Electrical control of the linear optical properties of particulate composite materials

The Bruggeman formalism for the homogenization of particulate composite materials is used to predict the effective permittivity dyadic of a two-constituent composite material with one constituent having the ability to display the Pockels effect. Scenarios wherein the constituent particles are randomly oriented, oriented spheres, and oriented spheroids are numerically explored. Thereby, homogenized composite materials (HCMs) are envisaged whose constitutive parameters may be continuously varied through the application of a low-frequency (dc) electric field. The greatest degree of control over the HCM constitutive parameters is achievable when the constituents comprise oriented and highly aspherical particles and have high electro-optic coefficients

Testing a Simplified Version of Einstein's Equations for Numerical Relativity

Solving dynamical problems in general relativity requires the full machinery of numerical relativity. Wilson has proposed a simpler but approximate scheme for systems near equilibrium, like binary neutron stars. We test the scheme on isolated, rapidly rotating, relativistic stars. Since these objects are in equilibrium, it is crucial that the approximation work well if we are to believe its predictions for more complicated systems like binaries. Our results are very encouraging.Comment: 9 pages (RevTeX 3.0 with 6 uuencoded figures), CRSR-107

Quasi-circular Orbits for Spinning Binary Black Holes

Using an effective potential method we examine binary black holes where the individual holes carry spin. We trace out sequences of quasi-circular orbits and locate the innermost stable circular orbit as a function of spin. At large separations, the sequences of quasi-circular orbits match well with post-Newtonian expansions, although a clear signature of the simplifying assumption of conformal flatness is seen. The position of the ISCO is found to be strongly dependent on the magnitude of the spin on each black hole. At close separations of the holes, the effective potential method breaks down. In all cases where an ISCO could be determined, we found that an apparent horizon encompassing both holes forms for separations well inside the ISCO. Nevertheless, we argue that the formation of a common horizon is still associated with the breakdown of the effective potential method.Comment: 13 pages, 10 figures, submitted to PR

Effects of visual and motion simulation cueing systems on pilot performance during takeoffs with engine failures

Data are presented that show the effects of visual and motion during cueing on pilot performance during takeoffs with engine failures. Four groups of USAF pilots flew a simulated KC-135 using four different cueing systems. The most basic of these systems was of the instrument-only type. Visual scene simulation and/or motion simulation was added to produce the other systems. Learning curves, mean performance, and subjective data are examined. The results show that the addition of visual cueing results in significant improvement in pilot performance, but the combined use of visual and motion cueing results in far better performance

Optimizing evacuation flow in a two-channel exclusion process

We use a basic setup of two coupled exclusion processes to model a stylised situation in evacuation dynamics, in which evacuees have to choose between two escape routes. The coupling between the two processes occurs through one common point at which particles are injected, the process can be controlled by directing incoming individuals into either of the two escape routes. Based on a mean-field approach we determine the phase behaviour of the model, and analytically compute optimal control strategies, maximising the total current through the system. Results are confirmed by numerical simulations. We also show that dynamic intervention, exploiting fluctuations about the mean-field stationary state, can lead to a further increase in total current.Comment: 16 pages, 6 figure

Approximate Killing Vectors on S^2

We present a new method for computing the best approximation to a Killing vector on closed 2-surfaces that are topologically S^2. When solutions of Killing's equation do not exist, this method is shown to yield results superior to those produced by existing methods. In addition, this method appears to provide a new tool for studying the horizon geometry of distorted black holes.Comment: 4 pages, 3 figures, submitted to Physical Review D, revtex

Enterohemorrhagic Escherichia coli colonization of human colonic epithelium in vitro and ex vivo

Enterohemorrhagic E. coli (EHEC) are important foodborne pathogens causing gastroenteritis and more severe complications such as hemorrhagic colitis and hemolytic uremic syndrome. Pathology is most pronounced in the colon, but to date there is no direct clinical evidence showing EHEC binding to colonic epithelium in patients. In this study, we investigated EHEC adherence to the human colon by using in vitro organ culture (IVOC) of colonic biopsies and polarized T84 colon carcinoma cells. We showed for the first time that EHEC colonized human colonic biopsies by forming typical attaching/effacing (A/E) lesions which were dependent on EHEC type III secretion (T3S) and binding of the outer membrane protein intimin to the Translocated intimin receptor (Tir). A/E lesion formation was dependent on oxygen levels and suppressed under oxygen-rich culture conditions routinely used for IVOC. In contrast, EHEC adherence to polarized T84 cells occurred independently of T3S and intimin and did not involve Tir translocation into the host cell membrane. Neither colonization of biopsies nor T84 cells was significantly affected by expression of Shiga toxins. Our study suggests that EHEC colonize and form stable A/E lesions on the human colon which is likely to contribute to intestinal pathology during infection. Furthermore, care needs to be taken when using cell culture models as they might not reflect the in vivo situation

The systematics, distribution and aspects of the ecology of the freshwater amphipod genus Paramelita (Crangonyctoidea: Paramelitidae)

Freshwater amphipods in southern Africa are divided into three distinct elements. Five ingolfiellid species occur in Namibia, six Sternophysinx species are found in the northern Cape Province and Transvaal, while 12 Paramelita species are restricted to the south-western Cape. The crangonyctoid genus Paramelita was last reviewed in 1981 based mainly on a reassessment of museum material. Thus, large scale field surveys of these amphipods have not been undertaken since they were first described by Barnard in 1916 and 1927. In addition, little is known about the ecology of these amphipods. This thesis examines the systematics, distribution and aspects of the ecology of Paramelita. After an extensive sampling programme in 1989 and 1990, several new species were discovered. Four new species, whose affinities with the known species were not immediately evident, were initially described, and later, after further analyses .of morphological and genetic differentiation amongst Paramelita populations, eight other new species were recognised. A phenetic analysis of the 24 Paramelita species revealed the existence of three distinct groups. The new genus Afrocrangonyx was proposed to accommodate seven of these species, whilst a single species was placed in the new genus Aquadulcaris. Sixteen species remained in Paramelita. Based on a cladistic analysis of morphological data, a fully resolved phylogenetic tree was obtained for Afrocrangonyx, but not for Paramelita, suggesting that the latter genus is not monophyletic. A population of P. nigroculus in Window Stream, Table Mountain, was selected for a detailed ecological study. P. nigroculus occurred at high densities, and showed no seasonal breeding patterns. An investigation of thermal acclimation and tolerances to high temperatures in P. nigroculus individuals showed that their acclimation rates and lethal limits were similar to many other crustaceans. P. nigroculus individuals feed mainly on allochthonous input. An investigation of the nature, timing and magnitude of this input indicated that litter was dominated by abscissed leaves which fall throughout the year, peaking in summer. Leaf fall values and benthic detritus standing stocks were within the range reported for headwaters worldwide. A study of leaf retention capabilities of two similar headwaters revealed that amphipod abundances were positively correlated with leaf retention characteristics. Leaf retention was strongly influenced by discharge, with increasing discharges resulting in decreases in retention and stream bed complexity. Thus the stream with higher discharges was less favourable for colonisation by amphipods. Since P. nigroculus occurred at such high densities, it was expected to play a significant role in breakdown of detritus. Breakdown of three riparian leaf species in litter bags submerged in two streams was investigated. Litter bags were designed to allow amphipod access, yet retain fragments which resulted from physical abrasion and feeding activity. Breakdown was rapid, and the decomposition rate of one of the leaf species was the highest ever recorded for a riparian leaf species. Thus, it was concluded that where they occur at high densities, Paramelita individuals play a major role in the detritus dynamics of the stream

Inner boundary conditions for black hole Initial Data derived from Isolated Horizons

We present a set of boundary conditions for solving the elliptic equations in the Initial Data Problem for space-times containing a black hole, together with a number of constraints to be satisfied by the otherwise freely specifiable standard parameters of the Conformal Thin Sandwich formulation. These conditions altogether are sufficient for the construction of a horizon that is instantaneously in equilibrium in the sense of the Isolated Horizons formalism. We then investigate the application of these conditions to the Initial Data Problem of binary black holes and discuss the relation of our analysis with other proposals that exist in the literature.Comment: 13 pages. Major general revision. Section V comparing with previous approaches restructured; discussion on the lapse boundary condition extended. Appendix with some technical details added. Version accepted for publication in Phys.Rev.