Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session L20: Turbulence: Particle-Laden Flows |
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Chair: Greg Voth, Wesleyan University Room: 208 |
Monday, November 23, 2015 4:05PM - 4:18PM |
L20.00001: Lagrangian Stretching and the Dynamics of Anisotropic Particles in Turbulence Greg Voth, Conor Hunt, Lydia Tierney, Stefan Kramel In the past few years, an extensive phenomenology has been developed that describes the orientation and rotation of small axisymmetric particles in turbulent flows. Many of the observed phenomena can be understood using a simple picture that considers the stretching a particle has experienced during the past few Kolmogorov times. The stretching can be quantified using the Cauchy-Green strain tensors. The eigenvectors of these tensors provide a convenient coordinate system in which to describe particle orientation and rotations. Consideration of stretching also offers new insights into the Lagrangian evolution of vorticity and strain in turbulence. Particles tend to align with a long axis along the maximum extensional eigenvector of the left Cauchy-Green tensor. The vorticity also aligns in this same direction leading to alignment between the particle rotation rate vector and a long axis. [Preview Abstract] |
Monday, November 23, 2015 4:18PM - 4:31PM |
L20.00002: Alignment of Disks with Lagrangian Stretching in Turbulence Conor Hunt, Lydia Tierney, Stefan Kramel, Greg Voth We study Lagrangian stretching in isotropic turbulence in order to understand both the rotations of disks and the preferential alignment of vorticity with the intermediate strain rate eigenvector. Using velocity gradient tensors from a numerical simulation of homogeneous isotropic turbulence at R$_{\lambda }=$ 180, we calculate the Cauchy-Green strain tensors whose eigenvectors provide a natural basis for studying stretching phenomenon. Previous work has shown that rods preferentially align with the vorticity as a result of both quantities independently aligning with the extensional Cauchy-Green eigenvector. In contrast, disks orient with their symmetry axis perpendicular to vorticity and preferentially align with the compressional Cauchy-Green eigenvector. We also find that the intermediate strain rate eigenvector is aligned with the extensional Cauchy-Green eigenvector. A natural consequence is that the intermediate strain rate eigenvector is aligned with the vorticity vector since conservation of angular momentum aligns vorticity with the direction it has been stretched. [Preview Abstract] |
Monday, November 23, 2015 4:31PM - 4:44PM |
L20.00003: Distribution and velocity of inertial particles in a turbulent channel flow Filippo Coletti, Kee Onn Fong, Andras Nemes, Nicholas Sloan The segregation of inertial particles in specific regions of turbulent fluid flows is a well known phenomenon, but experimental observations of its three-dimensional nature are lacking. Here we are concerned with the transport of small inertial particles in wall-bounded turbulence. In particular we consider a fully developed vertical channel flow. The working fluid is air laden with size-selected glass particles. The volume and mass loading are kept low and the particle diameter is smaller than the viscous length scale, so that the turbulence is unaffected by the dispersed phase. Tomographic particle image velocimetry is used to reconstruct the position and velocity of the inertial particles. In particular, the tendency of the particles to concentrate intermittently (turbulence clustering) and to drift towards the wall (turbophoresis) are quantitatively characterized by the instantaneous and mean concentration fields, respectively. The findings are discussed in relation to the results of previous studies which have used one-way coupled direct numerical simulations, and on which the current understanding of this class of flows is based. [Preview Abstract] |
Monday, November 23, 2015 4:44PM - 4:57PM |
L20.00004: Relative diffusion of a pair of inertial particles in the inertial sub-range of turbulence Kei Enohata, Koji Morishita, Takashi Ishihara Turbulent diffusion of a pair of inertial particles in 3-dimensional homogeneous and isotropic turbulence was studied using direct numerical simulation (DNS) with $2048^3$ grid points; the Taylor micro-scale Reynolds number in the DNS is approximately 425. For each set of the inertial particles with different values of the Stokes number ($St=0, 0.1, 0.2, 0.5, 1, 2, 5, 10$), $256^3$ particles are tracked using cubic spline interpolation for the velocity data in the DNS. Here $St=0$ corresponds to fluid particles. The DNS showed that for each value of $St$, the mean square of the distance $\delta x$ between the two inertial particles grows with time $t$ as $\langle \delta x^2 \rangle \sim C \epsilon t^3$ in the inertial subrange, which is in agreement with Richardson (1926) and Obukhov (1941). Here $\epsilon$ is the mean energy dissipation rate per unit mass, and $C$ is a constant of $O$(1) depending on the value of $St$ and the initial distance between the inertial particles. The DNS shows also that large clusters of strong vortices enhance relative diffusion of inertial particles of $St>1$. [Preview Abstract] |
Monday, November 23, 2015 4:57PM - 5:10PM |
L20.00005: Simulations of decaying turbulence laden with particles: how are statistics affected by two-way coupling numerical scheme? Jeremy Horwitz, Ali Mani This work builds on our recent analyses of two-way coupled particle-fluid systems under laminar conditions, in which we demonstrated that standard interpolation schemes in conjunction with the point-particle approximation can lead to significant underprediction of momentum exchange between the two phases. A simple interpolation correction scheme has been proposed which accounts for the difference between the disturbed and undisturbed fluid velocity at the location of each particle. The objective of this work is to demonstrate how different interpolation and projection schemes affect flow and/or particle statistics in a mean-stationary turbulent environment. We use direct numerical simulations of the Navier-Stokes equations to study decaying homogeneous isotropic turbulence laden with particles with different particle Stokes numbers, mass-loadings, and particle diameters to grid size. [Preview Abstract] |
Monday, November 23, 2015 5:10PM - 5:23PM |
L20.00006: Inertial Range Scaling of Rotation Rates of Particles in Turbulence Brendan Cole, Stefan Kramel, Greg Voth We measure mean-squared rotation rates of 3D-printed particles with sizes spanning the inertial range in a turbulent flow between oscillating grids. Tetrads, composed of four slender rods in tetrahedral symmetry, and triads, three slender rods in triangular planar symmetry, are tracked in a flow with $Re_\lambda = 156$ and $Re_\lambda = 214$ using four high-speed cameras. Tetrads rotate like spheres and triads rotate like disks. Measurements of tetrads' rotation rates as a function of particle size are direct measurements of the coarse-grained vorticity and provide a new way to measure inertial range scaling in turbulent flows. Similar measurements of rods, performed by Parsa and Voth, were consistent with the $<\omega^2> \ \sim r^{-\frac{4}{3}}$ scaling prediction, but the preferential alignment of rods affects their rotation rate and this preferential alignment could not be directly measured. Our triads allow measurement of the full solid-body rotation rate as well as the particle orientation and so we can quantify the preferential alignment for disks. [Preview Abstract] |
Monday, November 23, 2015 5:23PM - 5:36PM |
L20.00007: Settling of inertial particles through quiescent, weakly turbulent and strongly turbulent air Alec Petersen, Douglas Carter, Luci Baker, Filippo Coletti The fall speed of inertial particles suspended in a turbulent flow is an important parameter for numerous industrial applications and natural phenomena. Although this behavior has been subject to various investigations, precisely how the presence and strength of turbulent fluctuations affect the settling process is difficult to assess. Previous studies have provided several mechanisms which may either hinder or enhance the settling rate, but comparisons between them often show qualitative and quantitative discrepancies. We experimentally study the case of a suspension of size-selected microscopic particles falling through air. The mass fraction of the particles is low enough to neglect their influence on the fluid. Using randomly actuated jets of adjustable intensity, we gradually increase the mean velocity fluctuations of the air without introducing a significant mean flow. Because the integral length scale is kept approximately constant, the Kolmogorov scales become progressively smaller as the Reynolds number increases. We measure the average and fluctuating settling velocities for the different cases, and discuss the compatibility of our observations with the various proposed mechanisms of settling alteration by turbulence. [Preview Abstract] |
Monday, November 23, 2015 5:36PM - 5:49PM |
L20.00008: Wake-driven dynamics of finite-sized buoyant spheres in turbulence varghese mathai, vivekn prakash, jon Brons, Chao Sun, Detlef Lohse Particles suspended in turbulent flows are affected by the turbulence, and at the same time act back on the flow. The resulting coupling can give rise to rich variability in their dynamics. Here we report experimental results from an investigation on finite-sized buoyant spheres in turbulence. We find that even a marginal reduction in the particleâ€™s density from that of the fluid can result in strong modification of the particle dynamics. In contrast to classical spatial filtering arguments, we find that the particle acceleration variance increases with size. We trace this reversed trend back to the growing contribution from wake-induced forces. [Preview Abstract] |
Monday, November 23, 2015 5:49PM - 6:02PM |
L20.00009: Effects of ambient turbulence on a particle plume Adrian C.H. Lai, J.W. Er, Adrian W.K. Law, E. Eric Adams We investigated experimentally the effects of ambient turbulence on a particle plume. Homogeneous and isotropic turbulent ambient water was generated by a random jet array in a glass tank. Glass beads of different particle diameters were released continuously into this turbulent ambient using a submerged hourglass, forming particle plumes with a constant efflux velocity; different initial velocities were tested for each particle size. We focused on the region in which the integral length scale of the ambient eddies is larger than that of the particle plume size. Following the arguments of Hunt (1994) and the observation of Hubner (2004) on a single-phase plume, it is expected that in this region, the internal structure or Lagrangian spreading of the particle plume, will not be significantly affected, but the plume centerline would meander due to the ambient turbulence leading to an increase in the Eulerian width. In the presentation, first, we will present our preliminary experimental data which showed that this is also true for two-phase particle plumes. Second, based on this observation, we developed a theoretical framework using a stochastic approach to predict the spreading of the plume. Predictions of the model will be compared with our experimental data. [Preview Abstract] |
Monday, November 23, 2015 6:02PM - 6:15PM |
L20.00010: Dynamics of Small Inertia-Free Spheroidal Particles in a Turbulent Channel Flow Niranjan Reddy Challabotla, Lihao Zhao, Helge I. Andersson The study of small non-spherical particles suspended in turbulent fluid flows is of interest in view of the potential applications in industry and the environment. In the present work, we investigated the dynamics of inertia-free spheroidal particles suspended in fully-developed turbulent channel flow at Re$\tau \quad =$ 180 by using the direct numerical simulations (DNS) for the Eulerian fluid phase coupled with the Lagrangian point-particle tracking. We considered inertia-free spheroidal particles with a wide range of aspect ratios from 0.01 to 50, i.e. from flat disks to long rods. Although the spheroids passively translate along with the fluid, the particle orientation and rotation strongly depend on the particle shape. The flattest disks were preferentially aligned with their symmetry axis normal to the wall, whereas the longest rods aligned parallel to the wall. Strong mean rotational spin was observed for spherical particles and this has been damped with increasing asphericity both for rod-like and disk-like spheroids. The anisotropic mean and fluctuating fluid vorticity resulted in particle spin anisotropies which exhibited a complex dependence on the particle asphericty. [Preview Abstract] |
Monday, November 23, 2015 6:15PM - 6:28PM |
L20.00011: Single particle measurements of material line stretching in turbulence: Experiments Stefan Kramel, Saskia Tympel, Federico Toschi, Greg Voth We find that particles in the shape of chiral dipoles display a preferential rotation direction in three dimensional isotropic turbulence. The particles consist of two helical ends with opposite chirality that are connected by a straight rod. They are fabricated using 3D printing and have an aspect ratio of 10 and a length in the inertial range of our flow between oscillating grids. Due to their high aspect ratio, they move like material lines. Because material lines align with the extentional eigenvectors of the velocity gradient tensor they experience a mean stretching in turbulence. The stretching of a chiral dipole produces a rotation about the dipole axis and so chiral dipoles experience a non-zero mean spinning rate in turbulence. These results provide a first direct experimental measurement of the rate of material line stretching in turbulence. [Preview Abstract] |
Monday, November 23, 2015 6:28PM - 6:41PM |
L20.00012: Single particle measurements of material line stretching in turbulence: Numerics Saskia Tympel, Stefan Kramel, Federico Toschi, Greg Voth In three dimensional isotropic turbulence, particles in the shape of chiral dipoles display a preferential rotation direction. Chiral dipoles have two helical ends with opposite chirality that are connected by a straight rod. We assume particles to be small and neutrally buoyant, so that their centre of mass follows the same trajectories of tracer particles in homogeneous isotropic turbulence. First, we tune the model for dipoles spinning dynamics via Stokesian Dynamics simulations. Then, we compute the spinning dynamics around their preferential rotation axis in homogenous and isotropic turbulence using turbulent fields from high resolution DNS. Our numerics shows a very good agreement with experimental results and allow a deeper insight into the mechanisms of material line stretching in turbulence. [Preview Abstract] |
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