143 research outputs found
Constitutive Behavior of Aluminum Alloy Sheet At High Strain Rates
In this work, three aluminum sheet alloys, AA5754, AA5182 and AA6111, which are prime candidates for replacing mild steel in automobile structures, are tested in tension at quasi-static and high strain rates. In order to characterize the constitutive response of AA5754, AA5182 and AA6111 at high strain rates, tensile experiments were carried out at strain rates between 600 s-1 and 1500 s-1, and at temperatures between ambient and 300°C, using a tensile split Hopkinson bar (TSHB) apparatus. As part of this research, the apparatus was modified in order to provide an improved means of gripping the sheet specimens. Quasi-static experiments also were conducted using an Instron machine. The experimental data was fit to the Johnson-Cook and Zerilli-Armstrong constitutive models for all three alloys. The resulting fits were evaluated by numerically simulating the tensile experiments conducted using a finite element approach
A broken solar type II radio burst induced by a coronal shock propagating across the streamer boundary
We discuss an intriguing type II radio burst that occurred on 2011 March 27.
The dynamic spectrum was featured by a sudden break at about 43 MHz on the
well-observed harmonic branch. Before the break, the spectrum drifted gradually
with a mean rate of about -0.05 MHz/s. Following the break, the spectrum jumped
to lower frequencies. The post-break emission lasted for about three minutes.
It consisted of an overall slow drift which appeared to have a few fast drift
sub-bands. Simultaneous observations from the Solar TErrestrial RElations
Observatory (STEREO) and the Solar Dynamics Observatory (SDO) were also
available and are examined for this event. We suggest that the slow-drift
period before the break was generated inside a streamer by a coronal eruption
driven shock, and the spectral break as well as the relatively wide spectrum
after the break is a consequence of the shock crossing the streamer boundary
where density drops abruptly. It is suggested that this type of radio bursts
can be taken as a unique diagnostic tool for inferring the coronal density
structure, as well as the radio emitting source region.Comment: 12 pages, 5 figures, accepted by ApJ 201
Spatially resolved observations of a split-band coronal type-II radio burst
Context. The origin of coronal type-II radio bursts and of their
band-splitting are still not fully understood. Aims. To make progress in
solving this problem on the basis of one extremely well observed solar eruptive
event. Methods. The relative dynamics of multi-thermal eruptive plasmas,
observed in detail by the SDO/AIA and of the harmonic type-II burst sources,
observed by the NRH at ten frequencies from 445 to 151 MHz, is studied for the
partially behind the limb event on 3 November 2010. Special attention is given
to the band-splitting of the burst. Analysis is supplemented by investigation
of coronal hard X-ray (HXR) sources observed by the RHESSI. Results. It is
found that the flare impulsive phase was accompanied by the formation of a
double coronal HXR source, whose upper part coincided with the hot (T~10 MK)
eruptive plasma blob. The leading edge (LE) of the eruptive plasmas (T~1-2 MK)
moved upward from the flare region with the speed of v=900-1400 km/s. The type
II burst source initially appeared just above the LE apex and moved with the
same speed and in the same direction. After about 20 s it started to move about
twice faster, but still in the same direction. At any given moment the low
frequency component (LFC) source of the splitted type-II burst was situated
above the high frequency component (HFC) source, which in turn was situated
above the LE. It is also found that at a given frequency the HFC source was
located slightly closer to the photosphere than the LFC source. Conclusions.
The shock wave, which could be responsible for the observed type-II radio
burst, was initially driven by the multi-temperature eruptive plasmas, but
later transformed to a freely propagating blast shock wave. The most preferable
interpretation of the type-II burst splitting is that its LFC was emitted from
the upstream region of the shock, whereas the HFC - from the downstream region.Comment: 14 pages, 10 figure
Further constraints on electron acceleration in solar noise storms
We reexamine the energetics of nonthermal electron acceleration in solar
noise storms. A new result is obtained for the minimum nonthermal electron
number density required to produce a Langmuir wave population of sufficient
intensity to power the noise storm emission. We combine this constraint with
the stochastic electron acceleration formalism developed by Subramanian &
Becker (2005) to derive a rigorous estimate for the efficiency of the overall
noise storm emission process, beginning with nonthermal electron acceleration
and culminating in the observed radiation. We also calculate separate
efficiencies for the electron acceleration -- Langmuir wave generation stage
and the Langmuir wave -- noise storm production stage. In addition, we obtain a
new theoretical estimate for the energy density of the Langmuir waves in noise
storm continuum sources.Comment: Accepted for publication in Solar Physic
Observations of Low Frequency Solar Radio Bursts from the Rosse Solar-Terrestrial Observatory
The Rosse Solar-Terrestrial Observatory (RSTO; www.rosseobservatory.ie) was
established at Birr Castle, Co. Offaly, Ireland (53 05'38.9", 7 55'12.7") in
2010 to study solar radio bursts and the response of the Earth's ionosphere and
geomagnetic field. To date, three Compound Astronomical Low-cost Low-frequency
Instrument for Spectroscopy and Transportable Observatory (CALLISTO)
spectrometers have been installed, with the capability of observing in the
frequency range 10-870 MHz. The receivers are fed simultaneously by biconical
and log-periodic antennas. Nominally, frequency spectra in the range 10-400 MHz
are obtained with 4 sweeps per second over 600 channels. Here, we describe the
RSTO solar radio spectrometer set-up, and present dynamic spectra of a sample
of Type II, III and IV radio bursts. In particular, we describe fine-scale
structure observed in Type II bursts, including band splitting and rapidly
varying herringbone features
Broadband Radio Spectral Observations of Solar Eclipse on 2008-08-01 and Implications on the Quiet Sun Atmospheric Model
Based on the joint-observations of the radio broadband spectral emissions of
solar eclipse on August 1, 2008 at Jiuquan (total eclipse) and Huairou (partial
eclipse) at the frequencies of 2.00 -- 5.60 GHz (Jiuquan), 2.60 -- 3.80 GHZ
(Chinese solar broadband radiospectrometer, SBRS/Huairou), and 5.20 -- 7.60 GHz
(SBRS/Huairou), the authors assemble a successive series of broadband spectrum
with a frequency of 2.60 -- 7.60 GHz to observe the solar eclipse
synchronously. This is the first attempt to analyze the solar eclipse radio
emission under the two telescopes located at different places with broadband
frequencies in the periods of total and partial eclipse. With these analyses,
the authors made a new semiempirical model of the coronal plasma density of the
quiet Sun and made a comparison with the classic models.Comment: 10 pages, 4 figures, published on Sci. China Ser. G, 2009, Vol.52,
page 1765-177
Study of mechanical properties, microstructures and corrosion behavior of al 7075 t651 alloy with varying strain rate
Compression test of Al 7075 T651 was carried out at high strain rates (1138-2534 s-1) using Split Hopkinson Pressure Bar and at slow strain rate (10-4s-1) in 100KN Universal Testing machine to understand the improvement in mechanical properties and associated changes in microstructures. Cylindrical specimens of 6 mm height and 6 mm diameter were compressed dynamically. The influence of strain rates on mechanical properties, microstructure evolution and corrosion behavior after immersion test in 3.5% NaCl solution was also investigated. Strain rate, withdrawal stress and yield stress were observed to increase with impact velocity in high strain rate tests, while in slow strain rate tests, n value was observed to increase with increasing total strain. Microstructural observations revealed that after high strain rate test, grains of Al matrix were elongated. It was observed that corrosion resistance decreased with increase in impact velocity
On the relationship of shock waves to flares and coronal mass ejections
Context: Metric type II bursts are the most direct diagnostic of shock waves
in the solar corona.
Aims: There are two main competing views about the origin of coronal shocks:
that they originate in either blast waves ignited by the pressure pulse of a
flare or piston-driven shocks due to coronal mass ejections (CMEs). We studied
three well-observed type II bursts in an attempt to place tighter constraints
on their origins.
Methods: The type II bursts were observed by the ARTEMIS radio spectrograph
and imaged by the Nan\c{c}ay Radioheliograph (NRH) at least at two frequencies.
To take advantage of projection effects, we selected events that occurred away
from disk center.
Results: In all events, both flares and CMEs were observed. In the first
event, the speed of the shock was about 4200 km/s, while the speed of the CME
was about 850 km/s. This discrepancy ruled out the CME as the primary shock
driver. The CME may have played a role in the ignition of another shock that
occurred just after the high speed one. A CME driver was excluded from the
second event as well because the CMEs that appeared in the coronagraph data
were not synchronized with the type II burst. In the third event, the
kinematics of the CME which was determined by combining EUV and white light
data was broadly consistent with the kinematics of the type II burst, and,
therefore, the shock was probably CME-driven.
Conclusions: Our study demonstrates the diversity of conditions that may lead
to the generation of coronal shocks.Comment: 13 pages, 14 figures. "Astronomy and Astrophysics", in pres
A unified theory of the nonlinear oscillations of a cold plasma
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32294/1/0000361.pd
What is known about the patient's experience of medical tourism? A scoping review
<p>Abstract</p> <p>Background</p> <p>Medical tourism is understood as travel abroad with the intention of obtaining non-emergency medical services. This practice is the subject of increasing interest, but little is known about its scope.</p> <p>Methods</p> <p>A comprehensive scoping review of published academic articles, media sources, and grey literature reports was performed to answer the question: what is known about the patient's experience of medical tourism? The review was accomplished in three steps: (1) identifying the question and relevant literature; (2) selecting the literature; (3) charting, collating, and summarizing the information. Overall themes were identified from this process.</p> <p>Results</p> <p>291 sources were identified for review from the databases searched, the majority of which were media pieces (<it>n </it>= 176). A further 57 sources were included for review after hand searching reference lists. Of the 348 sources that were gathered, 216 were ultimately included in this scoping review. Only a small minority of sources reported on empirical studies that involved the collection of primary data (<it>n </it>= 5). The four themes identified via the review were: (1) decision-making (e.g., push and pull factors that operate to shape patients' decisions); (2) motivations (e.g., procedure-, cost-, and travel-based factors motivating patients to seek care abroad); (3) risks (e.g., health and travel risks); and (4) first-hand accounts (e.g., patients' experiential accounts of having gone abroad for medical care). These themes represent the most discussed issues about the patient's experience of medical tourism in the English-language academic, media, and grey literatures.</p> <p>Conclusions</p> <p>This review demonstrates the need for additional research on numerous issues, including: (1) understanding how multiple information sources are consulted and evaluated by patients before deciding upon medical tourism; (2) examining how patients understand the risks of care abroad; (3) gathering patients' prospective and retrospective accounts; and (4) the push and pull factors, as well as the motives of patients to participate in medical tourism. The findings from this scoping review and the knowledge gaps it uncovered also demonstrate that there is great potential for new contributions to our understanding of the patient's experience of medical tourism.</p
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