238 research outputs found
Absolute linear instability in laminar and turbulent gas/liquid two-layer channel flow
We study two-phase stratified flow where the bottom layer is a thin laminar
liquid and the upper layer is a fully-developed gas flow. The gas flow can be
laminar or turbulent. To determine the boundary between convective and absolute
instability, we use Orr--Sommerfeld stability theory, and a combination of
linear modal analysis and ray analysis. For turbulent gas flow, and for the
density ratio r=1000, we find large regions of parameter space that produce
absolute instability. These parameter regimes involve viscosity ratios of
direct relevance to oil/gas flows. If, instead, the gas layer is laminar,
absolute instability persists for the density ratio r=1000, although the
convective/absolute stability boundary occurs at a viscosity ratio that is an
order of magnitude smaller than in the turbulent case. Two further unstable
temporal modes exist in both the laminar and the turbulent cases, one of which
can exclude absolute instability. We compare our results with an
experimentally-determined flow-regime map, and discuss the potential
application of the present method to non-linear analyses.Comment: 33 pages, 20 figure
Inertial coalescence of droplets on a partially wetting substrate
We consider the growth rate of the height of the connecting bridge in rapid surface-tension-driven coalescence of two identical droplets attached on a partially wetting substrate. For a wide range of contact angle values, the height of the bridge grows with time following a power law with a universal exponent of 2/3, up to a threshold time, beyond which a 1/2 exponent results, that is known for coalescence of freely-suspended droplets. In a narrow range of contact angle values close to 90°, this threshold time rapidly vanishes and a 1/2 exponent results for a 90° contact angle. The argument is confirmed by three-dimensional numerical simulations based on a diffuse interface method with adaptive mesh refinement and a volume-of-fluid method
Spontaneous Charging and Crystallization of Water Droplets in Oil
We study the spontaneous charging and the crystallization of spherical
micron-sized water-droplets dispersed in oil by numerically solving, within a
Poisson-Boltzmann theory in the geometry of a spherical cell, for the density
profiles of the cations and anions in the system. We take into account
screening, ionic Born self-energy differences between oil and water, and
partitioning of ions over the two media. We find that the surface charge
density of the droplet as induced by the ion partitioning is significantly
affected by the droplet curvature and by the finite density of the droplets. We
also find that the salt concentration and the dielectric constant regime in
which crystallization of the water droplets is predicted is enhanced
substantially compared to results based on the planar oil-water interface,
thereby improving quantitative agreement with recent experiments.Comment: 10 pages, 7 figures, submitted for publicatio
Stability of additive-free water-in-oil emulsions
We calculate ion distributions near a planar oil-water interface within
non-linear Poisson-Boltzmann theory, taking into account the Born self-energy
of the ions in the two media. For unequal self-energies of cations and anions,
a spontaneous charge separation is found such that the water and oil phase
become oppositely charged, in slabs with a typical thickness of the Debye
screening length in the two media. From the analytical solutions, the
corresponding interfacial charge density and the contribution to the
interfacial tension is derived, together with an estimate for the
Yukawa-potential between two spherical water droplets in oil. The parameter
regime is explored where the plasma coupling parameter exceeds the
crystallization threshold, i.e. where the droplets are expected to form
crystalline structures due to a strong Yukawa repulsion, as recently observed
experimentally. Extensions of the theory that we discuss briefly include
numerical calculations on spherical water droplets in oil, and analytical
calculations of the linear PB-equation for a finite oil-water interfacial
width.Comment: 9 pages, 4 figures, accepted by JPCM for proceedings of LMC
The LHCb Outer Tracker Front End Electronics
This note provides an overview of the front-end electronics used to readout the drift-times of the LHCb Outer Tracker straw tube chambers. The main functional components of the readout are the ASDBLR ASIC for amplification and signal digitization, the OTIS ASIC for the time measurement and for the L0 buffering, and the GOL ASIC to serialize the digital data for the optical data transmission. The L1 buffer board used to receive the data which is sent via the optical link is a common LHCb development and is not described here. This note supersedes an earlier document [1]
Habitat use of urban-nesting lesser black-backed gulls during the breeding season
Increasing urbanisation is detrimental for some animal species and potentially advantageous for others. Urban-nesting populations of gulls have undergone rapid population increases worldwide, which has resulted in an increase in human-gull conflicts. In order to inform management and conservation decisions in relation to these populations, more information is needed about the behaviour of these birds in urban settings and how they utilise their environment. This study combined Global Positioning System (GPS) tracking data of 12 urban-nesting lesser black-backed gulls, Larus fuscus, with habitat and behaviour data over three breeding seasons (2016–2018). Despite the proximity of marine areas (~10 km), the birds only made significant use of terrestrial environments, spending two-thirds of their time away from the nest in suburban and urban areas, and one-third in rural green areas. The gulls utilised suburban and urban areas more as their chicks grew and appeared to use diverse foraging strategies to suit different habitats. These results indicate that the range of potential foraging areas available needs to be considered in management decisions and that urban bird populations may not use the resources they are expected to
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