26,217 research outputs found
Models for application of radiation boundary condition for MHD waves in collapse calculations
The problem of reflection of magnetohydrodynamic (MHD) waves at the boundary of a numerical grid has to be resolved in order to obtain reliable results for the end state of the (isothermal) collapse of a rotating, magnetic protostellar cloud. Since the goal of investigating magnetic braking in collapse simulations is to see if the transport of angular momentum via alfven waves is large enough to solve the angular momentum problem an approximation that artificially suppresses large amplitudes in the MHD waves can be self-defeating. For this reason, four alternate methods of handling reflected waves where no assumptions are made regarding the amplitudes of the waves were investigated. In order to study this problem (of reflection) without interference from other effects these methods were tried on two simpler cases. The four methods are discussed
Parity Effect in a Small Superconducting Particle
Matveev and Larkin calculated the parity effect on the ground state energy of
a small superconducting particle in the regimes where the mean level spacing is
either large or small compared to the bulk gap. We perform a numerical
calculation which extends their results into the intermediate regime, where the
level spacing is of the same order as the bulk gap.Comment: 6 LaTeX pages, including 2 EPS figures; corrected reference and
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Numerical studies of collapsing interstellar clouds
Numerical simulation of the structure and evolution of interstellar clouds was initiated. Steps were taken toward an integrated treatment of the dynamical, thermal, and chemical processes entering model calculations. A detailed study was made of radiative transfer in molecular lines to allow model predictions to be tested against empirical data. The calculations have successfully reproduced and explained several observed cloud properties, including abundances of complex molecular species and the apparent depletion of CO in dense cores
Light forces in ultracold photoassociation
We study the time-resolved photoassociation of ultracold sodium in an optical
dipole trap. The photoassociation laser excites pairs of atoms to molecular
states of large total angular momentum at high intensities (above 20
kW/cm). Such transitions are generally suppressed at ultracold
temperatures by the centrifugal barriers for high partial waves. Time-resolved
ionization measurements reveal that the atoms are accelerated by the dipole
potential of the photoassociation beam. We change the collision energy by
varying the potential depth, and observe a strong variation of the
photoassociation rate. These results demonstrate the important role of light
forces in cw photoassociation at high intensities.Comment: 7 pages, 3 figure
Microwave Remote Sensing of Ocean Surface Wind Speed and Rain Rates over Tropical Storms
The value of using narrowly spaced frequencies within a microwave band to measure wind speeds and rain rates over tropical storms with radiometers is reviewed. The technique focuses on results obtained in the overflights of Hurricane Allen during 5 and 8 of August, 1980
A benign, low Z electron capture agent for negative ion TPCs
We have identified nitromethane (CHNO) as an effective electron
capture agent for negative ion TPCs (NITPCs).
We present drift velocity and longitudinal diffusion measurements for
negative ion gas mixtures using nitromethane as the capture agent.
Not only is nitromethane substantially more benign than the only other
identified capture agent, CS, but its low atomic number will enable the use
of the NITPC as a photoelectric X{}-ray polarimeter in the 1{}-10 keV band
Evaluation of wind tunnel performance testings of an advanced 45 deg swept 8-bladed propeller at Mach numbers from 0.45 to 0.85
The increased emphasis of fuel conservation in the world and the rapid increase in the cost of jet fuel has stimulated a series of studies of both conventional and unconventional propulsion systems for commercial aircraft. The results of these studies indicate that a fuel saving of 15 to 30 percent may be realized by the use of an advanced high-speed turboprop (Prop-Fan) compared to aircraft equipped with high bypass turbofan engines of equivalent technology. The Prop-Fan propulsion system is being investigated as part of the NASA Aircraft Energy Efficient Program. This effort includes the wind tunnel testing of a series of 8 and 10-blade Prop-Fan models incorporate swept blades. Test results indicate efficiency levels near the goal of 80 percent at Mach 0.8 cruise and an altitude of 10.67 km (35,000 ft). Each successive swept model has shown improved efficiency relative to the straight blade model. The fourth model, with 45 deg swept blades reported herein, shows a net efficiency of 78.2 at the design point with a power loading of 301 kW/sq meter and a tip speed of 243.8 m/sec (800 ft/sec.)
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