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

2012 ◽  
Vol 712 ◽  
pp. 384-417 ◽  
Author(s):  
Mrugesh Shringarpure ◽  
Mariano I. Cantero ◽  
S. Balachandar
Keyword(s):  
Reynolds Stress ◽  
Suspended Sediments ◽  
Stable Stratification ◽  
Turbidity Currents ◽  
Vortical Structures ◽  
Threshold Size ◽  
Turbulence Suppression ◽  
Sediment Size ◽  
Ability To Regenerate ◽  
Monodisperse Suspensions

AbstractTurbidity 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.

scholarly journals Getting information on suspended sediments in a large river from acoustic backscatter

2018 ◽  
Vol 40 ◽  
pp. 04017
Author(s):  
Adrien Vergne ◽  
Céline Berni ◽  
Jérôme Le Coz
Keyword(s):  
Suspended Sediment Concentration ◽  
Spatial Information ◽  
Suspended Sediments ◽  
Sediment Concentration ◽  
Large River ◽  
Acoustic Backscatter ◽  
Qualitative Information ◽  
Sediment Size ◽  
The Common

There has been a growing interest in the last decade in extracting information on Suspended Sediment Concentration (SSC) from acoustic backscatter in rivers. Quantitative techniques are not yet effective, but acoustic backscatter already provides qualitative information on suspended sediments. In particular, in the common case of a bi-modal sediment size distribution, corrected acoustic backscatter can be used to look for sand particles in suspension and provide spatial information on their distribution throughout a river crosssection. This paper presents a case-study where these techniques have been applied.

scholarly journals Análise Montante-Jusante da Granulometria dos Sedimentos de Fundo e Suspenso do Rio Negro e Tributários (Bacia Amazônica, Brasil)

2021 ◽  
Vol 14 (2) ◽  
pp. 997
Author(s):  
Rogério Ribeiro Marinho ◽  
Antonio Fábio Sabbá Guimarães Vieira ◽  
Feliciano De Souza Maciel
Keyword(s):  
Sediment Transport ◽  
River Basin ◽  
Amazon Basin ◽  
Suspended Sediments ◽  
Fine Sand ◽  
Large River ◽  
Coarse Sand ◽  
Sediment Size ◽  
Negro River ◽  
Rio Negro

O conhecimento das características físicas de sedimentos transportados por grandes sistemas fluviais possui significativa importância para o entendimento de processos geomorfológicos e hidrológicos. O nível de conhecimento dos grandes sistemas fluviais da Amazônia e sua relação com o transporte de sedimentos ainda é limitado, resultando em lacunas de conhecimento sobre a dinâmica da paisagem nesta complexa região. Este trabalho teve como objetivo avaliar a distribuição espacial da granulometria de sedimentos de fundo e suspenso do Rio Negro e tributários. Realizou-se análise da distribuição do tamanho dos sedimentos em seções amostrais localizadas no alto, médio e baixo curso do Rio Negro. Os resultados da análise granulométrica da carga de fundo indicam a predominância de sedimentos com tamanho variando de 0,25 a 1,0 mm (principalmente areia fina, areia média e areia grossa) enquanto no baixo curso as amostram oscilaram de areia fina a partículas lamosas (< 0,50 mm). No Rio Negro os sedimentos suspensos são compostos principalmente de partículas finas de silte (90% menor que 80 µm) com diâmetro mediano (D50) de 25 µm. As características granulométricas apresentadas neste trabalho fornecem subsídios para o entendimento de processos hidrodinâmicos de transporte e deposição dos sedimentos de fundo e suspenso neste gigante sistema fluvial.    Upstream-downstream Granulometry Analysis of bed and suspended sediments in the Negro River Basin (Amazon Basin, Brazil)A B S T R A C TThe knowledge of the physical characteristics of sediments transported by large river systems has significant importance for the understanding of geomorphological and hydrological processes. The level of knowledge of the large rivers of the Amazon basin and their relationship with sediment transport is limited, resulting in gaps about the dynamics of the landscape in this complex region. This article analyzes the spatial distribution of granulometry of bed and suspended sediments in the Negro River and tributaries. An analysis of the sediment size distribution was carried out in sample sections located in the upper, middle and lower reaches of the Negro River. The results of the granulometric analysis of the bed load indicate the predominance of sediments with sizes ranging from 0.25 to 1.0 mm (mainly fine sand, medium sand and coarse sand) while in the low course they showed oscillated from fine sand to muddy particles (<0.50 mm). In the Negro River basin the suspended sediments are composed mainly of fine silt particles (90% less than 80 µm) with a median diameter (D50) of 25 µm. The granulometric characteristics presented in this work provide subsidies for the understanding of hydrodynamic processes of transport and deposition of bed and suspended sediments in this huge fluvial system.Keywords: sediment transport, Amazon floodplain, multichannel river, anabranching

Vortex pairing in a circular jet under controlled excitation. Part 2. Coherent structure dynamics

1980 ◽  
Vol 101 (3) ◽  
pp. 493-544 ◽  
Author(s):  
A. K. M. F. Hussain ◽  
K. B. M. Q. Zaman
Keyword(s):  
Reynolds Stress ◽  
Coherent Structure ◽  
Random Fluctuation ◽  
Spatial Distributions ◽  
Reynolds Stresses ◽  
Potential Core ◽  
Vortical Structures ◽  
Vortex Pairing ◽  
Structure Dynamics ◽  
Background Turbulence

The coherent structure dynamics in the near field of a circular jet has been experimentally explored by inducing ‘stable’ vortex pairing through controlled excitation (see Zaman & Hussain 1980) and applying phase-averaging techniques. Hot-wire measurements were made in a 7·62 cm air jet with laminar exit boundary layer at the Reynolds number ReD = 3·2 × 104, excited at the Strouhal number StD = 0·85. At a particular phase during the pairing process, spatial distributions of the phase-average longitudinal and lateral velocity perturbations (〈u)〉, 〈v〉), vorticity, streamlines, the coherent and background Reynolds stresses and turbulence intensities have been educed. These data have been obtained for four different locations occupied by the vortices at the same phase (preceding, during, and following the pairing event), in the region 0 < x/D < 5. Spatial distributions of these measures at four successive phases during the pairing process are also educed in an attempt to further understand the vortex-pairing dynamics. The flow physics is discussed on the basis of measurements over the physical extent of the vortical structures, phase-locked to specific phases of the pairing event and thus do not involve use of the Taylor hypothesis.The computed pseudostream functions at particular phases are compared with the corresponding streamlines drawn by the method of isoclines. Transition of the vortices is examined on the basis of vorticity diffusion, the superimposed random fluctuation field intensities and Reynolds stress and phase-locked circumferential correlation measurements. The peak vorticity drops rapidly owing to transition and interaction of the vortices during pairing but, farther downstream, the decay can be attributed to destruction of the coherent vorticity by the background turbulence Reynolds stress, especially at the locations of the latter's ‘saddle points’. Controlled excitation enhances the initial circumferential coherence of the vortical structures, but is ineffective in delaying turbulent breakdown near the end of the potential core; the breakdown appears to occur through evolution of the circumferential lobe structures. The coherent structure Reynolds stress is found to be much larger than the background turbulence Reynolds stress for 0 < x/D [lsim ] 3, but these two are comparable near the end of the jet potential core. The zone average of the coherent structure Reynolds stress over the cross-section of the merging vortex pair is much larger than that over a single vortical structure either before or after the completion of pairing. During the pairing process, such average correlations are found to be the largest at an early phase of the process while entrainment, turbulent breakdown as well as rapid diffusion of vorticity occur at a later phase. The regions of alternate positive and negative coherent Reynolds stresses associated with the structures and their interactions help explain ‘negative production’.

scholarly journals Investigation of Hydrodynamic Dispersion and Intra-pore Turbulence Effects in Porous Media

2019 ◽  
Vol 131 (2) ◽  
pp. 739-765
Author(s):  
Nima Fallah Jouybari ◽  
T. Staffan Lundström ◽  
J. Gunnar I. Hellström
Keyword(s):  
Porous Media ◽  
Reynolds Stress ◽  
Flow In Porous Media ◽  
Spherical Particles ◽  
Hydrodynamic Dispersion ◽  
Reynolds Stresses ◽  
Vortical Structures ◽  
Turbulence Structures ◽  
Unit Cells ◽  
Turbulence Effects

AbstractThe aim of the present paper is to evaluate and compare the pore level hydrodynamic dispersion and effects of turbulence during flow in porous media. In order to compute these quantities, large eddy simulations of turbulent flow in five unit cells comprised of spherical particles are performed and the results are averaged over the cells. Visualizations of vortical structures reveal that the size of the turbulence structures is of the size of the pores. Investigations furthermore yield that volume-averaged values of the hydrodynamic dispersion are of the same order as the Reynolds stress within the pores. It is also shown that the effect of intra-pore turbulence and hydrodynamic dispersion on the redistribution of macroscopic momentum within the porous medium is negligible compared to Forchheimer term. A discussion is provided on the accuracy of the eddy viscosity hypothesis in the modeling of the volume-averaged intra-pore Reynolds stresses. Finally, the effect of variation in the pore-scale geometry on the turbulence structures and averaged values of hydrodynamic dispersion and Reynolds stress is investigated.

Vortex Transport in Separating Flows and Role of Vortical Structures in Reynolds Stress Production and Distribution

2005 ◽  
Author(s):  
Miad Yazdani ◽  
Yasmin Khakpour
Keyword(s):  
Reynolds Stress ◽  
Momentum Equation ◽  
Navier Stokes ◽  
Viscous Effects ◽  
Navier Stokes Equation ◽  
Vortical Structures ◽  
Open Questions ◽  
Production And Distribution ◽  
History Of ◽  
Separating Flows

In this paper we will present some approaches on Reynolds stress production by vortex transport phenomena and nonlinear vorticity generation in momentum equation. First of all we represent a history of recent works to describe how fluid particle motions can be associated with Reynolds stress through either displacement or acceleration terms. In the next section we will describe how vortex stretching causes the Reynolds stress production and what is the dominant effect near and far from the boundary where viscous effects have to be considered. On the other hand, some vortex considered methodologies such as those synthesize boundary layer, as a collection of vortical objects seem to be inappropriate in general flow configuration. Therefore, there must be a moderate consideration in which both vortex and momentum transports come into account as it is done in LES. Furthermore since there exist open questions on Reynolds stress distribution in complex flows such as those with separation, our particular attention is paid to such effects due to vortical structures in separating flows. Further discussions include turbulence development caused by either vortex stretching or gradient terms that is determined by predominant conditions. However, it is seen that at the beginning, vorticity generators in Navier-Stokes equation contribute to dissipation effect. In addition, since such contribution corresponds to vorticity alignment, we investigate maximum vortex aligning and the effects of which causes the deviation of such alignment. The paper provides theoretical and numerical comparisons, where in the former, the vortical structure role is taken into account.

scholarly journals SEDIMENT TRANSPORT DUE TO CURRENT-SUPPORTED TURBIDITY CURRENTS OVER AN ERODIBLE BED

2018 ◽  
pp. 33
Author(s):  
Celalettin Emre Ozdemir ◽  
Sahar Haddadian
Keyword(s):  
Boundary Layer ◽  
Settling Velocity ◽  
Critical Shear Stress ◽  
Suspended Sediments ◽  
Turbidity Currents ◽  
Gravitational Acceleration ◽  
Boundary Layer Turbulence ◽  
Erosion Coefficient ◽  
Self Motion

Wave- and current-supported turbidity currents (WCSTCs), are one of the chief participants in shaping the marine geomorphology. What makes WCSTCs different from other turbidity currents is that boundary layer turbulence is required to suspend the sediments rather than the self-motion of the turbidity currents. In the presence of a mild slope, the gravitational acceleration drives the suspended sediments offshore (Sternberg et al., 1996; Wright et al., 2001). Depending on what dominates the boundary layer turbulence (BLT), we further define two major subclasses of WCSTCs: (i) wave-supported (WSTCs), and (ii) current-supported turbidity currents (CSTCs). Although significant advances have been made on the details of WSTCs (Ozdemir et al., 2011; Yu et al., 2014; Cheng et al., 2015), less is known about CSTCs. The objective of present study is to investigate the role of alongshore currents on CSTC dynamics over an erodible bottom boundary. The focus here is to identify the possible role of erosion on CSTC dynamics, and assess the coupling between current-induced BLT and suspended sediments for various bed erodibility parameters, i.e. critical shear stress, erosion coefficient, and settling velocity.

Dynamic Reynolds stress modeling of turbidity currents

1992 ◽  
Vol 97 (C6) ◽  
pp. 9645 ◽  
Author(s):  
Bård Brørs ◽  
Karl J. Eidsvik
Keyword(s):  
Reynolds Stress ◽  
Turbidity Currents ◽  
Stress Modeling
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