Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session BE: Multiphase Particle-Laden Flows I |
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Chair: S. Balachandar, University of Illinois, Urbana-Champaign Room: Hilton Chicago Continental B |
Sunday, November 20, 2005 10:56AM - 11:09AM |
BE.00001: Interaction of a finite-sized particle with wall turbulence Lanying Zeng, S. Balachandar, Fady Najjar, Paul Fischer Interaction of a {\em {finite-sized}} particle (diameter comparable or bigger than flow scales) with wall turbulence is quite complex. For example, in such situations the applicability of standard drag and lift correlations, which have been developed based on simple ambient flow conditions, can be questioned. Furthermore, the complex wake dynamics of a finite-sized particle has the potential significantly modify the carrier phase turbulence and in the presence of a nearby wall influences the wall shear stress and drag. In order to get the insight into this problem, we consider a turbulent channel flow with an embedded spherical particle of diameter varying from 2 to 20 times the Kolmogorov scale. The position of the particle is varied from near the wall, to within the buffer region and to the channel center. The particle Reynolds number in these cases varied from 40 to about 500. All relevant length and time scales of turbulence, attached boundary layers on the particles, and particle wakes are faithfully resolved. The results from the direct numerical simulation are compared with the corresponding predictions based on standard drag and lift relations, added-mass, and Basset history formulation. The details of wake dynamics and its influence on turbulence and wall shear stress is also considered. [Preview Abstract] |
Sunday, November 20, 2005 11:09AM - 11:22AM |
BE.00002: Fully-resolved numerical simulation of 1024 sedimenting spheres Andrea Prosperetti, Zhongzhen Zhang, Lorenzo Botto The dynamics of a suspension of finite-size particles settling under gravity in a Newtonian fluid is simulated. The ``Physalis'' numerical method is used to fully resolve the flow around the spheres at finite particle Reynolds number, with an elastic-collision model. Of interest in the investigation is the self-organization of the disperse phase and its effect on the sedimenting behavior. Particle clustering and anisotropy are found to be prominent features of the system. The suspension displays preferential orientation at scales comparable to the particle dimension. Fluctuations in the mean particle settling velocity are shown to be intimately linked to the anisotropy of the microstructure. The particle Lagrangian time scale in the direction gravity is larger than in the orthogonal directions and, as a consequence, a similar difference is found between the vertical and horizontal self-diffusion coefficients. [Preview Abstract] |
Sunday, November 20, 2005 11:22AM - 11:35AM |
BE.00003: Direct Numerical Simulation of Particles Dispersion in Turbulent Rayleigh-B\'{e}nard Flow in a Slender Closed Cylinder Paolo Oresta, Roberto Verzicco, Alfredo Soldati In this work, we use Direct Numerical Simulation of turbulence and Lagrangian tracking to investigate on dispersion and deposition of particles swarms in a Rayleigh-B\'{e}nard flows. We consider a closed, circular and slender cylinder heated from below and we solve the flow explicitly down to the smallest scales with a finite difference solver. We then track swarms of different size inertial particles to investigate on their dispersion and deposition. The problem is of fundamental significance in nuclear reactor safety issues and in environmental flows. The flow is of particular significants due to the interactions of Kolmogorov turbulence dynamics, characterizing the core region of the domain, with Bolgiano turbulence cascade mechanisms, characterizing the thermal layers. The particular statistics of this flow have an influence on particle dynamics. We will present results showing flow and particle statistics and we will examine particle instantaneous distribution and deposition mechanisms in connection with the dynamics of the flow structures. [Preview Abstract] |
Sunday, November 20, 2005 11:35AM - 11:48AM |
BE.00004: Consistent modeling of interphase turbulent kinetic energy transfer in particle--laden flows Ying Xu, Shankar Subramaniam This work is concerned with the mathematical modeling of particle-laden turbulent flows. An important term that needs to be modeled in turbulent two--phase flows is the interphase transfer of turbulent kinetic energy (TKE). Here we show that the sum of the transfer of turbulent kinetic energy (TKE) between the solid particle phase and the carrier fluid phase must equal the product of the mean slip and the mean interphase momentum source term. In the limit of zero mean slip this sum is zero, and the interphase TKE transfer is conservative, i.e., equal in magnitude but opposite in sign. We show that this constraint arises because of the interface boundary condition that requires the velocities in both phases to be the same at the interface, and because the instantaneous momentum transfer between the phases is equal and opposite in sign. This observation has important implications for modeling both the interphase TKE transfer term as well as the dissipation rate of TKE in the fluid phase. Representative multiphase turbulence models are analyzed from this perspective. [Preview Abstract] |
Sunday, November 20, 2005 11:48AM - 12:01PM |
BE.00005: Lagrangian Measurements of Inertial Particles in Wind Tunnel Turbulence Armann Gylfason, Sathya Ayyalasomayajula, Zellman Warhaft A large wind tunnel ($1 \times 1 \times 20$ m) with an active grid and a micro water spray system is being used to study the Lagrangian tracks of inertial particles ($0.1 < St < 10$) in high Reynolds number decaying turbulence ($50 < R_\lambda < 1000 $). We present results from 2D measurements of the inertial particle tracks, focusing on velocity structure functions and probability density functions, and discuss their extension to the 3D case. We describe the stringent specifications for such an experiment in which the high speed cameras are made to move with the mean velocity of the flow, and outline the methods used to correct for the vibrations of the equipment. The work is motivated by the need to understand the growth rate of water droplets in clouds and the clustering of aerosols in turbulent flows. Support is provided by the US National Science Foundation (NSF). [Preview Abstract] |
Sunday, November 20, 2005 12:01PM - 12:14PM |
BE.00006: Numerical simulation of scour around pipelines using an Euler-Euler coupling two-phase model Zhihe Zhao, Joe Fernando The scour around a long fixed pipeline initially located on a sandy bed is numerically simulated using an Eulerian two-phase model. This model implements Euler-Euler coupling of governing equations for both fluid and solid phases and a modified $k-\varepsilon $ turbulence closure for the fluid phase, the modeling system being a part of the computational fluid dynamics(CFD) software package Fluent. Both flow-particle and particle-particle interactions are considered in the model. During simulations, the interface between sand and water is specified using a threshold volume fraction of sand and the evolution of the bedforms is studied. The predictions of bedform evolution are in good agreement with previous laboratory measurements. Investigations into the mechanisms of scour reveal that the suspended-load and laminated-load transports are major contributors to the bedform evolution, and the former is the principal cause of scour. While some previously proposed scour development formulae for cylindrical objects are in good agreement with the simulations, scour predictions based on common operational mine-burial models show disparities with present simulations. [Preview Abstract] |
Sunday, November 20, 2005 12:14PM - 12:27PM |
BE.00007: Two-way Coupled Direct Simulation of Particle-laden Turbulent Flows Using Equilibrium Eulerian Approximation S. Balachandar, Babak Shotorban In the equilibrium Eulerian approximation, a velocity field is calculated for particles using a truncated series expansion in terms of the carrier-phase velocity field and its temporal and spatial derivatives (Ferry {\&} Balachandar, \textit{Int. J. Multiphase Flow }\textbf{27, }2001). Compared to other Eulerian-Eulerian approaches, this approach has the benefit of not solving the particle velocity differential equation; however, it is valid for sufficiently small particle time constants. The quilibrium Eulerian velocities can be used to solve the particle concentration equation (Rani {\&} Balachandar, \textit{Int. J. Multiphase Flow }\textbf{29}, 2003) and then the particle concentration is used to calculate the coupling term in the carrier-phase momentum equation. In order to assess the equilibrium Eulerian approximation in two-way coupling, the particle-laden homogenous turbulent shear flow is studied. Various statistics including the mean turbulent kinetic energy, its dissipation rate, the energy spectra and the Reynolds stresses are examined and compared against the results obtained through Eulerian-Lagrangian approach. [Preview Abstract] |
Sunday, November 20, 2005 12:27PM - 12:40PM |
BE.00008: A New Dual-timescale Langevin Model for Particle-laden Turbulent Flows Madhusudan Pai Gurpura, Shankar Subramaniam Accurate prediction of particle dispersion and interphase turbulent kinetic energy (TKE) transfer is important in modeling multiphase flows. Direct numerical simulations (DNS) of canonical particle-laden turbulent flows reveal that particle dispersion statistics and dynamics (TKE) evolve over different timescales. Furthermore, each timescale behaves differently with Stokes number St$_{\eta }$, a quantity that characterizes how quickly a particle responds to the carrier phase turbulent fluctuations at the Kolmogorov scale. In decaying turbulence, particles with large St$_{\eta}$ lose energy faster than particles with smaller St$_{\eta}$ while, in stationary turbulence, particles with larger St$_{\eta}$ lose correlation with their earlier velocities slower than particles with smaller St$_{\eta}$. It is desirable for two-phase turbulence models to capture these disparate timescales in canonical particle-laden flows in order to be predictive in more complex multiphase computations. A new dual-timescale Langevin model (DLM) is proposed that features different timescales for the drift and diffusion terms, and is successful in capturing the behavior of the disparate timescales associated with dispersion and dynamics in a two-phase flow. Model predictions are compared with results from DNS of canonical particle-laden turbulent flows. [Preview Abstract] |
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