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We investigate theoretically and experimentally the stability of two interlaced helical vortices with respect to displacement perturbations having wavelengths that are large compared to the size of the vortex cores. First, existing theoretical results are recalled and applied to the present configuration. Various modes of unstable perturbations, involving different phase relationships between the two vortices, are identified and their growth rates are calculated. They lead to a local pairing of neighbouring helix loops, or to a global pairing with one helix expanding and the other one contracting. A relation is established between this instability and the three-dimensional pairing of arrays of straight parallel vortices, and a striking quantitative agreement concerning the growth rates and frequencies is found. This shows that the local pairing of vortices is the driving mechanism behind the instability of the helix system. Second, an experimental study designed to observe these instabilities in a real flow is presented. Two helical vortices are generated by a two-bladed rotor in a water channel and characterised through dye visualisations and particle image velocimetry measurements. Unstable displacement modes are triggered individually, either by varying the rotation frequency of the rotor, or by imposing a small rotor eccentricity. The observed unstable mode structure, and the corresponding growth rates obtained from advanced processing of visualisation sequences, are in good agreement with theoretical predictions. The nonlinear late stages of the instability are also documented experimentally. Whereas local pairing leads to strong deformations and subsequent breakup of the vortices, global pairing results in a leapfrogging phenomenon, which temporarily restores the initial double-helix geometry, in agreement with recent observations from numerical simulations.
How do citizens evaluate democracy? Previous literature trying to address this question has often relied on single indicators to assess citizens’ assessment of democracy. This article contributes to this debate by using multiple indicators measuring different attributes to find a summary measure of citizens’ evaluations of democracy. Using the special module of the sixth round of the European Social Survey ‘Europeans’ understandings and evaluations of democracy’ and applying Bayesian factor analysis with country random effects, this article tests whether multiple indicators form an underlying trait measuring citizens’ evaluations of democracy. It finds the scores of this measure at the individual and country levels and validates this measure against other measures built at the system level, including the ‘satisfaction with democracy’ indicator, also illustrating its functioning as a dependent and an independent variable.
This work presents the capabilities of a novel L-shaped trailing-edge Gurney flap as a device for vibration reduction. The primary effect of this L-tab is represented by a modification of the reference aerofoil mean line shape through by two counter-rotating vortical structures created at the trailing edge. The comparison of the aerodynamic loads generated by the novel L-tab Gurney flap and a classical trailing-edge flap allows to estimate the ranges of reduced frequency where the L-tab is expected to perform better than a trailing edge flap and vice versa. Linear aerostructural models for a typical section representative of a helicopter blade equipped with a partial-span L-tab or a trailing-edge flap are built, and a higher harmonic control algorithm is applied. Performance are compared between the two devices to reduce separately the N/rev harmonics of the blade root rotating frame vertical force, flapping and feathering moments. The attainment of similar results with classical trailing-edge device is a further confirmation of the potential feasibility of this novel L-tab as an effective alternative means for vibration reduction on rotor blades.
We investigate the instability of a single helical vortex filament of small pitch with respect to displacement perturbations whose wavelength is large compared to the vortex core size. We first revisit previous theoretical analyses concerning infinite Rankine vortices, and consider in addition the more realistic case of vortices with Gausssian vorticity distributions and axial core flow. We show that the various instability modes are related to the local pairing of successive helix turns through mutual induction, and that the growth rate curve can be qualitatively and quantitatively predicted from the classical pairing of an array of point vortices. We then present results from an experimental study of a helical vortex filament generated in a water channel by a single-bladed rotor under carefully controlled conditions. Various modes of displacement perturbations could be triggered by suitable modulation of the blade rotation. Dye visualisations and particle image velocimetry allowed a detailed characterisation of the vortex geometry and the determination of the growth rate of the long-wave instability modes, showing good agreement with theoretical predictions for the experimental base flow. The long-term (downstream) development of the pairing instability leads to a grouping and swapping of helix loops. Despite the resulting complicated three-dimensional structure, the vortex filaments surprisingly remain mostly intact in our observation interval. The characteristic distance of evolution of the helical wake behind the rotor decreases with increasing initial amplitude of the perturbations; this can be predicted from the linear stability theory.
The influence of the airfoil thickness on aerodynamic loads is investigated numerically for harmonically pitching airfoils at low incidence, under the incompressible and inviscid flow approximation. Force coefficients obtained from finite-volume unsteady simulations of symmetrical 4-digit NACA airfoils are found to depart from the linear Theodorsen model of an oscillating flat plate. In particular, the value of the reduced frequency resulting in the inversion – from clockwise to counter-clockwise – of the lift/angle-of-attack hysteresis curve is found to increase with the airfoil thickness. Both the magnitude and direction of the velocity vector due to pitching over the airfoil surface differ from their flat-plate values. During the upstroke, namely nose-up rotation, phase, this results in a decrease (increase) of the normal velocity magnitude over the upper (lower) surface of the airfoil. The opposite occurs during the downstroke phase. This is confirmed by comparing the computed pressure distribution to the flat-plate linear Küssner model. Therefore, beyond the inversion frequency, the lift coefficient of a finite-thickness airfoil is higher during upstroke and lower during downstroke than its flat-plate counterpart. A similar dependence is also found for the quarter-chord moment coefficient. Accordingly, a modification to the classical Theodorsen model is proposed to take into account the effects of the airfoil thickness on unsteady loads. The new model is found to accurately predict the unsteady aerodynamics of a thick symmetric and a slightly cambered airfoil with a maximum thickness in the range 4–24 %. The limits of the present inviscid flow analysis are assessed by means of numerical simulation of high Reynolds number (
This article tests the cross-national equivalence of the political protest scale, as developed by Barnes and Kaase, in 20 Western European countries using a battery of items included in the fourth wave of the European Values Study. The scale measuring the concept of political protest is widely used, but no evidence of cross-country equivalence has yet been provided in the literature. The article illustrates the concept of political protest, the relationship between concept formation, operationalization, and measurement equivalence, and the possible consequences of a lack of equivalence. It is argued that comparative research may be threatened by a lack of measurement equivalence. The spread of international surveys eases comparative designs, but at the same time enlarges the chances that we compare what is not actually comparable. The article then outlines an empirical strategy to assess the political protest scale's measurement equivalence. To assess cross-country equivalence, Mokken Scale Analysis, a nonparametric scaling method within the family of Item Response Theory models, is used. This has been shown to work better than Confirmatory Factor Analysis when dealing with dichotomous and polytomous items forming ordinal scales. The results show that the cross-country equivalence of the political protest scale depends on the type of measure the scholar wishes to build and use.
A two-dimensional numerical scheme for the compressible Euler equations is presented and applied here to the simulation of exemplary compressible vortical flows. The proposed approach allows to perform computations on unstructured moving grids with adaptation, which is required to capture complex features of the flow-field. Grid adaptation is driven by suitable error indicators based on the Mach number and by element-quality constraints as well. At the new time level, the computational grid is obtained by a suitable combination of grid smoothing, edge-swapping, grid refinement and de-refinement. The grid modifications—including topology modification due to edge-swapping or the insertion/deletion of a new grid node—are interpreted at the flow solver level as continuous (in time) deformations of suitably-defined node-centered finite volumes. The solution over the new grid is obtained without explicitly resorting to interpolation techniques, since the definition of suitable interface velocities allows one to determine the new solution by simple integration of the Arbitrary Lagrangian-Eulerian formulation of the flow equations. Numerical simulations of the steady oblique-shock problem, of the steady transonic flow and of the start-up unsteady flow around the NACA 0012 airfoil are presented to assess the scheme capabilities to describe these flows accurately.
Several authors have recently shown that anterograde amnesia is often associated with semantic memory impairment in amnesic MCI patients. Similarly, after the MCI condition, some patients who convert to Alzheimer's disease (AD) show the classic onset (cAD) characterized by the impairment of memory and executive functions, whereas other AD patients show isolated defects of episodic and semantic memory without deficits in other cognitive domains. The latter have been considered an AD variant characterized by ‘focal Temporal Lobe Dysfunction’ (TLD). The aim of the present study was to assess the differences in disease progression between cAD and TLD. For this purpose a continuous series of newly diagnosed probable AD patients presenting as cAD (n = 30) and TLD (n = 25), matched for severity, and 65 healthy controls underwent a comprehensive neuropsychological evaluation at baseline; TLD and cAD were re-evaluated at a 24-month follow-up. At follow-up, TLD patients showed no significant worsening of cognitive functions, whereas cAD subjects displayed a significant worsening in all explored cognitive domains. In conclusion, our results confirm that probable AD presenting as TLD represents a specific onset of AD characterized by a slower rate of progression. (JINS, 2012, 18, 144–150)
This paper investigates the noise radiation produced by a rotor inside a duct, which is convected by a swirling-translating mean flow. The study is based on an extension of Gennaretti's and Morino's boundary element method to the frequency domain for scattering problems in conjunction with a spinning rotor source model in the presence of a swirl flow. The proposed formulation is validated against exact solutions and is further used to investigate the effects of the translating flow Mach number and swirling flow angular velocity on noise radiation to the far field. The scattered sound is highly affected by the convecting mean flow. The modal content of the scattered field increases when increasing the translating flow Mach number, while a swirling flow leads to a reduction of the mode propagation, if co-rotating with respect to the azimuthal order of the spinning source, or an increase of the modal content, if counter-rotating with respect to the source. In general, the mean translating flow moves the main lobes of the directivity patterns downstream, while in some cases the mean swirling flow neglects this effect and the downstream lobe is completely shifted.
5,10,15,20 meso-tetraphenyl porphyrin (H2TPP) and copper phthalocyanine (CuPc) films have been deposited by means of a recently developed plasma based technique named glow discharge induced sublimation (GDS). The two macrocycles have been deposited by vacuum evaporation (VE) and H2TPP by spin coating (SPIN) as well for comparison. The physical properties of the films have been characterized by means of scanning electron microscopy (SEM) and nitrogen physisorption measurements. SEM images and physisorption isotherms highlight both the much higher surface roughness and specific surface area of GDS samples with respect to the VE and SPIN ones. Optical sensing measurements, performed in differently concentrated ethyl alcohol atmospheres, highlight that GDS samples yield much higher response intensities than SPIN and VE films, short response times and complete recovery.
Since cancer is a complex phenomenon that incorporates events occurring on different
length and time scales, therefore multiscale models are needed if we hope to adequately address
cancer specific questions. In this paper we present three different multiscale individual-cell-based
models, each motivated by cancer-related problems emerging from each of the spatial scales: extracellular,
cellular or subcellular, but also incorporating relevant information from other levels.
We apply these hybrid models to investigate the influence of the microenvironement on tumour
invasion, cell-cell collaboration and competition leading to the initiation and growth of epithelial
tumours, and to evolution of cell phenotypes/genotypes arising in tumours growing in different
oxygen concentrations. We also discuss how these models relate to one another and can be used to
bridge biological scales relevant to cancer.
To perform a 3-year, prospective surveillance program for legionnaires disease (LD) in a large university hospital in Rome, and to assess the usefulness of the hospital water monitoring program in predicting the risk of nosocomial LD.
Samples from patients with new cases of nosocomial pneumonia were sent for legionella laboratory investigations. Meanwhile, water samples for bacteriological analysis were collected every 6 months from high- and medium-risk hospital wards (10 in total). Legionella pneumophila isolates collected were serotyped and analyzed by pulsed-field gel electrophoresis.
From June 2001 through May 2004, the pneumonia surveillance identified one case of nosocomial LD among 43 cases of nosocomial pneumonia (2.3%). Environmental investigations detected L. pneumophila in 12 (18.7%) of the 64 water samples, of which 50% belonged to serogroup 1. The L. pneumophila count and the percentage of positive locations never exceeded 102 colony-forming units/L and 20%, respectively, except when the LD nosocomial case occurred (positive water samples, 40%; I. pneumophila count, <102 colony-forming units/L). Genotyping showed 3 prevalent clones of L. pneumophila in the water distribution network, of which one persisted over the 3 years. One clone contained 3 different L. pneumophila serogroups (2, 4, and 6).
The low incidence of nosocomial cases of LD appears to be associated with a low percentage (<20%) of positive water samples per semester and with a low contamination level (<102 colony-forming units/L). An infection control system for nosocomial LD should, therefore, be based on both environmental and clinical surveillance, together with the appropriate maintenance of the hospital water distribution system.
Ternary W-Si-N thin films have been reactively sputter-deposited from a
W5Si3 target at different nitrogen partial pressures. The composition has
been determined by 2.2 MeV4He+ beam, the structure by
x-ray diffraction and transmission electron microscope, the chemical bonds
by Fourier transform - infrared spectroscopy and the surface morphology by
scanning electron microscopy. Electrical resistivity was measured by four
point probe technique on the as grown films. The film as-deposited is
amorphous with the Si/W ratio increasing from about 0.1 up to 0.55 with the
nitrogen content going from 0 to 60 at%. The heat treatments up to 980 °C
induce a loss of nitrogen in the nitrogen rich samples. Segregation of
metallic tungsten occurs in the sample with low nitrogen content
(W58Si21N21). Samples with high nitrogen
content preserve the amorphous structure, despite of the precipitation of a
more ordered phase inferred by FT-IR absorbance spectrum of the layer
treated at highest temperature. The surface morphology depends upon the
nitrogen content; the loss of nitrogen induces the formation of blistering
and in the most nitrogen rich sample the formation of holes. Electrical
resistivity preliminary results on the as grown layers range between 500 and
4750 μωcm passing from the lowest to the highest N concentration.
Primary cerebellopontine angle (CPA) cholesteatoma grows slowly and silently in the subarachnoidal spaces. The diagnosis is often late, when the lesion has reached large dimensions. Surgical removal is the only available therapy. Fifteen consecutive cases of CPA cholesteatoma managed at a tertiary otoneurosurgical referral unit between September 1985 and April 1999 were reviewed. The study population, consisting of seven males and eight females, had a mean age of 44 years of age (range 21–69) at the time of surgery. The clinical, audiological and radiological presentations were examined. The tumours were classified according to the Moffat classification of CPA cholesteatomas. In 67 per cent of cases the presenting symptom was related to the vestibulo-cochlear nerve. The average duration of symptoms was 23 months (ranging from one month–10 years). The hearing preservation approaches were utilized the most (11 cases), while the translabyrinthine approach alone, or in association with a middle fossa craniotomy, was performed in four cases. Tumour removal was total in 12 cases and subtotal in three cases. In cases undergoing hearing preservation surgery the mean pre-operative pure tone average (PTA) for the frequencies 0.5, 1, 2, and 3 kHz was 19.3 dB HL (SD 13.84) and the mean pre-operative speech discrimination score (SDS) was 89.8 per cent (SD 5.97). In 44.4 per cent of patients the hearing was preserved and the mean post-operative PTA was 20.29 dB HL (SD 15.84). In five patients post-operative complications occurred. No peri- or post-operative death occurred in this series, one patient developed a recurrence 15 years after the initial surgery.
Microcrystalline films of SiC.H have been deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) of SiH4+CH4+H2 mixtures with high power density and high H2 dilution, at variable CH4/SiH4 ratios. Their elemental composition, structural, optical and electrical properties have been investigated in order to evidence the influence of hydrogen and carbon on microcrystallinity.
A new PHOTO-CVD apparatus has been built in order to deposit a – Si : H films and other kinds of amorphous thin films by a technique which is both simple and versatile. This apparatus is composed of three chambers connected together: a load-lock chamber, a process chamber and a third chamber for in-situ analysis of deposited films. A peculiarity of the lamp, a dielectric discharge lamp which can work with noble gases like Xe or Kr, is that it can be completely dismounted without breaking the vacuum in order to clean the optical MgF2 window. By this method, the deposition chamber can be kept in very clean conditions. In this apparatus, we started to deposit a – SixC1−x: H of very good quality, taking their thickness into account. These films have been completely characterized by chemical (RBS, ERDA) and optical (PDS) methods. Their quality can be compared with quality of a – Si : H samples of the same thickness obtained by PECVD.