791 research outputs found
Chromospheric oscillations
We show results from SO/Sacramento Peak data to discuss three issues:
(i)--the spatial occurrence of chromospheric 3--min oscillations;
(ii)--the validity of Ca II H&K line-center Doppler Shift measurements;
(iii)--the signi ?cance of oscillation power and phase at frequencies above 10 mHz
Cancellation of the Chiral Anomaly in a Model with Spontaneous Symmetry Breaking
A perturbatively renormalized Abelian Higgs-Kibble model with a chirally
coupled fermion is considered. The Slavnov identity is fulfilled to all orders
of perturbation theory, which is crucial for renormalizability in models with
vector bosons. BRS invariance, i.e. the validity of the identity, forces the
chiral anomaly to be cancelled by Wess-Zumino counterterms. This procedure
preserves the renormalizability in the one-loop approximation but it violates
the Froissart bounds for partial wave amplitudes above some energy and destroys
renormalizability from the second order in h bar onwards due to the
counterterms. (The paper has 3 figs. in postscript which are not included; send
request to the author's e-mailbox with subject: figures . The author is willing
to mail hard copies of the paper.)Comment: 13 pages, plain TeX, SI 92-1
Combustion dynamics analysis of a pressurized airblast swirl burner using proper orthogonal decomposition
Jet fuel-fired combustors in aero gas turbine engines have switched to lean burn to decrease nitric oxide emissions in recent years as a result of strict emission regulations. Lean operating conditions, however, exhibit a heightened sensitivity to thermoacoustic instabilities and such burners require careful consideration in design and operation. Similar to natural gas-fired combustors, they exhibit thermoacoustic instabilities, but the characteristics are more complex and less well-studied. This paper presents a numerical investigation of an airblast jet fuel swirl burner operating with preheated air at lean pressurized conditions. In order to understand the acoustic characteristics of the in-house designed burner (Magister UT burner), detached eddy simulations are performed at relevant aero engine conditions. Simulation results are then analyzed by means of our internally developed parallel modal analysis package, PARAMOUNT, to perform proper orthogonal decomposition (POD) on large datasets. The resulting modes are inspected to highlight flow features of interest and their associated acoustic frequencies at unforced conditions. Single frequency acoustic forcing is employed to study the acoustic response of the burner to perturbations at similar frequencies to its precessing vortex core. We show that parallel computation of POD modes is a viable tool to investigate the main flow features of swirl burners and is suitable for highlighting the important acoustic frequencies without the need to employ fully compressible computational fluid dynamics solvers. Additionally, the analysis method reveals the ways in which various flow structures correlate with each other and how external perturbations modify them.</p
Non-linear numerical simulations of magneto-acoustic wave propagation in small-scale flux tubes
We present results of non-linear, 2D, numerical simulations of
magneto-acoustic wave propagation in the photosphere and chromosphere of
small-scale flux tubes with internal structure. Waves with realistic periods of
three to five minutes are studied, after applying horizontal and vertical
oscillatory perturbations to the equilibrium model. Spurious reflections of
shock waves from the upper boundary are minimized thanks to a special boundary
condition. This has allowed us to increase the duration of the simulations and
to make it long enough to perform a statistical analysis of oscillations. The
simulations show that deep horizontal motions of the flux tube generate a slow
(magnetic) mode and a surface mode. These modes are efficiently transformed
into a slow (acoustic) mode in the vA < cS atmosphere. The slow (acoustic) mode
propagates vertically along the field lines, forms shocks and remains always
within the flux tube. It might deposit effectively the energy of the driver
into the chromosphere. When the driver oscillates with a high frequency, above
the cut-off, non-linear wave propagation occurs with the same dominant driver
period at all heights. At low frequencies, below the cut-off, the dominant
period of oscillations changes with height from that of the driver in the
photosphere to its first harmonic (half period) in the chromosphere. Depending
on the period and on the type of the driver, different shock patterns are
observed.Comment: 22 pages 6 color figures, submitted to Solar Physics, proceeding of
SOHO 19/ GONG 2007 meeting, Melbourne, Australi
âMake no little plansâ: Impactful research to solve the next generation of transportation problems
The transportation science research community has contributed to numerous practical and intellectual innovations and improvements over the last decades. Technological advancements have broadened and amplified the potential impacts of our field.
At the same time, the world and its communities are facing greater and more serious
challenges than ever before. In this paper, we call upon the transportation science
research community to work on a research agenda that addresses some of the most
important of these challenges. This agenda is guided by the sustainable development
goals outlined by the United Nations and organized into three areas: (1) well-being,
(2) infrastructure, and, (3) natural environment. For each area, we identify current
and future challenges as well as research directions to address those challenges
Speckle-Tracking Echocardiography for Predicting Outcome in Chronic Aortic Regurgitation During Conservative Management and After Surgery
ObjectivesThe aim of this study was to test myocardial deformation imaging using speckle-tracking echocardiography for predicting outcomes in chronic aortic regurgitation.BackgroundIn chronic aortic regurgitation, left ventricular (LV) dysfunction must be detected early to allow timely surgery. Speckle-tracking echocardiography has been proposed for this purpose, but the clinical value of this method in aortic regurgitation has not been established.MethodsA longitudinal study was performed in 64 patients with moderate to severe aortic regurgitation. Thirty-five patients were managed conservatively with frequent clinical visits and sequential echocardiography and followed for an average of 19 ± 8 months, while 29 patients underwent surgery for the valve lesion and were followed for 6 months post-operatively. Baseline LV function by speckle-tracking and conventional echocardiography was compared with impaired outcome after surgery (defined as persisting symptoms or persisting LV dilation [LV end-diastolic volume index â„87 ml/m2] or dysfunction [LV ejection fraction <50%]) and with disease progression during conservative management (defined as development of symptoms, increase in LV volume >15%, or decrease in LV ejection fraction >10%).ResultsReduced myocardial systolic strain, systolic strain rate, and early diastolic strain rate by speckle-tracking echocardiography was associated with disease progression during conservative management (â16.3% vs. â19.0%, p = 0.02; â1.04 vs. â1.19 sâ1, p = 0.02; and 1.20 vs. 1.60 sâ1, p = 0.002, respectively) and with impaired outcome after surgery (â11.5% vs. â15.6%, p = 0.01; â0.88 vs. â1.01 sâ1, p = 0.04; and 0.98 vs. 1.33 sâ1, p = 0.01, respectively). Conventional parameters of LV function and size (LV ejection fraction and LV end-diastolic volume index) were associated with outcome after surgery (p = 0.04 and p = 0.01, respectively) but not with outcome during conservative management (p = 0.57 and p = 0.39, respectively).ConclusionsSpeckle-tracking echocardiography is useful for the early detection of LV systolic and diastolic dysfunction in chronic aortic regurgitation
Access to scientific literature by the conservation community
Access to the scientific literature is perceived to be a challenge to the biodiversity conservation community, but actual level of literature access relative to needs has never been assessed globally. We examined this question by surveying the constituency of the International Union for Conservation of Nature (IUCN) as a proxy for the conservation community, generating 2,285 responses. Of these respondents, âŒ97% need to use the scientific literature in order to support their IUCN-related conservation work, with âŒ50% needing to do so at least once per week. The crux of the survey revolved around the question, âHow easy is it for you currently to obtain the scientific literature you need to carry out your IUCN-related work?â and revealed that roughly half (49%) of the respondents find it not easy or not at all easy to access scientific literature. We fitted a binary logistic regression model to explore factors predicting ease of literature access. Whether the respondent had institutional literature access (55% do) is the strongest predictor, with region (Western Europe, the United States, Canada, Australia and New Zealand) and sex (male) also significant predictors. Approximately 60% of respondents from Western Europe, the United States, Canada, Australia and New Zealand have institutional access compared to âŒ50% in Asia and Latin America, and âŒ40% in Eastern Europe and in Africa. Nevertheless, accessing free online material is a popular means of accessing literature for both those with and without institutional access. The four journals most frequently mentioned when asked which journal access would deliver the greatest improvements to the respondentâs IUCN-related work were Conservation Biology, Biological Conservation, Nature, and Science. The majority prefer to read journal articles on screen but books in hard copy. Overall, it is apparent that access to the literature is a challenge facing roughly half of the conservation community worldwide
Enhanced Joule Heating in Umbral Dots
We present a study of magnetic profiles of umbral dots (UDs) and its
consequences on the Joule heating mechanisms. Hamedivafa (2003) studied Joule
heating using vertical component of magnetic field. In this paper UDs magnetic
profile has been investigated including the new azimuthal component of magnetic
field which might explain the relatively larger enhancement of Joule heating
causing more brightness near circumference of UD.Comment: 8 pages, 1 figure, accepted in Solar Physic
Properties of high-frequency wave power halos around active regions: an analysis of multi-height data from HMI and AIA onboard SDO
We study properties of waves of frequencies above the photospheric acoustic
cut-off of 5.3 mHz, around four active regions, through spatial maps
of their power estimated using data from Helioseismic and Magnetic Imager (HMI)
and Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory
(SDO). The wavelength channels 1600 {\AA} and 1700 {\AA} from AIA are now known
to capture clear oscillation signals due to helioseismic p modes as well as
waves propagating up through to the chromosphere. Here we study in detail, in
comparison with HMI Doppler data, properties of the power maps, especially the
so called 'acoustic halos' seen around active regions, as a function of wave
frequencies, inclination and strength of magnetic field (derived from the
vector field observations by HMI) and observation height. We infer possible
signatures of (magneto-)acoustic wave refraction from the observation height
dependent changes, and hence due to changing magnetic strength and geometry, in
the dependences of power maps on the photospheric magnetic quantities. We
discuss the implications for theories of p mode absorption and mode conversions
by the magnetic field.Comment: 22 pages, 12 figures, Accepted by journal Solar Physic
Burkhardt-Cottingham sum rule and forward spin polarizabilities in Heavy Baryon Chiral Perturbation Theory
We study spin-dependent sum rules for forward virtual Compton
scattering(VVCS) off the nucleon in heavy baryon chiral perturbation theory at
order . We show how these sum rules can be evaluated from low energy
expansions (in the virtual photon energy) of the forward VVCS amplitudes. We
study in particular the Burkhardt -Cottingham sum rule in HBChPT and higher
terms in the low energy expansion, which can be related to the generalized
forward spin polarizabilities of the nucleon. The dependence of these
observables on the photon virtuality can be accessed, at small and
intermediate values, from existing and forthcoming data at Jefferson Lab.Comment: 16 pages,4 fig
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