Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session CV: Particle Laden Flows II |
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Chair: Tim O'Hern, Sandia National Laboratories Room: Hyatt Regency Long Beach Regency B |
Sunday, November 21, 2010 1:00PM - 1:13PM |
CV.00001: Particle pair dispersion in turbulent boundary layer Cristian Marchioli, Enrico Pitton, Alfredo Soldati, Federico Toschi The rate at which two particles separate in turbulent flow is of central importance to predict the spatial distribution of inhomogeneities and to characterize mixing. Pair separation is analyzed for the specific case of small inertial particles in dispersed turbulent channel flow to determine the role of mean shear and small-scale structures. To this aim an Eulerian-Lagrangian approach based on pseudo-spectral direct numerical simulation of fully-developed gas-solid flow at friction Reynolds number $Re_{\tau}=150$ is used. Pair separation statistics were computed for particles with different inertia released from different regions of the channel. Results demonstrate (i) that shear-induced effects predominate in the near-wall region, where velocity gradients reach a maximum, whereas small-scale fluctuations predominate away from the wall, where turbulence becomes more homogeneous and isotropic; and (ii) that the modalities by which particles become affected depend strongly on inertia. Starting from these results, open modelling issues will be addressed. [Preview Abstract] |
Sunday, November 21, 2010 1:13PM - 1:26PM |
CV.00002: Self-ordering of microscopic particles in time-periodic flows Dmitri Pushkin, Denis Melnikov, Valentina Shevtsova Small macroscopic particles advected by fluid flows are generally believed to follow the surrounding fluid when their Stokes number, $St \ll 1$. We show that even when particles are as small as $St \sim 10^{-6}$, the inertia effects may lead to spontaneous self-organization of particles into dynamic coherent structures. The arising structures are typically spiral curves. They are a collective phenomenon observed when an ensemble of particles is evolved. While the structures are robust for a range of control parameters, they become sensitive quickly dissolve and particles mix with the fluid outside this range. We explain the structures' formation by the dynamical effect of phase-locking. It occurs for particle turn-over motions in vortical time-periodic flows. We show that this mechanism is responsible for the surprising assembly of particles into rotating spirals that was discovered experimentally in thermocapillary flows more than a decade ago and has remained unexplained until now. In our exposition we lean on the results of our numerical simulations, which reproduce the effect in physically realistic regimes. We expect that similarly to phase-locking in dynamical systems, this effect is subtle but generic and may cause localization and ordering of particles in time-periodic flows that abound in nature and applications. [Preview Abstract] |
Sunday, November 21, 2010 1:26PM - 1:39PM |
CV.00003: Effects of turbulence intensity and gravity on transport of inertial particles across a shearless turbulence interface Garrett Good, Sergiy Gerashchenko, Zellman Warhaft Water droplets of sub-Kolmogorov size are sprayed into the turbulence side of a shearless turbulent-non-turbulent interface (TNI) as well as a turbulent-turbulent interface (TTI). An active grid is used to form the mixing layer and a splitter plate separates the droplet-non droplet interface near the origin. Particle concentration, size and velocity are determined by Phase Doppler Particle Analyzer, the velocity field by hot wires, and the droplet accelerations by particle tracking. As for a passive scalar, for the TTI, the concentration profiles are described by an error function. For the TNI, the concentration profiles fall off more rapidly than for the TTI due to the large-scale intermittency. The profile evolution and effects of initial conditions are discussed, as are the relative importance of the large and small scales in the transport process. It is shown that the concentration statistics are better described in terms of the Stokes number based on the large scales than the small, but some features of the mixing are determined by the small scales, and these will be discussed. Sponsored by the U.S. NSF. [Preview Abstract] |
Sunday, November 21, 2010 1:39PM - 1:52PM |
CV.00004: Conditional statistics of inertial particle entrainment across a shearless turbulent interface Sergiy Gerashchenko, Garrett Good, Zellman Warhaft For the turbulent-non-turbulent shearless mixing layer described by Good et al. (``Effects of turbulence intensity and gravity on transport of inertial particles across a shearless turbulence interface.'' G. Good, S. Gerashchenko, Z. Warhaft, APS, DFD 2010), the large scales on the turbulent side showed a strong influence on the particle dynamics in the intermittent region of the mixing layer. Particle statistics conditioned on the turbulent bursts were measured and compared with the unconditioned statistics. The un-conditional statistics adequately describe the particle transport across the mixing layer. The conditional statistics show changes in particle concentration within the bursts as a function of penetration into the mixing layer and this is discussed in terms of the particle history. The effect of gravity on the conditioned particle dynamics, in particular on the time of arrival statistics and radial distribution function, is addressed. Sponsored by the U.S. NSF. [Preview Abstract] |
Sunday, November 21, 2010 1:52PM - 2:05PM |
CV.00005: Inertial particles in a shearless mixing layer: direct numerical simulations Peter Ireland, Lance Collins Entrainment, the drawing in of external fluid by a turbulent flow, is present in nearly all turbulent processes, from exhaust plumes to oceanic thermoclines to cumulus clouds. While the entrainment of fluid and of passive scalars in turbulent flows has been studied extensively, comparatively little research has been undertaken on inertial particle entrainment. We explore entrainment of inertial particles in a shearless mixing layer across a turbulent-non-turbulent interface (TNI) and a turbulent-turbulent interface (TTI) through direct numerical simulation (DNS). Particles are initially placed on one side of the interface and are advanced in time in decaying turbulence. Our results show that the TTI is more efficient in mixing droplets than the TNI. We also find that without the influence of gravity, over the range of Stokes numbers present in cumulus clouds, particle concentration statistics are essentially independent of the dissipation scale Stokes number. The DNS data agrees with results from experiments performed in a wind tunnel with close parametric overlap. We anticipate that a better understanding of the role of gravity and turbulence in inertial particle entrainment will lead to improved cloud evolution predictions and more accurate climate models. Sponsored by the U.S. NSF. [Preview Abstract] |
Sunday, November 21, 2010 2:05PM - 2:18PM |
CV.00006: On the relation between preferential concentration and radial relative velocity of inertial particles in homogeneous isotropic turbulence Baidurja Ray, Lance Collins \parindent = 0 pt{} The radial distribution function (RDF, a statistical measure of preferential concentration) and the PDF of the radial relative velocity ($w_r$) are the two statistical inputs to the collision kernel for inertial particles, which determines their collision rate in a turbulent flow. Although the relative velocity between the particles drives their spatial distribution (and hence the RDF), the relation between the two is not yet well-established. In this paper, we investigate this relationship using direct numerical simulation (DNS) of particle-laden homogeneous isotropic turbulence, with and without filtering. We show that the spatio-temporal variation of the skewness of the PDF of $w_r$ is qualitatively similar to that of the RDF. We then apply a low-pass sharp spectral filter to the DNS velocity field and use the filtered velocity field to calculate the RDF and the PDF of $w_r$. The first and second moments of $w_r$ are found to decrease monotonically with filtering for all separation distances irrespective of the particle Stokes number ($St$), whereas the skewness decreases with filtering for low $St$ and increases with filtering for high $St$. This non-monotonic response of the skewness to filtering is qualitatively similar to the response of the RDF to filtering, and points towards a connection between them. [Preview Abstract] |
Sunday, November 21, 2010 2:18PM - 2:31PM |
CV.00007: Settling of finite-size colliding particles in unbounded domains Julian Simeonov, Joseph Calantoni A numerical model for Direct Numerical Simulations of particle-laden flows is developed to investigate the bulk behavior of particle suspensions. The particle hydrodynamic forces are determined by solving the incompressible Navier-Stokes equations for the finite Reynolds number flow around individual particles. At grid resolutions permitting large-scale simulations, the pressure and viscous stress are resolved everywhere except in the gap of colliding particles where micro-scale lubrication effects become important just before contact. An analytical expression for the unresolved lubrication pressure force is added as a correction to the numerically resolved hydrodynamic force. The mechanical-contact interaction between particles is modeled with Hookean elasticity and friction. The model predicted dissipation of particle momentum during collisions is compared to experimental data for the coefficient of restitution of immersed binary collisions. We then consider the gravitational settling of particle suspensions in triply periodic domains and determine the dependence of the settling rate on the concentration and the domain size. The numerical results are compared to well-known empirical data for settling in pipes, which is often used in continuum models for particle-laden flow. [Preview Abstract] |
Sunday, November 21, 2010 2:31PM - 2:44PM |
CV.00008: Gravity influence on the clustering of charged particles in turbulence Jiang Lu, Hansen Nordsiek, Raymond Shaw We report results aimed at studying the interactions of bidisperse charged inertial particles in homogeneous, isotropic turbulence, under the influence of gravitational settling. We theoretically and experimentally investigate the impact of gravititational settling on particle clustering, which is quantified by the radial distribution function (RDF). The theory is based on a drift-diffusion (Fokker-Planck) model with gravitational settling appearing as a diffusive term depending on a dimensionless settling parameter. The experiments are carried out in a laboratory chamber with nearly homogeneous, isotropic turbulence in which the flow is seeded with charged particles and digital holography used to obtain 3D particle positions and velocities. The derived radial distribution function for bidisperse settling charged particles is compared to the experimental RDFs. [Preview Abstract] |
Sunday, November 21, 2010 2:44PM - 2:57PM |
CV.00009: Dust Settling in Protoplanetary Disks and the Onset of Kelvin-Helmholtz Instability Joseph Barranco, Aaron Lee, Eugene Chiang, Philip Marcus, Xylar Asay-Davis It is a remarkable fact that planets start out as microscopic grains within the protoplanetary disks of gas and dust in orbit around newly-formed protostars, somehow growing by a factor of $10^{40}$ in mass in a period no more than $10^7$ years. In the early stages of the planet formation process, small dust grains settle into the midplane of the disk in a few thousand years. As the dust layer gets thinner and denser, a vertical shear develops between the dust-rich layer at the midplane and the dust-poor gas above and below this layer. Of great interest is under what conditions such a layer will be unstable to Kelvin-Helmholtz instability (KHI), which will remix the dust with the gas, thwarting the formation of planets. In our previous work, we worked in the single-fluid limit in which the local dust-to gas ratio was an advectively conserved quantity (valid when the dust-gas friction time is very short). Here, we present new simulation in which this assumption is relaxed. We employ 2-fluid simulations of dust and gas to explore the evolution of a dust layer in the more general case in which the dust grains and gas can slip through each other. We will describe conditions that allow the dust layer to settle to sufficient density to gravitationally clump-up to form planetesimals before the onset of KHI. [Preview Abstract] |
Sunday, November 21, 2010 2:57PM - 3:10PM |
CV.00010: Dispersion of a cloud of particles in the accelerated flow behind a moving shock Gustaaf Jacobs, Thomas Dittmann, Wai-Sun Don We discuss the dynamics and dispersion of bronze particles that are initially arranged in varying cloud shapes and are accelerated in the supersonic flow behind a moving normal shock. Particle clouds with a particle volume concentration of 4\% are arranged initially in a rectangular, triangular and circular shape, whose angle with respect to the incoming flow are also varied. Simulations are performed with a recently developed high-order resolution Eulerian-Lagrangian method, that approximates the Euler equations governing the gas dynamics with the improved high order weighted essentially non-oscillatory scheme, while individual particles are traced in the Lagrangian frame using high-order time integration schemes. The purpose of these simulations is two-fold: we are aiming to match a published shocktube experiment of the dispersion of an initially, nominally rectangular cloud shape behind a moving shock and we are aiming to validate our high-order methods against these experiments. The dynamics and resulting dispersion patterns of the developing particle-laden flows are distinctly different between different cloud shapes but we will report statistical similarities and correlations between cloud spread and energy budgets of the particle phases. [Preview Abstract] |
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