A note on the stability of slip channel flows

We consider the influence of slip boundary conditions on the modal and non-modal stability of pressure-driven channel flows. In accordance with previous results by Gersting (1974) (Phys. Fluids, 17) but in contradiction with the recent investigation of Chu (2004) (C.R. Mecanique, 332), we show that slip increases significantly the value of the critical Reynolds number for linear instability. The non-modal stability analysis however reveals that the slip has a very weak influence on the maximum transient energy growth of perturbations at subcritical Reynolds numbers. Slip boundary conditions are therefore not likely to have a significant effect on the transition to turbulence in channel flows

Growth and instability of a laminar plume in a strongly stratified environment

Experimental studies of laminar plumes descending under gravity into stably stratified environments have shown the existence of a critical injection velocity beyond which the plume exhibits a bifurcation to a coiling instability in three dimensions or a sinuous instability in a Hele-Shaw flow. In addition, flow visualization has shown that, prior to the onset of the instability, a stable base flow is established in which the plume penetrates to a depth significantly smaller than the neutral buoyancy depth. Moreover, the fresh water that is viscously entrained by the plume recirculates within a ‘conduit’ whose boundary with the background stratification appears sharp. Beyond the bifurcation, the buckling plume takes the form of a travelling wave of varying amplitude, confined within the conduit, which disappears at the penetration depth. To determine the mechanisms underlying these complex phenomena, which take place at a strikingly low Reynolds number but a high Schmidt number, we study here a two-dimensional arrangement, as it is perhaps the simplest system which possesses all the key experimental features. Through a combination of numerical and analytical approaches, a scaling law is found for the plume’s penetration depth within the base flow (i.e. the flow where the instability is either absent or artificially suppressed), and the horizontal cross-stream velocity and concentration profile outside the plume are determined from an asymptotic analysis of a simplified model. Direct numerical simulations show that, with increasing flow rate, a sinuous global mode is destabilized giving rise to the self-sustained oscillations as in the experiment. The sinuous instability is shown to be a consequence of the baroclinic generation of vorticity, due to the strong horizontal gradients at the edge of the conduit, a mechanism that is relevant even at very low Reynolds numbers. Despite the strength of this instability, the penetration depth is not significantly affected by it, instead being determined by the properties of the plume in the vicinity of the source. This scenario is confirmed by a local stability analysis. A finite region of local absolute instability is found near the source for sinuous modes prior to the onset of the global instability. Sufficiently far from the source the flow is locally stable. Near the onset of the global instability, varicose modes are also found to be locally, but only convectively, unstable

1-Gb/s Transmission Over a Phosphorescent White LED by Using Rate-Adaptive Discrete Multitone Modulation

Light-emitting diodes (LEDs), which will be increasingly used in lighting technology, will also allow for distribution of broadband optical wireless signals. Visible-light communication (VLC) using white LEDs offers several advantages over the RF-based wireless systems, i.e., license-free spectrum, low power consumption, and higher privacy. Mostly, optical wireless can provide much higher data rates. In this paper, we demonstrate a VLC system based on a white LED for indoor broadband wireless access. After investigating the nonlinear effects of the LED and the power amplifier, a data rate of 1 Gb/s has been achieved at the standard illuminance level, by using an optimized discrete multitone modulation technique and adaptive bit- and power-loading algorithms. The bit-error ratio of the received data was 1.5 10^(-3), which is within the limit of common forward error correction (FEC) coding. These results twice the highest capacity that had been previously obtained

Chiral symmetry restoration, eigenvalue density of Dirac operator and axial U(1) anomaly at finite temperature

We reconsider constraints on the eigenvalue density of the Dirac operator in the chiral symmetric phase of 2 flavor QCD at finite temperature. To avoid possible ultra-violet(UV) divergences, we work on a lattice, employing the overlap Dirac operator, which ensures the exact "chiral" symmetry at finite lattice spacings. Studying multi-point correlation functions in various channels and taking their thermodynamical limit (and then taking the chiral limit), we obtain stronger constraints than those found in the previous studies: both the eigenvalue density at the origin and its first and second derivatives vanish in the chiral limit of 2 flavor QCD. In addition we show that the axial U(1) anomaly becomes invisible in susceptibilities of scalar and pseudo scalar mesons, suggesting that the 2nd order chiral phase transition with the O(4) scaling is not realized in 2 flavor QCD. Possible lattice artifacts when non-chiral lattice Dirac operator is employed are briefly discussed.Comment: 39 pages, 1 figure(2 eps files), a version published in PR

Hybrid Radio over Fiber and Visible Light (RoF-VLC)Communication System

We experimentally demonstrate the integration of Radio-over-Fiber and Visible Light Communication technologies, into a hybrid system for indoor communication. The system, realized according to IEEE 802.11g standard, works effectively at typical office luminance level

MR breast imaging: A comparative analysis of conventional and parallel imaging acquisition [RM delle mammelle: Confronto tra tecnica convenzionale ed imaging parallelo]

Purpose. The objective of this study was to compare conventional breast magnetic resonance imaging (MRI) with breast MRI acquired with the sensitivity-encoding (SENSE) technique on a 1.5-T MRI scanner in the same patient, on the basis of image quality and kinetics analysis. Materials and methods. Thirty-one patients with suspicious mammography and US findings were included in the study. Conventional breast MRI consisted of the following sequences: T1 (matrix, 288x512); T2 (matrix 225x512); short tau inversion recovery (STIR) (matrix 320x224) and dynamic T1 [2D fast-field echo (FFE)] (matrix 256x512; temporal resolution =80 s). The SENSE technique included the following sequences: T1 (matrix 512x512); T2 (matrix 512x512); short-tau inversion recovery (STIR) (matrix 320x224); dynamic T1 (3D FFE) (matrix 512x512, with a temporal resolution ≤70 s). Image quality was graded on a four-point scale, and the mean scores given to each sequence were compared between the two protocols. The relative enhancement rates and the qualitative features of the signal intensity (SI)/time curves were also compared between the two protocols. Results. The readers found 64 contrast-enhanced lesions in 31 patients. Nineteen patients had a total of 27 malignant lesions. In the remaining 12 patients, 37 benign lesions were found. No significant differences between the two protocols were observed with regard to the mean relative enhancement rates and the qualitative features of the SI/time curves. In detail, the mean image quality scores were higher for SENSE imaging (p<0.05). The mean image quality score for the T1 and T2 morphological sequences were comparable. In contrast, the quality scores for the STIR images differed significantly between the two protocols (p<0.001), and a significant difference was also observed when comparing the T1 postcontrast images (p<0.001). Conclusions. Our data suggest that the SENSE imaging protocol applied in our study is superior to conventional imaging with regard to image quality, especially for T1 postcontrast and STIR images. SENSE imaging protocols may provide an alternative to conventional sequences for contrast-enhanced MRI of the breast using 1.5-T MR scanners. © 2008 Springer-Verlag

Crucial role of α4 and α6 nicotinic acetylcholine receptor subunits from ventral tegmental area in systemic nicotine self-administration

The identification of the molecular mechanisms involved in nicotine addiction and its cognitive consequences is a worldwide priority for public health. Novel in vivo paradigms were developed to match this aim. Although the beta2 subunit of the neuronal nicotinic acetylcholine receptor (nAChR) has been shown to play a crucial role in mediating the reinforcement properties of nicotine, little is known about the contribution of the different alpha subunit partners of beta2 (i.e., alpha4 and alpha6), the homo-pentameric alpha7, and the brain areas other than the ventral tegmental area (VTA) involved in nicotine reinforcement. In this study, nicotine (8.7-52.6 microg free base/kg/inf) self-administration was investigated with drug-naive mice deleted (KO) for the beta2, alpha4, alpha6 and alpha7 subunit genes, their wild-type (WT) controls, and KO mice in which the corresponding nAChR subunit was selectively re-expressed using a lentiviral vector (VEC mice). We show that WT mice, beta2-VEC mice with the beta2 subunit re-expressed exclusively in the VTA, alpha4-VEC mice with selective alpha4 re-expression in the VTA, alpha6-VEC mice with selective alpha6 re-expression in the VTA, and alpha7-KO mice promptly self-administer nicotine intravenously, whereas beta2-KO, beta2-VEC in the substantia nigra, alpha4-KO and alpha6-KO mice do not respond to nicotine. We thus define the necessary and sufficient role of alpha4beta2- and alpha6beta2-subunit containing nicotinic receptors (alpha4beta2*- and alpha6beta2*-nAChRs), but not alpha7*-nAChRs, present in cell bodies of the VTA, and their axons, for systemic nicotine reinforcement in drug-naive mic

Conformality or confinement: (IR)relevance of topological excitations

We study aspects of the conformality to confinement transition for non-supersymmetric Yang-Mills theories with fermions in arbitrary chiral or vectorlike representations. We use the presence or absence of mass gap for gauge fluctuations as an identifier of the infrared behavior. Present-day understanding does not allow the mass gap for gauge fluctuations to be computed on R*4. However, recent progress allows its non-perturbative computation on R*3xS*1 by using either the twisted partition function or deformation theory, for a range of S*1 sizes depending on the theory. For small number of fermions, Nf, we show that the mass gap increases with increasing radius, due to the non-dilution of monopoles and bions, the topological excitations relevant for confinement on R*3xS*1. For sufficiently large Nf, we show that the mass gap decreases with increasing radius. In a class of theories, we claim that the decompactification limit can be taken while remaining within the region of validity of semi-classical techniques, giving the first examples of semiclassically solvable Yang-Mills theories at any size S*1. For general non-supersymmetric vectorlike or chiral theories, we conjecture that the change in the behavior of the mass gap on R*3xS*1 as a function of the radius occurs near the lower boundary of the conformal window and give non-perturbative estimates of its value. For vectorlike theories, we compare our estimates of the conformal window with existing lattice results, truncations of the Schwinger-Dyson equations, NSVZ beta function-inspired estimates, and degree of freedom counting criteria. For multi-generation chiral gauge theories, to the best of our knowledge, our estimates of the conformal window are the only known ones.Comment: 40 pages, 3 figures; modified various comments, reference adde