4,948 research outputs found
A Streamwise Constant Model of Turbulence in Plane Couette Flow
Streamwise and quasi-streamwise elongated structures have been shown to play
a significant role in turbulent shear flows. We model the mean behavior of
fully turbulent plane Couette flow using a streamwise constant projection of
the Navier Stokes equations. This results in a two-dimensional, three velocity
component () model. We first use a steady state version of the model to
demonstrate that its nonlinear coupling provides the mathematical mechanism
that shapes the turbulent velocity profile. Simulations of the model
under small amplitude Gaussian forcing of the cross-stream components are
compared to DNS data. The results indicate that a streamwise constant
projection of the Navier Stokes equations captures salient features of fully
turbulent plane Couette flow at low Reynolds numbers. A system theoretic
approach is used to demonstrate the presence of large input-output
amplification through the forced model. It is this amplification
coupled with the appropriate nonlinearity that enables the model to
generate turbulent behaviour under the small amplitude forcing employed in this
study.Comment: Journal of Fluid Mechanics 2010, in pres
Total synthesis of the post-translationally modified polyazole peptide antibiotic goadsporin
The structurally unique polyazole antibiotic goadsporin contains six heteroaromatic oxazole and thiazole rings integrated into a linear array of amino acids that also contains two dehydroalanine residues. An efficient total synthesis of goadsporin is reported in which the key steps are the use of rhodium(II)-catalyzed reactions of diazocarbonyl compounds to generate the four oxazole rings, which demonstrates the power of rhodium carbene chemistry in organic chemical synthesis
Motifs, Control, and Stability
The interactions of networks of transcription factors and signaling molecules can be understood, in part, through concepts from control theory and engineering
Natural recovery of genetic diversity by gene flow in reforested areas of the endemic Canary Island pine, Pinus canariensis
The endemic pine, Pinus canariensis, forms one of the main forest ecosystems
in the Canary Islands. In this archipelago, pine forest is a mosaic of natural
stands (remnants of past forest overexploitation) and artificial stands planted
from the 1940's. The genetic makeup of the artificially regenerated forest is
of some concern. The use of reproductive material with uncontrolled origin or
from a reduced number of parental trees may produce stands ill adapted to local
conditions or unable to adapt in response to environmental change. The genetic
diversity within a transect of reforested stands connecting two natural forest
fragments has been studied with nuclear and chloroplast microsatellites. Little
genetic differentiation and similar levels of genetic diversity to the
surrounding natural stands were found for nuclear markers. However, chloroplast
microsatellites presented lower haplotype diversity in reforested stands, and
this may be a consequence of the lower effective population size of the
chloroplast genome, meaning chloroplast markers have a higher sensitivity to
bottlenecks. Understory natural regeneration within the reforestation was also
analysed to study gene flow from natural forest into artificial stands.
Estimates of immigration rate into artificially regenerated forest were high
(0.68-0.75), producing a significant increase of genetic diversity (both in
chloroplast and nuclear microsatellites), which indicates the capacity for
genetic recovery for P. canariensis reforestations surrounded by larger natural
stands
Undergraduate Course in Environmental Design and Manufacturing
This paper is a progress report on the development of an undergraduate course in environmental design and manufacturing. The importance of the topic is clear from National Academy of Engineering statements regarding the need to incorporate content on sustainable technology. The multidisciplinary team includes academic faculty from engineering, science, liberal studies, and business in collaboration with industry partners. The team\u27s approach is to create a case-based course by adapting existing educational modules from Ford Motor Company. The group has received National Science Foundation funding for this project (DUE-0511322), and the project is currently in its beginning phases
Effects of antenatal betamethasone on preterm human and mouse ductus arteriosus: comparison with baboon data.
BackgroundAlthough studies involving preterm infants ≤34 weeks gestation report a decreased incidence of patent ductus arteriosus after antenatal betamethasone, studies involving younger gestation infants report conflicting results.MethodsWe used preterm baboons, mice, and humans (≤276/7 weeks gestation) to examine betamethasone's effects on ductus gene expression and constriction both in vitro and in vivo.ResultsIn mice, betamethasone increased the sensitivity of the premature ductus to the contractile effects of oxygen without altering the effects of other contractile or vasodilatory stimuli. Betamethasone's effects on oxygen sensitivity could be eliminated by inhibiting endogenous prostaglandin/nitric oxide signaling. In mice and baboons, betamethasone increased the expression of several developmentally regulated genes that mediate oxygen-induced constriction (K+ channels) and inhibit vasodilator signaling (phosphodiesterases). In human infants, betamethasone increased the rate of ductus constriction at all gestational ages. However, in infants born ≤256/7 weeks gestation, betamethasone's contractile effects were only apparent when prostaglandin signaling was inhibited, whereas at 26-27 weeks gestation, betamethasone's contractile effects were apparent even in the absence of prostaglandin inhibitors.ConclusionsWe speculate that betamethasone's contractile effects may be mediated through genes that are developmentally regulated. This could explain why betamethasone's effects vary according to the infant's developmental age at birth
- …