Session X46: Particle-Laden Flow: Non-Spherical Particles II

Inertial angular dynamics of non-spherical atmospheric particles

Cloud-ice crystals, volcanic ash, and microplastics are ubiquitous in the atmosphere. The orientation of these non-spherical particles influences their residence times, and the radiative properties of the atmosphere. These non-spherical particles are small, but their mass density is much greater than that of air. Studying the angular dynamics of such settling non-spherical particles is a major challenge. Therefore, previous studies have focused on particles settling in liquids. We demonstrate experimentally that the orientations of heavy, submillimetre-sized spheroids settling in air fluctuate considerably, in stark contrast to the very rapid orientation alignment observed in liquids. We establish theoretically that this behaviour is a consequence of large particle inertia. Our results highlight the central role of particle inertia in the angular dynamics of atmospheric particles, in an unexplored regime of parameters. This essential physical effect must be accounted for in models of atmospheric residence time, microplastic and volcanic ash dispersion, and the radiative properties of ice-laden clouds.   

The Effect of Particle Shape on the Dynamics of Suspensions of Spheroidal Particles

The effect of the shape on the dynamics of a suspension of non-spherical heavy particles is examined by fully resolved numerical simulations of oblate and prolate spheroids, as well as spheres, for a density ratio of ten, volume fractions ranging from 0.5% to 5%, and a Reynolds number between 20 and 30. The dynamics is determined both by the interactions of the particles with the fluid as well as by collisions, with the number and importance of collisions increasing with volume fractions. A single oblate and prolate spheroid fall broadside-on and at low volume fractions this is the predominant orientation. At higher volume fractions the orientation is more random. The various averaged quantities for prolate and oblate spheroids are similar and different from what is seen for spheres, particularly for low volume fractions. While the average slip velocity of the particles generally decreases with volume fraction, the orientation angle has an important role and as interactions and collisions orient some particles such that their smallest projected area faces the flow direction, they fall faster and the average particle slip velocity of the suspension is increased. The effect of particle orientation on the dynamics is obviously not captured by simulations with spherical particles.   

Uncovering the effect of morphology on snow particle settling dynamics under mild wind conditions

Natural snowflakes, with crystal categories including dendrites, plates, needles, graupels, and aggregates, exhibit complex settling kinematics due to their intricate shapes and atmospheric turbulence. To elucidate the impact of snow crystal shapes and atmospheric turbulence on these dynamics, we conduct a comprehensive study under various field conditions using a three-dimensional particle tracking velocimetry (3D PTV) system and a digital inline holography (DIH) system with a high-precision scale. These tools allow us to measure particle settling trajectories and simultaneously characterize snow size, shape, and density, thereby estimating particle aerodynamic properties. The current work focuses on the 3D kinematics of snow settling under low turbulence, where snow-shape effects are prominent. Our findings reveal diverse settling behaviors across crystal categories. Notably, we observed periodic fluctuations of the spanwise acceleration, suggesting the meandering motion of measured snow particle trajectories. The oscillation frequencies and amplitudes cover a wide range of scales and vary across crystal categories, with dendrites exhibiting the smallest frequency and largest amplitude and graupels showing the opposite. These variations likely depend on the particles' aerodynamic properties (e.g., drag coefficient, particle response time) and turbulence characteristics. Based on shape alone, needles settle faster than aggregates, followed by graupels and dendrites.   

Experimental study of perforated disks falling in turbulent air

Predicting the fall speed of frozen hydrometeors in the atmosphere is complicated by their unique geometries as well as by the air turbulence. Snowflakes and ice crystals often exhibit rimed branches resembling perforated shapes, which make them permeable to the air flow. This alters their wake dynamics in ways that have not been characterized in turbulence. As analogue to plate crystals, here we consider thin disks of 3 mm in diameter and compare geometries with and without perforations. These are dropped at controlled volume fractions into a large chamber generating a large region of approximately zero-mean-flow homogeneous air turbulence. Two turbulence levels are considered and contrasted with quiescent air conditions. The disks are imaged at 4300 Hz and their linear and rotational motion is reconstructed. Comparison of the solid and perforated disk behaviors in quiescent air shows that the perforated disks fall at a slower velocity than the solid disks. The perforated disks also fall more stably compared to the solid, which may either fall flat or tumble. In turbulence, both disk types experience a reduction in their average settling velocity, as compared to quiescent air, but the perforated disks are more mildly influenced by this effect.   

The effect of transverse gusts on free-falling plates

The kinematics of a free-falling body in an incompressible homogeneous quiescent flow depends on at least two dimensionless groups such as, for example, the nondimensional mass M ≡ m / ρ L² and the Galilei number Ga ≡ (m g / ρ)^1/2 / ν , where m and L are the mass and length of the body, ρ and ν are the density and the kinematic viscosity of the fluid. At low M and Ga, a two-dimensional body such as a plate falls steadily, while at higher M and Ga it flatters, tumbles, or combinations of the above. First, we investigate the effect of the permeability of a rectangular plate on the kinematics by coupling the two-dimensional Navier-Stokes equations and Euler's equation of motion, and by modelling the plate permeability with the Darcy equation. We found that the permeability increases the upper limit of M and Ga at which the plate falls steadily. Secondly, we investigate the transient effect of a transverse (horizontal) gust on the falling (vertical) velocity. The gust is defined by a smooth increase (through a cosine function) of the horizontal velocity from zero to u_G over a period t_G. We restrict our investigation to plates that fall steadily in the absence of a gust. We found that, for any u_G and t_G , the time-averaged terminal velocity over the transient is lower than the terminal velocity in quiescent flow. Hence, on average, a plate falling in a gusty flow falls slower than a plate falling in a quiescent flow. The effect of the gust increases with increasing M and Ga, and with increasing u_G . Conversely, the effect of the gusts plateaus for decreasing t_G and vanishes for increasing t_G . Overall these results suggest that the effect of horizontal gusts can be significant on slowing down the descent of free-falling bodies and may inform dispersal models of plant seeds such as, for example, the dandelion, and meteorological models on the transport of, for example, snowflakes.    

Orientation, displacement, and accumulation of anisotropic microswimmers under surface gravity waves

A three-dimensional numerical model is developed to investigate the instantaneous and time-averaged dynamics of anisotropic microswimmers under surface gravity waves over a range of particle shapes (characterized by eccentricity ε ϵ [-1 +1]) and particle motilities (vs). The particles are idealized as finite-sized prolate/oblate spheroids with small Stokes numbers and particle Reynolds numbers. We find that aspect ratio-dependent preferential alignment of the swimmers closely follows that of an inertial passive particle. However, there is a symmetry break in preferential orientation depending on whether the initial orientation is beyond some critical limit, and this is particularly consequential for microswimmers. Under the influence of surface gravity waves, negatively buoyant swimmers’ dynamics unfold the existence of regime changes from the domain wherein swimmers settle down despite their upward motility to the parameter domain where swimmers have vertically upward displacement defeating its negative buoyancy. When multiple swimmers are allowed to evolve, they tend to accumulate, which is primarily influenced by the velocity shear in the wave field. This result reveals that the thin layers of phytoplankton, as ecological activity hotspots in coastal oceans, can also be the result of swimmer-wave interaction.      

Transport dynamics of isotropic and anisotropic microparticles in inertio-elastic vortex flows

Spherical particles and fibers are common in biological and environmental flows and their transport dynamics are strongly affected by interaction with the flow structure. Here we focus on the response of spherical and elongated model particles to a single vortex flow field. For this purpose, we use a microfluidic cross-slot geometry, to generate a well characterized, stationary, three-dimensional streamwise vortex at moderate Reynolds number. Our experimental results, supported with numerical simulations, show that as the diameter of the spherical particles is increased, they are progressively expelled from the vortex core. This trend is further enhanced when the fluid's elasticity is slightly increased. Initial observations also indicate complex interactions between fibers and a vortical flow field, depending on fiber size and initial orientation. This work provides a fundamental contribution to the study of particle–flow interactions and for the improvement of particle sorting and transport techniques with possible environmental, industrial, and pharmaceutical applications.   

Merging experiments, theory and simulations of rheology, flow and microstructure of nanofiber dispersions to enable novel nanomaterials

Nanostructured materials provide unique mechanical, electrical, thermal and biological functions facilitating e.g. more sustainable material loops or improved health. Often. large scale production of promising material concepts depend on our ability to establish particular nanostructures such as aligned nanofibrils. In this context, well defined hydrodynamical manipulation of non-spherical nanoparticles is critical. Of course, accurate digital modelling is the key to design flow systems in which predefined structures can be produced with high accuracy and throughput.  With this need in mind, we present a combined experimental, numerical and theoretical investigation of the rheology, flow and nanostructure of cellulose nanofibrils (CNF) in a flow system that has been used to assemble very strong and stiff macroscale filaments by creating an aligned nanostructure. Rheometry, Optical Coherence Tomography (OCT), Polarized Optical Microscopy (POM) and Small Angle X-ray Scattering (SAXS) are combined with multiphase simulations for the flow and nanofibril alignment. The result is a digital twin that can be used both top evaluate the present process and develop new processes. However, it is also clear the there are hurdels in terms of fundamental understanding of nanodispersions that must be overcome in order to unleash the full potential of nanotechnology.   

Measurements of spinning and tumbling rates of Kolmogorov scale micro-plastic fibres

We measure the effect of the wall-normal location of micro-plastic fibres on spinning and tumbling rates in wall-bounded turbulence. The measurements are performed in a turbulent water channel at shear Reynolds number of 720. Fibres are 1.2mm long and 10μm in diameter (aspect ratio 120). Their length ranges from 4 to 12 Kolmogorov length scales. They are neutrally buoyant, inertial-less, and rigid in these flow conditions. 

Six high-speed cameras image the fibres at the channel centre and in a near-wall region. We employ and further refine a technique of tomographic fibre reconstruction and tracking. Their curved shape is used to define a fibre-fixed reference frame and measure its time-resolved orientation. Thus measurements of tumbling and spinning rates are enabled. We provide a discussion about the uncertainty on the rotation rates based on their shape and angular displacement between time-steps. Based on converged statistics, we observed that the mean and mean square spinning are higher than tumbling rates at both channel centre and near-wall region. Our results are original, because previous measurements are restricted to rotation rates of longer rods in homogeneous isotropic turbulence or to tumbling rates only.   

Slender flexible fibers in turbulent channel flow

The orientation and deformation of slender flexible fibers in turbulent channel flow is investigated numerically at varying fiber inertia, aspect ratio and bending stiffness. We use a Euler-Lagrangian approach based on DNS of the flow, combined to a rod-chain pointwise representation of the fibers. The simulations are performed using a pseudo-spectral solver at shear Reynolds numbers up to Re=1200 and tracking fibers with large aspect ratio (AR, up to 200), length L extending up to the inertial range of the turbulent flow, and Stokes numbers spanning two orders of magnitude (from St= 0.1 to St=11). For each combination of (AR, L, St) values, different fiber rigidities are considered to compare fibers with moderate stiffness (Young modulus E_Y=10⁴ in dimensionless units) with stiffless fibers (E_Y=0). We show that the fibers in the bulk of the flow orient with the local strain, align with the vorticity - as in homogeneous isotropic turbulence - and experience a tumbling rate comparable to that of rigid fibers. Near the walls, vorticity orients with the spanwise direction while flexible fibers align with the mean flow. This orthogonality determines a stronger contribution of the flow rotation to the tumbling rate. The most probable deformed shapes define a bi-variate probability space, suggesting that two main deformation patterns exist: eyelash bending and compressive buckling. The time spent by fibers in either bent state before being stretched again by the flow is found to scale with the fiber rotation timescale.   

Fiber Kinematics in the Near-Field of a Coaxial Jet

Using particle-laden jets, composite materials having specific properties may be fabricated. Using a coaxial jet that is characterized by two mixing layers, one can manipulate vortex generation by either changing the velocity ratio or the absolute value of the jet exit velocities. Here, nylon fiber kinematics (orientation, translation, tumbling rates) were measured in the near-field flow of a coaxial, water jet (one-way coupling) using high speed, planar particle image velocimetry (PIV). The fibers (length 1.6mm, diameter 52μm) were introduced in the center jet. Measurements were performed at three different ratios between   outer and inner jet exit velocities, r_u = 0,1,2.5. In addition, the effect of increasing the jet exit velocities was investigated for r_u = 1. Combined PIV and particle tracking velocimetry (PTV) data processing was done for the water velocities and fibers, respectively. Results indicate that fiber rotation is affected by large vortical structures of similar size as the fiber's length. The change in fiber orientation is especially pronounced at the point where the fiber becomes exposed to the inner shear layer. We will discuss both single fiber kinematics as well as orientation statistics in different regions downstream of the jet nozzle.