Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session GL: Particle Laden Flows II: Turbulence |
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Chair: Antonino Ferrante, University of Washington Room: 200A |
Monday, November 23, 2009 8:00AM - 8:13AM |
GL.00001: Hybrid RANS/LES of particle-laden turbulent flows Marcel Ilie, Stefan Llewellyn Smith One of the main issues in using large-eddy simulation (LES) for high Reynolds number flows in bounded domains is the requirement of very fine grid resolution near walls. We present a hybrid RANS/LES method in conjuction with a Lagrangian particle tracking algorithm, for the numerical prediction of particle-laden turbulent flows. The hybrid RANS/LES methodology aims to reduce the high computational effort of wall-resolved LES. This approach is based on the concept of dividing the simulation into a near-wall RANS part and an outer LES part, and allows the thickness of the near-wall RANS layer to be chosen freely. The near-wall layer is interfaced to the outer LES region using compatibility conditions for velocity and turbulent viscosity across the interface that are extracted dynamically as the simulation progresses. The influence of parameters such as particle shape and size, particle density and flow Reynolds number on the particle dispersion and total deposition is examined. Particles of fiber shape are more prone to deposition. Total particle deposition increases with the particle size, density and Reynolds number. [Preview Abstract] |
Monday, November 23, 2009 8:13AM - 8:26AM |
GL.00002: Simulation of turbulent flow laden with finite-size spherical particles Hui Gao, Lian-Ping Wang Particle-laden turbulent flow is of importance to many engineering applications and natural phenomena. Most previous studies utilize the point particle approach to study the effects of particles on the carrier turbulence, under the assumptions that the particle size is significantly smaller than the smallest turbulence length scale and the particle volume fraction is low. The present study focuses on the motion and hydrodynamic interactions of finite-size freely moving particles in a turbulent background flow. A mesoscopic lattice Boltzmann approach is applied to simulate a homogeneous isotropic turbulence and to realize the no-slip boundary condition on the boundary of each moving particle. The short-range lubrication force not resolved by the simulation is represented by a model in terms of particle relative location and velocity. The change of energy spectrum compared with the particle-free turbulence is discussed, as well as the time evolution of the turbulent kinetic energy and the dissipation rate. The effects of varying particle size, volume fraction, and particle-to-fluid density ratio will also be examined. [Preview Abstract] |
Monday, November 23, 2009 8:26AM - 8:39AM |
GL.00003: High-inertia particle acceleration statistics in a turbulent channel flow Valentina Lavezzo, Alfredo Soldati, Zellman Warhaft, Lance Collins Recent experiments in a turbulent boundary layer (Gerashchenko et al., 2008) have shown that the variance in the acceleration fluctuations of small, heavy particles in the near wall region increases with increasing inertia, contrary to the trend found for homogeneous and isotropic turbulence. In a previous study, we ran direct numerical simulations (DNS) of inertial particles in a channel flow to show how this phenomenon is related to the coupling of particle motion with shear and gravity. In this work, we extended the DNS to a much broader range of particle Stokes number (20, 40 and 100). The trend for the mean and variance of the particle-acceleration statistics at these much higher Stokes numbers are consistent with what previously was found for homogeneous and isotropic turbulence. We attribute this behavior to the inertial filtering by the particles of the underlying turbulent flow, as though at these higher Stokes numbers particles sample a more nearly isotropic flow field. The effect of gravity also has been considered and will be presented in detail. [Preview Abstract] |
Monday, November 23, 2009 8:39AM - 8:52AM |
GL.00004: Direct Numerical Simulation of dense particle-laden turbulent flows using immersed boundaries Fan Wang, Olivier Desjardins Dense particle-laden turbulent flows play an important role in many engineering applications, ranging from pharmaceutical coating and chemical synthesis to fluidized bed reactors. Because of the complexity of the physics involved in these flows, current computational models for gas-particle processes, such as drag and heat transfer, rely on empirical correlations and have been shown to lack accuracy. In this work, direct numerical simulations (DNS) of dense particle-laden flows are conducted, using immersed boundaries (IB) to resolve the flow around each particle. First, the accuracy of the proposed approach is tested on a range of 2D and 3D flows at various Reynolds numbers, and resolution requirements are discussed. Then, various particle arrangements and number densities are simulated, the impact on particle wake interaction is assessed, and existing drag models are evaluated in the case of fixed particles. In addition, the impact of the particles on turbulence dissipation is investigated. Finally, a strategy for handling moving and colliding particles is discussed. [Preview Abstract] |
Monday, November 23, 2009 8:52AM - 9:05AM |
GL.00005: Experimental Investigation of Charged Inertial Particles in Turbulence Hansen Nordsiek, Jiang Lu, Ewe Wei Saw, Raymond Shaw We report results from experiments aimed at studying the interactions of electrically charged inertial particles in homogeneous, isotropic turbulence. Conditions are selected to investigate the effects of mutual electrostatic repulsion of particles on their dynamics. We measure droplet clustering and relative velocities. The experiments are carried out in a laboratory chamber with nearly homogeneous, isotropic turbulence. The turbulence is characterized using LDV and 2-frame holographic particle tracking velocimetry. We seed the flow with charged particles and use digital holography to obtain 3D particle positions and velocities. From particle positions, we investigate the impact of mutual electrostatic repulsion on inertial clustering through the calculation of the radial distribution function (RDF). Specifically, repulsion overcomes inertial clustering below a shielding length as seen by a strong reduction in the RDF. [Preview Abstract] |
Monday, November 23, 2009 9:05AM - 9:18AM |
GL.00006: Effect of filtering on inertial particle clustering in homogeneous isotropic turbulence Baidurja Ray, Lance Collins The use of large-eddy simulation (LES) to represent inertial particles in a turbulent flow field requires a model for the effect of the subfilter eddies on the particle motion. A particularly challenging aspect of this modeling is correctly capturing particle clustering, which is driven principally by the small-scale eddies that have been filtered in a LES. In this paper, we investigate this problem by performing direct numerical simulations of homogeneous isotropic turbulence with inertial particles and compare the results to particles moving through a low-pass filtered velocity field. The filtering is done in wavenumber space and is akin to a `perfect' LES in that there is no subgrid model. We look at the two-particle radial distribution function (RDF) and the relative velocity probability density function (PDF) at different separation distances. We find that both the RDF and relative velocity PDF change substantially in response to the filtering. In particular, the level of clustering can be suppressed or enhanced depending on the value of the Stokes number. The spatial scales of the clustering are also affected. The results suggest requirements that a subfilter model should satisfy to correctly reproduce the RDF and relative velocity PDF. Such information will assist the future development of a LES model for inertial particles. [Preview Abstract] |
Monday, November 23, 2009 9:18AM - 9:31AM |
GL.00007: On the effects of Taylor-lengthscale size particles on isotropic turbulence F. Lucci, A. Ferrante, S. Elghobashi The effects of spherical particles of Taylor-lengthscale size ($ d \sim \lambda $) on isotropic turbulence are studied via DNS. A mesh of $256^3$ grid points is used with an initial microscale Reynolds number $Re_{\lambda 0}= 75 $. The flow around 6400 freely-moving particles is fully resolved using the Immersed Boundary method. The maximum volume fraction of the particles is $\phi_v = 0.1$. The maximum density ratio is $ \rho_p/\rho_f = 10 $ which corresponds to a mass fraction $\phi_m = 1.$ Our results show that particles with diameter $d \sim \lambda$ always reduce the turbulence kinetic energy (TKE), mostly by enhancing its dissipation rate, $\varepsilon(t)$. The augmented dissipation rate exceeds $\Psi_p(t)$, the rate of increase of TKE due to the two-way coupling force imparted by the particles on the surrounding fluid. The increased dissipation rate occurs close to the front of the particle surface due to the increased strain rates (both extensional and compressive) as the particles move through the surrounding turbulent eddies. For fixed volume fraction and diameter of the particles, the most pronounced effects on TKE, its dissipation rate and its rate of change due to two-way coupling occur by increasing the ratio $\rho_p/\rho_f$ which is directly proportional to the Stokes number, ($\tau_p/\tau_k$), and the particles mass fraction, $\phi_m $. [Preview Abstract] |
Monday, November 23, 2009 9:31AM - 9:44AM |
GL.00008: Mixing of inertial particles at a turbulent - non turbulent interface Sergiy Gerashchenko, Luis Ruelas, Zellman Warhaft Motivated by the problem of entrainment of dry air into clouds, water droplets are sprayed into the high turbulence side of a shearless turbulence mixing layer: a layer in which there is a step in turbulence intensity across the interface but there is negligible change in the mean velocity (Veeravelli and Warhaft, JFM, 1989, 208, 191). Active and passive grids are used to form the mixing layer. A splitter plate is used to separate droplet-non droplet interface near the origin. Particle concentration, size and velocity are determined by Phase Doppler Particle Analyzer, the velocity field by hot wires, and the droplet accelerations by particle tracking. The results are compared with injecting the particles into one side of homogeneous turbulence. We show that the particle number density is approximately an order of magnitude smaller on the low turbulence side of the turbulent-non-turbulent interface compared with that of a turbulent- turbulent interface with the same initial distribution of inertial particles on one side. Stokes and Froude number effects are investigated. Sponsored by the U.S. NSF. [Preview Abstract] |
Monday, November 23, 2009 9:44AM - 9:57AM |
GL.00009: Large scale accumulation of inertial particles in turbulent channel flow Luca Brandt, Philipp Schlatter, Gaetano Sardina, Carlo M. Casciola Spatially inhomogeneous turbulent flows induce peculiar phenomena on the transport of a dispersed phase of inertial particles. In channel flows the most striking effect is the spatial segregation of particles that may achieve a concentration at the wall largely exceeding that in the bulk. Here we approach the issue by considering direct numerical simulations in a channel seeded with different populations of diluted, tiny particles. The simulations at Re$_{\tau}$=180 have been performed using the largest domain size so far. The structures found in the fully developed stage of the process show strong spanwise correlations more intense than those found in the corresponding elongated structures of low and high fluid speed. The extremely regular spanwise organization corresponds to a mean spacing of about 120 plus units. The turbulent simulations with an increased size of the numerical box highlight some significant differences in the correlation of particle concentrations. A possible explanation of this feature can be related to large-scale structures of the velocity field, which might carry a considerable amount of energy. Correlations between turbulent events, sweep and ejections, and the particle motion to and from the wall will be also presented. [Preview Abstract] |
Monday, November 23, 2009 9:57AM - 10:10AM |
GL.00010: Self-similarity in particle laden flows at constant volume Matthew Mata, Natalie Grunewald, Rachel Levy, Thomas Ward, Andrea Bertozzi We consider constant volume thin film slurries on an incline. Clear fluids in this geometry are known to have a front position that moves according to a $t^{1/3}$ scaling law, based on similarity solution analysis [Huppert, Nature, 1982]. We investigate the same dynamics for particle laden flow using a recently proposed lubrication model for the slurry and physical experiments. Our analysis includes the role of a precursor in the model. We conclude that in the lubrication model, the height of the precursor significantly influences the speed of the fluid front, independent of particles settling in the direction of flow. By comparing theory with experiments we conclude that the $t^{1/3}$ scaling law persists, to leading order, for slurry flows with particle settling. However additional physics is needed in the existing lubrication models to quantitatively explain departures from clear-fluid self-similarity due to particle settling. [Preview Abstract] |
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