2,023 research outputs found
Dense bubble flow in a silo: an unusual flow of a dispersed medium
The dense flow of air bubbles in a two-dimensional silo (through an aperture
D) filled with a liquid is studied experimentally. A particle tracking
technique has been used to bring out the main properties of the flow:
displacements of the bubbles, transverse and axial velocities. The behavior of
the air bubbles is observed to present similarities with non-deformable solid
grains in a granular flow. Nevertheless, a correlation between the bubble
velocities and their deformations has been evidenced. Moreover, a new discharge
law (Beverloo-like) must be considered for such a system, where the flow rate
is observed to vary as D^{1/2} and depends on the deformability of the
particles.Comment: 5 pages, 6 figure
Bidirected minimum Manhattan network problem
In the bidirected minimum Manhattan network problem, given a set T of n
terminals in the plane, we need to construct a network N(T) of minimum total
length with the property that the edges of N(T) are axis-parallel and oriented
in a such a way that every ordered pair of terminals is connected in N(T) by a
directed Manhattan path. In this paper, we present a polynomial factor 2
approximation algorithm for the bidirected minimum Manhattan network problem.Comment: 14 pages, 16 figure
Certifying cost annotations in compilers
We discuss the problem of building a compiler which can lift in a provably
correct way pieces of information on the execution cost of the object code to
cost annotations on the source code. To this end, we need a clear and flexible
picture of: (i) the meaning of cost annotations, (ii) the method to prove them
sound and precise, and (iii) the way such proofs can be composed. We propose a
so-called labelling approach to these three questions. As a first step, we
examine its application to a toy compiler. This formal study suggests that the
labelling approach has good compositionality and scalability properties. In
order to provide further evidence for this claim, we report our successful
experience in implementing and testing the labelling approach on top of a
prototype compiler written in OCAML for (a large fragment of) the C language
The influence of planetesimal fragmentation on planet formation
Context. The effects of planetesimal fragmentation on planet formation has
been studied by various models on single embryos therefore neglecting
concurrent effects mostly in the outer disk. They show that planetesimal
fragmentation can both hinder or aid planet formation due to the introduction
of competing effects, namely speeding up accretion and depleting the feeding
zone of forming planets. Aims. We investigate the influence of the collisional
fragmentation of planetesimals on the planet formation process using a
population synthesis approach. We aim to investigate its effects for a large
set of initial conditions and also explore the consequences on the formation of
multiple embryos in the same disk. Methods. We run global planet formation
simulations including fragmentation, drift and an improved ice line
description. To do this we use a fragmentation model in our code. The initial
conditions for the simulations that are informed by observations are varied to
generate synthetic exoplanet populations. Results. Our synthetic populations
show that depending on the typical size of solids generated in collisions,
fragmentation in tandem with the radial drift can either enhance or hinder
planet formation. For larger fragments we see increased accretion throughout
the populations especially beyond the ice line. However, the shorter drift
timescale of smaller fragments, due to their stronger coupling to the gas, can
hinder the formation process. Furthermore, beyond the ice line fragmentation
promotes late growth when the damping by gas drag fades Conclusions.
Fragmentation significantly affects the planet formation process in various
ways for all types of planet and warrants further investigation.Comment: Accepted for publication in A&A, 22 pages, 16 figure
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