DNS of wall turbulence: dilute polymers and self-sustaining mechanisms

Angelis, E. De; Casciola, Carlo Massimo; Piva, R.

doi:10.1016/s0045-7930(01)00069-x

Cited by 141 publications

(127 citation statements)

References 11 publications

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“…This anticorrelation of polymer has also been found in the buffer layer via DNS of drag-reducing solutions. 50 The mechanism by which the polymer suppresses the streamwise vortices can be summarized as follows: Polymer molecules become stretched in the streamwise streak, then relax as they move from the streak into one of the streamwise vortices. The relaxation of the polymer molecules produces a force that directly opposes the motion of the fluid in the vortex and thus suppresses it.…”

Section: Given Whatmentioning

confidence: 99%

Polymer drag reduction in exact coherent structures of plane shear flow

et al. 2004

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Recently discovered traveling-wave solutions to the Navier-Stokes equations in plane shear geometries provide model flows for the study of turbulent drag reduction by polymer additives. These solutions, or "exact coherent states" (ECS), qualitatively capture the dominant structure of the near-wall buffer region of shear turbulence, i.e., counter-rotating pairs of streamwise-aligned vortices flanking a low-speed streak in the streamwise velocity. The optimum length scales for the ECS match well the length scales of the turbulent coherent structures and evidence suggests that the ECS underlie the dynamics of these structures. We study here the effect of viscoelasticity on these states. The changes to the velocity field for the viscoelastic ECS, where the FENE-P model calculates the polymer stress, mirror the modifications seen in experiments of fully turbulent flows of polymer solutions at low to moderate levels of drag reduction: drag is reduced, streamwise velocity fluctuations increase while wall-normal fluctuations decrease, and smaller wavelength structures are suppressed. These modifications to the ECS are due to the suppression of the streamwise vortices. The polymer molecules become highly stretched in the wavy, streamwise streaks, where the flow is predominately elongational, then relax as they move from the streaks into and around the streamwise vortices, where the flow is predominately rotational. This relaxation of the polymer molecules produces a force that directly opposes the fluid motion in the vortices, weakening them. Since the pressure fluctuations have their greatest magnitude (i.e., they are most negative) in the cores of the vortices, a reduction in vortex strength leads to a decrease in the magnitude of the pressure fluctuations. The pressure fluctuations redistribute energy from the streamwise velocity fluctuations to the Reynolds shear stress, so a decrease in their magnitude leads to a reduction in turbulent drag. For the viscoelastic ECS, we also find that after the onset of drag reduction (at Weissenberg number, WeϷ 7) there is a dramatic increase in the critical wall-normal length scale at which the ECS can exist. This sharp increase in length scale mirrors experimental observations and is also consistent with the observed shift to higher Reynolds numbers of the transition to turbulence in polymer solutions.

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Section: Given Whatmentioning

confidence: 99%

Polymer drag reduction in exact coherent structures of plane shear flow

et al. 2004

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“…The peak value of RMS(u′) also moves away from the wall as sediment concentration increases, linked to shear at the top of the thickened sublayer. It has been suggested that such thickening of the viscous sublayer and change in velocity gradient are a product of decreasing mixing in this near-bed flow that is caused by a stabilization of the boundary layer streaks and a reduction in the rate of bursting of low-momentum fluid upwards into the flow (de Angelis et al 2002;Li et al 2006). This in turn reduces interactions between the inner and outer regions of the flow (Kim & Sirviente 2005).…”

Section: Bedforms and Primary Current Stratification In Cohesive Sedimentioning

confidence: 99%

Predicting bedforms and primary current stratification in cohesive mixtures of mud and sand

Baas

Best

Peakall

2015

JGS

161

253

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The use of sedimentary structures as indicators of flow and sediment morphodynamics in ancient sediments lies at the very heart of sedimentology, and allows reconstruction of formative flow conditions generated in a wide range of grain sizes and sedimentary environments. However, the vast majority of past research has documented and detailed the range of bedforms generated in essentially cohesionless sediments that lack the presence of mud within the flow and within the sediment bed itself. Yet most sedimentary environments possess fine-grained sediments and recent work has shown how the presence of this fine sediment may substantially modify the fluid dynamics of such flows. It is increasingly evident that understanding the influence of mud, and the presence of cohesive forces, is essential to permit a fuller interpretation of many modern and ancient sedimentary successions. In this paper, the present state of knowledge on the stability of current- and wave-generated bedforms and their primary current stratification is reviewed, and a new extended bedform phase diagram is presented that summarizes the bedforms generated in mixtures of sand and mud under rapidly decelerated flows. This diagram provides a phase space using the variables of yield strength and grain mobility as the abscissa and ordinate axes, respectively, and defines the stability fields of a range of bedforms generated under flows that have modified fluid dynamics owing to the presence of suspended sediment within the flow. Our results also present unique data on a range of bedforms generated in such flows, whose recognition is essential to help interpret such deposits in the ancient sedimentary record, including the following: (1) heterolithic stratification, comprising alternating laminae or layers of sand and mud; (2) the preservation of low-amplitude bed-waves, large current ripples and bed scours with intrascour composite bedforms; (3) low-angle cross-lamination and long lenses and streaks of sand and mud formed by bed-waves; (4) complex stacking of reverse bedforms, mud layers and low-angle cross-lamination on the upstream face of bed scours; (5) planar bedding comprising stacked mud–sand couplets. Furthermore, the results shown herein demonstrate that flow variability is not required to produce deposits consisting of interbedded sand and muds, and that the nature of flaser, wavy and lenticular bedding ( sensu Reineck & Wunderlich 1968) may also need reconsideration in the deposits of such sediment-laden flows.

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“…The underlying mechanism that causes this drag reduction ͑DR͒ has been intensively studied over the last 50 years, and the original assumptions made by Lumley 1 and Metzner 2 have been verified by the numerical models, especially the direct numerical simulation ͑DNS͒. [3][4][5][6][7][8][9][10] Beginning with the study of Wells and Spangler, 11 it has been found that the polymer must be effective in the nearwall region for DR to occur. Petrie et al 12 confirmed Wells and Spangler's conclusion by carrying out experiments in a flat-plate boundary layer.…”

Section: Introductionmentioning

confidence: 99%

Modeling of viscoelastic turbulent flow in channel and pipe

Yang

Dou

2008

Physics of Fluids

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This paper investigates turbulent flows with or without polymer additives in open channels and pipes. Equations of mean velocity, root mean square of velocity fluctuations, and energy spectrum are derived, in which the shear stress deficit model is used and the non-Newtonian properties are represented by the viscoelasticity ␣ * . The obtained results show that, with ␣ * increment, ͑1͒ the streamwise velocity fluctuations is increased, ͑2͒ the wall-normal velocity fluctuation is attenuated, ͑3͒ the Reynolds stress is reduced, and ͑4͒ there is a redistribution of energy from high frequencies to the low frequencies for the streamwise component, but dimensionless distribution over all frequencies almost remains the same as that in Newtonian fluid flows. Good agreement between the derived equations and experimental data in small drag-reduction regime is achieved, which indicates that the present model is workable for Newtonian/non-Newtonian fluid turbulent flows.

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DNS of wall turbulence: dilute polymers and self-sustaining mechanisms

Cited by 141 publications

References 11 publications

Polymer drag reduction in exact coherent structures of plane shear flow

Polymer drag reduction in exact coherent structures of plane shear flow

Predicting bedforms and primary current stratification in cohesive mixtures of mud and sand

Modeling of viscoelastic turbulent flow in channel and pipe

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