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

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.  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. 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, 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

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. 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: Balachandar, S.. University of Florida; Estados Unido

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

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

Equivalence of turbulence statistics between monodisperse and polydisperse turbidity currents

Turbidity currents are buoyancy driven submarine flows where the source of buoyancy is typically a polydisperse sediment suspension. Sustained propagation of such flows depend on the ability of turbulence in the flow to keep the settling sediments in suspension. Recent studies by Cantero et al. (2012b) and Shringarpure et al. (2012) have investigated the interaction of monodisperse sediment suspension and turbulence in turbidity currents on smooth sloping beds. These studies showed that stable stratification of sediment suspension damps turbulence and in some cases can be fully suppress turbulence. Furthermore, it was shown that the turbulence damping effect of a monodisperse sediment suspension can be quantified by the product of shear Richardson number and the sediment settling velocity. In this study we generalize this result for a polydisperse sediment suspension. We compare the turbulence statistics of turbidity currents driven by different polydisperse suspensions and show that as long as the total amount of sediment and the product of shear Richardson number and effective settling velocity (a value representing the polydisperse suspension) are the same, the turbulent velocity statistics of the different polydisperse suspensions nearly collapse. Furthermore, if the effective settling velocity is chosen to be depth-dependent (a function of height from the bed) then the turbulence statistics involving sediment concentration also collapses between different polydisperse suspensions. These results suggest the possibility of modeling polydisperse currents with an equivalent monodisperse suspension whose total sediment load and depth-dependent settling velocity match those of the polydisperse suspension.Fil: Shringarpure, Mrugesh. University of Florida; Estados Unidos. ExxonMobil Upstream Research Company; Estados UnidosFil: Cantero, Mariano Ignacio. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Balachandar, S.. University of Florida; Estados Unido

Mechanisms of Complete Turbulence Suppression in Turbidity Currents Driven by Mono-Disperse and Bi-Disperse Suspensions of Sediment

Turbidity currents are submarine flows where the sediment fluid mixture (heavy current) drives along the sloping ocean floor displacing the surrounding clear fluid (light ambient). Under the influence of gravity, the suspended sediments drive the current and at the same time settle down on the ocean bed. The interplay of turbulent mixing and settling sediments leads to stable stratification of sediments in the turbidity current. In previous studies (Cantero et al. 2009b; Cantero et al., 2009a; Cantero et al., 2012a; Talling et al., 2007) it was observed that strong settling tendency (large sediment sizes) could cause complete turbulence suppression. In this study, we will analyse this process of complete turbulence suppression by means of direct numerical simulations (DNS) of turbidity currents. In wall bounded unstratified flows, it has been long established that turbulence is sustained by the process of auto-generation of near-wall hairpin like and quasi-streamwise turbulent vortical structures. It was also identified that auto-generation is possible only when the strength of the turbulent structures is greater than a threshold value (Zhou et. al., 1996). Through quadrant analysis of Reynolds stress events and visualization of turbulent vortical structures, we observe that stratification by sediments lead to damping and spatial re-distribution of turbulent vortical structures in the flow. We propose that complete turbulence suppression is brought about by a total shutdown in the auto-generation process of the existing turbulent structures in the flow. We also identify three parameters – Reynolds number (Reτ), Richardson number (Riτ) and sediment settling velocity (V˜z) that quantify the process of turbulence suppression. A criterion for complete turbulence suppression is also proposed which can be defined as a critical value for RiτV˜z. This critical value is a function of Ret and based on simulations, experiments and field observations it appears to have a logarithmic dependence on Reτ (Cantero et al. 2012). DNS of turbidity currents driven by bi-disperse suspension of sediments is also carried out and compared with the results of mono-disperse suspensions.Fil: Shringarpure, Mrugesh S.. University of Florida; Estados UnidosFil: Cantero, Mariano Ignacio. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Selvakumar, Balachandar. University of Florida; Estados Unido