M. Klein; N. Chakraborty; R. S. Cant
2008-01-01
The effects of mean flame radius and turbulence on self-sustained combustion of turbulent premixed spherical flames in decaying\\u000a turbulence have been investigated using three-dimensional direct numerical simulations (DNS) with single step Arrhenius chemistry.\\u000a Several flame kernels with different initial radius or initial turbulent field have been studied for identical conditions\\u000a of thermo-chemistry. It has been found that for very small
Takao Suzuki; Hui Ji; Fujio Yamamoto
2009-01-01
We develop a hybrid unsteady-flow simulation technique combining direct numerical simulation (DNS) and particle tracking velocimetry\\u000a (PTV) and demonstrate its capabilities by investigating flows past an airfoil. We rectify instantaneous PTV velocity fields\\u000a in a least-squares sense so that they satisfy the equation of continuity, and feed them to the DNS by equating the computational\\u000a time step with the frame
DNS and LES of some engineering flows
Wolfgang Rodi
2006-01-01
In this paper, direct numerical simulations (DNS) and large eddy simulations (LES) of three engineering flows carried out in the author's research group are presented. The first example, simulated both with DNS and LES, is the flow in a low-pressure turbine cascade with wakes passing periodically through the cascade channel. In this situation, the laminar–turbulent transition of the boundary layers
Detailed characteristics of drop-laden mixing layers: LES predictions compared to DNS
NASA Technical Reports Server (NTRS)
Okong'o, N.; Leboissetier, A.; Bellan, J.
2004-01-01
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.
Direct numerical simulations of spray spark ignition
Neophytou, Alexandre; Cant, Stewart R; Mastorakos, Epaminondas
2010-09-09
Direct Numerical Simulations (DNS) of a mono-disperse spray in a turbulent flow have been used to explore the nature of flame kernels when a spark is deposited in the spray. The simulations use complicated chemistry and are done with the code SENGA2...
Direct numerical simulation of incompressible turbulent flows
R Friedrich; T. J Hüttl; M Manhart; C Wagner
2001-01-01
The paper discusses recent achievements of direct numerical simulation (DNS) of incompressible flows. The various spatial discretization techniques which can be used in the case of simple or complex geometry are referred to, along with suitable time advancement schemes. The advantage of using a staggered variable arrangement and efficient Poisson solvers is stressed before initial and boundary conditions for inflow,
Xiaolin Zhong
1998-01-01
Direct numerical simulation (DNS) has become a powerful tool in studying fundamental phenomena of laminar-turbulent transition of high-speed boundary layers. Previous DNS studies of supersonic and hypersonic boundary layer transition have been limited to perfect-gas flow over flat-plate boundary layers without shock waves. For hypersonic boundary layers over realistic blunt bodies, DNS studies of transition need to consider the effects
E. Serre; P. Bontoux; R. Kotarba
2001-01-01
The purpose of this article is the direct numerical simulation (DNS) of the complex phenomena that precede the transition to turbulence inside a cavity subjected to rotation. The configurations of cylindrical cavities subjected to a radial throughflow or to a differential rotation of the walls are relevant to rotating machinery devices. At a high rotation rate, the DNS exhibits instability
Yukio Kaneda; Takashi Ishihara; Mitsuo Yokokawa; Ken'ichi Itakura; Atsuya Uno
2003-01-01
High-resolution direct numerical simulations (DNSs) of incompressible homogeneous turbulence in a periodic box with up to 40963 grid points were performed on the Earth Simulator computing system. DNS databases, including the present results, suggest that the normalized mean energy dissipation rate per unit mass tends to a constant, independent of the fluid kinematic viscosity nu as nu-->0. The DNS results
Direct numerical simulation of hot jets
NASA Technical Reports Server (NTRS)
Jacob, Marc C.
1993-01-01
The ultimate motivation of this work is to investigate the stability of two dimensional heated jets and its implications for aerodynamic sound generation from data obtained with direct numerical simulations (DNS). As pointed out in our last report, these flows undergo two types of instabilities, convective or absolute, depending on their temperature. We also described the limits of earlier experimental and theoretical studies and explained why a numerical investigation could give us new insight into the physics of these instabilities. The aeroacoustical interest of these flows was also underlined. In order to reach this goal, we first need to succeed in the DNS of heated jets. Our past efforts have been focused on this issue which encountered several difficulties. Our numerical difficulties are directly related to the physical problem we want to investigate since these absolutely or almost absolutely unstable flows are by definition very sensitive to the smallest disturbances and are very likely to reach nonlinear saturation through a numerical feedback mechanism. As a result, it is very difficult to compute a steady laminar solution using a spatial DNS. A steady state was reached only for strongly co-flowed jets, but these flows are almost equivalent to two independent mixing layers. Thus they are far from absolute instability and have much lower growth rates.
Prediction of dynamic and mixing characteristics of drop-laden mixing layers using DNS and LES
NASA Technical Reports Server (NTRS)
Okong'o, N.; Leboissetier, A.; Bellan, J.
2004-01-01
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.
NASA Astrophysics Data System (ADS)
Suzuki, Takao; Ji, Hui; Yamamoto, Fujio
2009-12-01
We develop a hybrid unsteady-flow simulation technique combining direct numerical simulation (DNS) and particle tracking velocimetry (PTV) and demonstrate its capabilities by investigating flows past an airfoil. We rectify instantaneous PTV velocity fields in a least-squares sense so that they satisfy the equation of continuity, and feed them to the DNS by equating the computational time step with the frame rate of the time-resolved PTV system. As a result, we can reconstruct unsteady velocity fields that satisfy the governing equations based on experimental data, with the resolution comparable to numerical simulation. In addition, unsteady pressure distribution can be solved simultaneously. In this study, particle velocities are acquired on a laser-light sheet in a water tunnel, and unsteady flow fields are reconstructed with the hybrid algorithm solving the incompressible Navier-Stokes equations in two dimensions. By performing the hybrid simulation, we investigate nominally two-dimensional flows past the NACA0012 airfoil at low Reynolds numbers. In part 1, we introduce the algorithm of the proposed technique and discuss the characteristics of hybrid velocity fields. In particular, we focus on a vortex shedding phenomenon under a deep stall condition (? = 15°) at Reynolds numbers of Re = 1000 and 1300, and compare the hybrid velocity fields with those computed with two-dimensional DNS. In part 2, the extension to higher Reynolds numbers is considered. The accuracy of the hybrid simulation is evaluated by comparing with independent experimental results at various angles of attack and Reynolds numbers up to Re = 104. The capabilities of the hybrid simulation are also compared with two-dimensional unsteady Reynolds-Averaged Navier-Stokes (URANS) solutions in part 2. In the first part of these twin papers, we demonstrate that the hybrid velocity field approaches the PTV velocity field over time. We find that intensive alternate vortex shedding past the airfoil, which is predicted by the two-dimensional DNS, is substantially suppressed in the hybrid simulation and the resultant flow field is similar to the PTV velocity field, which is projection of the three-dimensional velocity field on the streamwise plane. We attempt to identify the motion that originates three-dimensional flow patterns by highlighting the deviation of the PTV velocity field from the two-dimensional governing equations at each snapshot. The results indicate that the intensive spots of the deviation appear in the regions in which three-dimensional instabilities are induced in the shear layer separated from the pressure side.
A NOVEL ALGORITHM FOR DNS/LES OF COMPRESSIBLE TURBULENT FLOWS
Mahesh, Krishnan
A NOVEL ALGORITHM FOR DNS/LES OF COMPRESSIBLE TURBULENT FLOWS A DISSERTATION SUBMITTED is proposed for direct numerical and largeeddy simulation of compressible turbulent flows. The algorithm-scale. These properties make it well-suited for DNS/LES of compressible turbulent flows. A parallel, structured grid flow
Applications of direct numerical simulation of turbulence in second order closures
NASA Technical Reports Server (NTRS)
Shih, Tsan-Hsing; Lumley, John L.
1995-01-01
This paper discusses two methods of developing models for the rapid pressure-strain correlation term in the Reynolds stress transport equation using direct numerical simulation (DNS) data. One is a perturbation about isotropic turbulence, the other is a perturbation about two-component turbulence -- an extremely anisotropic turbulence. A model based on the latter method is proposed and is found to be very promising when compared with DNS data and other models.
Vitaly G. Soudakov; Ivan V. Egorov; Alexander V. Fedorov
2010-01-01
\\u000a Two-dimensional direct numerical simulation (DNS) of receptivity to acoustic disturbances radiating onto a flat plate with\\u000a a sharp leading edge in the Mach 6 free stream is carried out. Different angles of incidence of fast and slow acoustic waves\\u000a are considered. DNS results are compared with theoretical modeling of leading-edge receptivity and downstream propagation\\u000a of boundary-layer disturbances.
Direct Numerical Simulation of a Hypersonic Turbulent Boundary Layer on a Large Domain
Martín, Pino
Direct Numerical Simulation of a Hypersonic Turbulent Boundary Layer on a Large Domain Stephan Priebe , M. Pino Mart´in The direct numerical simulation (DNS) of a spatially-developing hypersonic There are few studies of hypersonic flows at Mach number greater than 5 and few involve the measurement of mean
Numerical Aerodynamic Simulation
NASA Technical Reports Server (NTRS)
1989-01-01
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.
PDF turbulence modeling and DNS
NASA Technical Reports Server (NTRS)
Hsu, A. T.
1992-01-01
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.
Ron Aitchison
\\u000a This chapter describes the operation of a DNS system. Namely, you’ll learn about the following topics:\\u000a \\u000a \\u000a \\u000a \\u000a • \\u000a \\u000a \\u000a DNS queries: How does your browser find www.example.com? How does your mail software know where to send your outgoing e-mail? Such operations\\u000a use DNS queries.\\u000a \\u000a \\u000a \\u000a \\u000a • \\u000a \\u000a \\u000a Reverse mapping: How does your mail software determine your identity? How do you find out who is
DNS of turbulent wall bounded flows with a passive scalar
Juan Guillermo Araya
2008-01-01
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)
Direct numerical simulation of leading edge receptivity to sound
H. L. Reed; D. A. Fuciarelli; I. J. Lyttle
1998-01-01
Numerical simulations of leading-edge acoustic receptivity are performed for a flat plate with a modified-super-elliptic (MSE) leading edge. For small freestream amplitude, the agreement between Branch I receptivity coefficients predicted from the DNS and the experiments of Saric and White (AIAA-98-2645, 1998) for acoustic waves at zero incidence is excellent. The effect of angle of incidence of the impinging wave
Haibo Dong
2003-01-01
Due to the progress in computer technology in recent years, distributed memory parallel computer systems are rapidly gaining importance in direct numerical simulation (DNS) of the stability and transition of compressible boundary layers. In most works, explicit methods have mainly been used in such simulations to advance the compressible Navier-Stokes equations in time. However, the small wall-normal grid sizes for
Ron Aitchison
\\u000a This chapter describes a number of common configurations when working with zone files and, in some cases, with BIND. These\\u000a solutions are presented to assist you in quickly implementing some commonly used features, recovering from errors, and illustrating\\u000a some of the more subtle uses of the DNS. The following topics are covered:\\u000a \\u000a \\u000a \\u000a \\u000a • \\u000a \\u000a \\u000a How to delegate a subdomain: This configuration
DNS of a Turbulent Boundary Layer with Surface Roughness
Yi Chen; James Cardillo; Guillermo Araya; Luciano Castillo; Kenneth Jansen
2010-01-01
A Direct numerical simulation (DNS) of a high Reynolds number, zero pressure gradient, turbulent boundary layer (Retheta= 2400) subjected to sandpaper surface roughness is performed. The surface roughness is modeled with a roughness parameter k^+ ˜ 25 to match the experiments at similar Reynolds number and roughness distribution. The employed computational method involves a synergy of the multi-scale dynamic approach
ACIDBASE TITRATION NUMERICAL SIMULATOR
Lorentz JÄNTSCHI; Delia GLIGOR; Mihaela Ligia
2003-01-01
The analytical methods of qualitative and quantitative determination of ions in solutions are very flexible to automation. The present work is focus on modeling the process of titration and presents a numerical simulation of acid-base titration. A PHP program to compute all iterations in titration process that solves a 3 rd rank equation to find value of pH for was
Numerical Propulsion System Simulation
NASA Technical Reports Server (NTRS)
Naiman, Cynthia
2006-01-01
The NASA Glenn Research Center, in partnership with the aerospace industry, other government agencies, and academia, is leading the effort to develop an advanced multidisciplinary analysis environment for aerospace propulsion systems called the Numerical Propulsion System Simulation (NPSS). NPSS is a framework for performing analysis of complex systems. The initial development of NPSS focused on the analysis and design of airbreathing aircraft engines, but the resulting NPSS framework may be applied to any system, for example: aerospace, rockets, hypersonics, power and propulsion, fuel cells, ground based power, and even human system modeling. NPSS provides increased flexibility for the user, which reduces the total development time and cost. It is currently being extended to support the NASA Aeronautics Research Mission Directorate Fundamental Aeronautics Program and the Advanced Virtual Engine Test Cell (AVETeC). NPSS focuses on the integration of multiple disciplines such as aerodynamics, structure, and heat transfer with numerical zooming on component codes. Zooming is the coupling of analyses at various levels of detail. NPSS development includes capabilities to facilitate collaborative engineering. The NPSS will provide improved tools to develop custom components and to use capability for zooming to higher fidelity codes, coupling to multidiscipline codes, transmitting secure data, and distributing simulations across different platforms. These powerful capabilities extend NPSS from a zero-dimensional simulation tool to a multi-fidelity, multidiscipline system-level simulation tool for the full development life cycle.
Direct numerical simulation of incompressible axisymmetric flows
NASA Astrophysics Data System (ADS)
Loulou, Patrick
1994-12-01
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.
An embedded upward flame spread model using 2D direct numerical simulations
Wei Xie; Paul E. DesJardin
2009-01-01
A fully coupled 2D fluid–solid direct numerical simulation (DNS) approach is used to simulate co-flow flame spread over poly(methyl methacrylate) (PMMA) at different angles of inclination. Comparison of simulations and experimental measurements are conducted over a range of flame spread rates. Results show that the heat flux to the preheating region varies considerably in time — contradicting often employed assumptions
Numerical Aerodynamic Simulation (NAS)
NASA Technical Reports Server (NTRS)
Peterson, V. L.; Ballhaus, W. F., Jr.; Bailey, F. R.
1983-01-01
The history of the Numerical Aerodynamic Simulation Program, which is designed to provide a leading-edge capability to computational aerodynamicists, is traced back to its origin in 1975. Factors motivating its development and examples of solutions to successively refined forms of the governing equations are presented. The NAS Processing System Network and each of its eight subsystems are described in terms of function and initial performance goals. A proposed usage allocation policy is discussed and some initial problems being readied for solution on the NAS system are identified.
Interpolation error in DNS simulations of turbulence: consequences for particle tracking
NASA Astrophysics Data System (ADS)
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
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.
Direct numerical simulation of downshift and inverse cascade for water wave turbulence.
Annenkov, S Y; Shrira, V I
2006-05-26
By means of direct numerical simulations (DNS) based on the integrodifferential Zakharov equation, we study the long-term evolution of nonlinear random water wave fields. For the first time, formation of powerlike Kolmogorov-type spectra corresponding to weak-turbulent inverse cascade is demonstrated by DNS, and the evolution in time of the resulting spectra is quantitatively investigated. The predictions of the statistical theory for water waves, both qualitative (formation of the direct and inverse cascades, self-similar behavior) and quantitative (the spectra exponents, specific shape of self-similar functions, the rate of time evolution) are found to be in good agreement with the DNS results, except for the initial part of the evolution, where the established statistical theory is not applicable yet and the evolution has a much faster time scale. PMID:16803176
Numerical simulations in turbomachinery
Hamed, A.A. (Cincinnati Univ., OH (United States))
1991-01-01
The continued increase in the complexity of modern turbomachinery designs dictates greater reliance computer simulations to analyze their flow fields and hardware. Turbomachinery flows are complicated by three dimensional effects, viscous-inviscid interactions, complex shock structures in transonic machines, and the injected coolant flows in gas turbines. It is important to be able to capture the complex flow structures associated with these phenomena, and with the three dimensional boundary layers, blade row interactions and tip clearance at both design and off-design conditions. With the advent of powerful computers, and the development of more efficient numerical codes, it is now possible in principal to model unsteady three dimensional flow fields of the type encountered in the successive turbomachinery blade passages of advanced configurations. However, as the experimental data base expands through improved measuring techniques, data processing and acquisition to include new, better and more detailed flow measurements, it is now possible to devote more effort to the physical modeling for improved turbomachinery performance prediction. In particular, the fidelity and reliability of the blade surface heat transfer and viscous and shock losses can be enhanced through such effort. As more rigorous and accurate models develop, they will enhance the design efforts through minimizing the required testing effort. The purpose of this symposium is to provide a forum for the dissemination of the recent turbomachinery flow simulations. The 17 papers in this volume cover a wide spectrum of turbomachinery applications involving incompressible, transonic, single and multiphase flows in axial and centrifugal machines. The presented methodologies also vary widely and range from design and optimization methods to three dimensional flow simulations.
A fast direct numerical simulation method for characterising hydraulic roughness
Chung, Daniel; MacDonald, Michael; Hutchins, Nicholas; Ooi, Andrew
2015-01-01
We describe a fast direct numerical simulation (DNS) method that promises to directly characterise the hydraulic roughness of any given rough surface, from the hydraulically smooth to the fully rough regime. The method circumvents the unfavourable computational cost associated with simulating high-Reynolds-number flows by employing minimal-span channels (Jimenez & Moin 1991). Proof-of-concept simulations demonstrate that flows in minimal-span channels are sufficient for capturing the downward velocity shift, that is, the Hama roughness function, predicted by flows in full-span channels. We consider two sets of simulations, first with modelled roughness imposed by body forces, and second with explicit roughness described by roughness-conforming grids. Owing to the minimal cost, we are able to conduct DNSs with increasing roughness Reynolds numbers while maintaining a fixed blockage ratio, as is typical in full-scale applications. The present method promises a practical, fast and accurate tool for character...
Direct numerical simulation of shockwave and turbulent boundary layer interactions
NASA Astrophysics Data System (ADS)
Wu, Minwei
Direct numerical simulations (DNS) of a shockwave/turbulent boundary layer interaction (STBLI) at Mach number 3 and Reynolds number based on the momentum thickness of 2300 are performed. A 4th-order accurate, bandwidth-optimized weighted-essentially-non-oscillatory (WENO) scheme is used and the method is found to be too dissipative for the STBLI simulation due to the over-adaptation properties of this original WENO scheme. In turn, a relative limiter is introduced to mitigate the problem. Tests on the Shu-Osher problem show that the modified WENO scheme decreases the numerical dissipation significantly. By utilizing a combination of the relative limiter and the absolute limiter described by Jiang & Shu [32], the DNS results are improved further. The DNS data agree well with the reference experiments of Bookey et al. [10] in the size of the separation bubble, the separation and reattachment point, the mean wall-pressure distribution, and the velocity profiles both upstream and downstream of the interaction region. The DNS data show that velocity profiles change dramatically along the streamwise direction. Downstream of the interaction, the velocity profiles show a characteristic "dip" in the logarithmic region, as shown by the experiments of Smits & Muck [66] at much higher Reynolds number. In the separation region, the velocity profiles are found to resemble those of a laminar flow, yet the flow does not fully relaminarize. The mass-flux turbulence intensity is amplified by a factor of about 5 throughout the interaction, which is consistent with that found in higher Reynolds experiments of Selig et al. [52]. All Reynolds stress components are greatly amplified by the interaction. Assuming that the ow is still two dimensional downstream of the interaction, the components rhou'u', rhov'v', rho w'w', and rho u'w' are amplified by factors of 6, 6, 12, and 24, respectively, where u is the streamwise and w is the wall-normal velocity. However, analyses of the turbulence structure show that the ow is not uniform in the spanwise direction downstream of the interaction. A pair of counter-rotating vortices is observed in streamwise-wall-normal planes in the mean ow downstream of the ramp corner. Taking the three-dimensionality into account, the amplification factors of the Reynolds stresses are greatly decreased. The component rhou'w' is amplified by a factor of about 10, which is comparable to that found in the experiments of Smits & Muck [66]. Strong Reynolds analogy (SRA) relations are also studied using the DNS data. The SRA is found to hold in the incoming boundary layer of the DNS. However, inside and downstream of the interaction region, the SRA relations are not satisfied. From the DNS analyses, the shock motion is characterized by a low frequency component (of order 0.01Uinfinity/delta). In addition, the motion of the shock is found to have two aspects: a spanwise wrinkling motion and a streamwise oscillatory motion. The spanwise wrinkling is observed to be a local feature with high frequencies (of order Uinfinity /delta). Two-point correlations reveal that the spanwise wrinkling is closely related to the low momentum motions in the incoming boundary layer as they convect through the shock. The low frequency shock motion is found to be a streamwise oscillation motion. Conditional statistics show that there is no significant difference in the mean properties of the incoming boundary layer when the shock is at an upstream or downstream location. However, analyses of the unsteadiness of the separation bubble reveal that the low frequency shock motion is driven by the downstream flow.
Direct numerical simulation of stagnation region flow and heat transfer with free-stream turbulence
Sungwon Bae; Sanjiva K. Lele; Hyung Jin Sung
2003-01-01
A direct numerical simulation is performed for stagnation-region flow with free-stream turbulence. A fully implicit second-order time-advancement scheme with fourth-order finite differences and an optimized scheme are employed. The optimized scheme is developed to save computational cost. The free-stream turbulence is a precomputed field of isotropic turbulence. The present DNS results in the ``damping'' and ``attached amplifying'' regimes are found
Direct numerical simulation of compressible homogeneous turbulence using natural initial conditions
NASA Astrophysics Data System (ADS)
Bhutoria, Vaibhav
Reynolds averaged Navier Stokes (RANS) solvers have become the workhorse for simulating turbulent flows for most practical purposes. While the incompressible turbulence models used with RANS equations have improved considerably in their predictive capability, significant breakthrough has not been achieved for their compressible counterparts. With the advancement in computing power, high resolution direct numerical simulation (DNS) of low Reynolds number turbulent flows has become feasible. DNS of simple turbulent flows provides a detailed database which can be used for developing and testing turbulence models. In this work, we perform DNS of compressible homogeneous turbulence---decaying isotropic turbulence and homogeneous shear flow---for a range of initial turbulent Mach numbers, (Mt 0 = 0.05--0.4) using the more natural initial conditions. Simulations were performed on grids with 1283 and 2563 points. Compressibility effects on the evolution of turbulent kinetic energy were studied. We found negligible compressibility effects for decaying isotropic turbulence, while homogeneous shear flow demonstrated compressibility effects in the growth rate of turbulent kinetic energy. Compressibility corrections to turbulence models in the form of the ratio epsilond/epsilon s, have been tested with the results from the simulations. For decaying isotropic turbulence a M4t scaling was found to be better than M2t while for homogeneous shear flow it was the opposite. The small value of the ratio epsilond/epsilons in decaying isotropic turbulence makes the M4t scaling less relevant. Based on the DNS results of homogeneous shear flow, a new correction parameterized by the gradient Mach number, Mg, is proposed. The parameter Cmu, which is assumed constant for incompressible two equation eddy viscosity models, is computed explicitly from the DNS data. An Mg, dependence of the parameter, Cmu, is proposed.
Direct numerical simulations of temporally developing turbulent reacting liquid-fueled jets
NASA Astrophysics Data System (ADS)
Shashank, Shashank; Pitsch, Heinz
2012-11-01
Liquid fueled engines are ubiquitous in the transportation industry because liquid fuel minimizes the weight and volume of propulsion systems. The combustion that occurs in these engines is an inherently multi-physics process, involving fuel evaporation, reaction kinetics, and high levels of turbulence. A desire for high fidelity data that explains complex interaction between different physical mechanisms motivates the consideration of direct numerical simulation (DNS) as an investigation tool. In this study three-dimensional DNS of a reacting n-heptane liquid fueled temporal jet have been performed to study auto-ignition and subsequent burning in conditions that are representative of a diesel engine environment. In these simulations the continuous phase is described using an Eulerian representation whereas Lagrangian particle tracking is used to model the dispersed phase. The results of this study will demonstrate the importance of unsteady effects, and of accounting for the interaction between different modes of combustion, when simulating spray combustion.
Numerical simulations of transonic wingtip
NASA Astrophysics Data System (ADS)
Chanrith, Suparat
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.
DNS of heat transfer in a high Reynolds number turbulent channel flow with local forcing
Guillermo Araya; Stefano Leonardi; Luciano Castillo
Direct Numerical Simulations (DNS) of the velocity\\/temperature fields at a Reynolds number of 10400 (based on the centerline velocity) in a turbulent channel flow, with periodic normal blowing\\/suction velocity disturbances located cyclically along the channel at both walls, are presented. In this opportunity, we include the analysis of the local forcing influence on the turbulent heat transfer as a continuation
C. David Pruett; Chau-Lyan Chang
1995-01-01
The laminar breakdown of the boundary-layer flow of an axisymmetric sharp cone in a Mach 8 flow is simulated by a synergistic approach that combines the parabolized stability equation (PSE) method and spatial direct numerical simulation (DNS). The transitional state is triggered by a symmetric pair of oblique second-mode disturbances whose nonlinear interactions generate strong streamwise vorticity, which leads in
Numerical Simulation of Gravity Waves
W. Craig; C. Sulem
1993-01-01
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
L. Duan; M. Pino Mart
This paper discusses the constitutive relations, surface catalytic model, and numerical methods for direct numerical simulation (DNS) of wall- bounded turbulence including nite-rate chemistry and gas-surface in- teraction. The validation of the code is provided for the mean o w by comparison to similarity solutions and laminar hypersonic boundary layer solutions, for the surface catalysis by comparison to analytic solutions
High-accuracy DNS of supersonic base flows and control of the near wake
Richard D. Sandberg; Hermann F. Fasel
2004-01-01
Large-scale numerical simulations of axisymmetric, supersonic base flows were conducted at various Reynolds numbers. Direct numerical simulations (DNS) were employed to investigate the hydrodynamic stability behavior of the near-wake region. As a consequence of physical flow instabilities, large coherent structures evolve that have a significant impact on the mean flow wand and are responsible for a considerable amount of base-drag.
NASA Technical Reports Server (NTRS)
Joslin, R. D.; Streett, C. L.; Chang, C.-L.
1991-01-01
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.
Inertial particles in a shearless mixing layer: direct numerical simulations
NASA Astrophysics Data System (ADS)
Ireland, Peter; Collins, Lance
2010-11-01
Entrainment, the drawing in of external fluid by a turbulent flow, is present in nearly all turbulent processes, from exhaust plumes to oceanic thermoclines to cumulus clouds. While the entrainment of fluid and of passive scalars in turbulent flows has been studied extensively, comparatively little research has been undertaken on inertial particle entrainment. We explore entrainment of inertial particles in a shearless mixing layer across a turbulent-non-turbulent interface (TNI) and a turbulent-turbulent interface (TTI) through direct numerical simulation (DNS). Particles are initially placed on one side of the interface and are advanced in time in decaying turbulence. Our results show that the TTI is more efficient in mixing droplets than the TNI. We also find that without the influence of gravity, over the range of Stokes numbers present in cumulus clouds, particle concentration statistics are essentially independent of the dissipation scale Stokes number. The DNS data agrees with results from experiments performed in a wind tunnel with close parametric overlap. We anticipate that a better understanding of the role of gravity and turbulence in inertial particle entrainment will lead to improved cloud evolution predictions and more accurate climate models. Sponsored by the U.S. NSF.
NASA Astrophysics Data System (ADS)
Hildings, Casper; Henningson, Dan
1996-11-01
Separation bubbles in laminar flow over a flat plate have been investigated using a modified version of the spectral DNS-code developed at FFA/KTH. By prescribing the velocity at the free-stream boundary, a varying pressure gradient was introduced, sufficiently strong to form a separation bubble. The numerical method chosen uses the so called ``fringe region'' technique to damp outflowing disturbances and return the flow to a prescribed inlet state. A detailed investigation of the efficiency of the disturbance damping has been made, since separation bubbles amplify disturbances several orders of magnitude. If these disturbances are not adequately damped they may reenter at the inflow and corrupt the computation. Guidelines to find the most efficient fringe are presented. Initially the 2D separation bubble investigated by Rist and Maucher (Rist, U., Maucher, U. AGARD-Symposium, 1994, Chania, Crete.) was calculated in order to verify the code. The characteristics of this bubble, including length and height as well as the growth rate of small disturbances compare well with those of Rist and Maucher. Presently a corresponding experimental study of separation bubbles is made at KTH. Comparisons between the ongoing numerical work and the experiments will be presented.
Requirements definition by numerical simulation
NASA Astrophysics Data System (ADS)
Hickman, James J.; Kostas, Chris; Tsang, Kang T.
1994-10-01
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.
Linear analysis and temporal DNS of compressible mixing layers
NASA Astrophysics Data System (ADS)
Karimi, Mona; Girimaji, Sharath
2012-11-01
We perform linear analysis and temporal direct numerical simulations (DNS) of high-speed compressible mixing layers. The DNS solver is based on Gas Kinetic Method (GKM) and has been validated in a variety of high-speed shear flows and against rapid distortion theory results. The objective is to examine the effect of compressibility on Kelvin-Helmholtz instability. We perform modal and statistical analysis. The difference in the behavior of two-dimensional and oblique modes is investigated. The action of pressure on 2D and oblique modes is differentiated. The stabilizing influence of compressibility on mixing layer growth (quantified by the so-called Langley Curve) in investigated from fundamental principles.
Terascale direct numerical simulations of turbulent combustion using S3D
NASA Astrophysics Data System (ADS)
Chen, J. H.; Choudhary, A.; de Supinski, B.; DeVries, M.; Hawkes, E. R.; Klasky, S.; Liao, W. K.; Ma, K. L.; Mellor-Crummey, J.; Podhorszki, N.; Sankaran, R.; Shende, S.; Yoo, C. S.
2009-01-01
Computational science is paramount to the understanding of underlying processes in internal combustion engines of the future that will utilize non-petroleum-based alternative fuels, including carbon-neutral biofuels, and burn in new combustion regimes that will attain high efficiency while minimizing emissions of particulates and nitrogen oxides. Next-generation engines will likely operate at higher pressures, with greater amounts of dilution and utilize alternative fuels that exhibit a wide range of chemical and physical properties. Therefore, there is a significant role for high-fidelity simulations, direct numerical simulations (DNS), specifically designed to capture key turbulence-chemistry interactions in these relatively uncharted combustion regimes, and in particular, that can discriminate the effects of differences in fuel properties. In DNS, all of the relevant turbulence and flame scales are resolved numerically using high-order accurate numerical algorithms. As a consequence terascale DNS are computationally intensive, require massive amounts of computing power and generate tens of terabytes of data. Recent results from terascale DNS of turbulent flames are presented here, illustrating its role in elucidating flame stabilization mechanisms in a lifted turbulent hydrogen/air jet flame in a hot air coflow, and the flame structure of a fuel-lean turbulent premixed jet flame. Computing at this scale requires close collaborations between computer and combustion scientists to provide optimized scaleable algorithms and software for terascale simulations, efficient collective parallel I/O, tools for volume visualization of multiscale, multivariate data and automating the combustion workflow. The enabling computer science, applied to combustion science, is also required in many other terascale physics and engineering simulations. In particular, performance monitoring is used to identify the performance of key kernels in the DNS code, S3D and especially memory intensive loops in the code. Through the careful application of loop transformations, data reuse in cache is exploited thereby reducing memory bandwidth needs, and hence, improving S3D's nodal performance. To enhance collective parallel I/O in S3D, an MPI-I/O caching design is used to construct a two-stage write-behind method for improving the performance of write-only operations. The simulations generate tens of terabytes of data requiring analysis. Interactive exploration of the simulation data is enabled by multivariate time-varying volume visualization. The visualization highlights spatial and temporal correlations between multiple reactive scalar fields using an intuitive user interface based on parallel coordinates and time histogram. Finally, an automated combustion workflow is designed using Kepler to manage large-scale data movement, data morphing, and archival and to provide a graphical display of run-time diagnostics.
High-resolution numerical simulation of turbulence in natural waterways
NASA Astrophysics Data System (ADS)
Kang, Seokkoo; Lightbody, Anne; Hill, Craig; Sotiropoulos, Fotis
2011-01-01
We develop an efficient and versatile numerical model for carrying out high-resolution simulations of turbulent flows in natural meandering streams with arbitrarily complex bathymetry. The numerical model solves the 3D, unsteady, incompressible Navier-Stokes and continuity equations in generalized curvilinear coordinates. The method can handle the arbitrary geometrical complexity of natural streams using the sharp-interface curvilinear immersed boundary (CURVIB) method of Ge and Sotiropoulos (2007) [1]. The governing equations are discretized with three-point, central, second-order accurate finite-difference formulas and integrated in time using an efficient, second-order accurate fractional step method. To enable efficient simulations on grids with tens of millions of grid nodes in long and shallow domains typical of natural streams, the algebraic multigrid (AMG) method is used to solve the Poisson equation for the pressure coupled with a matrix-free Krylov solver for the momentum equations. Depending on the desired level of resolution and available computational resources, the numerical model can either simulate, via direct numerical simulation (DNS), large-eddy simulation (LES), or unsteady Reynolds-averaged Navier-Stokes (URANS) modeling. The potential of the model as a powerful tool for simulating energetic coherent structures in turbulent flows in natural river reaches is demonstrated by applying it to carry out LES and URANS in a 50-m long natural meandering stream at resolution sufficiently fine to capture vortex shedding from centimeter-scale roughness elements on the bed. The accuracy of the simulations is demonstrated by comparisons with experimental data and the relative performance of the LES and URANS models is also discussed.
Detailed Comparison of DNS to PSE for Oblique Breakdown at Mach 3
NASA Technical Reports Server (NTRS)
Mayer, Christian S. J.; Fasel, Hermann F.; Choudhari, Meelan; Chang, Chau-Lyan
2010-01-01
A pair of oblique waves at low amplitudes is introduced in a supersonic flat-plate boundary layer. Their downstream development and the concomitant process of laminar to turbulent transition is then investigated numerically using Direct Numerical Simulations (DNS) and Parabolized Stability Equations (PSE). This abstract is the last part of an extensive study of the complete transition process initiated by oblique breakdown at Mach 3. In contrast to the previous simulations, the symmetry condition in the spanwise direction is removed for the simulation presented in this abstract. By removing the symmetry condition, we are able to confirm that the flow is indeed symmetric over the entire computational domain. Asymmetric modes grow in the streamwise direction but reach only small amplitude values at the outflow. Furthermore, this abstract discusses new time-averaged data from our previous simulation CASE 3 and compares PSE data obtained from NASA's LASTRAC code to DNS results.
? Yilmaz; L Davidson; F O Edis; H Saygin
2011-01-01
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
NASA Astrophysics Data System (ADS)
Yilmaz, ?.; Davidson, L.; Edis, F. O.; Saygin, H.
2011-12-01
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.
Xiaolin Zhong
1997-01-01
The receptivity of hypersonic boundary layers to freestream disturbances is altered considerably by the presence of the bow shocks. This paper studies, by direct numerical simulation (DNS), the receptivity of a hypersonic boundary layer to 2-D and 3-D freestream acoustic disturbances for a Mach 15 flow over a parabolic leading edge. The unsteady flow fields between the bow shock and
DepenDNS: Dependable Mechanism against DNS Cache Poisoning
NASA Astrophysics Data System (ADS)
Sun, Hung-Min; Chang, Wen-Hsuan; Chang, Shih-Ying; Lin, Yue-Hsun
DNS cache poisoning attacks have been proposed for a long time. In 2008, Kaminsky enhanced the attacks to be powerful based on nonce query method. By leveraging Kaminsky's attack, phishing becomes large-scale since victims are hard to detect attacks. Hence, DNS cache poisoning is a serious threat in the current DNS infrastructure. In this paper, we propose a countermeasure, DepenDNS, to prevent from cache poisoning attacks. DepenDNS queries multiple resolvers concurrently to verify an trustworthy answer while users perform payment transactions, e.g., auction, banking. Without modifying any resolver or authority server, DepenDNS is conveniently deployed on client side. In the end of paper, we conduct several experiments on DepenDNS to show its efficiency. We believe DepenDNS is a comprehensive solution against cache poisoning attacks.
Numerical Simulation of Black Holes
NASA Astrophysics Data System (ADS)
Teukolsky, Saul
2003-04-01
Einstein's equations of general relativity are prime candidates for numerical solution on supercomputers. There is some urgency in being able to carry out such simulations: Large-scale gravitational wave detectors are now coming on line, and the most important expected signals cannot be predicted except numerically. Problems involving black holes are perhaps the most interesting, yet also particularly challenging computationally. One difficulty is that inside a black hole there is a physical singularity that cannot be part of the computational domain. A second difficulty is the disparity in length scales between the size of the black hole and the wavelength of the gravitational radiation emitted. A third difficulty is that all existing methods of evolving black holes in three spatial dimensions are plagued by instabilities that prohibit long-term evolution. I will describe the ideas that are being introduced in numerical relativity to deal with these problems, and discuss the results of recent calculations of black hole collisions.
Nilanjan Chakraborty; E. Mastorakos
2008-01-01
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,
Simulating Reionization in Numerical Cosmology
Aaron Sokasian; Tom Abel; Lars E. Hernquist
2001-05-10
The incorporation of radiative transfer effects into cosmological hydrodynamical simulations is essential for understanding how the intergalactic medium (IGM) makes the transition from a neutral medium to one that is almost fully ionized. Here, we present an approximate numerical method designed to study in a statistical sense how a cosmological density field is ionized by a set of discrete point sources. A diffuse background radiation field is also computed self-consistently in our procedure. The method requires relatively few time steps and can be employed with simulations having high resolution. We describe the details of the algorithm and provide a description of how the method can be applied to the output from a pre-existing cosmological simulation to study the systematic reionization of a particular ionic species. As a first application, we compute the reionization of He II by quasars in the redshift range 3 to 6.
A fast direct numerical simulation method for characterising hydraulic roughness
NASA Astrophysics Data System (ADS)
Chung, D.; Chan, L.; MacDonald, M.; Hutchins, N.; Ooi, A.
2015-06-01
We describe a fast direct numerical simulation (DNS) method that promises to directly characterise the hydraulic roughness of any given rough surface, from the hydraulically smooth to the fully rough regime. The method circumvents the unfavourable computational cost associated with simulating high-Reynolds-number flows by employing minimal-span channels (Jimenez & Moin 1991). Proof-of-concept simulations demonstrate that flows in minimal-span channels are sufficient for capturing the downward velocity shift, that is, the Hama roughness function, predicted by flows in full-span channels. We consider two sets of simulations, first with modelled roughness imposed by body forces, and second with explicit roughness described by roughness-conforming grids. Owing to the minimal cost, we are able to conduct DNSs with increasing roughness Reynolds numbers while maintaining a fixed blockage ratio, as is typical in full-scale applications. The present method promises a practical, fast and accurate tool for characterising hydraulic resistance directly from profilometry data of rough surfaces.
Autoignition of hydrogen and air using direct numerical simulation
NASA Astrophysics Data System (ADS)
Doom, Jeffrey; Mahesh, Krishnan
2008-11-01
Direct numerical simulation (DNS) is used to study to auto--ignition in laminar vortex rings and turbulent diffusion flames. A novel, all--Mach number algorithm developed by Doom et al (J. Comput. Phys. 2007) is used. The chemical mechanism is a nine species, nineteen reaction mechanism for H2 and Air from Mueller at el (Int. J. Chem. Kinet. 1999). The vortex ring simulations inject diluted H2 at ambient temperature into hot air, and study the effects of stroke ratio, air to fuel ratio and Lewis number. At smaller stroke ratios, ignition occurs in the wake of the vortex ring and propagates into the vortex core. At larger stroke ratios, ignition occurs along the edges of the trailing column before propagating towards the vortex core. The turbulent diffusion flame simulations are three--dimensional and consider the interaction of initially isotropic turbulence with an unstrained diffusion flame. The simulations examine the nature of distinct ignition kernels, the relative roles of chemical reactions, and the relation between the observed behavior and laminar flames and the perfectly stirred reactor problem. These results will be discussed.
Numerical simulation of orographic rainbands
NASA Astrophysics Data System (ADS)
Anquetin, Sandrine; Minsicloux, Fabien; Creutin, Jean-Dominique; Cosma, StéPhanie
2003-04-01
This study, based on a statistical analysis of simulated warm rain event and radar data, aims at highlighting the main physical mechanisms that lead to organize shallow convection on the relief. The region of investigation, the Cévennes-Vivarais, is located in the southeast part of France. Radar images from the Cévennes experiment (fall 1986-1988) reveal a characteristic and repetitive structure of the rain distribution organized in narrow bands or plumes, oriented south-north in the case of stationary southerly Mediterranean flow. The event of 14 November 1986 has been selected and constitutes the data set of this numerical study. This work is closely associated with the previous work by [2001] which presents in detail the results of a geostatistical analysis of the radar data set extracted from the Cévennes experiment data base. The main results highlight the physical characteristics and the dynamics of the rain patterns. Following the recent work of [2002], high-resolution (? = 1 km) simulations have been continued with the nonhydrostatic three-dimensional (3D) atmospheric model MesoNH, in order to reproduce the observed rainbands over the Cévennes region. The numerical model correctly reproduces the structure and the dynamics of the rainbands. The geostatistical analysis has been applied for the simulated rain fields. The model slightly overestimates the northward advection velocity of the rain cells within the bands (75 km h-1 against 60 km h-1 for the observation), and the simulated rainbands are narrower and more organized around the N180° direction than the observed rain field. The comparison allows the qualification and validation of the choice of the numerical methodology and realism of the physical parameterizations. The analysis of the 3D simulated fields confirms the physical mechanisms responsible for the rain organization demonstrated by [2002] through idealized simulations. The statistical analysis highlights the presence of mean topographic features under low-level convergence zones composed of a succession of ridges and penetrating valleys orientated east-west. The rainbands are generated upstream of these topographic features and enhanced on the leeside due to the convergence created by the flow deflection around the obstacle and its penetration into the valleys. The simulated triggering takes place further to the south than the observed one, and the triggering is active as soon as the relief is suitably described in the model.
Wallace, J.M.; Bernard, P.S.; Balint, J.L.; Ong, L.
1992-01-01
Laboratory experiments and direct numerical simulations (DNS) of passive scalar contaminant dispersal in bounded shear flows have been carrried out. Both mass and heat transport have been experimentally studied. Statistical results for the temperature plume which develops from a line heat source at the wall are compared to the DNS results. The DNS results for this case and for the case of a uniform source with constant temperature boundaries are also compared to various model predictions.
Wallace, J.M.; Bernard, P.S.; Balint, J.L.; Ong, L.
1992-12-31
Laboratory experiments and direct numerical simulations (DNS) of passive scalar contaminant dispersal in bounded shear flows have been carrried out. Both mass and heat transport have been experimentally studied. Statistical results for the temperature plume which develops from a line heat source at the wall are compared to the DNS results. The DNS results for this case and for the case of a uniform source with constant temperature boundaries are also compared to various model predictions.
Direct Numerical Simulation of Transition in a Swept-Wing Boundary Layer
NASA Technical Reports Server (NTRS)
Duan, Lian; Choudhari, Meelan M.; Li, Fei
2013-01-01
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.
DNS of a passive scalar in a turbulent channel with local forcing at walls
Guillermo Araya; Stefano Leonardi; Luciano Castillo; Paolo Orlandi
2006-01-01
Direct Numerical Simulations (DNS) of the velocity and thermal fields in a fully developed turbulent channel, with normal periodic blowing\\/suction velocity disturbances at both walls, are presented. The governing equations have been discretized in an orthogonal coordinate system using a staggered central second-order finite-difference approximation. Results at low Reynolds number show a peak drag reduction of 60 percent and an
DNS of bifurcations in an air-filled rotating baroclinic annulus
Anthony Randriamampianina; Wolf-Gerrit Fruh; Peter L. Read; Pierre Maubert
2006-01-01
Three-dimensional Direct Numerical Simulation (DNS) on the nonlinear dynamics\\u000aand a route to chaos in a rotating fluid subjected to lateral heating is\\u000apresented here and discussed in the context of laboratory experiments in the\\u000abaroclinic annulus. Following two previous preliminary studies by Maubert and\\u000aRandriamampianina, the fluid used is air rather than a liquid as used in all\\u000aother
Liquid falling films: linear stability and direct numerical simulation
NASA Astrophysics Data System (ADS)
Schmidt, Patrick; O'Naraigh, Lennon; Valluri, Prashant; Lucquiaud, Mathieu
2013-11-01
Interfacial instability of falling liquid films in counter-current contact with a turbulent gas phase is investigated by means of an Orr-Sommerfeld analysis. This study is complemented by a full energy budget analysis, identifying the key mechanisms of the instability. This gives first insight into the dynamic behaviour of the two-phase system, which is relevant for a wide range of technical applications, such as absorption and distillation. The linear stability analysis is also used to identify the operating limits of a counter-current operation i.e. the so-called loading and flooding limits. In addition, the results of this analysis are benchmark for direct numerical simulations using the newly launched Two-Phase Level Set (http://sourceforge.net/projects/tpls/) solver. High resolution DNS is used to obtain detailed knowledge of important mechanisms at play, especially with regard to interfacial instability and transient system behaviour, which can help to design more efficient mass transfer equipment such as structured packings. Interfacial instability of falling liquid films in counter-current contact with a turbulent gas phase is investigated by means of an Orr-Sommerfeld analysis. This study is complemented by a full energy budget analysis, identifying the key mechanisms of the instability. This gives first insight into the dynamic behaviour of the two-phase system, which is relevant for a wide range of technical applications, such as absorption and distillation. The linear stability analysis is also used to identify the operating limits of a counter-current operation i.e. the so-called loading and flooding limits. In addition, the results of this analysis are benchmark for direct numerical simulations using the newly launched Two-Phase Level Set (http://sourceforge.net/projects/tpls/) solver. High resolution DNS is used to obtain detailed knowledge of important mechanisms at play, especially with regard to interfacial instability and transient system behaviour, which can help to design more efficient mass transfer equipment such as structured packings. Sulzer Chemtech Ltd, EPSRC, Energy Technology Partnership.
Numerical simulations of non-homogeneous viscoelastic turbulent channel flow
NASA Astrophysics Data System (ADS)
Housiadas, Kostas; Beris, Antony
2004-11-01
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.
Numerical simulation of glottal flow.
Hundertmark-Zaušková, A; Lehmann, R; Hess, M; Müller, F
2013-12-01
In cases of permanent immobility of both vocal folds patients have difficulties with breathing but rarely with voicing. However, clinical experience shows that the shape of the larynx (voice box) seems to have a significant influence on the degree of airflow and breathing pattern. In order to find an optimal geometry of the larynx in terms of easiness for breathing after the surgical change of vocal folds or false vocal cords (ventricular folds), a set of numerical simulations of glottal flow for weakly compressible Navier-Stokes equations has been performed. We compare airflow resistance and volumetric flow rate for several geometry concepts for inspiration as well as expiration. Finally, we discuss the optimal geometry with respect to the quality of breathing. PMID:24290934
M. Jangi; R. Yu; X. S. Bai
2012-01-01
A direct numerical simulation (DNS) coupling with multi-zone chemistry mapping (MZCM) is presented to simulate flame propagation and auto-ignition in premixed fuel\\/air mixtures. In the MZCM approach, the physical domain is mapped into a low-dimensional phase domain with a few thermodynamic variables as the independent variables. The approach is based on the fractional step method, in which the flow and
M. Jangi; R. Yu; X. S. Bai
2011-01-01
A direct numerical simulation (DNS) coupling with multi-zone chemistry mapping (MZCM) is presented to simulate flame propagation and auto-ignition in premixed fuel\\/air mixtures. In the MZCM approach, the physical domain is mapped into a low-dimensional phase domain with a few thermodynamic variables as the independent variables. The approach is based on the fractional step method, in which the flow and
Direct numerical simulations of curvature effects on shear layer transition over airfoils
NASA Astrophysics Data System (ADS)
Zhang, Wei; Cheng, Wan; Qamar, Adnan; Gao, Wei; Samtaney, Ravi
2013-11-01
Shear layer transition and subsequent turbulent flow development over the leeward section of airfoils are affected by the surface curvature in terms of its associated effects, such as laminar flow separation, adverse pressure gradient, and the interactions between separated flow and wake vortices, etc. We present direct numerical simulations (DNS) of shear layer transitions over two airfoils, NACA 4412 and NACA 0012-64, at 10 deg. angle of attack, and Rec = 104 based on uniform inflow velocity and chord length. The two airfoils chosen are geometrically almost the same with identical maximum thickness along with chordwise position but different cambers and hence different curvature. The curvature effects on the flow are presented by the unsteady evolution patterns of laminar flow separation; shear layer detachment, breakdown to turbulence, turbulent boundary layer reattachment and vortex shedding, and quantitative results on the development of turbulent boundary layer are emphasized. This DNS database is generated with an energy conservative fourth-order incompressible Navier-Stokes code with O(109) mesh points. Shear layer transition and subsequent turbulent flow development over the leeward section of airfoils are affected by the surface curvature in terms of its associated effects, such as laminar flow separation, adverse pressure gradient, and the interactions between separated flow and wake vortices, etc. We present direct numerical simulations (DNS) of shear layer transitions over two airfoils, NACA 4412 and NACA 0012-64, at 10 deg. angle of attack, and Rec = 104 based on uniform inflow velocity and chord length. The two airfoils chosen are geometrically almost the same with identical maximum thickness along with chordwise position but different cambers and hence different curvature. The curvature effects on the flow are presented by the unsteady evolution patterns of laminar flow separation; shear layer detachment, breakdown to turbulence, turbulent boundary layer reattachment and vortex shedding, and quantitative results on the development of turbulent boundary layer are emphasized. This DNS database is generated with an energy conservative fourth-order incompressible Navier-Stokes code with O(109) mesh points. Supported by a KAUST funded project on large eddy simulation of turbulent flows. The IBM Blue Gene P Shaheen at KAUST was utilized for the simulations.
Numerical simulation of electrokinetically driven micro flows
Hahm, Jungyoon
2005-11-01
Spectral element based numerical solvers are developed to simulate electrokinetically driven flows for micro-fluidic applications. Based on these numerical solvers, basic phenomena and devices for electrokinetic applications in micro and nano flows...
Numerical simulation of electrokinetically driven micro flows
Hahm, Jungyoon
2005-11-01
Spectral element based numerical solvers are developed to simulate electrokinetically driven flows for micro-fluidic applications. Based on these numerical solvers, basic phenomena and devices for electrokinetic applications in micro and nano flows...
NASA Astrophysics Data System (ADS)
Mueschke, Nicholas J.; Schilling, Oleg
2009-01-01
A 1152×760×1280 direct numerical simulation (DNS) using initial conditions, geometry, and physical parameters chosen to approximate those of a transitional, small Atwood number Rayleigh-Taylor mixing experiment [Mueschke et al., J. Fluid Mech. 567, 27 (2006)] is presented. In particular, the Atwood number is 7.5×10-4, and temperature diffusion is modeled by mass diffusion with an equivalent Schmidt number of 7. The density and velocity fluctuations measured just off of the splitter plate in this buoyantly unstable water channel experiment were parametrized to provide physically realistic, anisotropic initial conditions for the DNS. The methodology for parametrizing the measured data and numerically implementing the resulting perturbation spectra in the simulation is discussed in detail. The DNS is then validated by comparing quantities from the simulation to experimental measurements. In particular, large-scale quantities (such as the bubble front penetration hb and the mixing layer growth parameter ?b), higher-order statistics (such as velocity variances and the molecular mixing parameter ? on the center plane), and vertical velocity and density variance spectra from the DNS are shown to be in favorable agreement with the experimental data. The DNS slightly underestimates the growth of the bubble front hb but predicts ?b?0.07 at the latest time, in excellent agreement with the experimental measurement. While the molecular mixing parameter ? is also slightly underestimated by the DNS during the nonlinear and weakly turbulent growth phases, the late-time value ? ?0.55 compares favorably with the value ? ?0.6 measured in the experiment. The one-dimensional density and vertical velocity variance spectra are in excellent agreement between the DNS and experimental measurements. Differences between the quantities obtained from the DNS and from experimental measurements are related to limitations in the dynamic range of scales resolved in the DNS and other idealizations of the simulation. Specifically, the statistical convergence of the DNS results and confidence interval bounds are discussed. This work demonstrates that a parametrization of experimentally measured initial conditions can yield simulation data that quantitatively agrees well with experimentally measured low- and higher-order statistics in a Rayleigh-Taylor mixing layer. This study also provides resolution and initial conditions implementation requirements needed to simulate a physical Rayleigh-Taylor mixing experiment. In Paper II [Mueschke and Schilling, Phys. Fluids 21, 014107 (2009)], other quantities not measured in the experiment are obtained from the DNS and discussed, such as the integral- and Taylor-scale Reynolds numbers, Reynolds stress and dissipation anisotropy, two-dimensional density and velocity variance spectra, hypothetical chemical product formation measures, other local and global mixing parameters, and the statistical composition of mixed fluid. These quantities are valuable for assessing the predictions of Reynolds-averaged Navier-Stokes and large-eddy simulation models of Rayleigh-Taylor turbulent mixing.
NASA Technical Reports Server (NTRS)
Shih, Tsan-Hsing; Lumley, John L.
1991-01-01
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.
Gas-kinetic schemes for direct numerical simulations of compressible homogeneous turbulence.
Liao, Wei; Peng, Yan; Luo, Li-Shi
2009-10-01
We apply the gas-kinetic scheme (GKS) for the direct numerical simulations (DNSs) of compressible decaying homogeneous isotropic turbulence (DHIT). We intend to study the accuracy, stability, and efficiency of the gas-kinetic scheme for DNS of compressible homogeneous turbulence depending on both flow conditions and numerics. In particular, we study the GKS with multidimensional, quasi-one-dimensional, dimensional-splitting, and smooth-flow approximations. We simulate the compressible DHIT with the Taylor microscale Reynolds number Re(lambda)=72.0 and the turbulence Mach number Ma(t) between 0.1 and 0.6. We compute the low-order statistical quantities including the total kinetic energy K(t), the dissipation rate epsilon(t), the skewness S(u)(t), and the flatness F(u)(t) of the velocity field u(x,t). We assess the effects on the turbulence statistics due to the approximations made in the treatment of fluxes, the flux limiter, the accuracy of the interpolation, and the bulk viscosity. Our results show that the GKS is adequate for DNS of compressible homogeneous turbulence as far as the low-order turbulence statistics are concerned. PMID:19905477
NASA Astrophysics Data System (ADS)
Focke, C.; Bothe, D.
2012-07-01
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.
A study of aerosol activation at the cloud edge with high resolution numerical simulations
NASA Astrophysics Data System (ADS)
Babkovskaia, N.; Boy, M.; Smolander, S.; Romakkaniemi, S.; Rannik, U.; Kulmala, M.
2015-02-01
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.
Experimentation and direct numerical simulation of self-similar convergent detonation wave
NASA Astrophysics Data System (ADS)
Sorin, R.; Matignon, C.; Bozier, O.
The propagation of self similar convergent detonation wave in TATB-based explosive composition was studied both experimentally and numerically. The device constists in a 50 mm cylinder of TATB surrounded by an HMX tube. The detonation in HMX overdrives the detonation in TATB which adapts to the propagation velocity with a convergent front at centerline. We measured a curvature of ? = -21.2 m-1 for propagation velocity of 8750 m/s, which extends the knowledge of the (Dn,?) law. A wide ranged EOS/reaction rate model inspired from previous work of Wescott et al. was calibrated to reproduce both the run-to-detonation distance and the newly extended (Dn,?) law for the 1D sligthly curved detonation theory. 2D Direct Numerical Simulations (DNS) were made on fine resolved mesh grid for the experimental configuration and for various driver velocities. The simulation reproduces the experimental data both qualitatively (overall detonation structure) and quantitatively (? = -25.4 m-1).
Numerical Simulations of Thermobaric Explosions
Kuhl, A L; Bell, J B; Beckner, V E; Khasainov, B
2007-05-04
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.
Direct numerical simulation of soot formation and transport in turbulent nonpremixed ethylene flames
NASA Astrophysics Data System (ADS)
Lignell, David Owen
Combustion is central to society and accounts for the majority of the world's energy production. Soot formation, transport, and emission from turbulent flames are an important process in nonpremixed combustion. Soot is a major air pollutant with adverse health effects; its emission reduces combustion efficiencies associated with unburned fuel; and soot interacts strongly with the composition and temperature fields of flames, contributing to the bulk of radiative heat transfer. Simulation of combustion is an important and emerging discipline that compliments theoretical and experimental investigations and can provide fundamental insight into turbulent combustion environments and aid in engineering design of practical equipment. Simulations of practical combustion environments cannot fully resolve all flow and chemical phenomena due to the wide range of timescales and lengthscales present and must rely on models to capture the effects of unresolved turbulent transport and turbulence-chemistry interactions. Very little is know about soot formation in turbulent flames due to the difficulty of experimental measurements and the computational cost of simulation. Direct numerical simulation (DNS) resolves all relevant flow and chemical structures in turbulent flames, requiring no turbulence closure models. DNS of soot formation with realistic combustion chemistry and soot formation is presented in this dissertation. A series of increasingly complex flow configurations is investigated including one-dimensional relaxing diffusion flames, two-dimensional mixing layers and decaying turbulence simulations, and a three-dimensional temporally evolving jet flame. A reduced ethylene mechanism consisting of 19 transported species is coupled to a four-step soot model using the method of moments. The DNS are used to quantify soot formation and transport in turbulent flames. The proximity of soot to a flame is important, as this impacts the soot reaction and radiation rates. Differential diffusion between soot and the mixture fraction is very important. Multidimensional flame dynamic effects including flame curvature are shown to influence the direction of soot transport relative to a flame. The DNS provide a database by which turbulent combustion models may be validated and developed. Soot formation in the laminar flamelet and conditional moment closure models is examined.
Im, Hong G.
DNS of the effects of thermal stratification and turbulent mixing on H2/air ignition in a constant stratification on auto-ignition at constant volume and high pressure is studied by Direct Numerical Simulation of stratification lead to more front-like structures. Predictions of the multi-zone model are presented
Direct numerical simulations of low Reynolds number flow over airfoils with trailing-edge serrations
NASA Astrophysics Data System (ADS)
Sandberg, R. D.; Jones, L. E.
2011-08-01
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.
Numerical Simulation of a High Mach Number Jet Flow
NASA Technical Reports Server (NTRS)
Hayder, M. Ehtesham; Turkel, Eli; Mankbadi, Reda R.
1993-01-01
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.
Large eddy simulations and direct numerical simulations of high speed turbulent reacting flows
NASA Technical Reports Server (NTRS)
Givi, P.; Frankel, S. H.; Adumitroaie, V.; Sabini, G.; Madnia, C. K.
1993-01-01
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.
Direct numerical simulations of forced and unforced separation bubbles on an airfoil at incidence
NASA Astrophysics Data System (ADS)
Jones, L. E.; Sandberg, R. D.; Sandham, N. D.
Direct numerical simulations (DNS) of laminar separation bubbles on a NACA-0012 airfoil at Rec=5 are presented. Initially volume forcing is introduced in order to promote transition to turbulence. After obtaining sufficient data from this forced case, the explicitly added disturbances are removed and the simulation run further. With no forcing the turbulence is observed to self-sustain, with increased turbulence intensity in the reattachment region. A comparison of the forced and unforced cases shows that the forcing improves the aerodynamic performance whilst requiring little energy input. Classical linear stability analysis is performed upon the time-averaged flow field; however no absolute instability is observed that could explain the presence of self-sustaining turbulence. Finally, a series of simplified DNS are presented that illustrate a three-dimensional absolute instability of the two-dimensional vortex shedding that occurs naturally. Three-dimensional perturbations are amplified in the braid region of developing vortices, and subsequently convected upstream by local regions of reverse flow, within which the upstream velocity magnitude greatly exceeds that of the time-average. The perturbations are convected into the braid region of the next developing vortex, where they are amplified further, hence the cycle repeats with increasing amplitude. The fact that this transition process is independent of upstream disturbances has implications for modelling separation bubbles.
GPU accelerated flow solver for direct numerical simulation of turbulent flows
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
Graphical processing units (GPUs), characterized by significant computing performance, are nowadays very appealing for the solution of computationally demanding tasks in a wide variety of scientific applications. However, to run on GPUs, existing codes need to be ported and optimized, a procedure which is not yet standardized and may require non trivial efforts, even to high-performance computing specialists. In the present paper we accurately describe the porting to CUDA (Compute Unified Device Architecture) of a finite-difference compressible Navier–Stokes solver, suitable for direct numerical simulation (DNS) of turbulent flows. Porting and validation processes are illustrated in detail, with emphasis on computational strategies and techniques that can be applied to overcome typical bottlenecks arising from the porting of common computational fluid dynamics solvers. We demonstrate that a careful optimization work is crucial to get the highest performance from GPU accelerators. The results show that the overall speedup of one NVIDIA Tesla S2070 GPU is approximately 22 compared with one AMD Opteron 2352 Barcelona chip and 11 compared with one Intel Xeon X5650 Westmere core. The potential of GPU devices in the simulation of unsteady three-dimensional turbulent flows is proved by performing a DNS of a spatially evolving compressible mixing layer.
Comparing Aerodynamic Models for Numerical Simulation of
Peraire, Jaime
Comparing Aerodynamic Models for Numerical Simulation of Dynamics and Control of Aircraft and simulation of aircraft, yet other aerodynamics models exist that can provide more accurate results derivatives and other low fidelity models are frequently used in the design and flight simulation of aircraft
Rocket Engine Numerical Simulator (RENS)
NASA Technical Reports Server (NTRS)
Davidian, Kenneth O.
1997-01-01
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.
DNS and LES of a Shear-Free Mixing Layer
NASA Technical Reports Server (NTRS)
Knaepen, B.; Debliquy, O.; Carati, D.
2003-01-01
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.
NASA Technical Reports Server (NTRS)
Duan, Lian; Choudhari, Meelan M.
2014-01-01
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.
Numerical wind speed simulation model
Ramsdell, J.V.; Athey, G.F.; Ballinger, M.Y.
1981-09-01
A relatively simple stochastic model for simulating wind speed time series that can be used as an alternative to time series from representative locations is described in this report. The model incorporates systematic seasonal variation of the mean wind, its standard deviation, and the correlation speeds. It also incorporates systematic diurnal variation of the mean speed and standard deviation. To demonstrate the model capabilities, simulations were made using model parameters derived from data collected at the Hanford Meteorology Station, and results of analysis of simulated and actual data were compared.
A numerical study of flow-structure interactions with application to flow past a pair of cylinders
Papaioannou, Georgios (Georgios Vasilios), 1975-
2004-01-01
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 ...
NUMERICAL SIMULATION OF LARYNGEAL FLOW
In this study, we have investigated laryngeal air flows by numerically solving the corresponding Navier-Stokes equations expressed in a two-dimensional cylindrical coordinate system. The glottal aperture, defined by the geometry of the vocal folds was allowed to change with the v...
Numerical techniques of rigid body simulation
Eberle, David Michael
2001-01-01
bodies. Complicated simulations require the development of sophisticated collision detection systems and numerical techniques. Previous work in the field has almost exclusively been restricted to geometries that are convex or a union of convex pieces...
Numerical simulation of turbulent reacting flows
Cloutman, L.D.
1984-01-01
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.
High resolution DNS of jet stream generated tropopausal turbulence
NASA Astrophysics Data System (ADS)
Joseph, B.; Mahalov, A.; Nicolaenko, B.; Tse, K. L.
2003-05-01
Tropopausal turbulence (TT), generated by jet streams, is examined, for different nonhomogeneous basic state forcing profiles, through high resolution three-dimensional (3D) direct numerical simulations (DNS). Turbulent core regions, on either side of the jet, are characterized by a dramatic decrease in Brunt-Väisälä frequency (N), minima in gradient Richardson number (Rig) and shear length scales, and maxima in buoyancy Reynolds number and buoyancy length scales. The edges of turbulent mixing layers are marked by N2-bulges, which provides an alternate mechanism for multiple tropopauses. While temperature variance peaks in the vicinity of these N2-bulges, velocity variances peak within the turbulent core. Multiple branches in the scaling of flux Richardson number with Rig are found typical for TT, a result having implications to the problem of turbulence parameterization.
DNS of helicity-induced stratified turbulent flow
NASA Astrophysics Data System (ADS)
Chandy, Abhilash J.; Rahimi, Abbas
2013-11-01
Helical flows undergoing density stratification have wide applications in meteorological phenomena such as dust devils, tornadoes, and hurricanes due to the complexity and disasters caused by them. Direct numerical simulations (DNS) of transition to turbulence in a stably stratified Boussinesq fluid are presented for different rotation and stratification intensities. In order to understand the effect of velocity on the energy cascade, comparisons are made between helicity initiated and non-helical flows. Results show that stratification decelerates the helicity decay and causes velocity and vorticity to align with each other. With respect to the helical and non-helical flow comparisons, the total energy in the presence of stratification decays faster with helicity. In addition, the behavior of length scales were examined by comparing temporal variations of the vertical shearing of velocities. Results showed a growing asymmetry with time in the case of helical flow, while non-helical flow stayed close to begin symmetric.
NUMERICAL SIMULATION OF THE SMOLUCHOWSKI COAGULATION EQUATION
Filbet, Francis
NUMERICAL SIMULATION OF THE SMOLUCHOWSKI COAGULATION EQUATION FRANCIS FILBET AND PHILIPPE LAURENC2028 Abstract. In this paper, we develop a numerical scheme for the Smoluchowski coagulation equation, which. Smoluchowski coagulation equation, self-similar variables, finite volume method AMS subject classifications. 65
Numerical simulation of conservation laws
NASA Technical Reports Server (NTRS)
Chang, Sin-Chung; To, Wai-Ming
1992-01-01
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.
Syddansk Universitet Kaminsky DNS exploit
Boyar, Joan
begreber som der bliver benyttet i forbindelse med DNS. Top Level Domain (TLD): Eksempelvis .dk .com og TLD'er, og er s°aledes de første navneservere der bliver spurgt n°ar der laves en fore- spørgsel
Numerical Simulation For Supersonic Inlets
NASA Technical Reports Server (NTRS)
Varner, M. O.; Martindale, W. R.; Phares, W. J.; Kneile, K. R.; Adams, J. C., Jr.
1987-01-01
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.
Numerical Simulation of Bubble collisions with PRIME.
Cox, Simon
Numerical Simulation of Bubble collisions with PRIME. Institut fÃ¼r StrÃ¶mungsmechanik S. Heitkam1. Hoffmann2, W. Drenckhan1, D. Langevin1 J. FrÃ¶hlich2 LPS, Paris Sud XI1 and ISM, TU Dresden2 #12;Example CFD #12;Simulation of foam 3 Heitkam #12;Immersed Boundary Method Â· Forcing points on particle surface
NUMERICAL SIMULATIONS OF VIBRATION ASSISTED MACHINING
John A. Patten; Andre Williams
A two-dimensional (2-D) finite element model (FEM) has been developed to perform numerical simulations of vibration assisted machining (VAM). The model is based on an updated Lagrangian formulation, with adaptive remeshing. The model is capable of simulating the amplitude and frequency independently of the tool in the cutting (x) and thrust (y) force directions over a wide range of values.
Numerical simulation as applied to satellite radiation
NASA Astrophysics Data System (ADS)
Roques, Frederic
1992-12-01
Numerical simulation has recently become a basic step in the vulnerability analysis of space and military systems in EMC and ESD environments. This paper reviews the currently available mathematical modeling techniques suitable for the computerized simulation of the EMC and ESD environments. Particular attention is given to methods using the theory of lines, the finite-difference method, and the geometrical theory of diffraction.
Large eddy simulation and direct numerical simulation of high speed turbulent reacting flows
NASA Technical Reports Server (NTRS)
Adumitroaie, V.; Frankel, S. H.; Madnia, C. K.; Givi, P.
1993-01-01
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.
NASA Astrophysics Data System (ADS)
Koo, Heeseok; Raman, Venkat; Varghese, Philip L.
2013-11-01
Thermochemical nonequilibrium could be significant in scramjet engines due to intense shock-based compression in the pre-combustion isolator region. In particular, vibrational nonequilibrium could adversely affect ignition time and mixing efficiency. To understand the role of nonequilibrium in such flows, direct numerical simulation (DNS) of supersonic flows with vibrational excitation are studied. A linear time-scale model is used to describe the vibrational relaxation of excited species. Essentially, nonequilibrium alters the flow by changing the physical properties that are related to the translational temperature. Such changes introduce nonlinear effect on the scalar mixing process. Further, the redistribution of energy amongst the internal states affects chemical rates. An analysis of the impact of nonequilibrium on combustion is provided.
Direct Numerical Simulation of the generation of internal waves over a thin obstacle
NASA Astrophysics Data System (ADS)
Fraunie, Philippe; Houcine, Hatem; Chashechkin, Yuli; Gharbi, Adel; Lili, Taieb
2010-05-01
Numerical simulation of stratified flows past a thin obstacle are performed in comparison with laboratory experiments, allowing a detailed description of the transient processes occurring from the starting of the flow up to the formation of completed internal waves field. A high resolution finite differences scheme has been adapted to the low Reynolds Navier-Stokes equation with transport equation for density defined by salinity in the experiments. Details of the resolved flow pattern as obtained from DNS are enriching the quantitative description of this complex flow that can be expanded to the investigation of atmospheric and oceanic flows patterns on sharp topography. Acknowledgements : This work was partly financially supported by the RFBR (grants 08-05-00473 and 08-05-90434). PACA region grant for International Research in Mediterranean area.
NASA Astrophysics Data System (ADS)
Badreddine, Hassan; Vandewalle, Stefan; Meyers, Johan
2014-01-01
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.
Numerical Simulation of Nanostructure Growth
NASA Technical Reports Server (NTRS)
Hwang, Helen H.; Bose, Deepak; Govindan, T. R.; Meyyappan, M.
2004-01-01
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.
Numerically simulating the sandwich plate system structures
NASA Astrophysics Data System (ADS)
Feng, Guo-Qing; Li, Gang; Liu, Zhi-Hui; Niu, Huai-Lei; Li, Chen-Feng
2010-09-01
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.
THE SONIFICATION OF NUMERICAL FLUID FLOW SIMULATIONS
Edward Childs
2001-01-01
Computational Fluid Dynamics (CFD) software simulates fluid, air flow and heat transfer by solving the Navier-Stokes (N-S) equa- tions numerically. Realistic 3-D engineering simulations typically yield the values of 7 or more variables (e.g. fluid component ve- locities and temperatures) at hundreds of thousands of points in space, all as a function of time. It has been noted that solutions
Numerical simulation of unsteady viscous flows
NASA Technical Reports Server (NTRS)
Hankey, Wilbur L.
1987-01-01
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.
Numerical Simulation Of Silicon-Ribbon Growth
NASA Technical Reports Server (NTRS)
Woda, Ben K.; Kuo, Chin-Po; Utku, Senol; Ray, Sujit Kumar
1987-01-01
Mathematical model includes nonlinear effects. In development simulates growth of silicon ribbon from melt. Takes account of entire temperature and stress history of ribbon. Numerical simulations performed with new model helps in search for temperature distribution, pulling speed, and other conditions favoring growth of wide, flat, relatively defect-free silicon ribbons for solar photovoltaic cells at economically attractive, high production rates. Also applicable to materials other than silicon.
Numerical simulations of iced airfoils and wings
Jianping Pan
2004-01-01
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
Numerical Simulations of Major Barred Galaxies
Chien-Chang Yen; L. Lin; C. Yuan
2006-01-01
Galaxies with major bars such as NGC1300 and NGC1097, are characterized by straight dust lanes, central starburst ring and outer spirals. Recent IR observations also show there are star formation patches and spur-like substructures. Through numerical simulations, we can reproduce the dust lanes, the central starburst ring, and the outer spirals by imposing a single strong bar potential on a
A NUMERICAL SIMULATION OF HURRICANE HUGO
M. E. Nicholls; R. A. Pielke
Results of a simulation of Hurricane Hugo are presented. The numerical model used in this study is the Colorado State University Regional Atmospheric Modeling System (RAMS). The model contains a full set of non-hydrostatic com- pressible dynamic equations, a thermodynamic equation and a set of microphysics equations for water- and ice-phase clouds and precipitation. There are parame- terizations for longwave
A NUMERICAL SIMULATION OF HURRICANE ANDREW
M. E. Nicholls; R. A. Pielke
Results of a simulation of Hurricane Andrew are presented. The numerical model used in this study is the Colorado State University Regional Atmospheric Modeling System (RAMS). The model contains a full set of non-hydrostatic com- pressible dynamic equations, a thermodynamic equation and a set of microphysics equations for water- and ice-phase clouds and precipitation. There are parame- terizations for longwave
Simple Numerical Simulation of Strain Measurement
NASA Technical Reports Server (NTRS)
Tai, H.
2002-01-01
By adopting the basic principle of the reflection (and transmission) of a plane polarized electromagnetic wave incident normal to a stack of films of alternating refractive index, a simple numerical code was written to simulate the maximum reflectivity (transmittivity) of a fiber optic Bragg grating corresponding to various non-uniform strain conditions including photo-elastic effect in certain cases.
Numerical simulation of turbulent flow and heat
Numerical simulation of turbulent flow and heat transfer in multi-channel, narrow-gap fuel element in the fuel element should not surpass the designed value. It is, therefore, of great practical importance to investigate turbulent flow and heat transfer in the fuel element, experimentally or theoretical
Numerical simulations of flows around flame holders
NASA Astrophysics Data System (ADS)
Bai, X. S.; Fuchs, L.; Li, Y.
1992-01-01
The numerical simulations of turbulent flow around a flame holder model placed in a wind tunnel is presented. The calculations include both isothermal and reacting flows. The (cold) isothermal flow is modeled by Reynolds averaged Navier-Stokes N-S equations equipped with standard two-equation k-epsilon model and Large Eddy Simulations (LES). The (hot) reacting flow is calculated by a premixed flame model based on Magnussen's eddy dissipation concept, in addition to the N-S equations and k-epsilon equations. The system of equations is solved numerically on a system of global and locally refined grids, using a multigrid method. The results show that the numerical calculations compare well with the experimental results.
Direct numerical simulation of turbulent, chemically reacting flows
NASA Astrophysics Data System (ADS)
Doom, Jeffrey Joseph
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.
Numerical heating in hybrid plasma simulations
Rambo, P.W. [Lawrence Livermore National Lab., CA (United States)] [Lawrence Livermore National Lab., CA (United States)
1997-05-01
The numerical heating in hybrid particle-fluid simulations has been investigated with emphasis on the regime ZT{sub e}/T{sub i} {much_gt} 1, where Z is the charge state of the ions and T{sub e} and T{sub i} are the electron and ion temperatures, respectively. For the simple case of particle ions advanced in the ambipolar field due to quasineutral isothermal fluid electrons, the heating rate is observed to be weakly dependent on time step, inversely proportional to the number of simulation particles per grid cell and strongly increasing with increasing ZT{sub e}/T{sub i}. Additional smoothing, due to finite Debye length, or introduced through numerical means such as higher order particle interpolation or smoothing of grid quantities, is observed to significantly reduce this heating. Both one- and two-dimensional results are presented. These results are important to hybrid particle simulations of laser generated plasmas, a regime where ZT{sub e}/T{sub i} {much_gt} 1 is often encountered. As a relevant example, simulations of stimulated Brillouin scattering are presented illustrating the deleterious effect of numerical heating and attendant distortions to the particle distribution function. 15 refs., 6 figs.
Numerical simulation of groundwater flow on MPPs
Ashby, S.; Falgout, R.; Tompson, A. [Lawrence Livermore National Lab., CA (United States); Fogwell, T. [International Technology Corp., Martinez, CA (United States)
1994-03-01
Mathematical models are often used to aid in the design and management of engineered remediation procedures. This paper discusses the numerical simulation of groundwater flow in three-dimensional heterogeneous porous media. A portable and scalable code called PARFLOW is being developed for massively parallel computers to enable the detailed modeling of large sites. This code uses a turning bands algorithm to generate a statistically accurate subsurface realization, and preconditioned conjugate gradients to solve the linear system that yields the flow velocity field. Preliminary numerical results for the LLNL site are presented.
Numerical Investigations of the Leray-? Turbulence Model for Large Eddy Simulations
NASA Astrophysics Data System (ADS)
Frankel, S.; Kwan, Y.; Chandy, A.; Varghese, S.; Shen, J.; Fischer, P.
2007-11-01
Large eddy simulations (LES) of transition to turbulence for steady flow through a model eccentric stenotic blood vessel are reported featuring the use of the Leray-? model. The Leray-? model uses a filtered velocity field for fluid advection, modifying the nonlinear vortex stretching dynamics effectively suppressing scales smaller than ?, and reducing resolution requirements, in contrast to more traditional LES models which filter the entire velocity field and enhance viscous dissipation through a computed eddy viscosity. A Helmholtz differential filter, both with and without projection of the filtered field onto a divergence free space, is used to investigate the issue of incompressibility of the filtered field. The effect of filter size is also studied. The incompressible Navier-Stokes and differential filter equations are numerically integrated using an h/p spectral-element method on a grid with 2448 hexahedral cells on up to 1024 processors on the IBM Blue Gene/L at Argonne National Laboratory. Differences between instantaneous and statistical LES results (with polynomial order 7) and recent published direct numerical simulation (DNS) results (with polynomial order 13) are discussed. Additional results from Fourier pseudospectral homogeneous isotropic turbulence simulations may be employed to shed further light on the LES results.
Numerical simulation of centrifugal casting of pipes
NASA Astrophysics Data System (ADS)
Kaschnitz, E.
2012-07-01
A numerical simulation model for the horizontal centrifugal pipe casting process was developed with the commercial simulation package Flow3D. It considers - additionally to mass, energy and momentum conservation equations and free surface tracking - the fast radial and slower horizontal movement of the mold. The iron inflow is not steady state but time dependent. Of special importance is the friction between the liquid and the mold in connection with the viscosity and turbulence of the iron. Experiments with the mold at controlled revolution speeds were carried out using a high-speed camera. From these experiments friction coefficients for the description of the interaction between mold and melt were obtained. With the simulation model, the influence of typical process parameters (e.g. melts inflow, mold movement, melt temperature, cooling media) on the wall thickness of the pipes can be studied. The comparison to results of pipes from production shows a good agreement between simulation and reality.
Numerical Simulation of a Tornado Generating Supercell
NASA Technical Reports Server (NTRS)
Proctor, Fred H.; Ahmad, Nashat N.; LimonDuparcmeur, Fanny M.
2012-01-01
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.
NASA Astrophysics Data System (ADS)
Sloan, Gregory James
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.
Numerical simulations of iced airfoils and wings
NASA Astrophysics Data System (ADS)
Pan, Jianping
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.
Numerical simulations of compressible mixing layers
NASA Technical Reports Server (NTRS)
Normand, Xavier
1990-01-01
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.
Numerical simulation for hydrogen magnetic refrigeration
NASA Astrophysics Data System (ADS)
Zhu, Yiyin; Hattori, Hideyuki; Matsumoto, Koichi; Yanagisawa, Yoshinori; Nakagome, Hideki; Numazawa, Takenori
2012-06-01
We have built active magnetic regenerator (AMR) test apparatuses operated with a gas displacer to transfer the heat from magnetic material unit (AMR bed). Because finding an optimum parameter by experiment is not easy, numerical simulation is necessary to confirm the experimental conditions. As the first step of the project, we developed a 1-dimensional porous media model for hydrogen magnetic refrigerator with a Brayton-likeoperation cycle. This model has been calculated separately for heat exchange fluid and magnetic material. The results using two different magnetic materials have been compared.We confirmed that the simulation results agreed with experimental data of the internal gas displacer system.
Numerical Simulation of Compressible Plane Jets
NASA Astrophysics Data System (ADS)
Reichert, R. S.; Biringen, S.
1996-11-01
Spatial direct numerical simulation of compressible plane jets exhausting into a parallel stream has been performed. The mathematical model is inviscid and captures large-scale mixing events, such as vortex roll-up and multiple pairings. These are the most dynamically important features, even in the fully developed turbulent region in free shear flows such as the plane jet. Application and performance of inflow and outflow boundary conditions are discussed. Simulation of linear instabilities in shear layers and plane jets have been conducted as validation of the numerical methods. Random forcing is applied at inflow to produce nonlinear evolution in the plane jet flow. Time-averages over a long streamwise domain capture the mean field structure and allow computation of the jet growth and mean centerline velocity decay in the approximately self-preserving region. Convective Mach number is varied to assess compressibility effects on the nearly self-similar structure of the plane jet exhausting into a parallel stream.
Numerical simulations of catastrophic disruption: Recent results
NASA Technical Reports Server (NTRS)
Benz, W.; Asphaug, E.; Ryan, E. V.
1994-01-01
Numerical simulations have been used to study high velocity two-body impacts. In this paper, a two-dimensional Largrangian finite difference hydro-code and a three-dimensional smooth particle hydro-code (SPH) are described and initial results reported. These codes can be, and have been, used to make specific predictions about particular objects in our solar system. But more significantly, they allow us to explore a broad range of collisional events. Certain parameters (size, time) can be studied only over a very restricted range within the laboratory; other parameters (initial spin, low gravity, exotic structure or composition) are difficult to study at all experimentally. The outcomes of numerical simulations lead to a more general and accurate understanding of impacts in their many forms.
Numerical simulation and nasal air-conditioning.
Keck, Tilman; Lindemann, Jörg
2010-01-01
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
Numerical simulation and nasal air-conditioning
Keck, Tilman; Lindemann, Jörg
2011-01-01
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
Numerical simulation of earth's core formation
Rie Honda; Hitoshi Mizutani; Tetsuo Yamamoto
1993-01-01
Formation process of earth's core is studied by numerical simulations of flow field in a self-gravitating fluid sphere. The proto-earth was assumed to have gravitationally unstable three-layered structure initially, which consists of the uppermost silicate melt layer, the middle iron layer, and the central undifferentiated silicate-rich protocore. This structure of the heavy iron layer overlying the light protocore leads to
Numerical simulation of swept-wing flows
NASA Technical Reports Server (NTRS)
Reed, Helen L.
1991-01-01
The transition process characteristics of flows over swept wings were computationally modelled. The crossflow instability and crossflow/T-S wave interaction are analyzed through the numerical solution of the full three dimensional Navier-Stokes equations including unsteadiness, curvature, and sweep. The leading-edge region of a swept wing is considered in a three-dimensional spatial simulation with random disturbances as the initial conditions.
Numerical simulation of type III bursts
T. Takakura
1980-01-01
Results of numerical simulations for three models of type III solar radio bursts are discussed. These models include: (1) normal type III bursts on meter waves which show the maximum flux density in the meter-wavelength range; (2) low-frequency type III bursts which have a broad maximum of very high flux density at about 300 and 100 kHz; and (3) type
Numerical simulations of hyperfine transitions of antihydrogen
NASA Astrophysics Data System (ADS)
Kolbinger, B.; Capon, A.; Diermaier, M.; Lehner, S.; Malbrunot, C.; Massiczek, O.; Sauerzopf, C.; Simon, M. C.; Widmann, E.
2015-02-01
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.
Conditional statistics in a turbulent premixed flame derived from direct numerical simulation
NASA Technical Reports Server (NTRS)
Mantel, Thierry; Bilger, Robert W.
1994-01-01
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.
Numeric Simulation Tools of the IMPEx Infrastructure
NASA Astrophysics Data System (ADS)
Kallio, E. J.; Khodachenko, M. L.; Génot, V.; Schmidt, W.; Jarvinen, R.; Häkkinen, L.; Al-Ubaidi, T.; Topf, F.; Modolo, R.; Hess, S.; Alexeev, I. I.
2012-09-01
The EU-FP7 Project "Integrated Medium for Planetary Exploration" (IMPEx) was established as a result of scientific collaboration between institutions across Europe and is working on the integration of a set of interactive data analysis and modeling tools in the field of space plasma physics. These tools are comprised of numerical Hybrid/MHD and analytical Paraboloid magnetospheric models from the simulation sector as well as AMDA, ClWeb and 3DView from the data analysis and visualization sector. The basic feature of IMPEx consists in connection of different data sources, including archived computational simulation results and observational data, in order to analyse and visualize scientific data by means of interactive web-based tools. In this presentation we introduce Hybrid and Magnetohydrodynamic Modelling (HMM) environment which is an example of simulation services within IMPEx. HMM includes two global numerical hybrid models (a hybrid model HYB from FMI and a hybrid model from LATMOS) and magnetohydrodynamic model (GUMICS) from FMI. Especially, we introduce the web service, Hybrid Web Archive[1] which enables access to the simulation runs made by HYB and GUMICS models in the IMPEx HMM environment.
A direct numerical simulation study of vorticity transformation in weakly turbulent premixed flames
NASA Astrophysics Data System (ADS)
Lipatnikov, A. N.; Nishiki, S.; Hasegawa, T.
2014-10-01
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.
NEW NUMERICAL TECHNOLOGIES FOR THE SIMULATION OF ARC WELDING PROCESSES
Paris-Sud XI, Université de
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
Turbulent convection: comparison of Reynolds stress models with numerical simulations
Demoulin, Pascal
Turbulent convection: comparison of Reynolds stress models with numerical simulations Friedrich, University of Vienna, AÂ1090 Vienna, Austria ABSTRACT Numerical simulations of turbulent convection have of basic properties of compressible convection, and stellar atmospheres. Fully nonlocal convection models
Direct Numerical Simulation of Cosmological Reionization
NASA Astrophysics Data System (ADS)
So, Geoffrey C.
We examine the epoch of hydrogen reionization using a new numerical method that allows us to self-consistently couple all the relevant physical processes (gas dynamics, dark matter dynamics, self-gravity, star formation/feedback, radiative transfer, ionization, recombination, heating and cooling) and evolve the system of coupled equations on the same high resolution mesh. We refer to this approach as direct numerical simulation, in contrast to existing approaches which decouple and coarse-grain the radiative transfer and ionization balance calculations relative to the underlying dynamical calculation. Our method is scalable with respect to the number of radiation sources, size of the mesh, and the number of computer processors employed, and is described in Chapter 2 of this thesis. This scalability permits us to simulate cosmological reionization in large cosmological volumes (~100 Mpc) while directly modeling the sources and sinks of ionizing radiation, including radiative feedback effects such as photoevaporation of gas from halos, Jeans smoothing of the IGM, and enhanced recombination due to small scale clumping. With our fiducial simulation, we find that roughly 2 ionizing photons per baryon is needed to highly ionize the intergalactic medium. The complicated events during reionization that lead to this number can be generally described as inside-out, but in reality the narrative depends on the level of ionization of the gas one defines as ionized. We have updated the formula observers often use for estimating the ionized volume filling fraction formula with a delta b and trec,eff to get from O(10%) to O(1%) consistency with our simulation results. This improvement comes from not using the traditional clumping factor, but instead, considering the history and local effects which were neglected in formulating the original expression. And finally, we have a new upper limit for the escape fraction of ~0.6 from our simulation, which takes into account the photons in the energy density field and photons used to ionize H I.
Numerical reproducibility for implicit Monte Carlo simulations
Cleveland, M.; Brunner, T.; Gentile, N. [Lawrence Livermore National Laboratory, P. O. Box 808, Livermore CA 94550 (United States)
2013-07-01
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)
DNS of vibrating grid turbulence
NASA Astrophysics Data System (ADS)
Khujadze, G.; Oberlack, M.
Direct numerical simulation of the turbulence generated at a grid vibrating normally to itself using spectral code [1] is presented. Due to zero mean shear there is no production of turbulence apart from the grid. Action of the grid is mimiced by the function implemented in the middle of the simulation box:f_i (x_1 ,x_2 ) = {n^2 S}/2left\\{ {left| {{? _{i3} }/4\\cos left( {{2? }/Mx_1 } right)\\cos left. {left( {{2? }/Mx_2 } right)} right|} right.sin (nt) + {? _i }/4} right\\}, where M is the mesh size, S/2 - amplitude or stroke of the grid, n - frequency. ? i are random numbers with uniform distribution. The simulations were performed for the following parameters: x 1, x 2 ? [-?; ?], x 3 ? [-2?; 2?]; Re = nS 2/? = 1000; S/M = 2; Numerical grid: 128 × 128 × 256.
Comprehensive Numerical Simulation of Laser Materials Processing
NASA Astrophysics Data System (ADS)
Gross, Markus
The previous chapter concluded the physical discussion of laser materials processing (LMP). In this chapter no new physical concepts or theory of physical phenomena will be introduced. The objective of this chapter is rather to give an overview of how to analyse the complex system that is LaserMaterials Processing. As has been shown in great detail, the physical level of complexity is deep and analysis becomes extremely cumbersome if it is pursued on an analytical level. Albeit giving great insight into the detailed phenomena, whole processes and their sensitivity to ambient conditions and changes in process parameters or physical setup cannot be investigated using these methods alone. Here numerical simulation comes into play for the scientist investigating processes from an engineering point of view. Numerical simulation has an almost endless scope for system complexity and is only limited by the resources available and the time the investigator is prepared to wait for results. This final chapter should be read as a guide to how to get started.Some fundamental principles of discrete numerical modelling will be introduced and reference made to work by other authors. This, in the space available, can by no means be a comprehensive review, or a textbook of all the methods available and required. Nevertheless it should be seen as a starting point for investigators, at the doctoral student level, trying to get to terms with the task ahead, or for the researcher trying to move from practice to theory, from experiment to simulation, looking for a guide on what to look out for, where to go and which pitfalls to avoid.
Numerical simulation of viscous transonic airfoil flows
NASA Technical Reports Server (NTRS)
Coakley, Thomas J.
1987-01-01
Numerical simulations of transonic airfoil flows using the Reynolds-averaged Navier-Stokes equations and various turbulence models are presented and compared with experimental data. Three different airfoils were investigated under varying flow conditions ranging from subcritical unseparated flows to supercritical separated flows. The turbulence models investigated consisted of three zero-equation models and one two-equation model. For unseparated flows involving weak viscous-inviscid interactions, the four models were comparable in their agreement with experiment. For separated flows involving strong viscous-inviscid interactions, the nonequilibrium zero-equation model of Johnson and King gave the best overall agreement with experiment.
Numerical simulations of steel plate perforation
Chen, E.P.
1992-08-01
Numerical simulations of perforation in steel plates involve the treatment of material failure during the perforation process. One way to model physical material separation is to delete failed elements from the analysis based on an appropriate failure criterion. Different algorithms were used in different transient finite element codes to delete failed elements. This investigation compares the results of PRONTO 2D and LS-DYNA2D codes for a specific steel plate perforation problem. Influences of the deletion algorithms on material parameters are discussed.
Numerical simulations of steel plate perforation
Chen, E.P.
1992-01-01
Numerical simulations of perforation in steel plates involve the treatment of material failure during the perforation process. One way to model physical material separation is to delete failed elements from the analysis based on an appropriate failure criterion. Different algorithms were used in different transient finite element codes to delete failed elements. This investigation compares the results of PRONTO 2D and LS-DYNA2D codes for a specific steel plate perforation problem. Influences of the deletion algorithms on material parameters are discussed.
Sunspot decay numerical simulation and results
NASA Astrophysics Data System (ADS)
Thibault, Kim
We study the clusters formed by sunspot decay in a DLA numerical simulation. We analyze the spatial location and log-log size distributions of the clusters in the domain. The parameters we vary in order to quantify the results are the following: initial sunspot radius, moat flow cutoff radius, spot radius eroded at each time step and maximum number of flux tubes injected at each time step in the domain and around the sunspot. Random injection of flux tubes has both positive and negative polarities in the domain while a single polarity is used for injection around the sunspot. We use the quadratic sunspot decay law presented in Crouch et al. (2007).
Numerical simulation of swept-wing flows
NASA Technical Reports Server (NTRS)
Reed, Helen L.
1991-01-01
Efforts of the last six months to computationally model the transition process characteristics of flow over swept wings are described. Specifically, the crossflow instability and crossflow/Tollmien-Schlichting wave interactions are analyzed through the numerical solution of the full 3D Navier-Stokes equations including unsteadiness, curvature, and sweep. This approach is chosen because of the complexity of the problem and because it appears that linear stability theory is insufficient to explain the discrepancies between different experiments and between theory and experiment. The leading edge region of a swept wing is considered in a 3D spatial simulation with random disturbances as the initial conditions.
Numerical simulation of a cylinder in uniform
Saiki, E.M.; Biringen, S. [Univ. of Colorado, Boulder, CO (United States)] [Univ. of Colorado, Boulder, CO (United States)
1996-02-01
In this study, a virtual boundary technique is applied to the numerical simulation of stationary and moving cylinders in uniform flow. This approach readily allows the imposition of a no-slip boundary within the flow field by a feedback forcing term added to the momentum equations. In the present work, this technique is used with a high-order finite difference method, effectively eliminating spurious oscillations caused by the feedback forcing when used with spectrally discretized flow solvers. Very good agreement is found between the present calculations and previous computational and experimental results for steady and time-dependent flow at low Reynolds numbers. 32 refs., 15 figs., 3 tabs.
Direct numerical simulation of turbulent mixing.
Statsenko, V P; Yanilkin, Yu V; Zhmaylo, V A
2013-11-28
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
Numerical aspects of compressible turbulence simulations
NASA Astrophysics Data System (ADS)
Honein, Albert Edward
Nonlinear instabilities present a long standing hurdle for compact, high order, non dissipative, finite difference computation of compressible turbulence. The spectral-like accuracy of these schemes, while attractive, results in significant aliasing errors that corrupt the solution. As a result, successful simulations have been limited to moderate Reynolds numbers ( Re) and low-order or upwind schemes with inherent numerical dissipation. However, resorting to dissipative schemes in discretizing the nonlinear terms was shown to have a detrimental effect on turbulence. A recent LES approach is to abandon the subgrid model altogether and rely on the scheme dissipation to mimic the effect of small scales. A dissipative monotone integrated LES (MILES) algorithm based on a multidimensional flux-corrected transport (FCT) algorithm has been developed and tested for decaying compressible isotropic turbulence. Agreement with the benchmark experiments of Comte-Bellot and Corrsin is very sensitive to the parameters involved in the FCT algorithm, while the evolution of thermodynamic fluctuations do not compare well with direct numerical simulations. An under-resolved simulation of inviscid, compressible, isotropic turbulence at low Mach number is chosen as a severe benchmark to investigate the nonlinear stability properties of nondissipative schemes. The behavior of this benchmark is predicted by performing a fully de-aliased spectral simulation on a 32 3 grid with turbulent Mach number of Mto = 0.07. The kinetic energy and thermodynamic fluctuations are found to decay for finite Re, and remain constant at infinite Re for a long time before the occurrence of numerical shocks. Extending the proof of Kraichnan (Journal of the Acoustical Society of America, 27(3), 1955), this inviscid statistical equilibrium is demonstrated to be a consequence of the discrete equivalent of the Liouville theorem of classical statistical mechanics. Several existing non-dissipative methods are evaluated and instabilities are found to occur at finite Re, the value varying from one scheme to another. These instabilities arise due to violation of two conservation properties stemming from the entropy equation. New non-dissipative formulations respecting these conservation properties are proposed for schemes of any order, finite difference or spectral, and valid for regular and staggered grids. The numerical implementation is simply achieved using a conservative skew-symmetric splitting of the nonlinear terms. Furthermore, robust versions of the schemes in Nagarajan et al. (Journal of Computational Physics , 191(2), 2003) and Ducros et al. (Journal of Computational Physics, 161(1), 2000) have been formulated and found to be effective at very high Re.
Direct numerical simulation of compressible Kolmogorov flow
NASA Astrophysics Data System (ADS)
Bertsch, Rebecca; Girimaji, Sharath; Kumar, Gaurav
2012-11-01
Direct numerical simulations to investigate three-dimensional compressible turbulent Kolmogorov flow using the gas kinetic method are performed. The evolutions of single modes in isolation are examined in order to gain insight into the evolution of statistics of modes in collection. Single modes, or straight modes, are either stream-wise or span-wise and their combinations refer to oblique modes. Oblique modes (initially two-dimensional) are analyzed to isolate unstable modes and study the effect of gradient Mach number and Taylor-microscale Reynolds number. The competition between the Kelvin-Helmholtz and uniform shear instability is observed to determine the effect on flow-thermodynamic interactions. Simulations of turbulent Kolmogorov flow with varying amount of compressibility, determined by the gradient Mach number, are analyzed to support prior results of the stabilizing effect of compressibility in turbulent shear flows.
Stereo image visualization of numerically simulated turbulence
NASA Technical Reports Server (NTRS)
Robinson, Stephen K.; Hu, Kenneth C.
1989-01-01
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.
Analysis of end-to-end DNS quality
Feng Liu; Zhihui Luo; Zhenming Lei
2010-01-01
DNS is an important application as a part of the procedures of most Internet applications, which means quality of DNS directly influences quality of these applications. In order to monitor and analyze user perceived end-to-end quality of DNS applications, two key parameters of DNS quality were chosen. A distributed system NQM was used to monitor various types of practical DNS
Numerical simulations of Czochralski silicon growth
NASA Astrophysics Data System (ADS)
Chan, Y. T.; Gibeling, H. J.; Grubin, H. L.
1988-08-01
The present study involves the development and application of a numerical technique to the most common method of growing silicon crystals - the Czochralski process. The equations solved for the melt phase are the conservation equations for the mass, momentum, and energy. For the crystal phase, the energy balance equation is solved. The governing equations are formulated using primitive variables and are not thereby restricted to two-dimensional symmetry. The crystal and melt phases are coupled through thermal conditions applied along the melt-crystal interface. The shape of the melt-crystal interface and meniscus are determined through solutions to differential equations that govern the kinematic conditions at the specific interfaces, and are not assumed a priori. The numerical procedures are obtained by a well-documented procedure known as the consistently split linearized block implicit (LBI) scheme. Calculations, including one three-dimensional case, are performed for various growth conditions suitable for silicon crystal growth. The present study has successfully demonstrated the numerical capabilities for Czochralski crystal growth simulations.
DNS and LES of Separated Flows at Moderate Reynolds Numbers
NASA Astrophysics Data System (ADS)
Cadieux, F.; Domaradzki, J. A.; Sayadi, T.; Bose, S.; Duchaine, F.
2012-11-01
Flows in rotating machinery, for unmanned and micro aerial vehicles, wind turbines, and propellers consist of different flow regimes. First, a laminar boundary layer is followed by a laminar separation bubble with a shear layer on top of it that experiences transition to turbulence. Subsequently, the separated turbulent flow reattaches and evolves downstream from a nonequilibrium turbulent boundary layer to an equilibrium one. Typical RANS and LES turbulence modeling methods experience difficulties when simulating such flows because they were developed for fully developed turbulent flows. This currently leaves DNS as the only reliable but computationally expensive alternative. Our work assesses the capability of LES to reduce the resolution requirements for such flows. Flow over a flat plate with suitable velocity boundary conditions away from the plate to produce a separation bubble is considered. Benchmark DNS data for this configuration was generated with the resolution of 50 ×106 mesh points; also used was a different DNS database with 15 ×106 points reported by Spalart and Strelets in JFM 403 (2000). Employing two codes, one using structured and another unstructured mesh, we concluded that accurate LES are possible using O(1%) of the DNS resolution. Work performed during Stanford-CTR Summer Program 2012.
Numerical Simulations of Crater Formation with Dilatancy
NASA Astrophysics Data System (ADS)
Collins, G. S.; Melosh, H. J.; Wilson, C. R.; Wuennemann, K.
2011-12-01
The most characteristic geophysical signature of an impact crater is a circular negative gravity anomaly, centered over the crater. The cause of the gravity low is dilatancy: fracturing and brecciation, induced by the passage of the shock wave and comminution during crater formation, creates pore space between fragments and fractures, reducing the bulk density of the sub-crater material. Calculation of damage accumulation is routine in modern numerical impact simulations; accounting for dilatancy is not. As a result, most impact simulations do not correctly predict density changes beneath an impact crater, which limits the scope for comparison of model results with geophysical data. A simple approach to account for dilation during shear failure in impact simulations is to supplement the pressure computed by the equation of state with a "dilatancy pressure," representing the outward force of grains moving passed one another, in cells where shear failure has occurred (Johnson and Holmquist, 1994; doi:10.1063/1.46199). This additional pressure effectively shifts the pressure-density relationship for the dilatant material up (to a higher pressure) so that when the material unloads to atmospheric pressure the density drops to a (dilated) bulk density that is below the reference density of the pristine material. A limitation of this approach is that the bulk modulus of the dilated material is the same as that of the pristine material and, consequently, that an unrealistically large dilatancy pressure is required to achieve typical bulk densities of fractured rock. Here we propose an improvement to this approach where both the distension (porosity) and the pressure are modified during shear failure, which allows for the correct reduction in bulk modulus with increasing dilation. In our approach, shear failure leads to a prescribed decrease in the reference density of the dilatant material. The ratio of this reference density to the current density is used to compute a distension (porosity), which through the ?-? porosity model acts to increase the pressure by the amount required to shift the material from its current equation of state surface to that of the more distended, dilatant material. We show that simulations of crater formation using our dilatancy model are in good agreement with observed density and porosity variations beneath terrestrial simple craters and make predictions about the role of dilatancy in the formation of larger, complex craters. Our new dilatancy model will allow future numerical impact simulations to be directly compared with geophysical observations, such as gravity and seismic velocity anomalies, providing much greater observational constraint on simulation results. This is of particular significance for models of terrestrial craters where the surface expression has been removed by erosion and the geophysical signature is the only vestige of impact. Moreover, numerical simulations of cratering with dilatancy will aid in the interpretation of high-resolution gravity data soon to be collected over lunar craters by GRAIL.
Numerical simulation of pump-intake vortices
NASA Astrophysics Data System (ADS)
Rudolf, Pavel; Klas, Roman
2015-05-01
Pump pre-swirl or uneven flow distribution in front of the pump can induce pump-intake vortices. These phenomena result in blockage of the impeller suction space, deterioration of efficiency, drop of head curve and earlier onset of cavitation. Real problematic case, where head curve drop was documented, is simulated using commercial CFD software. Computational simulation was carried out for three flow rates, which correspond to three operating regimes of the vertical pump. The domain consists of the pump sump, pump itself excluding the impeller and the delivery pipe. One-phase approach is applied, because the vortex cores were not filled with air during observation of the real pump operation. Numerical simulation identified two surface vortices and one bottom vortex. Their position and strength depend on the pump flow rate. Paper presents detail analysis of the flow field on the pump intake, discusses influence of the vortices on pump operation and suggests possible actions that should be taken to suppress the intake vortices.
Numerical Simulations of Weak Lensing Measurements
David Bacon; Alexandre Refregier; Douglas Clowe; Richard Ellis
2000-07-03
(Abridged) Weak gravitational lensing induces distortions on the images of background galaxies, and thus provides a direct measure of mass fluctuations in the universe. Since the distortions induced by lensing on the images of background galaxies are only of a few percent, a reliable measurement demands very accurate galaxy shape estimation and a careful treatment of systematic effects. Here, we present a study of a shear measurement method using detailed simulations of artificial images. The images are produced using realisations of a galaxy ensemble drawn from the HST Groth strip. We consider realistic observational effects including atmospheric seeing, PSF anisotropy and pixelisation, incorporated in a manner to reproduce actual observations with the William Herschel Telescope. By applying an artificial shear to the simulated images, we test the shear measurement method proposed by Kaiser, Squires & Broadhurst (1995, KSB). Overall, we find the KSB method to be reliable with several provisos. To guide future weak lensing surveys, we study how seeing size, exposure time and pixelisation affect the sensitivity to shear. In addition, we study the impact of overlapping isophotes of neighboring galaxies, and find that this effect can produce spurious lensing signals on small scales. We discuss the prospects of using the KSB method for future, more sensitive, surveys. Numerical simulations of this kind are a required component of present and future analyses of weak lensing surveys.
51 (2006) APPLICATIONS OF MATHEMATICS No. 4, 321353 ON LARGE EDDY SIMULATION AND VARIATIONAL
John, Volker
2006-01-01
Dynamics (CFD). In general, Direct Numerical Simulation (DNS) is not feasible due to limited computer51 (2006) APPLICATIONS OF MATHEMATICS No. 4, 321353 ON LARGE EDDY SIMULATION AND VARIATIONAL MULTISCALE METHODS IN THE NUMERICAL SIMULATION OF TURBULENT INCOMPRESSIBLE FLOWS Volker John, Saarbrücken
Direct Numerical Simulation of dense particle-laden turbulent flows using immersed boundaries
NASA Astrophysics Data System (ADS)
Wang, Fan; Desjardins, Olivier
2009-11-01
Dense particle-laden turbulent flows play an important role in many engineering applications, ranging from pharmaceutical coating and chemical synthesis to fluidized bed reactors. Because of the complexity of the physics involved in these flows, current computational models for gas-particle processes, such as drag and heat transfer, rely on empirical correlations and have been shown to lack accuracy. In this work, direct numerical simulations (DNS) of dense particle-laden flows are conducted, using immersed boundaries (IB) to resolve the flow around each particle. First, the accuracy of the proposed approach is tested on a range of 2D and 3D flows at various Reynolds numbers, and resolution requirements are discussed. Then, various particle arrangements and number densities are simulated, the impact on particle wake interaction is assessed, and existing drag models are evaluated in the case of fixed particles. In addition, the impact of the particles on turbulence dissipation is investigated. Finally, a strategy for handling moving and colliding particles is discussed.
Numerical simulations of icing in turbomachinery
NASA Astrophysics Data System (ADS)
Das, Kaushik
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.
Numerical Simulations of Type Ia Supernova Explosions
F. K. Roepke; W. Hillebrandt; M. Gieseler; M. Reinecke; C. Travaglio
2006-09-15
We present a systematic study of the diversity of three-dimensional deflagration simulations of Type Ia supernova explosions arising from variations of the initial parameters. By altering the carbon mass fraction, the central density, and the metallicity of the progenitor white dwarf star, we set up a grid of numerical explosion models. While changing the central density has the largest impact on the explosion energy, the largest variation in the 56Ni production is found by changing the metallicity of the models. Varying the carbon mass fraction hardly affects the 56Ni synthesized although it alters the energetics of the explosion. Possible consequences for the shape of light curves of Type Ia supernovae are discussed.
Numerical simulations of jet- interstellar medium interactions
NASA Astrophysics Data System (ADS)
Ustamujic, S.; Gómez de Castro, A. I.; López-Santiago, J.
2015-05-01
The physical system formed by a very young star and its accretion disc is a scaled version of the compact object+accretion disc scenario observed in AGNs. For young stars with accretion discs (e.g. classical T Tauri stars), dense gas coming from the disc is collimated into a jet as explained in the context of the theory of magneto-centrifugal launching. We aim at studying the jet propagation and its interaction with the ambient medium. In particular, we are interested in determining the properties of the jet material in terms of density and temperature. Our objective is to understand the morphology of the jet at different wavelengths and the appearance of distinct structures such as blobs and Herbig-Haro objects and their relation with initial conditions. We performed a set of numerical model simulations of supersonic jet ramming into uniform ambient medium using the PLUTO code.
History of the numerical aerodynamic simulation program
NASA Technical Reports Server (NTRS)
Peterson, Victor L.; Ballhaus, William F., Jr.
1987-01-01
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.
Numerical relativity simulations of binary neutron stars
NASA Astrophysics Data System (ADS)
Thierfelder, Marcus; Bernuzzi, Sebastiano; Brügmann, Bernd
2011-08-01
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.
Direct numerical simulation of the turbulent channel flow of a polymer solution
NASA Astrophysics Data System (ADS)
Sureshkumar, R.; Beris, Antony N.; Handler, Robert A.
1997-03-01
In this work, we present from first principles a direct numerical simulation (DNS) of a fully turbulent channel flow of a dilute polymer solution. The polymer chains are modeled as finitely extensible and elastic dumbbells. The simulation algorithm is based on a semi-implicit, time-splitting technique which uses spectral approximations in the spatial coordinates. The computations are carried out on a CRAY T3D parallel computer. The simulations are carried out under fully turbulent conditions albeit, due to computational constraints, not at as high Reynolds number as that usually encountered in polymer-induced drag reduction experiments. In order to compensate for the lower Reynolds number, we simulate more elastic fluids than the ones encountered in drag reduction experiments resulting in Weissenberg numbers (a dimensionless number characterizing the flow elasticity) of similar magnitude. The simulations show that the polymer induces several changes in the turbulent flow characteristics, all of them consistent with available experimental results. In particular, we have observed, associated with drag reduction, a decrease in the streamwise vorticity fluctuations and an increase in the average spacing between the streamwise streaks of low speed fluid within the buffer layer. These findings suggest a partial inhibition of turbulence generating events within the buffer layer by the macromolecules after the onset of drag reduction. This inhibition is further shown to be associated with an enhanced effective viscosity attributed to the extensional thickening properties of polymer solutions, as proposed in the past by Metzner, Lumley and other investigators. Using the simulation results obtained for different sets of parameter values which modify the relaxational and extensional properties of the model, we propose a set of criteria for the onset of drag reduction.
DNS Measurements at the .CN TLD Servers
Yuchi Xuebiao; Wang Xin; Li Xiaodong; Yan Baoping
2009-01-01
The domain name system (DNS) is a fundamental Internet's infrastructure. There have been several studies on DNS measurements such as server workload and deployment characteristics. In this paper, based on the traffic collection spanning a two days period, we present a detailed study of the key characteristics of the .CN top level domain (TLD) name servers which are located at
DOMAIN NAME SYSTEM (DNS) SERVICES: NIST
June 2006 DOMAIN NAME SYSTEM (DNS) SERVICES: NIST RECOMMENDATIONS FOR SECURE DEPLOYMENT DOMAIN NAME and Technology Domain Name System (DNS) services have an important function in helping users readily access into a domain name that is easy for the user to remember. The IP address to which a user wishes to be connected
Numerical simulation of solar coronal magnetic fields
NASA Technical Reports Server (NTRS)
Dahlburg, Russell B.; Antiochos, Spiro K.; Zang, T. A.
1990-01-01
Many aspects of solar activity are believed to be due to the stressing of the coronal magnetic field by footpoint motions at the photosphere. The results are presented of a fully spectral numerical simulation which is the first 3-D time dependent simulation of footpoint stressing in a geometry appropriate for the corona. An arcade is considered that is initially current-free and impose a smooth footpoint motion that produces a twist in the field of approx 2 pi. The footprints were fixed and the evolution was followed until the field relaxes to another current-free state. No evidence was seen for any instability, either ideal or resistive and no evidence for current sheet formation. The most striking feature of the evolution is that in response to photospheric motions, the field expands rapidly upward to minimize the stress. The expansion has two important effects. First, it suppresses the development of dips in the field that could support dense, cool material. For the motions assumed, the magnetic field does not develop a geometry suitable for prominence formation. Second, the expansion inhibits ideal instabilities such as kinking. The results indicate that simple stearing of a single arcade is unlikely to lead to solar activity such as flares or prominences. Effects are discussed that might possibly lead to such activity.
Numerical simulation of premixed turbulent methane combustion
Day, Marc S.; Bell, John B.; Almgren, Ann S.; Beckner, Vincent E.; Lijewski, Michael J.; Cheng, Robert; Shepherd, Ian; Johnson, Matthew
2003-06-14
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.
Numerical simulation of the Polywell device
Simmons, K.H.; Santarius, J.F. [Univ. of Wisconsin, Madison, WI (United States). Fusion Technology Inst.
1995-12-31
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.
Numerical simulation of tulip flame dynamics
Cloutman, L.D.
1991-11-30
A finite difference reactive flow hydrodynamics program based on the full Navier-Stokes equations was used to simulate the combustion process in a homogeneous-charge, constant-volume combustion bomb in which an oddly shaped flame, known as a tulip flame'' in the literature, occurred. The tulip flame'' was readily reproduced in the numerical simulations, producing good agreement with the experimental flame shapes and positions at various times. The calculations provide sufficient detail about the dynamics of the experiment to provide some insight into the physical mechanisms responsible for the peculiar flame shape. Several factors seem to contribute to the tulip formation. The most important process is the baroclinic production of vorticity by the flame front, and this rate of production appears to be dramatically increased by the nonaxial flow generated when the initial semicircular flame front burns out along the sides of the chamber. The vorticity produces a pair of vortices behind the flame that advects the flame into the tulip shape. Boundary layer effects contribute to the details of the flame shape next to the walls of the chamber, but are otherwise not important. 24 refs.
Numerical simulation of tulip flame dynamics
Cloutman, L.D.
1991-11-30
A finite difference reactive flow hydrodynamics program based on the full Navier-Stokes equations was used to simulate the combustion process in a homogeneous-charge, constant-volume combustion bomb in which an oddly shaped flame, known as a ``tulip flame`` in the literature, occurred. The ``tulip flame`` was readily reproduced in the numerical simulations, producing good agreement with the experimental flame shapes and positions at various times. The calculations provide sufficient detail about the dynamics of the experiment to provide some insight into the physical mechanisms responsible for the peculiar flame shape. Several factors seem to contribute to the tulip formation. The most important process is the baroclinic production of vorticity by the flame front, and this rate of production appears to be dramatically increased by the nonaxial flow generated when the initial semicircular flame front burns out along the sides of the chamber. The vorticity produces a pair of vortices behind the flame that advects the flame into the tulip shape. Boundary layer effects contribute to the details of the flame shape next to the walls of the chamber, but are otherwise not important. 24 refs.
Numerical simulation of ballistic impact on composite laminates
M. A. G. Silva; C. Cisma?iu; C. G. Chiorean
2005-01-01
The paper reports experimental and numerical simulation of ballistic impact problems on thin composite laminated plates reinforced with Kevlar 29. Ballistic impact was imparted with simulated fragments designed in accordance with STANAG-2920 on plates of different thickness. Numerical modelling was developed and used to obtain an estimate for the limit perforation velocity (V50) and simulate failure modes and damage. Computations
Numerical Simulations and Design of Shearing Process for Aluminum Alloys
Aniruddha Khadke; Somnath Ghosh
2005-01-01
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.
Numerical simulation of micro-fluidic passive and active mixers
Kumar, Saurabh
2002-01-01
Numerical simulations of mixing using passive and active techniques are performed. For passive mixing, numerical modeling of a micro-fluidic device, build by Holden and Cremer, was performed. The micro-fluidic device consists of a Y...
A numerical simulation of the backward Raman amplifying in plasma
NASA Astrophysics Data System (ADS)
Wang, Hong-Yu; Huang, Zu-Qia
2005-12-01
This paper describe a numerical simulation method for the interaction between laser pulses and low density plasmas based on hydrodynamic approximation. We investigate Backward Raman Amplifying (BRA) experiments and their variants. The numerical results are in good agreement with experiments.
Numerical simulation of micro-fluidic passive and active mixers
Kumar, Saurabh
2002-01-01
Numerical simulations of mixing using passive and active techniques are performed. For passive mixing, numerical modeling of a micro-fluidic device, build by Holden and Cremer, was performed. The micro-fluidic device consists of a Y...
Numerical simulation of mesospheric gravity waves
NASA Astrophysics Data System (ADS)
Dörnbrack, A.; Rapp, M.; Latteck, R.
2012-12-01
Recently, first three-dimensionally resolved observations of polar mesospheric winter echeos (PMWEs) by a multi-beam experiment of the Middle Atmosphere Alomar Radar system (MAARSY) were published by Rapp et al. (2011). The observed PMWE at about 75 km altitude was tilted in the main flow direction (west to east). The origin of the PMWE was explained by two disparate concepts of gravity wave dynamics. On the one hand, the tilted PMWE was assumed to be aligned with the phase line of a linear gravity wave (? z ? 23 km, ? x ? 460 km) propagating at an intrinsic phase speed of -73 m/s against the westerly wind. On the other hand, turbulence generated by breaking gravity waves was a necessary element to explain the existence of backscattering fluctuations. In addition to a thoughrough analysis of the synoptic meteorological conditions, high-resolution numerical simulations are performed with the all-scale geophysical flow solver EULAG (Prusa et al., 2008). The anelastic and pseudo-incompressible approximated equations are solved in a 3D computational domain covers a 1500 km long slice of Scandinavia and spans from the surface to 100 km altitude. Multiple numerical experiments are performed to explore the origin of the observed PMWE. Various hypotheses are tested. The presentation will discuss if the PMWEs were the result of breaking mountain waves excited by the flow over Scandinavia or if dynamical instabilities occuring in the highly sheared mesospheric flow led to the observed turbulence. Rapp, M., R. Latteck, G. Stober, et al., 2011: First three-dimensional observations of polar mesosphere winter echoes: Resolving space-time ambiguity. J. Geophys. Res., 116, A11307, doi:10.1029/2011JA016858. Prusa, J.M., P.K. Smolarkiewicz, A.A. Wyszogrodzki, 2008: EULAG, a computational model for multiscale flows, Comput. Fluids 37, 1193-1207.
Numerical simulation of mesospheric gravity waves
NASA Astrophysics Data System (ADS)
Dörnbrack, Andreas; Rapp, Markus; Latteck, Ralph
2013-04-01
Recently, first three-dimensionally resolved observations of polar mesospheric winter echeos (PMWEs) by a multi-beam experiment of the Middle Atmosphere Alomar Radar system (MAARSY) were published by Rapp et al. (2011). The observed PMWE at about 75 km altitude was tilted in the main flow direction (west to east). The origin of the PMWE was explained by two disparate concepts of gravity wave dynamics. On the one hand, the tilted PMWE was assumed to be aligned with the phase line of a linear gravity wave (?z ? 23 km, ?x ? 460 km) propagating at an intrinsic phase speed of - 73 m/s against the westerly wind. On the other hand, turbulence generated by breaking gravity waves was a necessary element to explain the existence of backscattering fluctuations. In addition to a thorough analysis of the synoptic meteorological conditions, high-resolution numerical simulations are performed with the all-scale geophysical flow solver EULAG (Prusa et al., 2008). The anelastic and pseudo-incompressible approximated equations are solved in a 3D computational domain covers a 1500 km long slice of Scandinavia and spans from the surface to 100 km altitude. Multiple numerical experiments are performed to explore the origin of the observed PMWE. Various hypotheses are tested. The presentation will discuss if the PMWEs were the result of breaking mountain waves excited by the flow over Scandinavia or if dynamical instabilities occuring in the highly sheared mesospheric flow led to the observed turbulence. Rapp, M., R. Latteck, G. Stober, et al., 2011: First three-dimensional observations of polar mesosphere winter echoes: Resolving space-time ambiguity. J. Geophys. Res., 116, A11307, doi:10.1029/2011JA016858. Prusa, J.M., P.K. Smolarkiewicz, A.A. Wyszogrodzki, 2008: EULAG, a computational model for multiscale flows, Comput. Fluids 37, 1193-1207.
Numerical simulation of "An American Haboob"
NASA Astrophysics Data System (ADS)
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
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.
A mass-conserving volume of fluid method for DNS of droplet-laden isotropic turbulence
NASA Astrophysics Data System (ADS)
Ferrante, Antonino; Dodd, Michael
2012-11-01
We developed a mass-conserving wisps-free volume of fluid (VoF) method for direct numerical simulation (DNS) of droplet-laden turbulent flows. We used the continuous surface force (CSF) model to include the surface tension within a split-advection and mass-conserving VoF. The liquid-gas interface curvature is computed accurately using a variable-stencil height-function technique. We modified the sequence of the advection sweeps, and our results show that, in the case of non-zero Weber number, the algorithm is accurate and stable. We present DNS results of fully-resolved droplet-laden incompressible decaying isotropic turbulence at initial Re? = 190 using a computational mesh of 10243 grid points, droplet volume fraction 0.1 tracking the volumes of 7000 droplets of Weber number We = 0 . 5 based on the r.m.s. velocity fluctuation, droplet-to-fluid density ratio 10, and initial droplet diameter equal to the Taylor length-scale of turbulence.
Schilling, Oleg; Mueschke, Nicholas J.
2010-01-01
Data from a 1152X760X1280 direct numerical simulation (DNS) of a transitional Rayleigh-Taylor mixing layer modeled after a small Atwood number water channel experiment is used to comprehensively investigate the structure of mean and turbulent transport and mixing. The simulation had physical parameters and initial conditions approximating those in the experiment. The budgets of the mean vertical momentum, heavy-fluid mass fraction, turbulent kinetic energy, turbulent kinetic energy dissipation rate, heavy-fluid mass fraction variance, and heavy-fluid mass fraction variance dissipation rate equations are constructed using Reynolds averaging applied to the DNS data. The relative importance of mean and turbulent production, turbulent dissipationmore »and destruction, and turbulent transport are investigated as a function of Reynolds number and across the mixing layer to provide insight into the flow dynamics not presently available from experiments. The analysis of the budgets supports the assumption for small Atwood number, Rayleigh/Taylor driven flows that the principal transport mechanisms are buoyancy production, turbulent production, turbulent dissipation, and turbulent diffusion (shear and mean field production are negligible). As the Reynolds number increases, the turbulent production in the turbulent kinetic energy dissipation rate equation becomes the dominant production term, while the buoyancy production plateaus. Distinctions between momentum and scalar transport are also noted, where the turbulent kinetic energy and its dissipation rate both grow in time and are peaked near the center plane of the mixing layer, while the heavy-fluid mass fraction variance and its dissipation rate initially grow and then begin to decrease as mixing progresses and reduces density fluctuations. All terms in the transport equations generally grow or decay, with no qualitative change in their profile, except for the pressure flux contribution to the total turbulent kinetic energy flux, which changes sign early in time (a countergradient effect). The production-to-dissipation ratios corresponding to the turbulent kinetic energy and heavy-fluid mass fraction variance are large and vary strongly at small evolution times, decrease with time, and nearly asymptote as the flow enters a self-similar regime. The late-time turbulent kinetic energy production-to-dissipation ratio is larger than observed in shear-driven turbulent flows. The order of magnitude estimates of the terms in the transport equations are shown to be consistent with the DNS at late-time, and also confirms both the dominant terms and their evolutionary behavior. These results are useful for identifying the dynamically important terms requiring closure, and assessing the accuracy of the predictions of Reynolds-averaged Navier-Stokes and large-eddy simulation models of turbulent transport and mixing in transitional Rayleigh-Taylor instability-generated flow.« less
NASA Astrophysics Data System (ADS)
Druzhinin, Oleg; Troitskaya, Yuliya; Zilitinkevich, Sergej
2014-05-01
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.
Rotational threshold in global numerical dynamo simulations
NASA Astrophysics Data System (ADS)
Schrinner, M.
2013-04-01
Magnetic field observations of low-mass stars reveal an increase of magnetic activity with increasing rotation rate. The so-called activity-rotation relation is usually attributed to changes in the underlying dynamo processes generating the magnetic field. We examine the dependence of the field strength on rotation in global numerical dynamo models and interpret our results on the basis of energy considerations. In agreement with the scaling law proposed by Christensen and Aubert, the field strength in our simulations is set by the fraction of the available power used for the magnetic field generation. This is controlled by the dynamo efficiency calculated as the ratio of ohmic to total dissipation in our models. The dynamo efficiency grows strongly with increasing rotation rate at a Rossby number of 0.1 until it reaches its upper bound of 1 and becomes independent of rotation. This gain in efficiency is related to the strong rotational dependence of the mean electromotive force in this parameter regime. For multipolar models at Rossby numbers clearly larger than 0.1, on the other hand, we do not find a systematic dependence of the field strength on rotation. Whether the enhancement of the dynamo efficiency found in our dipolar models explains the observed activity-rotation relation needs to be further assessed.
Numerical Simulations of Heated Supersonic Rectangular Jets
NASA Astrophysics Data System (ADS)
Kolbe, R. L.; Kailasanath, K.; Boris, J. P.
1996-11-01
The heated supersonic flow from rectangular jets with paddles in the flow field have been simulated numerically to study the effects of heating on the flow field and near-field noise. The flapping motion across the narrow dimension of the jet, which is the dominant feature of the unheated jet, is also found to be present in heated jets with temperature ratios (temperature of the jet to that of the surroundings) from 2.17 to 5.0. With increasing jet temperature, the jet core extends further downstream towards the paddles and the shear layer development is also delayed. Furthermore, as the temperature ratio increases, the amplitude of the velocity fluctuations decrease and additional frequencies also begin to appear. The characteristic frequency also changes with temperature and the corresponding Strouhal number is found to decrease exponentially with increase in the temperature over the range studied. Several interesting observations on the modes of the heated jet will also be presented. footnote Sponsored by NASA-Lewis and NRL
NUMERICAL SIMULATIONS OF DRIVEN RELATIVISTIC MAGNETOHYDRODYNAMIC TURBULENCE
Zrake, Jonathan; MacFadyen, Andrew I. [Center for Cosmology and Particle Physics, Physics Department, New York University, New York, NY 10003 (United States)
2012-01-01
A wide variety of astrophysical phenomena involve the flow of turbulent magnetized gas with relativistic velocity or energy density. Examples include gamma-ray bursts, active galactic nuclei, pulsars, magnetars, micro-quasars, merging neutron stars, X-ray binaries, some supernovae, and the early universe. In order to elucidate the basic properties of the relativistic magnetohydrodynamical (RMHD) turbulence present in these systems, we present results from numerical simulations of fully developed driven turbulence in a relativistically warm, weakly magnetized and mildly compressible ideal fluid. We have evolved the RMHD equations for many dynamical times on a uniform grid with 1024{sup 3} zones using a high-order Godunov code. We observe the growth of magnetic energy from a seed field through saturation at {approx}1% of the total fluid energy. We compute the power spectrum of velocity and density-weighted velocity U = {rho}{sup 1/3} v and conclude that the inertial scaling is consistent with a slope of -5/3. We compute the longitudinal and transverse velocity structure functions of order p up to 11 and discuss their possible deviation from the expected scaling for non-relativistic media. We also compute the scale-dependent distortion of coherent velocity structures with respect to the local magnetic field, finding a weaker scale dependence than is expected for incompressible non-relativistic flows with a strong mean field.
Direct Numerical Simulation of Cell Printing
NASA Astrophysics Data System (ADS)
Qiao, Rui; He, Ping
2010-11-01
Structural cell printing, i.e., printing three dimensional (3D) structures of cells held in a tissue matrix, is gaining significant attention in the biomedical community. The key idea is to use desktop printer or similar devices to print cells into 3D patterns with a resolution comparable to the size of mammalian cells, similar to that in living organs. Achieving such a resolution in vitro can lead to breakthroughs in areas such as organ transplantation and understanding of cell-cell interactions in truly 3D spaces. Although the feasibility of cell printing has been demonstrated in the recent years, the printing resolution and cell viability remain to be improved. In this work, we investigate one of the unit operations in cell printing, namely, the impact of a cell-laden droplet into a pool of highly viscous liquids using direct numerical simulations. The dynamics of droplet impact (e.g., crater formation and droplet spreading and penetration) and the evolution of cell shape and internal stress are quantified in details.
Transonic aeroelastic numerical simulation in aeronautical engineering
NASA Astrophysics Data System (ADS)
Yang, Guowei
2006-06-01
A lower upper symmetric Gauss Seidel (LU-SGS) subiteration scheme is constructed for time-marching of the fluid equations. The Harten Lax van Leer Einfeldt Wada (HLLEW) scheme is used for the spatial discretization. The same subiteration formulation is applied directly to the structural equations of motion in generalized coordinates. Through subiteration between the fluid and structural equations, a fully implicit aeroelastic solver is obtained for the numerical simulation of fluid/structure interaction. To improve the ability for application to complex configurations, a multiblock grid is used for the flow field calculation and transfinite interpolation (TFI) is employed for the adaptive moving grid deformation. The infinite plate spline (IPS) and the principal of virtual work are utilized for the data transformation between the fluid and structure. The developed code was first validated through the comparison of experimental and computational results for the AGARD 445.6 standard aeroelastic wing. Then, the flutter character of a tail wing with control surface was analyzed. Finally, flutter boundaries of a complex aircraft configuration were predicted.
A numerical model simulation of longshore transport for Galveston Island
Gilbreath, Stephen Alexander
1995-01-01
The shoreline changes, deposition patterns, and longshore transport rates were calculated for the coast of Galveston Island using a numerical model simulation. The model only simulated changes due to waves creating longshore currents. East Beach...
3D DNS and LES of Breaking Inertia-Gravity Waves
NASA Astrophysics Data System (ADS)
Remmler, S.; Fruman, M. D.; Hickel, S.; Achatz, U.
2012-04-01
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.
NASA Astrophysics Data System (ADS)
Hackl, Jason F.
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
Direct numerical simulation of solid-liquid flow of Newtonian and viscoelastic fluids
NASA Astrophysics Data System (ADS)
Zhu, Mingyu
The main theme of this work is to enhance the understanding on the behavior of solid particles in flows of Newtonian or viscoelastic fluids by using both two-dimensional and three-dimensional direct numerical simulations (DNS). A large-scale state-of-the-art software package PARTMOVER3D is developed based on an Arbitrary Lagrangian-Eulerian (ALE) technique and an Elastic-Viscous-Stress-Split (EVSS) scheme. Our numerical results are extensively compared with analytical, experimental and numerical ones in the literature. We studied the motion of spheres sedimenting in a cylindrical tube filled with a Newtonian fluid. The hydrodynamic drag and lift on the particle are investigated under various conditions. The effects of the tube wall, in terms of the blockage ratio and the eccentricity ratio, on the particle terminal velocity, migration and rotation are studied. We also investigated the interaction between pair particles released in tandem or side by side at different Reynolds numbers. The migration of particles in a pressure driven flow is the heart of vast number of industrial applications. Using 3-D direct numerical simulations, we systematically investigated the independent parameters controlling the particle migration, which are the blockage ratio, the flow Reynolds number, and the solid-liquid density ratio. During the particle migration, the mechanisms of the fluid inertia, the wall confinement, the local flow shear rate, the particle slip velocity, the particle size, and the particle rotation were extensively examined through the stress distribution on the particle surface under different flow conditions. In the presence of a shear flow, an initially deposited bed of heavy particles will be entrained into the bulk fluid and convected away with the flow. We investigated the mechanism of this particle resuspension by using 2-D direct numerical simulations. Various effects on the lift force on the particle was analyzed by examining the distribution of the stress on the particle surface. We also studied the orientation of an elliptic particle during its resuspension, analyzed the interaction between a pair of particles, and presented the results of the resuspension of a layer of particles. There are striking differences of particle motions in viscoelastic and Newtonian fluids. We explained the mechanism of the anomalous particle behavior due to the elasticity of the viscoelastic fluid. The effects of the Deborah number, the Reynolds number, the retardation-relaxation time ratio, the blockage ratio, and the eccentricity rate on the behavior of the particles were also systemically investigated.
Low Reynolds number k-epsilon modelling with the aid of direct simulation data
W. Rodi; N. N. Mansour
1993-01-01
The constant C sub mu and the near-wall damping function f sub mu in the eddy-viscosity relation of the k-epsilon model are evaluated from direct numerical simulation (DNS) data for developed channel and boundary layer flow at two Reynolds numbers each. Various existing f sub mu model functions are compared with the DNS data, and a new function is fitted
Numerical Simulation of Complex Turbomachinery Flows
NASA Technical Reports Server (NTRS)
Chernobrovkin, A. A.; Lakshiminarayana, B.
1999-01-01
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.
Numerical Modeling and Simulation of a Cholesteric Liquid Crystal Polarizer
Jong Rak Park; Gihan Ryu; Jeongil Byun; Heenam Hwang; Sung Tae Kim; Insun Kim
2002-01-01
A numerical model based on Berreman’s 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
Numerical Simulation in Applied Geophysics. From the Mesoscale to the
Santos, Juan
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 layering, fractures and craks at the mesoscale (on the order of centimeters) are common in the earth
Pade Approximations in Inverse Homogenization and Numerical Simulation of Electromagnetic
Cherkaev, Elena
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
DNS of bifurcations in an air-filled rotating baroclinic annulus
Randriamampianina, A; Read, P L; Maubert, P; Randriamampianina, Anthony; Fruh, Wolf-Gerrit; Read, Peter L.; Maubert, Pierre
2006-01-01
Three-dimensional Direct Numerical Simulation (DNS) on the nonlinear dynamics and a route to chaos in a rotating fluid subjected to lateral heating is presented here and discussed in the context of laboratory experiments in the baroclinic annulus. Following two previous preliminary studies by Maubert and Randriamampianina, the fluid used is air rather than a liquid as used in all other previous work. This study investigated a bifurcation sequence from the axisymmetric flow to a number of complex flows. The transition sequence, on increase of the rotation rate, from the axisymmetric solution via a steady, fully-developed baroclinic wave to chaotic flow followed a variant of the classical quasi-periodic bifurcation route, starting with a subcritical Hopf and associated saddle-node bifurcation. This was followed by a sequence of two supercritical Hopf-type bifurcations, first to an amplitude vacillation, then to a three-frequency quasi-periodic modulated amplitude vacillation (MAV), and finally to a chaotic MAV\\...
DNS of a spatially evolving hypersonic turbulent boundary layer at Mach 8
NASA Astrophysics Data System (ADS)
Liang, Xian; Li, XinLiang
2013-07-01
This paper reports the direct numerical simulation (DNS) for hypersonic turbulent boundary layer over a flat-plate at Ma ?=8 with the ratio of wall-to-freestream temperature equal to 1.9, which indicates an extremely cold wall condition. It is primarily used to assess the wall temperature effects on the mean velocity profile, Walz equation, turbulent intensity, strong Reynolds analogy (SRA), and compressibility. The present high Mach number with cold wall condition induces strong compressibility effects. As a result, the Morkovin's hypothesis is not fully valid and so the classical SRA is also not fully consistent. However, some modified SRA is still valid at the far-wall region. It is also verified that the semi-local wall coordinate y* is better than conventional y + in analysis of statistics features in turbulent boundary layer (TBL) in hypersonic flow.
DNS of turbulent flow past a bluff body with a compliant tensegrity surface
NASA Astrophysics Data System (ADS)
Karandikar, Anish; Bewley, Thomas
2007-11-01
Direct numerical simulation (DNS) is used to study turbulent incompressible flow past a bluff body with a compliant surface. We use a 3D time-dependent coordinate transformation to account for the motion of the bluff body surface. Spatially, the flow domain is discretized using a dealiased pseudospectral method in the axial and azimuthal directions, while the radial (wall-normal) direction is discretized using a finite difference scheme. The grid is stretched in the azimuthal direction, which is handled spectrally. This leads to a unique challenge when solving the Poisson equation in the fractional step method for the time march, which we address with both multigrid and preconditioned BiCGStab algorithms. We are presently extending this flow code with a model for the compliant bluff body surface based on the ``tensegrity fabric'' paradigm which combines compressive members (bars) and tensile members (tendons) in a stable, flexible network.
Aggregation in an expanding cloud: experiments and numerical simulations
Françoise Dziedzinl; Robert Botet
1991-01-01
We have set-up an experimental device and a numerical model to study aggregation of an aerosol (titanium oxide) expanding in the atmosphere. By mean of scanning microscopic analysis, it is shown that agglomerated aerosol forms fractal clusters of fractal dimension of about 1.75. The numerical simulations quantitatively confirm this geometrical feature. We show how our numerical results can complete some
Numerical simulation of turbulent flow in a cyclonic separator
NASA Astrophysics Data System (ADS)
Bogdanov, Dmitry; Poniaev, Sergey
2014-12-01
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.
Direct numerical simulations of transition and turbulence in smooth-walled Stokes boundary layer
NASA Astrophysics Data System (ADS)
Ozdemir, Celalettin E.; Hsu, Tian-Jian; Balachandar, S.
2014-04-01
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.
Probing Strong Field Gravity Through Numerical Simulations
Choptuik, Matthew W; Pretorius, Frans
2015-01-01
This article is an overview of the contributions numerical relativity has made to our understanding of strong field gravity, to be published in the book "General Relativity and Gravitation: A Centennial Perspective", commemorating the 100th anniversary of general relativity.
Looking Back Molecular-dynamics simulations require numerical methods
Duisburg-Essen, Universität
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
NASA Astrophysics Data System (ADS)
Druzhinin, Oleg; Troitskaya, Yliya; Zilitinkevich, Sergej
2015-04-01
Detailed knowledge of the interaction of surface water waves with the wind flow is of primary importance for correct parameterization of turbulent momentum and heat fluxes which define the energy and momentum transfer between the atmosphere and hydrosphere. The objective of the present study is to investigate the properties of the stably stratified turbulent boundary-layer (BL) air-flow over waved water surface by direct numerical simulation (DNS) at a bulk Reynolds number varying from 15000 to 80000 and the surface-wave slope up to ka = 0.2. The DNS results show that the BL-flow remains in the statistically stationary, turbulent regime if the Reynolds number (ReL) based on the Obukhov length scale and friction velocity is sufficiently large (ReL > 100). In this case, mean velocity and temperature vertical profiles are well predicted by log-linear asymptotic solutions following from the Monin-Obukhov similarity theory provided the velocity and temperature roughness parameters, z0U and z0T, are appropriately prescribed. Both z0U and z0T increase for larger surface-wave slope. DNS results also show that turbulent momentum and heat fluxes and turbulent velocity and temperature fluctuations are increased for larger wave slope (ka) whereas the mean velocity and temperature derivatives remain practically the same for different ka. Thus, we conclude that the source of turbulence enhancement in BL-flow are perturbations induced by the surface wave, and not the shear instability of the bulk flow. On the other hand, if stratification is sufficiently strong, and the surface-wave slope is sufficiently small, the BL-flow over waved surface relaminarizes in the bulk of the domain. However, if the surface-wave slope exceeds a threshold value, the velocity and temperature fluctuations remain finite in the vicinity of the critical-layer level, where the surface-wave phase velocity coincides with the mean flow velocity. We call this new stably-stratified BL-flow regime observed in our DNS a "wave-pumping" regime. We develop a theoretical model and explain the occurrence of the wave-pumping regime observed in DNS as a result of the generation of two-dimensional (2D) disturbances in the air flow under the influence of the surface wave and secondary, parametric instability of these disturbances along the surface-wave front direction. The model predicts that the wave-pumping regime occurs only for sufficiently steep waves which is in agreement with DNS results. The model prediction for the amplitudes of the wave-induced 2D disturbances in the air flow is also in good qualitative and quantitative agreement with DNS results. The results also show that increasing the bulk Reynolds number of the air-flow leads to the development of a wide spectrum of the disturbances. At a sufficiently high super-criticality we expect a transition to occur from the wave-pumping regime to a fully-developed, turbulent BL-flow regime, even at high Richardson number when the air flow over a smooth surface relaminarizes. This work was supported by RFBR (project No. 14-05-00367) and by RSF (project No. 14-17 -0086).
Numerical simulation of Martian dust devils
Anthony D. Toigo; Mark I. Richardson; Shawn P. Ewald; Peter J. Gierasch
2003-01-01
Large eddy simulations of vertical convective vortices and dust devils in the Martian convective boundary layer are presented, employing a version of the Mars MM5 mesoscale model, adapted to use periodic boundary conditions and run at resolutions of 10 to 100 m. The effects of background horizontal wind speed and shear on dust devil development are studied in four simulations,
Malapaka, Shiva Kumar; Mueller, Wolf-Christian [Max-Planck Institute for Plasma Physics, Boltzmannstrasse 2, D-85748 Garching bei Muenchen (Germany)
2013-09-01
Statistical properties of the Sun's photospheric turbulent magnetic field, especially those of the active regions (ARs), have been studied using the line-of-sight data from magnetograms taken by the Solar and Heliospheric Observatory and several other instruments. This includes structure functions and their exponents, flatness curves, and correlation functions. In these works, the dependence of structure function exponents ({zeta}{sub p}) of the order of the structure functions (p) was modeled using a non-intermittent K41 model. It is now well known that the ARs are highly turbulent and are associated with strong intermittent events. In this paper, we compare some of the observations from Abramenko et al. with the log-Poisson model used for modeling intermittent MHD turbulent flows. Next, we analyze the structure function data obtained from the direct numerical simulations (DNS) of homogeneous, incompressible 3D-MHD turbulence in three cases: sustained by forcing, freely decaying, and a flow initially driven and later allowed to decay (case 3). The respective DNS replicate the properties seen in the plots of {zeta}{sub p} against p of ARs. We also reproduce the trends and changes observed in intermittency in flatness and correlation functions of ARs. It is suggested from this analysis that an AR in the onset phase of a flare can be treated as a forced 3D-MHD turbulent system in its simplest form and that the flaring stage is representative of decaying 3D-MHD turbulence. It is also inferred that significant changes in intermittency from the initial onset phase of a flare to its final peak flaring phase are related to the time taken by the system to reach the initial onset phase.
Numerical Simulation of Two Phase Flows
NASA Technical Reports Server (NTRS)
Liou, Meng-Sing
2001-01-01
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.
Direct numerical simulation of an isothermal reacting turbulent wall-jet
NASA Astrophysics Data System (ADS)
Pouransari, Zeinab; Brethouwer, Geert; Johansson, Arne V.
2011-08-01
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.
A numerical sensitivity analysis of streamline simulation
Chaban Habib, Fady Ruben
2005-02-17
ratios, mapping of saturation methods, number of streamlines, time step sizes, and gravity effects. This study provides a detailed investigation of some fundamental issues that are currently unresolved in streamline simulation....
Wavelet methods For the numerical simulation of incompressible fluids
Starck, Jean-Luc
Wavelet methods For the numerical simulation of incompressible fluids Erwan Deriaz Erwan Numerik, Seminar February 16th 2006 0-0 #12; Wavelets for the Navier-Stokes equations homogeneous or non homogeneous) With a wavelet discretization: £ ¦ ¨ 2 ©$# £ % ¦ ¨ 2 © & '' ( 1
Numerical Simulation of Nix's Rotation - Duration: 100 seconds.
This is a numerical simulation of the orientation of Nix as seen from the center of the Pluto system. It has been sped up so that one orbit of Nix around Pluto takes 2 seconds instead of 25 days. L...
Numerical Simulation Study on Transpired Solar Air Collector
Wang, C.; Guan, Z.; Zhao, X.; Wang, D.
2006-01-01
ICEBO2006, Shenzhen, China Re newable Energy Resources and a Greener Future Vol.VIII-3-4 Numerical Simulation Study on Transpired Solar Air Collector Chongjie Wang Zhenzhong Guan Xueyi Zhao Delin Wang Professor...
NUMERICAL METHODS FOR THE SIMULATION OF HIGH INTENSITY HADRON SYNCHROTRONS.
LUCCIO, A.; D'IMPERIO, N.; MALITSKY, N.
2005-09-12
Numerical algorithms for PIC simulation of beam dynamics in a high intensity synchrotron on a parallel computer are presented. We introduce numerical solvers of the Laplace-Poisson equation in the presence of walls, and algorithms to compute tunes and twiss functions in the presence of space charge forces. The working code for the simulation here presented is SIMBAD, that can be run as stand alone or as part of the UAL (Unified Accelerator Libraries) package.
Numerical simulation of tornado-like vortices around complex geometries
Frederique Drullion
2009-01-01
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
Floret Test, Numerical Simulations of the Dent, Comparison with Experiments
Lefran?ois, A; Cutting, J; Gagliardi, F; Tarver, C; Tran, T
2006-02-14
The Floret test has been developed as a screening test to study the performance of a small amount of HE. Numerical simulations have been performed recently using CTH. The objective of this study is to perform numerical simulations in order to better understand the shock waves interactions, involved in the dent formation. Different 3D wedge configurations have been tested using the Ignition and Growth reactive flow model for the HE receptor with Ls-Dyna.
Numerical simulation of transition, compressible turbulence, and reacting flows
NASA Technical Reports Server (NTRS)
Zang, T. A.; Drummond, J. P.; Erlebacher, G.; Speziale, C.; Hussaini, M. Y.
1987-01-01
Some of the recent work at NASA Langley on transition, turbulence, and reacting flows is summarized. Much of this effort has been motivated by outstanding technological problems in high-speed flow. A class of numerical algorithms for these problems has been developed and a variety of physical problems have been simulated. Descriptions are provided of the basic mathematical models, the nature of the numerical methods, and some of the recent simulations.
Numerical simulations of leading-edge acoustic receptivity
David A. Fuciarelli
1997-01-01
Numerical simulations of leading-edge acoustic receptivity are performed for a flat-plate with an elliptical leading-edge. The Modified Super Ellipse is chosen as the leading-edge geometry. The flow is simulated by solving the incompressible Navier-Stokes equations in a general curvilinear coordinate system in stream-function\\/vorticity form. The Modified Strongly Implicit Procedure is the second-order-accurate, robust, and memory-conservative numerical scheme used. A time-harmonic
Numerical simulation of tsunami waves generated by deformable submarine landslides
Kirby, James T.
Numerical simulation of tsunami waves generated by deformable submarine landslides Gangfeng Ma a 2013 Accepted 4 July 2013 Available online 15 July 2013 Keywords: Submarine landslide Nonhydrostatic wave model Tsunami wave Numerical modeling a b s t r a c t This paper presents a new submarine
Numerical Simulation of Solid Particle Propagation in the Atmosphere
P. Baltrenas; S. Vasarevicius; E. Petraitis
2003-01-01
Numerical models are currently mainly used to estimate air pollution in Lithuania. For example, the VARSA program has been used to determine pollution standards for industry, but this model was developed by a method itself devised 20 years ago, which has a narrow sphere of application and many shortcomings. Therefore, numerical simulation of gas and heavy aerosol propagation remains a
Numerical simulation of unsteady aerodynamic heating induced by shock reflections
Shigeru Aso; Kenichi Ohyama
1992-01-01
Numerical simulations on unsteady shock reflections by a ramp have been conducted in order to investigate unsteady aerodynamic heating due to shock reflection processes at a higher incident shock Mach number. The two-dimensional Navier-Stokes equations with a thin layer approximation are solved numerically by a Total Variation Diminishing (TVD) scheme. The effect of mesh refinement to the calculated results is
Applying numerical simulation results to LISA
NASA Astrophysics Data System (ADS)
Baker, John; Kelly, Bernard; McWilliams, Sean; Thorpe, James
2009-05-01
Binary black hole systems are key observational targets of both ground- and space-based gravitational wave observatories. Interpretation of these observations depends on a detailed understanding of the gravitational radiation waveforms predicted by General Relativity. Advances in numerical relativity are leading to an increasingly rich understanding of the strong radiation generated in the final moments of these mergers. This knowledge can now be applied to answer questions of gravitational wave data analysis. Using the Effective-One-Body formalism together with ideas from an implicit rotating source characterization of numerical relativity waveforms, we construct a parameterized analytic waveform model representing the complete gravitational wavetrain. Then using standard data analysis techniques we apply this model toward an improved understanding of how well the Laser Interferometer Space Antenna (LISA) will be able to measure the astrophysical parameters of massive black hole mergers.
Numerical simulation of in situ bioremediation
Travis, B.J.
1998-12-31
Models that couple subsurface flow and transport with microbial processes are an important tool for assessing the effectiveness of bioremediation in field applications. A numerical algorithm is described that differs from previous in situ bioremediation models in that it includes: both vadose and groundwater zones, unsteady air and water flow, limited nutrients and airborne nutrients, toxicity, cometabolic kinetics, kinetic sorption, subgridscale averaging, pore clogging and protozoan grazing.
Numerical simulation of subcooled flow boiling
Won Cheol Park
2003-01-01
Sub-cooled flow boiling in a U-bend has been examined using numerical methods. An Eulerian\\/Eulerian mathematical description was used with a multiphase computational algorithm to predict several types of flows and to examine sub-cooled flow boiling. As a prelude to the study of sub-cooled boiling and two-phase flows, single-phase laminar and turbulent flows in a U-bend were investigated. Air-water bubbly up
Numerical simulation of magmatic hydrothermal systems
NASA Astrophysics Data System (ADS)
Ingebritsen, S. E.; Geiger, S.; Hurwitz, S.; Driesner, T.
2010-03-01
The dynamic behavior of magmatic hydrothermal systems entails coupled and nonlinear multiphase flow, heat and solute transport, and deformation in highly heterogeneous media. Thus, quantitative analysis of these systems depends mainly on numerical solution of coupled partial differential equations and complementary equations of state (EOS). The past 2 decades have seen steady growth of computational power and the development of numerical models that have eliminated or minimized the need for various simplifying assumptions. Considerable heuristic insight has been gained from process-oriented numerical modeling. Recent modeling efforts employing relatively complete EOS and accurate transport calculations have revealed dynamic behavior that was damped by linearized, less accurate models, including fluid property control of hydrothermal plume temperatures and three-dimensional geometries. Other recent modeling results have further elucidated the controlling role of permeability structure and revealed the potential for significant hydrothermally driven deformation. Key areas for future research include incorporation of accurate EOS for the complete H2O-NaCl-CO2 system, more realistic treatment of material heterogeneity in space and time, realistic description of large-scale relative permeability behavior, and intercode benchmarking comparisons.
Numerical Simulation of a Thermoacoustic Refrigerator
Aniruddha S. Worlikar; Omar M. Knio
1996-01-01
A low Mach-number compressible flow model for the simulation of acoustically driven flow in a thermoacoustic stack is constructed. The model is based on the assumption that the acoustic wavelength is much larger than the characteristic hydrodynamic lengthscale. Thus, a simplified description of the flow is obtained which still retains the essential features of acoustically induced velocity oscillations near solid
Numerical simulation of superconducting accelerator magnets
Stefan Kurz; Stephan Russenschuck
2002-01-01
Modeling and simulation are key elements in assuring the fast and successful design of superconducting magnets. After a general introduction the paper focuses on electromagnetic field computations, which are an indispensable tool in the design process. A technique which is especially well suited for the accurate computation of magnetic fields in superconducting magnets is presented. This method couples boundary elements
Numerical simulation of Martian dust devils
Anthony D. Toigo; Mark I. Richardson; Shawn P. Ewald
2003-01-01
(1) Large eddy simulations of vertical convective vortices and dust devils in the Martian convective boundary layer are presented, employing a version of the Mars MM5 mesoscale model, adapted to use periodic boundary conditions and run at resolutions of 10 to 100 m. The effects of background horizontal wind speed and shear on dust devil development are studied in four
Numerical simulations of dwarf galaxy merger trees
NASA Astrophysics Data System (ADS)
Cloet-Osselaer, A.; De Rijcke, S.; Vandenbroucke, B.; Schroyen, J.; Koleva, M.; Verbeke, R.
2014-08-01
We investigate the evolution of dwarf galaxies using N-body/smoothed particle hydrodynamics simulations that incorporate their formation histories through merger trees constructed using the extended Press-Schechter formalism. The simulations are computationally cheap and have high spatial resolution. We compare the properties of galaxies with equal final mass but with different merger histories with each other and with those of observed dwarf spheroidals and irregulars. We show that the merger history influences many observable dwarf galaxy properties. We identify two extreme cases that make this influence stand out most clearly: (i) merger trees with one massive progenitor that grows through relatively few mergers and (ii) merger trees with many small progenitors that merge only quite late. At a fixed halo mass, a type (i) tree tends to produce galaxies with larger stellar masses, larger half-light radii, lower central surface brightness and, since fewer potentially angular momentum cancelling mergers are required to build up the final galaxy, a higher specific angular momentum, compared with a type (ii) tree. We do not perform full-fledged cosmological simulations and therefore cannot hope to reproduce all observed properties of dwarf galaxies. However, we show that the simulated dwarfs are similar to real ones.
Brush seal numerical simulation: Concepts and advances
NASA Technical Reports Server (NTRS)
Braun, M. J.; Kudriavtsev, V. V.
1994-01-01
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.
Numerical simulation of a controlled boundary layer
NASA Technical Reports Server (NTRS)
Zang, Thomas A.; Hussaini, M. Yousuff
1986-01-01
The problem of interest is the boundary layer over a flat plate. The three standard laminar flow control (LFC) techniques are pressure gradient, suction, and heating. The parameters used to describe the amount of control in the context of the boundary layer equations are introduced. The numerical method required to find the mean flow, the linear eigenvalues of the Orr-Sommerfeld equation, and the full, nonlinear, 3-D solution of the Navier-Stokes equations are outlined. A secondary instability exists for the parallel boundary subject to uniform pressure gradient, suction, or heating. Selective control of the spanwise mode reduces the secondary instability in the parallel boundary layer at low Reynolds number.
Numerical Simulation of a Gridded Inductive Output Amplifier
IOA MODEL
A set of codes for the numerical simulation of a gridded Inductive Output Amplifier has been developed in close collaboration between BINP and Philips RHW. The following problems are investigated: nonstationary gridded gun simulation, self-consistent solution for beam-cavity interaction and passband calculations. The Finite Element Method is used for the field calculation in the electron gun. Space charge forces are
Numerical simulation of a liquid propellant rocket motor
Nicolas M. C. Salvador; Marcelo M. Morales; Carlos E. S. S. Migueis; Demétrio Bastos-Netto
2001-01-01
This work presents a numerical simulation of the flow field in a liquid propellant rocket engine chamber and exit nozzle using techniques to allow the results to be taken as starting points for designing those propulsive systems. This was done using a Finite Volume method simulating the different flow regimes which usually take place in those systems. As the flow
Numerical Simulation of Pulsed Meander Coil Emat
NASA Astrophysics Data System (ADS)
Dhayalan, R.; Balasubramaniam, Krishnan; Krishnamurthy, C. V.
2010-02-01
Electro magnetic acoustic transducers (EMATs) are now being widely investigated for non-contact non-destructive testing (NDT) of solid materials. This type of transducer can generate and/or detect ultrasound in electrically conductive or magnetic materials through the Lorentz force principle and/or magneto-elastic effects. This work describes about the Meander coil EMAT that is modeled using finite element method. A 2-D finite element model was developed to calculate the induced current inside the medium, and subsequently the Lorentz force density in the medium. The calculated Lorentz force density values are applied for simulating the transient ultrasonic wave generation within the medium. Meander coil EMATs that were designed using the model were used for experimental studies. Several case studies will be reported which include Rayleigh waves, Shear waves, Longitudinal and Lamb wave modes using pulsed mode of excitation. The experimental results were agreed well with the simulation results.
Numerical simulation of inflatable membrane structures
NASA Astrophysics Data System (ADS)
Kolar, Ramesh; Whittinghill, Craig; Agrawal, Brij N.
2001-08-01
Inflatable structures are effective in space applications, as they are weight, volume and cost competitive. For certain space applications, higher gains are obtained for the antennas by increasing their size. Higher gains often result in increased data throughput. These and other advantages lead to inflatable structures being considered increasingly for building large space structures. However, large inflatable structures are prone to surface errors arising from environmental factors, among others. In this context, piezoelectric films are used for the active and passive control. In this paper, we discuss numerical approaches exploring piezoelectric film. In order to explore the applications of piezoelectric films, a circular diaphragm is subjected to varying pressures and displacements are measured using laser instrumentation. The effects of applying voltage on the shape of the piezoelectric film subjected to pressurization are studied. The piezoelectric film is modeled as a large displacement/large rotation membrane undergoing small strains. This paper presents experience gained in modeling the piezoelectric film subjected to both thermal and pressure loads. The numerical results are presented in the form of graphs. The response is studied for applied steady-state temperatures for various pressurization levels. Certain thermo-structural instabilities were encountered in the modeling and the paper presents procedures used in circumventing such instabilities for the piezoelectric type of thin inflatable membranes.
Numerical simulations of bent, disrupted radio jets
NASA Technical Reports Server (NTRS)
Loken, Chris; Burns, Jack O.
1993-01-01
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.
Numerical simulation of friction stir welding process
Dongun Kim; Harsha Badarinarayan; Ill Ryu; Ji Hoon Kim; Chongmin Kim; Kazutaka Okamoto; R. H. Wagoner; Kwansoo Chung
2009-01-01
Thermo-mechanical simulations of the friction stir butt welding and friction stir spot welding processes were performed for\\u000a AA5083-H18 sheets, utilizing commercial FVM codes which are based on the Eulerian formulation. For the friction stir butt\\u000a welding process, the computational fluid dynamics code, STAR-CCM+, was utilized under the steady state condition. Temperature\\u000a and strain rate histories along the material flow were
Numerical Simulations Using the Immersed Boundary Technique
NASA Technical Reports Server (NTRS)
Piomelli, Ugo; Balaras, Elias
1997-01-01
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.
Numerical simulation of complex turbomachinery flows
Andrey A. Chernobrovkin
1999-01-01
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
Numerical simulation of fundamental trapped sausage modes
M. Cécere; A. Costa; O. Reula
2011-01-01
Context: We integrate the 2D MHD ideal equations of a straight slab to simulate observational results associated with fundamental sausage trapped modes. Aims: Starting from a non-equilibrium state with a dense chromospheric layer, we analyse the evolution of the internal plasma dynamics of magnetic loops, subject to line-tying boundary conditions, and with the coronal parameters described in Asai et al.
Numerical Simulations of Fine Structures within Reconnecting
of the simulation domain are the open boundary. The magnetic field is line-tied at bottom. · The initial width w=0: Initial w = 0.1L, =0.1, Rm = 900. · Below: Rm = 500, 104, 5x104, 5x105. Magnetic field lines and Bx #12;3. · Plasma and magnetic fields on the two sides of the sheet commence to move to one another gradually
Towards the numerical verification of plasma simulation codes
NASA Astrophysics Data System (ADS)
Vukovic, Mirko
2012-10-01
To aid in verification of existing and new plasma simulation codes, we propose a suite of standard simulation problems against which a new code would be compared with. Each standard problem provides a detailed input specifications and results in forms of tables of numeric values. The problems use an idealized and simplified reaction cross-section and rates set. The problems are designed to verify individual numerical components of plasma simulation codes and the overall plasma simulation. The issue of establishing a ``correct'' plasma simulation result will be discussed. In addition, we will discuss the portability of these problems: the problems should be specified in a manner that can be read by simulation codes written in different languages, and executed on different platforms.
Scramjet Propulsive Flowpath Design and Numerical Simulation
NASA Astrophysics Data System (ADS)
Li, Jian-ping; Song, Wen-yan; Liu, Xin
2014-06-01
The integrated propulsive flowpath of scramjet configuration was preliminarily designed and analyzed in this paper. The flow-fields characteristics and performance of the designed two-dimensional integrated propulsive flowpath were numerically calculated under various equivalent fuel-air ratio conditions, using computational fluid dynamics methods. The calculation results were then compared with the experimental data on some typical conditions, and the flow-field and performance of the integrated scramjet flowpath with different equivalent fuel-air ratios were analyzed and discussed in detail. The investigation results from these efforts showed that: (1) the inlet function was beyond disturbances by combustion induced shock wave and pressure fluctuations under the equivalent fuel-air ratio condition of 1.0, which well satisfied the design requirements; (2) with the increasing equivalent fuel-air ratio, the combustion intensity in the combustor was significantly enhanced, resulting in an increasing net-thrust of the propulsive flowpath.
Numerical simulation of optically trapped particles
NASA Astrophysics Data System (ADS)
Volpe, Giorgio; Volpe, Giovanni
2014-07-01
Some randomness is present in most phenomena, ranging from biomolecules and nanodevices to financial markets and human organizations. However, it is not easy to gain an intuitive understanding of such stochastic phenomena, because their modeling requires advanced mathematical tools, such as sigma algebras, the Itô formula and martingales. Here, we discuss a simple finite difference algorithm that can be used to gain understanding of such complex physical phenomena. In particular, we simulate the motion of an optically trapped particle that is typically used as a model system in statistical physics and has a wide range of applications in physics and biophysics, for example, to measure nanoscopic forces and torques.
Fragmentation Instability of Molecular Clouds: Numerical Simulations
R. Indebetouw; E. G. Zweibel
2000-02-04
We simulate fragmentation and gravitational collapse of cold, magnetized molecular clouds. We explore the nonlinear development of an instability mediated by ambipolar diffusion, in which the collapse rate is intermediate to fast gravitational collapse and slow quasistatic collapse. Initially uniform stable clouds fragment into elongated clumps with masses largely determined by the cloud temperature, but substantially larger than the thermal Jeans mass. The clumps are asymmetric, with significant rotation and vorticity, and lose magnetic flux as they collapse. The clump shapes, intermediate collapse rates, and infall profiles may help explain observations not easily fit by contemporary slow or rapid collapse models.
Numerical and laboratory simulations of auroral acceleration
Gunell, H.; De Keyser, J. [1Belgian Institute for Space Aeronomy, Avenue Circulaire 3, B-1180 Brussels (Belgium)] [1Belgian Institute for Space Aeronomy, Avenue Circulaire 3, B-1180 Brussels (Belgium); Mann, I. [EISCAT Scientific Association, P.O. Box 812, SE-981 28 Kiruna, Sweden and Department of Physics, Umeå University, SE-901 87 Umeå (Sweden)] [EISCAT Scientific Association, P.O. Box 812, SE-981 28 Kiruna, Sweden and Department of Physics, Umeå University, SE-901 87 Umeå (Sweden)
2013-10-15
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.
Numerical and laboratory simulations of auroral acceleration
NASA Astrophysics Data System (ADS)
Gunell, H.; De Keyser, J.; Mann, I.
2013-10-01
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.
Numerical simulation of Martian dust devils
NASA Astrophysics Data System (ADS)
Toigo, Anthony D.; Richardson, Mark I.; Ewald, Shawn P.; Gierasch, Peter J.
2003-06-01
Large eddy simulations of vertical convective vortices and dust devils in the Martian convective boundary layer are presented, employing a version of the Mars MM5 mesoscale model, adapted to use periodic boundary conditions and run at resolutions of 10 to 100 m. The effects of background horizontal wind speed and shear on dust devil development are studied in four simulations, each extending over the daytime portion of one Martian day. The general vorticity development in all cases is similar, with roughly equal positive and negative vorticity extrema. Two dust devils were found to develop in the highest wind speed case and in a case run without background wind. The dust devil structures were found to agree well qualitatively with terrestrial dust devil observations, including the prediction of greatly diminished vertical velocities in the vortex core. Thermodynamic scaling theory of dust devils was found to provide good prediction of the relationship between central pressure and temperature in the modeled vortices. Examination of the turbulent kinetic energy budgets suggests balance between buoyancy generation and loss through dissipation and transport. The vorticity for the dust devils is provided by twisting of horizontal vorticity into the vertical. The horizontal vorticity originates from horizontal variations in temperature at the lower boundary (thermal buoyancy). While the horizontal winds generated by the modeled dust devils were likely insufficient to lift dust, this study provides a solid starting point for dynamic modeling of what may be an important component of the Martian dust cycle.
Numerical simulation of the SOFIA flow field
NASA Technical Reports Server (NTRS)
Klotz, Stephen P.
1995-01-01
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.
Numerical studies of boundary-layer receptivity
Helen L. Reed
1995-01-01
Direct numerical simulations (DNS) of the acoustic receptivity process on a semi-infinite flat plate with a modified-super-elliptic (MSE) leading edge are performed. The incompressible Navier-Stokes equations are solved in stream-function\\/vorticity form in a general curvilinear coordinate system. The steady basic-state solution is found by solving the governing equations using an alternating direction implicit (ADI) procedure which takes advantage of the
Numerical Simulation of Colliding Ion Acoustic Solitons
NASA Astrophysics Data System (ADS)
Nishimura, Y.; Chen, Y. H.; Cheng, C. Z.
2011-10-01
The ion acoustic wave dispersion relation ? = kCs 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)., 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. We discuss our studies on 1d-1v Vlasov-Poisson system employing the splitting scheme (by the method of characteristics). B.B. Kadomtsev, Doklady Akademii Nauk SSSR 192, 753 (1970).
Numerical simulations on the magnetopause current layer
Okuda, H.
1990-12-01
One-dimensional particle simulations are carried out in order to study the current layer between a plasma and magnetic field such as seen at the magnetopause boundary layer. When a subsonic solar wind plasma flow impinges upon a vacuum dipole magnetic field, the width of the current layer is found much smaller than the ion gyroradius and is close to theoretically predicted geometric mean of the ion and electron gyroradii. The width remains essentially the same when the magnetic field is filled with a thermal plasma whose density is smaller than the incoming solar wind density. The width, therefore, remains much smaller than the ion gyroradius. It is found that a similar sharp current layer develops in a plasma confined in a magnetic field such as seen in laboratory and space plasmas. 15 refs., 11 figs.
Numerical aerodynamic simulation facility. Preliminary study extension
NASA Technical Reports Server (NTRS)
1978-01-01
The production of an optimized design of key elements of the candidate facility was the primary objective of this report. This was accomplished by effort in the following tasks: (1) to further develop, optimize and describe the function description of the custom hardware; (2) to delineate trade off areas between performance, reliability, availability, serviceability, and programmability; (3) to develop metrics and models for validation of the candidate systems performance; (4) to conduct a functional simulation of the system design; (5) to perform a reliability analysis of the system design; and (6) to develop the software specifications to include a user level high level programming language, a correspondence between the programming language and instruction set and outline the operation system requirements.
Numerical simulation of supersonic gap flow.
Jing, Xu; Haiming, Huang; Guo, Huang; Song, Mo
2015-01-01
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
Numerical Simulation of Supersonic Gap Flow
Jing, Xu; Haiming, Huang; Guo, Huang; Song, Mo
2015-01-01
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
Numerical simulation of tides in Ontario Lacus
NASA Astrophysics Data System (ADS)
Vincent, David; Karatekin, Ozgür
2015-04-01
Hydrocarbons liquid filled lakes has been recently detected on Titan's surface. Most of these lakes are located in the northern latitudes but there is a substantial lake in the southern latitudes: Ontario Lacus. This lake gets our attention because of possible shoreline changes suggested by Cassini flybys over Ontario Lacus between September 2005 (T7) et January 2010 (T65). The shoreline changes could be due to evaporation-precipitation processes but could also be a consequence of tides. Previous studies showed that the maximal tidal amplitudes of Ontario Lacus would be about 0.2m (for an uniform bathymetry of 20m). In this study we simulate tidal amplitude and currents with SLIM (Second-generation Louvain-la-Neuve Ice-ocean Model, http://sites.uclouvain.be/slim/ ) which resolves 2D shallow water equation on an unstructured mesh. Unstructured mesh prevents problems like mesh discontinuities at poles and allows higher accuracy at some place like coast or straits without drastically increasing computing costs. The tide generating force modeled in this work is the gradient of tidal potential due to titan's obliquity and titan's orbital eccentricity around Saturn (other contribution such as sun tide generating force are unheeded). The uncertain input parameters such as the wind direction and amplitude, bottom friction and thermo-physical properties of hydrocarbons liquids are varied within their expected ranges. SAR data analysis can result in different bathymetry according to the method. We proceed simulations for different bathymetries: tidal amplitudes doesn't change but this is not the case for tidal currents. Using a recent bathymetry deduced from most recent RADAR/SAR observations and a finer mesh, the peak-to peak tidal amplitudes are calculated to be up to 0.6 m. which is more than a factor two larger than the previous results. The maximal offshore tidal currents magnitude is about 0.06 m/s.
Numerical simulation of porosity-free titanium dental castings.
Wu, M; Augthun, M; Schädlich-Stubenrauch, J; Sahm, P R; Spiekermann, H
1999-08-01
The objective of this research was to analyse, predict and control the porosity in titanium dental castings by the use of numerical simulation. A commercial software package (MAGMASOFT) was used. In the first part of the study, a model casting (two simplified tooth crowns connected by a connector bar) was simulated to analyse shrinkage porosity. Secondly, gas pores were numerically examined by means of a ball specimen with a "snake" sprue. The numerical simulation results were compared with the experimental casting results, which were made on a centrifugal casting machine. The predicted shrinkage levels coincided well with the experimentally determined levels. Based on the above numerical analyses, an optimised running and gating system design for the crown model was proposed. The numerical filling and solidification results of the ball specimen showed that this simulation model could be helpful for the explanation of the experimentally indicated gas pores. It was concluded that shrinkage porosity in titanium dental casting was predictable, and it could be minimised by improving the running and gating system design. Entrapped gas pores can be explained from the simulation results of the mould filling and solidification. PMID:10467947
Intermediate DNS Caching as an Attack Vector
Bill Manning
he Domain Name System (DNS) specification calls for the use of caching. Caching is expected to improve the overall responsiveness of the system by ensuring that answers to ques- tions are known and stored locally and that the query load placed on the authoritative servers is minimized. Certain presumptions are associated with caches that may no longer hold. This article
Numerical simulations of radiatively driven dusty winds
NASA Astrophysics Data System (ADS)
Krumholz, Mark R.; Thompson, Todd A.
2013-09-01
Radiation pressure on dust grains may be an important mechanism in driving winds in a wide variety of astrophysical systems. However, the efficiency of the coupling between the radiation field and the dusty gas is poorly understood in environments characterized by high optical depths like those in ultraluminous infrared galaxies (ULIRGs) and massive dense star clusters. We present a series of idealized numerical experiments, performed with the radiation-hydrodynamic code ORION, in which we study the dynamics of such winds and quantify their properties. We find that, after wind acceleration begins, radiation Rayleigh-Taylor instability forces the gas into a configuration that reduces the rate of momentum transfer from the radiation field to the gas by a factor of ˜10-100 compared to an estimate based on the optical depth at the base of the atmosphere; instead, the rate of momentum transfer from a driving radiation field of luminosity L to the gas is roughly L/c multiplied by one plus half the optical depth evaluated using the photospheric temperature, which is far smaller than the optical depth one would obtain using the interior temperature. When we apply our results to conditions appropriate to ULIRGs and star clusters, we find that the asymptotic wind momentum flux from such objects should not significantly exceed that carried by the direct radiation field, L/c. This result constrains the expected mass-loss rates from systems that exceed the Eddington limit to be of the order of the so-called `single-scattering' limit, and not significantly higher. We present an approximate fitting formula for the rate of momentum transfer from radiation to dusty gas through which it passes, which is suitable for implementation in sub-grid models of galaxy formation. Finally, we provide a first map of the column density distribution of gas in a radiatively driven wind as a function of velocity, and velocity dispersion.
Numerical simulation of the edge tone phenomenon
NASA Technical Reports Server (NTRS)
Dougherty, N. S.; Liu, B. L.; Ofarrell, J. M.
1994-01-01
Time accurate Navier-Stokes computations were performed to study a class 2 (acoustic) whistle, the edge tone, and to gain knowledge of the vortex-acoustic coupling mechanisms driving production of these tones. Results were obtained by solving the full Navier-Stokes equations for laminar compressible air flow of a two dimensional jet issuing from a slit interacting with a wedge. Cases considered were determined by varying the distance from the slit to the wedge. Flow speed was kept constant at 1,750 cm/s as was the slit thickness of 0.1 cm, corresponding to conditions in the experiments of Brown. The analytical computations revealed edge tones to be present in four harmonic stages of jet flow instability over the wedge as the jet length was varied from 0.3 to 1.6 cm. Excellent agreement was obtained in all four edge tone stage cases between the present computational results and the experimentally obtained frequencies and flow visualization results of Brown. Specific edge tone generation phenomena and further confirmation of certain theories and empirical formulas concerning these phenomena were brought to light in this analytical simulation of edge tones.
Probabilistic Approach to Numerical Simulation of Fracture
NASA Astrophysics Data System (ADS)
Gerasimov, Alexander
2013-06-01
The natural heterogeneity of real materials structure influencing on distribution of material physicomechanical characteristics (PMC) is one of the factors determining character of destruction. The introduction of the given factor in the equations of mechanics of a deformable solid is possible at use probabilistic laws of distribution PMC on volume of a considered design. There are problems where the fragmentation is mainly probabilistic process: explosive destruction axisymmetric shells where character of blasting fragmentation are beforehand unknown. Determining influence of heterogeneity of material structure is shown as well in problems punching thin barrier. In order that simulated process of a fragmentation reflected a real picture of behavior of the destroyed bodies, it is necessary to bring in casual distribution of initial deviations strength properties from rating value to PMC of a body. In work the explosive fragmentation of the shells, a fragmentation of a barrier and an shell after barrier piercing, punching thin barrier on a normal and under an angle, crushing of metal rings, process of high-speed impact of the laminated - spaced barrier with the steel spheres is considered.
Numerical simulation of field emission from silicon
Jensen, K.L.; Ganguly, A.K. [Naval Research Lab., Washington, DC (United States)] [Naval Research Lab., Washington, DC (United States)
1993-03-01
A common method for estimating current from semiconductor field emission is to assume that the potential inside the semiconductor is the same as that which would occur if no current were emitted [zero emitted current approximation (ZECA)], and then to use the calculated density at the surface in a standard Fowler-Nordheim analysis familiar from metallic field emission calculations. At high fields, it is obvious that this method fails, as the effect of current is no longer negligible in the calculation of the band bending effects. In this work, we shall present methods for calculating the band bending effects at arbitrary fields in the ZECA model; a comparison of the Wentzel-Kramers-Brillouin and Fowler-Nordheim approaches to field emission for silicon will be compared to the airy function approach of evaluating the transmission coefficient (TC) in the ZECA model; and finally, the limits of ZECA for field emission from silicon will be analyzed by comparing the TC calculations to Wigner distribution function (WDF) simulations performed with full band bending effects and scattering. As anticipated, the ZECA model, whether calculated by the TC or WDF approach, diverges from the full band bending WDF model for high fields. A discussion of this behavior is given in light of the density and current profiles. 20 refs., 8 figs.
Parallel Numerical Simulations of Water Reservoirs
NASA Astrophysics Data System (ADS)
Torres, Pedro; Mangiavacchi, Norberto
2010-11-01
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.
Numerical simulation of fundamental trapped sausage modes
Cécere, M; Reula, O
2011-01-01
Context: We integrate the 2D MHD ideal equations of a straight slab to simulate observational results associated with fundamental sausage trapped modes. Aims: Starting from a non-equilibrium state with a dense chromospheric layer, we analyse the evolution of the internal plasma dynamics of magnetic loops, subject to line-tying boundary conditions, and with the coronal parameters described in Asai et al. (2001) and Melnikov et al. (2002) to investigate the onset and damping of sausage modes. Methods: To integrate the equations we used a high resolution shock-capturing (HRSC) method specially designed to deal appropriately with flow discontinuities. Results: Due to non-linearities and inhomogeneities, pure modes are difficult to sustain and always occur coupled among them so as to satisfy, e.g., the line-tying constraint. We found that, in one case, the resonant coupling of the sausage fundamental mode with a slow one results in a non-dissipative damping of the former. Conclusions: In scenarios of thick and den...
Numerical Simulation of Ice Accretions Based on Unstructured Grids
NASA Astrophysics Data System (ADS)
Zhang, Q.; Gao, Z. H.
2011-09-01
An Eulerian method to numerically simulate ice accretions on airfoils based on unstructured grids has been presented. The Euler Equations are employed to solve the flowfields of the airfoils. The governing equations for droplets are solved based on the velocity distribution of the airflow to obtain the collection efficiency on the airfoil surfaces. The different freezing models are employed to simulate the freezing processes of the rime and glaze ice in order to calculate the ice amount. The ice accretions on a NACA 0012 airfoil have been simulated and the comparison with the experimental data indicates that the simulated method presented is feasible and effective.
Numerical simulation of steady and unsteady flows through plane cascades
NASA Astrophysics Data System (ADS)
Fo?t, J.; Hun?k, M.; Kozel, K.; Lain, J.; Šejna, M.; Vav?incová, M.
This paper of a few co-authors presents some works of the group of the Department of Technical Mathematics, Faculty of Mechanical Eng., TU Prague, which deals with numerical methods in fluid dynamics. We present numerical methods for a solution of different physical and mathematical models of flow through plane cascades. We use the Mac Cormack's scheme, Ron — Ho — Ni's scheme and Runge — Kutta schemes on H — type structured grid and upwind schemes on an unstructured triangular grid. This methods are used for simulation of steady or unsteady inviscid flow and for simulation of viscous laminar flow. We deal with comparison of different methods mutually and with experimental data and with comparison of different physical and mathematical models of flow used for numerical simulation.
Direct Numerical Simulation of Turbulent Compression Ramp Flow
N. A. Adams
1998-01-01
: A numerical procedure for the direct numerical simulation of compressible turbulent flow and shock–turbulence interaction\\u000a is detailed and analyzed. An upwind-biased finite-difference scheme with a compact centered stencil is used to discretize\\u000a the convective part of the Navier–Stokes equations. The scheme has a uniformly high approximation order and allows for a spectral-like\\u000a wave resolution while dissipating nonresolved wave numbers.
A numerical simulation on the perforation of reinforced concrete targets
Fenglei Huang; Haijun Wu; Qiankun Jin; Qingming Zhang
2005-01-01
The perforation of steel-bar-reinforced concrete target is studied numerically in this paper using LS-DYNA code and the modified Taylor–Chen–Kuszmaul continuum damage model. The crater diameters on both the front and back surfaces of the concrete target and the residual velocity of the projectile predicted by the numerical simulation are in good agreement with the experimental results reported by Hanchak et
Numerical Simulation of Low Mach Number Fluid - Phenomena
Scott H. Reitsma
1994-01-01
A method for the numerical simulation of low Mach number (M) fluid-acoustic phenomena is developed. This computational fluid-acoustic (CFA) methodology is based upon a set of conservation equations, termed finite-compressible, derived from the unsteady Navier-Stokes equations. The finite-compressible and more familiar pseudo-compressible equations are compared. The impact of derivation assumptions are examined theoretically and through numerical experimentation. The error associated
Numerical simulation of the countercurrent flow in a gas centrifuge
NASA Astrophysics Data System (ADS)
Cloutman, L. D.; Gentry, R. A.
1983-03-01
A finite difference method for the numerical simulation of the axisymmetric countercurrent flow in a gas centrifuge is presented. A time-marching technique is used to relay an arbitrary initial condition to the desired steady-state solution. All boundary layers can be resolved, and nonlinear effects may be included. Numerical examples are presented. It is concluded that this technique is capable of predicting accurately the performance of a wide variety of machines under all operating conditions of interest.
Industrial challenges for numerical simulation of crystal growth
NASA Astrophysics Data System (ADS)
Bogdanov, M.; Ofengeim, D.; Zhmakin, A.
2004-03-01
Numerical simulation of industrial crystal growth is difficult due to its multidisciplinary nature and the complex geometry of the real-life growth equipment. An attempt is made to itemize physical phenomena dominant in the different methods for growth of bulk crystals from the melt and the vapor phase as well as to review corresponding numerical approaches. Academic research and industrial applications are compared. Development of a computational engine and a graphic user interface of the industry-oriented codes is discussed. A simulator for the entire growth process of bulk crystals by sublimation method is described.
Numerical simulation of dynamic fracture and failure in solids
Chen, E.P.
1994-05-01
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.
Numerical simulation of electron heating in a tokamak
Bogdanova, N.E.; Larionov, M.M.; Fedorov, V.I.
1984-07-01
The electron temperature profiles resulting from localization of the auxiliary-heating power in various places have been simulated numerically. The effect of the auxiliary electron heating on the global plasma properties (the electron energy, the loop voltage, and the energy lifetime) has been studied. The results of the numerical simulation are compared with experimental data on electron cyclotron heating in the FT-1 tokamak. The method selected for solving the heat-conduction equation yields a universal electron temperature profile for the model used here. The results may thus prove useful for analyzing experiments on electron heating in other devices.
Spur-type instability observed on numerically simulated vortex filaments
NASA Technical Reports Server (NTRS)
Rossow, Vernon J.
1988-01-01
An instability observed on vortex filaments during numerical simulations of the three-dimensional, time-dependent dynamics of vortex wakes is studied to determine when and why it occurs. It is concluded that the observed instability is a consequence of the use of straight-line vortex segments of finite length to model continuously curving vortex filaments. The instability appears to occur only when the link length is a sizable fraction of the vortex span and, therefore, is not expected in an experiment. Guidelines are then given that help avoid numerical instabilities when vortex filaments are used in flow simulations.
Numerical simulation of tornado wind loading on structures
NASA Technical Reports Server (NTRS)
Maiden, D. E.
1976-01-01
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.
Numerical Simulations of a Possible Hypercomputational Quantum Algorithm
Sicard, A; Vélez, M; Sicard, Andr\\'es; Ospina, Juan; V\\'elez, Mario
2005-01-01
The hypercomputers compute functions or numbers, or more generally solve problems or carry out tasks, that cannot be computed or solved by a Turing machine. Several numerical simulations of a possible hypercomputational algorithm based on quantum computations previously constructed by the authors are presented. The hypercomputability of our algorithm is based on the fact that this algorithm could solve a classically non-computable decision problem, Hilbert's tenth problem. The numerical simulations were realized for three types of Diophantine equations: with and without solutions in non-negative integers, and without solutions by way of various traditional mathematical packages.
Numerical Simulations of a Possible Hypercomputational Quantum Algorithm
Andrés Sicard; Juan Ospina; Mario Vélez
2005-04-05
The hypercomputers compute functions or numbers, or more generally solve problems or carry out tasks, that cannot be computed or solved by a Turing machine. Several numerical simulations of a possible hypercomputational algorithm based on quantum computations previously constructed by the authors are presented. The hypercomputability of our algorithm is based on the fact that this algorithm could solve a classically non-computable decision problem, Hilbert's tenth problem. The numerical simulations were realized for three types of Diophantine equations: with and without solutions in non-negative integers, and without solutions by way of various traditional mathematical packages.
Numerical simulations of time-resolved quantum electronics
NASA Astrophysics Data System (ADS)
Gaury, Benoit; Weston, Joseph; Santin, Matthieu; Houzet, Manuel; Groth, Christoph; Waintal, Xavier
2014-01-01
Numerical simulation has become a major tool in quantum electronics both for fundamental and applied purposes. While for a long time those simulations focused on stationary properties (e.g. DC currents), the recent experimental trend toward GHz frequencies and beyond has triggered a new interest for handling time-dependent perturbations. As the experimental frequencies get higher, it becomes possible to conceive experiments which are both time-resolved and fast enough to probe the internal quantum dynamics of the system. This paper discusses the technical aspects-mathematical and numerical-associated with the numerical simulations of such a setup in the time domain (i.e. beyond the single-frequency AC limit). After a short review of the state of the art, we develop a theoretical framework for the calculation of time-resolved observables in a general multiterminal system subject to an arbitrary time-dependent perturbation (oscillating electrostatic gates, voltage pulses, time-varying magnetic fields, etc.) The approach is mathematically equivalent to (i) the time-dependent scattering formalism, (ii) the time-resolved non-equilibrium Green’s function (NEGF) formalism and (iii) the partition-free approach. The central object of our theory is a wave function that obeys a simple Schrödinger equation with an additional source term that accounts for the electrons injected from the electrodes. The time-resolved observables (current, density, etc.) and the (inelastic) scattering matrix are simply expressed in terms of this wave function. We use our approach to develop a numerical technique for simulating time-resolved quantum transport. We find that the use of this wave function is advantageous for numerical simulations resulting in a speed up of many orders of magnitude with respect to the direct integration of NEGF equations. Our technique allows one to simulate realistic situations beyond simple models, a subject that was until now beyond the simulation capabilities of available approaches.
Numerical Simulations of Galaxy Formation: Cooling, Heating, Star \\\\ Formation
NASA Astrophysics Data System (ADS)
Klypin, A. A.
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.
Large Eddy Simulations and Turbulence Modeling for Film Cooling
NASA Technical Reports Server (NTRS)
Acharya, Sumanta
1999-01-01
The objective of the research is to perform Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES) for film cooling process, and to evaluate and improve advanced forms of the two equation turbulence models for turbine blade surface flow analysis. The DNS/LES were used to resolve the large eddies within the flow field near the coolant jet location. The work involved code development and applications of the codes developed to the film cooling problems. Five different codes were developed and utilized to perform this research. This report presented a summary of the development of the codes and their applications to analyze the turbulence properties at locations near coolant injection holes.
Direct numerical simulation of compressible free shear flows
NASA Technical Reports Server (NTRS)
Lele, Sanjiva K.
1989-01-01
Direct numerical simulations of compressible free shear layers in open domains are conducted. Compact finite-difference schemes of spectral-like accuracy are used for the simulations. Both temporally-growing and spatially-growing mixing layers are studied. The effect of intrinsic compressibility on the evolution of vortices is studied. The use of convective Mach number is validated. Details of vortex roll up and pairing are studied. Acoustic radiation from vortex roll up, pairing and shape oscillations is studied and quantified.
Numerical simulation of double-diffusive finger convection
Joseph D. Hughes; Ward E. Sanford; H. Leonard Vacher
2005-01-01
A hybrid finite element, integrated finite difference numerical model is developed for the simulation of double-diffusive and multicomponent flow in two and three dimensions. The model is based on a multidimensional, density-dependent, saturated-unsaturated transport model (SUTRA), which uses one governing equation for fluid flow and another for solute transport. The solute-transport equation is applied sequentially to each simulated species. Density
Numerical simulation of a liquid propellant rocket motor
Nicolas M. C. Salvador; Marcelo M. Morales; Carlos E. S. S. Migueis; Demétrio Bastos-Netto
2001-01-01
This work presents a numerical simulation of the flow field in a liquid propellant rocket engine chamber and exit nozzle using\\u000a techniques to allow the results to be taken as starting points for designing those propulsive systems. This was done using\\u000a a Finite Volume method simulating the different flow regimes which usually take place in those systems. As the flow
Numerical simulation of deformation and figure quality of precise mirror
NASA Astrophysics Data System (ADS)
Vit, Tomáš; Melich, Radek; Sandri, Paolo
2015-01-01
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.
Numerical simulation of a premixed turbulent V-flame
Bell, John B.; Day, Marc S.; Grcar, Joseph F.; Lijewski, Michael J.; Johnson, Matt R.; Cheng, Robert K.; Shepherd, Ian G.
2003-07-27
We present three-dimensional, time-dependent simulations of a full-size laboratory-scale rod-stabilized premixed turbulent V-flame. The computations use an adaptive projection method based on a low Mach number formulation that incorporates detailed chemical kinetics and transport. The simulations are performed without introducing models for turbulence or turbulence chemistry interaction. We outline the numerical procedure and experimental setup, and compare computed results to mean flame location and surface wrinkling statistics gathered from experiment.
Numerical simulation of piezoelectric effect of bone under ultrasound irradiation
NASA Astrophysics Data System (ADS)
Hosokawa, Atsushi
2015-07-01
The piezoelectric effect of bone under ultrasound irradiation was numerically simulated using an elastic finite-difference time-domain method with piezoelectric constitutive equations (PE-FDTD method). First, to demonstrate the validity of the PE-FDTD method, the ultrasound propagation in piezoelectric ceramics was simulated and then compared with the experimental results. The simulated and experimental waveforms propagating through the ceramics were in good agreement. Next, the piezoelectric effect of human cortical bone on the ultrasound propagation was investigated by PE-FDTD simulation. The simulated result showed that the difference between the waveforms propagating through the bone without and with piezoelectricity was negligible. Finally, the spatial distributions of the electric fields in a human femur induced by ultrasound irradiation were simulated. The electric fields were changed by a bone fracture, which depended on piezoelectric anisotropy. In conclusion, the PE-FDTD method is considered to be useful for investigating the piezoelectric effect of bone.
Numerical simulations and modeling for stochastic biological systems with jumps
NASA Astrophysics Data System (ADS)
Zou, Xiaoling; Wang, Ke
2014-05-01
This paper gives a numerical method to simulate sample paths for stochastic differential equations (SDEs) driven by Poisson random measures. It provides us a new approach to simulate systems with jumps from a different angle. The driving Poisson random measures are assumed to be generated by stationary Poisson point processes instead of Lévy processes. Methods provided in this paper can be used to simulate SDEs with Lévy noise approximately. The simulation is divided into two parts: the part of jumping integration is based on definition without approximation while the continuous part is based on some classical approaches. Biological explanations for stochastic integrations with jumps are motivated by several numerical simulations. How to model biological systems with jumps is showed in this paper. Moreover, method of choosing integrands and stationary Poisson point processes in jumping integrations for biological models are obtained. In addition, results are illustrated through some examples and numerical simulations. For some examples, earthquake is chose as a jumping source which causes jumps on the size of biological population.
Research on Numerical Simulation of High Temperature Heat Pipe
Qing-fang Deng; Dongyi Zhou
2011-01-01
The paper researches the strong heat transfer numerical simulation to the transitional period of medium and low temperature of high temperature heat pipe heat exchanger. It sets the calculation model of the headquarters according to the characteristics of heat transfer of heat pipe heat exchanger, and forecasts the fluid temperature field distribution outside of pipe reasonably and accurately by FULENT
Numerical Simulation and Cold Modeling experiments on Centrifugal Casting
NASA Astrophysics Data System (ADS)
Keerthiprasad, Kestur Sadashivaiah; Murali, Mysore Seetharam; Mukunda, Pudukottah Gopaliengar; Majumdar, Sekhar
2011-02-01
In a centrifugal casting process, the fluid flow eventually determines the quality and characteristics of the final product. It is difficult to study the fluid behavior here because of the opaque nature of melt and mold. In the current investigation, numerical simulations of the flow field and visualization experiments on cold models have been carried out for a centrifugal casting system using horizontal molds and fluids of different viscosities to study the effect of different process variables on the flow pattern. The effects of the thickness of the cylindrical fluid annulus formed inside the mold and the effects of fluid viscosity, diameter, and rotational speed of the mold on the hollow fluid cylinder formation process have been investigated. The numerical simulation results are compared with corresponding data obtained from the cold modeling experiments. The influence of rotational speed in a real-life centrifugal casting system has also been studied using an aluminum-silicon alloy. Cylinders of different thicknesses are cast at different rotational speeds, and the flow patterns observed visually in the actual castings are found to be similar to those recorded in the corresponding cold modeling experiments. Reasonable agreement is observed between the results of numerical simulation and the results of cold modeling experiments with different fluids. The visualization study on the hollow cylinders produced in an actual centrifugal casting process also confirm the conclusions arrived at from the cold modeling experiments and numerical simulation in a qualitative sense.
Numerical simulations of atmospheric tides for solstice conditions
François Vial
1986-01-01
Numerical simulations are made to study the behavior of atmospheric tides in an atmosphere with arbitrary distribution of mean temperature and zonal wind. Owing to their importance at meteor heights, the parameterization of diffusive processes is discussed, and two kinds of models are used, depending on the particular mode studied. Specific attention is given to the (1, 1) mode of
Direct Numerical Simulation of a Quasilaminarized Boundary Layer
Luciano Castillo; Juan Guillermo Araya; Raul Bayoan Cal
2010-01-01
Direct Numerical Simulations of spatially-evolving turbulent boundary layers with strong favorable pressure gradients are performed. The driven force behind this investigation is elucidate the mechanisms responsible for the quasi-laminarization of the boundary layer. Budgets of the turbulent kinetic energy and the shear Reynolds stresses provide insight into the terms responsible for this phenomenon. The results also confirm the similarity analysis
Numerical simulations of shock-induced mixing and combustion
Bell, John B.; Day, Marc S.; Kuhl, A.
2003-07-27
In this paper we use numerical simulation to investigate shock-induced ignition and combustion of a hydrocarbon gas. The focus of this paper is on quantifying the effect of fidelity in the chemical kinetics on the overall solution. We model the system using the compressible Navier Stokes equations for a reacting mixture. These equations express conservation of species mass, momentum, total energy.
Numerical simulation of coal gasification in entrained flow coal gasifier
H. Watanabe; M. Otaka
2006-01-01
This paper presents modeling of a coal gasification reaction, and prediction of gasification performance for an entrained flow coal gasifier. The purposes of this study are to develop an evaluation technique for design and performance optimization of coal gasifiers using a numerical simulation technique, and to confirm the validity of the model. The coal gasification model suggested in this paper
Numerical simulations of volcanic jets: Importance of vent overpressure
is in stark contrast to the generally applied Gaussian or top-hat profile. We show that the magnitude; accepted 5 November 2007; published 29 February 2008. [1] Explosive volcanic eruption columns are generally shock waves. Using a pseudogas approximation for a mixture of tephra and gas, we numerically simulate
CLOUDY 90: Numerical Simulation of Plasmas and Their Spectra
G. J. Ferland; K. T. Korista; D. A. Verner; J. W. Ferguson; J. B. Kingdon; E. M. Verner
1998-01-01
CLOUDY is a large-scale spectral synthesis code designed to simulate fully physical conditions within an astronomical plasma and then predict the emitted spectrum. Here we describe version 90 (C90) of the code, paying particular attention to changes in the atomic database and numerical methods that have affected predictions since the last publicly available version, C84. The computational methods and uncertainties
Numerical Simulation of the December 26, 2004: Indian Ocean Tsunami
Kirby, James T.
Numerical Simulation of the December 26, 2004: Indian Ocean Tsunami J. Asavanant1, M. Ioualalen2, N. Kaewbanjak1, S. Grilli3, P. Watts4, and J. Kirby5 Abstract: The December 26, 2004 tsunami is one of the most devastating tsunami in recorded history. It was generated in the Indian Ocean off the western coast
Numerical simulation of tsunamis — Its present and near future
N. Shuto
1991-01-01
Hindcasting of a tsunami by numerical simulations is a process of lengthy and complicated deductions, knowing only the final results such as run-up heights and tide records, both of which are possibly biased due to an insufficient number of records and due to hydraulic and mechanical limitation of tide gauges. There are many sources of error. The initial profile, determined
Statistical palaeomagnetic field modelling and dynamo numerical simulation
C. Bouligand; G. Hulot; A. Khokhlov; G. A. Glatzmaier
2005-01-01
By relying on two numerical dynamo simulations for which such investigations are possible, we test the validity and sensitivity of a statistical palaeomagnetic field modelling approach known as the giant gaussian process (GGP) modelling approach. This approach is currently used to analyse palaeomagnetic data at times of stable polarity and infer some information about the way the main magnetic field
Numerical simulations of gas-particle ?ows with combustion
Julien NUSSBAUM; Philippe HELLUY; Alain CARRI
2005-01-01
This work is devoted to the numerical modelling of a reactive gas- particle ?ow that arises in internal ballistic. The model, proposed by Gough (1), takes into account complex physical phenomena such as mass transfer, drag force or intra granular stress. A non-conservative flnite volume approach adapted from (10) is applied in order to simulate the model. After an academic
Numerical simulations of gas-particle flows with combustion
Julien Nussbaum; Philippe Helluy; Jean-Marc Hérard; Alain Carriére
2006-01-01
This work is devoted to the numerical modelling of a reactive gas-particle flow that arises in internal ballistic. The model, proposed by Gough [2], takes into account complex physical phenomena such as mass transfer, drag force or intra granular stress. A non-conservative finite volume approach adapted from [11] is applied in order to simulate the model. After an academic validation
Particle acceleration in solar flares: observations versus numerical simulations
Particle acceleration in solar flares: observations versus numerical simulations A O Benz, P C processes such as isotropization and magnetic trapping are made. Keywords: Particle acceleration, hard X. As the electric field of reconnection with possible parallel component capable of particle acceleration is limited
Numerical Simulation of the Perrin-Like Experiments
ERIC Educational Resources Information Center
Mazur, Zygmunt; Grech, Dariusz
2008-01-01
A simple model of the random Brownian walk of a spherical mesoscopic particle in viscous liquids is proposed. The model can be solved analytically and simulated numerically. The analytic solution gives the known Einstein-Smoluchowski diffusion law r[superscript 2] = 2Dt, where the diffusion constant D is expressed by the mass and geometry of a…
A NUMERICAL SIMULATOR FOR PREDICTING CHEMICAL FLOOD PERFORMANCE
Michael Todd; Curtis Chase
1979-01-01
Intercomp has developed a finite difference based numerical reservoir simulator for predicting micellar\\/polymer flood performance. The model solves for N components in 3 fluid phases. All components may partition among the phases satisfying either pseudoternary phase behavior or a general, multicomponent, 3-phase flash. Polymer characteristics such as inaccessible pore volume, resistance, and residual resistance factors and retention hysteresis are included.
Numerical simulation of cementitious materials degradation under external sulfate attack
S. Sarkar; S. Mahadevan; J. C. L. Meeussen; H. van der Sloot; D. S. Kosson
2010-01-01
A numerical methodology is proposed in this paper to simulate the degradation of cementitious materials under external sulfate attack. The methodology includes diffusion of ions in and out of the structure, chemical reactions which lead to dissolution and precipitation of solids, and mechanical damage accumulation using a continuum damage mechanics approach. Diffusion of ions is assumed to occur under a
Numerical Simulations for spiral crystal growth with impurity, interlaced spiral
Ishii, Hitoshi
Numerical Simulations for spiral crystal growth with impurity, interlaced spiral and variable, then the crystal may need extremely high driving force around screw dislocation. #12;Hollow core type growth r ·Variable driving force by distance from a screw dislocation ·Wisker like growth ·Hollow core like growth
Experimental evaluation of numerical simulation of cavitating flow around hydrofoil
Matevž Dular; Rudolf Bachert; Bernd Stoffel; Brane Širok
2005-01-01
Cavitation in hydraulic machines causes different problems that can be related to its unsteady nature. An experimental and numerical study of developed cavitating flow was performed. Until now simulations of cavitating flow were limited to the self developed “in house” CFD codes. The goal of the work was to experimentally evaluate the capabilities of a commercial CFD code (Fluent) for
Numerical Simulations of Magnetically Confined Line Driven Winds
A. ud-Doula; S. Owocki
2001-01-01
We present fully dynamic numerical magnetohydrodynamic (MHD) simulations of line-driven winds from hot stars with assumed dipolar magnetic fields at the stellar surface. The magnetic fields can guide the wind outflows from higher latitudes towards the magnetic equator causing them to collide there with speeds of hundreds of km s-1. This may lead to significant equatorial density enhancements and wind
Acceleration and dissipation statistics of numerically simulated isotropic turbulence
particle velocity, acceleration is a fundamen- tal quantity in the study of turbulence in both Eulerian acceleration is a natural consequence of fluid particles moving in a local region of large velocity gradientsAcceleration and dissipation statistics of numerically simulated isotropic turbulence P. K. Yeunga
Numerical simulations of shock-induced mixing and combustion
John B. Bell; Marc S. Day; A. Kuhl
2003-01-01
In this paper we use numerical simulation to investigate shock-induced ignition and combustion of a hydrocarbon gas. The focus of this paper is on quantifying the effect of fidelity in the chemical kinetics on the overall solution. We model the system using the compressible Navier Stokes equations for a reacting mixture. These equations express conservation of species mass, momentum, total
Simulation of ductile crack growth using computational cells: numerical aspects
Haj-Ali, Rami
Simulation of ductile crack growth using computational cells: numerical aspects Arne S. Gulleruda the computational cell methodology to predict crack growth in ductile metals caused by void growth and coalescence 1999 Abstract This study explores key computational issues that aect analyses employing
Numerical simulations of a low power microchannel thermal cycling reactor
Erickson, David
of low cost batteries. In this study, a microchannel thermal cycling reactor design using inNumerical simulations of a low power microchannel thermal cycling reactor David Erickson, Dongqing were conducted to determine the conditions under which efficient, rapid thermal cycling can be achieved
Dynamic numerical simulation for ship-OWT collision
Nianxin Ren; Jinping Ou
2009-01-01
At present, more and more offshore wind farms have been built and numerous projects are on the drawing tables. Therefore, the study on the safety of collision between ships and offshore wind turbines (OWT) has great practical significance. The present study takes the advantage of the famous LS-DYNA explicit code to simulate the dynamic process of the collision between a
The numerical simulation of superhydrophobic surface's flow field characteristic
Qiaogao Huang; Haibao Hu; Guang Pan; Baowei Song
2012-01-01
Based on the flow field mathematical model of superhydrophobic surfaces, the numerical simulation of superhydrophobic surfaces with microcosmic topography in turbulence was carried out. The flow field characteristics of superhydrophobic surfaces were analyzed from the flow field structure, the shear stress distribution, the velocity distribution of gas-liquid interface and the turbulent kinetic energy distribution. The results show that the superhydrophobic
Numerical simulation of plugging failure in ballistic penetration
T. Børvik; O. S. Hopperstad; T. Berstad; M. Langseth
2001-01-01
A coupled computational material model of viscoplasticity and ductile damage has been developed and implemented in LS-DYNA. This model gives good agreement between numerical simulations and experimental observations of plugging failure in ballistic penetration, without the use of inverse modelling or predefined defects. However, even if the model constants can be determined from relatively simple uniaxial tensile tests, the computational
Numerical simulation of the LAGEOS thermal behavior and thermal accelerations
J. I. Andrés; R. Noomen; S. Vecellio None
2006-01-01
The temperature distribution throughout the LAGEOS satellites is simulated numerically with the objective to determine the resulting thermal force. The different elements and materials comprising the spacecraft, with their energy transfer, have been modeled with unprecedented detail. The radiation inputs on the satellites are direct solar (eclipse modulated), Earth albedo, and Earth infrared radiations. For each satellite the lifetime temperature
Direct numerical simulation of dispersed particles in a compressible fluid.
Tatsumi, Rei; Yamamoto, Ryoichi
2012-06-01
We present a direct numerical simulation method for investigating the dynamics of dispersed particles in a compressible solvent fluid. The validity of the simulation is examined by calculating the velocity relaxation of an impulsively forced spherical particle with a known analytical solution. The simulation also gives information about the fluid motion, which provides some insight into the particle motion. Fluctuations are also introduced by random stress, and the validity of this case is examined by comparing the calculation results with the fluctuation-dissipation theorem. PMID:23005240
Numerical relativity simulations in the era of the Einstein Telescope
Mark Hannam; Ian Hawke
2009-08-21
Numerical-relativity (NR) simulations of compact binaries are expected to be an invaluable tool in gravitational-wave (GW) astronomy. The sensitivity of future detectors such as the Einstein Telescope (ET) will place much higher demands on NR simulations than first- and second-generation ground-based detectors. We discuss the issues facing compact-object simulations over the next decade, with an emphasis on estimating where the accuracy and parameter space coverage will be sufficient for ET and where significant work is needed.
NUMERICAL SIMULATION OF NATURAL GAS-SWIRL BURNER
Ala Qubbaj
2005-03-01
A numerical simulation of a turbulent natural gas jet diffusion flame at a Reynolds number of 9000 in a swirling air stream is presented. The numerical computations were carried out using the commercially available software package CFDRC. The instantaneous chemistry model was used as the reaction model. The thermal, composition, flow (velocity), as well as stream function fields for both the baseline and air-swirling flames were numerically simulated in the near-burner region, where most of the mixing and reactions occur. The results were useful to interpret the effects of swirl in enhancing the mixing rates in the combustion zone as well as in stabilizing the flame. The results showed the generation of two recirculating regimes induced by the swirling air stream, which account for such effects. The present investigation will be used as a benchmark study of swirl flow combustion analysis as a step in developing an enhanced swirl-cascade burner technology.
Aggregation in an expanding cloud: experiments and numerical simulations
NASA Astrophysics Data System (ADS)
Dziedzinl, Françoise; Botet, Robert
1991-03-01
We have set-up an experimental device and a numerical model to study aggregation of an aerosol (titanium oxide) expanding in the atmosphere. By mean of scanning microscopic analysis, it is shown that agglomerated aerosol forms fractal clusters of fractal dimension of about 1.75. The numerical simulations quantitatively confirm this geometrical feature. We show how our numerical results can complete some points which are not available in experiments. Nous avons construit un dispositif expérimental et un modèle numérique pour étudier l'agrégation d'un aérosol (oxyde de titane) en expansion dans l'atmosphère. A partir de l'analyse de photographies prises au microscope électronique à balayage, on montre que l'aérosol aggloméré forme des amas fractals de dimension fractale de l'ordre de 1,75. Les simulations numériques confirment quantitativement cette particularité géométrique. Nous montrons comment nos résultats numériques peuvent compléter quelques points qui ne sont pas accessibles à l'expérience.
Contrasting of numerical uncertainties of climate models in simulating reversibility
NASA Astrophysics Data System (ADS)
Johnson, D. R.
2001-05-01
A challenge common to weather, climate and seasonal numerical prediction is the need to simulate accurately long range transport and reversible isentropic processes in combination with appropriate determination of sources/sinks of energy and entropy. With respect to the entropy of matter, this includes the distribution, transport and transformation of internal, gravitational and kinetic energies, the energies of water substances in all forms, and the related thermodynamic processes of phase changes involved with clouds including condensation, evaporation, precipitation, and cloud radiation interaction. A means to study a model's accuracy in simulating internal hydrologic processes is to determine its capability to simulate the appropriate conservation of potential and equivalent potential temperature as surrogates of dry and moist entropy under reversible moist adiabatic processes in which clouds form, evaporate and precipitate. An analysis of variance of model error from numerics is developed in which the sums of squares of the simulated differences between equivalent potential temperature and its proxy is partitioned in three components: the square of the deviations of differences from area mean difference, the square of the area mean deviation differences from the global mean difference and the square of the global difference. The three components provide information on the conservation of moist entropy and energy in relation to growth of bias and random components of model error horizontally, vertically and globally. The analysis of variance of the error sums of squares may also be applied to any regional climate domain. Results from a series of six global ten day simulations employing various versions of CCM2 and 3 --- all Eulerian spectral numerics, all semi-Lagrangian numerics, mixed Eulerian spectral and semi-Lagrangian numerics, --- and two UW hybrid isentropic models over a ten day period will be presented to detail differences in the simulation of reversible processes that develop from nonlinear numerical inaccuracies involving the exchange of energy and water substances. The partitioning of the global sum of squares isolate a structure of bias differences associated with the spurious vertical exchange of entropy and energy in all of the models except for the UW isentropic-sigma and isentropic-eta models. Furthermore, the empirical relative frequency distributions of the differences from the UW models were symmetric, triangular in form and equilibrated as expected from statistical theory in which the random variate is given by the difference of two variates, each of which is drawn from a rectangular distribution of rounding errors. The significance of these results in relation to the numerical difficulties encountered in the simulation of internal hydrologic processes, the timing and amount of precipitation in relation to the issue of reversibility will be discussed.
Numerical simulations for impact damage detection in composites using vibrothermography
NASA Astrophysics Data System (ADS)
Pieczonka, L. J.; Staszewski, W. J.; Aymerich, F.; Uhl, T.; Szwedo, M.
2010-06-01
Composite materials are widely used in many engineering applications due to their high strength-to-weight ratios. However, it is well known that composites are susceptible to impact damage. Detection of impact damage is an important issue in maintenance of composite structures. Various non-destructive image-based techniques have been developed for damage detection in composite materials. These include vibrothermography that detects surface temperature changes due to heating associated with frictional energy dissipation by damage. In the present paper numerical simulations are used to investigate heat generation in a composite plate with impact damage in order to support damage detection analysis with vibrothermography. Explicit finite elements are used to model ultrasonic wave propagation in the damaged plate. Simulated delamination and cracks induce frictional heating in the plate. Coupled thermo-mechanical simulations are performed in high frequencies using commercial LS-Dyna finite element code. Very good qualitative agreement between measurements and simulations has been obtained. The area of increased temperature corresponds very well with the damaged area in both experiments and simulations. Numerical model has to be further refined in order to quantitatively match the experiments. The main issues of concern are frictional and thermal properties of composites. The final goal of these research efforts is to predict damage detection sensitivity of vibrothermography in real engineering applications based on numerical models.
Numerical simulation of airfoil vibrations induced by turbulent flow
NASA Astrophysics Data System (ADS)
Dubcová, Lenka; Feistauer, Miloslav; Horácek, Jaroslav; Svácek, Petr
2008-08-01
The subject of this paper is the numerical simulation of the interaction between two-dimensional incompressible viscous flow and a vibrating airfoil. A solid elastically supported airfoil with two degrees of freedom, which can rotate around the elastic axis and oscillate in the vertical direction, is considered. The numerical simulation consists of the stabilized finite element solution of the Reynolds averaged Navier-Stokes equations with algebraic models of turbulence, coupled with the system of ordinary differential equations describing the airfoil motion. Since the computational domain is time dependent and the grid is moving, the Arbitrary Lagrangian-Eulerian (ALE) method is used. The developed method was applied to the simulation of flow-induced airfoil vibrations.
Configuration Management File Manager Developed for Numerical Propulsion System Simulation
NASA Technical Reports Server (NTRS)
Follen, Gregory J.
1997-01-01
One of the objectives of the High Performance Computing and Communication Project's (HPCCP) Numerical Propulsion System Simulation (NPSS) is to provide a common and consistent way to manage applications, data, and engine simulations. The NPSS Configuration Management (CM) File Manager integrated with the Common Desktop Environment (CDE) window management system provides a common look and feel for the configuration management of data, applications, and engine simulations for U.S. engine companies. In addition, CM File Manager provides tools to manage a simulation. Features include managing input files, output files, textual notes, and any other material normally associated with simulation. The CM File Manager includes a generic configuration management Application Program Interface (API) that can be adapted for the configuration management repositories of any U.S. engine company.
Numerical Propulsion System Simulation (NPSS) 1999 Industry Review
NASA Technical Reports Server (NTRS)
Lytle, John; Follen, Greg; Naiman, Cynthia; Evans, Austin
2000-01-01
The technologies necessary to enable detailed numerical simulations of complete propulsion systems are being developed at the NASA Glenn Research Center in cooperation with industry, academia, and other government agencies. Large scale, detailed simulations will be of great value to the nation because they eliminate some of the costly testing required to develop and certify advanced propulsion systems. In addition, time and cost savings will be achieved by enabling design details to be evaluated early in the development process before a commitment is made to a specific design. This concept is called the Numerical Propulsion System Simulation (NPSS). NPSS consists of three main elements: (1) engineering models that enable multidisciplinary analysis of large subsystems and systems at various levels of detail, (2) a simulation environment that maximizes designer productivity, and (3) a cost-effective, high-performance computing platform. A fundamental requirement of the concept is that the simulations must be capable of overnight execution on easily accessible computing platforms. This will greatly facilitate the use of large-scale simulations in a design environment. This paper describes the current status of the NPSS with specific emphasis on the progress made over the past year on air breathing propulsion applications. In addition, the paper contains a summary of the feedback received from industry partners in the development effort and the actions taken over the past year to respond to that feedback. The NPSS development was supported in FY99 by the High Performance Computing and Communications Program.
Thermal numerical simulator for laboratory evaluation of steamflood oil recovery
Sarathi, P.
1991-04-01
A thermal numerical simulator running on an IBM AT compatible personal computer is described. The simulator was designed to assist laboratory design and evaluation of steamflood oil recovery. An overview of the historical evolution of numerical thermal simulation, NIPER's approach to solving these problems with a desk top computer, the derivation of equations and a description of approaches used to solve these equations, and verification of the simulator using published data sets and sensitivity analysis are presented. The developed model is a three-phase, two-dimensional multicomponent simulator capable of being run in one or two dimensions. Mass transfer among the phases and components is dictated by pressure- and temperature-dependent vapor-liquid equilibria. Gravity and capillary pressure phenomena were included. Energy is transferred by conduction, convection, vaporization and condensation. The model employs a block centered grid system with a five-point discretization scheme. Both areal and vertical cross-sectional simulations are possible. A sequential solution technique is employed to solve the finite difference equations. The study clearly indicated the importance of heat loss, injected steam quality, and injection rate to the process. Dependence of overall recovery on oil volatility and viscosity is emphasized. The process is very sensitive to relative permeability values. Time-step sensitivity runs indicted that the current version is time-step sensitive and exhibits conditional stability. 75 refs., 19 figs., 19 tabs.
Numerical simulations of the thermoacoustic computed tomography breast imaging system
NASA Astrophysics Data System (ADS)
Kiser, William Lester, Jr.
A thermoacoustic wave is produced when an object absorbs energy and experiences a subsequent thermal expansion. We have developed a Thermoacoustic Computed Tomography (TACT) breast imaging system to exploit the thermoacoustic phenomena as a method of soft tissue imaging. By exposing the breast to short pulses of 434 MHz microwaves, ultrasonic pulses are generated and detected with a hemispherical transducer array submersed in a water bath. Filtering and back projecting the transducer signals generates a 3-D image that maps the localized microwave absorption properties of the breast. In an effort to understand the factors limiting image quality, the TACT system was numerically simulated. The simulations were used to generate the transducer signals that would be collected by the TACT system during a scan of an object. These simulated data streams were then fed into the system image reconstruction software to provide images of simulated phantoms. The effects of transducer diameter, transducer response, transducer array geometry and stimulating pulse width on the spatial and contrast resolution of the system were quantified using the simulations. The spatial resolution was highly dependent upon location in the imaging volume. This was due to the off axis response of transducers of finite aperture. Simulated data were compared with experimental data, obtained by imaging a parallel-piped resolution phantom, to verify the accuracy of the simulation code. A contrast-detail phantom was numerically simulated to determine the ability of the system to image spheres of diameters <1 cm with absorption values on the order of physiologic saline, when located in a background of noise. The results of the contrast-detail analysis were dependent on the location of the spheres in the imaging volume and the diameter of the simulated transducers. This work sets the foundation for the initial image quality studies of the TACT system. Improvements to the current imaging system, based on the results of the study, are suggested.
Image based numerical simulation of hemodynamics in a intracranial aneurysm
NASA Astrophysics Data System (ADS)
Le, Trung; Ge, Liang; Sotiropoulos, Fotis; Kallmes, David; Cloft, Harry; Lewis, Debra; Dai, Daying; Ding, Yonghong; Kadirvel, Ramanathan
2007-11-01
Image-based numerical simulations of hemodynamics in a intracranial aneurysm are carried out. The numerical solver based on CURVIB (curvilinear grid/immersed boundary method) approach developed in Ge and Sotiropoulos, JCP 2007 is used to simulate the blood flow. A curvilinear grid system that gradually follows the curved geometry of artery wall and consists of approximately 5M grid nodes is constructed as the background grid system and the boundaries of the investigated artery and aneurysm are treated as immersed boundaries. The surface geometry of aneurysm wall is reconstructed from an angiography study of an aneurysm formed on the common carotid artery (CCA) of a rabbit and discretized with triangular meshes. At the inlet a physiological flow waveform is specified and direct numerical simulations are used to simulate the blood flow. Very rich vortical dynamics is observed within the aneurysm area, with a ring like vortex sheds from the proximal side of aneurysm, develops and impinge onto the distal side of the aneurysm as flow develops, and destructs into smaller vortices during later cardiac cycle. This work was supported in part by the University of Minnesota Supercomputing Institute.
Numerical simulation of unsteady flow in an axisymmetric shear layer
NASA Astrophysics Data System (ADS)
Scott, J. N.; Hankey, W. L.
1986-01-01
Numerical simulation of unsteady flow in an axisymmetric subsonic shear layer is accomplished by solving the time-dependent compressible Navier-Stokes equations. The objective of the effort is to investigate by numerical means the influence of various flow parameters on the shear layer behavior. The parameters investigated include the velocity ratio of two streams, total temperature, and nozzle lip thickness. The computations are performed on a CRAY-IS computer using McCormack's explicit finite difference scheme. The computed results generally show qualitative agreement with experimental data.
Expert System Architecture for Rocket Engine Numerical Simulators: A Vision
NASA Technical Reports Server (NTRS)
Mitra, D.; Babu, U.; Earla, A. K.; Hemminger, Joseph A.
1998-01-01
Simulation of any complex physical system like rocket engines involves modeling the behavior of their different components using mostly numerical equations. Typically a simulation package would contain a set of subroutines for these modeling purposes and some other ones for supporting jobs. A user would create an input file configuring a system (part or whole of a rocket engine to be simulated) in appropriate format understandable by the package and run it to create an executable module corresponding to the simulated system. This module would then be run on a given set of input parameters in another file. Simulation jobs are mostly done for performance measurements of a designed system, but could be utilized for failure analysis or a design job such as inverse problems. In order to use any such package the user needs to understand and learn a lot about the software architecture of the package, apart from being knowledgeable in the target domain. We are currently involved in a project in designing an intelligent executive module for the rocket engine simulation packages, which would free any user from this burden of acquiring knowledge on a particular software system. The extended abstract presented here will describe the vision, methodology and the problems encountered in the project. We are employing object-oriented technology in designing the executive module. The problem is connected to the areas like the reverse engineering of any simulation software, and the intelligent systems for simulation.
Numerical simulations of internal wave generation by convection in water
NASA Astrophysics Data System (ADS)
Lecoanet, Daniel; Le Bars, Michael; Burns, Keaton J.; Vasil, Geoffrey M.; Brown, Benjamin P.; Quataert, Eliot; Oishi, Jeffrey S.
2015-06-01
Water's density maximum at 4 ?C makes it well suited to study internal gravity wave excitation by convection: an increasing temperature profile is unstable to convection below 4 ?C,but stably stratified above 4 ?C . We present numerical simulations of a waterlike fluid near its density maximum in a two-dimensional domain. We successfully model the damping of waves in the simulations using linear theory, provided we do not take the weak damping limit typically used in the literature. To isolate the physical mechanism exciting internal waves, we use the spectral code dedalus to run several simplified model simulations of our more detailed simulation. We use data from the full simulation as source terms in two simplified models of internal-wave excitation by convection: bulk excitation by convective Reynolds stresses, and interface forcing via the mechanical oscillator effect. We find excellent agreement between the waves generated in the full simulation and the simplified simulation implementing the bulk excitation mechanism. The interface forcing simulations overexcite high-frequency waves because they assume the excitation is by the "impulsive" penetration of plumes, which spreads energy to high frequencies. However, we find that the real excitation is instead by the "sweeping" motion of plumes parallel to the interface. Our results imply that the bulk excitation mechanism is a very accurate heuristic for internal-wave generation by convection.
NUMERICAL SIMULATION OF FLOW AROUND A BEND WITH GRAVEL BAR
NASA Astrophysics Data System (ADS)
Shibata, Ryoichi; Nakayama, Akihiko; Asami, Kayo
The flow around a bend of a real river reach during floods has been simulated by a Large-Eddy Simulation (LES) method in order to investigate the detailed flow characteristics and their effects on the gravel bars that exist on the inner banks of the meandering river reach. The hydrodynamic effects of flood flows near the gravel bar are thought to be closely related to the conditions for plant communities of diverse species to be maintained. The present numerical simulation, that represents the bed bathymetry to about 15 centimeters in the vertical direction, is found to reproduce the observed water level and slopes of a few hundred meter long reach for given discharges of a few hundred tons/sec. The simulated results include the three-dimensional velocity distributions and the bed shear stress along with the instantaneous secondary flow features and provide important information to study the hydrodynamic effects of flood flows on the gravel-bar plants.
Numerical Simulation of nZVI at the Field Scale
NASA Astrophysics Data System (ADS)
Chowdhury, A. I.; Krol, M.; Sleep, B. E.; O'Carroll, D. M.
2014-12-01
Nano-scale zero valent iron (nZVI) has been used at a number of contaminated sites over the last decade. At most of these sites, significant decreases in contaminant concentrations have resulted from the application of nZVI. However, limited work has been completed investigating nZVI mobility at the field-scale. In this study a three dimensional, three phase, finite difference numerical simulator (CompSim) was used to simulate nZVI and polymer transport in a variably saturated site. The model was able to accurately predict the field observed head data without parameter fitting. In addition, the numerical simulator estimated the amount of nZVI delivered to the saturated and unsaturated zones as well as the phase of nZVI (i.e., attached or aqueous phase). The simulation results showed that the injected slurry migrated radially outward from the injection well, and therefore nZVI transport was governed by injection velocity as well as viscosity of the injected solution. A suite of sensitivity analyses was performed to investigate the impact of different injection scenarios (e.g. different volume and injection rate) on nZVI migration. Simulation results showed that injection of a higher volume of nZVI delivered more iron particles at a given distance; however, not necessarily to a greater distance proportionate to the increase in volume. This study suggests that on-site synthesized nZVI particles are mobile in the subsurface and the numerical simulator can be a valuable tool for optimum design of nZVI applications.
Numerical Simulation of Multi-CME Events in the Heliosphere
NASA Astrophysics Data System (ADS)
Odstrcil, Dusan; Luhmann, Janet G.; Jian, Lan; Mays, Leila; Xie, Hong; Taktakishvilli, Aleksandre
The ENLIL-based modeling system enables faster-than-real time simulations of corotating and transient heliospheric disturbances. This “hybrid” system does not simulate origin of coronal mass ejections (CMEs) but uses appearance in coronagraphs, fits geometric and kinematic parameters, and launches a CME-like structure into the solar wind computed using the Wang-Sheeley-Arge (WSA) coronal model. Numerical heliospheric simulation then provides global context of CMEs propagating in the inner heliosphere and interacting with structured background solar wind and with other CMEs. In this presentation, we introduce the recent improvements that support modeling of the evolving background solar wind and continuous modeling of multiple-CME events. We simulated over 700 CMEs in 2011-2013 to validate and calibrate our new modeling system. In this presentation, we will show examples of multi-CME events in March 2012 and July 2012 periods of enhanced solar activity. We will present results of 3D numerical magnetohydrodynamic (MHD) simulations and compare them with remote white-light observations, with in-situ measurements of plasma parameters and detection of solar energetic particles (SEPs) at various spacecraft.
Numerical simulation of current induced sediment transport and resulting bedforms
NASA Astrophysics Data System (ADS)
Burkow, M.
2011-12-01
Usual technics for estimating the evolution of bedforms are accompanied by the use of laboratory flumes. The effort to construct and maintain this flumes are expensive and should be reduced by numerical simulations. In this talk we construct a numerical model to simulate sediment transport and resulting bedforms. We use NaSt3DGP as a Navier-Stokes Solver for incompressible single-phase flow problems in three dimensions. As a in-house research code NaSt3DGP uses finite difference discretization on staggered grids and Chorin's projection approach. Second order QUICK and VONOS schemes are applied for spatial discretization. Time discretizations are done with Adams-Bashforth and Runge-Kutta schemes of second and third order. Hence we computed a flow situation it is possible to use the simulated fluid properties like velocity and pressure as input values for our sediment model. Focusing on bed load transport we use the bed level equation derived by Exner to model sedimentary forms. After calculating necessary input values like shear stress and transport rates, we compute the new sediment surface. To show practicabilty we simulate the evolution of dunes and scour marks. In a further example we compare a simulated fluvial obstacle mark with measurements carried out in a laboratory flume.
Numerical Simulation of a Spatially Evolving Supersonic Turbulent Boundary Layer
NASA Technical Reports Server (NTRS)
Gatski, T. B.; Erlebacher, G.
2002-01-01
The results from direct numerical simulations of a spatially evolving, supersonic, flat-plate turbulent boundary-layer flow, with free-stream Mach number of 2.25 are presented. The simulated flow field extends from a transition region, initiated by wall suction and blowing near the inflow boundary, into the fully turbulent regime. Distributions of mean and turbulent flow quantities are obtained and an analysis of these quantities is performed at a downstream station corresponding to Re(sub x)= 5.548 x10(exp 6) based on distance from the leading edge.
Direct numerical simulation of compressible free shear flows
NASA Technical Reports Server (NTRS)
Lele, Sanjiva K.
1989-01-01
Direct numerical simulations of compressible free shear layers in open domains are conducted. Compact finite-difference schemes of spectral-like accuracy are used for the simulations. Both temporally-growing and spatially-growing mixing layers are studied. The effect of intrinsic compressibility on the evolution of vortices is studied. The use of convective Mach number is validated. Details of vortex roll up and pairing are studied. A simple explanation of the stabilizing effect of compressibility is offered. Acoustic radiation from vortex roll up, pairing and shape oscillations is studied and quantified.
Direct numerical simulation of compressible free shear flows
NASA Technical Reports Server (NTRS)
Lele, Sanjiva K.
1988-01-01
Direct numerical simulations of compressible free shear layers in open domains are conducted. Compact finite-difference schemes of spectral-like accuracy are used for the simulations. Both temporally-growing and spatially-growing mixing layers are studied. The effect of intrinsic compressibility on the evolution of vortices is studied. The use of convective Mach number is validated. Details of vortex roll up and pairing are studied. A simple explanation of the stabilizing effect of compressibility is offered. Acoustic radiation from vortex roll up, pairing and shape oscillations is studied and quantified.
A numerical technique for simulation of cloud seeding experiments
A Mary Selvam; A S Ramachandra Murty; Bh. V Ramana Murty
1978-01-01
Two numerical cloud seeding experiments, using historic rainfall for the Deccan plateau region in Maharashtra state, were\\u000a performed adopting different simulation techniques. The data used consisted of 1-day total rainfall for the 5-year period\\u000a 1951–55. A double-area cross-over design with area randomisation was adopted.\\u000a \\u000a \\u000a The first experiment, EXP-TR, was based on the simulation technique of Twomey and Robertson which involves
Numerical simulations of thin accretion discs with PLUTO
Parthasarathy, Varadarajan
2014-01-01
Our goal is to perform global simulations of thin accretion discs around compact bodies like neutron stars with dipolar magnetic profile and black holes by exploiting the facilities provided by state-of-the-art grid-based, high resolution shock capturing (HRSC) and finite volume codes. We have used the Godunov-type code PLUTO to simulate a thin disc around a compact object prescribed with a pseudo-Newtonian potential in a purely hydrodynamical (HD) regime, with numerical viscosity as a first step towards achieving our goal as mentioned above.
Numerical simulation study on the flow field of porous hydrofoil
NASA Astrophysics Data System (ADS)
Yu, F. R.; Zhang, L. X.
2012-11-01
Because cavitation and cavitation erosion will caused significant impact to the security and stability of hydro turbine, so changing geometric structure to reduce the risk of cavitation is considered. Punching many holes on the hydrofoil is adopted. By using RNG ? - ? turbulence model and SIMPLEC algorithm, the flow field around hydrofoil and porous hydrofoil are simulated based computational fluid dynamics(CFD). The numerical simulation result-velocity and pressure field of hydrofoil with different geometry are compared and analysed. This study introduces geometry optimization ideas to researchers for improving cavitation phenomenon in water turbine.
Numerical simulation of acoustic emission using acoustic contact elements
NASA Astrophysics Data System (ADS)
Suvorov, A. S.; Sokov, E. M.; Artel'nyi, P. V.
2014-11-01
A new method is described for nonconformal finite-element simulation of the region of interaction between an acoustic fluid and deformed solid bodies. The method is based on the use of special contact finite elements simulating the interpolation coupling between the acoustic pressure in the fluid and displacements of a two-phase surface. The test results of the method demonstrate a significant acceleration of grid convergence of computations. The numerical method is verified by the problem of predicting noise emission by a thin-wall inhomogeneous shell.
Numerical Simulation of Cast Distortion in Gas Turbine Engine Components
NASA Astrophysics Data System (ADS)
Inozemtsev, A. A.; Dubrovskaya, A. S.; Dongauser, K. A.; Trufanov, N. A.
2015-06-01
In this paper the process of multiple airfoilvanes manufacturing through investment casting is considered. The mathematical model of the full contact problem is built to determine stress strain state in a cast during the process of solidification. Studies are carried out in viscoelastoplastic statement. Numerical simulation of the explored process is implemented with ProCASTsoftware package. The results of simulation are compared with the real production process. By means of computer analysis the optimization of technical process parameters is done in order to eliminate the defect of cast walls thickness variation.
Numerical Relativity Simulations for Black Hole Merger Astrophysics
NASA Technical Reports Server (NTRS)
Baker, John G.
2010-01-01
Massive black hole mergers are perhaps the most energetic astronomical events, establishing their importance as gravitational wave sources for LISA, and also possibly leading to observable influences on their local environments. Advances in numerical relativity over the last five years have fueled the development of a rich physical understanding of general relativity's predictions for these events. Z will overview the understanding of these event emerging from numerical simulation studies. These simulations elucidate the pre-merger dynamics of the black hole binaries, the consequent gravitational waveform signatures ' and the resulting state, including its kick velocity, for the final black hole produced by the merger. Scenarios are now being considered for observing each of these aspects of the merger, involving both gravitational-wave and electromagnetic astronomy.
Probing Brownstein-Moffat Gravity via Numerical Simulations
C. S. S. Brandao; J. C. N. de Araujo
2010-06-04
In the standard scenario of the Newtonian gravity, a late-type galaxy (i.e., a spiral galaxy) is well described by a disk and a bulge embedded in a halo mainly composed by dark matter. In Brownstein-Moffat gravity, there is a claim that late-type galaxy systems would not need to have halos, avoiding as a result the dark matter problem, i.e., a modified gravity (non-Newtonian) would account for the galactic structure with no need of dark matter. In the present paper, we probe this claim via numerical simulations. Instead of using a "static galaxy," where the centrifugal equilibrium is usually adopted, we probe the Brownstein-Moffat gravity dynamically via numerical $N$-body simulations.
PROBING BROWNSTEIN-MOFFAT GRAVITY VIA NUMERICAL SIMULATIONS
Brandao, C. S. S.; De Araujo, J. C. N., E-mail: claudio@das.inpe.b, E-mail: jcarlos@das.inpe.b [Divisao de Astrofisica, Instituto Nacional de Pesquisas Espaciais, S. J. Campos, SP 12227-010 (Brazil)
2010-07-10
In the standard scenario of the Newtonian gravity, a late-type galaxy (i.e., a spiral galaxy) is well described by a disk and a bulge embedded in a halo mainly composed of dark matter. In Brownstein-Moffat gravity, there is a claim that late-type galaxy systems would not need to have halos, avoiding as a result the dark matter problem, i.e., a modified gravity (non-Newtonian) would account for the galactic structure with no need of dark matter. In the present paper, we probe this claim via numerical simulations. Instead of using a 'static galaxy', where the centrifugal equilibrium is usually adopted, we probe the Brownstein-Moffat gravity dynamically via numerical N-body simulations.
Numerical simulations of a diode laser BPH treatment system
Esch, V; London, R A; Papademetriou, S
1999-02-23
Numerical simulations are presented of the laser-tissue interaction of a diode laser system for treating benign prostate hyperplasia. The numerical model includes laser light transport, heat transport, cooling due to blood perfusion, thermal tissue damage, and enthalpy of tissue damage. Comparisons of the simulation results to clinical data are given. We report that a reasonable variation from a standard set of input data produces heating times which match those measured in the clinical trials. A general trend of decreasing damage volume with increasing heating time is described. We suggest that the patient-to- patient variability seen in the data can be explained by differences in fundamental biophysical properties such as the optical coefficients. Further work is identified, including the measurement and input to the model of several specific data parameters such as optical coefficients, blood perfusion cooling rate, and coagulation rates.
Numerical simulation study on earthquake-induced landslide stability evaluation
NASA Astrophysics Data System (ADS)
Li, C.; Tang, H.; Wang, L.; Xiong, C.
2009-12-01
Landslide is one of the most dangerous geological hazards in the world, and its stability is affected by many factors. Especially in the earthquake-prone area, the earthquake plays a quite significant role in the stability evaluation of landslide. A typical landslide in Western China area was taken as an example, from which the geological model of landslide with considering the earthquake effect was established. The dynamic analysis module of finite difference software FLAC3D was carried out the numerical simulation in order to evaluate the stability of the landslide under the condition of earthquake effect. The result indicates that the numerical simulation analysis is in concordance with the engineering practice of the landslide. This study can provided a significant scientific basis for the stability evaluation of landslide in earthquake-prone area of Western China.
Numerical simulation of weakly ionized hypersonic flow over reentry capsules
NASA Astrophysics Data System (ADS)
Scalabrin, Leonardo C.
The mathematical and numerical formulation employed in the development of a new multi-dimensional Computational Fluid Dynamics (CFD) code for the simulation of weakly ionized hypersonic flows in thermo-chemical non-equilibrium over reentry configurations is presented. The flow is modeled using the Navier-Stokes equations modified to include finite-rate chemistry and relaxation rates to compute the energy transfer between different energy modes. The set of equations is solved numerically by discretizing the flowfield using unstructured grids made of any mixture of quadrilaterals and triangles in two-dimensions or hexahedra, tetrahedra, prisms and pyramids in three-dimensions. The partial differential equations are integrated on such grids using the finite volume approach. The fluxes across grid faces are calculated using a modified form of the Steger-Warming Flux Vector Splitting scheme that has low numerical dissipation inside boundary layers. The higher order extension of inviscid fluxes in structured grids is generalized in this work to be used in unstructured grids. Steady state solutions are obtained by integrating the solution over time implicitly. The resulting sparse linear system is solved by using a point implicit or by a line implicit method in which a tridiagonal matrix is assembled by using lines of cells that are formed starting at the wall. An algorithm that assembles these lines using completely general unstructured grids is developed. The code is parallelized to allow simulation of computationally demanding problems. The numerical code is successfully employed in the simulation of several hypersonic entry flows over space capsules as part of its validation process. Important quantities for the aerothermodynamics design of capsules such as aerodynamic coefficients and heat transfer rates are compared to available experimental and flight test data and other numerical results yielding very good agreement. A sensitivity analysis of predicted radiative heating of a space capsule to several thermo-chemical non-equilibrium models is also performed.
Numerical simulations of the decay of primordial magnetic turbulence
Kahniashvili, Tina [McWilliams Center for Cosmology and Department of Physics, Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, Pennsylvania 15213 (United States); Department of Physics, Laurentian University, Ramsey Lake Road, Sudbury, ON P3E 2C (Canada); Abastumani Astrophysical Observatory, Ilia State University, 2A Kazbegi Ave, Tbilisi, GE-0160 (Georgia); Brandenburg, Axel [Nordita, AlbaNova University Center, Roslagstullsbacken 23, 10691 Stockholm (Sweden); Department of Astronomy, Stockholm University, SE 10691 Stockholm (Sweden); Tevzadze, Alexander G. [Abastumani Astrophysical Observatory, Ilia State University, 2A Kazbegi Ave, Tbilisi, GE-0160 (Georgia); Faculty of Exact and Natural Sciences, Tbilisi State University, 1 Chavchavadze Avenue Tbilisi, GE-0128 (Georgia); Ratra, Bharat [Department of Physics, Kansas State University, 116 Cardwell Hall, Manhattan, Kansas 66506 (United States)
2010-06-15
We perform direct numerical simulations of forced and freely decaying 3D magnetohydrodynamic turbulence in order to model magnetic field evolution during cosmological phase transitions in the early Universe. Our approach assumes the existence of a magnetic field generated either by a process during inflation or shortly thereafter, or by bubble collisions during a phase transition. We show that the final configuration of the magnetic field depends on the initial conditions, while the velocity field is nearly independent of initial conditions.
Numerical simulation of the extrusion of strongly compressible Newtonian liquids
Eleni Taliadorou; Georgios C. Georgiou; Evan Mitsoulis
2008-01-01
The axisymmetric and plane extrusion flows of a liquid foam are simulated assuming that the foam is a homogeneous compressible\\u000a Newtonian fluid that slips along the walls. Compressibility effects are investigated using both a linear and an exponential\\u000a equation of state. The numerical results confirm previous reports that the swelling of the extrudate decreases initially as\\u000a the compressibility of the
Numerical simulation of the countercurrent flow in a gas centrifuge
NASA Astrophysics Data System (ADS)
Cloutman, L. D.; Gentry, R. A.
A finite difference method is presented for the numerical simulation of the axisymmetric countercurrent flows in gas centrifuge. A time marching technique is used to relax an arbitrary initial condition to the desired steady state solution. All boundary layers may be resolved. It is concluded that this technique is capable of accurately predicting the performance of a wide variety of machines under all operating conditions of interest.
Numerical simulation of flow in the wet scrubber for desulfurization
NASA Astrophysics Data System (ADS)
Novosád, Jan; Vít, Tomáš
2015-05-01
This article deals with numerical simulation of flow and chemical reactions in absorber for desulfurization of flue-gas. The objective of the work is the investigation of effect of different nozzles types and their placement in spray layers. These nozzles distribute lime suspension into flue gas stream. The research includes two types of nozzles and four different arrangements of nozzles and spray layers. Conclusion describes the effect of nozzle types and their arrangements on the suspension concentration in absorber.
Remark on numerical simulation of 2D unsteady transonic flows
NASA Astrophysics Data System (ADS)
Fo?t, J.; Hülek, T.; Kozel, K.; Vavrincová, M.
The work deals with three numerical methods solving the system of Euler or Navier-Stokes equations. Mac Cormack cell centered and Ni cell vertex finite volume schemes were used for simulation of inviscid unsteady solution of transonic flows through a 2D cascade. Unsteady motion is caused by a periodic change of downstream pressure. The Runge-Kutta multistage cell centered finite volume scheme has been used for viscous laminar steady and unsteady transonic flows over NACA 0012.
Numerical simulations of oblique penetration into reinforced concrete targets
Yan Liu; Fenglei Huang; Aie Ma
2011-01-01
A dynamic constitutive model based on the tensile and the compressive damage models for concrete was developed and implemented into the three-dimensional finite element code, LS-DYNA. Numerical simulations of oblique penetration into reinforced concrete targets were performed using LS-DYNA. On the basis of the proposed model, the tensile and compressive damages of reinforced concrete after oblique penetration were observed and
EXTENDED SCALING LAWS IN NUMERICAL SIMULATIONS OF MAGNETOHYDRODYNAMIC TURBULENCE
Mason, Joanne; Cattaneo, Fausto [Department of Astronomy and Astrophysics, University of Chicago, 5640 S. Ellis Ave, Chicago, IL 60637 (United States); Perez, Jean Carlos; Boldyrev, Stanislav, E-mail: jmason@flash.uchicago.edu, E-mail: cattaneo@flash.uchicago.edu, E-mail: jcperez@wisc.edu, E-mail: boldyrev@wisc.edu [Department of Physics, University of Wisconsin at Madison, 1150 University Ave, Madison, WI 53706 (United States)
2011-07-10
Magnetized turbulence is ubiquitous in astrophysical systems, where it notoriously spans a broad range of spatial scales. Phenomenological theories of MHD turbulence describe the self-similar dynamics of turbulent fluctuations in the inertial range of scales. Numerical simulations serve to guide and test these theories. However, the computational power that is currently available restricts the simulations to Reynolds numbers that are significantly smaller than those in astrophysical settings. In order to increase computational efficiency and, therefore, probe a larger range of scales, one often takes into account the fundamental anisotropy of field-guided MHD turbulence, with gradients being much slower in the field-parallel direction. The simulations are then optimized by employing the reduced MHD equations and relaxing the field-parallel numerical resolution. In this work we explore a different possibility. We propose that there exist certain quantities that are remarkably stable with respect to the Reynolds number. As an illustration, we study the alignment angle between the magnetic and velocity fluctuations in MHD turbulence, measured as the ratio of two specially constructed structure functions. We find that the scaling of this ratio can be extended surprisingly well into the regime of relatively low Reynolds number. However, the extended scaling easily becomes spoiled when the dissipation range in the simulations is underresolved. Thus, taking the numerical optimization methods too far can lead to spurious numerical effects and erroneous representation of the physics of MHD turbulence, which in turn can affect our ability to identify correctly the physical mechanisms that are operating in astrophysical systems.
Numerical Simulations of the Metallicity Distribution in Dwarf Spheroidal Galaxies
Ripamonti, Emanuele; Tolstoy, E.; Helmi, A.; Battaglia, G.; /Kapteyn Astron. Inst., Groningen; Abel, T.; /KIPAC, Menlo Park
2006-12-12
Recent observations show that the number of stars with very low metallicities in the dwarf spheroidal satellites of the Milky Way is low, despite the low average metallicities of stars in these systems. We undertake numerical simulations of star formation and metal enrichment of dwarf galaxies in order to verify whether this result can be reproduced with ''standard'' assumptions. The answer is likely to be negative, unless some selection bias against very low metallicity stars is present in the observations.
Numerical simulation of Smith-Purcell free-electron lasers.
Kumar, V.; Kim, K.-J.; Accelerator Systems Division (APS)
2005-01-01
We present a one-dimensional time-dependent analysis and simulation of Smith-Purcell (SP) free-electron lasers (FELs). The coupled Maxwell-Lorentz equations are set up, and the details of numerical simulation are presented. At low electron beam energy, a SP-FEL is a backward wave oscillator (BWO), and oscillations can be achieved without the need for feedback mirrors. In the linear regime, we show that the optical power grows exponentially if the current is larger than a certain value, the start current. Results of our numerical calculation compare well with the analytic calculation in the linear regime and show saturation behavior in the nonlinear regime. An electron beam traveling close and parallel to a mettalic grating, with grating rulings perpendicular to the electron motion, gives off polarized electromagnetic radiation known as Smith-Purcell radiation. A Smith-Purcell free-electron laser (SP-FEL) based on this effect is interesting as a possible compact source of tunable, coherent THz radiation. Analytic theory of SP-FELs in the linear regime has been discussed by several authors under different approximations. Here, we present a fully self-consistent nonlinear analysis, which can be used to understand the saturation behavior and simulate the realistic effects; coupled Maxwell-Lorentz equations for an SP-FEL driven by sheet beam are derived and solved numerically to perform detailed analysis.
Numerical Homogenization of Jointed Rock Masses Using Wave Propagation Simulation
NASA Astrophysics Data System (ADS)
Gasmi, Hatem; Hamdi, Essaïeb; Bouden Romdhane, Nejla
2014-07-01
Homogenization in fractured rock analyses is essentially based on the calculation of equivalent elastic parameters. In this paper, a new numerical homogenization method that was programmed by means of a MATLAB code, called HLA-Dissim, is presented. The developed approach simulates a discontinuity network of real rock masses based on the International Society of Rock Mechanics (ISRM) scanline field mapping methodology. Then, it evaluates a series of classic joint parameters to characterize density (RQD, specific length of discontinuities). A pulse wave, characterized by its amplitude, central frequency, and duration, is propagated from a source point to a receiver point of the simulated jointed rock mass using a complex recursive method for evaluating the transmission and reflection coefficient for each simulated discontinuity. The seismic parameters, such as delay, velocity, and attenuation, are then calculated. Finally, the equivalent medium model parameters of the rock mass are computed numerically while taking into account the natural discontinuity distribution. This methodology was applied to 17 bench fronts from six aggregate quarries located in Tunisia, Spain, Austria, and Sweden. It allowed characterizing the rock mass discontinuity network, the resulting seismic performance, and the equivalent medium stiffness. The relationship between the equivalent Young's modulus and rock discontinuity parameters was also analyzed. For these different bench fronts, the proposed numerical approach was also compared to several empirical formulas, based on RQD and fracture density values, published in previous research studies, showing its usefulness and efficiency in estimating rapidly the Young's modulus of equivalent medium for wave propagation analysis.
Large Eddy Simulation Modeling and Flamelet Analysis of a Jet in Cross Flow
NASA Astrophysics Data System (ADS)
Chan, Wai Lee; Kolla, Hemanth; Ihme, Matthias; Chen, Jacqueline
2011-11-01
Jet in cross flow (JICF) configurations are frequently used as fuel injection strategies in combustion systems, such as gas turbines, boilers, and high-speed propulsion systems. Recently, direct numerical simulations (DNS) have been performed to investigate flame-stabilization mechanisms in a reactive JICF. By utilizing this DNS-database, fundamental modeling assumptions of flamelet-based large eddy simulation (LES) combustion models for application to JICF are evaluated. To this end, a priori and a posteriori studies are performed using steady and unsteady flamelet models to isolate and model combustion flow processes that control transient ignition events and flame stabilization.
Numerical simulation of supersonic wake flow with parallel computers
Wong, C.C. [Sandia National Labs., Albuquerque, NM (United States); Soetrisno, M. [Amtec Engineering, Inc., Bellevue, WA (United States)
1995-07-01
Simulating a supersonic wake flow field behind a conical body is a computing intensive task. It requires a large number of computational cells to capture the dominant flow physics and a robust numerical algorithm to obtain a reliable solution. High performance parallel computers with unique distributed processing and data storage capability can provide this need. They have larger computational memory and faster computing time than conventional vector computers. We apply the PINCA Navier-Stokes code to simulate a wind-tunnel supersonic wake experiment on Intel Gamma, Intel Paragon, and IBM SP2 parallel computers. These simulations are performed to study the mean flow in the near wake region of a sharp, 7-degree half-angle, adiabatic cone at Mach number 4.3 and freestream Reynolds number of 40,600. Overall the numerical solutions capture the general features of the hypersonic laminar wake flow and compare favorably with the wind tunnel data. With a refined and clustering grid distribution in the recirculation zone, the calculated location of the rear stagnation point is consistent with the 2D axisymmetric and 3D experiments. In this study, we also demonstrate the importance of having a large local memory capacity within a computer node and the effective utilization of the number of computer nodes to achieve good parallel performance when simulating a complex, large-scale wake flow problem.
Numerical Simulation of Delamination Growth in Composite Materials
NASA Technical Reports Server (NTRS)
Camanho, P. P.; Davila, C. G.; Ambur, D. R.
2001-01-01
The use of decohesion elements for the simulation of delamination in composite materials is reviewed. The test methods available to measure the interfacial fracture toughness used in the formulation of decohesion elements are described initially. After a brief presentation of the virtual crack closure technique, the technique most widely used to simulate delamination growth, the formulation of interfacial decohesion elements is described. Problems related with decohesion element constitutive equations, mixed-mode crack growth, element numerical integration and solution procedures are discussed. Based on these investigations, it is concluded that the use of interfacial decohesion elements is a promising technique that avoids the need for a pre-existing crack and pre-defined crack paths, and that these elements can be used to simulate both delamination onset and growth.
Numerical Simulation of Liquid Nitrogen Chilldown of a Vertical Tube
NASA Technical Reports Server (NTRS)
Darr, Samuel; Hu, Hong; Schaeffer, Reid; Chung, Jacob; Hartwig, Jason; Majumdar, Alok
2015-01-01
This paper presents the results of a one-dimensional numerical simulation of the transient chilldown of a vertical stainless steel tube with liquid nitrogen. The direction of flow is downward (with gravity) through the tube. Heat transfer correlations for film, transition, and nucleate boiling, as well as critical heat flux, rewetting temperature, and the temperature at the onset of nucleate boiling were used to model the convection to the tube wall. Chilldown curves from the simulations were compared with data from 55 recent liquid nitrogen chilldown experiments. With these new correlations the simulation is able to predict the time to rewetting temperature and time to onset of nucleate boiling to within 25% for mass fluxes ranging from 61.2 to 1150 kg/(sq m s), inlet pressures from 175 to 817 kPa, and subcooled inlet temperatures from 0 to 14 K below the saturation temperature.
Numerical simulation of experiments in the Giant Planet Facility
NASA Technical Reports Server (NTRS)
Green, M. J.; Davy, W. C.
1979-01-01
Utilizing a series of existing computer codes, ablation experiments in the Giant Planet Facility are numerically simulated. Of primary importance is the simulation of the low Mach number shock layer that envelops the test model. The RASLE shock-layer code, used in the Jupiter entry probe heat-shield design, is adapted to the experimental conditions. RASLE predictions for radiative and convective heat fluxes are in good agreement with calorimeter measurements. In simulating carbonaceous ablation experiments, the RASLE code is coupled directly with the CMA material response code. For the graphite models, predicted and measured recessions agree very well. Predicted recession for the carbon phenolic models is 50% higher than that measured. This is the first time codes used for the Jupiter probe design have been compared with experiments.
Numerical simulations of Kadomtsev-Petviashvili soliton interactions
NASA Astrophysics Data System (ADS)
Infeld, E.; Senatorski, A.; Skorupski, A. A.
1995-04-01
The Kadomtsev-Petviashvili equation generalizes that of Korteweg and de Vries to two space dimensions and arises in various weakly dispersive media. Two very different species of soliton solutions are known for one variant, KPI. The first species to be discovered are line solitons, the second are two dimensional lumps. This paper describes numerical simulations, consistent with all constraints of the equation, in which very distorted line solitons break up into smaller line solitons and arrays of lumps. The arrays can interact with one another. In some cases, aspects of the results of the simulations can be understood in the light of specially constructed exact solutions. Simulations in which initial conditions fail to satisfy the constraints of the equation are also described.
Numerical Simulations of Penetration and Overshoot in the Sun
Tamara M. Rogers; Gary A. Glatzmaier
2006-01-30
We present numerical simulations of convective overshoot in a two-dimensional model of the solar equatorial plane. The simulated domain extends from 0.001 R_sun to 0.93 R_sun, spanning both convective and radiative regions. We show that convective penetration leads to a slightly extended, mildly subadiabatic temperature gradient beneath the convection zone below which there is a rapid transition to a strongly subadiabatic region. A slightly higher temperature is maintained in the overshoot region by adiabatic heating from overshooting plumes. This enhanced temperature may partially account for the sound speed discrepancy between the standard solar model and helioseismology. Simulations conducted with tracer particles suggest that a fully mixed region exists down to at least 0.687 R_sun.
SCIT-DNS: Critical Infrastructure Protection through Secure DNS Server Dynamic Updates
Sood, Arun K.
framework of DNS servers that voids the above requirement. Our approach, called Self-Cleansing Intrusion rotation and self-cleansing cycles are in the range of minutes, restricting the damages of even undetected assurance, intrusion containment, self-cleansing systems I. INTRODUCTION As the world becomes ever more
Numerical simulation and experimental verification of extended source interferometer
NASA Astrophysics Data System (ADS)
Hou, Yinlong; Li, Lin; Wang, Shanshan; Wang, Xiao; Zang, Haijun; Zhu, Qiudong
2013-12-01
Extended source interferometer, compared with the classical point source interferometer, can suppress coherent noise of environment and system, decrease dust scattering effects and reduce high-frequency error of reference surface. Numerical simulation and experimental verification of extended source interferometer are discussed in this paper. In order to provide guidance for the experiment, the modeling of the extended source interferometer is realized by using optical design software Zemax. Matlab codes are programmed to rectify the field parameters of the optical system automatically and get a series of interferometric data conveniently. The communication technique of DDE (Dynamic Data Exchange) was used to connect Zemax and Matlab. Then the visibility of interference fringes can be calculated through adding the collected interferometric data. Combined with the simulation, the experimental platform of the extended source interferometer was established, which consists of an extended source, interference cavity and image collection system. The decrease of high-frequency error of reference surface and coherent noise of the environment is verified. The relation between the spatial coherence and the size, shape, intensity distribution of the extended source is also verified through the analysis of the visibility of interference fringes. The simulation result is in line with the result given by real extended source interferometer. Simulation result shows that the model can simulate the actual optical interference of the extended source interferometer quite well. Therefore, the simulation platform can be used to guide the experiment of interferometer which is based on various extended sources.
Numerical simulation and experimental validation of RF drying
Bringhurst, S.; White, M.J.; Iskander, M.F. [Univ. of Utah, Salt Lake City, UT (United States). Electrical Engineering Dept.
1996-12-31
The Finite-Difference Time-Domain (FDTD) method has been used by the group to simulate a wide variety of Radio Frequency (RF) and induction-drying processes and realistic, microwave-sintering experiments. Many results were presented and some guidelines towards the effective use of the microwave and RF heating technologies of ceramic ware were developed. In this paper the authors describe an experimental effort which was used to validate the FDTD simulation results. Specifically an experimental RF dryer, Thermax Model No. T3GB operating at 25 MHz, was used to dry ceramic ware of various materials, sizes, and shapes and the temperature distribution pattern was monitored using six fiber-optic temperature probes. The measured heating patterns were then compared with the FDTD simulation results. Many of the guidelines developed using the numerical simulations were confirmed experimentally. Results from various comparisons between simulation and experimental data will be presented. Additional results from the simulation efforts illustrating possible procedures for improving the efficiency and the uniformity of RF drying will also be described.
Numerical simulation of flow separation control by oscillatory fluid injection
NASA Astrophysics Data System (ADS)
Resendiz Rosas, Celerino
2005-07-01
In this work, numerical simulations of flow separation control are performed. The separation control technique studied is called "synthetic jet actuation". The developed code employs a cell centered finite volume scheme which handles viscous, steady and unsteady compressible turbulent flows. The pulsating zero mass jet flow is simulated by imposing a harmonically varying transpiration boundary condition on the airfoil's surface. Turbulence is modeled with the algebraic model of Baldwin and Lomax. The application of synthetic jet actuators is based in their ability to energize the boundary layer, thereby providing significant increase in the lift coefficient. This has been corroborated experimentally and it is corroborated numerically in this research. The performed numerical simulation investigates the flow over a NACA0015 airfoil. For this flow Re = 9 x 105 and the reduced frequency and momentum coefficient are F + = 1.1 and Cmu = 0.04 respectively. The oscillatory injection takes place at 12.27% chord from the leading edge. A maximum increase in the mean lift coefficient of 93% is predicted by the code. A discrepancy of approximately 10% is observed with corresponding experimental data from the literature. The general trend is, however, well captured. The discrepancy is attributed to the modeling of the injection boundary condition and to the turbulence model. A sensitivity analysis of the lift coefficient to different values of the oscillation parameters is performed. It is concluded that tangential injection, F+ ? O(1) and the utilized grid resolution around the site of injection are optimal. Streamline fields obtained for different angles of injection are analyzed. Flow separation and attachment as functions of the injection angle and of the velocity of injection can be observed. It is finally concluded that a reliable numerical tool has been developed which can be utilized as a support tool in the optimization of the synthetic jet operation and in the modeling of its operation.
Studying Turbulence Using Numerical Simulation Databases. No. 7; Proceedings of the Summer Program
NASA Technical Reports Server (NTRS)
1998-01-01
The Seventh Summer Program of the Center for Turbulence Research took place in the four-week period, July 5 to July 31, 1998. This was the largest CTR Summer Program to date, involving thirty-six participants from the U. S. and nine other countries. Thirty-one Stanford and NASA-Ames staff members facilitated and contributed to most of the Summer projects. A new feature, and perhaps a preview of the future programs, was that many of the projects were executed on non-NASA computers. These included supercomputers located in Europe as well as those operated by the Departments of Defense and Energy in the United States. In addition, several simulation programs developed by the visiting participants at their home institutions were used. Another new feature was the prevalence of lap-top personal computers which were used by several participants to carry out some of the work that in the past were performed on desk-top workstations. We expect these trends to continue as computing power is enhanced and as more researchers (many of whom CTR alumni) use numerical simulations to study turbulent flows. CTR's main role continues to be in providing a forum for the study of turbulence for engineering analysis and in facilitating intellectual exchange among the leading researchers in the field. Once again the combustion group was the largest. Turbulent combustion has enjoyed remarkable progress in using simulations to address increasingly complex and practically more relevant questions. The combustion group's studies included such challenging topics as fuel evaporation, soot chemistry, and thermonuclear reactions. The latter study was one of three projects related to the Department of Energy's ASCI Program (www.llnl.gov/asci); the other two (rocket propulsion and fire safety) were carried out in the turbulence modeling group. The flow control and acoustics group demonstrated a successful application of the so-called evolution algorithms which actually led to a previously unknown forcing strategy for jets yielding increased spreading rate. A very efficient algorithm for flow in complex geometries with moving boundaries based on the immersed boundary forcing technique was tested with very encouraging results. Also a new strategy for the destruction of aircraft trailing vortices was introduced and tested. The Reynolds Averaged Modeling (RANS) group demonstrated that the elliptic relaxation concept for RANS calculations is also applicable to transonic flows with shocks; however, prediction of laminar/turbulent transition remains an important pacing item. A large fraction of the LES effort was devoted to the development and testing of a new algorithmic procedure (as opposed to phenomenological model) for subgrid scale modeling based on regularized de-filtering of the flow variables. This appears to be a very promising approach, and a significant effort is currently underway to assess its robustness in high Reynolds number flows and in conjunction with numerical methods for complex flows. As part of the Summer Program two review tutorials were given on Turbulent structures in hydrocarbon pool fires (Sheldon Tieszen), and Turbulent combustion modeling: from RANS to LES via DNS (Luc Vervisch); and two seminars entitled Assessment of turbulence models for engineering applications (Paul Durbin) and Subgrid-scale modeling for non-premixed, turbulent reacting flows (James Riley) were presented. A number of colleagues from universities, government agencies, and industry attended the final presentations of the participants on July 31 and participated in the discussions. There are twenty-six papers in this volume grouped in five areas. Each group is preceded with an overview by its coordinator.
Efficient treatment of complex geometries for large eddy simulations of turbulent flows
D. G. E. Grigoriadis; J. G. Bartzis; A. Goulas
2004-01-01
Incompressible turbulent flow over a backward facing step at Reh=5100 is investigated by large eddy simulations (LES). The ratio of the oncoming boundary layer thickness ? to the step height h was set to 1.2. Additionally channel flows at various Re? numbers are presented for the validation of the numerical code. The results are compared with existing DNS and experimental
Filtered density function for large eddy simulation of turbulent reacting flows
Carlo solution and the performance of the closures employed in the FDF transport equation are assessed by comparisons with results obtained by direct numerical simulation DNS and by conventional LES procedures predictions of the large scale flow features. A survey of combustion literature reveals relatively little work
Testing a low-Reynolds number k-epsilon turbulence model based on direct simulation data
V. Michelassi; W. Rodi; J. Zhu
1993-01-01
A low Re k-epsilon model based on direct numerical simulation (DNS) data proposed by Rodi and Mansour (1993) (RM) is discussed. The model was complemented by a damping function multiplying the destruction term in the model epsilon equation. The modified version of the RM model yielded generally good predictions of all major quantities in developed channel and boundary layer flows
Mean Flow Generation and Inertial Wave Attractors in a Librating Annulus: DNS and Theory
NASA Astrophysics Data System (ADS)
Klein, Marten; Ghasemi V, Abouzar; Seelig, Torsten; Borcia, Ion Dan; Harlander, Uwe; Will, Andreas
2015-04-01
Rotation ?0 is one of the most important system parameters in geophysical fluid dynamics (GFD) due to stratification of angular momentum. Oscillatory motion (libration) of the fluid over solid surfaces is generated in many systems by harmonic forces, usually resulting from rotation of the fluid in a homogeneous gravitational and/or electromagnetic field. Small amplitudes of libration can excite inertial waves, which are shear waves, band-limited to frequencies 0 < ? < 2?0 and dispersive with respect to direction. Large amplitudes of libration can excite turbulent motion (Görtler vortices), which is transpored into the bulk by the Coriolis force. Waves and vortices may take part in the redistribution of kinetic energy and angular momentum. Complex system responses are possible so that it is of fundamental interest to understand inertial waves and related turbulent phenomena in order to quantify their relevance for applications. Direct numerical simulations (DNS) of an axially rotating annulus have been carried out for straight and inclined cylinder walls, with and without lids. The system was perturbed by libration of inner and/or outer cylinder walls and/or of the lids with small libration amplitude ?? 0.1 and frequency ?0.3 and frequencies ?
Numerical Simulation of Biodegradation Processes in Subsurface Systems
NASA Astrophysics Data System (ADS)
Hernandez-Rendon, M.
2007-12-01
In this work an evaluation of a numerical scheme to simulate mutispecies reactive transport undergoing biodegradation in porous media is presented. This procedure relies on the application of an operator splitting algorithm introduced by Glowinski [1]. It was used to simulate a system of two species transport with non linear reactions with good results. The main advantage in applying it is that the system of partial differential equations is fully decoupled. Approximation of time derivative is obtained using standard procedures. The spatial advection- diffusion part is solved by means of the standard Galerkin finite element method; nonlinear integrals are evaluated with Gaussian cuadrature. Numerical results are presented for two test cases that take into account the transport of five species; the first one with first order and sequential biotransformation is compared with the analytical solution and with the numerical approximation of the coupled system. In the second, nonlinear and simultaneous reactions are included. [1] Glowinski, R., (2000). Operator-splitting methods for initial value problems: Application to the atmospheric dispersion equations, Lectures 6-8, University of Houston.
Numerical resolution effects on simulations of massive black hole seeds
NASA Astrophysics Data System (ADS)
Regan, John A.; Johansson, Peter H.; Haehnelt, Martin G.
2014-03-01
We have performed high-resolution numerical simulations with the hydrodynamical adaptive mesh refinement code ENZO to investigate the formation of massive seed black holes in a sample of six dark matter haloes above the atomic cooling threshold. The aim of this study is to illustrate the effects of varying the maximum refinement level on the final object formed. The virial temperatures of the simulated haloes range from T ˜ 10 000 to ˜16 000 K and they have virial masses in the range M ˜ 2 × 107 to ˜7 × 107 M? at z ˜ 15. The outcome of our six fiducial simulations is both generic and robust. A rotationally supported, marginally gravitationally stable, disc forms with an exponential profile. The mass and scale length of this disc depends strongly on the maximum refinement level used. Varying the maximum refinement level by factors between 1/64 and 256 times the fiducial level illustrates the care that must be taken in interpreting the results. The lower resolution simulations show tentative evidence that the gas may become rotationally supported out to 20 pc while the highest resolution simulations show only weak evidence of rotational support due to the shorter dynamical times for which the simulation runs. The higher resolution simulations do, however, point to fragmentation at small scales of the order of ˜100 au. In the highest resolution simulations a central object of a few times 102 M? forms with multiple strongly bound, Jeans unstable, clumps of ?10 M? and radii of 10-20 au suggesting the formation of dense star clusters in these haloes.
An Object Model for a Rocket Engine Numerical Simulator
NASA Technical Reports Server (NTRS)
Mitra, D.; Bhalla, P. N.; Pratap, V.; Reddy, P.
1998-01-01
Rocket Engine Numerical Simulator (RENS) is a packet of software which numerically simulates the behavior of a rocket engine. Different parameters of the components of an engine is the input to these programs. Depending on these given parameters the programs output the behaviors of those components. These behavioral values are then used to guide the design of or to diagnose a model of a rocket engine "built" by a composition of these programs simulating different components of the engine system. In order to use this software package effectively one needs to have a flexible model of a rocket engine. These programs simulating different components then should be plugged into this modular representation. Our project is to develop an object based model of such an engine system. We are following an iterative and incremental approach in developing the model, as is the standard practice in the area of object oriented design and analysis of softwares. This process involves three stages: object modeling to represent the components and sub-components of a rocket engine, dynamic modeling to capture the temporal and behavioral aspects of the system, and functional modeling to represent the transformational aspects. This article reports on the first phase of our activity under a grant (RENS) from the NASA Lewis Research center. We have utilized Rambaugh's object modeling technique and the tool UML for this purpose. The classes of a rocket engine propulsion system are developed and some of them are presented in this report. The next step, developing a dynamic model for RENS, is also touched upon here. In this paper we will also discuss the advantages of using object-based modeling for developing this type of an integrated simulator over other tools like an expert systems shell or a procedural language, e.g., FORTRAN. Attempts have been made in the past to use such techniques.
Dynamic failure mechanisms of ceramic bars : Experiments and numerical simulations
NASA Astrophysics Data System (ADS)
Espinosa, H. D.; Brar, N. S.
1995-10-01
Failure mechanisms in ceramics are investigated by means of bar impact experiments and numerical simulations of the wave propagation event. Stress histories are measured by embedding manganin stress gauges in the ceramic bars. The fracture event is examined by high speed photography. A violent radial expansion, in a region close to the impact surface, followed by a cloud of debris is observed. Numerical simulations of the inelastic wave propagation event are performed with a multiple-plane microcracking model. The simulations show that when the impact stress exceeds a material threshold, the stress wave in the bar has a relatively short duration which is controlled by the rate of unconfined compressive damage. A nonzero inelastic strain rate at the wave front is required in the simulations to properly capture the measured stress attenuation with propagation distance. This feature is related to a heterogeneous material microstructure which is a common occurrence in ceramics. Furthermore, the simulations predict a radial expansion of the bar as a result of not only compressive but also tensile damage. The radial velocity histories on the bar surface are functions of wave propagation distance and damage rate. Tensile damage is induced by stress release from the rod surface and is restricted to the bar core, due to wave focusing, and to the bar free end. In the latest case, reflection of the compressive pulse produces bar spallation. The two dimensional distribution of tensile and compressive damage is assessed by means of contour plots of volumetric strain and the second invariant of the inelastic strain tensor.
Numerical simulation of multi-layered textile composite reinforcement forming
NASA Astrophysics Data System (ADS)
Wang, P.; Hamila, N.; Boisse, P.
2011-05-01
One important perspective in aeronautics is to produce large, thick or/and complex structural composite parts. The forming stage presents an important role during the whole manufacturing process, especially for LCM processes (Liquid Composites Moulding) or CFRTP (Continuous Fibre Reinforcements and Thermoplastic resin). Numerical simulations corresponding to multi-layered composite forming allow the prediction for a successful process to produce the thick parts, and importantly, the positions of the fibres after forming to be known. This paper details a set of simulation examples carried out by using a semi-discrete shell finite element made up of unit woven cells. The internal virtual work is applied on all woven cells of the element taking into account tensions, in-plane shear and bending effects. As one key problem, the contact behaviours of tool/ply and ply/ply are described in the numerical model. The simulation results not only improve our understanding of the multi-layered composite forming process but also point out the importance of the fibre orientation and inter-ply friction during formability.
Numerical simulation of free water surface in pump intake
NASA Astrophysics Data System (ADS)
Zhao, L. J.; Nohmi, M.
2012-11-01
The purpose of this paper is to verify the volume of fluid (VOF) method for simulating the free water surface flow in pump intake. With the increasing computer power, VOF method has been becoming a more flexible and accurate choice to replace the conventional fixed water surface method, because it does not require assumptions on the nature of air-water interface. Two examples are presented in this paper. The first example is presented for simulating the growth of air-entrained vortices. LES (Large Eddy Simulation) model, instead of RANS (Reynolds averaged Navier-Stokes) turbulence model, is used to capture the peak of circular velocity around the vortex core. Numerical result shows good agreement with the benchmark experiment carried by the Turbomachinery Society of Japan. The second example predicts the flow rate distribution in the pump intake consisting of one opened and two closed channels. VOF result is compared with the conventional fixed water surface method assuming free-slip boundary condition on the fluid interface. The difference of flow pattern in the opened channel indicates that numerical flow field is affected remarkably by the setup of boundary condition at air-water interface.
Numerical simulation of the non-Newtonian mixing layer
NASA Technical Reports Server (NTRS)
Azaiez, Jalel; Homsy, G. M.
1993-01-01
This work is a continuing effort to advance our understanding of the effects of polymer additives on the structures of the mixing layer. In anticipation of full nonlinear simulations of the non-Newtonian mixing layer, we examined in a first stage the linear stability of the non-Newtonian mixing layer. The results of this study show that, for a fluid described by the Oldroyd-B model, viscoelasticity reduces the instability of the inviscid mixing layer in a special limit where the ratio (We/Re) is of order 1 where We is the Weissenberg number, a measure of the elasticity of the flow, and Re is the Reynolds number. In the present study, we pursue this project with numerical simulations of the non-Newtonian mixing layer. Our primary objective is to determine the effects of viscoelasticity on the roll-up structure. We also examine the origin of the numerical instabilities usually encountered in the simulations of non-Newtonian fluids.
Numerical simulation of pressure pulsations in Francis turbines
NASA Astrophysics Data System (ADS)
Magnoli, M. V.; Schilling, R.
2012-11-01
In the last decades, hydraulic turbines have experienced the increase of their power density and the extension of their operating range, leading the fluid and mechanical dynamic effects to become significantly more pronounced. The understanding of the transient fluid flow and of the associated unsteady effects is essential for the reduction of the pressure pulsation level and improvement of the machine dynamic behaviour. In this study, the instationary fluid flow through the complete turbine was numerically calculated for an existing Francis machine with high specific speed. The hybrid turbulence models DES (detached eddy simulation) and SAS (scale adaptive simulation) allowed the accurate simulation of complex dynamic flow effects, such as the rotor-stator-interaction and the draft tube instabilities. Different operating conditions, as full load, part load, higher part load and deep part load, were successfully simulated and showed very tight agreement with the experimental results from the model tests. The transient pressure field history, obtained from the CFD (computational fluid dynamics) simulation and stored for each time step, was used as input for the full instationary FEA (finite element analysis) of turbine components. The assessment of the machine dynamic motion also offered the possibility to contribute to the understanding of the pressure pulsation effects and to further increase the turbine stability. This research project was developed at the Institute of Fluid Mechanics of the TU München.
Numerical simulations for plasma-based dry reforming
NASA Astrophysics Data System (ADS)
Snoeckx, Ramses; Aerts, Robby; Bogaerts, Annemie
2012-10-01
The conversion of greenhouse gases (CO2 and CH4) to more valuable chemicals is one of the challenges of the 21st century. The aim of this study is to describe the plasma chemistry occurring in a DBD for the dry reforming of CO2/CH4 mixtures, via numerical simulations. For this purpose we apply the 0D simulation code ``Global/kin,'' developed by Kushner, in order to simulate the reaction chemistry and the actual reaction conditions for a DBD, including the occurrence of streamers. For the chemistry part, we include a chemistry set consisting of 62 species taking part in 530 reactions. First we describe the reaction chemistry during one streamer, by simulating one discharge pulse and its afterglow, to obtain a better understanding of the reaction kinetics. Subsequently, we expand these results to real time scale simulations, i.e., 1 to 10 seconds, where we analyze the effects of the multiple discharges (streamers) and input energy on the conversion and the selectivity of the reaction products, as well as on the energy efficiency of the process. The model is validated based on experimental data from literature.
A Numerical simulation of transition in plane channel flow
NASA Technical Reports Server (NTRS)
Goglia, G.; Biringen, S.
1982-01-01
A numerical simulation of the final stages of transition to turbulence in plane channel flow at a Reynolds number of 7500 is described. Three dimensional, incompressible Navier-Stokes equations are numerically integrated to obtain the time evolution of two and three dimensional finite amplitude disturbances. Computations are performed on the CYBER-203 vector processor for a 32 by 33 by 32 grid. Solutions indicate the existence of structures similar to those observed in the laboratory and which are characteristic of various stages of transition that lead to final breakdown. Details of the resulting flow field after breakdown indicate the evolution of streak-like formations found in turbulent flows. Although the flow field does approach a steady state (turbulent channel flow), implementation of subgrid-scale terms are necessary to obtain proper turbulent statistics.
A numerical simulation of transition in plane channel flow
NASA Technical Reports Server (NTRS)
Biringen, S.
1983-01-01
This paper involves a numerical simulation of the final stages of transition to turbulence in plane channel flow at a Reynolds number of 7500. Three-dimensional, incompressible Navier-Stokes equations are numerically integrated to obtain the time-evolution of two- and three-dimensional finite-amplitude disturbances. Computations are performed on the CYBER-203 vector processor for a 32 x 33 x 32 grid. Solutions indicate the existence of structures similar to those observed in the laboratory and which are characteristic of various stages of transition that lead to final breakdown. Details of the resulting flow field after breakdown indicate the evolution of streak-like formations found in turbulent flows. Although the flow field does approach a steady-state (turbulent channel flow), implementation of subgrid-scale terms are necesary to obtain proper turbulent statistics.
Hydrodynamics of Hypersonic Jets: Experiments and Numerical Simulations
Belan, Marco; Tordella, Daniela; Massaglia, Silvano; Ferrari, Attilio; Mignone, Andrea; Bodenschatz, Eberhard
2011-01-01
Stars form in regions of the galaxy that are denser and cooler than the mean interstellar medium. These regions are called Giant Molecular Clouds. At the beginning of their life, up to $10^5-10^6$ years, stars accrete matter from their rich surrounding environment and are origin of a peculiar phenomenon that is the jet emission. Jets from Young Stellar Objects (YSOs) are intensively studied by the astrophysical community by observations at different wavelengths, analytical and numerical modeling and laboratory experiments. Indications about the jet propagation and its resulting morphologies are here obtained by means of a combined study of hypersonic jets carried out both in the laboratory and by numerical simulations.
Numerical simulation of MHD shock waves in the solar wind
NASA Technical Reports Server (NTRS)
Steinolfson, R. S.; Dryer, M.
1978-01-01
The effects of the interplanetary magnetic field on the propagation speed of shock waves through an ambient solar wind are examined by numerical solutions of the time-dependent nonlinear equations of motion. The magnetic field always increases the velocity of strong shocks. Although the field may temporarily slow down weak shocks inside 1 AU, it eventually also causes weak shocks to travel faster than they would without the magnetic field at larger distances. Consistent with the increase in the shock velocity, the gas pressure ratio across a shock is reduced considerably in the presence of the magnetic field. The numerical method is used to simulate (starting at 0.3 AU) the large deceleration of a shock observed in the lower corona by ground-based radio instrumentation and the more gradual deceleration of the shock in the solar wind observed by the Pioneer 9 and Pioneer 10 spacecraft.
Delay Fast Packets (DFP): Prevention of DNS Cache Poisoning
Dolev, Danny
Delay Fast Packets (DFP): Prevention of DNS Cache Poisoning Shimrit Tzur-David Kiril Lashchiver, or any other network resource. This paper presents a solution for the cache poisoning attack in which the attacker inserts incorrect data into the DNS cache. In order to successfully poison the cache, the attacker
Numerical simulation of a liquid propellant rocket motor
NASA Astrophysics Data System (ADS)
Salvador, Nicolas M. C.; Morales, Marcelo M.; Migueis, Carlos E. S. S.; Bastos-Netto, Demétrio
2001-03-01
This work presents a numerical simulation of the flow field in a liquid propellant rocket engine chamber and exit nozzle using techniques to allow the results to be taken as starting points for designing those propulsive systems. This was done using a Finite Volume method simulating the different flow regimes which usually take place in those systems. As the flow field has regions ranging from the low subsonic to the supersonic regimes, the numerical code used, initially developed for compressible flows only, was modified to work proficiently in the whole velocity range. It is well known that codes have been developed in CFD, for either compressible or incompressible flows, the joint treatment of both together being complex even today, given the small number of references available in this area. Here an existing code for compressible flow was used and primitive variables, the pressure, the Cartesian components of the velocity and the temperature instead of the conserved variables were introduced in the Euler and Navier-Stokes equations. This was done to permit the treatment at any Mach number. Unstructured meshes with adaptive refinements were employed here. The convective terms were treated with upwind first and second order methods. The numerical stability was kept with artificial dissipation and in the spatial coverage one used a five stage Runge-Kutta scheme for the Fluid Mechanics and the VODE (Value of Ordinary Differential Equations) scheme along with the Chemkin II in the chemical reacting solution. During the development of this code simulating the flow in a rocket engine, comparison tests were made with several different types of internal and external flows, at different velocities, seeking to establish the confidence level of the techniques being used. These comparisons were done with existing theoretical results and with other codes already validated and well accepted by the CFD community.
Numerical simulation of morphodynamic diversity in the World's largest rivers
NASA Astrophysics Data System (ADS)
Nicholas, A.
2012-04-01
The World's largest rivers share many common properties, including gentle longitudinal bed gradients (~ 0.01-0.1 m per km), high mean annual discharges (~ >10,000 cumecs), and sand-sized bed sediment (D50 ~ 0.1-0.4 mm), yet despite these similarities they are characterised by diverse planform patterns and morphodynamic behaviour (including meandering, braided and anabranching river styles). Recent studies have shown that this diversity cannot be explained using existing channel pattern classification schemes that apply to small rivers. Indeed at present, the causes of morphodynamic diversity in the World's largest rivers remain unclear. Moreover, elucidation of process-form interactions in large rivers is hampered by logistical difficulties involved in field data collection, and by the time period over which satellite imagery is available, which is short given the slow rates of channel change in many large rivers. Numerical models provide a further possible approach for investigating large river morphodynamics. However, although many such models exist, they have generally been developed or applied to simulate either meandering or braided rivers, rather than to investigate a range of channel styles. This paper aims to address this shortcoming using a new numerical simulation model, which is applied to explore the controls on morphodynamic diversity in large sand-bed rivers. This model is based on the 2D shallow water equations with secondary circulation correction, with model components representing total sand transport, suspended transport of silt and clay, bank erosion, vegetation growth and floodplain development. Numerical simulations representing time periods of c. 200 years illustrate how a wide range of channel morphologies, including meandering, braided and anabranching channels, may develop from the same initial conditions and external forcing (valley gradient and discharge regime). These results shed light on the process controls on morphodynamic diversity in the World's largest rivers and provide a possible explanation for the failure of channel pattern classification schemes when applied to such rivers.
Numerical simulation of blood with fluid-structure interactions using the lattice-Boltzmann method
Reasor, Daniel A; Yun, Brian M; Aidun, Cyrus K
2010-01-01
The fluid dynamics video presented here outlines recent advances in the simulation of multiphase cellular blood flow through the direct numerical simulations of deformable red blood cells (RBCs) demonstrated through several numerical experiments. Videos show RBC deformations in variety of numerical simulations, relative viscosity of a suspension of RBCs in shear, and the cell-depleted wall layer for blood Hagen--Poiseuille flow.
Numerical simulation of wear process and reliability lifetime for cylindrical roller bearing
Ying Shi; Qinyu Jiang; Baoliang Li
2010-01-01
In order to solve the wear numerical simulation and the prediction of reliability lifetime of cylindrical roller bearing, a numerical simulation model of wear for cylindrical roller bearing is established by the method for wear numerical simulation of components. According to the discrete theory, computer technology and dynamic theory of roller bearing, with the wear state of cylindrical roller bearing
Numerically-simulated formation and propagation of interplanetary shocks
NASA Technical Reports Server (NTRS)
Wu, S. T.
1985-01-01
The present numerical method for simulating the formation and propagation of interplanetary shocks is based on the shock-capturing finite difference scheme of Lax (1950) and Lax and Wendroff (1960), as well as the recent method of NEAR characteristics of Nakagawa (1980, 1981). Attention is given to examples which strongly suggest that all the shocked solar wind plasma parameters due to given physical perturbations, such as flare-generated shocks, can be predicted through the use of this method; the method is, however, limited to the supersonic and super-Alfvenic flow.
Numerical simulations of the accretion-ejection instability
Stuart Caunt; Michel Tagger
2000-11-17
The Accretion-Ejection Instability (AEI) is explored numerically using a global 2d model of the inner region of a magnetised accretion disk. The disk is initially currentless but threaded by an external vertical magnetic field created by external currents, and frozen in the flow. In agreement with the theory a spiral instability, similar in many ways to those observed in self-gravitating disks, but driven by magnetic stresses, develops when the magnetic field is close to equipartition with the disk thermal pressure. The present non-linear simulations give good evidence that such an instability can occur in the inner region of accretion disks.
General Physics Motivations for Numerical Simulations of Quantum Field Theory
Rajan Gupta
1999-05-20
In this introductory article a brief description of Quantum Field Theories (QFT) is presented with emphasis on the distinction between strongly and weakly coupled theories. A case is made for using numerical simulations to solve QCD, the regnant theory describing the interactions between quarks and gluons. I present an overview of what these calculations involve, why they are hard, and why they are tailor made for parallel computers. Finally, I try to communicate the excitement amongst the practitioners by giving examples of the quantities we will be able to calculate to within a few percent accuracy in the next five years.
Numerical Simulation of Low-Density Shock-Wave Interactions
NASA Technical Reports Server (NTRS)
Glass, Christopher E.
1999-01-01
Computational Fluid Dynamics (CFD) numerical simulations of low-density shock-wave interactions for an incident shock impinging on a cylinder have been performed. Flow-field density gradient and surface pressure and heating define the type of interference pattern and corresponding perturbations. The maximum pressure and heat transfer level and location for various interaction types (i.e., shock-wave incidence with respect to the cylinder) are presented. A time-accurate solution of the Type IV interference is employed to demonstrate the establishment and the steadiness of the low-density flow interaction.
Numerical simulation of fluid flow around a scramaccelerator projectile
NASA Technical Reports Server (NTRS)
Pepper, Darrell W.; Humphrey, Joseph W.; Sobota, Thomas H.
1991-01-01
Numerical simulations of the fluid motion and temperature distribution around a 'scramaccelerator' projectile are obtained for Mach numbers in the 5-10 range. A finite element method is used to solve the equations of motion for inviscid and viscous two-dimensional or axisymmetric compressible flow. The time-dependent equations are solved explicitly, using bilinear isoparametric quadrilateral elements, mass lumping, and a shock-capturing Petrov-Galerkin formulation. Computed results indicate that maintaining on-design performance for controlling and stabilizing oblique detonation waves is critically dependent on projectile shape and Mach number.
Numerical simulation of carbon arc discharge for nanoparticle synthesis
Kundrapu, M.; Keidar, M. [Department of Mechanical and Aerospace Engineering, George Washington University, Washington, DC 20052 (United States)
2012-07-15
Arc discharge with catalyst-filled carbon anode in helium background was used for the synthesis of carbon nanoparticles. In this paper, we present the results of numerical simulation of carbon arc discharges with arc current varying from 10 A to 100 A in a background gas pressure of 68 kPa. Anode sublimation rate and current voltage characteristics are compared with experiments. Distribution of temperature and species density, which is important for the estimation of the growth of nanoparticles, is obtained. The probable location of nanoparticle growth region is identified based on the temperature range for the formation of catalyst clusters.
Accurate numerical simulation of short fiber optical parametric amplifiers.
Marhic, M E; Rieznik, A A; Kalogerakis, G; Braimiotis, C; Fragnito, H L; Kazovsky, L G
2008-03-17
We improve the accuracy of numerical simulations for short fiber optical parametric amplifiers (OPAs). Instead of using the usual coarse-step method, we adopt a model for birefringence and dispersion which uses fine-step variations of the parameters. We also improve the split-step Fourier method by exactly treating the nonlinear ellipse rotation terms. We find that results obtained this way for two-pump OPAs can be significantly different from those obtained by using the usual coarse-step fiber model, and/or neglecting ellipse rotation terms. PMID:18542454
Numerical simulation of convective boundary layer above polynyas and leads.
NASA Astrophysics Data System (ADS)
Debolskiy, Andrey; Stepanenko, Victor
2013-04-01
Arctic region is very important as one of drivers for global atmosphere circulation. Meanwhile, results of modern global atmospheric models, both climatic and weather forecasting differs significantly from each other and observations in this region. One of the reasons for these uncertainties can be inaccurate simulation of ice and snow cover distribution, which accuracy depends in turn on variety of factors. Among others, appropriate parameterizations of atmospheric boundary layer over inhomogeneous surface, not explicitly resolved at the atmospheric model grid, can decrease these inaccuracies. The main objective of these parameterizations is to calculate surface heat and water vapor fluxes, averaged over the whole model cell. However, due to great differences in structure of boundary layers formed over cold ice and relatively warm open water, which cause nonlinear dependencies,the parameterizations suggested to the moment can hardly be regarded as applicable for "complete" set of synoptic scenarios . The present paper attempts to improve standard mosaic method of flux aggregation, which is still common in climate models [1]. The main idea is to derive heat fluxes using data from numerical experiments, explicitly reproducing most of sub grid (for global models) turbulence motions spectra, and compare with fluxes calculated using mosaic method implying the part of model domain to be a global model cell. The study is based on idealized high resolution (~10 m) experiments with typically observed surface parameters (temperature and roughness), ice-open water distribution, initial temperature and wind profiles distribution included in Large Eddy Simulation model of Insitute of Numerical Mathematics RAS [2],[3]. Analysis of other boundary layer characteristics such as its height, eddy diffusivity profiles, kinetic energy is presented. The modeling results are compared with field experiments' data gathered at White Sea. References: 1. V.M. Stepanenko, P.M. Miranda, V.N. Lykosov. Numerical simulation of mesoscale iteration of atmosphere and hydrological inhomogeneous surface (in Russian). Computational technologies,2006, vol. 11 No.7: p.118-127 2. A.V. Glazunov, V.N. Lykossov. Large eddy simulation of interaction of ocean and atmospheric boundary layers. Russian Journal of Numerical Analysis and Mathematical Modeling. 2003 Vol.18, No. 4: p.279-295 3. Glazunov A.V. Modeling of neutral-stratified turbulent flow over horizontal rough surface(in Russian) Izvestiya. Atmospheric and Oceanic Physics vol.42, No3: p.307-325
Numerical simulations of separatrix instabilities in collisionless magnetic reconnection
NASA Astrophysics Data System (ADS)
Divin, A.; Lapenta, G.; Markidis, S.; Newman, D. L.; Goldman, M. V.
2012-04-01
Electron scale dynamics of magnetic reconnection separatrix jets is studied in this paper. Instabilities developing in directions both parallel and perpendicular to the magnetic field are investigated. Implicit particle-in-cell simulations with realistic electron-to-ion mass ratio are complemented by a set of small scale high resolution runs having the separatrix force balance as the initial condition. A special numerical procedure is developed to introduce the force balance into the small scale runs. Simulations show the development of streaming instabilities and consequent formation of electron holes in the parallel direction. A new electron jet instability develops in the perpendicular direction. The instability is closely related to the electron MHD Kelvin-Helmholtz mode and is destabilized by a flow, perpendicular to magnetic field at the separatrix. Tearing instability of the separatrix electron jet is modulated strongly by the electron MHD Kelvin-Helmholtz mode.
Numerical simulations of energy transfer in counter-streaming plasmas
NASA Astrophysics Data System (ADS)
Davis, S. P.; Capdessus, R.; d'Humières, E.; Jequier, S.; Andriyash, I.; Tikhonchuk, V.
2013-03-01
Collisionless shock formation is investigated with large scale fully electromagnetic two-dimensional Particle-in-Cell numerical simulations. Two plasmas are colliding in the center of mass reference frame at sub-relativistic velocities. Their interaction leads to collisionless stochastic electron heating, ion slowing down and formation of a shock front. We focus here on the initial stage of evolution where electron heating is due to the Weibel-like micro-instability driven by the high-speed ion flow. A two stage process is described in the detailed analysis of our simulation results. Filament generation, followed by turbulent mixing, constitute the dominant mechanism for energy repartition. The global properties are illustrated by examination of single filament evolution in terms of energy/particle density and fields.
Computational aeroacoustics and numerical simulation of supersonic jets
NASA Technical Reports Server (NTRS)
Morris, Philip J.; Long, Lyle N.
1996-01-01
The research project has been a computational study of computational aeroacoustics algorithms and numerical simulations of the flow and noise of supersonic jets. During this study a new method for the implementation of solid wall boundary conditions for complex geometries in three dimensions has been developed. In addition, a detailed study of the simulation of the flow in and noise from supersonic circular and rectangular jets has been conducted. Extensive comparisons have been made with experimental measurements. A summary of the results of the research program are attached as the main body of this report in the form of two publications. Also, the report lists the names of the students who were supported by this grant, their degrees, and the titles of their dissertations. In addition, a list of presentations and publications made by the Principal Investigators and the research students is also included.
Numerical simulation of transient hypervelocity flow in an expansion tube
NASA Technical Reports Server (NTRS)
Jacobs, P. A.
1992-01-01
Several numerical simulations of the transient flow of helium in an expansion tube are presented in an effort to identify some of the basic mechanisms which cause the noisy test flows seen in experiments. The calculations were performed with an axisymmetric Navier-Stokes code based on a finite volume formulation and upwinding techniques. Although laminar flow and ideal bursting of the diaphragms was assumed, the simulations showed some of the important features seen in experiments. In particular, the discontinuity in tube diameter of the primary diaphragm station introduced a transverse perturbation to the expanding driver gas and this perturbation was seen to propagate into the test gas under some flow conditions. The disturbances seen in the test flow can be characterized as either small amplitude, low frequency noise possibly introduced during shock compression or large amplitude, high frequency noise associated with the passage of the reflected head of the unsteady expansion.
Numerical simulation of transient hypervelocity flow in an expansion tube
NASA Technical Reports Server (NTRS)
Jacobs, P. A.
1992-01-01
Several numerical simulations of the transient flow of helium in an expansion tube are presented. The aim of the exercise is to provide further information on the operational problems of the NASA Langley expansion tube. The calculations were performed with an axisymmetric Navier-Stokes code based on a finite-volume formulation and upwinding techniques. Although laminar flow and ideal bursting of the diaphragms was assumed, the simulations showed some of the important features seen in the experiments. In particular, the discontinuity in the tube diameter at the primary diaphragm station introduced a transverse perturbation to the expanding driver gas, and this perturbation was seen to propagate into the test gas under some flow conditions. The disturbances seen in the test flow can be characterized as either 'small-amplitude' noise possibly introduced during shock compression or 'large-amplitude' noise associated with the passage of the reflected head of the unsteady expansion.
NAS - Supercomputing master tool for aeronautics. [Numerical Aerodynamic Simulation
NASA Technical Reports Server (NTRS)
Bailey, F. R.
1985-01-01
Features, performance objectives and applications planned for the NASA National Aerodynamics Simulator (NAS) are outlined. NAS was conceived in the 1970s as a means to performing numerical aerodynamic simulations beyond the scope of wind tunnel testing for high-speed flight. Present supercomputers cannot deal with problems exhibiting strongly coupled viscous effects, which are being increasingly more accurately represented by approximations to the full Navier-Stokes equations. Located at the NASA-Ames Center, the NAS will by 1990 comprise a distributed computer network capable of a 4 Gflop computing rate and have a memory capacity of 1 billion 64-bit words. Remote access to the system through UNIX-based microcomputers will be available through land lines and satellite links. New supercomputers will be tested on the system without disturbing ongoing work. The core machine will be a Cray-2 with a 2 Gflop rate.
DSMC numerical simulation of lateral jet interaction with rarefied atmosphere
NASA Astrophysics Data System (ADS)
Liang, Jie; Li, Zhihui; Li, Xuguo; Fang, Ming
2014-12-01
Hybrid Cartesian grids and surface unstructured triangular cells are used in the DSMC method to simulate lateral jet interaction flow with rarefied atmosphere in transitional regime. The self-adaption of uniform Cartesian grid is conducted according to the local density gradient. The parallel DSMC code is used to compute the lateral jet interaction of a three-dimensional flat plate model on low density wind tunnel test conditions. The results of complex flow field structures, surface flow characteristics and separation lengths agree well with experimental data. The supersonic and hypersonic lateral jet interactions with hypersonic rarefied incoming flow on a slender blunt double-cone are investigated. The effects of flight altitudes, free stream velocities and angles of attack on plume-atmosphere interaction flow fields are numerically analyzed. The influences of parameter distributions with uniform /non-uniform nozzle exit on the separation distance and vortex structure near the jet are simulated in detail.
Numerical Simulations of Plasma Based Flow Control Applications
NASA Technical Reports Server (NTRS)
Suzen, Y. B.; Huang, P. G.; Jacob, J. D.; Ashpis, D. E.
2005-01-01
A mathematical model was developed to simulate flow control applications using plasma actuators. The effects of the plasma actuators on the external flow are incorporated into Navier Stokes computations as a body force vector. In order to compute this body force vector, the model solves two additional equations: one for the electric field due to the applied AC voltage at the electrodes and the other for the charge density representing the ionized air. The model is calibrated against an experiment having plasma-driven flow in a quiescent environment and is then applied to simulate a low pressure turbine flow with large flow separation. The effects of the plasma actuator on control of flow separation are demonstrated numerically.
Numerical Simulation of Low Mach Number Fluid - Phenomena.
NASA Astrophysics Data System (ADS)
Reitsma, Scott H.
A method for the numerical simulation of low Mach number (M) fluid-acoustic phenomena is developed. This computational fluid-acoustic (CFA) methodology is based upon a set of conservation equations, termed finite-compressible, derived from the unsteady Navier-Stokes equations. The finite-compressible and more familiar pseudo-compressible equations are compared. The impact of derivation assumptions are examined theoretically and through numerical experimentation. The error associated with these simplifications is shown to be of O(M) and proportional to the amplitude of unsteady phenomena. A computer code for the solution of the finite -compressible equations is developed from an existing pseudo -compressible code. Spatial and temporal discretization issues relevant in the context of near field fluid-acoustic simulations are discussed. The finite volume code employs a MUSCL based third order upwind biased flux difference splitting algorithm for the convective terms. An explicit, three stage, second order Runge-Kutta temporal integration is employed for time accurate simulations while an implicit, approximately factored time quadrature is available for steady state convergence acceleration. The CFA methodology is tested in a series of problems which examine the appropriateness of the governing equations, the exacerbation of spatial truncation errors and the degree of temporal accuracy. Characteristic based boundary conditions employing a spatial formulation are developed. An original non-reflective boundary condition based upon the generalization and extension of existing methods is derived and tested in a series of multi-dimensional problems including those involving viscous shear flows and propagating waves. The final numerical experiment is the simulation of boundary layer receptivity to acoustic disturbances. This represents the first simulation of receptivity at a surface inhomogeneity in which the acoustic phenomena is modeled using physically appropriate wavelengths. Required steady solution accuracy, convergence acceleration techniques, boundary condition/flow field interactions and the challenges of scale resolution are addressed. The computed results are in agreement with linear stability theory and experimental measurements. The overall conclusion is that this methodology, once optimized, represents a potentially valuable tool for studying the physics of fluid-acoustic phenomena including edgetone feedback, cavity resonance, transition phenomena and eventually turbulent flow noise generation.
Numerical integration methods for large-scale biophysical simulations
NASA Astrophysics Data System (ADS)
Milotti, Edoardo; Del Fabbro, Alessio; Chignola, Roberto
2009-11-01
Simulations of biophysical systems inevitably include steps that correspond to time integrations of ordinary differential equations. These equations are often related to enzyme action in the synthesis and destruction of molecular species, and in the regulation of transport of molecules into and out of the cell or cellular compartments. Enzyme action is almost invariably modeled with the quasi-steady-state Michaelis-Menten formula or its close relative, the Hill formula: this description leads to systems of equations that may be stiff and hard to integrate, and poses unusual computational challenges in simulations where a smooth evolution is interrupted by the discrete events that mark the cells' lives, like initiation and termination of DNA synthesis or mitosis. These discrete events lead to abrupt parameter changes and to variable system size. This is the case of a numerical model (Virtual Biophysics Lab - VBL) that we are developing to simulate the growth of three-dimensional tumor cell aggregates (spheroids). The underlying cellular events have characteristic timescales that span approximately 12 orders of magnitude, and thus the program must be robust and stable. Moreover the program must be able to manage a very large number of equations (of the order of 107-108), and finally it must be able to accept frequent modifications of the underlying theoretical model. Here we study the applicability of known integration methods to this context - quite unusual from the point of view of the standard theory of differential equations, but extremely relevant to biophysics - and we describe the results of numerical tests in situations similar to those found in actual simulations.
Numerical simulation of roll vortices in the convective boundary layer
NASA Astrophysics Data System (ADS)
Liu, Huizhi; Sang, Jianguo
2011-05-01
Roll vortices, which often appear when cold air outbreaks over warm ocean surfaces, are an important system for energy and substance exchange between the land surface and atmosphere. Numerical simulations were carried out in the study to simulate roll vortices in the convective boundary layer (CBL). The results indicate, that with proper atmospheric conditions, such as thermal instability in the CBL, stable stratification in the overlying layer and suitable wind shear, and a temperature jump between the two layers in a two-layer atmosphere, convective bands appear after adding initial pulses in the atmosphere. The simulated flow and temperature fields presented convective bands in the horizontal and roll vortices in the crosswind section. The structure of the roll vortices were similar to those observed in the cloud streets, as well as those from analytical solutions. Simulations also showed the influence of depth and instability strength of the CBL, as well as the stratification above the top of the CBL, on the orientation spacing and strength of the roll vortices. The fluxes caused by the convective rolls were also investigated, and should perhaps be taken into account when explaining the surface energy closure gap in the CBL.
Numerical simulation of a 100-ton ANFO detonation
NASA Astrophysics Data System (ADS)
Weber, P. W.; Millage, K. K.; Crepeau, J. E.; Happ, H. J.; Gitterman, Y.; Needham, C. E.
2015-03-01
This work describes the results from a US government-owned hydrocode (SHAMRC, Second-Order Hydrodynamic Automatic Mesh Refinement Code) that simulated an explosive detonation experiment with 100,000 kg of Ammonium Nitrate-Fuel Oil (ANFO) and 2,080 kg of Composition B (CompB). The explosive surface charge was nearly hemispherical and detonated in desert terrain. Two-dimensional axisymmetric (2D) and three-dimensional (3D) simulations were conducted, with the 3D model providing a more accurate representation of the experimental setup geometry. Both 2D and 3D simulations yielded overpressure and impulse waveforms that agreed qualitatively with experiment, including the capture of the secondary shock observed in the experiment. The 2D simulation predicted the primary shock arrival time correctly but secondary shock arrival time was early. The 2D-predicted impulse waveforms agreed very well with the experiment, especially at later calculation times, and prediction of the early part of the impulse waveform (associated with the initial peak) was better quantitatively for 2D compared to 3D. The 3D simulation also predicted the primary shock arrival time correctly, and secondary shock arrival times in 3D were closer to the experiment than in the 2D results. The 3D-predicted impulse waveform had better quantitative agreement than 2D for the later part of the impulse waveform. The results of this numerical study show that SHAMRC may be used reliably to predict phenomena associated with the 100-ton detonation. The ultimate fidelity of the simulations was limited by both computer time and memory. The results obtained provide good accuracy and indicate that the code is well suited to predicting the outcomes of explosive detonations.
Prandtl number dependence of Nusselt number in direct numerical simulations
NASA Astrophysics Data System (ADS)
Kerr, Robert M.; Herring, Jackson R.
2000-09-01
The dependence of the Nusselt number Nu on the Rayleigh Ra and Prandtl Pr number is determined for 104 < Ra < 107 and 0.07 < Pr < 7 using DNS with no-slip upper and lower boundaries and free-slip sidewalls in a 8 × 8 × 2 box. Nusselt numbers, velocity scales and boundary layer thicknesses are calculated. For Nu there are good comparisons with experimental data and scaling laws for all the cases, including Ra2/7 laws at Pr = 0.7 and Pr = 7 and at low Pr, a Ra1/4 regime. Calculations at Pr = 0.3 predict a new Nu [similar] Ra2/7 regime at slightly higher Ra than the Pr = 0.07 calculations reported here and the mercury Pr = 0.025 experiments.
Numerical Simulation of Rocket Exhaust Interaction with Lunar Soil
NASA Technical Reports Server (NTRS)
Liever, Peter; Tosh, Abhijit; Curtis, Jennifer
2012-01-01
This technology development originated from the need to assess the debris threat resulting from soil material erosion induced by landing spacecraft rocket plume impingement on extraterrestrial planetary surfaces. The impact of soil debris was observed to be highly detrimental during NASA s Apollo lunar missions and will pose a threat for any future landings on the Moon, Mars, and other exploration targets. The innovation developed under this program provides a simulation tool that combines modeling of the diverse disciplines of rocket plume impingement gas dynamics, granular soil material liberation, and soil debris particle kinetics into one unified simulation system. The Unified Flow Solver (UFS) developed by CFDRC enabled the efficient, seamless simulation of mixed continuum and rarefied rocket plume flow utilizing a novel direct numerical simulation technique of the Boltzmann gas dynamics equation. The characteristics of the soil granular material response and modeling of the erosion and liberation processes were enabled through novel first principle-based granular mechanics models developed by the University of Florida specifically for the highly irregularly shaped and cohesive lunar regolith material. These tools were integrated into a unique simulation system that accounts for all relevant physics aspects: (1) Modeling of spacecraft rocket plume impingement flow under lunar vacuum environment resulting in a mixed continuum and rarefied flow; (2) Modeling of lunar soil characteristics to capture soil-specific effects of particle size and shape composition, soil layer cohesion and granular flow physics; and (3) Accurate tracking of soil-borne debris particles beginning with aerodynamically driven motion inside the plume to purely ballistic motion in lunar far field conditions. In the earlier project phase of this innovation, the capabilities of the UFS for mixed continuum and rarefied flow situations were validated and demonstrated for lunar lander rocket plume flow impingement under lunar vacuum conditions. Applications and improvements to the granular flow simulation tools contributed by the University of Florida were tested against Earth environment experimental results. Requirements for developing, validating, and demonstrating this solution environment were clearly identified, and an effective second phase execution plan was devised. In this phase, the physics models were refined and fully integrated into a production-oriented simulation tool set. Three-dimensional simulations of Apollo Lunar Excursion Module (LEM) and Altair landers (including full-scale lander geometry) established the practical applicability of the UFS simulation approach and its advanced performance level for large-scale realistic problems.
Numerical Simulations For the F-16XL Aircraft Configuration
NASA Technical Reports Server (NTRS)
Elmiligui, Alaa A.; Abdol-Hamid, Khaled; Cavallo, Peter A.; Parlette, Edward B.
2014-01-01
Numerical simulations of flow around the F-16XL are presented as a contribution to the Cranked Arrow Wing Aerodynamic Project International II (CAWAPI-II). The NASA Tetrahedral Unstructured Software System (TetrUSS) is used to perform numerical simulations. This CFD suite, developed and maintained by NASA Langley Research Center, includes an unstructured grid generation program called VGRID, a postprocessor named POSTGRID, and the flow solver USM3D. The CRISP CFD package is utilized to provide error estimates and grid adaption for verification of USM3D results. A subsonic high angle-of-attack case flight condition (FC) 25 is computed and analyzed. Three turbulence models are used in the calculations: the one-equation Spalart-Allmaras (SA), the two-equation shear stress transport (SST) and the ke turbulence models. Computational results, and surface static pressure profiles are presented and compared with flight data. Solution verification is performed using formal grid refinement studies, the solution of Error Transport Equations, and adaptive mesh refinement. The current study shows that the USM3D solver coupled with CRISP CFD can be used in an engineering environment in predicting vortex-flow physics on a complex configuration at flight Reynolds numbers.
Numerical simulation of flow induced airfoil vibrations with large amplitudes
NASA Astrophysics Data System (ADS)
Svá?ek, P.; Feistauer, M.; Horá?ek, J.
2007-04-01
The subject of this paper is the numerical simulation of the interaction of two-dimensional incompressible viscous flow and a vibrating airfoil. A solid airfoil with two degrees of freedom, which can rotate around the elastic axis and oscillate in the vertical direction, is considered. The numerical simulation consists of the finite element solution of the Navier Stokes equations, coupled with the system of ordinary differential equations describing the airfoil motion. The high Reynolds numbers considered 10 10 require the application of a suitable stabilization of the finite element discretization. The method presented in this paper is based on the laminar model and the turbulence modelling is not applied here. The time-dependent computational domain and a moving grid are taken into account with the aid of the arbitrary Lagrangian Eulerian (ALE) formulation of the Navier Stokes equations. Special attention is paid to the time discretization and the solution of the nonlinear discrete problem on each time level is performed. As a result, a sufficiently accurate and robust method is developed, which is applied to the case of flow-induced airfoil vibrations with large amplitudes after the loss of aeroelastic stability. The computational results are compared with known aerodynamical data and with results of aeroelastic calculations obtained by NASTRAN code for a linear approximation.
Numerical simulation of premixed flame propagation in a closed tube
NASA Astrophysics Data System (ADS)
Kuzuu, Kazuto; Ishii, Katsuya; Kuwahara, Kunio
1996-08-01
Premixed flame propagation of methane-air mixture in a closed tube is estimated through a direct numerical simulation of the three-dimensional unsteady Navier-Stokes equations coupled with chemical reaction. In order to deal with a combusting flow, an extended version of the MAC method, which can be applied to a compressible flow with strong density variation, is employed as a numerical method. The chemical reaction is assumed to be an irreversible single step reaction between methane and oxygen. The chemical species are CH 4, O 2, N 2, CO 2, and H 2O. In this simulation, we reproduce a formation of a tulip flame in a closed tube during the flame propagation. Furthermore we estimate not only a two-dimensional shape but also a three-dimensional structure of the flame and flame-induced vortices, which cannot be observed in the experiments. The agreement between the calculated results and the experimental data is satisfactory, and we compare the phenomenon near the side wall with the one in the corner of the tube.
Numerical Simulations of an Isolated Microburst. Part II: Sensitivity Experiments.
NASA Astrophysics Data System (ADS)
Proctor, Fred H.
1989-07-01
Isolated and stationary microburst are simulated using a time-dependent, high-resolution, axisymmetric numerical model. A microburst downdraft is initiated by specifying a distribution of precipitation at the top boundary of the model and allowing it to fall into the domain. Part II of this series examines numerous experiments in order to evaluate the sensitivity of microbursts to the environment and other factors. The model experiments provide valuable insight into the characteristics of microbursts. Specifically, the numerical simulations indicate that microburst intensity is sensitive to 1) the vertical distribution of ambient temperature and humidity, 2) the horizontal width of either the precipitation shaft or downdraft, 3) the magnitude of the precipitation loading, 4) the type of precipitation (i.e., rain, hail, graupel, or snow), and 5) the duration of the precipitation. The environmental conditions were found to be extremely important and the horizontal scale of the precipitation shaft played a significant role in determining the strength and structure of the microburst. The presence of a ground-based stable layer weakened the ring vortex, suppressed the outflow expansion rate, and excited gravity oscillations when penetrated by a microburst. Additional experiments suggested that rotating microbursts have weaker low-level downdrafts and outflows than the nonrotating variety.Several interesting scenarios are discovered for the most elective generation of an intense microburst. In one of these, snow was found to be very effective in generating intense low-reflectivity microbursts within a typical dry-microburst environment. The structure of the snow-driven microburst was unique compared to those driven by other precipitation types, having a relatively narrow stalactite-shaped radar echo, an intense downdraft, modest cooling, and strong shear. Cooling of the air from the sublimation of snow was found to be the dominant driving process for the dry microburst with snow.Several applications of the results were investigated also. Based on the model experiments which used a variety of observed environments, an index was developed for predicting the potential for wet microbursts. The important environmental parameters included in the index are: the height of the melting level, the mean lapse rate for temperature below the melting level, and the humidity at both the melting level and 1 km above the ground. Also examined from the numerical simulations is a possible relationship between the microburst temperature drop and outflow speed.
Numerical simulations of NASA research instrumentation in hurricane environments
NASA Astrophysics Data System (ADS)
Albers, Cerese M.
Tropical cyclone intensity prediction is an issue at the forefront of mesoscale numerical weather prediction efforts because it is an area where historically there have been only small improvements, and yet much more progress is needed to improve advance warnings for land- falling tropical cyclones (TCs). In recent years, research instrumentation has been developed for deployment aboard aircraft that remotely study tropical cyclones in order to answer critical intensity questions about TCs. One such instrument is the NASA Hurricane Imaging Radiometer (HIRAD) that has been developed to observe hurricane surface wind speeds and rain rates. This study explores the expected benefits of this instrument's data to numerical simulations of tropical cyclones using two different data assimilation methods within the experimental framework of Observing System Simulation Experiments (OSSE). The HIRAD instrument performed its inaugural hurricane flights during the summer 2010 NASA Genesis and Rapid Intensification Processes (GRIP) field program, when it first studied Hurricane Karl undergoing Rapid Intensification (RI) during its brief transit over the southern Gulf of Mexico. RI events such as this one are particularly difficult to forecast given the short duration and distance over water between landmasses. The aims of this study are four-fold: first, the creation of two Nature Run simulations of Hurricane Karl as a strong and a weak hurricane; second, the accurate simulation of the HIRAD instrument's rain rate and wind speed observations; third, the development and use of two data assimilation schemes for use with the Weather Research and Forecasting (WRF) model using simulated HIRAD data for both Nature Runs; and fourth, the improvement of Hurricane Karl's intensity forecast at the end of the data assimilation period due to the inclusion of HIRAD observations and potential use for aiding the forecast of landfalling intensity. The two data assimilation schemes in this study include the creation of an update to the Krishnamurti et. al, technique of Physical Initialization of rain rates for a mesoscale model, and the adjustment and use of an Ensemble Kalman Filter (EnKF) data assimilation scheme developed by Zhang et al. for use with tropical cyclone wind speeds. Additionally, since HIRAD currently flies in the NASA HS3 field campaign on the same Global Hawk as the NASA High-altitude Radar (HIWRAP)- a dual-frequency, dual-beam conical scanning Doppler radar system- a few OSSE experiments are performed combining simulated data from both instruments. These assess the potential benefits of assimilating HIRAD wind speeds alongside HIWRAP radial velocities to further improve the initialization of TC intensity.
Pulsatile albumin transport in large arteries: a numerical simulation study.
Rappitsch, G; Perktold, K
1996-11-01
Albumin transport in a stenosed artery configuration is analyzed numerically under steady and pulsatile flow conditions. The flow dynamics is described applying the incompressible Navier-Stokes equations for Newtonian fluids, the mass transport is modelled using the convection diffusion equation. The boundary conditions describing the solute wall flux take into account the concept of endothelial resistance to albumin flux by means of a shear dependent permeability model based on experimental data. The study concentrates on the influence of steady and pulsatile flow patterns and of regional variations in vascular geometry on the solute wall flux and on the ratio of endothelial resistance to concentration boundary layer resistance. The numerical solution of the Navier-Stokes equations and of the transport equation applies the finite element method where stability of the convection dominated transport process is achieved by using an upwind procedure and a special subelement technique. Numerical simulations are carried out for albumin transport in a stenosed artery segment with 75 percent area reduction representing a late stage in the progression of an atherosclerotic disease. It is shown that albumin wall flux varies significantly along the arterial section, is strongly dependent upon the different flow regimes and varies considerably during a cardiac cycle. The comparison of steady results and pulsatile results shows differences up to 30 percent between time-averaged flux and steady flux in the separated flow region downstream the stenosis. PMID:8950655
A numerical algorithm for simulating two species plasma
NASA Astrophysics Data System (ADS)
Datwyler, Richard F.
An algorithm for modeling two species plasmas, which evolves the number density, flow velocity, and temperature equations coupled to Maxwell's electric and magnetic field equations, is discussed. Charge separation effects and the displacement current are retained. Mathematical derivations of normal modes in cold and hot plasmas, as represented by dispersion relations resulting from a linear analysis of the two fluid equations, are presented. In addition, numerical theory in relation to the ideas of geometry, temporal and spatial discretization, linearization of the fluid equations, and an expansion using finite elements is given. Numerical results generated by this algorithm compare favorably to analytical results and other published work. Specifically, we present numerical results, which agree with electrostatics, plasma oscillations at zero pressure, finite temperature acoustic waves, electromagnetic waves, whistler waves, and magnetohydrodynamics (MHD) waves, as well as a Fourier analysis showing fidelity to multiple dispersion relations in a single simulation. Final consideration is given to two species plasma stability calculations with a focus on the force balance of the initial conditions for a resistive MHD tearing mode benchmark and a static minimum energy plasma state.
Numerical simulation of supersonic flow past reentry capsules
NASA Astrophysics Data System (ADS)
Mehta, R. C.
2006-03-01
The flow fields over ARD (ESA's atmospheric reentry demonstrator), OREX (orbital reentry experiments) and spherically blunted cone-flare reentry configurations are numerically obtained by solving time-dependent, axisymmetric, compressible Navier-Stokes equations for freestream Mach numbers range of 1.2-6.0. The fluid dynamics are discretized in spatial coordinates employing a finite volume approach which reduces the governing equations to semi discretized ordinary differential equations. Temporal integration is performed using the multistage Runge-Kutta time-stepping scheme. A local time step is used to achieve steady-state solution. The numerical simulation is carried out on a structured grid. The flow-field features around the reentry capsule, such as bow shock wave, sonic line, expansion fan and recirculating flow in the base region are obtained. A good agreement is found between the calculated value of aerodynamic drag coefficient of the spherically blunted cone/fare reentry configuration with the experimental data. The effects of geometrical parameters, such as radius of the spherical cap, half cone angle, with sharp shoulder edge and with smooth shoulder edge on the flow-field have been numerically investigated for various reentry configuration which will be useful for optimization of the reentry capsule.
Detection and thermal description of medicanes from numerical simulation
NASA Astrophysics Data System (ADS)
Picornell, M. A.; Campins, J.; Jansà, A.
2014-05-01
Tropical-like cyclones rarely affect the Mediterranean region but they can produce strong winds and heavy precipitations. These warm-core cyclones, called MEDICANES (MEDIterranean hurriCANES), are small in size, develop over the sea and are infrequent. For these reasons, the detection and forecast of medicanes are a difficult task and many efforts have been devoted to identify them. The goals of this work are to contribute to a proper description of these structures and to develop some criteria to identify medicanes from numerical weather prediction (NWP) model outputs. To do that, existing methodologies for detecting, characterizating and tracking cyclones have been adapted to small-scale intense cyclonic perturbations. First, a mesocyclone detection and tracking algorithm has been modified to select intense cyclones. Next, the parameters that define the Hart's cyclone phase diagram are tuned and calculated to examine their thermal structure. Four well-known medicane events have been described from numerical simulation outputs of the European Centre for Medium-Range Weather Forecast (ECMWF) model. The predicted cyclones and their evolution have been validated against available observational data and numerical analyses from the literature.
Study on Numerical Simulation of Fouling in Compressor Rotor
NASA Astrophysics Data System (ADS)
Chen, Shao-wen; Shi, Hui; Zhang, Chen; Wang, Song-tao; Wang, Zhong-qi
2012-09-01
The effects of fouling on the performance of an axial compressor rotor were investigated numerically. The NASA Rotor 37 was considered to perform a numerical investigation by means of a commercial computational fluid dynamic code. The numerical model was validated by comparing with the experimental data available from literatures. The computed performance maps and exit parameter distributions showed a good agreement with experimental data. The model was then used to simulate the effect of fouling on compressor rotor by various fouling configurations including added thickness and surface roughness levels. The mechanism of the compressor deterioration due to fouling was discussed in detail. As a result, fouling causes a significant reduction in rotor total pressure ratio and isentropic efficiency. Increased roughness was found to have a greater influence on the rotor characteristic parameters than increased blade thickness. Increased wall roughness has a weaker influence on the operation range at stable conditions than that of increased blade thickness. Increasing the blade thickness significantly reduces the operation range of the rotor at stable conditions and has a stronger influence on the stable operation of the compressor. The interaction of shock wave/boundary layer was one of the main factors that influenced the rotor characteristics.
Kinetic theory and numerical simulations of two-species coagulation
Carlos Escudero; Fabricio Macia; Raul Toral; Juan J. L. Velazquez
2014-04-19
In this work we study the stochastic process of two-species coagulation. This process consists in the aggregation dynamics taking place in a ring. Particles and clusters of particles are set in this ring and they can move either clockwise or counterclockwise. They have a probability to aggregate forming larger clusters when they collide with another particle or cluster. We study the stochastic process both analytically and numerically. Analytically, we derive a kinetic theory which approximately describes the process dynamics. One of our strongest assumptions in this respect is the so called well-stirred limit, that allows neglecting the appearance of spatial coordinates in the theory, so this becomes effectively reduced to a zeroth dimensional model. We determine the long time behavior of such a model, making emphasis in one special case in which it displays self-similar solutions. In particular these calculations answer the question of how the system gets ordered, with all particles and clusters moving in the same direction, in the long time. We compare our analytical results with direct numerical simulations of the stochastic process and both corroborate its predictions and check its limitations. In particular, we numerically confirm the ordering dynamics predicted by the kinetic theory and explore properties of the realizations of the stochastic process which are not accessible to our theoretical approach.
Numerical Simulation of Non-Thermal Food Preservation
NASA Astrophysics Data System (ADS)
Rauh, C.; Krauss, J.; Ertunc, Ö.; Delgado, a.
2010-09-01
Food preservation is an important process step in food technology regarding product safety and product quality. Novel preservation techniques are currently developed, that aim at improved sensory and nutritional value but comparable safety than in conventional thermal preservation techniques. These novel non-thermal food preservation techniques are based for example on high pressures up to one GPa or pulsed electric fields. in literature studies the high potential of high pressures (HP) and of pulsed electric fields (PEF) is shown due to their high retention of valuable food components as vitamins and flavour and selective inactivation of spoiling enzymes and microorganisms. for the design of preservation processes based on the non-thermal techniques it is crucial to predict the effect of high pressure and pulsed electric fields on the food components and on the spoiling enzymes and microorganisms locally and time-dependent in the treated product. Homogenous process conditions (especially of temperature fields in HP and PEF processing and of electric fields in PEF) are aimed at to avoid the need of over-processing and the connected quality loss and to minimize safety risks due to under-processing. the present contribution presents numerical simulations of thermofluiddynamical phenomena inside of high pressure autoclaves and pulsed electric field treatment chambers. in PEF processing additionally the electric fields are considered. Implementing kinetics of occurring (bio-) chemical reactions in the numerical simulations of the temperature, flow and electric fields enables the evaluation of the process homogeneity and efficiency connected to different process parameters of the preservation techniques. Suggestions to achieve safe and high quality products are concluded out of the numerical results.
Xiaoliang Wang; Ashok Gidwani; Steven L. Girshick; Peter H. McMurry
2005-01-01
We have developed a numerical simulation methodology that is able to accurately characterize the focusing performance of aerodynamic lens systems. The commercial computational fluid dynamics (CFD) software FLUENT was used to simulate the gas flow field. Particle trajectories were tracked using the Lagrangian approach. Brownian motion of nanoparticles was successfully incorporated in our numerical simulations. This simulation tool was then
Hybrid numerical methods for multiscale simulations of subsurface biogeochemical processes
NASA Astrophysics Data System (ADS)
Scheibe, T. D.; Tartakovsky, A. M.; Tartakovsky, D. M.; Redden, G. D.; Meakin, P.
2007-07-01
Many subsurface flow and transport problems of importance today involve coupled non-linear flow, transport, and reaction in media exhibiting complex heterogeneity. In particular, problems involving biological mediation of reactions fall into this class of problems. Recent experimental research has revealed important details about the physical, chemical, and biological mechanisms involved in these processes at a variety of scales ranging from molecular to laboratory scales. However, it has not been practical or possible to translate detailed knowledge at small scales into reliable predictions of field-scale phenomena important for environmental management applications. A large assortment of numerical simulation tools have been developed, each with its own characteristic scale. Important examples include 1. molecular simulations (e.g., molecular dynamics); 2. simulation of microbial processes at the cell level (e.g., cellular automata or particle individual-based models); 3. pore-scale simulations (e.g., lattice-Boltzmann, pore network models, and discrete particle methods such as smoothed particle hydrodynamics); and 4. macroscopic continuum-scale simulations (e.g., traditional partial differential equations solved by finite difference or finite element methods). While many problems can be effectively addressed by one of these models at a single scale, some problems may require explicit integration of models across multiple scales. We are developing a hybrid multi-scale subsurface reactive transport modeling framework that integrates models with diverse representations of physics, chemistry and biology at different scales (sub-pore, pore and continuum). The modeling framework is being designed to take advantage of advanced computational technologies including parallel code components using the Common Component Architecture, parallel solvers, gridding, data and workflow management, and visualization. This paper describes the specific methods/codes being used at each scale, techniques used to directly and adaptively couple across model scales, and preliminary results of application to a multi-scale model of mineral precipitation at a solute mixing interface.
Numerical simulation of the impeller tip clearance effect on centrifugal compressor performance
Hoenninger, Corbett Reed
2001-01-01
This thesis presents the numerical simulation of flow in centrifugal compressors. A three-dimensional Navier-Stokes solver was employed to simulate flow through two centrifugal compressors. The first compressor simulated was the NASA low speed...
Numerical simulation of the impeller tip clearance effect on centrifugal compressor performance
Hoenninger, Corbett Reed
2001-01-01
This thesis presents the numerical simulation of flow in centrifugal compressors. A three-dimensional Navier-Stokes solver was employed to simulate flow through two centrifugal compressors. The first compressor simulated was the NASA low speed...
Finite Element Numerical Simulation for 2-D Ground Water Groundwater Movement in Confined Aquifer
Mengling Zhao
\\u000a According to the plane suppose of groundwater movement in confined aquifer, the numerical model of 2-D ground water flow in\\u000a confined aquifer with variable h was established and was calculated by the finite element numerical simulation method. For a instance is validated by using\\u000a matlab.the numerical simulation shows that the calculated results of the finite element numerical simulation method is
Numerical Simulation of Sickle Cell Blood Flow in the Microcirculation
NASA Astrophysics Data System (ADS)
Berger, Stanley A.; Carlson, Brian E.
2001-11-01
A numerical simulation of normal and sickle cell blood flow through the transverse arteriole-capillary microcirculation is carried out to model the dominant mechanisms involved in the onset of vascular stasis in sickle cell disease. The transverse arteriole-capillary network is described by Strahler's network branching method, and the oxygen and blood transport in the capillaries is modeled by a Krogh cylinder analysis utilizing Lighthill's lubrication theory, as developed by Berger and King. Poiseuille's law is used to represent blood flow in the arterioles. Applying this flow and transport model and utilizing volumetric flow continuity at each network bifurcation, a nonlinear system of equations is obtained, which is solved iteratively using a steepest descent algorithm coupled with a Newton solver. Ten different networks are generated and flow results are calculated for normal blood and sickle cell blood without and with precapillary oxygen loss. We find that total volumetric blood flow through the network is greater in the two sickle cell blood simulations than for normal blood owing to the anemia associated with sickle cell disease. The percentage of capillary blockage in the network increases dramatically with decreasing pressure drop across the network in the sickle cell cases while there is no blockage when normal blood flows through simulated networks. It is concluded that, in sickle cell disease, without any vasomotor dilation response to decreasing oxygen concentrations in the blood, capillary blockage will occur in the microvasculature even at average pressure drops across the transverse arteriole-capillary networks.
Numerical simulation of polymerization in interdigital multilamination micromixers.
Serra, Christophe; Sary, Nicolas; Schlatter, Guy; Hadziioannou, Georges; Hessel, Volker
2005-09-01
Free radical polymerization in microfluidic devices modeled with the help of numerical simulations is discussed. The simulation method used allows the simultaneous solvation of partial differential equations resulting from the hydrodynamics, thermal and mass transfer (convection, diffusion and chemical reaction). Three microfluidic devices are modeled, two interdigital multilamination micromixers respectively with a large and short focusing section, and a simple T-junction followed by a microtube reactor together considered as a bilamination micromixer with a large focusing section. The simulations show that in spite of the heat released by the polymerization reaction, the thermal transfer in such microfluidic devices is high enough to ensure isothermal conditions. Moreover, for low radial Peclet number, microfluidic devices with a large focusing section can achieve better control over the polymerization than a laboratory scale reactor as the polydispersity index obtained is very close to the theoretical limiting value. As the characteristic dimension of the microfluidic device increases, i.e. for high radial Peclet number, the reactive medium cannot be fully homogenized by the diffusion transport before leaving the system resulting in a high polydispersity index and a loss in the control of the polymerization. PMID:16100581
Numerical Relativity in Spherical Polar Coordinates: Off-center Simulations
Baumgarte, Thomas W; Müller, Ewald
2015-01-01
We have recently presented a new approach for numerical relativity simulations in spherical polar coordinates, both for vacuum and for relativistic hydrodynamics. Our approach is based on a reference-metric formulation of the BSSN equations, a factoring of all tensor components, as well as a partially implicit Runge-Kutta method, and does not rely on a regularization of the equations, nor does it make any assumptions about the symmetry across the origin. In order to demonstrate this feature we present here several off-centered simulations, including simulations of single black holes and neutron stars whose center is placed away from the origin of the coordinate system, as well as the asymmetric head-on collision of two black holes. We also revisit our implementation of relativistic hydrodynamics and demonstrate that a reference-metric formulation of hydrodynamics together with a factoring of all tensor components avoids problems related to the coordinate singularities at the origin and on the axes. As a parti...
Numerical Relativity in Spherical Polar Coordinates: Off-center Simulations
Thomas W. Baumgarte; Pedro J. Montero; Ewald Müller
2015-06-03
We have recently presented a new approach for numerical relativity simulations in spherical polar coordinates, both for vacuum and for relativistic hydrodynamics. Our approach is based on a reference-metric formulation of the BSSN equations, a factoring of all tensor components, as well as a partially implicit Runge-Kutta method, and does not rely on a regularization of the equations, nor does it make any assumptions about the symmetry across the origin. In order to demonstrate this feature we present here several off-centered simulations, including simulations of single black holes and neutron stars whose center is placed away from the origin of the coordinate system, as well as the asymmetric head-on collision of two black holes. We also revisit our implementation of relativistic hydrodynamics and demonstrate that a reference-metric formulation of hydrodynamics together with a factoring of all tensor components avoids problems related to the coordinate singularities at the origin and on the axes. As a particularly demanding test we present results for a shock wave propagating through the origin of the spherical polar coordinate system.
Comparing Numerical Spall Simulations with a Nonlinear Spall Formation Model
NASA Astrophysics Data System (ADS)
Ong, L.; Melosh, H. J.
2012-12-01
Spallation accelerates lightly shocked ejecta fragments to speeds that can exceed the escape velocity of the parent body. We present high-resolution simulations of nonlinear shock interactions in the near surface. Initial results show the acceleration of near-surface material to velocities up to 1.8 times greater than the peak particle velocity in the detached shock, while experiencing little to no shock pressure. These simulations suggest a possible nonlinear spallation mechanism to produce the high-velocity, low show pressure meteorites from other planets. Here we pre-sent the numerical simulations that test the production of spall through nonlinear shock interactions in the near sur-face, and compare the results with a model proposed by Kamegai (1986 Lawrence Livermore National Laboratory Report). We simulate near-surface shock interactions using the SALES_2 hydrocode and the Murnaghan equation of state. We model the shock interactions in two geometries: rectangular and spherical. In the rectangular case, we model a planar shock approaching the surface at a constant angle phi. In the spherical case, the shock originates at a point below the surface of the domain and radiates spherically from that point. The angle of the shock front with the surface is dependent on the radial distance of the surface point from the shock origin. We model the target as a solid with a nonlinear Murnaghan equation of state. This idealized equation of state supports nonlinear shocks but is tem-perature independent. We track the maximum pressure and maximum velocity attained in every cell in our simula-tions and compare them to the Hugoniot equations that describe the material conditions in front of and behind the shock. Our simulations demonstrate that nonlinear shock interactions in the near surface produce lightly shocked high-velocity material for both planar and cylindrical shocks. The spall is the result of the free surface boundary condi-tion, which forces a pressure gradient from the peak shock pressure to the zero pressure boundary. The nonlinear shock interactions occur where the pressure contours curve to accommodate the free surface. The material within this spall zone is ejected at speeds up to 1.8 km s-1 for an imposed pulse of 1 km s-1. Where the ejection velocities are highest, the maximum pressure attained in each cell is effectively zero. We compare our simulation results with a model for nonlinear shock interactions proposed by Kamegai (1986). This model recognizes that the material behind the shock is compressed and has a higher soundspeed than the mate-rial in front of the shock. As the rarefaction wave moves behind the shock, its increased velocity through the com-pressed material combines with the residual particle velocity behind the shock to "catch up" with the shock. This occurs in the near surface where the sum of the compressed sound speed and the residual particle velocity is greater than or equal to the shock velocity. Initial results for the spherical shocks qualitatively match the volume described by this model, but differ significantly in the quantitative slope of the curve defining the region of interaction. We continue to test the Kamegai model with high-resolution numerical simulations of shock interactions to determine its potential application to planetary spallation.
Performance of subgrid-scale models in coarse large eddy simulations of a laminar separation bubble
NASA Astrophysics Data System (ADS)
Cadieux, Francois; Domaradzki, Julian A.
2015-04-01
The flow over many blades and airfoils at moderate angles of attack and Reynolds numbers ranging from 104 to 105 undergoes separation due to the adverse pressure gradient generated by surface curvature. In many cases, the separated shear layer then transitions to turbulence and reattaches, closing off a recirculation region—the laminar separation bubble. An equivalent problem is formulated by imposing suitable boundary conditions for flow over a flat plate to avoid numerical and mesh generation issues. Recent work demonstrated that accurate large eddy simulation (LES) of such a flow is possible using only O(1%) of the direct numerical simulation (DNS) resolution but the performance of different subgrid-scale models could not be properly assessed because of the effects of unquantified numerical dissipation. LES of a laminar separation bubble flow over a flat plate is performed using a pseudo-spectral Navier-Stokes solver at resolutions corresponding to 3% and 1% of the chosen DNS benchmark by Spalart and Strelets (2000). The negligible numerical dissipation of the pseudo-spectral code allows an unambiguous assessment of the performance of subgrid-scale models. Three explicit subgrid-scale models—dynamic Smagorinsky, ?, and truncated Navier-Stokes (TNS)—are compared to a no-model simulation (under-resolved DNS) and evaluated against benchmark DNS data focusing on two quantities of critical importance to airfoil and blade designers: time-averaged pressure (Cp) and skin friction (Cf) predictions used in lift and drag calculations. Results obtained with explicit subgrid-scale models confirm that accurate LES of laminar separation bubble flows is attainable with as low as 1% of DNS resolution, and the poor performance of the no-model simulation underscores the necessity of subgrid-scale modeling in coarse LES with low numerical dissipation.
Auroral magnetospheric cyclotron emission processes: numerical and experimental simulations
NASA Astrophysics Data System (ADS)
Ronald, K.; Speirs, D. C.; McConville, S. L.; Gillespie, K. M.; Phelps, A. D. R.; Bingham, R.; Vorgul, I.; Cairns, R. A.; Cross, A. W.; Robertson, C. W.; Whyte, C. G.; He, W.; Kellett, B. J.
2011-07-01
Satellites have observed powerful radio waves (up to 1 GW peaked at 300 kHz) radiating from regions of reduced plasma density about 3200 km above the Earth's surface in the polar magnetosphere. The emission is associated with the observation of horseshoe or 'shell' distributions in the velocity space of the Earthbound flux of electrons, arising from magnetic compression. It has been postulated that this distribution holds the free energy required to explain the radiation emission. To verify this proposition, a series of numerical and experimental simulations of the mechanism scaled to microwave frequencies have been conducted in concert with a theoretical analysis of the growth rate and propagation of the resultant radiation. The numerical simulations were conducted with electron beams of 75 keV and currents of 18 and 34 A gyrating in a magnetic field of 0.18 T, and 85 keV and 18 A in a magnetic field of 0.48 T. The simulations predicted that the radiation would be emitted close to the cyclotron frequency (4.42 and 11.7 GHz) in near cut-off TE modes (TE0,1 and TE0,3/TE2,3, respectively) with efficiencies of 2% and 1.3%. The experimental measurements demonstrated mode content very close to that predicted by the simulations with output powers of 19 kW and 35 kW from electron beams of 75 keV energy and 12 A and 34 A of current, respectively, at 4.42 GHz. Powers of 9.4 kW and 30 kW from 85 keV electron beams of 12 and 30 A were observed at 11.7 GHz. The spectral, polarization and propagation properties of the radiation are similar to those observed in the magnetosphere. A scheme will be outlined for introducing a background plasma into the apparatus, with ?pe < ?ce/10 to enhance the comparison with the auroral magnetosphere.
Stabilized Quantum Gravity: Stochastic Interpretation and Numerical Simulation
J. Greensite
1992-06-29
Following the reasoning of Claudson and Halpern, it is shown that "fifth-time" stabilized quantum gravity is equivalent to Langevin evolution (i.e. stochastic quantization) between fixed non-singular, but otherwise arbitrary, initial and final states. The simple restriction to a fixed final state at $t_5 \\rightarrow \\infty$ is sufficient to stabilize the theory. This equivalence fixes the integration measure, and suggests a particular operator-ordering, for the fifth-time action of quantum gravity. Results of a numerical simulation of stabilized, latticized Einstein-Cartan theory on some small lattices are reported. In the range of cosmological constant $\\l$ investigated, it is found that: 1) the system is always in the broken phase $ \
Direct Numerical Simulation of Chemical Non-equilibrium Turbulent Flow
NASA Astrophysics Data System (ADS)
Chen, Xiao-Ping; Li, Xin-Liang
2013-06-01
Temporally evolving high-temperature turbulent channel flows (at Ma? = 6 and 10 and Re? = 12000) are performed by using direct numerical simulation with the assumption of local thermal equilibrium and chemical non-equilibrium. The turbulent statistical characteristics are studied. We find that the Morkovin theory for the Van Direst transformed velocity remains valid, while the compressibility effects need to be considered since the turbulent Mach number is high enough, especially for the higher Mach number case. The dissociation/recombination reactions are excited, which are proved by the mean temperature, mass fractions and specific heat ratio. The importance of the mean property variations is studied from the rms velocity and mass fraction fluctuations.
Stability analysis and numerical simulation of simplified solid rocket motors
NASA Astrophysics Data System (ADS)
Boyer, G.; Casalis, G.; Estivalèzes, J.-L.
2013-08-01
This paper investigates the Parietal Vortex Shedding (PVS) instability that significantly influences the Pressure Oscillations of the long and segmented solid rocket motors. The eigenmodes resulting from the stability analysis of a simplified configuration, namely, a cylindrical duct with sidewall injection, are presented. They are computed taking into account the presence of a wall injection defect, which is shown to induce hydrodynamic instabilities at discrete frequencies. These instabilities exhibit eigenfunctions in good agreement with the measured PVS vortical structures. They are successfully compared in terms of temporal evolution and frequencies to the unsteady hydrodynamic fluctuations computed by numerical simulations. In addition, this study has shown that the hydrodynamic instabilities associated with the PVS are the driving force of the flow dynamics, since they are responsible for the emergence of pressure waves propagating at the same frequency.
Optimization of vortex pinning by nanoparticles using numerical simulations
NASA Astrophysics Data System (ADS)
Koshelev, Alexei; Sadovskyy, Ivan; Phillips, Carolyn; Glatz, Andreas
2015-03-01
Vortex pinning by self-assembled nanoparticles has been established as an efficient route to enhance current-carrying capability of practical superconductors. We explore vortex pinning by randomly distributed metallic spherical nanoparticles using large-scale numerical simulations of time-dependent Ginzburg-Landau equations. We found optimal size and density of particles at which the highest critical current realizes for fixed magnetic field. For every particle size the critical current reaches maximum value at certain particle density, typically corresponding to 15-22% of the volume fraction filled by the particles, which is close to the percolation concentration. This optimal particle density increases with the magnetic field. We also found that the optimal particle diameter is close to 4 coherence lengths. Our results provide guidance for pinning optimization in practical superconductors. The work was supported by the SciDAC program funded by U.S. DOE, Office of Science, Advanced Scientific Computing Research and Basic Energy Science.
Vision detection of weld pool shape parameters and numerical simulation
NASA Astrophysics Data System (ADS)
Yuan, Youzhi; Liu, Nansheng; Wang, Yanfeng; Luo, Wei; Liu, Xiaorui
2008-12-01
An active vision sensing system which established by low power laser and common CCD is used to shoot GTAW welding pool images. Based on choose the appropriate LASER and filters, projected angle of laser and shoot angle of CCD, we avoid the disturbance of arc effectively and obtain clear images of the welding pool. The shape parameters of welding pool can obtain by demarcated and calculated. And then a three-dimentional numerical model of GTAW welding pool is established based on the theories of fluid dynamics and heat transfer. It considered the convection heat transfer of liquid metal and heat conduction of solid metal. And FLUENT software is used for simulate the GTAW welding pool. Extract the weld pool shape parameters and done comparative study with the vision detection of pool. This will be beneficial to avoid repeat experiments and improve work efficiency.
Fully-resolved numerical simulation of 1024 sedimenting spheres
NASA Astrophysics Data System (ADS)
Prosperetti, Andrea
2005-11-01
The dynamics of a suspension of finite-size particles settling under gravity in a Newtonian fluid is simulated. The ``Physalis'' numerical method is used to fully resolve the flow around the spheres at finite particle Reynolds number, with an elastic-collision model. Of interest in the investigation is the self-organization of the disperse phase and its effect on the sedimenting behavior. Particle clustering and anisotropy are found to be prominent features of the system. The suspension displays preferential orientation at scales comparable to the particle dimension. Fluctuations in the mean particle settling velocity are shown to be intimately linked to the anisotropy of the microstructure. The particle Lagrangian time scale in the direction gravity is larger than in the orthogonal directions and, as a consequence, a similar difference is found between the vertical and horizontal self-diffusion coefficients.
Response maxima in time-modulated turbulence: Direct numerical simulations
NASA Astrophysics Data System (ADS)
Kuczaj, A. K.; Geurts, B. J.; Lohse, D.
2006-03-01
The response of turbulent flow to time-modulated forcing is studied by direct numerical simulations of the Navier-Stokes equations. The large-scale forcing is modulated via periodic energy input variations at frequency ?. The response is maximal for frequencies in the range of the inverse of the large eddy turnover time, confirming the mean-field predictions of von der Heydt, Grossmann and Lohse (Phys. Rev. E, 67 (2003) 046308). In accordance with the theory the response maximum shows only a small dependence on the Reynolds number. At sufficiently high frequencies the amplitude of the kinetic energy response decreases as 1/?. For frequencies beyond the range of maximal response, a significant change in the phase-shift relative to the time-modulated forcing is observed. For large ? the phase shift approaches roughly 90° for the total energy and 180° for the energy dissipation rate.
Particle Acceleration by Compressive Pump Mechanism: Numerical Simulations
NASA Astrophysics Data System (ADS)
Kota, J.; Giacalone, J.; Jokipii, J. R.
2013-12-01
We study particle acceleration my compressive velocity and density fluctuations in a simple one-dimensional system similar that suggested by Fisk and Gloeckler (2008,2012). We solve the diffusive Parker-equation numerically assuming a periodic velocity and corresponding density fluctuations. The simulation is time-dependent and discuss adiabatic cooling/heating from large-scale expansion/compression and the back-reaction of accelerated particles on the core fluid. The predicted momentum spectra will be discussed. We also investigate how effective the compressive acceleration mechanism can be in producing seed particles for acceleration in the lower corona. Fisk, L.A. and G. Gloeckler, 2008: Astrophys. J., 686, 1466 Fisk, L.A. and G. Gloeckler, 2012: Space Sci. Rev., 173, 433
Numerical simulation for hydrodynamic characteristics of a bionic flapping hydrofoil
NASA Astrophysics Data System (ADS)
Su, Yu-min; Wang, Zhao-li; Zhang, Xi; Guo, Bing-jie
2012-06-01
In order to study the propulsion mechanism of the bionic flapping hydrofoil (BFH), a 2-DoF (heave and pitch) motion model is formulated. The hydrodynamic performance of BFH with a series of kinematical parameters is explored via numerical simulation based on FLUENT. The calculated result is compared with the experimental value of MIT and that by the panel method. Moreover, the effect of inlet velocity, the angle of attack, the heave amplitude, the pitch amplitude, the phase difference, the heave biased angle, the pitch biased angle and the oscillating frequency are investigated. The study is useful for guiding the design of bionic underwater vehicle based on flapping propulsion. It is indicated that the optimal parameters combination is v = 0.5 m/s, ? 0 = 40°, ? 0 = 30°, ? = 90°, ? bias = 0°, ? bias = 0° and f = 0.5 Hz.
Numerical simulation of the passive gas mixture flow
NASA Astrophysics Data System (ADS)
Kyncl, Martin; Pelant, Jaroslav
2015-05-01
We work with the system of equations describing non-stationary compressible turbulent multicomponent flow in the gravitational field, and we focus on the numerical solution of these equations. In these computations we assume the mixture of perfect inert gases. The thermodynamic constants are functions in time and space. The finite volume method is used. In order to solve the local boundary problem at each mesh face, we use the original analysis of the exact solution of the Riemann problem. The roughness of the surface is simulated via the specific dissipation at the wall. We show the computational results obtained with the own-developed code (C,FORTRAN) for the solution of the 3D compressible turbulent mixture flow. The originality of this work lies with the special handling of the boundary conditions, which shows superior behavior, and own computational code.
Direct numerical simulations of vortex breakdown in swirling jets†
NASA Astrophysics Data System (ADS)
Kollmann, W.; Ooi, A. S. H.; Chong, M. S.; Soria, J.
2001-04-01
Vortex breakdown in swirling free jets exhibits a rich variety of flow structures in a compact subdomain of the flow field. Model equations assuming steady and rotationally symmetric motion above a conical stream surface show bi- and multi-stability (Shtern and Hussain 1996 J. Fluid Mech.309 1-44) of several flow forms, hysteresis and negative thrust phenomena. Direct numerical simulations are used to investigate the development of the conical jet layer and its break up into three-dimensional vortex structures for moderate Reynolds numbers. Three-dimensional flows for swirl numbers at and above the threshold value observed experimentally (Billant et al 1999 J. Fluid Mech.376 183-219) are considered.
The Beam Forming Numerical Simulation for High Power Neutral Injector
Sorokin, A.; Deichuli, P.; Ivanov, A.; Mishagin, V. [Budker Institute of Nuclear Physics (Russian Federation)
2005-01-15
High power neutral beam injector START-4 for plasma heating has been described. The distinctive features of the injector are comparatively large initial beam aperture (200 mm) and multi holes grids with the large numbers of the holes (more than 3000). A significant focusing is realized to a beam diameter 50 mm at a length 1.2 m. The disadvantage of the multi holes optic is low transparency, which decreases the efficiency of plasma source and makes worse vacuum conditions in the source. The possible decisions of these problems are using ion-optical systems (IOS) with enlarged diameter of holes and, also, application IOS with the azimuthal-slit holes structure. Numerical simulation and test experiments have been carried out for investigation of the ability such IOS geometries.
The Numerical Simulation of Ship Waves using Cartesian Grid Methods
Sussman, Mark
2014-01-01
Two different cartesian-grid methods are used to simulate the flow around the DDG 5415. The first technique uses a "coupled level-set and volume-of-fluid" (CLS) technique to model the free-surface interface. The no-flux boundary condition on the hull is imposed using a finite-volume technique. The second technique uses a level-set technique (LS) to model the free-surface interface. A body-force technique is used to impose the hull boundary condition. The predictions of both numerical techniques are compared to whisker-probe measurements of the DDG 5415. The level-set technique is also used to investigate the breakup of a two-dimensional spray sheet.
Numerical aerodynamic simulation facility feasibility study, executive summary
NASA Technical Reports Server (NTRS)
1979-01-01
There were three major issues examined in the feasibility study. First, the ability of the proposed system architecture to support the anticipated workload was evaluated. Second, the throughput of the computational engine (the flow model processor) was studied using real application programs. Third, the availability, reliability, and maintainability of the system were modeled. The evaluations were based on the baseline systems. The results show that the implementation of the Numerical Aerodynamic Simulation Facility, in the form considered, would indeed be a feasible project with an acceptable level of risk. The technology required (both hardware and software) either already exists or, in the case of a few parts, is expected to be announced this year.
Numerical simulations for a variable order fractional Schnakenberg model
NASA Astrophysics Data System (ADS)
Hammouch, Z.; Mekkaoui, T.; Belgacem, F. B. M.
2014-12-01
This paper is concerned with the numerical solutions of a variable-order space-time fractional reaction-diffusion model. The space-time fractional derivative is considered in the sense of Riesz-Feller, the system is defined by replacing the second order space derivatives with the variable Riesz-Feller derivatives. The problem is solved by an explicit finite difference method. Finally, simulation results to this problem are presented and discussed. In the original article PDF file, as supplied to AIP Publishing, the name and affiliation of author F. B. M. Belgacem was missing due to a Latex compiling error. This article was updated on 29 January 2015 to correct that error.
Numerical Simulation and Optimazation of Small Scale LNG Plant
NASA Astrophysics Data System (ADS)
Li, H. Y.; Jia, L. X.; Fan, Q. H.; Yin, Q. S.
2006-04-01
The LNG20 is a small-scale natural gas liquefier. Its capacity is 20 cube meters LNG per day. This liquefier could be used for the pipeline gas, coalbed gas, oil field gas liquefaction and peakshaving plant for town gas gate station and natural gas power plant. Two processing cycles are applied to LNG20, nitrogen expander cycle and mixed refrigerant cycle. In this report, two feed gases are the target sources; one is the pipeline gas in "West-to-east pipeline gas" in a gate station in Zhejiang province and coalbed gas in Northeast China. The numerical simulation and optimization for the LNG20 were carried out to obtain the design parameters.
Numerical simulation of flow characteristics in micro shock tubes
NASA Astrophysics Data System (ADS)
Zhang, Guang; Setoguchi, Toshiaki; Kim, Heuy Dong
2015-06-01
Recently micro shock tubes have been widely used in many engineering and industrial fields, but the characteristics of unsteady flow are not well known to date in micro shock tubes. Compared to conventional shock tubes with macro scales, flows related to shock waves in micro shock tubes are highly complicated. Stronger viscous and dissipative interactions make shock wave dynamic behaviors significantly different from theoretical predictions. In the present study, a CFD work was applied to the unsteady compressible Navier-Stokes equations which were solved using a fully implicit finite volume scheme. The diaphragm pressure ratio and shock tube diameter were varied to investigate their effects on micro shock tube flows. Different wall boundary conditions were also performed to observe shock wave and contact surface propagation with no slip and slip walls. Detailed flow characteristics at the foot of shock wave and contact surface propagation were known from the present numerical simulations.
Observation and numerical simulation of a convective initiation during COHMEX
NASA Technical Reports Server (NTRS)
Song, J. Aaron; Kaplan, Michael L.
1991-01-01
Under a synoptically undisturbed condition, a dual-peak convective lifecycle was observed with the COoperative Huntsville Meteorological EXperiment (COHMEX) observational network over a 24-hour period. The lifecycle included a multicell storm, which lasted about 6 hours, produced a peak rainrate exceeding 100 mm/hr, and initiated a downstream mesoscale convective system. The 24-hour accumulated rainfall of this event was the largest during the entire COHMEX. The downstream mesoscale convective system, unfortunately, was difficult to investigate quantitatively due to the lack of mesoscale observations. The dataset collected near the time of the multicell storm evolution, including its initiation, was one of the best datasets of COHMEX. In this study, the initiation of this multicell storm is chosen as the target of the numerical simulations.
Numerical simulation of winds behaviour in Macón site
NASA Astrophysics Data System (ADS)
Gonzalez, E.; Sacco, C.; Vrech, R.; Renzi, V.; García Lambas, D.; Recabarren, P.
The set up of a large telescope requires a previous comprehensive study of the local meteorological and seeing conditions. The south ridge of mount Macón, province of Salta, Argentina has been studied for more than a year and good seeing conditions have been found. However, strong wind condi- tions drastically affect the selection of the best place where a large telescope may be installed. In the present work, a CFD (Computed Flow Dynamic) study is performed to analyze the wind speed distribution and flow pattern over the site. A GPS based high resolution topographical model was de- veloped. The numerical formulation is a large eddy simulation (LES) of the incompressible Navier-Stokes equations approximated using the finite element method and results are confirmed with the experimental measure- ments.
Numerical simulation of equatorial plasma bubbles over Cachimbo: COPEX campaign
NASA Astrophysics Data System (ADS)
Carrasco, A. J.; Batista, I. S.; Abdu, M. A.
2014-08-01
The problem of day-to-day variability in onset of equatorial spread F (ESF) is addressed using data from the 2002 COPEX observational campaign in Brazil and numerical modeling. The observational results show that for values of virtual height of the F layer base less than 355 km at around 18:35 LT, and for the prereversal peak enhancement of the vertical plasma drift (Vp) less than 30 m/s, the spread-F (ESF) was absent on four nights over Cachimbo (9.5°S, 54.8°W, dip latitude = -2.1°). In this work we analyze the geophysical conditions for the generation of the irregularities by comparing the nights with and without the ESF. In the comparison a numerical code is used to simulate plasma irregularity development in an extended altitude range from the bottom of the equatorial F layer. The code uses the flux corrected transport method with Boris-Book’s flux limiter for the spatial integration and a predictor-corrector method for the direct time integration of the continuity equation for O+ and the SOR (Successive-Over-Relaxation) method for electric potential equation. The code is tested with different evening eastward electric fields (or vertical drifts Vp < 30 m/s and Vp > 30 m/s) in order to study the influence of the prereversal enhancement in the zonal electric field on plasma bubble formation and development. The code also takes into account the zonal wind, the vertical electric field and the collision frequency of ions with neutrals and the amplitude of initial perturbation. The simulation shows a good agreement with the observational results of the ESF. The results of the code suggest that the instability can grow at the F layer bottomside by the Rayleigh-Taylor mechanism only when the Vp > 30 m/s. In the analyzed cases we have considered the competition of other geophysical parameters in the generation of plasma structures.
DoX: A Peer-to-Peer Antidote for DNS Cache Poisoning Attacks
California at Davis, University of
DoX: A Peer-to-Peer Antidote for DNS Cache Poisoning Attacks Lihua Yuan ECE, UC Davis lyuan cache poisoning, which causes the DNS to return false name-to-IP mappings and can be used as a foothold inaccurate DNS records caused by cache poisoning attacks. DoX also helps DNS servers to improve cache
Optimal Taylor-Couette flow: direct numerical simulations
NASA Astrophysics Data System (ADS)
Ostilla, Rodolfo; Stevens, Richard J. A. M.; Grossmann, Siegfried; Verzicco, Roberto; Lohse, Detlef
2013-03-01
We numerically simulate turbulent Taylor-Couette flow for independently rotating inner and outer cylinders, focusing on the analogy with turbulent Rayleigh-B\\'enard flow. Reynolds numbers of $Re_i=8\\cdot10^3$ and $Re_o=\\pm4\\cdot10^3$ of the inner and outer cylinders, respectively, are reached, corresponding to Taylor numbers Ta up to $10^8$. Effective scaling laws for the torque and other system responses are found. Recent experiments with the Twente turbulent Taylor-Couette ($T^3C$) setup and with a similar facility in Maryland at very high Reynolds numbers have revealed an optimum transport at a certain non-zero rotation rate ratio $a = -\\omega_o / \\omega_i$ of about $a_{opt}=0.33-0.35$. For large enough $Ta$ in the numerically accessible range we also find such an optimum transport at non-zero counter-rotation. The position of this maximum is found to shift with the driving, reaching a maximum of $a_{opt}=0.15$ for $Ta=2.5\\cdot10^7$. An explanation for this shift is elucidated, consistent with the experimental result that $a_{opt}$ becomes approximately independent of the driving strength for large enough Reynolds numbers. We furthermore numerically calculate the angular velocity profiles and visualize the different flow structures for the various regimes. By writing the equations in a frame co-rotating with the outer cylinder a link is found between the local angular velocity profiles and the global transport quantities.
Experimental and numerical simulation for swirl flow in a combustor
NASA Astrophysics Data System (ADS)
Dulin, V. M.; Markovich, D. M.; Minakov, A. V.; Hanjalic, K.; Chikishev, L. M.
2013-12-01
Results of the experimental and numerical simulation for swirl flow in combustion of a lean methane-air mixture in a model combustor at atmospheric pressure are represented. The panoramic method for the flow velocity measurement and the calculation by a large eddy method were used for the investigation of the nonstationary turbulent flow. The numerical modeling for the breakdown of the vortex core of the flow and the topology of large-scale vortex structures forming in it showed the close fit to the experiment. The analysis of obtained data showed that for the case of the intensive swirl of the flow as well as in the case of the flow without combustion, dynamics of the flow with combustion was determined by the global azimuthal instability mode corresponding to the intensive precession of the vortex core. The flame had the similar characteristics of the stability and compactness in the case of stabilization by the low swirl; however, velocity pulsations in the flow corresponded to the development of only local instability modes. Thus, the other kind of vortex breakdown in the case of the low swirl, for which the central recirculation zone is lacking, is not only favorable in view of the reduction of the NO x emission, but also remains a possibility for the effective use of the active control method for the flow and combustion. In particular, the given result may be used for the elimination of the thermoacoustic resonance in combustors.
Direct numerical simulation of curved turbulent channel flow
NASA Technical Reports Server (NTRS)
Moser, R. D.; Moin, P.
1984-01-01
Low Reynolds number, mildly curved, turbulent channel flow has been simulated numerically without subgrid scale models. A new spectral numerical method developed for this problem was used, and the computations were performed with 2 million degrees of freedom. A variety of statistical and structural information has been extracted from the computed flow fields. These include mean velocity, turbulence stresses, velocity skewness, and flatness factors, space time correlations and spectra, all the terms in the Reynolds stress balance equations, and contour and vector plots of instantaneous velocity fields. The effects of curvature on this flow were determined by comparing the concave and convex sides of the channel. The observed effects are consistent with experimental observations for mild curvature. The most significant difference in the turbulence statistics between the concave and convex sides was in the Reynolds shear stress. This was accompanied by significant differences in the terms of the Reynolds shear stress balance equations. In addition, it was found that stationary Taylor-Gortler vortices were present and that they had a significant effect on the flow by contributing to the mean Reynolds shear stress, and by affecting the underlying turbulence.
Quantifying paleosecular variation: Insights from numerical dynamo simulations
NASA Astrophysics Data System (ADS)
Lhuillier, F.; Gilder, S. A.
2013-12-01
Numerical dynamo simulations can be used to investigate paleosecular variation of Earth-like magnetic fields over several million-year timescales. Using a set of five numerical models integrated over the equivalent of 40-50 Myr, we generated synthetic data analogous to paleomagnetic data. We show that paleosecular variation among the five models is best discriminated by the relative variability in paleointensity (?_F) and the precision parameter (k) of directions or poles. Whether the geodynamo operated in different regimes in its past can be best tested with these parameters in combination. Roughly one million years of time with 200 time-independent samples is required to achieve convergence of ?_F and k. The quantities ?_F and k correlate well with the average chron duration (?_chr), which suggests that excursions and reversals are an integral part of palaeosecular variation. If applicable to the geodynamo, the linear dependence of k on ?_chr could help to predict ?_chr for the Earth during geologic times with no available reversal frequency data; it also predicts much higher average k for directions during superchrons (k ? 2500 for the Cretaceous normal superchron) than during actively reversing times (k ? 35 for the last 80 Myr). As such high k values are not observed, either this family of dynamo models is not applicable to the geodynamo, or the geodynamo regime acting during superchrons lies statistically within the same energy state as at present.
Numerical Simulation of Wall Heat Load in Combustor Flow
NASA Astrophysics Data System (ADS)
Panara, D.; Hase, M.; Krebs, W.; Noll, B.
2007-09-01
Due to the major mechanism of NOx generation, there is generally a temperature trade off between improved cycle efficiency, material constraints and low NOx emission. The cycle efficiency is proportional to the highest cycle temperature, but unfortunately also the NOx production increases with increasing combustion temperature. For this reason, the modern combustion chamber design has been oriented towards lean premixed combustion system and more and more attention must be focused on the cooling air management. The challenge is to ensure sufficiently low temperature of the combustion liner with very low amount of film or effusion cooling air. Correct numerical prediction of temperature fields and wall heat load are therefore of critical interest in the modern combustion chamber design. Moreover, lean combustion technology has shown the appearance of thermo-acoustic instabilities which have to be taken into account in the simulation and, more in general, in the design of reliable combustion systems. In this framework, the present investigation addresses the capability of a commercial multiphysics code (ANSYS CFX) to correctly predict the wall heat load and the core flow temperature field in a scaled power generation combustion chamber with a simplified ceramic liner. Comparison are made with the experimental results from the ITS test rig at the University of Karlsruhe [1] and with a previous numerical campaign from [2]. In addition the effect of flow unsteadyness on the wall heat load is discussed showing some limitations of the traditional steady state flow thermal design.
Numerical simulations of the blood flow through vertebral arteries.
Jozwik, Krzysztof; Obidowski, Damian
2010-01-19
Vertebral arteries are two arteries whose structure and location in human body result in development of special flow conditions. For some of the arteries, one can observe a significant difference between flow rates in the left and the right arteries during ultrasonography diagnosis. Usually the reason of such a difference was connected with pathology of the artery in which a smaller flow rate was detected. Simulations of the flow through the selected type of the vertebral artery geometry for twenty five cases of artery diameters have been carried out. The main aim of the presented experiment was to visualize the flow in the region of vertebral arteries junction in the origin of the basilar artery. It is extremely difficult to examine this part of human circulation system, thus numerical experiments may be helpful in understanding the phenomena occurring when two relatively large arteries join together to form one vessel. The obtained results have shown that an individual configuration and diameters of particular arteries can exert an influence on the flow in them and affect a significant difference between flow rates for vertebral arteries. It has been assumed in the investigations that modelled arteries were absolutely normal, without any pathology. In the numerical experiment, the non-Newtonian model of blood was employed. PMID:19909956
Direct Numerical Simulation of Solid Deformation During Dendritic Solidification
NASA Astrophysics Data System (ADS)
Yamaguchi, M.; Beckermann, C.
2014-08-01
Deformation of the semisolid mush during solidification is a common phenomenon in metal casting and can lead to defects such as hot tears, macrosegregation, and porosity. The morphology of the solidifying mush, including the shape of the dendrites and the distribution of grain boundaries, plays a key role in determining its mechanical behavior. In the current study, a polycrystalline phase-field model is combined with a material point method stress analysis to numerically simulate the fully coupled dendritic solidification and elasto-viscoplastic deformation behavior of a pure substance in two dimensions. It is shown that solid compressive and shear deformations result in variations in the crystallographic orientation angle within a single dendrite that, in turn, affect the subsequent solidification behavior. Shearing of a dendritic structure occurs primarily in relatively narrow bands near or inside the grain boundaries or the thin junctions between different dendrite arms. The deformations can cause the formation of low-angle tilt grain boundaries inside of individual dendrite arms. In addition, grain boundaries form when different arms of a deformed single dendrite impinge. During compression of a high-solid-fraction dendritic structure, the deformations are limited to a relatively thin layer along the compressing boundary. The compression causes consolidation of this layer into a fully solid structure that consists of numerous subgrains.
Direct Numerical Simulation of turbulent flows generated by fractal grids
NASA Astrophysics Data System (ADS)
Laizet, Sylvain; Christos Vassilicos, John; Lamballais, Eric
2008-11-01
Recently at Imperial College London, experiments of turbulence generated by fractal grids placed at the entrance of a wind tunnel have shown that complex multiscale boundaries/initial conditions can generate a far downstream decaying homogeneous isotropic turbulence with broad power law (approximately -5/3) energy spectra but laminar-like dissipation (Hurst & Vassilicos, Seoud & Vassilicos 2007 in PoF). Although the wind tunnel measurements have provided invaluable time-resolved informations on the unique properties of multiscale generated turbulent flows, understanding the spatial structure of these flows is necessary to discover the origins of these properties. The goal of the present numerical study is to investigate the spatio-temporal flow structure and the properties of the turbulent flow generated by these fractal objects. To solve the incompressible Navier-Stokes equations, we use a numerical code (called ``Incompact3d'') based on sixth-order compact schemes for spatial discretization and second order Adams-Bashforth scheme for time advancement. These are very large simulations, in particular because of the multi-scale nature of the fractal turbulence generators, and require state-of-the art top-end parallel computing.
Numerical simulation of a cross flow Marine Hydrokinetic turbine.
NASA Astrophysics Data System (ADS)
Hall, Taylor; Aliseda, Alberto
2011-11-01
In the search for alternative sources of energy, the kinetic energy of water currents in oceans, rivers and estuaries is being explored as predictable and environmentally benign. We are investigating the flow past a cross flow turbine in which a helical blade under hydrodynamic forces turns around a shaft perpendicular to the free stream. This type of turbine, while very different from the classical horizontal axis turbine commonly used in the wind energy field, presents advantages for stacking in very narrow constricted channels where the water currents are consistently high and therefore turbine installation may be economically feasible. We use a model of a helical four-bladed turbine in cross flow to investigate the efficiency of the energy capture and the dynamics of the turbulent wake. Scale model experiments in a flume are used to validate the numerical results on a stationary configuration as an initial step towards creating an accurate numerical model of the turbine. The simulation of the rotating turbine provides a full perspective on the effect of angular position on flow detachment and vortex shedding from the blade, as well as on the fluctuations of the shaft torque produced (a problematic feature of this type of turbine). The results are analyzed in terms of hydrodynamic optimization of the blade and its structural loading. Supported by DOE through the Northwest National Marine Renewable Energy Center.
Numerical Simulation of a Mist Singlet Oxygen Generator
NASA Astrophysics Data System (ADS)
Endo, Masamori; Muto, Shigeki; Fujioka, Tomoo; Nanri, Kenzo
2002-01-01
A numerical simulation code for a mist singlet oxygen generator (SOG) is developed. Unlike previous SOGs, a mist SOG utilizes fine droplets of basic hydrogen peroxide (BHP) to achieve a stoichiometric reaction with chlorine gas in a single pass through a reaction zone. The numerical model presented in the present paper deals with the depletion of superficial HO2- density and the diffusive redistribution of each droplet, water evaporation, temperature variation of the droplet due to chemical reaction and evaporation, and heat exchange between the gas and liquid phases. Under identical initial conditions, the calculated results are consistent with the results from previous experiments. The heterogeneous quenching probability of O2(1?) to the BHP surface (?) was determined by a comparison between the experimental and calculated results, and was found to be 2× 10-3. The process conditions were then varied to establish the theoretical limit of BHP utilization. For a very small (15 ?m) droplet diameter, it was shown that 50% BHP could be utilized with an output of 64% O2(1?) yield and 88% Cl2 utilization.
Numerical Simulation of a Slow Streamer-Blowout CME
NASA Astrophysics Data System (ADS)
Lynch, Benjamin J.; Masson, Sophie; Li, Yan; DeVore, C. Richard; Luhmann, Janet; Antiochos, Spiro K.
2014-06-01
We present a 3D numerical MHD simulation of the 2008 Jun 2 gradual streamer blowout CME that had virtually no identifiable low coronal signatures. We energize the field by simple footpoint shearing along the source region's polarity inversion line and model the background solar wind structure using an ?2MK isothermal wind and a low-order potential field source surface representation of the CR2070 synoptic magnetogram. Our results show that the CME ``initiation’’ is obtained by slowly disrupting the quasi-steady-state configuration of the helmet streamer, resulting in the standard eruptive flare picture that ejects the sheared fields, but very slowly, on a relatively large scale, and with very little magnetic energy release. We obtain a relatively slow CME eruption of order the background solar wind speed and argue that these slow streamer blowout CMEs (now also known as ``stealth CMEs’’) are simply at the lowest end of the CME energy distribution. We present comparisons of the CME propagation through the corona (?15Rs) in synthetic white-light images derived from the simulation density structure with multi-spacecraft coronagraph data from STEREO/SECCHI and SOHO/LASCO.
Numerical simulation of dissolved silica in the San Fancisco Bay
Peterson, D.H.; Festa, J.F.; Conomos, T.J.
1978-01-01
A two-dimensional (vertical) steady-state numerical model that simulates water circulation and dissolved-silica distributions is applied to northern San Francisco Bay. The model (1) describes the strong influence of river inflow on estuarine circulation and, in turn, on the biologically modulated silica concentration, and (2) shows how rates of silica uptake relate to silica supply and mixing rates in modifying a conservative behavior. Longitudinal silica distributions influenced by biological uptake (assuming both vertically uniform and vertically decreasing uptake situations) show that uptake rates of 1 to 10 ??g-at. l-1 day-1 are sufficient to depress silica concentrations at river inflows of 100-400 m3 s-1, respectively, and that the higher rates appear ineffective at inflows above 400 m3 s-1. The simulations further indicate that higher silica utilization in the null zone is not essential to depress silica concentrations strongly there. Advective water-replacement times at river inflows of 400, 200 and 100 m3 s-1 are computed to be less than 25, 45 and 75 days, respectively, for a 120-km estuary-river system. ?? 1978.
Numerical simulations of self-gravitating magnetized disks
NASA Astrophysics Data System (ADS)
Fromang, S.; de Villiers, J.-P.; Balbus, S.
In the early phases of their evolution, protoplanetary disks are massive enough for self-gravitating effects to be important. This is for example the case of disks around Class 0 and some Class I protostars, but other peculiar and more evolved cases may exist, like the circumbinary disk of GG Tau. On larger scales, the central parts of our galaxy show an m=1 density wave, and the disks feeding supermassive black holes in the heart of AGNs are certainly self-gravitating. At the same time, these objects are prone to the development of the magnetorotational instability (MRI) if a weak magnetic field is present. It is therefore relevant to address the question of the evolution of a system under the simultaneous effects of the MRI and gravitational instabilities. These instabilities may dramatically influence the early evolution of protoplanetary disks, and, as a result, planet formation itself. In this talk, I will present a first view on these issues, by means of 2D and 3D MHD numerical simulations of self-gravitating magnetized torii. To date, these simulations are the first examples of the behaviour of the MRI in a self-gravitating environment.
Numerical Simulation of Flow Through an Artificial Heart
NASA Technical Reports Server (NTRS)
Rogers, Stuart E.; Kutler, Paul; Kwak, Dochan; Kiris, Cetin
1989-01-01
A solution procedure was developed that solves the unsteady, incompressible Navier-Stokes equations, and was used to numerically simulate viscous incompressible flow through a model of the Pennsylvania State artificial heart. The solution algorithm is based on the artificial compressibility method, and uses flux-difference splitting to upwind the convective terms; a line-relaxation scheme is used to solve the equations. The time-accuracy of the method is obtained by iteratively solving the equations at each physical time step. The artificial heart geometry involves a piston-type action with a moving solid wall. A single H-grid is fit inside the heart chamber. The grid is continuously compressed and expanded with a constant number of grid points to accommodate the moving piston. The computational domain ends at the valve openings where nonreflective boundary conditions based on the method of characteristics are applied. Although a number of simplifing assumptions were made regarding the geometry, the computational results agreed reasonably well with an experimental picture. The computer time requirements for this flow simulation, however, are quite extensive. Computational study of this type of geometry would benefit greatly from improvements in computer hardware speed and algorithm efficiency enhancements.
Numerical simulation of drifting snow sublimation in the saltation layer
Dai, Xiaoqing; Huang, Ning
2014-01-01
Snow sublimation is an important hydrological process and one of the main causes of the temporal and spatial variation of snow distribution. Compared with surface sublimation, drifting snow sublimation is more effective due to the greater surface exposure area of snow particles in the air. Previous studies of drifting snow sublimation have focused on suspended snow, and few have considered saltating snow, which is the main form of drifting snow. In this study, a numerical model is established to simulate the process of drifting snow sublimation in the saltation layer. The simulated results show 1) the average sublimation rate of drifting snow particles increases linearly with the friction velocity; 2) the sublimation rate gradient with the friction velocity increases with increases in the environmental temperature and the undersaturation of air; 3) when the friction velocity is less than 0.525?m/s, the snowdrift sublimation of saltating particles is greater than that of suspended particles; and 4) the snowdrift sublimation in the saltation layer is less than that of the suspended particles only when the friction velocity is greater than 0.625?m/s. Therefore, the drifting snow sublimation in the saltation layer constitutes a significant portion of the total snow sublimation. PMID:25312383
Numerical Simulation of the Detonation Propagation in Silicon Carbide Shell
NASA Astrophysics Data System (ADS)
Balagansky, Igor; Terechov, Anton
2013-06-01
Last years it was experimentally shown that in condensed high explosive charges (HE) placed in silicon carbide shell with sound velocity greater than the detonation velocity in HE, there may be observed interesting phenomena. Depending on the conditions, as an increase or decrease of the detonation velocity and pressure on the detonation front can be observed. There is also the distortion of the detonation front until the formation of a concave front. For a detailed explanation of the physical nature of the phenomenon we have provided numerical simulation of detonation wave propagation in Composition B HE charge, which was placed in silicon carbide shell. Modeling was performed with Ansys Autodyn in 2D-axis symmetry posting on an Eulerian mesh. Special attention was paid to selection of the parameters values in Lee-Tarver kinetic equation for HE and choice of constants to describe behavior of the ceramics. For comparison, also we have carried out the modeling of propagation of detonation in a completely similar assembly with brass shell. The simulation results agree well with the experimental data. In particular, in silicon carbide shell distortion of the detonation front was observed. A characteristic feature of the process is the pressure waves propagating in the direction of the axis of symmetry on the back surface of the detonation front.
Numerical Simulation of Regional Circulation in the Monterey Bay Region
NASA Technical Reports Server (NTRS)
Tseng, Y. H.; Dietrich, D. E.; Ferziger, J. H.
2003-01-01
The objective of this study is to produce a high-resolution numerical model of Mon- terey Bay area in which the dynamics are determined by the complex geometry of the coastline, steep bathymetry, and the in uence of the water masses that constitute the CCS. Our goal is to simulate the regional-scale ocean response with realistic dynamics (annual cycle), forcing, and domain. In particular, we focus on non-hydrostatic e ects (by comparing the results of hydrostatic and non-hydrostatic models) and the role of complex geometry, i.e. the bay and submarine canyon, on the nearshore circulation. To the best of our knowledge, the current study is the rst to simulate the regional circulation in the vicinity of Monterey Bay using a non-hydrostatic model. Section 2 introduces the high resolution Monterey Bay area regional model (MBARM). Section 3 provides the results and veri cation with mooring and satellite data. Section 4 compares the results of hydrostatic and non-hydrostatic models.
Numerical simulation of linear fiction welding (LFW) processes
Fratini, L.; La Spisa, D. [University of Palermo-Dept. of Industrial engineering (Italy)
2011-05-04
Solid state welding processes are becoming increasingly important due to a large number of advantages related to joining ''unweldable'' materials and in particular light weight alloys. Linear friction welding (LFW) has been used successfully to bond non-axisymmetric components of a range of materials including titanium alloys, steels, aluminum alloys, nickel, copper, and also dissimilar material combinations. The technique is useful in the research of quality of the joints and in reducing costs of components and parts of the aeronautic and automotive industries.LFW involves parts to be welded through the relative reciprocating motion of two components under an axial force. In such process the heat source is given by the frictional forces work decaying into heat determining a local softening of the material and proper bonding conditions due to both the temperature increase and the local pressure of the two edges to be welded. This paper is a comparative test between the numerical model in two dimensions, i.e. in plane strain conditions, and in three dimensions of a LFW process of AISI1045 steel specimens. It must be observed that the 3D model assures a faithful simulation of the actual threedimensional material flow, even if the two-dimensional simulation computational times are very short, a few hours instead of several ones as the 3D model. The obtained results were compared with experimental values found out in the scientific literature.
Numerical simulations of free-electron laser oscillators
McVey, B.D.; Goldstein, J.C.; Tokar, R.L.; Elliott, C.J.; Gitomer, S.J.; Schmitt, M.J.; Thode, L.E.
1988-01-01
A numerical simulation capability has been developed to model the physics and realistic design constraints of free electron laser oscillators driven by rf linear accelerators. Two computer codes have been written FELEX and FELP. The code FELP is a one spatial dimension code with essentially unlimited time or spectral resolution. The codes are complementary and their use is dependent upon the problem being addressed. The code FELP is used to model optical and electron micropulse structure, broadband noise, and the sideband instability. The code FELEX models accelerator generated electron beam distributions, the transport of these distributions through wigglers with misalignments and field errors, self-consistent interaction with the optical field, and propagation of the optical field through resonators with realistically modelled components. FELEX is routinely used to match resonator designs to the optical parameters of the electron beam, and used to investigate the physics of 3-D micropulse effects. Some details of the codes will be presented along with various examples of simulation results. 22 refs., 10 figs., 1 tab.
Sunspot Seismology: Testing Surface Effects with Numerical Simulations
NASA Astrophysics Data System (ADS)
Braun, Douglas; Birch, A. C.; Hanasoge, S. M.
2007-05-01
The discovery that sunspots absorb acoustic waves was first announced twenty years ago at a previous SPD meeting in Honolulu. A considerable effort has been made to understand the physics of the interaction between acoustic waves and sunspots. However, the implications of this two-decade old discovery are still being explored in helioseismology. An ongoing controversy involves the role of surface effects, including absorption, in modeling the subsurface structure of sunspots. Braun and Birch recently suggested that observed frequency variations, at fixed phase speeds, of acoustic travel-time perturbations through sunspots offers evidence for a strong contribution to travel times from structures with vertical scales smaller than about one Mm near the solar surface. We test this suggestion with the numerical simulations of acoustic-wave propagation hrough specified sound-speed perturbations of a background solar model. An important finding is that travel times measured using helioseismic holography from simulations employing sound-speed perturbations typical of recent time-distance inversions do not predict the strong frequency variations observed in with solar data. We are in the process of evaluating whether shallow sound-speed perturbations, such as that proposed by Fan, Braun and Chou to explain the acoustic scattering propertis of sunspots observed with Hankel analysis, can reproduce the frequency variations observed in sunspots. This work is supported by contracts NAS5-02139, NNH05CC76C and NNH04CC05C from NASA, and grant AST-0406225 from the NSF.
Dayside reconnection in 3D global Hall MHD numerical simulations
NASA Astrophysics Data System (ADS)
Lin, L.; Germaschewski, K.; Bhattacharjee, A.; Maynard, K.; Sullivan, B. P.; Raeder, J.
2012-12-01
We investigate magnetic reconnection at the dayside magnetopause using three dimensional global resistive Hall MHD numerical simulations with the new code, Hall OpenGGCM. Runs are performed with constant spatially uniform resistivity and steady southward IMF conditions at various values of Lundquist number and ion-skin depth to determine scaling. Our results show that in the high Lundquist number limit, Hall physics can allow magnetic flux-pileup to be locally suppressed. The pileup scalings obtained are compared with the stagnation point flow solutions of Sonnerup and Priest [J. Plasma Phys., 14, 1975], and the Hall mediated flux pileup analysis of Dorelli [Phys. Plasmas, 10, 2003]. We also investigate how asymmetric reconnection manifests itself in 3D Hall simulations with particular attention to the 2D analysis of Cassak and Shay [Phys. Plasmas, 14, 2007]. While the theory appears to give reasonable predictions for the offset locations of the x-point and stagnation points, the expressions given for the reconnection electric field and outflow velocities do not agree with what we observe and likely require remediation to account for realistic global geometry. Much like what is observed in 2D collisionless reconnection studies, Hall physics in these global simulations gives rise to more compact dissipation regions with bifurcations in current density extending polewards (when viewed in the GSEx-GSEz plane) which bulge outwards into the magnetosheath. We note also that at larger Lundquist numbers, macroscopic dissipation region structures appear to filament along the flanks of the magnetopause due to the development of Kelvin-Helmholtz instability. The bearing of Hall physics on the relative frequency and character of poleward propagating flux transfer events is also discussed.
Study of Underexpanded Sonic Jets by Numerical Simulation
NASA Astrophysics Data System (ADS)
Thanigaiarasu, S.; Karthick, R.; Arunprasad, R.; SyedMusthafa, H.; Elangovan, S.; Rathakrishnan, E.
2013-03-01
This paper deals with the numerical simulation of sonic underexpanded axi-symmetric jets, issuing from a convergent nozzle, at different nozzle pressure ratios. The simulations have been carried out by solving the unsteady Reynolds Average Navier-Stokes (RANS) equations utilizing the realizable
Deploying and Monitoring DNS Security (DNSSEC) Eric Osterweil
California at Los Angeles, University of
Spider as well as several general lessons that stem from its design and implementation. I. INTRODUCTION The DNS and the root zone itself have announced plans to deploy DNSSEC. At the same time, the increasing awareness
The direct numerical simulations of the turbulent wakes of axisymmetric bodies
NASA Technical Reports Server (NTRS)
Riley, J. J.; Metcalfe, R. W.
1978-01-01
Results of direct numerical simulations of turbulence are compared with both laboratory data and self-similarity theory for the case of the turbulent wakes of towed, axisymmetric bodies. In general, the agreement of the simulation results with both the laboratory data and the self-similarity theory is good, although the comparisons are hampered by inadequate procedures for initializing the numerical simulations.
Numerical simulation of a closed rotor-stator system using Large Eddy Simulation
NASA Astrophysics Data System (ADS)
Amouyal, Solal Abraham Teva
A large eddy simulation of an enclosed annular rotor stator cavity is presented. The geometry is characterized by a large aspect ratio G = (b-a)/h = 18.32 and a small radius ratio a/b = 0.152, where a and b are the inner and outer radii of the rotating disk and h is the interdisk spacing. The rotation rate o under consideration is equivalent to the rotational Reynolds number Re = o b2 /nu= 9.5x104 , where nu is the kinematic viscosity. The main objective of this study is to correctly simulate the rotor stator cavity using a low order numerical scheme on unstructured grids. The numerical simulations were run on the software AVBP developed by the Centre Europeen de Recherche et de Formation Avancee en Calcul Scientific. The results were compared to the experimental results obtained by Sebastien Poncet of Universit e Aix-Marseille. Two large eddy simulations techniques were used: the Smagorinsky and Wall-adapting local eddy-viscosity models. The simulations were run on three set of grids, each with a different cell resolution-14, 35 and 50- along the thickness of the system. Results from each mesh show a good qualitative agreement of the mean velocity field with Poncet's experimental results. It was found that the Samgorinsky model to be more appropriate for this configuration.
The CONV-3D code for DNS CFD calculation
NASA Astrophysics Data System (ADS)
Chudanov, Vladimir; ALCF ThermHydraX Team
2014-03-01
The CONV-3D code for DNS CFD calculation of thermal and hydrodynamics on Fast Reactor with use of supercomputers is developed. This code is highly effective in a scalability at the high performance computers such as ``Chebyshev'', ``Lomonosov'' (Moscow State University, Russia), Blue Gene/Q(ALCF MIRA, ANL). The scalability is reached up to 106 processors. The code was validated on a series of the well known tests in a wide range of Rayleigh (106-1016) and Reynolds (103-105. Such code was validated on the blind tests OECD/NEA of the turbulent intermixing in horizontal subchannels of the fuel assembly at normal pressure and temperature (Matis-H), of the flows in T-junction and the report IBRAE/ANL was published. The good coincidence of numerical predictions with experimental data was reached, that specifies applicability of the developed approach for a prediction of thermal and hydrodynamics in a boundary layer at small Prandtl that is characteristic of the liquid metal reactors. Project Name: ThermHydraX. Project Title: U.S.-Russia Collaboration on Cross-Verification and Validation in Thermal Hydraulics.
Design and Evaluation of DNS as Location Manager for HIP
Shuigen Yang; Hongbin Luo; Yajuan Qin; Hongke Zhang
2009-01-01
Host Identity Protocol (HIP) is designed to provide secure and continuous communication by separating the identifier and locator\\u000a roles of the Internet Protocol (IP) address. HIP also has efficient solutions to support host mobility. In this paper, we\\u000a propose a location management scheme based on Domain Name System (DNS) for HIP. In the proposed scheme, a new DNS HIP resource
Prctica 7: Configuraci de DNS Objectius de la prctica
PolitÃ¨cnica de Catalunya, Universitat
lÃnia de comandes. El segÃ¼ent exemple fa la peticiÃ³ fent servir el servidor de DNS 147 perquÃ¨ la recarregui mitjanÃ§ant la comanda rndc. Quina comanda heu fet servir? Ara proveu a resoldre una.conf perquÃ¨ faci servir el vostre servidor de DNS. (Opcional) Aquest canvi nomÃ©s durarÃ fins que feu un reboot
Low Reynolds number k-epsilon modelling with the aid of direct simulation data
NASA Technical Reports Server (NTRS)
Rodi, W.; Mansour, N. N.
1993-01-01
The constant C sub mu and the near-wall damping function f sub mu in the eddy-viscosity relation of the k-epsilon model are evaluated from direct numerical simulation (DNS) data for developed channel and boundary layer flow at two Reynolds numbers each. Various existing f sub mu model functions are compared with the DNS data, and a new function is fitted to the high-Reynolds-number channel flow data. The epsilon-budget is computed for the fully developed channel flow. The relative magnitude of the terms in the epsilon-equation is analyzed with the aid of scaling arguments, and the parameter governing this magnitude is established. Models for the sum of all source and sink terms in the epsilon-equation are tested against the DNS data, and an improved model is proposed.
Numerical Simulations of Astrophysical Jets from Keplerian Accretion Disks
NASA Astrophysics Data System (ADS)
Ouyed, Rachid
This thesis presents a series of magnetohydrodynamic (MHD) simulations which were designed to study the origin and evolution of astrophysical jets (galactic and extra-galactic). We developed and extended a version of the ZEUS-2D code which served as the numerical basis of our simulations and attached to it a complete analysis package. With our version of the code, we established an initial state which consists of an accretion disk and its cold corona in stable equilibrium around a central object. No softening parameter was used to model the Newtonian gravitational potential of the central object. The corona and accretion disk are initially in pressure balance with one another. These initial states were constructed so as to be numerically stable. In this thesis, we only considered magnetic configurations for which the Lorentz force is initially zero (J × B = 0). In particular initial J = 0 configurations are studied. We set the boundary conditions to be open conditions to avoid collimation due to grid reflection effects. To test the theory of winds centrifugally driven from the surface of Keplerian accretion disks, we started with an open magnetic field line configuration. We found that a steady jet is quickly established, allowing direct comparison with the theory. We find the gas to be centrifugally accelerated through the Alfven and the fast magnetosonic surfaces and collimated into cylinders parallel to the disk's axis. The velocities achieved in our simulations are of the order of 250 km/s for our standard young stellar object (a 0.5 Msolar proto-star) and of the order or 108 km/s for our standard active galactic nuclei (a 108Msolar black hole). Our jet solutions are very efficient in magnetically extracting angular momentum and energy from the disk. The second magnetic configuration we have studied consists of a uniform vertical structure wherein the magnetic field lines are parallel to the disk's axis. Here, the rotation of the disk twists the magnetic field lines. Because of the Keplerian scaling of the rotational velocity with the disk radius, the twisting of the field lines is higher in the inner parts of the disk. The strong magnetic gradient thus generated opens up the initial magnetic configuration in a narrow region. Within this narrow region, a wind is ejected from the field lines that have opened to less than the critical angle (?60o). The strong toroidal magnetic field generated recollimates the flow towards the disk's axis and, through MHD shocks, produces knots. No special initial magnetic field structure is required in order to launch episodic outflows in our simulations. Rather, conditions favorable for the formation of an outflow set themselves up automatically through the production of a toroidal magnetic field whose pressure readjusts the structure of the field above the disk. The knot generator is episodic, and is inherent to the jet. (Abstract shortened by UMI.)
NUMERICAL SIMULATIONS OF CONVERSION TO ALFVEN WAVES IN SUNSPOTS
Khomenko, E. [Instituto de Astrofisica de Canarias, 38205 La Laguna, Tenerife (Spain); Cally, P. S., E-mail: khomenko@iac.es, E-mail: paul.cally@monash.edu [School of Mathematical Sciences and Monash Centre for Astrophysics, Monash University, Clayton, Victoria 3800 (Australia)
2012-02-10
We study the conversion of fast magnetoacoustic waves to Alfven waves by means of 2.5D numerical simulations in a sunspot-like magnetic configuration. A fast, essentially acoustic, wave of a given frequency and wave number is generated below the surface and propagates upward through the Alfven/acoustic equipartition layer where it splits into upgoing slow (acoustic) and fast (magnetic) waves. The fast wave quickly reflects off the steep Alfven speed gradient, but around and above this reflection height it partially converts to Alfven waves, depending on the local relative inclinations of the background magnetic field and the wavevector. To measure the efficiency of this conversion to Alfven waves we calculate acoustic and magnetic energy fluxes. The particular amplitude and phase relations between the magnetic field and velocity oscillations help us to demonstrate that the waves produced are indeed Alfven waves. We find that the conversion to Alfven waves is particularly important for strongly inclined fields like those existing in sunspot penumbrae. Equally important is the magnetic field orientation with respect to the vertical plane of wave propagation, which we refer to as 'field azimuth'. For a field azimuth less than 90 Degree-Sign the generated Alfven waves continue upward, but above 90 Degree-Sign downgoing Alfven waves are preferentially produced. This yields negative Alfven energy flux for azimuths between 90 Degree-Sign and 180 Degree-Sign . Alfven energy fluxes may be comparable to or exceed acoustic fluxes, depending upon geometry, though computational exigencies limit their magnitude in our simulations.
Numerical simulations of volcanic jets: Importance of vent overpressure
NASA Astrophysics Data System (ADS)
Ogden, Darcy E.; Wohletz, Kenneth H.; Glatzmaier, Gary A.; Brodsky, Emily E.
2008-02-01
Explosive volcanic eruption columns are generally subdivided into a gas-thrust region and a convection-dominated plume. Where vents have greater than atmospheric pressure, the gas-thrust region is overpressured and develops a jet-like structure of standing shock waves. Using a pseudogas approximation for a mixture of tephra and gas, we numerically simulate the effects of shock waves on the gas-thrust region. These simulations are of free-jet decompression of a steady state high-pressure vent in the absence of gravity or a crater. Our results show that the strength and position of standing shock waves are strongly dependent on the vent pressure and vent radius. These factors control the gas-thrust region's dimensions and the character of vertical heat flux into the convective plume. With increased overpressure, the gas-thrust region becomes wider and develops an outer sheath in which the erupted mixture moves at higher speeds than it does near the column center. The radius of this sheath is linearly dependent on the vent radius and the square root of the overpressure. The sheath structure results in an annular vertical heat flux profile at the base of the convective plume, which is in stark contrast to the generally applied Gaussian or top-hat profile. We show that the magnitude of expansion is larger than that predicted from previous 1D analyses, resulting in much slower average vertical velocities after expansion. These new relationships between vent pressure and plume expansion may be used with observations of plume diameter to constrain the pressure at the vent.