Scholars Hangar (United States Air Force Academy)
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Upward Flame Spread Over Thin Solids in Partial Gravity
Full text linkThe effects of partial-gravity, reduced pressure, and sample width on upward flame spread over a thin cellulose fuel were studied experimentally and the results were compared to a numerical flame spread simulation. Fuel samples 1-cm, 2-cm, and 4-cm wide were burned in air at reduced pressures of 0.2 to 0.4 atmospheres in simulated gravity environments of 0.1-G, 0.16-G (Lunar), and 0.38-G (Martian) onboard the NASA KC-135 aircraft and in normal-gravity tests. Observed steady flame propagation speeds and pyrolysis lengths were approximately proportional to the gravity level. Flames spread more quickly and were longer with the wider samples and the variations with gravity and pressure increased with sample width. A numerical simulation of upward flame spread was developed including three-dimensional Navier-Stokes equations, one-step Arrhenius kinetics for the gas phase flame and for the solid surface decomposition, and a fuel-surface radiative loss. The model provides detailed structure of flame temperatures, the flow field interactions with the flame, and the solid fuel mass disappearance. The simulation agrees with experimental flame spread rates and their dependence on gravity level but predicts a wider flammable region than found by experiment. Some unique three-dimensional flame features are demonstrated in the model results
Searching to National Security in an NBC World: Four Papers on Changing Nuclear, Biological, and Chemical Threats in U.S. Government Policy in the Post-cold War International Security Environment
No full textiv, 200 pages : illustrations ; 23 cm
Dynamic Loading of Shallow Foundations: Theory Versus Experiment
No full textIn soil-structure interaction problems in soil dynamics, an important subject is the dynamic behavior of embedded foundations. Presented in this thesis is a basic study of the vertical and lateral vibration characteristics of shallow foundations. To obtain a physical understanding of the problem for granular soils, a series of forced vibration tests of square foundations embedded to various depths in sand is performed in a geotechnical centrifuge. A parallel boundary element analysis which can handle nonuniform soil profiles is also used to develop an analytical framework for the problem. By comparing the experimental results with the analytical solution, it is shown that the modeling of the experimental results from vertical vibrations with the homogeneous half-space or square-root half-space theory is directly feasible, while the lateral vibration case requires the use of appropriate Impedance Modification Factors IMFs to capture the observed dynamic foundation stiffnesses. Support is also shown for a power law dependence of the embedded foundation equivalent homogeneous shear modulus on the prototype footing half-width and average contact pressure. Available online at: https://apps.dtic.mil/sti/pdfs/ADA379985.pd
The Poisson Alignment Reference System Implementation at the Advanced Photon Source
Full text linkThe Poisson spot was established using a collimated laser beam from a 3-mW diode laser. It was monitored on a quadrant detector and found to be very sensitive to vibration and air disturbances. Therefore, for future work we strongly recommend a sealed vacuum tube in which the Poisson line may be propagated. A digital single-axis feedback system was employed to generate an straight line reference (SLR) on the X axis. Pointing accuracy was better than 8 {+-} 2 microns at a distance of 5 m. The digital system was found to be quite slow with a maximum bandwidth of 47 {+-} 9 Hz. Slow drifts were easily corrected but any vibration over 5 Hz was not. We recommend an analog proportional-integral-derivative (PID) controller for high bandwidth and smooth operation of the kinematic mirror. Although the Poisson alignment system (PAS) at the Advanced Photon Source is still in its infancy, it already shows great promise as a possible alignment system for the low-energy undulator test line (LEUTL). Since components such as wigglers and quadruples will initially be aligned with respect to each other using conventional means and mounted on some kind of rigid rail, the goal would be to align six to ten such rails over a distance of about 30 m. The PAS could be used to align these rails by mounting a sphere at the joint between two rails. These spheres would need to be in a vacuum pipe to eliminate the refractive effects of air. Each sphere would not be attached to either rail but instead to a flange connecting the vacuum pipes of each rail. Thus the whole line would be made up of straight, rigid segments that could be aligned by moving the joints. Each sphere would have its own detector, allowing the operators to actively monitor the position of each joint and therefore the overall alignment of the system