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1

Numerical simulation of primary break-up and atomization: DNS and modelling study

This work deals with numerical simulations of atomization with high Weber and Reynolds values. A special attention has been devoted to the modelling of primary break-up. Due to progress of direct numerical simulations (DNS) of two phase flows it is now possible to simulate the primary break-up of a Diesel spray [Menard, T., Tanguy, S., Berlemont, A., 2007. Coupling level

R. Lebas; T. Menard; P. A. Beau; A. Berlemont; F. X. Demoulin

2009-01-01

2

NASA Technical Reports Server (NTRS)

The principal objective is to extend the boundaries within which large eddy simulations (LES) and direct numerical simulations (DNS) can be applied in computational analyses of high speed reacting flows. A summary of work accomplished during the last six months is presented.

Givi, Peyman; Madnia, Cyrus K.; Steinberger, C. J.; Frankel, S. H.

1992-01-01

3

The effects of mean flame radius and turbulence on self-sustained combustion of turbulent premixed spherical flames in decaying\\u000a turbulence have been investigated using three-dimensional direct numerical simulations (DNS) with single step Arrhenius chemistry.\\u000a Several flame kernels with different initial radius or initial turbulent field have been studied for identical conditions\\u000a of thermo-chemistry. It has been found that for very small

M. Klein; N. Chakraborty; R. S. Cant

2008-01-01

4

Direct numerical simulation of compressible turbulent flows

This paper reviews the authors’ recent studies on compressible turbulence by using direct numerical simulation (DNS), including\\u000a DNS of isotropic (decaying) turbulence, turbulent mixing-layer, turbulent boundary-layer and shock\\/boundary-layer interaction.\\u000a Turbulence statistics, compressibility effects, turbulent kinetic energy budget and coherent structures are studied based\\u000a on the DNS data. The mechanism of sound source in turbulent flows is also analyzed. It shows

Xin-Liang Li; De-Xun Fu; Yan-Wen Ma; Xian Liang

2010-01-01

5

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

6

Direct numerical simulation of free falling sphere in creeping flow

In the present study, direct numerical simulations (DNS) are performed on single and a swarm of particles settling under the action of gravity. The simulations have been carried out in the creeping flow range of Reynolds number from 0.01 to 1 for understanding the hindrance effect, of the other particles, on the settling velocity and drag coefficient. The DNS code

Rupesh K. Reddy; Shi Jin; K. Nandakumar; Peter D. Minev; Jyeshtharaj B. Joshi

2010-01-01

7

Discussion of DNS: Past, Present, and Future

NASA Technical Reports Server (NTRS)

This paper covers the review, status, and projected future of direct numerical simulation (DNS) methodology relative to the state-of-the-art in computer technology, numerical methods, and the trends in fundamental research programs.

Joslin, Ronald D.

1997-01-01

8

An experimental and DNS numerical study of multi-hole cooling

NASA Astrophysics Data System (ADS)

New integrally woven ceramic matrix composites (CMC), which can be multi-hole cooled, offer the prospect of substantial combustion gain. Little is known about the fluid mechanics and heat transfer in the application of such multi-hole cooled CMC systems. In this thesis, multi-hole cooling for two types of ceramic composites (oxide/oxide and SiC/SiC) with different hole geometries is studied both experimentally and numerically. With a unique heat transfer tunnel system, effects on the cooling effectiveness of parameters including blowing ratio, momentum ratio, Reynolds number, temperature ratio and hole geometry and wall material, are studied. In addition, profiles of the mean velocity, temperature and rms temperature fluctuation over the cooling surface are measured to provide further understanding of the cooling process. Duplication of the key parameters for multi-hole cooling for a real combustor flow condition is achieved with parameter scaling and the results show the efficiency of multi-hole cooling especially for the oblique hole, SiC/SiC specimen. In parallel, a 3D heat transfer model, which includes the mean solution and an unsteady solution for the wall temperature and heat flux, is developed to fully couple the heat transfer in the primary flow, in the backside flow and flow in the holes with the heat conduction in the multi-hole wall. With model tests for laminar and turbulent cooling, the 3D model is found to be very efficient for the mean temperature solution and for the unsteady solution. The 3D model is applied with DNS calculations for the simulations of multi-hole cooling at low Reynolds number. The cooling effectiveness predicted by the 3D heat transfer model is significantly different from that predicted by an adiabatic wall model and the 3D model results agree well with the experimental results. The DNS calculation of the primary flow with multi-hole cooling provides detailed information about the physics of the cooling process including the velocity and temperature fluctuations and their correlations, the vortical structures near the surface, the dominant frequencies, and the temperature, heat flux fluctuations on the cooling surface.

Zhong, Fengquan

9

Hybrid LES\\/DNS Methods for Turbulence Control

Opposition control and optimal control for turbulent channel flow (Re_tau=100) based on Large Eddy Simulation (LES) has been successfully implemented and good agreement is achieved with prior Direct Numerical Simulation (DNS) under the same conditions. However, since LES is an approximation of DNS, we need to establish the viability of the controls predicted by LES optimization. Thus, we have constructed

Yong Chang; S. Scott Collis

1999-01-01

10

Hydroacoustic forcing function modeling using DNS database

NASA Technical Reports Server (NTRS)

A wall pressure frequency spectrum model (Blake 1971 ) has been evaluated using databases from Direct Numerical Simulations (DNS) of a turbulent boundary layer (Na & Moin 1996). Good agreement is found for moderate to strong adverse pressure gradient flows in the absence of separation. In the separated flow region, the model underpredicts the directly calculated spectra by an order of magnitude. The discrepancy is attributed to the violation of the model assumptions in that part of the flow domain. DNS computed coherence length scales and the normalized wall pressure cross-spectra are compared with experimental data. The DNS results are consistent with experimental observations.

Zawadzki, I.; Gershfield, J. L.; Na, Y.; Wang, M.

1996-01-01

11

Efficient Parallel Algorithm For Direct Numerical Simulation of Turbulent Flows

NASA Technical Reports Server (NTRS)

A distributed algorithm for a high-order-accurate finite-difference approach to the direct numerical simulation (DNS) of transition and turbulence in compressible flows is described. This work has two major objectives. The first objective is to demonstrate that parallel and distributed-memory machines can be successfully and efficiently used to solve computationally intensive and input/output intensive algorithms of the DNS class. The second objective is to show that the computational complexity involved in solving the tridiagonal systems inherent in the DNS algorithm can be reduced by algorithm innovations that obviate the need to use a parallelized tridiagonal solver.

Moitra, Stuti; Gatski, Thomas B.

1997-01-01

12

National Technical Information Service (NTIS)

The two- and three-dimensional development of leading-edge receptivity and transition in a 2-D Joukowsky airfoil boundary layer are investigated by direct numerical simulation (DNS) using the incompressible Navier-Stokes equations. The numerical investiga...

C. Liu Z. Liu G. Xiong

1996-01-01

13

Direct Numerical Simulations of Turbulent Flow in a Wavy Channel

A spectrally preconditioned biconjugate gradient algorithm (Bi-CGSTAB) has been developed that enabled us to perform high accuracy (spectral) efficient Direct Numerical Simulations (DNS) of Newtonian turbulent flow in an undulating channel geometry. The DNS of have been performed in a channel geometry involving a single sinusoidal solid wavy wall with amplitude\\/half width ratio of 0.1 and a wave length of

Luo Wang; Kostas Housiadas; Antony Beris

2006-01-01

14

Direct Numerical Simulation of Flame\\/Acoustic Interactions

Combustion phenomena are of high scientific and technological interest, in particular for energy generation and transportation\\u000a systems. Direct Numerical Simulations (DNS) have become an essential and well-established research tool to investigate the\\u000a structure of turbulent flames, since they do not rely on any approximate turbulence models. In this project the DNS code ?\\u000a 3C is employed to investigate different flame

Hemdan Shalaby; Gábor Janiga; Dominique Thévenin

15

Direct numerical simulation (DNS) has become a powerful tool in studying fundamental phenomena of laminar-turbulent transition of high-speed boundary layers. Previous DNS studies of supersonic and hypersonic boundary layer transition have been limited to perfect-gas flow over flat-plate boundary layers without shock waves. For hypersonic boundary layers over realistic blunt bodies, DNS studies of transition need to consider the effects

Xiaolin Zhong

1998-01-01

16

A spectrally preconditioned biconjugate gradient algorithm has been developed to perform efficiently Direct Numerical Simulations (DNS) of Newtonian turbulent flow in a wavy channel. A transformation involving the shear direction only is applied to map the wavy geometry into a rectangular one so that a spectral approximation can be applied. DNS of Newtonian turbulent flow over a single sinusoidal wavy

Luo Wang; Peter Wapperom; Antony Beris

2005-01-01

17

Dynamo in the Taylor-Green vortex: Direct numerical simulations and modeling of MHD flows

Direct numerical simulations (DNS) and Lagragian-averaged model runs (LAMHD) of three-dimensional magnetohydrodynamic turbulence are presented. The model allows for a significant reduction of computer resources at given Reynolds numbers. It correctly reproduces the growth rate of magnetic energy and captures the nonlinear saturation level; intermittency is recovered as well. Low magnetic Prandlt number dynamos are then explored combining DNS, LAMHD

Annick Pouquet; Pablo Mininni; David Montgomery; Jean-Francois Pinton; Helene Politano; Yannick Ponty

2005-01-01

18

NASA Astrophysics Data System (ADS)

Wall jets over a curved wall geometry (Coanda flows) are investigated using DNS and turbulence modeling. In experiments large coherent structures have enhanced the effectiveness of wall jets in delaying or preventing flow separation on airfoils. Understanding the behavior of these structures is essential for utilizing wall jets for separation control. The research objective is to investigate curvature effects on large coherent structures, in particular the development of longitudinal (Goertler-type) vortices and their interaction with 2D vortices. The focus is on Coanda cylinders using two computational approaches. With the Flow Simulation Methodology (FSM), a turbulent wall jet is computed over a cylinder segment on a body-fitted grid. In FSM, the contribution of the turbulence model depends on the grid resolution relative to a local turbulent length scale. For a flat-plate reference case, FSM is employed as DNS, LES, and URANS. In all cases the large 2D vortices are captured. For the curved-wall geometry, FSM is employed as a DNS. Goertler-type vortices emerge in the simulation but remain weak due to the narrow computational domain. In the second approach, Coanda flows including nozzle and separated region are computed using immersed boundary techniques (IBT). The feasibility of IBT for Coanda Flows is established.

Fasel, Hermann F.

2002-07-01

19

Direct Numerical Simulations of Decaying Isotropic Compressible Turbulence

NASA Astrophysics Data System (ADS)

We describe 256^3 direct numerical simulations (DNS) of decaying compressible isotropic turbulence at fluctuation Mach numbers of M_t ~ 0.5 and at Taylor Reynolds numbers Re_? = O(100). Regions of high-negative divergence are indicative of the presence of eddy shocklets in these simulations. A quantitative analysis of shocklets will be presented. Furthermore, the issue of initial conditions for these simulations will be discussed. Comparisons of the decay of the turbulent kinetic energy obtained from the DNS with large-eddy simulations at 32^3 resolution are the subject of another presentation at this conference.

Samtaney, Ravi; Pullin, D. I.; Kosovic, Branko; Meiron, D. I.

1999-11-01

20

Direct numerical simulation of stenotic flows, Part 1: Steady flow

Direct numerical simulations (DNS) of steady and pulsatile flow through 75% (by area reduction) stenosed tubes have been performed, with the motivation of understanding the biofluid dynamics of actual stenosed arteries. The spectral-element method, providing geometric flexibility and high-order spectral accuracy, was employed for the simulations. The steady flow results are examined here while the pulsatile flow analysis is dealt

SONU S. VARGHESE; STEVEN H. FRANKEL

2005-01-01

21

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

22

Unsteady dynamics and organized structures from DNS over an idealized building canopy

A numerical study is performed to elucidate the dominant turbulent processes that occur in urban areas. Comprehensive data from direct numerical simulations (DNS) over idealized three-dimensional arrays of buildings are analysed to study the unsteady and organized aspects of the turbulent flow. The accuracy of the DNS is evaluated by comparing turbulence statistics with a high quality wind-tunnel dataset. The

Omduth Coceal; Adrian Dobre; T. G. Thomas

2007-01-01

23

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

24

Direct numerical simulation of turbulent flow in elliptical ducts

Direct numerical simulation (DNS) of fully developed turbulent flow in elliptical ducts is performed. The mean cross-stream secondary flows exhibited by two counter- rotating vortices which are symmetrical about the major ellipse's axis are examined. The mean flow characteristics and turbulence statistics are obtained. The variation of the statistical quantities such as the Reynolds stresses and turbulence intensities along the

N IKOLAY N IKITIN; ALEXANDER Y AKHOT

2005-01-01

25

Direct Numerical Simulation of Shock-wave\\/Isotropic Turbulence Interaction

We conduct direct numerical simulations (DNS) of shock\\/isotropic-turbulence interac- tions (SITI), in which the turbulence is highly compressible. We find, consistent with previous studies using weakly compressible turbulence, that turbulent kinetic energy and spanwise vorticity fluctuations become persistently amplified upon passage through a shock wave and that the spanwise Taylor microscale becomes persistently diminished. In addi- tion, we find that

Nathan E. Grube; Ellen M. Taylor; M. Pino Mart

26

This paper describes in detail a numerical scheme designed for direct numerical simulation (DNS) of turbulent drag reduction. The hybrid spatial scheme includes Fourier spectral accuracy in two directions and sixth-order compact finite differences for first and second-order wall-normal derivatives, while time marching can be up to fourth-order accurate. High-resolution and high-drag reduction viscoelastic DNS are made possible through domain

Laurent Thais; Andrés E. Tejada-Mart?´nez; Thomas B. Gatski; Gilmar Mompean

2011-01-01

27

Direct Simulation of Turbulent Combustion.

National Technical Information Service (NTIS)

Direct Numerical Simulation (DNS) results obtained for premixed turbulent flames are presented and discussed to provide a description of the DNS possibilities and limitations. Numerical issues and boundary conditions are discussed. The basis of flamelet m...

T. J. Poinsot

1992-01-01

28

Direct Numerical Simulations of Decaying Isotropic Compressible Turbulence

We describe 256^3 direct numerical simulations (DNS) of decaying compressible isotropic turbulence at fluctuation Mach numbers of M_t ~ 0.5 and at Taylor Reynolds numbers Re_lambda = O(100). Regions of high-negative divergence are indicative of the presence of eddy shocklets in these simulations. A quantitative analysis of shocklets will be presented. Furthermore, the issue of initial conditions for these simulations

Ravi Samtaney; D. I. Pullin; Branko Kosovic; D. I. Meiron

1999-01-01

29

In order to understand the statistics and physics of turbulence, we performed large-scale direct numerical simulations (DNS's) of canonical incompressible turbulent flows on the Earth Simulator, including those of (i) turbulence in a periodic box, (ii) multi-phase turbulent shear flows, (iii) turbulent Ekman boundary layer, and (iv) passive scalar turbulence. The DNS data were analyzed to study (i) the energy

Chuichi Arakawa; Makoto Iida; Yukio Kaneda; Takashi Ishihara; Hiroshi Kawamura; Yoshinobu Yamamoto; Toshiyuki Gotoh; Takeshi Watanabe

30

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

31

Applications of direct numerical simulation of turbulence in second order closures

NASA Technical Reports Server (NTRS)

This paper discusses two methods of developing models for the rapid pressure-strain correlation term in the Reynolds stress transport equation using direct numerical simulation (DNS) data. One is a perturbation about isotropic turbulence, the other is a perturbation about two-component turbulence -- an extremely anisotropic turbulence. A model based on the latter method is proposed and is found to be very promising when compared with DNS data and other models.

Shih, Tsan-Hsing; Lumley, John L.

1995-01-01

32

DNS and LES of some engineering flows

NASA Astrophysics Data System (ADS)

In this paper, direct numerical simulations (DNS) and large eddy simulations (LES) of three engineering flows carried out in the author's research group are presented. The first example, simulated both with DNS and LES, is the flow in a low-pressure turbine cascade with wakes passing periodically through the cascade channel. In this situation, the laminar-turbulent transition of the boundary layers on the blade surfaces, which is strongly influenced by the passing wakes, is of special interest. Next, LES of the flow past the Ahmed body is presented, which is a car model with slant back. In spite of the fairly simple geometry, the flow around the model has many features of the complex, fully 3D flow around real cars. The third example, for which LES is presented, is the flow past a surface mounted circular cylinder of height-to-diameter ratio of 2.5. In this case also complex 3D flow develops with interaction of various vortices behind the cylinder. By means of these examples, the paper shows that complex turbulent flows of engineering relevance can be predicted realistically by DNS and LES, albeit at large cost. The methods are particularly suited and superior to RANS methods for situations where unsteadiness like shedding and large-scale structures dominate the flow, and DNS has evolved into an important tool for studying transition mechanisms.

Rodi, Wolfgang

2006-02-01

33

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

34

Direct numerical simulation of transitions toward turbulence in complex channel flows

A new numerical tool for the direct numerical simulation (DNS) of instability and transition to turbulence is presented. The Navier-Stokes equations for incompressible flow are solved in generalized curvilinear coordinates so that channel flows may be investigated in which the walls of the channel are both curved and wavy. The channel geometry and the flow solution are assumed to be

Bradley D. Duncan

1998-01-01

35

\\u000a This chapter describes a number of common configurations when working with zone files and, in some cases, with BIND. These\\u000a solutions are presented to assist you in quickly implementing some commonly used features, recovering from errors, and illustrating\\u000a some of the more subtle uses of the DNS. The following topics are covered:\\u000a \\u000a \\u000a \\u000a \\u000a • \\u000a \\u000a \\u000a How to delegate a subdomain: This configuration

Ron Aitchison

36

Direct numerical simulation of nonpremixed flame-wall interactions

The objective of the present study is to use detailed numerical modeling to obtain basic information on the interaction of nonpremixed flames with cold wall surfaces. The questions of turbulent fuel-air-temperature mixing, flame extinction, and wall-surface heat transfer are studied using direct numerical simulation (DNS). The DNS configuration corresponds to an ethylene-air diffusion flame stabilized in the near-wall region of a chemically inert solid surface. Simulations are performed with adiabatic or isothermal wall boundary conditions and with different turbulence intensities. The simulations feature flame extinction events resulting from excessive wall cooling and convective heat transfer rates up to 90 kW/m{sup 2}. The structure of the simulated wall flames is studied in terms of a classical mass-mixing variable, the fuel-air based mixture fraction, and a less familiar heat loss variable, the excess enthalpy variable, introduced to provide a measure of nonadiabatic behavior due to wall cooling. In addition to the flame structure, extinction events are also studied in detail and a modified flame extinction criterion that combines the concepts of mixture fraction and excess enthalpy is proposed and then tested against the DNS data. (author)

Wang, Yi [Department of Mechanical Engineering, University of Maryland, College Park, MD 20742 (United States); Trouve, Arnaud [Department of Fire Protection Engineering, University of Maryland, College Park, MD 20742 (United States)

2006-02-01

37

Direct numerical simulation of flow separation around a NACA 0012 airfoil

Direct numerical simulation (DNS) for the flow separation and transition around a NACA 0012 airfoil with an attack angle of 4° and Reynolds number of 105 based on free-stream velocity and chord length is presented. The details of the flow separation, detached shear layer, vortex shedding, breakdown to turbulence, and re-attachment of the boundary layer are captured in the simulation.

Hua Shan; Li Jiang; Chaoqun Liu

2005-01-01

38

NASA Astrophysics Data System (ADS)

We present results of Lagrangian statistical quantities for direct numerical simulation (DNS) of turbulent channel flow at Reynolds number Re? = 950 based on shear velocity and channel half-height. Attention is focused on time correlations of fluid particle velocity and on the wall-normal diffusivity as a function of the wall-normal distance. Away from the wall region the DNS results compare favorably with the results of recent statistical models based on Kolmogorov theory and Onsager symmetry relations. It is found that a value for the Kolmogorov constant of C0 = 6 gives optimal agreement between DNS results and results of the statistical models for all quantities considered.

Kuerten, J. G. M.; Brouwers, J. J. H.

2013-10-01

39

Numerical Errors: Reliable Numerical Simulations.

National Technical Information Service (NTIS)

Understanding numerical errors in long calculations is a very subtle science and is critical to understanding the reliability of the final answer. We will carefully examine the accumulation of numerical errors over time and discuss how these can lead to r...

L. Jameson

2001-01-01

40

A further study of numerical errors in large-eddy simulations

Numerical errors in large-eddy simulations (LES) arise from aliasing and discretization errors, and errors in the subfilter-scale (SFS) turbulence model. Using a direct numerical simulation (DNS) dataset of stably stratified shear flow to perform a priori tests, we compare the numerical error from several finite difference schemes to the magnitude of the SFS force. This is an extension of Ghosal’s

Fotini Katopodes Chow; Parviz Moin

2003-01-01

41

The Use of DNS in Turbulence Modeling

NASA Technical Reports Server (NTRS)

The use of Direct numerical simulations (DNS) data in developing and testing turbulence models is reviewed. The data is used to test turbulence models at all levels: algebraic, one-equation, two-equation and full Reynolds stress models were tested. Particular examples on the development of models for the dissipation rate equation are presented. Homogeneous flows are used to test new scaling arguments for the various terms in the dissipation rate equation. The channel flow data is used to develop modifications to the equation model that take into account near-wall effects. DNS of compressible flows under mean compression are used in testing new compressible modifications to the two-equation models.

Mansour, Nagi N.; Merriam, Marshal (Technical Monitor)

1997-01-01

42

Direct numerical simulations (DNS) are conducted of turbulent flow passing an infinitely thin trailing edge. The objective is to investigate the turbulent flow field in the vicinity of the trailing edge and the associated broadband noise generation. To generate a turbulent boundary layer a short distance from the inflow boundary, high- amplitude lifted streaks and disturbances that can be associated

RICHARD D. S ANDBERG; NEIL D. S ANDHAM

2008-01-01

43

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.

44

NASA Astrophysics Data System (ADS)

Batteries and fuel cells are widely used to generate electrical energy, especially in recent applications to electric and hybrid vehicles. To simulate a porous electrode for batteries and fuel cells, macro-homogeneous models are often employed in which the actual morphology of the electrode is ignored, thereby making computations much easier. However, such models are based on the volume-averaging technique, which smears the microscopically complex interfacial structures and has to invoke empirical correlations for describing the effective transport properties in a multiphase system. In this work, a methodology is developed to achieve the description on the pore level based on direct numerical simulation (DNS) method. The DNS solves the accurate point-wise conservation equations on a real micro-structure of the porous electrode and hence utilizes the intrinsic transport properties for each phase. To demonstrate the DNS method, an idealized morphology and further a random microstructure are constructed to represent all the phases composing the porous electrode. A single set of conservation equations of charge and species valid in all phases are developed and numerically solved using a finite volume technique. The present DNS model is first applied to simulate the behavior of an intercalative carbon electrode in the widely used lithium-ion cell. The concentration and potential distributions in both solid and electrolyte phases at the pore level are obtained across the electrode during the discharge. The species and charge transport processes, as well as the electrochemical reactions, are computationally visualized when discharging the electrode. In addition, empirical correlations in porous electrode theory, which describe the dependency of effective properties (diffusion coefficient, conductivity, etc.) on the porosity, are corroborated using the fundamental DNS data. Then the discharge processes of a full lithium ion cell at various rates are simulated with DNS approach and verified by the experimental data. In the application to the cathode catalyst layer of PEM fuel cells, DNS is employed to identify three characteristic voltage losses: kinetics losses, ohmic losses and O2 transport losses. On a constructed random microstructure, DNS is also utilized to optimize the inlet air humidity and the composition design and hence achieve the minimum voltage loss during operation. In summary, the newly developed DNS method has provided an effective method to simulate behavior of thin porous electrodes with microscopically complicated geometries and the fundamentals insight into structure-performance relationships of porous electrodes for the first time.

Wang, Guoqing

45

CoDNS: Improving DNS Performance and Reliability via Cooperative Lookups

The Domain Name System (DNS) is a ubiquitous part of everyday computing, translating human-friendly ma- chine names to numeric IP addresses. Most DNS re- search has focused on server-side infrastructure, with the assumption that the aggressive caching and redundancy on the client side are sufficient. However, through sys- tematic monitoring, we find that client-side DNS fail- ures are widespread and

Vivek S. Pai; Larry L. Peterson; Zhe Wang

2004-01-01

46

Direct Numerical Simulation of Tilted Rayleigh-Taylor Instability

NASA Astrophysics Data System (ADS)

The tilted Rayleigh-Taylor instability, where the initial interface is not perpendicular to the driving acceleration, is investigated using Direct Numerical Simulations (DNS). In this configuration, the inclination of the initial interface results in a large-scale overturning motion in addition to the buoyancy driven instability. The DNS results are compared to the rocket-rig experiments of Smeeton and Youngs (AWE Report No. 35/87) at several Atwood numbers (A=0.267, 0.48, and 0.90). Since the initial conditions in these experiments are largely unknown, an extensive range of initial conditions have been explored to match the mixing layer growth between DNS and experiments. The evolution of the mixing layer was found to be strongly influenced, for the duration of the experiments, by the initial spectrum shape and peak location, as well as the perturbation amplitude. A set of initial conditions matching the experimental growth rates has been determined. Results are also presented on the interaction between shear and buoyancy, including the parameters influencing the overturning and mixing.

Wei, Tie; Livescu, Daniel

2011-11-01

47

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

48

In a recent study we showed that the two-fluid (TF) formulation can be used in the direct numerical simulation (DNS) of bubble- (or particle-) laden decaying isotropic turbulence with considerable saving in CPU-time and memory as compared to the trajectory approach employed by many researchers. In the present paper, we develop a Lagrangian-Eulerian mapping (LEM) solver for DNS of bubble-laden

O. A. Druzhinin; S. E. Elghobashi

1999-01-01

49

In a recent study we showed that the two-fluid (TF) formulation can be used in the direct numerical simulation (DNS) of bubble- (or particle-) laden decaying isotropic turbulence with considerable saving in CPU-time and memory as compared to the trajectory approach employed by many researchers. In the present paper, we develop a Lagrangian–Eulerian mapping (LEM) solver for DNS of bubble-laden

O. A Druzhinin; S. E Elghobashi

1999-01-01

50

DNS-DERIVED FORCE DISTRIBUTION ON FLEXIBLE CYLINDERS SUBJECT TO VORTEX-INDUCED VIBRATION

We use direct numerical simulation (DNS) based on spectral methods to simulate turbulent flow past rigid and flexible cylinders subject to vortex-induced vibrations (VIV). We present comparisons of amplitude, and lift and drag forces, at Reynolds number 1000 for a short and a long cylinder, and we examine differences between a traveling wave response and a standing wave response. The

C. Evangelinos; D. Lucor; G. E. KARNIADAKIS

2000-01-01

51

DNS of Turbulent Boundary Layers under Highenthalpy Conditions

NASA Astrophysics Data System (ADS)

To study real-gas effects and turbulence-chemistry interaction, direct numerical simulations (DNS) of hypersonic boundary layers are conducted under typical hypersonic conditions. We consider the boundary layer on a lifting-body consisting of a flat plate at an angle of attack, which flies at altitude 30km with a Mach number 21. Two different inclined angles, 35^o and 8^o, are considered,representing blunt and slender bodies. Both noncatalytic and supercatalytic wall conditions are considered. The DNS data are studied to assess the validity of Morkovin's hypothesis, the strong Reynolds analogy, as well as the behaviors of turbulence structures under high-enthalpy conditions.Relative to low-enthalpy conditions [1], significant differences in typical scalings are observed. [4pt] [1] L. Duan and I. Beekman and M. P. Mart'in, Direct numerical simulation of hypersonic turbulent boundary layers. Part 2: Effect of temperature, J. Fluid Mech. 655 (2010), 419-445.

Duan, Lian; Martín, Pino

2010-11-01

52

Temporal numerical simulations of turbulent Coanda wall jets

NASA Astrophysics Data System (ADS)

In a novel application of the temporal numerical simulation, an investigation of turbulence modeling techniques is carried for the turbulent wall jet over a convex surface (Coanda wall jet.) The simultaneous presence of multiple instability mechanisms and the interaction with the turbulence dynamics at the solid boundary produces a unique combination of different large turbulent coherent structures that constitutes both a consistent challenge for numerical simulations and an ideal test bed for turbulence models. The Temporal Direct Numerical Simulation (TDNS) of the Coanda wall jet restricts the focus from the global turbulent Coanda wall jet to a smaller, local portion of the flow and offers a wide array of advantages to the present work. In particular, the size of the computational domain can be arbitrarily chosen in both the spanwise and the streamwise directions. This allows to either suppress or enhance individual physical mechanisms and, consequently, to selectively reproduce different large coherent structures within the local flow. In the first part, temporal numerical simulations are employed to reproduce four different flow scenarios of the local Coanda wall jet with a level of numerical resolution that, because of the reduced size of the computational domain, cannot be matched by standard DNS of the entire physical flow (spatial DNS, or SDNS .) The TDNS of these four flow scenarios are then used in the second part for an a-posteriori analysis of different turbulence models in order to address common shortcomings shown by Hybrid Turbulence Models (HTM). For each flow scenario, the turbulent flow field is deliberately decomposed in resolved and unresolved flows by the application of different filters in space corresponding to different grid resolution. The behavior of turbulence models can be reproduced from the resolved flow and compared to the turbulent stress tensor directly calculated from the unresolved part of the flow field. Starting from the RANS limit, turbulence models with different levels of complexity are studied. Successively, the performance of these models is analyzed at intermediate numerical resolutions between RANS, LES, and DNS . Finally, an improved formulation of the Flow Simulation Methodology (FSM) is proposed.

Valsecchi, Pietro

53

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

54

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

55

DNS investigation of late-stage transition in hypersonic channel flow

NASA Astrophysics Data System (ADS)

We perform direct numerical simulations (DNS) of normal-mode evolution in hypersonic channel flows to investigate late-stage transition physics. A well-validated compressible flow solver based on Gas-Kinetic Method (GKM) is used in the computations. In this temporal DNS, periodic boundary condition is employed in the streamwise direction and wall conditions at the normal boundaries. The DNS code is first validated against analytical transition (Orr-Sommerfeld) results in the incompressible flow regime. In the compressible regime, the code is validated against homogeneous shear flow rapid distortion theory (RDT) data. Direct numerical simulation of normal modes in laminar channel flow at very high Mach number shows that the evolution exhibits a three-stage behavior similar to that observed in many hypersonic boundary layer experiments and RDT of homogeneous shear flow. The physics associated with each transition stage is investigated in great detail and a physical picture of late-stage transition is proposed.

Xie, Zhimin; Girimaji, Sharath

2011-11-01

56

NASA Astrophysics Data System (ADS)

A numerical algorithm and code are developed and applied to direct numerical simulation (DNS) of unsteady two-dimensional flow fields relevant to stability of the hypersonic boundary layer. An implicit second-order finite-volume technique is used for solving the compressible Navier-Stokes equations. Numerical simulation of disturbances generated by a periodic suction-blowing on a flat plate is performed at free-stream Mach number 6. For small forcing amplitudes, the second-mode growth rates predicted by DNS agree well with the growth rates resulted from the linear stability theory (LST) including nonparallel effects. This shows that numerical method allows for simulation of unstable processes despite its dissipative features. Calculations at large forcing amplitudes illustrate nonlinear dynamics of the disturbance flow field. DNS predicts a nonlinear saturation of fundamental harmonic and rapid growth of higher harmonics. These results are consistent with the experimental data of Stetson and Kimmel obtained on a sharp cone at the free-stream Mach number 8.

Egorov, I. V.; Fedorov, A. V.; Soudakov, V. G.

2006-02-01

57

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

58

High speed turbulent reacting flows: DNS and LES

NASA Technical Reports Server (NTRS)

Work on understanding the mechanisms of mixing and reaction in high speed turbulent reacting flows was continued. Efforts, in particular, were concentrated on taking advantage of modern computational methods to simulate high speed turbulent flows. In doing so, two methodologies were used: large eddy simulations (LES) and direct numerical simulations (DNS). In the work related with LES the objective is to study the behavior of the probability density functions (pdfs) of scalar properties within the subgrid in reacting turbulent flows. The data base obtained by DNS for a detailed study of the pdf characteristics within the subgrid was used. Simulations are performed for flows under various initializations to include the effects of compressibility on mixing and chemical reactions. In the work related with DNS, a two-dimensional temporally developing high speed mixing layer under the influence of a second-order non-equilibrium chemical reaction of the type A + B yields products + heat was considered. Simulations were performed with different magnitudes of the convective Mach numbers and with different chemical kinetic parameters for the purpose of examining the isolated effects of the compressibility and the heat released by the chemical reactions on the structure of the layer. A full compressible code was developed and utilized, so that the coupling between mixing and chemical reactions is captured in a realistic manner.

Givi, Peyman

1990-01-01

59

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

60

Direct numerical simulation of turbulent non-Newtonian flow using a spectral element method

A spectral element—Fourier method (SEM) for Direct Numerical Simulation (DNS) of the turbulent flow of non-Newtonian fluids is described and the particular requirements for non-Newtonian rheology are discussed. The method is implemented in parallel using the MPI message passing kernel, and execution times scale somewhat less than linearly with the number of CPUs, however this is more than compensated by

M. Rudman; H. M. Blackburn

2006-01-01

61

Direct numerical simulations of supercritical fluid mixing layers applied to heptane{nitrogen

Direct numerical simulations (DNS) are conducted of a model hydrocarbon{nitrogen mixing layer under supercritical conditions. The temporally developing mixing layer conguration is studied using heptane and nitrogen supercritical fluid streams at a pressure of 60 atm as a model system related to practical hydrocarbon-fuel\\/air systems. An entirely self-consistent cubic Peng{Robinson equation of state is used to describe all thermodynamic mixture

JOSETTE B ELLAN

2001-01-01

62

DNS for Flow Separation Control Around Airfoil by Steady and Pulsed Jets.

National Technical Information Service (NTIS)

This work consists of two parts. The first part is direct numerical simulation (DNS) for flow separation and transition around a NACA 0012 airfoil with an attack angle of 4 degrees and Reynolds number of 100,000. The details of the flow separation, format...

S. Deng L. Jiang C. Liu

2004-01-01

63

Direct numerical simulations of a turbulent boundary layer in an adverse pressure gradient

NASA Astrophysics Data System (ADS)

Direct numerical simulations (DNS) of the Navier-Stokes equations has been carried out with the objective of studying a turbulent boundary layer in an adverse pressure gradient (APG). Two APG cases has been simulated as well as one zero pressure gradient case (ZPG). The program uses spectral methods and utilizes the 'fringe method'. The simulations start with a laminar boundary layer at the inflow which is tripped by a random volume force trip near the wall. Self-similarity is obtained and different scalings are investigated. Two approaches to the analysis of the turbulent boundary layer equations found in the literature are compared. In this work we conclude that the nonlinear equations are required in order to obtain mean flow characteristics in agreement with the DNS. The analysis of turbulent statistics gives some hints to the mechanism of separation and the proper modeling required for APG boundary layers.

Skote, Martin; Henningson, Dan

1996-11-01

64

Numerical simulations of transonic wingtip

NASA Astrophysics Data System (ADS)

This report presents numerical simulations using ANSYS FLUENT for a NACA 0012 wing both in two- and three-dimensional cases to compare with an experimental data. This comparison also allowed for any wall interference in wind tunnels to be examined. Moreover, the report also presents numerical simulation of a NACA 0012 wingtip. The simulations were conducted at Mach 0.5, 0.7, 0.75, and 0.8 at various chord Reynolds number and the angle of attack of 0, 2, 4, and 5. The numerical and experimental data were in good agreement for the full wing and the wingtip cases.

Chanrith, Suparat

65

DNS of Decelerating Turbulent Boundary Layers

NASA Astrophysics Data System (ADS)

We conduct Direct Numerical Simulation (DNS) of turbulent plane-channel flow subjected to bulk deceleration and to a uniform strain field consisting of streamwise compression (partial U/partial x < 0) and wall-normal stretching (partial V/partial y > 0). This creates a time-developing flow containing most of the essential physics of spatially developing APG boundary layers, particularly in the outer layer. The logistics are much more favorable than those of a true spatial case, both for the DNS and for the testing of turbulence models. The computations are performed at two Reynolds numbers, with initial Re_? = 180 and 395, and advanced past the point when the deceleration causes the mean skin friction to change sign. This parallel-flow analog of ``separation'' isolates outer-layer features of separated boundary layers that are due to mean strain and vanishing surface stress from those caused by streamline curvature. Files are available to allow comparisons at the level of skin friction, velocity and stress profiles, and Reynolds-stress budgets. Changes in the velocity--pressure-gradient term ?_ij are found to dominate the initial evolution of the flow.

Coleman, G. N.; Kim, J.; Spalart, P. R.

1998-11-01

66

Direct numerical simulations (DNSs) are performed in order to study acoustic emissions generated during the transition of isothermal and non-isothermal mixing layers. The sound from temporally evolving mixing layers is computed directly using DNS for a computational domain, which includes both aerodynamic and acoustic fields. Good precision of the computed acoustic field is ensured by using a numerical code based

Véronique Fortuné; Éric Lamballais; Yves Gervais

2004-01-01

67

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

68

High-accuracy DNS of supersonic base flows and control of the near wake

Large-scale numerical simulations of axisymmetric, supersonic base flows were conducted at various Reynolds numbers. Direct numerical simulations (DNS) were employed to investigate the hydrodynamic stability behavior of the near-wake region. As a consequence of physical flow instabilities, large coherent structures evolve that have a significant impact on the mean flow wand and are responsible for a considerable amount of base-drag.

Richard D. Sandberg; Hermann F. Fasel

2004-01-01

69

Direct numerical simulation of leading edge receptivity to sound

NASA Astrophysics Data System (ADS)

Numerical simulations of leading-edge acoustic receptivity are performed for a flat plate with a modified-super-elliptic (MSE) leading edge. For small freestream amplitude, the agreement between Branch I receptivity coefficients predicted from the DNS and the experiments of Saric and White (AIAA-98-2645, 1998) for acoustic waves at zero incidence is excellent. The effect of angle of incidence of the impinging wave is investigated and found to produce higher receptivity coefficients than in the symmetric case. The slope of leading-edge receptivity coefficient versus angle of incidence of the impinging wave is found to be less than 1/4 of the slope predicted by zero-thickness flat-plate theory. However, there is excellent agreement between the DNS and finite-nose-radius theory of Hammerton and Kerschen (J. Fluid Mech. 310, 243-267, 1996). These results clearly demonstrate the importance of including the effects of the finite nose radius in any receptivity study. Finally, downstream of the leading-edge region, linear stability theory is found to accurately reproduce the characteristics of the instability waves. At higher freestream forcing, an instability wave generated by nonlinear interaction is found at double the frequency of the forcing.

Reed, H. L.; Fuciarelli, D. A.; Lyttle, I. J.

1998-11-01

70

Numerical Simulation of Gravity Waves

We present a new spectral method to simulate numerically the waterwave problem in a channel for a fluid of finite or infinite depth. It is formulated in terms of the free surface elevation eta and the velocity potential varphi. The numerical method is based on the reduction of this problem to a lower-dimensional computation involving surface variables alone. To accomplish

W. Craig; C. Sulem

1993-01-01

71

Numerical simulation of turbulent flows

The paper surveys recent progress in numerical simulation of turbulent flows involving solution of the Navier-Stokes equations for incompressible flow in the form of truncated Fourier series with periodic boundary conditions (spectral methods). Types of expansion functions for different types of boundary conditions are suggested. Isotropic turbulence simulations are examined, in which averages were obtained as arithmetic means over bands

S. A. Orszag

1977-01-01

72

Three dimensional, fully compressible direct numerical simulations (DNS) of premixed turbulent flames are carried out in a\\u000a V-flame configuration. The governing equations and the numerical implementation are described in detail, including modifications\\u000a made to the Navier–Stokes Characteristic Boundary Conditions (NSCBC) to accommodate the steep transverse velocity and composition\\u000a gradients generated when the flame crosses the boundary. Three cases, at turbulence

Thomas D. Dunstan; Nedunchezhian Swaminathan; Ken N. C. Bray; R. Stewart Cant

73

A Numerical Study of Continuous Data Assimilation for the 2D-NS Equations Using Nodal Points

NASA Astrophysics Data System (ADS)

This thesis conducts a number of numerical experiments using massively parallel GPU computations to study a new continuous data assimilation algorithm. We test the algorithm on two-dimensional incompressible fluid flows given by the Navier--Stokes equations. In this context, observations of the Eulerian velocity field given at a finite resolution of nodal points in space may be used to recover the exact velocity field over time. We also consider nodal measurements of the vorticity field and stream function. The main difference between this new algorithm and previous continuous data assimilation methods is the inclusion of a relaxation parameter micro that controls the rate at which the approximate solution is forced toward the observational measurements. If micro is too small, the approximate solution obtained by data assimilation may not converge to the reference solution; however, if micro is too large then high frequency spill-over from the observations may contaminate the approximate solution. Our focus is on the resolution of the nodal points necessary for the algorithm to recover the exact velocity field and how best to choose the parameter micro.

Gesho, Masakazu

74

Terascale direct numerical simulations of turbulent combustion using S3D

Computational science is paramount to the understanding of underlying processes in internal combustion engines of the future that will utilize non-petroleum-based alternative fuels, including carbon-neutral biofuels, and burn in new combustion regimes that will attain high efficiency while minimizing emissions of particulates and nitrogen oxides. Next-generation engines will likely operate at higher pressures, with greater amounts of dilution and utilize alternative fuels that exhibit a wide range of chemical and physical properties. Therefore, there is a significant role for high-fidelity simulations, direct numerical simulations (DNS), specifically designed to capture key turbulence-chemistry interactions in these relatively uncharted combustion regimes, and in particular, that can discriminate the effects of differences in fuel properties. In DNS, all of the relevant turbulence and flame scales are resolved numerically using high-order accurate numerical algorithms. As a consequence terascale DNS are computationally intensive, require massive amounts of computing power and generate tens of terabytes of data. Recent results from terascale DNS of turbulent flames are presented here, illustrating its role in elucidating flame stabilization mechanisms in a lifted turbulent hydrogen/air jet flame in a hot air coflow, and the flame structure of a fuel-lean turbulent premixed jet flame. Computing at this scale requires close collaborations between computer and combustion scientists to provide optimized scaleable algorithms and software for terascale simulations, efficient collective parallel I/O, tools for volume visualization of multiscale, multivariate data and automating the combustion workflow. The enabling computer science, applied to combustion science, is also required in many other terascale physics and engineering simulations. In particular, performance monitoring is used to identify the performance of key kernels in the DNS code, S3D and especially memory intensive loops in the code. Through the careful application of loop transformations, data reuse in cache is exploited thereby reducing memory bandwidth needs, and hence, improving S3D's nodal performance. To enhance collective parallel I/O in S3D, an MPI-I/O caching design is used to construct a two-stage write-behind method for improving the performance of write-only operations. The simulations generate tens of terabytes of data requiring analysis. Interactive exploration of the simulation data is enabled by multivariate time-varying volume visualization. The visualization highlights spatial and temporal correlations between multiple reactive scalar fields using an intuitive user interface based on parallel coordinates and time histogram. Finally, an automated combustion workflow is designed using Kepler to manage large-scale data movement, data morphing, and archival and to provide a graphical display of run-time diagnostics.

Chen, Jackie [Sandia National Laboratories (SNL); Klasky, Scott A [ORNL; Hawkes, Evatt R [Sandia National Laboratories (SNL); Sankaran, Ramanan [ORNL; Choudhary, Alok [Northwestern University, Evanston; Yoo, Chun S [Sandia National Laboratories (SNL); Liao, Wei-keng [Northwestern University, Evanston; Podhorszki, Norbert [ORNL

2009-01-01

75

Numerical simulation of Mach reflections

NASA Astrophysics Data System (ADS)

In this paper, the “FLIC” difference method with triangular mesh is adopted to numerically simulate the regular and Mach reflections that occur when a shock wave pass around a wedge. The compuational result is compared with the shock tube experimental results of G. Ben-Dor and I. I. Glass. The comparison shows that the position, shape of shock wave and height of Mach stem all show a good agreement. Consequently, the “FLIC” difference method with triangular mesh is quite satisfactory in numerical simulation of the regular and Mach reflections.

Yinfan, Li; Deliang, Zhang; Yiming, Cao

1985-06-01

76

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

77

NASA Astrophysics Data System (ADS)

High resolution direct numerical simulations (DNS) 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 mesh performance in capturing the range of dynamics. FE models scale well over many thousands of processors and do not impose restrictions on domain shape, but they are computationally expensive. Use of discontinuous discretisations and adaptive unstructured meshing technologies, which reduce the required element count by approximately two orders of magnitude, results in high resolution DNS models of turbidity currents at a fraction of the cost of traditional FE models. The benefits of this technique will enable simulation of turbidity currents in complex and large domains where DNS modelling was previously unachievable.

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

2014-05-01

78

Detailed Comparison of DNS to PSE for Oblique Breakdown at Mach 3

NASA Technical Reports Server (NTRS)

A pair of oblique waves at low amplitudes is introduced in a supersonic flat-plate boundary layer. Their downstream development and the concomitant process of laminar to turbulent transition is then investigated numerically using Direct Numerical Simulations (DNS) and Parabolized Stability Equations (PSE). This abstract is the last part of an extensive study of the complete transition process initiated by oblique breakdown at Mach 3. In contrast to the previous simulations, the symmetry condition in the spanwise direction is removed for the simulation presented in this abstract. By removing the symmetry condition, we are able to confirm that the flow is indeed symmetric over the entire computational domain. Asymmetric modes grow in the streamwise direction but reach only small amplitude values at the outflow. Furthermore, this abstract discusses new time-averaged data from our previous simulation CASE 3 and compares PSE data obtained from NASA's LASTRAC code to DNS results.

Mayer, Christian S. J.; Fasel, Hermann F.; Choudhari, Meelan; Chang, Chau-Lyan

2010-01-01

79

Multiscale Problems in Fluid and MHD: Combining direct numerical simulations and models

NASA Astrophysics Data System (ADS)

Geophysical flows, with a huge number of interacting scales, cannot be studied with direct numerical simulations (DNS) without proper modeling. In this context, DNS, Lagragian-averaged (LAMHD) and Large-Eddy Simulations (LES) runs of magnetohydrodynamics are presented. The models allow for a significant reduction of computer resources at given Reynolds numbers; e.g., with LAMHD, one reproduces the growth rate of magnetic energy and captures the saturation level of the dynamo instability. Combining DNS, LAMHD and LES, low magnetic Prandlt number dynamos have then been explored. Several forcing, from Beltrami to fully non-helical, are used and give similar though not identical results. In the case of the Taylor-Green vortex with a well defined structure at large scales and strong turbulent fluctuations, dynamos are observed down to the lowest PM=0.01 that can be modeled accurately; the critical magnetic Reynolds number increases sharply with PM as turbulence sets in and then saturates; in the linear phase, the most unstable magnetic modes move to small scales as PM is decreased; a Kazantsev 3/2 spectrum develops with strong non-local nonlinear transfer.

Pouquet, Annick; Montgomery, David

2005-10-01

80

Direct Numerical Simulations of Boiling

NASA Astrophysics Data System (ADS)

For flow problem of practical interest it is frequently necessary to account for phase change between liquid and vapor. Boiling, in particular, is one of the most efficient ways of removing heat from a solid surface. It is therefore commonly used in energy generation and refrigeration. The large volume change and the high temperatures involved can make the consequences of design or operational errors catastrophic and accurate predictions are highly desirable. For numerical simulations of boiling it is necessary to solve the energy equation, in addition to conservation equations for mass and momentum, and to account for the release/absorption of latent heat at the phase boundary. We describe a numerical method for direct numerical simulations of boiling and show results from simulations of explosive boiling of a vapor bubble in an initially superheated liquids. As the vapor bubble grows, its surface becomes unstable, developing wrinkles that increase the surface area significantly. The increased surface area does, however, have relatively little impact on the growth rate for the parameters examined due to a relatively thick thermal boundary layer. We have also examined film boiling and find relatively good agreement with experimental correlations. Research supported by NASA.

Tryggvason, Gretar; Esmaeeli, Asghar

2003-11-01

81

Laboratory experiments and direct numerical simulations (DNS) of passive scalar contaminant dispersal in bounded shear flows have been carrried out. Both mass and heat transport have been experimentally studied. Statistical results for the temperature plume which develops from a line heat source at the wall are compared to the DNS results. The DNS results for this case and for the case of a uniform source with constant temperature boundaries are also compared to various model predictions.

Wallace, J.M.; Bernard, P.S.; Balint, J.L.; Ong, L.

1992-12-31

82

Laboratory experiments and direct numerical simulations (DNS) of passive scalar contaminant dispersal in bounded shear flows have been carrried out. Both mass and heat transport have been experimentally studied. Statistical results for the temperature plume which develops from a line heat source at the wall are compared to the DNS results. The DNS results for this case and for the case of a uniform source with constant temperature boundaries are also compared to various model predictions.

Wallace, J.M.; Bernard, P.S.; Balint, J.L.; Ong, L.

1992-01-01

83

Numerical simulation of wave collapses

NASA Astrophysics Data System (ADS)

This paper deals with the results of numerical simulation of two problems in wave collapse theory. First, employing a two-dimensional model an attempt is made to investigate the influence of the self-focusing instability development on the formation of a multifocus structure during the self-focusing of a high-power quasi-optical wave beam (having much higher power than the critical self-focusing one, p ? pcr). Second, it is demonstrated that the dissipative stabilization of the transverse instability of a Langmuir soliton is possible within the frames of the vector equations taking into account the electric field vortex component generation during a Langmuir collapse.

Litvak, A. G.; Petrova, T. A.; Fraiman, G. M.; Sher, E. M.; Yunakovsky, A. D.

1991-09-01

84

Numerical simulation of active separation control by a synthetic jet

NASA Astrophysics Data System (ADS)

Direct numerical simulation (DNS) and large-eddy simulation (LES) are carried out to investigate the frequency effect of zero-net-mass-flux forcing (synthetic jet) on a generic separated flow. The selected test case is a rounded ramp at a Reynolds number based on the step height of 28 275. The incoming boundary layer is fully turbulent with R_theta {=} 1410. The whole flow in the synthetic jet cavity is computed to ensure an accurate description of the actuator effect on the flow field. In a first step, DNS is used to validate LES of this particular flow. In a second step, the effect of a synthetic jet at two reduced frequencies of 0.5 and 4 (based on the separation length of the uncontrolled case and the free-stream velocity) is investigated using LES. It is demonstrated that, with a proper choice of the oscillating frequency, separation can be drastically reduced for a velocity ratio between the jet and the flow lower than one. The low frequency is close to the natural vortex shedding frequency. Two different modes of the synthetic jet have been identified. A vorticity-dominated mode is observed in the low-frequency forcing case for which the separation length is reduced by 54%, while an acoustic-dominated mode is identified in the high-frequency forcing case for which the separation length is increased by 43%. The decrease of the separation length in the low-frequency forcing case is correlated with an increase of the turbulent kinetic energy level and consequently with an increase of the entrainment in the separated zone. A linear inviscid stability analysis shows that the increase of the separation length in the high-frequency forcing case is due to a modification of the mean velocity profile suggested by Stanek and coworkers. The result is a lower amplification of the perturbations and consequently, a lower entrainment into the mixing layer. To our knowledge, it is the first time that Stanek's hypothesis has been assessed, thanks to numerical simulations of fully turbulent flow.

Dandois, Julien; Garnier, Eric; Sagaut, Pierre

85

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

86

DNS of Turbulent Natural Convection Flows on the MareNostrum supercomputer

A code for the direct numerical simulation (DNS) of incompressible turbulent flows that provides a fairly good scalability\\u000a for a wide range of computer architectures has been developed. The spatial discretization of the incompressible Navier-Stokes\\u000a equations is carried out using a fourth-order symmetry-preserving discretization. Since the code is fully explicit, from a\\u000a parallel point of view, the main bottleneck is

F. X. Trias; A. Gorobets; M. Soria; A. Oliva

87

NASA Astrophysics Data System (ADS)

In a recent study we showed that the two-fluid (TF) formulation can be used in the direct numerical simulation (DNS) of bubble- (or particle-) laden decaying isotropic turbulence with considerable saving in CPU-time and memory as compared to the trajectory approach employed by many researchers. In the present paper, we develop a Lagrangian-Eulerian mapping (LEM) solver for DNS of bubble-laden turbulent shear flows using TF. The purpose of LEM is to resolve the large gradients of bubble velocity and concentration which result from the absence of the diffusion terms in the equations of bubble-phase motion and the preferential accumulation of bubbles. A standard finite-difference scheme (FDS) fails to resolve these gradients. We examine the performance of the new method in DNS of a bubble-laden Taylor-Green vortex, spatially developing plane mixing layer, and homogeneous shear turbulent flow.

Druzhinin, O. A.; Elghobashi, S. E.

1999-09-01

88

DNS of Shock / Isotropic Turbulence Interaction

NASA Astrophysics Data System (ADS)

We discuss DNS of Shock / Isotropic Turbulence Interactions (SITI). We vary the incoming turbulence Mach number up to 0.8 and the convective Mach number up to 5 in order to determine their effects on the interaction. These cases are challenging due to the presence of shocklets in the incoming turbulence as well as significant motion of the main shock. Shock-capturing must be used at all points while still maintaining low enough numerical dissipation to preserve the turbulent fluctuations. We use the linearly- and nonlinearly-optimized Weighted Essentially Non-Oscillatory (WENO) method[1,2]. Particular attention is paid to the inflow boundary condition, where we find the use of snapshots of "frozen" turbulence from decaying isotropic box simulations to be unsatisfactory. We instead use time-varying inflow data generated by a separate forced isotropic turbulence simulation with a specified convection speed. This allows us to access flow conditions where the assumptions of Taylor's Hypothesis are not met. 1.) Mart'in, M.P., Taylor, E.M., Wu, M., and Weirs, V.G., JCP 220(1) 270-89, 2006. 2.) Taylor, E.M., Wu, M., and Mart'in, M.P., JCP 223(1) 384-97, 2007.

Grube, Nathan; Taylor, Ellen; Martín, Pino

2010-11-01

89

Compressible Turbulent Channel Flows: DNS Results and Modeling

NASA Technical Reports Server (NTRS)

The present paper addresses some topical issues in modeling compressible turbulent shear flows. The work is based on direct numerical simulation of two supersonic fully developed channel flows between very cold isothermal walls. Detailed decomposition and analysis of terms appearing in the momentum and energy equations are presented. The simulation results are used to provide insights into differences between conventional time-and Favre-averaging of the mean-flow and turbulent quantities. Study of the turbulence energy budget for the two cases shows that the compressibility effects due to turbulent density and pressure fluctuations are insignificant. In particular, the dilatational dissipation and the mean product of the pressure and dilatation fluctuations are very small, contrary to the results of simulations for sheared homogeneous compressible turbulence and to recent proposals for models for general compressible turbulent flows. This provides a possible explanation of why the Van Driest density-weighted transformation is so successful in correlating compressible boundary layer data. Finally, it is found that the DNS data do not support the strong Reynolds analogy. A more general representation of the analogy is analysed and shown to match the DNS data very well.

Huang, P. G.; Coleman, G. N.; Bradshaw, P.; Rai, Man Mohan (Technical Monitor)

1994-01-01

90

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

91

Direct numerical simulations of homogeneous turbulence subject to periodic shear

NASA Astrophysics Data System (ADS)

We perform direct numerical simulations (DNS) of homogeneous turbulence subject to periodic shear S {=} S_{scriptsizemax} sin (omega t), where omega is the forcing frequency and S_{scriptsizemax} is the maximum shear. The lattice Boltzmann method (LBM) is employed in our simulations and a periodic body force is introduced to produce the required shear. We find that the turbulence behaviour is a strong function of the forcing frequency. There exists a critical frequency omega_{cr}/S_{scriptsizemax} {?} 0.5 at which the observed behaviour bifurcates. At lower forcing frequencies (omega {<} omega_{cr}), turbulence is sustained and the kinetic energy grows. At higher frequencies, the kinetic energy decays. It is shown that the phase difference between the applied strain and the Reynolds stress decreases monotonically from pi in the constant shear case to pi/2 in very high frequency shear cases. As a result, the net turbulence production per cycle decreases with increasing frequency. In fact, at omega/S_{scriptsizemax} {?} 10, decaying isotropic turbulence results are recovered. The frequency-dependence of anisotropy and Reynolds stress budget are also investigated in detail. It is shown that inviscid rapid distortion theory (RDT) does not capture the observed features: it predicts purely oscillatory behaviour at all forcing frequencies. Second moment closure models do predict growth at low frequencies and decay at high frequencies, but the critical frequency value is underestimated. The challenges posed by this flow to turbulence closure modelling are identified.

Yu, Dazhi; Girimaji, Sharath S.

2006-11-01

92

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

93

Numerical simulation of glottal flow.

In cases of permanent immobility of both vocal folds patients have difficulties with breathing but rarely with voicing. However, clinical experience shows that the shape of the larynx (voice box) seems to have a significant influence on the degree of airflow and breathing pattern. In order to find an optimal geometry of the larynx in terms of easiness for breathing after the surgical change of vocal folds or false vocal cords (ventricular folds), a set of numerical simulations of glottal flow for weakly compressible Navier-Stokes equations has been performed. We compare airflow resistance and volumetric flow rate for several geometry concepts for inspiration as well as expiration. Finally, we discuss the optimal geometry with respect to the quality of breathing. PMID:24290934

Hundertmark-Zaušková, A; Lehmann, R; Hess, M; Müller, F

2013-12-01

94

Drag reduction by polymer additives in a turbulent pipe flow: numerical and laboratory experiments

In order to study the roles of stress anisotropy and of elasticity in the mechanism of drag reduction by polymer additives we investigate a turbulent pipe flow of a dilute polymer solution. The investigation is carried out by means of direct numerical simulation (DNS) and laser Doppler velocimetry (LDV). In our DNS two dierent models are used to describe the

M. A. H ULSEN; F. T. M. N IEUWSTADT; J. M. Burgers

1997-01-01

95

The performance of second-order conditional moment closure (CMC) depends on models to evaluate conditional variances and covariances of temperature and species mass fractions. In this paper the closure schemes based on the steady laminar flamelet model (SLFM) are validated against direct numerical simulation (DNS) involving extinction and ignition. Scaling is performed to reproduce proper absolute magnitudes, irrespective of the origin of mismatch between local flamelet structures and scalar dissipation rates. DNS based on the pseudospectral method is carried out to study hydrogen-air combustion with a detailed kinetic mechanism, in homogeneous, isotropic, and decaying turbulent media. Lewis numbers are set equal to unity to avoid complication of differential diffusion. The SLFM-based closures for correlations among fluctuations of reaction rate, scalar dissipation rate, and species mass fractions show good comparison with DNS. The variance parameter in lognormal PDF and the constants in the dissipation term have been estimated from DNS results. Comparison is made for the resulting conditional profiles from DNS, first-order CMC, and second-order CMC with correction to the most critical reaction step according to sensitivity analysis. Overall good agreement ensures validity of the SLFM-based closures for modeling conditional variances and covariances in second-order CMC.

Sreedhara, S.; Huh, Kang Y. [Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784 (Korea, Republic of)

2005-12-01

96

Simulating reionization in numerical cosmology

NASA Astrophysics Data System (ADS)

The incorporation of radiative transfer effects into cosmological hydrodynamical simulations is essential for understanding how the intergalactic medium (IGM) makes the transition from a neutral medium to one that is almost fully ionized. I present an approximate numerical method designed to study in a statistical sense how a cosmological density field is ionized by various sets of sources. The method requires relatively few time steps and can be employed with simulations of high resolution. First, I explore the reionization history of Helium II by z < 6 quasars. Comparisons between HeII opacities measured observationally and inferred from our analysis reveal that the uncertainties in the empirical luminosity function provide enough leeway to provide a satisfactory match. A property common to all the calculations is that the epoch of Helium II reionization must have occurred between 3?

Sokasian, Aaron

2003-11-01

97

NASA Astrophysics Data System (ADS)

Turbulence structure in an open-channel flow with a zero-shear gas-liquid interface was numerically investigated by a three-dimensional direct numerical simulation (DNS) based on a fifth-order finite-difference formulation, and the relationship between scalar transfer across a zero-shear gas-liquid interface and organized motion near the interface was discussed. The numerical predictions of turbulence quantities were also compared with the measurements by means of a two-color laser Doppler velocimeter. The results by the DNS show that the vertical motion is restrained in the interfacial region and there the turbulence energy is redistributed from the vertical direction to the streamwise and spanwise directions through the pressure fluctuation. The large-scale eddies are generated by bursting phenomena in the wall region and they are lifted up toward the interfacial region. Then, the eddies renew the interface and promote the scalar transfer across the gas-liquid interface. Both the damping effect and the generation process of the surface-renewal motions predicted by the DNS explain well the experimental results deduced in previously published studies. Furthermore, the predicted bursting frequency and mass transfer coefficient are in good agreement with the measurements.

Komori, Satoru; Nagaosa, Ryuichi; Murakami, Yasuhiro; Chiba, Satoshi; Ishii, Katsuya; Kuwahara, Kunio

1993-01-01

98

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

99

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

100

NASA Astrophysics Data System (ADS)

The turbulent channel flow with streamwise rotation has been investigated by means of several different analytical, numerical, and modelling approaches. Lie group analysis of the two-point correlation equations led to linear scaling laws for the streamwise mean velocity. In addition it was found that a cross-flow in the spanwise direction is induced, which may also exhibit a linear region. By further analysis of the two-point correlation equation, it is shown that all six components of the Reynolds stress tensor are non-zero. In addition certain symmetries and skew-symmetries about the centreline have been established for all flow quantities. All these findings of the analysis have been verified very well by means of direct numerical simulations (DNS). The flow has also been calculated with large-eddy simulations (LES) and second-moment closure models. The dynamic LES captured most of the theoretical and DNS findings quantitatively. Except for one stress component the second-moment closure model was able to capture most of the basic trends, but no quantitative agreement could be achieved.

Oberlack, M.; Cabot, W.; Pettersson Reif, B. A.; Weller, T.

2006-09-01

101

Characteristics of a Numerical Fluid Dynamics Simulator.

National Technical Information Service (NTIS)

John von Neumann envisioned scientists and mathematicians analyzing and controlling their numerical experiments on nonlinear dynamic systems interactively. We describe our concept of a real-time Numerical Fluid Dynamics Simulator NFDS, and derive its char...

K. H. A. Winkler M. L. Norman J. L. Norton

1985-01-01

102

Numerical Simulations of the Slingatron

NASA Technical Reports Server (NTRS)

The slingatron mass accelerator is described for several track configurations (shapes), and numerical simulations of this accelerating mass traversing a given track configuration are presented. The sled is modeled as a point mass that interacts with the slingatron track using both a conventional and a new empirical velocity dependent friction law. The closed loop circular slingatron was found to produce high maximum sled velocities provided the gyration angular speed is always increasing. In contrast several spiral shaped slingatron tracks reveal that high maximum sled velocities are obtainable with the gyration speed held constant. In fact, a slingatron constructed out of semi-circles is shown capable of generating high velocity sleds in such a way that no initial sled injection is necessary. Choosing the proper initial gyration phase with an empirically determined friction model allows the mass sled to gain ever-increasing velocities when placed in a semi-circle slingatron. The sled bearing pressure and its total acceleration are examined and presented.

Cooper, Gene R.; Tidman, Derek A.; Bundy, Mark L.; Wilkerson, Stephen

2001-01-01

103

Inflow and initial conditions for direct numerical simulation based on adjoint data assimilation

NASA Astrophysics Data System (ADS)

A method for generating inflow conditions for direct numerical simulations (DNS) of spatially-developing flows is presented. The proposed method is based on variational data assimilation and adjoint-based optimization. The estimation is conducted through an iterative process involving a forward integration of a given dynamical model followed by a backward integration of an adjoint system defined by the adjoint of the discrete scheme associated to the dynamical system. The approach's robustness is evaluated on two synthetic velocity field sequences provided by numerical simulation of a mixing layer and a wake flow behind a cylinder. The performance of the technique is also illustrated in a real world application by using noisy large scale PIV measurements. This method denoises experimental velocity fields and reconstructs a continuous trajectory of motion fields from discrete and unstable measurements.

Gronskis, A.; Heitz, D.; Mémin, E.

2013-06-01

104

The multiscale formulation of large eddy simulation: Decay of homogeneous isotropic turbulence

The variational multiscale method is applied to the large eddy simulation (LES) of homogeneous, isotropic flows and compared with the classical Smagorinsky model, the dynamic Smagorinsky model, and direct numerical simulation (DNS) data. Overall, the multiscale method is in better agreement with the DNS data than both the Smagorinsky model and the dynamic Smagorinsky model. The results are somewhat remarkable

Thomas J. R. Hughes; Luca Mazzei; Assad A. Oberai; Alan A. Wray

2001-01-01

105

Numerical simulation of direct injection gasoline engine model

Direct Injection Gasoline different numerical simulation models were analyzed. There are three kinds of the numerical simulation of turbulent: direct numerical simulation, large eddy simulation and Reynolds averaged simulation. Direct numerical simulation of turbulence modeling without using the numerical calculation solves the flow equations directly. Large eddy simulation of the main idea is: use of large-scale direct numerical solution of

Yongfeng Liu; Pucheng Pei; Yong Lu

2011-01-01

106

NASA Technical Reports Server (NTRS)

Analysis of Direct Numerical Simulations (DNS) transitional states of temporal, supercritical mixing layers for C7H16/N2 and O2/H2 shows that the evolution of all layers is characterized by the formation of high-density-gradient magnitude (HDGM) regions.

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

2003-01-01

107

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

108

Resolution requirements for numerical simulations of transition

NASA Technical Reports Server (NTRS)

The resolution requirements for direct numerical simulations of transition to turbulence are investigated. A reliable resolution criterion is determined from the results of several detailed simulations of channel and boundary-layer transition.

Zang, Thomas A.; Krist, Steven E.; Hussaini, M. Yousuff

1989-01-01

109

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

110

NASA Astrophysics Data System (ADS)

A direct numerical simulation (DNS) has been performed of turbulent channel flow over a three-dimensional Cartesian grid of 30×20×9 cubes in, respectively, the streamwise, spanwise, and wall-normal direction. The grid of cubes mimics a permeable wall with a porosity of 0.875. The flow field is resolved with 600×400×400 mesh points. To enforce the no-slip and no-penetration conditions on the cubes, an immersed boundary method is used. The results of the DNS are compared with a second DNS in which a continuum approach is used to model the flow through the grid of cubes. The continuum approach is based on the volume-averaged Navier-Stokes (VANS) equations [S. Whitaker, ``The Forchheimer equation: a theoretical development,'' Transp. Porous Media 25, 27 (1996)] for the volume-averaged flow field. This method has the advantage that it requires less computational power than the direct simulation of the flow through the grid of cubes. More in general, for complex porous media one is usually forced to use the VANS equations, because a direct simulation would not be possible with present-day computer facilities. A disadvantage of the continuum approach is that in order to solve the VANS equations, closures are needed for the drag force and the subfilter-scale stress. For porous media, the latter can often be neglected. In the present work, a relation for the drag force is adopted based on the Irmay [``Modèles théoriques d'écoulement dans les corps poreux,'' Bulletin Rilem 29, 37 (1965)] and the Burke-Plummer model [R. B. Bird, W. E. Stewart, and E. N. Lightfoot, Transport Phenomena (Wiley, New York, 2002)], with the model coefficients determined from simulations reported by W. P. Breugem, B. J. Boersma, and R. E. Uittenbogaard [``Direct numerical simulation of plane channel flow over a 3D Cartesian grid of cubes,'' Proceedings of the Second International Conference on Applications of Porous Media, edited by A. H. Reis and A. F. Miguel (Évora Geophysics Center, Évora, 2004), p. 27]. The results of the DNS with the grid of cubes and the second DNS in which the continuum approach is used, agree very well.

Breugem, W. P.; Boersma, B. J.

2005-02-01

111

Predicting Drag-Reducing Wall Turbulence of Surfactant Solution by Direct Numerical Simulation

NASA Astrophysics Data System (ADS)

We performed direct numerical simulations (DNS) of turbulent channel flow of surfactant solution to investigate characteristics of modulated wall turbulence and turbulent drag reduction. In the simulations, the effect of surfactant solution was represented by the modified Bird-Carreau model for shear viscosity. Our simulations reproduced experimental features of turbulent flow of surfactant solution. One result was a drag reduction rate of 37%, which is in the range of low drag reduction. Considering instantaneous structures and turbulence statistics, our scale analysis found that modulation of turbulent flow of surfactant solution for the Newtonian fluid could be generally normalized with the local variable viscosity. Some additional corrections are, however, needed to apply this to predicting the dissipation rate, which the local viscosity affects directly in turbulent flow.

Ohta, Takashi; Usui, Yuto; Yasoshima, Hiroshi

112

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

113

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

114

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

115

Direct Numerical Simulation of Supersonic Jet Flow

A numerical method is given for direct numerical simulation of the nonlinear evolution of instability waves in supersonic round jets, with spatial discretisation based on high-order compact finite differences. The numerical properties of a class of symmetric and asymmetric schemes are analysed. Implementation for the Navier–Stokes equations in cylindrical polar coordinates is discussed with particular attention given to treatment of

K. H. Luo; N. D. Sandham

1997-01-01

116

Numerical Simulation of Unsteady Aerodynamic Models

NASA Technical Reports Server (NTRS)

This report documents the results of the numerical simulations of unsteady aerodynamic models. The results focus on numerical accuracy and efficiency, and the robustness of the numerical methods. The aerodynamic models includes the classical Wagner and Kussner functions and the Leishman-Beddoes dynamic stall model. The simulations includes the numerical approximations of the Duhamel's integrals using both indicial (step) and impulse responses, the numerical integrations of the state-space models, and the exact solutions. The report also presents the conversion among different model representations.

Nguyen, Khanh Q.; Warmbrodt, William (Technical Monitor)

1997-01-01

117

In this paper an efficient split-type Finite-Difference (FD) scheme with high modal resolution – most important for the streamwise convection terms that cause wave transport and interaction – is derived for a mixed Fourier-spectral\\/FD method that is designed for the spatial direct numerical simulation (DNS) of boundary-layer transition and turbulence. Using a relatively simple but thorough and instructive modal analysis

Markus J. Kloker

1997-01-01

118

GPU accelerated flow solver for direct numerical simulation of turbulent flows

NASA Astrophysics Data System (ADS)

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; Bernardini, Matteo; Botti, Michela

2013-02-01

119

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

120

Dynamo in the Taylor-Green vortex: Direct numerical simulations and modeling of MHD flows

NASA Astrophysics Data System (ADS)

Direct numerical simulations (DNS) and Lagragian-averaged model runs (LAMHD) of three-dimensional magnetohydrodynamic turbulence are presented. The model allows for a significant reduction of computer resources at given Reynolds numbers. It correctly reproduces the growth rate of magnetic energy and captures the nonlinear saturation level; intermittency is recovered as well. Low magnetic Prandlt number dynamos are then explored combining DNS, LAMHD and Large-Eddy Simulations. The flow is forced with a Taylor-Green non-helical vortex with a well-defined structure at large scales and strong turbulent fluctuations. Dynamos are observed down to the lowest PM=0.01 that can be modeled accurately for this flow; the critical magnetic Reynolds number increases sharply with PM as turbulence sets in and then saturates; in the linear phase, the most unstable magnetic modes move to small scales as PM is decreased; a Kazantsev 3/2 spectrum develops with strong non-local nonlinear transfer; then the dynamo grows at large scales and modifies the turbulent velocity fluctuations. Other forcing including Beltrami flows are found to behave in a similar fashion.

Pouquet, Annick; Mininni, Pablo; Montgomery, David; Pinton, Jean-Francois; Politano, Helene; Ponty, Yannick

2005-11-01

121

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

122

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

Peter Carlsson; Mats Tinnsten; Mats Ainegren

2011-01-01

123

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

124

Numerical Simulation of Gluey Particles

We propose here a model and a numerical scheme to compute the motion of rigid particles interacting through the lubrication force. In the case of a particle approaching a plane, we propose an algorithm and prove its convergence towards the solutions to the gluey particle model proposed by B. Maury. We propose a multi-particle version of this gluey model which

Aline Lefebvre

2008-01-01

125

Numerical simulation of conservation laws

NASA Technical Reports Server (NTRS)

A new numerical framework for solving conservation laws is being developed. This new approach differs substantially from the well established methods, i.e., finite difference, finite volume, finite element and spectral methods, in both concept and methodology. The key features of the current scheme include: (1) direct discretization of the integral forms of conservation laws, (2) treating space and time on the same footing, (3) flux conservation in space and time, and (4) unified treatment of the convection and diffusion fluxes. The model equation considered in the initial study is the standard one dimensional unsteady constant-coefficient convection-diffusion equation. In a stability study, it is shown that the principal and spurious amplification factors of the current scheme, respectively, are structurally similar to those of the leapfrog/DuFort-Frankel scheme. As a result, the current scheme has no numerical diffusion in the special case of pure convection and is unconditionally stable in the special case of pure diffusion. Assuming smooth initial data, it will be shown theoretically and numerically that, by using an easily determined optimal time step, the accuracy of the current scheme may reach a level which is several orders of magnitude higher than that of the MacCormack scheme, with virtually identical operation count.

Chang, Sin-Chung; To, Wai-Ming

1992-01-01

126

NAL (National Aerospace Laboratory) Numerical Simulator System.

National Technical Information Service (NTIS)

The Numerical Simulator System installed at NAL Computational Sciences Division in February 1987 is working well for computational research activities. The background and the course of the system installation, the illustration of the system configuration ...

H. Miyoshi M. Fukuda

1987-01-01

127

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

128

The direct numerical simulation, extended to boundary - fitted coordinate, has been carried out for a fully-developed turbulent flow thermal hydraulics in a triangular rod bundle. The rod bundle is premised to be an infinite array. The spacer grid effects are ignored. The purpose of this work is to verify DNS methodology to be applied for deriving coefficients for inter-subchannel turbulent mixing and heat transfer on a rod. These coefficients are incorporated in subchannel analysis codes. To demonstrate the validity of this methodology, numerical calculation was performed for the bundle with the pitch to diameter ratio 1.2, at friction Reynolds number of 600 and Prandtl number of 1. The results for the hydraulic parameters are compared with published DNS data, and the results for the heat exchange coefficients -- with those obtained using semi-empirical correlations. (authors)

Yudov, Yury V. [Alexandrov Research Institute of Technology - NITI, Sosnovy Bor, Leningrad reg., 188537 (Russian Federation)

2006-07-01

129

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

130

NASA Astrophysics Data System (ADS)

We present a comprehensive analysis of transport processes associated with electrohydrodynamic chaos in electrokinetic systems containing an ion-selective surface. The system considered is an aqueous symmetric binary electrolyte between an ion-selective surface and a stationary reservoir. Transport is driven by an external electric field. Using direct numerical simulations (DNS) of the coupled Poisson-Nernst-Planck and Navier-Stokes equations in 2D we show significant transitions in flow behavior from coherent vortex pairs to fully chaotic multi-layer vortex structures with a broadband energy spectrum. Additionally, we demonstrate that these vortices can eject both positive and negative free charge density into the bulk of the domain and completely disrupt the structure of the traditionally described extended space charge region. The resulting dynamical behavior poses a challenge for traditional asymptotic modeling that relies on the quasi-electroneutral bulk assumption. Furthermore, we quantify for the first time the relative importance of energy dissipation due to viscous effects in various transport regimes. Finally, we present a framework for the development of ensemble-averaged models (similar to Reynolds Averaged Navier-Stokes equations) and assess the importance of the unclosed terms based on our DNS data.

Druzgalski, C. L.; Andersen, M. B.; Mani, A.

2013-11-01

131

Aerospace Numerical Simulation and Digital Prototyping Technologies

NASA Astrophysics Data System (ADS)

A High End Digital Prototyping system (HEDP) designed for aerospace numerical simulation is introduced in this paper. This system is a problem solving environment equipped with capability of parallel mesh generation, immersive visual steering, large-scale visualization and parallel computation. All enabling technologies are realized as separate modules and coupled through a software bus, which makes them integrated seamlessly. Detailed design principles and a numerical simulation of turbulent combustion in the HyShot Scramjet whitin the HEDP system is addressed.

Zheng, Yao; Xie, Lijun; Zou, Jianfeng; Chen, Jianjun; Zhang, Jifa

2010-05-01

132

NASA Astrophysics Data System (ADS)

Due to the progress in computer technology in recent years, distributed memory parallel computer systems are rapidly gaining importance in direct numerical simulation (DNS) of the stability and transition of compressible boundary layers. In most works, explicit methods have mainly been used in such simulations to advance the compressible Navier-Stokes equations in time. However, the small wall-normal grid sizes for viscous flow simulations impose severe stability restriction on the allowable time steps in simulations using explicit method. This requires implicit treatment to the numerical methods. Although fully implicit methods are often used in steady-flow calculations to remove the stability restriction on time steps, they are seldom used in transient flow simulations because the time steps used in time-accurate calculations are often not large enough to offset high computational cost of using fully implicit methods. In this thesis, we present an efficient high-order semi-implicit method, which only treats the stiff terms implicitly, for the DNS study the hypersonic boundary-layer receptivity to freestream disturbances over blunt bodies. It is shown that the semi-implicit method can meet the requirements for both computational efficiency and numerical accuracy in the DNS studies. However, we can not implement our semi-implicit method on single computer to solve unsteady Navier-Stokes equations for the direct numerical simulation of supersonic and hypersonic boundary layer flows on parallel computers directly. The semi-implicit algorithm has to be modified to achieve the communications among processors in solving the global block linear systems. In this thesis, a divide and conquer (DAC) method is used to parallelly solve the block linear system from the semi-implicit method. A parallel Fourier collocation method is also implemented in the periodic spanwise direction. It is shown that by implementing the new parallel semi-implicit scheme the simulations of compressible transient flow can benefit greatly from parallel computer systems by increasing both simulation sizes and speed while maintaining high temporal accuracy. To implement our new numerical methods on the numerical studies of compressible boundary layer stability and transitions, numerical simulations of the receptivity process of hypersonic boundary layer flows over 3-D blunt leading edges are chosen to be investigated because the receptivity phenomena are much more complex and currently not well understood. In this thesis, parametric simulations of receptivity freestream disturbances which includes fast acoustic waves, vorticity waves and entropy waves for Mach 15 flow over 3-D blunt leading edges have been carried out by using our new methods. The results show that initial transient growth generated and developed inside the hypersonic boundary layer near the leading edge can be observed in the receptivity of freestream standing vorticity or entropy waves, but not acoustic waves or traveling waves. It has been shown that this initial transient growth near the leading edge can be possibly explained by the transient growth theory. Additionally, cooling the surface will increase the growth. By adding inhomogeneous boundary conditions or random roughness on the surface can strongly increase the magnitude of growth.

Dong, Haibo

133

Numerical simulation of coextrusion and film casting

SUMMARY In the first part of this paper a numerical strategy is developed for the numerical simulation of the coextrusion process. Coextrusion consists of extruding many polymers in the same die in order to combine their respective properties. The die is generally flat and quite large and consequently a two-dimensional approximation is sufficient. The main difficulty is to accurately predict

A. Fortin; P. Carrier; Y. Demay

1995-01-01

134

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

135

Numerical Simulations of HH 555

NASA Astrophysics Data System (ADS)

We present three-dimensional (3D) gasdynamic simulations of the Herbig Haro object HH 555. HH 555 is a bipolar jet emerging from the tip of an elephant trunk entering the Pelican Nebula from the adjacent molecular cloud. Both beams of HH 555 are curved away from the center of the H II region. This indicates that they are being deflected by a sidewind probably coming from a star located inside the nebula or by the expansion of the nebula itself. HH 555 is most likely an irradiated jet emerging from a highly embedded protostar, which has not yet been detected. In our simulations we vary the incident photon flux, which in one of our models is equal to the flux coming from a star 1 pc away emitting 5×1048 ionizing (i.e., with energies above the H Lyman limit) photons per second. An external, plane-parallel flow (a ``sidewind'') is coming from the same direction as the photoionizing flux. We have made four simulations, decreasing the photon flux by a factor of 10 in each simulation. We discuss the properties of the flow, and we compute H? emission maps (integrated along lines of sight). We show that the level of the incident photon flux has an important influence on the shape and visibility of the jet. If the flux is very high, it causes a strong evaporation of the neutral clump, producing a photoevaporated wind traveling in the direction opposite to the incident flow. The interaction of the two flows creates a double shock ``working surface'' around the clump, protecting it and the jet from the external flow. The jet only starts to curve when it penetrates through the working surface.

Kajdi?, P.; Raga, A. C.

2007-12-01

136

Numerical simulation of vortex breakdown

NASA Technical Reports Server (NTRS)

The breakdown of an isolated axisymmetric vortex embedded in an unbounded uniform flow is examined by numerical integration of the complete Navier-Stokes equations for unsteady axisymmetric flow. Results show that if the vortex strength is small, the solution approaches a steady flow and the vortex is stable. If the strength is large enough, the solution remains unsteady and a recirculating zone will appear near the axis, its form and internal structure resembling those of the axisymmetric breakdown bubbles with multi-cells observed by Faler and Leibovich (1978). For apppropriate combinations of flow parameters, the flow reveals quasi-periodicity. Parallel calculations with the quasi-cylindrical approximation indicate that so far as predicting of breakdown is concerned, its results coincide quite well with the results mentioned above. Both show that the vortex breakdown has little concern with the Reynolds number or with the critical classification of the upstream flow, at least for the lower range of Reynolds numbers.

Shi, X.

1985-01-01

137

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

NASA Technical Reports Server (NTRS)

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.

1993-01-01

138

Numerical simulations of disordered superconductors

This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The authors carried out Monte Carlo studies of the critical behavior of superfluid {sup 4}He in aerogel. They found the superfluid density exponent increases in the presence of fractal disorder with a value roughly consistent with experimental results. They also addressed the localization of flux lines caused by splayed columnar pins. Using a Sine-Gordon-type of renormalization group study they obtained an analytic form for the critical temperature. They also determined the critical temperature from I-V characteristics obtained from a molecular dynamics simulation. The combined studies enabled one to construct the phase diagram as a function of interaction strength, temperature, and disorder. They also employed the recently developed mapping between boson world-lines and the flux motion to use quantum Monte Carlo simulations to analyze localization in the presence of disorder. From measurements of the transverse flux line wandering, they determined the critical ratio of columnar to point disorder strength needed to localize the bosons.

Bedell, K.S.; Gubernatis, J.E. [Los Alamos National Lab., NM (United States); Scalettar, R.T.; Zimanyi, G.T. [Univ. of California, Davis, CA (United States)

1997-12-01

139

Numerical Simulations of Drop Collisions

NASA Technical Reports Server (NTRS)

Three-dimensional simulations of the off-axis collisions of two drops are presented. The full Navier-Stokes equations are solved by a Front-Tracking/Finite-Difference method that allows a fully deformable fluid interface and the inclusion of surface tension. The drops are accelerated towards each other by a body force that is turned off before the drops collide. Depending on whether the interface between the drops is ruptured or not, the drops either bounce or coalesce. For drops that coalesce, the impact parameter, which measures how far the drops are off the symmetry line, determines the eventual outcome of the collision. For low impact parameters, the drops coalesce permanently, but for higher impact parameters, a grazing collision, where the drops coalesce and then stretch apart again is observed. The results are in agreement with experimental observations.

Nobari, M. R. H.; Tryggvason, G.

1994-01-01

140

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

141

Direct numerical simulations of mack-mode damping on porous coated cones

NASA Astrophysics Data System (ADS)

The flow field over a 3 degree blunt cone is investigated with respect to a hypersonic stability analysis of the boundary-layer flow at Mach 6 with porous as well as smooth walls by comparing local direct numerical simulations (DNS) and linear stability theory (LST) data. The original boundary-layer profile is generated by a finite volume solver, using shock capturing techniques to generate an axisymmetric flow field. Local boundary-layer profiles are extracted from this flow field and hypersonic Mack-modes are superimposed for cone-walls with and without a porous surface used as a passive transition-reduction device. Special care is taken of curvature effects of the wall on the mode development over smooth and porous walls.

Lüdeke, H.; Wartemann, V.

2013-06-01

142

Multistage Turbomachinery Flows Simulated Numerically

NASA Technical Reports Server (NTRS)

At the NASA Lewis Research Center, a comprehensive assessment was made of the predictive capability of the average passage flow model as applied to multistage axial-flow compressors. This model, which describes the time-averaged flow field within a typical passage of a blade row embedded in a multistage configuration, is being widely used throughout U.S. aircraft industry as an integral part of their design systems. Rotor flow-angle deviation. In this work, detailed data taken within a four and one-half stage large low-speed compressor were used to assess the weaknesses and strengths of the predictive capabilities of the average passage flow model. The low-speed compressor blading is of modern design and employs stator end-bends. Measurements were made with slow- and high response instrumentation. The high-response measurements revealed the velocity components of both the rotor and stator wakes. From the measured wake profiles, we found that the flow exiting the rotors deviated from the rotor exit metal angle to a lesser degree than was predicted by the average passage flow model. This was found to be due to blade boundary layer transition, which recently has been shown to exist on multistage axial compressor rotor and stator blades, but was not accounted for in the average passage model. Consequently, a model that mimics the effects of blade boundary layer transition, Shih k-epsilon model, was incorporated into the average passage model. Simulations that incorporated this transition model showed a dramatic improvement in agreement with data. The altered model thus improved predictive capability for multistage axial-flow compressors, and this was verified by detailed experimental measurement.

Hathaway, Michael D.; Adamczyk, John J.; Shabbir, Aamir; Wellborn, Steven R.

1999-01-01

143

DNS PseudoRandom Number Generators Weakness

\\u000a In article [1] we presented the results of analysis of well-known weakness of some DNS servers: poor quality of pseudo-random\\u000a numbers generators (PRNG), which makes it possible to hack them using the birthday attack. In this article we present extended\\u000a analysis of current DNS servers: DNS build in Windows 2003 server with SP2, DNS from Windows 2008 server with SP2 and

Maciej Szmit; Anna Szmit

144

Numerical simulations of strong incompressible magnetohydrodynamic turbulence

Magnetised plasma turbulence pervades the universe and is likely to play an important role in a variety of astrophysical settings. Magnetohydrodynamics (MHD) provides the simplest theoretical framework in which phenomenological models for the turbulent dynamics can be built. Numerical simulations of MHD turbulence are widely used to guide and test the theoretical predictions; however, simulating MHD turbulence and accurately measuring its scaling properties is far from straightforward. Computational power limits the calculations to moderate Reynolds numbers and often simplifying assumptions are made in order that a wider range of scales can be accessed. After describing the theoretical predictions and the numerical approaches that are often employed in studying strong incompressible MHD turbulence, we present the findings of a series of high-resolution direct numerical simulations. We discuss the effects that insufficiencies in the computational approach can have on the solution and its physical interpretation.

Mason, J.; Cattaneo, F. [Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637 (United States); Perez, J. C. [Space Science Center and Department of Physics, University of New Hampshire, Durham, New Hampshire 03824 (United States); Boldyrev, S. [Department of Physics, University of Wisconsin at Madison, 1150 University Ave, Madison, Wisconsin 53706 (United States)

2012-05-15

145

Study of Cardiac Defibrillation Through Numerical Simulations

NASA Astrophysics Data System (ADS)

Three-dimensional numerical simulations of the defibrillation problem are presented. In particular, in this study we use the rabbit ventricular geometry as a realistic model system for evaluating the efficacy of defibrillatory shocks. Statistical data obtained from the simulations were analyzed in term of a dose-response curve. Good quantitative agreement between our numerical results and clinically relevant values is obtained. An electric field strength of about 6.6 V/cm indicates a fifty percent probability of successful defibrillation for a 12-ms monophasic shock. Our validated model will be useful for optimizing defibrillation protocols.

Bragard, J.; Marin, S.; Cherry, E. M.; Fenton, F. H.

146

Numerical propulsion system simulation: An interdisciplinary approach

NASA Technical Reports Server (NTRS)

The tremendous progress being made in computational engineering and the rapid growth in computing power that is resulting from parallel processing now make it feasible to consider the use of computer simulations to gain insights into the complex interactions in aerospace propulsion systems and to evaluate new concepts early in the design process before a commitment to hardware is made. Described here is a NASA initiative to develop a Numerical Propulsion System Simulation (NPSS) capability.

Nichols, Lester D.; Chamis, Christos C.

1991-01-01

147

Numerical propulsion system simulation - An interdisciplinary approach

NASA Technical Reports Server (NTRS)

The tremendous progress being made in computational engineering and the rapid growth in computing power that is resulting from parallel processing now make it feasible to consider the use of computer simulations to gain insights into the complex interactions in aerospace propulsion systems and to evaluate new concepts early in the design process before a commitment to hardware is made. Described here is a NASA initiative to develop a Numerical Propulsion System Simulation (NPSS) capability.

Nichols, Lester D.; Chamis, Christos C.

1991-01-01

148

Simulation of Nonlinear Instabilities in an Attachment-Line Boundary Layer.

National Technical Information Service (NTIS)

The linear and the nonlinear stability of disturbances that propagate along the attachment line of a three-dimensional boundary layer is considered. The spatially evolving disturbances in the boundary layer are computed by direct numerical simulation (DNS...

R. D. Joslin

1996-01-01

149

Numerical Simulations of Solar Acoustic Field

NASA Astrophysics Data System (ADS)

We present numerical simulations of propagation of acoustic waves in the upper convection zone using a standard solar model and realistic equation of state (OPAL model). The main goals are to study properties of solar waves for various excitation sources and interaction of these waves with spatial inhomogeneities, and also to generate artificial wave fields for testing local helioseismic diagnostics of the solar interior, currently used for SOHO/MDI and GONG data. In our numerical model, non-reflecting boundary conditions based on absorbing 3D perfectly matched layer (PML) are imposed at all boundaries of the computational domain in Cartesian geometry. This prevents spurious reflection of acoustic waves from boundaries back to the computational domain. The top non-reflecting boundary is set in the solar atmosphere above the temperature minimum. This allowed us to realistically model the wave reflection from the solar atmosphere. We have developed a special PLM model, numerically stable in the case of a stratified medium with gravity, and investigated and tested various numerical schemes (including high-order dispersion-relation-preserving scheme). Numerical simulations have been carried out on parallel computers for different kinds of acoustic sources(force and energy sources). Single point sources are used to calculate realistic Green functions required for holographic seismic imaging. Simulated acoustic field from multiple sources randomly distributed below the photosphere is used as artificial data for testing helioseismic inversions, accuracy of Born and ray approximations.

Parchevsky, K. V.; Kosovichev, A. G.

2005-12-01

150

Digital elevation models in numerical rockfall simulations

NASA Astrophysics Data System (ADS)

The current state of the art in rigid body rockfall modelling permits full three-dimensional simulation of real rock shapes and their interactions with the terrain. The terrain is represented by digital elevation models DEM, providing the geometric terrain information on which the spatial model parameters are assigned. This is fundamental to numerical simulations of mass movements. DEM's can be obtained from a number of sources and offer spatial resolutions ranging from centimetres up to 90m. The spatial resolution representing the terrain morphology can have a strong bearing on modelling results. In particular if finer scale morphologies (centimetres to meters) such as the terrain roughness of a scree slope or boulder field are included in the DEM. The issue occurs if the meso- to micro- scale roughness is included that would normally be loose surface material, because in the modelling domain the terrain surface is a rigid body. It is at these scales a crossover between representing terrain properties as either model parameters or terrain geometry occurs. Little is known about the optimal resolution to represent terrain in rockfall simulations. In this contribution we present the results of numerical simulations with different DEM resolutions. We sampled the terrain morphology of a highly rockfall active area in Matter valley in Switzerland using LiDAR with a maximum resolution of 50cm. The DEM was resampled at resolutions of 1m, 5m and 20m and rockfall simulations were performed where the model ground impact parameters were held constant. To induce the naturally stochastic initial conditions of rock fall release we vary the rock shapes and release orientation, while the potential energy was held constant. We compare the different simulation results and discuss the influence of the DEM resolution on fully three dimensional rockfall simulations. We find the DEM resolution has a strong influence on the simulation results demonstrating that the selection of the DEM is a crucial step in numerical simulation of rockfalls.

Bühler, Yves; Glover, James; Christen, Marc; Bartelt, Perry

2014-05-01

151

Numerical Simulations of Solar Acoustic Field

We present numerical simulations of propagation of acoustic waves in the upper convection zone using a standard solar model and realistic equation of state (OPAL model). The main goals are to study properties of solar waves for various excitation sources and interaction of these waves with spatial inhomogeneities, and also to generate artificial wave fields for testing local helioseismic diagnostics

K. V. Parchevsky; A. G. Kosovichev

2005-01-01

152

IRIS Spectrum Line Plot - Numeric Simulation

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...

153

Using Numerical Simulation to Analyze Ship Collision

Nonlinear finite element method (FEM) is a powerful tool for analyzing ship collision problem and has seen more and more applications in recent years. The reliability of the numerical simulation results largely depends on the proper definition of the problem and careful control of some critical parameters. As part of a benchmarking exercise for a ship-to-ship collision project, the work

Fuqiang Wu; Ge Wang

154

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

155

Numerical simulations on ion acoustic double layers

A numerical study of ion-acoustic double layer in the upper atmosphere has been performed for both periodic and 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 and the ionospheric Pedersen resistance. It is found that the number of double layers and the associated

T. Sato; H. Okuda

1981-01-01

156

Direct numerical simulation of supercritical annular electroconvection.

We use direct numerical simulation to study electrically driven convection in an annular thin film. The simulation models a laboratory experiment that consists of a weakly conducting, submicron thick liquid crystal film suspended between two concentric electrodes. The film is driven to convect by imposing a sufficiently large voltage across it. The flow is driven by a surface charge density inversion which is unstable to the imposed electrical force. This mechanism is closely analogous to the mass density inversion which is unstable to the buoyancy force in conventional, thermally driven Rayleigh-Bénard convection. The simulation uses a pseudospectral method with Chebyshev polynomials in the radial direction and Fourier modes in the azimuthal direction. The numerical results, which are in good agreement with previous experimental data and theoretical predictions, reveal several insights. The mode competition near a codimension-two point exhibits hysteresis. The primary bifurcation is supercritical for a broad range of fluid and geometrical parameters. PMID:17930140

Tsai, Peichun; Daya, Zahir A; Deyirmenjian, Vatche B; Morris, Stephen W

2007-08-01

157

NASA Astrophysics Data System (ADS)

We combine experimental, theoretical and numerical efforts to investigate the turbulent wake far behind a surface ship at model scales. Experimental measurements using digital particle image velocimetry (DPIV) are performed for the wakes of three towed hulls with beam-to-draught ratios b/d = 1, 2, 6. Based on model speed and beam, the Reynolds and Froude numbers are O(103) and O(10[minus sign]2) respectively. Distinct surface features associated with persistent surface-normal vorticity have been identified, which are characterized by large-scale meandering structures. Both lateral and longitudinal scales of the meandering are quantified, with the former found to increase as b/d decreases and the latter independent of b/d. Based on measurements at multiple horizontal and vertical planes, profiles of the mean flow and fluctuation intensity for each velocity component are obtained. To understand the turbulence transition mechanism, an Orr Sommerfeld stability analysis (OS) is formulated for the wake flow with free-surface boundary conditions, and solved by using a fourth-order finite-difference scheme. Unstable modes antisymmetric to the wake centre-plane are identified. Consistent with the experimental results, the growth rates of unstable modes increase substantially as b/d decreases, while the dependence of meandering wavelengths on b/d is found to be weak. Finally, we perform direct numerical simulation (DNS) of Navier Stokes equations for the wake flow. The growth rates of unstable modes agree well with the predictions by OS analysis. Compared with experiments, DNS accurately captures the surface-normal vorticity signatures, the meandering features, as well as statistics of turbulence intensity. We also obtain from DNS a detailed description of enstrophy, turbulence length scales, and vortex structures for the wake flow.

Shen, Lian; Zhang, Chiong; Yue, Dick K. P.

2002-10-01

158

Direct Numerical Simulation of a Dry Shear-free Convective Boundary Layer

NASA Astrophysics Data System (ADS)

Due to the thinness of the inversion layer, entrainment in the Convective Boundary Layer (CBL) is not explicitly resolved in models and is still a major source of uncertainty. Recent work using Large Eddy Simulations (LES) shows lack of convergence in the inversion layer with further grid refinement, even for a vertical resolution of 2 meters. Observational studies of entrainment in the CBL are even more problematic, whether they be field observations or their low Reynolds number analogs in the laboratory, since fine measurements of the three-dimensional flow field at the inversion layer are practically unattainable. As an alternative, we use Direct Numerical Simulations (DNS), which resolves the three-dimensional flow field down to the scale of molecular diffusion. Faithful representation of the whole range of turbulent scales would mean that attainable Reynolds numbers are orders of magnitude lower than that in the atmosphere because of limited computational resources. However, the significant increase in computing power now allows for simulations that are comparable in size to tank experiments. Furthermore, we can invoke Reynolds number similarity to justify the use of DNS to study an idealized convective boundary layer. As a first step, we consider here the dry, shear-free case with constant surface buoyancy flux B0 working against a stable background stratification with constant buoyancy frequency N. Fixing the Prandlt number Pr = ?/? to 1, where ? is the molecular kinematic viscosity and ? is the molecular diffusivity, the problem is characterized by a single non-dimensional parameter (B0/?)/N2 which can be interpreted as the ratio between a reference well-mixed layer height and the diffusive layer thickness. In the atmosphere, (B0/?)/N2 is at least O(106), while for our first simulation, (B0/?)/N2 ~ 40. We have done one simulation with a 1024x1024x541 grid that uses vertical grid stretching, and another that is twice as wide (2048x2048x541) for assessing statistical convergence and the effect of the computational domain size. Even with vertical grid stretching, the grid spacing is smaller than the Kolmogorov length scale. Despite the low Reynolds number, we obtain qualitatively comparable vertical structure as in LES and observations. Relative values **max/B0 ~ 0.8 - 0.9, and TKEmax/w*2 ~ 0.3 - 0.38 are within the range found in literature. The entrainment ratio A = - min/B0 fluctuates in time but has an increasing trend from 0.08 to 0.12, smaller than the canonical value (A = 0.2) but close to the result from fine-resolution LES (A ~ 0.14). We explored different definitions of the mean inversion height zi and chose the vertical location of the buoyancy fluctuation peak at the inversion (max(brms)) because it proved to be more robust. As a function of time, zi is approximated well by a ?t curve within 10%. The corresponding Richardson number Ribrms = (max(brms)zi)/w*2 approaches Ribrms ~ O(1) and is slightly increasing in time. To check for low Reynolds number effects, we do a simulation that is twice as high (2048x2048x1024), therefore increasing (B0/?)/N2 to approximately 100 and the physical domain to approximately a 2-meter box. After establishing DNS as a feasible tool for studying the dry shear-free CBL, we will then use DNS data to investigate the physics of entrainment.**

Garcia, J. R.; Mellado, J. P.

2012-04-01

159

Direct numerical simulations of bubble-laden turbulent flows using the two-fluid formulation

NASA Astrophysics Data System (ADS)

Direct numerical simulations (DNS) of bubble-laden isotropic decaying turbulence are performed using the two-fluid approach (TF) instead of the Eulerian-Lagrangian approach (EL). The motivation for the study is that EL requires considerable computational resources, especially for the case of two-way coupling, where the instantaneous trajectories of a large number of individual bubbles need to be computed. The TF formulation is developed by spatially averaging the instantaneous equations of the carrier flow and bubble phase over a scale of the order of the Kolmogorov length scale, which, in our case, is much larger than the bubble diameter. On that scale, the bubbles are treated as a continuum (without molecular diffusivity) characterized by the bubble phase velocity field and concentration (volume fraction). The bubble concentration, C, is assumed small enough (C<=10-3) to neglect the bubble-bubble interactions. As a test case, direct simulation of a bubble-laden Taylor-Green vortex with one-way coupling is performed with a bubble response time of the order of the flow time scale (inverse of the mean vorticity). This simple flow allows a direct examination of the effects of the preferential accumulation of bubbles in the high-enstrophy regions of the flow on the accuracy of the two-fluid formulation. The temporal development of the maximum bubble concentration obtained from DNS agrees well with the analytical solution. DNS of the bubble-laden decaying turbulence are also performed for both cases of one-way and two-way coupling. Here, the bubble diameter and response time are much smaller than the Kolmogorov length and time scales, respectively. In this case, as expected, the effects of the preferential accumulation of the bubbles are not pronounced. The results also show that the bubble-laden flow is analogous to a stratified flow with an effective density =(1-C)?f. Thus, due to the two-way interaction between the bubbles and carrier flow, the turbulence decay is enhanced with stable stratification, and reduced with unstable stratification.

Druzhinin, O. A.; Elghobashi, S.

1998-03-01

160

Experiments and Numerical Simulations of Electrodynamic Tether

NASA Astrophysics Data System (ADS)

As an effective means of suppressing space debris growth, the Aerospace Research and Development Directorate of the Japan Aerospace Exploration Agency (JAXA) has been investigating an active space debris removal system that employs highly efficient electrodynamic tether (EDT) technology for orbital transfer. This study investigates tether deployment dynamics by means of on-ground experiments and numerical simulations of an electrodynamic tether system. Some key parameters used in the numerical simulations, such as the elastic modulus and damping ratio of the tether, the spring constant of the coiling of the tether, and deployment friction, must be estimated, and various experiments are conducted to determine these values. As a result, the following values were obtained: The elastic modulus of the tether was 40 GPa, and the damping ratio of the tether was 0.02. The spring constant and the damping ratio of the tether coiling were 10-4 N/m and 0.025 respectively. The deployment friction was 0.038? + 0.005 N. In numerical simulations using a multiple mass tether model, tethers with lengths of several kilometers are deployed and the attitude dynamics of satellites attached to the end of the tether and tether libration are calculated. As a result, the simulations confirmed successful deployment of the tether with a length of 500 m using the electrodynamic tether system.

Iki, Kentaro; Kawamoto, Satomi; Takahashi, Ayaka; Ishimoto, Tomori; Yanagida, Atsushi; Toda, Susumu

161

Numerical simulation of groundwater flow on MPPs

Mathematical models are often used to aid in the design and management of engineered remediation procedures. This paper discusses the numerical simulation of groundwater flow in three-dimensional heterogeneous porous media. A portable and scalable code called PARFLOW is being developed for massively parallel computers to enable the detailed modeling of large sites. This code uses a turning bands algorithm to generate a statistically accurate subsurface realization, and preconditioned conjugate gradients to solve the linear system that yields the flow velocity field. Preliminary numerical results for the LLNL site are presented.

Ashby, S.; Falgout, R.; Tompson, A. [Lawrence Livermore National Lab., CA (United States); Fogwell, T. [International Technology Corp., Martinez, CA (United States)

1994-03-01

162

Towards an Automated Full-Turbofan Engine Numerical Simulation.

National Technical Information Service (NTIS)

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-...

J. A. Reed M. G. Turner A. Norris J. P. Veres

2003-01-01

163

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

164

Conditional statistics in a turbulent premixed flame derived from direct numerical simulation

NASA Technical Reports Server (NTRS)

The objective of this paper is to briefly introduce conditional moment closure (CMC) methods for premixed systems and to derive the transport equation for the conditional species mass fraction conditioned on the progress variable based on the enthalpy. Our statistical analysis will be based on the 3-D DNS database of Trouve and Poinsot available at the Center for Turbulence Research. The initial conditions and characteristics (turbulence, thermo-diffusive properties) as well as the numerical method utilized in the DNS of Trouve and Poinsot are presented, and some details concerning our statistical analysis are also given. From the analysis of DNS results, the effects of the position in the flame brush, of the Damkoehler and Lewis numbers on the conditional mean scalar dissipation, and conditional mean velocity are presented and discussed. Information concerning unconditional turbulent fluxes are also presented. The anomaly found in previous studies of counter-gradient diffusion for the turbulent flux of the progress variable is investigated.

Mantel, Thierry; Bilger, Robert W.

1994-01-01

165

Numerical Simulation in a Supercirtical CFB Boiler

NASA Astrophysics Data System (ADS)

The dimension of the hot circulation loop of the supercritical CFB boiler is large, and there are many unknowns and challenges that should be identified and resolved during the development. In order to realize a reasonable and reliable design of the hot circulation loop, numerical simulation of gas-solid flow in a supercritical CFB boiler was conducted by using FLUENT software. The working condition of hot circulation loop flow field, gas-solid flow affected by three unsymmetrical cyclones, air distribution and pressure drop in furnace were analyzed. The simulation results showed that the general arrangement of the 600MWe supercritical CFB boiler is reasonable.

Zhang, Yanjun; Gaol, Xiang; Luo, Zhongyang; Jiang, Xiaoguo

166

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

167

Numerical simulations of compressible mixing layers

NASA Technical Reports Server (NTRS)

Direct numerical simulations of two-dimensional temporally growing compressible mixing layers are presented. The Kelvin-Helmholtz instability is initially excited by a white-noise perturbation superimposed onto a hyperbolic tangent meanflow profile. The linear regime is studied at low resolution in the case of two flows of equal temperatures, for convective Mach numbers from 0.1 to 1 and for different values of the Reynolds number. At higher resolution, the complete evolution of a two-eddy mixing layer between two flows of different temperatures is simulated at moderate Reynolds number. Similarities and differences between flows of equal convective Mach numbers are discussed.

Normand, Xavier

1990-01-01

168

Direct numerical simulations and modeling of jets in crossflow

NASA Astrophysics Data System (ADS)

Jets in crossflow are central to a variety of applications such as fuel injection, gas turbine combustion and film-cooling. Direct Numerical Simulations are used to study the different aspects of round jets in a crossflow. The first problem studies the effect of jet and crossflow velocity profiles on jet trajectories and the near-field. A new scaling law for the jet trajectory is proposed, that accounts for these parameters. The proposed scaling is shown to be a significant improvement over current scaling laws. DNS of a turbulent jet in crossflow is performed at conditions corresponding to an experiment (Su & Mungal 2004). Detailed comparison shows good agreement with experiment, and additional quantities, not available experimentally, are presented. Turbulent kinetic energy budget is computed, and is used to suggest possible reasons for the difficulty experienced by current engineering models in predicting this complex flow. A predictor-corrector approach is implemented to compute passive scalar transport. This ensures that the local scalar concentration is always within bounds. The passive scalar is introduced along with the jet fluid, once the velocity field is statistically stationary. Mean scalar profiles show a good agreement when compared to the experiment. The scalar field is used to compute entrainment of the crossflow fluid by the jet, which is greater than that in a regular jet. The reasons for a transverse jet's enhanced entrainment are explained in terms of the pressure field in the vicinity of the jet. A two-dimensional model problem is used to study jet cross-section deformation. The model jet deforms at its trailing edge, exhibits the Kelvin-Helmholtz instability at its outer edges, and---later---yields a counter-rotating vortex pair (CVP). The model jet experiences constant acceleration in its initial stages, and moves at constant velocity at longer times. Deformation of the jet cross-section may be explained in terms of the pressure field that the crossflow fluid imposes on the jet, and the acceleration that the jet experiences. It is shown that the CVP is formed even in two dimensions, and that the pipe is not necessary.

Muppidi, Suman

169

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

170

Numerical simulation of gridded electrostatic lens

Gridded electrostatic lenses are frequently used in extraction systems and low energy ion beam transport line. Typically, for numerical simulation the grid is treated as a metal plate transparent for beam particles. The influence of real grid geometry on the beam dynamics in the gridded lens has been investigated by KOBRA-3d code. Beam emittance growth for different lens parameters has been investigated. Approximating expressions for obtained results are presented. The grid geometry providing minimal beam distortions is proposed.

Kropachev, G. N.; Alexeev, N. N.; Balabin, A. I.; Kulevoy, T. V.; Nikolaev, V. I [Institute for Theoretical and Experimental Physics, Moscow (Russian Federation)

2012-02-15

171

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.

172

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.

Keck, Tilman; Lindemann, Jorg

2011-01-01

173

Numerical Simulations of Boundary-Driven Dynamos

NASA Astrophysics Data System (ADS)

An important topic of physics research is how magnetic fields are generated and maintained in the many astrophysical bodies where they are ubiquitously observed. Of particular interest, are reversals of magnetic fields of planets and stars, especially those of the Earth and the Sun. In an attempt to provide intuition on this problem, numerous physical dynamo experiments have been performed in different configurations. Recently, a tremendous breakthrough was made in the Von Karman sodium (VKS) experiments in France when the most realistic laboratory fluid dynamo to date was produced by driving an unconstrained flow in a cylinder of liquid sodium (Monchaux et al, 2007, PRL). One of the curiosities of the VKS experiment however is the effect of the composition of the impellers that drive the flow. Steel blades failed to produce a dynamo, but soft iron impellers, which have much higher magnetic permeability, succeeded. The role of the magnetic properties of the boundaries in boundary-driven dynamos is therefore clearly of interest. Kinematic and laminar numerical dynamo simulations (Giesecke et al, 2010, PRL & Gissinger et al, 2008 EPL) have shed some light but turbulent, nonlinear simulations are necessary. Roberts, Glatzmaier & Clune 2010 created a simplified model of the VKS setup by using three-dimensional numerical simulations in a spherical geometry with differential zonal motions of the boundary replacing the driving impellers of the VKS experiment. We have extended these numerical simulations further towards a more complete understanding of such boundary-forced dynamos. In particular, we have examined the effect of the magnetic boundary conditions - changes in the wall thickness, the magnetic permeability, and the electrical conductivity - on the mechanisms responsible for dynamo generation. Enhanced permeability, conductivity and wall thickness all help dynamo action to different degrees. We are further extending our investigations to asymmetric forcing to examine the possible existence of solutions incorporating field reversals. Asymmetry can quench dynamo action by destroying the complex correlations that are necessary to regenerate axisymmetric poloidal field.

White, K.; Brummell, N.; Glatzmaier, G. A.

2012-12-01

174

Numerical simulation of binary liquid droplet collision

NASA Astrophysics Data System (ADS)

A numerical investigation of binary droplet collision has been conducted. The complete process of the collision of two liquid droplets is dynamically simulated by solving the incompressible Navier-Stokes equations coupled with the convective equation of the level set function that captures the interface between the liquid and the gas phases. The simulations cover four major regimes of binary collision: bouncing, coalescence, reflexive separation, and stretching separation. For water droplets in air, the numerical results are compared with the experiments by and Ashgriz and Poo [J. Fluid Mech. 221, 183 (1990)] on collision consequences. For hydrocarbon (C14H30) droplets in nitrogen gas, the simulated results are compared in detail with the time-resolved photographic images of the collision processes obtained by Qian and Law [J. Fluid Mech. 331, 59 (1997)] in every collision regime. The present numerical results suggest that the mechanism of a bouncing collision is governed by the macroscopic dynamics. However, the fact that the present macroscopic numerical model is unable to capture the collision regime of coalescence after minor deformation supports the speculation that its mechanism is related to the microscopic dynamics. Furthermore, the transition from bouncing to coalescence collisions has been predicted and agrees well with the analytical model. The mechanism of satellite droplet formation for head-on collision and stretching separation collision is also studied based on the detailed time-resolved dynamic simulation results. It is then confirmed that end pinching is the main cause of satellite formation in head-on collisions whereas the capillary-wave instability becomes dominant in large impact parameter cases. In the case of an intermediate impact parameter, the effects of twisting and stretching due to the angular momentum and the inertia of the colliding droplets are significant for the satellite formation.

Pan, Yu; Suga, Kazuhiko

2005-08-01

175

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

176

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

177

Numerical simulation of a multilevel atom interferometer

We present a comprehensive numerical simulation of an echo-type atom interferometer. The simulation confirms an interesting theoretical description of this interferometer that includes effects due to spontaneous emission and magnetic sublevels. Both the simulation and the theoretical model agree with the results of experiments. These developments provide an improved understanding of several observable effects. The evolution of state populations due to stimulated emission and absorption during the standing-wave interaction imparts a time-dependent phase on each atomic momentum state. This manifests itself as an asymmetry in the signal shape that depends on the strength of the interaction as well as spontaneous emission due to a nonzero population in the excited states. The degree of asymmetry is a measure of a nonzero relative phase between interfering momentum states.

Barrett, B.; Beattie, S.; Kumarakrishnan, A. [Department of Physics and Astronomy, York University, Toronto, Ontario M3J 1P3 (Canada); Yavin, I. [Center of Cosmology and Particle Physics, New York University, New York, New York 10003 (United States)

2010-08-15

178

Direct numerical simulation of turbulent flow in a channel with different types of surface roughness

NASA Astrophysics Data System (ADS)

Direct numerical simulation (DNS) was performed for turbulent channel flow (Re? = 400) for two types of wall surface roughness and well as smooth walls. The roughness elements of first type were assumed to be two-dimensional, transverse square rods positioned on both walls in a non-staggered arrangement. The height of the rods corresponds to y^+ = 13.6 and thus extends in the buffer layer. The second type of roughness was represented by a set of hemispherical obstacles (height of y^ + = 10) located on both channel walls and arranged on a square lattice. The presented simulations are part of benchmark problems defined by thermal-hydraulics focus area of the Consortium for Advanced Simulations of Light Water Reactors (CASL). This problem simulates the effect of the presence of growing bubbles on the walls of nuclear reactor fuel rods and aimed on evaluating CFD capabilities of various codes before applying them to more advanced problems. Mean turbulent quantities were computed and compared with available analytical and experimental results. The results of this work will be used to evaluate the performance of other LES and RANS codes on this benchmark problem.

Bolotnov, Igor A.

2011-11-01

179

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

180

NASA Astrophysics Data System (ADS)

Transition prediction in two-dimensional laminar boundary layers developing on airfoil sections at subsonic speeds and very low turbulence levels is still a challenge. The commonly used semi-empirical prediction tools are mainly based on linear stability theory and do not account for nonlinear effects present unavoidably starting with certain stages of transition. One reason is the lack of systematic investigations of the weakly nonlinear stages of transition, especially of the strongest interactions of the instability modes predominant in non-self-similar boundary layers. The present paper is devoted to the detailed experimental, numerical, and theoretical study of weakly nonlinear subharmonic resonances of Tollmien-Schlichting waves in an airfoil boundary layer, representing main candidates for the strongest mechanism of these initial nonlinear stages. The experimental approach is based on phase-locked hot-wire measurements under controlled disturbance conditions using a new disturbance source being capable to produce well-defined, complex wave compositions in a wide range of streamwise and spanwise wave numbers. The tests were performed in a low-turbulence wind tunnel at a chord Reynolds number of Re = 0.7 × 106. Direct numerical simulations (DNS) were utilized to provide a detailed comparison for the test cases. The results of weakly nonlinear theory (WNT) enabled a profound understanding of the underlying physical mechanisms observed in the experiments and DNS. The data obtained in experiment, DNS and WNT agree basically and provide a high degree of reliability of the results. Interactions occurring between components of various initial frequency-wavenumber spectra of instability waves are investigated by systematic variation of parameters. It is shown that frequency-detuned and spanwise-wavenumber-detuned subharmonic-type resonant interactions have an extremely large spectral width. Similar to results obtained for self-similar base flows it is found that the amplification factors in the frequency-detuned resonances can be even higher than in tuned cases, in spite of the strong base-flow non-self-similarity. An explanation of this unusual phenomenon is found based on the theoretical analysis and comparison of experimental, theoretical, and DNS data.

Würz, W.; Sartorius, D.; Kloker, M.; Borodulin, V. I.; Kachanov, Y. S.; Smorodsky, B. V.

2012-09-01

181

Numerical recipes for mold filling simulation

Has the ability to simulate the filling of a mold progressed to a point where an appropriate numerical recipe achieves the desired results? If results are defined to be topological robustness, computational efficiency, quantitative accuracy, and predictability, all within a computational domain that faithfully represents complex three-dimensional foundry molds, then the answer unfortunately remains no. Significant interfacial flow algorithm developments have occurred over the last decade, however, that could bring this answer closer to maybe. These developments have been both evolutionary and revolutionary, will continue to transpire for the near future. Might they become useful numerical recipes for mold filling simulations? Quite possibly. Recent progress in algorithms for interface kinematics and dynamics, linear solution methods, computer science issues such as parallelization and object-oriented programming, high resolution Navier-Stokes (NS) solution methods, and unstructured mesh techniques, must all be pursued as possible paths toward higher fidelity mold filling simulations. A detailed exposition of these algorithmic developments is beyond the scope of this paper, hence the authors choose to focus here exclusively on algorithms for interface kinematics. These interface tracking algorithms are designed to model the movement of interfaces relative to a reference frame such as a fixed mesh. Current interface tracking algorithm choices are numerous, so is any one best suited for mold filling simulation? Although a clear winner is not (yet) apparent, pros and cons are given in the following brief, critical review. Highlighted are those outstanding interface tracking algorithm issues the authors feel can hamper the reliable modeling of today`s foundry mold filling processes.

Kothe, D.; Juric, D.; Lam, K.; Lally, B.

1998-07-01

182

Numerical methods for large-eddy simulation in general co-ordinates

NASA Astrophysics Data System (ADS)

Large scale unsteady motions in many practical engineering flows play a very important role and it is very unlikely that these unsteady flow features can be captured within the framework of Reynolds averaged Navier-Stokes approach. Large-eddy simulation (LES) has become, arguably, the only practical numerical tool for predicting those flows more accurately since it is still not realistic to apply DNS to practical engineering flows with the current and near future available computing power.Numerical methods for the LES of turbulent flows in complex geometry have been developed and applied to predict practical engineering flows successfully. The method is based on body-fitted curvilinear co-ordinates with the contravariant velocity components of the general Navier-Stokes equations discretized on a staggered orthogonal mesh. For incompressible flow simulations the main source of computational expense is due to the solution of a Poisson equation for pressure. This is especially true for flows in complex geometry. A multigrid 3D pressure solver is developed to speed up the solution. In addition, the Poisson equation for pressure takes a simpler form with no cross-derivatives when orthogonal mesh is used and hence resulting in increased convergence rate and producing more accurate solutions.

Tang, Gefeng; Yang, Zhiyin; McGuirk, James J.

2004-09-01

183

Numerical Simulation of Fluid Mud Gravity Currents

NASA Astrophysics Data System (ADS)

Fluid mud bottom gravity currents are simulated numerically using a commercial computational fluid dynamics software, ANSYS-Fluent. In this study, Eulerian-Eulerian multi-fluid method is selected since this method treats all phases in a multiphase system as interpenetrated continua. There are three different phases in the computational model constructed for this study: water, fluid mud, and air. Water and fluid mud are defined as two miscible fluids and the mass and momentum transfers between these two phases are taken into account. Fluid mud, which is a dense suspension of clay particles and water, is defined as a single-phase non-Newtonian fluid via user-defined-functions. These functions define the physical characteristics (density, viscosity, etc.) of the fluid mud and these characteristics vary with changing suspension concentration due to mass transfer between the fluid mud and the water phase. Results of this two-dimensional numerical model are verified with data obtained from experiments conducted in a laboratory flume with a lock-release set-up. Numerical simulations are currently being conducted to elucidate turbulent entrainment of ambient water into fluid mud gravity currents. This study is motivated by coastal dredge disposal operations.

Yilmaz, N. A.; Testik, F. Y.

2011-12-01

184

Direct Numerical Simulations of Capillary Wave Turbulence

NASA Astrophysics Data System (ADS)

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.

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

2014-06-01

185

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

186

Numerical simulation of industrial superplastic forming

Superplastic forming is a metal forming process that allows a variety of components with very complex geometries to be produced at one tenth the cost of conventional machining. The industrial superplastic forming process can be optimized with the application of the finite element method to predict the optimal applied pressure history and the final part thickness distribution. This paper discusses the application the nonlinear implicit, three dimensional finite element code, NIKE3D to the problem of numerically simulating and optimizing the superplastic forming of Ti-6AI-4V components.

Haberman, K.S.; Bennett, J.G.; Miller, E.L.; Piltch, M.S. [Los Alamos National Lab., NM (United States); Leyer, L.K.; Leodolter, W. [Jet Die, Lansing, MI (United States)]|[Flame Co., Ogden, UT (United States)

1995-03-01

187

Numerical simulation of plasma double layers

NASA Technical Reports Server (NTRS)

Numerical simulation results are presented for a plasma double layer, the computer model being a finite one-dimensional particle-in-cell plasma with specified potential difference across the system. A single pulse is formed which crosses the system with constant velocity; this is followed by the formation of a potential drop across a limited region of the plasma. An approximate expression relating the spatial extent of the double layer and the potential drop is presented. Electron and ion beams are generated which tend to lead to instabilities in the upstream and downstream regions.

Joyce, G.; Hubbard, R. F.

1978-01-01

188

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

189

Numerical simulations unravel the cosmic web.

The universe is permeated by a network of filaments, sheets, and knots collectively forming a "cosmic web." The discovery of the cosmic web, especially through its signature of absorption of light from distant sources by neutral hydrogen in the intervening intergalactic medium, exemplifies the interplay between theory and experiment that drives science and is one of the great examples in which numerical simulations have played a key and decisive role. We recount the milestones in our understanding of cosmic structure; summarize its impact on astronomy, cosmology, and physics; and look ahead by outlining the challenges faced as we prepare to probe the cosmic web at new wavelengths. PMID:18174431

Faucher-Giguère, Claude-André; Lidz, Adam; Hernquist, Lars

2008-01-01

190

Numerical simulation of cloud-aerosol interactions

NASA Astrophysics Data System (ADS)

The Lagrangian Cloud Model (LCM) is a Large Eddy Simulation model with a two way coupling between the Eulerian (dynamics and thermodynamics) and Lagrangian (microphysics) parts. Since Lagrangian representation of microphysics does not suffer from numerical diffusion in the radius and physical space, and solves the full condensation model, it may be an alternative for the bin approach. In this talk implementation of the collision process in a Lagrangian framework will be discussed and the ability of the model to assess the impact of changes in the aerosol size distribution on the cloud droplet size distributions in stratocumulus clouds observed during VOCALS will be presented.

Andrejczuk, M.; Gadian, A.; Blyth, A. M.

2009-12-01

191

Numerical simulation of streaklines in unsteady flows

NASA Astrophysics Data System (ADS)

A common practice in experimental fluid dynamics is the injection of smoke into a wind tunnel or dye into a water tank for the purpose of flow visualization. Photographs of the resulting streak-line patterns play an important role in both the qualitative and quantitative interpretation of the flow. In this paper, the streak-line patterns for several unsteady flows of known flowfields are simulated numerically. The computed results for flows around moving vortices and for those past an elliptical cylinder in various angular motions show that streak-line patterns in such flows are sometimes misleading.

Chow, Chuen-Yen; Leben, Robert R.; Gea, Lie-Mine

1989-01-01

192

Numerical simulation of flow through biofluid devices

NASA Technical Reports Server (NTRS)

The results of a numerical simulation of flow through an artificial heart and through an artificial tilting-disk heart valve are presented. The simulation involves solving the incompressible Navier-Stokes equations; the solution process is described. The details and difficulties of modeling these particular geometries are discussed. The artificial heart geometry uses a single moving grid, and the valve computation uses an overlaid-grid approach with one moving grid and one stationary grid. The equations must be solved iteratively for each discrete time step of the computations, requiring a significant amount of computing time. It is particularly difficult to analyze and present the fluid physics represented by these calculations because of the time-varying nature of the flow, and because the flows are internal. Three-dimensional graphics and scientific visualization techniques have become instrumental in solving these problems.

Rogers, Stuart E.; Kwak, Dochan; Kiris, Cetin; Chang, I-Dee

1990-01-01

193

Numerical simulation of flow through biofluid devices

NASA Astrophysics Data System (ADS)

The results of a numerical simulation of flow through an artificial heart and through an artificial tilting-disk heart valve are presented. The simulation involves solving the incompressible Navier-Stokes equations; the solution process is described. The details and difficulties of modeling these particular geometries are discussed. The artificial heart geometry uses a single moving grid, and the valve computation uses an overlaid-grid approach with one moving grid and one stationary grid. The equations must be solved iteratively for each discrete time step of the computations, requiring a significant amount of computing time. It is particularly difficult to analyze and present the fluid physics represented by these calculations because of the time-varying nature of the flow, and because the flows are internal. Three-dimensional graphics and scientific visualization techniques have become instrumental in solving these problems.

Rogers, Stuart E.; Kwak, Dochan; Kiris, Cetin; Chang, I.-Dee

194

Numerical simulation of large fabric filter

NASA Astrophysics Data System (ADS)

Fabric filters are used in the wide range of industrial technologies for cleaning of incoming or exhaust gases. To achieve maximal efficiency of the discrete phase separation and long lifetime of the filter hoses, it is necessary to ensure uniform load on filter surface and to avoid impacts of heavy particles with high velocities to the filter hoses. The paper deals with numerical simulation of two phase flow field in a large fabric filter. The filter is composed of six chambers with approx. 1600 filter hoses in total. The model was simplified to one half of the filter, the filter hoses walls were substituted by porous zones. The model settings were based on experimental data, especially on the filter pressure drop. Unsteady simulations with different turbulence models were done. Flow field together with particles trajectories were analyzed. The results were compared with experimental observations.

Sedlá?ek, Jan; Kova?ík, Petr

2012-04-01

195

Numerical scene simulation for cloudy atmospheric background

NASA Astrophysics Data System (ADS)

Methods for cloud scene simulation are analyzed and studied based on the properties of real cloud edge and distribution of the atmospheric background radiance. A new method is proposed to simulate cloud scene by means of the fractal geometry algorithm after effects of clouds on the atmospheric background radiation are analyzed. Firstly, spectral radiance of cloudless and cloudy atmospheric background is respectively calculated according to the radiative transfer theory. Secondly, a simulation method for cloud edge is developed based on the improved random generator after studying the properties of real cloud edge and self-similar character of the fractal principle. Finally, an algorithm implemented to create cloud texture is designed by employing the radiance distribution of cloudy atmospheric background, quadric random method and Diamond-Square interpolation. All 2-D atmospheric background radiance within given view filed has been computed with the mixed modified delta-Eddington approximation method. Some numerical scenes for cloudy atmospheric background are triumphantly simulated with the radiative transfer theory and the fractal geometry algorithm.

Yang, Chunping; Zhang, Yan; Kang, Meilin; Guo, Jing; Wu, Jian

2009-10-01

196

Secure Domain Name System (DNS) Deployment Guide.

National Technical Information Service (NTIS)

The Domain Name System (DNS) is a distributed computing system that enables access to Internet resources by user-friendly domain names rather than IP addresses, by translating domain names to IP addresses and back. The DNS infrastructure is made up of com...

R. Chandramouli S. Rose

2013-01-01

197

Evolution of Planetesimals. II. Numerical Simulations

NASA Astrophysics Data System (ADS)

We continue our investigation of the dynamical evolution and coagulation process of planetesimals With a numerical N-body scheme, we simulate gravitational scattering and physical collisions among a system of planetesimals. The results of these simulations confirm our earlier analytical results that dynamical equilibrium is attained with a velocity dispersion comparable to the surface escape velocity of those planetesimals which contribute most of the system mass. In such an equilibrium, the rate of energy transfer from the systematic shear to dispersive motion, induced by gravitational scattering, is balanced by the rate of energy dissipation resulting from physical collisions. We also confirm that dynamical friction can lead to energy equipartition between an abundant population of low-mass field planetesimals and a few collisionally induced mergers with larger masses. These effects produce mass segregation in phase space and runaway coagulation. Collisions also lead to coagulation and evolution of the mass spectrum. The mergers of two field planetesimals can provide sufficient mass differential with other planetesimals for dynamical friction to induce energy equipartition and mass segregation. For small velocity dispersions, the more massive planetesimals produce relatively large gravitational focusing factors. Consequently, the growth time scale decreases with mass and runaway coagulation is initiated. Our numerical simulations show that, provided there is sufficient supply of low-mass planetesimals, runaway coagulation can lead to the formation of protoplanetary cores with masses comparable to a significant fraction of an Earth mass. We estimate that, at 1 AU, the characteristic time scale for the initial stages of planetesimal growth is ˜104 yr and ˜105 yr for the growth to protoplanetary cores. At Jupiter's present distance, these time scales are an order of magnitude longer.

Aarseth, S. J.; Lin, D. N. C.; Palmer, P. L.

1993-01-01

198

a Study on Correlation Moments of Two-Phase Fluctuating Velocity Using Direct Numerical Simulation

NASA Astrophysics Data System (ADS)

Existing models of two-phase fluctuating velocity correlation moments are unsatisfactory because of their inability to clearly identify the dependency of two-phase velocity covariance on fluid- and particle-phase velocity second moments. This is especially true of wall-bounded turbulent flows. In this paper, the statistical fluctuating velocity of both phases in particle-laden turbulent channel flows were obtained numerically by means of direct numerical simulation (DNS) coupled to the Lagrangian particle trajectory method. The effects of particle Stokes number on the scaling of two-phase fluctuating velocity correlation moments were analyzed considering effects of flow inhomogeneity. An improved two-phase correlation closure model of exponential decay with emphasis on the particle-phase kinetic energy was then proposed based on the results of an evaluation of five existing models. This new model was found to be better than previous models, which used local equilibrium assumption. The present investigations may facilitate understanding of two-phase flow physics and the construction of models capable of predicting the movements of particle-laden turbulent flows accurately using Reynolds-averaged Navier-Stokes (RANS) methods.

Wang, Bing; Wei, Wei; Zhang, Huiqiang

2013-10-01

199

Direct Numerical Simulation of turbulent channel flow with V-shape turbulators

NASA Astrophysics Data System (ADS)

Direct Numerical Simulations (DNS) are carried out to study the channel flow V-shaped roughness on both wall. The roughness is modeled by the immerse boundary method for passive heat transport in a turbulent channel flow with V-shape square ribs for w/k = 3, 8, 10, 15 (w being the pitch, k the height of the ribs turbulators) with a k/h = 0.25, 0.1 (h being the mid-height of the channel). The angle of inclination of the V-shape turbulators is 45 degrees. Numerical results show that V-shape square ribs are more efficient than square ribs in maximizing the heat transfer. The configuration with w/k = 3, k/h = 0.25 presents the largest heat flux. The increase in the heat transfer is due to a secondary motion which is generated by the V-shape turbulators. Secondary motions at the location of the sidewalls transport the heat out of the cavity of the turbulators to the crest pane.

Toro Medina, Jaime A.

200

Numerical simulation of premixed turbulent methane combustion

In this paper we study the behavior of a premixed turbulent methane flame in three dimensions using numerical simulation. The simulations are performed using an adaptive time-dependent low Mach number combustion algorithm based on a second-order projection formulation that conserves both species mass and total enthalpy. The species and enthalpy equations are treated using an operator-split approach that incorporates stiff integration techniques for modeling detailed chemical kinetics. The methodology also incorporates a mixture model for differential diffusion. For the simulations presented here, methane chemistry and transport are modeled using the DRM-19 (19-species, 84-reaction) mechanism derived from the GRIMech-1.2 mechanism along with its associated thermodynamics and transport databases. We consider a lean flame with equivalence ratio 0.8 for two different levels of turbulent intensity. For each case we examine the basic structure of the flame including turbulent flame speed and flame surface area. The results indicate that flame wrinkling is the dominant factor leading to the increased turbulent flame speed. Joint probability distributions are computed to establish a correlation between heat release and curvature. We also investigate the effect of turbulent flame interaction on the flame chemistry. We identify specific flame intermediates that are sensitive to turbulence and explore various correlations between these species and local flame curvature. We identify different mechanisms by which turbulence modulates the chemistry of the flame.

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

2001-12-14

201

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

NASA Technical Reports Server (NTRS)

The objective 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 analysis of high-speed reacting turbulent flows. In the first phase of this research, conducted within the past six months, focus was in three directions: RANS of turbulent reacting flows by Probability Density Function (PDF) methods, RANS of non-reacting turbulent flows by advanced turbulence closures, and LES of mixing dominated reacting flows by a dynamics subgrid closure. A summary of our efforts within the past six months of this research is provided in this semi-annual progress report.

Givi, Peyman

1994-01-01

202

Numerical simulations of black-hole spacetimes

NASA Astrophysics Data System (ADS)

This thesis covers various aspects of the numerical simulation of black-hole spacetimes according to Einstein's general theory of relativity, using the Spectral Einstein Code developed by the Caltech-Cornell-CITA collaboration. The first topic is improvement of binary-black-hole initial data. One such issue is the construction of binary-black-hole initial data with nearly extremal spins that remain nearly constant during the initial relaxation in an evolution. Another concern is the inclusion of physically realistic tidal deformations of the black holes to reduce the high-frequency components of the spurious gravitational radiation content, and represents a first step in incorporating post-Newtonian results in constraint-satisfying initial data. The next topic is the evolution of black-hole binaries and the gravitational waves they emit. The first spectral simulation of two inspiralling black holes through merger and ringdown is presented, in which the black holes are nonspinning and have equal masses. This work is extended to perform the first spectral simulations of two inspiralling black holes with moderate spins and equal masses, including the merger and ringdown. Two configurations are considered, in which both spins are either anti-aligned or aligned with the orbital angular momentum. Highly accurate gravitational waveforms are computed for all these cases, and are used to calibrate waveforms in the effective-one-body model. The final topic is the behavior of quasilocal black-hole horizons in highly dynamical situations. Simulations of a rotating black hole that is distort ed by a pulse of ingoing gravitational radiation are performed. Multiple marginally outer trapped surfaces are seen to appear and annihilate with each other during the evolution, and the world tubes th ey trace out are all dynamical horizons. The dynamical horizon and angular momentum flux laws are evaluated in this context, and the dynamical horizons are contrasted with the event horizon. The format ion of multiple marginally outer trapped surfaces in the Vaidya spacetime is also treated.

Chu, Tony

203

3D Numerical simulations of oblique subduction

NASA Astrophysics Data System (ADS)

In the past 2D numerical studies (e.g. Gerya et al., 2002; Gorczyk et al., 2007; Malatesta et al., 2012) provided evidence that during intraoceanic subduction a serpentinite channel forms above the downgoing plate. This channel forms as a result of hydration of the mantle wedge by uprising slab-fluids. Rocks buried at high depths are finally exhumed within this buoyant low-viscosity medium. Convergence rate in these 2D models was described by a trench-normal component of velocity. Several present and past subduction zones worldwide are however driven by oblique convergence between the plates, where trench-normal motion of the subducting slab is coupled with trench-parallel displacement of the plates. Can the exhumation mechanism and the exhumation rates of high-pressure rocks be affected by the shear component of subduction? And how uprise of these rocks can vary along the plate margin? We tried to address these questions performing 3D numerical models that simulate an intraoceanic oblique subduction. The models are based on thermo-mechanical equations that are solved with finite differences method and marker-in-cell techniques combined with multigrid approach (Gerya, 2010). In most of the models a narrow oceanic basin (500 km-wide) surrounded by continental margins is depicted. The basin is floored by either layered or heterogeneous oceanic lithosphere with gabbro as discrete bodies in serpentinized peridotite and a basaltic layer on the top. A weak zone in the mantle is prescribed to control the location of subduction initiation and therefore the plate margins geometry. Finally, addition of a third dimension in the simulations allowed us to test the role of different plate margin geometries on oblique subduction dynamics. In particular in each model we modified the dip angle of the weak zone and its "lateral" geometry (e.g. continuous, segmented). We consider "continuous" weak zones either parallel or increasingly moving away from the continental margins. Moreover, we tested the effect on subduction/exhumation dynamics of several values of the trench-parallel component of convergence-rate vector. Gerya T., Stöckhert B., Perchuk A.L. (2002). Exhumation of high-pressure metamorphic rocks in a subduction channel: a numerical simulation. Tectonics, vol. 21, n. 6, 1056. Gerya, T. V., 2010. Introduction to numerical geodynamic modelling. Cambridge University Press, Cambridge. Gorczyk W., Guillot S., Gerya T.V., Hattori K. (2007a). Asthenospheric upwelling, oceanic slab retreat, and exhumation of UHP mantle rocks: insights from Greater Antilles. Geophysical research letters, vol. 34, L21309. Malatesta C., Gerya T., Scambelluri M., Federico L., Crispini L., Capponi G. (2012). Intraoceanic subduction of "heterogeneous" oceanic lithosphere in narrow basins: 2D numerical modeling. Lithos, http://dx.doi.org/10.1016/j.lithos.2012.01.003

Malatesta, C.; Gerya, T.; Scambelluri, M.; Crispini, L.; Federico, L.; Capponi, G.

2012-04-01

204

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

205

Modeling Scalar variance from Direct Numerical Simulations of a turbulent mixing layer

NASA Astrophysics Data System (ADS)

Many studies have focused on analyzing and predicting the mixing of a scalar such as fuel concentration in turbulent flows. However, the subfilter scalar variance in Large Eddy Simulations (LES) still requires additional considerations. The present work aims at obtaining results for the turbulent mixture of a scalar in configurations relevant to reactive flows, i.e. in the presence of mean velocity/scalar gradients. A Direct Numerical Simulation (DNS) of a turbulent mixing layer has been performed by initially combining two boundary layers. The high order conservative finite difference low Mach number NGA code was used together with the BQuick scheme for the transport of mixture fraction. The self-similar nature of the flow and energy spectra have been considered to analyze the turbulent flow field. High order velocity schemes (4th order) were found to play an important role in capturing accurately the mixing of fuel and air. The scalar variance has been calculated by filtering the solution and has been compared to various models usually used in LES. Following an earlier study by Balarac et al. [Phys. Fluids 20 (2008)], the concept of optimal estimators has been considered to identify the set of parameters most suitable to express the subfilter variance. Finally, the quality of the standard dynamic approach has been assessed.

Ravinel, Baptiste; Blanquart, Guillaume

2010-11-01

206

Numerical simulations of explosive-type generator

NASA Astrophysics Data System (ADS)

Explosive-type generator (ETG) was elaborated by Yu. Kiselev (IDG RAS) for generation of high velocity gaseous jets. It differs from previous versions of similar devices (being developed by Voytenko) by very high efficiency (about 5-10% at maximum jet velocity of around 40 km/s). The operation of the instrument is based on porous matter compression under action of cylindrical detonation wave. Plasma acceleration is intensified due to cummulation effect. The generator was successfully used in active space experiments conducted by IDG and APL and other applications. Numerical simulations of ETG operation is based on using the SOVA multi-material hydrocode which is similar to the CTH system. Explosive thermodynamics is described by empirical equation of state, ANEOS is applied for other constructional materials (including porous agent). The results allow to reproduce all stages of ETG action and obtain jet parameters closely matched experimental measurements.

Artem'eva, Nataly; Shuvalov, Valery

1999-06-01

207

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

208

Numerical simulations of convectively excited gravity waves

NASA Astrophysics Data System (ADS)

Magneto-convection and gravity waves are numerically simulated with a nonlinear, three-dimensional, time-dependent model of a stratified, rotating, spherical fluid shell heated from below. A Solar-like reference state is specified while global velocity, magnetic field, and thermodynamic perturbations are computed from the anelastic magnetohydrodynamic equations. Convective overshooting from the upper (superadiabatic) part of the shell excites gravity waves in the lower (subadiabatic) part. Due to differential rotation and Coriolis forces, convective cell patterns propagate eastward with a latitudinally dependent phase velocity. The structure of the excited wave motions in the stable region is more time-dependent than of the convective motions above. The magnetic field tends to be concentrated over giant-cell downdrafts in the convective zone but is affected very little by the wave motion in the stable region.

Glatzmaier, G. A.

1984-12-01

209

Numerical simulations to study solar wind turbulence

Numerical simulation of coupled equations of kinetic Alfven wave (KAW) and ion acoustic wave is presented in the solar wind. The nonlinear dynamical equations satisfy the modified Zakharov system of equations by taking the nonadiabatic response of the background density. The ponderomotive nonlinearity is incorporated in the wave dynamics. The effect of Landau damping of KAW is taken into account. Localization of magnetic field intensity and the wavenumber spectra (perpendicular and parallel) of magnetic fluctuations are studied in solar plasmas around 1 a.u. Our results reveal the formation of damped localized structures and the steeper spectra that are in good agreement with the observations. These damped structures and steeper turbulent spectra can be responsible for plasma heating and particle acceleration in solar wind.

Sharma, R. P.; Sharma, Nidhi; Kumar, Sanjay [Centre for Energy Studies, Indian Institute of Technology Delhi, Delhi 110016 (India); Kumar, Sachin [Department of Applied Sciences and Humanities, Ajay Kumar Garg Engineering College, Ghaziabad 201009 (India); Singh, H. D. [Department of Physical Sciences, School of Physical and Chemical Sciences, Sikkim University, Sikkim 737102 (India)

2011-02-15

210

Numerical simulation of double-diffusive convection.

NASA Astrophysics Data System (ADS)

Semi-convective mixing, as an example of double-diffusive convection, is of general importance in multicomponent fluid mixing processes. In astrophysics it occurs when a mean molecular weight gradient caused by a mixture of light material on top of heavier one counteracts the convective instability caused by a temperature gradient. Previous theoretical work has disagreed on the presence or absence of layer formation. To settle this question high resolution numerical simulations of such a semi-convective system have been performed for the 2D case. A detailed parameter study with varying Prandtl-, Lewis- and Rayleighnumber has been done. Theoretical estimations for the layering process could be validated. It is shown that semi-convection layers form under realistic stability conditions also for a parameter regime relevance to stellar astrophysics.

Zaussinger, Florian

2010-05-01

211

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

212

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

213

Numerical simulations of convectively excited gravity waves

Magneto-convection and gravity waves are numerically simulated with a nonlinear, three-dimensional, time-dependent model of a stratified, rotating, spherical fluid shell heated from below. A Solar-like reference state is specified while global velocity, magnetic field, and thermodynamic perturbations are computed from the anelastic magnetohydrodynamic equations. Convective overshooting from the upper (superadiabatic) part of the shell excites gravity waves in the lower (subadiabatic) part. Due to differential rotation and Coriolis forces, convective cell patterns propagate eastward with a latitudinally dependent phase velocity. The structure of the excited wave motions in the stable region is more time-dependent than that of the convective motions above. The magnetic field tends to be concentrated over giant-cell downdrafts in the convective zone but is affected very little by the wave motion in the stable region.

Glatzmaier, G.A.

1983-01-01

214

Numerical simulation of back discharge ignition

NASA Astrophysics Data System (ADS)

Back discharge refers to any discharges initiated at or near a dielectric layer covering a passive electrode (Czech et al 2011 Eur. Phys. J. D 65 459-74). Back discharge activity is commonly observed in electrostatic precipitators. This study aims to contribute to increasing the fundamental understanding of back discharge phenomena by using a plasma fluid model. The modelling strategy only considers the region of back discharge development as a first approach, and the numerical simulation is complemented by an experimental study. Back discharge ignition is studied with a pinhole of radius 100 µm set in a dielectric layer. First, we have considered the criterion for back discharge ignition from an electrostatic point of view, and the numerical results confirm the major role of the surface charge density deposited on the dielectric layer. Then the dynamics of back discharge in the ‘onset-streamer’ regime (Masuda and Mizuno 1977/1978 J. Electrostat. 2 375-96) is described: the discharge ignites inside the pinhole, develops outside as a cathode-directed ionizing wave, before stopping. This regime is characterized by a current pulse and the corresponding optical emission. Results obtained in experiments and simulations are in good agreement. Furthermore, this discharge regime is independent of the pinhole radius (ranging from 75 to 150 µm) despite a change in the discharge shape. Finally, an increase in the initial negative ion density or Laplacian electric field is found to be responsible for the transition from ‘onset-streamer’ to ‘space streamer’ regime, which corresponds well with experimental observations.

Jánský, Jaroslav; Gaychet, Sylvain; Bessières, Delphine; Soulem, Nicolas; Paillol, Jean; Lemont, Florent

2014-02-01

215

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

216

Reconnection Diffusion, Star Formation, and Numerical Simulations

NASA Astrophysics Data System (ADS)

We consider fast magnetic reconnection that takes place within turbulent magnetic flux and show that the process results in diffusion of magnetic fields and matter, which we term reconnection diffusion. The process of reconnection diffusion is based on the model of 3D reconnection of weakly turbulent magnetic fields and is applicable to both fully ionized and partially ionized gas. The rate of reconnection diffusion does not depend on the level of ionization and therefore the usually employed ambipolar diffusion idea gets irrelevant for magnetic field transport in turbulent fluids. We claim that the reconnection diffusion process is a manifestation of the violation of flux conservation in highly conducting turbulent fluids. We discuss the consequences of reconnection diffusion for star formation and stress. We show that reconnection diffusion on large scales is independent of small scale magnetic field dynamics of magnetic fields. We conclude that numerical simulations correctly represents the diffusion of actual astrophysical magnetic fields in flows with substantially larger Lundquist numbers if these simulated regions regions are turbulent.

Lazarian, A.

2013-04-01

217

Numerical simulation of premixed turbulent methane combustion

With adaptive-grid computational methodologies and judicious use of compressible and low Mach number combustion models, we are carrying out three-dimensional, time-dependent direct numerical simulations of a laboratory-scale turbulent premixed methane burner. In the laboratory experiment, turbulence is generated by a grid located in the throat of a 50mm diameter circular nozzle; swirl is be introduced by four tangential air jets spaced uniformly around the circumference of the nozzle just above the turbulence grid. A premixed methane flame is stabilized above the nozzle in the central core region where a velocity deficit is induced7the swirling flow. The time-dependent flow field inside the nozzle, from the turbulence grid and the high-speed jets, to the nozzle exit plane is simulated using an adaptive-grid embedded-boundary compressible Navier-Stokes solver. The compressible calculation then provides time-dependent boundary conditions for an adaptive low Mach number model of the swirl-stabilized premixed flame. The low Mach model incorporates detailed chemical kinetics and species transport using 20 species and 84 reactions. Laboratory diagnostics available for comparisons include characterizations of the flow field just down stream of the nozzle exit plane, and flame surface statistics, such as mean location, wrinkling and crossing frequencies.

Day, Marc S.; Bell, John B.; Almgren, Ann S.; Beckner, Vincent E.; Lijewski, Michael J.; Cheng, Robert; Shepherd, Ian; Johnson, Matthew

2003-06-14

218

Collisionless microinstabilities in stellarators. II. Numerical simulations

NASA Astrophysics Data System (ADS)

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.

2013-12-01

219

Direct numerical simulation of sheared turbulent flow

NASA Technical Reports Server (NTRS)

The summer assignment to study sheared turbulent flow was divided into three phases which were: (1) literature survey, (2) computational familiarization, and (3) pilot computational studies. The governing equations of fluid dynamics or Navier-Stokes equations describe the velocity, pressure, and density as functions of position and time. In principle, when combined with conservation equations for mass, energy, and thermodynamic state of the fluid a determinate system could be obtained. In practice the Navier-Stokes equations have not been solved due to the nonlinear nature and complexity of these equations. Consequently, the importance of experiments in gaining insight for understanding the physics of the problem has been an ongoing process. Reasonable computer simulations of the problem have occured as the computational speed and storage of computers has evolved. The importance of the microstructure of the turbulence dictates the need for high resolution grids in extracting solutions which contain the physical mechanisms which are essential to a successful simulation. The recognized breakthrough occurred as a result of the pioneering work of Orzag and Patterson in which the Navier-Stokes equations were solved numerically utilizing a time saving toggling technique between physical and wave space, known as a spectral method. An equally analytically unsolvable problem, containing the same quasi-chaotic nature as turbulence, is known as the three body problem which was studied computationally as a first step this summer. This study was followed by computations of a two dimensional (2D) free shear layer.

Harris, Vascar G.

1994-01-01

220

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

221

Numerical simulation of LIGO input optics

NASA Astrophysics Data System (ADS)

Numerical analysis has been carried out to understand the performance of the Input Optics used in the first generation of LIGO (Laser Interferometer Gravitational-wave Observatory) detector. The input optics is a subsystem consisting of a mode cleaner and mode-matching telescope, where all the optics are suspended and installed in vacuum. Using the end-to-end package (LIGO programming language), computer codes have been made to simulate the input optics. Giving realistic seismic noise to the suspension point of the optics and using the length sensing/alignment sensing control for the mode cleaner, the performance of the input optics has been simulated under various scenarios such as with an order of magnitude higher seismic noise than the normal level, and with/without the alignment sensing control feedback from the arm cavity to the mode-matching telescope. The results are assessed in terms of the beam pointing fluctuation of the laser beam going into the arm cavities, and its influence on the optical coupling to the arm cavities and the noise level at the gravitational wave port signal.

None, Shivanand; Jamal, Nafis; Yoshida, Sanichiro

2005-11-01

222

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

223

When the detonation reaction-zone length, {eta}{sub r}, is short in comparison to the dimensions of the explosive piece being burnt, the detonation can be viewed as a propagating surface (or front) separating burnt from unburnt material. If the product of the shock curvature, {kappa} and {eta}{sub r} is small (i.e., the scaled shock curvature satisfies the {vert_bar}{kappa}{eta}{sub r}{vert_bar} {much_lt} 1), then to leading order the speed of this surface, D{sub n}({kappa}) is a function only of {kappa}. It is in this limit that the original version of the asymptotic detonation front theory, called detonation shock dynamics (DSD), derives the propagation law, D{sub n}({kappa}). In this lecture, the authors compare D{sub n}({kappa})-theory with the results obtained with high-resolution direct numerical simulations (DNS), and then use the DNS results to guide the development of extended asymptotic front theories with enhanced predictive capabilities.

Aslam, T.D.; Bdzil, J.B.

1998-02-01

224

A numerical simulation of the Catalina Eddy

A shallow cyclonic eddy termed the Catalina Eddy has occasionally been observed during summer in the bight of southern California. The Catalina Eddy occurs within {approximately}100 km from the coastal mountains with a depth typically extending up to the marine inversion level of several hundred meters above sea level and a diameter on the order of 100--200 km. The Catalina Eddy is produced by the interaction between the synoptic-scale northerly flow and the formidable topography along the southern California coast. A favorable synoptic situation that enhances the increased low-level climatological northerly flow along the central California coastline is the presence of the prominent east-west pressure gradient between the subtropical East Pacific high and the inland thermal low over California. Increased northerlies impinging on the San Rafael mountains north of Santa Barbara result in enhanced mesoscale lee troughing in the bight and establishment of a narrow ridge alongshore, leading to establishment of cyclonic vorticity in the bight. This paper describes numerical simulations and predictions of a Catalina Eddy event with a high-resolution multi-level limited area model. The model is initialized and forced at the lateral boundaries by the National Meteorological Center`s (NMC) 2.5{degree} {times} 2.5{degree} global objective analysis and also by NMC`s medium range forecast model (MRF) 1--10 day forecasts. In the authors previous effort to simulate mesoscale disturbances such as the Catalina Eddy the integrations were performed up to 1 model-day utilizing the NMC analysis as fixed lateral boundary conditions. In this paper they describe the results of continuous 5- to 7-day simulations of the Catalina Eddy event of 26--30 June 1988 by utilizing time-dependent lateral boundary conditions obtained from NMC`s global objective analysis as well as NMC`s MRF forecasts.

Ueyoshi, Kyozo; Roads, J.O. [Scripps Institution of Oceanography, La Jolla, CA (United States); Alpert, J. [National Meteorological Center, Washington, DC (United States)

1991-12-31

225

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

226

On the problem of optimization of DNS root servers' placement

The Domain Name System (DNS) is a critical component of the modern Internet. It provides a critical link between human users and Internet routing infrastructure by map- ping host names to IP addresses. The DNS is a hierarchy of distributed system of servers anchored at 13 DNS root servers. In this paper we examine the macroscopic connectivity between the DNS

Tony Lee; Bradley Huffaker; Marina Fomenkov

2003-01-01

227

Numerical simulations of interacting disk galaxies

NASA Technical Reports Server (NTRS)

Galaxy-galaxy interactions have long attracted many extragalactic astronomers in various aspects. A number of computer simulations performed in the 1970s have successfully reproduced the peculiar morphologies observed in interacting disk galaxies and clarified that tidal deformation explains most of the observed global peculiarities. However, most of these simulations have used test particles in modelling the disk component. Tidal response of a self-gravitating disk remains to be further clarified. Another topic which is intensely discussed at present is the relation between galaxy-galaxy interactions and activity. Many observations suggest that interactions trigger strong starbursts and possibly active galactic nuclei (AGN). However, the detailed mechanism of triggering is not yet clear. It is vital here to understand the dynamics of interstellar gas. In order to understand various phenomena related to galaxy-galaxy interactions (mainly for disk galaxies), the author performed a series of numerical simulations on close galaxy encounters which includes both interstellar gas and self-gravitating disk components. In these simulations, the galaxy model to be perturbed (target galaxy) consists of a halo and a disk. The halo was treated as a rigid spherical gravitational field which is assumed to remain fixed during the interaction. The disk is composed of stars and gas. The stellar disk was constructed by 20000 collisionless particles of the same mass. Those particles move in the halo gravitational field, interacting with each other and with the perturber. Therefore, the self-graviy of the disk is properly taken into account. Stellar particles were initially given circular velocities with small random motions required to stabilize the disk against local axisymmetric disturbances. The gravitational field of the stellar disk was calculated by the particle-mesh scheme (e.g. Hockney and Eastwood 1981). The gaseous component was modelled by the cloud-particle scheme (e.g. Roberts and Hausman 1984). Here, the authors represent the gas as an ensemble of small spheres (i.e. clouds) and include the creation of an OB star in a cloud-cloud collision and subsequent velocity push on nearby clouds due to a supernova explosion.

Noguchi, Masafumi

1990-01-01

228

Numerical simulations of interacting disk galaxies

NASA Astrophysics Data System (ADS)

Galaxy-galaxy interactions have long attracted many extragalactic astronomers in various aspects. A number of computer simulations performed in the 1970s have successfully reproduced the peculiar morphologies observed in interacting disk galaxies and clarified that tidal deformation explains most of the observed global peculiarities. However, most of these simulations have used test particles in modelling the disk component. Tidal response of a self-gravitating disk remains to be further clarified. Another topic which is intensely discussed at present is the relation between galaxy-galaxy interactions and activity. Many observations suggest that interactions trigger strong starbursts and possibly active galactic nuclei (AGN). However, the detailed mechanism of triggering is not yet clear. It is vital here to understand the dynamics of interstellar gas. In order to understand various phenomena related to galaxy-galaxy interactions (mainly for disk galaxies), the author performed a series of numerical simulations on close galaxy encounters which includes both interstellar gas and self-gravitating disk components. In these simulations, the galaxy model to be perturbed (target galaxy) consists of a halo and a disk. The halo was treated as a rigid spherical gravitational field which is assumed to remain fixed during the interaction. The disk is composed of stars and gas. The stellar disk was constructed by 20000 collisionless particles of the same mass. Those particles move in the halo gravitational field, interacting with each other and with the perturber. Therefore, the self-graviy of the disk is properly taken into account. Stellar particles were initially given circular velocities with small random motions required to stabilize the disk against local axisymmetric disturbances. The gravitational field of the stellar disk was calculated by the particle-mesh scheme (e.g. Hockney and Eastwood 1981).

Noguchi, Masafumi

1990-11-01

229

Numerical simulation of mesospheric gravity waves

NASA Astrophysics Data System (ADS)

Recently, first three-dimensionally resolved observations of polar mesospheric winter echeos (PMWEs) by a multi-beam experiment of the Middle Atmosphere Alomar Radar system (MAARSY) were published by Rapp et al. (2011). The observed PMWE at about 75 km altitude was tilted in the main flow direction (west to east). The origin of the PMWE was explained by two disparate concepts of gravity wave dynamics. On the one hand, the tilted PMWE was assumed to be aligned with the phase line of a linear gravity wave (?z ? 23 km, ?x ? 460 km) propagating at an intrinsic phase speed of - 73 m/s against the westerly wind. On the other hand, turbulence generated by breaking gravity waves was a necessary element to explain the existence of backscattering fluctuations. In addition to a thorough analysis of the synoptic meteorological conditions, high-resolution numerical simulations are performed with the all-scale geophysical flow solver EULAG (Prusa et al., 2008). The anelastic and pseudo-incompressible approximated equations are solved in a 3D computational domain covers a 1500 km long slice of Scandinavia and spans from the surface to 100 km altitude. Multiple numerical experiments are performed to explore the origin of the observed PMWE. Various hypotheses are tested. The presentation will discuss if the PMWEs were the result of breaking mountain waves excited by the flow over Scandinavia or if dynamical instabilities occuring in the highly sheared mesospheric flow led to the observed turbulence. Rapp, M., R. Latteck, G. Stober, et al., 2011: First three-dimensional observations of polar mesosphere winter echoes: Resolving space-time ambiguity. J. Geophys. Res., 116, A11307, doi:10.1029/2011JA016858. Prusa, J.M., P.K. Smolarkiewicz, A.A. Wyszogrodzki, 2008: EULAG, a computational model for multiscale flows, Comput. Fluids 37, 1193-1207.

Dörnbrack, Andreas; Rapp, Markus; Latteck, Ralph

2013-04-01

230

Numerical simulation of mesospheric gravity waves

NASA Astrophysics Data System (ADS)

Recently, first three-dimensionally resolved observations of polar mesospheric winter echeos (PMWEs) by a multi-beam experiment of the Middle Atmosphere Alomar Radar system (MAARSY) were published by Rapp et al. (2011). The observed PMWE at about 75 km altitude was tilted in the main flow direction (west to east). The origin of the PMWE was explained by two disparate concepts of gravity wave dynamics. On the one hand, the tilted PMWE was assumed to be aligned with the phase line of a linear gravity wave (? z ? 23 km, ? x ? 460 km) propagating at an intrinsic phase speed of -73 m/s against the westerly wind. On the other hand, turbulence generated by breaking gravity waves was a necessary element to explain the existence of backscattering fluctuations. In addition to a thoughrough analysis of the synoptic meteorological conditions, high-resolution numerical simulations are performed with the all-scale geophysical flow solver EULAG (Prusa et al., 2008). The anelastic and pseudo-incompressible approximated equations are solved in a 3D computational domain covers a 1500 km long slice of Scandinavia and spans from the surface to 100 km altitude. Multiple numerical experiments are performed to explore the origin of the observed PMWE. Various hypotheses are tested. The presentation will discuss if the PMWEs were the result of breaking mountain waves excited by the flow over Scandinavia or if dynamical instabilities occuring in the highly sheared mesospheric flow led to the observed turbulence. Rapp, M., R. Latteck, G. Stober, et al., 2011: First three-dimensional observations of polar mesosphere winter echoes: Resolving space-time ambiguity. J. Geophys. Res., 116, A11307, doi:10.1029/2011JA016858. Prusa, J.M., P.K. Smolarkiewicz, A.A. Wyszogrodzki, 2008: EULAG, a computational model for multiscale flows, Comput. Fluids 37, 1193-1207.

Dörnbrack, A.; Rapp, M.; Latteck, R.

2012-12-01

231

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

232

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

233

Advanced in turbulence physics and modeling by direct numerical simulations

NASA Technical Reports Server (NTRS)

The advent of direct numerical simulations of turbulence has opened avenues for research on turbulence physics and turbulence modeling. Direct numerical simulation provides values for anything that the scientist or modeler would like to know about the flow. An overview of some recent advances in the physical understanding of turbulence and in turbulence modeling obtained through such simulations is presented.

Reynolds, W. C.

1987-01-01

234

Numerical simulation of a magnetron plasma sputtering system using VORPAL

Three-dimensional numerical simulation is conducted for a magnetron sputtering plasma using the particle in cell code VORPAL. Numerical simulation of sputtering process requires accurate models of nuclear stopping in materials, particle dynamics and self-consistent electromagnetic fields. VORPAL can simulate cold dense plasma sputtering system under many different electromagnetic configurations. The dynamics of both incident particles and sputtered neutral atoms are

Chuandong Zhou; Peter H. Stoltz

2010-01-01

235

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

236

Analysis and modeling of subgrid scalar mixing using numerical data

NASA Technical Reports Server (NTRS)

Direct numerical simulations (DNS) of passive scalar mixing in isotropic turbulence is used to study, analyze and, subsequently, model the role of small (subgrid) scales in the mixing process. In particular, we attempt to model the dissipation of the large scale (supergrid) scalar fluctuations caused by the subgrid scales by decomposing it into two parts: (1) the effect due to the interaction among the subgrid scales; and (2) the effect due to interaction between the supergrid and the subgrid scales. Model comparisons with DNS data show good agreement. This model is expected to be useful in the large eddy simulations of scalar mixing and reaction.

Girimaji, Sharath S.; Zhou, YE

1995-01-01

237

Numerical Simulation of Magnetized Target Fusion

NASA Astrophysics Data System (ADS)

Magnetized Target Fusion (MTF), an Alternative Fusion Concept, requires (1) a high density (10^17-18 cm-3), warm plasma (100-300 eV) as a target, (2) a stable, high-velocity solid liner to compress the target, and (3) magnetized energy-loss inhibition for only a few 10's of microseconds. Experiments by Los Alamos on a Field Reversed Configuration and by VNIIEF on a Z-pinch are being conducted to produce the target plasma. Liner implosions suitable for a long-lived FRC target have been demonstrated in a joint Los Alamos-AFRL (Albuquerque) set of experiments. Faster liner implosions, for Z-pinch targets, have been performed jointly by VNIIEF and Los Alamos. These are close to full MTF-capability. Using these liner capabilities, we have conducted MHD simulations of typical MTF compressions. The imploding wall interaction with a hot plasma has been studied numerically, including accurate treatments of the liner material dynamics. Liner surfaces in contact with 100-1000 Tesla fields melt, but not until late in the implosion phase. Evaporation of the liner material occurs even later: little mix into the hot plasma in 1-D models.

Faehl, R. J.; Sheehey, P. T.; Kirkpatrick, R. C.; Lindemuth, I. R.

2000-10-01

238

Numerical simulations of drainage flows on Mars

NASA Technical Reports Server (NTRS)

Data collected by Viking Landers have shown that the meteorology of the near surface Martian environment is analogous to desertlike terrestrial conditions. Geological evidence such as dunes and frost streaks indicate that the surface wind is a potentially important factor in scouring of the martian landscape. In particular, the north polar basin shows erosional features that suggest katabatic wind convergence into broad valleys near the margin of the polar cap. The pattern of katabatic wind drainage off the north polar cap is similar to that observed on Earth over Antarctica or Greenland. The sensitivity is explored of Martian drainage flows to variations in terrain slope and diurnal heating using a numerical modeling approach. The model used is a 2-D sigma coordinate primitive equation system that has been used for simulations of Antarctic drainage flows. Prognostic equations include the flux forms of the horizontal scalar momentum equations, temperature, and continuity. Parameterization of both longwave (terrestrial) and shortwave (solar) radiation is included. Turbulent transfer of heat and momentum in the Martian atmosphere remains uncertain since relevant measurements are essentially nonexistent.

Parish, Thomas R.; Howard, Alan D.

1992-01-01

239

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

240

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

241

Configuration Management File Manager Developed for Numerical Propulsion System Simulation.

National Technical Information Service (NTIS)

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 Configuratio...

G. J. Follen

1997-01-01

242

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

243

Numerical Simulation of Comoving Frame Transport Equation

NASA Astrophysics Data System (ADS)

According to the fireball model, Gamma-ray Bursts (GRBs) and their afterglows are thought to be radiated from the relativistically moving fluid with Lorentz factor of tens to hundreds. To simulate these relativistic radiation phenomena, we have been developing a relativistic radiation hydrodynamics code. In our code, the radiation transport equation is solved in the comoving frame which moves with the fluid. The advantage of using the comoving frame equation is that the fundamental properties of radiation such as emissivity, absorption and scattering can be treated in the same way as when the fluid is at rest. Starting from the lab frame transport equation, we have derived the comoving frame transport equation in 2-D Cartesian coordinates. The structure of the comoving frame transport equation is similar to that of the lab frame equation except that the comoving frame equation has additional terms of radiation intensity variation over angle and energy. We numerically solve this comoving frame transport equation using an implicit method based upon the sparse linear solver which has been used to solve the lab frame transport equation. Our transport code is implemented in an AMR capable fashion using PARAMESH, a parallel adaptive mesh refinement library. We exhibit the results of some verification tests in this presentation. The comoving frame transport code will be combined with the relativistic hydrodynamics code to build up the relativistic radiation hydrodynamics code. For our radiation hydrodynamics code, we have already developed a 2-D Cartesian relativistic hydrodynamics code using the Flux Corrected Transport (FCT) algorithm with Zalesak's multi-dimensional limiter.

Kwak, Kyujin; Swesty, F.

2006-06-01

244

Effects of Computational Domain on Numerical Simulation of Building Fires

Computational fluid dynamics (CFD) modeling (or field modeling) is becoming the main method for numerical simulation of building fires. Among many factors that influence the validity and accuracy of CFD simulation results, the computational domain is sometimes overlooked. In this article, the effects of computational domain on simulation results are analyzed. Simulation results from the use of different domains are

Xiaocui Zhang; Manjiang Yang; Jian Wang; Yaping He

2010-01-01

245

Numerical control machining simulation: a comprehensive survey

Since the first numerical control (NC) machine tool was created at Massachusetts Institute of Technology in the 1950s, productivity and quality of machined parts have been increased through using NC and later computer numerical control (CNC) machine tools. Like other computer programs, errors may occur in a CNC program, which may lead to scraps or even accidents. Therefore, NC programs

Yu Zhang; Xun Xu; Yongxian Liu

2011-01-01

246

NUMERICAL NOISE PM SIMULATION IN CMAQ

We have found that numerical noise in the latest release of CMAQ using the yamo advection scheme when compiled on Linux cluster with pgf90 (5.0 or 6.0). We recommend to use -C option to eliminate the numerical noise....

247

Direct numerical simulations of transition and turbulence in smooth-walled Stokes boundary layer

NASA Astrophysics Data System (ADS)

Stokes boundary layer (SBL) is a time-periodic canonical flow that has several environmental, industrial, and physiological applications. Understanding the hydrodynamic instability and turbulence in SBL, therefore, will shed more light on the nature of such flows. Unlike its steady counterpart, the flow in a SBL varies both in space and time, which makes hydrodynamic instability and transition from laminar to turbulent state highly complicated. In this study, we utilized direct numerical simulations (DNS) to understand the characteristics of hydrodynamic instability, the transition from laminar to turbulent state, and the characteristics of intermittent turbulence in a smooth SBL for Re_? in the range of 500-1000. Simulation results show that nonlinear growth plays a critical role on the instability at Re_? = 500 and 600. However, the nonlinear growth does not warrant sustainable transition to turbulence and the outcome is highly dependent on the amplitude and spatial distribution of the initial velocity disturbance in addition to Re_? . Simulation results at Re_? = 500 confirm that instability and subsequent transitional flow will eventually decay. At Re_? = 600 nonlinear growth recurs at every modulation period but such transition does not evolve into fully developed turbulence at any time in the modulation cycle. At Re_? = 700, the flow shows features of fully developed turbulence during some modulation periods and the transitional character of Re_? = 600 at the remaining. Therefore, we conclude that flow in the range of Re_? = 600-700 is to be classified as self-sustaining transitional flow. For higher Reynolds number the flow indeed exhibits features of fully developed boundary layer turbulence for a portion of the wave period, which is known as the intermittently turbulent regime in the literature.

Ozdemir, Celalettin E.; Hsu, Tian-Jian; Balachandar, S.

2014-04-01

248

Determination of vibration-related spinal loads by numerical simulation

Objective. Dynamic spinal loads due to human whole body vibrations are extremely difficult to determine experimentally. However, they can be predicted by numerical simulation. This paper presents an approach for the prediction of dynamic spinal loads caused by whole body vibrations, as well as some basic considerations concerning the process of numerical simulation.Background. Long-term whole body vibrations have been found

Steffen Pankoke; Jörg Hofmann; Horst P. Wölfel

2001-01-01

249

glsim: A general library for numerical simulation

NASA Astrophysics Data System (ADS)

We describe glsim, a C++ library designed to provide routines to perform basic housekeeping tasks common to a very wide range of simulation programs, such as reading simulation parameters or reading and writing self-describing binary files with simulation data. The design also provides a framework to add features to the library while preserving its structure and interfaces.

Grigera, Tomás S.

2011-10-01

250

Numerical simulation of vortex structures in a near wake

NASA Astrophysics Data System (ADS)

Numerical simulation of steady and unsteady regimes for subsonic and transonic flows is realized for the classical problem concerned with the flow near a cylinder. The conservative numerical schemes are used. The large-scale vortex structures in a near-wake bluff body observed experimentally in real streams are investigated within the inviscid gas framework. For the small and moderate Reynolds number, the numerical simulation of nonseparated and separated stationary and unsteady regimes is based on the Navier-Stokes model.

Babakov, A. V.

251

Numerical Simulation of Evaporating Capillary Jets.

National Technical Information Service (NTIS)

A detailed numerical study of evaporating capillary jets is presented. The analysis is performed through use of a Galerkin finite element method with penalty formulation for solving the equations of motion and a flux method for tracking the free surface. ...

J. D. Zeda

1999-01-01

252

NASA Astrophysics Data System (ADS)

Direct Numerical Simulations (DNS) are conducted for temporally developing reacting H2/O2 shear layers at an ambient pressure of 100atm. The compressible form of the governing equations are coupled with the Peng Robinson real gas equation of state and are solved using eighth order central finite differences and fourth order Runge Kutta time integration with resolutions up to ˜3/4 billion grid points. The formulation includes a detailed pressure dependent kinetics mechanism having 8 species and 19 steps, detailed property models, and generalized forms of the multicomponent heat and mass diffusion vectors derived from nonequilibrium thermodynamics and fluctuation theory. The DNS is performed over a range of Reynolds numbers up to 4500 based on the free stream velocity difference and initial vorticity thickness. The results are then analyzed in an a priori manner to illustrate the role of the subgrid mass flux vector within the filtered form of the governing equations relevant to Large Eddy Simulations. The subgrid mass flux vector is found to be a significant term; particularly within localized regions of the flame.

Foster, Justin; Miller, Richard

2011-11-01

253

High-Reynolds number turbulent boundary layers studied by numerical simulation

NASA Astrophysics Data System (ADS)

Direct and large-eddy simulations (DNS and LES) of spatially developing high-Reynolds number turbulent boundary layers (Re? up to 4300) under zero pressure gradient are studied. The inflow of the computational domain and the tripping of the boundary layer is located at low Reynolds numbers Re? 350, a position where natural transition to turbulence can be expected. The simulation thus includes the spatial evolution of the boundary layer for an extended region, providing statistics and budget terms at each streamwise position. The data is obtained with up to O(10^10) grid points using a parallelised, fully spectral method. The DNS and LES results are critically evaluated and validated, in comparison with other relevant data, e.g. the experiments by "Osterlund et al. (1999). Quantities difficult or even impossible to measure, e.g. pressure fluctuations and complete Reynolds stress budgets, shall be discussed. In addition, special emphasis is put on a further quantification of the large-scale structures appearing in the flow, and their relation to other wall-bounded flow as e.g. channel flow. The results clearly show that with today's computer power Reynolds numbers relevant for industrial applications can be within reach for DNS/LES.

Schlatter, Philipp; Li, Qiang; Brethouwer, Geert; Johansson, Arne V.; Henningson, Dan S.

2009-11-01

254

Numerical Simulation Of Cutting Of Gear Teeth

NASA Technical Reports Server (NTRS)

Shapes of gear teeth produced by gear cutters of specified shape simulated computationally, according to approach based on principles of differential geometry. Results of computer simulation displayed as computer graphics and/or used in analyses of design, manufacturing, and performance of gears. Applicable to both standard and non-standard gear-tooth forms. Accelerates and facilitates analysis of alternative designs of gears and cutters. Simulation extended to study generation of surfaces other than gears. Applied to cams, bearings, and surfaces of arbitrary rolling elements as well as to gears. Possible to develop analogous procedures for simulating manufacture of skin surfaces like automobile fenders, airfoils, and ship hulls.

Oswald, Fred B.; Huston, Ronald L.; Mavriplis, Dimitrios

1994-01-01

255

Numerical simulation of supercritical shock wave in channel contraction

The shock wave is a discontinuous profile of rapidly varied flow. A series of shock waves in channel contraction are numerically simulated and their results are verified using experimental data. Based on time- and space-marching approaches, two high-resolution finite-difference numerical schemes are applied for simulation of two-dimensional oblique shock waves. To initiate the simulation, the body-fitted coordinate transformation is employed

Ming-Hsi Hsu; Wei-Hsien Teng; Chintu Lai

1998-01-01

256

Numerical Simulation of Two Phase Flows

NASA Technical Reports Server (NTRS)

Two phase flows can be found in broad situations in nature, biology, and industry devices and can involve diverse and complex mechanisms. While the physical models may be specific for certain situations, the mathematical formulation and numerical treatment for solving the governing equations can be general. Hence, we will require information concerning each individual phase as needed in a single phase. but also the interactions between them. These interaction terms, however, pose additional numerical challenges because they are beyond the basis that we use to construct modern numerical schemes, namely the hyperbolicity of equations. Moreover, due to disparate differences in time scales, fluid compressibility and nonlinearity become acute, further complicating the numerical procedures. In this paper, we will show the ideas and procedure how the AUSM-family schemes are extended for solving two phase flows problems. Specifically, both phases are assumed in thermodynamic equilibrium, namely, the time scales involved in phase interactions are extremely short in comparison with those in fluid speeds and pressure fluctuations. Details of the numerical formulation and issues involved are discussed and the effectiveness of the method are demonstrated for several industrial examples.

Liou, Meng-Sing

2001-01-01

257

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

258

Numerical simulation of a collapsible nonlinear structure subjected to impacts

A numerical model for the simulation of dynamic compression of a co llapsible multilayer structure subjected to transient compressive excitation is described and tested. It is shown that impacts induce a chaotic collapse of crumple layers. The simulations predict shock pulses and intralaminar forces. Simulated shock pulses that a free -falling impactor is subjected to, and the corresponding animated deformation

Michael A. Sek

259

Computational Tools for the Numerical Simulation of Welding Processes

This paper discusses various computational tools developed for the numerical simulation of welding processes. The tools can be broadly classified into two groups: Process related tools that facilitate simulation accuracy; Speed and ease-of-use related tools that facilitate simulation and operational efficiency. Process related tools that are discussed include the weld heat flux specification, the weld path and weld fillers that

Shivakumar Padmanaban; Sanjay Choudhry

2004-01-01

260

Direct numerical simulation of an isothermal reacting turbulent wall-jet

NASA Astrophysics Data System (ADS)

In the present investigation, Direct Numerical Simulation (DNS) is used to study a binary irreversible and isothermal reaction in a plane turbulent wall-jet. The flow is compressible and a single-step global reaction between an oxidizer and a fuel species is solved. The inlet based Reynolds, Schmidt, and Mach numbers of the wall-jet are Re = 2000, Sc = 0.72, and M = 0.5, respectively, and a constant coflow velocity is applied above the jet. At the inlet, fuel and oxidizer enter the domain separately in a non-premixed manner. The turbulent structures of the velocity field show the common streaky patterns near the wall, while a somewhat patchy or spotty pattern is observed for the scalars and the reaction rate fluctuations in the near-wall region. The reaction mainly occurs in the upper shear layer in thin highly convoluted reaction zones, but it also takes place close to the wall. Analysis of turbulence and reaction statistics confirms the observations in the instantaneous snapshots, regarding the intermittent character of the reaction rate near the wall. A detailed study of the probability density functions of the reacting scalars and comparison to that of the passive scalar throughout the domain reveals the significance of the reaction influence as well as the wall effects on the scalar distributions. The higher order moments of both the velocities and the scalar concentrations are analyzed and show a satisfactory agreement with experiments. The simulations show that the reaction can both enhance and reduce the dissipation of fuel scalar, since there are two competing effects; on the one hand, the reaction causes sharper scalar gradients and thus a higher dissipation rate, on the other hand, the reaction consumes the fuel scalar thereby reducing the scalar dissipation.

Pouransari, Zeinab; Brethouwer, Geert; Johansson, Arne V.

2011-08-01

261

Simulation of crossflow instability on a supersonic highly swept wing

NASA Technical Reports Server (NTRS)

A direct numerical simulation (DNS) algorithm has been developed and validated for use in the investigation of crossflow instability on supersonic swept wings, an application of potential relevance to the design of the High-Speed Civil Transport (HSCT). The algorithm is applied to the investigation of stationary crossflow instability on an infinitely long 77-degree swept wing in Mach 3.5 flow. The results of the DNS are compared with the predictions of linear parabolized stability equation (PSE) methodology. In-general, the DNS and PSE results agree closely in terms of modal growth rate, structure, and orientation angle. Although further validation is needed for large-amplitude (nonlinear) disturbances, the close agreement between independently derived methods offers preliminary validation of both DNS and PSE approaches.

Pruett, C. David

1995-01-01

262

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

263

Numerical Reference Models for Optical Metrology Simulation

Optical modeling on the computer can aid R&D efforts to enhance metrology methods, and simi- larly for lithography, alignment, and particulate monitoring. However, full exploitation of optical modeling is hindered by the lack of appropriate benchmarks for verifying algorithms and evaluat- ing approximations. To help remedy this situation we describe a preliminary set of scalar, 2D numerical reference models (NRMs).

Gregory L. Wojcik; John Mould; Egon Marx; Mark P. Davidson

264

Numerical simulation of woven fabric wrinkling

Fabric’s tendency to wrinkle is crucially important to the textile industry as it impacts on the visual appeal of apparels. Current methods of grading this characteristic are very subjective and inadequate. As such, a quantitative method for assessing fabric wrinkling is of the utmost importance for the textile community. To that end, this paper reports on a numerical study of

B. El Abed; S. Msahli; H. Bel Hadj Salah; R. Zaouali; F. Sakli

2011-01-01

265

Polarization transmission at RHIC, numerical simulations

Typical tracking simulations regarding the transmission of the polarization in the proton-proton collider RHIC are discussed. They participate in general studies aimed at understanding and improving polarization performances during polarized proton-proton runs.

Meot F.; Bai, M.; Liu, C.; Minty, M.; Ranjbar, V.

2012-05-20

266

Numerical simulations for MHD coronal seismology

NASA Astrophysics Data System (ADS)

Magnetohydrodynamic (MHD) processes are important for the transfer of energy over large scales in plasmas and so are essential to understanding most forms of dynamical activity in the solar atmosphere. The introduction of transverse structuring into models for the corona modifies the behavior of MHD waves through processes such as dispersion and mode coupling. Exploiting our understanding of MHD waves with the diagnostic tool of coronal seismology relies upon the development of sufficiently detailed models to account for all the features in observations. The development of realistic models appropriate for highly structured and dynamical plasmas is often beyond the domain of simple mathematical analysis and so numerical methods are employed. This paper reviews recent numerical results for seismology of the solar corona using MHD.

Pascoe, David James

2014-07-01

267

Numerical simulations of thin film thermal flow

NASA Astrophysics Data System (ADS)

The thin film thermal flow process in long trenches is analyzed using a simulator which solves the equations which govern viscous, incompressible fluid flow. The total thermal baking process is divided into small time steps. At each time step, we solve the governing equations using the penalty function formulation and the Galerkin finite element method to obtain local velocity vectors. The free surface of the flowing film is updated according to these local velocity vectors. As an example application, we simulate the flow of boron and phosphorus doped silicon dioxide glass films in 2 micrometer high by 2 micrometer wide, infinitely long trenches, for which two-dimensional profile evolution is appropriate. The simulated film profiles show that the local leveling rate of a film is a sensitive function of surface curvature. The simulation program predicts that lower viscosity and thicker films have superior planarization properties compared with higher viscosity and thinner films. These trends are in agreement with empirical observations and previous modeling and simulation work on glass film planarization processes.

Liao, Hung; Cale, Timothy S.

1994-12-01

268

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

269

Numerical Simulation Of Buckling In Waffle Plants

NASA Technical Reports Server (NTRS)

Accurate results obtained when fillet radii considered. Two reports describe numerical and experimental study of application of PASCO and WAFFLE computer programs to analysis of buckling in integrally machined, biaxially stiffened panel. PASCO (Panal Analysis and Sizing Code) is finite-element stress-and-strain code written for analysis and sizing of uniaxially stiffened panels. WAFFLE program provides comprehensive stress analysis of waffle panel, used to determine bending moments at interfaces.

Yin, Dah N.; Tran, Vu M.

1990-01-01

270

Numerical simulation of magmatic hydrothermal systems

The dynamic behavior of magmatic hydrothermal systems entails coupled and nonlinear multiphase flow, heat and solute transport, and deformation in highly heterogeneous media. Thus, quantitative analysis of these systems depends mainly on numerical solution of coupled partial differential equations and complementary equations of state (EOS). The past 2 decades have seen steady growth of computational power and the development of numerical models that have eliminated or minimized the need for various simplifying assumptions. Considerable heuristic insight has been gained from process-oriented numerical modeling. Recent modeling efforts employing relatively complete EOS and accurate transport calculations have revealed dynamic behavior that was damped by linearized, less accurate models, including fluid property control of hydrothermal plume temperatures and three-dimensional geometries. Other recent modeling results have further elucidated the controlling role of permeability structure and revealed the potential for significant hydrothermally driven deformation. Key areas for future reSearch include incorporation of accurate EOS for the complete H2O-NaCl-CO2 system, more realistic treatment of material heterogeneity in space and time, realistic description of large-scale relative permeability behavior, and intercode benchmarking comparisons. Copyright 2010 by the American Geophysical Union.

Ingebritsen, S. E.; Geiger, S.; Hurwitz, S.; Driesner, T.

2010-01-01

271

Numerical simulation of magmatic hydrothermal systems

NASA Astrophysics Data System (ADS)

The dynamic behavior of magmatic hydrothermal systems entails coupled and nonlinear multiphase flow, heat and solute transport, and deformation in highly heterogeneous media. Thus, quantitative analysis of these systems depends mainly on numerical solution of coupled partial differential equations and complementary equations of state (EOS). The past 2 decades have seen steady growth of computational power and the development of numerical models that have eliminated or minimized the need for various simplifying assumptions. Considerable heuristic insight has been gained from process-oriented numerical modeling. Recent modeling efforts employing relatively complete EOS and accurate transport calculations have revealed dynamic behavior that was damped by linearized, less accurate models, including fluid property control of hydrothermal plume temperatures and three-dimensional geometries. Other recent modeling results have further elucidated the controlling role of permeability structure and revealed the potential for significant hydrothermally driven deformation. Key areas for future research include incorporation of accurate EOS for the complete H2O-NaCl-CO2 system, more realistic treatment of material heterogeneity in space and time, realistic description of large-scale relative permeability behavior, and intercode benchmarking comparisons.

Ingebritsen, S. E.; Geiger, S.; Hurwitz, S.; Driesner, T.

2010-03-01

272

Numerical simulations of plasma double layers

NASA Technical Reports Server (NTRS)

The results of analytical studies of quasi-static electric fields along geomagnetic field lines are discussed. The calculations were targeted at the structure, generation mechanisms and stability parameters. The field consists of two oppositely charged layers, either weakly or strongly charged, with an electric field between. Existence conditions are defined for the double layer field and balancing requirements are explored. Details of the simulation techniques, i.e., particle in cell and Vlasov simulations, for studying the double layer are outlined, noting that both periodic and quasi-periodic simulations are used. Solutions to Poisson's equation for fixed and floating point boundary conditions are generated. Finally, attention is also given to oblique and two-dimensional magnetic double layers.

Goertz, C. K.; Borovsky, J. E.

1983-01-01

273

Numerical simulations on electrostatic hydrogen cyclotron instabilities

Both one- and two-dimensional particle simulation models have been used to study the nonlinear behavior of the electrostatic hydrogen cyclotron instabilities driven by the electron current along magnetic field. It is found that the instability saturates as a result of electron velocity space diffusion along magnetic field. The cyclotron waves remain highly coherent in the nonlinear stage. When the electron drift speed is comparable to thermal speed, substantial ion heating as well as particle cross-field diffusion comparable to Bohm diffusion has been observed. Comparisons of the simulation results with the theoretical predictions and the observations in both laboratory and space plasmas are discussed.

Okuda, H.; Cheng, C.Z.; Lee, W.W.

1981-02-01

274

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

275

Numerical simulation of compressible flows with application to noise control

Spatial direct numerical simulations of compressible plane jets exhausting into parallel streams are examined. The inviscid mathematical model captures the most dynamically important, large-scale mixing events, such as vortex roll-up and pairings. Two-dimensional simulations employ the fully explicit (2-4) method, while three- dimensional simulations use implicit spatial compact differencing with Runge-Kutta time-advancement. All simulations utilize characteristic-based boundary conditions. Shear layer

Robert Stephen Reichert

1998-01-01

276

Numerical simulation of interacting vortex tubes

The structure of the cores of interacting vortex tubes in three-dimensional incompressible hydrodynamics has been simulated by a pseudospectral method. A fast reconnection is observed for Reynolds numbers of order 1000. At higher Reynolds numbers, the core tends to flatten, suggesting the formation of vortex ribbons.

Alain Pumir; Robert M. Kerr

1987-01-01

277

Numerical simulation of interacting vortex tubes

The structure of the cores of interacting vortex tubes in three-dimensional incompressible hydrodynamics has been simulated by a pseudospectral method. A fast reconnection is observed for Reynolds numbers of order 1 000. At higher Reynolds numbers, the core tends to flatten, suggesting the formation of vortex ribbons.

Pumir, A.; Kerr, R.M.

1987-04-20

278

Numerical Simulation of Wood Chip Combustion

Wood combustion is a complex physicochemical process which involves moisture evaporation, solid pyrolysis, component transportation, gas combustion, radiation feedback and turbulent flow. In this work, the combustion of wood was modeled by means of large eddy simulation (LES), considering all the processes mentioned above. The wood mass loss rate and surface temperature of calculation agreed well with experimental data. The

Huawen Yang; Li Zhang; Kai Li

2010-01-01

279

Numerical simulations of dwarf galaxy merger trees

NASA Astrophysics Data System (ADS)

We investigate the evolution of dwarf galaxies using N-body/smoothed particle hydrodynamics simulations that incorporate their formation histories through merger trees constructed using the extended Press-Schechter formalism. The simulations are computationally cheap and have high spatial resolution. We compare the properties of galaxies with equal final mass but with different merger histories with each other and with those of observed dwarf spheroidals and irregulars. We show that the merger history influences many observable dwarf galaxy properties. We identify two extreme cases that make this influence stand out most clearly: (i) merger trees with one massive progenitor that grows through relatively few mergers and (ii) merger trees with many small progenitors that merge only quite late. At a fixed halo mass, a type (i) tree tends to produce galaxies with larger stellar masses, larger half-light radii, lower central surface brightness and, since fewer potentially angular momentum cancelling mergers are required to build up the final galaxy, a higher specific angular momentum, compared with a type (ii) tree. We do not perform full-fledged cosmological simulations and therefore cannot hope to reproduce all observed properties of dwarf galaxies. However, we show that the simulated dwarfs are similar to real ones.

Cloet-Osselaer, A.; De Rijcke, S.; Vandenbroucke, B.; Schroyen, J.; Koleva, M.; Verbeke, R.

2014-08-01

280

Numerical simulation of the plasma double layer

A one-dimensional particle-in-cell computer simulation is used to model the formation of an electrostatic double layer. The conditions for the onset of the layer formation are explored and a relation between the length of the layer and the electrostatic potential difference across is found.

Christoph K. Goertz; Glenn Joyce

1975-01-01

281

Numerical Simulation of Current Sheet Creation Above Real Active Regions

NASA Astrophysics Data System (ADS)

The current sheet creation above the preflare active region 20 MAY, 1991 has been numerically simulated in 3D MHD approximation. A current sheet development, and plasma ejection along the sheet is shown

Podgorny, A. I.; Podgorny, I. M.; Minami, S.

282

Numerical Simulation of Reactive Flow in Internal Combustion Engines.

National Technical Information Service (NTIS)

Multidimensional numerical simulations of the reactive fluid flow in an internal combustion engine cylinder are useful in helping engine designers obtain insight into the physical mechanisms governing efficiency and pollutant formation. A comprehensive nu...

L. D. Cloutman J. K. Dukowicz J. D. Ramshaw

1980-01-01

283

Characterizing Electron Temperature Gradient Turbulence Via Numerical Simulation.

National Technical Information Service (NTIS)

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 t...

W. M. Nevins J. Candy S. Cowley T. Dannert A. Dimits

2006-01-01

284

Numerical Simulation of the Thermally Generated Mountain-Foreland Circulation.

National Technical Information Service (NTIS)

A hydrostatic mesoscale model is used for numerical simulation of the thermally generated mountain-foreland circulation. For an idealized large mountain valley especially the influence of different mountain shapes and various surface properties is investi...

F. Somieski

1987-01-01

285

Numerical simulation of cross field amplifiers

Cross field amplifiers (CFA) have been used in many applications where high power, high frequency microwaves are needed. Although these tubes have been manufactured for decades, theoretical analysis of their properties is not as highly developed as for other microwave devices such as klystrons. One feature distinguishing cross field amplifiers is that the operating current is produced by secondary emission from a cold cathode. This removes the need for a heater and enables the device to act as a switch tube, drawing no power until the rf drive is applied. However, this method of generating the current does complicate the simulation. We are developing a simulation model of cross field amplifiers using the PIC code CONDOR. We simulate an interaction region, one traveling wavelength long, with periodic boundary conditions. An electric field with the appropriate phase velocity is imposed on the upper boundary of the problem. Evaluation of the integral of E{center dot}J gives the power interchanged between the wave and the beam. Given the impedance of the structure, we then calculate the change in the traveling wave field. Thus we simulate the growth of the wave through the device. The main advance of our model over previous CFA simulations is the realistic tracking of absorption and secondary emission. The code uses experimental curves to calculate secondary production as a function of absorbed energy, with a theoretical expression for the angular dependence. We have used this code to model the 100 MW X-band CFA under construction at SLAC, as designed by Joseph Feinstein and Terry Lee. We are examining several questions of practical interest, such as the power and spectrum of absorbed electrons, the minimum traveling wave field needed to initiate spoke formation, and the variation of output power with dc voltage, anode-cathode gap, and magnetic field. 5 refs., 8 figs.

Eppley, K.

1990-01-01

286

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

287

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

288

NUMERICAL METHODS FOR THE SIMULATION OF HIGH INTENSITY HADRON SYNCHROTRONS.

Numerical algorithms for PIC simulation of beam dynamics in a high intensity synchrotron on a parallel computer are presented. We introduce numerical solvers of the Laplace-Poisson equation in the presence of walls, and algorithms to compute tunes and twiss functions in the presence of space charge forces. The working code for the simulation here presented is SIMBAD, that can be run as stand alone or as part of the UAL (Unified Accelerator Libraries) package.

LUCCIO, A.; D'IMPERIO, N.; MALITSKY, N.

2005-09-12

289

Numerical simulation of 2-D oblique shock-wave reflections

The transport phenomena of oblique shock-wave 2D nonstationary oblique shock-wave reflections over compressive wedges in air are studied using numerical simulation. The operator splitting method, the MacCormack scheme, the FCT modification, and adaptive grid generation techniques are used in the numerical simulation. Calculations of the oblique wall angles show that the three reflection shapes obtained by classical pseudostationary theoretical analysis

Genwang Mao

1991-01-01

290

Numerical Methods for the Simulation of High Intensity Hadron Synchrotrons.

National Technical Information Service (NTIS)

Numerical algorithms for PIC simulation of beam dynamics in a high intensity synchrotron on a parallel computer are presented. We introduce numerical solvers of the Laplace-Poisson equation in the presence of walls, and algorithms to compute tunes and twi...

A. Luccio N. D'Imperio N. Malitsky

2005-01-01

291

Transverse waves in numerical simulations of cellular detonations

In this paper the structure of strong transverse waves in two-dimensional numerical simulations of cellular detonations is investigated. Resolution studies are performed and it is shown that much higher resolutions than those generally used are required to ensure that the flow and burning structures are well resolved. Resolutions of less than about 20 numerical points in the characteristic reaction length

Gary J. Sharpe

2001-01-01

292

Numerical simulation of shock waves generated by an opposing jet

This paper describes the numerical simulation of the shock waves generated by an opposing sonic jet exhausting counter to a supersonic free stream at Mach number 3. The flow considered is axisymmetric and inviscid, and the jet and free stream are composed of the same kind of gas. The numerical method employs the Harten-Yee second order upwind TVD scheme. The

M. Nishida; H. Nomura; S. Aso

1992-01-01

293

Numerical simulation of grating couplers for mode multiplexed systems

NASA Astrophysics Data System (ADS)

A numerical investigation of a two dimensional integrated fiber grating coupler capable of exciting several LP fiber modes in both TE and TM polarization is presented. Simulation results and an assessment of the numerical complexity of the 3D, fully vectorial finite element model of the device are shown.

Wohlfeil, Benjamin; Burger, Sven; Stamatiadis, Christos; Pomplun, Jan; Schmidt, Frank; Zimmermann, Lars; Petermann, Klaus

2014-03-01

294

Numerical Simulations of Acoustically Driven, Burning Droplets

NASA Astrophysics Data System (ADS)

The burning characteristics of fuel droplets exposed to external acoustical excitation within a microgravity environment are investigated numerically. The issue of acoustic excitation of flames in microgravity is especially pertinent to understanding the behavior of accidental fires which could occur in spacecraft crew quarters and which could be affected by pressure perturbations as result from ventilation fans or engine vibrations. Combustion of methanol fuel droplets is considered here using a full chemical reaction mechanism.(Marchese, A.J., et al., 26th Symp. (Int.) on Comb., p. 1209, 1997) The droplet and surrounding diffusion flame are situated within a cylindrical acoustic waveguide where standing waves are generated with varying frequency and amplitude. Applied sound pressure levels are limited at present to magnitudes for which the droplet shape remains spherical. A third order accurate, essentially-non-oscillatory (ENO) numerical scheme is employed to accurately resolve the spatial and temporal evolution of the flame front. Acoustically excited vs. non-excited external conditions for the burning droplet in microgravity are compared, and the effects of acoustic frequency, sound pressure level, and relative position of the droplet with respect to pressure and velocity nodes are explored.

Kim, Heon-Chang; Karagozian, Ann R.; Smith, Owen I.

1999-11-01

295

Schumann resonances on Mars: Numerical simulations

NASA Astrophysics Data System (ADS)

The study of the propagation of Extremely Low Frequency (ELF) waves is essential for the electromagnetic sounding investigations planned for some of the future Martian missions, e.g. NetLander. Future surface stations will have the possibility of continuous recording of the low frequency electromagnetic field fluctuations. Natural electromagnetic waves may be produced near the surface by electrostatic discharges in dust storms (dust devils) or geological activity and can be trapped in the resonant cavity formed by the surface and upper ionosphere. Low frequency electromagnetic waves can also travel along the magnetic field lines of the recently discovered magnetic anomalies from the magnetosphere to the surface and may produce resonant structures in the cavity. The measurements of resonant frequencies, also called Schumann frequencies, by ground stations can be used for remote sensing of the electrical conductivity of the lower ionosphere/atmosphere. The structure of the Schumann resonances is mainly determined by the geometry of the cavity and the global electrical conductivity of the ionosphere/atmosphere. We present a numerical model of electromagnetic wave propagation based on the Transmission Line method (TLM) with the aim of calculating the resonance frequencies on Mars. First the model is validated by application to the terrestrial case where the numerical results obtained are very close to the experimental ones which supports our predictions on Mars. The same model can be used to estimate the global electrical conductivity using future real measurements of ground stations or even balloons on Mars.

Molina-Cuberos, G. J.; Morente, J. A.; Porti, J.; Salinas, A.; Schwingenschuh, K.; Lichtenegger, H. I. M.; Besser, B. P.; Eichelberger, H. U.; Margineda, J.

2003-04-01

296

Numerical Simulation of Pulsed Meander Coil Emat

NASA Astrophysics Data System (ADS)

Electro magnetic acoustic transducers (EMATs) are now being widely investigated for non-contact non-destructive testing (NDT) of solid materials. This type of transducer can generate and/or detect ultrasound in electrically conductive or magnetic materials through the Lorentz force principle and/or magneto-elastic effects. This work describes about the Meander coil EMAT that is modeled using finite element method. A 2-D finite element model was developed to calculate the induced current inside the medium, and subsequently the Lorentz force density in the medium. The calculated Lorentz force density values are applied for simulating the transient ultrasonic wave generation within the medium. Meander coil EMATs that were designed using the model were used for experimental studies. Several case studies will be reported which include Rayleigh waves, Shear waves, Longitudinal and Lamb wave modes using pulsed mode of excitation. The experimental results were agreed well with the simulation results.

Dhayalan, R.; Balasubramaniam, Krishnan; Krishnamurthy, C. V.

2010-02-01

297

NUMERICAL SIMULATIONS OF FLUX COMPRESSION GENERATOR SYSTEMS

This paper presents simulation results of high voltage systems powered by a high-explosives' driven helical flux compression generator, FCG. The FCG model was benchmarked using experiments performed with a small generator with initial inductance 23 µH and final inductance 0.2 µH. The generator design allows for a very fast inductance reduction in the final stages of the compression. The armature

P. Appelgren; A. Larsson; S. E. Nyholm

298

Numerical Simulations Using the Immersed Boundary Technique

NASA Technical Reports Server (NTRS)

The immersed-boundary method can be used to simulate flows around complex geometries within a Cartesian grid. This method has been used quite extensively in low Reynolds-number flows, and is now being applied to turbulent flows more frequently. The technique will be discussed, and three applications of the method will be presented, with increasing complexity. to illustrate the potential and limitations of the method, and some of the directions for future work.

Piomelli, Ugo; Balaras, Elias

1997-01-01

299

Numerical simulations of bent, disrupted radio jets

NASA Technical Reports Server (NTRS)

We present preliminary results from three-dimensional hydrodynamical simulations designed to investigate the physics of jet bending and disruption. The specific scenario considered here involves a mildly supersonic jet crossing a contact discontinuity at the interface between the interstellar medium (ISM) and the intercluster medium (ICM) and then encountering a cross-wind in the ICM. The resultant morphologies show many of the features observed in radio sources including jet flaring, bending, and extended tails.

Loken, Chris; Burns, Jack O.

1993-01-01

300

Numerical simulation of magma chamber dynamics

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

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

2010-01-01

301

Numerical Simulations of Europa Hydrothermal Plumes

NASA Astrophysics Data System (ADS)

The liquid water interiors of Europa and other icy moons of the outer solar system are likely to be driven by geothermal heating from the sea floor, leading to the development of buoyant hydrothermal plumes. These plumes potentially control icy surface geomorphology, and are of interest to astrobiologists. We have performed a series of simulations of these plumes using the MITGCM. We assume in this experiment that Europa's ocean is deep (of order 100 km) and unstratified, and that plume buoyancy is controlled by temperature, not composition. A series of experiments was performed to explore a limited region of parameter space, with ocean depth H ranging from 50 to 100 km deep, source heat flux Q between 1 and 10 GW, and values of the Coriolis parameter f between 30% and 90% of the Europa average value. As predicted by earlier work, the plumes in our simulations form narrow cylindrical chimneys (a few km across) under the influence of the Coriolis effect. These plumes broaden over time until they become baroclinically unstable, breaking up into cone-shaped eddies when they become 20-35 km in diameter; the shed eddies are of a similar size. Large-scale currents in the region of the plume range between 1.5 and 5 cm/s; temperature anomalies in the plume far from the seafloor are tiny, varying between 30 and 160 microkelvin. Variations in plume size, shape, speed, and temperature are in excellent agreement with previous laboratory tank experiments, and in rough agreement with theoretical predictions. Plume dynamics and geometry are controlled by a "natural Rossby number" which depends strongly on depth H and Coriolis parameter f, but only weakly on source heat flux Q. However, some specific theoretical predictions are not borne out by these simulations. The time elapsed between startup of the source and the beginning of eddy-shedding is much less variable than predicted; also, the plume temperature varies with ocean depth H when our theory says it should not. Both of these results can be explained by noting that the theory assumes that mixing between plume fluid and ambient fluid occurs only very near the heat source, but this does not appear to be true in the simulations. 3-d view of simulated Europa plume. Boundary indicated by 3-d surface; flat surfaces at left and top show temperature in sections through the plume.

Goodman, J. C.; Lenferink, E.

2009-12-01

302

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

303

Numerical simulation of unsteady thermal Marangoni flows

NASA Astrophysics Data System (ADS)

A two-dimensional numerical code for the analysis of unsteady thermal Marangoni flows which solves the full Navier-Stokes equation for natural, Marangoni, or combined free convection in plane and axisymmetric geometries and arbitrary variations of surface tension with temperature is developed. The equations are discretized using a semi-implicit finite difference scheme that preserves conservative and transport properties within cell mesh and allows the use of time-steps much larger than those relative to a purely explicit technique. Results pertain to a plane configuration with surface tension varying linearly with temperature. The time evolution of the velocity and temperature fields, and the assessement of the range of validity of the so-called shallow-cavity approximation for steady state are discussed.

Napolitano, L. G.; Golia, C.; Viviani, A.

1984-12-01

304

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

305

Scramjet Propulsive Flowpath Design and Numerical Simulation

NASA Astrophysics Data System (ADS)

The integrated propulsive flowpath of scramjet configuration was preliminarily designed and analyzed in this paper. The flow-fields characteristics and performance of the designed two-dimensional integrated propulsive flowpath were numerically calculated under various equivalent fuel-air ratio conditions, using computational fluid dynamics methods. The calculation results were then compared with the experimental data on some typical conditions, and the flow-field and performance of the integrated scramjet flowpath with different equivalent fuel-air ratios were analyzed and discussed in detail. The investigation results from these efforts showed that: (1) the inlet function was beyond disturbances by combustion induced shock wave and pressure fluctuations under the equivalent fuel-air ratio condition of 1.0, which well satisfied the design requirements; (2) with the increasing equivalent fuel-air ratio, the combustion intensity in the combustor was significantly enhanced, resulting in an increasing net-thrust of the propulsive flowpath.

Li, Jian-ping; Song, Wen-yan; Liu, Xin

2014-06-01

306

Numerical simulation for cosmological fluid flows

NASA Astrophysics Data System (ADS)

We describe a hydrodynamical simulation of the evolution of cosmological perturbations throughout recombination up to gravitational collapse. The cosmological model is the standard one with different values of the density parameter Omega0 and a nonbaryonic component. We follow the evolution of the mixture of radiation and matter by solving the radiation transfer and hydrodynamical equations. Thomson scattering is here the dominating process and we use a formalism close to the one proposed by Hsieh & Spiegel (1976). This fully nonlinear code allows us to give a coherent description from the initial prerecombination conditions until the formation of the first nonlinear gravitational objects, and shows the behavior of shocks before recombination.

Ducloux, E.; Leorat, J.; Gerbal, D.; Alecian, G.

1992-04-01

307

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

308

Numerical simulation of electromagnetic turbulence in tokamaks

Nonlinear two- and three-fluid equations are written for the time evolution of the perturbed electrostatic potential, densities, vector potential, and parallel ion motion of collisional and trapped electron plasmas in tokamak geometry. The nonlinear terms arise from the E x B/sub 0/ convection (d/dt = partial/partialt+v/sub E/ x del/sub perpendicular/) and magnetic flutter (del-tilde/sub parallel/ = del/sub parallel/+(B/sub perpendicular//B/sub 0/) x del/sub perpendicular/). Simplified two-dimensional (k/sub perpendicular/) mode coupling simulations with a fixed average parallel wavenumber (k/sub parallel/ = 1/Rq) and curvature drift (..omega../sub g/ = (L/sub n//R)..omega../sub asterisk/ ) characteristic of outward ballooning are performed. Homogeneous stationary turbulent states of the dissipative drift and interchange modes from 0< or =..beta..<..beta../sub crit/ for both the collisional and trapped electron plasmas are obtained. Transport coefficients associated with E x B and magnetic motions are calculated. The problem of simulating plasmas with high viscous Reynolds number is treated with an absorbing mantle at the largest wavenumbers.

Waltz, R.E.

1985-02-01

309

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

310

Numerical Simulation of Driven Electron Acoustic Waves.

NASA Astrophysics Data System (ADS)

Electron-acoustic waves (EAW's) are nonlinear modes that can exist even at low amplitude. Within linear theory, EAW's would be heavily Landau damped because the wave phase velocity is comparable to the electron thermal velocity (? 1.3 k vth). However, the nonlinearity (trapped particles) effectively turns off Landau damping. This paper uses Eulerian and PIC simulations to investigate the excitation and stability of EAW's. Successful excitation occurs when a relatively low amplitude driver field is applied resonantly for a sufficiently long time (many trapping periods). The excited EAW rings at nearly constant amplitude long after the driver is turned off, provided that the EAW has the longest wavelength that fits into the simulation domain. Otherwise, the EAW decays to a longer wavelength EAW. In phase space, this decay to a longer wavelength EAW appears as a merger of the vortex-like trapped particle distributions. In recent experiments with pure electron plasma columns (see poster by Kabantsev and Driscoll), EAW’s were successfully excited at the predicted resonant frequency, and the predicted decay to longer wavelength was observed. J.P. Holloway and J.J. Dorning, Phys Rev A 44 3856 (1991). F. Valentini, T.M. O'Neil, D.H.E. Dubin, Phys Plas 13 052303 (2006).

Valentini, F.; Dubin, D. H. E.; O'Neil, T. M.

2006-10-01

311

Linking Intrinsic GRB Properties with Numerical Simulations

NASA Astrophysics Data System (ADS)

Since their discovery in the late 1960s, gamma-ray bursts (GRBs) have established a reputation of being mysterious astrophysical phenomena. This well-deserved reputation is a result of complex physical processes hidden from observation, and extreme energies previously unobserved. GRB observations have provided measurements of several physical parameters, including information about the energy and the dynamics of these events. From the several decades of observational data, it is widely believed that GRBs have beamed emission, leading to their association with relativistic jets. The progenitor of the observed emission is still unknown, although recent clues have advanced our understanding. Due to observational limitations, we must rely on theoretical studies and computational simulations to further understand the complex physical processes. In this paper we discuss the link between GRB observable parameters and those required for computational simulations. Following the spirit of this conference on AGN, Microquasar and GRB jets, this information is intended for an audience with a wide appreciation of jets, but without detailed knowledge of GRBs.

Richardson, G. A.

2006-09-01

312

Asteroid families: observational results versus numerical simulations

NASA Astrophysics Data System (ADS)

Asteroids families are groups of small bodies that share certain orbital and spectral properties. Families are concentrations in the proper elements phase space. It is believed that they are the result of energetic collisions that break-up and disperse the original parent bodies. In this presentation we compare the rotational periods of family members measured by observations made in Observatorio Pico dos Dias, Estacion Astrofisica Bosque Alegre and Casleo observatories to the rotational periods resulting from simulations of the collisional breakup of a rubble pile asteroid using a model recently developed by us. We used the observed light curves of asteroids members of the Maria and Themis families to estimate the period and shape of each object. Themis and Maria families have around 300 and 150 members, respectively. Our observation consist of 10 members of the Maria Family and 12 from Themis family, adding this to the data from the literature we have a data set of 33 objects for Themis and 20 for Maria. The estimated shapes of the objects are fed into the model in order to obtain a distribution of rotational periods after break-up. This distribution is then compared to the observed distribution. The observed and simulated distributions show some differences due to the fact that the observed distribution were affected by the subsequent collisional evolution of the family after its formation, but overall shapes are comparable.

Duffard, R.; Alvarez-Candal, A.; Angeli, C.; Roig, F.; Lazzaro, D.

2003-08-01

313

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

314

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

315

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

316

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

317

Numerical Simulation on Spray Characteristics in Direct Injection Gasoline Engine

In this paper, a simulation model was built for in- cylinder stratified charged and lean combustion GDI engine, and the spray model was validated. Based on this, numerical simulation of mixture stratified combustion was performed in a gasoline direct injection (GDI) engine with AVL FIRE. The effects of different injection timings on the mixture quality near the spark plug were

Shuhua Li; Ying Gao; Jun Li; Shichun Yang

2011-01-01

318

Numerical Simulation of Coal Floor Fault Activation Influenced by Mining

By means of the numerical simulation software ANSYS, the activation regularity of coal floor faults caused by mining is simulated. The results indicate that the variation in horizontal, vertical and shear stresses, as well as the horizontal and vertical displacements in the upper and the lower fault blocks at the workface are almost identical. Influenced by mining of the floor

Lian-guo WANG; Xie-xing MIAO

2006-01-01

319

Numerical Simulation of One and Two-Dimensional ESEEM Experiments

Numerical simulation has become an indispensable tool for the interpretation of pulse EPR experiments. In this work it is shown how automatic orientation selection, grouping of operator factors, and direct selection and elimination of coherences can be used to improve the efficiency of time-domain simulations of one- and two-dimensional electron spin echo envelope modulation (ESEEM) spectra. The program allows for

Z. L. Madi; S. Van Doorslaer; A. Schweiger

2002-01-01

320

NUMERICAL SIMULATION OF GROUND MOVEMENTS CAUSED BY PIPE

Results of ground movements obtained from numerical and physical simulations of pipe bursting are compared. Physical simulation involves the specific case where an existing intact clay pipe with an external diameter of 184 mm backfilled with poorly-graded dense sand was replaced with a polyethylene pipe with an outside diameter of 165 mm. A commercially available burst head was used featuring

Ian D. Moore

321

Numerical simulation of focusing process of reflected shock waves

The focusing effect of concave parabolic reflectors on shock-wave propagation in a duct is investigated experimentally and by means of numerical simulations. The simulation results are presented graphically and compared with shadowgraphs. It is found that pressure amplification increases with initial shock strength but saturates at Mach 2 or above. At higher incident-shock Mach numbers, the ratio of gasdynamic to

M. Nishida; H. Kishige

1988-01-01

322

Numerical Simulation of Thermal Process in an Industrial Rotary Furnace

A numerical simulation was performed for the complex thermal processes of heating steel bars in a rotary furnace, which involve both the momentum transfer and the energy transfer mainly by radiation and combustion. A CFD commercial software CFX was employed to solve the proposed 2-D mathematical model. The boundary conditions for the simulation were initially chosen basing on on-line measured

Zeyi Jiang

2005-01-01

323

Numerical Simulations of Shock Wave-driven Chromospheric Jets

We present the results of numerical simulations of shock wave-driven jets in the solar atmosphere. The dependence of observable quantities such as maximum velocity and deceleration on parameters such as the period and amplitude of initial disturbances and the inclination of the magnetic field is investigated. Our simulations show excellent agreement with observations, and shed new light on the correlation

L. Heggland; B. De Pontieu; V. H. Hansteen

2007-01-01

324

Fast numerical simulation for full bore rupture of pressurized pipelines

An efficient numerical simulation (CNGS-MOC), based on the method of characteristics for simulating full bore rupture of long pipelines containing two-phase hydrocarbons, was developed. The use of curved characteristics, in conjunction with a compound nested grid system, as well as a fast mathematical algorithm, lead to a significant reduction of CPU time, while improving accuracy. The model is validated extensively

Haroun Mahgerefteh; Pratik Saha; Ioannis G. Economou

1999-01-01

325

Numerical Simulation of Propeller Performance with an Inclined Shaft Arrangement.

National Technical Information Service (NTIS)

A propeller in an inclined shaft arrangement has been simulated using a Reynolds-Averaged Navier-Stokes solver. The commercially available codes Ansys' Fluent and IcemCFD were used for the numerical simulation. The method has been demonstrated through a s...

C. Dai S. Schroeder

2010-01-01

326

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

327

NASA Astrophysics Data System (ADS)

The present study addresses the capability of a large set of shock-capturing schemes to recover the basic interactions between acoustic, vorticity and entropy in a direct numerical simulation (DNS) framework. The basic dispersive and dissipative errors are first evaluated by considering the advection of a Taylor vortex in a uniform flow. Two transonic cases are also considered. The first one consists of the interaction between a temperature spot and a weak shock. This test emphasizes the capability of the schemes to recover the production of vorticity through the baroclinic process. The second one consists of the interaction of a Taylor vortex with a weak shock, corresponding to the framework of the linear theory of Ribner. The main process in play here is the production of an acoustic wave. The results obtained by using essentially non-oscillatory (ENO), total variation diminishing (TVD), compact-TVD and MUSCL schemes are compared with those obtained by means of a sixth-order accurate Hermitian scheme, considered as reference. The results are as follows; the ENO schemes agree pretty well with the reference scheme. The second-order accurate Upwind-TVD scheme exhibits a strong numerical diffusion, while the MUSCL scheme behavior is very sensitive to the value on the parameter in the limiter function minmod. The compact-TVD schemes do not yield improvement over the standard TVD schemes. Copyright

Tenaud, C.; Garnier, E.; Sagaut, P.

2000-05-01

328

Numerical simulation of rough-surface aerodynamics

NASA Astrophysics Data System (ADS)

Computational fluid dynamics (CFD) simulations of flow over surfaces with roughness in which the details of the surface geometry must be resolved pose major challenges. The objective of this study is to address these challenges through two important engineering problems, where roughness play a critical role---flow over airfoils with accrued ice and flow and heat transfer over turbine blade surfaces roughened by erosion and/or deposition. CFD simulations of iced airfoils face two major challenges. The first is how to generate high-quality single- and multi-block structured grids for highly convoluted convex and concave surface geometries with multiple scales. In this study, two methods were developed for the generation of high-quality grids for such geometries. The method developed for single-block grids involves generating a grid about the clean airfoil, carving out a portion of that grid about the airfoil, replacing that portion with a grid that accounts for the accrued ice geometry, and performing elliptic smoothing. The method developed for multi-block grids involves a transition-layer grid to ensure jaggedness in the ice geometry does not propagate into the domain. It also involves a "thick" wrap-around grid about the ice to ensure grid lines clustered next to solid surfaces do not propagate as streaks of tightly packed grid lines into the domain along block boundaries. For multi-block grids, this study also developed blocking topologies that ensure solutions to multi-block grids converge to steady state as quickly as single-block grids. The second major challenge in CFD simulations of iced airfoils is not knowing when it will predict reliably because of uncertainties in the turbulence modeling. In this study, the effects of turbulence models in predicting lift, drag, and moment coefficients were examined for airfoils with rime ice (i.e., ice with jaggedness only) and with glaze ice (i.e., ice with multiple protruding horns and surface jaggedness) as a function of angle of attack. In this examination, three different CFD codes---WIND, FLUENT, and PowerFLOW were used to examine a variety of turbulence models, including Spalart-Allmaras, RNG k-epsilon, shear-stress transport, v2-f, and differential Reynolds stress with and without non-equilibrium wall functions. The accuracy of the CFD predictions was evaluated by comparing grid-independent solutions with measured experimental data. Results obtained show CFD with WIND and FLUENT to predict the aerodynamics of airfoils with rime ice reliably up to near stall for all turbulence models investigated. (Abstract shortened by UMI.)

Chi, Xingkai

329

Numerical Simulation of Bioparticle Manipulation Using Dielectrophoresis

NASA Astrophysics Data System (ADS)

The manipulation of biological particles (cells, viruses DNA, proteins) is an important research subject in microfluidics. The dielectrophoresis (DEP) based on AC electrokinetics became one of the most widely used tools in particle separation, due to the low operating voltage (1-20 V), which significantly diminishes the Joule heating of the suspension. The present paper presents a 2-D model for the DEP-based separation microsystem consisting of a microchannel controlled with an interdigitated electrode array. The dielectrophoretic force produced by the nonuniform electric field acts on the bioparticles in suspension in fluid medium inducing spatial movement. The force depends on a number of parameters such as the geometry of the electrodes, the frequency of the applied electric field, as well as dielectric properties of the particles and the surrounding medium. The behavior of this type of system is numerically investigated. The electrical field distribution, the DEP force and the concentration profile in the microchannel are computed using a code based on the finite element technique. The analysis of the role of different problem parameters confirms the potential applications of DEP in handling and separation of particles.

Neculae, A.; Bunoiu, O. M.; Lungu, M.

2010-08-01

330

Boundary layer control on magnetohydrodynamic numerical simulations

NASA Astrophysics Data System (ADS)

In the context of planetary interiors the geometry and magnitude of magnetic fields has been found to be controlled by the heat flux through the outer boundary (e.g., Olson & Christensen 2006). However, there is indication, based on experimental work, that the balance between thermal, viscous, and Coriolis effects defines the flow behaviour in non-magnetic fluids by means of boundary layer control (King et al. 2009). Using numerical models of magnetohydrodynamic fluids in spherical shells, we find that the force balances at the top boundary may be determining the overall behaviour of the flow. Similar to flow control by boundary layers in non-magnetic convective systems, we find that boundary layers associated with temperature, viscosity, Coriolis forces, and magnetic fields play an important role in determining the large scale flow. Boundary layer relative thicknesses are found to correlate with the internal force balances. For planetary dynamos, this implies that the boundary conditions at the top of the dynamo region - not only the heat flux through the boundary - control the mode of convection as well as magnetic field magnitude, geometry and secular variation.

Gomez Perez, N.; Heimpel, M. H.

2012-12-01

331

Towards the numerical verification of plasma simulation codes

NASA Astrophysics Data System (ADS)

To aid in verification of existing and new plasma simulation codes, we propose a suite of standard simulation problems against which a new code would be compared with. Each standard problem provides a detailed input specifications and results in forms of tables of numeric values. The problems use an idealized and simplified reaction cross-section and rates set. The problems are designed to verify individual numerical components of plasma simulation codes and the overall plasma simulation. The issue of establishing a ``correct'' plasma simulation result will be discussed. In addition, we will discuss the portability of these problems: the problems should be specified in a manner that can be read by simulation codes written in different languages, and executed on different platforms.

Vukovic, Mirko

2012-10-01

332

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

333

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

334

Relevance of numerical simulations to booming sand

NASA Astrophysics Data System (ADS)

We have performed a simulation study of three-dimensional cohesionless granular flows down an inclined chute. We find that the oscillations observed in [L. E. Silbert, Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.94.098002 94, 098002 (2005)] near the angle of repose are harmonic vibrations of the lowest normal mode. Their frequencies depend on the contact stiffness as well as on the depth of the flow. Could these oscillations account for the phenomena of “booming sand”? We estimate an effective contact stiffness from the Hertz law, but this leads to frequencies that are several times higher than observed. However, the Hertz law also predicts interpenetrations of a few nanometers, indicating that the oscillations frequencies are governed by the surface stiffness, which can be much lower than the bulk one. This is in agreement with previous studies ascribing the ability to sing to the presence of a soft coating on the grain surface.

Richard, Patrick; McNamara, Sean; Tankeo, Merline

2012-01-01

335

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

336

SIRTF controller simulation - Instability masked by numerical integration

NASA Technical Reports Server (NTRS)

In the course of a simulation study of a candidate design for the Space Infrared Telescope Facility (SIRTF), an unusual phenomenon was observed. The uncompensated control system was unstable, but a numerical simulation with the fixed-step-size classical fourth-order Runge-Kutta method gave a stable response. This phenomenon is described in the setting in which it occurred. The Runge-Kutta simulation model is analyzed as a discrete linear system and shown to be stable, thus corroborating the numerical results.

Rajan, N.; Alexandro, F. J.; Hirata, J.

1988-01-01

337

Numerical Simulation of Cold Dense Plasma Sputtering with VORPAL

NASA Astrophysics Data System (ADS)

Sputtering is an evaporation process that physically removes atoms from a solid target material. This process takes place under bombardment of the target surface by energetic ions. Sputtering is widely applied in material processing and coating, such as etching and thin film deposition. Numerical simulation of sputtering process requires both accurate models of nuclear stopping in materials, particle dynamics and consistent electromagnetic fields. The particle in cell code VORPAL can simulate cold dense plasma under many different electromagnetic configurations. The dynamics of both incident particles and sputtered neutral atoms are simulated in VORPAL, and the sputtering yield is calculated from a standalone numerical library for a variety of materials that are commonly used in industrial applications. Numerical simulation of the spatial distribution of sputtering resulting from a cold dense plasma under externally applied magnetic field and self-consistent electric field is presented.

Zhou, Chuandong; Stoltz, Peter; Veitzer, Seth

2009-10-01

338

Batman-cracks. Observations and numerical simulations

NASA Astrophysics Data System (ADS)

To ensure mechanical strength of fiber reinforced plastics (FRP), good adhesion between fibers and the matrix is considered to be an essential requirement. An efficient test of fiber-matrix interface characterization is the fragmentation test which provides information about the interface slip mechanism. This test consists of the longitudinal loading of a single fiber which is embedded in a matrix specimen. At critical loads the fiber experiences fragmentation. This fragmentation will terminate depending upon the shear-slip strength of the fiber-matrix adhesion, which is inversely proportional to average fragment lengths. Depending upon interface strength characteristics either bond or slip matrix fracture can occur at the onset of fiber fracture. Certain particular features of matrix fracture are observed at the locations of fiber fracture in situations where there is sufficient interface bond strength. These refer to the development of fractures with a complex surface topography. The experimental procedure involved in the fragmentation tests is discussed and the boundary element technique to examine the development of multiple matrix fractures at the fiber fracture locations is examined. The mechanics of matrix fracture is examined. When bond integrity is maintained, a fiber fracture results in a matrix fracture. The matrix fracture topography in a fragmentation test is complex; however, simplified conoidal fracture patterns can be used to investigate the crack extension phenomena. Via a mixed-mode fracture criterion, the generation of a conoidal fracture pattern in the matrix is investigated. The numerical results compare favorably with observed experimental data derived from tests conducted on fragmentation test specimens consisting of a single glass fiber which is embedded in a polyester matrix.

Selvadurai, A. P. S.; Busschen, A. Ten; Ernst, L. J.

1991-05-01

339

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

340

Numerical simulations of radiatively driven dusty winds

NASA Astrophysics Data System (ADS)

Radiation pressure on dust grains may be an important mechanism in driving winds in a wide variety of astrophysical systems. However, the efficiency of the coupling between the radiation field and the dusty gas is poorly understood in environments characterized by high optical depths like those in ultraluminous infrared galaxies (ULIRGs) and massive dense star clusters. We present a series of idealized numerical experiments, performed with the radiation-hydrodynamic code ORION, in which we study the dynamics of such winds and quantify their properties. We find that, after wind acceleration begins, radiation Rayleigh-Taylor instability forces the gas into a configuration that reduces the rate of momentum transfer from the radiation field to the gas by a factor of ˜10-100 compared to an estimate based on the optical depth at the base of the atmosphere; instead, the rate of momentum transfer from a driving radiation field of luminosity L to the gas is roughly L/c multiplied by one plus half the optical depth evaluated using the photospheric temperature, which is far smaller than the optical depth one would obtain using the interior temperature. When we apply our results to conditions appropriate to ULIRGs and star clusters, we find that the asymptotic wind momentum flux from such objects should not significantly exceed that carried by the direct radiation field, L/c. This result constrains the expected mass-loss rates from systems that exceed the Eddington limit to be of the order of the so-called `single-scattering' limit, and not significantly higher. We present an approximate fitting formula for the rate of momentum transfer from radiation to dusty gas through which it passes, which is suitable for implementation in sub-grid models of galaxy formation. Finally, we provide a first map of the column density distribution of gas in a radiatively driven wind as a function of velocity, and velocity dispersion.

Krumholz, Mark R.; Thompson, Todd A.

2013-09-01

341

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

342

Phoenix data analysis and numerical simulation

NASA Astrophysics Data System (ADS)

The Phoenix 2 data analysis project was aimed at developing techniques for applying Doppler radar to observations of the turbulent boundary layer and at defining the important statistical properties and the large eddy structure more completely than is generally possible with direct sensors. An array of direct sensors was applied for comparison and to observe some of the finer scales of turbulence not accessible to the radar. A large eddy simulation experiment was also carried out for the purposes of observational comparison and improved interpretation. The original emphasis was on technique development in an environment thought to be moderately well understood. In outcome the technological developments are mixed in quality and some remain incomplete, while the scientific results are much more interesting than originally expected. Important improvements were made in single and dual Doppler processing, and satisfactory comparisons made between them. The errors in vertical velocity from the dual Doppler remain unpleasantly large in the upper levels and the effective resolution for some fields disappointingly coarse.

Lilly, Douglas K.

1990-06-01

343

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

344

Numerical Simulation of SNCR Technology with Simplified Chemical Kinetics Model

NASA Astrophysics Data System (ADS)

The paper deals with numerical simulation of SNCR method. For numerical modelling was used CFD code Ansys/CFX. SNCR method was described by dominant chemical reaction, which were look up NIST Chemical database. The reactions including reduction of NOx and concentration change of pollutants, like N2O and CO in flue gas too. Proposed chemical kinetics and CFD model was applied to two boilers. Both simulations were compared with experimental measurements. First simulation was used to validation of chemical mechanism. Second simulation was based on first simulation and it was used to verification of compiled SNCR chemical mechanism. Next the new variant of the reagent penetration lance was proposed and compared with the original variants.

Blejcha?, T.; Dolní?ková, D.

2013-04-01

345

This paper reports a simple novel technique for the numerical simulation of hot collision-free plasmas. The method is termed Vlasov hybrid simulation (VHS). A time varying phase space simulation box and grid are defined, and the phase fluid within the box is filled with simulation particles. The distribution function F (or deltaF) is defined on the phase trajectory of each

D. Nunn

1993-01-01

346

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

347

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

348

A numerical simulation on the perforation of reinforced concrete targets

The perforation of steel-bar-reinforced concrete target is studied numerically in this paper using LS-DYNA code and the modified Taylor–Chen–Kuszmaul continuum damage model. The crater diameters on both the front and back surfaces of the concrete target and the residual velocity of the projectile predicted by the numerical simulation are in good agreement with the experimental results reported by Hanchak et

Fenglei Huang; Haijun Wu; Qiankun Jin; Qingming Zhang

2005-01-01

349

Direct Numerical Simulation of Turbulent Compression Ramp Flow

: A numerical procedure for the direct numerical simulation of compressible turbulent flow and shock–turbulence interaction\\u000a is detailed and analyzed. An upwind-biased finite-difference scheme with a compact centered stencil is used to discretize\\u000a the convective part of the Navier–Stokes equations. The scheme has a uniformly high approximation order and allows for a spectral-like\\u000a wave resolution while dissipating nonresolved wave numbers.

N. A. Adams

1998-01-01

350

Numerical simulations of shock wave propagation in condensed multiphase materials

We propose to find out numerical solutions of a travelling shock wave in condensed mixtures by using a direct numerical simulation.\\u000a Condensed multiphase materials under shock wave conditions are mechanically characterized by a unique pressure and a unique\\u000a velocity. In this study, the mixture is considered as a collection of grains separated by interface between each material:\\u000a this problem of

Eric Daniel; Jacques Massoni

2007-01-01

351

Vortex Locking in Direct Numerical Simulations of Quantum Turbulence

NASA Astrophysics Data System (ADS)

Direct numerical simulations are used to examine the locking of quantized superfluid vortices and normal fluid vorticity in evolving turbulent flows. The superfluid is driven by the normal fluid, which undergoes either a decaying Taylor-Green flow or a linearly forced homogeneous isotropic turbulent flow, although the back reaction of the superfluid on the normal fluid flow is omitted. Using correlation functions and wavelet transforms, we present numerical and visual evidence for vortex locking on length scales above the intervortex spacing.

Morris, Karla; Koplik, Joel; Rouson, Damian W. I.

2008-07-01

352

Numerical simulation of two phase flows by random vortex method

An application of the random vortex method to the numerical simulation of two-phase flows and the physical model and algorithm of two-phase flows is presented. Two-phase turbulent flow past a cylinder has been calculated. Numerical results have reproduced the coherent structure and the Karman vortex street which exist in two-phase turbulent flows. The calculations are in good agreement with known

C. Yan

1986-01-01

353

On the elimination of numerical Cerenkov radiation in PIC simulations

NASA Astrophysics Data System (ADS)

Particle-in-cell (PIC) simulations are a useful tool in modeling plasma in physical devices. The Yee finite difference time domain (FDTD) method is commonly used in PIC simulations to model the electromagnetic fields. However, in the Yee FDTD method, poorly resolved waves at frequencies near the cut off frequency of the grid travel slower than the physical speed of light. These slowly traveling, poorly resolved waves are not a problem in many simulations because the physics of interest are at much lower frequencies. However, when high energy particles are present, the particles may travel faster than the numerical speed of their own radiation, leading to non-physical, numerical Cerenkov radiation. Due to non-linear interaction between the particles and the fields, the numerical Cerenkov radiation couples into the frequency band of physical interest and corrupts the PIC simulation. There are two methods of mitigating the effects of the numerical Cerenkov radiation. The computational stencil used to approximate the curl operator can be altered to improve the high frequency physics, or a filtering scheme can be introduced to attenuate the waves that cause the numerical Cerenkov radiation. Altering the computational stencil is more physically accurate but is difficult to implement while maintaining charge conservation in the code. Thus, filtering is more commonly used. Two previously published filters by Godfrey and Friedman are analyzed and compared to ideally desired filter properties.

Greenwood, Andrew D.; Cartwright, Keith L.; Luginsland, John W.; Baca, Ernest A.

2004-12-01

354

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

355

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

356

Astrophysical jets: Observations, numerical simulations, and laboratory experiments

This paper provides summaries of ten talks on astrophysical jets given at the HEDP/HEDLA-08 International Conference in St. Louis. The talks are topically divided into the areas of observation, numerical modeling, and laboratory experiment. One essential feature of jets, namely, their filamentary (i.e., collimated) nature, can be reproduced in both numerical models and laboratory experiments. Another essential feature of jets, their scalability, is evident from the large number of astrophysical situations where jets occur. This scalability is the reason why laboratory experiments simulating jets are possible and why the same theoretical models can be used for both observed astrophysical jets and laboratory simulations.

Bellan, P. M. [Caltech, Pasadena, California 91125 (United States); Livio, M. [Space Telescope Science Institute, Baltimore, Maryland 21218 (United States); Kato, Y. [University of Tsukuba, Ibaraki 3058577 (Japan); Lebedev, S. V. [Blackett Laboratory, Imperial College, London SW7 2BW (United Kingdom); Ray, T. P. [Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2 (Ireland); Ferrari, A. [Dipartimento di Fisica, Universita di Torino, via Pietro Giuria 1, 10125 Torino, Italy and Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637 (United States); Hartigan, P. [Department of Physics and Astronomy, Rice University, Houston, Texas 77251-1892 (United States); Frank, A. [Department of Physics and Astronomy and Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14627 (United States); Foster, J. M. [AWE Aldermaston, Reading RG7 4PR (United Kingdom); Nicolaie, P. [Centre Lasers Intenses et Applications, Universite Bordeaux 1-CEA-CNRS, 33405 Talence (France)

2009-04-15

357

Numerical simulation of graphene in an external magnetic field

NASA Astrophysics Data System (ADS)

In this paper the results of numerical simulation of graphene effective field theory in external magnetic field are presented. The numerical simulation is performed using noncompact (3+1)-dimensional Abelian lattice gauge fields and (2+1)-dimensional staggered lattice fermions. The dependences of fermion condensate and conductivity on the dielectric permittivity of the substrate for different values of external magnetic field are calculated. It is found that magnetic field shifts insulator-semimetal phase transition to larger values of the dielectric permittivity of the substrate. The phase diagram of graphene in external magnetic field is drawn.

Boyda, D. L.; Braguta, V. V.; Valgushev, S. N.; Polikarpov, M. I.; Ulybyshev, M. V.

2014-06-01

358

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

359

Numerical simulations of time-resolved quantum electronics

NASA Astrophysics Data System (ADS)

Numerical simulation has become a major tool in quantum electronics both for fundamental and applied purposes. While for a long time those simulations focused on stationary properties (e.g. DC currents), the recent experimental trend toward GHz frequencies and beyond has triggered a new interest for handling time-dependent perturbations. As the experimental frequencies get higher, it becomes possible to conceive experiments which are both time-resolved and fast enough to probe the internal quantum dynamics of the system. This paper discusses the technical aspects-mathematical and numerical-associated with the numerical simulations of such a setup in the time domain (i.e. beyond the single-frequency AC limit). After a short review of the state of the art, we develop a theoretical framework for the calculation of time-resolved observables in a general multiterminal system subject to an arbitrary time-dependent perturbation (oscillating electrostatic gates, voltage pulses, time-varying magnetic fields, etc.) The approach is mathematically equivalent to (i) the time-dependent scattering formalism, (ii) the time-resolved non-equilibrium Green’s function (NEGF) formalism and (iii) the partition-free approach. The central object of our theory is a wave function that obeys a simple Schrödinger equation with an additional source term that accounts for the electrons injected from the electrodes. The time-resolved observables (current, density, etc.) and the (inelastic) scattering matrix are simply expressed in terms of this wave function. We use our approach to develop a numerical technique for simulating time-resolved quantum transport. We find that the use of this wave function is advantageous for numerical simulations resulting in a speed up of many orders of magnitude with respect to the direct integration of NEGF equations. Our technique allows one to simulate realistic situations beyond simple models, a subject that was until now beyond the simulation capabilities of available approaches.

Gaury, Benoit; Weston, Joseph; Santin, Matthieu; Houzet, Manuel; Groth, Christoph; Waintal, Xavier

2014-01-01

360

Accuracy evaluation of a numerical simulation model of nasal airflow.

Abstract Conclusion: Our numerical simulation model provides an accurate reflection of nasal airflow, and the results were validated by clinical measurements. Objectives: To evaluate the accuracy of a numerical simulation model of nasal airflow. Methods: Ten volunteers with normal nasal cavities underwent CT, acoustic rhinometry, and rhinomanometry. CT data were uploaded into Mimics, ICEM-CFD, Fluent, and CFD-Post software for three-dimensional modeling, finite element grid division, transient calculations, and analysis, respectively. Velocity and pressure data of airflow were obtained during the normal respiratory cycle. The accuracy of the simulation was evaluated by two methods: acoustic rhinometry measurements were used to evaluate the accuracy of the anatomic model, and rhinomanometry measurements were used to evaluate the accuracy of the nasal resistance values obtained by numerical simulation. Results: There were no significant differences between the values describing the model and the acoustic rhinometry measurements, the nasal resistance values obtained by numerical simulation. The airflow through the nasal cavity was mainly laminar. The maximum velocities were measured at the nasal valve, the amplitudes of all velocity curves at locations beyond the nasal valve were reduced. The amplitudes of the pressure curves increased from the front to the back of the airway. PMID:24702230

Lu, Jiuxing; Han, Demin; Zhang, Luo

2014-05-01

361

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

362

Building Blocks for Reliable Complex Nonlinear Numerical Simulations

NASA Technical Reports Server (NTRS)

This talk 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)

2002-01-01

363

Building Blocks for Reliable Complex Nonlinear Numerical Simulations

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.

Yee, H. C.

2005-01-01

364

Direct numerical simulation of diluted combustion by evaporating droplets

Diluted combustion has been studied using DNS in a three-dimensional temporally developing reacting shear-layer with the oxidizer stream laden with evaporating droplets. The gaseous phase is described in the Eulerian frame while the discrete droplet phase is treated in the Lagrangian frame, with strong two-way coupling between the two phases through mass, momentum and energy exchange. Grid-resolution-independent results have been

J. Xia; K. H. Luo

2009-01-01

365

Numerical simulations of very large impacts on the Earth

Vertical impacts on the Earth of asteroids 500–3000km in diameter at 15km\\/s have been numerically modelled using the hydrodynamic SOVA code. This code has been modified for the spherical system of coordinates well suited for simulations of very large impacts when the entire Earth is involved in motion. The simulations include cratering process, upward motion of deep mantle layers, fall

Vladimir V. Svetsov

2005-01-01

366

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

367

Numerical Simulations of Shock Wave-Driven Jets

We present the results of numerical simulations of shock wave-driven jets in\\u000athe solar atmosphere. The dependence of observable quantities like maximum\\u000avelocity and deceleration on parameters such as the period and amplitude of\\u000ainitial disturbances and the inclination of the magnetic field is investigated.\\u000aOur simulations show excellent agreement with observations, and shed new light\\u000aon the correlation between

L. Heggland; B. De Pontieu; V. H. Hansteen

2007-01-01

368

Parallel adaptive numerical simulation of dry avalanches over natural terrain

High-fidelity computational simulation can be an invaluable tool in planning strategies for hazard risk mitigation. The accuracy and reliability of the predictions are crucial elements of these tools being successful. We present here a new simulation tool for dry granular avalanches using several new techniques for enhancing numerical solution accuracy.Highlights of our new methodology are the use of a depth-averaged

A. K. Patra; A. C. Bauer; C. C. Nichita; E. B. Pitman; M. F. Sheridan; M. Bursik; B. Rupp; A. Webber; A. J. Stinton; L. M. Namikawa; C. S. Renschler

2005-01-01

369

Numerical Simulation of Helicopter Aeromechanics in Slow Descent Flight

\\u000a In this paper we present numerical simulation results for a generic helicopter configuration in slow descent flight. The well\\u000a known HART-II test case has been chosen as the experimental reference, especially the baseline case. This test case is characterized\\u000a by the occurrence of Blade-Vortex Interactions (BVI) and can thus be considered as very ambitious with respect to the aerodynamic\\u000a simulation.

M. Embacher; M. Keßler; F. Bensing; E. Krämer

370

GPU Accelerated Numerical Simulation of Viscous Flow Down a Slope

NASA Astrophysics Data System (ADS)

Numerical simulations are an effective tool in natural risk analysis. They are useful to determine the propagation and the runout distance of gravity driven movements such as debris flows or landslides. To evaluate these processes an approach on analogue laboratory experiments and a GPU accelerated numerical simulation of the flow of a viscous liquid down an inclined slope is considered. The physical processes underlying large gravity driven flows share certain aspects with the propagation of debris mass in a rockslide and the spreading of water waves. Several studies have shown that the numerical implementation of the physical processes of viscous flow produce a good fit with the observation of experiments in laboratory in both a quantitative and a qualitative way. When considering a process that is this far explored we can concentrate on its numerical transcription and the application of the code in a GPU accelerated environment to obtain a 3D simulation. The objective of providing a numerical solution in high resolution by NVIDIA-CUDA GPU parallel processing is to increase the speed of the simulation and the accuracy on the prediction. The main goal is to write an easily adaptable and as short as possible code on the widely used platform MATLAB, which will be translated to C-CUDA to achieve higher resolution and processing speed while running on a NVIDIA graphics card cluster. The numerical model, based on the finite difference scheme, is compared to analogue laboratory experiments. This way our numerical model parameters are adjusted to reproduce the effective movements observed by high-speed camera acquisitions during the laboratory experiments.

Gygax, Remo; Räss, Ludovic; Omlin, Samuel; Podladchikov, Yuri; Jaboyedoff, Michel

2014-05-01

371

Direct numerical simulations of supercritical fluid mixing layers applied to heptane nitrogen

NASA Astrophysics Data System (ADS)

Direct numerical simulations (DNS) are conducted of a model hydrocarbon nitrogen mixing layer under supercritical conditions. The temporally developing mixing layer configuration is studied using heptane and nitrogen supercritical fluid streams at a pressure of 60 atm as a model system related to practical hydrocarbon-fuel/air systems. An entirely self-consistent cubic Peng Robinson equation of state is used to describe all thermodynamic mixture variables, including the pressure, internal energy, enthalpy, heat capacity, and speed of sound along with additional terms associated with the generalized heat and mass transport vectors. The Peng Robinson formulation is based on pure-species reference states accurate to better than 1% relative error through comparisons with highly accurate state equations over the range of variables used in this study (600 [less-than-or-eq, slant] T [less-than-or-eq, slant] 1100 K, 40 [less-than-or-eq, slant] p [less-than-or-eq, slant] 80 atm) and is augmented by an accurate curve fit to the internal energy so as not to require iterative solutions. The DNS results of two-dimensional and three-dimensional layers elucidate the unique thermodynamic and mixing features associated with supercritical conditions. Departures from the perfect gas and ideal mixture conditions are quantified by the compression factor and by the mass diffusion factor, both of which show reductions from the unity value. It is found that the qualitative aspects of the mixing layer may be different according to the specification of the thermal diffusion factors whose value is generally unknown, and the reason for this difference is identified by examining the second-order statistics: the constant Bearman Kirkwood (BK) thermal diffusion factor excites fluctuations that the constant Irwing Kirkwood (IK) one does not, and thus enhances overall mixing. Combined with the effect of the mass diffusion factor, constant positive large BK thermal diffusion factors retard diffusional mixing, whereas constant moderate IK factors tend to promote diffusional mixing. Constant positive BK thermal diffusion factors also tend to maintain density gradients, with resulting greater shear and vorticity. These conclusions about IK and BK thermal diffusion factors are species-pair dependent, and therefore are not necessarily universal. Increasing the temperature of the lower stream to approach that of the higher stream results in increased layer growth as measured by the momentum thickness. The three-dimensional mixing layer exhibits slow formation of turbulent small scales, and transition to turbulence does not occur even for a relatively long non-dimensional time when compared to a previous, atmospheric conditions study. The primary reason for this delay is the initial density stratification of the flow, while the formation of strong density gradient regions both in the braid and between-the-braid planes may constitute a secondary reason for the hindering of transition through damping of emerging turbulent eddies.

Miller, Richard S.; Harstad, Kenneth G.; Bellan, Josette

2001-06-01

372

NASA Astrophysics Data System (ADS)

Direct numerical simulations (DNS) of a supercritical temporal mixing layer are conducted for the purpose of exploring the characteristics of high-pressure transitional mixing behaviour. The conservation equations are formulated according to fluctuation-dissipation (FD) theory, which is consistent with non-equilibrium thermodynamics and converges to kinetic theory in the low-pressure limit. According to FD theory, complementing the low-pressure typical transport properties (viscosity, diffusivity and thermal conductivity), the thermal diffusion factor is an additional transport property which may play an increasingly important role with increasing pressure. The Peng Robinson equation of state with appropriate mixing rules is coupled to the dynamic conservation equations to obtain a closed system. The boundary conditions are periodic in the streamwise and spanwise directions, and of non-reflecting outflow type in the cross-stream direction. Due to the strong density stratification, the layer is considerably more difficult to entrain than equivalent gaseous or droplet-laden layers, and exhibits regions of high density gradient magnitude that become very convoluted at the transitional state. Conditional averages demonstrate that these regions contain predominantly the higher-density, entrained fluid, with small amounts of the lighter, entraining fluid, and that in these regions the mixing is hindered by the thermodynamic properties of the fluids. During the entire evolution of the layer, the dissipation is overwhelmingly due to species mass flux followed by heat flux effects with minimal viscous contribution, and there is a considerable amount of backscatter in the flow. Most of the species mass flux dissipation is due to the molecular diffusion term with significant contributions from the cross-term proportional to molecular and thermal diffusion. These results indicate that turbulence models for supercritical fluids should primarily focus on duplicating the species mass flux rather than the typical momentum flux, which constitutes the governing dissipation in atmospheric mixing layers. Examination of the passive-scalar probability density functions (PDFs) indicates that neither the Gaussian, nor the beta PDFs are able to approximate the evolution of the DNS-extracted PDF from its inception through transition. Furthermore, the temperature species PDFs are well correlated, meaning that their joint PDF is not properly approximated by the product of their marginal PDFs; this indicates that the traditional reactive flow modelling based on replacing the joint PDF representing the reaction rate by the product of the marginal PDFs is not appropriate. Finally, the subgrid-scale temperature species PDFs are also well correlated, and the species PDF exhibits important departures from the Gaussian. These results suggest that classic PDFs used in atmospheric pressure flows would not capture the physics of this supercritical mixing layer, either in an assumed PDF model at the larger scale, or at the subgrid scale.

Okong'o, Nora A.; Bellan, Josette

2002-08-01

373

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

374

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

375

Direct numerical simulations of three-dimensional bubbly flows

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

Bernard Bunner; Grétar Tryggvason

1999-01-01

376

Numerical simulation of cohesive powder behavior in a fluidized bed

A numerical simulation model was developed for wet powder fluidization in the scope of investigation on cohesive powder behavior. The model was developed based on the discrete element method (DEM) with the inter-particle cohesive interaction due to liquid bridging. To take into account the liquid bridge force between particles and between a particle and a wall, a regression expression for

Takafumi Mikami; Hidehiro Kamiya; Masayuki Horio

1998-01-01

377

Numerical simulation of the double slit interference with ultracold atoms

We present a numerical simulation of the double slit interference experiment realized by F. Shimizu, K. Shimizu and H. Takuma with ultracold atoms. We show how the Feynman path integral method enables the calculation of the time-dependent wave function. Because the evolution of the probability density of the wave packet just after it exits the slits raises the issue of

Michel Gondran; Alexandre Gondran

2005-01-01

378

Numerical Simulation of the Double Slit Interference with Ultracold Atoms

We present a numerical simulation of the double slit interference experiment realized by F. Shimizu, K. Shimizu and H. Takuma with ultracold atoms. We show how the Feynman path integral method enables the calculation of the time-dependent wave function. Because the evolution of the probability density of the wave packet just after it exits the slits raises the issue of

Michel Gondran; Alexandre Gondran

2007-01-01

379

Numerical Simulations of Impact Tests on a Rigid Wall.

National Technical Information Service (NTIS)

The behavior at impact of steel 30NCD16 is evaluated using the isothermal Johnson-Cook and Lindholm dynamic models. The numerical simulation of the impact tests against a rigid wall and the comparison to the experimental results highlight the interest of ...

D. Bois A. Grave M. L. Meyniel

1987-01-01

380

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

381

NUMERICAL SIMULATION OF THREE-DIMENSIONAL TUFT CORONA AND ELECTROHYDRODYNAMICS

The numerical simulation of three-dimensional tuft corona and electrohydrodynamics (EHD) is discussed. The importance of high-voltage and low-current operation in the wire-duct precipitator has focused attention on collecting high-resistivity dust. The local current density of in...

382

Numerical simulation of the transient moisture transfer through porous insulation

This paper develops a numerical model for simulating heat and moisture transfer through a porous insulation, with impermeable, adiabatic vertical boundaries, and with one horizontal boundary facing a warm humid ambient and the other facing a cold impermeable surface. The problem is modeled as one-dimensional, transient, multi phase flow with variable physical properties. The analysis has identified four phases of

N. E. Wijeysundera; B. F. Zheng; M. Iqbal; E. G. Hauptmann

1996-01-01

383

The numerical simulation of superhydrophobic surface's flow field characteristic

Based on the flow field mathematical model of superhydrophobic surfaces, the numerical simulation of superhydrophobic surfaces with microcosmic topography in turbulence was carried out. The flow field characteristics of superhydrophobic surfaces were analyzed from the flow field structure, the shear stress distribution, the velocity distribution of gas-liquid interface and the turbulent kinetic energy distribution. The results show that the superhydrophobic

Qiaogao Huang; Haibao Hu; Guang Pan; Baowei Song

2012-01-01

384

Direct Numerical Simulations of Turbulent Flows over Superhydrophobic Surfaces

Direct numerical simulations are used to investigate the drag reducing performance of superhydrophobic surfaces in turbulent channel flow. Slip velocities, wall shear stresses, and Reynolds stresses are considered for a variety of superhydrophobic surface micro-feature geometry configurations at a friction Reynolds number of Retau = 180. For the largest micro-feature spacing of 90mum an average slip velocity over 75% of

Michael B. Martell; J. Blair Perot; Jonathan P. Rothstein

2008-01-01

385

Numerical Simulation of Shock Wave Propagation in Fractured Cortical Bone

Shock waves (SW) are considered a promising method to treat bone non unions, but the associated mechanisms of action are not well understood. In this study, numerical simulations are used to quantify the stresses induced by SWs in cortical bone tissue. We use a 3D FDTD code to solve the linear lossless equations that describe wave propagation in solids and

Fre´de´ric Padilla; Robin Cleveland

2009-01-01

386

A NUMERICAL MODEL FOR THE SIMULATION OF EXTERNAL GEAR PUMPS

External gear pumps are the most widely employed source of power in hydraulic applications, allowing good performance and low manufacturing costs. The present paper reports the description of a numerical model for the simulation of these kind of machines, developed by the authors. The model has been implemented using the software AMESim ® , introducing new in-house C++ models, with

Paolo CASOLI; Andrea VACCA; Germano FRANZONI

2005-01-01

387

Numerical simulation of convections in the microgravity environment

NASA Astrophysics Data System (ADS)

A numerical simulation program to study g-jitter and Marangoni convection in the liquid phase under microgravity were produced. The program solves the Navier-Stokes equations in the derived variable configuration. An alternative direction implicit method is used to solve the finite differences equations representing flow in the cavity. The resulting flow field and mixing characteristic are analyzed at different dimensionless times.

Boudreault, R.

1984-12-01

388

Numerical simulation of liquid sloshing in three-dimensional tanks

NASA Astrophysics Data System (ADS)

Liquid sloshing in 3D dynamically excited containers is simulated by means of the panel method derived from the boundary-integral technique. Free-surface profiles and tank-wall pressures are determined for spherical tanks under forced sway- and pitch-mode excitation. An incremental multistep treatment gives results on the effects of sloshing that are numerically stable.

Hwang, J. H.; Kim, I. S.; Seol, Y. S.; Lee, S. C.; Chon, Y. K.

1992-07-01

389

Numerical Simulation of Cold Dense Plasma Sputtering with VORPAL

Sputtering is an evaporation process that physically removes atoms from a solid target material. This process takes place under bombardment of the target surface by energetic ions. Sputtering is widely applied in material processing and coating, such as etching and thin film deposition. Numerical simulation of sputtering process requires both accurate models of nuclear stopping in materials, particle dynamics and

Chuandong Zhou; Peter Stoltz; Seth Veitzer

2009-01-01

390

Interference microscopy of subnanometer depth resolution: Numerical simulation

NASA Astrophysics Data System (ADS)

We present results of numerical simulation performed with the aim to study the possibility of measuring subnanometer-range roughness parameters using an automated interference Linnik microscope with a white-light source. It is shown that reconstruction noises of phase images can be reduced to 0.1 nm when using averaging of interferograms and phase images.

Vishnyakov, G. N.; Levin, G. G.; Minaev, V. L.; Tsel'mina, I. Yu.

2013-12-01

391

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

392

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

393

Numerical Simulations of RC Hollow Piers Under Horizontal Cyclic Loading

This article addresses numerical simulations of experimentally tested RC hollow-section piers prone to shear problems under cyclic loading. Detailed nonlinear modeling was applied with 3D FEM discretization using a damage model for concrete and a cyclic behavior model for steel bars represented by uniaxial elements Analyses were supported by parameter calibration based on experimental pier test data. The results confirmed

P. Delgado; A. Monteiro; A. Arêde; N. Vila Pouca; R. Delgado; A. Costa

2011-01-01

394

Numerical simulation of coal gasification in entrained flow coal gasifier

This paper presents modeling of a coal gasification reaction, and prediction of gasification performance for an entrained flow coal gasifier. The purposes of this study are to develop an evaluation technique for design and performance optimization of coal gasifiers using a numerical simulation technique, and to confirm the validity of the model. The coal gasification model suggested in this paper

H. Watanabe; M. Otaka

2006-01-01

395

Fluid–structure interactions of a cross parachute: numerical simulation

The dynamics of parachutes involves complex interaction between the parachute structure and the surrounding flow field. Accurate representation of parachute systems requires treatment of the problem as a fluid–structure interaction (FSI). In this paper we present the numerical simulations we performed for the purpose of comparison to a series of cross-parachute wind tunnel experiments. The FSI model consists of a

Keith Stein; Richard Benney; Tayfun Tezduyar; Jean Potvin

2001-01-01

396

Numerical simulation of standing detonation induced by oblique shock wave

The induction of standing detonation in a gaseous mixture in a convergent-divergent nozzle by an oblique shock wave is investigated theoretically by means of numerical simulations using a multilevel-grid method. The approaches used to model the chemical reactions, the thermodynamic properties, the transport properties, and the combustion chamber is described; the derivation of the fundamental equations are outlined; and the

Shiro Taki

1989-01-01

397

Numerical simulation of viscous flows over transonic aircraft configurations

NASA Technical Reports Server (NTRS)

Numerical simulation of compressible viscous flow fields is performed for a transonic transport configuration. A single structured grid system is constructed using analytical transformations such as conformal mapping, shearing/twisting/rotating/clustering/stretching transformations. The Reynolds-averaged, thin-layer Navier-Stokes equations are solved on a supercomputer, FACOM VP-400, using the LU-ADI factorization method.

Takanashi, Susumu; Yoshida, Masahiro; Fujii, Kozo; Matsushima, Kisa; Obayashi, Shigeru

1987-01-01

398

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

399

Numerical simulation of thermal and kinetic evolution of overthrusted structures.

National Technical Information Service (NTIS)

A 2D numerical simulation of the formation of overthrusted structures is proposed. It rests on the hypotheses of balanced cross-sections: conservation of volume in a section parallel to the transport direction with a parallel folding. Each balanced sectio...

L. Endignoux

1989-01-01

400

Numerical Simulations of Turbulent Mixing in Eruption Clouds

Eruption clouds in explosive volcanic eruptions are a kind of free boundary shear flow with very high Reynolds numbers (Re > 108), and their dynamics are governed by the entrainment of ambient air into eruption clouds by turbulent mixing and the density change of eruption clouds accompanied by turbulent mix- ing. We developed a numerical pseudo-gas model which correctly simulates

Yujiro J. Suzuki; Takehiro Koyaguchi

2007-01-01

401

Numerical simulation of shock waves in cavity of excimer laser

To clarify the characteristics of the generation and propagation of shock waves generated by pulse discharges in an excimer laser, numerical simulation using a TVD scheme and a grid- distortion splitting method is carried out. The calculations are conducted in conditions corresponding to our visualization experiments and high-frequency operations. The propagation and attenuation of the shock waves in the high-frequency

Shinichiroh Kosugi; Kazuo Maeno; Hiroki Honma

1995-01-01

402

Effect of schemes on numerical simulations of shock wave reflection

The full Navier-Stokes equations were solved numerically by using a parallel computer at NAL to simulate oblique reflection of plane shock waves over a rigid wall. The dependent hyperbolic conservation forms for finite volume cells were integrated by applying a second order TVD scheme. The three stage Runge-Kutta method was used for time integration. Various types of Riemann solvers of

Fumio Higashino; Yoko Takakura; Masahiro Yoshida; Satoru Ogawa

1994-01-01

403

Numerical simulation of shock wave diffraction by TVD schemes

An upwind total variation diminishing (TVD) scheme and a predictor-corrector symmetric TVD scheme were used to numerically simulate the blast wave diffraction on a stationary object. The objective is to help design an optimum configuration so that lateral motion is minimized and at the same time vortex shedding and flow separation are reduced during a blast wave encounter. Results are

Victor Y. C. Young; H. C. Yee

1987-01-01

404

The numerical simulation of shock wave motion in confined space

A finite volume TVD scheme is used to numerically simulate the motion of a shock wave projecting into a confined space. The calculated results are in agreement with experimental ones when the shock wave propagates in atmosphere. Following the shock wave, vortex rings and several shock waves appear in the flowfield. Calculations indicate that the characteristics of pressure pulses near

Jia-Ling Le; Tie-Suo Gao; Wen-Xiang Cao

1993-01-01

405

Numerical simulation of shock wave focusing over parabolic reflectors

The problem of a plane shock wave that propagates in an air media and then is reflected from a parabolic concave reflector and focuses at some region is considered. The shock focusing can greatly magnify the pressure and the temperature. The purpose of this study is to numerically simulate the shock focusing process of the reflection of shock waves from

S. M. Liang; C. S. Wu; F. M. Yu; L. N. Wu

1995-01-01

406

Numerical Simulation of I-type Spherule Formation

NASA Astrophysics Data System (ADS)

Numerical simulations for atmospheric entry of spherules are performed to calculate the time dependence of velocity, altitude, spherules' radii and temperature. They indicate that the strong apparent gravitational acceleration due to atmospheric drag results in exposure of metallic cores to the air during melting and oxidation of spherules. Two major types of internal structures of Fe-Ni spherules are explained by this model.

Yada, T.; Sekiya, M.; Nakamura, T.; Takaoka, N.

1996-03-01

407

Numerical simulations of a fully submerged propeller subject to ventilation

Numerical simulations aimed at modeling the phenomenon of ventilation on a fully submerged propeller were performed. Ventilation occurs on thruster propellers operating at high loadings and heavy sea states, experiencing continuous cycles in and out-of water. This leads to sudden thrust losses and violent impact loads, which can damage shaft bearings and gears of azimuth and tunnel thrusters. Damages in

A. Califano; S. Steen

2011-01-01

408

Experimental evaluation of numerical simulation of cavitating flow around hydrofoil

Cavitation in hydraulic machines causes different problems that can be related to its unsteady nature. An experimental and numerical study of developed cavitating flow was performed. Until now simulations of cavitating flow were limited to the self developed “in house” CFD codes. The goal of the work was to experimentally evaluate the capabilities of a commercial CFD code (Fluent) for

Matevž Dular; Rudolf Bachert; Bernd Stoffel; Brane Širok

2005-01-01

409

Numerical approaches for multidimensional simulations of stellar explosions

NASA Astrophysics Data System (ADS)

We introduce numerical algorithms for initializing multidimensional simulations of stellar explosions with 1D stellar evolution models. The initial mapping from 1D profiles onto multidimensional grids can generate severe numerical artifacts, one of the most severe of which is the violation of conservation laws for physical quantities. We introduce a numerical scheme for mapping 1D spherically-symmetric data onto multidimensional meshes so that these physical quantities are conserved. We verify our scheme by porting a realistic 1D Lagrangian stellar profile to the new multidimensional Eulerian hydro code CASTRO. Our results show that all important features in the profiles are reproduced on the new grid and that conservation laws are enforced at all resolutions after mapping. We also introduce a numerical scheme for initializing multidimensional supernova simulations with realistic perturbations predicted by 1D stellar evolution models. Instead of seeding 3D stellar profiles with random perturbations, we imprint them with velocity perturbations that reproduce the Kolmogorov energy spectrum expected for highly turbulent convective regions in stars. Our models return Kolmogorov energy spectra and vortex structures like those in turbulent flows before the modes become nonlinear. Finally, we describe approaches to determining the resolution for simulations required to capture fluid instabilities and nuclear burning. Our algorithms are applicable to multidimensional simulations besides stellar explosions that range from astrophysics to cosmology.

Chen, Ke-Jung; Heger, Alexander; Almgren, Ann S.

2013-11-01

410

NUMERICAL SIMULATION OF THE FAST DENSE GAS EXPERIMENT

The preliminary design of a Ludwieg tube experiment for the verification of the existence of nonclassical rarefaction shock waves in dense vapors is here critically analyzed by means of real gas numerical simulations of the experimental setup. The Flexible Asym- metric Shock Tube (FAST) setup is a dense gas Ludwieg-type tube in which a rarefaction shock is produced after the

C. Zamfirescu; A. Guardone; P. Colonna

2006-01-01

411

Numerical simulation of a surface barrier discharge in air

The development of a surface barrier discharge in air at atmospheric pressure under the action of a constant voltage of different polarity is simulated numerically. When the polarity of the high-voltage electrode is negative, the discharge develops as an ionization wave that moves along the dielectric surface. When the polarity is positive, the discharge develops as a streamer that first moves above the dielectric surface and then comes into contact with and continues to develop along it. In the case of a high-voltage electrode of positive polarity, the discharge zone above the dielectric surface is approximately five times thicker than that in the case of negative polarity. The characteristic aspects of numerical simulation of the streamer phase of a surface barrier discharge are discussed. The numerical results on the density of the charge stored at the dielectric surface and on the length of the discharge zone agree with the experimental data.

Solov'ev, V. R.; Konchakov, A. M.; Krivtsov, V. M.; Aleksandrov, N. L. [Moscow Institute of Physics and Technology (Russian Federation)

2008-07-15

412

NUMERICAL SIMULATION OF NATURAL GAS-SWIRL BURNER

A numerical simulation of a turbulent natural gas jet diffusion flame at a Reynolds number of 9000 in a swirling air stream is presented. The numerical computations were carried out using the commercially available software package CFDRC. The instantaneous chemistry model was used as the reaction model. The thermal, composition, flow (velocity), as well as stream function fields for both the baseline and air-swirling flames were numerically simulated in the near-burner region, where most of the mixing and reactions occur. The results were useful to interpret the effects of swirl in enhancing the mixing rates in the combustion zone as well as in stabilizing the flame. The results showed the generation of two recirculating regimes induced by the swirling air stream, which account for such effects. The present investigation will be used as a benchmark study of swirl flow combustion analysis as a step in developing an enhanced swirl-cascade burner technology.

Ala Qubbaj

2005-03-01

413

Aggregation in an expanding cloud: experiments and numerical simulations

NASA Astrophysics Data System (ADS)

We have set-up an experimental device and a numerical model to study aggregation of an aerosol (titanium oxide) expanding in the atmosphere. By mean of scanning microscopic analysis, it is shown that agglomerated aerosol forms fractal clusters of fractal dimension of about 1.75. The numerical simulations quantitatively confirm this geometrical feature. We show how our numerical results can complete some points which are not available in experiments. Nous avons construit un dispositif expérimental et un modèle numérique pour étudier l'agrégation d'un aérosol (oxyde de titane) en expansion dans l'atmosphère. A partir de l'analyse de photographies prises au microscope électronique à balayage, on montre que l'aérosol aggloméré forme des amas fractals de dimension fractale de l'ordre de 1,75. Les simulations numériques confirment quantitativement cette particularité géométrique. Nous montrons comment nos résultats numériques peuvent compléter quelques points qui ne sont pas accessibles à l'expérience.

Dziedzinl, Françoise; Botet, Robert

1991-03-01

414

Numerical relativity simulations in the era of the Einstein Telescope

NASA Astrophysics Data System (ADS)

Numerical-relativity (NR) simulations of compact binaries are expected to be an invaluable tool in gravitational-wave 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.

Hannam, Mark; Hawke, Ian

2011-02-01

415

Collapse of a Liquid Column: Numerical Simulation and Experimental Validation

NASA Astrophysics Data System (ADS)

This paper is focused on the numerical and experimental analyses of the collapse of a liquid column. The measurements of the interface position in a set of experiments carried out with shampoo and water for two different initial column aspect ratios are presented together with the corresponding numerical predictions. The experimental procedure was found to provide acceptable recurrence in the observation of the interface evolution. Basic models describing some of the relevant physical aspects, e.g. wall friction and turbulence, are included in the simulations. Numerical experiments are conducted to evaluate the influence of the parameters involved in the modeling by comparing the results with the data from the measurements. The numerical predictions reasonably describe the physical trends.

Cruchaga, Marcela A.; Celentano, Diego J.; Tezduyar, Tayfun E.

2007-03-01

416

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

417

Benchmarks and numerical methods for the simulation of boiling flows

NASA Astrophysics Data System (ADS)

Comparisons of different numerical methods suited to the simulations of phase changes are presented in the framework of interface capturing computations on structured fixed computational grids. Due to analytical solutions, we define some reference test-cases that every numerical technique devoted to phase change should succeed. Realistic physical properties imply some drastic interface jump conditions on the normal velocity or on the thermal flux. The efficiencies of Ghost Fluid and Delta Function Methods are compared to compute the normal velocity jump condition. Next, we demonstrate that high order extrapolation methods on the thermal field allow performing accurate and robust simulations for a thermally controlled bubble growth. Finally, some simulations of the growth of a rising bubble are presented, both for a spherical bubble and a deformed bubble.

Tanguy, Sébastien; Sagan, Michaël; Lalanne, Benjamin; Couderc, Frédéric; Colin, Catherine

2014-05-01

418

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

419

Simulation for the expansion of the wildfire with numerical weather simulation MM5

NASA Astrophysics Data System (ADS)

1. Background Frequent occurrence of wildfires all over the world is considered as one of major resources of greenhouse gases. For example, a lot of wildfires in Alaska occur in summer. Now, the satellites of NOAA and Terra/Aqua are watching the earth and the wildfire are detected. Of course, to detection wildfire is very important, but the influence on inhabitants is more important. Our purpose is to make the numerical simulation of the wildfire spread in the small area with numerical weather simulation MM5. We think this will be useful to help fire fighting and global environment such as the replace of CO2. 2. Numerical Wildfire Spread Simulation There are many type of the numerical simulation of wildfire spread. In our simulation, the wildfire velocity is based on the Rhothermel equation and other parts are made of the cell automata. The area of the wildfire is the uniform vegetation consisted of the boreal forest (Picea mariana). The main factor of the expansion speed is wind velocity and speed. The continuous change of the weather is simulated with regional meteorological simulation MM5. The real spread of the Boundary Fire are observed by Alaska Fire Service. In this study, we validate the simulation result with the AFS data. 3. The Simulation Results We are constructing the simulation with Boundary Fire in 2004 in central Alaska. MM5 is very useful to reconstruct or forecast the distribution of local weather. We show the examples of the results in the poster. 4. Conclusion We constructed the numerical simulation model of wildfire spread with numerical weather simulation MM5. The result of simulation is being verified by the observed data by AFS .

Kimura, K.; Honma, T.

2008-12-01

420

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

421

Constraints on interpretation of the Eltanin impact from numerical simulations

NASA Astrophysics Data System (ADS)

The results of numerical simulations of the Eltanin impact are combined with the available geological data in order to reconstruct the impact dynamics and to get some constraints on the impact parameters. Numerical simulations show that the Eltanin projectile size should be less than 2 km for a 45° oblique impact and less than 1.5 km for a vertical impact. On the other hand, we demonstrate that the projectile diameter cannot be considerably smaller than 1 km; otherwise, the impact-induced water flow cannot transport eroded sediments across large distances. The maximum displacement approximately equals the water crater radius and rapidly decreases with increasing distances. Numerical simulations also show that ejecta deposits strongly depend on impact angle and projectile size and, therefore, cannot be used for reliable estimates of the initial projectile mass. The initial amplitudes of tsunami-like waves are estimated. The presence of clay-rich sediments, typical for the abyssal basins in cores PS2709 and PS2708 on the Freeden Seamounts (Bellingshausen Sea, Southern Ocean) combined with numerical data allow us to suggest a probable point of impact to the east of the seamounts. The results do not exclude the possibility that a crater in the ocean bottom may exist, but such a structure has not been found yet.

Shuvalov, Valery; Gersonde, Rainer

2014-06-01

422

Constraints on interpretation of the Eltanin impact from numerical simulations

NASA Astrophysics Data System (ADS)

The results of numerical simulations of the Eltanin impact are combined with the available geological data in order to reconstruct the impact dynamics and to get some constraints on the impact parameters. Numerical simulations show that the Eltanin projectile size should be less than 2 km for a 45° oblique impact and less than 1.5 km for a vertical impact. On the other hand, we demonstrate that the projectile diameter cannot be considerably smaller than 1 km; otherwise, the impact-induced water flow cannot transport eroded sediments across large distances. The maximum displacement approximately equals the water crater radius and rapidly decreases with increasing distances. Numerical simulations also show that ejecta deposits strongly depend on impact angle and projectile size and, therefore, cannot be used for reliable estimates of the initial projectile mass. The initial amplitudes of tsunami-like waves are estimated. The presence of clay-rich sediments, typical for the abyssal basins in cores PS2709 and PS2708 on the Freeden Seamounts (Bellingshausen Sea, Southern Ocean) combined with numerical data allow us to suggest a probable point of impact to the east of the seamounts. The results do not exclude the possibility that a crater in the ocean bottom may exist, but such a structure has not been found yet.

Shuvalov, Valery; Gersonde, Rainer

2014-07-01

423

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

424

Numerical simulation of the unsteady behaviour of cavitating flows

NASA Astrophysics Data System (ADS)

A 2D numerical model is proposed to simulate unsteady cavitating flows. The Reynolds-averaged Navier-Stokes equations are solved for the mixture of liquid and vapour, which is considered as a single fluid with variable density. The vapourization and condensation processes are controlled by a barotropic state law that relates the fluid density to the pressure variations. The numerical resolution is a pressure-correction method derived from the SIMPLE algorithm, with a finite volume discretization. The standard scheme is slightly modified to take into account the cavitation phenomenon. That numerical model is used to calculate unsteady cavitating flows in two Venturi type sections. The choice of the turbulence model is discussed, and the standard RNG k-model is found to lead to non-physical stable cavities. A modified k-model is proposed to improve the simulation. The influence of numerical and physical parameters is presented, and the numerical results are compared to previous experimental observations and measurements. The proposed model seems to describe the unsteady cavitation behaviour in 2D geometries well.

Coutier-Delgosha, O.; Reboud, J. L.; Delannoy, Y.

2003-06-01

425

Thermal numerical simulator for laboratory evaluation of steamflood oil recovery

A thermal numerical simulator running on an IBM AT compatible personal computer is described. The simulator was designed to assist laboratory design and evaluation of steamflood oil recovery. An overview of the historical evolution of numerical thermal simulation, NIPER's approach to solving these problems with a desk top computer, the derivation of equations and a description of approaches used to solve these equations, and verification of the simulator using published data sets and sensitivity analysis are presented. The developed model is a three-phase, two-dimensional multicomponent simulator capable of being run in one or two dimensions. Mass transfer among the phases and components is dictated by pressure- and temperature-dependent vapor-liquid equilibria. Gravity and capillary pressure phenomena were included. Energy is transferred by conduction, convection, vaporization and condensation. The model employs a block centered grid system with a five-point discretization scheme. Both areal and vertical cross-sectional simulations are possible. A sequential solution technique is employed to solve the finite difference equations. The study clearly indicated the importance of heat loss, injected steam quality, and injection rate to the process. Dependence of overall recovery on oil volatility and viscosity is emphasized. The process is very sensitive to relative permeability values. Time-step sensitivity runs indicted that the current version is time-step sensitive and exhibits conditional stability. 75 refs., 19 figs., 19 tabs.

Sarathi, P.

1991-04-01

426