98,969 research outputs found
The Impact of Dual Enrollment on College Degree Attainment: Do Low-SES Students Benefit?
Dual enrollment in high school is viewed by many as one mechanism for increasing college admission and completion of low-income students. However, little evidence demonstrates that these students discretely benefit from dual enrollment and whether these programs narrow attainment gaps vis-Ă -vis students from middle-class or affluent family backgrounds. Using the National Education Longitudinal Study (N = 8,800), this study finds significant benefits in boosting rates of college degree attainment for low-income students while holding weaker effects for peers from more affluent backgrounds. These results remain even with analyses from newer data of college freshman of 2004. This report conducts sensitivity analyses and found that these results are robust to relatively large unobserved confounders. However, expanding dual enrollment programs would modestly reduce gaps in degree attainment
Autonomous Deployment of a Solar Panel Using an Elastic Origami and Distributed Shape Memory Polymer Actuators
Deployable mechanical systems such as space solar panels rely on the
intricate stowage of passive modules, and sophisticated deployment using a
network of motorized actuators. As a result, a significant portion of the
stowed mass and volume are occupied by these support systems. An autonomous
solar panel array deployed using the inherent material behavior remains
elusive. In this work, we develop an autonomous self-deploying solar panel
array that is programmed to activate in response to changes in the surrounding
temperature. We study an elastic "flasher" origami sheet embedded in a circle
of scissor mechanisms, both printed with shape memory polymers. The scissor
mechanisms are optimized to provide the maximum expansion ratio while
delivering the necessary force for deployment. The origami sheet is also
optimized to carry the maximum number of solar panels given space constraints.
We show how the folding of the "flasher" origami exhibits a bifurcation
behavior resulting in either a cone or disk shape both numerically and in
experiments. A folding strategy is devised to avoid the undesired cone shape.
The resulting design is entirely 3D printed, achieves an expansion ratio of
1000% in under 40 seconds, and shows excellent agreement with simulation
prediction both in the stowed and deployed configurations.Comment: 12 pages, 12 figure
Effects of Pore Walls and Randomness on Phase Transitions in Porous Media
We study spin models within the mean field approximation to elucidate the
topology of the phase diagrams of systems modeling the liquid-vapor transition
and the separation of He--He mixtures in periodic porous media. These
topologies are found to be identical to those of the corresponding random field
and random anisotropy spin systems with a bimodal distribution of the
randomness. Our results suggest that the presence of walls (periodic or
otherwise) are a key factor determining the nature of the phase diagram in
porous media.Comment: REVTeX, 11 eps figures, to appear in Phys. Rev.
Advanced liner-cooling techniques for gas turbine combustors
Component research for advanced small gas turbine engines is currently underway at the NASA Lewis Research Center. As part of this program, a basic reverse-flow combustor geometry was being maintained while different advanced liner wall cooling techniques were investigated. Performance and liner cooling effectiveness of the experimental combustor configuration featuring counter-flow film-cooled panels is presented and compared with two previously reported combustors featuring: splash film-cooled liner walls; and transpiration cooled liner walls (Lamilloy)
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