363 research outputs found
Computational study of jetlet structures in perforate silencers
Reynolds Averaged Navier Stokes allow for the steady state solutions while Large eddy
simulation is a useful tool for predicting the spatial and temporal behavior of flow structures
generated by complex geometries. However, many predictions suffer from poor grid
resolution and initial conditions resulting in poor development of the initial jet shear layer
and consequent incorrect prediction of critical flow behaviour. In this work, two 1mm thick
perforate plate geometries of 23% and 40% porosity with 2mm diameter holes at an overall
pressure ratio of 1.45 have been investigated. Results presented in this paper show the
initial jetlet and fully merged jet flow-field to be sensitive to the porosity and the presence
of partial holes around the circumference of the plate. The increase in porosity reduces
the available entrainment flow, and increases the local jetlet interaction and resultant turbulence
levels. This interaction fundamentally changes the flow structure from coherent
vortex rings (found at low porosity) to a helical structure. The 2nd and 4th order spatiotemporal
correlation Rij and Rij,kl are presented as evidence of the associated impact on
acoustic source modeling
Computational study of aero-acoustic sources in perforate silencers
Reynolds Averaged Navier Stokes and Large Eddy Simulations of two perforate plates
at a overall pressure ratio of 1.45 have been performed to allow analysis of the sensitivity
of acoustic noise sources to porosity. Two geometries are presented: A 23% porosity and a
40% porosity 1mm plate with 2mm diameter holes. Results presented in this paper show the
initial jetlet and fully merged jet flow-field to be sensitive to the porosity and the presence
of partial holes around the circumference of the plate. The increase in porosity reduces
the available entrainment flow, and increases the local jetlet interaction and resultant
turbulence levels. This interaction fundamentally changes the flow structure from coherent
vortex rings (found at low porosity) to a helical structure. The 2nd and 4th order spatio-
temporal correlation Rij and Rij,kl are presented as suggested validation data for acoustic
source modeling together with far-field noise spectra obtained via a Ffowcs-Williams &
Hawkings surface integral method
Large eddy simulation to extract fourth order space time turbulence correlations in jets including microjet injection
Jet noise is still a major component of overall aircraft noise emission at take-off, and its reduction is
important to sustain the continuing growth of air transport. Computationally expensive Large Eddy
Simulations can be used to assess the four-order spatio-temporal correlations so as to provide input and
guidance to cheaper jet noise models. Large Eddy Simulations are presented for an isothermal Mach 0.75
jet at a Reynolds number of 1 million with and without microjet injection. The imposition of a numerical
boundary layer trip inside the jet nozzle ensures that the shear layer is fully turbulent immediately
downstream of the nozzle lip. The eight high-pressure microjets penetrate the shear layer producing
streamwise vorticity on the inside of the jet. This dissipates before the end of the potential core and there is
no effect on potential core length. The peak turbulence intensity within the shear layer is reduced, with the
greatest reduction at locations aligned with the microjet injection points. The shapes of the fourth order
correlation envelopes are little changed by the microjets, but there is a significant difference in the absolute
magnitudes. Compared to a clean jet all significant correlation terms are reduced, with the reduction still
occurring at x/Dj=6.5 where the effect of the microjets on the mean flow has dissipated. This reduction
could be used to calibrate a jet noise model in order to take account of the microjets
Comment on: âPeatland carbon stocks and burn history: Blanket bog peat core evidence highlights charcoal impacts on peat physical properties and long-term carbon storageâ, by A. Heinemeyer, Q. Asena, W.L. Burn and A.L. Jones (Geo: Geography and Environment. 2018; e00063)
A recent paper by Heinemeyer et al. (2018) in this journal has suggested that the use of prescribed fire may enhance carbon accumulation in UK upland blanket bogs. We challenge this finding based on a number of concerns with the original manuscript including the lack of an unburned control, insufficient replication, unrecognised potential confounding factors, and potentially large inaccuracies in the core dating approach used to calculate carbon accumulation rates. We argue that burnâmanagement of peatlands is more likely to lead to carbon loss than carbon gain
Greenhouse gas emission factors associated with rewetting of organic soils
Drained organic soils are a significant source of greenhouse gas (GHG) emissions to the atmosphere. Rewetting these soils may reduce GHG emissions and could also create suitable conditions for return of the carbon (C) sink function characteristic of undrained organic soils. In this article we expand on the work relating to rewetted organic soils that was carried out for the 2014 Intergovernmental Panel on Climate Change (IPCC) Wetlands Supplement. We describe the methods and scientific approach used to derive the Tier 1 emission factors (the rate of emission per unit of activity) for the full suite of GHG and waterborne C fluxes associated with rewetting of organic soils. We recorded a total of 352 GHG and waterborne annual flux data points from an extensive literature search and these were disaggregated by flux type (i.e. CO2, CH4, N2O and DOC), climate zone and nutrient status. Our results showed fundamental differences between the GHG dynamics of drained and rewetted organic soils and, based on the 100 year global warming potential of each gas, indicated that rewetting of drained organic soils leads to: net annual removals of CO2 in the majority of organic soil classes; an increase in annual CH4 emissions; a decrease in N2O and DOC losses; and a lowering of net GHG emissions. Data published since the Wetlands Supplement (n = 58) generally support our derivations. Significant data gaps exist, particularly with regard to tropical organic soils, DOC and N2O. We propose that the uncertainty associated with our derivations could be significantly reduced by the development of country specific emission factors that could in turn be disaggregated by factors such as vegetation composition, water table level, time since rewetting and previous land use history
Responsible agriculture must adapt to the wetland character of midâlatitude peatlands
Drained, lowland agricultural peatlands are greenhouse gas (GHG) emission hotspots and a large but vulnerable store of irrecoverable carbon. They exhibit soil loss rates of ~2.0 cm yrâ1 and are estimated to account for 32% of global cropland emissions while producing only 1.1% of crop kilocalories. Carbon dioxide emissions account for >80% of their terrestrial GHG emissions and are largely controlled by water table depth. Reducing drainage depths is, therefore, essential for responsible peatland management. Peatland restoration can substantially reduce emissions. However, this may conflict with societal needs to maintain productive use, to protect food security and livelihoods. Wetland agriculture strategies will, therefore, be required to adapt agriculture to the wetland character of peatlands, and balance GHG mitigation against productivity, where halting emissions is not immediately possible. Paludiculture may substantially reduce GHG emissions but will not always be viable in the current economic landscape. Reduced drainage intensity systems may deliver partial reductions in the rate of emissions, with smaller modifications to existing systems. These compromise systems may face fewer hurdles to adoption and minimize environmental harm until societal conditions favour strategies that can halt emissions. Wetland agriculture will face agronomic, socio-economic and water management challenges, and careful implementation will be required. Diversity of values and priorities among stakeholders creates the potential for conflict. Successful implementation will require participatory research approaches and co-creation of workable solutions. Policymakers, private sector funders and researchers have key roles to play but adoption risks would fall predominantly on land managers. Development of a robust wetland agriculture paradigm is essential to deliver resilient production systems and wider environmental benefits. The challenge of responsible use presents an opportunity to rethink peatland management and create thriving, innovative and green wetland landscapes for everyone's future benefit, while making a vital contribution to global climate change mitigation
Constructive and destructive use of compilers in elliptic curve cryptography
Although cryptographic software implementation is often performed by expert programmers, the range of performance and security driven options, as well as more mundane software engineering issues, still make it a challenge. The use of domain specific language and compiler techniques to assist in description and optimisation of cryptographic software is an interesting research challenge. In this paper we investigate two aspects of such techniques, focusing on Elliptic Curve Cryptography (ECC) in particular. Our constructive results show that a suitable language allows description of ECC based software in a manner close to the original mathematics; the corresponding compiler allows automatic production of an executable whose performance is competitive with that of a hand-optimised implementation. In contrast, we study the worrying potential for naĂŻve compiler driven optimisation to render cryptographic software insecure. Both aspects of our work are set within the context of CACE, an ongoing EU funded project on this general topic
Primordial black holes in braneworld cosmologies: Formation, cosmological evolution and evaporation
We consider the population evolution and evaporation of primordial black
holes in the simplest braneworld cosmology, Randall-Sundrum type II. We
demonstrate that black holes forming during the high-energy phase of this
theory (where the expansion rate is proportional to the density) have a
modified evaporation law, resulting in a longer lifetime and lower temperature
at evaporation, while those forming in the standard regime behave essentially
as in the standard cosmology. For sufficiently large values of the AdS radius,
the high-energy regime can be the one relevant for primordial black holes
evaporating at key epochs such as nucleosynthesis and the present. We examine
the formation epochs of such black holes, and delimit the parameter regimes
where the standard scenario is significantly modified.Comment: 9 pages RevTeX4 file with four figures incorporated, minor changes to
match published versio
Negative Parity 70-plet Baryon Masses in the 1/Nc Expansion
The masses of the negative parity SU(6) 70-plet baryons are analyzed in the
1/Nc expansion to order 1/Nc and to first order in SU(3) breaking. At this
level of precision there are twenty predictions. Among them there are the well
known Gell-Mann Okubo and equal spacing relations, and four new relations
involving SU(3) breaking splittings in different SU(3) multiplets. Although the
breaking of SU(6) symmetry occurs at zeroth order in 1/Nc, it turns out to be
small. The dominant source of the breaking is the hyperfine interaction which
is of order 1/Nc. The spin-orbit interaction, of zeroth order in 1/Nc, is
entirely fixed by the splitting between the singlet states Lambda(1405) and
Lambda(1520), and the spin-orbit puzzle is solved by the presence of other
zeroth order operators involving flavor exchange.Comment: 31 pages, 3 figure
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