Evolution of isolated turbulent trailing vortices

In this work, the temporal evolution of a low swirl-number turbulent Batchelor vortex is studied using pseudospectral direct numerical simulations. The solution of the governing equations in the vorticity-velocity form allows for accurate application of boundary conditions. The physics of the evolution is investigated with an emphasis on the mechanisms that influence the transport of axial and angular momentum. Excitation of normal mode instabilities gives rise to coherent large scale helical structures inside the vortical core. The radial growth of these helical structures and the action of axial shear and differential rotation results in the creation of a polarized vortex layer. This vortex layer evolves into a series of hairpin-shaped structures that subsequently breakdown into elongated fine scale vortices. Ultimately, the radially outward propagation of these structures results in the relaxation of the flow towards a stable high-swirl configuration. Two conserved quantities, based on the deviation from the laminar solution, are derived and these prove to be useful in characterizing the polarized vortex layer and enhancing the understanding of the transport process. The generation and evolution of the Reynolds stresses is also addressed

Electrical determination of the valence-band discontinuity in HgTe-CdTe heterojunctions

Current-voltage behavior is studied experimentally in a Hg0.78Cd0.22Te-CdTe-Hg0.78Cd0.22Te heterostructure grown by molecular beam epitaxy. At temperatures above 160 K, energy-band diagrams suggest that the dominant low-bias current is thermionic hole emission across the CdTe barrier layer. This interpretation yields a direct determination of 390±75 meV for the HgTe-CdTe valence-band discontinuity at 300 K. Similar analyses of current-voltage data taken at 190–300 K suggest that the valence-band offset decreases at low temperatures in this heterojunction

Inquiry project-based learning with wiki at Primary Five level

Inquiry project-based learning (inquiry PjBL) has been regarded as an innovative educational approach among local primary schools in Hong Kong, replacing gradually the more traditional didactic teaching approach. Many studies have been conducted to investigate the effectiveness of inquiry PjBL in equipping students with skills needed by the 21st century. However, the lack of an effective delivery method hinders its widespread implementation. This study examined the suitability of using wiki, a form of Web 2.0 technologies, as a platform for the implementation of inquiry PjBL among Primary Five students in Hong Kong. Four schools were recruited and students (n=420) were given an inquiry group project to work on, using Google Sites as an online collaborative platform. The outcomes were evaluated using an online survey designed to assess the effectiveness of wiki in four domains: learning/pedagogy, motivation, group interaction, and technology. Focused group interviews were also conducted to obtain students’ insights regarding the learning experiences. The study found that the use of a wiki in supporting inquiry PjBL was perceived positively by students. Students found the learning experience enjoyable and the various functionality of the wiki enhanced their acquisition of knowledge and skills. Findings of this study may help provide practical suggestions to education practitioners to incorporate wiki into inquiry PjBL and help demonstrate the potential of wiki in facilitating the development of knowledge and skills among students.postprin

Threshold effects in excited charmed baryon decays

Motivated by recent results on charmed baryons from CLEO and FOCUS, we reexamine the couplings of the orbitally excited charmed baryons. Due to its proximity to the [Sigma_c pi] threshold, the strong decays of the Lambda_c(2593) are sensitive to finite width effects. This distorts the shape of the invariant mass spectrum in Lambda_{c1}-> Lambda_c pi^+pi^- from a simple Breit-Wigner resonance, which has implications for the experimental extraction of the Lambda_c(2593) mass and couplings. We perform a fit to unpublished CLEO data which gives M(Lambda_c(2593)) - M(Lambda_c) = 305.6 +- 0.3 MeV and h2^2 = 0.24^{+0.23}_{-0.11}, with h2 the Lambda_{c1}-> Sigma_c pi strong coupling in the chiral Lagrangian. We also comment on the new orbitally excited states recently observed by CLEO.Comment: 9 pages, 3 figure

ROBOT VISION: CALIBRATION OF WIDE-ANGLE LENS CAMERAS USING COLLINEARITY CONDITION AND K-NEAREST NEIGHBOUR REGRESSION

Visual perception is regularly used by humans and robots for navigation. By either implicitly or explicitly mapping the environment, ego-motion can be determined and a path of actions can be planned. The process of mapping and navigation are delicately intertwined; therefore, improving one can often lead to an improvement of the other. Both processes are sensitive to the interior orientation parameters of the camera system and mathematically modelling these systematic errors can often improve the precision and accuracy of the overall solution. This paper presents an automatic camera calibration method suitable for any lens, without having prior knowledge about the sensor. Statistical inference is performed to map the environment and localize the camera simultaneously. K-nearest neighbour regression is used to model the geometric distortions of the images. A normal-angle lens Nikon camera and wide-angle lens GoPro camera were calibrated using the proposed method, as well as the conventional bundle adjustment with self-calibration method (for comparison). Results showed that the mapping error was reduced from an average of 14.9 mm to 1.2 mm (i.e. a 92 % improvement) and 66.6 mm to 1.5 mm (i.e. a 98 % improvement) using the proposed method for the Nikon and GoPro cameras, respectively. In contrast, the conventional approach achieved an average 3D error of 0.9 mm (i.e. 94 % improvement) and 6 mm (i.e. 91 % improvement) for the Nikon and GoPro cameras, respectively. Thus, the proposed method performs more consistently, irrespective of the lens/sensor used: it yields results that are comparable to the conventional approach for normal-angle lens cameras, and it has the additional benefit of improving calibration results for wide-angle lens cameras

Hydrodynamics of the Kuramoto-Sivashinsky Equation in Two Dimensions

The large scale properties of spatiotemporal chaos in the 2d Kuramoto-Sivashinsky equation are studied using an explicit coarse graining scheme. A set of intermediate equations are obtained. They describe interactions between the small scale (e.g., cellular) structures and the hydrodynamic degrees of freedom. Possible forms of the effective large scale hydrodynamics are constructed and examined. Although a number of different universality classes are allowed by symmetry, numerical results support the simplest scenario, that being the KPZ universality class.Comment: 4 pages, 3 figure

Influence of the anion potential on the charge ordering in quasi-one dimensional charge transfer salts

We examine the various instabilities of quarter-filled strongly correlated electronic chains in the presence of a coupling to the underlying lattice. To mimic the physics of the (TMTTF)$_2$X Bechgaard-Fabre salts we also include electrostatic effects of intercalated anions. We show that small displacements of the anion can stabilize new mixed Charged Density Wave-Bond Order Wave phases in which central symmetry centers are suppressed. This finding is discussed in the context of recent experiments. We suggest that the recently observed charge ordering is due to a cooperative effect between the Coulomb interaction and the coupling of the electronic stacks to the anions. On the other hand, the Spin-Peierls instability at lower temperature requires a Peierls-like lattice coupling.Comment: Latex, 4 pages, 4 postscript figure

Propagating Wave Patterns in a Derivative Nonlinear Schr\"odinger System with Quintic Nonlinearity

Exact expressions are obtained for a diversity of propagating patterns for a derivative nonlinear Schr\"odinger equation with a quintic nonlinearity. These patterns include bright pulses, fronts and dark solitons. The evolution of the wave envelope is determined via a pair of integrals of motion, and reduction is achieved to Jacobi elliptic cn and dn function representations. Numerical simulations are performed to establish the existence of parameter ranges for stability. The derivative quintic nonlinear Schr\"odinger model equations investigated here are important in the analysis of strong optical signals propagating in spatial or temporal waveguides.Comment: J. Phys. Soc. Jpn. in pres