Simplified combustion noise theory yielding a prediction of fluctuating pressure level

The first order equations for the conservation of mass and momentum in differential form are combined for an ideal gas to yield a single second order partial differential equation in one dimension and time. Small perturbation analysis is applied. A Fourier transformation is performed that results in a second order, constant coefficient, nonhomogeneous equation. The driving function is taken to be the source of combustion noise. A simplified model describing the energy addition via the combustion process gives the required source information for substitution in the driving function. This enables the particular integral solution of the nonhomogeneous equation to be found. This solution multiplied by the acoustic pressure efficiency predicts the acoustic pressure spectrum measured in turbine engine combustors. The prediction was compared with the overall sound pressure levels measured in a CF6-50 turbofan engine combustor and found to be in excellent agreement

Three-dimensional cometary dust coma modelling in the collisionless regime: strengths and weaknesses

Inverse coma and tail modelling of comets based on the method developed by Finson & Probstein is commonly used to analyse cometary coma images. Models of this type often contain a large number of assumptions that may not be constrained unless wide temporal or spectral coverage is available and the comets are bright and at relatively small geocentric distance. They are used to predict physical parameters, such as the mass distribution of the dust, but rarely give assessments of the accuracy of the estimate. A three-dimensional cometary dust coma model in the collisionless regime has been developed to allow the effectiveness of such models to constrain dust coma properties to be tested. The model is capable of simulating the coma morphology for the following input parameters: the comet nucleus shape, size, rotation, emission function (including active fraction and jets), grain velocity distribution (and dispersion), size distribution, dust production rate, grain material and light scattering from the cometary dust. Characterization of the model demonstrates that the mass distribution cannot be well constrained as is often assumed; the cumulative mass distribution index ? can only be constrained to within ±0.15. The model is highly sensitive to the input grain terminal velocity distribution so model input can be tested with a large degree of confidence. Complex secondary parameters such as jets, rotation and grain composition all have an effect on the structure of the coma in similar ways, so unique solutions for these parameters cannot be derived from a single optical image alone. Multiple images at a variety of geometries close in time can help constrain these effects. The model has been applied to photometric observations of comets 126P/IRAS and 46P/Wirtanen to constrain a number of physical properties including the dust production rate and mass distribution index. The derived dust production rate (Qdust) for 46P/Wirtanen was 3+7/1.5 kg s1 at a pre-perihelion heliocentric distance of 1.8 au, and for P/IRAS was 50+100/20 kg s1 at a pre-perihelion heliocentric distance of 1.7 au; both comets exhibited a mass distribution index ? = 0.8 ± 0.15

Dust Grain Orbital Behavior Around Ceres

Many asteroids show indications they have undergone impacts with meteoroid particles having radii between 0.01 m and 1 m. During such impacts, small dust grains will be ejected at the impact site. The possibility of these dust grains (with radii greater than 2.2x10-6 m) forming a halo around a spherical asteroid (such as Ceres) is investigated using standard numerical integration techniques. The orbital elements, positions, and velocities are determined for particles with varying radii taking into account both the influence of gravity, radiation pressure, and the interplanetary magnetic field (for charged particles). Under the influence of these forces it is found that dust grains (under the appropriate conditions) can be injected into orbits with lifetimes in excess of one year. The lifetime of the orbits is shown to be highly dependent on the location of the ejection point as well as the angle between the surface normal and the ejection path. It is also shown that only particles ejected within 10 degrees relative to the surface tangential survive more than a few hours and that the longest-lived particles originate along a line perpendicular to the Ceres-Sun line.Comment: 8 pages, Presented at COSPAR '0

The Propagation and Survival of Interstellar Grains

In this paper we discuss the propagation of dust through the interstellar medium (ISM), and describe the destructive effects of stellar winds, jets, and supernova shock waves on interstellar dust. We review the probability that grains formed in stellar outflows or supernovae survive processing in and propagation through the ISM, and incorporate themselves relatively unprocessed into meteoritic bodies in the solar system. We show that very large (radii >= 5 micron) and very small grains (radii <= 100 Angstrom) with sizes similar to the pre-solar SiC and diamond grains extracted from meteorites, can survive the passage through 100\kms shock waves relatively unscathed. High velocity (>= 250 km/s) shocks destroy dust efficiently. However, a small (~10%) fraction of the stardust never encountered such fast shocks before incorporation into the solar system. All grains should therefore retain traces of their passage through interstellar shocks during their propagation through the ISM. The grain surfaces should show evidence of processing due to sputtering and pitting due to small grain cratering collisions on the micron-sized grains. This conclusion seems to be in conflict with the evidence from the large grains recovered from meteorites which seem to show little interstellar processing.Comment: 19 pages, 5 figures (.eps), LaTeX, to appear in "Astrophysical Implications of the Laboratory Study of Presolar Materials" AIP Conference Proceedings, 1997 T.J. Bernatowicz and E. Zinner (eds.

A bimodal dust grain distribution in the IC 434 HII region

Recent studies of dust in the interstellar medium have challenged the capabilities and validity of current dust models, indicating that the properties of dust evolve as it transits between different phases of the interstellar medium. We conduct a multi-wavelength study of the dust emission from the ionized gas of the IC 434 emission nebula, and combine this with modeling, from large scales that provide insight into the history of the IC 434/L1630 region, to small scales that allow us to infer quantitative properties of the dust content inside the H II region. The dust enters the H II region through momentum transfer with a champagne flow of ionized gas, set up by a chance encounter between the L1630 molecular cloud and the star cluster of $\sigma$ Ori. We observe two clearly separated dust populations inside the ionized gas, that show different observational properties, as well as contrasting optical properties. Population A is colder ($\sim$ 25 K) than predicted by widely-used dust models, its temperature is insensitive to an increase of the impinging radiation field, is momentum-coupled to the gas, and efficiently absorbs radiation pressure to form a dust wave at 1.0 pc ahead of $\sigma$ Ori AB. Population B is characterized by a constant [20/30] flux ratio throughout the HII region, heats up to $\sim$ 75 K close to the star, and is less efficient in absorbing radiation pressure, forming a dust wave at 0.1 pc from the star. We conclude that the dust inside IC 434 is bimodal. The characteristics of population A are remarkable and can not be explained by current dust models. Population B are grains that match the classical description of spherical, compact dust. Our results confirm recent work that stress the importance of variations in the dust properties between different regions of the interstellar medium.Comment: 18 pages, 10 figures, proposed for acceptance in A&