Small-Scale Anisotropy in Turbulent Shearless Mixing

Tordella, Daniela; Iovieno, Michele

doi:10.1103/physrevlett.107.194501

Cited by 27 publications

(32 citation statements)

References 20 publications

(66 reference statements)

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“…The scalar layer thickness θ is defined as the distance between the points where θ is equal to 0.25 and 0.75. The energy layer thickness is defined as the distance between the points where the normalised turbulent kinetic energy (E − E 2 )/(E 1 − E 2 ) is equal to 0.25 and 0.75 as in [14,15]. The exponents of the power-law fitting of the scalar thickness growth are indicated.…”

Section: Statisticsmentioning

confidence: 99%

“…The intensity of the intermittency and the penetration of this layer are controlled by the kinetic energy gradient and, when present, by the integral scale gradient. The Reynolds number has a minor effect on the large-scale velocity intermittency but has a major impact on the level of small-scale intermittency [15].…”

Section: Passive Scalar Transport Across the Interfacementioning

confidence: 99%

“…It should be noticed that this particular fluctuation field features a compression of the fluid filaments normal to the interface, that is along the direction of the mean gradient of both the scalar and kinetic energy fields. This is accompanied by the stretching of the fluid filaments parallel to the interface and is signature of the smallscale anisotropy [15]. It should be noticed that local compression in the straining motion is also a feature associated with the ramp and cliff structure of the scalar field observed in the case of an imposed mean scalar gradient in anisotropic turbulent-like flow field [3,10,17].…”

Section: Introductionmentioning

confidence: 97%

“…The field that transports it is formed by one high kinetic energy turbulent isotropic field which is left to convectively diffuse into a lower energy one. In this shear-free velocity field, the region where the two turbulences interact is preceded by a highly intermittent thin layer -where the energy flux is maximum -that propagates into the low-energy region (see [13][14][15][16]). It should be noticed that this particular fluctuation field features a compression of the fluid filaments normal to the interface, that is along the direction of the mean gradient of both the scalar and kinetic energy fields.…”

Section: Introductionmentioning

confidence: 99%

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Mixing of a passive scalar across a thin shearless layer: concentration of intermittency on the sides of the turbulent interface

Iovieno

Savino

Gallana

et al. 2014

Journal of Turbulence

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The advection of a passive scalar through an initial flat interface separating two different isotropic decaying turbulent fields is investigated in two and three dimensions. Simulations have been performed for a range of Taylor's microscale Reynolds numbers from 45 to 250 and for a Schmidt number equal to 1. Different to the case where the transport involves the momentum and kinetic energy only and one intermittency layer is formed in the low-turbulent energy side of the system, in the passive scalar concentration field two intermittent layers are observed to develop at the sides of the interface. The layers move normally to the interface in opposite directions. The dimensionality produces different time scaling of the passive scalar diffusion, which is much faster in the two-dimensional case. In two dimensions, the propagation of the intermittent layers exhibits a significant asymmetry with respect to the initial position of the interface and is deeper for the layer which moves towards the high kinetic energy side of the system. In three dimensions, the two intermittent layers propagate nearly symmetrically with respect the centre of the mixing region. During the temporal decay, inside the mixing, which is both inhomogeneous and anisotropic but devoid of a mean velocity shear, the passive scalar spectra are computed. In three dimensions, the exponent in the scaling range gets in time a value close to that of the kinetic energy spectrum of isotropic turbulence (−5/3). In two dimensions, instead the exponent settles down to a value that is about one-half of the corresponding isotropic case. By means of an analysis based on simple wavy perturbations of the interface we show that the formation of the double layer of intermittency is a dynamic general feature not specific to the turbulent transport. These results of our numerical study are discussed in the context of experimental results and numerical simulations.

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Section: Statisticsmentioning

confidence: 99%

Section: Passive Scalar Transport Across the Interfacementioning

confidence: 99%

Section: Introductionmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Mixing of a passive scalar across a thin shearless layer: concentration of intermittency on the sides of the turbulent interface

Iovieno

Savino

Gallana

et al. 2014

Journal of Turbulence

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“…There are at least two reasons for choosing this approach to determine c D : First, ∆(t) is a function of only one dimension (time), making it a much simpler candidate for determining c D than say K, which depends on both y and t. Second, in Tordella & Iovieno [22] that the BHRZ closure model should be most appropriate. In §IV we will discuss the choice of c D further and the value we obtain for the SFML.…”

Section: B Model Constantsmentioning

confidence: 99%

Model of non-stationary, inhomogeneous turbulence

Bragg¹,

Kurien²,

Clark³

2016

Theor. Comput. Fluid Dyn.

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We compare results from a spectral model for non-stationary, inhomogeneous turbulence (Besnard et al., Theor. Comp. Fluid. Dyn., vol. 8, pp 1-35, 1996) with Direct Numerical Simulation (DNS) data of a shear-free mixing layer (SFML) (Tordella et al., Phys. Rev. E, vol. 77, 016309, 2008). The SFML is used as a test case in which the efficacy of the model closure for the physical-space transport of the fluid velocity field can be tested in a flow with inhomogeneity, without the additional complexity of mean-flow coupling. The model is able to capture certain features of the SFML quite well for intermediate to long-times, including the evolution of the mixing-layer width and turbulent kinetic energy. At short-times, and for more sensitive statistics such as the generation of the velocity field anisotropy, the model is less accurate. We present arguments, supported by the DNS data, that a significant cause of the discrepancies is the local approximation to the intrinsically non-local pressure-transport in physical-space that was made in the model, the effects of which would be particularly strong at short-times when the inhomogeneity of the SFML is strongest. *

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Investigation of Turbulent Entrainment‐Mixing Processes With a New Particle‐Resolved Direct Numerical Simulation Model

Gao

Liu

et al. 2018

JGR Atmospheres

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A new particle‐resolved three‐dimensional direct numerical simulation model is developed that combines Lagrangian droplet tracking with the Eulerian field representation of turbulence near the Kolmogorov microscale. Six numerical experiments are performed to investigate the processes of entrainment of clear air and subsequent mixing with cloudy air and their interactions with cloud microphysics. The experiments are designed to represent different combinations of three configurations of initial cloudy area and two turbulence modes (decaying and forced turbulence). Five existing measures of microphysical homogeneous mixing degree are examined, modified, and compared in terms of their ability as a unifying measure to represent the effect of various entrainment‐mixing mechanisms on cloud microphysics. Also examined and compared are the conventional Damköhler number and transition scale number as a dynamical measure of different mixing mechanisms. Relationships between the various microphysical measures and dynamical measures are investigated to search for a unified parameterization of entrainment‐mixing processes. The results show that even with the same cloud water fraction, the thermodynamic and microphysical properties are different, especially for the decaying cases. Further analysis confirms that despite the detailed differences in cloud properties among the six simulation scenarios, the variety of turbulent entrainment‐mixing mechanisms can be reasonably represented with power law relationships between the microphysical homogeneous mixing degrees and the dynamical measures.

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Small-Scale Anisotropy in Turbulent Shearless Mixing

Cited by 27 publications

References 20 publications

Mixing of a passive scalar across a thin shearless layer: concentration of intermittency on the sides of the turbulent interface

Mixing of a passive scalar across a thin shearless layer: concentration of intermittency on the sides of the turbulent interface

Model of non-stationary, inhomogeneous turbulence

Investigation of Turbulent Entrainment‐Mixing Processes With a New Particle‐Resolved Direct Numerical Simulation Model

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