16,026 research outputs found
Bose-Einstein Condensation of Excitons: Reply to Tikhodeev's Criticism
The extended version of our reply to Comment on ``Critical Velocities in
Exciton Superfluidity'' by S. G. Tikhodeev (Phys. Rev. Lett., 84 (2000), 3502
or from http://prl.aps.org/) is presented here. The principal question is
discussed: does the moving exciton-phonon packet contain the coherent
`nucleus', or the exciton-phonon condensate?Comment: 3 pages in LaTe
Controlling flexible structures: A survey of methods
Most of the presently available control system design techniques applicable to flexible structure problems were developed to design controllers for rigid body systems. Although many of these design methods can be applied to flexible dynamics problems, recently developed techniques may be more suitable for flexible structure controller design. The purpose of this presentation is to examine briefly the peculiarities of the dynamics of flexible structures and to stimulate discussion about top level controller design approaches when designing controllers for flexible structures. Presented here is a suggestion of a set of categories of design methods for designing controllers for flexible structures as well as a discussion of the advantages and disadvantages of each category. No attempt has been made herein to select one category of design techniques as the best for flexible structure controller design. Instead, it is hoped that the structure suggested by these categories will facilitate further discussion on the merits of particular methods that will eventually point to those design techniques suitable for further development
Modeling and control system design and analysis tools for flexible structures
Described here are Boeing software tools used for the development of control laws of flexible structures. The Boeing Company has developed a software tool called Modern Control Software Package (MPAC). MPAC provides the environment necessary for linear model development, analysis, and controller design for large models of flexible structures. There are two features of MPAC which are particularly appropriate for use with large models: (1) numerical accuracy and (2) label-driven nature. With the first feature MPAC uses double precision arithmetic for all numerical operations and relies on EISPAC and LINPACK for the numerical foundation. With the second feature, all MPAC model inputs, outputs, and states are referenced by user-defined labels. This feature allows model modification while maintaining the same state, input, and output names. In addition, there is no need for the user to keep track of a model variable's matrix row and colunm locations. There is a wide range of model manipulation, analysis, and design features within the numerically robust and flexible environment provided by MPAC. Models can be built or modified using either state space or transfer function representations. Existing models can be combined via parallel, series, and feedback connections; and loops of a closed-loop model may be broken for analysis
Antigen-presenting cells and antigen presentation in tertiary lymphoid organs
Tertiary lymphoid organs (TLOs) form in territorialized niches of peripheral tissues characterized by the presence of antigens; however, little is known about mechanism(s) of antigen handling by ectopic lymphoid structures. In this mini review, we will discuss the role of antigen-presenting cells and mechanisms of antigen presentation in TLOs, summarizing what is currently known about this facet of the formation and function of these tissues as well as identifying questions yet to be addressed
Terrestrial planets across space and time
The study of cosmology, galaxy formation and exoplanets has now advanced to a
stage where a cosmic inventory of terrestrial planets may be attempted. By
coupling semi-analytic models of galaxy formation to a recipe that relates the
occurrence of planets to the mass and metallicity of their host stars, we trace
the population of terrestrial planets around both solar-mass (FGK type) and
lower-mass (M dwarf) stars throughout all of cosmic history. We find that the
mean age of terrestrial planets in the local Universe is Gyr for FGK
hosts and Gyr for M dwarfs. We estimate that hot Jupiters have
depleted the population of terrestrial planets around FGK stars by no more than
, and that only of the terrestrial planets at the
current epoch are orbiting stars in a metallicity range for which such planets
have yet to be confirmed. The typical terrestrial planet in the local Universe
is located in a spheroid-dominated galaxy with a total stellar mass comparable
to that of the Milky Way. When looking at the inventory of planets throughout
the whole observable Universe, we argue for a total of and terrestrial planets around FGK and M
stars, respectively. Due to light travel time effects, the terrestrial planets
on our past light cone exhibit a mean age of just Gyr. These
results are discussed in the context of cosmic habitability, the Copernican
principle and searches for extraterrestrial intelligence at cosmological
distances.Comment: 11 pages, 8 figures. v.2: Accepted for publication in ApJ. Some
changes in quantitative results compared to v.1, mainly due to differences in
IMF assumption
The Conformal Anomaly in General Rank 1 Symmetric Spaces and Associated Operator Product
We compute the one-loop effective action and the conformal anomaly associated
with the product of the Laplace type operators , acting in irreducible rank 1 symmetric spaces of non-compact
type. The explicit form of the zeta functions and the conformal anomaly of the
stress-energy momentum tensor is derived.Comment: 10 pages, LaTe
Application of remote sensing to selected problems within the state of California
Specific case studies undertaken to demonstrate the usefulness of remote sensing technology to resource managers in California are highlighted. Applications discussed include the mapping and quantization of wildland fire fuels in Mendocino and Shasta Counties as well as in the Central Valley; the development of a digital spectral/terrain data set for Colusa County; the Forsythe Planning Experiment to maximize the usefulness of inputs from LANDSAT and geographic information systems to county planning in Mendocino County; the development of a digital data bank for Big Basin State Park in Santa Cruz County; the detection of salinity related cotton canopy reflectance differences in the Central Valley; and the surveying of avocado acreage and that of other fruits and nut crops in Southern California. Special studies include the interpretability of high altitude, large format photography of forested areas for coordinated resource planning using U-2 photographs of the NASA Bucks Lake Forestry test site in the Plumas National Forest in the Sierra Nevada Mountains
Application of remote sensing to selected problems within the state of California
There are no author-identified significant results in this report
Design of integrated pitch axis for autopilot/autothrottle and integrated lateral axis for autopilot/yaw damper for NASA TSRV airplane using integral LQG methodology
Two designs are presented for control systems for the NASA Transport System Research Vehicle (TSRV) using integral Linear Quadratic Gaussian (LQG) methodology. The first is an integrated longitudinal autopilot/autothrottle design and the second design is an integrated lateral autopilot/yaw damper/sideslip controller design. It is shown that a systematic top-down approach to a complex design problem combined with proper application of modern control synthesis techniques yields a satisfactory solution in a reasonable period of time
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