1,482 research outputs found
External Operators and Anomalous Dimensions in Soft-Collinear Effective Theory
It has recently been argued that soft-collinear effective theory for
processes involving both soft and collinear partons contains a new
soft-collinear mode, which can communicate between the soft and collinear
sectors of the theory. The formalism incorporating the corresponding fields
into the effective Lagrangian is extended to include external current and
four-quark operators relevant to weak interactions. An explicit calculation of
the anomalous dimensions of these operators reveals that soft-collinear modes
are needed for correctly describing the ultraviolet behavior of the effective
theory.Comment: 15 pages, 2 figure
Large probe arrays for measuring mean and time dependent local oil volume fraction and local oil velocity component distributions in inclined oil-in-water flows
Arrays of dual-sensor and four-sensor needle conductance probes have been used to measure the mean and time dependent local properties of upward inclined, bubbly oil-in-water flows (also known as dispersed oil-in-water flows) in a 153mm diameter pipe. The flow properties that were measured were (i) the local in-situ oil volume fraction ; (ii) the local oil velocity in the axial direction of the pipe (the direction); and (iii) the local oil velocity in the direction from the lower side of the inclined pipe to its upper side (the direction). Oil velocities in the direction (orthogonal to the and directions) were found to be negligible. For all of the flow conditions investigated it was found that the mean value of varied from a maximum value at the upper side of the inclined pipe to a minimum value at the lower side, and that the rate of decrease of this mean value of with distance in the direction became greater as the pipe inclination angle from the vertical was increased. It was also found that the mean value of was greatest at the upper side of the inclined pipe and decreased towards the lower side of the inclined pipe, the rate of decrease with distance in the direction again becoming greater as was increased. For , a water volumetric flow rate , an oil volumetric flow rate and using a sampling period over a total time interval of , it was found that at the upper side of the inclined pipe the standard deviation in was 31.6% of the mean value of . Furthermore for , , and it was found that the standard deviation in the cross-pipe oil velocity component was approximately equal to the standard deviation in the axial velocity component . These large temporal variations in the local flow properties have been attributed to the presence of large scale Kelvin-Helmholtz waves which intermittently appear in the flow. It is believed that the techniques outlined in this paper for measuring the standard deviation of local flow properties as a function of the sampling period will be of considerable value in validating mathematical models of time dependent oil-water flows. It should be noted that the principal focus of this paper is on the measurement techniques that were used and the methods of data analysis rather than the presentation of exhaustive experimental results at numerous different flow conditions
Combustion in a heavy-duty direct-injection engine using hydrogen–methane blend fuels
Adding hydrogen to the fuel in a direct injection natural gas engine offers the
potential significantly to reduce local and global air pollutant emissions. This work reports on
the effects of fuelling a heavy-duty engine with late-cycle direct injection of blended hydrogen–
methane fuels and diesel pilot ignition over a range of engine operating conditions. The effect
of hydrogen on the combustion event varies with operating condition, providing insight into
the fundamental factors limiting the combustion process. Combustion stability is enhanced at
all conditions studied; this leads directly to a significant reduction in emissions of combustion
byproducts, including carbon monoxide, particulate matter, and unburned fuel. Carbon
dioxide emissions are also significantly reduced by the lower carbon–energy ratio of the fuel.
The results suggest that this technique can significantly reduce both local and global pollutant
emissions associated with heavy-duty transport applications while requiring minimal changes
to the fuelling system
Hydrogen-methane blend fuelling of a heavy-duty, direct-injection engine
Combining hydrogen with natural gas as a fuel for internal
combustion engines provides an early opportunity to introduce
hydrogen into transportation applications. This study
investigates the effects of fuelling a heavy-duty engine with a
mixture of hydrogen and natural gas injected directly into the
combustion chamber. The combustion system, developed for
natural gas fuelling, is not modified for blended hydrogen
operation. The results demonstrate that hydrogen can have a
significant beneficial effect in reducing emissions without
affecting efficiency or requiring significant engine
modifications. Combustion stability is enhanced through the
higher reactivity of the hydrogen, resulting in reduced emissions
of unburned methane. The fuel’s lower carbon:energy ratio also
reduces CO2 emissions. These results combine to significantly
reduce tailpipe greenhouse gas (GHG) emissions. However, the
effect on net GHG’s, including both tailpipe and fuelproduction
emissions, depends on the source of the hydrogen.
Cleaner sources, such as electrolysis based on renewables and
hydro-electric power, generate a significant net reduction in
GHG emissions. Hydrogen generated by steam-methane
reforming is essentially GHG neutral, while electrolysis using
electricity from fossil-fuel power plants significantly increases
net GHG emissions compared to conventional natural gas
fuelling
The influence of fuel composition on a heavy-duty, natural-gas direct-injection engine
This work investigates the implications of natural gas composition on the combustion in a heavy-duty
natural gas engine and on the associated pollutant emissions. In this engine system, natural gas is injected
into the combustion chamber shortly before the end of the compression stroke; a diesel pilot that precedes
the natural gas injection provides the ignition source. The effects of adding ethane, propane, hydrogen, and
nitrogen to the fuel are reported here. The results indicate that these additives had no significant effect on
the engine’s power or fuel consumption. Emissions of unburned fuel are reduced for all additives through
either enhanced ignition or combustion processes. Black carbon particulate matter emissions are increased
by ethane and propane, but are virtually eliminated by including nitrogen or hydrogen in the fuel
The effects of fuel dilution in a natural-gas direct-injection engine
This study reports the effects of fuelling a heavy-duty single-cylinder research
engine with pilot-ignited late-cycle direct-injected natural gas diluted with 0, 20, and 40 per
cent nitrogen. The combustion duration is unaffected while its intensity is reduced and its
stability is increased. Emissions of nitrogen oxides, particulate matter, hydrocarbons, and
carbon monoxide are all reduced, with no effect on the engine’s performance and efficiency.
The results indicate the benefits of increased in-cylinder turbulence and are of particular
relevance when considering fuel composition variations with non-conventional sources of
gaseous fuels
Sudakov Resummation for Subleading SCET Currents and Heavy-to-Light Form Factors
The hard-scattering contributions to heavy-to-light form factors at large
recoil are studied systematically in soft-collinear effective theory (SCET).
Large logarithms arising from multiple energy scales are resummed by matching
QCD onto SCET in two stages via an intermediate effective theory. Anomalous
dimensions in the intermediate theory are computed, and their form is shown to
be constrained by conformal symmetry. Renormalization-group evolution equations
are solved to give a complete leading-order analysis of the hard-scattering
contributions, in which all single and double logarithms are resummed. In two
cases, spin-symmetry relations for the soft-overlap contributions to form
factors are shown not to be broken at any order in perturbation theory by
hard-scattering corrections. One-loop matching calculations in the two
effective theories are performed in sample cases, for which the relative
importance of renormalization-group evolution and matching corrections is
investigated. The asymptotic behavior of Sudakov logarithms appearing in the
coefficient functions of the soft-overlap and hard-scattering contributions to
form factors is analyzed.Comment: 50 pages, 10 figures; minor corrections, version to appear in JHE
Modularity of plant metabolic gene clusters: a trio of linked genes that are collectively required for acylation of triterpenes in oat
Operon-like gene clusters are an emerging phenomenon in the field of plant natural products. The genes encoding some of the best-characterized plant secondary metabolite biosynthetic pathways are scattered across plant genomes. However, an increasing number of gene clusters encoding the synthesis of diverse natural products have recently been reported in plant genomes. These clusters have arisen through the neo-functionalization and relocation of existing genes within the genome, and not by horizontal gene transfer from microbes. The reasons for clustering are not yet clear, although this form of gene organization is likely to facilitate co-inheritance and co-regulation. Oats (Avena spp) synthesize antimicrobial triterpenoids (avenacins) that provide protection against disease. The synthesis of these compounds is encoded by a gene cluster. Here we show that a module of three adjacent genes within the wider biosynthetic gene cluster is required for avenacin acylation. Through the characterization of these genes and their encoded proteins we present a model of the subcellular organization of triterpenoid biosynthesis
Gravitational field of domain wall in Lyra geometry
In this paper, we study the domain wall with time dependent displacement
vectors based on Lyra geometry in normal gauge i.e. displacement vector . The field theoretic energy momentum tensor is
considered with zero pressure perpendicular to the wall. We find an exact
solutions of Einstein equation for a scalar field with a potential
describing the gravitational field of a plane symmetric domain wall.
We have seen that the hyper surfaces parallel to the wall ()
are three dimensional de-sitter spaces. It is also shown that the gravitational
field experienced by test particle is attractive.Comment: 11 pages, 6 figures; Submitted in Astrophysics and Space Science
after minor revisio
Strong Phases and Factorization for Color Suppressed Decays
We prove a factorization theorem in QCD for the color suppressed decays B0->
D0 M0 and B0-> D*0 M0 where M is a light meson. Both the color-suppressed and
W-exchange/annihilation amplitudes contribute at lowest order in LambdaQCD/Q
where Q={mb, mc, Epi}, so no power suppression of annihilation contributions is
found. A new mechanism is given for generating non-perturbative strong phases
in the factorization framework. Model independent predictions that follow from
our results include the equality of the B0 -> D0 M0 and B0 -> D*0 M0 rates, and
equality of non-perturbative strong phases between isospin amplitudes,
delta(DM) = delta(D*M). Relations between amplitudes and phases for M=pi,rho
are also derived. These results do not follow from large Nc factorization with
heavy quark symmetry.Comment: 38 pages, 6 figs, typos correcte
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