7,113 research outputs found
Compensation for nonlinear effects due to high heat flux in thin-film thermometry
Compensation for nonlinear effects due to high heat flux in thin-film thermometr
Convection and heat transfer in layered sloping warm-water\ud aquifers
What convective flow is induced if a geologically-tratified groundwater aquifer is subject to a vertical temperature gradient? How strong is the flow? What is the nett heat transfer? Is the flow stable? How does the convection affect the subsequent species distribution if a pollutant finds its way into the aquifer? This paper begins to address such questions. Quantitative models for buoyancy-driven fluid flow in long, sloping warm-water aquifers with both smoothly- and discretely-layered structures are formulated. The steady-state profiles are calculated for the temperature and for the fluid specific volume flux (Darcy velocity) parallel to the boundaries in a sloping system subjected to a perpendicular temperature gradient, at low Rayleigh numbers. The conducted and advected heat fluxes are compared and it is shown that the system acts somewhat like a heat pipe. The maximum possible ratio of naturally advected-to-conducted heat transfer is determined, together with the corresponding permeability and thermal conductivity profiles
Concurrent processing simulation of the space station
The development of a new capability for the time-domain simulation of multibody dynamic systems and its application to the study of a large angle rotational maneuvers of the Space Station is described. The effort was divided into three sequential tasks, which required significant advancements of the state-of-the art to accomplish. These were: (1) the development of an explicit mathematical model via symbol manipulation of a flexible, multibody dynamic system; (2) the development of a methodology for balancing the computational load of an explicit mathematical model for concurrent processing; and (3) the implementation and successful simulation of the above on a prototype Custom Architectured Parallel Processing System (CAPPS) containing eight processors. The throughput rate achieved by the CAPPS operating at only 70 percent efficiency, was 3.9 times greater than that obtained sequentially by the IBM 3090 supercomputer simulating the same problem. More significantly, analysis of the results leads to the conclusion that the relative cost effectiveness of concurrent vs. sequential digital computation will grow substantially as the computational load is increased. This is a welcomed development in an era when very complex and cumbersome mathematical models of large space vehicles must be used as substitutes for full scale testing which has become impractical
BiSON data preparation: A correction for differential extinction and the weighted averaging of contemporaneous data
The Birmingham Solar Oscillations Network (BiSON) has provided high-quality
high-cadence observations from as far back in time as 1978. These data must be
calibrated from the raw observations into radial velocity and the quality of
the calibration has a large impact on the signal-to-noise ratio of the final
time series. The aim of this work is to maximise the potential science that can
be performed with the BiSON data set by optimising the calibration procedure.
To achieve better levels of signal-to-noise ratio we perform two key steps in
the calibration process: we attempt a correction for terrestrial atmospheric
differential extinction; and the resulting improvement in the calibration
allows us to perform weighted averaging of contemporaneous data from different
BiSON stations. The improvements listed produce significant improvement in the
signal-to-noise ratio of the BiSON frequency-power spectrum across all
frequency ranges. The reduction of noise in the power spectrum will allow
future work to provide greater constraint on changes in the oscillation
spectrum with solar activity. In addition, the analysis of the low-frequency
region suggests we have achieved a noise level that may allow us to improve
estimates of the upper limit of g-mode amplitudes.Comment: Accepted for publication in MNRAS; 10 pages, 7 figure
The Sun in transition? Persistence of near-surface structural changes through Cycle 24
We examine the frequency shifts in low-degree helioseismic modes from the
Birmingham Solar-Oscillations Network (BiSON) covering the period from 1985 -
2016, and compare them with a number of global activity proxies well as a
latitudinally-resolved magnetic index. As well as looking at frequency shifts
in different frequency bands, we look at a parametrization of the shift as a
cubic function of frequency. While the shifts in the medium- and highfrequency
bands are very well correlated with all of the activity indices (with the best
correlation being with the 10.7 cm radio flux), we confirm earlier findings
that there appears to have been a change in the frequency response to activity
during solar cycle 23, and the low frequency shifts are less correlated with
activity in the last two cycles than they were in Cycle 22. At the same time,
the more recent cycles show a slight increase in their sensitivity to activity
levels at medium and higher frequencies, perhaps because a greater proportion
of activity is composed of weaker or more ephemeral regions. This lends weight
to the speculation that a fundamental change in the nature of the solar dynamo
may be in progress.Comment: 9 pages, 6 figures. Accepted by MNRAS 24 May 201
Performance of the Birmingham Solar-Oscillations Network (BiSON)
The Birmingham Solar-Oscillations Network (BiSON) has been operating with a
full complement of six stations since 1992. Over 20 years later, we look back
on the network history. The meta-data from the sites have been analysed to
assess performance in terms of site insolation, with a brief look at the
challenges that have been encountered over the years. We explain how the
international community can gain easy access to the ever-growing dataset
produced by the network, and finally look to the future of the network and the
potential impact of nearly 25 years of technology miniaturisation.Comment: 31 pages, 19 figures. Accepted by Solar Physics: 2015 October 20.
First online: 2015 December 7. Open Acces
Measurements at low energies of the polarization-transfer coefficient Kyy' for the reaction 3H(p,n)3He at 0 degrees
Measurements of the transverse polarization coefficient Kyy' for the reaction
3H(p,n)3He are reported for outgoing neutron energies of 1.94, 5.21, and 5.81
MeV. This reaction is important both as a source of polarized neutrons for
nuclear physics experiments, and as a test of theoretical descriptions of the
nuclear four-body system. Comparison is made to previous measurements,
confirming the 3H(p,n)3He reaction can be used as a polarized neutron source
with the polarization known to an accuracy of approximately 5%. Comparison to
R-matrix theory suggests that the sign of the 3F3 phase-shift parameter is
incorrect. Changing the sign of this parameter dramatically improves the
agreement between theory and experiment.Comment: 12 pages, RevTeX, 5 eps figures, submitted to Phys. Rev.
The environment and host haloes of the brightest z~6 Lyman-break galaxies
By studying the large-scale structure of the bright high-redshift Lyman-break
galaxy (LBG) population it is possible to gain an insight into the role of
environment in galaxy formation physics in the early Universe. We measure the
clustering of a sample of bright (-22.7<M_UV<-21.125) LBGs at z~6 and use a
halo occupation distribution (HOD) model to measure their typical halo masses.
We find that the clustering amplitude and corresponding HOD fits suggests that
these sources are highly biased (b~8) objects in the densest regions of the
high-redshift Universe. Coupled with the observed rapid evolution of the number
density of these objects, our results suggest that the shape of high luminosity
end of the luminosity function is related to feedback processes or dust
obscuration in the early Universe - as opposed to a scenario where these
sources are predominantly rare instances of the much more numerous M_UV ~ -19
population of galaxies caught in a particularly vigorous period of star
formation. There is a slight tension between the number densities and
clustering measurements, which we interpret this as a signal that a refinement
of the model halo bias relation at high redshifts or the incorporation of
quasi-linear effects may be needed for future attempts at modelling the
clustering and number counts. Finally, the difference in number density between
the fields (UltraVISTA has a surface density ~1.8 times greater than UDS) is
shown to be consistent with the cosmic variance implied by the clustering
measurements.Comment: 19 pages, 8 figures, accepted MNRAS 23rd March 201
Equatorial X-rays and their effect on the lower mesosphere
On the night of May 23/24, 1975, a sequence of rocket and balloon experiments was launched from Chilca Base, Peru (12.5 deg S, 76.8 deg W, magnetic dip = - 0.7 deg). Detailed analysis and comparisons of the data yielded the first direct measurement of lower mesospheric response to a galactic X-ray source. This result could only have been determined at the equator, where cosmic ray background effects are minimal. The objective of the experiments was to seek out the equatorial energetic electron belt, sporadically reported to contain fluxes near auroral levels, measure the bremsstrahlung radiation produced by this particle belt, and determine the influence of this radiation on the middle atmosphere. High altitude rocket payloads (Nike Tomahawk 18.170 and 18.171) were launched to probe the thermosphere during and following the anticipated downward drift period. Each carried an on-axis X-ray scintillation detector and Geiger Mueller energetic electron detectors. Magnetometers and lunar sensors were used to determine payload aspect
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