NASA Technical Reports Server (NTRS)
Givi, Peyman; Madnia, Cyrus K.; Steinberger, C. J.; Frankel, S. H.
1992-01-01
The principal objective is to extend the boundaries within which large eddy simulations (LES) and direct numerical simulations (DNS) can be applied in computational analyses of high speed reacting flows. A summary of work accomplished during the last six months is presented.
Direct numerical simulations (DNS) of particles in spatially varying electric fields
NASA Astrophysics Data System (ADS)
Amah, E.; Janjua, M.; Fischer, I. S.; Singh, P.
2013-11-01
We have developed a direct numerical simulation (DNS) scheme to simulate the motion of dielectric particles suspended in a dielectric liquid in nonuniform electric fields. The motion of particles is tracked using a distributed Lagrange multiplier method (DLM) and the electric forces acting on the particles are calculated by an efficient scheme in which the Maxwell stress tensor (MST) is integrated over the surfaces of the particles to obtain the force. The code is validated by performing a convergence study and by comparing the particle trajectories in a dielectrophoretic cage with those given by the point-dipole method. We also show that the trajectories of the two or more interacting particles given by the MST method can be different from those obtained using the point-dipole method since the latter does not consider particle-particle interactions.
Direct Numerical Simulation and Large Eddy Simulation of Compressible Turbulence
Erlebacher, Gordon
12 Direct Numerical Simulation and Large Eddy Simulation of Compressible Turbulence GORDON simulation (DNS) and the largeÂeddy simulation (LES) of homogeneous compressÂ ible turbulent flows. Some DNS. In his seminal paper on supersonic turbulence, Kovasznay (1957) deÂ scribed the decomposition
Numerical simulation of supersonic boundary layer transition
NASA Technical Reports Server (NTRS)
Guo, Y.; Adams, N. A.; Sandham, N. D.; Kleiser, L.
1994-01-01
The present contribution reviews some of the recent progress obtained at our group in the direct numerical simulation (DNS) of compressible boundary layer transition. Elements of the different simulation approaches and numerical techniques employed are surveyed. Temporal and spatial simulations, as well as comparisons with results obtained from Parabolized Stability Equations, are discussed. DNS results are given for flat plate boundary layers in the Mach number range 1.6 to 4.5. A temporal DNS at Mach 4.5 has been continued through breakdown all the way to the turbulent stage. In addition results obtained with a recently developed extended temporal DNS approach are presented, which takes into account some nonparallel effects of a growing boundary layer. Results from this approach are quite close to those of spatial DNS, while preserving the efficiency of the temporal DNS.
Numerical Investigation of Forced Strong and Weak Wall Jets %Using DNS and LES
NASA Astrophysics Data System (ADS)
Wernz, S.; Fasel, H. F.
1997-11-01
Wall jets are technically important flows that are used, for example, for boundary layer control on airfoils. Recent experiments by Wygnanski et al. have demonstrated the effectiveness of pulsed wall jets to control separation for flows over single-element or segmented airfoils. For gaining insight into the fundamental mechanisms responsible for the often striking effect of periodic forcing on wall jets, in the present research forced transitional and turbulent wall jets are investigated using Direct Numerical Simulation (DNS) and Large-Eddy Simulation (LES). Both DNS and LES are performed using a three-dimensional Navier-Stokes code based on the incompressible vorticity-velocity formulation. For the LES, a Smagorinsky-type subgrid-scale turbulence model has been incorporated into the code. Two types of base flows are considered. Strong wall jets are represented by a Glauert-type wall jet with a small free stream component added, weak wall jets are generated by wall-tangential blowing through a slot into a boundary layer. In the simulations, the base flows are periodically forced using a blowing and suction slot in the wall. For the LES of turbulent wall jets, rapid breakdown to turbulence is triggered by large amplitude three-dimensional random forcing. The focus of the present study is the influence of the free steam component on the development of large coherent structures. This research is funded by AFOSR, Grant Nr. F49620-97-1-0274, and is also supported by a grant of HPC time from the DoD HPC Shared Resource Center, CEWES.
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.
Discussion of DNS: Past, Present, and Future
NASA Technical Reports Server (NTRS)
Joslin, Ronald D.
1997-01-01
This paper covers the review, status, and projected future of direct numerical simulation (DNS) methodology relative to the state-of-the-art in computer technology, numerical methods, and the trends in fundamental research programs.
An experimental and DNS numerical study of multi-hole cooling
NASA Astrophysics Data System (ADS)
Zhong, Fengquan
New integrally woven ceramic matrix composites (CMC), which can be multi-hole cooled, offer the prospect of substantial combustion gain. Little is known about the fluid mechanics and heat transfer in the application of such multi-hole cooled CMC systems. In this thesis, multi-hole cooling for two types of ceramic composites (oxide/oxide and SiC/SiC) with different hole geometries is studied both experimentally and numerically. With a unique heat transfer tunnel system, effects on the cooling effectiveness of parameters including blowing ratio, momentum ratio, Reynolds number, temperature ratio and hole geometry and wall material, are studied. In addition, profiles of the mean velocity, temperature and rms temperature fluctuation over the cooling surface are measured to provide further understanding of the cooling process. Duplication of the key parameters for multi-hole cooling for a real combustor flow condition is achieved with parameter scaling and the results show the efficiency of multi-hole cooling especially for the oblique hole, SiC/SiC specimen. In parallel, a 3D heat transfer model, which includes the mean solution and an unsteady solution for the wall temperature and heat flux, is developed to fully couple the heat transfer in the primary flow, in the backside flow and flow in the holes with the heat conduction in the multi-hole wall. With model tests for laminar and turbulent cooling, the 3D model is found to be very efficient for the mean temperature solution and for the unsteady solution. The 3D model is applied with DNS calculations for the simulations of multi-hole cooling at low Reynolds number. The cooling effectiveness predicted by the 3D heat transfer model is significantly different from that predicted by an adiabatic wall model and the 3D model results agree well with the experimental results. The DNS calculation of the primary flow with multi-hole cooling provides detailed information about the physics of the cooling process including the velocity and temperature fluctuations and their correlations, the vortical structures near the surface, the dominant frequencies, and the temperature, heat flux fluctuations on the cooling surface.
NASA Astrophysics Data System (ADS)
Suzuki, Takao; Yamamoto, Fujio
2015-10-01
Data-assimilation capabilities of hybrid-type simulations integrating time-resolved particle image velocimetry with unsteady computational fluid dynamics (CFD) are characterized, and a series of algorithms developed previously are evaluated in terms of four criteria: (i) compatibility with the governing equations; (ii) completeness of a set of flow quantities; (iii) temporal and spatial filtering functions; and (iv) spatial resolution. This study specifically introduces a hierarchy of three hybrid simulations combining time-resolved particle tracking velocimetry (PTV) and direct numerical simulation (DNS) from low to high fidelities: the proper orthogonal decomposition-Galerkin-projection approach with proportional feedback of PTV data, the DNS solver with similar feedback, and the DNS solver with the extended Kalman filter. By solving a planar-jet problem at {Re}? 2000, we demonstrate that the resultant hybrid flow fields essentially (i) satisfy the governing equations spatially and approximately temporally, and (ii) can provide instantaneous pressure fields (iii) with the noise levels substantially lower than those of the original PTV data and (iv) the resolution comparable to CFD. The results show that increasing the feedback gain improves replicability, i.e. the agreement between the simulation and the data; however, it degrades temporal compatibility and filtering functions. On the other hand, the fidelity enhances both replicability and spatial filtering, but increases computational cost.
LES versus DNS: A comparative study
NASA Technical Reports Server (NTRS)
Shtilman, L.; Chasnov, J. R.
1992-01-01
We have performed Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES) of forced isotropic turbulence at moderate Reynolds numbers. The subgrid scale model used in the LES is based on an eddy viscosity which adjusts instantaneously the energy spectrum of the LES to that of the DNS. The statistics of the large scales of the DNS (filtered DNS field or fDNS) are compared to that of the LES. We present results for the transfer spectra, the skewness and flatness factors of the velocity components, the PDF's of the angle between the vorticity and the eigenvectors of the rate of strain, and that between the vorticity and the vorticity stretching tensor. The above LES statistics are found to be in good agreement with those measured in the fDNS field. We further observe that in all the numerical measurements, the trend was for the LES field to be more gaussian than the fDNS field. Future research on this point is planned.
Numerical simulation of turbulent jet primary breakup in Diesel engines
Helluy, Philippe
Numerical simulation of turbulent jet primary breakup in Diesel engines Peng Zeng1 Marcus Herrmann and Aerospace Engineering Arizona State University "Micro-Macro Modelling and Simulation of Liquid-Vapour Flows" IRMA Strasbourg, 23.Jan.2008 #12;Introduction DNS of Primary Breakup in Diesel Injection Phase
Direct numerical simulations of turbulent lean premixed combustion
NASA Astrophysics Data System (ADS)
Sankaran, Ramanan; Hawkes, Evatt R.; Chen, Jacqueline H.; Lu, Tianfeng; Law, Chung K.
2006-09-01
In recent years, due to the advent of high-performance computers and advanced numerical algorithms, direct numerical simulation (DNS) of combustion has emerged as a valuable computational research tool, in concert with experimentation. The role of DNS in delivering new Scientific insight into turbulent combustion is illustrated using results from a recent 3D turbulent premixed flame simulation. To understand the influence of turbulence on the flame structure, a 3D fully-resolved DNS of a spatially-developing lean methane-air turbulent Bunsen flame was performed in the thin reaction zones regime. A reduced chemical model for methane-air chemistry consisting of 13 resolved species, 4 quasi-steady state species and 73 elementary reactions was developed specifically for the current simulation. The data is analyzed to study possible influences of turbulence on the flame thickness. The results show that the average flame thickness increases, in qualitative agreement with several experimental 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.
DNS of autoignition in turbulent diffusion H2/air and Krishnan Mahesh
Mahesh, Krishnan
DNS of autoignition in turbulent diffusion H2/air flames Jeff Doom and Krishnan Mahesh University of Minnesota, Minneapolis, MN, 55455, USA Direct numerical simulation (DNS) is used to study auto. The chemical mechanism is a nine species, nineteen reaction mechanism for H2 and Air from Mueller at el.2
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.
Terascale Direct Numerical Simulations of Turbulent Combustion: Capabilities and Limits (PReSS Talk)
Yoo, Chun Sang
2009-03-26
The rapid growth in computational capabilities has provided great opportunities for direct numerical simulations (DNS) of turbulent combustion, a type of simulations without any turbulence model. With the help of terascale high performance supercomputing (HPC) resources, we are now able to provide fundamental insight into turbulence-chemistry interaction in simple laboratory-scale turbulent flames with detailed chemistry using three-dimensional (3D) DNS. However, the actual domain size of 3D-DNS is still limited within {approx} O(10 cm{sup 3}) due to its tremendously high grid resolution required to resolve the smallest turbulent length scale as well as flame structures. Moreover, 3D-DNS will require more computing powers to investigate next-generation engines, of which operating conditions will be characterized by higher pressures, lower temperatures, and higher levels of dilution. In this talk, I will discuss the capabilities and limits of DNS of turbulent combustion and present some results of ignition/extinction characteristics of a highly diluted hydrogen flame counter-flowing against heated air. The results of our recent 3D-DNS of a spatially-developing turbulent lifted hydrogen jet flame in heated coflow will also be presented. The 3D-DNS was performed at a jet Reynolds number of 11,000 with {approx} 1 billion grid points, which required 3.5 million CPU hours on Cray XT3/XT4 at Oak Ridge National Laboratories.
Direct Numerical Simulation and Theories of Wall Turbulence with a Range of Pressure Gradients
NASA Technical Reports Server (NTRS)
Coleman, G. N.; Garbaruk, A.; Spalart, P. R.
2014-01-01
A new Direct Numerical Simulation (DNS) of Couette-Poiseuille flow at a higher Reynolds number is presented and compared with DNS of other wall-bounded flows. It is analyzed in terms of testing semi-theoretical proposals for universal behavior of the velocity, mixing length, or eddy viscosity in pressure gradients, and in terms of assessing the accuracy of two turbulence models. These models are used in two modes, the traditional one with only a dependence on the wall-normal coordinate y, and a newer one in which a lateral dependence on z is added. For pure Couette flow and the Couette-Poiseuille case considered here, this z-dependence allows some models to generate steady streamwise vortices, which generally improves the agreement with DNS and experiment. On the other hand, it complicates the comparison between DNS and models.
Rocket engine numerical simulation
NASA Technical Reports Server (NTRS)
Davidian, Ken
1993-01-01
The topics are presented in view graph form and include the following: a definition of the rocket engine numerical simulator (RENS); objectives; justification; approach; potential applications; potential users; RENS work flowchart; RENS prototype; and conclusions.
DNS and LES of Turbulent Channel Flow with Hydrophobic Surface
NASA Astrophysics Data System (ADS)
Yang, X. L.; He, G. W.; Zhang, X.
Hydrophobic surface benefits for drag reduction. Min and Kim[1] do the first Direct Numerical Simulation on drag reduction in turbulent channel flow. And Fukagata and Kasagi[2] make some theoretical analysis based on Dean[3]’s formula and some observations in the DNS results. Using their theory, they conclude that drag reduction is possible in large Reynolds number. Both Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) are performed in our research. How the LES behaving in the turbulent channel flow with hydrophobic surface is examined. Original Smagorinsky model and its Dynamical model are used in LES. The slip velocities predicted by LES using Dynamical model are in good agreement with DNS as shown in the Figure. Although the percentage of drag reduction predicted by LES shows some discrepancies, it is in the error limit for industrial flow. First order and second order moments of LES are also examined and compared with DNS’s results. The first-order moments is calculated well by LES. But there are some discrepancies of second-order moments between LES and DNS.
Acceleration Statistics of Inertial Particles from High Resolution DNS Turbulence
Cencini, Massimo
Acceleration Statistics of Inertial Particles from High Resolution DNS Turbulence Federico Toschi1 the statistics of particle acceleration. We focus on the probability density function of the normalised acceleration. 2 Heavy Particle Dynamics and Numerical Simulations The equations of motion of a small, rigid
Large eddy simulations and direct numerical simulations of high speed turbulent reacting flows
NASA Technical Reports Server (NTRS)
Givi, Peyman; Madnia, C. K.; Steinberger, C. J.; Tsai, A.
1991-01-01
This research is involved with the implementations of advanced computational schemes based on large eddy simulations (LES) and direct numerical simulations (DNS) to study the phenomenon of mixing and its coupling with chemical reactions in compressible turbulent flows. In the efforts related to LES, a research program was initiated to extend the present capabilities of this method for the treatment of chemically reacting flows, whereas in the DNS efforts, focus was on detailed investigations of the effects of compressibility, heat release, and nonequilibrium kinetics modeling in high speed reacting flows. The efforts to date were primarily focussed on simulations of simple flows, namely, homogeneous compressible flows and temporally developing hign speed mixing layers. A summary of the accomplishments is provided.
Martín, Pino
A bandwidth-optimized WENO scheme for the effective direct numerical simulation of compressible May 2006 Available online 12 July 2006 Abstract Two new formulations of a symmetric WENO method-oscillatory (WENO) schemes provide a means for the DNS of compressible turbulent flow. In WENO schemes [5
Numerical simulation of thixoforming
Zavaliangos, A.; Lawley, A.
1995-02-01
Processing of alloys and composites in the solid plus liquid range has advantages over casting, forging, and powder metallurgy techniques. The sensitivity of semisolid slurries to temperature variations and their history and rate dependent behavior, however, make process design and control difficult. Precise selection of die velocity, process temperature, and die design is necessary to produce satisfactory products. Therefore, a computational capability for the prediction of the rheological behavior of semisolid materials would be an invaluable tool in process design. This work presents preliminary results on numerical simulations of thixoforming operations. Constitutive models that are able to describe qualitatively the transient flow behavior of semisolid materials are implemented in a finite element program. Simple but realistic thixoforming operations are simulated. Weaknesses of currently available constitutive models and numerical techniques are identified and discussed.
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.
Numerical Simulations of Bubble Dispersion over a Hydrofoil
NASA Astrophysics Data System (ADS)
Zhu, Shuang; Ooi, Andrew; Blackburn, Hugh; Anderson, Brendon
2009-11-01
The production and entrainment of bubbles in ship wakes is not completely understood despite the fact that it has many practical applications. For example, bubbles trapped in the large vortical structures in the ship wake can form clusters that are able to persist for large distances leaving a long trail of bubbles, which increases the ship's signature; an important consideration in the defence environment. The fundamental mechanisms behind the complicated bubbly flow can be understood using data from numerical simulations. The objective of the study is to investigate the accuracy of current state-of-art numerical models for simulating bubbly flows. A spectral element-Fourier code will be used to carry out direct numerical simulations (DNS) with Lagrangian particle tracking to study the interaction of the upstream bubble distribution with a hydrofoil at different angles of attack and Reynolds numbers, and the effect on the resulting downstream bubble distribution.
DNS and Combined Laser Diagnostics of Turbulent Combustion
NASA Astrophysics Data System (ADS)
Tanahashi, Mamoru; Sato, Makoto; Shimura, Masayasu; Miyauchi, Toshio
With the developments of computer technologies, three-dimensional direct numerical simulations (DNS) of turbulent combustion have been realized with a detailed or reduced kinetic mechanism. The 3D DNS gives detailed information about turbulent flames, while there are few experimental techniques which have high accuracy enough to compare with DNS. In this paper, after showing summary of recent DNS of turbulent premixed flames, newly-developed laser diagnostics are presented. Simultaneous CH-OH planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (PIV) are used to investigate the local flame structure of the turbulent premixed flames. From CH-OH PLIF and PIV measurements, flame fronts are identified, and the curvature of the flame front and the tangential strain rate at the flame front are evaluated. The experimental results are compared with 3D DNS of hydrogen-air and methane-air turbulent premixed flames. The flame displacement speeds in turbulent premixed flames have been measured directly by the CH double-pulsed PLIF. Since the time interval of the successive CH PLIF can be selected arbitrarily, both of the large scale dynamics and local displacement of the flame front can be obtained. As an application of laser diagnostics for development of high-efficient and low-emission combustors, reconstruction of 3D flame structure is shown by using multiple-plane OH PLIF.
Confidence in Numerical Simulations
Hemez, Francois M.
2015-02-23
This PowerPoint presentation offers a high-level discussion of uncertainty, confidence and credibility in scientific Modeling and Simulation (M&S). It begins by briefly evoking M&S trends in computational physics and engineering. The first thrust of the discussion is to emphasize that the role of M&S in decision-making is either to support reasoning by similarity or to “forecast,” that is, make predictions about the future or extrapolate to settings or environments that cannot be tested experimentally. The second thrust is to explain that M&S-aided decision-making is an exercise in uncertainty management. The three broad classes of uncertainty in computational physics and engineering are variability and randomness, numerical uncertainty and model-form uncertainty. The last part of the discussion addresses how scientists “think.” This thought process parallels the scientific method where by a hypothesis is formulated, often accompanied by simplifying assumptions, then, physical experiments and numerical simulations are performed to confirm or reject the hypothesis. “Confidence” derives, not just from the levels of training and experience of analysts, but also from the rigor with which these assessments are performed, documented and peer-reviewed.
T. Gastine; B. Dintrans
2008-09-29
We present a purely-radiative hydrodynamic model of the kappa-mechanism that sustains radial oscillations in Cepheid variables. We determine the physical conditions favourable for the kappa-mechanism to occur by the means of a configurable hollow in the radiative conductivity profile. By starting from these most favourable conditions, we complete nonlinear direct numerical simulations (DNS) and compare them with the results given by a linear-stability analysis of radial modes. We find that well-defined instability strips are generated by changing the location and shape of the conductivity hollow. For a given position in the layer, the hollow amplitude and width stand out as the key parameters governing the appearance of unstable modes driven by the kappa-mechanism. The DNS confirm both the growth rates and structures of the linearly-unstable modes. The nonlinear saturation that arises is produced by intricate couplings between the excited fundamental mode and higher damped overtones. These couplings are measured by projecting the DNS fields onto an acoustic subspace built from regular and adjoint eigenvectors and a 2:1 resonance is found to be responsible for the saturation of the kappa-mechanism instability.
Entropy Splitting for High Order Numerical Simulation of Compressible Turbulence
NASA Technical Reports Server (NTRS)
Sandham, N. D.; Yee, H. C.; Kwak, Dochan (Technical Monitor)
2000-01-01
A stable high order numerical scheme for direct numerical simulation (DNS) of shock-free compressible turbulence is presented. The method is applicable to general geometries. It contains no upwinding, artificial dissipation, or filtering. Instead the method relies on the stabilizing mechanisms of an appropriate conditioning of the governing equations and the use of compatible spatial difference operators for the interior points (interior scheme) as well as the boundary points (boundary scheme). An entropy splitting approach splits the inviscid flux derivatives into conservative and non-conservative portions. The spatial difference operators satisfy a summation by parts condition leading to a stable scheme (combined interior and boundary schemes) for the initial boundary value problem using a generalized energy estimate. A Laplacian formulation of the viscous and heat conduction terms on the right hand side of the Navier-Stokes equations is used to ensure that any tendency to odd-even decoupling associated with central schemes can be countered by the fluid viscosity. A special formulation of the continuity equation is used, based on similar arguments. The resulting methods are able to minimize spurious high frequency oscillation producing nonlinear instability associated with pure central schemes, especially for long time integration simulation such as DNS. For validation purposes, the methods are tested in a DNS of compressible turbulent plane channel flow at a friction Mach number of 0.1 where a very accurate turbulence data base exists. It is demonstrated that the methods are robust in terms of grid resolution, and in good agreement with incompressible channel data, as expected at this Mach number. Accurate turbulence statistics can be obtained with moderate grid sizes. Stability limits on the range of the splitting parameter are determined from numerical tests.
Direct numerical simulation of turbulent reacting flows
Chen, J.H.
1993-12-01
The development of turbulent combustion models that reflect some of the most important characteristics of turbulent reacting flows requires knowledge about the behavior of key quantities in well defined combustion regimes. In turbulent flames, the coupling between the turbulence and the chemistry is so strong in certain regimes that is is very difficult to isolate the role played by one individual phenomenon. Direct numerical simulation (DNS) is an extremely useful tool to study in detail the turbulence-chemistry interactions in certain well defined regimes. Globally, non-premixed flames are controlled by two limiting cases: the fast chemistry limit, where the turbulent fluctuations. In between these two limits, finite-rate chemical effects are important and the turbulence interacts strongly with the chemical processes. This regime is important because industrial burners operate in regimes in which, locally the flame undergoes extinction, or is at least in some nonequilibrium condition. Furthermore, these nonequilibrium conditions strongly influence the production of pollutants. To quantify the finite-rate chemistry effect, direct numerical simulations are performed to study the interaction between an initially laminar non-premixed flame and a three-dimensional field of homogeneous isotropic decaying turbulence. Emphasis is placed on the dynamics of extinction and on transient effects on the fine scale mixing process. Differential molecular diffusion among species is also examined with this approach, both for nonreacting and reacting situations. To address the problem of large-scale mixing and to examine the effects of mean shear, efforts are underway to perform large eddy simulations of round three-dimensional jets.
Validation of Direct Numerical Simulations in 3D pore geometries and Large-Eddy Simulations
NASA Astrophysics Data System (ADS)
Naumov, Dmitri
2013-04-01
Numerical solutions of the Navier-Stokes Equations became more popular in recent decades with increasingly accessible and powerful computational resources. Simulations in reconstructed or artificial pore geometries are often performed to gain insight into microscopic fluid flow structures or are used for upscaling quantities of interest, like hydraulic conductivity. A physically adequate representation of pore scale flow fields requires analysis of large domains in combination with turbulence models. We solve incompressible Navier-Stokes Equations in a cubic lattice and cubic close packing of spheres placed in a square duct with Direct Numerical Simulations (DNS) and analyze the validity of the results. The influence of the number of spheres and mesh discretization is investigated for fluid flow up to Reynolds numbers of 5000 based on the spheres' diameter. The numerical simulations are performed with the OpenFOAM open-source CFD software. We statistically investigate spatial and temporal properties of the resulting fluid flow field and its kinetic energy spectra, and compare them to Large-Eddy Simulations (LES) performed for the same geometries. Differences between the DNS and LES are discussed together with upscaled hydraulic properties with respect to the number of spheres and the Reynolds number.
Direct Numerical Simulation of a Weakly Stratified Turbulent Wake
NASA Technical Reports Server (NTRS)
Redford, J. A.; Lund, T. S.; Coleman, Gary N.
2014-01-01
Direct numerical simulation (DNS) is used to investigate a time-dependent turbulent wake evolving in a stably stratified background. A large initial Froude number is chosen to allow the wake to become fully turbulent and axisymmetric before stratification affects the spreading rate of the mean defect. The uncertainty introduced by the finite sample size associated with gathering statistics from a simulation of a time-dependent flow is reduced, compared to earlier simulations of this flow. The DNS reveals the buoyancy-induced changes to the turbulence structure, as well as to the mean-defect history and the terms in the mean-momentum and turbulence-kinetic-energy budgets, that characterize the various states of this flow - namely the three-dimensional (essentially unstratified), non-equilibrium (or 'wake-collapse') and quasi-two-dimensional (or 'two-component') regimes observed elsewhere for wakes embedded in both weakly and strongly stratified backgrounds. The wake-collapse regime is not accompanied by transfer (or 'reconversion') of the potential energy of the turbulence to the kinetic energy of the turbulence, implying that this is not an essential feature of stratified-wake dynamics. The dependence upon Reynolds number of the duration of the wake-collapse period is demonstrated, and the effect of the details of the initial/near-field conditions of the wake on its subsequent development is examined.
Turbulence analysis of rough wall channel flows based on direct numerical simulation
Mishra, A. V.; Bolotnov, I. A.
2012-07-01
Direct numerical simulation (DNS) of rough wall channel flows was performed for various surface roughnesses. The goal of the presented research is to investigate the effect of nucleating bubbles in subcooled boiling conditions on the turbulence. The nucleating bubbles are represented by hemispherical roughness elements at the wall. The stabilized finite element based code, PHASTA, is used to perform the simulations. Validation against theoretical, experimental and numerical data is performed for smooth channel flow and rectangular rod type of roughness. The presence of roughness elements affects the flow structure within the roughness sublayer, which is estimated to be 5 times the height of roughness elements. DNS observations are consistent with this result and demonstrate the flow homogeneity above 50 viscous units. The influence of roughness elements layout and density on the turbulence parameters is also demonstrated and analyzed. (authors)
Numerical simulation of dusty plasmas
Winske, D.
1995-09-01
The numerical simulation of physical processes in dusty plasmas is reviewed, with emphasis on recent results and unresolved issues. Three areas of research are discussed: grain charging, weak dust-plasma interactions, and strong dust-plasma interactions. For each area, we review the basic concepts that are tested by simulations, present some appropriate examples, and examine numerical issues associated with extending present work.
Numerical simulation of shock/turbulent boundary layer interaction
NASA Technical Reports Server (NTRS)
Biringen, Sedat; Hatay, Ferhat F.
1993-01-01
Most flows of aerodynamic interest are compressible and turbulent. However, our present knowledge on the structures and mechanisms of turbulence is mostly based on incompressible flows. In the present work, compressibility effects in turbulent, high-speed, boundary layer flows are systematically investigated using the Direct Numerical Simulation (DNS) approach. Three-dimensional, time-dependent, fully nonlinear, compressible Navier-Stokes equations were numerically integrated by high-order finite-difference methods; no modeling for turbulence is used during the solution because the available resolution is sufficient to capture the relevant scales. The boundary layer problem deals with fully-turbulent compressible flows over flat geometries. Apart from its practical relevance to technological flows, turbulent compressible boundary layer flow is the simplest experimentally realizable turbulent compressible flow. Still, measuring difficulties prohibit a detailed experimental description of the flow, especially in the near-wall region. DNS studies provide a viable means to probe the physics of compressible turbulence in this region. The focus of this work is to explore the paths of energy transfer through which compressible turbulence is sustained. The structural similarities and differences between the incompressible and compressible turbulence are also investigated. The energy flow patterns or energy cascades are found to be directly related to the evolution of vortical structures which are generated in the near-wall region. Near-wall structures, and mechanisms which are not readily accessible through physical experiments are analyzed and their critical role on the evolution and the behavior of the flow is documented extensively.
Direct Numerical Simulation of the Mixing Layer past Serrated Nozzle Ends
NASA Astrophysics Data System (ADS)
Babucke, Andreas; Kloker, Markus J.; Rist, Ulrich
The effect of serrations on the mixing layer past a thin splitter plate is investigated using spatial direct numerical simulation (DNS) with direct sound computation. Two different geometries are considered which yield a spanwise deformation of the Kelvin-Helmholtz rollers, streamwise vortices and subsequent breakdown of large-scale coherent structures, strongly affecting the noise emission. The results reveal that the spanwise extent of the serration is the driving parameter for sound reduction while its actual shape is less important.
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.
Numerical simulation of Bootstrap Current
Wu, Yanlin; White, R.B.
1993-05-01
The neoclassical theory of Bootstrap Current in toroidal systems is calculated in magnetic flux coordinates and confirmed by numerical simulation. The effects of magnetic ripple, loop voltage, and magnetic and electrostatic perturbations on bootstrap current for the cases of zero and finite plasma pressure are studied. The numerical results are in reasonable agreement with analytical estimates.
Direct numerical simulation of Coriolis effects on cylindrical gravity currents
NASA Astrophysics Data System (ADS)
Cantero, Mariano; Salinas, Jorge; Bonometti, Thomas; Dari, Enzo
2013-11-01
Gravity currents are generated by the action of gravity (or other volumetric force) on changes in fluid density. When they appear in turbulent regime, gravity currents are of a non-linear nature and have a wide range of temporal and spatial scales. In these systems there is a strong coupling between turbulence and stratification effects, with important consequences in the exchange of mass, momentum and energy. At geophysical scale, the analysis of these type of flows is further complicated by the influence of rotation effects by the Coriolis forces originated by earth's rotation. In this work we address the rotational effects in gravity currents with cylindrical initial condition by means of direct numerical simulations (DNS). We report results on five three dimensional DNS with grid resolutions up to 166-million points, with different boundary conditions, Reynolds numbers (Re=4000 and Re=8000), and different conditions of rotation. The results focus mainly on the distance of propagation of the fronts, frequency of the successive outward fronts, and the turbulent structures present in the currents and their influence in flow dynamics. Support from CONICET, CNEA, ANPCyT, UPS and IMFT is greatly acknowledged.
Masada, Youhei; Sano, Takayoshi E-mail: sano@ile.osaka-u.ac.jp
2014-10-10
The mechanism of large-scale dynamos in rigidly rotating stratified convection is explored by direct numerical simulations (DNS) in Cartesian geometry. A mean-field dynamo model is also constructed using turbulent velocity profiles consistently extracted from the corresponding DNS results. By quantitative comparison between the DNS and our mean-field model, it is demonstrated that the oscillatory ?{sup 2} dynamo wave, excited and sustained in the convection zone, is responsible for large-scale magnetic activities such as cyclic polarity reversal and spatiotemporal migration. The results provide strong evidence that a nonuniformity of the ?-effect, which is a natural outcome of rotating stratified convection, can be an important prerequisite for large-scale stellar dynamos, even without the ?-effect.
Numerical simulation of fracture
Margolin, L.G.
1983-01-01
A constructive model for brittle, and quasi-brittle materials is described. The Bedded Crack Model contains a microphysical description of fracture based on Griffith theory. The effect of cracks on material properties is described by effective modulus theory. Underlying the model is a statistical framework in which the evolution in time of a statistical distribution of cracks is calculated. The theory upon which the model is based is described. The model is implemented in a finite difference computer code. Our model is contrasted with the phenomenologic models usually found in computer codes. A computational simulation of the strain rate dependence of failure stress is presented and compared with laboratory data. A simulation of a gas gun experiment is presented, and the mechanism of spall described.
Direct numerical simulation of turbulent channel flow over porous walls
NASA Astrophysics Data System (ADS)
Rosti, Marco E.; Cortelezzi, Luca; Quadrio, Maurizio
2015-12-01
We perform direct numerical simulations (DNS) of a turbulent channel flow over porous walls. In the fluid region the flow is governed by the incompressible Navier--Stokes (NS) equations, while in the porous layers the Volume-Averaged Navier--Stokes (VANS) equations are used, which are obtained by volume-averaging the microscopic flow field over a small volume that is larger than the typical dimensions of the pores. In this way the porous medium has a continuum description, and can be specified without the need of a detailed knowledge of the pore microstructure by indipendently assigning permeability and porosity. At the interface between the porous material and the fluid region, momentum-transfer conditions are applied, in which an available coefficient related to the unknown structure of the interface can be used as an error estimate. To set up the numerical problem, the velocity-vorticity formulation of the coupled NS and VANS equations is derived and implemented in a pseudo-spectral DNS solver. Most of the simulations are carried out at $Re_\\tau=180$ and consider low-permeability materials; a parameter study is used to describe the role played by permeability, porosity, thickness of the porous material, and the coefficient of the momentum-transfer interface conditions. Among them permeability, even when very small, is shown to play a major role in determining the response of the channel flow to the permeable wall. Turbulence statistics and instantaneous flow fields, in comparative form to the flow over a smooth impermeable wall, are used to understand the main changes introduced by the porous material. A simulations at higher Reynolds number is used to illustrate the main scaling quantities.
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.
Under consideration for publication in J. Fluid Mech. 1 DNS of the thermal effects of laser energy
Mahesh, Krishnan
, it poses an interesting fluid dynamic problem. The blast wave is initially teardrop shaped but becomesUnder consideration for publication in J. Fluid Mech. 1 DNS of the thermal effects of laser energy with isotropic turbulence is studied using numerical simulations. The simulations use air as the working fluid
DNS of premixed turbulent V-flame: coupling spectral and finite difference methods
NASA Astrophysics Data System (ADS)
Hauguel, Raphael; Vervisch, Luc; Domingo, Pascale
2005-01-01
To allow for a reliable examination of the interaction between velocity fluctuations, acoustics and combustion, a novel numerical procedure is discussed in which a spectral solution of the Navier-Stokes equations is directly associated to a high-order finite difference fully compressible DNS solver (sixth order PADE). Using this combination of high-order solvers with accurate boundary conditions, simulations have been performed where a turbulent premixed V-shape flame develops in grid turbulence. In the light of the DNS results, a sub-model for premixed turbulent combustion is analyzed. To cite this article: R. Hauguel et al., C. R. Mecanique 333 (2005).
DNS of aerosol evolution in a turbulent jet
NASA Astrophysics Data System (ADS)
Zhou, Kun; Attili, Antonio; Bisetti, Fabrizio
2011-11-01
The effects of turbulence on the evolution of aerosols are not well understood. In this work, the interaction of aerosol dynamics and turbulence are studied in a canonical flow configuration by numerical means. The configuration consists of a hot nitrogen stream saturated with dibutyl phthalate (DBP) vapor mixing with cool air in a shear layer. A direct numerical simulation (DNS) for the momentum and scalar fields is coupled with the direct quadrature method of moments (DQMOM) for the condensing liquid phase. The effects of turbulent mixing on aerosol processes (nucleation, condensation, and coagulation) are quantified by analyzing the statistics of number density and droplet sizes.
Subgrid model evaluation through lockstep DNS/LES of a turbulent jet
NASA Astrophysics Data System (ADS)
Bhagatwala, Ankit; Raman, Venkat; Chen, Jacqueline
2013-11-01
The aim of this study is to analyze the validity of the common Smagorinsky type closure assumptions employed in LES scalar mixing and scalar dissipation rate models. This is done using a unique DNS-LES lockstep methodology, wherein a DNS is run simultaneously with several LES instances. The LES only solves for the scalar fields and obtains the velocity fields directly from the filtered DNS solution at every substep of time. The LES is also solved on the same grid as the DNS. This eliminates two primary sources of error in LES, numerical error associated with a coarser grid and modeling error arising from the modeled velocity field. The only source of error then, is from the closure assumption made for the LES model. One instance of DNS and three LES instances of a 3D turbulent slot jet at a Reynolds number of 7500 are simulated. The three LES simulations correspond to three different filter widths. Predictions of resolved and subgrid contributions of scalar second moment, scalar variance and scalar dissipation rate are compared. Implications for turbulent combustion models that heavily rely on these parameters are discussed.
Mueschke, N; Schilling, O
2008-07-23
A 1152 x 760 x 1280 direct numerical simulation (DNS) using initial conditions, geometry, and physical parameters chosen to approximate those of a transitional, small Atwood number Rayleigh-Taylor mixing experiment [Mueschke, Andrews and Schilling, J. Fluid Mech. 567, 27 (2006)] is presented. The density and velocity fluctuations measured just off of the splitter plate in this buoyantly unstable water channel experiment were parameterized to provide physically-realistic, anisotropic initial conditions for the DNS. The methodology for parameterizing the measured data and numerically implementing the resulting perturbation spectra in the simulation is discussed in detail. The DNS model of the experiment is then validated by comparing quantities from the simulation to experimental measurements. In particular, large-scale quantities (such as the bubble front penetration hb and the mixing layer growth parameter {alpha}{sub b}), higher-order statistics (such as velocity variances and the molecular mixing parameter {theta}), and vertical velocity and density variance spectra from the DNS are shown to be in favorable agreement with the experimental data. Differences between the quantities obtained from the DNS and from experimental measurements are related to limitations in the dynamic range of scales resolved in the simulation and other idealizations of the simulation model. This work demonstrates that a parameterization of experimentally-measured initial conditions can yield simulation data that quantitatively agrees well with experimentally-measured low- and higher-order statistics in a Rayleigh-Taylor mixing layer. This study also provides resolution and initial conditions implementation requirements needed to simulate a physical Rayleigh-Taylor mixing experiment. In Part II [Mueschke and Schilling, Phys. Fluids (2008)], other quantities not measured in the experiment are obtained from the DNS and discussed, such as the integral- and Taylor-scale Reynolds numbers, Reynolds stress anisotropy and two-dimensional density and velocity variance spectra, hypothetical chemical product formation measures, other local and global mixing parameters, and the statistical composition of mixed fluid.
NASA Astrophysics Data System (ADS)
Parkinson, S. D.; Hill, J.; Piggott, M. D.; Allison, P. A.
2014-05-01
High resolution direct numerical simulations (DNS) are an important tool for the detailed analysis of turbidity current dynamics. Models that resolve the vertical structure and turbulence of the flow are typically based upon the Navier-Stokes equations. Two-dimensional simulations are known to produce unrealistic cohesive vortices that are not representative of the real three-dimensional physics. The effect of this phenomena is particularly apparent in the later stages of flow propagation. The ideal solution to this problem is to run the simulation in three dimensions but this is computationally expensive. This paper presents a novel finite-element (FE) DNS turbidity current model that has been built within Fluidity, an open source, general purpose, computational fluid dynamics code. The model is validated through re-creation of a lock release density current at a Grashof number of 5 × 106 in two, and three-dimensions. Validation of the model considers the flow energy budget, sedimentation rate, head speed, wall normal velocity profiles and the final deposit. Conservation of energy in particular is found to be a good metric for measuring mesh performance in capturing the range of dynamics. FE models scale well over many thousands of processors and do not impose restrictions on domain shape, but they are computationally expensive. Use of discontinuous discretisations and adaptive unstructured meshing technologies, which reduce the required element count by approximately two orders of magnitude, results in high resolution DNS models of turbidity currents at a fraction of the cost of traditional FE models. The benefits of this technique will enable simulation of turbidity currents in complex and large domains where DNS modelling was previously unachievable.
Numerical simulations of generic singularities.
Garfinkle, David
2004-10-15
Numerical simulations of the approach to the singularity in vacuum spacetimes are presented here. The spacetimes examined have no symmetries and can be regarded as representing the general behavior of singularities. It is found that the singularity is spacelike and that, as it is approached, the spacetime dynamics becomes local and oscillatory. PMID:15524970
DNS of Flows over Periodic Hills using a Discontinuous-Galerkin Spectral-Element Method
NASA Technical Reports Server (NTRS)
Diosady, Laslo T.; Murman, Scott M.
2014-01-01
Direct numerical simulation (DNS) of turbulent compressible flows is performed using a higher-order space-time discontinuous-Galerkin finite-element method. The numerical scheme is validated by performing DNS of the evolution of the Taylor-Green vortex and turbulent flow in a channel. The higher-order method is shown to provide increased accuracy relative to low-order methods at a given number of degrees of freedom. The turbulent flow over a periodic array of hills in a channel is simulated at Reynolds number 10,595 using an 8th-order scheme in space and a 4th-order scheme in time. These results are validated against previous large eddy simulation (LES) results. A preliminary analysis provides insight into how these detailed simulations can be used to improve Reynoldsaveraged Navier-Stokes (RANS) modeling
A Review of Direct Numerical Simulations of Astrophysical Detonations and Their Implications
Parete-Koon, Suzanne T; Messer, Bronson; Smith, Chris R; Papatheodore, Thomas L
2013-01-01
Multi-dimensional direct numerical simulations (DNS) of astrophysical detonations in degenerate matter have revealed that the nuclear burning is typically characterized by cellular structure caused by transverse instabilities in the detonation front. Type Ia supernova modelers often use one- dimensional DNS of detonations as inputs or constraints for their whole star simulations. While these one-dimensional studies are useful tools, the true nature of the detonation is multi-dimensional. The multi-dimensional structure of the burning influences the speed, stability, and the composition of the detonation and its burning products, and therefore, could have an impact on the spectra of Type Ia supernovae. Considerable effort has been expended modeling Type Ia supernovae at densities above 1 107 g cm 3 where the complexities of turbulent burning dominate the flame propagation. However, most full star models turn the nuclear burning schemes off when the density falls below 1 107 g cm 3 and distributed burning begins. The deflagration to detonation transition (DDT) is believed to occur at just these densities and consequently they are the densities important for studying the properties of the subsequent detonation. This work will review the status of DNS studies of detonations and their possible implications for Type Ia supernova models. It will cover the development of Detonation theory from the first simple Chapman-Jouguet (CJ) detonation models to the current models based on the time-dependent, compressible, reactive flow Euler equations of fluid dynamics.
A review of direct numerical simulations of astrophysical detonations and their implications
NASA Astrophysics Data System (ADS)
Parete-Koon, Suzanne T.; Smith, Christopher R.; Papatheodore, Thomas L.; Messer, O. E. Bronson
2013-04-01
Multi-dimensional direct numerical simulations (DNS) of astrophysical detonations in degenerate matter have revealed that the nuclear burning is typically characterized by cellular structure caused by transverse instabilities in the detonation front. Type Ia supernova modelers often use onedimensional DNS of detonations as inputs or constraints for their whole star simulations.While these one-dimensional studies are useful tools, the true nature of the detonation is multi-dimensional. The multi-dimensional structure of the burning influences the speed, stability, and the composition of the detonation and its burning products, and therefore, could have an impact on the spectra of Type Ia supernovae. Considerable effort has been expended modeling Type Ia supernovae at densities above 1×107 g·cm-3 where the complexities of turbulent burning dominate the flame propagation. However, most full star models turn the nuclear burning schemes off when the density falls below 1×107 g·cm-3 and distributed burning begins. The deflagration to detonation transition (DDT) is believed to occur at just these densities and consequently they are the densities important for studying the properties of the subsequent detonation. This work will review the status of DNS studies of detonations and their possible implications for Type Ia supernova models. It will cover the development of Detonation theory from the first simple Chapman-Jouguet (CJ) detonation models to the current models based on the time-dependent, compressible, reactive flow Euler equations of fluid dynamics.
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.
Direct Numerical Simulations of Boiling
NASA Astrophysics Data System (ADS)
Tryggvason, Gretar; Esmaeeli, Asghar
2003-11-01
For flow problem of practical interest it is frequently necessary to account for phase change between liquid and vapor. Boiling, in particular, is one of the most efficient ways of removing heat from a solid surface. It is therefore commonly used in energy generation and refrigeration. The large volume change and the high temperatures involved can make the consequences of design or operational errors catastrophic and accurate predictions are highly desirable. For numerical simulations of boiling it is necessary to solve the energy equation, in addition to conservation equations for mass and momentum, and to account for the release/absorption of latent heat at the phase boundary. We describe a numerical method for direct numerical simulations of boiling and show results from simulations of explosive boiling of a vapor bubble in an initially superheated liquids. As the vapor bubble grows, its surface becomes unstable, developing wrinkles that increase the surface area significantly. The increased surface area does, however, have relatively little impact on the growth rate for the parameters examined due to a relatively thick thermal boundary layer. We have also examined film boiling and find relatively good agreement with experimental correlations. Research supported by NASA.
Numerical Propulsion System Simulation Architecture
NASA Technical Reports Server (NTRS)
Naiman, Cynthia G.
2004-01-01
The Numerical Propulsion System Simulation (NPSS) is a framework for performing analysis of complex systems. Because the NPSS was developed using the object-oriented paradigm, the resulting architecture is an extensible and flexible framework that is currently being used by a diverse set of participants in government, academia, and the aerospace industry. NPSS is being used by over 15 different institutions to support rockets, hypersonics, power and propulsion, fuel cells, ground based power, and aerospace. Full system-level simulations as well as subsystems may be modeled using NPSS. The NPSS architecture enables the coupling of analyses at various levels of detail, which is called numerical zooming. The middleware used to enable zooming and distributed simulations is the Common Object Request Broker Architecture (CORBA). The NPSS Developer's Kit offers tools for the developer to generate CORBA-based components and wrap codes. The Developer's Kit enables distributed multi-fidelity and multi-discipline simulations, preserves proprietary and legacy codes, and facilitates addition of customized codes. The platforms supported are PC, Linux, HP, Sun, and SGI.
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.
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.
On locating the obstruction in the upper airway via numerical simulation
Wang, Yong; Elghobashi, S.
2014-01-01
The fluid dynamical properties of the air flow in the upper airway (UA) are not fully understood at present due to the three-dimensional (3D) patient-specific complex geometry of the airway, flow transition from laminar to turbulent and flow-structure interaction during the breathing cycle. It is quite difficult at present to experimentally measure the instantaneous velocity and pressure at specific points in the human airway. On the other hand, direct numerical simulation (DNS) can predict all the flow properties and resolve all its relevant length- and time-scales. We developed a DNS solver with the state-of-the-art lattice Boltzmann method (LBM), and used it to investigate the flow in two patient-specific UAs reconstructed from CT scan data. Inspiration and expiration flows through these two airways are studied. The time-averaged first spatial derivative of pressure (pressure gradient), ?p/?z, is used to locate the region of the UA obstruction. But the time-averaged second spatial derivative, ?2p/?z2, is used to pinpoint the exact location of the obstruction. The present results show that the DNS-LBM solver can be used to obtain accurate flow details in the UA and is a powerful tool to locate its obstruction. PMID:24389271
Statistically Steady Turbulence in Soap Films: Direct Numerical Simulations with Ekman Friction
Prasad Perlekar; Rahul Pandit
2008-11-09
We present a detailed direct numerical simulation (DNS) designed to investigate the combined effects of walls and Ekman friction on turbulence in forced soap films. We concentrate on the forward-cascade regime and show how to extract the isotropic parts of velocity and vorticity structure functions and thence the ratios of multiscaling exponents. We find that velocity structure functions display simple scaling whereas their vorticity counterparts show multiscaling; and the probability distribution function of the Weiss parameter $\\Lambda$, which distinguishes between regions with centers and saddles, is in quantitative agreement with experiments.
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.
Direct numerical simulation for incompressible channel flow at Re? = 5200
NASA Astrophysics Data System (ADS)
Lee, Myoungkyu; Malaya, Nicholas; Moser, Robert D.
2013-11-01
High-resolution direct numerical simulation (DNS) of wall-bounded canonical channel flow at Re? = 5200 is performed. The computational domain is 8 ?? × 2 ? × 3 ?? with 10240 × 1536 × 7680 grid points in streamwise (x), wall-normal direction (y), and spanwise (z) directions, respectively. Fourier spectral method (x, and z) and B-splines (y) are used for the the computation of derivatives. In this presentation we demonstrate that the simulation exhibits several features of high Reynolds number wall-bounded turbulence. The value of von Kármán constant appears to be ? = 0.384 in the region of y+=300 ~ y = 0 . 2 ? where the mean velocity profile shows logarithmic variation. Also, distinct inner (?x+= 800, ?z+= 120) and outer (?x= 8 ?, ?z= ?) peaks in one-dimensional premultiplied spectra of the velocity variance are observed. Finally, the kx- 1 region is observed in the range of y+= 120 ~ 150 and kx= 6 ~ 10 . This work is supported by NSF PetaApps grants: OCI-0749223 and NSF PRAC Grant 0832634.
Compressible Turbulent Channel Flows: DNS Results and Modeling
NASA Technical Reports Server (NTRS)
Huang, P. G.; Coleman, G. N.; Bradshaw, P.; Rai, Man Mohan (Technical Monitor)
1994-01-01
The present paper addresses some topical issues in modeling compressible turbulent shear flows. The work is based on direct numerical simulation of two supersonic fully developed channel flows between very cold isothermal walls. Detailed decomposition and analysis of terms appearing in the momentum and energy equations are presented. The simulation results are used to provide insights into differences between conventional time-and Favre-averaging of the mean-flow and turbulent quantities. Study of the turbulence energy budget for the two cases shows that the compressibility effects due to turbulent density and pressure fluctuations are insignificant. In particular, the dilatational dissipation and the mean product of the pressure and dilatation fluctuations are very small, contrary to the results of simulations for sheared homogeneous compressible turbulence and to recent proposals for models for general compressible turbulent flows. This provides a possible explanation of why the Van Driest density-weighted transformation is so successful in correlating compressible boundary layer data. Finally, it is found that the DNS data do not support the strong Reynolds analogy. A more general representation of the analogy is analysed and shown to match the DNS data very well.
Direct numerical simulation of supersonic combustion with finite-rate chemistry
NASA Astrophysics Data System (ADS)
Saghafian, Amirreza; Pitsch, Heinz
2011-11-01
Three-dimensional direct numerical simulations (DNS) of reacting and inert compressible turbulent mixing layers have been performed. The simulations cover convective Mach numbers from subsonic to supersonic. A detailed chemistry mechanism with 9 species and 29 reactions for hydrogen is used in the reacting simulations. Effects of different initial conditions on the structure of the mixing layer, and time required to reach self-similarity are studied. Flame/turbulence interaction is analyzed by studying turbulent kinetic energy, Reynolds stresses, and their budgets in the reacting and inert simulations. The effects of different reactions on the heat release and mixture composition especially in the regions where shocklets impinge the flame are studied. These DNS databases will provide a better understanding for the compressibility effects on the combustion, and will be used to assess the accuracy of Flamelet/Progress variable approach in supersonic regime. This material is based upon work supported by the Department of Energy under the Predictive Science Academic Alliance Program (PSAAP) at Stanford University, Award Number(s)DE-FC52-08NA28614.
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.
Numerical Simulation of Protoplanetary Vortices
NASA Technical Reports Server (NTRS)
Lin, H.; Barranco, J. A.; Marcus, P. S.
2003-01-01
The fluid dynamics within a protoplanetary disk has been attracting the attention of many researchers for a few decades. Previous works include, to list only a few among many others, the well-known prescription of Shakura & Sunyaev, the convective and instability study of Stone & Balbus and Hawley et al., the Rossby wave approach of Lovelace et al., as well as a recent work by Klahr & Bodenheimer, which attempted to identify turbulent flow within the disk. The disk is commonly understood to be a thin gas disk rotating around a central star with differential rotation (the Keplerian velocity), and the central quest remains as how the flow behavior deviates (albeit by a small amount) from a strong balance established between gravitational and centrifugal forces, transfers mass and momentum inward, and eventually forms planetesimals and planets. In earlier works we have briefly described the possible physical processes involved in the disk; we have proposed the existence of long-lasting, coherent vortices as an efficient agent for mass and momentum transport. In particular, Barranco et al. provided a general mathematical framework that is suitable for the asymptotic regime of the disk; Barranco & Marcus (2000) addressed a proposed vortex-dust interaction mechanism which might lead to planetesimal formation; and Lin et al. (2002), as inspired by general geophysical vortex dynamics, proposed basic mechanisms by which vortices can transport mass and angular momentum. The current work follows up on our previous effort. We shall focus on the detailed numerical implementation of our problem. We have developed a parallel, pseudo-spectral code to simulate the full three-dimensional vortex dynamics in a stably-stratified, differentially rotating frame, which represents the environment of the disk. Our simulation is validated with full diagnostics and comparisons, and we present our results on a family of three-dimensional, coherent equilibrium vortices.
The AGE method for direct numerical simulation of turbulent shear flow
NASA Astrophysics Data System (ADS)
Bisset, David K.
1998-11-01
The advected grid explicit (AGE) method for direct numerical simulation of incompressible turbulent shear flows is presented. The Navier-Stokes equations are used for momentum in a velocity-pressure formulation. Mass continuity and an equation of state link pressure with density (which is not assumed identically constant). Time advancement is entirely explicit, and spatial representation is localized (e.g. finite difference) and centred. Magnitudes of non-linear terms are reduced on advected grid(s), and numerical instabilities are efficiently reduced by targeted diffusion. Computation time scales directly on the number of grid points (virtual memory issues aside), and is very short for a DNS method. A spatially developing two-stream mixing layer was simulated as an example, reaching a vorticity thickness Reynolds number >20 000. Comparison with experimental results from self-similar mixing layers is satisfactory in terms of growth rate and Reynolds stress profiles. Turbulent vortical structures are visualized by means of pressure surfaces.
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.
LES, DNS and RANS for the analysis of high-speed turbulent reacting flows
NASA Technical Reports Server (NTRS)
Adumitroaie, V.; Colucci, P. J.; Taulbee, D. B.; Givi, P.
1995-01-01
The purpose of this research is to continue our efforts in advancing the state of knowledge in large eddy simulation (LES), direct numerical simulation (DNS), and Reynolds averaged Navier Stokes (RANS) methods for the computational analysis of high-speed reacting turbulent flows. In the second phase of this work, covering the period 1 Aug. 1994 - 31 Jul. 1995, we have focused our efforts on two programs: (1) developments of explicit algebraic moment closures for statistical descriptions of compressible reacting flows and (2) development of Monte Carlo numerical methods for LES of chemically reacting flows.
DNS of fully-resolved droplet-laden decaying isotropic turbulence
NASA Astrophysics Data System (ADS)
Ferrante, A.; Dodd, M.
2013-11-01
We investigated the effects of finite-size droplets on decaying isotropic turbulence by performing direct numerical simulation (DNS). We performed DNS using our new pressure-correction/volume-of-fluid method that is mass-conservative and second-order accurate. The simulations were performed at Re?0 = 75 on a 10243 grid such to resolve each droplet with 32 grid points per diameter. We fully resolve all the relevant scales of turbulence around thousands of freely-moving droplets of Taylor length-scale size as well as the fluid motion inside the droplets. We will discuss the effects of the droplets on the temporal development of turbulence kinetic energy and its dissipation rate. Also, we will present the effects on turbulence of the droplet Weber number and of the density ratio between the droplet and the surrounding fluid. NSF CAREER #1054591.
A direct numerical simulation study of higher order statistics in a turbulent round jet
NASA Astrophysics Data System (ADS)
Taub, G. N.; Lee, Hyungoo; Balachandar, S.; Sherif, S. A.
2013-11-01
Up until recently direct numerical simulation (DNS) studies involving round turbulent jets have focused on first and second order statistics and vortical behavior near the source of the jet. The third order statistics necessary to compute the turbulent kinetic energy and Reynolds stress transport equations have been examined using LES studies. However, further examination with DNS is important as, on the subgrid scale, LES uses models for Reynolds stress. In this study a DNS of a turbulent free jet with a Reynolds number equal to ReJ = 2000 is computed using a second order accurate, time splitting finite volume scheme. First, second, and third order statistics are compared with previous experimental and numerical studies. All terms of the turbulent kinetic energy balance are calculated directly. The results are compared to experimental studies such as those of Hussein et al. ["Velocity measurements in a high-Reynolds-number, momentum-conserving, axisymmetric, turbulent jet," J. Fluid Mech. 258, 31-75 (1994)], Panchapakesan and Lumley ["Turbulence measurements in axisymmetric jets of air and helium. Part 1. Air jet," J. Fluid Mech. 246, 197-233 (1993)], and others. The assumptions made by the various experimental studies in order to solve the dissipation and pressure diffusion terms are discussed and examined using the data from the current study. The Reynolds stress transport equations are also calculated and discussed. Vortical structures are visualized by the ?ci method and discussed along with entrainment of ambient fluid into the jet. The one dimensional energy spectra in the azimuthal direction are calculated directly and are also discussed.
Numerical Simulations of Granular Processes
NASA Astrophysics Data System (ADS)
Richardson, Derek C.; Michel, Patrick; Schwartz, Stephen R.; Ballouz, Ronald-Louis; Yu, Yang; Matsumura, Soko
2014-11-01
Spacecraft images and indirect observations including thermal inertia measurements indicate most small bodies have surface regolith. Evidence of granular flow is also apparent in the images. This material motion occurs in very low gravity, therefore in a completely different gravitational environment than on the Earth. Understanding and modeling these motions can aid in the interpretation of imaged surface features that may exhibit signatures of constituent material properties. Also, upcoming sample-return missions to small bodies, and possible future manned missions, will involve interaction with the surface regolith, so it is important to develop tools to predict the surface response. We have added new capabilities to the parallelized N-body gravity tree code pkdgrav [1,2] that permit the simulation of granular dynamics, including multi-contact physics and friction forces, using the soft-sphere discrete-element method [3]. The numerical approach has been validated through comparison with laboratory experiments (e.g., [3,4]). Ongoing and recently completed projects include: impacts into granular materials using different projectile shapes [5]; possible tidal resurfacing of asteroid Apophis during its 2029 encounter [6]; the Brazil-nut effect in low gravity [7]; and avalanche modeling.Acknowledgements: DCR acknowledges NASA (grants NNX08AM39G, NNX10AQ01G, NNX12AG29G) and NSF (AST1009579). PM acknowledges the French agency CNES. SRS works on the NEOShield Project funded under the European Commission’s FP7 program agreement No. 282703. SM acknowledges support from the Center for Theory and Computation at U Maryland and the Dundee Fellowship at U Dundee. Most simulations were performed using the YORP cluster in the Dept. of Astronomy at U Maryland and on the Deepthought High-Performance Computing Cluster at U Maryland.References: [1] Richardson, D.C. et al. 2000, Icarus 143, 45; [2] Stadel, J. 2001, Ph.D. Thesis, U Washington; [3] Schwartz, S.R. et al. 2012, Gran. Matt. 14, 363. [4] Schwartz, S.R. et al. 2013, Icarus 226, 67; [5] Schwartz, S.R. et al. 2014, P&SS, 10.1016/j.pss.2014.07.013; [6] Yu, Y. et al. 2014, Icarus, 10.1016/j.icarus.2014.07.027; [7] Matsumura, S. et al. 2014, MNRAS, 10.1093/mnras/stu1388.
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.
Threedimensional numerical simulation for various geometries
Herbin, Raphaèle
modelling and numerical simulation of natural gasfed solid oxide cells (Solid Oxide Fuel Cell, SOFC pollution rate. Moreover, the electrolyte in a SOFC is solid rather than liquid (as for instanceThreedimensional numerical simulation for various geometries of solid oxide fuel cells J
NASA Astrophysics Data System (ADS)
Kuhl, J. M.; Desjardin, P. E.
2012-01-01
Two-dimensional, fully coupled direct numerical simulations (DNS) are conducted to examine the local energy dynamics of a flexible cantilevered plate in the wake of a two-dimensional circular cylinder. The motion of the cantilevered plate is described using a finite element formulation and a fully compressible, finite volume Navier Stokes solver is used to compute the flow field. A sharp interface level set method is employed in conjunction with a ghost fluid method to describe the immersed boundaries of the bluff body and flexible plate. DNS is first conducted to validate the numerical methodology and compared with previous studies of flexible cantilevered plates and flow over bluff bodies; excellent agreement with previous results is observed. A newly defined power production/loss geometry metric is introduced based on surface curvature and plate velocity. The metric is found to be useful for determining which sections of the plate will produce energy based on curvature and deflection rate. Scatter plots and probability measures are presented showing a high correlation between the direction of energy transfer (i.e., to or from the plate) and the sign of the newly defined curvature-deflection-rate metric. The findings from this study suggest that a simple local geometry/kinematic based metric can be devised to aid in the development and design of flexible wind energy harvesting flutter mills.
Numerical simulations of impact penetration tests
NASA Astrophysics Data System (ADS)
Shibue, T.; Nakayama, E.; Natsumura, T.; Tanaka, T.; Asano, T.
1994-07-01
This paper gives one way to estimate critical strength of a steel plate against perforation failure by combinations of impact penetration tests and numerical simulations. The simulation tool employed is DYNA3D with a function to evaluate strain-rate dependencey of material strength. At first, numerical simulations of a test give a set of material properties, then a series of numerical simulations reproduces a series of impact penetration tests to estimate the critical perforation velocity. The esimated critical perforation velocity is found to be 12% less than the measured value, whereas, the deformations of projectlies show a good agreement with those measured.
Mean-field and direct numerical simulations of magnetic flux concentrations from vertical field
NASA Astrophysics Data System (ADS)
Brandenburg, A.; Gressel, O.; Jabbari, S.; Kleeorin, N.; Rogachevskii, I.
2014-02-01
Context. Strongly stratified hydromagnetic turbulence has previously been found to produce magnetic flux concentrations if the domain is large enough compared with the size of turbulent eddies. Mean-field simulations (MFS) using parameterizations of the Reynolds and Maxwell stresses show a large-scale negative effective magnetic pressure instability and have been able to reproduce many aspects of direct numerical simulations (DNS) regarding growth rate, shape of the resulting magnetic structures, and their height as a function of magnetic field strength. Unlike the case of an imposed horizontal field, for a vertical one, magnetic flux concentrations of equipartition strength with the turbulence can be reached, resulting in magnetic spots that are reminiscent of sunspots. Aims: We determine under what conditions magnetic flux concentrations with vertical field occur and what their internal structure is. Methods: We use a combination of MFS, DNS, and implicit large-eddy simulations (ILES) to characterize the resulting magnetic flux concentrations in forced isothermal turbulence with an imposed vertical magnetic field. Results: Using DNS, we confirm earlier results that in the kinematic stage of the large-scale instability the horizontal wavelength of structures is about 10 times the density scale height. At later times, even larger structures are being produced in a fashion similar to inverse spectral transfer in helically driven turbulence. Using ILES, we find that magnetic flux concentrations occur for Mach numbers between 0.1 and 0.7. They occur also for weaker stratification and larger turbulent eddies if the domain is wide enough. Using MFS, the size and aspect ratio of magnetic structures are determined as functions of two input parameters characterizing the parameterization of the effective magnetic pressure. DNS, ILES, and MFS show magnetic flux tubes with mean-field energies comparable to the turbulent kinetic energy. These tubes can reach a length of about eight density scale heights. Despite being ?1% equipartition strength, it is important that their lower part is included within the computational domain to achieve the full strength of the instability. Conclusions: The resulting vertical magnetic flux tubes are being confined by downflows along the tubes and corresponding inflow from the sides, which keep the field concentrated. Application to sunspots remains a viable possibility.
GPU accelerated flow solver for direct numerical simulation of turbulent flows
Salvadore, Francesco; Botti, Michela
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.
Simulating climate with two different numerical schemes
Gutowski, W.J.; Iacono, M.J. ); Xin-Zhong, Liang ); Wang, Wei-Chyung )
1990-06-01
We compare climate simulations from two models that are identical except that one uses gridpoint techniques and the other spectral techniques for the numerical treatment of atmospheric dynamics. Our purpose is not to determine which technique is better but to indicate the dependence of climate simulation on model numerics. Simulations of a perpetual July are performed using spectral and gridpoint dynamics codes coupled to the Oregon State University model's two-layer atmospheric physics. 31 refs., 28 figs.
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 for certain simulations without a large increase in computational time. In this paper, sev- eral aerodynamics
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.
NASA Astrophysics Data System (ADS)
Sandberg, Richard; Fasel, Hermann
2001-11-01
A high-order accurate compressible Navier-Stokes code was developed to perform Direct Numerical Simulations (DNS) and Unsteady RANS (URANS) of a supersonic turbulent, axisymmetric wake. Bluff bodies in supersonic flows have been investigated for several decades motivated by the desire to obtain efficient methods to reduce the base drag which results from the low pressure in the recirculation region. Because of the tremendous resolution requirements, DNS have been restricted to moderate Reynolds numbers and so far, RANS calculations have failed to predict the correct mean flow behavior in the near wake region mainly because of not being able to capture relevant large structures. For the present work the full compressible Navier-Stokes equations in cylindrical coordinates are solved using sixth-order split compact differences for the downstream and radial directions featuring a state of the art treatment of the axis, and pseudospectral discretization in the azimuthal direction. To ensure an accurate time advancement, a fourth-order Runge-Kutta scheme (R-K) is employed. For Unsteady RANS calculations, the k and e equations are solved either with a first order ADI -scheme or using a fourth order R-K integration. The Reynolds stresses are computed with an Algebraic Stress Model (ASM). Several DNS are presented which are compared to experiments and allow us to obtain 3-D data for Reynolds numbers up to 100,000 that can be used as a benchmark for the ongoing URANS. Also, validation calculations for the turbulence model were performed and are presented.
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 Simulations of Bouncing Jets
Lee, Sanghyun
2014-07-18
using Finite Elements . . . . . . . . . . . . . . . 71 4.3 Time discretization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 4.4 Surface Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 4.5 Numerical results... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 4.5.1 Rayleigh-Taylor instability . . . . . . . . . . . . . . . . . . . . 77 4.5.2 Rising Bubble benchmark problems . . . . . . . . . . . . . . . 78 4.5.3 Buckling fluid . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 5. NON...
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...
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 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.
A numerical sensitivity analysis of streamline simulation
Chaban Habib, Fady Ruben
2005-02-17
-1 A NUMERICAL SENSITIVITY ANALYSIS OF STREAMLINE SIMULATION A Thesis by FADY RUBEN CHABAN HABIB Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree... of MASTER OF SCIENCE December 2004 Major Subject: Petroleum Engineering A NUMERICAL SENSITIVITY ANALYSIS OF STREAMLINE SIMULATION A Thesis by FADY RUBEN CHABAN HABIB Submitted to the Office of Graduate Studies...
Bisetti, Fabrizio; Attili, Antonio; Pitsch, Heinz
2014-01-01
Combustion of fossil fuels is likely to continue for the near future due to the growing trends in energy consumption worldwide. The increase in efficiency and the reduction of pollutant emissions from combustion devices are pivotal to achieving meaningful levels of carbon abatement as part of the ongoing climate change efforts. Computational fluid dynamics featuring adequate combustion models will play an increasingly important role in the design of more efficient and cleaner industrial burners, internal combustion engines, and combustors for stationary power generation and aircraft propulsion. Today, turbulent combustion modelling is hindered severely by the lack of data that are accurate and sufficiently complete to assess and remedy model deficiencies effectively. In particular, the formation of pollutants is a complex, nonlinear and multi-scale process characterized by the interaction of molecular and turbulent mixing with a multitude of chemical reactions with disparate time scales. The use of direct numerical simulation (DNS) featuring a state of the art description of the underlying chemistry and physical processes has contributed greatly to combustion model development in recent years. In this paper, the analysis of the intricate evolution of soot formation in turbulent flames demonstrates how DNS databases are used to illuminate relevant physico-chemical mechanisms and to identify modelling needs. PMID:25024412
Utilizing Direct Numerical Simulations of Transition and Turbulence in Design Optimization
NASA Technical Reports Server (NTRS)
Rai, Man M.
2015-01-01
Design optimization methods that use the Reynolds-averaged Navier-Stokes equations with the associated turbulence and transition models, or other model-based forms of the governing equations, may result in aerodynamic designs with actual performance levels that are noticeably different from the expected values because of the complexity of modeling turbulence/transition accurately in certain flows. Flow phenomena such as wake-blade interaction and trailing edge vortex shedding in turbines and compressors (examples of such flows) may require a computational approach that is free of transition/turbulence models, such as direct numerical simulations (DNS), for the underlying physics to be computed accurately. Here we explore the possibility of utilizing DNS data in designing a turbine blade section. The ultimate objective is to substantially reduce differences between predicted performance metrics and those obtained in reality. The redesign of a typical low-pressure turbine blade section with the goal of reducing total pressure loss in the row is provided as an example. The basic ideas presented here are of course just as applicable elsewhere in aerodynamic shape optimization as long as the computational costs are not excessive.
Direct Numerical Simulations of High-Speed Turbulent Boundary Layers over Riblets
NASA Technical Reports Server (NTRS)
Duan, Lian; Choudhari, Meelan, M.
2014-01-01
Direct numerical simulations (DNS) of spatially developing turbulent boundary layers over riblets with a broad range of riblet spacings are conducted to investigate the effects of riblets on skin friction at high speeds. Zero-pressure gradient boundary layers under two flow conditions (Mach 2:5 with T(sub w)/T(sub r) = 1 and Mach 7:2 with T(sub w)/T(sub r) = 0:5) are considered. The DNS results show that the drag-reduction curve (delta C(sub f)/C(sub f) vs l(sup +)(sub g )) at both supersonic speeds follows the trend of low-speed data and consists of a `viscous' regime for small riblet size, a `breakdown' regime with optimal drag reduction, and a `drag-increasing' regime for larger riblet sizes. At l l(sup +)(sub g) approx. 10 (corresponding to s+ approx 20 for the current triangular riblets), drag reduction of approximately 7% is achieved at both Mach numbers, and con rms the observations of the few existing experiments under supersonic conditions. The Mach- number dependence of the drag-reduction curve occurs for riblet sizes that are larger than the optimal size, with smaller slopes of (delta C(sub f)/C(sub f) for larger freestream Mach numbers. The Reynolds analogy holds with 2(C(sub h)=C(sub f) approximately equal to that of at plates for both drag-reducing and drag-increasing configurations.
Numerical simulation of jet noise
NASA Astrophysics Data System (ADS)
Paliath, Umesh
In the present work, computational aeroacoustics and parallel computers are used to conduct a study of flow-induced noise from different jet nozzle geometries. The nozzle is included as part of the computational domain. This is important to predict jet noise from nozzles associated with military aircraft engines. The Detached Eddy Simulation (DES) approach is used to simulate both the jet nozzle internal and external flows as well as the jet plume. This methodology allows the turbulence model to transition from an unsteady Reynolds Averaged Navier-Stokes (URANS) method for attached boundary layers to a Large Eddy Simulation (LES) in separated regions. Thus, it is ideally suited to jet flow simulations where the nozzle is included. Both cylindrical polar and Cartesian coordinate systems are used. A spectral method is used to avoid the centerline singularity when using the cylindrical coordinate system. The one equation Spalart-Allmaras turbulence model, in DES mode, is used to describe the evolution of the turbulent eddy viscosity. An explicit 4th order Runge-Kutta time marching scheme is used. For spatial discritization the Dispersion Relation Preserving scheme(DRP) is used. The farfield sound is evaluated using the Ffowcs Williams-Hawkings permeable surface wave extrapolation method. This permits the noise to be predicted at large distances from the jet based on fluctuations in the jets near field. The present work includes a study of the effect of different nozzle geometries such as axisymmetric/non-axisymmetric and planar/non-planar exits on the far field noise predictions. Also the effect of operating conditions such as a heated/unheated jet, the effect of forward flight, a jet flow at an angle of attack, and the effect of a supersonic exit Mach number, are included in the study.
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)
Yu, R.; Lipatnikov, A. N.; Bai, X. S.
2014-08-01
In order to gain further insight into (i) the use of conditioned quantities for characterizing turbulence within a premixed flame brush and (ii) the influence of front propagation on turbulent scalar transport, a 3D Direct Numerical Simulation (DNS) study of an infinitely thin front that self-propagates in statistically stationary, homogeneous, isotropic, forced turbulence was performed by numerically integrating Navier-Stokes and level set equations. While this study was motivated by issues relevant to premixed combustion, the density was assumed to be constant in order (i) to avoid the influence of the front on the flow and, therefore, to know the true turbulence characteristics as reference quantities for assessment of conditioned moments and (ii) to separate the influence of front propagation on turbulent transport from the influence of pressure gradient induced by heat release. Numerical simulations were performed for two turbulence Reynolds numbers (50 and 100) and four ratios (1, 2, 5, and 10) of the rms turbulent velocity to the front speed. Obtained results show that, first, the mean front thickness is decreased when a ratio of the rms turbulent velocity to the front speed is decreased. Second, although the gradient diffusion closure yields the right direction of turbulent scalar flux obtained in the DNS, the diffusion coefficient Dt determined using the DNS data depends on the mean progress variable. Moreover, Dt is decreased when the front speed is increased, thus, indicating that the front propagation affects turbulent scalar transport even in a constant-density case. Third, conditioned moments of the velocity field differ from counterpart mean moments, thus, disputing the use of conditioned velocity moments for characterizing turbulence when modeling premixed turbulent combustion. Fourth, computed conditioned enstrophies are close to the mean enstrophy in all studied cases, thus, suggesting the use of conditioned enstrophy for characterizing turbulence within a premixed flame brush.
Histogram Comparison via Numerical Simulations
NASA Astrophysics Data System (ADS)
Cardiel, N.
2015-09-01
Although the use of histograms implies loss of information due to the fact that the actual data are replaced by the central values of the considered intervals, these graphical representations are commonly employed in scientific communication, particularly in astrophysics. This work explores the possibility of applying the Anderson-Darling test, a well-known test suitable for the comparison of continuous data sets, to the comparison of data in histogram format. For that purpose the data within each histogram interval are resampled, using the information provided by the frequencies of the adjacent intervals. Several resampling strategies have been examined by the comparison of histograms built from simulated data following a normal distribution.
Numerical simulations of disordered superconductors
Bedell, K.S.; Gubernatis, J.E.; Scalettar, R.T.; Zimanyi, G.T.
1997-12-01
This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The authors carried out Monte Carlo studies of the critical behavior of superfluid {sup 4}He in aerogel. They found the superfluid density exponent increases in the presence of fractal disorder with a value roughly consistent with experimental results. They also addressed the localization of flux lines caused by splayed columnar pins. Using a Sine-Gordon-type of renormalization group study they obtained an analytic form for the critical temperature. They also determined the critical temperature from I-V characteristics obtained from a molecular dynamics simulation. The combined studies enabled one to construct the phase diagram as a function of interaction strength, temperature, and disorder. They also employed the recently developed mapping between boson world-lines and the flux motion to use quantum Monte Carlo simulations to analyze localization in the presence of disorder. From measurements of the transverse flux line wandering, they determined the critical ratio of columnar to point disorder strength needed to localize the bosons.
Direct Numerical Simulation of Elastically Modified Turbulent Taylor-Couette Flow
NASA Astrophysics Data System (ADS)
Liu, Nansheng; Khomami, Bamin
2012-11-01
Direct Numerical Simulations (DNS) of elastically modified turbulent Taylor-Couette (TC) flow are carried out to study the effect of polymer additives on the dynamics of the flow, using a fully spectral method in conjunction with the FENE-P model for the description of polymer chain dynamics. Significant polymer-induced drag increase is observed for the TC flow, which is strikingly different from the findings of drag reduction in the turbulent viscoelastic channel flow. Careful examination of turbulent, viscous and elastic stresses show that the elastically modified wall structures are mainly responsible for the polymer-induced drag increase. In addition, turbulence statistics are analyzed to develop the correlations between the polymer body force and velocity. The probability density functions (PDFs) of the velocity and polymer stress fluctuations are illustrated to reveal the stochastic characteristics of the flow. This work was supported by the NSF grant CBET-0755269 and NSFC grant NO. 10972211.
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.
NASA Astrophysics Data System (ADS)
Baba, Yuya; Kurose, Ryoichi
The detailed behaviors of gaseous and spray lifted flames are studied by two- dimensional direct numerical simulations (DNS), and the characteristics of the flamelets are investigated in terms of two key variables for flamelet modeling, namely mixture fraction and scalar G. The results show that both the gaseous and spray lifted flames are partially premixed flames, in which premixed and diffusion flames co-exist and the premixed flame stabilizing the flames precedes to the diffusion flame. The non-combusting and combusting regions can be generally discriminated by the scalar G, and the premixed and diffusion flames in the combusting region can be predicted by flame index, respectively. Although the flamelets in the diffusion flame of the gaseous lifted flame are characterized by the mixture fraction and scalar dissipation rate, those on the spray lifted flame are not. To account for the flamelet characteristics of the spray lifted flame, flamelet/progress-variable approach needs to be introduced.
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.
NASA Astrophysics Data System (ADS)
Vukoslav?evi?, Petar V.; Wallace, James M.
2013-11-01
Multi-sensor, hot-wire probes of various configurations have been used for 25 years to simultaneously measure the velocity vector and the velocity gradient tensor in turbulent flows. This is the same period in which direct numerical simulations (DNS) were carried out to investigate these flows. Using the first DNS of a turbulent boundary layer, Moin and Spalart ["Contributions of numerical simulation data bases to the physics, modeling and measurement of turbulence," NASA Technical Memorandum 100022 (1987)] examined, virtually, the performance of a two-sensor X-array probe with the sensors idealized as points in the numerical grid. Subsequently, several investigators have used DNS for similar studies. In this paper we use a highly resolved minimal channel flow DNS, following Jiménez and Moin ["The minimal flow unit in near-wall turbulence," J. Fluid Mech. 225, 213 (1991)], to study the performance of an 11-sensor probe. Our previous studies of this type have indicated that, on balance, a probe of the design described here may provide the most accurate measurements of many of the statistics formed from the velocity vector and the velocity gradient tensor (rms and skewness values of the velocity and vorticity components as well as the Reynolds shear stress and the dissipation and production rates). The results of the present study show that, indeed, the sensor and array configurations of a probe of this design are considerably better than previous designs that have been used, and they are likely to give reasonably satisfactory results for such measurements with a real probe in a real bounded flow.
Two- and three-dimensional Direct Numerical Simulation of particle-laden gravity currents
NASA Astrophysics Data System (ADS)
Espath, L. F. R.; Pinto, L. C.; Laizet, S.; Silvestrini, J. H.
2014-02-01
In this numerical study, we are interested in the prediction of a mono-disperse dilute suspension particle-laden flow in the typical lock-exchange configuration. The main originality of this work is that the deposition of particles is taken into account for high Reynolds numbers up to 10 000, similar to the experimental ones. Unprecedented two- and three-dimensional Direct Numerical Simulations (DNS) are undertaken with the objective to investigate the main features of the flow such as the temporal evolution of the front location, the sedimentation rate, the resulting streamwise deposit profiles, the wall shear velocity as well as the complete energy budget calculated without any approximations for the first time. It is found that the Reynolds number can influence the development of the current front. Comparisons between the 2D and 3D simulations for various Reynolds numbers allow us to assess which quantities of interest for the geoscientist could be evaluated quickly with a 2D simulation. We find that a 2D simulation is not able to predict accurately the previously enumerated features obtained in a 3D simulation, with maybe the exception of the sedimentation rate for which a qualitative agreement can be found.
DNS, Enstrophy Balance, and the Dissipation Equation in a Separated Turbulent Channel Flow
NASA Technical Reports Server (NTRS)
Balakumar, Ponnampalam; Rubinstein, Robert; Rumsey, Christopher L.
2013-01-01
The turbulent flows through a plane channel and a channel with a constriction (2-D hill) are numerically simulated using DNS and RANS calculations. The Navier-Stokes equations in the DNS are solved using a higher order kinetic energy preserving central schemes and a fifth order accurate upwind biased WENO scheme for the space discretization. RANS calculations are performed using the NASA code CFL3D with the komega SST two-equation model and a full Reynolds stress model. Using DNS, the magnitudes of different terms that appear in the enstrophy equation are evaluated. The results show that the dissipation and the diffusion terms reach large values at the wall. All the vortex stretching terms have similar magnitudes within the buffer region. Beyond that the triple correlation among the vorticity and strain rate fluctuations becomes the important kinematic term in the enstrophy equation. This term is balanced by the viscous dissipation. In the separated flow, the triple correlation term and the viscous dissipation term peak locally and balance each other near the separated shear layer region. These findings concur with the analysis of Tennekes and Lumley, confirming that the energy transfer terms associated with the small-scale dissipation and the fluctuations of the vortex stretching essentially cancel each other, leaving an equation for the dissipation that is governed by the large-scale motion.
Study of Cardiac Defibrillation Through Numerical Simulations
NASA Astrophysics Data System (ADS)
Bragard, J.; Marin, S.; Cherry, E. M.; Fenton, F. H.
Three-dimensional numerical simulations of the defibrillation problem are presented. In particular, in this study we use the rabbit ventricular geometry as a realistic model system for evaluating the efficacy of defibrillatory shocks. Statistical data obtained from the simulations were analyzed in term of a dose-response curve. Good quantitative agreement between our numerical results and clinically relevant values is obtained. An electric field strength of about 6.6 V/cm indicates a fifty percent probability of successful defibrillation for a 12-ms monophasic shock. Our validated model will be useful for optimizing defibrillation protocols.
Reducing numerical diffusion in magnetospheric simulations
NASA Astrophysics Data System (ADS)
Tóth, Gábor; Meng, Xing; Gombosi, Tamas I.; Ridley, Aaron J.
2011-07-01
Physics-based global magnetosphere modeling requires large computational resources. It is still impractical to resolve the computational domain to the point where numerical errors become negligible. One possible way of reducing numerical diffusion is the “Boris correction”: the semirelativistic magnetohydrodynamics equations are solved with an artificially reduced speed of light. Here we introduce a new alternative approach, an Implicit Scheme with Limited Numerical Dissipation (ISLND). The fully implicit time stepping provides stability, and the wave speeds are limited in the dissipative numerical fluxes only. This limiting only affects the numerical scheme, and it does not modify the equations being solved. This approach can be employed for most total variation diminishing schemes. The differences between the Boris and ISLND schemes are demonstrated in simple numerical tests. We also perform several simulations for two magnetic storms using the global magnetosphere, the ionosphere electrodynamics, and the inner magnetosphere models of the Space Weather Modeling Framework, and we compare the Boris scheme with the limited numerical dissipation method and also with the unmodified base scheme at various grid resolutions. We find that for these particular simulations the Boris scheme and the ISLND scheme produce comparable results, both being significantly less diffusive than the unmodified scheme.
Numerical propulsion system simulation: An interdisciplinary approach
NASA Technical Reports Server (NTRS)
Nichols, Lester D.; Chamis, Christos C.
1991-01-01
The tremendous progress being made in computational engineering and the rapid growth in computing power that is resulting from parallel processing now make it feasible to consider the use of computer simulations to gain insights into the complex interactions in aerospace propulsion systems and to evaluate new concepts early in the design process before a commitment to hardware is made. Described here is a NASA initiative to develop a Numerical Propulsion System Simulation (NPSS) capability.
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 Aerated Powder Consolidation1
properties. Over time the excess air diffuses through the powder and eventually escapes through the top discharge from a hopper of a fine powder at a much greater rate than that of the flow of ordinary granularNumerical Simulation of Aerated Powder Consolidation1 Kristy A. Coffey and Pierre A. Gremaud
Numerical Simulation of Aerated Powder Consolidation 1
properties. Over time the excess air diffuses through the powder and eventually escapes through the top discharge from a hopper of a fine powder at a much greater rate than that of the flow of ordinary granularNumerical Simulation of Aerated Powder Consolidation 1 Kristy A. Coffey and Pierre A. Gremaud
Numerical Simulation of a Convective Turbulence Encounter
NASA Technical Reports Server (NTRS)
Proctor, Fred H.; Hamilton, David W.; Bowles, Roland L.
2002-01-01
A numerical simulation of a convective turbulence event is investigated and compared with observational data. The numerical results show severe turbulence of similar scale and intensity to that encountered during the test flight. This turbulence is associated with buoyant plumes that penetrate the upper-level thunderstorm outflow. The simulated radar reflectivity compares well with that obtained from the aircraft's onboard radar. Resolved scales of motion as small as 50 m are needed in order to accurately diagnose aircraft normal load accelerations. Given this requirement, realistic turbulence fields may be created by merging subgrid-scales of turbulence to a convective-cloud simulation. A hazard algorithm for use with model data sets is demonstrated. The algorithm diagnoses the RMS normal loads from second moments of the vertical velocity field and is independent of aircraft motion.
Numerical simulation pollutant transport with LBM
NASA Astrophysics Data System (ADS)
qiao, yanchao; guo, ziqi
2013-04-01
Numerical simulation is important at pollutants transport. In this research we will use the Lattice Boltzmann Method (LBM) modelling the pollutants transport in Tai Lake in China. The LBM has emerged as a powerful tool for simulating the behaviour of multi-component fluid systems in complex pore networks. We will build a quick response simulation system, which is base on the high resolution GIS figure, using the LBM numerical method. When we get the necessary parameter, we can use the system to predict the pollutants transport in the river and find out the pollutants spatial and temporal distribution in the pollution incidents. This is a powerful tool and method for water quality prediction, management and planning decisions.
Direct Numerical Simulation of a Dry Shear-free Convective Boundary Layer
NASA Astrophysics Data System (ADS)
Garcia, J. R.; Mellado, J. P.
2012-04-01
Due to the thinness of the inversion layer, entrainment in the Convective Boundary Layer (CBL) is not explicitly resolved in models and is still a major source of uncertainty. Recent work using Large Eddy Simulations (LES) shows lack of convergence in the inversion layer with further grid refinement, even for a vertical resolution of 2 meters. Observational studies of entrainment in the CBL are even more problematic, whether they be field observations or their low Reynolds number analogs in the laboratory, since fine measurements of the three-dimensional flow field at the inversion layer are practically unattainable. As an alternative, we use Direct Numerical Simulations (DNS), which resolves the three-dimensional flow field down to the scale of molecular diffusion. Faithful representation of the whole range of turbulent scales would mean that attainable Reynolds numbers are orders of magnitude lower than that in the atmosphere because of limited computational resources. However, the significant increase in computing power now allows for simulations that are comparable in size to tank experiments. Furthermore, we can invoke Reynolds number similarity to justify the use of DNS to study an idealized convective boundary layer. As a first step, we consider here the dry, shear-free case with constant surface buoyancy flux B0 working against a stable background stratification with constant buoyancy frequency N. Fixing the Prandlt number Pr = ?/? to 1, where ? is the molecular kinematic viscosity and ? is the molecular diffusivity, the problem is characterized by a single non-dimensional parameter (B0/?)/N2 which can be interpreted as the ratio between a reference well-mixed layer height and the diffusive layer thickness. In the atmosphere, (B0/?)/N2 is at least O(106), while for our first simulation, (B0/?)/N2 ~ 40. We have done one simulation with a 1024x1024x541 grid that uses vertical grid stretching, and another that is twice as wide (2048x2048x541) for assessing statistical convergence and the effect of the computational domain size. Even with vertical grid stretching, the grid spacing is smaller than the Kolmogorov length scale. Despite the low Reynolds number, we obtain qualitatively comparable vertical structure as in LES and observations. Relative values
Characterizing Dark DNS Behavior Jon Oberheide1
Mao, Zhuoqing Morley
these addresses. In this paper, we introduce the concept of dark DNS, the DNS queries associated with darknet DNS authority when deploying darknet sensors to prevent attackers from easily evading monitored darknets. Finally, we present honeydns, a tool that complements existing network sensors and low
Direct Numerical Simulations of Transitional/Turbulent Wakes
NASA Technical Reports Server (NTRS)
Rai, Man Mohan
2011-01-01
The interest in transitional/turbulent wakes spans the spectrum from an intellectual pursuit to understand the complex underlying physics to a critical need in aeronautical engineering and other disciplines to predict component/system performance and reliability. Cylinder wakes have been studied extensively over several decades to gain a better understanding of the basic flow phenomena that are encountered in such flows. Experimental, computational and theoretical means have been employed in this effort. While much has been accomplished there are many important issues that need to be resolved. The physics of the very near wake of the cylinder (less than three diameters downstream) is perhaps the most challenging of them all. This region comprises the two detached shear layers, the recirculation region and wake flow. The interaction amongst these three components is to some extent still a matter of conjecture. Experimental techniques have generated a large percentage of the data that have provided us with the current state of understanding of the subject. More recently computational techniques have been used to simulate cylinder wakes, and the data from such simulations are being used to both refine our understanding of such flows as well as provide new insights. A few large eddy and direct numerical simulations (LES and DNS) of cylinder wakes have appeared in the literature in the recent past. These investigations focus on the low Reynolds number range where the cylinder boundary layer is laminar (sub-critical range). However, from an engineering point of view, there is considerable interest in the situation where the upper and/or lower boundary layer of an airfoil is turbulent, and these turbulent boundary layers separate from the airfoil to contribute to the formation of the wake downstream. In the case of cylinders, this only occurs at relatively large unit Reynolds numbers. However, in the case of airfoils, the boundary layer has the opportunity to transition to turbulence on the airfoil surface at a relatively lower unit Reynolds number because the characteristic length of the airfoil is typically one to two orders of magnitude larger than the trailing edge diameter. This transition to turbulence would occur unless there is a strong favorable pressure gradient that results in the boundary layer remaining laminar or transitional over the surface of the airfoil. This presentation will focus on two direct numerical simulations that have been performed at NASA ARC. The first is of a cylinder wake with laminar separating boundary layers. The second is the wake of a flat plate with a circular trailing edge. The upper and lower plate surface boundary layers are both turbulent and statistically identical. Thus the computed wake is symmetric in a statistical sense. This flow is more representative of airfoil wakes than cylinder wakes. Results from the two simulations including flow visualization and turbulence statistics in the near wake will be presented at the seminar.
Numerical simulation of spacecraft charging phenomena
NASA Technical Reports Server (NTRS)
Laframboise, J. G.; Prokopenko, S. M. L.
1977-01-01
A numerical simulation program is being constructed having the following features: (1) infinite circular cylindrical geometry with angle-dependence, (2) inclusion of incident particles, photoelectrons, secondary electrons, backscattered electrons, any gun emissions, and any internal current pathways including surface conductive layers, (3) quasistatic time-dependent iteration, in which sheath potential changes during particle transit times are ignored, (4) use of approximate, locally-dependent space charge density expressions in solving Poisson's equation for sheath potentials, with use of numerical orbit-following to determine surface currents, and (5) incident particle velocity distributions isotropic or beam-like, or some superposition of these. Rationales for each of these features are discussed.
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.
Direct numerical simulation of turbulent mixing in grid-generated turbulence
NASA Astrophysics Data System (ADS)
Nagata, Kouji; Suzuki, Hiroki; Sakai, Yasuhiko; Hayase, Toshiyuki; Kubo, Takashi
2008-12-01
Turbulent mixing of passive scalar (heat) in grid-generated turbulence (GGT) is simulated by means of direct numerical simulation (DNS). A turbulence-generating grid, on which the velocity components are set to zero, is located downstream of the channel entrance, and it is numerically constructed on the staggered mesh arrangement using the immersed boundary method. The grid types constructed are: (a) square-mesh biplane grid, (b) square-mesh single-plane grid, (c) composite grid consisting of parallel square-bars and (d) fractal grid. Two fluids with different temperatures are provided separately in the upper and lower streams upstream of the turbulence-generating grids, generating the thermal mixing layer behind the grids. For the grid (a), simulations for two different Prandtl numbers of 0.71 and 7.1, corresponding to air and water flows, are conducted to investigate the effect of the Prandtl number. The results show that the typical grid turbulence and shearless mixing layer are generated downstream of the grids. The results of the scalar field show that a typical thermal mixing layer is generated as well, and the effects of the Prandtl numbers on turbulent heat transfer are observed.
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.
Issues in Numerical Simulation of Fire Suppression
Tieszen, S.R.; Lopez, A.R.
1999-04-12
This paper outlines general physical and computational issues associated with performing numerical simulation of fire suppression. Fire suppression encompasses a broad range of chemistry and physics over a large range of time and length scales. The authors discuss the dominant physical/chemical processes important to fire suppression that must be captured by a fire suppression model to be of engineering usefulness. First-principles solutions are not possible due to computational limitations, even with the new generation of tera-flop computers. A basic strategy combining computational fluid dynamics (CFD) simulation techniques with sub-grid model approximations for processes that have length scales unresolvable by gridding is presented.
Numerical Simulation in a Supercirtical CFB Boiler
NASA Astrophysics Data System (ADS)
Zhang, Yanjun; Gaol, Xiang; Luo, Zhongyang; Jiang, Xiaoguo
The dimension of the hot circulation loop of the supercritical CFB boiler is large, and there are many unknowns and challenges that should be identified and resolved during the development. In order to realize a reasonable and reliable design of the hot circulation loop, numerical simulation of gas-solid flow in a supercritical CFB boiler was conducted by using FLUENT software. The working condition of hot circulation loop flow field, gas-solid flow affected by three unsymmetrical cyclones, air distribution and pressure drop in furnace were analyzed. The simulation results showed that the general arrangement of the 600MWe supercritical CFB boiler is reasonable.
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.
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.
Numerical simulation of orbiting black holes.
Brügmann, Bernd; Tichy, Wolfgang; Jansen, Nina
2004-05-28
We present numerical simulations of binary black hole systems which for the first time last for about one orbital period for close but still separate black holes as indicated by the absence of a common apparent horizon. An important part of the method is the construction of comoving coordinates, in which both the angular and the radial motion are minimized through a dynamically adjusted shift condition. We use fixed mesh refinement for computational efficiency. PMID:15245270
Direct numerical simulation of a recorder.
Giordano, N
2013-02-01
The aeroacoustics of a recorder are studied using a direct numerical simulation based on the Navier-Stokes equations in two dimensions. Spatial maps for the air pressure and velocity give a detailed picture of vortex shedding near the labium. Changes in the spectrum as a result of variations in the blowing speed are also investigated. The results are in good semi-quantitative agreement with general results for these phenomena from experiments. PMID:23363126
NASA Astrophysics Data System (ADS)
Li, Feng-Chen; Cai, Wei-Hua; Zhang, Hong-Na; Wang, Yue
2012-11-01
Direct numerical simulations (DNS) were performed for the forced homogeneous isotropic turbulence (FHIT) with/without polymer additives in order to elaborate the characteristics of the turbulent energy cascading influenced by drag-reducing effects. The finite elastic non-linear extensibility-Peterlin model (FENE-P) was used as the conformation tensor equation for the viscoelastic polymer solution. Detailed analyses of DNS data were carried out in this paper for the turbulence scaling law and the topological dynamics of FHIT as well as the important turbulent parameters, including turbulent kinetic energy spectra, enstrophy and strain, velocity structure function, small-scale intermittency, etc. A natural and straightforward definition for the drag reduction rate was also proposed for the drag-reducing FHIT based on the decrease degree of the turbulent kinetic energy. It was found that the turbulent energy cascading in the FHIT was greatly modified by the drag-reducing polymer additives. The enstrophy and the strain fields in the FHIT of the polymer solution were remarkably weakened as compared with their Newtonian counterparts. The small-scale vortices and the small-scale intermittency were all inhibited by the viscoelastic effects in the FHIT of the polymer solution. However, the scaling law in a fashion of extended self-similarity for the FHIT of the polymer solution, within the presently simulated range of Weissenberg numbers, had no distinct differences compared with that of the Newtonian fluid case.
2001 Numerical Propulsion System Simulation Review
NASA Technical Reports Server (NTRS)
Lytle, John; Follen, Gregory; Naiman, Cynthia; Veres, Joseph; Owen, Karl; Lopez, Isaac
2002-01-01
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. Major accomplishments include the first formal release of the NPSS object-oriented architecture (NPSS Version 1) and the demonstration of a one order of magnitude reduction in computing cost-to-performance ratio using a cluster of personal computers. The paper also describes the future NPSS milestones, which include the simulation of space transportation propulsion systems in response to increased emphasis on safe, low cost access to space within NASA's Aerospace Technology Enterprise. In addition, the paper contains a summary of the feedback received from industry partners on the fiscal year 2000 effort and the actions taken over the past year to respond to that feedback. NPSS was supported in fiscal year 2001 by the High Performance Computing and Communications Program.
2000 Numerical Propulsion System Simulation Review
NASA Technical Reports Server (NTRS)
Lytle, John; Follen, Greg; Naiman, Cynthia; Veres, Joseph; Owen, Karl; Lopez, Isaac
2001-01-01
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. Major accomplishments include the first formal release of the NPSS object-oriented architecture (NPSS Version 1) and the demonstration of a one order of magnitude reduction in computing cost-to-performance ratio using a cluster of personal computers. The paper also describes the future NPSS milestones, which include the simulation of space transportation propulsion systems in response to increased emphasis on safe, low cost access to space within NASA'S Aerospace Technology Enterprise. In addition, the paper contains a summary of the feedback received from industry partners on the fiscal year 1999 effort and the actions taken over the past year to respond to that feedback. NPSS was supported in fiscal year 2000 by the High Performance Computing and Communications Program.
NASA Astrophysics Data System (ADS)
Yang, Juan-Cheng; Li, Feng-Chen; Cai, Wei-Hua; Zhang, Hong-Na; Yu, Bo
2015-08-01
Our previous experimental studies have confirmed that viscoelastic-fluid-based nanofluid (VFBN) prepared by suspending nanoparticles in a viscoelastic base fluid (VBF, behaves drag reduction at turbulent flow state) can reduce turbulent flow resistance as compared with water and enhance heat transfer as compared with VBF. Direct numerical simulation (DNS) is performed in this study to explore the mechanisms of heat transfer enhancement (HTE) and flow drag reduction (DR) for the VFBN turbulent flow. The Giesekus model is used as the constitutive equation for VFBN. Our previously proposed thermal dispersion model is adopted to take into account the thermal dispersion effects of nanoparticles in the VFBN turbulent flow. The DNS results show similar behaviors for flow resistance and heat transfer to those obtained in our previous experiments. Detailed analyses are conducted for the turbulent velocity, temperature, and conformation fields obtained by DNSs for different fluid cases, and for the friction factor with viscous, turbulent, and elastic contributions and heat transfer rate with conductive, turbulent and thermal dispersion contributions of nanoparticles, respectively. The mechanisms of HTE and DR of VFBN turbulent flows are then discussed. Based on analogy theory, the ratios of Chilton-Colburn factor to friction factor for different fluid flow cases are investigated, which from another aspect show the significant enhancement in heat transfer performance for some cases of water-based nanofluid and VFBN turbulent flows. Project supported by the National Natural Science Foundation of China (Grant No. 51276046), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20112302110020), the China Postdoctoral Science Foundation (Grant No. 2014M561037), and the President Fund of University of Chinese Academy of Sciences, China (Grant No. Y3510213N00).
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.
Does the choice of the forcing term affect flow statistics in DNS of turbulent channel flow?
Quadrio, Maurizio; Hasegawa, Yosuke
2015-01-01
We seek possible statistical consequences of the way a forcing term is added to the Navier--Stokes equations in the Direct Numerical Simulation (DNS) of incompressible channel flow. Simulations driven by constant flow rate, constant pressure gradient and constant power input are used to build large databases, and in particular to store the complete temporal trace of the wall-shear stress for later analysis. As these approaches correspond to different dynamical systems, it can in principle be envisaged that these differences are reflect by certain statistics of the turbulent flow field. The instantaneous realizations of the flow in the various simulations are obviously different, but, as expected, the usual one-point, one-time statistics do not show any appreciable difference. However, the PDF for the fluctuations of the streamwise component of wall friction reveals that the simulation with constant flow rate presents lower probabilities for extreme events of large positive friction. The low probability value ...
Numerical recipes for mold filling simulation
Kothe, D.; Juric, D.; Lam, K.; Lally, B.
1998-07-01
Has the ability to simulate the filling of a mold progressed to a point where an appropriate numerical recipe achieves the desired results? If results are defined to be topological robustness, computational efficiency, quantitative accuracy, and predictability, all within a computational domain that faithfully represents complex three-dimensional foundry molds, then the answer unfortunately remains no. Significant interfacial flow algorithm developments have occurred over the last decade, however, that could bring this answer closer to maybe. These developments have been both evolutionary and revolutionary, will continue to transpire for the near future. Might they become useful numerical recipes for mold filling simulations? Quite possibly. Recent progress in algorithms for interface kinematics and dynamics, linear solution methods, computer science issues such as parallelization and object-oriented programming, high resolution Navier-Stokes (NS) solution methods, and unstructured mesh techniques, must all be pursued as possible paths toward higher fidelity mold filling simulations. A detailed exposition of these algorithmic developments is beyond the scope of this paper, hence the authors choose to focus here exclusively on algorithms for interface kinematics. These interface tracking algorithms are designed to model the movement of interfaces relative to a reference frame such as a fixed mesh. Current interface tracking algorithm choices are numerous, so is any one best suited for mold filling simulation? Although a clear winner is not (yet) apparent, pros and cons are given in the following brief, critical review. Highlighted are those outstanding interface tracking algorithm issues the authors feel can hamper the reliable modeling of today`s foundry mold filling processes.
Numerical simulation of real-world flows
NASA Astrophysics Data System (ADS)
Hayase, Toshiyuki
2015-10-01
Obtaining real flow information is important in various fields, but is a difficult issue because measurement data are usually limited in time and space, and computational results usually do not represent the exact state of real flows. Problems inherent in the realization of numerical simulation of real-world flows include the difficulty in representing exact initial and boundary conditions and the difficulty in representing unstable flow characteristics. This article reviews studies dealing with these problems. First, an overview of basic flow measurement methodologies and measurement data interpolation/approximation techniques is presented. Then, studies on methods of integrating numerical simulation and measurement, namely, four-dimensional variational data assimilation (4D-Var), Kalman filters (KFs), state observers, etc are discussed. The first problem is properly solved by these integration methodologies. The second problem can be partially solved with 4D-Var in which only initial and boundary conditions are control parameters. If an appropriate control parameter capable of modifying the dynamical structure of the model is included in the formulation of 4D-Var, unstable modes are properly suppressed and the second problem is solved. The state observer and KFs also solve the second problem by modifying mathematical models to stabilize the unstable modes of the original dynamical system by applying feedback signals. These integration methodologies are now applied in simulation of real-world flows in a wide variety of research fields. Examples are presented for basic fluid dynamics and applications in meteorology, aerospace, medicine, etc.
DNS of Viscoelastic Turbulent Channel Flow at High Drag Reduction
NASA Astrophysics Data System (ADS)
Beris, Antony; Housiadas, Kostas; Wang, Luo
2006-03-01
A new method has been developed to enable Direct Numerical Simulations (DNS) of viscoelastic turbulent channel flow with high accuracy spectral methods at high values of drag reduction (HDR), when the polymer molecules undergo high extensional deformation. To faithfully represent that we have expressed the conformation tensor, c, as the exponential of another tensor a, c=exp(a) and we solve for a instead of c. Thus, by construction, the positive definite property of c is always preserved. In addition, a stabilizing artificial diffusion has been added to the viscoelastic constitutive model and efficiently implemented numerically using a multigrid method. The Finite-Elasticity Non-Linear Elastic Dumbbell model with the Peterlin approximation (FENE-P) is then used to represent the effect of polymer molecules in solution. To achieve HDR we used high values of the key model parameters: (a) the maximum extensional viscosity, which for the FENE-P constitutive model is proportional to the quantity (1-?)*L?2, where ? is the solvent viscosity ratio and L is the maximum extensibility parameter and (b) the friction Weissenberg number, We?.
Numerical simulation of cross-country skiing.
Carlsson, Peter; Tinnsten, Mats; Ainegren, Mats
2011-08-01
A program for numerical simulation of a whole ski race, from start to finish, is developed in MATLAB. The track is modelled by a set of cubical splines in two dimensions and can be used to simulate a track in a closed loop or with the start and finish at different locations. The forces considered in the simulations are gravitational force, normal force between snow and skis, drag force from the wind, frictional force between snow and ski and driving force from the skier. The differential equations of motion are solved from start to finish with the Runge-Kutta method. Different wind situations during the race can be modelled, as well as different glide conditions on different parts of the track. It is also possible to vary the available power during the race. The simulation program's output is the total time of the race, together with the forces and speed during different parts of the race and intermediate times at selected points. Some preliminary simulations are also presented. PMID:21607888
Numerical simulation of platelet margination in microcirculation
NASA Astrophysics Data System (ADS)
Zhao, Hong; Shaqfeh, Eric
2009-11-01
The adhesion of platelets to vascular walls is the first step in clotting. This process critically depends on the preferential concentration of platelets near walls. The presence of red blood cells, which are the predominant blood constituents, is known to affect the steady state platelet concentration and the dynamic platelet margination, but the underlying mechanism is not well understood to-day. We use a direct numerical simulation to study the platelet margination process, with particular emphasis on the Stokesian hydrodynamic interactions among red cells, platelets, and vessel walls. Well-known mechanical models are used for the shearing and bending stiffness of red cell membranes, and the stiffer platelets are modeled as rigid discoids. A boundary integral formulation is used to solve the flow field, where the numerical solution procedure is accelerated by a parallel O(N N) smooth particle-mesh Ewald method. The effects of red cell hematocrit and deformability will be discussed.
Turbulent convection: comparison of Reynolds stress models with numerical simulations
Meyer-Vernet, Nicole
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 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 simulations unravel the cosmic web.
Faucher-Giguère, Claude-André; Lidz, Adam; Hernquist, Lars
2008-01-01
The universe is permeated by a network of filaments, sheets, and knots collectively forming a "cosmic web." The discovery of the cosmic web, especially through its signature of absorption of light from distant sources by neutral hydrogen in the intervening intergalactic medium, exemplifies the interplay between theory and experiment that drives science and is one of the great examples in which numerical simulations have played a key and decisive role. We recount the milestones in our understanding of cosmic structure; summarize its impact on astronomy, cosmology, and physics; and look ahead by outlining the challenges faced as we prepare to probe the cosmic web at new wavelengths. PMID:18174431
NASA Astrophysics Data System (ADS)
Tryggvason, Gretar; Esmaeeli, Asghar; Biswas, Souvik
2004-11-01
Recent stuies of bubbly flows, using direct numerical simulations, are discussed. The goal of this study is to examine the collective behavior of many bubbles as the rise Reynolds number is increased and and a single bubble rises unsteadily, as well as to examine the motion of bubbles in channels. A front-tracking/finite volume method is used to fully resolve all flow scales, including the bubbles and the flow around them. Two cases are simulated, for one the bubbles remain nearly spherical and for the other case the bubbles are deformable and wobble. The wobbly bubbles remains relatively uniformly distributed and are not susceptible to the streaming instability found by Bunner and Tryggvason (2003) for deformable bubbles at lower rise Reynolds numbers. The more spherical bubbles, on the other hand, form transients ``rafts'' somewhat similar to those seen in potential flow simulation of many bubbles. For channel flow we compare results from direct numerical simulations of bubbly flow with prediction of the steady-state two-fluid model of Antal, Lahey, and Flaherty (1991). The simulations are done assuming a two-dimensional system and the model coefficients are adjusted slightly to match the data for upflow. The results generally agree reasonably well, even though the simulated void fraction is considerably higher than the one assumed in the derivation of the model. Research supported by DOE.
DNS of Laminar-Turbulent Transition in Swept-Wing Boundary Layers
NASA Technical Reports Server (NTRS)
Duan, L.; Choudhari, M.; Li, F.
2014-01-01
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 and the mode selected for forcing corresponds to the most amplified secondary instability mode that, in this case, derives a majority of its growth from energy production mechanisms associated with the wall-normal shear of the stationary basic state. An inlet boundary condition is carefully designed to allow for accurate injection of instability wave modes and minimize acoustic reflections at numerical boundaries. Nonlinear parabolized stability equation (PSE) predictions compare well with the DNS in terms of modal amplitudes and modal shape during the strongly nonlinear phase of the secondary instability mode. During the transition process, the skin friction coefficient rises rather rapidly and the wall-shear distribution shows a sawtooth pattern that is analogous to the previously documented surface flow visualizations of transition due to stationary crossflow instability. Fully turbulent features are observed in the downstream region of the flow.
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 rarefied nozzle plume impingements
NASA Astrophysics Data System (ADS)
Hyakutake, Toru; Nishida, Michio
2001-08-01
This paper describes numerical simulation of rarefied nozzle plume impingements. Two different reservoir pressures 400 kPa are considered. In the case of 400 kPa, the simulation of the nozzle flow was conducted by using the Navier-Stokes equation, and then the analysis of the plume flow was carried out by the DSMC method, employing the nozzle exit conditions obtained by Navier-Stokes equation. On the other hand, for 4 kPa, both the nozzle flow and the plume impingement have been calculated using the DSMC method. Concerning the angle between the nozzle axis and the flat plate, three kinds of angle are selected, that is, 90°, 45° and 0°. In addition, we considered the case where there exists a flat plate behind the nozzle. Simulated results have been compared with the existing experiments for the pressure and shear stress distributions on the flat plate. A good agreement between the DSMC results and the experiments are shown. In the case of the oblique and parallel impingements, the location of the impingement pressure peak and the stagnation point shifted upstream with increasing rarefaction.
Numerical Simulation of an Idealized Thermoacoustic Engine
NASA Astrophysics Data System (ADS)
Worlikar, A.; Knio, O.; Klein, R.
1996-11-01
In its simplest form, a thermoacoustic device consists of two main components: a resonance tube where the flow is characterized by lengthscales of the order of the acoustic wavelength, and a stack of plates which are separated by distances much smaller than the acoustic wavelength. This paper discusses the development and implementation of a numerical simulation scheme which efficiently overcomes the scale disparity of the problem. The approach relies on a combination of simplified analytical expressions and multi-dimensional flow simulations of vortical flow dynamics in the neighborhood of the stack in order to construct a quasi-one-dimensional representation of the effects of the stack. The stack model is then embedded into a low-Mach-number simulation scheme which is used to compute weakly-nonlinear acoustics within the resonance tube. The low-Mach-number scheme is based on a multiple-pressure-variable approach which enables us, in an accurate and efficient fashion, to overcome an otherwise stiff CFL restriction. The resulting model is used to analyze the response of thermally-driven pressure oscillations to the configuration of the thermoacoustic stack.
Numerical Simulations of High Enthalpy Pulse Facilities
NASA Technical Reports Server (NTRS)
Wilson, Gregory J.; Edwards, Thomas A. (Technical Monitor)
1995-01-01
Axisymmetric flows within shock tubes and expansion tubes are simulated including the effects of finite rate chemistry and both laminar and turbulent boundary layers. The simulations demonstrate the usefulness of computational fluid dynamics for characterizing the flows in high enthalpy pulse facilities. The modeling and numerical requirements necessary to simulate these flows accurately are also discussed. Although there is a large body of analysis which explains and quantifies the boundary layer growth between the shock and the interface in a shock tube, there is a need for more detailed solutions. Phenomena such as thermochemical nonequilibrium. or turbulent transition behind the shock are excluded in the assumptions of Mirels' analysis. Additionally there is inadequate capability to predict the influence of the boundary layer on the expanded gas behind the interface. Quantifying the gas in this region is particularly important in expansion tubes because it is the location of the test gas. Unsteady simulations of the viscous flow in shock tubes are computationally expensive because they must follow features such as a shock wave over the length of the facility and simultaneously resolve the small length scales within the boundary layer. As a result, efficient numerical algorithms are required. The numerical approach of the present work is to solve the axisymmetric gas dynamic equations using an finite-volume formulation where the inviscid fluxes are computed with a upwind TVD scheme. Multiple species equations are included in the formulation so that finite-rate chemistry can be modeled. The simulations cluster grid points at the shock and interface and translate this clustered grid with these features to minimize numerical errors. The solutions are advanced at a CFL number of less than one based on the inviscid gas dynamics. To avoid limitations on the time step due to the viscous terms, these terms are treated implicitly. This requires a block tri-diagonal matrix inversion along each line of cells normal to the wall. The cost of this inversion is more than offset by the larger allowable time step. The source terms representing the finite-rate chemical kinetics are also treated implicitly. An algebraic turbulence model for compressible flow is used. The flow in a low pressure shock tube is computed and the results are compared with Mirels'analysis. The driven gas is nitrogen at 70 Pa, and the incident shock speed is approximately 2.9 km/sec so that there is little dissociation. The simulations include a laminar boundary layer and are run until the limiting flow regime is achieved. At this limit, the shock and interface travel at the same velocity because the amount of driven gas between these two features remains the same: the mass flow across the shock is equal to the mass of gas being entrained at the interface by the boundary layer. Simulations with several grids are presented to establish the grid independence of the solution, Good agreement is achieved between Mirels' correlations and the computations. This is expected since the flow conditions are chosen to be consistent with the assumptions used in Mirels' analysis. This comparison adds credibility to the numerical approach and highlights some of the differences between the theory and the detailed simulations. In addition, simulations of the HYPULSE expansion tube are presented for two operating conditions and the computations are compared to experimental data. The operating gas for both cases is nitrogen. One test condition is at a total enthalpy of 15.2 MJ/Kg and a relatively low pressure of 2 kPa. This case is characterized by a laminar boundary layer and significant chemical nonequilibrium. in the acceleration gas. The second test condition is at a total enthalpy of 10.2 MJ/Kg and a pressure of 38 kPa and is characterized by a turbulent boundary layer. The simulations compare well with experiment and reveal that the nonuniformity in pressure observed during the test time is related to variations in the boundary layer displacement thickness.
Huson, Peter N.
2012-01-01
Blast Simulator Facility 2.4 Numerical Simulation . . . . .the Blast Simulator tests. Next, the numerical simulation ofblast loadings. With no observed delamination of the overlaminates, these simulations
Numerical simulations of giant planetary core formation
NASA Astrophysics Data System (ADS)
Ngo, Henry Hoang Khoi
In the widely accepted core accretion model of planet formation, small rocky and/or icy bodies (planetesimals) accrete to form protoplanetary cores. Gas giant planets are believed to have solid cores that must reach a critical mass, ˜10 Earth masses (M?), after which there is rapid inflow of gas from the gas disk. In order to accrete the gas giants' massive atmospheres, this step must occur within the gas disk's lifetime (1 -- 10 million years). Numerical simulations of solid body accretion in the outer Solar System are performed using two integrators. The goal of these simulations is to investigate the effects of important dynamical processes instead of specifically recreating the formation of the Solar System's giant planets. The first integrator uses the Symplectic Massive Body Algorithm (SyMBA) with a modification to allow for planetesimal fragmentation. Due to computational constraints, this code has some physical limitations, specifically that the planetesimals themselves cannot grow, so protoplanets must be seeded in the simulations. The second integrator, the Lagrangian Integrator for Planetary Accretion and Dynamics (LIPAD), is more computationally expensive. However, its treatment of planetesimals allows for growth of potential giant planetary cores from a disk consisting only of planetesimals. Thus, this thesis' preliminary simulations use the first integrator to explore a wider range of parameters while the main simulations use LIPAD to further investigate some specific processes. These simulations are the first use of LIPAD to study giant planet formation and they identify a few important dynamical processes affecting core formation. Without any fragmentation, cores tend to grow to ˜2M ?. When planetesimal fragmentation is included, the resulting fragments are easier to accrete and larger cores are formed (˜4 M?). But, in half of the runs, the fragments force the entire system to migrate towards the Sun. In other half, outward migration via scattering off a large number of planetesimal helps the protoplanets grow and survive. However, in a preliminary set of simulations including protoplanetary fragmentation, very few collisions are found to result in accretion so it is difficult for any cores to form.
Numerical simulations of two-phase Taylor-Couette turbulence using an Euler-Lagrange approach
Spandan, Vamsi; Verzicco, Roberto; Lohse, Detlef
2015-01-01
Two-phase turbulent Taylor-Couette (TC) flow is simulated using an Euler-Lagrange approach to study the effects of a secondary phase dispersed into a turbulent carrier phase (here bubbles dispersed into water). The dynamics of the carrier phase is computed using Direct Numerical Simulations (DNS) in an Eulerian framework, while the bubbles are tracked in a Lagrangian manner by modelling the effective drag, lift, added mass and buoyancy force acting on them. Two-way coupling is implemented between the dispersed phase and the carrier phase which allows for momentum exchange among both phases and to study the effect of the dispersed phase on the carrier phase dynamics. The radius ratio of the TC setup is fixed to $\\eta=0.833$, and a maximum inner cylinder Reynolds number of $Re_i=8000$ is reached. We vary the Froude number ($Fr$), which is the ratio of the centripetal to the gravitational acceleration of the dispersed phase and study its effect on the net torque required to drive the TC system. In a two-phase TC...
Computing abstraction hierarchies by numerical simulation
Bundy, A.; Giunchiglia, F.; Sebastiani, R.; Walsh, T.
1996-12-31
We present a novel method for building ABSTRIPS-style abstraction hierarchies in planning. The aim of this method is to minimize the amount of backtracking between abstraction levels. Previous approaches have determined the criticality of operator preconditions by reasoning about plans directly. Here, we adopt a simpler and faster approach where we use numerical simulation of the planning process. We demonstrate the theoretical advantages of our approach by identifying some simple properties lacking in previous approaches but possessed by our method. We demonstrate the empirical advantages of our approach by a set of four benchmark experiments using the ABTWEAK system. We compare the quality of the abstraction hierarchies generated with those built by the ALPINE and HIGHPOINT algorithms.
Numerical simulation of sudden stratospheric warmings
NASA Technical Reports Server (NTRS)
Schoeberl, M. R.; Strobel, D. F.
1980-01-01
The paper presents a mechanistic, quasi-geostrophic, semi-spectral model with a self-consistent calculation of the mean zonal flow fields used to numerically simulate sudden stratospheric warmings generated by a single zonal harmonic (m) planetary wave. The development of a warming depends on the strength of the westerly winds in the lower stratosphere, and the growth rate of a m=2 warming is accelerated during the westerly zonal wind phase of the quasi-biennial oscillation. An m=1 warming is characterized by oscillation of wave amplitude and mean flow from resonantly trapped, westward, propagating planetary waves moving in and out of phase with the tropospherically forced stationary planetary wave. An m=2 critical level develops in the equatorial region and advances poleward, and its warming is characterized by a sudden intensification after an initially slow growth in contrast to slowly developing m=1 warmings.
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 aerodynamic simulation facility feasibility study
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. Facets of the work described include the hardware configuration, software, user language, and fault tolerance.
Numerical simulation and modelling of experimental dynamos
NASA Astrophysics Data System (ADS)
Christophe, G.; Emmanuel, D.; Stephan, F.
2008-12-01
The recent success of the VKS dynamo provides a new way to investigate dynamo action in turbulent conducting flows. We present results of 3D direct numerical simulations for different flow geometries used to try to generate experimental dynamos. In the case of the VKS experiment, we show how boundaries with a high magnetic permeability lead to a significant decrease of the critical magnetic Reynolds number, thus allowing the observation of dynamo action. We also understand the mechanism leading to experimentally observed geometries of the magnetic field without using any ad hoc mean field equation. In the case of the geometry of the Madison experiment, we show different mechanisms by which Cowling theorem can be by-passed, thus leading to a mean magnetic field with a strong axial dipolar component instead of the predicted equatorial dipole. This competition between axial and equatorial dipoles could also account for secular variations of the Earth magnetic field.
Numerical simulation of three dimensional transonic flows
NASA Technical Reports Server (NTRS)
Sahu, Jubaraj; Steger, Joseph L.
1987-01-01
The three-dimensional flow over a projectile has been computed using an implicit, approximately factored, partially flux-split algorithm. A simple composite grid scheme has been developed in which a single grid is partitioned into a series of smaller grids for applications which require an external large memory device such as the SSD of the CRAY X-MP/48, or multitasking. The accuracy and stability of the composite grid scheme has been tested by numerically simulating the flow over an ellipsoid at angle of attack and comparing the solution with a single grid solution. The flowfield over a projectile at M = 0.96 and 4 deg angle-of-attack has been computed using a fine grid, and compared with experiment.
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.
Direct Numerical Simulation of Automobile Cavity Tones
NASA Technical Reports Server (NTRS)
Kurbatskii, Konstantin; Tam, Christopher K. W.
2000-01-01
The Navier Stokes equation is solved computationally by the Dispersion-Relation-Preserving (DRP) scheme for the flow and acoustic fields associated with a laminar boundary layer flow over an automobile door cavity. In this work, the flow Reynolds number is restricted to R(sub delta*) < 3400; the range of Reynolds number for which laminar flow may be maintained. This investigation focuses on two aspects of the problem, namely, the effect of boundary layer thickness on the cavity tone frequency and intensity and the effect of the size of the computation domain on the accuracy of the numerical simulation. It is found that the tone frequency decreases with an increase in boundary layer thickness. When the boundary layer is thicker than a certain critical value, depending on the flow speed, no tone is emitted by the cavity. Computationally, solutions of aeroacoustics problems are known to be sensitive to the size of the computation domain. Numerical experiments indicate that the use of a small domain could result in normal mode type acoustic oscillations in the entire computation domain leading to an increase in tone frequency and intensity. When the computation domain is expanded so that the boundaries are at least one wavelength away from the noise source, the computed tone frequency and intensity are found to be computation domain size independent.
NASA Astrophysics Data System (ADS)
Sullivan, Peter P.; McWilliams, James C.; Melville, W. Kendall
2004-05-01
We devise a stochastic model for the effects of breaking waves and fit its distribution functions to laboratory and field data. This is used to represent the space time structure of momentum and energy forcing of the oceanic boundary layer in turbulence-resolving simulations. The aptness of this breaker model is evaluated in a direct numerical simulation (DNS) of an otherwise quiescent fluid driven by an isolated breaking wave, and the results are in good agreement with laboratory measurements. The breaker model faithfully reproduces the bulk features of a breaking event: the mean kinetic energy decays at a rate approaching t(-1) , and a long-lived vortex (eddy) is generated close to the water surface. The long lifetime of this vortex (more than 50 wave periods) makes it effective in energizing the surface region of oceanic boundary layers. Next, a comparison of several different DNS of idealized oceanic boundary layers driven by different surface forcing (i.e. constant current (as in Couette flow), constant stress, or a mixture of constant stress plus stochastic breakers) elucidates the importance of intermittent stress transmission to the underlying currents. A small amount of active breaking, about 1.6% of the total water surface area at any instant in time, significantly alters the instantaneous flow patterns as well as the ensemble statistics. Near the water surface a vigorous downwelling upwelling pattern develops at the head and tail of each three-dimensional breaker. This enhances the vertical velocity variance and generates both negative- and positive-signed vertical momentum flux. Analysis of the mean velocity and scalar profiles shows that breaking effectively increases the surface roughness z_o by more than a factor of 30; for our simulations z_o/lambda {?} 0.04 to 0.06, where lambda is the wavelength of the breaking wave. Compared to a flow driven by a constant current, the extra mixing from breakers increases the mean eddy viscosity by more than a factor of 10 near the water surface. Breaking waves alter the usual balance of production and dissipation in the turbulent kinetic energy (TKE) budget; turbulent and pressure transports and breaker work are important sources and sinks in the budget. We also show that turbulent boundary layers driven by constant current and constant stress (i.e. with no breaking) differ in fundamental ways. The additional freedom provided by a constant-stress boundary condition permits finite velocity variances at the water surface, so that flows driven by constant stress mimic flows with weakly and statistically homogeneous breaking waves.
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 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.
Collisionless microinstabilities in stellarators. II. Numerical simulations
Proll, J. H. E.; Xanthopoulos, P.; Helander, P.
2013-12-15
Microinstabilities exhibit a rich variety of behavior in stellarators due to the many degrees of freedom in the magnetic geometry. It has recently been found that certain stellarators (quasi-isodynamic ones with maximum-J geometry) are partly resilient to trapped-particle instabilities, because fast-bouncing particles tend to extract energy from these modes near marginal stability. In reality, stellarators are never perfectly quasi-isodynamic, and the question thus arises whether they still benefit from enhanced stability. Here, the stability properties of Wendelstein 7-X and a more quasi-isodynamic configuration, QIPC, are investigated numerically and compared with the National Compact Stellarator Experiment and the DIII-D tokamak. In gyrokinetic simulations, performed with the gyrokinetic code GENE in the electrostatic and collisionless approximation, ion-temperature-gradient modes, trapped-electron modes, and mixed-type instabilities are studied. Wendelstein 7-X and QIPC exhibit significantly reduced growth rates for all simulations that include kinetic electrons, and the latter are indeed found to be stabilizing in the energy budget. These results suggest that imperfectly optimized stellarators can retain most of the stabilizing properties predicted for perfect maximum-J configurations.
Numerical simulations of phase change in microgravity
Juric, D.; Tryggvason, G.
1996-12-31
Direct numerical simulations of liquid-solid and liquid-vapor phase change are conducted under microgravity conditions. The time-dependent governing equations are solved using a two-dimensional finite-difference/front-tracking method. Large interface deformations, topology change, latent heat, surface tension and unequal material properties between the phases are included in the simulations. Results are presented for two specific problems: directional solidification of a dilute binary alloy and the rapid evaporation of a superheated liquid (vapor explosion). For the directional solidification problem, solution of the fully coupled solute and energy equations reveals the evolution of morphologically complex structures such as tip splitting, coarsening and droplet detachment from deep intercellular grooves. A variety of important solute segregation patterns such as necking, coring and banding are also observed. The boiling problem couples the phase change with fluid flow. This requires the solution of the Navier-Stokes and energy equations with interphase mass transfer. The energetic growth of instabilities on planar and circular interfaces during the unstable explosive evaporation of a superheated liquid in microgravity is demonstrated.
Numerical simulations of ultrafast pulse measurements
NASA Astrophysics Data System (ADS)
Liu, Xuan
Recently, an extremely sensitive technique---OPA-XFROG has been developed. A short pump pulse serves as the gate by parametrically amplifying a short segment of the signal pulse in a nonlinear crystal. High optical parametric gain makes possible the complete measurement of ultraweak, ultrashort light pulses. Unlike interferometric methods, it does not carry prohibitively restrictive requirements, such as perfect mode-matching, perfect spatial coherence, highly stable absolute phase, and a same-spectrum reference pulse. We simulate the OPA-XFROG technique and show that by a proper choice of the nonlinear crystal and the noncollinear mixing geometry it is possible to match the group velocities of the pump, signal, and idler pulses, which permits the use of relatively thick crystals to achieve high gain without measurement distortion. Gain bandwidths of ˜100 nm are possible, limited by group velocity dispersion. In the second part of the thesis, we numerically simulate the performance of the ultrasimple ultrashort laser pulse measurement device---GRENOUILLE. While simple in practice, GRENOUILLE has many theoretical subtleties because it involves the second-harmonic generation of relatively tightly focused and broadband pulses. In addition, these processes occur in a thick crystal, in which the phase-matching bandwidth is deliberately made narrow compared to the pulse bandwidth. We developed a model that include all sum-frequency-generation processes, both collinear and noncollinear. We also include dispersion using the Sellmeier equation for the crystal BBO. Working in the frequency domain, we compute the GRENOUILLE trace for practical---and impractical---examples and show that accurate measurements are easily obtained for properly designed devices. For pulses far outside a GRENOUILLE's operating range (on the long side), we numerically deconvolve the GRENOUILLE trace with the response function of GRENOUILLE to improve its spectral resolution. In the last part of the thesis, we simulate the second harmonic generation with tightly focused beams by use of lens. Thus, we are able to explain the 'weird' focusing effect that has been a 'puzzles' for us in the GRENOUILLE measurement.
NASA Technical Reports Server (NTRS)
Joslin, Ronald D.; Streett, Craig L.; Chang, Chau-Lyan
1992-01-01
Spatially evolving instabilities in a boundary layer on a flat plate are computed by direct numerical simulation (DNS) of the incompressible Navier-Stokes equations. In a truncated physical domain, a nonstaggered mesh is used for the grid. A Chebyshev-collocation method is used normal to the wall; finite difference and compact difference methods are used in the streamwise direction; and a Fourier series is used in the spanwise direction. For time stepping, implicit Crank-Nicolson and explicit Runge-Kutta schemes are used to the time-splitting method. The influence-matrix technique is used to solve the pressure equation. At the outflow boundary, the buffer-domain technique is used to prevent convective wave reflection or upstream propagation of information from the boundary. Results of the DNS are compared with those from both linear stability theory (LST) and parabolized stability equation (PSE) theory. Computed disturbance amplitudes and phases are in very good agreement with those of LST (for small inflow disturbance amplitudes). A measure of the sensitivity of the inflow condition is demonstrated with both LST and PSE theory used to approximate inflows. Although the DNS numerics are very different than those of PSE theory, the results are in good agreement. A small discrepancy in the results that does occur is likely a result of the variation in PSE boundary condition treatment in the far field. Finally, a small-amplitude wave triad is forced at the inflow, and simulation results are compared with those of LST. Again, very good agreement is found between DNS and LST results for the 3-D simulations, the implication being that the disturbance amplitudes are sufficiently small that nonlinear interactions are negligible.
LES, DNS and RANS for the analysis of high-speed turbulent reacting flows
NASA Technical Reports Server (NTRS)
Givi, Peyman; Taulbee, Dale B.; Adumitroaie, Virgil; Sabini, George J.; Shieh, Geoffrey S.
1994-01-01
The purpose of this research is to continue our efforts in advancing the state of knowledge in large eddy simulation (LES), direct numerical simulation (DNS), and Reynolds averaged Navier Stokes (RANS) methods for the computational analysis of high-speed reacting turbulent flows. In the second phase of this work, covering the period 1 Sep. 1993 - 1 Sep. 1994, we have focused our efforts on two research problems: (1) developments of 'algebraic' moment closures for statistical descriptions of nonpremixed reacting systems, and (2) assessments of the Dirichlet frequency in presumed scalar probability density function (PDF) methods in stochastic description of turbulent reacting flows. This report provides a complete description of our efforts during this past year as supported by the NASA Langley Research Center under Grant NAG1-1122.
NASA Astrophysics Data System (ADS)
Parkinson, S. D.; Hill, J.; Piggott, M. D.; Allison, P. A.
2014-09-01
High-resolution direct numerical simulations (DNSs) are an important tool for the detailed analysis of turbidity current dynamics. Models that resolve the vertical structure and turbulence of the flow are typically based upon the Navier-Stokes equations. Two-dimensional simulations are known to produce unrealistic cohesive vortices that are not representative of the real three-dimensional physics. The effect of this phenomena is particularly apparent in the later stages of flow propagation. The ideal solution to this problem is to run the simulation in three dimensions but this is computationally expensive. This paper presents a novel finite-element (FE) DNS turbidity current model that has been built within Fluidity, an open source, general purpose, computational fluid dynamics code. The model is validated through re-creation of a lock release density current at a Grashof number of 5 × 106 in two and three dimensions. Validation of the model considers the flow energy budget, sedimentation rate, head speed, wall normal velocity profiles and the final deposit. Conservation of energy in particular is found to be a good metric for measuring model performance in capturing the range of dynamics on a range of meshes. FE models scale well over many thousands of processors and do not impose restrictions on domain shape, but they are computationally expensive. The use of adaptive mesh optimisation is shown to reduce the required element count by approximately two orders of magnitude in comparison with fixed, uniform mesh simulations. This leads to a substantial reduction in computational cost. The computational savings and flexibility afforded by adaptivity along with the flexibility of FE methods make this model well suited to simulating turbidity currents in complex domains.
Laizet, S; Vassilicos, J C
2014-01-01
We focus in this paper on the effect of the resolution of Direct Numerical Simulations (DNS) on the spatio-temporal development of the turbulence downstream of a single square grid. The aims of this study are to validate our numerical approach by comparing experimental and numerical one-point statistics downstream of a single square grid and then investigate how the resolution is impacting the dynamics of the flow. In particular, using the Q-R diagram, we focus on the interaction between the strain-rate and rotation tensors, the symmetric and skew-symmetric parts of the velocity gradient tensor respectively. We first show good agreement between our simulations and hot-wire experiment for one-point statistics on the centreline of the single square grid. Then, by analysing the shape of the Q-R diagram for various streamwise locations, we evaluate the ability of under-resolved DNS to capture the main features of the turbulence downstream of the single square grid.
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.
2014-04-01
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 (Non-hydrostatic Mesoscale Model on E grid, Janjic et al., 2001; Dust REgional Atmospheric Model, Nickovic et al., 2001; Pérez et al., 2006) with 4 km horizontal resolution. A mask of the potentially dust productive regions is obtained from the land cover and the normalized difference vegetation index (NDVI) data from the Moderate Resolution Imaging Spectroradiometer (MODIS). The scope of this paper is validation of the dust model performance, and not use of the model as a tool to investigate mechanisms related to the storm. Results demonstrate the potential technical capacity and availability of the relevant data to build an operational system for dust storm forecasting as a part of a warning system. Model results are compared with radar and other satellite-based images and surface meteorological and PM10 observations. The atmospheric model successfully hindcasted the position of the front in space and time, with about 1 h late arrival in Phoenix. The dust model predicted the rapid uptake of dust and high values of dust concentration in the ensuing storm. South of Phoenix, over the closest source regions (~25 km), the model PM10 surface dust concentration reached ~2500 ?g m-3, but underestimated the values measured by the PM10 stations within the city. Model results are also validated by the MODIS aerosol optical depth (AOD), employing deep blue (DB) algorithms for aerosol loadings. Model validation included Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), equipped with the lidar instrument, to disclose the vertical structure of dust aerosols as well as aerosol subtypes. Promising results encourage further research and application of high-resolution modeling and satellite-based remote sensing to warn of approaching severe dust events and reduce risks for safety and health.
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.
Numerical simulation of carpet cloaking device in terahertz frequency range
NASA Astrophysics Data System (ADS)
Gill, V. V.; Vozianova, A. V.; Khodzitsky, M. K.
2015-11-01
This work is devoted to the numerical calculation of the effective constitutive parameters of the carpet cloaking device and to the numerical simulation of this cloak using finite element method (FEM) for the terahertz frequency range.
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.
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.
Numerical simulations of unsteady flows in turbomachines
NASA Astrophysics Data System (ADS)
Dorney, Daniel Joseph
The performance of axial and centrifugal turbomachines is significantly affected by the presence of unsteady and viscous flow mechanisms. Most contemporary design systems, however, use steady or linearized unsteady inviscid flow analyses to generate new blade shapes. In an effort to increase the understanding of unsteady viscous flows in turbomachinery blade rows, and to determine the limitations of linearized inviscid flow analyses, a two-part investigation was conducted. In the first portion of this investigation, a nonlinear viscous flow analysis was developed for the prediction of unsteady flows in two dimensional axial turbomachinery blade rows. The boundary conditions were formulated to allow the specification of vortical, entropic and acoustic excitations at the inlet, and acoustic excitations at exit, of a cascade. Numerical simulations were performed for flat plate and compressor exit guide vane cascades, and the predicted results were compared with solutions from classical linearized theory and linearized inviscid flow analysis. The unsteady pressure fields predicted with the current analysis showed close agreement with the linearized solutions for low to moderate temporal frequency vortical and acoustic excitations. As the temporal frequency of the excitations was increased, nonlinear effects caused discrepancies to develop between the linearized and Navier-Stokes solution sets. The inclusion of viscosity had a significant impact on the unsteady vorticity field, but only a minimal effect on the unsteady pressure field. In the second part of this investigation, a quasi-three-dimensional Navier-Stokes analysis was modified and applied to flows in centrifugal turbomachinery blade rows. Inviscid and viscous flow simulations were performed for a centrifugal impeller at three operating conditions. By comparing the predicted and experimental circumferential distributions of the relative frame velocity and flow angle downstream of the impeller, it was hypothesized that in the experiments the end secondary flows energize the impeller suction surface boundary making the local flow behave like an inviscid fluid. The performance curve generated from the viscous calculations showed close agreement with the experimental data.
Numerical Simulations of Saturn's Polar Cyclones
NASA Astrophysics Data System (ADS)
Brueshaber, Shawn R.; Sayanagi, Kunio M.
2014-11-01
Shawn R. Brueshaber, Department of Mechanical Engineering, Western Michigan UniversityKunio M. Sayanagi, Atmospheric and Planetary Sciences, Hampton UniversityCassini mission to Saturn has revealed evidences of a warm core cyclone centered on each of the poles of the planet. The morphology of the clouds in these cyclones resembles that of a terrestrial hurricane. The formation and maintenance mechanisms of these large polar cyclones are yet to be explained. Scott (2011, Astrophys. Geophys. Fluid Dyn) proposed that cyclonic vortices beta-drifting poleward can result in a polar cyclone, and demonstrated that beta-drifting cyclonic vortices can indeed cause accumulation of cyclonic vorticity at the pole using a 1-layer quasi-geostrophic model.The objectives of our project is to test Scott's hypothesis using a 1.5-layer shallow-water model and many-layer primitive equations model. We use the Explicit Planetary Isentropic Coordinate (EPIC) model (Dowling et al. 1998, 2004, Icarus) to perform direct numerical simulations of Saturn's polar atmosphere. To date, our project has focused on modifying the model to construct a polar rectangular model grid in order to avoid the problem of polar singularity associated with the conventional latitude-longitude grids employed in many general circulation models. We present our preliminary simulations, which show beta-drifting cyclones cause a poleward flux of cyclonic vorticity, which is consistent with Scott's results.Our study is partially supported by NASA Outer Planets Research Grant NNX12AR38G and NSF Astronomy and Astrophysics Grant 1212216 to KMS.
Schilling, Oleg; Mueschke, Nicholas J.
2010-10-18
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. Thus, 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
Schilling, Oleg; Mueschke, Nicholas J.
2010-10-18
Data from a 1152X760X1280 direct numerical simulation (DNS) of a transitional Rayleigh-Taylor mixing layer modeled after a small Atwood number water channel experiment is used to comprehensively investigate the structure of mean and turbulent transport and mixing. The simulation had physical parameters and initial conditions approximating those in the experiment. The budgets of the mean vertical momentum, heavy-fluid mass fraction, turbulent kinetic energy, turbulent kinetic energy dissipation rate, heavy-fluid mass fraction variance, and heavy-fluid mass fraction variance dissipation rate equations are constructed using Reynolds averaging applied to the DNS data. The relative importance of mean and turbulent production, turbulent dissipation and destruction, and turbulent transport are investigated as a function of Reynolds number and across the mixing layer to provide insight into the flow dynamics not presently available from experiments. The analysis of the budgets supports the assumption for small Atwood number, Rayleigh/Taylor driven flows that the principal transport mechanisms are buoyancy production, turbulent production, turbulent dissipation, and turbulent diffusion (shear and mean field production are negligible). As the Reynolds number increases, the turbulent production in the turbulent kinetic energy dissipation rate equation becomes the dominant production term, while the buoyancy production plateaus. Distinctions between momentum and scalar transport are also noted, where the turbulent kinetic energy and its dissipation rate both grow in time and are peaked near the center plane of the mixing layer, while the heavy-fluid mass fraction variance and its dissipation rate initially grow and then begin to decrease as mixing progresses and reduces density fluctuations. All terms in the transport equations generally grow or decay, with no qualitative change in their profile, except for the pressure flux contribution to the total turbulent kinetic energy flux, which changes sign early in time (a countergradient effect). The production-to-dissipation ratios corresponding to the turbulent kinetic energy and heavy-fluid mass fraction variance are large and vary strongly at small evolution times, decrease with time, and nearly asymptote as the flow enters a self-similar regime. The late-time turbulent kinetic energy production-to-dissipation ratio is larger than observed in shear-driven turbulent flows. The order of magnitude estimates of the terms in the transport equations are shown to be consistent with the DNS at late-time, and also confirms both the dominant terms and their evolutionary behavior. Thus, 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.
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.
Structure of the turbulent/non-turbulent interface of turbulent boundary layers - DNS results
NASA Astrophysics Data System (ADS)
Ishihara, Takashi; Ogasawara, Hiroki; Hunt, Julian C. R.
2013-11-01
Direct numerical simulations (DNS) of turbulent boundary layers (TBL) along a flat plate are used to study the properties of turbulent/non-turbulent (T/NT) interface of the TBL. The values of the momentum-thickness-based Reynolds numbers, Re? , used for this study, are 500 - 2200 . Analysis of the conditional statistics near the interface of the TBL shows that there is a small peak in the span-wise vorticity, and an associated small jump in stream-wise velocity. It is shown that the interfacial layer has a double structure which consists of a turbulent sub-layer with thickness of the order of the Taylor micro scale and its outer boundary (super layer) with thickness of the order of the Kolmogorov length scale. An approximate profile of the conditional average of span-wise vorticity near the interface fits well to the DNS data. The velocity jump near the T/NT interface of the TBL is of the order of the rms value of velocity fluctuations near the interface. Conditional cross correlations of the stream-wise or the wall-normal velocity fluctuations change sharply across the interface, which are consistent with the blocking mechanism of the interface (Hunt and Durbin 1999).
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.
Numerical simulations of drainage flows on Mars
NASA Technical Reports Server (NTRS)
Parish, Thomas R.; Howard, Alan D.
1992-01-01
Data collected by Viking Landers have shown that the meteorology of the near surface Martian environment is analogous to desertlike terrestrial conditions. Geological evidence such as dunes and frost streaks indicate that the surface wind is a potentially important factor in scouring of the martian landscape. In particular, the north polar basin shows erosional features that suggest katabatic wind convergence into broad valleys near the margin of the polar cap. The pattern of katabatic wind drainage off the north polar cap is similar to that observed on Earth over Antarctica or Greenland. The sensitivity is explored of Martian drainage flows to variations in terrain slope and diurnal heating using a numerical modeling approach. The model used is a 2-D sigma coordinate primitive equation system that has been used for simulations of Antarctic drainage flows. Prognostic equations include the flux forms of the horizontal scalar momentum equations, temperature, and continuity. Parameterization of both longwave (terrestrial) and shortwave (solar) radiation is included. Turbulent transfer of heat and momentum in the Martian atmosphere remains uncertain since relevant measurements are essentially nonexistent.
Direct numerical simulations of aeolian sand ripples
Durán, Orencio; Claudin, Philippe; Andreotti, Bruno
2014-01-01
Aeolian sand beds exhibit regular patterns of ripples resulting from the interaction between topography and sediment transport. Their characteristics have been so far related to reptation transport caused by the impacts on the ground of grains entrained by the wind into saltation. By means of direct numerical simulations of grains interacting with a wind flow, we show that the instability turns out to be driven by resonant grain trajectories, whose length is close to a ripple wavelength and whose splash leads to a mass displacement toward the ripple crests. The pattern selection results from a compromise between this destabilizing mechanism and a diffusive downslope transport which stabilizes small wavelengths. The initial wavelength is set by the ratio of the sediment flux and the erosion/deposition rate, a ratio which increases linearly with the wind velocity. We show that this scaling law, in agreement with experiments, originates from an interfacial layer separating the saltation zone from the static sand bed, where momentum transfers are dominated by midair collisions. Finally, we provide quantitative support for the use of the propagation of these ripples as a proxy for remote measurements of sediment transport. PMID:25331873
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
Numerical simulation of the flow over Barchan dunes
NASA Astrophysics Data System (ADS)
Omidyeganeh, Mohammad; Piomelli, Ugo; Christensen, Kenneth T.; Best, Jim
2012-11-01
We performed large-eddy simulation of the turbulent flow over a typical barchan dune model. The configuration is similar to that of experiments carried out at the University of Illinois, but the Reynolds number based on the free-surface velocity and the dune height is one fifth of the experiment. The simulation adopts the volume-of-fluid technique to model the dune. The use of periodic boundary conditions in the streamwise and spanwise directions implies that we are considering a fully developed flow over one dune in an infinite array. The height of the domain is close to the thickness of the approaching boundary layer, upstream of the dunes in the experiment. The resolution used is close to a typical DNS; ?x+ < 20 . 7 , ?y+ < 0 . 8 , and ?z+ < 10 . 3 . The approaching flow to the dune accelerates over the stoss (upstream) side and rises up to the crest, while at the same time diverging slowly in the spanwise direction toward the closest horn. The separated flow either reattaches on the plane or moves helically inside the recirculation zone toward the closest horn. The separated shear-layer extends downstream and toward the free-surface and contribute to downstream dunes. The agreement of the turbulence statistics with the experiment is good.
NASA Astrophysics Data System (ADS)
Dutta, S.; Cantero, M. I.; Garcia, M. H.
2014-08-01
Sediment transport in nature comprises of bedload and suspended load, and precise modelling of these processes is essential for accurate sediment flux estimation. Traditionally, non-cohesive suspended sediment has been modelled using the advection-diffusion equation (Garcia, 2008), where the success of the model is largely dependent on accurate approximation of the sediment diffusion coefficients. The current study explores the effect of self-stratification on sediment diffusivity using suspended sediment concentration data from direct numerical simulations (DNS) of flows subjected to different levels of stratification, where the level of stratification is dependent on the particle size (parameterized using particle fall velocity ? and volume-averaged sediment concentration (parameterized using shear Richardson number Ri?. Two distinct configurations were explored, first the channel flow configuration (similar to flow in a pipe or a duct) and second, a boundary-layer configuration (similar to open-channel flow). Self-stratification was found to modulate the turbulence intensity (Cantero et al., 2009b), which in turn was found to reduce vertical sediment diffusivity in portions of the domain exposed to turbulence damping. The effect of particle size on vertical sediment diffusivity has been studied in the past by several authors (Rouse, 1937; Coleman, 1970; Nielsen and Teakle, 2004); so in addition to the effect of particle size, the current study also explores the effect of sediment concentration on vertical sediment diffusivity. The results from the DNS simulations were compared with experiments (Ismail, 1952; Coleman, 1986) and field measurements (Coleman, 1970), and were found to agree qualitatively, especially for the case of channel flows. The aim of the study is to understand the effect of stratification due to suspended sediment on vertical sediment diffusivity for different flow configurations, in order to gain insight of the underlying physics, which will eventually help us to improve the existing models for sediment diffusivity.
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.
NASA Astrophysics Data System (ADS)
Tiselj, Iztok
2014-12-01
Channel flow DNS (Direct Numerical Simulation) at friction Reynolds number 180 and with passive scalars of Prandtl numbers 1 and 0.01 was performed in various computational domains. The "normal" size domain was ˜2300 wall units long and ˜750 wall units wide; size taken from the similar DNS of Moser et al. The "large" computational domain, which is supposed to be sufficient to describe the largest structures of the turbulent flows was 3 times longer and 3 times wider than the "normal" domain. The "very large" domain was 6 times longer and 6 times wider than the "normal" domain. All simulations were performed with the same spatial and temporal resolution. Comparison of the standard and large computational domains shows the velocity field statistics (mean velocity, root-mean-square (RMS) fluctuations, and turbulent Reynolds stresses) that are within 1%-2%. Similar agreement is observed for Pr = 1 temperature fields and can be observed also for the mean temperature profiles at Pr = 0.01. These differences can be attributed to the statistical uncertainties of the DNS. However, second-order moments, i.e., RMS temperature fluctuations of standard and large computational domains at Pr = 0.01 show significant differences of up to 20%. Stronger temperature fluctuations in the "large" and "very large" domains confirm the existence of the large-scale structures. Their influence is more or less invisible in the main velocity field statistics or in the statistics of the temperature fields at Prandtl numbers around 1. However, these structures play visible role in the temperature fluctuations at low Prandtl number, where high temperature diffusivity effectively smears the small-scale structures in the thermal field and enhances the relative contribution of large-scales. These large thermal structures represent some kind of an echo of the large scale velocity structures: the highest temperature-velocity correlations are not observed between the instantaneous temperatures and instantaneous streamwise velocities, but between the instantaneous temperatures and velocities averaged over certain time interval.
Zhi-Gang Feng
2012-05-31
The simulation of particulate flows for industrial applications often requires the use of two-fluid models, where the solid particles are considered as a separate continuous phase. One of the underlining uncertainties in the use of the two-fluid models in multiphase computations comes from the boundary condition of the solid phase. Typically, the gas or liquid fluid boundary condition at a solid wall is the so called no-slip condition, which has been widely accepted to be valid for single-phase fluid dynamics provided that the Knudsen number is low. However, the boundary condition for the solid phase is not well understood. The no-slip condition at a solid boundary is not a valid assumption for the solid phase. Instead, several researchers advocate a slip condition as a more appropriate boundary condition. However, the question on the selection of an exact slip length or a slip velocity coefficient is still unanswered. Experimental or numerical simulation data are needed in order to determinate the slip boundary condition that is applicable to a two-fluid model. The goal of this project is to improve the performance and accuracy of the boundary conditions used in two-fluid models such as the MFIX code, which is frequently used in multiphase flow simulations. The specific objectives of the project are to use first principles embedded in a validated Direct Numerical Simulation particulate flow numerical program, which uses the Immersed Boundary method (DNS-IB) and the Direct Forcing scheme in order to establish, modify and validate needed energy and momentum boundary conditions for the MFIX code. To achieve these objectives, we have developed a highly efficient DNS code and conducted numerical simulations to investigate the particle-wall and particle-particle interactions in particulate flows. Most of our research findings have been reported in major conferences and archived journals, which are listed in Section 7 of this report. In this report, we will present a brief description of these results.
NUMERICAL SIMULATION OF AIR POLLUTION DYNAMICS DUE TO
Olszewski Jr., Edward A.
NUMERICAL SIMULATION OF AIR POLLUTION DYNAMICS DUE TO POINT SOURCE EMISSIONS FROM AN INDUSTRIAL · Works Cited #12;Statement of the Problem · Numerical simulation of air pollution from a point source threat of air pollution Major health hazard to humans and ecosystem Billions effected, mainly
DNS of soot formation in three-dimensional turbulent non-premixed jet flames
NASA Astrophysics Data System (ADS)
Attili, Antonio; Bisetti, Fabrizio; Mueller, Michael E.; Pitsch, Heinz
2012-11-01
A set of three-dimensional Direct Numerical Simulations (DNS) of soot formation in a three-dimensional n-heptane/air turbulent non-premixed jet flame has been performed to investigate the coupling between turbulence, chemistry, and soot dynamics with varying Damköhler number. Finite rate chemistry of Polycyclic Aromatic Hydrocarbons (PAH) is included in the chemistry model. Soot is described with a bivariate distribution in volume-surface sample space, and a selected number of moments of the distribution are transported via a recently proposed transport Lagrangian scheme. Closure of the soot moment equations is achieved via the Hybrid Method of Moments (HMOM). It is observed that, for smaller Damköhler number, the mass fraction of soot particles decreases while the number density stays approximately constant. In addition, Lagrangian statistics are used to study the evolution and transport of soot aggregates during their movement in physical and mixture fraction space.
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.
NUMERICAL NOISE PM SIMULATION IN CMAQ
We have found that numerical noise in the latest release of CMAQ using the yamo advection scheme when compiled on Linux cluster with pgf90 (5.0 or 6.0). We recommend to use -C option to eliminate the numerical noise.
Numerical simulations of the intergalactic medium
Tom Theuns
2002-09-05
The intergalactic medium at redshifts 2--6 can be studied observationally through the absorption features it produces in the spectra of background quasars. Most of the UV-absorption lines arise in mildly overdense regions, which can be simulated reliably with current hydrodynamical simulations. Comparison of observed and simulated spectra allows one to put contraints on the model's parameters.
Numerical simulation of seasonal groundwater pumping
NASA Astrophysics Data System (ADS)
Filimonova, Elena; Baldenkov, Mikhail
2015-04-01
Increasing scarcity and contamination of water recourses require innovative water management strategies such as combined water system. The combined water system is a complex technology comprising two separate wells, major catchment-zone well and compensation pumping well, located inside a single stream basin. The major well is supplied by the well's catchment zone or surface flow, thus depleting the stream flow. The pumping rate of a major well is determined by the difference between the current stream flow and the minimum permissible stream flow. The deficiency of the stream flow in dry seasons can be compensated for by the short-term pumping of groundwater. The compensation pumping rate is determined by the difference between water demand and the permissible water withdrawal of the major well. The source for the compensation well is the aquifer storage. The estimation of streamflow depletion caused by compensation pumping is major question to evaluate the efficiency of the combined water system. Short-term groundwater pumping can use aquifer storage instead of catchment-zone water until the drawdown reaches the edge of the stream. Traditionally pumping simulation calculates in two-step procedure. Natural conditions, an aquifer system is in an approximate dynamic equilibrium, describe by steady-state model. A steady-state solution provides an initial heads, a set of flows through boundaries, and used as initial state for transient solutions, when pumping is imposed on an aquifer system. The transient solutions provide the total change in flows through the boundaries. A difference between the transient and steady-state solutions estimates the capture and the streamflow depletion. Numerical modeling of cyclical compensation pumping has special features: the periodic solution, the seasonal changes through the boundaries and the importance even small drawdown of stream level. When seasonality is a modeling feature, traditional approach leads to mistaken values of streamflow depletion. In this case three-step procedure is used. The first step is usual construction steady-state model. Then steady oscillatory model is constructed in which heads and flows through boundaries vary through the seasons but repeat from year to year (from cycle to cycle). Steady oscillatory solutions are used as initial conditions for transient pumping model. The stream flow depletion is estimated by difference between the transient solution and steady oscillatory solution. The purpose of these investigations was to evaluate the error, caused by using non-periodic solution as initial conditions for transient pumping model and to determine number of cycles required to reach steady oscillatory solution. For this study seasonal numerical models were constructed using ModTECH 2.3 and MODFLOW-2000. The developed models showed significant errors of stream depletion value, when non-periodic solution is used, miscalculation exceed 70 percent and more. It was obtained equations to estimate required number of cycles (N): for confined aquiferN = 0.2 - z + 9 for unconfined aquiferN = 0.0051 - z - 0.3 (L-+L-')2 -S z = T where T and S are transmissivity and specific yield of the aquifer (or storage coefficient for a confined aquifer), L' is stream leakance and L is riverbank size.
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.
NASA Astrophysics Data System (ADS)
Troitskaya, Yuliya; Druzhinin, Oleg
2013-04-01
Interaction of surface water waves with the wind flow is of primary importance for the wave modeling. The most difficult case for modeling is that of steep waves, when the strongly non-linear effects (e.g. sheltering, flow separation, vortex formation etc.) are encountered in the airflow over waves. Of special interest is also the influence of the wind flow stratification on the wind-wave interaction. In this work the preliminary results of direct numerical simulation (DNS)of structure and statistical characteristics of a turbulent, stably stratified atmospheric boundary layer over waved water surface are presented. In the experiments two-dimensional water waves with different wave age parameters (c/u* = 0-10, where u* is the friction velocity and c is the wave celerity), wave slope ka = 0-0.2 and at a bulk Reynolds number Re = 15000 and different values of the bulk Richardson number Ri (based on the buoyancy jump, bulk velocity and the surface wave length) are considered. The shape of the water wave is prescribed and does not evolve under the action of the wind. The full, 3D Navier-Stokes equations under the Boussinesq approximation are solved in curvilinear coordinates in a frame of reference moving the phase velocity of the wave. The shear driving the flow is created by an upper plane boundary moving horizontally with a bulk velocity in the x-direction. Periodic boundary conditions are considered in the horizontal (x) and lateral (y) directions, and no-slip boundary condition is considered in the vertical z-direction. The grid of nodes in the x, y, and z directions is used. The Adams-Bashforth method is employed to advance the integration in time and the equation for the pressure is solved iteratively by using FFT in the x and y directions and the Gauss method in the z-direction. Ensemble-averaged velocity and pressure fields are evaluated by averaging over time and the spanwise coordinate. Profiles of the mean velocity and turbulent stresses are obtained by averaging over wavelength. The preliminary DNS results show that the wind flow is significantly affected by the stratification. If the Richardson number is sufficiently small, the instantaneous vector velocity fields manifest considerable airflow separation at the crests of the surface waves similar to that observed in physical experiments by PIV-technique. Alternatively the ensemble averaged velocity fields are non-separating and have typical structures similar to those observed in shear flows near critical levels, where the phase velocity of the disturbance coincides with the flow velocity. On the other hand, for large Richardson numbers the wind flow turbulence is superseded by internal lee waves radiated from the wave crests and dissipating at a critical level, at some distance above the crests. The DNS results are compared with the prediction of a theoretical model of a turbulent boundary layer, based on the system of Reynolds-averaged equations with the first-order closure hypothesis. The wind-wave interaction is considered within the quasi-linear approximation, i.e., wave-induced disturbances in the air flow are considered in the linear approximation, but the resistive effect of the wave momentum flux on the mean flow velocity profile is taken into account. This paper was supported by RFBR (project codes 10-05-00339-A, 10-05-91177-GFEN_A, 09-05-00779-A;, 11-05-00455-A).
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).
DNS of Drag Reduction by Dilute Polymer Solutions at MDR
NASA Astrophysics Data System (ADS)
Lee, Dong-Hyun
2005-11-01
The phenomenon of Maximum Drag Reduction (MDR) by dilute polymer solutions is investigated by DNS. The objective is to establish the flow features and critical parameters needed to reach MDR. Simulations are performed in turbulent channel flows at Re?o 230 & 570 using a pseudo-spectral mixed Eulerian/Lagrangian scheme with either the FENE-P dumbbell or the FENE-LSMR chain models of the polymer and realistic polymer parameters. It is observed that the Weissenberg number (We?o) is the critical parameter for achieving high drag reduction, with the other parameters, such as concentration, having little effect. Results to date suggest that a We?o˜Re?o is required to achieve MDR. At Re?o 230, Virk's MDR asymptote is reached at We?o˜100. The flow statistics here agree with the experimental data of Ptasinski et al. [Flow Turbulence Combust. 66, 159--182 (2001)]. At higher We?o, DNS shows a relaminarization of the flow. The flow statistics during the intermediate states leading to relaminarization agree with the experimental data of Warholic et al. [Exp. Fluids. 27, 461--472 (1999)]. At Re?o 570, the highest We?o we have so far achieved in DNS (We?o˜300) is not sufficient to achieve MDR. These results suggest that in poly-disperse polymer solutions, it is the trace amount of the highest molecular weight polymers which contribute most to drag reduction.
Numerical simulation of the 1988 midwestern drought
Chern, Jiun-Dar; Sun, Wen-Yih
1997-11-01
In this study, the Purdue Regional Model (PRM) is utilized to simulate the monthly evolution of the weather patterns during the summer of 1988. The primary goal of this study is to develop and validate the PRM. The PRM, a regional climate model, is a hydrostatic primitive-equation model that uses the Arakawa C staggered grid in the horizontal and a terrain-following vertical coordinate. The model was used to simulate the 1988 drought for one month with lateral boundary conditions. The simulation reproduced the driest events in the Midwest; however, the simulated precipitation along the Gulf coast and Florida was underestimated. This suggests that the 60 km model resolution used in the simulation was not high enough to simulate the convective precipitation associated with the sea breeze circulations. 10 refs., 5 figs.
Simulation of crossflow instability on a supersonic highly swept wing
NASA Technical Reports Server (NTRS)
Pruett, C. David
1995-01-01
A direct numerical simulation (DNS) algorithm has been developed and validated for use in the investigation of crossflow instability on supersonic swept wings, an application of potential relevance to the design of the High-Speed Civil Transport (HSCT). The algorithm is applied to the investigation of stationary crossflow instability on an infinitely long 77-degree swept wing in Mach 3.5 flow. The results of the DNS are compared with the predictions of linear parabolized stability equation (PSE) methodology. In-general, the DNS and PSE results agree closely in terms of modal growth rate, structure, and orientation angle. Although further validation is needed for large-amplitude (nonlinear) disturbances, the close agreement between independently derived methods offers preliminary validation of both DNS and PSE approaches.
Numerical simulation of turbulent combustion: Scientific challenges
NASA Astrophysics Data System (ADS)
Ren, ZhuYin; Lu, Zhen; Hou, LingYun; Lu, LiuYan
2014-08-01
Predictive simulation of engine combustion is key to understanding the underlying complicated physicochemical processes, improving engine performance, and reducing pollutant emissions. Critical issues as turbulence modeling, turbulence-chemistry interaction, and accommodation of detailed chemical kinetics in complex flows remain challenging and essential for high-fidelity combustion simulation. This paper reviews the current status of the state-of-the-art large eddy simulation (LES)/prob-ability density function (PDF)/detailed chemistry approach that can address the three challenging modelling issues. PDF as a subgrid model for LES is formulated and the hybrid mesh-particle method for LES/PDF simulations is described. Then the development need in micro-mixing models for the PDF simulations of turbulent premixed combustion is identified. Finally the different acceleration methods for detailed chemistry are reviewed and a combined strategy is proposed for further development.
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.
NASA Astrophysics Data System (ADS)
Angeli, D.; Stalio, E.; Corticelli, M. A.; Barozzi, G. S.
2015-11-01
A parallel algorithm is presented for the Direct Numerical Simulation of buoyancy- induced flows in open or partially confined periodic domains, containing immersed cylindrical bodies of arbitrary cross-section. The governing equations are discretized by means of the Finite Volume method on Cartesian grids. A semi-implicit scheme is employed for the diffusive terms, which are treated implicitly on the periodic plane and explicitly along the homogeneous direction, while all convective terms are explicit, via the second-order Adams-Bashfort scheme. The contemporary solution of velocity and pressure fields is achieved by means of a projection method. The numerical resolution of the set of linear equations resulting from discretization is carried out by means of efficient and highly parallel direct solvers. Verification and validation of the numerical procedure is reported in the paper, for the case of flow around an array of heated cylindrical rods arranged in a square lattice. Grid independence is assessed in laminar flow conditions, and DNS results in turbulent conditions are presented for two different grids and compared to available literature data, thus confirming the favorable qualities of the method.
Polarization transmission at RHIC, numerical simulations
Meot F.; Bai, M.; Liu, C.; Minty, M.; Ranjbar, V.
2012-05-20
Typical tracking simulations regarding the transmission of the polarization in the proton-proton collider RHIC are discussed. They participate in general studies aimed at understanding and improving polarization performances during polarized proton-proton runs.
Detailed numerical simulations of laser cooling processes
NASA Technical Reports Server (NTRS)
Ramirez-Serrano, J.; Kohel, J.; Thompson, R.; Yu, N.
2001-01-01
We developed a detailed semiclassical numerical code of the forces applied on atoms in optical and magnetic fields to increase the understanding of the different roles that light, atomic collisions, background pressure, and number of particles play in experiments with laser cooled and trapped atoms.
High order hybrid numerical simulations of two dimensional detonation waves
NASA Technical Reports Server (NTRS)
Cai, Wei
1993-01-01
In order to study multi-dimensional unstable detonation waves, a high order numerical scheme suitable for calculating the detailed transverse wave structures of multidimensional detonation waves was developed. The numerical algorithm uses a multi-domain approach so different numerical techniques can be applied for different components of detonation waves. The detonation waves are assumed to undergo an irreversible, unimolecular reaction A yields B. Several cases of unstable two dimensional detonation waves are simulated and detailed transverse wave interactions are documented. The numerical results show the importance of resolving the detonation front without excessive numerical viscosity in order to obtain the correct cellular patterns.
Numerical simulation of ultrasonic tomography inspections of highly heterogeneous materials
NASA Astrophysics Data System (ADS)
Molero, M.; Medina, L.; Lluveras, D.; Izquierdo, M. A. G.; Anaya, J. J.
2012-12-01
This paper deals with the simulation of ultrasonic transmission tomography systems in water-immersed to nondestructively inspect highly heterogeneous materials with fractures. The time-domain Elastodynamic Finite Integration Technique (EFIT) was employed for all numerical simulations because is able to reliably simulate this type of ultrasonic problems. The EFIT code was implemented using OpenCL and PyOpenCL. Several ultrasonic tomography inspection setups were numerically simulated under different conditions varying the number of ultrasonic sources and their size and number and different operation schemes. Sinograms of concrete scenarios were computed and compared for each configuration, using homogeneous materials with similar fracture types and experimentally validated.
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 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 of flow separation control by oscillatory fluid injection
Resendiz Rosas, Celerino
2005-08-29
In this work, numerical simulations of flow separation control are performed. The sep-aration control technique studied is called 'synthetic jet actuation'. The developed code employs a cell centered finite volume scheme which handles viscous...
Numerical Simulation Study on Transpired Solar Air Collector
Wang, C.; Guan, Z.; Zhao, X.; Wang, D.
2006-01-01
The unglazed transpired solar air collector is now a well-recognized solar air heater for heating outside air directly. In this article, researchers introduced numerical simulation tools into the solar air collector research area, analyzed...
Parallel numerical reservoir simulation: A feasibility study
Michielse, P.H.
1994-12-31
This paper discusses a feasibility study to implement a parallel reservoir simulator on parallel computers. The basis of this study is a reservoir simulator that models an injection-production mechanism. The simulator implements a multigrid solver for the elliptic part of the equations, and uses adaptive local grid refinement to rack moving fronts in the reservoir. The parallelization method is based on a domain decomposition method, which assigns the subdomains to the processors. In order to obtain a correct solution, communication across the internal boundaries between the subdomains is required. The implementation of the multigrid method imposes restrictions on the domain decomposition. Furthermore, the adaptive local grid refinement may cause the work load distribution over the processors to be out of balance. Hence, some load balancing technique is required to ensure parallel efficiency. This parallel efficiency is illustrated by experiments on a Convex MetaSeries system.
Numerical Simulation of Physical and Chemical Processes in Fluidized Bed
NASA Astrophysics Data System (ADS)
Baturin, D. A.; Gil, A. V.
2015-10-01
The paper presents a numerical simulation of the furnace with a circulating fluidized bed. Numerical study carried out for the bottom of the combustion chamber with the varying heights of volume filling. The results contours of particulate matter concentration and of velocities, as well as a graphical representation of changes in the concentration of particles on the bed height are shown. Simulation performed in Eulerian - Eulerian representation on a 2D model.
Numerical simulation of a stroke: computational problems and methodology.
Descombes, Stéphane; Dumont, Thierry
2008-05-01
We discuss the difficulties of the numerical simulation of a stroke, and we describe the numerical methods which we have developed and used to obtain some realistic results. Nowadays, the computations are performed in two-dimensional slices of a brain, but the strategies to obtain full three-dimensional simulations are explored. This paper is written so as to be understandable by non-mathematicians. PMID:18067953
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 the Gailitis dynamo David Moss1
Moss, David
Numerical simulation of the Gailitis dynamo David Moss1 School of Mathematics University to be inconsistent with the work of Moss (1990), who in the context of Cp star magnetism obtained a positive growth illustrated numerically for the first time by Brandenburg, Moss and 1e-mail: moss@ma.man.ac.uk 1 #12;Soward
Numerical simulation of turbulent flows around airfoil and wing
NASA Technical Reports Server (NTRS)
Marx, Yves P.
1990-01-01
During the last years the simulation of compressible viscous flows has received much attention. While the numerical methods were improved drastically, a satisfactory modeling of the Reynolds stresses is still missing. In this paper, after a short description of the numerical procedure used for solving the Reynolds equations, experiments with a promising simple turbulence model are discussed.
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 wave model Tsunami wave Numerical modeling a b s t r a c t This paper presents a new submarine of landslide motion and associated tsunami wave generation on parameters including sediment settling velocity
Numerical Simulation of a Natural Circulation Steam Generator
Weinmüller, Ewa B.
Numerical Simulation of a Natural Circulation Steam Generator W. Linzer \\Lambda , K. Ponweiser circulation steam generator. We focus on a model with a simple geometry consisting of two vertical pipes properties of water and steam. We present a numerical algorithm based on an explicit upwind discretization
Numerical simulations of cardiovascular diseases and global matter transport
Simakov, S S; Evdokimov, A V; Kholodov, Y A
2007-01-01
Numerical model of the peripheral circulation and dynamical model of the large vessels and the heart are discussed in this paper. They combined together into the global model of blood circulation. Some results of numerical simulations concerning matter transport through the human organism and heart diseases are represented in the end.
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.
Brush seal numerical simulation: Concepts and advances
NASA Astrophysics Data System (ADS)
Braun, M. J.; Kudriavtsev, V. V.
1994-07-01
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 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.
The Numerical Simulation of Orographic Storms
NASA Astrophysics Data System (ADS)
Bradley, Michael Morgan
Recent observational studies of winter storms over the Glamorgan Hills of south Wales (Browning et al., 1974, and Hill et al., 1981) have confirmed the importance of the seeder-feeder mechanism of orographic precipitation enhancement, proposed by Bergeron (1965). However, the latter study also indicated that the sensitivity of the enhancement to the low-level wind speed is much greater than predicted by the seeder-feeder model of Bader and Roach (1977), which simulates only the microphysical processes. A new, dynamic mountain model was used to investigate the hypothesis that mountain wave effects were responsible for this increased wind speed sensitivity. The model, which evolved from the cloud model of Klemp and Wilhelmson (1978), is formulated in terrain-following coordinates. In addition to representing convective and stratiform cloud processes, the model is also capable of accurately simulating moderately nonlinear mountain waves, and is suitable for studying many aspects of orographic storms. The simulations show that orographic clouds significantly alter the airflow over hills and, conversely, that mountain waves strongly influence the moist processes. Wind speed, wind shear, atmospheric stability, and relative humidity are all important factors in the complex scale interactions which occur during orographic storms. The results strongly support the hypothesis that certain mountain wave effects greatly increase the wind speed sensitivity of the Bergeron seeder-feeder mechanism.
Numerical Simulation of Ion Thruster Optics
NASA Technical Reports Server (NTRS)
Rawlin, Vincent K. (Technical Monitor); Farnell, Cody C.; Williams, John D.; Wilbur, Paul J.
2003-01-01
A three-dimensional simulation code (ffx) designed to analyze ion thruster optics is described. It is an extension of an earlier code and includes special features like the ability to model a wide range of grid geometries, cusp details, and mis-aligned aperture pairs to name a few. However, the principle reason for advancing the code was in the study of ion optics erosion. Ground based testing of ion thruster optics, essential to the understanding of the processes of grid erosion, can be time consuming and costly. Simulation codes that can accurately predict grid lifetimes and the physical mechanisms of grid erosion can be of great utility in the development of future ion thruster optics designed for more ambitious applications. Results of simulations are presented that describe wear profiles for several standard and nonstandard aperture geometries, such as those grid sets with square- or slotted-hole layout patterns. The goal of this paper will be to introduce the methods employed in the ffx code and to briefly demonstrate their use.
Numerical simulation of electrospray in the cone-jet mode.
Herrada, M A; López-Herrera, J M; Gañán-Calvo, A M; Vega, E J; Montanero, J M; Popinet, S
2012-08-01
We present a robust and computationally efficient numerical scheme for simulating steady electrohydrodynamic atomization processes (electrospray). The main simplification assumed in this scheme is that all the free electrical charges are distributed over the interface. A comparison of the results with those calculated with a volume-of-fluid method showed that the numerical scheme presented here accurately describes the flow pattern within the entire liquid domain. Experiments were performed to partially validate the numerical predictions. The simulations reproduced accurately the experimental shape of the liquid cone jet, providing correct values of the emitted electric current even for configurations very close to the cone-jet stability limit. PMID:23005852
NASA Technical Reports Server (NTRS)
Okong'o, Nora; Bellan, Josette
2005-01-01
Models for large eddy simulation (LES) are assessed on a database obtained from direct numerical simulations (DNS) of supercritical binary-species temporal mixing layers. The analysis is performed at the DNS transitional states for heptane/nitrogen, oxygen/hydrogen and oxygen/helium mixing layers. The incorporation of simplifying assumptions that are validated on the DNS database leads to a set of LES equations that requires only models for the subgrid scale (SGS) fluxes, which arise from filtering the convective terms in the DNS equations. Constant-coefficient versions of three different models for the SGS fluxes are assessed and calibrated. The Smagorinsky SGS-flux model shows poor correlations with the SGS fluxes, while the Gradient and Similarity models have high correlations, as well as good quantitative agreement with the SGS fluxes when the calibrated coefficients are used.
Numerical simulations of traffic data via fluid dynamic approach
Bretti, Gabriella
traffic assignment or within-day models, thus rendering necessary the use of traffic simulation models. Such models, principally created from static network traffic assignments, can be divided in microscopicNumerical simulations of traffic data via fluid dynamic approach S. Blandin a , G. Bretti b , A
Numerical Simulation of a Laboratory-Scale Turbulent Slot Flame
Bell, John B.
interaction. See Vervisch et al. [6] and Bell et al. [7] for simulations of turbulent V-flames. In this paperNumerical Simulation of a Laboratory-Scale Turbulent Slot Flame John B. Bella , Marcus S. Daya dynamically resolves the flame and turbulent structures. Detailed comparisons with experimental measurements
Numerical simulation of icing, deicing, and shedding
NASA Technical Reports Server (NTRS)
Wright, W. B.; Dewitt, K. J.; Keith, T. G., Jr.
1991-01-01
An algorithm has been developed to numerically model the concurrent phenomena of two-dimensional transient heat transfer, ice accretion, ice shedding and ice trajectory which arise from the use of electrothermal pad. The Alternating Direction Implicit method is used to simultaneously solve the heat transfer and accretion equations occurring in the multilayered body covered with ice. In order to model the phase change between ice and water, a technique was used which assumes a phase for each node. This allows the equations to be linearized such that a direct solution is possible. This technique requires an iterative procedure to find the correct phase at each node. The computer program developed to find this solution has been integrated with the NASA-Lewis flow/trajectory code LEWICE.
Numerical simulation of electrophoresis separation processes
NASA Technical Reports Server (NTRS)
Ganjoo, D. K.; Tezduyar, T. E.
1986-01-01
A new Petrov-Galerkin finite element formulation has been proposed for transient convection-diffusion problems. Most Petrov-Galerkin formulations take into account the spatial discretization, and the weighting functions so developed give satisfactory solutions for steady state problems. Though these schemes can be used for transient problems, there is scope for improvement. The schemes proposed here, which consider temporal as well as spatial discretization, provide improved solutions. Electrophoresis, which involves the motion of charged entities under the influence of an applied electric field, is governed by equations similiar to those encountered in fluid flow problems, i.e., transient convection-diffusion equations. Test problems are solved in electrophoresis and fluid flow. The results obtained are satisfactory. It is also expected that these schemes, suitably adapted, will improve the numerical solutions of the compressible Euler and the Navier-Stokes equations.
Numerical simulation of the Beta II experiment
Shumaker, D.E.; Boyd, J.K.; McNamara, B.; Turner, W.C.
1981-10-01
The transport code FRT which is a 1-1/2-D transport-equilibrium code for an axisymmetric plasma was used to simulate the decay of the plasma and magnetic fields of the Beta II experiment. A comparison is made between the experimentally determined decay times for the magnetic fields and particle confinement times and the computed decay times. It is found that 1% oxygen impurity is enough to clamp the electron temperature below the radiation barrier, which is in agreement with the experiment.
Feasibility study for a numerical aerodynamic simulation facility. Volume 1
NASA Technical Reports Server (NTRS)
Lincoln, N. R.; Bergman, R. O.; Bonstrom, D. B.; Brinkman, T. W.; Chiu, S. H. J.; Green, S. S.; Hansen, S. D.; Klein, D. L.; Krohn, H. E.; Prow, R. P.
1979-01-01
A Numerical Aerodynamic Simulation Facility (NASF) was designed for the simulation of fluid flow around three-dimensional bodies, both in wind tunnel environments and in free space. The application of numerical simulation to this field of endeavor promised to yield economies in aerodynamic and aircraft body designs. A model for a NASF/FMP (Flow Model Processor) ensemble using a possible approach to meeting NASF goals is presented. The computer hardware and software are presented, along with the entire design and performance analysis and evaluation.
Numerical and laboratory simulations of auroral acceleration
Gunell, H.; De Keyser, J.; Mann, I.
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 simulation of electromagnetic turbulence in tokamaks
Waltz, R.E.
1985-02-01
Nonlinear two- and three-fluid equations are written for the time evolution of the perturbed electrostatic potential, densities, vector potential, and parallel ion motion of collisional and trapped electron plasmas in tokamak geometry. The nonlinear terms arise from the E x B/sub 0/ convection (d/dt = partial/partialt+v/sub E/ x del/sub perpendicular/) and magnetic flutter (del-tilde/sub parallel/ = del/sub parallel/+(B/sub perpendicular//B/sub 0/) x del/sub perpendicular/). Simplified two-dimensional (k/sub perpendicular/) mode coupling simulations with a fixed average parallel wavenumber (k/sub parallel/ = 1/Rq) and curvature drift (..omega../sub g/ = (L/sub n//R)..omega../sub asterisk/ ) characteristic of outward ballooning are performed. Homogeneous stationary turbulent states of the dissipative drift and interchange modes from 0< or =..beta..<..beta../sub crit/ for both the collisional and trapped electron plasmas are obtained. Transport coefficients associated with E x B and magnetic motions are calculated. The problem of simulating plasmas with high viscous Reynolds number is treated with an absorbing mantle at the largest wavenumbers.
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 simulation of the SOFIA flowfield
NASA Technical Reports Server (NTRS)
Klotz, Stephen P.
1994-01-01
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 run the telescope is mounted behind the wings. Results suggest that the cavity markedly influences the mean pressure distribution on empennage surfaces and that 110-140 decibel (db) sound pressure levels are typical in the cavity and on the horizontal and vertical stabilizers. A strong source of sound was found to exist on the rim of the open telescope cavity. The presence of this source suggests that additional design work needs to be performed in order to minimize the sound emanating from that location. A fluid dynamic analysis of the engine plumes is also contained in this report. The analysis was part of an effort to quantify the degradation of telescope performance resulting from the proximity of the port engine exhaust plumes to the open telescope bay.
Presented by High Fidelity Direct Numerical
;3 Managed by UT-Battelle for the U.S. Department of Energy Chen_S3D_SC10 S3D--first-principles combustion;5 Managed by UT-Battelle for the U.S. Department of Energy Chen_S3D_SC10 Combustion science enabled by NCCS;2 Managed by UT-Battelle for the U.S. Department of Energy Chen_S3D_SC10 Direct numerical simulation (DNS
Numerical simulation of rough-surface aerodynamics
NASA Astrophysics Data System (ADS)
Chi, Xingkai
Computational fluid dynamics (CFD) simulations of flow over surfaces with roughness in which the details of the surface geometry must be resolved pose major challenges. The objective of this study is to address these challenges through two important engineering problems, where roughness play a critical role---flow over airfoils with accrued ice and flow and heat transfer over turbine blade surfaces roughened by erosion and/or deposition. CFD simulations of iced airfoils face two major challenges. The first is how to generate high-quality single- and multi-block structured grids for highly convoluted convex and concave surface geometries with multiple scales. In this study, two methods were developed for the generation of high-quality grids for such geometries. The method developed for single-block grids involves generating a grid about the clean airfoil, carving out a portion of that grid about the airfoil, replacing that portion with a grid that accounts for the accrued ice geometry, and performing elliptic smoothing. The method developed for multi-block grids involves a transition-layer grid to ensure jaggedness in the ice geometry does not propagate into the domain. It also involves a "thick" wrap-around grid about the ice to ensure grid lines clustered next to solid surfaces do not propagate as streaks of tightly packed grid lines into the domain along block boundaries. For multi-block grids, this study also developed blocking topologies that ensure solutions to multi-block grids converge to steady state as quickly as single-block grids. The second major challenge in CFD simulations of iced airfoils is not knowing when it will predict reliably because of uncertainties in the turbulence modeling. In this study, the effects of turbulence models in predicting lift, drag, and moment coefficients were examined for airfoils with rime ice (i.e., ice with jaggedness only) and with glaze ice (i.e., ice with multiple protruding horns and surface jaggedness) as a function of angle of attack. In this examination, three different CFD codes---WIND, FLUENT, and PowerFLOW were used to examine a variety of turbulence models, including Spalart-Allmaras, RNG k-epsilon, shear-stress transport, v2-f, and differential Reynolds stress with and without non-equilibrium wall functions. The accuracy of the CFD predictions was evaluated by comparing grid-independent solutions with measured experimental data. Results obtained show CFD with WIND and FLUENT to predict the aerodynamics of airfoils with rime ice reliably up to near stall for all turbulence models investigated. (Abstract shortened by UMI.)
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 the world ocean circulation
NASA Technical Reports Server (NTRS)
Takano, K.; Mintz, Y.; Han, Y. J.
1973-01-01
A multi-level model, based on the primitive equations, is developed for simulating the temperature and velocity fields produced in the world ocean by differential heating and surface wind stress. The model ocean has constant depth, free slip at the lower boundary, and neglects momentum advection; so that there is no energy exchange between the barotropic and baroclinic components of the motion, although the former influences the latter through temperature advection. The ocean model was designed to be coupled to the UCLA atmospheric general circulation model, for the study of the dynamics of climate and climate changes. But here, the model is tested by prescribing the observed seasonally varying surface wind stress and the incident solar radiation, the surface air temperature and humidity, cloudiness and the surface wind speed, which, together with the predicted ocean surface temperature, determine the surface flux of radiant energy, sensible heat and latent heat.
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
Primitive numerical simulation of circular Couette flow
Hasiuk, J.F.
1988-01-01
The azimuthal-invariant, 3-d cylindrical, incompressible Navier-Stokes equations are solved to steady state for a finite-length, physically realistic model. The numerical method relies on an alternating-direction implicit (ADI) scheme that is formally second-order accurate in space and first-order accurate in time. The equations are linearized and uncoupled by evaluating variable coefficients at the previous time iteration. Wall grid clustering is provided by a Roberts transformation in radial and axial directions. A vorticity-velocity formulation is found to be preferable to a vorticity-stream function approach. Subject to no-slip, Dirichlet boundary conditions, except for the inner-cylinder rotation velocity (impulsive start-up) and zero-flow initial conditions, nonturbulent solutions are obtained for sub- and supercritical Reynolds numbers of 100 to 400 for a finite geometry. An axially-stretched model solution is shown to asymptotically approach the 1-d analytic Couette solution at the cylinder midheight. Flowfield change from laminar to Taylor-vortex flow is discussed as a function of Reynolds number.
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.
Mahalingam, S.; Chen, J.H.; Vervisch, L.
1994-01-01
Three-dimensional Direct Numerical Simulations (DNS) of turbulent non-premixed flames including finite-rate chemistry and heat release effects were performed. Two chemical reaction models were considered: (1) a single-step global reaction model in which the heat release and activation energy parameters are chosen to model methane-air combustion, and (2) a two-step reaction model to simulate radical production and consumption and to compare against the single-step model. The model problem consists of the interaction between an initially unstrained laminar diffusion flame and a three-dimensional field of homogeneous turbulence. Conditions ranging from fast chemistry to the pure mixing limit were studied by varying a global Damkoehler number. Results suggest that turbulence-induced mixing acting along the stoichiometric line leads to a strong modification of the inner structure of the turbulent flame compared with a laminar strained flame, resulting in intermediate species concentrations well above the laminar prediction. This result is consistent with experimental observations. Comparison of the response of the turbulent flame structure due to changes in the scalar dissipation rate with a steady strained laminar flame reveals that unsteady strain rates experienced by the turbulent flame may be responsible for the observed high concentrations of reaction intermediates.
Protecting BGP Routes to Top-Level DNS Servers
Wang, Lan
--Fault-tolerance, DNS infrastructure protection, route hijacking, BGP path filtering. Ã¦ 1 INTRODUCTION THE Domain NameProtecting BGP Routes to Top-Level DNS Servers Lan Wang, Xiaoliang Zhao, Member, IEEE, Dan Pei of potential faults and attacks. In particular, false routing announcements can deny access to the DNS service
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.
DNS ADVISING NOTES: Didactic Program in Dietetics
Lim, Seonhee
DNS ADVISING NOTES: Didactic Program in Dietetics Minimum Academic Requirements 2015-2016 DivisionEE 1610/1780 Physiology: NS 3410 (4, Spring, lecture) AND NS 3420 (2, Spring, Lab, concurrent registration: Adolescence and Emerging Adulthood (3, Spring) OR Psych 1101 Introduction to Psychology (3, Fall
Yoo, Chun S
2011-01-01
Direct numerical simulation (DNS) of the near-field of a three-dimensional spatially-developing turbulent ethylene jet flame in highly-heated coflow is performed with a reduced mechanism to determine the stabilization mechanism. The DNS was performed at a jet Reynolds number of 10,000 with over 1.29 billion grid points. The results show that auto-ignition in a fuel-lean mixture at the flame base is the main source of stabilization of the lifted jet flame. The Damkoehler number and chemical explosive mode (CEM) analysis also verify that auto-ignition occurs at the flame base. In addition to auto-ignition, Lagrangian tracking of the flame base reveals the passage of large-scale flow structures and their correlation with the fluctuations of the flame base similar to a previous study (Yoo et al., J. Fluid Mech. 640 (2009) 453-481) with hydrogen/air jet flames. It is also observed that the present lifted flame base exhibits a cyclic 'saw-tooth' shaped movement marked by rapid movement upstream and slower movement downstream. This is a consequence of the lifted flame being stabilized by a balance between consecutive auto-ignition events in hot fuel-lean mixtures and convection induced by the high-speed jet and coflow velocities. This is confirmed by Lagrangian tracking of key variables including the flame-normal velocity, displacement speed, scalar dissipation rate, and mixture fraction at the stabilization point.
Yoo, C. S.; Richardson, E.; Sankaran, R.; Chen, J. H.
2011-01-01
Direct numerical simulation (DNS) of the near-field of a three-dimensional spatially-developing turbulent ethylene jet flame in highly-heated coflow is performed with a reduced mechanism to determine the stabilization mechanism. The DNS was performed at a jet Reynolds number of 10,000 with over 1.29 billion grid points. The results show that auto-ignition in a fuel-lean mixture at the flame base is the main source of stabilization of the lifted jet flame. The Damköhler number and chemical explosive mode (CEM) analysis also verify that auto-ignition occurs at the flame base. In addition to auto-ignition, Lagrangian tracking of the flame base reveals the passage of large-scale flow structures and their correlation with the fluctuations of the flame base similar to a previous study (Yoo et al., J. Fluid Mech. 640 (2009) 453–481) with hydrogen/air jet flames. It is also observed that the present lifted flame base exhibits a cyclic ‘saw-tooth’ shaped movement marked by rapid movement upstream and slower movement downstream. This is a consequence of the lifted flame being stabilized by a balance between consecutive auto-ignition events in hot fuel-lean mixtures and convection induced by the high-speed jet and coflow velocities. This is confirmed by Lagrangian tracking of key variables including the flame-normal velocity, displacement speed, scalar dissipation rate, and mixture fraction at the stabilization point.
Numerical simulation of surface barriers for shrub-steppe ecoregions
White, Mark D.; Ward, Andy L.
2006-02-11
Surface barriers, constructed of earthen materials, are being proposed for the long-term management of vadose-zone buried waste and subsurface contamination for sites within the shrub-steppe ecoregion of North America. Field experiments of a prototype barrier on a shrub-steppe site have been ongoing since 1994, providing water balance data, which includes drainage from the sideslopes. Design and licensing of surface barriers will require a demonstrated understanding of the nonisothermal geohydrologic and coupled ground surface to atmosphere water mass and energy transport processes that control water infiltration to the subsurface. As a prelude to inverse numerical modeling to estimate critical parameters for the prototype barrier, this paper describes and demonstrates a numerical simulator for modeling the prototype barrier for shrub-steppe environments. The numerical simulator comprises a nonisothermal multifluid subsurface flow and transport simulator fully coupled to a modified nonlinear sparsely vegetated (bare substrate to closed canopy) evapotranspiration module that mechanistically predicts evaporation.
Numerical Convergence of Hydrodynamical SPH Simulations of Cooling Clusters
R. Valdarnini
2001-11-05
The results from hydrodynamical TREESPH simulations of galaxy clusters are used to investigate the dependence of the final cluster X-ray properties upon the numerical resolution and the assumed star formation models for the cooled gas. A comparison between runs with different star formation methods shows that the results of simulations, based on star formation methods in which gas conversion into stars is controlled by an efficiency parameter c_{star}, are sensitive to the simulation numerical resolution. In this respect star formation methods based instead on a local density threshold, are shown to give more stable results. Final X-ray luminosities are found to be numerically stable, with uncertainties of a factor 2.
Numerical simulation of propagating concentration profiles in renal tubules.
Pitman, E B; Layton, H E; Moore, L C
1994-05-01
Method-dependent mechanisms that may affect dynamic numerical solutions of a hyperbolic partial differential equation that models concentration profiles in renal tubules are described. Some numerical methods that have been applied to the equation are summarized, and ways by which the methods may misrepresent true solutions are analysed. Comparison of these methods demonstrates the need for thoughtful application of computational mathematics when simulating complicated time-dependent phenomena. PMID:8087082
Numerical and laboratory simulation of fault motion and earthquake occurrence
NASA Technical Reports Server (NTRS)
Cohen, S. C.
1979-01-01
This paper reviews the simulation of earthquake occurrence by numerical and laboratory mechanical block models. Simple linear rheological elements are used with elastic forces driving the main events and viscoelastic forces being important for aftershock and creep occurrence. Friction and its dependence on velocity, stress, and displacement also play a key role in determining how, when, and where fault motion occurs. The discussion of the qualitative behavior of the simulators focuses on the manner in which energy is stored in the system and released by the unstable and stable sliding processes. The numerical results emphasize the statistics of earthquake occurrence and the correlations among source parameters.
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 simulation of wall-bounded turbulent shear flows
NASA Technical Reports Server (NTRS)
Moin, P.
1982-01-01
Developments in three dimensional, time dependent numerical simulation of turbulent flows bounded by a wall are reviewed. Both direct and large eddy simulation techniques are considered within the same computational framework. The computational spatial grid requirements as dictated by the known structure of turbulent boundary layers are presented. The numerical methods currently in use are reviewed and some of the features of these algorithms, including spatial differencing and accuracy, time advancement, and data management are discussed. A selection of the results of the recent calculations of turbulent channel flow, including the effects of system rotation and transpiration on the flow are included.
Numerical simulation of water flow around a rigid fishing net
Roger Lewandowski; Géraldine Pichot
2006-12-20
This paper is devoted to the simulation of the flow around and inside a rigid axisymmetric net. We describe first how experimental data have been obtained. We show in detail the modelization. The model is based on a Reynolds Averaged Navier-Stokes turbulence model penalized by a term based on the Brinkman law. At the out-boundary of the computational box, we have used a "ghost" boundary condition. We show that the corresponding variational problem has a solution. Then the numerical scheme is given and the paper finishes with numerical simulations compared with the experimental data.
Direct numerical simulation of auto-ignition of a hydrogen vortex ring reacting with hot air
Doom, Jeff; Mahesh, Krishnan
2009-04-15
Direct numerical simulation (DNS) is used to study chemically reacting, laminar vortex rings. A novel, all-Mach number algorithm developed by Doom et al. [J. Doom, Y. Hou, K. Mahesh, J. Comput. Phys. 226 (2007) 1136-1151] is used. The chemical mechanism is a nine species, nineteen reaction mechanism for H{sub 2}/air combustion proposed by Mueller et al. [M.A. Mueller, T.J. Kim, R.A. Yetter, F.L. Dryer, Int. J. Chem. Kinet. 31 (1999) 113-125]. Diluted H{sub 2} at ambient temperature (300 K) is injected into hot air. The simulations study the effect of fuel/air ratios, oxidizer temperature, Lewis number and stroke ratio (ratio of piston stroke length to diameter). Results show that auto-ignition occurs in fuel lean, high temperature regions with low scalar dissipation at a 'most reactive' mixture fraction, {zeta}{sub MR} (Mastorakos et al. [E. Mastorakos, T.A. Baritaud, T.J. Poinsot, Combust. Flame 109 (1997) 198-223]). Subsequent evolution of the flame is not predicted by {zeta}{sub MR}; a most reactive temperature T{sub MR} is defined and shown to predict both the initial auto-ignition as well as subsequent evolution. For stroke ratios less than the formation number, ignition in general occurs behind the vortex ring and propagates into the core. At higher oxidizer temperatures, ignition is almost instantaneous and occurs along the entire interface between fuel and oxidizer. For stroke ratios greater than the formation number, ignition initially occurs behind the leading vortex ring, then occurs along the length of the trailing column and propagates toward the ring. Lewis number is seen to affect both the initial ignition as well as subsequent flame evolution significantly. Non-uniform Lewis number simulations provide faster ignition and burnout time but a lower maximum temperature. The fuel rich reacting vortex ring provides the highest maximum temperature and the higher oxidizer temperature provides the fastest ignition time. The fuel lean reacting vortex ring has little effect on the flow and behaves similar to a non-reacting vortex ring. (author)
Numerical simulation of the Langevin equation for skewed turbulence
Ermak, D. L.; Nasstrom, J. S.
1994-12-01
In this paper the authors present a numerical method for the generalized Langevin equation of motion with skewed random forcing for the case of homogeneous, skewed turbulence. The authors begin by showing how the analytic solution to the Langevin equation for this case can be used to determine the relationship between the particle velocity moments and the properties of the skewed random force. They then present a numerical method that uses simple probability distribution functions to simulate the effect of the random force. The numerical solution is shown to be exact in the limit of infinitesimal time steps, and to be within acceptable error limits when practical time steps are used.
Numerical simulation of tethered DNA in shear flow.
Litvinov, S; Hu, X Y; Adams, N A
2011-05-11
The behavior of tethered DNA in shear flow is investigated numerically by the smoothed dissipative particle dynamics (SDPD) method. Unlike numerical methods used in previous studies, SDPD models the solvent explicitly, takes into account the fully coupled hydrodynamic interactions and is free of the numerical artifact of wall sticking. Based on numerical simulations the static and dynamic properties of a tethered DNA is studied both qualitatively and quantitatively. The observed properties are in general agreement with previous experimental, numerical and theoretical work. Furthermore, the cyclic-motion phenomenon is studied by power spectrum density and cross-correlation function analysis, which suggest that there is only a very weak coherent motion of tethered DNA for a characteristic timescale larger than the relaxation time. Cyclic motion is more likely relevant as an isolated event than a typical mode of DNA motion. PMID:21508485
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.
Numerical simulation of double-diffusive finger convection
Hughes, J.D.; Sanford, W.E.; Vacher, H.L.
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 coupling of the flow and solute-transport equations is accounted for and handled using a sequential implicit Picard iterative scheme. High-resolution data from a double-diffusive Hele-Shaw experiment, initially in a density-stable configuration, is used to verify the numerical model. The temporal and spatial evolution of simulated double-diffusive convection is in good agreement with experimental results. Numerical results are very sensitive to discretization and correspond closest to experimental results when element sizes adequately define the spatial resolution of observed fingering. Numerical results also indicate that differences in the molecular diffusivity of sodium chloride and the dye used to visualize experimental sodium chloride concentrations are significant and cause inaccurate mapping of sodium chloride concentrations by the dye, especially at late times. As a result of reduced diffusion, simulated dye fingers are better defined than simulated sodium chloride fingers and exhibit more vertical mass transfer. Copyright 2005 by the American Geophysical Union.
Building Blocks for Reliable Complex Nonlinear Numerical Simulations
NASA Technical Reports Server (NTRS)
Yee, H. C.
2005-01-01
This chapter describes some of the building blocks to ensure a higher level of confidence in the predictability and reliability (PAR) of numerical simulation of multiscale complex nonlinear problems. The focus is on relating PAR of numerical simulations with complex nonlinear phenomena of numerics. To isolate sources of numerical uncertainties, the possible discrepancy between the chosen partial differential equation (PDE) model and the real physics and/or experimental data is set aside. The discussion is restricted to how well numerical schemes can mimic the solution behavior of the underlying PDE model for finite time steps and grid spacings. The situation is complicated by the fact that the available theory for the understanding of nonlinear behavior of numerics is not at a stage to fully analyze the nonlinear Euler and Navier-Stokes equations. The discussion is based on the knowledge gained for nonlinear model problems with known analytical solutions to identify and explain the possible sources and remedies of numerical uncertainties in practical computations.
Building Blocks for Reliable Complex Nonlinear Numerical Simulations
NASA Technical Reports Server (NTRS)
Yee, H. C.; Mansour, Nagi N. (Technical Monitor)
2002-01-01
This talk describes some of the building blocks to ensure a higher level of confidence in the predictability and reliability (PAR) of numerical simulation of multiscale complex nonlinear problems. The focus is on relating PAR of numerical simulations with complex nonlinear phenomena of numerics. To isolate sources of numerical uncertainties, the possible discrepancy between the chosen partial differential equation (PDE) model and the real physics and/or experimental data is set aside. The discussion is restricted to how well numerical schemes can mimic the solution behavior of the underlying PDE model for finite time steps and grid spacings. The situation is complicated by the fact that the available theory for the understanding of nonlinear behavior of numerics is not at a stage to fully analyze the nonlinear Euler and Navier-Stokes equations. The discussion is based on the knowledge gained for nonlinear model problems with known analytical solutions to identify and explain the possible sources and remedies of numerical uncertainties in practical computations. Examples relevant to turbulent flow computations are included.
Building Blocks for Reliable Complex Nonlinear Numerical Simulations. Chapter 2
NASA Technical Reports Server (NTRS)
Yee, H. C.; Mansour, Nagi N. (Technical Monitor)
2001-01-01
This chapter describes some of the building blocks to ensure a higher level of confidence in the predictability and reliability (PAR) of numerical simulation of multiscale complex nonlinear problems. The focus is on relating PAR of numerical simulations with complex nonlinear phenomena of numerics. To isolate sources of numerical uncertainties, the possible discrepancy between the chosen partial differential equation (PDE) model and the real physics and/or experimental data is set aside. The discussion is restricted to how well numerical schemes can mimic the solution behavior of the underlying PDE model for finite time steps and grid spacings. The situation is complicated by the fact that the available theory for the understanding of nonlinear behavior of numerics is not at a stage to fully analyze the nonlinear Euler and Navier-Stokes equations. The discussion is based on the knowledge gained for nonlinear model problems with known analytical solutions to identify and explain the possible sources and remedies of numerical uncertainties in practical computations. Examples relevant to turbulent flow computations are included.
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.
NASA Astrophysics Data System (ADS)
Liu, J.; Zhou, Q.; Akhavan, R.
2002-11-01
Direct numerical simulations of dilute polymer solutions in a turbulent channel flow have been performed to investigate the role of polymer extensibility, polymer concentration, and Weissenberg number in polymer drag reduction. The calculations are based on DNS of turbulence using standard pseudo-spectral methods and a FENE-P dumbbell constitutive model for the polymer. The base Newtonian flow had a Re_? ˜ 215, based on channel half width and wall friction velocity. Weissenberg numbers ranging from We_? ˜ 10 to We ? ˜ 200 and polymer extensibility parameters ranging from b=10,000 to b=100,000, corresponding to PEO or PAM of molecular weight in the range of one million to ten million, are studied. Different regimes of drag reduction observed experimentally, ranging from small drag reduction to maximum drag reduction (MDR), are captured in these computations. Drag reduction is shown to be due to patches of biaxial elongational flow encountered in the buffer layer. The Weissenberg number is the critical parameter determining the magnitude of drag reduction in a given polymer/solvent system. Effective drag reduction requires a high polymer extensibility and We_? ˜ O(50-100) or higher. Available experimental data are re-analyzed in light of these results and are shown to fit the trends proposed by these scalings.
Direct Numerical Simulation of Solid Deformation During Dendritic Solidification
Beckermann, Christoph
Direct Numerical Simulation of Solid Deformation During Dendritic Solidification M. YAMAGUCHI1 solidification is a common phe- nomenon in metal casting and can lead to defects such as hot tears, macro in the crystallographic orientation angle within a single dendrite that, in turn, affect the subsequent solidification
Stress and diffusion induced interface motion: Modelling and numerical simulations
Styles, Vanessa
Stress and diffusion induced interface motion: Modelling and numerical simulations Harald Garcke of Mathematics, University of Sussex, Brighton, BN1 9QH, U.K. Abstract We propose a phase field model for stress stress effects. In this paper we will demonstrate that the model can also be used to describe other
Numerical simulation of a generalized Zakharov system Peter A. Markowich
Markowich, Peter A.
Numerical simulation of a generalized Zakharov system Shi Jin Peter A. Markowich and Chunxiong-Technological Cooperation Agreement funded by the OEAD (Austria). Department of Mathematical Science, Tsinghua University, A-1000, Vienna, Austria. Website: www.peter-markowich.net. Â§ Department of Mathematical Science
IRIS Spectrum Line Plot - Numeric Simulation - Duration: 13 seconds.
This video is similar to the IRIS Spectrum Line Plot video at http://www.youtube.com/watch?v=E4V_vF3qMSI, but now as derived from a numerical simulation of the Sun by the University of Oslo. Credit...
Numerical Simulation of Laminar Reacting Flows with Complex Chemistry
Bell, John B.
Numerical Simulation of Laminar Reacting Flows with Complex Chemistry M S Day and J B Bell Lawrence number reacting ows with complex chemistry. Our approach uses a form of the low Mach number equations represents a generalization of the Pember et al methodology to incorporate complex chemistry and the e#11
Numerical Simulation of Laminar Reacting Flows with Complex Chemistry
Bell, John B.
Numerical Simulation of Laminar Reacting Flows with Complex Chemistry M S Day and J B Bell Lawrence number reacting flows with complex chemistry. Our approach uses a form of the low Mach number equations represents a generalization of the Pember et al methodology to incorporate complex chemistry and the effects
Detonation shock dynamics and comparisons with direct numerical simulation
Aslam, Tariq
observed to change by as much as 40% due to multi-dimensional effects [3]. Failure of detonation waves hasDetonation shock dynamics and comparisons with direct numerical simulation Tariq D. Aslam , and D- nation and detonation shock dynamics (DSD) is made. The theory of DSD defines the motion
Detonation shock dynamics and comparisons with direct numerical simulation
Aslam, Tariq
]. Failure of detonation waves has also been observed experimentally. Other dynamics, such as pulsatingDetonation shock dynamics and comparisons with direct numerical simulation Tariq D. Aslam # , and D nation and detonation shock dynamics (DSD) is made. The theory of DSD defines the motion
The Numerical Simulation of Ship Waves Using Cartesian Grid Methods
Sussman, Mark
The Numerical Simulation of Ship Waves Using Cartesian Grid Methods M. Sussman (Florida State-dimensional spray sheet. 1 Introduction At moderate to high speed, the turbulent flow along the hull of a ship, and complex interac- tions between the ship hull and the free surface, such as transom-stern flows
A review of numerical simulation of hydrothermal systems.
Mercer, J.W.; Faust, C.R.
1979-01-01
Many advances in simulating single and two-phase fluid flow and heat transport in porous media have recently been made in conjunction with geothermal energy research. These numerical models reproduce system thermal and pressure behaviour and can be used for other heat-transport problems, such as high-level radioactive waste disposal and heat-storage projects. -Authors
Simulated Composite Baseball Bat Impacts Using Numerical and Experimental Techniques
Smith, Lloyd V.
Simulated Composite Baseball Bat Impacts Using Numerical and Experimental Techniques L. V. Smith, M, WA 99164-2920 ABSTRACT A study has been undertaken to develop techniques for assessing baseball bat-wood baseball bats were approved for collegiate play over 25 years ago. The primary motivation was to reduce
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…
NUMERICAL SIMULATION OF A VISCOELASTIC FLUID WITH ANISOTROPIC HEAT CONDUCTION
Wapperom, Peter
NUMERICAL SIMULATION OF A VISCOELASTIC FLUID WITH ANISOTROPIC HEAT CONDUCTION P. WAPPEROM and M contraction. Key words: shift factors, dissipation, anisotropic heat conduction, finite elements, streamline with Fourier's law, where the heat conduction may be anisotropic. In the derivation of a temperature equation
Simulation of ductile crack growth using computational cells: numerical aspects
Haj-Ali, Rami
rights reserved. Keywords: Finite elements; Crack growth; Cell elements; Damage mechanics; FractureSimulation of ductile crack growth using computational cells: numerical aspects Arne S. Gulleruda , Xiaosheng Gaoa , Robert H. Dodds Jra, *, R. Haj-Alib a Department of Civil Engineering, University
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 granular flows Pierre A. Gremaud
Numerical simulation of granular flows Pierre A. Gremaud John V. Matthews Dept. of Math. & CRSC. Typically, the size of the particles range from 10 m, for a very fine powder, to 10 m, in case of grains. The mechanical properties of the grains are described by a small number of assumptions and parameters. Friction
NUMERICAL SIMULATION OF TWO-DIMENSIONAL MELTING AND RESOLIDIFICATION OF
Zhang, Yuwen
uses two types of the metal powders possessing significantly different melting points [3, 4]. The high-melting-point necessary to avoid ``balling.'' Solidification of the low-melting-point metal bonds the high-melting- pointNUMERICAL SIMULATION OF TWO-DIMENSIONAL MELTING AND RESOLIDIFICATION OF A TWO-COMPONENT METAL
Direct Numerical Simulation of Unsteady Decelerating Flows Yongmann M. Chung
Chung, Yongmann M.
, the interaction between the large-scale unsteady motions and the small- scale turbulence is not well understood applied in many engineering applications where large-scale unsteady (mean flow) motions are frequently.M.Chung@warwick.ac.uk SUMMARY Direct numerical simulations are performed for a turbulent flow subjected to a sudden change
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.
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.
Benchmarks and numerical methods for the simulation of boiling flows
NASA Astrophysics Data System (ADS)
Tanguy, Sébastien; Sagan, Michaël; Lalanne, Benjamin; Couderc, Frédéric; Colin, Catherine
2014-05-01
Comparisons of different numerical methods suited to the simulations of phase changes are presented in the framework of interface capturing computations on structured fixed computational grids. Due to analytical solutions, we define some reference test-cases that every numerical technique devoted to phase change should succeed. Realistic physical properties imply some drastic interface jump conditions on the normal velocity or on the thermal flux. The efficiencies of Ghost Fluid and Delta Function Methods are compared to compute the normal velocity jump condition. Next, we demonstrate that high order extrapolation methods on the thermal field allow performing accurate and robust simulations for a thermally controlled bubble growth. Finally, some simulations of the growth of a rising bubble are presented, both for a spherical bubble and a deformed bubble.
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.
NASA Astrophysics Data System (ADS)
Dutta, S.; Cantero, M. I.; Garcia, M. H.
2013-11-01
Sediment transport in nature comprises of bed-load and suspended load, and precise modelling of suspended load transport is essential for accurate sediment flux estimation. Traditionally, non-cohesive suspended sediment has been modelled using the advection-diffusion equation (Garcia, 2008), where the success of the model is largely dependent on accurate approximation of the sediment diffusion coefficients. The current study explores the effect of self-stratification on sediment diffusivity using suspended sediment concentration data from Direct Numerical Simulations (DNS) of flows subjected to different levels of stratification, where the level of stratification is dependent on the particle size (parameterized using particle fall velocity V~) and volume-averaged sediment concentration (parameterized using shear Richardson number Ri?). Two distinct configurations were explored, first the channel flow configuration (similar to flow in a pipe or a duct) and second, a boundary layer configuration (similar to open-channel flow). Self-stratification was found to modulate the turbulence intensity (Cantero et al., 2009), which in turn was found to reduce vertical sediment diffusivity in portions of the domain exposed to turbulence damping. Effect of particle size on vertical sediment diffusivity has been studied in the past by several authors (Rouse, 1937; Coleman, 1970; Nielsen and Teakle, 2004); so in addition to the effect of particle size, the current study also explores the effect of sediment concentration on vertical sediment diffusivity. The results from the DNS simulations were compared with experiments (Ismail, 1952; Coleman, 1986) and field measurements (Coleman, 1970); and were found to agree qualitatively especially for the case of channel flows. The aim of the study was to understand the effect of stratification due to suspended sediment on vertical sediment diffusivity for different flow configurations, in order to gain insight of the underlying physics, which will eventually help us to improve the existing models for sediment diffusivity.
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.
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.
Graphics interfaces and numerical simulations: Mexican Virtual Solar Observatory
NASA Astrophysics Data System (ADS)
Hernández, L.; González, A.; Salas, G.; Santillán, A.
2007-08-01
Preliminary results associated to the computational development and creation of the Mexican Virtual Solar Observatory (MVSO) are presented. Basically, the MVSO prototype consists of two parts: the first, related to observations that have been made during the past ten years at the Solar Observation Station (EOS) and at the Carl Sagan Observatory (OCS) of the Universidad de Sonora in Mexico. The second part is associated to the creation and manipulation of a database produced by numerical simulations related to solar phenomena, we are using the MHD ZEUS-3D code. The development of this prototype was made using mysql, apache, java and VSO 1.2. based GNU and `open source philosophy'. A graphic user interface (GUI) was created in order to make web-based, remote numerical simulations. For this purpose, Mono was used, because it is provides the necessary software to develop and run .NET client and server applications on Linux. Although this project is still under development, we hope to have access, by means of this portal, to other virtual solar observatories and to be able to count on a database created through numerical simulations or, given the case, perform simulations associated to solar phenomena.
Numerical simulations of internal wave generation by convection in water.
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. PMID:26172801
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 simulations of zero-Prandtl-number thermohaline convection
NASA Astrophysics Data System (ADS)
Prat, V.; Lignières, F.; Lagarde, N.
2015-12-01
Thermohaline (or fingering) convection has been used to explain chemical anomalies at the surface of red giant stars. However, recent numerical simulations suggest that the efficiency of thermohaline convection is lower than expected, and thus not sufficient to explain the observations. One of the uncertainties of these simulations is that they have been performed in a parameter range for the Prandtl number (i.e. the ratio between viscosity and thermal diffusivity) which is far from what can be found in stellar interiors. Using the small-Péclet-number approximation, we are able for the first time to perform simulations of thermohaline convection in a parameter domain which is relevant for stellar physics. In the present paper, we discuss the validity of this approximation and compare our results with previous simulations and models.
Numerical simulation of cavitating turbulent flow through a Francis turbine
NASA Astrophysics Data System (ADS)
Zhang, L.; Liu, J. T.; Wu, Y. L.; Liu, S. H.
2012-11-01
The unsteady cavitating turbulent flow in a Francis turbine is simulated based on governing equations of the mixture model for cavity-liquid two-phase flows with the RNG k-epsilon turbulence model in the present paper. An improved mass transfer expression in the mixture model is obtained based on evaporation and condensation mechanics with considering the effects of the non-dissolved gas, the turbulence, the tension of interface at cavity and the effect of phase change rate and so on. The governing equations of the mixture model for the unsteady cavitating-liquid flow is solved by a direct coupling method numerically with the finite volume method (FVM) using the unstructured tetrahedron grid and the structured hexahedral grid system. This direct coupling simulation was successfully applied to simulate the cavitating two-phase turbulent flow through a Francis turbine. The simulated external results agreed well with the experimental results.
Design and numerical simulation of thermionic electron gun
Hosseinzadeh, M
2015-01-01
This paper reports the simulation of an electron gun. The effects of some parameters on the beam quality were studied and optimal choices were identified. It gives numerical beam qualities in common electrostatic triode gun, and the dependences on design parameters such as electrode geometries and bias voltages to these electrodes are shown. An electron beam of diameter 5 mm with energy of five kilo electron volt was assumed for simulation process. Some design parameters were identified as variable parameters in the presence of space charge. These parameters are the inclination angle of emission electrode, the applied voltage to focusing electrode, the gap width between the emission electrode and the focusing electrode and the diameter of the focusing electrode. The triode extraction system is designed and optimized by using CST software (for Particle Beam Simulations). The physical design of the extraction system is given in this paper. From the simulation results, it is concluded that the inclination angle ...
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.
Characterizing Electron Temperature Gradient Turbulence Via Numerical Simulation
Nevins, W M; Candy, J; Cowley, S; Dannert, T; Dimits, A; Dorland, W; Estrada-Mila, C; Hammett, G W; Jenko, F; Pueschel, M J; Shumaker, D E
2006-05-22
Numerical simulations of electron temperature gradient (ETG) turbulence are presented which characterize the ETG fluctuation spectrum, establish limits to the validity of the adiabatic ion model often employed in studying ETG turbulence, and support the tentative conclusion that plasmaoperating regimes exist in which ETG turbulence produces sufficient electron heat transport to be experimentally relevant. We resolve prior controversies regarding simulation techniques and convergence by benchmarking simulations of ETG turbulence from four microturbulence codes, demonstrating agreement on the electron heat flux, correlation functions, fluctuation intensity, and rms flow shear at fixed simulation cross section and resolution in the plane perpendicular to the magnetic field. Excellent convergence of both continuum and particle-in-cell codes with time step and velocity-space resolution is demonstrated, while numerical issues relating to perpendicular (to the magnetic field) simulation dimensions and resolution are discussed. A parameter scan in the magnetic shear, s, demonstrates that the adiabatic ion model is valid at small values of s (s < 0.4 for the parameters used in this scan) but breaks down at higher magnetic shear. A proper treatment employing gyrokinetic ions reveals a steady increase in the electron heat transport with increasing magnetic shear, reaching electron heat transport rates consistent with analyses of experimental tokamak discharges.
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 jet noise from different jet nozzle geometries
NASA Astrophysics Data System (ADS)
Paliath, Umesh; Morris, Philip J.
2005-09-01
This paper describes the numerical simulation of flow-induced noise from jets with different nozzle geometries. The nozzles considered include axisymmetric and nonaxisymmetric nozzles, such as circular and rectangular. Also the study is extended to examine the differences between noise radiated from nozzles with planar exists and those with nonplanar exist, such as beveled nozzles. The detached-eddy simulation (DES) approach is used to simulate both the jet nozzle internal and external flows as well as the jet plume. This methodology allows the turbulence model to transition from an unsteady Reynolds averaged Navier-Stokes (URANS) method for attached boundary layers to a large-eddy simulation (LES) in separated regions. Thus, it is ideally suited to jet flow simulations when the nozzle is included. Both cylindrical polar and Cartesian coordinate systems are used as the basis for grid generation. The one equation Spalart-Allmaras turbulence model is used to describe the evo! lution of the turbulent eddy viscosity. Dispersion relation preserving algorithms are used for spatial discretization and an explicit 4th order Runge-Kutta scheme is used for time marching. The far-field sound is evaluated using the Ffowcs Williams-Hawkings permeable surface acoustic analogy. This permits the noise to be predicted at large distances from the jet based on fluctuations in the jet's near field. This provides a good compromise between numerical accuracy and computational cost. The results are compared with experimental data for both unheated and heated jet cases.
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.
Extrapolating gravitational-wave data from numerical simulations
Boyle, Michael; Mroue, Abdul H.
2009-12-15
Two complementary techniques are developed for obtaining the asymptotic form of gravitational-wave data at large radii from numerical simulations, in the form of easily implemented algorithms. It is shown that, without extrapolation, near-field effects produce errors in extracted waveforms that can significantly affect LIGO data analysis. The extrapolation techniques are discussed in the context of Newman-Penrose data applied to extrapolation of waveforms from an equal-mass, nonspinning black-hole binary simulation. The results of the two methods are shown to agree within error estimates. The various benefits and deficiencies of the methods are discussed.
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 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 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 Simulations of Forming Aluminum Beverage Can End Shells
NASA Astrophysics Data System (ADS)
Han, Jing; Yamazaki, Koetsu; Hasegawa, Takashi; Itoh, Ryouiti; Nishiyama, Sadao
2011-08-01
Forming simulations of can end shell have been implemented and compared with the experimental observations of the shell actual forming process. The influences of the loads applied to tools, the clearances between tools, the shapes of the tool profiles and the positions of tools, on the shell forming quality, have then been investigated numerically. The design optimization method based on the numerical simulations has been applied to search optimum design points, in order to reduce the amount of thinning subjected to the constraints of the shell geometric shape and the suppression of wrinkles. The optimization results show that the amount of thinning can be reduced up to 4% by optimizing the forming route, adjusting the clearances and the loads, and modifying the tool shapes.
Simulating Prosthetic Heart Valve Hemodynamics: Numerical Model Development
NASA Astrophysics Data System (ADS)
Ge, Liang
2005-11-01
Since the first successful implantation of a prosthetic heart valve four decades ago, over 50 different designs have been developed including both mechanical and bio-prosthetic valves. Valve implants, however, are associated with increased risk of blood clotting, a trend which is believed to be linked to the complex hemodynamics induced by the prosthesis. To understand prosthetic valve hemodynamics under physiological conditions, we develop a numerical method capable of simulating flows in realistic prosthetic heart valves in anatomical geometries. The method employs a newly developed hybrid numerical technique that integrates the chimera overset grid approach with a Cartesian, sharp-interface immersed boundary methodology. The capabilities of the method are demonstrated by applying it to simulate pulsatile flow in both bileaflet and tri-leaflet valves moving with prescribed leaflet kinematics.
Numerical aerodynamic simulation program long haul communications prototype
NASA Technical Reports Server (NTRS)
Cmaylo, Bohden K.; Foo, Lee
1987-01-01
This document is a report of the Numerical Aerodynamic Simulation (NAS) Long Haul Communications Prototype (LHCP). It describes the accomplishments of the LHCP group, presents the results from all LHCP experiments and testing activities, makes recommendations for present and future LHCP activities, and evaluates the remote workstation accesses from Langley Research Center, Lewis Research Center, and Colorado State University to Ames Research Center. The report is the final effort of the Long Haul (Wideband) Communications Prototype Plan (PT-1133-02-N00), 3 October 1985, which defined the requirements for the development, test, and operation of the LHCP network and was the plan used to evaluate the remote user bandwidth requirements for the Numerical Aerodynamic Simulation Processing System Network.
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
Brandao, C. S. S.; De Araujo, J. C. N. E-mail: jcarlos@das.inpe.b
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.
Morphodynamic-numerical Simulation of River Bars and Dunes
NASA Astrophysics Data System (ADS)
Mewis, P.
2003-04-01
It is well accepted in Hydromechnics, Meteorology and Oceanography that instabilities are responsible for the generation of turbulence, cyclones and Golf Stream rings. In the morphodynamic behavior several instabilities have been identified that generate ripples, dunes, antidunes, alternate bars and tidal ridges respectively (Callander, Kennedy, Fredsoe, Hulscher a.o.). These can be modelled numericaly using the right set of equations and appropriate numerical schemes. The instabilities predicted mostly by linear stability analysis are simulated fully nonlinear using a numerical model. Problems arise due to a decoupling within the numerical solution of the system of first order partial differential equations resulting in short waves with a length of twice the mesh spacing that grow unbounded. This decoupling badly distorts the physically sound generation of bedforms. It is shown that, like in the case of pressure coupled equations, special care is needed in the numerical formulation of the model. Upwinding techniques are usefull to prevent the decoupling and yield good results. Other methods to cope with the decoupling problem are shown and discussed shortly. The strong influence of commonly applied smoothing techniques including the upwinding on the numerical result is demonstrated. Using a threedimensional hydrostatic flow model, coupled with a morphodynamic module alternate bars and also dunes can be simulated. The morphodynamic-numerical model SMOR is applied to simulate the generation of alternate bars like in Tubinos experiment. The same model is used to simulate scour overdeepening in river curves for the experiment of Odgaard. The generation of dunes is a more complicated mechanism that is nevertheless inherent in threedimensional models. Thus the generation of dunes has been simulated. The shape and dimensions of the dunes seem to be reasonable. The results are compared with observations and discussed. A very simple extension for depth integrated (2D) morphodynamic models to account for secondary flow by the water level gradient is presented. Applying this extention not only in the transverse but also in the streamwise direction leads to the generation of dunes in 2D models.
Numerical model for learning concepts of streamflow simulation
DeLong, L.L.
1993-01-01
Numerical models are useful for demonstrating principles of open-channel flow. Such models can allow experimentation with cause-and-effect relations, testing concepts of physics and numerical techniques. Four PT is a numerical model written primarily as a teaching supplement for a course in one-dimensional stream-flow modeling. Four PT options particularly useful in training include selection of governing equations, boundary-value perturbation, and user-programmable constraint equations. The model can simulate non-trivial concepts such as flow in complex interconnected channel networks, meandering channels with variable effective flow lengths, hydraulic structures defined by unique three-parameter relations, and density-driven flow.The model is coded in FORTRAN 77, and data encapsulation is used extensively to simplify maintenance and modification and to enhance the use of Four PT modules by other programs and programmers.
Numerical simulation for the high performance industrial reheating furnace design
Murakami, Hideki; Saito, Toshiaki; Hayashi, Junnichi; Hida, Atsushi
1999-07-01
The high performance industrial furnace, providing significant energy savings, low pollutant emission and high control ability on heating, has been developing. For designing the process, numerical simulations of a slab reheating furnace, with the advanced combustion system adopting highly preheated air have been performed, using a three-dimensional unsteady mathematical model. An essential feature of the model is the incorporation of the three-dimensional turbulent model (LES) and the Radiative Energy Absorption Distribution (READ) method. Numerical results has been verified with experimental results, velocity data of a water-model and heat flux data of a large unit furnace. The results have been, also, visualized by the thermal particle method. The numerical results lead to the conclusion that the regenerative burner system developed has the large advantage of heating slabs uniformly in a wide reheating furnace, and suggest possibility of a compact reheating furnace.
Numerical simulation of shock wave diffraction by TVD schemes
NASA Technical Reports Server (NTRS)
Young, Victor Y. C.; Yee, H. C.
1987-01-01
An upwind total variation diminishing (TVD) scheme and a predictor-corrector symmetric TVD scheme were used to numerically simulate the blast wave diffraction on a stationary object. The objective is to help design an optimum configuration so that lateral motion is minimized and at the same time vortex shedding and flow separation are reduced during a blast wave encounter. Results are presented for a generic configuration for both a coarse grid and a fine grid to illustrate the global and local diffraction flow fields. Numerical experiments for the shock wave reflection on a wedge are also included to validate the current approach. Numerical study indicated that these TVD schemes are more stable and produced higher shock resolution than classical shock capturing methods such as the explicit MacCormack scheme.
A mesh-free approach to numerical rock mechanics simulations
NASA Astrophysics Data System (ADS)
Jansen, Gunnar; Galvan, Boris; Miller, Stephen
2014-05-01
Numerical simulation of the nucleation, growth, and coalescence of fracture networks is a fundamental aspect of lithospheric geodynamics and engineering applications such as enhanced geothermal systems, hydraulic fracturing and CO2 sequestration. Modeling the underlying mechanics is challenging because of several numerical difficulties. In particular, fracture path evolution predicted by mesh-based models can be heavily affected by numerical resolution of the chosen discretization scheme. Additionally, large deformations can lead to numerical errors associated with highly deformed elements. We are developing algorithms that simulate fracture nucleation and growth using mesh-free methods that overcome the difficulties arising from the mesh-sensitivity of conventional mesh-based methods. We implemented a mesh-free local Petrov-Galerkin method (MLPG), which is based on the local weak form of the problem under consideration. This method requires no mesh for interpolation or integration, and thus may be well-suited to handle strain localization occurring during fracture development. Interpolation is performed using moving least squares approximation (MLS) shape functions, and since nodal integration is performed locally, this approach can be parallelized efficiently. We present a mesh-free 2D elasto-plastic model for geomaterials that includes frictional hardening and cohesion softening using the Mohr-Coulomb failure criterion to simulate fracture network evolution and dynamic fracture propagation. Model performance is further enhanced through parallelization by utilising a hybrid CPU/GPU cluster using the PETSc library. We outline the implementation of the developed code, and evaluate its performance from a series of benchmark simulations.
Utah, University of
Feature-Based Statistical Analysis of Combustion Simulation Data Janine C. Bennett, Member, IEEE framework for feature-based statistical analysis of large-scale scientific data and demonstrate its effec- tiveness by analyzing features from Direct Numerical Simulations (DNS) of turbulent combustion. Turbulent
Transient productivity index for numerical well test simulations
Blanc, G.; Ding, D.Y.; Ene, A.
1997-08-01
The most difficult aspect of numerical simulation of well tests is the treatment of the Bottom Hole Flowing (BHF) Pressure. In full field simulations, this pressure is derived from the Well-block Pressure (WBP) using a numerical productivity index which accounts for the grid size and permeability, and for the well completion. This productivity index is calculated assuming a pseudo-steady state flow regime in the vicinity of the well and is therefore constant during the well production period. Such a pseudo-steady state assumption is no longer valid for the early time of a well test simulation as long as the pressure perturbation has not reached several grid-blocks around the well. This paper offers two different solutions to this problem: (1) The first one is based on the derivation of a Numerical Transient Productivity Index (NTPI) to be applied to Cartesian grids; (2) The second one is based on the use of a Corrected Transmissibility and Accumulation Term (CTAT) in the flow equation. The representation of the pressure behavior given by both solutions is far more accurate than the conventional one as shown by several validation examples which are presented in the following pages.
Detecting Botnet Activities Based on Abnormal DNS traffic
Manasrah, Ahmed M; Abouabdalla, Omar Amer; Ramadass, Sureswaran
2009-01-01
IThe botnet is considered as a critical issue of the Internet due to its fast growing mechanism and affect. Recently, Botnets have utilized the DNS and query DNS server just like any legitimate hosts. In this case, it is difficult to distinguish between the legitimate DNS traffic and illegitimate DNS traffic. It is important to build a suitable solution for botnet detection in the DNS traffic and consequently protect the network from the malicious Botnets activities. In this paper, a simple mechanism is proposed to monitors the DNS traffic and detects the abnormal DNS traffic issued by the botnet based on the fact that botnets appear as a group of hosts periodically. The proposed mechanism is also able to classify the DNS traffic requested by group of hosts (group behavior) and single hosts (individual behavior), consequently detect the abnormal domain name issued by the malicious Botnets. Finally, the experimental results proved that the proposed mechanism is robust and able to classify DNS traffic, and effi...
A Posteriori Study of a DNS Database Describing Super critical Binary-Species Mixing
NASA Technical Reports Server (NTRS)
Bellan, Josette; Taskinoglu, Ezgi
2012-01-01
Currently, the modeling of supercritical-pressure flows through Large Eddy Simulation (LES) uses models derived for atmospheric-pressure flows. Those atmospheric-pressure flows do not exhibit the particularities of high densitygradient magnitude features observed both in experiments and simulations of supercritical-pressure flows in the case of two species mixing. To assess whether the current LES modeling is appropriate and if found not appropriate to propose higher-fidelity models, a LES a posteriori study has been conducted for a mixing layer that initially contains different species in the lower and upper streams, and where the initial pressure is larger than the critical pressure of either species. An initially-imposed vorticity perturbation promotes roll-up and a double pairing of four initial span-wise vortices into an ultimate vortex that reaches a transitional state. The LES equations consist of the differential conservation equations coupled with a real-gas equation of state, and the equation set uses transport properties depending on the thermodynamic variables. Unlike all LES models to date, the differential equations contain, additional to the subgrid scale (SGS) fluxes, a new SGS term that is a pressure correction in the momentum equation. This additional term results from filtering of Direct Numerical Simulation (DNS) equations, and represents the gradient of the difference between the filtered pressure and the pressure computed from the filtered flow field. A previous a priori analysis, using a DNS database for the same configuration, found this term to be of leading order in the momentum equation, a fact traced to the existence of high-densitygradient magnitude regions that populated the entire flow; in the study, models were proposed for the SGS fluxes as well as this new term. In the present study, the previously proposed constantcoefficient SGS-flux models of the a priori investigation are tested a posteriori in LES, devoid of or including, the SGS pressure correction term. The present pressure-correction model is different from, and more accurate as well as less computationally intensive than that of the a priori study. The constant-coefficient SGS-flux models encompass the Smagorinsky (SMC), in conjunction with the Yoshizawa (YO) model for the trace, the Gradient (GRC) and the Scale Similarity (SSC) models, all exercised with the a priori study constant coefficients calibrated at the transitional state. The LES comparison is performed with the filtered- and-coarsened (FC) DNS, which represents an ideal LES solution. Expectably, an LES model devoid of SGS terms is shown to be considerably inferior to models containing SGS effects. Among models containing SGS effects, those including the pressure-correction term are substantially superior to those devoid of it. The sensitivity of the predictions to the initial conditions and grid size are also investigated. Thus, it has been discovered that, additional to the atmospheric-pressure models currently used, a new model is necessary to simulate supercritical-pressure flows. This model depends on the thermodynamic characteristics of the chemical species involved.
Towards an Automated Full-Turbofan Engine Numerical Simulation
NASA Technical Reports Server (NTRS)
Reed, John A.; Turner, Mark G.; Norris, Andrew; Veres, Joseph P.
2003-01-01
The objective of this study was to demonstrate the high-fidelity numerical simulation of a modern high-bypass turbofan engine. The simulation utilizes the Numerical Propulsion System Simulation (NPSS) thermodynamic cycle modeling system coupled to a high-fidelity full-engine model represented by a set of coupled three-dimensional computational fluid dynamic (CFD) component models. Boundary conditions from the balanced, steady-state cycle model are used to define component boundary conditions in the full-engine model. Operating characteristics of the three-dimensional component models are integrated into the cycle model via partial performance maps generated automatically from the CFD flow solutions using one-dimensional meanline turbomachinery programs. This paper reports on the progress made towards the full-engine simulation of the GE90-94B engine, highlighting the generation of the high-pressure compressor partial performance map. The ongoing work will provide a system to evaluate the steady and unsteady aerodynamic and mechanical interactions between engine components at design and off-design operating conditions.
Numerical simulation of supersonic wake flow with parallel computers
Wong, C.C.; Soetrisno, M.
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 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 radiative heat loss in an experimental burner
Cloutman, L.D.; Brookshaw, L.
1993-09-01
We describe the numerical algorithm used in the COYOTE two-dimensional, transient, Eulerian hydrodynamics program to allow for radiative heat losses in simulations of reactive flows. The model is intended primarily for simulations of industrial burners, but it is not confined to that application. It assumes that the fluid is optically thin and that photons created by the fluid immediately escape to free space or to the surrounding walls, depending upon the application. The use of the model is illustrated by simulations of a laboratory-scale experimental burner. We find that the radiative heat losses reduce the local temperature of the combustion products by a modest amount, typically on the order of 50 K. However, they have a significant impact on NO{sub x} production.
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.
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.
Chen, Jackie; Sankaran, Ramanan; Yoo, Chun S
2009-01-01
Direct numerical simulation (DNS) of the near field of a three-dimensional spatially developing turbulent lifted hydrogen jet flame in heated coflow is performed with a detailed mechanism to determine the stabilization mechanism and the flame structure. The DNS was performed at a jet Reynolds number of 11,000 with over 940 million grid points. The results show that auto-ignition in a fuel-lean mixture at the flame base is the main source of stabilization of the lifted jet flame. A chemical flux analysis shows the occurrence of near-isothermal chemical chain branching preceding thermal runaway upstream of the stabilization point, indicative of hydrogen auto-ignition in the second limit. The Damkoehler number and key intermediate-species behaviour near the leading edge of the lifted flame also verify that auto-ignition occurs at the flame base. At the lifted-flame base, it is found that heat release occurs predominantly through ignition in which the gradients of reactants are opposed. Downstream of the flame base, both rich-premixed and non-premixed flames develop and coexist with auto-ignition. In addition to auto-ignition, Lagrangian tracking of the flame base reveals the passage of large-scale flow structures and their correlation with the fluctuations of the flame base. In particular, the relative position of the flame base and the coherent flow structure induces a cyclic motion of the flame base in the transverse and axial directions about a mean lift-off height. This is confirmed by Lagrangian tracking of key scalars, heat release rate and velocity at the stabilization point.
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 simulations of pyroclastic surge in eruptions of Usu Volcano
NASA Astrophysics Data System (ADS)
Saba, M.; Saito, T.; Oshima, H.
2006-12-01
Pyroclastic surge developed during explosive volcanic eruption can cause hazardous damage as serious as other phenomena such as blast waves or eruptive fragments. A numerical code is developed for simulating the pyroclastic surge. The code is an extension of a shock capturing code that treats shock wave propagation and other complex wave interactions (Saito, 2002). It is applied for simulating imaginary eruptions of Usu volcano, one of the most active volcano in Japan. The eruption is modeled as a high-speed jet ejected from a vent opened at the ground into a stationary atomosphere. The governing equations of mass, momentum and energy are solved by finite volume methods known as WAF(Weighed Averaged Flux) method with the HLLC approximate Riemann solver(Toro, 1997). The numerical grids of the terrain are generated from a digital elevation map. Preliminary numerical tests are carried out and followings are found: a) At the beginning of eruption, a blast wave is generated and propagates rather quickly(supersonic propagation). b) After the blast wave, a column of of high density erupting as developes. c) In about 100 sec from the beginning of the eruption, it is observed that high density gas flow spreads out along the ground as pyroclastic surge. The correlation between the gas ejection speed and the speed of the pyroclastic surge is also investigated. Saito, T. (2002), Numerical analysis of dusty-gas flows, J. Comput. Phys., 176, 129-144. Toro, E. F. (1997), Riemann solvers and numerical methods for fluid dynamics, Springer-Verlag, Heidelberg.
Urban-breeze circulation during the CAPITOUL experiment: numerical simulations
NASA Astrophysics Data System (ADS)
Hidalgo, J.; Masson, V.; Pigeon, G.
2008-12-01
In this study we present a numerical simulation of the urban-breeze circulation observed in Toulouse, South-West of France, during the Intensive Observation Period number 5 (IOP5, 3rd and 4th July 2004) of the CAPITOUL experiment (Feb. 2004-2005). The numerical simulation is performed with the non-hydrostatic atmospheric model MesoNH (Lafore et al. 1998) coupled with the urban surface scheme TEB (Masson 2000). Four two-way, grid-nested models with horizontal grid resolution of 12 km, 3 km, 1 km and 0.25 km are used. The diurnal cycle of temperature, the nocturnal heat island and the early morning cool island are reproduced by the model. For the urban-breeze period, between 12.00 UTC to 18.00 UTC, the heat island structure and the simulated turbulent fluxes are discussed based on the observed surface energy balance and urban canopy temperature. The numerical simulations confirm the presence of a convergent circulation at the surface towards the city centre and a divergent counter-current 1500 m above the ground. The intensity of the urban-breeze circulation is of the order of 1.5 m s-1 and its extension, in the mean wind axis, is two times the diameter of the city. The dynamical perturbation on the ABL due to the roughness of the city is only significant up to 50 m of height, the urban breeze circulation being caused by the pressure gradient due to the UHI-induced thermal effects. An evaluation of the improvement on the ABL thermodynamics representation when going down to 250 m of horizontal resolution instead of 1 km is also presented.
Stochastic algorithms for the analysis of numerical flame simulations
Bell, John B.; Day, Marcus S.; Grcar, Joseph F.; Lijewski, Michael J.
2004-04-26
Recent progress in simulation methodologies and high-performance parallel computers have made it is possible to perform detailed simulations of multidimensional reacting flow phenomena using comprehensive kinetics mechanisms. As simulations become larger and more complex, it becomes increasingly difficult to extract useful information from the numerical solution, particularly regarding the interactions of the chemical reaction and diffusion processes. In this paper we present a new diagnostic tool for analysis of numerical simulations of reacting flow. Our approach is based on recasting an Eulerian flow solution in a Lagrangian frame. Unlike a conventional Lagrangian view point that follows the evolution of a volume of the fluid, we instead follow specific chemical elements, e.g., carbon, nitrogen, etc., as they move through the system . From this perspective an ''atom'' is part of some molecule of a species that is transported through the domain by advection and diffusion. Reactions cause the atom to shift from one chemical host species to another and the subsequent transport of the atom is given by the movement of the new species. We represent these processes using a stochastic particle formulation that treats advection deterministically and models diffusion and chemistry as stochastic processes. In this paper, we discuss the numerical issues in detail and demonstrate that an ensemble of stochastic trajectories can accurately capture key features of the continuum solution. The capabilities of this diagnostic are then demonstrated by applications to study the modulation of carbon chemistry during a vortex-flame interaction, and the role of cyano chemistry in rm NO{sub x} production for a steady diffusion flame.
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 simulation of turbulent gas flames in tubes.
Salzano, E; Marra, F S; Russo, G; Lee, J H S
2002-12-01
Computational fluid dynamics (CFD) is an emerging technique to predict possible consequences of gas explosion and it is often considered a powerful and accurate tool to obtain detailed results. However, systematic analyses of the reliability of this approach to real-scale industrial configurations are still needed. Furthermore, few experimental data are available for comparison and validation. In this work, a set of well documented experimental data related to the flame acceleration obtained within obstacle-filled tubes filled with flammable gas-air mixtures, has been simulated. In these experiments, terminal steady flame speeds corresponding to different propagation regimes were observed, thus, allowing a clear and prompt characterisation of the numerical results with respect to numerical parameters, as grid definition, geometrical parameters, as blockage ratio and to mixture parameters, as mixture reactivity. The CFD code AutoReagas was used for the simulations. Numerical predictions were compared with available experimental data and some insights into the code accuracy were determined. Computational results are satisfactory for the relatively slower turbulent deflagration regimes and became fair when choking regime is observed, whereas transition to quasi-detonation or Chapman-Jogouet (CJ) were never predicted. PMID:12423940
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.
Using Numerical Modeling to Simulate Space Capsule Ground Landings
NASA Technical Reports Server (NTRS)
Heymsfield, Ernie; Fasanella, Edwin L.
2009-01-01
Experimental work is being conducted at the National Aeronautics and Space Administration s (NASA) Langley Research Center (LaRC) to investigate ground landing capabilities of the Orion crew exploration vehicle (CEV). The Orion capsule is NASA s replacement for the Space Shuttle. The Orion capsule will service the International Space Station and be used for future space missions to the Moon and to Mars. To evaluate the feasibility of Orion ground landings, a series of capsule impact tests are being performed at the NASA Langley Landing and Impact Research Facility (LandIR). The experimental results derived at LandIR provide means to validate and calibrate nonlinear dynamic finite element models, which are also being developed during this study. Because of the high cost and time involvement intrinsic to full-scale testing, numerical simulations are favored over experimental work. Subsequent to a numerical model validated by actual test responses, impact simulations will be conducted to study multiple impact scenarios not practical to test. Twenty-one swing tests using the LandIR gantry were conducted during the June 07 through October 07 time period to evaluate the Orion s impact response. Results for two capsule initial pitch angles, 0deg and -15deg , along with their computer simulations using LS-DYNA are presented in this article. A soil-vehicle friction coefficient of 0.45 was determined by comparing the test stopping distance with computer simulations. In addition, soil modeling accuracy is presented by comparing vertical penetrometer impact tests with computer simulations for the soil model used during the swing tests.
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 multi-layered textile composite reinforcement forming
Wang, P.; Hamila, N.; Boisse, P.
2011-05-04
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 Simulations of Coronal Heating through Footpoint Braiding
NASA Astrophysics Data System (ADS)
Hansteen, V.; Guerreiro, N.; De Pontieu, B.; Carlsson, M.
2015-10-01
Advanced three-dimensional (3D) radiative MHD simulations now reproduce many properties of the outer solar atmosphere. When including a domain from the convection zone into the corona, a hot chromosphere and corona are self-consistently maintained. Here we study two realistic models, with different simulated areas, magnetic field strength and topology, and numerical resolution. These are compared in order to characterize the heating in the 3D-MHD simulations which self-consistently maintains the structure of the atmosphere. We analyze the heating at both large and small scales and find that heating is episodic and highly structured in space, but occurs along loop-shaped structures, and moves along with the magnetic field. On large scales we find that the heating per particle is maximal near the transition region and that widely distributed opposite-polarity field in the photosphere leads to a greater heating scale height in the corona. On smaller scales, heating is concentrated in current sheets, the thicknesses of which are set by the numerical resolution. Some current sheets fragment in time, this process occurring more readily in the higher-resolution model leading to spatially highly intermittent heating. The large-scale heating structures are found to fade in less than about five minutes, while the smaller, local, heating shows timescales of the order of two minutes in one model and one minutes in the other, higher-resolution, model.
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.
The ballistic transport instability in Saturn's rings - III. Numerical simulations
NASA Astrophysics Data System (ADS)
Latter, Henrik N.; Ogilvie, Gordon I.; Chupeau, Marie
2014-07-01
Saturn's inner B-ring and its C-ring support wavetrains of contrasting amplitudes but with similar length-scales, 100-1000 km. In addition, the inner B-ring is punctuated by two intriguing `flat' regions between radii 93 000 and 98 000 km in which the waves die out, whereas the C-ring waves coexist with a forest of plateaus, narrow ringlets, and gaps. In both regions, the waves are probably generated by a large-scale linear instability whose origin lies in the meteoritic bombardment of the rings: the ballistic transport instability. In this paper, the third in a series, we numerically simulate the long-term non-linear evolution of this instability in a convenient local model. Our C-ring simulations confirm that the unstable system forms low-amplitude wavetrains possessing a preferred band of wavelengths. B-ring simulations, on the other hand, exhibit localized non-linear wave `packets' separated by linearly stable flat zones. Wave packets travel slowly while spreading in time, a result that suggests the observed flat regions in Saturn's B-ring are shrinking. Finally, we present exploratory runs of the inner B-ring edge which reproduce earlier numerical results: ballistic transport can maintain the sharpness of a spreading edge while building a `ramp' structure at its base. Moreover, the ballistic transport instability can afflict the ramp region, but only in low-viscosity runs.
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 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 Simulation of Cellular Blood Flow through a Rigid Artery
NASA Astrophysics Data System (ADS)
Reasor, Daniel; Clausen, Jonathan; Aidun, Cyrus
2009-11-01
In blood flow, red blood cells (RBCs), the most numerous constituent of blood, influence continuum-level measures by altering the suspension at microscopic scales. The presence of RBCs alters the stress and diffusion individual cells experience, which can influence cardiovascular diseases by affecting other cells present in blood like platelets and white blood cells. Simulations of blood at a cellular level provide a tool that allows exploration of both the rheology and the stress and diffusion of individual suspended cells. In this work, a hybrid lattice-Boltzmann/finite element method is used to simulate suspension flows characteristic of blood with deformable RBCs at realistic hematocrit values. We have shown the ability to simulate thousands deformable suspensions capturing non-Newtonian flow characteristics such as shear thinning, and the results agree well with experimental observations. Simulations through rigid arteries have been deformed with as many as 2500 RBCs. This work outlines results obtained for pressure-gradient driven blood flow through a rigid artery with 20%, 30%, 40%, and 50% hematocrit values. Results include the effect these deformable RBCs have on mean velocity, flow rate, radial variation of RBC concentration, and the effective viscosity for simulations at moderate to low cell capillary numbers, Ca <=0.08.
Numerical Simulations of Unsteady Natural Convection in Interconnected Systems
NASA Astrophysics Data System (ADS)
Rivera-Solorio, Carlos Ivan; Ramirez-Tijerina, Ramon
2007-11-01
Numerical simulations are performed to study the process of unsteady natural convection in a configuration formed by two interconnected systems. In this configuration, one of the systems has a heat source that increases the temperature of the fluid. By natural convection, this fluid moves to a second system, which works as a radiator. The fluid cools off and descends to return to the first system. The process studied applies to oil heaters, power oil transformers, electrical devices and electronic equipment. The evolution of the velocities and temperature fields of the fluid are analyzed for different configurations and operating conditions of the interconnected systems. The effect in the time response of the heat transfer process is studied for the conditions considered. Conclusions drawn from the numerical results are presented.
DNS Zones Revisited Ward van Wanrooij, Aiko Pras
Pras, Aiko
DNS Zones Revisited Ward van Wanrooij, Aiko Pras Abstract - Recent research suggests that, due investigates the correct configuration of DNS zones, by checking if main configuration requirements, recommendations and best- practices rules have been followed. Our research shows that almost one out of four zones
Geographic Web Usage Estimation By Monitoring DNS Huseyin Akcan
Suel, Torsten
usage statistics are only available locally to the website administrators. Alexa [1] was oneGeographic Web Usage Estimation By Monitoring DNS Caches H¨useyin Akcan CIS Department Polytechnic gather usage statistics ranging from a single DNS server to global scale. In particular, this enables
Unsteady numerical simulations of the stability and dynamics of flames
NASA Technical Reports Server (NTRS)
Kailasanath, K.; Patnaik, G.; Oran, E. S.
1995-01-01
In this report we describe the research performed at the Naval Research Laboratory in support of the NASA Microgravity Science and Applications Program over the past three years (from Feb. 1992) with emphasis on the work performed since the last microgravity combustion workshop. The primary objective of our research is to develop an understanding of the differences in the structure, stability, dynamics and extinction of flames in earth gravity and in microgravity environments. Numerical simulations, in which the various physical and chemical processes can be independently controlled, can significantly advance our understanding of these differences. Therefore, our approach is to use detailed time-dependent, multi-dimensional, multispecies numerical models to perform carefully designed computational experiments. The basic issues we have addressed, a general description of the numerical approach, and a summary of the results are described in this report. More detailed discussions are available in the papers published which are referenced herein. Some of the basic issues we have addressed recently are (1) the relative importance of wall losses and gravity on the extinguishment of downward-propagating flames; (2) the role of hydrodynamic instabilities in the formation of cellular flames; (3) effects of gravity on burner-stabilized flames, and (4) effects of radiative losses and chemical-kinetics on flames near flammability limits. We have also expanded our efforts to include hydrocarbon flames in addition to hydrogen flames and to perform simulations in support of other on-going efforts in the microgravity combustion sciences program. Modeling hydrocarbon flames typically involves a larger number of species and a much larger number of reactions when compared to hydrogen. In addition, more complex radiation models may also be needed. In order to efficiently compute such complex flames recent developments in parallel computing have been utilized to develop a state-of-the-art parallel flame code. This is discussed below in some detail after a brief discussion of the numerical models.
Computer-based numerical simulations of adsorption in nanostructures
NASA Astrophysics Data System (ADS)
Khashimova, Diana
2014-08-01
Zeolites are crystalline oxides with uniform, molecular-pore diameters of 3-14Å. Significant developments since 1950 made production of synthetic zeolites with high purity and controlled chemical composition possible. In powder-form, zeolites are major role-players in high-tech, industrial catalysis, adsorption, and ion exchange applications. Understanding properties of thin-film zeolites has been a focus of recent research. The ability to fine-tune desired macroscopic properties by controlled alteration at the molecular level is paramount. The relationships between macroscopic and molecular-level properties are established by experimental research. Because generating macroscopic, experimental data in a controlled laboratory can be prohibitively costly and time-consuming, reliable numerical simulations, which remove such difficulties, are an attractive alternative. Using a Configurational Biased Monte Carlo (CBMC) approach in grand canonical ensemble, numerical models for pure component and multicomponent adsorption processes were developed. Theoretical models such as ideal (IAST) and real adsorbed solution theory (RAST) to predict mixture adsorption in nanopores were used for comparison. Activity coefficients used in RAST calculations were determined from the Wilson, spreading pressure and COSMO-RS models. Investigative testing of the method on known materials, represented by all-silica zeolites such as MFI (channel type) and DDR (cage type), proved successful in replicating experimental data on adsorption of light hydrocarbons - alkanes, such as methane, ethane, propane and butane. Additionally, adsorption of binary and ternary mixtures was simulated. The given numerical approach developed can be a powerful, cost and time saving tool to predict process characteristics for different molecular-structure configurations. The approach used here for simulating adsorption properties of nanopore materials including process characteristics, may have great potential for other properties of interest.
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 and laboratory simulation of fault motion and earthquake occurrence
NASA Technical Reports Server (NTRS)
Cohen, S. C.
1978-01-01
Simple linear rheologies were used with elastic forces driving the main events and viscoelastic forces being important for aftershock and creep occurrence. Friction and its dependence on velocity, stress, and displacement also plays a key role in determining how, when, and where fault motion occurs. The discussion of the qualitative behavior of the simulators focuses on the manner in which energy was stored in the system and released by the unstable and stable sliding processes. The numerical results emphasize the statistics of earthquake occurrence and the correlations among source parameters.
Numerical simulation of lava flows: Applications to the terrestrial planets
NASA Technical Reports Server (NTRS)
Zimbelman, James R.; Campbell, Bruce A.; Kousoum, Juliana; Lampkin, Derrick J.
1993-01-01
Lava flows are the visible expression of the extrusion of volcanic materials on a variety of planetary surfaces. A computer program described by Ishihara et al. appears to be well suited for application to different environments, and we have undertaken tests to evaluate their approach. Our results are somewhat mixed; the program does reproduce reasonable lava flow behavior in many situations, but we have encountered some conditions common to planetary environments for which the current program is inadequate. Here we present our initial efforts to identify the 'parameter space' for reasonable numerical simulations of lava flows.
Numerical simulation of radiant ceiling panels for indoor cooling
NASA Astrophysics Data System (ADS)
Cammarata, Giuliano; Petrone, Giuseppe; Masi, Filippo
2015-11-01
The aim of this work is to analyse the thermal performance in cooling of two different models of radiative ceiling panels, by varying the discharge temperature and the velocity of heatcarrying fluid. The first one geometrical configuration refers to a coil- embedded panel, while the second one is made by a matrix of tubes. Numerical simulations were carried-out by using a FE-approach to solve governing equations for the physical system. Thermal performances were analysed as a function of the inlet temperature and the inlet velocity of the fluid. Then interpolation functions are proposed in order to assess thermal performances for both geometrical configurations against several working conditions.
Numerical simulation of realistic high-temperature superconductors
1997-07-01
One of the main obstacles in the development of practical high-temperature superconducting (HTS) materials is dissipation, caused by the motion of magnetic flux quanta called vortices. Numerical simulations provide a promising new approach for studying these vortices. By exploiting the extraordinary memory and speed of massively parallel computers, researchers can obtain the extremely fine temporal and spatial resolution needed to model complex vortex behavior. The results may help identify new mechanisms to increase the current-capability capabilities and to predict the performance characteristics of HTS materials intended for industrial applications.
Diffusive mesh relaxation in ALE finite element numerical simulations
Dube, E.I.
1996-06-01
The theory for a diffusive mesh relaxation algorithm is developed for use in three-dimensional Arbitary Lagrange/Eulerian (ALE) finite element simulation techniques. This mesh relaxer is derived by a variational principle for an unstructured 3D grid using finite elements, and incorporates hourglass controls in the numerical implementation. The diffusive coefficients are based on the geometric properties of the existing mesh, and are chosen so as to allow for a smooth grid that retains the general shape of the original mesh. The diffusive mesh relaxation algorithm is then applied to an ALE code system, and results from several test cases are discussed.
Numerical simulations of large-scale solar magnetic fields
NASA Technical Reports Server (NTRS)
Devore, C. R.; Boris, J. P.; Young, T. R., Jr.; Sheeley, N. R.; Harvey, K. L.
1985-01-01
A transport equation which describes the evolution of the large-scale magnetic field of the sun was solved numerically. Data derived from solar magnetic observations are used to initialize the computations and to account for the emergence of new magnetic flux during the sunspot cycle. The objective is to assess the ability of the model to reproduce the observed evolution of the field patterns. Recent results from simulations of individual active regions over a few solar rotations and of the magnetic field of the sun over sunspot cycle 21 are discussed.
Numerical simulation of compact intracloud discharge and generated electromagnetic pulse
NASA Astrophysics Data System (ADS)
Babich, L. P.; Bochkov, E. I.; Kutsyk, I. M.
2015-06-01
Using the concept of the relativistic runaway electron avalanche, numerical simulation of compact intracloud discharge as a generator of powerful natural electromagnetic pulses (EMPs) in the HF-UHF range was conducted. We evaluated the numbers of electrons initiating the avalanche, with which the calculated EMP characteristics are consistent with measured ones. The discharge capable of generating EMPs produces runaway electrons in numbers close to those in the source of terrestrial ?-flashes (TGF) registered in the nearest space, which may be an argument for a joint EMP and TGF source.
NASA Astrophysics Data System (ADS)
Tanahashi, Mamoru; Kikuta, Satoshi; Miyauchi, Toshio
2004-11-01
Three-dimensional DNS of methane-air turbulent premixed flames have been conducted to investigate local extinction mechanism of turbulent premixed flames. A reduced kinetic mechanism (MeCH-19), which is created from GRI-Mech. 2.11 and includes 23 reactive species and 19 step reactions, are used to simulate CH_4-O_2-N2 reaction in turbulence. The effectiveness of this reduced kinetic mechanism has been conformed by preliminary two-dimensional DNS with the reduced kinetic mechanism and two detailed kinetic mechanisms; GRI-Mech. 2.11 and Miller & Bowman. Flame structures of methane-air turbulent premixed flames are compared with those of hydrogen-air turbulent premixed flames which have been obtained by 3D-DNS with a detailed kinetic mechanism in our previous study. Local extinctions occur in methane-air turbulent premixed flames, whereas no extinction is observed for hydrogen-air flames in nearly same turbulence condition. The local extinction mechanism is discussed based on eddy/flame interaction in small scales.
Numerical simulation of the evolution of glacial valley cross sections
Seddik, Hakime; Sugiyama, Shin; Naruse, Renji
2009-01-01
A numerical model was developed for simulating the formation of U-shaped glacial valleys by coupling a two-dimensional ice flow model with an erosion model for a transverse cross section. The erosion model assumes that the erosion rate varies quadratically with sliding speed. We compare the two-dimensional model with a simple shallow-ice approximation model and show the differences in the evolution of a pre-glacial V-shaped valley profile using the two models. We determine the specific role of the lateral shear stresses acting on the glacier side walls in the formation of glacial valleys. By comparing the model results with field data, we find that U-shaped valleys can be formed within 50 ka. A shortcoming of the model is that it primarily simulates the formation of glacial valleys by deepening, whereas observed valleys apparently have formed mainly by widening.
Numerical solutions of atmospheric flow over semielliptical simulated hills
NASA Technical Reports Server (NTRS)
Shieh, C. F.; Frost, W.
1981-01-01
Atmospheric motion over obstacles on plane surfaces to compute simulated wind fields over terrain features was studied. Semielliptical, two dimensional geometry and numerical simulation of flow over rectangular geometries is also discussed. The partial differential equations for the vorticity, stream function, turbulence kinetic energy, and turbulence length scale were solved by a finite difference technique. The mechanism of flow separation induced by a semiellipse is the same as flow over a gradually sloping surface for which the flow separation is caused by the interaction between the viscous force, the pressure force, and the turbulence level. For flow over bluff bodies, a downstream recirculation bubble is created which increases the aspect ratio and/or the turbulence level results in flow reattachment close behind the obstacle.
Numerical simulation of frontogenesis in a moist atmosphere
NASA Technical Reports Server (NTRS)
Hsie, E.-Y.; Anthes, R. A.; Keyser, D.
1984-01-01
This paper describes the effects of condensation and evaporation on mesoscale frontal circulations in a two-dimensional numerical model. Utilizing an explicit scheme for the prediction of water vapor, cloud water and rainwater, the model is used to investigate the interactions between convection and the larger-scale environment. The model results are qualitatively compared with results of theoretical and observational studies, including those from the recent Severe Environmental Storms and Mesoscale Experiment-Atmospheric Variability Experiment (SESAME-AVE). Three major differences are observed in a comparison of the moist and dry simulations: (1) The speed of the upper- and lower-level jets was significantly higher in the moist case, (2) The intensity of the ageostrophic circulations in the moist simulation was much stronger, (3) The vertical velocity field in the moist case was characterized by a banded structure not present in the dry case.
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.
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.
High Resolution Numerical Simulation of Detonation Diffraction of Condensed Explosives
NASA Astrophysics Data System (ADS)
Wang, Cheng
2015-06-01
In this paper, A high resolution large scale parallel computation software is developed based on positivity preserving for finite difference WENO method, high order boundary treatment method, multi-medium interface treatment. A new method for deriving the partial derivative of pressure in respect of every conserved quantity is proposed. The software can simulate detonation diffraction problems for two-dimensional condensed explosives. The numerical simulation results revealed the forming reasons of the low-pressure region, the low-density region, the ``vortex'' region and the ``dead zone'' in the vicinity of the corner. Furthermore, it demonstrated that the retonation will generate along the inner wall, and it plays an important role in the process of detonation diffraction. Finally, we obtain that the propagating state of detonation wave around the corner is generally determined by two factors: the transverse shock wave along the inner wall downwards and the extending curved detonation wave.
Fast numerical simulation for full bore rupture of pressurized pipelines
Mahgerefteh, H.; Saha, P.; Economou, I.G.
1999-06-01
An efficient numerical simulation (CNGS-MOC), based on the method of characteristics for simulating full bore rupture of long pipelines containing two-phase hydrocarbons, was developed. The use of curved characteristics, in conjunction with a compound nested grid system, as well as a fast mathematical algorithm, lead to a significant reduction of CPU time, while improving accuracy. The model is validated extensively against field data including those obtained during the Piper Alpha tragedy, as well as the Isle of Grain depressurization tests. Its predictions are compared with those based on other mathematical models including PLAC, META-HEM, MSM-CS, as well as BLOWDOWN. Both CNGS-MOC and META-HEM produce reasonably accurate predictions with the remaining models assessed performing relatively poorly.
Numerical simulation of a catastrophe model for coronal mass ejections
NASA Technical Reports Server (NTRS)
Forbes, T. G.
1990-01-01
Van Tend and Kuperus (1978) proposed a simple catastrophe model for magnetically driving coronal mass ejections, prominence eruptions, and two-ribbon flares. Their model, which is based on simple circuit concepts, suggests that a stable configuration containing a current filament will lose equilibrium when the filament current exceeds a critical value. Here, a two-dimensional numerical simulation is used to test how the Van Tend-Kuperus model works in an ideal MHD fluid. The simulation exhibits the expected loss of mechanical equilibrium near the predicted critical value, but the current filament moves only a short distance upward before coming to rest at a new equilibrium. However, this new equilibrium contains a current sheet which is resistively unstable to magnetic reconnection, and if magnetic reconnection occurs rapidly, the filament can continue to move upward at Alfvenic speeds.
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.
Mean-field concept and direct numerical simulations of rotating magnetoconvection and the geodynamo
M. Schrinner; K. -H. Raedler; D. Schmitt; M. Rheinhardt; U. R. Christensen
2006-09-27
A comparison is made between mean-field models and direct numerical simulations of rotating magnetoconvection and the geodynamo. The mean-field coefficients are calculated with the fluid velocity taken from the direct numerical simulations. The magnetic fields resulting from mean-field models are then compared with the mean magnetic field from the direct numerical simulations.
Numerical Simulation Modelling for Velocity Measurement of Electromagnetic Flow Meter
NASA Astrophysics Data System (ADS)
Wang, J. Z.; Gong, C. L.; Tian, G. Y.; Lucas, G. P.
2006-10-01
An induced voltage EMF in the area of measuring single-phase flow rate in pipes has been used in many industrial areas. To measure the continuous phase velocity profile in multiphase flows where the continuous phase is an electrical conductor, Electrical capacitance and resistance tomography has been comprehensively investigated, except for continuous phase velocity profile measurement. This paper tries to design the numerical simulation model according to the basic electromagnetic induction law and to investigate the relationship between induced electric potential or potential drop and the velocity distribution of the conductive continuous phase in the flow. First, the 3-Dimenssion simulating module for EMF is built. Given the most simple velocity profile of the fluid in the pipe, the value of the induced potential difference between electrodes is obtained by simulation and theoretical computation according to J A Shercliff's weight function. The relative error is 6.066 . This proves that the simulation model is accurate enough to investigate the characteristic of the induced potential difference of EMF. Finally, the relationship between induced potential difference and the velocity profile is analysed in detail where the complicated velocity profile is expressed as vz = 1m/s when 0.022
Numerical simulation of a plasma actuator based on ion transport
Yamamoto, Seiya; Fukagata, Koji
2013-06-28
Two-dimensional numerical simulation of ion transport and flow around a single dielectric barrier discharge plasma actuator (PA) is performed. Spatial distributions of ions and electrons as well as their time evolution are obtained by solving the transport equations of monovalent positive ions, monovalent negative ions, and electrons. Voltage and frequency of the driving alternating-current signal are assumed to be 8 kV and 5 kHz, respectively. Special focus is laid upon the effect of voltage gradient dV/dt on the magnitude of the body force. The validity of steady force models often used in flow simulation is also examined. The simulation results show that the magnitude of the body force induced by the PA increases as the voltage gradient dV/dt increases and its increase rate becomes milder at higher voltage. The mechanism of body force generation is explained from the time evolution of number density fields of ions and electrons. A comparison between flow simulations using a time-resolved body force and its time-averaged counterpart demonstrates that the time-averaged model gives sufficiently accurate results when the time scale of the flow is more than 30 times greater than that of the PA.
SIMATB: Numerical Simulation in Support to Spacecraft AIT/AIV
NASA Astrophysics Data System (ADS)
Pasquier, F.
2007-08-01
Astrium Satellites, as major integrator of Space vehicles for Earth observation, Navigation, Science and Telecommunications is facing a continuous challenge to improve its operational excellence and industrial processes for reducing time cycles and costs while maintaining high quality standards. A contributor to this constant improvement is the increased use of full software simulations in support to sub-systems / system integration and validation, up to in-orbit operations and exploitation. To achieve this objective, Astrium is deploying through the company the so-called Model-Based Development and Verification (MDV) approach, which implements optimised industrial processes supported by dedicated simulation tools and test facilities. This MDV approach takes benefit from Astrium past experience in the use of full numerical simulation for on-board software validation. MDV approach is now extended to a wider range of spacecraft AIT/AIV processes that allows reducing the amount of hardware (onboard equipment models and EGSE) necessary to support these tasks by ensuring proper representativeness of the simulation models.
Characteristics of the Tropical Tropopause: Observations and Numerical Simulations.
NASA Astrophysics Data System (ADS)
Sijikumar, S.; Sunilkumar, S. V.; Parameswaran, Krishnaswamy; Rajeev, Kunjukrishnapillai
2012-07-01
The Tropical Tropopause plays a pivotal role in climate system through radiative forcing by thin cirrus clouds, convective anvils and regulation of the atmospheric constituents in the stratosphere-troposphere exchange processes. The Tropical Tropopause Layer (TTL) is a source region for stratospheric water vapour. Variations in the stratospheric water vapour in total and ozone to a certain extend is governed by the characteristics of TTL. The upper air data during the field campaign of Tropical Tropopause Dynamics (TTD) experiment shows a periodic disturbance in the upper troposphere/lower stratosphere region. Investigate the spatio-temporal structure of the observed disturbances numerical simulations are carried out. Simulations using the mesoscale model Weather Research Forecast (WRF) are compared with the upper air data during the field campaign of TTD experiments and the simulated profiles showed an excellent agreement with observed profiles. This study showed that the observed features of the periodic disturbance in the measured temperature and wind could be faithfully reproduced by the model and simulations throw more light into the actual mechanism responsible for these observed periodic disturbances.
Simulating Australian Urban Climate in a Mesoscale Atmospheric Numerical Model
NASA Astrophysics Data System (ADS)
Thatcher, Marcus; Hurley, Peter
2012-01-01
We develop an urban canopy scheme coupled to a mesoscale atmospheric numerical model and evaluate the simulated climate of an Australian city. The urban canopy scheme is based on the Town Energy Budget approach, but is modified to efficiently represent the predominately suburban component of Australian cities in regional climate simulations. Energy conservation is improved by adding a simple model of air-conditioning to prevent the urban parametrization acting as an energy sink during the Australian summer. In-canyon vegetation for suburban areas is represented by a big-leaf model, but with a largely reduced set of prognostic variables compared to previous approaches. Although we have used a recirculation/venting based parametrization of in-canyon turbulent heat fluxes that employs two canyon wall energy budgets, we avoid using a fixed canyon orientation by averaging the canyon fluxes after integrating over 180° of possible canyon orientations. The urban canopy scheme is evaluated by simulating the climate for Melbourne, Australia after coupling it to The Air Pollution Model. The combined system was found to predict a realistic climatology of air temperatures and winds when compared with observations from Environmental Protection Authority monitoring stations. The model also produced a plausible partitioning of the urban energy budget when compared to urban flux-tower studies. Overall, the urban canyon parametrization appears to have reasonable potential for studying present and predicting changes in future Australian urban climates in regional climate simulations.
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 of Floodplain Heterogeneity Effects on Meanders Migration
NASA Astrophysics Data System (ADS)
Bogoni, M.; Lanzoni, S.; Putti, M.
2014-12-01
Floodplains and sinuous rivers have a close relationship with each other, mutually influencing their evolutions in time and space. The heterogeneity in erosional resistance has a crucial role on meander planform evolution. It depends on external factors, like land use and cover, but also on the composition of the floodplain, which is due to the ancient geological composition and to the processes associated to long-term river migration. In particular, banks erosion and deposition cause a variation of the superficial composition of the soil, therefore the river patterns are influenced by the previous trends. Based on some recent works, the aim of this contribution is to collect numerical information on the relations between meander migration and the heterogeneity of floodplains caused by oxbow lakes. Numerical simulations have been performed to analyze the temporal and spatial behavior of meanders with a range of values of the erosional resistance of the plain. These values are set as a function of some factors: the characteristic grain size of sediment transported by the flow, the deposition age of the sediments, the eventual presence of vegetation on the banks. The statistical analysis of characteristic geometrical quantities of meanders are able to show the dependence of the simulation results on the meander history. In particular we try to answer to the following questions: how do the rivers affect themselves during their spatial and temporal evolution, modifying the distribution of the floodplain erodibility? Do the migration history plays a main role on the meanders migration modeling?
Numerical simulation on weight function of electromagnetic flow meter
NASA Astrophysics Data System (ADS)
Wang, Jingzhuo; Lu, Rongsheng
2006-11-01
A new finite element procedure for the solution of the weight function, which represents the degree of the contribution of the fluid velocity to the signal in the cross section of a pipe of electromagnetic flow meter, at every point of the cross section of flow pipe of multiphase flow was presented. The solution of the weight function is important for the velocity profile inversion process. First, a numerical simulation model was built up with Femlab. Then a comparison study was investigated using the numerical simulation and Shercliff's weight function. It proved that the finite element methodology is correct for solving the weight function. Based on this result, a new kind of weight quantity solution scheme was carried out when the electrodes are located on the internal circumference of flow pipe and the connecting line of electrodes is parallel to the pipe diameter that is superposition with the coordinate axis. This methodology is effective to obtain the weight function value of electromagnetic flow meter and will be used in the invert reconstruction of the velocity profile imaging, which is significantly important for volumetric flow rate measurement.
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 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 Spray Atomization in Supersonic Flows
NASA Astrophysics Data System (ADS)
Wang, Jiangfeng; Liu, Chen; Wu, Yizhao
With the rapid development of the air-breathing hypersonic vehicle design, an accurate description of the combustion properties becomes more and more important, where one of the key techniques is the procedure of the liquid fuel mixing, atomizing and burning coupled with the supersonic crossflow in the combustion chamber. The movement and distribution of the liquid fuel droplets in the combustion chamber will influence greatly the combustion properties, as well as the propulsion performance of the ramjet/scramjet engine. In this paper, numerical simulation methods on unstructured hybrid meshes were carried out for liquid spray atomization in supersonic crossflows. The Kelvin-Helmholtz/Rayleigh-Taylor hybrid model was used to simulate the breakup process of the liquid spray in a supersonic crossflow with Mach number 1.94. Various spray properties, including spray penetration height, droplet size distribution, were quantitatively compared with experimental results. In addition, numerical results of the complex shock wave structure induced by the presence of liquid spray were illustrated and discussed.
Numerical simulation of flow through the Langley parametric scramjet engine
NASA Technical Reports Server (NTRS)
Srinivasan, Shivakumar; Kamath, Pradeep S.; Mcclinton, Charles R.
1989-01-01
The numerical simulation of a three-dimensional turbulent, reacting flow through the entire Langley parametric scramjet engine has been obtained using a piecewise elliptic approach. The last section in the combustor has been analyzed using a parabolized Navier-Stokes code. The facility nozzle flow was analyzed as a first step. The outflow conditions from the nozzle were chosen as the inflow conditions of the scramjet inlet. The nozzle and the inlet simulation were accomplished by solving the three-dimensional Navier-Stokes equations with a perfect gas assumption. The inlet solution downstream of the scramjet throat was used to provide inflow conditions for the combustor region. The first two regions of the combustor were analyzed using the MacCormack's explicit scheme. However, the source terms in the species equations were solved implicitly. The finite rate chemistry was modeled using the two-step reaction model of Rogers and Chinitz. A complete reaction model was used in the PNS code to solve the last combustor region. The numerical solutions provide an insight of the flow details in a complete hydrogen-fueled scramjet engine module.
Numerical simulation of plasticity at high strain rate
Margolin, L. ); Flower, E.C. )
1991-01-01
In numerical simulations of solid dynamics, the total strain rates are calculated from the velocity fields and are input to the constitutive laws. For an elastic-plastic material, the strain rate for a computational cycle may cause the elastic stress to exceed the specified yield surface. This implies that the stress path must move along the yield surface, and some of the strain is realized as plastic (irreversible) strain. The usual numerical method of integrating the elastic-plastic stress-strain relations is a construction procedure due to Wilkins. The virtue of this construction is that it is simple. Sometimes the procedure is inaccurate and leads to incorrect final stress states and plastic strain. In the computer programs, the computational time step is controlled so that the total strain rate can be assumed to be constant over a cycle. With this assumption, it is possible to find a simple yet exact expression for the final stress states for a large variety of yield surfaces. We will derive this expression assuming only that the yield surface does not explicitly depend on the deviator stresses themselves. A numerical program is used to compare the results of Wilkins' construction with new results for particular strain paths. 5 refs., 4 figs.
Numerical simulation of nonlinear dynamical systems driven by commutative noise
Carbonell, F. Biscay, R.J.; Jimenez, J.C.; Cruz, H. de la
2007-10-01
The local linearization (LL) approach has become an effective technique for the numerical integration of ordinary, random and stochastic differential equations. One of the reasons for this success is that the LL method achieves a convenient trade-off between numerical stability and computational cost. Besides, the LL method reproduces well the dynamics of nonlinear equations for which other classical methods fail. However, in the stochastic case, most of the reported works has been focused in Stochastic Differential Equations (SDE) driven by additive noise. This limits the applicability of the LL method since there is a number of interesting dynamics observed in equations with multiplicative noise. On the other hand, recent results show that commutative noise SDEs can be transformed into a random differential equation (RDE) by means of a random diffeomorfism (conjugacy). This paper takes advantages of such conjugacy property and the LL approach for defining a LL scheme for SDEs driven by commutative noise. The performance of the proposed method is illustrated by means of numerical simulations.
Numerical simulation of drop breakup and coalescence with soluble
NASA Astrophysics Data System (ADS)
Cristini, Vittorio; Lowengrub, John; Zhou, Hua; Macosko, Chris
2003-11-01
In the processing of emulsions and polymer blends, the drop size distributions are determined by two coexisting processes: drop breakup and coalescence. Here we study the effects of surfactants, e.g. block copolymers, on these phenomena and on the shear and normal stress in dilute blends by direct numerical simulation. We use a newly developed 3D adaptive algorithm. A nonlinear equation of state for the surfactant is used and van der Waals forces, which are responsible for coalescence, are included in the numerical method. Surfactants are transported by convection-diffusion on the drop/matrix interface and between the interface and the bulk phases. Our accurate and robust numerical method features parallel computation and adaptive reconstruction of the finite element meshes describing the bulk phases and the interface. We find that surfactants affect strongly the breakup and coalescence mechanisms by introducing nonuniformities in surface tension. The related Marangoni (tangential) stresses at the interface greatly inhibit coalescence but in a nontrivial fashion. At small coverages of surfactant at the interface, the critical capillary number for coalescence (below which coalescence will occur) decreases. However, at larger coverages, the critical capillary number reaches a minimum and then increases again and tends to the value for clean (surfactant-free) interfaces. This behavior was first observed experimentally by Leal and coworkers. In this talk, we demonstrate that this behavior is a consequence of a nontrivial evolution of the Marangoni stresses. We also demonstrate that under certain conditions surfactants enhance coalescence by a totally different mechanism. This surfactant induced coalescence occurs when drops are separating and the surfactant-enriched highly-stretched drop tips interact. Finally, we present preliminary results of simulations that indicate that surfactants have a strong effect on the size of the fragments resulting from drop breakup events, and on the total stress in dilute blends.
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.
Numerical simulations of turbulent thermal convection with differential rotation
NASA Technical Reports Server (NTRS)
Cabot, William H.
1991-01-01
The solar nebula, from which the planets in our solar system formed, featured a disk of gas and dust grains in rapid, differential rotation, and at some stage was likely to have been unstable to thermal convection. This situation is suspected by many to lead to significant turbulent Reynolds stress production and angular momentum transport in such systems, and estimates of transport rates have been attempted from unsubstantiated phenomenological models. In order to determine the circumstances and physical conditions under which our own planetary system formed and to explain recent observations of young stellar systems, it is necessary to develop realistic models of heat and angular momentum transport for such flows. Developing an understanding of complicated flows featuring thermal convection, rotation, and shear is also of wide interest in stellar astrophysics and in planetary and terrestrial atmospheric studies. The ultimate objective is to develop workable models based on the numerical simulations for constructing global solar nebula models; viz., relatively simple prescriptions for heat and angular momentum fluxes from given system parameters (e.g., ratios of rotation, shear, and convective lapse rates) are characterized, quantified, and developed. Toward this end, our program has been to attempt to understand the behavior of the direct numerical simulations of Boussinesq convection, which, despite the complexity of the results, is still an overly simplified approximation to the real system and should be more amenable to analysis. These results are also intended to be tested against turbulence models, especially those designed for atmospheric boundary layers, and may provide a basis for subgrid-scale models. In order to make the numerical simulations more realistic with regard to the solar nebula problem, a fully compressible code that will allow incorporation of large density stratifications and realistic thermodynamic and radiative properties is developed. In order to explore the properties of these flows at the very high values of Re found in natural systems and the very low values of Pr found in most astrophysical contexts, we will need to employ large-eddy simulations for which we want to determine the most appropriate subgrid-scale model to incorporate.
Outflow Driven Cavities: Numerical Simulations of Intermediaries of Protostellar Turbulence
Andrew J. Cunningham; Adam Frank; Alice C. Quillen; Eric G. Blackman
2006-10-27
We investigate the evolution of fossil cavities produced by extinct YSO jets and wide angle outflows. Fossil cavities are ellipsoidal or cylindrical shells of swept-up of ambient (molecular cloud) material moving at low velocities. The cavities form when the momentum in a YSO jet or wide angle outflow decays in time allowing the bowshock or swept-up shell to decelerate to velocities near the turbulent speed in the cloud. It has been suggested in previous studies that cavities provide efficient coupling between the jets/outflows and the cloud and, as such, are the agents by which cloud turbulence is can be re-energized. In this paper we carry forward a series of numerical simulations of jets and outflows whose momentum flux decrease in time. We compare simulations with decaying momentum fluxes to those with constant flux. We show that decaying flux models exhibit deceleration of the outflow head and backfilling via expansion off of the cavity walls. They also have lower density contrast, are longer lived and wider than their continuously driven counterparts. The simulations recover the basic properties of observed fossil cavities. In addition, we provide synthetic observations in terms of P-V diagrams which demonstrate that fossil cavities form both jets and wide angle outflows are characterized by linear "Hubble-law" expansions patterns superimposed on "spur" patterns indicative of the head of a bow shock.
A simplified DEM numerical simulation of vibroflotation without backfill
NASA Astrophysics Data System (ADS)
Jiang, M. J.; Liu, W. W.; He, J.; Sun, Y.
2015-09-01
Vibroflotation is one of the deep vibratory compaction techniques for ground reinforcement. This method densities the soil and improves its mechanical properties, thus helps to protect people's lives and property from geological disasters. The macro reinforcement mechanisms of vibroflotation method have been investigated by numerical simulations, laboratory and in-situ experiments. However, little attention has been paid on its micro - mechanism, which is essential to fully understand the principle of the ground reinforcement. Discrete element method (DEM), based on discrete mechanics, is more powerful to solve large deformation and failure problems. This paper investigated the macro-micro mechanism of vibroflotation without backfill under two conditions, i.e., whether or not the ground water was considered, by incorporating inter-particle rolling resistance model in the DEM simulations. Conclusions obtained are as follows: The DEM simulations incorporating rolling resistance well replicate the mechanical response of the soil assemblages and are in line with practical observations. The void ratio of the granular soil fluctuates up and down in the process of vibroflotation, and finally reduces to a lower value. It is more efficient to densify the ground without water compared to the ground with water.
The ballistic transport instability in Saturn's rings III: numerical simulations
Latter, Henrik; Chupeau, Marie
2014-01-01
Saturn's inner B-ring and its C-ring support wavetrains of contrasting amplitudes but with similar length scales, 100-1000 km. In addition, the inner B-ring is punctuated by two intriguing `flat' regions between radii 93,000 km and 98,000 km in which the waves die out, whereas the C-ring waves coexist with a forest of plateaus, narrow ringlets, and gaps. In both regions the waves are probably generated by a large-scale linear instability whose origin lies in the meteoritic bombardment of the rings: the ballistic transport instability. In this paper, the third in a series, we numerically simulate the long-term nonlinear evolution of this instability in a convenient local model. Our C-ring simulations confirm that the unstable system forms low-amplitude wavetrains possessing a preferred band of wavelengths. B-ring simulations, on the other hand, exhibit localised nonlinear wave `packets' separated by linearly stable flat zones. Wave packets travel slowly while spreading in time, a result that suggests the obser...
Numerical simulations of Mach stem formation via intersecting bow shocks
NASA Astrophysics Data System (ADS)
Hansen, E. C.; Frank, A.; Hartigan, P.; Yirak, K.
2015-12-01
Hubble Space Telescope observations show bright knots of H? emission within outflowing young stellar jets. Velocity variations in the flow create secondary bow shocks that may intersect and lead to enhanced emission. When the bow shocks intersect at or above a certain critical angle, a planar shock called a Mach stem is formed. These shocks could produce brighter H? emission since the incoming flow to the Mach stem is parallel to the shock normal. In this paper we report first results of a study using 2-D numerical simulations designed to explore Mach stem formation at the intersection of bow shocks formed by hypersonic "bullets" or "clumps". Our 2-D simulations show how the bow shock shapes and intersection angles change as the adiabatic index ? changes. We show that the formation or lack of a Mach stem in our simulations is consistent with the steady-state Mach stem formation theory. Our ultimate goal, which is part of an ongoing research effort, is to characterize the physical and observational consequences of bow shock intersections including the formation of Mach stems.
Numerical Simulation of a Solar Domestic Hot Water System
NASA Astrophysics Data System (ADS)
Mongibello, L.; Bianco, N.; Di Somma, M.; Graditi, G.; Naso, V.
2014-11-01
An innovative transient numerical model is presented for the simulation of a solar Domestic Hot Water (DHW) system. The solar collectors have been simulated by using a zerodimensional analytical model. The temperature distributions in the heat transfer fluid and in the water inside the tank have been evaluated by one-dimensional models. The reversion elimination algorithm has been used to include the effects of natural convection among the water layers at different heights in the tank on the thermal stratification. A finite difference implicit scheme has been implemented to solve the energy conservation equation in the coil heat exchanger, and the energy conservation equation in the tank has been solved by using the finite difference Euler implicit scheme. Energy conservation equations for the solar DHW components models have been coupled by means of a home-made implicit algorithm. Results of the simulation performed using as input data the experimental values of the ambient temperature and the solar irradiance in a summer day are presented and discussed.
Numerical Simulation of Flow Field Within Parallel Plate Plastometer
NASA Technical Reports Server (NTRS)
Antar, Basil N.
2002-01-01
Parallel Plate Plastometer (PPP) is a device commonly used for measuring the viscosity of high polymers at low rates of shear in the range 10(exp 4) to 10(exp 9) poises. This device is being validated for use in measuring the viscosity of liquid glasses at high temperatures having similar ranges for the viscosity values. PPP instrument consists of two similar parallel plates, both in the range of 1 inch in diameter with the upper plate being movable while the lower one is kept stationary. Load is applied to the upper plate by means of a beam connected to shaft attached to the upper plate. The viscosity of the fluid is deduced from measuring the variation of the plate separation, h, as a function of time when a specified fixed load is applied on the beam. Operating plate speeds measured with the PPP is usually in the range of 10.3 cm/s or lower. The flow field within the PPP can be simulated using the equations of motion of fluid flow for this configuration. With flow speeds in the range quoted above the flow field between the two plates is certainly incompressible and laminar. Such flows can be easily simulated using numerical modeling with computational fluid dynamics (CFD) codes. We present below the mathematical model used to simulate this flow field and also the solutions obtained for the flow using a commercially available finite element CFD code.
Numerical relativity in spherical polar coordinates: Off-center simulations
NASA Astrophysics Data System (ADS)
Baumgarte, Thomas W.; Montero, Pedro J.; Müller, Ewald
2015-03-01
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 Baumgarte-Shapiro-Shibata-Nakamura 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.
Numerical Simulations of Coronal Heating through Footpoint Braiding
Hansteen, Viggo; De Pontieu, Bart; Carlsson, Mats
2015-01-01
Advanced 3D radiative MHD simulations now reproduce many properties of the outer solar atmosphere. When including a domain from the convection zone into the corona, a hot chromosphere and corona are self-consistently maintained. Here we study two realistic models, with different simulated area, magnetic field strength and topology, and numerical resolution. These are compared in order to characterize the heating in the 3D-MHD simulations which self-consistently maintains the structure of the atmosphere. We analyze the heating at both large and small scales and find that heating is episodic and highly structured in space, but occurs along loop shaped structures, and moves along with the magnetic field. On large scales we find that the heating per particle is maximal near the transition region and that widely distributed opposite-polarity field in the photosphere leads to a greater heating scale height in the corona. On smaller scales, heating is concentrated in current sheets, the thicknesses of which are set ...
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.
DNS of Viscoelastic Turbulent Channel Flow at Maximum Drag Reduction
NASA Astrophysics Data System (ADS)
Zhou, Qiang; Liu, Jin; Akhavan, Rayhaneh
2003-11-01
Maximum Drag Reduction (MDR) by dilute polymer solutions is investigated in a turbulent channel flow by DNS. The simulations were performed using a semi-Lagrangian scheme employing pseudo-spectral methods for the solvent and a Backward tracking Lagrangian Particle Method (BLPM) with a multimode (FENE-LSMR) constitutive model for the polymer. The polymer and flow parameters (Re_? ˜ 225, b=45000, nk_BT/? u^2_?=1.15× 10-3) were selected to replicate one of the experimental data points of Virk (1975). Simulations were performed for We_? ranging from 35 to 200, and resulted in drag reductions of 30-70% (MDR). The computed turbulence statistics agree with the experimental data of Warholic et al.(1999) for comparable drag reduction. All simulations with We_? ? 100 result in maximum drag reduction and a mean velocity profile in agreement with Virk's asymptote. However, the condition of zero Reynolds shear stress is achieved only for We_? ? 150. The physics of drag reduction at MDR is currently under study and will be discussed.
Numerical simulation of high intensity laser-plasma interaction
NASA Astrophysics Data System (ADS)
Fomyts'kyi, Mykhailo
In this work two different areas of high intensity laser-plasma interaction are considered. The first part of the dissertation describes the dynamics of laser-irradiated clusters. It addresses two different regimes of laser-cluster interactions. In the so-called Coulomb regime, the laser pulse removes a significant part of the electrons from the cluster. The remaining electrons form a cold electron core inside a positively charged ion shell. The ion shell expands due to its space charge. A different situation occurs in the so-called hydrodynamic regime. In this case, a two-component electron distribution is formed in the cluster due to stochastic vacuum heating. The cluster remains quasi-neutral and it expands due to the hot electron pressure. Understanding electron and ion dynamics in both these regimes is the main goal of the first part of the dissertation. Stochastic vacuum heating of the electrons is demonstrated in the hydrodynamic regime. Anisotropy in cluster expansion is predicted and the sign of the anisotropy is found to depend on the laser intensity. A model of harmonic generation in clusters is developed. Resonant enhancement of harmonic generation during cluster expansion is demonstrated. Our theoretical models are verified and extended via numerical simulations using a newly-developed particle-in-cell axisymmetric electrostatic code. The second part of the dissertation deals with laser wakefield acceleration in the self-modulation regime seeded by a Raman shifted low amplitude laser pulse. Raman seeding provides means of coherent control of the excited wakefield. The energy threshold for pulse modulation in the diffraction limited regime is derived. The relative roles of the seed and the leading edge of the pulse in creating an initial perturbation are compared. One dimensional and two dimensional particle-in-cell simulations are employed to model the effects of the seed pulse. Examples of coherent control are demonstrated. Numerical simulations show that a 38 mJ Raman seeded pulse can generate relativistic bunches of ˜1 nC. Conventional (unseeded) self-modulated laser wakefield acceleration would require significantly more energetic pulses at relativistic intensities for generating similar electron bunches. Our results indicate that a pulse repetition rate of ˜1 kHz may be feasible with proper Raman seeding. The simulation also demonstrate the possibility of Raman-seeded acceleration by pulses of subcritical power (P = 1/2 Pc, 19 mJ) in a plasma channel.
Formation of relativistic MHD jets: stationary state solutions & numerical simulations
Fendt, Christian
2008-01-01
We discuss numerical results of relativistic magnetohydrodynamic (MHD) jet formation models. We first review some examples of stationary state solutions treating the collimation and acceleration process of relativistic MHD jets. We provide an a posteriori check for the MHD condition in highly magnetized flows, namely the comparison of particle density to Goldreich-Julian density. Using the jet dynamical parameters calculated from the MHD model we show the rest-frame thermal X-ray spectra of the jet, from which we derive the overall spectrum taking into account a variation of Doppler boosting and Doppler shift of emission lines along the outflow. Finally, we present preliminary results of relativistic MHD simulations of jet formation demonstrating the acceleration of a low velocity (0.01c) disk wind to a collimated high velocity (0.8c).
Formation of relativistic MHD jets: stationary state solutions & numerical simulations
Christian Fendt; Elisabetta Memola
2008-11-20
We discuss numerical results of relativistic magnetohydrodynamic (MHD) jet formation models. We first review some examples of stationary state solutions treating the collimation and acceleration process of relativistic MHD jets. We provide an a posteriori check for the MHD condition in highly magnetized flows, namely the comparison of particle density to Goldreich-Julian density. Using the jet dynamical parameters calculated from the MHD model we show the rest-frame thermal X-ray spectra of the jet, from which we derive the overall spectrum taking into account a variation of Doppler boosting and Doppler shift of emission lines along the outflow. Finally, we present preliminary results of relativistic MHD simulations of jet formation demonstrating the acceleration of a low velocity (0.01c) disk wind to a collimated high velocity (0.8c).
Numerical simulation of a radially injected barium cloud
NASA Technical Reports Server (NTRS)
Swift, D. W.; Wescott, E. M.
1981-01-01
Electrostatic two-dimensional numerical simulations of a radially symmetric barium injection experiment demonstrate that ions created by solar UV irradiation are electrostatically bound to the electrons which remain tied to the field lines on which they are created. Two possible instabilities are identified, but neither of them causes the barium plasma cloud to polarize in a way that would permit the plasma to keep up with the neutrals. In a second model, the velocity of the neutrals is allowed to be a function of the azimuthal angle. Here, a portion of the cloud does polarize in a way that allows a portion of the plasma to detach and move outward at the approximate speed of the neutrals. No rapid detachment is found when only the density of the neutrals is given an azimuthal asymmetry.
Numerical Simulation of Explosive Consolidation of Superconducting Bulk Components
NASA Astrophysics Data System (ADS)
Mamalis, A. G.; Vottea, I. N.; Manolakos, D. E.; Szalay, A.; Kladas, A.
The explosive compaction technique has been used for manufacturing bulk superconducting components, e.g. three dimensional plates and axisymmetric billets, rods, discs and tubes, with high density and good electrical and magnetic characteristics. The high pressures and temperatures developed in a very short time result in sintering and in fracturing of the original grains, inducing primarily line defects that would provide flux pinning centers in Type II superconductors. Densified superconducting ceramic YBCO bulk components of various geometries were produced by this dynamic technique and the whole process was simulated by the explicit finite element code LS-DYNA3D. In this paper, experimental and numerical results of the fabricated superconducting bulks are reported and discussed. Applications are also briefly outlined.
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.
Numerical simulation of ion rings and ion beam propagation
NASA Astrophysics Data System (ADS)
Manofsky, A.
The development of numerical simulation techniques for studying the physics of ion beams and rings in a background plasma as applicable to certain problems in magnetic and inertial confinement fusion is presented. Two codes were developed for these purposes: RINGA and CIDER. The 2 and 1/2 dimensional particle code RINGA follows the trajectories of ions in their self consistent magnetic field. The code assumes strict charge neutrality and admits currents only in the azimuthal direction. The injection and resistive trapping of ion rings was with RINGA. Modifications to RINGA to include finite pressure of confined plasma and beam ion electron slowing down collisions are discussed. In the CIDER hybrid code, ions are represented by particles and electrons by an inertialess thermal fluid which obeys a generalized Ohm's law. Fields are solved in the quasineutral Darwin approximation. Several collisional and atomic processes are included.
Numerical simulation of vehicle crashworthiness and occupant protection
NASA Technical Reports Server (NTRS)
Saha, Nripen K.
1993-01-01
Numerical simulation of vehicle crashworthiness and occupant protection are addressed. The vehicle crashworthiness design objectives are to design the vehicle structure for optimum impact energy absorption, and to design the restraint system (seatbelts, airbags, bolsters, etc.) for optimum occupant protection. The following approaches are taken; a major part of the impact energy is to be absorbed by the vehicle structure; the restraint components will provide protection against the remaining crash energy; certain vehicle components are designed to deform under specific types and speeds of impact in a desired mode for sound energy management; structural components such as front side rails, rear rails, door structure and pillars undergo large amounts of deformation; and with properly designed geometry and material these components assist in mitigating the effects of impact.
Numerical simulation of vehicle crashworthiness and occupant protection
NASA Astrophysics Data System (ADS)
Saha, Nripen K.
1993-08-01
Numerical simulation of vehicle crashworthiness and occupant protection are addressed. The vehicle crashworthiness design objectives are to design the vehicle structure for optimum impact energy absorption, and to design the restraint system (seatbelts, airbags, bolsters, etc.) for optimum occupant protection. The following approaches are taken; a major part of the impact energy is to be absorbed by the vehicle structure; the restraint components will provide protection against the remaining crash energy; certain vehicle components are designed to deform under specific types and speeds of impact in a desired mode for sound energy management; structural components such as front side rails, rear rails, door structure and pillars undergo large amounts of deformation; and with properly designed geometry and material these components assist in mitigating the effects of impact.
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 and immersive visualization of hairpin vortices.
Tufo, H.M.; Fischer, P.F.; Papka, M.E.; Blom, K.
1999-08-28
To better understand the vortex dynamics of coherent structures in turbulent and transitional boundary layers, we consider direct numerical simulation of the interaction between a flat-plate-boundary-layer flow and an isolated hemispherical roughness element. Of principal interest is the evolution of hairpin vortices that form an interlacing pattern in the wake of the hemisphere, lift away from the wall, and are stretched by the shearing action of the boundary layer. Using animations of unsteady three-dimensional representations of this flow, produced by the vtk toolkit and enhanced to operate in a CAVE virtual environment, we identify and study several key features in the evolution of this complex vortex topology not previously observed in other visualization formats.
Direct numerical simulation of the three-dimensional vortex street
NASA Astrophysics Data System (ADS)
Karniadakis, George Em; Triantafyllou, George S.
1990-01-01
A numerical study of three-dimensional vortex streets is performed through direct simulation of the incompressible Navier-Stokes equations. A three-dimensional spectral-element method is employed with a viscous outflow boundary condition. It is shown that there exists a critical Reynolds number value, close to 180, above which two-dimensional vortex streets, while stable to two-dimensional perturbations, become unstable to three-dimensional perturbations. This instability amplifies exponentially the spanwise velocity component and leads to the formation of three-dimensional flow patterns. A new frequency of oscillation (Strouhal number) is thus selected, which is lower than the frequency of the two-dimensional vortex street. The implications of these findings for the issues of transition to turbulence and the Strouhal versus Reynolds number relation are discussed.
Direct numerical simulation of transition to turbulence in Goertler flow
NASA Astrophysics Data System (ADS)
Liu, Wei; Domaradzki, J. A.
1993-01-01
The transition to turbulence in a boundary-layer-flow counterrotating vortices with axes aligned in the streamwise direction was invesitgated using direct numerical simulation techniques. The vortices were assumed to have been generated by the Goertler instability mechanism operating in boundary-layer flows over concave walls. Results indicate that instability oscillations started in the spanwise direction and were followed later by oscillations in the vertical direction. Regions of high perturbation velocity correlated well with the regions of high spanwise shear, while no obvious correlation with the vertical shear regions was observed. The analysis of the kinetic energy balance equation revealed that most of the perturbation energy production in the initial stages of transition occurs in the region characterized by large spanwise shear created by the action of vortices moving low-speed fluid away from the wall.
Numerical Simulation of Dual-Mode Scramjet Combustors
NASA Technical Reports Server (NTRS)
Rodriguez, C. G.; Riggins, D. W.; Bittner, R. D.
2000-01-01
Results of a numerical investigation of a three-dimensional dual-mode scramjet isolator-combustor flow-field are presented. Specifically, the effect of wall cooling on upstream interaction and flow-structure is examined for a case assuming jet-to-jet symmetry within the combustor. Comparisons are made with available experimental wall pressures. The full half-duct for the isolator-combustor is then modeled in order to study the influence of side-walls. Large scale three-dimensionality is observed in the flow with massive separation forward on the side-walls of the duct. A brief review of convergence-acceleration techniques useful in dual-mode simulations is presented, followed by recommendations regarding the development of a reliable and unambiguous experimental data base for guiding CFD code assessments in this area.
The thermal stability of coronal loops - Numerical simulations
NASA Technical Reports Server (NTRS)
Mok, Y.; Schnack, D. D.; Van Hoven, G.
1991-01-01
The radiative stability of thermally isolated coronal loops with free-flow boundary conditions by nonlinear numerical simulation are studied. A chromosphere-to-corona loop equilibrium (including the option of a deep chromosphere) is first established by following the nonlinear evolution from an initial isothermal state with rigid boundaries. The end conditions are then changed, to allow free flow and to fix the temperature, and investigate the response to nonisobaric perturbations. Within a family of loops of the same pressure, long hot loops to be stable and short cool loops to be unstable to the thermal chromospheric-expansion mode are found. The stable cases remain so, even when long chromospheric ends and/or gravity are added. In those cases which are unstable, the subsequent nonlinear evolution which exhibits swelling of the chromosphere until the entire loop becomes cool and dense are followed.
3D Numerical Simulation on the Rockslide Generated Tsunamis
NASA Astrophysics Data System (ADS)
Chuang, M.; Wu, T.; Wang, C.; Chu, C.
2013-12-01
The rockslide generated tsunami is one of the most devastating nature hazards. However, the involvement of the moving obstacle and dynamic free-surface movement makes the numerical simulation a difficult task. To describe both the fluid motion and solid movement at the same time, we newly developed a two-way fully-coupled moving solid algorithm with 3D LES turbulent model. The free-surface movement is tracked by volume of fluid (VOF) method. The two-step projection method is adopted to solve the Navier-Stokes type government equations. In the new moving solid algorithm, a fictitious body force is implicitly prescribed in MAC correction step to make the cell-center velocity satisfied with the obstacle velocity. We called this method the implicit velocity method (IVM). Because no extra terms are added to the pressure Poission correction, the pressure field of the fluid part is stable, which is the key of the two-way fluid-solid coupling. Because no real solid material is presented in the IVM, the time marching step is not restricted to the smallest effective grid size. Also, because the fictitious force is implicitly added to the correction step, the resulting velocity is accurate and fully coupled with the resulting pressure field. We validated the IVM by simulating a floating box moving up and down on the free-surface. We presented the time-history obstacle trajectory and compared it with the experimental data. Very accurate result can be seen in terms of the oscillating amplitude and the period (Fig. 1). We also presented the free-surface comparison with the high-speed snapshots. At the end, the IVM was used to study the rock-slide generated tsunamis (Liu et al., 2005). Good validations on the slide trajectory and the free-surface movement will be presented in the full paper. From the simulation results (Fig. 2), we observed that the rockslide generated waves are manly caused by the rebounding waves from two sides of the sliding rock after the water is dragging down by the solid downward motion. We also found that the turbulence has minor effect to the main flow field. The rock size, rock density, and the steepness of the slope were analyzed to understand their effects to the maximum runup height. The detailed algorithm of IVM, the validation, the simulation and analysis of rockslide tsunami will be presented in the full paper. Figure 1. Time-history trajectory of obstacle for the floating obstacle simulation. Figure 2. Snapshots of the free-surface elevation with streamlines for the rockslide tsunami simulation.
Numerical Simulation of Real Debris-Flow Events
NASA Astrophysics Data System (ADS)
Fraccarollo, L.; Papa, M.
2000-09-01
A one-dimensional model is presented to predict debris-flow runouts. The model is based on shallow water type assumptions. The fluid is assumed to be homogeneous and the original bed of the flow domain to be unerodible. The fluid is characterized by a rheology of Bingham type. A numerical tool able to cope with the nature of debris flows has been worked out. It represents an extension of a second order accurate and conservative method of Godunov type. Special care has been devoted to the influence of the source terms and of the geometrical representation of the natural cross sections, which play a fundamental role. The application concerns a monitored event in the Dolomites in Italy, where field analyses allowed a characterization of the behavior of solid-liquid mixture as a yield stress material. The comparison between numerical simulations and field observations highlights the impossibility of representing all phases of the flow with constant values of the rheological parameters. Nevertheless the results show that it is possible to separately represent the phase of the flow in the upstream reach and the phase of the deposition in the alluvial fan, with a good agreement with field observations.
Numerical Simulation of Bubble Dynamics in Deformable Vessels
NASA Astrophysics Data System (ADS)
Coralic, Vedran; Colonius, Tim
2011-11-01
The growth and collapse of cavitation bubbles has been implicated as a potential damage mechanism leading to the rupture of blood vessels in shock wave lithotripsy (SWL). While this phenomenon has been investigated numerically, the resulting simulations have often assumed some degree of symmetry and have often failed to include a large number of influential physics, such as viscosity, compressibility, surface tension, phase change and fluid-structure interactions. We present here our efforts to explore the role that cavitation bubbles play in the rupture of blood vessels in SWL and to improve upon the current state of the numerical approach. We have developed a three-dimensional, high-order accurate, shock- and interface-capturing, multicomponent flow algorithm that accounts for the effects of viscosity and surface tension. At this time, we omit any effects due to elasticity and instead, as a first step, model tissue as a viscous and stiffened gas. We discuss preliminary results for the Rayleigh and shock-induced collapse of a gas bubble within a blood vessel and characterize the increase in vessel deformation with increasing bubble confinement and proximity to the vessel wall. This research was supported by the National Institutes of Health grant No. 2PO1DK43881.
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.
Numerical Simulations of Particle Deposition in Metal Foam Heat Exchangers
NASA Astrophysics Data System (ADS)
Sauret, Emilie; Saha, Suvash C.; Gu, Yuantong
2013-01-01
Australia is a high-potential country for geothermal power with reserves currently estimated in the tens of millions of petajoules, enough to power the nation for at least 1000 years at current usage. However, these resources are mainly located in isolated arid regions where water is scarce. Therefore, wet cooling systems for geothermal plants in Australia are the least attractive solution and thus air-cooled heat exchangers are preferred. In order to increase the efficiency of such heat exchangers, metal foams have been used. One issue raised by this solution is the fouling caused by dust deposition. In this case, the heat transfer characteristics of the metal foam heat exchanger can dramatically deteriorate. Exploring the particle deposition property in the metal foam exchanger becomes crucial. This paper is a numerical investigation aimed to address this issue. Two-dimensional (2D) numerical simulations of a standard one-row tube bundle wrapped with metal foam in cross-flow are performed and highlight preferential particle deposition areas.
Numerical Simulation on Zonal Disintegration in Deep Surrounding Rock Mass
Chen, Xuguang; Wang, Yuan; Mei, Yu; Zhang, Xin
2014-01-01
Zonal disintegration have been discovered in many underground tunnels with the increasing of embedded depth. The formation mechanism of such phenomenon is difficult to explain under the framework of traditional rock mechanics, and the fractured shape and forming conditions are unclear. The numerical simulation was carried out to research the generating condition and forming process of zonal disintegration. Via comparing the results with the geomechanical model test, the zonal disintegration phenomenon was confirmed and its mechanism is revealed. It is found to be the result of circular fracture which develops within surrounding rock mass under the high geostress. The fractured shape of zonal disintegration was determined, and the radii of the fractured zones were found to fulfill the relationship of geometric progression. The numerical results were in accordance with the model test findings. The mechanism of the zonal disintegration was revealed by theoretical analysis based on fracture mechanics. The fractured zones are reportedly circular and concentric to the cavern. Each fracture zone ruptured at the elastic-plastic boundary of the surrounding rocks and then coalesced into the circular form. The geometric progression ratio was found to be related to the mechanical parameters and the ground stress of the surrounding rocks. PMID:24592166
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.
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.
Finite-difference numerical simulations of underground explosion cavity decoupling
NASA Astrophysics Data System (ADS)
Aldridge, D. F.; Preston, L. A.; Jensen, R. P.
2012-12-01
Earth models containing a significant portion of ideal fluid (e.g., air and/or water) are of increasing interest in seismic wave propagation simulations. Examples include a marine model with a thick water layer, and a land model with air overlying a rugged topographic surface. The atmospheric infrasound community is currently interested in coupled seismic-acoustic propagation of low-frequency signals over long ranges (~tens to ~hundreds of kilometers). Also, accurate and efficient numerical treatment of models containing underground air-filled voids (caves, caverns, tunnels, subterranean man-made facilities) is essential. In support of the Source Physics Experiment (SPE) conducted at the Nevada National Security Site (NNSS), we are developing a numerical algorithm for simulating coupled seismic and acoustic wave propagation in mixed solid/fluid media. Solution methodology involves explicit, time-domain, finite-differencing of the elastodynamic velocity-stress partial differential system on a three-dimensional staggered spatial grid. Conditional logic is used to avoid shear stress updating within the fluid zones; this approach leads to computational efficiency gains for models containing a significant proportion of ideal fluid. Numerical stability and accuracy are maintained at air/rock interfaces (where the contrast in mass density is on the order of 1 to 2000) via a finite-difference operator "order switching" formalism. The fourth-order spatial FD operator used throughout the bulk of the earth model is reduced to second-order in the immediate vicinity of a high-contrast interface. Current modeling efforts are oriented toward quantifying the amount of atmospheric infrasound energy generated by various underground seismic sources (explosions and earthquakes). Source depth and orientation, and surface topography play obvious roles. The cavity decoupling problem, where an explosion is detonated within an air-filled void, is of special interest. A point explosion source located at the center of a spherical cavity generates only diverging compressional waves. However, we find that shear waves are generated by an off-center source, or by a non-spherical cavity (e.g. a tunnel). Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
Numerical simulation of linear fiction welding (LFW) processes
Fratini, L.; La Spisa, D.
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 Detonation Wave - Magnetic Field Interactions
NASA Astrophysics Data System (ADS)
Cole, Lord; Karagozian, Ann
2012-11-01
Numerical simulations of one- and two-dimensional detonation waves subjected to an applied magnetic field are performed, with applications to flow control and MHD thrust augmentation in Pulse Detonation Engines and their design variations. The evolution of the ionization processes and the diffusive and convective transport of the magnetic field are examined in the context of their effect on detonation dynamics. As with prior studies on hydrogen-air detonation dynamics, the present studies explore hydrogen-air-cesium detonations via high order shock capturing schemes and complex reaction kinetics, in addition to a two-temperature relaxation model for the plasma. One-dimensional simulations examining the non-coupled effect of the magnetic field on the unsteady detonation indicate that the stabilizing effect of the dilluent, cesium, becomes less effective when it becomes an active participant under the influence of strong magnetic fields. Two-dimensional dynamics allow a more complete coupling between the magnetic field and the detonation kinetics to be represented, with implications for an alteration in stability characteristics. Supported by the US Air Force/ERC, Inc. under subcontract RS100226.
Numerical simulation of spall failure in metals under shock compression
NASA Astrophysics Data System (ADS)
Bayandin, Yuriy; Naimark, Oleg; Uvarov, Sergey
2009-06-01
Nonlinear aspects of damage localization and damage-failure transition are the subject of long-term research and arise both fundamental and applied interest related to multiscale nature of failure mechanism. The developed statistical model of solid with mesoscopic defects allowed the formulation of phenomenological model in terms of two independent variables - the defect density tensor and structural scaling parameter and the simulation of shock wave propagation in the linkage with structural relaxation phenomena. Presented investigation is related to the study of damage-failure scenario in the plate impact test (spall failure) taking into account long-range correlation properties induced by the collective modes of defects. Numerical simulation of plane shock wave propagation was carried out to establish spall conditions and to propose the mechanism of damage-failure transitions as a specific form of self-organized criticality in the ensemble of mesoscopic defects - structural-scaling transition. Characteristic features of this transition are the generation of collective modes in mesodefect ensemble that are responsible for damage localization and transition to failure.
Numerical simulation of a turning alpine ski during recreational skiing.
Hirano, Y; Tada, N
1996-09-01
While downhill snow skiing, recreational alpine skiers enjoy making turning motions with their skis. These motions are mainly induced by skidding, while turning by alpine ski racers is made by carving a trace in the snow. In the present study we treat the turning motions by recreational alpine skiers. This "skidding" turning motion is made possible by centripetal forces acting on the ski and skier dynamic motion systems, with these forces arising due to the skier placing the ski's longitudinal axis at an angle that is inclined away from the velocity vector and edging the ski into the snow. When snow is soft, the edged ski creates a snow impacting force, whereas a snow cutting force occurs when it is hard. Here, we calculate the former force using a three-dimensional water jet analogy, while the latter one using conventional metal cutting theory, after which the corresponding equations of motion for each system are derived and numerically solved. This methodology enables simulating the curvilinear and rotational motion of the ski and skier systems. Resultant simulations quantitatively show for the first time that the resultant radius of curvature of a ski track while downhill skiing is strongly dependent on the location of the ski boot on the ski's longitudinal axis and also on its side-cut (midlength taper). PMID:8883012
Numerical simulations of the mountain iron tracer data
Yamada, T.; Bunker, S.
1990-01-01
Extensive field experiments were conducted during 1965 and 1966 near Vandenberg Air Force Base. The experiments included wind speed and wind direction measurements at several towers, upper air soundings by radiosondes, and fluorescent particle releases to characterize the diffusion processes. The data provide a unique opportunity to test numerical models under realistic boundary conditions: land-sea contrast and complex topography. We used HOTMAC (High Order Turbulence Model for Air Circulations), a three-dimensional mesoscale model based on simplified turbulence-closure equations to simulate temporal and spatial variations of wind, temperature, mixing ratio of water vapor, and turbulence distributions. Surface concentrations were computed by using a three-dimensional transport and diffusion model RAPTAD (Random Puff Transport and Diffusion). RAPTAD is a Lagrangian puff code based on the Monte Carlo statistical diffusion process. The center location and standard deviation of concentration distribution for each puff are computed by using wind and turbulence modeled by HOTMAC. Then, the concentration at any location is computed by summing concentrations contributed by all the puffs. The purpose of this study is to simulate the transport and dispersion of atmospheric pollutants in the complex terrain surroundings at Vandenberg Air Force Base by using the Los Alamos national Laboratory (LANL) atmospheric models HOTMAC and RAPTAD.
Numerical simulation of respiratory flow patterns within human lung.
Calay, R K; Kurujareon, Jutarat; Holdø, Arne Erik
2002-04-01
A computational fluid dynamics (CFD) modelling approach is used to study the unsteady respiratory airflow dynamics within a human lung. The three-dimensional asymmetric bifurcation model of the central airway based on the morphological data given by Horsfield et al. (J. Appl. Physiol. 67 (1971) 207) was used in the present study to simulate the oscillatory respiratory. The single bifurcation was found to be sufficient to give a number of results which both qualitatively and quantitatively agreed well with other published experimental and CFD results. Numerical simulation were made for two breathing conditions: (a) resting or normal breathing condition and (b) maximal exercise condition. The respiratory flow results for the both conditions are found strongly dependent on the convective effect and the viscous effect with some contribution of the unsteadiness effect. The secondary motions were stronger for the normal breathing condition as compared with the maximal exercise condition. The difference between the two cases is the flow separation regions found close to the carinal ridge for maximal exercise condition. For normal breathing condition no separation regions was observed in this region. PMID:12380010
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.
Numerical simulation of hypoxic cell regulation in avascular tumor growth
NASA Astrophysics Data System (ADS)
Mohd Said, Norfarizan; Ibrahim, Arsmah; Alias, Norma
2013-04-01
Avascular tumor is an early stage of tumor which does not have the blood vessels themselves and depends entirely on the cells around them to get the supply of nutrients such as oxygen and glucose. Hypoxia is a condition in which living cells are deprived of oxygen needed to maintain metabolism and growth. In avascular tumor, the hypoxic environment inhibits the cells proliferation and distinguishes the cellular dynamics into proliferative, quiescent and necrotic cells. In this paper, we present a numerical simulation of mathematical model describing these cellular dynamics using Matlab software with R2009a version. The model formulated in the form of one-dimensional parabolic partial differential equations depending on time and space. The discretization is based on forward differential respect to time and central differential respect to space of finite difference approximation. The results of simulation show that the distribution of proliferating, quiescent and necrotic cells within a tumor spheroid with respect to time and the cells regulation under different rates of nutrients consumptions in one-dimensional computational domain. In conclusion, in the hypoxic environment, the proliferative and quiescent cells grow slowly dependent on some parameter changes and the necrotic cells emerged at the tumor core.
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 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.
Magnetohydrodynamic Numerical Simulations of Magnetic Reconnection in Interstellar Medium
NASA Astrophysics Data System (ADS)
Tanuma, Syuniti
2000-03-01
In this thesis, we perform two-dimensional (2D) resistive magnetohydrodynamic (MHD) numerical simulations of the magnetic reconnection in interstellar medium. Part I is introduction. The motivation of the study is to investigate the origin of hot gas in interstellar medium. A scenario for generating X-ray gas in Galaxy is proposed, and examined by performing 2D MHD simulations with simple assumptions (Part II). The magnetic reconnection triggered by a supernova (Part III) and Parker instability (Part IV) are studied in detail, by performing 2D MHD simulations. Furthermore, the magnetic reconnection is also studied by performing three-dimensional (3D) MHD numerical simulation in (Part V). % Finally, we discuss and summarize the thesis (Parts VI and VII). Part I First, we review observation of Galactic Ridge X-ray Emission (GRXE) and its problems. Second, we describe observation of interstellar magnetic field briefly. Third, we review magnetic reconnection, theoretical models, numerical simulations, observations and experiments, and tearing instability. Forth, Parker instability (undular mode of magnetobuoyancy instability) is mentioned. Finally, we show the purpose of this thesis. Part II We present a scenario for the origin of the hot plasma in Galaxy as a model of strong X-ray emission [sim 3-10 keV; LX(2-10 keV) sim 1038 erg s-1], called GRXE, which has been observed near to the galactic plane. GRXE is thermal emission from a hot component (sim 7 keV) and a cool component (sim 0.8 keV). Observations suggest that the hot component is diffuse, and that it is not escaping away freely. Both what heats the hot component and what confines it in Galactic ridge still remain puzzling, while the cool component is believed to be created by supernovae. We propose a new scenario: the hot component is heated by magnetic reconnection, and confined by a helical magnetic field produced by magnetic reconnection. We solved 2D MHD equations numerically to study how magnetic reconnection, triggered by a supernova explosion, creates hot plasmas and magnetic islands (helical tubes), and how the magnetic islands confine the hot plasmas in Galaxy. The supernova shock is one of the possible mechanisms to trigger reconnection in Galaxy. We conclude that magnetic reconnection is able to heat the GRXE plasma if the magnetic field is localized in an intense flux tube with Blocal sim 30 muG. Part III This is the main part of the thesis. We examine the magnetic reconnection triggered by a supernova shock (or a point explosion) in interstellar medium, by performing 2D MHD numerical simulations with high spatial resolution. The magnetic reconnection starts long after the supernova shock (fast-mode MHD shock wave) passes a current sheet. The current sheet evolves as follows: (i) The tearing-mode instability is excited by the supernova shock. The current sheet becomes thin in the nonlinear phase of tearing instability. (ii) The current-sheet thinning is saturated when the current-sheet thickness becomes comparable to that of Sweet-Parker current sheet. After that, Sweet-Parker type reconnection starts, and the current-sheet length increases. (iii) The secondary tearing-mode instability occurs in the thin Sweet-Parker current sheet. (iv) As a result, further current-sheet thinning occurs, because gas density decreases in the current sheet. The anomalous resistivity sets in, and Petschek type reconnection starts. The interstellar gas is accelerated and heated. The magnetic energy is released quickly while magnetic islands are moving in the current sheet during Petschek type reconnection. (v) Magnetic reconnection stops because the gas pressure increases in the current sheet near left and right boundaries. The released magnetic energy is determined by the interstellar magnetic field strength, not by the energy of initial supernova nor distance between the supernova and the current sheet. We suggest that magnetic reconnection is a possible mechanism to generate X-ray gas in Galaxy. Part IV We examine the magnetic reconnection triggered by Parker instability
NASA Astrophysics Data System (ADS)
Tamura, Yuichi; Nakamura, Hiroaki; Ohno, Nobuaki
In numerical simulation research, visualization is indispensable method, and virtual reality (VR) is also very effective for visualizing numerical data. But VR is not friendly for simulation researchers, since computer for simulation is different from visualization computer in general. In this research, we propose seamless and interactive simulation environment by network communication. Under this environment, the researchers need not to recognize these computers and can analyze the simulation result interactively.
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.
Torres-Verdín, Carlos
1 Efficient Numerical Simulation of Axisymmetric Electromagnetic Induction Data using a High-well borehole induction measurements. #12;2 1. INTRODUCTION Accurate and rapid simulation of electromagnetic (EM-Order Generalized Extended Born Approximation (Ho- GEBA) for the numerical simulation of electromagnetic scattering
A three-dimensional numerical simulation of spreading umbrella clouds
NASA Astrophysics Data System (ADS)
Suzuki, Y. J.; Koyaguchi, T.
2009-03-01
During explosive volcanic eruptions, an eruption column buoyantly rises as a turbulent plume, and an umbrella cloud spreads laterally as a gravity current at the neutral buoyancy level. The source conditions of explosive eruptions such as mass discharge rates of magma at vents have been estimated from the field observations (e.g., satellite images) on the height of the eruption columns and the spreading rate of umbrella clouds on the basis of the one-dimensional (1-D) plume model and the gravity current model. However, these simplified models contain empirical constants (entrainment coefficient of turbulent plume, k, and Froude number of gravity current, ?) that should be justified. We developed a time-dependent three-dimensional (3-D) numerical model of eruption clouds to independently determine the values of these constants. The 3-D model is designed to calculate quantitative features of turbulent mixing between an eruption cloud and the ambient air without any a priori empirical constants by applying a sufficiently fine grid size with a third-order accuracy scheme. It has reproduced the fundamental features of eruption clouds including eruption columns, pyroclastic flows, coignimbrite ash clouds, and umbrella clouds. The altitudes of the spreading umbrella clouds in the 3-D simulations are consistent with those of the neutral buoyancy level of eruption columns estimated by the 1-D plume model with the entrainment coefficient k ˜ 0.1. The relationship between the volumetric flow rate and the spreading rate of the umbrella cloud in the 3-D simulations is explained by the axisymmetrical gravity current model with ? ˜ 0.2 for eruption clouds in the tropical atmosphere and ? ˜ 0.1 for those in the midlatitude atmosphere. On the other hand, the total column height in the 3-D simulations strongly oscillates even for a constant mass discharge rate, and its time average tends to be substantially greater than the column height estimated by the 1-D plume model with k ˜ 0.1, particularly for large-scale eruption clouds in the midlatitude atmosphere. We recommend a method to estimate the mass discharge rate from the observation of the column height or the spreading rate of umbrella cloud. The method and the accuracy of the 3-D simulations are tested on the basis of the field data (e.g., the total height of the eruption column, the altitude and the spreading rate of the umbrella cloud, and the mass discharge rate) of the Pinatubo 1991 eruption.
Numerical simulation of turbidity current flow and sedimentation
Zeng, J.
1992-01-01
A computer-based numerical model of turbidity-current flow and sedimentation has been developed by integrating geological observations with basic equations for fluid and sediment motion. The model emphasizes water mixing across the upper boundary, particle-concentration controls on sediment support and flow dynamics, and sediment fractionation during sedimentation. The model includes three numerical components: (1) a sedimentation/fluidization model for quantifying sediment-size fractionation in sedimenting multi-component suspensions; (2) a concentration-viscosity model for quantifying the changes in density and viscosity of high-concentrated sediment suspensions; and (3) a layer-averaged flow model for tracing downslope flow evolution using continuity and momentum equations. The resulting simulation monitors the sedimentation history of layer-averaged turbidity flows over submarine slopes in terms of the evolution of flow velocity, thickness, and sediment concentration and the resulting rate of sedimentation and sediment size fractionation in a longitudinal section of the flow. The model generates turbidites and outputs downslope variations in their thickness and grain-size structuring. The model is tested by reference to modern turbidity currents in Bute Inlet, British Columbia. Using initial and boundary conditions approximating those of Bute Inlet yields model flows that show downslope evolutions and deposit turbidites closely resembling their natural counterparts. Additional flow experiments provide quantitative evaluation of the effects of basin geometry, sediment concentration, and sediment sources on the formation and properties of turbidites. Experimental high-concentration flows show much higher downslope velocities and lower sediment-setting velocities than more dilute flows, resulting in longer sediment-transport. Model turbidites formed by high-concentration and low-concentration flows show both distribution and coarse-tail grading.
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 simulation of endocytosis: Viscous flow driven by membranes with non-uniformly distributed
Numerical simulation of endocytosis: Viscous flow driven by membranes with non- ing endocytosis and other cell processes are typically orchestrated by curvature- inducing molecules;Keywords: Endocytosis, Navier-Stokes flow, Helfrich energy, Phase-field model, Clathrin, Numerical
On spurious behavior of CFD simulations
NASA Technical Reports Server (NTRS)
Yee, H.C.; Torczynski, J. R.; Morton, S. A.; Visbal, M. R.; Sweby, P. K.
1997-01-01
Spurious behavior in underresolved grids and/or semi-implicit temporal discretizations for four computational fluid dynamics (CFD) simulations are studied. The numerical simulations consist of (a) a 1-D chemically relaxed nonequilibrium model, (b) the direct numerical simulation (DNS) of 2-D incompressible flow over a backward facing step, (c) a loosely-coupled approach for a 2-D fluid-structure interaction, and (d) a 3-D compressible unsteady flow simulation of vortex breakdown in delta wings. Using knowledge from dynamical systems theory, various types of spurious behaviors that are numerical artifacts were systematically identified. These studies revealed the various possible dangers of misinterpreting numerical simulation of realistic complex flows that are constrained by the available computing power. In large scale computations underresolved grids, semi-implicit procedures, loosely-coupled implicit procedures, and insufficiently long time integration in DNS are most often unavoidable. Consequently, care must be taken in both computation and in interpretation of the numerical data. The results presented confirm the important role that dynamical systems theory can play in the understanding of the nonlinear behavior of numerical algorithms and in aiding the identification of the sources of numerical uncertainties in CFD.
On spurious behavior of CFD simulations
Yee, H.C.; Torczynski, J.R.; Morton, S.A.; Visbal, M.R.; Sweby, P.K.
1997-05-01
Spurious behavior in underresolved grids and/or semi-implicit temporal discretizations for four computational fluid dynamics (CFD) simulations are studied. The numerical simulations consist of (a) a 1-D chemically relaxed nonequilibrium model, (b) the direct numerical simulation (DNS) of 2-D incompressible flow over a backward facing step, (c) a loosely-coupled approach for a 2-D fluid-structure interaction, and (d) a 3-D compressible unsteady flow simulation of vortex breakdown in delta wings. Using knowledge from dynamical systems theory, various types of spurious behaviors that are numerical artifacts were systematically identified. These studies revealed the various possible dangers of misinterpreting numerical simulation of realistic complex flows that are constrained by the available computing power. In large scale computations underresolved grids, semi-implicit procedures, loosely-coupled implicit procedures, and insufficiently long time integration in DNS are most often unavoidable. Consequently, care must be taken in both computation and in interpretation of the numerical data. The results presented confirm the important role that dynamical systems theory can play in the understanding of the nonlinear behavior of numerical algorithms and in aiding the identification of the sources of numerical uncertainties in CFD.
Polyelectrolyte gels as bending actuators: modeling and numerical simulation
NASA Astrophysics Data System (ADS)
Wallmersperger, Thomas; Keller, Karsten; Attaran, Abdolhamid
2013-04-01
Polyelectrolyte gels are ionic electroactivematerials. They have the ability to react as both, sensors and actuators. As actuators they can be used e.g. as artificial muscles or drug delivery control; as sensors they may be used for measuring e.g. pressure, pH or other ion concentrations in the solution. In this research both, anionic and cationic polyelectrolyte gels placed in aqueous solution with mobile anions and cations are investigated. Due to external stimuli the polyelectrolyte gels can swell or shrink enormously by the uptake or delivery of solvent. In the present research a coupled multi-field problem within a continuum mechanics framework is proposed. The modeling approach introduces a set of equations governing multiple fields of the problem, including the chemical field of the ionic species, the electrical field and the mechanical field. The numerical simulation is performed by using the Finite Element Method. Within the study some test cases will be carried out to validate our model. In the works by Gülch et al., the application of combined anionic-cationic gels as grippers was shown. In the present research for an applied electric field, the change of the concentrations and the electric potential in the complete polymer is simulated by the given formulation. These changes lead to variations in the osmotic pressure resulting in a bending of different polyelectrolyte gels. In the present research it is shown that our model is capable of describing the bending behavior of anionic or cationic gels towards the different electrodes (cathode or anode).
Ab initio molecular simulations with numeric atom-centered orbitals
NASA Astrophysics Data System (ADS)
Blum, Volker; Gehrke, Ralf; Hanke, Felix; Havu, Paula; Havu, Ville; Ren, Xinguo; Reuter, Karsten; Scheffler, Matthias
2009-11-01
We describe a complete set of algorithms for ab initio molecular simulations based on numerically tabulated atom-centered orbitals (NAOs) to capture a wide range of molecular and materials properties from quantum-mechanical first principles. The full algorithmic framework described here is embodied in the Fritz Haber Institute "ab initio molecular simulations" (FHI-aims) computer program package. Its comprehensive description should be relevant to any other first-principles implementation based on NAOs. The focus here is on density-functional theory (DFT) in the local and semilocal (generalized gradient) approximations, but an extension to hybrid functionals, Hartree-Fock theory, and MP2/GW electron self-energies for total energies and excited states is possible within the same underlying algorithms. An all-electron/full-potential treatment that is both computationally efficient and accurate is achieved for periodic and cluster geometries on equal footing, including relaxation and ab initio molecular dynamics. We demonstrate the construction of transferable, hierarchical basis sets, allowing the calculation to range from qualitative tight-binding like accuracy to meV-level total energy convergence with the basis set. Since all basis functions are strictly localized, the otherwise computationally dominant grid-based operations scale as O(N) with system size N. Together with a scalar-relativistic treatment, the basis sets provide access to all elements from light to heavy. Both low-communication parallelization of all real-space grid based algorithms and a ScaLapack-based, customized handling of the linear algebra for all matrix operations are possible, guaranteeing efficient scaling (CPU time and memory) up to massively parallel computer systems with thousands of CPUs.
Numerical Simulation Study of the Sanchiao Fault Earthquake Scenarios
NASA Astrophysics Data System (ADS)
Wang, Yi-Min; Lee, Shiann-Jong
2015-04-01
Sanchiao fault is a western boundary fault of the Taipei basin located in northern Taiwan, close to the densely populated Taipei metropolitan area. Recent study indicated that there is about 40 km of the fault trace extended to the marine area offshore northern Taiwan. Combining the marine and terrestrial parts, the total fault length of Sanchiao fault could be nearly 70 kilometers which implies that this fault has potential to produce a big earthquake. In this study, we analyze several Sanchiao fault earthquake scenarios based on the recipe for predicting strong ground motion. The characterized source parameters include fault length, rupture area, seismic moment, asperity, and slip pattern on the fault plane. According to the assumption of the characterized source model, Sanchiao fault has been inferred to have the potential to produce an earthquake with moment magnitude (Mw) larger than 7.0. Three-dimensional seismic simulation results based upon spectral-element method (SEM) indicate that peak ground acceleration (PGA) is significantly stronger along the fault trace. The basin effect also plays an important role when wave propagates in the Taipei basin which cause seismic wave amplified and prolong the shaking for a very long time. Among all rupture scenarios, the rupture propagated from north to south is the most serious one. Owing to the rupture directivity as well as the basin effects, large PGA (>1g) was observed in the Taipei basin, especially in the northwest side. The results of these scenario earthquake simulations will provide important physically-based numerical data for earthquake mitigation and seismic hazard assessment.
Numerical simulation of non-Newtonian free shear flows
NASA Technical Reports Server (NTRS)
Homsy, G. M.; Azaiez, J.
1993-01-01
Free shear flows, like those of mixing layers, are encountered in aerodynamics, in the atmosphere, and in the ocean as well as in many industrial applications such as flow reactors or combustion chambers. It is, therefore, crucial to understand the mechanisms governing the process of transition to turbulence in order to predict and control the evolution of the flow. Delaying transition to turbulence as far downstream as possible allows a gain in energy expenditure while accelerating the transition can be of interest in processes where high mixing is desired. Various methods, including the use of polymer additives, can be effective in controlling fluid flows. The drag reduction obtained by the addition of small amounts of high polymers has been an active area of research for the last three decades. It is now widely believed that polymer additives can affect the stability of a large variety of flows and that dilute solutions of these polymers have been shown to produce drag reductions of over 80 percent in internal flows and over 60 percent in external flows under a wide range of conditions. The major thrust of this work is to study the effects of polymer additives on the stability of the incompressible mixing layer through large scale numerical simulations. In particular, we focus on the two dimensional flow and examine how the presence of viscoelasticity may affect the typical structures of the flow, namely roll-up and pairing of vortices.
Numerical Simulation of Particle Distribution in Capillary Membrane during Backwash
Mansour, Hussam; Keller, Anik; Gimbel, Rolf; Kowalczyk, Wojciech
2013-01-01
The membrane filtration with inside-out dead-end driven UF-/MF- capillary membranes is an effective process for particle removal in water treatment. Its industrial application increased in the last decade exponentially. To date, the research activities in this field were aimed first of all at the analysis of filtration phenomena disregarding the influence of backwash on the operation parameters of filtration plants. However, following the main hypothesis of this paper, backwash has great potential to increase the efficiency of filtration. In this paper, a numerical approach for a detailed study of fluid dynamic processes in capillary membranes during backwash is presented. The effect of particle size and inlet flux on the backwash process are investigated. The evaluation of these data concentrates on the analysis of particle behavior in the cross sectional plane and the appearance of eventually formed particle plugs inside the membrane capillary. Simulations are conducted in dead-end filtration mode and with two configurations. The first configuration includes a particle concentration of 10% homogeneously distributed within the capillary and the second configuration demonstrates a cake layer on the membrane surface with a packing density of 0.6. Analyzing the hydrodynamic forces acting on the particles shows that the lift force plays the main role in defining the particle enrichment areas. The operation parameters contribute in enhancing the lift force and the heterogeneity to anticipate the clogging of the membrane. PMID:24957056
Numerical Simulation and Chaotic Analysis of an Aluminum Holding Furnace
NASA Astrophysics Data System (ADS)
Wang, Ji-min; Zhou, Yuan-yuan; Lan, Shen; Chen, Tao; Li, Jie; Yan, Hong-jie; Zhou, Jie-min; Tian, Rui-jiao; Tu, Yan-wu; Li, Wen-ke
2014-12-01
To achieve high heat efficiency, low pollutant emission and homogeneous melt temperature during thermal process of secondary aluminum, taking into account the features of aluminum alloying process, a CFD process model was developed and integrated with heat load and aluminum temperature control model. This paper presented numerical simulation of aluminum holding furnaces using the customized code based on FLUENT packages. Thermal behaviors of aluminum holding furnaces were investigated by probing into main physical fields such as flue gas temperature, velocity, and concentration, and combustion instability of aluminum holding process was represented by chaos theory. The results show that aluminum temperature uniform coefficient firstly decreases during heating phase, then increases and reduces alternately during holding phase, lastly rises during standing phase. Correlation dimension drops with fuel velocity. Maximal Lyapunov exponent reaches to a maximum when air-fuel ratio is close to 1. It would be a clear comprehension about each phase of aluminum holding furnaces to find new technology, retrofit furnace design, and optimize parameters combination.
Numerical Simulation and Scaling Analysis of Cell Printing
NASA Astrophysics Data System (ADS)
Qiao, Rui; He, Ping
2011-11-01
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 inkjet printer or similar devices to print cells into 3D patterns with a resolution comparable to the size of mammalian cells. Achieving such a resolution in vitro can lead to breakthroughs in areas such as organ transplantation. Although the feasibility of cell printing has been demonstrated recently, the printing resolution and cell viability remain to be improved. Here we investigate a unit operation in cell printing, namely, the impact of a cell-laden droplet into a pool of highly viscous liquids. The droplet and cell dynamics are quantified using both direct numerical simulation and scaling analysis. These studies indicate that although cell experienced significant stress during droplet impact, the duration of such stress is very short, which helps explain why many cells can survive the cell printing process. These studies also revealed that cell membrane can be temporarily ruptured during cell printing, which is supported by indirect experimental evidence.
Rapid Numerical Simulation of Viscous Axisymmetric Flow Fields
NASA Technical Reports Server (NTRS)
Tweedt, Daniel L.; Chima, Rodrick V.
1995-01-01
A two-dimensional Navier-Stokes code has been developed for rapid numerical simulation of axisymmetric flow fields, including flow fields with an azimuthal velocity component. The azimuthal-invariant Navier-Stokes equations in a cylindrical coordinate system are mapped to a general body-fitted coordinate system, with the streamwise viscous terms then neglected by applying the thin-layer approximation. Turbulence effects are modeled using an algebraic model, typically the Baldwin-Lomax turbulence model, although a modified Cebeci-Smith model can also be used. The equations are discretized using central finite differences and solved using a multistage Runge-Kutta algorithm with a spatially varying time step and implicit residual smoothing. Results are presented for calculations of supersonic flow over a waisted body-of-revolution, transonic flow through a normal shock wave in a straight circular duct of constant cross sectional area, swirling supersonic (inviscid) flow through a strong shock in a straight radial duct, and swirling subsonic flow in an annular-to-circular diffuser duct. Comparisons between computed and experimental results are in fair to good agreement, demonstrating that the viscous code can be a useful tool for practical engineering design and analysis work.
Numerical simulation of drift waves and trapped ion modes
Kingsbury, O.T.; Waltz, R.E. )
1994-07-01
Two-dimensional numerical simulations are used to study the interaction of trapped electron drift waves (DW) and trapped ion modes (TIM). Wave-number ([ital k]) space is divided into long and short wave regions at a poloidal wave number corresponding to the ion bounce frequency. Two field models are used to describe trapped electron drift wave dynamics at short waves and trapped ion mode dynamics for long waves. The standard case has curvature effects and collisionality. The nonlinearity that couples the two regions includes a trapped ion banana width effect analogous to finite Larmor radius (FLR) polarization drift. The principal result of this study is that the TIM do not contribute to the diffusion significantly, regardless of the model for the nonlinear coupling to the DW. This conclusion is supported by a more general four field model that includes pressure dynamics and which allows ion temperature gradient (ITG) driven drift modes. When the collisionality is varied, the diffusion deviates from the [gamma]/[ital k][sup 2][sub [ital x
Numerical simulations of turbulent jet ignition and combustion
NASA Astrophysics Data System (ADS)
Validi, Abdoulahad; Irannejad, Abolfazl; Jaberi, Farhad
2013-11-01
The ignition and combustion of a homogeneous lean hydrogen-air mixture by a turbulent jet flow of hot combustion products injected into a colder gas mixture are studied by a high fidelity numerical model. Turbulent jet ignition can be considered as an efficient method for starting and controlling the reaction in homogeneously charged combustion systems used in advanced internal combustion and gas turbine engines. In this work, we study in details the physics of turbulent jet ignition in a fundamental flow configuration. The flow and combustion are modeled with the hybrid large eddy simulation/filtered mass density function (LES/FMDF) approach, in which the filtered form the compressible Navier-Stokes equations are solved with a high-order finite difference scheme for the turbulent velocity and the FMDF transport equations are solved with a Lagrangian stochastic method to obtain the scalar (temperature and species mass fractions) field. The hydrogen oxidation is described by a detailed reaction mechanism with 37 elementary reactions and 9 species.
A numerical simulation of external heat transfer around turbine blades
NASA Astrophysics Data System (ADS)
Maaroofi, K.; Nobari, M. R. H.; Shirani, E.
2007-11-01
External heat transfer prediction is performed in two-dimensional turbine blade cascades using the Reynolds-averaged Navier-Stokes equations. For this purpose, six different turbulence models including the algebraic Baldwin-Lomax (AIAA paper 78-257, 1978), three low- Re k-? models (Chien in AIAA J 20:33-38, 1982; Launder and Sharma in Lett Heat Mass Transf 1(2):131-138, 1974; Biswas and Fukuyama in J Turbomach 116:765-773, 1994), and two k-? models (Wilcox in AIAA J 32(2):247-255, 1994) are taken into account. The computer code developed employs a finite volume method to solve governing equations based on an explicit time marching approach with capability to simulate subsonic, transonic and supersonic flows. The Roe method is used to decompose the inviscid fluxes and the gradient theorem to decompose viscous fluxes. The performance of different turbulence models in prediction of heat transfer is examined. To do so, the effect of Reynolds and Mach numbers along with the turbulent intensity are taken into account, and the numerical results obtained are compared with the experimental data available.
Numerical Simulations of Driven Supersonic Relativistic MHD Turbulence
NASA Astrophysics Data System (ADS)
Zrake, Jonathan; MacFadyen, Andrew
2011-08-01
Models for GRB outflows invoke turbulence in relativistically hot magnetized fluids. In order to investigate these conditions we have performed high-resolution three-dimensional numerical simulations of relativistic magneto-hydrodynamical (RMHD) turbulence. We find that magnetic energy is amplified to several percent of the total energy density by turbulent twisting and folding of magnetic field lines. Values of ?B>~0.01 are thus naturally expected. We study the dependence of saturated magnetic field energy fraction as a function of Mach number and relativistic temperature. We then present power spectra of the turbulent kinetic and magnetic energies. We also present solenoidal (curl-like) and dilatational (divergence-like) power spectra of kinetic energy. We propose that relativistic effects introduce novel couplings between these spectral components. The case we explore in most detail is for equal amounts of thermal and rest mass energy, corresponding to conditions after collisions of shells with relative Lorentz factors of several. These conditions are relevant in models for internal shocks, for the late afterglow phase, for cocoon material along the edge of a relativistic jet as it propagates through a star, as well neutron stars merging with each other and with black hole companions. We find that relativistic turbulence decays extremely quickly, on a sound crossing time of an eddy. Models invoking sustained relativistic turbulence to explain variability in GRB prompt emission are thus strongly disfavored unless a persistant driving of the turbulence is maintained for the duration of the prompt emission.
Numerical simulation of spin-glass transition phenomena (invited)
NASA Astrophysics Data System (ADS)
Walstedt, R. E.; Walker, L. R.
1982-11-01
Results are presented from a continuing investigation of spin-glass transition effects by numerical simulation. The model is one of classical unit vectors randomly distributed on an fcc lattice, with RKKY exchange coupling and nearest-neighbor dipolar coupling in an approximate representation of dilute Mn in Cu. Results for system energy and for dipolar energy as a function of temperature show no resolvable features at the freezing temperature TG* in accord with the high-precision specific heat data of Fogle, Boyer, Phillips, and Van Curen. Additional details of shattered susceptibility behavior in the vicinity of the ground-state transition are presented. Further data on freezing temperatures as a function of dipolar coupling strength D show a trend toward macroscopic behavior (i.e., TG* independent of D) with a sample of 4928 spins. Finally, a moderately sharp onset of freezing transverse to an applied magnetic field is found, as predicted by the mean-field calculations of Toulouse and Gabay. As expected, this transition is unrelated to transverse susceptibilities.
Numerical Simulation of a Small-Scale Mild Combustor
NASA Astrophysics Data System (ADS)
Veríssimo, A.; Oliveira, R.; Coelho, P. J.; Costa, M.
2012-11-01
This work reports numerical simulations of a small-scale cylindrical combustor operating in the mild combustion regime. Preheated air is supplied by a central nozzle, while the fuel (methane) is injected through 16 holes placed equidistantly in a circumference concentric with the air nozzle. The calculations were carried out using the commercial code Ansys-Fluent. Turbulence was modelled using the realizable k-epsilon model. Two different combustion models were employed, namely the eddy dissipation concept and the joint composition pdf transport model. In both cases, a chemical mechanism comprising 13 transported species and 73 chemical reactions was used, as well as a global single-step reaction. A thorough comparison of the predictions obtained using the pdf transport model and the eddy dissipation concept with detailed experimental data is presented. Both models are able to accurately predict the temperature and the O2 and CO2 molar fractions over most of the combustor, but the temperature field is overestimated in the vicinity of the burner. Discrepancies are found in the prediction of the CO molar fraction, particularly when the eddy dissipation concept is used.
Numerical simulation of incoherent optical wave propagation in nonlinear fibers
NASA Astrophysics Data System (ADS)
Fernandez, Arnaud; Balac, Stéphane; Mugnier, Alain; Mahé, Fabrice; Texier-Picard, Rozenn; Chartier, Thierry; Pureur, David
2013-11-01
The present work concerns the study of pulsed laser systems containing a fiber amplifier for boosting optical output power. In this paper, this fiber amplification device is included into a MOPFA laser, a master oscillator coupled with fiber amplifier, usually a cladding-pumped high-power amplifier often based on an ytterbium-doped fiber. An experimental study has established that the observed nonlinear effects (such as Kerr effect, four waves mixing, Raman effect) could behave very differently depending on the characteristics of the optical source emitted by the master laser. However, it has not yet been possible to determine from the experimental data if the statistics of the photons is alone responsible for the various nonlinear scenarios observed. Therefore, we have developed a numerical simulation software for solving the generalized nonlinear Schrödinger equation with a stochastic source term in order to validate the hypothesis that the coherence properties of the master laser are mainly liable for the behavior of the observed nonlinear effects. Contribution to the Topical Issue "Numelec 2012", Edited by Adel Razek.
Numerical Simulations of Instabilities in Single-Hole Office Elements
NASA Technical Reports Server (NTRS)
Ahuja, Vineet; Hosangadi, Ashvin; Hitt, Matthew A.; Lineberry, David M.
2013-01-01
An orifice element is commonly used in liquid rocket engine test facilities either as a flow metering device, a damper for acoustic resonance or to provide a large reduction in pressure over a very small distance in the piping system. While the orifice as a device is largely effective in stepping down pressure, it is also susceptible to a wake-vortex type instability that generates pressure fluctuations that propagate downstream and interact with other elements of the test facility resulting in structural vibrations. Furthermore in piping systems an unstable feedback loop can exist between the vortex shedding and acoustic perturbations from upstream components resulting in an amplification of the modes convecting downstream. Such was the case in several tests conducted at NASA as well as in the Ariane 5 strap-on P230 engine in a static firing test where pressure oscillations of 0.5% resulted in 5% thrust oscillations. Exacerbating the situation in cryogenic test facilities, is the possibility of the formation of vapor clouds when the pressure in the wake falls below the vapor pressure leading to a cavitation instability that has a lower frequency than the primary wake-vortex instability. The cavitation instability has the potential for high amplitude fluctuations that can cause catastrophic damage in the facility. In this paper high-fidelity multi-phase numerical simulations of an orifice element are used to characterize the different instabilities, understand the dominant instability mechanisms and identify the tonal content of the instabilities.
Three-Dimensional Numerical Simulation of Airflow in Nasopharynx.
NASA Astrophysics Data System (ADS)
Shome, Biswadip; Wang, Lian-Ping; Santare, Michael H.; Szeri, Andras Z.; Prasad, Ajay K.; Roberts, David
1996-11-01
A three-dimensional numerical simulation of airflow in nasopharynx (from the soft palate to the epiglottis) was conducted, using anatomically accurate model and finite element method, to study the influence of flow characteristics on obstructive sleep apnea (OSA). The results showed that the pressure drop in the nasopharynx is in the range 200-500 Pa. Ten different nasopharynx geometries resulting from three OSA treatment therapies (CPAP, mandibular repositioning devices, and surgery) were compared. The results confirmed that the airflow in the nasopharynx lies in the transitional flow regime and thus, a subtle change in the morphology caused by these treatment therapies has a large effect on the airflow. The onset of turbulence can cause as much as 40% of increase in pressure drop. For the transitional flow regime, the k-? turbulence model was found to be the most appropriate model, when compared to the mixing length and the k-? model, as it correctly reproduces the limiting laminar behavior. In addition, the pressure drop increased approximately as the square of the volumetric flow rate. Supported by NIH.
Direct Numerical Simulation of Flow Over Passive Geometric Disturbances
NASA Astrophysics Data System (ADS)
Vizard, Alexander
It is well understood that delaying flow separation on a bluff body allows significant drag reduction, which is attractive in many applications. With this in mind, many separation control mechanisms, both active and passive, have been developed and tested to optimize the effects of this phenomenon. Although this idea is generally accepted, the physical occurrences in the near-wall region during transition that lead to separation delay are not well understood. The current study evaluates the impact of both spherical dimples, and sandgrain style roughness on downstream flow by performing direct numerical simulations over such geometries on a zero pressure gradient flat plate. It is shown that although dimples and random roughness of similar characteristic length scales exhibit similar boundary layer characteristics, dimples are more successful in developing high momentum in the vicinity of the wall. Additionally it is shown that increasing the relative size of the rough elements does not increase the near-wall momentum, and is undesirable in controlling separation. Finally, it is shown that the impact of roughness elements on the flow is more immediate, and that, for the case of one row of dimples and an equivalent area of roughness, the roughness patch is more successful in transitioning the near-wall region to a non-laminar state. It can be concluded from variation in the span of the flowfield for a single row of dimples that the size and orientation of the disturbance region is significant to the results.
Advanced Numerical Methods and Software Approaches for Semiconductor Device Simulation
Carey, Graham F.; Pardhanani, A. L.; Bova, S. W.
2000-01-01
In this article we concisely present several modern strategies that are applicable to driftdominated carrier transport in higher-order deterministic models such as the driftdiffusion, hydrodynamic, and quantum hydrodynamic systems. The approaches include extensions of “upwind” and artificial dissipation schemes, generalization of the traditional Scharfetter – Gummel approach, Petrov – Galerkin and streamline-upwind Petrov Galerkin (SUPG), “entropy” variables, transformations, least-squares mixed methods and other stabilized Galerkin schemes such as Galerkin least squares and discontinuous Galerkin schemes. The treatment is representative rather than an exhaustive review and several schemes are mentioned only briefly with appropriate reference to the literature. Some of themore »methods have been applied to the semiconductor device problem while others are still in the early stages of development for this class of applications. We have included numerical examples from our recent research tests with some of the methods. A second aspect of the work deals with algorithms that employ unstructured grids in conjunction with adaptive refinement strategies. The full benefits of such approaches have not yet been developed in this application area and we emphasize the need for further work on analysis, data structures and software to support adaptivity. Finally, we briefly consider some aspects of software frameworks. These include dial-an-operator approaches such as that used in the industrial simulator PROPHET, and object-oriented software support such as those in the SANDIA National Laboratory framework SIERRA.« less
Advanced numerical methods and software approaches for semiconductor device simulation
CAREY,GRAHAM F.; PARDHANANI,A.L.; BOVA,STEVEN W.
2000-03-23
In this article the authors concisely present several modern strategies that are applicable to drift-dominated carrier transport in higher-order deterministic models such as the drift-diffusion, hydrodynamic, and quantum hydrodynamic systems. The approaches include extensions of upwind and artificial dissipation schemes, generalization of the traditional Scharfetter-Gummel approach, Petrov-Galerkin and streamline-upwind Petrov Galerkin (SUPG), entropy variables, transformations, least-squares mixed methods and other stabilized Galerkin schemes such as Galerkin least squares and discontinuous Galerkin schemes. The treatment is representative rather than an exhaustive review and several schemes are mentioned only briefly with appropriate reference to the literature. Some of the methods have been applied to the semiconductor device problem while others are still in the early stages of development for this class of applications. They have included numerical examples from the recent research tests with some of the methods. A second aspect of the work deals with algorithms that employ unstructured grids in conjunction with adaptive refinement strategies. The full benefits of such approaches have not yet been developed in this application area and they emphasize the need for further work on analysis, data structures and software to support adaptivity. Finally, they briefly consider some aspects of software frameworks. These include dial-an-operator approaches such as that used in the industrial simulator PROPHET, and object-oriented software support such as those in the SANDIA National Laboratory framework SIERRA.
Numerical simulations of thermal convection at high Prandtl numbers
NASA Astrophysics Data System (ADS)
Silano, Gabriella
2008-11-01
Direct numerical simulations of thermal convection are conducted for a cylindrical cell of aspect ratio .5ex1-.1em/ -.15em.25ex2 . The Prandtl number (Pr) varies from 10^0 to 10^4 and the Rayleigh numbers (Ra) are moderate (10^5 < Ra < 10^9). This study is motivated by the fact that the role of the Prandtl number in convective heat transport is not yet fully understood. The three-dimensional behaviors of the temperature and velocity fields, of the viscous and thermal dissipation fields, and of the diffusive and convective heat fluxes are explored. In the ranges of Pr and Ra considered, we find steady, periodic and chaotic regimes, and large-scale structures which are more complex than the single recirculation cell filling the whole volume. Multiple flow structures are found to be associated with a given set of conditions. The multiple solutions seem to be more probable at higher Pr numbers and could explain the scatter in some data trends. In collaboration with Katepalli Raju Sreenivasan, The Abdus Salam International Centre for Theoretical Physics - Trieste, and Roberto Verzicco, DIM, Universitàdegli Studi di Roma Tor Vergata - Roma.
Entrainment and Detrainment in Numerically Simulated Cumulus Congestus Clouds.
NASA Astrophysics Data System (ADS)
Carpenter, Richard L., Jr.
In order to better understand the mechanisms for cumulus entrainment, we have numerically simulated cumulus congestus clouds in three dimensions, using one level of grid nesting to achieve grid spacings as small as 50 m. The model does not allow precipitation or ice formation, and achieves saturation through bulk condensation. The environment is that of New Mexican cumulus clouds that were observed on 9-10 August 1987. The topographical surface heating is simulated by a Gaussian surface heat flux profile. The clouds are 1-4 km in vertical extent, and undergo episodes of growth and decay. Model results agree nicely with available observations. We find that cumulus entrainment is primarily the result not of plume-like processes or penetrative downdrafts, but rather is associated with the toroidal circulation of rising thermals. These thermals contain cores of undilute subcloud air that play a vital role in cumulus entrainment. Thermals that have exhausted their buoyancy due to entrainment or overshooting may continue upward for a significant distance due to their momentum. Their subsequent collapse is responsible for a large portion of the entrainment. Our findings are consistent with the shedding thermal model proposed by Blyth et al. (1988). A significant detrainment layer was found in one of the simulations. The height of the layer was correlated with the level at which the buoyancy of actual parcels decreased most rapidly with height. The presence and strength of detrainment layers is evidently quite sensitive to the vertical buoyancy profile. It is not possible to generate realistic convective clouds (of the scale considered here) from a quiescent model atmosphere; turbulent motion is needed in order to broaden the clouds and reduce their buoyancy. One means for accomplishing this is presented. Spatial resolution at least as fine as that used here (50 m) appears necessary in order to capture the details of cumulus entrainment. Clouds generated on coarser grids do not entrain at a sufficient rate, leading to updrafts that are too strong and cloud tops that are too high.
NASA Technical Reports Server (NTRS)
Reed, John A.; Afjeh, Abdollah A.
1995-01-01
A major difficulty in designing aeropropulsion systems is that of identifying and understanding the interactions between the separate engine components and disciplines (e.g., fluid mechanics, structural mechanics, heat transfer, material properties, etc.). The traditional analysis approach is to decompose the system into separate components with the interaction between components being evaluated by the application of each of the single disciplines in a sequential manner. Here, one discipline uses information from the calculation of another discipline to determine the effects of component coupling. This approach, however, may not properly identify the consequences of these effects during the design phase, leaving the interactions to be discovered and evaluated during engine testing. This contributes to the time and cost of developing new propulsion systems as, typically, several design-build-test cycles are needed to fully identify multidisciplinary effects and reach the desired system performance. The alternative to sequential isolated component analysis is to use multidisciplinary coupling at a more fundamental level. This approach has been made more plausible due to recent advancements in computation simulation along with application of concurrent engineering concepts. Computer simulation systems designed to provide an environment which is capable of integrating the various disciplines into a single simulation system have been proposed and are currently being developed. One such system is being developed by the Numerical Propulsion System Simulation (NPSS) project. The NPSS project, being developed at the Interdisciplinary Technology Office at the NASA Lewis Research Center is a 'numerical test cell' designed to provide for comprehensive computational design and analysis of aerospace propulsion systems. It will provide multi-disciplinary analyses on a variety of computational platforms, and a user-interface consisting of expert systems, data base management and visualization tools, to allow the designer to investigate the complex interactions inherent in these systems. An interactive programming software system, known as the Application Visualization System (AVS), was utilized for the development of the propulsion system simulation. The modularity of this system provides the ability to couple propulsion system components, as well as disciplines, and provides for the ability to integrate existing, well established analysis codes into the overall system simulation. This feature allows the user to customize the simulation model by inserting desired analysis codes. The prototypical simulation environment for multidisciplinary analysis, called Turbofan Engine System Simulation (TESS), which incorporates many of the characteristics of the simulation environment proposed herein, is detailed.
Mathematical Study and Numerical Simulation of Multispectral Bioluminescence Tomography
Cong, Wenxiang; Wang, Ge
2006-01-01
Multispectral bioluminescence tomography (BLT) attracts increasingly more attention in the area of optical molecular imaging. In this paper, we analyze the properties of the solutions to the regularized and discretized multispectral BLT problems. First, we show the solution existence, uniqueness, and its continuous dependence on the data. Then, we introduce stable numerical schemes and derive error estimates for numerical solutions. We report some numerical results to illustrate the performance of the numerical methods on the quality of multispectral BLT reconstruction. PMID:23165038
Direct Numerical Simulation of Liquid Transport Through Fibrous Porous Media
NASA Astrophysics Data System (ADS)
Palakurthi, Nikhil Kumar
Fluid flow through fibrous media occurs in many industrial processes, including, but not limited, to fuel cell technology, drug delivery patches, sanitary products, textile reinforcement, filtration, heat exchangers, and performance fabrics. Understanding the physical processes involved in fluid flow through fibrous media is essential for their characterization as well as for the optimization and development of new products. Macroscopic porous-media equations require constitutive relations, which account for the physical processes occurring at the micro-scale, to predict liquid transport at the macro-scale. In this study, micro-scale simulations were conducted using conventional computational fluid dynamics (CFD) technique (finite-volume method) to determine the macroscopic constitutive relations. The first part of this thesis deals with the single-phase flow in fibrous media, following which multi-phase flow through fibrous media was studied. Darcy permeability is an important parameter that characterizes creeping flow through a fibrous porous medium. It has a complex dependence on the medium's properties such as fibers' in-plane and through-plane orientation, diameter, aspect ratio, curvature, and porosity. A suite of 3D virtual fibrous structures with a wide range of geometric properties were constructed, and the permeability values of the structures were calculated by solving the 3D incompressible Navier-Stokes equations. The through-plane permeability was found to be a function of only the fiber diameter, the fibers' through-plane orientation, and the porosity of the medium. The numerical results were used to extend a permeability-porosity relation, developed in literature for 3D isotropic fibrous media, to a wide range of fibers' through-plane orientations. In applications where rate of capillary penetration is important, characterization of porous media usually involves determination of either the effective pore radius from capillary penetration experiments or a representative pore radius (R50) from pore-size distribution data. The relationship between effective and representative pore radii was studied by performing direct simulations of capillary penetration of a wetting liquid using a finite-volume-based volume-of-fluid (VOF) method. The simulated unidirectional liquid penetration through fibrous media followed Lucas-Washburn kinetics (L ˜ t1/2), except during the initial stages, which are dominated by inertial forces. Even though fluid properties and contact angle were kept constant in the simulations, the effective pore radii were found to be quite different from the representative radii. It can be concluded that the differences between effective and representative pore radii did not arise from contact angle variations. The unsaturated flow through fibrous media at the macro-scale is typically described using Richard's equation which requires constitutive relations: capillary pressure and permeability as a function of liquid saturation. In the present study, the quasi-static capillary pressure-saturation (P c-S) relationship for the primary drainage in a 3D isotropic fibrous medium was determined by performing micro-scale simulations using a VOF method. The Pc-S relationship obtained from the VOF method was compared with the results from the full-morphology (FM) method. Good agreement was observed between the results from the VOF and FM methods, thus suggesting that the FM method, a computationally less intensive method as compared to VOF method, may be sufficient for estimating the Pc-S relationship for primary drainage.
A Parallel, Finite-Volume Algorithm for Large-Eddy Simulation of Turbulent Flows
NASA Technical Reports Server (NTRS)
Bui, Trong T.
1999-01-01
A parallel, finite-volume algorithm has been developed for large-eddy simulation (LES) of compressible turbulent flows. This algorithm includes piecewise linear least-square reconstruction, trilinear finite-element interpolation, Roe flux-difference splitting, and second-order MacCormack time marching. Parallel implementation is done using the message-passing programming model. In this paper, the numerical algorithm is described. To validate the numerical method for turbulence simulation, LES of fully developed turbulent flow in a square duct is performed for a Reynolds number of 320 based on the average friction velocity and the hydraulic diameter of the duct. Direct numerical simulation (DNS) results are available for this test case, and the accuracy of this algorithm for turbulence simulations can be ascertained by comparing the LES solutions with the DNS results. The effects of grid resolution, upwind numerical dissipation, and subgrid-scale dissipation on the accuracy of the LES are examined. Comparison with DNS results shows that the standard Roe flux-difference splitting dissipation adversely affects the accuracy of the turbulence simulation. For accurate turbulence simulations, only 3-5 percent of the standard Roe flux-difference splitting dissipation is needed.
Eisenberg, Michael A.
1991-05-01
The Kineticist's Workbench is a program that simulates chemical reaction mechanisms by predicting, generating, and interpreting numerical data. Prior to simulation, it analyzes a given mechanism to predict that ...
Fringer, Oliver B.
Threedimensional, nonhydrostatic numerical simulation of nonlinear internal wave generation of threedimensional, nonhydrostatic simulations of internal tides and waves in the South China Sea (SCS) using in the SCS. Internal wave amplitudes are underpredicted which causes underprediction of internal wave speeds
Theoretical and Numerical Simulation of Non-Newtonian Fluid Flow in Propped Fractures
Ouyang, Liangchen
2013-12-10
model, and then modified the model based on numerical simulation results. In the simulations, I developed a micro pore-scale model to mimic the real porous structure of flow channel in propped fractures. The correlation of pressure gradient...
Numerical Simulation of Flow and Determination of Aerodynamic Forces in the Balanced Control Valve
NASA Astrophysics Data System (ADS)
Matas, R.; Straka, F.; Hoznedl, M.
2013-04-01
The contribution subscribes a numerical simulation of a steam flow through a balanced control valve. The influence of some parameters in simulations were tested, analyzed and discussed. As a result of the simulations a graph of aerodynamics forces for a specific turbine characteristic was obtained. The results from numerical simulations were compared with results from experiments. The experiment was performed with an air flow, but the final data were converted with a criterion to steam flow.
Numerical Simulation of Ion Rings and Ion Beam Propagation.
NASA Astrophysics Data System (ADS)
Mankofsky, Alan
This thesis presents the development of numerical simulation techniques for studying the physics of ion beams and rings in a background plasma as applicable to certain problems in magnetic and inertial confinement fusion. Two codes have been developed for these purposes: RINGA and CIDER. The 2 and 1/2-dimensional (r,z,v(,r),v(,(theta)),v(,z); (PAR-DIFF)/(PAR-DIFF)(theta) = 0) particle code RINGA follows the trajectories of ions in their self-consistent magnetic field. The code assumes strict charge neutrality and admits currents only in the azimuthal direction, i.e., (PHI) = J(,r) = J(,z) = 0. The injection and resistive trapping of ion rings has been studied with RINGA. The number of particles trapped as a fraction of the total number injected N is found to be strongly dependent upon (1) N (in the range 2.85 x 10('16) - 3.99 x 10('17)) and (2) mirror ratios in the system (1.05 -1.14), and more weakly dependent upon (3) wall resistance per unit length (0.72 (OMEGA)/cm - 1.80 (OMEGA)/cm) and (4) beam divergence (0(DEGREES)-6(DEGREES)). Fractions of trapped particles in excess of 0.9 have been observed. Modifications to RINGA to include finite pressure of confined plasma and beam ion-electron slowing down collisions are discussed. Finite plasma pressure leads to a diamagnetic current which increases the field reversal factor in ion ring equilibria, while causing the closed flux surfaces to expand outward. The ideal magnetohydrodynamic stability of the plasma is analyzed in the high toroidal mode number limit, where the beam ions are noninteracting. The existence of stable high-(beta) equilibria is demonstrated. One such equilibrium, stable to both ideal interchange and ballooning modes, has <(beta)> (TBOND) 8(pi) / (DBLTURN) 55%. In the CIDER hybrid code, ions are represented by particles and electrons by an inertialess thermal fluid which obeys a generalized Ohm's law. Fields are solved in the quasineutral Darwin approximation. Several collisional and atomic processes are included. CIDER has been used to simulate the propagation of an intense ion beam through a z-pinch plasma channel. We find that transport efficiencies in the 75% range are possible using 5-7 MeV, 1-2 MA proton beams with initial divergences in the range 1.5(DEGREES)-7(DEGREES) in a 4 m hydrogen channel. Current neutralization in excess of 99% is found, and no gross axisymmetric instabilities are observed.
Applications of granular-dynamics numerical simulations to asteroid surfaces
NASA Astrophysics Data System (ADS)
Richardson, D. C.; Michel, P.; Schwartz, S. R.; Yu, Y.; Ballouz, R.-L.; Matsumura, S.
2014-07-01
Spacecraft images and indirect observations including thermal inertia measurements indicate most small bodies have surface regolith. Evidence of granular flow is also apparent in the images. This material motion occurs in very low gravity, therefore in a totally different gravitational environment than on the Earth. Upcoming sample-return missions to small bodies, and possible future manned missions, will involve interaction with the surface regolith, so it is important to develop tools to predict the surface response. We have added new capabilities to the N-body gravity tree code pkdgrav [1,2] that permit the simulation of granular dynamics, including multi-contact physics and friction forces, using the soft-sphere discrete-element method [3]. The numerical approach has been validated through comparison with laboratory experiments (e.g., [3,4]). (1) We carried out impacts into granular materials using different projectile shapes under Earth's gravity [5] and compared the results to laboratory experiments [6] in support of JAXA's Hayabusa 2 asteroid sample-return mission. We tested different projectile shapes and confirmed that the 90-degree cone was the most efficient at excavating mass when impacting 5-mm-diameter glass beads. Results are sensitive to the normal coefficient of restitution and the coefficient of static friction. Preliminary experiments in micro-gravity for similar impact conditions show both the amount of ejected mass and the timescale of the impact process increase, as expected. (2) It has been found (e.g., [7,8]) that ''fresh'' (unreddened) Q-class asteroids have a high probability of recent planetary encounters (˜1 Myr; also see [9]), suggesting that surface refreshening may have occurred due to tidal effects. As an application of the potential effect of tidal interactions, we carried out simulations of Apophis' predicted 2029 encounter with the Earth to see whether regolith motion might occur, using a range of plausible material parameters [10]. We confirm that global shape modification of the asteroid will be negligible [11] and focus on the external perturbations affecting sand piles at various surface locations. We predict that no major landslides will occur, but slight disturbances resulting in regolith motion may take place, depending on the details of the sand-pile structure. (3) The OSIRIS-REx asteroid sample-return mission will use a sampling device designed to penetrate the surface regolith and collect up to 60 g of material by propelling it into the device using compressed nitrogen gas. We conducted numerical experiments for the expected encounter conditions, including the spacecraft touchdown speed and mechanical properties, that show the surface compliance is a strong function of material parameters, even in the micro-gravity environment. (4) A large intruder in a container filled with smaller particles will rise to the top when the container is shaken vigorously, a phenomenon known as the Brazil-nut effect. This effect has been invoked as a possible explanation for the origin of exposed boulders on small-asteroid surfaces [12]. We studied the effect over a wide range of parameters and compared against experiments [13]. We find the Brazil-nut effect is not sensitive to coefficients of restitution but is sensitive to friction constants and the oscillation frequency. We find the rise speed of the intruder is proportional to the square root of the ambient gravitational acceleration. The critical oscillation speeds for the effect to take place are comparable to expected seismic events from non-destructive impacts on small bodies.
Numerical simulation of a meteorological regime of Pontic region
NASA Astrophysics Data System (ADS)
Toropov, P.; Silvestrova, K.
2012-04-01
The Black Sea Coast of Caucasus is one of priority in sense of meteorological researches. It is caused both strategic and economic importance of coast, and current development of an infrastructure for the winter Olympic Games «Sochi-2014». During the winter period at the Black Sea Coast of Caucasus often there are the synoptic conditions leading to occurrence of the dangerous phenomena of weather: «northeast», ice-storms, strong rains, etc. The Department of Meteorology (Moscow State University) throughout 8 years spends regular measurements on the basis of Southern Department of Institute of Oenology of the Russian Academy of Sciences in July and February. They include automatically measurements with the time resolution of 5 minutes in three points characterizing landscape or region (coast, steppe plain, top of the Markothsky ridge), measurements of flux of solar radiation, measurements an atmospheric precipitation in 8 points, which remoteness from each other - 2-3 km. The saved up material has allowed to reveal some features of a meteorological mode of coast. But an overall objective of measurements - an estimation of quality of the numerical forecast by means of «meso scale» models (for example - model WRF). The first of numerical experiments by WRF model were leaded in 2007 year and were devoted reproduction of a meteorological mode of the Black Sea coast. The second phase of experiments has been directed on reproduction the storm phenomena (Novorossiysk nord-ost). For estimation of the modeling data was choused area witch limited by coordinates 44,1 - 44,75 (latitude) and 37,6 - 39 (longitude). Estimations are spent for the basic meteorological parameters - for pressure, temperature, speed of a wind. As earlier it was marked, 8 meteorological stations are located in this territory. Their values are accepted for the standard. Errors are calculated for February 2005, 2006, 2008, 2011 years, because in these periods was marked a strong winds. As the initial data in WRF model are used FNL the analysis, pumped up each six hours. The data is in the open access (http://nomad3.ncep.noaa.gov/pub/) in a grib format. Spatial step FNL of the FNL analysis is 1 degree. In the experiment 1-3 February 2011, was made the assimilation of station data located within the territory or identified during our expeditions. It is shown that the model WRF successfully reproduces the meteorological regime the Black Sea coast. The average error of simulation n without learning station data is as follows: for a temperature of 1.5 s for wind speed - 2 m / sec. The maximum error for the temperature is 5 C, and for wind speed 10 m / sec. To experiment with the assimilation of station data the error is reduced by an average of 20%. The spatial structure of temperature and wind fields close to the actually observed. Thus, it can be argued that the model WRF can be successfully applied to numerical forecast a dangerous phenomenon, such as «Novorossiysk nord-ost». The work is done in Natural Risk Assessment Laboratory under contract G.34.31.0007.
Numerical Simulation of Floating Bodies in Extreme Free Surface Waves
NASA Astrophysics Data System (ADS)
Hu, Zheng Zheng; Causon, Derek; Mingham, Clive; Qiang, Ling
2010-05-01
A task of the EPSRC funded research project 'Extreme Wave loading on Offshore Wave Energy Devices: a Hierarchical Team Approach' is to investigate the survivability of two wave energy converter (WEC) devices Pelamis and the Manchester Bobber using different CFD approaches. Both devices float on the water surface, generating the electricity from the motion of the waves. In this paper, we describe developments of the AMAZON-SC 3D numerical wave tank (NWT) to study extreme wave loading of a fixed or floating (in Heave motion) structure. The extreme wave formulation as an inlet condition is due to Dalzell (1999) and Ning et. al. (2009) in which a first or second-order Stokes focused wave can be prescribed. The AMAZON-SC 3D code (see e.g. Hu et al. (2009)) uses a cell centred finite volume method of the Godunov-type for the space discretization of the Euler and Navier Stokes equations. The computational domain includes both air and water regions with the air/water boundary captured as a discontinuity in the density field thereby admitting the break up and recombination of the free surface. Temporal discretisation uses the artificial compressibility method and a dual time stepping strategy to maintain a divergence free velocity field. Cartesian cut cells are used to provide a fully boundary-fitted gridding capability on an regular background Cartesian grid. Solid objects are cut out of the background mesh leaving a set of irregularly shaped cells fitted to the boundary. The advantages of the cut cell approach have been outlined previously by Causon et al. (2000, 2001) including its flexibility for dealing with complex geometries whether stationary or in relative motion. The field grid does not need to be recomputed globally or even locally for moving body cases; all that is necessary is to update the local cut cell data at the body contour for as long as the motion continues. The handing of numerical wave paddles and device motion in a NWT is therefore straightforward and efficient. Firstly, extreme design wave conditions are generated in an empty NWT and compared with physical experiments as a precursor to calculations to investigate the survivability of the Bobber device operating in a challenging wave climate. Secondly, we consider a bench-mark test case involving in a first order regular wave maker acting on a fixed cylinder and Pelamis. Finally, a floating Bobber has been simulated under extreme wave conditions. These results will be reported at the meeting. Causon D.M., Ingram D.M., Mingham C.G., Yang G. Pearson R.V. (2000). Calculation of shallow water flows using a Cartesian cut cell approach. Advances in Water resources, 23: 545-562. Causon D.M., Ingram D.M., Mingham C.G. (2000). A Cartesian cut cell method for shallow water flows with moving boundaries. Advances in Water resources, 24: 899-911. Dalzell J.F. 1999 A note on finite depth second-order wave-wave interactions. Appl. Ocean Res. 21, 105-111. Ning D.Z., Zang J., Liu S.X. Eatock Taylor R. Teng B. & Taylor P.H. 2009 Free surface and wave kinematics for nonlinear focused wave groups. J. Ocean Engineering. Accepted. Hu Z.Z., Causon D.M., Mingham C.M. and Qian L.(2009). Numerical wave tank study of a wave energy converter in heave. Proceedlings 19th ISOPE conference, Osaka, Japan Qian L., Causon D.M. & Mingham C.G., Ingram D.M. 2006 A free-surface capturing method for two fluid flows with moving bodies. Proc. Roy. Soc. London, Vol. A 462 21-42.
Numerical simulations of superluminous supernovae of type IIn
NASA Astrophysics Data System (ADS)
Dessart, Luc; Audit, Edouard; Hillier, D. John
2015-06-01
We present numerical simulations that include 1D Eulerian multigroup radiation-hydrodynamics, 1D non-local thermodynamic equilibrium (non-LTE) radiative transfer, and 2D polarized radiative transfer for superluminous interacting supernovae (SNe). Our reference model is a ˜10 M? inner shell with 1051 erg ramming into an ˜3 M? cold outer shell (the circumstellar medium, or CSM) that extends from 1015 to 2 × 1016 cm and moves at 100 km s-1. We discuss the light-curve evolution, which cannot be captured adequately with a grey approach. In this type of interactions, the shock-crossing time through the optically thick CSM is much longer than the photon diffusion time. Radiation is thus continuously leaking from the shock through the CSM. This configuration is distinct from the shell-shocked model. Our spectra redden with time, with a peak distribution in the near-UV during the first month gradually shifting to the optical range over the following year. Initially, Balmer lines exhibit a narrow line core and the broad line wings that are characteristic of electron scattering in the SNe IIn atmospheres (CSM). At later times, they also exhibit a broad blue-shifted component which arises from the cold dense shell. Our model results are broadly consistent with the bolometric light curve and spectral evolution observed for SN 2010jl. Invoking a prolate pole-to-equator density ratio in the CSM, we can also reproduce the ˜2 per cent continuum polarization, and line depolarization, observed in SN 2010jl. By varying the inner shell kinetic energy and the mass and extent of the outer shell, a large range of peak luminosities and durations, broadly compatible with superluminous SNe IIn like 2010jl or 2006gy, can be produced.
Hybrid Numerical Methods for Multiscale Simulations of Subsurface Biogeochemical Processes
Scheibe, Timothy D.; Tartakovsky, Alexandre M.; Tartakovsky, Daniel M.; Redden, George D.; Meakin, Paul
2007-08-01
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 including molecular (e.g., molecular dynamics), microbial (e.g., cellular automata or particle individual-based models), pore (e.g., lattice-Boltzmann, pore network models, and discrete particle methods such as smoothed particle hydrodynamics) and continuum scales (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 lithospheric plate dynamics and seismicity
NASA Astrophysics Data System (ADS)
Ismail-Zadeh, A.; Rozenberg, V.; Melnikova, L.; Soloviev, A.
2009-12-01
We model the lithospheric plate dynamics as an interaction of rigid blocks (spherical segments) and faults separating them. The spherical block-and-fault dynamics (BAFD) model consists of 15 blocks approximating tectonic plates and 5 additional blocks representing parts of the plates, where deep seismicity is observed. The blocks move as a consequence of the prescribed underlying mantle motion. The block displacements at any time are defined so that the system of the blocks is in a quasi-static equilibrium state. Because of the block rigidity, all deformations take place in the fault zones. The interaction between the spherical blocks is visco-elastic (a state of stress accumulation), so long as the ratio of the stress to the pressure is below a certain strength level. When this level is exceeded in some part of a fault, a stress-drop (a synthetic earthquake) occurs in accordance with the dry friction law. Immediately following the earthquake and for some period of time, the corresponding parts of the faults are in a state of creep. Catalogs of synthetic earthquakes are produced as results of numerical simulations. Using the catalogs of synthetic events we can study frequency-magnitude relationships, clustering of the events, long-range interaction of earthquakes, earthquake mechanisms, and fault slips. The model catalogs obtained reflect important features of global seismicity: (i) two large seismic belts, the circum-Pacific and Alpine-Himalayan; (ii) extensive, but less pronounced, seismicity at mid-oceanic ridges; and (iii) increased seismic activity associated with triple junctions of plate boundaries. The model results are consistent with the observations: Nazca/South America, Cocos/Caribbean, India/Eurasia, California region, Arabia/Eurasia, northern Australia, and the Philippine plate margin are marked in the model as the regions prone to strong earthquakes. The modeled seismic activity is moderate at the boundaries such as the southern Pacific plate, Nazca/Pacific, east and southwest of Africa, India/Australia, and North America/Eurasia.
Numerical simulation of human orientation perception during lunar landing
NASA Astrophysics Data System (ADS)
Clark, Torin K.; Young, Laurence R.; Stimpson, Alexander J.; Duda, Kevin R.; Oman, Charles M.
2011-09-01
In lunar landing it is necessary to select a suitable landing point and then control a stable descent to the surface. In manned landings, astronauts will play a critical role in monitoring systems and adjusting the descent trajectory through either supervisory control and landing point designations, or by direct manual control. For the astronauts to ensure vehicle performance and safety, they will have to accurately perceive vehicle orientation. A numerical model for human spatial orientation perception was simulated using input motions from lunar landing trajectories to predict the potential for misperceptions. Three representative trajectories were studied: an automated trajectory, a landing point designation trajectory, and a challenging manual control trajectory. These trajectories were studied under three cases with different cues activated in the model to study the importance of vestibular cues, visual cues, and the effect of the descent engine thruster creating dust blowback. The model predicts that spatial misperceptions are likely to occur as a result of the lunar landing motions, particularly with limited or incomplete visual cues. The powered descent acceleration profile creates a somatogravic illusion causing the astronauts to falsely perceive themselves and the vehicle as upright, even when the vehicle has a large pitch or roll angle. When visual pathways were activated within the model these illusions were mostly suppressed. Dust blowback, obscuring the visual scene out the window, was also found to create disorientation. These orientation illusions are likely to interfere with the astronauts' ability to effectively control the vehicle, potentially degrading performance and safety. Therefore suitable countermeasures, including disorientation training and advanced displays, are recommended.
Assisted Sonication vs Conventional Transesterification Numerical Simulation and Sensitivity Study
NASA Astrophysics Data System (ADS)
Janajreh, Isam; Noorul Hussain, Mohammed; El Samad, Tala
2015-10-01
Transeterification is known as slow reaction that can take over several hours to complete as the two immiscible liquid reactants combine to form biodiesel and the less favorable glycerol. The quest of finding the perfect catalyst, optimal operational conditions, and reactor configuration to accelerate the reaction in mere few minutes that ensures high quality biodiesel, in economically viable way is coming along with sonication. This drastic reduction is a key enabler for the development of a continuous processing that otherwise is fairly costly and low throughput using conventional method. The reaction kinetics of sonication assisted as inferred by several authors is several time faster and this work implements these rates in a high fidelity numerical simulation model. This flow model is based on Navier-Stokes equations coupled with energy equation for non-isothermal flow and the transport equations of the multiple reactive species. The model is initially validated against experimental data from previous work of the authors using an annular reactor configuration. Following the validation, comparison of the reaction rate is shown to gain more insight to the distribution of the reaction and its attained rates. The two models (conventional and sonication) then compared on the basis of their sensitivity to the methane to oil molar ratio as the most pronounced process parameter. Both the exit reactor yield and the distribution of the species are evaluated with favorable yield under sonication process. These results pave the way to build a more robust process intensified reactor having an integrated selective heterogeneous catalyst to steer the reaction. This can avoid the downstream cleaning processes, cutting reaction time, and render economic benefit to the process.
Linking numerical simulations of molecular cloud structure with observations.
NASA Astrophysics Data System (ADS)
Kainulainen, Jouni
2015-08-01
Understanding the physical processes that control the life-cycle of the cold interstellar medium (ISM) is one of the key themes in the astrophysics of galaxies today. This importance derives from the role of the cold ISM as the birthplace of new stars, and consequently, as an indivisible constituent of galaxy evolution. In the current paradigm of turbulence-regulated ISM, star formation is controlled by the internal structure of individual molecular clouds, which in turn is set by a complex interplay of turbulence, gravity, and magnetic fields in the clouds. It is in the very focus of the field to determine how these processes give rise to the observed structure of molecular clouds. In this talk, I will review our current efforts to confront this paradigm with the goal of observationally constraining how different processes regulate molecular cloud structure and star formation. At the heart of these efforts lies the use of numerical simulations of gravo-turbulent media to A) define physically meaningful characteristics that are sensitive to the different cloud-shaping processes, and B) determine if and how such characteristics can be recovered by observations. I will show in my talk how this approach has recently led to new constraints for some fundamental measures of the molecular cloud structure. Such constraints allow us to assess the roles of turbulence and gravity in controlling the ISM structure and star formation. I will also highlight specific recent results, focusing on the nature of filamentary structures within molecular clouds. These results may provide a novel set of observational constraints with which to challenge the turbulence-regulated ISM paradigm. Finally, I will discuss the current challenges and open questions in understanding the link between molecular cloud structure and star formation, and speculate on key directions to aim the near-future studies.
Numerical simulation of ion rings and ion beam propagation
Mankofsky, A.
1982-01-01
This thesis presents the development of numerical simulation techniques for studying the physics of ion beams and rings in a background plasma as applicable to certain problems in magnetic and inertial confinement fusion. Two codes have been developed for these purposes: RINGA and CIDER. The 2 and 1/2-dimensional (r,z,v/sub r/, v/sub theta/, v/sub z/, par. delta/par. delta theta = 0) particle code RINGA follows the trajectories of ions in their self-consistent magnetic field. The code assumes strict charge neutrality and admits currents only in the azimuthal direction, i.e., PHI = J/sub r/ = J/sub z/ = 0. The injection and resistive trapping of ion rings has been studied with RINGA. The number of particles trapped as a fraction of the total number injected N is found to be strongly dependent upon (1) N (in the range 2.85 x 10/sup 16/ - 3.99 x 10/sup 17/) and (2) mirror ratios in the system (1.05 to 1.14), and more weakly dependent upon (3) wall resistance per unit length (0.72 ..cap omega../cm - 1.80 ..cap omega../cm) and (4) beam divergence (0/sup 0/ to 6/sup 0/). Fractions of trapped particles in excess of 0.9 have been observed. Modifications to RINGA to include finite pressure of confined plasma and beam ion-electron slowing down collisions are discussed. Finite plasma pressure leads to a diamagnetic current which increases the field reversal factor in ion ring equilibria, while causing the closed flux surfaces to expand outward.
Numerical Simulations Of Jupiter's 5-micron Hot Spots
NASA Astrophysics Data System (ADS)
Choi, David S.; Showman, A. P.; Dowling, T. E.; Palotai, C. J.
2007-10-01
Jupiter's 5-micron hot spots are equatorial regions that emit strongly in the 5-micron infrared window, indicating that the radiation emanates from regions as deep as 8 bars, well below the vertical location of the main ammonia cloud deck (0.6 bar). Thus, these areas lack the cloud cover that would otherwise prevent the deep thermal radiation from emanating to space. Because the Galileo Probe measured conditions in a 5-micron hot spot, fully understanding data from the probe requires a complete dynamical understanding of the hot spots. Showman and Dowling (2000) presented numerical simulations demonstrating that hot spots were areas of descending air, helping to explain the dryness measured by the Galileo probe, and were successful in reproducing other observed characteristics of the hot spots such as their periodic spacing, westward propagation, and vertical wind shear. Showman and Dowling (2000) and Friedson (2005) concluded that hot spots are a manifestation of an equatorially trapped Rossby wave. We re-examine this atmospheric phenomenon using the newest version of the Explicit Planetary Isentropic Coordinate (EPIC) general circulation model in an effort to address several shortcomings of Showman and Dowling's original study. The technical advances made in the GCM allow us to test the sensitivity of hotspots to static stability, vertical wind shear, and varying equatorial wave modes with greater vertical resolution than previously possible. We discuss what ambient atmospheric conditions are required for the hot spots to persist. Furthermore, we examine how the abyssal atmosphere affects the morphology, strength, and evolution of the hot spots. Finally, we address the possibility that variations in the static stability profile of the Jovian atmosphere are responsible for the fact that hot spots exhibit minimal signature in the stratosphere. This research is supported by NASA's Planetary Atmospheres program.
Studying Turbulence Using Numerical Simulation Databases - X Proceedings of the 2004 Summer Program
NASA Technical Reports Server (NTRS)
Moin, Parviz; Mansour, Nagi N.
2004-01-01
This Proceedings volume contains 32 papers that span a wide range of topics that reflect the ubiquity of turbulence. The papers have been divided into six groups: 1) Solar Simulations; 2) Magnetohydrodynamics (MHD); 3) Large Eddy Simulation (LES) and Numerical Simulations; 4) Reynolds Averaged Navier Stokes (RANS) Modeling and Simulations; 5) Stability and Acoustics; 6) Combustion and Multi-Phase Flow.
Massively-Parallel Spectral Element Large Eddy Simulation of a Ring-Type Gas Turbine Combustor
Camp, Joshua Lane
2012-07-16
c Work CFD Computational Fluid Dynamics DNS Direct Numerical Simulation DOF Degree of Freedom FEM Finite Element Method GLL Gauss-Lobatto-Legendre GT Gas Turbine LES Large Eddy Simulation PVC Precessing Vortex Core RANS Reynolds... that causes the rotor to spin. A similar example of this type of power generation machine is a hydroelectric dam. In this case, the potential energy from the height of the water is extracted. These machines are great concepts because of the fact...
Numerical simulation of the 1993 midwestern flood: Real data simulation and verification
Sun, Wen-Yih; Bosilovich, M.G.
1997-11-01
This work presents the first part of a study of the regional forcing on the 1993 extreme precipitation event in the Midwestern United States. A 30 day numerical simulation for each month, June and July, is presented from an analysis using the Purdue Regional Model. The veracity of the model, especially with respect to the hydrologic cycle and the difference between the transient (June) and stationary (July) wave patterns, is examined. The model is able to reproduce each months` general longwave circulation and precipitation patterns. The circulation during July seems to include the same features; however, the primary precipitation feature is displaced too far to the north. The model is able to distinguish between the different atmospheric circulations in June (synoptic scale cyclones) and July (mesoscale convective systems). Overall, the model simulations are reasonable. 9 refs., 7 figs.
Numerical simulation of rarefied flow through a slit. I - Direct simulation Monte Carlo results
NASA Technical Reports Server (NTRS)
Wadsworth, D. C.; Erwin, D. A.
1993-01-01
The pressure-driven flow of a rarefied monatomic gas through a two-dimensional slit is simulated using the direct simulation Monte Carlo technique. Of particular interest is the change in flow field structure as pressure ratio and Knudsen number are varied. Comparisons are made to quantify the limits of validity of free-molecular theory and approximate, nearly free-molecular iterative methods. Also addressed is the sensitivity of the numerical solutions to grid structure and boundary conditions. The free-molecular theory is found to predict quantitative flow field properties (e.g., centerline velocities or downstream flux profiles) reasonably well for large finite Knudsen number with the error dependent on the pressure ratio. The nearly free-molecular corrections are shown to have limited range of applicability. A previously derived parameter is found to correlate total mass flux well as a function of pressure ratio and Knudsen number over a large portion of the transitional regime.
Energetics diagnosis of numerical simulation of atmospheric blocking
NASA Technical Reports Server (NTRS)
Kung, Ernest C.
1990-01-01
A series of systematic comprehensive diagnoses of Goddard Laboratory for Atmospheres (GLA) General Circulation Model (GCM) simulation experiments was performed in reference to predictability and energetics of the Northern Hemisphere blocking circulation. The simulation experiments were performed. The following subject areas are also covered: an analysis of simulated summer blocking episodes; energetics examination of winter blocking simulations in the Northern Hemisphere; normal mode energetic and error analysis of GLA GCM simulations with the different horizontal resolutions during a winter month; and simulations of winter blocking episodes using observed sea surface temperatures.
NUMERICAL SIMULATION OF HEAT TRANSFER IN MATERIALS WITH ANISOTROPIC THERMAL CONDUCTIVITY
NUMERICAL SIMULATION OF HEAT TRANSFER IN MATERIALS WITH ANISOTROPIC THERMAL CONDUCTIVITY: A FINITE/626-7370 URL: http://www.ima.umn.edu #12;Numerical simulation of heat transfer in materials with anisotropic, and Peter Philip May 24, 2005 Abstract We devise a finite volume scheme for nonlinear heat transfer
Advanced numerical methods for the simulation of alloy solidification with high
Jimack, Peter
26 Advanced numerical methods for the simulation of alloy solidification with high Lewis number J research. Keywords: Numerical methods, alloy solidification. 1. Introduction In order to model and simulate Conference on Solidification Processing, Sheffield, July 2007 demonstrate that it is possible, and indeed
The Shape Distribution of Splash-form Tektites Predicted by Numerical Simulations of
Spiteri, Raymond J.
The Shape Distribution of Splash-form Tektites Predicted by Numerical Simulations of Rotating Fluid Distribution of Splash-form Tektites Predicted by Numerical Simulations of Rotating Fluid Drops S. L. B U T L E September 2010) Splash-form tektites are glassy rocks ranging in size from roughly 1 to 100 mm
The Influence of Airborne Doppler Radar Data Quality on Numerical Simulations of a Tropical Cyclone
Pu, Zhaoxia
The Influence of Airborne Doppler Radar Data Quality on Numerical Simulations of a Tropical Cyclone The impact of airborne Doppler radar data assimilation on improving numerical simulations of tropical cy of the radar data quality control on assimilation of the airborne Doppler radar reflectivity and radial
Method for numerical simulation of two-term exponentially correlated colored noise
Yilmaz, B.; Ayik, S.; Abe, Y.; Gokalp, A.; Yilmaz, O.
2006-04-15
A method for numerical simulation of two-term exponentially correlated colored noise is proposed. The method is an extension of traditional method for one-term exponentially correlated colored noise. The validity of the algorithm is tested by comparing numerical simulations with analytical results in two physical applications.
OBJECT ORIENTED PROGRAMMING TECHNIQUES AND FAC METHOD IN NUMERICAL RESERVOIR SIMULATION \\Lambda
OBJECT ORIENTED PROGRAMMING TECHNIQUES AND FAC METHOD IN NUMERICAL RESERVOIR SIMULATION \\Lambda in numerical simulation of flow through hydrocarbon reservoirs within limitations in computing time and memory. These consist of solution of the conservation equations whichs govern the motion of fluid through the reservoir
Numerical Simulations of Interactions between Gravity Waves and Deep Moist Convection
Collett Jr., Jeffrey L.
Numerical Simulations of Interactions between Gravity Waves and Deep Moist Convection ZACHARY A, in final form 2 September 2004) ABSTRACT This study uses a numerical model to simulate deep convection both into the stratosphere, allowing for the simultaneous examination of the convection and the vertically propagating
Numerical Simulation of a single emitter colloid thruster in pure droplet cone-jet mode
Numerical Simulation of a single emitter colloid thruster in pure droplet cone-jet mode Jorge of a single emitter colloid thruster in pure droplet cone-jet mode by Jorge Alejandro Carretero Benignos by ............................................................................................. Lalit Anand Chair, Committee on Graduate Students #12;Numerical Simulation of a single emitter colloid
NUMERICAL SIMULATION OF POOL BOILING FOR STEADY STATE AND TRANSIENT HEATING
Maruyama, Shigeo
1 NUMERICAL SIMULATION OF POOL BOILING FOR STEADY STATE AND TRANSIENT HEATING Ying He, Masahiro role in nucleate and transition boiling heat transfer at high heat flux. Many experiments have been in the numerical simulation of boiling heat transfer. In this study, based on the macrolayer evaporation model
Numerical Simulation of Turbulent Flows in Complex Geometries using the Coherent Vortex
École Normale Supérieure
Approach based on Orthonormal Wavelet Decomposition Henning Bockhorn, Jordan A. Denev, Margarete Domingues turbulence from a Direct Numerical Simulation. Henning Bockhorn Institute for Technical Chemistry and Polymer;2 H. Bockhorn et al. Even more than in the incompressible regime, the numerical simulation of fully
Direct Numerical Simulation of Supersonic Turbulent Boundary Layer over a Compression Ramp
MartÃn, Pino
Direct Numerical Simulation of Supersonic Turbulent Boundary Layer over a Compression Ramp M. Wu numerical simulation of shock wave and turbulent boundary layer interaction for a 24 deg compression ramp boundary layer, the mean wall-pressure distribution, the size of the separation bubble, and the velocity