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 B37: Turbulence: Particle Laden Flows 
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Chair: Bernhard Mehlig, University of Gothenburg Room: 619 
Saturday, November 23, 2019 4:40PM  4:53PM 
B37.00001: Effect of fluid inertia on the orientation of a small spheroid settling in turbulence Bernhard Mehlig We study the angular dynamics of small nonspherical particles settling in a turbulent flow. Most solid particles encountered in Nature are not spherical, and their orientations affect their settling speeds, as well as their collision and aggregation rates in suspensions. Whereas the random action of turbulent eddies favours an isotropic distribution of orientations, gravitational settling breaks the rotational symmetry. We demonstrate here that the fluidinertia torque plays a dominant role in the problem. As a consequence rodlike particles tend to settle horizontally in turbulence, the more so the larger the settling number ${\rm Sv}$ (a dimensionless measure of the settling speed). For large ${\rm Sv}$ we determine the fluctuations around this preferential horizontal orientation for prolate particles with arbitrary aspect ratios, assuming small Stokes number ${\rm St}$ (a dimensionless measure of particle inertia). This overdamped theory predicts that the orientation distribution is very narrow at large ${\rm Sv}$, with a variance proportional to ${\rm Sv}^{4}$ for rods and ${\rm Sv}^{8}$ for disks. The abstract is based mainly on arXiv:1904.00481 (New Journal of Physics https://doi.org/10.1088/13672630/ab3062). [Preview Abstract] 
(Author Not Attending)

B37.00002: Collision efficiency of rapidly settling particles in a turbulent flow Pijush Patra, Anubhab Roy, Donald L. Koch We calculate the collision rate constant of hydrodynamically interacting low inertia spherical particle pairs sedimenting in a homogeneous isotropic turbulent flow in the rapid settling limit where the settling time across a Kolmogorov eddy is short compared with Kolmogorov time scale. Due to the subKolmogorov particle sizes, we approximate the fluid motion in the vicinity of the particle pair locally as a linear flow with a fluctuating velocity gradient that appears from the background turbulent flow. The response of the relative particle position is small over the correlation time of the flow and therefore, a diffusive process characterizes the relative motion with a diffusivity $D_{ij}^H$ and the hydrodynamic interactions lead to a net drift, $V_i^H$, toward small interparticle separations. The driftdiffusion fluxes are expressed in terms of the velocity gradient autocorrelation function along the settling trajectory and in this particular problem due to rapid settling assumption we are able to relate it with the turbulence energy spectrum. A convectiondiffusion equation for pair probability density function, $P(\textbf{r}, t)$, is derived in terms of the hydrodynamic turbulent pair diffusion and drift and then solved numerically to calculate the collision rate constant. [Preview Abstract] 
Saturday, November 23, 2019 5:06PM  5:19PM 
B37.00003: Measurement of Inertial Particle Collision Statistics in Isotropic Turbulence Using 3D Particle Tracking Velocimetry Adam Hammond, Zach Liang, Hui Meng We experimentally investigate the effects of turbulence and particle inertia on three particle collision statistics in particleladen isotropic turbulence: radial distribution function (RDF), radial particlepair relative velocity (RV), and for the first time, geometric collision kernel for which RDF and RV are factors. Experimentally obtaining these statistics has been difficult in the past especially as particle separation distance $r$ decreases below the Kolmogorov length scale $\eta $ to nearcontact, where the physics becomes obscure and theoretical models may not hold. Using ShaketheBox 3D particle tracking, we are able to resolve particle positions and velocities at $r$\textless $\eta $, which enables estimations of RDF and RV at nearcontact and calculation of the collision kernel. Experiments are performed in a truncated icosahedron isotropic turbulence chamber at ten Stokes numbers 0.2\textless \textit{St}\textless 2.3. The \textit{St} was changed by varying the fan speed (which also changes 246$\le $\textit{Re}$_{\lambda }\le $357) and particle diameter d between 15$\mu $m and 45$\mu $m. RDF increases as $r$/$\eta $ decreases to $O$(1), and increases with \textit{St}. RV decreases as $r$/$\eta $ decreases until $r$/$\eta =$1, recapitulating previous DNS and experimental results (Dou et al, 2018, DOI: 10.1017/jfm.2017.813); however, as $r$/$\eta $ \textless 1, RV exhibits a sharp upturn. This first simultaneous nearcontact estimation of RDF, RV, and collision kernel calculated from real turbulence enables examination of theory and DNS, and may reveal new phenomena not previously accounted for in prior models. [Preview Abstract] 
Saturday, November 23, 2019 5:19PM  5:32PM 
B37.00004: Fall velocities of Hydrometeors in Turbulent Flow Ahmad Talaei, Tim Garrett An understanding of the interactions of precipitating aerosols, droplets and ice crystals within an atmospheric turbulent flow is fundamental to predictions of atmospheric weather and climate. Here we examine the mean settling velocity of a hydrometeor falling into a random Gaussian turbulent flow using hydrometeor images and velocities captured by the MultiAngle Snowflake Camera (MASC) at Oliktok Point, Alaska. Analyses reveal hydrometeor Reynolds numbers ranging from 1 to 1000, sharply peaked at 200. Due to mathematical difficulties, previous analytical solutions of the equation of motion of a falling sphere in a viscous liquid have been constrained to the slowly falling particles in the Stokes regime with Reynolds numbers less than unity. In this study, we introduce an analytical solution for higher Reynolds numbers and develop an equation of motion for studying the interaction of atmospheric turbulence and hydrometers. The results show settling velocity reduction in weak turbulence and enhancement in strong turbulence. [Preview Abstract] 
Saturday, November 23, 2019 5:32PM  5:45PM 
B37.00005: Inertial particle velocity and distribution in vertical turbulent channel flow: a numerical and experimental comparison David Richter, Guiquan Wang, Kee Onn Fong, Filippo Coletti, Jesse Capecelatro This study is concerned with the statistics of vertical turbulent channel flow laden with inertial particles for two different volume concentrations ($\Phi_{V} = 3 \times 10^{6}$ and $\Phi_{V} = 5 \times 10^{5}$) at a Stokes number of $St^{+} = 58.6$ based on viscous units. Two independent direct numerical simulation models utilizing the pointparticle approach are compared to recent experimental measurements, where all relevant nondimensional parameters are directly matched. While both numerical models are built on the same general approach, details of the implementations are different. At low volume loading, both numerical models are in general agreement with the experimental measurements, with certain exceptions near the walls. At high loading, these discrepancies are increased, and it is found that particle clustering is overpredicted in the simulations as compared to the experimental observations. Potential reasons for the discrepancies are discussed. As this study is among the first to perform onetoone comparisons of particleladen flow statistics between numerical models and experiments, it suggests that continued efforts are required to reconcile differences between the observed behavior and numerical predictions. [Preview Abstract] 
Saturday, November 23, 2019 5:45PM  5:58PM 
B37.00006: Simultaneous tracking of suspended particles and timeresolved PIV in a turbulent boundary layer Filippo Coletti, Lucia Baker A detailed picture of the interaction between suspended sediment and the carrier fluid has only recently begun to emerge due to recent advances in experimental and numerical methods. Here we investigate experimentally the dynamics of spherical particles in a turbulent boundary layer in a saltationsuspension transport regime. Particle image velocimetry and particle tracking velocimetry are used to obtain simultaneous, timeresolved fluid velocity fields and particle trajectories. Statistics of particle velocity, particle acceleration, and fluid velocity at particle locations are computed to characterize particle behavior and investigate mechanisms for particle deposition and resuspension. Fluid ejection events near the wall appear to be a main mechanism for particle suspension, while fluid sweeps contribute less to particle deposition. Particle acceleration variance is found to peak markedly near the wall, in response to passing turbulent structures. Resuspension is preceded on average by an increase in streamwise fluid velocity at particle location, while deposition is preceded by a decrease in fluid velocity. Resuspending particles experience much stronger wallnormal acceleration magnitude than depositing particles. [Preview Abstract] 
Saturday, November 23, 2019 5:58PM  6:11PM 
B37.00007: Shape and scaledependent coupling between inertialess spheroids and velocity gradients in turbulence Nimish Pujara, Cristian Lalescu, Michael Wilczek Particles of different shapes and sizes are commonly found suspended in a turbulent flow in the environment and in industrial processes. To better understand the dynamics of neutrallybuoyant, nonspherical particles in dilute concentrations, we compute the motion of inertialess spheroids in direct numerical simulations of turbulence using oneway forcing. Particles of different sizes are modelled as as tracers after the velocity field has been coarsegrained at different filter scales. The focus is on the statistics of particle rotations and what they reveal about the interaction between the particles and velocity gradients. While particle rotations in the coordinate axes fixed to each particle show interesting variations with particle shape and filter scale, the meansquare value of particle angular velocity in the global coordinate axes is nearly constant across all shapes and scales. These trends are further probed by examining the particle alignment with fluid vorticity and how this depends on particle shape and filter scale. Finally, a comparison between these results and laboratory experiments provides insights into how particle inertia may influence particleturbulence coupling for large, anisotropic particles. [Preview Abstract] 
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