Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session D39: Particle-Laden and Multiphase Flows |
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Chair: Changhoon Lee, Yonsei University, Korea Room: Georgia World Congress Center Ballroom 3/4 |
Sunday, November 18, 2018 2:30PM - 2:43PM |
D39.00001: Multiscale particle motion in rotating turbulence Xiaolong Zhang, Rohit Dhariwal, Andrew D Bragg Using Direct Numerical Simulations (DNS) and theory, we examine the multiscale motion of particles (with and without inertia) in rotating turbulence. Rapid rotation affects particle motion directly, through Coriolis and centrifugal forces, and also indirectly through the way it modifies the spatiotemporal structure of the turbulence. The DNS are designed to enable exploration of particle motion in both the Zeman regime (where Zeman phenomenology is expected to hold) and the inverse cascade regime of the flow. Asymptotic predictions for the particle relative dispersion in the plane normal to the rotation axis are derived for both the Zeman and inverse cascade regimes, capturing the leading order effects of particle inertia at these scales. For fluid particles, the results show that rotation leads to strong anisotropy of their relative dispersion, with dispersion parallel to the rotation axis being not only smaller, but also growing much more slowly in time than that in the plane normal to rotation. Further, high-order moments of the dispersion show a suppression of extreme events in the relative dispersion in the parallel direction due to rotation. We also find that in certain regimes, particle inertia leads to some unexpected modifications to the relative dispersion behavior. |
Sunday, November 18, 2018 2:43PM - 2:56PM |
D39.00002: Experimental studies of deformation and breakup of bubbles under strong turbulence Ashik Ullah Mohammad Masuk, Ashwanth Salibindla, Shiyong Tan, Rui Ni We have developed a vertical water tunnel facility to create turbulence with a strong energy dissipation rate. It allows us to investigate deformation and breakup of bubbles driven mostly by turbulence rather than by the buoyancy force. In this system, we simultaneously acquire the Lagrangian trajectories of bubbles with their reconstructed 3D shapes, as well as a high-concentration of tracer particles around these gas bubbles to calculate the surrounding flow field. Their spatio-temporal statistics enable us to study the couplings and the energy transfer between the two phases during deformation and breakup. The results may potentially help us to develop a better closure model for multiphase flow in a turbulent environment. |
Sunday, November 18, 2018 2:56PM - 3:09PM |
D39.00003: Effects of Flow Structures in Homogeneous Isotropic Turbulence on Particle Clustering Rachael Hager, Ömer Savaş In clouds, the main growth mechanism of droplets with diameters 10-50 µm, known as the size-gap, is collision and coalescence. Atmospheric turbulence is known to increase the droplet growth rate in this range by enhancing the relative velocity between droplets and the formation of droplet clustering, thus, increasing the droplet collision rate. The focus here is to further understand droplet clustering by experimentally exploring the relationship between homogeneous isotropic turbulent flow structures and particle clustering. A 40-cm Eaton box is used to generate a homogeneous turbulent flow in which aluminum-oxide and fluorescent polymer particles are injected. Images of the aluminum-oxide particles with diameters of 0.5 µm are taken and analyzed using Particle Image Velocimetry to determine the flow structures present. Simultaneously, a second camera is used to capture images through a filter of the fluorescent polymer particles with diameters of 15 µm. Various optical tools are used to overlap the region of interest in the two sets of images. The relationship between particle concentration and flow structures is examined for a range of flow conditions, where clustering is quantified using a Voronoi cell analysis and the particle number density. |
Sunday, November 18, 2018 3:09PM - 3:22PM |
D39.00004: On the interaction of homogeneous shear turbulence and droplets of Taylor length-scale size Pablo I Trefftz-Posada, Antonio Ferrante Our objective is to determine the effects of linear mean shear flow on the interaction of droplets with homogeneous turbulence. We performed DNS of 3130 finite-size droplets of diameter approximately equal to the Taylor lengthscale and with 5\% droplet volume fraction in homogeneous shear turbulence at initial Taylor-scale Reynolds number Re$_\lambda$=75. We studied the effects of varying the Weber number and the shear number on the droplet/turbulence interaction. Following the derivation of the turbulence kinetic energy (TKE) equation for two-fluid flow found in Dodd, Ferrante [J. Fluid Mechanics, Vol. 806, pp. 356-412], we derived the TKE equations for two-fluid flow in homogeneous shear turbulence. We will present the numerical methods we used to simulate two-fluid homogeneous shear turbulence, as well as the effects on droplet/turbulence interaction caused by the mean shear. |
Sunday, November 18, 2018 3:22PM - 3:35PM |
D39.00005: Increase in scale of fluid motion and decrease in fluid turbulence by addition of particles Yoichi Mito Large contributions of large-scale fluid motions, that are characterized by their large Lagrangian time scales, to turbulent transport and sustenance of wall turbulence are presented using a direct numerical simulation of fluid turbulence flowing downward through a vertical channel in which a small amount of particles are added from uniformly-distributed point sources. Feedback effect of particles on fluid turbulence is considered using a point force method. Lagrangian measurements are done by releasing passive particles, that do not exert forces on the fluid, and fluid particles from point sources, located at several distances from the wall and several times. The objective of the present study is to reconcile the Lagrangian measurement with the Eulerian measurement and to have an insight into the mechanism, that sustains wall turbulence, and how it is attenuated by the addition of particles. The measurements indicate that wall turbulence is sustained by large-scale fluid motions with multiple flow directions, of which variety is due to small-scale turbulence structures, such as vortices, and that the dissipation of small-scale structures, caused by particles, reduces wall-normal velocity components of large-scale fluid motions, which results in damping of fluid turbulence. |
Sunday, November 18, 2018 3:35PM - 3:48PM |
D39.00006: Abstract Withdrawn
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Sunday, November 18, 2018 3:48PM - 4:01PM |
D39.00007: Behavior of heavy particles in shear turbulence under gravity Seulgi Lee, Changhoon Lee This study is aimed at investigation of particle-laden shear turbulence under gravity using direct numerical simulation. The Navier-Stokes equation was numerically solved on 1283 grids using a pseudo-spectral scheme on shear-periodic cubic domain to describe homogeneous shear turbulence at Reλ=67 and the shear parameter S* ~ 7. The equation of particle motion was solved by taking into account the linear Stokes drag and gravity together. The twenty-five cases were simulated for the Stokes number, St = 0.1, 0.5, 1, 5, 10 and the gravity factor, W= 0, 10, 20, 30, 40. Clustering of particles occurs when Stokes number is around 1 without gravity by shearing motion. We found, however, as gravity is applied to particles of St =1, the clustering effect seems to weaken slightly, and the particles tend to align with shear direction. When Stokes number is higher than 1 without gravity, the particles are almost randomly distributed. However, when heavy particles with St=5 settle under strong gravity (W=40), a string pattern of particle clustering is clearly observed. A different pattern of clustering due to gravity is clearly discernible from the cases without gravity. Detailed statistics of particles motion and a plausible physical mechanism will be discussed in the presentation. |
Sunday, November 18, 2018 4:01PM - 4:14PM |
D39.00008: The interaction between the finite-size particles and stably stratified turbulent channel flow. Juwon Jang, Changhoon Lee The interaction between finite-size particles and stably stratified turbulent channel flow was investigated using direct numerical simulation. The parameters of the fluid flow have been chosen as $Pr=7$, $Re_\tau=180$ and $Ri_\tau=0$ or $20$. The properties of the solid particles are such that the density ratio $1.00098$ or $1$, and they have an adiabatic surface. The diameter of the particle is $D_p^{+}=37.8$. The stratification is known to interfere with the transfer of heat and momentum in the vertical direction. The particles become trapped between the wall and the center of the channel because of the internal gravity wave (IGW) formed by the buoyancy. Because of this IGW, which acts as a wall, the particles have a locally low concentration in the channel center. This stratification effect is further enhanced by particles as the volume fraction of the particles increases, and the enhanced stratification again has a stronger influence on the distribution of the particles. Near the wall, the particles experience primarily high-speed streak regions. We present statistical descriptions for this interaction, namely the residence time, the PDF of the particle velocity and so on. |
Sunday, November 18, 2018 4:14PM - 4:27PM |
D39.00009: Surfacing of gyrotactic micro swimmers in free surface turbulence. Harshit Bhatia, Cristian Marchioli, Alfredo Soldati We investigate the surfacing of gyrotactic swimmers in free-surface turbulence. This setup mimics an environmentally-plausible situation (dynamic behavior involving different types of phytoplankton) in water bodies when surface waves and ripples are smooth or absent. We perform direct numerical simulation of open-channel turbulence at varying shear Reynolds number (from 170 to 1018 based on the channel height) and we track inertia less swimmers with stability number covering 2 orders of magnitude, which orient themselves according to Jeffery equation. We consider both spherical and elongated swimmers, the latter being modeled as prolate ellipsoids with aspect ratio up to 10. We show that vertical migration of the swimmers, which are initially distributed randomly throughout the flow, depends on both their re-orientation ability and shape. This effect reflects in the velocity, orientation and concentration statistics. Once at the surface, swimmers tend to stay there and organize themselves into filamentary clusters, which are in direct correlation with the topology of surface turbulence. This indicates that free-surface turbulence has a strong influence on the swimmers’ spatial distribution and, in turn, on the carbon-dioxide exchange cycle across the water/atmosphere interface. |
Sunday, November 18, 2018 4:27PM - 4:40PM |
D39.00010: Electric field driven phase transition in a confined suspension of negatively polarized particles Boris Khusid, Qian Lei, Ezinwa Elele We study the effect of a combination of a strong AC field and a weak DC field on the formation of cellular patterns in suspensions of neutrally-buoyant negatively polarized particles. An unexpected appearance of cellular patterns in such suspensions was observed by Kumar, Khusid, Acrivos, PRL95, 2005 and Agarwal, Yethiraj, PRL102, 2009. Following the application of a sufficiently strong AC field, particles aggregated head-to-tail into chains that bridged the gap between two electrodes and then formed a cellular pattern, in which large-scale particle-free domains were enclosed by particle-rich thin walls. These cellular structures were not observed in numerous experiments and MD simulations of field induced phase transitions in polarized suspensions. We will present data on the phase diagram and formation kinetics of cellular patterns by single-particle and multi-particle chains. Depending on applied fields, cellular patterns can be formed in two steps: 2D-> 3D and 3D-> 3D. We also will discuss the design of experiments to study phase transitions in suspensions of non-buoyancy-matched polarized particles in microgravity on the International Space Station. |
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