First Instability and Structural Sensitivity of the Flow Past Two Side-by-Side Cylinders

The onset of two-dimensional instabilities in the flow past two side-by-side circular cylinders is numerically investigated in the ranges 0.1 <= 6 <= 3 and Re < 100, with g being the non-dimensional gap spacing between the surfaces of the two cylinders and Re the Reynolds number. A comprehensive, global stability analysis of the symmetric base flow is carried out, indicating that three harmonic modes and one steady antisymmetric mode become unstable at different values of g and Re. These modes are known to promote distinct flow regimes at increasing values of g: single bluff-body, asymmetric, in-phase and antiphase synchronized vortex shedding. For each mode, the inherent structural sensitivity is examined in order to identify the core region of the related instability mechanism. In addition, by exploiting the structural sensitivity analysis to base flow modifications, a passive control strategy is proposed for the simultaneous suppression of the two synchronized shedding modes using two small secondary cylinders. Its effectiveness is then validated a posteriori by means of direct numerical simulations

Centre-Manifold Reduction of Bifurcating Flows

In this paper we describe a general and systematic approach to the centre-manifold reduction and normal form computation of flows undergoing complicated bifurcations. The proposed algorithm is based on the theoretical work of Coullet & Spiegel (SIAM J. Appl. Maths, vol. 43(4), 1983, pp. 776821) and can be used to approximate centre manifolds of arbitrary dimension for large-scale dynamical systems depending on a scalar parameter. Compared with the classical multiple-scale technique frequently employed in hydrodynamic stability, the proposed method can be coded in a rather general way without any need to resort to the introduction and tuning of additional time scales. The method is applied to the dynamical system described by the incompressible NavierStokes equations showing that high-order, weakly nonlinear models of bifurcating flows can be derived automatically, even for multiple codimension bifurcations. We first validate the method on the primary Hopf bifurcation of the flow past a circular cylinderand after we illustrate its application to a codimension-two bifurcation arising in the flow past two side-by-side circular cylinder

Photometric and spectroscopic study of the intermediate-age open cluster NGC 2355

In this paper we analyse the evolutionary status and properties of the old open cluster NGC 2355, located in the Galactic anticentre direction, as a part of the long term programme BOCCE. NGC 2355 was observed with LBC@LBT using the Bessel $B$, $V$, and $I_c$ filters. The cluster parameters have been obtained using the synthetic colour-magnitude diagram (CMD) method, as done in other papers of this series. Additional spectroscopic observations with FIES@NOT of three giant stars were used to determine the chemical properties of the cluster. Our analysis shows that NGC 2355 has metallicity slightly less than solar, with [Fe/H]$=-0.06$ dex, age between 0.8 and 1 Gyr, reddening $E(B-V)$ in the range 0.14 and 0.19 mag, and distance modulus $(m-M)_0$ of about 11 mag. We also investigated the abundances of O, Na, Al, $\alpha$, iron-peak, and neutron capture elements, showing that NGC 2355 falls within the abundance distribution of similar clusters (same age and metallicity). The Galactocentric distance of NGC~2355 places it at the border between two regimes of metallicity distribution; this makes it an important cluster for the study of the chemical properties and evolution of the disc.Comment: 20 pages, 11 figures, Accepted on MNRA

On the Origin of the Flip-Flop Instability of two Side-by-Side Cylinder Wakes

In this work the flip-flop instability occurring in the flow past two side-by-side circular cylinders is numerically investigated within the range of non-dimensional gap spacing 0.6<g<1.4 and Reynolds number 50<Re\leq 90. The inherent two-dimensional flow pattern is characterized by an asymmetric unsteady wake (with respect to the horizontal axis of symmetry) with the gap flow being deflected alternatively toward one of the cylinders. Such behaviour has been ascribed by other authors to a bistability of the flow, and therefore termed flip-flop. In contrast, the simulations performed herein provide new evidence that at low Reynolds numbers the flip-flopping state develops through an instability of the in-phase synchronized vortex shedding between the two cylinder wakes. This new scenario is confirmed and explained by means of a linear global stability investigation of the in-phase periodic base flow. The Floquet analysis reveals indeed that a pair of complex-conjugate multipliers becomes unstable having the same low frequency as the gap flow flip-over. The neutral curve of this secondary instability is tracked within the above range of gap spacing. The spatiotemporal shape of the unstable Floquet mode is then analysed and its structural sensitivity is considered in order to identify the 'core' region of the flip-flop instability mechanism

Ameloblastoma plexiforme del maxilar: manejo quirúrgico y protético. A propósito de un caso

El ameloblastoma es un tumor odontógeno benigno de derivación epitelial, de elevada invasión local, crecimiento intermitente y con alta probabilidad de recidiva. Puede dividirse en los siguientes tipos histológicos: folicular, plexiforme, acantomatoso, a células basales y granulares. El ameloblastoma folicular es el subtipo histológico más común. Se manifiesta prevalentemente durante la tercera, cuarta y quinta década de vida, sin predilección para el sexo, aun así se puede encontrar en cualquier grupo de edad incluyendo niños. La mayoría de los ameloblastomas se encuentra sobretodo en mandíbula (al nivel del ángulo y rama). Las características clínicas, incluso si complementadas con radiografías y muestras histológicas, no son útiles a la hora de predeterminar el comportamiento biológico y por lo tanto el pronóstico de un ameloblastoma individual. Los autores presentan un caso localizado de ameloblastoma plexiforme localizado en la parte posterior del maxilar de un hombre de 30 años; el término "plexiforme" se refiere al aspecto de anastomosis de los islotes del epitelio odontógeno, en contraste con el tipo «folicular». En este artículo se discuten importantes aspectos de la patología, del tratamiento quirúrgico y protésico. En conclusión, es necesaria una cooperación estricta a largo plazo de un grupo de clínicos, patólogos, cirujanos y prostodoncistas, para ofrecer el mejor tratamiento individualizado en cada caso. The ameloblastoma is a clinically persistent benign tumor derived from odontogenic epithelium, locally invasive, intermittent in growth and with high a probability of recurrence. The following histologic patterns may be distinguished: follicular, plexiform, spindle cell, acanthomatous, basal cell type and granular cell. The follicular ameloblastoma is the most common histologic tipe. The majority of patients are in 3th, 4th and 5th decade, but can be found in any age group including children. The majority of ameloblastomas are observed in the mandible (mostly angle or ramus). Clinical features, even if combined with radiology or histology findings, are not useful when trying to determine the biological behaviour and therefore the prognosis of an individual ameloblastoma. The authors report a case of plexiform ameloblastoma in the posterior maxilla, in male 30 year old otherwise healthy; the term "plexiform" refers to the appearance of anastomosing islands of odontogenic epithelium in contrast to a follicular pattern. Important questions on pathology, surgical and prosthetic therapy are discussed in this paper. In conclusion, a close cooperation of clinicians, pathologists, surgeons and prosthesists is necessary and very important over a long period of time for the best management of each individual case

Multiple bound states in scissor-shaped waveguides

We study bound states of the two-dimensional Helmholtz equations with Dirichlet boundary conditions in an open geometry given by two straight leads of the same width which cross at an angle $\theta$. Such a four-terminal junction with a tunable $\theta$ can realized experimentally if a right-angle structure is filled by a ferrite. It is known that for $\theta=90^o$ there is one proper bound state and one eigenvalue embedded in the continuum. We show that the number of eigenvalues becomes larger with increasing asymmetry and the bound-state energies are increasing as functions of $\theta$ in the interval $(0,90^o)$. Moreover, states which are sufficiently strongly bent exist in pairs with a small energy difference and opposite parities. Finally, we discuss how with increasing $\theta$ the bound states transform into the quasi-bound states with a complex wave vector.Comment: 6 pages, 6 figure

Large Binocular Telescope observations of PSR J2043+2740

We present the results of deep optical imaging of the radio/$\gamma$-ray pulsar PSR J2043+2740, obtained with the Large Binocular Telescope (LBT). With a characteristic age of 1.2 Myr, PSR J2043+2740 is one of the oldest (non recycled) pulsars detected in $\gamma$-rays, although with still a quite high rotational energy reservoir ($\dot{E}_{\rm rot} = 5.6 \times 10^{34}$ erg s$^{-1}$). The presumably close distance (a few hundred pc), suggested by the hydrogen column density ($N_{\rm H} \lesssim 3.6 \times 10^{20}$ cm$^{-2}$), would make it a viable target for deep optical observations, never attempted until now. We observed the pulsar with the Large Binocular Camera of the LBT. The only object (V=25.44$\pm$0.05) detected within ~3" from the pulsar radio coordinates is unrelated to it. PSR J2043+2740 is, thus, undetected down to V~26.6 (3-$\sigma$), the deepest limit on its optical emission. We discuss the implications of this result on the pulsar emission properties.Comment: 4 pages, 3 figures, accepted for publication on MNRA