924 research outputs found
Numerical Investigation of the Aerodynamic Properties of a Flying Wing Configuration
The numerical investigations of a generic UCAV configuration are presented. These investigations are part of the DLR internal project UCAV-2010. Compressible speed conditions are considered and presented. The DLR-F17E UCAV configuration is a flying lambda delta wing with sweep angle of 53° and varying leading edge radius. The flow field of
this UCAV configuration is dominated by vortex structures and vortex-to-vortex interaction. The paper aims to give a comparison between numerical- and experimental investigations in order to gain a deeper understanding of the complex flow physics. Furthermore, it will highlight the influence of Mach- and Reynolds number change on the flow and the overall aerodynamic behavior of the configuration. The DLR TAU-Code is used to simulate the flow field, using an unstructured grid and the turbulence model of Spalart-Allmaras. Forces and moment measurements taken in the DNW-TWG, Göttingen, on the DLR-F17E configuration serve as the experimental basis to validate the numerical findings. Findings on the SACCON configuration serve as a comparison case aiming to show possible portability between different model scales but also to find analogies between low speed (M=0.15) and compressible speed (M=0.5) scenarios. This paper builds up upon the finding within the NATO/RTO AVT-161 Research Task Group on “Assessment and Control Predictions for NATO Air and Sea Vehicles” and its findings shall serve as a basis for further experimental investigations of medium to high speed wind tunnel experiments. Furthermore, this paper addresses the importance of understanding and the ability to predict controlled- and
uncontrolled flow separation and the interaction of vortex systems in order to estimate the aerodynamic behavior within the entire flight envelope and to meet Stability- and Control
needs
Solidarity between generations: a five-country study of the social process of aging
"This paper examines three questions: the level and intensity of the tangible services that adult children provide their aged parents; the content of what they give and get from each other; and the consequences of these activities for the sense of well-being that the aged feel about their own lives. I present some data that is relevant for an exploration of these questions. The data are based on a Harris poll conducted for the Commonwealth Fund in the Spring of 1991. The term »solidarity« is based on the idea that the feeling of togetherness, based on close family ties provides a basis for identification which, in turn, leads to a willingness to provide mutual assistance. Solidarity also occurs in the public sector via the institutional arrangements of social security. which is commonly referred to as the »generational contract«. The public and familial represent two different forms for the expression of solidarity between generations. One of the most interesting and important policy questions concerns the impact of these two systems of generational transfers on each other." [author's abstract
The role of spatial averaging in the precision of gene expression patterns
During embryonic development, differentiating cells respond via gene
expression to positional cues from morphogen gradients. While gene expression
is often highly erratic, embryonic development is precise. We show by theory
and simulations that diffusion of the expressed protein can enhance the
precision of its expression domain. While diffusion lessens the sharpness of
the expression boundary, it also reduces super-Poissonian noise by washing out
bursts of gene expression. Balancing these effects yields an optimal diffusion
constant maximizing the precision of the expression domain.Comment: 5 pages, 4 EPS figure
Feasibility of UV lasing without inversion in mercury vapor
We investigate the feasibility of UV lasing without inversion at a wavelength
of nm utilizing interacting dark resonances in mercury vapor. Our
theoretical analysis starts with radiation damped optical Bloch equations for
all relevant 13 atomic levels. These master equations are generalized by
considering technical phase noise of the driving lasers. From the Doppler
broadened complex susceptibility we obtain the stationary output power from
semiclassical laser theory. The finite overlap of the driving Gaussian laser
beams defines an ellipsoidal inhomogeneous gain distribution. Therefore, we
evaluate the intra-cavity field inside a ring laser self-consistently with
Fourier optics. This analysis confirms the feasibility of UV lasing and reveals
its dependence on experimental parameters.Comment: changes were made according to reviewer comments (accepted for
publication in JOSA B
Fundamental Limits to Position Determination by Concentration Gradients
Position determination in biological systems is often achieved through
protein concentration gradients. Measuring the local concentration of such a
protein with a spatially-varying distribution allows the measurement of
position within the system. In order for these systems to work effectively,
position determination must be robust to noise. Here, we calculate fundamental
limits to the precision of position determination by concentration gradients
due to unavoidable biochemical noise perturbing the gradients. We focus on
gradient proteins with first order reaction kinetics. Systems of this type have
been experimentally characterised in both developmental and cell biology
settings. For a single gradient we show that, through time-averaging, great
precision can potentially be achieved even with very low protein copy numbers.
As a second example, we investigate the ability of a system with oppositely
directed gradients to find its centre. With this mechanism, positional
precision close to the centre improves more slowly with increasing averaging
time, and so longer averaging times or higher copy numbers are required for
high precision. For both single and double gradients, we demonstrate the
existence of optimal length scales for the gradients, where precision is
maximized, as well as analyzing how precision depends on the size of the
concentration measuring apparatus. Our results provide fundamental constraints
on the positional precision supplied by concentration gradients in various
contexts, including both in developmental biology and also within a single
cell.Comment: 24 pages, 2 figure
Ideas of Reform: Like Buddhist Souls
In 1967 Martin Rein and Peter Marris wrote an important book exploring the projects leading to the development of community action and related programs of the Great Society. In it they describe reform as a diffuse process in which preferences clash and evolve. Purposeful reform rarely has the intended consequences. The selection below is taken from the concluding remarks of their book, The Dilemmas of Social Reform, copyright University of Chicago Press, and is reprinted here with permission
Tau Appearance In Atmospheric Neutrino Interactions
If the correct interpretation of the Super-Kamiokande atmospheric neutrino
data is \nu_\mu -> \nu_\tau oscillation, the contained data sample should
already have more than 10 tau appearance events. We study the challenging task
of detecting the tau, focussing on the decay chain \tau^\pm -> \rho^\pm ->
\pi^\pm \pi^0 in events with quasi-elastic tau production. The background
level, which is currently quite uncertain because of a lack of relevant neutral
current data, can be measured by the near detector in the K2K experiment. Our
estimates of the background suggest that it may be possible to detect tau
appearance in Super-Kamiokande with 5-10 years of running.Comment: 13 pages, uses psfi
The optogenetic (r)evolution
Optogenetics is a rapidly evolving field of technology that allows optical control of genetically targeted biological systems at high temporal and spatial resolution. By heterologous expression of light-sensitive microbial membrane proteins, opsins, cell type-specific depolarization or silencing can be optically induced on a millisecond time scale. What started in a petri dish is applicable today to more complex systems, ranging from the dissection of brain circuitries in vitro to behavioral analyses in freely moving animals. Persistent technical improvement has focused on the identification of new opsins, suitable for optogenetic purposes and genetic engineering of existing ones. Optical stimulation can be combined with various readouts defined by the desired resolution of the experimental setup. Although recent developments in optogenetics have largely focused on neuroscience it has lately been extended to other targets, including stem cell research and regenerative medicine. Further development of optogenetic approaches will not only highly increase our insight into health and disease states but might also pave the way for a future use in therapeutic applications
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