2,243 research outputs found
Wheel–rail contact: experimental study of the creep forces–creepage relationships
The wheel–rail contact problem plays an important role in the simulation methods used to solve railway dynamics problems. As a consequence, many different mathematical models have been developed to calculate wheel–rail contact forces. However, most of them tackle this problem purely from a theoretical point of view and need to be experimentally validated. Such validation could also reveal the influence of certain parameters not taken into account in the mathematical developments. This paper presents the steps followed in building a scaled test-bench to experimentally characterise the wheel–rail contact problem. The results of the longitudinal contact force as a function of the longitudinal creepage are obtained and the divergences with respect to Kalker's simplified theory are analysed. The influence of lateral creepage, angular velocity and certain contaminants such as cutting fluid or high positive friction modifier is also discussed
Colgajo libre córtico-perióstico de la arteria genicular descendente en el tratamiento de la seudoartritis distal de tibia
Presentamos un caso de un varón de 35 años con pseudoartrosis aséptica de tibia distal con escaso defecto óseo, intervenido previamente mediante recambio del material de osteosíntesis e injerto de cresta iliaca, que fracasó. Tras estudio de las diferentes opciones de tratamiento, se le propuso estabilizar el foco mediante enclavado endomedular y aumentar la capacidad osteogénica con un colgajo córtico-perióstico de la arteria genicular descendente, según la técnica de Sakai. Discusión: El manejo terapéutico de la pseudoartrosis distal de tibia no está del todo resuelto. Cuando otras opciones no han sido efectivas, el clavo endomedular asociado al colgajo libre córtico-perióstico de la arteria genicular descendente puede ser una opción válida para el tratamiento. Esta técnica quirúrgica supone un trabajo coordinado entre los especialistas en traumatología y cirugía plástica.We present a case of 35 years old man presented with aseptic distal tibial nonunion without bone defect. Previous unsuccessful surgical attempt, consisting on revision of the internal fixation and iliac crest bone grafting, was performed. After studying the different treatment options, we suggested him to carry out an endomedular nail stabilization of the nonunion site in combination with a descending genicular corticoperiosteal free flap to provide osteogenetic enhancement, as described by Sakai. Discussion: The treatment protocol of the distal tibial nonunion still remains controversial. When other surgical attempts are unsuccessful, the combination of endomedular nailing and descending genicular corticoperiosteal free flap, may be useful. This surgical technique may require a coordinated work between orthopaedic and plastic surgeons
Ranking and clustering of nodes in networks with smart teleportation
Random teleportation is a necessary evil for ranking and clustering directed
networks based on random walks. Teleportation enables ergodic solutions, but
the solutions must necessarily depend on the exact implementation and
parametrization of the teleportation. For example, in the commonly used
PageRank algorithm, the teleportation rate must trade off a heavily biased
solution with a uniform solution. Here we show that teleportation to links
rather than nodes enables a much smoother trade-off and effectively more robust
results. We also show that, by not recording the teleportation steps of the
random walker, we can further reduce the effect of teleportation with dramatic
effects on clustering.Comment: 10 pages, 7 figure
Ranking structured documents using utility theory in the Bayesian network retrieval model
In this paper a new method based on Utility and Decision theory is presented to deal with structured documents. The aim of the application of these methodologies is to refine a first ranking of structural units, generated by means of an Information Retrieval Model based on Bayesian Networks. Units are newly arranged in the new ranking by combining their posterior probabilities, obtained in the first stage, with the expected utility of retrieving them. The experimental work has been developed using the Shakespeare structured collection and the results show an improvement of the effectiveness of this new approach
Fast Searching in Packed Strings
Given strings and the (exact) string matching problem is to find all
positions of substrings in matching . The classical Knuth-Morris-Pratt
algorithm [SIAM J. Comput., 1977] solves the string matching problem in linear
time which is optimal if we can only read one character at the time. However,
most strings are stored in a computer in a packed representation with several
characters in a single word, giving us the opportunity to read multiple
characters simultaneously. In this paper we study the worst-case complexity of
string matching on strings given in packed representation. Let be
the lengths and , respectively, and let denote the size of the
alphabet. On a standard unit-cost word-RAM with logarithmic word size we
present an algorithm using time O\left(\frac{n}{\log_\sigma n} + m +
\occ\right). Here \occ is the number of occurrences of in . For this improves the bound of the Knuth-Morris-Pratt algorithm.
Furthermore, if our algorithm is optimal since any
algorithm must spend at least \Omega(\frac{(n+m)\log
\sigma}{\log n} + \occ) = \Omega(\frac{n}{\log_\sigma n} + \occ) time to
read the input and report all occurrences. The result is obtained by a novel
automaton construction based on the Knuth-Morris-Pratt algorithm combined with
a new compact representation of subautomata allowing an optimal
tabulation-based simulation.Comment: To appear in Journal of Discrete Algorithms. Special Issue on CPM
200
Dynamical measurements of deviations from Newton's law
In a previous work (arXiv:1609.05654v2), an experimental setup aiming at the
measurement of deviations from the Newtonian distance dependence of
gravitational interactions was proposed. The theoretical idea behind this setup
was to study the trajectories of a "Satellite" with a mass around a "Planet" with mass , looking for precession of the orbit. The
observation of such feature induced by gravitational interactions would be an
unambiguous indication of a gravitational potential with terms different from
and, thus, a powerful tool to detect deviations from Newton's
law. In this paper we optimize the proposed setup in order to achieve maximal
sensitivity to look for {\em Beyond-Newtonian} corrections. We study in detail
possible background sources that could induce precession and quantify their
impact on the achievable sensitivity. We conclude that a dynamical measurement
of deviations from newtonianity can test Yukawa-like corrections to the
potential with strength as low as for distances as small
as .Comment: Two-column format (26 pages), 18 figure
Order preserving pattern matching on trees and DAGs
The order preserving pattern matching (OPPM) problem is, given a pattern
string and a text string , find all substrings of which have the
same relative orders as . In this paper, we consider two variants of the
OPPM problem where a set of text strings is given as a tree or a DAG. We show
that the OPPM problem for a single pattern of length and a text tree
of size can be solved in time if the characters of are
drawn from an integer alphabet of polynomial size. The time complexity becomes
if the pattern is over a general ordered alphabet. We
then show that the OPPM problem for a single pattern and a text DAG is
NP-complete
La regeneració natural després dels incendis forestals a la Comunitat Valenciana
Ecosystem regeneration after wildfire in the Valencia region. The response to wildfires of representative forest ecosystems from the Valencia region is analysed in the present paper. The regenerative strategy of the species that dominate these eco systems is shown to be relevant, as are the physical and nutritional characteristics of the soils where they usually grow. The repercussions of the increasing fire recurrence in the loss of ecosystem regenerative capacity and complexity are discussed. En el present article s'analitza la resposta dels diferents ecosistemes forestals més freqüentment afectats per incendis a la Comunitat Valenciana. S'observa la relevancia de l' estratègia regenerativa després del foc de les espècies que dominen aquestes formacions vegetals, així com les característiques fisiques i nutricionals dels diferents tipus de sòls on generalment es desenvolupen. Es discuteix la repercussió de l'increment de la recurrència d' incendis en la pèrdua de la capacitat regenerativa i de la complexitat dels ecosistemes analitzats
Adaptive query-based sampling of distributed collections
As part of a Distributed Information Retrieval system a de-scription of each remote information resource, archive or repository is usually stored centrally in order to facilitate resource selection. The ac-quisition ofprecise resourcedescriptionsistherefore animportantphase in Distributed Information Retrieval, as the quality of such represen-tations will impact on selection accuracy, and ultimately retrieval per-formance. While Query-Based Sampling is currently used for content discovery of uncooperative resources, the application of this technique is dependent upon heuristic guidelines to determine when a sufficiently accurate representation of each remote resource has been obtained. In this paper we address this shortcoming by using the Predictive Likelihood to provide both an indication of thequality of an acquired resource description estimate, and when a sufficiently good representation of a resource hasbeen obtained during Query-Based Sampling
Revisiting the Problem of Searching on a Line
We revisit the problem of searching for a target at an unknown location on a
line when given upper and lower bounds on the distance D that separates the
initial position of the searcher from the target. Prior to this work, only
asymptotic bounds were known for the optimal competitive ratio achievable by
any search strategy in the worst case. We present the first tight bounds on the
exact optimal competitive ratio achievable, parameterized in terms of the given
bounds on D, along with an optimal search strategy that achieves this
competitive ratio. We prove that this optimal strategy is unique. We
characterize the conditions under which an optimal strategy can be computed
exactly and, when it cannot, we explain how numerical methods can be used
efficiently. In addition, we answer several related open questions, including
the maximal reach problem, and we discuss how to generalize these results to m
rays, for any m >= 2
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