Session M27: Particle-laden Flows: Non-Spherical Particles I

Three-dimensional particle tracking velocimetry measurements of flow around freely falling ice-particles

Aerodynamics of an ice-particle is influenced by its fall speed, orientation and shape. Three dimensional particle tracking velocimetry measurements were performed for freely falling 3D printed ice-particle analogues and their aggregates in a vertical tank of quiescent water-glycerine mixture. Plate-like and columnar ice-particle crystals along with their aggregates were used in the present investigation. 3D printed analogues of ice-particle aggregates closely mimic complex shapes of hydrometeors in clouds and atmosphere due to the varying  number and orientation of the constituent particles forming the aggregates. Flow physics and the fall attitude of particles and their aggregates have been investigated. The critical Reynold numbers (Re) for the onset of unsteadiness in the flow differs due to the flow separation behaviour at the edges of singular particles and those at the sides of aggregates. The wake is a signature of geometry of the particle and its fall speed. Three dimensional wakes were noted with a larger wake width for the aggregates. Three regimes: (i) Steady, (ii) Shedding, (iii) Chaotic regimes were identified for the falling particles based on nature of the wake at various Re. The flow field and vortical structures are compared for the particles and their aggregates. The flow characteristics of the wake and the fall attitude of hydrometeors are coupled. The fall trajectory of the particles along with the  nature of orientation in both the steady and unsteady regimes was compared with the behaviour of their aggregates. The crystals oriented themselves with approximately the maximum projected cross-section facing the fall direction, while aggregates had a less predictable orientation, which varied with Re in some cases. The crystals flutter, rotate and spiral during the descent depending on their geometry and terminal velocity.   

Measuring the Translation-Rotation Coupling of an Isotropic Helicoid

In 1871, Lord Kelvin proposed the isotropic helicoid, a chiral particle whose translation-rotation coupling is independent of orientation. Recent experiments have not been able to observe the expected chiral coupling, while new theory has shown that hydrodynamic interactions between the vanes of an isotropic should allow a nonzero but small translation-rotation coupling. We are performing the next generation of experiments in search of this nonzero-coupling. The effort involves two experiments. In the first experiment, we suspend the test particle and measure torque generated by fluid flowing past it. In the second experiment, we measure the rotation of a sedimenting particle in a quiescent fluid.   

Mass Transfer from Non-Spherical Particles in Turbulence

Inertial, non-spherical particles in turbulent flows show interesting kinematic properties such as the difference between tumbling and spinning rates depending on size, and overall enstrophy being shape independent. While kinematics have been studied for many different types of Taylor-scale, non-spherical particles such as fibers, flat particles, and cuboids, to our knowledge, no dissolving non-spherical particles have been studied in the same size range. In this study, we present results for rod-like and disc-like neutrally buoyant particles that dissolve in water. The particles we tested were designed to explore the shape-motion-flux relationship through a variety of surface areas, volumes, and aspect ratios. Throughout the parameter space tested, we found that between rods and discs, disc-like particles dissolved faster than rod-like particles. However, within the parameter spaces of rod-shaped particles that were tested, the rods’ dissolution rate decreased as the surface-area-to-volume ratio decreased. This suggests that the surface-area-to-volume ratio could be the governing factor characterizing dissolution rates within particles of the same shape, but not between particles of different shapes.   

Orientation and tumbling of inertial rod and disk particles in a turbulent boundary layer

The behavior of inertial disk and rod particles in an open channel flow are investigated experimentally in order to examine the effects of shape on their orientation and rotation in turbulence. The friction Reynolds number of the flow is Re_τ~600, and the particle Stokes number based on the viscous timescale is St⁺~O(10). Particle tracking velocimetry is used to obtain time-resolved particle trajectories. 3D orientations and tumbling rates of the rods and disks are reconstructed from their silhouettes projected onto the imaging plane. Rods tend to orient in the streamwise direction, while disks prefer to align their symmetry axis normal to the wall. This alignment is much more stable for disks. Rods undergo stronger tumbling in the near-wall region, and they tumble freely in response to the mean shear and turbulent fluid velocity fluctuations, whereas disks wobble about their preferential wall-normal orientation. In addition, ascending rods have higher tumbling rates than descending rods; this trend is not observed for disks. This implies a correlation between the turbulence events that resuspend rods and those that cause them to tumble.   

Influence of shear Reynolds number on the dynamics of non-axisymmetric fibres in turbulent channel flow

We investigate experimentally the effect of the shear Reynolds number on the behaviour of long, non-axisymmetric fibres in turbulent channel flow. Fibres are slender, curved and neutrally buoyant particles and their shape ranges from low to moderate curvature. Experiments are performed in the TU Wien Turbulent Water Channel for three values of shear Reynolds number, namely 180, 360 and 720. Time-resolved recordings of the laser-illuminated measurement volume are obtained from four high-speed cameras and used to infer fibres dynamics. With the aid of multiplicative algebraic reconstruction techniques, fibres position and orientation are determined in time. Measurements allow a complete characterization of the fibres dynamics in all the regions of the channel. We observe that the fibres dynamics is strongly influenced by their curvature. Through a comparison between measurements of near-wall dynamics of fibres and near-wall dynamics of flow, we identify a causal relationship between fibre velocity and orientation, and the near-wall turbulence dynamics. Finally, we provide original measurements of the tumbling rate of the fibres, for which we report the influence of fibres curvature, and we confirm previous findings obtained in numerical and experimental works.   

Long non-axisymmetric fibres in channel flow turbulence

Effect of shape asymmetry on the behavior of the long fibers is investigated in this study. The experimental facility consists of a closed water channel (aspect ratio 10). The fibers used are characterized by a length-to-diameter ratio of 120 and moderately curved. Fibers are recorded by four high-speed cameras in a fully developed flow section. We propose a new 3D reconstruction and tracking method based on the light intensity distribution, consisting of time-resolved multiple algebraic reconstruction technique (MART) combined with an advanced imaging analysis technique. With the experimental database produced, we are able to link the fibers dynamics to their shape, i.e. to the average fiber curvature. Fibers concentration, orientation and rotational dynamics are investigated all along the channel height, which includes the near-wall flow region. We observed that small deviations from the straight shape (i.e. small values of the fibers curvature) can produce significant modifications of fibers concentration and orientation. Moreover, from the analysis of the mean squared tumbling, we show that also the rotational dynamics of the fibers are controlled by their shape and the magnitude of the mean squared tumbling increases as a function of the shape asymmetry.   

Fluttering and Tumbling of Two-Dimensional Concave/Convex Bodies

We study the orientation dynamics of two-dimensional concavo-convex solid bodies denser than the fluid through which they fall under gravity. We show that the orientation dynamics of the body, quantified in terms of the angle Φ relative to the horizontal, undergoes a transcritical bifurcation at a Reynolds number Re_c⁽¹⁾  and a subcritical pitchfork bifurcation at a critical Reynolds number Re_c⁽²⁾ > Re_c⁽¹⁾ . For  Re < Re_c⁽¹⁾  , the concave-downwards orientation ( Φ = 0 ) is unstable and bodies overturn into the  Φ = π  orientation. For Re_c⁽¹⁾ < Re < Re_c⁽²⁾ , the falling body has two stable equilibria at Φ = 0  and  Φ = π  for steady descent. For Re > Re_c⁽²⁾ , the concave-downwards   Φ = 0  orientation is again unstable, and bodies that start concave-downwards exhibit overstable oscillations about the unstable fixed point, eventually tumbling into the stable  Φ = π   orientation. The Re_c⁽²⁾ ≈ 15 at which the subcritical pitchfork bifurcation occurs is distinct from the Re or the onset of vortex shedding, which causes the  Φ = π  equilibrium to also become unstable, with bodies fluttering about  Φ = π .  The complex orientation dynamics of irregularly shaped bodies evidenced here is relevant in a wide range of physical scenarios.   

Buckling and temporal order of flexible fibers in shear flows

Dynamics of flexible fibers in shear flow are studied experimentally and numerically for initially straight configurations at different 3D orientations with respect to the flow. The Reynolds number is much smaller than unity. We focus on time scales of the order of a few Jeffery periods, and analyze dependence of the dynamics on the ratio A of bending to shear forces. In the experiments, we observe fibers in the flow-vorticity plane, which gives insight into the motion out of the shear plane. In the simulations, we use the multipole expansion corrected for lubrication and implemented in the precise Hydromultipole numerical codes. We observe that for a very limited range of initial orientations from the compressional region of the shear flow, excluding those from the flow-vorticity plane, fibers undergo a compressional buckling, with a pronounced but very short deformation of shape along their whole length, which is in contrast to the typical local bending that originates over a long time from the fiber ends. Since fibers straighten out in the flow-vorticity plane while tumbling, the compressional buckling is transient -- it does not appear for times longer than 1/4 of the Jeffery period. For larger times, bending of fibers is always driven by their ends.