2,060 research outputs found
A Non-Mainstream Viewpoint on Apparent Superluminal Phenomena in AGN Jet
The group velocity of light in material around the AGN jet is acquiescently
one (c as a unit), but this is only a hypothesis. Here, we re-derive apparent
superluminal and Doppler formulas for the general case (it is assumed that the
group velocity of light in the uniform and isotropic medium around a jet (a
beaming model) is not necessarily equal to one, e.g., Araudo et al. (2010)
thought that there may be dense clouds around AGN jet base), and show that the
group velocity of light close to one could seriously affect apparent
superluminal phenomena and Doppler effect in the AGN jet (when the viewing
angle and Lorentz factor take some appropriate values).Comment: 4 pages, 2 figures, new version accepted for publication in Journal
of Astrophysics and Astronom
An asymmetrical synchrotron model for knots in the 3C 273 jet
To interpret the emission of knots in the 3C 273 jet from radio to X-rays, we
propose a synchrotron model in which, owing to the shock compression effect,
the injection spectra from a shock into the upstream and downstream emission
regions are asymmetric. Our model could well explain the spectral energy
distributions of knots in the 3C 273 jet, and predictions regarding the knots
spectra could be tested by future observations.Comment: 9 pages, 1 figure, 1 table, new version accepted for publication in
Ap
Q-enhanced fold-and-bond MEMS inductors
This work presents a novel coil fabrication technology
to enhance quality factor (Q factor) of microfabricated inductors
for implanted medical wireless sensing and data/power transfer
applications. Using parylene as a flexible thin-film device
substrate, a post-microfabrication substrate folding-and-bonding
method is developed to effectively increase the metal thickness of
the surface-micromachined inductors, resulting in their lower
self-resistance so their higher quality factor. One-fold-and-bond
coils are successfully demonstrated as an example to verify the
feasibility of the fabrication technology with measurement results
in good agreements with device simulation. Depending on target
specifications, multiple substrate folding-and-bonding can be
extensively implemented to facilitate further improved electrical
characteristics of the coils from single fabrication batch. Such Q-enhanced
inductors can be broadly utilized with great potentials
in flexible integrated wireless devices/systems for intraocular
prostheses and other biomedical implants
Lidar-based wake tracking for closed-loop wind farm control
This work presents two advancements towards closed-loop wake redirection of a wind turbine. First, a model-based wake-tracking approach is presented, which uses a nacelle-based lidar system facing downwind to obtain information about the wake. The method uses a reduced-order wake model to track the wake. The wake tracking is demonstrated with lidar measurement data from an offshore campaign and with simulated lidar data from a simulation with the Simulator fOr Wind Farm Applications (SOWFA). Second, a controller for closed-loop wake steering is presented. It uses the wake-tracking information to set the yaw actuator of the wind turbine to redirect the wake to a desired position. Altogether, the two approaches enable a closed-loop wake redirection
Efficient preliminary floating offshore wind turbine design and testing methodologies and application to a concrete spar design
The current key challenge in the floating offshore wind turbine industry and research is on designing economic floating systems that can compete with fixed-bottom offshore turbines in terms of levelized cost of energy. The preliminary platform design, as well as early experimental design assessments, are critical elements in the overall design process. In this contribution, a brief review of current floating offshore wind turbine platform pre-design and scaled testing methodologies is provided, with a focus on their ability to accommodate the coupled dynamic behaviour of floating offshore wind systems. The exemplary design and testing methodology for a monolithic concrete spar platform as performed within the European KIC AFOSP project is presented. Results from the experimental tests compared to numerical simulations are presented and analysed and show very good agreement for relevant basic dynamic platform properties. Extreme and fatigue loads and cost analysis of the AFOSP system confirm the viability of the presented design process. In summary, the exemplary application of the reduced design and testing methodology for AFOSP confirms that it represents a viable procedure during pre-design of floating offshore wind turbine platforms.Peer ReviewedPostprint (author’s final draft
An hourglass model for the flare of HST-1 in M87
To explain the multi-wavelength light curves (from radio to X-ray) of HST-1
in the M87 jet, we propose an hourglass model that is a modified two-zone
system of Tavecchio & Ghisellini (hereafter TG08): a slow hourglass-shaped or
Laval nozzle-shaped layer connected by two revolving exponential surfaces
surrounding a fast spine, through which plasma blobs flow. Based on the
conservation of magnetic flux, the magnetic field changes along the axis of the
hourglass. We adopt the result of TG08---the high-energy emission from GeV to
TeV can be produced through inverse Compton by the two-zone system, and the
photons from radio to X-ray are mainly radiated by the fast inner zone system.
Here, we only discuss the light curves of the fast inner blob from radio to
X-ray. When a compressible blob travels down the axis of the first bulb in the
hourglass, because of magnetic flux conservation, its cross section experiences
an adiabatic compression process, which results in particle acceleration and
the brightening of HST-1. When the blob moves into the second bulb of the
hourglass, because of magnetic flux conservation, the dimming of the knot
occurs along with an adiabatic expansion of its cross section. A similar broken
exponential function could fit the TeV peaks in M87, which may imply a
correlation between the TeV flares of M87 and the light curves from radio to
X-ray in HST-1. The Very Large Array (VLA) 22 GHz radio light curve of HST-1
verifies our prediction based on the model fit to the main peak of the VLA 15
GHz radio light curve.Comment: 14 pages, 2 figures, accepted for publication in A
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