3,537 research outputs found
Active stabilization to prevent surge in centrifugal compression systems
This report documents an experimental and analytical study of the active stabilization of surge in a centrifugal engine. The aims of the research were to extend the operating range of a compressor as far as possible and to establish the theoretical framework for the active stabilization of surge from both an aerodynamic stability and a control theoretic perspective. In particular, much attention was paid to understanding the physical limitations of active stabilization and how they are influenced by control system design parameters. Previously developed linear models of actively stabilized compressors were extended to include such nonlinear phenomena as bounded actuation, bandwidth limits, and robustness criteria. This model was then used to systematically quantify the influence of sensor-actuator selection on system performance. Five different actuation schemes were considered along with four different sensors. Sensor-actuator choice was shown to have a profound effect on the performance of the stabilized compressor. The optimum choice was not unique, but rather shown to be a strong function of some of the non-dimensional parameters which characterize the compression system dynamics. Specifically, the utility of the concepts were shown to depend on the system compliance to inertia ratio ('B' parameter) and the local slope of the compressor speedline. In general, the most effective arrangements are ones in which the actuator is most closely coupled to the compressor, such as a close-coupled bleed valve inlet jet, rather than elsewhere in the flow train, such as a fuel flow modulator. The analytical model was used to explore the influence of control system bandwidth on control effectiveness. The relevant reference frequency was shown to be the compression system's Helmholtz frequency rather than the surge frequency. The analysis shows that control bandwidths of three to ten times the Helmholtz frequency are required for larger increases in the compressor flow range. This has important implications for implementation in gas turbine engines since the Helmholtz frequencies can be over 100 Hz, making actuator design extremely challenging
Feeding and Feedback in the Inner Kiloparsec of the Active Galaxy NGC2110
We present two-dimensional gaseous kinematics of the inner 1.1 x 1.6kpc^2 of
the Seyfert 2 galaxy NGC2110, from optical spectra obtained with the GMOS
integral field spectrograph on the Gemini South telescope at a spatial
resolution of 100pc. Gas emission is observed over the whole field-of-view,
with complex - and frequently double - emission-line profiles. We have
identified four components in the emitting gas, according to their velocity
dispersion (sigma), which we refer to as: (1) warm gas disk (sigma =
100-220km/s); (2) cold gas disk (sigma = 60-90km/s); (3) nuclear component
(sigma = 220-600km/s); and (4) northern cloud (sigma = 60-80km/s). Both the
cold and warm disk components are dominated by rotation and have similar gas
densities, but the cold gas disk has lower velocity dispersions and reaches
higher rotation velocities. We attribute the warm gas disk to a thick gas layer
which encompasses the cold disk as observed in some edge-on spiral galaxies.
After subtraction of a rotation model from the cold disk velocity field, we
observe excess blueshifts of 50km/s in the far side of the galaxy as well as
similar excess redshifts in the near side. These residuals can be interpreted
as due to nuclear inflow in the cold gas, with an estimated ionized gas mass
inflow rate of 2.2 x 10^(-2)Msun/yr. We have also subtracted a rotating model
from the warm disk velocity field and found excess blueshifts of 100km/s to the
SW of the nucleus and excess redshifts of 40km/s to the NE, which we attribute
to gas disturbed by an interaction with a nuclear spherical outflow. This
nuclear outflow is the origin of the nuclear component observed within the
inner 300pc and it has a mass outflow rate of 0.9Msun/yr. In a region between
1" and 4" north of the nucleus we find a new low sigma component of ionized gas
which we attribute to a high latitude cloud photoionized by the nuclear source.Comment: 17 pages, 13 figures, 1 table; accepted for publication in MNRA
Outflows in the Narrow Line Region of Bright Seyfert Galaxies - I: GMOS-IFU Data
We present two-dimensional maps of emission-line fluxes and kinematics, as
well as of the stellar kinematics of the central few kpc of five bright nearby
Seyfert galaxies -- Mrk\,6, Mrk\,79, Mrk\,348, Mrk\,607 and Mrk\,1058 --
obtained from observations with the Gemini Multi-Object Spectrograph (GMOS)
Integral Field Unit (IFU) on the Gemini North Telescope. The data cover the
inner 3\farcs55\farcs0 -- corresponding to physical scales in the range
0.60.9 to 1.52.2\,kpc -- at a spatial resolution ranging
from 110 to 280 pc with a spectral coverage of 4300 -- 7100\,\AA\ and velocity
resolution of 90\,km\,s. The gas excitation is Seyfert like
everywhere but show excitation, but show excitation gradients that are
correlated with the gas kinematics, reddening and/or the gas density. The gas
kinematics show in all cases two components: a rotation one similar to that
observed in the stellar velocity field, and an outflow component. In the case
of Mrk607, the gas is counter-rotating relative to the stars. Enhanced gas
velocity dispersion is observed in association to the outflows according to two
patterns: at the locations of the highest outflow velocities along the
ionization axis or perpendicularly to it in a strip centered at the nucleus
that we attribute to an equatorial outflow. Bipolar outflows are observed in
Mrk\,348 and Mrk\,79, while in Mrk\,1058 only the blueshifted part is clearly
observed, while in the cases of Mrk\,6 and Mrk\,607 the geometry of the outflow
needs further constraints from modeling to be presented in a forthcoming study,
where the mass flow rate and powers will also be obtained.Comment: 20 pages, accepted by MNRA
Evaluation of the Effect of Pollution on the Level of Antioxidant Enzymes in Freshwater Fishes
To assess the impact of polluted water on fishes, biochemical studies were carried out to determine the biochemical alterations in tissues of fishes collected from the control and upstream, midstream and downstream sites of the Varahanadhi river in Tamil Nadu, India. Two fish species such as Catla (Catla catla) and Mrigal (Cirrhinus mrigala) were taken and the tissues such as liver and kidney were excised and tissue homogenate was prepared. Using the tissue homogenate lipid peroxide and lipid hydroperoxide, CAT and SOD activity, GPx activity, GR and GST activities and non-enzymatic antioxidants such as vitamin C and vitamin E was estimated. An increase in lipid peroxide and lipid hydroperoxide level was observed in all the organs of fishes collected from the downstream site when compared with the control, upstream and midstream site of the river. A decrease in CAT and SOD activity and in non-enzymatic antioxidants such as vitamin C and vitamin E, increased Glutathione reductase and GST activities was observed in the fishes collected from the downstream site. These changes might be due to the exposure of fish to pollutants, industrial waste and heavy metals present in water sample causing adverse effects on fishes inhabiting the river
Survival by the Fittest: Hospital‐Level Variation in Quality of Resuscitation Care
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/139113/1/jah3453.pd
Atmospheric forcing during active convection in the Labrador Sea and its impact on mixed-layer depth
Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 6978–6992, doi:10.1002/2015JC011607.Hydrographic data from the Labrador Sea collected in February–March 1997, together with atmospheric reanalysis fields, are used to explore relationships between the air-sea fluxes and the observed mixed-layer depths. The strongest winds and highest heat fluxes occurred in February, due to the nature and tracks of the storms. While greater numbers of storms occurred earlier and later in the winter, the storms in February followed a more organized track extending from the Gulf Stream region to the Irminger Sea where they slowed and deepened. The canonical low-pressure system that drives convection is located east of the southern tip of Greenland, with strong westerly winds advecting cold air off the ice edge over the warm ocean. The deepest mixed layers were observed in the western interior basin, although the variability in mixed-layer depth was greater in the eastern interior basin. The overall trend in mixed-layer depth through the winter in both regions of the basin was consistent with that predicted by a 1-D mixed-layer model. We argue that the deeper mixed layers in the west were due to the enhanced heat fluxes on that side of the basin as opposed to oceanic preconditioning.National Science Foundation (RP); Natural Science and Engineering Research Council of Canada Grant Number: OCE-12596182017-03-2
The complex gas kinematics in the nucleus of the Seyfert 2 galaxy NGC 1386: rotation, outflows and inflows
We present optical integral field spectroscopy of the circum-nuclear gas of
the Seyfert 2 galaxy NGC 1386. The data cover the central 7 (530 680 pc) at a spatial resolution of 0.9"
(68 pc), and the spectral range 5700-7000 \AA\ at a resolution of 66 km
s. The line emission is dominated by a bright central component, with
two lobes extending 3 north and south of the
nucleus. We identify three main kinematic components. The first has low
velocity dispersion ( 90 km s), extends over the
whole field-of-view, and has a velocity field consistent with gas rotating in
the galaxy disk. We interpret the lobes as resulting from photoionization of
disk gas in regions where the AGN radiation cones intercept the disk. The
second has higher velocity dispersion ( 200 km s)
and is observed in the inner 150 pc around the continuum peak. This component
is double peaked, with redshifted and blueshifted components separated by
500 km s. Together with previous HST imaging, these features
suggest the presence of a bipolar outflow for which we estimate a mass outflow
rate of 0.1 M yr. The third
component is revealed by velocity residuals associated with enhanced velocity
dispersion and suggests that outflow and/or rotation is occurring approximately
in the equatorial plane of the torus. A second system of velocity residuals may
indicate the presence of streaming motions along dusty spirals in the disk.Comment: 24 pages, 16 figures, 3 tables, interesting results, accepted for
publication in Ap
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