Variable-viscosity mixing in the very near field of a round jet

Talbot, B; Danaila, Luminita; Renou, Bruno

doi:10.1088/0031-8949/2013/t155/014006

Cited by 14 publications

(10 citation statements)

References 27 publications

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“…Most studies reported in literature have focused on the impact of variable viscosity on the large scales. Chhabra et al (2005) and Talbot et al (2013) studied turbulent jet flows, where the viscosity between jet fluid and host fluid differs. They observed, compared to a flow with uniform viscosity, that the relationship between production and dissipation of turbulent energy is altered and that the entrainment and the spreading rates of the jet flow are changed.…”

Section: Introductionmentioning

confidence: 99%

One-point and two-point statistics of homogeneous isotropic decaying turbulence with variable viscosity

Gauding

Danaila

Varéa

2018

International Journal of Heat and Fluid Flow

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The decay of homogeneous isotropic turbulence in a variable viscosity fluid with a viscosity ratio up to 15 is analyzed by means of highly resolved direct numerical simulations (DNS) at low Reynolds numbers. The question addressed by the present work is how quantities such as the kinetic energy and the associated dissipation rate, as well as the inter-scale transport mechanism of turbulence are changed by local fluctuations of the viscosity. From the one-point budget equation of the turbulent kinetic energy, it is shown that the mean dissipation rate is nearly unchanged by variable viscosity effects. This result is explained by a negative correlation between the local viscosity and the local velocity gradients. However, the dissipation is a highly fluctuating quantity with a strong level of intermittency. From a statistical analysis it is shown that turbulent flows with variable viscosity are characterized by an enhanced level of small-scale intermittency, which results in the presence of smaller length scales and a modified turbulent mixing. The effect of variable viscosity on the turbulent cascade is analyzed by a budget equation for the velocity structure function. From DNS it is shown that viscosity gradients contribute to the inter-scale transport mechanism in the form of an inverse transport, where information propagates from the small scales to the large scales.

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Section: Introductionmentioning

confidence: 99%

One-point and two-point statistics of homogeneous isotropic decaying turbulence with variable viscosity

Gauding

Danaila

Varéa

2018

International Journal of Heat and Fluid Flow

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“…Such situations occur in turbulent jets, and the concentration-dependent viscosity may modify the entrainment mechanism during the transition to turbulence. 8 A strong gradient of viscosity may also relaminarize mixing zones at late stage in magma fountains. 9 Still, the precise role of variable viscosity is not completely understood as it is entangled with the complex features of inhomogeneous and anisotropic turbulent flows.…”

Section: Introductionmentioning

confidence: 99%

The effects of variable viscosity on the decay of homogeneous isotropic turbulence

2014

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In this paper, we investigate the decay of incompressible homogeneous isotropic turbulence in a variable viscosity fluid. The viscosity coefficient is assumed to depend linearly on a scalar, representing either a temperature or a concentration, and obeying a simple advection-diffusion equation. At high Reynolds numbers, Direct Numerical Simulations (DNS) allow us to confirm the validity of Taylor's postulate that the dissipation is independent from the viscosity and its fluctuations. At low Reynolds numbers, we report the presence of extra energy at small scales due to these variable viscosity effects. This implies that the turbulent kinetic energy decreases less rapidly as a function of time in variable viscosity fluids. In order to explain this phenomenon and quantify its importance on the turbulent flow, we propose a statistical approach based on an eddy-damped quasi-normal Markovian (EDQNM) spectral closure which takes into account the nonlinearity introduced by variable viscosity. It is shown that this latter additional term is of constant sign in the energy spectrum equation and reduces the dissipation of the flow as observed. Also, by assuming the dominance of distant interactions between wave numbers, we can propose a simple formula expressing that variable viscosity effects lead to an effective reduction of the mean viscosity proportional to the variance of viscosity fluctuations.

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“…gas. Talbot, Danaila & Renou (2013) experimentally studied mixing in the near field of a round jet with (viscosity ratio of the outer and inner flows) and found that VVM evolves to a self-similar solution faster compared with CVM. In the same premise, for , Voivenel et al.…”

Section: Introductionmentioning

confidence: 99%

“…gas. Talbot, Danaila & Renou (2013) experimentally studied mixing in the near field of a round jet with m = 5.5 (viscosity ratio of the outer and inner flows) and found that VVM evolves to a self-similar solution faster compared with CVM. In the same premise, for m = 3.5, Voivenel et al (2016) observed self-similar profiles of velocity starting from x/d ≥ 4.5 while the Taylor microscale Reynolds number was not self-similar.…”

Section: Introductionmentioning

confidence: 99%

Coaxial jets with disparate viscosity: mixing and laminarization characteristics

et al. 2023

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Mixing of fluids in a coaxial jet is studied under four distinct viscosity ratios, $m=1$ , $10$ , $20$ and $40$ , using highly resolved large-eddy simulations (LES), particle image velocimetry and planar laser-induced fluorescence. The accuracy of predictions is tested against data obtained by the simultaneous experimental measurements of velocity and concentration fields. For the highest and lowest viscosity ratios, standard RANS models with unclosed terms pertaining to viscosity variations are employed. We show that the standard Reynolds-averaged Navier–Stokes (RANS) approach with no explicit modelling for variable-viscosity terms is not applicable whereas dynamic LES models provide high-quality agreement with the measurements. To identify the underlying mixing physics and sources of discrepancy in RANS predictions, two distinct mixing modes are defined based on the viscosity ratio. Then, for each mode, the evolution of mixing structures, momentum budget analysis with emphasis on variable-viscosity terms, analysis of the turbulent activity and decay of turbulence are investigated using highly resolved LES data. The mixing dynamics is found to be quite distinct in each mixing mode. Variable viscosity manifests multiple effects that are working against each other. Viscosity gradients induce additional instabilities while increasing overall viscosity decreases the effective Reynolds number leading to laminarization of the turbulent jet, explaining the lack of dispersion and turbulent diffusion. Momentum budget analysis reveals that variable-viscosity terms are significant to be neglected. The scaling of the energy spectrum cascade suggests that in the TLL mode the unsteady laminar shedding is responsible for the eddies observed.

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Variable-viscosity mixing in the very near field of a round jet

Cited by 14 publications

References 27 publications

One-point and two-point statistics of homogeneous isotropic decaying turbulence with variable viscosity

One-point and two-point statistics of homogeneous isotropic decaying turbulence with variable viscosity

The effects of variable viscosity on the decay of homogeneous isotropic turbulence

Coaxial jets with disparate viscosity: mixing and laminarization characteristics

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