974 research outputs found
Characteristics of the wavelength of ripples on icicles
It is known that the wavelength of the ripples on icicles in nature is of
centimeter-scale. Such study on morphological instability of ice-water
interface during ice growth from flowing supercooled water film with one side
being a free surface has recently been made [K. Ueno, Phys. Rev. E 68, 021603
(2003)]. This is a first theoretical study taking into account the influence of
the shape of the water-air surface on the growth condition of infinitesimal
disturbances of the ice-water interface. A simpler formula to determine the
wavelength of the ripples than that in the previous paper is derived. It seems
that the wavelength of ripples is insensitive to the water supply rates,
diameters of the icicles and surrounding air temperatures. The details of
dependence of the wavelengh of ripples on these parameters are investigated.Comment: 15 pages, 6 figure
Morphological instability of the solid-liquid interface in crystal growth under supercooled liquid film flow and natural convection airflow
Ring-like ripples on the surface of icicles are an example of morphological
instability of the ice-water interface during ice growth under supercooled
water film flow. The surface of icicles is typically covered with ripples of
about 1 cm in wavelength, and the wavelength appears to be almost independent
of external temperature, icicle radius, and volumetric water flow rate. One
side of the water layer consists of the water-air surface and growing ice is
the other. This is one of the more complicated moving phase boundary problems
with two interfaces. A recent theoretical work [K. Ueno, Phys. Rev. E 68,
(2003) 021603] to address the underlying instability that produces ripples is
based on the assumption of the absence of airflow around icicles. In this
paper, we extend the previous theoretical framework to include a natural
convection airflow ahead of the water-air surface and consider whether the
effect of natural convection airflow on the wavelength of ripples produced on
an ice surface is essential or not.Comment: 19 pages, 5 figure
Social media adoption in higher education: a case study involving IT/IS students
This paper discusses the adoption and use of social media in Higher Education (HE). The aim of the research reported in this paper was to identify the main factors and problem areas in the adoption and use of social media in HE. Our study included a survey involving students of Information Technology and Information Systems in Greece and in Finland. In order to verify the findings from the survey, a follow-up survey was also undertaken. The unified technology adoption approach was identified to be a suitable underlying theory for this study. The analysis of viewpoints of students was needed in order to understand converging and diverging viewpoints. The results showed that infrastructure is the most important issue in the planning of learning/teaching activities based on social media, followed by the role of social influence. Based on the analysis, guidelines for planning social-media-based learning activities are proposed. Indications of further work complete the paper
Energetics of positron states trapped at vacancies in solids
We report a computational first-principles study of positron trapping at
vacancy defects in metals and semiconductors. The main emphasis is on the
energetics of the trapping process including the interplay between the positron
state and the defect's ionic structure and on the ensuing annihilation
characteristics of the trapped state. For vacancies in covalent semiconductors
the ion relaxation is a crucial part of the positron trapping process enabling
the localization of the positron state. However, positron trapping does not
strongly affect the characteristic features of the electronic structure, e.g.,
the ionization levels change only moderately. Also in the case of metal
vacancies the positron-induced ion relaxation has a noticeable effect on the
calculated positron lifetime and momentum distribution of annihilating
electron-positron pairs.Comment: Submitted to Physical Review B on 17 April 2007. Revised version
submitted on 6 July 200
Modeling the momentum distributions of annihilating electron-positron pairs in solids
Measuring the Doppler broadening of the positron annihilation radiation or
the angular correlation between the two annihilation gamma quanta reflects the
momentum distribution of electrons seen by positrons in the
material.Vacancy-type defects in solids localize positrons and the measured
spectra are sensitive to the detailed chemical and geometric environments of
the defects. However, the measured information is indirect and when using it in
defect identification comparisons with theoretically predicted spectra is
indispensable. In this article we present a computational scheme for
calculating momentum distributions of electron-positron pairs annihilating in
solids. Valence electron states and their interaction with ion cores are
described using the all-electron projector augmented-wave method, and atomic
orbitals are used to describe the core states. We apply our numerical scheme to
selected systems and compare three different enhancement (electron-positron
correlation) schemes previously used in the calculation of momentum
distributions of annihilating electron-positron pairs within the
density-functional theory. We show that the use of a state-dependent
enhancement scheme leads to better results than a position-dependent
enhancement factor in the case of ratios of Doppler spectra between different
systems. Further, we demonstrate the applicability of our scheme for studying
vacancy-type defects in metals and semiconductors. Especially we study the
effect of forces due to a positron localized at a vacancy-type defect on the
ionic relaxations.Comment: Submitted to Physical Review B on September 1 2005. Revised
manuscript submitted on November 14 200
Numerical and experimental verification of a theoretical model of ripple formation in ice growth under supercooled water film flow
Little is known about morphological instability of a solidification front
during the crystal growth of a thin film of flowing supercooled liquid with a
free surface: for example, the ring-like ripples on the surface of icicles. The
length scale of the ripples is nearly 1 cm. Two theoretical models for the
ripple formation mechanism have been proposed. However, these models lead to
quite different results because of differences in the boundary conditions at
the solid-liquid interface and liquid-air surface. The validity of the
assumption used in the two models is numerically investigated and some of the
theoretical predictions are compared with experiments.Comment: 30 pages, 9 figure
BVOC-aerosol-climate feedbacks investigated using NorESM
Both higher temperatures and increased CO2 concentrations are (separately) expected to increase the emissions of biogenic volatile organic compounds (BVOCs). This has been proposed to initiate negative climate feedback mechanisms through increased formation of secondary organic aerosol (SOA). More SOA can make the clouds more reflective, which can provide a cooling. Furthermore, the increase in SOA formation has also been proposed to lead to increased aerosol scattering, resulting in an increase in diffuse radiation. This could boost gross primary production (GPP) and further increase BVOC emissions. In this study, we have used the Norwegian Earth System Model (NorESM) to investigate both these feedback mechanisms. Three sets of experiments were set up to quantify the feedback with respect to (1) doubling the CO2, (2) increasing temperatures corresponding to a doubling of CO2 and (3) the combined effect of both doubling CO2 and a warmer climate. For each of these experiments, we ran two simulations, with identical setups, except for the BVOC emissions. One simulation was run with interactive BVOC emissions, allowing the BVOC emissions to respond to changes in CO2 and/or climate. In the other simulation, the BVOC emissions were fixed at present-day conditions, essentially turning the feedback off. The comparison of these two simulations enables us to investigate each step along the feedback as well as estimate their overall relevance for the future climate. We find that the BVOC feedback can have a significant impact on the climate. The annual global BVOC emissions are up to 63 % higher when the feedback is turned on compared to when the feedback is turned off, with the largest response when both CO2 and climate are changed. The higher BVOC levels lead to the formation of more SOA mass (max 53 %) and result in more particles through increased new particle formation as well as larger particles through increased condensation. The corresponding changes in the cloud properties lead to a -0.43 W m(-2) stronger net cloud forcing. This effect becomes about 50 % stronger when the model is run with reduced anthropogenic aerosol emissions, indicating that the feedback will become even more important as we decrease aerosol and precursor emissions. We do not find a boost in GPP due to increased aerosol scattering on a global scale. Instead, the fate of the GPP seems to be controlled by the BVOC effects on the clouds. However, the higher aerosol scattering associated with the higher BVOC emissions is found to also contribute with a potentially important enhanced negative direct forcing (-0.06 W m(-2)). The global total aerosol forcing associated with the feedback is -0.49 W m(-2), indicating that it has the potential to offset about 13 % of the forcing associated with a doubling of CO2.Peer reviewe
Analysis of electron-positron momentum spectra of metallic alloys as supported by first-principles calculations
Electron-positron momentum distributions measured by the coincidence Doppler
broadening method can be used in the chemical analysis of the annihilation
environment, typically a vacancy-impurity complex in a solid. In the present
work, we study possibilities for a quantitative analysis, i.e., for
distinguishing the average numbers of different atomic species around the
defect. First-principles electronic structure calculations self-consistently
determining electron and positron densities and ion positions are performed for
vacancy-solute complexes in Al-Cu, Al-Mg-Cu, and Al-Mg-Cu-Ag alloys. The
ensuing simulated coincidence Doppler broadening spectra are compared with
measured ones for defect identification. A linear fitting procedure, which uses
the spectra for positrons trapped at vacancies in pure constituent metals as
components, has previously been employed to find the relative percentages of
different atomic species around the vacancy [A. Somoza et al. Phys. Rev. B 65,
094107 (2002)]. We test the reliability of the procedure by the help of
first-principles results for vacancy-solute complexes and vacancies in
constituent metals.Comment: Submitted to Physical Review B on September 19 2006. Revised version
submitted on November 8 2006. Published on February 14 200
Vacancy-Impurity Complexes in Highly Sb-Doped Si Grown by Molecular Beam Epitaxy
Positron annihilation measurements, supported by first-principles electron-structure calculations, identify vacancies and vacancy clusters decorated by 1–2 dopant impurities in highly Sb-doped Si. The concentration of vacancy defects increases with Sb doping and contributes significantly to the electrical compensation. Annealings at low temperatures of 400–500 K convert the defects to larger complexes where the open volume is neighbored by 2–3 Sb atoms. This behavior is attributed to the migration of vacancy-Sb pairs and demonstrates at atomic level the metastability of the material grown by epitaxy at low temperature.Peer reviewe
BVOC–aerosol–climate feedbacks investigated using NorESM
Both higher temperatures and increased CO2 concentrations are
(separately) expected to increase the emissions of biogenic volatile organic
compounds (BVOCs). This has been proposed to initiate negative climate
feedback mechanisms through increased formation of secondary organic aerosol
(SOA). More SOA can make the clouds more reflective, which can provide a
cooling. Furthermore, the increase in SOA formation has also been proposed to
lead to increased aerosol scattering, resulting in an increase in diffuse
radiation. This could boost gross primary production (GPP) and further
increase BVOC emissions. In this study, we have used the Norwegian Earth
System Model (NorESM) to investigate both these feedback mechanisms. Three
sets of experiments were set up to quantify the feedback with respect to (1)Â doubling
the CO2, (2)Â increasing temperatures corresponding to a doubling of
CO2 and (3)Â the combined effect of both doubling CO2 and a
warmer climate. For each of these experiments, we ran two simulations, with
identical setups, except for the BVOC emissions. One simulation was run with
interactive BVOC emissions, allowing the BVOC emissions to respond to changes
in CO2 and/or climate. In the other simulation, the BVOC emissions
were fixed at present-day conditions, essentially turning the feedback off.
The comparison of these two simulations enables us to investigate each step
along the feedback as well as estimate their overall relevance for the future
climate.
We find that the BVOC feedback can have a significant impact on the climate.
The annual global BVOC emissions are up to 63 % higher when the feedback
is turned on compared to when the feedback is turned off, with the largest
response when both CO2 and climate are changed. The higher BVOC
levels lead to the formation of more SOA mass (max 53 %) and result in
more particles through increased new particle formation as well as larger
particles through increased condensation. The corresponding changes in the
cloud properties lead to a −0.43 W m−2 stronger net cloud forcing.
This effect becomes about 50 % stronger when the model is run with
reduced anthropogenic aerosol emissions, indicating that the feedback will
become even more important as we decrease aerosol and precursor emissions. We
do not find a boost in GPP due to increased aerosol scattering on a global
scale. Instead, the fate of the GPP seems to be controlled by the BVOC effects
on the clouds. However, the higher aerosol scattering associated with the
higher BVOC emissions is found to also contribute with a potentially
important enhanced negative direct forcing (−0.06 W m−2). The global
total aerosol forcing associated with the feedback is −0.49 W m−2,
indicating that it has the potential to offset about 13 % of the forcing
associated with a doubling of CO2.</p
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