4,184 research outputs found
An integral formulation for wave propagation on weakly non-uniform potential flows
An integral formulation for acoustic radiation in moving flows is presented.
It is based on a potential formulation for acoustic radiation on weakly
non-uniform subsonic mean flows. This work is motivated by the absence of
suitable kernels for wave propagation on non-uniform flow. The integral
solution is formulated using a Green's function obtained by combining the
Taylor and Lorentz transformations. Although most conventional approaches based
on either transform solve the Helmholtz problem in a transformed domain, the
current Green's function and associated integral equation are derived in the
physical space. A dimensional error analysis is developed to identify the
limitations of the current formulation. Numerical applications are performed to
assess the accuracy of the integral solution. It is tested as a means of
extrapolating a numerical solution available on the outer boundary of a domain
to the far field, and as a means of solving scattering problems by rigid
surfaces in non-uniform flows. The results show that the error associated with
the physical model deteriorates with increasing frequency and mean flow Mach
number. However, the error is generated only in the domain where mean flow
non-uniformities are significant and is constant in regions where the flow is
uniform
Quasi-periodic Oscillations in the X-ray Light Curves from Relativistic Tori
We use a relativistic ray-tracing code to analyze the X-ray emission from a
pressure-supported oscillating relativistic torus around a black hole. We show
that a strong correlation exists between the {\it intrinsic} frequencies of the
torus normal modes and the {\it extrinsic} frequencies seen in the observed
light curve power spectrum. This correlation demonstrates the feasibility of
the oscillating-torus model to explain the multiple peaks seen in black hole
high-frequency quasi-periodic oscillations. Using an optically thin,
monochromatic emission model, we also determine how a relativistically
broadened emission line and the amplitude of the X-ray modulations are
dependent on the observer's inclination angle and on the torus oscillation
amplitudes. Observations of these features can provide important information
about the torus as well as the black hole.Comment: 4 pages, 3 figures, submitted to ApJ
Ultracold collisions involving heteronuclear alkali metal dimers
We have carried out the first quantum dynamics calculations on ultracold
atom-diatom collisions in isotopic mixtures. The systems studied are
spin-polarized 7Li + 6Li7Li, 7Li + 6Li2, 6Li + 6Li7Li and 6Li + 7Li2. Reactive
scattering can occur for the first two systems even when the molecules are in
their ground rovibrational states, but is slower than vibrational relaxation in
homonuclear systems. Implications for sympathetic cooling of heteronuclear
molecules are discussed.Comment: 4 pages, 3 figure
Enzyme activity below the dynamical transition at 220 K
Enzyme activity requires the activation of anharmonic motions, such as jumps between potential energy wells. However, in general, the forms and time scales of the functionally important anharmonic dynamics coupled to motion along the reaction coordinate remain to be determined. In particular, the question arises whether the temperature-dependent dynamical transition from harmonic to anharmonic motion in proteins, which has been observed experimentally and using molecular dynamics simulation, involves the activation of motions required for enzyme function. Here we present parallel measurements of the activity and dynamics of a cryosolution of glutamate dehydrogenase as a function of temperature. The dynamical atomic fluctuations faster than ~100 ps were determined using neutron scattering. The results show that the enzyme remains active below the dynamical transition observed at ~220 K, i.e., at temperatures where no anharmonic motion is detected. Furthermore, the activity shows no significant deviation from Arrhenius behavior down to 190 K. The results indicate that the observed transition in the enzyme's dynamics is decoupled from the rate-limiting step along the reaction coordinate
Arctic in Rapid Transition (ART) : science plan
The Arctic is undergoing rapid transformations that have brought the Arctic Ocean to the top of international political agendas. Predicting future conditions of the Arctic Ocean system requires scientific knowledge of its present status as well as a process-based understanding of the mechanisms of change. The Arctic in Rapid Transition (ART) initiative is an integrative, international, interdisciplinary pan-Arctic program to study changes and feedbacks among the physical and biogeochemical components of the Arctic Ocean and their ultimate impacts on biological productivity. The goal of ART is to develop priorities for Arctic marine science over the next decade. Three overarching questions form the basis of the ART science plan:
(1) How were past transitions in sea ice connected to energy flows, elemental cycling, biological diversity and productivity, and how do these compare to present and
projected shifts?
(2) How will biogeochemical cycling respond to transitions in terrestrial, gateway and shelf-to-basin fluxes?
(3) How do Arctic Ocean organisms and ecosystems respond to environmental transitions including temperature, stratification, ice conditions, and pH?
The integrated approach developed to answer the ART key scientific questions comprises: (a) process studies and observations to reveal mechanisms, (b) the establishment of links to existing monitoring programs, (c) the evaluation of geological records to extend time-series, and (d) the improvement of our modeling capabilities of climate-induced transitions. In order to develop an implementation plan for the ART initiative, an international and interdisciplinary workshop is currently planned to take place in Winnipeg, Canada in October 2010
Texture as a Diagnostic Signal in Mammograms
Radiologists can discriminate between normal and abnormal breast tissue at a glance, suggesting that radiologists might be using some âglobal signalâ of abnormality. Our study investigated whether texture descriptions can be used to characterize the global signal of abnormality and whether radiologists use this information during interpretation. Synthetic images were generated using a texture synthesis algorithm trained on texture descriptions extracted from sections of mammograms. Radiologists completed a task that required rating the abnormality of briefly presented tissue sections. When the abnormal tissue had no visible lesion, radiologists seemed to use texture descriptions; performance was similar across real and synthesized tissue sections. However, when the abnormal tissue had a visible lesion, radiologists seemed to rely on additional mechanisms beyond the texture descriptions; performance increased for the real tissue sections. These findings suggest that radiologists can use texture descriptions as global signals of abnormality in interpretation of breast tissue
A survey of self-management in dynamic software architecture specifications
As dynamic software architecture use becomes more widespread, a variety of formal specification languages have been developed to gain a better understanding of the foundations of this type of software evolutionary change. In this paper we survey 14 formal specification approaches based on graphs, process algebras, logic, and other formalisms. Our survey will evaluate the ability of each approach to specify self-managing systems as well as the ability to address issues regarding expressiveness and scalability. Based on the results of our survey we will provide recommendations on future directions for improving the specification of dynamic software architectures, specifically self-managed architectures
Glucose-containing peritoneal dialysis fluids regulate leptin secretion from 3T3-L1 adipocytes
Background. A marked elevation of serum leptin is observed soon after the start of peritoneal dialysis (PD), suggesting that leptin production may be stimulated by this treatment. Glucose metabolism is the major factor regulating leptin. The current study was designed to test if glucose-based PD fluids might regulate leptin production in vitro. Methods. 3T3-L1 adipocytes were exposed to a 50:50 mixture of dialysis solutions and medium M199 containing 10% serum for â€48âh. Leptin secretion in culture cell supernatants was measured by enzyme-linked immunosorbent assay and leptin mRNA content by northern blot analysis. Results. The high glucose-based commercial dialysate PD4 produced a higher leptin secretion compared with an identical laboratory-manufactured dialysate (Lab-D), but with a physiological glucose concentration of 5âmM (P<0.05). Raising glucose concentration from 2.75 to 40âmM in Lab-D induced a dose-dependent increase in leptin secretion of â€110±12% at 48âh (P<0.001) and leptin mRNA (P<0.05; glucose 2.75 vs 40âmM). Inhibition of UDP-N-acetylglucosamine biosynthesis, with 6-diazo-5-oxo-norleucine added to Lab-D, abolished most of the glucose-stimulated leptin release and downregulated leptin gene expression. Furthermore, glucose-free Lab-D supplemented with 1âmM glucosamine, an intermediate product in UDP-N-acetylglucosamine biosynthesis, increased leptin secretion by 28±11% over control (P<0.05), although without effect on leptin mRNA, after 48âh of culture. Conclusions. These results suggest that the PD-induced hyperleptinaemia could, in part, be mediated by the effect of glucose-based dialysis fluids on leptin production by adipocytes via activation of the hexosamine biosynthetic pathwa
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