Variance reduction by importance sampling and the method of splitting in Monte Carlo calculations
Variance reduction by importance sampling and method of splitting in Monte Carlo calculations
Rosenbaum, B. M.↗
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On the rate at which comets split
We use time-resolved charge-coupled device (CCD) images to assess the frequency of splitting of comets. When blinked on a computer, the CCD images provide relatively high and uniform sensitivity to comoving companions. We find that 3 comets are split in a sample of 49, and estimate that the cometary splitting rate is S approximately 0.01 per year per comet or larger. This large rate suggests that splitting may be an important destructive process for the cometary nuclei.
Chen, Jun↗
Convective dynamics
In addressing meteorological processes, including convection, it is convenient to separate these processes into their kinematic, dynamic, and microphysical components. Dynamic processes deal with the forces that initiate and sustain or alter motion, while kinematic processes describe the motion field. Microphysical processes address the distribution of water substance. This review of convective dynamics is organized along the following lines. First is an assessment of radar capability, particularly the use of one or more Doppler radars, to portray the wind field. Then various types of precipitating moist convection are reviewed. Each type or mode of convection results from a distinct environment. The principal interest of dynamics here is its role in the convection's evolution, structure, and movement. Thus, the important processes of storm rotation, splitting and propagation, including tornadogenesis, are examined.
Ray, Peter S.↗
A new Lagrangian method for real gases at supersonic speed
With the renewed interest in high speed flights, the real gas effect is of theoretical as well as practical importance. In the past decade, upwind splittings or Godunov-type Riemann solutions have received tremendous attention and as a result significant progress has been made both in the ideal and non-ideal gas. In this paper, we propose a new approach that is formulated using the Lagrangian description, for the calculation of supersonic/hypersonic real gas inviscid flows. This new formulation avoids the grid generation step which is automatically obtained as the solution procedure marches in the 'time-like' direction. As a result, no remapping is required and the accuracy is faithfully maintained in the Lagrangian level. In this paper, we give numerical results for a variety of real gas problems consisting of essential elements in high speed flows, such as shock waves, expansion waves, slip surfaces and their interactions. Finally, calculations for flows in a generic inlet and nozzle are presented.
Loh, C. Y.↗
The 100th Anniversary of the Fall of Nakhla: The Subdivision of BM1913,25
This year marks the 100th anniversary of the fall of Nakhla, a cumulate clinopyroxenite of martian origin that fell near Alexandria, Egypt in 1911. Multiple fragments of the meteorite were seen to fall over an area of 4.5 km in diameter. Approximately 40 stones were recovered with a combined weight of about 10 kg. Most of the larger specimens found their way to museums and meteorite collections in Cairo, Paris, Berlin, and the Smithsonian, to name a few. In 1998, the British Museum sent a 641g (BM1913,25), fully fusion crusted stone of Nakhla to the Johnson Space Center (JSC) for processing in the Antarctic Meteorite Lab in order to allocate samples to the scientific community. The stone was split in half in a dry nitrogen glove box. One half of the stone was sent back to the museum and the other half (346g) was used for sample allocations. From 1998-2001, 37 scientists requested 65 separate samples of Nakhla, including 2 thin sections This set of allocations was especially important in that all of the sample splits are from the same piece of Nakhla and it had a known history since it was acquired by the museum in 1913. With the multiple fragments of Nakhla, it is not known from what pieces the main bulk of research has been done, what variation may exist between all the pieces and to what contaminants the fragments may have been exposed, (i.e. water, solvents or cutting fluids, etc.). All of the allocations prepared at JSC were processed in a nitrogen cabinet using only stainless steel, aluminum, and Teflon tools and containers to reduce the chance of introducing any new contaminants. The focused effort to subdivide and distribute samples of Nakhla to the meteorite community resulted in enhanced under-standing of Nakhla and nakhlites in general: organic geochemistry, weathering, sulfur isotopes, radiometric age, and magmatic history. There are 13 Nakhlites that have been recovered to date: Nakhla, Layfayette, Governador Valdares, three from NW Africa, three from Yamato and four from Miller Range regions in Antarctica. The Yamato Nakhlites are paired as are the Miller Range samples.
McBridge, Kathleen M.↗
Reynolds-stress and dissipation rate budgets in a turbulent channel flow
The budgets for the Reynolds stresses and for the dissipation rate of the turbulence kinetic energy are computed using direct simulation data of a turbulent channel flow. The budget data reveal that all the terms in the budget become important close to the wall. For inhomogeneous pressure boundary conditions, the pressure-strain term is split into a return term, a rapid term, and a Stokes term. The Stokes term is important close to the wall. The rapid and return terms play different roles depending on the component of the term. A split of the velocity pressure-gradient term into a redistributive term and a diffusion term is proposed, which should be simpler to model. The budget data is used to test existing closure models for the pressure-strain term, the dissipation rate, and the transport rate. In general, further work is needed to improve the models.
Mansour, N. N.↗
Reynolds-stress and dissipation-rate budgets in a turbulent channel flow
The budgets for the Reynolds stresses and for the dissipation rate of the turbulence kinetic energy are computed using direct simulation data of a turbulent channel flow. The budget data reveal that all the terms in the budget become important close to the wall. For inhomogeneous pressure boundary conditions, the pressure-strain term is split into a return term, a rapid term, and a Stokes term. The Stokes term is important close to the wall. The rapid and return terms play different roles depending on the component of the term. A split of the velocity pressure-gradient term into a redistributive term and a diffusion term is proposed, which should be simpler to model. The budget data is used to test existing closure models for the pressure-strain term, the dissipation rate, and the transport rate. In general, further work is needed to improve the models.
Mansour, N. N.↗
Threshold kinetics of a solar-simulator-pumped iodine laser
A model of the chemical kinetics of the n-C3F7I solar-simulator-pumped iodine laser is utilized to study the major kinetic processes associated with the threshold behavior of this experimental system. Excited-state diffusion to the cell wall is the dominant limiting factor below 5 torr. Excited-state diffusion to the cell wall is the dominant limiting factor below 5 torr. Excited-state recombination with the alkyl radical and quenching by the parent gas control threshold at higher pressures. Treatment of the hyperfine splitting and uncertainty in the pressure broadening are important factors in fixing the threshold level. In spite of scatter in the experimental data caused by instabilities in the simulator high-pressure high-pressure arc, reasonable agreement is achieved between the model and experiment. Model parameters arrived at are within the uncertainty range of values found in the literature.
Wilson, J. W.↗
Voyager 2 Uranus and Neptune targeting
Targeting strategies are developed for the Voyager 2 flybys of Uranus and Neptune/Triton. The need to maximize science return, conserve propellant, and maintain spacecraft safety presents a challenge, given the difficulty in estimating the spacecraft orbit relative to these outer planets. Expected propellant usage, science return, and targeting complexity are presented for each targeting strategy. For the dual encounter of Neptune and its satellite Triton, split targeting conditions are proposed to fix the most important conditions at each body, and thus minimize science losses resulting from Triton ephemeris uncertainties.
Gray, D. L.↗
Oscillations of a rotating liquid drop
The effect of rotation on the oscillation frequencies of a liquid drop is investigated under the assumptions that the drop is imbedded in a fluid of the same or different density and that the interface between drop and fluid is acted on by constant surface tension. While rotation influences the oscillations through both Coriolis force and the centrifugal distortion of the drop, only the former is important for nonaxisymmetric oscillations in first approximation, causing the predicted splitting of the frequency for the two modes that differ in circular polarization sign with respect to the axis of rotation. In axisymmetric oscillations, the centrifugal distortion and the Coriolis force combine to increase frequency in the cases where drop density exceeds that of the fluid.
Busse, F. H.↗
Atomic oxygen fine-structure splittings with tunable far-infrared spectroscopy
Fine-structure splittings of atomic oxygen (O-16) in the ground state have been accurately measured using a tunable far-infrared spectrometer. The 3P0-3pl splitting is 2,060,069.09 (10) MHz, and the 3Pl-3P2 splitting is 4,744,777.49 (16) MHz. These frequencies are important for measuring atomic oxygen concentration in earth's atmosphere and the interstellar medium.
Zink, Lyndon R.↗
Recent advances in LPE GaAs-based solar cells
Several modifications were made to an infinite solution liquid phase epitaxy system that help fabricate both GaAs-based cells and thin cells that effectively reduce power-to-weight ratio for space applications. The most important development is the multiwell crucible for multilayer growth. Using a split crucible in one system, as many as five layers were grown in succession with varying Al levels and dopants. The structures grown were used to produce thin GaAs cells only 10 to 20 microns thick and also to grow cascade cell components. Results of these studies are presented and their applications to the future development of GaAs-based cells are discussed.
Kamath, G. S.↗
Mobile terminal equipment design utilising split-loop phase-lock techniques
The design and resultant performance of the terminal equipment in a mobile satellite system is vitally important in respect to the overall cost/performance compromise of the whole system. Improvements in system performance which also result in a reduction of the equipment cost are rare. However, this paper details a significant advance in terminal design, utilizing a novel form of 'split-loop' phase locked receiver/downconverter system to enable an accurate, stable and wide coverage terminal to be realized at a reduced cost. The system has the capability of automatically locking onto any carrier within a complete transponder, and can cope with severe amplitude modulation and fading effects.
Kenington, P. B.↗
Development of new flux splitting schemes
Maximizing both accuracy and efficiency has been the primary objective in designing a numerical algorithm for CFD. This is especially important for solution of complex three-dimensional systems of Navier-Stokes equations which often include turbulence modeling and chemistry effects. Recently, upwind schemes have been well received for both their capability of resolving discontinuities and their sound theoretical basis in characteristic theory for hyperbolic systems. With this in mind, two new flux splitting techniques are presented for upwind differencing.
Liou, MENG-S.↗
Microcracking, microcrack-induced delamination, and longitudinal splitting of advanced composite structures
A combined analytical and experimental study was conducted to analyze microcracking, microcrack-induced delamination, and longitudinal splitting in polymer matrix composites. Strain energy release rates, calculated by a variational analysis, were used in a failure criterion to predict microcracking. Predictions and test results were compared for static, fatigue, and cyclic thermal loading. The longitudinal splitting analysis accounted for the effects of fiber bridging. Test data are analyzed and compared for longitudinal splitting and delamination under mixed-mode loading. This study emphasizes the importance of using fracture mechanics analyses to understand the complex failure processes that govern composite strength and life.
Nairn, John A.↗
Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team
The Reference Mission was developed over a period of several years and was published in NASA Special Publication 6107 in July 1997. The purpose of the Reference Mission was to provide a workable model for the human exploration of Mars, which is described in enough detail that alternative strategies and implementations can be compared and evaluated. NASA is continuing to develop the Reference Mission and expects to update this report in the near future. It was the purpose of the Reference Mission to develop scenarios based on the needs of scientists and explorers who want to conduct research on Mars; however, more work on the surface-mission aspects of the Reference Mission is required and is getting under way. Some aspects of the Reference Mission that are important for the consideration of the surface mission definition include: (1) a split mission strategy, which arrives at the surface two years before the arrival of the first crew; (2) three missions to the outpost site over a 6-year period; (3) a plant capable of producing rocket propellant for lifting off Mars and caches of water, O, and inert gases for the life-support system; (4) a hybrid physico-chemical/bioregenerative life-support system, which emphasizes the bioregenerative system more in later parts of the scenario; (5) a nuclear reactor power supply, which provides enough power for all operations, including the operation of a bioregenerative life-support system as well as the propellant and consumable plant; (6) capability for at least two people to be outside the habitat each day of the surface stay; (7) telerobotic and human-operated transportation vehicles, including a pressurized rover capable of supporting trips of several days' duration from the habitat; (7) crew stay times of 500 days on the surface, with six-person crews; and (8) multiple functional redundancies to reduce risks to the crews on the surface. New concepts are being sought that would reduce the overall cost for this exploration program and reducing the risks that are indigenous to Mars exploration. Among those areas being explored are alternative space propulsion approaches, solar vs. nuclear power, and reductions in the size of crews.
Connolly, John↗
The Precambrian Biogeochemical Carbon Isotopic Record: Contributions of Thermal Versus Biological Processes
Superplumes offer a new approach for understanding global C cycles. Isotopes help to discern the impacts of geological, environmental and biological processes ujpun the evolution of these cycles. For example, C-13/C-12 values of coeval sedimentary organics and carbonates give global estimates of the fraction of C buried as organics (Forg), which today lies near 0.2. Before Oxygenic photosynthesis arose, our biosphere obtained reducing power for biosynthesis solely from thermal volatiles and rock alteration. Thus Forg was dominated by the mantle redox state, which has remained remarkably constant for greater than Gy. Recent data confirm that the long-term change in Forg had been small, indicating that the mantle redox buffer remains important even today. Oxygenic photosynthesis enabled life to obtain additional reducing power by splitting the water molecule. Accordingly, biological organic production rose above the level constrained by the mantle-derived flux of reduced species. For example, today, chemoautotrophs harvesting energy from hydrothermal emanations can synthesize at most between 0.2 x 10(exp 12) and 2x 10(exp 12) mol C yr-1 of organic C globally. In contrast, global photosynthetic productivity is estimated at 9000 x 10(exp 12) mol C yr-1. Occasionally photosynthetic productivity did contribute to dramatically -elevated Forg values (to 0.4 or more) as evidenced by very high carbonate C-13/C-12. The interplay between biological, tectonic and other environmental factors is illustrated by the mid-Archean to mid-Proterozoic isotopic record. The relatively constant C-13/C-12 values of Archean carbonates support the view that photosynthetically-driven Forg increases were not yet possible. In contrast, major excursions in C-13/C-12, and thus also in Forg, during the early Proterozoic confirmed the global importance of oxygenic photosynthesis by that time. Remarkably, the superplume event at 1.9 Ga did not trigger another major Forg increase, despite the favorable conditions for organic burial that were evidenced by elevated sea levels and higher black shale abundances. Perhaps those superplume-rated processes that favored enhanced organic burial were offset by the ability of mantle-derived redox buffering, which was enhanced during the superplume event, to limit excursions in Forg.
DesMarais, David J.↗
Development of new flux splitting schemes
Maximizing both accuracy and efficiency has been the primary objective in designing a numerical algorithm for computational fluid dynamics (CFD). This is especially important for solutions of complex three dimensional systems of Navier-Stokes equations which often include turbulence modeling and chemistry effects. Recently, upwind schemes have been well received for their capability in resolving discontinuities. With this in mind, presented are two new flux splitting techniques for upwind differencing. The first method is based on High-Order Polynomial Expansions (HOPE) of the mass flux vector. The second new flux splitting is based on the Advection Upwind Splitting Method (AUSM). The calculation of the hypersonic conical flow demonstrates the accuracy of the splitting in resolving the flow in the presence of strong gradients. A second series of tests involving the two dimensional inviscid flow over a NACA 0012 airfoil demonstrates the ability of the AUSM to resolve the shock discontinuity at transonic speed. A third case calculates a series of supersonic flows over a circular cylinder. Finally, the fourth case deals with tests of a two dimensional shock wave/boundary layer interaction.
Liou, Meng-Sing↗