Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session D32: Particle-Laden Flows: Radiation and Gravity |
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Chair: Mrugesh Shringarpure, University of Florida Room: 2020 |
Sunday, November 23, 2014 2:15PM - 2:28PM |
D32.00001: Towards understanding of particle-based solar receivers: impact of preferential concentration on heat transfer statistics Hadi Pouransari, Ali Mani This work aims to develop characterization of heating non-uniformities when a particle-laden fluid is heated via a radiative source. We consider a numerical setting with an inflow-outflow configuration in which a premixed turbulent stream is subject to uniform radiation intensity in the heating section. Direct numerical simulation of fluid-particle mixture is developed using finite difference approximation to the low-Mach Navier-Stokes equations and Lagrangian tracking to represent particle transport. The medium is considered optically thin with all radiation absorbed primarily by solid particles and then exchanged conductively to a gaseous carrier phase. In such setting preferential concentration of the particles leads to heating non-uniformities, which can impact system performance. We will present a characterization of mean versus rms temperature fields for a wide range of Stokes numbers. [Preview Abstract] |
Sunday, November 23, 2014 2:28PM - 2:41PM |
D32.00002: Settling of hot particles through turbulence Filippo Coletti, Ari Frankel, Hadi Pouransari, Ali Mani Particle-laden flows in which the dispersed phase is not isothermal with the continuous phase are common in a wealth of natural and industrial setting. In this study we consider the case of inertial particles heated by thermal radiation while settling through a turbulent transparent gas. Particles much smaller than the minimum flow scales are considered. The particle Stokes number (based on the Kolmogorov time scale) and the nominal settling velocity (normalized by the root-mean-square fluid velocity fluctuation) are both of order unity. In the considered dilute and optically thin regime, each particle receives the same heat flux. Numerical simulations are performed in which the two-way coupling between dispersed and continuous phase is taken into account. The momentum and energy equations are solved in a triply periodic domain, resolving all spatial and temporal scales. While falling, the heated particles shed plumes of buoyant gas, modifying the turbulence structure and enhancing velocity fluctuations in the vertical direction. The radiative forcing does not affect preferential concentration (clustering of particles in low vorticity regions), but reduces preferential sweeping (particle sampling regions of downward fluid motion). Overall, the mean settling velocity varies slightly when heating the particles, while its variance is greatly increased. [Preview Abstract] |
Sunday, November 23, 2014 2:41PM - 2:54PM |
D32.00003: Turbulence-radiation interactions in a particle-laden flow Ari Frankel, Hadi Pouransari, Gianluca Iaccarino, Ali Mani Turbulent fluctuations in a radiatively participating medium can significantly alter the mean heat transfer characteristics in a manner that current RANS models cannot accurately capture. While turbulence-radiation interaction has been studied extensively in traditional combustion systems, such interactions have not yet been studied in the context of particle-laden flows. This work is motivated by applications in particle-based solar receivers in which external radiation is primarily absorbed by a dispersed phase and conductively exchanged with the carrier fluid. Direct numerical simulations of turbulence with Lagrangian particles subject to a collimated radiation source are performed with a flux-limited diffusion approximation to radiative transfer. The dependence of the turbulence-radiation interaction statistics on the particle Stokes number will be demonstrated. [Preview Abstract] |
Sunday, November 23, 2014 2:54PM - 3:07PM |
D32.00004: Influence of thermal radiation on turbulent kinetic energy spectrum in particle-laden flows Jacqueline Chen, Hemanth Kolla, Hadi Pouransari, Ali Mani We investigate density-weighted spectra of turbulent kinetic energy of the gas phase in particle-laden flows with thermal radiation. Compressible DNS of three-dimensional homogeneous isotropic decaying turbulence laden with point particles reveal that thermal radiation alters the spectrum of the total turbulent kinetic energy by introducing dilatational velocity fluctuations at large wavenumbers, while the spectrum of the divergence-free modes remains unaffected. With increasing time the magnitude of the energy content in the dilatational modes increases while the wavenumber range over which they are active also broadens. Pressure-dilatation correlations, which are source-like terms in the balance equation for density-weighted energy spectrum, confirm the high-wavenumber influence of radiation-induced dilatation. [Preview Abstract] |
Sunday, November 23, 2014 3:07PM - 3:20PM |
D32.00005: Spreading of non-planar non-axisymmetric gravity and turbidity currents Nadim Zgheib, Thomas Bonometti, S. Balachandar The dynamics of non-axisymmetric turbidity currents is considered here. The study comprises a series of experiments for which a finite volume of particle-laden solution is released into fresh water. A mixture of water and polystyrene particles of diameter $280 < D_{p} < 315\mu m$ and density $\rho _{c} =1007Kg/m^{3}$ is initially confined in a hollow cylinder at the center of a large tank filled with fresh water. Cylinders with four different cross-sections are examined: a circle, a plus-shape, a rectangle and a rounded rectangle in which the sharp corners are smoothened. The time evolution of the front is recorded as well the spatial distribution of the thickness of the final deposit via the use of a laser triangulation technique. The dynamics of the front and final deposit are significantly influenced by the initial geometry, displaying substantial azimuthal variation especially for the rectangular case where the current extends farther and deposits more particles along the initial minor axis of the rectangular cross section. Interestingly, this departure from axisymmetry cannot be predicted by current theoretical methods such as the Box Model. Several parameters are varied to assess the dependence on the settling velocity, initial height aspect ratio, local curvature and mixture density. [Preview Abstract] |
Sunday, November 23, 2014 3:20PM - 3:33PM |
D32.00006: Process of entrainment in particulate gravity currents Mrugesh Shringarpure, Jorge Salinas, Mariano Cantero, S. Balachandar Various geophysical flows like turbidity currents, river flows, dust storms etc transport huge quantities of dispersed phase over large distances. Typically in such flows a dispersed phase rich layer is swept along with the flow. The amount of dispersed phase carried depends on the dynamics of this layer which are governed by a strong coupling between turbulence and suspended particles. This layer evolves, i.e., grows/shrinks in size, due to entrainment/detrainment of surrounding clear fluid at its interface (where a sharp change from particle rich fluid to surrounding clear fluid occurs). Also in many applications there is entrainment and detainment of particles at the bottom boundary due to settling and resuspension. The entrainment processes that occur here have important consequences. Consistent entrainment means the flow is energetic enough to mix/distribute the dispersed phase layer in the bulk flow. To study these processes, we introduce a layer of suspended particles into a fully turbulent channel flow and capture the entrainment processes in detail. Three parameters - Reynolds number, particle size and Richardson number dictate the entrainment process. Various simulations have been performed that explores this parametric space and identifies various entrainment regimes. [Preview Abstract] |
Sunday, November 23, 2014 3:33PM - 3:46PM |
D32.00007: Dynamics of bidensity suspensions in gravity-driven thin film flows Jeffrey Wong, Andrea Bertozzi We study bidensity suspensions of a viscous fluid on an incline, where the particles migrate due to a combination of gravity-induced settling and shear induced migration. The physical problem is modeled by a hyperbolic system of conservation laws for the height and particle concentrations. We consider the constant flux problem and show that the system exhibits three-shock solutions corresponding to distinct fronts of particles and liquid traveling at different speeds, as well as singular shock solutions for sufficiently large concentrations, for which the mechanism predicted by the model is similar the single-species case. We also consider initial conditions corresponding to a finite reservoir of fluid, where solutions are rarefaction-shock pairs, and compare to experiments. [Preview Abstract] |
Sunday, November 23, 2014 3:46PM - 3:59PM |
D32.00008: Periodic oscillations of particles settling under gravity in a viscous fluid Maria L. Ekiel-Jezewska New periodic solutions of three spherical particles settling under gravity in a viscous fluid are found and their relation to chaotic dynamics of a cluster of three randomly distributed particles is shown. An analogue of this solution has not been detected in the point-particle approximation. However, the existence of such an unstable periodic orbit was previously suggested by Janosi et al., Phys.Rev. E,\textbf{56}, 2858 (1997) and was claimed to be responsible for the numerically observed chaotic scattering of three point-particles settling under gravity. The significance of periodic orbits for dynamics of sedimenting particles in other non-regular configurations is also illustrated by other examples. [Preview Abstract] |
Sunday, November 23, 2014 3:59PM - 4:12PM |
D32.00009: Inertial particle clustering, relative velocity, and collision statistics in the presence of gravity Peter J. Ireland, Andrew D. Bragg, Lance R. Collins We use direct numerical simulations to investigate the dynamics of inertial particles in the presence of gravitational forces over a large range of Taylor-scale Reynolds numbers ($90 \leq R_\lambda \leq 597$). The particle inertial and gravitational forces are parameterized to provide insight into the motion and growth of water droplets in warm, cumulus clouds. We perform a detailed analysis of the effect of the periodic boundary conditions in the simulations and find that extended domain lengths are needed for accurate particle statistics, especially at low Reynolds numbers. While gravity reduces the relative velocities of all particle classes, it has a bifurcated effect on particle clustering, suppressing (enhancing) clustering for weakly (strongly) inertial particles. We provide a physical explanation of these trends by extending the model of Zaichik \& Alipchenkov (New J. Phys., 11:103018, 2009) to account for gravitational effects. The particle statistics are strongly anisotropic, and we use spherical harmonic decomposition to quantify this anisotropy. Finally, we compare collision statistics of inertial particles with gravity to those without gravity and suggest practical implications for the onset of precipitation in cumulus clouds. [Preview Abstract] |
Sunday, November 23, 2014 4:12PM - 4:25PM |
D32.00010: Velocity and acceleration statistics from direct numerical simulations (DNS) of particle-laden homogeneous turbulent shear flow Parvez Sukheswalla, Andrew Bragg, Lance Collins We study the effects of imposed mean shear and gravity (acting normal to shear) on the velocity and acceleration statistics of inertial particles in homogeneous turbulent shear flow. Single- and two-particle statistics from high-resolution DNS are analyzed for various particle sizes, shear rates, and settling parameters. We find that mean particle settling speeds are enhanced by turbulence, and are sensitive to changes in shear and gravity. Also, the net drift of particles due to gravity increases their locally averaged velocities in the mean-flow direction. Particle-pair relative velocities are anisotropic, with the radial inward component and the second-order structure function both maximal along the mean-strain contractional axis. Stronger shear shifts this orientation towards the mean flow streamlines, reflecting the larger particle fluctuating velocities in that direction. Gravity and shear together cause particle r.m.s. accelerations to increase with increasing inertia, in contrast to isotropic turbulence, but consistent with past turbulent boundary layer experiments. Analytical predictions for particle mean velocities and accelerations agree well with the DNS data. These results have important implications for the anisotropic collision kernel in shear flow. [Preview Abstract] |
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