Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session H37: Particle Laden Flows: NonSpherical Particles 
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Chair: S. Balachandar, University of Florida Room: 619 
Monday, November 25, 2019 8:00AM  8:13AM 
H37.00001: Comprehensive modeling of hydrodynamic forces and torques on nonspherical particles using the PIEP model W. C. Moore, S. Balachandar, Yunchao Yang The first aim of this work is to present a general formulation for the force and torque experienced by an isolated nonspherical particle. Then, a working model is introduced for a spheroid subject to uniform flow. To construct this model, particle resolved direct numerical simulations (PRDNS) of flow around a spheroid are performed for subcritical Reynolds numbers. These PRDNS consider various angles of attack, rotational Reynolds numbers, and axes of rotation. The pairwise interaction extended pointparticle (PIEP) model is then utilized to model a spheroid as three spheres, and extensions to other geometries, such as cubes as superposition of nine spheres, are being considered. Lastly, this model is used to predict the dynamics of a settling spheroid. The model's results are compared to PRDNS results. [Preview Abstract] 
(Author Not Attending)

H37.00002: Rotation rate of fibers in turbulence Gautier Verhille, Ankur Bordoloi, Theresa Oehmke Since the last decade , more and more studies are devoted to the dynamics of anisotropic particles in turbulence. It has been shown theoretically and numerically that fibers smaller than the Kolmogorov length tend to align preferentially with the vorticity. More recently, Pujara et al. shows that longer fibers tend to align with the most extensional direction of the coarse grained velocity greadient. This difference of preferential alignment should have major impact on the rotational dynamics of particles: small fibers are expected to spin whereas long fibers are expected to tumble. We present here an experimental investigation on the global rotational dynamics (tumbling and spinning) of fibers smaller and larger than the Kolmogorov length. In this talk we will quantify the spinning and tumbling of fibers smaller and longer than the Kolmogorov length [Preview Abstract] 
Monday, November 25, 2019 8:26AM  8:39AM 
H37.00003: Dynamics and wakes of freely settling and rising cubes Anthony Wachs, Arman SeyedAhmadi In this investigation, we present numerical simulations of freely settling and rising cubes in a quiescent Newtonian fluid for various values of the Galileo number $70 \leq Ga \leq 250$ and of the solidtofluid density ratio 0.2 $\leq m \leq 7$. Ultimately, we obtain a comprehensive twoparameter flow map for a freely moving cube and characterize prominent features of each regime of motion such as trajectories and wake structure. Unlike the case of a sphere, helical motion is observed for all density ratios, marking it as a characteristic type of motion for a cube. Furthermore, we present an indepth force analysis relevant to the induced lateral motions, and we show that there is a significant jump in the drag coefficient coincident with the onset of the helical regime where largeamplitude lateral displacements appear. The enhancement of the drag coefficient is explained to be a combined effect of the vortexinduced drag and the orientation angle of the cube. [Preview Abstract] 
Monday, November 25, 2019 8:39AM  8:52AM 
H37.00004: Shape Dependence of Settling Velocities for Particles in Wavy Flows Laura Clark, Michelle DiBenedetto, Nicholas Ouellette, Jeffrey Koseff We investigated the effect of particle shape and oscillatory flow on the settling velocities of negatively buoyant particles with intermediate particle Reynolds numbers. In a laboratory wave tank, we tested a range of particle shapes, including discs, rods, and spheres, and wave characteristics, including shallow, deep, and intermediate surface gravity waves. By simultaneously measuring the velocities of the nonspherical particles and the fluid flow field, we extracted~the instantaneous particle slip velocities, the statistics of how the particles preferentially sample the wave field, and their net settling velocities. We find that all of these~quantities~display a significant shape dependence. [Preview Abstract] 
Monday, November 25, 2019 8:52AM  9:05AM 
H37.00005: On the freefall dynamics of highly inertial ellipsoids at $Re_p$ of $O(1)$ Johannes Milan G{\"u}ttler, Gholamhossein Bagheri Much is still unknown about the dynamics of inertial ellipsoids as inertial nonspherical particles in turbulence lag behind the flow to adjust their orientation. In particular, it is not quantitatively understood how particles of different shapes orient themselves in turbulent flows and how fast their orientation responds to flow fluctuations. Experiments on single particles in a quiescent medium are the first step to characterize this. We present our work on freely falling ellipsoidal particles in the 'intermediate' regime of particle Reynolds number 110 using shadowgraphy in an airfilled column setup. The particles are welldefined and have volumes equivalent to a diameter 140 um sphere, which became possible thanks to utilizing 2PhotonPolymerization. The density ratio between the particles and the medium (air) is approx. 1000. The experiments were performed using three highspeed cameras to capture both the transient and the terminal state. This allows us to look into rotation/tumbling rates, terminal velocities and the transient dynamics: do stable fixed points in the orientation or oscillatory motions exist and is there a stable orientation that the particle will take? For particles with unstable transient orientation, what is the steadystate orientation? [Preview Abstract] 
Monday, November 25, 2019 9:05AM  9:18AM 
H37.00006: TaylorAris Dispersion of Elongated Rods Ajay Harishankar Kumar, Thomas Powers, Daniel Harris Particles transported in fluid flows, such as cells or nanorods, are rarely spherical in nature. ~ In this study, we numerically and theoretically investigate the dispersion of an initial concentration of elongated rods in 2D pressuredriven shear flow.~ The rods translate due to diffusion and advection, and rotate due to rotational diffusion as well as their classical Jeffery's orbit in shear flow. When rotational diffusion dominates, we approach the classical Taylor Dispersion result for the longitudinal spreading rate by using an orientationally averaged translational diffusivity for the rods.~ However, in the high shear limit, the rods tend to align with the flow and ultimately spread faster as a direct consequence of their anisotropic diffusivities. The relative importance of the shearinduced orbit and rotational diffusivity can be represented by a rotational Peclet number, and allows us to bridge these two regimes. [Preview Abstract] 
Monday, November 25, 2019 9:18AM  9:31AM 
H37.00007: Tuning the hydrodynamic interactions between nonuniform sedimenting particles Kavinda Nissanka, Xiaolei Ma, Justin Burton Sedimentation dynamics of nonBrownian particles with nonuniform density distributions have recently been shown to exhibit hyperuniformity (Goldfriend et al., Phys. Rev. Lett., 2017). These predictions are tested here experimentally using spheroidal particles. Particles are classified by two parameters, their aspect ratio $\kappa $ and center of mass offset $\chi $. They were created from 2mm diameter spheres glued in various configurations. Spheres of different densities are used to vary $\chi $. We record the positions of particles using a highspeed camera, as they sediment in a quasi2D tank at Re $=$ 10$^{\mathrm{3}}$. Using tracking algorithms, we reconstruct the sedimentation trajectories. Pair interaction between particles is characterized by particle center separation vs. vertical distance traveled. Particles with $\kappa $\textgreater 1 exhibit repulsive behavior, while particles with $\kappa $\textless 1 attract each other. Interaction strength is shown to be inversely proportional to $\chi $. For 3 or more simultaneously sedimenting particles, repulsive interactions lead to a more uniform distribution of particles positions, whereas attractive interactions lead to particle clustering. [Preview Abstract] 
Monday, November 25, 2019 9:31AM  9:44AM 
H37.00008: Clustering Instability of Fiber Suspensions in Turbulent Shear Flows Lee Walsh, Greg Voth In a suspension of nonattractive rigid rods in a turbulent shear flow, we observe spontaneous aggregation into traveling clusters of high concentration. The fibers first accumulate in the turbulent boundary layer where they then break the symmetry of the apparatus by collecting into a few traveling clusters. The experimental apparatus is a TaylorCouette cell whose floor and inner wall are rotating, and ceiling and outer wall are fixed. The heavy particles sediment downward and outward, so their concentration is highest near the junction of the rotating floor and the fixed outer cylindrical wall. With a multicamera imaging system we can measure 3D fluid and fiber motion in the entire volume, and we are able to resolve individual fiber dynamics in the highconcentration regime where fiberfiber interactions become important. [Preview Abstract] 
Monday, November 25, 2019 9:44AM  9:57AM 
H37.00009: Time scale of rotation of inertial fibers in isotropic turbulence Ankur Bordoloi, Gautier Verhille, Evan Variano The past few years of research has significantly advanced our understanding of the rotational kinematics of inertial fibers in turbulence and their scaling with size. However, the question related to the time scale of rotational dispersion remains open. The time scale of rotation of inertial fibers is integral to processes such as papermaking, turbulent dragreduction, fiberglass blowing, and locomotion of planktonic organisms in the ocean. Based on timeresolved measurements of the orientation of rigid inertial fibers in a turbulencetank, we compute a time scale ($\tau _{\mathrm{d}})$ fiber rotation. We show that this timescale can be predicted by Kolmogorov's inertialrange scaling based on fiber length ($L)$ only when the diameter ($d)$ is small. For fibers with large diameters, we invoke our previous theoretical model designed specifically for the variance of fiber rotation and find that it fails to predict $\tau_{\mathrm{d}}$. We propose herein a new model that successfully predicts both these parameters for a wide range of $L$ and $d$. [Preview Abstract] 
Monday, November 25, 2019 9:57AM  10:10AM 
H37.00010: Inertial Settling of Flexible Fiber Suspensions Mona Rahmani, Arash Alizad Banaei, Mark Martinez, Luca Brandt The inertial settling of suspensions of flexible and rigid fibers are investigated using an Immersed Boundary Method. For the settling of a suspension of inextensible and slender fibers at a Galileo number of $Ga=160$, we examine a range of dimensionless bending rigidities and fiber concentrations that span dilute and semidilute regimes ($0.5 
Monday, November 25, 2019 10:10AM  10:23AM 
H37.00011: Tumbling rate of anisotropic particles in turbulent convection Linfeng Jiang, Chao Sun, Enrico Calzavarini The rotational dynamics of small anisotropic material particles ($e.g. $fibers or disks) in turbulent flows has been the focus of a series of recent studies. Experiments as well as numerical simulations have highlighted their complex behavior, which is inherited from the nontrivial dynamics of the velocity gradient tensor along the particle trajectories. We report the investigation of orientation dynamics of neutrally buoyant anisotropic particles as they are advected in the RayleighBenard convection by means of experiments and simulations. Compared with the homogeneous isotropic turbulence, the global rotation rate square for particles reveals a similar distribution whereas the averaged value as a function of aspect ratio shows a significant decrease. We propose a simple model to qualitatively understand the phenomenon. It is found that the large scale circulation significantly changes the flow topology into a bidimensional state so as to be responsible for the rotation rate variation. [Preview Abstract] 
Monday, November 25, 2019 10:23AM  10:36AM 
H37.00012: A finite Re slender body theory Anubhab Roy, Donald L. Koch The effects of fluid inertia on the settling motion of fibers is studied theoretically. Khayat \& Cox (1989) were the first to give a theory of hydrodynamic forces and torques on a slender body when fluid inertia is nonzero. Their theory uses a matched asymptotic expansion with a viscous inner flow and Oseenâ€™s approximation for the outer flow. This restricts the analysis to cases where Re defined based on fiber diameter ($Re_D$) is zero. We develop a novel finite Re slender body theory that allows the inner flow to be described by steady NavierStokes and thus provide better comparisons of drag and torque with realistic scenarios where the $Re_D\neq0$. [Preview Abstract] 
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