Home  About us  Contact
 

Previous Simulations (previous + simulation)

Distribution by Scientific Domains
Physics and Astronomy40%

Selected Abstracts

Importance of the accretion process in asteroid thermal evolution: 6 Hebe as an example

METEORITICS & PLANETARY SCIENCE, Issue 5 2003
Amitabha Ghosh
Previous simulations of asteroid heat transfer have assumed that accretion was instantaneous. For the first time, we present a thermal model that assumes a realistic (incremental) accretion scenario and takes into account the heat budget produced by decay of 26Al during the accretion process. By modeling 6 Hebe (assumed to be the H chondrite parent body), we show that, in contrast to results from instantaneous accretion models, an asteroid may reach its peak temperature during accretion, the time at which different depth zones within the asteroid attain peak metamorphic temperatures may increase from the center to the surface, and the volume of high-grade material in the interior may be significantly less than that of unmetamorphosed material surrounding the metamorphic core. We show that different times of initiation and duration of accretion produce a spectrum of evolutionary possibilities, and thereby, highlight the importance of the accretion process in shaping an asteroid's thermal history. Incremental accretion models provide a means of linking theoretical models of accretion to measurable quantities (peak temperatures, cooling rates, radioisotope closure times) in meteorites that were determined by their thermal histories. [source]

Influence of reaction mechanisms, grid spacing, and inflow conditions on the numerical simulation of lifted supersonic flames

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2010
P. Gerlinger
Abstract The simulation of supersonic combustion requires finite-rate chemistry because chemical and fluid mechanical time scales may be of the same order of magnitude. The size of the chosen reaction mechanism (number of species and reactions involved) has a strong influence on the computational time and thus should be chosen carefully. This paper investigates several hydrogen/air reaction mechanisms frequently used in supersonic combustion. It is shown that at low flight Mach numbers of a supersonic combustion ramjet (scramjet), some kinetic schemes can cause highly erroneous results. Moreover, extremely fine computational grids are required in the lift-off region of supersonic flames to obtain grid-independent solutions. The fully turbulent Mach 2 combustion experiment of Cheng et al. (Comb. Flame 1994; 99: 157,173) is chosen to investigate the influences of different reaction mechanisms, grid spacing, and inflow conditions (contaminations caused by precombustion). A detailed analysis of the experiment will be given and errors of previous simulations are identified. Thus, the paper provides important information for an accurate simulation of the Cheng et al. experiment. The importance of this experiment results from the fact that it is the only supersonic combustion test case where temperature and species fluctuations have been measured simultaneously. Such data are needed for the validation of probability density function methods. Copyright © 2009 John Wiley & Sons, Ltd. [source]

A new force field for simulating phosphatidylcholine bilayers

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2010
David Poger
Abstract A new force field for the simulation of dipalmitoylphosphatidylcholine (DPPC) in the liquid-crystalline, fluid phase at zero surface tension is presented. The structure of the bilayer with the area per lipid (0.629 nm2; experiment 0.629,0.64 nm2), the volume per lipid (1.226 nm3; experiment 1.229,1.232 nm3), and the ordering of the palmitoyl chains (order parameters) are all in very good agreement with experiment. Experimental electron density profiles are well reproduced in particular with regard to the penetration of water into the bilayer. The force field was further validated by simulating the spontaneous assembly of DPPC into a bilayer in water. Notably, the timescale on which membrane sealing was observed using this model appears closer to the timescales for membrane resealing suggested by electroporation experiments than previous simulations using existing models. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 [source]

Mass loss and yield uncertainty in low-mass asymptotic giant branch stars

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
Richard J. Stancliffe
ABSTRACT We investigate the uncertainty in surface abundances and yields of asymptotic giant branch (AGB) stars. We apply three different mass-loss laws to a 1.5-M, star of metallicity Z= 0.008 at the beginning of the thermally pulsing-asymptotic giant branch (TP-AGB) phase. Efficient third dredge-up is found even at very low envelope mass, contrary to previous simulations with other evolution codes. We find that the yield of carbon is uncertain by about 15 per cent and for most other light elements the yield is uncertain at the level of 20,80 per cent. For iron group elements, the uncertainty varies from around 30 per cent for the more-abundant species to over a factor of 2 for the less-abundant radioactive species, like 60Fe. The post-AGB surface abundances for this mass and metallicity are much more uncertain due to the dilution of dredged-up material in differing envelope masses in the later stages of the models. Our results are compared to known planetary nebula and post-AGB abundances. We find that the models are mostly consistent with observations but we are unable to reproduce observations of some of the isotopes. [source]

Moisture,convection feedback in the tropics

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 604 2004
W. W. Grabowski
Abstract This paper discusses the large-scale moisture,convection feedback in the tropics, where spatial fluctuations of deep convection cause perturbations of free-tropospheric moisture which, in turn, affect the spatial distribution of deep convection. A simple heuristic argument using the timescale of free-tropospheric humidity change explains why moisture,convection feedback is particularly relevant for tropical intraseasonal oscillations. The large-scale dynamical context for moisture,convection feedback is investigated in idealized rotating constant-sea-surface-temperature (,tropics everywhere') aquaplanet using cloud-resolving convection parametrization (CRCP; super-parametrization) and a traditional convective parametrization (the Emanuel scheme). The large-scale organization of convection within the equatorial waveguide takes the form of MJO-like (Madden,Julian Oscillation) coherent structures. First, CRCP simulations are performed in which development of large-scale free-tropospheric moisture perturbations is artificially suppressed using relaxation with a timescale of one day. As in previous simulations where much shorter relaxation timescale was used, MJO-like coherences do not develop and, if already present, they disintegrate rapidly. Second, CRCP simulations that start from planetary-scale moisture perturbation in the free troposphere are conducted. The ensuing large-scale velocity perturbations have e-folding times of five and seven days, respectively, for interactive and prescribed radiation simulations. This supports the conjecture that interactive radiation enhances moisture,convection feedback; an enhanced large-scale circulation results from differences in radiative cooling between areas having enhanced and suppressed convectively-generated moisture and cloudiness. Additional support for the role of moisture,convection feedback in intraseasonal oscillations is seen in simulations that apply the Emanuel scheme. The standard configuration of the Emanuel scheme is insensitive to free-tropospheric humidity and results in weak MJO-like coherences. A simple modification of the Emanuel scheme that enhances its sensitivity to free-tropospheric humidity dramatically improves the simulated MJO-like coherences. Copyright © 2004 Royal Meteorological Society [source]