1,721,462 research outputs found
On the mother bodies of steady polygonal uniform vortices. Part I: numerical experiments
No full textThe existence of an integral relation between self-induced velocity of a uniform, planar vortex and Schwarz function of its boundary opens the way to understand the kinematics of the vortex by analysing the internal singularities of that function. In general, they are branch cuts and form the so-called "mother body" of the vortex, because they generate the same external velocities of the vortex, by means of a relation identical to the Biot-Savart law for a vortex sheet. The jump of the Schwarz function across the cuts plays the role of the (complex) density of circulation. This paper investigates the singularities of polygonal vortices, which are highly nontrivial steady vortices widely present in Nature, and having fascinating properties, some of them still not well understood. By means of the equation of the dynamics of the Schwarz function specialised for steady vortices, a numerical tool based on elementary properties of the holomorphic functions is used for detecting the internal singularities and evaluating their strengths. In this way, it is shown that an nagonal vortex possesses n internal branch cuts. In a reference system having origin on the centre of vorticity of the vortex and real axis crossing one of its vertices, these cuts start from the origin and are directed along the n roots of the unity, so that they are aligned with the vertices. The positions of the branch points and the values assumed by the Schwarz function in these points are calculated by evaluating this function just outside the vortex boundary. Once the conditions on the branch points are defined, a power series representation of the Schwarz function is proposed, that is able to explain the behaviour of its real and imaginary parts in neighbourhoods of these points. Some conjectures about the external singularities are also discussed
The time evolution of the mother body of a planar uniform vortex moving in an inviscid fluid
No full textThe time evolution of the mother body of a planar, uniform vortex that moves in an incompressible, inviscid fluid is investigated. The vortex is isolated, so that its motion is just due to self-induced velocities. Its mother body is defined as the part internal to the vortex of the singular set of the Schwarz function of its boundary. In the present analysis, it is an arc of curve (branch cut), starting and ending in the two internal branch points of this function, across any point of which the Schwarz function experiences a finite jump. By looking at the mother body from the outside of the vortex, it behaves as a vortex sheet having density of circulation given by the jump of the Schwarz function. Its name (mother body) is taken from Geophysics, and it is here used due to its property of generating, outside the vortex and on its boundary, the same velocity as the vortex itself. The shape of the branch cut and the jump of the Schwarz function across any point of it change in time, by following the motion of the vortex boundary. As it happens for a physical vortex sheet, the mother body is not a material line, so that it does not move according to the velocities induced by the vortex. In the present paper, the cut shape, the above jumps, as well as the cut velocities are deduced from the time evolution equation of the Schwarz function. Numerical experiments, carried out by building the branch cut and calculating the limit values of the Schwarz function on its sides during the vortex motion, confirm the analytical calculations. Some global quantities (circulation, first and second order moments) are here rewritten as integrals on the cut, and their conservation during the vortex motion is analytically and numerically verified. Indeed, the numerical simulations show that they behave in the same way as their classical contour dynamics forms, written in terms of integrals on the vortex boundary. This proves that the shape of the cut, as well as the limit values of the Schwarz function on its sides, are correctly calculated during the motion
Rising bubble in the presence of a wall or a flat free surface
No full textThe dynamics of a gas/vapour bubble rising under the action of gravity in an isochoric, inviscid liquid bounded by flat solid wall or free surface is investigated by developing simplified analytical models, based on the assumption that the bubble remains spherical during its motion.The bounding surface is assumed orthogonal to the gravity. Effects of viscosity are neglected and the fluid is initially at rest, so that theresulting flow is a potential one. Suitable forms of the classical Rayleigh–Plesset equation for the time evolution of radius and center of mass are deduced.The bubble dynamics resulting from the models is compared with numerical simulationperformed with a boundary integral code, in which the bubble shape is assumed symmetricabout an axis aligned with gravity
Antigiudaismo e antisemitismo: continuità e frattura
No full textIl saggio intende riprendere in considerazione le diverse interpretazione dei rapporti tra antigiudaismo e antisemitismo sottolineando le origini diverse dei due fenomeni, che pur si intrecciano nella storia, contamonandosi a vicenda
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
Fascismo anticristiano?
No full textIl saggio intende mettere in evidenza come nel fascismo fossero presenti correnti in cui era forte un carattere anticristiano, proveniente soprattutto dagli ambienti nazionalisti e del sindacalismo rivoluzionario. Un significativo salto di qualità in questo senso ha rappresentato la campagna antisemita, legata al mito neopagano del sangue, con il conseguente avvicinamento, anche sul piano ideologico, al nazismo tedesco
Are Europeans moving towards dietary habits more suitable for reducing cardiovascular disease risk?
Full text linkAims: Cardiovascular diseases are the main cause of death in Europe. Food choices represent the most important factors undermining health and well-being; they account for as much as half of all CVD deaths in Europe. The aim of this viewpoint is to evaluate food choices of the European population and their temporal trends in relation to possible effects on the cardiovascular disease risk. Data synthesis: The CVD death rate attributable to diet-related factors has fallen in Europe over the last 25 years; however, the pace of the reduction has slowed down in the last ten years. In parallel, in the last decade unfavorable changes in the dietary habits associated with CVD risk have occurred. Conclusions: A mismatch exists between the available evidence on the health-promoting potential of the diet and the relatively modest and inconsistent improvements of dietary habits in the European population observed in recent years. Nutritional education alone will not be enough to improve the lifestyle of people in Europe. Policy options to be considered to reach this aim should necessarily include also initiatives for facilitating production, marketing, availability and affordability of healthy foods in each and every European country
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