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 M4: Particle-Laden Flows: Clustering and Dispersion II |
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Chair: Rodney O. Fox, Iowa State University Room: 103 |
Tuesday, November 24, 2015 8:00AM - 8:13AM |
M4.00001: Strongly coupled turbulent gas-particle flows in vertical channels Rodney O. Fox, Jesse Capecelatro, Olivier Desjardins Eulerian-Lagrangian (EL) simulations of strongly coupled (high mass loading) gas-particle flows in vertical channels are performed with the purpose of exploring the fundamental physics of fully developed, wall-bounded multiphase turbulence. An adaptive spatial filter is developed that accurately decomposes the total granular energy of the particles into correlated and uncorrelated components at each location in the wall-normal direction of the flow. In this manner, Reynolds- and phase-averaged (PA) two-phase turbulence statistics up to second order are reported for both phases and for three values of the PA mean fluid velocity. As expected due to the high mass loading, in all cases the turbulence production due to mean drag dominates production due to mean shear. A multiphase LRR-IP Reynolds-stress turbulence model is developed to predict the turbulent flow statistics as a function of the wall-normal distance. Using a correlation for the vertical drift velocity developed from the EL data, the turbulence model predictions agree satisfactorily with all of one-point EL statistics for the vertical channel flows, as well as for the homogeneous cluster-induced turbulence (CIT) statistics reported previously. [Preview Abstract] |
Tuesday, November 24, 2015 8:13AM - 8:26AM |
M4.00002: Analysis and Comparison with DNS of a Stochastic Model for the Relative Motion of High-Stokes-Number Particles in Isotropic Turbulence Rohit Dhariwal, Sarma Rani, Donald Koch In an earlier work, Rani, Dhariwal, and Koch (JFM, Vol.~756, 2014) developed an analytical closure for the diffusion current in the PDF transport equation describing the relative motion of high-Stokes-number particle pairs in isotropic turbulence. In this study, an improved closure was developed for the diffusion coefficient, such that the motion of the particle-pair center of mass is taken into account. Using the earlier and the new analytical closures, Langevin simulations of pair relative motion were performed for four particle Stokes numbers, $St_\eta = 10,~20,~40,~80$ and at two Taylor micro-scale Reynolds numbers $Re_\lambda = 76,~131$. Detailed comparisons of the analytical model predictions with those of DNS were undertaken. It is seen that the pair relative motion statistics obtained from the improved theory show excellent agreement with the DNS statistics. The radial distribution functions (RDFs), and relative velocity PDFs obtained from the improved-closure-based Langevin simulations are found to be in very good agreement with those from DNS. It was found that the RDFs and relative velocity RMS increased with $Re_\lambda$ for all $St_\eta$. The collision kernel also increased strongly with $Re_\lambda $, since it depended on the RDF and the radial relative velocities. [Preview Abstract] |
Tuesday, November 24, 2015 8:26AM - 8:39AM |
M4.00003: Experimental studies of gas-particle mixtures under sudden expansion Heather Zunino, Ronald Adrian, Amanda Clarke High-speed video cameras and pressure sensors were used to capture the movement of a particle bed due to a passing expansion fan created by a diaphragm burst in a shock tube. The particle bed is placed on the high-pressure side (\textit{p4}) of the shock tube. Once the diaphragm bursts, it expands upward into the low-pressure region (\textit{p1}). Several interesting structures are captured and examined, including instabilities located at the top surface of the particle bed and particle vacant regions within the bed. These features are discussed along with their relevance to the spikes of material seen radially ejected outward during a cylindrical explosion. The characteristics of this flow are compared for several different pressure regimes. Two-dimensional and three-dimensional Fourier analyses are used to further explore and measure the frequency of the features imaged. [Preview Abstract] |
Tuesday, November 24, 2015 8:39AM - 8:52AM |
M4.00004: Study of snow-atmosphere interactions over an Antarctic surface using large eddy simulations coupled with a Lagrangian stochastic model Francesco Comola, Marco Giometto, Ernesto Trujillo, Katherine Leonard, Ted Maksym, Marc Parlange, Michael Lehning The need for a better understanding of fluid and morphodynamic processes over Antarctic sea ice motivates the development of detailed models of small-scale snow-atmosphere interactions. At large scales, these interactions drive spatial patterns of snow distribution and snow transport from the marginal ice to the sea. However, challenges arise when representing the detailed sequence of processes involved, such as aerodynamic entrainment, particle dynamics, feedback on fluid momentum and particle impacts. We use a Lagrangian stochastic model coupled to large eddy simulations to represent particle trajectories in turbulent flows. An immersed boundary method is used to represent the underlying surface and a dynamic surface roughness model is used to account for the drag induced by the subgrid-scale roughness. The model is set up for an Antarctic sea ice floe over which pre- and post-storm snow distributions were measured using a terrestrial laser scanner. The dataset, collected as part of the Sea Ice Physics and Ecosystem Experiment 2, indicates marked changes in the snow distribution as a result of snow drift, providing valuable testing grounds for the model. Model results are in agreement with blowing snow concentrations at different heights and with the observed patterns of erosion and deposition. [Preview Abstract] |
Tuesday, November 24, 2015 8:52AM - 9:05AM |
M4.00005: Preferential accumulation and enhanced relative velocity of inertial droplets due to interactions with homogeneous isotropic turbulence Colin Bateson, Alberto Aliseda We present results from wind tunnel experiments on the evolution of small inertial ($d\approx 10-200~\mu m$) water droplets in homogeneous, isotropic, slowly decaying grid turbulence. High-speed imaging and a Particle Tracking algorithm are used to calculate relative velocity distributions. We analyze the preferential concentration, via the 2D Radial Distribution Function, and enhanced relative velocity of droplets resulting from their inertial interactions with the underlying turbulence. The two-dimensional particle velocities, measured from multi-image tracks along a streamwise plane, are conditionally analyzed with respect to the distance from the nearest particle. We focus on the non-normality of the statistics for the particle-particle separation velocity component to examine the influence of the inertial interaction with the turbulence on the dynamics of the droplets. We observe a negative bias (in the mean and mode) in the separation velocity of particles for short separations, signaling a tendency of particles to collide more frequently than a random agitation by turbulence would predict. The tails of the distribution are interpreted in terms of the collision/coalescence process and the probability of collisions that do not lead to coalescence. [Preview Abstract] |
Tuesday, November 24, 2015 9:05AM - 9:18AM |
M4.00006: Bringing Clouds into Our Lab! - The Influence of Turbulence on the Early Stage Rain Droplets Mehmet Altug Yavuz, Rudie Kunnen, GertJan Heijst, Herman Clercx We are investigating a droplet-laden flow in an air-filled turbulence chamber, forced by speaker-driven air jets. The speakers are running in a random manner; yet they allow us to control and define the statistics of the turbulence. We study the motion of droplets with tunable size (Stokes numbers $\sim$ 0.13 - 9) in a turbulent flow, mimicking the early stages of raindrop formation. 3D Particle Tracking Velocimetry (PTV) together with Laser Induced Fluorescence (LIF) methods are chosen as the experimental method to track the droplets and collect data for statistical analysis. Thereby it is possible to study the spatial distribution of the droplets in turbulence using the so-called Radial Distribution Function (RDF), a statistical measure to quantify the clustering of particles. Additionally, 3D-PTV technique allows us to measure velocity statistics of the droplets and the influence of the turbulence on droplet trajectories, both individually and collectively. In this contribution, we will present the clustering probability quantified by the RDF for different Stokes numbers. We will explain the physics underlying the influence of turbulence on droplet cluster behavior. [Preview Abstract] |
Tuesday, November 24, 2015 9:18AM - 9:31AM |
M4.00007: Quantification of statistical phenomena in turbulent dispersions Matthew Yates, David Hann, Buddhika Hewakandamby Understanding of turbulent dispersions is of great importance for environmental and industrial applications. This includes developing a greater understanding of particle movement in atmospheric flows, and providing data that can be used to validate CFD models aimed at producing more accurate simulations of dispersed turbulent flows, aiding design of many industrial components. Statistical phenomena in turbulent dispersions were investigated using Particle Image Velocimetry. Experiments were carried out in a two dimensional channel over a Reynolds number range of 10000-30000, using water and 500 micron hydrogel particles. Particles were injected at the channel entrance, and dispersion properties were characterised at different distances downstream from the injection point. Probability density functions were compiled for the velocity components of the hydrogels for differing flow conditions. Higher order PDFs were constructed to investigate the behaviour of particle pairs. Dispersed phase data was also used to investigate the mechanics of collisions between hydrogel particles, allowing for calculation of the co-efficient of restitution. PIV algorithms were used to create velocity maps for the continuous phase for varying dispersed phase fractions. [Preview Abstract] |
Tuesday, November 24, 2015 9:31AM - 9:44AM |
M4.00008: Turbulent Soret Effect Dhrubaditya Mitra, Nils Erland L. Haugen, Igor Rogachevskii We study, turbophoresis--the clustering properties of heavy inertial passive particles in a inhomogeneous turbulent flow--by direct numerical simulation of inhomogeneously forced turbulence in a periodic box without walls. The forcing is a periodic function of one coordinate direction. The inertial particles cluster near the minima of the turbulent kinetic energy. Draw- ing analogy with Soret effect in near-equilibrium thermodynamics, we can describe the flux of particles as a sum of two fluxes, described by two turbulent transport coefficients, turbulent diffusion of particles and turbophoretic coefficient. The second (turbophoretic) flux is assumed to be proportional to the gradient of turbulent intensity. The ratio of these two coefficients would be analogous to Soret coefficient, hence we call this the turbulent Soret coefficient. Our numerical calculation show that such a description is a good description of our data. Furthermore, we find that the turbulent Soret coefficient is a non-monotonic function of the particle inertia (described the the Stokes number); i.e. beyond a critical Stokes number the clustering of the particles decreases, but in a smooth manner. [Preview Abstract] |
Tuesday, November 24, 2015 9:44AM - 9:57AM |
M4.00009: Caustics and collisions of inertial particles Rama Govindarajan, S. Ravichandran, Samriddhi Ray, P. Deepu Caustics are formed when inertial particles of very different velocities collide in a flow, and are a consequence of the dissipative nature of particle motion in a suspension. Using a simple model for vortex-dominated flow with heavy particles, we suggested that sling caustics form only within a neighbourhood around a vortex, the square of whose radius is proportional to the product of circulation and particle inertia. Particles starting close to this critical radius congregate close together, resulting in large spikes in (Lagrangian) number density. We test these predictions by counting the number of collisions of particles in a randomly forced flow and correlating the collision locations with vorticity. We also study the effect of caustics on droplet growth in a super-saturated environment. We hope that these studies will be of interest in long-standing problems of physical interest such as the mechanism of broadening of droplet spectra in a turbulent flow. [Preview Abstract] |
Tuesday, November 24, 2015 9:57AM - 10:10AM |
M4.00010: Condensing aerosol Dynamics in homogeneous isotropic turbulence Amjad Alshaarawi, Antonio Attili, Fabrizio Bisetti The interaction of a condensing aerosol with homogeneous isotropic turbulence is simulated at Re$_{\lambda} \approx 95$. The simulation consists of a three-dimensional direct numerical simulation of homogeneous isotropic turbulence with a statistically stationary forced velocity field. Patches of dry and cold gas mix with patches of hot carrier gas saturated with vapor of a condensable species, inducing the homogeneous nucleation of particles due to supersaturation. An approach based on the quadrature method of moments and a Lagrangian numerical scheme is adopted for the transport and dynamics of the liquid droplets [Attili \& Bisetti, Comp. Fluids 84, 2013; Zhou {\it et al.}, Phys. Fluids 26, 2014]. Two regimes related to the eddy turnover timescale are observed, i.e., a nucleation regime and a consumption regime [Alshaarawi \& Bisetti, J. Aerosol Sci. 81, 2015]. In the nucleation regime, at short eddy turnover timescales, mixing is fast enough to suppress nucleation by mixing the fluid to the mean state at which nucleation vanishes. In the consumption regime, at long eddy turnover timescales, mixing is slow and nucleation continues until it is suppressed by the consumption of the vapor phase due to the growth of the droplets. [Preview Abstract] |
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