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1

Direct Numerical Simulation of Turbulent Flows

Direct numerical simulation (DNS) of turbulent flo ws is reviewed here. Back- ground of DNS is presented and the importance of DNS in turbulence is highlighted. Further, we discuss related numerical issues such as the available methods, bound- ary conditions and spatial and temporal discretizations. Finally, impact of DNS on turbulence modeling is described and the future possibilities of this

Anirudh Modi

2

NASA Astrophysics Data System (ADS)

Conceptually simple and computationally most efficient polygonal computational grains with voids/inclusions are proposed for the direct numerical simulation of the micromechanics of piezoelectric composite/porous materials with non-symmetrical arrangement of voids/inclusions. These are named "Multi-Physics Computational Grains" (MPCGs) because each "mathematical grain" is geometrically similar to the irregular shapes of the physical grains of the material in the micro-scale. So each MPCG element represents a grain of the matrix of the composite and can include a pore or an inclusion. MPCG is based on assuming independent displacements and electric-potentials in each cell. The trial solutions in each MPCG do not need to satisfy the governing differential equations, however, they are still complete, and can efficiently model concentration of electric and mechanical fields. MPCG can be used to model any generally anisotropic material as well as nonlinear problems. The essential idea can also be easily applied to accurately solve other multi-physical problems, such as complex thermal-electro-magnetic-mechanical materials modeling. Several examples are presented to show the capabilities of the proposed MPCGs and their accuracy.

Bishay, Peter L.; Dong, Leiting; Atluri, Satya N.

2014-11-01

3

Scalable DNS code for high Reynolds number channel flow simulation on BG/Q

Scalable DNS code for high Reynolds number channel flow simulation on BG/Q MyoungKyu Lee mk on BG/Q MiraCon Mar, 2013 2 / 35 #12;Project Overview Project Title Petascale Direct Numerical Simulations of Turbulent Channel Flow Goal Expand our understand of wall-bounded turbulence Personnel P

Kemner, Ken

4

Detailed characteristics of drop-laden mixing layers: LES predictions compared to DNS

NASA Technical Reports Server (NTRS)

Results have been compared from Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) of a temporal mixing layer laden with evaporating drops, to assess the ability of LES to reproduce detailed characteristics of DNS.

Okong'o, N.; Leboissetier, A.; Bellan, J.

2004-01-01

5

Turbulent partially premixed combustion: DNS analysis and RANS simulation

) which can lead to formation of acid rain; un-burnt hydrocarbons (UHC), soots and carbon monoxide (CO) are other examples. In recent years, it has been widely recognised that greenhouse gases (GHG), mainly consisting of carbon dioxide (CO2), are having... premixed combustion [142]. . . . . 23 3.1 Schematic of DNS configuration. Instantaneous temperature (K) field in mid z-y plane. Stoichiometric mixture fraction contour Zst = 0.03 (Black solid). . . . . . . . . . . . . . . . . . . . . . . . 40 3...

Ruan, S.

2013-03-12

6

Direct numerical simulation of incompressible turbulent flows

The paper discusses recent achievements of direct numerical simulation (DNS) of incompressible flows. The various spatial discretization techniques which can be used in the case of simple or complex geometry are referred to, along with suitable time advancement schemes. The advantage of using a staggered variable arrangement and efficient Poisson solvers is stressed before initial and boundary conditions for inflow,

R Friedrich; T. J Hüttl; M Manhart; C Wagner

2001-01-01

7

Applications of direct numerical simulation to complex turbulent flows

The technique of direct numerical simulation (DNS) is discussed with emphasis on application to complex turbulent flows. Several\\u000a specific DNS schemes for time-integration of the governing three-dimensional Navier-Stokes equations are briefly described.\\u000a The application of the methods reviewed here involve first the simulation of a spherical particle in a flat-plate boundary\\u000a layer providing insight into the processes of bypass transition.

Sedat Biringen; Robert S. Reichert

1997-01-01

8

Contribution of Direct Numerical Simulation to Understanding and Modelling Turbulent Transport

With the advances in large scale computers, reliable numerical methods and efficient post-processing environment, direct numerical simulation (DNS) has become a valuable and indispensable resource for fundamental turbulence research, although DNS is possible only when the turbulent Reynolds (or Peclet) number remains small to moderate. This paper reviews the contribution that various DNSS have made to understanding and modelling turbulent

Nobuhide Kasagi; Naoki Shikazono

1995-01-01

9

Direct numerical simulation of turbulent flow over a backward-facing step

NASA Astrophysics Data System (ADS)

Turbulent flow in a channel with a sudden expansion(backward-facing step) is studied by direct numerical simulation (DNS) of incompressible Navier-Stokes equations. Initial results are presented for a 3D DNS of a backward-facing step flow with Reynolds number 6000, based on average bulk upstream velocity and step height. The expansion ratio is 2. Turbulent inflow is provided by regeneration of velocity and pressure fields from a plane downstream from the inflow. Simulations are made using the Semtex DNS spectral element solver. The goal is to generate hi-resolution DNS data of a high Reynolds number flow over a backward-facing step for LES comparisons.

Kopera, Michal A.; Cantwell, Christopher; Kerr, Robert M.; Barkley, Dwight; Blackburn, Hugh

2009-11-01

10

LES versus DNS: A comparative study

NASA Technical Reports Server (NTRS)

We have performed Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES) of forced isotropic turbulence at moderate Reynolds numbers. The subgrid scale model used in the LES is based on an eddy viscosity which adjusts instantaneously the energy spectrum of the LES to that of the DNS. The statistics of the large scales of the DNS (filtered DNS field or fDNS) are compared to that of the LES. We present results for the transfer spectra, the skewness and flatness factors of the velocity components, the PDF's of the angle between the vorticity and the eigenvectors of the rate of strain, and that between the vorticity and the vorticity stretching tensor. The above LES statistics are found to be in good agreement with those measured in the fDNS field. We further observe that in all the numerical measurements, the trend was for the LES field to be more gaussian than the fDNS field. Future research on this point is planned.

Shtilman, L.; Chasnov, J. R.

1992-01-01

11

Direct numerical simulation of turbulence\\/radiation interaction in premixed combustion systems

An important fundamental issue in chemically reacting turbulent flows is turbulence\\/radiation interaction (TRI); TRI arises from highly nonlinear coupling between temperature and composition fluctuations. Here, a photon Monte Carlo method for the solution of the radiative transfer equation has been integrated into a turbulent combustion direct numerical simulation (DNS) code. DNS has been used to investigate TRI in a canonical

Y. Wu; D. C. Haworth; M. F. Modest; B. Cuenot

2005-01-01

12

Numerical simulation of turbulent jet primary breakup in Diesel engines

Numerical simulation of turbulent jet primary breakup in Diesel engines Peng Zeng1 Marcus Herrmann and Aerospace Engineering Arizona State University "Micro-Macro Modelling and Simulation of Liquid-Vapour Flows" IRMA Strasbourg, 23.Jan.2008 #12;Introduction DNS of Primary Breakup in Diesel Injection Phase

Helluy, Philippe

13

Direct numerical simulation of hot jets

NASA Technical Reports Server (NTRS)

The ultimate motivation of this work is to investigate the stability of two dimensional heated jets and its implications for aerodynamic sound generation from data obtained with direct numerical simulations (DNS). As pointed out in our last report, these flows undergo two types of instabilities, convective or absolute, depending on their temperature. We also described the limits of earlier experimental and theoretical studies and explained why a numerical investigation could give us new insight into the physics of these instabilities. The aeroacoustical interest of these flows was also underlined. In order to reach this goal, we first need to succeed in the DNS of heated jets. Our past efforts have been focused on this issue which encountered several difficulties. Our numerical difficulties are directly related to the physical problem we want to investigate since these absolutely or almost absolutely unstable flows are by definition very sensitive to the smallest disturbances and are very likely to reach nonlinear saturation through a numerical feedback mechanism. As a result, it is very difficult to compute a steady laminar solution using a spatial DNS. A steady state was reached only for strongly co-flowed jets, but these flows are almost equivalent to two independent mixing layers. Thus they are far from absolute instability and have much lower growth rates.

Jacob, Marc C.

1993-01-01

14

DNS of Compressible Turbulent Flows

\\u000a In the first part of this paper direct numerical simulation (DNS) is used to explore similarities and differences between\\u000a fully developed supersonic turbulent plane channel and axisymmetric pipe flow bounded by isothermal walls. The comparison\\u000a is based on equal friction Mach and Reynolds numbers. In the second part a comparison between supersonic turbulent nozzle\\u000a and diffuser flow is undertaken based

Rainer Ghosh; Somnath Ghosh

15

Prediction of dynamic and mixing characteristics of drop-laden mixing layers using DNS and LES

NASA Technical Reports Server (NTRS)

Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) have been conducted of a temporal mixing layer laden with evaporating drops, in order to assess the ability of LES to reproduce dynamic and mixing aspects of the DNS which affect combustion, independently of combustion models.

Okong'o, N.; Leboissetier, A.; Bellan, J.

2004-01-01

16

Large eddy simulations and direct numerical simulations of high speed turbulent reacting flows

NASA Technical Reports Server (NTRS)

This research is involved with the implementation of advanced computational schemes based on large eddy simulations (LES) and direct numerical simulations (DNS) to study the phenomenon of mixing and its coupling with chemical reactions in compressible turbulent flows. In the efforts related to LES, a research program to extend the present capabilities of this method was initiated for the treatment of chemically reacting flows. In the DNS efforts, the focus is on detailed investigations of the effects of compressibility, heat release, and non-equilibrium kinetics modelings in high speed reacting flows. Emphasis was on the simulations of simple flows, namely homogeneous compressible flows, and temporally developing high speed mixing layers.

Givi, Peyman; Madnia, Cyrus K.; Steinberger, Craig J.

1990-01-01

17

The advent of petascale computing applied to direct numerical simulation (DNS) of turbulent combustion has transformed our ability to interrogate fine-grained ‘turbulence-chemistry’ interactions in canonical and laboratory configurations. In particular, three-dimensional DNS, at moderate Reynolds numbers and with complex chemistry, is providing unprecedented levels of detail to isolate and reveal fundamental causal relationships between turbulence, mixing and reaction. This information

Jacqueline H. Chen

2011-01-01

18

Direct Numerical Simulation Modeling of Bilayer Cathode Catalyst Layers in Polymer Electrolyte Fuel CL of a polymer electrolyte fuel cell using a direct numerical simulation DNS model is presented. Two- mance limitation in polymer electrolyte fuel cells PEFCs is still primarily owing to the sluggish

19

Terascale Direct Numerical Simulations of Turbulent Combustion: Capabilities and Limits (PReSS Talk)

The rapid growth in computational capabilities has provided great opportunities for direct numerical simulations (DNS) of turbulent combustion, a type of simulations without any turbulence model. With the help of terascale high performance supercomputing (HPC) resources, we are now able to provide fundamental insight into turbulence-chemistry interaction in simple laboratory-scale turbulent flames with detailed chemistry using three-dimensional (3D) DNS. However, the actual domain size of 3D-DNS is still limited within {approx} O(10 cm{sup 3}) due to its tremendously high grid resolution required to resolve the smallest turbulent length scale as well as flame structures. Moreover, 3D-DNS will require more computing powers to investigate next-generation engines, of which operating conditions will be characterized by higher pressures, lower temperatures, and higher levels of dilution. In this talk, I will discuss the capabilities and limits of DNS of turbulent combustion and present some results of ignition/extinction characteristics of a highly diluted hydrogen flame counter-flowing against heated air. The results of our recent 3D-DNS of a spatially-developing turbulent lifted hydrogen jet flame in heated coflow will also be presented. The 3D-DNS was performed at a jet Reynolds number of 11,000 with {approx} 1 billion grid points, which required 3.5 million CPU hours on Cray XT3/XT4 at Oak Ridge National Laboratories.

Yoo, Chun Sang (Combustion Research Facility, SNL) [Combustion Research Facility, SNL

2009-03-26

20

Equations for direct numerical simulation of sound propagation in a moving atmosphere

Most previous analytical and numerical studies of sound propagation in a moving atmosphere have been based on wave equations for the sound pressure and on various parabolic approximations to the wave equations. However, these equations cannot be used as starting equations for recently proposed direct numerical simulation (DNS) of sound propagation outdoor since such starting equations should be first-order differential

Vladimir E. Ostashev; Lanbo Liu; D. Keith Wilson; Mark L. Moran; David F. Aldridge; David Marlin

2003-01-01

21

PDF turbulence modeling and DNS

NASA Technical Reports Server (NTRS)

The problem of time discontinuity (or jump condition) in the coalescence/dispersion (C/D) mixing model is addressed in probability density function (pdf). A C/D mixing model continuous in time is introduced. With the continuous mixing model, the process of chemical reaction can be fully coupled with mixing. In the case of homogeneous turbulence decay, the new model predicts a pdf very close to a Gaussian distribution, with finite higher moments also close to that of a Gaussian distribution. Results from the continuous mixing model are compared with both experimental data and numerical results from conventional C/D models. The effect of Coriolis forces on compressible homogeneous turbulence is studied using direct numerical simulation (DNS). The numerical method used in this study is an eight order compact difference scheme. Contrary to the conclusions reached by previous DNS studies on incompressible isotropic turbulence, the present results show that the Coriolis force increases the dissipation rate of turbulent kinetic energy, and that anisotropy develops as the Coriolis force increases. The Taylor-Proudman theory does apply since the derivatives in the direction of the rotation axis vanishes rapidly. A closer analysis reveals that the dissipation rate of the incompressible component of the turbulent kinetic energy indeed decreases with a higher rotation rate, consistent with incompressible flow simulations (Bardina), while the dissipation rate of the compressible part increases; the net gain is positive. Inertial waves are observed in the simulation results.

Hsu, A. T.

1992-01-01

22

DNS simulations of lean premixed flame kernels with flamelet generated manifolds; an overview

In this paper recent developments in the application of the method of ?amelet generated manifolds (FGM) to simulations of turbulent premixed combustion are consid- ered. Initially the method was developed for kinetic reduction for simulations of laminar combustion using stationary solvers. Recently the method was extended to direct numer- ical simulations of turbulent combustion. A lean premixed ?ame kernel was

Rob J. M. Bastiaans; Jeroen A. van Oijen; Goey de LPH

2006-01-01

23

Direct Numerical Simulation and Theories of Wall Turbulence with a Range of Pressure Gradients

NASA Technical Reports Server (NTRS)

A new Direct Numerical Simulation (DNS) of Couette-Poiseuille flow at a higher Reynolds number is presented and compared with DNS of other wall-bounded flows. It is analyzed in terms of testing semi-theoretical proposals for universal behavior of the velocity, mixing length, or eddy viscosity in pressure gradients, and in terms of assessing the accuracy of two turbulence models. These models are used in two modes, the traditional one with only a dependence on the wall-normal coordinate y, and a newer one in which a lateral dependence on z is added. For pure Couette flow and the Couette-Poiseuille case considered here, this z-dependence allows some models to generate steady streamwise vortices, which generally improves the agreement with DNS and experiment. On the other hand, it complicates the comparison between DNS and models.

Coleman, G. N.; Garbaruk, A.; Spalart, P. R.

2014-01-01

24

We devise a stochastic model for the effects of breaking waves and fit its distribution functions to laboratory and field data. This is used to represent the space time structure of momentum and energy forcing of the oceanic boundary layer in turbulence-resolving simulations. The aptness of this breaker model is evaluated in a direct numerical simulation (DNS) of an otherwise

Peter P. Sullivan; James C. McWilliams; W. Kendall Melville

2004-01-01

25

Rocket engine numerical simulator

NASA Technical Reports Server (NTRS)

The topics are presented in viewgraph form and include the following: a rocket engine numerical simulator (RENS) definition; objectives; justification; approach; potential applications; potential users; RENS work flowchart; RENS prototype; and conclusion.

Davidian, Ken

1993-01-01

26

Rocket engine numerical simulation

NASA Technical Reports Server (NTRS)

The topics are presented in view graph form and include the following: a definition of the rocket engine numerical simulator (RENS); objectives; justification; approach; potential applications; potential users; RENS work flowchart; RENS prototype; and conclusions.

Davidian, Ken

1993-01-01

27

\\u000a This chapter describes the operation of a DNS system. Namely, you’ll learn about the following topics:\\u000a \\u000a \\u000a \\u000a \\u000a • \\u000a \\u000a \\u000a DNS queries: How does your browser find www.example.com? How does your mail software know where to send your outgoing e-mail? Such operations\\u000a use DNS queries.\\u000a \\u000a \\u000a \\u000a \\u000a • \\u000a \\u000a \\u000a Reverse mapping: How does your mail software determine your identity? How do you find out who is

Ron Aitchison

28

Direct Numerical Simulations of Multiphase Flows

NASA Astrophysics Data System (ADS)

Many natural and industrial processes, such as rain and gas exchange between the atmosphere and oceans, boiling heat transfer, atomization and chemical reactions in bubble columns, involve multiphase flows. Often the mixture can be described as a disperse flow where one phase consists of bubbles or drops. Direct numerical simulations (DNS) of disperse flow have recently been used to study the dynamics of multiphase flows with a large number of bubbles and drops, often showing that the collective motion results in relatively simple large-scale structure. Here we review simulations of bubbly flows in vertical channels where the flow direction, as well as the bubble deformability, has profound implications on the flow structure and the total flow rate. Results obtained so far are summarized and open questions identified. The resolution for DNS of multiphase flows is usually determined by a dominant scale, such as the average bubble or drop size, but in many cases much smaller scales are also present. These scales often consist of thin films, threads, or tiny drops appearing during coalescence or breakup, or are due to the presence of additional physical processes that operate on a very different time scale than the fluid flow. The presence of these small-scale features demand excessive resolution for conventional numerical approaches. However, at small flow scales the effects of surface tension are generally strong so the interface geometry is simple and viscous forces dominate the flow and keep it simple also. These are exactly the conditions under which analytical models can be used and we will discuss efforts to combine a semi-analytical description for the small-scale processes with a fully resolved simulation of the rest of the flow. We will, in particular, present an embedded analytical description to capture the mass transfer from bubbles in liquids where the diffusion of mass is much slower than the diffusion of momentum. This results in very thin mass-boundary layers that are difficult to resolve, but the new approach allows us to simulate the mass transfer from many freely evolving bubbles and examine the effect of the interactions of the bubbles with each other and the flow. We will conclude by attempting to summarize the current status of DNS of multiphase flows.

Tryggvason, Gretar

2013-03-01

29

Direct numerical simulation of a statistically stationary, turbulent reacting flow

NASA Astrophysics Data System (ADS)

An inhomogeneous, non-premixed, stationary, turbulent, reacting model flow that is accessible to direct numerical simulation (DNS) is described for investigating the effects of mixing on reaction and for testing mixing models. The mixture-fraction-progress-variable approach of Bilger is used, with a model, finite-rate, reversible, single-step thermochemistry, yielding non-trivial stationary solutions corresponding to stable reaction and also allowing local extinction to occur. There is a uniform mean gradient in the mixture fraction, which gives rise to stationarity as well as a flame brush. A range of reaction zone thicknesses and Damkohler numbers are examined, yielding a broad spectrum of behaviour, including thick and thin flames, local extinction and near equilibrium. Based on direct numerical simulations, results from the conditional moment closure (CMC) and the quasi-equilibrium distributed reaction (QEDR) model are evaluated. Large intermittency in the scalar dissipation leads to local extinction in the DNS. In regions of the flow where local extinction is not present, CMC and QEDR based on the local scalar dissipation give good agreement with the DNS.

Overholt, M. R.; Pope, S. B.

1999-06-01

30

ACIDBASE TITRATION NUMERICAL SIMULATOR

The analytical methods of qualitative and quantitative determination of ions in solutions are very flexible to automation. The present work is focus on modeling the process of titration and presents a numerical simulation of acid-base titration. A PHP program to compute all iterations in titration process that solves a 3 rd rank equation to find value of pH for was

Lorentz JÄNTSCHI; Delia GLIGOR; Mihaela Ligia

2003-01-01

31

NASA Astrophysics Data System (ADS)

Experimental observation of particle clustering in turbulent flow is often complicated by particles of non-uniform in size, and the accuracy of most diagnostic techniques is sensitive to the particle size. To make meaningful comparisons with experiments direct numerical simulations (DNS) must take into consideration the properties of the particles and model the limitations introduced by the diagnostics. We present a series of DNS of inertial particles in turbulence that were designed to match the measurements of Cao et al. In their experiments, the particle radial distribution function (RDF) was obtained from 3-D holographic images of particles in a turbulence box. DNS was done using the same particle size distribution as the particles in the experiments. The parameters of the flow (Re #) and the particles (St #) were matched. In the DNS, we eliminated particles below a specified size and recalculated the RDF to better match the experiments. Trends in the variation of the RDF with size cutoff were found to be non-intuitive, but can be explained based on an extension of the theory of Chun et al. (2005) to polydisperse particles. We optimized the cutoff size based on the comparison of the experimental and numerical RDF at one fan speed and applied the same cutoff to other fan speeds. The results showed good agreement. The comparison highlights the complexity of matching DNS and experimental observations.

Salazar, Juan; de Jong, Jeremy; Cao, Lujie; Woodward, Scott; Meng, Hui; Collins, Lance

2006-11-01

32

Towards Petascale DNS of High Reynolds-Number Turbulent Boundary Layer

NASA Astrophysics Data System (ADS)

In flight vehicles, a large portion of fuel consumption is due to skin-friction drag. Reduction of this drag will significantly reduce the fuel consumption of flight vehicles and help our nation to reduce CO 2 emissions. In order to reduce skin-friction drag, an increased understanding of wall-turbulence is needed. Direct numerical simulation (DNS) of spatially developing turbulent boundary layers (SDTBL) can provide the fundamental understanding of wall-turbulence in order to produce models for Reynolds averaged Navier-Stokes (RANS) and large-eddy simulations (LES). DNS of SDTBL over a flat plate at Retheta = 1430 - 2900 were performed. Improvements were made to the DNS code allowing for higher Reynolds number simulations towards petascale DNS of turbulent boundary layers. Mesh refinement and improvements to the inflow and outflow boundary conditions have resulted in turbulence statistics that match more closely to experimental results. The Reynolds stresses and the terms of their evolution equations are reported.

Webster, Keegan R.

33

Semiconvection: numerical simulations

NASA Astrophysics Data System (ADS)

A grid of numerical simulations of double-diffusive convection is presented for the astrophysical case where viscosity (Prandtl number Pr) and solute diffusivity (Lewis number Le) are much lower than the thermal diffusivity. As in laboratory and geophysical cases, convection takes place in a layered form. The proper translation of subsonic flows in a stellar interior and an incompressible (Boussinesq) fluid is given, and the validity of the Boussinesq approximation for the semiconvection problem is checked by comparison with fully compressible simulations. The predictions of a simplified theory of mixing in semiconvection given in a companion paper are tested against the numerical results, and used to extrapolate these to astrophysical conditions. The predicted effective He-diffusion coefficient is nearly independent of the double-diffusive layering thickness d. For a fiducial main sequence model (15 M?) the inferred mixing time scale is of the order of 1010 yr. An estimate for the secular increase in d during the semiconvective phase is given. It can potentially reach a significant fraction of the pressure scale height. Movies associated to Figs. 5 and 7 are available in electronic form at http://www.aanda.org

Zaussinger, F.; Spruit, H. C.

2013-06-01

34

Entropy Splitting for High Order Numerical Simulation of Compressible Turbulence

NASA Technical Reports Server (NTRS)

A stable high order numerical scheme for direct numerical simulation (DNS) of shock-free compressible turbulence is presented. The method is applicable to general geometries. It contains no upwinding, artificial dissipation, or filtering. Instead the method relies on the stabilizing mechanisms of an appropriate conditioning of the governing equations and the use of compatible spatial difference operators for the interior points (interior scheme) as well as the boundary points (boundary scheme). An entropy splitting approach splits the inviscid flux derivatives into conservative and non-conservative portions. The spatial difference operators satisfy a summation by parts condition leading to a stable scheme (combined interior and boundary schemes) for the initial boundary value problem using a generalized energy estimate. A Laplacian formulation of the viscous and heat conduction terms on the right hand side of the Navier-Stokes equations is used to ensure that any tendency to odd-even decoupling associated with central schemes can be countered by the fluid viscosity. A special formulation of the continuity equation is used, based on similar arguments. The resulting methods are able to minimize spurious high frequency oscillation producing nonlinear instability associated with pure central schemes, especially for long time integration simulation such as DNS. For validation purposes, the methods are tested in a DNS of compressible turbulent plane channel flow at a friction Mach number of 0.1 where a very accurate turbulence data base exists. It is demonstrated that the methods are robust in terms of grid resolution, and in good agreement with incompressible channel data, as expected at this Mach number. Accurate turbulence statistics can be obtained with moderate grid sizes. Stability limits on the range of the splitting parameter are determined from numerical tests.

Sandham, N. D.; Yee, H. C.; Kwak, Dochan (Technical Monitor)

2000-01-01

35

Particle-Resolved Direct Numerical Simulation for Gas-Solid Flow Model Development

NASA Astrophysics Data System (ADS)

Gas-solid flows in nature and industrial applications are characterized by multiscale and nonlinear interactions that manifest as rich flow physics and pose unique modeling challenges. In this article, we review particle-resolved direct numerical simulation (PR-DNS) of the microscale governing equations for understanding gas-solid flow physics and obtaining quantitative information for model development. A clear connection between a microscale realization and meso/macroscale representation is necessary for PR-DNS to be used effectively for model development at the meso- and macroscale. Furthermore, the design of PR-DNS must address the computational challenges of parameterizing models in a high-dimensional parameter space and obtaining accurate statistics of flow properties from a finite number of realizations at acceptable grid resolution. This review also summarizes selected recent insights into the physics of momentum, kinetic energy, and heat transfer in gas-solid flows obtained from PR-DNS. Promising future applications of PR-DNS include the study of the effect of number fluctuations on hydrodynamics, instabilities in gas-solid flow, and wall-bounded flows.

Tenneti, Sudheer; Subramaniam, Shankar

2014-01-01

36

Direct Numerical Simulation of Transitional Turbulent Flow in a Closed Rotor-Stator Cavity

The transitional turbulent regime in confined flow between a rotating and a stationary disc is studied using direct numerical\\u000a simulation. Besides its fundamental importance as a three-dimensional prototype flow, such flows frequently arise in many\\u000a industrial devices, especially in turbomachinary applications. The present contribution extends the DNS simulation into the\\u000a turbulent flow regime, to a rotational Reynolds number Re =3

E. Serre; P. Bontoux; B. E. Launder

2002-01-01

37

Numerical Simulations of Fission

NASA Astrophysics Data System (ADS)

In this paper, we use the term fission to refer to the breakup of an equilibrium celestial body driven by rapid rotation. Historically, it was conjectured that fission would lead to splitting of a body directly into two or more pieces. Numerical hydrodynamic simulation techniques have now become sufficiently powerful to study the outcome of dynamic fission instabilities. We summarize recent work and present new simulations spanning a range of rotation rates and fluid compressibility. In the best resolved cases dynamic fission instability always leads to ejection of a ring or disk of debris rather thin one or a few discrete bodies. In this case, just as in most other lunar origin theories, a fission-product Moon must accrete out of a geocentric swarm of material. Intrinsic nonaxisymmetry of the remnant Earth after fission would prevent rapid recollapse of the swarm. The revised picture aleviates some of the problems associated with earlier versions of the fission theory. The two most serious remaining objections are that it is difficult to make the proto-Earth rotate fast enough to undergo fission and that the proto-Earth must be largely molten at the time it fissions. To overcome the first objection, it may be necessary to combine fission with the planetesimal impact theory. Some advantages of such a hybrid theory are discussed. The second objection cannot be fully assessed until more is known about the fission history and accretion of the proto-Earth.

Durisen, Richard H.; Gingold, Robert A.

1987-01-01

38

Direct numerical simulation of shockwave and turbulent boundary layer interactions

NASA Astrophysics Data System (ADS)

Direct numerical simulations (DNS) of a shockwave/turbulent boundary layer interaction (STBLI) at Mach number 3 and Reynolds number based on the momentum thickness of 2300 are performed. A 4th-order accurate, bandwidth-optimized weighted-essentially-non-oscillatory (WENO) scheme is used and the method is found to be too dissipative for the STBLI simulation due to the over-adaptation properties of this original WENO scheme. In turn, a relative limiter is introduced to mitigate the problem. Tests on the Shu-Osher problem show that the modified WENO scheme decreases the numerical dissipation significantly. By utilizing a combination of the relative limiter and the absolute limiter described by Jiang & Shu [32], the DNS results are improved further. The DNS data agree well with the reference experiments of Bookey et al. [10] in the size of the separation bubble, the separation and reattachment point, the mean wall-pressure distribution, and the velocity profiles both upstream and downstream of the interaction region. The DNS data show that velocity profiles change dramatically along the streamwise direction. Downstream of the interaction, the velocity profiles show a characteristic "dip" in the logarithmic region, as shown by the experiments of Smits & Muck [66] at much higher Reynolds number. In the separation region, the velocity profiles are found to resemble those of a laminar flow, yet the flow does not fully relaminarize. The mass-flux turbulence intensity is amplified by a factor of about 5 throughout the interaction, which is consistent with that found in higher Reynolds experiments of Selig et al. [52]. All Reynolds stress components are greatly amplified by the interaction. Assuming that the ow is still two dimensional downstream of the interaction, the components rhou'u', rhov'v', rho w'w', and rho u'w' are amplified by factors of 6, 6, 12, and 24, respectively, where u is the streamwise and w is the wall-normal velocity. However, analyses of the turbulence structure show that the ow is not uniform in the spanwise direction downstream of the interaction. A pair of counter-rotating vortices is observed in streamwise-wall-normal planes in the mean ow downstream of the ramp corner. Taking the three-dimensionality into account, the amplification factors of the Reynolds stresses are greatly decreased. The component rhou'w' is amplified by a factor of about 10, which is comparable to that found in the experiments of Smits & Muck [66]. Strong Reynolds analogy (SRA) relations are also studied using the DNS data. The SRA is found to hold in the incoming boundary layer of the DNS. However, inside and downstream of the interaction region, the SRA relations are not satisfied. From the DNS analyses, the shock motion is characterized by a low frequency component (of order 0.01Uinfinity/delta). In addition, the motion of the shock is found to have two aspects: a spanwise wrinkling motion and a streamwise oscillatory motion. The spanwise wrinkling is observed to be a local feature with high frequencies (of order Uinfinity /delta). Two-point correlations reveal that the spanwise wrinkling is closely related to the low momentum motions in the incoming boundary layer as they convect through the shock. The low frequency shock motion is found to be a streamwise oscillation motion. Conditional statistics show that there is no significant difference in the mean properties of the incoming boundary layer when the shock is at an upstream or downstream location. However, analyses of the unsteadiness of the separation bubble reveal that the low frequency shock motion is driven by the downstream flow.

Wu, Minwei

39

Direct numerical simulation of turbulent reacting flows

The development of turbulent combustion models that reflect some of the most important characteristics of turbulent reacting flows requires knowledge about the behavior of key quantities in well defined combustion regimes. In turbulent flames, the coupling between the turbulence and the chemistry is so strong in certain regimes that is is very difficult to isolate the role played by one individual phenomenon. Direct numerical simulation (DNS) is an extremely useful tool to study in detail the turbulence-chemistry interactions in certain well defined regimes. Globally, non-premixed flames are controlled by two limiting cases: the fast chemistry limit, where the turbulent fluctuations. In between these two limits, finite-rate chemical effects are important and the turbulence interacts strongly with the chemical processes. This regime is important because industrial burners operate in regimes in which, locally the flame undergoes extinction, or is at least in some nonequilibrium condition. Furthermore, these nonequilibrium conditions strongly influence the production of pollutants. To quantify the finite-rate chemistry effect, direct numerical simulations are performed to study the interaction between an initially laminar non-premixed flame and a three-dimensional field of homogeneous isotropic decaying turbulence. Emphasis is placed on the dynamics of extinction and on transient effects on the fine scale mixing process. Differential molecular diffusion among species is also examined with this approach, both for nonreacting and reacting situations. To address the problem of large-scale mixing and to examine the effects of mean shear, efforts are underway to perform large eddy simulations of round three-dimensional jets.

Chen, J.H. [Sandia National Laboratories, Livermore, CA (United States)

1993-12-01

40

A fast direct numerical simulation method for characterising hydraulic roughness

We describe a fast direct numerical simulation (DNS) method that promises to directly characterise the hydraulic roughness of any given rough surface, from the hydraulically smooth to the fully rough regime. The method circumvents the unfavourable computational cost associated with simulating high-Reynolds-number flows by employing minimal-span channels (Jimenez & Moin 1991). Proof-of-concept simulations demonstrate that flows in minimal-span channels are sufficient for capturing the downward velocity shift, that is, the Hama roughness function, predicted by flows in full-span channels. We consider two sets of simulations, first with modelled roughness imposed by body forces, and second with explicit roughness described by roughness-conforming grids. Owing to the minimal cost, we are able to conduct DNSs with increasing roughness Reynolds numbers while maintaining a fixed blockage ratio, as is typical in full-scale applications. The present method promises a practical, fast and accurate tool for character...

Chung, Daniel; MacDonald, Michael; Hutchins, Nicholas; Ooi, Andrew

2015-01-01

41

Validation of Direct Numerical Simulations in 3D pore geometries and Large-Eddy Simulations

NASA Astrophysics Data System (ADS)

Numerical solutions of the Navier-Stokes Equations became more popular in recent decades with increasingly accessible and powerful computational resources. Simulations in reconstructed or artificial pore geometries are often performed to gain insight into microscopic fluid flow structures or are used for upscaling quantities of interest, like hydraulic conductivity. A physically adequate representation of pore scale flow fields requires analysis of large domains in combination with turbulence models. We solve incompressible Navier-Stokes Equations in a cubic lattice and cubic close packing of spheres placed in a square duct with Direct Numerical Simulations (DNS) and analyze the validity of the results. The influence of the number of spheres and mesh discretization is investigated for fluid flow up to Reynolds numbers of 5000 based on the spheres' diameter. The numerical simulations are performed with the OpenFOAM open-source CFD software. We statistically investigate spatial and temporal properties of the resulting fluid flow field and its kinetic energy spectra, and compare them to Large-Eddy Simulations (LES) performed for the same geometries. Differences between the DNS and LES are discussed together with upscaled hydraulic properties with respect to the number of spheres and the Reynolds number.

Naumov, Dmitri

2013-04-01

42

NASA Technical Reports Server (NTRS)

Transitional databases from Direct Numerical Simulation (DNS) of three-dimensional mixing layers for single-phase flows and two-phase flows with evaporation are analyzed and used to examine the typical hypothesis that the scalar dissipation Probability Distribution Function (PDF) may be modeled as a Gaussian. The databases encompass a single-component fuel and four multicomponent fuels, two initial Reynolds numbers (Re), two mass loadings for two-phase flows and two free-stream gas temperatures. Using the DNS calculated moments of the scalar-dissipation PDF, it is shown, consistent with existing experimental information on single-phase flows, that the Gaussian is a modest approximation of the DNS-extracted PDF, particularly poor in the range of the high scalar-dissipation values, which are significant for turbulent reaction rate modeling in non-premixed flows using flamelet models. With the same DNS calculated moments of the scalar-dissipation PDF and making a change of variables, a model of this PDF is proposed in the form of the (beta)-PDF which is shown to approximate much better the DNS-extracted PDF, particularly in the regime of the high scalar-dissipation values. Several types of statistical measures are calculated over the ensemble of the fourteen databases. For each statistical measure, the proposed (beta)-PDF model is shown to be much superior to the Gaussian in approximating the DNS-extracted PDF. Additionally, the agreement between the DNS-extracted PDF and the (beta)-PDF even improves when the comparison is performed for higher initial Re layers, whereas the comparison with the Gaussian is independent of the initial Re values. For two-phase flows, the comparison between the DNS-extracted PDF and the (beta)-PDF also improves with increasing free-stream gas temperature and mass loading. The higher fidelity approximation of the DNS-extracted PDF by the (beta)-PDF with increasing Re, gas temperature and mass loading bodes well for turbulent reaction rate modeling.

Selle, L. C.; Bellan, Josette

2006-01-01

43

Direct Numerical Simulation of a Weakly Stratified Turbulent Wake

NASA Technical Reports Server (NTRS)

Direct numerical simulation (DNS) is used to investigate a time-dependent turbulent wake evolving in a stably stratified background. A large initial Froude number is chosen to allow the wake to become fully turbulent and axisymmetric before stratification affects the spreading rate of the mean defect. The uncertainty introduced by the finite sample size associated with gathering statistics from a simulation of a time-dependent flow is reduced, compared to earlier simulations of this flow. The DNS reveals the buoyancy-induced changes to the turbulence structure, as well as to the mean-defect history and the terms in the mean-momentum and turbulence-kinetic-energy budgets, that characterize the various states of this flow - namely the three-dimensional (essentially unstratified), non-equilibrium (or 'wake-collapse') and quasi-two-dimensional (or 'two-component') regimes observed elsewhere for wakes embedded in both weakly and strongly stratified backgrounds. The wake-collapse regime is not accompanied by transfer (or 'reconversion') of the potential energy of the turbulence to the kinetic energy of the turbulence, implying that this is not an essential feature of stratified-wake dynamics. The dependence upon Reynolds number of the duration of the wake-collapse period is demonstrated, and the effect of the details of the initial/near-field conditions of the wake on its subsequent development is examined.

Redford, J. A.; Lund, T. S.; Coleman, Gary N.

2014-01-01

44

Direct numerical simulation of supersonic combustion with finite-rate chemistry

NASA Astrophysics Data System (ADS)

Three-dimensional direct numerical simulations (DNS) of reacting and inert compressible turbulent mixing layers have been performed. The simulations cover convective Mach numbers from subsonic to supersonic. A detailed chemistry mechanism with 9 species and 29 reactions for hydrogen is used in the reacting simulations. Effects of different initial conditions on the structure of the mixing layer, and time required to reach self-similarity are studied. Flame/turbulence interaction is analyzed by studying turbulent kinetic energy, Reynolds stresses, and their budgets in the reacting and inert simulations. The effects of different reactions on the heat release and mixture composition especially in the regions where shocklets impinge the flame are studied. These DNS databases will provide a better understanding for the compressibility effects on the combustion, and will be used to assess the accuracy of Flamelet/Progress variable approach in supersonic regime.

Saghafian, Amirreza; Pitsch, Heinz

2011-11-01

45

Numerical simulation of dusty plasmas

The numerical simulation of physical processes in dusty plasmas is reviewed, with emphasis on recent results and unresolved issues. Three areas of research are discussed: grain charging, weak dust-plasma interactions, and strong dust-plasma interactions. For each area, we review the basic concepts that are tested by simulations, present some appropriate examples, and examine numerical issues associated with extending present work.

Winske, D.

1995-09-01

46

Numerical Simulation of Transpiration Cooling

Numerical Simulation of Transpiration Cooling through Porous Material Wolfgang Dahmen 1 , Thomas Gotzen 1 and Siegfried MÂ¨uller 1 Bericht Nr. 374 September 2013 Key words: Transpiration cooling, porous School AICES (GSC 111). #12;Numerical simulation of transpiration cooling through porous material W

47

Numerical Simulations in Cosmology I

The purpose of these lectures is to give a short introduction into a very vast field of numerical simulations for cosmological applications. I focus on major features of the simulations: the equations, main numerical techniques, effects of resolution, and methods of halo identification.

A. Klypin

1996-05-30

48

Turbulence analysis of rough wall channel flows based on direct numerical simulation

Direct numerical simulation (DNS) of rough wall channel flows was performed for various surface roughnesses. The goal of the presented research is to investigate the effect of nucleating bubbles in subcooled boiling conditions on the turbulence. The nucleating bubbles are represented by hemispherical roughness elements at the wall. The stabilized finite element based code, PHASTA, is used to perform the simulations. Validation against theoretical, experimental and numerical data is performed for smooth channel flow and rectangular rod type of roughness. The presence of roughness elements affects the flow structure within the roughness sublayer, which is estimated to be 5 times the height of roughness elements. DNS observations are consistent with this result and demonstrate the flow homogeneity above 50 viscous units. The influence of roughness elements layout and density on the turbulence parameters is also demonstrated and analyzed. (authors)

Mishra, A. V.; Bolotnov, I. A. [Dept. of Nuclear Engineering, North Carolina State Univ., Campus Box 7909, Raleigh, NC 27695-7909 (United States)

2012-07-01

49

Results are used from direct numerical simulation (DNS) of incompressible plane-channel flow subjected to a uniform straining field typical of a two-dimensional adverse pressure gradient (APG) to investigate the accuracy of three second-moment closures specially designed to account for wall-bounded turbulence. Since the DNS statistics satisfy a one-dimensional unsteady problem with rigorously defined boundary and initial conditions, and since the

M. A. Sciberras; G. N. Coleman

2007-01-01

50

Direct numerical simulation of compressible homogeneous turbulence using natural initial conditions

NASA Astrophysics Data System (ADS)

Reynolds averaged Navier Stokes (RANS) solvers have become the workhorse for simulating turbulent flows for most practical purposes. While the incompressible turbulence models used with RANS equations have improved considerably in their predictive capability, significant breakthrough has not been achieved for their compressible counterparts. With the advancement in computing power, high resolution direct numerical simulation (DNS) of low Reynolds number turbulent flows has become feasible. DNS of simple turbulent flows provides a detailed database which can be used for developing and testing turbulence models. In this work, we perform DNS of compressible homogeneous turbulence---decaying isotropic turbulence and homogeneous shear flow---for a range of initial turbulent Mach numbers, (Mt 0 = 0.05--0.4) using the more natural initial conditions. Simulations were performed on grids with 1283 and 2563 points. Compressibility effects on the evolution of turbulent kinetic energy were studied. We found negligible compressibility effects for decaying isotropic turbulence, while homogeneous shear flow demonstrated compressibility effects in the growth rate of turbulent kinetic energy. Compressibility corrections to turbulence models in the form of the ratio epsilond/epsilon s, have been tested with the results from the simulations. For decaying isotropic turbulence a M4t scaling was found to be better than M2t while for homogeneous shear flow it was the opposite. The small value of the ratio epsilond/epsilons in decaying isotropic turbulence makes the M4t scaling less relevant. Based on the DNS results of homogeneous shear flow, a new correction parameterized by the gradient Mach number, Mg, is proposed. The parameter Cmu, which is assumed constant for incompressible two equation eddy viscosity models, is computed explicitly from the DNS data. An Mg, dependence of the parameter, Cmu, is proposed.

Bhutoria, Vaibhav

51

Statistically planar turbulent premixed and partially premixed flames for different initial turbulence intensities are simulated\\u000a for global equivalence ratios ??>?=?0.7 and ??>?=?1.0 using three-dimensional Direct Numerical Simulations (DNS) with simplified chemistry. For the simulations of partially\\u000a premixed flames, a random distribution of equivalence ratio following a bimodal distribution of equivalence ratio is introduced\\u000a in the unburned reactants ahead of the

Sean P. Malkeson; Nilanjan Chakraborty

2011-01-01

52

Numerical simulation of hydraulic fracturing

NUMERICAL SIMULATION OF HYDRAULIC FRACTURING A Thesis by JOSEPH BARNES WARNER Submitted to the Graduate College of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE May 1987 Maj or Subj ect...: Petroleum Engineering NUMERICAL SIMULATION OF HYDRAULIC FRACTURING A Thesis by JOSEPH BARNES WARNER Approved as to style and content by: S. A. Holditch (Chairman of Committee) D. D. Van Fleet (member) J. E. Russell (m be ) W. D. Von onten ( ead...

Warner, Joseph Barnes

1987-01-01

53

Inertial particles in a shearless mixing layer: direct numerical simulations

NASA Astrophysics Data System (ADS)

Entrainment, the drawing in of external fluid by a turbulent flow, is present in nearly all turbulent processes, from exhaust plumes to oceanic thermoclines to cumulus clouds. While the entrainment of fluid and of passive scalars in turbulent flows has been studied extensively, comparatively little research has been undertaken on inertial particle entrainment. We explore entrainment of inertial particles in a shearless mixing layer across a turbulent-non-turbulent interface (TNI) and a turbulent-turbulent interface (TTI) through direct numerical simulation (DNS). Particles are initially placed on one side of the interface and are advanced in time in decaying turbulence. Our results show that the TTI is more efficient in mixing droplets than the TNI. We also find that without the influence of gravity, over the range of Stokes numbers present in cumulus clouds, particle concentration statistics are essentially independent of the dissipation scale Stokes number. The DNS data agrees with results from experiments performed in a wind tunnel with close parametric overlap. We anticipate that a better understanding of the role of gravity and turbulence in inertial particle entrainment will lead to improved cloud evolution predictions and more accurate climate models. Sponsored by the U.S. NSF.

Ireland, Peter; Collins, Lance

2010-11-01

54

NASA Astrophysics Data System (ADS)

The mechanism of large-scale dynamos in rigidly rotating stratified convection is explored by direct numerical simulations (DNS) in Cartesian geometry. A mean-field dynamo model is also constructed using turbulent velocity profiles consistently extracted from the corresponding DNS results. By quantitative comparison between the DNS and our mean-field model, it is demonstrated that the oscillatory ?2 dynamo wave, excited and sustained in the convection zone, is responsible for large-scale magnetic activities such as cyclic polarity reversal and spatiotemporal migration. The results provide strong evidence that a nonuniformity of the ?-effect, which is a natural outcome of rotating stratified convection, can be an important prerequisite for large-scale stellar dynamos, even without the ?-effect.

Masada, Youhei; Sano, Takayoshi

2014-10-01

55

Numerical simulation of fracture

A constructive model for brittle, and quasi-brittle materials is described. The Bedded Crack Model contains a microphysical description of fracture based on Griffith theory. The effect of cracks on material properties is described by effective modulus theory. Underlying the model is a statistical framework in which the evolution in time of a statistical distribution of cracks is calculated. The theory upon which the model is based is described. The model is implemented in a finite difference computer code. Our model is contrasted with the phenomenologic models usually found in computer codes. A computational simulation of the strain rate dependence of failure stress is presented and compared with laboratory data. A simulation of a gas gun experiment is presented, and the mechanism of spall described.

Margolin, L.G.

1983-01-01

56

A DNS study of turbulent mixing of two passive scalars A. Junejaa)

of turbulent motion is its ability to mix and to transport passive scalars at rates much higher than those dueA DNS study of turbulent mixing of two passive scalars A. Junejaa) and S. B. Pope Sibley School; accepted 25 April 1996 We employ direct numerical simulations to study the mixing of two passive scalars

57

EXAMPLES OF THE POTENTIAL OF DNS FOR THE UNDERSTANDING OF REACTIVE

in the combustion chamber is today a necessary stage when developing or improving engines. Model development is 1 of the multiphase flow DNS (Direct Numerical Simulation) to help to understand basic physics and to interpret some to experimental results. It includes droplet dispersion, lami- nar spray flame instability, spray combustion

Paris-Sud XI, Université de

58

Requirements definition by numerical simulation

NASA Astrophysics Data System (ADS)

We are investigating the issues involved in requirements definition for narcotics interdiction: how much of a particular signature is possible, how does this amount change for different conditions, and what is the temporal relationship in various scenarios. Our approach has been to simulate numerically the conditions that arise during vapor or particulate transport. The advantages of this approach are that (1) a broad range of scenarios can be rapidly and inexpensively analyzed by simulation, and (2) simulations can display quantities that are difficult or impossible to measure. The drawback of this approach is that simulations cannot include all of the phenomena present in a real measurement, and therefore the fidelity of the simulation results is always an issue. To address this limitation, we will ultimately combine the results of numerical simulations with measurements of physical parameters for inclusion in the simulation. In this paper, we discuss these issues and how they apply to the current problems in narcotics interdictions, especially cargo containers. We also show the results of 1D and 3D numerical simulations, and compare these results with analytical solutions. The results indicate that this approach is viable. We also present data from 3D simulations of vapor transport in a loaded cargo container and some of the issues present in this ongoing work.

Hickman, James J.; Kostas, Chris; Tsang, Kang T.

1994-10-01

59

DNS of MHD turbulent flow via the HELIOS supercomputer system at IFERC-CSC

NASA Astrophysics Data System (ADS)

The simulation plays an important role to estimate characteristics of cooling in a blanket for such high heating plasma in ITER-BA. An objective of this study is to perform large -scale direct numerical simulation (DNS) on heat transfer of magneto hydro dynamic (MHD) turbulent flow on coolant materials assumed from Flibe to lithium. The coolant flow conditions in ITER-BA are assumed to be Reynolds number and Hartmann number of a higher order. The maximum target of the DNS assumed by this study based on the result of the benchmark of Helios at IFERC-CSC for Project cycle 1 is 116 TB (2048 nodes). Moreover, we tested visualization by ParaView to visualize directly the large-scale computational result. If this large-scale DNS becomes possible, an essential understanding and modelling of a MHD turbulent flow and a design of nuclear fusion reactor contributes greatly.

Satake, Shin-ichi; Kimura, Masato; Yoshimori, Hajime; Kunugi, Tomoaki; Takase, Kazuyuki

2014-06-01

60

A 1152 x 760 x 1280 direct numerical simulation (DNS) using initial conditions, geometry, and physical parameters chosen to approximate those of a transitional, small Atwood number Rayleigh-Taylor mixing experiment [Mueschke, Andrews and Schilling, J. Fluid Mech. 567, 27 (2006)] is presented. The density and velocity fluctuations measured just off of the splitter plate in this buoyantly unstable water channel experiment were parameterized to provide physically-realistic, anisotropic initial conditions for the DNS. The methodology for parameterizing the measured data and numerically implementing the resulting perturbation spectra in the simulation is discussed in detail. The DNS model of the experiment is then validated by comparing quantities from the simulation to experimental measurements. In particular, large-scale quantities (such as the bubble front penetration hb and the mixing layer growth parameter {alpha}{sub b}), higher-order statistics (such as velocity variances and the molecular mixing parameter {theta}), and vertical velocity and density variance spectra from the DNS are shown to be in favorable agreement with the experimental data. Differences between the quantities obtained from the DNS and from experimental measurements are related to limitations in the dynamic range of scales resolved in the simulation and other idealizations of the simulation model. This work demonstrates that a parameterization of experimentally-measured initial conditions can yield simulation data that quantitatively agrees well with experimentally-measured low- and higher-order statistics in a Rayleigh-Taylor mixing layer. This study also provides resolution and initial conditions implementation requirements needed to simulate a physical Rayleigh-Taylor mixing experiment. In Part II [Mueschke and Schilling, Phys. Fluids (2008)], other quantities not measured in the experiment are obtained from the DNS and discussed, such as the integral- and Taylor-scale Reynolds numbers, Reynolds stress anisotropy and two-dimensional density and velocity variance spectra, hypothetical chemical product formation measures, other local and global mixing parameters, and the statistical composition of mixed fluid.

Mueschke, N; Schilling, O

2008-07-23

61

NASA Astrophysics Data System (ADS)

Advances in high-performance computational capabilities enable scientific simulations with increasingly realistic physical representations. This situation is especially true of turbulent combustion involving multiscale interactions between turbulent flow, complex chemical reaction, and scalar transport. A fundamental understanding of combustion processes is crucial to the development and optimization of next-generation combustion technologies operating with alternative fuels, at higher pressures, and under less stable operating conditions, such as highly dilute, stratified mixtures. Direct numerical simulations (DNS) of turbulent combustion resolving all flow and chemical features in canonical configurations are used to improve fundamental understanding of complex flow processes and to provide a database for the development and validation of combustion models. A description of the DNS solver and its optimization for use in massively parallel simulations is presented. Recent DNS results from a series of three combustion configurations are presented: soot formation and transport in a nonpremixed ethylene jet flame, the effect of fuel stratification in methane Bunsen flames, and extinction and reignition processes in nonpremixed ethylene jet flames.

Lignell, D. O.; Chen, J. H.; Richardson, E. S.

2008-07-01

62

Equations for direct numerical simulation of sound propagation in a moving atmosphere

NASA Astrophysics Data System (ADS)

Most previous analytical and numerical studies of sound propagation in a moving atmosphere have been based on wave equations for the sound pressure and on various parabolic approximations to the wave equations. However, these equations cannot be used as starting equations for recently proposed direct numerical simulation (DNS) of sound propagation outdoor since such starting equations should be first-order differential equations with respect to time. In the present paper, we derive two closed sets of the first-order differential equations for the sound pressure and fluctuations in medium velocity and density due to a propagating sound wave. These sets can be used as starting equations for DNS of sound propagation in a moving atmosphere. The ranges of applicability of these sets are studied by comparing them with the equations for the sound pressure used previously. Note that both sets can also be employed for analytical studies of sound propagation in a moving atmosphere. Examples of the use of these sets for DNS and analytical studies of sound propagation in a moving atmosphere are presented. [Work partially supported by a DoD High-Performance Computing Modernization Office grant and U.S. Army Research Office Grant No. DAAG19-01-1-0640.

Ostashev, Vladimir E.; Liu, Lanbo; Wilson, D. Keith; Moran, Mark L.; Aldridge, David F.; Marlin, David

2003-04-01

63

A Review of Direct Numerical Simulations of Astrophysical Detonations and Their Implications

Multi-dimensional direct numerical simulations (DNS) of astrophysical detonations in degenerate matter have revealed that the nuclear burning is typically characterized by cellular structure caused by transverse instabilities in the detonation front. Type Ia supernova modelers often use one- dimensional DNS of detonations as inputs or constraints for their whole star simulations. While these one-dimensional studies are useful tools, the true nature of the detonation is multi-dimensional. The multi-dimensional structure of the burning influences the speed, stability, and the composition of the detonation and its burning products, and therefore, could have an impact on the spectra of Type Ia supernovae. Considerable effort has been expended modeling Type Ia supernovae at densities above 1 107 g cm 3 where the complexities of turbulent burning dominate the flame propagation. However, most full star models turn the nuclear burning schemes off when the density falls below 1 107 g cm 3 and distributed burning begins. The deflagration to detonation transition (DDT) is believed to occur at just these densities and consequently they are the densities important for studying the properties of the subsequent detonation. This work will review the status of DNS studies of detonations and their possible implications for Type Ia supernova models. It will cover the development of Detonation theory from the first simple Chapman-Jouguet (CJ) detonation models to the current models based on the time-dependent, compressible, reactive flow Euler equations of fluid dynamics.

Parete-Koon, Suzanne T [ORNL; Messer, Bronson [ORNL; Smith, Chris R [ORNL; Papatheodore, Thomas L [ORNL

2013-01-01

64

A review of direct numerical simulations of astrophysical detonations and their implications

NASA Astrophysics Data System (ADS)

Multi-dimensional direct numerical simulations (DNS) of astrophysical detonations in degenerate matter have revealed that the nuclear burning is typically characterized by cellular structure caused by transverse instabilities in the detonation front. Type Ia supernova modelers often use onedimensional DNS of detonations as inputs or constraints for their whole star simulations.While these one-dimensional studies are useful tools, the true nature of the detonation is multi-dimensional. The multi-dimensional structure of the burning influences the speed, stability, and the composition of the detonation and its burning products, and therefore, could have an impact on the spectra of Type Ia supernovae. Considerable effort has been expended modeling Type Ia supernovae at densities above 1×107 g·cm-3 where the complexities of turbulent burning dominate the flame propagation. However, most full star models turn the nuclear burning schemes off when the density falls below 1×107 g·cm-3 and distributed burning begins. The deflagration to detonation transition (DDT) is believed to occur at just these densities and consequently they are the densities important for studying the properties of the subsequent detonation. This work will review the status of DNS studies of detonations and their possible implications for Type Ia supernova models. It will cover the development of Detonation theory from the first simple Chapman-Jouguet (CJ) detonation models to the current models based on the time-dependent, compressible, reactive flow Euler equations of fluid dynamics.

Parete-Koon, Suzanne T.; Smith, Christopher R.; Papatheodore, Thomas L.; Messer, O. E. Bronson

2013-04-01

65

Numerical Simulation of Deoilin Hydrocyclones

Abstract—In this research the separation efficiency of deoiling hydrocyclone is evaluated using three-dimensional simulation of multiphase flow based on Eulerian-Eulerian finite volume method. The mixture approach of Reynolds Stress Model is also employed to capture the features of turbulent multiphase swirling flow. The obtained separation efficiency of Colman's design is compared with available experimental data and showed that the separation curve of deoiling hydrocyclones can be predicted using numerical simulation. Keywords—Deoiling hydrocyclone, Eulerian-Eulerian Model,

Reza Maddahian; Bijan Farhanieh; Simin Dokht Saemi

66

Numerical simulation of late wakes in stratified and sheared flows

We employ high-resolution DNS methods to examine the dynamics, energetics, and structure of submarine wakes evolving to late times. Because of the computational focus of the project, our efforts are twofold: both computational efficiency and numerical accuracy. Thus our codes are highly optimized on the DoD computational platforms, scale linearly with increasing CPUs, and employ methods to achieve high computational

D. Fritts; M. Gourlay; W. Orlando; C. Meyer; J. Werne; T. Lund

2003-01-01

67

Statistically planar turbulent premixed and partially premixed flames for different initial turbulence intensity are simulated for global equivalence ratio ??? = 0.7 and 1.0 using three-dimensional simplified chemistry based Direct Numerical Simulations (DNS). For the simulations of partially premixed flames a bimodal distribution of equivalence ratio variation about the prescribed value of ??? is introduced in the fresh reactants. The simulation parameters

Sean P. Malkeson; Nilanjan Chakraborty

2010-01-01

68

Numerical Propulsion System Simulation Architecture

NASA Technical Reports Server (NTRS)

The Numerical Propulsion System Simulation (NPSS) is a framework for performing analysis of complex systems. Because the NPSS was developed using the object-oriented paradigm, the resulting architecture is an extensible and flexible framework that is currently being used by a diverse set of participants in government, academia, and the aerospace industry. NPSS is being used by over 15 different institutions to support rockets, hypersonics, power and propulsion, fuel cells, ground based power, and aerospace. Full system-level simulations as well as subsystems may be modeled using NPSS. The NPSS architecture enables the coupling of analyses at various levels of detail, which is called numerical zooming. The middleware used to enable zooming and distributed simulations is the Common Object Request Broker Architecture (CORBA). The NPSS Developer's Kit offers tools for the developer to generate CORBA-based components and wrap codes. The Developer's Kit enables distributed multi-fidelity and multi-discipline simulations, preserves proprietary and legacy codes, and facilitates addition of customized codes. The platforms supported are PC, Linux, HP, Sun, and SGI.

Naiman, Cynthia G.

2004-01-01

69

DNS of Flows over Periodic Hills using a Discontinuous-Galerkin Spectral-Element Method

NASA Technical Reports Server (NTRS)

Direct numerical simulation (DNS) of turbulent compressible flows is performed using a higher-order space-time discontinuous-Galerkin finite-element method. The numerical scheme is validated by performing DNS of the evolution of the Taylor-Green vortex and turbulent flow in a channel. The higher-order method is shown to provide increased accuracy relative to low-order methods at a given number of degrees of freedom. The turbulent flow over a periodic array of hills in a channel is simulated at Reynolds number 10,595 using an 8th-order scheme in space and a 4th-order scheme in time. These results are validated against previous large eddy simulation (LES) results. A preliminary analysis provides insight into how these detailed simulations can be used to improve Reynoldsaveraged Navier-Stokes (RANS) modeling

Diosady, Laslo T.; Murman, Scott M.

2014-01-01

70

Numerical simulation of adjustable nozzles

NASA Astrophysics Data System (ADS)

This paper presents the optimal combination of the nozzle and needle valve in adjustable nozzles. With numerical simulation tool of FLUENT, water jet out of different nozzles with contraction angle 30°, 40°, 50°, 60°, 70°, 80°are simulated, and within nozzles there are different needle valves with contraction angle 20°, 30°, 40°, 50°, 60°, the simulation results are compared and analyzed. The needle valves locate at the same position in the nozzles in different combinations. After water jet flows out of the nozzle, along the direction of flow there are three flow stages: initial stage, transitional stage and basic stage. The sectional area of bundle of jet shrinks near the exit of the nozzle when fluid flows out of the nozzle. Ignoring the transitional stage, the shrinkage of the bundle of jet occurs in the initial stage. The severer the contraction of jet is, the bigger the maximum velocity of the jet is, the faster the axial velocity decays. When the contraction of the jet is slight, jet flow is stable, the attenuation of the axial velocity is slower. The flow field is investigated by two-dimensional numerical analysis. Different combinations are analyzed to find the performance variation in order to select the best combination. The study offers references for the selection of the nozzle shape and needle valve shape for adjustable nozzle. It has important significance in the design and manufacture of adjustable jet pump.

Zhang, S. B.; Zhu, J. M.

2013-12-01

71

Electrochimica Acta 51 (2006) 31393150 Direct numerical simulation (DNS) modeling of PEFC electrodes

takes place at the triple-phase boundary forming an active catalyzed interface, where oxygen is consumed. The fuel (i.e. hydrogen) and oxidant (i.e. oxygen) react electrochemically in the active catalyst layers catalyst layer, while oxygen reduc- tion reaction (ORR) takes place in the cathode catalyst layer. Despite

72

On locating the obstruction in the upper airway via numerical simulation

The fluid dynamical properties of the air flow in the upper airway (UA) are not fully understood at present due to the three-dimensional (3D) patient-specific complex geometry of the airway, flow transition from laminar to turbulent and flow-structure interaction during the breathing cycle. It is quite difficult at present to experimentally measure the instantaneous velocity and pressure at specific points in the human airway. On the other hand, direct numerical simulation (DNS) can predict all the flow properties and resolve all its relevant length- and time-scales. We developed a DNS solver with the state-of-the-art lattice Boltzmann method (LBM), and used it to investigate the flow in two patient-specific UAs reconstructed from CT scan data. Inspiration and expiration flows through these two airways are studied. The time-averaged first spatial derivative of pressure (pressure gradient), ?p/?z, is used to locate the region of the UA obstruction. But the time-averaged second spatial derivative, ?2p/?z2, is used to pinpoint the exact location of the obstruction. The present results show that the DNS-LBM solver can be used to obtain accurate flow details in the UA and is a powerful tool to locate its obstruction. PMID:24389271

Wang, Yong; Elghobashi, S.

2014-01-01

73

Influence of bubbles on liquid turbulence based on the direct numerical simulation of channel flows

NASA Astrophysics Data System (ADS)

It is well known that the bubbles in turbulent flow can modify the structure and intensity of the turbulence. Recent progress in two-phase direct numerical simulation (DNS) provides a new level of detailed information about the two-phase turbulence. The availability of DNS data for single and two-phase turbulent channel flows makes it possible to compute the bubble-induced source terms in the turbulent kinetic energy equation. The turbulent kinetic energy equation, including turbulence production, dissipation and viscous and turbulent diffusion can be derived from the Navier-Stokes equations. Those exact analytical expressions can be applied to the instantaneous pressure and velocity fluctuating fields found in the single and two-phase DNS data to obtain the time-averaged lateral distribution for each term. By analyzing both single-phase and two-phase turbulent channel flows we can estimate the difference in those terms for various gas volume fraction flows. This information can be used to quantify the influence of the bubbles on turbulence in gas-liquid two-phase flows. The results will include an assessment of the currently used models of bubble-induced turbulence, as well as a validation of the new models developed in the current study.

Bolotnov, Igor; Drew, Donald; Lahey, Richard, Jr.; Podowski, Michael

2010-11-01

74

Statistically Steady Turbulence in Soap Films: Direct Numerical Simulations with Ekman Friction

We present a detailed direct numerical simulation (DNS) designed to investigate the combined effects of walls and Ekman friction on turbulence in forced soap films. We concentrate on the forward-cascade regime and show how to extract the isotropic parts of velocity and vorticity structure functions and thence the ratios of multiscaling exponents. We find that velocity structure functions display simple scaling whereas their vorticity counterparts show multiscaling; and the probability distribution function of the Weiss parameter $\\Lambda$, which distinguishes between regions with centers and saddles, is in quantitative agreement with experiments.

Prasad Perlekar; Rahul Pandit

2008-11-09

75

Statistically Steady Turbulence in Soap Films: Direct Numerical Simulations with Ekman Friction

We present a detailed direct numerical simulation (DNS) designed to investigate the combined effects of walls and Ekman friction on turbulence in forced soap films. We concentrate on the forward-cascade regime and show how to extract the isotropic parts of velocity and vorticity structure functions and thence the ratios of multiscaling exponents. We find that velocity structure functions display simple scaling whereas their vorticity counterparts show multiscaling; and the probability distribution function of the Weiss parameter $\\Lambda$, which distinguishes between regions with centers and saddles, is in quantitative agreement with experiments.

Perlekar, Prasad

2008-01-01

76

Direct Numerical Simulation of Transition in a Swept-Wing Boundary Layer

NASA Technical Reports Server (NTRS)

Direct numerical simulation (DNS) is performed to examine laminar to turbulent transition due to high-frequency secondary instability of stationary crossflow vortices in a subsonic swept-wing boundary layer for a realistic natural-laminar-flow airfoil configuration. The secondary instability is introduced via inflow forcing derived from a two-dimensional, partial-differential-equation based eigenvalue computation; and the mode selected for forcing corresponds to the most amplified secondary instability mode which, in this case, derives a majority of its growth from energy production mechanisms associated with the wall-normal shear of the stationary basic state. Both the growth of the secondary instability wave and the resulting onset of laminar-turbulent transition are captured within the DNS computations. The growth of the secondary instability wave in the DNS solution compares well with linear secondary instability theory when the amplitude is small; the linear growth is followed by a region of reduced growth resulting from nonlinear effects before an explosive onset of laminar breakdown to turbulence. The peak fluctuations are concentrated near the boundary layer edge during the initial stage of transition, but rapidly propagates towards the surface during the process of laminar breakdown. Both time-averaged statistics and flow visualization based on the DNS reveal a sawtooth transition pattern that is analogous to previously documented surface flow visualizations of transition due to stationary crossflow instability. The memory of the stationary crossflow vortex is found to persist through the transition zone and well beyond the location of the maximum skin friction.

Duan, Lian; Choudhari, Meelan M.; Li, Fei

2013-01-01

77

DepenDNS: Dependable Mechanism against DNS Cache Poisoning

NASA Astrophysics Data System (ADS)

DNS cache poisoning attacks have been proposed for a long time. In 2008, Kaminsky enhanced the attacks to be powerful based on nonce query method. By leveraging Kaminsky's attack, phishing becomes large-scale since victims are hard to detect attacks. Hence, DNS cache poisoning is a serious threat in the current DNS infrastructure. In this paper, we propose a countermeasure, DepenDNS, to prevent from cache poisoning attacks. DepenDNS queries multiple resolvers concurrently to verify an trustworthy answer while users perform payment transactions, e.g., auction, banking. Without modifying any resolver or authority server, DepenDNS is conveniently deployed on client side. In the end of paper, we conduct several experiments on DepenDNS to show its efficiency. We believe DepenDNS is a comprehensive solution against cache poisoning attacks.

Sun, Hung-Min; Chang, Wen-Hsuan; Chang, Shih-Ying; Lin, Yue-Hsun

78

Numerical Simulations of Homogeneous Turbulence Using Lagrangian-Averaged Navier-Stokes Equations

NASA Technical Reports Server (NTRS)

The Lagrangian-averaged Navier-Stokes (LANS) equations are numerically evaluated as a turbulence closure. They are derived from a novel Lagrangian averaging procedure on the space of all volume-preserving maps and can be viewed as a numerical algorithm which removes the energy content from the small scales (smaller than some a priori fixed spatial scale alpha) using a dispersive rather than dissipative mechanism, thus maintaining the crucial features of the large scale flow. We examine the modeling capabilities of the LANS equations for decaying homogeneous turbulence, ascertain their ability to track the energy spectrum of fully resolved direct numerical simulations (DNS), compare the relative energy decay rates, and compare LANS with well-accepted large eddy simulation (LES) models.

Mohseni, Kamran; Shkoller, Steve; Kosovic, Branko; Marsden, Jerrold E.; Carati, Daniele; Wray, Alan; Rogallo, Robert

2000-01-01

79

Numerical simulation of sprites halo

NASA Astrophysics Data System (ADS)

In the framework of C. Wilson's hypothesis substantiating a possibility of electric discharge development in the Earth's atmosphere at high altitudes above thunderclouds, numerical simulations were executed of the discharge exciting the sprite halo with realistic variations of thundercloud dipole moment transferred to the ground by positive lightning discharge. For various values of time and altitude, at which the avalanche-to-streamer transition occurs, optical radiation was calculated in the 1 P, 2 P, and 1 N bands of the nitrogen molecule and Meinel's band of the N{2/+} ion. The calculated brightness and space-time evolution of the luminescence are consistent with the data of the field observations of the halo luminescence.

Bochkov, E. I.; Babich, L. P.; Kutsyk, I. M.

2014-03-01

80

Relativistic Positioning Systems: Numerical Simulations

The motion of satellite constellations similar to GPS and Galileo is numerically simulated and, then, the region where bifurcation (double positioning) occurs is appropriately represented. In the cases of double positioning, the true location may be found using additional information (angles or times). The zone where the Jacobian, J, of the transformation from inertial to emission coordinates vanishes is also represented and interpreted. It is shown that the uncertainties in the satellite world lines produce positioning errors, which depend on the value of |J|. The smaller this quantity the greater the expected positioning errors. Among all the available 4-tuples of satellites, the most appropriate one -for a given location- should minimize positioning errors (large enough |J| values) avoiding bifurcation. Our study is particularly important to locate objects which are far away from Earth, e.g., satellites.

Neus Puchades; Diego Sáez

2014-04-03

81

Numerical Simulation on Laser Fusion in China

Numerical simulation is a powerful tool to get insight into the physics of laser fusion. Much effort has been devoted to develop the numerical simulation code series named LARED in China. The code series LARED are composed of six parts and enable us to have the simulation capability for the key processes in laser fusion. In recent years, a number

Shaoping Zhu; Wenbing Pei; Yan Xu; Peijun Gu; Ke Lan; Wenhua Ye; Junfeng Wu; Jinghong Li; Yaoming Gao; Chunyang Zheng; Shuanggui Li; Zeyao Mo; Jun Yan; Weiyan Zhang

2009-01-01

82

NASA Astrophysics Data System (ADS)

A 1152×760×1280 direct numerical simulation (DNS) using initial conditions, geometry, and physical parameters chosen to approximate those of a transitional, small Atwood number Rayleigh-Taylor mixing experiment [Mueschke et al., J. Fluid Mech. 567, 27 (2006)] is presented. In particular, the Atwood number is 7.5×10-4, and temperature diffusion is modeled by mass diffusion with an equivalent Schmidt number of 7. The density and velocity fluctuations measured just off of the splitter plate in this buoyantly unstable water channel experiment were parametrized to provide physically realistic, anisotropic initial conditions for the DNS. The methodology for parametrizing the measured data and numerically implementing the resulting perturbation spectra in the simulation is discussed in detail. The DNS is then validated by comparing quantities from the simulation to experimental measurements. In particular, large-scale quantities (such as the bubble front penetration hb and the mixing layer growth parameter ?b), higher-order statistics (such as velocity variances and the molecular mixing parameter ? on the center plane), and vertical velocity and density variance spectra from the DNS are shown to be in favorable agreement with the experimental data. The DNS slightly underestimates the growth of the bubble front hb but predicts ?b?0.07 at the latest time, in excellent agreement with the experimental measurement. While the molecular mixing parameter ? is also slightly underestimated by the DNS during the nonlinear and weakly turbulent growth phases, the late-time value ? ?0.55 compares favorably with the value ? ?0.6 measured in the experiment. The one-dimensional density and vertical velocity variance spectra are in excellent agreement between the DNS and experimental measurements. Differences between the quantities obtained from the DNS and from experimental measurements are related to limitations in the dynamic range of scales resolved in the DNS and other idealizations of the simulation. Specifically, the statistical convergence of the DNS results and confidence interval bounds are discussed. This work demonstrates that a parametrization of experimentally measured initial conditions can yield simulation data that quantitatively agrees well with experimentally measured low- and higher-order statistics in a Rayleigh-Taylor mixing layer. This study also provides resolution and initial conditions implementation requirements needed to simulate a physical Rayleigh-Taylor mixing experiment. In Paper II [Mueschke and Schilling, Phys. Fluids 21, 014107 (2009)], other quantities not measured in the experiment are obtained from the DNS and discussed, such as the integral- and Taylor-scale Reynolds numbers, Reynolds stress and dissipation anisotropy, two-dimensional density and velocity variance spectra, hypothetical chemical product formation measures, other local and global mixing parameters, and the statistical composition of mixed fluid. These quantities are valuable for assessing the predictions of Reynolds-averaged Navier-Stokes and large-eddy simulation models of Rayleigh-Taylor turbulent mixing.

Mueschke, Nicholas J.; Schilling, Oleg

2009-01-01

83

NASA Technical Reports Server (NTRS)

Recently, several second order closure models have been proposed for closing the second moment equations, in which the velocity-pressure gradient (and scalar-pressure gradient) tensor and the dissipation rate tensor are two of the most important terms. In the literature, these correlation tensors are usually decomposed into a so called rapid term and a return-to-isotropy term. Models of these terms have been used in global flow calculations together with other modeled terms. However, their individual behavior in different flows have not been fully examined because they are un-measurable in the laboratory. Recently, the development of direct numerical simulation (DNS) of turbulence has given us the opportunity to do this kind of study. With the direct numerical simulation, we may use the solution to exactly calculate the values of these correlation terms and then directly compare them with the values from their modeled formulations (models). Here, we make direct comparisons of five representative rapid models and eight return-to-isotropy models using the DNS data of forty five homogeneous flows which were done by Rogers et al. (1986) and Lee et al. (1985). The purpose of these direct comparisons is to explore the performance of these models in different flows and identify the ones which give the best performance. The modeling procedure, model constraints, and the various evaluated models are described. The detailed results of the direct comparisons are discussed, and a few concluding remarks on turbulence models are given.

Shih, Tsan-Hsing; Lumley, John L.

1991-01-01

84

NASA Astrophysics Data System (ADS)

Despite tremendous progress in development of numerical techniques and constitutive theories for polymeric fluids in the past decade, Direct Numerical Simulation (DNS) of elastic turbulence has posed tremendous challenges to researchers engaged in developing first principles models and simulations that can accurately and robustly predict the dynamical behavior of polymeric flows. In this presentation, we report the first DNS of elastic turbulence in the Taylor-Couette (TC) flow. Specifically, our computations with prototypical constitutive equations for dilute polymeric solutions, such as the FENE-P model are capable of reproducing the essential features of the experimentally observed elastic turbulence in TC flow of this class of fluids, namely, randomly fluctuating fluid motion excited in a broad range of spatial and temporal scales, and a significant increase of the flow resistance. Moreover, the experimentally measured Power Spectral Density of radial velocity fluctuations, i.e., two contiguous regions of power-law decay, -1.1 at lower frequencies and -2.2 at high-frequencies is accurately computed.

Liu, Nansheng; Khomami, Bamin

2011-11-01

85

The state-of-the-art for Direct Numerical Simulation (DNS) of boiling multiphase flows is reviewed, focussing on potential of available computational techniques, the level of current success for their applications to model several basic flow regimes (film, pool-nucleate and wall-nucleate boiling -- FB, PNB and WNB, respectively). Then, we discuss multiphysics and multiscale nature of practical boiling flows in LWR reactors, requiring high-fidelity treatment of interfacial dynamics, phase-change, hydrodynamics, compressibility, heat transfer, and non-equilibrium thermodynamics and chemistry of liquid/vapor and fluid/solid-wall interfaces. Finally, we outline the framework for the {\\sf Fervent} code, being developed at INL for DNS of reactor-relevant boiling multiphase flows, with the purpose of gaining insight into the physics of multiphase flow regimes, and generating a basis for effective-field modeling in terms of its formulation and closure laws.

Nourgaliev R.; Knoll D.; Mousseau V.; Berry R.

2007-04-01

86

Numerical Simulations of Thermobaric Explosions

A Model of the energy evolution in thermobaric explosions is presented. It is based on the two-phase formulation: conservation laws for the gas and particle phases along with inter-phase interaction terms. It incorporates a Combustion Model based on the mass conservation laws for fuel, air and products; source/sink terms are treated in the fast-chemistry limit appropriate for such gas dynamic fields. The Model takes into account both the afterburning of the detonation products of the booster with air, and the combustion of the fuel (Al or TNT detonation products) with air. Numerical simulations were performed for 1.5-g thermobaric explosions in five different chambers (volumes ranging from 6.6 to 40 liters and length-to-diameter ratios from 1 to 12.5). Computed pressure waveforms were very similar to measured waveforms in all cases - thereby proving that the Model correctly predicts the energy evolution in such explosions. The computed global fuel consumption {mu}(t) behaved as an exponential life function. Its derivative {dot {mu}}(t) represents the global rate of fuel consumption. It depends on the rate of turbulent mixing which controls the rate of energy release in thermobaric explosions.

Kuhl, A L; Bell, J B; Beckner, V E; Khasainov, B

2007-05-04

87

A study of aerosol activation at the cloud edge with high resolution numerical simulations

NASA Astrophysics Data System (ADS)

High resolution numerical simulations are used to study the structure of the cloud edge area. We consider an aerosol distribution function with a similar aerosol core size (12 nm). The aerosol composition is assumed to be water soluble NaCl. Depending on the specific conditions in the investigated cloud edge area, water is evaporated or activated from the aerosol surface. We use a publicly available high order domain code for direct numerical simulation (DNS) in combination with the Smagorinsky subgrid scale model. We compare 2D and 3D model results of turbulent air motion of aerosol particles with varying grid cell sizes. We show that a 2D model with high resolution gives a more realistic number of activated particles than the corresponding 3D model with lower resolution. We also study the effects of aerosol dynamics on turbulent fields and show that water vapor condensation and evaporation have significant effects on temperature and supersaturation fields.

Babkovskaia, N.; Boy, M.; Smolander, S.; Romakkaniemi, S.; Rannik, U.; Kulmala, M.

2015-02-01

88

Direct numerical simulation of soot formation and transport in turbulent nonpremixed ethylene flames

NASA Astrophysics Data System (ADS)

Combustion is central to society and accounts for the majority of the world's energy production. Soot formation, transport, and emission from turbulent flames are an important process in nonpremixed combustion. Soot is a major air pollutant with adverse health effects; its emission reduces combustion efficiencies associated with unburned fuel; and soot interacts strongly with the composition and temperature fields of flames, contributing to the bulk of radiative heat transfer. Simulation of combustion is an important and emerging discipline that compliments theoretical and experimental investigations and can provide fundamental insight into turbulent combustion environments and aid in engineering design of practical equipment. Simulations of practical combustion environments cannot fully resolve all flow and chemical phenomena due to the wide range of timescales and lengthscales present and must rely on models to capture the effects of unresolved turbulent transport and turbulence-chemistry interactions. Very little is know about soot formation in turbulent flames due to the difficulty of experimental measurements and the computational cost of simulation. Direct numerical simulation (DNS) resolves all relevant flow and chemical structures in turbulent flames, requiring no turbulence closure models. DNS of soot formation with realistic combustion chemistry and soot formation is presented in this dissertation. A series of increasingly complex flow configurations is investigated including one-dimensional relaxing diffusion flames, two-dimensional mixing layers and decaying turbulence simulations, and a three-dimensional temporally evolving jet flame. A reduced ethylene mechanism consisting of 19 transported species is coupled to a four-step soot model using the method of moments. The DNS are used to quantify soot formation and transport in turbulent flames. The proximity of soot to a flame is important, as this impacts the soot reaction and radiation rates. Differential diffusion between soot and the mixture fraction is very important. Multidimensional flame dynamic effects including flame curvature are shown to influence the direction of soot transport relative to a flame. The DNS provide a database by which turbulent combustion models may be validated and developed. Soot formation in the laminar flamelet and conditional moment closure models is examined.

Lignell, David Owen

89

Direct numerical simulations of low Reynolds number flow over airfoils with trailing-edge serrations

NASA Astrophysics Data System (ADS)

Direct numerical simulations (DNS) have been conducted of NACA-0012 with serrated and straight flat-plate trailing-edge extensions using a purposely developed immersed boundary method. For the low Reynolds number airfoil flows accessible by DNS, laminar separation bubbles involving laminar-turbulent transition and turbulent reattachment occurs. Comparing results from simulations with serrated and un-serrated trailing-edge extensions, noise reduction for higher frequencies is shown using power spectra and one-third octave averaged pressure contours. The effect of the trailing-edge serrations on an acoustic feedback loop observed in previous simulations and the subsequent effect on the laminar separation bubble is studied via cross-correlations, probability density functions of skin friction and spanwise wavenumber spectra. The results show that the presence of serrations leads to some spanwise variation of transitional structures in the separated shear layer, but does not significantly affect the overall hydrodynamic field on the airfoil upstream of the serrations. Two reasons for why the hydrodynamic field is not considerably affected by the presence of serrations are suggested.

Sandberg, R. D.; Jones, L. E.

2011-08-01

90

Numerical Simulations of Granular Processes

NASA Astrophysics Data System (ADS)

Spacecraft images and indirect observations including thermal inertia measurements indicate most small bodies have surface regolith. Evidence of granular flow is also apparent in the images. This material motion occurs in very low gravity, therefore in a completely different gravitational environment than on the Earth. Understanding and modeling these motions can aid in the interpretation of imaged surface features that may exhibit signatures of constituent material properties. Also, upcoming sample-return missions to small bodies, and possible future manned missions, will involve interaction with the surface regolith, so it is important to develop tools to predict the surface response. We have added new capabilities to the parallelized N-body gravity tree code pkdgrav [1,2] that permit the simulation of granular dynamics, including multi-contact physics and friction forces, using the soft-sphere discrete-element method [3]. The numerical approach has been validated through comparison with laboratory experiments (e.g., [3,4]). Ongoing and recently completed projects include: impacts into granular materials using different projectile shapes [5]; possible tidal resurfacing of asteroid Apophis during its 2029 encounter [6]; the Brazil-nut effect in low gravity [7]; and avalanche modeling.Acknowledgements: DCR acknowledges NASA (grants NNX08AM39G, NNX10AQ01G, NNX12AG29G) and NSF (AST1009579). PM acknowledges the French agency CNES. SRS works on the NEOShield Project funded under the European Commission’s FP7 program agreement No. 282703. SM acknowledges support from the Center for Theory and Computation at U Maryland and the Dundee Fellowship at U Dundee. Most simulations were performed using the YORP cluster in the Dept. of Astronomy at U Maryland and on the Deepthought High-Performance Computing Cluster at U Maryland.References: [1] Richardson, D.C. et al. 2000, Icarus 143, 45; [2] Stadel, J. 2001, Ph.D. Thesis, U Washington; [3] Schwartz, S.R. et al. 2012, Gran. Matt. 14, 363. [4] Schwartz, S.R. et al. 2013, Icarus 226, 67; [5] Schwartz, S.R. et al. 2014, P&SS, 10.1016/j.pss.2014.07.013; [6] Yu, Y. et al. 2014, Icarus, 10.1016/j.icarus.2014.07.027; [7] Matsumura, S. et al. 2014, MNRAS, 10.1093/mnras/stu1388.

Richardson, Derek C.; Michel, Patrick; Schwartz, Stephen R.; Ballouz, Ronald-Louis; Yu, Yang; Matsumura, Soko

2014-11-01

91

Numerical Simulation of a High Mach Number Jet Flow

NASA Technical Reports Server (NTRS)

The recent efforts to develop accurate numerical schemes for transition and turbulent flows are motivated, among other factors, by the need for accurate prediction of flow noise. The success of developing high speed civil transport plane (HSCT) is contingent upon our understanding and suppression of the jet exhaust noise. The radiated sound can be directly obtained by solving the full (time-dependent) compressible Navier-Stokes equations. However, this requires computational storage that is beyond currently available machines. This difficulty can be overcome by limiting the solution domain to the near field where the jet is nonlinear and then use acoustic analogy (e.g., Lighthill) to relate the far-field noise to the near-field sources. The later requires obtaining the time-dependent flow field. The other difficulty in aeroacoustics computations is that at high Reynolds numbers the turbulent flow has a large range of scales. Direct numerical simulations (DNS) cannot obtain all the scales of motion at high Reynolds number of technological interest. However, it is believed that the large scale structure is more efficient than the small-scale structure in radiating noise. Thus, one can model the small scales and calculate the acoustically active scales. The large scale structure in the noise-producing initial region of the jet can be viewed as a wavelike nature, the net radiated sound is the net cancellation after integration over space. As such, aeroacoustics computations are highly sensitive to errors in computing the sound sources. It is therefore essential to use a high-order numerical scheme to predict the flow field. The present paper presents the first step in a ongoing effort to predict jet noise. The emphasis here is in accurate prediction of the unsteady flow field. We solve the full time-dependent Navier-Stokes equations by a high order finite difference method. Time accurate spatial simulations of both plane and axisymmetric jet are presented. Jet Mach numbers of 1.5 and 2.1 are considered. Reynolds number in the simulations was about a million. Our numerical model is based on the 2-4 scheme by Gottlieb & Turkel. Bayliss et al. applied the 2-4 scheme in boundary layer computations. This scheme was also used by Ragab and Sheen to study the nonlinear development of supersonic instability waves in a mixing layer. In this study, we present two dimensional direct simulation results for both plane and axisymmetric jets. These results are compared with linear theory predictions. These computations were made for near nozzle exit region and velocity in spanwise/azimuthal direction was assumed to be zero.

Hayder, M. Ehtesham; Turkel, Eli; Mankbadi, Reda R.

1993-01-01

92

NASA Astrophysics Data System (ADS)

In a recent direct numerical simulation (DNS) study [P. K. Yeung and K. R. Sreenivasan, "Spectrum of passive scalars of high molecular diffusivity in turbulent mixing," J. Fluid Mech. 716, R14 (2013)] with Schmidt number as low as 1/2048, we verified the essential physical content of the theory of Batchelor, Howells, and Townsend ["Small-scale variation of convected quantities like temperature in turbulent fluid. 2. The case of large conductivity," J. Fluid Mech. 5, 134 (1959)] for turbulent passive scalar fields with very strong diffusivity, decaying in the absence of any production mechanism. In particular, we confirmed the existence of the -17/3 power of the scalar spectral density in the so-called inertial-diffusive range. In the present paper, we consider the DNS of the same problem, but in the presence of a uniform mean gradient, which leads to the production of scalar fluctuations at (primarily) the large scales. For the parameters of the simulations, the presence of the mean gradient alters the physics of mixing fundamentally at low Peclet numbers. While the spectrum still follows a -17/3 power law in the inertial-diffusive range, the pre-factor is non-universal and depends on the magnitude of the mean scalar gradient. Spectral transfer is greatly reduced in comparison with those for moderately and weakly diffusive scalars, leading to several distinctive features such as the absence of dissipative anomaly and a new balance of terms in the spectral transfer equation for the scalar variance, differing from the case of zero gradient. We use the DNS results to present an alternative explanation for the observed scaling behavior, and discuss a few spectral characteristics in detail.

Yeung, P. K.; Sreenivasan, K. R.

2014-01-01

93

Large eddy simulations and direct numerical simulations of high speed turbulent reacting flows

NASA Technical Reports Server (NTRS)

The primary objective of this research is to extend current capabilities of Large Eddy Simulations (LES) and Direct Numerical Simulations (DNS) for the computational analyses of high speed reacting flows. Our efforts in the first two years of this research have been concentrated on a priori investigations of single-point Probability Density Function (PDF) methods for providing subgrid closures in reacting turbulent flows. In the efforts initiated in the third year, our primary focus has been on performing actual LES by means of PDF methods. The approach is based on assumed PDF methods and we have performed extensive analysis of turbulent reacting flows by means of LES. This includes simulations of both three-dimensional (3D) isotropic compressible flows and two-dimensional reacting planar mixing layers. In addition to these LES analyses, some work is in progress to assess the extent of validity of our assumed PDF methods. This assessment is done by making detailed companions with recent laboratory data in predicting the rate of reactant conversion in parallel reacting shear flows. This report provides a summary of our achievements for the first six months of the third year of this program.

Givi, P.; Frankel, S. H.; Adumitroaie, V.; Sabini, G.; Madnia, C. K.

1993-01-01

94

The Numerical Simulation of Turbulence

In this contribution, I give an overview of the various approaches toward the numerical modelling of turbulence, particularly, in the interstellar medium. The discussion is placed in a physical context, i. e. computational problems are motivated from basic physical considerations. Presenting selected examples for solutions to these problems, I introduce the basic ideas of the most commonly used numerical methods.

W. Schmidt

2007-12-06

95

NASA Astrophysics Data System (ADS)

Two-dimensional, fully coupled direct numerical simulations (DNS) are conducted to examine the local energy dynamics of a flexible cantilevered plate in the wake of a two-dimensional circular cylinder. The motion of the cantilevered plate is described using a finite element formulation and a fully compressible, finite volume Navier Stokes solver is used to compute the flow field. A sharp interface level set method is employed in conjunction with a ghost fluid method to describe the immersed boundaries of the bluff body and flexible plate. DNS is first conducted to validate the numerical methodology and compared with previous studies of flexible cantilevered plates and flow over bluff bodies; excellent agreement with previous results is observed. A newly defined power production/loss geometry metric is introduced based on surface curvature and plate velocity. The metric is found to be useful for determining which sections of the plate will produce energy based on curvature and deflection rate. Scatter plots and probability measures are presented showing a high correlation between the direction of energy transfer (i.e., to or from the plate) and the sign of the newly defined curvature-deflection-rate metric. The findings from this study suggest that a simple local geometry/kinematic based metric can be devised to aid in the development and design of flexible wind energy harvesting flutter mills.

Kuhl, J. M.; Desjardin, P. E.

2012-01-01

96

LES, DNS and RANS for the analysis of high-speed turbulent reacting flows

NASA Technical Reports Server (NTRS)

The purpose of this research is to continue our efforts in advancing the state of knowledge in large eddy simulation (LES), direct numerical simulation (DNS), and Reynolds averaged Navier Stokes (RANS) methods for the computational analysis of high-speed reacting turbulent flows. In the second phase of this work, covering the period 1 Aug. 1994 - 31 Jul. 1995, we have focused our efforts on two programs: (1) developments of explicit algebraic moment closures for statistical descriptions of compressible reacting flows and (2) development of Monte Carlo numerical methods for LES of chemically reacting flows.

Adumitroaie, V.; Colucci, P. J.; Taulbee, D. B.; Givi, P.

1995-01-01

97

Mean-field and direct numerical simulations of magnetic flux concentrations from vertical field

NASA Astrophysics Data System (ADS)

Context. Strongly stratified hydromagnetic turbulence has previously been found to produce magnetic flux concentrations if the domain is large enough compared with the size of turbulent eddies. Mean-field simulations (MFS) using parameterizations of the Reynolds and Maxwell stresses show a large-scale negative effective magnetic pressure instability and have been able to reproduce many aspects of direct numerical simulations (DNS) regarding growth rate, shape of the resulting magnetic structures, and their height as a function of magnetic field strength. Unlike the case of an imposed horizontal field, for a vertical one, magnetic flux concentrations of equipartition strength with the turbulence can be reached, resulting in magnetic spots that are reminiscent of sunspots. Aims: We determine under what conditions magnetic flux concentrations with vertical field occur and what their internal structure is. Methods: We use a combination of MFS, DNS, and implicit large-eddy simulations (ILES) to characterize the resulting magnetic flux concentrations in forced isothermal turbulence with an imposed vertical magnetic field. Results: Using DNS, we confirm earlier results that in the kinematic stage of the large-scale instability the horizontal wavelength of structures is about 10 times the density scale height. At later times, even larger structures are being produced in a fashion similar to inverse spectral transfer in helically driven turbulence. Using ILES, we find that magnetic flux concentrations occur for Mach numbers between 0.1 and 0.7. They occur also for weaker stratification and larger turbulent eddies if the domain is wide enough. Using MFS, the size and aspect ratio of magnetic structures are determined as functions of two input parameters characterizing the parameterization of the effective magnetic pressure. DNS, ILES, and MFS show magnetic flux tubes with mean-field energies comparable to the turbulent kinetic energy. These tubes can reach a length of about eight density scale heights. Despite being ?1% equipartition strength, it is important that their lower part is included within the computational domain to achieve the full strength of the instability. Conclusions: The resulting vertical magnetic flux tubes are being confined by downflows along the tubes and corresponding inflow from the sides, which keep the field concentrated. Application to sunspots remains a viable possibility.

Brandenburg, A.; Gressel, O.; Jabbari, S.; Kleeorin, N.; Rogachevskii, I.

2014-02-01

98

GPU accelerated flow solver for direct numerical simulation of turbulent flows

Graphical processing units (GPUs), characterized by significant computing performance, are nowadays very appealing for the solution of computationally demanding tasks in a wide variety of scientific applications. However, to run on GPUs, existing codes need to be ported and optimized, a procedure which is not yet standardized and may require non trivial efforts, even to high-performance computing specialists. In the present paper we accurately describe the porting to CUDA (Compute Unified Device Architecture) of a finite-difference compressible Navier–Stokes solver, suitable for direct numerical simulation (DNS) of turbulent flows. Porting and validation processes are illustrated in detail, with emphasis on computational strategies and techniques that can be applied to overcome typical bottlenecks arising from the porting of common computational fluid dynamics solvers. We demonstrate that a careful optimization work is crucial to get the highest performance from GPU accelerators. The results show that the overall speedup of one NVIDIA Tesla S2070 GPU is approximately 22 compared with one AMD Opteron 2352 Barcelona chip and 11 compared with one Intel Xeon X5650 Westmere core. The potential of GPU devices in the simulation of unsteady three-dimensional turbulent flows is proved by performing a DNS of a spatially evolving compressible mixing layer.

Salvadore, Francesco [CASPUR – via dei Tizii 6/b, 00185 Rome (Italy)] [CASPUR – via dei Tizii 6/b, 00185 Rome (Italy); Bernardini, Matteo, E-mail: matteo.bernardini@uniroma1.it [Department of Mechanical and Aerospace Engineering, University of Rome ‘La Sapienza’ – via Eudossiana 18, 00184 Rome (Italy)] [Department of Mechanical and Aerospace Engineering, University of Rome ‘La Sapienza’ – via Eudossiana 18, 00184 Rome (Italy); Botti, Michela [CASPUR – via dei Tizii 6/b, 00185 Rome (Italy)] [CASPUR – via dei Tizii 6/b, 00185 Rome (Italy)

2013-02-15

99

Numerical Simulation of Turbulent Fluid Flows

NASA Technical Reports Server (NTRS)

Numerical simulation of turbulent flows is discussed. Computational requirements for the direct simulaton of turbulence, simulation of arbitrary homogeneous flows, an expansion technique for wall bounded flows with application to pipe flow, and possibilities of flow representations or modeling techniques that allow the simulation of high Reynolds number flows with a relatively small number of dependent variables are included.

Leonard, A.

1983-01-01

100

Rocket Engine Numerical Simulator (RENS)

NASA Technical Reports Server (NTRS)

Work is being done at three universities to help today's NASA engineers use the knowledge and experience of their Apolloera predecessors in designing liquid rocket engines. Ground-breaking work is being done in important subject areas to create a prototype of the most important functions for the Rocket Engine Numerical Simulator (RENS). The goal of RENS is to develop an interactive, realtime application that engineers can utilize for comprehensive preliminary propulsion system design functions. RENS will employ computer science and artificial intelligence research in knowledge acquisition, computer code parallelization and objectification, expert system architecture design, and object-oriented programming. In 1995, a 3year grant from the NASA Lewis Research Center was awarded to Dr. Douglas Moreman and Dr. John Dyer of Southern University at Baton Rouge, Louisiana, to begin acquiring knowledge in liquid rocket propulsion systems. Resources of the University of West Florida in Pensacola were enlisted to begin the process of enlisting knowledge from senior NASA engineers who are recognized experts in liquid rocket engine propulsion systems. Dr. John Coffey of the University of West Florida is utilizing his expertise in interviewing and concept mapping techniques to encode, classify, and integrate information obtained through personal interviews. The expertise extracted from the NASA engineers has been put into concept maps with supporting textual, audio, graphic, and video material. A fundamental concept map was delivered by the end of the first year of work and the development of maps containing increasing amounts of information is continuing. Find out more information about this work at the Southern University/University of West Florida. In 1996, the Southern University/University of West Florida team conducted a 4day group interview with a panel of five experts to discuss failures of the RL10 rocket engine in conjunction with the Centaur launch vehicle. The discussion was recorded on video and audio tape. Transcriptions of the entire proceedings and an abbreviated video presentation of the discussion highlights are under development. Also in 1996, two additional 3year grants were awarded to conduct parallel efforts that would complement the work being done by Southern University and the University of West Florida. Dr. Prem Bhalla of Jackson State University in Jackson, Mississippi, is developing the architectural framework for RENS. By employing the Rose Rational language and Booch Object Oriented Programming (OOP) technology, Dr. Bhalla is developing the basic structure of RENS by identifying and encoding propulsion system components, their individual characteristics, and cross-functionality and dependencies. Dr. Ruknet Cezzar of Hampton University, located in Hampton, Virginia, began working on the parallelization and objectification of rocket engine analysis and design codes. Dr. Cezzar will use the Turbo C++ OOP language to translate important liquid rocket engine computer codes from FORTRAN and permit their inclusion into the RENS framework being developed at Jackson State University. The Southern University/University of West Florida grant was extended by 1 year to coordinate the conclusion of all three efforts in 1999.

Davidian, Kenneth O.

1997-01-01

101

Numerical wind speed simulation model

A relatively simple stochastic model for simulating wind speed time series that can be used as an alternative to time series from representative locations is described in this report. The model incorporates systematic seasonal variation of the mean wind, its standard deviation, and the correlation speeds. It also incorporates systematic diurnal variation of the mean speed and standard deviation. To demonstrate the model capabilities, simulations were made using model parameters derived from data collected at the Hanford Meteorology Station, and results of analysis of simulated and actual data were compared.

Ramsdell, J.V.; Athey, G.F.; Ballinger, M.Y.

1981-09-01

102

NASA Technical Reports Server (NTRS)

Direct numerical simulations (DNS) of Mach 6 turbulent boundary layer with nominal freestream Mach number of 6 and Reynolds number of Re(sub T) approximately 460 are conducted at two wall temperatures (Tw/Tr = 0.25, 0.76) to investigate the generated pressure fluctuations and their dependence on wall temperature. Simulations indicate that the influence of wall temperature on pressure fluctuations is largely limited to the near-wall region, with the characteristics of wall-pressure fluctuations showing a strong temperature dependence. Wall temperature has little influence on the propagation speed of the freestream pressure signal. The freestream radiation intensity compares well between wall-temperature cases when normalized by the local wall shear; the propagation speed of the freestream pressure signal and the orientation of the radiation wave front show little dependence on the wall temperature.

Duan, Lian; Choudhari, Meelan M.

2014-01-01

103

NUMERICAL SIMULATION OF LARYNGEAL FLOW

In this study, we have investigated laryngeal air flows by numerically solving the corresponding Navier-Stokes equations expressed in a two-dimensional cylindrical coordinate system. The glottal aperture, defined by the geometry of the vocal folds was allowed to change with the v...

104

Complex Computer Simulations, Numerical Artifacts, and Numerical Phenomena

The study of some typical complex computer simulations, pre-senting one or more Complexity features, as the: a) symmetry breaking, b) nonlinear properties, c) dissipative processes, d) high-logical depth, e) self-organizing processes, etc allows to point out some several numerical artifacts, namely the: (i) distortions, (ii) scattering, (iii) pseudo-convergence, (iv) insta-bility, (v) mis-leading (false) symmetry-breaking simulations and others. The detailed analysis

Dan-Alexandru Iordache; Paul Sterian; Florin Pop; Andreea-Rodica Sterian

2010-01-01

105

A numerical study of flow-structure interactions with application to flow past a pair of cylinders

Flow-structure interaction is a generic problem for many engineering applications, such as flow--induced oscillations of marine risers and cables. In this thesis a Direct Numerical Simulation (DNS) approach based on ...

Papaioannou, Georgios (Georgios Vasilios), 1975-

2004-01-01

106

Numerical simulation of turbulent reacting flows

A multidimensional, transient computational fluid dynamics algorithm has been developed for simulation of reacting flows in automotive engines. We describe the basic features of the method and present several numerical solutions obtained with it.

Cloutman, L.D.

1984-01-01

107

Drugs, Herbicides and Numerical Simulations Hermann Mena

Drugs, Herbicides and Numerical Simulations Hermann Mena University of Innsbruck Glyphosate is one of the herbicides used by the Colombian government to spray coca fields. Sprays took place for a number of years

Matijevic, Domagoj

108

CFD Numerical Simulation of Exhaust Muffler

Based on the physical numerical modeling of the flow field of the muffler in this paper, the author simulated the field by numerical method with Fluent and analyzed the effect which the internal flow field has on the performance of the muffler, which may be a credible guidance of the muffler structural design.

Jun Chen; Xiong Shi

2011-01-01

109

Numerical Simulation of Deoilin Hydrocyclones

Abstract—In this research the separation efficiency of deoiling hydrocyclone is evaluated using three-dimensional simulation of multiphase flow based on Eulerian-Eulerian finite volume method. The mixture approach of Reynolds Stress Model is also employed to capture the features of turbulent

Reza Maddahian; Bijan Farhanieh; Simin Dokht Saemi

110

DNS of laminar-turbulent boundary layer transition induced by solid obstacles

Results of numerical simulations obtained by a staggered finite difference scheme together with an efficient immersed boundary method are presented to understand the effects of the shape of three-dimensional obstacles on the transition of a boundary layer from a laminar to a turbulent regime. Fully resolved Direct Numerical Simulations (DNS), highlight that the closer to the obstacle the symmetry is disrupted the smaller is the transitional Reynolds number. It has been also found that the transition can not be related to the critical roughness Reynolds number used in the past. The simulations highlight the differences between wake and inflectional instabilities, proving that two-dimensional tripping devices are more efficient in promoting the transition. Simulations at high Reynolds number demonstrate that the reproduction of a real experiment with a solid obstacle at the inlet is an efficient tool to generate numerical data bases for understanding the physics of boundary layers. The quality of the numerical ...

Orlandi, Paolo; Bernardini, Matteo

2015-01-01

111

Combustion of fossil fuels is likely to continue for the near future due to the growing trends in energy consumption worldwide. The increase in efficiency and the reduction of pollutant emissions from combustion devices are pivotal to achieving meaningful levels of carbon abatement as part of the ongoing climate change efforts. Computational fluid dynamics featuring adequate combustion models will play an increasingly important role in the design of more efficient and cleaner industrial burners, internal combustion engines, and combustors for stationary power generation and aircraft propulsion. Today, turbulent combustion modelling is hindered severely by the lack of data that are accurate and sufficiently complete to assess and remedy model deficiencies effectively. In particular, the formation of pollutants is a complex, nonlinear and multi-scale process characterized by the interaction of molecular and turbulent mixing with a multitude of chemical reactions with disparate time scales. The use of direct numerical simulation (DNS) featuring a state of the art description of the underlying chemistry and physical processes has contributed greatly to combustion model development in recent years. In this paper, the analysis of the intricate evolution of soot formation in turbulent flames demonstrates how DNS databases are used to illuminate relevant physico-chemical mechanisms and to identify modelling needs. PMID:25024412

Bisetti, Fabrizio; Attili, Antonio; Pitsch, Heinz

2014-08-13

112

Direct Numerical Simulations of High-Speed Turbulent Boundary Layers over Riblets

NASA Technical Reports Server (NTRS)

Direct numerical simulations (DNS) of spatially developing turbulent boundary layers over riblets with a broad range of riblet spacings are conducted to investigate the effects of riblets on skin friction at high speeds. Zero-pressure gradient boundary layers under two flow conditions (Mach 2:5 with T(sub w)/T(sub r) = 1 and Mach 7:2 with T(sub w)/T(sub r) = 0:5) are considered. The DNS results show that the drag-reduction curve (delta C(sub f)/C(sub f) vs l(sup +)(sub g )) at both supersonic speeds follows the trend of low-speed data and consists of a `viscous' regime for small riblet size, a `breakdown' regime with optimal drag reduction, and a `drag-increasing' regime for larger riblet sizes. At l l(sup +)(sub g) approx. 10 (corresponding to s+ approx 20 for the current triangular riblets), drag reduction of approximately 7% is achieved at both Mach numbers, and con rms the observations of the few existing experiments under supersonic conditions. The Mach- number dependence of the drag-reduction curve occurs for riblet sizes that are larger than the optimal size, with smaller slopes of (delta C(sub f)/C(sub f) for larger freestream Mach numbers. The Reynolds analogy holds with 2(C(sub h)=C(sub f) approximately equal to that of at plates for both drag-reducing and drag-increasing configurations.

Duan, Lian; Choudhari, Meelan, M.

2014-01-01

113

Numerical Simulations of Bouncing Jets

of Committee, Andrea Bonito Committee Members, Jean-Luc Guermond Raytcho Lazarov Kumbakonam Rajagopal Head of Department, Emil Straube August 2014 Major Subject: Mathematics Copyright 2014 Sanghyun Lee ABSTRACT The Kaye effect is a fascinating phenomenon of a....D. I also appreciate all his contributions of time, ideas and financial support to continue my Ph.D. Also, my thesis committee and numerical analysis group greatly supported me through all these years. Thank you to Jean-Luc Guermond for sharing so much...

Lee, Sanghyun

2014-07-18

114

Numerical Simulations of Bouncing Jets

Bouncing jets are fascinating phenomenons occurring under certain conditions when a jet impinges on a free surface. This effect is observed when the fluid is Newtonian and the jet falls in a bath undergoing a solid motion. It occurs also for non-Newtonian fluids when the jets falls in a vessel at rest containing the same fluid. We investigate numerically the impact of the experimental setting and the rheological properties of the fluid on the onset of the bouncing phenomenon. Our investigations show that the occurrence of a thin lubricating layer of air separating the jet and the rest of the liquid is a key factor for the bouncing of the jet to happen. The numerical technique that is used consists of a projection method for the Navier-Stokes system coupled with a level set formulation for the representation of the interface. The space approximation is done with adaptive finite elements. Adaptive refinement is shown to be very important to capture the thin layer of air that is responsible for the bouncing.

Bonito, Andrea; Lee, Sanghyun

2015-01-01

115

Numerical simulation of shrouded propellers

NASA Technical Reports Server (NTRS)

A numerical model was developed for the evaluation of the performance characteristics of a shrouded propeller. Using this model, a computational study was carried out to investigate the feasibility of improving the aerodynamic performance of a propeller by encasing it in a shroud. The propeller blade was modeled by a segmented bound vortex positioned along the span of the blade at its quarter-chord-line. The shroud was modeled by a number of discrete vortex rings. Due to the mutual dependence of shroud and propeller vortex strengths and the propeller vortex wake an iterative scheme was employed. Three shroud configurations were considered: a cylindrical and two conical shrouds. The computed performance of the shrouded propeller was compared with that of a free propeller of identical propeller geometry. The numerical results indicated that the cylindrical shroud outperformed the conical shroud configurations for the cases considered. Furthermore, when compared to the free propeller performance, the cylindrical shroud showed a considerable performance enhancement over the free propeller. However, the improvements were found to decrease with an increase in the advance ratio and to virtually diminish at advance ratios of about 2.5.

Afjeh, Abdollah A.

1991-01-01

116

Numerical simulation of jet noise

NASA Astrophysics Data System (ADS)

In the present work, computational aeroacoustics and parallel computers are used to conduct a study of flow-induced noise from different jet nozzle geometries. The nozzle is included as part of the computational domain. This is important to predict jet noise from nozzles associated with military aircraft engines. The Detached Eddy Simulation (DES) approach is used to simulate both the jet nozzle internal and external flows as well as the jet plume. This methodology allows the turbulence model to transition from an unsteady Reynolds Averaged Navier-Stokes (URANS) method for attached boundary layers to a Large Eddy Simulation (LES) in separated regions. Thus, it is ideally suited to jet flow simulations where the nozzle is included. Both cylindrical polar and Cartesian coordinate systems are used. A spectral method is used to avoid the centerline singularity when using the cylindrical coordinate system. The one equation Spalart-Allmaras turbulence model, in DES mode, is used to describe the evolution of the turbulent eddy viscosity. An explicit 4th order Runge-Kutta time marching scheme is used. For spatial discritization the Dispersion Relation Preserving scheme(DRP) is used. The farfield sound is evaluated using the Ffowcs Williams-Hawkings permeable surface wave extrapolation method. This permits the noise to be predicted at large distances from the jet based on fluctuations in the jets near field. The present work includes a study of the effect of different nozzle geometries such as axisymmetric/non-axisymmetric and planar/non-planar exits on the far field noise predictions. Also the effect of operating conditions such as a heated/unheated jet, the effect of forward flight, a jet flow at an angle of attack, and the effect of a supersonic exit Mach number, are included in the study.

Paliath, Umesh

117

Large eddy simulation and direct numerical simulation of high speed turbulent reacting flows

NASA Astrophysics Data System (ADS)

The objective of this research is to make use of Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) for the computational analyses of high speed reacting flows. Our efforts in the first phase of this research conducted within the past three years have been directed in several issues pertaining to intricate physics of turbulent reacting flows. In our previous 5 semi-annual reports submitted to NASA LaRC, as well as several technical papers in archival journals, the results of our investigations have been fully described. In this progress report which is different in format as compared to our previous documents, we focus only on the issue of LES. The reason for doing so is that LES is the primary issue of interest to our Technical Monitor and that our other findings were needed to support the activities conducted under this prime issue. The outcomes of our related investigations, nevertheless, are included in the appendices accompanying this report. The relevance of the materials in these appendices are, therefore, discussed only briefly within the body of the report. Here, results are presented of a priori and a posterior analyses for validity assessments of assumed Probability Density Function (PDF) methods as potential subgrid scale (SGS) closures for LES of turbulent reacting flows. Simple non-premixed reacting systems involving an isothermal reaction of the type A + B yields Products under both chemical equilibrium and non-equilibrium conditions are considered. A priori analyses are conducted of a homogeneous box flow, and a spatially developing planar mixing layer to investigate the performance of the Pearson Family of PDF's as SGS models. A posteriori analyses are conducted of the mixing layer using a hybrid one-equation Smagorinsky/PDF SGS closure. The Smagorinsky closure augmented by the solution of the subgrid turbulent kinetic energy (TKE) equation is employed to account for hydrodynamic fluctuations, and the PDF is employed for modeling the effects of scalar fluctuations. The implementation of the model requires the knowledge of the local values of the first two SGS moments. These are provided by additional modeled transport equations. In both a priori and a posteriori analyses, the predicted results are appraised by comparison with subgrid averaged results generated by DNS. Based on these results, the paths to be followed in future investigations are identified.

Adumitroaie, V.; Frankel, S. H.; Madnia, C. K.; Givi, P.

118

A Numerical Simulation of Cyclic Mesocyclogenesis

A three-dimensional nonhydrostatic numerical model, the Advanced Regional Prediction System, is used to study the process of cyclic mesocyclogenesis in a classic supercell thunderstorm. During the 4-h simulation, the storm's mesocyclone undergoes two distinct occlusions, with the beginning of a third indicated at the end of the simulation. The occlusion process exhibits a period of approximately 60 min and is

Edwin J. Adlerman; Kelvin K. Droegemeier; Robert Davies-Jones

1999-01-01

119

Cost-effective numerical simulation of SEU

A highly modified version of the PISCES 2D simulator has been developed which allows simultaneous solution of the charge collection and circuit problems and the optional use of cylindrical coordinates. The new code has been designed for use in industry and employs robust numerical methods. The program runs on a VAX-11-780 and a typical simulation takes about 4 hours of

J. G. Rollins; T. K. Tsubota; W. A. Kolasinski; N. F. Haddad; L. Rockett; M. Cerrila; W. B. Hennley

1988-01-01

120

DNS and LES of a Shear-Free Mixing Layer

NASA Technical Reports Server (NTRS)

The purpose of this work is twofold. First, given the computational resources available today, it is possible to reach, using DNS, higher Reynolds numbers than in Briggs et al.. In the present study, the microscale Reynolds numbers reached in the low- and high-energy homogeneous regions are, respectively, 32 and 69. The results reported earlier can thus be complemented and their robustness in the presence of increased turbulence studied. The second aim of this work is to perform a detailed and documented LES of the shear-free mixing layer. In that respect, the creation of a DNS database at higher Reynolds number is necessary in order to make meaningful LES assessments. From the point of view of LES, the shear-free mixing-layer is interesting since it allows one to test how traditional LES models perform in the presence of an inhomogeneity without having to deal with difficult numerical issues. Indeed, as argued in Briggs et al., it is possible to use a spectral code to study the shear-free mixing layer and one can thus focus on the accuracy of the modelling while avoiding contamination of the results by commutation errors etc. This paper is organized as follows. First we detail the initialization procedure used in the simulation. Since the flow is not statistically stationary, this initialization procedure has a fairly strong influence on the evolution. Although we will focus here on the shear-free mixing layer, the method proposed in the present work can easily be used for other flows with one inhomogeneous direction. The next section of the article is devoted to the description of the DNS. All the relevant parameters are listed and comparison with the Veeravalli & Warhaft experiment is performed. The section on the LES of the shear-free mixing layer follows. A detailed comparison between the filtered DNS data and the LES predictions is presented. It is shown that simple eddy viscosity models perform very well for the present test case, most probably because the flow seems to be almost isotropic in the small-scale range that is not resolved by the LES.

Knaepen, B.; Debliquy, O.; Carati, D.

2003-01-01

121

Direct numerical simulations of turbulence

NASA Astrophysics Data System (ADS)

Due to exceptional possibilities offered by large scientific calculators, the direct, unstationary resolution of Navier-Stokes equations in situations pertaining to turbulence and transition is now at hand. In this paper, we develop very high resolution simulations of three-dimensional incompressible and compressible mixing layers, developed isotropic turbulence, and rotating turbulence. We focus on coherent structures and large-scale intermittency. In the mixing-layer case, we show that Kelvin-Helmholtz vortices may be submitted to helical pairings, resulting into dislocations of the field. Afterwards, we look at the effect of compressibility: it is shown that, above a convective Mach number of 0.8, the helical pairing is inhibited, and yields to a staggered array of large lambda-shaped vortices, resembling the structures of a Mach 5 boundary layer computed previously. In the isotropic turbulence case, we show also the existence of large coherent low-pressure vortices. This large-scale intermittency is characterized by exponential tails of various partial differential equations (pdfs). We develop also a new subgrid-scale model allowing to reach very large Reynolds numbers through Large-eddy simulations. This model is applied with success to the backward-facing step, and to the compressible boundary layer. Finally, one looks at the influence of solid-body rotation upon free-shear flows or homogeneous turbulence. At low Rossby number, rotation is found to stabilize both cyclonic and anticyclonic vorticies. At Rossby numbers of the order of 1, cyclonic vortices are stabilized, while anticyclonic vortices are violently disrupted.

Lesieur, M.; Comte, P.; Metals, O.

122

Numerical Simulation of Nanostructure Growth

NASA Technical Reports Server (NTRS)

Nanoscale structures, such as nanowires and carbon nanotubes (CNTs), are often grown in gaseous or plasma environments. Successful growth of these structures is defined by achieving a specified crystallinity or chirality, size or diameter, alignment, etc., which in turn depend on gas mixture ratios. pressure, flow rate, substrate temperature, and other operating conditions. To date, there has not been a rigorous growth model that addresses the specific concerns of crystalline nanowire growth, while demonstrating the correct trends of the processing conditions on growth rates. Most crystal growth models are based on the Burton, Cabrera, and Frank (BCF) method, where adatoms are incorporated into a growing crystal at surface steps or spirals. When the supersaturation of the vapor is high, islands nucleate to form steps, and these steps subsequently spread (grow). The overall bulk growth rate is determined by solving for the evolving motion of the steps. Our approach is to use a phase field model to simulate the growth of finite sized nanowire crystals, linking the free energy equation with the diffusion equation of the adatoms. The phase field method solves for an order parameter that defines the evolving steps in a concentration field. This eliminates the need for explicit front tracking/location, or complicated shadowing routines, both of which can be computationally expensive, particularly in higher dimensions. We will present results demonstrating the effect of process conditions, such as substrate temperature, vapor supersaturation, etc. on the evolving morphologies and overall growth rates of the nanostructures.

Hwang, Helen H.; Bose, Deepak; Govindan, T. R.; Meyyappan, M.

2004-01-01

123

NUMERICAL SIMULATIONS OF SPICULE ACCELERATION

Observations in the H{alpha} line of hydrogen and the H and K lines of singly ionized calcium on the solar limb reveal the existence of structures with jet-like behavior, usually designated as spicules. The driving mechanism for such structures remains poorly understood. Sterling et al. shed some light on the problem mimicking reconnection events in the chromosphere with a one-dimensional code by injecting energy with different spatial and temporal distributions and tracing the thermodynamic evolution of the upper chromospheric plasma. They found three different classes of jets resulting from these injections. We follow their approach but improve the physical description by including non-LTE cooling in strong spectral lines and non-equilibrium hydrogen ionization. Increased cooling and conversion of injected energy into hydrogen ionization energy instead of thermal energy both lead to weaker jets and smaller final extent of the spicules compared with Sterling et al. In our simulations we find different behavior depending on the timescale for hydrogen ionization/recombination. Radiation-driven ionization fronts also form.

Guerreiro, N.; Carlsson, M.; Hansteen, V., E-mail: n.m.r.guerreiro@astro.uio.no, E-mail: mats.carlsson@astro.uio.no, E-mail: viggo.hansteen@astro.uio.no [Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, N-0315 Oslo (Norway)

2013-04-01

124

NASA Astrophysics Data System (ADS)

The current work focuses on the development and application of an efficient algorithm for optimization of three-dimensional turbulent flows, simulated using Direct Numerical Simulation (DNS) or Large-Eddy Simulations, and further characterized by large-dimensional optimization-parameter spaces. The optimization algorithm is based on Sequential Quadratic Programming (SQP) in combination with a damped formulation of the limited-memory BFGS method. The latter is suitable for solving large-scale constrained optimization problems whose Hessian matrices cannot be computed and stored at a reasonable cost. We combine the algorithm with a line-search merit function based on an L1-norm to enforce the convergence from any remote point. It is first shown that the proposed form of the damped L-BFGS algorithm is suitable for solving equality constrained Rosenbrock type functions. Then, we apply the algorithm to an optimal-control test problem that consists of finding the optimal initial perturbations to a turbulent temporal mixing layer such that mixing is improved at the end of a simulation time horizon T. The controls are further subject to a non-linear equality constraint on the total control energy. DNSs are used to resolve all turbulent scales of motion, and a continuous adjoint formulation is employed to calculate the gradient of the cost functionals. We compare the convergence speed of the SQP L-BFGS algorithm to a conventional non-linear conjugate-gradient method (i.e. the current standard in DNS-based optimal control), and find that the SQP algorithm is more than an order of magnitude faster than the conjugate-gradient method.

Badreddine, Hassan; Vandewalle, Stefan; Meyers, Johan

2014-01-01

125

Numerical simulation of gravitational lenses

NASA Astrophysics Data System (ADS)

Gravitational lens is a massive body or system of bodies with gravitational field that bends directions of light rays propagating nearby. This may cause an observer to see multiple images of a light source, e.g. a star, if there is a gravitational lens between the star and the observer. Light rays that form each individual image may have different distances to travel, which creates time delays between them. In complex gravitational fields generated by the system of stars, analytical calculation of trajectories and light intensities is virtually impossible. Gravitational lens of two massive bodies, one behind another, are able to create four images of a light source. Furthermore, the interaction between the four light beams can form a complicated interference pattern. This article provides a brief theory of light behavior in a gravitational field and describes the algorithm for constructing the trajectories of light rays in a gravitational field, calculating wave fronts and interference pattern of light. If you set gravitational field by any number of transparent and non- transparent objects (stars) and set emitters of radio wave beams, it is possible to calculate the interference pattern in any region of space. The proposed method of calculation can be applied even in the case of the lack of continuity between the position of the emitting stars and position of the resulting image. In this paper we propose methods of optimization, as well as solutions for some problems arising in modeling of gravitational lenses. The simulation of light rays in the sun's gravitational field is taken as an example. Also caustic is constructed for objects with uniform mass distribution.

Cherniak, Yakov

126

Numerical simulation of planar Bragg waveguide bends

An analytical form was found and transparency boundary conditions were numerically approximated for the parabolic wave equation\\u000a in curvilinear coordinates. It was shown that the solutions obtained by the parabolic equation method are in agreement with\\u000a the solutions to the spectral problem defining the Bragg waveguide modes. The field amplitude and bending loss were numerically\\u000a simulated depending on the curvature

D. V. Prokopovich; A. V. Popov; A. V. Vinogradov

2008-01-01

127

Contribution to Numerical Simulation of Laser Welding

Contribution deals with numerical simulation of thermal and stress fields in welding tubes made of austenitic stainless CrNi steel type AISI 304 with a pulsed Nd:YAG laser. Process simulation was realised by use of ANSYS 10 software. Experiments were aimed at solution of asymptotic, standard and the so-called shell model. Thermally dependent properties of AISI 304 steel were considered. Thermal

Milan Tur?a; Bohumil Taraba; Petr Ambrož; Miroslav Sahul

2011-01-01

128

Numerical simulation of unsteady viscous flows

NASA Technical Reports Server (NTRS)

Most unsteady viscous flows may be grouped into two categories, i.e., forced and self-sustained oscillations. Examples of forced oscillations occur in turbomachinery and in internal combustion engines while self-sustained oscillations prevail in vortex shedding, inlet buzz, and wing flutter. Numerical simulation of these phenomena was achieved due to the advancement of vector processor computers. Recent progress in the simulation of unsteady viscous flows is addressed.

Hankey, Wilbur L.

1987-01-01

129

Numerical Simulation Of Silicon-Ribbon Growth

NASA Technical Reports Server (NTRS)

Mathematical model includes nonlinear effects. In development simulates growth of silicon ribbon from melt. Takes account of entire temperature and stress history of ribbon. Numerical simulations performed with new model helps in search for temperature distribution, pulling speed, and other conditions favoring growth of wide, flat, relatively defect-free silicon ribbons for solar photovoltaic cells at economically attractive, high production rates. Also applicable to materials other than silicon.

Woda, Ben K.; Kuo, Chin-Po; Utku, Senol; Ray, Sujit Kumar

1987-01-01

130

Numerical Simulations of Strong Incompressible Magnetohydrodynamic Turbulence

NASA Astrophysics Data System (ADS)

Magnetized turbulence pervades the universe and is likely to play an important role in a variety of astrophysical processes. Magnetohydrodynamics provides the simplest theoretical framework in which phenomenological models for the turbulent dynamics can be built. Numerical simulations are widely used to guide and test the theoretical predictions; however, simulating MHD turbulence is not without its difficulties. Computational power limits the simulations to parameter regimes that are much less extreme than those in astrophysics and often simplifying assumptions are made in order that a wider range of scales can be accessed. After describing the competing theoretical predictions and the numerical approaches that are often employed in studying strong incompressible MHD turbulence, I will present the findings of a series of high-resolution direct numerical simulations. I will discuss the effects that physically motivated simplifying assumptions can have on the numerical solution and its physical interpretation.[4pt] Collaborators: Stanislav Boldyrev (U. Wisconsin - Madison), Fausto Cattaneo (U. Chicago), Jean C. Perez (U. New Hampshire).

Mason, Joanne

2011-11-01

131

Numerical simulations of stiff fluid gravitational singularities

Numerical simulations of the approach to the singularity in spacetimes with stiff fluid matter are presented here. The spacetimes examined have no symmetries and can be regarded as representing the general behavior of singularities in the presence of such matter. It is found that the singularity is spacelike and that as it is approached, the spacetime dynamics becomes local and non-oscillatory.

Joshua Curtis; David Garfinkle

2005-06-21

132

Simple Numerical Simulation of Strain Measurement

NASA Technical Reports Server (NTRS)

By adopting the basic principle of the reflection (and transmission) of a plane polarized electromagnetic wave incident normal to a stack of films of alternating refractive index, a simple numerical code was written to simulate the maximum reflectivity (transmittivity) of a fiber optic Bragg grating corresponding to various non-uniform strain conditions including photo-elastic effect in certain cases.

Tai, H.

2002-01-01

133

Numerical simulation of tubular blown film processing

Tubular film blowing is perhaps one of the most important and economical industrial processes used for the production of thin, biaxially oriented films. A numerical simulation of the film blowing process was performed using the Runge-Kutta scheme. The kinematic and force balance equations governing the process are derived, and the constitutive model proposed by Cao and Campbell is utilized. The model accounts for liquidlike behavior at the freeze line; it alters the demarcation between liquidlike behavior and solidlike behavior from the suggested kinematically based constraint to a rheologically based constraint, the plastic-elastic transition (PET). The paper presents a detailed discussion on how the numerical models were developed and implemented. The numerical simulation was successful in duplicating Cao and Campbell's results. Recommendations are made to gain some insight into the problem.

Pontaza, J.P.; Reddy, J.N.

2000-02-25

134

Numerical simulation for differential absorption lidar system

NASA Astrophysics Data System (ADS)

In the recent years, the most country in the word attach importance to the environmental and the protection of environmental doubly. The differential absorption lidar (DIAL)operating in the infrared wavelengths is a powerful standoff sensor for rapid remote detection of chemical emissions. It represents also a powerful technique for pollution monitoring of the atmosphere environment. Whereas, the numerical simulation system of DIAL has been shown that is a powerful tool for system design and performance evaluation and improved performance of system, along with research new information processing algorithm provided with the laboratory environment. In this Paper, a DIAL operating in the infrared(LWIR)numerical simulated system is established. It can simulate both traditional two-wavelength DIAL and multi-wavelength DIAL. It simulates the directional or scanning two operational modes. One can obtain information by it such as gas kind, concentration and distribution and verify the information processing algorithms visually. It can generate the return signals and can calculate their SNR value for various simulated environment and weather and system conditions visually. In the paper, first review laser light atmospheric propagation characteristic and then, the environmental models is ascertained including the effects of the atmosphere attenuates and scatters, the atmospheric turbulence and the roughness target producing reflective speckle and so on. Especially, the DIAL simulated system includes some new information processing algorithms of discriminating target gases and estimating their concentration. By now, the DIAL simulated system combined with this information processing algorithms has not been reported.

Wang, WeiRan; Yin, ShiRong

2007-09-01

135

DNS, Enstrophy Balance, and the Dissipation Equation in a Separated Turbulent Channel Flow

NASA Technical Reports Server (NTRS)

The turbulent flows through a plane channel and a channel with a constriction (2-D hill) are numerically simulated using DNS and RANS calculations. The Navier-Stokes equations in the DNS are solved using a higher order kinetic energy preserving central schemes and a fifth order accurate upwind biased WENO scheme for the space discretization. RANS calculations are performed using the NASA code CFL3D with the komega SST two-equation model and a full Reynolds stress model. Using DNS, the magnitudes of different terms that appear in the enstrophy equation are evaluated. The results show that the dissipation and the diffusion terms reach large values at the wall. All the vortex stretching terms have similar magnitudes within the buffer region. Beyond that the triple correlation among the vorticity and strain rate fluctuations becomes the important kinematic term in the enstrophy equation. This term is balanced by the viscous dissipation. In the separated flow, the triple correlation term and the viscous dissipation term peak locally and balance each other near the separated shear layer region. These findings concur with the analysis of Tennekes and Lumley, confirming that the energy transfer terms associated with the small-scale dissipation and the fluctuations of the vortex stretching essentially cancel each other, leaving an equation for the dissipation that is governed by the large-scale motion.

Balakumar, Ponnampalam; Rubinstein, Robert; Rumsey, Christopher L.

2013-01-01

136

Interpreting Observations of GRBs with Numerical Simulations

NASA Astrophysics Data System (ADS)

We show how numerical simulations have triggered the interpretation of GRB 101225A, so-called, the “Christmas burst.” This event is unusual because of its extremely long ?-ray emission and optical counterpart. The X-ray spectrum shows a black-body component which is present on a handful of nearby gamma-ray bursts (GRBs). Numerical models have shown that the atypical properties of this GRB can be explained by the interaction between an ultrarelativistic jet and high-density ejecta, which naturally results after the dynamical common-envelope phase of the merger between a neutron star and the He core of a red giant binary system.

Aloy, M. A.; Cuesta-Martínez, C.; Mimica, P.; Obergaulinger, M.; Thöne, C. C.; Ugarte Postigo, A.; Fryer, C.; Page, K. L.; Gorosabel, J.; Perley, D. A.; Kouveliotou, C.; Janka, H. T.; Racusin, J. L.; Christmas Burts Collaboration

2013-04-01

137

Numerical Investigations of the Leray-? Turbulence Model for Large Eddy Simulations

NASA Astrophysics Data System (ADS)

Large eddy simulations (LES) of transition to turbulence for steady flow through a model eccentric stenotic blood vessel are reported featuring the use of the Leray-? model. The Leray-? model uses a filtered velocity field for fluid advection, modifying the nonlinear vortex stretching dynamics effectively suppressing scales smaller than ?, and reducing resolution requirements, in contrast to more traditional LES models which filter the entire velocity field and enhance viscous dissipation through a computed eddy viscosity. A Helmholtz differential filter, both with and without projection of the filtered field onto a divergence free space, is used to investigate the issue of incompressibility of the filtered field. The effect of filter size is also studied. The incompressible Navier-Stokes and differential filter equations are numerically integrated using an h/p spectral-element method on a grid with 2448 hexahedral cells on up to 1024 processors on the IBM Blue Gene/L at Argonne National Laboratory. Differences between instantaneous and statistical LES results (with polynomial order 7) and recent published direct numerical simulation (DNS) results (with polynomial order 13) are discussed. Additional results from Fourier pseudospectral homogeneous isotropic turbulence simulations may be employed to shed further light on the LES results.

Frankel, S.; Kwan, Y.; Chandy, A.; Varghese, S.; Shen, J.; Fischer, P.

2007-11-01

138

NASA Astrophysics Data System (ADS)

The direct numerical simulation (DNS) offers the most accurate approach to modeling the behavior of a physical system, but carries an enormous computation cost. There exists a need for an accurate DNS to model the coupled solid-fluid system seen in targeted drug delivery (TDD), nanofluid thermal energy storage (TES), as well as other fields where experiments are necessary, but experiment design may be costly. A parallel DNS can greatly reduce the large computation times required, while providing the same results and functionality of the serial counterpart. A D2Q9 lattice Boltzmann method approach was implemented to solve the fluid phase. The use of domain decomposition with message passing interface (MPI) parallelism resulted in an algorithm that exhibits super-linear scaling in testing, which may be attributed to the caching effect. Decreased performance on a per-node basis for a fixed number of processes confirms this observation. A multiscale approach was implemented to model the behavior of nanoparticles submerged in a viscous fluid, and used to examine the mechanisms that promote or inhibit clustering. Parallelization of this model using a masterworker algorithm with MPI gives less-than-linear speedup for a fixed number of particles and varying number of processes. This is due to the inherent inefficiency of the master-worker approach. Lastly, these separate simulations are combined, and two-way coupling is implemented between the solid and fluid.

Sloan, Gregory James

139

Linear stability analysis and direct numerical simulation of a miscible two-fluid channel flow

NASA Astrophysics Data System (ADS)

The temporal evolution of an initially laminar two-fluid channel flow is investigated using linear stability analysis and direct numerical simulation. The stability of a two-fluid shear flow is encountered in numerous situations, including water wave generation by wind, atomization of fuels, aircraft deicing and nuclear reactor cooling. The application of particular interest in this study is liquefying hybrid combustion, for which the two-fluid channel flow is used as a model problem to characterize the relevant mixing and entrainment mechanisms. The two fluids are miscible with dissimilar densities and viscosities. The thickness of one of the fluid layers is much smaller than that of the other, with the denser and more viscous fluid comprising the thin layer. Linear stability analysis is used to identify possibly unstable modes in the two-fluid configuration. The analysis is considered for two different situations. In one case, the fluid density and viscosity change discontinuously across a sharp interface, while in the other, the fluids are separated by a finite thickness transition layer, over which the fluid properties vary continuously. In the sharp interface limit, the linear stability is governed by an Orr-Sommerfeld equation in each fluid layer, coupled by boundary conditions at the interface. A numerical solution of the system of equations is performed using a Chebyshev spectral collocation method. In the case where the fluids are separated by a finite thickness transition zone, an Orr-Sommerfeld-type equation is solved with the compound matrix method. The non-linear stages of the flow evolution are investigated by direct numerical simulation. In a temporal simulation, two of the three spatial dimensions are periodic. Fourier spectral discretization is used in these dimensions, while a compact finite difference scheme is utilized in the non-periodic direction. The time advancement is performed by a projection method with a third order Adams-Bashforth-Moulton predictor-corrector scheme. Initial conditions for the DNS are supplied by the linear stability analysis. Linear stability analysis indicates that the thickness of the transition zone between the fluids has a significant impact on the amplification of the two least stable modes present in the two-fluid channel flow. Compared to the sharp interface limit, one of the modes is damped and the other one is either amplified or damped depending on the Reynolds number. Two dissimilar entrainment mechanisms are observed in the DNS calculations, corresponding to these two modes. One results in significantly more entrainment and mixing between the two fluids. This mechanism exhibits a greater degree of vorticity generation, particularly due to the baroclinic effect.

Haapanen, Siina Ilona

140

Numerical simulation of centrifugal casting of pipes

NASA Astrophysics Data System (ADS)

A numerical simulation model for the horizontal centrifugal pipe casting process was developed with the commercial simulation package Flow3D. It considers - additionally to mass, energy and momentum conservation equations and free surface tracking - the fast radial and slower horizontal movement of the mold. The iron inflow is not steady state but time dependent. Of special importance is the friction between the liquid and the mold in connection with the viscosity and turbulence of the iron. Experiments with the mold at controlled revolution speeds were carried out using a high-speed camera. From these experiments friction coefficients for the description of the interaction between mold and melt were obtained. With the simulation model, the influence of typical process parameters (e.g. melts inflow, mold movement, melt temperature, cooling media) on the wall thickness of the pipes can be studied. The comparison to results of pipes from production shows a good agreement between simulation and reality.

Kaschnitz, E.

2012-07-01

141

Numerical Simulation of a Tornado Generating Supercell

NASA Technical Reports Server (NTRS)

The development of tornadoes from a tornado generating supercell is investigated with a large eddy simulation weather model. Numerical simulations are initialized with a sounding representing the environment of a tornado producing supercell that affected North Carolina and Virginia during the Spring of 2011. The structure of the simulated storm was very similar to that of a classic supercell, and compared favorably to the storm that affected the vicinity of Raleigh, North Carolina. The presence of mid-level moisture was found to be important in determining whether a supercell would generate tornadoes. The simulations generated multiple tornadoes, including cyclonic-anticyclonic pairs. The structure and the evolution of these tornadoes are examined during their lifecycle.

Proctor, Fred H.; Ahmad, Nashat N.; LimonDuparcmeur, Fanny M.

2012-01-01

142

Issues in Numerical Simulation of Fire Suppression

This paper outlines general physical and computational issues associated with performing numerical simulation of fire suppression. Fire suppression encompasses a broad range of chemistry and physics over a large range of time and length scales. The authors discuss the dominant physical/chemical processes important to fire suppression that must be captured by a fire suppression model to be of engineering usefulness. First-principles solutions are not possible due to computational limitations, even with the new generation of tera-flop computers. A basic strategy combining computational fluid dynamics (CFD) simulation techniques with sub-grid model approximations for processes that have length scales unresolvable by gridding is presented.

Tieszen, S.R.; Lopez, A.R.

1999-04-12

143

Cost-effective numerical simulation of SEU

A highly modified version of the PISCES 2D simulator has been developed which allows simultaneous solution of the charge collection and circuit problems and the optional use of cylindrical coordinates. The new code has been designed for use in industry and employs robust numerical methods. The program runs on a VAX-11-780 and a typical simulation takes about 4 hours of CPU time. Fortran source code will be distributed at low cost. Test runs have been conducted on an advanced submicron IC process and excellent agreement with cyclotron test data was obtained.

Rollins, J.G.; Tsubota, T.K.; Kolasinski, W.A.; Haddad, N.F.; Rockett, L.; Cerrila, M.; Hennley, W.B.

1988-12-01

144

Numerical simulation of magma energy extraction

The Magma Energy Program is a speculative endeavor regarding practical utility of electrical power production from the thermal energy which reside in magma. The systematic investigation has identified an number of research areas which have application to the utilization of magma energy and to the field of geothermal energy. Eight topics were identified which involve thermal processes and which are areas for the application of the techniques of numerical simulation. These areas are: (1) two-phase flow of the working fluid in the wellbore, (2) thermodynamic cycles for the production of electrical power, (3) optimization of the entire system, (4) solidification and fracturing of the magma caused by the energy extraction process, (5) heat transfer and fluid flow within an open, direct-contact, heat-exchanger, (6) thermal convection in the overlying geothermal region, (7) thermal convection within the magma body, and (8) induced natural convection near the thermal energy extraction device. Modeling issues have been identified which will require systematic investigation in order to develop the most appropriate strategies for numerical simulation. It appears that numerical simulations will be of ever increasing importance to the study of geothermal processes as the size and complexity of the systems of interest increase. It is anticipated that, in the future, greater emphasis will be placed on the numerical simulation of large-scale, three-dimensional, transient, mixed convection in viscous flows and porous media. Increased computational capabilities, e.g.; massively parallel computers, will allow for the detailed study of specific processes in fractured media, non-Darcy effects in porous media, and non-Newtonian effects. 23 refs., 13 figs., 1 tab.

Hickox, C.E.

1991-01-01

145

Numerical simulation of orbiting black holes

We present numerical simulations of binary black hole systems which for the first time last for about one orbital period for close but still separate black holes as indicated by the absence of a common apparent horizon. An important part of the method is the construction of comoving coordinates, in which both the angular and radial motion is minimized through a dynamically adjusted shift condition. We use fixed mesh refinement for computational efficiency.

Bernd Bruegmann; Wolfgang Tichy; Nina Jansen

2004-05-12

146

Numerical simulation and nasal air-conditioning.

Heating and humidification of the respiratory air are the main functions of the nasal airways in addition to cleansing and olfaction. Optimal nasal air conditioning is mandatory for an ideal pulmonary gas exchange in order to avoid desiccation and adhesion of the alveolar capillary bed. The complex three-dimensional anatomical structure of the nose makes it impossible to perform detailed in vivo studies on intranasal heating and humidification within the entire nasal airways applying various technical set-ups. The main problem of in vivo temperature and humidity measurements is a poor spatial and time resolution. Therefore, in vivo measurements are feasible only to a restricted extent, solely providing single temperature values as the complete nose is not entirely accessible. Therefore, data on the overall performance of the nose are only based on one single measurement within each nasal segment. In vivo measurements within the entire nose are not feasible. These serious technical issues concerning in vivo measurements led to a large number of numerical simulation projects in the last few years providing novel information about the complex functions of the nasal airways. In general, numerical simulations merely calculate predictions in a computational model, e.g. a realistic nose model, depending on the setting of the boundary conditions. Therefore, numerical simulations achieve only approximations of a possible real situation. The aim of this review is the synopsis of the technical expertise on the field of in vivo nasal air conditioning, the novel information of numerical simulations and the current state of knowledge on the influence of nasal and sinus surgery on nasal air conditioning. PMID:22073112

Keck, Tilman; Lindemann, Jörg

2010-01-01

147

Numerical simulations on ion acoustic double layers

A comprehensive numerical study of ion acoustic double layers has been performed for both periodic as well as for nonperiodic systems by means of one-dimensional particle simulations. For a nonperiodic system, an external battery and a resistance are used to model the magnetospheric convection potential and the ionospheric Pedersen resistance. It is found that the number of double layers and the associated potential buildup across the system increases with the system length.

Sato, T.; Okuda, H.

1980-07-01

148

Numerical simulation of magma energy extraction

NASA Astrophysics Data System (ADS)

The Magma Energy Program is a speculative endeavor regarding practical utility of electrical power production from the thermal energy which resides in magma. The systematic investigation has identified a number of research areas which have application to the utilization of magma energy and to the field of geothermal energy. Eight topics were identified which involve thermal processes and which are areas for the application of the techniques of numerical simulation. These areas are (1) two-phase flow of the working fluid in the wellbore, (2) thermodynamic cycles for the production of electrical power, (3) optimization of the entire system, (4) solidification and fracturing of the magma caused by the energy extraction process, (5) heat transfer and fluid flow within an open, direct-contact, heat-exchanger, (6) thermal convection in the overlying geothermal region, (7) thermal convection within the magma body, and (8) induced natural convection near the thermal energy extraction device. Modeling issues have been identified which will require systematic investigation in order to develop the most appropriate strategies for numerical simulation. It appears that numerical simulations will be of ever increasing importance to the study of geothermal processes as the size and complexity of the systems of interest increase. It is anticipated that, in the future, greater emphasis will be placed on the numerical simulation of large-scale, three-dimensional, transient, mixed convection in viscous flows and porous media. Increased computational capabilities, e.g.; massively parallel computers, will allow for the detailed study of specific processes in fractured media, non-Darcy effects in porous media, and non-Newtonian effects.

Hickox, C. E.

149

Numerical simulation and nasal air-conditioning

Heating and humidification of the respiratory air are the main functions of the nasal airways in addition to cleansing and olfaction. Optimal nasal air conditioning is mandatory for an ideal pulmonary gas exchange in order to avoid desiccation and adhesion of the alveolar capillary bed. The complex three-dimensional anatomical structure of the nose makes it impossible to perform detailed in vivo studies on intranasal heating and humidification within the entire nasal airways applying various technical set-ups. The main problem of in vivo temperature and humidity measurements is a poor spatial and time resolution. Therefore, in vivo measurements are feasible only to a restricted extent, solely providing single temperature values as the complete nose is not entirely accessible. Therefore, data on the overall performance of the nose are only based on one single measurement within each nasal segment. In vivo measurements within the entire nose are not feasible. These serious technical issues concerning in vivo measurements led to a large number of numerical simulation projects in the last few years providing novel information about the complex functions of the nasal airways. In general, numerical simulations merely calculate predictions in a computational model, e.g. a realistic nose model, depending on the setting of the boundary conditions. Therefore, numerical simulations achieve only approximations of a possible real situation. The aim of this review is the synopsis of the technical expertise on the field of in vivo nasal air conditioning, the novel information of numerical simulations and the current state of knowledge on the influence of nasal and sinus surgery on nasal air conditioning. PMID:22073112

Keck, Tilman; Lindemann, Jörg

2011-01-01

150

NASA Astrophysics Data System (ADS)

Large eddy simulations (LES) of low-speed, wall-bounded turbulent flows were conducted by numerically integrating the compressible Navier-Stokes equations in a generalized curvilinear coordinate system. An efficient numerical scheme based on a third-order additive semi-implicit Runge-Kutta method for time advancement and a sixth-order accurate, compact finite-difference scheme for spatial discretization were used. The convective terms in the wall-normal direction were treated implicitly to remove the time-step limitation associated with the use of fine meshes in the near-wall region for high Reynolds number viscous flows. The dynamic Smagorinsky subgrid-scale eddy viscosity model was used to close the filtered equations. Generalized characteristic-based non-reflecting boundary conditions were used together with grid stretching and enhanced damping in the exit zone. The accuracy and efficiency of the numerical scheme was assessed by simple acoustic model problems and by comparing LES predictions for fully developed turbulent channel flow and turbulent separated flow in an asymmetric diffuser to previous direct numerical simulation (DNS) and experimental data, respectively. LES predictions for both flows were in reasonable agreement with the DNS and experimental mean velocity and turbulence statistics. The findings suggest that the numerical approach employed here offers comparable accuracy to similar recent studies at approximately one-third of the computational cost and may provide both an accurate and efficient way to conduct computational aeroacoustics studies for low Mach number, confined turbulent flows.

Suh, Jungsoo; Frankel, Steven H.; Mongeau, Luc; Plesniak, Michael W.

2006-07-01

151

2000 Numerical Propulsion System Simulation Review

NASA Technical Reports Server (NTRS)

The technologies necessary to enable detailed numerical simulations of complete propulsion systems are being developed at the NASA Glenn Research Center in cooperation with industry, academia, and other government agencies. Large scale, detailed simulations will be of great value to the nation because they eliminate some of the costly testing required to develop and certify advanced propulsion systems. In addition, time and cost savings will be achieved by enabling design details to be evaluated early in the development process before a commitment is made to a specific design. This concept is called the Numerical Propulsion System Simulation (NPSS). NPSS consists of three main elements: (1) engineering models that enable multidisciplinary analysis of large subsystems and systems at various levels of detail, (2) a simulation environment that maximizes designer productivity, and (3) a cost-effective. high-performance computing platform. A fundamental requirement of the concept is that the simulations must be capable of overnight execution on easily accessible computing platforms. This will greatly facilitate the use of large-scale simulations in a design environment. This paper describes the current status of the NPSS with specific emphasis on the progress made over the past year on air breathing propulsion applications. Major accomplishments include the first formal release of the NPSS object-oriented architecture (NPSS Version 1) and the demonstration of a one order of magnitude reduction in computing cost-to-performance ratio using a cluster of personal computers. The paper also describes the future NPSS milestones, which include the simulation of space transportation propulsion systems in response to increased emphasis on safe, low cost access to space within NASA'S Aerospace Technology Enterprise. In addition, the paper contains a summary of the feedback received from industry partners on the fiscal year 1999 effort and the actions taken over the past year to respond to that feedback. NPSS was supported in fiscal year 2000 by the High Performance Computing and Communications Program.

Lytle, John; Follen, Greg; Naiman, Cynthia; Veres, Joseph; Owen, Karl; Lopez, Isaac

2001-01-01

152

2001 Numerical Propulsion System Simulation Review

NASA Technical Reports Server (NTRS)

The technologies necessary to enable detailed numerical simulations of complete propulsion systems are being developed at the NASA Glenn Research Center in cooperation with industry, academia and other government agencies. Large scale, detailed simulations will be of great value to the nation because they eliminate some of the costly testing required to develop and certify advanced propulsion systems. In addition, time and cost savings will be achieved by enabling design details to be evaluated early in the development process before a commitment is made to a specific design. This concept is called the Numerical Propulsion System Simulation (NPSS). NPSS consists of three main elements: (1) engineering models that enable multidisciplinary analysis of large subsystems and systems at various levels of detail, (2) a simulation environment that maximizes designer productivity, and (3) a cost-effective, high-performance computing platform. A fundamental requirement of the concept is that the simulations must be capable of overnight execution on easily accessible computing platforms. This will greatly facilitate the use of large-scale simulations in a design environment. This paper describes the current status of the NPSS with specific emphasis on the progress made over the past year on air breathing propulsion applications. Major accomplishments include the first formal release of the NPSS object-oriented architecture (NPSS Version 1) and the demonstration of a one order of magnitude reduction in computing cost-to-performance ratio using a cluster of personal computers. The paper also describes the future NPSS milestones, which include the simulation of space transportation propulsion systems in response to increased emphasis on safe, low cost access to space within NASA's Aerospace Technology Enterprise. In addition, the paper contains a summary of the feedback received from industry partners on the fiscal year 2000 effort and the actions taken over the past year to respond to that feedback. NPSS was supported in fiscal year 2001 by the High Performance Computing and Communications Program.

Lytle, John; Follen, Gregory; Naiman, Cynthia; Veres, Joseph; Owen, Karl; Lopez, Isaac

2002-01-01

153

NASA Astrophysics Data System (ADS)

The motion of small particles in turbulent conditions is influenced by the entire range of length- and time-scales of the flow. At high Reynolds numbers this range of scales is too broad for direct numerical simulation (DNS). Such flows can only be approached using large-eddy simulation (LES), which requires the introduction of a sub-filter model for the momentum dynamics. Likewise, for the particle motion the effect of sub-filter scales needs to be reconstructed approximately, as there is no explicit access to turbulent sub-filter scales. To recover the dynamic consequences of the unresolved scales, partial reconstruction through approximate deconvolution of the LES-filter is combined with explicit stochastic forcing in the equations of motion of the particles. We analyze DNS of high-Reynolds turbulent channel flow to a priori extract the ideal forcing that should be added to retain correct statistical properties of the dispersed particle phase in LES. The probability density function of the velocity differences that need to be included in the particle equations and their temporal correlation display a striking and simple structure with little dependence on Reynolds number and particle inertia, provided the differences are normalized by their RMS, and the correlations expressed in wall units. This is key to the development of a general "stand-alone" stochastic forcing for inertial particles in LES.

Geurts, B. J.; Kuerten, J. G. M.

2012-08-01

154

Numeric Simulation Tools of the IMPEx Infrastructure

NASA Astrophysics Data System (ADS)

The EU-FP7 Project "Integrated Medium for Planetary Exploration" (IMPEx) was established as a result of scientific collaboration between institutions across Europe and is working on the integration of a set of interactive data analysis and modeling tools in the field of space plasma physics. These tools are comprised of numerical Hybrid/MHD and analytical Paraboloid magnetospheric models from the simulation sector as well as AMDA, ClWeb and 3DView from the data analysis and visualization sector. The basic feature of IMPEx consists in connection of different data sources, including archived computational simulation results and observational data, in order to analyse and visualize scientific data by means of interactive web-based tools. In this presentation we introduce Hybrid and Magnetohydrodynamic Modelling (HMM) environment which is an example of simulation services within IMPEx. HMM includes two global numerical hybrid models (a hybrid model HYB from FMI and a hybrid model from LATMOS) and magnetohydrodynamic model (GUMICS) from FMI. Especially, we introduce the web service, Hybrid Web Archive[1] which enables access to the simulation runs made by HYB and GUMICS models in the IMPEx HMM environment.

Kallio, E. J.; Khodachenko, M. L.; Génot, V.; Schmidt, W.; Jarvinen, R.; Häkkinen, L.; Al-Ubaidi, T.; Topf, F.; Modolo, R.; Hess, S.; Alexeev, I. I.

2012-09-01

155

Numerical Simulation of Flowing Blood Cells

NASA Astrophysics Data System (ADS)

The cellular detail of blood is an essential factor in its flow, especially in vessels or devices with size comparable to that of its suspended cells. This article motivates and reviews numerical simulation techniques that provide a realistic description of cell-scale blood flow by explicitly representing its coupled fluid and solid mechanics. Red blood cells are the principal focus because of their importance and because of their remarkable deformability, which presents particular simulation challenges. Such simulations must couple discretizations of the large-deformation elasticity of the cells with the viscous flow mechanics of the suspension. The Reynolds numbers are low, so the effectively linear fluid mechanics is amenable to a wide range of simulation methods, although the constitutive models and geometric factors of the coupled system introduce challenging nonlinearity. Particular emphasis is given to the relative merits of several fundamentally different simulation methods. The detailed description provided by such simulations is invaluable for advancing our scientific understanding of blood flow, and their ultimate impact will be in the design of biomedical tools and interventions.

Freund, Jonathan B.

2014-01-01

156

Direct numerical simulations of the double scalar mixing layer. Part II: Reactive scalars

The reacting double scalar mixing layer (RDSML) is investigated as a canonical multistream flow and a model problem for simple piloted diffusion flames. In piloted diffusion flames, the reacting fuel and oxidizer streams are initially separated by a central pilot stream at stoichiometric composition. The primary purpose of this pilot is to delay the mixing of the pure streams until a stable flame base can develop. In such multistream systems, the modeling of turbulent scalar mixing is complicated by the multiple feed streams, leading to more complex fine-scale statistics, which remain as yet an unmet modeling challenge compared to the simpler two-feed system. In Part I we described how multimodal mixture fraction probability density functions (PDFs) and conditional scalar dissipation rates can be modeled with a presumed mapping function approach. In this work we present an efficient and robust extension of the modeling to a general multistream reacting flow and compare predictions to three-dimensional direct numerical simulations (DNS) of the RDSML with a single-step reversible chemistry model and varying levels of extinction. With high extinction levels, the interaction with the pilot stream is described. Additionally, state-of-the-art combustion modeling calculations including conditional moment closure (CMC) and stationary laminar flamelet modeling (SLFM) are performed with the newly developed mixing model. Excellent agreement is found between the DNS and modeling predictions, even where the PDF is essentially a triple-delta shape near the flame base, so long as extinction levels are moderate to low. The suggested approach outlined in this paper is strictly valid only for flows that can be described by a single mixture fraction. For these flows the approach should provide engineers with fine-scale models that are of accuracy comparable to those already available for binary mixing, at only marginally higher complexity and cost. (author)

Mortensen, Mikael [School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, 2006 Sydney NSW (Australia); de Bruyn Kops, Stephen M. [Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA 01003 (United States); Cha, Chong M. [Department of Mechanical Engineering, Indiana University-Purdue University, Indianapolis, IN 46202 (United States)

2007-06-15

157

A direct numerical simulation study of vorticity transformation in weakly turbulent premixed flames

NASA Astrophysics Data System (ADS)

Database obtained earlier in 3D Direct Numerical Simulations (DNS) of statistically stationary, 1D, planar turbulent flames characterized by three different density ratios ? is processed in order to investigate vorticity transformation in premixed combustion under conditions of moderately weak turbulence (rms turbulent velocity and laminar flame speed are roughly equal to one another). In cases H and M characterized by ? = 7.53 and 5.0, respectively, anisotropic generation of vorticity within the flame brush is reported. In order to study physical mechanisms that control this phenomenon, various terms in vorticity and enstrophy balance equations are analyzed, with both mean terms and terms conditioned on a particular value c of the combustion progress variable being addressed. Results indicate an important role played by baroclinic torque and dilatation in transformation of average vorticity and enstrophy within both flamelets and flame brush. Besides these widely recognized physical mechanisms, two other effects are documented. First, viscous stresses redistribute enstrophy within flamelets, but play a minor role in the balance of the mean enstrophy overline{? } within turbulent flame brush. Second, negative correlation overline{mathbf {u}^' } \\cdot nabla ? ^' }} between fluctuations in velocity u and enstrophy gradient contributes substantially to an increase in the mean overline{? } within turbulent flame brush. This negative correlation is mainly controlled by the positive correlation between fluctuations in the enstrophy and dilatation and, therefore, dilatation fluctuations substantially reduce the damping effect of the mean dilatation on the vorticity and enstrophy fields. In case L characterized by ? = 2.5, these effects are weakly pronounced and overline{? } is reduced mainly due to viscosity. Under conditions of the present DNS, vortex stretching plays a minor role in the balance of vorticity and enstrophy within turbulent flame brush in all three cases.

Lipatnikov, A. N.; Nishiki, S.; Hasegawa, T.

2014-10-01

158

Numerical reproducibility for implicit Monte Carlo simulations

We describe and compare different approaches for achieving numerical reproducibility in photon Monte Carlo simulations. Reproducibility is desirable for code verification, testing, and debugging. Parallelism creates a unique problem for achieving reproducibility in Monte Carlo simulations because it changes the order in which values are summed. This is a numerical problem because double precision arithmetic is not associative. In [1], a way of eliminating this roundoff error using integer tallies was described. This approach successfully achieves reproducibility at the cost of lost accuracy by rounding double precision numbers to fewer significant digits. This integer approach, and other extended reproducibility techniques, are described and compared in this work. Increased precision alone is not enough to ensure reproducibility of photon Monte Carlo simulations. A non-arbitrary precision approaches required a varying degree of rounding to achieve reproducibility. For the problems investigated in this work double precision global accuracy was achievable by using 100 bits of precision or greater on all unordered sums which where subsequently rounded to double precision at the end of every time-step. (authors)

Cleveland, M.; Brunner, T.; Gentile, N. [Lawrence Livermore National Laboratory, P. O. Box 808, Livermore CA 94550 (United States)

2013-07-01

159

Numerical simulation of cross-country skiing.

A program for numerical simulation of a whole ski race, from start to finish, is developed in MATLAB. The track is modelled by a set of cubical splines in two dimensions and can be used to simulate a track in a closed loop or with the start and finish at different locations. The forces considered in the simulations are gravitational force, normal force between snow and skis, drag force from the wind, frictional force between snow and ski and driving force from the skier. The differential equations of motion are solved from start to finish with the Runge-Kutta method. Different wind situations during the race can be modelled, as well as different glide conditions on different parts of the track. It is also possible to vary the available power during the race. The simulation program's output is the total time of the race, together with the forces and speed during different parts of the race and intermediate times at selected points. Some preliminary simulations are also presented. PMID:21607888

Carlsson, Peter; Tinnsten, Mats; Ainegren, Mats

2011-08-01

160

Direct numerical simulations of capillary wave turbulence.

This work presents direct numerical simulations of capillary wave turbulence solving the full three-dimensional Navier-Stokes equations of a two-phase flow. When the interface is locally forced at large scales, a statistical stationary state appears after few forcing periods. Smaller wave scales are generated by nonlinear interactions, and the wave height spectrum is found to obey a power law in both wave number and frequency, in good agreement with weak turbulence theory. By estimation of the mean energy flux from the dissipated power, the Kolmogorov-Zakharov constant is evaluated and found to be compatible with the exact theoretical value. The time scale separation between linear, nonlinear interaction, and dissipative times is also observed. These numerical results confirm the validity of the weak turbulence approach to quantify out-of equilibrium wave statistics. PMID:24972211

Deike, Luc; Fuster, Daniel; Berhanu, Michael; Falcon, Eric

2014-06-13

161

Direct numerical simulations of capillary wave turbulence

This work presents Direct Numerical Simulations of capillary wave turbulence solving the full 3D Navier Stokes equations of a two-phase flow. When the interface is locally forced at large scales, a statistical stationary state appears after few forcing periods. Smaller wave scales are generated by nonlinear interactions, and the wave height spectrum is found to obey a power law in both wave number and frequency in good agreement with weak turbulence theory. By estimating the mean energy flux from the dissipated power, the Kolmogorov-Zakharov constant is evaluated and found to be compatible with the exact theoretical value. The time scale separation between linear, nonlinear interaction and dissipative times is also observed. These numerical results confirm the validity of weak turbulence approach to quantify out-of equilibrium wave statistics.

Luc Deike; Daniel Fuster; Michaël Berhanu; Eric Falcon

2014-06-03

162

Numerical simulation of platelet margination in microcirculation

NASA Astrophysics Data System (ADS)

The adhesion of platelets to vascular walls is the first step in clotting. This process critically depends on the preferential concentration of platelets near walls. The presence of red blood cells, which are the predominant blood constituents, is known to affect the steady state platelet concentration and the dynamic platelet margination, but the underlying mechanism is not well understood to-day. We use a direct numerical simulation to study the platelet margination process, with particular emphasis on the Stokesian hydrodynamic interactions among red cells, platelets, and vessel walls. Well-known mechanical models are used for the shearing and bending stiffness of red cell membranes, and the stiffer platelets are modeled as rigid discoids. A boundary integral formulation is used to solve the flow field, where the numerical solution procedure is accelerated by a parallel O(N N) smooth particle-mesh Ewald method. The effects of red cell hematocrit and deformability will be discussed.

Zhao, Hong; Shaqfeh, Eric

2009-11-01

163

Turbulent convection: comparison of Reynolds stress models with numerical simulations

Turbulent convection: comparison of Reynolds stress models with numerical simulations Friedrich, University of Vienna, AÂ1090 Vienna, Austria ABSTRACT Numerical simulations of turbulent convection have of basic properties of compressible convection, and stellar atmospheres. Fully nonlocal convection models

Demoulin, Pascal

164

Direct numerical simulation of turbulent mixing.

The results of three-dimensional numerical simulations of turbulent flows obtained by various authors are reviewed. The paper considers the turbulent mixing (TM) process caused by the development of the main types of instabilities: those due to gravitation (with either a fixed or an alternating-sign acceleration), shift and shock waves. The problem of a buoyant jet is described as an example of the mixed-type problem. Comparison is made with experimental data on the TM zone width, profiles of density, velocity and turbulent energy and degree of homogeneity. PMID:24146009

Statsenko, V P; Yanilkin, Yu V; Zhmaylo, V A

2013-11-28

165

Numerical simulation of swept-wing flows

NASA Technical Reports Server (NTRS)

Efforts of the last six months to computationally model the transition process characteristics of flow over swept wings are described. Specifically, the crossflow instability and crossflow/Tollmien-Schlichting wave interactions are analyzed through the numerical solution of the full 3D Navier-Stokes equations including unsteadiness, curvature, and sweep. This approach is chosen because of the complexity of the problem and because it appears that linear stability theory is insufficient to explain the discrepancies between different experiments and between theory and experiment. The leading edge region of a swept wing is considered in a 3D spatial simulation with random disturbances as the initial conditions.

Reed, Helen L.

1991-01-01

166

The numerical simulation of accelerator components

The techniques of the numerical simulation of plasmas can be readily applied to problems in accelerator physics. Because the problems usually involve a single component ''plasma,'' and times that are at most, a few plasma oscillation periods, it is frequently possible to make very good simulations with relatively modest computation resources. We will discuss the methods and illustrate them with several examples. One of the more powerful techniques of understanding the motion of charged particles is to view computer-generated motion pictures. We will show several little movie strips to illustrate the discussions. The examples will be drawn from the application areas of Heavy Ion Fusion, electron-positron linear colliders and injectors for free-electron lasers. 13 refs., 10 figs., 2 tabs.

Herrmannsfeldt, W.B.; Hanerfeld, H.

1987-05-01

167

Eruptions of Coronal Jets in Numerical Simulations

NASA Astrophysics Data System (ADS)

Observations by Yohkoh, SOHO, Hinode and STEREO show that X-ray jets in the coronal holes, with an inverse-Y shape, occur at a rate of about 60 jets/day, and have a typical outward velocity of 160km/s, height of 50 Mm and width of 8 Mm, and liftime of 10 min (Savcheva et al., 2007). Numerical simulations suggest that jets are related to the reconnection between the newly emerging and pre-existing magnetic fields. We therefore here study the eruption of coronal jets by MHD simulations of flux emergence into the corona with ambient fields. To examine the role of the ambient fields, we carry out a set of simulations with fields of varying directions and strength. Reconnection of the emerging and ambient fields converts magnetic energy into thermal and kinetic energy of the plasma. The simulation results here captures the statistical properties of coronal jets, with temperature up to 2.5 MK, and speed up to 180 km/s. Heat conduction along the field lines and radiative cooling are also implemented to produce a more realistic thermal structure along the field lines, which allows us to compare the synthetic images with AIA observations.

Fang, F.; Fan, Y.

2013-12-01

168

Terascale High-Fidelity Simulations of Turbulent Combustion with Detailed Chemistry

This SciDAC project enabled a multidisciplinary research consortium to develop a high fidelity direct numerical simulation (DNS) software package for the simulation of turbulent reactive flows. Within this collaboration, the authors, based at CMU's Pittsburgh Supercomputing Center (PSC), focused on extensive new developments in Sandia National Laboratories' "S3D" software to address more realistic combustion features and geometries while exploiting Terascale computational possibilities. This work significantly advances the state-of-the-art of DNS of turbulent reacting flows.

Raghurama Reddy; Roberto Gomez; Junwoo Lim; Yang Wang; Sergiu Sanielevici

2004-10-15

169

Direct numerical simulations of bidisperse bubbly flows

NASA Astrophysics Data System (ADS)

Three-dimensional direct numerical simulations of homogeneous bidisperse bubbly flows at finite Reynolds numbers are presented. A front-tracking/finite-difference method is used to accurately follow the motion of N1 large bubbles of volume V1 and N2 small bubbles of volume V_2. Periodic boundary conditions are used to approximate homogeneous flows. The effect of the interaction between the large bubbles and the small bubbles on the statistical properties of the flow is studied. These statistical properties include the rise velocity, fluctuation velocity, and dispersion cofficient of the bubbles as well as the fluctuation velocity and kinetic energy spectrum of the liquid. To characterize the respective position and motion of the bubbles, the pair probability density is examined. The void fraction ranges between 2% and 12%. Two cases are considered. In the first case, V_1=2V2 and N_1=N_2. The results are found to be quantitatively similar to the results of the monodisperse simulations. In the second case, V_1=8V2 and N1 < N_2. The results exhibit strong differences from the monodisperse simulation results. The small bubbles experience much higher fluctuations relative to their rise velocity tham the large bubbles. The liquid kinetic energy is larger than in the monodisperse case. It appears that the small bubbles hinder the motion of the large bubbles and the large bubbles pull the small bubbles along as they rise.

Bunner, Bernard; Göz, Manfred; Sommerfeld, Martin; Tryggvason, Gretar

2001-11-01

170

Numerical Simulations of Weak Lensing Measurements

(Abridged) Weak gravitational lensing induces distortions on the images of background galaxies, and thus provides a direct measure of mass fluctuations in the universe. Since the distortions induced by lensing on the images of background galaxies are only of a few percent, a reliable measurement demands very accurate galaxy shape estimation and a careful treatment of systematic effects. Here, we present a study of a shear measurement method using detailed simulations of artificial images. The images are produced using realisations of a galaxy ensemble drawn from the HST Groth strip. We consider realistic observational effects including atmospheric seeing, PSF anisotropy and pixelisation, incorporated in a manner to reproduce actual observations with the William Herschel Telescope. By applying an artificial shear to the simulated images, we test the shear measurement method proposed by Kaiser, Squires & Broadhurst (1995, KSB). Overall, we find the KSB method to be reliable with several provisos. To guide future weak lensing surveys, we study how seeing size, exposure time and pixelisation affect the sensitivity to shear. In addition, we study the impact of overlapping isophotes of neighboring galaxies, and find that this effect can produce spurious lensing signals on small scales. We discuss the prospects of using the KSB method for future, more sensitive, surveys. Numerical simulations of this kind are a required component of present and future analyses of weak lensing surveys.

David Bacon; Alexandre Refregier; Douglas Clowe; Richard Ellis

2000-07-03

171

Numerical Simulations of High Enthalpy Pulse Facilities

NASA Technical Reports Server (NTRS)

Axisymmetric flows within shock tubes and expansion tubes are simulated including the effects of finite rate chemistry and both laminar and turbulent boundary layers. The simulations demonstrate the usefulness of computational fluid dynamics for characterizing the flows in high enthalpy pulse facilities. The modeling and numerical requirements necessary to simulate these flows accurately are also discussed. Although there is a large body of analysis which explains and quantifies the boundary layer growth between the shock and the interface in a shock tube, there is a need for more detailed solutions. Phenomena such as thermochemical nonequilibrium. or turbulent transition behind the shock are excluded in the assumptions of Mirels' analysis. Additionally there is inadequate capability to predict the influence of the boundary layer on the expanded gas behind the interface. Quantifying the gas in this region is particularly important in expansion tubes because it is the location of the test gas. Unsteady simulations of the viscous flow in shock tubes are computationally expensive because they must follow features such as a shock wave over the length of the facility and simultaneously resolve the small length scales within the boundary layer. As a result, efficient numerical algorithms are required. The numerical approach of the present work is to solve the axisymmetric gas dynamic equations using an finite-volume formulation where the inviscid fluxes are computed with a upwind TVD scheme. Multiple species equations are included in the formulation so that finite-rate chemistry can be modeled. The simulations cluster grid points at the shock and interface and translate this clustered grid with these features to minimize numerical errors. The solutions are advanced at a CFL number of less than one based on the inviscid gas dynamics. To avoid limitations on the time step due to the viscous terms, these terms are treated implicitly. This requires a block tri-diagonal matrix inversion along each line of cells normal to the wall. The cost of this inversion is more than offset by the larger allowable time step. The source terms representing the finite-rate chemical kinetics are also treated implicitly. An algebraic turbulence model for compressible flow is used. The flow in a low pressure shock tube is computed and the results are compared with Mirels'analysis. The driven gas is nitrogen at 70 Pa, and the incident shock speed is approximately 2.9 km/sec so that there is little dissociation. The simulations include a laminar boundary layer and are run until the limiting flow regime is achieved. At this limit, the shock and interface travel at the same velocity because the amount of driven gas between these two features remains the same: the mass flow across the shock is equal to the mass of gas being entrained at the interface by the boundary layer. Simulations with several grids are presented to establish the grid independence of the solution, Good agreement is achieved between Mirels' correlations and the computations. This is expected since the flow conditions are chosen to be consistent with the assumptions used in Mirels' analysis. This comparison adds credibility to the numerical approach and highlights some of the differences between the theory and the detailed simulations. In addition, simulations of the HYPULSE expansion tube are presented for two operating conditions and the computations are compared to experimental data. The operating gas for both cases is nitrogen. One test condition is at a total enthalpy of 15.2 MJ/Kg and a relatively low pressure of 2 kPa. This case is characterized by a laminar boundary layer and significant chemical nonequilibrium. in the acceleration gas. The second test condition is at a total enthalpy of 10.2 MJ/Kg and a pressure of 38 kPa and is characterized by a turbulent boundary layer. The simulations compare well with experiment and reveal that the nonuniformity in pressure observed during the test time is related to variations in the boundary layer displacement thickness.

Wilson, Gregory J.; Edwards, Thomas A. (Technical Monitor)

1995-01-01

172

Multi-Scale Simulations of Stirred Liquid–Liquid Dispersions

Direct numerical simulations (DNS) of liquid–liquid dispersions in a three-dimensional periodic domain have been performed by means of the lattice–Boltzmann method. The dispersion was agitated by a random force field such as to produce isotropic turbulence. The turbulence levels and its history were based on a fluid parcel in a turbulently stirred tank traversing the impeller swept region. The DNS

J. J. Derksen

2007-01-01

173

Direct Numerical Simulation of dense particle-laden turbulent flows using immersed boundaries

NASA Astrophysics Data System (ADS)

Dense particle-laden turbulent flows play an important role in many engineering applications, ranging from pharmaceutical coating and chemical synthesis to fluidized bed reactors. Because of the complexity of the physics involved in these flows, current computational models for gas-particle processes, such as drag and heat transfer, rely on empirical correlations and have been shown to lack accuracy. In this work, direct numerical simulations (DNS) of dense particle-laden flows are conducted, using immersed boundaries (IB) to resolve the flow around each particle. First, the accuracy of the proposed approach is tested on a range of 2D and 3D flows at various Reynolds numbers, and resolution requirements are discussed. Then, various particle arrangements and number densities are simulated, the impact on particle wake interaction is assessed, and existing drag models are evaluated in the case of fixed particles. In addition, the impact of the particles on turbulence dissipation is investigated. Finally, a strategy for handling moving and colliding particles is discussed.

Wang, Fan; Desjardins, Olivier

2009-11-01

174

Numerical simulation on hypervelocity collision fragmentation

NASA Astrophysics Data System (ADS)

Fragmentation events are the most important source of space debris. The hypervelocity col-lision fragmentation is one of the main categories and is divided into catastrophic and non-catastrophic collisions. Experimental studies have shown that there exists a fragmentation threshold value which is usually characterized by the ratio of impact kinetic energy with the target mass. The target may undergo catastrophic collision if the ratio exceeds the threshold value, otherwise non-catastrophic collision will be expected. The main objective of this paper is to study the hypervelocity collision fragmentation initiated by numerical simulations. For different aerospace materials, projectiles with different shapes and collision velocity to different targets, dozens of cases are performed to determine the threshold value. The effect of material, shape and collision velocities on the results is analyzed to verify the threshold value for the fragmentation.

Song, Weidong; Wang, Ronglan

175

Computing abstraction hierarchies by numerical simulation

We present a novel method for building ABSTRIPS-style abstraction hierarchies in planning. The aim of this method is to minimize the amount of backtracking between abstraction levels. Previous approaches have determined the criticality of operator preconditions by reasoning about plans directly. Here, we adopt a simpler and faster approach where we use numerical simulation of the planning process. We demonstrate the theoretical advantages of our approach by identifying some simple properties lacking in previous approaches but possessed by our method. We demonstrate the empirical advantages of our approach by a set of four benchmark experiments using the ABTWEAK system. We compare the quality of the abstraction hierarchies generated with those built by the ALPINE and HIGHPOINT algorithms.

Bundy, A.; Giunchiglia, F.; Sebastiani, R.; Walsh, T.

1996-12-31

176

Direct Numerical Simulations of PBX 9501

NASA Astrophysics Data System (ADS)

We have explicitly gridded HMX crystals in PBX 9501 from 25 ?m in diameter up to ˜.5 mm. We used HMX particle size distributions found in the literature to determine the relative numbers of different sized particulates. We applied our modified Mori-Tanaka theory to model the smaller crystals embedded in the plasticized estane binder (the dirty-binder). This model was modified to accommodate the large amount of HMX in the dirty binder. We then subjected the ˜1 million element PBX 9501 realization to boundary conditions commensurate with a Split Hopkinson Pressure Bar experiment. We compare results to experiment and a micro-mechanical model we have reported on earlier. We also discuss the information which can be extracted from these direct numerical simulations.

Mas, E. M.; Clements, B. E.; George, D. C.

2004-07-01

177

Numerical aerodynamic simulation facility feasibility study

NASA Technical Reports Server (NTRS)

There were three major issues examined in the feasibility study. First, the ability of the proposed system architecture to support the anticipated workload was evaluated. Second, the throughput of the computational engine (the flow model processor) was studied using real application programs. Third, the availability reliability, and maintainability of the system were modeled. The evaluations were based on the baseline systems. The results show that the implementation of the Numerical Aerodynamic Simulation Facility, in the form considered, would indeed be a feasible project with an acceptable level of risk. The technology required (both hardware and software) either already exists or, in the case of a few parts, is expected to be announced this year. Facets of the work described include the hardware configuration, software, user language, and fault tolerance.

1979-01-01

178

Numerical Simulations of Acoustically Driven, Burning Droplets

NASA Technical Reports Server (NTRS)

This computational study focuses on understanding and quantifying the effects of external acoustical perturbations on droplet combustion. A one-dimensional, axisymmetric representation of the essential diffusion and reaction processes occurring in the vicinity of the droplet stagnation point is used here in order to isolate the effects of the imposed acoustic disturbance. The simulation is performed using a third order accurate, essentially non-oscillatory (ENO) numerical scheme with a full methanol-air reaction mechanism. Consistent with recent microgravity and normal gravity combustion experiments, focus is placed on conditions where the droplet is situated at a velocity antinode in order for the droplet to experience the greatest effects of fluid mechanical straining of flame structures. The effects of imposed sound pressure level and frequency are explored here, and conditions leading to maximum burning rates are identified.

Kim, H.-C.; Karagozian, A. R.; Smith, O. I.; Urban, Dave (Technical Monitor)

1999-01-01

179

History of the numerical aerodynamic simulation program

NASA Technical Reports Server (NTRS)

The Numerical Aerodynamic Simulation (NAS) program has reached a milestone with the completion of the initial operating configuration of the NAS Processing System Network. This achievement is the first major milestone in the continuing effort to provide a state-of-the-art supercomputer facility for the national aerospace community and to serve as a pathfinder for the development and use of future supercomputer systems. The underlying factors that motivated the initiation of the program are first identified and then discussed. These include the emergence and evolution of computational aerodynamics as a powerful new capability in aerodynamics research and development, the computer power required for advances in the discipline, the complementary nature of computation and wind tunnel testing, and the need for the government to play a pathfinding role in the development and use of large-scale scientific computing systems. Finally, the history of the NAS program is traced from its inception in 1975 to the present time.

Peterson, Victor L.; Ballhaus, William F., Jr.

1987-01-01

180

Direct Numerical Simulation of Automobile Cavity Tones

NASA Technical Reports Server (NTRS)

The Navier Stokes equation is solved computationally by the Dispersion-Relation-Preserving (DRP) scheme for the flow and acoustic fields associated with a laminar boundary layer flow over an automobile door cavity. In this work, the flow Reynolds number is restricted to R(sub delta*) < 3400; the range of Reynolds number for which laminar flow may be maintained. This investigation focuses on two aspects of the problem, namely, the effect of boundary layer thickness on the cavity tone frequency and intensity and the effect of the size of the computation domain on the accuracy of the numerical simulation. It is found that the tone frequency decreases with an increase in boundary layer thickness. When the boundary layer is thicker than a certain critical value, depending on the flow speed, no tone is emitted by the cavity. Computationally, solutions of aeroacoustics problems are known to be sensitive to the size of the computation domain. Numerical experiments indicate that the use of a small domain could result in normal mode type acoustic oscillations in the entire computation domain leading to an increase in tone frequency and intensity. When the computation domain is expanded so that the boundaries are at least one wavelength away from the noise source, the computed tone frequency and intensity are found to be computation domain size independent.

Kurbatskii, Konstantin; Tam, Christopher K. W.

2000-01-01

181

Numerical simulation of solar coronal magnetic fields

NASA Technical Reports Server (NTRS)

Many aspects of solar activity are believed to be due to the stressing of the coronal magnetic field by footpoint motions at the photosphere. The results are presented of a fully spectral numerical simulation which is the first 3-D time dependent simulation of footpoint stressing in a geometry appropriate for the corona. An arcade is considered that is initially current-free and impose a smooth footpoint motion that produces a twist in the field of approx 2 pi. The footprints were fixed and the evolution was followed until the field relaxes to another current-free state. No evidence was seen for any instability, either ideal or resistive and no evidence for current sheet formation. The most striking feature of the evolution is that in response to photospheric motions, the field expands rapidly upward to minimize the stress. The expansion has two important effects. First, it suppresses the development of dips in the field that could support dense, cool material. For the motions assumed, the magnetic field does not develop a geometry suitable for prominence formation. Second, the expansion inhibits ideal instabilities such as kinking. The results indicate that simple stearing of a single arcade is unlikely to lead to solar activity such as flares or prominences. Effects are discussed that might possibly lead to such activity.

Dahlburg, Russell B.; Antiochos, Spiro K.; Zang, T. A.

1990-01-01

182

Numerical Simulations of the G Ring Arc

NASA Astrophysics Data System (ADS)

We conducted numerical simulations of 3,830 massless test particles in the Saturn system near the 7:6 corotation eccentricity resonance (CER) with Mimas, in response to the recent observations of an arc in the faint and dusty G Ring (see Hedman et al, this conference). We simulated 80 years of evolution, taking into account the perturbations of Saturn's nine most massive moons, the Sun, Jupiter, and Saturn's J2 and J4 gravity harmonics. Perturber trajectories were obtained at every timestep from JPL ephemera DE414 and SAT252. We find that particles are efficiently trapped into librations about any of the six corotation sites, consistent with the observed arc which extends 30o in longitude. We find forced eccentricities that are consistent with the observed radial width of the arc, and a long libration period (1273 days) which indicates we may have yet to observe the turning point in the arc's libration. The potentially disruptive effects of inter-particle collisions were not taken into account, due to the G Ring's very low optical depth. Leakage out of the resonance is probably dominated instead by non-gravitational forces, which preferentially drive smaller (dust) grains outward, plausibly accounting for the observed radial profile of the G Ring.

Tiscareno, Matthew S.; Hedman, M. M.; Burns, J. A.; Nicholson, P. D.

2007-07-01

183

Collisionless microinstabilities in stellarators. II. Numerical simulations

Microinstabilities exhibit a rich variety of behavior in stellarators due to the many degrees of freedom in the magnetic geometry. It has recently been found that certain stellarators (quasi-isodynamic ones with maximum-J geometry) are partly resilient to trapped-particle instabilities, because fast-bouncing particles tend to extract energy from these modes near marginal stability. In reality, stellarators are never perfectly quasi-isodynamic, and the question thus arises whether they still benefit from enhanced stability. Here, the stability properties of Wendelstein 7-X and a more quasi-isodynamic configuration, QIPC, are investigated numerically and compared with the National Compact Stellarator Experiment and the DIII-D tokamak. In gyrokinetic simulations, performed with the gyrokinetic code GENE in the electrostatic and collisionless approximation, ion-temperature-gradient modes, trapped-electron modes, and mixed-type instabilities are studied. Wendelstein 7-X and QIPC exhibit significantly reduced growth rates for all simulations that include kinetic electrons, and the latter are indeed found to be stabilizing in the energy budget. These results suggest that imperfectly optimized stellarators can retain most of the stabilizing properties predicted for perfect maximum-J configurations.

Proll, J. H. E.; Xanthopoulos, P.; Helander, P. [Max-Planck-Institut für Plasmaphysik, EURATOM Association, Teilinstitut Greifswald, Wendelsteinstraße 1, 17491 Greifswald, Germany and Max-Planck/Princeton Research Center for Plasma Physics, 17491 Greifswald (Germany)] [Max-Planck-Institut für Plasmaphysik, EURATOM Association, Teilinstitut Greifswald, Wendelsteinstraße 1, 17491 Greifswald, Germany and Max-Planck/Princeton Research Center for Plasma Physics, 17491 Greifswald (Germany)

2013-12-15

184

Numerical simulation of the Polywell device

Recent ideas concerning inertial-electrostatic confinement (IEC) of fusion plasmas coupled with recent experimental results have motivated looking at the problem of confinement of these plasmas in both the gridded (pure electrostatic) and magnetically assisted (via confinement of high beta plasmas in a magnetic cusp) configuration. Questions exist as to the nature of the potential well structure and the confinement properties of high beta plasmas in magnetic cusp configurations. This work focuses on the magnetically assisted concept known as the Polywell{trademark}. Results are reported on the numerical simulation of IEC plasmas aimed at answering some of these questions. In particular the authors focus on two aspects of the Polywell, namely the structure of the magnetic cusp field in the Polywell configuration and the nature of the confinement of a high beta plasma in a magnetic cusp field. The existence of line cusps in the Polywell is still in dispute. A computer code for modeling the magnetic field structure and mod-B surface has been written and results are presented for the Polywell. Another source of controversy is the nature of the confinement of a high beta plasma in a magnetic cusp, and in particular in the polywell. Results from 2-D Particle In Cell (PIC) simulations aimed at answering some of these questions are presented.

Simmons, K.H.; Santarius, J.F. [Univ. of Wisconsin, Madison, WI (United States). Fusion Technology Inst.

1995-12-31

185

NASA Astrophysics Data System (ADS)

An innovative method for prescribing turbulent thermal inflow information in spatially developing boundary layers under streamwise pressure gradients is introduced for attached flows. The approach is tested and validated in a suite of Direct Numerical Simulations (DNS) of thermal boundary layers for zero (ZPG) and adverse (APG) pressure gradients with momentum thickness Reynolds numbers (Re?) up to 3000. The turbulent thermal data are generated based on the dynamic multi-scale approach proposed by Araya et al. ["A dynamic multi-scale approach for turbulent inflow boundary conditions in spatially evolving flows," J. Fluid Mech. 670, 581-605 (2011)], which is extended to include thermal field simulations in the present article. The approach is based on the original rescaling-recycling method developed by Lund, Wu, and Squires ["Generation of turbulent inflow data for spatially developing boundary layer simulations," J. Comput. Phys. 140, 233-258 (1998)] for ZPG flows. Isothermal walls are considered for the thermal field and the molecular Prandtl number is 0.71. In addition, only inlet momentum/thermal boundary layer thicknesses must be prescribed while other flow parameters such as the inlet friction velocity, u?, and friction temperature, ??, are computed dynamically based on the flow solution obtained downstream by means of a test plane. This plane is located between the inlet and recycle stations. Based on the unique and extensive DNS results of heat transfer obtained in this investigation, the effects of Reynolds numbers and adverse pressure gradients on the flow and thermal parameters are also explored and visualized. The principal outcome of adverse pressure gradient on the flow parameters has been determined as a secondary peak, particularly on the streamwise velocity fluctuations in the outer region, which shows clear evidence of energy production in the outer flow and not only in the buffer layer as traditionally known. Nevertheless, this peak is not so obvious on the thermal fluctuations but it is hypothesized that the reason is mainly attributed to the absence of a freestream thermal gradient, as imposed in the velocity field. Furthermore, the high-speed streaks in the buffer layer are observed to be notably shorter and wider in a Strong APG than in the ZPG case. Finally, a significant decrease of the turbulent Prandtl number is attributed to the presence of a Strong APG.

Araya, Guillermo; Castillo, Luciano

2013-09-01

186

Numerical simulation of cloud rise phenomena associated with nuclear bursts

We present numerical simulations of cloud evolution from nuclear explosions using high-resolution numerical methods. Our numerical approach includes a fluid mechanical model that is a combination of a compressible code (GEODYN) and a low Mach code (LMC). Early stages of nuclear explosions that are characterized by the blust wave propagation are simulated with an explicit code (GEODYN) that solves the

Y. Kanarska; I. Lomov; L. Glenn; T. Antoun

2009-01-01

187

Numerical simulations of ultrafast pulse measurements

NASA Astrophysics Data System (ADS)

Recently, an extremely sensitive technique---OPA-XFROG has been developed. A short pump pulse serves as the gate by parametrically amplifying a short segment of the signal pulse in a nonlinear crystal. High optical parametric gain makes possible the complete measurement of ultraweak, ultrashort light pulses. Unlike interferometric methods, it does not carry prohibitively restrictive requirements, such as perfect mode-matching, perfect spatial coherence, highly stable absolute phase, and a same-spectrum reference pulse. We simulate the OPA-XFROG technique and show that by a proper choice of the nonlinear crystal and the noncollinear mixing geometry it is possible to match the group velocities of the pump, signal, and idler pulses, which permits the use of relatively thick crystals to achieve high gain without measurement distortion. Gain bandwidths of ˜100 nm are possible, limited by group velocity dispersion. In the second part of the thesis, we numerically simulate the performance of the ultrasimple ultrashort laser pulse measurement device---GRENOUILLE. While simple in practice, GRENOUILLE has many theoretical subtleties because it involves the second-harmonic generation of relatively tightly focused and broadband pulses. In addition, these processes occur in a thick crystal, in which the phase-matching bandwidth is deliberately made narrow compared to the pulse bandwidth. We developed a model that include all sum-frequency-generation processes, both collinear and noncollinear. We also include dispersion using the Sellmeier equation for the crystal BBO. Working in the frequency domain, we compute the GRENOUILLE trace for practical---and impractical---examples and show that accurate measurements are easily obtained for properly designed devices. For pulses far outside a GRENOUILLE's operating range (on the long side), we numerically deconvolve the GRENOUILLE trace with the response function of GRENOUILLE to improve its spectral resolution. In the last part of the thesis, we simulate the second harmonic generation with tightly focused beams by use of lens. Thus, we are able to explain the 'weird' focusing effect that has been a 'puzzles' for us in the GRENOUILLE measurement.

Liu, Xuan

188

The Numerical Simulation of Thunderstorm Outflow Dynamics

NASA Astrophysics Data System (ADS)

Two high-resolution, two-dimensional numerical models are developed and used to investigate the dynamics of thunderstorm outflows. The first model employs the set of unapproximated, inviscid, fully compressible hydrodynamical equations, while the second, more economical model is based on a simplified set of inviscid, "quasi-compressible" equations. Neither model includes moist microphysical processes. The outflow models are designed with the flexibility needed to address many aspects of outflow dynamics with a high degree of physical realism. An outflow may be initialized in the models as a purely horizontal flow issuing from a lateral boundary, as a cold vertical downdraft imposed at the upper boundary (which is assumed to represent cloud base), or as a downdraft parameterized by a heat sink. The sensitivity of modeled outflow properties to these initialization techniques is addressed. Turbulent mixing in outflows, manifest as Kelvin -Helmholtz shearing instability, is successfully simulated with the outflow models. Although this type of hydrodynamical instability has long been observed in laboratory density currents (the dynamical analog of outflows), this is the first time it has ever been reproduced in modeled outflows. The characteristics of the Kelvin-Helmholtz instability are compared with linear theory and laboratory results, and the sensitivity of the associated turbulent mixing to several physical and computational parameters is discussed. The characteristics of the ambient environment are found to play key roles in the dynamics of simulated outflows. Several vertical wind shear and static stability profiles are examined in the model, and their influence on outflow behavior is addressed. Surface frictional effects are also shown to significantly alter the internal flow structure of outflows. Results of simulations with and without surface friction are compared, and several features in the modeled flows are shown to be similar to structure observed in laboratory density currents and thunderstorm outflows.

Droegemeier, Kelvin Kay

189

NASA Technical Reports Server (NTRS)

Spatially evolving instabilities in a boundary layer on a flat plate are computed by direct numerical simulation (DNS) of the incompressible Navier-Stokes equations. In a truncated physical domain, a nonstaggered mesh is used for the grid. A Chebyshev-collocation method is used normal to the wall; finite difference and compact difference methods are used in the streamwise direction; and a Fourier series is used in the spanwise direction. For time stepping, implicit Crank-Nicolson and explicit Runge-Kutta schemes are used to the time-splitting method. The influence-matrix technique is used to solve the pressure equation. At the outflow boundary, the buffer-domain technique is used to prevent convective wave reflection or upstream propagation of information from the boundary. Results of the DNS are compared with those from both linear stability theory (LST) and parabolized stability equation (PSE) theory. Computed disturbance amplitudes and phases are in very good agreement with those of LST (for small inflow disturbance amplitudes). A measure of the sensitivity of the inflow condition is demonstrated with both LST and PSE theory used to approximate inflows. Although the DNS numerics are very different than those of PSE theory, the results are in good agreement. A small discrepancy in the results that does occur is likely a result of the variation in PSE boundary condition treatment in the far field. Finally, a small-amplitude wave triad is forced at the inflow, and simulation results are compared with those of LST. Again, very good agreement is found between DNS and LST results for the 3-D simulations, the implication being that the disturbance amplitudes are sufficiently small that nonlinear interactions are negligible.

Joslin, Ronald D.; Streett, Craig L.; Chang, Chau-Lyan

1992-01-01

190

DNS of Laminar-Turbulent Transition in Swept-Wing Boundary Layers

NASA Technical Reports Server (NTRS)

Direct numerical simulation (DNS) is performed to examine laminar to turbulent transition due to high-frequency secondary instability of stationary crossflow vortices in a subsonic swept-wing boundary layer for a realistic natural-laminar-flow airfoil configuration. The secondary instability is introduced via inflow forcing and the mode selected for forcing corresponds to the most amplified secondary instability mode that, in this case, derives a majority of its growth from energy production mechanisms associated with the wall-normal shear of the stationary basic state. An inlet boundary condition is carefully designed to allow for accurate injection of instability wave modes and minimize acoustic reflections at numerical boundaries. Nonlinear parabolized stability equation (PSE) predictions compare well with the DNS in terms of modal amplitudes and modal shape during the strongly nonlinear phase of the secondary instability mode. During the transition process, the skin friction coefficient rises rather rapidly and the wall-shear distribution shows a sawtooth pattern that is analogous to the previously documented surface flow visualizations of transition due to stationary crossflow instability. Fully turbulent features are observed in the downstream region of the flow.

Duan, L.; Choudhari, M.; Li, F.

2014-01-01

191

Numerical Simulation Of Landslide-generated Waves

NASA Astrophysics Data System (ADS)

Landslide-generated water waves have been of great interest to ocean and coastal engi- neers, as well as to dams-keeping engineers. Many researchers, therefore, have studied generation mechanism and propagation characteristics of the water waves generated by landslides. Recently, Watts (1997) carried out experiments on propagations of the landslide-generated waves generated by using some kind of particles and investigated the relationship between particle diameters and volumes and wave characteristic. Fritz (2000) attempted to measure fluid motion induced the landslides by using the PIV method. However, the generation mechanism of the landslide-generated waves has not been understood yet. Most analytical and numerical studies are based on the shallow water approximation. However, neglecting vertical accelerations is inaccurate in the generation zone where water depth change rapidly and granular particles, such as rock and sand, run down. In addition, most studies have been dealt with motion of particles as a block motion. Heinrich (1992) simulated a block motion slipping down along an inclined slope and the fluid motion around the block by using the Volume of Fluid (VOF) method. But his method can not predict the granular motion such as a landslide. As mentioned above, the generation mechanics and propagation characteristics of landslide generated water waves have not been investigated with enough accuracy. Because of the assumptions used in each approaches, it can be said that those ap- proaches give incorrect answers and predictions for the region, which the assumptions are supposed to be impropriety. The aim of this study is to develope a numerical model for analyzing generation and propagation process of water waves by granular landslide with taking granular motion run-down a slope and nonlinearity of fluid motion induced by granular motion into consideration. The three-dimensional discrete element method (DEM) is applied to analyze granu- lar landslides. Campbell et al.(1995) carried out large-scale landslide simulations by using the DEM. In their calculations, two-dimensional discs were used. In this study, applying the 3-D DEM makes calculation and prediction of the real granular particle motions possible. The fluid motion induced by the granular motion is solved by calculating mass con- servation equation and the three-dimensional Navier-Stokes equations with taking particle-fluid interactions and volume of the particles into consideration. The water surface is traced by using the volume of fluid method (VOF) extended to analysis of liquid-solid flow. Using the developed numerical models, some landslide-generated waves were pre- dicted. The calculated results on the time history of the surface elevations at the given locations were compared with the experimental results performed by Watts (1997). The comparisons show the reasonable agreement between numerical and experimen- tal results. Furthermore, the fluid motion induced by the particle motion shows the similar tendency with measurement result by Fritz using the PIV system.

Shigematsu, T.; Hirose, M.; Oda, K.

192

NASA Astrophysics Data System (ADS)

High-resolution direct numerical simulations (DNSs) are an important tool for the detailed analysis of turbidity current dynamics. Models that resolve the vertical structure and turbulence of the flow are typically based upon the Navier-Stokes equations. Two-dimensional simulations are known to produce unrealistic cohesive vortices that are not representative of the real three-dimensional physics. The effect of this phenomena is particularly apparent in the later stages of flow propagation. The ideal solution to this problem is to run the simulation in three dimensions but this is computationally expensive. This paper presents a novel finite-element (FE) DNS turbidity current model that has been built within Fluidity, an open source, general purpose, computational fluid dynamics code. The model is validated through re-creation of a lock release density current at a Grashof number of 5 × 106 in two and three dimensions. Validation of the model considers the flow energy budget, sedimentation rate, head speed, wall normal velocity profiles and the final deposit. Conservation of energy in particular is found to be a good metric for measuring model performance in capturing the range of dynamics on a range of meshes. FE models scale well over many thousands of processors and do not impose restrictions on domain shape, but they are computationally expensive. The use of adaptive mesh optimisation is shown to reduce the required element count by approximately two orders of magnitude in comparison with fixed, uniform mesh simulations. This leads to a substantial reduction in computational cost. The computational savings and flexibility afforded by adaptivity along with the flexibility of FE methods make this model well suited to simulating turbidity currents in complex domains.

Parkinson, S. D.; Hill, J.; Piggott, M. D.; Allison, P. A.

2014-09-01

193

Numerical simulation of "an American haboob"

NASA Astrophysics Data System (ADS)

A dust storm of fearful proportions hit Phoenix in the early evening hours of 5 July 2011. This storm, an American haboob, was predicted hours in advance because numerical, land-atmosphere modeling, computing power and remote sensing of dust events have improved greatly over the past decade. High-resolution numerical models are required for accurate simulation of the small scales of the haboob process, with high velocity surface winds produced by strong convection and severe downbursts. Dust productive areas in this region consist mainly of agricultural fields, with soil surfaces disturbed by plowing and tracks of land in the high Sonoran Desert laid barren by ongoing draught. Model simulation of the 5 July 2011 dust storm uses the coupled atmospheric-dust model NMME-DREAM (Non-hydrostatic Mesoscale Model on E grid, Janjic et al., 2001; Dust REgional Atmospheric Model, Nickovic et al., 2001; Pérez et al., 2006) with 4 km horizontal resolution. A mask of the potentially dust productive regions is obtained from the land cover and the normalized difference vegetation index (NDVI) data from the Moderate Resolution Imaging Spectroradiometer (MODIS). The scope of this paper is validation of the dust model performance, and not use of the model as a tool to investigate mechanisms related to the storm. Results demonstrate the potential technical capacity and availability of the relevant data to build an operational system for dust storm forecasting as a part of a warning system. Model results are compared with radar and other satellite-based images and surface meteorological and PM10 observations. The atmospheric model successfully hindcasted the position of the front in space and time, with about 1 h late arrival in Phoenix. The dust model predicted the rapid uptake of dust and high values of dust concentration in the ensuing storm. South of Phoenix, over the closest source regions (~25 km), the model PM10 surface dust concentration reached ~2500 ?g m-3, but underestimated the values measured by the PM10 stations within the city. Model results are also validated by the MODIS aerosol optical depth (AOD), employing deep blue (DB) algorithms for aerosol loadings. Model validation included Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), equipped with the lidar instrument, to disclose the vertical structure of dust aerosols as well as aerosol subtypes. Promising results encourage further research and application of high-resolution modeling and satellite-based remote sensing to warn of approaching severe dust events and reduce risks for safety and health.

Vukovic, A.; Vujadinovic, M.; Pejanovic, G.; Andric, J.; Kumjian, M. R.; Djurdjevic, V.; Dacic, M.; Prasad, A. K.; El-Askary, H. M.; Paris, B. C.; Petkovic, S.; Nickovic, S.; Sprigg, W. A.

2014-04-01

194

Numerical simulation of "An American Haboob"

NASA Astrophysics Data System (ADS)

A dust storm of fearful proportions hit Phoenix in the early evening hours of 5 July 2011. This storm, an American haboob, was predicted hours in advance because numerical, land-atmosphere modeling, computing power and remote sensing of dust events have improved greatly over the past decade. High resolution numerical models are required for accurate simulation of the small-scales of the haboob process, with high velocity surface winds produced by strong convection and severe downbursts. Dust productive areas in this region consist mainly of agricultural fields, with soil surfaces disturbed by plowing and tracks of land in the high Sonoran desert laid barren by ongoing draught. Model simulation of the 5 July 2011 dust storm uses the coupled atmospheric-dust model NMME-DREAM with 3.5 km horizontal resolution. A mask of the potentially dust productive regions is obtained from the land cover and the Normalized Difference Vegetation Index (NDVI) data from the Moderate Resolution Imaging Spectroradiometer (MODIS). Model results are compared with radar and other satellite-based images and surface meteorological and PM10 observations. The atmospheric model successfully hindcasted the position of the front in space and time, with about 1 h late arrival in Phoenix. The dust model predicted the rapid uptake of dust and high values of dust concentration in the ensuing storm. South of Phoenix, over the closest source regions (~ 25 km), the model PM10 surface dust concentration reached ~ 2500 ?g m-3, but underestimated the values measured by the PM10stations within the city. Model results are also validated by the MODIS aerosol optical depth (AOD), employing deep blue (DB) algorithms for aerosol loadings. Model validation included Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), equipped with the lidar instrument, to disclose the vertical structure of dust aerosols as well as aerosol subtypes. Promising results encourage further research and application of high-resolution modeling and satellite-based remote sensing to warn of approaching severe dust events and reduce risks for safety and health.

Vukovic, A.; Vujadinovic, M.; Pejanovic, G.; Andric, J.; Kumjian, M. R.; Djurdjevic, V.; Dacic, M.; Prasad, A. K.; El-Askary, H. M.; Paris, B. C.; Petkovic, S.; Nickovic, S.; Sprigg, W. A.

2013-10-01

195

The difficulty of experimental measurements of the scalar dissipation rate in turbulent flames has required researchers to estimate the true three-dimensional (3D) scalar dissipation rate from one-dimensional (1D) or two-dimensional (2D) gradient measurements. In doing so, some relationship must be assumed between the true values and their lower dimensional approximations. We develop these relationships by assuming a form for the statistics of the gradient vector orientation, which enables several new results to be obtained and the true 3D scalar dissipation PDF to be reconstructed from the lower-dimensional approximations. We use direct numerical simulations (DNS) of turbulent plane jet flames to examine the orientation statistics, and verify our assumptions and final results. We develop and validate new theoretical relationships between the lower-dimensional and true moments of the scalar dissipation PDF assuming a log-normal true PDF. We compare PDFs reconstructed from lower-dimensional gradient projections with the true values and find an excellent agreement for a 2D simulated measurement and also for a 1D simulated measurement perpendicular to the mean flow variations. Comparisons of PDFs of thermal dissipation from DNS with those obtained via reconstruction from 2D experimental measurements show a very close match, indicating this PDF is not unique to a particular flame configuration. We develop a technique to reconstruct the joint PDF of the scalar dissipation and any other scalar, such as chemical species or temperature. Reconstructed conditional means of the hydroxyl mass fraction are compared with the true values and an excellent agreement is obtained.

Chen, Jackie [Sandia National Laboratories (SNL); Sankaran, Ramanan [ORNL; Hawkes, Evatt R [Sandia National Laboratories (SNL)

2009-05-01

196

Numerical simulation of magma chamber dynamics.

NASA Astrophysics Data System (ADS)

Magma chambers are characterized by periodic arrivals of deep magma batches that give origin to complex patterns of magma convection and mixing, and modify the distribution of physical quantities inside the chamber. We simulate the transient, 2D, multi-component homogeneous dynamics in geometrically complex dyke+chamber systems, by means of GALES, a finite element parallel C++ code solving mass, momentum and energy equations for multi-component homogeneous gas-liquid (± crystals) mixtures in compressible-to-incompressible flow conditions. Code validation analysis includes several cases from the classical engineering literature, corresponding to a variety of subsonic to supersonic gas-liquid flow regimes (see http://www.pi.ingv.it/~longo/gales/gales.html). The model allows specification of the composition of the different magmas in the domain, in terms of ten major oxides plus the two volatile species H2O and CO2. Gas-liquid thermodynamics are modeled by using the compositional dependent, non-ideal model in Papale et al. (Chem.. Geol., 2006). Magma properties are defined in terms of local pressure, temperature, and composition including volatiles. Several applications are performed within domains characterized by the presence of one or more magma chambers and one or more dykes, with different geometries and characteristic size from hundreds of m to several km. In most simulations an initial compositional interface is placed at the top of a feeding dyke, or at larger depth, with the deeper magma having a lower density as a consequence of larger volatile content. The numerical results show complex patterns of magma refilling in the chamber, with alternating phases of magma ingression and magma sinking from the chamber into the feeding dyke. Intense mixing takes place in feeding dykes, so that the new magma entering the chamber is always a mixture of the deep and the initially resident magma. Buoyant plume rise occurs through the formation of complex convective patterns, giving origin to a density-stratified magma chamber.

Longo, Antonella; Papale, Paolo; Montagna, Chiara Paola; Vassalli, Melissa; Giudice, Salvatore; Cassioli, Andrea

2010-05-01

197

Numerical Simulations of Saturn's Polar Cyclones

NASA Astrophysics Data System (ADS)

Shawn R. Brueshaber, Department of Mechanical Engineering, Western Michigan UniversityKunio M. Sayanagi, Atmospheric and Planetary Sciences, Hampton UniversityCassini mission to Saturn has revealed evidences of a warm core cyclone centered on each of the poles of the planet. The morphology of the clouds in these cyclones resembles that of a terrestrial hurricane. The formation and maintenance mechanisms of these large polar cyclones are yet to be explained. Scott (2011, Astrophys. Geophys. Fluid Dyn) proposed that cyclonic vortices beta-drifting poleward can result in a polar cyclone, and demonstrated that beta-drifting cyclonic vortices can indeed cause accumulation of cyclonic vorticity at the pole using a 1-layer quasi-geostrophic model.The objectives of our project is to test Scott's hypothesis using a 1.5-layer shallow-water model and many-layer primitive equations model. We use the Explicit Planetary Isentropic Coordinate (EPIC) model (Dowling et al. 1998, 2004, Icarus) to perform direct numerical simulations of Saturn's polar atmosphere. To date, our project has focused on modifying the model to construct a polar rectangular model grid in order to avoid the problem of polar singularity associated with the conventional latitude-longitude grids employed in many general circulation models. We present our preliminary simulations, which show beta-drifting cyclones cause a poleward flux of cyclonic vorticity, which is consistent with Scott's results.Our study is partially supported by NASA Outer Planets Research Grant NNX12AR38G and NSF Astronomy and Astrophysics Grant 1212216 to KMS.

Brueshaber, Shawn R.; Sayanagi, Kunio M.

2014-11-01

198

NASA Astrophysics Data System (ADS)

Parameterization of turbulent momentum and heat fluxes in a turbulent, stably stratified boundary layer flow over water surface is important for numerical climate modeling and weather prediction. In this work, the detailed structure and statistical characteristics of a turbulent, stably stratified atmospheric boundary layer flow over water surface is studied by direct numerical simulation (DNS). The most difficult case for modeling is that of flows at high Reynolds numbers and sufficiently steep surface waves, when strongly non-linear effects (e.g. sheltering, boundary layer separation, vortex formation etc.) are encountered. Of special interest is the influence of the wind flow stratification on the properties of boundary-layer turbulence and the turbulent momentum and heat fluxes. In DNS a two-dimensional water wave with different wave age parameters (c/u*, where u* is the friction velocity and c is the wave celerity), wave slope ka varying from 0 to 0.2 and bulk Reynolds number Re (from 15000 to 80000) and different Richardson numbers are considered. The shape of the water wave is prescribed and does not evolve under the action of the wind. The full, 3D Navier-Stokes equations under the Boussinesq approximation are solved in curvilinear coordinates in a frame of reference moving the phase velocity of the wave. The shear driving the flow is created by an upper plane boundary moving horizontally with a bulk velocity in the x-direction. Periodic boundary conditions are considered in the horizontal (x) and lateral (y) directions, and no-slip boundary condition is considered in the vertical z-direction. The grid of 360 x 240 x 360 nodes in the x, y, and z directions is used. The Adams-Bashforth method is employed to advance the integration in time and the equation for the pressure is solved iteratively. Ensemble-averaged velocity and pressure fields are evaluated by averaging over time and the spanwise coordinate. Profiles of the mean velocity and turbulent stresses are obtained by averaging over wavelength. The DNS results show that the properties of the boundary layer flow are significantly affected by stratification. If the Richardson number Ri is sufficiently small, the flow remains turbulent and qualitatively similar to the non-stratified case. On the other hand, at high Ri turbulent fluctuations and momentum and heat fluxes decay to zero at low wave slope but remain finite at sufficiently large ka (>0.15). Parameterization of turbulent and heat production, diffusion and dissipation is also performed by a closure procedure and compared with the results of DNS. The criteria in terms of the product of the Kolmogorov time scale and local buoyancy frequency or/and the ratio of the Kolmogorov vs. Ozmidov lengh scales is proposed to characterize the different flow regimes observed in DNS. This work was supported by RFBR (project Nos. 10-05-91177, 14-05-00367) and by the grant from the Government of the Russian Federation under contract No. 11.G34.31.0048.

Druzhinin, Oleg; Troitskaya, Yuliya; Zilitinkevich, Sergej

2014-05-01

199

Data from a 1152X760X1280 direct numerical simulation (DNS) of a transitional Rayleigh-Taylor mixing layer modeled after a small Atwood number water channel experiment is used to comprehensively investigate the structure of mean and turbulent transport and mixing. The simulation had physical parameters and initial conditions approximating those in the experiment. The budgets of the mean vertical momentum, heavy-fluid mass fraction, turbulent kinetic energy, turbulent kinetic energy dissipation rate, heavy-fluid mass fraction variance, and heavy-fluid mass fraction variance dissipation rate equations are constructed using Reynolds averaging applied to the DNS data. The relative importance of mean and turbulent production, turbulent dissipation and destruction, and turbulent transport are investigated as a function of Reynolds number and across the mixing layer to provide insight into the flow dynamics not presently available from experiments. The analysis of the budgets supports the assumption for small Atwood number, Rayleigh/Taylor driven flows that the principal transport mechanisms are buoyancy production, turbulent production, turbulent dissipation, and turbulent diffusion (shear and mean field production are negligible). As the Reynolds number increases, the turbulent production in the turbulent kinetic energy dissipation rate equation becomes the dominant production term, while the buoyancy production plateaus. Distinctions between momentum and scalar transport are also noted, where the turbulent kinetic energy and its dissipation rate both grow in time and are peaked near the center plane of the mixing layer, while the heavy-fluid mass fraction variance and its dissipation rate initially grow and then begin to decrease as mixing progresses and reduces density fluctuations. All terms in the transport equations generally grow or decay, with no qualitative change in their profile, except for the pressure flux contribution to the total turbulent kinetic energy flux, which changes sign early in time (a countergradient effect). The production-to-dissipation ratios corresponding to the turbulent kinetic energy and heavy-fluid mass fraction variance are large and vary strongly at small evolution times, decrease with time, and nearly asymptote as the flow enters a self-similar regime. The late-time turbulent kinetic energy production-to-dissipation ratio is larger than observed in shear-driven turbulent flows. The order of magnitude estimates of the terms in the transport equations are shown to be consistent with the DNS at late-time, and also confirms both the dominant terms and their evolutionary behavior. These results are useful for identifying the dynamically important terms requiring closure, and assessing the accuracy of the predictions of Reynolds-averaged Navier-Stokes and large-eddy simulation models of turbulent transport and mixing in transitional Rayleigh-Taylor instability-generated flow.

Schilling, Oleg [Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States); Mueschke, Nicholas J. [Texas A and M Univ., College Station, TX (United States)

2010-01-01

200

Direct Numerical Simulation of Cell Printing

NASA Astrophysics Data System (ADS)

Structural cell printing, i.e., printing three dimensional (3D) structures of cells held in a tissue matrix, is gaining significant attention in the biomedical community. The key idea is to use desktop printer or similar devices to print cells into 3D patterns with a resolution comparable to the size of mammalian cells, similar to that in living organs. Achieving such a resolution in vitro can lead to breakthroughs in areas such as organ transplantation and understanding of cell-cell interactions in truly 3D spaces. Although the feasibility of cell printing has been demonstrated in the recent years, the printing resolution and cell viability remain to be improved. In this work, we investigate one of the unit operations in cell printing, namely, the impact of a cell-laden droplet into a pool of highly viscous liquids using direct numerical simulations. The dynamics of droplet impact (e.g., crater formation and droplet spreading and penetration) and the evolution of cell shape and internal stress are quantified in details.

Qiao, Rui; He, Ping

2010-11-01

201

Direct numerical simulations of aeolian sand ripples

Aeolian sand beds exhibit regular patterns of ripples resulting from the interaction between topography and sediment transport. Their characteristics have been so far related to reptation transport caused by the impacts on the ground of grains entrained by the wind into saltation. By means of direct numerical simulations of grains interacting with a wind flow, we show that the instability turns out to be driven by resonant grain trajectories, whose length is close to a ripple wavelength and whose splash leads to a mass displacement towards the ripple crests. The pattern selection results from a compromise between this destabilizing mechanism and a diffusive downslope transport which stabilizes small wavelengths. The initial wavelength is set by the ratio of the sediment flux and the erosion/deposition rate, a ratio which increases linearly with the wind velocity. We show that this scaling law, in agreement with experiments, originates from an interfacial layer separating the saltation zone from the static sand bed, where momentum transfers are dominated by mid-air collisions. Finally, we provide quantitative support for the use the propagation of these ripples as a proxy for remote measurements of sediment transport.

Orencio Duran; Philippe Claudin; Bruno Andreotti

2014-11-07

202

Cloud interactions and merging - Numerical simulations

NASA Technical Reports Server (NTRS)

A total of 48 numerical experiments have been performed to study cloud interactions adn merging by means of a two-dimensional multi-cell model. Two soundings of deep convection during GATE and two different magnitudes of large-scale lifting have been used as the initial conditions and as the main forcing on the model. Over two hundred groups of cloud systems with a life history of over sixty minutes have been generated under the influence of different combinations of the stratification and large-scale lifting. The results demonstrate the increase in convective activity and in amount of precipitation with increased intensity of large-scale lifting. The results also show increased occurrence of cloud merger with increased intensity of large-scale lifting. The most unfavorable environmental conditions for cloud merging are (1) less unstable stratification of the atmosphere and (2) weaker large-scale lifting. A total of fourteen cloud systems qualify as mergers. Two selected cases will be described dynamically and thermodynamically in this paper. Although these cloud mergers have been simulated under the influence of different synoptic-scale conditions, the major physical mechanism related to the cloud merging process is the same as that proposed by Simpson. Cumulus downdrafts and associated cold outflows play a dominant role in the merging process in all cases studied.

Tao, W.-K.; Simpson, J.

1984-01-01

203

Numerical simulation of valveless pulsed combustors

NASA Astrophysics Data System (ADS)

The paper describes a mathematical model for simulating the cyclic operation of valveless pulsed combustors. The flows in the inlet and tail pipe were assumed to be one-dimensional while the combustion chamber was treated as a large reservoir with uniform thermodynamic properties. The effects of wall friction, heat transfer, gradual area changes, variable entropy and composition changes due to chemical reaction were included in the modelling. The set of first order quasi-linear hyperbolic partial differential equations describing the inlet and tail pipe flows were solved by a numerical method of characteristics. The results predicted with the model are in good agreement with some experimental results. The model showed that intermittent combustion in the combustion chamber is established through variations in the concentrations of carbon dioxide and oxygen. Lower and upper fuel flow rate extinction limits which have been observed experimentally, were found, from the results of the model, to be due to leanness and richness of the fresh charge in the combustion chamber, respectively.

Olorunmaiye, J. A.; Kentfield, J. A. C.

1989-08-01

204

Direct numerical simulations of aeolian sand ripples.

Aeolian sand beds exhibit regular patterns of ripples resulting from the interaction between topography and sediment transport. Their characteristics have been so far related to reptation transport caused by the impacts on the ground of grains entrained by the wind into saltation. By means of direct numerical simulations of grains interacting with a wind flow, we show that the instability turns out to be driven by resonant grain trajectories, whose length is close to a ripple wavelength and whose splash leads to a mass displacement toward the ripple crests. The pattern selection results from a compromise between this destabilizing mechanism and a diffusive downslope transport which stabilizes small wavelengths. The initial wavelength is set by the ratio of the sediment flux and the erosion/deposition rate, a ratio which increases linearly with the wind velocity. We show that this scaling law, in agreement with experiments, originates from an interfacial layer separating the saltation zone from the static sand bed, where momentum transfers are dominated by midair collisions. Finally, we provide quantitative support for the use of the propagation of these ripples as a proxy for remote measurements of sediment transport. PMID:25331873

Durán, Orencio; Claudin, Philippe; Andreotti, Bruno

2014-11-01

205

Numerical Simulations of Heated Supersonic Rectangular Jets

NASA Astrophysics Data System (ADS)

The heated supersonic flow from rectangular jets with paddles in the flow field have been simulated numerically to study the effects of heating on the flow field and near-field noise. The flapping motion across the narrow dimension of the jet, which is the dominant feature of the unheated jet, is also found to be present in heated jets with temperature ratios (temperature of the jet to that of the surroundings) from 2.17 to 5.0. With increasing jet temperature, the jet core extends further downstream towards the paddles and the shear layer development is also delayed. Furthermore, as the temperature ratio increases, the amplitude of the velocity fluctuations decrease and additional frequencies also begin to appear. The characteristic frequency also changes with temperature and the corresponding Strouhal number is found to decrease exponentially with increase in the temperature over the range studied. Several interesting observations on the modes of the heated jet will also be presented. footnote Sponsored by NASA-Lewis and NRL

Kolbe, R. L.; Kailasanath, K.; Boris, J. P.

1996-11-01

206

Numerical simulation of condensation on structured surfaces.

Condensation of liquid droplets on solid surfaces happens widely in nature and industrial processes. This phase-change phenomenon has great effect on the performance of some microfluidic devices. On the basis of micro- and nanotechnology, superhydrophobic structured surfaces can be well-fabricated. In this work, the nucleating and growth of droplets on different structured surfaces are investigated numerically. The dynamic behavior of droplets during the condensation is simulated by the multiphase lattice Boltzmann method (LBM), which has the ability to incorporate the microscopic interactions, including fluid-fluid interaction and fluid-surface interaction. The results by the LBM show that, besides the chemical properties of surfaces, the topography of structures on solid surfaces influences the condensation process. For superhydrophobic surfaces, the spacing and height of microridges have significant influence on the nucleation sites. This mechanism provides an effective way for prevention of wetting on surfaces in engineering applications. Moreover, it suggests a way to prevent ice formation on surfaces caused by the condensation of subcooled water. For hydrophilic surfaces, however, microstructures may be submerged by the liquid films adhering to the surfaces. In this case, microstructures will fail to control the condensation process. Our research provides an optimized way for designing surfaces for condensation in engineering systems. PMID:25347594

Fu, Xiaowu; Yao, Zhaohui; Hao, Pengfei

2014-11-25

207

A high-order public domain code for direct numerical simulations of turbulent combustion

NASA Astrophysics Data System (ADS)

A high-order scheme for direct numerical simulations of turbulent combustion is discussed. Its implementation in the massively parallel and publicly available PENCIL CODE is validated with the focus on hydrogen combustion. This is the first open source DNS code with detailed chemistry available. An attempt has been made to present, for the first time, the full set of evolution and auxiliary equations required for a complete description of single phase non-isothermal fluid dynamics with detailed chemical reactions. Ignition delay times (0D) and laminar flame velocities (1D) are calculated and compared with results from the commercially available Chemkin code. The scheme is verified to be fifth order in space. Upon doubling the resolution, a 32-fold increase in the accuracy of the flame front is demonstrated. Finally, also turbulent and spherical flame front velocities are calculated and the implementation of the non-reflecting so-called Navier-Stokes Characteristic Boundary Condition is validated in all three directions.

Babkovskaia, N.; Haugen, N. E. L.; Brandenburg, A.

2011-01-01

208

A numerical model simulation of longshore transport for Galveston Island

The shoreline changes, deposition patterns, and longshore transport rates were calculated for the coast of Galveston Island using a numerical model simulation. The model only simulated changes due to waves creating longshore currents. East Beach...

Gilbreath, Stephen Alexander

1995-01-01

209

LES, DNS and RANS for the analysis of high-speed turbulent reacting flows

NASA Technical Reports Server (NTRS)

The purpose of this research is to continue our efforts in advancing the state of knowledge in large eddy simulation (LES), direct numerical simulation (DNS), and Reynolds averaged Navier Stokes (RANS) methods for the computational analysis of high-speed reacting turbulent flows. In the second phase of this work, covering the period 1 Sep. 1993 - 1 Sep. 1994, we have focused our efforts on two research problems: (1) developments of 'algebraic' moment closures for statistical descriptions of nonpremixed reacting systems, and (2) assessments of the Dirichlet frequency in presumed scalar probability density function (PDF) methods in stochastic description of turbulent reacting flows. This report provides a complete description of our efforts during this past year as supported by the NASA Langley Research Center under Grant NAG1-1122.

Givi, Peyman; Taulbee, Dale B.; Adumitroaie, Virgil; Sabini, George J.; Shieh, Geoffrey S.

1994-01-01

210

Hierarchical formulations for numerical flow simulations

NASA Astrophysics Data System (ADS)

A new hierarchical formulation for the equations of fluid motion is developed. The hierarchical nature of the new formulation is due to its ability to simulate all levels of fluid flow approximations, namely, inviscid irrotational isentropic flows (potential flow formulation), inviscid rotational non-isentropic flows (Euler formulation) and viscous heat conducting flows (Navier-Stokes formulation). The new formulation uses a potential flow solver as a base solver which is evaluated everywhere in the flow field, while convection/diffusion equations for entropy, vorticity and total enthalpy are only evaluated within limited domains of the flow field where rotational effects are present such as in regions containing shocks, boundary layers and/or wakes. This is accomplished by using a Helmholtz decomposition of the velocity vector into the gradient of a potential function plus a rotational component. The density and pressure are reformulated in terms of the speed and entropy. The new formulation identifies an acoustic mode, governed by the potential equation, from the convection/diffusion mode governing the entropy, vorticity and total enthalpy. This identification of modes together with the ability to restrict the evaluation of entropy, vorticity and total enthalpy to relatively small domains within the flow field offers several advantages over the traditional Euler and Navier-Stokes formulations from the point of view of upwinding, multigrid and the incompressible flow limit. To test the robustness, efficiency and accuracy of this new approach, several flow problems are simulated. These problems include 2-D shock wave/boundary layer interaction, 2-D inviscid and viscous flows over cylinders, 2-D inviscid and viscous flows over airfoils and 3-D inviscid and viscous flows over wings. The results obtained using the new formulation agree well with both experimental results and numerical results obtained from traditional Euler and Navier-Stokes formulations. Fast convergence rates are also achieved through the implementation of multigrid to the augmented potential equation which results in an order of magnitude reduction in work units as compared to single grid computations.

Wahba, Essam Moustafa

211

Numerical Simulation of Complex Turbomachinery Flows

NASA Technical Reports Server (NTRS)

An unsteady, multiblock, Reynolds Averaged Navier Stokes solver based on Runge-Kutta scheme and Pseudo-time step for turbo-machinery applications was developed. The code was validated and assessed against analytical and experimental data. It was used to study a variety of physical mechanisms of unsteady, three-dimensional, turbulent, transitional, and cooling flows in compressors and turbines. Flow over a cylinder has been used to study effects of numerical aspects on accuracy of prediction of wake decay and transition, and to modify K-epsilon models. The following simulations have been performed: (a) Unsteady flow in a compressor cascade: Three low Reynolds number turbulence models have been assessed and data compared with Euler/boundary layer predictions. Major flow features associated with wake induced transition were predicted and studied; (b) Nozzle wake-rotor interaction in a turbine: Results compared to LDV data in design and off-design conditions, and cause and effect of unsteady flow in turbine rotors were analyzed; (c) Flow in the low-pressure turbine: Assessed capability of the code to predict transitional, attached and separated flows at a wide range of low Reynolds numbers and inlet freestream turbulence intensity. Several turbulence and transition models have been employed and comparisons made to experiments; (d) leading edge film cooling at compound angle: Comparisons were made with experiments, and the flow physics of the associated vortical structures were studied; and (e) Tip leakage flow in a turbine. The physics of the secondary flow in a rotor was studied and sources of loss identified.

Chernobrovkin, A. A.; Lakshiminarayana, B.

1999-01-01

212

NASA Astrophysics Data System (ADS)

Sediment transport in nature comprises of bedload and suspended load, and precise modelling of these processes is essential for accurate sediment flux estimation. Traditionally, non-cohesive suspended sediment has been modelled using the advection-diffusion equation (Garcia, 2008), where the success of the model is largely dependent on accurate approximation of the sediment diffusion coefficients. The current study explores the effect of self-stratification on sediment diffusivity using suspended sediment concentration data from direct numerical simulations (DNS) of flows subjected to different levels of stratification, where the level of stratification is dependent on the particle size (parameterized using particle fall velocity ? and volume-averaged sediment concentration (parameterized using shear Richardson number Ri?. Two distinct configurations were explored, first the channel flow configuration (similar to flow in a pipe or a duct) and second, a boundary-layer configuration (similar to open-channel flow). Self-stratification was found to modulate the turbulence intensity (Cantero et al., 2009b), which in turn was found to reduce vertical sediment diffusivity in portions of the domain exposed to turbulence damping. The effect of particle size on vertical sediment diffusivity has been studied in the past by several authors (Rouse, 1937; Coleman, 1970; Nielsen and Teakle, 2004); so in addition to the effect of particle size, the current study also explores the effect of sediment concentration on vertical sediment diffusivity. The results from the DNS simulations were compared with experiments (Ismail, 1952; Coleman, 1986) and field measurements (Coleman, 1970), and were found to agree qualitatively, especially for the case of channel flows. The aim of the study is to understand the effect of stratification due to suspended sediment on vertical sediment diffusivity for different flow configurations, in order to gain insight of the underlying physics, which will eventually help us to improve the existing models for sediment diffusivity.

Dutta, S.; Cantero, M. I.; Garcia, M. H.

2014-08-01

213

DNS and Multi-Scale Modeling of Multi-Phase Flows

NASA Astrophysics Data System (ADS)

Direct numerical simulations (DNS) of multiphase flows, where every continuum length and time scale is fully resolved, have now advanced to the point where it is possible to study in considerable detail fairly complex systems, such as the flow of hundreds of bubbles, drops, and solid particles. Here we discuss such simulations from a multi-scale perspective, focusing on two aspects: First of all, DNS results can help with the development of closure relations of unresolved processes in simulations of large-scale ``industrial'' systems. As an example we discuss recent results for deformable bubbles in weakly turbulent channel flows. The lift induced lateral migration of the bubbles controls the flow, but the lift is very different for nearly spherical and more deformable bubbles, resulting in different flow structures and flow rates. Nevertheless, the results show that the collective motion of many bubbles leads to relatively simple flow structure in both cases, emphasizing the need to examine as large a range of scales as possible. The other multi-scale aspect results from the fact that multiphase flows often produce ``features'' such as thin films, filaments, and drops that are much smaller than the ``dominant'' flow scales. The geometry of these features is usually simple, since surface tension effects are strong and inertia effects are relatively small. In isolation these features are therefore often well described by analytical or semi-analytical models. Recent efforts to capture thin films using classical thin film theory, and to compute mass transfer in high Schmidt number flows using boundary layer approximations, in combination with direct numerical simulations of the rest of the flow, are described.

Tryggvason, Gretar

2011-11-01

214

NASA Astrophysics Data System (ADS)

Channel flow DNS (Direct Numerical Simulation) at friction Reynolds number 180 and with passive scalars of Prandtl numbers 1 and 0.01 was performed in various computational domains. The "normal" size domain was ˜2300 wall units long and ˜750 wall units wide; size taken from the similar DNS of Moser et al. The "large" computational domain, which is supposed to be sufficient to describe the largest structures of the turbulent flows was 3 times longer and 3 times wider than the "normal" domain. The "very large" domain was 6 times longer and 6 times wider than the "normal" domain. All simulations were performed with the same spatial and temporal resolution. Comparison of the standard and large computational domains shows the velocity field statistics (mean velocity, root-mean-square (RMS) fluctuations, and turbulent Reynolds stresses) that are within 1%-2%. Similar agreement is observed for Pr = 1 temperature fields and can be observed also for the mean temperature profiles at Pr = 0.01. These differences can be attributed to the statistical uncertainties of the DNS. However, second-order moments, i.e., RMS temperature fluctuations of standard and large computational domains at Pr = 0.01 show significant differences of up to 20%. Stronger temperature fluctuations in the "large" and "very large" domains confirm the existence of the large-scale structures. Their influence is more or less invisible in the main velocity field statistics or in the statistics of the temperature fields at Prandtl numbers around 1. However, these structures play visible role in the temperature fluctuations at low Prandtl number, where high temperature diffusivity effectively smears the small-scale structures in the thermal field and enhances the relative contribution of large-scales. These large thermal structures represent some kind of an echo of the large scale velocity structures: the highest temperature-velocity correlations are not observed between the instantaneous temperatures and instantaneous streamwise velocities, but between the instantaneous temperatures and velocities averaged over certain time interval.

Tiselj, Iztok

2014-12-01

215

THREE-DIMENSIONAL NUMERICAL SIMULATION OF DOUGH KNEADING

In this article we report on work concerned with the three-dimensional numerical simulation of dough mixing that arises in the food processing industry. Two dough mixers at various rotation speeds are studied, one with one stirrer and the other with two stirrers. Various types of fluid models are incorporated in this work. Numerical simulations are based on three dimensions in

D. DING; M. F. WEBSTER

2000-01-01

216

Direct Numerical Simulation of Solid Deformation During Dendritic Solidification

, a polycrystalline phase-field model is combined with a material point method stress analysis to numerically simulateDirect Numerical Simulation of Solid Deformation During Dendritic Solidification M. YAMAGUCHI1 solidification is a common phe- nomenon in metal casting and can lead to defects such as hot tears, macro

Beckermann, Christoph

217

Direct Numerical Simulation of Cosmological Reionization /

high redshift SFRD to calibrate our reionization simulations. To calculate the radiation feedback, we define an emissivityhigh redshift SFRD to calibrate our reionization simulations. To calculate the radiation feedback, we define an emissivity

So, Geoffrey C.

2013-01-01

218

On Robust Covert Channels Inside DNS

NASA Astrophysics Data System (ADS)

Covert channels inside DNS allow evasion of networks which only provide a restricted access to the Internet. By encapsulating data inside DNS requests and replies exchanged with a server located outside the restricted network, several existing implementations provide either an IP over DNS tunnel, or a socket-like service (TCP over DNS). This paper contributes a detailed overview of the challenges faced by the design of such tunnels, and describes the existing implementations. Then, it introduces TUNS, our prototype of an IP over DNS tunnel, focused on simplicity and protocol compliance. Comparison of TUNS and the other implementations showed that this approach is successful: TUNS works on all the networks we tested, and provides reasonable performance despite its use of less efficient encapsulation techniques, especially when facing degraded network conditions.

Nussbaum, Lucas; Neyron, Pierre; Richard, Olivier

219

Numerical simulation of turbulent flow in a cyclonic separator

NASA Astrophysics Data System (ADS)

Numerical simulation of a turbulent flow of air with dispersed particles through a cyclonic separator is presented. Because of a high streamline curvature in the separator it is difficult to simulate the flow by using the conventional turbulent models. In this work the curvature correction term was included into the k – ? – SST turbulence model implemented in the OpenFOAM® software. Experimental data and results of numerical simulation by the commercial ANSYS Fluent® solver for a turbulent flow in a U-duct were used to validate the model. The numerical simulation of the flow in the cyclonic separator demonstrates that the implemented turbulence model successfully predicts the cyclonic separator efficiency.

Bogdanov, Dmitry; Poniaev, Sergey

2014-12-01

220

Numerical simulation results for the eddy current benchmark problem

NASA Astrophysics Data System (ADS)

This paper presents the numerical simulation results on an eddy current NDE benchmark problem proposed by the World Federation of NDE Centers. The problem involves the calculation of the impedance of a pair of differentially connected coils scanning an infinitely long tube. The excitation source is provided by feeding the coils with an AC current. An edge-based finite element method is used to simulate the problem numerically. Preliminary results obtained using the simulation model are presented.

Li, Y.; Zhang, Z.; Sun, Y.; Udpa, L.; Udpa, S.

2002-05-01

221

The simulation of particulate flows for industrial applications often requires the use of two-fluid models, where the solid particles are considered as a separate continuous phase. One of the underlining uncertainties in the use of the two-fluid models in multiphase computations comes from the boundary condition of the solid phase. Typically, the gas or liquid fluid boundary condition at a solid wall is the so called no-slip condition, which has been widely accepted to be valid for single-phase fluid dynamics provided that the Knudsen number is low. However, the boundary condition for the solid phase is not well understood. The no-slip condition at a solid boundary is not a valid assumption for the solid phase. Instead, several researchers advocate a slip condition as a more appropriate boundary condition. However, the question on the selection of an exact slip length or a slip velocity coefficient is still unanswered. Experimental or numerical simulation data are needed in order to determinate the slip boundary condition that is applicable to a two-fluid model. The goal of this project is to improve the performance and accuracy of the boundary conditions used in two-fluid models such as the MFIX code, which is frequently used in multiphase flow simulations. The specific objectives of the project are to use first principles embedded in a validated Direct Numerical Simulation particulate flow numerical program, which uses the Immersed Boundary method (DNS-IB) and the Direct Forcing scheme in order to establish, modify and validate needed energy and momentum boundary conditions for the MFIX code. To achieve these objectives, we have developed a highly efficient DNS code and conducted numerical simulations to investigate the particle-wall and particle-particle interactions in particulate flows. Most of our research findings have been reported in major conferences and archived journals, which are listed in Section 7 of this report. In this report, we will present a brief description of these results.

Zhi-Gang Feng

2012-05-31

222

Numerical simulation of turbulent combustion: Scientific challenges

NASA Astrophysics Data System (ADS)

Predictive simulation of engine combustion is key to understanding the underlying complicated physicochemical processes, improving engine performance, and reducing pollutant emissions. Critical issues as turbulence modeling, turbulence-chemistry interaction, and accommodation of detailed chemical kinetics in complex flows remain challenging and essential for high-fidelity combustion simulation. This paper reviews the current status of the state-of-the-art large eddy simulation (LES)/prob-ability density function (PDF)/detailed chemistry approach that can address the three challenging modelling issues. PDF as a subgrid model for LES is formulated and the hybrid mesh-particle method for LES/PDF simulations is described. Then the development need in micro-mixing models for the PDF simulations of turbulent premixed combustion is identified. Finally the different acceleration methods for detailed chemistry are reviewed and a combined strategy is proposed for further development.

Ren, ZhuYin; Lu, Zhen; Hou, LingYun; Lu, LiuYan

2014-08-01

223

Coupled numerical simulation of hot stamping process and experimental verification

NASA Astrophysics Data System (ADS)

Hot stamping process is a high non-linear process showing the effect on thermal, mechanical and metallurgical phenomena as they relate to each other. In order to carry out this coupled numerical simulation, fundamental thermal properties such as interfacial heat transfer coefficient and convection heat transfer coefficient as well as crucial mechanics properties were first investigated. Hot stamping tools with cooling system which has been optimized by genetic algorithm were employed in the simulation. The coupled numerical simulation to the whole hot stamping process was built with the ABAQUS/Explicit and FLUENT. Experiment was setup and the results of blank temperature and spring-back were compared with the results of coupled numerical simulation. The comparisons show that the simulation results of numerical model are consistent with experimental results.

Li, Ye; Ying, Liang; Hu, Ping; Shi, Dongyong; Zhao, Xi; Dai, Minghua

2013-05-01

224

Statistical properties of the Sun's photospheric turbulent magnetic field, especially those of the active regions (ARs), have been studied using the line-of-sight data from magnetograms taken by the Solar and Heliospheric Observatory and several other instruments. This includes structure functions and their exponents, flatness curves, and correlation functions. In these works, the dependence of structure function exponents ({zeta}{sub p}) of the order of the structure functions (p) was modeled using a non-intermittent K41 model. It is now well known that the ARs are highly turbulent and are associated with strong intermittent events. In this paper, we compare some of the observations from Abramenko et al. with the log-Poisson model used for modeling intermittent MHD turbulent flows. Next, we analyze the structure function data obtained from the direct numerical simulations (DNS) of homogeneous, incompressible 3D-MHD turbulence in three cases: sustained by forcing, freely decaying, and a flow initially driven and later allowed to decay (case 3). The respective DNS replicate the properties seen in the plots of {zeta}{sub p} against p of ARs. We also reproduce the trends and changes observed in intermittency in flatness and correlation functions of ARs. It is suggested from this analysis that an AR in the onset phase of a flare can be treated as a forced 3D-MHD turbulent system in its simplest form and that the flaring stage is representative of decaying 3D-MHD turbulence. It is also inferred that significant changes in intermittency from the initial onset phase of a flare to its final peak flaring phase are related to the time taken by the system to reach the initial onset phase.

Malapaka, Shiva Kumar; Mueller, Wolf-Christian [Max-Planck Institute for Plasma Physics, Boltzmannstrasse 2, D-85748 Garching bei Muenchen (Germany)

2013-09-01

225

Numerical simulation of quasi-multifractal diffusion process

The properties of quasi-multifractal diffusion process are discussed. A discrete model of the process is constructed, and a method is proposed for calculating the quasi-multifractal spectrum, based on statistical processing of its realizations. An analysis of multifractal properties performed by numerical simulation of the quasi-multifractal spectrum is qualitatively substantiated by examining realizations of the simulated process. The results of numerical simulations suggest that there are three distinct scaling regions. Special attention is given to comparative analyses between numerical and analytical results and between realizations of the proposed process and the well-known multifractal random walk.

Saichev, A. I., E-mail: saichev@hotmail.ru; Filimonov, V. A. [Nizhni Novgorod State University (Russian Federation)], E-mail: vladimir.a.filimonov@gmail.com

2008-08-15

226

Numerical Simulation of Baroclinic Jovian Vortices

We examine the evolution of baroclinic vortices in a time-dependent, nonlinear numerical model of a Jovian atmosphere. The model uses a normal-mode expansion in the vertical, using the barotropic and first two baroclinic modes. Results for the stability of baroclinic vortices on an f plane in the absence of a mean zonal flow are similar to results of Earth vortex

Richard K. Achterberg; Andrew P. Ingersoll

1994-01-01

227

The Numerical Simulation of Thunderstorm Outflow Dynamics

Two high-resolution, two-dimensional numerical models are developed and used to investigate the dynamics of thunderstorm outflows. The first model employs the set of unapproximated, inviscid, fully compressible hydrodynamical equations, while the second, more economical model is based on a simplified set of inviscid, \\

Kelvin Kay Droegemeier

1985-01-01

228

Unsteady Flow Simulation: A Numerical Challenge

The prediction of unsteady flow field in turbine blades as well as in the turbomachinery stages is now an affordable item, and is required by the reduced margin for increasing efficiency, stability and life of propulsion components. The numerical tools are now capable to run within reasonable time 3D unsteady calculation for full stage, and the new techniques on the

Francesco Martelli; Elisabetta Belardini; Paolo Adami

229

High order hybrid numerical simulations of two dimensional detonation waves

NASA Technical Reports Server (NTRS)

In order to study multi-dimensional unstable detonation waves, a high order numerical scheme suitable for calculating the detailed transverse wave structures of multidimensional detonation waves was developed. The numerical algorithm uses a multi-domain approach so different numerical techniques can be applied for different components of detonation waves. The detonation waves are assumed to undergo an irreversible, unimolecular reaction A yields B. Several cases of unstable two dimensional detonation waves are simulated and detailed transverse wave interactions are documented. The numerical results show the importance of resolving the detonation front without excessive numerical viscosity in order to obtain the correct cellular patterns.

Cai, Wei

1993-01-01

230

Numerical simulation of ultrasonic tomography inspections of highly heterogeneous materials

NASA Astrophysics Data System (ADS)

This paper deals with the simulation of ultrasonic transmission tomography systems in water-immersed to nondestructively inspect highly heterogeneous materials with fractures. The time-domain Elastodynamic Finite Integration Technique (EFIT) was employed for all numerical simulations because is able to reliably simulate this type of ultrasonic problems. The EFIT code was implemented using OpenCL and PyOpenCL. Several ultrasonic tomography inspection setups were numerically simulated under different conditions varying the number of ultrasonic sources and their size and number and different operation schemes. Sinograms of concrete scenarios were computed and compared for each configuration, using homogeneous materials with similar fracture types and experimentally validated.

Molero, M.; Medina, L.; Lluveras, D.; Izquierdo, M. A. G.; Anaya, J. J.

2012-12-01

231

Airborne laser pressure recovery system - Numerical simulations

NASA Astrophysics Data System (ADS)

A numerical method capable of accurately predicting flowfields in a radial cylindrical supersonic diffusion laser (SDL) is developed by incorporating a modified two-layer Cebeci-Smith (Cebeci et al., 1970) algebraic eddy viscosity turbulence model into the compressible Navier-Stokes equations. The required modifications to the model are extremely sensitive to the von Karman universal mixing length constant, the sublayer thickness parameter, the Clausser outer region constant, and the downstream location in the diffuser duct at which these modifications are implemented. The experimental tests were conducted at a diffuser entrance unit Reynolds number 1.6 million per foot. It is shown that the diffuser performance is contrained by the source nozzle mixing losses, the duct length, and the requirement for wall boundary layer energization if the design requires a relatively short duct. The numerical solutions confirm the Neumann and Lustwerk (1949) experimental conclusions regarding minimum diffuser duct length if no wall boundary layer energization is employed.

Horkovich, J. A.

1993-07-01

232

Numerical simulations of inviscid capillary pinchoff

NASA Astrophysics Data System (ADS)

Inviscid capillary pinchoff is studied numerically, for an axisymmetric model problem in which a bubble pinches at two points on the symmetry plane, breaking into two symmetric end-bubbles and a satellite bubble in between. Results are presented for a range of density jumps across the bubble. The numerical method uses a formulation in terms of arclength and tangent angle, and incorporates a new procedure to redistribute the computational points dynamically, in order to maintain resolution in regions of high curvature. The results are compared with alternative computations by Lepinnen and Lister [Phys. Fluids 15 (2003) 568], where available. New results include details about the cone-crater structure near pinchoff, and the dependence of satellite bubble volume and pinchoff time on the density ratio.

Nitsche, Monika; Steen, Paul H.

2004-10-01

233

Numerical simulations of inviscid capillary pinchoff

NASA Astrophysics Data System (ADS)

Inviscid capillary pinchoff is studied numerically for an axisymmetric model problem in which a bubble pinches at two points on the symmetry plane, breaking into two symmetric end-bubbles and a satellite bubble in between. The interface is represented by a vortex sheet. The numerical method tracks the vortex sheet using a formulation in terms of arclength and tangent angle, and incorporates a new procedure to redistribute the computational points dynamically, in order to maintain resolution in regions of high curvature. Results are presented for a range of density jumps across the bubble. The results are compared with alternative computations by Lepinnen and Lister (Phys. Fluids 15, 2003), where available. New results include details about the cone-crater structure near pinchoff, and the dependence of satellite bubble volume and pinchoff time on the density ratio.

Nitsche, Monika; Steen, Paul

2004-11-01

234

Numerical Simulation of Grain Boundary Grooving By Level Set Method

is to develop and test a fast numerical approach for the simulation of the formation development propagation in the presence of applied or internal fields. Com mon examples are growth of GB grooves and cavities [13, 14

Averbuch, Amir

235

Numerical Simulation Study on Transpired Solar Air Collector

The unglazed transpired solar air collector is now a well-recognized solar air heater for heating outside air directly. In this article, researchers introduced numerical simulation tools into the solar air collector research area, analyzed...

Wang, C.; Guan, Z.; Zhao, X.; Wang, D.

2006-01-01

236

Advances in numerical simulation of turbulent flows

Although significant progress has been made in predicting turbulent flows, there still remain challenging problems related, particularly, to modeling the thin-wall layers, low Reynolds number and transitional turbulent flows, there-dimensional flows, and large-scale industrial flows. The overall objective of the symposium was to focus on advances in the numerical prediction of physical phenomena relevant to turbulence and its origin, as

I. Celik; T. Kobayashi; K. N. Ghia; J. Kurokawa

1991-01-01

237

Numerical simulation of in situ bioremediation

Models that couple subsurface flow and transport with microbial processes are an important tool for assessing the effectiveness of bioremediation in field applications. A numerical algorithm is described that differs from previous in situ bioremediation models in that it includes: both vadose and groundwater zones, unsteady air and water flow, limited nutrients and airborne nutrients, toxicity, cometabolic kinetics, kinetic sorption, subgridscale averaging, pore clogging and protozoan grazing.

Travis, B.J.

1998-12-31

238

DNS of turbulent wall bounded flows with a passive scalar

NASA Astrophysics Data System (ADS)

In this thesis, Direct Numerical Simulations (DNS) of the velocity and temperature fields are performed for incompressible turbulent flows in plane channels and spatially-developing boundary layers. The main goal is to numerically analyze the behavior of the momentum and thermal boundary layers subjected to different external and upstream conditions, the main focus is given to: (i) local flow perturbations, (ii) different Reynolds numbers, and, (iii) external pressure gradient. Two types of turbulent wall-bounded flows are examined in this investigation. One of them consists of the fully developed turbulent channel. Furthermore, after the developing section, the boundary layers generated by the lower and upper walls collapse. From this point to downstream, periodic boundary conditions are applicable due to the existent homogeneity. The second type of wall bounded flow explored possesses no restriction in the upper zone; consequently, the boundary layer may grow infinitely downstream. This streamwise non-homogeneous state does not allow to prescribe periodic boundary conditions along the direction of the flow. Therefore, time-dependent turbulent information must be assigned at the domain inlet, turning the numerical problem into a very challenging one. The spatially-developing turbulent boundary layer in a flat plate is a typical example of non-homogeneous flow. In the first part of this thesis, the influence of local forcing on an incompressible turbulent channel flow is numerically investigated. The extensive information provided by the DNS enable us to have a better understanding of the physical mechanism responsible for local heat transfer enhancement and drag reduction. Time-periodic blowing/suction is applied by means of thin spanwise slots located at the lower and upper walls of the channel at several forcing frequencies. It was found in Araya et al. (2008-a) the existence of a characteristic frequency, i.e. of f = 0.64 or f* = 0.044, at which maximum local augmentation of the molecular and turbulent heat transfer rates were obtained downstream from the local forcing source. Furthermore, the key role of pressure fluctuations in the energy exchange and redistribution of energy among the components was confirmed by Araya et al. (2008-b) by analyzing the budget of wall-normal turbulent heat fluxes in locally forced turbulent flows at the characteristic frequency. Additionally, the analysis of power spectra and cospectra of fluctuations in Araya et al. (2008-b) demonstrated that the largest energy increases due to periodic blowing/suction are attained by the wall-normal velocity fluctuations and wall-normal turbulent heat fluxes at very low wavenumbers or large scales. The latter part of this work is principally devoted to the analysis of the rescaling-recycling method on the generation of time-dependent turbulent inflow conditions on spatially evolving boundary layers in zero (ZPG) and adverse (APG) pressure gradient flows. The rescaling-recycling method shows promising features as a turbulent inflow generator, particularly on pressure gradient (PG) flows. Its simplicity permits to avoid the calculation of the laminar-transition stage, and, as a consequence, a huge amount of computational time can be saved. Not to mention that the computational domain is drastically reduced due to the short developing section needed. Nevertheless, the original procedure proposed by Lund et al. (1998) was limited to flows without streamwise pressure gradients due to the single scaling assumption. This is indeed the first time that a recycling approach successfully worked for PG flows. In this study, an alternative multi-scale similarity method for the generation of inflow turbulent momentum/thermal information is introduced for flows with and without streamwise pressure gradients for Reynolds numbers up to 2300 based on the momentum thickness, i.e. Retheta. The velocity scaling laws for the mean flow are based on the works by George and Castillo (1997) and Castillo and George (2001). In the same way, the mean temperature scaling is der

Araya, Juan Guillermo

239

Numerical simulations of dense collisional systems

NASA Astrophysics Data System (ADS)

The present use of a local simulation method akin to that of Wisdom and Tremaine (1988) to examine the viscous stability characteristics of dense planetary rings confirms that the viscous instability of the standard elastic model of icy particles should not occur for systems of identical, meter-sized particles, but may indeed occur in dense systems composed of cm-sized ones. In the case of nonidentical particles, small particles become more easily unstable. The layered structure of Wisdom and Tremaine's simulation with self-gravity can be substantially modified if the vertical field is calculated self-consistently; in some cases, a flattening to the central plane may be virtually complete.

Salo, H.

1991-04-01

240

Numerical simulation of tornado-like vortices around complex geometries

In this paper, numerical simulations of tornadoes are presented. The problem considered is to first simulate numerically a tornado-like vortex passing through a populated area and then analyse the paths as it impacts structures. This study was invigorated after a tornado, classified F2 on the Fujita scale, striked Embry-Riddle Aeronautical University Daytona Beach Campus in December 2006. The computational domain

Frederique Drullion

2009-01-01

241

On the Numerical Simulation of Waterflooding of Heterogeneous

On the Numerical Simulation of Waterflooding of Heterogeneous Petroleum Reservoirs Jim Douglas, Jr displacement in petroleum reservoirs. A very detailed description of the numerical method is presented. FollowÂ970 Campinas, SP, Brazil z Laborat'orio Nacional de ComputaÂ¸c~ao Cient'ifica/CNPq, Rua Lauro MÂ¨uller, 455

Douglas Jr., Jim

242

Numerical simulations of cardiovascular diseases and global matter transport

Numerical model of the peripheral circulation and dynamical model of the large vessels and the heart are discussed in this paper. They combined together into the global model of blood circulation. Some results of numerical simulations concerning matter transport through the human organism and heart diseases are represented in the end.

Simakov, S S; Evdokimov, A V; Kholodov, Y A

2007-01-01

243

Numerical simulations of a low power microchannel thermal cycling reactor

A roadblock to the development of stand-alone microscale biomedical and chemical analysis systems is the relatively high heating power requirement of the microscale reactors, which typically exceeds the capability of low cost batteries. In this study, a microchannel thermal cycling reactor design using in-channel heating and passive cooling is proposed and a numerical model has been developed. Numerical simulations were

David Erickson; Dongqing Li

2002-01-01

244

NUMERICAL SIMULATION OF COMPRESSIBLE TWO-PHASE FLUID FLOW

states in the ghost cells and modify real fluid by interfacial states compute the reconstructionNUMERICAL SIMULATION OF COMPRESSIBLE TWO-PHASE FLUID FLOW : Ghost fluid method vs Saurel Abgrall Discretization Saurel-Abgrall Approach "Real" Ghost Fluid Method · Numerical Results Shock

Helluy, Philippe

245

Experimental and numerical simulation of dough kneading in filled geometries

Experimental and numerical simulation of dough kneading in filled geometries D.M. Binding, M numerical model and experimental studies for rotating flows associated with dough kneading, and validate is part of a broader study aimed at improving understanding of the technology involved in dough mixing

Grant, P. W.

246

Modeling and Numerical Simulation Of Tethered Buoy Dynamics

In this article we deal with the numerical simulation of the dynamics of a tethered buoy, which is a mechanical system for marine applications consisting of a rigid floating body (buoy) connected by an elastic cable to the bottom of the fluid environment. A novel mixed finite element formulation is proposed for the spatial numerical approximation of the equa- tions

Antonio Montano; Marco Restelli; Riccardo Sacco

2005-01-01

247

Numerical simulation of tsunami waves generated by deformable submarine landslides

Numerical simulation of tsunami waves generated by deformable submarine landslides Gangfeng Ma a 2013 Accepted 4 July 2013 Available online 15 July 2013 Keywords: Submarine landslide Nonhydrostatic wave model Tsunami wave Numerical modeling a b s t r a c t This paper presents a new submarine

Kirby, James T.

248

Numerical simulations of astrophysical jets: comparison of integration schemes

In this paper we model an astrophysical jet and simulate its evolution numerically by means of different numerical schemes, focusing in particular on Godunov-type methods (Roe, HLLE, PPM) but considering also central methods (local Lax-Friedrichs). The results yielded by various codes are then compared, on the basis of various parameters, such as jet morphology, the statistical distributions of basic physical

S. Massaglia; N. Zurlo; G. Bodo

2003-01-01

249

Numerical simulation of laser full penetration welding

A three dimensional finite element model has been developed to dynamically simulate the laser full penetration welding process. The parametric design capabilities of the finite element code ANSYS (revision 5.4) were employed for this purpose. The model calculates the transient temperature profile and the dimensions of the fusion zone during the welding process. The heat source was parameterized by the

Komeil Kazemi; John A. Goldak

2009-01-01

250

Numerical Simulations of Aqueous Ozone Decomposition

The decomposition of ozone in basic aqueous solutions was simulated on the basis of two previously reported detailed mechanisms. According to the calculations, the mechanism given by Tomiyasu, Fukutomi and Gordon is an appropriate model for the ozone decomposition, both in the presence and the absence of hydrogen peroxide and\\/or carbonate ion in basic solution. On the basis of this

K. Chelkowska; D. Grasso; I. Fábián; G. Gordon

1992-01-01

251

Numerical simulation of air-cooling tower

NASA Astrophysics Data System (ADS)

A mathematic model for packed air-cooling tower thermodynamic calculation is set up in this paper on the basis of fundamental heat and mass transfer equations. Based on the Double Film theory, direct equation-solving method is used to simulate air-cooling tower, and variation of parameters is taken to analyze the data and results of the program.

Hao, Lijuan; Li, Huanzhi; Sun, Zhaohu; Tong, Lige

2003-08-01

252

Brush seal numerical simulation: Concepts and advances

NASA Technical Reports Server (NTRS)

The development of the brush seal is considered to be most promising among the advanced type seals that are presently in use in the high speed turbomachinery. The brush is usually mounted on the stationary portions of the engine and has direct contact with the rotating element, in the process of limiting the 'unwanted' leakage flows between stages, or various engine cavities. This type of sealing technology is providing high (in comparison with conventional seals) pressure drops due mainly to the high packing density (around 100 bristles/sq mm), and brush compliance with the rotor motions. In the design of modern aerospace turbomachinery leakage flows between the stages must be minimal, thus contributing to the higher efficiency of the engine. Use of the brush seal instead of the labyrinth seal reduces the leakage flow by one order of magnitude. Brush seals also have been found to enhance dynamic performance, cost less, and are lighter than labyrinth seals. Even though industrial brush seals have been successfully developed through extensive experimentation, there is no comprehensive numerical methodology for the design or prediction of their performance. The existing analytical/numerical approaches are based on bulk flow models and do not allow the investigation of the effects of brush morphology (bristle arrangement), or brushes arrangement (number of brushes, spacing between them), on the pressure drops and flow leakage. An increase in the brush seal efficiency is clearly a complex problem that is closely related to the brush geometry and arrangement, and can be solved most likely only by means of a numerically distributed model.

Braun, M. J.; Kudriavtsev, V. V.

1994-01-01

253

Brush seal numerical simulation: Concepts and advances

NASA Astrophysics Data System (ADS)

The development of the brush seal is considered to be most promising among the advanced type seals that are presently in use in the high speed turbomachinery. The brush is usually mounted on the stationary portions of the engine and has direct contact with the rotating element, in the process of limiting the 'unwanted' leakage flows between stages, or various engine cavities. This type of sealing technology is providing high (in comparison with conventional seals) pressure drops due mainly to the high packing density (around 100 bristles/sq mm), and brush compliance with the rotor motions. In the design of modern aerospace turbomachinery leakage flows between the stages must be minimal, thus contributing to the higher efficiency of the engine. Use of the brush seal instead of the labyrinth seal reduces the leakage flow by one order of magnitude. Brush seals also have been found to enhance dynamic performance, cost less, and are lighter than labyrinth seals. Even though industrial brush seals have been successfully developed through extensive experimentation, there is no comprehensive numerical methodology for the design or prediction of their performance. The existing analytical/numerical approaches are based on bulk flow models and do not allow the investigation of the effects of brush morphology (bristle arrangement), or brushes arrangement (number of brushes, spacing between them), on the pressure drops and flow leakage. An increase in the brush seal efficiency is clearly a complex problem that is closely related to the brush geometry and arrangement, and can be solved most likely only by means of a numerically distributed model.

Braun, M. J.; Kudriavtsev, V. V.

1994-07-01

254

Numerical simulation of a controlled boundary layer

NASA Technical Reports Server (NTRS)

The problem of interest is the boundary layer over a flat plate. The three standard laminar flow control (LFC) techniques are pressure gradient, suction, and heating. The parameters used to describe the amount of control in the context of the boundary layer equations are introduced. The numerical method required to find the mean flow, the linear eigenvalues of the Orr-Sommerfeld equation, and the full, nonlinear, 3-D solution of the Navier-Stokes equations are outlined. A secondary instability exists for the parallel boundary subject to uniform pressure gradient, suction, or heating. Selective control of the spanwise mode reduces the secondary instability in the parallel boundary layer at low Reynolds number.

Zang, Thomas A.; Hussaini, M. Yousuff

1986-01-01

255

Direct numerical simulations in material processing

NASA Astrophysics Data System (ADS)

A unified approach for direct simulations of fluid flow, heat transfer, and phase changes is discussed. The method is based on writing one set of conservation equation for all phases involved, allowing arbitrary changes in material properties and adding singular terms at phase boundary to ensure that the correct boundary conditions are incorporated. This approach allows the conservation equations to ge solved on a fixed grid in a very efficient way. By explicit tracking of the phase boundary by a lower dimensional moving grid, the method is capable of producing accurate solutions for complex phase boundaries. Examples of simulations of the solidification of pure materials, binary alloys, and drops impinging on a solid surface are shown.

Tryggvason, Getar; Juric, Damir; Han, Jaehoon; Ceccio, Steven L.

1996-07-01

256

Numerical Simulations of Astrophysical Jets from Keplerian Accretion Disks

This thesis presents a series of magnetohydrodynamic (MHD) simulations whichwere designed to study the origin and evolution of astrophysical jets (galactic andextra-galactic). \\\\Ve developed and extended a version of the ZEUS-2D code whichserved as the numerical basis of our simulations and attached to it a complete analysispackage that was developed in order to make contact with the theory and observationsof

Rachid Ouyed

1996-01-01

257

Direct Numerical Simulation of Unsteady Decelerating Flows Yongmann M. Chung

in developing advanced turbulence models for unsteady flow problems. RESULTS AND DISCUSSION The calculations.M.Chung@warwick.ac.uk SUMMARY Direct numerical simulations are performed for a turbulent flow subjected to a sudden change. INTRODUCTION Due to the progress in recent computer technology, unsteady turbulent flow simulations have been

Chung, Yongmann M.

258

Towards a universal numerical simulation model for laser material processing

This paper gives an overview on a new but still quite universal numerical simulation model for laser material processing. After a short introduction into the model, simulation results on several processes like laser beam deep penetration welding, drilling or cutting are presented. Finally an outlook on planned further developments is given and possible applications of the model are discussed.

Andreas Otto; Michael Schmidt

2010-01-01

259

Numerical simulation of laser--induced breakdown of air

The laser--induced breakdown of air is studied using numerical simulation. When focused onto a small volume of air, a laser beam heats and ionizes the air, causing a plasma to form. Three models of air with varing levels of physical complexity are considered. The simulations are challenging due to presence of very strong shock waves and very low densities in

Shankar Ghosh; Krishnan Mahesh

2007-01-01

260

Numerical simulation of a liquid propellant rocket motor

This work presents a numerical simulation of the flow field in a liquid propellant rocket engine chamber and exit nozzle using techniques to allow the results to be taken as starting points for designing those propulsive systems. This was done using a Finite Volume method simulating the different flow regimes which usually take place in those systems. As the flow

Nicolas M. C. Salvador; Marcelo M. Morales; Carlos E. S. S. Migueis; Demétrio Bastos-Netto

2001-01-01

261

A numerical simulation for a pumped storage reservoir. Research report

Hydroelectric pumped storage power generation is a method of providing electrical energy for peak demand periods now being used by many companies around the world. A computer program was developed to numerically simulate the velocity and temperature fields in an idealized two-dimensional pumped storage reservoir. The reservoir model includes a submerged inlet and a rising upper boundary to simulate rising

J. D. Faber; R. A. Bajur; S. H. Schwartz

1977-01-01

262

Numerical simulation of ice accretions on an aircraft wing

An approach based on the Eulerian two-phase flow theory to numerically simulate ice accretions on an aircraft wing is developed. The air flowfield is obtained through Euler flow computation. The water droplets? flowfield is solved through proposing a permeable wall to simulate the droplets impingement. The droplets collection efficiency is calculated according to the droplets velocity and apparent density distribution.

Yihua Cao; Chao Ma; Qiang Zhang; John Sheridan

263

Particle acceleration in solar flares: observations versus numerical simulations

Particle acceleration in solar flares: observations versus numerical simulations A O Benz, P C@astro.phys.ethz.ch Abstract. Solar flares are generally agreed to be impulsive releases of magnetic energy. ReconnectionÂray observations, transitÂtime damping simulation, reconnection, astrophysics #12; 2 1. Introduction Solar flares

264

Numerical simulation of icing, deicing, and shedding

NASA Technical Reports Server (NTRS)

An algorithm has been developed to numerically model the concurrent phenomena of two-dimensional transient heat transfer, ice accretion, ice shedding and ice trajectory which arise from the use of electrothermal pad. The Alternating Direction Implicit method is used to simultaneously solve the heat transfer and accretion equations occurring in the multilayered body covered with ice. In order to model the phase change between ice and water, a technique was used which assumes a phase for each node. This allows the equations to be linearized such that a direct solution is possible. This technique requires an iterative procedure to find the correct phase at each node. The computer program developed to find this solution has been integrated with the NASA-Lewis flow/trajectory code LEWICE.

Wright, W. B.; Dewitt, K. J.; Keith, T. G., Jr.

1991-01-01

265

Numerical simulation of electrophoresis separation processes

NASA Technical Reports Server (NTRS)

A new Petrov-Galerkin finite element formulation has been proposed for transient convection-diffusion problems. Most Petrov-Galerkin formulations take into account the spatial discretization, and the weighting functions so developed give satisfactory solutions for steady state problems. Though these schemes can be used for transient problems, there is scope for improvement. The schemes proposed here, which consider temporal as well as spatial discretization, provide improved solutions. Electrophoresis, which involves the motion of charged entities under the influence of an applied electric field, is governed by equations similiar to those encountered in fluid flow problems, i.e., transient convection-diffusion equations. Test problems are solved in electrophoresis and fluid flow. The results obtained are satisfactory. It is also expected that these schemes, suitably adapted, will improve the numerical solutions of the compressible Euler and the Navier-Stokes equations.

Ganjoo, D. K.; Tezduyar, T. E.

1986-01-01

266

Numerical simulation of an axial blood pump.

The axial blood pump with a magnetically suspended impeller is superior to other artificial blood pumps because of its small size. In this article, the distributions of velocity, path line, pressure, and shear stress in the straightener, the rotor, and the diffuser of the axial blood pump, as well as the gap zone were obtained using the commercial software, Fluent (version 6.2). The main focus was on the flow field of the blood pump. The numerical results showed that the axial blood pump could produce 5.14 L/min of blood at 100 mm Hg through the outlet when rotating at 11,000 rpm. However, there was a leakage flow of 1.06 L/min in the gap between the rotor cylinder and the pump housing, and thus the overall flow rate the impeller could generate was 6.2 L/min. The numerical results showed that 75% of the scalar shear stresses (SSs) were less than 250 Pa, and 10% were higher than 500 Pa within the whole pump. The high SS region appeared around the blade tip where a large variation of velocity direction and magnitude was found, which might be due to the steep angle variation at the blade tip. Because the exposure time of the blood cell at the high SS region within the pump was relatively short, it might not cause serious damage to the blood cells, but the improvement of blade profile should be considered in the future design of the axial pump. PMID:17584481

Chua, Leok Poh; Su, Boyang; Lim, Tau Meng; Zhou, Tongming

2007-07-01

267

NASA Technical Reports Server (NTRS)

Models for large eddy simulation (LES) are assessed on a database obtained from direct numerical simulations (DNS) of supercritical binary-species temporal mixing layers. The analysis is performed at the DNS transitional states for heptane/nitrogen, oxygen/hydrogen and oxygen/helium mixing layers. The incorporation of simplifying assumptions that are validated on the DNS database leads to a set of LES equations that requires only models for the subgrid scale (SGS) fluxes, which arise from filtering the convective terms in the DNS equations. Constant-coefficient versions of three different models for the SGS fluxes are assessed and calibrated. The Smagorinsky SGS-flux model shows poor correlations with the SGS fluxes, while the Gradient and Similarity models have high correlations, as well as good quantitative agreement with the SGS fluxes when the calibrated coefficients are used.

Okong'o, Nora; Bellan, Josette

2005-01-01

268

Numerical simulation of optically trapped particles

NASA Astrophysics Data System (ADS)

Some randomness is present in most phenomena, ranging from biomolecules and nanodevices to financial markets and human organizations. However, it is not easy to gain an intuitive understanding of such stochastic phenomena, because their modeling requires advanced mathematical tools, such as sigma algebras, the Itô formula and martingales. Here, we discuss a simple finite difference algorithm that can be used to gain understanding of such complex physical phenomena. In particular, we simulate the motion of an optically trapped particle that is typically used as a model system in statistical physics and has a wide range of applications in physics and biophysics, for example, to measure nanoscopic forces and torques.

Volpe, Giorgio; Volpe, Giovanni

2014-07-01

269

Feasibility study for a numerical aerodynamic simulation facility. Volume 1

NASA Technical Reports Server (NTRS)

A Numerical Aerodynamic Simulation Facility (NASF) was designed for the simulation of fluid flow around three-dimensional bodies, both in wind tunnel environments and in free space. The application of numerical simulation to this field of endeavor promised to yield economies in aerodynamic and aircraft body designs. A model for a NASF/FMP (Flow Model Processor) ensemble using a possible approach to meeting NASF goals is presented. The computer hardware and software are presented, along with the entire design and performance analysis and evaluation.

Lincoln, N. R.; Bergman, R. O.; Bonstrom, D. B.; Brinkman, T. W.; Chiu, S. H. J.; Green, S. S.; Hansen, S. D.; Klein, D. L.; Krohn, H. E.; Prow, R. P.

1979-01-01

270

Numerical and laboratory simulations of auroral acceleration

The existence of parallel electric fields is an essential ingredient of auroral physics, leading to the acceleration of particles that give rise to the auroral displays. An auroral flux tube is modelled using electrostatic Vlasov simulations, and the results are compared to simulations of a proposed laboratory device that is meant for studies of the plasma physical processes that occur on auroral field lines. The hot magnetospheric plasma is represented by a gas discharge plasma source in the laboratory device, and the cold plasma mimicking the ionospheric plasma is generated by a Q-machine source. In both systems, double layers form with plasma density gradients concentrated on their high potential sides. The systems differ regarding the properties of ion acoustic waves that are heavily damped in the magnetosphere, where the ion population is hot, but weakly damped in the laboratory, where the discharge ions are cold. Ion waves are excited by the ion beam that is created by acceleration in the double layer in both systems. The efficiency of this beam-plasma interaction depends on the acceleration voltage. For voltages where the interaction is less efficient, the laboratory experiment is more space-like.

Gunell, H.; De Keyser, J. [1Belgian Institute for Space Aeronomy, Avenue Circulaire 3, B-1180 Brussels (Belgium)] [1Belgian Institute for Space Aeronomy, Avenue Circulaire 3, B-1180 Brussels (Belgium); Mann, I. [EISCAT Scientific Association, P.O. Box 812, SE-981 28 Kiruna, Sweden and Department of Physics, Umeå University, SE-901 87 Umeå (Sweden)] [EISCAT Scientific Association, P.O. Box 812, SE-981 28 Kiruna, Sweden and Department of Physics, Umeå University, SE-901 87 Umeå (Sweden)

2013-10-15

271

Numerical simulations of moon-ringlet interaction

NASA Astrophysics Data System (ADS)

Nonaxisymmetric ring features excited by perturbations of shepherd satellites are studied in terms of direct particle simulations using Aarseth's N-body integrator combined with the calculation of particle-particle impacts. Interaction parameters typical to Saturn's F-ring are investigated. The generation of clumps by external satellites is verified, but the interparticle collisions tend to smooth sharp features. Using F-ring parameters the clumps are observed to cover the total azimuthal length, but it is not clear whether these azimuthally overlapping clumps would be detectable in the actual F-ring. Gravitational scattering by ring particles increases the velocity dispersion, smearing regular azimuthal features at least in the rings of low optical depths. Considerable accretion is observed to occur, particles sticking pairwise to each other, even if the tendency of the particles to accrete is artificially reduced in the simulations. A new explanation for the braided appearance of the F-ring is proposed, based on the interaction between the shepherding satellites and the ring containing embedded moonlets. In our model the braiding is a dynamic phenomenon: the braids are destroyed and recreated in a cyclical manner.

Hanninen, J.

1993-05-01

272

Numerical simulation of the SOFIA flowfield

NASA Technical Reports Server (NTRS)

This report provides a concise summary of the contribution of computational fluid dynamics (CFD) to the SOFIA (Stratospheric Observatory for Infrared Astronomy) project at NASA Ames and presents results obtained from closed- and open-cavity SOFIA simulations. The aircraft platform is a Boeing 747SP and these are the first SOFIA simulations run with the aircraft empennage included in the geometry database. In the open-cavity run the telescope is mounted behind the wings. Results suggest that the cavity markedly influences the mean pressure distribution on empennage surfaces and that 110-140 decibel (db) sound pressure levels are typical in the cavity and on the horizontal and vertical stabilizers. A strong source of sound was found to exist on the rim of the open telescope cavity. The presence of this source suggests that additional design work needs to be performed in order to minimize the sound emanating from that location. A fluid dynamic analysis of the engine plumes is also contained in this report. The analysis was part of an effort to quantify the degradation of telescope performance resulting from the proximity of the port engine exhaust plumes to the open telescope bay.

Klotz, Stephen P.

1994-01-01

273

LSST Astrometry: Simulations and Numerical Studies

NASA Astrophysics Data System (ADS)

Astrometry is an important part of the Large Synoptic Survey Telescope (LSST; http://lsst.org) program. This is reflected in the requirement in the Science Requirements Document for a maximum of 10mas differential astrometric error from a single measure of a star with high signal-to-noise ratio. Assuming that this requirement will be met, the LSST will obtain parallax and proper-motion measurements of comparable accuracy to those of Gaia at its faint limit (r<20) and smoothly extend the error versus magnitude curve deeper by about 5 mag. Recent efforts to reduce the risk for this requirement have concentrated in three areas. First, the LSST Image Simulator has been used to generate sequences of images containing stars with various astrometric parameters and a range of simulated observing conditions. Second, the digital archive of the scans of photographic sky survey plates in the region of SDSS Stripe 82 have been reprocessed so that they can be compared to the LSST Data Management astrometric solutions to find stars with significant proper motions. Third, short exposure observations from data archives have been processed, and new observations have been requested from various telescopes, including the Dark Energy Camera and the Space Surveillance Telescope. Results from these investigations will be presented, and the predictions for the astrometric performance of LSST will be discussed.

Ivezic, Zeljko; Monet, D. G.; Claver, C. F.; Axelrod, T. S.; Gizis, J.; Lupton, R.

2013-01-01

274

Numerical and laboratory simulations of auroral acceleration

NASA Astrophysics Data System (ADS)

The existence of parallel electric fields is an essential ingredient of auroral physics, leading to the acceleration of particles that give rise to the auroral displays. An auroral flux tube is modelled using electrostatic Vlasov simulations, and the results are compared to simulations of a proposed laboratory device that is meant for studies of the plasma physical processes that occur on auroral field lines. The hot magnetospheric plasma is represented by a gas discharge plasma source in the laboratory device, and the cold plasma mimicking the ionospheric plasma is generated by a Q-machine source. In both systems, double layers form with plasma density gradients concentrated on their high potential sides. The systems differ regarding the properties of ion acoustic waves that are heavily damped in the magnetosphere, where the ion population is hot, but weakly damped in the laboratory, where the discharge ions are cold. Ion waves are excited by the ion beam that is created by acceleration in the double layer in both systems. The efficiency of this beam-plasma interaction depends on the acceleration voltage. For voltages where the interaction is less efficient, the laboratory experiment is more space-like.

Gunell, H.; De Keyser, J.; Mann, I.

2013-10-01

275

Numerical simulation of the SOFIA flow field

NASA Technical Reports Server (NTRS)

This report provides a concise summary of the contribution of computational fluid dynamics (CFD) to the SOFIA (Stratospheric Observatory for Infrared Astronomy) project at NASA Ames and presents results obtained from closed- and open-cavity SOFIA simulations. The aircraft platform is a Boeing 747SP and these are the first SOFIA simulations run with the aircraft empennage included in the geometry database. In the open-cavity runs the telescope is mounted behind the wings. Results suggest that the cavity markedly influences the mean pressure distribution on empennage surfaces and that 110-140 decibel (db) sound pressure levels are typical in the cavity and on the horizontal and vertical stabilizers. A strong source of sound was found to exist on the rim of the open telescope cavity. The presence of this source suggests that additional design work needs to be performed in order to minimize the sound emanating from that location. A fluid dynamic analysis of the engine plumes is also contained in this report. The analysis was part of an effort to quantify the degradation of telescope performance resulting from the proximity of the port engine exhaust plumes to the open telescope bay.

Klotz, Stephen P.

1995-01-01

276

Numerical simulation of the world ocean circulation

NASA Technical Reports Server (NTRS)

A multi-level model, based on the primitive equations, is developed for simulating the temperature and velocity fields produced in the world ocean by differential heating and surface wind stress. The model ocean has constant depth, free slip at the lower boundary, and neglects momentum advection; so that there is no energy exchange between the barotropic and baroclinic components of the motion, although the former influences the latter through temperature advection. The ocean model was designed to be coupled to the UCLA atmospheric general circulation model, for the study of the dynamics of climate and climate changes. But here, the model is tested by prescribing the observed seasonally varying surface wind stress and the incident solar radiation, the surface air temperature and humidity, cloudiness and the surface wind speed, which, together with the predicted ocean surface temperature, determine the surface flux of radiant energy, sensible heat and latent heat.

Takano, K.; Mintz, Y.; Han, Y. J.

1973-01-01

277

Numerical aerodynamic simulation facility. Preliminary study extension

NASA Technical Reports Server (NTRS)

The production of an optimized design of key elements of the candidate facility was the primary objective of this report. This was accomplished by effort in the following tasks: (1) to further develop, optimize and describe the function description of the custom hardware; (2) to delineate trade off areas between performance, reliability, availability, serviceability, and programmability; (3) to develop metrics and models for validation of the candidate systems performance; (4) to conduct a functional simulation of the system design; (5) to perform a reliability analysis of the system design; and (6) to develop the software specifications to include a user level high level programming language, a correspondence between the programming language and instruction set and outline the operation system requirements.

1978-01-01

278

Numerical simulations on the magnetopause current layer

One-dimensional particle simulations are carried out in order to study the current layer between a plasma and magnetic field such as seen at the magnetopause boundary layer. When a subsonic solar wind plasma flow impinges upon a vacuum dipole magnetic field, the width of the current layer is found much smaller than the ion gyroradius and is close to theoretically predicted geometric mean of the ion and electron gyroradii. The width remains essentially the same when the magnetic field is filled with a thermal plasma whose density is smaller than the incoming solar wind density. The width, therefore, remains much smaller than the ion gyroradius. It is found that a similar sharp current layer develops in a plasma confined in a magnetic field such as seen in laboratory and space plasmas. 15 refs., 11 figs.

Okuda, H.

1990-12-01

279

Numerical Simulation of Supersonic Gap Flow

Various gaps in the surface of the supersonic aircraft have a significant effect on airflows. In order to predict the effects of attack angle, Mach number and width-to-depth ratio of gap on the local aerodynamic heating environment of supersonic flow, two-dimensional compressible Navier-Stokes equations are solved by the finite volume method, where convective flux of space term adopts the Roe format, and discretization of time term is achieved by 5-step Runge-Kutta algorithm. The numerical results reveal that the heat flux ratio is U-shaped distribution on the gap wall and maximum at the windward corner of the gap. The heat flux ratio decreases as the gap depth and Mach number increase, however, it increases as the attack angle increases. In addition, it is important to find that chamfer in the windward corner can effectively reduce gap effect coefficient. The study will be helpful for the design of the thermal protection system in reentry vehicles. PMID:25635395

Jing, Xu; Haiming, Huang; Guo, Huang; Song, Mo

2015-01-01

280

Numerical simulation of supersonic gap flow.

Various gaps in the surface of the supersonic aircraft have a significant effect on airflows. In order to predict the effects of attack angle, Mach number and width-to-depth ratio of gap on the local aerodynamic heating environment of supersonic flow, two-dimensional compressible Navier-Stokes equations are solved by the finite volume method, where convective flux of space term adopts the Roe format, and discretization of time term is achieved by 5-step Runge-Kutta algorithm. The numerical results reveal that the heat flux ratio is U-shaped distribution on the gap wall and maximum at the windward corner of the gap. The heat flux ratio decreases as the gap depth and Mach number increase, however, it increases as the attack angle increases. In addition, it is important to find that chamfer in the windward corner can effectively reduce gap effect coefficient. The study will be helpful for the design of the thermal protection system in reentry vehicles. PMID:25635395

Jing, Xu; Haiming, Huang; Guo, Huang; Song, Mo

2015-01-01

281

DNS Name Service based on Secure Multicast DNS for IPv6 Mobile Ad Hoc Networks

DNS Name Service based on Secure Multicast DNS for IPv6 Mobile Ad Hoc Networks Jaehoon Jeong which can provide mobile nodes in IPv6 mobile ad hoc network with secure name-to-address resolution, mobile ad hoc network, multicast, IPv6. I. INTRODUCTION Mobile Ad Hoc Network (MANET) is the network

Jeong, Jaehoon "Paul"

282

Numerical simulation of baroclinic Jovian vortices

NASA Astrophysics Data System (ADS)

We examine the evolution of baroclinic vortices in a time-dependent, nonlinear numerical model of a Jovian atmosphere. The model uses a normal-mode expansion in the vertical, using the barotropic and first two baroclinic modes. Results for the stability of baroclinic vortices on an f plane in the absence of a mean zonal flow are similar to results of Earth vortex models, although the presence of a fluid interior on the Jovian planets shifts the stability boundaries to smaller length scales. The presence of a barotropic mean zonal flow in the interior stabilizes vortices against instability and significantly modifies the finite amplitude form of baroclinic instabilities. The effect of a zonal flow on a form of barotropic instability produces periodic oscillations in the latitude and longitude of the vortex as observed at the level of the cloud tops. This instability may explain some, but not all, observations of longitudinal oscillations of vortices on the outer planets. Oscillations in aspect ratio and orientation of stable vortices in a zonal shear flow are observed in this baroclinic model, as in simpler two-dimensional models. Such oscillations are also observed in the atmospheres of Jupiter and Neptune. The meridional propagation and decay of vortices on a beta plane is inhibited by the presence of a mean zonal flow. The direction of propagation of a vortex relative to the mean zonal flow depends upon the sign of the meridional potential vorticity gradient; combined with observations of vortex drift rates, this may provide a constraint on model assumption for the flow in the deep interior of the Jovian planets.

Achterberg, R. K.; Ingersoll, A. P.

1994-02-01

283

Direct numerical simulation of flow and heat transfer in a turbine cascade with incoming wakes

NASA Astrophysics Data System (ADS)

Direct numerical simulations (DNS) of flow in a turbine cascade with heat transfer have been performed. The set-up of the simulations was chosen in close accordance with previous experiments. Three of the experimental situations were simulated: one without free-stream turbulence and two with periodically incoming wakes of different frequency and with different levels of background fluctuation. Hence, the calculations allow us to study the influence of impinging wakes and background fluctuations on the development of the boundary layers and the local Nusselt number along the surfaces of the heated blade. Along the suction side, the pressure gradient is first favourable and then turns adverse near the trailing edge and the boundary layer remains laminar for the case without free-stream turbulence with the Nusselt number showing the typical decay from the leading to the trailing edge. With periodic wakes and background turbulence, transition occurs when the pressure gradient turns adverse, but intermittency persists so that the boundary layer is not fully turbulent when the trailing edge is reached. In this region, the heat transfer is increased significantly by an amount comparable to that found in the experiments. In the pre-transitional region with favourable pressure gradient, the flow acceleration stretches the free-stream vortices, aligning their axis with the flow direction, thereby forming streamwise vortical structures. These increase the laminar heat transfer in this region by 20 30%, which is, however, much less than observed in the experiments. On the pressure side, the pressure gradient is favourable along the entire blade so that the boundary layer remains laminar. Here, the wakes, through their impingement, also generate streamwise vortical structures which, because of the low convection speed on this side, have a very long lifetime compared to the structures along the suction side. Also these structures increase the laminar heat transfer by about 30%, which for the case with the highest wake frequency is again much less than in the experiments. The simulated average level of fluctuations in the laminar parts of the boundary layers is comparable or even higher than that in the experiments so that it seems likely that a difference in the spectral contents causes the discrepancies. The wake turbulence entering the calculation domain corresponds to that in far wakes with relatively small-scale structures, whereas in the experiments the wakes most probably still carried some large-scale fluctuations of the size of the wake width, which have been found to be more effective in increasing laminar heat transfer.

Wissink, Jan G.; Rodi, Wolfgang

2006-12-01

284

Numerical simulation of porosity-free titanium dental castings.

The objective of this research was to analyse, predict and control the porosity in titanium dental castings by the use of numerical simulation. A commercial software package (MAGMASOFT) was used. In the first part of the study, a model casting (two simplified tooth crowns connected by a connector bar) was simulated to analyse shrinkage porosity. Secondly, gas pores were numerically examined by means of a ball specimen with a "snake" sprue. The numerical simulation results were compared with the experimental casting results, which were made on a centrifugal casting machine. The predicted shrinkage levels coincided well with the experimentally determined levels. Based on the above numerical analyses, an optimised running and gating system design for the crown model was proposed. The numerical filling and solidification results of the ball specimen showed that this simulation model could be helpful for the explanation of the experimentally indicated gas pores. It was concluded that shrinkage porosity in titanium dental casting was predictable, and it could be minimised by improving the running and gating system design. Entrapped gas pores can be explained from the simulation results of the mould filling and solidification. PMID:10467947

Wu, M; Augthun, M; Schädlich-Stubenrauch, J; Sahm, P R; Spiekermann, H

1999-08-01

285

Numerical simulation of the edge tone phenomenon

NASA Technical Reports Server (NTRS)

Time accurate Navier-Stokes computations were performed to study a class 2 (acoustic) whistle, the edge tone, and to gain knowledge of the vortex-acoustic coupling mechanisms driving production of these tones. Results were obtained by solving the full Navier-Stokes equations for laminar compressible air flow of a two dimensional jet issuing from a slit interacting with a wedge. Cases considered were determined by varying the distance from the slit to the wedge. Flow speed was kept constant at 1,750 cm/s as was the slit thickness of 0.1 cm, corresponding to conditions in the experiments of Brown. The analytical computations revealed edge tones to be present in four harmonic stages of jet flow instability over the wedge as the jet length was varied from 0.3 to 1.6 cm. Excellent agreement was obtained in all four edge tone stage cases between the present computational results and the experimentally obtained frequencies and flow visualization results of Brown. Specific edge tone generation phenomena and further confirmation of certain theories and empirical formulas concerning these phenomena were brought to light in this analytical simulation of edge tones.

Dougherty, N. S.; Liu, B. L.; Ofarrell, J. M.

1994-01-01

286

Numerical simulation of the Filchner overflow

NASA Astrophysics Data System (ADS)

The plume of Ice Shelf Water (ISW) flowing into the Weddell Sea over the Filchner sill contributes to the formation of Antarctic Bottom Water. The Filchner overflow is simulated using a hydrostatic, primitive equation three-dimensional ocean model with a 0.5-2 Sv ISW influx above the Filchner sill. The best fit to mooring temperature observations is found with influxes of 0.5 and 1 Sv, below a previous estimate of 1.6 ± 0.5 Sv based on sparse mooring velocities. The plume first moves north over the continental shelf, and then turns west, along slope of the continental shelf break where it breaks up into subplumes and domes, some of which then move downslope. Other subplumes run into the eastern submarine ridge and propagate along the ridge downslope in a chaotic manner. The next, western ridge is crossed by the plume through several paths. Despite a number of discrepancies with observational data, the model reproduces many attributes of the flow. In particular, we argue that the temporal variability shown by the observations can largely be attributed to the unstable structure of the flow, where the temperature fluctuations are determined by the motion of the domes past the moorings. Our sensitivity studies show that while thermobaricity plays a role, its effect is small for the flows considered. Smoothing the ridges out demonstrate that their presence strongly affects the plume shape around the ridges. An increase in the bottom drag or viscosity leads to slowing down, and hence thickening and widening of the plume.

Wilchinsky, Alexander V.; Feltham, Daniel L.

2009-12-01

287

Numerical Simulations of the Wardle Instability

NASA Astrophysics Data System (ADS)

In dense interstellar clouds, the ionisation fraction is so low that the material may be considered to be made up of two fluids: a perfectly conducting fluid consisting of the ions and electrons and a neutral fluid consisting of atomic hydrogen. These interact via collisions, but the imperfect coupling leads to a finite resistivity (ambi-polar diffusion). Under these conditions, there exist shock structures, called C-shocks, in which the dissipation is due to resistivity rather than viscosity (Draine 1980). Wardle (1990, 1991a,b) showed that C-shocks with Alfven Mach numbers greater than ? 5 are subject to a transverse corrugation instability and nonlinear calculations have shown that this leads to the formation of dense fingers of neutral gas (Toth 1995a,b; Stone 1997; Neufeld & Stone 1997; MacLow & Smith 1997). However, the instability relies on a separation between the conducting fluid and the neutral fluid, which does not occur if timescale for ionisation equilibrium is short compared to the flow time through the shock structure. The ionisation fraction is then simply a function of neutral density and our simulations show that this does indeed suppress the instability. Since the timescale for ionisation equilibrium is always short compared to the flow time in dense clouds, this means that the instability does not occur unless charged grains play a significant role. Instability is possible in this case because a fluid composed of charged grains does undergo separation from the neutrals and the grain mass fraction influences the ionisation fraction. We use the multi-fluid code described in Falle (2003), which includes the grain fluid, to show that the instability can occur in such cases.

Falle, S. A. E. G.; Hartquist, T. W.; van Loo, S.

2009-04-01

288

Geometric issues in reverse osmosis: numerical simulation and experimentation.

This investigation is a synergistic combination of laboratory experimentation and numerical simulation to quantify the practical impact of geometric imperfections in the flow channels of a reverse osmosis (RO) system. To this end, carefully executed experiments are performed to quantify the fluid flow in a system containing feed spacers which are embedded in the RO membrane. In a complementary activity, numerical simulations were performed both for an ideal geometric situation (without embedments) and the actual geometric configuration including the embedments. It was found that the presence of unaccounted embedments affected the pressure drop predictions for the system by 14-19%. When account was taken of the embedments, the simulation results were found to be virtually coincident with the experimental results. This outcome suggests that deviations between experimental and simulation results encountered in the literature might well have been due to geometrical deviations of the type investigated here. The numerical simulation of the feedwater fluid flow was based on the often-used but unverified assumption that the velocity field experiences the geometric periodicity of the feed spacer. This assumption was lent support by results from a non-periodic simulation model and by the excellent agreement between the numerically based predictions and the experimental data. PMID:25353939

Srivathsan, G; Sparrow, Ephraim; Gorman, John

2014-01-01

289

Numerical Simulation of Ice Accretions Based on Unstructured Grids

NASA Astrophysics Data System (ADS)

An Eulerian method to numerically simulate ice accretions on airfoils based on unstructured grids has been presented. The Euler Equations are employed to solve the flowfields of the airfoils. The governing equations for droplets are solved based on the velocity distribution of the airflow to obtain the collection efficiency on the airfoil surfaces. The different freezing models are employed to simulate the freezing processes of the rime and glaze ice in order to calculate the ice amount. The ice accretions on a NACA 0012 airfoil have been simulated and the comparison with the experimental data indicates that the simulated method presented is feasible and effective.

Zhang, Q.; Gao, Z. H.

2011-09-01

290

Numerical Convergence of Hydrodynamical SPH Simulations of Cooling Clusters

The results from hydrodynamical TREESPH simulations of galaxy clusters are used to investigate the dependence of the final cluster X-ray properties upon the numerical resolution and the assumed star formation models for the cooled gas. A comparison between runs with different star formation methods shows that the results of simulations, based on star formation methods in which gas conversion into stars is controlled by an efficiency parameter c_{star}, are sensitive to the simulation numerical resolution. In this respect star formation methods based instead on a local density threshold, are shown to give more stable results. Final X-ray luminosities are found to be numerically stable, with uncertainties of a factor 2.

R. Valdarnini

2001-11-05

291

Direct numerical simulations of gas/liquid multiphase flows

NASA Astrophysics Data System (ADS)

Direct numerical simulations of bubbly flows are reviewed and recent progress is discussed. Simulations, of homogeneous bubble distribution in fully periodic domains at relatively low Reynolds numbers have already yielded considerable insight into the dynamics of such flows. Many aspects of the evolution converge rapidly with the size of the systems and results for the rise velocity, the velocity fluctuations, as well as the average relative orientation of bubble pairs have been obtained. The challenge now is to examine bubbles at higher Reynolds numbers, bubbles in channels and confined geometry, and bubble interactions with turbulent flows. We briefly review numerical methods used for direct numerical simulations of multiphase flows, with a particular emphasis on methods that use the so-called "one-field" formulation of the governing equations, and then discuss studies of bubbles in periodic domains, along with recent work on wobbly bubbles, bubbles in laminar and turbulent channel flows, and bubble formation in boiling.

Tryggvason, Gretar; Esmaeeli, Asghar; Lu, Jiacai; Biswas, Souvik

2006-09-01

292

Probing Physics of Magnetohydrodynamic Turbulence Using Direct Numerical Simulation

The energy spectrum and the nolinear cascade rates of MHD turbulence is not clearly understood. We have addressed this problem using direct numerical simulation and analytical calculations. Our numerical simulations indicate that Kolmogorov-like phenomenology with $k^{-5/3}$ energy spectrum, rather than Kraichnan's $k^{-3/2}$, appears to be applicable in MHD turbulence. Here, we also construct a self-consistent renomalization group procedure in which the mean magnetic field gets renormalized, which in turns yields $k^{-5/3}$ energy spectrum. The numerical simulations also show that the fluid energy is transferred to magnetic energy. This result could shed light on the generation magnetic field as in dynamo mechanism.

Mahendra K. Verma; Gaurav Dar

1998-03-21

293

Numerical simulation of surface barriers for shrub-steppe ecoregions

Surface barriers, constructed of earthen materials, are being proposed for the long-term management of vadose-zone buried waste and subsurface contamination for sites within the shrub-steppe ecoregion of North America. Field experiments of a prototype barrier on a shrub-steppe site have been ongoing since 1994, providing water balance data, which includes drainage from the sideslopes. Design and licensing of surface barriers will require a demonstrated understanding of the nonisothermal geohydrologic and coupled ground surface to atmosphere water mass and energy transport processes that control water infiltration to the subsurface. As a prelude to inverse numerical modeling to estimate critical parameters for the prototype barrier, this paper describes and demonstrates a numerical simulator for modeling the prototype barrier for shrub-steppe environments. The numerical simulator comprises a nonisothermal multifluid subsurface flow and transport simulator fully coupled to a modified nonlinear sparsely vegetated (bare substrate to closed canopy) evapotranspiration module that mechanistically predicts evaporation.

White, Mark D.; Ward, Andy L.

2006-02-11

294

Numerical simulations of rarefied gas flows in thin film processes

Many processes exist in which a thin film is deposited from the gas phase, e.g. Chemical Vapor Deposition (CVD). These processes are operated at ever decreasing reactor operating pressures and with ever decreasing wafer feature dimensions, reaching into the rarefied flow regime.\\u000aAs numerical simulation tools are frequently used to design and improve reactors, there is a need for numerical

R. Dorsman

2007-01-01

295

Numerical simulation of convection depth in shear cell under microgravity

By applying the idea of an isotope trace experiment, the concept of isotope trace in hydrodynamics simulation is proposed. The isotope concentration is conceived as a numerical marker to trace the convection and estimate numerically the convection depth due to shearing in shear cell under microgravity. Two different shearing rates, 2mm\\/s and 20mm\\/s, are investigated, and results demonstrate that the

Z. Zeng; H. Mizuseki; K. Ichinoseki; K. Higashino; Y. Kawazoe

1999-01-01

296

The numerical simulation of vanadium RedOx flow batteries

In this article a theoretical model of a RedOx flow cell (RFC), based on an equation system of fluid dynamics and of electrochemistry,\\u000a is presented. A numerical algorithm of the solution of the equation system is developed. The results of numerical simulation\\u000a of processes in the RFC are analyzed and validated in a test cell. The effects of different electrode

I. M. Bayanov; R. Vanhaelst

297

Industrial Challenges for Numerical Simulation of Crystal Growth

Numerical simulation of industrial crystal growth is dificult due to its multidisciplinary nature and complex geometry of\\u000a real-life growth equipment. An attempt is made to itemize physical phenomena dominant in different methods for growth of bulk\\u000a crystals from melt and from vapour phase and to review corresponding numerical approaches. Academic research and industrial\\u000a applications are compared. Development of computational engine

Dmitry K. Ofengeim; Alexander I. Zhmakin

2003-01-01

298

Numerical simulation and experimental validation of coiled adiabatic capillary tubes

The objective of this study is to extend and validate the model developed and presented in previous works [O. García-Valladares, C.D. Pérez-Segarra, A. Oliva, Numerical simulation of capillary tube expansion devices behaviour with pure and mixed refrigerants considering metastable region. Part I: mathematical formulation and numerical model, Applied Thermal Engineering 22 (2) (2002) 173–182; O. García-Valladares, C.D. Pérez-Segarra, A. Oliva,

O. García-Valladares

2007-01-01

299

Numerical simulations of astrophysical jets: comparison of integration schemes

NASA Astrophysics Data System (ADS)

In this paper we model an astrophysical jet and simulate its evolution numerically by means of different numerical schemes, focusing in particular on Godunov-type methods (Roe, HLLE, PPM) but considering also central methods (local Lax-Friedrichs). The results yielded by various codes are then compared, on the basis of various parameters, such as jet morphology, the statistical distributions of basic physical quantities, cocoon features, and the integral value of significant quantities.

Massaglia, S.; Zurlo, N.; Bodo, G.

300

Numerical simulation of random composite dielectrics. II. Simulations including dissipation

A simulation model presented earlier for the permittivity of a composite material has now been extended to include the dissipation of energy inside the medium. It is employed to calculate the complex dielectric function of so-called effective media, i.e. randomly distributed lossy particles in an insulating matrix. A general equation formulated before for lossless media proves to be valid also

S. Stölzle; A. Enders; G. Nimtz

1992-01-01

301

Numerical and laboratory simulation of fault motion and earthquake occurrence

NASA Technical Reports Server (NTRS)

This paper reviews the simulation of earthquake occurrence by numerical and laboratory mechanical block models. Simple linear rheological elements are used with elastic forces driving the main events and viscoelastic forces being important for aftershock and creep occurrence. Friction and its dependence on velocity, stress, and displacement also play a key role in determining how, when, and where fault motion occurs. The discussion of the qualitative behavior of the simulators focuses on the manner in which energy is stored in the system and released by the unstable and stable sliding processes. The numerical results emphasize the statistics of earthquake occurrence and the correlations among source parameters.

Cohen, S. C.

1979-01-01

302

Numerical simulation of tornado wind loading on structures

NASA Technical Reports Server (NTRS)

A numerical simulation of a tornado interacting with a building was undertaken in order to compare the pressures due to a rotational unsteady wind with that due to steady straight winds used in design of nuclear facilities. The numerical simulations were performed on a two-dimensional compressible hydrodynamics code. Calculated pressure profiles for a typical building were then subjected to a tornado wind field and the results were compared with current quasisteady design calculations. The analysis indicates that current design practices are conservative.

Maiden, D. E.

1976-01-01

303

Numerical simulation of water flow around a rigid fishing net

This paper is devoted to the simulation of the flow around and inside a rigid axisymmetric net. We describe first how experimental data have been obtained. We show in detail the modelization. The model is based on a Reynolds Averaged Navier-Stokes turbulence model penalized by a term based on the Brinkman law. At the out-boundary of the computational box, we have used a "ghost" boundary condition. We show that the corresponding variational problem has a solution. Then the numerical scheme is given and the paper finishes with numerical simulations compared with the experimental data.

Roger Lewandowski; Géraldine Pichot

2006-12-20

304

Numerical simulation of dynamic fracture and failure in solids

Numerical simulation of dynamic fracture and failure processes in solid continua using Lagrangian finite element techniques is the subject of discussion in this investigation. The specific configurations in this study include penetration of steel projectiles into aluminum blocks and concrete slabs. The failure mode in the aluminum block is excessive deformation while the concrete slab fails by hole growth, spallation, and scabbing. The transient dynamic finite element code LS-DYNA2D was used for the numerical analysis. The erosion capability in LS-DYNA2D was exercised to carry out the fracture and failure simulations. Calculated results were compared to the experimental data. Good correlations were obtained.

Chen, E.P.

1994-05-01

305

DNS AND LES WITH STOCHASTIC MODELLING OF SUBGRID ACCELERATION APPLIED TO SOLID PARTICLES IN A HIGH

DNS AND LES WITH STOCHASTIC MODELLING OF SUBGRID ACCELERATION APPLIED TO SOLID PARTICLES IN A HIGH implications for many environmental systems, from sediment transport to at- mospheric dispersion of pollutants or solid deposition in ma- rine flows. Previous experimental (Kaftori et al., 1995) and numerical

Boyer, Edmond

306

Direct numerical simulation of auto-ignition of a hydrogen vortex ring reacting with hot air

Direct numerical simulation (DNS) is used to study chemically reacting, laminar vortex rings. A novel, all-Mach number algorithm developed by Doom et al. [J. Doom, Y. Hou, K. Mahesh, J. Comput. Phys. 226 (2007) 1136-1151] is used. The chemical mechanism is a nine species, nineteen reaction mechanism for H{sub 2}/air combustion proposed by Mueller et al. [M.A. Mueller, T.J. Kim, R.A. Yetter, F.L. Dryer, Int. J. Chem. Kinet. 31 (1999) 113-125]. Diluted H{sub 2} at ambient temperature (300 K) is injected into hot air. The simulations study the effect of fuel/air ratios, oxidizer temperature, Lewis number and stroke ratio (ratio of piston stroke length to diameter). Results show that auto-ignition occurs in fuel lean, high temperature regions with low scalar dissipation at a 'most reactive' mixture fraction, {zeta}{sub MR} (Mastorakos et al. [E. Mastorakos, T.A. Baritaud, T.J. Poinsot, Combust. Flame 109 (1997) 198-223]). Subsequent evolution of the flame is not predicted by {zeta}{sub MR}; a most reactive temperature T{sub MR} is defined and shown to predict both the initial auto-ignition as well as subsequent evolution. For stroke ratios less than the formation number, ignition in general occurs behind the vortex ring and propagates into the core. At higher oxidizer temperatures, ignition is almost instantaneous and occurs along the entire interface between fuel and oxidizer. For stroke ratios greater than the formation number, ignition initially occurs behind the leading vortex ring, then occurs along the length of the trailing column and propagates toward the ring. Lewis number is seen to affect both the initial ignition as well as subsequent flame evolution significantly. Non-uniform Lewis number simulations provide faster ignition and burnout time but a lower maximum temperature. The fuel rich reacting vortex ring provides the highest maximum temperature and the higher oxidizer temperature provides the fastest ignition time. The fuel lean reacting vortex ring has little effect on the flow and behaves similar to a non-reacting vortex ring. (author)

Doom, Jeff; Mahesh, Krishnan [Department of Aerospace Engineering and Mechanics, University of Minnesota, 107 Akerman Hall, Minneapolis, MN (United States)

2009-04-15

307

Building Blocks for Reliable Complex Nonlinear Numerical Simulations. Chapter 2

NASA Technical Reports Server (NTRS)

This chapter describes some of the building blocks to ensure a higher level of confidence in the predictability and reliability (PAR) of numerical simulation of multiscale complex nonlinear problems. The focus is on relating PAR of numerical simulations with complex nonlinear phenomena of numerics. To isolate sources of numerical uncertainties, the possible discrepancy between the chosen partial differential equation (PDE) model and the real physics and/or experimental data is set aside. The discussion is restricted to how well numerical schemes can mimic the solution behavior of the underlying PDE model for finite time steps and grid spacings. The situation is complicated by the fact that the available theory for the understanding of nonlinear behavior of numerics is not at a stage to fully analyze the nonlinear Euler and Navier-Stokes equations. The discussion is based on the knowledge gained for nonlinear model problems with known analytical solutions to identify and explain the possible sources and remedies of numerical uncertainties in practical computations. Examples relevant to turbulent flow computations are included.

Yee, H. C.; Mansour, Nagi N. (Technical Monitor)

2001-01-01

308

Large Eddy Simulations and Turbulence Modeling for Film Cooling

NASA Technical Reports Server (NTRS)

The objective of the research is to perform Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES) for film cooling process, and to evaluate and improve advanced forms of the two equation turbulence models for turbine blade surface flow analysis. The DNS/LES were used to resolve the large eddies within the flow field near the coolant jet location. The work involved code development and applications of the codes developed to the film cooling problems. Five different codes were developed and utilized to perform this research. This report presented a summary of the development of the codes and their applications to analyze the turbulence properties at locations near coolant injection holes.

Acharya, Sumanta

1999-01-01

309

Numerical Simulation of the Gulf Stream and the Deep Circulation

The Gulf Stream system has been numerically simulated with relatively high resolution and realistic forcing. The surface fluxes\\u000a of the simulation were obtained from archives of calculations from the Eta-29 km model which is an National Center for Environment\\u000a Prediction (NCEP) operational atmospheric prediction model; synoptic fields are available every 3 hour. A comparison between\\u000a experiments with and without surface

Hyun-Chul Lee; George L. Mellor

2003-01-01

310

Numerical simulation of a premixed turbulent V-flame

We present three-dimensional, time-dependent simulations of a full-size laboratory-scale rod-stabilized premixed turbulent V-flame. The computations use an adaptive projection method based on a low Mach number formulation that incorporates detailed chemical kinetics and transport. The simulations are performed without introducing models for turbulence or turbulence chemistry interaction. We outline the numerical procedure and experimental setup, and compare computed results to mean flame location and surface wrinkling statistics gathered from experiment.

Bell, John B.; Day, Marc S.; Grcar, Joseph F.; Lijewski, Michael J.; Johnson, Matt R.; Cheng, Robert K.; Shepherd, Ian G.

2003-07-27

311

Numerical simulation of deformation and figure quality of precise mirror

NASA Astrophysics Data System (ADS)

The presented paper shows results and a comparison of FEM numerical simulations and optical tests of the assembly of a precise Zerodur mirror with a mounting structure for space applications. It also shows how the curing of adhesive film can impact the optical surface, especially as regards deformations. Finally, the paper shows the results of the figure quality analysis, which are based on data from FEM simulation of optical surface deformations.

Vit, Tomáš; Melich, Radek; Sandri, Paolo

2015-01-01

312

Numerical simulation of protein stamping process driven by capillary force.

Numerical simulations based on first-principle conservation laws of mass and momentum are performed to observe flow characteristics during the stamping process. The protein solution is transferred by a new design of microstamps with microchannels and printed on a bottom substrate. Furthermore, key physics of the stamping process and the control factors achieving uniform spot size can be identified and optimized after these simulations. PMID:16696302

Lin, Shih-Chang; Tseng, Fangang; Chieng, Ching-Chang

2002-09-01

313

Numerical simulation of a liquid propellant rocket motor

This work presents a numerical simulation of the flow field in a liquid propellant rocket engine chamber and exit nozzle using\\u000a techniques to allow the results to be taken as starting points for designing those propulsive systems. This was done using\\u000a a Finite Volume method simulating the different flow regimes which usually take place in those systems. As the flow

Nicolas M. C. Salvador; Marcelo M. Morales; Carlos E. S. S. Migueis; Demétrio Bastos-Netto

2001-01-01

314

Direct numerical simulation of compressible free shear flows

NASA Technical Reports Server (NTRS)

Direct numerical simulations of compressible free shear layers in open domains are conducted. Compact finite-difference schemes of spectral-like accuracy are used for the simulations. Both temporally-growing and spatially-growing mixing layers are studied. The effect of intrinsic compressibility on the evolution of vortices is studied. The use of convective Mach number is validated. Details of vortex roll up and pairing are studied. Acoustic radiation from vortex roll up, pairing and shape oscillations is studied and quantified.

Lele, Sanjiva K.

1989-01-01

315

Numerical simulations and modeling for stochastic biological systems with jumps

NASA Astrophysics Data System (ADS)

This paper gives a numerical method to simulate sample paths for stochastic differential equations (SDEs) driven by Poisson random measures. It provides us a new approach to simulate systems with jumps from a different angle. The driving Poisson random measures are assumed to be generated by stationary Poisson point processes instead of Lévy processes. Methods provided in this paper can be used to simulate SDEs with Lévy noise approximately. The simulation is divided into two parts: the part of jumping integration is based on definition without approximation while the continuous part is based on some classical approaches. Biological explanations for stochastic integrations with jumps are motivated by several numerical simulations. How to model biological systems with jumps is showed in this paper. Moreover, method of choosing integrands and stationary Poisson point processes in jumping integrations for biological models are obtained. In addition, results are illustrated through some examples and numerical simulations. For some examples, earthquake is chose as a jumping source which causes jumps on the size of biological population.

Zou, Xiaoling; Wang, Ke

2014-05-01

316

Data from a 1152X760X1280 direct numerical simulation (DNS) [N. J. Mueschke and O. Schilling, Phys. Fluids 21, 014106 (2009)] of a transitional Rayleigh-Taylor mixing layer modeled after a small Atwood number water channel experiment is used to comprehensively investigate the structure of mean and turbulent transport and mixing. The simulation had physical parameters and initial conditions approximating those in the experiment. The budgets of the mean vertical momentum, heavy-fluid mass fraction, turbulent kinetic energy, turbulent kinetic energy dissipation rate, heavy-fluid mass fraction variance, and heavy-fluid mass fraction variance dissipation rate equations are constructed using Reynolds averaging applied to the DNS data. The relative importance of mean and turbulent production, turbulent dissipation and destruction, and turbulent transport are investigated as a function of Reynolds number and across the mixing layer to provide insight into the flow dynamics not presently available from experiments. The analysis of the budgets supports the assumption for small Atwood number, Rayleigh/Taylor driven flows that the principal transport mechanisms are buoyancy production, turbulent production, turbulent dissipation, and turbulent diffusion (shear and mean field production are negligible). As the Reynolds number increases, the turbulent production in the turbulent kinetic energy dissipation rate equation becomes the dominant production term, while the buoyancy production plateaus. Distinctions between momentum and scalar transport are also noted, where the turbulent kinetic energy and its dissipation rate both grow in time and are peaked near the center plane of the mixing layer, while the heavy-fluid mass fraction variance and its dissipation rate initially grow and then begin to decrease as mixing progresses and reduces density fluctuations. All terms in the transport equations generally grow or decay, with no qualitative change in their profile, except for the pressure flux contribution to the total turbulent kinetic energy flux, which changes sign early in time (a countergradient effect). The production-to-dissipation ratios corresponding to the turbulent kinetic energy and heavy-fluid mass fraction variance are large and vary strongly at small evolution times, decrease with time, and nearly asymptote as the flow enters a self-similar regime. The late-time turbulent kinetic energy production-to-dissipation ratio is larger than observed in shear-driven turbulent flows. The order of magnitude estimates of the terms in the transport equations are shown to be consistent with the DNS at late-time, and also confirms both the dominant terms and their evolutionary behavior. These results are useful for identifying the dynamically important terms requiring closure, and assessing the accuracy of the predictions of Reynolds-averaged Navier-Stokes and large-eddy simulation models of turbulent transport and mixing in transitional Rayleigh-Taylor instability-generated flow.

Schilling, Oleg [Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States); Mueschke, Nicholas J. [Texas A and M Univ., College Station, TX (United States)

2010-10-18

317

Simulation of ductile crack growth using computational cells: numerical aspects

Simulation of ductile crack growth using computational cells: numerical aspects Arne S. Gulleruda the computational cell methodology to predict crack growth in ductile metals caused by void growth and coalescence, and the porosity for cell deletion Ð can adversely aect predicted crack growth resistance (R ) curves and/or hinder

Haj-Ali, Rami

318

Numerical simulation of a variable speed refrigeration system

This work presents two numerical models to simulate the transient and steady state behavior of a vapor compression refrigeration system. The condenser and the evaporator were divided into a number of control volumes. Time dependent partial differential equations system was obtained from the mass, energy and momentum balances for each control volume. As the expansion valve and the compressor both

R. N. N. Koury; L. Machado; K. A. R. Ismail

2001-01-01

319

On the Numerical Simulation of Waterflooding of Heterogeneous

On the Numerical Simulation of Waterflooding of Heterogeneous Petroleum Reservoirs Jim Douglas, Jr consider as a model problem the two-phase immiscible displacement in petroleum reservoirs. A very detailed of Mathematics, University of Campinas, 13081-970 Campinas, SP, Brazil LaboratÂ´orio Nacional de Computa

Furtado, Fred

320

Numerical simulation of a supersonic flow CO chemical laser

The flow field of a supersonic flow CO chemical laser is simulated numerically. The 2D compressible Navier-Stokes equations which include species transport and vibrational relaxation equations are solved by a finite volume method utilizing a point implicit scheme. The effects of the nozzle boundary layers, diffusive mixing, and oblique shock waves on the laser performance are discussed.

Hirokazu Yamada; Koh-Ichi Masuo; Wataru Masuda; Ken-Ichi Matsuno

1991-01-01

321

IRIS Spectrum Line Plot - Numeric Simulation - Duration: 13 seconds.

This video is similar to the IRIS Spectrum Line Plot video at http://www.youtube.com/watch?v=E4V_vF3qMSI, but now as derived from a numerical simulation of the Sun by the University of Oslo. Credit...

322

Numerical simulation of transpiration cooling through porous , T. Gotzen1

Numerical simulation of transpiration cooling through porous material W. Dahmen1 , T. Gotzen1 and S 55, 52056 Aachen SUMMARY Transpiration cooling using ceramic matrix composite (CMC) materials with experiments. KEY WORDS: Transpiration cooling, porous media flow, Darcy-Forchheimer equation, coupled finite

323

Numerical simulations of acoustic fields on boundary-fitted grids

In this paper we propose a numerical method for the simulation of acoustic fields, in presence of an obstacle in the physical domain, by finite difference solution of a two-dimensional differential model on boundary-fitted grids. The computational process gathers the elliptic grid generation, the linear Euler equations in curvilinear coordinates, the associated bicharacteristic equation and a fully explicit finite difference

Maria M. Cerimele; Francesca Pistella; Rosa Maria Spitaleri

2008-01-01

324

Numerical Simulation of the December 26, 2004: Indian Ocean Tsunami

Numerical Simulation of the December 26, 2004: Indian Ocean Tsunami J. Asavanant1, M. Ioualalen2, N. Kaewbanjak1, S. Grilli3, P. Watts4, and J. Kirby5 Abstract: The December 26, 2004 tsunami is one of the most devastating tsunami in recorded history. It was generated in the Indian Ocean off the western coast

Kirby, James T.

325

Numerical simulation of tsunamis — Its present and near future

Hindcasting of a tsunami by numerical simulations is a process of lengthy and complicated deductions, knowing only the final results such as run-up heights and tide records, both of which are possibly biased due to an insufficient number of records and due to hydraulic and mechanical limitation of tide gauges. There are many sources of error. The initial profile, determined

N. Shuto

1991-01-01

326

Dissipative state formulations and numerical simulation of a porous medium

of such a material which describes the wave prop- agation inside the porous medium, is given on (, z, x) RÃ?]0, 1Dissipative state formulations and numerical simulation of a porous medium for boundary absorbing: The problem under consideration relates to a model of porous wall devoted to aircraft motors noise reduction

327

A Mobile Computing Architecture for Numerical Simulation Cyril Dumont

A Mobile Computing Architecture for Numerical Simulation Cyril Dumont Paris 12 University LACL, a mobile computing architecture, based on mobile agents and JavaSpace. At the end of this paper, we apply. For example, in the International Conference on Mobile Ubiquitous Computing, Systems, Services and Technol

Paris-Sud XI, Université de

328

Numerical Simulation of the Perrin-Like Experiments

ERIC Educational Resources Information Center

A simple model of the random Brownian walk of a spherical mesoscopic particle in viscous liquids is proposed. The model can be solved analytically and simulated numerically. The analytic solution gives the known Einstein-Smoluchowski diffusion law r[superscript 2] = 2Dt, where the diffusion constant D is expressed by the mass and geometry of a…

Mazur, Zygmunt; Grech, Dariusz

2008-01-01

329

Numerical aerodynamic simulation facility preliminary study: Executive study

NASA Technical Reports Server (NTRS)

A computing system was designed with the capability of providing an effective throughput of one billion floating point operations per second for three dimensional Navier-Stokes codes. The methodology used in defining the baseline design, and the major elements of the numerical aerodynamic simulation facility are described.

1977-01-01

330

Generating Airplane Wings for Numerical Simulation and Manufacturing

Generating Airplane Wings for Numerical Simulation and Manufacturing Karl-Heinz Brakhage Philipp of multi-parted airplane wings from a cross-section given by a point cloud and the top view of the wing-Structure Interaction at Airplane Wings", the aerodynamics of high lift and cruise configurations and the interaction

331

Numerical simulations of autoignition in turbulent mixing flows

Two-dimensional direct numerical simulations have been performed of the autoignition of (i) laminar and turbulent shearless mixing layers between fuel and hotter air, (ii) thin slabs of fuel exposed to air from both sides, and (iii) homogeneous stagnant adiabatic mixtures. It has been found that the time for the first appearance of an ignition site is almost independent of the

T. J. Poinsot

1997-01-01

332

Numerical Simulations of a RoofTop Wind Turbine

Unsteady numerical simulations of a high efficiency roof-top wind turbine have been performed. The wind turbine cross section design was based on geometrical optimization study of Rahai and Hefazi for increasing contributions of the lift force to the torque, resulting in significant improvements in the performance of a vertical axis wind turbine. The wind turbine was 30 cm in diameter

Shahab Moayedian; Hamid Rahai

2010-01-01

333

Numerical simulation of plugging failure in ballistic penetration

A coupled computational material model of viscoplasticity and ductile damage has been developed and implemented in LS-DYNA. This model gives good agreement between numerical simulations and experimental observations of plugging failure in ballistic penetration, without the use of inverse modelling or predefined defects. However, even if the model constants can be determined from relatively simple uniaxial tensile tests, the computational

T. Børvik; O. S. Hopperstad; T. Berstad; M. Langseth

2001-01-01

334

Numerical Simulation of Interfacial Transport Processes using OpenFOAM

Numerical Simulation of Interfacial Transport Processes using OpenFOAMÂ® Fundamentals/matrix coupling) Figure 1: Growing droplet at sin- gle capillary; interface covered by multiple surfactants. OpenFOAM to the conventional mathematical notation for tensors and partial differential equations. Thus, OpenFOAM renders

Heermann, Dieter W.

335

Numerical simulation of selective laser sintering transient temperature field

A numerical simulation pattern based on finite element algorithm is proposed for calculation of selective laser sintering transient temperature field. The patter is based on the transient thermal radiation and the boundary conditions is concerned soundly, particularly during the transient sintering the relation between thermal conductivity and transient sintering temperature is set up on the basis of practical test to

Jian Xing; Xiaogang Sun

2009-01-01

336

Numerical simulation of pulsed laser ablation in air

Pulsed laser ablation is important in a variety of engineering applications involving precise removal of materials in laser micromachining and laser treatment of bio-materials. Particularly, detailed numerical simulation of complex laser ablation phenomena in air, taking the interaction between ablation plume and air into account, is required for many practical applications. In this paper, high-power pulsed laser ablation under atmospheric

Bukuk Oh; Dongsik Kim; Wonseok Jang; Bosung Shin

2003-01-01

337

Numerical simulation of high-intensity standing wave fields

The numerical simulation of high-intensity ultrasound sources is considered based on a finite element scheme, which has been developed by the authors during the last years. Having previously dealt with pulsed devices, we here consider ultrasound sources operating in cw-mode. In this case additional effects due to the formation of nonlinear standing wave fields can be observed. First, we will

Annekathrin Doehle; Johann Hoffelner; Hermann Landes; Reinhard Lerch

2002-01-01

338

Numerical simulation of cementitious materials degradation under external sulfate attack

A numerical methodology is proposed in this paper to simulate the degradation of cementitious materials under external sulfate attack. The methodology includes diffusion of ions in and out of the structure, chemical reactions which lead to dissolution and precipitation of solids, and mechanical damage accumulation using a continuum damage mechanics approach. Diffusion of ions is assumed to occur under a

S. Sarkar; S. Mahadevan; J. C. L. Meeussen; H. van der Sloot; D. S. Kosson

2010-01-01

339

Random copolymer adsorption: Morita approximation compared to exact numerical simulations

We study the adsorption of ideal random lattice copolymers with correlations in the sequences on homogeneous substrates with two different methods: An analytical solution of the problem based on the constrained annealed approximation introduced by Morita in 1964 and the generating function technique, and direct numerical simulations of lattice chains averaged over many realizations of random sequences. Both methods allow

Alexey Polotsky; Andreas Degenhard; Friederike Schmid

2009-01-01

340

NEW NUMERICAL TECHNOLOGIES FOR THE SIMULATION OF ARC WELDING PROCESSES

of welding processes, namely: o Modelling of material deposit (filler material), o Control of the mesh, o Antipolis, France; soudage@transvalor.com Keywords: welding, finite elements, material deposit, adaptiveNEW NUMERICAL TECHNOLOGIES FOR THE SIMULATION OF ARC WELDING PROCESSES Michel Bellet 1 , Makhlouf

Paris-Sud XI, Université de

341

Numerical simulation of the stamping process through microchannels

This study proposes a stamper array chip with embedded microchannels that delivers fixed size and shape liquid samples to a bottom chip for quantitative biodiagnosis and bioassays. The transfer process and physics are analyzed by solving first-principle equations numerically. The simulation proves that the surface tension force inside a microchannel plays an important role in driving the liquid fluid from

Shih-Chang Lin; Fangang Tseng; Ching-Chang Chieng

2003-01-01

342

Numerical simulation of protein stamping process driven by capillary force

Numerical simulations based on first-principle conservation laws of mass and momentum are performed to observe flow characteristics during the stamping process. The protein solution is transferred by a new design of microstamps with microchannels and printed on a bottom substrate. Furthermore, key physics of the stamping process and the control factors achieving uniform spot size can be identified and optimized

Shih-Chang Lin; Fangang Tseng; Ching-Chang Chieng

2002-01-01

343

Numerical Simulation for Asset-Liability Management in Life Insurance

Numerical Simulation for Asset-Liability Management in Life Insurance T. Gerstner1 , M. Griebel1 regulations and stronger competitions have increased the demand for stochastic asset-liability management (ALM of low effective dimension. 1 Introduction The scope of asset-liability management is the responsible

Bartels, Soeren

344

Numerical Simulation and Cold Modeling experiments on Centrifugal Casting

NASA Astrophysics Data System (ADS)

In a centrifugal casting process, the fluid flow eventually determines the quality and characteristics of the final product. It is difficult to study the fluid behavior here because of the opaque nature of melt and mold. In the current investigation, numerical simulations of the flow field and visualization experiments on cold models have been carried out for a centrifugal casting system using horizontal molds and fluids of different viscosities to study the effect of different process variables on the flow pattern. The effects of the thickness of the cylindrical fluid annulus formed inside the mold and the effects of fluid viscosity, diameter, and rotational speed of the mold on the hollow fluid cylinder formation process have been investigated. The numerical simulation results are compared with corresponding data obtained from the cold modeling experiments. The influence of rotational speed in a real-life centrifugal casting system has also been studied using an aluminum-silicon alloy. Cylinders of different thicknesses are cast at different rotational speeds, and the flow patterns observed visually in the actual castings are found to be similar to those recorded in the corresponding cold modeling experiments. Reasonable agreement is observed between the results of numerical simulation and the results of cold modeling experiments with different fluids. The visualization study on the hollow cylinders produced in an actual centrifugal casting process also confirm the conclusions arrived at from the cold modeling experiments and numerical simulation in a qualitative sense.

Keerthiprasad, Kestur Sadashivaiah; Murali, Mysore Seetharam; Mukunda, Pudukottah Gopaliengar; Majumdar, Sekhar

2011-02-01

345

Numerical simulation of single crystal growth by submerged heater method

A novel method of crystal growth which utilizes an axial submerged heater is proposed and numerically simulated. Single crystals should be grown by directional solidification in vertical bottom seeded crucibles. Submerged in the melt, the heater supplies the heat axially downward, enclosing and stratifying a small active portion of the melt.

Aleksandar G. Ostrogorsky

1990-01-01

346

Modeling and Numerical Simulation of a Complete Hydroelectric Production Site

For many years the EPFL Laboratory for Electrical Machines develops SIMSEN, a numerical software package for the simulation in transient and steady-state conditions of electrical power systems and adjustable speed drives, having an arbitrary topology. SIMSEN is based on a collection of modules, each corresponding to one system component (machines, converters, transformers, control devices, etc.). This contribution presents the extension

C. Nicolet; P. Allenbach; J.-J. Simond; F. Avellan

2007-01-01

347

Numerical simulation of flow separation control by oscillatory fluid injection

in this research. The performed numerical simulation investigates the flow over a NACA0015 air-foil. For this flow Re = 9??105 and the reduced frequency and momentum coefficient are F+ = 1:1 and C?? = 0:04 respectively. The oscillatory injection takes place at 12...

Resendiz Rosas, Celerino

2005-08-29

348

Acoustic waves in the solar chromosphere Numerical simulations with COBOLD

Acoustic waves in the solar chromosphere Â Numerical simulations with COBOLD Sven Wedemeyer 1 from our preliminary 3ÂD model at a height of 1000 km (middle chromosphere): Hot wave fronts and cool of the convection zone, photosphere and low/middle chromosphere with propaÂ gating shock waves. 500 1000 1500 Height

Wedemeyer-BÃ¶hm, Sven

349

Chromospheric Dynamics ---What Can Be Learnt from Numerical Simulations

Chromospheric Dynamics --- What Can Be Learnt from Numerical Simulations Mats Carlsson 1 and Robert. Observations of the solar chromosphere are often interpreted using methods derived from static modeling (e. It is found that enhanced chromospheric emission, which corresponds to an outwardly increasing semi

Stein, Robert

350

Numerical Simulation of Mud-Filtrate Invasion in Deviated Wells

, capillary pressure, permeability anisotropy, dipping layers, and degree of hydraulic communication between the spatial distribution of fluids in the near- borehole region caused by mud-filtrate invasion is necessary- fluid-flow simulator that is widely used in large-scale reservoir applications. Checks of numerical

Torres-VerdÃn, Carlos

351

Acceleration and dissipation statistics of numerically simulated isotropic turbulence

. As a result, it is useful to study the statistical relationships be- tween acceleration and velocity gradientsAcceleration and dissipation statistics of numerically simulated isotropic turbulence P. K. Yeunga the statistics of acceleration in a Eulerian frame. A major emphasis is on the use of conditional averaging

352

A review of numerical simulation of hydrothermal systems.

Many advances in simulating single and two-phase fluid flow and heat transport in porous media have recently been made in conjunction with geothermal energy research. These numerical models reproduce system thermal and pressure behaviour and can be used for other heat-transport problems, such as high-level radioactive waste disposal and heat-storage projects. -Authors

Mercer, J.W.; Faust, C.R.

1979-01-01

353

Numerical simulation of a radial diffuser turbulent airflow

In the present work are presented results from numerical simulations performed with the ANSYS-CFX® code. We have studied a radial diffuser flow case, which is the main academic problem used to study the flow behavior on flat plate valves. The radial flow inside the diffuser has important behavior such as the turbulence decay downstream and recirculation regions inside the valve

Alysson Kennerly Colaciti; Luis Miguel Valdés López; Hélio Aparecido Navarro; Luben Cabezas-gómez

2007-01-01

354

Direct numerical simulation of a transitional rectangular jet

A well-resolved numerical simulation of a Mach 0.5 jet exiting from a rectangular shaped nozzle with an aspect ratio of 5 into a quiescent ambient was performed at a Reynolds number of 2000 based on the narrow side of the nozzle. The transition process was triggered by the most unstable symmetric linear eigenmode of a parallel laminar jet profile at

B. Rembold; N. A. Adams; L. Kleiser

2002-01-01

355

Numerical simulation and experimental observations of initial friction transients

Experiments were performed to better understand the sliding frictional behavior between metals under relatively high shear and normal forces. Microstructural analyses were done to estimate local near-surface stress and strain gradients. The numerical simulation of the observed frictional behavior was based on a constitutive model that uses a state variable approach.

Hughes, D.A.; Weingarten, L.I.; Dawson, D.B.

1995-07-01

356

Numerical simulation of incoherent optical wave propagation in nonlinear fibres

nonlinear effects (such as Kerr effect, four waves mixing, Raman effect) could behave very differentlyNumerical simulation of incoherent optical wave propagation in nonlinear fibres A. Fernandez1,2 , S such as Kerr effect, four waves mixing, Raman effect, for 3 different pulsed fibre laser sources (100 ns

Boyer, Edmond

357

Direct Numerical Simulation of Polymer Electrolyte Fuel Cell Catalyst

285 6 Direct Numerical Simulation of Polymer Electrolyte Fuel Cell Catalyst Layers Partha P O2, H2O, N2 x y Figure 1. Schematic diagram of a polymer electrolyte fuel cell. (MEA). The anode devices for mobile, stationary and portable power. Among the several types of fuel cells, polymer

358

Numerical simulations of large impacts in thick atmospheres

We present numerical simulations of asteroids and comets striking the atmosphere of Venus. We consider a wide variety of objects, both comets and asteroids, ranging in size from 1 to 4 km diameter, striking at impact angles ranging from the vertical to 60 degrees, at velocities from 20 to 90 km\\/s, in a variety of shapes (including some lumpy spheroidal

K. Zahnle; D. Korycansky

1998-01-01

359

AI-Based Simulation: An Alternative to Numerical Simulation and Modeling

AI-Based Simulation: An Alternative to Numerical Simulation and Modeling Shahab D. Mohaghegh1,2 1 technology based on pattern recognition capabilities of artificial intelli- gence and data mining. Keywords Simulation. 1 Introduction In this paper a new class of reservoir models that are developed based on the pat

Mohaghegh, Shahab

360

MULTI-SCALE SIMULATIONS OF STIRRED LIQUID -- LIQUID DISPERSIONS

Direct numerical simulations (DNS) of liquid -liquid dispersions in a three-dimensional periodic domain have been performed by means of the lattice -Boltzmann method. The dis- persion was agitated by a random force field such as to produce isotropic turbulence. The turbu- lence levels and its history were based on a fluid parcel in a turbulently stirred tank traversing the impeller

J. J. Derksenand

2007-01-01

361

Numerical relativity simulations in the era of the Einstein Telescope

Numerical-relativity (NR) simulations of compact binaries are expected to be an invaluable tool in gravitational-wave (GW) astronomy. The sensitivity of future detectors such as the Einstein Telescope (ET) will place much higher demands on NR simulations than first- and second-generation ground-based detectors. We discuss the issues facing compact-object simulations over the next decade, with an emphasis on estimating where the accuracy and parameter space coverage will be sufficient for ET and where significant work is needed.

Mark Hannam; Ian Hawke

2009-08-21

362

DNS Based Spam Bots Detection in a University

We carried out an entropy study on the DNS query traffic from the outside of a university campus network to the top domain DNS server when querying about reverse resolution on the PC room terminals through April 1st, 2007 to April 30th, 2008. The following interesting results are given: (1) In January 17th, 2008, the DNS query traffic is mainly

D. A. L. Romaa; S. Kubota; K. Sugitani; Y. Musashi

2008-01-01

363

Evaluation of a vortex-based subgrid stress model using DNS databases

NASA Technical Reports Server (NTRS)

The performance of a SubGrid Stress (SGS) model for Large-Eddy Simulation (LES) developed by Misra k Pullin (1996) is studied for forced and decaying isotropic turbulence on a 32(exp 3) grid. The physical viability of the model assumptions are tested using DNS databases. The results from LES of forced turbulence at Taylor Reynolds number R(sub (lambda)) approximately equals 90 are compared with filtered DNS fields. Probability density functions (pdfs) of the subgrid energy transfer, total dissipation, and the stretch of the subgrid vorticity by the resolved velocity-gradient tensor show reasonable agreement with the DNS data. The model is also tested in LES of decaying isotropic turbulence where it correctly predicts the decay rate and energy spectra measured by Comte-Bellot & Corrsin (1971).

Misra, Ashish; Lund, Thomas S.

1996-01-01

364

Numerical simulation of reactivity measurements in WER-1000 reactor

Reactivity is one of the most used and important concepts in physics and nuclear reactor calculations carried out for the safety analysis. Currently, design calculation of Russian WER reactors are carried out with modern coupled time-dependent neutronic and heat hydraulic codes. They allow to perform numerical simulation of reactivity measurements. However, point kinetic model used for simulation of large reactivity insertion leads to some issues. The paper discusses the numerical simulation of reactivity measurement, shows that the reactivity obtained from the steady state solution does not always correspond to the measured value. Comparison of scram system reactivity worth calculated and measured during physical start-up of unit 3, Kalinin NPP is presented. (authors)

Popykin, A.; Kavun, O.; Shevchenko, S.; Shevchenko, R. [Scientific and Engineering Center for Nuclear and Radiation Safety, Malaya Krasnoselskaya St., 2/8, bid. 5, 107140, Moscow (Russian Federation)

2012-07-01

365

Configuration Management File Manager Developed for Numerical Propulsion System Simulation

NASA Technical Reports Server (NTRS)

One of the objectives of the High Performance Computing and Communication Project's (HPCCP) Numerical Propulsion System Simulation (NPSS) is to provide a common and consistent way to manage applications, data, and engine simulations. The NPSS Configuration Management (CM) File Manager integrated with the Common Desktop Environment (CDE) window management system provides a common look and feel for the configuration management of data, applications, and engine simulations for U.S. engine companies. In addition, CM File Manager provides tools to manage a simulation. Features include managing input files, output files, textual notes, and any other material normally associated with simulation. The CM File Manager includes a generic configuration management Application Program Interface (API) that can be adapted for the configuration management repositories of any U.S. engine company.

Follen, Gregory J.

1997-01-01

366

Accuracy of Large-Eddy Simulation of Premixed Turbulent Combustion

The accuracy of large-eddy simulation (LES) of a turbulent premixed Bunsen flame is investigated in this paper. To distinguish\\u000a between discretization and modeling errors, multiple large-eddy simulations, using different grid size h but the same filterwidth\\u000a ?, are compared with the direct numerical simulation (DNS). In addition, large-eddy simulations using multiple ? but the same\\u000a ratio ?\\/h are compared. The

A. W. Vreman; R. J. M. Bastiaans; B. J. Geurts

367

Numerical Propulsion System Simulation (NPSS) 1999 Industry Review

NASA Technical Reports Server (NTRS)

The technologies necessary to enable detailed numerical simulations of complete propulsion systems are being developed at the NASA Glenn Research Center in cooperation with industry, academia, and other government agencies. Large scale, detailed simulations will be of great value to the nation because they eliminate some of the costly testing required to develop and certify advanced propulsion systems. In addition, time and cost savings will be achieved by enabling design details to be evaluated early in the development process before a commitment is made to a specific design. This concept is called the Numerical Propulsion System Simulation (NPSS). NPSS consists of three main elements: (1) engineering models that enable multidisciplinary analysis of large subsystems and systems at various levels of detail, (2) a simulation environment that maximizes designer productivity, and (3) a cost-effective, high-performance computing platform. A fundamental requirement of the concept is that the simulations must be capable of overnight execution on easily accessible computing platforms. This will greatly facilitate the use of large-scale simulations in a design environment. This paper describes the current status of the NPSS with specific emphasis on the progress made over the past year on air breathing propulsion applications. In addition, the paper contains a summary of the feedback received from industry partners in the development effort and the actions taken over the past year to respond to that feedback. The NPSS development was supported in FY99 by the High Performance Computing and Communications Program.

Lytle, John; Follen, Greg; Naiman, Cynthia; Evans, Austin

2000-01-01

368

A unifying framework for intelligent DNS management

The Domain Name System (DNS) is a special kind of distributed directory service for people to create and access the network information systems by (1) allowing local control of its segments and (2) making each segment’s data available on the Internet using a client–server scheme. However, few administrators have the expertise to do the jobs well since this distributed mechanism

Chang-sheng Chen; Shian-shyong Tseng; Chien-liang Liu

2003-01-01

369

Resolución de consultas anónimas sobre DNS

Resumen—La utilización de DNS como mecanismo base de nuevos servicios telemáticos basados en resolución de nombres puede suponer riesgos en seguridad y privacidad. La información intercambiada entre clientes y servidores viaja sin ningún tipo de protección. Dicha información puede ser capturada por malware o servidores de acceso deshonestos y acabar siendo vendida para su utilización en técnicas de spamming o

Joaquín García-Alfaro; Sergio Castillo-Pérez

370

NASA Astrophysics Data System (ADS)

Sediment transport in nature comprises of bed-load and suspended load, and precise modelling of suspended load transport is essential for accurate sediment flux estimation. Traditionally, non-cohesive suspended sediment has been modelled using the advection-diffusion equation (Garcia, 2008), where the success of the model is largely dependent on accurate approximation of the sediment diffusion coefficients. The current study explores the effect of self-stratification on sediment diffusivity using suspended sediment concentration data from Direct Numerical Simulations (DNS) of flows subjected to different levels of stratification, where the level of stratification is dependent on the particle size (parameterized using particle fall velocity V~*) and volume-averaged sediment concentration (parameterized using shear Richardson number Ri?). Two distinct configurations were explored, first the channel flow configuration (similar to flow in a pipe or a duct) and second, a boundary layer configuration (similar to open-channel flow). Self-stratification was found to modulate the turbulence intensity (Cantero et al., 2009), which in turn was found to reduce vertical sediment diffusivity in portions of the domain exposed to turbulence damping. Effect of particle size on vertical sediment diffusivity has been studied in the past by several authors (Rouse, 1937; Coleman, 1970; Nielsen and Teakle, 2004); so in addition to the effect of particle size, the current study also explores the effect of sediment concentration on vertical sediment diffusivity. The results from the DNS simulations were compared with experiments (Ismail, 1952; Coleman, 1986) and field measurements (Coleman, 1970); and were found to agree qualitatively especially for the case of channel flows. The aim of the study was to understand the effect of stratification due to suspended sediment on vertical sediment diffusivity for different flow configurations, in order to gain insight of the underlying physics, which will eventually help us to improve the existing models for sediment diffusivity.*

Dutta, S.; Cantero, M. I.; Garcia, M. H.

2013-11-01

371

Graphics interfaces and numerical simulations: Mexican Virtual Solar Observatory

NASA Astrophysics Data System (ADS)

Preliminary results associated to the computational development and creation of the Mexican Virtual Solar Observatory (MVSO) are presented. Basically, the MVSO prototype consists of two parts: the first, related to observations that have been made during the past ten years at the Solar Observation Station (EOS) and at the Carl Sagan Observatory (OCS) of the Universidad de Sonora in Mexico. The second part is associated to the creation and manipulation of a database produced by numerical simulations related to solar phenomena, we are using the MHD ZEUS-3D code. The development of this prototype was made using mysql, apache, java and VSO 1.2. based GNU and `open source philosophy'. A graphic user interface (GUI) was created in order to make web-based, remote numerical simulations. For this purpose, Mono was used, because it is provides the necessary software to develop and run .NET client and server applications on Linux. Although this project is still under development, we hope to have access, by means of this portal, to other virtual solar observatories and to be able to count on a database created through numerical simulations or, given the case, perform simulations associated to solar phenomena.

Hernández, L.; González, A.; Salas, G.; Santillán, A.

2007-08-01

372

Numerical simulation of the interaction between two bubbles

NASA Astrophysics Data System (ADS)

Different evolution patterns of two bubbles may be observed for different values of the phase difference and the inter-bubble distance. Based on potential flow theory, a boundary element method (BEM) is adopted to simulate the interaction between two bubbles and the toroidal bubble after the jet impact is also investigated by placing a vortex ring within the bubble. Meanwhile, some numerical techniques are used to deal with problems like coalescence and collapse. Typical phenomena like coalescence, jet towards and jet away are investigated numerically in this paper. The mechanisms underlying the various phenomena are given through the analysis of the velocity and pressure fields.

Han, R.; Yao, X. L.; Zhang, A. M.

2015-01-01

373

Expert System Architecture for Rocket Engine Numerical Simulators: A Vision

NASA Technical Reports Server (NTRS)

Simulation of any complex physical system like rocket engines involves modeling the behavior of their different components using mostly numerical equations. Typically a simulation package would contain a set of subroutines for these modeling purposes and some other ones for supporting jobs. A user would create an input file configuring a system (part or whole of a rocket engine to be simulated) in appropriate format understandable by the package and run it to create an executable module corresponding to the simulated system. This module would then be run on a given set of input parameters in another file. Simulation jobs are mostly done for performance measurements of a designed system, but could be utilized for failure analysis or a design job such as inverse problems. In order to use any such package the user needs to understand and learn a lot about the software architecture of the package, apart from being knowledgeable in the target domain. We are currently involved in a project in designing an intelligent executive module for the rocket engine simulation packages, which would free any user from this burden of acquiring knowledge on a particular software system. The extended abstract presented here will describe the vision, methodology and the problems encountered in the project. We are employing object-oriented technology in designing the executive module. The problem is connected to the areas like the reverse engineering of any simulation software, and the intelligent systems for simulation.

Mitra, D.; Babu, U.; Earla, A. K.; Hemminger, Joseph A.

1998-01-01

374

Theory and Numerical Simulations of Self-Gravitating Core Formation

NASA Astrophysics Data System (ADS)

In star-forming molecular clouds, dense cores grow and evolve due to a combination of supersonic turbulent compression and self-gravity, with the details of the dynamical processes mediated by magnetic stresses and ion-neutral drift. In classical theory, cores with sufficiently high gravitational energy compared to thermal and magnetic support undergo outside-in collapse to reach a state in which the density profile approaches a singular r^-2 power law. This collapse is evident in numerical simulations, for a wide range of initial and environmental conditions. Classical theory predicts a subsequent outside-in infall stage, which is also seen in simulations. I will discuss numerical hydrodynamic and magnetohydrodynamic simulations of core formation and evolution, concentrating on evolution up to the stage of singularity formation. Observations show that cores are found to lie within larger-scale filaments, and simulations indicate that these filaments grow at the same time as cores develop within them. Although magnetic fields are often been thought of as a significant barrier to star formation that must be surmounted via ambipolar diffusion, recent simulations show that the properties of cores formed in hydrodynamic, ideal MHD, and diffusive models are quite similar. I will discuss how this can be understood in terms of anisotropic core formation models.

Ostriker, Eve C.

2014-07-01

375

Characterizing Electron Temperature Gradient Turbulence Via Numerical Simulation

Numerical simulations of electron temperature gradient (ETG) turbulence are presented which characterize the ETG fluctuation spectrum, establish limits to the validity of the adiabatic ion model often employed in studying ETG turbulence, and support the tentative conclusion that plasmaoperating regimes exist in which ETG turbulence produces sufficient electron heat transport to be experimentally relevant. We resolve prior controversies regarding simulation techniques and convergence by benchmarking simulations of ETG turbulence from four microturbulence codes, demonstrating agreement on the electron heat flux, correlation functions, fluctuation intensity, and rms flow shear at fixed simulation cross section and resolution in the plane perpendicular to the magnetic field. Excellent convergence of both continuum and particle-in-cell codes with time step and velocity-space resolution is demonstrated, while numerical issues relating to perpendicular (to the magnetic field) simulation dimensions and resolution are discussed. A parameter scan in the magnetic shear, s, demonstrates that the adiabatic ion model is valid at small values of s (s < 0.4 for the parameters used in this scan) but breaks down at higher magnetic shear. A proper treatment employing gyrokinetic ions reveals a steady increase in the electron heat transport with increasing magnetic shear, reaching electron heat transport rates consistent with analyses of experimental tokamak discharges.

Nevins, W M; Candy, J; Cowley, S; Dannert, T; Dimits, A; Dorland, W; Estrada-Mila, C; Hammett, G W; Jenko, F; Pueschel, M J; Shumaker, D E

2006-05-22

376

Characterizing electron temperature gradient turbulence via numerical simulation

Numerical simulations of electron temperature gradient (ETG) turbulence are presented that characterize the ETG fluctuation spectrum, establish limits to the validity of the adiabatic ion model often employed in studying ETG turbulence, and support the tentative conclusion that plasma-operating regimes exist in which ETG turbulence produces sufficient electron heat transport to be experimentally relevant. We resolve prior controversies regarding simulation techniques and convergence by benchmarking simulations of ETG turbulence from four microturbulence codes, demonstrating agreement on the electron heat flux, correlation functions, fluctuation intensity, and rms flow shear at fixed simulation cross section and resolution in the plane perpendicular to the magnetic field. Excellent convergence of both continuum and particle-in-cell codes with time step and velocity-space resolution is demonstrated, while numerical issues relating to perpendicular (to the magnetic field) simulation dimensions and resolution are discussed. A parameter scan in the magnetic shear, s, demonstrates that the adiabatic ion model is valid at small values of s (s<0.4 for the parameters used in this scan) but breaks down at higher magnetic shear. A proper treatment employing gyrokinetic ions reveals a steady increase in the electron heat transport with increasing magnetic shear, reaching electron heat transport rates consistent with analyses of experimental tokamak discharges.

Nevins, W. M.; Candy, J.; Cowley, S.; Dannert, T.; Dimits, A.; Dorland, W.; Estrada-Mila, C.; Hammett, G. W.; Jenko, F.; Pueschel, M. J.; Shumaker, D. E. [Lawrence Livermore National Laboratory, Livermore, California 94551 (United States); General Atomics, San Diego, California 92186 (United States); Department of Physics and Astronomy, UCLA, Los Angeles, California 90095-1547 (United States); Centre de Recherches en Physique des Plasmas (CRPP), Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne (Switzerland); Lawrence Livermore National Laboratory, Livermore, California 94551 (United States); University of Maryland, College Park, Maryland 20742 (United States); Department of Mechanical and Aerospace Engineering, UCSD, San Diego, California 92093 (United States); Princeton Plasma Physics Laboratory, Princeton, New Jersey 08536 (United States); Max-Planck Institut fuer Plasmaphysik, D-85748 Garching (Germany); Lawrence Livermore National Laboratory, Livermore, California 94551 (United States)

2006-12-15

377

Numerical simulation of acoustic emission using acoustic contact elements

NASA Astrophysics Data System (ADS)

A new method is described for nonconformal finite-element simulation of the region of interaction between an acoustic fluid and deformed solid bodies. The method is based on the use of special contact finite elements simulating the interpolation coupling between the acoustic pressure in the fluid and displacements of a two-phase surface. The test results of the method demonstrate a significant acceleration of grid convergence of computations. The numerical method is verified by the problem of predicting noise emission by a thin-wall inhomogeneous shell.

Suvorov, A. S.; Sokov, E. M.; Artel'nyi, P. V.

2014-11-01

378

Numerical Simulation of Impact Effects on Multilayer Fabrics

NASA Astrophysics Data System (ADS)

High strength fabrics provide lightweight impact protection and are employed in a wide range of applications. Examples include body armor for law enforcement and military personnel and orbital debris shielding for the International Space Station. Numerical simulation of impact effects on fabric protection systems is difficult, due to the complex woven structure of the fabric layers and the typical application of fabrics in a multilayer configuration. Recent research has applied a new particle-element method to the simulation of impact effects on multilayer fabrics, applicable over a wide range of impact velocities, for use in body armor and orbital debris shielding design applications.

Fahrenthold, Eric; Rabb, Robert; Bohannan, April

2007-12-01

379

New prescriptions of turbulent transport from local numerical simulations

NASA Astrophysics Data System (ADS)

Massive stars often experience fast rotation, which is known to induce turbulent mixing with a strong impact on the evolution of these stars. Local direct numerical simulations of turbulent transport in stellar radiative zones are a promising way to constrain phenomenological transport models currently used in many stellar evolution codes. We present here the results of such simulations of stably-stratified sheared turbulence taking notably into account the effects of thermal diffusion and chemical stratification. We also discuss the impact of theses results on stellar evolution theory.

Prat, V.; Lignières, F.; Lesur, G.

2015-01-01

380

Numerical Simulation of Cavitation Flow in Tandem Cascade

\\u000a Complex cavitation phenomenon in tandem cascade is investigated numerically. Three-dimensional multi-block structural mesh,\\u000a Finite volume method(FVM), Shear stress transport(SST) turbulence model are employed. Single phase interface tracking model\\u000a is used as the cavitation model. Flow field without cavitation involved is firstly computed as the initial field for further\\u000a cavitation flow simulation. Some results are obtained through the simulation:(1) the position

Can Kang

381

Numerical simulations of a diode laser BPH treatment system

Numerical simulations are presented of the laser-tissue interaction of a diode laser system for treating benign prostate hyperplasia. The numerical model includes laser light transport, heat transport, cooling due to blood perfusion, thermal tissue damage, and enthalpy of tissue damage. Comparisons of the simulation results to clinical data are given. We report that a reasonable variation from a standard set of input data produces heating times which match those measured in the clinical trials. A general trend of decreasing damage volume with increasing heating time is described. We suggest that the patient-to- patient variability seen in the data can be explained by differences in fundamental biophysical properties such as the optical coefficients. Further work is identified, including the measurement and input to the model of several specific data parameters such as optical coefficients, blood perfusion cooling rate, and coagulation rates.

Esch, V; London, R A; Papademetriou, S

1999-02-23

382

Numerical aerodynamic simulation program long haul communications prototype

NASA Technical Reports Server (NTRS)

This document is a report of the Numerical Aerodynamic Simulation (NAS) Long Haul Communications Prototype (LHCP). It describes the accomplishments of the LHCP group, presents the results from all LHCP experiments and testing activities, makes recommendations for present and future LHCP activities, and evaluates the remote workstation accesses from Langley Research Center, Lewis Research Center, and Colorado State University to Ames Research Center. The report is the final effort of the Long Haul (Wideband) Communications Prototype Plan (PT-1133-02-N00), 3 October 1985, which defined the requirements for the development, test, and operation of the LHCP network and was the plan used to evaluate the remote user bandwidth requirements for the Numerical Aerodynamic Simulation Processing System Network.

Cmaylo, Bohden K.; Foo, Lee

1987-01-01

383

Numerical Relativity Simulations for Black Hole Merger Astrophysics

NASA Technical Reports Server (NTRS)

Massive black hole mergers are perhaps the most energetic astronomical events, establishing their importance as gravitational wave sources for LISA, and also possibly leading to observable influences on their local environments. Advances in numerical relativity over the last five years have fueled the development of a rich physical understanding of general relativity's predictions for these events. Z will overview the understanding of these event emerging from numerical simulation studies. These simulations elucidate the pre-merger dynamics of the black hole binaries, the consequent gravitational waveform signatures ' and the resulting state, including its kick velocity, for the final black hole produced by the merger. Scenarios are now being considered for observing each of these aspects of the merger, involving both gravitational-wave and electromagnetic astronomy.

Baker, John G.

2010-01-01

384

Numerical model for learning concepts of streamflow simulation

Numerical models are useful for demonstrating principles of open-channel flow. Such models can allow experimentation with cause-and-effect relations, testing concepts of physics and numerical techniques. Four PT is a numerical model written primarily as a teaching supplement for a course in one-dimensional stream-flow modeling. Four PT options particularly useful in training include selection of governing equations, boundary-value perturbation, and user-programmable constraint equations. The model can simulate non-trivial concepts such as flow in complex interconnected channel networks, meandering channels with variable effective flow lengths, hydraulic structures defined by unique three-parameter relations, and density-driven flow.The model is coded in FORTRAN 77, and data encapsulation is used extensively to simplify maintenance and modification and to enhance the use of Four PT modules by other programs and programmers.

DeLong, L.L.

1993-01-01

385

MHD turbulent dynamo in astrophysics: Theory and numerical simulation

NASA Astrophysics Data System (ADS)

This thesis treats the physics of dynamo effects through theoretical modeling of magnetohydrodynamic (MHD) systems and direct numerical simulations of MHD turbulence. After a brief introduction to astrophysical dynamo research in Chapter 1, the following issues in developing dynamic models of dynamo theory are addressed: In Chapter 2, nonlinearity that arises from the back reaction of magnetic field on velocity field is considered in a new model for the dynamo ?-effect. The dependence of ?-coefficient on magnetic Reynolds number, kinetic Reynolds number, magnetic Prandtl number and statistical properties of MHD turbulence is studied. In Chapter 3, the time-dependence of magnetic helicity dynamics and its influence on dynamo effects are studied with a theoretical model and 3D direct numerical simulations. The applicability of and the connection between different dynamo models are also discussed. In Chapter 4, processes of magnetic field amplification by turbulence are numerically simulated with a 3D Fourier spectral method. The initial seed magnetic field can be a large-scale field, a small-scale magnetic impulse, and a combination of these two. Other issues, such as dynamo processes due to helical Alfvénic waves and the implication and validity of the Zeldovich relation, are also addressed in Appendix B and Chapters 4 & 5, respectively. Main conclusions and future work are presented in Chapter 5. Applications of these studies are intended for astrophysical magnetic field generation through turbulent dynamo processes, especially when nonlinearity plays central role. In studying the physics of MHD turbulent dynamo processes, the following tools are developed: (1)A double Fourier transform in both space and time for the linearized MHD equations (Chapter 2 and Appendices A & B). (2)A Fourier spectral numerical method for direct simulation of 3D incompressible MHD equations (Appendix C).

Chou, Hongsong

2001-10-01

386

Direct numerical simulations of three-dimensional bubbly flows

NASA Astrophysics Data System (ADS)

Direct numerical simulations of the motion of many buoyant bubbles are presented. The Navier-Stokes equation is solved by a front tracking/finite difference method that allows a fully deformable interface. The evolution of 91 nearly spherical bubbles at a void fraction of 6% is followed as the bubbles rise over 100 bubble diameters. While the individual bubble velocities fluctuate, the average motion reaches a statistical steady state with a rise Reynolds number of about 25.

Bunner, Bernard; Tryggvason, Grétar

1999-08-01

387

NUMERICAL SIMULATION OF UNCONFINED TURBULENT LIFTED JET FLAMES

The goal of the present work is to report the numer ical simulation of a turbulent lifted methane-air jet flame using a partial premix ed combustion model. This model is based on the transport of a non-reacting variable G, which represents the normal distance to the flame front. The G-equation is derived from the local kinematics rela tion between the

Amândio J. Rebola; Pedro J. Coelho

388

Extended Scaling Laws in Numerical Simulations of Magnetohydrodynamic Turbulence

NASA Astrophysics Data System (ADS)

Magnetized turbulence is ubiquitous in astrophysical systems, where it notoriously spans a broad range of spatial scales. Phenomenological theories of MHD turbulence describe the self-similar dynamics of turbulent fluctuations in the inertial range of scales. Numerical simulations serve to guide and test these theories. However, the computational power that is currently available restricts the simulations to Reynolds numbers that are significantly smaller than those in astrophysical settings. In order to increase computational efficiency and, therefore, probe a larger range of scales, one often takes into account the fundamental anisotropy of field-guided MHD turbulence, with gradients being much slower in the field-parallel direction. The simulations are then optimized by employing the reduced MHD equations and relaxing the field-parallel numerical resolution. In this work we explore a different possibility. We propose that there exist certain quantities that are remarkably stable with respect to the Reynolds number. As an illustration, we study the alignment angle between the magnetic and velocity fluctuations in MHD turbulence, measured as the ratio of two specially constructed structure functions. We find that the scaling of this ratio can be extended surprisingly well into the regime of relatively low Reynolds number. However, the extended scaling easily becomes spoiled when the dissipation range in the simulations is underresolved. Thus, taking the numerical optimization methods too far can lead to spurious numerical effects and erroneous representation of the physics of MHD turbulence, which in turn can affect our ability to identify correctly the physical mechanisms that are operating in astrophysical systems.

Mason, Joanne; Perez, Jean Carlos; Cattaneo, Fausto; Boldyrev, Stanislav

2011-07-01

389

Numerical simulation of thermal behavior during laser direct metal deposition

To discuss the influence of thin-walled blade’s curvature change and accumulating layer number on the temperature field distribution\\u000a in laser direct metal deposition and obtain a uniform thickness of a thin-walled blade, the temperature field distribution\\u000a was calculated by numerical simulation. The thin-walled blade’s curvature change and accumulating layer number can be studied,\\u000a respectively. The effect of accumulating layer number

Gangxian Zhu; Anfeng Zhang; Dichen Li; Yiping Tang; Zhiqiang Tong; Qiaopan Lu

2011-01-01

390

Lake Dynamics: Observation and High-Resolution Numerical Simulation

We have demonstrated the rich set of hydrodynamic phenomena that operate in a natural lake. State-of-the-art experimental\\u000a methods yeild a wealth of data that are highly resolved in time and in one space axis. The orthogonal space is sampled only\\u000a very sparsely, however. Still, such data allow the estimation of crucial effective parameters like vertical diffusion coefficients.\\u000a Numerical simulations, on

C. Rohden; A. Hauser; K. Wunderle; J. Ilmberger; G. Wittum; K. Roth

391

Numerical simulation of ion-beam formation (invited)

The methods of numerical simulation of particle optics problems are examined with respect to their accuracy and stability. Principles of space-charge optics and of beam formation are recalled to address questions like the general validity of the 3\\/2-power law and the Pierce angle. For the simplest mathematical model of the plasma sheath the ‘‘correct’’ shape of the wall electrode is

Reinard Becker

1996-01-01

392

Experimental data base for numerical simulations of turbulent diffusion flames

The objective of the presented measurements is to provide an experimental data base for comparison with numerical simulation\\u000a results of turbulent H2-air diffusion flames. Additionally, the date base may also be used for a proof of new measurement techniques, when the same\\u000a flame conditions are applied. The data base contains time and spatial resolved data on all three velocity components,

Tsung-Chieh Cheng; Gerhard Fruechtel; Andreas Neuber; Friedrich Lipp; Egon P. Hassel; Johannes Janicka

1995-01-01

393

Direct numerical simulation of passive scalar transport in transverse jets

Direct numerical simulation is used to study passive scalar transport and mixing in a round turbulent jet, in a laminar crossflow. The ratio of the jet velocity to that of the crossflow is 5.7, the Schmidt number of the scalar is 1.49, and the jet-exit Reynolds number is 5000. The scalar field is used to compute entrainment of the crossflow

Suman Muppidi; Krishnan Mahesh

2008-01-01

394

Numerical simulation of the extrusion of strongly compressible Newtonian liquids

The axisymmetric and plane extrusion flows of a liquid foam are simulated assuming that the foam is a homogeneous compressible\\u000a Newtonian fluid that slips along the walls. Compressibility effects are investigated using both a linear and an exponential\\u000a equation of state. The numerical results confirm previous reports that the swelling of the extrudate decreases initially as\\u000a the compressibility of the

Eleni Taliadorou; Georgios C. Georgiou; Evan Mitsoulis

2008-01-01

395

Numerical Simulations of the Metallicity Distribution in Dwarf Spheroidal Galaxies

Recent observations show that the number of stars with very low metallicities in the dwarf spheroidal satellites of the Milky Way is low, despite the low average metallicities of stars in these systems. We undertake numerical simulations of star formation and metal enrichment of dwarf galaxies in order to verify whether this result can be reproduced with ''standard'' assumptions. The answer is likely to be negative, unless some selection bias against very low metallicity stars is present in the observations.

Ripamonti, Emanuele; Tolstoy, E.; Helmi, A.; Battaglia, G.; /Kapteyn Astron. Inst., Groningen; Abel, T.; /KIPAC, Menlo Park

2006-12-12

396

Numerical simulation of unsteady turbulent flow through a Francis turbine

This paper introduces the 3D numerical simulation of unsteady turbulent flow in the entire flow passage of a Francis turbine\\u000a with computational fluid dynamics (CFD) technology. The boundary conditions have been implemented based on the 3D averaged\\u000a Navier-Stokes equations. The governing equations are discreted on space by the finite volume method and on time step by the\\u000a finite difference method.

Ying Hu; Heming Cheng; Ji Hu; Xirong Li

2011-01-01

397

A numerical simulation of the formation of the terrestrial planets

A numerical simulation of the accumulation of the terrestrial planets was performed, starting with 200 lunar-size planetesimals distributed uniformly in a plane; these were placed in circular orbits around the sun between 0.5 and 1.5 AU, with the aim that they would form Venus and earth by inelastic collisions. The rule was that when two bodies physically collided, they coalesced

M. Lecar; S. J. Aarseth

1986-01-01

398

Fluid dynamic numerical simulation of a gas phase polymerization reactor

SUMMARY This article presents preliminaryuid dynamic simulation results of ethylene polymerization dense ?uidized bed using the two-phaseow numerical code ESTET-ASTRID developed by Electricited e France for CFB boilers and based on the two-?uid modelling approach. The continuous phase consists of gas and the dispersed phase consists of catalyst particles. The particleuctuating motion is modelled using two-separate transport equations, on the

Anne Gobin; Hervé Neau; Olivier Simonin; Jean-Richard Llinas; Vince Reiling; Jean-Lofc Sélo

2003-01-01

399

Numerical Simulation of Microbiological Growth in the Capillary Fringe

NASA Astrophysics Data System (ADS)

The capillary fringe (CF) is a highly dynamic zone in a porous media at the interface between water-saturated aquifer and vadose zone, where steep biogeochemical gradients and thus high bioactivities are expected. In recent years, considerable effort has been undertaken to deepen the understanding of the physical (flow, diffusion, dispersion), geochemical (dissolution, precipitation) and biological (metabolism, excretion, biofilm formation) processes in the CF. We developed a numerical simulator for multiphase multicomponent flow in porous media which is able to consider simultaneously multiphase flow, component transport, phase exchange, geochemical reactions and microbiological processes. A splitting approach for phase transport, component transport and reaction/phase exchanges allows the usage of higher-order discretizations for the component transport. This reduces numerical dispersion significantly, which is especially important in the simulation of reactive flow. In a flow-through laboratory experiment performed at the Karlsruhe Institute of Technology, Germany, within the project "Dynamic Capillary Fringes - A Multidisciplinary Approach", the oxygen phase transfer, the growth and the transport of a bacteria (green fluorescent Escherichia coli) were investigated. The results of numerical simulations of the E. coli growth in the CF with a high nutrient supply under steady-state and transient flow conditions are compared to the experimental data.

Hron, P.; Jost, D.; Engwer, C.; Ippisch, O.; Bastian, P.

2012-04-01

400

Numerical Homogenization of Jointed Rock Masses Using Wave Propagation Simulation

NASA Astrophysics Data System (ADS)

Homogenization in fractured rock analyses is essentially based on the calculation of equivalent elastic parameters. In this paper, a new numerical homogenization method that was programmed by means of a MATLAB code, called HLA-Dissim, is presented. The developed approach simulates a discontinuity network of real rock masses based on the International Society of Rock Mechanics (ISRM) scanline field mapping methodology. Then, it evaluates a series of classic joint parameters to characterize density (RQD, specific length of discontinuities). A pulse wave, characterized by its amplitude, central frequency, and duration, is propagated from a source point to a receiver point of the simulated jointed rock mass using a complex recursive method for evaluating the transmission and reflection coefficient for each simulated discontinuity. The seismic parameters, such as delay, velocity, and attenuation, are then calculated. Finally, the equivalent medium model parameters of the rock mass are computed numerically while taking into account the natural discontinuity distribution. This methodology was applied to 17 bench fronts from six aggregate quarries located in Tunisia, Spain, Austria, and Sweden. It allowed characterizing the rock mass discontinuity network, the resulting seismic performance, and the equivalent medium stiffness. The relationship between the equivalent Young's modulus and rock discontinuity parameters was also analyzed. For these different bench fronts, the proposed numerical approach was also compared to several empirical formulas, based on RQD and fracture density values, published in previous research studies, showing its usefulness and efficiency in estimating rapidly the Young's modulus of equivalent medium for wave propagation analysis.

Gasmi, Hatem; Hamdi, Essaïeb; Bouden Romdhane, Nejla

2014-07-01

401

Transient productivity index for numerical well test simulations

The most difficult aspect of numerical simulation of well tests is the treatment of the Bottom Hole Flowing (BHF) Pressure. In full field simulations, this pressure is derived from the Well-block Pressure (WBP) using a numerical productivity index which accounts for the grid size and permeability, and for the well completion. This productivity index is calculated assuming a pseudo-steady state flow regime in the vicinity of the well and is therefore constant during the well production period. Such a pseudo-steady state assumption is no longer valid for the early time of a well test simulation as long as the pressure perturbation has not reached several grid-blocks around the well. This paper offers two different solutions to this problem: (1) The first one is based on the derivation of a Numerical Transient Productivity Index (NTPI) to be applied to Cartesian grids; (2) The second one is based on the use of a Corrected Transmissibility and Accumulation Term (CTAT) in the flow equation. The representation of the pressure behavior given by both solutions is far more accurate than the conventional one as shown by several validation examples which are presented in the following pages.

Blanc, G.; Ding, D.Y.; Ene, A. [Institut Francais du Petrole, Pau (France)] [and others

1997-08-01

402

DNS benchmark solution of the fully developed turbulent channel flow with heat transfer

NASA Astrophysics Data System (ADS)

In the present paper direct numerical simulation (DNS) of the fully developed turbulent non-isothermal flow has been study for Re?=150 and for Pr=1.0. The focus is on the role of the thermal boundary condition type on the results. Various types of thermal boundary conditions presented in literature has been considered in this work: isoflux wall boundary conditions, symmetrical isofluxes wall boundary conditions and isothermal b.c. also with combination with adiabatic or isothermal second wall. Turbulence statistics for the fluid flow and thermal field as well turbulence structures are presented and compared. Numerical analysis assuming both zero and non-zero temperature fluctuations at the wall and zero and non-zero temperature gradient in the channel centre shows that thermal structures may differ depend on case and region. Results shows that the type of thermal boundary conditions significantly influence temperature fluctuations while the mean temperature is not affected. Difference in temperature fluctuation generate the difference in turbulent heat fluxes. Presented results are prepared in the form of the benchmark solution data and will be available in the digital form on the website http://home.agh.edu.pl/jaszczur.

Jaszczur, M.

2014-08-01

403

Towards an Automated Full-Turbofan Engine Numerical Simulation

NASA Technical Reports Server (NTRS)

The objective of this study was to demonstrate the high-fidelity numerical simulation of a modern high-bypass turbofan engine. The simulation utilizes the Numerical Propulsion System Simulation (NPSS) thermodynamic cycle modeling system coupled to a high-fidelity full-engine model represented by a set of coupled three-dimensional computational fluid dynamic (CFD) component models. Boundary conditions from the balanced, steady-state cycle model are used to define component boundary conditions in the full-engine model. Operating characteristics of the three-dimensional component models are integrated into the cycle model via partial performance maps generated automatically from the CFD flow solutions using one-dimensional meanline turbomachinery programs. This paper reports on the progress made towards the full-engine simulation of the GE90-94B engine, highlighting the generation of the high-pressure compressor partial performance map. The ongoing work will provide a system to evaluate the steady and unsteady aerodynamic and mechanical interactions between engine components at design and off-design operating conditions.

Reed, John A.; Turner, Mark G.; Norris, Andrew; Veres, Joseph P.

2003-01-01

404

Numerical Simulation of Delamination Growth in Composite Materials

NASA Technical Reports Server (NTRS)

The use of decohesion elements for the simulation of delamination in composite materials is reviewed. The test methods available to measure the interfacial fracture toughness used in the formulation of decohesion elements are described initially. After a brief presentation of the virtual crack closure technique, the technique most widely used to simulate delamination growth, the formulation of interfacial decohesion elements is described. Problems related with decohesion element constitutive equations, mixed-mode crack growth, element numerical integration and solution procedures are discussed. Based on these investigations, it is concluded that the use of interfacial decohesion elements is a promising technique that avoids the need for a pre-existing crack and pre-defined crack paths, and that these elements can be used to simulate both delamination onset and growth.

Camanho, P. P.; Davila, C. G.; Ambur, D. R.

2001-01-01

405

Common envelope numerical simulations: What are we missing?

NASA Astrophysics Data System (ADS)

The common envelope (CE) interaction is a very fast astrophysical process (it lasts about a year) whereby a close binary star temporarily becomes one large star. What happens next is anybody's guess. The two stars may merge, explode or transform into a compact binary. The main way to study this phenomenon is through numerical simulations, but nowadays the works that have been performed in this context fail to explain various important questions about the CE. By carrying out three-dimensional (3D) simulations of the CE interaction using a 3D hydrodynamic plus gravity code, we are trying to push the simulations a step further. We are adding additional ingredients that could be relevant to solve those important questions. In particular, we are analysing the effects of the interaction preceding the CE on the CE itself and of the stellar rotation.

Iaconi, Roberto

2014-09-01

406

NASA Astrophysics Data System (ADS)

Results at Ekman Reynolds numbers Re ranging from 1000 to 2828 expand the direct numerical simulation (DNS) contribution to the theory of wall-bounded turbulence. An established spectral method is used, with rules for domain size and grid resolution at each Reynolds number derived from the theory. The Re increase is made possible by better computers and by optimizing the grid in relation to the wall shear-stress direction. The boundary-layer thickness in wall units ?+ varies here by a factor of about 5.3, and reaches values near 5000, or 22 times the minimum at which turbulence has been sustained. An equivalent channel Reynolds number, based on the pressure gradient in wall units, would reach about Re?=1250. The principal goal of the analysis, the impartial identification of a log law, is summarized in the local "Karman measure" d(ln z+)/dU+. The outcome differs from that for Hoyas and Jiménez [Phys. Fluids 18, 011702 (2006)] and for Hu et al. [AIAA J. 44, 1541 (2006)] in channel-flow DNS at similar Reynolds numbers, for reasons unknown: Here, the law of the wall is gradually established up to a z+ around 400, with little statistical scatter. To leading order, it is consistent with the experiments of Österlund et al. [Phys. Fluids 12, 1 (2000)] in boundary layers. With the traditional expression, a logarithmic law is not present, in that the Karman measure drifts from about 0.41 at z+?70 to the 0.37-0.38 range for z+?500, with Re=2828. However, if a virtual origin is introduced with a shift of a+=7.5 wall units, the data support a long logarithmic layer with ? =0.38 a good fit to d(ln[z++a+])/dU+. A determination of the Karman constant from the variation of the skin-friction coefficients with Reynolds numbers also yields values near 0.38. The uncertainty is about ±0.01. These values are close to the boundary-layer experiments, but well below the accepted range of [0.40,0.41] and the experimental pipe-flow results near 0.42. The virtual-origin concept is also controversial, although nonessential at transportation or atmospheric Reynolds numbers. Yet, this series may reflect some success in verifying the law of the wall and investigating the logarithmic law by DNS, redundantly and with tools more impartial than the visual fit of a straight line to a velocity profile.

Spalart, Philippe R.; Coleman, Gary N.; Johnstone, Roderick

2008-10-01

407

Studying Turbulence Using Numerical Simulation Databases. No. 7; Proceedings of the Summer Program

NASA Technical Reports Server (NTRS)

The Seventh Summer Program of the Center for Turbulence Research took place in the four-week period, July 5 to July 31, 1998. This was the largest CTR Summer Program to date, involving thirty-six participants from the U. S. and nine other countries. Thirty-one Stanford and NASA-Ames staff members facilitated and contributed to most of the Summer projects. A new feature, and perhaps a preview of the future programs, was that many of the projects were executed on non-NASA computers. These included supercomputers located in Europe as well as those operated by the Departments of Defense and Energy in the United States. In addition, several simulation programs developed by the visiting participants at their home institutions were used. Another new feature was the prevalence of lap-top personal computers which were used by several participants to carry out some of the work that in the past were performed on desk-top workstations. We expect these trends to continue as computing power is enhanced and as more researchers (many of whom CTR alumni) use numerical simulations to study turbulent flows. CTR's main role continues to be in providing a forum for the study of turbulence for engineering analysis and in facilitating intellectual exchange among the leading researchers in the field. Once again the combustion group was the largest. Turbulent combustion has enjoyed remarkable progress in using simulations to address increasingly complex and practically more relevant questions. The combustion group's studies included such challenging topics as fuel evaporation, soot chemistry, and thermonuclear reactions. The latter study was one of three projects related to the Department of Energy's ASCI Program (www.llnl.gov/asci); the other two (rocket propulsion and fire safety) were carried out in the turbulence modeling group. The flow control and acoustics group demonstrated a successful application of the so-called evolution algorithms which actually led to a previously unknown forcing strategy for jets yielding increased spreading rate. A very efficient algorithm for flow in complex geometries with moving boundaries based on the immersed boundary forcing technique was tested with very encouraging results. Also a new strategy for the destruction of aircraft trailing vortices was introduced and tested. The Reynolds Averaged Modeling (RANS) group demonstrated that the elliptic relaxation concept for RANS calculations is also applicable to transonic flows with shocks; however, prediction of laminar/turbulent transition remains an important pacing item. A large fraction of the LES effort was devoted to the development and testing of a new algorithmic procedure (as opposed to phenomenological model) for subgrid scale modeling based on regularized de-filtering of the flow variables. This appears to be a very promising approach, and a significant effort is currently underway to assess its robustness in high Reynolds number flows and in conjunction with numerical methods for complex flows. As part of the Summer Program two review tutorials were given on Turbulent structures in hydrocarbon pool fires (Sheldon Tieszen), and Turbulent combustion modeling: from RANS to LES via DNS (Luc Vervisch); and two seminars entitled Assessment of turbulence models for engineering applications (Paul Durbin) and Subgrid-scale modeling for non-premixed, turbulent reacting flows (James Riley) were presented. A number of colleagues from universities, government agencies, and industry attended the final presentations of the participants on July 31 and participated in the discussions. There are twenty-six papers in this volume grouped in five areas. Each group is preceded with an overview by its coordinator.

1998-01-01

408

Stochastic algorithms for the analysis of numerical flame simulations

Recent progress in simulation methodologies and high-performance parallel computers have made it is possible to perform detailed simulations of multidimensional reacting flow phenomena using comprehensive kinetics mechanisms. As simulations become larger and more complex, it becomes increasingly difficult to extract useful information from the numerical solution, particularly regarding the interactions of the chemical reaction and diffusion processes. In this paper we present a new diagnostic tool for analysis of numerical simulations of reacting flow. Our approach is based on recasting an Eulerian flow solution in a Lagrangian frame. Unlike a conventional Lagrangian view point that follows the evolution of a volume of the fluid, we instead follow specific chemical elements, e.g., carbon, nitrogen, etc., as they move through the system . From this perspective an ''atom'' is part of some molecule of a species that is transported through the domain by advection and diffusion. Reactions cause the atom to shift from one chemical host species to another and the subsequent transport of the atom is given by the movement of the new species. We represent these processes using a stochastic particle formulation that treats advection deterministically and models diffusion and chemistry as stochastic processes. In this paper, we discuss the numerical issues in detail and demonstrate that an ensemble of stochastic trajectories can accurately capture key features of the continuum solution. The capabilities of this diagnostic are then demonstrated by applications to study the modulation of carbon chemistry during a vortex-flame interaction, and the role of cyano chemistry in rm NO{sub x} production for a steady diffusion flame.

Bell, John B.; Day, Marcus S.; Grcar, Joseph F.; Lijewski, Michael J.

2004-04-26

409

Three-dimensional numerical simulation of cell deformation

NASA Astrophysics Data System (ADS)

Blood is a multiphase suspension of various deformable cells. The particulate nature of blood is absent in large blood vessels making a numerical/theoretical analysis somewhat easier. The analysis is also simplified for the flow through small capillaries, where blood cells flow in an ordered, `single-file' fashion. The main difficulty arises for the vessels of ˜10--500 micron diameter, where the cells move in a 'multi-file' fashion. The Casson fluid model, used to describe blood flow in such vessels, often fails to elucidate many microrheological events. In order to perform accurate and detailed numerical simulations of blood flow at microscales, we are developing 3D simulation techniques for multiple deformable cells using immersed boundary method. In this method, the cells are modeled as capsules, that is, liquid drops surrounded by elastic membranes. The model allows us to include various constitutive laws for the cell membrane, as well as the rheological properties of the liquid inside the cell. It also allows inclusion of the cell nucleus, as in case of a white blood cell or a neonatal red blood cell. In this talk we will describe the numerical techniques, and then explore the deformation dynamics of a nucleated/non-nucleated cell in a shear flow.

Doddi, Sai

2005-11-01

410

Water and heat fluxes in desert soils: 2. Numerical simulations

NASA Astrophysics Data System (ADS)

Transient one-dimensional fluxes of soil water (liquid and vapor) and heat in response to 1 year of atmospheric forcing were simulated numerically for a site in the Chihuahuan Desert of Texas. The model was initialized and evaluated using the monitoring data presented in a companion paper (Scanlon, this issue). Soil hydraulic and thermal properties were estimated a priori from a combination of laboratory measurements, models, and other published information. In the first simulation, the main drying curves were used to describe soil water retention, and hysteresis was ignored. Remarkable consistency was found between computed and measured water potentials and temperatures. Attenuation and phase shift of the seasonal cycle of water potentials below the shallow subsurface active zone (0.0- to 0.3-m depth) were similar to those of temperatures, suggesting that water potential fluctuations were driven primarily by temperature changes. Water fluxes in the upper 0.3 m of soil were dominated by downward and upward liquid fluxes that resulted from infiltration of rain and subsequent evaporation from the surface. Upward flux was vapor dominated only in the top several millimeters of the soil during periods of evaporation. Below a depth of 0.3 m, water fluxes varied slowly and were dominated by downward thermal vapor flux that decreased with depth, causing a net accumulation of water. In a second simulation, nonhysteretic water retention was instead described by the estimated main wetting curves; the resulting differences in fluxes were attributed to lower initial water contents (given fixed initial water potential) and unsaturated hydraulic conductivities that were lower than they were in the first simulation. Below a depth of 0.3 m, the thermal vapor fluxes dominated and were similar to those in the first simulation. Two other simulations were performed, differing from the first only in the prescription of different (wetter) initial water potentials. These three simulations yielded identical solutions in the upper 0.2 m of soil after infiltration of summer rain; however, the various initial water potentials were preserved throughout the year at depths greater than 0.2 m. Comparison of all four simulations showed that the predominantly upward liquid fluxes below a depth of 0.2 m were very sensitive to the differences in water retention functions and initial water potentials among simulations, because these factors strongly affected hydraulic conductivities. Comparison of numerical modeling results with chemical tracer data showed that values of downward vapor flux below the surface evaporation zone were of the same order of magnitude as those previously estimated by analysis of depth distributions of bomb 3H (volatile) and bomb 36Cl (nonvolatile).

Scanlon, Bridget R.; Milly, P. C. D.

1994-03-01

411

An Object Model for a Rocket Engine Numerical Simulator

NASA Technical Reports Server (NTRS)

Rocket Engine Numerical Simulator (RENS) is a packet of software which numerically simulates the behavior of a rocket engine. Different parameters of the components of an engine is the input to these programs. Depending on these given parameters the programs output the behaviors of those components. These behavioral values are then used to guide the design of or to diagnose a model of a rocket engine "built" by a composition of these programs simulating different components of the engine system. In order to use this software package effectively one needs to have a flexible model of a rocket engine. These programs simulating different components then should be plugged into this modular representation. Our project is to develop an object based model of such an engine system. We are following an iterative and incremental approach in developing the model, as is the standard practice in the area of object oriented design and analysis of softwares. This process involves three stages: object modeling to represent the components and sub-components of a rocket engine, dynamic modeling to capture the temporal and behavioral aspects of the system, and functional modeling to represent the transformational aspects. This article reports on the first phase of our activity under a grant (RENS) from the NASA Lewis Research center. We have utilized Rambaugh's object modeling technique and the tool UML for this purpose. The classes of a rocket engine propulsion system are developed and some of them are presented in this report. The next step, developing a dynamic model for RENS, is also touched upon here. In this paper we will also discuss the advantages of using object-based modeling for developing this type of an integrated simulator over other tools like an expert systems shell or a procedural language, e.g., FORTRAN. Attempts have been made in the past to use such techniques.

Mitra, D.; Bhalla, P. N.; Pratap, V.; Reddy, P.

1998-01-01

412

Using Numerical Modeling to Simulate Space Capsule Ground Landings

NASA Technical Reports Server (NTRS)

Experimental work is being conducted at the National Aeronautics and Space Administration s (NASA) Langley Research Center (LaRC) to investigate ground landing capabilities of the Orion crew exploration vehicle (CEV). The Orion capsule is NASA s replacement for the Space Shuttle. The Orion capsule will service the International Space Station and be used for future space missions to the Moon and to Mars. To evaluate the feasibility of Orion ground landings, a series of capsule impact tests are being performed at the NASA Langley Landing and Impact Research Facility (LandIR). The experimental results derived at LandIR provide means to validate and calibrate nonlinear dynamic finite element models, which are also being developed during this study. Because of the high cost and time involvement intrinsic to full-scale testing, numerical simulations are favored over experimental work. Subsequent to a numerical model validated by actual test responses, impact simulations will be conducted to study multiple impact scenarios not practical to test. Twenty-one swing tests using the LandIR gantry were conducted during the June 07 through October 07 time period to evaluate the Orion s impact response. Results for two capsule initial pitch angles, 0deg and -15deg , along with their computer simulations using LS-DYNA are presented in this article. A soil-vehicle friction coefficient of 0.45 was determined by comparing the test stopping distance with computer simulations. In addition, soil modeling accuracy is presented by comparing vertical penetrometer impact tests with computer simulations for the soil model used during the swing tests.

Heymsfield, Ernie; Fasanella, Edwin L.

2009-01-01

413

Numerical simulation of three-dimensional bubbly flows.

NASA Astrophysics Data System (ADS)

Multiphase flow researchers have long recognized the need for direct numerical simulations of the motion of bubbles in order to complement experimental studies, provide insight into the small-scale, short-time phenomena occuring in these flows, and validate engineering models. A finite-difference/front-tracking method for the representation of deformable fluid interfaces is parallelized and large three-dimensional direct numerical simulations of buoyant bubbly flows are conducted. Results are presented for simulations of up to 864 bubbles in a periodic domain at a Reynolds number of about 30 and a volume fraction of 6%. The flow structure is studied to understand the mechanisms that drive the interactions between the bubbles. Nearly spherical bubbles show a weak trend to align themselves horizontally with repect to each other whereas more deformable bubbles move in a tandem configuration. The effects of system size, volume fraction, and interface deformability on the flow structure and the rise velocity, velocity fluctuations, and Reynolds stresses are investigated. Some figures and animations of results can be seen on the author's webpage.

Bunner, Bernard; Tryggvason, Gretar

1998-11-01

414

Multiscale considerations in direct numerical simulations of multiphase flowsa)

NASA Astrophysics Data System (ADS)

Direct Numerical Simulations of multiphase flows have progressed rapidly over the last decade and it is now possible to simulate, for example, the motion of hundreds of deformable bubbles in turbulent flows. The availability of results from such simulations should help advance the development of new and improved closure relations and models of the average or large-scale flows. We review recent results for bubbly flow in vertical channels, discuss the difference between upflow and downflow and the effect of the bubble deformability and how the resulting insight allowed us to produce a simple description of the large scale flow, for certain flow conditions. We then discuss the need for the development of numerical methods for more complex situations, such as where the flow creates spontaneous thin films and threads, or where additional physical processes take place at a rate that is very different from the fluid flow. Recent work on capturing localized small-scale processes using embedded analytical models, focusing on the mass transfer from bubbles in liquids with low mass diffusivity, suggests one approach. We conclude by discussing immediate needs for progress on the theoretical framework for describing the large-scale motion of multiphase flows and the need for multiscale methods to capture physical processes taking place at diverse length and time scales.

Tryggvason, Gretar; Dabiri, Sadegh; Aboulhasanzadeh, Bahman; Lu, Jiacai

2013-03-01

415

Numerical simulation of multi-layered textile composite reinforcement forming

One important perspective in aeronautics is to produce large, thick or/and complex structural composite parts. The forming stage presents an important role during the whole manufacturing process, especially for LCM processes (Liquid Composites Moulding) or CFRTP (Continuous Fibre Reinforcements and Thermoplastic resin). Numerical simulations corresponding to multi-layered composite forming allow the prediction for a successful process to produce the thick parts, and importantly, the positions of the fibres after forming to be known. This paper details a set of simulation examples carried out by using a semi-discrete shell finite element made up of unit woven cells. The internal virtual work is applied on all woven cells of the element taking into account tensions, in-plane shear and bending effects. As one key problem, the contact behaviours of tool/ply and ply/ply are described in the numerical model. The simulation results not only improve our understanding of the multi-layered composite forming process but also point out the importance of the fibre orientation and inter-ply friction during formability.

Wang, P.; Hamila, N.; Boisse, P. [University of Lyon, INSA-Lyon, LaMCos, CNRS UMR 5259, F-69621 (France)

2011-05-04

416

Numerical simulation of the non-Newtonian mixing layer

NASA Technical Reports Server (NTRS)

This work is a continuing effort to advance our understanding of the effects of polymer additives on the structures of the mixing layer. In anticipation of full nonlinear simulations of the non-Newtonian mixing layer, we examined in a first stage the linear stability of the non-Newtonian mixing layer. The results of this study show that, for a fluid described by the Oldroyd-B model, viscoelasticity reduces the instability of the inviscid mixing layer in a special limit where the ratio (We/Re) is of order 1 where We is the Weissenberg number, a measure of the elasticity of the flow, and Re is the Reynolds number. In the present study, we pursue this project with numerical simulations of the non-Newtonian mixing layer. Our primary objective is to determine the effects of viscoelasticity on the roll-up structure. We also examine the origin of the numerical instabilities usually encountered in the simulations of non-Newtonian fluids.

Azaiez, Jalel; Homsy, G. M.

1993-01-01

417

Numerical simulations for plasma-based dry reforming

NASA Astrophysics Data System (ADS)

The conversion of greenhouse gases (CO2 and CH4) to more valuable chemicals is one of the challenges of the 21st century. The aim of this study is to describe the plasma chemistry occurring in a DBD for the dry reforming of CO2/CH4 mixtures, via numerical simulations. For this purpose we apply the 0D simulation code ``Global/kin,'' developed by Kushner, in order to simulate the reaction chemistry and the actual reaction conditions for a DBD, including the occurrence of streamers. For the chemistry part, we include a chemistry set consisting of 62 species taking part in 530 reactions. First we describe the reaction chemistry during one streamer, by simulating one discharge pulse and its afterglow, to obtain a better understanding of the reaction kinetics. Subsequently, we expand these results to real time scale simulations, i.e., 1 to 10 seconds, where we analyze the effects of the multiple discharges (streamers) and input energy on the conversion and the selectivity of the reaction products, as well as on the energy efficiency of the process. The model is validated based on experimental data from literature.

Snoeckx, Ramses; Aerts, Robby; Bogaerts, Annemie

2012-10-01

418

Direct numerical simulation of turbulent counterflow nonpremixed flames

NASA Astrophysics Data System (ADS)

This paper presents our recent progress in terascale three-dimensional simulations of turbulent nonpremixed flames in the presence of a mean flow strain and fine water droplets. Under the ongoing university collaborative project supported by the DOE SciDAC Program [1] along with the INCITE 2007 Project [2], the study aims at bringing the state-of-the-art high-fidelity simulation capability to the next level by incorporating various advanced physical models for soot formation, radiative heat transfer, and lagrangian spray dynamics, to an unprecedented degree of detail in high-fidelity simulation application. The targeted science issue is fundamental characteristics of flame suppression by the complex interaction between turbulence, chemistry, radiation, and water spray. The high quality simulation data with full consideration of multi-physics processes will allow fundamental understanding of the key physical and chemical mechanisms in the flame quenching behavior. In this paper, recent efforts on numerical algorithms and model development toward the targeted terascale 3D simulations are discussed and some preliminary results are presented.

Im, Hong G.; Trouvé, Arnaud; Rutland, Christopher J.; Arias, Paul G.; Narayanan, Praveen; Srinivasan, Seshasai; Yoo, Chun Sang

2007-07-01

419

Numerical Simulation of Cellular Blood Flow through a Rigid Artery

NASA Astrophysics Data System (ADS)

In blood flow, red blood cells (RBCs), the most numerous constituent of blood, influence continuum-level measures by altering the suspension at microscopic scales. The presence of RBCs alters the stress and diffusion individual cells experience, which can influence cardiovascular diseases by affecting other cells present in blood like platelets and white blood cells. Simulations of blood at a cellular level provide a tool that allows exploration of both the rheology and the stress and diffusion of individual suspended cells. In this work, a hybrid lattice-Boltzmann/finite element method is used to simulate suspension flows characteristic of blood with deformable RBCs at realistic hematocrit values. We have shown the ability to simulate thousands deformable suspensions capturing non-Newtonian flow characteristics such as shear thinning, and the results agree well with experimental observations. Simulations through rigid arteries have been deformed with as many as 2500 RBCs. This work outlines results obtained for pressure-gradient driven blood flow through a rigid artery with 20%, 30%, 40%, and 50% hematocrit values. Results include the effect these deformable RBCs have on mean velocity, flow rate, radial variation of RBC concentration, and the effective viscosity for simulations at moderate to low cell capillary numbers, Ca <=0.08.

Reasor, Daniel; Clausen, Jonathan; Aidun, Cyrus

2009-11-01

420

Numerical simulations of electrostatically driven jets from nonviscous droplets

NASA Astrophysics Data System (ADS)

The evolution of a perfectly conducting and nonviscous fluid, under the action of an electric field (uniform at infinity), is studied numerically. Level set techniques are employed to develop an Eulerian potential flow model that can follow the drop evolution past breakup, while the free surface fluid velocity and the electric field force are obtained via axisymmetric boundary integral calculations. Numerical results are presented for neutral and charged droplets and for free charged droplets. In all cases, the evolution droplet aspect ratio, progeny droplet size, Taylor cone angles, jet shapes, and self-similar scaling exponents are reported. In particular, for free charged water droplets, the bursting frequency and other jetting characteristics have been carefully analyzed. Wherever possible, these results are compared with previously reported experiments and simulations.

Garzon, M.; Gray, L. J.; Sethian, J. A.

2014-03-01

421

Numerical simulations of a diode laser BPH treatment system

NASA Astrophysics Data System (ADS)

Numerical simulations are presented of the laser-tissue interaction of a diode laser system for treating benign prostate hyperplasia. The numerical model includes laser light transport, heat transport, cooling due to blood perfusion, thermal tissue damage, and enthalpy of tissue damage. Comparisons of the stimulation results to clinical data are given. We report that a reasonable variation from a standard set of input data produces heating times which match those measured in the clinical trials. A general trend of decreasing damage volume with increasing heating time is described. We suggest that the patient-to-patient variability seen in the data can be explained by differences in fundamental biophysical properties such as the optical coefficients. Further work is identified, including the measurement and input to the model of several specific data parameters such as optical coefficients, blood perfusion cooling rate, and coagulation rates.

London, Richard A.; Esch, Victor C.; Papademetriou, Stephanos

1999-06-01

422

Numerical simulation of reconnection in an emerging magnetic flux region

NASA Technical Reports Server (NTRS)

Numerical solutions in two dimensions are presented for the resistive MHD equations of an initial boundary value problem, simulating reconnection between an emerging magnetic flux region and an overlying coronal magnetic field. This numerical model displays four main phases, which are interpreted in terms of (1) a slowly evolving quasi-steady phase during which most of the magnetic flux emerges, with approximate equilibrium between magnetic and pressure forces; (2) an impulsive phase in which either the reconnection or continuing emergence of the first phase comes to disrupt the equilibrium, and extensive acceleration occurs as the high pressure region within the emerging region drives the fluid upwards and outwards; (3) a second quasi-steady phase; and (4) a potential-static phase in which continuing reconnection, ohmic dissipation, and fluid transport through boundaries depletes the system of all currents and flows.

Forbes, T. G.; Priest, E. R.

1984-01-01

423

Numerical Simulations of Relativistic Magnetic Reconnection with Galerkin Methods

NASA Astrophysics Data System (ADS)

We present the results of two-dimensional magnetohydrodynamical numerical simulations of relativistic magnetic reconnection, with particular emphasis on the dynamics of Petschek-type configurations with high Lundquist numbers, S˜ 105 - 108. The numerical scheme adopted, allowing for unprecedented accuracy for this type of calculations, is based on high order finite volume and discontinuous Galerkin methods as recently proposed by Dumbser & Zanotti (2009). The possibility of producing high Lorentz factors is discussed, by studying the effects produced on the dynamics by different magnetization and resistivity regimes. We show that Lorentz factors close to ˜ 4 can be produced for a plasma parameter ? = 0.05. Moreover, we find that the Sweet-Parker layers are unstable, generating secondary magnetic islands, but only for S > Sc ˜ 108, much larger than what is reported in the Newtonian regime.

Zanotti, O.; Dumbser, M.

2012-07-01

424

Experimental and 3D Numerical Simulation of Reinforced Shear Joints

NASA Astrophysics Data System (ADS)

The load transfer capacity and failure mechanism of a fully grouted bolt installed across a joint in shear is investigated, both experimentally and numerically, in five types of bolt. The double-shearing testing of bolts were studied in concrete blocks of 20, 40 and 100 MPa strengths, subjected to different pretension loads of 0, 5, 10, 20, 50 and 80 KN, respectively. The parameters examined include: shear resistance, shear displacement, induced strains and stresses during the bolt-bending process, and its ultimate failure across the sheared joint planes. The conclusions drawn from the study were that the strength of the concrete, bolt profile configuration and bolt pretension load played a significant influence on the shear resistance, shear displacement and failure mechanism of the reinforced medium. Experimental and numerical simulations showed that the failure occurs as a result of the induced axial and shear stresses acting between the hinge point distances in the vicinity of the shear joint plane.

Jalalifar, Hossein; Aziz, N.

2010-02-01

425

Computer-based numerical simulations of adsorption in nanostructures

NASA Astrophysics Data System (ADS)

Zeolites are crystalline oxides with uniform, molecular-pore diameters of 3-14Å. Significant developments since 1950 made production of synthetic zeolites with high purity and controlled chemical composition possible. In powder-form, zeolites are major role-players in high-tech, industrial catalysis, adsorption, and ion exchange applications. Understanding properties of thin-film zeolites has been a focus of recent research. The ability to fine-tune desired macroscopic properties by controlled alteration at the molecular level is paramount. The relationships between macroscopic and molecular-level properties are established by experimental research. Because generating macroscopic, experimental data in a controlled laboratory can be prohibitively costly and time-consuming, reliable numerical simulations, which remove such difficulties, are an attractive alternative. Using a Configurational Biased Monte Carlo (CBMC) approach in grand canonical ensemble, numerical models for pure component and multicomponent adsorption processes were developed. Theoretical models such as ideal (IAST) and real adsorbed solution theory (RAST) to predict mixture adsorption in nanopores were used for comparison. Activity coefficients used in RAST calculations were determined from the Wilson, spreading pressure and COSMO-RS models. Investigative testing of the method on known materials, represented by all-silica zeolites such as MFI (channel type) and DDR (cage type), proved successful in replicating experimental data on adsorption of light hydrocarbons - alkanes, such as methane, ethane, propane and butane. Additionally, adsorption of binary and ternary mixtures was simulated. The given numerical approach developed can be a powerful, cost and time saving tool to predict process characteristics for different molecular-structure configurations. The approach used here for simulating adsorption properties of nanopore materials including process characteristics, may have great potential for other properties of interest.

Khashimova, Diana

2014-08-01

426

Numerical simulation of the integrated space shuttle vehicle in ascent

NASA Technical Reports Server (NTRS)

A simulation of the flow about the integrated space shuttle vehicle in ascent mode has been undertaken for various flight conditions using the Chimera composite grid discretization approach. Overset body-conforming grids were used to represent each geometric component, and an implicit approximately factored finite-difference procedure was used to solve the three-dimensional thin-layer Navier-Stokes equations. The computational results have been compared with both wind tunnel and flight test data. Although relatively good agreement is obtained with the experimental data, further refinement and evaluation of numerical error is under way.

Buning, P. G.; Chiu, I. T.; Obayashi, S.; Rizk, Y. M.; Steger, J. L.

1988-01-01

427

Numerical and laboratory simulation of fault motion and earthquake occurrence

NASA Technical Reports Server (NTRS)

Simple linear rheologies were used with elastic forces driving the main events and viscoelastic forces being important for aftershock and creep occurrence. Friction and its dependence on velocity, stress, and displacement also plays a key role in determining how, when, and where fault motion occurs. The discussion of the qualitative behavior of the simulators focuses on the manner in which energy was stored in the system and released by the unstable and stable sliding processes. The numerical results emphasize the statistics of earthquake occurrence and the correlations among source parameters.

Cohen, S. C.

1978-01-01

428

General Physics Motivations for Numerical Simulations of Quantum Field Theory

In this introductory article a brief description of Quantum Field Theories (QFT) is presented with emphasis on the distinction between strongly and weakly coupled theories. A case is made for using numerical simulations to solve QCD, the regnant theory describing the interactions between quarks and gluons. I present an overview of what these calculations involve, why they are hard, and why they are tailor made for parallel computers. Finally, I try to communicate the excitement amongst the practitioners by giving examples of the quantities we will be able to calculate to within a few percent accuracy in the next five years.

Rajan Gupta

1999-05-20

429

Numerical simulation of carbon arc discharge for nanoparticle synthesis

Arc discharge with catalyst-filled carbon anode in helium background was used for the synthesis of carbon nanoparticles. In this paper, we present the results of numerical simulation of carbon arc discharges with arc current varying from 10 A to 100 A in a background gas pressure of 68 kPa. Anode sublimation rate and current voltage characteristics are compared with experiments. Distribution of temperature and species density, which is important for the estimation of the growth of nanoparticles, is obtained. The probable location of nanoparticle growth region is identified based on the temperature range for the formation of catalyst clusters.

Kundrapu, M.; Keidar, M. [Department of Mechanical and Aerospace Engineering, George Washington University, Washington, DC 20052 (United States)

2012-07-15

430

Numerical Simulations of Interacting Gas-Rich Barred Galaxies

Using an N-body+SPH code we have performed numerical simulations to investigate the dynamical effects of an interaction between an initially barred galaxy and a small spherical companion. In the models described here the small companion passes through the disk of the larger galaxy perpendicular to its plane. The impact positions and times are varied with respect to the evolutionary phase of the bar and disc. The interactions produce expanding ring structures, offset bars, spokes, and other asymmetries in the stars and gas. They also affect the strength and pattern speed of the bar.

I. Berentzen; C. H. Heller; E. Athanassoula; K. J. Fricke

1999-04-12

431

ISL 500 kJ 15 mm railgun numerical simulation

NASA Astrophysics Data System (ADS)

The study presents a numerical simulation of the mechanical and electromagnetic behavior of a 15-mm round bore railgun. The electromagnetic calculations were performed with the MSC/EMAS finite element code. The calculated electromagnetic forces acting on the structure serve as an input for the MSC/DYNA finite element code. The theoretical results are compared to the experimental measurements on the 400-kJ drive railgun. This model is used to compare the behavior of a solid armature with the current input at the backside to front-fed solid armatures. Several optimizations of the present design are proposed.

Kienner, Philippe; Kitzinger, Klaus

1993-01-01

432

Numerical aerodynamic simulation facility preliminary study, volume 1

NASA Technical Reports Server (NTRS)

A technology forecast was established for the 1980-1985 time frame and the appropriateness of various logic and memory technologies for the design of the numerical aerodynamic simulation facility was assessed. Flow models and their characteristics were analyzed and matched against candidate processor architecture. Metrics were established for the total facility, and housing and support requirements of the facility were identified. An overview of the system is presented, with emphasis on the hardware of the Navier-Stokes solver, which is the key element of the system. Software elements of the system are also discussed.

1977-01-01

433

Direct numerical simulation of equilibrium turbulent boundary layers

NASA Technical Reports Server (NTRS)

This paper describes the simulation of turbulent boundary layers by direct numerical solution of the three-dimensional, time-dependent Navier-Stokes equations, using a spectral method. The flow is incompressible, with Re sub delta = 1000. The equations are written in the self-similar coordinate system and periodic streamwise and spanwise boundary conditions are imposed. A family of nine 'equilibrium' boundary layers, from the strongly accelerated 'sink' flow to Stratford's separating boundary layer is treated. Good general agreement with experiments is observed. The effects of pressure gradients on the structures and statistics, both in the wall and wake regions are discussed.

Spalart, P. R.; Leonard, A.

1985-01-01

434

Numerical Simulation of Low-Density Shock-Wave Interactions

NASA Technical Reports Server (NTRS)

Computational Fluid Dynamics (CFD) numerical simulations of low-density shock-wave interactions for an incident shock impinging on a cylinder have been performed. Flow-field density gradient and surface pressure and heating define the type of interference pattern and corresponding perturbations. The maximum pressure and heat transfer level and location for various interaction types (i.e., shock-wave incidence with respect to the cylinder) are presented. A time-accurate solution of the Type IV interference is employed to demonstrate the establishment and the steadiness of the low-density flow interaction.

Glass, Christopher E.

1999-01-01

435

Diffusive mesh relaxation in ALE finite element numerical simulations

The theory for a diffusive mesh relaxation algorithm is developed for use in three-dimensional Arbitary Lagrange/Eulerian (ALE) finite element simulation techniques. This mesh relaxer is derived by a variational principle for an unstructured 3D grid using finite elements, and incorporates hourglass controls in the numerical implementation. The diffusive coefficients are based on the geometric properties of the existing mesh, and are chosen so as to allow for a smooth grid that retains the general shape of the original mesh. The diffusive mesh relaxation algorithm is then applied to an ALE code system, and results from several test cases are discussed.

Dube, E.I.

1996-06-01

436

A Posteriori Study of a DNS Database Describing Super critical Binary-Species Mixing

NASA Technical Reports Server (NTRS)

Currently, the modeling of supercritical-pressure flows through Large Eddy Simulation (LES) uses models derived for atmospheric-pressure flows. Those atmospheric-pressure flows do not exhibit the particularities of high densitygradient magnitude features observed both in experiments and simulations of supercritical-pressure flows in the case of two species mixing. To assess whether the current LES modeling is appropriate and if found not appropriate to propose higher-fidelity models, a LES a posteriori study has been conducted for a mixing layer that initially contains different species in the lower and upper streams, and where the initial pressure is larger than the critical pressure of either species. An initially-imposed vorticity perturbation promotes roll-up and a double pairing of four initial span-wise vortices into an ultimate vortex that reaches a transitional state. The LES equations consist of the differential conservation equations coupled with a real-gas equation of state, and the equation set uses transport properties depending on the thermodynamic variables. Unlike all LES models to date, the differential equations contain, additional to the subgrid scale (SGS) fluxes, a new SGS term that is a pressure correction in the momentum equation. This additional term results from filtering of Direct Numerical Simulation (DNS) equations, and represents the gradient of the difference between the filtered pressure and the pressure computed from the filtered flow field. A previous a priori analysis, using a DNS database for the same configuration, found this term to be of leading order in the momentum equation, a fact traced to the existence of high-densitygradient magnitude regions that populated the entire flow; in the study, models were proposed for the SGS fluxes as well as this new term. In the present study, the previously proposed constantcoefficient SGS-flux models of the a priori investigation are tested a posteriori in LES, devoid of or including, the SGS pressure correction term. The present pressure-correction model is different from, and more accurate as well as less computationally intensive than that of the a priori study. The constant-coefficient SGS-flux models encompass the Smagorinsky (SMC), in conjunction with the Yoshizawa (YO) model for the trace, the Gradient (GRC) and the Scale Similarity (SSC) models, all exercised with the a priori study constant coefficients calibrated at the transitional state. The LES comparison is performed with the filtered- and-coarsened (FC) DNS, which represents an ideal LES solution. Expectably, an LES model devoid of SGS terms is shown to be considerably inferior to models containing SGS effects. Among models containing SGS effects, those including the pressure-correction term are substantially superior to those devoid of it. The sensitivity of the predictions to the initial conditions and grid size are also investigated. Thus, it has been discovered that, additional to the atmospheric-pressure models currently used, a new model is necessary to simulate supercritical-pressure flows. This model depends on the thermodynamic characteristics of the chemical species involved.

Bellan, Josette; Taskinoglu, Ezgi

2012-01-01

437

Computational aeroacoustics and numerical simulation of supersonic jets

NASA Technical Reports Server (NTRS)

The research project has been a computational study of computational aeroacoustics algorithms and numerical simulations of the flow and noise of supersonic jets. During this study a new method for the implementation of solid wall boundary conditions for complex geometries in three dimensions has been developed. In addition, a detailed study of the simulation of the flow in and noise from supersonic circular and rectangular jets has been conducted. Extensive comparisons have been made with experimental measurements. A summary of the results of the research program are attached as the main body of this report in the form of two publications. Also, the report lists the names of the students who were supported by this grant, their degrees, and the titles of their dissertations. In addition, a list of presentations and publications made by the Principal Investigators and the research students is also included.

Morris, Philip J.; Long, Lyle N.

1996-01-01

438

Numerical simulation of MPD thruster flows with anomalous transport

Anomalous transport effects in an Ar self-field coaxial MPD thruster are presently studied by means of a fully 2D two-fluid numerical code; its calculations are extended to a range of typical operating conditions. An effort is made to compare the spatial distribution of the steady state flow and field properties and thruster power-dissipation values for simulation runs with and without anomalous transport. A conductivity law based on the nonlinear saturation of lower hybrid current-driven instability is used for the calculations. Anomalous-transport simulation runs have indicated that the resistivity in specific areas of the discharge is significantly higher than that calculated in classical runs. 21 refs.

Caldo, G.; Choueiri, E.Y.; Kelly, A.J.; Jahn, R.G. (Princeton University, NJ (United States))

1992-07-01

439

Numerical simulation of transient hypervelocity flow in an expansion tube

NASA Technical Reports Server (NTRS)

Several numerical simulations of the transient flow of helium in an expansion tube are presented in an effort to identify some of the basic mechanisms which cause the noisy test flows seen in experiments. The calculations were performed with an axisymmetric Navier-Stokes code based on a finite volume formulation and upwinding techniques. Although laminar flow and ideal bursting of the diaphragms was assumed, the simulations showed some of the important features seen in experiments. In particular, the discontinuity in tube diameter of the primary diaphragm station introduced a transverse perturbation to the expanding driver gas and this perturbation was seen to propagate into the test gas under some flow conditions. The disturbances seen in the test flow can be characterized as either small amplitude, low frequency noise possibly introduced during shock compression or large amplitude, high frequency noise associated with the passage of the reflected head of the unsteady expansion.

Jacobs, P. A.

1992-01-01

440

Numerical simulations of ultrasimple ultrashortlaser-pulse measurement

NASA Astrophysics Data System (ADS)

We numerically simulate the performance of the ultrasimple frequency-resolved-optical-gating (FROG) technique, GRENOUILLE, for measuring ultrashort laser pulses. While simple in practice, GRENOUILLE has many theoretical subtleties because it involves the second-harmonic generation of relatively tightly focused and broadband pulses. In addition, these processes occur in a thick crystal, in which the phase-matching bandwidth is deliberately made narrow compared to the pulse bandwidth. In these simulations, we include all sum-frequency-generation processes, both collinear and noncollinear. We also include dispersion using the Sellmeier equation for the crystal BBO. Working in the frequency domain, we compute the GRENOUILLE trace for practical—and impractical— examples and show that accurate measurements are easily obtained for properly designed devices.

Liu, Xuan; Trebino, Rick; Smith, Arlee V.

2007-04-01