115,099 research outputs found
Shape optimization of damping layers
Shape optimization of unconstrained and constrained damping layers is completed. The specific problem analyzed is a cantilever beam loaded at its tip by a harmonic force. Finite element modeling and mathematical programming techniques are used to obtain the solution. Performance measures are taken to be reduction of maximum diplacement and increase in fatigue lifetime. Results include the improvement, over the uniform treatment case, of these measures when the profile of the damping layer is optimized
Catastrophe versus instability for the eruption of a toroidal solar magnetic flux rope
The onset of a solar eruption is formulated here as either a magnetic
catastrophe or as an instability. Both start with the same equation of force
balance governing the underlying equilibria. Using a toroidal flux rope in an
external bipolar or quadrupolar field as a model for the current-carrying flux,
we demonstrate the occurrence of a fold catastrophe by loss of equilibrium for
several representative evolutionary sequences in the stable domain of parameter
space. We verify that this catastrophe and the torus instability occur at the
same point; they are thus equivalent descriptions for the onset condition of
solar eruptions.Comment: V2: update to conform to the published article; new choice for
internal inductance of torus; updated Fig. 2; new Figs. 3, 5, and
Probing molecular frame photoionization via laser generated high-order harmonics from aligned molecules
Present photoionization experiments cannot measure molecular frame
photoelectron angular distributions (MFPAD) from the outermost valence
electrons of molecules. We show that details of the MFPAD can be retrieved with
high-order harmonics generated by infrared lasers from aligned molecules. Using
accurately calculated photoionization transition dipole moments for
fixed-in-space molecules, we show that the dependence of the magnitude and
phase of the high-order harmonics on the alignment angle of the molecules
observed in recent experiments can be quantitatively reproduced. This result
provides the needed theoretical basis for ultrafast dynamic chemical imaging
using infrared laser pulses.Comment: 5 pages, 4 figure
Quantitative Rescattering Theory for high-order harmonic generation from molecules
The Quantitative Rescattering Theory (QRS) for high-order harmonic generation
(HHG) by intense laser pulses is presented. According to the QRS, HHG spectra
can be expressed as a product of a returning electron wave packet and the
photo-recombination differential cross section of the {\em laser-free}
continuum electron back to the initial bound state. We show that the shape of
the returning electron wave packet is determined mostly by the laser only. The
returning electron wave packets can be obtained from the strong-field
approximation or from the solution of the time-dependent Schr\"odinger equation
(TDSE) for a reference atom. The validity of the QRS is carefully examined by
checking against accurate results for both harmonic magnitude and phase from
the solution of the TDSE for atomic targets within the single active electron
approximation. Combining with accurate transition dipoles obtained from
state-of-the-art molecular photoionization calculations, we further show that
available experimental measurements for HHG from partially aligned molecules
can be explained by the QRS. Our results show that quantitative description of
the HHG from aligned molecules has become possible. Since infrared lasers of
pulse durations of a few femtoseconds are easily available in the laboratory,
they may be used for dynamic imaging of a transient molecule with femtosecond
temporal resolutions.Comment: 50 pages, 15 figure
A three-dimensional viscous flow analysis for the helicopter tip vortex generation problem
The tip vortex flow field occurring in the vicinity of the tip region of a a helicopter rotor blade is a very complicated three-dimensional, viscous flow phenomenon. The details of the flow in the tip region can have a major effect in determining the generated rotor noise and can significantly affect the performance and dynamic loading of the rotor blade. The three-dimensional viscous subsonic tip vortex generation processes is investigated by a numerical procedure which allows spatial forward-marching integration, utilizing flow approximations from the velocity-decomposition approach of Briley and McDonald. The approach has been applied to compute the laminar and turbulent tip vortex flows for a constant thickness slab airfoil with a square tip, a constant thickness slab airfoil with a half round tip and a NACA 0012 airfoil with a half round tip. The basic mechanism of the tip vortex generation process as well as the prediction of vortex appearance, strength and secondary flow shown by the calculations are in qualitative agreement with experimental results
Investigation of charge coupled device correlation techniques
Analog Charge Transfer Devices (CTD's) offer unique advantages to signal processing systems, which often have large development costs, making it desirable to define those devices which can be developed for general system's use. Such devices are best identified and developed early to give system's designers some interchangeable subsystem blocks, not requiring additional individual development for each new signal processing system. The objective of this work is to describe a discrete analog signal processing device with a reasonably broad system use and to implement its design, fabrication, and testing
Nonuniversal Effects in the Homogeneous Bose Gas
Effective field theory predicts that the leading nonuniversal effects in the
homogeneous Bose gas arise from the effective range for S-wave scattering and
from an effective three-body contact interaction. We calculate the leading
nonuniversal contributions to the energy density and condensate fraction and
compare the predictions with results from diffusion Monte Carlo calculations by
Giorgini, Boronat, and Casulleras. We give a crude determination of the
strength of the three-body contact interaction for various model potentials.
Accurate determinations could be obtained from diffusion Monte Carlo
calculations of the energy density with higher statistics.Comment: 24 pages, RevTex, 5 ps figures, included with epsf.te
Gamma-Ray Bursts are Produced Predominately in the Early Universe
It is known that some observed gamma-ray bursts (GRBs) are produced at
cosmological distances and that the GRB production rate may follow the star
formation rate. We model the BATSE-detected intensity distribution of long GRBs
in order to determine their space density distribution and opening angle
distribution. Our main results are: the lower and upper distance limits to the
GRB production are z 0.24 and >10, respectively; the GRB opening angle follows
an exponential distribution and the mean opening angle is about 0.03 radians;
and the peak luminosity appears to be a better standard candle than the total
energy of a GRB.Comment: 12 pages, 2 figur
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