Upscale energy transfer and flow topology in free-surface turbulence

Lovecchio, Salvatore; Zonta, Francesco; Soldati, Alfredo

doi:10.1103/physreve.91.033010

Cited by 14 publications

(12 citation statements)

References 24 publications

(32 reference statements)

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“…First, no evident −5/3 range is observed except for few of the lowest wave numbers: This can be attributed to the intermittent nature of turbulence associated with spatial fluctuations in the rate of energy dissipation. A relatively larger range of high wave numbers can be identified over which spectra exhibit a −3 scaling: In the present flow configuration, however, this corresponds to up-cascading of energy from large to small wave numbers, namely, to merging of smaller flow structures into larger structures [29]. Such findings cannot be reconciled with the KLB theory for 2D turbulence.…”

Section: A Flow Field Characterizationcontrasting

confidence: 58%

“…The effect of imposing the flat surface does not alter the turbulence in the bulk of flow, as it is constantly generated from the bottom wall (source of shear). The choice of imposing a flat free surface has been discussed in Lovecchio et al [18] and is motivated by previous findings [24,[29][30][31], which have shown that light particles moving at the deformed free surface of a turbulent flow are subject to clustering mechanisms that come from the horizontal divergence at the surface: These mechanisms induce a compressible effect similar to the one observed over flat surfaces.…”

Section: Physical Problem and Methodologymentioning

confidence: 99%

“…The most evident macroscopic manifestation of floaters and motile particle dynamics in freesurface turbulence is the formation of highly concentrated clusters near the surface [13,20,24,29]. These originate from the interaction between individual cells and surface flow structures.…”

Section: B Reynolds Number and Stability Number Effects On Surface Cmentioning

confidence: 99%

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Role of large-scale advection and small-scale turbulence on vertical migration of gyrotactic swimmers

et al. 2019

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In this work, we use direct-numerical-simulation-based Eulerian-Lagrangian simulations to investigate the dynamics of small gyrotactic swimmers in free-surface turbulence. We consider open-channel flow turbulence in which bottom-heavy swimmers are dispersed. Swimmers are characterized by different vertical stability, so that some realign to swim upward with a characteristic time smaller than the Kolmogorov timescale, while others possess a reorientation time longer than the Kolmogorov timescale. We cover one order of magnitude in the flow Reynolds number and two orders of magnitude in the stability number, which is a measure of bottom heaviness. We observe that large-scale advection dominates vertical motion when the stability number, scaled on the local Kolmogorov timescale of the flow, is larger than unity: This condition is associated to enhanced migration toward the surface, particularly at low Reynolds number, when swimmers can rise through surface renewal motions that originate directly from the bottomboundary turbulent bursts. Conversely, small-scale effects become more important when the Kolmogorov-based stability number is below unity: Under this condition, migration toward the surface is hindered, particularly at high Reynolds, when bottom-boundary bursts are less effective in bringing bulk fluid to the surface. In an effort to provide scaling arguments to improve predictions of models for motile microorganisms in turbulent water bodies, we demonstrate that a Kolmogorov-based stability number around unity represents a threshold beyond which swimmer capability to reach the free surface and form clusters saturates.

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Section: A Flow Field Characterizationcontrasting

confidence: 58%

Section: Physical Problem and Methodologymentioning

confidence: 99%

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Role of large-scale advection and small-scale turbulence on vertical migration of gyrotactic swimmers

et al. 2019

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“…also [43]. Furthermore, recent investigations [42] show a three-dimensional character of the turbulent flow at free surfaces; however, implications with regard to boundary conditions for the bulk flow seem to be uncertain at present. For the present analysis, it may be seen as reassuring that the application of the conventional boundary conditions (16a) and (16b) in previous work has led to reasonable agreement with measurements of surface elevation as well as shear stress distribution for both undular jumps [25,35,36,61] and stationary solitary waves [62].…”

Section: (Xh )mentioning

confidence: 99%

“…m , according to (37) and (42), respectively. a Plane ramp with height ψ ∞ and length L. First-order eigenvalue ψ ∞ = 12. b Bump with isosceles triangular cross section of height ψ(0) and half-length L…”

Section: Solution Of the First Kind (Solitary-wave Type) For A Plane mentioning

confidence: 99%

Near-critical turbulent open-channel flows over bumps and ramps

2018

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Steady two-dimensional turbulent free-surface flow in a channel with a slightly uneven bottom is considered. The shape of the unevenness of the bottom can be in the form of a bump or a ramp of very small height. The slope of the channel bottom is assumed to be small, and the bottom roughness is assumed to be constant. Asymptotic expansions for very large Reynolds numbers and Froude numbers close to the critical value Fr = 1, respectively, are performed. The relative order of magnitude of two small parameters, i.e. the bottom slope and (Fr − 1), is defined such that no turbulence modelling is required. The result is a steady-state version of an extended Korteweg-de Vries equation for the surface elevation. Other flow quantities, such as pressure, flow velocity components, and bottom shear stress, are expressed in terms of the surface elevation. An exact solution describing stationary solitary waves of the classical shape is obtained for a bottom of a particular shape. For more general shapes of ramps and bumps, stationary solitary waves of the classical shape are also obtained as a first approximation in the limit of small, but nonzero, dissipation. With the exception of an eigensolution for a ramp, an outer region has to be introduced. The outer solution describes a 'tail' that is attached to the stationary solitary wave. In addition to the solutions of the solitary-wave type, solutions of smaller amplitudes are obtained both numerically and analytically. Experiments in a water channel confirm the existence of both types of stationary single waves. 1 Introduction Gravity driven, plane (2D) free-surface flow of an incompressible liquid over a bottom containing an obstacle is a fundamental problem of hydraulics; cf. [12,14]. The classical approach is a one-dimensional flow approximation, based on the assumption of a hydrostatic pressure distribution. However, that is insufficient for many applications; cf. the monograph [11] on non-hydrostatic free-surface flows. In particular, the one-dimensional flow approximation together with the assumption of a hydrostatic pressure distribution leads to equations that Dedicated to Professor Alfred Kluwick on the occasion of his 75th birthday.

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