741 research outputs found
Stability of Relativistic Matter with Magnetic Fields for Nuclear Charges up to the Critical Value
We give a proof of stability of relativistic matter with magnetic fields all
the way up to the critical value of the nuclear charge .Comment: LaTeX2e, 12 page
Reshocked Richtmyer-Meshkov instability: Numerical study and modeling of random multi-mode experiments
On the Order of Plates Providing the Maximum Ballistic Limit Velocity of a Layered Armor
Abstract The high-velocity impact of a three-dimensional rigid sharp impactor penetrating into a ductile layered armor is studied using a simpli"ed model for an impactor}armor interaction. The goal of the study is to determine the order of the plates in the armor that provides the maximum ballistic limit velocity. It is found that the ratio of the distortion pressure to the density of the plate is the parameter that determines the order of the plates in the multi-layered armor that provides the maximum ballistic limit velocity, namely, the plates must be placed in the order of increasing this parameter, regardless of their thickness. The right order of the plates can be of prime importance if the values of this parameter vary strongly for di!erent plates. Numerical simulation is performed in order to verify the theoretical "ndings and to obtain a quantitative estimate for the increase in the ballistic limit that results from changing the order of the plates in the armor
Estimating specific surface area of fine stream bed sediments from geochemistry
Specific surface area (SSA) of headwater stream bed sediments is a fundamental property which determines the nature of sediment surface reactions and influences ecosystem-level, biological processes. Measurements of SSA â commonly undertaken by BET nitrogen adsorption â are relatively costly in terms of instrumentation and operator time. A novel approach is presented for estimating fine (2.5 mg kgâ1), four elements were identified as significant predictors of SSA (ordered by decreasing predictive power): V > Ca > Al > Rb. The optimum model from these four elements accounted for 73% of the variation in bed sediment SSA (range 6â46 m2 gâ1) with a root mean squared error of prediction â based on leave-one-out cross-validation â of 6.3 m2 gâ1. It is believed that V is the most significant predictor because its concentration is strongly correlated both with the quantity of Fe-oxides and clay minerals in the stream bed sediments, which dominate sediment SSA. Sample heterogeneity in SSA â based on triplicate measurements of sub-samples â was a substantial source of variation (standard error = 2.2 m2 gâ1) which cannot be accounted for in the regression model.
The model was used to estimate bed sediment SSA at the other 1792 sites and at 30 duplicate sites where an extra sediment sample had been collected, 25 m from the original site. By delineating sub-catchments for the headwater sediment sites only those sub-catchments were selected with a dominant (>50% of the sub-catchment area) bedrock formation and land use type; the bedrock and land use classes accounted for 39% and 7% of the variation in bed sediment SSA, respectively. Variation in estimated, fine bed sediment SSA from the paired, duplicate sediment sites was small (2.7 m2 gâ1), showing that local variation in SSA at stream sites is modest when compared to that between catchments. How the approach might be applied in other environments and its potential limitations are discussed
Local Guarantees in Graph Cuts and Clustering
Correlation Clustering is an elegant model that captures fundamental graph
cut problems such as Min Cut, Multiway Cut, and Multicut, extensively
studied in combinatorial optimization. Here, we are given a graph with edges
labeled or and the goal is to produce a clustering that agrees with the
labels as much as possible: edges within clusters and edges across
clusters. The classical approach towards Correlation Clustering (and other
graph cut problems) is to optimize a global objective. We depart from this and
study local objectives: minimizing the maximum number of disagreements for
edges incident on a single node, and the analogous max min agreements
objective. This naturally gives rise to a family of basic min-max graph cut
problems. A prototypical representative is Min Max Cut: find an cut
minimizing the largest number of cut edges incident on any node. We present the
following results: an -approximation for the problem of
minimizing the maximum total weight of disagreement edges incident on any node
(thus providing the first known approximation for the above family of min-max
graph cut problems), a remarkably simple -approximation for minimizing
local disagreements in complete graphs (improving upon the previous best known
approximation of ), and a -approximation for
maximizing the minimum total weight of agreement edges incident on any node,
hence improving upon the -approximation that follows from
the study of approximate pure Nash equilibria in cut and party affiliation
games
Compressible flow structures interaction with a two-dimensional ejector: a cold-flow study
An experimental study has been conducted to examine the interaction of compressible ïŹow structures such as
shocks and vortices with a two-dimensional ejector geometry using a shock-tube facility. Three diaphragm pressure
ratios ofP4
=P1 = 4, 8, and 12 have been employed, whereP4
is the driver gas pressure andP1
is the pressure within
the driven compartment of the shock tube. These lead to incident shock Mach numbers of Ms = 1:34, 1.54, and 1.66,
respectively. The length of the driver section of the shock tube was 700 mm. Air was used for both the driver and
driven gases. High-speed shadowgraphy was employed to visualize the induced ïŹowïŹeld. Pressure measurements
were taken at different locations along the test section to study theïŹow quantitatively. The induced ïŹow is unsteady
and dependent on the degree of compressibility of the initial shock wave generated by the rupture of the diaphragm
Mapping water infiltration rate using ground and uav hyperspectral data: A case study of Alento, italy
Water infiltration rate (WIR) into the soil profile was investigated through a comprehensive study harnessing spectral information of the soil surface. As soil spectroscopy provides inval-uable information on soil attributes, and as WIR is a soil surfaceâdependent property, field spectroscopy may model WIR better than traditional laboratory spectral measurements. This is because sampling for the latter disrupts the soilâsurface status. A field soil spectral library (FSSL), consisting of 114 samples with different textures from six different sites over the Mediterranean basin, combined with traditional laboratory spectral measurements, was created. Next, partial least squares regression analysis was conducted on the spectral and WIR data in different soil texture groups, showing better performance of the field spectral observations compared to traditional laboratory spectroscopy. Moreover, several quantitative spectral properties were lost due to the sampling pro-cedure, and separating the samples according to texture gave higher accuracies. Although the visible nearâinfraredâshortwave infrared (VNIRâSWIR) spectral region provided better accuracy, we resampled the spectral data to the resolution of a Cubert hyperspectral sensor (VNIR). This hyper-spectral sensor was then assembled on an unmanned aerial vehicle (UAV) to apply one selected spectralâbased model to the UAV data and map the WIR in a semiâvegetated area within the Alento catchment, Italy. Comprehensive spectral and WIR groundâtruth measurements were carried out simultaneously with the UAVâCubert sensor flight. The results were satisfactorily validated on the ground using field samples, followed by a spatial uncertainty analysis, concluding that the UAV with hyperspectral remote sensing can be used to map soil surfaceârelated soil properties
Reshocked Richtmyer-Meshkov instability: Numerical study and modeling of random multi-mode experiments
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