140,041 research outputs found
Comparative Analysis of Decision Tree Algorithms for Data Warehouse Fragmentation
One of the main problems faced by Data Warehouse designers is fragmentation.Several studies have proposed data mining-based horizontal fragmentation methods.However, not exists a horizontal fragmentation technique that uses a decision tree. This paper presents the analysis of different decision tree algorithms to select the best one to implement the fragmentation method. Such analysis was performed under version 3.9.4 of Weka, considering four evaluation metrics (Precision, ROC Area, Recall and F-measure) for different selected data sets using the Star Schema Benchmark. The results showed that the two best algorithms were J48 and Random Forest in most cases; nevertheless, J48 was selected because it is more efficient in building the model.One of the main problems faced by Data Warehouse designers is fragmentation.Several studies have proposed data mining-based horizontal fragmentation methods.However, not exists a horizontal fragmentation technique that uses a decision tree. This paper presents the analysis of different decision tree algorithms to select the best one to implement the fragmentation method. Such analysis was performed under version 3.9.4 of Weka, considering four evaluation metrics (Precision, ROC Area, Recall and F-measure) for different selected data sets using the Star Schema Benchmark. The results showed that the two best algorithms were J48 and Random Forest in most cases; nevertheless, J48 was selected because it is more efficient in building the model
Crossover of the weighted mean fragment mass scaling in 2D brittle fragmentation
We performed vertical and horizontal sandwich 2D brittle fragmentation
experiments. The weighted mean fragment mass was scaled using the multiplicity
. The scaling exponent crossed over at . In the
small regime, the binomial multiplicative (BM) model was
suitable and the fragment mass distribution obeyed log-normal form. However, in
the large regime, in which a clear power-law cumulative
fragment mass distribution was observed, it was impossible to describe the
scaling exponent using the BM model. We also found that the scaling exponent of
the cumulative fragment mass distribution depended on the manner of impact
(loading conditions): it was 0.5 in the vertical sandwich experiment, and
approximately 1.0 in the horizontal sandwich experiment.Comment: 5 pages, 3 figure
The Three-dimensional Evolution of Rising, Twisted Magnetic Flux Tubes in a Gravitationally Stratified Model Convection Zone
We present three-dimensional numerical simulations of the rise and
fragmentation of twisted, initially horizontal magnetic flux tubes which evolve
into emerging Omega-loops. The flux tubes rise buoyantly through an
adiabatically stratified plasma that represents the solar convection zone. The
MHD equations are solved in the anelastic approximation, and the results are
compared with studies of flux tube fragmentation in two dimensions. We find
that if the initial amount of field line twist is below a critical value, the
degree of fragmentation at the apex of a rising Omega-loop depends on its
three-dimensional geometry: the greater the apex curvature of a given
Omega-loop, the lesser the degree of fragmentation of the loop as it approaches
the photosphere. Thus, the amount of initial twist necessary for the loop to
retain its cohesion can be reduced substantially from the two-dimensional
limit. The simulations also suggest that as a fragmented flux tube emerges
through a relatively quiet portion of the solar disk, extended crescent-shaped
magnetic features of opposite polarity should form and steadily recede from one
another. These features eventually coalesce after the fragmented portion of the
Omega-loop emerges through the photosphere.Comment: 17 pages, 17 figures, uses AAS LaTeX macros v5.0. ApJ, in pres
Tracking granules on the Sun's surface and reconstructing horizontal velocity fields: I. the CST algorithm
Determination of horizontal velocity fields on the solar surface is crucial
for understanding the dynamics of structures like mesogranulation or
supergranulation or simply the distribution of magnetic fields.
We pursue here the development of a method called CST for coherent structure
tracking, which determines the horizontal motion of granules in the field of
view.
We first devise a generalization of Strous method for the segmentation of
images and show that when segmentation follows the shape of granules more
closely, granule tracking is less effective for large granules because of
increased sensitivity to granule fragmentation. We then introduce the
multi-resolution analysis on the velocity field, based on Daubechies wavelets,
which provides a view of this field on different scales. An algorithm for
computing the field derivatives, like the horizontal divergence and the
vertical vorticity, is also devised. The effects from the lack of data or from
terrestrial atmospheric distortion of the images are also briefly discussed.Comment: in press in Astronomy and Astrophysics, 9 page
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