1,720,967 research outputs found
Propulsive efficiency of oscillating foils
No full textWe determine the flow field generated by a two-dimensional foil in
steady forward motion and a combination of harmonic heaving
and pitching oscillations, by numerically solving momentum and
continuity equations in the stream function-vorticity formulation.
Attention is focused on moderate values of the Reynolds number
and Mach numbers much smaller than one. The versatile tool which
has been developed has allowed the achievements of results for a
large set of values of the paramenters and the identification of
those giving rise to optimal propulsive performances. Conditions
for optimal production of thrust are:
-operation at Strouhal number between 0.25 and 0.40;
-large amplitude of heave motion-to-chord ratio (of order one);
-large maximum angle of attack, between 15 and 25 degrees;
-phase angle between heave and pitch (pitch leading heave) of about 75
degrees when the reference point for heave motion is at the one-third
chord length from the leading edge
Numerical experiments on the transient motions of a flapping foil
No full textThe flow field generated by a moving foil which mimics a fish tail during
a fast-start maneuver is investigated by means of numerical experiments.
The numerical simulations have some advantages with respect to laboratory
experiments. Indeed, having access to the velocity and pressure fields both
in space and in time, it is possible to 'measure' quantities like vorticity, forces
and torques which are quite difficult to obtain in laboratory. Moreover,
data can be easily gained for different foil kinematics. The obtained results
show that the time history of the propulsive force strongly depends on the
kinematics of the foil. Moreover, the numerical simulations have allowed
to identify the values of the parameters providing optimal propulsive performances and to understand the mechanism originating a positive thrust
A simple model of propulsive oscillating foils
No full textThe design of thrusters inspired by the locomotion of fishes is
currently investigated in many research centres for unmanned
underwater vehicles.
Fast fishes propel themselves in water through the rhytmic motion
of their tail. Propulsion is achieved by means of the periodic
shedding of vortex structures by the edges of the tail. Thrust is
produced because the vortices give rise to a steady jet of fluid
which leaves the tail in the direction which is opposite the forward
motion of the fish. Assuming that the fish tail can be modelled by a
2-D plate in steady forward motion and oscillating with a combination
of harmonic heaving and pitching movements, Brown and Michael's
model is presently used to determine the dynamics of the vortex
structures shed by plate edges. Numerical simulations have been
carried out to investigate the effects on the flow field of varying
the physical parameters of the phenomenon. The knowledge of
the strength and trajectory of the vortex structures shed by the
plate allows the characteristics of the jet producing the thrust to
be quantified
Chaotic flow generated by an oscillating foil
No full textThe present paper describes the results of a numerical study
of the flow field generated by the oscillations of a thrusting foil.
The investigation is carried out to see whether a transition from a
periodic flow to an aperiodic chaotic flow is present as observed around
oscillating cylinders. Moreover, the transition process is analysed in detail
and the characteristics of the system attractor are determined as function
of the parameters of the problem. The approach considers a two-dimensional
problem. Such purely two-dimensional method of calculation describes
the main features of the phenomenon but does not provide a complete
and accurate description of the flow, because it takes into account only
the cross-stream wake vorticity (at right angles to the direction of motion)
although in reality trailing vorticity (parallel to the direction of motion)
can appear even when a 2-D foil is considered. Notwithstanding the
above limitations, a two-dimensional approach can provide useful
information on the overall vorticity dynamics and on the forces acting
on the foil and it is the necessary introduction to full 3-D simulations
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
- …
