Thermal instability in a time-dependent base state due to sudden heating

When a large body of fluid is heated from below at a horizontal surface the heat diffuses into the fluid, giving rise to a gravitationally unstable layer adjacent to the boundary. A consideration of the instantaneous Rayleigh number using the thickness of this buoyant layer as a length scale would lead one to expect that the heated fluid is initially stable, and only becomes unstable after a finite time. This transition would also apply to other situations, such as heating a large body of fluid from the side, where a buoyant upward flow develops near the boundary. In such cases when the evolving thermal boundary layer first becomes unstable the time-scale for the growth of the instabilities may be comparable to the time-scale of the evolution of the background temperature profile, and so analytical approximations such as the quasi-static approximation, where the time-evolution of the background state is ignored, are not strictly appropriate. We develop a numerical scheme where we find the optimal growth of linear perturbations to the background flow over a given time interval. Part of this problem is to determine an appropriate measure of the amplitude to the disturbances, as inappropriate choices can lead to apparent growth of disturbances over finite time intervals even when the fluid is stable. By considering the Rayleigh–B´enard problem, we show these problems can be avoided by choosing a measure of the amplitude that uses both the velocity and temperature perturbations, and which minimizes the maximum growth. We apply our analysis to the problems of heating a semi-infinite body of fluid from horizontal and vertical boundaries. We will show that for heating from a vertical boundary there are large and small Prandtl number modes. For some Prandtl numbers both modes may play a role in the growth of instabilities. In some cases there is transition during the evolution of the most unstable instabilities in fluids such as water, where initially the instabilities are large Prandtl number modes and then morph into small Prandtl number modes part of the way through their evolution

Dissolved noble gases and stable isotopes as tracers of preferential fluid flow along faults in the Lower Rhine Embayment, Germany

Groundwater in shallow unconsolidated sedimentary aquifers close to the Bornheim fault in the Lower Rhine Embayment (LRE), Germany, has relatively low δ2H and δ18O values in comparison to regional modern groundwater recharge, and 4He concentrations up to 1.7 × 10−4 cm3 (STP) g–1 ± 2.2 % which is approximately four orders of magnitude higher than expected due to solubility equilibrium with the atmosphere. Groundwater age dating based on estimated in situ production and terrigenic flux of helium provides a groundwater residence time of ∼107 years. Although fluid exchange between the deep basal aquifer system and the upper aquifer layers is generally impeded by confining clay layers and lignite, this study’s geochemical data suggest, for the first time, that deep circulating fluids penetrate shallow aquifers in the locality of fault zones, implying  that sub-vertical fluid flow occurs along faults in the LRE. However, large hydraulic-head gradients observed across many faults suggest that they act as barriers to lateral groundwater flow. Therefore, the geochemical data reported here also substantiate a conduit-barrier model of fault-zone hydrogeology in unconsolidated sedimentary deposits, as well as corroborating the concept that faults in unconsolidated aquifer systems can act as loci for hydraulic connectivity between deep and shallow aquifers. The implications of fluid flow along faults in sedimentary basins worldwide are far reaching and of particular concern for carbon capture and storage (CCS) programmes, impacts of deep shale gas recovery for shallow groundwater aquifers, and nuclear waste storage sites where fault zones could act as potential leakage pathways for hazardous fluids

Umatilla Virus Genome Sequencing and Phylogenetic Analysis: Identification of Stretch Lagoon Orbivirus as a New Member of the Umatilla virus Species

The genus Orbivirus, family Reoviridae, includes 22 species of viruses with genomes composed of ten segments of linear dsRNA that are transmitted between their vertebrate hosts by insects or ticks, or with no identified vectors. Full-genome sequence data are available for representative isolates of the insect borne mammalian orbiviruses (including bluetongue virus), as well as a tick borne avian orbivirus (Great Island virus). However, no sequence data are as yet available for the mosquito borne avian orbiviruses

Elective minimally invasive coronary artery bypass: Shunt or tournique occlusion? Assessment of a protective role of perioperative left anterior descending shunting on myocardial damage. A prospective randomized study

<p>Abstract</p> <p>Background</p> <p>To determine impact of intraluminal-left anterior descending shunt to prevent myocardial damage in minimally invasive coronary artery bypass.</p> <p>Methods</p> <p>38 patients were randomly assigned to external tournique occlusion (n = 19) or intraluminal-left anterior descending shunt group (n = 19). Blood samples for cardiac troponin T were collected at 30 minutes prior to, 6 and 24 hours after surgery.</p> <p>Results</p> <p>One patient in external tournique occlusion and two patients in intraluminal-left anterior descending shunt group were excluded from futher analysis due to preoperative cardiac troponin T level above the 99th-percentile. Postoperatively, each six patients in external tournique occlusion (33.3%) and intraluminal-left anterior descending shunt (35.3%) group were above the 99th-percentile. Two patients from each group (external tournique occlusion group 11.1% vs. intraluminal-left anterior descending shunt group 11.8%) had peak values above 10-% coeficient of variation cutoff (<it>p</it> = 1). There were no significant differences in between both groups at all studied timepoints.</p> <p>Conclusion</p> <p>There was no protective effect of intraluminal shunting on myocardial damage compared to short-term tournique occlusion. It is upon the surgeon's discretion which method may preferrably be used to achieve a bloodless field in grafting of the non-occluded left anterior descending in minimally invasive coronary artery bypass.</p