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.
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.
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.
Progress in direct numerical simulation of turbulent heat transfer
Kasagi, Nobuhide; Iida, Oaki
1999-07-01
With high performance computers, reliable numerical methods and efficient post-processing environment, direct numerical simulation (DNS) offers valuable numerical experiments for turbulent heat transfer research. In particular, one can extensively study the turbulence dynamics and transport mechanism by visualizing any physical variable in space and time. It is also possible to establish detailed database of various turbulence statistics of turbulent transport phenomena, while systematically changing important flow and scalar field parameters. The present paper illustrates these novelties of DNS by introducing several examples in recent studies. Future directions of DNS for turbulence and heat transfer research are also discussed.
An algorithm for fast DNS cavitating flows simulations using homogeneous mixture approach
NASA Astrophysics Data System (ADS)
Žnidarčič, A.; Coutier-Delgosha, O.; Marquillie, M.; Dular, M.
2015-12-01
A new algorithm for fast DNS cavitating flows simulations is developed. The algorithm is based on Kim and Moin projection method form. Homogeneous mixture approach with transport equation for vapour volume fraction is used to model cavitation and various cavitation models can be used. Influence matrix and matrix diagonalisation technique enable fast parallel computations.
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.
Hydroacoustic forcing function modeling using DNS database
NASA Technical Reports Server (NTRS)
Zawadzki, I.; Gershfield, J. L.; Na, Y.; Wang, M.
1996-01-01
A wall pressure frequency spectrum model (Blake 1971 ) has been evaluated using databases from Direct Numerical Simulations (DNS) of a turbulent boundary layer (Na & Moin 1996). Good agreement is found for moderate to strong adverse pressure gradient flows in the absence of separation. In the separated flow region, the model underpredicts the directly calculated spectra by an order of magnitude. The discrepancy is attributed to the violation of the model assumptions in that part of the flow domain. DNS computed coherence length scales and the normalized wall pressure cross-spectra are compared with experimental data. The DNS results are consistent with experimental observations.
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.
A numerical method for DNS/LES of turbulent reacting flows
Doom, Jeff; Hou, Yucheng; Mahesh, Krishnan
2007-09-10
A spatially non-dissipative, implicit numerical method to simulate turbulent reacting flows over a range of Mach numbers, is described. The compressible Navier-Stokes equations are rescaled so that the zero Mach number equations are discretely recovered in the limit of zero Mach number. The dependent variables are co-located in space, and thermodynamic variables are staggered from velocity in time. The algorithm discretely conserves kinetic energy in the incompressible, inviscid, non-reacting limit. The chemical source terms are implicit in time to allow for stiff chemical mechanisms. The algorithm is readily extended to complex chemical mechanisms. Numerical examples using both simple and complex chemical mechanisms are presented.
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.
Large eddy simulations and direct numerical simulations of high speed turbulent reacting flows
NASA Technical Reports Server (NTRS)
Givi, Peyman; Madnia, Cyrus K.; Steinberger, Craig J.
1990-01-01
This research is involved with the implementation of advanced computational schemes based on large eddy simulations (LES) and direct numerical simulations (DNS) to study the phenomenon of mixing and its coupling with chemical reactions in compressible turbulent flows. In the efforts related to LES, a research program to extend the present capabilities of this method was initiated for the treatment of chemically reacting flows. In the DNS efforts, the focus is on detailed investigations of the effects of compressibility, heat release, and non-equilibrium kinetics modelings in high speed reacting flows. Emphasis was on the simulations of simple flows, namely homogeneous compressible flows, and temporally developing high speed mixing layers.
Numerical Aerodynamic Simulation
NASA Technical Reports Server (NTRS)
1989-01-01
An overview of historical and current numerical aerodynamic simulation (NAS) is given. The capabilities and goals of the Numerical Aerodynamic Simulation Facility are outlined. Emphasis is given to numerical flow visualization and its applications to structural analysis of aircraft and spacecraft bodies. The uses of NAS in computational chemistry, engine design, and galactic evolution are mentioned.
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.
PDF turbulence modeling and DNS
NASA Technical Reports Server (NTRS)
Hsu, A. T.
1992-01-01
The problem of time discontinuity (or jump condition) in the coalescence/dispersion (C/D) mixing model is addressed in probability density function (pdf). A C/D mixing model continuous in time is introduced. With the continuous mixing model, the process of chemical reaction can be fully coupled with mixing. In the case of homogeneous turbulence decay, the new model predicts a pdf very close to a Gaussian distribution, with finite higher moments also close to that of a Gaussian distribution. Results from the continuous mixing model are compared with both experimental data and numerical results from conventional C/D models. The effect of Coriolis forces on compressible homogeneous turbulence is studied using direct numerical simulation (DNS). The numerical method used in this study is an eight order compact difference scheme. Contrary to the conclusions reached by previous DNS studies on incompressible isotropic turbulence, the present results show that the Coriolis force increases the dissipation rate of turbulent kinetic energy, and that anisotropy develops as the Coriolis force increases. The Taylor-Proudman theory does apply since the derivatives in the direction of the rotation axis vanishes rapidly. A closer analysis reveals that the dissipation rate of the incompressible component of the turbulent kinetic energy indeed decreases with a higher rotation rate, consistent with incompressible flow simulations (Bardina), while the dissipation rate of the compressible part increases; the net gain is positive. Inertial waves are observed in the simulation results.
Significance of DNS in combustion science
NASA Astrophysics Data System (ADS)
Takeno, Tadao; Mizobuchi, Yasuhiro
2006-08-01
The recent advancement in numerical calculations provides us with a new scientific approach to combustion phenomena, that is, numerical experiment. The authors have succeeded in capturing a hydrogen jet lifted flame by DNS (Direct Numerical Simulation) approach with detailed chemistry and exact transport properties. The simulation made clear that the flame is not a single flame, but a complex flame composed of three flame elements. Some aspects of the flame elements showed the properties of laminar flames and some showed very complicated and unsteady nature of turbulent flames that cannot be described by the conventional laminar flame theory. In this article, the problems of this kind of study are identified and the direction of study is suggested throughout the DNS studies on the hydrogen jet lifted flame. To cite this article: T. Takeno, Y. Mizobuchi, C. R. Mecanique 334 (2006).
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.
Estimating Uncertainties in Statistics Computed from DNS
NASA Astrophysics Data System (ADS)
Malaya, Nicholas; Ulerich, Rhys; Oliver, Todd; Moser, Robert
2012-11-01
Direct numerical simulation (DNS) of turbulence is a critical tool for investigating the physics of turbulent flows and for informing and developing engineering turbulence models. For instance, flow statistics obtained from DNS are commonly used as ``truth data'' for the calibration and evaluation of turbulence models. Thus, like experimental data, uncertainty estimates are a necessary component of the reported output. In DNS, uncertainties in the computed statistics arise from two sources: finite sampling and the discretization of the Navier-Stokes equations. Here, we apply estimators for both sources of error. Finite sampling errors are estimated using the ``effective sample size,'' which accounts for the fact that the instantaneous data are correlated. Discretization errors are estimated using data from simulations with varying time step and mesh spacing. The performance of these estimators is tested for several statistics using DNS of turbulent channel flow at low Reynolds number (Reτ ~ 180). This work is supported by the Department of Energy National Nuclear Security Administration under Award Number [DE-FC52-08NA28615].
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.
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.
Direct numerical simulation of nonpremixed flame-wall interactions
Wang, Yi; Trouve, Arnaud
2006-02-01
The objective of the present study is to use detailed numerical modeling to obtain basic information on the interaction of nonpremixed flames with cold wall surfaces. The questions of turbulent fuel-air-temperature mixing, flame extinction, and wall-surface heat transfer are studied using direct numerical simulation (DNS). The DNS configuration corresponds to an ethylene-air diffusion flame stabilized in the near-wall region of a chemically inert solid surface. Simulations are performed with adiabatic or isothermal wall boundary conditions and with different turbulence intensities. The simulations feature flame extinction events resulting from excessive wall cooling and convective heat transfer rates up to 90 kW/m{sup 2}. The structure of the simulated wall flames is studied in terms of a classical mass-mixing variable, the fuel-air based mixture fraction, and a less familiar heat loss variable, the excess enthalpy variable, introduced to provide a measure of nonadiabatic behavior due to wall cooling. In addition to the flame structure, extinction events are also studied in detail and a modified flame extinction criterion that combines the concepts of mixture fraction and excess enthalpy is proposed and then tested against the DNS data. (author)
Direct Numerical Simulations of Multiphase Flows
NASA Astrophysics Data System (ADS)
Tryggvason, Gretar
2013-03-01
Many natural and industrial processes, such as rain and gas exchange between the atmosphere and oceans, boiling heat transfer, atomization and chemical reactions in bubble columns, involve multiphase flows. Often the mixture can be described as a disperse flow where one phase consists of bubbles or drops. Direct numerical simulations (DNS) of disperse flow have recently been used to study the dynamics of multiphase flows with a large number of bubbles and drops, often showing that the collective motion results in relatively simple large-scale structure. Here we review simulations of bubbly flows in vertical channels where the flow direction, as well as the bubble deformability, has profound implications on the flow structure and the total flow rate. Results obtained so far are summarized and open questions identified. The resolution for DNS of multiphase flows is usually determined by a dominant scale, such as the average bubble or drop size, but in many cases much smaller scales are also present. These scales often consist of thin films, threads, or tiny drops appearing during coalescence or breakup, or are due to the presence of additional physical processes that operate on a very different time scale than the fluid flow. The presence of these small-scale features demand excessive resolution for conventional numerical approaches. However, at small flow scales the effects of surface tension are generally strong so the interface geometry is simple and viscous forces dominate the flow and keep it simple also. These are exactly the conditions under which analytical models can be used and we will discuss efforts to combine a semi-analytical description for the small-scale processes with a fully resolved simulation of the rest of the flow. We will, in particular, present an embedded analytical description to capture the mass transfer from bubbles in liquids where the diffusion of mass is much slower than the diffusion of momentum. This results in very thin mass-boundary layers that are difficult to resolve, but the new approach allows us to simulate the mass transfer from many freely evolving bubbles and examine the effect of the interactions of the bubbles with each other and the flow. We will conclude by attempting to summarize the current status of DNS of multiphase flows. Support by NSF and DOE (CASL)
DNS of turbulent channel flow with a higher Reynolds number
NASA Astrophysics Data System (ADS)
Kawamura, Hiroshi; Abe, Hiroyuki; Matsuo, Yuichi
1998-11-01
With an aid of recent developments in the super and parallel computers, the direct numerical simulation (DNS) of turbulence is now being increasingly performed. It is already more than 10 years ago when Kim-Moin-Moser published their DNS on the turbulent channel flow. Their Reynolds number based on the friction velocity and channel half width was Re_τ=180. In a couple of years they made also a DNS with a higher Re_τ of 395. To the author's knowledge, DNS of a higher Reynolds number than 400 has not been published. The author's group has attempted to perform the DNS of Re_τ=640 using a vectorized parallel computer NWT (Numerical Wind Tunnel). The calculation was made with 512×256×256 grids on 64 processors. The numerical integration was made by the finite difference method. The discretization scheme was a so-called 'consistent scheme', in which a special attention was paid for the consistency between the analytical and the numerical differentiations. The mean velocity and the Reynolds stress components as well as the budget of their transport equations were obtained. They are compared with those of lower Reynolds number flows and the effect of the Reynolds number is discussed.
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 DNSs results. The first-order moments is calculated well by LES. But there are some discrepancies of second-order moments between LES and DNS.
The Use of DNS in Turbulence Modeling
NASA Technical Reports Server (NTRS)
Mansour, Nagi N.; Merriam, Marshal (Technical Monitor)
1997-01-01
The use of Direct numerical simulations (DNS) data in developing and testing turbulence models is reviewed. The data is used to test turbulence models at all levels: algebraic, one-equation, two-equation and full Reynolds stress models were tested. Particular examples on the development of models for the dissipation rate equation are presented. Homogeneous flows are used to test new scaling arguments for the various terms in the dissipation rate equation. The channel flow data is used to develop modifications to the equation model that take into account near-wall effects. DNS of compressible flows under mean compression are used in testing new compressible modifications to the two-equation models.
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.
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.
Dynamic stiffness removal for direct numerical simulations
Lu, Tianfeng; Law, Chung K.; Yoo, Chun Sang; Chen, Jacqueline H.
2009-08-15
A systematic approach was developed to derive non-stiff reduced mechanisms for direct numerical simulations (DNS) with explicit integration solvers. The stiffness reduction was achieved through on-the-fly elimination of short time-scales induced by two features of fast chemical reactivity, namely quasi-steady-state (QSS) species and partial-equilibrium (PE) reactions. The sparse algebraic equations resulting from QSS and PE approximations were utilized such that the efficiency of the dynamic stiffness reduction is high compared with general methods of time-scale reduction based on Jacobian decomposition. Using the dimension reduction strategies developed in our previous work, a reduced mechanism with 52 species was first derived from a detailed mechanism with 561 species. The reduced mechanism was validated for ignition and extinction applications over the parameter range of equivalence ratio between 0.5 and 1.5, pressure between 10 and 50 atm, and initial temperature between 700 and 1600 K for ignition, and worst-case errors of approximately 30% were observed. The reduced mechanism with dynamic stiffness removal was then applied in homogeneous and 1-D ignition applications, as well as a 2-D direct numerical simulation of ignition with temperature inhomogeneities at constant volume with integration time-steps of 5-10 ns. The integration was numerically stable and good accuracy was achieved. (author)
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 Aerodynamic Simulation (NAS)
NASA Technical Reports Server (NTRS)
Peterson, V. L.; Ballhaus, W. F., Jr.; Bailey, F. R.
1983-01-01
The history of the Numerical Aerodynamic Simulation Program, which is designed to provide a leading-edge capability to computational aerodynamicists, is traced back to its origin in 1975. Factors motivating its development and examples of solutions to successively refined forms of the governing equations are presented. The NAS Processing System Network and each of its eight subsystems are described in terms of function and initial performance goals. A proposed usage allocation policy is discussed and some initial problems being readied for solution on the NAS system are identified.
Numerical propulsion system simulation
NASA Technical Reports Server (NTRS)
Lytle, John K.; Remaklus, David A.; Nichols, Lester D.
1990-01-01
The cost of implementing new technology in aerospace propulsion systems is becoming prohibitively expensive. One of the major contributors to the high cost is the need to perform many large scale system tests. Extensive testing is used to capture the complex interactions among the multiple disciplines and the multiple components inherent in complex systems. The objective of the Numerical Propulsion System Simulation (NPSS) is to provide insight into these complex interactions through computational simulations. This will allow for comprehensive evaluation of new concepts early in the design phase before a commitment to hardware is made. It will also allow for rapid assessment of field-related problems, particularly in cases where operational problems were encountered during conditions that would be difficult to simulate experimentally. The tremendous progress taking place in computational engineering and the rapid increase in computing power expected through parallel processing make this concept feasible within the near future. However it is critical that the framework for such simulations be put in place now to serve as a focal point for the continued developments in computational engineering and computing hardware and software. The NPSS concept which is described will provide that framework.
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.
Direct Numerical Simulation of Mach 3 Compression Ramp Flow
NASA Astrophysics Data System (ADS)
Priebe, Stephan; Martin, Pino
2010-11-01
We present the direct numerical simulation (DNS) of a shockwave and turbulent boundary layer interaction (STBLI) generated by a compression ramp. The flow conditions are Mach 2.9 and Re?=2900, and the ramp angle is 24 degrees. STBLI flows are known to display low-frequency unsteadiness, typically at frequencies 1-2 orders of magnitude lower than that of the incoming undisturbed boundary layer. The presence of these low-frequency motions in the DNS data and their relationship with the upstream and downstream flow regions have been demonstrated (Priebe and Martin, AIAA paper 2010-108). The DNS data show that the low-frequency shock motion is significantly correlated with the downstream flow. A statistically significant but small correlation is also found with the upstream flow. In the present paper, we investigate the flow structure associated with the downstream flow regions and study the time-and-space resolved dynamics of the shock motion, shear layer and separated flow regions.
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.
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.
Direct numerical simulation of shockwave and turbulent boundary layer interactions
NASA Astrophysics Data System (ADS)
Wu, Minwei
Direct numerical simulations (DNS) of a shockwave/turbulent boundary layer interaction (STBLI) at Mach number 3 and Reynolds number based on the momentum thickness of 2300 are performed. A 4th-order accurate, bandwidth-optimized weighted-essentially-non-oscillatory (WENO) scheme is used and the method is found to be too dissipative for the STBLI simulation due to the over-adaptation properties of this original WENO scheme. In turn, a relative limiter is introduced to mitigate the problem. Tests on the Shu-Osher problem show that the modified WENO scheme decreases the numerical dissipation significantly. By utilizing a combination of the relative limiter and the absolute limiter described by Jiang & Shu [32], the DNS results are improved further. The DNS data agree well with the reference experiments of Bookey et al. [10] in the size of the separation bubble, the separation and reattachment point, the mean wall-pressure distribution, and the velocity profiles both upstream and downstream of the interaction region. The DNS data show that velocity profiles change dramatically along the streamwise direction. Downstream of the interaction, the velocity profiles show a characteristic "dip" in the logarithmic region, as shown by the experiments of Smits & Muck [66] at much higher Reynolds number. In the separation region, the velocity profiles are found to resemble those of a laminar flow, yet the flow does not fully relaminarize. The mass-flux turbulence intensity is amplified by a factor of about 5 throughout the interaction, which is consistent with that found in higher Reynolds experiments of Selig et al. [52]. All Reynolds stress components are greatly amplified by the interaction. Assuming that the ow is still two dimensional downstream of the interaction, the components rhou'u', rhov'v', rho w'w', and rho u'w' are amplified by factors of 6, 6, 12, and 24, respectively, where u is the streamwise and w is the wall-normal velocity. However, analyses of the turbulence structure show that the ow is not uniform in the spanwise direction downstream of the interaction. A pair of counter-rotating vortices is observed in streamwise-wall-normal planes in the mean ow downstream of the ramp corner. Taking the three-dimensionality into account, the amplification factors of the Reynolds stresses are greatly decreased. The component rhou'w' is amplified by a factor of about 10, which is comparable to that found in the experiments of Smits & Muck [66]. Strong Reynolds analogy (SRA) relations are also studied using the DNS data. The SRA is found to hold in the incoming boundary layer of the DNS. However, inside and downstream of the interaction region, the SRA relations are not satisfied. From the DNS analyses, the shock motion is characterized by a low frequency component (of order 0.01Uinfinity/delta). In addition, the motion of the shock is found to have two aspects: a spanwise wrinkling motion and a streamwise oscillatory motion. The spanwise wrinkling is observed to be a local feature with high frequencies (of order Uinfinity /delta). Two-point correlations reveal that the spanwise wrinkling is closely related to the low momentum motions in the incoming boundary layer as they convect through the shock. The low frequency shock motion is found to be a streamwise oscillation motion. Conditional statistics show that there is no significant difference in the mean properties of the incoming boundary layer when the shock is at an upstream or downstream location. However, analyses of the unsteadiness of the separation bubble reveal that the low frequency shock motion is driven by the downstream flow.
NASA Technical Reports Server (NTRS)
Selle, L. C.; Bellan, Josette
2006-01-01
Transitional databases from Direct Numerical Simulation (DNS) of three-dimensional mixing layers for single-phase flows and two-phase flows with evaporation are analyzed and used to examine the typical hypothesis that the scalar dissipation Probability Distribution Function (PDF) may be modeled as a Gaussian. The databases encompass a single-component fuel and four multicomponent fuels, two initial Reynolds numbers (Re), two mass loadings for two-phase flows and two free-stream gas temperatures. Using the DNS calculated moments of the scalar-dissipation PDF, it is shown, consistent with existing experimental information on single-phase flows, that the Gaussian is a modest approximation of the DNS-extracted PDF, particularly poor in the range of the high scalar-dissipation values, which are significant for turbulent reaction rate modeling in non-premixed flows using flamelet models. With the same DNS calculated moments of the scalar-dissipation PDF and making a change of variables, a model of this PDF is proposed in the form of the (beta)-PDF which is shown to approximate much better the DNS-extracted PDF, particularly in the regime of the high scalar-dissipation values. Several types of statistical measures are calculated over the ensemble of the fourteen databases. For each statistical measure, the proposed (beta)-PDF model is shown to be much superior to the Gaussian in approximating the DNS-extracted PDF. Additionally, the agreement between the DNS-extracted PDF and the (beta)-PDF even improves when the comparison is performed for higher initial Re layers, whereas the comparison with the Gaussian is independent of the initial Re values. For two-phase flows, the comparison between the DNS-extracted PDF and the (beta)-PDF also improves with increasing free-stream gas temperature and mass loading. The higher fidelity approximation of the DNS-extracted PDF by the (beta)-PDF with increasing Re, gas temperature and mass loading bodes well for turbulent reaction rate modeling.
DNS of Turbulent Boundary Layers under Highenthalpy Conditions
NASA Astrophysics Data System (ADS)
Duan, Lian; Martn, Pino
2010-11-01
To study real-gas effects and turbulence-chemistry interaction, direct numerical simulations (DNS) of hypersonic boundary layers are conducted under typical hypersonic conditions. We consider the boundary layer on a lifting-body consisting of a flat plate at an angle of attack, which flies at altitude 30km with a Mach number 21. Two different inclined angles, 35^o and 8^o, are considered,representing blunt and slender bodies. Both noncatalytic and supercatalytic wall conditions are considered. The DNS data are studied to assess the validity of Morkovin's hypothesis, the strong Reynolds analogy, as well as the behaviors of turbulence structures under high-enthalpy conditions.Relative to low-enthalpy conditions [1], significant differences in typical scalings are observed. [4pt] [1] L. Duan and I. Beekman and M. P. Mart'in, Direct numerical simulation of hypersonic turbulent boundary layers. Part 2: Effect of temperature, J. Fluid Mech. 655 (2010), 419-445.
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.
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.
Direct numerical simulation of superfluid turbulence
NASA Astrophysics Data System (ADS)
Morris, Karla
At low temperatures, as quantum effects become increasingly apparent, helium (He4) transforms into a superfluid. The motion of superfluid helium (He II) can be decomposed into two interpenetrating components: (1) an inviscid (superfluid) liquid containing line vortices with quantized circulation and (2) a (normal fluid) gas of elementary thermal excitations. At sufficiently high driving velocities, the motion of He II becomes unstable and transitions to turbulence, commonly termed superfluid turbulence or quantum turbulence. A growing body of empirical evidence suggests that the macroscopic statistical behavior of quantum turbulence closely matches that of classical turbulence despite considerable differences in the physics at the mesoscopic scale of the inter-vortex spacing and the microscopic scale of the vortex core diameters [47,50]. Although a commonly used phenomenology involving quantum-vortex/normal-vortex locking has achieved some success in explaining the macroscopic similarities, current laboratory measurements lack sufficient spatial resolution to verify vortex locking. The work presented here investigates the detailed mechanisms underlying quantum turbulence via direct numerical simulations (DNS) of superfluid vortex interactions with interpenetrating normal fluid turbulence. The driving fluid is the normal component which behaves as a statistically homogeneous isotropic turbulent flow, and both forced and decaying cases are simulated. The data obtained from the simulation is analyzed using wavelet transforms and velocity correlations. The normal fluid calculation employs a Navier-Stokes (NS) solver developed by Rouson and Xu [31] in a manner that facilitates rapid integration of new physics by expressing dynamical equations in forms very closely mirroring their analytical expression. The superfluid calculation employs a vortex filament method originated by Schwarz [39,40,41]. The Navier-Stokes and vortex filament equations are marched in time using a software module developed by Rouson, Morris and Xu [33] which facilitates rapid implementation of time advancement algorithms for coupled multi-physics problems.
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)
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.
Investigation of Hill's optical turbulence model by means of direct numerical simulation.
Muschinski, Andreas; de Bruyn Kops, Stephen M
2015-12-01
For almost four decades, Hill's "Model 4" [J. Fluid Mech.88, 541 (1978)JFLSA70022-112010.1017/S002211207800227X] has played a central role in research and technology of optical turbulence. Based on Batchelor's generalized Obukhov-Corrsin theory of scalar turbulence, Hill's model predicts the dimensionless function h(κl_{0},Pr) that appears in Tatarskii's well-known equation for the 3D refractive-index spectrum in the case of homogeneous and isotropic turbulence, Φ_{n}(κ)=0.033Cn2κ^{-11/3}h(κl_{0},Pr). Here we investigate Hill's model by comparing numerical solutions of Hill's differential equation with scalar spectra estimated from direct numerical simulation (DNS) output data. Our DNS solves the Navier-Stokes equation for the 3D velocity field and the transport equation for the scalar field on a numerical grid containing 4096^{3} grid points. Two independent DNS runs are analyzed: one with the Prandtl number Pr=0.7 and a second run with Pr=1.0. We find very good agreement between h(κl_{0},Pr) estimated from the DNS output data and h(κl_{0},Pr) predicted by the Hill model. We find that the height of the Hill bump is 1.79 Pr^{1/3}, implying that there is no bump if Pr<0.17. Both the DNS and the Hill model predict that the viscous-diffusive "tail" of h(κl_{0},Pr) is exponential, not Gaussian. PMID:26831396
Direct numerical simulations of temporally developing turbulent reacting liquid-fueled jets
NASA Astrophysics Data System (ADS)
Shashank, Shashank; Pitsch, Heinz
2012-11-01
Liquid fueled engines are ubiquitous in the transportation industry because liquid fuel minimizes the weight and volume of propulsion systems. The combustion that occurs in these engines is an inherently multi-physics process, involving fuel evaporation, reaction kinetics, and high levels of turbulence. A desire for high fidelity data that explains complex interaction between different physical mechanisms motivates the consideration of direct numerical simulation (DNS) as an investigation tool. In this study three-dimensional DNS of a reacting n-heptane liquid fueled temporal jet have been performed to study auto-ignition and subsequent burning in conditions that are representative of a diesel engine environment. In these simulations the continuous phase is described using an Eulerian representation whereas Lagrangian particle tracking is used to model the dispersed phase. The results of this study will demonstrate the importance of unsteady effects, and of accounting for the interaction between different modes of combustion, when simulating spray combustion.
Numerical 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.
High speed turbulent reacting flows: DNS and LES
NASA Technical Reports Server (NTRS)
Givi, Peyman
1990-01-01
Work on understanding the mechanisms of mixing and reaction in high speed turbulent reacting flows was continued. Efforts, in particular, were concentrated on taking advantage of modern computational methods to simulate high speed turbulent flows. In doing so, two methodologies were used: large eddy simulations (LES) and direct numerical simulations (DNS). In the work related with LES the objective is to study the behavior of the probability density functions (pdfs) of scalar properties within the subgrid in reacting turbulent flows. The data base obtained by DNS for a detailed study of the pdf characteristics within the subgrid was used. Simulations are performed for flows under various initializations to include the effects of compressibility on mixing and chemical reactions. In the work related with DNS, a two-dimensional temporally developing high speed mixing layer under the influence of a second-order non-equilibrium chemical reaction of the type A + B yields products + heat was considered. Simulations were performed with different magnitudes of the convective Mach numbers and with different chemical kinetic parameters for the purpose of examining the isolated effects of the compressibility and the heat released by the chemical reactions on the structure of the layer. A full compressible code was developed and utilized, so that the coupling between mixing and chemical reactions is captured in a realistic manner.
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.
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.
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 α{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 α-effect, which is a natural outcome of rotating stratified convection, can be an important prerequisite for large-scale stellar dynamos, even without the Ω-effect.
DNS of MHD turbulent flow via the HELIOS supercomputer system at IFERC-CSC
NASA Astrophysics Data System (ADS)
Satake, Shin-ichi; Kimura, Masato; Yoshimori, Hajime; Kunugi, Tomoaki; Takase, Kazuyuki
2014-06-01
The simulation plays an important role to estimate characteristics of cooling in a blanket for such high heating plasma in ITER-BA. An objective of this study is to perform large -scale direct numerical simulation (DNS) on heat transfer of magneto hydro dynamic (MHD) turbulent flow on coolant materials assumed from Flibe to lithium. The coolant flow conditions in ITER-BA are assumed to be Reynolds number and Hartmann number of a higher order. The maximum target of the DNS assumed by this study based on the result of the benchmark of Helios at IFERC-CSC for Project cycle 1 is 116 TB (2048 nodes). Moreover, we tested visualization by ParaView to visualize directly the large-scale computational result. If this large-scale DNS becomes possible, an essential understanding and modelling of a MHD turbulent flow and a design of nuclear fusion reactor contributes greatly.
Fish Pectoral Fin Hydrodynamics; Part II: Numerical Simulations and Analysis
NASA Astrophysics Data System (ADS)
Dong, H.; Madden, P. G.
2005-11-01
High-fidelity numerical simulations are being used to examine the key hydrodynamic features and thrust performance of the pectoral fin of a bluegill sunfish which is moving at a constant forward velocity. The numerical modeling approach employs a parallelized immersed boundary solver which can perform direct (DNS) or large-eddy simulation (LES) of flow past highly deformable bodies such as fish pectoral fins. The three-dimensional, time-dependent fin kinematics is obtained via a stereo-videographic technique and experiments also provide PIV data which is used to validate the numerical simulations. The primary objectives of the CFD effort are to quantify the thrust performance of the bluegill sunfish pectoral fin as well as to establish the mechanisms responsible for thrust production. Simulations show that the pectoral fin produces a relatively large amount of thrust at all phases in the fin motion while limiting the magnitude of the transverse forces. The motion of the fin produces a distinct system of connected vortices which are examined in detail in order to gain insight into the thrust producing mechanisms.
Estimating uncertainties in statistics computed from direct numerical simulation
NASA Astrophysics Data System (ADS)
Oliver, Todd A.; Malaya, Nicholas; Ulerich, Rhys; Moser, Robert D.
2014-03-01
Rigorous assessment of uncertainty is crucial to the utility of direct numerical simulation (DNS) results. Uncertainties in the computed statistics arise from two sources: finite statistical sampling and the discretization of the Navier-Stokes equations. Due to the presence of non-trivial sampling error, standard techniques for estimating discretization error (such as Richardson extrapolation) fail or are unreliable. This work provides a systematic and unified approach for estimating these errors. First, a sampling error estimator that accounts for correlation in the input data is developed. Then, this sampling error estimate is used as part of a Bayesian extension of Richardson extrapolation in order to characterize the discretization error. These methods are tested using the Lorenz equations and are shown to perform well. These techniques are then used to investigate the sampling and discretization errors in the DNS of a wall-bounded turbulent flow at Re? ? 180. Both small (Lx/? Lz/? = 4? 2?) and large (Lx/? Lz/? = 12? 4?) domain sizes are investigated. For each case, a sequence of meshes was generated by first designing a "nominal" mesh using standard heuristics for wall-bounded simulations. These nominal meshes were then coarsened to generate a sequence of grid resolutions appropriate for the Bayesian Richardson extrapolation method. In addition, the small box case is computationally inexpensive enough to allow simulation on a finer mesh, enabling the results of the extrapolation to be validated in a weak sense. For both cases, it is found that while the sampling uncertainty is large enough to make the order of accuracy difficult to determine, the estimated discretization errors are quite small. This indicates that the commonly used heuristics provide adequate resolution for this class of problems. However, it is also found that, for some quantities, the discretization error is not small relative to sampling error, indicating that the conventional wisdom that sampling error dominates discretization error for this class of simulations needs to be reevaluated.
Numerical simulations of hot spots
NASA Astrophysics Data System (ADS)
Norman, Michael L.
Numerical simulations of hot spots and their associated jets are examined with emphasis on their dynamical variability. Attention is given to two-dimensional simulations, which incorporate dynamically passive and important magnetic fields in the ideal MHD limit. Distributions of total and polarized radio brightness have been derived for comparison with observations. The move toward three-dimensional simulations is documented, and hydrodynamical models for multiple hot spots are discussed. It is suggested that useful insights can be obtained from two-dimensional slab jet simulation, which relax the axisymmetric constraints while allowing high numerical resolution. In particular the dentist-drill model of Scheuer (1982) for working-surface variability is substantiated, and it is shown to result from self-excited jet instabilities near the working surface.
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.
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
Terascale direct numerical simulations of turbulent combustion using S3D.
Sankaran, Ramanan; Mellor-Crummy, J.; DeVries, M.; Yoo, Chun Sang; Ma, K. L.; Podhorski, N.; Liao, W. K.; Klasky, S.; de Supinski, B.; Choudhary, A.; Hawkes, Evatt R.; Chen, Jacqueline H.; Shende, Sameer
2008-08-01
Computational science is paramount to the understanding of underlying processes in internal combustion engines of the future that will utilize non-petroleum-based alternative fuels, including carbon-neutral biofuels, and burn in new combustion regimes that will attain high efficiency while minimizing emissions of particulates and nitrogen oxides. Next-generation engines will likely operate at higher pressures, with greater amounts of dilution and utilize alternative fuels that exhibit a wide range of chemical and physical properties. Therefore, there is a significant role for high-fidelity simulations, direct numerical simulations (DNS), specifically designed to capture key turbulence-chemistry interactions in these relatively uncharted combustion regimes, and in particular, that can discriminate the effects of differences in fuel properties. In DNS, all of the relevant turbulence and flame scales are resolved numerically using high-order accurate numerical algorithms. As a consequence terascale DNS are computationally intensive, require massive amounts of computing power and generate tens of terabytes of data. Recent results from terascale DNS of turbulent flames are presented here, illustrating its role in elucidating flame stabilization mechanisms in a lifted turbulent hydrogen/air jet flame in a hot air co-flow, and the flame structure of a fuel-lean turbulent premixed jet flame. Computing at this scale requires close collaborations between computer and combustion scientists to provide optimized scaleable algorithms and software for terascale simulations, efficient collective parallel I/O, tools for volume visualization of multiscale, multivariate data and automating the combustion workflow. The enabling computer science, applied to combustion science, is also required in many other terascale physics and engineering simulations. In particular, performance monitoring is used to identify the performance of key kernels in the DNS code, S3D and especially memory intensive loops in the code. Through the careful application of loop transformations, data reuse in cache is exploited thereby reducing memory bandwidth needs, and hence, improving S3D's nodal performance. To enhance collective parallel I/O in S3D, an MPI-I/O caching design is used to construct a two-stage write-behind method for improving the performance of write-only operations. The simulations generate tens of terabytes of data requiring analysis. Interactive exploration of the simulation data is enabled by multivariate time-varying volume visualization. The visualization highlights spatial and temporal correlations between multiple reactive scalar fields using an intuitive user interface based on parallel coordinates and time histogram. Finally, an automated combustion workflow is designed using Kepler to manage large-scale data movement, data morphing, and archival and to provide a graphical display of run-time diagnostics.
Terascale direct numerical simulations of turbulent combustion using S3D
NASA Astrophysics Data System (ADS)
Chen, J. H.; Choudhary, A.; de Supinski, B.; DeVries, M.; Hawkes, E. R.; Klasky, S.; Liao, W. K.; Ma, K. L.; Mellor-Crummey, J.; Podhorszki, N.; Sankaran, R.; Shende, S.; Yoo, C. S.
2009-01-01
Computational science is paramount to the understanding of underlying processes in internal combustion engines of the future that will utilize non-petroleum-based alternative fuels, including carbon-neutral biofuels, and burn in new combustion regimes that will attain high efficiency while minimizing emissions of particulates and nitrogen oxides. Next-generation engines will likely operate at higher pressures, with greater amounts of dilution and utilize alternative fuels that exhibit a wide range of chemical and physical properties. Therefore, there is a significant role for high-fidelity simulations, direct numerical simulations (DNS), specifically designed to capture key turbulence-chemistry interactions in these relatively uncharted combustion regimes, and in particular, that can discriminate the effects of differences in fuel properties. In DNS, all of the relevant turbulence and flame scales are resolved numerically using high-order accurate numerical algorithms. As a consequence terascale DNS are computationally intensive, require massive amounts of computing power and generate tens of terabytes of data. Recent results from terascale DNS of turbulent flames are presented here, illustrating its role in elucidating flame stabilization mechanisms in a lifted turbulent hydrogen/air jet flame in a hot air coflow, and the flame structure of a fuel-lean turbulent premixed jet flame. Computing at this scale requires close collaborations between computer and combustion scientists to provide optimized scaleable algorithms and software for terascale simulations, efficient collective parallel I/O, tools for volume visualization of multiscale, multivariate data and automating the combustion workflow. The enabling computer science, applied to combustion science, is also required in many other terascale physics and engineering simulations. In particular, performance monitoring is used to identify the performance of key kernels in the DNS code, S3D and especially memory intensive loops in the code. Through the careful application of loop transformations, data reuse in cache is exploited thereby reducing memory bandwidth needs, and hence, improving S3D's nodal performance. To enhance collective parallel I/O in S3D, an MPI-I/O caching design is used to construct a two-stage write-behind method for improving the performance of write-only operations. The simulations generate tens of terabytes of data requiring analysis. Interactive exploration of the simulation data is enabled by multivariate time-varying volume visualization. The visualization highlights spatial and temporal correlations between multiple reactive scalar fields using an intuitive user interface based on parallel coordinates and time histogram. Finally, an automated combustion workflow is designed using Kepler to manage large-scale data movement, data morphing, and archival and to provide a graphical display of run-time diagnostics.
A high-order photon Monte Carlo method for radiative transfer in direct numerical simulation
Wu, Y.; Modest, M.F.; Haworth, D.C. . E-mail: dch12@psu.edu
2007-05-01
A high-order photon Monte Carlo method is developed to solve the radiative transfer equation. The statistical and discretization errors of the computed radiative heat flux and radiation source term are isolated and quantified. Up to sixth-order spatial accuracy is demonstrated for the radiative heat flux, and up to fourth-order accuracy for the radiation source term. This demonstrates the compatibility of the method with high-fidelity direct numerical simulation (DNS) for chemically reacting flows. The method is applied to address radiative heat transfer in a one-dimensional laminar premixed flame and a statistically one-dimensional turbulent premixed flame. Modifications of the flame structure with radiation are noted in both cases, and the effects of turbulence/radiation interactions on the local reaction zone structure are revealed for the turbulent flame. Computational issues in using a photon Monte Carlo method for DNS of turbulent reacting flows are discussed.
High-resolution numerical simulation of turbulence in natural waterways
NASA Astrophysics Data System (ADS)
Kang, Seokkoo; Lightbody, Anne; Hill, Craig; Sotiropoulos, Fotis
2011-01-01
We develop an efficient and versatile numerical model for carrying out high-resolution simulations of turbulent flows in natural meandering streams with arbitrarily complex bathymetry. The numerical model solves the 3D, unsteady, incompressible Navier-Stokes and continuity equations in generalized curvilinear coordinates. The method can handle the arbitrary geometrical complexity of natural streams using the sharp-interface curvilinear immersed boundary (CURVIB) method of Ge and Sotiropoulos (2007) [1]. The governing equations are discretized with three-point, central, second-order accurate finite-difference formulas and integrated in time using an efficient, second-order accurate fractional step method. To enable efficient simulations on grids with tens of millions of grid nodes in long and shallow domains typical of natural streams, the algebraic multigrid (AMG) method is used to solve the Poisson equation for the pressure coupled with a matrix-free Krylov solver for the momentum equations. Depending on the desired level of resolution and available computational resources, the numerical model can either simulate, via direct numerical simulation (DNS), large-eddy simulation (LES), or unsteady Reynolds-averaged Navier-Stokes (URANS) modeling. The potential of the model as a powerful tool for simulating energetic coherent structures in turbulent flows in natural river reaches is demonstrated by applying it to carry out LES and URANS in a 50-m long natural meandering stream at resolution sufficiently fine to capture vortex shedding from centimeter-scale roughness elements on the bed. The accuracy of the simulations is demonstrated by comparisons with experimental data and the relative performance of the LES and URANS models is also discussed.
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.
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.
DepenDNS: Dependable Mechanism against DNS Cache Poisoning
NASA Astrophysics Data System (ADS)
Sun, Hung-Min; Chang, Wen-Hsuan; Chang, Shih-Ying; Lin, Yue-Hsun
DNS cache poisoning attacks have been proposed for a long time. In 2008, Kaminsky enhanced the attacks to be powerful based on nonce query method. By leveraging Kaminsky's attack, phishing becomes large-scale since victims are hard to detect attacks. Hence, DNS cache poisoning is a serious threat in the current DNS infrastructure. In this paper, we propose a countermeasure, DepenDNS, to prevent from cache poisoning attacks. DepenDNS queries multiple resolvers concurrently to verify an trustworthy answer while users perform payment transactions, e.g., auction, banking. Without modifying any resolver or authority server, DepenDNS is conveniently deployed on client side. In the end of paper, we conduct several experiments on DepenDNS to show its efficiency. We believe DepenDNS is a comprehensive solution against cache poisoning attacks.
Validation of an LES Model for Soot Evolution against DNS Data in Turbulent Jet Flames
NASA Astrophysics Data System (ADS)
Mueller, Michael
2012-11-01
An integrated modeling approach for soot evolution in turbulent reacting flows is validated against three-dimensional Direct Numerical Simulation (DNS) data in a set of n-heptane nonpremixed temporal jet flames. As in the DNS study, the evolution of the soot population is described statistically with the Hybrid Method of Moments (HMOM). The oxidation of the fuel and formation of soot precursors are described with the Radiation Flamelet/Progress Variable (RFPV) model that includes an additional transport equation for Polycyclic Aromatic Hydrocarbons (PAH) to account for the slow chemistry governing these species. In addition, the small-scale interactions between soot, chemistry, and turbulence are described with a presumed subfilter PDF approach that accounts for the very large spatial intermittency characterizing soot in turbulent reacting flows. The DNS dataset includes flames at three different Damkhler numbers to study the influence of global mixing rates on the evolution of PAH and soot. In this work, the ability of the model to capture these trends quantitatively as Damkhler number varies is investigated. In order to reliably assess the LES approach, the LES is initialized from the filtered DNS data after an initial transitional period in an effort to minimize the hydrodynamic differences between the DNS and the LES.
Direct Numerical Simulation of Supersonic Turbulent Boundary Layers
NASA Astrophysics Data System (ADS)
Guarini, Stephen; Moser, Robert; Shariff, Karim; Wray, Alan
1997-11-01
Initial results from the direct numerical simulation (DNS) of compressible turbulent boundary layers will be presented. The spatially developing boundary layer is first transformed to a parallel shear layer using a transformation similar to that used by Spalart for an incompressible boundary layer. This allows us to avoid inflow and outflow boundary conditions, and to apply periodic boundary conditions in the streamwise and spanwise directions. The resulting equations are then solved using a mixed Fourier B-spline Galerkin method. One challenge to these highly accurate and non-dissipative numerics has been the occurrence of sharp density gradients, which require significantly more resolution than the incompressible case, especially during transients. The first simulation is at Mach 2.5 with a momentum thickness Reynolds number based on wall viscosity of R_?'=825. The simulations are used to examine the physics of the compressible boundary layer and to compute turbulence statistics and terms in the budget equations. The turbulence statistics include: rms and mean profiles, energy spectra, and two-point correlations.
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.
A review of direct numerical simulations of astrophysical detonations and their implications
Parete-Koon, Suzanne T.; Smith, Christopher R.; Papatheodore, Thomas L.; Bronson Messer, O. E.
2013-04-11
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. Considerablemore » effort has been expended modeling Type Ia supernovae at densities above 1x107 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 1x107 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 reviews 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.« less
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 1107 gcm-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 1107 gcm-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.
Detailed Comparison of DNS to PSE for Oblique Breakdown at Mach 3
NASA Technical Reports Server (NTRS)
Mayer, Christian S. J.; Fasel, Hermann F.; Choudhari, Meelan; Chang, Chau-Lyan
2010-01-01
A pair of oblique waves at low amplitudes is introduced in a supersonic flat-plate boundary layer. Their downstream development and the concomitant process of laminar to turbulent transition is then investigated numerically using Direct Numerical Simulations (DNS) and Parabolized Stability Equations (PSE). This abstract is the last part of an extensive study of the complete transition process initiated by oblique breakdown at Mach 3. In contrast to the previous simulations, the symmetry condition in the spanwise direction is removed for the simulation presented in this abstract. By removing the symmetry condition, we are able to confirm that the flow is indeed symmetric over the entire computational domain. Asymmetric modes grow in the streamwise direction but reach only small amplitude values at the outflow. Furthermore, this abstract discusses new time-averaged data from our previous simulation CASE 3 and compares PSE data obtained from NASA's LASTRAC code to DNS results.
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
Autoignition of hydrogen and air using direct numerical simulation
NASA Astrophysics Data System (ADS)
Doom, Jeffrey; Mahesh, Krishnan
2008-11-01
Direct numerical simulation (DNS) is used to study to auto--ignition in laminar vortex rings and turbulent diffusion flames. A novel, all--Mach number algorithm developed by Doom et al (J. Comput. Phys. 2007) is used. The chemical mechanism is a nine species, nineteen reaction mechanism for H2 and Air from Mueller at el (Int. J. Chem. Kinet. 1999). The vortex ring simulations inject diluted H2 at ambient temperature into hot air, and study the effects of stroke ratio, air to fuel ratio and Lewis number. At smaller stroke ratios, ignition occurs in the wake of the vortex ring and propagates into the vortex core. At larger stroke ratios, ignition occurs along the edges of the trailing column before propagating towards the vortex core. The turbulent diffusion flame simulations are three--dimensional and consider the interaction of initially isotropic turbulence with an unstrained diffusion flame. The simulations examine the nature of distinct ignition kernels, the relative roles of chemical reactions, and the relation between the observed behavior and laminar flames and the perfectly stirred reactor problem. These results will be discussed.
Direct Numerical Simulation of Supersonic Turbulent Boundary Layers
NASA Technical Reports Server (NTRS)
Guarini, Stephen; Moser, R.; Shariff, K.; Wray, A.; Merriam, Marshal (Technical Monitor)
1997-01-01
The talk will present some initial results from the direct numerical simulation (DNS) of compressible turbulent boundary layers. We solve numerically the compressible Navier-Stokes equations using a method based on Spalart's transformation for the incompressible turbulent boundary layer. This allows the spatially developing boundary layer to be transformed to a calculation with periodic boundary conditions in the streamwise and spanwise directions. The equations are solved using Fourier expansions in the horizontal directions and B-splines in the wall-normal direction. The first simulation is at Mach 2.5 with a momentum thickness Reynolds number based on wall viscosity of R(sub theta(sup 1)) = 825. We are examining the physics of the compressible boundary layer using turbulence statistics and budget equations. The turbulence statistics include: rms (root mean square) and mean profiles, energy spectra, and two-point correlations. It is found that there are large density gradients which require significantly more resolution than the incompressible case.
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
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.
Numerical Simulations of Homogeneous Turbulence Using Lagrangian-Averaged Navier-Stokes Equations
NASA Technical Reports Server (NTRS)
Mohseni, Kamran; Shkoller, Steve; Kosovic, Branko; Marsden, Jerrold E.; Carati, Daniele; Wray, Alan; Rogallo, Robert
2000-01-01
The Lagrangian-averaged Navier-Stokes (LANS) equations are numerically evaluated as a turbulence closure. They are derived from a novel Lagrangian averaging procedure on the space of all volume-preserving maps and can be viewed as a numerical algorithm which removes the energy content from the small scales (smaller than some a priori fixed spatial scale alpha) using a dispersive rather than dissipative mechanism, thus maintaining the crucial features of the large scale flow. We examine the modeling capabilities of the LANS equations for decaying homogeneous turbulence, ascertain their ability to track the energy spectrum of fully resolved direct numerical simulations (DNS), compare the relative energy decay rates, and compare LANS with well-accepted large eddy simulation (LES) models.
Numerical simulations of non-homogeneous viscoelastic turbulent channel flow
NASA Astrophysics Data System (ADS)
Housiadas, Kostas; Beris, Antony
2004-11-01
The effect of the polymer mixing in turbulent channel flow is studied through numerical simulations, using a spectral technique. In particular, we simulate injection of polymeric material through a slit very close to the wall and parallel to it in pre-established Newtonian turbulent flow. The governing equations consist of the mass conservation, the modified Navier-Stokes equation (in order to take into account the polymer extra-stress), the evolution equation for the conformation tensor and an advection-diffusion equation for the polymer concentration. The injection process is simulated by dividing the computational domain in three different regions: (a) the entrance region where the polymer is introduced (b) the developing region where the polymer is allowed to convect freely interacting/modifying the turbulent flow and (c) the recovering region where we use a reacting sink to force the removal of the polymer from the solvent in order to re-establish the inlet conditions. A fully spectral method is used in order to solve the set of governing equations similar to that developed for homogenous viscoelastic turbulent DNS (Housiadas & Beris, Phys. Fluids, 15, (2003)). Although a significantly improved numerical algorithm has been successfully used before (Housiadas & Beris, to appear in J. Non-Newt. Fluid Mech. (2004)) a further improved version of that algorithm is presented in this work. The new algorithm has enabled us to extend the simulations for much wider range of viscoelasticity parameter values as well as for many viscoelastic models like the FENE-P, Giesekus, Oldroyd-B and the modified Giesekus/FENE-P model. Results for illustrative sets of parameter values are going to be presented.
DNS of H2/Air Combustion using Complex Chemistry
NASA Astrophysics Data System (ADS)
Doom, Jeff; Mahesh, Krishnan
2007-11-01
Direct numerical simulation (DNS) is used to study reacting, laminar, vortex rings and turbulent diffusion flames. A novel, all--Mach number algorithm developed by Doom et al (J. Comput. Phys. 2007) is used. The chemical mechanism is a nine species, nineteen reaction mechanism for H2 and Air from Mueller at el (Int. J. Chem. Kinet. 1999) and the extended Zel'dovich mechanism was used to account for the formation of NO. Simulations were performed for three dimensional vortex rings where diluted H2 at ambient temperature (300 K) is injected into hot air (1200 K). The effect of Damkohler number and stroke length will be discussed. Simulations of a three dimensional turbulent diffusion flames were performed. Isotropic turbulence is superimposed on an unstrained diffusion flame where diluted H2 at ambient temperature interacts with hot air. Results of the simulation will be discussed.
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.
Particle-Based Direct Numerical Simulation of Contaminant Transport and Deposition in Porous Flow
Ray A. Berry; Richard C. Martineau; Thomas R. Wood
2004-02-01
This work describes an approach to porous flow modeling in which the "micro-length scale to macro-length scale" physical descriptions are addressed as Lagrangian, pore-level flow and transport. The flow features of the physical domain are solved by direct numerical simulation (DNS) with a grid-free, hybrid smoothed particle hydrodynamics (SPH) numerical method (Berry, 2002) based on a local Riemann solution. In addition to being able to handle the large deformation, fluid–fluid and fluid–solid interactions within the contorted geometries of intra- and inter-pore-scale modeling, this Riemann–SPH method should be able to simulate other complexities, such as multiple fluid phases and chemical, particulate, and microbial transport with volumetric and surface reactions. A simple model is presented for the transfer of a contaminant from a carrier fluid to solid surfaces and is demonstrated for flow in a simulated porous media
DNS of Shock / Isotropic Turbulence Interaction
NASA Astrophysics Data System (ADS)
Grube, Nathan; Taylor, Ellen; Martn, Pino
2010-11-01
We discuss DNS of Shock / Isotropic Turbulence Interactions (SITI). We vary the incoming turbulence Mach number up to 0.8 and the convective Mach number up to 5 in order to determine their effects on the interaction. These cases are challenging due to the presence of shocklets in the incoming turbulence as well as significant motion of the main shock. Shock-capturing must be used at all points while still maintaining low enough numerical dissipation to preserve the turbulent fluctuations. We use the linearly- and nonlinearly-optimized Weighted Essentially Non-Oscillatory (WENO) method[1,2]. Particular attention is paid to the inflow boundary condition, where we find the use of snapshots of "frozen" turbulence from decaying isotropic box simulations to be unsatisfactory. We instead use time-varying inflow data generated by a separate forced isotropic turbulence simulation with a specified convection speed. This allows us to access flow conditions where the assumptions of Taylor's Hypothesis are not met. 1.) Mart'in, M.P., Taylor, E.M., Wu, M., and Weirs, V.G., JCP 220(1) 270-89, 2006. 2.) Taylor, E.M., Wu, M., and Mart'in, M.P., JCP 223(1) 384-97, 2007.
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.
Numerical Simulations of Thermal Turbulence
NASA Astrophysics Data System (ADS)
Christie, Steven Lee
Direct numerical simulations of turbulent Rayleigh -Benard convection were conducted to study the soft turbulence regime. This regime was defined in a 1988 experiment by the rate at which the Nusselt number scaled with Rayleigh number, the probability distribution (pdf) of temperature fluctuations, and the type of thermal structures present. All of these properties changed abruptly in the experiment as the Rayleigh number was increased above a threshold value where a new regime called hard turbulence was defined. By conducting our simulations in a lateral domain larger than the experiments, we were able to show that these properties do not all change at the same Rayleigh number. Visualization and lateral wavenumber spectra confirmed the structural portrait of soft turbulence. However, the Nusselt number scaling conformed to that of hard turbulence, and the temperature statistics showed an unclear mixture of both regimes' signatures. Prompted by this apparent effect of lateral scale upon the turbulence, we focussed on the temperature pdfs and repeated one of our simulations with nothing changed except a reduction of this dimension in half. The pdf changed from Gaussian, the purported soft turbulence distribution, to the exponential profile associated with hard turbulence. Further study of the lateral scales' influence was then pursued by separating such scales out of the flowfields themselves. Using the same two cases with disparate lateral box sizes, we filtered-out high and low bands of horizontal wavenumber for each and constructed pdfs. The results were that, for both of these flow with distinct exponential and Gaussian statistics, each one contained Gaussianity among its largest horizontal scales and exponentiality (and flatter distributions) among its smallest ones. We therefore conclude that these distributions are indicative of predominant scales, and that thermal turbulence cannot be universally characterized by Rayleigh number; aspect ratio, and probably other parameters must be included. However, at extremely high Rayleigh number, it may remain true that hard turbulence is a universal asymptotic state regardless of other parameters. (Copies available exclusively from Micrographics Department, Doheny Library, USC, Los Angeles, CA 90089-0182.).
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.
Turbulent flame-wall interaction: a DNS study
Chen, Jackie; Hawkes, Evatt R; Sankaran, Ramanan; Gruber, Andrea
2010-01-01
A turbulent flame-wall interaction (FWI) configuration is studied using three-dimensional direct numerical simulation (DNS) and detailed chemical kinetics. The simulations are used to investigate the effects of the wall turbulent boundary layer (i) on the structure of a hydrogen-air premixed flame, (ii) on its near-wall propagation characteristics and (iii) on the spatial and temporal patterns of the convective wall heat flux. Results show that the local flame thickness and propagation speed vary between the core flow and the boundary layer, resulting in a regime change from flamelet near the channel centreline to a thickened flame at the wall. This finding has strong implications for the modelling of turbulent combustion using Reynolds-averaged Navier-Stokes or large-eddy simulation techniques. Moreover, the DNS results suggest that the near-wall coherent turbulent structures play an important role on the convective wall heat transfer by pushing the hot reactive zone towards the cold solid surface. At the wall, exothermic radical recombination reactions become important, and are responsible for approximately 70% of the overall heat release rate at the wall. Spectral analysis of the convective wall heat flux provides an unambiguous picture of its spatial and temporal patterns, previously unobserved, that is directly related to the spatial and temporal characteristic scalings of the coherent near-wall turbulent structures.
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.
Direct numerical simulation of the sea flows around blunt bodies
NASA Astrophysics Data System (ADS)
Matyushin, Pavel V.; Gushchin, Valentin A.
2015-11-01
The aim of the present paper is the demonstration of the opportunities of the mathematical modeling of the separated flows of the sea water around blunt bodies on the basis of the Navier-Stokes equations (NSE) in the Boussinesq approximation. The 3D density stratified incompressible viscous fluid flows around a sphere have been investigated by means of the direct numerical simulation (DNS) on supercomputers and the visualization of the 3D vortex structures in the wake. For solving of NSE the Splitting on physical factors Method for Incompressible Fluid flows (SMIF) with hybrid explicit finite difference scheme (second-order accuracy in space, minimum scheme viscosity and dispersion, capable for work in wide range of the Reynolds (Re) and the internal Froude (Fr) numbers and monotonous) has been developed and successfully applied. The different transitions in sphere wakes with increasing of Re (10 < Re < 500) and decreasing of Fr (0.005 < Fr < 100) have been investigated in details. Thus the classifications of the viscous fluid flow regimes around a sphere have been refined.
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 accelerated interfaces
Marcus, D.L.; Bell, J.B. ); Puckett, E.G. ); Saltzman, J. )
1991-10-21
We describe two numerical methods that have been designed to model the dynamics of accelerated interfaces. One method is based on the numerical approximation of the incompressible, variable-density Navier-Stokes equations while the other is based on a numerical approximation of the Euler equations. One feature that these methods share is that the numerical approximation to the governing fluid flow equations is accomplished in part by means of a second order extension of Godunov's method. We present the results of computations to model Rayleigh-Taylor instability and Richtmyer-Meshkov instability. 22 refs., 11 figs.
Numerical simulation of accelerated interfaces
Marcus, D.L.; Bell, J.B.; Puckett, E.G.; Saltzman, J.
1991-10-21
We describe two numerical methods that have been designed to model the dynamics of accelerated interfaces. One method is based on the numerical approximation of the incompressible, variable-density Navier-Stokes equations while the other is based on a numerical approximation of the Euler equations. One feature that these methods share is that the numerical approximation to the governing fluid flow equations is accomplished in part by means of a second order extension of Godunov`s method. We present the results of computations to model Rayleigh-Taylor instability and Richtmyer-Meshkov instability. 22 refs., 11 figs.
Sreedhara, S.; Huh, Kang Y.
2005-12-01
The performance of second-order conditional moment closure (CMC) depends on models to evaluate conditional variances and covariances of temperature and species mass fractions. In this paper the closure schemes based on the steady laminar flamelet model (SLFM) are validated against direct numerical simulation (DNS) involving extinction and ignition. Scaling is performed to reproduce proper absolute magnitudes, irrespective of the origin of mismatch between local flamelet structures and scalar dissipation rates. DNS based on the pseudospectral method is carried out to study hydrogen-air combustion with a detailed kinetic mechanism, in homogeneous, isotropic, and decaying turbulent media. Lewis numbers are set equal to unity to avoid complication of differential diffusion. The SLFM-based closures for correlations among fluctuations of reaction rate, scalar dissipation rate, and species mass fractions show good comparison with DNS. The variance parameter in lognormal PDF and the constants in the dissipation term have been estimated from DNS results. Comparison is made for the resulting conditional profiles from DNS, first-order CMC, and second-order CMC with correction to the most critical reaction step according to sensitivity analysis. Overall good agreement ensures validity of the SLFM-based closures for modeling conditional variances and covariances in second-order CMC.
Sankaran, Ramanan; Chen, Jacqueline H.; Hawkes, Evatt R.; Pebay, Philippe Pierre
2005-01-01
The influence of thermal stratification on autoignition at constant volume and high pressure is studied by direct numerical simulation (DNS) with detailed hydrogen/air chemistry. Parametric studies on the effect of the initial amplitude of the temperature fluctuations, the initial length scales of the temperature and velocity fluctuations, and the turbulence intensity are performed. The combustion mode is characterized using the diagnostic measures developed in Part I of this study. Specifically, the ignition front speed and the scalar mixing timescales are used to identify the roles of molecular diffusion and heat conduction in each case. Predictions from a multizone model initialized from the DNS fields are presented and differences are explained using the diagnostic tools developed.
Hawkes, Evatt R.; Sankaran, Ramanan; Pebay, Philippe P.; Chen, Jacqueline H.
2006-04-15
The influence of thermal stratification on autoignition at constant volume and high pressure is studied by direct numerical simulation (DNS) with detailed hydrogen/air chemistry. Parametric studies on the effect of the initial amplitude of the temperature fluctuations, the initial length scales of the temperature and velocity fluctuations, and the turbulence intensity are performed. The combustion mode is characterized using the diagnostic measures developed in Part I of this study. Specifically, the ignition front speed and the scalar mixing timescales are used to identify the roles of molecular diffusion and heat conduction in each case. Predictions from a multizone model initialized from the DNS fields are presented and differences are explained using the diagnostic tools developed. (author)
Nourgaliev R.; Knoll D.; Mousseau V.; Berry R.
2007-04-01
The state-of-the-art for Direct Numerical Simulation (DNS) of boiling multiphase flows is reviewed, focussing on potential of available computational techniques, the level of current success for their applications to model several basic flow regimes (film, pool-nucleate and wall-nucleate boiling -- FB, PNB and WNB, respectively). Then, we discuss multiphysics and multiscale nature of practical boiling flows in LWR reactors, requiring high-fidelity treatment of interfacial dynamics, phase-change, hydrodynamics, compressibility, heat transfer, and non-equilibrium thermodynamics and chemistry of liquid/vapor and fluid/solid-wall interfaces. Finally, we outline the framework for the {\\sf Fervent} code, being developed at INL for DNS of reactor-relevant boiling multiphase flows, with the purpose of gaining insight into the physics of multiphase flow regimes, and generating a basis for effective-field modeling in terms of its formulation and closure laws.
Sankaran, Ramanan; Mason, Scott D.; Chen, Jacqueline H.; Hawkes, Evatt R.; Im, Hong G.
2005-01-01
The influence of thermal stratification on autoignition at constant volume and high pressure is studied by direct numerical simulation (DNS) with detailed hydrogen/air chemistry. Parametric studies on the effect of the initial amplitude of the temperature fluctuations, the initial length scales of the temperature and velocity fluctuations, and the turbulence intensity are performed. The combustion mode is characterized using the diagnostic measures developed in Part I of this study. Specifically, the ignition front speed and the scalar mixing timescales are used to identify the roles of molecular diffusion and heat conduction in each case. Predictions from a multizone model initialized from the DNS fields are presented and differences are explained using the diagnostic tools developed.
Inflow and initial conditions for direct numerical simulation based on adjoint data assimilation
NASA Astrophysics Data System (ADS)
Gronskis, A.; Heitz, D.; Mmin, E.
2013-06-01
A method for generating inflow conditions for direct numerical simulations (DNS) of spatially-developing flows is presented. The proposed method is based on variational data assimilation and adjoint-based optimization. The estimation is conducted through an iterative process involving a forward integration of a given dynamical model followed by a backward integration of an adjoint system defined by the adjoint of the discrete scheme associated to the dynamical system. The approach's robustness is evaluated on two synthetic velocity field sequences provided by numerical simulation of a mixing layer and a wake flow behind a cylinder. The performance of the technique is also illustrated in a real world application by using noisy large scale PIV measurements. This method denoises experimental velocity fields and reconstructs a continuous trajectory of motion fields from discrete and unstable measurements.
Relativistic positioning systems: Numerical simulations
NASA Astrophysics Data System (ADS)
Puchades Colmenero, Neus
The position of users located on the Eart&hacute;s surface or near it may be found with the classic positioning systems (CPS). Certain information broadcast by satellites of global navigation systems, as GPS and GALILEO, may be used for positioning. The CPS are based on the Newtonian formalism, although relativistic post-Newtonian corrections are done when they are necessary. This thesis contributes to the development of a different positioning approach, which is fully relativistic from the beginning. In the relativistic positioning systems (RPS), the space-time position of any user (ship, spacecraft, and so on) can be calculated with the help of four satellites, which broadcast their proper times by means of codified electromagnetic signals. In this thesis, we have simulated satellite 4-tuples of the GPS and GALILEO constellations. If a user receives the signals from four satellites simultaneously, the emission proper times read -after decoding- are the user "emission coordinates". In order to find the user "positioning coordinates", in an appropriate almost inertial reference system, there are two possibilities: (a) the explicit relation between positioning and emission coordinates (broadcast by the satellites) is analytically found or (b) numerical codes are designed to calculate the positioning coordinates from the emission ones. Method (a) is only viable in simple ideal cases, whereas (b) allows us to consider realistic situations. In this thesis, we have designed numerical codes with the essential aim of studying two appropriate RPS, which may be generalized. Sometimes, there are two real users placed in different positions, which receive the same proper times from the same satellites; then, we say that there is bifurcation, and additional data are needed to choose the real user position. In this thesis, bifurcation is studied in detail. We have analyzed in depth two RPS models; in both, it is considered that the satellites move in the Schwarzschil&dacute;s space-time (ST) corresponding to an ideal static spherically symmetric Earth. Since this ST is asymptotically Minkowskian, as soon as Schwarzschild's metric is assumed, there is a reference system with origin in the Earth center, which is inertial from the theoretical point of view, and almost inertial - since the acceleration due to the Sun and other small effects are being neglected- from the physical point of view. In one of the models, the 0-order RPS, photons move in the Minkowski ST asymptotic to Schwarzschild, whereas in the second model (1-order RPS), photons move - as the satellites - in Schwarzschild's ST. In the context of RPS, error estimations seem to be more systematic than in the framework of CPS. In the scheme presented in this thesis, there are two types of positioning errors. The U-errors are due to deviations of the actual satellite world lines with respect to the nominal lines (which have been adequately defined); whereas the S-errors arise when some sources of the gravitational field are neglected and, consequently, the metric and the photon null geodesics are not fully accurate. For every user, the total error is the addition of the U and the S errors. We have found, represented, and studied, the distributions of bifurcation points, U-errors, and S-errors, inside a wide spherical region with a radius of 100.000 km, which is located around Earth and centered in an arbitrary point on Eart&hacute;s surface; in this way, we have discussed - for the first time - spacecraft positioning in this region based on GPS and GALILEO 4-tuples. (Abstract shortened by UMI.).
NASA Astrophysics Data System (ADS)
Kaul, C. M.; Raman, V.
2011-03-01
Subfilter scalar variance is a critical indicator of small scale mixing in large eddy simulation (LES) of turbulent combustion and is an important parameter of conserved scalar based combustion models. Realistic combustion models have a highly nonlinear dependence on the conserved scalar, making the prediction of flow thermochemistry sensitive to errors in subfilter variance modeling, including errors due to numerical discretization. Large numerical errors can result from the use of grid-based filtering and the resulting under-resolution of the smallest filtered scales, which are a key to variance modeling. Hence, the development of variance models should take into account this sensitivity to numerical discretization. In this work, a novel coupled direct numerical simulation (DNS)-LES a posteriori method is used to study the role of discretization errors in variance prediction for the two most widely used types of models: algebraic dynamic models and transport equation-based models. Algebraic models are found to be ill-suited to discretization due to their dependence on filtered scalar gradient values. Additionally, the use of dynamic modeling procedures enhances their sensitivity to filtered scalar errors. The accuracy of transport equation models primarily rests on the accuracy of the scalar dissipation rate closure with numerical error having a secondary effect. The influence of dissipation rate modeling error is investigated using the unique information provided by the combined DNS-LES simulation method. Overall, transport equation models are found to offer a more powerful approach to variance modeling due to more complete model physics and reduced effects of discretization error.
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.
Numerical Simulation of a High Mach Number Jet Flow
NASA Technical Reports Server (NTRS)
Hayder, M. Ehtesham; Turkel, Eli; Mankbadi, Reda R.
1993-01-01
The recent efforts to develop accurate numerical schemes for transition and turbulent flows are motivated, among other factors, by the need for accurate prediction of flow noise. The success of developing high speed civil transport plane (HSCT) is contingent upon our understanding and suppression of the jet exhaust noise. The radiated sound can be directly obtained by solving the full (time-dependent) compressible Navier-Stokes equations. However, this requires computational storage that is beyond currently available machines. This difficulty can be overcome by limiting the solution domain to the near field where the jet is nonlinear and then use acoustic analogy (e.g., Lighthill) to relate the far-field noise to the near-field sources. The later requires obtaining the time-dependent flow field. The other difficulty in aeroacoustics computations is that at high Reynolds numbers the turbulent flow has a large range of scales. Direct numerical simulations (DNS) cannot obtain all the scales of motion at high Reynolds number of technological interest. However, it is believed that the large scale structure is more efficient than the small-scale structure in radiating noise. Thus, one can model the small scales and calculate the acoustically active scales. The large scale structure in the noise-producing initial region of the jet can be viewed as a wavelike nature, the net radiated sound is the net cancellation after integration over space. As such, aeroacoustics computations are highly sensitive to errors in computing the sound sources. It is therefore essential to use a high-order numerical scheme to predict the flow field. The present paper presents the first step in a ongoing effort to predict jet noise. The emphasis here is in accurate prediction of the unsteady flow field. We solve the full time-dependent Navier-Stokes equations by a high order finite difference method. Time accurate spatial simulations of both plane and axisymmetric jet are presented. Jet Mach numbers of 1.5 and 2.1 are considered. Reynolds number in the simulations was about a million. Our numerical model is based on the 2-4 scheme by Gottlieb & Turkel. Bayliss et al. applied the 2-4 scheme in boundary layer computations. This scheme was also used by Ragab and Sheen to study the nonlinear development of supersonic instability waves in a mixing layer. In this study, we present two dimensional direct simulation results for both plane and axisymmetric jets. These results are compared with linear theory predictions. These computations were made for near nozzle exit region and velocity in spanwise/azimuthal direction was assumed to be zero.
Numerical Simulation of Unsteady Aerodynamic Models
NASA Technical Reports Server (NTRS)
Nguyen, Khanh Q.; Warmbrodt, William (Technical Monitor)
1997-01-01
This report documents the results of the numerical simulations of unsteady aerodynamic models. The results focus on numerical accuracy and efficiency, and the robustness of the numerical methods. The aerodynamic models includes the classical Wagner and Kussner functions and the Leishman-Beddoes dynamic stall model. The simulations includes the numerical approximations of the Duhamel's integrals using both indicial (step) and impulse responses, the numerical integrations of the state-space models, and the exact solutions. The report also presents the conversion among different model representations.
NASA Technical Reports Server (NTRS)
Okong'o, N. A.; Bellan, J.
2003-01-01
Analysis of Direct Numerical Simulations (DNS) transitional states of temporal, supercritical mixing layers for C7H16/N2 and O2/H2 shows that the evolution of all layers is characterized by the formation of high-density-gradient magnitude (HDGM) regions.
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.
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.
NASA Astrophysics Data System (ADS)
Breugem, W. P.; Boersma, B. J.
2005-02-01
A direct numerical simulation (DNS) has been performed of turbulent channel flow over a three-dimensional Cartesian grid of 30209 cubes in, respectively, the streamwise, spanwise, and wall-normal direction. The grid of cubes mimics a permeable wall with a porosity of 0.875. The flow field is resolved with 600400400 mesh points. To enforce the no-slip and no-penetration conditions on the cubes, an immersed boundary method is used. The results of the DNS are compared with a second DNS in which a continuum approach is used to model the flow through the grid of cubes. The continuum approach is based on the volume-averaged Navier-Stokes (VANS) equations [S. Whitaker, "The Forchheimer equation: a theoretical development," Transp. Porous Media 25, 27 (1996)] for the volume-averaged flow field. This method has the advantage that it requires less computational power than the direct simulation of the flow through the grid of cubes. More in general, for complex porous media one is usually forced to use the VANS equations, because a direct simulation would not be possible with present-day computer facilities. A disadvantage of the continuum approach is that in order to solve the VANS equations, closures are needed for the drag force and the subfilter-scale stress. For porous media, the latter can often be neglected. In the present work, a relation for the drag force is adopted based on the Irmay ["Modles thoriques d'coulement dans les corps poreux," Bulletin Rilem 29, 37 (1965)] and the Burke-Plummer model [R. B. Bird, W. E. Stewart, and E. N. Lightfoot, Transport Phenomena (Wiley, New York, 2002)], with the model coefficients determined from simulations reported by W. P. Breugem, B. J. Boersma, and R. E. Uittenbogaard ["Direct numerical simulation of plane channel flow over a 3D Cartesian grid of cubes," Proceedings of the Second International Conference on Applications of Porous Media, edited by A. H. Reis and A. F. Miguel (vora Geophysics Center, vora, 2004), p. 27]. The results of the DNS with the grid of cubes and the second DNS in which the continuum approach is used, agree very well.
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.
Bansal, Gaurav; Mascarenhas, Ajith; Chen, Jacqueline H.
2014-10-01
In our paper, two- and three-dimensional direct numerical simulations (DNS) of autoignition phenomena in stratified dimethyl-ether (DME)/air turbulent mixtures are performed. A reduced DME oxidation mechanism, which was obtained using rigorous mathematical reduction and stiffness removal procedure from a detailed DME mechanism with 55 species, is used in the present DNS. The reduced DME mechanism consists of 30 chemical species. This study investigates the fundamental aspects of turbulence-mixing-autoignition interaction occurring in homogeneous charge compression ignition (HCCI) engine environments. A homogeneous isotropic turbulence spectrum is used to initialize the velocity field in the domain. Moreover, the computational configuration corresponds to amore » constant volume combustion vessel with inert mass source terms added to the governing equations to mimic the pressure rise due to piston motion, as present in practical engines. DME autoignition is found to be a complex three-staged process; each stage corresponds to a distinct chemical kinetic pathway. The distinct role of turbulence and reaction in generating scalar gradients and hence promoting molecular transport processes are investigated. Then, by applying numerical diagnostic techniques, the different heat release modes present in the igniting mixture are identified. In particular, the contribution of homogeneous autoignition, spontaneous ignition front propagation, and premixed deflagration towards the total heat release are quantified.« less
Bansal, Gaurav; Mascarenhas, Ajith; Chen, Jacqueline H.
2014-10-01
In our paper, two- and three-dimensional direct numerical simulations (DNS) of autoignition phenomena in stratified dimethyl-ether (DME)/air turbulent mixtures are performed. A reduced DME oxidation mechanism, which was obtained using rigorous mathematical reduction and stiffness removal procedure from a detailed DME mechanism with 55 species, is used in the present DNS. The reduced DME mechanism consists of 30 chemical species. This study investigates the fundamental aspects of turbulence-mixing-autoignition interaction occurring in homogeneous charge compression ignition (HCCI) engine environments. A homogeneous isotropic turbulence spectrum is used to initialize the velocity field in the domain. Moreover, the computational configuration corresponds to a constant volume combustion vessel with inert mass source terms added to the governing equations to mimic the pressure rise due to piston motion, as present in practical engines. DME autoignition is found to be a complex three-staged process; each stage corresponds to a distinct chemical kinetic pathway. The distinct role of turbulence and reaction in generating scalar gradients and hence promoting molecular transport processes are investigated. Then, by applying numerical diagnostic techniques, the different heat release modes present in the igniting mixture are identified. In particular, the contribution of homogeneous autoignition, spontaneous ignition front propagation, and premixed deflagration towards the total heat release are quantified.
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 wind speed simulation model
Ramsdell, J.V.; Athey, G.F.; Ballinger, M.Y.
1981-09-01
A relatively simple stochastic model for simulating wind speed time series that can be used as an alternative to time series from representative locations is described in this report. The model incorporates systematic seasonal variation of the mean wind, its standard deviation, and the correlation speeds. It also incorporates systematic diurnal variation of the mean speed and standard deviation. To demonstrate the model capabilities, simulations were made using model parameters derived from data collected at the Hanford Meteorology Station, and results of analysis of simulated and actual data were compared.
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...
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.
NASA Astrophysics Data System (ADS)
Ling, Y.; Haselbacher, A.; Balachandar, S.; Najjar, F. M.; Stewart, D. S.
2013-01-01
The interaction of shock waves with deformable particles is an important fundamental problem. In some applications, e.g., the detonation of explosives loaded with metal particles, the pressure behind the shock wave can be significantly larger than the yield strength of the particle material. This means that particles can deform severely during their interaction with the shock wave. The experimental and theoretical studies of shock interaction with deformable particles (SIDP) are extremely challenging because of its highly transient nature. As a result, no accurate model exists yet that can be used in simulations. The objective of this paper is to develop a simple point-particle model that accurately captures the unsteady force and heat-transfer in SIDP. In the development of this model, we build on earlier models by Ling et al. (Int. J. Multiphase Flow 37, 1026-1044 (2011)) for the unsteady force and heat-transfer contributions for rigid particles. Insights gained from direct numerical simulations (DNS) guide the extension of these models to deforming particles. Results obtained with the extended model for the interaction of a deforming particle with a shock wave and a Chapman-Jouguet detonation wave compare well with DNS results and therefore offer significant improvements over standard models.
NASA Astrophysics Data System (ADS)
Druzhinin, O.; Troitskaya, Yu; Zilitinkevich, S.
2016-02-01
The processes of turbulent mixing and momentum and heat exchange occur in the upper ocean at depths up to several dozens of meters and in the atmospheric boundary layer within interval of millimeters to dozens of meters and can not be resolved by known large- scale climate models. Thus small-scale processes need to be parameterized with respect to large scale fields. This parameterization involves the so-called bulk coefficients which relate turbulent fluxes with large-scale fields gradients. The bulk coefficients are dependent on the properties of the small-scale mixing processes which are affected by the upper-ocean stratification and characteristics of surface and internal waves. These dependencies are not well understood at present and need to be clarified. We employ Direct Numerical Simulation (DNS) as a research tool which resolves all relevant flow scales and does not require closure assumptions typical of Large-Eddy and Reynolds Averaged Navier-Stokes simulations (LES and RANS). Thus DNS provides a solid ground for correct parameterization of small-scale mixing processes and also can be used for improving LES and RANS closure models. In particular, we discuss the problems of the interaction between small-scale turbulence and internal gravity waves propagating in the pycnocline in the upper ocean as well as the impact of surface waves on the properties of atmospheric boundary layer over wavy water surface.
Numerical simulation of heat exchanger
Sha, W.T.
1985-01-01
Accurate and detailed knowledge of the fluid flow field and thermal distribution inside a heat exchanger becomes invaluable as a large, efficient, and reliable unit is sought. This information is needed to provide proper evaluation of the thermal and structural performance characteristics of a heat exchanger. It is to be noted that an analytical prediction method, when properly validated, will greatly reduce the need for model testing, facilitate interpolating and extrapolating test data, aid in optimizing heat-exchanger design and performance, and provide scaling capability. Thus tremendous savings of cost and time are realized. With the advent of large digital computers and advances in the development of computational fluid mechanics, it has become possible to predict analytically, through numerical solution, the conservation equations of mass, momentum, and energy for both the shellside and tubeside fluids. The numerical modeling technique will be a valuable, cost-effective design tool for development of advanced heat exchangers.
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.
Coincidental match of numerical simulation and physics
NASA Astrophysics Data System (ADS)
Pierre, B.; Gudmundsson, J. S.
2010-08-01
Consequences of rapid pressure transients in pipelines range from increased fatigue to leakages and to complete ruptures of pipeline. Therefore, accurate predictions of rapid pressure transients in pipelines using numerical simulations are critical. State of the art modelling of pressure transient in general, and water hammer in particular include unsteady friction in addition to the steady frictional pressure drop, and numerical simulations rely on the method of characteristics. Comparison of rapid pressure transient calculations by the method of characteristics and a selected high resolution finite volume method highlights issues related to modelling of pressure waves and illustrates that matches between numerical simulations and physics are purely coincidental.
Numerical Simulation For Supersonic Inlets
NASA Technical Reports Server (NTRS)
Varner, M. O.; Martindale, W. R.; Phares, W. J.; Kneile, K. R.; Adams, J. C., Jr.
1987-01-01
Flows calculated for realistic engine-inlet conditions. Computer code LAPIN, large-perturbation inlet, developed to analyze large-perturbation, transient-flow fields in supersonic inlets. Robust, quick-running code capable of solving unsteady quasi-one-dimensional, inviscid-flow problems in mixed subsonic and supersonic regimes for inlets. Approach based upon quasi-one-dimensional, inviscid, unsteady formulation including engineering models of unstart/restart, bleed, bypass, and geometrical effects. Numerical solution of governing time-dependent equations of motion accomplished through shock-capturing, finite-difference algorithm. Program written in FORTRAN IV.
The Asteroid Shape: Numerical Simulation
NASA Astrophysics Data System (ADS)
Mohamed, R. A.; Lupishko, D. F.; Shevchenko, V. G.
1996-01-01
The numerical model of an asteroid, which shape is assumed to be a triaxial ellipsoid, is applied to real asteroids. Eleven asteroids (15, 20, 22, 23, 28, 31, 41, 43, 130, 354, and 532) are selected with reliably determined pole coordinates and axes ratios of the approximating ellipsoid, which may be used as test objects for the evaluation of new methods of asteroid pole coordinate determination. On the other hand, the shapes of asteroids 2, 3, 7, 29, 39, 44, 87, 216, 349, and 624 do not fit the triaxial ellipsoids with the available axes ratios. Their pole coordinates need to be redefined by the methods free from the asteroid shape influence.
DNS and LES of a Shear-Free Mixing Layer
NASA Technical Reports Server (NTRS)
Knaepen, B.; Debliquy, O.; Carati, D.
2003-01-01
The purpose of this work is twofold. First, given the computational resources available today, it is possible to reach, using DNS, higher Reynolds numbers than in Briggs et al.. In the present study, the microscale Reynolds numbers reached in the low- and high-energy homogeneous regions are, respectively, 32 and 69. The results reported earlier can thus be complemented and their robustness in the presence of increased turbulence studied. The second aim of this work is to perform a detailed and documented LES of the shear-free mixing layer. In that respect, the creation of a DNS database at higher Reynolds number is necessary in order to make meaningful LES assessments. From the point of view of LES, the shear-free mixing-layer is interesting since it allows one to test how traditional LES models perform in the presence of an inhomogeneity without having to deal with difficult numerical issues. Indeed, as argued in Briggs et al., it is possible to use a spectral code to study the shear-free mixing layer and one can thus focus on the accuracy of the modelling while avoiding contamination of the results by commutation errors etc. This paper is organized as follows. First we detail the initialization procedure used in the simulation. Since the flow is not statistically stationary, this initialization procedure has a fairly strong influence on the evolution. Although we will focus here on the shear-free mixing layer, the method proposed in the present work can easily be used for other flows with one inhomogeneous direction. The next section of the article is devoted to the description of the DNS. All the relevant parameters are listed and comparison with the Veeravalli & Warhaft experiment is performed. The section on the LES of the shear-free mixing layer follows. A detailed comparison between the filtered DNS data and the LES predictions is presented. It is shown that simple eddy viscosity models perform very well for the present test case, most probably because the flow seems to be almost isotropic in the small-scale range that is not resolved by the LES.
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.
Eruption Morphologies from Numerical Simulations
NASA Astrophysics Data System (ADS)
Gisler, Galen
2013-04-01
Eruptive processes in nature produce a wide variety of morphologies, including cone sheets, dykes, sills, and pipes. The choice of a particular eruptive style is determined partly by local inhomogeneities, and partly by the gross overall properties of the country rock and the physical properties of the eruptive fluid. We have performed two-dimensional simulations designed to capture a range of morphologies in an eruptive system, using the finite-volume code Sage, originally developed at Science Applications International. In these simulations, we supply a mixture of basaltic magma, supercritical water, and carbon dioxide at a given pressure and zero velocity into a 2-km deep fill of basaltic country rock. We vary the supply pressure and the material properties of the country rock in a parameter study. All simulation runs are followed until the volatile-rich mixture breaks out at the surface. Pipes are produced at high pressures with softer backgrounds, cone sheets at lower pressures and stiffer backgrounds, while sills are produced in intermediate regimes.
Numerical simulations of thermographic responses in composites
NASA Astrophysics Data System (ADS)
Winfree, William P.; Cramer, K. Elliott; Zalameda, Joseph N.; Howell, Patricia A.
2016-02-01
Numerical simulations of thermographic responses in composite materials have been useful for evaluating and optimizing thermographic analysis techniques. Numerical solutions are particularly beneficial for thermographic techniques, since the fabrication of specimens with realistic flaws is difficult. A quadrupole method for performing the simulations in two dimensions is presented. The results are compared to a finite element simulation of the same geometry. The technique is shown to be in good agreement with a finite element simulation of the same geometry, however, it requires about one hundredth of the computational time.
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.
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.
Numerical Simulations of Cavitating Flows in Venturi
NASA Astrophysics Data System (ADS)
Goncalvès, Eric; Champagnac, Maxime; Patella, Regiane Fortes
2008-09-01
Different computational fluid dynamics (CFD) strategies were developed to analyze and to better understand the cavitation phenomenon. Based on homogeneous models, two numerical approaches were explored to capture large density variations and unsteady behaviors of cavitating flows. Several simulations were performed in a two-dimensional Venturi geometry. Local and global analyses were proposed based on comparisons between experimental and numerical results.
DNS of helicity-induced stratified turbulent flow
NASA Astrophysics Data System (ADS)
Chandy, Abhilash J.; Rahimi, Abbas
2013-11-01
Helical flows undergoing density stratification have wide applications in meteorological phenomena such as dust devils, tornadoes, and hurricanes due to the complexity and disasters caused by them. Direct numerical simulations (DNS) of transition to turbulence in a stably stratified Boussinesq fluid are presented for different rotation and stratification intensities. In order to understand the effect of velocity on the energy cascade, comparisons are made between helicity initiated and non-helical flows. Results show that stratification decelerates the helicity decay and causes velocity and vorticity to align with each other. With respect to the helical and non-helical flow comparisons, the total energy in the presence of stratification decays faster with helicity. In addition, the behavior of length scales were examined by comparing temporal variations of the vertical shearing of velocities. Results showed a growing asymmetry with time in the case of helical flow, while non-helical flow stayed close to begin symmetric.
Bisetti, Fabrizio; Attili, Antonio; Pitsch, Heinz
2014-08-13
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
Direct Numerical Simulation of Turbulent Pipe Flows subjected to Transverse Oscillations
NASA Astrophysics Data System (ADS)
Czajkowski, Mark; Desjardins, Olivier
2009-11-01
Fundamental effects of transverse oscillations on a turbulent pipe are being studied using direct numerical simulations (DNS). Previous studes of pipes subjected to oscillations around their central axis have shown a reduction in pressure drop. This study is a generalization of previous work to the case of an oscillation around an axis parallel to but not coinciding with the pipe centerline.The role of oscillation frequency, amplitude, and radius on the statistics of turbulent pipe flows, as well as bulk properties like pressure drop are investigated. Two key non-dimensional numbers are identified. The first relates the oscillation amplitude to a turbulent length scale (Taylor micro-scale); and the second number, a Strouhal number, compares the oscillation frequency to a turbulent time scale. These non-dimensional numbers, along with oscillation radius, are varied over a range of values, and DNS were performed using the arbitrarily high order accurate code NGA [Desjardins, et al., JCP 2008]. The resulting turbulent flows were analysed using a variety of turbulent statistics and compared to stationary and pipes rotating around their central axis.
Comparison of direct numerical simulation databases of turbulent channel flow at Re? = 180
NASA Astrophysics Data System (ADS)
Vreman, A. W.; Kuerten, J. G. M.
2014-01-01
Direct numerical simulation (DNS) databases are compared to assess the accuracy and reproducibility of standard and non-standard turbulence statistics of incompressible plane channel flow at Re? = 180. Two fundamentally different DNS codes are shown to produce maximum relative deviations below 0.2% for the mean flow, below 1% for the root-mean-square velocity and pressure fluctuations, and below 2% for the three components of the turbulent dissipation. Relatively fine grids and long statistical averaging times are required. An analysis of dissipation spectra demonstrates that the enhanced resolution is necessary for an accurate representation of the smallest physical scales in the turbulent dissipation. The results are related to the physics of turbulent channel flow in several ways. First, the reproducibility supports the hitherto unproven theoretical hypothesis that the statistically stationary state of turbulent channel flow is unique. Second, the peaks of dissipation spectra provide information on length scales of the small-scale turbulence. Third, the computed means and fluctuations of the convective, pressure, and viscous terms in the momentum equation show the importance of the different forces in the momentum equation relative to each other. The Galilean transformation that leads to minimum peak fluctuation of the convective term is determined. Fourth, an analysis of higher-order statistics is performed. The skewness of the longitudinal derivative of the streamwise velocity is stronger than expected (-1.5 at y+ = 30). This skewness and also the strong near-wall intermittency of the normal velocity are related to coherent structures.
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.
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
Direct numerical simulation of a turbulent stably stratified air flow above a wavy water surface
NASA Astrophysics Data System (ADS)
Druzhinin, O. A.; Troitskaya, Yu. I.; Zilitinkevich, S. S.
2016-01-01
The influence of the roughness of the underlaying water surface on turbulence is studied in a stably stratified boundary layer (SSBL). Direct numerical simulation (DNS) is conducted at various Reynolds (Re) and Richardson (Ri) numbers and the wave steepness ka. It is shown that, at constant Re, the stationary turbulent regime is set in at Ri below the threshold value Ri c depending on Re. At Ri > Ri c , in the absence of turbulent fluctuations near the wave water surface, three-dimensional quasiperiodical structures are identified and their threshold of origin depends on the steepness of the surface wave on the water surface. This regime is called a wave pumping regime. The formation of three-dimensional structures is explained by the development of parametric instability of the disturbances induced by the surface water in the air flow. The DNS results are quite consistent with prediction of the theoretical model of the SSBL flow, in which solutions for the disturbances of the fields of velocity and temperature in the wave pumping regime are found to be a solution of a two-dimensional linearized system with the heterogeneous boundary condition, which is caused by the presence of the surface wave. In addition to the turbulent fluctuations, the three-dimensional structures in the wave pumping regime provide for the transfer of impulse and heat, i.e., the increase in the roughness of the water-air boundary caused by the presence of waves intensifies the exchange in the SSBL.
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.
NUMERICAL SIMULATIONS OF SPICULE ACCELERATION
Guerreiro, N.; Carlsson, M.; Hansteen, V. E-mail: mats.carlsson@astro.uio.no
2013-04-01
Observations in the H{alpha} line of hydrogen and the H and K lines of singly ionized calcium on the solar limb reveal the existence of structures with jet-like behavior, usually designated as spicules. The driving mechanism for such structures remains poorly understood. Sterling et al. shed some light on the problem mimicking reconnection events in the chromosphere with a one-dimensional code by injecting energy with different spatial and temporal distributions and tracing the thermodynamic evolution of the upper chromospheric plasma. They found three different classes of jets resulting from these injections. We follow their approach but improve the physical description by including non-LTE cooling in strong spectral lines and non-equilibrium hydrogen ionization. Increased cooling and conversion of injected energy into hydrogen ionization energy instead of thermal energy both lead to weaker jets and smaller final extent of the spicules compared with Sterling et al. In our simulations we find different behavior depending on the timescale for hydrogen ionization/recombination. Radiation-driven ionization fronts also form.
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.
Numerical Simulations of Spicule Acceleration
NASA Astrophysics Data System (ADS)
Guerreiro, N.; Carlsson, M.; Hansteen, V.
2013-04-01
Observations in the H? line of hydrogen and the H and K lines of singly ionized calcium on the solar limb reveal the existence of structures with jet-like behavior, usually designated as spicules. The driving mechanism for such structures remains poorly understood. Sterling et al. shed some light on the problem mimicking reconnection events in the chromosphere with a one-dimensional code by injecting energy with different spatial and temporal distributions and tracing the thermodynamic evolution of the upper chromospheric plasma. They found three different classes of jets resulting from these injections. We follow their approach but improve the physical description by including non-LTE cooling in strong spectral lines and non-equilibrium hydrogen ionization. Increased cooling and conversion of injected energy into hydrogen ionization energy instead of thermal energy both lead to weaker jets and smaller final extent of the spicules compared with Sterling et al. In our simulations we find different behavior depending on the timescale for hydrogen ionization/recombination. Radiation-driven ionization fronts also form.
Numerical Simulation of Turbulence Control with Surface-Mounted Actuators
NASA Astrophysics Data System (ADS)
Mohd-Yusof, J.; Koumoutsakos, P.
1998-11-01
A numerical scheme is developed which provides a framework to study a variety of actuator configurations without adaption of the underlying mesh, using body forces to enforce the wall boundary conditions. The forcing scheme does not impose time step restrictions on the underlying flow solver, in this case a Fourier/B-spline spectral method. The actuator configuration is coupled to a machine-learning algorithm to optimize the actuator geometry and feedback parameters to maximize drag reduction. The method is tested in DNS of a turbulent channel flow with a time-dependent bumpy wall. Preliminary results indicate that significant drag modification can be obtained.
Yudov, Yury V.
2006-07-01
The direct numerical simulation, extended to boundary - fitted coordinate, has been carried out for a fully-developed turbulent flow thermal hydraulics in a triangular rod bundle. The rod bundle is premised to be an infinite array. The spacer grid effects are ignored. The purpose of this work is to verify DNS methodology to be applied for deriving coefficients for inter-subchannel turbulent mixing and heat transfer on a rod. These coefficients are incorporated in subchannel analysis codes. To demonstrate the validity of this methodology, numerical calculation was performed for the bundle with the pitch to diameter ratio 1.2, at friction Reynolds number of 600 and Prandtl number of 1. The results for the hydraulic parameters are compared with published DNS data, and the results for the heat exchange coefficients -- with those obtained using semi-empirical correlations. (authors)
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.
DNS of Flow in a Low-Pressure Turbine Cascade Using a Discontinuous-Galerkin Spectral-Element Method
NASA Technical Reports Server (NTRS)
Garai, Anirban; Diosady, Laslo Tibor; Murman, Scott; Madavan, Nateri
2015-01-01
A new computational capability under development for accurate and efficient high-fidelity direct numerical simulation (DNS) and large eddy simulation (LES) of turbomachinery is described. This capability is based on an entropy-stable Discontinuous-Galerkin spectral-element approach that extends to arbitrarily high orders of spatial and temporal accuracy and is implemented in a computationally efficient manner on a modern high performance computer architecture. A validation study using this method to perform DNS of flow in a low-pressure turbine airfoil cascade are presented. Preliminary results indicate that the method captures the main features of the flow. Discrepancies between the predicted results and the experiments are likely due to the effects of freestream turbulence not being included in the simulation and will be addressed in the final paper.
NASA Astrophysics Data System (ADS)
Koo, Heeseok; Raman, Venkat; Varghese, Philip L.
2013-11-01
Thermochemical nonequilibrium could be significant in scramjet engines due to intense shock-based compression in the pre-combustion isolator region. In particular, vibrational nonequilibrium could adversely affect ignition time and mixing efficiency. To understand the role of nonequilibrium in such flows, direct numerical simulation (DNS) of supersonic flows with vibrational excitation are studied. A linear time-scale model is used to describe the vibrational relaxation of excited species. Essentially, nonequilibrium alters the flow by changing the physical properties that are related to the translational temperature. Such changes introduce nonlinear effect on the scalar mixing process. Further, the redistribution of energy amongst the internal states affects chemical rates. An analysis of the impact of nonequilibrium on combustion is provided.
Direct Numerical Simulation of 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.
Numerical simulations of cryogenic cavitating flows
NASA Astrophysics Data System (ADS)
Kim, Hyunji; Kim, Hyeongjun; Min, Daeho; Kim, Chongam
2015-12-01
The present study deals with a numerical method for cryogenic cavitating flows. Recently, we have developed an accurate and efficient baseline numerical scheme for all-speed water-gas two-phase flows. By extending such progress, we modify the numerical dissipations to be properly scaled so that it does not show any deficiencies in low Mach number regions. For dealing with cryogenic two-phase flows, previous EOS-dependent shock discontinuity sensing term is replaced with a newly designed EOS-free one. To validate the proposed numerical method, cryogenic cavitating flows around hydrofoil are computed and the pressure and temperature depression effect in cryogenic cavitation are demonstrated. Compared with Hord's experimental data, computed results are turned out to be satisfactory. Afterwards, numerical simulations of flow around KARI turbopump inducer in liquid rocket are carried out under various flow conditions with water and cryogenic fluids, and the difference in inducer flow physics depending on the working fluids are examined.
Direct numerical simulation of a turbulent reactive plume on a parallel computer
Cook, A.W.; Riley, J.J.
1996-12-01
A computational algorithm is described for direct numerical simulation (DNS) of a reactive plume in spatially evolving grid turbulence. The algorithm uses sixth-order compact differencing in conjunction with a fifth-order compact boundary scheme which has been developed and found to be stable. A compact filtering method is discussed as a means of stabilizing calculations where the viscous/diffusive terms are differenced in their conservative form. This approach serves as an alternative to nonconservative differencing, previously advocated as a means of damping the 2{delta} waves. In numerically solving the low Mach number equations the time derivative of the density field in the pressure Poisson equation was found to be the most destabilizing part of the calculation. Even-ordered finite difference approximations to this derivative were found to be more stable than odd-ordered approximations. Turbulence at the inlet boundary is generated by scanning through an existing three-dimensional field of fully developed turbulence. In scanning through the inlet field, it was found that a high order interpolation is necessary in order to provide continuous velocity derivatives. Regarding pressure, a Neumann inlet condition combined with a Dirichlet outlet condition was found to work well. The chemistry follows the single-step, irreversible, global reaction: Fuel + (r) Oxidizer {yields} (1 + r)Product + Heat, with parameters chosen to match experimental data as far as allowed by resolution constraints. Simulation results are presented for four different cases in order to examine the effects of heat release, Damkoehler number, and Arrhenius kinetics on the flow physics. Statistical data from the DNS are compared to theory and wind tunnel data and found in reasonable agreement with regard to growth of turbulent length scales, decay of turbulent kinetic energy, decay of centerline scalar concentration, decrease in scalar rms, and spread of plume profile.
Contribution to Numerical Simulation of Laser Welding
NASA Astrophysics Data System (ADS)
Tur?a, Milan; Taraba, Bohumil; Ambro, Petr; Sahul, Miroslav
Contribution deals with numerical simulation of thermal and stress fields in welding tubes made of austenitic stainless CrNi steel type AISI 304 with a pulsed Nd:YAG laser. Process simulation was realised by use of ANSYS 10 software. Experiments were aimed at solution of asymptotic, standard and the so-called shell model. Thermally dependent properties of AISI 304 steel were considered. Thermal fields developed in the course of welding process and also shape of weld pool were assessed. Contribution is aimed at simulation of technological welding process with input parameters regarding the thermal and strain task and comparison of attained results with real experiment. The achieved results of numerical simulation were almost identical with a real weldment thermally affected by welding process.
Numerical Simulations of Precession Driven Flow
NASA Astrophysics Data System (ADS)
Lorenzani, S.; Tilgner, A.
We present numerical simulations of precession driven flow at parameters at which the flow is unstable. The instabilities can be classified into viscous and inertial insta- bilities. It will be shown that a viscous instability may start at the boundaries or in the bulk of the fluid. The simulations also reveal resonant collapses of the excited modes. The possible implications for the geodynamo will be discussed.
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.
NASA Astrophysics Data System (ADS)
Vukoslavčević, 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.
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.
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 plasma opening switches
Mason, R.J.; Jones, M.E.; Bergman, C.D.
1989-01-01
Plasma Opening Switches have been examined numerically with the aid of the ANTHEM plasma simulation model. A generic bi-cylindrical switch is studied. The switching of generator pulses ranging from 50 ns to 1 ..mu..sec is reviewed, for a variety of plasma fill lengths and densities, and for a range of resistive loads. 7 refs., 9 figs.
A numerical simulation of galaxy subcluster mergers
NASA Technical Reports Server (NTRS)
Roettiger, Kurt; Burns, Jack O.; Loken, Chris
1993-01-01
We present preliminary results of a 3-D numerical simulation of two merging subclusters of galaxies. By self-consistently modelling the intracluster gas and dark matter dynamics, we hope to gain insight as to how the dynamics of both relate to such observables as the cluster x-ray emission, radio source morphology, and velocity dispersions.
Direct numerical simulations and modeling of a spatially-evolving turbulent wake
NASA Astrophysics Data System (ADS)
Cimbala, John M.
1994-12-01
Understanding of turbulent free shear flows (wakes, jets, and mixing layers) is important, not only for scientific interest, but also because of their appearance in numerous practical applications. Turbulent wakes, in particular, have recently received increased attention by researchers at NASA Langley. The turbulent wake generated by a two-dimensional airfoil has been selected as the test-case for detailed high-resolution particle image velocimetry (PIV) experiments. This same wake has also been chosen to enhance NASA's turbulence modeling efforts. Over the past year, the author has completed several wake computations, while visiting NASA through the 1993 and 1994 ASEE summer programs, and also while on sabbatical leave during the 1993-94 academic year. These calculations have included two-equation (K-omega and K-epsilon) models, algebraic stress models (ASM), full Reynolds stress closure models, and direct numerical simulations (DNS). Recently, there has been mutually beneficial collaboration of the experimental and computational efforts. In fact, these projects have been chosen for joint presentation at the NASA Turbulence Peer Review, scheduled for September 1994. DNS calculations are presently underway for a turbulent wake at Re(sub theta) = 1000 and at a Mach number of 0.20. (Theta is the momentum thickness, which remains constant in the wake of a two dimensional body.) These calculations utilize a compressible DNS code written by M. M. Rai of NASA Ames, and modified for the wake by J. Cimbala. The code employs fifth-order accurate upwind-biased finite differencing for the convective terms, fourth-order accurate central differencing for the viscous terms, and an iterative-implicit time-integration scheme. The computational domain for these calculations starts at x/theta = 10, and extends to x/theta = 610. Fully developed turbulent wake profiles, obtained from experimental data from several wake generators, are supplied at the computational inlet, along with appropriate noise. After some adjustment period, the flow downstream of the inlet develops into a fully three-dimensional turbulent wake. Of particular interest in the present study is the far wake spreading rate and the self-similar mean and turbulence profiles. At the time of this writing, grid resolution studies are underway, and a code is being written to calculate turbulence statistics from these wake calculations; the statistics will be compared to those from the ongoing PIV wake measurements, those of previous experiments, and those predicted by the various turbulence models. These calculations will lead to significant long-term benefits for the turbulence modeling effort. In particular, quantities such as the pressure-strain correlation and the dissipation rate tensor can be easily calculated from the DNS results, whereas these quantities are nearly impossible to measure experimentally. Improvements to existing turbulence models (and development of new models) require knowledge about flow quantities such as these.
Direct numerical simulation of turbulent, chemically reacting flows
NASA Astrophysics Data System (ADS)
Doom, Jeffrey Joseph
This dissertation: (i) develops a novel numerical method for DNS/LES of compressible, turbulent reacting flows, (ii) performs several validation simulations, (iii) studies auto-ignition of a hydrogen vortex ring in air and (iv) studies a hydrogen/air turbulent diffusion flame. The numerical method is spatially non-dissipative, implicit and applicable over a range of Mach numbers. The compressible Navier-Stokes equations are rescaled so that the zero Mach number equations are discretely recovered in the limit of zero Mach number. The dependent variables are co--located in space, and thermodynamic variables are staggered from velocity in time. The algorithm discretely conserves kinetic energy in the incompressible, inviscid, non--reacting limit. The chemical source terms are implicit in time to allow for stiff chemical mechanisms. The algorithm is readily applicable to complex chemical mechanisms. Good results are obtained for validation simulations. The algorithm is used to study auto-ignition in laminar vortex rings. A nine species, nineteen reaction mechanism for H2/air combustion proposed by Mueller et al. [37] is used. Diluted H 2 at ambient temperature (300 K) is injected into hot air. The simulations study the effect of fuel/air ratio, oxidizer temperature, Lewis number and stroke ratio (ratio of piston stroke length to diameter). Results show that auto--ignition occurs in fuel lean, high temperature regions with low scalar dissipation at a 'most reactive' mixture fraction, zeta MR (Mastorakos et al. [32]). Subsequent evolution of the flame is not predicted by zetaMR; a most reactive temperature TMR is defined and shown to predict both the initial auto-ignition as well as subsequent evolution. For stroke ratios less than the formation number, ignition in general occurs behind the vortex ring and propagates into the core. At higher oxidizer temperatures, ignition is almost instantaneous and occurs along the entire interface between fuel and oxidizer. For stroke ratios greater than the formation number, ignition initially occurs behind the leading vortex ring, then occurs along the length of the trailing column and propagates towards the ring. Lewis number is seen to affect both the initial ignition as well as subsequent flame evolution significantly. Non-uniform Lewis number simulations provide faster ignition and burnout time but a lower maximum temperature. The fuel rich reacting vortex ring provides the highest maximum temperature and the higher oxidizer temperature provides the fastest ignition time. The fuel lean reacting vortex ring has little effect on the flow and behaves similar to a non--reacting vortex ring. We then study auto-ignition of turbulent H2/air diffusion flames using the Mueller et al. [37] mechanism. Isotropic turbulence is superimposed on an unstrained diffusion flame where diluted H 2 at ambient temperature interacts with hot air. Both, unity and non-unity Lewis number are studied. The results are contrasted to the homogeneous mixture problem and laminar diffusion flames. Results show that auto-ignition occurs in fuel lean, low vorticity, high temperature regions with low scalar dissipation around a most reactive mixture fraction, zetaMR (Mastorakos et al. [32]). However, unlike the laminar flame where auto-ignition occurs at zetaMR, the turbulent flame auto-ignites over a very broad range of zeta around zetaMR, which cannot completely predict the onset of ignition. The simulations also study the effects of three-dimensionality. Past two--dimensional simulations (Mastorakos et al. [32]) show that when flame fronts collide, extinction occurs. However, our three dimensional results show that when flame fronts collide; they can either increase in intensity, combine without any appreciable change in intensity or extinguish. This behavior is due to the three--dimensionality of the flow.
NASA Technical Reports Server (NTRS)
Card, J. M.; Chen, J. H.; Day, M.; Mahalingam, S.
1994-01-01
Turbulent non-premixed stoichiometric methane-air flames modeled with reduced kinetics have been studied using the direct numerical simulation approach. The simulations include realistic chemical kinetics, and the molecular transport is modeled with constant Lewis numbers for individual species. The effect of turbulence on the internal flame structure and extinction characteristics of methane-air flames is evaluated. Consistent with earlier DNS with simple one-step chemistry, the flame is wrinkled and in some regions extinguished by the turbulence, while the turbulence is weakened in the vicinity of the flame due to a combination of dilatation and an increase in kinematic viscosity. Unlike previous results, reignition is observed in the present simulations. Lewis number effects are important in determining the local stoichiometry of the flame. The results presented in this work are preliminary but demonstrate the feasibility of incorporating reduced kinetics for the oxidation of methane with direct numerical simulations of homogeneous turbulence to evaluate the limitations of various levels of reduction in the kinetics and to address the formation of thermal and prompt NO(x).
NASA Astrophysics Data System (ADS)
Bake, S.; Meyer, D. G. W.; Rist, U.
2002-05-01
The mechanism of turbulence development in periodic Klebanoff transition in a boundary layer has been studied experimentally and in a direct numerical simulation (DNS) with controlled disturbance excitation. In order to compare the results quantitatively, the flow parameters were matched in both methods, thus providing complementary data with which the origin of turbulence in the transition process could be explained. Good agreement was found for the development of the amplitude and shape of typical disturbance structures, the [Lambda]-vortices, including the development of ring-like vortices and spikes in the time traces. The origin and the spatial development of random velocity perturbations were measured in the experiment, and are shown together with the evolution of local high-shear layers. Since the DNS is capable of providing the complete velocity and vorticity fields, further conclusions are drawn based on the numerical data. The mechanisms involved in the flow randomization process are presented in detail. It is shown how the random perturbations which initially develop at the spike-positions in the outer part of the boundary layer influence the flow randomization process close to the wall. As an additional effect, the interaction of vortical structures and high-shear layers of different disturbance periods was found to be responsible for accelerating the transition to a fully developed turbulent flow. These interactions lead to a rapid intensification of a high-shear layer very close to the wall that quickly breaks down because of the modulation it experiences through interactions with vortex structures from the outer part of the boundary layer. The final breakdown process will be shown to be dominated by locally appearing vortical structures and shear layers.
Numerical Simulation of Aircraft Trailing Vortices
NASA Technical Reports Server (NTRS)
Proctor, Fred H.; Switzer, George F.
2000-01-01
The increase in air traffic is currently outpacing the development of new airport runways. This is leading to greater air traffic congestion, resulting in costly delays and cancellations. The National Aeronautics and Space Administration (NASA) under its Terminal Area Productivity (TAP) program is investigating new technologies that will allow increased airport capacity while maintaining the present standards for safety. As an element of this program, the Aircraft Vortex Spacing System (AVOSS) is being demonstrated in July 2000, at Dallas Ft-Worth Airport. This system allows reduced aircraft separations, thus increasing the arrival and departure rates, while insuring that wake vortices from a leading aircraft do not endanger trailing aircraft. The system uses predictions or wake vortex position and strength based on input from the current weather state. This prediction is accomplished by a semi-empirical model developed from theory, field observations, and relationships derived from numerical wake vortex simulations. Numerical experiments with a Large Eddy Simulation (LES) model are being conducted in order to provide guidance for the enhancement of these prediction algorithms. The LES Simulations of wake vortices are carried out with NASA's Terminal Area Simulation System (TASS). Previous wake vortex investigations with TASS are described. The primary objective of these numerical studies has been to quantify vortex transport and decay in relation to atmospheric variables. This paper summarizes many of the previous investigations with the TASS model and presents some new results regarding the onset of wake vortex decay.
Numerical simulation of an electroweak oscillon
Graham, N.
2007-10-15
Numerical simulations of the bosonic sector of the SU(2)xU(1) electroweak standard model in 3+1 dimensions have demonstrated the existence of an oscillon--an extremely long-lived, localized, oscillatory solution to the equations of motion--when the Higgs mass is equal to twice the W{sup {+-}} boson mass. It contains total energy roughly 30 TeV localized in a region of radius 0.05 fm. A detailed description of these numerical results is presented.
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
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 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.
Numerical Simulation of Two-Phase Flow in Severely Damaged Core Geometries
Meekunnasombat, Phongsan; Fichot, Florian; Quintard, Michel
2006-07-01
In the event of a severe accident in a nuclear reactor, the oxidation, dissolution and collapse of fuel rods is likely to change dramatically the geometry of the core. A large part of the core would be damaged and would look like porous medium made of randomly distributed pellet fragments, broken claddings and relocated melts. Such a complex medium must be cooled in order to stop the accident progression. IRSN investigates the effectiveness of the water re-flooding mechanism in cooling this medium where complex two-phase flows are likely to exist. A macroscopic model for the prediction of the cooling sequence was developed for the ICARE/CATHARE code (IRSN mechanistic code for severe accidents). It still needs to be improved and assessed. It appears that a better understanding of the flow at the pore scale is necessary. As a result, a direct numerical simulation (DNS) code was developed to investigate the local features of a two-phase flow in complex geometries. In this paper, the Cahn-Hilliard model is used to simulate flows of two immiscible fluids in geometries representing a damaged core. These geometries are synthesized from experimental tomography images (PHEBUS-FP project) in order to study the effects of each degradation feature, such as displacement and fragmentation of the fuel rods and claddings, on the two-phase flow. For example, the presence of fragmented fuel claddings is likely to enhance the trapping of the residual phase (either steam or water) within the medium which leads to less flow fluctuations in the other phase. Such features are clearly shown by DNS calculations. From a series of calculations where the geometry of the porous medium is changed, conclusions are drawn for the impact of rods damage level on the characteristics of two-phase flow in the core. (authors)
Numerical simulations of iced airfoils and wings
NASA Astrophysics Data System (ADS)
Pan, Jianping
A numerical study was conducted to understand the effects of simulated ridge and leading-edge ice shapes on the aerodynamic performance of airfoils and wings. In the first part of this study, a range of Reynolds numbers and Mach numbers, as well as ice-shape sizes and ice-shape locations were examined for various airfoils with the Reynolds-Averaged Navier-Stokes approach. Comparisons between simulation results and experimental force data showed favorable comparison up to stall conditions. At and past stall condition, the aerodynamic forces were typically not predicted accurately for large upper-surface ice shapes. A lift-break (pseudo-stall) condition was then defined based on the lift curve slope change. The lift-break angles compared reasonably with experimental stall angles, and indicated that the critical ice-shape location tended to be near the location of minimum pressure and the location of the most adverse pressure gradient. With the aim of improving the predictive ability of the stall behavior for iced airfoils, simulations using the Detached Eddy Simulation (DES) approach were conducted in the second part of this numerical investigation. Three-dimensional DES computations were performed for a series of angles of attack around stall for the iced NACA 23012 and NLF 0414 airfoils. The simulations for both iced airfoils provided the maximum lift coefficients and stall behaviors qualitatively consistent with experiments.
Numerical 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.
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 and nasal air-conditioning
Keck, Tilman; Lindemann, Jrg
2011-01-01
Heating and humidification of the respiratory air are the main functions of the nasal airways in addition to cleansing and olfaction. Optimal nasal air conditioning is mandatory for an ideal pulmonary gas exchange in order to avoid desiccation and adhesion of the alveolar capillary bed. The complex three-dimensional anatomical structure of the nose makes it impossible to perform detailed in vivo studies on intranasal heating and humidification within the entire nasal airways applying various technical set-ups. The main problem of in vivo temperature and humidity measurements is a poor spatial and time resolution. Therefore, in vivo measurements are feasible only to a restricted extent, solely providing single temperature values as the complete nose is not entirely accessible. Therefore, data on the overall performance of the nose are only based on one single measurement within each nasal segment. In vivo measurements within the entire nose are not feasible. These serious technical issues concerning in vivo measurements led to a large number of numerical simulation projects in the last few years providing novel information about the complex functions of the nasal airways. In general, numerical simulations merely calculate predictions in a computational model, e.g. a realistic nose model, depending on the setting of the boundary conditions. Therefore, numerical simulations achieve only approximations of a possible real situation. The aim of this review is the synopsis of the technical expertise on the field of in vivo nasal air conditioning, the novel information of numerical simulations and the current state of knowledge on the influence of nasal and sinus surgery on nasal air conditioning. PMID:22073112
Numerical Simulations of Boundary-Driven Dynamos
NASA Astrophysics Data System (ADS)
White, K.; Brummell, N.; Glatzmaier, G. A.
2012-12-01
An important topic of physics research is how magnetic fields are generated and maintained in the many astrophysical bodies where they are ubiquitously observed. Of particular interest, are reversals of magnetic fields of planets and stars, especially those of the Earth and the Sun. In an attempt to provide intuition on this problem, numerous physical dynamo experiments have been performed in different configurations. Recently, a tremendous breakthrough was made in the Von Karman sodium (VKS) experiments in France when the most realistic laboratory fluid dynamo to date was produced by driving an unconstrained flow in a cylinder of liquid sodium (Monchaux et al, 2007, PRL). One of the curiosities of the VKS experiment however is the effect of the composition of the impellers that drive the flow. Steel blades failed to produce a dynamo, but soft iron impellers, which have much higher magnetic permeability, succeeded. The role of the magnetic properties of the boundaries in boundary-driven dynamos is therefore clearly of interest. Kinematic and laminar numerical dynamo simulations (Giesecke et al, 2010, PRL & Gissinger et al, 2008 EPL) have shed some light but turbulent, nonlinear simulations are necessary. Roberts, Glatzmaier & Clune 2010 created a simplified model of the VKS setup by using three-dimensional numerical simulations in a spherical geometry with differential zonal motions of the boundary replacing the driving impellers of the VKS experiment. We have extended these numerical simulations further towards a more complete understanding of such boundary-forced dynamos. In particular, we have examined the effect of the magnetic boundary conditions - changes in the wall thickness, the magnetic permeability, and the electrical conductivity - on the mechanisms responsible for dynamo generation. Enhanced permeability, conductivity and wall thickness all help dynamo action to different degrees. We are further extending our investigations to asymmetric forcing to examine the possible existence of solutions incorporating field reversals. Asymmetry can quench dynamo action by destroying the complex correlations that are necessary to regenerate axisymmetric poloidal field.
Numerical simulation of binary liquid droplet collision
NASA Astrophysics Data System (ADS)
Pan, Yu; Suga, Kazuhiko
2005-08-01
A numerical investigation of binary droplet collision has been conducted. The complete process of the collision of two liquid droplets is dynamically simulated by solving the incompressible Navier-Stokes equations coupled with the convective equation of the level set function that captures the interface between the liquid and the gas phases. The simulations cover four major regimes of binary collision: bouncing, coalescence, reflexive separation, and stretching separation. For water droplets in air, the numerical results are compared with the experiments by and Ashgriz and Poo [J. Fluid Mech. 221, 183 (1990)] on collision consequences. For hydrocarbon (C14H30) droplets in nitrogen gas, the simulated results are compared in detail with the time-resolved photographic images of the collision processes obtained by Qian and Law [J. Fluid Mech. 331, 59 (1997)] in every collision regime. The present numerical results suggest that the mechanism of a bouncing collision is governed by the macroscopic dynamics. However, the fact that the present macroscopic numerical model is unable to capture the collision regime of coalescence after minor deformation supports the speculation that its mechanism is related to the microscopic dynamics. Furthermore, the transition from bouncing to coalescence collisions has been predicted and agrees well with the analytical model. The mechanism of satellite droplet formation for head-on collision and stretching separation collision is also studied based on the detailed time-resolved dynamic simulation results. It is then confirmed that end pinching is the main cause of satellite formation in head-on collisions whereas the capillary-wave instability becomes dominant in large impact parameter cases. In the case of an intermediate impact parameter, the effects of twisting and stretching due to the angular momentum and the inertia of the colliding droplets are significant for the satellite formation.
Numerical simulations on ion acoustic double layers
Sato, T.; Okuda, H.
1980-07-01
A comprehensive numerical study of ion acoustic double layers has been performed for both periodic as well as for nonperiodic systems by means of one-dimensional particle simulations. For a nonperiodic system, an external battery and a resistance are used to model the magnetospheric convection potential and the ionospheric Pedersen resistance. It is found that the number of double layers and the associated potential buildup across the system increases with the system length.
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.
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.
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.
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.
Chen, Jacqueline H.; Hawkes, Evatt R.; Sankaran, Ramanan; Mason, Scott D.; Im, Hong G.
2006-04-15
The influence of thermal stratification on autoignition at constant volume and high pressure is studied by direct numerical simulation (DNS) with detailed hydrogen/air chemistry with a view to providing better understanding and modeling of combustion processes in homogeneous charge compression-ignition engines. Numerical diagnostics are developed to analyze the mode of combustion and the dependence of overall ignition progress on initial mixture conditions. The roles of dissipation of heat and mass are divided conceptually into transport within ignition fronts and passive scalar dissipation, which modifies the statistics of the preignition temperature field. Transport within ignition fronts is analyzed by monitoring the propagation speed of ignition fronts using the displacement speed of a scalar that tracks the location of maximum heat release rate. The prevalence of deflagrative versus spontaneous ignition front propagation is found to depend on the local temperature gradient, and may be identified by the ratio of the instantaneous front speed to the laminar deflagration speed. The significance of passive scalar mixing is examined using a mixing timescale based on enthalpy fluctuations. Finally, the predictions of the multizone modeling strategy are compared with the DNS, and the results are explained using the diagnostics developed. (author)
Numerical simulation of precipitation induced downbursts
NASA Technical Reports Server (NTRS)
Proctor, F. H.
1985-01-01
Using the Terminal Area Simulation System (TASS), numerical simulations of downburst structure and sensitivity, based on vertical profiles of environmental temperature, humidity and wind velocity observed during the June and August 1982 JAWS project, are presented. Two-dimensional axisymmetric simulations examining downburst evolution, structure and sensitivity, assume a 40-m constant grid size on a 10-km diameter 5-km deep cylindrical domain. The three-dimensional experiment, examining the effects of vertical wind shear and other asymmetrical aspects of the downpour, assumed a 500-m constant horizontal grid size and a 35 x 35 x 18.5-km area. In the downburst primary structure depicted, outflow speeds were found to be sensitive to environmental temperature and humidity, as well as to precipitation radius and intensity. A vortex ring was found to propagate downwards, and maximum outflow winds occur when the vortex ring first reaches the surface.
Compact bone: numerical simulation of mechanical characteristics.
Crolet, J M; Aoubiza, B; Meunier, A
1993-06-01
One of the main difficulties encountered in the numerical simulation of the anisotropic elastic characteristics of compact bone is to account for the Haversian microstructure when determining the overall macroscopic behavior. Engineering analyses of such problems are usually based on 'homogenized approximations'. Compact bone is not exactly a composite material, but rather a heterogeneous medium which exhibits a multiscale composite structure. If the homogenized approximation is precise enough (and this is true for the mathematical theory of homogenization), it is then possible to simulate the macroscopic behavior from the microscopic mechanical characteristics. The present paper is devoted to such mathematical developments. Moreover, the 'inverse simulation' allows the computation of the microscopic stress fields in the haversian structure from the macroscopic stress fields, taking into account bone microstructure. PMID:8390470
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 Extreme European Windstorms
NASA Astrophysics Data System (ADS)
Mitas, C.
2010-12-01
Extreme synoptic storms present a significant economic risk in Europe mostly due to high wind gusts. This study focuses on understanding the most extreme storms that hit the European continent in terms of their dynamic and thermodynamic properties, utilizing high resolution numerical simulations of storms extracted from GCM runs. We have performed an ensemble of GCM simulations of over 2000 years, extracted tracks of storms produced by the GCM, downscaled these storms with WRF at a relatively high resolution (50km), and finally estimated their incurred economic loss using our proprietary catastrophe model. We choose a small set of the most extreme storms (in terms of economic loss) to analyze in more detail in the following way: 1. Identify common characteristics between extreme storms in terms of their evolution, vertical and horizontal structure, and the synoptic situation in which they are embedded. 2. Perform a series of numerical experiments with increasing WRF resolution and investigate changes in the storm intensity, size, and structure. The overall objective of this paper is to provide a clear understanding of the nature of extreme events and the numerical means needed to investigate them.
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.
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.
Numerical reproducibility for implicit Monte Carlo simulations
Cleveland, M.; Brunner, T.; Gentile, N.
2013-07-01
We describe and compare different approaches for achieving numerical reproducibility in photon Monte Carlo simulations. Reproducibility is desirable for code verification, testing, and debugging. Parallelism creates a unique problem for achieving reproducibility in Monte Carlo simulations because it changes the order in which values are summed. This is a numerical problem because double precision arithmetic is not associative. In [1], a way of eliminating this roundoff error using integer tallies was described. This approach successfully achieves reproducibility at the cost of lost accuracy by rounding double precision numbers to fewer significant digits. This integer approach, and other extended reproducibility techniques, are described and compared in this work. Increased precision alone is not enough to ensure reproducibility of photon Monte Carlo simulations. A non-arbitrary precision approaches required a varying degree of rounding to achieve reproducibility. For the problems investigated in this work double precision global accuracy was achievable by using 100 bits of precision or greater on all unordered sums which where subsequently rounded to double precision at the end of every time-step. (authors)
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).
Direct numerical simulations of the double scalar mixing layer. Part II: Reactive scalars
Mortensen, Mikael; de Bruyn Kops, Stephen M.; Cha, Chong M.
2007-06-15
The reacting double scalar mixing layer (RDSML) is investigated as a canonical multistream flow and a model problem for simple piloted diffusion flames. In piloted diffusion flames, the reacting fuel and oxidizer streams are initially separated by a central pilot stream at stoichiometric composition. The primary purpose of this pilot is to delay the mixing of the pure streams until a stable flame base can develop. In such multistream systems, the modeling of turbulent scalar mixing is complicated by the multiple feed streams, leading to more complex fine-scale statistics, which remain as yet an unmet modeling challenge compared to the simpler two-feed system. In Part I we described how multimodal mixture fraction probability density functions (PDFs) and conditional scalar dissipation rates can be modeled with a presumed mapping function approach. In this work we present an efficient and robust extension of the modeling to a general multistream reacting flow and compare predictions to three-dimensional direct numerical simulations (DNS) of the RDSML with a single-step reversible chemistry model and varying levels of extinction. With high extinction levels, the interaction with the pilot stream is described. Additionally, state-of-the-art combustion modeling calculations including conditional moment closure (CMC) and stationary laminar flamelet modeling (SLFM) are performed with the newly developed mixing model. Excellent agreement is found between the DNS and modeling predictions, even where the PDF is essentially a triple-delta shape near the flame base, so long as extinction levels are moderate to low. The suggested approach outlined in this paper is strictly valid only for flows that can be described by a single mixture fraction. For these flows the approach should provide engineers with fine-scale models that are of accuracy comparable to those already available for binary mixing, at only marginally higher complexity and cost. (author)
Late-Stage Transitional Boundary-Layer Structures. Direct Numerical Simulation and Experiment
NASA Astrophysics Data System (ADS)
Borodulin, V. I.; Gaponenko, V. R.; Kachanov, Y. S.; Meyer, D. G. W.; Rist, U.; Lian, Q. X.; Lee, C. B.
This paper is devoted to direct comparisons of related, detailed experimental and numerical studies of the non-linear, late stages of laminar-turbulenttransition in a boundary layer including flow breakdown and the beginning offlow randomization. Preceding non-linear stages of the transition process arealso well documented and compared with previous studies. The experiments wereconducted with the help of a hot-wire anemometer. The numerical study wascarried out by direct numerical simulation (DNS) of the flow employing theso-called spatial approach. Both the experiments and the DNS were performed atcontrolled disturbance conditions with an excitation of instability waves inthe flat-plate boundary layer. In the two cases, the primary disturbanceconsists of a time-harmonic, two-dimensional Tollmien--Schlichting wave thathas a very weak initial spanwise modulation. Despite somewhat differentinitial disturbance conditions used in the experiment and simulation, thesubsequent flow evolution at late non-linear stages is found to be practicallythe same. Detailed qualitative and quantitative comparisons of theinstantaneous velocity and vorticity fields are performed for twocharacteristic stages of the non-linear flow breakdown: (i) ``one-spike stage'' and (ii) ``three-spike stage.'' The twoapproaches clearly show in detail the process of development of the ?-structure, a periodical formation of ring-like vortices, the evolution of the surrounding flow field, and the beginning of flowrandomization. In particular, it is found experimentally and numerically thatthe ring-like vortices (associated with the well-known spikes) induce somerather intensive positive velocity fluctuations (positive spikes) in thenear-wall region which have the same scales as the ring-like vortices and propagate downstream with the same high (almost free-stream) speed. The positive spikes form a new high-shear layer in the near-wall region. In the experiment the induced near-wall perturbationshave a significant irregular low-frequency component. These non-periodicalmotions play an important role in the process of flow randomization and finaltransition to turbulence that starts under the ring-like vortices in thevicinity of the peak position.
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.
NASA Technical Reports Server (NTRS)
Hanebutte, Ulf R.; Joslin, Ronald D.; Zubair, Mohammad
1994-01-01
The implementation and the performance of a parallel spatial direct numerical simulation (PSDNS) code are reported for the IBM SP1 supercomputer. The spatially evolving disturbances that are associated with laminar-to-turbulent in three-dimensional boundary-layer flows are computed with the PS-DNS code. By remapping the distributed data structure during the course of the calculation, optimized serial library routines can be utilized that substantially increase the computational performance. Although the remapping incurs a high communication penalty, the parallel efficiency of the code remains above 40% for all performed calculations. By using appropriate compile options and optimized library routines, the serial code achieves 52-56 Mflops on a single node of the SP1 (45% of theoretical peak performance). The actual performance of the PSDNS code on the SP1 is evaluated with a 'real world' simulation that consists of 1.7 million grid points. One time step of this simulation is calculated on eight nodes of the SP1 in the same time as required by a Cray Y/MP for the same simulation. The scalability information provides estimated computational costs that match the actual costs relative to changes in the number of grid points.
Comprehensive Numerical Simulation of Laser Materials Processing
NASA Astrophysics Data System (ADS)
Gross, Markus
The previous chapter concluded the physical discussion of laser materials processing (LMP). In this chapter no new physical concepts or theory of physical phenomena will be introduced. The objective of this chapter is rather to give an overview of how to analyse the complex system that is LaserMaterials Processing. As has been shown in great detail, the physical level of complexity is deep and analysis becomes extremely cumbersome if it is pursued on an analytical level. Albeit giving great insight into the detailed phenomena, whole processes and their sensitivity to ambient conditions and changes in process parameters or physical setup cannot be investigated using these methods alone. Here numerical simulation comes into play for the scientist investigating processes from an engineering point of view. Numerical simulation has an almost endless scope for system complexity and is only limited by the resources available and the time the investigator is prepared to wait for results. This final chapter should be read as a guide to how to get started.Some fundamental principles of discrete numerical modelling will be introduced and reference made to work by other authors. This, in the space available, can by no means be a comprehensive review, or a textbook of all the methods available and required. Nevertheless it should be seen as a starting point for investigators, at the doctoral student level, trying to get to terms with the task ahead, or for the researcher trying to move from practice to theory, from experiment to simulation, looking for a guide on what to look out for, where to go and which pitfalls to avoid.
NASA Astrophysics Data System (ADS)
Wrz, W.; Sartorius, D.; Kloker, M.; Borodulin, V. I.; Kachanov, Y. S.; Smorodsky, B. V.
2012-09-01
Transition prediction in two-dimensional laminar boundary layers developing on airfoil sections at subsonic speeds and very low turbulence levels is still a challenge. The commonly used semi-empirical prediction tools are mainly based on linear stability theory and do not account for nonlinear effects present unavoidably starting with certain stages of transition. One reason is the lack of systematic investigations of the weakly nonlinear stages of transition, especially of the strongest interactions of the instability modes predominant in non-self-similar boundary layers. The present paper is devoted to the detailed experimental, numerical, and theoretical study of weakly nonlinear subharmonic resonances of Tollmien-Schlichting waves in an airfoil boundary layer, representing main candidates for the strongest mechanism of these initial nonlinear stages. The experimental approach is based on phase-locked hot-wire measurements under controlled disturbance conditions using a new disturbance source being capable to produce well-defined, complex wave compositions in a wide range of streamwise and spanwise wave numbers. The tests were performed in a low-turbulence wind tunnel at a chord Reynolds number of Re = 0.7 106. Direct numerical simulations (DNS) were utilized to provide a detailed comparison for the test cases. The results of weakly nonlinear theory (WNT) enabled a profound understanding of the underlying physical mechanisms observed in the experiments and DNS. The data obtained in experiment, DNS and WNT agree basically and provide a high degree of reliability of the results. Interactions occurring between components of various initial frequency-wavenumber spectra of instability waves are investigated by systematic variation of parameters. It is shown that frequency-detuned and spanwise-wavenumber-detuned subharmonic-type resonant interactions have an extremely large spectral width. Similar to results obtained for self-similar base flows it is found that the amplification factors in the frequency-detuned resonances can be even higher than in tuned cases, in spite of the strong base-flow non-self-similarity. An explanation of this unusual phenomenon is found based on the theoretical analysis and comparison of experimental, theoretical, and DNS data.
Numerical simulation of riblet controlled oblique transition
NASA Astrophysics Data System (ADS)
Klumpp, S.; Meinke, M.; Schrder, W.
To analyze the fundamental physical mechanism which determines the damping effect of a riblet surface on three-dimensional oblique transition numerical simulations of a spatial evolving zero-pressure gradient boundary layer above a clean and a riblet wall are performed. The laminar flow is excited by two oblique waves to force the oblique transition scenario. The occurring three-dimensional structures, i.e, ? and streamwisely aligned vortices are found to be damped and their breakdown to turbulence is damped by the riblets compared to a clean surface. The investigation of the near-wall flow structures reveals secondary flows induced by the riblets.
Numerical simulation of industrial superplastic forming
Haberman, K.S.; Bennett, J.G.; Miller, E.L.; Piltch, M.S.; Leyer, L.K.; Leodolter, W. |
1995-03-01
Superplastic forming is a metal forming process that allows a variety of components with very complex geometries to be produced at one tenth the cost of conventional machining. The industrial superplastic forming process can be optimized with the application of the finite element method to predict the optimal applied pressure history and the final part thickness distribution. This paper discusses the application the nonlinear implicit, three dimensional finite element code, NIKE3D to the problem of numerically simulating and optimizing the superplastic forming of Ti-6AI-4V components.
Numerical simulation of swept-wing flows
NASA Technical Reports Server (NTRS)
Reed, Helen L.
1991-01-01
Efforts of the last six months to computationally model the transition process characteristics of flow over swept wings are described. Specifically, the crossflow instability and crossflow/Tollmien-Schlichting wave interactions are analyzed through the numerical solution of the full 3D Navier-Stokes equations including unsteadiness, curvature, and sweep. This approach is chosen because of the complexity of the problem and because it appears that linear stability theory is insufficient to explain the discrepancies between different experiments and between theory and experiment. The leading edge region of a swept wing is considered in a 3D spatial simulation with random disturbances as the initial conditions.
Numerical simulations unravel the cosmic web.
Faucher-Gigure, Claude-Andr; 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
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
Direct Numerical Simulation of Turbulence and Mixing in Highly Compressible Flows
NASA Astrophysics Data System (ADS)
Tian, Yifeng; Jaberi, Farhad; Li, Zhaorui; Livescu, Daniel
2015-06-01
The effects of normal shock waves on isotropic turbulence and scalar mixing are studied by direct numerical simulation (DNS) of fully compressible equations with high-order monotonicity-preserving and compact finite-difference numerical schemes for various flow and scalar conditions. Detailed examinations of the turbulence and scalar statistics such as the turbulent kinetic energy and scalar variance indicate that the numerical method is accurate and is able to correctly capture the shock-turbulence interactions and scalar mixing near and away from the shock even at very high Mach numbers. As expected, the shock wave increases the small-scale turbulence and the skewness and flatness of the turbulent velocity fluctuations, but the turbulent compressibility is actually decreased by the shock. The effect of shock on the turbulence was to found be strongly dependent on the pre-shock turbulence parameters such as the turbulence intensity. The enhancement of scalar mixing by the shock is also found to be dependent on the pre-shock scalar structure. The mechanisms responsible for the modification of turbulence and scalar mixing are identified by analyzing the flow structure and the transport equations for the Reynolds stress, vorticity and scalar variance inside and outside the shock zone.
Numerical simulation of corotating interaction regions
NASA Technical Reports Server (NTRS)
Kota, J.; Jokipii, J. R.
1995-01-01
We report on a numerical simulation of Corotating Interaction Regions (CIRs) in the solar wind. We employ an MHD calculation with a tilted dipole model: slow wind is assumed around the tilted current sheet. As a simplifying assumption the solar wind is kept radial, meridional and azimuthal deflections are neglected. Despite of this simplification the model reproduces several observed features of CIR-s. CIR-s turn out most prominent at mid-latitudes. Toward increasing heliographic latitudes, reverse shocks become dominant, forward shocks tend to become weaker then disappear. The frozen in magnetic field is calculated and applied for the simulation of the transport of cosmic rays in the heliosphere. Some consequences on recurrent cosmic ray phenomena and on the acceleration of energetic particles will be discussed.
Numerical Simulations of Disc-Planet Interactions
NASA Astrophysics Data System (ADS)
Nelson, Richard P.; Paardekooper, Sijme-Jan
2011-02-01
The gravitational interaction between a protoplanetary disc and planetary sized bodies that form within it leads to the exchange of angular momentum, resulting in migration of the planets and possible gap formation in the disc for more massive planets. In this article, we review the basic theory of disc-planet interactions, and discuss the results of recent numerical simulations of planets embedded in protoplanetary discs. We consider the migration of low mass planets and recent developments in our understanding of so-called type I migration when a fuller treatment of the disc thermodynamics is included. We discuss the runaway migration of intermediate mass planets (so-called type III migration), and the migration of giant planets (type II migration) and the associated gap formation in this disc. The availability of high performance computing facilities has enabled global simulations of magnetised, turbulent discs to be computed, and we discuss recent results for both low and high mass planets embedded in such discs.
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 simulations of a turbulent axial vortex
NASA Astrophysics Data System (ADS)
Qin, Jim Hongxin
Although the vortex is present in most flows of engineering interest, the turbulent structure of the vortex is not well understood. Current prediction capabilities are especially weak for the vortex as well as other strongly rotating flows. The objective of this work is to aid the development of turbulence models for the vortex as well as strongly rotating flows in general by using direct numerical simulations of the vortex. The present study focuses on the turbulent axial vortex with and without an external strain field. The numerical simulations of a turbulent axial vortex without strain, i.e. an isolated vortex, have been performed by using a pseudo spectral method for compressible flow. The results qualitatively match well with the experimental data. The isolated vortex is stable unless the mean axial wake flow has sufficient magnitude. During the period of decay of disturbances, the mean tangential velocity profile exhibits anti-diffusion because a negative eddy viscosity develops near the center of the vortex. With the disturbance growth, the isolated vortex develops large-scale helical vortex structures, but they eventually disappear during the period of relaminarization. The details of turbulent statistics have been examined. The turbulent structure is related to the in stability of the isolated vortex. The budgets for the Reynolds stresses reveal that the production term is the primary source term, but the pressure strain, pressure transport, and turbulent transport terms also make a large contribution to the budgets for the Reynolds stresses.
Direct Numerical Simulations of Transient Dispersion
NASA Astrophysics Data System (ADS)
Porter, M.; Valdes-Parada, F.; Wood, B.
2008-12-01
Transient dispersion is important in many engineering applications, including transport in porous media. A common theoretical approach involves upscaling the micro-scale mass balance equations for convection- diffusion to macro-scale equations that contain effective medium quantities. However, there are a number of assumptions implicit in the various upscaling methods. For example, results obtained from volume averaging are often dependent on a given set of length and time scale constraints. Additionally, a number of the classical models for dispersion do not fully capture the early-time dispersive behavior of the solute for a general set of initial conditions. In this work, we present direct numerical simulations of micro-scale transient mass balance equations for convection-diffusion in both capillary tubes and porous media. Special attention is paid to analysis of the influence of a new time- decaying coefficient that filters the effects of the initial conditions. The direct numerical simulations were compared to results obtained from solving the closure problem associated with volume averaging. These comparisons provide a quantitative measure of the significance of (1) the assumptions implicit in the volume averaging method and (2) the importance of the early-time dispersive behavior of the solute due to various initial conditions.
Numerical simulation of swash zone fluid accelerations
NASA Astrophysics Data System (ADS)
Puleo, Jack A.; Farhadzadeh, Ali; Kobayashi, Nobuhisa
2007-07-01
A volume-of-fluid Navier-Stokes solver incorporating a k-ɛ closure model, NumErical Water FLUME (NEWFLUME) (Lin and Xu, 2005) is used to investigate the temporal and spatial structure of accelerations in the swash zone of steep beaches for surging, plunging, and nearly spilling waves. Simulations for idealized swash over planar slopes show that shoreward directed local accelerations exist only for up to 22% of the swash cycle depending on wave type. In general, shoreward directed local accelerations were predicted to occur only near the wave run down limit (Baldock and Holmes, 1997). Similarly, convective accelerations had the largest magnitudes during this time and tended to be either near zero or shoreward directed for the remainder of the swash cycle. This finding is in direct contrast to a modified ballistic theory for swash motion arising as a particular solution to the depth-averaged nonlinear shallow water equations (Peregrine and Williams, 2001) that predicts the convective acceleration maintains the sign of the fluid velocity. Near-bed pressure gradients are found to be poorly correlated to both the depth-averaged and near-bed local acceleration. These numerical findings indicate that local fluid accelerations are not a proxy for pressure gradients in the swash zone for enhanced sediment transporting mechanisms or parameterizations. However, the numerical results require corroboration with highly resolved swash zone data.
Evidence for hairpin packet structure in DNS channel flow
NASA Astrophysics Data System (ADS)
Liu, Zichao; Adrian, Ronold J.
1999-11-01
2-D PIV measurements of boundary layers and channel flows have revealed that wall turbulence is thickly populated with hairpin vortices that mostly appear in groups as packets. A DNS of a single hairpin vortex evolution into a packet has disclosed the packet generation mechanism. However, no DNS studies have revealed hairpin packets. The present DNS study demonstrates, for the first time, the existence of hairpin packets in turbulent flow. They are found to appear frequently in DNS of fully developed turbulent channel flows at low and moderate Reynolds numbers, Re = 150 and 300. The packet consisting of many asymmetric hairpins travel downstream with little dispersion. Due to the collective pumping effect of individual hairpins the packet generates a long low-momentum streak of about 1000 viscous length and creates a strong Q2 event and large Reynolds stress. This evidence substantiates the view that vortex packets are a universal feature of wall turbulence, independent of effects due to boundary layer trips or critical conditions in the aforementioned numerical studies.
Aspect ratio effects in turbulent duct flows studied with DNS
NASA Astrophysics Data System (ADS)
Vinuesa, R.; Noorani, A.; Lozano-Durn, A.; Schlatter, P.; Fischer, P.; Nagib, H.
2012-11-01
Three-dimensional effects present in turbulent duct flows, i.e., side-wall boundary layers and secondary motions, are studied by means of direct numerical simulations (DNS). The spectral element code Nek5000, developed by Fischer et. al. (2008), is used to compute turbulent duct flows with aspect ratios 1 and 3 in streamwise-periodic boxes of length 25 h (long enough to capture the longest streamwise structures). The total number of grid points is 28 and 62 million respectively, and the inflow conditions were adjusted iteratively in order to keep the same bulk Reynolds number at the centerplane (Reb , c = 2800) in both cases. Spanwise variations in wall shear, mean-flow profiles and turbulence statistics were analyzed with aspect ratio, and also compared with the 2D channel. The simulations were started from a laminar duct profile, and transition to turbulence was triggered by means of trip-forcing in the wall-normal direction, applied at the two horizontal walls. In addition, we developed a convergence criterion aimed at assessing the necessary averaging time TA for converged statistics. We find that econdary motions present in duct flows require longer averaging times and the total shear-stress profile is not necessarily linear.
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.
Numerical simulation of premixed turbulent methane combustion
Bell, John B.; Day, Marcus S.; Grcar, Joseph F.
2001-12-14
In this paper we study the behavior of a premixed turbulent methane flame in three dimensions using numerical simulation. The simulations are performed using an adaptive time-dependent low Mach number combustion algorithm based on a second-order projection formulation that conserves both species mass and total enthalpy. The species and enthalpy equations are treated using an operator-split approach that incorporates stiff integration techniques for modeling detailed chemical kinetics. The methodology also incorporates a mixture model for differential diffusion. For the simulations presented here, methane chemistry and transport are modeled using the DRM-19 (19-species, 84-reaction) mechanism derived from the GRIMech-1.2 mechanism along with its associated thermodynamics and transport databases. We consider a lean flame with equivalence ratio 0.8 for two different levels of turbulent intensity. For each case we examine the basic structure of the flame including turbulent flame speed and flame surface area. The results indicate that flame wrinkling is the dominant factor leading to the increased turbulent flame speed. Joint probability distributions are computed to establish a correlation between heat release and curvature. We also investigate the effect of turbulent flame interaction on the flame chemistry. We identify specific flame intermediates that are sensitive to turbulence and explore various correlations between these species and local flame curvature. We identify different mechanisms by which turbulence modulates the chemistry of the flame.
The Numerical Propulsion System Simulation: An Overview
NASA Technical Reports Server (NTRS)
Lytle, John K.
2000-01-01
Advances in computational technology and in physics-based modeling are making large-scale, detailed simulations of complex systems possible within the design environment. For example, the integration of computing, communications, and aerodynamics has reduced the time required to analyze major propulsion system components from days and weeks to minutes and hours. This breakthrough has enabled the detailed simulation of major propulsion system components to become a routine part of designing systems, providing the designer with critical information about the components early in the design process. This paper describes the development of the numerical propulsion system simulation (NPSS), a modular and extensible framework for the integration of multicomponent and multidisciplinary analysis tools using geographically distributed resources such as computing platforms, data bases, and people. The analysis is currently focused on large-scale modeling of complete aircraft engines. This will provide the product developer with a "virtual wind tunnel" that will reduce the number of hardware builds and tests required during the development of advanced aerospace propulsion systems.
Numerical simulation of pump-intake vortices
NASA Astrophysics Data System (ADS)
Rudolf, Pavel; Klas, Roman
2015-05-01
Pump pre-swirl or uneven flow distribution in front of the pump can induce pump-intake vortices. These phenomena result in blockage of the impeller suction space, deterioration of efficiency, drop of head curve and earlier onset of cavitation. Real problematic case, where head curve drop was documented, is simulated using commercial CFD software. Computational simulation was carried out for three flow rates, which correspond to three operating regimes of the vertical pump. The domain consists of the pump sump, pump itself excluding the impeller and the delivery pipe. One-phase approach is applied, because the vortex cores were not filled with air during observation of the real pump operation. Numerical simulation identified two surface vortices and one bottom vortex. Their position and strength depend on the pump flow rate. Paper presents detail analysis of the flow field on the pump intake, discusses influence of the vortices on pump operation and suggests possible actions that should be taken to suppress the intake vortices.
Numerical simulations of 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 Propulsion System Simulation: An Overview
NASA Technical Reports Server (NTRS)
Lytle, John K.
2000-01-01
The cost of implementing new technology in aerospace propulsion systems is becoming prohibitively expensive and time consuming. One of the main contributors to the high cost and lengthy time is the need to perform many large-scale hardware tests and the inability to integrate all appropriate subsystems early in the design process. The NASA Glenn Research Center is developing the technologies required to enable simulations of full aerospace propulsion systems in sufficient detail to resolve critical design issues early in the design process before hardware is built. This concept, called the Numerical Propulsion System Simulation (NPSS), is focused on the integration of multiple disciplines such as aerodynamics, structures and heat transfer with computing and communication technologies to capture complex physical processes in a timely and cost-effective manner. The vision for NPSS, as illustrated, is to be a "numerical test cell" that enables full engine simulation overnight on cost-effective computing platforms. There are several key elements within NPSS that are required to achieve this capability: 1) clear data interfaces through the development and/or use of data exchange standards, 2) modular and flexible program construction through the use of object-oriented programming, 3) integrated multiple fidelity analysis (zooming) techniques that capture the appropriate physics at the appropriate fidelity for the engine systems, 4) multidisciplinary coupling techniques and finally 5) high performance parallel and distributed computing. The current state of development in these five area focuses on air breathing gas turbine engines and is reported in this paper. However, many of the technologies are generic and can be readily applied to rocket based systems and combined cycles currently being considered for low-cost access-to-space applications. Recent accomplishments include: (1) the development of an industry-standard engine cycle analysis program and plug 'n play architecture, called NPSS Version 1, (2) A full engine simulation that combines a 3D low-pressure subsystem with a 0D high pressure core simulation. This demonstrates the ability to integrate analyses at different levels of detail and to aerodynamically couple components, the fan/booster and low-pressure turbine, through a 3D computational fluid dynamics simulation. (3) Simulation of all of the turbomachinery in a modern turbofan engine on parallel computing platform for rapid and cost-effective execution. This capability can also be used to generate full compressor map, requiring both design and off-design simulation. (4) Three levels of coupling characterize the multidisciplinary analysis under NPSS: loosely coupled, process coupled and tightly coupled. The loosely coupled and process coupled approaches require a common geometry definition to link CAD to analysis tools. The tightly coupled approach is currently validating the use of arbitrary Lagrangian/Eulerian formulation for rotating turbomachinery. The validation includes both centrifugal and axial compression systems. The results of the validation will be reported in the paper. (5) The demonstration of significant computing cost/performance reduction for turbine engine applications using PC clusters. The NPSS Project is supported under the NASA High Performance Computing and Communications Program.
3D Numerical simulations of oblique subduction
NASA Astrophysics Data System (ADS)
Malatesta, C.; Gerya, T.; Scambelluri, M.; Crispini, L.; Federico, L.; Capponi, G.
2012-04-01
In the past 2D numerical studies (e.g. Gerya et al., 2002; Gorczyk et al., 2007; Malatesta et al., 2012) provided evidence that during intraoceanic subduction a serpentinite channel forms above the downgoing plate. This channel forms as a result of hydration of the mantle wedge by uprising slab-fluids. Rocks buried at high depths are finally exhumed within this buoyant low-viscosity medium. Convergence rate in these 2D models was described by a trench-normal component of velocity. Several present and past subduction zones worldwide are however driven by oblique convergence between the plates, where trench-normal motion of the subducting slab is coupled with trench-parallel displacement of the plates. Can the exhumation mechanism and the exhumation rates of high-pressure rocks be affected by the shear component of subduction? And how uprise of these rocks can vary along the plate margin? We tried to address these questions performing 3D numerical models that simulate an intraoceanic oblique subduction. The models are based on thermo-mechanical equations that are solved with finite differences method and marker-in-cell techniques combined with multigrid approach (Gerya, 2010). In most of the models a narrow oceanic basin (500 km-wide) surrounded by continental margins is depicted. The basin is floored by either layered or heterogeneous oceanic lithosphere with gabbro as discrete bodies in serpentinized peridotite and a basaltic layer on the top. A weak zone in the mantle is prescribed to control the location of subduction initiation and therefore the plate margins geometry. Finally, addition of a third dimension in the simulations allowed us to test the role of different plate margin geometries on oblique subduction dynamics. In particular in each model we modified the dip angle of the weak zone and its "lateral" geometry (e.g. continuous, segmented). We consider "continuous" weak zones either parallel or increasingly moving away from the continental margins. Moreover, we tested the effect on subduction/exhumation dynamics of several values of the trench-parallel component of convergence-rate vector. Gerya T., Stöckhert B., Perchuk A.L. (2002). Exhumation of high-pressure metamorphic rocks in a subduction channel: a numerical simulation. Tectonics, vol. 21, n. 6, 1056. Gerya, T. V., 2010. Introduction to numerical geodynamic modelling. Cambridge University Press, Cambridge. Gorczyk W., Guillot S., Gerya T.V., Hattori K. (2007a). Asthenospheric upwelling, oceanic slab retreat, and exhumation of UHP mantle rocks: insights from Greater Antilles. Geophysical research letters, vol. 34, L21309. Malatesta C., Gerya T., Scambelluri M., Federico L., Crispini L., Capponi G. (2012). Intraoceanic subduction of "heterogeneous" oceanic lithosphere in narrow basins: 2D numerical modeling. Lithos, http://dx.doi.org/10.1016/j.lithos.2012.01.003
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 simulations to study solar wind turbulence
Sharma, R. P.; Sharma, Nidhi; Kumar, Sanjay; Kumar, Sachin; Singh, H. D.
2011-02-15
Numerical simulation of coupled equations of kinetic Alfven wave (KAW) and ion acoustic wave is presented in the solar wind. The nonlinear dynamical equations satisfy the modified Zakharov system of equations by taking the nonadiabatic response of the background density. The ponderomotive nonlinearity is incorporated in the wave dynamics. The effect of Landau damping of KAW is taken into account. Localization of magnetic field intensity and the wavenumber spectra (perpendicular and parallel) of magnetic fluctuations are studied in solar plasmas around 1 a.u. Our results reveal the formation of damped localized structures and the steeper spectra that are in good agreement with the observations. These damped structures and steeper turbulent spectra can be responsible for plasma heating and particle acceleration in solar wind.
Numerical simulation of excited jet mixing layers
NASA Technical Reports Server (NTRS)
Scott, J. N.; Hankey, W. L.
1987-01-01
A numerical simulation of unsteady flow in jet mixing layers, both with and without external excitation, has been performed by solving the time-dependent compressible Navier-Stokes equations. Computations were performed on a CRAY X-MP computer using MacCormick's explicit finite difference algorithm. Different excitation methods were investigated and were shown to be very effective in controlling the well organized periodic production, shedding and pairing of large scale vortex structures. It is found that pressure excitation was generally more effective than temperature excitation, and that grid refinement results in substantial improvement in the resolution of unsteady features. The location and orientation, in addition to the frequency, of the excitation source are shown to have a significant influence on the production and interaction of large scale vortex structures in the jet mixing layer.
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 Simulations of Acoustically Driven, Burning Droplets
NASA Technical Reports Server (NTRS)
Kim, H.-C.; Karagozian, A. R.; Smith, O. I.; Urban, Dave (Technical Monitor)
1999-01-01
This computational study focuses on understanding and quantifying the effects of external acoustical perturbations on droplet combustion. A one-dimensional, axisymmetric representation of the essential diffusion and reaction processes occurring in the vicinity of the droplet stagnation point is used here in order to isolate the effects of the imposed acoustic disturbance. The simulation is performed using a third order accurate, essentially non-oscillatory (ENO) numerical scheme with a full methanol-air reaction mechanism. Consistent with recent microgravity and normal gravity combustion experiments, focus is placed on conditions where the droplet is situated at a velocity antinode in order for the droplet to experience the greatest effects of fluid mechanical straining of flame structures. The effects of imposed sound pressure level and frequency are explored here, and conditions leading to maximum burning rates are identified.
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.
Polarimetric bio-aerosol detection: numerical simulation
NASA Astrophysics Data System (ADS)
Snow, J. William; Bicknell, W. Ed; Burke, Hsiao-hua K.
2005-11-01
This paper examines the use of bi-static lidar to remotely detect the release of aerosolized biological agent. The detection scheme exploits bio-aerosol induced changes in the Stokes parameters of scattered radiation in comparison to scattered radiation from ambient background aerosols alone. A polarization distance metric is introduced to discriminate between changes caused by the two types of aerosols. Scattering code computations are the information source. Three application scenarios are considered: outdoor arena, indoor auditorium, and building heating-ventilation-air-conditioning (HVAC) system. Numerical simulations are employed to determine sensitivity of detection to laser wavelength and to particle physical properties. Results of the study are described and details are given for the specific example of a 1.50 ?m lidar system operating outdoors over a 1000-m range.
Direct Numerical Simulations of PBX 9501
NASA Astrophysics Data System (ADS)
Mas, E. M.; Clements, B. E.; George, D. C.
2004-07-01
We have explicitly gridded HMX crystals in PBX 9501 from 25 ?m in diameter up to .5 mm. We used HMX particle size distributions found in the literature to determine the relative numbers of different sized particulates. We applied our modified Mori-Tanaka theory to model the smaller crystals embedded in the plasticized estane binder (the dirty-binder). This model was modified to accommodate the large amount of HMX in the dirty binder. We then subjected the 1 million element PBX 9501 realization to boundary conditions commensurate with a Split Hopkinson Pressure Bar experiment. We compare results to experiment and a micro-mechanical model we have reported on earlier. We also discuss the information which can be extracted from these direct numerical simulations.
Numerical Simulations of a Lobed Fuel Injector
NASA Technical Reports Server (NTRS)
Strickland, J. H.; Selerland, T.; Karagozian, A. R.
1998-01-01
Numerical modeling of the nonreactive mixing processes associated with a lobed fuel injector in a coflowing air stream is presented. The lobed fuel injector is a device which generates strong streamwise vorticity, producing locally high strain rates which can enhance the molecular mixing of reactants while delaying ignition in a controlled manner. Vortex element modeling is used to simulate flow field evolution and fuel element mixing characteristics for this lobed fuel injector. Quantitative predictions for vorticity generation and qualitative results for streamwise rollup compare well qualitatively with recent experimental investigations of this flow field [Smith et al, Phys. Fluids 9, 667 (1997)]. Parametric studies of the effects of lobe amplitude-to-wavelength ratio, lobe angle, and lobe shape for given flow conditions suggest that geometrical features may be optimized to enhance mixing and control reaction processes.
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.
Numerical simulation of flexible blank drawer formation
NASA Astrophysics Data System (ADS)
Li, L. C.; Li, M. Z.
2015-12-01
This paper presents a finite element model for a technology of Fexible Blank Drawer Formation (FBDF) for the forming process.In order to verify the feasibility and versatility of the FBDF technology, we performed numerical simulations for multi-point saddle-shaped parts, and for convex curved surface and hemispherical parts. We analysed the effect of different forming methods on wrinkling and cracking, stress and strain distribution as well as circulating and spring back. Also, we studied the effect on the FBDF result caused by the chucking power, blank drawer force, friction coefficient and material parameters. The results showed that under the same conditions, the parts formed by FBDF technology showed uniform stress and strain distribution with little spring back. The blank drawer force can efficiently restrain the defects such as wrinkling and crack.
DNS of low Reynolds number turbulent flows in dimpled channels
NASA Astrophysics Data System (ADS)
Wang, Zhengyi; Yeo, K. S.; Khoo, B. C.
Direct numerical simulation (DNS) is performed to study turbulent flows over dimpled surfaces in a channel. Results on mean field and second-order quantities are obtained. Horseshoe vortices can be observed in the dimples of sparse arrays. As inter-dimple separation is reduced, the feet of the horseshoe vortices are gradually lifted off the dimple surface, and the resulting flow structures in the cavities become flattened and stretched to become something akin to two-dimensional separation bubbles. At the higher dimple density, the stream traces near the surface also develop a distinct formation similar to what had been observed in earlier Reynolds-averaged Navier Stokes (RANS) simulations (Isaev, S.A., Leont'ev, A.I. and Baranov, P.A., 2000, Technical Physics Letters, 26, 15; Lin, Y.L., Shih, T.I.-P. and Chyu, M.K., 1999, ASME paper, 99-GT-263; Lin, Y.L. Shih, T.I.-P., 2001, International Journal of Transfer Phenomena, 3, 1). Regions of high turbulence intensity are found above the downstream half of the dimples and along their side edges. These regions coincide with the locations of vortex shedding found in the experiments of Ligrani et al. (2001, Physics of Fluids, 13, 3442) and the locations of vorticity concentrations observed in Park et al. (2004, Numerical Heat Transfer, Part A (Applications), 45(1), 1) and Won and Ligrani (2004, Numerical Heat Transfer, Part A (Applications), 46(6), 549). For a fixed mean pressure gradient, it is observed that the flow rates through the channels are reduced by the presence of dimples. This indicates that the dimpled channels we have studied so far have larger drag than flat-wall channels. Computed friction coefficients for dimpled channels also confirmed the conclusion.
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.
Direct Numerical Simulation of a Film Cooling Configuration with a Micro-ramp Vortex Generator
NASA Astrophysics Data System (ADS)
Shinn, Aaron; Pratap Vanka, S.
2010-11-01
A Direct Numerical Simulation (DNS) of an inclined turbulent jet interacting with a cross-flow in a film cooling configuration is performed. The inclined turbulent jet represents the coolant flow and the cross-flow represents the hot combustion gases. In this configuration, it is known that the coolant jet tends to lift off the wall that is to be cooled, thus decreasing heat transfer effectiveness. The micro-ramp vortex generator is placed downstream of the coolant jet and is used to modify the trajectory of the coolant jet such that it remains closer to the wall, thus enhancing heat transfer. The purpose of this study is to examine the micro-ramp's effect on both the flowfield and heat transfer of the film cooling problem. The coolant jet is inclined at an angle of 35 degrees to the freestream, the blowing ratio is 1.5, and the Reynolds number based on the jet diameter and freestream cross-flow velocity is 8000. The incompressible Navier-Stokes equations are solved numerically using a 3D finite volume solver (CU-FLOW) implemented on a Graphics Processing Unit (GPU).
NASA Astrophysics Data System (ADS)
Fasel, Hermann F.
2002-04-01
A new compressible Navier-Stokes code in cylindrical coordinates was developed for investigating axisymmetric wakes of bluff-based bodies in supersonic flows. In this code, high-order compact finite differences derived for non-equidistant grids are employed and a new state-of-the-art axis treatment is incorporated. Additionally, the fully three-dimensional transport equations for turbulent kinetic energy and turbulent dissipation are implemented to enable (steady or unsteady) Reynolds Averaged Navier Stokes (RANS) simulations. Furthermore, a new "Flow Simulation Methodology" (FSM) was developed for computing complex compressible flows. The centerpiece of FSM is a strategy to provide the proper amount of modeling of the subgrid scales. This is accomplished by a "contribution function" which locally and instantaneously compares the smallest relevant scales to the local grid size. The contribution function is designed such that it provides no modeling if the computation is locally well resolved so that the computation approaches a Direct Numerical Simulation (DNS) in the fine grid limit, or provides modeling of all scales in the coarse grid limit and thus approaches an unsteady RANS (URANS) calculation. In between these resolution limits, the contribution function adjusts the necessary modeling for the unresolved scales while the larger (resolved) scales are computed as in traditional Large Eddy Simulations (LES). Preliminary results have shown that the new high order code has great advantages for supersonic base flow simulations and that calculations, in particular together with FSM, will allow simulations of supersonic base flows at much higher Reynolds numbers than possible with conventional LES.
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 simulation of solar coronal magnetic fields
NASA Technical Reports Server (NTRS)
Dahlburg, Russell B.; Antiochos, Spiro K.; Zang, T. A.
1990-01-01
Many aspects of solar activity are believed to be due to the stressing of the coronal magnetic field by footpoint motions at the photosphere. The results are presented of a fully spectral numerical simulation which is the first 3-D time dependent simulation of footpoint stressing in a geometry appropriate for the corona. An arcade is considered that is initially current-free and impose a smooth footpoint motion that produces a twist in the field of approx 2 pi. The footprints were fixed and the evolution was followed until the field relaxes to another current-free state. No evidence was seen for any instability, either ideal or resistive and no evidence for current sheet formation. The most striking feature of the evolution is that in response to photospheric motions, the field expands rapidly upward to minimize the stress. The expansion has two important effects. First, it suppresses the development of dips in the field that could support dense, cool material. For the motions assumed, the magnetic field does not develop a geometry suitable for prominence formation. Second, the expansion inhibits ideal instabilities such as kinking. The results indicate that simple stearing of a single arcade is unlikely to lead to solar activity such as flares or prominences. Effects are discussed that might possibly lead to such activity.
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.
Hybrid Numerical Simulations Of Planetesimal Accretion
NASA Astrophysics Data System (ADS)
Marzari, Francesco; Weidenschilling, S. J.
2006-09-01
The multi-zone simulation code modelling the accretion of planetesimals into planets (Spaute et al. 1991, Icarus 92, 147; Weidenschilling et al. 1997, ICARUS 128, 429) includes a statistical continuum of small bodies in logarithmic mass bins, while large bodies are discrete objects with individual masses and orbits. Formerly, gravitational interactions between large planetary embryos were treated by statistical scattering. The code has now been updated to properly handle the orbits of protoplanets in a deterministic way. The trajectories of the larger bodies are numerically computed with the symplectic integrator SyYMBA. The additional forces acting on the protoplanets due to collisions with smaller planetesimals and their gravitational perturbations, including dynamical friction, as well as gas drag and tidal interaction with the solar nebula, are incorporated in the N-body algorithm by applying a further step in the leap-frog structure of the SyMBA integrator. The changes in the orbital elements of the large bodies, computed in the stochastic part of the code with a Monte Carlo approach, are applied for half a timestep before and after the N-body Hamiltonian propagation as suggested in Lee & Peale (ApJ 567, 596, 2002). With this code we intend to study the effect of dynamical friction on terrestrial planet formation and the accretion of planetary cores in the outer solar system. We will present preliminary results of simulations performed with the updated code.
Collisionless microinstabilities in stellarators. II. Numerical simulations
NASA Astrophysics Data System (ADS)
Proll, J. H. E.; Xanthopoulos, P.; Helander, P.
2013-12-01
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 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 Simulations of Hypersonic Boundary Layer Transition
NASA Astrophysics Data System (ADS)
Bartkowicz, Matthew David
Numerical schemes for supersonic flows tend to use large amounts of artificial viscosity for stability. This tends to damp out the small scale structures in the flow. Recently some low-dissipation methods have been proposed which selectively eliminate the artificial viscosity in regions which do not require it. This work builds upon the low-dissipation method of Subbareddy and Candler which uses the flux vector splitting method of Steger and Warming but identifies the dissipation portion to eliminate it. Computing accurate fluxes typically relies on large grid stencils or coupled linear systems that become computationally expensive to solve. Unstructured grids allow for CFD solutions to be obtained on complex geometries, unfortunately, it then becomes difficult to create a large stencil or the coupled linear system. Accurate solutions require grids that quickly become too large to be feasible. In this thesis a method is proposed to obtain more accurate solutions using relatively local data, making it suitable for unstructured grids composed of hexahedral elements. Fluxes are reconstructed using local gradients to extend the range of data used. The method is then validated on several test problems. Simulations of boundary layer transition are then performed. An elliptic cone at Mach 8 is simulated based on an experiment at the Princeton Gasdynamics Laboratory. A simulated acoustic noise boundary condition is imposed to model the noisy conditions of the wind tunnel and the transitioning boundary layer observed. A computation of an isolated roughness element is done based on an experiment in Purdue's Mach 6 quiet wind tunnel. The mechanism for transition is identified as an instability in the upstream separation region and a comparison is made to experimental data. In the CFD a fully turbulent boundary layer is observed downstream.
A numerical simulation of the Catalina Eddy
Ueyoshi, Kyozo; Roads, J.O.; Alpert, J.
1991-12-31
A shallow cyclonic eddy termed the Catalina Eddy has occasionally been observed during summer in the bight of southern California. The Catalina Eddy occurs within {approximately}100 km from the coastal mountains with a depth typically extending up to the marine inversion level of several hundred meters above sea level and a diameter on the order of 100--200 km. The Catalina Eddy is produced by the interaction between the synoptic-scale northerly flow and the formidable topography along the southern California coast. A favorable synoptic situation that enhances the increased low-level climatological northerly flow along the central California coastline is the presence of the prominent east-west pressure gradient between the subtropical East Pacific high and the inland thermal low over California. Increased northerlies impinging on the San Rafael mountains north of Santa Barbara result in enhanced mesoscale lee troughing in the bight and establishment of a narrow ridge alongshore, leading to establishment of cyclonic vorticity in the bight. This paper describes numerical simulations and predictions of a Catalina Eddy event with a high-resolution multi-level limited area model. The model is initialized and forced at the lateral boundaries by the National Meteorological Center`s (NMC) 2.5{degree} {times} 2.5{degree} global objective analysis and also by NMC`s medium range forecast model (MRF) 1--10 day forecasts. In the authors previous effort to simulate mesoscale disturbances such as the Catalina Eddy the integrations were performed up to 1 model-day utilizing the NMC analysis as fixed lateral boundary conditions. In this paper they describe the results of continuous 5- to 7-day simulations of the Catalina Eddy event of 26--30 June 1988 by utilizing time-dependent lateral boundary conditions obtained from NMC`s global objective analysis as well as NMC`s MRF forecasts.
DNS Study of Transient Disturbance Growth and Bypass Transition
NASA Astrophysics Data System (ADS)
Stephani, Kelly; Goldstein, David
2008-11-01
Direct numerical simulation was used to investigate the detailed flow past a periodic array of cylindrical roughness elements. The problem was constructed as channel flow over a flat plate surface with roughness elements formed using an immersed boundary technique with a spectral method approach. Solutions were obtained for two roughness heights corresponding to Reynolds numbers (Rek) of 189 and 350, and results are presented for both cases. Cylindrical roughness elements with Rek=189 produced minimal disturbances and the flow remained laminar in the wake downstream of the roughness elements. Flow past cylindrical roughness elements corresponding to Rek=350 was found to transition as soon as 2-3 cylinder diameters downstream and had developed into fully turbulent flow by the end of the domain. Results were found to compare reasonably well with a similar set of DNS computations by Rizzetta and Visbal using a sixth-order-accurate centered compact finite difference scheme as well as experimental results obtained by Ergin and White using time-averaged hotwire measurements of the velocity components.
Numerical Simulations of Falling Sphere Viscometry Experiments.
NASA Astrophysics Data System (ADS)
O Dwyer, L.; Kellogg, L. H.; Lesher, C. E.
2007-12-01
The falling sphere technique based on Stokes' law is widely used to determine the viscosities of geologically relevant melts at high pressures. Stokes' law is valid when a rigid sphere falls slowly and steadily through a stationary and infinite Newtonian medium of uniform properties. High-pressure falling sphere experiments however, usually involve dropping a dense, refractory sphere through a liquid contained by a cylindrical capsule of finite size. The sphere velocity is influenced by the walls (Faxen correction) and ends of the capsule, and possible convective motion of the fluid. Efforts are made to minimize thermal gradients in laboratory experiments, but small temperature differences within the capsule can lead to convection complicating interpretation. We utilize GALE (Moresi et al., 2003;), a finite element particle-in-cell code, to examine these factors in numerical models of conditions similar to those of high-pressure experiments. Our modeling considers a three- dimensional box or cylinder containing a cluster of particles that represent the dense sphere in laboratory experiments surrounded by low viscosity particles representing the melt. GALE includes buoyancy forces, heat flow, and viscosity variations so our model can be used to assess the effects of the capsule's walls and ends, and the consequences of thermal gradients on the sphere's velocity and trajectory. Comparisons between our numerical simulations and real-time falling sphere experiments involving lower viscosity molten komatiite are made to assess the validity of Stokes' law with the standard Faxen correction included, and formulations considering end effects. The modeling also permits an evaluation of the uncertainties in recovering accurate liquid viscosities from Stokes' law when a dense sphere falls through a convecting low viscosity melt. It also allows us to assess acceleration to a terminal velocity that can provide constraints on melt viscosity in experiments in which the terminal velocity was not reached.
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 "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; Prez 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 Simulations of Relativistic Jets in Collapsars
NASA Astrophysics Data System (ADS)
Zhang, W.; Woosley, S. E.; Heger, A.; MacFadyen, A. I.
2003-12-01
The propagation and break out of relativistic jets in collapsars, which are believed to give rise to outburst of high-energy emission known as gamma-ray bursts (GRBs), are examined in multi-dimensional numerical simulations using a special relativistic hydrodynamics code. If powered long enough, a relativistic jet from a collapsar can break out of a massive Wolf-Rayet star. During its propagation, the jet is collimated by the passage through the stellar mantle. Starting with an initial half-angle of up to 20 degrees, it emerges with an half-angle that, though variable with time, is around 5 degrees. Interaction of the jet with the star and its own cocoon also causes mixing that sporadically decelerates the flow. As it erupts, the highly relativistic jet core (3 to 5 degrees) is surrounded by a cocoon of less energetic, but still moderately relativistic ejecta (Γ ˜ 15) that expands and becomes visible at larger polar angles ( ˜ 10 degrees). We predict a distribution of energy and Lorentz factor with viewing angle in the jet beam and its cocoon. These results have important implications for the observed light curves and energies of GRBs and imply that what is seen may vary greatly with viewing angle. In particular, we predict the existence of a large number of low energy GRBs with mild Lorentz factors that may be related to GRB 980425/SN 1998bw and to the recently recognized XRFs. Jet stability is also examined in three-dimensional calculations. It is found that a three-dimensional jet undergoes a kink instability. Processing jets are also examined in three-dimensional simulations. If the jet changes angle by more than three degrees in several seconds, it will dissipate, producing a broad beam with inadequate Lorentz factor to make a common gamma-ray burst.
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.
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.
Numerical simulation of ball-racket impact
NASA Astrophysics Data System (ADS)
Yu, Yingpang
The collision of a ball with a tennis racket is usually modeled in terms of rigid body dynamics or an elastic system involving only a few springs. In this paper, we study the impact between a tennis ball and racket, by modeling the tennis ball in two different yaws. One method models the tennis ball as a Hertz elastic body and the other one models the ball by a more accurate finite element analysis. In the first model, we assume that the elastic properties of the ball obeys Hertz's law. In the finite element model, we consider the tennis ball as a shell witch is a elastic system constructed out of many isotropic small linear flat, elements, witch have both elastic and damping properties. The damping in each way is approximated as viscous term. In both methods, we study the static condition of deformation against a rigid surface before applying these models to dynamical processes. We compare these two methods and eventually determine how the racket parameters effect the performance of the racket, using numerical simulations. Comparison with experiment are show to confirm the general conclusion of the model.
Transonic aeroelastic numerical simulation in aeronautical engineering
NASA Astrophysics Data System (ADS)
Yang, Guowei
2006-06-01
A lower upper symmetric Gauss Seidel (LU-SGS) subiteration scheme is constructed for time-marching of the fluid equations. The Harten Lax van Leer Einfeldt Wada (HLLEW) scheme is used for the spatial discretization. The same subiteration formulation is applied directly to the structural equations of motion in generalized coordinates. Through subiteration between the fluid and structural equations, a fully implicit aeroelastic solver is obtained for the numerical simulation of fluid/structure interaction. To improve the ability for application to complex configurations, a multiblock grid is used for the flow field calculation and transfinite interpolation (TFI) is employed for the adaptive moving grid deformation. The infinite plate spline (IPS) and the principal of virtual work are utilized for the data transformation between the fluid and structure. The developed code was first validated through the comparison of experimental and computational results for the AGARD 445.6 standard aeroelastic wing. Then, the flutter character of a tail wing with control surface was analyzed. Finally, flutter boundaries of a complex aircraft configuration were predicted.
The numerical simulation of subsonic flutter
NASA Technical Reports Server (NTRS)
Strganac, Thomas W.; Mitchum, Maria V.; Mook, Dean T.
1987-01-01
The present paper describes a numerical simulation of unsteady, subsonic aeroelastic responses. The technique accounts for aerodynamic nonlinearities associated with angles of attack, vortex-dominated flow, static deformations, and unsteady behavior. The fluid and the wing together are treated as a single dynamic system, and the equations of motion for the structure and flowfield are integrated simultaneously and interactively in the time domain. The method employs an iterative scheme based on a predictor-corrector technique. The aerodynamic loads are computed by the general unsteady vortex-lattice method and are determined simultaneously with the motion of the wing. Two models are used to demonstrate the technique: a rigid wing on an elastic support experiencing plunge and pitch about the elastic axis, and a continuous wing rigidly supported at the root chord experiencing spanwise bending and twisting. The time domain solution coupled with the unsteady vortex-lattice method provides the capability of graphically depicting wing and wake motion. Several graphs that illustrate the time domain behavior of the wing and wake are presented.
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.
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
LES, DNS and RANS for the analysis of high-speed turbulent reacting flows
NASA Technical Reports Server (NTRS)
Givi, Peyman
1994-01-01
The objective of this research is to continue our efforts in advancing the state of knowledge in Large Eddy Simulation (LES), Direct Numerical Simulation (DNS), and Reynolds Averaged Navier Stokes (RANS) methods for the analysis of high-speed reacting turbulent flows. In the first phase of this research, conducted within the past six months, focus was in three directions: RANS of turbulent reacting flows by Probability Density Function (PDF) methods, RANS of non-reacting turbulent flows by advanced turbulence closures, and LES of mixing dominated reacting flows by a dynamics subgrid closure. A summary of our efforts within the past six months of this research is provided in this semi-annual progress report.
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.
Terascale High-Fidelity Simulations of Turbulent Combustion with Detailed Chemistry
Raghurama Reddy; Roberto Gomez; Junwoo Lim; Yang Wang; Sergiu Sanielevici
2004-10-15
This SciDAC project enabled a multidisciplinary research consortium to develop a high fidelity direct numerical simulation (DNS) software package for the simulation of turbulent reactive flows. Within this collaboration, the authors, based at CMU's Pittsburgh Supercomputing Center (PSC), focused on extensive new developments in Sandia National Laboratories' "S3D" software to address more realistic combustion features and geometries while exploiting Terascale computational possibilities. This work significantly advances the state-of-the-art of DNS of turbulent reacting flows.
Advanced in turbulence physics and modeling by direct numerical simulations
NASA Technical Reports Server (NTRS)
Reynolds, W. C.
1987-01-01
The advent of direct numerical simulations of turbulence has opened avenues for research on turbulence physics and turbulence modeling. Direct numerical simulation provides values for anything that the scientist or modeler would like to know about the flow. An overview of some recent advances in the physical understanding of turbulence and in turbulence modeling obtained through such simulations is presented.
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.
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.
A numerical simulation of the backward Raman amplifying in plasma
NASA Astrophysics Data System (ADS)
Wang, Hong-Yu; Huang, Zu-Qia
2005-12-01
This paper describe a numerical simulation method for the interaction between laser pulses and low density plasmas based on hydrodynamic approximation. We investigate Backward Raman Amplifying (BRA) experiments and their variants. The numerical results are in good agreement with experiments.
Numerical simulation of 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.
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.
Chen, Jackie; Sankaran, Ramanan; Hawkes, Evatt R
2009-05-01
The difficulty of experimental measurements of the scalar dissipation rate in turbulent flames has required researchers to estimate the true three-dimensional (3D) scalar dissipation rate from one-dimensional (1D) or two-dimensional (2D) gradient measurements. In doing so, some relationship must be assumed between the true values and their lower dimensional approximations. We develop these relationships by assuming a form for the statistics of the gradient vector orientation, which enables several new results to be obtained and the true 3D scalar dissipation PDF to be reconstructed from the lower-dimensional approximations. We use direct numerical simulations (DNS) of turbulent plane jet flames to examine the orientation statistics, and verify our assumptions and final results. We develop and validate new theoretical relationships between the lower-dimensional and true moments of the scalar dissipation PDF assuming a log-normal true PDF. We compare PDFs reconstructed from lower-dimensional gradient projections with the true values and find an excellent agreement for a 2D simulated measurement and also for a 1D simulated measurement perpendicular to the mean flow variations. Comparisons of PDFs of thermal dissipation from DNS with those obtained via reconstruction from 2D experimental measurements show a very close match, indicating this PDF is not unique to a particular flame configuration. We develop a technique to reconstruct the joint PDF of the scalar dissipation and any other scalar, such as chemical species or temperature. Reconstructed conditional means of the hydroxyl mass fraction are compared with the true values and an excellent agreement is obtained.
A Numerical Simulation of Cyclic Mesocyclogenesis.
NASA Astrophysics Data System (ADS)
Adlerman, Edwin J.; Droegemeier, Kelvin K.; Davies-Jones, Robert
1999-07-01
A three-dimensional nonhydrostatic numerical model, the Advanced Regional Prediction System, is used to study the process of cyclic mesocyclogenesis in a classic supercell thunderstorm. During the 4-h simulation, the storm's mesocyclone undergoes two distinct occlusions, with the beginning of a third indicated at the end of the simulation. The occlusion process exhibits a period of approximately 60 min and is qualitatively similar in each case.Initial midlevel (3-7 km) mesocyclogenesis proceeds according to the `classic' picture, that is, via tilting of streamwise environmental vorticity. The development of an evaporatively driven rear-flank downdraft (RFD) signals the beginning of the occlusion process. The developing RFD wraps cyclonically around the mesocyclone, causing the gust front to surge outward. Simultaneously, the occluding mesocyclone rapidly intensifies near the surface. Trajectory analyses demonstrate that this intensification follows from the tilting and stretching of near-ground (<500 m) streamwise vorticity produced by baroclinic generation, crosswise exchange, and streamwise stretching along descending parcel trajectories in the RFD. The surging gust front also initiates updraft development on the downshear flank at midlevels, resulting in a two-celled updraft structure. As the near-ground mesocyclone becomes detached from the gust front due to the developing occlusion downdraft, the upshear updraft flank weakens as its conditionally unstable inflow is cut off at low levels; at the same time, the downshear updraft flank continues to develop eastward. The end of the occlusion process is signaled as the old near-ground mesocyclone becomes completely embedded near the surface in divergent outflow beneath the decaying updraft and is advected away by the mean flow.Near-ground mesocyclogenesis is initiated in the new updraft in a process nearly identical to that of the initial mesocyclone. However, after the first occlusion, near-ground equivalent potential temperature and buoyancy contours are fortuitously oriented such that streamwise baroclinic generation can proceed without delay. Thus, although the initial occlusion requires two hours to become fully organized, the second occurs only one hour later. In effect, the occlusion appears to set the stage for more rapid development of subsequent mesocyclones.
Numerical simulations of relativistic jets in collapsars
NASA Astrophysics Data System (ADS)
Zhang, Weiqun
Relativistic jets in collapsars are studied in this thesis. Shocks within these jets are believed to give rise to outbursts of high-energy emission known as gamma-ray bursts (GRBs). The propagation and break out of the jets are examined in multi-dimensional numerical simulations using a special relativistic hydrodynamics code. If powered long enough, a relativistic jet from a collapsar can break out of a massive Wolf-Rayet star. During its propagation, the jet is collimated by the passage through the stellar mantle. Starting with an initial half-angle of up to 20 degrees, it emerges with an half-angle that, though variable with time, is around 5 degrees. Interaction of the jet with the star and its own cocoon also causes mixing that sporadically decelerates the flow. We speculate that this mixing instability is chiefly responsible for the variable Lorentz factor needed in the internal shock model and for the complex light curves seen in many gamma-ray bursts. As it erupts, the highly relativistic jet core (3 to 5 degrees) is surrounded by a cocoon of less energetic, but still moderately relativistic ejecta (Gamma ˜ 15) that expands and becomes visible at larger polar angles (˜10 degrees). We predict a distribution of energy and Lorentz factor with viewing angle in the jet beam and its cocoon. These results have important implications for the observed light curves and energies of GRBs and imply that what is seen may vary greatly with viewing angle. In particular, we predict the existence of a large number of low energy GRBs with mild Lorentz factors that may be related to GRB 980425/SN 1998bw and to the recently recognized cosmological X-ray flashes (XRFs). Jet stability is also examined in three-dimensional calculations. It is found that a three-dimensional jet undergoes a kink instability. Processing jets are also examined in three-dimensional simulations. If the jet changes angle by more than three degrees in several seconds, it will dissipate, producing a broad beam with inadequate Lorentz factor to make a common GRB.
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.
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.
Direct Numerical Simulation of Vertical Particulate Channel Flow in the Turbulent Regime
NASA Astrophysics Data System (ADS)
Uhlmann, M.; Pinelli, A.
We have conducted a DNS study of dilute turbulent particulate flow in a vertical plane channel, considering up to 8192 finite-size rigid particles with numerically resolved phase interfaces. The particle diameter corresponds to approximately 9 wall units and their terminal Reynolds number is set to 136. The fluid flow with bulk Reynolds number 2700 is directed upward, which maintains the particles suspended upon average. Two different density ratios were simulated, varying by a factor of 4.5. The corresponding Stokes numbers of the two particles were O(10) in the near-wall region and O(1) in the outer flow. We have observed the formation of large-scale elongated streak-like structures with streamwise dimensions of the order of 8 channel half-widths and cross-stream dimensions of the order of one half-width. At the same time, we have found no evidence of significant formation of particle clusters, which suggests that the large structures are due to an mtxinsic instability of the flow, triggered by the presence of the particles. It was found that the mean flow velocity profile tends towards a concave shape, and the turbulence intensity as well as the normal stress anisotropy are strongly increased. The effect of varying the Stokes number while keeping the buoyancy, particle size and volume fraction constant was relatively weak. More details about part of this work can be found in (2008).
Direct numerical simulations of drag reduction due to bubble injection into a turbulent channel flow
NASA Astrophysics Data System (ADS)
Lu, Jiacai; Tryggvason, Gretar
2004-11-01
Experimentally it has found that it is possible to reduce the friction drag of submerged bodies by the injection of a modest amount of air into the turbulent boundary layer. The mechanism by which bubbles reduce drag is, however, essentially unknown. Here, the effect of the bubbles on the wall drag in a turbulent channel flow is examined by direct numerical simulations (DNS). A front-tracking/finite volume method is used to fully resolve all flow scales, including the bubbles and the flow around them, for bubbles injected near the walls in the so-called ``minimum turbulent channel.'' The result show that slightly deformable bubbles located near the walls can lead to a significant reduction of the wall drag. Drag reduction is seen even when a few bubbles still remain near the wall. The mechanism for the drag reduction seems to be the suppression of streamwise vorticity as the bubbles pass over them, forcing mutual cancellation with wall bounded vortices of the opposite sign. Spherical bubbles, on the other hand, lead to a large increase in drag when they collide with the wall. The size of the bubbles is about fifty wall units, comparable to what is found in experiments where the bubbles are injected through the wall. The effect of the bubbles on the various averaged quantities describing the turbulent flow are discussed. Research supported by DARPA.
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 electrochemical reactions in a turbulent electrolyte
NASA Astrophysics Data System (ADS)
Doche, Olivier; Bauer, Frederic; Tardu, Sedat
2010-11-01
In electrochemical processes, such as industrial electrodeposition, the flow state can influence the mass transfer of the active chemical species in solution. This could lead to significant modifications of reaction kinetics at the electrode and obviously affects the global performance of the system. We aim here to describe via DNS the behavior of a turbulent electrolyte in a channel configuration where electrode are placed at each wall. Since the whole problem is governed by a full multiphysic coupling, we resolve in 3D and at each time step a set of equations constituted by 2 turbulent transport equations -momentum and a passive scalar- completed by the potential distribution resolution. These 3 distinct physics are coupled through the Butler-Volmer boundary condition which acts at the electrode/electrolyte interface and governs the whole electrochemical activity. We present the numerical methodology used in this work and all the quantitative results obtained. We also report significant differences with the literature, mainly on the mass transfer statistics, which tend to confirm that a fully coupled approach is necessary to obtain a reliable description of the physic involved in such electrochemical transformations.
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 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.
Analysis and modeling of subgrid scalar mixing using numerical data
NASA Technical Reports Server (NTRS)
Girimaji, Sharath S.; Zhou, YE
1995-01-01
Direct numerical simulations (DNS) of passive scalar mixing in isotropic turbulence is used to study, analyze and, subsequently, model the role of small (subgrid) scales in the mixing process. In particular, we attempt to model the dissipation of the large scale (supergrid) scalar fluctuations caused by the subgrid scales by decomposing it into two parts: (1) the effect due to the interaction among the subgrid scales; and (2) the effect due to interaction between the supergrid and the subgrid scales. Model comparisons with DNS data show good agreement. This model is expected to be useful in the large eddy simulations of scalar mixing and reaction.
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.
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 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.
Numerical simulation of steam condensation in a nozzle
NASA Astrophysics Data System (ADS)
Halama, Jan; Fort, Jaroslav
2012-04-01
This paper describes a model of compressible flow including homogenous nucleation and consequent growth and evaporation of droplets. Considered fluid is water vapor with relatively small mass fraction of dispersed water droplets. Main issues of numerical simulation based on this model are discussed. Results of numerical simulation are compared with experimental data.
Numerical Simulation of Electroosmotic Flow through Triangular Microchannel
NASA Astrophysics Data System (ADS)
Gnanaraj, Vaitheeswaran; Mohan, V.
2007-11-01
Numerical simulation electroosmotic flow through triangular microchannels has been developed in this paper. The governing equations consist of a 2D Poisson-Boltzman equation and a 2D Navier Stoke's with Electric Double Layer (EDL) field and velocity field in the cross-section of triangular microchannel are solved analytically. The effects of channel height, electrolyte concentration, surface potential, EDL thickness and externally applied elctric field on the velocity profile of traiangular microchannels are numerically studied. The comparison of numerical simulation results shows excellent agreement with the corresponding analytical solution. The numerical simulation shows significant influences of channel cross-section geometry and volumetric flow rate.
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.
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 Damkhler 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 Damkhler 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.
DNS of a spatially evolving hypersonic turbulent boundary layer at Mach 8
NASA Astrophysics Data System (ADS)
Liang, Xian; Li, XinLiang
2013-07-01
This paper reports the direct numerical simulation (DNS) for hypersonic turbulent boundary layer over a flat-plate at Ma ∞=8 with the ratio of wall-to-freestream temperature equal to 1.9, which indicates an extremely cold wall condition. It is primarily used to assess the wall temperature effects on the mean velocity profile, Walz equation, turbulent intensity, strong Reynolds analogy (SRA), and compressibility. The present high Mach number with cold wall condition induces strong compressibility effects. As a result, the Morkovin's hypothesis is not fully valid and so the classical SRA is also not fully consistent. However, some modified SRA is still valid at the far-wall region. It is also verified that the semi-local wall coordinate y* is better than conventional y + in analysis of statistics features in turbulent boundary layer (TBL) in hypersonic flow.
DNS of 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?oRe?o is required to achieve MDR. At Re?o 230, Virk's MDR asymptote is reached at We?o100. 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?o300) 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 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 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.
Large-eddy simulation of transitional channel flow
NASA Technical Reports Server (NTRS)
Piomelli, Ugo; Zang, Thomas A.
1990-01-01
A large-eddy simulation (LES) of transition in plane channel flow was carried out. The LES results were compared with those of a fine direct numerical simulation (DNS), and with those of a coarse DNS that uses the same mesh as the LES, but does not use a residual stress model. While at the early stages of transition, LES and coarse DNS give the same results: the presence of the residual stress model was found to be necessary to predict accurately mean velocity and Reynolds stress profiles during the late stages of transition (after the second spike stage). The evolution of single Fourier modes is also predicted more accurately by the LES than by the DNS. As small scales are generated, the dissipative character of the residual stress starts to reproduce correctly the energy cascade. As transition progresses, the flow approaches its fully developed turbulent state, the subgrid scales tend towards equilibrium, and the model becomes more accurate.
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.
High-Reynolds number turbulent boundary layers studied by numerical simulation
NASA Astrophysics Data System (ADS)
Schlatter, Philipp; Li, Qiang; Brethouwer, Geert; Johansson, Arne V.; Henningson, Dan S.
2009-11-01
Direct and large-eddy simulations (DNS and LES) of spatially developing high-Reynolds number turbulent boundary layers (Reθ up to 4300) under zero pressure gradient are studied. The inflow of the computational domain and the tripping of the boundary layer is located at low Reynolds numbers Reθ 350, a position where natural transition to turbulence can be expected. The simulation thus includes the spatial evolution of the boundary layer for an extended region, providing statistics and budget terms at each streamwise position. The data is obtained with up to O(10^10) grid points using a parallelised, fully spectral method. The DNS and LES results are critically evaluated and validated, in comparison with other relevant data, e.g. the experiments by "Osterlund et al. (1999). Quantities difficult or even impossible to measure, e.g. pressure fluctuations and complete Reynolds stress budgets, shall be discussed. In addition, special emphasis is put on a further quantification of the large-scale structures appearing in the flow, and their relation to other wall-bounded flow as e.g. channel flow. The results clearly show that with today's computer power Reynolds numbers relevant for industrial applications can be within reach for DNS/LES.
NASA Astrophysics Data System (ADS)
Druzhinin, Oleg; Troitskaya, Yliya; Zilitinkevich, Sergej
2015-04-01
Detailed knowledge of the interaction of surface water waves with the wind flow is of primary importance for correct parameterization of turbulent momentum and heat fluxes which define the energy and momentum transfer between the atmosphere and hydrosphere. The objective of the present study is to investigate the properties of the stably stratified turbulent boundary-layer (BL) air-flow over waved water surface by direct numerical simulation (DNS) at a bulk Reynolds number varying from 15000 to 80000 and the surface-wave slope up to ka = 0.2. The DNS results show that the BL-flow remains in the statistically stationary, turbulent regime if the Reynolds number (ReL) based on the Obukhov length scale and friction velocity is sufficiently large (ReL > 100). In this case, mean velocity and temperature vertical profiles are well predicted by log-linear asymptotic solutions following from the Monin-Obukhov similarity theory provided the velocity and temperature roughness parameters, z0U and z0T, are appropriately prescribed. Both z0U and z0T increase for larger surface-wave slope. DNS results also show that turbulent momentum and heat fluxes and turbulent velocity and temperature fluctuations are increased for larger wave slope (ka) whereas the mean velocity and temperature derivatives remain practically the same for different ka. Thus, we conclude that the source of turbulence enhancement in BL-flow are perturbations induced by the surface wave, and not the shear instability of the bulk flow. On the other hand, if stratification is sufficiently strong, and the surface-wave slope is sufficiently small, the BL-flow over waved surface relaminarizes in the bulk of the domain. However, if the surface-wave slope exceeds a threshold value, the velocity and temperature fluctuations remain finite in the vicinity of the critical-layer level, where the surface-wave phase velocity coincides with the mean flow velocity. We call this new stably-stratified BL-flow regime observed in our DNS a "wave-pumping" regime. We develop a theoretical model and explain the occurrence of the wave-pumping regime observed in DNS as a result of the generation of two-dimensional (2D) disturbances in the air flow under the influence of the surface wave and secondary, parametric instability of these disturbances along the surface-wave front direction. The model predicts that the wave-pumping regime occurs only for sufficiently steep waves which is in agreement with DNS results. The model prediction for the amplitudes of the wave-induced 2D disturbances in the air flow is also in good qualitative and quantitative agreement with DNS results. The results also show that increasing the bulk Reynolds number of the air-flow leads to the development of a wide spectrum of the disturbances. At a sufficiently high super-criticality we expect a transition to occur from the wave-pumping regime to a fully-developed, turbulent BL-flow regime, even at high Richardson number when the air flow over a smooth surface relaminarizes. This work was supported by RFBR (project No. 14-05-00367) and by RSF (project No. 14-17 -0086).
Numerical simulator for superconducting integrated circuits
Rollins, J.G. )
1991-02-01
Recent advances in materials technology may greatly reduce the cost of producing and operating superconducting (SC) integrated circuits (IC's). In anticipation of the development of these new IC's, this paper describes a computer program and models for simulation of Josephson junction switching circuits. The program uses SPICE like input syntax and is capable of both static and dynamic analysis. The basic operation of Josephson logic is explained and several example simulations are given.
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 error in groundwater flow and solute transport simulation
NASA Astrophysics Data System (ADS)
Woods, Juliette A.; Teubner, Michael D.; Simmons, Craig T.; Narayan, Kumar A.
2003-06-01
Models of groundwater flow and solute transport may be affected by numerical error, leading to quantitative and qualitative changes in behavior. In this paper we compare and combine three methods of assessing the extent of numerical error: grid refinement, mathematical analysis, and benchmark test problems. In particular, we assess the popular solute transport code SUTRA [Voss, 1984] as being a typical finite element code. Our numerical analysis suggests that SUTRA incorporates a numerical dispersion error and that its mass-lumped numerical scheme increases the numerical error. This is confirmed using a Gaussian test problem. A modified SUTRA code, in which the numerical dispersion is calculated and subtracted, produces better results. The much more challenging Elder problem [Elder, 1967; Voss and Souza, 1987] is then considered. Calculation of its numerical dispersion coefficients and numerical stability show that the Elder problem is prone to error. We confirm that Elder problem results are extremely sensitive to the simulation method used.
An idealized numerical simulation of mammatus-like clouds
NASA Astrophysics Data System (ADS)
Kanak, Katharine M.; Straka, Jerry M.
2006-01-01
A three-dimensional numerical simulation of mammatus-like clouds is presented. A portion of a cirrus outflow anvil cloud is simulated including cloud ice and snow microphysical representations. The simulated mammatus clouds appear in a cellular pattern and are compared with the few available previously published physical observations of mammatus.
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 powered-lift flows
NASA Technical Reports Server (NTRS)
Van Dalsem, William R.; Chawla, Kalpana; Smith, Merritt H.; Abeloff, Patricia A.
1990-01-01
This article presents work performed at NASA's Ames Research Center involving the application of Computational Fluid Dynamics (CFD) to the prediction of flows encountered by powered-lift aircraft operating in ground effect. These flows are characterized by jet and jet-induced flows interacting with the ground and aerodynamic surfaces. Over the last five years, work has progressed from simulating the interaction of a single jet impacting on a ground plane, through the simulation of a delta planform with multiple jets in ground effect, to an ongoing effort to simulate the complete flow about a Harrier AV-8B in ground effect. Efforts have also been made to predict the thermal interaction between hot propulsive jets and a landing surface of arbitrary thermal properties. Progress to date in each of these areas will be outlined.
Numerical simulations of plasma double layers
NASA Technical Reports Server (NTRS)
Goertz, C. K.; Borovsky, J. E.
1983-01-01
The results of analytical studies of quasi-static electric fields along geomagnetic field lines are discussed. The calculations were targeted at the structure, generation mechanisms and stability parameters. The field consists of two oppositely charged layers, either weakly or strongly charged, with an electric field between. Existence conditions are defined for the double layer field and balancing requirements are explored. Details of the simulation techniques, i.e., particle in cell and Vlasov simulations, for studying the double layer are outlined, noting that both periodic and quasi-periodic simulations are used. Solutions to Poisson's equation for fixed and floating point boundary conditions are generated. Finally, attention is also given to oblique and two-dimensional magnetic double layers.
Malapaka, Shiva Kumar; Mueller, Wolf-Christian
2013-09-01
Statistical properties of the Sun's photospheric turbulent magnetic field, especially those of the active regions (ARs), have been studied using the line-of-sight data from magnetograms taken by the Solar and Heliospheric Observatory and several other instruments. This includes structure functions and their exponents, flatness curves, and correlation functions. In these works, the dependence of structure function exponents ({zeta}{sub p}) of the order of the structure functions (p) was modeled using a non-intermittent K41 model. It is now well known that the ARs are highly turbulent and are associated with strong intermittent events. In this paper, we compare some of the observations from Abramenko et al. with the log-Poisson model used for modeling intermittent MHD turbulent flows. Next, we analyze the structure function data obtained from the direct numerical simulations (DNS) of homogeneous, incompressible 3D-MHD turbulence in three cases: sustained by forcing, freely decaying, and a flow initially driven and later allowed to decay (case 3). The respective DNS replicate the properties seen in the plots of {zeta}{sub p} against p of ARs. We also reproduce the trends and changes observed in intermittency in flatness and correlation functions of ARs. It is suggested from this analysis that an AR in the onset phase of a flare can be treated as a forced 3D-MHD turbulent system in its simplest form and that the flaring stage is representative of decaying 3D-MHD turbulence. It is also inferred that significant changes in intermittency from the initial onset phase of a flare to its final peak flaring phase are related to the time taken by the system to reach the initial onset phase.
Coupled numerical simulation of hot stamping process and experimental verification
NASA Astrophysics Data System (ADS)
Li, Ye; Ying, Liang; Hu, Ping; Shi, Dongyong; Zhao, Xi; Dai, Minghua
2013-05-01
Hot stamping process is a high non-linear process showing the effect on thermal, mechanical and metallurgical phenomena as they relate to each other. In order to carry out this coupled numerical simulation, fundamental thermal properties such as interfacial heat transfer coefficient and convection heat transfer coefficient as well as crucial mechanics properties were first investigated. Hot stamping tools with cooling system which has been optimized by genetic algorithm were employed in the simulation. The coupled numerical simulation to the whole hot stamping process was built with the ABAQUS/Explicit and FLUENT. Experiment was setup and the results of blank temperature and spring-back were compared with the results of coupled numerical simulation. The comparisons show that the simulation results of numerical model are consistent with experimental results.
Numerical simulation of instability and transition physics
NASA Technical Reports Server (NTRS)
Streett, C. L.
1990-01-01
The study deals with the algorithm technology used in instability and transition simulations. Discretization methods are outlined, and attention is focused on high-order finite-difference methods and high-order centered-difference formulas. One advantage of finite-difference methods over spectral methods is thought to be in implementation of nonrigorous boundary conditions. It is suggested that the next significant advances in the understanding of transition physics and the ability to predict transition will come with more physically-realistic simulations. Compressible-flow algorithms are discussed, and it is noted that with further development, exploration of bypass mechanism on simple bodies at high speed would be possible.
Numerical simulations on electrostatic hydrogen cyclotron instabilities
Okuda, H.; Cheng, C.Z.; Lee, W.W.
1981-02-01
Both one- and two-dimensional particle simulation models have been used to study the nonlinear behavior of the electrostatic hydrogen cyclotron instabilities driven by the electron current along magnetic field. It is found that the instability saturates as a result of electron velocity space diffusion along magnetic field. The cyclotron waves remain highly coherent in the nonlinear stage. When the electron drift speed is comparable to thermal speed, substantial ion heating as well as particle cross-field diffusion comparable to Bohm diffusion has been observed. Comparisons of the simulation results with the theoretical predictions and the observations in both laboratory and space plasmas are discussed.
Numerical simulation of interacting vortex tubes
NASA Astrophysics Data System (ADS)
Pumir, Alain; Kerr, Robert M.
1987-04-01
The structure of the cores of interacting vortex tubes in three-dimensional incompressible hydrodynamics has been simulated by a pseudospectral method. A fast reconnection is observed for Reynolds numbers of order 1000. At higher Reynolds numbers, the core tends to flatten, suggesting the formation of vortex ribbons.
Numerical simulation of quasi-multifractal diffusion process
Saichev, A. I. Filimonov, V. A.
2008-08-15
The properties of quasi-multifractal diffusion process are discussed. A discrete model of the process is constructed, and a method is proposed for calculating the quasi-multifractal spectrum, based on statistical processing of its realizations. An analysis of multifractal properties performed by numerical simulation of the quasi-multifractal spectrum is qualitatively substantiated by examining realizations of the simulated process. The results of numerical simulations suggest that there are three distinct scaling regions. Special attention is given to comparative analyses between numerical and analytical results and between realizations of the proposed process and the well-known multifractal random walk.
Numerical simulation of cross field amplifiers
Eppley, K.
1990-01-01
Cross field amplifiers (CFA) have been used in many applications where high power, high frequency microwaves are needed. Although these tubes have been manufactured for decades, theoretical analysis of their properties is not as highly developed as for other microwave devices such as klystrons. One feature distinguishing cross field amplifiers is that the operating current is produced by secondary emission from a cold cathode. This removes the need for a heater and enables the device to act as a switch tube, drawing no power until the rf drive is applied. However, this method of generating the current does complicate the simulation. We are developing a simulation model of cross field amplifiers using the PIC code CONDOR. We simulate an interaction region, one traveling wavelength long, with periodic boundary conditions. An electric field with the appropriate phase velocity is imposed on the upper boundary of the problem. Evaluation of the integral of E{center dot}J gives the power interchanged between the wave and the beam. Given the impedance of the structure, we then calculate the change in the traveling wave field. Thus we simulate the growth of the wave through the device. The main advance of our model over previous CFA simulations is the realistic tracking of absorption and secondary emission. The code uses experimental curves to calculate secondary production as a function of absorbed energy, with a theoretical expression for the angular dependence. We have used this code to model the 100 MW X-band CFA under construction at SLAC, as designed by Joseph Feinstein and Terry Lee. We are examining several questions of practical interest, such as the power and spectrum of absorbed electrons, the minimum traveling wave field needed to initiate spoke formation, and the variation of output power with dc voltage, anode-cathode gap, and magnetic field. 5 refs., 8 figs.
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 simulation of oil pool boundary evolution
NASA Astrophysics Data System (ADS)
Khudobina, Yulia; Bubenchikov, Aleksey; Bubenchikov, Mikhail; Matvienko, Oleg; Libin, Eduard
2016-01-01
The study of spatial distribution of hydrocarbon resources and forecasting their geographical location is of great importance for the most complete recovery of hydrocarbons from deposits. The present study gives new mathematical results in the theory of stratified fluid flow in a porous medium. This paper analyzes the evolution of oil pool boundary basing on vortex numerical model for movement of the boundary separating fluids of different densities. It presents the investigation of how the location of light fluid regarding the heavier fluid influences on the changes in the boundary between two media in case of various shifting of the well.
Numerical Simulation of Taylor Cone-Jet
NASA Astrophysics Data System (ADS)
Toledo, Ronne
The Taylor cone-jet is a particular type of electrohydrodynamic phenomenon where electrostatic stresses and surface tension effects shape the interface of the jet in a peculiar conical shape. A thin jet is issued from the cone apex that further breaks up into a fine aerosol. Due to its monodispersive properties, this fine aerosol has found a number of applications, ranging from mass spectrometry, colloidal space propulsion, combustion, nano-fabrication, coating/painting, and many others. In this study, a general non-dimensional analysis is performed to derive the governing equations and boundary conditions. In accordance with the observations of Gamero-Castano (2010), noting that droplet electric potential is insensitive to the flow rate conditions, a particular set of characteristic parameters is proposed, based on the terminal jet diameter. In order to solve the non-dimensional set of governing equations and boundary conditions, a numerical method combining the Boundary Element Method and the Finite Volume Method is developed. Results of electric current have shown good agreement with numerical and experimental data available in the literature. The main feature of the algorithm developed is related to the decoupling of the electrostatic from the hydrodynamic problem, allowing us to accurately prescribe the far field electric potential boundary conditions away from the hydrodynamic computational domain used to solve the hydrodynamics of the transition region near the cone apex.
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.
Direct Numerical Simulations of Whole Blood
NASA Astrophysics Data System (ADS)
Macmeccan, Robert; Clausen, Jonathan; Neitzel, Paul; Aidun, Cyrus
2007-11-01
Hundreds of three-dimensional, deformable red blood cells and platelets are simulated at physiologic hematocrit using the recently developed lattice-Boltzmann--finite-element method. This method provides the efficiency and versatility to simulate large suspensions while accurately modeling biconcave red blood cells as elastic membranes with bending stiffness and an internal solution of hemoglobin. Bulk rheology of whole blood at continuum-level scales is quantitatively described with effective viscosity and shear-thinning behavior. Suspension microstructure and red-blood-cell deformation compares well with experimental measures. The local stress environment that platelets experience in whole blood is described as it pertains to shear-mediated platelet adhesion with 25% of platelets experiencing a surface shear stress greater than twice the effective suspension stress.
Numerical simulations of bent, disrupted radio jets
NASA Technical Reports Server (NTRS)
Loken, Chris; Burns, Jack O.
1993-01-01
We present preliminary results from three-dimensional hydrodynamical simulations designed to investigate the physics of jet bending and disruption. The specific scenario considered here involves a mildly supersonic jet crossing a contact discontinuity at the interface between the interstellar medium (ISM) and the intercluster medium (ICM) and then encountering a cross-wind in the ICM. The resultant morphologies show many of the features observed in radio sources including jet flaring, bending, and extended tails.
Numerical Simulation of 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.
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.
Scramjet Propulsive Flowpath Design and Numerical Simulation
NASA Astrophysics Data System (ADS)
Li, Jian-ping; Song, Wen-yan; Liu, Xin
2014-06-01
The integrated propulsive flowpath of scramjet configuration was preliminarily designed and analyzed in this paper. The flow-fields characteristics and performance of the designed two-dimensional integrated propulsive flowpath were numerically calculated under various equivalent fuel-air ratio conditions, using computational fluid dynamics methods. The calculation results were then compared with the experimental data on some typical conditions, and the flow-field and performance of the integrated scramjet flowpath with different equivalent fuel-air ratios were analyzed and discussed in detail. The investigation results from these efforts showed that: (1) the inlet function was beyond disturbances by combustion induced shock wave and pressure fluctuations under the equivalent fuel-air ratio condition of 1.0, which well satisfied the design requirements; (2) with the increasing equivalent fuel-air ratio, the combustion intensity in the combustor was significantly enhanced, resulting in an increasing net-thrust of the propulsive flowpath.
Numerical simulation of 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 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 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 optically trapped particles
NASA Astrophysics Data System (ADS)
Volpe, Giorgio; Volpe, Giovanni
2014-07-01
Some randomness is present in most phenomena, ranging from biomolecules and nanodevices to financial markets and human organizations. However, it is not easy to gain an intuitive understanding of such stochastic phenomena, because their modeling requires advanced mathematical tools, such as sigma algebras, the It formula and martingales. Here, we discuss a simple finite difference algorithm that can be used to gain understanding of such complex physical phenomena. In particular, we simulate the motion of an optically trapped particle that is typically used as a model system in statistical physics and has a wide range of applications in physics and biophysics, for example, to measure nanoscopic forces and torques.
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 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.
Floret Test, Numerical Simulations of the Dent, Comparison with Experiments
Lefran?ois, A; Cutting, J; Gagliardi, F; Tarver, C; Tran, T
2006-02-14
The Floret test has been developed as a screening test to study the performance of a small amount of HE. Numerical simulations have been performed recently using CTH. The objective of this study is to perform numerical simulations in order to better understand the shock waves interactions, involved in the dent formation. Different 3D wedge configurations have been tested using the Ignition and Growth reactive flow model for the HE receptor with Ls-Dyna.
Numerical 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 simulations of moon-ringlet interaction
NASA Astrophysics Data System (ADS)
Hanninen, J.
1993-05-01
Nonaxisymmetric ring features excited by perturbations of shepherd satellites are studied in terms of direct particle simulations using Aarseth's N-body integrator combined with the calculation of particle-particle impacts. Interaction parameters typical to Saturn's F-ring are investigated. The generation of clumps by external satellites is verified, but the interparticle collisions tend to smooth sharp features. Using F-ring parameters the clumps are observed to cover the total azimuthal length, but it is not clear whether these azimuthally overlapping clumps would be detectable in the actual F-ring. Gravitational scattering by ring particles increases the velocity dispersion, smearing regular azimuthal features at least in the rings of low optical depths. Considerable accretion is observed to occur, particles sticking pairwise to each other, even if the tendency of the particles to accrete is artificially reduced in the simulations. A new explanation for the braided appearance of the F-ring is proposed, based on the interaction between the shepherding satellites and the ring containing embedded moonlets. In our model the braiding is a dynamic phenomenon: the braids are destroyed and recreated in a cyclical manner.
Numerical and laboratory simulations of auroral acceleration
NASA Astrophysics Data System (ADS)
Gunell, H.; De Keyser, J.; Mann, I.
2013-10-01
The existence of parallel electric fields is an essential ingredient of auroral physics, leading to the acceleration of particles that give rise to the auroral displays. An auroral flux tube is modelled using electrostatic Vlasov simulations, and the results are compared to simulations of a proposed laboratory device that is meant for studies of the plasma physical processes that occur on auroral field lines. The hot magnetospheric plasma is represented by a gas discharge plasma source in the laboratory device, and the cold plasma mimicking the ionospheric plasma is generated by a Q-machine source. In both systems, double layers form with plasma density gradients concentrated on their high potential sides. The systems differ regarding the properties of ion acoustic waves that are heavily damped in the magnetosphere, where the ion population is hot, but weakly damped in the laboratory, where the discharge ions are cold. Ion waves are excited by the ion beam that is created by acceleration in the double layer in both systems. The efficiency of this beam-plasma interaction depends on the acceleration voltage. For voltages where the interaction is less efficient, the laboratory experiment is more space-like.
Numerical simulation of 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.
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 Simulations of a Flux Rope Ejection
NASA Astrophysics Data System (ADS)
Pagano, P.; Mackay, D. H.; Poedts, S.
2015-03-01
Coronal mass ejections (CMEs) are the most violent phenomena observed on the Sun. One of the most successful models to explain CMEs is the flux rope ejection model, where a magnetic flux rope is expelled from the solar corona after a long phase along which the flux rope stays in equilibrium while magnetic energy is being accumulated. However, still many questions are outstanding on the detailed mechanism of the ejection and observations continuously provide new data to interpret and put in the context. Currently, extreme ultraviolet (EUV) images from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) are providing new insights into the early phase of CME evolution. In particular, observations show the ejection of magnetic flux ropes from the solar corona and how they evolve into CMEs. However, these observations are difficult to interpret in terms of basic physical mechanisms and quantities, thus, we need to compare equivalent quantities to test and improve our models. In our work, we intend to bridge the gap between models and observations with our model of flux rope ejection where we consistently describe the full life span of a flux rope from its formation to ejection. This is done by coupling the global non-linear force-free model (GNLFFF) built to describe the slow low- β formation phase, with a full MHD simulation run with the software MPI-AMRVAC, suitable to describe the fast MHD evolution of the flux rope ejection that happens in a heterogeneous β regime. We also explore the parameter space to identify the conditions upon which the ejection is favoured (gravity stratification and magnetic field intensity) and we produce synthesised AIA observations (171 Å and 211 Å). To carry this out, we run 3D MHD simulation in spherical coordinates where we include the role of thermal conduction and radiative losses, both of which are important for determining the temperature distribution of the solar corona during a CME. Our model of flux rope ejection is successful in realistically describing the entire life span of a flux rope and we also set some conditions for the backgroud solar corona to favour the escape of the flux rope, so that it turns into a CME. Furthermore, our MHD simulation reproduces many of the features found in the AIA observations.
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 simulation of supersonic gap flow.
Jing, Xu; Haiming, Huang; Guo, Huang; Song, Mo
2015-01-01
Various gaps in the surface of the supersonic aircraft have a significant effect on airflows. In order to predict the effects of attack angle, Mach number and width-to-depth ratio of gap on the local aerodynamic heating environment of supersonic flow, two-dimensional compressible Navier-Stokes equations are solved by the finite volume method, where convective flux of space term adopts the Roe format, and discretization of time term is achieved by 5-step Runge-Kutta algorithm. The numerical results reveal that the heat flux ratio is U-shaped distribution on the gap wall and maximum at the windward corner of the gap. The heat flux ratio decreases as the gap depth and Mach number increase, however, it increases as the attack angle increases. In addition, it is important to find that chamfer in the windward corner can effectively reduce gap effect coefficient. The study will be helpful for the design of the thermal protection system in reentry vehicles. PMID:25635395
Numerical Simulation of Supersonic Gap Flow
Jing, Xu; Haiming, Huang; Guo, Huang; Song, Mo
2015-01-01
Various gaps in the surface of the supersonic aircraft have a significant effect on airflows. In order to predict the effects of attack angle, Mach number and width-to-depth ratio of gap on the local aerodynamic heating environment of supersonic flow, two-dimensional compressible Navier-Stokes equations are solved by the finite volume method, where convective flux of space term adopts the Roe format, and discretization of time term is achieved by 5-step Runge-Kutta algorithm. The numerical results reveal that the heat flux ratio is U-shaped distribution on the gap wall and maximum at the windward corner of the gap. The heat flux ratio decreases as the gap depth and Mach number increase, however, it increases as the attack angle increases. In addition, it is important to find that chamfer in the windward corner can effectively reduce gap effect coefficient. The study will be helpful for the design of the thermal protection system in reentry vehicles. PMID:25635395
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 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.
Relevance of numerical simulations to booming sand
NASA Astrophysics Data System (ADS)
Richard, Patrick; McNamara, Sean; Tankeo, Merline
2012-01-01
We have performed a simulation study of three-dimensional cohesionless granular flows down an inclined chute. We find that the oscillations observed in [L. E. Silbert, Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.94.098002 94, 098002 (2005)] near the angle of repose are harmonic vibrations of the lowest normal mode. Their frequencies depend on the contact stiffness as well as on the depth of the flow. Could these oscillations account for the phenomena of booming sand? We estimate an effective contact stiffness from the Hertz law, but this leads to frequencies that are several times higher than observed. However, the Hertz law also predicts interpenetrations of a few nanometers, indicating that the oscillations frequencies are governed by the surface stiffness, which can be much lower than the bulk one. This is in agreement with previous studies ascribing the ability to sing to the presence of a soft coating on the grain surface.
Relevance of numerical simulations to booming sand.
Richard, Patrick; McNamara, Sean; Tankeo, Merline
2012-01-01
We have performed a simulation study of three-dimensional cohesionless granular flows down an inclined chute. We find that the oscillations observed in [L. E. Silbert, Phys. Rev. Lett. 94, 098002 (2005)] near the angle of repose are harmonic vibrations of the lowest normal mode. Their frequencies depend on the contact stiffness as well as on the depth of the flow. Could these oscillations account for the phenomena of "booming sand"? We estimate an effective contact stiffness from the Hertz law, but this leads to frequencies that are several times higher than observed. However, the Hertz law also predicts interpenetrations of a few nanometers, indicating that the oscillations frequencies are governed by the surface stiffness, which can be much lower than the bulk one. This is in agreement with previous studies ascribing the ability to sing to the presence of a soft coating on the grain surface. PMID:22400502
Numerical simulation of tides in Ontario Lacus
NASA Astrophysics Data System (ADS)
Vincent, David; Karatekin, Ozgr
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.
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.
Batman-cracks. Observations and numerical simulations
NASA Astrophysics Data System (ADS)
Selvadurai, A. P. S.; Busschen, A. Ten; Ernst, L. J.
1991-05-01
To ensure mechanical strength of fiber reinforced plastics (FRP), good adhesion between fibers and the matrix is considered to be an essential requirement. An efficient test of fiber-matrix interface characterization is the fragmentation test which provides information about the interface slip mechanism. This test consists of the longitudinal loading of a single fiber which is embedded in a matrix specimen. At critical loads the fiber experiences fragmentation. This fragmentation will terminate depending upon the shear-slip strength of the fiber-matrix adhesion, which is inversely proportional to average fragment lengths. Depending upon interface strength characteristics either bond or slip matrix fracture can occur at the onset of fiber fracture. Certain particular features of matrix fracture are observed at the locations of fiber fracture in situations where there is sufficient interface bond strength. These refer to the development of fractures with a complex surface topography. The experimental procedure involved in the fragmentation tests is discussed and the boundary element technique to examine the development of multiple matrix fractures at the fiber fracture locations is examined. The mechanics of matrix fracture is examined. When bond integrity is maintained, a fiber fracture results in a matrix fracture. The matrix fracture topography in a fragmentation test is complex; however, simplified conoidal fracture patterns can be used to investigate the crack extension phenomena. Via a mixed-mode fracture criterion, the generation of a conoidal fracture pattern in the matrix is investigated. The numerical results compare favorably with observed experimental data derived from tests conducted on fragmentation test specimens consisting of a single glass fiber which is embedded in a polyester matrix.
Numerical simulation of baroclinic Jovian vortices
NASA Astrophysics Data System (ADS)
Achterberg, R. K.; Ingersoll, A. P.
1994-02-01
We examine the evolution of baroclinic vortices in a time-dependent, nonlinear numerical model of a Jovian atmosphere. The model uses a normal-mode expansion in the vertical, using the barotropic and first two baroclinic modes. Results for the stability of baroclinic vortices on an f plane in the absence of a mean zonal flow are similar to results of Earth vortex models, although the presence of a fluid interior on the Jovian planets shifts the stability boundaries to smaller length scales. The presence of a barotropic mean zonal flow in the interior stabilizes vortices against instability and significantly modifies the finite amplitude form of baroclinic instabilities. The effect of a zonal flow on a form of barotropic instability produces periodic oscillations in the latitude and longitude of the vortex as observed at the level of the cloud tops. This instability may explain some, but not all, observations of longitudinal oscillations of vortices on the outer planets. Oscillations in aspect ratio and orientation of stable vortices in a zonal shear flow are observed in this baroclinic model, as in simpler two-dimensional models. Such oscillations are also observed in the atmospheres of Jupiter and Neptune. The meridional propagation and decay of vortices on a beta plane is inhibited by the presence of a mean zonal flow. The direction of propagation of a vortex relative to the mean zonal flow depends upon the sign of the meridional potential vorticity gradient; combined with observations of vortex drift rates, this may provide a constraint on model assumption for the flow in the deep interior of the Jovian planets.
Numerical simulation of electrospray in the cone-jet mode.
Herrada, M A; Lpez-Herrera, J M; Gan-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
Numerical simulation of reversing buoyancy gravity currents
NASA Astrophysics Data System (ADS)
Radhakrishnan, Senthil; Lenk, Erik; Boekels, Michael; Meiburg, Eckart
2012-11-01
Sediment laden fluid propagates as an underflow when its bulk density is higher than the density of the ambient fluid. If the density of the interstitial fluid in gravity current is smaller than the density of the ambient fluid, the gravity current can become positively buoyant after sufficient particles have settled. The current then lifts off from the bottom surface and travels as a surface gravity current over the heavier ambient fluid. These types of currents, where the buoyancy reverses its direction, have been observed when sediment laden fresh water enters the sea or during volcanic eruption that creates a pyroclastic flow. We use a lock-exchange configuration with mono-disperse and bi-disperse particles to study the lofting characteristics of reversing buoyancy currents. This talk will focus on results obtained from Large-eddy Simulation of high Reynolds number currents. In particular, the deposit profiles show a sharp decay at the lift-off point unlike a ground hugging turbidity current whose deposit profile has a slow monotonic decay from the lock region.
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.
Numerical simulation of thermocapillary wetting suppression
NASA Astrophysics Data System (ADS)
Chen, Jyh-Chen; Kuo, C.-W.; Neitzel, G. Paul
2002-11-01
The commercial code FIDAP, based on the finite-element method, is used to investigate a nonwetting phenomenon that occurs when a liquid drop is pressed against a solid wall held at a sufficiently lower temperature. In this situation, an interstitial gas film is induced by thermocapillary convection and separates the drop from the wall, forming a self-lubricating system. The flow in both the gas and liquid phases must be computed to simulate the non-wetting phenomenon. We explore the velocity and thermal fields of both the interstitial film and the liquid drop. A steady-state solution is discussed, with many parameters being considered, i.e., drop/wall temperature differences and relative displacement from the point of first apparent contact, as well as varying drop liquids. The results of the present study indicate that a silicone-oil drop may experience nonwetting while a water drop may not. The mechanism promoting the existence or non-existence of the nonwetting state is also discussed.
Numerical simulation of aluminum extrusion processes
NASA Astrophysics Data System (ADS)
Hughes, T. J.; Muller, A.
1995-04-01
This presentation describes a research program directed towards the development of automated design procedures for aluminum extrusion technology. The objective is to eliminate costly trial and error by being able to simultaneously design the product, die, billet, and process (e.g.. extrusion temperatures and speeds, uniformizing metal flow, etc.), within constraints of feasibility, and satisfying objectives including, but not limited to, optimizing shape, surface finish, and properties of the product, processing costs, time to market, and full utilization of capabilities. The approach is based on the development of efficient and effective analysis of the whole processing system employing newly developed finite element solution technologies for complex, multi region, multiphysical behavior. Generalizations of these methodologies to include Arbitrary Lagrangian-Eulerian (ALE) mesh descriptions for nonlinear, elastic viscoplastic mechanical constitution equations will allow the faithful modeling of the metal flow within the die system and the accurate attainment of final shape upon exit. Automatic meshing and adaptive remeshing will insure efficient and accurate simulation of the entire forming process. New element technologies facilitating the use of general meshing procedures for difficult metal-forming processes involving a variety of kinematical constraints, such as incompressibility, contact, etc., are utilized. Feature based design methodologies, parametric modeling, and knowledge-based engineering techniques will constitute the fundamental methodologies for representing designs, managing the hierarchy of analysis models, performing model reduction and feature removal, and effectively utilizing design knowledge.
Probabilistic Approach to Numerical Simulation of Fracture
NASA Astrophysics Data System (ADS)
Gerasimov, Alexander
2013-06-01
The natural heterogeneity of real materials structure influencing on distribution of material physicomechanical characteristics (PMC) is one of the factors determining character of destruction. The introduction of the given factor in the equations of mechanics of a deformable solid is possible at use probabilistic laws of distribution PMC on volume of a considered design. There are problems where the fragmentation is mainly probabilistic process: explosive destruction axisymmetric shells where character of blasting fragmentation are beforehand unknown. Determining influence of heterogeneity of material structure is shown as well in problems punching thin barrier. In order that simulated process of a fragmentation reflected a real picture of behavior of the destroyed bodies, it is necessary to bring in casual distribution of initial deviations strength properties from rating value to PMC of a body. In work the explosive fragmentation of the shells, a fragmentation of a barrier and an shell after barrier piercing, punching thin barrier on a normal and under an angle, crushing of metal rings, process of high-speed impact of the laminated - spaced barrier with the steel spheres is considered.
Validated numerical simulation model of a dielectric elastomer generator
NASA Astrophysics Data System (ADS)
Foerster, Florentine; Moessinger, Holger; Schlaak, Helmut F.
2013-04-01
Dielectric elastomer generators (DEG) produce electrical energy by converting mechanical into electrical energy. Efficient operation requires homogeneous deformation of each single layer. However, by different internal and external influences like supports or the shape of a DEG the deformation will be inhomogeneous and hence negatively affect the amount of the generated electrical energy. Optimization of the deformation behavior leads to improved efficiency of the DEG and consequently to higher energy gain. In this work a numerical simulation model of a multilayer dielectric elastomer generator is developed using the FEM software ANSYS. The analyzed multilayer DEG consists of 49 active dielectric layers with layer thicknesses of 50 ?m. The elastomer is silicone (PDMS) while the compliant electrodes are made of graphite powder. In the simulation the real material parameters of the PDMS and the graphite electrodes need to be included. Therefore, the mechanical and electrical material parameters of the PDMS are determined by experimental investigations of test samples while the electrode parameters are determined by numerical simulations of test samples. The numerical simulation of the DEG is carried out as coupled electro-mechanical simulation for the constant voltage energy harvesting cycle. Finally, the derived numerical simulation model is validated by comparison with analytical calculations and further simulated DEG configurations. The comparison of the determined results show good accordance with regard to the deformation of the DEG. Based on the validated model it is now possible to optimize the DEG layout for improved deformation behavior with further simulations.
Numerical Simulations of the Wardle Instability
NASA Astrophysics Data System (ADS)
Falle, S. A. E. G.; Hartquist, T. W.; van Loo, S.
2009-04-01
In dense interstellar clouds, the ionisation fraction is so low that the material may be considered to be made up of two fluids: a perfectly conducting fluid consisting of the ions and electrons and a neutral fluid consisting of atomic hydrogen. These interact via collisions, but the imperfect coupling leads to a finite resistivity (ambi-polar diffusion). Under these conditions, there exist shock structures, called C-shocks, in which the dissipation is due to resistivity rather than viscosity (Draine 1980). Wardle (1990, 1991a,b) showed that C-shocks with Alfven Mach numbers greater than ? 5 are subject to a transverse corrugation instability and nonlinear calculations have shown that this leads to the formation of dense fingers of neutral gas (Toth 1995a,b; Stone 1997; Neufeld & Stone 1997; MacLow & Smith 1997). However, the instability relies on a separation between the conducting fluid and the neutral fluid, which does not occur if timescale for ionisation equilibrium is short compared to the flow time through the shock structure. The ionisation fraction is then simply a function of neutral density and our simulations show that this does indeed suppress the instability. Since the timescale for ionisation equilibrium is always short compared to the flow time in dense clouds, this means that the instability does not occur unless charged grains play a significant role. Instability is possible in this case because a fluid composed of charged grains does undergo separation from the neutrals and the grain mass fraction influences the ionisation fraction. We use the multi-fluid code described in Falle (2003), which includes the grain fluid, to show that the instability can occur in such cases.
Numerical simulation of photoexcited polaron states in water
NASA Astrophysics Data System (ADS)
Zemlyanaya, E. V.; Volokhova, A. V.; Lakhno, V. D.; Amirkhanov, I. V.; Puzynin, I. V.; Puzynina, T. P.; Rikhvitskiy, V. S.; Atanasova, P. Kh.
2015-10-01
We consider the dynamic polaron model of the hydrated electron state on the basis of a system of three nonlinear partial differential equations with appropriate initial and boundary conditions. A parallel numerical algorithm for the numerical solution of this system has been developed. Its effectiveness has been tested on a few multi-processor systems. A numerical simulation of the polaron states formation in water under the action of the ultraviolet range laser irradiation has been performed. The numerical results are shown to be in a reasonable agreement with experimental data and theoretical predictions.
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.
NASA Astrophysics Data System (ADS)
Druzhinin, Oleg; Ostrovsky, Lev
2015-04-01
The interaction between small-scale turbulence and internal gravity waves (IWs) plays an important role in the processes of mixing which have direct impact on the dynamics of seasonal pycnocline in the ocean. Among many interesting and practically important aspects of this interaction are the effects of damping of IWs by turbulence on the one hand, and the possibility of the enhancement of turbulence by IWs on the other hand. Previously these effects were studied mostly in laboratory experiments. The present study presents the results of direct numerical simulation (DNS) of the IW-turbulence interaction. We perform DNS of the dynamics of small-scale turbulence near a pycnocline in the presence of monochromatic internal gravity wave propagating along a pycnocline. Small-scale turbulence is induced in a horizontal layer at some distance above the pycnocline. The velocity and density fields of IW propagating in the pycnocline are also prescribed as initial condition, and the IW wavelength is considered to be by the order of magnitude larger as compared to the initial turbulence integral length scale. Stratification in the pycnocline is considered to be sufficiently strong so that the effects of turbulent mixing remain negligible. In order to study the effect of damping of IW by turbulence, we firstly consider a stationary forced turbulence. The DNS results show that the observed IW damping rate is well predicted by a theory based on the semi-empirical approach, but only in the case where turbulence is sufficiently strong to be only weakly perturbed by the internal wave. However, the theory overestimates the damping rate almost by the order of magnitude if IW amplitude is of the order or larger as compared to the turbulence amplitude. The effect of the IW on the turbulence dynamics is further studied in the case where IW amplitude is of the order of the initial turbulence amplitude. In this case, turbulence is not supported by additional forcing and the effect of damping of IW by turbulence remains negligible. The DNS results show that in the absence of IW turbulence decays, but its decay rate is reduced in the vicinity of the pycnocline where stratification effects are significant. In this case, at sufficiently late times most of turbulent energy is located in a layer close to the pycnocline center. Here turbulent eddies are collapsed in the vertical direction and acquire the "pancake" shape. IW modifies turbulence dynamics, in that the turbulence kinetic energy (TKE) is significantly enhanced as compared to the TKE in the absence of IW. As in the case without IW, most of turbulent energy is localized in the vicinity of the pycnocline center. Here the TKE spectrum is considerably enhanced in the entire wavenumber range as compared to the TKE spectrum in the absence of IW. This work was supported by RFBR (project No. 14-05-00367).
Numerical simulation of porosity-free titanium dental castings.
Wu, M; Augthun, M; Schdlich-Stubenrauch, J; Sahm, P R; Spiekermann, H
1999-08-01
The objective of this research was to analyse, predict and control the porosity in titanium dental castings by the use of numerical simulation. A commercial software package (MAGMASOFT) was used. In the first part of the study, a model casting (two simplified tooth crowns connected by a connector bar) was simulated to analyse shrinkage porosity. Secondly, gas pores were numerically examined by means of a ball specimen with a "snake" sprue. The numerical simulation results were compared with the experimental casting results, which were made on a centrifugal casting machine. The predicted shrinkage levels coincided well with the experimentally determined levels. Based on the above numerical analyses, an optimised running and gating system design for the crown model was proposed. The numerical filling and solidification results of the ball specimen showed that this simulation model could be helpful for the explanation of the experimentally indicated gas pores. It was concluded that shrinkage porosity in titanium dental casting was predictable, and it could be minimised by improving the running and gating system design. Entrapped gas pores can be explained from the simulation results of the mould filling and solidification. PMID:10467947
Evaluation of a vortex-based subgrid stress model using DNS databases
NASA Technical Reports Server (NTRS)
Misra, Ashish; Lund, Thomas S.
1996-01-01
The performance of a SubGrid Stress (SGS) model for Large-Eddy Simulation (LES) developed by Misra k Pullin (1996) is studied for forced and decaying isotropic turbulence on a 32(exp 3) grid. The physical viability of the model assumptions are tested using DNS databases. The results from LES of forced turbulence at Taylor Reynolds number R(sub (lambda)) approximately equals 90 are compared with filtered DNS fields. Probability density functions (pdfs) of the subgrid energy transfer, total dissipation, and the stretch of the subgrid vorticity by the resolved velocity-gradient tensor show reasonable agreement with the DNS data. The model is also tested in LES of decaying isotropic turbulence where it correctly predicts the decay rate and energy spectra measured by Comte-Bellot & Corrsin (1971).
Compressible Turbulent Flow Numerical Simulations of Tip Vortex Cavitation
NASA Astrophysics Data System (ADS)
Khatami, F.; van der Weide, E.; Hoeijmakers, H.
2015-12-01
For an elliptic Arndt's hydrofoil numerical simulations of vortex cavitation are presented. An equilibrium cavitation model is employed. This single-fluid model assumes local thermodynamic and mechanical equilibrium in the mixture region of the flow, is employed. Furthermore, for characterizing the thermodynamic state of the system, precomputed multiphase thermodynamic tables containing data for the appropriate equations of state for each of the phases are used and a fast, accurate, and efficient look-up approach is employed for interpolating the data. The numerical simulations are carried out using the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations for compressible flow. The URANS equations of motion are discretized using an finite volume method for unstructured grids. The numerical simulations clearly show the formation of the tip vortex cavitation in the flow about the elliptic hydrofoil.
Numerical simulation of a laser-acoustic landmine detection system
NASA Astrophysics Data System (ADS)
Lancranjan, Ion I.; Miclos, Sorin; Savastru, Dan; Savastru, Roxana; Opran, Constantin
2012-06-01
The preliminary numerical simulation results obtained in the analysis of a landmine detection system based on laser excitation of acoustic - seismic waves in the soil and observing its surface vibration above the embedded landmine are presented. The presented numerical simulations comprise three main parts: 1) Laser oscillator and laser beam propagation and absorption in soil; a laser oscillator operated in Q-switched regime is considered; different laser wavelengths are investigated. 2) Acoustic - seismic wave generation by absorption in soil of laser pulse energy; 3) Evaluation of acoustic - seismic wave generation by the buried in soil landmine; 4) Comparison of Distributed Feed- Back Fiber Laser (DFB-FL) and Laser Doppler Vibrometer (LDV) detector used for soil vibrations evaluation. The above mentioned numerical simulation is dedicated for evaluation of an integrated portable detection system.
Study on the numerical schemes for hypersonic flow simulation
NASA Astrophysics Data System (ADS)
Nagdewe, S. P.; Shevare, G. R.; Kim, Heuy-Dong
2009-10-01
Hypersonic flow is full of complex physical and chemical processes, hence its investigation needs careful analysis of existing schemes and choosing a suitable scheme or designing a brand new scheme. The present study deals with two numerical schemes Harten, Lax, and van Leer with Contact (HLLC) and advection upstream splitting method (AUSM) to effectively simulate hypersonic flow fields, and accurately predict shock waves with minimal diffusion. In present computations, hypersonic flows have been modeled as a system of hyperbolic equations with one additional equation for non-equilibrium energy and relaxing source terms. Real gas effects, which appear typically in hypersonic flows, have been simulated through energy relaxation method. HLLC and AUSM methods are modified to incorporate the conservation laws for non-equilibrium energy. Numerical implementation have shown that non-equilibrium energy convect with mass, and hence has no bearing on the basic numerical scheme. The numerical simulation carried out shows good comparison with experimental data available in literature. Both numerical schemes have shown identical results at equilibrium. Present study has demonstrated that real gas effects in hypersonic flows can be modeled through energy relaxation method along with either AUSM or HLLC numerical scheme.
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.
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.
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 graphene in an external magnetic field
NASA Astrophysics Data System (ADS)
Boyda, D. L.; Braguta, V. V.; Valgushev, S. N.; Polikarpov, M. I.; Ulybyshev, M. V.
2014-06-01
In this paper the results of numerical simulation of graphene effective field theory in external magnetic field are presented. The numerical simulation is performed using noncompact (3+1)-dimensional Abelian lattice gauge fields and (2+1)-dimensional staggered lattice fermions. The dependences of fermion condensate and conductivity on the dielectric permittivity of the substrate for different values of external magnetic field are calculated. It is found that magnetic field shifts insulator-semimetal phase transition to larger values of the dielectric permittivity of the substrate. The phase diagram of graphene in external magnetic field is drawn.
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.
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 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
Vortical flow aerodynamics - Physical aspects and numerical simulation
NASA Technical Reports Server (NTRS)
Newsome, Richard W.; Kandil, Osama A.
1987-01-01
Progress in the numerical simulation of vortical flow due to three-dimensional flow separation about flight vehicles at high angles of attack and quasi-steady flight conditions is surveyed. Primary emphasis is placed on Euler and Reynolds-averaged Navier-Stokes methods where the vortices are 'captured' as a solution to the governing equations. A discussion of the relevant flow physics provides a perspective from which to assess numerical solutions. Current numerical prediction capabilities and their evolutionary development are surveyed. Future trends and challenges are identified and discussed.
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 simulations of time-resolved quantum electronics
NASA Astrophysics Data System (ADS)
Gaury, Benoit; Weston, Joseph; Santin, Matthieu; Houzet, Manuel; Groth, Christoph; Waintal, Xavier
2014-01-01
Numerical simulation has become a major tool in quantum electronics both for fundamental and applied purposes. While for a long time those simulations focused on stationary properties (e.g. DC currents), the recent experimental trend toward GHz frequencies and beyond has triggered a new interest for handling time-dependent perturbations. As the experimental frequencies get higher, it becomes possible to conceive experiments which are both time-resolved and fast enough to probe the internal quantum dynamics of the system. This paper discusses the technical aspects-mathematical and numerical-associated with the numerical simulations of such a setup in the time domain (i.e. beyond the single-frequency AC limit). After a short review of the state of the art, we develop a theoretical framework for the calculation of time-resolved observables in a general multiterminal system subject to an arbitrary time-dependent perturbation (oscillating electrostatic gates, voltage pulses, time-varying magnetic fields, etc.) The approach is mathematically equivalent to (i) the time-dependent scattering formalism, (ii) the time-resolved non-equilibrium Greens function (NEGF) formalism and (iii) the partition-free approach. The central object of our theory is a wave function that obeys a simple Schrdinger equation with an additional source term that accounts for the electrons injected from the electrodes. The time-resolved observables (current, density, etc.) and the (inelastic) scattering matrix are simply expressed in terms of this wave function. We use our approach to develop a numerical technique for simulating time-resolved quantum transport. We find that the use of this wave function is advantageous for numerical simulations resulting in a speed up of many orders of magnitude with respect to the direct integration of NEGF equations. Our technique allows one to simulate realistic situations beyond simple models, a subject that was until now beyond the simulation capabilities of available approaches.
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
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. 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 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.
Processing biobased polymers using plasticizers: Numerical simulations versus experiments
NASA Astrophysics Data System (ADS)
Desplentere, Frederik; Cardon, Ludwig; Six, Wim; Erkoç, Mustafa
2016-03-01
In polymer processing, the use of biobased products shows lots of possibilities. Considering biobased materials, biodegradability is in most cases the most important issue. Next to this, bio based materials aimed at durable applications, are gaining interest. Within this research, the influence of plasticizers on the processing of the bio based material is investigated. This work is done for an extrusion grade of PLA, Natureworks PLA 2003D. Extrusion through a slit die equipped with pressure sensors is used to compare the experimental pressure values to numerical simulation results. Additional experimental data (temperature and pressure data along the extrusion screw and die are recorded) is generated on a dr. Collin Lab extruder producing a 25mm diameter tube. All these experimental data is used to indicate the appropriate functioning of the numerical simulation tool Virtual Extrusion Laboratory 6.7 for the simulation of both the industrial available extrusion grade PLA and the compound in which 15% of plasticizer is added. Adding the applied plasticizer, resulted in a 40% lower pressure drop over the extrusion die. The combination of different experiments allowed to fit the numerical simulation results closely to the experimental values. Based on this experience, it is shown that numerical simulations also can be used for modified bio based materials if appropriate material and process data are taken into account.
Numerical simulation of impact cratering on granular material
NASA Astrophysics Data System (ADS)
Wada, Koji; Senshu, Hiroki; Matsui, Takafumi
2006-02-01
A new numerical code based on the Distinct Element Method (DEM) is developed to study the impact cratering processes on granular material. This code has a potential advantage to simulate the cratering process on granular material, since the movement of discrete particles can be treated. To show the physical plausibility of this code, we conduct 3-D numerical simulations of vertical impact into granular material targets that consist of 384,000 particles, and compare the results with those from experimental studies. It is shown that the excavation stage of cratering derived from experimental studies is represented well by our simulation: the size of the crater cavity, and the ejecta velocity and angle distributions are consistent with those obtained in laboratory experiments. The impact simulation code developed in this study is thus suggested to be useful for the analysis of the impact cratering process on granular material.
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)
3D numerical simulation of transient processes in hydraulic turbines
NASA Astrophysics Data System (ADS)
Cherny, S.; Chirkov, D.; Bannikov, D.; Lapin, V.; Skorospelov, V.; Eshkunova, I.; Avdushenko, A.
2010-08-01
An approach for numerical simulation of 3D hydraulic turbine flows in transient operating regimes is presented. The method is based on a coupled solution of incompressible RANS equations, runner rotation equation, and water hammer equations. The issue of setting appropriate boundary conditions is considered in detail. As an illustration, the simulation results for runaway process are presented. The evolution of vortex structure and its effect on computed runaway traces are analyzed.
Numerical Simulations of the Digital Microfluidic Manipulation of Single Microparticles.
Lan, Chuanjin; Pal, Souvik; Li, Zhen; Ma, Yanbao
2015-09-01
Single-cell analysis techniques have been developed as a valuable bioanalytical tool for elucidating cellular heterogeneity at genomic, proteomic, and cellular levels. Cell manipulation is an indispensable process for single-cell analysis. Digital microfluidics (DMF) is an important platform for conducting cell manipulation and single-cell analysis in a high-throughput fashion. However, the manipulation of single cells in DMF has not been quantitatively studied so far. In this article, we investigate the interaction of a single microparticle with a liquid droplet on a flat substrate using numerical simulations. The droplet is driven by capillary force generated from the wettability gradient of the substrate. Considering the Brownian motion of microparticles, we utilize many-body dissipative particle dynamics (MDPD), an off-lattice mesoscopic simulation technique, in this numerical study. The manipulation processes (including pickup, transport, and drop-off) of a single microparticle with a liquid droplet are simulated. Parametric studies are conducted to investigate the effects on the manipulation processes from the droplet size, wettability gradient, wetting properties of the microparticle, and particle-substrate friction coefficients. The numerical results show that the pickup, transport, and drop-off processes can be precisely controlled by these parameters. On the basis of the numerical results, a trap-free delivery of a hydrophobic microparticle to a destination on the substrate is demonstrated in the numerical simulations. The numerical results not only provide a fundamental understanding of interactions among the microparticle, the droplet, and the substrate but also demonstrate a new technique for the trap-free immobilization of single hydrophobic microparticles in the DMF design. Finally, our numerical method also provides a powerful design and optimization tool for the manipulation of microparticles in DMF systems. PMID:26241832
GPU Accelerated Numerical Simulation of Viscous Flow Down a Slope
NASA Astrophysics Data System (ADS)
Gygax, Remo; Rss, Ludovic; Omlin, Samuel; Podladchikov, Yuri; Jaboyedoff, Michel
2014-05-01
Numerical simulations are an effective tool in natural risk analysis. They are useful to determine the propagation and the runout distance of gravity driven movements such as debris flows or landslides. To evaluate these processes an approach on analogue laboratory experiments and a GPU accelerated numerical simulation of the flow of a viscous liquid down an inclined slope is considered. The physical processes underlying large gravity driven flows share certain aspects with the propagation of debris mass in a rockslide and the spreading of water waves. Several studies have shown that the numerical implementation of the physical processes of viscous flow produce a good fit with the observation of experiments in laboratory in both a quantitative and a qualitative way. When considering a process that is this far explored we can concentrate on its numerical transcription and the application of the code in a GPU accelerated environment to obtain a 3D simulation. The objective of providing a numerical solution in high resolution by NVIDIA-CUDA GPU parallel processing is to increase the speed of the simulation and the accuracy on the prediction. The main goal is to write an easily adaptable and as short as possible code on the widely used platform MATLAB, which will be translated to C-CUDA to achieve higher resolution and processing speed while running on a NVIDIA graphics card cluster. The numerical model, based on the finite difference scheme, is compared to analogue laboratory experiments. This way our numerical model parameters are adjusted to reproduce the effective movements observed by high-speed camera acquisitions during the laboratory experiments.
Numerical Simulation of Flow-Induced Structure in Complex Fluids
NASA Astrophysics Data System (ADS)
Yamamoto, Takehiro
2007-04-01
It is important to investigate the flow-induced structure for the analysis of the mechanism of flow behavior of complex fluids. The present paper includes two topics in which the flow-induced structure is numerically investigated. The first topic treats the suspensions of disc-like particles under simple shear flows. Disc-like particles were modeled by oblate spheroid particles, and the Brownian dynamics simulation was performed for suspensions of the particles interacting via the Gay-Berne potential. This simulation confirmed that this model system was applicable to the analysis of flow of suspension of disc-like particles. The second one is the numerical simulation of the deformation behavior of a droplet in shear flows. The present simulation is the first step for the numerical simulation of the flow-induced structure in emulsions. This simulation can demonstrate the deformation behavior of droplet observed in experiments and predict effects of non-Newtonian property of fluids on the droplet deformation.
Numerical simulations and modeling for stochastic biological systems with jumps
NASA Astrophysics Data System (ADS)
Zou, Xiaoling; Wang, Ke
2014-05-01
This paper gives a numerical method to simulate sample paths for stochastic differential equations (SDEs) driven by Poisson random measures. It provides us a new approach to simulate systems with jumps from a different angle. The driving Poisson random measures are assumed to be generated by stationary Poisson point processes instead of Lvy processes. Methods provided in this paper can be used to simulate SDEs with Lvy noise approximately. The simulation is divided into two parts: the part of jumping integration is based on definition without approximation while the continuous part is based on some classical approaches. Biological explanations for stochastic integrations with jumps are motivated by several numerical simulations. How to model biological systems with jumps is showed in this paper. Moreover, method of choosing integrands and stationary Poisson point processes in jumping integrations for biological models are obtained. In addition, results are illustrated through some examples and numerical simulations. For some examples, earthquake is chose as a jumping source which causes jumps on the size of biological population.
NUMERICAL SIMULATION OF THREE-DIMENSIONAL TUFT CORONA AND ELECTROHYDRODYNAMICS
The numerical simulation of three-dimensional tuft corona and electrohydrodynamics (EHD) is discussed. The importance of high-voltage and low-current operation in the wire-duct precipitator has focused attention on collecting high-resistivity dust. The local current density of in...
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 approaches for multidimensional simulations of stellar explosions
NASA Astrophysics Data System (ADS)
Chen, Ke-Jung; Heger, Alexander; Almgren, Ann S.
2013-11-01
We introduce numerical algorithms for initializing multidimensional simulations of stellar explosions with 1D stellar evolution models. The initial mapping from 1D profiles onto multidimensional grids can generate severe numerical artifacts, one of the most severe of which is the violation of conservation laws for physical quantities. We introduce a numerical scheme for mapping 1D spherically-symmetric data onto multidimensional meshes so that these physical quantities are conserved. We verify our scheme by porting a realistic 1D Lagrangian stellar profile to the new multidimensional Eulerian hydro code CASTRO. Our results show that all important features in the profiles are reproduced on the new grid and that conservation laws are enforced at all resolutions after mapping. We also introduce a numerical scheme for initializing multidimensional supernova simulations with realistic perturbations predicted by 1D stellar evolution models. Instead of seeding 3D stellar profiles with random perturbations, we imprint them with velocity perturbations that reproduce the Kolmogorov energy spectrum expected for highly turbulent convective regions in stars. Our models return Kolmogorov energy spectra and vortex structures like those in turbulent flows before the modes become nonlinear. Finally, we describe approaches to determining the resolution for simulations required to capture fluid instabilities and nuclear burning. Our algorithms are applicable to multidimensional simulations besides stellar explosions that range from astrophysics to cosmology.
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 Simulations of Wing-Body Junction Flows
NASA Technical Reports Server (NTRS)
Krishnamurthy, R.; Cagle, C.; Chandra, S.
1996-01-01
The goal of the research project is to contribute to the optimized design of fan bypass systems in advanced turbofan engines such as the Advanced Ducted Propulsors (ADP). The immediate objective is to perform numerical simulation of duct-strut interactions to elucidate the loss mechanisms associated with this configuration that is characteristic of ADP. These numerical simulations would complement an experimental study being undertaken at Purdue University. As the first step in the process, a numerical study of wing-body junction flow is being undertaken as it shares a number of characteristics with the duct-strut interaction flow. The presence of the characteristic horseshoe vortex and the associated secondary flow are the salient features that contribute to making this flow a challenge to predict numerically. The simulations will be performed with the NPARC code on the CRAY Y-MP platform at LeRC. The grids for the simulation have been generated using an algebraic mapping technique with a multisurface algorithm.
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
Numerical simulation of a HgCdTe solidification process
NASA Astrophysics Data System (ADS)
Alexiades, V.; Geist, G. A.; Solomon, A. D.
1985-08-01
The solidification of a cylindrical ingot of mercury-cadmium-telluride is modeled taking into account both heat conduction and solute diffusion. Values of the relevant thermophysical parameters of the pseudo-binary HgTe-CdTe are compiled. The model is implemented numerically by a finite-difference discretization and results of the simulation of a freezing experiment are reported.
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 cavitating flow past axisymmetric body
NASA Astrophysics Data System (ADS)
Kim, Dong-Hyun; Park, Warn-Gyu; Jung, Chul-Min
2012-09-01
Cavitating flow simulation is of practical importance for many engineering systems, such as marine propellers, pump impellers, nozzles, torpedoes, etc. The present work has developed the base code to solve the cavitating flows past the axisymmetric bodies with several forebody shapes. The governing equation is the Navier-Stokes equation based on homogeneous mixture model. The momentum is in the mixture phase while the continuity equation is solved in liquid and vapor phase, separately. The solver employs an implicit preconditioning algorithm in curvilinear coordinates. The computations have been carried out for the cylinders with hemispherical, 1-caliber, and 0-caliber forebody and, then, compared with experiments and other numerical results. Fairly good agreements with experiments and numerical results have been achieved. It has been concluded that the present numerical code has successfully accounted for the cavitating flows past axisymmetric bodies. The present code has also shown the capability to simulate ventilated cavitation.
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.
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
An a priori DNS study of the shadow-position mixing model
Zhao, Xin -Yu; Bhagatwala, Ankit; Chen, Jacqueline H.; Haworth, Daniel C.; Pope, Stephen B.
2016-01-15
In this study, the modeling of mixing by molecular diffusion is a central aspect for transported probability density function (tPDF) methods. In this paper, the newly-proposed shadow position mixing model (SPMM) is examined, using a DNS database for a temporally evolving di-methyl ether slot jet flame. Two methods that invoke different levels of approximation are proposed to extract the shadow displacement (equivalent to shadow position) from the DNS database. An approach for a priori analysis of the mixing-model performance is developed. The shadow displacement is highly correlated with both mixture fraction and velocity, and the peak correlation coefficient of themore » shadow displacement and mixture fraction is higher than that of the shadow displacement and velocity. This suggests that the composition-space localness is reasonably well enforced by the model, with appropriate choices of model constants. The conditional diffusion of mixture fraction and major species from DNS and from SPMM are then compared, using mixing rates that are derived by matching the mixture fraction scalar dissipation rates. Good qualitative agreement is found, for the prediction of the locations of zero and maximum/minimum conditional diffusion locations for mixture fraction and individual species. Similar comparisons are performed for DNS and the IECM (interaction by exchange with the conditional mean) model. The agreement between SPMM and DNS is better than that between IECM and DNS, in terms of conditional diffusion iso-contour similarities and global normalized residual levels. It is found that a suitable value for the model constant c that controls the mixing frequency can be derived using the local normalized scalar variance, and that the model constant a controls the localness of the model. A higher-Reynolds-number test case is anticipated to be more appropriate to evaluate the mixing models, and stand-alone transported PDF simulations are required to more fully enforce localness and to assess model performance.« less
Collapse of a Liquid Column: Numerical Simulation and Experimental Validation
NASA Astrophysics Data System (ADS)
Cruchaga, Marcela A.; Celentano, Diego J.; Tezduyar, Tayfun E.
2007-03-01
This paper is focused on the numerical and experimental analyses of the collapse of a liquid column. The measurements of the interface position in a set of experiments carried out with shampoo and water for two different initial column aspect ratios are presented together with the corresponding numerical predictions. The experimental procedure was found to provide acceptable recurrence in the observation of the interface evolution. Basic models describing some of the relevant physical aspects, e.g. wall friction and turbulence, are included in the simulations. Numerical experiments are conducted to evaluate the influence of the parameters involved in the modeling by comparing the results with the data from the measurements. The numerical predictions reasonably describe the physical trends.
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.
Configuration Management File Manager Developed for Numerical Propulsion System Simulation
NASA Technical Reports Server (NTRS)
Follen, Gregory J.
1997-01-01
One of the objectives of the High Performance Computing and Communication Project's (HPCCP) Numerical Propulsion System Simulation (NPSS) is to provide a common and consistent way to manage applications, data, and engine simulations. The NPSS Configuration Management (CM) File Manager integrated with the Common Desktop Environment (CDE) window management system provides a common look and feel for the configuration management of data, applications, and engine simulations for U.S. engine companies. In addition, CM File Manager provides tools to manage a simulation. Features include managing input files, output files, textual notes, and any other material normally associated with simulation. The CM File Manager includes a generic configuration management Application Program Interface (API) that can be adapted for the configuration management repositories of any U.S. engine company.
Numerical simulation of the unsteady behaviour of cavitating flows
NASA Astrophysics Data System (ADS)
Coutier-Delgosha, O.; Reboud, J. L.; Delannoy, Y.
2003-06-01
A 2D numerical model is proposed to simulate unsteady cavitating flows. The Reynolds-averaged Navier-Stokes equations are solved for the mixture of liquid and vapour, which is considered as a single fluid with variable density. The vapourization and condensation processes are controlled by a barotropic state law that relates the fluid density to the pressure variations. The numerical resolution is a pressure-correction method derived from the SIMPLE algorithm, with a finite volume discretization. The standard scheme is slightly modified to take into account the cavitation phenomenon. That numerical model is used to calculate unsteady cavitating flows in two Venturi type sections. The choice of the turbulence model is discussed, and the standard RNG k-model is found to lead to non-physical stable cavities. A modified k-model is proposed to improve the simulation. The influence of numerical and physical parameters is presented, and the numerical results are compared to previous experimental observations and measurements. The proposed model seems to describe the unsteady cavitation behaviour in 2D geometries well.
Thermal numerical simulator for laboratory evaluation of steamflood oil recovery
Sarathi, P.
1991-04-01
A thermal numerical simulator running on an IBM AT compatible personal computer is described. The simulator was designed to assist laboratory design and evaluation of steamflood oil recovery. An overview of the historical evolution of numerical thermal simulation, NIPER's approach to solving these problems with a desk top computer, the derivation of equations and a description of approaches used to solve these equations, and verification of the simulator using published data sets and sensitivity analysis are presented. The developed model is a three-phase, two-dimensional multicomponent simulator capable of being run in one or two dimensions. Mass transfer among the phases and components is dictated by pressure- and temperature-dependent vapor-liquid equilibria. Gravity and capillary pressure phenomena were included. Energy is transferred by conduction, convection, vaporization and condensation. The model employs a block centered grid system with a five-point discretization scheme. Both areal and vertical cross-sectional simulations are possible. A sequential solution technique is employed to solve the finite difference equations. The study clearly indicated the importance of heat loss, injected steam quality, and injection rate to the process. Dependence of overall recovery on oil volatility and viscosity is emphasized. The process is very sensitive to relative permeability values. Time-step sensitivity runs indicted that the current version is time-step sensitive and exhibits conditional stability. 75 refs., 19 figs., 19 tabs.
Numerical 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.
Graphics interfaces and numerical simulations: Mexican Virtual Solar Observatory
NASA Astrophysics Data System (ADS)
Hernndez, L.; Gonzlez, A.; Salas, G.; Santilln, 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.
Image based numerical simulation of hemodynamics in a intracranial aneurysm
NASA Astrophysics Data System (ADS)
Le, Trung; Ge, Liang; Sotiropoulos, Fotis; Kallmes, David; Cloft, Harry; Lewis, Debra; Dai, Daying; Ding, Yonghong; Kadirvel, Ramanathan
2007-11-01
Image-based numerical simulations of hemodynamics in a intracranial aneurysm are carried out. The numerical solver based on CURVIB (curvilinear grid/immersed boundary method) approach developed in Ge and Sotiropoulos, JCP 2007 is used to simulate the blood flow. A curvilinear grid system that gradually follows the curved geometry of artery wall and consists of approximately 5M grid nodes is constructed as the background grid system and the boundaries of the investigated artery and aneurysm are treated as immersed boundaries. The surface geometry of aneurysm wall is reconstructed from an angiography study of an aneurysm formed on the common carotid artery (CCA) of a rabbit and discretized with triangular meshes. At the inlet a physiological flow waveform is specified and direct numerical simulations are used to simulate the blood flow. Very rich vortical dynamics is observed within the aneurysm area, with a ring like vortex sheds from the proximal side of aneurysm, develops and impinge onto the distal side of the aneurysm as flow develops, and destructs into smaller vortices during later cardiac cycle. This work was supported in part by the University of Minnesota Supercomputing Institute.
Numerical simulation of the interaction between two bubbles
NASA Astrophysics Data System (ADS)
Han, R.; Yao, X. L.; Zhang, A. M.
2015-01-01
Different evolution patterns of two bubbles may be observed for different values of the phase difference and the inter-bubble distance. Based on potential flow theory, a boundary element method (BEM) is adopted to simulate the interaction between two bubbles and the toroidal bubble after the jet impact is also investigated by placing a vortex ring within the bubble. Meanwhile, some numerical techniques are used to deal with problems like coalescence and collapse. Typical phenomena like coalescence, jet towards and jet away are investigated numerically in this paper. The mechanisms underlying the various phenomena are given through the analysis of the velocity and pressure fields.
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 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 4C makes it well suited to study internal gravity wave excitation by convection: an increasing temperature profile is unstable to convection below 4C, but stably stratified above 4C. 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 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.
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.
Mueschke, N; Schilling, O; Andrews, M
2007-01-09
A spectral/compact finite-difference method with a third-order Adams-Bashforth-Moulton time-evolution scheme is used to perform a direct numerical simulation (DNS) of Rayleigh-Taylor flow. The initial conditions are modeled by parameterizing the multi-mode velocity and density perturbations measured just off of the splitter plate in water channel experiments. Parameters in the DNS are chosen to match the experiment as closely as possible. The early-time transition from a weakly-nonlinear to a strongly-nonlinear state, as well as the onset of turbulence, is examined by comparing the DNS and experimental results. The mixing layer width, molecular mixing parameter, vertical velocity variance, and density variance spectrum obtained from the DNS are shown to be in good agreement with the corresponding experimental values.
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 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.
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.
2006-12-15
Numerical simulations of electron temperature gradient (ETG) turbulence are presented that characterize the ETG fluctuation spectrum, establish limits to the validity of the adiabatic ion model often employed in studying ETG turbulence, and support the tentative conclusion that plasma-operating regimes exist in which ETG turbulence produces sufficient electron heat transport to be experimentally relevant. We resolve prior controversies regarding simulation techniques and convergence by benchmarking simulations of ETG turbulence from four microturbulence codes, demonstrating agreement on the electron heat flux, correlation functions, fluctuation intensity, and rms flow shear at fixed simulation cross section and resolution in the plane perpendicular to the magnetic field. Excellent convergence of both continuum and particle-in-cell codes with time step and velocity-space resolution is demonstrated, while numerical issues relating to perpendicular (to the magnetic field) simulation dimensions and resolution are discussed. A parameter scan in the magnetic shear, s, demonstrates that the adiabatic ion model is valid at small values of s (s<0.4 for the parameters used in this scan) but breaks down at higher magnetic shear. A proper treatment employing gyrokinetic ions reveals a steady increase in the electron heat transport with increasing magnetic shear, reaching electron heat transport rates consistent with analyses of experimental tokamak discharges.
Numerical simulation of acoustic emission using acoustic contact elements
NASA Astrophysics Data System (ADS)
Suvorov, A. S.; Sokov, E. M.; Artel'nyi, P. V.
2014-11-01
A new method is described for nonconformal finite-element simulation of the region of interaction between an acoustic fluid and deformed solid bodies. The method is based on the use of special contact finite elements simulating the interpolation coupling between the acoustic pressure in the fluid and displacements of a two-phase surface. The test results of the method demonstrate a significant acceleration of grid convergence of computations. The numerical method is verified by the problem of predicting noise emission by a thin-wall inhomogeneous shell.
Numerical Simulations of the Rayleigh-Taylor Instability
NASA Astrophysics Data System (ADS)
Tryggvason, Grtar
1988-04-01
Two Lagrangian-Eulerian vortex methods to simulate the motion of an interface between inviscid fluids of different densities are presented. The representation of the interface as a vortex sheet eliminates numerical diffusion, and by coupling the tracked interface with a stationary grid (using the well-known vortex-in-cell method) the high cost associated with traditional vortex methods is reduced. These methods are applied to the Rayleigh-Taylor instability. For finite density ratios the appearance of rolled up vortices and a possible singularity formation has limited simulations in the past. By providing proper regularization our methods overcome some of these difficulties.
Numerical simulation of the final stages of terrestrial planet formation
NASA Technical Reports Server (NTRS)
Cox, L. P.; Lewis, J. S.
1980-01-01
Three representative numerical simulations of the growth of the terrestrial planets by accretion of large protoplanets are considered. The mass and relative-velocity distributions of the bodies are free to evolve simultaneously in response to close gravitational encounters and occasional collisions between bodies. The collisions between bodies arise therefore in a natural way and the assumption of expressions for the relative-velocity distribution and the gravitational collision cross section is unnecessary. These simulations indicate that the growth of bodies with final masses approaching those of Venus and earth is possible, at least for the case of a two-dimensional system
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.
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.
New prescriptions of turbulent transport from local numerical simulations
NASA Astrophysics Data System (ADS)
Prat, V.; Lignires, F.; Lesur, G.
2015-01-01
Massive stars often experience fast rotation, which is known to induce turbulent mixing with a strong impact on the evolution of these stars. Local direct numerical simulations of turbulent transport in stellar radiative zones are a promising way to constrain phenomenological transport models currently used in many stellar evolution codes. We present here the results of such simulations of stably-stratified sheared turbulence taking notably into account the effects of thermal diffusion and chemical stratification. We also discuss the impact of theses results on stellar evolution theory.
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.
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.
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 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.
Numerical simulation of LVI test onto composite plates
NASA Astrophysics Data System (ADS)
Caputo, Francesco; Lamanna, Giuseppe; De Luca, Alessandro; Lopresto, Valentina
2014-05-01
The aim of the proposed research activity is to investigate on the structural behaviour of laminated composite plates under low velocity impacts. Analytical closed-form methods are generally unable to describe simultaneously different composite failure modes, as well as experimental tests are unable to be applied to complex boundary conditions or component geometries. Within this work it is shown that numerical simulation appears a reliable tool to describe and forecast the damage onset and growth in composite components. A numerical procedure, based on explicit finite element methods, has been proposed and applied to the simulation of drop mass impacts on composite plates, by taking in account both intra-lamina and inter-laminates damages onset and propagation up to component failure. A global/local modelling approach has been employed to create the model and its validation has been performed by considering results from different sessions of experimental tests.
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.
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 Simulations of the Geodynamo and Scaling Laws
NASA Astrophysics Data System (ADS)
Oruba, L.; Dormy, E.
2013-12-01
State of the art numerical models of the Geodynamo are still performed in a parameter regime extremely remote from the values relevant to the physics of the Earth core. In order to establish a connection between dynamo modeling and the geophysical motivation, it is necessary to use scaling laws. Such laws establish the dependency of essential quantities (such as the magnetic field strength) on measured or controlled quantities. They allow for a direct confrontation of advanced models with geophysical constraints. We will present a detailed analysis of scaling laws based on a wide database of 185 direct numerical simulations (courtesy of U. Christensen) and test various existing scaling laws. Our main concern is to stress the risks of a direct numerical fit free from physical insight. We show that different a priori hypothesis can yield contradictory dependences, in particular concerning the dependence of the magnetic field strength on the rotation rate as well as on the viscosity.
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 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.
Analysis of mass transfer in dissipative nonideal systems: Numerical simulation
Vaulina, O. S.; Adamovich, K. G.
2008-05-15
The results of a numerical analysis of mass transfer in extended quasi-two-dimensional and three-dimensional dissipative nonideal systems are presented. Pair interaction between particles is modeled by isotropic repulsive potentials represented by combinations of power laws and exponentials. Simulations are performed for parameter values characteristic of laboratory dusty plasmas. It is shown that short-time particle dynamics in nonideal liquid systems is similar to evolution of thermal oscillations at crystal lattice sites.
Numerical simulation of melting ice around a floating by microwaves
NASA Astrophysics Data System (ADS)
Lakzian, Esmail; Parsian, Armin; Lakzian, Kazem
2015-04-01
In this paper a new method in using microwaves is provided for melting the ice around a floating equipment in a freezing condition in cold regions. The numerical simulation's results for validation are compared with the simple model's experimental data. Using microwave in melting the ice around a floating equipment is caused by lack of the mechanical wear, low energy dissipation factor and acceptable defrosting process speed in small lakes.
Numerical simulation of melting ice around a floating by microwaves
NASA Astrophysics Data System (ADS)
Lakzian, Esmail; Parsian, Armin; Lakzian, Kazem
2016-03-01
In this paper a new method in using microwaves is provided for melting the ice around a floating equipment in a freezing condition in cold regions. The numerical simulation's results for validation are compared with the simple model's experimental data. Using microwave in melting the ice around a floating equipment is caused by lack of the mechanical wear, low energy dissipation factor and acceptable defrosting process speed in small lakes.
Numerical simulations of a pulsed detonation wave augmentation device
NASA Technical Reports Server (NTRS)
Cambier, Jean-Luc; Adelman, Henry; Menees, Gene P.
1993-01-01
We present here the concept of a hybrid engine for Single Stage To Orbit (SSTO) air-breathing hypersonic vehicle. This concept relies on the use of pulsed detonation waves, both for thrust generation and mixing/combustion augmentation. We describe the principles behind the engine concept, which we call the Pulsed Detonation Wave Augmentor (PDWA). We demonstrate the principles of operation for two possible configurations through numerical simulations. We also attempt a first approximation to engine design, and propose various applications.
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.
EXTENDED SCALING LAWS IN NUMERICAL SIMULATIONS OF MAGNETOHYDRODYNAMIC TURBULENCE
Mason, Joanne; Cattaneo, Fausto; Perez, Jean Carlos; Boldyrev, Stanislav E-mail: cattaneo@flash.uchicago.edu E-mail: boldyrev@wisc.edu
2011-07-10
Magnetized turbulence is ubiquitous in astrophysical systems, where it notoriously spans a broad range of spatial scales. Phenomenological theories of MHD turbulence describe the self-similar dynamics of turbulent fluctuations in the inertial range of scales. Numerical simulations serve to guide and test these theories. However, the computational power that is currently available restricts the simulations to Reynolds numbers that are significantly smaller than those in astrophysical settings. In order to increase computational efficiency and, therefore, probe a larger range of scales, one often takes into account the fundamental anisotropy of field-guided MHD turbulence, with gradients being much slower in the field-parallel direction. The simulations are then optimized by employing the reduced MHD equations and relaxing the field-parallel numerical resolution. In this work we explore a different possibility. We propose that there exist certain quantities that are remarkably stable with respect to the Reynolds number. As an illustration, we study the alignment angle between the magnetic and velocity fluctuations in MHD turbulence, measured as the ratio of two specially constructed structure functions. We find that the scaling of this ratio can be extended surprisingly well into the regime of relatively low Reynolds number. However, the extended scaling easily becomes spoiled when the dissipation range in the simulations is underresolved. Thus, taking the numerical optimization methods too far can lead to spurious numerical effects and erroneous representation of the physics of MHD turbulence, which in turn can affect our ability to identify correctly the physical mechanisms that are operating in astrophysical systems.
Numerical simulation of reconnection in an emerging magnetic flux region
NASA Technical Reports Server (NTRS)
Forbes, T. C.
1984-01-01
The resistive MHD equations are numerically solved in two dimensions for an initial-boundary-value problem which simulates reconnection between an emerging magnetic flux region and an overlying coronal magnetic field. The solution involves both ideal-MHD and resistive-MHD processes, and the solution shows an evolution which is remarkably suggestive of the preflare, impulsive, and main phases of the flare-cycle.
Numerical simulation of flow in the wet scrubber for desulfurization
NASA Astrophysics Data System (ADS)
Novosád, Jan; Vít, Tomáš
2015-05-01
This article deals with numerical simulation of flow and chemical reactions in absorber for desulfurization of flue-gas. The objective of the work is the investigation of effect of different nozzles types and their placement in spray layers. These nozzles distribute lime suspension into flue gas stream. The research includes two types of nozzles and four different arrangements of nozzles and spray layers. Conclusion describes the effect of nozzle types and their arrangements on the suspension concentration in absorber.
The calibration of numerically simulated color and material change processes
NASA Astrophysics Data System (ADS)
Szke, L.; Wortberg, J.
2014-05-01
As shown in the past, the first steps of product changing processes within extrusion dies can be observed through numerical simulations using transient calculations and the volume of fluid (VOF) approach. However, in the later part of the changing process, influences from the system, such as surface properties of the die channel or particle types in the polymer, govern the progress of the melt flow directly at the wall. Recently an approach allows these effects to be implemented into the numerical simulation despite the complexity of the whole flow system. In general it is common to assume a zero velocity at the channel wall in fluid dynamic calculations. In reality the exchange of materials during the extrusion process can be observed. Therefore, a finite velocity at the wall has to exist. Although this velocity is very low, it cannot be ignored for product changes as for most other calculations, because the velocity of the source material at the wall dominates the time or the amount of target material needed to complete the change. To calibrate the numerical calculation considering the effects near the channel wall a correcting function based on the experimental data is used. Therefore a wall velocity is calculated analytically and implemented as boundary condition in the numerical computation. This function is based on experimental data from color and material changes of several low density polyethylene (LDPE) types.
NASA Technical Reports Server (NTRS)
Lee, Theodore K.; Zhong, Xiao-Lin
1998-01-01
The numerical simulation of transitional and turbulent processes in hypersonic boundary layers often involves a physical process of a shock-disturbance wave interaction in complex two-dimensional and three-dimensional flow fields. For such simulations, it is required that there be a high order of accuracy in capturing both the shock and the small disturbances. The purpose of this paper is to evaluate the viability of using high order shock capturing schemes to track small disturbances in a multi-dimensional steady hypersonic flow. The numerical methods that are to be studied are the Total Variation Diminishing (TVD) scheme, and Essentially Non-Oscillatory (ENO) scheme. This paper shows that the presence of numerical oscillations in the flow field solution may drastically hinder any attempt at tracking the propagation of any physical disturbances. It has been found that the numerical oscillations that exist for shock capturing methods may be significant enough to pollute a flow field containing small physical disturbances. The effects of the refinement of the grid do not reduce the oscillations, but rather they decrease the wavelength of the oscillations. It is shown that by aligning the shock with the grid, the amplitude of these spurious oscillations may be greatly reduced.
Numerical simulations of asteroids modelled as gravitational aggregates with cohesion
NASA Astrophysics Data System (ADS)
Richardson, D. C.; Michel, P.; Walsh, K. J.; Flynn, K. W.
2009-02-01
Evidence is mounting that asteroids larger than a few hundred metres in diameter are gravitational aggregates of smaller, cohesive pieces. For example, images of 25143 Itokawa show a boulder-strewn surface reminiscent of what might be expected following gravitational reaccumulation of material ejected from a catastrophic impact into a larger body. We have developed a new numerical approach to modelling gravitational aggregates that includes for the first time several prescriptions for variable material strength/cohesion while preserving the desirable features of fast and accurate computation from our prior methods. The new model can be used to construct non-idealized rubble piles made up of irregular, competent pieces, or to preserve shape and spin information of reaccumulated bodies in high-resolution simulations of asteroid family formation, by allowing fragments to stick on contact (and optionally bounce or cause further fragmentation, depending on user-selectable parameters). We detail the numerical method, which involves solving the rigid-body equations of motion and handling non-central/off-axis impacts, and present simulations of collisional and rotational disruption of asteroids as illustrative examples. This work is part of an ongoing effort to improve the realism and applicability of numerical simulations to the collisional and dynamical evolution of asteroids and other small solar system bodies.
Numerical simulation of a colloidal thruster in the droplet regime
NASA Astrophysics Data System (ADS)
Carretero, Jorge; Martnez-Snchez, Manuel
2004-12-01
Experiments with colloid thrusters have shown their ability to operate in various flow regimes extending from the pure droplet to the mixed ion-droplet regime, and in some cases to pure ion emission. Experimental research on this issue has been done using time of flight techniques which have characterized the specific charge of the extracted species, combined with stopping-potential energy analyses. We address this issue from the numerical perspective by developing a single emitter colloid simulation. In previous papers we have presented results from our numerical model for an axisymmetric colloidal jet [J.A. Carretero, M. Martnez-Snchez, in: 38th Joint Propulsion Conf. and Exhibit, 7-10 July, 2002, Indianapolis, IN, AIAA-2002-3812, 2002; J.A. Carretero, M. Martnez-Snchez, in: 28th Internat. Electric Propulsion Conf., 17-21 March, 2003, Toulouse France, 2003]. The numerical simulation models the cone-jet transition region of the colloid jet; starting from the needle to the extractor grid, thus reproducing the typical electrospray experimental configuration. The liquid is modelled as an incompressible viscous fluid with a constant conductivity. A Surface charge relaxation equation is included in the model, and the potential and electric fields in the fluid are solved for. The equations have been simplified by employing a slenderness approximation except for the free surface boundary conditions where the terms have been kept exact. Simulation results have been shown to compare well to experimental data for a variety of liquids, for flows 1
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.
Numerical prediction of microstructure and hardness in multicycle simulations
Oddy, A.S.; McDill, J.M.J.
1996-06-01
Thermal-microstructural predictions are made and compared to physical simulations of heat-affected zones in multipass and weaved welds. The microstructural prediction algorithm includes reaustenitization kinetics, grain growth, austenite decomposition kinetics, hardness, and tempering. Microstructural simulation of weaved welds requires that the algorithm include transient reaustenitization, austenite decomposition for arbitrary thermal cycles including during reheating, and tempering. Material properties for each of these phenomena are taken from the best available literature. The numerical predictions are compared with the results of physical simulations made at the Metals Technology Laboratory, CANMET, on a Gleeble 1500 simulator. Thermal histories used in the physical simulations included single-pass welds, isothermal tempering, two-cycle, and three-cycle welds. The two- and three-cycle welds include temper-bead and weaved-weld simulations. A recurring theme in the analysis is the significant variation found in the material properties for the same grade of steel. This affected all the material properties used including those governing reaustenitization, austenite grain growth, austenite decomposition, and hardness. Hardness measurements taken from the literature show a variation of {+-}5 to 30 HV on the same sample. Alloy differences within the allowable range also led to hardness variations of {+-}30 HV for the heat-affected zone of multipass welds. The predicted hardnesses agree extremely well with those taken from the physical simulations.
Numerical simulations of localized high field 1H MR spectroscopy
NASA Astrophysics Data System (ADS)
Kaiser, Lana G.; Young, Karl; Matson, Gerald B.
2008-11-01
The limited bandwidths of volume selective RF pulses in localized in vivo MRS experiments introduce spatial artifacts that complicate spectral quantification of J-coupled metabolites. These effects are commonly referred to as a spatial interference or "four compartment" artifacts and are more pronounced at higher field strengths. The main focus of this study is to develop a generalized approach to numerical simulations that combines full density matrix calculations with 3D localization to investigate the spatial artifacts and to provide accurate prior knowledge for spectral fitting. Full density matrix calculations with 3D localization using experimental pulses were carried out for PRESS (TE = 20, 70 ms), STEAM (TE = 20, 70 ms) and LASER (TE = 70 ms) pulse sequences and compared to non-localized simulations and to phantom solution data at 4 T. Additional simulations at 1.5 and 7 T were carried out for STEAM and PRESS (TE = 20 ms). Four brain metabolites that represented a range from weak to strong J-coupling networks were included in the simulations (lactate, N-acetylaspartate, glutamate and myo-inositol). For longer TE, full 3D localization was necessary to achieve agreement between the simulations and phantom solution spectra for the majority of cases in all pulse sequence simulations. For short echo time (TE = 20 ms), ideal pulses without localizing gradients gave results that were in agreement with phantom results at 4 T for STEAM, but not for PRESS (TE = 20). Numerical simulations that incorporate volume localization using experimental RF pulses are shown to be a powerful tool for generation of accurate metabolic basis sets for spectral fitting and for optimization of experimental parameters.
Restoration of cloud contaminated ocean color images using numerical simulation
NASA Astrophysics Data System (ADS)
Yang, Xuefei; Mao, Zhihua; Chen, Jianyu; Huang, Haiqing
2015-10-01
It is very hard to access cloud-free remote sensing data, especially for the ocean color images. A cloud removal approach from ocean color satellite images based on numerical modeling is introduced. The approach removes cloud-contaminated portions and then reconstructs the missing data utilizing model simulated values. The basic idea is to create the relationship between cloud-free patches and cloud-contaminated patches under the assumption that both of them are influenced by the same marine hydrodynamic conditions. Firstly, we find cloud-free GOCI (the Geostationary Ocean Color Imager) retrieved suspended sediment concentrations (SSC) in the East China Sea before and after the time of cloudy images, which are set as initial field and validation data for numerical model, respectively. Secondly, a sediment transport model based on COHERENS, a coupled hydrodynamic-ecological ocean model for regional and shelf seas, is configured. The comparison between simulated results and validation images show that the sediment transport model can be used to simulate actual sediment distribution and transport in the East China Sea. Then, the simulated SSCs corresponding to the cloudy portions are used to remove the cloud and replace the missing values. Finally, the accuracy assessments of the results are carried out by visual and statistical analysis. The experimental results demonstrate that the proposed method can effectively remove cloud from GOCI images and reconstruct the missing data, which is a new way to enhance the effectiveness and availability of ocean color data, and is of great practical significance.
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.
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 Delamination Growth in Composite Materials
NASA Technical Reports Server (NTRS)
Camanho, P. P.; Davila, C. G.; Ambur, D. R.
2001-01-01
The use of decohesion elements for the simulation of delamination in composite materials is reviewed. The test methods available to measure the interfacial fracture toughness used in the formulation of decohesion elements are described initially. After a brief presentation of the virtual crack closure technique, the technique most widely used to simulate delamination growth, the formulation of interfacial decohesion elements is described. Problems related with decohesion element constitutive equations, mixed-mode crack growth, element numerical integration and solution procedures are discussed. Based on these investigations, it is concluded that the use of interfacial decohesion elements is a promising technique that avoids the need for a pre-existing crack and pre-defined crack paths, and that these elements can be used to simulate both delamination onset and growth.
Numerical Simulation of Liquid Nitrogen Chilldown of a Vertical Tube
NASA Technical Reports Server (NTRS)
Darr, Samuel; Hu, Hong; Schaeffer, Reid; Chung, Jacob; Hartwig, Jason; Majumdar, Alok
2015-01-01
This paper presents the results of a one-dimensional numerical simulation of the transient chilldown of a vertical stainless steel tube with liquid nitrogen. The direction of flow is downward (with gravity) through the tube. Heat transfer correlations for film, transition, and nucleate boiling, as well as critical heat flux, rewetting temperature, and the temperature at the onset of nucleate boiling were used to model the convection to the tube wall. Chilldown curves from the simulations were compared with data from 55 recent liquid nitrogen chilldown experiments. With these new correlations the simulation is able to predict the time to rewetting temperature and time to onset of nucleate boiling to within 25% for mass fluxes ranging from 61.2 to 1150 kg/(sq m s), inlet pressures from 175 to 817 kPa, and subcooled inlet temperatures from 0 to 14 K below the saturation temperature.
3d Numerical Simulation of Flow Structure in Confluence River
NASA Astrophysics Data System (ADS)
Qing-yuan, Yang; Yi, Sun; Xian-ye, Wang; Wei-zhen, Lu; Xie-kang, Wang
2010-05-01
Confluence zones in rivers are common occurrence in natural rivers, and its flow structure, especially secondary flow, has much impact on sediment transport and pollutant dispersion in confluence region. Flume experiment studies have proved the variation of separation zone from the water surface to the bottom, but there are little numerical simulation studies on the scale of separation zone. As the developing of computational fluid dynamics, there are several models to simulate the turbulence properties in the river. This paper uses the standard k-e, RNG k-e and RSM turbulence model to simulate the secondary flow and separation zone in the confluence river, and compared the results with the experiment data quantification ally.
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 space debris impacts on the Whipple shield
NASA Astrophysics Data System (ADS)
Katayama, M.; Toda, S.; Kibe, S.
1997-06-01
The authors carried out three series of experimental tests of the first bumper perforation and main wall cratering processes directly caused by three types of projectiles with about 2, 4 and 7 km s -1 impact velocities but comparable initial kinetic energies, by using three different accelerators (one-stage powder gun, two-stage light-gas gun and rail gun), for the purpose of investigating space debris hypervelocity impacts onto single-walled Whipple bumper shields [1]. In the present study, after reviewing the numerical simulation method of hydrocode for both Eulerian and Lagrangian descriptions, a number of parametric numerical simulation analyses using multiple material Eulerian methods were performed in order to optimize the material properties of bumper and main wall materials through comparison with experimental results of single target impacts by the projectiles. In particular, the material data on the dynamic fracture phenomena are discussed in detail in the first part. Then a couple of numerical calculations using the interactive Lagrangian rezoning method to simulate the overall impact process against the single walled Whipple shield were performed and compared with the corresponding experimental results. Both results indicated fairly good agreement with each other. Moreover, it was demonstrated that the present method is helpful and efficient in understanding the impact phenomena and fracture mechanism in the space debris hypervelocity impact problem. Finally the multiple material Eulerian method was applied to the same problems modeled by the interactive Lagrangian rezoning method used previously, because the former is much easier to use for almost all users, although it is more diffusive and unclear of material boundaries than the latter. Those two kinds of numerical results also indicated fairly good agreements with each other.
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.
Numerical Simulation of a Reactive Flow in an Overexpanded Nozzle
NASA Astrophysics Data System (ADS)
Sainte-Rose, B.; Bertier, N.; Dupoitieux, F.
2009-01-01
In this paper we present numerical simulations of a reactive flow in an overexpanded cryotechnic planar nozzle. This work tends to show the limits of classic Reynolds Averaged Navier Stokes (RANS) approaches to predict a post-combustion region which was experimentally evidenced inside the nozzle extension. To cope with these limitations, we propose an hybrid RANS-Large Eddy Simulation (LES) method called Delayed Detached Eddy Simulation (DDES) which has not often been used to simulate reactive flows. Indeed this model has been created to treat near wall flows and is an affordable solution to simulate complex unsteady compressible flows, and to have access to accurate skin friction and wall thermal fluxes. The test case studied was proposed in the frame of the ATAC1 program, the simulations presented here were made using a bi-dimensional grid; however, to be fully relevant in agreement with the physics of turbulence, such methods would require a tri-dimensional grid nevertheless interesting remarks can be drawn.
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.
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.
Numerical simulation and parameterization of orographic gravity waves
Kim, Y.J.
1992-01-01
The problem of parameterizing gravity waves generated by subgridscale orography has recently been recognized important for large-scale models of the atmosphere. It has been pointed out that excessively strong westerlies may appear in the mid-latitude northern hemisphere winter if subgrid-scale orographic gravity-wave drag is neglected in high-resolution simulations. Parameterization schemes for orographic gravity waves have been evaluated only on merits of overall improvement of simulated large-scale fields. This study follows a direct way of assessing parameterization schemes by using a numerical mesoscale gravity-wave model: Regarding a subdomain of the mesoscale model as a horizontal grid interval of a large-scale model, the author compares two vertical drag profiles-one estimated from a parameterization scheme applied to the subdomain-averaged variables and the other calculated directly from the variables at individual grid points of the mesoscale model. The author reviews studies of gravity waves over mesoscale orography and the ideas behind the parameterization of orographic gravity waves. A two-dimensional numerical anelastic nonhydrostatic gravity-wave model the author has developed is described for this study and presents results simulated by the model. Through comparisons with the numerically simulated results, it is found the treatment of low-level wave-breaking and the choice of parameters representing orography in existing parameterization schemes should be improved: It was found that not only the standard deviation of orography, which is the only information regarding orography in existing schemes, but also higher moments of orography, such as the asymmetry and convexity of orography, should be considered. In an attempt to correct these problems, the author proposes a revised scheme that uses the additional orographic statistics that was devised. The author first tests the scheme for isolated orography cases and finds encouraging results.
Numerical simulations of solar and stellar convection and oscillations
NASA Astrophysics Data System (ADS)
Giorgobiani, Dali
The Sun has a resonant cavity between the surface where the density decreases rapidly due to its low temperature and the interior where the sound speed increases with increasing temperature. Sound waves are trapped in this resonant cavity, and thousands of these p-mode oscillations are observed in the velocity Doppler shift and intensity variations of the lines in the solar spectrum. These resonant modes are excited by convection near the solar surface and are used to probe both the local and global structure of the Sun. The question of interest here is excitation of acoustic waves by convection, and the interaction between convection and the resonant p-modes. Turbulent motions stochastically excite the resonant modes via Reynolds stresses and entropy fluctuations. Interaction between the convective motions and the waves modifies the mode frequencies, spectrum and amplitudes. We investigate turbulence and its interaction with oscillations by means of the realistic three-dimensional numerical simulations of the shallow upper layer of the solar convective zone. We use the numerical code of Stein & Nordlund, which solves 3D system of the compressible (magneto) hydrodynamic equations and includes LTE radiative transfer near the visible surface. The properties of oscillation modes in the simulation closely match their observed characteristics. This means that our numerical model reproduces the basic properties of solar oscillations. This is an important step in studying the physical properties of solar oscillations and their interaction with turbulence. The similarity of the oscillation mode properties in the simulation and observations means that the simulations can be used to investigate the origin of mode behavior and its interaction with turbulent plasma. The frequency spectra of the solar acoustic modes are asymmetric. We study the corresponding asymmetry of the simulation modes in order to understand its origin and its relation to the excitation sources. We find that radiative transfer on top of the convection zone can be responsible for the acoustic mode asymmetry reversal. Acoustic mode excitation rates depend on the details of the turbulent energy spectra. We analyze spatial and temporal components of the spectra in the simulations to learn more about stellar turbulent convection and its role in mode driving. We use similar simulations of convection in other stars to calculate their mode excitation rates, and determine how p-mode driving depends on stellar parameters.
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.
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
Numerical resolution effects on simulations of massive black hole seeds
NASA Astrophysics Data System (ADS)
Regan, John A.; Johansson, Peter H.; Haehnelt, Martin G.
2014-03-01
We have performed high-resolution numerical simulations with the hydrodynamical adaptive mesh refinement code ENZO to investigate the formation of massive seed black holes in a sample of six dark matter haloes above the atomic cooling threshold. The aim of this study is to illustrate the effects of varying the maximum refinement level on the final object formed. The virial temperatures of the simulated haloes range from T 10 000 to 16 000 K and they have virial masses in the range M 2 107 to 7 107 M? at z 15. The outcome of our six fiducial simulations is both generic and robust. A rotationally supported, marginally gravitationally stable, disc forms with an exponential profile. The mass and scale length of this disc depends strongly on the maximum refinement level used. Varying the maximum refinement level by factors between 1/64 and 256 times the fiducial level illustrates the care that must be taken in interpreting the results. The lower resolution simulations show tentative evidence that the gas may become rotationally supported out to 20 pc while the highest resolution simulations show only weak evidence of rotational support due to the shorter dynamical times for which the simulation runs. The higher resolution simulations do, however, point to fragmentation at small scales of the order of 100 au. In the highest resolution simulations a central object of a few times 102 M? forms with multiple strongly bound, Jeans unstable, clumps of ?10 M? and radii of 10-20 au suggesting the formation of dense star clusters in these haloes.
Impact of multi-component diffusion in turbulent combustion using direct numerical simulations
Bruno, Claudio; Sankaran, Vaidyanathan; Kolla, Hemanth; Chen, Jacqueline H.
2015-08-28
This study presents the results of DNS of a partially premixed turbulent syngas/air flame at atmospheric pressure. The objective was to assess the importance and possible effects of molecular transport on flame behavior and structure. To this purpose DNS were performed at with two proprietary DNS codes and with three different molecular diffusion transport models: fully multi-component, mixture averaged, and imposing the Lewis number of all species to be unity.
Numerical Simulation of Seimic Behavior of Pipeline in Liquefiable Soil
NASA Astrophysics Data System (ADS)
Zou, Degao; Kong, Xianjing; Xu, Bin
This study focused on the behavior of a burial pipe with special reference to its stability against floatation subject to soil liquefaction. The excess pore water pressure response behaviors of soil foundations, and the effectiveness of different types of drainage or reinforcement measures were investigated using Finite Element Method (FEM). FEM numerical model is a coupled stress-flow finite element procedure, based on u-p formulation of dynamic Biot's equations (Zienkiewicz, 1982). The hyperbolic stress and strain relationship was used in the numerical model, which takes into account the stiffness and strength degradation. Pore pressure generation due to earthquake loading was calculated via the pore pressure model (Seed et al, 1979). Performance of the numerical models was studied by simulating a series of shake table tests. Excess pore pressures predicted by numerical models were compared with the pore pressure transducer records during experiments. Also, the effectiveness of different drainage measures against uplifting of pipelines was compared. It was demonstrated that the models were able to provide results in agreement with experiments.
Does the choice of the forcing term affect flow statistics in DNS of turbulent channel flow?
NASA Astrophysics Data System (ADS)
Quadrio, Maurizio; Frohnapfel, Bettina; Hasegawa, Yosuke
2016-01-01
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 of such events explains their negligible contribution to the commonly computed statistics; however, the very existence of a difference in the PDF demonstrates that the forcing term is not entirely uninfluential. Other statistics for wall-based quantities (the two components of friction and pressure) are examined; in particular spatio-temporal autocorrelations show small differences at large temporal separations, where unfortunately the residual statistical uncertainty is still of the same order of the observed difference. Hence we suggest that the specific choice of the forcing term does not produce important statistical consequences, unless one is interested in the strongest events of high wall friction, that are underestimated by a simulation run at constant flow rate.
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 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
NASA Astrophysics Data System (ADS)
Wang, P.; Hamila, N.; Boisse, P.
2011-05-01
One important perspective in aeronautics is to produce large, thick or/and complex structural composite parts. The forming stage presents an important role during the whole manufacturing process, especially for LCM processes (Liquid Composites Moulding) or CFRTP (Continuous Fibre Reinforcements and Thermoplastic resin). Numerical simulations corresponding to multi-layered composite forming allow the prediction for a successful process to produce the thick parts, and importantly, the positions of the fibres after forming to be known. This paper details a set of simulation examples carried out by using a semi-discrete shell finite element made up of unit woven cells. The internal virtual work is applied on all woven cells of the element taking into account tensions, in-plane shear and bending effects. As one key problem, the contact behaviours of tool/ply and ply/ply are described in the numerical model. The simulation results not only improve our understanding of the multi-layered composite forming process but also point out the importance of the fibre orientation and inter-ply friction during formability.
Numerical 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.
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.
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.
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.
Numerical simulations of drop impact on superhydrophobic structured surfaces
NASA Astrophysics Data System (ADS)
Guzzetti, Davide; Larentis, Stefano; Pugno, Nicola
2011-11-01
During the last decade drop impact dynamics on superhydrophobic surfaces has been intensively investigated because of the incredible properties of water repellency exhibited by this kind of surfaces, mostly inspired by biological examples such as Lotus leave. Thanks to the recent progress in micro-fabrication technology is possible to tailor surfaces wettability defining specific pillar-like structured surfaces. In this work, the behavior of impinging drops on these pillar-like surfaces is simulated, characterizing temporal evolution of droplets contact radius and drop maximal deformation dependence on Weber number. Numerical simulations results are compared with theoretical and experimental results guaranteeing simulation reliability. Fingering patterns obtained from drop impact has been studied obtaining a correlation between number of fingers and Weber number. Drop fragmentation pattern obtained from simulations supports the proposed correlation. Different drop impact outcomes (e.g. rebound, fragmentation) on structured superhydrophobic surfaces are simulated, focusing on the influence of micro-structured surface geometrical pattern. This investigation is relevant in order to define design rules for possible reliable non wettable surfaces. Financial support by Alta Scuola Politecnica.
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.
Numerical simulation of MHD shock waves in the solar wind
NASA Technical Reports Server (NTRS)
Steinolfson, R. S.; Dryer, M.
1978-01-01
The effects of the interplanetary magnetic field on the propagation speed of shock waves through an ambient solar wind are examined by numerical solutions of the time-dependent nonlinear equations of motion. The magnetic field always increases the velocity of strong shocks. Although the field may temporarily slow down weak shocks inside 1 AU, it eventually also causes weak shocks to travel faster than they would without the magnetic field at larger distances. Consistent with the increase in the shock velocity, the gas pressure ratio across a shock is reduced considerably in the presence of the magnetic field. The numerical method is used to simulate (starting at 0.3 AU) the large deceleration of a shock observed in the lower corona by ground-based radio instrumentation and the more gradual deceleration of the shock in the solar wind observed by the Pioneer 9 and Pioneer 10 spacecraft.
Numerical simulations of electrostatically driven jets from nonviscous droplets
NASA Astrophysics Data System (ADS)
Garzon, M.; Gray, L. J.; Sethian, J. A.
2014-03-01
The evolution of a perfectly conducting and nonviscous fluid, under the action of an electric field (uniform at infinity), is studied numerically. Level set techniques are employed to develop an Eulerian potential flow model that can follow the drop evolution past breakup, while the free surface fluid velocity and the electric field force are obtained via axisymmetric boundary integral calculations. Numerical results are presented for neutral and charged droplets and for free charged droplets. In all cases, the evolution droplet aspect ratio, progeny droplet size, Taylor cone angles, jet shapes, and self-similar scaling exponents are reported. In particular, for free charged water droplets, the bursting frequency and other jetting characteristics have been carefully analyzed. Wherever possible, these results are compared with previously reported experiments and simulations.
Numerical simulation of reconnection in an emerging magnetic flux region
NASA Technical Reports Server (NTRS)
Forbes, T. G.; Priest, E. R.
1984-01-01
Numerical solutions in two dimensions are presented for the resistive MHD equations of an initial boundary value problem, simulating reconnection between an emerging magnetic flux region and an overlying coronal magnetic field. This numerical model displays four main phases, which are interpreted in terms of (1) a slowly evolving quasi-steady phase during which most of the magnetic flux emerges, with approximate equilibrium between magnetic and pressure forces; (2) an impulsive phase in which either the reconnection or continuing emergence of the first phase comes to disrupt the equilibrium, and extensive acceleration occurs as the high pressure region within the emerging region drives the fluid upwards and outwards; (3) a second quasi-steady phase; and (4) a potential-static phase in which continuing reconnection, ohmic dissipation, and fluid transport through boundaries depletes the system of all currents and flows.
Direct numerical simulation of turbulent flow with an impedance condition
NASA Astrophysics Data System (ADS)
Olivetti, Simone; Sandberg, Richard D.; Tester, Brian J.
2015-05-01
DNS solutions for a pipe/jet configuration are re-computed with the pipe alone to investigate suppression of previously identified internal noise source(s) with an acoustic liner, using a time domain acoustic liner model developed by Tam and Auriault (AIAA Journal, 34 (1996) 913-917). Liner design parameters are chosen to achieve up to 30 dB attenuation of the broadband pressure field over the pipe length without affecting the velocity field statistics. To understand the effect of the liner on the acoustic and turbulent components of the unsteady wall pressure, an azimuthal/axial Fourier transform is applied and the acoustic and turbulent wavenumber regimes clearly identified. It is found that the spectral component occupying the turbulent wavenumber range is unaffected by the liner whereas the acoustic wavenumber components are strongly attenuated, with individual radial modes being evident as each cuts on with increasing Strouhal number.
Numerical simulation of compressible flows with application to noise control
NASA Astrophysics Data System (ADS)
Reichert, Robert Stephen
1998-09-01
Spatial direct numerical simulations of compressible plane jets exhausting into parallel streams are examined. The inviscid mathematical model captures the most dynamically important, large-scale mixing events, such as vortex roll-up and pairings. Two-dimensional simulations employ the fully explicit (2-4) method, while three- dimensional simulations use implicit spatial compact differencing with Runge-Kutta time-advancement. All simulations utilize characteristic-based boundary conditions. Shear layer linear instability computations validate the numerical methods. Time averages over long streamwise jet domains capture the mean field and turbulence structure. The convective Mach number Mc is varied to assess compressibility effects on the nearly self-similar region. The two-dimensional simulations demonstrate that the model is capable of capturing the correct jet spreading and decay. Mean streamwise excess velocity profiles are unaffected by compressibility, but normal entrainment velocity is essentially shut off at low-to-moderate compressibility (Mc /le 0.4). Two-dimensional computations are unable to capture the turbulence structure of the jet due to their artificial vortex organization in the fully developed region. The three-dimensional computations examine one identical case, Mc = 0.4, to assess the validity of the two- dimensional computations. Additionally, the three- dimensional model allows extension to a higher compressibility, Mc = 0.7, at which smaller three- dimensional structures dominate. Excess velocity profiles are again the same, but entrainment velocity is not completely eliminated like in two dimensions. The three- dimensional inviscid model adequately captures the turbulence structure. Good qualitative agreement is obtained between the computed turbulent statistics and available (incompressible) experimental data. Two-point correlations in the normal direction provide insight into the spatial evolution of organized structures. The (2-4) scheme and characteristic boundary conditions are also employed to compute acoustic wave propagation in an acoustically lined duct. The motivation is to assess the effect on sound attenuation of bias flow passed through the liner for application to noise suppression in jet engine nacelles. The mathematical model lumps the liner presence into a continuous empirical momentum equation source term which mimics the liner's acoustic impedance behavior. This source term deter- mines the time-domain effects of the nonlinear impedance of the liner's component porous sheets. The source term constants are matched to complex impedance data via a one-dimensional numerical impedance tube simulation. Sound pressure levels and axially transmitted power in a two-dimensional duct are computed to examine attenuation with different bias flows and frequencies. A final case considers noise propagation in the presence of a complicated hydrodynamic field, computed using the two- dimensional jet code.
Numerical Simulation of Tangling in Jet Engine Turbines
NASA Astrophysics Data System (ADS)
Cendn, David A.; Erice, Borja; Glvez, Francisco; Snchez-Glvez, Vicente
2012-12-01
The numerical analysis of certain safety related problems presents serious difficulties, since the large number of components present leads to huge finite element models that can only be solved by using large and expensive computers or by making rough approaches to the problem. Tangling, or clashing, in the turbine of a jet engine airplane is an example of such problems. This is caused by the crash and friction between rotor and stator blades in the turbine after an eventual shaft failure. When facing the study of an event through numerical modelling, the accurate simulation of this problem would require the engineer to model all the rotor and stator blades existing in the turbine stage, using a small element size in all pieces. Given that the number of stator and rotor blades is usually around 200, such simulations would require millions of elements. This work presents a new numerical methodology, specifically developed for the accurate modelling of the tangling problem that, depending on the turbine configuration, is able to reduce the number of nodes up to an order of magnitude without losing accuracy. The methodology, which benefits from the cyclic configuration of turbines, is successfully applied to the numerical analysis of a hypothetical tangling event in a turbine, providing valuable data such as the rotating velocity decrease of the turbine, the braking torque and the damage suffered by the blades. The methodology is somewhat general and can be applied to any problem in which damage caused by the interaction between a rotating and static piece is to be analysed.
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 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 aerodynamic simulation facility preliminary study, volume 1
NASA Technical Reports Server (NTRS)
1977-01-01
A technology forecast was established for the 1980-1985 time frame and the appropriateness of various logic and memory technologies for the design of the numerical aerodynamic simulation facility was assessed. Flow models and their characteristics were analyzed and matched against candidate processor architecture. Metrics were established for the total facility, and housing and support requirements of the facility were identified. An overview of the system is presented, with emphasis on the hardware of the Navier-Stokes solver, which is the key element of the system. Software elements of the system are also discussed.
Numerical simulation of fluid flow around a scramaccelerator projectile
NASA Technical Reports Server (NTRS)
Pepper, Darrell W.; Humphrey, Joseph W.; Sobota, Thomas H.
1991-01-01
Numerical simulations of the fluid motion and temperature distribution around a 'scramaccelerator' projectile are obtained for Mach numbers in the 5-10 range. A finite element method is used to solve the equations of motion for inviscid and viscous two-dimensional or axisymmetric compressible flow. The time-dependent equations are solved explicitly, using bilinear isoparametric quadrilateral elements, mass lumping, and a shock-capturing Petrov-Galerkin formulation. Computed results indicate that maintaining on-design performance for controlling and stabilizing oblique detonation waves is critically dependent on projectile shape and Mach number.
Numerical simulation of low Prandtl number turbulent mixing
NASA Technical Reports Server (NTRS)
Gibson, C.; Rogers, M.; Chasnov, J.; Petresky, J.
1990-01-01
Numerical simulations of turbulent mixing of strongly diffusive scalar fields were carried out with and without subgrid-scale modeling of the small-scale strain field. For low Reynolds number flows, when the rate of strain field (determined primarily by the small scales) is fully resolved, the scalar microstructure was found to collapse under Batchelor rate-of-strain scaling even for small Prandtl numbers, in agreement with Kerr. For high Reynolds number flows, when small-scale straining is modeled with a subgrid-scale model, the scalar microstructure follows the Batchelor, Howells, and Townsend predictions that the small-scale rate-of-strain is irrelevant.
Achieving better cooling of turbine blades using numerical simulation methods
NASA Astrophysics Data System (ADS)
Inozemtsev, A. A.; Tikhonov, A. S.; Sendyurev, C. I.; Samokhvalov, N. Yu.
2013-02-01
A new design of the first-stage nozzle vane for the turbine of a prospective gas-turbine engine is considered. The blade's thermal state is numerically simulated in conjugate statement using the ANSYS CFX 13.0 software package. Critical locations in the blade design are determined from the distribution of heat fluxes, and measures aimed at achieving more efficient cooling are analyzed. Essentially lower (by 50-100°C) maximal temperature of metal has been achieved owing to the results of the performed work.
Numerical simulation of flash radiography on a dense object
NASA Astrophysics Data System (ADS)
Zou, Zhigao; Liang, Decong; Li, Kaibo
1999-06-01
This thesis uses Monte Carlo method to numerically simulate the course of electron and photon transportion. We have Computed and analyzed the image which is formed at film when high energy X-ray irradiates dense object. It analyzes two kinds of calumniators. We have analyzed influences which result from the front and back window. We have got the object attenuation factor of exposure from X-ray source to film, have gained space distribution of exposure at film and have gained that every element contributes to scattered radiation.
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 Moving Vehicle Across the Obstacle
NASA Astrophysics Data System (ADS)
Daniel, ?ubo; Valakov, Veronika; Korti, Jn
2014-12-01
Vehicle excitation by passing over pavement unevenness represents the actual problem which is solved on many departments in this time. The numerical methods are mainly applied for the solutions of the interaction of similar systems. The Finite Element Method (FEM) is the best-known and widely used. It provides appropriate accuracy. In this article, commercial computer software ADINA based on the FEM is used to solve kinematic excitation of vehicle. The 2D model of vehicle which represents real type of lorry T-815 is excited by simulation of passage across obstacle. There are compared dynamic responses of the vehicle on the kinematic excitation at different speeds.
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 γ-flashes (TGF) registered in the nearest space, which may be an argument for a joint EMP and TGF source.
Numerical simulation of carbon arc discharge for nanoparticle synthesis
Kundrapu, M.; Keidar, M.
2012-07-15
Arc discharge with catalyst-filled carbon anode in helium background was used for the synthesis of carbon nanoparticles. In this paper, we present the results of numerical simulation of carbon arc discharges with arc current varying from 10 A to 100 A in a background gas pressure of 68 kPa. Anode sublimation rate and current voltage characteristics are compared with experiments. Distribution of temperature and species density, which is important for the estimation of the growth of nanoparticles, is obtained. The probable location of nanoparticle growth region is identified based on the temperature range for the formation of catalyst clusters.
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.
Numerical Simulation of Low-Density Shock-Wave Interactions
NASA Technical Reports Server (NTRS)
Glass, Christopher E.
1999-01-01
Computational Fluid Dynamics (CFD) numerical simulations of low-density shock-wave interactions for an incident shock impinging on a cylinder have been performed. Flow-field density gradient and surface pressure and heating define the type of interference pattern and corresponding perturbations. The maximum pressure and heat transfer level and location for various interaction types (i.e., shock-wave incidence with respect to the cylinder) are presented. A time-accurate solution of the Type IV interference is employed to demonstrate the establishment and the steadiness of the low-density flow interaction.
2D numerical simulation of the resistive reconnection layer
D. A. Uzdensky; R. M. Kulsrud
2000-07-21
In this paper the authors present a two-dimensional numerical simulation of a reconnection current layer in incompressible resistive magnetohydrodynamics with uniform resistivity in the limit of very large Lundquist numbers. They use realistic boundary conditions derived consistently from the outside magnetic field, and they also take into account the effect of the backpressure from flow into the separatrix region. They find that within a few Alfven times the system reaches a steady state consistent with the Sweet-Parker model, even if the initial state is Petschek-like.
Numerical simulations of an isolated microbursts. II - Sensitivity experiments
NASA Technical Reports Server (NTRS)
Proctor, Fred H.
1989-01-01
A time-dependent, high-resolution, axisymmetric numerical model simulating isolated and stationary microbursts is used to evaluate the sensitivity of microbursts to the environment and other factors. It is found that microbursts intensity is sensitive to ther vertical distirbution of ambient temperature and humidity, the horiontal width of the precipitation shaft or downdraft, the magnitutde of precipitation loading, and the type and duration of precipitation. Scenarios for the generation of an intense microburst and the possible applications of the results are examined.
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 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.
Mean Flow Generation and Inertial Wave Attractors in a Librating Annulus: DNS and Theory
NASA Astrophysics Data System (ADS)
Klein, Marten; Ghasemi V, Abouzar; Seelig, Torsten; Borcia, Ion Dan; Harlander, Uwe; Will, Andreas
2015-04-01
Rotation Ω0 is one of the most important system parameters in geophysical fluid dynamics (GFD) due to stratification of angular momentum. Oscillatory motion (libration) of the fluid over solid surfaces is generated in many systems by harmonic forces, usually resulting from rotation of the fluid in a homogeneous gravitational and/or electromagnetic field. Small amplitudes of libration can excite inertial waves, which are shear waves, band-limited to frequencies 0 < ω < 2Ω0 and dispersive with respect to direction. Large amplitudes of libration can excite turbulent motion (Görtler vortices), which is transpored into the bulk by the Coriolis force. Waves and vortices may take part in the redistribution of kinetic energy and angular momentum. Complex system responses are possible so that it is of fundamental interest to understand inertial waves and related turbulent phenomena in order to quantify their relevance for applications. Direct numerical simulations (DNS) of an axially rotating annulus have been carried out for straight and inclined cylinder walls, with and without lids. The system was perturbed by libration of inner and/or outer cylinder walls and/or of the lids with small libration amplitude ɛ≤ 0.1 and frequency ω0.3 and frequencies ω