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

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

Accepted versio

Interaction of temperature, salinity and extracellular polymeric substances controls trace element incorporation into tufa calcite

The influence of extracellular polymeric substances on carbonate mineral growth in natural settings remains one of the most poorly understood contributors to the growth of non-marine carbonate sediments. The influences of these materials are complicated by their association with living cells creating local microenvironments via metabolism and enzyme production, and by our uncertainty about the extracellular polymeric substances materials themselves. Different mixtures of extracellular polymeric substance molecules may behave in different ways, and differences in the local physical environment may alter how the mixtures influence mineral formation, and even result in different patterns of polymerization. Here, the influence of extracellular polymeric substances on calcite precipitation rate and Mg/Cacalcite in the absence of cells is investigated using extracts of extracellular polymeric substances from temperate fluvial tufa biofilm. The influence is complex, with the concentration of extracellular polymeric substances in solution altering deposition rate and trace element incorporation. Moreover, the results show interaction of the presence/absence of extracellular polymeric substances and both temperature and salinity. However, despite extracting extracellular polymeric substances from the same parent sample, a uniform influence was not found in these experiments, implying that the mixture is sufficiently variable within a sample for microenvironments within the biofilm to either promote or inhibit mineralization. As sedimentologists, we can no longer take the view that extracellular polymeric substances are a bystander material, or that they have a single set of coherent and predictable or intuitive influences. Rather, the emphasis must be on investigating the specific mixtures present in nature, and their complex and dynamic interaction with both mineral surfaces and hydrochemical conditions

Financing strategies for growth in the renewable energy industry in South Africa

This study analyses empirical qualitative data collected from key stakeholders in the renewable energy industry in South Africa. As a step on the path towards developing better success in financing for renewable energy entrepreneurs, a financing framework is proposed and used to create a holistic view of the financing process in the renewable energy sector. It allows stakeholders to get an understanding of all the motives, barriers, sources of capital and possible destinations of capital in one system. Many good reasons exist for South Africa to invest in renewable energy with motives dominated by environmental concerns, diversity of supply, job creation and economic development. Internationally, investment in renewable energy projects has been growing despite a significant slump in overall global investment trends. In a decentralised business model, smaller renewable energy based businesses will continue to have difficulty in raising finance in South Africa. Key barriers include the high price of energy and equipment resulting in poor profitability, the reliability and quality of government policy, a lack of awareness and experience and a lack of innovative financing solutions. The study finds there are many expectations for government to address the needs of the industry within the context of its current regulated status. There appears to be a preference for demand side interventions, which rely on levies, subsidies and tax incentives. This paper strives to offer new ways of looking at the financing problems currently being experienced in the industry and proposes an innovative framework to assist the stakeholders in the industry in structuring financing for renewable energy ventures

Financing strategies for growth in the renewable energy industry in South Africa

This study analyses empirical qualitative data collected from key stakeholders in the renewable energy industry in South Africa. As a step on the path towards developing better success in financing for renewable energy entrepreneurs, a financing framework is proposed and used to create a holistic view of the financing process in the renewable energy sector. It allows stakeholders to get an understanding of all the motives, barriers, sources of capital and possible destinations of capital in one system. Many good reasons exist for South Africa to invest in renewable energy with motives dominated by environmental concerns, diversity of supply, job creation and economic development. Internationally, investment in renewable energy projects has been growing despite a significant slump in overall global investment trends. In a decentralised business model, smaller renewable energy based businesses will continue to have difficulty in raising finance in South Africa. Key barriers include the high price of energy and equipment resulting in poor profitability, the reliability and quality of government policy, a lack of awareness and experience and a lack of innovative financing solutions. The study finds there are many expectations for government to address the needs of the industry within the context of its current regulated status. There appears to be a preference for demand side interventions, which rely on levies, subsidies and tax incentives. This paper strives to offer new ways of looking at the financing problems currently being experienced in the industry and proposes an innovative framework to assist the stakeholders in the industry in structuring financing for renewable energy ventures

NEOGENE AND QUATERNARY SEDIMENTATION PATTERNS IN THE NORTHWESTERN HYBLEAN PLATEAU (SE SICILY): THE EFFECTS OF A COLLISIONAL PROCESS ON A FORELAND MARGIN

Miocene carbonates and marls in the northwestern margin of the Hyblean Plateau (SE Sicily) exhibit considerable thickness variations consistent with synsedimentary tectonism. In the Early-Middle Mio­cene carbonates (Ragusa Formation) a series of parallel swells dose to the platform edge encouraged the development of a coarse-grained bioclastic facies. The overlying Middle-Late Miocene marls (Tellaro Formation) thin over these seafloor highs and associated mafie Tortonian volcanics are particularly developed along the hinge zones between swells and local sea-floor depressions. The basic extrusives appear to have followed in­cipient NE-SW fault lines propagated from deeper basement fractures. During the succeeding Messinian these faults were activated to produce horst structures (Vallone Lamia horst, Monte Caratabia horst and Cameme horst). These separate locai NE-SW oriented narrow grabens in which thick Messinian evaporites were deposited, probably connected with the main Centrai Sicilian Basin. Further subacrial deposits and volcanics levelled the Hyblean horst and graben topography prior to the major Early Pliocene marine high-stand. The ensuing transgression covered the study area with a thick pelagic Early Pliocene chalk. Local regressions, the product of Middle Pliocene and Early Pleistocene major tectonic activity along the margins of the plateau, are associated with local unconformities. Final emergence along the margins of the plateau occurred at the close of Early Pleistocene times when the emplacement of the Gela Nappe, from the NE, partly filled the foredeep basin. The withdrawal of the sea from the foredeep occurred shortly afterwards

The heats of formation of the haloacetylenes XCCY [X, Y = H, F, Cl]: basis set limit ab initio results and thermochemical analysis

The heats of formation of haloacetylenes are evaluated using the recent W1 and W2 ab initio computational thermochemistry methods. These calculations involve CCSD and CCSD(T) coupled cluster methods, basis sets of up to spdfgh quality, extrapolations to the one-particle basis set limit, and contributions of inner-shell correlation, scalar relativistic effects, and (where relevant) first-order spin-orbit coupling. The heats of formation determined using W2 theory are: \hof(HCCH) = 54.48 kcal/mol, \hof(HCCF) = 25.15 kcal/mol, \hof(FCCF) = 1.38 kcal/mol, \hof(HCCCl) = 54.83 kcal/mol, \hof(ClCCCl) = 56.21 kcal/mol, and \hof(FCCCl) = 28.47 kcal/mol. Enthalpies of hydrogenation and destabilization energies relative to acetylene were obtained at the W1 level of theory. So doing we find the following destabilization order for acetylenes: FCCF $>$ ClCCF $>$ HCCF $>$ ClCCCl $>$ HCCCl $>$ HCCH. By a combination of W1 theory and isodesmic reactions, we show that the generally accepted heat of formation of 1,2-dichloroethane should be revised to -31.8$\pm$0.6 kcal/mol, in excellent agreement with a very recent critically evaluated review. The performance of compound thermochemistry schemes such as G2, G3, G3X and CBS-QB3 theories has been analyzed.Comment: Mol. Phys., in press (E. R. Davidson issue

Oxygen Concentration Flammability Thresholds of Selected Aerospace Materials Considered for the Constellation Program

Materials selection for spacecraft is based on an upward flammability test conducted in a quiescent environment in the highest expected oxygen concentration environment. The test conditions and its pass/fail test logic do not provide sufficient quantitative materials flammability information for an advanced space exploration program. A modified approach has been suggested determination of materials self-extinguishment limits. The flammability threshold information will allow NASA to identify materials with increased flammability risk from oxygen concentration and total pressure changes, minimize potential impacts, and allow for development of sound requirements for new spacecraft and extraterrestrial landers and habitats. This paper provides data on oxygen concentration self-extinguishment limits under quiescent conditions for selected materials considered for the Constellation Program