7,620 research outputs found
Flow rate of polygonal grains through a bottleneck: Interplay between shape and size
We report two-dimensional simulations of circular and polygonal grains
passing through an aperture at the bottom of a silo. The mass flow rate for
regular polygons is lower than for disks as observed by other authors. We show
that both the exit velocity of the grains and the packing fraction are lower
for polygons, which leads to the reduced flow rate. We point out the importance
of the criteria used to define when two objects of different shape are
considered to be of the same size. Depending on this criteria, the mass flow
rate may vary significantly for some polygons. Moreover, the particle flow rate
is non-trivially related to a combination of mass flow rate, particle shape and
particle size. For some polygons, the particle flow rate may be lower or higher
than that of the corresponding disks depending on the size comparison criteria.Comment: 9 pages, 8 figure
Exact predictions from Edwards ensemble vs. realistic simulations of tapped narrow two-dimensional granular columns
We simulate via a Discrete Element Method the tapping of a narrow column of
disk under gravity. For frictionless disks, this system has a simple analytic
expression for the density of states in the Edwards volume ensemble. We compare
the predictions of the ensemble at constant compactivity against the results
for the steady states obtained in the simulations. We show that the steady
states cannot be properly described since the microstates sampled are not in
correspondence with the predicted distributions, suggesting that the postulates
of flat measure and ergodicity are, either or both, invalid for this simple
realization of a static granular system. However, we show that certain
qualitative features of the volume fluctuations difficult to predict from
simple arguments are captured by the theory.Comment: 11 pages, 6 figure
Refinement by interpretation in {\pi}-institutions
The paper discusses the role of interpretations, understood as multifunctions
that preserve and reflect logical consequence, as refinement witnesses in the
general setting of pi-institutions. This leads to a smooth generalization of
the refinement-by-interpretation approach, recently introduced by the authors
in more specific contexts. As a second, yet related contribution a basis is
provided to build up a refinement calculus of structured specifications in and
across arbitrary pi-institutions.Comment: In Proceedings Refine 2011, arXiv:1106.348
Sistemas Contextuales Interpretados
El tratamiento lógico de los contextos y del conocimiento han recorrido carriles paralelos. Los
contextos comienzan ha ser tomados con atención desde la lógica, a partir de una tradición
dedicada a dar cuenta de los problemas que quedaron sin resolver al sumir el concepto de
referencia en clave fregeana; esta tradición está encabezada por Perry y Kaplan. La inteligencia
artificial aborda los contextos a partir de los trabajos de McCarthy sobre el problema de la
generalidad. Una perspectiva computacional guiada por la idea de construir bases de datos que
funcionen contextuahnente
String equations in Whitham hierarchies: tau-functions and Virasoro constraints
A scheme for solving Whitham hierarchies satisfying a special class of string
equations is presented. The tau-function of the corresponding solutions is
obtained and the differential expressions of the underlying Virasoro
constraints are characterized. Illustrative examples of exact solutions of
Whitham hierarchies are derived and applications to conformal maps dynamics are
indicated.Comment: 26 pages, 2 figure
Kinematics of the Outflow From The Young Star DG Tau B: Rotation in the vicinities of an optical jet
We present CO(2-1) line and 1300 m continuum observations made
with the Submillimeter Array (SMA) of the young star DG Tau B. We find, in the
continuum observations, emission arising from the circumstellar disk
surrounding DG Tau B. The CO(2-1) line observations, on the other hand,
revealed emission associated with the disk and the asymmetric outflow related
with this source. Velocity asymmetries about the flow axis are found over the
entire length of the flow. The amplitude of the velocity differences is of the
order of 1 -- 2 km s over distances of about 300 -- 400 AU. We interpret
them as a result of outflow rotation. The sense of the outflow and disk
rotation is the same. Infalling gas from a rotating molecular core cannot
explain the observed velocity gradient within the flow. Magneto-centrifugal
disk winds or photoevaporated disk winds can produce the observed rotational
speeds if they are ejected from a keplerian disk at radii of several tens of
AU. Nevertheless, these slow winds ejected from large radii are not very
massive, and cannot account for the observed linear momentum and angular
momentum rates of the molecular flow. Thus, the observed flow is probably
entrained material from the parent cloud. DG Tau B is a good laboratory to
model in detail the entrainment process and see if it can account for the
observed angular momentum.Comment: Accepted to Ap
Seismic site classification from the horizontal-to-vertical response spectral ratios: use of the Spanish strong-motion database
Normally, the average of the horizontal-to-vertical (H/V) ratios of the 5% damped response spectra of ground motions is used to classify the site of strong-motion stations. In these cases, only the three-orthogonal as-recorded acceleration components are used in the analysis, and all the vector compositions that can generate a different response for each period oscillator are excluded. In this study, the Spanish strong-motion database was used to classify the sites of accelerometric stations based on the predominant periods through the average horizontal-to-vertical spectral ratios (HVSR) of recorded ground motions. Moreover, the directionality effects using the vector composition of the horizontal components of ground motions were also considered in the estimations of H/V ratios. This consideration is a relevant novelty compared to the traditional H/V ratios methods. Only earthquakes with magnitudes above 3.5 and hypocentral distances below 200 km were selected, which resulted in 692 ground-motion records, corresponding to 86 stations, from events in the period between 1993 and 2017. After the analysis, a predominant-period site classification was assigned to each station. On the whole, the obtained mean and standard deviation values of the spectral ratios are comparable to those shown by other researchers. Therefore, the advantages of the proposed procedure, which takes the directionality effects into account, can be summarized as follows: (a) The obtained information is richer and gives enables more sophisticated and realistic analyses on the basis of percentiles and (b) it is easier to detect anomalous stations, sites, and/or accelerograms. Moreover, the method eliminates the effect of directionality as a contributor to epistemic uncertainty.Peer ReviewedPostprint (published version
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