Session L41: Particle-Laden Flows: Particle-Turbulence Interactions II

Sedimentation of microplastics due to their aggregation with biogenic particles in turbulent flows

In this study we investigate the enhancement of sedimentation of microplastics due to their aggregation with biogenic particles in homogeneous and isotropic turbulence using direct numerical simulations and Lagrangian tracking of point particles. The prevalence of microplastics (plastic particles less than 5 mm in size) in oceans and lakes has been a growing environmental concern in the past decade. Only a small fraction of the microplastics that are estimated to enter the ocean, or generated in the oceans through the fragmentation of larger pieces, are found to be floating on the ocean surface. The missing of microplastics from the ocean surface has been attributed to the aggregation of microplastics with heavier biogenic particles. In this regard, we investigate how the combined effects of differential settling/rising of microplastics and biogenic particles and turbulent flow motion contribute to the aggregation of microplastic and biogenic particles for different settling numbers and Stokes numbers. Additionally, we quantify the composition of the aggregates and the rate of settling of microplastics due to the formation of the aggregates.   

Roughness effect on particle laden turbulent channel flow

Particle-laden turbulent suspensions of inertial particles are ubiquitous, occurring naturally as well as in industries with highly inertial particles. Fluid Turbulence gets attenuated with relatively small size particles, whereas it gets amplified for larger size particles. It has been reported that turbulent disruption happens discontinuously for wall-bounded flows. The objective of the present study is to quantify the effect of particle and wall roughness on turbulence modulation. Point-particle Direct numerical simulations (PP-DNS) are carried out for a vertical channel, including the roughness in particle-particle and particle-wall interactions. The Reynolds number based on the channel width and average airflow velocity and viscosity Re = 5600 is considered. The particle Reynolds number based on the particle diameter and root mean square of the difference in the particle and fluid velocities is in the range of 4–15. The particle volume fractions are 0–3.5×10^-3, and the particle Stokes number is 200. It is observed that irrespective of the nature of particle-particle interactions, only the particle-wall interaction majorly controls the dynamics of the flow inside the system. With an increase in the roughness between wall-particle interaction, the second moments of the velocity fluctuations increase. However, the particle mean velocity decreases, and fluid velocity becomes more flat at the center core. The rough particle-wall interaction leads to an early collapse in the fluid phase turbulence.   

Settling dynamics of Kolmogorov scale sediment particles in homogeneous isotropic turbulence

Settling dynamics of slightly heavier-than-fluid particles in turbulence is of critical importance for suspended sediment transport. Of particular interest is the mean particle settling velocity in turbulence which can be influenced by fast tracking, vortex trapping, and loitering mechanisms. A point-particle model based on the complete Maxey-Riley equation is used to investigate settling dynamics of particles in vortical flows. First, particles settling in a stationary Taylor-Green vortex is investigated by varying the vortex strength relative to the particle settling speed in quiescent flow. For medium vortex strength, the settling speeds are observed to decrease due to loitering and vortex trapping effects. Fast tracking dominates and increases the settling speed for higher vortex strengths. This behavior is further investigated in a two-way coupled, forced, homogeneous isotropic turbulence by varying the turbulence intensity relative to the settling speed in quiescent flow for multiple Stokes numbers, in order to characterize the settling dynamics of Kolmogorov-scale particles. Mechanisms that influence the settling dynamics for a range of turbulence intensity and particle Stokes numbers are identified.   

Inertial chiral particles affect homogeneous turbulence vorticity

We present results from direct numerical simulations of chiral finite-size inertial particles, under the effect of gravity in a tri-periodic domain with homogenous isotropic turbulence. Although the interest in turbulent inertial particle-laden flows has increased in the last decade, most of the previous studies have mainly focused on highly symmetrical particles (i.e. spheres or ellipsoids) either with or without inertia. In this study we introduce chiral and convex particles whose size is in the turbulence inertial range. 

Chirality is a property of anisotropy for which an object is distinguishable from its mirror image. Thanks to this feature, our particles brake spatial reflexion symmetry, coupling translational and rotational degrees of freedom. Moreover, in all our simulations, the carrier and the dispersed phases are two-way coupled and collisions among particles are also taken into account.

We observe a turbulence modulation that increases with the increase of particles volume fraction and the same energy spectra scaling of turbulent bubbly flow. Chiral particles, falling under gravity, show a preferential angular velocity both in quiescent and turbulent flows. The most important evidence of this behaviour is observed in the vorticity statistics, highlighting a Froude number effect. Indeed, we observe a skewed distribution of the vertical vorticity component with a positive or negative peak depending on the particular chirality chosen.   

The Effect of Particle Collisions on Heat Transfer in a Thermally Developing Mixing Layer in a Dilute Turbulent Particle-Laden Flow

We present a study on the influence of inter-particle collisions on the turbulent heat flux within a temporally developing thermal mixing layer between two homothermal regions in a homogeneous and isotropic turbulent velocity field. We carry out two-way thermally coupled Eulerian-Lagrangian Direct Numerical Simulations across a wide range of particle Stokes numbers, ranging from 0.2 to 3, at a Taylor microscale Reynolds number from 56 up to 124, while maintaining a thermal to kinetic relaxation times ratio of 4.43. The particle concentration in the dilute regime is set at 4x10^-4 volume fraction.

The initial temperature difference between the two regions correlates temperature and velocity fluctuations in the mixing layer. However, scattering due to particle collisions tends to reduce these correlations. We quantitatively evaluate the overall impact on the average heat transfer and single point statistics of fluids and particles by comparing them with a collisionless regime. Remarkably, even at the highest simulated Stokes numbers, the effect remains minor.  In addition, we present statistical data on the temperature difference between colliding particles under various flow conditions and collision rates. In particular, our results indicate that collisions do not significantly hinder the ability of particles to modulate fluid temperature fluctuations.   

Turbulence modification of particle-laden flow in horizontal rectangular duct

Pneumatic conveying is used to transport solids in many chemical and pharmaceutical industries. Despite its high industrial relevance, the different types of interactions, like the particle–turbulence, particle-particle, and particle–wall for wall-bounded flows, are not well understood. The objective of the present work is to understand the two-phase dynamics of the particle-laden flow in a horizontal rectangular duct. Simulations are performed using the Eulerian-Lagrangian framework considering turbulent fluid medium as the continuum, and the particles are discrete Lagrangian point particles. Although point particle approximation is used to calculate drag force on the particles, their finite size effect is considered to simulate particle-particle and particle-wall interactions. The duct used in the present investigation has a cross-section of 4:1. Two-way coupling is used with the De-Felice equation to model the drag. Dynamic one equation LES is coupled with DEM for the particle phase. Simulations are performed using both perfectly smooth and rough particles, where linear spring-daspot model is used considering normal coefficient of restitution of 0.9. The mean velocity profiles for the gas and particle phases and the second moments of the velocity fluctuations are computed. It is observed that the fluid Reynolds stress decreases with the increase in the mass loading ratio, and a turbulence collapse is observed for the mass loading ratio of 1.0 when the particle Stokes number is 82.   

The Effect of Subgrid-scale Turbulence on LES of Inertial Particle Transport in a Boundary Layer Flow

Investigation of particle dispersion in boundary layer flows is of great importance in several environmental applications, such as atmospheric pollutant dispersion and ember tracking during wildfires. In these and many practical cases, the accurate tracking of particle trajectories across large spatial and temporal scales requires the use of Large Eddy Simulations (LES). However, one of the main limitations of LES is its inherent filtering of smaller-than-filter size (subgrid-scale (SGS)) flow features, which can compromise the accuracy of particle transport models. These filtering-induced inaccuracies accumulate over time, leading to increasingly higher errors as the spatial and temporal scales of the simulation increase. In this work, we employ a structural SGS model to account for and investigate the effects of sub-grid turbulence on the transport of particles in a boundary layer flow. We focus our analysis on settling particles of different sizes, while identifying the mechanisms governing their distribution within the flow. When comparing against Direct Numerical Simulations, the SGS model improves both flow and particle statistics, leading to a better prediction of landing distributions across a wide range of particle sizes.   

A continuous random walk approach for stochastic modeling of particles dispersion and deposition using LES

A stochastic continuous random walk approach (CRW) method is used to model the subgrid-scale (SGS) velocity fluctuations seen by particles for large-eddy simulation of particle-laden flow. An Eulerian-Lagrangian approach was used for particles with the range of Stokes numbers from St = 1 to 24. First, particle statistics are reported where the filtered direct-numerical simulation (FDNS) velocity is used in the Lagrangian equations of solid particles without a subgrid-scale velocity fluctuation model. The DNS data are filtered by applying a sharp spectral filter to the turbulent velocity field. Comparative analysis of particulate phase results before and after filtering revealed a significant impact of SGS velocity fluctuations on particle dispersion and deposition within the channel. Subsequently, SGS fluctuations generated by the stochastic CRW model were added to the filtered velocity in the Lagrangian equation of particles. With the inclusion of a drift term and a proper Lagrangian time scale in the CRW equation, the method can accurately predict particle statistics. The particle deposition velocities for the range of Stokes number are validated against experimental data and DNS simulations. The results showed that the appropriate stochastic CRW model could correctly predict the SGS velocity fluctuations and enhance the accuracy of the LES of turbulent particle-laden flows.   

Turbulence transport in particle-laden compressible flows

Turbulence transport in compressible dispersed two-phase flows is complicated by strong coupling between velocity fluctuations, shock waves, and particles. In this talk, highly-resolved numerical simulations are employed to quantify turbulence transport mechanisms and guide the development of a reduced-order model. Two canonical flows are considered: (i) compressible flow through homogeneous suspension of particles and (ii) a planar shock interacting with a cloud of particles. Turbulent kinetic energy (TKE) is found to contribute to a significant portion of the resolved kinetic energy, as much as 100% at the highest volume fractions considered. The TKE budget reveals turbulence is primarily produced via drag and is hindered by gas-phase compressibility. A two-equation model is proposed and implemented within a hyperbolic Eulerian-based two-fluid model. The model is found to be accurate across a wide range of volume fractions and Mach numbers in both homogeneous and inhomogeneous flows.   

Experimental investigation on dynamics of charged bidisperse particles in turbulence

From tribocharged particles in sandstorms, the dust lightening after volcanic eruptions, to the removal of particulate pollutants in the flue gas, the dynamics of charged particles in turbulence is important to a wide range of natural and industrial applications. Compared with neutral particles, the presence of electrostatic force could significantly modulate particles dynamics and cause abundant phenomena. In this talk, we present an experimental study of the dynamics of oppositely charged bidisperse particles in homogeneous isotropic turbulence. The spatial distribution of charged particles is first measured to show the global effects of electrostatic force on particle distribution in turbulence. For particle trajectories close to each other, the relative motion is further analyzed to show the dominant influence of the electrostatic force over short distances. Finally, a theoretical analysis is presented to quantify the competition between the electrostatic force and the hydrodynamic interaction, which contributes to a more complete physical understanding of charged particle-laden flows.   

The Settling Rates of Particles in Rayleigh–Bénard Turbulence

It is widely recognized that better understanding the interaction between clouds and aerosols is among the most pressing and essential endeavors in climate and atmospheric sciences. One particular process of interest is the settling rate of heavy particles and how this controls changes in rain formation and droplet size distribution. While much work has been done understanding particle settling rates in homogeneous isotropic turbulence and the potential for particles to settle significantly faster than their predicted Stokes settling velocity, more work needs to be done in other environments. Towards this aim, a set of experiments were conducted utilizing the Pi Chamber experimental facility at Michigan Technological University to investigate the physics dictating particle settling time in Rayleigh-Bénard turbulence; knowledge of droplet settling and lifetime is vital for characterizing how long droplets are exposed to a modeled turbulent environment.

The Pi Chamber is a rectangular chamber with an inner working volume of 3.14 m³ built for generating clouds. Among other features, it can maintain a statistically steady, turbulent Rayleigh-Bénard flow at a Rayleigh number of approximately 10⁹. In this set of experiments, solid, non-evaporating spheres (namely oil, glass, and hollow glass) of varying sizes were added to the chamber. After the initial injection, the decreasing concentration of particles over time was measured, and from this, a decay rate was calculated. An ideal Stokes settling velocity would predict that these decay rates vary as the square of the particle diameter, and a primary goal of this work is to quantify any deviation from this theoretical behavior. In this talk, we will show the results across a broad range of particle sizes (and thus Stokes and settling numbers), highlighting the multiple physical mechanisms dictating settling rate across particle size and density. In future work, this knowledge will be combined with condensational growth and decay in a cloudy environment to improve our understanding of cloud droplet growth and size distribution evolution.