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Turbulence

Description: Current theories for material mixing include multiphase interpenetration and single-field turbulence transport with large density variations. Neither approach by itself is adequate for current problem-solving needs, but in combination they offer tremendous opportunities for the analysis of complex material dynamics. Multiphase theory contributes the ''ordered'' jets or particulate trajectories that penetrate in wave-like fashion; turbulence transport superimposes the important nonlinear diffusive component to the mixing. Shear impedance and energy transport arise naturally in this combined analysis.
Date: April 1, 1987
Creator: Harlow, F.H.
Partner: UNT Libraries Government Documents Department

Effects of finite size on the critical behavior of fluid films

Description: This thesis presents the results of refractive index studies performed on films of a critical mixture of 2,6-lutidine+water near the lower critical point. The films ranged in thickness from 0.46 ..mu..m to approx. 300 ..mu..m and were confined between the highly reflecting surfaces of a pair of optical flats in an interferometer. Above the critical temperatures of these films, in the two-phase region, ..delta..n was measured, the difference between the refractive indexes of the two phases. Since to a close approximation ..delta..n is proportional to the order parameter, measurements of ..delta..n as a function of temperature map out the coexistence curves of the films.
Date: January 1, 1978
Creator: Scheibner, B A
Partner: UNT Libraries Government Documents Department

Adaptive mesh refinement for 1-dimensional gas dynamics

Description: We consider the solution of the one-dimensional equation of gas-dynamics. Accurate numerical solutions are difficult to obtain on a given spatial mesh because of the existence of physical regions where components of the exact solution are either discontinuous or have large gradient changes. Numerical methods treat these phenomena in a variety of ways. In this paper, the method of adaptive mesh refinement is used. A thorough description of this method for general hyperbolic systems is given elsewhere and only properties of the method pertinent to the system are elaborated.
Date: January 1, 1982
Creator: Hedstrom, G.; Rodrigue, G.; Berger, M. & Oliger, J.
Partner: UNT Libraries Government Documents Department

The relaxation of the second moments in rapid shear flows of smooth disks

Description: This paper compares the results of numerical simulations for two- dimensional, rapid, homogeneous shear flows of identical, smooth, inelastic disks with the predictions of Jenkins and Richman (JFM 192, 313-328 (1988)) for the relaxation of the second moments of the velocity distribution function following a homogeneous, but anisotropic disturbance of their steady values. For nearly elastic disks, the time-history of the relaxation is in excellent agreement with the theory in both its dense and dilute limits. However, deviations are observed in the case of inelastic particles. 2 refs., 8 figs.
Date: January 1, 1991
Creator: Louge, M.Y. (Cornell Univ., Ithaca, NY (United States). Sibley School of Mechanical and Aerospace Engineering); Jenkins, J.T. (Cornell Univ., Ithaca, NY (United States). Dept. of Theoretical and Applied Mechanics) & Hopkins, M.A. (Army Cold Regions Research and Engineering Lab., Hanover, NH (United States))
Partner: UNT Libraries Government Documents Department

Production of optically thin free-standing oil films from the edge of a rotating disc

Description: A method is described for forming thin free-standing oil films which are spun from the edge of a sharp-edged rotating disc. The films can be made thin enough to show strong optical interference colors when viewed in white light. The thinnest films have areal densities down to about 10 to 20 ..mu..gm/cm/sup 2/. A stable roughly triangular film with an area of about 10 cm/sup 2/ and fairly uniform thickness can be readily produced. Much larger films having either greater thickness or less stability are also possible. Films have been produced both in air and in vacuum.
Date: January 1, 1980
Creator: Cramer, J.G.; Burch, D.F.; Rodenberg, R. & Cramer, P.B.
Partner: UNT Libraries Government Documents Department

Hybrid simulations

Description: The philosophy and numerical implementation of hybrid algorithms are reviewed. In the hybrid approximation, a plasma is described by a set of discrete equations, equivalent to a Vlasov-fluid system. The dynamics of one or more species are modeled using moment equations, while the remaining species are treated as a large number of individual macro-particles. In this paper the hybrid method will be compared to fluid and particle-in-cell algorithms, and the strengths and weaknesses of the various methods will be discussed. A specific limit of the hybrid model, that of macro-particle ions and massless, charge-neutralizing fluid electrons, will be analyzed in detail with particular emphasis on multi-dimensional codes.
Date: January 1, 1987
Creator: Quest, K.B.
Partner: UNT Libraries Government Documents Department

Instability of a supersonic shock free elliptic jet

Description: This paper presents a comparison of the measured and the computed spatial stability properties of an aspect ratio 2 supersonic shock free elliptic jet. The shock free nature of the elliptic jet provides an ideal test of validity of modeling the large scale coherent structures in the initial mixing region of noncircular supersonic jets with linear hydrodynamic stability theory. Both aerodynamic and acoustic data were measured. The data are used to compute the mean velocity profiles and to provide a description of the spatial composition of pressure waves in the elliptic jet. A hybrid numerical scheme is applied to solve the Rayleigh problem governing the inviscid linear spatial stability of the jet. The measured mean velocity profiles are used to provide a qualitative model for the cross sectional geometry and the smooth velocity profiles used in the stability analysis. Computational results are presented for several modes of instability at two jet cross sections. The acoustic measurements show that a varicose instability is the jet's perferred mode of motion. The stability analysis predicts that the Strouhal number varies linearly as a function of axial distance in the jet's initial mixing region, which is in good qualitative agreement with previous measurements. 18 refs., 18 figs., 1 tab.
Date: January 1, 1990
Creator: Baty, R.S. (Sandia National Labs., Albuquerque, NM (USA)); Seiner, J.M. & Ponton, M.K. (National Aeronautics and Space Administration, Hampton, VA (USA). Langley Research Center)
Partner: UNT Libraries Government Documents Department

A fast solver for systems of axisymmetric ring vortices

Description: A method which is capable of efficient calculation of the axisymmetric flow field produced by a large system of ring vortices is presented in this report. The system of ring vortices can, in turn, be used to model body surfaces and wakes in incompressible unsteady axisymmetric flow fields. This method takes advantage of source point and field point series expansions which enables one to make calculations for interactions between groups of vortices which are in well separated spatial domains rather than having to consider interactions between every pair of vortices. In this work, series expansions for the stream function of the ring vortex system are obtained. Such expansions explicitly contain the radial and axial velocity components. A Fortran computer code RSOLV has been written to execute the fast solution technique to calculate the stream function and the axial and radial velocity components at points in the flow field. Test cases have been run to optimize the code and to benchmark the truncation errors and CPU time savings associated with the method. Non-dimensional truncation errors for the stream function and total velocity field are on the order of 5 {times} 10{sup {minus}5} and 3 {times} 10{sup {minus}3} respectively. Single precision accuracy produces errors in these quantities up to about 1 {times} 10{sup {minus}5}. For 100 vortices in the field, there is virtually no CPU time savings with the fast solver. For 10,000 vortices in the flow, the fast solver obtains solutions in about 1% to 3% of the time required for the direct solution technique. Simulations of vortices with square and circular cores were run in order to obtain expressions for the self-induced velocities of such vortices. 8 refs., 26 figs.
Date: September 1, 1990
Creator: Strickland, J.H. & Amos, D.E.
Partner: UNT Libraries Government Documents Department

Acoustic damping for explicit calculations of fluid flow at low Mach number

Description: A method is proposed for damping the sound waves in explicit calculations of fluid flow at low Mach number, where sound waves are usually not of interest but may distract attention from other flow features. The method is based on the introduction of an artificial pressure q of the form q = - q/sub 0/rhoc/sup 2/..delta..t(del x u - del x u/sub 0/), where q/sub 0/ is a coefficient of order unity, rho is the density, c is the sound speed, ..delta..t is the time step, and u/sub 0/ is the velocity field that would obtain at zero Mach number. When del x u/sub 0/ is zero, the method becomes equivalent to the use of an artificial bulk viscosity q/sub 0/rhoc/sup 2/..delta..t. However, del x u/sub 0/ can be substantially different from zero in problems with heat or mass sources (e.g., combustion), and its inclusion is then essential to obtain the correct pressure field. The method is well suited for use in conjunction with explicit numerical schemes that employ acoustic subcycling or artificial reduction of the sound speed for improved efficiency at low Mach number. The beneficial effects of the method are illustrated by means of calculations with an acoustic subcycling computer program. 9 refs., 1 fig.
Date: January 1, 1986
Creator: Ramshaw, J.D.; O'Rourke, P.J. & Amsden, A.A.
Partner: UNT Libraries Government Documents Department

Computational method for free surface hydrodynamics

Description: There are numerous flow phenomena in pressure vessel and piping systems that involve the dynamics of free fluid surfaces. For example, fluid interfaces must be considered during the draining or filling of tanks, in the formation and collapse of vapor bubbles, and in seismically shaken vessels that are partially filled. To aid in the analysis of these types of flow phenomena, a new technique has been developed for the computation of complicated free-surface motions. This technique is based on the concept of a local average volume of fluid (VOF) and is embodied in a computer program for two-dimensional, transient fluid flow called SOLA-VOF. The basic approach used in the VOF technique is briefly described, and compared to other free-surface methods. Specific capabilities of the SOLA-VOF program are illustrated by generic examples of bubble growth and collapse, flows of immiscible fluid mixtures, and the confinement of spilled liquids.
Date: January 1, 1980
Creator: Hirt, C.W. & Nichols, B.D.
Partner: UNT Libraries Government Documents Department

Fractional volume of fluid method for free boundary dynamics

Description: The volume of fluid (VOF) technique is presented as a simple and efficient means for numerically treating free boundaries embedded in a calculational mesh of Eulerian or Arbitrary-Lagrangian-Eulerian cells. It is particularly useful because it uses a minimum of stored information, treats intersecting free boundaries automatically, and can be readily extended to three-dimensional calculations.
Date: January 1, 1980
Creator: Nichols, B.D.; Hirt, C.W. & Hotchkiss, R.S.
Partner: UNT Libraries Government Documents Department

Concepts in strong Langmuir turbulence theory

Description: Some of the basic concepts of strong Langmuir turbulence (SLT) theory are reviewed. In SLT system, a major fraction of the turbulent energy is carried by local, time-dependent, nonlinear excitations called cavitons. Modulational instability, localization of Langmuir fields by density fluctuations, caviton nucleation, collapse, and burnout and caviton correlations are reviewed. Recent experimental evidence will be presented for SLT phenomena in the interaction of powerful HF waves with the ionosphere and in laser-plasma interaction experiments. 38 refs., 11 figs.
Date: January 1, 1990
Creator: DuBois, D.F. & Rose, H.A.
Partner: UNT Libraries Government Documents Department

Equation of state of strongly coupled plasma mixtures

Description: Thermodynamic properties of strongly coupled (high density) plasmas of mixtures of light elements have been obtained by Monte Carlo simulations. For an assumed uniform charge background the equation of state of ionic mixtures is a simple extension of the one-component plasma EOS. More realistic electron screening effects are treated in linear response theory and with an appropriate electron dielectric function. Results have been obtained for the ionic pair distribution functions, and for the electric microfield distribution.
Date: February 3, 1984
Creator: DeWitt, H.E.
Partner: UNT Libraries Government Documents Department

Transition of fractal dimension in a latticed dynamical system

Description: We study a recursion relation that manifests two distinct routes to turbulence, both of which reproduce commonly observed phenomena: the Feigenbaum route, with period-doubling frequencies; and a much more general route with noncommensurate frequencies and frequency entrainment, and locking. Intermittency and large-scale aperiodic spatial patterns are reproduced in this new route. In the oscillatory instability regime the fracal dimension saturates at D/sub F/ approx. = 2.6 with imbedding dimensions while in the turbulent regime D/sub F/ saturates at 6.0. 19 refs., 3 figs.
Date: March 1, 1986
Creator: Duong-van, M.
Partner: UNT Libraries Government Documents Department

New approach to multiphase equilibria: application to high-pressure physics problems

Description: A multiphase, multicomponent equation-of-state (EOS) model based on first principles of statistical mechanics is described. The model has been used to study fluid-fluid phase separations in binary (H/sub 2/-He, Ar-Ne, Xe-He, and N/sub 2/-H/sub 2/O) and ternary or more complex systems involving species with C, H, N, and O atoms. Results of these calculations and a brief description of a new theory which can simultaneously describe both solid and fluid EOS properties are given. 26 refs., 4 figs.
Date: June 1, 1985
Creator: Ree, F.H.
Partner: UNT Libraries Government Documents Department

Flow and plasticity via nonequilibrium molecular dynamics

Description: The viscous flow of fluids and the plastic flow of solids, such as metals, are interesting from both the practical and the theoretical points of view. Atomistic molecular dynamics simulations provide a way of visualizing and understanding these flows in a detailed microscopic way. Simulations are necessarily carried out at relatively high rates of strain. For this reason they are ideally suited to the study of nonlinear flow phenomena: normal stresses induced by shear deformation, stress rotation, and the coupling of stress with heat flow, for instance. The simulations require appropriate boundary conditions, forces, and equations of motion. Newtonian mechanics is relatively inefficient for this simulation task. A modification, Nonequilibrium Molecular Dynamics, has been developed to simulate nonequilibrium flows. By now, many high-strain-rate rheological studies of flowing (viscous) fluids and (plastic) solids have been carried out. Here I describe the new methods used in the simulations and some results obtained in this way. A three-body shear-flow exercise is appended to make these ideas more concrete.
Date: June 11, 1984
Creator: Hoover, W.G.
Partner: UNT Libraries Government Documents Department

Hybrid simulation codes with application to shocks and upstream waves

Description: Hybrid codes in which part of the plasma is represented as particles and the rest as fluid are discussed. In the past few years such codes with particle ions and massless, fluid electrons have been applied to space plasmas, especially to collisionless shocks. All of these simulation codes are one-dimensional and similar in structure, except for how the field equation are solved. We describe in detail the various approaches that are used (resistive Ohm's law, predictor-corrector, Hamiltonian) and compare results from the various codes with examples taken from collisionless shocks and low frequency wave phenomena upstream of shocks.
Date: February 3, 1985
Creator: Winske, D.
Partner: UNT Libraries Government Documents Department

PIC (Particle-in-Cell) and its progeny

Description: The Particle-in-Cell (PIC) method was developed to solve numerically for the strongly contorting dynamics of several materials in two or three space dimensions. Related numerical techniques with Lagrangian marker particles are also discussed in this brief survey. An extensive bibliography lists many of the earlier publications describing these techniques.
Date: January 1, 1987
Creator: Harlow, F.H.
Partner: UNT Libraries Government Documents Department

Particle code simulations with injected particles

Description: As problems we are interested in become more complex, we often find our simulations stretching the limits of available computer resources. For example, an interesting problem is simulation of dissipation processes in sub-critical collisionless shocks. To simulate this system our simulation box must contain the shock and its upstream and downstream regions over the entire length of a run. If the shock moves with any appreciable speed the box must then be considerably larger than the shock thickness making it hard to resolve the shock front itself with a reasonable number of grid points. A solution to this problem is to run the simulation in the frame of reference of the shock. Particles are injected upstream of the shock and leave the simulation box downstream. With the shock stationary in the simulation box, we only need to contain enough of the up and downstream regions for the fields, etc., to settle down and separate the shock from the box boundaries. In this tutorial we consider some basic algorithms used in a practical particle injection code, such as the two dimensional WAVE code used at Los Alamos. We will try to present these ideas in a simple format general enough to be easily included in any particle code. Topics covered are: smoothly injecting particles, generating the distribution functions, time dependent injection density, and boundary conditions on fields and particles.
Date: January 1, 1985
Creator: Aldrich, C.H.
Partner: UNT Libraries Government Documents Department

Front tracking and two dimensional Riemann problems: a conference report

Description: A substantial improvement in resolution has been achieved for the computation of jump discontinuities in gas dynamics using the method of front tracking. The essential feature of this method is that a lower dimensional grid is fitted to and follows the discontinuous waves. At the intersection points of these discontinuities, two-dimensional Riemann problems occur. In this paper we study such two-dimensional Riemann problems from both numerical and theoretical points of view. Specifically included is a numerical solution for the Mach reflection, a general classification scheme for two-dimensional elementary waves, and a discussion of problems and conjectures in this area.
Date: January 1, 1984
Creator: Glimm, J.; Klingenberg, C.; McBryan, O.; Plohr, B.; Sharp, D. & Yaniv, S.
Partner: UNT Libraries Government Documents Department

Exact partial solution to the steady-state, compressible fluid flow problems of jet formation and jet penetration

Description: This report treats analytically the problem of the symmetric impact of two compressible fluid streams. The flow is assumed to be steady, plane, inviscid, and subsonic and that the compressible fluid is of the Chaplygin (tangent gas) type. In the analysis, the governing equations are first transformed to the hodograph plane where an exact, closed-form solution is obtained by standard techniques. The distributions of fluid properties along the plane of symmetry as well as the shapes of the boundary streamlines are exactly determined by transforming the solution back to the physical plane. The problem of a compressible fluid jet penetrating into an infinite target of similar material is also exactly solved by considering a limiting case of this solution. This new compressible flow solution reduces to the classical result of incompressible flow theory when the sound speed of the fluid is allowed to approach infinity. Several illustrations of the differences between compressible and incompressible flows of the type considered are presented.
Date: October 1, 1980
Creator: Karpp, R.R.
Partner: UNT Libraries Government Documents Department