Experimental evidence of settling retardation in a turbulence column

Akutina, Yulia; Revil-Baudard, Thibaud; Chauchat, Julien; Eiff, Olivier

doi:10.1103/physrevfluids.5.014303

Cited by 9 publications

(23 citation statements)

References 35 publications

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“…The measured quantity shows that the effective settling velocity is about ten times smaller than the one in quiescent water i.e. C w ≈ 0.1 consistent with measurements of [23] (see Appendix D for details). The two-fluid LES also predicts settling retardation even though the reduction is less strong (C w = 0.4) than in RB15 experiments.…”

supporting

confidence: 81%

“…In other words, the settling velocity in turbulent water has to be lower than that in quiescent water to explain RB15 measurements. In a recent paper, using the exact same particles as in RB15, [23] have shown that the settling velocity of an isolated particle falling in an homogeneous isotropic turbulent flow field can be as low as 50% of its value in quiescent water.…”

mentioning

confidence: 99%

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On the physical origin of enhanced turbulent-dispersion of 'heavy particles' in suspended-load

Chauchat¹,

Revil-Baudard²,

Cheng³

et al. 2020

Preprint

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In the state-of-the-art for suspended-load modeling it is commonly assumed that the concentration profile results from a balance between a settling flux, in which the settling velocity is considered as equal to its value for a single settling particle in quiescent water, and an upward turbulent flux modeled using a Fickian gradient diffusion approximation. While this model provides a general framework, comparison with experiments reveals that the concentration diffusivity is not equal to the eddy viscosity and a turbulent Schmidt number needs to be introduced. Based on Coleman (1970,1981) data, van Rijn (1984) proposed an empirical model which suggests that the Schmidt number is a decreasing function of Ws/u*. This result is intriguing as it suggests that the turbulent dispersion of sediment concentration is enhanced when the particle&#8217;s settling velocity increases relative to the bed friction velocity. Van Rijn suggested that this is due to centrifugal forces that tends to throw inertial particles out of the turbulent vortices leading to an enhanced particle dispersion compared to momentum. In the present contribution, we use high-resolution experimental data and turbulence resolving two-phase flow simulations that directly resolve the turbulent momentum and particle fluxes and the flow turbulence to investigate the different terms appearing on the mass balance mentioned above.&#160; Both the experimental and the numerical results show that the actual turbulent Schmidt number based on the resolved sediment flux is higher than unity meaning that turbulent dispersion efficiency of &#171;&#160;heavy particles&#160;&#187; is reduced. This contradicts van Rijn&#8217;s prediction model of the Schmidt number. One plausible explanation is that the settling velocity of particles is reduced in highly turbulent flows. Using the experimental and numerical results, the actual settling velocity in the turbulent flow is retrieved from the mass balance at steady state. It is found that it is significantly retarded compared with the value in quiescent water (10 to 40%). This result is in good agreement with the one obtained in recent experiments performed in a turbulent grid at KIT (Germany) using the same particles (Akutina et al., 2020). The authors found a settling retardation of 16% for the same turbulent intensities as in the present experiments. The results presented herein completely change the paradigm for turbulent suspension load modeling and open new perspectives on the development of new, physical process-based, parametrizations required for large-scale models. This, of course, will require to extend the proposed methodology to a wider range of flow and sediment conditions.

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supporting

confidence: 81%

mentioning

confidence: 99%

On the physical origin of enhanced turbulent-dispersion of 'heavy particles' in suspended-load

Chauchat¹,

Revil-Baudard²,

Cheng³

et al. 2020

Preprint

View full text Add to dashboard Cite

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“…Interestingly, previous experiments [18,29], as well as ours, reveal that the particle velocity is hindered (slowed down with respect to the still fluid terminal velocity) as the Re λ increases above a certain threshold (see figure 6). Other experiments, e.g., Akutina et al [51] have also reported hindering for particles falling inside a turbulent column. Although particles with small Rouse and Stokes numbers have settling velocities (magnitudes) that depend strongly on the liquid fraction φ v and Re λ , we observe that for Ro > O(0.1) (after the peak of maximum settling enhancement) the normalized particle settling (∆V /u) seems to have a quasi-linear behavior (see figure 6).…”

Section: Normalized Settling Velocitymentioning

confidence: 80%

Effect of Reλ and Rouse numbers on the settling of inertial droplets in homogeneous isotropic turbulence

et al. 2021

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We present an experimental study on the settling velocity of dense sub-Kolmogorov particles in active-grid-generated turbulence in a wind tunnel. Using phase Doppler interferometry, we observe that the modifications of the settling velocity of inertial particles, under homogeneous isotropic turbulence and dilute conditions (i.e., small liquid fraction φv ≤ O(10) −5 ), is controlled by the Taylor-Reynolds number Re λ of the carrier flow. Meanwhile, we did not find a strong influence of the ratio between the fluid and gravity accelerations on the particle settling behavior. Remarkably, we find that that the degree of hindering experienced by the particles (i.e., the measured particle settling velocity is smaller in magnitude than its respective one in still fluid conditions) increases with Re λ . This observation is contrary to previous works at intermediate values of Re λ that report the opposite effect: settling enhancement. Nonetheless, our trend is observed at all particle sizes investigated, and when previous experimental data is included into the analysis, our data suggest that the particle settling behavior may be non-monotonic with Re λ : inducing enhancement at moderate values of Re λ , and at promoting hindering at higher values of Re λ . Moreover, at the highest Re λ studied, the settling enhancement regime ceases to exist. Finally, we find that the difference between the measured particle settling velocity (Vp) and the particle terminal velocity in still fluid conditions (VT ), normalized by the carrier phase rms fluctuations, (Vp − VT )/u scales linearly with the Rouse number Ro = VT /u (i.e., the ratio between the particles settling velocity and the fluid rms fluctuations). However, such behavior (Vp − VT )/u ∼ −Ro appears only to be valid for moderate values of the Rouse number.

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“…2014; Akutina et al. 2020). These features can be also seen using a model cellular flow of counter-rotating vortices.…”

Section: Introductionmentioning

confidence: 99%

“…Preferential sweeping is generally considered responsible for the turbulence increasing the settling velocity of inertial particles but other mechanisms, such as vortex trapping and loitering or nonlinear drag effects, have also been proposed for, on the contrary, reducing the settling velocity (see e.g. Nielsen 1993; Mei 1994;Good et al 2014;Akutina et al 2020). These features can be also seen using a model cellular flow of counter-rotating vortices.…”

Section: Introductionmentioning

confidence: 99%