804 research outputs found
Control of plume interference effects on axisymmetric afterbodies
Plume interference effects on the axisymmetric flowfields around powered missiles are investigated using computational techniques. The study is mainly to understand the physics of the plume-induced shock and separation particularly at high plume to exit pressure ratios with and without shock-turbulent boundary layer control methods
A general pressure equation based method for incompressible two-phase flows
We present a fully-explicit, iteration-free, weakly-compressible method to
simulate immiscible incompressible two-phase flows. To update pressure, we
circumvent the computationally expensive Poisson equation and use the general
pressure equation. In addition, the volume-of-fluid approach is used for
interface capturing under the operator-split methodology. Our method is
fully-explicit and stable with simple local spatial discretization, and hence,
it is easy to implement. Several two- and three-dimensional canonical two-phase
flows are simulated. The qualitative and quantitative results prove that our
method is capable of accurately handling problems involving a range of density
and viscosity ratios and surface tension effects
Microscopic Model for Photoinduced Magnetism in the Molecular Complex Perchlorate
A theoretical model for understanding photomagnetism in the heptanuclear
complex perchlorate is developed. It is a
many-body model involving the active orbitals on the transition metal ions. The
model is exactly solved using a valence bond approach. The ground state
solution of the model is highly degenerate and is spanned by five S=0 states,
nine S=1 states, five S=2 states and one S=3 state. The orbital occupancies in
all these states correspond to six ions and one diamagnetic
ion. The optically excited charge-transfer (CT) state in each spin sector occur
at nearly the same excitation energy of 2.993 eV for the physically reasonable
parameter values. The degeneracy of the CT states is largest in the S=3 sector
and so is the transition dipole moment from the ground state to these excited
states. Thus laser irradiation with light of this energy results in most
intense absorption in the S=3 sector. The life-time of the S=3 excited states
is also expected to be the largest as the number of states below that energy is
very sparse in this spin sector when compared to other spin sectors. These twin
features of our model explain the observed photomagnetism in the
complex.Comment: 8 pages, 6 figures and 1 tabl
Fast and accurate predictions of covalent bonds in chemical space
We assess the predictive accuracy of perturbation theory based estimates of changes in covalent bonding due to linear alchemical interpolations among molecules. We have investigated σ bonding to hydrogen, as well as σ and π bonding between main-group elements, occurring in small sets of iso-valence-electronic molecules with elements drawn from second to fourth rows in the p-block of the periodic table. Numerical evidence suggests that first order Taylor expansions of covalent bonding potentials can achieve high accuracy if (i) the alchemical interpolation is vertical (fixed geometry), (ii) it involves elements from the third and fourth rows of the periodic table, and (iii) an optimal reference geometry is used. This leads to near linear changes in the bonding potential, resulting in analytical predictions with chemical accuracy (∼1 kcal/mol). Second order estimates deteriorate the prediction. If initial and final molecules differ not only in composition but also in geometry, all estimates become substantially worse, with second order being slightly more accurate than first order. The independent particle approximation based second order perturbation theory performs poorly when compared to the coupled perturbed or finite difference approach. Taylor series expansions up to fourth order of the potential energy curve of highly symmetric systems indicate a finite radius of convergence, as illustrated for the alchemical stretching of H2 (+). Results are presented for (i) covalent bonds to hydrogen in 12 molecules with 8 valence electrons (CH4, NH3, H2O, HF, SiH4, PH3, H2S, HCl, GeH4, AsH3, H2Se, HBr); (ii) main-group single bonds in 9 molecules with 14 valence electrons (CH3F, CH3Cl, CH3Br, SiH3F, SiH3Cl, SiH3Br, GeH3F, GeH3Cl, GeH3Br); (iii) main-group double bonds in 9 molecules with 12 valence electrons (CH2O, CH2S, CH2Se, SiH2O, SiH2S, SiH2Se, GeH2O, GeH2S, GeH2Se); (iv) main-group triple bonds in 9 molecules with 10 valence electrons (HCN, HCP, HCAs, HSiN, HSiP, HSiAs, HGeN, HGeP, HGeAs); and (v) H2 (+) single bond with 1 electron
Constrained semi-analytical models of Galactic outflows
We present semi-analytic models of galactic outflows, constrained by
available observations on high redshift star formation and reionization.
Galactic outflows are modeled in a manner akin to models of stellar wind blown
bubbles. Large scale outflows can generically escape from low mass halos
(M<10^9 M_sun) for a wide range of model parameters but not from high mass
halos (M> 10^{11} M_sun). The gas phase metallicity of the outflow and within
the galaxy are computed. Ionization states of different metal species are
calculated and used to examine the detectability of metal lines from the
outflows. The global influence of galactic outflows is also investigated.
Models with only atomic cooled halos significantly fill the IGM at z~3 with
metals (with -2.5>[Z/Z_sun]>-3.7), the actual extent depending on the
efficiency of winds, the IMF, the fractional mass that goes through star
formation and the reionization history of the universe. In these models, a
large fraction of outflows at z~3 are supersonic, hot (T> 10^5 K) and have low
density, making metal lines difficult to detect. They may also result in
significant perturbations in the IGM gas on scales probed by the Lyman-alpha
forest. On the contrary, models including molecular cooled halos with a normal
mode of star formation can potentially volume fill the universe at z> 8 without
drastic dynamic effects on the IGM, thereby setting up a possible metallicity
floor (-4.0<[Z/Z_sun]<-3.6). Interestingly, molecular cooled halos with a
``top-heavy'' mode of star formation are not very successful in establishing
the metallicity floor because of the additional radiative feedback, that they
induce. (Abridged)Comment: 27 pages, 31 figures, 2 tables, pdflatex. Accepted for publication in
MNRA
Combining data from primary and ancillary surveys to assess the association between neighborhood-level characteristics and health outcomes: the Multi-Ethnic Study of Artherosclerosis
There is increasing interest in understanding the role of neighborhood-level factors on the health of individuals. Many large-scale epidemiological studies that accurately measure health status of individuals and individual risk factors exist. Sometimes these studies are linked to area-level databases (e.g. census) to assess the association between crude area-level characteristics and health. However, information from such databases may not measure the neighborhood-level constructs of interest. More recently, large-scale epidemiological studies have begun collecting data to measure specific features of neighborhoods using ancillary surveys. The ancillary surveys are composed of a separate, typically larger, set of individuals. The challenge is then to combine information from these two surveys to assess the role of neighborhood-level factors. We propose a method for combining information from the two data sources using a likelihood-based framework. We compare it with currently used ad hoc approaches via a simulation study. The simulation study shows that the proposed approach yields estimates with better sampling properties (less bias and better coverage probabilities) compared with the other approaches. However, there are cases where some ad hoc approaches may provide adequate estimates. We also compare the methods by applying them to the Multi-Ethnic Study of Atherosclerosis and its Neighborhood Ancillary Survey. Copyright © 2008 John Wiley & Sons, Ltd.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/61232/1/3384_ftp.pd
Sharing Social Network Data: Differentially Private Estimation of Exponential-Family Random Graph Models
Motivated by a real-life problem of sharing social network data that contain
sensitive personal information, we propose a novel approach to release and
analyze synthetic graphs in order to protect privacy of individual
relationships captured by the social network while maintaining the validity of
statistical results. A case study using a version of the Enron e-mail corpus
dataset demonstrates the application and usefulness of the proposed techniques
in solving the challenging problem of maintaining privacy \emph{and} supporting
open access to network data to ensure reproducibility of existing studies and
discovering new scientific insights that can be obtained by analyzing such
data. We use a simple yet effective randomized response mechanism to generate
synthetic networks under -edge differential privacy, and then use
likelihood based inference for missing data and Markov chain Monte Carlo
techniques to fit exponential-family random graph models to the generated
synthetic networks.Comment: Updated, 39 page
A proof of the Grothendieck-Serre conjecture on principal bundles over regular local rings containing infinite fields
Let R be a regular local ring, containing an infinite field. Let G be a
reductive group scheme over R. We prove that a principal G-bundle over R is
trivial, if it is trivial over the fraction field of R.Comment: Section "Formal loops and affine Grassmannians" is removed as this is
now covered in arXiv:1308.3078. Exposition is improved and slightly
restructured. Some minor correction
Effect of dimples on glancing shock wave turbulent boundary layer interactions
An experimental study has been conducted to examine the control effectiveness of dimples on the glancing shock wave turbulent boundary layer interaction produced by a series of hemi-cylindrically blunted fins at Mach numbers 0.8 and 1.4, and at angles of sweep 0°, 15°, 30° and 45°. Schlieren photography, oil flow, pressure sensitive paints, and pressure tappings were employed to examine the characteristics of the induced flow field. The passive control technique used a series of 2 mm diameter, 1 mm deep indents drilled across the hemi-cylindrical leading edge at angles 0°, 45° and 90°. The effects of dimples were highly dependent on their orientation relative to the leading edge apex, and the local boundary layer properties
Simulating Particle Dispersions in Nematic Liquid-Crystal Solvents
A new method is presented for mesoscopic simulations of particle dispersions
in nematic liquid crystal solvents. It allows efficient first-principle
simulations of the dispersions involving many particles with many-body
interactions mediated by the solvents. A simple demonstration is shown for the
aggregation process of a two dimentional dispersion.Comment: 5 pages, 5 figure
- …