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 Q36: Particle-Laden Flows: Turbulence IIParticles Turbulence
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Chair: Qing Wang, Stanford University Room: 302 |
Tuesday, November 21, 2017 12:50PM - 1:03PM |
Q36.00001: Direct Numerical Simulations of Particle-Laden Turbulent Channel Flow Anand Samuel Jebakumar, Kannan Premnath, John Abraham In a recent experimental study, Lau and Nathan (2014) reported that the distribution of particles in a turbulent pipe flow is strongly influenced by the Stokes number (St). At St lower than 1, particles migrate toward the wall and at St greater than 10 they tend to migrate toward the axis. It was suggested that this preferential migration of particles is due to two forces, the Saffman lift force and the turbophoretic force. Saffman lift force represents a force acting on the particle as a result of a velocity gradient across the particle when it leads or lags the fluid flow. Turbophoretic force is induced by turbulence which tends to move the particle in the direction of decreasing turbulent kinetic energy. In this study, the Lattice Boltzmann Method (LBM) is employed to simulate a particle-laden turbulent channel flow through Direct Numerical Simulations (DNS). We find that the preferential migration is a function of particle size in addition to the St. We explain the effect of the particle size and St on the Saffman lift force and turbophoresis and present how this affects particle concentration at different conditions. [Preview Abstract] |
Tuesday, November 21, 2017 1:03PM - 1:16PM |
Q36.00002: Verification of Eulerian--Eulerian and Eulerian--Lagrangian simulations for fluid--particle flows Bo Kong, Ravi G. Patel, Jesse Capecelatro, Olivier Desjardins, Rodney O. Fox In this work, we study the performance of three simulation techniques for fluid-particle flows: (1) a volume--filtered Euler--Lagrange approach (EL), (2) a quadrature--based moment method using the anisotropic Gaussian closure (AG), and (3) a traditional two-fluid model. By simulating two problems: particles in frozen homogeneous isotropic turbulence (HIT), and cluster--induced turbulence (CIT), the convergence of the methods under grid refinement is found to depend on the simulation method and the specific problem, with CIT simulations facing fewer difficulties than HIT. Although EL converges under refinement for both HIT and CIT, its statistical results exhibit dependence on the techniques used to extract statistics for the particle phase. For HIT, converging both EE methods (TFM and AG) poses challenges, while for CIT, AG and EL produce similar results. Overall, all three methods face challenges when trying to extract converged, parameter-independent statistics due to the presence of shocks in the particle phase. [Preview Abstract] |
Tuesday, November 21, 2017 1:16PM - 1:29PM |
Q36.00003: Rayleigh-Benard turbulence modified by two-way coupled particles Hyungwon Park, Kevin O'Keefe, David Richter Direct numerical simulation (DNS) with Lagrangian point particles is used to study Rayleigh-Bénard convection to understand modifications due to the interaction of inertial particle, gauged by the turbulent kinetic energy (TKE) and Nusselt number (Nu). The initially dispersed particles experience gravitational settling, and become introduced at the lower wall such that turbulence must overcome the settling velocity for the particles to vertically distribute throughout the domain. The particle properties of interest are inertia, as characterized by the Stokes number, and settling velocity. Furthermore, individual contributions by the momentum-coupling and thermal-coupling are studied to see which most significantly changes Nu and TKE. Our results show that particles with Stokes number of order unity maximize Nu, corresponding to a peak of clustering and attenuation of TKE. It is also shown that particles two-way coupled only through momentum attenuate Nu and weaken TKE, while thermal-only coupling also weakens TKE but enhances Nu. When both couplings are present, however, thermal coupling overwhelms the attenuation caused by momentum coupling and the net result is an enhancement of Nu. [Preview Abstract] |
Tuesday, November 21, 2017 1:29PM - 1:42PM |
Q36.00004: The Rapid Distortion of Two-Way Coupled Particle-Laden Turbulence Mohamed Kasbaoui, Donald Koch, Olivier Desjardins The modulation of sheared turbulence by dispersed particles is addressed in the two-way coupling regime. The preferential sampling of the straining regions of the flow by inertial particles in turbulence leads to the formation of clusters. These fast sedimenting particle structures cause the anisotropic alteration of turbulence at small scales in the direction of gravity. These effects are investigated in a revisited Rapid Distortion Theory (RDT), extended for two-way coupled particle-laden flows. To make the analysis tractable, we assume that particles have small but non-zero inertia. In the classical results for single-phase flows, the RDT assumption of fast shearing compared to the turbulence time scales leads to the distortion of “frozen” turbulence. In particle-laden turbulence, the coupling between the two phases remains strong even under fast shearing and leads to a dynamic modulation of the turbulence spectrum. Turbulence statistics obtained from RDT are compared with Euler-Lagrange simulations of homogeneously sheared particle-laden turbulence. [Preview Abstract] |
Tuesday, November 21, 2017 1:42PM - 1:55PM |
Q36.00005: Assessment of sub-grid scale dispersion closure with regularized deconvolution method in a particle-laden turbulent jet Qing Wang, Xinyu Zhao, Matthias Ihme Particle-laden turbulent flows are important in numerous industrial applications, such as spray combustion engines, solar energy collectors etc. It is of interests to study this type of flows numerically, especially using large-eddy simulations (LES). However, capturing the turbulence-particle interaction in LES remains challenging due to the insufficient representation of the effect of sub-grid scale (SGS) dispersion. In the present work, a closure technique for the SGS dispersion using regularized deconvolution method (RDM) is assessed. RDM was proposed as the closure for the SGS dispersion in a counterflow spray that is studied numerically using finite difference method on a structured mesh. A presumed form of LES filter is used in the simulations. In the present study, this technique has been extended to finite volume method with an unstructured mesh, where no presumption on the filter form is required. The method is applied to a series of particle-laden turbulent jets. Parametric analyses of the model performance are conducted for flows with different Stokes numbers and Reynolds numbers. The results from LES will be compared against experiments and direct numerical simulations (DNS). [Preview Abstract] |
Tuesday, November 21, 2017 1:55PM - 2:08PM |
Q36.00006: Preliminary investigation of the effect of electric charge on particle-pair relative velocity in isotropic turbulence Adam Hammond, Zhongwang Dou, Tushar Kailu, Zach Liang, Hui Meng In many particle-laden turbulent flows including thunderstorm clouds and aerosol sprays, the particles may be electrically charged. How the Coulomb force between charged particles competes with the turbulence forces on particle motion is not yet fully understood. Mean inward particle pair relative velocity (particle RV), a quantity relevant for particle collision in isotropic turbulence, is expected to be affected by charge. We extend our recent particle tracking velocimetry (PTV) study on particle pair relative velocity in fan-driven isotropic turbulence to particles with charge. To accomplish this, we established a method to independently vary particle charge distributions by balancing particle density and size while keeping constant Re$_{\mathrm{\lambda }}$ and St, developed a unique instrument to measure particle charge using in-line holography, and measured particle RV using PTV at three levels of charge under a single flow condition. We present charged particle RV measurements from the experiments at Re$_{\mathrm{\lambda }}=$343, St$\approx $1.19, and charge of order 10$^{\mathrm{-15\thinspace }}$Coulombs, which show that particle RV increases with magnitude of bipolar charge. This study paves the way for a comprehensive exploration of relative motion of charged particle in isotropic turbulence. [Preview Abstract] |
Tuesday, November 21, 2017 2:08PM - 2:21PM |
Q36.00007: Temperature fluctuations in numerical simulations of particle-laden isotropic turbulence with two-way coupling Maurizio Carbone, Andrew Bragg, Michele Iovieno The two-way coupling between fluid and particle temperature fluctuations in forced steady isotropic turbulence is investigated by means of direct numerical simulations with sub-Kolmogorov inertial particle Lagrangian tracking. The aim is to determine the sensitivity of the temperature field statistics to the presence of particles, parametrized by the Stokes number ($St$) and the thermal Stokes number ($St_\theta$). As shown by B\'ec et al.\ (\textit{PRL}, 2014), the inertia of particles enhances mixing and heat transfer because particles cluster in regions of sharp temperature gradients out of the Lagrangian coherent structures. This trend strongly affects the small-scale dynamics of the temperature field and the dissipation rate of the temperature variance. Moreover, in presence of a two-way coupling, the finite mass and heat capacity of particles allows them to carry temperature increments across the scales of the flow influencing the temperature statistics at all scales. The resulting non trivial behavior is highlighted by means of two-particle statistics and two-point statistics of the temperature field. [Preview Abstract] |
Tuesday, November 21, 2017 2:21PM - 2:34PM |
Q36.00008: Experimental study of dense suspension of large particles in a turbulent boundary layer Lucia Baker, Filippo Coletti Turbulent flows laden with high concentrations of particles are encountered in many natural and industrial settings, from river sediment to fluidized beds. In many of these situations the size of the particles is comparable to the energetic flow scales, violating basic assumptions of standard numerical models and calling for careful experimental observations. We experimentally investigate an open channel flow with a bulk Reynolds number of 70,000 laden with spherical particles slightly denser than the fluid. The particle diameter is around one tenth of the boundary layer thickness, and particle volume fractions between five and twenty percent are examined. Refractive index matching allows us to simultaneously characterize both fluid and particle motion by laser imaging. Because the particles are slightly heavier than the fluid, a vertical gradient of volume fraction exists, and the particles form stable layers above the floor. Even at the lowest volume fractions, the particles drastically alter the boundary layer structure: the fluid velocity profile is almost linear, suggesting that the effective fluid viscosity is greatly augmented by the particle presence. Moreover, increasing the volume fraction enhances the streamwise and wall-normal velocity fluctuations of both phases. [Preview Abstract] |
Tuesday, November 21, 2017 2:34PM - 2:47PM |
Q36.00009: Effect of gravitational settling of small heavy particles on two-way interactions in near-wall turbulence Junghoon Lee, Changhoon Lee We investigate particle-fluid interactions during settling of small heavy particles in shear, using direct numerical simulation of turbulent channel flow with a point-force approach. The particle parameters considered are chosen to be identical to those in the available experiment on micrometer-sized water droplets in a horizontal turbulent boundary layer in air (Gerashchenko et al., JFM 2008). In Lagrangian frame, we track each individual particle that is introduced at the edge of the viscous sublayer in the upper part of the channel until its arrival at the channel bottom. Once a particle reaches the bottom wall, a new particle is considered to keep a constant particle volume fraction. Our results indicate that the settling particles enhance the fluid velocity fluctuations associated with downward motions of the fluid. As a result, more vortices are generated farther away from the upper wall, while in the lower side of the channel vortex suppression is observed. Furthermore, we show the development of large-scale streamwise circulations which span almost the channel half width in the upper part of the channel. We also discuss the difference in the two-way interactions between inhomogeneous shear and homogeneous isotropic turbulence. [Preview Abstract] |
Tuesday, November 21, 2017 2:47PM - 3:00PM |
Q36.00010: Measurements of orientation, sedimentation, and dispersal of ramified particles in isotropic turbulence Greg A. Voth, Stefan Kramel, Udayshankar K Menon, Donald L. Koch We experimentally measure the sedimentation of non-spherical particles in isotropic turbulence. We obtain time-resolved 3D orientations of the particles along with the fluid velocity field around them in a vertical water tunnel. An active jet array with 40 individually controllable jets enables us to adjust the turbulence intensity and observe the transition from strongly aligned to randomized particle orientations. We focus on the orientation statistics of ramified particles formed from several slender arms, including fibers and particles with three arms in planar symmetry (triads), which allows us to study alignment of both fibers and disk-like particles. We can predict the turbulent intensity at which the transition from aligned to randomized particle orientations occurs using a non-dimensional settling factor given by the ratio of rotation timescale of the turbulence at the scale of the particle to the rotation timescale of a particles in quiescent flow due to inertial torques. A model of ramified particle motion based on slender body theory provides accurate predictions of the vertical and horizontal particle velocities relative to the turbulent fluid. [Preview Abstract] |
Tuesday, November 21, 2017 3:00PM - 3:13PM |
Q36.00011: Theoretical predictions of the orientation distribution of high-aspect-ratio, inertial particles settling in isotropic turbulence Udayshankar Menon, Anubhab Roy, Stefan Kramel, Greg Voth, Donald Koch When anisotropic particles settle in isotropic turbulence, the inertial torque due to their settling favors broadside alignment while turbulence favors orientational dispersion. This process leads, for example, to the anisotropic scattering of electromagnetic radiation in icy clouds due to the orientation distribution of ice crystals which can have needle-like or disk-like shapes. We study two types of particles amenable to the use of slender-body theory (Batchelor 1970, Khayat and Cox 1989): fibers and planar triads consisting of three connected rods. For particles smaller than the Kolmogorov scale, the effect of turbulence can be described in terms of a temporally fluctuating local linear flow field following the motion of the particle. When the settling velocity is small compared with the Kolmogorov velocity, the particle samples the fluid velocity gradients along a Lagrangian path and our simulations employ the stochastic velocity gradient model of Girimaji and Pope (1990). When the settling velocity is large compared with the Kolmogorov velocity, the large inertial torque causes the particle to achieve a quasi-steady orientation with respect to the local velocity gradient allowing analytical predictions of the small orientational dispersion away from the preferred horizontal alignment. Supported by Army Research Office grant W911NF1510205 [Preview Abstract] |
Tuesday, November 21, 2017 3:13PM - 3:26PM |
Q36.00012: Particle migration and stresslet from fully resolved simulations of particles in turbulent pipe flows Federico Toschi, Abhineet Gupta, Herman J.H. Clercx Particle-laden turbulent flows occur in a variety of natural and industrial flows. The numerical simulation of such flows still remains challenging and relatively fewer studies were conducted to investigate the coupling between fully resolved particles and turbulent flows. Here we will present fully-resolved numerical simulations (based on the Lattice Boltzmann Method) to investigate turbulent pipe flows laden with large neutrally-buoyant particles at low Reynolds number and under dilute conditions. In our study the energy input was kept fixed resulting in Reynolds numbers, based on friction velocity, around 250. Two different particle radii were used with particle to pipe diameters ratios of 0.05 and 0.075, respectively. Both Eulerian and Lagrangian statistical properties were quantified along with the stresslet exerted by the fluid on the spherical particles. The high particle-to-fluid slip velocity close to the wall corresponds, locally, to events of high energy dissipation which are absent in the single-phase turbulent flow. The migration of particles from inner to outer region of the pipe, the dependence of the stresslet on the particle radial positions and the fragmentation rate of particles, estimated using the stresslet, have also been investigated. [Preview Abstract] |
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