23,674 research outputs found
The construction and operation of a water tunnel in application to flow visualization studies of an oscillating airfoil
The water tunnel which was constructed at the NASA Ames Research Center is described along with the flow field adjacent to an oscillating airfoil. The design and operational procedures of the tunnel are described in detail. Hydrogen bubble and thymol blue techniques are used to visualize the flow field. Results of the flow visualizations are presented in a series of still pictures and a high speed movie. These results show that time stall is more complicated than simple shedding from the leading edge or the trailing edge, particularly at relatively low frequency oscillations comparable to those of a helicopter blade. Therefore, any successful theory for predicting the stall loads on the helicopter blades must treat an irregular separated region rather than a discrete vortex passing over each blade surface
Long-Period Building Response to Earthquakes in the San Francisco Bay Area
This article reports a study of modeled, long-period building responses to ground-motion simulations of earthquakes in the San Francisco Bay Area. The earthquakes include the 1989 magnitude 6.9 Loma Prieta earthquake, a magnitude 7.8 simulation of the 1906 San Francisco earthquake, and two hypothetical magnitude 7.8 northern San Andreas fault earthquakes with hypocenters north and south of San Francisco. We use the simulated ground motions to excite nonlinear models of 20-story, steel, welded moment-resisting frame (MRF) buildings. We consider MRF buildings designed with two different strengths and modeled with either ductile or brittle welds. Using peak interstory drift ratio (IDR) as a performance measure, the stiffer, higher strength building models outperform the equivalent more flexible, lower strength designs. The hypothetical magnitude 7.8 earthquake with hypocenter north of San Francisco produces the most severe ground motions. In this simulation, the responses of the more flexible, lower strength building model with brittle welds exceed an IDR of 2.5% (that is, threaten life safety) on 54% of the urban area, compared to 4.6% of the urban area for the stiffer, higher strength building with ductile welds. We also use the simulated ground motions to predict the maximum isolator displacement of base-isolated buildings with linear, single-degree-of-freedom (SDOF) models. For two existing 3-sec isolator systems near San Francisco, the design maximum displacement is 0.5 m, and our simulations predict isolator displacements for this type of system in excess of 0.5 m in many urban areas. This article demonstrates that a large, 1906-like earthquake could cause significant damage to long-period buildings in the San Francisco Bay Area
vbyCaHbeta CCD Photometry of Clusters. VI. The Metal-Deficient Open Cluster NGC 2420
CCD photometry on the intermediate-band vbyCaHbeta system is presented for
the metal-deficient open cluster, NGC 2420. Restricting the data to probable
single members of the cluster using the CMD and the photometric indices alone
generates a sample of 106 stars at the cluster turnoff. The average E(b-y) =
0.03 +/- 0.003 (s.e.m.) or E(B-V) = 0.050 +/- 0.004 (s.e.m.), where the errors
refer to internal errors alone. With this reddening, [Fe/H] is derived from
both m1 and hk, using b-y and Hbeta as the temperature index. The agreement
among the four approaches is reasonable, leading to a final weighted average of
[Fe/H] = -0.37 +/- 0.05 (s.e.m.) for the cluster, on a scale where the Hyades
has [Fe/H] = +0.12. When combined with the abundances from DDO photometry and
from recalibrated low-resolution spectroscopy, the mean metallicity becomes
[Fe/H] = -0.32 +/- 0.03. It is also demonstrated that the average cluster
abundances based upon either DDO data or low-resolution spectroscopy are
consistently reliable to 0.05 dex or better, contrary to published attempts to
establish an open cluster metallicity scale using simplistic offset corrections
among different surveys.Comment: scheduled for Jan. 2006 AJ; 33 pages, latex, includes 7 figures and 2
table
Conditional expectations associated with quantum states
An extension of the conditional expectations (those under a given subalgebra
of events and not the simple ones under a single event) from the classical to
the quantum case is presented. In the classical case, the conditional
expectations always exist; in the quantum case, however, they exist only if a
certain weak compatibility criterion is satisfied. This compatibility criterion
was introduced among others in a recent paper by the author. Then,
state-independent conditional expectations and quantum Markov processes are
studied. A classical Markov process is a probability measure, together with a
system of random variables, satisfying the Markov property and can equivalently
be described by a system of Markovian kernels (often forming a semigroup). This
equivalence is partly extended to quantum probabilities. It is shown that a
dynamical (semi)group can be derived from a given system of quantum observables
satisfying the Markov property, and the group generators are studied. The
results are presented in the framework of Jordan operator algebras, and a very
general type of observables (including the usual real-valued observables or
self-adjoint operators) is considered.Comment: 10 pages, the original publication is available at http://www.aip.or
Monopole Vector Spherical Harmonics
Eigenfunctions of total angular momentum for a charged vector field
interacting with a magnetic monopole are constructed and their properties
studied. In general, these eigenfunctions can be obtained by applying vector
operators to the monopole spherical harmonics in a manner similar to that often
used for the construction of the ordinary vector spherical harmonics. This
construction fails for the harmonics with the minimum allowed angular momentum.
These latter form a set of vector fields with vanishing covariant curl and
covariant divergence, whose number can be determined by an index theorem.Comment: 21 pages, CU-TP-60
Simplified methods of assessing the impact of grid frequency dynamics upon generating plants
The frequency of the national electricity grid is affected by fluctuations in supply and demand, and so continually "judders" in an essentially unpredictable fashion around 50 Hz. At present such perturbations do not seemingly affect Nuclear Electric as most of their plant is run at more or less constant load, but they would like to be able to offer the national grid a mode of operation in which they "followed" the grid frequency: i.e., as the frequency rose above or fell below 50 Hz, the plant's output would be adjusted so as to tend to restore the frequency to 50 Hz. The aim is to maintain grid frequency within 0.2 Hz of its notional value. Such a mode of operation, however, would cause a certain amount of damage to plant components owing to the consequent continual changes in temperature and pressure within them.
Nuclear Electric currently have complex computational models of how plants will behave under these conditions, which allows them to compute plant data (e.g., reactor temperatures) from given grid frequency data. One approach to damage assessment would require several years'-worth of real grid data to be fed into this model and the corresponding damage computed (via "cycle distributions" created by their damage experts). The results of this analysis would demonstrate one of three possibilities: the damage may be acceptable under all reasonable operating conditions; or it may be acceptable except in the case of an exceptional abrupt change in grid frequency (caused by power transmission line failure, or another power station suddenly going off-line, for instance), in which case some kind of backup supply (e.g., gas boilers) would be required; or it may simply be unacceptable.
However, their current model runs in approximately real time, making it inappropriate for such a large amount of data: our problem was to suggest alternative approaches. Specifically, we were asked the following questions:
- Can component damage be reliably estimated directly from cycle distributions of grid frequency? i.e., are there maps from frequency cycle distributions to plant parameter cycle distributions?
- Can a simple model of plant dynamics be used to assess the potential for such maps?
- What methods can be used to select representative samples of grid frequency behaviour?
- What weightings should be applied to the selections?
- Is it possible to construct a "cycle transform" (Fourier transform) which will capture the essential features of grid frequency and which can then be inverted to generate simulated frequency transients?
We did not consider this last question, other than to say "probably not".
We were supplied with data of the actual grid frequency measurements for the evening of 29/7/95, and the corresponding plant responses (obtained using Nuclear Electric's current computational model). A simplified nonlinear mathematical model of the plant was also provided.
Two main approaches were considered: statistical prediction and analytical modelling via a reduction of the simplified plant model
Frequency Dependent Specific Heat from Thermal Effusion in Spherical Geometry
We present a novel method of measuring the frequency dependent specific heat
at the glass transition applied to 5-polyphenyl-4-ether. The method employs
thermal waves effusing radially out from the surface of a spherical thermistor
that acts as both a heat generator and thermometer. It is a merit of the method
compared to planar effusion methods that the influence of the mechanical
boundary conditions are analytically known. This implies that it is the
longitudinal rather than the isobaric specific heat that is measured. As
another merit the thermal conductivity and specific heat can be found
independently. The method has highest sensitivity at a frequency where the
thermal diffusion length is comparable to the radius of the heat generator.
This limits in practise the frequency range to 2-3 decades. An account of the
3omega-technique used including higher order terms in the temperature
dependency of the thermistor and in the power generated is furthermore given.Comment: 17 pages, 15 figures, Substantially revised versio
Higher-dimensional multifractal value sets for conformal infinite graph directed Markov systems
We give a description of the level sets in the higher dimensional
multifractal formalism for infinite conformal graph directed Markov systems. If
these systems possess a certain degree of regularity this description is
complete in the sense that we identify all values with non-empty level sets and
determine their Hausdorff dimension. This result is also partially new for the
finite alphabet case.Comment: 20 pages, 1 figur
3D-Matched-Filter Galaxy Cluster Finder I: Selection Functions and CFHTLS Deep Clusters
We present an optimised galaxy cluster finder, 3D-Matched-Filter (3D-MF),
which utilises galaxy cluster radial profiles, luminosity functions and
redshift information to detect galaxy clusters in optical surveys. This method
is an improvement over other matched-filter methods, most notably through
implementing redshift slicing of the data to significantly reduce line-of-sight
projections and related false positives. We apply our method to the
Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) Deep fields, finding ~170
galaxy clusters per square degree in the 0.2 <= z <= 1.0 redshift range. Future
surveys such as LSST and JDEM can exploit 3D-MF's automated methodology to
produce complete and reliable galaxy cluster catalogues. We determine the
reliability and accuracy of the statistical approach of our method through a
thorough analysis of mock data from the Millennium Simulation. We detect
clusters with 100% completeness for M_200 >= 3.0x10^(14)M_sun, 88% completeness
for M_200 >= 1.0x10^(14)M_sun, and 72% completeness well into the 10^(13)M_sun
cluster mass range. We show a 36% multiple detection rate for cluster masses >=
1.5x10^(13)M_sun and a 16% false detection rate for galaxy clusters >~
5x10^(13)M_sun, reporting that for clusters with masses <~ 5x10^(13)M_sun false
detections may increase up to ~24%. Utilising these selection functions we
conclude that our galaxy cluster catalogue is the most complete CFHTLS Deep
cluster catalogue to date.Comment: 18 pages, 17 figures, 5 tables; v2: added Fig 5, minor edits to match
version published in MNRA
Gamma-Ray Burst Sequences in Hardness Ratio-Peak Energy Plane
The narrowness of the distribution of the peak energy of
spectrum of gamma-ray bursts (GRBs) and the unification of GRB population are
great puzzles yet to be solved. We investigate the two puzzles based on the
global spectral behaviors of different GRB population in the
plane (HR the spectral hardness ratio) with BATSE and HETE-2 observations. It
is found that long GRBs and XRFs observed by HETE-2 seem to follow the same
sequence in the plane, with the XRFs at the low end of this
sequence. The long and short GRBs observed by BATSE follow significantly
different sequences in the plane, with most of the short GRBs
having a larger hardness ratio than the long GRBs at a given .
These results indicate that the global spectral behaviors of the long GRB
sample and the XRF sample are similar, while that of short GRBs is different.
The short GRBs seem to be a unique subclass of GRBs, and they are not the
higher energy extension of the long GRBs (abridged).Comment: 9 pages, 3 figure
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