Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session M36: ParticleLaden Flows: Let's Get Together (Clustering)Particles

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Chair: Michelle Driscoll, Northwestern University Room: 302 
Tuesday, November 21, 2017 8:00AM  8:13AM 
M36.00001: Stable clusters emerge from unstable fronts Michelle Driscoll, Blaise Delmotte, Mena Youssef, Wenjie Fei, Stefano Sacanna, Kyle Bishop, Aleksandar Donev, Paul Chaikin Rotating colloidal particles near a surface create strong advective flows, which can lead to a rich variety of collective effects. It has recently been shown that longlived compact motile structures, called ``critters'', emerge naturally from a fingering instability in this microroller system. We identified these new structures using largescale 3D simulations, and have recently made promising steps towards producing them in the lab. Our simulations and experiments suggest that these critters are a stable state of the system, move much faster than individual rollers, and quickly respond to a changing drive. We believe that critters are unique in that they are clusters which are formed only with hydrodynamic interactions. Furthermore, as compact, selfassembled structures which can easily be remotely guided, critters may offer a promising tool for microscopic transport. [Preview Abstract] 
Tuesday, November 21, 2017 8:13AM  8:26AM 
M36.00002: Nonlinear dynamics of clustering in particleladen turbulent flows Mahdi Esmaily, Ali Mani Heavy inertial particles in spatially and temporally varying flows can form clusters if their relaxation time is on the order of the dissipation time scale of the flow. This regime, identified by $\rm St = \mathcal O(1)$, is investigated in this study using analytical tools. We show that the nonlinear variation of segregation versus St can be explained by considering a onedimensional canonical setting where particles are subjected to an oscillatory velocity gradient that is constant in space. Our analysis shows that the Lyapunov exponent, as a measure of particle segregation, reaches a minimum at $\rm St = \mathcal O(1)$ and becomes positive at $\rm St \gg 1$ and approaches zero as $\rm St \to 0$ or $\infty$. These predictions, which are corroborated by the numerical results, are directly linked and compared against measurements of the dispersion and segregation in threedimensional turbulence. Our analysis reveals a strongly nonlinear behavior of the Lyapunov exponents in the straining regimes of strong oscillations. [Preview Abstract] 
(Author Not Attending)

M36.00003: Clustering of small bubbles in homogeneous isotropic turbulence Changhoon Lee, Itzhak Fouxon, Gihun Shim, Seulgi Lee Transport of small bubbles in isotropic turbulence is numerically and theoretically investigated. The difference of the bubble's motion from the motion of heavy particles is determined completely by the lift force. The socalled flow of bubbles exists in the whole range of valid parameters of typically used equation of bubble motion. Therefore, the bubble velocity can be explicitly expressed in terms of the local fluid velocity and its derivatives. The spectrum of Lyapunov exponents and fractal dimension can be theoretically estimated. For very strong gravity, the bubble distribution shows a very strong multi fractal structure. Direct numerical simulation of bubbleladen isotropic turbulence has been performed. The Lyapunov exponents and fractal dimension are favorably compared between simulation and theory. Strong multi fractal structure takes the form of vertical columnar clustering. [Preview Abstract] 
Tuesday, November 21, 2017 8:39AM  8:52AM 
M36.00004: Wavelet investigation of preferential concentration in particleladen turbulence Maxime Bassenne, Javier Urzay, Kai Schneider, Parviz Moin Direct numerical simulations of particleladen homogeneousisotropic turbulence are employed in conjunction with wavelet multiresolution analyses to study preferential concentration in both physical and spectral spaces. Spatiallylocalized energy spectra for velocity, vorticity and particlenumber density are computed, along with their spatial fluctuations that enable the quantification of scaledependent probability density functions, intermittency and interphase conditional statistics. The main result is that particles are found in regions of lower turbulence spectral energy than the corresponding mean. This suggests that modeling the subgridscale turbulence intermittency is required for capturing the smallscale statistics of preferential concentration in largeeddy simulations. Additionally, a method is defined that decomposes a particle numberdensity field into the sum of a coherent and an incoherent components. The coherent component representing the clusters can be sparsely described by at most 1.6{\%} of the total number of wavelet coefficients. An application of the method, motivated by radiativeheattransfer simulations, is illustrated in the form of a gridadaptation algorithm that results in nonuniform meshes refined around particle clusters. It leads to a reduction of the number of control volumes by one to two orders of magnitude. [Preview Abstract] 
Tuesday, November 21, 2017 8:52AM  9:05AM 
M36.00005: Aggregation of magnetic particles in turbulence Hector De La Rosa, Gautier Verhille, Patrice Le Gal The formation of particle aggregates in flows is an ubiquitous process in industrial and environmental contexts. The aim of our study is to describe the saturation of the aggregation process of inertial particles in turbulence, i.e. when the particles are larger than the Kolmogorov dissipative scale $\eta_k$. For this purpose, we seeded a high Reynolds number turbulent von Karman flow ($Re\sim10^6$, $\eta_K\sim10~\mu$m) with millimeter size nearly neutrally buoyant magnetic particles. Each magnetic dipole imposes a torque and a force on the other magnets at the origin of the cohesion of the aggregates. On the contrary turbulent fluctuations impose an external stress which may fragment the aggregates. We study the statistics of the size distribution of the aggregates. Assuming a Kolmogorov inertial scaling for the turbulent velocity fluctuations, we predict theoretically the average size of the aggregates as a function of the turbulence intensity. A scaling law is deduced from the theoretical model and is verified by our experimental results. [Preview Abstract] 
Tuesday, November 21, 2017 9:05AM  9:18AM 
M36.00006: Investigations on a novel photoacoustofluidic effect Gabriel Dumy, Mauricio Hoyos, JeanLuc Aider Acoustic manipulation of microobjects (particles, cells, bacteria) can be achieved using ultrasonic standing waves in a fluidic or microfluidic resonator. By matching resonator dimensions and acoustic field frequency it is possible to use acoustic radiation force (ARF) to gather the particles in the pressure nodal (or antinodal) plane, creating one or several aggregates. In standard operating conditions, they are stable for as long as needed in acoustic levitation at this position. In this study, we present a new unexpected phenomenon. After creating an aggregate of lightabsorbing particles, we show that it is possible to force the breakup of the aggregate when lighting it with an electromagnetic wave of adequate wavelength and intensity. While the particles remain in levitation, they are rejected and propelled away from the aggregate, leading to its destruction. We show that this phenomenon depends on both amplitude of the ultrasonic field and lighting intensity. Various experiments with different types of particles and concentrations are used to discuss the possible phenomenon explanations. Moreover, investigations showed that this phenomenon applies to biological compounds such as red blood cells and stem cells, suggesting potential biomedical applications. [Preview Abstract] 
Tuesday, November 21, 2017 9:18AM  9:31AM 
M36.00007: Particle dispersion and segregation in suspension flows with bidispersed particle sizes Amanda Howard, Martin Maxey Suspensions of neutrally buoyant, nonBrownian particles with monodispersed size in a low Reynolds number pressure driven flow display an irreversible net flux of particles towards the center of the channel, leading to tightly packed particles at the core of the channel and a low concentration of particles near the walls. When the particles have bidispersed sizes, the large particles on average migrate to the center of the channel faster than the smaller particles, which can lead to separation of the particles by size. We will present a series of numerical simulations for dense suspensions of bidispersed particles in a planar channel with a range of size ratios. The particles segregate by size across the channel when both the size ratio of large to small particles and the initial volume fraction of large particles are sufficiently large. We will discuss the dynamics behind this segregation and the role of particle contact pressure and compare the volume fraction and stress profiles to those of monodispersed suspensions and suspension balance models. [Preview Abstract] 
Tuesday, November 21, 2017 9:31AM  9:44AM 
M36.00008: Mixing Behaviors of Wet Granular Materials in a Pulsating Fluidized Bed Eldin Wee Chuan Lim The Discrete Element Method combined with Computational Fluid Dynamics was coupled with a capillary liquid bridge force model for computational studies of mixing behaviors in a gas fluidized bed containing wet granular materials. There was a high tendency for wet particles to form large agglomerates within which relative motions between adjacent particles were hindered. This resulted in much lower mixing efficiencies compared with fluidization of dry particles. Capillary liquid bridge forces were on average stronger than both fluid drag forces and particleparticle collision forces. Particle exchange between agglomerates was necessary for mixing to occur during fluidization of wet granular materials but required strong capillary liquid bridge forces to be overcome. When pulsation of the inlet gas flow was applied, voidage waves comprising regions of high and low particle concentration formed within the fluidized bed. This allowed particles to cluster and disperse repeatedly, thus facilitating exchange of particles between agglomerates and promoting mixing of particles throughout the fluidized bed. This points towards the possibility of utilizing pulsed fluidization as an effective means of improving mixing efficiencies in fluidized bed systems containing wet granular materials. [Preview Abstract] 
Tuesday, November 21, 2017 9:44AM  9:57AM 
M36.00009: Unsteady sedimentation of flocculating nonBrownian suspensions Alexander Zinchenko Microstructural evolution and temporal dynamics of the sedimentation rate $U(t)$ are studied for a monodisperse suspension of nonBrownian spherical particles subject to van der Waals attraction and electrostatic repulsion in the realistic range of colloidal parameters (Hamaker constant, surface potential, double layer thickness etc.). A novel economical highorder multipole algorithm is used to fully resolve hydrodynamical interactions in the dynamical simulations with up to 500 spheres in a periodic box and O(10$^{\mathrm{6}})$ time steps, combined with geometry perturbation (Zinchenko A.Z. Phil. Trans. R. Soc. Lond. A (1998), vol. 356, 29532998) to incorporate lubrication and extend the solution to arbitrarily small particle separations. The total colloidal force near the secondary minimum often greatly exceeds the effective gravity/buoyancy force, resulting in the formation of strong but flexible bonds and large clusters as the suspension evolves from an initial wellmixed state of nonaggregated spheres. Ensemble averaging over many initial configurations is used to predict $U(t)$ for particle volume fractions between 0.1 and 0.25. The results are fully convergent, systemsize independent and cover a 22.5 fold growth of $U(t)$ after a latency time. [Preview Abstract] 

M36.00010: ABSTRACT WITHDRAWN 
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