LES of additive and non-additive pulsatile flows in a model arterial stenosis

Transition of additive and non-additive pulsatile flows through a simple 3D model of arterial stenosis is investigated by using a large eddy simulation (LES) technique. We find in both the pulsatile cases that the interaction of the two shear layers, one of which separates from the nose of the stenosis and the another one from its opposite wall, causes recirculation in the flow downstream of the stenosis where the nature of the transient flow becomes turbulent. The strength of this recirculation is found to be quite high from the non-additive pulsations when the flow Reynolds numbers, Re ≥ 1500, for which both the pressure and shearing stresses take on an oscillating form at the post-stenotic region. Potential medical consequences of these results are discussed in the paper. In addition, some comparisons of the non-additive pulsatile results are given with those of both the additive pulsatile and steady flows. The capability of using LES to simulate the pulsatile transitional flow is also assessed, and the present results show that the smaller (subgrid) scales (SGS) contributes about 78% energy dissipation to the flow when the Reynolds number is taken as 2000. The level of SGS dissipation decreases as the Reynolds number is decreased. The numerical results are validated with the experimental data available in literature where a quite good agreement is found

Effects of experimental disturbance on multi-taxa assemblages and traits: conservation implication in a forest-open landscape mosaic

Overcoming fragmentation and isolation requires innovative solutions if cohesive biodiversity networks are to be created in modernised landscapes. Within Europe much of the biodiversity interest is in semi-natural habitats that exist as isolated reserves. This thesis aimed to test the connectivity potential of open habitat for lowland heathland biodiversity within a mosaic forest landscape. A range of experimental management treatments were implemented covering a gradient of disturbance intensity intended to enhance connectivity through plantation forest for early-successional biodiversity. Both species composition and life history traits were investigated enabling a comprehensive interpretation of response across multiple species. Sampling programs identified over 87000 invertebrates, comprising 38188 spiders from 183 species, 41531 ants from 20 species and 7564 carabids from 93 species, and recorded 23241 observations of 222 vascular plant species. Initial investigations revealed forestry trackways contained a component of the regional heathland spider assemblage, but this was significantly degraded as adjacent forest matured. Experiments to augment heathland biodiversity in trackways resulted in contrasting responses between taxa. Specialist carabids and vascular plants (associated with heathland or early-successional habitats), increased in abundance and richness with high intensity disturbance. Spider assemblages were left depauperate and did not completely recover after two seasons; ants did not respond at any disturbance level. Trait-based analysis showed that the abundance of aerial dispersers increased and size decreased with disturbance intensity for carabids and plants. In contrast, spider body size increased with greater disturbance and aerial dispersal was not significant. For spiders, ephemeral stepping stones, in the form young restock coupes, support the majority of the heath assemblage, whereas open linear habitat in the form of trackways, suffer from edge effects and are dominated by generalist and woodland spiders. Network cohesion will benefit from intensive disturbance management and a combination of connectivity elements to incorporate contrasting dispersal abilities

Mg/Ca ratios in freshwater microbial carbonates: Thermodynamic, kinetic and vital effects

The ratio of magnesium to calcium (Mg/Ca) in carbonate minerals in an abiotic setting is conventionally assumed to be predominantly controlled by (Mg/Ca)solution and a temperature dependant partition coefficient. This temperature dependence suggests that both marine (e.g. foraminiferal calcite and corals) and freshwater (e.g. speleothems and surface freshwater deposits, “tufas”) carbonate deposits may be important archives of palaeotemperature data. However, there is considerable uncertainty in all these settings. In surface freshwater deposits this uncertainty is focussed on the influence of microbial biofilms. Biogenic or “vital” effects may arise from microbial metabolic activity and/or the presence of extracellular polymeric substances (EPS). This study addresses this key question for the first time, via a series of unique through-flow microcosm and agitated flask experiments where freshwater calcite was precipitated under controlled conditions. These experiments reveal there is no strong relationship between (Mg/Ca)calcite and temperature, so the assumption of thermodynamic fractionation is not viable. However, there is a pronounced influence on (Mg/Ca)calcite from precipitation rate, so that rapidly forming precipitates develop with very low magnesium content indicating kinetic control on fractionation. Calcite precipitation rate in these experiments (where the solution is only moderately supersaturated) is controlled by biofilm growth rate, but occurs even when light is excluded indicating that photosynthetic influences are not critical. Our results thus suggest the apparent kinetic fractionation arises from the electrochemical activity of EPS molecules, and are therefore likely to occur wherever these molecules occur, including stromatolites, soil and lake carbonates and (via colloidal EPS) speleothems

Arthropod traits and assemblages differ between core patches, transient stepping-stones and landscape corridors

Context Restoring landscape connectivity can mitigate fragmentation and improve population resilience, but functional equivalence of contrasting elements is poorly understood. Evaluating biodiversity outcomes requires examining assemblage-responses across contrasting taxa. Objectives We compared arthropod species and trait composition between contrasting open-habitat network elements: core patches, corridors (allowing individual dispersal and population percolation), and transient stepping-stones (potentially enhancing metapopulation dynamics). Methods Carabids and spiders were sampled from core patches of grass-heath habitat (n = 24 locations across eight sites), corridors (trackways, n = 15) and recently-replanted clear-fells (transient patches, n = 19) set in a forest matrix impermeable to openhabitat arthropods. Species and trait (habitat association, diet, body size, dispersal ability) composition were compared by ordination and fourth corner analyses. Results Each network element supported distinct arthropod assemblages with differing functional trait composition. Core patches were dominated by specialist dry-open habitat species while generalist and woodland species contributed to assemblages in connectivity elements. Nevertheless, transient patches (and to a lesser degree, corridors) supported dry-open species characteristic of the focal grass-heath sites. Trait associations differed markedly among the three elements. Dispersal mechanisms and their correlates differed between taxa, but dry-open species in transient patches were characterised by traits favouring dispersal (large running hunter spiders and large, winged, herbivorous carabids), in contrast to wingless carabids in corridors. Conclusions Core patches, dispersal corridors and transient stepping-stones are not functionally interchangeable within this system. Semi-natural core patches supported a filtered subset of the regional fauna. Evidence for enhanced connectivity through percolation (corridors) or meta-population dynamics (stepping stones) differed between the two taxa

Taylor dispersion of gyrotactic swimming micro-organisms in a linear flow

The theory of generalized Taylor dispersion for suspensions of Brownian particles is developed to study the dispersion of gyrotactic swimming micro-organisms in a linear shear flow. Such creatures are bottom-heavy and experience a gravitational torque which acts to right them when they are tipped away from the vertical. They also suffer a net viscous torque in the presence of a local vorticity field. The orientation of the cells is intrinsically random but the balance of the two torques results in a bias toward a preferred swimming direction. The micro-organisms are sufficiently large that Brownian motion is negligible but their random swimming across streamlines results in a mean velocity together with diffusion. As an example, we consider the case of vertical shear flow and calculate the diffusion coefficients for a suspension of the alga <i>Chlamydomonas nivalis</i>. This rational derivation is compared with earlier approximations for the diffusivity

Linking mineralisation process and sedimentary product in terrestrial carbonates using a solution thermodynamic approach

Determining the processes which generate terrestrial carbonate deposits (tufas, travertines and to a lesser extent associated chemical sediments such as calcretes and speleothems) is a long-standing problem. Precipitation of mineral products from solution reflects a complex combination of biological, equilibrium and kinetic processes, and the different morphologies of carbonate sediment produced by different processes have yet to be clearly demarked. Building on the groundbreaking work of previous authors, we propose that the underlying control on the processes leading to the deposition of these products can be most parsimoniously understood from the thermodynamic properties of their source solutions. Here, we report initial observations of the differences in product generated from spring and lake systems spanning a range of temperature–supersaturation space. We find that at high supersaturation, biological influences are masked by high rates of physico-chemical precipitation, and sedimentary products from these settings infrequently exhibit classic "biomediated" fabrics such as clotted micrite. Likewise, at high temperature (>40 °C) exclusion of vascular plants and complex/diverse biofilms can significantly inhibit the magnitude of biomediated precipitation, again impeding the likelihood of encountering the "bio-type" fabrics. <br></br> Conversely, despite the clear division in product between extensive tufa facies associations and less spatially extensive deposits such as oncoid beds, no clear division can be identified between these systems in temperature–supersaturation space. We reiterate the conclusion of previous authors, which demonstrate that this division cannot be made on the basis of physico-chemical characteristics of the solution alone. We further provide a new case study of this division from two adjacent systems in the UK, where tufa-like deposition continuous on a metre scale is happening at a site with lower supersaturation than other sites exhibiting only discontinuous (oncoidal) deposition. However, a strong microbiological division is demonstrated between these sites on the basis of suspended bacterial cell distribution, which reach a prominent maximum where tufa-like deposits are forming. <br></br> We conclude that at high supersaturation, the thermodynamic properties of solutions provide a highly satisfactory means of linking process and product, raising the opportunity of identifying water characteristics from sedimentological/petrological characteristics of ancient deposits. At low supersaturation, we recommend that future research focuses on geomicrobiological processes rather than the more traditional, inorganic solution chemistry approach dominant in the past

Dispersion of biased swimming microorganisms in a fluid flowing through a tube

Classical Taylor-Aris dispersion theory is extended to describe the transport of suspensions of self-propelled dipolar cells in a tubular flow. General expressions for the mean drift and effective diffusivity are determined exactly in terms of axial moments, and compared with an approximation a la Taylor. As in the Taylor-Aris case, the skewness of a finite distribution of biased swimming cells vanishes at long times. The general expressions can be applied to particular models of swimming microorganisms, and thus be used to predict swimming drift and diffusion in tubular bioreactors, and to elucidate competing unbounded swimming drift and diffusion descriptions. Here, specific examples are presented for gyrotactic swimming algae.Comment: 20 pages, 4 figures. Published version available at http://rspa.royalsocietypublishing.org/content/early/2010/02/09/rspa.2009.0606.short?rss=

The Anglosphere Core as a Pluralistic Security Community

This thesis posits that in the post World War 2 era, a nexus of relationships has given rise to a transnational group of five states that form a Deutschian security community. This Anglospheric security community of the US, UK, Australia and NZ, is examined by utilising Adler and Barnett’s security community model. The model is adapted to give greater weight to the role of memes/culture. It finds that it is culture factors (meme-complexes) related to political values and social behaviour that inform the nature, and modus operandi of this Anglospheric security community. The Brexit debate on the UK’s future is found to have illuminated these issues by exposing aspects the contradictions between the competing meme-complexes of the wider Anglospheric community and the EU. The Anglospheric security community’s durability and progress is found to be directly related to notions of legitimacy. This conclusion is informed by revisiting Deutsch’s original writings on the difference between naturally developing communities and the dangers of policy-elites creating political constructs that run counter to cultural considerations. A values-based meme-complex found to provides not just a common identity but to inform the nature of the Anglospheric security community from which it accrues legitimacy. It is further posited that the Adler and Barnett model’s standard categorisations of pluralistic security community types do not adequately describe certain features of the Anglospheric security community. The research in this thesis has uncovered new institutions and fora and established that members do assist one another in conflict and confirms it to be a tightly-coupled version. However, the Anglospheric security community displays an actorship not implicit in Adler and Barnett’s categorisation. This thesis offers the terms ‘synergic’ and ‘hemiplegic’ to describe functional and dysfunctional communities. The Anglospheric security community is held to be synergic since it exhibits actorship on defence and security matters externally. In contrast the European Union is held up to be hemiplegic due to endemic problems to function cohesively on external defence issues

Instability modes and transition of pulsatile stenotic flow: pulse-period dependence

Accepted versio