35,283 research outputs found
Renormalization of Drift and Diffusivity in Random Gradient Flows
We investigate the relationship between the effective diffusivity and
effective drift of a particle moving in a random medium. The velocity of the
particle combines a white noise diffusion process with a local drift term that
depends linearly on the gradient of a gaussian random field with homogeneous
statistics. The theoretical analysis is confirmed by numerical simulation. For
the purely isotropic case the simulation, which measures the effective drift
directly in a constant gradient background field, confirms the result
previously obtained theoretically, that the effective diffusivity and effective
drift are renormalized by the same factor from their local values. For this
isotropic case we provide an intuitive explanation, based on a {\it spatial}
average of local drift, for the renormalization of the effective drift
parameter relative to its local value. We also investigate situations in which
the isotropy is broken by the tensorial relationship of the local drift to the
gradient of the random field. We find that the numerical simulation confirms a
relatively simple renormalization group calculation for the effective
diffusivity and drift tensors.Comment: Latex 16 pages, 5 figures ep
Gamow-Teller GT+ distributions in nuclei with mass A=90-97
We investigate the Gamow-Teller strength distributions in the
electron-capture direction in nuclei having mass A=90-97, assuming a 88Sr core
and using a realistic interaction that reasonably reproduces nuclear
spectroscopy for a wide range of nuclei in the region as well as experimental
data on Gamow-Teller strength distributions. We discuss the systematics of the
distributions and their centroids. We also predict the strength distributions
for several nuclei involving stable isotopes that should be experimentally
accessible for one-particle exchange reactions in the near future.Comment: 9 pages, 10 figures (from 17 eps files), to be submitted to
Phys.Rev.C; corrected typos, minor language change
Poissonian bursts in e-mail correspondence
Recent work has shown that the distribution of inter-event times for e-mail
communication exhibits a heavy tail which is statistically consistent with a
cascading Poisson process. In this work we extend the analysis to higher-order
statistics, using the Fano and Allan factors to quantify the extent to which
the empirical data depart from the known correlations of Poissonian statistics.
The analysis shows that the higher-order statistics from the empirical data is
indistinguishable from that of randomly reordered time series, thus
demonstrating that e-mail correspondence is no more bursty or correlated than a
Poisson process. Furthermore synthetic data sets generated by a cascading
Poisson process replicate the burstiness and correlations observed in the
empirical data. Finally, a simple rescaling analysis using the best-estimate
rate of activity, confirms that the empirically observed correlations arise
from a non-homogeneus Poisson process
Boundary Effects in the One Dimensional Coulomb Gas
We use the functional integral technique of Edwards and Lenard to solve the
statistical mechanics of a one dimensional Coulomb gas with boundary
interactions leading to surface charging. The theory examined is a one
dimensional model for a soap film. Finite size effects and the phenomenon of
charge regulation are studied. We also discuss the pressure of disjunction for
such a film. Even in the absence of boundary potentials we find that the
presence of a surface affects the physics in finite systems. In general we find
that in the presence of a boundary potential the long distance disjoining
pressure is positive but may become negative at closer interplane separations.
This is in accordance with the attractive forces seen at close separations in
colloidal and soap film experiments and with three dimensional calculations
beyond mean field. Finally our exact results are compared with the predictions
of the corresponding Poisson-Boltzmann theory which is often used in the
context of colloidal and thin liquid film systems.Comment: 28 pages, LATEX2e, 11 figures, uses styles[12pt] resubmission because
of minor corrections to tex
Beneficiation of ilmenite from lumar analog
The results reported were obtained on a meteoric eucrite sample called Millbillillie Sample no. 173. Optical microscopy studies of the sample showed it to consist of ilmenite, troilite, and transparent gangue. The transparent gangue consisted of feldspar (anorthite), pyroxenes, olivines, and opaques. Troilite was present in minor quantities. Screen assay analyses of the 30, 100, 200, and 400 US mesh screen fractions showed that minor concentration of titanium occurred in the 200 x 400 and -400 mesh screen fractions. Scanning electron microscopy (SEM) studies of the bulk sample showed the presence of a variety of ilmenite grains, ranging from 50 microns down to less than 1 micron without any evidence of liberation. Electron Diffraction Scans (EDS) confirmed the ratio of Fe to Ti in the ilmenite grains. Dry magnetic separation in a Frantz Isodynamic Separator was found to be effective only at sizes finer than 150 microns (100 US mesh) and more so at 200 mesh (74 microns). In each case, dedusting of the sample to remove -400 mesh (-0.037 microns) fines was required. Liberation size was determined to be 200 mesh and finer. The highest grade concentrate assaying 3.45 percent Ti was produced by magnetic separation of the -200 + 400 mesh screen fraction assaying 0.44 Ti (from a -30 mesh sample) at a current setting of 0.35 AMP. This concentrate contained 21.2 percent of the Ti values in the screen fraction with 2.72 weight percent of feed to test. The results can be projected to a sample stage ground to -200 mesh. Magnetic separation of the 200 + 400 mesh (-0.074 + 0.037 microns) should produce a concentrate accounting for 1.41 weight percent of the feed. This concentrate will analyze 3.45 percent Ti and contain 10.3 percent of the Ti values in the feed. By changing the Frantz Magnetic Separator settings, a lower grade concentrate analyzing 0.98 percent Ti can be produced at an increased recovery of 25.4 percent. The concentrate weight will be 11.7 percent of the feed. It must be emphasized that improved grades and recoveries can be obtained with the -400 mesh fines. However, beneficiation of these extremely fine materials is not possible in a practical process scheme
Skill set profile clustering based on student capability vectors computed from online tutoring data
In educational research, a fundamental goal is identifying which skills students have mastered, which skills they have not, and which skills they are in the process of mastering. As the number of examinees, items, and skills increases, the estimation of even simple cognitive diagnosis models becomes difficult. To address this, we introduce a capability matrix showing for each skill the proportion correct on all items tried by each student involving that skill. We apply variations of common clustering methods to this matrix and discuss conditioning on sparse subspaces. We demonstrate the feasibility and scalability of our method on several simulated datasets and illustrate the difficulties inherent in real data using a subset of online mathematics tutor data. We also comment on the interpretability and application of the results for teachers
- …