Numerical Simulation Capability Research Articles

Aspects of acoustics in MEMS devices

In this talk we will present an overview of aspects of structural acoustics in MEMS devices at Sandia. Recent results concerning viscous wave motion in a rotational micromechanical system will be presented. In addition, experimental approaches for the characterization of acoustic effects in MEMS devices will be discussed. We will also present an overview of massively parallel numerical simulation capabilities for large-scale and small-scale (MEMS) structural acoustic analysis. Since large numbers of degrees of freedom are typically present in structural acoustic analysis, massively parallel computations are essential in solving practical application problems. Two formulations for the fluid will be considered: a standard linear velocity potential formulation, and a nonlinear wave formulation. The applications of the two formulations will be discussed. A parallelization scheme will be presented that allows for arbitrary decomposition of the wet interface. Finally, numerical results of application problems will be presented. [Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.]

Discrete Element Modeling Applied to Laboratory Simulation of Near-Wellbore Mechanics

Simulation results of near-wellbore failure phenomena are presented from a joint experimental-numerical study directed at developing a robust numerical simulation capability for the exploration and prediction of near-wellbore mechanics. An experimental procedure was developed for the laboratory simulation of slurry injection. A true-triaxial vessel, which applied realistic, three-dimensional stress conditions, was used to perform slurry injection into Berea sandstone. Under anisotropic horizontal stress conditions, vertical hydraulic fractures initiated and propagated in the direction of the maximum horizontal stress. Under isotropic horizontal stress conditions, multiple vertical fractures were induced and propagated in random orientation. A computationally efficient numerical model based on the discrete element method (DEM) is described and applied to simulate various wellbore phenomena. Radially graded, two-dimensional DEM models of the near-wellbore region were created of bonded disk elements. Source ...

Asynchronous Fast Adaptive Composite-Grid Methods for Elliptic Problems: Theoretical Foundations

Accurate numerical modeling of complex physical, chemical, and biological systems requires numerical simulation capability over a large range of length scales, with the ability to capture rapidly varying phenomena localized in space and/or time. Adaptive mesh refinement (AMR) is a numerical process for dynamically introducing local fine resolution on computational grids during the solution process, in response to unresolved error in a computation. Fast adaptive composite-grid (FAC) methods are a class of algorithms that exploit the multilevel structure of AMR grids to solve elliptic problems efficiently. This paper develops a theoretical foundation for AFACx, an asynchronous FAC method. A new multilevel condition number estimate establishes that the convergence rate of the AFACx algorithm does not degrade as the number of refinement levels in the AMR hierarchy increases.

Mathematical modeling of industrial furnaces considering detailed oil spray characteristics

Results from atomization experiments and mathematical modeling of heavy fuel oil combustion in industrial furnaces are presented. A phase Doppler particle analyzer (PDPA) was used to characterize a heavy fuel oil spray supplied by an atomizer designed for glass melting furnaces. A unique calculation method for oil spray was developed to apply the results from the PDPA measurement as boundary inlet conditions for the computational fluid dynamics (CFD) simulation. Additional studies of the atomizer provided detailed in-flame measurements in a semi-industrial-scale glass melting furnace. The modeling results were compared with the measured in-flame data. The objective of this study was to improve the numerical simulation capabilities by incorporating atomizer-specific spray characteristics into a CFD program and to predict the effects of spray characteristics on the heavy fuel oil combustion.

The validity of 2D numerical simulations of vortical structures around a bridge deck

This paper deals with a computational study for evaluating the capability of 2D numerical simulation for predicting the vortical structure around a quasibluff bridge deck. The laminar form, a number of BANS equation models, and the LES approach are evaluated. The study was applied to the deck section of the Great Belt East Bridge. The results are compared with wind-tunnel data and previously conducted computational simulations. Sensitivity of the results with regard to the computational approaches applied for each model is discussed. Finally, the study confirms the importance of safety-barrier modelling in the analysis of bridge aerodynamics.

Numerical Simulation of Glass Forming and Conditioning

Advances in numerical simulation capabilities have made the modeling of both glass‐conditioning and glass‐forming processes feasible. Glass‐forming operations include large free surface deformations, conjugate heat transfer, and complex contact phenomena. In this paper, glass‐container forming processes are modeled to provide insight into the impacts of the various stages of forming and conditioning on final container quality. The model uses finite elements and includes the effects of viscoelasticity, surface tension, and time‐varying heat transfer. Special attention is given to areas that require further developments in numerical capabilities and an increased knowledge of boundary conditions and material properties.

SAPWIN-a symbolic simulator as a support in electrical engineering education

Gaining an insight into circuit properties in electrical engineering classes can be achieved by using computer based tools. A computer program which combines symbolic and numerical simulation capabilities is of great help, because such a program provides students with automatic analysis tools. This paper presents the program SAPWIN, which has been developed to perform an automatic symbolic and numerical analysis of linear circuits. The paper presents program features, their development lines and fundamental aspects. Also, the educational purposes which are contained in the use of the program itself are presented.

Analyzing irreversibilities in Stokes flows containing suspended particles using the traction boundary integral equation method

Irreversibilities in suspension flows at low Reynolds numbers have been observed experimentally. These irreversibilities can lead to particle migration, normal stress differences, and complex microstructure including particle chains and agglomerations. The morphology of the microstructure formation and the relationship of the microstructure to the continuum properties of the suspension are current topics of research interest. Detailed information concerning the microstructure is hard to obtain experimentally, and hence, accurate numerical simulation capabilities are becoming relied on more heavily to fill the gap between the mesoscale and continuum properties. In this research, a traction boundary integral equation (TBIE) method is used to analyze irreversibilities in Stokes flow containing suspended particles. The TBIE is particularly well-suited for this type of analysis since the associated discretized sets of linear equations are better conditioned than the more prevalent velocity boundary integral equation (VBIE) method especially as particles tend to aggregate.

Techniques for metal hydride thermal energy conversion and their optimization

To achieve the most natural features of metal hydride thermal energy conversion systems, both physical analysis of system components and optimization of the operation process are required, which can be solved by numerical simulation without arduous works of trial and error. The conventional and new cycling performance of a system can be predicted also by numerical simulation and easily proved by bench-scale techniques constructed with simulation results. In order to realize this procedure for the system development, it is necessary to set a computer simulation program which is based on detailed information about equilibrium properties of metal hydride and operating conditions. In the series of numerical runs of the simulation program, it was shown how metal hydride thermal energy conversion techniques could be optimized through the choice of a proper hydride pair from Zr-based alloys and several operation parameters. Especially, it was found that the heat transfer problem played a decisive roll in cooling power of the system. Finally, the unlimited capability of numerical simulation was proved by a good correspondence of simulation data with real experimental data.

Multigrid techniques: A fast and efficient method for the numerical simulation of elastohydrodynamically lubricated point contact problems

The introduction and further development of algorithms based on multigrid (multilevel) techniques has resulted in fast and efficient solvers for elastohydrodynamically lubricated (EHL) point contact problems. As a result the capability to simulate steady state and transient point contact problems numerically has greatly increased in the past decade. This has led to an increased understanding of the mechanisms controlling film formation and pressure in the contact in relation to the operating conditions. In this article an overview is given of the essential ingredients of an efficient multigrid algorithm for an EHL point contact problem. Results are presented for some characteristic problems that illustrate today's numerical simulation capability. It is concluded that, with respect to EHL, one of the main questions of the last part of the century has been answered, i.e. how to solve efficiently the modelling equations such that realistic and highly loaded situations can be simulated. The challenge that researchers and engineers now face is how to translate the new insights into simple rules for design and engineering, and to establish clearly the limits of validity of the models that are used in simulations both on the scale of the contact as a whole as well as on a subcontact scale.

Turbulent Transient Gas Injections

Compressible transient turbulent gaseous jets are formed when natural gas is injected directly into a diesel engine. Multi-dimensional simulations are used to analyze the penetration, mixing, and combustion of such gaseous fuel jets. The capability of multi-dimensional numerical simulations, based on the k-ε turbulence model, to reproduce the experimentally verified penetration rate of free transient jets is evaluated. The model is found to reproduce the penetration rate dependencies on momentum, time, and density, but is more accurate when one of the k-ε coefficients is modified. The paper discusses other factors affecting the accuracy of the calculations, in particular, the mesh density and underexpanded injection conditions. Simulations are then used to determine the impact of chamber turbulence, injection duration, and wall contact on transient jet penetration. The model also shows that gaseous jets and evaporating diesel sprays with small droplet size mix at much the same rate when injected with equivalent momentum injection rate. [S0098-2202(00)02304-X]

Simulation of Unsteady Flow in Nozzle-Ejector Mixer

A numerical-simulation capability is developed with emphasis on capturing the  ow and acoustic disturbances in internal  ows. The conŽ guration considered is that of a circular ejector with single-element primary nozzle. The Favre-Ž ltered Navier–Stokes equations with a subgrid model are used to simulate the large-scale structure. The high-order dispersion-relation-preserving scheme is used to minimize dispersion and dissipation errors. Special boundary treatments are adopted for in ow, out ow, and solid walls to eliminate nonphysical re ections. Results show the growth of the disturbances, the formation of weak shock-cell structures, and the propagation of acoustic waves.

Mechanics of nonlinear short-wave generation by a moored near-surface buoy

We consider the nonlinear interaction problem of surface waves with a tethered near-surface buoy. Our objective is to investigate mechanisms for nonlinear short surface wave generation in this complete coupled wave–buoy–cable dynamical system. We develop an effective numerical simulation capability coupling an efficient and high-resolution high-order spectral method for the nonlinear wave–buoy interaction problem with a robust implicit finite-difference method for the cable–buoy dynamics. The numerical scheme accounts for nonlinear wave–wave and wave–body interactions up to an arbitrary high order in the wave steepness and is able to treat extreme motions of the cable including conditions of negative cable tension. Systematic simulations show that beyond a small threshold value of the incident wave amplitude, the buoy performs chaotic motions, characterized by the snapping of the cable. The root cause of the chaotic response is the interplay between the snapping of the cable and the generation of surface waves, which provides a source of strong (radiation) damping. As a result of this interaction, the chaotic buoy motion switches between two competing modes of snapping response: one with larger average peak amplitude and lower characteristic frequency, and the other with smaller amplitude and higher frequency. The generated high-harmonic/short surface waves are greatly amplified once the chaotic motion sets in. Analyses of the radiated wave spectra show significant energy at higher frequencies which is orders of magnitude larger than can be expected from nonlinear generation under regular motion.

Stochastic effects in ocean acoustics. Advances in theory and experiment

As the need to operate in the kilohertz range increased in the 1960s, existing approaches to ocean acoustics, based on sound-speed profiles and ray-tracing, became quite inadequate. The raggedness of acoustic transmissions, previously regarded simply as noise, was so pronounced at the higher frequencies that the stochastic properties themselves became an urgent area of research. Several major ocean transmission trials were carried out to obtain fluctuation data. The trial at Cobb Seamount produced results that could not be explained by existing random-wave propagation theory. The research that followed led to significant advances in several areas. New propagation theory emerged based on the parabolic equations for the acoustic field moments. In particular it was the construction of analytical solutions of the equation for the fourth moment that provided the first satisfactory explanation of the Cobb intensity fluctuation spectra. Advances in numerical simulation methods and capability have allowed pictures of the full stochastic acoustic field in an ocean transmission trial to be produced. These have not only confirmed experimental variances and cross correlations but have also revealed the spatial structure of the field, including interesing features such as the elongated sound ribbons that arise in some cases.

On the Interface Debonding Models

Interface debonding models offer the possibility to simulate numerically the interface crack growth, and the eventual contact friction behaviour on the cracked area. The paper considers models in the framework of an Interface Damage Mechanics, relating displacement discontinuities across the interface to the corresponding tractions, through various constitutive and damage equations. The approach is based on the initial works by Needleman (1987) and Tvergaard (1990a, 1990b). After a brief review of the existing models, two new versions are proposed and discussed in detail, with the purpose of simulating mixed mode progressive decohesion, followed by a contact/friction behaviour after complete separation, and to guarantee the continuity between the two phases. The first modified model considers coupling terms in the elastic behaviour with damage. The second one introduces the possibility for sliding effects, obeying the Coulomb criterion, during the decohesion phase. The initial Tvergaard model and the second modified model are shown to be derivable from a state potential. Moreover, the thermodynamic framework is used to choose some characteristics of the Model II under a compressive behaviour. Under tension, the models reduce to the classical one. A three-dimensional generalization is given in the Appendix. In addition, the numerical simulation capabilities of the interface debonding models are illustrated by two examples: the fibre-matrix-interface decohesion in a Metal Matrix composite with a bridged matrix crack and the delamination of a DCB (Double Cantilever Beam) specimen.

Assessment of numerical simulation capabilities for medium-structure interaction systems under explosive loads

This study was aimed at assessing the capabilities to numerically simulate a dynamic medium-structure interaction system composed of geologic backfills, a shock insulation material, an explosive charge, and a partially-buried structure. Conventional high explosive charges were detonated at the same depth, but at various stand off distances from the structure. The finite element code DYNA3D was employed for estimating the relationships between the physical parameters of the problem and the behavior of the system. The results of the finite element analysis were then compared to test data gathered from field experiments and predictions from simpler computational methods.

Constitutive model of creep in rock salt applied to underground room closure

A multimechanism constitutive model of the creep of polycrystalline rock salt has been developed based on steady state creep as modified to incorporate transient creep through workhardening and recovery. Application of the model is in calculation of the closure of underground rooms excavated in a natural, bedded evaporite salt deposit. This requires an integrated technology consisting of not only the proper constitutive model, but also a range of other system elements relating to the site geology, initial and boundary conditions, and numerical methods peculiar to the application. Through finite element calculations, this model, with appropriate laboratory material parameters and a Tresca flow potential, has predicted the closure of a number of large, in situ experimental rooms at the Waste Isolation Pilot Plant (WIPP). In this paper, an overview of the model development, laboratory parameter generation and numerical simulation capability is given, together with a summary of the comparisons of the calculated and measured creep closure results. The comparisons encompass a range of test room configurations that exercise the predictive capability for stratigraphic setting, room geometry, thermal heating, stress field interaction and two and three-dimensional deformation fields. The simulations have proven satisfactory and suggest the general adequacy of the predictive technology.

Space‐time finite element computation of compressible flows between moving components

AbstractA numerical simulation capability for the injector flow of a regenerative liquid propellant gun (RLPG) is presented. The problem involves fairly complex geometries and two pistons in relative motion; therefore a stabilized space‐time finite element formulation developed earlier and capable of handling flows with moving mechanical components is used. In addition to the specifics of the numerical method, its application to a 30 mm RLPG test firing is discussed. The computational data from the simulation of this test case are interpreted to provide information on flow characteristics, with emphasis on the tendency of the flow to separate from the injection orifice boundary of the test problem. In addition, the computations provided insight into the behaviour of the flow entering the combustion chamber.

Numerical simulation of the dynamics of turbulent boundary layers: perspectives of a transition simulator

The current and prospective capabilities of numerical simulations of turbulent boundary layers are discussed. The stringent resolution requirements for resolving instantaneous structures and dynamics (rather than just for producing statistics) are emphasized. Improvements and alternatives to the prevailing simulation methodology are proposed.

Numerical simulations of free electron laser oscillators

A numerical simulation capability has been developed to model the physics and realistic design constraints of free electron laser oscillators driven by rf linear accelerators. Two computer codes have been written, FELEX and FELP. The code FELP is a one spatial dimension code with essentially unlimited time or spectral resolution. The codes are complementary and their use is dependent upon the problem being addressed. The code FELP is used to model optical and electron micropulse structure, broadband noise, and the sideband instability. The code FELEX models accelerator generated electron beam distributions, the transport of these distributions through wigglers with misalignments and field errors, self-consistent interaction with the optical field, and propagation of the optical field through resonators with realistically modelled components. FELEX is routinely used to match resonator designs to the optical parameters of the electron beam, and used to investigate the physics of 3-D micropulse effects. Some details of the codes will be presented along with various examples of simulation results.