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Secondary flows drive triboelectric powder charging in pneumatic conveying

JOURNAL OF FLUID MECHANICS 2025年 1007卷 A52-A52页

作者： Ozler, Gizem Grosshans, Holger Phys Tech Bundesanstalt PTB Anal & Simulat Explos Protect Braunschweig Germany Otto von Guericke Univ Inst Aparatus & Environm Technol Magdeburg Germany

Highly resolved simulations reveal the fundamental influence of a carrier fluid's flow dynamics on triboelectric powder charging. We found that particles transported through a square-shaped duct charge faster than in a channel flow caused by secondary flows that led to more severe particle-wall collisions. Specifically, particles with a Stokes number of 4.69 achieve 85 % of their equilibrium charge approximately 1.5 times faster in duct flow than in channel flow. Also, charge distribution is more uniform in a duct cross-section compared with a channel cross-section. In channel flow, particles are trapped near the walls and collide frequently due to limited movement in the wall-normal direction, causing localized charge buildup. In contrast, duct flow promotes better mixing through secondary flows, reducing repeating collisions and providing uniform charge distribution across the cross-section. Upon charging, electrostatic forces significantly reshape particle behaviour and distribution. Once the powder achieves half of its equilibrium charge, particles increasingly accumulate at the wall, leading to a reduced concentration in the central region. These changes in particle distribution have a noticeable impact on the surrounding fluid phase and alter the overall flow dynamics. These findings open the possibility for a new measure to control powder charging by imposing a specific pattern.

关键词： multiphase and particle-laden flows aerodynamics turbulent flows

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Modelling of filtered drag force for clustered particle-laden flows based on interface-resolved simulation data

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JOURNAL OF FLUID MECHANICS 2024年 998卷 A57-A57页

作者： Xia, Yan Lin, Zhaowu Guo, Yu Yu, Zhaosheng Zhejiang Univ Dept Mech State Key Lab Fluid Power & Mechatron Syst Hangzhou 310027 Peoples R China

Interface-resolved direct numerical simulations (DNS) of clustered settling suspensions in a periodic domain are performed to study the filtered drag force for clustered particle-laden flows. Our results show that, for the homogeneous system, the filtered drag is independent of the filter size, whereas for the clustered particle-laden flows, the averaged drag becomes smaller than the homogeneous drag at the filter size above 4 particle diameters. The drag reduction saturates at the filter size being comparable to the cluster size in the horizontal direction in our simulations. A new correlation is proposed to account for the mesoscale effect on the filtered drag force by using drift velocity and variance of the solid volume fraction, based on the modification of existing subgrid drag models for the inhomogeneous system. The existing models for the drift velocity and the variance of the solid volume fraction are assessed using our DNS data. A new model for the drift velocity and the variance of the solid volume fraction is proposed, based on the combination and modification of the previous models. All mesoscale models considered can predict well the filtered drag with comparable accuracy, and are superior to the homogeneous drag model for the clustered system. Our models with the same parameter values obtained from the large-scale system can also predict well the filtered drag for smaller computational domain sizes.

关键词： multiphase and particle-laden flows

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Attenuation of turbulence in a periodic cube by anisotropic solid particles

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JOURNAL OF FLUID MECHANICS 2025年 1008卷 A6-A6页

作者： Awai, Hideto Motoori, Yutaro Goto, Susumu Osaka Univ Grad Sch Engn Sci 1-3 Machikaneyama Toyonaka Osaka 5608531 Japan

We conduct direct numerical simulations (DNS) to investigate the attenuation of turbulence in a periodic cube due to the addition of prolate spheroidal solid particles. Even with a dilute volume fraction of $O(10<^>{-2})$ , particles can drastically attenuate the turbulence. Our DNS show that the turbulent kinetic energy reduces more significantly when the particles' Stokes number is larger, size is smaller or aspect ratio is larger. We can explain these results based on the formula proposed by Oka and Goto (2022 J. Fluid Mech. 949, A45), which relates the turbulence attenuation rate to the energy dissipation rate $\epsilon _p$ around particles. More precisely, under the condition that the volume fraction of particles is fixed, $\epsilon _p$ is larger when the Stokes number and, therefore, the relative velocity between fluid and particles are larger, the particle size is smaller or the aspect ratio is larger. These results also imply that the rotation of the anisotropic particles plays only a limited role in the attenuation of turbulence when the Stokes number of particles is sufficiently large, because the main cause of the attenuation is the relative translational velocity between fluid and particles.

关键词： turbulent flows multiphase and particle-laden flows

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An analytical model for the slip velocity of particles in turbulence

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JOURNAL OF FLUID MECHANICS 2024年 996卷 A1-A1页

作者： Berk, Tim Coletti, Filippo Utah State Univ Dept Mech & Aerosp Engn Logan UT 84322 USA Swiss Fed Inst Technol Dept Mech & Proc Engn CH-8092 Zurich Switzerland

Predicting the magnitude of the slip velocity of non-tracer particles with respect to the surrounding fluid is crucial to address both fundamental and practical questions involving dispersed turbulent flows. Here we derive an analytical model to predict the slip velocity of spherical particles in homogeneous isotropic turbulence. We modulate the particle equation of motion according to the inertial filtering framework, and obtain closed-form expressions for the mean slip velocity magnitude as a function of the governing parameters. These are compared against laboratory measurements and direct numerical simulations, demonstrating close agreement for both light and heavy particles, both smaller and larger than the Kolmogorov scales. The predictive value of the model and its implications are discussed, as well as the range of validity of the underlying assumptions.

关键词： multiphase and particle-laden flows turbulent flows

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On the relevance of lift force modelling in turbulent wall flows with small inertial particles

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JOURNAL OF FLUID MECHANICS 2024年 988卷 A47-A47页

作者： Gao, Wei Shi, Pengyu Parsani, Matteo Costa, Pedro King Abdullah Univ Sci & Technol Mech Engn Phys Sci & Engn Div Thuwal 239556900 Saudi Arabia King Abdullah Univ Sci & Technol Extreme Comp Res Ctr Comp Elect & Math Sci & Engn Div Appl Math & Computat Sci Thuwal 239556900 Saudi Arabia Univ Toulouse Inst Mecan Fluides Toulouse IMFT CNRS Toulouse France Helmholtz Zentrum Dresden Rossendorf Inst Fluid Dynam D-01328 Darmstadt Germany Delft Univ Technol Proc & Energy Dept Leeghwaterstr 39 NL-2628 CB Delft Netherlands

In particle-laden turbulent wall flows, lift forces can influence the near-wall turbulence. This has been observed recently in particle-resolved simulations, which, however, are too expensive to be used in upscaled models. Instead, point-particle simulations have been the method of choice to simulate the dynamics of these flows during the last decades. While this approach is simpler, cheaper and physically sound for small inertial particles in turbulence, some issues remain. In the present work, we address challenges associated with lift force modelling in turbulent wall flows and the impact of lift forces in the near-wall flow. We performed direct numerical simulations of small inertial point particles in turbulent channel flow for fixed Stokes number and mass loading while varying the particle size. Our results show that the particle dynamics in the buffer region, causing the apparent particle-to-fluid slip velocity to vanish, raises major challenges for modelling lift forces accurately. While our results confirm that lift forces have little influence on particle dynamics for sufficiently small particle sizes, for inner-scaled diameters of order one and beyond, lift forces become quite important near the wall. The different particle dynamics under lift forces results in the modulation of streamwise momentum transport in the near-wall region. We analyse this lift-induced turbulence modulation for different lift force models, and the results indicate that realistic models are critical for particle-modelled simulations to correctly predict turbulence modulation by particles in the near-wall region.

关键词： multiphase and particle-laden flows

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An experimental study on the settling velocity of inertial particles in different homogeneous isotropic turbulent flows

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JOURNAL OF FLUID MECHANICS 2023年第1期970卷 A23-A23页

作者： Ferran, Amelie Machicoane, Nathanael Aliseda, Alberto Obligado, Martin Univ Grenoble Alpes CNRS Grenoble INP LEGI F-38000 Grenoble France Univ Washington Dept Mech Engn Seattle WA 98195 USA

We propose an experimental study on the gravitational settling velocity of dense, sub-Kolmogorov inertial particles under different background turbulent flows. We report phase Doppler particle analyser measurements in a low-speed wind tunnel uniformly seeded with micrometre scale water droplets. Turbulence is generated with three different grids (two consisting of different active-grid protocols while the third is a regular static grid), allowing us to cover a very wide range of turbulence conditions in terms of Taylor-scale-based Reynolds numbers (Re-lambda is an element of [30-520]), Rouse numbers (Ro is an element of[0-5]) and volume fractions (phi(v) is an element of [0.5 x 10(-5)- 2.0 x 10(-5)]). We find, in agreement with previous works, that enhancement of the settling velocity occurs at low Rouse number, while hindering of the settling occurs at higher Rouse number for decreasing turbulence energy levels. The wide range of flow parameters explored allowed us to observe that enhancement decreases significantly with the Taylor-Reynolds number and is significantly affected by the volume fraction phi(v). We also studied the effect of large-scale forcing on settling velocity modification. The possibility of changing the inflow conditions by using different grids allowed us to test cases with fixed Re-lambda and turbulent intensity but with different integral length scale. Finally, we assess the existence of secondary flows in the wind tunnel and their role on particle settling. This is achieved by characterising the settling velocity at two different positions, the centreline and close to the wall, with the same streamwise coordinate.

关键词： turbulent flows multiphase and particle-laden flows

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Investigating the parametric dependence of the impact of two-way coupling on inertial particle settling in turbulence

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JOURNAL OF FLUID MECHANICS 2024年 987卷 A17-A17页

作者： Bhattacharjee, Soumak Tom, Josin Carbone, Maurizio Bragg, Andrew D. Duke Univ Dept Civil & Environm Engn Durham NC 27708 USA Univ Bayreuth Theoret Phys 1 Univ Str 30 D-95447 Bayreuth Germany Max Planck Inst Dynam & Selforg Fassberg 17 D-37077 Gottingen Germany Convergent Sci 6400 Enterprise Lane Madison WI 53719 USA

Tom et al. (J. Fluid Mech., vol. 947, 2022, p. A7) investigated the impact of two-way coupling (2WC) on particle settling velocities in turbulence. For the limited parameter choices explored, it was found that (i) 2WC substantially enhances particle settling compared with the one-way coupled case, even at low mass loading Fm and (ii) preferential sweeping remains the mechanism responsible for the particles settling faster than the Stokes settling velocity in 2WC flows. However, significant alterations to the flow structure that can occur at higher mass loadings mean that the conclusions from Tom et al. (J. Fluid Mech., vol. 947, 2022, p. A7) may not generalise. Indeed, even under very low mass loadings, the influence of 2WC on particle settling might persist, challenging the conventional assumption. We therefore explore a much broader portion of the parameter space, with simulations covering cases where the impact of 2WC on the global fluid statistics ranges from negligible to strong. We find that, even for Fm = 7.5 x 10(-3), 2WC can noticeably increase the settling for some choices of the Stokes and Froude numbers. When Fm is large enough for the global fluid statistics to be strongly affected, we show that preferential sweeping continues to be the mechanism that enhances particle settling rates. Finally, we compare our results with previous numerical and experimental studies. While in some cases there is reasonable agreement, discrepancies exist even between different numerical studies and between different experiments. Future studies must seek to understand this before the discrepancies between numerical and experimental results can be adequately addressed.

关键词： multiphase and particle-laden flows turbulent flows

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Parameterization of turbulence modulation by finite-size solid particles in forced homogeneous isotropic turbulence

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JOURNAL OF FLUID MECHANICS 2023年第1期963卷 A6-A6页

作者： Peng, Cheng Sun, Qichao Wang, Lian-Ping Shandong Univ Sch Mech Engn Key Lab High Efficiency & Clean Mech Manufacture Minist Educ Jinan 250061 Peoples R China Southern Univ Sci & Technol Ctr Complex Flows & Soft Matter Res Guangdong Prov Key Lab Turbulence Res & Applicat Shenzhen 518055 Guangdong Peoples R China Southern Univ Sci & Technol Dept Mech & Aerosp Engn Shenzhen 518055 Guangdong Peoples R China Southern Univ Sci & Technol Guangdong Hong Kong Macao Joint Lab Data Driven Fl Shenzhen 518055 Guangdong Peoples R China

Turbulence modulation by finite-size particles in homogeneous isotropic turbulence (HIT) has been investigated numerically and experimentally in many studies, but its controlling parameters are not fully clear. In this work, four non-dimensional parameters governing the turbulent modulation by non-settling particles, i.e. Re-? of the background HIT, the particle-to-fluid density ratio p(p)/p(f), the relative particle size d(p)/? and the particle volume fraction f(v), are identified through dimensional analysis. Then, a parameterization study is conducted based on results from fully resolved direct numerical simulations to investigate the influence of the above non-dimensional parameters on the modulation of turbulent kinetic energy (TKE) and viscous dissipation rate. Empirical models that quantitatively predict the modulation of TKE and dissipation rate are then developed by fitting in the simulation results. These models are used to examine the turbulence modulation results reported in the literature. The model predictions and the data points of TKE modulation show reasonable agreement, but the model predicting the modulation of dissipation rate needs further deliberation as the credibility of the available data points is currently difficult to assess. The generality and the physics behind these empirical models also require further investigation.

关键词： multiphase and particle-laden flows

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Taylor-Couette flow of hard-sphere suspensions: overview of current understanding

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PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES 2023年第2243期381卷 20220125-20220125页

作者： Baroudi, Lina Majji, Madhu, V Peluso, Stephen Morris, Jeffrey F. Manhattan Coll Dept Mech Engn Bronx NY 10471 USA MIT Dept Chem Engn Cambridge MA 02139 USA CUNY Levich Inst City Coll New York New York NY 10031 USA CUNY Dept Chem Engn City Coll New York New York NY 10031 USA

Although inertial particle-laden flows occur in a wide range of industrial and natural processes, there is both a lack of fundamental understanding of these flows and continuum-level governing equations needed to predict transport and particle distribution. Towards this effort, the Taylor-Couette flow (TCF) system has been used recently to study the flow behaviour of particle-laden fluids under inertia. This article provides an overview of experimental, theoretical and computational work related to the TCF of neutrally buoyant non-Brownian suspensions, with an emphasis on the effect of finite-sized particles on the series of flow transitions and flow structures. particles, depending on their size and concentration, cause several significant deviations from Newtonian fluid behaviour, including shifting the Reynolds number corresponding to transitions in flow structure and changing the possible structures present in the flow. Furthermore, particles may also migrate depending on the flow structure, leading to hysteretic effects that further complicate the flow behaviour. The current state of theoretical and computational modelling efforts to describe the experimental observations is discussed, and suggestions for potential future directions to improve the fundamental understanding of inertial particle-laden flows are provided. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (part 1)'.

关键词： Taylor-Couette flow suspension multiphase and particle-laden flows

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Effects of surfactants on bubble-induced turbulence

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JOURNAL OF FLUID MECHANICS 2023年第1期970卷 A13-A13页

作者： Ma, Tian Hessenkemper, Hendrik Lucas, Dirk Bragg, Andrew D. Helmholtz Zentrum Dresden Rossendorf Inst Fluid Dynam D-01328 Dresden Germany Duke Univ Dept Civil & Environm Engn Durham NC 27708 USA

We use experiments to explore the effect of surfactants on bubble-induced turbulence (BIT) at different scales, considering how the bubbles affect the flow kinetic energy, anisotropy and extreme events. To this end, high-resolution particle shadow velocimetry measurements are carried out in a bubble column in which the flow is generated by bubble swarms rising in water for two different bubble diameters (3 and 4 mm) and moderate gas volume fractions (0.5 %-1.3 %). We use tap water as the base liquid and add 1-Pentanol as an additional surfactant with varying bulk concentration, leading to different bubble shapes and surface boundary conditions. The results reveal that with increasing surfactant concentration, the BIT generated increases in strength, even though bubbles of a given size rise more slowly with surfactants. We also find that the level of anisotropy in the flow is enhanced with increasing surfactant concentration for bubbles of the same size, and that for the same surfactant concentration, smaller bubbles generate stronger anisotropy in the flow. Concerning the intermittency quantified by the normalized probability density functions of the fluid velocity increments, our results indicate that extreme values in the velocity increments become more probable with decreasing surfactant concentration for cases with smaller bubbles and low gas void fraction, while the effect of the surfactant is much weaker for cases with larger bubble and higher void fractions.

关键词： drops and bubbles multiphase and particle-laden flows turbulent flows

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