7,542 research outputs found
Catastrophic Evaporation of Rocky Planets
Short-period exoplanets can have dayside surface temperatures surpassing 2000
K, hot enough to vaporize rock and drive a thermal wind. Small enough planets
evaporate completely. We construct a radiative-hydrodynamic model of
atmospheric escape from strongly irradiated, low-mass rocky planets, accounting
for dust-gas energy exchange in the wind. Rocky planets with masses < 0.1
M_Earth (less than twice the mass of Mercury) and surface temperatures > 2000 K
are found to disintegrate entirely in < 10 Gyr. When our model is applied to
Kepler planet candidate KIC 12557548b --- which is believed to be a rocky body
evaporating at a rate of dM/dt > 0.1 M_Earth/Gyr --- our model yields a
present-day planet mass of < 0.02 M_Earth or less than about twice the mass of
the Moon. Mass loss rates depend so strongly on planet mass that bodies can
reside on close-in orbits for Gyrs with initial masses comparable to or less
than that of Mercury, before entering a final short-lived phase of catastrophic
mass loss (which KIC 12557548b has entered). Because this catastrophic stage
lasts only up to a few percent of the planet's life, we estimate that for every
object like KIC 12557548b, there should be 10--100 close-in quiescent
progenitors with sub-day periods whose hard-surface transits may be detectable
by Kepler --- if the progenitors are as large as their maximal, Mercury-like
sizes (alternatively, the progenitors could be smaller and more numerous).
According to our calculations, KIC 12557548b may have lost ~70% of its
formation mass; today we may be observing its naked iron core.Comment: Accepted to MNRAS with minor edits compared to version
Atmospheric Heat Redistribution on Hot Jupiters
Infrared lightcurves of transiting hot Jupiters present a trend in which the
atmospheres of the hottest planets are less efficient at redistributing the
stellar energy absorbed on their daysides---and thus have a larger day-night
temperature contrast---than colder planets. No predictive atmospheric model has
been published that identifies which dynamical mechanisms determine the
atmospheric heat redistribution efficiency on tidally locked exoplanets. Here
we present a two-layer shallow water model of the atmospheric dynamics on
synchronously rotating planets that explains the observed trend. Our model
shows that planets with weak friction and weak irradiation exhibit a banded
zonal flow with minimal day-night temperature differences, while models with
strong irradiation and/or strong friction exhibit a day-night flow pattern with
order-unity fractional day-night temperature differences. To interpret the
model, we develop a scaling theory that shows that the timescale for gravity
waves to propagate horizontally over planetary scales, t_wave, plays a dominant
role in controlling the transition from small to large temperature contrasts.
This implies that heat redistribution is governed by a wave-like process,
similar to the one responsible for the weak temperature gradients in the
Earth's tropics. When atmospheric drag can be neglected, the transition from
small to large day-night temperature contrasts occurs when t_wave ~
sqrt(t_rad/Omega), where t_rad is the radiative relaxation time and Omega is
the planetary rotation frequency. Alternatively, this transition criterion can
be expressed as t_rad ~ t_vert, where t_vert is the timescale for a fluid
parcel to move vertically over the difference in day-night thickness. These
results subsume the commonly used timescale comparison for estimating heat
redistribution efficiency between t_rad and the global horizontal advection
timescale, t_adv.Comment: Accepted to ApJ with minor edits compared to version 1; 17 pages, 11
figure
Objective analysis of toric intraocular lens rotation and centration
PURPOSE: To assess the repeatability of an objective image analysis technique to determine intraocular lens (IOL) rotation and centration. SETTING: Six ophthalmology clinics across Europe. METHODS: One-hundred seven patients implanted with Akreos AO aspheric IOLs with orientation marks were imaged. Image quality was rated by a masked observer. The axis of rotation was determined from a line bisecting the IOL orientation marks. This was normalized for rotation of the eye between visits using the axis bisecting 2 consistent conjunctival vessels or iris features. The center of ovals overlaid to circumscribe the IOL optic edge and the pupil or limbus were compared to determine IOL centration. Intrasession repeatability was assessed in 40 eyes and the variability of repeated analysis examined. RESULTS: Intrasession rotational stability of the IOL was ±0.79 degrees (SD) and centration was ±0.10 mm horizontally and ±0.10 mm vertically. Repeated analysis variability of the same image was ±0.70 degrees for rotation and ±0.20 mm horizontally and ±0.31 mm vertically for centration. Eye rotation (absolute) between visits was 2.23 ± 1.84 degrees (10%>5 degrees rotation) using one set of consistent conjunctival vessels or iris features and 2.03 ± 1.66 degrees (7%>5 degrees rotation) using the average of 2 sets (P =.13). Poorer image quality resulted in larger apparent absolute IOL rotation (r =-0.45,P<.001). CONCLUSIONS: Objective analysis of digital retroillumination images allows sensitive assessment of IOL rotation and centration stability. Eye rotation between images can lead to significant errors if not taken into account. Image quality is important to analysis accuracy
Reinforcement Learning for Wind Turbine Load Control: How AI can drive tomorrow‘s wind turbines
Load control strategies for wind turbines are used to reduce the structural wear of the turbine without reducing energy yield. Until now, these control strategies are almost exclusively built up-on linear approaches like PID and model-based controllers due to their robustness. However, advances in turbine size and capabilities create a need for more complex control strategies that can effectively address design challenges in modern turbines.
This work presents WINDL, a load control policy based on a neural network, which is trained through model-free Reinforcement Learning (RL) on a simulated wind turbine. While RL has achieved great success in the past on games and simple simulation benchmarks, applications to more complex control problems are starting to emerge just recently.
We show that through the usage of regularization techniques and signal transformations, such an application to the field of wind turbine load control is possible. Using a smoothness regularizer, we incentivize the highly non-linear neural network policy to output control actions that are safe to apply to a wind turbine.
The Coleman transformation, a common tool for the design of traditional PID-based load control strategies, is used to project signals into a stationary coordinate space, increasing robustness and final policy performance.
Trained to control a large offshore turbine in a model-free fashion, WINDL finds a control policy that outperforms a state-of-the-art controller based on the IPC strategy with respect to the prima-ry optimization goal blade loads. Using the DEL metric, we measure 54.1% lower blade loads in the steady wind and 13.45% lower blade loads in the turbulent wind scenario.
While such levels of blade reduction come with slightly worse performance on secondary optimi-zation goals like pitch wear and power production, we demonstrate the ability to control the trade-off between different optimization goals on the example of pitch versus blade loads. To comple-ment our findings, we perform a qualitative analysis of the policy behavior and learning process.
We believe our work to be the first application of RL to wind turbine load control that exceeds baseline performance in the primary optimization metric, opening up the possibility of including specialized load controllers for targeting critical design driving scenarios of modern large wind turbines.:Problem
Method
Aim
Results
Conclusio
Engaging stakeholder communities as body image intervention partners: The Body Project as a case example
© 2015 Elsevier Ltd Despite recent advances in developing evidence-based psychological interventions, substantial changes are needed in the current system of intervention delivery to impact mental health on a global scale (Kazdin & Blase, 2011). Prevention offers one avenue for reaching large populations because prevention interventions often are amenable to scaling-up strategies, such as task-shifting to lay providers, which further facilitate community stakeholder partnerships. This paper discusses the dissemination and implementation of the Body Project, an evidence-based body image prevention program, across 6 diverse stakeholder partnerships that span academic, non-profit and business sectors at national and international levels. The paper details key elements of the Body Project that facilitated partnership development, dissemination and implementation, including use of community-based participatory research methods and a blended train-the-trainer and task-shifting approach. We observed consistent themes across partnerships, including: sharing decision making with community partners, engaging of community leaders as gatekeepers, emphasizing strengths of community partners, working within the community's structure, optimizing non-traditional and/or private financial resources, placing value on cost-effectiveness and sustainability, marketing the program, and supporting flexibility and creativity in developing strategies for evolution within the community and in research. Ideally, lessons learned with the Body Project can be generalized to implementation of other body image and eating disorder prevention programs
Decision Making in Supply Chains with Waste Considerations
As global population and income levels have increased, so has the waste generated as a byproduct of our production and consumption processes. Approximately two billion tons of municipal solid waste are generated globally every year – that is, more than half a kilogram per person each day. This waste, which is generated at various stages of the supply chain, has negative environmental effects and often represents an inefficient use or allocation of limited resources.
With the growing concern about waste, many governments are implementing regulations to reduce waste. Waste is a often consequence of the inventory decisions of different players in a supply chain. As such, these regulations aim to reduce waste by influencing inventory decisions. However, determining the inventory decisions of players in a supply chain is not trivial. Modern supply chains often consist of numerous players, who may each differ in their objectives and in the factors they consider when making decisions such as how much product to buy and when. While each player
makes unilateral inventory decisions, these decisions may also affect the decisions of other players. This complexity makes it difficult to predict how a policy will affect profit and waste outcomes for individual players and the supply chain as a whole.
This dissertation studies the inventory decisions of players in a supply chain when faced with policy interventions to reduce waste. In particular, the focus is on food supply chains, where food waste and packaging waste are the largest waste components.
Chapter 2 studies a two-period inventory game between a seller (e.g., a wholesaler) and a buyer (e.g., a retailer) in a supply chain for a perishable food product with uncertain demand from a downstream market. The buyer can differ in whether he considers factors affecting future periods or the seller’s supply availability in his period purchase decisions – that is, in his degree of strategic behavior. The focus is on understanding how the buyer’s degree of strategic behavior affects inventory outcomes. Chapter 3 builds on this understanding by investigating waste outcomes and how policies that penalize waste affect individual and supply chain profits and waste.
Chapter 4 studies the setting of a restaurant that uses reusable containers instead of single-use ones to serve its delivery and take-away orders. With policy-makers discouraging the use of single-use containers through surcharges or bans, reusable containers have emerged as an alternative. Managing inventories of reusable containers is challenging for a restaurant as both demand and returns of containers are uncertain and the restaurant faces various customers types. This chapter investigates how the proportion of each customer type affects the restaurant’s inventory decisions and costs
Surface Layer Accretion in Conventional and Transitional Disks Driven by Far-Ultraviolet Ionization
Whether protoplanetary disks accrete at observationally significant rates by
the magnetorotational instability (MRI) depends on how well ionized they are.
Disk surface layers ionized by stellar X-rays are susceptible to charge
neutralization by small condensates, ranging from ~0.01-micron-sized grains to
angstrom-sized polycyclic aromatic hydrocarbons (PAHs). Ion densities in
X-ray-irradiated surfaces are so low that ambipolar diffusion weakens the MRI.
Here we show that ionization by stellar far-ultraviolet (FUV) radiation enables
full-blown MRI turbulence in disk surface layers. Far-UV ionization of atomic
carbon and sulfur produces a plasma so dense that it is immune to ion
recombination on grains and PAHs. The FUV-ionized layer, of thickness 0.01--0.1
g/cm^2, behaves in the ideal magnetohydrodynamic limit and can accrete at
observationally significant rates at radii > 1--10 AU. Surface layer accretion
driven by FUV ionization can reproduce the trend of increasing accretion rate
with increasing hole size seen in transitional disks. At radii < 1--10 AU,
FUV-ionized surface layers cannot sustain the accretion rates generated at
larger distance, and unless turbulent mixing of plasma can thicken the
MRI-active layer, an additional means of transport is needed. In the case of
transitional disks, it could be provided by planets.Comment: Final proofed version. Corrects X-ray-driven accretion rates in the
high PAH case for Figures 8 and
Accretion through the inner hole of transitional disks: What happens to the dust?
We study the effect of radiation pressure on the dust in the inner rim of
transitional disks with large inner holes. In particular, we evaluate whether
radiation pressure can be responsible for keeping the inner holes dust-free,
while allowing gas accretion to proceed. This has been proposed in a paper by
Chiang and Murray-Clay (2007, Nature Physics 3, p. 604) who explain the
formation of these holes as an inside-out evacuation due to X- ray-triggered
accretion of the innermost layer of the disk rim outside of the hole. We show
that radiation pressure is clearly incapable of stopping dust from flowing into
the hole because of dust pile-up and optical depth effects, and also because of
viscous mixing. Other mechanisms need to be found to explain the persistence of
the opacity hole in the presence of accretion, and we speculate on possible
solutions.Comment: 6 pages, 3 figures, Accepted for publication by Astronomy and
Astrophysic
Is the Blade Element Momentum theory overestimating wind turbine loads? – An aeroelastic comparison between OpenFAST's AeroDyn and QBlade's Lifting-Line Free Vortex Wake method
Abstract. Load calculations play a key role in determining the design loads of different wind turbine components. To obtain the aerodynamic loads for these calculations, the industry relies heavily on the Blade Element Momentum (BEM) theory. BEM methods use several engineering correction models to capture the aerodynamic phenomena present in Design Load Cases (DLCs) with turbulent wind. Because of this, BEM methods can overestimate aerodynamic loads under challenging conditions when compared to higher-order aerodynamic methods – such as the Lifting-Line Free Vortex Wake (LLFVW) method – leading to unnecessarily high design loads and component costs. In this paper, we give a quantitative answer to the question of load overestimation of a particular BEM implementation by comparing the results of aeroelastic load calculations done with the BEM-based OpenFAST code and the QBlade code, which uses a particular implementation of the LLFVW method. We compare extreme and fatigue load
predictions from both codes using sixty-six 10 min load simulations of the Danish Technical University (DTU) 10 MW Reference Wind Turbine according to the IEC 61400-1 power production DLC group. Results from both codes show differences in fatigue and extreme load estimations for the considered sensors of the turbine. LLFVW simulations predict 9 % lower lifetime damage equivalent loads (DELs) for the out-of-plane blade root and the tower base fore–aft bending moments
compared to BEM simulations. The results also show that lifetime DELs for the yaw-bearing tilt and yaw moments are 3 % and 4 % lower when calculated with the LLFVW code. An ultimate state analysis shows that extreme loads of the blade root out-of-plane bending moment predicted by the LLFVW simulations are 3 % lower than the moments predicted by BEM simulations. For the maximum tower base fore–aft bending moment, the LLFVW simulations predict an increase of 2 %. Further analysis reveals that there are two main contributors to these load differences. The first is the different way both codes treat the effect of the nonuniform wind field on the local blade aerodynamics. The second is the higher average aerodynamic torque in the LLFVW simulations. It influences the transition between operating modes of the controller and changes the aeroelastic behavior of the turbine, thus affecting the loads
Engaging Stakeholder Communities as Body Image Intervention Partners: The Body Project as a Case Example
Despite recent advances in developing evidence-based psychological interventions, substantial changes are needed in the current system of intervention delivery to impact mental health on a global scale (Kazdin & Blase, 2011). Prevention offers one avenue for reaching large populations because prevention interventions often are amenable to scaling-up strategies, such as task-shifting to lay providers, which further facilitate community stakeholder partnerships. This paper discusses the dissemination and implementation of the Body Project, an evidence-based body image prevention program, across 6 diverse stakeholder partnerships that span academic, non-profit and business sectors at national and international levels. The paper details key elements of the Body Project that facilitated partnership development, dissemination and implementation, including use of community-based participatory research methods and a blended train-the-trainer and task-shifting approach. We observed consistent themes across partnerships, including: sharing decision making with community partners, engaging of community leaders as gatekeepers, emphasizing strengths of community partners, working within the community\u27s structure, optimizing non-traditional and/or private financial resources, placing value on cost-effectiveness and sustainability, marketing the program, and supporting flexibility and creativity in developing strategies for evolution within the community and in research. Ideally, lessons learned with the Body Project can be generalized to implementation of other body image and eating disorder prevention programs
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