10,014 research outputs found
The NASA Electric Propulsion Program
The NASA OAST Propulsion, Power, and Energy Division supports an electric propulsion program aimed at providing benefits to a broad class of missions. Concepts which have the potential to enable or significantly benefit space exploration and exploitation are identified and advanced toward application in the near and far term. This paper summarizes recent program progress in mission/system analysis; in electrothermal, electrostatic, and electromagnetic propulsion technologies; and in propulsion/spacecraft integration
Eroding ribbon thermocouples: impulse response and transient heat flux analysis
We have investigated a particular type of fast-response surface thermocouple to determine if it is appropriate to use a one dimensional transient heat conduction model to derive the transient surface heat flux from the measurements of surface temperature. With these sensors, low thermal inertia thermocouple junctions are formed near the surface by abrasive wear. Using laser excitation, we obtained the impulse response of these commercially available devices. The response of particular sensors can vary if new junctions are created by abrasive wear. Furthermore, the response of these sensors was found to deviate substantially from the one dimensional model and varied from sensor to sensor. The impulse response was simulated with greater fidelity using a two dimensional finite element model, but three dimensional effects also appear to be significant. The impact of these variations on the derived heat flux is assessed for the case of measurements in an internal combustion engine. When the measured impulse response is used to derive the surface heat flux, the apparent reversal of heat flux during the expansion stroke does not occur
Spatial gene drives and pushed genetic waves
Gene drives have the potential to rapidly replace a harmful wild-type allele
with a gene drive allele engineered to have desired functionalities. However,
an accidental or premature release of a gene drive construct to the natural
environment could damage an ecosystem irreversibly. Thus, it is important to
understand the spatiotemporal consequences of the super-Mendelian population
genetics prior to potential applications. Here, we employ a reaction-diffusion
model for sexually reproducing diploid organisms to study how a locally
introduced gene drive allele spreads to replace the wild-type allele, even
though it possesses a selective disadvantage . Using methods developed by
N. Barton and collaborators, we show that socially responsible gene drives
require , a rather narrow range. In this "pushed wave" regime, the
spatial spreading of gene drives will be initiated only when the initial
frequency distribution is above a threshold profile called "critical
propagule", which acts as a safeguard against accidental release. We also study
how the spatial spread of the pushed wave can be stopped by making gene drives
uniquely vulnerable ("sensitizing drive") in a way that is harmless for a
wild-type allele. Finally, we show that appropriately sensitized drives in two
dimensions can be stopped even by imperfect barriers perforated by a series of
gaps
Are Pschological Principles Useful?(A Guid to the Study of Human Learning)
To some extent, each person is his own doctor, his own economist, his own historian, his own counselor, his own psychologist, and his own teacher.
It is said that a man who is his own lawyer has a fool for a client. Can the same be said of a man\u27s other roles? At least for learning (which professionally intersects education and psychology) any person may have the goal of helping himself to appreciate and evaluate the role of the learning professional.
A knowledge explosion has taken place in the area of learning, as in the many fields of science in the university. It is not now a sufficient answer to teach a person how to think or how to learn. We must give a guide to his selective attenion for continued study. In this way the fantastic expansion of detail may be related to sets or structures of basic knowledge, so that the subsumptive organized pattern may give a sufficient context for adequate comprehension and evaluation.
In this presentation, we would like to name and illustrate five dusters of factors which are now known to influence learning behavior. Within these clusters are to be found specific independent variables now proven to affect behavior in measurable ways. These clusters include current material on environment stimulus control, readiness, aptitude, self-concept, perception, cognition, mode of attack, transfer, reinforcement, and feedback. the concept of thinking as a particularly important human behavior will be stressed
Case records of the Massachusetts General Hospital. Case 4-2006. A 79-year-old woman with myalgias, fatigue, and shortness of breath
Socially-distributed cognition and cognitive architectures: towards an ACT-R-based cognitive social simulation capability
ACT-R is one of the most widely used cognitive architectures, and it has been used to model hundreds of phenomena described in the cognitive psychology literature. In spite of this, there are relatively few studies that have attempted to apply ACT-R to situations involving social interaction. This is an important omission since the social aspects of cognition have been a growing area of interest in the cognitive science community, and an understanding of the dynamics of collective cognition is of particular importance in many organizational settings. In order to support the computational modeling and simulation of socially-distributed cognitive processes, a simulation capability based on the ACT-R architecture is described. This capability features a number of extensions to the core ACT-R architecture that are intended to support social interaction and collaborative problem solving. The core features of a number of supporting applications and services are also described. These applications/services support the execution, monitoring and analysis of simulation experiments. Finally, a system designed to record human behavioral data in a collective problem-solving task is described. This system is being used to undertake a range of experiments with teams of human subjects, and it will ultimately support the development of high fidelity ACT-R cognitive models. Such models can be used in conjunction with the ACT-R simulation capability to test hypotheses concerning the interaction between cognitive, social and technological factors in tasks involving socially-distributed information processing
A Kerr-microresonator optical clockwork
Kerr microresonators generate interesting and useful fundamental states of
electromagnetic radiation through nonlinear interactions of continuous-wave
(CW) laser light. Using photonic-integration techniques, functional devices
with low noise, small size, low-power consumption, scalable fabrication, and
heterogeneous combinations of photonics and electronics can be realized. Kerr
solitons, which stably circulate in a Kerr microresonator, have emerged as a
source of coherent, ultrafast pulse trains and ultra-broadband
optical-frequency combs. Using the f-2f technique, Kerr combs support
carrier-envelope-offset phase stabilization for optical synthesis and
metrology. In this paper, we introduce a Kerr-microresonator optical clockwork
based on optical-frequency division (OFD), which is a powerful technique to
transfer the fractional-frequency stability of an optical clock to a lower
frequency electronic clock signal. The clockwork presented here is based on a
silicon-nitride (SiN) microresonator that supports an optical-frequency
comb composed of soliton pulses at 1 THz repetition rate. By electro-optic
phase modulation of the entire SiN comb, we arbitrarily generate
additional CW modes between the SiN comb modes; operationally, this
reduces the pulse train repetition frequency and can be used to implement OFD
to the microwave domain. Our experiments characterize the residual frequency
noise of this Kerr-microresonator clockwork to one part in , which
opens the possibility of using Kerr combs with high performance optical clocks.
In addition, the photonic integration and 1 THz resolution of the SiN
frequency comb makes it appealing for broadband, low-resolution liquid-phase
absorption spectroscopy, which we demonstrate with near infrared measurements
of water, lipids, and organic solvents
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