Control of turbulent transport in supercritical currents by three families of hairpin vortices

We elucidate the turbulent structure of supercritical gravity currents by means of highly resolved simulations that employ up to a billion grid points. Three families of hairpin vortices dominate the near-bed and interface layers of the current, with their generation mechanisms dictating their shape, orientation, and number density. The interaction of near-bed and lower-interface hairpins explains the weak inviscid lidlike behavior of the streamwise velocity maximum, while the upper-interface vortices are responsible for the substantially lower entrainment compared to turbulent wall jets.Fil: Salinas, Jorge Sebastián. University of Florida; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Balachandar, S.. University of Florida; Estados UnidosFil: Cantero, Mariano Ignacio. Comisión Nacional de Energía Atómica. Gerencia de Área de Aplicaciones de la Tecnología Nuclear. Gerencia de Investigación Aplicada; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentin

Force and torque model sensitivity and coarse graining for bedload-dominated sediment transport

We present results from Euler–Lagrange simulations of turbulent flow over an erodible monodisperse bed of particles at a shear Reynolds number of R e τ = 180 . The Galileo number G a and the ratio of Shields to the critical Shields number Θ / θ c r were varied in the range 11.4 to 29.8 and 1.32 to 5.98, respectively. Two drag force models were investigated along with the influence of lift, particle rotation, and tangential collision forces for each model. Both models give similar results as far as mean particle flux and Shields stress are concerned, however we find that excluding particle rotation, without ignoring the associated tangential collisional force, significantly reduces the particle flux with little influence on Shields stress. On the other hand, when both particle rotation and tangential collision forces are not taken into account, the particle flux is practically unchanged, but the excess Shields stress slightly increases compared to the case where both effects are considered. We also find the lift force to substantially influence particle flux. Additionally, we show the importance of spatial coarse graining and time averaging for Euler–Euler simulations and quantify the reduction in scatter for space and time dependent variables such as sediment flux, Shields stress, and bed surface fluid velocities

Improved guidelines of indoor airborne transmission taking into account departure from the well-mixed assumption

The well-mixed assumption has been widely used in predicting the spread of infectious diseases in indoor spaces. It is to be expected that a perfect well-mixed state will not be achieved in an indoor space at any reasonable level of ventilation. This work evaluates the well-mixed assumption by comparing the theory with results from large eddy simulations. The robustness of the well-mixed theory is established by comparing at four different levels. The comparison also points out systematic departures in pathogen concentration which can be accurately accounted for with an easily implementable correction factor to quantities such as cumulative exposure time. With the well-mixed model as the baseline, the correction factor can be used to account for additional important problem-specific details. We demonstrate that more accurately accounting for variability in pathogen concentration can help obtain improved estimates for enhanced guidelines of indoor airborne transmission. We further demonstrate that at source-sink separation distances smaller than 5 m, the well-mixed theory on average underestimates the risk of contagion, while for distances larger than about 5 m, the well-mixed theory\u27s prediction, on average, is overly restrictive

Universal nature of rapid evolution of conservative gravity and turbidity currents perturbed from their self-similar state

The results of six highly resolved direct and large eddy simulations of gravity and conservative turbidity currents are presented to illustrate the point that the rapidly varying evolution of these currents follow a universal cyclic sequence of four states. The demarcation between these four states is determined by the bulk Richardson number and the acceleration/deceleration of the flow. We describe in detail the identification process of these states, together with the associated three-dimensional structure of the current. The exact depth-averaged momentum balance is computed and used to explain the intricate details of the nonmonotonic rapid evolution of the current between the different states. Finally, the balance of turbulent kinetic energy and concentration flux are computed to explain how and why the current evolves through the cyclic sequence of states. We also explore the mechanism by which the current can exit the cyclic sequence and slowly evolve towards self-similar supercritical or subcritical states.Fil: Zúñiga, Santiago Luciano. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Salinas, Jorge S.. University of Florida; Estados UnidosFil: Balachandar, S.. University of Florida; Estados UnidosFil: Cantero, Mariano Ignacio. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentin

Peering inside a cough or sneeze to explain enhanced airborne transmission under dry weather

High-fidelity simulations of coughs and sneezes that serve as virtual experiments are presented, and they offer an unprecedented opportunity to peer into the chaotic evolution of the resulting airborne droplet clouds. While larger droplets quickly fall-out of the cloud, smaller droplets evaporate rapidly. The non-volatiles remain airborne as droplet nuclei for a long time to be transported over long distances. The substantial variation observed between the different realizations has important social distancing implications, since probabilistic outlier-events do occur and may need to be taken into account when assessing the risk of contagion. Contrary to common expectations, we observe dry ambient conditions to increase by more than four times the number of airborne potentially virus-laden nuclei, as a result of reduced droplet fall-out through rapid evaporation. The simulation results are used to validate and calibrate a comprehensive multiphase theory, which is then used to predict the spread of airborne nuclei under a wide variety of ambient conditions

Entrainment in temporally evolving turbidity currents

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: Mecánica Computacional vol. XXXV, no. 19Facultad de Ingenierí

Cerebrospinal fluid shunting for the management of cryptococcal meningitis: a scoping review

Objective: This scoping review aimed to describe studies that evaluate the management of cryptococcal meningitis (CM) using cerebrospinal fluid (CSF) shunts, types of shunts used, and clinically relevant patient outcomes. Methods: We searched in the following databases: PubMed, Web of Science/Core collection, Embase, the Cochrane Library, and clinicaltrials.gov on 1 April 2022. We included two-arm and one-arm cohort studies that evaluated clinically relevant patient outcomes. Case reports were used to describe the type of CSF shunts used and the rationale behind its selection. The selection and extraction processes were independently performed by two authors. Results: This study included 20 cohort studies and 26 case reports. Only seven cohort studies compared two groups. Ventriculoperitoneal shunt was the most commonly used type of shunt (82.1%). The main indications for placing a shunt were persistently high opening pressure (57.1%) and persisting neurological symptoms or deterioration (54.3%). Cohort studies suggest that patients with shunt showed improvement in some outcomes such as neurological symptoms and hospital stay length. The most common shunt complications were post-operative fever (1–35.6%) and shunt obstruction (7–16%). Conclusion: CSF shunts may improve some clinically relevant outcomes in patients with CM, but the evidence is very uncertain.Revisión por pare

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

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 especifíca;Fil: Fedele, Juan. No especifíca;Fil: Hoyal, David. No especifíca;Fil: Cantero, Mariano Ignacio. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentin

Slope dependence of self-similar structure and entrainment in gravity currents

Results from seven direct and large-eddy simulations of gravity currents on slopes ranging from 0.14° to 2.86° that span from the subcritical to the supercritical regime are studied. By considering a long domain, attention is focused on the near-self-similar state approached by these currents far downstream. In the self-similar limit, the various shape factors, local Richardson number, entrainment coefficient, velocity scale and basal drag coefficient reach a constant value, while the current height, volume and momentum fluxes continue to increase linearly. Their dependence on slope is presented.Fil: Salinas, Jorge S.. University of Florida; Estados UnidosFil: Zúñiga, Santiago Luciano. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Cantero, Mariano Ignacio. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Shringarpure, M.. No especifíca;Fil: Fedele, J.. No especifíca;Fil: Hoyal, D.. No especifíca;Fil: Balachandar, S.. University of Florida; Estados Unido

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

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: Zúñiga, Santiago Luciano. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Shringarpure, M.. No especifíca;Fil: Fedele, J.. No especifíca;Fil: Hoyal, D.. No especifíca;Fil: Cantero, Mariano Ignacio. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin