7,648 research outputs found
Universal Scaling Laws for Dense Particle Suspensions in Turbulent Wall-Bounded Flows
The macroscopic behavior of dense suspensions of neutrally-buoyant spheres in
turbulent plane channel flow is examined. We show that particles larger than
the smallest turbulence scales cause the suspension to deviate from the
continuum limit in which its dynamics is well described by an effective
suspension viscosity. This deviation is caused by the formation of a particle
layer close to the wall with significant slip velocity. By assuming two
distinct transport mechanisms in the near-wall layer and the turbulence in the
bulk, we define an effective wall location such that the flow in the bulk can
still be accurately described by an effective suspension viscosity. We thus
propose scaling laws for the mean velocity profile of the suspension flow,
together with a master equation able to predict the increase in drag as
function of the particle size and volume fraction.Comment: Accepted for publication in PRL. Supplemental material include
Effects of the finite particle size in turbulent wall-bounded flows of dense suspensions
We use interface-resolved simulations to study finite-size effects in
turbulent channel flow of neutrally-buoyant spheres. Two cases with particle
sizes differing by a factor of 2, at the same solid volume fraction of 20% and
bulk Reynolds number are considered. These are complemented with two reference
single-phase flows: the unladen case, and the flow of a Newtonian fluid with
the effective suspension viscosity of the same mixture in the laminar regime.
As recently highlighted in Costa et al. (PRL 117, 134501), a particle-wall
layer is responsible for deviations of the statistics from what is observed in
the continuum limit where the suspension is modeled as a Newtonian fluid with
an effective viscosity. Here we investigate the fluid and particle dynamics in
this layer and in the bulk. In the particle-wall layer, the near wall
inhomogeneity has an influence on the suspension micro-structure over a
distance proportional to the particle size. In this layer, particles have a
significant (apparent) slip velocity that is reflected in the distribution of
wall shear stresses. This is characterized by extreme events (both much higher
and much lower than the mean). Based on these observations we provide a scaling
for the particle-to-fluid apparent slip velocity as a function of the flow
parameters. We also extend the flow scaling laws in to second-order Eulerian
statistics in the homogeneous suspension region away from the wall. Finite-size
effects in the bulk of the channel become important for larger particles, while
negligible for lower-order statistics and smaller particles. Finally, we study
the particle dynamics along the wall-normal direction. Our results suggest that
1-point dispersion is dominated by particle-turbulence (and not
particle-particle) interactions, while differences in 2-point dispersion and
collisional dynamics are consistent with a picture of shear-driven
interactions
Ethereum blockchain as a decentralized and autonomous key server: storing and extracting public keys through smart contracts
Ethereum is an open-source, public, blockchain-based distributed computing platform featuring smart contract functionality. It provides a decentralized Turing-complete virtual machine which can execute scripts using an international network of public nodes.
The purpose of this thesis is to build a decentralized and autonomous key server using Ethereum smart contracts to store and retrieve information. We did an overall introduction of Bitcoin and Ethereum to provide a background of the study. We then analyzed the current problems of key discovery with traditional servers and web-of-trust. We designed, built and tested an application that can verify contact cards (email address, PGP public key, domain address, Facebook account), link them to an Ethereum address and store them on a public contract running on the Ethereum blockchain. Finally we made an analysis of the costs and limitations of such solution and proposed some future improvements. The results show that Ethereum is a good choice for storing public keys, thanks to the immutability and irreversibility of the blockchain
An Inkjet Printed Chipless RFID Sensor for Wireless Humidity Monitoring
A novel chipless RFID humidity sensor based on a finite Artificial Impedance
Surface (AIS) is presented. The unit cell of the AIS is composed of three
concentric loops thus obtaining three deep and high Q nulls in the
electromagnetic response of the tag. The wireless sensor is fabricated using
low-cost inkjet printing technology on a thin sheet of commercial coated paper.
The patterned surface is placed on a metal backed cardboard layer. The relative
humidity information is encoded in the frequency shift of the resonance peaks.
Varying the relative humidity level from 50% to 90%, the frequency shift has
proven to be up to 270MHz. The position of the resonance peaks has been
correlated to the relative humidity level of the environment on the basis of a
high number of measurements performed in a climatic chamber, specifically
designed for RF measurements of the sensor. A very low error probability of the
proposed sensor is demonstrated when the device is used with a 10% RH humidity
level discrimination
Low Rank Vector Bundles on the Grassmannian G(1,4)
Here we define the concept of -regularity for coherent sheaves on the
Grassmannian G(1,4) as a generalization of Castelnuovo-Mumford regularity on
. In this setting we prove analogs of some classical properties. We
use our notion of -regularity in order to prove a splitting criterion for
rank 2 vector bundles with only a finite number of vanishing conditions. In the
second part we give the classification of rank 2 and rank 3 vector bundles
without "inner" cohomology (i.e. H^i_*(E)=H^i(E\otimes\Q)=0 for any
) on G(1,4) by studying the associated monads.Comment: 11 pages, no figure
Structure and phase behavior of colloidal dumbbells with tunable attractive interactions
We investigate thermodynamic and structural properties of colloidal dumbbells
in the framework provided by the Reference Interaction Site Model (RISM) theory
of molecular fluids and Monte Carlo simulations. We consider two different
models: in the first one we set identical square-well attractions on the two
tangent spheres composing the molecule (SW-SW model); in the second scheme, one
of square-well interactions is switched off (HS-SW model). Appreciable
differences emerge between the physical properties of the two models.
Specifically, the behavior of SW-SW structure factors points
to the presence of a gas-liquid coexistence, as confirmed by subsequent fluid
phase equilibria calculations. Conversely, the HS-SW develops a low-
peak, signaling the presence of aggregates; such a process destabilizes the
gas-liquid phase separation, promoting at low temperatures the formation of a
cluster phase, whose structure depends on the system density. We further
investigate such differences by studying the phase behavior of a series of
intermediate models, obtained from the original SW-SW by progressively reducing
the depth of one square-well interaction. RISM structural predictions
positively reproduce the simulation data, including the rise of ) in
the SW-SW model and the low- peak in the HS-SW structure factor. As for the
phase behavior, RISM agrees with Monte Carlo simulations in predicting a
gas-liquid coexistence for the SW-SW model (though the critical parameters
appears overestimated by the theory) and its progressive disappearance moving
toward the HS-SW model.Comment: 12 pages, 13 figures, 1 table, 78 reference
Novel directions for the management of dual antiplatelet therapy in patients with coronary artery disease
Dual antiplatelet therapy has been extensively studied in the last two decades, giving opportunities for thorough evidence-based recommendations. The ESC/EACTS guidelines on DAPT in CAD provide guidance for DAPT type and duration, maintaining a patient-centred focus. Patient-specific and dynamic evaluation of the ischaemic and bleeding risks is key for optimal treatment decisions
Excava(c)tion in Vignale. Archaeology on stage, archaeology on the Web
As an orchestra or a rock star, archaeologists have their audience too. This paper wants to highlight an integrated approach between fieldwork, its account and its dissemination to the public in different ways, including social media. This potential integration has come to life in the 2011 excavation of the Roman mansio of Vignale (Italy) and it has been named “Excava(c)tion”. It doesn’t mean a new way of digging but another way of approaching the excavation, an approach integrated toward and with the public, both on site and on the social Web. “Excava(c)tion” conceives the site as a stage and digging as a performance, through a continuous dialogue between archaeologists and the public. Archaeologists share their work in the form of guided tours (live, theatrical-like performances), communicative diaries and videos (edited, motion-picture performances) and on a blog (www.uominiecoseavignale.it). They receive back comments and oral accounts from the local community about the main themes of common interest. “Excava(c)tion” means engagement both of archaeologists and the public in the pursuit of a global multivocality during archaeological excavation
On the Bike Spreading Problem
A free-floating bike-sharing system (FFBSS) is a dockless rental system where an individual can borrow a bike and returns it anywhere, within the service area. To improve the rental service, available bikes should be distributed over the entire service area: a customer leaving from any position is then more likely to find a near bike and then to use the service. Moreover, spreading bikes among the entire service area increases urban spatial equity since the benefits of FFBSS are not a prerogative of just a few zones. For guaranteeing such distribution, the FFBSS operator can use vans to manually relocate bikes, but it incurs high economic and environmental costs. We propose a novel approach that exploits the existing bike flows generated by customers to distribute bikes. More specifically, by envisioning the problem as an Influence Maximization problem, we show that it is possible to position batches of bikes on a small number of zones, and then the daily use of FFBSS will efficiently spread these bikes on a large area. We show that detecting these zones is NP-complete, but there exists a simple and efficient 1-1/e approximation algorithm; our approach is then evaluated on a dataset of rides from the free-floating bike-sharing system of the city of Padova
Freeze-in at stronger coupling
Predictivity of many non-thermal dark matter (DM) models is marred by the
gravitational production background. This problem is ameliorated in models with
lower reheating temperature , which allows for dilution of gravitationally
produced relics. We study the freeze-in dark matter production mechanism in the
thermal bath with the electroweak scale temperature. The process is
Boltzmann-suppressed if the dark matter mass is above . In this case, the
coupling to the thermal bath has to be significant to account for the observed
dark matter relic density. As a result, the direct DM detection experiments
already probe such freeze-in models, excluding significant parts of parameter
space. The forthcoming experiments will explore this framework further,
extending to lower couplings and higher reheating temperatures.Comment: 10 page
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