528 research outputs found

    The Structure and Entrainment Characteristics of Partially Confined Gravity Currents

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    Seafloor channels are the main conduit for turbidity currents transporting sediment to the deep ocean, and they can extend for thousands of kilometers along the ocean floor. Although it is common for channel‐traversing turbidity currents to spill onto levees and other out‐of‐channel areas, the associated flow development and channel‐current interaction remain poorly understood; much of our knowledge of turbidity current dynamics comes from studies of fully confined scenarios. Here we investigate the role that partial lateral confinement may play in affecting turbidity current dynamics. We report on laboratory experiments of partially confined, dilute saline flows of variable flux rate traversing fixed, straight channels with cross‐sectional profiles representative of morphologies found in the field. Complementary numerical experiments, validated against high‐resolution laboratory velocity data, extend the scope of the analysis. The experiments show that partial confinement exerts a first‐order control on flow structure. Overbank and downstream discharges rapidly adjust over short length scales, providing a mechanism via which currents of varying sizes can be tuned by a channel and conform to a given channel geometry. Across a wide range of flow magnitudes and states of flow equilibration to the channel, a high‐velocity core remains confined within the channel with a constant ratio of velocity maximum height to channel depth. Ongoing overbank flow prevents any flow thickening due to ambient entrainment, allowing stable downstream flow evolution. Despite dynamical differences, the entrainment rates of partially confined and fully confined flows remain comparable for a given Richardson number

    Scaling Analysis of Multipulsed Turbidity Current Evolution With Application to Turbidite Interpretation

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    Deposits of submarine turbidity currents, turbidites, commonly exhibit upward‐fining grain size profiles reflecting deposition under waning flow conditions. However, more complex grading patterns such as multiple cycles of inverse‐to‐normal grading are also seen and interpreted as recording deposition under cycles of waxing and waning flow. Such flows are termed multipulsed turbidity currents, and their deposits pulsed or multipulsed turbidites. Pulsing may arise at flow initiation, or following downstream flow combination. Prior work has shown that individual pulses within multipulsed flows are advected forward and merge, such that complex longitudinal velocity profiles eventually become monotonically varying, although transition length scales in natural settings could not be predicted. Here we detail the first high frequency spatial (vertical, streamwise) and temporal measurements of flow velocity and density distribution in multipulsed gravity current experiments. The data support both a process explanation of pulse merging and a phase‐space analysis of transition length scales; in prototype systems, the point of merging corresponds to the transition in any deposit from multipulsed to normally graded turbidites. The scaling analysis is limited to quasi‐horizontal natural settings in which multipulsed flows are generated by sequences of relatively short sediment failures (10 km) sequences of breaches or where pulsing arises from combination at confluences of single‐pulsed flows, such flows may be responsible for the pulsing signatures seen in some distal turbidites, >100 km from source

    Interactions between downslope flows and a developing cold-air pool

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    A numerical model has been used to characterize the development of a region of enhanced cooling in an alpine valley with a width of order (Formula presented.) km, under decoupled stable conditions. The region of enhanced cooling develops largely as a region of relatively dry air which partitions the valley atmosphere dynamics into two volumes, with airflow partially trapped within the valley by a developing elevated inversion. Complex interactions between the region of enhanced cooling and the downslope flows are quantified. The cooling within the region of enhanced cooling and the elevated inversion is almost equally partitioned between radiative and dynamic effects. By the end of the simulation, the different valley atmospheric regions approach a state of thermal equilibrium with one another, though this cannot be said of the valley atmosphere and its external environment.Peer reviewe

    Optimisation of flow resistance and turbulent mixing over bed forms

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    Previous work on the interplay between turbulent mixing and flow resistance for flows over periodic rib roughness elements is extended to consider the flow over idealized shapes representative of naturally occurring sedimentary bed forms. The primary motivation is to understand how bed form roughness affects the carrying capacity of sediment-bearing flows in environmental fluid dynamics applications, and in engineering applications involving the transport of particulate matter in pipelines. For all bed form shapes considered, it is found that flow resistance and turbulent mixing are strongly correlated, with maximum resistance coinciding with maximum mixing, as was previously found for the special case of rectangular roughness elements. Furthermore, it is found that the relation between flow resistance to eddy viscosity collapses to a single monotonically increasing linear function for all bed form shapes considered, indicating that the mixing characteristics of the flows are independent of the detailed morphology of individual roughness elements

    Interactions between the night time valley-wind system and a developing cold-air pool

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    This is a pre-copyedited, author-produced PDF of an article accepted for publication in Boundary-Layer Meteorology following peer review. The version of record [Arduini, G., Staquet, C & Chemel, C., ‘Interactions between the night time valley-wind system and a developing cold-air pool’, Boundary-Layer Meteorol (2016) 161:1 (49-72), first published online June 2, 2016, is available at Springer online at doi: 10.1007/s10546-016-0155-8The Weather Research and Forecast (WRF) numerical model is used to characterize the influence of a thermally-driven down-valley flow on a developing cold-air pool in an idealized alpine valley decoupled from the atmosphere above. Results for a three-dimensional (3D) valley, which allows for the formation of a down-valley flow, and for a two-dimensional (2D) valley, where the formation of a down-valley flow is inhibited, are analyzed and compared. A key result is that advection leads to a net cooling in the 2D valley and to a warming in the 3D valley, once the down-valley flow is fully developed. This difference stems from the suppression of the slope-flow induced upward motions over the valley centre in the 3D valley. As a result, the downslope flows develop a cross-valley circulation within the cold-air pool, the growth of the cold-air pool is reduced and the valley atmosphere is generally warmer than in the 2D valley. A quasi-steady state is reached for which the divergence of the down-valley flow along the valley is balanced by the convergence of the downslope flows at the top of the cold-air pool, with no net contribution of subsiding motions far from the slope layer. More precisely, the inflow of air at the top of the cold-air pool is found to be driven by an interplay between the return flow from the plain region and subsidence over the plateaux. Finally, the mechanisms that control the structure of the cold-air pool and its evolution are found to be independent of the valley length as soon as the quasi-steady state is reached and the down-valley flow is fully developed.Peer reviewedFinal Accepted Versio

    Identification of polyhydroxyalkanoates in Halococcus and other haloarchaeal species

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    Polyhydroxyalkanoates (PHAs) are accumulated in many prokaryotes. Several members of the Halobacteriaceae produce poly-3-hydroxybutyrate (PHB), but it is not known if this is a general property of the family. We evaluated identification methods for PHAs with 20 haloarchaeal species, three of them isolates from Permian salt. Staining with Sudan Black B, Nile Blue A, or Nile Red was applied to screen for the presence of PHAs. Transmission electron microscopy and 1H-nuclear magnetic resonance spectroscopy were used for visualization of PHB granules and chemical confirmation of PHAs in cell extracts, respectively. We report for the first time the production of PHAs by Halococcus sp. (Halococcus morrhuae DSM 1307T, Halococcus saccharolyticus DSM 5350T, Halococcus salifodinae DSM 8989T, Halococcus dombrowskii DSM 14522T, Halococcus hamelinensis JCM 12892T, Halococcus qingdaonensis JCM 13587T), Halorubrum sp. (Hrr. coriense DSM 10284T, Halorubrum chaoviator DSM 19316T, Hrr. chaoviator strains NaxosII and AUS-1), haloalkaliphiles (Natronobacterium gregoryi NCMB 2189T, Natronococcus occultus DSM 3396T) and Halobacterium noricense DSM 9758T. No PHB was detected in Halobacterium salinarum NRC-1 ATCC 700922, Hbt. salinarum R1 and Haloferax volcanii DSM 3757T. Most species synthesized PHAs when growing in synthetic as well as in complex medium. The polyesters were generally composed of PHB and poly-ß-hydroxybutyrate-co-3-hydroxyvalerate (PHBV). Available genomic data suggest the absence of PHA synthesis in some haloarchaea and in all other Euryarchaeota and Crenarchaeota. Homologies between haloarchaeal and bacterial PHA synthesizing enzymes had indicated to some authors probable horizontal gene transfer, which, considering the data obtained in this study, may have occurred already before Permian times

    Differences in the Tumor Microenvironment between African-American and European-American Breast Cancer Patients

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    Background: African-American breast cancer patients experience higher mortality rates than European-American patients despite having a lower incidence of the disease. We tested the hypothesis that intrinsic differences in the tumor biology may contribute to this cancer health disparity. Methods and Results: Using laser capture microdissection, we examined genome-wide mRNA expression specific to tumor epithelium and tumor stroma in 18 African-American and 17 European-American patients. Numerous genes were differentially expressed between these two patient groups and a two-gene signature in the tumor epithelium distinguished between them. To identify the biological processes in tumors that are different by race/ethnicity, Gene Ontology and disease association analyses were performed. Several biological processes were identified which may contribute to enhanced disease aggressiveness in African-American patients, including angiogenesis and chemotaxis. African-American tumors also contained a prominent interferon signature. The role of angiogenesis in the tumor biology of African-American
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