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  • NUMERICAL ANALYSIS  (3)
  • Hale-Bopp  (2)
  • Astrophysics  (1)
  • COMPUTER PROGRAMMING AND SOFTWARE  (1)
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Keywords
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  • 1
    Electronic Resource
    Electronic Resource
    Dust-Gas Interrelations In Comets: Observations And Theory (1997)
    Springer
    Earth, moon and planets 79 (1997), S. 275-306 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; Hale-Bopp ; dynamics ; photochemistry
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The development of the expanding atmosphere from the evaporating cometary nucleus has traditionally focused on observing and modeling the separate development of two distinct components, gas and dust,which are coupled dynamically with one another at distances out to a few tens of cometary radii. In the last decade or so, however, direct evidence from observations and suggestions from theory suggest that the dusty-gas coma is a tightly coupled system where material is transferred between the solid and gaseous phase as an important integral part of the basic development of the coma. Comet Hale-Bopp (C/1995 O1) was discovered far from the sun and is the largest and most productive comet, in the sense of release of gas and dust in modern times. This has permitted observations to be made over an unprecedented range of heliocentric distance. This paper presents a review of a range of important issues regarding interrelations between dust and gas in comets, a description of the gas and dust environment for Hale-Bopp, and a summary of the preliminary results from Hale-Bopp which are relevant to these issues. Particular topics include dusty-gas models, dust fading and fragmentation, the role of dust and gas jets, the day/night distribution of gas and dust, and extended sources of gas.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006257922294
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    MHD Simulation of Comets: The Plasma Environment of Comet Hale-Bopp (1997)
    Springer
    Earth, moon and planets 79 (1997), S. 179-207 
    Details
    ISSN: 1573-0794
    Keywords: MHD ; comets ; Hale-Bopp ; cometary x-rays
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract MHD simulation results of the interaction of the expanding atmosphere of comet Hale-Bopp with the magnetized solar wind are presented. At the upstream boundary a supersonic and superalfvénic solar wind enters into the simulation box 25 million km upstream of the nucleus. The solar wind is continuously mass loaded with cometary ions originating from the nucleus. The effects of photoionization, recombination and ion-neutral frictional drag are taken into account in the model. The governing equations are solved on an adaptively refined unstructured Cartesian grid using our MUSCL-type upwind numerical technique, MAUS-MHD (Multiscale Adaptive Upwind Scheme for MHD). The combination of the adaptive refinement with the MUSCL-scheme allows the entire cometary atmosphere to be modeled, while still resolving both the shock and the diamagnetic cavity of the comet. Detailed simulation results for the plasma environment of comet Hale-Bopp for slow and fast solar wind conditions are presented. We also calculate synthetic H2O+, CO+ and soft x-ray images for observing conditions on April 11, 1997.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006289418660
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  • 3
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    Cartesian-cell based grid generation and adaptive mesh refinement (1993)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: Viewgraphs on Cartesian-cell based grid generation and adaptive mesh refinement are presented. Topics covered include: grid generation; cell cutting; data structures; flow solver formulation; adaptive mesh refinement; and viscous flow.
    Keywords: COMPUTER PROGRAMMING AND SOFTWARE
    Type: NASA. Langley Research Center, Unstructured Grid Generation Techniques and Software; p 193-201
    Format: application/pdf
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  • 4
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    An approximate Riemann solver for magnetohydrodynamics (that works in more than one dimension) (1994)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: An approximate Riemann solver is developed for the governing equations of ideal magnetohydrodynamics (MHD). The Riemann solver has an eight-wave structure, where seven of the waves are those used in previous work on upwind schemes for MHD, and the eighth wave is related to the divergence of the magnetic field. The structure of the eighth wave is not immediately obvious from the governing equations as they are usually written, but arises from a modification of the equations that is presented in this paper. The addition of the eighth wave allows multidimensional MHD problems to be solved without the use of staggered grids or a projection scheme, one or the other of which was necessary in previous work on upwind schemes for MHD. A test problem made up of a shock tube with rotated initial conditions is solved to show that the two-dimensional code yields answers consistent with the one-dimensional methods developed previously.
    Keywords: NUMERICAL ANALYSIS
    Type: AD-280296 , NASA-CR-194902 , NAS 1.26:194902 , ICASE-94-24
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  • 5
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    An accuracy assessment of Cartesian-mesh approaches for the Euler equations (1995)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A critical assessment of the accuracy of Cartesian-mesh approaches for steady, transonic solutions of the Euler equations of gas dynamics is made. An exact solution of the Euler equations (Ringleb's flow) is used not only to infer the order of the truncation error of the Cartesian-mesh approaches, but also to compare the magnitude of the discrete error directly to that obtained with a structured mesh approach. Uniformly and adaptively refined solutions using a Cartesian-mesh approach are obtained and compared to each other and to uniformly refined structured mesh results. The effect of cell merging is investigated as well as the use of two different K-exact reconstruction procedures. The solution methodology of the schemes is explained and tabulated results are presented to compare the solution accuracies.
    Keywords: NUMERICAL ANALYSIS
    Type: NASA-TM-111200 , NAS 1.15:111200 , NIPS-96-07173 , (ISSN 0021-9991)
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  • 6
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    A genuinely multi-dimensional upwind cell-vertex scheme for the Euler equations (1989)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The solution of the two-dimensional Euler equations is based on the two-dimensional linear convection equation and the Euler-equation decomposition developed by Hirsch et al. The scheme is genuinely two-dimensional. At each iteration, the data are locally decomposed into four variables, allowing convection in appropriate directions. This is done via a cell-vertex scheme with a downwind-weighted distribution step. The scheme is conservative, and third-order accurate in space. The derivation and stability analysis of the scheme for the convection equation, and the derivation of the extension to the Euler equations are given. Preconditioning techniques based on local values of the convection speeds are discussed. The scheme for the Euler equations is applied to two channel-flow problems. It is shown to converge rapidly to a solution that agrees well with that of a third-order upwind solver.
    Keywords: NUMERICAL ANALYSIS
    Type: NASA-TM-102029 , E-4772 , NAS 1.15:102029 , ICOMP-89-13 , AIAA PAPER 89-0095 , Aerospace Sciences Meeting; Jan 09, 1989 - Jan 12, 1989; Reno, NV; United States
    Format: application/pdf
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  • 7
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    Three-Dimensional Multiscale MHD Model of Cometary Plasma Environments (1996)
    In:  CASI
    Details
    Publication Date: 2019-08-15
    Description: First results of a three-dimensional multiscale MHD model of the interaction of an expanding cometary atmosphere with the magnetized solar wind are presented. The model starts with a supersonic and super-Alfvenic solar wind far upstream of the comet (25 Gm upstream of the nucleus) with arbitrary interplanetary magnetic field orientation. The solar wind is continuously mass loaded with cometary ions originating from a 10-km size nucleus. The effects of photoionization, electron impact ionization, recombination, and ion-neutral frictional drag are taken into account in the model. The governing equations are solved on an adaptively refined unstructured Cartesian grid using our new multiscale upwind scalar conservation laws-type numerical technique (MUSCL). We have named this the multiscale adaptive upwind scheme for MHD (MAUS-MHD). The combination of the adaptive refinement with the MUSCL-scheme allows the entire cometary atmosphere to be modeled, while still resolving both the shock and the diamagnetic cavity of the comet. The main findings are the following: (1) Mass loading decelerates the solar wind flow upstream of the weak cometary shock wave (M approximately equals 2, M(sub A) approximately equals 2), which forms at a subsolar standoff distance of about 0.35 Gm. (2) A cometary plasma cavity is formed at around 3 x 10(exp 3) km from the nucleus. Inside this cavity the plasma expands outward due to the frictional interaction between ions and neutrals. On the nightside this plasma cavity considerably narrows and a relatively fast and dense cometary plasma beam is ejected into the tail. (3) Inside the plasma cavity a teardrop-shaped inner shock is formed, which is terminated by a Mach disk on the nightside. Only the region inside the inner shock is the 'true' diamagnetic cavity. (4) The model predicts four distinct current systems in the inner coma: the density peak current, the cavity boundary current, the inner shock current, and finally the cross-tail current. (5) The calculated plasma parameters (magnetic field, plasma density, speed, and temperature) are in very good agreement with published Giotto observations.
    Keywords: Astrophysics
    Type: NASA-CR-204732 , NAS 1.26:204732 , Paper-96JA01075 , Journal of Geophysical Research (ISSN 0148-0227); 101; A7; 15,233-15,253
    Format: text
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