23,005 research outputs found
Effect of a simulated engine jet blowing above an arrow wing at Mach 2.0
The effects of a gas jet simulating a turbojet engine exhaust blowing above a cambered and twisted arrow wing were investigated. Tests were conducted in the Langley 4-foot supersonic pressure tunnel at a Mach number of 2.0. Nozzle pressure ratios from 1 to 64 were tested with both helium and air used as jet gases. The tests were conducted at angles of attack from -2 deg to 8 deg at a Reynolds number of 9,840,000 per meter. Only the forces and moments on the wing were measured. Results of the investigation indicated that the jet blowing over the wing caused reductions in maximum lift-drag ratio of about 4 percent for helium and 6 percent for air at their respective design nozzle pressure ratios, relative to jet-off data. Moderate changes in the longitudinal, vertical, or angular positions of the jet relative to the wing had little effect on the wing aerodynamic characteristics
High power phase locked laser oscillators
The feasibility of mechanizing an adaptive array of independent laser oscillators for generation of a high power coherent output was experimentally investigated. Tests were structured to evaluate component/system requirements for delivery of energy to a low-earth orbit satellite. Initial experiments addressed the control issues of phase locking unstable resonators at low power levels. A successful phase lock demonstration formed the basis for the design and fabrication of the high power, water-cooled, control mirror subsequently installed in the NASA LeRC high power laser. Tests were performed to characterize the operational limits of the laser system and included quantitative assessment of the frequency stability, noise sources, and optical properties of the beam
Splitting Sensitivity of the Ground and 7.6 eV Isomeric States of 229Th
The lowest-known excited state in nuclei is the 7.6 eV isomer of 229Th. This
energy is within the range of laser-based investigations that could allow
accurate measurements of possible temporal variation of this energy splitting.
This in turn could probe temporal variation of the fine-structure constant or
other parameters in the nuclear Hamiltonian. We investigate the sensitivity of
this transition energy to these quantities. We find that the two states are
predicted to have identical deformations and thus the same Coulomb energies
within the accuracy of the model (viz., within roughly 30 keV). We therefore
find no enhanced sensitivity to variation of the fine-structure constant. In
the case of the strong interaction the energy splitting is found to have a
complicated dependence on several parameters of the interaction, which makes an
accurate prediction of sensitivity to temporal changes of fundamental constants
problematical. Neither the strong- nor Coulomb-interaction contributions to the
energy splitting of this doublet can be constrained within an accuracy better
than a few tens of keV, so that only upper limits can be set on the possible
sensitivity to temporal variations of the fundamental constants.Comment: 4 pages, 2 figure
Optimal cooling strategies for magnetically trapped atomic Fermi-Bose mixtures
We discuss cooling efficiency for different-species Fermi-Bose mixtures in
magnetic traps. A better heat capacity matching between the two atomic species
is achieved by a proper choice of the Bose cooler and the magnetically
trappable hyperfine states of the mixture. When a partial spatial overlap
between the two species is also taken into account, the deepest Fermi
degeneracy is obtained for an optimal value of the trapping frequency ratio.
This can be achieved by assisting the magnetic trap with a deconfining light
beam, as shown in the case of fermionic 6Li mixed with 23Na, 87Rb, and 133Cs,
with optimal conditions found for the not yet explored 6Li-87Rb mixture.Comment: 5 pages, 3 figures, to appear in Physical Review
The role of sign in students' modeling of scalar equations
We describe students revising the mathematical form of physics equations to
match the physical situation they are describing, even though their revision
violates physical laws. In an unfamiliar air resistance problem, a majority of
students in a sophomore level mechanics class at some point wrote Newton's
Second Law as F = -ma; they were using this form to ensure that the sign of the
force pointed in a direction consistent with the chosen coordinate system while
assuming that some variables have only positive value. We use one student's
detailed explanation to suggest that students' issues with variables are
context-dependent, and that much of their reasoning is useful for productive
instruction.Comment: 5 pages, 1 figure, to be published in The Physics Teache
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