1,721,165 research outputs found
Oscillatory and steady streaming flow in the anterior chamber of the moving eye
No full textWe study the flow induced by eye rotations in the anterior chamber (AC) of the eye,
the region between the cornea and the iris. We model the geometry of the AC as
a thin domain sitting on the surface of a sphere, and study both the simpler case
of a constant-height domain as well as a more realistic AC shape. We model eye
rotations as harmonic in time with prescribed frequency ω f and amplitude β, and use
lubrication theory to simplify the governing equations. We write the equations in a
reference frame moving with the domain and show that fluid motion is governed by
three dimensionless parameters: the aspect ratio of the AC, the angular amplitude
β and the Womersley number α. We simplify the equations under the physiologically
realistic assumptions that is small and α large, leading to a linear system that can be
decomposed into three harmonics: a dominant frequency component, with frequency
ω f , and a steady streaming component and a third component with frequency 2ω f .
We solve the problem analytically for the constant-height domain and numerically as
the solution of ordinary differential equations in the more realistic geometry. Both the
primary flow and the steady streaming are shown to have a highly three-dimensional
structure, which has not been highlighted in previous numerical works. We show
that the steady streaming is particularly relevant from the clinical point of view, as
it induces fluid mixing in the AC. Furthermore, the steady flow component is the
dominant mixing mechanism during the night, when the thermal flow induced by
temperature variations across the AC is suppressed
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
Aqueous humour flow in the posterior chamber of the eye and its modifications due to pupillary block and iridotomy
No full textThe anterior chamber (AC) and posterior chamber (PC) of the eye are connected through the pupil and are filled with aqueous humour. The aqueous flows from the posterior to the AC at an approximately constant rate, and the intraocular pressure is governed by this rate and the resistance to aqueous outflow. In some patients the iris and lens come into contact, leading to pressure build-up in the PC, peripheral axial shallowing of the AC and, possibly, to angle-closure glaucoma. This can lead to blindness, which may be prevented by surgically creating an iridotomy, that is a hole through the iris to facilitate the flow from the posterior to the AC. The problem of optimal size and location of an iridotomy is still poorly understood. In this article, we study aqueous flow in the PC and investigate how it is modified in the presence of an iridotomy. Our approach is based on the lubrication theory, which allows us to solve the problem semi-analytically. We treat the iridotomy as a point sink and assume that the flux through it is proportional to the pressure. We find that the ideal size and location of an iridotomy are influenced by various geometrical and fluid mechanical factors, the most relevant of which are the size of the hole and the length and height of the iris-lens channel. For certain iridotomy diameters, we find that the jet velocity through the iridotomy might be large enough to cause possible corneal damage
Traction on the retina induced by saccadic movements in the presence of Posterior Vitreous Detachment
No full textVitreous humor, which fills most part of the eye's interior, is a gel-like substance in the young healthy man.
With aging, it undergoes a liquefaction process that usually leads to vitreous separation from the retina, which is called Posterior Vitreous Detachment (PVD).
Sites of strong adherence between the vitreous and the retina may prevent a complete detachment.
In such circumstances the adherent vitreous fibers may pull so hard as to produce a retinal break.
This rupture mechanism may possibly be induced by eye rotations.
Our aim is to study the dynamic stress concentration on the retina induced by saccadic eye movements. While our formulation applies to a general three-dimensional problem, we consider a plane strain problem just to simplify the computational setting.
The solid vitreous is modeled as a hyperelastic incompressible solid, endowed with a Mooney-Rivlin response function, and the liquefied vitreous as a Newtonian fluid. Moreover, we account for finite-amplitude eye rotations.
We implemented our model in Comsol by using the PDE Application Mode in weak form and the Moving Mesh Application Mode. The numerical simulations show that very high values of the traction are attained at the boundary of the interface during eye rotations
A mechanical model of ocular bulb vibrations and implications for acoustic tonometry
No full textIn this study, we propose a comprehensive mechanical model of ocular bulb vibrations and discuss its implications for acoustic tonometry. The model describes the eye wall as a spherical, pre-stressed elastic shell containing a viscoelastic material and accounts for the interaction between the elastic corneoscleral shell and the viscoelastic vitreous humor. We investigate the natural frequencies of the system and the corresponding vibration modes, expanding the solution in terms of scalar and vector spherical harmonics. From a quantitative point of view, our findings reveal that the eyebulb vibration frequencies significantly depend on IOP. This dependency has two origins: “geometric” stiffening, due to an increase of the pre-stress, and “material” stiffening, due to the nonlinearity of the stress-strain curve of the sclera. The model shows that the second effect is by far dominant. We also find that the oscillation frequencies depend on ocular rigidity, but this dependency is important only at relatively large values of IOP. Thus close to physiological conditions, IOP is the main determinant of ocular vibration frequencies. The vitreous rheological properties are found to mostly influence vibration damping. This study contributes to the understanding of the mechanical behavior of the eye under dynamic conditions and thus has implications for non-contact intraocular pressure measurement techniques, such as acoustic tonometry. The model can also be relevant for other ocular pathological conditions, such as traumatic retinal detachment, which are believed to be influenced by the dynamic behavior of the eye
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