Low-speed flow hydrodynamics

This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The objective of this project was the inception and development of a new casting simulation tool that is founded in modern, high-order numerical algorithms, accurate physical models, and advanced computational science constructs needed to execute efficiently on parallel architectures. This project has therefore led to the development and application of a new simulation tool (known as Telluride) for the modeling of casting processes used in the manufacture of metal alloy components needed for various Department of Energy (DOE) and Defense Programs (DP) projects. As a result of the efforts undertaken in this project, Telluride can now model key foundry processes in the DOE/DP and in industry. Successes realized over the course of this project have secured funding for further Telluride development by the DOE Accelerated Strategic Computing Initiative (ASCI) Program

JTpack90: A parallel, object-based, Fortran 90 linear algebra package

The authors have developed an object-based linear algebra package, currently with emphasis on sparse Krylov methods, driven primarily by needs of the Los Alamos National Laboratory parallel unstructured-mesh casting simulation tool Telluride. Support for a number of sparse storage formats, methods, and preconditioners have been implemented, driven primarily by application needs. They describe the object-based Fortran 90 approach, which enhances maintainability, performance, and extensibility, the parallelization approach using a new portable gather/scatter library (PGSLib), current capabilities and future plans, and present preliminary performance results on a variety of platforms

A high resolution finite volume method for efficient parallel simulation of casting processes on unstructured meshes

We discuss selected aspects of a new parallel three-dimensional (3-D) computational tool for the unstructured mesh simulation of Los Alamos National Laboratory (LANL) casting processes. This tool, known as {bold Telluride}, draws upon on robust, high resolution finite volume solutions of metal alloy mass, momentum, and enthalpy conservation equations to model the filling, cooling, and solidification of LANL castings. We briefly describe the current {bold Telluride} physical models and solution methods, then detail our parallelization strategy as implemented with Fortran 90 (F90). This strategy has yielded straightforward and efficient parallelization on distributed and shared memory architectures, aided in large part by new parallel libraries {bold JTpack9O} for Krylov-subspace iterative solution methods and {bold PGSLib} for efficient gather/scatter operations. We illustrate our methodology and current capabilities with source code examples and parallel efficiency results for a LANL casting simulation

Amplitude measurements of Faraday waves

A light reflection technique is used to measure quantitatively the surface elevation of Faraday waves. The performed measurements cover a wide parameter range of driving frequencies and sample viscosities. In the capillary wave regime the bifurcation diagrams exhibit a frequency independent scaling proportional to the wavelength. We also provide numerical simulations of the full Navier-Stokes equations, which are in quantitative agreement up to supercritical drive amplitudes of 20%. The validity of an existing perturbation analysis is found to be limited to 2.5% overcriticaly.Comment: 7 figure

RIPPLE: A new model for incompressible flows with free surfaces

A new free surface flow model, RIPPLE, is summarized. RIPPLE obtains finite difference solutions for incompressible flow problems having strong surface tension forces at free surfaces of arbitrarily complex topology. The key innovation is the Continuum Surface Force (CSF) model which represents surface tension as a (strongly) localized volume force. Other features include a high-order momentum advection model, a volume-of-fluid free surface treatment, and an efficient two-step projection solution method. RIPPLE'S unique capabilities are illustrated with two example problems: low-gravity jet-induced tank flow, and the collision and coalescence of two cylindrical rods. 17 refs., 7 figs

A comparison of interface tracking methods

In this Paper we provide a direct comparison of several important algorithms designed to track fluid interfaces. In the process we propose improved criteria by which these methods are to be judged. We compare and contrast the behavior of the following interface tracking methods: high order monotone capturing schemes, level set methods, volume-of-fluid (VOF) methods, and particle-based (particle-in-cell, or PIC) methods. We compare these methods by first applying a set of standard test problems, then by applying a new set of enhanced problems designed to expose the limitations and weaknesses of each method. We find that the properties of these methods are not adequately assessed until they axe tested with flows having spatial and temporal vorticity gradients. Our results indicate that the particle-based methods are easily the most accurate of those tested. Their practical use, however, is often hampered by their memory and CPU requirements. Particle-based methods employing particles only along interfaces also have difficulty dealing with gross topology changes. Full PIC methods, on the other hand, do not in general have topology restrictions. Following the particle-based methods are VOF volume tracking methods, which are reasonably accurate, physically based, robust, low in cost, and relatively easy to implement. Recent enhancements to the VOF methods using multidimensional interface reconstruction and improved advection provide excellent results on a wide range of test problems

High resolution finite volume parallel simulations of mould filling and binary alloy solidification on unstructured 3-D meshes

The Los Alamos National Laboratory (LANL) is currently developing a new casting simulation tool (known as Telluride) that employs robust, high-resolution finite volume algorithms for incompressible fluid flow, volume tracking of interfaces, and solidification physics on three-dimensional (3-D) unstructured meshes. Their finite volume algorithms are based on colocated cell-centered schemes that are formally second order in time and space. The flow algorithm is a 3-D extension of recent work on projection method solutions of the Navier-Stokes (NS) equations. Their volume tracking algorithm can accurately track topologically complex interfaces by approximating the interface geometry as piecewise planar. Coupled to their fluid flow algorithm is a comprehensive binary alloy solidification model that incorporates macroscopic descriptions of heat transfer, solute redistribution, and melt convection as well as a microscopic description of segregation. The finite volume algorithms, which are efficient, parallel, and robust, can yield high-fidelity solutions on a variety of meshes, ranging from those that are structured orthogonal to fully unstructured (finite element). The authors discuss key computer science issues that have enabled them to efficiently parallelize their unstructured mesh algorithms on both distributed and shared memory computing platforms. These include their functionally object-oriented use of Fortran 90 and new parallel libraries for gather/scatter functions (PGSLib) and solutions of linear systems of equations (JTpack90). Examples of their current capabilities are illustrated with simulations of mold filling and solidification of complex 3-D components currently being poured in LANL foundries

Convergence and accuracy of kernel-based continuum surface tension models

Numerical models for flows of immiscible fluids bounded by topologically complex interfaces possessing surface tension inevitably start with an Eulerian formulation. Here the interface is represented as a color function that abruptly varies from one constant value to another through the interface. This transition region, where the color function varies, is a thin O(h) band along the interface where surface tension forces are applied in continuum surface tension models. Although these models have been widely used since the introduction of the popular CSF method [BKZ92], properties such as absolute accuracy and uniform convergence are often not exhibited in interfacial flow simulations. These properties are necessary if surface tension-driven flows are to be reliably modeled, especially in three dimensions. Accuracy and convergence remain elusive because of difficulties in estimating first and second order spatial derivatives of color functions with abrupt transition regions. These derivatives are needed to approximate interface topology such as the unit normal and mean curvature. Modeling challenges are also presented when formulating the actual surface tension force and its local variation using numerical delta functions. In the following they introduce and incorporate kernels and convolution theory into continuum surface tension models. Here they convolve the discontinuous color function into a mollified function that can support accurate first and second order spatial derivatives. Design requirements for the convolution kernel and a new hybrid mix of convolution and discretization are discussed. The resulting improved estimates for interface topology, numerical delta functions, and surface force distribution are evidenced in an equilibrium static drop simulation where numerically-induced artificial parasitic currents are greatly mitigated

A parallel, volume-tracking algorithm for unstructured meshes

Many diverse areas of industry benefit from the use of volume of fluid methods to predict the movement of materials. Casting is a common method of part fabrication. The accurate prediction of the casting process is pivotal to industry. Mold design and casting is currently considered an art by industry. It typically involves many trial mold designs, and the rejection of defective parts is costly. Failure of cast parts, because residual stresses reduce the part`s strength, can be catastrophic. Cast parts should have precise geometric details that reduce or eliminate the need for machining after casting. Volume of fluid codes will help designers predict how the molten metal fills a mold and where ay trapped voids remain. Prediction of defects due to thermal contraction or expansion will eliminate defective, trial mold designs and speed the parts to market with fewer rejections. Increasing the predictability and therefore the accuracy of the casting process will reduce the art that is involved in mold design and parts casting. Here, recent enhancements to multidimensional volume-tracking algorithms are presented. Illustrations in two dimensions are given. The improvements include new, local algorithms for interface normal constructions and a new full remapping algorithm for time integration. These methods are used on structured and unstructured grids