Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session L24: Particle-Laden Flows: Nonspherical Particles |
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Chair: Lina Baroudi, Manhattan College Room: 251 B |
Monday, November 25, 2024 8:00AM - 8:13AM |
L24.00001: Orientational dynamics of sedimenting prolate spheroidal particles in a viscoelastic fluid Lina Baroudi, Arjun Sharma, Donald Lyle Koch We investigate the orientational dynamics of a prolate spheroid sedimenting in an inertia-less viscoelastic fluid using a combination of theory and numerical simulations. While a fore-aft, axisymmetric particle sedimenting in a quiescent Newtonian fluid with negligible inertia maintains its initial orientation, fluid viscoelasticity causes the particle to rotate toward a stable equilibrium orientation. Previous studies have suggested that the stable equilibrium for a prolate particle is longside-on, regardless of the particle's aspect ratio, initial orientation, or the rheology of the polymer solution. In this work, we demonstrate that the longside-on orientation is not the only possible orientation for a sedimenting prolate particle. In the limit of small Deborah number, $De$, our theory predicts a transition from longside-on to broadside-on orientation for aspect ratios $\kappa$ beyond $\kappa = 123$ for an Oldroyd-B fluid. Numerical simulations enable us to examine the effects of other properties. Increasing $De$ lowers the transition $\kappa$ so that broadside orientation can be observed for aspect ratios as low as $\kappa=20$. The polymer concentration and maximum extensibility are also found to have profound qualitative effects on the particle dynamics. The detailed physical mechanism behind the reversal in the sign of the viscoelastic rotation rate is presented, with findings indicating that polymer-induced solvent stress is the primary cause of the change in stable orientation. |
Monday, November 25, 2024 8:13AM - 8:26AM |
L24.00002: The dispersion of polydisperse non-spherical droplets in homogeneous isotropic turbulence Yushu Lin, John Palmore Predicting the motion of droplets in turbulence is essential in spray combustion. To model the spray in numerical simulations, the Lagrangian particle tracking method is widely used in research, which represents the spray as a discrete collection of spherical particles. One limitation of this approach is that it neglects the importance of droplet deformation. Past work from several authors has demonstrated the importance of droplet shape on droplet vaporization, combustion, and drag. Recently, we investigated non-spherical droplet dispersion in homogeneous isotropic turbulence (HIT), and found that non-spherical droplets have higher dispersion than spherical droplets for the same parameters [Lin and Palmore, ASME IMECE 2024]. However, that work only focuses on droplets of a particular size and does not investigate why this dispersion occurs. Real sprays consist of droplets in various sizes, shapes, and flow conditions. It is unknown how the dispersion and accumulation are related to these facts. Hence, this work investigates droplet-laden HIT flow for polydisperse non-spherical droplets utilizing an in-house code developed for multiphase flows. For each droplet diameter class, we directly extract relevant information such as average droplet deformation (via aspect ratio), droplet dispersion (via droplet velocity autocorrelation), and the preferential concentration (via Q-criterion value). This information is used gain insight into why dispersion is higher for non-spherical droplets. |
Monday, November 25, 2024 8:26AM - 8:39AM |
L24.00003: Settling spheroids, and spheroids in linear flow fields, in a fluid with spatial viscosity variations Arjun Sharma, Peter A Bosler, PhD, Rama Govindarajan, Donald Koch A generalized reciprocal theorem is used to relate the force and torque induced on an arbitrary particle by an arbitrary viscosity variation in an inertia-less fluid to integrals involving Stokes flow fields and the spatial dependence of viscosity. These expressions are analytically evaluated using spheroidal harmonics and then used to obtain the mobility of the particle during sedimentation, and in a linear flow, of a fluid with linear viscosity stratification. The coupling between the rotational and translational motion induced by stratification causes a particle to fall under gravity along a curved trajectory and rotates the spheroid’s centerline, creating a variety of rotational and translational dynamics. The settling dynamics are independent of the initial orientation but depend on the particle’s aspect ratio and the alignment of gravity with the stratification direction. Spheroids with an aspect ratio between 0.55 and 2.0 exhibit the largest variety of settling behaviors. Interestingly, this range covers most microplastics and typical microorganisms. In a simple shear flow, cross-streamline migration occurs due to the stratification-induced force generated on a rotating particle. Similarly, a particle no longer stays at the stagnation point of a uniaxial extensional flow and its axis does not align with the extensional axis. While fully analytical results are obtained, numerical simulations provide a source of validation. These simulations also provide additional insights into the stratification-induced force- and torque-producing mechanisms through the stratification-induced stress, which is not accessed in the analytical calculations based on the reciprocal theorem. |
Monday, November 25, 2024 8:39AM - 8:52AM |
L24.00004: Rotational Dynamics of Ellipsoidal Particles in Turbulent Channel Flows Domenico Zaza, Michele Iovieno, Alfredo Soldati Turbulent flows carrying small non-spherical particles are ubiquitous in both industrial and natural systems. In this study, we employ Direct Numerical Simulations (DNS) coupled with Lagrangian particle tracking to investigate the dynamics of elongated ellipsoidal particles in turbulent channel flows. The Navier-Stokes equations are solved on a fixed Eulerian grid using pseudo-spectral methods, while the motion (translation and rotation) of Np = 2,000,000 prolate ellipsoids is tracked by solving a set of ordinary differential equations for each particle. The equations describing particle dynamics account solely for the hydrodynamic drag force and torque, consistent with relevant literature. Considering friction Reynolds numbers ranging from Reτ = 180 to Reτ = 720, the simulations aim to benchmark and complement the experimental results on the rotational dynamics of microplastic slender fibers recently obtained at the TU Wien Turbulent Water Channel. With a focus on particles with small inertia (St+ ~ O(10-2)) and large aspect ratio (λ ~ O(102)), we explore the influence of the mean shear and vorticity in the near-wall region on particle rotation rates around their longitudinal axis (spinning rate) and transversal axes (tumbling rates). We further analyze the influence of the Reynolds number on the statistical relation between particle orientation and the local flow topology, specifically examining particle alignment with the local fluid's main directions of compression/expansion and rotation. |
Monday, November 25, 2024 8:52AM - 9:05AM |
L24.00005: Relaxation time of rod-like particle alignment under shear or extensional flow: the effect of particle polydispersity Yuto Yokoyama, Vincenzo Calabrese, Simon J Haward, Amy Q Shen Understanding how rod-like particles align and relax under the flow is crucial for improving the quality of compound materials and their manufacturing processes. It is predicted that monodisperse rod-like particles have a single time scale for relaxation, while polydisperse rod-like particles have been reported to exhibit multiple time scales. However, the relationship between these multiple time scales and flow conditions remains unclear. We studied the relaxation time of polydisperse cellulose nanocrystals (CNC) using a glass microchannel designed to generate simple shear and extensional flows. The birefringence induced by CNC alignment was measured with a high-speed polarization camera. After reaching a steady state, the flow was stopped, and the relaxation time was measured. The relaxation time decreased non-linearly with increasing strain rate in the low strain rate region and became constant in the high strain rate region. The CNC polydispersity qualitatively explained these results. Further experiments using samples with monodispersity and different polydispersity are needed for a better understanding. This research provides fundamental knowledge on the alignment control of rod-like particles. |
Monday, November 25, 2024 9:05AM - 9:18AM |
L24.00006: Transport of rod-shaped particles in a canopy flow with a buoyant plume Laura Sunberg, Hayoon Chung, Erika MacDonald, Nicholas T Ouellette, Jeffrey R Koseff Spot fire spread is a mode of wildfire spread in which a main wildfire launches smoldering firebrands, or embers, that ignite new "spot fires" upon landing at some distance downwind. Firebrands can be a wide array of shapes, and previous studies have shown that shape may impact their transport. To better understand the transport of these firebrands, and specifically the effect of firebrand shape, we experimentally investigated the transport of particles in a flow containing both canopy turbulence and a heated plume. We compared the transport of sphere- and rod-shaped particles with the same settling velocity to analyze the possible impacts of firebrand shape on spot fire spread. Rod behavior was characterized by examining orientation and spin rate. Cases with higher turbulence showed a greater distribution of rod orientations than cases with lower turbulence. Rod spin rates were similar in all cases except the short canopy, no plume case, where they were lower. These differences notwithstanding, we found that the spheres and rods landed with similar spatial distributions so long as either a plume or upstream canopy added turbulence to the system. If there was no plume and the upstream canopy was short, then the rods traveled farther and dispersed more than the spheres. These results suggest that it is not always necessary to account for firebrand shape to accurately predict spot fire spread, but they also reveal characteristics of how rod-shaped particles are transported in such a system. |
Monday, November 25, 2024 9:18AM - 9:31AM |
L24.00007: Free-settling dynamics of irregular microplastic particles in water Simon Eberhard, Christian Lundgaard, Jens H Walther, Knud Erik Meyer Irregular particles play a crucial role in numerous industrial and natural processes, such as manufacturing and the oceanic transport of microplastics. In many cases the settling dynamics is of fundamental importance. Previous studies focus on spherical or regular-shaped particles (such as ellipsoids or discs). In this work irregular particles are generated from an industrial granulation process of five different polymers. The variation in mechanical properties among the polymers result in a wide range irregular shapes. The study investigates settling in the intermediate settling regime at particle Reynolds number 100-1000. The characteristic length of the particles is 2-10 mm and the densities between 1030-1400 kg/m3. A stereo vision technique is developed that allows for simultaneous tracking of both the 3 translational and the 3 rotational degrees of freedom of the particle. Additionally, a particle resolved CFD model is validated with the experiment and used to extend the range of densities and Reynolds numbers while keeping the shape constant. The subsequent analysis focuses on the coupling between the rotational and translational dynamics. It is observed that the variation in shape resulting from the granulation process, of the same polymer, yields particles settling in all possible regimes. Furthermore, the study investigates the impact of settling regime on velocity, instantaneous cross-sectional area, and drag coefficient. |
Monday, November 25, 2024 9:31AM - 9:44AM |
L24.00008: Numerical study of the dynamics and mass transport of a helically twisted rectangular prism in simple shear flow Yanxing Wang, Ruben Gonzalez Pizarro, Dafne Sotelo Andana, Hui Wan, Tie Wei, Fangjun Shu Utilizing high-fidelity numerical simulations based on the lattice Boltzmann method, the dynamics of a helically twisted rectangular prism in simple shear flow and the transport of mass released from its surface are comprehensively studied. The findings reveal that the helical rectangular prism undergoes lateral movement when subjected to shear flow. This results in unique dynamic characteristics and mass transport behaviors compared to non-helical particles. These insights offer a mechanistic foundation for controlling particle motion by modifying particle shape. The fluid and particle dynamics, along with mass transport characteristics, are analyzed both qualitatively and quantitatively. |
Monday, November 25, 2024 9:44AM - 9:57AM |
L24.00009: Particle-Resolved Direct Numerical Simulations of Fractal Aggregates Settling through Density-Stratified Fluid Zachary Maches, Eckart Heinz Meiburg The settling of fractal aggregates with complex geometries through fluid of varying density plays a crucial role in many industrial and environmental systems. To study the influence that aggregate shape and particle density have on settling dynamics, by performing Particle-Resolved Direct Numerical Simulations (PR-DNS) we investigate the settling of fractal aggregates through a density interface. We find that the velocity of the aggregates passing through the interface is determined by the fractal dimension, the relative density of the particles to the fluid, and the Galileo number. Crucial to determining the velocity is the retention of fluid from the less dense upper layer in the aggregates' pore spaces, which imposes an upwards buoyancy force on the aggregate as it passes into the denser lower layer. The relative density between the internal fluid and the aggregate itself, as well as the timescale over which the denser external fluid diffuses and convects into the aggregate's pore space, can greatly affect the relationship between aggregate compactness and the settling velocity, and whether a more porous aggregate settles faster or slower than a less porous one. From this, we introduce general relationships governing the settling velocity of aggregates based on the fractal dimension, Galileo number, and particle and fluid densities. |
Monday, November 25, 2024 9:57AM - 10:10AM |
L24.00010: Inertial torques on curved atmospheric fibres Fabien Candelier, Linus Sundberg, Alain Pumir, Kristian Gustavsson, Bernhard Mehlig Atmospheric fibres accelerate the air as they move. This results in fluid-inertia torques that rotate the fibres, influencing their transport through the air. Little is known about this effect, beyond the general consensus that it is significant, because most theory of inertial torques is for axisymmetric particles. However, atmospheric fibres tend to lack this symmetry. Here we determine theoretically how breaking of axisymmetry changes the dynamics of rigid curved fibres in air. We find that planar fibres still align in quiescent air, but settle at an oblique angle with gravity. Our results make it clear that inertial alignment is a general and thus important factor for the transport of atmospheric particles. |
Monday, November 25, 2024 10:10AM - 10:23AM |
L24.00011: Transport and clogging of fibers in millifluidic channels Thomas Minh Huu Nguyen, Justin Maddox, Nathan Vani, Sebastien Kuchly, Alban Sauret, Harishankar Manikantan The transport of particles in strongly confined geometries is a complex process, especially when the particle size becomes comparable to the channel size. An interplay of particle shape, hydrodynamics, and steric and colloidal forces can lead to clogged channels. In this work, we explore the transport and clogging of fibers in millifluidic channels using a combination of experiments, theory and computations. We quantify the clogging probability of a fiber when flowed through a bend as a function of fiber length, channel width, bend angle and wall curvature, which we translate to a geometric condition for clogging. Using numerical simulations based on resistive-force theory coupled to a frictional model for wall contact, we then map out a phase space of particle configurations and lengths that demarcate cases where the fiber is clogged, finding qualitative agreement with experiments. We then turn to more complex geometries such as constrictions to illustrate the role of fiber length relative to gap size and the curvature of the boundaries in inducing a clog. Put together, these insights build toward a mechanistic understanding of clogging and transport of anisotropic particles in porous media, and in developing guidelines for the design of clog-resilient fluid systems. |
Monday, November 25, 2024 10:23AM - 10:36AM |
L24.00012: A Simplified Theory for a Light, Rigid, and Thin Fiber's Orientation Probability Distribution in Viscous Flows Bchara Sidnawi, Qianhong Wu In composite materials applications where the directionality of a component's mechanical, thermal, and electrical properties is of special interest, control of fiber orientation in the matrix is paramount. The dynamics of flexible fibers in viscous flows are extensively covered in the literature. In this study, however, we present a theoretical examination of a rigid, light, and small (local) fiber in a viscous matrix flow, where we focus on the flow field's normal component to the fiber as the main driver of its rotation. From there, we extend the analysis to a collection of initially scattered fibers then derive the equation governing the evolution of the fiber orientation's probability density function by visualizing it as an intensive property being transported by the time derivative of the orientation vector on a unit sphere. The governing equation is then solved numerically in a given flow field and an animation of the solution is presented. This study may provide a useful tool for enhanced control of the fiber orientation distribution in precursor matrix flows leading up to the fiber configuration at which the matrix cures. |
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