283 research outputs found

    Towards a universal criteria for turbulence suppression in dilute turbidity currents with non-cohesive sediments

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    Turbidity currents exhibit fascinating physics as their sustained propagation depends on a tight interplay between the suspended sediments and turbulence. If resuspension dominates over deposition the intensity of the flow will increase, while if deposition dominates the flow turbulence can be completely damped inducing rapid settling of sediments and, eventually, flow extinction. This work explores the phenomenon whereby turbulence in a dilute turbidity current with non-cohesive sediments is abruptly extinguished owing to increasedsuspended sediment stratification.聽 Three parameters control the flow dynamics: Reynolds number (Re_t), Richardson number (Ri_t) and sediment settling velocity (V_z). The condition for complete turbulence suppression can be expressed as a critical value for Ri_t V_z. Based on simulations, limited experiments and limited field data, the critical value appears to have a logarithmic dependence on Re_t.Fil: Cantero, Mariano Ignacio. Comisi贸n Nacional de Energ铆a At贸mica. Gerencia del 脕rea de Energ铆a Nuclear. Instituto Balseiro. Archivo Hist贸rico del Centro At贸mico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Hist贸rico del Centro At贸mico Bariloche e Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Cient铆ficas y T茅cnicas. Centro Cient铆fico Tecnol贸gico Conicet - Patagonia Norte; ArgentinaFil: Shringarpure, Mrugesh. University of Florida; Estados UnidosFil: Balachandar, S.. University of Florida; Estados Unido

    Dynamics of complete turbulence suppression in turbidity currents driven by monodisperse suspensions of sediment

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    Turbidity currents derive their motion from the excess density imposed by suspended sediments. The settling tendency of sediments is countered by flow turbulence, which expends energy to keep them in suspension. This interaction leads to downward increasing concentration of suspended sediments (stable stratification) in the flow. Thus in a turbidity current sediments play the dual role of sustaining turbulence by driving the flow and damping turbulence due to stable stratification. By means of direct numerical simulations, it has been shown previously that stratification above a threshold can substantially reduce turbulence and possibly extinguish it. This study expands the simplified model by Cantero et al. (J. Geophys. Res., vol.114, 2009a, C03008), and puts forth a proposition that explains the mechanism of complete turbulence suppression due to suspended sediments. In our simulations it is observed that suspensions of larger sediments lead to stronger stratification and, above a threshold size, induce an abrupt transition in the flow to complete turbulence suppression. It has been widely accepted that hairpin and quasi-streamwise vortices are key to sustaining turbulence in wall-bounded flows, and that only vortices of sufficiently strong intensity can spawn the next generation of vortices. This auto-generation mechanism keeps the flow populated with hairpin and quasi-streamwise vortical structures and thus sustains turbulence. From statistical analysis of Reynolds stress events and visualization of flow structures, it is observed that settling sediments damp the Reynolds stress events (Q2 events), which means a reduction in both the strength and spatial distribution of vortical structures. Beyond the threshold sediment size, the existing vortical structures in the flow are damped to an extent where they lose their ability to regenerate the subsequent generation of turbulent vortical structures, which ultimately leads to complete turbulence suppression.Fil: Shringarpure, Mrugesh. University of Florida; Estados UnidosFil: Cantero, Mariano Ignacio. Comisi贸n Nacional de Energ铆a At贸mica. Gerencia del 脕rea de Energ铆a Nuclear. Instituto Balseiro. Archivo Hist贸rico del Centro At贸mico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Hist贸rico del Centro At贸mico Bariloche e Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Cient铆ficas y T茅cnicas. Centro Cient铆fico Tecnol贸gico Conicet - Patagonia Norte; ArgentinaFil: Balachandar, S.. University of Florida; Estados Unido

    Prevalence of Vitamin D3 deficiency among pediatric patients with idiopathic nephrotic syndrome in remission - A cross-sectional observational study from Vadodara, Gujarat

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    Background: Vitamin D deficiency occurs in nephrotic syndrome (NS) through various mechanisms, resulting in loss of both, Vitamin D binding protein and 25-(OH) D, in the urine leading to the risk of bone disorders. Objectives: The objectives of the study were to detect the prevalence of Vitamin D deficiency in children with idiopathic NS during remission. Methods: This study was conducted from April to November 2016 at the pediatric nephrology clinic at a tertiary care hospital in Vadodara. A total of 34 children were enrolled with idiopathic NS in remission, of which 14 had first attack of NS and ten of Frequently Relapsing Nephrotic Syndrome (FRNS) and Infrequently Relapsing Nephrotic Syndrome each. Vitamin D levels were measured using serum levels of 25-(OH) D by chemiluminescence method. Results: Vitamin D deficiency was observed in 28 of 34 (82%) children; of which, 16 (47%) had severe deficiency and 12 (35.2%) had mild to moderate deficiency. Children with the first attack of NS had a median Vitamin D level of 8.17 ng/ml (interquartile range [IQR] 2.9鈥28), IFRNS had a median of 6.8 ng/ml (IQR 2.9鈥33), and FRNS had the lowest median of 5.3 ng/ml (IQR 2.9鈥16). Although there were differences among all the 3 groups, differences were not statistically significant (Kruskal鈥揥allis 4.89, p=0.08) which showed decreased levels of Vitamin D. Conclusion: High prevalence of Vitamin D deficiency was observed in all 3 groups of idiopathic NS; the lowest being in FRNS. There was no significant association with lower levels of Vitamin D and relapses in NS. More research is needed to assess Vitamin D deficiency and to ensure the effect of Vitamin D supplementation for children with NS

    Medical research and ethics - Revisited

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    Ethics in the medical research is known since many years; however, there have been new developments in this area recently. A phenomenal improvement in the health-care system, leading to increased life expectancy, and thereby, newer lifestyle and other health-related diseases has opened avenues for newer drugs and health-care technology. However, these have to be tried and tested in the context of the disease epidemiology, health-care delivery and of course, medical ethics. Monitoring and evaluation of the treatment regimes of well documented effective medicines is also required. This is the core of medical research. With the ever increasing concept of evidence-based medical system, and thereby, a rapid rise in the number of clinical trials; the role of medical ethics is potentially increasing to keep the patient

    Entrainment in temporally evolving turbidity currents

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    Turbidity currents are sediment laden shear flows that run along a sloping bed, often sub- merged beneath a deep layer of quiescent fluid, driven by the excess hydrostatic pressure due to the suspended sediments. Turbidity currents are always turbulent since the suspended sediment particles that drive the flow cannot remain in suspension under laminar conditions. As the turbidity current travels downslope, the flow interacts with the bed at the bottom and with the ambient fluid layer at the top. Ambient fluid entrainment is a fascinating fluid mechanical phenomenon where quiescent ambient fluid is ingested into the current to an active shear flow. As the turbidity current flows downstream over the sloping bed, under a deep ambient of clear fluid, clear ambient fluid is continuously entrained into the turbidity current and the thickness of the current increases. In this work we study the entrainment mech- anism taking place between the ambient fluid layer and the turbidity current by means of fully resolved direct numerical simulations. Entrainment is a function of both the local Richardson number, Ri, and the non-dimensional settling velocity of the sediments. Here we consider a model turbidity current that is homogeneous in the streamwise direction. Thus, the effect of entrainment of clear fluid at the top of the turbidity current results in a temporal growth of the current height. With the assumption of streamwise homogeneity we investigate a non-stationary problem where the temporal growth of the height of the turbidity current is monitored in order to evaluate the role of entrainment of clear fluid.Publicado en: Mec谩nica Computacional vol. XXXV, no. 19Facultad de Ingenier铆

    Soft transition between subcritical and supercritical currents through intermittent cascading interfacial instabilities

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    Long-running gravity currents are flows that are submerged beneath a deep layer of quiescent fluid and they travel over long distances along inclined or horizontal surfaces. They are driven by the density difference between the current and the clear ambient fluid above. In this work we present results on highly resolved direct numerical simulations of turbid underflows that involve nearly 1 billion degrees of freedom. We assess the effect of bed slope on the flow statistics. We explore the turbulence dynamics of the interface in the classical sub-A nd supercritical regimes. We investigate the structure of interfacial turbulence and its relation to the turbulence statistic. A transcritical regime is identified where intermittent cascading interfacial instabilities appear. We investigate how departure from the self-sustaining equilibrium state may be the mechanism responsible for this cyclic evolution of the transcritical regime.Fil: Salinas, Jorge. University of Florida; Estados UnidosFil: Balachandar, S.. University of Florida; Estados UnidosFil: Shringarpure, Mrugesh. No especif铆ca;Fil: Fedele, Juan. No especif铆ca;Fil: Hoyal, David. No especif铆ca;Fil: Cantero, Mariano Ignacio. Comisi贸n Nacional de Energ铆a At贸mica. Gerencia del 脕rea de Energ铆a Nuclear. Instituto Balseiro. Archivo Hist贸rico del Centro At贸mico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Hist贸rico del Centro At贸mico Bariloche e Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Cient铆ficas y T茅cnicas. Centro Cient铆fico Tecnol贸gico Conicet - Patagonia Norte; Argentin

    On the definition, evolution, and properties of the outer edge of gravity currents: A direct-numerical and large-eddy simulation study

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    Gravity currents are flows driven by the action of gravity over fluids with different densities. Here, we focus on gravity currents where heavier fluid travels along the bottom of a sloping bed, underneath a large body of stagnant lighter ambient fluid. The thickness of the current increases due to entrainment of ambient fluid into the current. Direct numerical and large eddy simulations of gravity currents and a wall-jet transporting a passive scalar field are performed. We focus on the rate of penetration of mean momentum and mean concentration of the agent responsible for the density difference (temperature, salinity, or sediment volume fraction) into the ambient fluid. The rates of penetration of turbulence-related quantities (i.e., turbulent kinetic energy, Reynolds flux, and stress) into the ambient are analyzed. A robust methodology for defining the upper edge of these quantities and thereby defining the current thickness using these different mean and turbulent quantities is presented. A comparison between downstream evolution of the gravity current with the corresponding behaviors of canonical wall-bounded turbulent flows is presented. The present understanding of turbulent/non-turbulent interface (TNTI) is extended to include subcritical flows where, due to the strong effect of stratification, the TNTI is buried well within the upper edge of the current and confined right above the inner near-bed layer. The present work sheds light on the striking difference between the different definitions of thickness (momentum, concentration, turbulence, etc.) in subcritical gravity currents, where stratification suppresses turbulence in the upper region of the current.Fil: Salinas, Jorge S.. University of Florida; Estados UnidosFil: Balachandar, S.. University of Florida; Estados UnidosFil: Z煤帽iga, Santiago Luciano. Comisi贸n Nacional de Energ铆a At贸mica. Centro At贸mico Bariloche; Argentina. Comisi贸n Nacional de Energ铆a At贸mica. Gerencia del 脕rea de Energ铆a Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Cient铆ficas y T茅cnicas; ArgentinaFil: Shringarpure, M.. No especif铆ca;Fil: Fedele, J.. No especif铆ca;Fil: Hoyal, D.. No especif铆ca;Fil: Cantero, Mariano Ignacio. Comisi贸n Nacional de Energ铆a At贸mica. Gerencia del 脕rea de Energ铆a Nuclear. Instituto Balseiro; Argentina. Comisi贸n Nacional de Energ铆a At贸mica. Centro At贸mico Bariloche; Argentina. Consejo Nacional de Investigaciones Cient铆ficas y T茅cnicas; Argentin
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