821 research outputs found
Colloidal diffusion and hydrodynamic screening near boundaries
The hydrodynamic interactions between colloidal particles in small ensembles are measured at varying distances from a no-slip surface over a range of inter-particle separations. The diffusion tensor for motion parallel to the wall of each ensemble is calculated by analyzing thousands of particle trajectories generated by blinking holographic optical tweezers and by dynamic simulation. The Stokesian
Dynamics simulations predict similar particle dynamics. By separating the dynamics into three classes of modes: self, relative and collective diffusion, we observe qualitatively different behavior depending on the relative magnitudes of the distance of the ensemble from the wall and the inter-particle separation. A simple picture of the pair-hydrodynamic interactions is developed, while many-body-hydrodynamic interactions give rise to more complicated behavior. The results demonstrate that the
effect of many-body hydrodynamic interactions in the presence of a wall is much richer than the single
particle behavior and that the multiple-particle behavior cannot be simply predicted by a superposition of pair interactions
Strain-Rate Frequency Superposition in Large-Amplitude Oscillatory Shear
In a recent work, Wyss, {\it et.al.} [Phys. Rev. Lett., {\bf 98}, 238303
(2007)] have noted a property of `soft solids' under oscillatory shear, the
so-called strain-rate frequency superposition (SRFS). We extend this study to
the case of soft solids under large-amplitude oscillatory shear (LAOS). We show
results from LAOS studies in a monodisperse hydrogel suspension, an aqueous
gel, and a biopolymer suspension, and show that constant strain-rate frequency
sweep measurements with soft solids can be superimposed onto master curves for
higher harmonic moduli, with the {\it same} shift factors as for the linear
viscoelastic moduli. We show that the behavior of higher harmonic moduli at low
frequencies in constant strain-rate frequency sweep measurements is similar to
that at large strain amplitudes in strain-amplitude sweep tests. We show
surface plots of the harmonic moduli and the energy dissipation rate per unit
volume in LAOS for soft solids, and show experimentally that the energy
dissipated per unit volume depends on the first harmonic loss modulus alone, in
both the linear and the nonlinear viscoelastic regime.Comment: 10 pages, 25 figures, accepted for publication in Physical Review E.
Incorporates referee comment
Strengthening value and risk culture using a real-time logical tool
While many lessons were learned from the 2007
global financial crisis, the Basel Committee on
Banking Supervision identifies as one of the most
significant the fact that the IT and data architectures
used by banks were inadequate to support
comprehensive management of financial risk. Many
banks lacked the ability to aggregate risk exposures
and identify concentrations quickly and accurately
at the bank group level, across business lines or
between legal entities. Some banks were unable
to manage risk properly because of weak risk data
aggregation capabilities and risk reporting practices.
These weaknesses had severe consequences for
banks and for the stability of the financial system as
a whole.peer-reviewe
Hydromagnetic turbulence in computer simulations
In ordinary turbulence research it has been a long standing tradition to
solve the equations in spectral space giving the best possible accuracy. This
is indeed a natural choice for incompressible problems with periodic
boundaries, but it is no longer optimal in many astrophysical circumstances. It
is argued that lower order spatial derivatives schemes are unacceptable in view
of their low overall accuracy, even when mass, momentum, and energy are
conserved to machine accuracy. High order finite difference schemes are
therefore found to be quite efficient and physically appropriate. They are also
easily and efficiently implemented on massively parallel computers. High order
schemes also yield sufficient overall accuracy. Our code uses centered finite
differences which make the adaptation to other problems simple. Since the code
is not written in conservative form, conservation of mass, energy and momentum
can be used to monitor to quality of the solution. A third order Runge-Kutta
scheme with 2N-storage is used for calculating the time advance.Comment: 6 pages, 4 figures (Proceedings of CCP2001 in Aachen
Hedgehog signaling patterns the outgrowth of unpaired skeletal appendages in zebrafish
Background: Little is known about the control of the development of vertebrate unpaired appendages such as the caudal fin, one of the key morphological specializations of fishes. Recent analysis of lamprey and dogshark median fins suggests the co-option of some molecular mechanisms between paired and median in Chondrichthyes. However, the extent to which the molecular mechanisms patterning paired and median fins are shared remains unknown.Results: Here we provide molecular description of the initial ontogeny of the median fins in zebrafish and present several independent lines of evidence that Sonic hedgehog signaling emanating from the embryonic midline is essential for establishment and outgrowth of the caudal fin primordium. However, gene expression analysis shows that the primordium of the adult caudal fin does not harbor a Sonic hedgehog-expressing domain equivalent to the Shh secreting zone of polarizing activity ( ZPA) of paired appendages.Conclusion: Our results suggest that Hedgehog proteins can regulate skeletal appendage outgrowth independent of a ZPA and demonstrates an unexpected mechanism for mediating Shh signals in a median fin primordium. The median fins evolved before paired fins in early craniates, thus the patterning of the median fins may be an ancestral mechanism that controls the outgrowth of skeletogenic appendages in vertebrates
Clinically Undiagnosed Prostate Carcinoma Metastatic to Renal Oncocytoma
Tumors-to-tumor metastasis is an uncommon occurrence and can be a source of great diagnostic difficulty, especially when the donor tumor is undiagnosed. Here we report a case of a kidney resected for a primary neoplasm (oncocytoma) that harbored metastases from a clinically undiagnosed prostatic adenocarcinoma. The presence of the poorly differentiated metastasis within an otherwise typical oncocytoma in the absence of metastases in the surrounding nonneoplastic renal parenchyma resulted in a diagnostic dilemma. To our knowledge, this is the first report of a case in the English literature of a clinically undiagnosed prostatic adenocarcinoma metastatic to a renal oncocytoma identified on examination of the resected renal neoplasm
Synchronization of organ pipes: experimental observations and modeling
We report measurements on the synchronization properties of organ pipes.
First, we investigate influence of an external acoustical signal from a
loudspeaker on the sound of an organ pipe. Second, the mutual influence of two
pipes with different pitch is analyzed. In analogy to the externally driven, or
mutually coupled self-sustained oscillators, one observes a frequency locking,
which can be explained by synchronization theory. Further, we measure the
dependence of the frequency of the signals emitted by two mutually detuned
pipes with varying distance between the pipes. The spectrum shows a broad
``hump'' structure, not found for coupled oscillators. This indicates a complex
coupling of the two organ pipes leading to nonlinear beat phenomena.Comment: 24 pages, 10 Figures, fully revised, 4 big figures separate in jpeg
format. accepted for Journal of the Acoustical Society of Americ
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Simulation Strategies for Shock-Turbulence Interactions
The computational challenge of predicting shock-turbulence interactions stems from the fundamentally different physics at play. Shock waves are microscopically thin regions wherein flow properties change rapidly over a distance roughly equal to the molecular mean free path; hence, they are essentially strong discontinuities in the flow field. Turbulence, on the other hand, is a chaotic phenomenon with broadband spatial and temporal scales of motion. Most shock-capturing methods rely on strong numerical dissipation to artificially smooth the discontinuity, such that it can be resolved on the computational grid. Unfortunately, the artificial dissipation necessary for capturing shocks has a deleterious effect on turbulence. An additional problem is the fact that shock-capturing schemes are typically based on one-dimensional Riemann solutions that are not strictly valid in multiple dimensions. This can lead to anisotropy errors and grid-seeded perturbations. Other complications arising from upwinding, flux limiting, operator splitting etc., can seriously degrade performance and generate significant errors, especially in multiple dimensions. The purpose of this work is to design improved algorithms, capable of capturing both shocks and turbulence, which also scale to tens of thousands of processors. We have evaluated two new hydrodynamic algorithms, in relation to the widely used WENO method, on a suite of test cases. The new methods, referred to as the 'Compact' and 'Hybrid' schemes, show very promising results
Beyond Paradise—Meeting the Challenges in Tropical Biology in the 21st Century
Tropical ecosystems support a diversity of species and ecological processes that are unparalleled anywhere else on Earth.Despite their tremendous social and scientific importance, tropical ecosystems are rapidly disappearing. To help tropical ecosystems and the human communities dependent upon them better face the challenges of the 21st century, tropical biologists must provide critical knowledge in three areas: (1) the structure and functioning of tropical ecosystems; (2)the nature and magnitude of anthropogenic effects on tropical ecosystems; and (3) the socio-economic drivers of these anthropogenic effects. To develop effective strategies for conservation, restoration, and sustainable management of tropical ecosystems, scientific perspectives must be integrated with social necessities. Three principles for guiding tropical biological research are suggested: (1) broadening the set of concerns; (2) integration of biological knowledge with the social sciences and traditional knowledge; and (3) linking science to policy and action. Four broad recommendations are proposed for immediate action in tropical biology and conservation that are fundamental to all biological and social disciplines in the tropics: (1) assemble and disseminate information on life’s diversity in the tropics; (2) enhance tropical field stations and build a worldwide network to link them with tropical field biologists at their field sites; (3) bring the field of tropical biology to the tropics by strengthening institutions in tropical countries through novel partnerships between tropical and temperate zone institutions and scientists; and (4) create concrete
mechanisms to increase interactions between tropical biologists, social scientists, and policy makers
Cognitive Function, Physical Performance, Health, and Disease: Norms From the Georgia Centenarian Study
This study provides, for the first time, normative data on cognitive functioning and physical performance, health and health behaviors, and diseases from a population-based sample of 244 centenarians and near-centenarians (M age = 100.5 years, range 98–108, 84.8% women, 21.3% African American) from the Georgia Centenarian Study. Data are presented by the four key dimensions of gender, race, residence, and educational attainment. Results illustrate the profound range of functioning in this age group and indicate considerable differences as a function of each dimension. Bivariate models generally suggest that cognitive functioning and physical performance is higher for men than women; whites than African Americans; community than facility residents; and those with more than high school education than those with less than high school education. Multivariate models elaborate that differences in educational attainment generally account for the largest proportion of variance in cognitive functioning and residential status generally accounts for the largest proportion of variance in physical performance measures. Addition of health variables seldom increases variance accounted for in each domain beyond these four dimensions
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