416 research outputs found
Statistical Analysis of Small Ellerman Bomb Events
The properties of Ellerman bombs (EBs), small-scale brightenings in the
H-alpha line wings, have proved difficult to establish due to their size being
close to the spatial resolution of even the most advanced telescopes. Here, we
aim to infer the size and lifetime of EBs using high-resolution data of an
emerging active region collected using the Interferometric BIdimensional
Spectrometer (IBIS) and Rapid Oscillations of the Solar Atmosphere (ROSA)
instruments as well as the Helioseismic and Magnetic Imager (HMI) onboard the
Solar Dynamics Observatory (SDO). We develop an algorithm to track EBs through
their evolution, finding that EBs can often be much smaller (around 0.3") and
shorter lived (less than 1 minute) than previous estimates. A correlation
between G-band magnetic bright points and EBs is also found. Combining SDO/HMI
and G-band data gives a good proxy of the polarity for the vertical magnetic
field. It is found that EBs often occur both over regions of opposite polarity
flux and strong unipolar fields, possibly hinting at magnetic reconnection as a
driver of these events.The energetics of EB events is found to follow a
power-law distribution in the range of "nano-flare" (10^{22-25} ergs).Comment: 19 pages. 7 Figure
Discovery of close companions to the nearby young stars HD 199143 and HD 358623
Young stellar systems in the solar neighborhood provide valuable laboratories
for detailed studies of star and planet formation. The bright F8V star HD
199143 and the Li-rich late-type emission line star HD 358623 are among the
nearest young stars identified to date, and may be members of a young
association in Capricornus. We present high-resolution near-infrared images of
these two sources, obtained using the adaptive optics system on the 3.6-meter
telescope at the European Southern Observatory in La Silla, Chile. Our
observations reveal that both are in fact close binary systems. The newly
discovered companion at a separation of 1'' may account for the unusual
characteristics of HD 199143 --rapid rotation, emission lines, ultraviolet
variability, and excess infrared emission-- recently discussed by van den
Ancker and co-workers. HD 199143 may be a rare example of a close binary with
only a circum{\it secondary} disk. With the detection of a 2'' companion,
HD 358623 is now possibly one of the closest known T Tauri binaries. Both
binary systems are prime targets for follow-up spectroscopic and astrometric
observations.Comment: 9 pages, 1 PostScript figure, to appear in The Astrophysical Journal
Letter
Propagation of Waves above a Plage as Observed by IRIS and SDO
Context. MHD waves are proposed to transport sufficient energy from the
photosphere to heat the transition-region (TR) and corona. However, various
aspects of these waves such as their nature, propagation characteristics and
role in the atmospheric heating process remain poorly understood and are a
matter of further investigation. Aims. We aim to investigate wave propagation
within an active-region (AR) plage using IRIS and AIA observations. The main
motivation is to understand the relationship between photospheric and TR
oscillations. We plan to identify the locations in the plage region where
magnetic flux tubes are essentially vertical, and further our understanding of
the propagation and nature of these waves. Methods. We have used photospheric
observations from AIA (i.e., AIA 1700 {\AA}) as well as TR imaging observations
(IRIS/SJI Si iv 1400.0 {\AA}). We have investigated propagation of the waves
into the TR from the photosphere using wavelet analysis (e.g., cross power,
coherence and phase difference) with inclusion of a customized noise model.
Results. Fast Fourier Transform(FFT) shows the distribution of wave power at
photospheric & TR heights. Waves with periods between 2.0- and 9.0-minutes
appear to be correlated between the photosphere and TR. We exploited a
customized noise model to estimate 95% confidence levels for IRIS observations.
On the basis of the sound speed in the TR and estimated propagation speed,
these waves are best interpreted as the slow magneto acoustic waves (SMAW). It
is found that almost all locations show correlation/propagation of waves over
broad range of period from photosphere to TR. It suggests the wave's
correlation/propagation spatial occurrence frequency is very high within the
plage area.Comment: 20 Pages, 9 figures, Accepted for Publication in A&
IDENTIFICATION OF MECHANICAL ALTERATIONS FROM THEIR EFFECT ON PERFORMANCE OF A RADIAL COMPRESSOR
ABSTRACT An experimental study of the influence of mechanical alterations in a stage of a radial compressor with a vaned diffuser is presented. The mechanical alterations considered correspond to changes which can be produced by the occurrence of faults or deterioration of the compressor. They include the insertion of an inlet obstruction, an obstruction in a diffuser passage, increase of impeller tip clearance, and impeller fouling. The change in the compressor performance parameters, from the reference condition, is established from the experimental results. These changes are referred to the overall stage performance but also to its components, impeller, and diffuser. In order to establish diagnostic abilities, appropriate indices are introduced. The behaviour of these indices is related to the altered stage conditions and the possibility of using them for identifying the stage condition is demonstrated
The Influence of Magnetic Field on Oscillations in the Solar Chromosphere
Two sequences of solar images obtained by the Transition Region and Coronal
Explorer in three UV passbands are studied using wavelet and Fourier analysis
and compared to the photospheric magnetic flux measured by the Michelson
Doppler Interferometer on the Solar Heliospheric Observatory to study wave
behaviour in differing magnetic environments. Wavelet periods show deviations
from the theoretical cutoff value and are interpreted in terms of inclined
fields. The variation of wave speeds indicates that a transition from dominant
fast-magnetoacoustic waves to slow modes is observed when moving from network
into plage and umbrae. This implies preferential transmission of slow modes
into the upper atmosphere, where they may lead to heating or be detected in
coronal loops and plumes.Comment: 8 pages, 6 figures (4 colour online only), accepted for publication
in The Astrophysical Journa
Development of Gas Turbine Performance Models Using a Generic Simulation Toolâ,
ABSTRACT An approach for gas turbine engine modelling using a general purpose object-oriented simulation tool is described. A commercially available such tool that can be adapted to different fields, through the creation of reusable modelling component libraries representing parts or equipment of a physical system, is employed. Libraries are developed using an object-oriented language. The possibility for quick implementation of new models and rapid analysis of results, through the use of a graphical user interface is demonstrated. A turbofan model, developed for both steady state and transient performance simulation, is used to illustrate the advantages offered by this approach. Results are presented and compared to those produced by an industry-accepted model. The flexibility of incorporating particular features into a model is demonstrated by presenting the implementation of adaptive features and a study of engine frequency response
COMBINING ADVANCED DATA ANALYSIS METHODS FOR THE CONSTITUTION OF AN INTEGRATED GAS TURBINE CONDITION MONITORING AND DIAGNOSTIC SYSTEM
ABSTRACT This paper presents principles for the constitution of gas turbine monitoring and diagnostic systems which: a. are integrated, namely manage all the tasks essential for achieving a diagnosis (measurement, analysis, interpretation, historical data management etc.) b. employ different kind of processing methods in order to cover an extensive range of engine conditions, including direct data evaluation and data consistency checks, thermodynamic analysis, vibration analysis. The requirements to be fulfilled by an industrial gas turbine monitoring system are briefly reviewed and ways to achieve them are discussed, indicating how they can be materialized by implementation of specific techniques. Techniques previously derived by the group of the authors are implemented, and the merits they offer when used in combination are discussed. Features of a system, materialized according to the principles discussed, into an operating industrial gas turbine is presented. On-line application of advanced analysis techniques, such as adaptive performance modeling is discussed, on the basis of observations of the collected data. Data collected from an engine operating in the field are presented to substantiate the matters discussed, and cases of successful fault identification are shown
Performance analysis of twin-spool water injected gas turbines using adaptive modeling.
ABSTRACT The development of an adaptive performance model for multi-spool gas turbines, equipped with the possibility of water injection is described. The model covers water injection at engine inlet, between the compressors and at the compressor exit. The selection of modification factors and the procedure for adapting component characteristics to overall performance data is discussed. A case of adaptation to overall performances is presented. The use of the model for studying overall engine and components performance is demonstrated. It is shown how operation with water injection modifies component operation, a fact that allows the identification of a wider range of the performance characteristics, in comparison to dry operation. This fact may also increase the diagnostic ability of techniques employing adaptive models. The sensitivity of diagnostic procedures to the different modes of operation of a gas turbine of the type described in the paper is also discussed
Velocity Response of the Observed Explosive Events in the Lower Solar Atmosphere: I. Formation of the Flowing Cool Loop System
We observe plasma flows in cool loops using the Slit-Jaw Imager (SJI) onboard
the Interface Region Imaging Spectrometer (IRIS). Huang et al. (2015) observed
unusually broadened Si IV 1403 angstrom line profiles at the footpoints of such
loops that were attributed to signatures of explosive events (EEs). We have
chosen one such uni-directional flowing cool loop system observed by IRIS where
one of the footpoints is associated with significantly broadened Si IV line
profiles. The line profile broadening indirectly indicates the occurrence of
numerous EEs below the transition region (TR), while it directly infers a large
velocity enhancement /perturbation further causing the plasma flows in the
observed loop system. The observed features are implemented in a model
atmosphere in which a low-lying bi-polar magnetic field system is perturbed in
the chromosphere by a velocity pulse with a maximum amplitude of 200 km/s. The
data-driven 2-D numerical simulation shows that the plasma motions evolve in a
similar manner as observed by IRIS in the form of flowing plasma filling the
skeleton of a cool loop system. We compare the spatio-temporal evolution of the
cool loop system in the framework of our model with the observations, and
conclude that their formation is mostly associated with the velocity response
of the transient energy release above their footpoints in the chromosphere/TR.
Our observations and modeling results suggest that the velocity responses most
likely associated to the EEs could be one of the main candidates for the
dynamics and energetics of the flowing cool loop systems in the lower solar
atmosphere.Comment: In Press; The Astrophysical Journal; 14 Pages; 9 Figure
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