On the Reynolds number scaling of vorticity production at no-slip walls during vortex-wall collisions

Recently, numerical studies revealed two different scaling regimes of the peak enstrophy Z and palinstrophy P during the collision of a dipole with a no-slip wall [Clercx and van Heijst, Phys. Rev. E 65, 066305, 2002]: Z ∝ Re0.8 and P ∝ Re2.25 for 5 × 102 ≤ Re ≤ 2 × 104 and Z ∝ Re0.5 and P ∝ Re1.5 for Re ≥ 2 × 104 (with Re based on the velocity and size of the dipole). A critical Reynolds number Rec(here, Rec ≈ 2 × 104) is identified below which the interaction time of the dipole with the boundary layer depends on the kinematic viscosity ν. The oscillating plate as a boundary-layer problem can then be used to mimick the vortex-wall interaction and the following scaling relations are obtained: Z ∝ Re^3/4, P ∝ Re^9/4, and dP/dt ∝ Re11/4 in agreement with the numerically obtained scaling laws. For Re ≥ Rec the interaction time of the dipole with the boundary layer becomes independent of the kinematic viscosity and, applying flat-plate boundary-layer theory, this yields: Z ∝ Re1/2 and P ∝ Re3/2

A Method for Estimating Caribou Consumption by Northern Canadians

Caribou is an important source of protein in the diet of northern Canadians. It is also an important pathway for airborne environmental contaminants that concentrate in the lichen - caribou - human food chain. We present a method for estimating caribou consumption that is independent of questionnaires and dietary surveys. The method is based on direct, whole-body measurements of fallout radiocesium in northern caribou consumers and on measurements of the concentrations of radiocesium in the meat. From the 1989-90 surveys of five Arctic communities, we obtained the following mean (90th percentile) intakes of caribou meat in grams per day: Baker Lake - males 65 (141), females 41 (88); Rae-Edzo - males 42 (103), females 31 (80); Old Crow - males 41 (108), females 23 (59); Fort McPherson - males 41 (77), females 32 (68); Aklavik - males 20 (47), females 15 (37). Compared with surveys carried out in the late 1960s, these values indicate a twofold to fourfold decrease in caribou consumption over a period of 20 years. A dietary survey questionnaire administered during the 1989-90 survey provided useful information on the consumption of various caribou organs, methods of meat preparation, and consumption of other traditional foods.Le caribou constitue une importante source de protéines dans le régime alimentaire des Canadiens. Il représente également une voie d'entrée majeure pour les contaminants environnementaux en suspension dans l'air, qui se concentrent dans la chaîne alimentaire lichen - caribou - être humain. Nous présentons une méthode d'évaluation de la consommation de caribou qui n'est pas fondée sur des questionnaires et sondages alimentaires. La méthode s'appuie sur des mesures directes du césium radioactif provenant de retombées, mesures effectuées sur le corps entier de consommateurs nordiques de caribou, ainsi que sur des mesures de la concentration de césium radioactif dans la viande. À partir des enquêtes menées en 1989 et 1990 dans cinq communautés de l'Arctique, nous avons obtenu les apports moyens suivants (90e percentile) de viande de caribou en grammes par jour: Baker Lake - hommes 65 (141), femmes 41 (88); Rae-Edzo - hommes 42 (103), femmes 31 (80); Old Crow - hommes 41 (108), femmes 23 (59); Fort McPherson - hommes 41 (77), femmes 32 (68); Aklavik - hommes 20 (47), femmes 15 (37). Quand on les compare aux sondages menés à la fin des années 1960, ces valeurs révèlent une baisse de 50 à 75 p. cent dans la consommation de caribou sur une période de 20 ans. Une enquête alimentaire réalisée de 1989 à 1990 à l'aide d'un questionnaire a fourni des renseignements utiles sur la consommation de divers abats, les méthodes d'apprêt de la viande ainsi que la consommation d'autres aliments traditionnels

ac-Field-Controlled Anderson Localization in Disordered Semiconductor Superlattices

An ac field, tuned exactly to resonance with the Stark ladder in an ideal tight binding lattice under strong dc bias, counteracts Wannier-Stark localization and leads to the emergence of extended Floquet states. If there is random disorder, these states localize. The localization lengths depend non-monotonically on the ac field amplitude and become essentially zero at certain parameters. This effect is of possible relevance for characterizing the quality of superlattice samples, and for performing experiments on Anderson localization in systems with well-defined disorder.Comment: 10 pages, Latex; figures available on request from [email protected]

Growth, microstructure, and failure of crazes in glassy polymers

We report on an extensive study of craze formation in glassy polymers. Molecular dynamics simulations of a coarse-grained bead-spring model were employed to investigate the molecular level processes during craze nucleation, widening, and breakdown for a wide range of temperature, polymer chain length $N$, entanglement length $N_e$ and strength of adhesive interactions between polymer chains. Craze widening proceeds via a fibril-drawing process at constant drawing stress. The extension ratio is determined by the entanglement length, and the characteristic length of stretched chain segments in the polymer craze is $N_e/3$. In the craze, tension is mostly carried by the covalent backbone bonds, and the force distribution develops an exponential tail at large tensile forces. The failure mode of crazes changes from disentanglement to scission for $N/N_e\sim 10$, and breakdown through scission is governed by large stress fluctuations. The simulations also reveal inconsistencies with previous theoretical models of craze widening that were based on continuum level hydrodynamics

The effect of gamma irradiation on selected growth factors and receptors mRNA in glycerol cryopreserved human amniotic membrane

Human amniotic membrane (HAM), due to its high biocompatibility, low immunogenicity, anti-microbial, anti-viral properties as well as the presence of its growth factors, has been used in various clinical applications. These growth factors are key factors in regulating many cellular processes such as cellular growth, proliferation and cellular differentiation. The current study aimed to explore the effect of glycerol cryopreservation and gamma irradiation on the selected growth factors and receptors mRNA present in HAM. Eight growth factors, namely, EGF, HGF, KGF, TGF-α, TGF-β1, TGF-β2, TGF-β3 and bFGF and two growth factor receptors, HGFR and KGFR were evaluated in this study. The total RNA was extracted and converted to complimentary DNA using commercial kits. Subsequently, the mRNA expressions of these growth factors were evaluated using quantitative PCR and the results were statistically analyzed using REST-MCS software. This study indicated the presence of these growth factors and receptors mRNA in fresh, glycerol cryopreserved and irradiated glycerol cryopreserved HAM. In glycerol cryopreserved HAM, the mRNA expression showed up-regulation of HGF and bFGF and down-regulation of the rest of 8 genes which were EGF, HGFR, KGF, KGFR, TGF-α, TGF-β1, TGF-β2 and TGF-β3. Interestingly, the glycerol cryopreserved HAM radiated with 15 kGy showed up-regulation in the mRNA expression of 7 genes, namely, EGF, HGF, KGF, KGFR, TGF-β1, TGF-β2 and TGF-β3 and down-regulated mRNA expression of HGFR, TGF-α and bFGF. However, these mRNA expressions did not show a statistically significant difference compared to control groups. Thus, it can be concluded that the glycerol cryopreservation did not have an effect on the growth factors’ and receptors’ mRNA expression levels in HAM. Similarly, 15 kGy gamma irradiation did not have an effect on the growth factors’ and receptors’ mRNA expression in glycerol cryopreserved HAM. This finding provides a useful information to clinicians and surgeons to choose the best method for HAM preservation that could benefit patients in their treatment

Presidential Popularity and Reputation

This paper reports on the results of an empirical study of relationships between the popularity of US presidents and economic variables. Traditionally, these relationships are based on the hypothesis that voters hold the incumbent President responsible for the economic situation. We derive an alternative specification of popularity, based on the hypothesis that political parties perform better on different issues. Empirical evidence turns out to be strongly in favour of our hypothesis. Our findings have important implications for studies on government behaviour in which it is assumed that one of the objectives of administrations is to maximise votes