Phonon transport in large scale carbon-based disordered materials: Implementation of an efficient order-N and real-space Kubo methodology

We have developed an efficient order-N real-space Kubo approach for the calculation of the phonon conductivity which outperforms state-of-the-art alternative implementations based on the Green's function formalism. The method treats efficiently the time-dependent propagation of phonon wave packets in real space, and this dynamics is related to the calculation of the thermal conductance. Without loss of generality, we validate the accuracy of the method by comparing the calculated phonon mean free paths in disordered carbon nanotubes (isotope impurities) with other approaches, and further illustrate its upscalability by exploring the thermal conductance features in large width edge-disordered graphene nanoribbons (up to ~20 nm), which is out of the reach of more conventional techniques. We show that edge-disorder is the most important scattering mechanism for phonons in graphene nanoribbons with realistic sizes and thermal conductance can be reduced by a factor of ~10.Comment: Accepted for publication in Physical Review B - Rapid Communication

Specific pollinator attraction and the diversification of sexually deceptive Chiloglottis (Orchidaceae)

Abstract.: Evidence indicates that sexually deceptive Chiloglottis R.Br. (Orchidaceae) taxa specifically attract their thynnine wasp (Tiphiidae) pollinators through the floral odour mimicry of female wasp sex pheromones. We use amplified fragment length polymorphisms (AFLPs) to reconstruct the species-level phylogeny of Chiloglottis, make a preliminary evaluation of genetic distinctions between species, and compare the historical association among orchids and their pollinators using wasp sequence data from a previous study. AFLPs show large differences between three sub-generic clades relative to that found among species within each clade. Interspecific genetic barriers are indicated by AFLP discontinuities among species unlike in previously reported DNA sequence data. However, such barriers are demonstrated clearly in only one of the two pairs of sympatric species sampled more intensively. We interpret these patterns as indicating either (i) a rapid and recent radiation of species within each clade following histories of stasis or extinction, or (ii) alternating cycles of divergence and gene flow acting to homogenize genetic differences among species within each of the three clade

Giant scour-fills in ancient channel-lobe transition zones: Formative processes and depositional architecture

Scours are common features of modern deep-marine seascapes, particularly downstream of the mouths of slope channels within channel-lobe transition zones (CLTZs). Their dimensions can exceed hundreds of metres in width and length, and tens of metres in depth. However, the stratigraphic architecture of the infill of these erosional bedforms is rarely described from the rock record and no large (>100 m width) scours have been described in detail from exhumed CLTZs. Here, the infill of two erosional features (0.5-1 km long and 15-20 m thick) from the Permian Karoo Basin succession, South Africa, are presented from palaeogeographically well- constrained CLTZs; one from Fan 3 in the Tanqua depocentre and one from Unit A5 in the Laingsburg depocentre. The basal erosion surfaces of the features are asymmetric with steep, undulating, and composite upstream margins, and low gradient simple downstream margins. The basal infill consists of thin-bedded siltstone and sandstone beds cut by closely-spaced scours; these beds are interpreted as partially reworked fine grained tails of bypassing flows with evidence for flow deflection. The erosional features are interpreted as giant scour-fills. The internal architecture suggests different evolutionary histories for each case. The Unit A5 scour-fill shows a simple cut-and-fill history with lateral and upward transitions from siltstone- to sandstone-prone deposits. In contrast, the Fan 3 scour-fill shows headward erosion and lengthening of the scour surface suggesting temporal changes in the interaction between turbidity currents and the scour surface. This relationship could support the occurrence of a hydraulic jump during part of the fill history, while the majority of the fill represents deposition from subcritical flows. Diversity in scour preservation mechanisms could explain the variety in depositional histories. The architecture, sedimentary facies and palaeoflow patterns of the scour-fills are distinctly different to well documented adjacent basin-floor channel-fills at the same stratigraphic levels. The recognition of scour-fills helps to constrain their sedimentological and stratigraphic expression in the subsurface, and to improve our understanding of the stratigraphic architecture of channel-lobe transition zones

The Geomorphology of Submarine Channel Systems of the Northern Line Islands Ridge, Central Equatorial Pacific Ocean

More than 844,000 km² of the northern Line Islands Ridge mapped with multibeam bathymetry and backscatter provide unprecedented views of the geomorphology of this isolated area in the central equatorial Pacific Ocean. A compilation of all available multibeam data in the area reveals six extensive submarine dendritic channel systems that encompass a combined drainage area that exceeds 60,000 km². The channel systems occur in a predominately carbonate environment and are the longest calciclastic submarine channel systems mapped in the oceans to date. The channel systems occur in a carbonate-dominated region well above the carbonate compensation depth and have developed into the surface of basins that are surrounded by small guyots and seamounts that make up a discontinuous rim around the summit of the northern Line Island Ridge. The channels have mostly straight or gently curved well-developed tributaries and main reaches. Although the Line Island Ridge has been dated at 86 to 68 Ma old, the channels occur on the surface and are not buried by any significant sediment accumulations. Levees are very rare along the channel banks and no bathymetric expression of submarine fans was found where the channels exit onto the adjacent abyssal basins. There is sparse evidence of landslide deposits throughout the ridge although the flanks of the guyots exhibit numerous headwall scarps. The presence of plunge pools below the northwest escarpment, together with well-defined channels meters to hundreds of meters deep relative to the surrounding seafloor, suggests the channels might be relatively recent (perhaps late Neogene or even younger) features developed long after the ridge subsided more than a kilometer below sea level

Utilisation of underwater acoustic backscatter systems to characterise nuclear waste suspensions remotely

This paper reports application of ABS (Acoustic Backscatter Systems) to address nuclear waste management within the UK. ABS offers a route towards an online monitoring system to characterise wastes safely and remotely in real-time, during pipeline transportation; resulting in reduced hazard reduction timescales and taxpayer cost savings. Here, an ultrasonic velocimetry profiler (UVP) was used to analyse glass dispersions of varying concentrations, to assess online waste monitoring applicability. Acoustic backscatter profiles were collected to establish attenuation coefficients for two glass sizes (∼40 and 80 µm) with 2 and 4 MHz probes. The 4 MHz probes were the most highly attenuating while transducer active radius had a negligible effect on probe sensitivity with either glass. A calibration procedure was used to measure sediment attenuation coefficients, which were compared to model estimates and experimental literature. For most systems, measured coefficients were close to estimated values, highlighting improved calibration accuracy, due to the mixing tank used. However, values for the smaller glass and 2 MHz probes overestimated model predictions, due to additional viscous attenuation. The measured value for the larger glass with 4 MHz probes was underestimated, this was caused by high attenuation reaching the instrument noise-floor, limiting the region available for analysis

Relationship between bowl-shaped clastic injectites and parent sand depletion; implications for their scale invariant morphology and composition

3D seismic reflection data provides a means to assess the impact of injection on parent sands, and to quantify the character of the resulting injectite networks. The morphology of a series of large injectite structures hosted in the Palaeocene Lower Lista Formation were mapped using broadband 3D seismic data from the North Sea to investigate their relationship with parent sands. Fourteen bowl-shaped structures were identified within the Lista Formation in the study area (60-85 m in height, and 200-900 m in width). Sand is absent (below resolution) below these large-scale bowls, suggesting that the parent sand is the underlying Maureen Formation and sand 'welds' formed, rather than sand-prone channelised deposits within the Lista Formation. Identification of injectite networks can be ambiguous, which impacts geological model development. Observations from exhumed systems and core, offers high resolution insights into the complexity of injectite networks. To advance our understanding of this scale gap, we argue for injectites being scale invariant in their shape and grain-size. This permits the application of outcrop-scale knowledge to seismic-scale interpretation. The demonstrable depletion of parent sands, and their scale invariance, can be applied to basin-fills worldwide to reduce uncertainties of the impact of sand injectites on hydrocarbon reservoirs

In situ characterization of mixing and sedimentation dynamics in an impinging jet ballast tank via acoustic backscatter

Impinging jets are utilized in numerous applications, including nuclear waste treatment, for both the erosion of sediment beds and maintaining particulates in suspension. Pulse-echo ultrasonic methods offer great potential for the in situ monitoring of critical mixing and settling dynamics, in concentrated dispersions. A non-active scaled version of a Highly Active Storage Tank at Sellafield, UK, was profiled with an acoustic backscatter system under various jet firing conditions. An advanced analysis technique enabled the direct quantification of dispersion concentration changes from the converted backscatter attenuation. Hence, the erosion and mixing capability of the jets, and settling kinetics were characterized. It was found that jet operation alone provided inadequate localized mixing of eroded sediment. An additional air-lift process operation was required to hinder the rapid re-settling of dispersed particulates

A unifying computational fluid dynamics investigation on the river-like to river-reversed secondary circulation in submarine channel bends

A numerical model of saline density currents across a triple-bend sinuous submerged channel enclosed by vertical sidewalls is developed. The unsteady, non-Boussinesq, turbulent form of the Reynolds Averaged Navier-Stokes equations is employed to study the flow structure in a quasi-steady state. Recursive tests are performed with axial slopes of 0.08°, 0.43°, 1.5°, and 2.5°. For each numerical experiment, the downstream and vertical components of the fluid velocity, density, and turbulent kinetic energy are presented at four distinct locations within the channel cross section. It is observed that a crucial change in the flow pattern at the channel bends is observed as the axial slope is increased. At low values of the axial slope a typical river-like pattern is found. At an inclination of 1.5°a transition starts to occur. When the numerical test is repeated with an axial slope of 2.5°, a clearly visible river-reversed secondary circulation is achieved. The change in the cross-sectional flow pattern appears to be associated with the spatial displacement of the core of the maximum downstream fluid velocity. Therefore, the axial slope in this series of experiments is linked to the velocity structure of the currents, with the height of the velocity maximum decreasing as a function of increasing slope. As such, the axial slope should be regarded also as a surrogate for flows with enhanced density or sediment stratification and higher Froude numbers. The work unifies the apparently paradoxical experimental and numerical results on secondary circulation in submarine channels

Numerical modelling of turbulent particle-laden sonic CO2 jets with experimental validation

Under-expanded particle-laden flows resulting in velocities greater than the local speed of sound are a feature of a wide number of applications in aviatic, astronautical, and process engineering scenarios including those relating to the accidental release of high-pressure fluids from reservoirs or pipelines. Such pipelines are considered to be the most likely method for transportation of captured carbon dioxide (CO2) from power plants and other industries prior to subsequent storage in carbon capture and storage (CCS) applications. Their safe operation is of paramount importance as their contents are likely to be in the region of several thousand tonnes. CO2 poses a number of dangers upon release due to its physical properties. It is a colourless and odourless asphyxiant which has a tendency to sublimation and solid formation, and is directly toxic if inhaled in air at concentrations around 5%, and likely to be fatal at concentrations around 10%. The developments presented in this paper concern the formulation of a multi-phase homogeneous discharge and dispersion model capable of predicting the near-field fluid dynamic, phase and particle behaviour of such CO2 releases, with validation against measurements of laboratory-scale jet releases of CO2 recently obtained by our group