3,247 research outputs found
An electronic system for measuring thermophysical properties of wind tunnel models
An electronic system is described which measures the surface temperature of a small portion of the surface of the model or sample at high speeds using an infrared radiometer. This data is processed along with heating rate data from the reference heat gauge in a small computer and prints out the desired thermophysical properties, time, surface temperature, and reference heat rate. This system allows fast and accurate property measurements over thirty temperature increments. The technique, the details of the apparatus, the procedure for making these measurements, and the results of some preliminary tests are presented
Modeling Course-Based Undergraduate Research Experiences: An Agenda for Future Research and Evaluation
Course-based undergraduate research experiences (CUREs) are being championed as scalable ways of involving undergraduates in science research. Studies of CUREs have shown that participating students achieve many of the same outcomes as students who complete research internships. However, CUREs vary widely in their design and implementation, and aspects of CUREs that are necessary and sufficient to achieve desired student outcomes have not been elucidated. To guide future research aimed at understanding the causal mechanisms underlying CURE efficacy, we used a systems approach to generate pathway models representing hypotheses of how CURE outcomes are achieved. We started by reviewing studies of CUREs and research internships to generate a comprehensive set of outcomes of research experiences, determining the level of evidence supporting each outcome. We then used this body of research and drew from learning theory to hypothesize connections between what students do during CUREs and the outcomes that have the best empirical support. We offer these models as hypotheses for the CURE community to test, revise, elaborate, or refute. We also cite instruments that are ready to use in CURE assessment and note gaps for which instruments need to be developed.Howard Hughes Medical InstituteScience and Mathematics Educatio
Extensions of Johnson's and Morita's homomorphisms that map to finitely generated abelian groups
We extend each higher Johnson homomorphism to a crossed homomorphism from the
automorphism group of a finite-rank free group to a finite-rank abelian group.
We also extend each Morita homomorphism to a crossed homomorphism from the
mapping class group of once-bounded surface to a finite-rank abelian group.
This improves on the author's previous results [Algebr. Geom. Topol. 7
(2007):1297-1326]. To prove the first result, we express the higher Johnson
homomorphisms as coboundary maps in group cohomology. Our methods involve the
topology of nilpotent homogeneous spaces and lattices in nilpotent Lie
algebras. In particular, we develop a notion of the "polynomial straightening"
of a singular homology chain in a nilpotent homogeneous space.Comment: 34 page
Thermodynamic phase-field model for microstructure with multiple components and phases: the possibility of metastable phases
A diffuse-interface model for microstructure with an arbitrary number of
components and phases was developed from basic thermodynamic and kinetic
principles and formalized within a variational framework. The model includes a
composition gradient energy to capture solute trapping, and is therefore suited
for studying phenomena where the width of the interface plays an important
role. Derivation of the inhomogeneous free energy functional from a Taylor
expansion of homogeneous free energy reveals how the interfacial properties of
each component and phase may be specified under a mass constraint. A diffusion
potential for components was defined away from the dilute solution limit, and a
multi-obstacle barrier function was used to constrain phase fractions. The
model was used to simulate solidification via nucleation, premelting at phase
boundaries and triple junctions, the intrinsic instability of small particles,
and solutal melting resulting from differing diffusivities in solid and liquid.
The shape of metastable free energy surfaces is found to play an important role
in microstructure evolution and may explain why some systems premelt at phase
boundaries and phase triple junctions while others do not.Comment: 14 pages, 8 figure
Apparatus for determining thermophysical properties of test specimens
Apparatus is described for directly measuring the quantity square root of pck of a test specimen such as a wind tunnel model where p is density, c is the specific heat and k is the thermal conductivity of the specimen. The test specimen and a reference specimen are simultaneously subjected to the heat from a heat source. A thermocouple is attached to the reference specimen for producing a first electrical analog signal proportional to the heat rate Q that the test specimen is subjected to and an infrared radiometer that is aimed at the test specimen produces a second electrical analog signal proportional to the surface temperature T of the test specimen. An analog-to-digital converter converts the first and second electrical analog signals to digital signals. These digital signals are applied to a computer for determining the quantity
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