show its application for simulations of flow over a backward facing step and flow past a cylinder.Master of Science
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Li, Yuepeng. "Development of an unstructured grid, finite volume eutrophication model for the shallow water coastal bay: Application in the Lynnhaven River Inlet system." W&M ScholarWorks, 2006. https://scholarworks.wm.edu/etd/1539616741.
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The shallow water region is an important portion of the estuarine and coastal waters, since it encompasses the entire land-water margin as the buffer zone and supports one of the most productive ecosystems. When light can penetrate to the sediment, it triggers the benthic microalgae community to perform photosynthesis, resulting in a benthic-pelagic exchange flux different from that of the deeper water. This study utilized the laboratory-measured benthic flux, and a suite of well-calibrated numerical models to examine the eutrophication process in the Lynnhaven River Inlet system with special emphasis on: the role played by benthic microalgae, and nutrient budgets (sources, sinks, and pathways) of the system. An unstructured grid hydrodynamic model UnTRIM developed for the shallow water environment was applied to the Lynnhaven to quantify the transport time scale and as the input for the water quality model. Based on the skill assessment result, it was clear that the presence of benthic microalgae is indispensable for an accurate and realistic calibration of the water quality model. Analysis of field samples in the laboratory experiments demonstrated that benthic microalgae performed photosynthesis under light conditions in surficial sediments, resulting in the net uptake of nutrients and the release of oxygen both to the overlying water column and down to the sediment. Based on the results of annual nutrient budgets, it was shown that the major external source for nitrogen and phosphorus was from nonpoint source loadings. There were three comparable sinks: export to the Bay, burial in the deep sediment, and ditrification in the case of nitrogen. One of the major pathways for nitrogen and phosphorus was the internal recycling. The regenerated dissolved nutrients that were recycled in the water column were more than two times larger than the current total nutrient external loadings. Sensitivity tests showed that, due to their retention capacity, benthic microalgae's presence could decrease the overall export to the Bay, enhance the internal recycling, and increase the denitrification rate in the sediment.
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Athanasiadis, Aristotelis. "Three-dimensional hybrid grid generation with application to high Reynolds number viscous flows." Doctoral thesis, Universite Libre de Bruxelles, 2004. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/211130.
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In this thesis, an approach is presented for the generation of grids suitable for the simulation of high Reynolds number viscous flows in complex three-dimensional geometries. The automatic and reliable generation of such grids is today on the biggest bottlenecks in the industrial CFD simulation environment.In the proposed approach, unstructured tetrahedral grids are employed for the regions far from the viscous boundaries of the domain, while semi-structured layers of high aspect ratio prismatic and hexahedral elements are used to provide the necessary grid resolution inside the boundary layers and normal to the viscous walls. The definition of the domain model is based on the STEP ISO standard and the topological information contained in the model is used for applying the hierarchical grid generation parameters defined by the user. An efficient, high-quality and robust algorithm is presented for the generation of the unstructured simplicial (triangular of tetrahedral) part of the grid. The algorithm is based on the Delaunay triangulation and the internal grid points are created following a centroid or frontal approach. For the surface grid generation, a hybrid approach is also proposed similar to the volume.
Semi-structured grids are generated on the surface grid (both on the edges and faces of the domain) to improve the grid resolution around convex and concave ridges and corners, by aligning the grid elements in the directions of high solution gradients along the surface. A method is also developed for automatically setting the grid generation parameters related to the surface grid generation based on the curvature of the surface in order to obtain an accurate and smooth surface grid. Finally, a semi-structured prismatic/hexahedral grid generation algorithm is presented for the generation of the part of grid close to the viscous walls of the domain. The algorithm is further extended with improvements meant to increase the grid quality around concave and convex ridges of the domain, where the semi-structured grids are known to be inadequate.
The combined methodology is demonstrated on a variety of complex examples mainly from the automotive and aeronautical industry.
Doctorat en sciences appliquées
info:eu-repo/semantics/nonPublished
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Basa, Mustafa Mazhar. "Determination Of Computational Domain Boundaries For Viscous Flow Around Two Dimensional Bodies." Master's thesis, METU, 2006. http://etd.lib.metu.edu.tr/upload/12607878/index.pdf.
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Borders of flow field around immersed bodies can be extended to long distances
since there are no physical boundaries. In computational practice however, the flow
domain must be restricted to a reasonable size by imposing appropriate boundary
conditions at the edges of the computational space. In this thesis work, streamlines
obtained from potential flow solution in a relatively large spatial domain are utilized
to specify the boundaries and boundary conditions for a more restricted
computational domain to be used for detailed viscous flow computations. A grid
generation code is adopted for generation of unstructured triangular grid systems for
domains involving multiple immersed bodies of any shape at arbitrary orientations
such as a group of tall buildings in horizontal plane. Finite volume method is used in
the solution of Laplace equation for the stream function. A deformation modulus is
introduced as a probe parameter to aid locating the viscous flow boundaries. The
computer code acts as a preprocessor for viscous flow computations, specifying the
computational boundaries, the boundary conditions and generating the computational
grid.
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Lee, Jae-doo. "Development of an Efficient Viscous Approach in a Cartesian Grid Framework and Application to Rotor-Fuselage Interaction." Diss., Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/11474.
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Despite the high cost of memory and CPU time required to resolve the boundary layer, a viscous unstructured grid solver has many advantages over a structured grid solver such as the convenience in automated grid generation and shock or vortex capturing by solution adaption. Since the geometry and flow phenomenon of a helicopter are very complex, unstructured grid-based methods are well-suited to model properly the rotor-fuselage interaction than the structured grid solver. In present study, an unstructured Cartesian grid solver is developed on the basis of the existing solver, NASCART-GT. Instead of cut-cell approach, immersed boundary approach is applied with ghost cell boundary condition, which increases the accuracy and minimizes unphysical fluctuations of the flow properties. The standard k-epsilon model by Launder and Spalding is employed for the turbulence modeling, and a new wall function approach is devised for the unstructured Cartesian grid solver. It is quite challenging and has never done before to apply wall function approach to immersed Cartesian grid. The difficulty lies in the inability to acquire smooth variation of y+ in the desired range due to the non-body-fitted cells near the solid wall. The wall function boundary condition developed in this work yields stable and reasonable solution within the accuracy of the turbulence model. The grid efficiency is also improved with respect to the conventional method. The turbulence modeling is validated and the efficiency of the developed boundary condition is tested in 2-D flow field around a flat plate, NACA0012 airfoil, axisymmetric hemispheroid, and rotorcraft applications.
For rotor modeling, an actuator disk model is chosen, since it is efficient and is widely verified in the study of the rotor-fuselage interaction. This model considers the rotor as an infinitely thin disk, which carries pressure jump across the disk and allows flow to pass through it. The full three dimensional calculations of Euler and RANS equations are performed for the GT rotor model and ROBIN configuration to test implemented actuator disk model along with the developed turbulence modeling. Finally, the characteristics of the rotor-fuselage interaction are investigated by comparing the numerical solutions with the experiments.
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Acikgoz, Nazmiye. "Adaptive and Dynamic Meshing Methods for Numerical Simulations." Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/14521.
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For the numerical simulation of many problems of engineering interest, it is desirable to have an automated mesh adaption tool. This is important especially for problems characterized by anisotropic features and require mesh clustering in the direction of high gradients. Another significant issue in meshing emerges in unsteady simulations with moving boundaries, where the boundary motion has to be accommodated by deforming the computational grid. Similarly, there exist problems where current mesh needs to be adapted to get more accurate solutions. To solve these problems, we propose three novel procedures.
In the first part of this work, we present an optimization procedure for three-dimensional anisotropic tetrahedral grids based on metric-driven h-adaptation. Through the use of topological and geometrical operators, the mesh is iteratively adapted until the final mesh minimizes a given objective function. We propose an optimization process based on an ad-hoc application of the simulated annealing technique, which improves the likelihood of removing poor elements from the grid. Moreover, a local implementation of the simulated annealing is proposed to reduce the computational cost.
Many challenging unsteady multi-physics problems are characterized by moving boundaries and/or interfaces. When the boundary displacements are large, degenerate elements are easily formed in the grid such that frequent remeshing is required. We propose a new r-adaptation technique that is valid for all types of elements (e.g., triangle, tet, quad, hex, hybrid) and deforms grids that undergo large imposed displacements at their boundaries. A grid is deformed using a network of linear springs composed of edge springs and a set of virtual springs. The virtual springs are constructed in such a way as to oppose element collapsing.
Both frequent remeshing, and exact-pinpointing of clustering locations are great challenges of numerical simulations, which can be overcome by adaptive meshing algorithms. Therefore, we conclude this work by defining a novel mesh adaptation technique where the entire mesh is adapted upon application of a force field in order to comply with the target mesh or to get more accurate solutions. The method has been tested for two-dimensional problems of a-priori metric definitions as well as for oblique shock clusterings.
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Zheng, Yun. "Aerodynamic computation on unstructured-grids." Thesis, Durham University, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.398570.
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Johnson, Anne. "Parallelising implicit methods on unstructured grids." Thesis, University of Oxford, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.320231.
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Saito, Olga Harumi. "Esquema numérico com reconstrução mínimos quadrados de alta ordem em malhas não-estruturadas para a formulação euleriana do transporte de partículas." Universidade de São Paulo, 2008. http://www.teses.usp.br/teses/disponiveis/45/45132/tde-15042008-165404/.
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O estudo do transporte de partículas tem uma importância fundamental em diversas áreas de pesquisas como, por exemplo, na formação de gelo em uma aeronave pois pode afetar a sua sustentação e estabilidade. Tamanha é a preocupação com a segurança de vôo que diversos estudos têm sido realizados, resultando em códigos computacionais como o LEWICE nos Estados Unidos, TRAJICE no Reino Unido, ONERA na França e CANICE no Canadá. No Brasil, um dos estudo é feito pela EMBRAER em parceria com algumas instituições. O objetivo deste trabalho é desenvolver um algoritmo que possa ser empregado na trajetória das partículas, utilizando uma formulação euleriana que elimina a dificuldade da semeadura de partículas específica da formulação lagrangiana na determinação da fração de volume da partícula. O método empregado é dos volumes finitos em malhas não-estruturadas cuja principal chave está na reconstrução mínimos quadrados de alta ordem com restrição nos contornos. O desenvolvimento do trabalho engloba 3 etapas: definição da geometria e geração das malhas; utilização de um solver para o tratamento do escoamento do ar e obtenção do campo de velocidade; implementação e utilização do esquema numérico com reconstrução mínimos quadrados de alta ordem para simular o cálculo da fração de volume com imposição de condições limites apropriadas no contorno do corpo. Os resultados dos testes realizados mostram que o esquema numérico com reconstrução mínimos quadrados pode ser empregado na resolução de equações que apresentam uma região de descontinuidade, como é o caso da região de sombra, reduzindo a largura da banda de difusão numérica e overshoots.The particle transport study has a fundamental importance in diverse research area like in the icing accretion on an aircraft because that can affect its sustentation and stability. The concern is so big that many researches have been carried through, resulting in computational codes like the LEWICE in the United States, TRAJICE in the United Kingdom, ONERA in France and CANICE in Canada. In Brazil, one of the study has been made by the EMBRAER with some institutes. The goal of this work is to develop an algorithm that can be used in the particles trajectory study, using an Eulerian method that eliminates the difficulty particle sowing, particular of the Lagrangian method, in the determination of the droplet fraction volume. This is made by the finite volume method on unstructured meshes whose main key is the high order reconstruction with restriction on the boundary. The development of the work involves 3 stages: geometry definition and mesh generation; using code for the treatment of the air flow and obtained flow velocity; use of the high order numerical scheme least square reconstruction to simulate the droplet fraction volume result with imposition of appropriate limit conditions in the body contour. The realized simulations shown that Least Square method can be used in problem resolution that present descontinuos region like is shadow region reducing numerical diffusion and overshoots.
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Globisch, Gerhard. "The hierarchical preconditioning having unstructured threedimensional grids." Universitätsbibliothek Chemnitz, 2005. http://nbn-resolving.de/urn:nbn:de:swb:ch1-200501021.
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Continuing the previous work in the preprint 97-11 done for the 2D-approach in this paper we describe the Yserentant preconditioned conjugate gradient method as well as the BPX-preconditioned cg-iteration fastly solving 3D-elliptic boundary value problems on unstructured quasi uniform grids. These artificially constructed hierarchical methods have optimal computational costs. In the case of the sequential computing several numerical examples demonstrate their efficiency not depending on the finite element types used for the discretiziation of the original potential problem. Moreover, implementing the methods in parallel first results are given.
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Bakosi, József. "PDF modeling of turbulent flows on unstructured grids." Fairfax, VA : George Mason University, 2008. http://hdl.handle.net/1920/3083.
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Thesis (Ph.D.)--George Mason University, 2008.Vita: p. 178. Thesis director: Zafer Boybeyi. Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Computational Sciences and Informatics. Title from PDF t.p. (viewed June 30, 2008). Includes bibliographical references (p. 168-177). Also issued in print.
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Lin, F. P. "Multigrid method for compressible flows on unstructured grids." Thesis, Swansea University, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.637921.
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This thesis presents procedures of the multigrid method using unstructured grids for solutions of inviscid compressible flows. The work concentrates on the components of the procedures, such as flow solvers (smoothers), intergrid interpolation, multigrid cycles, nested iterations and data structures. The performance and comparison of different smoothers are in particular emphasised. Some related aspects, e.g. the boundary treatment, are also discussed. Several two dimensional examples, for problems of external and internal inviscid compressible flows, are given to demonstrate the ability of the methods in improving the efficiency of simulating steady compressible flows.
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Iqbal, Kashif H. "Comparison of high-order methods on unstructured grids." Thesis, Cranfield University, 2013. http://dspace.lib.cranfield.ac.uk/handle/1826/8274.
Full textAbstract:
A high-order Discontinuous Galerkin (DG) method is formulated and implemented on the Cranfield University’s 3D unstructured Finite Volume Method (FVM) code (UCNS3D), for both linear and non-linear hyperbolic conservation laws and for test-cases which exhibit both smooth and discontinuous solutions. As both DG and FVM are developed on the same solver platform, this enables the use of any procedures which are common to both the methods, thus, ensuring the closest possible compari-son.
The initial part of the thesis details the basic concepts and derivation of the discon-tinuous Galerkin method in the 1D space for the advection equation, which is then extended to the 3D space for a hyperbolic system.
Prior to comparing the FVM and DG methods, the DG method implementation is verified. The verification is a combination of a theoretical and numerical approach which endeavours to minimize any potential programming errors.
Following the verification of the DG method, the FVM and DG methods are compared for numerous flows: the linear advection equation and Euler equations, sufficiently smooth testcases, and testcases which require a limiter to suppress Gibb’s oscillations.
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Bruner, Christopher William Stuteville. "Parallelization of the Euler Equations on Unstructured Grids." Diss., Virginia Tech, 1996. http://hdl.handle.net/10919/30397.
Full textAbstract:
Several different time-integration algorithms for the Euler equations are investigated on two distributed-memory parallel computers using an explicit message-passing paradigm: these are classic Euler Explicit, four-stage Jameson-style Runge-Kutta, Block Jacobi, Block Gauss-Seidel, and Block Symmetric Gauss-Seidel. A finite-volume formulation is used for the spatial discretization of the physical domain. Both two- and three-dimensional test cases are evaluated against five reference solutions to demonstrate accuracy of the fundamental sequential algorithms. Different schemes for communicating or approximating data that are not available on the local compute node are discussed and it is shown that complete sharing of the evolving solution to the inner matrix problem at every iteration is faster than the other schemes considered. Speedup and efficiency issues pertaining to the various time-integration algorithms are then addressed for each system. Of the algorithms considered, Symmetric Block Gauss-Seidel has the overall best performance. It is also demonstrated that using parallel efficiency as the sole means of evaluating performance of an algorithm often leads to erroneous conclusions; the clock time needed to solve a problem is a much better indicator of algorithm performance. A general method for extending one-dimensional limiter formulations to the unstructured case is also discussed and applied to Van Albada’s limiter as well as Roe’s Superbee limiter. Solutions and convergence histories for a two-dimensional supersonic ramp problem using these limiters are presented along with computations using the limiters of Barth & Jesperson and Venkatakrishnan — the Van Albada limiter has performance similar to Venkatakrishnan’s.Ph. D.
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Bin, Mohamad Badry Ahmad Badarudin. "Synthetic Turbulence Generation for LES on Unstructured Cartesian Grids." Thesis, Cranfield University, 2007. http://hdl.handle.net/1826/3172.
Full textAbstract:
A parallel CFD code to solve incompressible fluid flow on unstructured Cartesian
meshes has been developed almost from ground up. Turbulence statistics have been
computed using the Large Eddy Simulation technique. The new code was subjected to
some validation where results are compared to available reference data. An analysis
on the iteration and discretisation errors was carried out.
This code was then applied to predict the lid driven cubical cavity flow in at a bulk
Reynolds number of 10,000. Three different mesh sizes were used to investigate
independence of results on grid size. Amongst others, turbulence statistics were
checked against Kolmogorov -5/3 law.
A detailed study of synthetic turbulence methods was carried out and applied to the
prediction of flow in a duct with square cross section using an inlet and outflow
boundaries. Three different turbulence generation methods were investigated namely
the artificial turbulence generation method, random perturbation method and a novel
hybrid particle-wave method also termed as the enhanced vortex particle method in
this study. The mean and instantaneous field variables together with the turbulence
statistics from each method were compared and analysed.
Finally, the code was used to solve turbulent flow over arrays of wall-mounted
obstacles with mesh densities comparable to previous studies. The velocity profiles
and vector fields at various locations in the domain were compared to data obtained
from recent LES simulations. The artificial turbulence generation case was applied for
the first time to produce turbulence at the inlet. The turbulence kinetic energy
spectrum distribution agrees well with reference data.
Important findings from this study are clarified and some suggestions for future work
are given in the conclusions section.
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Tsoutsanis, Panagiotis. "Very high-order methods for 3D arbitrary unstructured grids." Thesis, Cranfield University, 2009. http://hdl.handle.net/1826/4511.
Full textAbstract:
Understanding the motion of fluids is crucial for the development and analysis of new designs
and processes in science and engineering. Unstructured meshes are used in this context
since they allow the analysis of the behaviour of complicated geometries and configurations
that characterise the designs of engineering structures today. The existing numerical methods
developed for unstructured meshes suffer from poor computational efficiency, and their applicability
is not universal for any type of unstructured meshes. High-resolution high-order
accurate numerical methods are required for obtaining a reasonable guarantee of physically
meaningful results and to be able to accurately resolve complicated flow phenomena that
occur in a number of processes, such as resolving turbulent flows, for direct numerical simulation
of Navier-Stokes equations, acoustics etc.
The aim of this research project is to establish and implement universal, high-resolution, very
high-order, non-oscillatory finite-volume methods for 3D unstructured meshes. A new class
of linear and WENO schemes of very high-order of accuracy (5
th
) has been developed. The
key element of this approach is a high-order reconstruction process that can be applied to any
type of meshes. The linear schemes which are suited for problems with smooth solutions,
employ a single reconstruction polynomial obtained from a close spatial proximity. In the
WENO schemes the reconstruction polynomials, arising from different topological regions,
are non-linearly combined to provide high-order of accuracy and shock capturing features.
The performance of the developed schemes in terms of accuracy, non-oscillatory behaviour
and flexibility to handle any type of 3D unstructured meshes has been assessed in a series of
test problems. The linear and WENO schemes presented achieve very high-order of accuracy
(5
th
). This is the first class of WENO schemes in the finite volume context that possess highorder
of accuracy and robust non-oscillatory behaviour for any type of unstructured meshes.
The schemes have been employed in a newly developed 3D unstructured solver (UCNS3D).
UCNS3D utilises unstructured grids consisted of tetrahedrals, pyramids, prisms and hexahedral
elements and has been parallelised using the MPI framework. The high parallel efficiency
achieved enables the large scale computations required for the analysis of new designs and
processes in science and engineering.