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 AU: Multiphase Flows I |
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Chair: Amy Shen, University of Washington Room: Hyatt Regency Long Beach Regency A |
Sunday, November 21, 2010 8:00AM - 8:13AM |
AU.00001: Effect of microstructural anisotropy on the fluid-particle drag force William Holloway, Jin Sun, Sankaran Sundaresan The permeabilities of particle assemblies with anisotropic microstructures have been determined through lattice-Boltzmann simulations. Such assemblies were created by subjecting them to simple shear in a periodic domain. The extent of anisotropy depends on the scaled rate of deformation of the particle assembly ${\left| {\underline{\underline D} } \right|d} \mathord{\left/ {\vphantom {{\left| {\underline{\underline D} } \right|d} {\sqrt T }}} \right. \kern-\nulldelimiterspace} {\sqrt T }$, where$d$is the particle diameter, $\left| {\underline{\underline D} } \right|$ is the magnitude of the rate of strain tensor, and $T$is the granular temperature. The anisotropy of the permeability tensor increases with the scaled rate of deformation and the particle volume fraction, and it can readily be rationalized in terms of the structure tensor of the assembly. A model for the anisotropic permeability is proposed in terms of mean free path of the deformed assembly, and the rate of strain tensor. [Preview Abstract] |
Sunday, November 21, 2010 8:13AM - 8:26AM |
AU.00002: Influence of bubbles on liquid turbulence based on the direct numerical simulation of channel flows Igor Bolotnov, Donald Drew, Richard Lahey, Jr., Michael Podowski It is well known that the bubbles in turbulent flow can modify the structure and intensity of the turbulence. Recent progress in two-phase direct numerical simulation (DNS) provides a new level of detailed information about the two-phase turbulence. The availability of DNS data for single and two-phase turbulent channel flows makes it possible to compute the bubble-induced source terms in the turbulent kinetic energy equation. The turbulent kinetic energy equation, including turbulence production, dissipation and viscous and turbulent diffusion can be derived from the Navier-Stokes equations. Those exact analytical expressions can be applied to the instantaneous pressure and velocity fluctuating fields found in the single and two-phase DNS data to obtain the time-averaged lateral distribution for each term. By analyzing both single-phase and two-phase turbulent channel flows we can estimate the difference in those terms for various gas volume fraction flows. This information can be used to quantify the influence of the bubbles on turbulence in gas-liquid two-phase flows. The results will include an assessment of the currently used models of bubble-induced turbulence, as well as a validation of the new models developed in the current study. [Preview Abstract] |
Sunday, November 21, 2010 8:26AM - 8:39AM |
AU.00003: Lagrangian statistics of bubbles in a turbulent boundary layer Michael Mattson, Krishnan Mahesh We are developing the capability to simulate bubbly flows in complex geometries using unstructured grids and an Euler--Lagrangian methodology. In the Lagrangian bubble model, the bubbles are treated as a dispersed phase in the carrier fluid, and individual bubbles are point--particles governed by an equation for bubble motion. The behavior of the bubble radius is determined by integrating the Rayleigh--Plesset equation. For this talk, direct numerical simulation is used to solve the Navier--Stokes equations for a spatially--evolving turbulent boundary layer ($Re_{\theta}\!=\!600\!-\!1800$) and bubbles are injected into the near-wall region. Except for the Reynolds number, the simulation matches all parameters of an experiment by Sanders, {\it et al.}\ (J. Fluid Mech., 2006). The bubbly suspension is dilute and one--way coupled equations are used. The temporal evolution of the bubble dispersion, probability density functions of the forces on a bubble and void--fraction profiles will be presented, and the impact of bubble behavior on drag reduction and the effect of cavitation number will be discussed. [Preview Abstract] |
Sunday, November 21, 2010 8:39AM - 8:52AM |
AU.00004: Interfacial Force Model Development for Turbulent Bubbly Flows Dillon Shaver, Igor Bolotnov, Steven Antal, Michael Podowski Typically, a Reynolds averaged Navier-Stokes (RANS) simulation of turbulent bubbly flows makes use of interfacial force models which represent the interaction between the bubbles and the continuous liquid. The modeled forces include drag, virtual mass, turbulent dispersion, and lift. A direct numerical simulation (DNS) fully resolves turbulent fluctuations in velocity and, when coupled with the level set method, can simulate a two-phase flow without relying on interfacial force models. Results from DNS can provide a level of insight into flow characteristics not easily achievable with traditional experimental methods. This makes DNS ideal for developing interfacial force models for use with RANS codes. Turbulent, air/water, bubbly flows in a channel have been previously simulated using the DNS code, PHASTA. Utilizing the time-averaging concept, average velocities of the two phases, void fraction, turbulent kinetic energy, and turbulence dissipation rate distributions are calculated from the DNS data. This information is then used to develop and calibrate the interfacial force models used in the RANS code, NPHASE-CMFD. Two cases are analyzed. The first is of many small, spherical bubbles of 0.9 mm diameter. The other is of a single, large, cap bubble of 3.625 mm equivalent diameter. Both simulations correspond to the liquid Reynolds number of 11,200, based on the hydraulic diameter. [Preview Abstract] |
Sunday, November 21, 2010 8:52AM - 9:05AM |
AU.00005: Light Particles in Turbulence: acceleration statistics Julian Martinez Mercado, Vivek Nagendra Prakash, Yoshiyuki Tagawa, Chao Sun, Detlef Lohse Three-dimensional Lagrangian Particle Tracking experiments are used to study acceleration statistics of light particles ($\beta=3\rho_f/(\rho_f+2\rho_p)=3$) in isotropic turbulence. Microbubbles of size comparable to Kolmogorov's lengthscale are injected in a turbulent water channel. By varying $Re$ we study the effect of changing the turbulent lengthscale on the statistics for a fixed particle size. We compare our results with previous experimental and numerical data on particles in turbulence. We find that acceleration PDFs show stretched exponential tails, the shape being independent of $Re$. The acceleration autocorrelation shows that light particles decorrelate faster than tracer or heavy particles. The correlation drops rapidly to zero in less than one Kolmogorov's timescale. The decorrelation time increases with $Re$. This trend is in agreement with previous experimental data for different flows and with numerical simulations. [Preview Abstract] |
Sunday, November 21, 2010 9:05AM - 9:18AM |
AU.00006: Light particles in turbulence: velocity statistics Vivek Nagendra Prakash, Julian Martinez Mercado, Yoshiyuki Tagawa, Chao Sun, Detlef Lohse We conduct experiments to study light particles in turbulence using Particle Tracking Velocimetry (PTV) in three-dimensions. Microbubbles are dispersed in a homogenous and isotropic turbulent flow in the Twente water tunnel. The size of the microbubbles is fixed and is comparable to the Kolmogorov length scale of the flow. The Lagrangian velocity statistics of the microbubbles are obtained from the trajectories captured using PTV. The velocity statistics (PDF, autocorrelation and structure functions) of microbubbles are studied at different $Re$ and compared with previous experiments and numerics for particles in turbulence. The velocity PDF of the 3 velocity components (x, y and z) show a robust gaussian profile (independent of Re) with flatness values between 2.74 to 3.25. We calculate the velocity autocorrelation and find that the decorrelation time increases with increasing $Re$. We also calculate the second and fourth - order velocity structure functions and find a reasonable agreement with previous numerical simulations. [Preview Abstract] |
Sunday, November 21, 2010 9:18AM - 9:31AM |
AU.00007: Local bubble distribution in bubbly turbulent Taylor-Couette flow Daniela Narezo, Dennis van Gils, Chao Sun, Detlef Lohse In turbulent Taylor-Couette flow, the injection of bubbles reduces the global drag on the cylinder surfaces. The previous bubbly turbulent drag reduction measurements in TC flow were mainly based on the global torque, which is not sufficient to understand the mechanism of bubbly drag reduction. One of the key issues is the actual bubble distribution inside the TC gap. Using optical fibers placed inside the TC gap, we scanned the local bubble distribution in the radial direction. An extension of this technique is a four-point optical fiber probe, which enables to retrieve the bubble velocity vector and aspect ratio. [Preview Abstract] |
Sunday, November 21, 2010 9:31AM - 9:44AM |
AU.00008: An arbitrary-Lagrangian-Eulerian method for simulating particle-bubble interactions Tong Qin, Pengtao Yue, Saad Ragab Particle-bubble interaction is an important process in flotation. This problem is difficult in that it involves three phases: solid particles, gas bubbles, and surrounding liquid. In this work, an arbitrary-Lagrangian-Eulerian (ALE) approach is developed for the direct numerical simulation. A moving triangular mesh is used to track the surfaces of rigid solid particles and deformable gas bubbles. The gas motion inside each bubble is neglected, and the pressure is determined by the isothermal gas law. The equations for the particle motion and the Navier-Stokes equations for the liquid motion are solved in a unified finite element framework. The whole system is solved by an implicit scheme which is second order in time. In the end, we will show results on the head-on collision between a bubble and a particle and the subsequent film drainage process. Depending on the collision conditions, the particle may attach to the bubble or be bounced back. Comparisons with experiments will also be presented. [Preview Abstract] |
Sunday, November 21, 2010 9:44AM - 9:57AM |
AU.00009: Simulation of particle motion at interface Young Joon Choi, Patrick Anderson A diffuse interface model is presented to describe the motion and interaction of particles in two-phase flows. In the diffuse interface model, the interface is considered to have a small but finite thickness, which circumvents explicit tracking of the interface. For the direct numerical simulation of the particle motion, we incorporate an extended finite element method, in which the particle domain is decoupled from the fluid domain while using a regular mesh for the whole computational domain. By combining the diffuse interface method and the extended finite element method, the particle motion at an interface can be simulated on a fixed Eulerian mesh without any need of re-meshing. We apply a small disturbance on a particle resting at an interface between two fluids, and study the particle movement towards its equilibrium position. In particular, we investigate the effect of surface tension, interfacial thickness and particle size on the time required to reach its equilibrium position. [Preview Abstract] |
Sunday, November 21, 2010 9:57AM - 10:10AM |
AU.00010: A multi-Gaussian quadrature method of moments for gas-particle flows Rodney Fox, Christophe Chalons, Marc Massot Gas-particle flows with finite Stokes number can be modeled at the mesoscopic level using a kinetic description for the particle velocity. In the context of large-eddy simulation (LES), the filtered kinetic equation contains a velocity diffusion term and a spatial flux term that depend on the sub-grid-scale (SGS) gas-phase velocity fluctuations. In very dilute (i.e. non-collisional) flows, the SGS terms drive the particle velocity distribution function towards a Gaussian form. On the other hand, particle trajectory crossing of inertial particles lead to highly non-Gaussian distributions. In the context of moment methods for solving the kinetic equation, the latter can be successfully modeled using quadrature-based moment methods. In this work, such methods are extended to allow for a continuous transition between a delta-function representation of the quadrature nodes and a multi-Gaussian representation. Examples with 1D and 2D velocity phase spaces are presented to illustrate the advantages of multi-Gaussian quadrature. In particular, the representation of the spatial fluxes is considerably improved for cases where the velocity distribution is close to Gaussian. [Preview Abstract] |
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