1,721,124 research outputs found
An assessment of some effects of the nonsmoothness of the Leishman-Beddoes dynamic stall model on the nonlinear dynamics of a typical aerofoil section
No full textWe investigate the dynamic behaviour of the typical airfoil section modelled structurally by linear springs in pitch id plunge with the aerodynamic loading represented by our interpretation of the state-space version of the Leishman-Beddoes semi-empirical model. Similarly to other semi-empirical models of dynamic stall, this model presents the nonlinear component of the unsteady aerodynamic loading on the airfoil by a series of equations, with impirical coefficients, devised specifically for each of the relevant dynamic stall flow states. Given this piecewise definition of the loading, we pay particular attention to the description of the discontinuities of the model and to their effect on the dynamics of the system through phase plots, Poincare sections and bifurcation diagrams. These results how that the model is sensitive to small variations of some of the parameters of the model. They also show that prohibitively small timesteps are required to obtain numerically converged Poincare maps. We advocate the use of event detection techniques for the numerical integration of the equations of motion to reduce this severe timestep restriction
Remarks on the nonlinear dynamics of a typical aerofoil section in dynamic stall
No full textWe use standard tools of the theory of dynamical systems such as phase plots, bifurcation diagrams and basins of attraction to analyse and understand the dynamic behaviour of a typical aerofoil section under dynamic stall conditions. The structural model is linear and the aerodynamic loading is represented by the Leishman-Beddoes semi- empirical dynamic stall model. The loads given by this model are non- linear and non-smooth, therefore we have integrated the equation of motion using a Runge-Kutta-Fehlberg (RKF45) algorithm equipped with event detection. We perform simulations of the motion for a range of Mach numbers and show that the model is very sensitive to small variations. This is evidenced by the presence in the bifurcation diagram of co-existing attractors or, in other words, by the existence of more than one steady-state motion for a given Mach number. The mecha- nisms for the appearance and disappearance of the co-exisiing attractors are elucidated by analysing the evolution of their basins of attraction as the Mach number changes
Approximation schemes for districting problems with probabilistic constraints
No full textIn this work a districting problem with stochastic demand is investigated. Chance-constraints are used to model the balancing requirements. Explicit contiguity constraints are also considered. After motivating the problem and discussing several modeling aspects, an approximate deterministic counterpart is proposed which is the core of new solution algorithms devised. The latter are based upon a location-allocation scheme, whose first step consists of considering either a problem with a sample of scenarios or a sample of single-scenario problems. This leads to two variants of a new heuristic. The second version calls for the use of a so-called attractiveness function as a means to find a good trade-off between the (approximate) solutions obtained for the single-scenario problems. Different definitions of such functions are discussed. Extensive computational tests were performed whose results are reported
Solutions for districting problems with chance-constrained balancing requirements
No full textIn this paper, a districting problem with stochastic demands is investigated. The goal is to divide a geographic area into p contiguous districts such that, with some given probability, the districts are balanced with respect to some given lower and upper thresholds. The problem is cast as a p-median problem with contiguity constraints that is further enhanced with chance-constrained balancing requirements. The total assignment cost of the territorial units to the representatives of the corresponding districts is used as a surrogate compactness measure to be optimized. Due to the tantalizing purpose of deriving a deterministic equivalent for the problem, a two-phase heuristic is developed. In the first phase, the chance-constraints are ignored and a feasible solution is constructed for the relaxed problem; in the second phase, the solution is corrected if it does not meet the chance-constraints. In this case, a simulation procedure is proposed for estimating the probability of a given solution to yield a balanced districting. That procedure also provides information for guiding the changes to make in the solution. The results of a series of computational tests performed are discussed based upon a set of testbed instances randomly generated. Different families of probability distributions for the demands are also investigated, namely: Uniform, Log-normal, Exponential, and Poisson
Shape reconstruction from medical images and quality mesh generation via implicit surfaces
No full textAssessment of added mass effects on flutter boundaries using the Leishman-Beddoes dynamic stall model
No full textWe consider the dynamics of a typical airfoil section both in forced and free oscillations and investigate the
importance of the added mass terms, i.e. the second derivatives in time of the pitch angle and plunge displacement. The structural behaviour is modelled by linear springs in pitch and plunge and the aerodynamic loading represented by our
interpretation of the state-space version of the Leishman–Beddoes semi-empirical model. The added mass terms are
often neglected since this leads to an explicit system of ODEs amenable for solution using standard ODEsolvers. We
analyse the effect of neglecting the added mass terms in forced oscillations about a set of mean angles of incidence by
comparing the solutions obtained with the explicit and implicit systems of ODEs and conclude that their differences
amount to a time lag that increases at a constant rate with increases of the reduced frequency
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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