1,979 research outputs found
On stability and spiral patterns in polar disks
To investigate the stability properties of polar disks we performed
two-dimensional hydrodynamical simulations for flat polytropic gaseous
self-gravitating disks which were perturbed by a central S0-like component. Our
disk was constructed to resemble that of the proto-typical galaxy NGC 4650A.
This central perturbation induces initially a stationary two-armed
tightly-wound leading spiral in the polar disk. For a hot disk (Toomre
parameter Q>1.7), the structure does not change over the simulation time of 4.5
Gyr. In case of colder disks, the self-gravity of the spiral becomes dominant,
it decouples from the central perturbation and grows, until reaching a
saturation stage in which an open trailing spiral is formed, rather similar to
that observed in NGC4650A. The timescale for developing non-linear structures
is 1-2 Gyr; saturation is reached within 2-3 Gyr. The main parameter
controlling the structure formation is the Toomre parameter. The results are
surprisingly insensitive to the properties of the central component. If the
polar disk is much less massive than that in NGC4650A, it forms a weaker
tightly-wound spiral, similar to that seen in dust absorption in the dust disk
of NGC2787. Our results are derived for a polytropic equation of state, but
appear to be generic as the adiabatic exponent is varied between \gamma = 1
(isothermal) and \gamma = 2 (very stiff).Comment: 14 pages including 23 figures, accepted for publication in Astronomy
& Astrophysic
An Experimental and Analytical Approach to Understanding the Dynamic Leaching from Municipal Solid Waste Combustion Residue
This paper describes an experimental technique involving the use of small columns for generating significant quantities of leachate data from municipal solid waste (MSW) solid residues within a relatively short amount of time. Data analysis using the discretized mass balance equations descriptive of the system results in best estimates of governing transport parameters that can, in turn, be used to predict the long-term release of leachable components (As, Cd, Cu, Fe, Ni, Pb, Zn, Ca, Mg, Na, K, Cl, SO4) from the solid matrix. Results indicate that both chemical solubility and physical transport are important factors affecting the flux of contaminants from the solid to the solution phase
Counteracting systems of diabaticities using DRAG controls: The status after 10 years
The task of controlling a quantum system under time and bandwidth limitations
is made difficult by unwanted excitations of spectrally neighboring energy
levels. In this article we review the Derivative Removal by Adiabatic Gate
(DRAG) framework. DRAG is a multi-transition variant of counterdiabatic
driving, where multiple low-lying gapped states in an adiabatic evolution can
be avoided simultaneously, greatly reducing operation times compared to the
adiabatic limit. In its essence, the method corresponds to a convergent version
of the superadiabatic expansion where multiple counterdiabaticity conditions
can be met simultaneously. When transitions are strongly crowded, the system of
equations can instead be favorably solved by an average Hamiltonian (Magnus)
expansion, suggesting the use of additional sideband control. We give some
examples of common systems where DRAG and variants thereof can be applied to
improve performance.Comment: 7 pages, 2 figure
Dipole Bound Excited States of Polycyclic Aromatic Hydrocarbons containing Nitrogen and their Relation to the Interstellar Medium
Polycyclic aromatic hydrocarbons (PAHs) are the most abundant type of molecule present in the interstellar medium (ISM). It has been hypothesized that nitrogen replacement within a ring is likely for PAHs present in the ISM. Additionally, electrons, protons, and hydrogen atoms are readily added to or removed from PAHs creating a truly diverse set of chemistries in various interstellar regions. The presence of a nitrogen within a PAH (called a PANH herein) that is additionally dehydrogenated leads to a neutral radical with a large dipole moment. It has recently been shown through the use of high-level quantum chemical computations for small molecules that the corresponding closed-shell anions support electronically excited states within a dipole- bound formalism as a result of the interaction between the loosely bound excited electron and the dipole moment of the neutral radical. We are extending this methodology to larger molecules, PANHs in this case. If a dipole-bound excited state of a PANH anion exists, it could possess unique features that could shed light on various unresolved interstellar spectra potentially including even the diffuse interstellar bands
Engineering adiabaticity at an avoided crossing with optimal control
We investigate ways to optimize adiabaticity and diabaticity in the
Landau-Zener model with non-uniform sweeps. We show how diabaticity can be
engineered with a pulse consisting of a linear sweep augmented by an
oscillating term. We show that the oscillation leads to jumps in populations
whose value can be accurately modeled using a model of multiple,
photon-assisted Landau-Zener transitions, which generalizes work by Wubs et al.
[New J. Phys. 7, 218 (2005)]. We extend the study on diabaticity using methods
derived from optimal control. We also show how to preserve adiabaticity with
optimal pulses at limited time, finding a non-uniform quantum speed limit
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Stereometric Design for Desk-Top SFF Fabrication
Solid Freeform Fabrication (SFF) technologies refer to the fabrication of physical parts directly from
computer based solid models described by STL (Stereo Lithography) or VRML (Virtual Reality Modeling
Language) files generated by Computer-Aided Design (CAD) systems. Most of the SFF processes produce parts
by building them layer by layer using a row by row pattern, though it is possible to build the part using other
patterns. The SFF technology represents a challenge to designers who, in addition to making decisions
concerning optimum shape and functionality of the entire part, have'to take under consideration several other
manufacturing factors. These factors cover a wide range of technical issues such as Computer-Aided Design
model generation, part description and model slicing files, laser path files, precision of part design, rendering
patterns, manufacturing tolerances, thermal expansion and residual stress phenomena.
This paper investigates the effect of rendering patterns on the integrity, material characteristics and
mechanical properties of the parts prepared by a desk-top SFF device using diode lasers. Fe - Bronze (Cu - Sn)
premixed metal powders were used as the starting material. The particle size was about 100 /lm to 200 /lm.
Density, tensile strength and microstructure of the parts prepared using different rendering patterns were
characterized. The results were analyzed to seek optimal rendering patterns. It was noticed that the samples
were strong along the laser scanning direction, while they were weak perpendicular to the scanning direction.
These results suggest that the laser scanning patterns should be designed to minimize the warping and maximize
the strength of the part in the direction depending on the part's function.Mechanical Engineerin
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