A Voronoï analysis of preferential concentration in a vertical channel flow

Nilsen, Christopher; Andersson, Helge I.; Zhao, Lihao

doi:10.1063/1.4830435

Cited by 47 publications

(37 citation statements)

References 34 publications

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“…Using our DNS data, we perform a 3D Voronoï analysis of the particles, and then compute a range of statistical quantities using the information from the analysis. In order to address the issue of open Voronoï cells at the boundaries of the domain, the particles across the boundaries are periodically repeated to ensure that all the cells are closed and the sum of Voronoï volumes is equal to the domain volume ( [42,43,44]). Therefore, the Voronoï volumes associated with the particles adjacent to the boundaries are ill-defined and so are ignored in our analysis.…”

Section: Computational Detailsmentioning

confidence: 99%

Local analysis of the clustering, velocities, and accelerations of particles settling in turbulence

Momenifar

Bragg

2020

Phys. Rev. Fluids

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Using 3D Voronoï analysis, we explore the local dynamics of small, settling, inertial particles in isotropic turbulence using Direct Numerical Simulations (DNS). We independently vary the Taylor Reynolds number R λ ∈ [90, 398], Froude number F r ≡ a η /g ∈ [0.052, ∞] (where a η is the Kolmogorov acceleration, and g is the acceleration due to gravity), and Kolmogorov scale Stokes number St ≡ τ p /τ η ∈ [0, 3]. In agreement with previous results using global measures of particle clustering, such as the Radial Distribution Function (RDF), we find that for small Voronoï volumes (corresponding to the most clustered particles), the behavior is strongly dependent upon St and F r, but only weakly dependent upon R λ , unless St > 1. However, larger Voronoï volumes (void regions) exhibit a much stronger dependence on R λ , even when St ≤ 1, and we show that this, rather than the behavior at small volumes, is the cause of the sensitivity of the standard deviation of the Voronoï volumes that has been previously reported. We also show that the largest contribution to the particle settling velocities is associated with increasingly larger Voronoï volumes as the settling parameter Sv ≡ St/F r is increased.Our local analysis of the acceleration statistics of settling inertial particles shows that clustered particles experience a net acceleration in the direction of gravity, while particles in void regions experience the opposite. The particle acceleration variance, however, is a convex function of the Voronoï volumes, with or without gravity, which seems to indicate a non-trivial relationship between the Voronoï volumes and the sizes of the turbulent flow scales. Results for the variance of the fluid acceleration at the inertial particle positions are of the order of the square of the Kolmogorov acceleration and depend only weakly on Voronoï volumes. These results call into question the "sweep-stick" mechanism for particle clustering in turbulence which would lead one to expect that clustered particles reside in the special regions where the fluid acceleration is zero (or at least small).We then consider the properties of particles in clusters, which are regions of connected Voronoï cells whose volume is less than a certain threshold. The results show self-similarity of the clusters, and that the statistics of the cluster volumes depends only weakly on St, with a stronger dependance on F r and R λ . Finally, we compare the average settling velocities of all particles in the flow with those in clusters, and show that those in the clusters settle much faster, in agreement with previous work. However, we also find that this difference grows significantly with increasing R λ and exhibits a non-monotonic dependence on F r. The kinetic energy of the particles, however, are almost the

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Section: Computational Detailsmentioning

confidence: 99%

Local analysis of the clustering, velocities, and accelerations of particles settling in turbulence

Momenifar

Bragg

2020

Phys. Rev. Fluids

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“…The variance is defined as σ 2 2 for each wall-parallel kth bin, where V is the Voronoï volume for each particle and V m is the local mean Voronoï volume in this bin. The Voronoï volumes were calculated using the same method as that described and validated by Nilsen et al 6 For randomly distributed particles σ 2 V /V 2 m ≈ 0.18, and larger σ 2 V indicates stronger inhomogeneity of the preferential concentration. As shown in Fig.…”

Section: A Characteristics Of Wall-parallel Preferential Concentrationmentioning

confidence: 99%

“…Quantification of preferential clustering and characterizing clusters/voids can be challenging, but there are some successful approaches. For a complete review of those methods, the readers are referred to the comprehensive review paper by Monchaux et al 1 One commonly used method for measuring inhomogeneities in particle distributions is the Voronoï diagram, 6 the advantage of which is its independence of a pre-defined grid, as in box counting schemes. For a 3D particle field, the Voronoï volume divides the whole domain into sub-volumes consisting of all points being closer to one particular particle than others and thus provides a specific volume that is the inverse of the local particle number density.…”

Section: Introductionmentioning

confidence: 99%

“…This method has been used to quantify particle clustering in both homogeneous flows 4,7,8 and wall-bounded flow. 6,9,10 Another commonly encountered flow scenario is the wallbounded flow. Studies on particle dispersion in turbulent channel flows (Poiseuille flows) are mostly focused on wall-normal particle segregation near the walls, 11,12 which is attributed to turbophoresis.…”

Section: Introductionmentioning

confidence: 99%

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Preferential particle concentration in wall-bounded turbulence with zero skin friction

2017

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Inertial particles dispersed in turbulence distribute themselves unevenly. Besides their tendency to segregate near walls, they also concentrate preferentially in wall-parallel planes. We explore the latter phenomenon in a tailor-made flow with the view to examine the homogeneity and anisotropy of particle clustering in the absence of mean shear as compared with conventional, i.e., sheared, wall turbulence. Inertial particles with some different Stokes numbers are suspended in a turbulent Couette-Poiseuille flow, in which one of the walls moves such that the shear rate vanishes at that wall. The anisotropies of the velocity and vorticity fluctuations are therefore qualitatively different from those at the opposite non-moving wall, along which quasi-coherent streaky structures prevail, similarly as in turbulent pipe and channel flows. Preferential particle concentration is observed near both walls. The inhomogeneity of the concentration is caused by the strain-vorticity selection mechanism, whereas the anisotropy originates from coherent flow structures. In order to analyse anisotropic clustering, a two-dimensional Shannon entropy method is developed. Streaky particle structures are observed near the stationary wall where the flow field resembles typical wall-turbulence, whereas particle clusters near the moving friction-free wall are similar to randomly oriented clusters in homogeneous isotropic turbulence, albeit with a modest streamwise inclination. In the absence of mean-shear and near-wall streaks, the observed anisotropy is ascribed to the imprint of large-scale flow structures which reside in the bulk flow and are global in nature.

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“…Like many authors (Monchaux et al 2010;Garcia-Villalba, Kidanemariam & Uhlmann 2012;Monchaux, Bourgoin & Cartellier 2012;Tagawa et al 2012Tagawa et al , 2013Dejoan & Monchaux 2013;Nicolai, Jacob & Piva 2013;Nilsen, Andersson & Zhao 2013;Uhlmann & Doychev 2014), we base our analysis of the particle clustering on Voronoï tessellation of the particle positions. Each of the elementary cells of the Voronoï tessellation is attributed to a particle.…”

Section: A1 Reference Statementioning

confidence: 99%

Turbulent thermal convection driven by heated inertial particles

et al. 2016

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This is an author-deposited version published in: http://oatao.univ-toulouse.fr/ Eprints ID: 18470Open Archive Toulouse Archive Ouverte (OATAO) OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. The heating of particles in a dilute suspension, for instance by radiation, chemical reactions or radioactivity, leads to local temperature fluctuations in the fluid due to the non-uniformity of the disperse phase. In the presence of a gravity field, the fluid is set in motion by the resulting buoyancy forces. When the particle density is different than that of the fluid, the fluid motion alters the spatial distribution of the particles and possibly strengthens their concentration inhomogeneities. This in turn causes more intense local heating. Direct numerical simulations in the Boussinesq limit show this feedback loop. Various regimes are identified depending on the particle inertia. For very small particle inertia, the macroscopic behaviour of the system is the result of many thermal plumes that are generated independently of each other. For significant particle inertia, clusters of particles are observed and their dynamics controls the flow. The emergence of very intermittent turbulent fluctuations shows that the flow is influenced by the larger structures (turbulent convection) as well as by the small-scale dynamics that affect particle segregation and thus the flow forcing. Assuming thermal equilibrium between the particles and the fluid (i.e. infinitely fast thermal relaxation of the particle), we investigate the evolution of statistical observables with the change of the main control parameters (namely the particle number density, the particle inertia and the domain size), and propose a scaling argument for these trends. Concerning the energy density in the spectral space, it is observed that the turbulent energy and temperature spectra follow a power law, the exponent of which varies continuously with the Stokes number. Furthermore, the study of the spectra of the temperature and momentum forcing (and thus of the concentration/temperature and velocity/temperature correlations) gives strong support to the proposed feedback loop mechanism. We then discuss the intermittency of the flow, and analyse the effect of relaxing some of the simplifying assumptions, thus assessing the relevance of the original studied configuration. To cite this version:

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A Voronoï analysis of preferential concentration in a vertical channel flow

Cited by 47 publications

References 34 publications

Local analysis of the clustering, velocities, and accelerations of particles settling in turbulence

Local analysis of the clustering, velocities, and accelerations of particles settling in turbulence

Preferential particle concentration in wall-bounded turbulence with zero skin friction

Turbulent thermal convection driven by heated inertial particles

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