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1

Use of direct numerical simulation (DNS) data to investigate spatial resolution issues in measurements of wall-bounded turbulence  

E-print Network

Use of direct numerical simulation (DNS) data to investigate spatial resolution issues of direct numerical simulation (DNS) data to investigate spatial resolution issues in measurements of wall-dimensional energy spectra from direct numerical simulation (DNS) of turbulent channel flow at Re 950. Various

Marusic, Ivan

2

Large Eddy Simulations (LES) and Direct Numerical Simulations (DNS) for the computational analyses of high speed reacting flows  

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

A numerical method for DNS of turbulent reacting flows using complex chemistry  

E-print Network

A numerical method for DNS of turbulent reacting flows using complex chemistry Rajapandiyan reacting flows.1 Large Eddy Simulations (LES) and Direct Numerical Simulations (DNS) are very accurate the capability to perform DNS/LES of turbulent reacting flows in complex geometries. The challenge in simulating

Mahesh, Krishnan

4

A Numerical Method for DNS/LES of Turbulent Reacting Flows Jeff Doom, Yucheng Hou & Krishnan Mahesh  

E-print Network

A Numerical Method for DNS/LES of Turbulent Reacting Flows Jeff Doom, Yucheng Hou & Krishnan Mahesh mechanisms are presented. Keywords: DNS; LES; Turbulent reacting flows; Non-dissipative; Implicit 1 Introduction The direct numerical simulation (DNS) and large-eddy simulation (LES) of turbulent reacting flows

Mahesh, Krishnan

5

Direct Numerical Simulation DNS: Maximum Error as a Function of Mode Number  

SciTech Connect

Numerical errors can be characterized in terms of algebraic polynomial approximation of a sine wave. The magnitude of the error will, therefore, depend on the energy at each mode in a Fourier expansion. Flows with a great deal of energy in the highest modes, such as Turbulence, are therefore the most difficult to approximate.

Jameson, L.

2000-06-01

6

44th AIAA Aerospace Sciences Meeting and Exhibit, Jan 9Jan 12, Reno, Nevada Assessment of Numerical Methods for DNS of  

E-print Network

of Numerical Methods for DNS of Shockwave/Turbulent Boundary Layer Interaction M. Wu and M.P. Martin Mechanical of error for the discrepancy between previous direct numerical simulation (DNS) results of Wu et al.1 and experimental data of Bookey et al.2 are investigated. Deficiencies of current numerical methods for DNS

Martín, Pino

7

Measurements and Laboratory Simulations of the Upper DNS Hierarchy  

E-print Network

Measurements and Laboratory Simulations of the Upper DNS Hierarchy Duane Wessels1 , Marina Fomenkov}@caida.org Abstract. Given that the global DNS system, especially at the higher root and top-levels, experiences significant query loads, we seek to answer the following questions: (1) How does the choice of DNS caching

California at San Diego, University of

8

Measurements and Laboratory Simulations of the Upper DNS Hierarchy  

E-print Network

Measurements and Laboratory Simulations of the Upper DNS Hierarchy Duane Wessels 1 , Marina}@caida.org Abstract. Given that the global DNS system, especially at the higher root and top­levels, experiences significant query loads, we seek to answer the following questions: (1) How does the choice of DNS caching

California at San Diego, University of

9

Multi-physics computational grains (MPCGs) for direct numerical simulation (DNS) of piezoelectric composite/porous materials and structures  

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

10

Large Eddy Simulations Of A Turbulent Auto-Igniting C2H4 Flame DNS  

Microsoft Academic Search

Large eddy simulations of a turbulent auto-igniting flame are performed to analyze the interaction of different combustion regimes in a flamelet modeling framework. The case that is considered is a direct numerical simulation (DNS) of a non-premixed jet flame at Re=10,000 with heated co-flow. This DNS was performed by Yoo et al. (Proc. Comb. Inst., 2010) using 1.29 billion cells

Edward Knudsen; Shashank; Heinz Pitsch; Ed Richardson; Jackie Chen

2010-01-01

11

Numerical simulation of supersonic boundary layer transition  

NASA Technical Reports Server (NTRS)

The present contribution reviews some of the recent progress obtained at our group in the direct numerical simulation (DNS) of compressible boundary layer transition. Elements of the different simulation approaches and numerical techniques employed are surveyed. Temporal and spatial simulations, as well as comparisons with results obtained from Parabolized Stability Equations, are discussed. DNS results are given for flat plate boundary layers in the Mach number range 1.6 to 4.5. A temporal DNS at Mach 4.5 has been continued through breakdown all the way to the turbulent stage. In addition results obtained with a recently developed extended temporal DNS approach are presented, which takes into account some nonparallel effects of a growing boundary layer. Results from this approach are quite close to those of spatial DNS, while preserving the efficiency of the temporal DNS.

Guo, Y.; Adams, N. A.; Sandham, N. D.; Kleiser, L.

1994-01-01

12

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

E-print Network

Scalable DNS code for high Reynolds number channel flow simulation on BG/Q MyoungKyu Lee mk, Austin) DNS code for high Re flow on BG/Q MiraCon Mar, 2013 1 / 35 #12;Contents Project Overview Performance Optimization Early Result Conclusion M.K. Lee (Univ of Texas, Austin) DNS code for high Re flow

Kemner, Ken

13

Numerical Errors in DNS: Total Run-Time Error  

SciTech Connect

Understanding numerical errors in simulations is critical for many reasons. First and foremost, one must some estimate concerning the reliability of the final result. Simply put, numerical errors add up over time and in most cases the increase is a linear process. It is quite possible that running a code for a very long time can lead to a solution which is completely meaningless even though it may look reasonable. This manuscript will begin a technical discussion on these issues.

Jameson, L.

2000-06-06

14

SIMULATION NUMERIQUE DIRECTE (DNS) DU CHANGEMENT DE PHASE LIQUIDE-VAPEUR  

E-print Network

SIMULATION NUM´ERIQUE DIRECTE (DNS) DU CHANGEMENT DE PHASE LIQUIDE-VAPEUR Contribution à l'étude de multiphasique 4 Méthode numérique 5 Résultats numériques 6 On going & To do G. Faccanoni DNS DU CHANGEMENT DE numérique 5 Résultats numériques 6 On going & To do G. Faccanoni DNS DU CHANGEMENT DE PHASE LIQUIDE-VAPEUR 3

Faccanoni, Gloria

15

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

16

DNS OF A CHANNEL FLOW WITH VARIABLE PROPERTIES Franck Nicoud  

E-print Network

DNS OF A CHANNEL FLOW WITH VARIABLE PROPERTIES Franck Nicoud Center for Turbulence Research describes Direct Numerical Simulations (DNS) of a low speed channel ow with strong heat transfer consid- ered in the DNS study of a supersonic channel ow performed by Coleman et al. (1995). These data

Nicoud, Franck

17

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

18

DNS of autoignition in turbulent diffusion H2/air and Krishnan Mahesh  

E-print Network

DNS of auto­ignition in turbulent diffusion H2/air flames Jeff Doom and Krishnan Mahesh University of Minnesota, Minneapolis, MN, 55455, USA Direct numerical simulation (DNS) is used to study auto and diffusion, then auto­ignite due to the high temperatures of the oxidizer. Direct numerical simulation (DNS

Mahesh, Krishnan

19

DNS of Reacting H2/Air Laminar Vortex Rings and Krishnan Mahesh  

E-print Network

DNS of Reacting H2/Air Laminar Vortex Rings Jeff Doom and Krishnan Mahesh University of Minnesota, Minneapolis, MN, 55455, USA Direct numerical simulation (DNS) is used to study reacting, laminar vortex rings

Mahesh, Krishnan

20

Impact of the floating-point precision and interpolation scheme on the results of DNS of  

E-print Network

Impact of the floating-point precision and interpolation scheme on the results of DNS of turbulence of the floating-point precision and interpo- lation scheme on the results of direct numerical simulations (DNS

Grauer, Rainer

21

DNS and LES of decaying isotropic turbulence with and without frame rotation using lattice Boltzmann method  

E-print Network

DNS and LES of decaying isotropic turbulence with and without frame rotation using lattice for performing direct numerical simulations (DNS) and large-eddy simulations (LES) of turbulent flows. Decaying. We perform three categories of simulations. The first category involves LBE-DNS of HIT

Luo, Li-Shi

22

DNS Simulation of the Arizona Drop-tank Experiment  

NASA Astrophysics Data System (ADS)

The Miranda code has been employed to investigate the flow generated in the drop-tank experiments of Niederhaus and Jacobs. Their experiments consisted of a small tank containing a heavy fluid and a lighter fluid, mounted on vertical rails. The tank was oscillated horizontally, creating standing waves on the heavy/light fluid interface. The tank was then dropped, bounced on a spring, and allowed to coast up and back down until it came to a rest. Video images documented the development of the impulsively-accelerated density interface. We have made use of Miranda to explore the dynamics and mixing behavior of this flow. Miranda is a spectral/compact-difference DNS code that resolves the flow's viscous and species diffusion scales. After simulating the initial conditions and the pre-bounce development, we calculate the post-bounce development and validate the calculations with the data. We then further explore the dynamics of the flow. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory, under contract No. W-7405-Eng-48.

Miller, Paul; Cook, Andrew

2001-11-01

23

Validation of a DNS code for wall-bounded turbulence including  

E-print Network

Validation of a DNS code for wall-bounded turbulence including finite-rate reactions and surface, and numerical methods for direct numerical simulation (DNS) of wall- bounded turbulence including finite has been done using DNS databases of incompressible flows or using perfect gas wind tunnel data

Martín, Pino

24

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

25

Terascale Direct Numerical Simulations of Turbulent Combustion  

Microsoft Academic Search

The rapid growth in computational power in the past decade has presented both opportunities and challenges for high-fidelity simulations of turbulent reacting flows. The advent of terascale computing power has made it possible to glean fundamental physical insight into fine-grained ``turbulence-chemistry'' interactions in simple laboratory-scale turbulent flames from direct numerical simulation at moderate Reynolds numbers with detailed chemistry. Recent DNS

Jacqueline Chen

2009-01-01

26

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

27

Turbulent partially premixed combustion: DNS analysis and RANS simulation  

E-print Network

for different modelling cases. . . . . . . . . . . . . 142 xviii Nomenclature Roman Symbols c Reaction progress variable D Diameter of fuel jet nozzle, D = 2 mm dh Uniform grid spacing in the DNS dr Ring width in the ring averaging procedure Ka Karlovitz number... xix LIST OF TABLES Z Mixture fraction z Cross stream direction of the jet Zi Mass fraction of element i Dc Diffusivity of the progress variable DZ Diffusivity of the mixture fraction F complex function k Turbulent kinetic energy Greek Symbols ?...

Ruan, S.

2013-03-12

28

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

E-print Network

A DNS study of turbulent mixing of two passive scalars A. Junejaa) and S. B. Pope Sibley School, direct numerical simulations DNS of the Navier-Stokes equations have emerged as a leading research tool. Conse- quently, the present work employs DNS to study the mixing of two decaying scalar fields

29

Low-Frequency Unsteadiness in DNS of Shock Wave/Turbulent Boundary Layer Interaction  

E-print Network

Low-Frequency Unsteadiness in DNS of Shock Wave/Turbulent Boundary Layer Interaction Stephan Priebe , M. Pino Mart´in The direct numerical simulation (DNS) of a compression ramp shock wave/ turbulent in the context of a compression ramp DNS against experiments at matching flow conditions.1, 2 The low

Martín, Pino

30

Predictive Inner-Outer Model for Turbulent Boundary Layers Applied to Hypersonic DNS Data  

E-print Network

Predictive Inner-Outer Model for Turbulent Boundary Layers Applied to Hypersonic DNS Data Clara numerical simulation (DNS) data of supersonic and hypersonic turbulent boundaries with Mach 3 and Mach 7 are calibrated from each of the DNS data sets. The model is extended to include near-wall predictions

Martín, Pino

31

Numerical simulation of turbulent jet primary breakup in Diesel engines  

E-print Network

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

Helluy, Philippe

32

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

NASA Technical Reports Server (NTRS)

The basic objective of this research is to extend the capabilities of Large Eddy Simulations (LES) and Direct Numerical Simulations (DNS) for the computational analyses of high speed reacting flows. In the efforts related to LES, we were primarily involved with assessing the performance of the various modern methods based on the Probability Density Function (PDF) methods for providing closures for treating the subgrid fluctuation correlations of scalar quantities in reacting turbulent flows. In the work on DNS, we concentrated on understanding some of the relevant physics of compressible reacting flows by means of statistical analysis of the data generated by DNS of such flows. In the research conducted in the second year of this program, our efforts focused on the modeling of homogeneous compressible turbulent flows by PDF methods, and on DNS of non-equilibrium reacting high speed mixing layers. Some preliminary work is also in progress on PDF modeling of shear flows, and also on LES of such flows.

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

1992-01-01

33

Direct numerical simulation of salt sheets and turbulence in a double-diffusive shear layer  

E-print Network

Direct numerical simulation of salt sheets and turbulence in a double-diffusive shear layer Satoshi. [1] We describe three-dimensional direct numerical simulations (DNS) of double-diffusively stratified: Kimura, S., and W. Smyth (2007), Direct numerical simulation of salt sheets and turbulence in a double

Smyth, William David

34

LES versus DNS: A comparative study  

NASA Technical Reports Server (NTRS)

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

Shtilman, L.; Chasnov, J. R.

1992-01-01

35

35th Fluid Dynamics Meeting,Toronto, Canada, June 2005 Assessment of STBLI DNS Data and Comparison  

E-print Network

35th Fluid Dynamics Meeting,Toronto, Canada, June 2005 Assessment of STBLI DNS Data and Comparison experiments and direct numerical simulations (DNS) of shock/turbulent boundary layer interaction (STBLI) under, as observed experimentally,5, 6 are found in the DNS data. Namely, the 24 compression corner produces the same

Martín, Pino

36

Numerical Aerodynamic Simulation  

NASA Technical Reports Server (NTRS)

An overview of historical and current numerical aerodynamic simulation (NAS) is given. The capabilities and goals of the Numerical Aerodynamic Simulation Facility are outlined. Emphasis is given to numerical flow visualization and its applications to structural analysis of aircraft and spacecraft bodies. The uses of NAS in computational chemistry, engine design, and galactic evolution are mentioned.

1989-01-01

37

Analysis of numerical errors in large eddy simulation using statistical closure theory  

E-print Network

Analysis of numerical errors in large eddy simulation using statistical closure theory Noma Park develops a dynamic error analysis procedure for the numerical errors arising from spatial discretization, LES solutions are more sensitive to numerical errors than DNS. Therefore, the analysis and control

Mahesh, Krishnan

38

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

39

From Large-Eddy Simulation to Direct Numerical Simulation of a lean premixed swirl flame: Filtered laminar flame-PDF modeling  

Microsoft Academic Search

Large-Eddy Simulations (LES) and Direct Numerical Simulation (DNS) are applied to the analysis of a swirl burner operated with a lean methaneair mixture and experimentally studied by Meier et al. [19]. LES is performed for various mesh refinements, to study unsteady and coherent large-scale behavior and to validate the simulation tool from measurements, while DNS enables to gain insight into

V. Moureau; P. Domingo; L. Vervisch

2011-01-01

40

Direct numerical simulations of two-phase immiscible wakes  

NASA Astrophysics Data System (ADS)

Tammisola et al (2012 J. Fluid Mech. 713 632-58) have observed a counter-intuitive destabilizing effect of the surface tension in planar wakes by means of a global linear analysis. In the present study, we conduct direct numerical simulation (DNS) of wakes of two immiscible fluids. The numerical scheme is based on a level set approach to track the interface position. We simulate both sinuous and varicose perturbations of wake flows. DNS confirms a destabilization on the sinuous perturbations in presence of a moderate amount of the surface tension, while wakes are stabilized when the surface tension is further increased. Varicose perturbations present in contrast an intermittent low-amplitude oscillatory regime which does not significantly affect the position of the interface.

Biancofiore, Luca; Gallaire, Franois; Laure, Patrice; Hachem, Elie

2014-08-01

41

Rocket engine numerical simulation  

NASA Technical Reports Server (NTRS)

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

Davidian, Ken

1993-01-01

42

Rocket engine numerical simulator  

NASA Technical Reports Server (NTRS)

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

Davidian, Ken

1993-01-01

43

CoDNS: Improving DNS Performance and Reliability via Cooperative Lookups  

E-print Network

CoDNS: Improving DNS Performance and Reliability via Cooperative Lookups KyoungSoo Park, Vivek S 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

Rivière, Etienne

44

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

NASA Technical Reports Server (NTRS)

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

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

2014-01-01

45

Direct numerical simulation of flow separation around a NACA 0012 airfoil  

Microsoft Academic Search

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

46

A numerical method for DNS/LES of turbulent reacting flows  

SciTech Connect

A spatially non-dissipative, implicit numerical method to simulate turbulent reacting flows over a range of Mach numbers, is described. The compressible Navier-Stokes equations are rescaled so that the zero Mach number equations are discretely recovered in the limit of zero Mach number. The dependent variables are co-located in space, and thermodynamic variables are staggered from velocity in time. The algorithm discretely conserves kinetic energy in the incompressible, inviscid, non-reacting limit. The chemical source terms are implicit in time to allow for stiff chemical mechanisms. The algorithm is readily extended to complex chemical mechanisms. Numerical examples using both simple and complex chemical mechanisms are presented.

Doom, Jeff; Hou, Yucheng [Department of Aerospace Engineering and Mechanics, University of Minnesota (United States); Mahesh, Krishnan [Department of Aerospace Engineering and Mechanics, University of Minnesota (United States)], E-mail: mahesh@aem.umn.edu

2007-09-10

47

A numerical method for DNS/LES of high--enthalpy turbulent flows  

NASA Astrophysics Data System (ADS)

A numerical method is developed for simulation of high-- enthalpy turbulent flows. A non-dissipative algorithm is used for accurate flux reconstruction at the cell faces. The method is combined with a predictor corrector based shock capturing scheme to simulate strong shock waves encountered in high-- enthalpy flows. A non--linear limiter is used to limit the application of shock capturing only to the vicinity of the shock wave to minimize dissipation. The Navier-Stokes equations are suitably modified to represent various thermo--chemical processes occurring in high--enthalpy flows. A five species model for air is considered. To account for finite rate chemical reactions, individual mass conservation equations are solved for every species. An equation for conservation of vibrational energy is also solved to account for vibrational excitation. Species diffusion is modeled through Fick's law. Transport properties are computed taking high temperature effects into account. The numerical method is evaluated using test problems.

Ghosh, Shankar; Mahesh, Krishnan

2011-11-01

48

Numerical simulation of fracture  

SciTech Connect

The Bedded Crack Model (BCM) is a constitutive model for brittle materials. It is based on effective modulus theory and makes use of a generalized Griffith criterion for crack growth. It is used in a solid dynamic computer code to simulate stress wave propagation and fracture in rock. A general description of the model is given and then the theoretical basis for it is presented. Some effects of finite cell size in numerical simulations are discussed. The use of the BCM is illustrated in simulations of explosive fracture of oil shale. There is generally good agreement between the calculations and data from field experiments.

Margolin, L.G.; Adams, T.F.

1982-01-01

49

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

NASA Technical Reports Server (NTRS)

This research is involved with the implementations of advanced computational schemes based on large eddy simulations (LES) and direct numerical simulations (DNS) to study the phenomenon of mixing and its coupling with chemical reactions in compressible turbulent flows. In the efforts related to LES, a research program was initiated to extend the present capabilities of this method for the treatment of chemically reacting flows, whereas in the DNS efforts, focus was on detailed investigations of the effects of compressibility, heat release, and nonequilibrium kinetics modeling in high speed reacting flows. The efforts to date were primarily focussed on simulations of simple flows, namely, homogeneous compressible flows and temporally developing hign speed mixing layers. A summary of the accomplishments is provided.

Givi, Peyman; Madnia, C. K.; Steinberger, C. J.; Tsai, A.

1991-01-01

50

High performance computation for DNS\\/LES  

Microsoft Academic Search

The paper is focused on high-order compact schemes for direct numerical simulation (DNS) and large eddy simulation (LES) for flow separation, transition, tip vortex, and flow control. A discussion is given for several fundamental issues such as high quality grid generation, high-order schemes for curvilinear coordinates, the CFL condition for complex geometry, and high-order weighted compact schemes for shock capturing

Chaoqun Liu

2006-01-01

51

Numerical simulation of an asymptotically reduced system for rotationally constrained convection  

Microsoft Academic Search

For rotationally constrained convection, the Taylor Proudman theorem enforces an organization of nonlinear flows into tall columnar or compact plume structures. While coherent structures in convection under moderate rotation are exclusively cyclonic, recent experiments for rapid rotation have revealed a transition to equal populations of cyclonic and anticyclonic structures. Direct numerical simulation (DNS) of this regime is expensive, however, and

Michael Sprague; Keith Julien; Edgar Knobloch; Joseph Werne

2006-01-01

52

Direct numerical simulation of particle wall transfer and deposition in upward turbulent pipe flow  

Microsoft Academic Search

Transfer and deposition of inertial particles or droplets in turbulent pipe flow are crucial processes in a number of industrial and environmental applications. In this work, we use direct numerical simulation (DNS) and Lagrangian tracking to study turbulent transfer and deposition of inertial particles in vertical upward circular pipe flow. Our objects are: (i) to quantify turbulent transfer of heavy

Cristian Marchioli; Andrea Giusti; Maria Vittoria Salvetti; Alfredo Soldati

2003-01-01

53

Direct numerical simulations of trailing-edge noise generated by boundary-layer instabilities  

Microsoft Academic Search

Direct numerical simulations (DNS) are conducted of noise generated at an infinitely thin trailing edge (TE). The aim is to predict the far-field sound and the near-field hydrodynamics, thereby providing an insight into the physical mechanisms of sound generation at airfoil TEs and potentially helping to validate acoustic theories. One of the theories widely used is the classical inviscid theory

R. D. Sandberg; N. D. Sandham; P. F. Joseph

2007-01-01

54

DNS of turbulent wall bounded flows with a passive scalar  

Microsoft Academic Search

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)

Juan Guillermo Araya

2008-01-01

55

DNS-LES Validation of an Algebraic Second-Order-Moment Combustion Model  

Microsoft Academic Search

Direct numerical simulation (DNS) of three-dimensional turbulent reacting channel flows with buoyancy is carried out using a spectral method. Statistical results from the DNS database are used to validate an algebraic second-order-moment sub-grid-scale (ASOM-SGS) combustion model and show that the ASOM-SGS model is reasonable. Furthermore, a methaneair jet flame is simulated by largeeddy simulation (LES) using the ASOM-SGS model and

F. Wang; L. X. Zhou; C. X. Xu; C. K. Chan

2009-01-01

56

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

57

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

NASA Technical Reports Server (NTRS)

The main 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. In the efforts related to LES, we were concerned with developing reliable subgrid closures for modeling of the fluctuation correlations of scalar quantities in reacting turbulent flows. In the work on DNS, we focused our attention to further investigation of the effects of exothermicity in compressible turbulent flows. In our previous work, in the first year of this research, we have considered only 'simple' flows. Currently, we are in the process of extending our analyses for the purpose of modeling more practical flows of current interest at LaRC. A summary of our accomplishments during the third six months of the research is presented.

Givi, P.; Madnia, C. K.; Steinberger, C. J.; Frankel, S. H.; Vidoni, T. J.

1991-01-01

58

Direct numerical simulation of incompressible axisymmetric flows  

NASA Technical Reports Server (NTRS)

In the present work, we propose to conduct direct numerical simulations (DNS) of incompressible turbulent axisymmetric jets and wakes. The objectives of the study are to understand the fundamental behavior of axisymmetric jets and wakes, which are perhaps the most technologically relevant free shear flows (e.g. combuster injectors, propulsion jet). Among the data to be generated are various statistical quantities of importance in turbulence modeling, like the mean velocity, turbulent stresses, and all the terms in the Reynolds-stress balance equations. In addition, we will be interested in the evolution of large-scale structures that are common in free shear flow. The axisymmetric jet or wake is also a good problem in which to try the newly developed b-spline numerical method. Using b-splines as interpolating functions in the non-periodic direction offers many advantages. B-splines have local support, which leads to sparse matrices that can be efficiently stored and solved. Also, they offer spectral-like accuracy that are C(exp O-1) continuous, where O is the order of the spline used; this means that derivatives of the velocity such as the vorticity are smoothly and accurately represented. For purposes of validation against existing results, the present code will also be able to simulate internal flows (ones that require a no-slip boundary condition). Implementation of no-slip boundary condition is trivial in the context of the b-splines.

Loulou, Patrick

1994-01-01

59

Semiconvection: numerical simulations  

NASA Astrophysics Data System (ADS)

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

Zaussinger, F.; Spruit, H. C.

2013-06-01

60

Terascale Direct Numerical Simulations of Turbulent Combustion  

NASA Astrophysics Data System (ADS)

The rapid growth in computational power in the past decade has presented both opportunities and challenges for high-fidelity simulations of turbulent reacting flows. The advent of terascale computing power has made it possible to glean fundamental physical insight into fine-grained ``turbulence-chemistry'' interactions in simple laboratory-scale turbulent flames from direct numerical simulation at moderate Reynolds numbers with detailed chemistry. Recent DNS results are presented to elucidate the role of autoignition and large-eddy mixing on the stabilization of a lifted ethylene-air jet flame in a heated coflow. The role of scalar dissipation rate on modulating ignition delays or lift-off heights is discussed. The simulations were performed at a jet Reynolds number of 10,000 and required 1.3 billion grid points to resolve the turbulence and flame structure. In a second related topic, the morphology of the scalar dissipation rate field in a turbulent jet flame is examined using topological methods, in particular the Morse-Smale Complex, which provides a natural segmentation of dissipation rate elements or ``features.'' These features are tracked in time, and conditional feature statistics are presented.

Chen, Jacqueline

2009-03-01

61

Direct Numerical Simulation of Mach 3 Compression Ramp Flow  

NASA Astrophysics Data System (ADS)

We present the direct numerical simulation (DNS) of a shockwave and turbulent boundary layer interaction (STBLI) generated by a compression ramp. The flow conditions are Mach 2.9 and Re?=2900, and the ramp angle is 24 degrees. STBLI flows are known to display low-frequency unsteadiness, typically at frequencies 1-2 orders of magnitude lower than that of the incoming undisturbed boundary layer. The presence of these low-frequency motions in the DNS data and their relationship with the upstream and downstream flow regions have been demonstrated (Priebe and Martin, AIAA paper 2010-108). The DNS data show that the low-frequency shock motion is significantly correlated with the downstream flow. A statistically significant but small correlation is also found with the upstream flow. In the present paper, we investigate the flow structure associated with the downstream flow regions and study the time-and-space resolved dynamics of the shock motion, shear layer and separated flow regions.

Priebe, Stephan; Martin, Pino

2010-11-01

62

DNS of Rough Surface Turbulent Boundary Layer  

NASA Astrophysics Data System (ADS)

A dynamic method for prescribing realistic inflow boundary conditions is presented for simulations of spatially developing turbulent boundary layers subject to surface roughness. Direct Numerical Simulation (DNS) of a moderate Reynolds number, zero pressure gradient (ZPG) turbulent boundary layer was performed. The boundary layer was subjected to transitional, 24-grit sand grain surface roughness, modeled with a roughness parameter of k^+ 12 and a Reynolds number of R?= 2400. The computational method involves a synergy of the multi-scale dynamic approach and a new methodology for mapping high-resolution topographical surface data into a computational fluid dynamics environment. It is shown here that the multi-scale dynamic approach can be successfully extended to simulations, which incorporate surface roughness. In terms of the mean velocity and Reynolds stresses, the DNS results are encouraging as they demonstrate good agreement with the LDA measurements performed under similar conditions.

Cardillo, James; Araya, Guillermo; Chen, Yi; Jansen, Kenneth; Sahni, Onkar; Castillo, Luciano

2011-11-01

63

Double-diffusive interfaces in Lake Kivu reproduced by direct numerical simulations  

NASA Astrophysics Data System (ADS)

diffusion transforms uniform background gradients of temperature and salinity into "staircases" of homogeneous mixed layers that are separated by high-gradient interfaces. Direct numerical simulations (DNS) and microstructure measurements are two independent methods of estimating double-diffusive fluxes. By performing DNS under similar conditions as found in our measurements in Lake Kivu, we are able to compare results from both methods for the first time. We find that (i) the DNS reproduces the measured interface thicknesses of in situ microstructure profiles, (ii) molecular heat fluxes through interfaces capture the total vertical heat fluxes for density ratios larger than three, and (iii) the commonly used heat flux parameterization underestimates the total fluxes by a factor of 1.3 to 2.2.

Sommer, Tobias; Carpenter, Jeffrey R.; West, Alfred

2014-07-01

64

Numerical simulation of turbulence over tensegrity fabric  

NASA Astrophysics Data System (ADS)

In this research we aim to reduce turbulent skin friction by designing and optimizing tensegrity fabrics. Such fabrics form a new class of compliant surfaces consisting of a weave of both members under tension and members under compression. Boundary conditions on the flow are handled with a time-dependent coordinate transformation. We first note that, when designing the numerical algorithm for approximating the Navier-Stokes equation in the flow domain (with moving boundaries), special care (intrinsic differentiation of a contravariant vector) is needed to handle the temporal differentiation of the momentum term when using a contravariant formulation. A Cartesian-based formulation may also be used, and has proven to be more tractable in the 3D setting. The spectral DNS flow code is coupled with a tensegrity simulation code to compute the flow/structure interaction; recent simulation results will be presented. A complex-step derivative (CSD) technique may then be used to optimize the response characteristics of the tensegrity structure in order to minimize the drag at the flow/structure interface; this strategy will also be discussed.

Luo, Haoxiang; Bewley, Thomas

2003-11-01

65

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

NASA Astrophysics Data System (ADS)

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

Tenneti, Sudheer; Subramaniam, Shankar

2014-01-01

66

Direct numerical simulation of turbulent reacting flows  

SciTech Connect

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

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

1993-12-01

67

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

68

Numerical simulation of dusty plasmas  

SciTech Connect

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

69

DNS and the theory of receptivity of a supersonic boundary layer to free-stream disturbances  

NASA Astrophysics Data System (ADS)

Direct numerical simulation (DNS) of receptivity of a boundary layer over flat plate is carried out. The free stream Mach number is equal to 6. The following two-dimensional disturbances are introduced into the free-stream flow: fast and slow acoustic waves, temperature spottiness. A theoretical model describing the excitation of unstable waves in the boundary layer is developed using the biorthogonal eigenfunction decomposition method. The DNS results agree with the theoretical predictions.

Soudakov, Vitaly; Fedorov, Alexander; Ryzhov, Alexander

2011-12-01

70

Evaluation of some high-order shock capturing schemes for direct numerical simulation of unsteady two-dimensional free flows  

Microsoft Academic Search

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

C. Tenaud; E. Garnier; P. Sagaut

2000-01-01

71

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.

72

Scalar Dissipation Modeling for Passive and Active Scalars: a priori Study Using Direct Numerical Simulation  

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

73

Numerical simulation of shock/turbulent boundary layer interaction  

NASA Technical Reports Server (NTRS)

Most flows of aerodynamic interest are compressible and turbulent. However, our present knowledge on the structures and mechanisms of turbulence is mostly based on incompressible flows. In the present work, compressibility effects in turbulent, high-speed, boundary layer flows are systematically investigated using the Direct Numerical Simulation (DNS) approach. Three-dimensional, time-dependent, fully nonlinear, compressible Navier-Stokes equations were numerically integrated by high-order finite-difference methods; no modeling for turbulence is used during the solution because the available resolution is sufficient to capture the relevant scales. The boundary layer problem deals with fully-turbulent compressible flows over flat geometries. Apart from its practical relevance to technological flows, turbulent compressible boundary layer flow is the simplest experimentally realizable turbulent compressible flow. Still, measuring difficulties prohibit a detailed experimental description of the flow, especially in the near-wall region. DNS studies provide a viable means to probe the physics of compressible turbulence in this region. The focus of this work is to explore the paths of energy transfer through which compressible turbulence is sustained. The structural similarities and differences between the incompressible and compressible turbulence are also investigated. The energy flow patterns or energy cascades are found to be directly related to the evolution of vortical structures which are generated in the near-wall region. Near-wall structures, and mechanisms which are not readily accessible through physical experiments are analyzed and their critical role on the evolution and the behavior of the flow is documented extensively.

Biringen, Sedat; Hatay, Ferhat F.

1993-01-01

74

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

75

Direct numerical simulation of a turbulent wind over a wavy water surface  

NASA Astrophysics Data System (ADS)

Parameterization of the wind-water-waves interaction is a key problem of the air-sea system modeling. Of most importance are water waves with sufficiently large steepness, when nonlinear effects related to the boundary layer separation and vortex generation in the wind flow are well pronounced. Known experimental techniques (contact methods and particle image velocimetry) are not yet able to provide a full, detailed understanding of the wind flow in the viscous sublayer and the buffer region. As an alternative, we consider direct numerical simulations (DNS). In the present paper we discuss numerical algorithm and results of DNS of a turbulent wind flow over a wavy water surface. Waves with maximum steepness of ka = 0.2, wave age 0 < c/u* < 10, and Reynolds number Re = 15,000 are considered. Full, 3-D Navier-Stokes equations are solved in curvilinear coordinates in a reference frame moving with the wave phase speed c. DNS results show that an instantaneous velocity field is characterized by the presence of well-pronounced separation zones in the vicinity of the wave crests whereas the average velocity field is nonseparating. We also perform a comparison of the DNS results with the predictions of a theoretical quasi-linear model of the wind-wave interaction.

Druzhinin, O. A.; Troitskaya, Y. I.; Zilitinkevich, S. S.

2012-11-01

76

DNS of the kappa-mechanism  

E-print Network

We present a purely-radiative hydrodynamic model of the kappa-mechanism that sustains radial oscillations in Cepheid variables. We determine the physical conditions favourable for the kappa-mechanism to occur by the means of a configurable hollow in the radiative conductivity profile. By starting from these most favourable conditions, we complete nonlinear direct numerical simulations (DNS) and compare them with the results given by a linear-stability analysis of radial modes. We find that well-defined instability strips are generated by changing the location and shape of the conductivity hollow. For a given position in the layer, the hollow amplitude and width stand out as the key parameters governing the appearance of unstable modes driven by the kappa-mechanism. The DNS confirm both the growth rates and structures of the linearly-unstable modes. The nonlinear saturation that arises is produced by intricate couplings between the excited fundamental mode and higher damped overtones. These couplings are measu...

Gastine, T

2008-01-01

77

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

78

Direct numerical simulation of flame stabilization downstream of a transverse fuel jet in cross-flow  

Microsoft Academic Search

A reactive transverse fuel jet in cross-flow (JICF) configuration is studied using three-dimensional direct numerical simulation (DNS) with detailed chemical kinetics in order to investigate the mechanism of flame stabilization in the near field of a fuel jet nozzle. JICF configurations are used in practical applications where high mixing rates are desirable between the jet and the cross-flow fluids such

R. W. Grout; A. Gruber; C. S. Yoo; J. H. Chen

2011-01-01

79

Direct numerical simulation of turbulent flows in a wall-normal rotating channel  

Microsoft Academic Search

Direct numerical simulation (DNS) is carried out to study turbulence characteristics in a wall-normal rotating channel with the rotation number N? from 0 to 0.12 and the Reynolds number 194 based on the friction velocity of the non-rotating case and the half-height of the channel. Based on the present calculated results, two typical rotation regimes are identified. When in weak

Nan-Sheng Liu; Xi-Yun Lu

2005-01-01

80

Resilience of helical fields to turbulent diffusion - II. Direct numerical simulations  

NASA Astrophysics Data System (ADS)

Blackman and Subramanian (Paper I) found that sufficiently strong large-scale helical magnetic fields are resilient to turbulent diffusion, decaying on resistively slow rather than turbulently fast time-scales. This bolsters fossil field origins for magnetic fields in some astrophysical objects. Here, we study direct numerical simulations (DNS) of decaying large-scale helical magnetic fields in the presence of non-helical turbulence for two cases: (1) the initial helical field is large enough to decay resistively but transitions to fast decay; (2) the case of Paper I, wherein the transition energy for the initial helical field to decay fast directly is sought. Simulations and two-scale modelling (based on Paper 1) reveal the transition energy, Ec1 to be independent of the turbulent forcing scale, within a small range of RM. For case (2), the two-scale theory predicts a large-scale helical transition energy of Ec2 = (k1/kf)2Meq, where k1 and kf are the large-scale and small turbulent forcing scale, respectively, and Meq is the equipartition magnetic energy. The DNS agree qualitatively with this prediction but the RM, currently achievable, is too small to satisfy a condition 3/RM ? (k1/kf)2, necessary to robustly reveal the transition, Ec2. That two-scale theory and DNS agree wherever they can be compared suggests that Ec2 of Paper I should be identifiable at higher RM in DNS.

Bhat, Pallavi; Blackman, Eric G.; Subramanian, Kandaswamy

2014-03-01

81

Numerical Simulation of Gravity Waves  

Microsoft Academic Search

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

82

Numerical simulation of Bootstrap Current  

SciTech Connect

The neoclassical theory of Bootstrap Current in toroidal systems is calculated in magnetic flux coordinates and confirmed by numerical simulation. The effects of magnetic ripple, loop voltage, and magnetic and electrostatic perturbations on bootstrap current for the cases of zero and finite plasma pressure are studied. The numerical results are in reasonable agreement with analytical estimates.

Wu, Yanlin; White, R.B.

1993-05-01

83

Numerical simulation of fracture  

SciTech Connect

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

Margolin, L.G.

1983-01-01

84

AIAA 033464 Preliminary Work on DNS and LES  

E-print Network

AIAA 03­3464 Preliminary Work on DNS and LES of STBLI M. Pino Martin, Sheng Xu and Minwei Wu­4344 #12;Preliminary Work on DNS and LES of STBLI M. Pino Martin, Sheng Xu and Minwei Wu Department simulation (DNS) and large-eddy simulation (LES). In addition, the initialization procedure and boundary

Martín, Pino

85

Requirements definition by numerical simulation  

NASA Astrophysics Data System (ADS)

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

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

1994-10-01

86

Spatial Direct Numerical Simulation of Boundary-Layer Transition Mechanisms: Validation of PSE Theory  

NASA Technical Reports Server (NTRS)

A study of instabilities in incompressible boundary-layer flow on a flat plate is conducted by spatial direct numerical simulation (DNS) of the Navier-Stokes equations. Here, the DNS results are used to critically evaluate the results obtained using parabolized stability equations (PSE) theory and to study mechanisms associated with breakdown from laminar to turbulent flow. Three test cases are considered: two-dimensional Tollmien-Schlichting wave propagation, subharmonic instability breakdown, and oblique-wave break-down. The instability modes predicted by PSE theory are in good quantitative agreement with the DNS results, except a small discrepancy is evident in the mean-flow distortion component of the 2-D test problem. This discrepancy is attributed to far-field boundary- condition differences. Both DNS and PSE theory results show several modal discrepancies when compared with the experiments of subharmonic breakdown. Computations that allow for a small adverse pressure gradient in the basic flow and a variation of the disturbance frequency result in better agreement with the experiments.

Joslin, R. D.; Streett, C. L.; Chang, C.-L.

1991-01-01

87

An electrowetting microvalve: numerical simulation.  

PubMed

Numerical simulation of a zero-leakage microvalve is investigated where a liquid droplet is used as a gate to regulate the flow in a T junction. The droplet gate is activated by changing its surface tension via an applied electric field. Numerical simulation of the droplet actuation is considered where the effect of electrowetting is imposed in the form of a modified boundary condition at the contact line. Numerical simulation is used to predict the droplet behavior and to design the valve. It is found that the pressure breakdown of the microvalve is significantly affected by the geometry of the T junction corners. It is expected that such a microvalve design will improve the sensitivity and performance of a wide variety of microfluidic devices. PMID:17124138

Mohseni, Kamran; Dolatabadi, Ali

2006-09-01

88

Numerical methods for unsteady compressible multi-component reacting flows on fixed and moving grids  

Microsoft Academic Search

Deriving high precision schemes to compute turbulent flows on fixed or moving complex grids is becoming a central issue in the direct numerical simulation (DNS) and large eddy simulation (LES) community. The step between classical DNS\\/LES codes on fixed structured grids and future methods on moving unstructured grids is a significant evolution in terms of numerical methods. For reacting flows,

V. Moureau; G. Lartigue; Y. Sommerer; C. Angelberger; O. Colin; T. Poinsot

2005-01-01

89

Interpolation error in DNS simulations of turbulence: consequences for particle tracking  

NASA Astrophysics Data System (ADS)

An important aspect in numerical simulations of particle-laden turbulent flows is the interpolation of the flow field. Interpolations are needed in many applications, for example, simulations of turbulent aerosol transport, transport of bed load sediments and marine species. For the interpolation different approaches are used. Where some studies use low-order linear interpolation, others use high-order spline methods. This study focuses on estimating the interpolation error and compares it with the discretisation error of the flow field. In this way one can balance the errors in order to achieve an optimal result. Algorithms have been developed for the approximation of the interpolation error. As a spin-off of the theoretical analysis a practical method is proposed which enables direct estimation of the interpolation error from the energy spectrum of the flow. Furthermore it is shown how this energy spectrum is affected by the interpolation. Our results suggest that a coherent choice of the interpolation method and the value of kmax? should be made in order to balance the errors. Our approach may provide a quantitative indicator for this purpose.

van Hinsberg, Michel; Ten Thije Boonkkamp, Jan; Toschi, Federico; Clercx, Herman

2011-11-01

90

DNS of the kappa-mechanism  

E-print Network

We present a purely-radiative hydrodynamic model of the kappa-mechanism that sustains radial oscillations in Cepheid variables. We determine the physical conditions favourable for the kappa-mechanism to occur by the means of a configurable hollow in the radiative conductivity profile. By starting from these most favourable conditions, we complete nonlinear direct numerical simulations (DNS) and compare them with the results given by a linear-stability analysis of radial modes. We find that well-defined instability strips are generated by changing the location and shape of the conductivity hollow. For a given position in the layer, the hollow amplitude and width stand out as the key parameters governing the appearance of unstable modes driven by the kappa-mechanism. The DNS confirm both the growth rates and structures of the linearly-unstable modes. The nonlinear saturation that arises is produced by intricate couplings between the excited fundamental mode and higher damped overtones. These couplings are measured by projecting the DNS fields onto an acoustic subspace built from regular and adjoint eigenvectors and a 2:1 resonance is found to be responsible for the saturation of the kappa-mechanism instability.

T. Gastine; B. Dintrans

2008-09-29

91

Estimating uncertainties in statistics computed from direct numerical simulation  

NASA Astrophysics Data System (ADS)

Rigorous assessment of uncertainty is crucial to the utility of direct numerical simulation (DNS) results. Uncertainties in the computed statistics arise from two sources: finite statistical sampling and the discretization of the Navier-Stokes equations. Due to the presence of non-trivial sampling error, standard techniques for estimating discretization error (such as Richardson extrapolation) fail or are unreliable. This work provides a systematic and unified approach for estimating these errors. First, a sampling error estimator that accounts for correlation in the input data is developed. Then, this sampling error estimate is used as part of a Bayesian extension of Richardson extrapolation in order to characterize the discretization error. These methods are tested using the Lorenz equations and are shown to perform well. These techniques are then used to investigate the sampling and discretization errors in the DNS of a wall-bounded turbulent flow at Re? ? 180. Both small (Lx/? Lz/? = 4? 2?) and large (Lx/? Lz/? = 12? 4?) domain sizes are investigated. For each case, a sequence of meshes was generated by first designing a "nominal" mesh using standard heuristics for wall-bounded simulations. These nominal meshes were then coarsened to generate a sequence of grid resolutions appropriate for the Bayesian Richardson extrapolation method. In addition, the small box case is computationally inexpensive enough to allow simulation on a finer mesh, enabling the results of the extrapolation to be validated in a weak sense. For both cases, it is found that while the sampling uncertainty is large enough to make the order of accuracy difficult to determine, the estimated discretization errors are quite small. This indicates that the commonly used heuristics provide adequate resolution for this class of problems. However, it is also found that, for some quantities, the discretization error is not small relative to sampling error, indicating that the conventional wisdom that sampling error dominates discretization error for this class of simulations needs to be reevaluated.

Oliver, Todd A.; Malaya, Nicholas; Ulerich, Rhys; Moser, Robert D.

2014-03-01

92

CoDNS: Improving DNS Performance and Reliability via Cooperative Lookups  

Microsoft Academic Search

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

93

Terascale direct numerical simulations of turbulent combustion using S3D.  

SciTech Connect

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 co-flow, 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.

Sankaran, Ramanan; Mellor-Crummy, J.; DeVries, M.; Yoo, Chun Sang; Ma, K. L.; Podhorski, N.; Liao, W. K.; Klasky, S.; de Supinski, B.; Choudhary, A.; Hawkes, Evatt R.; Chen, Jacqueline H.; Shende, Sameer

2008-08-01

94

Interpolation error in DNS simulations of turbulence: consequences for particle tracking  

NASA Astrophysics Data System (ADS)

An important aspect in numerical simulations of particle laden turbulent flows is the interpolation of the flow field. For the interpolation different approaches are used. Where some studies use low order linear interpolation others use high order spline methods. We compare several interpolation methods and conclude that interpolation based on B-spline functions has several advantages compared with traditional methods. First, B-spline interpolation can be executed very efficiently by optimal use of the pseudo-spectral code, only one FFT needs to be executed where Hermite spline needs multiple FFTs for computing the derivatives. Second, the smoothness of the interpolated field is higher than that of Hermite spline interpolation. Finally, the interpolation error almost matches the one of Hermite spline which is not reached by the other methods investigated. Further, we focus on estimating the interpolation error and compare it with the discretisation error of the flow field. In this way one can balance the errors in order to achieve an optimal result. Algorithms have been developed for the approximation of the interpolation error. As a spin-off of the theoretical analysis a practical method is proposed which enables direct estimation of the interpolation error from the energy spectrum, which may provide a quantitative indicator for this purpose.

van Hinsberg, M. A. T.; ten Thije Boonkkamp, J. H. M.; van de Wiel, B. J. H.; Toschi, F.; Clercx, H. J. H.

2011-12-01

95

Numerical simulation of turbulent duct flows with constant power input  

E-print Network

The numerical simulation of a flow through a duct requires an externally specified forcing that makes the fluid flow against viscous friction. To this aim, it is customary to enforce a constant value for either the flow rate (CFR) or the pressure gradient (CPG). When comparing a laminar duct flow before and after a geometrical modification that induces a change of the viscous drag, both approaches (CFR and CPG) lead to a change of the power input across the comparison. Similarly, when carrying out the (DNS and LES) numerical simulation of unsteady turbulent flows, the power input is not constant over time. Carrying out a simulation at constant power input (CPI) is thus a further physically sound option, that becomes particularly appealing in the context of flow control, where a comparison between control-on and control-off conditions has to be made. We describe how to carry out a CPI simulation, and start with defining a new power-related Reynolds number, whose velocity scale is the bulk flow that can be atta...

Hasegawa, Yosuke; Frohnapfel, Bettina

2014-01-01

96

A zonal grid algorithm for DNS of turbulent boundary layers  

Microsoft Academic Search

A zonal grid algorithm for direct numerical simulation (DNS) of incompressible turbulent flows within a Finite-Volume framework is presented. The algorithm uses fully coupled embedded grids and a conservative treatment of the grid-interface variables. A family of conservative prolongation operators is tested in a 2D vortex dipole and a 3D turbulent boundary layer flow. These tests show that both, first-

Michael Manhart

2004-01-01

97

Direct numerical simulations of particle-laden density currents with adaptive, discontinuous finite elements  

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

98

LES and DNS of Shock-Boundary Layer Interactions.  

NASA Astrophysics Data System (ADS)

Large-eddy simulations (LES) of an incident oblique shock wave interacting with a flat-plate supersonic turbulent boundary layer at various flow/shock conditions are performed and the results are compared with the direct numerical simulation (DNS) data. The objectives are to evaluate the performance of compressible subgrid-scale (SGS) models in shock-turbulence flow regions and to study the effects of shock angle, Mach number and other parameters on the shock-boundary layer interactions. The filtered compressible Navier-Stokes equations are solved with a seventh-order Monotonicity-Preserving scheme for the Euler fluxes and a sixth-order compact scheme for the viscous terms. Comparison of DNS and LES results reveal the significance of the SGS model in supersonic boundary layer flow, particularly in the shock-turbulence regions. Due to its excessive dissipative nature, the standard Smagorinsky and gradient type models are found to predict a significantly larger mean separation bubble size when compared to the DNS data. In contrast, the predicted results by the scale-similarity, mixed or dynamic Smagorinsky models are found to be in reasonably good agreement with the DNS. Similar trends are observed for all the major flow variables. In general, the dynamic models, though computationally expensive, are found to generate better results when compared to other models.

Jammalamadaka, Avinash; Li, Zhaorui; Jaberi, Farhad

2009-11-01

99

Numerical Simulation of Deoilin Hydrocyclones  

E-print Network

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

Reza Maddahian; Bijan Farhanieh; Simin Dokht Saemi

100

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

101

Numerical Simulation of Secondary Instability in Hypersonic Boundary Layers  

NASA Astrophysics Data System (ADS)

Secondary Gortler instability in a Mach 15 flow over a blunt wedge with a concave surface is investigated using direct numerical simulation (DNS). Initial forcing disturbances in the simulation are obtained from linear stability theory (LST), and subsequent linear and nonlinear development of the hypersonic Gortler vortices and their secondary instability are studied by computing the full Navier-Stokes equations using a fifth order finite difference upwind scheme and a shock fitting method. The nonlinear development of Gortler vortices distorts the mean flow and leads to highly inflectional profiles not only in wall normal direction, but also in spanwise direction which induce the secondary instability. In the break-down process of Gortler vortices, unsteady fluctuations appear in the vortices. Such a process is through a secondary instability mechanism. Nonlinear development of Gortler vortices in the Mach 15 flow has been studied by imposing strong disturbances obtained from LST at the inlet of computational domain. A two-dimensional linear stability code is applied for the distorted mean flow in order to find secondary modes of hypersonic Gortler vortices. The mode obtained by linear stability analysis is imposed at the entrance of the computational domain. Subsequent development of the secondary mode is carried out by solving the full Navier-Stokes equations. The numerical results of nonlinear development of hypersonic gortler vortices show the inflectional profiles in boundary layers. The numerical results of secondary instability show that the interaction of Grtler vortices with the strong varicose mode leads to the development of a horseshoe vortex.

Whang, Chong; Zhong, Xiaolin

2001-11-01

102

Calculation of Inter-Subchannel Turbulent Mixing Rate and Heat Transfer in a Triangular-Arrayed Rod Bundle Using Direct Numerical Simulation  

Microsoft Academic Search

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

Yudov; Yury V

2006-01-01

103

Numerical Propulsion System Simulation Architecture  

NASA Technical Reports Server (NTRS)

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

Naiman, Cynthia G.

2004-01-01

104

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

SciTech Connect

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

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

2013-01-01

105

DNS-based predictive control of turbulence: an optimal benchmark for feedback algorithms  

Microsoft Academic Search

Direct numerical simulations (DNS) and optimal control theory are used in a predictive control setting to determine controls that effectively reduce the turbulent kinetic energy and drag of a turbulent flow in a plane channel at Re[tau] = 100 and Re[tau] = 180. Wall transpiration (unsteady blowing\\/suction) with zero net mass flux is used as the control. The algorithm used

Thomas R. Bewley; Parviz Moin; Roger Temam

2001-01-01

106

DNS of separating, low Reynolds number flow in a turbine cascade with incoming wakes  

Microsoft Academic Search

Three-dimensional direct numerical simulations (DNS) of flow in a low-pressure turbine cascade at high angle of attack have been performed. The large angle of attack is found to cause separation at the leading edge and somewhat upstream of the trailing edge along the suction side of the blade. The separation bubble at the leading edge is small and any disturbances

J. G. Wissink

2003-01-01

107

DNS of turbulent heat transfer in channel flow with respect to Reynolds and Prandtl number effects  

Microsoft Academic Search

The direct numerical simulation (DNS) of turbulent heat transfer in a channel flow has been carried out to investigate the Reynolds and Prandtl number effects on the turbulent heat transport. The configuration is a fully developed turbulent channel flow with uniform heating from both walls. The Reynolds numbers based on the friction velocity and the channel half width are 180

Hiroshi Kawamura; Hiroyuki Abe; Yuichi Matsuo

1999-01-01

108

Prediction of boundary layer sound radiation from wall shear stresses using DNS data  

Microsoft Academic Search

Sound radiation from a plane turbulent boundary layer is investigated using databases from direct numerical simulations (DNS) of plane turbulent Poiseuille flow up to Reynolds number Re = 1440. Correlation areas for fluctuating wall shear stresses are found to collapse on viscous scaling over the current Reynolds number range. The power spectral density of radiated pressure and the spectrum of

Zhiwei Hu; Christopher L. Morfey; Neil D. Sandham

109

DNS of heat transfer increase in a cylinder stagnation region due to wake-induced turbulence  

Microsoft Academic Search

Heat transfer in the stagnation point area of a heated cylinder is investigated using Direct Numerical Simulation (DNS). The heated cylinder is subjected to the turbulent wake of a smaller cylinder placed upstream. Two Reynolds numbers based on the diameter of the heated cylinder of 13,200 and 48,000 are chosen. In accordance with correlations in the literature, an increase in

L. Venema; D. von Terzi; H.-J. Bauer; W. Rodi

2011-01-01

110

NUMERICAL SIMULATION OF MULTIPOLE CONFINEMENT (Examples)  

E-print Network

NUMERICAL SIMULATION OF MULTIPOLE CONFINEMENT (Examples) by J. R. Patau and J. C. Sprott April 1974 in a zero-dimensional calculation. l "Numerical Simulation of r..1ultipole Confinement, " J. R. Patau and J

Sprott, Julien Clinton

111

Outline o DNS resolution  

E-print Network

Outline o DNS resolution o IPv6; DNS protocol overview o a server is authoritative for names in its domain. o for all the address of another server o DNS can run on top of UDP: - overall time out - time out

Biagioni, Edoardo S.

112

Direct Numerical Simulations of Localised Forced Ignition in Turbulent Mixing Layers: The Effects of Mixture Fraction and Its Gradient  

Microsoft Academic Search

The effects of mixture fraction value ? and the magnitude of its gradient |??| at the ignitor location on the localised forced ignition of turbulent mixing layers under decaying turbulence is studied\\u000a based on three-dimensional compressible Direct Numerical Simulations (DNS) with simplified chemistry. The localised ignition\\u000a is accounted for by a spatial Gaussian power distribution in the energy transport equation,

Nilanjan Chakraborty; E. Mastorakos

2008-01-01

113

Numerical Simulations in Particle Physics  

E-print Network

Numerical simulations have become an important tool to understand and predict non-perturbative phenomena in particle physics. In this article we attempt to present a general overview over the field. First, the basic concepts of lattice gauge theories are described, including a discussion of currently used algorithms and the reconstruction of continuum physics from lattice results. We then proceed to present some results for QCD, both at low energies and at high temperatures, as well as for the electro-weak sector of the standard model.

F. Karsch; E. Laermann

1993-04-14

114

Analysis of DNS Cache Effects on Query Distribution  

PubMed Central

This paper studies the DNS cache effects that occur on query distribution at the CN top-level domain (TLD) server. We first filter out the malformed DNS queries to purify the log data pollution according to six categories. A model for DNS resolution, more specifically DNS caching, is presented. We demonstrate the presence and magnitude of DNS cache effects and the cache sharing effects on the request distribution through analytic model and simulation. CN TLD log data results are provided and analyzed based on the cache model. The approximate TTL distribution for domain name is inferred quantificationally. PMID:24396313

2013-01-01

115

Analysis of DNS cache effects on query distribution.  

PubMed

This paper studies the DNS cache effects that occur on query distribution at the CN top-level domain (TLD) server. We first filter out the malformed DNS queries to purify the log data pollution according to six categories. A model for DNS resolution, more specifically DNS caching, is presented. We demonstrate the presence and magnitude of DNS cache effects and the cache sharing effects on the request distribution through analytic model and simulation. CN TLD log data results are provided and analyzed based on the cache model. The approximate TTL distribution for domain name is inferred quantificationally. PMID:24396313

Wang, Zheng

2013-01-01

116

DNS of shock-turbulent boundary layer interaction  

NASA Astrophysics Data System (ADS)

A novel DNS/LES capability for high speed viscous flows on unstructured grids is being developed. Shock-turbulent boundary layer interaction results in flow separation, shock unsteadiness, and increased aerodynamic and thermal loads. This paper focuses on the DNS of a Mach 3 turbulent boundary layer flow past a 24 degree compression corner. In our simulations, the upstream turbulent boundary layer is obtained by roughness--induced transition of a laminar boundary layer, and not by rescaling methods. The simulations are performed at the conditions of experiments by Bookey et al (AIAA Paper 2005). The results will be compared to experimental data. We will present the evolution of the boundary layer flow across the shock, low frequency unsteadiness, and the upstream influence of the corner. Both numerical issues and their physical implications will be discussed.

Muppidi, Suman; Mahesh, Krishnan

2010-11-01

117

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

118

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

119

On locating the obstruction in the upper airway via numerical simulation.  

PubMed

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

Wang, Yong; Elghobashi, S

2014-03-01

120

Direct numerical simulation of non-premixed flame-wall interactions  

NASA Astrophysics Data System (ADS)

The overall objective of this paper is to illustrate how detailed numerical modelling may be used to bring basic information on fundamental problems in combustion science. We consider in the following the interaction of non-premixed flames with cold solid wall surfaces. Flame-wall interactions are an important feature in many combustion systems, that result in significant changes in the flame and wall dynamics: the flame strength is reduced near cold wall surfaces, leading possibly to (partial or total) quenching, while the gassolid heat flux takes peak values at flame contact. The questions of turbulent fuel-air-temperature mixing, flame extinction and wall surface heat transfer are here 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 up to 90 kW/m2. The structure of the simulated wall flames is studied in terms of a classical mass mixing variable, i.e. the fuel-airbased mixture fraction, and a less familiar heat loss variable, i.e. the excess enthalpy variable, introduced to provide a measure of non-adiabatic behavior due to wall cooling.

Wang, Y.; Trouv, A.

2005-01-01

121

Relativistic Positioning Systems: Numerical Simulations  

E-print Network

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

Neus Puchades; Diego Sez

2014-04-03

122

Numerical simulation of cocontinuous morphologies  

NASA Astrophysics Data System (ADS)

In strongly sheared emulsions, experiments (e.g., Galloway and Macosko 2001) have shown that systems consisting of one continuous (matrix) and one dispersed (drops) phase may undergo a coalescence cascade leading to a system in which both phases are continuous, i.e., cocontinuous, (sponge-like). Such configurations may have desirable diffusional, mechanical and electrical properties and thus have wide-ranging applications. Using a diffuse interface method developed by Kim and Lowengrub 2001, we perform numerical simulations of the interface length per unit area as a function of volume fractions in 2-d. In this approach, interfaces have small but finite thickness and limited chemical diffusion is used to change the topology of interfaces. In this presentation, we discuss the effects of the viscosity ratio, surface tension, and flow on interface length per unit area and compare it with experiment results. The use of adaptive mesh refinement techniques recently developed by Kim, Wise and Lowengrub will also be discussed.

Kim, Junseok

2005-11-01

123

Numerical simulations of magnetized jets  

NASA Technical Reports Server (NTRS)

The present axisymmetric numerical simulations of light hypersonic jets allow unmagnetized jets and jets carrying a dynamically important magnetic field to be contrasted. After decelerating a weakly magnetized jet through a series of weak, oblique shocks, a Mach disk and a strong annular shock are encountered near the outer edges of the contact discontinuity separating the shocked fluid from the shocked ambient gas. Upon passing the annular shock, the gas quickly expands and enters a backflowing cocoon surrounding the jet. The overall speed of advance of the jet is reduced; matter near the jet axis which passes through the terminal Mach disk accumulates in a plug, and gas is discharged into the cocoon by the intermittent shedding of vortices. When magnetic stresses dominate, however, the jet is rapidly decelerated via a Mach disk and strong annular shock.

Lind, Kevin R.; Payne, David G.; Meier, David L.; Blandford, Roger D.

1989-01-01

124

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

125

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

126

Numerical simulations of non-homogeneous viscoelastic turbulent channel flow  

NASA Astrophysics Data System (ADS)

The effect of the polymer mixing in turbulent channel flow is studied through numerical simulations, using a spectral technique. In particular, we simulate injection of polymeric material through a slit very close to the wall and parallel to it in pre-established Newtonian turbulent flow. The governing equations consist of the mass conservation, the modified Navier-Stokes equation (in order to take into account the polymer extra-stress), the evolution equation for the conformation tensor and an advection-diffusion equation for the polymer concentration. The injection process is simulated by dividing the computational domain in three different regions: (a) the entrance region where the polymer is introduced (b) the developing region where the polymer is allowed to convect freely interacting/modifying the turbulent flow and (c) the recovering region where we use a reacting sink to force the removal of the polymer from the solvent in order to re-establish the inlet conditions. A fully spectral method is used in order to solve the set of governing equations similar to that developed for homogenous viscoelastic turbulent DNS (Housiadas & Beris, Phys. Fluids, 15, (2003)). Although a significantly improved numerical algorithm has been successfully used before (Housiadas & Beris, to appear in J. Non-Newt. Fluid Mech. (2004)) a further improved version of that algorithm is presented in this work. The new algorithm has enabled us to extend the simulations for much wider range of viscoelasticity parameter values as well as for many viscoelastic models like the FENE-P, Giesekus, Oldroyd-B and the modified Giesekus/FENE-P model. Results for illustrative sets of parameter values are going to be presented.

Housiadas, Kostas; Beris, Antony

2004-11-01

127

Numerical Simulations of High Speed Turbulent Jets in Crossflow  

NASA Astrophysics Data System (ADS)

This dissertation studies high speed jets in crossflow using numerical simulations. The complexity of this flow makes detailed measurements difficult, and only limited information is provided by past experimental studies. Traditional engineering simulation tools also have difficulties in simulating such flows. Therefore, the current study: 1) develops Large-Eddy Simulation (LES) capability and novel subgrid-scale (SGS) models for high speed flows in complex geometries; 2) realizes multiple methods to generate realistic turbulent boundary layer inflow condition for unstructured compressible flow solver; 3) explores the detailed physics of high speed jets in crossflow; 4) investigates the jet trajectory, entrainment and coherent vortical motions. Large-eddy simulation capability is developed for the base numerical scheme developed by Park & Mahesh (2007) for solving the compressible Navier-Stokes equations on unstructured grids. Large-eddy simulations are performed to study an under-expanded sonic jet injected into a supersonic crossflow and an over-expanded supersonic jet injected into a subsonic crossflow, where the flow conditions are based on Santiago et al.'s (1997) and Beresh et al.'s (2005) experiments, respectively. The simulations successfully reproduce experimentally observed shock systems and vortical structures. The time averaged flow fields are compared to the experimental results, and reasonable agreement is observed. The behavior of the flow is discussed, and the similarities and differences between the two regimes are studied. The trajectory and entrainment of the transverse jet is investigated. A modification to Schetz & Billig's theory (1966) theory is proposed, which yields good prediction of the jet trajectories in the current simulations in the near field. Along the jet center streamline, the jet entrainment grow faster compared to turbulent free jets. In the very far field, the growth rates of jet entrainment decrease to values that are close to those for turbulent free jets. The Strouhal numbers of relevant flow structures are computed from power spectral densities. Many flow motions are observed to be correlated in sonic jet in supersonic crossflow. The frequencies in supersonic injection are observed to be higher than those in sonic injection. A novel eddy-viscosity model, "dynamic k-equation model'', is proposed for the LES of compressible flows with complex flow geometries, where the transport equation for sub-grid scale (SGS) kinetic energy is introduced to predict SGS kinetic energy, instead of using Yoshizawa's model as in standard Dynamic Smagorinsky Model (DSM). The exact SGS kinetic energy transport equation for compressible flows is derived formally. Each of the unclosed terms in the SGS kinetic energy equation is modeled separately and dynamically closed, instead of being grouped into production and dissipation terms, as in the Reynolds Averaged Navier-Stokes (RANS) equations. A priori test using Direct Numerical Simulation (DNS) of decaying isotropic turbulence shows that for Smagorinsky type eddy viscosity model, the correlation between SGS stress and the model is comparable to that from the original model. Also, the suggested model for pressure dilatation term in the SGS kinetic energy equation is found to have high correlation with its actual value. In a posteriori tests, the proposed dynamic k-equation model is applied to decaying isotropic turbulence and normal shock/isotropic turbulence interaction, and yields good agreement with available experimental and DNS data. Compared with the results of Dynamic Smagorinsky Model (DSM), the k-equation model predicts better energy spectra at high wave numbers, similar kinetic energy decay and fluctuations of thermodynamic quantities for decaying isotropic turbulence. For shock/turbulent interaction, &#

Chai, Xiaochuan

128

DNS of turbulent channel flow at very low Reynolds numbers  

E-print Network

Direct numerical simulations (DNS) of fully-developed turbulent channel flows for very low Reynolds numbers have been performed with a larger computational box sizes than those of existing DNS. The friction Reynolds number was decreased down to 60, where the friction Reynolds number is based on the friction velocity and the channel half width. When the Reynolds number was decreased to 60 with small computational box size, the flow became laminar. Using a large box, we found that a localized turbulence was observed to sustain in the form of periodic oblique band. This type of locally disordered flow is similar to a equilibrium turbulent puff in a transitional pipe flow. Various turbulence statistics such as turbulence intensities, vorticity fluctuations and Reynolds stresses are provided. Especially, their near-wall asymptotic behavior and budget terms of turbulence kinetic energy were discussed with respect to the Reynolds-number dependence and an influence of the computational box size. Other detailed charac...

Tsukahara, Takahiro

2014-01-01

129

DNS of aerosol evolution in a turbulent jet  

NASA Astrophysics Data System (ADS)

The effects of turbulence on the evolution of aerosols are not well understood. In this work, the interaction of aerosol dynamics and turbulence are studied in a canonical flow configuration by numerical means. The configuration consists of a hot nitrogen stream saturated with dibutyl phthalate (DBP) vapor mixing with cool air in a shear layer. A direct numerical simulation (DNS) for the momentum and scalar fields is coupled with the direct quadrature method of moments (DQMOM) for the condensing liquid phase. The effects of turbulent mixing on aerosol processes (nucleation, condensation, and coagulation) are quantified by analyzing the statistics of number density and droplet sizes.

Zhou, Kun; Attili, Antonio; Bisetti, Fabrizio

2011-11-01

130

Numerical Simulation of Protoplanetary Vortices  

NASA Technical Reports Server (NTRS)

The fluid dynamics within a protoplanetary disk has been attracting the attention of many researchers for a few decades. Previous works include, to list only a few among many others, the well-known prescription of Shakura & Sunyaev, the convective and instability study of Stone & Balbus and Hawley et al., the Rossby wave approach of Lovelace et al., as well as a recent work by Klahr & Bodenheimer, which attempted to identify turbulent flow within the disk. The disk is commonly understood to be a thin gas disk rotating around a central star with differential rotation (the Keplerian velocity), and the central quest remains as how the flow behavior deviates (albeit by a small amount) from a strong balance established between gravitational and centrifugal forces, transfers mass and momentum inward, and eventually forms planetesimals and planets. In earlier works we have briefly described the possible physical processes involved in the disk; we have proposed the existence of long-lasting, coherent vortices as an efficient agent for mass and momentum transport. In particular, Barranco et al. provided a general mathematical framework that is suitable for the asymptotic regime of the disk; Barranco & Marcus (2000) addressed a proposed vortex-dust interaction mechanism which might lead to planetesimal formation; and Lin et al. (2002), as inspired by general geophysical vortex dynamics, proposed basic mechanisms by which vortices can transport mass and angular momentum. The current work follows up on our previous effort. We shall focus on the detailed numerical implementation of our problem. We have developed a parallel, pseudo-spectral code to simulate the full three-dimensional vortex dynamics in a stably-stratified, differentially rotating frame, which represents the environment of the disk. Our simulation is validated with full diagnostics and comparisons, and we present our results on a family of three-dimensional, coherent equilibrium vortices.

Lin, H.; Barranco, J. A.; Marcus, P. S.

2003-01-01

131

Numerical Simulations of Thermobaric Explosions  

SciTech Connect

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

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

2007-05-04

132

Multiscale Issues in DNS of Multiphase Flows  

NASA Astrophysics Data System (ADS)

In direct numerical simulations (DNS) of multiphase flows it is frequently found that features much smaller than the "dominant" flow scales emerge. Those features consist of thin films, filaments, drops, and boundary layers, and usually surface tension is strong so the geometry is simple. Inertia effects are also small so the flow is simple and often there is a clear separation of scales between those features and the rest of the flow. Thus it is often possible to describe the evolution of this flow by analytical models. Here we discuss two examples of the use of analytical models to account for small-scale features in DNS of multiphase flows. For the flow in the film beneath a drop sliding down a sloping wall, we capture the evolution of films that are too thin to be accurately resolved using a grid that is sufficient for the rest of the flow by a thin film model. The other example is the mass transfer from a gas bubbly rising in a liquid. Since diffusion of mass is much slower than the diffusion of momentum, the mass transfer boundary layer is very thin and can be captured by a simple boundary layer model. The coupling of the model for the unresolved features to the rest of the flow is discussed for both examples.

Tryggvason, Gretar; Thomas, Siju; Lu, Jiacai; Aboulhasanzadeh, Bahman

2010-11-01

133

DNS of the kappa-mechanism. I. Radial modes in the purely radiative case  

E-print Network

Context: Hydrodynamical model of the kappa-mechanism in a purely radiative case. Aims: First, to determine the physical conditions propitious to kappa-mechanism in a layer with a configurable conductivity hollow and second, to perform the (nonlinear) direct numerical simulations (DNS) from the most favourable setups. Methods: A linear stability analysis applied to radial modes using a spectral solver and DNS thanks to a high-order finite difference code are compared. Results: Changing the hollow properties (location and shape) lead to well-defined instability strips. For a given position in the layer, the amplitude and width of the hollow appear to be the key parameters to get unstable modes driven by kappa-mechanism. The DNS achieved from these more auspicious configurations confirm the growth rates as well as structures of linearly unstable modes. The nonlinear saturation follows through intricate couplings between the excited fundamental mode and higher damped overtones.

T. Gastine; B. Dintrans

2007-11-08

134

DNS of the kappa-mechanism. I. Radial modes in the purely radiative case  

E-print Network

Context: Hydrodynamical model of the kappa-mechanism in a purely radiative case. Aims: First, to determine the physical conditions propitious to kappa-mechanism in a layer with a configurable conductivity hollow and second, to perform the (nonlinear) direct numerical simulations (DNS) from the most favourable setups. Methods: A linear stability analysis applied to radial modes using a spectral solver and DNS thanks to a high-order finite difference code are compared. Results: Changing the hollow properties (location and shape) lead to well-defined instability strips. For a given position in the layer, the amplitude and width of the hollow appear to be the key parameters to get unstable modes driven by kappa-mechanism. The DNS achieved from these more auspicious configurations confirm the growth rates as well as structures of linearly unstable modes. The nonlinear saturation follows through intricate couplings between the excited fundamental mode and higher damped overtones.

Gastine, T

2007-01-01

135

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

136

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

NASA Technical Reports Server (NTRS)

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

Diosady, Laslo T.; Murman, Scott M.

2014-01-01

137

Numerical simulation package for speckle metrology  

NASA Astrophysics Data System (ADS)

A computer program package for numerical simulation of speckle phenomena has been developed. It is suitable for simulating both objective and subjective speckle effects in various optical setups. Several simulation results are presented in this paper. The simulations was made in UNIX and Windows NT environment.

Kornis, Janos; Bokor, Nandor; Nemeth, Attila

1998-09-01

138

Numerical Simulations of Granular Processes  

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

139

A critical comparison of second order closures with direct numerical simulation of homogeneous turbulence  

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

140

Group analysis, direct numerical simulation and modelling of a turbulent channel flow with streamwise rotation  

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

141

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

NASA Astrophysics Data System (ADS)

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

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

2015-02-01

142

DNS Study of Effects of Suddenly-Vanishing Wall Heating in Turbulent Boundary Layer  

NASA Astrophysics Data System (ADS)

The objective of this study is to investigate a thermal field in the turbulent boundary layer by means of direct numerical simulation (DNS), in which the wall heating has suddenly vanished in the downstream region, i.e., the wall is heated by a constant temperature condition followed by an adiabatic condition. The DNS of spatially developing boundary layers with heat transfer using the generation of turbulent inflow data method has been conducted. In this study, two types of flow field with heat transfer are investigated via DNS. One is a turbulent boundary layer along flat plate, and the other is a turbulent boundary layer over the forward-facing step. In both cases, constant temperature wall followed by adiabatic wall condition is adopted. In particular, the turbulent heat transfer phenomena around suddenly-changing wall thermal condition are revealed. In the case of forward-facing step flow, since the adiabatic wall thermal condition is applied on the step, a peculiar phenomenon is observed in comparison with the case of flat plate. DNS results clearly show the statistics and structure of turbulent heat transfer in a constant temperature wall followed by an adiabatic wall. Also, DNS clearly shows the wall-limiting behaviour of turbulence in thermal field whose index number with reference to the distance from the wall changes due to the modification of wall thermal conditions, which may be useful for the turbulence modelling.

Hattori, Hirofumi; Yamada, Syohei; Houra, Tomoya

143

DNS of turbulent heat transfer in channel flow with low to medium-high Prandtl number fluid  

Microsoft Academic Search

The direct numerical simulation (DNS) of the turbulent heat transfer for various Prandtl numbers ranging from 0.025 to 5 are performed to obtain statistical quantities such as turbulent heat flux, temperature variance and their budget terms. The configuration is the fully developed channel flow with uniform heating from both walls. The Reynolds number based on the friction velocity and the

Hiroshi Kawamura; Kouichi Ohsaka; Hiroyuki Abe; Kiyoshi Yamamoto

1998-01-01

144

Direct numerical simulation of binary off-center collisions of shear thinning droplets at high Weber numbers  

NASA Astrophysics Data System (ADS)

Binary droplet collisions, a prototype elementary subprocess inside sprays, are investigated by direct numerical simulations (DNS) based on an extended volume of fluid method. We focus on shear-thinning droplet collisions. In order to capture the dynamics of droplet collisions with different outcomes, we account for off-center collisions at high Weber numbers. Such collision conditions lead to the formation of extremely thin fluid lamellae. It turns out that these thin lamellae determine the smallest length scales which must be resolved in a DNS. A stabilization algorithm is presented which prevents the lamellae from rupturing. It is validated by comparison with experimental data and applied for a droplet collision study of shear-thinning liquids. The results show that, independent of the off-set of the colliding droplets, a collision of Newtonian liquid droplets with appropriately chosen viscosity can reproduce the collision dynamics of the shear-thinning liquid droplets. This includes temporal evolution of shapes and energy.

Focke, C.; Bothe, D.

2012-07-01

145

The Numerical Simulation of Turbulence  

E-print Network

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

W. Schmidt

2007-12-06

146

Numerical Simulation of a High Mach Number Jet Flow  

NASA Technical Reports Server (NTRS)

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

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

1993-01-01

147

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

148

Characteristics of supercritical turbulence from Direct Numerical Simulations of C(sub 7)H(sub 16)/N(sub 2) and O(sub 2)/H(sub 2)  

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

149

Numerical simulation of Kelvin-Helmholtz instability using an implicit, non-dissipative DNS algorithm  

NASA Astrophysics Data System (ADS)

An in-house, fully parallel compressible Navier-Stokes solver was developed based on an implicit, non-dissipative, energy conserving, finite-volume algorithm. PETSc software was utilized for this purpose. To be able to handle occasional instances of slow convergence due to possible oscillating pressure corrections on successive iterations in time, a fixing procedure was adopted. To demonstrate the algorithms ability to evolve a linear perturbation into nonlinear hydrodynamic turbulence, temporal Kelvin-Helmholtz Instability problem is studied. KHI occurs when a perturbation is introduced into a system with a velocity shear. The theory can be used to predict the onset of instability and transition to turbulence in fluids moving at various speeds. In this study, growth rate of the instability was compared to predictions from linear theory using a single mode perturbation in the linear regime. Effect of various factors on growth rate was also discussed. Compressible KHI is most unstable in subsonic/transonic regime. High Reynolds number (low viscosity) allows perturbations to develop easily, in consistent with the nature of KHI. Higher wave numbers (shorter wavelengths) also grow faster. These results match with the findings of stability analysis, as well as other results presented in the literature.

Yilmaz, ?.; Davidson, L.; Edis, F. O.; Saygin, H.

2011-12-01

150

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

151

Direct numerical simulation of turbulent mixing at very low Schmidt number with a uniform mean gradient  

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

152

Numerical Simulation of Turbulent Fluid Flows  

NASA Technical Reports Server (NTRS)

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

Leonard, A.

1983-01-01

153

Numerical simulation of jet noise  

Microsoft Academic Search

In the present work, computational aeroacoustics and parallel computers are used to conduct a study of flow-induced noise from different jet nozzle geometries. The nozzle is included as part of the computational domain. This is important to predict jet noise from nozzles associated with military aircraft engines. The Detached Eddy Simulation (DES) approach is used to simulate both the jet

Umesh Paliath

2006-01-01

154

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

155

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

NASA Astrophysics Data System (ADS)

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

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

2014-02-01

156

GPU accelerated flow solver for direct numerical simulation of turbulent flows  

SciTech Connect

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 NavierStokes 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

157

Effects of Lewis number on conditional fluid velocity statistics in low Damkhler number turbulent premixed combustion: A direct numerical simulation analysis  

NASA Astrophysics Data System (ADS)

The effects of global Lewis number Le on the statistics of fluid velocity components conditional in unburned reactants and fully burned products in the context of Reynolds Averaged Navier Stokes simulations have been analysed using a Direct Numerical Simulations (DNS) database of statistically planar turbulent premixed flames with a low Damkhler number and Lewis number ranging from 0.34 to 1.2. The conditional velocity statistics extracted from DNS data have been analysed with respect to the well-known Bray-Moss-Libby (BML) expressions which were derived based on bi-modal probability density function of reaction progress variable for high Damkhler number flames. It has been shown that the Lewis number substantially affects the mean velocity and the velocity fluctuation correlation conditional in products, with the effect being particularly pronounced for low Le. As far as the mean velocity and the velocity fluctuation correlation conditional in reactants are concerned, the BML expressions agree reasonably well with the DNS data reported in the present work. Based on a priori analysis of present and previously reported DNS data, the BML expressions have been empirically modified here in order to account for Lewis number effects, and the non-bimodal distribution of reaction progress variable. Moreover, it has been demonstrated for the first time that surface averaged velocity components and Reynolds stresses conditional in unburned reactants can be modelled without invoking expressions involving the Lewis number, as these surface averaged conditional quantities remain approximately equal to their conditionally averaged counterparts in the unburned mixture.

Chakraborty, Nilanjan; Lipatnikov, Andrei N.

2013-04-01

158

Direct Numerical Simulation of Two Shock Wave/Turbulent Boundary Layer Interactions  

NASA Astrophysics Data System (ADS)

Direct numerical simulations (DNSs) of two shock wave/turbulent boundary layer interactions (STBLIs) are presented in this thesis. The first interaction is a 24 compression ramp at Mach 2.9, and the second interaction is an 8 compression ramp at Mach 7.2. The large-scale low-frequency unsteadiness in the Mach 2.9 DNS is investigated with the aim of shedding some light on its physical origin. Previous experimental and computational works have linked the unsteadiness either to fluctuations in the incoming boundary layer or to a mechanism in the downstream separated flow. Consistent with experimental observations, the shock in the DNS is found to undergo streamwise oscillations, which are broadband and occur at frequencies that are about two orders of magnitude lower than the characteristic frequency of the energy-containing turbulent scales in the incoming boundary layer. Based on a coherence and phase analysis of signals at the wall and in the flow field, it is found that the low frequency shock unsteadiness is statistically linked to pulsations of the downstream separated flow. The statistical link with fluctuations in the upstream boundary layer is also investigated. A weak link is observed: the value of the low-frequency coherence with the upstream flow is found to lie just above the limit of statistical significance, which is determined by means of a Monte Carlo study. The dynamics of the downstream separated flow are characterized further based on low-pass filtered DNS fields. The results suggest that structural changes occur in the downstream separated flow during the low-frequency motions, including the breaking-up of the separation bubble, which is observed when the shock moves downstream. The structural changes are described based on the Cf distribution through the interaction, as well as the velocity and vorticity fields. The possible link between the low-frequency dynamics observed in the DNS and results from global instability theory is explored. It is observed that the structural changes in the downstream separated flow are reminiscent of certain global linear instability modes reported in the literature, suggesting that an inherent instability of the separated flow could be the driving mechanism for the unsteadiness. The separated shear layer in the DNS is characterized: the self-similarity of the shear layer profiles, the formation of vortical structures in the shear layer, and the low-frequency behavior of the shear layer are investigated. Based on the results, possible low-frequency mechanisms involving the shear layer are discussed. The second DNS presented in this thesis is of an attached hypersonic STBLI (8 compression ramp at Mach 7.2). The flow is described based on flow visualizations, distributions of wall quantities, as well as mean and fluctuating fields. Wall heat transfer scalings and the turbulence amplification in the interaction are discussed. The DNS results are compared to experiments performed by Smits and co-workers at similar flow conditions, and excellent qualitative agreement is observed.

Priebe, Stephan

159

Drugs, Herbicides and Numerical Simulations Hermann Mena  

E-print Network

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

Matijevic, Domagoj

160

Numerical simulation of turbulent reacting flows  

SciTech Connect

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

161

Numerical simulation of heat exchanger  

SciTech Connect

Accurate and detailed knowledge of the fluid flow field and thermal distribution inside a heat exchanger becomes invaluable as a large, efficient, and reliable unit is sought. This information is needed to provide proper evaluation of the thermal and structural performance characteristics of a heat exchanger. It is to be noted that an analytical prediction method, when properly validated, will greatly reduce the need for model testing, facilitate interpolating and extrapolating test data, aid in optimizing heat-exchanger design and performance, and provide scaling capability. Thus tremendous savings of cost and time are realized. With the advent of large digital computers and advances in the development of computational fluid mechanics, it has become possible to predict analytically, through numerical solution, the conservation equations of mass, momentum, and energy for both the shellside and tubeside fluids. The numerical modeling technique will be a valuable, cost-effective design tool for development of advanced heat exchangers.

Sha, W.T.

1985-01-01

162

DNS of a Mach 4 Boundary Layer with Chemical Reactions M. Pino Martin  

E-print Network

DNS of a Mach 4 Boundary Layer with Chemical Reactions M. Pino Mart´in Graham V. Candler Aerospace simulation (DNS) database is used to develop a greater understanding of the turbulence-chemistry interaction by endothermic reactions and destabilized by exother- mic reactions. In our previous work,2,3 we used DNS

Martín, Pino

163

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

164

Numerical Simulations of Hypervelocity Plasma Jets  

Microsoft Academic Search

Numerical simulations with comparisons to experiments of hypervelocity plasma jets in development at HyperV Technologies Corp. are presented. The focus will be on the new plasma jet designed to drive rotation in the University of Maryland MCX experiment. Performance of coaxial plasma jets is typically limited by the blow-by instability. Extensive numerical modeling with the Mach 2 code was used

M. W. Phillips; F. D. Witherspoon; A. Case; S. J. Messer; T. P. Hughes; D. R. Welch; I. N. Bogatu; S. R. Galkin; J. S. Kim

2007-01-01

165

Analysis of Numerical Simulation Database for Pressure Fluctuations Induced by High-Speed Turbulent Boundary Layers  

NASA Technical Reports Server (NTRS)

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

Duan, Lian; Choudhari, Meelan M.

2014-01-01

166

Numerical Simulation For Supersonic Inlets  

NASA Technical Reports Server (NTRS)

Flows calculated for realistic engine-inlet conditions. Computer code LAPIN, large-perturbation inlet, developed to analyze large-perturbation, transient-flow fields in supersonic inlets. Robust, quick-running code capable of solving unsteady quasi-one-dimensional, inviscid-flow problems in mixed subsonic and supersonic regimes for inlets. Approach based upon quasi-one-dimensional, inviscid, unsteady formulation including engineering models of unstart/restart, bleed, bypass, and geometrical effects. Numerical solution of governing time-dependent equations of motion accomplished through shock-capturing, finite-difference algorithm. Program written in FORTRAN IV.

Varner, M. O.; Martindale, W. R.; Phares, W. J.; Kneile, K. R.; Adams, J. C., Jr.

1987-01-01

167

Numerical simulation of hydraulic transients  

NASA Astrophysics Data System (ADS)

A numerical method suitable for the analysis of hydraulic transients in one-dimensional pipelines as well as some applications of the method are presented in this thesis. In the present method one-dimensional flow equations are solved in a characteristic form using a finite difference technique. A nonequilibrium two-phase flow model is used, which makes it possible to analyze the effect of vaporization. The motion of the pipe-wall, which is important in some types of hydraulic transients, can be taken into account approximately. The main application of the method has been the piping of nuclear reactors.

Siikonen, T.

168

Numerical simulations of quasar absorbers  

E-print Network

The physical state of the intergalactic medium can be probed in great detail with the intervening absorption systems seen in quasar spectra. The properties of the Hydrogen absorbers depend on many cosmological parameters, such as the matter-power spectrum, reionisation history, ionising background and the nature of the dark matter. The spectra also contain metal lines, which can be used to constrain the star formation history and the feedback processes acting in large and small galaxies. Simulations have been instrumental in investigating to what extent these parameters can be unambiguously constrained with current and future data. This paper is meant as an introduction to this subject, and reviews techniques and methods for simulating the intergalactic medium.

Tom Theuns

2005-07-25

169

NUMERICAL SIMULATIONS OF CHROMOSPHERIC MICROFLARES  

SciTech Connect

With gravity, ionization, and radiation being considered, we perform 2.5 dimensional (2.5D) compressible resistive magnetohydrodynamic (MHD) simulations of chromospheric magnetic reconnection using the CIP-MOCCT scheme. The temperature distribution of the quiet-Sun atmospheric model VALC and the helium abundance (10%) are adopted. Our 2.5D MHD simulation reproduces qualitatively the temperature enhancement observed in chromospheric microflares. The temperature enhancement DELTAT is demonstrated to be sensitive to the background magnetic field, whereas the total evolution time DELTAt is sensitive to the magnitude of the anomalous resistivity. Moreover, we found a scaling law, which is described as DELTAT/DELTAt {approx} n{sub H} {sup -1.5} B {sup 2.1}eta{sub 0} {sup 0.88}. Our results also indicate that the velocity of the upward jet is much greater than that of the downward jet, and the X-point may move up or down.

Jiang, R. L.; Fang, C.; Chen, P. F., E-mail: fangc@nju.edu.c [Department of Astronomy, Nanjing University, Nanjing 210093 (China)

2010-02-20

170

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

171

Numerical Simulations of Bouncing Jets  

E-print Network

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

Bonito, Andrea; Lee, Sanghyun

2015-01-01

172

Chapter 3 Epoch Making Simulation Direct Numerical Simulations of Fundamental Turbulent Flows with the Largest Grid Numbers in the World and its Application of Modeling for Engineering Turbulent Flows Project Representative  

E-print Network

In order to understand universal nature of turbulence, we performed large-scale direct numerical simulations (DNS's) of canonical incompressible turbulence on the Earth Simulator, including those of (i) turbulence in a periodic box and (ii) turbulent Ekman boundary layer. The DNS data were analyzed to study (i) the decay of isotropic turbulence and (ii) Reynolds number dependence and the three-dimensional characteristics in the turbulent Ekman boundary layer. Data analyses based on a wavelet-based method and those based on the multifractal model of turbulence were also made by using data obtained by the high-resolution DNS of forced incompressible turbulence in a periodic box. We also performed numerical simulation of turbulent flows on the ES from the view point of engineering applications. We made a large eddy simulation for urban turbulent boundary layer. The information obtained by the LES will be utilized to solve the recent environmental problems such as air pollution and heat island in the urban area, also to establish the secure and safe society against the hazardous gasses at the center of city. We also made a detached eddy simulation (DES) for wake flow fields of a wind turbine. Aerodynamic performance and structure of wake flow behind a wind turbine were predicted for some wind speeds.

Chuichi Arakawa

173

Numerical simulation of semiconductor devices  

SciTech Connect

An extension of Gummel's method to calculate the electron density and the potential in unipolar devices is presented. How Gummel's algorithm is implemented to solve the system of partial differential equations is explained. The current continuity equation is solved directly to obtain the electron density. Some comparisons with the stream function method to solve the current continuity equation are also discussed. It should be noted that the stream function method can only be used for time-independent problems with no recombination or generation terms included, whereas the method described can be generalized to time-dependent problems or problems with recombination and generation terms included. Finally numerical results for a standard MOSFET and JFET are given.

Slamet, S.

1981-09-01

174

DNS of a Laminar Separation Bubble in the Presence of Oscillating External Flow  

Microsoft Academic Search

A three-dimensional Direct Numerical Simulation (DNS) of a laminar separation bubble in the presence of oscillating flow is\\u000a performed. The oscillating flow induces a streamwise pressure gradient varying in time. The special shape of the upper boundary\\u000a of the computational domain, together with the oscillating pressure gradient causes the boundary layer flow to alternately\\u000a separate and re-attach. When the inflow

J. G. Wissink; W. Rodi

2003-01-01

175

Numerical simulation of transitional flow  

NASA Technical Reports Server (NTRS)

The applicability of active control of transition by periodic suction-blowing is investigated via direct simulations of the Navier-Stokes equations. The time-evolution of finite-amplitude disturbances in plane channel flow is compared in detail with and without control. The analysis indicates that, for relatively small three-dimensional amplitudes, a two-dimensional control effectively reduces disturbance growth rates even for linearly unstable Reynolds numbers. After the flow goes through secondary instability, three-dimensional control seems necessary to stabilize the flow. An investigation of the temperature field suggests that passive temperature contamination is operative to reflect the flow dynamics during transition.

Biringen, Sedat

1986-01-01

176

Numerical simulations of disordered superconductors  

SciTech Connect

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

177

Numerical Simulation of Cavitating Flows  

NASA Astrophysics Data System (ADS)

We are developing the capability to simulate cavitating flows (e.g sheet to cloud cavitation transition) in complex geometries. The compressible flow solver (Park & Mahesh, AIAA Paper 2007-0722) has been extended to solve for multiphase flows on unstructured meshes. A multi-phase medium is constructed using a homogenous equilibrium model that assumes thermal equilibrium between the liquid phase and the vapor phase. The algorithm solves the compressible Navier Stokes Equations for the liquid/vapor medium along with the transport equation for the liquid's mass fraction. A characteristic-based shock capturing scheme is extended to handle non-ideal gases and mixtures, and applied in a predictor-corrector approach. The base scheme is non-dissipative and this approach ensures that the shock-capturing is active only in the regions of discontinuity. We will present details of this algorithm, its implementation, and validation.

Gnanaskandan, Aswin; Mahesh, Krishnan

2011-11-01

178

Resilience of helical fields to turbulent diffusion II: direct numerical simulations  

E-print Network

The recent study of Blackman and Subramanian (Paper I) indicates that large scale helical magnetic fields are resilient to turbulent diffusion in the sense that helical fields stronger than a critical value, decay on slow (~resistively mediated), rather than fast ($\\sim$ turbulent) time scales. This gives more credence to potential fossil field origin models of the magnetic fields in stars, galaxies and compact objects. We analyze a suite of direct numerical simulations (DNS) of decaying large scale helical magnetic fields in the presence of non-helical turbulence to further study the physics of helical field decay. We study two separate cases: (1) the initial field is large enough to decay resistively, is tracked until it transitions to decay fast, and the critical large scale helical field at that transition is sought; (2) the case of Paper I, wherein there is a critical initial helical field strength below which the field undergoes fast decay right from the beginning. In case (1), both DNS and solutions of...

Bhat, Pallavi; Subramanian, Kandaswamy

2013-01-01

179

Advancing predictive models for particulate formation in turbulent flames via massively parallel direct numerical simulations.  

PubMed

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

180

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

NASA Technical Reports Server (NTRS)

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

Duan, Lian; Choudhari, Meelan, M.

2014-01-01

181

Comparison of direct numerical simulation databases of turbulent channel flow at Re? = 180  

NASA Astrophysics Data System (ADS)

Direct numerical simulation (DNS) databases are compared to assess the accuracy and reproducibility of standard and non-standard turbulence statistics of incompressible plane channel flow at Re? = 180. Two fundamentally different DNS codes are shown to produce maximum relative deviations below 0.2% for the mean flow, below 1% for the root-mean-square velocity and pressure fluctuations, and below 2% for the three components of the turbulent dissipation. Relatively fine grids and long statistical averaging times are required. An analysis of dissipation spectra demonstrates that the enhanced resolution is necessary for an accurate representation of the smallest physical scales in the turbulent dissipation. The results are related to the physics of turbulent channel flow in several ways. First, the reproducibility supports the hitherto unproven theoretical hypothesis that the statistically stationary state of turbulent channel flow is unique. Second, the peaks of dissipation spectra provide information on length scales of the small-scale turbulence. Third, the computed means and fluctuations of the convective, pressure, and viscous terms in the momentum equation show the importance of the different forces in the momentum equation relative to each other. The Galilean transformation that leads to minimum peak fluctuation of the convective term is determined. Fourth, an analysis of higher-order statistics is performed. The skewness of the longitudinal derivative of the streamwise velocity is stronger than expected (-1.5 at y+ = 30). This skewness and also the strong near-wall intermittency of the normal velocity are related to coherent structures.

Vreman, A. W.; Kuerten, J. G. M.

2014-01-01

182

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

183

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

E-print Network

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

Papaioannou, Georgios (Georgios Vasilios), 1975-

2004-01-01

184

Numerical Simulation of Nanostructure Growth  

NASA Technical Reports Server (NTRS)

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

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

2004-01-01

185

Numerical simulation of gravitational lenses  

NASA Astrophysics Data System (ADS)

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

Cherniak, Yakov

186

Three-dimensional direct numerical simulation study of conditioned moments associated with front propagation in turbulent flows  

NASA Astrophysics Data System (ADS)

In order to gain further insight into (i) the use of conditioned quantities for characterizing turbulence within a premixed flame brush and (ii) the influence of front propagation on turbulent scalar transport, a 3D Direct Numerical Simulation (DNS) study of an infinitely thin front that self-propagates in statistically stationary, homogeneous, isotropic, forced turbulence was performed by numerically integrating Navier-Stokes and level set equations. While this study was motivated by issues relevant to premixed combustion, the density was assumed to be constant in order (i) to avoid the influence of the front on the flow and, therefore, to know the true turbulence characteristics as reference quantities for assessment of conditioned moments and (ii) to separate the influence of front propagation on turbulent transport from the influence of pressure gradient induced by heat release. Numerical simulations were performed for two turbulence Reynolds numbers (50 and 100) and four ratios (1, 2, 5, and 10) of the rms turbulent velocity to the front speed. Obtained results show that, first, the mean front thickness is decreased when a ratio of the rms turbulent velocity to the front speed is decreased. Second, although the gradient diffusion closure yields the right direction of turbulent scalar flux obtained in the DNS, the diffusion coefficient Dt determined using the DNS data depends on the mean progress variable. Moreover, Dt is decreased when the front speed is increased, thus, indicating that the front propagation affects turbulent scalar transport even in a constant-density case. Third, conditioned moments of the velocity field differ from counterpart mean moments, thus, disputing the use of conditioned velocity moments for characterizing turbulence when modeling premixed turbulent combustion. Fourth, computed conditioned enstrophies are close to the mean enstrophy in all studied cases, thus, suggesting the use of conditioned enstrophy for characterizing turbulence within a premixed flame brush.

Yu, R.; Lipatnikov, A. N.; Bai, X. S.

2014-08-01

187

ICS 351: Today's plan a DNS client sends a query to any DNS server  

E-print Network

ICS 351: Today's plan DNS WiFi #12;DNS Query a DNS client sends a query to any DNS server usually, every client is configured with the IP address of one or more DNS servers the servers a translation since the DNS name space and the zones are arranged hierarchically, a DNS server always knows

Biagioni, Edoardo S.

188

Assessment of closure schemes in second-order conditional moment closure against DNS with extinction and ignition  

SciTech Connect

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

189

Numerically simulating the sandwich plate system structures  

NASA Astrophysics Data System (ADS)

Sandwich plate systems (SPS) are advanced materials that have begun to receive extensive attention in naval architecture and ocean engineering. At present, according to the rules of classification societies, a mixture of shell and solid elements are required to simulate an SPS. Based on the principle of stiffness decomposition, a new numerical simulation method for shell elements was proposed. In accordance with the principle of stiffness decomposition, the total stiffness can be decomposed into the bending stiffness and shear stiffness. Displacement and stress response related to bending stiffness was calculated with the laminated shell element. Displacement and stress response due to shear was calculated by use of a computational code write by FORTRAN language. Then the total displacement and stress response for the SPS was obtained by adding together these two parts of total displacement and stress. Finally, a rectangular SPS plate and a double-bottom structure were used for a simulation. The results show that the deflection simulated by the elements proposed in the paper is larger than the same simulated by solid elements and the analytical solution according to Hoff theory and approximate to the same simulated by the mixture of shell-solid elements, and the stress simulated by the elements proposed in the paper is approximate to the other simulating methods. So compared with calculations based on a mixture of shell and solid elements, the numerical simulation method given in the paper is more efficient and easier to do.

Feng, Guo-Qing; Li, Gang; Liu, Zhi-Hui; Niu, Huai-Lei; Li, Chen-Feng

2010-09-01

190

DNS-based measurement of the mean impulse response of homogeneous isotropic turbulence  

E-print Network

A technique for measuring the mean impulse response function of stationary homogeneous isotropic turbulence is proposed. Such measurement is carried out here on the basis of Direct Numerical Simulation (DNS). A zero-mean white-noise volume forcing is used to probe the turbulent flow, and the response function is obtained by accumulating the space-time correlation between the white forcing and the velocity field. This technique to measure the turbulent response in a DNS numerical experiment is a new research tool in that field of spectral closures where the linear response concept is invoked either by resorting to renormalized perturbations theories or by introducing the well-known Fluctuation-Dissipation Relation (FDR). Though the results obtained in the present work are limited to relatively low values of the Reynolds number, a preliminary analysis is possible. Both the characteristic form and the time scaling properties of the response function are investigated in the universal subrange of dissipative waven...

Carini, Marco

2010-01-01

191

Distributed DNS Troubleshooting Vasileios Pappas  

E-print Network

Distributed DNS Troubleshooting Vasileios Pappas UCLA Computer Science vpappas@cs.ucla.edu Patrik designed to iden- tify a number of DNS configuration errors. These errors range from commonly seen misconfigurations that are well known among DNS operators, such as lame delegations, to less known ones

California at Los Angeles, University of

192

Syddansk Universitet Kaminsky DNS exploit  

E-print Network

Syddansk Universitet DM829 Kaminsky DNS exploit Jan Christensen - 241189 Anders Knudsen 150885 12. maj 2012 #12;Indhold 1 Indledning 2 2 Introduktion til DNS 2 2.1 Cache . . . . . . . . . . . . . . . . . . . . . . . . . 7 1 #12;2 INTRODUKTION TIL DNS 1 Indledning Denne rapport beskriver det sikkerhedshul som Dan

Boyar, Joan

193

Secure Distributed DNS Christian Cachin  

E-print Network

Secure Distributed DNS Christian Cachin IBM Research Zurich Research Laboratory CH-8803 R A correctly working Domain Name System (DNS) is essen- tial for the Internet. Due to its significance and because of deficiencies in its current design, the DNS is vulnerable to a wide range of attacks

Cachin, Christian

194

Diversity in DNS performance measures  

Microsoft Academic Search

DNS is a critical component of the operation of Internet applications. However, DNS performance in the wide-area is not well understood. A number of studies present DNS performance measurements [1], [2], [3], [4], but the measurements are out of date, are not collected at client locations (e.g., they are taken at root servers), or are collected at very few client

Richard Liston; Sridhar Srinivasan; Ellen W. Zegura

2002-01-01

195

DNS and BIND Security Issues  

Microsoft Academic Search

Abstract Efforts are underway,to add security to the DNS protocol. We have observed that if BIND would just do what the DNS specifications say it should do, stop crashing, and start checking its inputs, then most of the existing security holes in DNS as practiced would go away. To be sure, attackers would still have a pretty easy time co-opting

Paul Vixie

1995-01-01

196

DNS Performance Impact of misconfigurations  

E-print Network

DNS Performance Impact of misconfigurations During the past twenty years the Domain Name System (DNS) has sustained phenomenal growth while maintaining satisfactory performance. However, the original of misconfigurations in DNS today: lame delegation, diminished server redundancy, and cyclic zone dependency. Zones

Amir, Yair

197

Security of Patched DNS  

E-print Network

In spite of the availability of DNSSEC, which protects against cache poisoning even by MitM attackers, many caching DNS resolvers still rely for their security against poisoning on merely validating that DNS responses contain some 'unpredictable' values, copied from the re- quest. These values include the 16 bit identifier field, and other fields, randomised and validated by different 'patches' to DNS. We investigate the prominent patches, and show how attackers can circumvent all of them, namely: - We show how attackers can circumvent source port randomisation, in the (common) case where the resolver connects to the Internet via different NAT devices. - We show how attackers can circumvent IP address randomisation, using some (standard-conforming) resolvers. - We show how attackers can circumvent query randomisation, including both randomisation by prepending a random nonce and case randomisation (0x20 encoding). We present countermeasures preventing our attacks; however, we believe that our attacks provide ...

Herzberg, Amir

2012-01-01

198

Numerical simulation of unsteady viscous flows  

NASA Technical Reports Server (NTRS)

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

Hankey, Wilbur L.

1987-01-01

199

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

200

Sequential Quadratic Programming (SQP) for optimal control in direct numerical simulation of turbulent flow  

NASA Astrophysics Data System (ADS)

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

Badreddine, Hassan; Vandewalle, Stefan; Meyers, Johan

2014-01-01

201

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

202

Two- and three-dimensional Direct Numerical Simulation of particle-laden gravity currents  

NASA Astrophysics Data System (ADS)

In this numerical study, we are interested in the prediction of a mono-disperse dilute suspension particle-laden flow in the typical lock-exchange configuration. The main originality of this work is that the deposition of particles is taken into account for high Reynolds numbers up to 10 000, similar to the experimental ones. Unprecedented two- and three-dimensional Direct Numerical Simulations (DNS) are undertaken with the objective to investigate the main features of the flow such as the temporal evolution of the front location, the sedimentation rate, the resulting streamwise deposit profiles, the wall shear velocity as well as the complete energy budget calculated without any approximations for the first time. It is found that the Reynolds number can influence the development of the current front. Comparisons between the 2D and 3D simulations for various Reynolds numbers allow us to assess which quantities of interest for the geoscientist could be evaluated quickly with a 2D simulation. We find that a 2D simulation is not able to predict accurately the previously enumerated features obtained in a 3D simulation, with maybe the exception of the sedimentation rate for which a qualitative agreement can be found.

Espath, L. F. R.; Pinto, L. C.; Laizet, S.; Silvestrini, J. H.

2014-02-01

203

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

204

Turbulent flame-wall interaction: a DNS study  

SciTech Connect

A turbulent flame-wall interaction (FWI) configuration is studied using three-dimensional direct numerical simulation (DNS) and detailed chemical kinetics. The simulations are used to investigate the effects of the wall turbulent boundary layer (i) on the structure of a hydrogen-air premixed flame, (ii) on its near-wall propagation characteristics and (iii) on the spatial and temporal patterns of the convective wall heat flux. Results show that the local flame thickness and propagation speed vary between the core flow and the boundary layer, resulting in a regime change from flamelet near the channel centreline to a thickened flame at the wall. This finding has strong implications for the modelling of turbulent combustion using Reynolds-averaged Navier-Stokes or large-eddy simulation techniques. Moreover, the DNS results suggest that the near-wall coherent turbulent structures play an important role on the convective wall heat transfer by pushing the hot reactive zone towards the cold solid surface. At the wall, exothermic radical recombination reactions become important, and are responsible for approximately 70% of the overall heat release rate at the wall. Spectral analysis of the convective wall heat flux provides an unambiguous picture of its spatial and temporal patterns, previously unobserved, that is directly related to the spatial and temporal characteristic scalings of the coherent near-wall turbulent structures.

Chen, Jackie [Sandia National Laboratories (SNL); Hawkes, Evatt R [Sandia National Laboratories (SNL); Sankaran, Ramanan [ORNL; Gruber, Andrea [SINTEF Energy Research

2010-01-01

205

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.

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

2014-05-01

206

Numerical simulations on ion acoustic double layers  

Microsoft Academic Search

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

207

Numerical Simulation of Pulse Detonation Engine Phenomena  

Microsoft Academic Search

This paper describes one- and two-dimensional numerical simulations, with simplified as well as full reaction kinetics, of a single cycle pulse detonation engine (PDE). The present studies explore the igni- tion energies associated with the initiation of a det- onation in the PDE tube, and quantify reactive flow phenomena, performance parameters, and noise gen- eration associated with full and simplified

Xing He; Ann R. Karagozian

2003-01-01

208

Numerical Simulation of a Convective Turbulence Encounter  

NASA Technical Reports Server (NTRS)

A numerical simulation of a convective turbulence event is investigated and compared with observational data. The numerical results show severe turbulence of similar scale and intensity to that encountered during the test flight. This turbulence is associated with buoyant plumes that penetrate the upper-level thunderstorm outflow. The simulated radar reflectivity compares well with that obtained from the aircraft's onboard radar. Resolved scales of motion as small as 50 m are needed in order to accurately diagnose aircraft normal load accelerations. Given this requirement, realistic turbulence fields may be created by merging subgrid-scales of turbulence to a convective-cloud simulation. A hazard algorithm for use with model data sets is demonstrated. The algorithm diagnoses the RMS normal loads from second moments of the vertical velocity field and is independent of aircraft motion.

Proctor, Fred H.; Hamilton, David W.; Bowles, Roland L.

2002-01-01

209

Direct numerical simulations of turbulent non-premixed methane-air flames modeled with reduced kinetics  

NASA Technical Reports Server (NTRS)

Turbulent non-premixed stoichiometric methane-air flames modeled with reduced kinetics have been studied using the direct numerical simulation approach. The simulations include realistic chemical kinetics, and the molecular transport is modeled with constant Lewis numbers for individual species. The effect of turbulence on the internal flame structure and extinction characteristics of methane-air flames is evaluated. Consistent with earlier DNS with simple one-step chemistry, the flame is wrinkled and in some regions extinguished by the turbulence, while the turbulence is weakened in the vicinity of the flame due to a combination of dilatation and an increase in kinematic viscosity. Unlike previous results, reignition is observed in the present simulations. Lewis number effects are important in determining the local stoichiometry of the flame. The results presented in this work are preliminary but demonstrate the feasibility of incorporating reduced kinetics for the oxidation of methane with direct numerical simulations of homogeneous turbulence to evaluate the limitations of various levels of reduction in the kinetics and to address the formation of thermal and prompt NO(x).

Card, J. M.; Chen, J. H.; Day, M.; Mahalingam, S.

1994-01-01

210

Direct numerical simulation of turbulent, chemically reacting flows  

NASA Astrophysics Data System (ADS)

This dissertation: (i) develops a novel numerical method for DNS/LES of compressible, turbulent reacting flows, (ii) performs several validation simulations, (iii) studies auto-ignition of a hydrogen vortex ring in air and (iv) studies a hydrogen/air turbulent diffusion flame. The numerical method is spatially non-dissipative, implicit and applicable over a range of Mach numbers. The compressible Navier-Stokes equations are rescaled so that the zero Mach number equations are discretely recovered in the limit of zero Mach number. The dependent variables are co--located in space, and thermodynamic variables are staggered from velocity in time. The algorithm discretely conserves kinetic energy in the incompressible, inviscid, non--reacting limit. The chemical source terms are implicit in time to allow for stiff chemical mechanisms. The algorithm is readily applicable to complex chemical mechanisms. Good results are obtained for validation simulations. The algorithm is used to study auto-ignition in laminar vortex rings. A nine species, nineteen reaction mechanism for H2/air combustion proposed by Mueller et al. [37] is used. Diluted H 2 at ambient temperature (300 K) is injected into hot air. The simulations study the effect of fuel/air ratio, 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 MR (Mastorakos et al. [32]). Subsequent evolution of the flame is not predicted by zetaMR; a most reactive temperature TMR 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 towards 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. We then study auto-ignition of turbulent H2/air diffusion flames using the Mueller et al. [37] mechanism. Isotropic turbulence is superimposed on an unstrained diffusion flame where diluted H 2 at ambient temperature interacts with hot air. Both, unity and non-unity Lewis number are studied. The results are contrasted to the homogeneous mixture problem and laminar diffusion flames. Results show that auto-ignition occurs in fuel lean, low vorticity, high temperature regions with low scalar dissipation around a most reactive mixture fraction, zetaMR (Mastorakos et al. [32]). However, unlike the laminar flame where auto-ignition occurs at zetaMR, the turbulent flame auto-ignites over a very broad range of zeta around zetaMR, which cannot completely predict the onset of ignition. The simulations also study the effects of three-dimensionality. Past two--dimensional simulations (Mastorakos et al. [32]) show that when flame fronts collide, extinction occurs. However, our three dimensional results show that when flame fronts collide; they can either increase in intensity, combine without any appreciable change in intensity or extinguish. This behavior is due to the three--dimensionality of the flow.

Doom, Jeffrey Joseph

211

Numerical Simulation of Aircraft Trailing Vortices  

NASA Technical Reports Server (NTRS)

The increase in air traffic is currently outpacing the development of new airport runways. This is leading to greater air traffic congestion, resulting in costly delays and cancellations. The National Aeronautics and Space Administration (NASA) under its Terminal Area Productivity (TAP) program is investigating new technologies that will allow increased airport capacity while maintaining the present standards for safety. As an element of this program, the Aircraft Vortex Spacing System (AVOSS) is being demonstrated in July 2000, at Dallas Ft-Worth Airport. This system allows reduced aircraft separations, thus increasing the arrival and departure rates, while insuring that wake vortices from a leading aircraft do not endanger trailing aircraft. The system uses predictions or wake vortex position and strength based on input from the current weather state. This prediction is accomplished by a semi-empirical model developed from theory, field observations, and relationships derived from numerical wake vortex simulations. Numerical experiments with a Large Eddy Simulation (LES) model are being conducted in order to provide guidance for the enhancement of these prediction algorithms. The LES Simulations of wake vortices are carried out with NASA's Terminal Area Simulation System (TASS). Previous wake vortex investigations with TASS are described. The primary objective of these numerical studies has been to quantify vortex transport and decay in relation to atmospheric variables. This paper summarizes many of the previous investigations with the TASS model and presents some new results regarding the onset of wake vortex decay.

Proctor, Fred H.; Switzer, George F.

2000-01-01

212

Numerical simulation of an electroweak oscillon  

SciTech Connect

Numerical simulations of the bosonic sector of the SU(2)xU(1) electroweak standard model in 3+1 dimensions have demonstrated the existence of an oscillon--an extremely long-lived, localized, oscillatory solution to the equations of motion--when the Higgs mass is equal to twice the W{sup {+-}} boson mass. It contains total energy roughly 30 TeV localized in a region of radius 0.05 fm. A detailed description of these numerical results is presented.

Graham, N. [Department of Physics, Middlebury College, Middlebury, Vermont 05753 (United States)

2007-10-15

213

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

214

Direct numerical simulations of tonal noise generated by laminar flow past airfoils  

NASA Astrophysics Data System (ADS)

A numerical investigation is presented of noise generated by flow past symmetric NACA airfoils with different thickness and at various angles of attack at M=0.4 and a Reynolds number based on chord of Re=50,000. Direct numerical simulations (DNS) are employed to directly compute both the near-field hydrodynamics and the far-field sound. The DNS data are then used to investigate whether the approach of determining tonal noise radiation based on the surface pressure difference, as done in the classical trailing-edge theory of Amiet, yields satisfactory results for finite thickness airfoils subject to mean loading effects. In addition, the accuracy of Amiet's surface pressure jump function is evaluated. Overall, the modified theory of Amiet appears to be suitable for finite thickness airfoils up to moderate incidence. However, when increasing the airfoil thickness to 12% chord, which corresponds to a trailing-edge angle of 16.8?, an unexpected phase change between the incident and scattered pressure is found at the frequency of the forced instability waves. This phase change is attributed to the flow oscillating around the trailing edge at a separate wake frequency. For the largest incidence investigated, Amiet's response function does not predict the total surface pressure difference as accurately as for zero or small incidence at the vortex shedding frequency, resulting in a poor prediction of the directivity and amplitude of the acoustic pressure. Moreover, predicting the airfoil self-noise based on the surface pressure difference appears not to be generally applicable at higher angles of attack because the radiated sound is only partly due to classical trailing-edge noise mechanisms. In these cases, it appears as if volume sources in the flow cannot be neglected.

Sandberg, R. D.; Jones, L. E.; Sandham, N. D.; Joseph, P. F.

2009-03-01

215

Numerical simulations of iced airfoils and wings  

NASA Astrophysics Data System (ADS)

A numerical study was conducted to understand the effects of simulated ridge and leading-edge ice shapes on the aerodynamic performance of airfoils and wings. In the first part of this study, a range of Reynolds numbers and Mach numbers, as well as ice-shape sizes and ice-shape locations were examined for various airfoils with the Reynolds-Averaged Navier-Stokes approach. Comparisons between simulation results and experimental force data showed favorable comparison up to stall conditions. At and past stall condition, the aerodynamic forces were typically not predicted accurately for large upper-surface ice shapes. A lift-break (pseudo-stall) condition was then defined based on the lift curve slope change. The lift-break angles compared reasonably with experimental stall angles, and indicated that the critical ice-shape location tended to be near the location of minimum pressure and the location of the most adverse pressure gradient. With the aim of improving the predictive ability of the stall behavior for iced airfoils, simulations using the Detached Eddy Simulation (DES) approach were conducted in the second part of this numerical investigation. Three-dimensional DES computations were performed for a series of angles of attack around stall for the iced NACA 23012 and NLF 0414 airfoils. The simulations for both iced airfoils provided the maximum lift coefficients and stall behaviors qualitatively consistent with experiments.

Pan, Jianping

216

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

217

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

218

Direct Numerical Simulations of Transitional/Turbulent Wakes  

NASA Technical Reports Server (NTRS)

The interest in transitional/turbulent wakes spans the spectrum from an intellectual pursuit to understand the complex underlying physics to a critical need in aeronautical engineering and other disciplines to predict component/system performance and reliability. Cylinder wakes have been studied extensively over several decades to gain a better understanding of the basic flow phenomena that are encountered in such flows. Experimental, computational and theoretical means have been employed in this effort. While much has been accomplished there are many important issues that need to be resolved. The physics of the very near wake of the cylinder (less than three diameters downstream) is perhaps the most challenging of them all. This region comprises the two detached shear layers, the recirculation region and wake flow. The interaction amongst these three components is to some extent still a matter of conjecture. Experimental techniques have generated a large percentage of the data that have provided us with the current state of understanding of the subject. More recently computational techniques have been used to simulate cylinder wakes, and the data from such simulations are being used to both refine our understanding of such flows as well as provide new insights. A few large eddy and direct numerical simulations (LES and DNS) of cylinder wakes have appeared in the literature in the recent past. These investigations focus on the low Reynolds number range where the cylinder boundary layer is laminar (sub-critical range). However, from an engineering point of view, there is considerable interest in the situation where the upper and/or lower boundary layer of an airfoil is turbulent, and these turbulent boundary layers separate from the airfoil to contribute to the formation of the wake downstream. In the case of cylinders, this only occurs at relatively large unit Reynolds numbers. However, in the case of airfoils, the boundary layer has the opportunity to transition to turbulence on the airfoil surface at a relatively lower unit Reynolds number because the characteristic length of the airfoil is typically one to two orders of magnitude larger than the trailing edge diameter. This transition to turbulence would occur unless there is a strong favorable pressure gradient that results in the boundary layer remaining laminar or transitional over the surface of the airfoil. This presentation will focus on two direct numerical simulations that have been performed at NASA ARC. The first is of a cylinder wake with laminar separating boundary layers. The second is the wake of a flat plate with a circular trailing edge. The upper and lower plate surface boundary layers are both turbulent and statistically identical. Thus the computed wake is symmetric in a statistical sense. This flow is more representative of airfoil wakes than cylinder wakes. Results from the two simulations including flow visualization and turbulence statistics in the near wake will be presented at the seminar.

Rai, Man Mohan

2011-01-01

219

Numerical simulation of a turbulent magnetic dynamo.  

PubMed

We present numerical simulations of a turbulent magnetic dynamo mimicking closely the Riga-dynamo experiment at Re approximately 3.5x10(6) and 15< or =Rem< or =20. The Reynolds-averaged Navier-Stokes equations for the fluid flow and turbulence field are solved simultaneously with the direct numerical solution of the magnetic field equations. The fully integrated two-way-coupled simulations reproduced all features of the magnetic self-excitation detected by the Riga experiment, with frequencies and amplitudes of the self-generated magnetic field in good agreement with the experimental records, and provided full insight into the unsteady magnetic and velocity fields and the mechanisms of the dynamo action. PMID:17358541

Kenjeres, S; Hanjali?, K

2007-03-01

220

Numerical simulation of solidification processes in enclosures  

NASA Astrophysics Data System (ADS)

The present work deals with the development and application of numerical models for the simulation of solidification problems liquid/solid taking diffusion and convection into account. For the calculation of the thermal coupled flow process the finite element method is applied. In order to improve the numerical stability of the free convection problems, the streamline-upwind/Petrov-Galerkin method is used. Solidification processes are moving boundary problems. Three different models are set up which consider latent heat at the solidification front respectively in the mixed zone during the phase transition. Moreover, numerical methods are investigated in order to describe the behaviour of the flow at the boundary of the moving phase. Three examples serve illustrations; the technical example - casting of a transport and storage container - was provided by the company Siempelkamp Gieerei GmbH.

Szimmat, J.

221

Numerical simulations on ion acoustic double layers  

SciTech Connect

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

222

Numerical simulation of magma energy extraction  

SciTech Connect

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

Hickox, C.E.

1991-01-01

223

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

224

Direct numerical simulation of a recorder.  

PubMed

The aeroacoustics of a recorder are studied using a direct numerical simulation based on the Navier-Stokes equations in two dimensions. Spatial maps for the air pressure and velocity give a detailed picture of vortex shedding near the labium. Changes in the spectrum as a result of variations in the blowing speed are also investigated. The results are in good semi-quantitative agreement with general results for these phenomena from experiments. PMID:23363126

Giordano, N

2013-02-01

225

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.

226

Numerical simulation and nasal air-conditioning  

PubMed Central

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

Keck, Tilman; Lindemann, Jrg

2011-01-01

227

Numerical Simulations of Hyperfine Transitions of Antihydrogen  

E-print Network

One of the ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) collaboration's goals is the measurement of the ground state hyperfine transition frequency in antihydrogen, the antimatter counterpart of one of the best known systems in physics. This high precision experiment yields a sensitive test of the fundamental symmetry of CPT. Numerical simulations of hyperfine transitions of antihydrogen atoms have been performed providing information on the required antihydrogen events and the achievable precision.

Kolbinger, B; Diermaier, M; Lehner, S; Malbrunot, C; Massiczek, O; Sauerzopf, C; Simon, M C; Widmann, E

2015-01-01

228

A numerical sensitivity analysis of streamline simulation  

E-print Network

Studies of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Approved as to style and content by: _______________________________ Akhil Datta-Gupta (Chair of Committee... Numerical Sensitivity Analysis of Streamline Simulation. (December 2004) Fady Ruben Chaban Habib, B.S., Universidad Bicentenaria de Aragua, Venezuela Chair of Advisory Committee: Dr. Akhil Datta-Gupta Nowadays, field development strategy has become...

Chaban Habib, Fady Ruben

2005-02-17

229

RFCCD Microthruster Performance via Numerical Simulation  

Microsoft Academic Search

Particle-in-cell\\/Monte Carlo (PIC\\/MCC) and Direct Simulation Monte Carlo (DSMC) algorithms comprise a numerical model to assess the propulsive capability of a RF plasma microthruster concept. This thruster concept is an electrothermal device and exploits RF capacitively coupled discharge (RFCCD) to heat a propellant. This RF plasma mi- crothruster has potential to alleviate some severe constraints on microsatellite propulsion systems such

William B. Stein; Alina A. Alexeenko; Ivana Hrbud

2008-01-01

230

Numerical relativity simulations of binary neutron stars  

Microsoft Academic Search

We present a new numerical relativity code designed for simulations of compact binaries involving matter. The code is an upgrade of the BAM code to include general relativistic hydrodynamics and implements state-of-the-art high-resolution-shock-capturing schemes on a hierarchy of mesh refined Cartesian grids with moving boxes. We test and validate the code in a series of standard experiments involving single neutron

Marcus Thierfelder; Sebastiano Bernuzzi; Bernd Brgmann

2011-01-01

231

Numerical simulation of non-circular jets  

Microsoft Academic Search

Results are presented of numerical simulations of spatially developing, three-dimensional jets issued from circular and non-circular nozzles of identical equivalent diameters. Elliptic, rectangular and triangular jets are considered with aspect-ratios of 1:1 and 2:1. Flow visualization results show that large scale coherent structures are formed in both cornered and non-cornered jets. The axis-switching phenomenon is captured in all non-unity aspect-ratio

R. S. Miller; C. K. Madnia; P. Givi

1995-01-01

232

Numerical simulation of primary and secondary atomization  

NASA Astrophysics Data System (ADS)

The physics of the atomization process involves many complex phenomena, which occur at different scales of space and time. The numerical study of such a problem is a great challenge for different reasons. Large density ratios, presence of a singular surface tension force, interface localization and transport, mass conservation, all of these make accurate numerical simulation difficult to perform. Several strategies have been investigated at ONERA in order to find an optimal method to simulate the atomization process with the CEDRE code. Both interface capturing and diffuse interface algorithms have been tested. The present development consists in the implementation of a multi-fluid version of the current gas solver of CEDRE, which conserves all its original features. One of the principal axes of research is based on a method which couples the new multi-fluid method with one of the dispersed phase solvers of CEDRE. The long term purpose is to be able to perform numerical simulation of both primary and secondary atomization.

Zuzio, Davide; Estivalezes, Jean-Luc; Villedieu, Philippe; Blanchard, Ghislain

2013-01-01

233

Direct numerical simulation of turbulent mixing in grid-generated turbulence  

NASA Astrophysics Data System (ADS)

Turbulent mixing of passive scalar (heat) in grid-generated turbulence (GGT) is simulated by means of direct numerical simulation (DNS). A turbulence-generating grid, on which the velocity components are set to zero, is located downstream of the channel entrance, and it is numerically constructed on the staggered mesh arrangement using the immersed boundary method. The grid types constructed are: (a) square-mesh biplane grid, (b) square-mesh single-plane grid, (c) composite grid consisting of parallel square-bars and (d) fractal grid. Two fluids with different temperatures are provided separately in the upper and lower streams upstream of the turbulence-generating grids, generating the thermal mixing layer behind the grids. For the grid (a), simulations for two different Prandtl numbers of 0.71 and 7.1, corresponding to air and water flows, are conducted to investigate the effect of the Prandtl number. The results show that the typical grid turbulence and shearless mixing layer are generated downstream of the grids. The results of the scalar field show that a typical thermal mixing layer is generated as well, and the effects of the Prandtl numbers on turbulent heat transfer are observed.

Nagata, Kouji; Suzuki, Hiroki; Sakai, Yasuhiko; Hayase, Toshiyuki; Kubo, Takashi

2008-12-01

234

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

NASA Astrophysics Data System (ADS)

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

Haapanen, Siina Ilona

235

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

236

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

237

Numerical simulation of complex turbomachinery flows  

NASA Astrophysics Data System (ADS)

An unsteady viscous flow solver based on the Runge-Kutta scheme has been developed. Pseudo-time step technique has been incorporated to provide efficient simulation of unsteady flows. Utilization of the pseudo-time approach reduces the computational time by a factor varying from 5 to 25 times in comparison with the original solver. The results of the stability analysis of the dual time step scheme are used to establish an optimum pseudo-time step based on the local CFL, VonNeuman numbers and the ratio of pseudo-to-physical time steps. The code has been validated against the analytical and experimental data. The influence of the numerical aspects (artificial dissipation, grid density etc.) on the accuracy of the prediction of the wake decay, transition, flow over a cylinder has been analyzed. Based on the results of the test cases, modifications to the k-epsilon model to improve the accuracy in the regions with dominant normal stresses are incorporated. Developed solver has been used for the numerical simulation of the complex steady and unsteady turbomachinery flows, including unsteady transitional flows in compressor and turbine, leading edge film cooling and secondary flow simulations. Three low Re k-epsilon turbulence models have been assessed for their ability to predict the unsteady transitional flows. Good agreement with the measured data has been achieved. The numerical solver was able to predict major features, associated with the wake-induced transition on blade (wake induced transitional strip, wake induced turbulent strip, etc.). Analysis and interpretation of the results from the flow simulation have been carried out to understand additional flow physics associated with the transitional, film cooling and secondary flows.

Chernobrovkin, Andrey A.

1999-11-01

238

Numerical recipes for mold filling simulation  

SciTech Connect

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

239

DNS of Unsteady Heat Transfer Increase on aCurved Surface Due to Wake-Induced Turbulence  

Microsoft Academic Search

\\u000a From experimental studies it is well established that free-stream and wake-induced turbulence can increase the heat transfer\\u000a on curved surfaces such as turbine blades or cylinders even if the mean flow matches corresponding laminar velocity profiles.\\u000a Here, preliminary results of an investigation using (embedded) Direct Numerical Simulation (DNS) are reported that aim at\\u000a identifying the responsible physical mechanisms for this

D. von Terzi; L. Venema; H.-J. Bauer; W. Rodi

240

Development and parallelization of a direct numerical simulation to study the formation and transport of nanoparticle clusters in a viscous fluid  

NASA Astrophysics Data System (ADS)

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

Sloan, Gregory James

241

Direct Numerical Simulation of Spatial Transition to Turbulence Using Fourth-Order Vertical Velocity Second-Order Vertical Vorticity Formulation  

NASA Astrophysics Data System (ADS)

A highly accurate algorithm has been developed to study the process of spatial transition to turbulence. The algorithmic details of the direct numerical simulation (DNS) of transition to turbulence in a boundary layer based on a formulation in terms of vertical velocity and vertical vorticity are presented. Issues concerning the boundary conditions are discussed. The linear viscous terms are discretized using an implicit Crank-Nicholson scheme, and a low-storage Runge-Kutta method is used for the nonlinear terms. For the spatial discretization, fourth-order compact finite differences have been used, as these have been found to have better resolution compared to explicit differencing schemes of comparable order. The number of grid points that are needed per wavelength is close to the theoretical optimum for any numerical scheme. The resulting time-discretized fourth-order equations are split up into two second-order equations, resulting in Helmholtz- and Poisson-type equations. The boundary conditions for the Laplacian of the vertical velocity are determined using an influence matrix method. A robust multigrid algorithm has been developed to solve the resulting anisotropic elliptical equations. For the outflow boundary, a buffer domain method, which smoothly reduces the disturbances to zero, in conjunction with parabolization of the Navier-Stokes equations has been used. The validation of the results for the DNS solver is made both for linear and weakly nonlinear cases.

Bhaganagar, Kiran; Rempfer, Dietmar; Lumley, John

2002-07-01

242

Spatial characterization of the numerically simulated vorticity fields of a flow in a flume  

Microsoft Academic Search

The topology of large scale structures in a turbulent boundary layer is investigated numerically. Spatial characteristics of the large scale structure are presented through an original method, proper orthogonal decomposition (POD) of the three-dimensional vorticity fields. The DNS results, obtained by Tiselj et al. [23] for a fully developed turbulent flow in a flume, are used in the present work

Alex Liberzon; Roi Gurka; Iztok Tiselj; Gad Hetsroni

2005-01-01

243

Untangling the Web from DNS  

Microsoft Academic Search

The Web relies on the Domain Name System (DNS) to resolve the hostname portion of URLs into IP addresses. This marriage-of-convenience enabled the Web's mete- oric rise, but the resulting entanglement is now hinder- ing both infrastructuresthe Web is overly constrained by the limitations of DNS, and DNS is unduly burdened by the demands of the Web. There has been

Michael Walfish; Hari Balakrishnan; Scott J. Shenker

2004-01-01

244

Numerical reproducibility for implicit Monte Carlo simulations  

SciTech Connect

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

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

2013-07-01

245

Characteristics of a numerical fluid dynamics simulator  

SciTech Connect

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 characteristics adopting the following as guiding principles: (a) that the performance characteristics (information output, throughput and storage) of the NFDS be defined by the maximum rate at which the researcher can absorb the results of his/her simulation, thus establishing an impedance match between man and machine, (b) that the NFDS off-load from the researcher's brain in every way possible the routine tasks of data analysis that can be done automatically, and (c) that the NFDS is operated as a dedicated experimental device, much as a wind tunnel, and that the researcher has complete control over the apparatus and experiment. We envision the NFDS to be composed of simulation processors, data storage devices, and image processing devices of extremely high power and capacity, interconnected by very high throughput communication channels. We present individual component performance requirements for both real-time and playback operating modes of the NFDS, using problems of current interest in fluid dynamics as examples.

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

1985-01-01

246

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

247

Direct numerical simulations of capillary wave turbulence.  

PubMed

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

248

NEW NUMERICAL TECHNOLOGIES FOR THE SIMULATION OF ARC WELDING PROCESSES  

E-print Network

NEW NUMERICAL TECHNOLOGIES FOR THE SIMULATION OF ARC WELDING PROCESSES Michel Bellet 1 , Makhlouf Antipolis, France; soudage@transvalor.com Keywords: welding, finite elements, material deposit, adaptive for arc welding simulation and analysis. The new numerical technologies essentially consist first

Paris-Sud XI, Université de

249

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

250

Scalability study of parallel spatial direct numerical simulation code on IBM SP1 parallel supercomputer  

NASA Technical Reports Server (NTRS)

The implementation and the performance of a parallel spatial direct numerical simulation (PSDNS) code are reported for the IBM SP1 supercomputer. The spatially evolving disturbances that are associated with laminar-to-turbulent in three-dimensional boundary-layer flows are computed with the PS-DNS code. By remapping the distributed data structure during the course of the calculation, optimized serial library routines can be utilized that substantially increase the computational performance. Although the remapping incurs a high communication penalty, the parallel efficiency of the code remains above 40% for all performed calculations. By using appropriate compile options and optimized library routines, the serial code achieves 52-56 Mflops on a single node of the SP1 (45% of theoretical peak performance). The actual performance of the PSDNS code on the SP1 is evaluated with a 'real world' simulation that consists of 1.7 million grid points. One time step of this simulation is calculated on eight nodes of the SP1 in the same time as required by a Cray Y/MP for the same simulation. The scalability information provides estimated computational costs that match the actual costs relative to changes in the number of grid points.

Hanebutte, Ulf R.; Joslin, Ronald D.; Zubair, Mohammad

1994-01-01

251

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

252

Numerical Simulation of Drilling with Pulsed Beams  

NASA Astrophysics Data System (ADS)

Electron beam drilling is an efficient method for microperforation. The technique using an electron beam for melting up and blowing out the material works in vacuum. Ablation is mainly based on the expulsion of the molten material by two competing mechanisms: evaporation of the workpiece as well as of the backing material. In this contribution a numerical finite volume OpenFOAM simulation model for beam-matter interaction in electron beam drilling is presented showing the transient dynamics of the process as well as the influence of various processing parameters.

Leitz, Karl-Heinz; Koch, Holger; Otto, Andreas; Maaz, Alexander; Lwer, Thorsten; Schmidt, Michael

253

Direct numerical simulation of turbulent mixing.  

PubMed

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

254

Numerical Simulations Unravel the Cosmic Web  

E-print Network

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 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.

C. -A. Faucher-Giguere; A. Lidz; L. Hernquist

2008-03-03

255

Numerical Simulation of Axisymmetric Free Surface Flows  

NASA Astrophysics Data System (ADS)

This paper describes an extension of the GENSMAC code for solving two-dimen-sional free surface flows to axisymmetric flows. Like GENSMAC the technique is finite difference based and embodies, but considerably extends, the SMAC (simplified marker and cell) ideas. It incorporates adaptive time stepping and an accurate representation of the free surfaces while at the same time only uses surface particles to define the free surfaces, greatly increasing the computational speed; in addition, it employs a graphic interface with solid modeling techniques to provide enhanced three-dimensional visualization. Various simulations are undertaken to illustrate and validate typical flows. Both G. I. Taylor's viscous jet plunging into a fluid and a liquid drop splashing onto a fluid are simulated. Also, the important industrial application of container filling is illustrated. Finally, a comparison is made with the linear theory of standing waves and the code is validated by a numerical convergence study.

Tome, M. F.; Castelo, A.; Murakami, J.; Cuminato, J. A.; Minghim, R.; Oliveira, M. C. F.; Mangiavacchi, N.; McKee, S.

2000-01-01

256

Stereo image visualization of numerically simulated turbulence  

NASA Technical Reports Server (NTRS)

Stereo imaging techniques are used in the current study to analyze the results of a direct Navier-Stokes simulation of a turbulent boundary layer. Coherent structures embedded within the unsteady, highly three-dimensional turbulent flow fields are significantly enhanced through the use of stereo computer graphic renderings. In the paper, the geometrical foundations of two different methods for generating stereo images are reviewed. Sufficient detail is provided to enable readers to code stereo display algorithms from scratch. The effects of varying the geometric parameters of the stereo algorithm are demonstrated. Sample image pairs from animations of the turbulence numerical simulation are presented in color. Stereo imagery, especially when used with time-evolving data, provides new insight into the physical processes responsible for the generation an maintenance of turbulence within boundary layers.

Robinson, Stephen K.; Hu, Kenneth C.

1989-01-01

257

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

258

Dns Extensions To Network Address Translators (dns_alg)  

Microsoft Academic Search

Abstract: Domain Name Service (DNS) provides name to address mapping within arouting class (ex: IP). Network Address Translators (NATs) attempt toprovide transparent routing between hosts in disparate address realmsof the same routing class. Typically, NATs exist at the border of astub domain, hiding private addresses from external addresses. Thisdocument identifies the need for DNS extensions to NATs and outlineshow a

P Srisuresh; G Tsirtsis; P Akkiraju; A Heffernan

1999-01-01

259

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

260

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

261

High resolution DNS studies of long-time behavior of homogeneous turbulent shear flow  

NASA Astrophysics Data System (ADS)

As discussed in Isaza & Collins [J. Fluid Mech. 678:14--40, 2011], the shear parameter S^* has a pronounced effect on velocity gradient statistics for homogeneous turbulent shear flow (HTSF). Due to the importance of this effect, especially for higher S^*, we extended those studies to higher resolution using a new direct numerical simulation (DNS) code based on a pseudospectral algorithm that avoids remeshing [Brucker et al., J. Comp. Phys. 225:20--32, 2007], and decomposes the domain into ``pencils''. We present DNS with 2048x1024x1024 grid points, achieving a maximum Taylor microscale Reynolds number of 300. The peak in the initial energy spectrum, viscosity, and box configuration also have been optimized to maximize the time window for well-resolved simulations (up to S t=20), ensuring we are well into the asymptotic regime. The DNS runs confirm the sensitivity of the large- and small-scale statistics to S^*, as was found by Isaza & Collins. We also investigated the interaction between the fluctuating vorticity vector and rate-of-strain tensor as a function of scale, and find alignments vary dramatically, suggesting the primary source of enstrophy is at large scales, followed by a forward cascade to small scales. This helps explain the persistent sensitivity of the velocity gradient statistics to S^*. The combination of results suggests a new framework for modeling HTSF at high values of S^*.

Sukheswalla, Parvez; Vaithianathan, T.; Collins, Lance R.

2011-11-01

262

Numerical simulation of glass fogging and defogging  

NASA Astrophysics Data System (ADS)

A numerical procedure for the prediction of fogging and defogging phenomena is presented. The simulation involves the solution of an air flow field along a cold solid surface, the evaluation of the unsteady conduction through the solid itself, and a model for the heat and mass transfer within the thin water layer on the fogged surface. A suite of routines for the unsteady simulation of the water layer evolution is coupled with an equal order finite element Navier Stokes solver and a finite volume conduction code. The procedure is fully independent of the numerical details of the solid and fluid domain solvers. Two different coupling approaches may be followed: A loose one, where the Navier Stokes solution is used only for a steady state estimate of the heat transfer coefficient, or a close one, where the Navier Stokes, conduction and water layer codes are iterated simultaneously. The latter is required for the problem of natural convection, where temperature (and thus the energy balance of the water layer) and flow field are coupled. The water layer is modelled as a collection of closely packed tiny droplets, leaving a portion of dry area among them. The effect of the contact angle is taken into account, and physical assumptions allow to define the local ratio between wet and dry surface for both the fogging and defogging process. As a case study, a comparison with experimental data for a complete fogging and defogging cycle of a glass lens in natural convection is presented.

Croce, Giulio; D'Agaro, Paola; Della Mora, Francesca

2005-08-01

263

Numerical simulations of rotating axisymmetric sunspots  

E-print Network

A numerical model of axisymmetric convection in the presence of a vertical magnetic flux bundle and rotation about the axis is presented. The model contains a compressible plasma described by the nonlinear MHD equations, with density and temperature gradients simulating the upper layer of the sun's convection zone. The solutions exhibit a central magnetic flux tube in a cylindrical numerical domain, with convection cells forming collar flows around the tube. When the numerical domain is rotated with a constant angular velocity, the plasma forms a Rankine vortex, with the plasma rotating as a rigid body where the magnetic field is strong, as in the flux tube, while experiencing sheared azimuthal flow in the surrounding convection cells, forming a free vortex. As a result, the azimuthal velocity component has its maximum value close to the outer edge of the flux tube. The azimuthal flow inside the magnetic flux tube and the vortex flow are prograde relative to the rotating cylindrical reference frame. A retrograde flow appears at the outer wall. The most significant convection cell outside the flux tube is the location for the maximum value of the azimuthal magnetic field component. The azimuthal flow and magnetic structure are not generated spontaneously, but decay exponentially in the absence of any imposed rotation of the cylindrical domain.

G. J. J. Botha; F. H. Busse; N. E. Hurlburt; A. M. Rucklidge

2008-04-28

264

Elimination of numerical oscillations in power system dynamic simulation  

Microsoft Academic Search

This paper addresses numerical oscillations encountered in power system dynamic simulation resulting from trapezoidal numerical integration rule. Two methods are presented to eliminate the numerical oscillations: trapezoidal with numerical stabilizer method and Gear's second order method. A detailed comparison is given regarding the accuracy of the trapezoidal rule, trapezoidal with numerical stabilizer method, and Gear's method. The validity of the

Wenzhong Gao; Eugene Solodovnik; Roger Dougal; George Cokkinides; A. P. Sakis Meliopoulos

2003-01-01

265

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

266

Numerical simulation of Reynolds number effects on velocity shear flow around a circular cylinder  

NASA Astrophysics Data System (ADS)

Three-dimensional Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) are performed to investigate the shear effects on flow around a circular cylinder at Reynolds numbers of Re=60-1000. The shear parameter, ?, which is based on the velocity gradient, cylinder diameter and upstream mean velocity at the center plane of the cylinder, varies from 0 to 0.30. Variations of Strouhal number, drag and lift coefficients, and unsteady wake structures with shear parameter are studied, along with their dependence on Reynolds number. The presented simulation provides detailed information for the flow field around a circular cylinder in shear flow. This study shows that the Strouhal number exhibits no significant variation with shear parameter. The stagnation point moves to the high-velocity side almost linearly with shear parameter, and this result mainly influences the aerodynamic forces acting on a circular cylinder in shear flow. Both the Reynolds number and shear parameter influence the movement of the stagnation point and separation point. Mode A wake instability is suppressed into parallel vortex shedding mode at a certain shear parameter. The lift force increases with increasing shear parameter and acts from the high-velocity side to the low-velocity side. In addition, a simple method to estimate the lift force coefficient in shear flow is provided.

Cao, Shuyang; Ozono, Shigehira; Tamura, Yukio; Ge, Yaojun; Kikugawa, Hironori

2010-07-01

267

Numerical simulation of premixed turbulent methane combustion  

SciTech Connect

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

268

A numerical study of the turbulent Ekman layer  

NASA Technical Reports Server (NTRS)

The three-dimensional time-dependent turbulent flow in a neutrally stratified Ekman layer over a smooth flat surface was numerically simulated by directly solving the Navier-Stokes equations. Issues addressed using the direct numerical simulation (DNS) fields include the presence or absence of large-scale coherent structures ('longitudinal' or 'roll' vortices) in neutrally stratified Ekman-layer turbulence, the effects of the horizontal component of the angular velocity vector (i.e., latitude), and implications for models of the PBL. Experimental and DNS profiles are compared.

Coleman, G. N.; Ferziger, J. H.; Spalart, P. R.

1990-01-01

269

Numerical Propulsion System Simulation: An Overview  

NASA Technical Reports Server (NTRS)

The cost of implementing new technology in aerospace propulsion systems is becoming prohibitively expensive and time consuming. One of the main contributors to the high cost and lengthy time is the need to perform many large-scale hardware tests and the inability to integrate all appropriate subsystems early in the design process. The NASA Glenn Research Center is developing the technologies required to enable simulations of full aerospace propulsion systems in sufficient detail to resolve critical design issues early in the design process before hardware is built. This concept, called the Numerical Propulsion System Simulation (NPSS), is focused on the integration of multiple disciplines such as aerodynamics, structures and heat transfer with computing and communication technologies to capture complex physical processes in a timely and cost-effective manner. The vision for NPSS, as illustrated, is to be a "numerical test cell" that enables full engine simulation overnight on cost-effective computing platforms. There are several key elements within NPSS that are required to achieve this capability: 1) clear data interfaces through the development and/or use of data exchange standards, 2) modular and flexible program construction through the use of object-oriented programming, 3) integrated multiple fidelity analysis (zooming) techniques that capture the appropriate physics at the appropriate fidelity for the engine systems, 4) multidisciplinary coupling techniques and finally 5) high performance parallel and distributed computing. The current state of development in these five area focuses on air breathing gas turbine engines and is reported in this paper. However, many of the technologies are generic and can be readily applied to rocket based systems and combined cycles currently being considered for low-cost access-to-space applications. Recent accomplishments include: (1) the development of an industry-standard engine cycle analysis program and plug 'n play architecture, called NPSS Version 1, (2) A full engine simulation that combines a 3D low-pressure subsystem with a 0D high pressure core simulation. This demonstrates the ability to integrate analyses at different levels of detail and to aerodynamically couple components, the fan/booster and low-pressure turbine, through a 3D computational fluid dynamics simulation. (3) Simulation of all of the turbomachinery in a modern turbofan engine on parallel computing platform for rapid and cost-effective execution. This capability can also be used to generate full compressor map, requiring both design and off-design simulation. (4) Three levels of coupling characterize the multidisciplinary analysis under NPSS: loosely coupled, process coupled and tightly coupled. The loosely coupled and process coupled approaches require a common geometry definition to link CAD to analysis tools. The tightly coupled approach is currently validating the use of arbitrary Lagrangian/Eulerian formulation for rotating turbomachinery. The validation includes both centrifugal and axial compression systems. The results of the validation will be reported in the paper. (5) The demonstration of significant computing cost/performance reduction for turbine engine applications using PC clusters. The NPSS Project is supported under the NASA High Performance Computing and Communications Program.

Lytle, John K.

2000-01-01

270

Direct Numerical Simulation of Drag Reduction in Turbulent Pipe Flow with Spanwise Wall Oscillation Using a Spectral Element Method  

NASA Astrophysics Data System (ADS)

Results of a direct numerical simulation of turbulent pipe flow with spanwise wall oscillation, using NEK5000, a spectral element Navier-Stokes solver, are presented. The polar-cylindrical coordinate singularity at the pipe axis is avoided by solving the flow in Cartesian coordinates with a stadium-like element cross-section. Near the center of the pipe, a Cartesian configuration is used, while near the wall, the elements are mapped to a polar configuration. Each element uses 10th order Legendre Lagrangian interpolants in each direction, with a local Jacobi/Conjugate Gradient solver and a global Schwarz Multigrid solver. Validation with previous DNS and experiments is performed for Re?=180 using 960 elements and a length of 10 R, and the drag reduction studies are performed at Re?=150 using 2560 elements and a length of 20 R. Comparisons will also be made with previous DNS and drag reduction studies. Results showing better correlation with experiments using a spectral method compared to a 2nd order finite difference radial discretizations will be presented.

Duggleby, Andrew

2005-11-01

271

Numerical simulations of icing in turbomachinery  

NASA Astrophysics Data System (ADS)

Safety concerns over aircraft icing and the high experimental cost of testing have spurred global interest in numerical simulations of the ice accretion process. Extensive experimental and computational studies have been carried out to understand the icing on external surfaces. No parallel initiatives were reported for icing on engine components. However, the supercooled water droplets in moist atmosphere that are ingested into the engine can impinge on the component surfaces and freeze to form ice deposits. Ice accretion could block the engine passage causing reduced airflow. It raises safety and performance concerns such as mechanical damage from ice shedding as well as slow acceleration leading to compressor stall. The current research aims at developing a computational methodology for prediction of icing phenomena on turbofan compression system. Numerical simulation of ice accretion in aircraft engines is highly challenging because of the complex 3-D unsteady turbomachinery flow and the effects of rotation on droplet trajectories. The aim of the present research focuses on (i) Developing a computational methodology for ice accretion in rotating turbomachinery components; (ii) Investigate the effect of inter-phase heat exchange; (iii) Characterize droplet impingement pattern and ice accretion at different operating conditions. The simulations of droplet trajectories are based on a Eulerian-Lagrangian approach for the continuous and discrete phases. The governing equations are solved in the rotating blade frame of reference. The flow field is computed by solving the 3-D solution of the compressible Reynolds Averaged Navier Stokes (RANS) equations. One-way interaction models simulate the effects of aerodynamic forces and the energy exchange between the flow and the droplets. The methodology is implemented in the cool, TURBODROP and applied to the flow field and droplet trajectories in NASA Roto-67r and NASA-GE E3 booster rotor. The results highlight the variation of impingement location and temperature with droplet size. It also illustrates the effect of rotor speed on droplet temperature rise. The computed droplet impingement statistics and flow properties are used to calculate ice shapes. It was found that the mass of accreted ice and maximum thickness is highly sensitive to rotor speed and radial location.

Das, Kaushik

272

T-DNS: Connection-Oriented DNS to Improve Privacy and Security  

E-print Network

T-DNS: Connection-Oriented DNS to Improve Privacy and Security USC/ISI Technical Report ISI-TR-688: University of Southern California 2: Verisign ABSTRACT This paper explores connection-oriented DNS to improve DNS security and privacy. DNS is the canonical example of a connectionless, single packet, request

Heidemann, John

273

Assignment 4 -DNS and Mail In this assignment you will configure DNS and Sendmail.  

E-print Network

Assignment 4 - DNS and Mail In this assignment you will configure DNS and Sendmail. To complete are finished with the exercise. 2. Configure DNS To run DNS you will be running BIND. Make sure named.lab.cse.ucsc.edu. The master DNS server should be the machine you where assigned. Name your computer with the same name it has

Obraczka, Katia

274

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

275

History of the numerical aerodynamic simulation program  

NASA Technical Reports Server (NTRS)

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

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

1987-01-01

276

Computing abstraction hierarchies by numerical simulation  

SciTech Connect

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

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

1996-12-31

277

Jurgen Geiser Numerical Simulations of Sublimation  

E-print Network

apparatus SiC growth by physical vapor transport (PVT) SiC-seed-crystal , Gas : 2000 ­ 3000 K , Si A S ¡ ¢ ¡ £ ¤¥ £ ¦ ¤ ¤§¨ ¤¥ ¢¥© ¥¡ £ ¢ ¡ ¨ ¦ ¤ ¢ ¤ ¦ ¨ ¥ ¦ ¤¥ ¢ ¤ ¢ ¨ ! £ ¤ © ¨ " ¤#¡ £ $ © ¨ % ¡ £ ¢ ¨ ¡ & ' & J¨urgen Geiser Numerical Simulations of Sublimation Growth for SiC single Crystal : Anisotropy of Sublimation Growth for SiC single Crystal: Anisotropy Materials, Radiation and Transient Heat Transfer. · Task

Geiser, Juergen

278

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 m 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 m) 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.

Jnsk, Jaroslav; Gaychet, Sylvain; Bessires, Delphine; Soulem, Nicolas; Paillol, Jean; Lemont, Florent

2014-02-01

279

Dark Matter Subhaloes in Numerical Simulations  

E-print Network

We use cosmological LCDM numerical simulations to model the evolution of the substructure population in sixteen dark matter haloes with resolutions of up to seven million particles within the virial radius. The combined substructure circular velocity distribution function (VDF) for hosts of 10^11 to 10^14 Msun at redshifts from zero to two or higher has a self-similar shape, is independent of host halo mass and redshift, and follows the relation: dn/dv=(1/8)(v_cmax/v_cmax,host)^-4. Halo to halo variance in the VDF is a factor of roughly two to four. At high redshifts, we find preliminary evidence for fewer large substructure haloes (subhaloes). Specific angular momenta are significantly lower for subhaloes nearer the host halo centre where tidal stripping is more effective. The radial distribution of subhaloes is marginally consistent with the mass profile for r >~ 0.3r_vir, where the possibility of artificial numerical disruption of subhaloes can be most reliably excluded by our convergence study, although a subhalo distribution that is shallower than the mass profile is favoured. Subhalo masses but not circular velocities decrease toward the host centre. Subhalo velocity dispersions hint at a positive velocity bias at small radii. There is a weak bias toward more circular orbits at lower redshift, especially at small radii. We additionally model a cluster in several power law cosmologies of P ~ k^n, and demonstrate that a steeper spectral index, n, results in significantly less substructure.

Darren Reed; Fabio Governato; Thomas Quinn; Jeffrey Gardner; Joachim Stadel; George Lake

2004-06-02

280

Direct Numerical Simulation of Automobile Cavity Tones  

NASA Technical Reports Server (NTRS)

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

Kurbatskii, Konstantin; Tam, Christopher K. W.

2000-01-01

281

Numerical relativity simulations of binary neutron stars  

NASA Astrophysics Data System (ADS)

We present a new numerical relativity code designed for simulations of compact binaries involving matter. The code is an upgrade of the BAM code to include general relativistic hydrodynamics and implements state-of-the-art high-resolution-shock-capturing schemes on a hierarchy of mesh refined Cartesian grids with moving boxes. We test and validate the code in a series of standard experiments involving single neutron star spacetimes. We present test evolutions of quasiequilibrium equal-mass irrotational binary neutron star configurations in quasicircular orbits which describe the late inspiral to merger phases. Neutron star matter is modeled as a zero-temperature fluid; thermal effects can be included by means of a simple ideal gas prescription. We analyze the impact that the use of different values of damping parameter in the Gamma-driver shift condition has on the dynamics of the system. The use of different reconstruction schemes and their impact in the post-merger dynamics is investigated. We compute and characterize the gravitational radiation emitted by the system. Self-convergence of the waves is tested, and we consistently estimate error bars on the numerically generated waveforms in the inspiral phase.

Thierfelder, Marcus; Bernuzzi, Sebastiano; Brgmann, Bernd

2011-08-01

282

Implications of Netalyzr's DNS Measurements Nicholas Weaver  

E-print Network

Implications of Netalyzr's DNS Measurements Nicholas Weaver ICSI Christian Kreibich ICSI Boris IP addresses. One of the primary focus areas of Netalyzr is DNS behavior, including DNS resolver. Additional tests detect and categorize the behavior of any DNS proxies in the users' gateways or firewalls

Paxson, Vern

283

Characterizing Dark DNS Behavior Jon Oberheide1  

E-print Network

Characterizing Dark DNS Behavior Jon Oberheide1 , Manish Karir2 , and Z. Morley Mao1 1 Electrical such deployments has been thoroughly scrutinized, little attention has been paid to DNS queries targeting these addresses. In this paper, we introduce the concept of dark DNS, the DNS queries associated with darknet

Mao, Zhuoqing Morley

284

Quantifying DNS Namespace Influence6 Casey Deccioa,  

E-print Network

Quantifying DNS Namespace Influence6 Casey Deccioa, , Jeff Sedayaob , Krishna Kantc , PrasantUniversity of California Davis, 1 Shields Ave., Davis, CA, USA Abstract Name resolution using the Domain Name System (DNS the control of the domain's owner. In this article we review the DNS protocol and several DNS server

California at Davis, University of

285

" Recursive and Iterative Queries " Resource record and DNS query  

E-print Network

9/9/14 1 Chapter 2 Outline DNS " Overview " Recursive and Iterative Queries " Resource record and DNS query " DNS Protocol " DNS Caching " DNS Services " Reverse DNS lookup #12;9/9/14 2 DNS (Domain Name System) Internet host's and router interfaces: " IPv4 address (32 bit): used

286

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

287

Numerical simulation of the Polywell device  

SciTech Connect

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

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

1995-12-31

288

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

289

Numerical Simulation of Randomly Forced Turbulent Flows  

NASA Astrophysics Data System (ADS)

Several authors have proposed studying randomly forced turbulent flows (e.g., E. A. Novikov, Soviet Physics JETP, 20(5), 1290 1965). More recently, theoretical investigations (e.g., renormalization group) have focused on white-noise forced Navier-Stokes equations (V. Yakhot and S. A. Orszag, J. Sci. Comput.1(1), 3 1986). The present article aims to provide an appropriate numerical method for the simulation of randomly forced turbulent systems. The spatial discretization is based on the classical Fourier spectral method. The time integration is performed by a second-order Runge-Kutta scheme. The consistency of an extension of this scheme to treat additive noise stochastic differential equations is proved. The random number generator is based on lagged Fibonacci series. Results are presented for two randomly forced problems: the Burgers and the incompressible Navier-Stokes equations with a white-noise in time forcing term characterized by a power-law correlation function in spectral space. A variety of statistics are computed for both problems, including the structure functions. The third-order structure functions are compared with their exact expressions in the inertial subrange. The influence of the dissipation mechanism (viscous or hyperviscous) on the inertial subrange is discussed. In particular, probability density functions of velocity increments are computed for the Navier-Stokes simulation. Finally, for both Burgers and Navier-Stokes problems, our results support the view that random sweeping is the dominant effect of the large-scale motion on the small-scales.

Machiels, L.; Deville, M. O.

1998-09-01

290

Collisionless microinstabilities in stellarators. II. Numerical simulations  

SciTech Connect

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 fr Plasmaphysik, EURATOM Association, Teilinstitut Greifswald, Wendelsteinstrae 1, 17491 Greifswald, Germany and Max-Planck/Princeton Research Center for Plasma Physics, 17491 Greifswald (Germany)] [Max-Planck-Institut fr Plasmaphysik, EURATOM Association, Teilinstitut Greifswald, Wendelsteinstrae 1, 17491 Greifswald, Germany and Max-Planck/Princeton Research Center for Plasma Physics, 17491 Greifswald (Germany)

2013-12-15

291

Numerical simulation of premixed turbulent methane combustion  

SciTech Connect

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

292

Numerical simulations of ultrafast pulse measurements  

NASA Astrophysics Data System (ADS)

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

Liu, Xuan

293

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

NASA Technical Reports Server (NTRS)

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

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

2013-01-01

294

Numerical Simulations and Design of Shearing Process for Aluminum Alloys  

Microsoft Academic Search

This work combines experimental studies with finite element simulations to develop a reliable numerical model for the simulation of shearing process in aluminum alloys. The critical concern with respect to product quality in this important process is burr forma- tion. Numerical simulations are aimed at understanding the role of process variables on burr formation and for recommending process design parameters.

Aniruddha Khadke; Somnath Ghosh

2005-01-01

295

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS Int. J. Numer. Meth. Fluids 2000; 33: 249278  

E-print Network

­278 Evaluation of some high-order shock capturing schemes for direct numerical simulation of unsteady two & Sons, Ltd. KEY WORDS: acoustic wave propagation; CFD; DNS; high-order shock-capturing scheme; vorticity^tillon Cedex, France SUMMARY The present study addresses the capability of a large set of shock-capturing

296

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

297

Numerical Simulations of Saturn's Polar Cyclones  

NASA Astrophysics Data System (ADS)

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

Brueshaber, Shawn R.; Sayanagi, Kunio M.

2014-11-01

298

DNS of the effects of thermal stratification and turbulent mixing on H2/air ignition in a constant volume, and comparison with the  

E-print Network

DNS of the effects of thermal stratification and turbulent mixing on H2/air ignition in a constant (DNS) with complex H2/air chemistry with a view to providing better understanding of combustion from an initial spectrum of "hot spots" at high pressure is simulated using two-dimensional DNS

Im, Hong G.

299

DNS and LES of two-phase flows with cavitation  

E-print Network

We report on recent progress in the physical and numerical modeling of compressible two-phase flows that involve phase transition between the liquid and gaseous state of the fluid. The high-speed dynamics of cavitation bubbles is studied in well-resolved simulations (DNS) with a sharp-interface numerical model on a micro scale. The underlying assumption of the employed evaporation/condensation model is that phase change occurs in thermal non-equilibrium and that the associated timescale is larger than that of the wave dynamics. Results for the collapse of a spherical vapor bubble close to a solid wall are discussed for three different bubble-wall configurations. The major challenge for such numerical investigations is to accurately reproduce the dynamics of the interface between liquid and vapor during the entire collapse process, including the high-speed dynamics of the late stages, where compressibility of both phases plays a decisive role. Direct interface resolving simulations are intractable for real wor...

Hickel, Stefan

2014-01-01

300

A Lagrangian VOF tensorial penalty method for the DNS of resolved particle-laden flows  

NASA Astrophysics Data System (ADS)

The direct numerical simulation of particle flows is investigated by a Lagrangian VOF approach and penalty methods of second order convergence in space for incompressible flows interacting with resolved particles on a fixed structured grid. A specific Eulerian volume of fluid method is developed with a Lagrangian tracking of the phase function while the solid and divergence free constraints are ensured implicitly in the motion equations thanks to fictitious domains formulations, adaptive augmented Lagrangian approaches and viscous penalty methods. A specific strategy for handling particle collisions and lubrication effects is also presented. Various dilute particle laden flows are considered for validating the models and numerical methods. Convergence studies are proposed for estimating the time and space convergence orders of the global DNS approach. Finally, two dense particle laden flows are simulated, namely the flow across a fixed array of cylinders and the fluidization of 2133 particles in a vertical pipe. The numerical solutions are compared to existing theoretical and experimental results with success.

Vincent, Stphane; Brndle de Motta, Jorge Csar; Sarthou, Arthur; Estivalezes, Jean-Luc; Simonin, Olivier; Climent, Eric

2014-01-01

301

Numerical simulation of hot-electron phenomena  

SciTech Connect

An accurate two-dimensional numerical model for MOS transistors incorporating avalanche processes is presented. The Laplace and Poisson equations for the electrostatic potential in the gate oxide and bulk and the current-continuity equations for the electron and hole densities are solved using finite-difference techniques. The current-continuity equations incorporate terms modeling avalanche generation, bulk and surface Shockley-Read-Hall thermal generation-recombination, and Auger recombination processes. The simulation is performed to a depth in the substrate sufficient to include the depletion region, and the remaining substrate is modeled as a parasitic resistance. The increase in the substrate potential caused by the substrate current flowing through the substrate resistance is also included. The hot-electon distribution function is modeled using Baraff's maximum anisotropy distribution function. The model is used to study hot-electron phenomena including negative-resistance avalanche breakdown in short-channel MOSFET's and electron injection into the gate oxide. The model accurately predicts the positive-resistance branch of the drain current-voltage characteristic and could, in principle, predict the negative-resistance branch and the sustain voltage.

Watanabe, D.S.; Slamet, S.

1983-09-01

302

Numerical simulation of condensation on structured surfaces.  

PubMed

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

Fu, Xiaowu; Yao, Zhaohui; Hao, Pengfei

2014-11-25

303

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

304

Cloud interactions and merging - Numerical simulations  

NASA Technical Reports Server (NTRS)

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

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

1984-01-01

305

Direct numerical simulations of aeolian sand ripples.  

PubMed

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

Durn, Orencio; Claudin, Philippe; Andreotti, Bruno

2014-11-01

306

Numerical Simulations of The Mantle Lithosphere Delamination  

NASA Astrophysics Data System (ADS)

Sudden uplift, extension, and increased igneous activity are often explained by rapid mechanical thinning of the lithospheric mantle (e.g., Kay and Kay (1993)). Two main thinning mechanisms have been proposed, convective removal of a thickened litho- spheric root or delamination at the favor of a crustal decoupling level. The latter mechanism was first defined by Bird (1979) as the whole mantle lithosphere peel- ing suddenly away from the crust by the propagation of a strongly localised shear zone, and sinking into the mantle. We perform 2-D numerical simulations of convection using a viscoplastic rheol- ogy with a viscosity depending strongly on temperature, depth, and composition (crust/mantle). The simulations develop in three steps. (1) We first obtain "classical" sublithospheric convection for a long period of time (100 Myrs), yielding a slightly heterogeneous lithospheric temperature structure. (2) At some time, in some simu- lations, a strong thinning of the mantle occurs progressively (in about 20 Myrs) in a small area (100 km wide). This process puts the asthenosphere in direct contact with the lower crust. (3) Large pieces of mantle lithosphere then quickly sink into the man- tle, by the horizontal propagation of a detachment level away from the "asthenospheric conduit". We determine the favourable parameters (crustal thicknesses, activation energies, and friction coefficients) to the initiation of delamination (step 2). We find that delamina- tion initiates in places where the Moho temperature is the higher, as soon as the crust and mantle viscosities are sufficiently low. Delamination may thus occur on Earth in orogenic settings when the crust is thick and the Moho temperature exceeds 8000C. We also perform simulations with variable crustal thicknesses, and show that if de- lamination occurs at the base of the deeper crust, it does not propagate along the base of the shallower crust. References: Bird, P., Continental delamination and the Colorado Plateau, J. Geophys. Res., 84, 7561-7571, 1979. Kay, R. W. and Kay, S. M., Delamination and delamination magmatism, Tectonophysics, 219, 177-189, 1993.

Morency, C.; Doin, M.-P.

307

Numerical simulations of weak lensing measurements  

NASA Astrophysics Data System (ADS)

Weak gravitational lensing induces distortions on the images of background galaxies, and thus provides a direct measure of mass fluctuations in the Universe. The distortion signature from large-scale structure has recently been detected by several groups for the first time, opening promising prospects for the near future. Since the distortions induced by lensing on the images of background galaxies are only of the order of a few per cent, a reliable measurement demands very accurate galaxy shape estimation and a careful treatment of systematic effects. Here, we present a study of a shear measurement method using detailed simulations of artificial images. The images are produced using realizations of a galaxy ensemble drawn from the Hubble Space Telescope Groth strip. We consider realistic observational effects including atmospheric seeing, point spread function (PSF) anisotropy and pixelization, incorporated in such a manner as to reproduce actual observations with the William Herschel Telescope. By applying an artificial shear to the simulated images, we test the shear measurement method proposed by Kaiser, Squires & Broadhurst (KSB). Overall, we find the KSB method to be reliable with the following provisos. First, although the recovered shear is linearly related to the input shear, we find a coefficient of proportionality of about 0.8. In addition, we find a residual anti-correlation between the PSF ellipticity and the corrected ellipticities of faint galaxies. To guide future weak lensing surveys, we study the ways in which seeing size, exposure time and pixelization affect the sensitivity to shear. We find that worsened seeing linearly increases the noise in the shear estimate, while the sensitivity depends only weakly on exposure time. The noise is dramatically increased if the pixel scale is larger than that of the seeing. In addition, we study the impact both of overlapping isophotes between neighbouring galaxies, and of PSF correction residuals: together these are foundQ13 to produce spurious lensing signals on small scales. We discuss the prospects of using the KSB method for future, more sensitive, surveys. Numerical simulations of this kind are a required component of present and future analyses of weak lensing surveys.

Bacon, David J.; Refregier, Alexandre; Clowe, Douglas; Ellis, Richard S.

2001-08-01

308

Direct numerical simulations of particle-laden density currents with adaptive, discontinuous finite elements  

NASA Astrophysics Data System (ADS)

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

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

2014-09-01

309

DNS of three-dimensional separation in turbulent diffuser flows  

NASA Astrophysics Data System (ADS)

The spectral-element method (SEM), is a high-order numerical method with the ability to accurately simulate fluid flows in complex geometries. SEM has opened the possibility to study - in great detail - fluid phenomena known to be very sensitive to discretization errors, e.g. flows undergoing pressureinduced separation [4]. Recently, Cherry et al. [1] performed experiments using Magnetic Resonance Velocimetry (MRV) of turbulent diffuser flow exhibiting unsteady three-dimensional separation at Re = 10 000 based on bulk velocity and height of the inflow duct. Two geometries with different opening angles were investigated and it was found that the flow was extremely sensitive to slight changes in the geometrical setup. To understand this sensitivity and to analyze its causes, we present a direct numerical simulation (DNS) of one of these cases (denoted by "Diffuser 1" in [1]) by means of the SEM. Here, we focus on careful analysis of mean flow results in order to assess the quality of the simulation data.

Ohlsson, Johan; Schlatter, Philipp; Fischer, Paul F.; Henningson, Dan S.

310

Validation of three-dimensional incompressible spatial direct numerical simulation code: A comparison with linear stability and parabolic stability equation theories for boundary-layer transition on a flat plate  

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

311

Numerical Simulation of Complex Turbomachinery Flows  

NASA Technical Reports Server (NTRS)

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

Chernobrovkin, A. A.; Lakshiminarayana, B.

1999-01-01

312

Seismic response of fractures by numerical simulation  

NASA Astrophysics Data System (ADS)

Fractures occur on a wide range of scales and are important in the study of hydrocarbon reservoirs. Seismic simulation by finite-difference modelling using an equivalent medium method can devise the elastic parameters of a cell intersected by a fracture as those of a medium with an equivalent seismic response. Numerical experiments confirm that diffractions from the fracture tips are a strong component of the total wavefield. However, a comparison of boxcar, linear, angular and elliptic tapering suggests that there is little dependence on shape because the energy involved in a single diffraction is much lower than the incident energy. An open, fluid filled fracture has stronger effect on the wavefield than the wet and dry, multiple crack models, because an open fracture would have a stronger dissimilarity to the background rock. The density of microcracks within a fracture also has strong effect on the seismic response, however the properties of those cracks are not significant to the overall seismic response. Considering a distribution of a large number of fractures, even when the overall density of fractures is held constant, longer fractures attenuate seismic energy more than smaller ones. For the orientation effect, fractures oriented in the direction of propagation seem to affect the wavefield more than those perpendicular because of the incident wave striking the fracture at an angle greater than the critical angle. Experiments on clustering of the fractures indicate that although clusters which are large compared to the wavelength may attenuate and 'shield' more than for a uniform distribution, smaller ones in fact attenuate less, because of the 'healing' effect. These are important results when trying to characterize the fracture properties including density, clustering, size and orientation of a fractured reservoir from field seismic data.

Hall, Fiona; Wang, Yanghua

2012-04-01

313

The study of the properties of turbulent stably stratified air flow over water surface by direct numerical simulation  

NASA Astrophysics Data System (ADS)

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

Druzhinin, Oleg; Troitskaya, Yuliya; Zilitinkevich, Sergej

2014-05-01

314

Pade Approximations in Inverse Homogenization and Numerical Simulation of Electromagnetic  

E-print Network

Pad´e Approximations in Inverse Homogenization and Numerical Simulation of Electromagnetic Fields and in numerical simulation of time- domain electromagnetic fields in composites. It is assumed that the scale governing the electromagnetic fields are of convolution type. We use rational Pad´e approximation to derive

Cherkaev, Elena

315

NUMERICAL SIMULATION OF SURFACE WATER WAVES ON THE SPHERE  

E-print Network

­surface potential flow model of water waves. The equations of motion have a Hamiltonian structure, are nonNUMERICAL SIMULATION OF SURFACE WATER WAVES ON THE SPHERE Panayotis Panayotaros Department for the numerical simulation of small amplitude water waves in spherical geometry. The method is based on a free

316

Numerical simulation of flow through microchannels with designed roughness  

Microsoft Academic Search

A three-dimensional numerical simulation of flow through serpentine microchannels with designed roughness in form of obstructions placed along the channels walls is conducted here. CFD-ACE+ is used for the numerical simulations. The effect of the roughness height (surface roughness), geometry, Reynolds number on the friction factor is investigated. It is found that the friction factor increases in a nonlinear fashion

A. S. Rawool; Sushanta K. Mitra; S. G. Kandlikar

2006-01-01

317

Numerical Modeling and Simulation of a Cholesteric Liquid Crystal Polarizer  

Microsoft Academic Search

A numerical model based on Berremans 4 4 matrix approach was developed and used for computational simulation of a cholesteric liquid crystal (CLC) polarizer. Explicit expressions of the 4 4 propagation matrices for several optical films, which constitute a CLC polarizer, were presented. Numerical simulations for optical properties of a CLC layer, a linear polarizer and a CLC

Jong Rak Park; Gihan Ryu; Jeongil Byun; Heenam Hwang; Sung Tae Kim; Insun Kim

2002-01-01

318

Direct numerical simulation of transition: The spatial approach  

NASA Technical Reports Server (NTRS)

The objective of this paper is to review the efforts in spatial direct numerical simulations for transition modeling. Much recent success has been realized in the development of more efficient numerical algorithms as well as a robust downstream boundary condition. Efforts at explaining complex physical phenomena through the use of simulations are reviewed.

Reed, Helen L.

1994-01-01

319

Numerical Simulation in Applied Geophysics. From the Mesoscale to the  

E-print Network

Numerical Simulation in Applied Geophysics. From the Mesoscale to the Macroscale Juan E. Santos Numerical Simulation in Applied Geophysics. From the Mesoscale to the Macroscale ­ p. #12;Introduction. I Seismic wave propagation is a common technique used in hydrocarbon exploration geophysics, mining

Santos, Juan

320

Fixed-scale statistics and the geometry of turbulent dispersion at high Reynolds number via numerical simulation  

NASA Astrophysics Data System (ADS)

The relative dispersion of one uid particle with respect to another is fundamentally related to the transport and mixing of contaminant species in turbulent flows. The most basic consequence of Kolmogorov's 1941 similarity hypotheses for relative dispersion, the Richardson-Obukhov law that mean-square pair separation distance grows with the cube of time t3 at intermediate times in the inertial subrange, is notoriously difficult to observe in the environment, laboratory, and direct numerical simulations (DNS). Inertial subrange scaling in size parameters like requires careful adjustment for the initial conditions of the dispersion process as well as a very wide range of scales (high Reynolds number) in the flow being studied. However, the statistical evolution of the shapes of clusters of more than two particles has already exhibited statistical invariance at intermediate times in existing DNS. This invariance is identified with inertial-subrange scaling and is more readily observed than inertial-subrange scaling for the seemingly simpler quantity . Results from dispersion of clusters of four particles (called tetrads) in large-scale DNS at grid resolutions up to 40963 and Taylor-scale Reynolds numbers Rlambda from 140 to 1000 are used to explore the question of statistical universality in measures of the size and shape of tetrahedra in homogeneous isotropic turbulence in distinct scaling regimes at very small times (ballistic), intermediate times (inertial) and very late times (diffusive). Derivatives of 1/3 with respect to time normalized by the characteristic time scale at the initial tetrad size r0 constitute a powerful technique in isolating t3 scaling in . This technique is applied to the eigenvalues of a moment-of-inertia-like tensor formed from the separation vectors between particles in the tetrad. Estimates of the proportionality constant g in the Richardson-Obukhov t3 law from DNS at Rlambda ? 1000 converge towards the value g ? 0.56 reported in previous studies. The exit time taken by a particle pair to first reach successively larger thresholds of fixed separation distance is also brie y discussed and found to have unexplained dependence on initial separation distance for negative moments, but good inertial range scaling for positive moments. The use of diffusion models of relative dispersion in the inertial subrange to connect mean exit time to g is also tested and briefly discussed in these simulations. Mean values and probability density functions of shape parameters including the triangle aspect ratio w, tetrahedron volume-to-gyration radius ratio V2/3/R 2 and normalized moment-of-inertia eigenvalues are all found to approach invariant forms in the inertial subrange for a wider range of initial separations than size parameters such as mean-square gyration radius. These results constitute the clearest evidence to date that turbulence has a tendency to distort and elongate multiparticle configurations more severely in the inertial subrange than it does in the diffusive regime at asymptotically late time. Triangle statistics are found to be independent of initial shape for all time beyond the ballistic regime. The development and testing of different schemes for parallelizing the cubic spline interpolation procedure for particle velocities needed to track particles in DNS is also covered. A "pipeline" method of moving batches of particles from processor to processor is adopted due to its low memory overhead, but there are challenges in achieving good performance scaling.

Hackl, Jason F.

321

Twenty years of experimental and direct numerical simulation access to the velocity gradient tensor: What have we learned about turbulence?a)  

NASA Astrophysics Data System (ADS)

Twenty years ago there was no experimental access to the velocity gradient tensor for turbulent flows. Without such access, knowledge of fundamental and defining properties of turbulence, such as vorticity dissipation, and strain rates and helicity, could not be studied in the laboratory. Although a few direct simulations at very low Reynolds numbers had been performed, most of these did not focus on properties of the small scales of turbulence defined by the velocity gradient tensor. In 1987 the results of the development and first successful use of a multisensor hot-wire probe for simultaneous measurements of all the components of the velocity gradient tensor in a turbulent boundary layer were published by Balint et al. [Advances in Turbulence: Proceedings of the First European Turbulence Conference (Springer-Verlag, New York, 1987), p. 456]. That same year measurements of all but one of the terms in the velocity gradient tensor were carried out, although not simultaneously, in the self-preserving region of a turbulent circular cylinder wake by Browne et al. [J. Fluid Mech. 179, 307 (1987)], and the first direct numerical simulation (DNS) of a turbulent channel flow was successfully carried out and reported by Kim et al. [J. Fluid Mech. 177, 133 (1987)], including statistics of the vorticity field. Also in that year a DNS of homogeneous shear flow by Rogers and Moin [J. Fluid Mech. 176, 33 (1987)] was published in which the authors examined the structure of the vorticity field. Additionally, Ashurst et al. [Phys. Fluids 30, 2343 (1987)] examined the alignment of the vorticity and strainrate fields using this homogeneous shear flow data as well as the DNS of isotropic turbulence of Kerr [J. Fluid Mech. 153, 31 (1985)] who had initiated such studies. Furthermore, Metcalfe et al. [J. Fluid Mech. 184, 207 (1987)] published results from their direct simulation of a temporally developing planar mixing layer in which they examined coherent vortical states resulting from secondary instabilities. Since then several experimentalists have used multisensor hot-wire probes of increasing complexity in turbulent boundary layers, wakes, jets, mixing layers, and grid flows. Numerous computationalists have employed DNS in a wide variety of turbulent flows at ever increasing Reynolds numbers. Particle image velocimetry and other optical methods have been rapidly developed and advanced during these two decades which have provided other means of access to these fundamental properties of turbulence. This paper reviews highlights of these remarkable developments and points out some of the most important things we have learned about turbulence as a result.

Wallace, James M.

2009-02-01

322

NUMERICAL NOISE PM SIMULATION IN CMAQ  

EPA Science Inventory

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....

323

Numerical simulation of turbulent flow in a cyclonic separator  

NASA Astrophysics Data System (ADS)

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

Bogdanov, Dmitry; Poniaev, Sergey

2014-12-01

324

T-DNS: Connection-Oriented DNS to Improve Privacy and Security (extended)  

E-print Network

T-DNS: Connection-Oriented DNS to Improve Privacy and Security (extended) USC/ISI Technical Report Somaiya 1 1: University of Southern California 2: Verisign Labs ABSTRACT DNS is the canonical protocol for connectionless UDP. Yet DNS today is challenged by eavesdropping that compromises privacy, source

Heidemann, John

325

The Autoconfiguration of Recursive DNS Server and the Optimization of DNS Name Resolution  

E-print Network

The Autoconfiguration of Recursive DNS Server and the Optimization of DNS Name Resolution provides the mechanism for the auto- configuration of recursive DNS server in mobile node and the optimization of DNS name resolution in the hierarchical mobile IPv6 network. Whenever the mobile node moves

Jeong, Jaehoon "Paul"

326

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

E-print Network

DNS Name Service based on Secure Multicast DNS for IPv6 Mobile Ad Hoc Networks Jaehoon Jeong: http://www.adhoc.6ants.net/ Abstract-- In this paper, we propose an architecture of secure DNS system and service discovery. Because mobile ad hoc network has dynamic topology, the current DNS is inappropriate

Jeong, Jaehoon "Paul"

327

Effect of self-stratification on sediment diffusivity in channel flows and boundary layers: a study using direct numerical simulations  

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

328

Tracking of large-scale structures in turbulent channel with direct numerical simulation of low Prandtl number passive scalar  

NASA Astrophysics Data System (ADS)

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

Tiselj, Iztok

2014-12-01

329

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

330

Looking Back Molecular-dynamics simulations require numerical methods  

E-print Network

Looking Back · Molecular-dynamics simulations require numerical methods for the integration, Heun, and Runge-Kutta. However, these methods are not suitable for molecular-dynamics simulations. · Frequently used methods for the integration of the equations in a molecular-dynamics simulation are the Gear

Duisburg-Essen, Universität

331

Numerical simulation study of a tree windbreak  

Microsoft Academic Search

In this study, computational fluid dynamics (CFD) was utilised to investigate the flow characteristics around tree windbreaks. The efficiency of windbreaks depends on many factors which can be investigated in field experiments, though this is limited due to several reasons such as unstable weather conditions, few measuring points, etc. Fortunately, the investigation is possible via computer simulations. The simulation technique

J. P. Bitog; I.-B. Lee; H.-S. Hwang; M.-H. Shin; S.-W. Hong; I.-H. Seo; K.-S. Kwon; E. Mostafa; Z. Pang

332

Numerical simulation of turbulent combustion: Scientific challenges  

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

333

Detection of Kaminsky DNS Cache Poisoning Attack  

Microsoft Academic Search

We statistically investigated the total inbound standard DNS resolution traffic from the Internet to the top domain DNS server in a university campus network through January 1st to December 31st, 2010. The following results are obtained: (1) We found five Kaminsky DNS Cache Poisoning (Kaminsky) attacks in observation of rapid decrease in the unique source IP address based entropy of

Yasuo Musashi; Masaya Kumagai; Shinichiro Kubota; Kenichi Sugitani

2011-01-01

334

DOMAIN NAME SYSTEM (DNS) SERVICES: NIST  

E-print Network

June 2006 DOMAIN NAME SYSTEM (DNS) SERVICES: NIST RECOMMENDATIONS FOR SECURE DEPLOYMENT DOMAIN NAME SYSTEM (DNS) SERVICES: NIST RECOMMENDATIONS FOR SECURE DEPLOYMENT Shirley Radack, EditorShirley Radack and Technology Domain Name System (DNS) services have an important function in helping users readily access

335

Visualizing DNS Traffic Department of EECS,  

E-print Network

Visualizing DNS Traffic Pin Ren Department of EECS, Northwestern University p (DNS) security challenges, such as dis- tributed denial of service (DDoS) and cache poisoning at- tacks nature of DNS queries. The addition of visual metaphors such as Stacking Graphs, Two Tone Pseudo Color

Gooch, Bruce

336

DNS Resolvers Considered Harmful Kyle Schomp  

E-print Network

DNS Resolvers Considered Harmful Kyle Schomp , Mark Allman , and Michael Rabinovich Case Western Reserve University, International Computer Science Institute Abstract-- The Domain Name System (DNS, shared DNS resolvers are a notorious security weak spot in the system. We propose an unorthodox approach

Rabinovich, Michael "Misha"

337

Reducing DNS Caching Saleem N. Bhatti  

E-print Network

Reducing DNS Caching Saleem N. Bhatti University of St Andrews, UK saleem of traffic control (e.g. multi-homing). Currently, the Domain Name System (DNS) is used to resolve names to DNS records, with relatively large time-to-live (TTL) values (several thousands of seconds

Bhatti, Saleem N.

338

Wireshark Lab: DNS Version: 2.0  

E-print Network

Wireshark Lab: DNS Version: 2.0 © 2007 J.F. Kurose, K.W. Ross. All Rights Reserved Computer System (DNS) translates hostnames to IP addresses, fulfilling a critical role in the Internet infrastructure. In this lab, we'll take a closer look at the client side of DNS. Recall that the client's role

Di Mauro, Nicola

339

AIAA 033726 Preliminary DNS Database of  

E-print Network

AIAA 03­3726 Preliminary DNS Database of Hypersonic Turbulent Boundary Layers M. Pino Martin­4344 #12;Preliminary DNS Database of Hypersonic Turbulent Boundary Layers M. Pino Martin Department. Based on a better understanding of the real flow physics and using DNS data, accurate turbulence models

Martín, Pino

340

Quantifying and Improving DNS Availability Casey Deccio  

E-print Network

Quantifying and Improving DNS Availability By Casey Deccio B.S. (Brigham Young University) 2002 M;Copyright by CASEY DECCIO 2010 #12;Abstract Quantifying and Improving DNS Availability by Casey Deccio The Domain Name System (DNS) is one of the components most critical to Internet functionality. Nearly all

California at Davis, University of

341

Security Monitoring of DNS traffic Bojan Zdrnja  

E-print Network

Security Monitoring of DNS traffic Bojan Zdrnja CompSci 780, University of Auckland, May 2006. b.zdrnja@auckland.ac.nz Abstract The Domain Name System (DNS) is a critical part of the Internet. This paper analyzes methods for passive DNS replication and describes the replication setup at the University of Auckland. Analysis

Goodman, James R.

342

Semantic based DNS Forensics Samuel Marchal1  

E-print Network

Semantic based DNS Forensics Samuel Marchal1 , J´er^ome Franc¸ois1 , Radu State1 , Thomas Engel1 1.lastname@uni.lu Abstract--In network level forensics, Domain Name Service (DNS) is a rich source of information. This paper describes a new approach to mine DNS data for forensic purposes. We propose a new technique that leverages

Boyer, Edmond

343

Pollution Resilience for DNS Resolvers Andrew Kalafut  

E-print Network

Pollution Resilience for DNS Resolvers Andrew Kalafut Computer Science Dept Indiana University Bloomington, IN Email: minaxi@cs.indiana.edu Abstract--The DNS is a cornerstone of the Internet. Unfortu- nately, no matter how securely an organization provisions and guards its own DNS infrastructure

Gupta, Minaxi

344

TP n1 -Gnralits, DNS Services rseau  

E-print Network

TP n°1 - Généralités, DNS Services réseau Licence Professionnelle Réseaux et Télécommunications M;3 Installation et configuration d'un serveur DNS Nous allons maintenant installer et configurer un serveur DNS. Q

Hauspie, Michaël

345

Common DNS Operational and Configuration Errors  

Microsoft Academic Search

This memo describes errors often found in both the operation of Domain Name System (DNS) servers, and in the data that these DNS servers contain. This memo tries to summarize current Internet requirements as well as common practice in the operation and configuration of the DNS. This memo also tries to summarize or expand upon issues raised in (RFC 1537).

D. Barr

1996-01-01

346

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

347

Numerical Simulations of MHD Turbulence in Accretion Disks  

E-print Network

We review numerical simulations of MHD turbulence. The last decade has witnessed fundamental advances both in the technical capabilities of direct numerical simulation, and in our understanding of key physical processes. Magnetic fields tap directly into the free energy sources in a sufficiently ionized gas. The result is that adverse angular velocity and adverse temperature gradients, not the classical angular momentum and entropy gradients, destabilize laminar and stratified flow. This has profound consequences for astrophysical accretion flows, and has opened the door to a new era of numerical simulation experiments.}

Steven A. Balbus; John F. Hawley

2002-03-20

348

Investigation of statistical parameters of turbulent air flow over waved water surface by direct numerical simulation  

NASA Astrophysics Data System (ADS)

Interaction of surface water waves with the wind flow is of primary importance for the wave modeling. The most difficult case for modeling is that of steep waves, when the strongly non-linear effects (e.g. sheltering, flow separation, vortex formation etc.) are encountered in the airflow over waves. Of special interest is also the influence of the wind flow stratification on the wind-wave interaction. In this work the preliminary results of direct numerical simulation (DNS)of structure and statistical characteristics of a turbulent, stably stratified atmospheric boundary layer over waved water surface are presented. In the experiments two-dimensional water waves with different wave age parameters (c/u* = 0-10, where u* is the friction velocity and c is the wave celerity), wave slope ka = 0-0.2 and at a bulk Reynolds number Re = 15000 and different values of the bulk Richardson number Ri (based on the buoyancy jump, bulk velocity and the surface wave length) are considered. The shape of the water wave is prescribed and does not evolve under the action of the wind. The full, 3D Navier-Stokes equations under the Boussinesq approximation are solved in curvilinear coordinates in a frame of reference moving the phase velocity of the wave. The shear driving the flow is created by an upper plane boundary moving horizontally with a bulk velocity in the x-direction. Periodic boundary conditions are considered in the horizontal (x) and lateral (y) directions, and no-slip boundary condition is considered in the vertical z-direction. The grid of nodes in the x, y, and z directions is used. The Adams-Bashforth method is employed to advance the integration in time and the equation for the pressure is solved iteratively by using FFT in the x and y directions and the Gauss method in the z-direction. Ensemble-averaged velocity and pressure fields are evaluated by averaging over time and the spanwise coordinate. Profiles of the mean velocity and turbulent stresses are obtained by averaging over wavelength. The preliminary DNS results show that the wind flow is significantly affected by the stratification. If the Richardson number is sufficiently small, the instantaneous vector velocity fields manifest considerable airflow separation at the crests of the surface waves similar to that observed in physical experiments by PIV-technique. Alternatively the ensemble averaged velocity fields are non-separating and have typical structures similar to those observed in shear flows near critical levels, where the phase velocity of the disturbance coincides with the flow velocity. On the other hand, for large Richardson numbers the wind flow turbulence is superseded by internal lee waves radiated from the wave crests and dissipating at a critical level, at some distance above the crests. The DNS results are compared with the prediction of a theoretical model of a turbulent boundary layer, based on the system of Reynolds-averaged equations with the first-order closure hypothesis. The wind-wave interaction is considered within the quasi-linear approximation, i.e., wave-induced disturbances in the air flow are considered in the linear approximation, but the resistive effect of the wave momentum flux on the mean flow velocity profile is taken into account. This paper was supported by RFBR (project codes 10-05-00339-A, 10-05-91177-GFEN_A, 09-05-00779-A;, 11-05-00455-A).

Troitskaya, Yuliya; Druzhinin, Oleg

2013-04-01

349

Example of NDN-DNS lookup for ndnsim.netNDN-DNS lookupNDN-DNS server roles Encapsulating in NDN Mapping in NDN  

E-print Network

Example of NDN-DNS lookup for ndnsim.netNDN-DNS lookupNDN-DNS server roles Encapsulating in NDN mapping service for NDN Alexander Afanasyev, Yingdi Yu, Lixia Zhang App App User app Interest/DNS query Data/DNS response App Interest with hint App Data Local stub resolver, remote stub resolver, or remote

California at Los Angeles, University of

350

Polarization transmission at RHIC, numerical simulations  

SciTech Connect

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

351

High order hybrid numerical simulations of two dimensional detonation waves  

NASA Technical Reports Server (NTRS)

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

Cai, Wei

1993-01-01

352

Numerical simulation of ultrasonic tomography inspections of highly heterogeneous materials  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

353

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

354

Numerical simulation of laserinduced breakdown in Shankar Ghosh  

E-print Network

Numerical simulation of laser­induced breakdown in air Shankar Ghosh and Krishnan Mahesh University and Exhibit 7 - 10 January 2008, Reno, Nevada AIAA 2008-1069 Copyright © 2008 by Shankar Ghosh. Published

Mahesh, Krishnan

355

Numerical simulation of hot-electron phenomena  

Microsoft Academic Search

An accurate two-dimensional numerical model for MOS transistors incorporating avalanche processes is presented. The Laplace and Poisson equations for the electrostatic potential in the gate oxide and bulk and the current-continuity equations for the electron and hole densities are solved using finite-difference techniques. The current-continuity equations incorporate terms modeling avalanche generation, bulk and surface Shockley-Read-Hall thermal generation-recombination, and Auger recombination

D. S. Watanabe; S. Slamet

1983-01-01

356

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

357

Numerical simulation and physical aspects of supersonic vortex breakdown  

NASA Technical Reports Server (NTRS)

Existing numerical simulations and physical aspects of subsonic and supersonic vortex-breakdown modes are reviewed. The solution to the problem of supersonic vortex breakdown is emphasized in this paper and carried out with the full Navier-Stokes equations for compressible flows. Numerical simulations of vortex-breakdown modes are presented in bounded and unbounded domains. The effects of different types of downstream-exit boundary conditions are studied and discussed.

Liu, C. H.; Kandil, O. A.; Kandil, H. A.

1993-01-01

358

Numerical simulation of tornado-like vortices around complex geometries  

Microsoft Academic Search

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

Frederique Drullion

2009-01-01

359

Numerical simulation of instability and transition physics  

NASA Technical Reports Server (NTRS)

The study deals with the algorithm technology used in instability and transition simulations. Discretization methods are outlined, and attention is focused on high-order finite-difference methods and high-order centered-difference formulas. One advantage of finite-difference methods over spectral methods is thought to be in implementation of nonrigorous boundary conditions. It is suggested that the next significant advances in the understanding of transition physics and the ability to predict transition will come with more physically-realistic simulations. Compressible-flow algorithms are discussed, and it is noted that with further development, exploration of bypass mechanism on simple bodies at high speed would be possible.

Streett, C. L.

1990-01-01

360

Numerical simulations of cardiovascular diseases and global matter transport  

E-print Network

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

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

2007-01-01

361

Numerical simulation of the Gailitis dynamo David Moss1  

E-print Network

Numerical simulation of the Gailitis dynamo David Moss1 School of Mathematics University to be inconsistent with the work of Moss (1990), who in the context of Cp star magnetism obtained a positive growth illustrated numerically for the first time by Brandenburg, Moss and 1e-mail: moss@ma.man.ac.uk 1 #12;Soward

Moss, David

362

Numerical simulation of electroosmotic effect in serpentine channels  

Microsoft Academic Search

Numerical simulation of flow through a three-dimensional serpentine microchannel, subjected to a voltage perpendicular to the flow direction is presented here. Commercial CFD software CFD-ACE+ is used for the numerical analysis. A parametric study is conducted to investigate the effect of radius of curvature, Reynolds number, zeta potential and Debye length on the pressure drop and friction factor. Each case

A. S. Rawool; Sushanta K. Mitra

2006-01-01

363

NUMERICAL SIMULATIONS OF TRANSVERSE COMPRESSION AND DENSIFICATION IN WOOD  

E-print Network

NUMERICAL SIMULATIONS OF TRANSVERSE COMPRESSION AND DENSIFICATION IN WOOD John A. Nairn1 Professor- terials is a useful tool for stress analysis and for failure modeling. Although FEA of wood as an anisotropic continuum is used, numerical modeling of realistic wood structures, including details of wood

Nairn, John A.

364

Numerical simulations of a low power microchannel thermal cycling reactor  

Microsoft Academic Search

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

David Erickson; Dongqing Li

2002-01-01

365

Numerical Simulations of Subsonic and Transonic Open-Cavity Flows  

E-print Network

Numerical Simulations of Subsonic and Transonic Open-Cavity Flows Yiyang Sun , Aditya G. Nair-dimensional open-cavity flows for free stream Mach number of 0.1 to 1.6 are investi- gated with direct numerical the fundamental problem of flow over open cavities can provide insights into the behavior of the flow over

366

Numerical simulation of the plasma double layer  

Microsoft Academic Search

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

367

Numerical simulation of rough-surface aerodynamics  

Microsoft Academic Search

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

Xingkai Chi

2005-01-01

368

Numerical simulation of collapsing volcanic columns  

Microsoft Academic Search

A complex thermofluid dynamic two-phase and two-component flow model is implemented to simulate collapsing volcanic columns. The model involves only one particle size class and accounts for water vapor mixing with air in the atmosphere. The effect of particle collisions in the model is accounted for by a kinetic theory model that solves for the granular temperature and therefore predicts

Flavio Dobran; Augusto Neri; Giovanni Macedonio

1993-01-01

369

Numerical Simulation of Free Standing Hybrid Risers  

E-print Network

.......................................................................... 50 4.2.3 Fatigue Damage Estimation of the Vertical Riser ............................... 59 4.3 Dynamic Simulations of FSHR due to Buoyancy Can VIM ..................... 65 4.3.1 Natural Frequency... ................................................. 74 Figure 4.43 Stress variation under different current speeds ............................................. 75 Figure 4.44 Stress range versus current speed ................................................................. 75 Figure 4.45 Fatigue...

Hou, Tiancong

2014-08-13

370

Numerical simulation of laser full penetration welding  

Microsoft Academic Search

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

Komeil Kazemi; John A. Goldak

2009-01-01

371

Numerical simulation of cross field amplifiers  

SciTech Connect

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

372

DNS Security Introduction and Requirements  

Microsoft Academic Search

The Domain Name System Security Extensions (DNSSEC) add data origin authentication and data integrity to the Domain Name System. This document introduces these extensions and describes their capabilities and limitations. This document also discusses the services that the DNS security extensions do and do not provide. Last, this document describes the interrelationships between the documents that collectively describe DNSSEC.

R. Austein; M. Larson

2003-01-01

373

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

374

Numerical simulation of spatially-evolving instability  

NASA Technical Reports Server (NTRS)

A computational study of the spatial stability of plane Poiseuille flow is presented. The numerical scheme employs a time-splitting method to integrate the full Navier-Stokes equations using spectral collocation/finite-difference discretization on a non-staggered mesh. The eigenvalue decomposition procedure is applied for the solution of the Poisson equations using the capacitance matrix technique. The buffer domain method is incorporated for the outflow boundary conditions. The input perturbation velocities are obtained by solving the Orr-Sommerfeld equation for the nonlinear eigenvalue problem employing the companion matrix method. Computational results are compared with the linear theory for two-dimensional disturbances.

Danabasoglu, G.; Biringen, Sedat; Streett, C. L.

1990-01-01

375

Numerical simulation of the 1993 midwestern flood  

NASA Astrophysics Data System (ADS)

During the summer of 1993, persistent and heavy precipitation caused a long-lived catastrophic flood in the midwestern United States. In this paper, Midwest hydrology, atmospheric circulation of the 1993 summer, and feedback between the surface and precipitating systems were investigated using the Purdue Regional Model (PRM). Observational analyses of the monthly mean circulation have identified several differences between June and July 1993 that should be considered when studying the interaction of the surface and precipitation systems. The PRM is utilized to simulate the weather during each thirty day period. The PRM is verified against European Centre for Medium Range Weather Forecasts (ECMWF) monthly mean observational analyses. Key features, such as the upper level jet stream and trough, moisture transport and heavy precipitation are reproduced by the PRM. Furthermore, the model is able to simulate the different daily precipitation patterns observed during each month. Some model biases are identified and discussed. The verisimilitude between the model and observed circulations allows for sensitivity experiments to investigate the interaction between the surface and precipitation systems. In general, the results indicated that the June (transient cyclone period) simulations were not as sensitive to the surface anomalies as the July (stationary convective period) simulations. While a large-scale dry soil anomaly slightly decreased the moist convective instability of the planetary boundary layer (PBL), a large scale surface pressure perturbation occurred and weakened the low level jet and moisture transport from the Gulf of Mexico. Both wet and dry soil moisture anomalies, imposed across the southern Great Plains, caused a reduction in the low level jet and Midwestern precipitation. Differential heating at the surface in the control simulations associated with the gradient of soil moisture in the southern Great Plains enhanced the mean southerly wind in the PBL. The influence of 'recirculation of water' is quantified by model simulation and separate sensitivity experiments. The month of July was more sensitive to the local water source than June (-20% and -12%, respectively). This may be related to the larger amount of convective precipitation that occurred in July. The influence of topography on the upper level trough and low level jet is also discussed.

Bosilovich, Michael Gerard

376

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

377

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

378

The Numerical Simulation of Orographic Storms  

NASA Astrophysics Data System (ADS)

Recent observational studies of winter storms over the Glamorgan Hills of south Wales (Browning et al., 1974, and Hill et al., 1981) have confirmed the importance of the seeder-feeder mechanism of orographic precipitation enhancement, proposed by Bergeron (1965). However, the latter study also indicated that the sensitivity of the enhancement to the low-level wind speed is much greater than predicted by the seeder-feeder model of Bader and Roach (1977), which simulates only the microphysical processes. A new, dynamic mountain model was used to investigate the hypothesis that mountain wave effects were responsible for this increased wind speed sensitivity. The model, which evolved from the cloud model of Klemp and Wilhelmson (1978), is formulated in terrain-following coordinates. In addition to representing convective and stratiform cloud processes, the model is also capable of accurately simulating moderately nonlinear mountain waves, and is suitable for studying many aspects of orographic storms. The simulations show that orographic clouds significantly alter the airflow over hills and, conversely, that mountain waves strongly influence the moist processes. Wind speed, wind shear, atmospheric stability, and relative humidity are all important factors in the complex scale interactions which occur during orographic storms. The results strongly support the hypothesis that certain mountain wave effects greatly increase the wind speed sensitivity of the Bergeron seeder-feeder mechanism.

Bradley, Michael Morgan

379

Numerical Simulation of Ion Thruster Optics  

NASA Technical Reports Server (NTRS)

A three-dimensional simulation code (ffx) designed to analyze ion thruster optics is described. It is an extension of an earlier code and includes special features like the ability to model a wide range of grid geometries, cusp details, and mis-aligned aperture pairs to name a few. However, the principle reason for advancing the code was in the study of ion optics erosion. Ground based testing of ion thruster optics, essential to the understanding of the processes of grid erosion, can be time consuming and costly. Simulation codes that can accurately predict grid lifetimes and the physical mechanisms of grid erosion can be of great utility in the development of future ion thruster optics designed for more ambitious applications. Results of simulations are presented that describe wear profiles for several standard and nonstandard aperture geometries, such as those grid sets with square- or slotted-hole layout patterns. The goal of this paper will be to introduce the methods employed in the ffx code and to briefly demonstrate their use.

Rawlin, Vincent K. (Technical Monitor); Farnell, Cody C.; Williams, John D.; Wilbur, Paul J.

2003-01-01

380

Numerical simulation of jet noise from different jet nozzle geometries  

Microsoft Academic Search

This paper describes the numerical simulation of flow-induced noise from jets with different nozzle geometries. The nozzles considered include axisymmetric and nonaxisymmetric nozzles, such as circular and rectangular. Also the study is extended to examine the differences between noise radiated from nozzles with planar exists and those with nonplanar exist, such as beveled nozzles. The detached-eddy simulation (DES) approach is

Umesh Paliath; Philip J. Morris

2005-01-01

381

A numerical simulation for a pumped storage reservoir. Research report  

Microsoft Academic Search

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

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

1977-01-01

382

Numerical simulations of collapsing, isothermal, magnetic clouds  

NASA Astrophysics Data System (ADS)

The numerical method of Gingold and Monaghan (1977) has been extended to include the effects of a magnetic field. The method allows for fully three dimensional motions of a self-graviting gas threaded by a magnetic field. An idealized picture of a magnetic gas cloud was used in which a nonrotating sphere of gas with uniform density is imbedded in a similar intercloud region. Both the cloud and the intercloud are threaded by a uniform magnetic field, and a pressure balance exists across their interface. The intercloud medium is considered to be a sphere enveloping the cloud with a radius twice that of the cloud and a density one tenth of it. The process of the calculated collapse is described.

Phillips, G. J.

383

Numerical Simulations of Relativistic Shock Acceleration  

E-print Network

We review the present status of the cosmic ray acceleration theory in mildly relativistic shock waves. Due to the involved substantial particle anisotropies analytical methods can tackle only simple situations involving weakly turbulent conditions near the shock. The numerical Monte Carlo methods are used to study the acceleration process in more general conditions. Contrary to non-relativistic shocks, the cosmic ray spectra at relativistic shock waves depend substantially on the magnetic field configurations and turbulence spectra. Depending on these conditions both very flat and very steep spectra can be created, with the characteristic acceleration time scales changing non-monotonously with the turbulence amplitude. Thus, the discussed theory is not able to uniquely explain the observed synchrotron spectra of relativistic shocks. We also mention an interesting possibility of particle acceleration at incompressible flow discontinuities (shear layers) at boundaries of relativistic jets.

M. Ostrowski

2001-04-06

384

DNS and RANS Modeling of Dispersion in the Wake of an Obstacle  

NASA Astrophysics Data System (ADS)

We present a numerical study of the dispersion of a passive scalar in turbulent separated flows to establish the predictive capabilities of algebraic flux models against the standard eddy-diffusivity representation. The scalar dispersion from a point source over a wavy wall is initially investigated to carefully evaluate scalar flux models through comparisons with DNS data. The roof-top release of a passive plume from a wall-mounted cube in a turbulent boundary layer is then presented to demonstrate that algebraic models can also be applied successfully to atmospheric dispersion at street-scale. Despite the questionable validity of local-equilibrium conditions, the numerical experiments show that algebraic models provide a significant improvement for scalar dispersion simulations of complex flows with respect to the standard eddy-diffusivity model.

Philips, David; Rossi, Riccardo; Iaccarino, Gianluca

2009-11-01

385

DNS Performance and the Effectiveness of Caching Jaeyeon Jung, Emil Sit, Hari Balakrishnan, Member, IEEE, and Robert Morris  

E-print Network

tolerance, or route client requests to servers topologically close to the clients. Because cached DNS clients at these institutions interact with the wide-area domain name system, focusing on performance-driven simulations that ex- plore the effect of varying TTLs and varying degrees of cache sharing on DNS cache hit

Akella, Aditya

386

The numerical simulation of multistage turbomachinery flows  

NASA Technical Reports Server (NTRS)

The need to account for momentum and energy transport by the unsteady deterministic flow field in modeling the time-averaged flow state within a blade row passage embedded in a multistage compressor is assessed. It was found that, within the endwall regions, large-scale three-dimensional unsteady structures existed which caused significant transport of momentum and energy across the time-averaged stream surface of a stator flow field. These experiments confirmed that the tranport process is dominated by turbulent diffusion in the midspan region. A model was then proposed for simulating this transport process, and a limited study was undertaken to assess its validity.

Adamczyk, J. J.; Beach, T. A.; Celestina, M. L.; Mulac, R. A.; To, W. M.

1990-01-01

387

Evidence for hairpin packet structure in DNS channel flow  

NASA Astrophysics Data System (ADS)

2-D PIV measurements of boundary layers and channel flows have revealed that wall turbulence is thickly populated with hairpin vortices that mostly appear in groups as packets. A DNS of a single hairpin vortex evolution into a packet has disclosed the packet generation mechanism. However, no DNS studies have revealed hairpin packets. The present DNS study demonstrates, for the first time, the existence of hairpin packets in turbulent flow. They are found to appear frequently in DNS of fully developed turbulent channel flows at low and moderate Reynolds numbers, Re = 150 and 300. The packet consisting of many asymmetric hairpins travel downstream with little dispersion. Due to the collective pumping effect of individual hairpins the packet generates a long low-momentum streak of about 1000 viscous length and creates a strong Q2 event and large Reynolds stress. This evidence substantiates the view that vortex packets are a universal feature of wall turbulence, independent of effects due to boundary layer trips or critical conditions in the aforementioned numerical studies.

Liu, Zichao; Adrian, Ronold J.

1999-11-01

388

Numerical and laboratory simulations of auroral acceleration  

SciTech Connect

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, Ume University, SE-901 87 Ume (Sweden)] [EISCAT Scientific Association, P.O. Box 812, SE-981 28 Kiruna, Sweden and Department of Physics, Ume University, SE-901 87 Ume (Sweden)

2013-10-15

389

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

390

Numerical simulations of moon-ringlet interaction  

NASA Astrophysics Data System (ADS)

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

Hanninen, J.

1993-05-01

391

Numerical simulation of electromagnetic turbulence in tokamaks  

SciTech Connect

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

392

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

393

Comparing DNS Resolvers in the Wild Bernhard Ager Wolfgang Muhlbauer  

E-print Network

Comparing DNS Resolvers in the Wild Bernhard Ager Wolfgang M¨uhlbauer Georgios Smaragdakis Steve/ETH) Comparing DNS Resolvers in the Wild Nov 1 2010 1 #12;Motivation Domain Name System (DNS) DNS: resolve www of the Internet Its performance is critical Ager et al (TUB/DT Labs/ETH) Comparing DNS Resolvers in the Wild Nov

Smaragdakis, Georgios

394

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

395

Numerical Simulation of Supersonic Gap Flow  

PubMed Central

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

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

2015-01-01

396

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

397

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

NASA Technical Reports Server (NTRS)

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

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

1994-01-01

398

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

399

DNS of Forced Gas Flows in Circular Tubes at Various Heating Ratios  

NASA Astrophysics Data System (ADS)

A direct numerical simulation (DNS) with turbulent transport of a variable p roperty has been carried out to grasp and understand a laminarization phenom ena caused by a strong heating. In this study, the inlet Reynolds number bas ed on a bulk velocity and a pipe diameter was set to be constant; Re=6000. T he temperature distribution taken from the experiments by Shehata and McElig ot (1998) was applied to the wall as a thermal boundary condition. Their exp eriments concentrated on three characteristics cases with gas property varia tion: turbulent, laminarizing and intermediate or "subturbulent". In our pre vious study regarding the variable property turbulent pipe flow (Satake et a l., 2000), the DNS result has been shown in good agreement with one case for their experimental data (RUN445, laminarizing case). In present results, th e flow parameter was set to be RUN635 and RUN618 by Shehata and McEligot (19 98). The number of computational grids used in this study was 768 x 64 x 128 in the z-, r- and ?-directions, respectively. The turbulent quantities such as the mean flow, temperature fluctuations, turbulent stresses and the turbulent statistics were obtained via present DNS. The turbulent drag for RUN635 decreases along the streamwise direction. The turbulent intensities also reduced for RUN635. But in RUN618, the turbulent intensity for streamwi se component still remains to the stream direction. The reason of these phen omena can be considered that the fluid property cannot change for developing temperature boundary layer. That is, the developing the velocity boundary l ayer delay for the developing thermal boundary layer.

Satake, Shin-Ichi; Kunugi, Tomoaki; Shehata, A. Mohsen; McEligot, Donald M.

2001-11-01

400

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

SciTech Connect

Numerical models of varying complexity have been used to simulate nonisothermal liquid and vapor flow. Development of these models has been motivated by problems such as evaluation of insaturated zones, geothermal reservoirs, and nuclear waste disposal sites. The objective of this study was to evaluate and explain liquid and vapor fluxes in the shallow unsaturated zone of the Chihuahuan Desert of Texas in response to an annual climate cycle as opposed to shorter, restricted periods. The approach was to use numerical simulations to interpret observed field data. Good agreement was found between NHD (nonhysteretic drying water retention function)-simulated and field-measured water potentials and temperatures. This simulation research provides a greater understanding of unsaturated zone processes in desert soils. Agreement between computed and measured parameters are attributed to the robustness of the thermal calculations. These simulations also indicate sone of the main sources of uncertainty, particularly in the estimated hydraulic conductivities.

Scanlon, B.R. (Univ. of Texas, Austin (United States)); Milly, P.C.D. (Geological Survey, Princeton, NJ (United States))

1994-03-01

401

Betelgeuse - Numerical Simulations of an Entire Supergiant  

NASA Astrophysics Data System (ADS)

The red supergiant Betelgeuse varies in visual brightness on time-scales of weeks, months, and years. In 1975 Martin Schwarzschild attributed these fluctuations to huge convection cells, each of them covering a significant fraction of the stellar surface, so that the individual brightness changes result in a non-vanishing variation of the total luminosity. Starting about 11 years ago, interferometric observations in the visible wavelength regime revealed the existence of large-scale inhomogeneities on the surface of Betelgeuse, typically described as 0 to 3 unresolved ``hot spots'' on a cooler circular stellar disk, varying with time in number, intensity, and position. Nevertheless, the observations still have a poor resolution, resulting in surface ``images'' with only a handful of pixels. And there has been some debate about the nature of the detected surface features: Are they of convective origin due to the action of granules or supergranules? And what is the role of shocks, stellar rotation, or magnetic fields? To improve the theoretical understanding of the surface phenomena of Betelgeuse, a new radiation hydrodynamics code (``COBOLD'') has been written with the aim to include the entire star in the computational box. It employs special inner (for the stellar core) and outer boundaries appropriate for this particular geometry. The simulated star shows indeed huge surface convection cells and high photospheric velocities. But the cells look different from solar granulation and produce features which might be interpreted as ``hot spots'' in low-resolution observations.

Freytag, Bernd

402

Parallel Numerical Simulations of Water Reservoirs  

NASA Astrophysics Data System (ADS)

The study of the water flow and scalar transport in water reservoirs is important for the determination of the water quality during the initial stages of the reservoir filling and during the life of the reservoir. For this scope, a parallel 2D finite element code for solving the incompressible Navier-Stokes equations coupled with scalar transport was implemented using the message-passing programming model, in order to perform simulations of hidropower water reservoirs in a computer cluster environment. The spatial discretization is based on the MINI element that satisfies the Babuska-Brezzi (BB) condition, which provides sufficient conditions for a stable mixed formulation. All the distributed data structures needed in the different stages of the code, such as preprocessing, solving and post processing, were implemented using the PETSc library. The resulting linear systems for the velocity and the pressure fields were solved using the projection method, implemented by an approximate block LU factorization. In order to increase the parallel performance in the solution of the linear systems, we employ the static condensation method for solving the intermediate velocity at vertex and centroid nodes separately. We compare performance results of the static condensation method with the approach of solving the complete system. In our tests the static condensation method shows better performance for large problems, at the cost of an increased memory usage. Performance results for other intensive parts of the code in a computer cluster are also presented.

Torres, Pedro; Mangiavacchi, Norberto

2010-11-01

403

Numerical Simulation of Colliding Ion Acoustic Solitons  

NASA Astrophysics Data System (ADS)

The ion acoustic wave dispersion relation ?= k Cs we are familiar with, is in the long wave length limit. Inclusion of short wave-length (Debye length) scale through Poisson equation gives rise to the Korteweg de Vries (KdV) equation. We simulate propagation of solitary waves by solving the KdV equation in one dimensional and two dimensional planer geometries (Kadomtsev-Petviashvili equation).ootnotetext B.B.Kadomtsev, Doklady Akademii Nauk SSSR 192, 753 (1970).^,ootnotetextY.Nishida and T.Nagasawa, Phys. Rev. Lett. 42, 379 (1979). On the other hand, a different nonlinear term, ponderomotive force gives rise to Langmuir solitons by the interaction between high frequency Langmuir waves and low frequency ion acoustic waves.ootnotetextV.E.Zhaharov, Sov. Phys. JETP 35, 908 (1972). We discuss our studies on 1d-1v Vlasov-Poisson system employing the splitting schemeootnotetextC.Z.Cheng and G.Knorr, J. Comput. Phys. 22, 330 (1976). (by the method of characteristics).

Nishimura, Y.; Chen, Y. H.; Cheng, C. Z.

2011-11-01

404

Evaluation of some high-order shock capturing schemes for direct numerical simulation of unsteady two-dimensional free flows  

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

405

DNS and Multi-Scale Modeling of Multi-Phase Flows  

NASA Astrophysics Data System (ADS)

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

Tryggvason, Gretar

2011-11-01

406

A novel algorithm for DNS/LES of compressible turbulent flows  

NASA Astrophysics Data System (ADS)

A non-dissipative, robust, implicit algorithm is proposed for direct numerical and large-eddy simulation of compressible turbulent flows. The algorithm addresses the problems caused by low Mach numbers and under-resolved high Reynolds numbers. It collocates variables in space to allow easy extension to unstructured grids, and discretely conserves mass, momentum and total energy. The Navier-Stokes equations are non-dimensionalized using an incompressible scaling for pressure, and the energy equation is used to obtain an expression for the velocity divergence. A pressure-correction approach is used to solve the resulting equations such that the discrete divergence is constrained by the energy equation. As a result, the discrete equations analytically reduce to the incompressible equations at very low Mach number; i.e. the algorithm overcomes the acoustic time-scale limit without preconditioning or solution of an implicit system of equations. The algorithm discretely conserves kinetic energy in the incompressible inviscid limit, and is robust for inviscid compressible turbulence on the convective time-scale. These properties make it well-suited for DNS/LES of compressible turbulent flows. A parallel, structured grid flow solver was developed and used to apply the algorithm to acoustic propagation, the incompressible Taylor problem, periodic shock tube problem, laminar compressible boundary layer, isotropic turbulence and turbulent channel flow. The results are compared to analytical solution and experiment as appropriate. The simulations suggest that the algorithm is very promising for DNS/LES of compressible turbulent flows. Keywords: compressible turbulence, direct numerical simulation, large-eddy simulation, all-Mach number, non-dissipative, discrete energy conservation.

Hou, Yucheng

407

Numerical Convergence of Hydrodynamical SPH Simulations of Cooling Clusters  

E-print Network

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

R. Valdarnini

2001-11-05

408

Direct Numerical Simulation of Turbulent Compression Ramp Flow  

NASA Astrophysics Data System (ADS)

A numerical procedure for the direct numerical simulation of compressible turbulent flow and shock-turbulence interaction is detailed and analyzed. An upwind-biased finite-difference scheme with a compact centered stencil is used to discretize the convective part of the Navier-Stokes equations. The scheme has a uniformly high approximation order and allows for a spectral-like wave resolution while dissipating nonresolved wave numbers. When hybridized with an essentially nonoscillatory scheme near discontinuities, the scheme becomes shock-capturing and its resolution properties are preserved. Diffusive parts are discretized with symmetric compact finite differences and an explicit Runge-Kutta scheme is used for time-advancement. The peculiarities of efficient upwinding and coupling procedures are described and validation results are given. Using direct numerical simulation data, some aspects of turbulent supersonic compression ramp flow are studied to demonstrate the effectiveness of the simulation procedure.

Adams, N. A.

409

Numerical simulation of convection depth in shear cell under microgravity  

Microsoft Academic Search

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

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

1999-01-01

410

Numerical simulation of separated boundary-layer flow  

Microsoft Academic Search

The numerical simulation of time-dependent, 2-D compressible boundary-layer flow containing a region of separation is studied. The separation is generated by the introduction of an adverse pressure gradient along the freestream boundary. In order to validate the numerical method, a low Mach-number laminar separation bubble flow is considered, which enables an extensive comparison with incompressible results. The generation of an

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

1997-01-01

411

Direct Numerical Simulation of Turbulent Compression Ramp Flow  

Microsoft Academic Search

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

N. A. Adams

1998-01-01

412

Direct Numerical Simulation of Turbulent Compression Ramp Flow  

Microsoft Academic Search

: A numerical procedure for the direct numerical simulation of compressible turbulent flow and shockturbulence 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 NavierStokes 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

413

Numerical simulation of imbibition process in fractured cores  

E-print Network

of the equations describing the imbibition of pure and carbonated water and the countercurrent flow of oil in porous rocks. Comparison between numerical solutions and experimental observations indicates that the models do simulate water imbibition and the COp... diffusion process of laboratory experiments. Predictions with these models can be made for the purpose of design and evaluation of carbonated waterflooding projects. The numerical calculation procedure in this work is for imbibition of water into a one...

Jiang, Chunhuan

2012-06-07

414

Direct Numerical Simulation of decaying two-dimensional turbulence in a no-slip square box using Smoothed Particle Hydrodynamics  

E-print Network

This paper explores the application of SPH to a Direct Numerical Simulation (DNS) of decaying turbulence in a two-dimensional no-slip wall-bounded domain. In this bounded domain, the inverse energy cascade, and a net torque exerted by the boundary, result in a spontaneous spin up of the fluid, leading to a typical end state of a large monopole vortex that fills the domain. The SPH simulations were compared against published results using a high accuracy pseudo-spectral code. Ensemble averages of the kinetic energy, enstrophy and average vortex wavenumber compared well against the pseudo-spectral results, as did the evolution of the total angular momentum of the fluid. However, while the pseudo-spectral results emphasised the importance of the no-slip boundaries as generators of long lived coherent vortices in the flow, no such generation was seen in the SPH results. Vorticity filaments produced at the boundary were always dissipated by the flow shortly after separating from the boundary layer. The kinetic ene...

Robinson, Martin

2011-01-01

415

Finite-rate chemistry and transient effects in Direct Numerical Simulations of turbulent non-premixed flames  

SciTech Connect

Three-dimensional Direct Numerical Simulations (DNS) of turbulent non-premixed flames including finite-rate chemistry and heat release effects were performed. Two chemical reaction models were considered: (1) a single-step global reaction model in which the heat release and activation energy parameters are chosen to model methane-air combustion, and (2) a two-step reaction model to simulate radical production and consumption and to compare against the single-step model. The model problem consists of the interaction between an initially unstrained laminar diffusion flame and a three-dimensional field of homogeneous turbulence. Conditions ranging from fast chemistry to the pure mixing limit were studied by varying a global Damkoehler number. Results suggest that turbulence-induced mixing acting along the stoichiometric line leads to a strong modification of the inner structure of the turbulent flame compared with a laminar strained flame, resulting in intermediate species concentrations well above the laminar prediction. This result is consistent with experimental observations. Comparison of the response of the turbulent flame structure due to changes in the scalar dissipation rate with a steady strained laminar flame reveals that unsteady strain rates experienced by the turbulent flame may be responsible for the observed high concentrations of reaction intermediates.

Mahalingam, S. [Colorado Univ., Boulder, CO (United States). Dept. of Mechanical Engineering; Chen, J.H. [Sandia National Labs., Livermore, CA (United States); Vervisch, L. [Institut de Mecanique des Fluides, Numeriques (France)

1994-01-01

416

Impulscytophotometrische DNS-Untersuchungen bei Blasenkarzinomen  

Microsoft Academic Search

Zusammenfassung Die Anwendbarkeit und die Bedeutung quantitativer zellularer DNS-Bestimmungen mit Hilfe rascher durchfluzytofluorometrischer Technik werden bei neudiagnostizierten Tumoren sowie der Behandlung des Carcinoma in situ der Blase diskutiert. Aus den DNS-Histogram-men lassen sich Ploidiegrad u?d Anteil von S-Phase-Zellen, welcher die Proliferationsaktivitt des Tumors wiedergibt, bestimmen. Diploi-de DNS-Verteilungen finden sich bei 60 % von T 1-Tumoren, whrend alle anderen Tumoren einschlielich

B. Tribukait; H. Gustafson

1980-01-01

417

Detecting DNS Tunnels Using Character Frequency Analysis  

Microsoft Academic Search

High-bandwidth covert channels pose significant risks to sensitive and\\u000aproprietary information inside company networks. Domain Name System (DNS)\\u000atunnels provide a means to covertly infiltrate and exfiltrate large amounts of\\u000ainformation passed network boundaries. This paper explores the possibility of\\u000adetecting DNS tunnels by analyzing the unigram, bigram, and trigram character\\u000afrequencies of domains in DNS queries and responses. It

Kenton Born; David Gustafson

2010-01-01

418

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

419

Numerical time-domain simulation of diffusive ultrasound in concrete.  

PubMed

Certain aspects of diffusive ultrasound fields in concrete are still unknown and thus, systematic parameter studies using numerical time-domain simulations of the ultrasonic propagation process could lead to further insights into theoretical and experimental questions. In the present paper, the elastodynamic finite integration technique (EFIT) is used to simulate a diffusive reverberation measurement at a concrete specimen taking aggregates, pores, and viscoelastic damping explicitly into account. The numerical results for dissipation and diffusivity are compared with theoretical models. Moreover, the influence of air-filled pores in the cement matrix is demonstrated. PMID:15047383

Schubert, Frank; Koehler, Bernd

2004-04-01

420

Astrophysical jets: Observations, numerical simulations, and laboratory experiments  

SciTech Connect

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

421

Numerical simulation of transition in wall-bounded shear flows  

NASA Technical Reports Server (NTRS)

The current status of numerical simulation techniques for the transition to turbulence in incompressible channel and boundary-layer flows is surveyed, and typical results are presented graphically. The focus is on direct numerical simulations based on the full nonlinear time-dependent Navier-Stokes equations without empirical closure assumptions for prescribed initial and boundary conditions. Topics addressed include the vibrating ribbon problem, space and time discretization, initial and boundary conditions, alternative methods based on the triple-deck approximation, two-dimensional channel and boundary-layer flows, three-dimensional boundary layers, wave packets and turbulent spots, compressible flows, transition control, and transition modeling.

Kleiser, Leonhard; Zang, Thomas A.

1991-01-01

422

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

423

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

424

Numerical simulation of dynamic fracture and failure in solids  

SciTech Connect

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

425

Numerical Simulations of Galaxy Formation: Cooling, Heating, Star \\\\ Formation  

NASA Astrophysics Data System (ADS)

Formation of luminous matter in the Universe is a complicated process, which includes many processes and components. It is the vastly different scales involved in the process (from star formation on few parsec scales to galaxy clusters and superclusters on megaparsecs scales) and numerous ill-understood processes, which make the whole field a maze of unsolved, but exciting problems. We present new approximations for numerical treatment of multiphase ISM forming stars. The approximations were tested and calibrated using N-body+fluid numerical simulations. We specifically target issues related with effects of unresolved lumpinesses of the gas.

Klypin, A. A.

426

A numerical study of strained three-dimensional wall-bounded turbulence  

Microsoft Academic Search

Channel flow, initially fully developed and two-dimensional, is subjected to mean strains that emulate the eect of rapid changes of streamwise and spanwise pressure gradients in three-dimensional boundary layers, ducts, or diusers. As in previous studies of homogeneous turbulence, this is done by deforming the domain of a direct numerical simulation (DNS); here however the domain is periodic in only

G. N. C OLEMAN

2000-01-01

427

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

428

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

429

Detonation front theories: Using high-resolution DNS to define extended asymptotic scalings and models  

SciTech Connect

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

430

Numerical simulation of deformation and figure quality of precise mirror  

NASA Astrophysics Data System (ADS)

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

Vit, Tom; Melich, Radek; Sandri, Paolo

2015-01-01

431

A numerical method for the direct simulation of compressible turbulence  

Microsoft Academic Search

Compressible turbulence is characterized by a large range of length scales and the presence of shockwaves. Because turbulence is inherently unsteady and three-dimensional, and because direct simulations resolve all turbulent scales, direct simulations are computationally intensive. Therefore, a well-designed numerical method will combine accurate resolution of a large range of length scales (or bandwidth), robust shock-capturing, and efficient implementation on

Vincent Gregory Weirs

1998-01-01

432

Numerical simulation of friction stir welding by natural element methods  

Microsoft Academic Search

In this work we address the problem of numerically simulating the Friction Stir Welding process. Due to the special characteristics\\u000a of this welding method (i.e., high speed of the rotating pin, very large deformations, etc.) finite element methods (FEM)\\u000a encounter several difficulties. While Lagrangian simulations suffer from mesh distortion, Eulerian or Arbitrary Lagrangian\\u000a Eulerian (ALE) ones still have difficulties due

I. Alfaro; G. Racineux; A. Poitou; E. Cueto; F. Chinesta

2009-01-01

433

Numerical simulation of dispersed gas-liquid flows  

Microsoft Academic Search

This paper reviews the available information on numerical simulation of dispersed gas-liquid flows. Emphasis is on informing\\u000a the reader about various aspects of constructing simulation models rather than giving an exhaustive literature review. The\\u000a information is organised in a way so as to provide answers to the following questions: how to formulate model equations? how\\u000a to select suitable algorithms and

V V Ranade

1992-01-01

434

A Priori Analysis of Subgrid-Scale Models for Large Eddy Simulations of Supercritical Binary-Species Mixing Layers  

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

435

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; Rss, Ludovic; Omlin, Samuel; Podladchikov, Yuri; Jaboyedoff, Michel

2014-05-01

436

Use of an Accurate DNS Particulate Flow Method to Supply and Validate Boundary Conditions for the MFIX Code  

SciTech Connect

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

437

3D DNS and LES of Breaking Inertia-Gravity Waves  

NASA Astrophysics Data System (ADS)

As inertia-gravity waves we refer to gravity waves that have a sufficiently low frequency and correspondingly large horizontal wavelength to be strongly influenced by the Coriolis force. Inertia-gravity waves are very active in the middle atmosphere and their breaking is potentially an important influence on the circulation in this region. The parametrization of this process requires a good theoretical understanding, which we want to enhance with the present study. Primary linear instabilities of an inertia-gravity wave and "2.5-dimensional" nonlinear simulations (where the spatial dependence is two dimensional but the velocity and vorticity fields are three-dimensional) with the wave perturbed by its leading primary instabilities by Achatz [1] have shown that the breaking differs significantly from that of high-frequency gravity waves due to the strongly sheared component of velocity perpendicular to the plane of wave-propagation. Fruman & Achatz [2] investigated the three-dimensionalization of the breaking by computing the secondary linear instabilities of the same waves using singular vector analysis. These secondary instabilities are variations perpendicular to the direction of the primary perturbation and the wave itself, and their wavelengths are an order of magnitude shorter than both. In continuation of this work, we carried out fully three-dimensional nonlinear simulations of inertia-gravity waves perturbed by their leading primary and secondary instabilities. The direct numerical simulation (DNS) was made tractable by restricting the domain size to the dominant scales selected by the linear analyses. The study includes both convectively stable and unstable waves. To the best of our knowledge, this is the first fully three-dimensional nonlinear direct numerical simulation of inertia-gravity waves at realistic Reynolds numbers with complete resolution of the smallest turbulence scales. Previous simulations either were restricted to high frequency gravity waves (e. g. Fritts et al. [3]), or the ratio N/f was artificially reduced (e. g. Lelong & Dunkerton [4]). The present simulations give us insight into the three-dimensional breaking process as well as the emerging turbulence. We assess the possibility of reducing the computational costs of three-dimensional simulations by using an implicit turbulence subgrid-scale parametrization based on the Adaptive Local Deconvolution Method (ALDM) for stratified turbulence [5]. In addition, we have performed ensembles of nonlinear 2.5-dimensional DNS, like those in Achatz [1] but with a small amount of noise superposed to the initial state, and compared the results with coarse-resolution simulations using either ALDM as well as with standard LES schemes. We found that the results of the models with parametrized turbulence, which are orders of magnitude more computationally economical than the DNS, compare favorably with the DNS in terms of the decay of the wave amplitude with time (the quantity most important for application to gravity-wave drag parametrization) suggesting that they may be trusted in future simulations of gravity wave breaking.

Remmler, S.; Fruman, M. D.; Hickel, S.; Achatz, U.

2012-04-01

438

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

SciTech Connect

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

439

Numerical Simulation of Polymer Injection in Turbulent Flow Past  

E-print Network

Numerical Simulation of Polymer Injection in Turbulent Flow Past a Circular Cylinder David Richter) dimensionless using the cylinder diameter D as the relevant length scale, the convective time U1=D as the time scale, and the total solution viscosity lT;0 at the injector location as the viscosity scale

Shaqfeh, Eric

440

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