1,514 research outputs found
Empirical Study of Ground Proximity Effects for Small-scale Electroaerodynamic Thrusters
Electroaerodynamic (EAD) propulsion, where thrust is produced by collisions
between electrostatically-accelerated ions and neutral air, is a potentially
transformative method for indoor flight owing to its silent and solid-state
nature. Like rotors, EAD thrusters exhibit changes in performance based on
proximity to surfaces. Unlike rotors, they have no fragile and quickly spinning
parts that have to avoid those surfaces; taking advantage of the efficiency
benefits from proximity effects may be a route towards longer-duration indoor
operation of ion-propelled fliers. This work presents the first empirical study
of ground proximity effects for EAD propulsors, both individually and as
quad-thruster arrays. It focuses on multi-stage ducted centimeter-scale
actuators suitable for use on small robots envisioned for deployment in
human-proximal and indoor environments. Three specific effects (ground,
suckdown, and fountain lift), each occurring with a different magnitude at a
different spacing from the ground plane, are investigated and shown to have
strong dependencies on geometric parameters including thruster-to-thruster
spacing, thruster protrusion from the fuselage, and inclusion of flanges or
strakes. Peak thrust enhancement ranging from 300 to 600% is found for certain
configurations operated in close proximity (0.2 mm) to the ground plane and as
much as a 20% increase is measured even when operated centimeters away
On the role of the UV and X-ray radiation in driving a disk wind in X-ray binaries
X-ray heating of the photosphere of an accretion disk is a possible mechanism
to produce strong, broad UV emission lines in low mass X-ray binaries (LMXBs).
However, detailed photoionization calculations show that this mechanism fails
to produce sufficient emission measure. We present the results of
hydrodynamical calculations of the disk photosphere irradiated by strong
X-rays. We attempt to determine whether LMXBs can harbor significant UV-driven
disk winds despite the effects of X-ray ionization. Such winds would be a
likely candidate for the site of emission of UV lines and may better explain
the observations than the X-ray heated disk photosphere. We find that the local
disk radiation cannot launch a wind from the disk because of strong ionizing
radiation from the central object. Unphysically high X-ray opacities would be
required to shield the UV emitting disk and allow the line force to drive a
disk wind. However the same X-ray radiation that inhibits line driving heats
the disk and can produce a hot bipolar wind or corona above the disk. To assess
the impact of X-ray heating upon driving of a disk wind by the line force in
any system with an accretion disk we derive analytic formulae. In particular,
we compare results of line-driven disk wind models for accretion disks in LMXBs
and active galactic nuclei. The latter show spectral features associated with a
strong and fast disk wind. The key parameter determining the role of the line
force is not merely the presence of the luminous UV zone in the disk and the
presence of the X-rays, but also the distance of this UV zone from the center.Comment: LaTeX, 34 pages, contains color figures, to appear in Ap
Toward a Millimeter-Scale Tendon-Driven Continuum Wrist with Integrated Gripper for Microsurgical Applications
Microsurgery is a particularly impactful yet challenging form of surgery.
Robot assisted microsurgery has the potential to improve surgical dexterity and
enable precise operation on such small scales in ways not previously possible.
Intraocular microsurgery is a particularly challenging domain in part due to
the lack of dexterity that is achievable with rigid instruments inserted
through the eye. In this work, we present a new design for a millimeter-scale,
dexterous wrist intended for microsurgery applications. The wrist is created
via a state-of-the-art two-photon-polymerization (2PP) microfabrication
technique, enabling the wrist to be constructed of flexible material with
complex internal geometries and critical features at the micron-scale. The
wrist features a square cross section with side length of 1.25 mm and total
length of 3.75 mm. The wrist has three tendons routed down its length which,
when actuated by small-scale linear actuators, enable bending in any plane. We
present an integrated gripper actuated by a fourth tendon routed down the
center of the robot. We evaluate the wrist and gripper by characterizing its
bend-angle. We achieve more than 90 degrees bending in both axes. We
demonstrate out of plane bending as well as the robot's ability to grip while
actuated. Our integrated gripper/tendon-driven continuum robot design and
meso-scale assembly techniques have the potential to enable small-scale wrists
with more dexterity than has been previously demonstrated. Such a wrist could
improve surgeon capabilities during teleoperation with the potential to improve
patient outcomes in a variety of surgical applications, including intraocular
surgery.Comment: 2 pages, 3 figures, submitted to the 2023 Hamlyn Symposium on Medical
Robotics conferenc
How tidal processes impact the transfer of sediment from source to sink : Mekong River collaborative studies
Author Posting. © Oceanography Society, 2017. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 30, no. 3 (2017): 22–33, doi:10.5670/oceanog.2017.311.Significant sediment transformation and trapping occur along the tidal and estuarine reaches of large rivers, complicating sediment source signals transmitted to the coastal ocean. The collaborative Mekong Tropical Delta Study explored the tidally influenced portion of the Mekong River to investigate processes that impact mud- and sand-sized sediment transport and deposition associated with varying fluvial and marine influences. Researchers participating in this 2014–2015 project found that as sand and mud progress down the tidal portion of the river, sands in suspension can settle during reduced or slack flows as river discharge becomes progressively more affected by tides in the seaward direction. Consequently, deposits on the tidal river bed are connected to sand transport in the channel. In contrast, fine mud particles remain in suspension until they reach an interface zone where waters are still fresh, but the downstream saline estuary nonetheless impacts the flows. In this interface zone, as within the estuary, fine particles tend to settle, draping the sand beds with mud and limiting the connection between the bed and suspended sand. In the Mekong system, the interface and estuarine zones migrate along the distributary channels seasonally, resulting in variable trapping dynamics and channel bed texture. Therefore, the signature of fluvial-sediment discharge is altered on its path to the coastal ocean, and the disconnected mud and sand supply functions at the river mouth should result in distinct offshore depositional signatures.This research was funded by the US Office of Naval
Research (grant numbers: N00014-15-1-2011, N00014-
13-1-0127, N00014-13-1-0781, N00014-14-1-0145)
HST Imaging of fading AGN candidates. I. Host-galaxy properties and origin of the extended gas*
We present narrow- and medium-band Hubble Space Telescope imaging, with additional supporting ground-based imaging, spectrophotometry, and Fabry–Perot interferometric data, for eight galaxies identified as hosting a fading active galactic nucleus (AGN). These are selected to have AGN-ionized gas projected kpc from the nucleus and energy budgets with a significant shortfall of ionizing radiation between the requirement to ionize the distant gas and the AGN as observed directly, indicating fading of the AGN on ≈50,000 yr timescales. This paper focuses on the host-galaxy properties and origin of the gas. In every galaxy, we identify evidence of ongoing or past interactions, including tidal tails, shells, and warped or chaotic dust structures; a similarly selected sample of obscured AGNs with extended ionized clouds shares this high incidence of disturbed morphologies. Several systems show multiple dust lanes in different orientations, broadly fit by differentially precessing disks of accreted material viewed ~1.5 Gyr after its initial arrival. The host systems are of early Hubble type; most show nearly pure de Vaucouleurs surface brightness profiles and Sérsic indices appropriate for classical bulges, with one S0 and one SB0 galaxy. The gas has a systematically lower metallicity than the nuclei; three systems have abundances uniformly well below solar, consistent with an origin in tidally disrupted low-luminosity galaxies, while some systems have more nearly solar abundances (accompanied by such signatures as multiple Doppler components), which may suggest redistribution of gas by outflows within the host galaxies themselves. These aspects are consistent with a tidal origin for the extended gas in most systems, although the ionized gas and stellar tidal features do not always match closely. Unlike extended emission regions around many radio-loud AGNs, these clouds are kinematically dominated by rotation, in some cases in warped disks. Outflows can play important kinematic roles only in localized regions near some of the AGNs. We find only a few sets of young star clusters potentially triggered by AGN outflows. In UGC 7342 and UGC 11185, multiple luminous star clusters are seen just within the projected ionization cones, potentially marking star formation triggered by outflows. As in the discovery example, Hanny\u27s Voorwerp/IC 2497, there are regions in these clouds where the lack of a strong correlation between Hα surface brightness and ionization parameter indicates that there is unresolved fine structure in the clouds. Together with thin coherent filaments spanning several kpc, persistence of these structures over their orbital lifetimes may require a role for magnetic confinement. Overall, we find that the sample of fading AGNs occur in interacting and merging systems, that the very extended ionized gas is composed of tidal debris rather than galactic winds, and that these host systems are bulge-dominated and show no strong evidence of triggered star formation in luminous clusters
Joint spatiotemporal models to predict seabird densities at sea
Introduction: Seabirds are abundant, conspicuous members of marine ecosystems worldwide. Synthesis of distribution data compiled over time is required to address regional management issues and understand ecosystem change. Major challenges when estimating seabird densities at sea arise from variability in dispersion of the birds, sampling effort over time and space, and differences in bird detection rates associated with survey vessel type.
Methods: Using a novel approach for modeling seabirds at sea, we applied joint dynamic species distribution models (JDSDM) with a vector-autoregressive spatiotemporal framework to survey data collected over nearly five decades and archived in the North Pacific Pelagic Seabird Database. We produced monthly gridded density predictions and abundance estimates for 8 species groups (77% of all birds observed) within Cook Inlet, Alaska. JDSDMs included habitat covariates to inform density predictions in unsampled areas and accounted for changes in observed densities due to differing survey methods and decadal-scale variation in ocean conditions.
Results: The best fit model provided a high level of explanatory power (86% of deviance explained). Abundance estimates were reasonably precise, and consistent with limited historical studies. Modeled densities identified seasonal variability in abundance with peak numbers of all species groups in July or August. Seabirds were largely absent from the study region in either fall (e.g., murrelets) or spring (e.g., puffins) months, or both periods (shearwaters).
Discussion: Our results indicated that pelagic shearwaters (Ardenna spp.) and tufted puffin (Fratercula cirrhata) have declined over the past four decades and these taxa warrant further investigation into underlying mechanisms explaining these trends. JDSDMs provide a useful tool to estimate seabird distribution and seasonal trends that will facilitate risk assessments and planning in areas affected by human activities such as oil and gas development, shipping, and offshore wind and renewable energy
High-resolution, H band Spectroscopy of Be Stars with SDSS-III/APOGEE: I. New Be Stars, Line Identifications, and Line Profiles
APOGEE has amassed the largest ever collection of multi-epoch,
high-resolution (R~22,500), H-band spectra for B-type emission line (Be) stars.
The 128/238 APOGEE Be stars for which emission had never previously been
reported serve to increase the total number of known Be stars by ~6%. We focus
on identification of the H-band lines and analysis of the emission peak
velocity separations (v_p) and emission peak intensity ratios (V/R) of the
usually double-peaked H I and non-hydrogen emission lines. H I Br11 emission is
found to preferentially form in the circumstellar disks at an average distance
of ~2.2 stellar radii. Increasing v_p toward the weaker Br12--Br20 lines
suggests these lines are formed interior to Br11. By contrast, the observed IR
Fe II emission lines present evidence of having significantly larger formation
radii; distinctive phase lags between IR Fe II and H I Brackett emission lines
further supports that these species arise from different radii in Be disks.
Several emission lines have been identified for the first time including
~16895, a prominent feature in the spectra for almost a fifth of the sample
and, as inferred from relatively large v_p compared to the Br11-Br20, a tracer
of the inner regions of Be disks. Unlike the typical metallic lines observed
for Be stars in the optical, the H-band metallic lines, such as Fe II 16878,
never exhibit any evidence of shell absorption, even when the H I lines are
clearly shell-dominated. The first known example of a quasi-triple-peaked Br11
line profile is reported for HD 253659, one of several stars exhibiting intra-
and/or extra-species V/R and radial velocity variation within individual
spectra. Br11 profiles are presented for all discussed stars, as are full
APOGEE spectra for a portion of the sample.Comment: accepted in A
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