Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session FP: Particle-Laden Flows IV |
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Chair: Alan Kerstein, Sandia National Laboratories Room: Salt Palace Convention Center 251 D |
Monday, November 19, 2007 8:00AM - 8:13AM |
FP.00001: Inertial particle behavior in unsteady separated flow Gustaaf Jacobs We study the inertial particle dynamics in particle-laden unsteady separated flows inspired by the recent findings of an innovative, kinematic fluid particle separation theory. It was established that a flow that moderately fluctuates around an averaged flow state features separation of fluid particles along a material surface that emanates from a fixed starting location. Particles with small mass and size behave like fluid particles and their dispersion behavior near separation is the same. Larger and heavier particles (or droplets) with significant inertia have a slower response to the carrier fluid than fluid particles. The unresponsiveness of the fluid particles' ejection location to a fluctuating flow field is thus surely emulated by the even less responsive inertial particles. In flows with large (turbulent) fluctuations, fluid particles no longer separate from a fixed location. However, the slower responding inertial particles are likely to feature fixed separation. We show that inertial particles in a turbulent separated particle-laden flow eject away from the wall along distinct surfaces at a fixed location that coincides with the averaged carrier flow separation location. We illustrate this behavior by means of Direct Numerical Simulation of particle-laden two-dimensional shedding flows over backward-facing step and in a diffuser. [Preview Abstract] |
Monday, November 19, 2007 8:13AM - 8:26AM |
FP.00002: The Average Stress in Fluid-Particle Flows Andrea Prosperetti, Quan Zhang We present an analysis of the average stress in a disperse fluid-particle flow. It is shown that, in addition to the well-known stresslet contribution to the symmetric mixture stress, other contributions arise whenever the disperse-phase volume fraction or the particle-fluid relative velocity is non-uniform. Furthermore, even in the absence of external couples acting on the particles, in general the stress also acquires a non-symmetric contribution. The analysis is general and applicable in any Reynolds number regime. As an example, the general expressions are evaluated for the case of particles in Stokes flow. The dilute limit is treated by extending to the non-uniform case Batchelor's renormalization procedure, while computational ensemble averaging is used for dense systems. Estimates of the importance of the new terms are presented. [Preview Abstract] |
Monday, November 19, 2007 8:26AM - 8:39AM |
FP.00003: Particle clustering in turbulence: Bridging the gap between experiments and Simulations E.W. Saw, Juan Salazar, L.R. Collins, R.A. Shaw Direct numerical simulations (DNS) of inertial clustering in turbulence are usually performed with monodisperse particles having uniform-random initial spatial distribution. In these simulations, clustering statistics (e.g. radial distribution function) are obtained using full 3D spatial distribution. However, due to realistic constraints, most experiments to date are done with significant polydispersity, non-uniform particle initial spatial distribution and lower dimensional sampling. Here we pick as an example a recent wind-tunnel study of spray injected droplets using phase Doppler interferometers (PDI). Comparisons of results from both sources are often ambiguous. In view of this, we ran two DNS with polydisperse particles (matching experimental conditions). One of these initiates with a uniform-random spatial distribution while the other initiates with particle concentrated in the center of the simulation volume to imitate spray injection. We study the effect of polydispersity and initial inhomogeneity on the 3D radial distribution function. In addition, we resample the particles in 1D to simulate measurement by a PDI and compare the results with the wind-tunnel data. We will present our findings, with emphasis on how they will ease analysis of experimental results. [Preview Abstract] |
Monday, November 19, 2007 8:39AM - 8:52AM |
FP.00004: A priori test of particle clustering effect on subgrid modeling Wen Wang, Shankar Subramaniam, Richard H. Pletcher, Ying Xu, Rahul Garg, Madhusudan Pai Large eddy simulation has been widely used to study turbulent particle-laden flows in isotropic turbulence, channel flow and even complex engine flows. Subgrid scale models of interphase momentum transfer depend on particle size and response time. However, in most flows preferential concentration causes particle clustering, which introduces new length scales that need to be considered in a two-phase LES subgrid model. In this talk, we will first present the new length scales introduced by particle clusters and their relationship with flow and filter scales in a homogeneous isotropic turbulent flow field. Then the filtered flow field and interphase force field based on both cluster length scales and flow scales will be analyzed and the effect of particle Stokes number will be addressed. This result will provide a basis for a new subgrid scale model for large eddy simulation of turbulent particle-laden flows. [Preview Abstract] |
Monday, November 19, 2007 8:52AM - 9:05AM |
FP.00005: Spatio-Temporal Segregation-Pattern Drift in Particle-Laden Rimming Flow Estelle Guyez, Peter J. Thomas In Ref. [1] we described a new banding pattern developing from particle segregation in particle-laden flow inside a partially fluid-filled, horizontal, rotating cylinder. Hitherto we believed that the pattern was quasi stationary once developed. However, long-term observations have revealed that this is not the case. The system can display an extremely rich spatio- temporal behaviour that emerges as the patterns drift extremely slowly along the axis of rotation. Due to these low average pattern-drift velocities the complex system dynamics often only reveal themselves when conducting experiments extending over several weeks. Here we discuss some of the observed typical aspects of the long-term behaviour of the system for the first time. \newline \newline [1] Boote, O.A.M. \& Thomas, P.J. 1999 Effects of granular additives on transition boundaries between flow states of rimming flow, Phys. Fluids vol. 11, 2020-2029. [Preview Abstract] |
Monday, November 19, 2007 9:05AM - 9:18AM |
FP.00006: Measurements of the radial distribution function of inertial particles in turbulent flows. Stephanie Neuscamman, Sathyanarayana Ayyalasomayajula, Juan Salazar, Sergiy Gerashchenko, Lance Collins, Zellman Warhaft Inertial~particles (particles with a higher density than that of the surrounding fluid) in turbulent flows cluster: they are ejected from regions of high vorticity and concentrate in regions of high strain. The Radial Distribution Function (RDF) [S. Sundaram and L.R. Collins, J. Fluid Mech. 335, 1997; R. A.~Shaw et al, Q. J.R. Met. Soc. 128, 2002]~is used to quantify clustering. Computations show that it peaks for particle separations in the dissipation range. Here we describe~measurements of~two-dimensional RDFs in decaying high Reynolds number wind tunnel turbulence. The turbulence is produced using an active grid and the inertial particles are generated by water sprays. For this poly-dispersed droplet distribution the size distribution is determined using a phase doppler particle analyzer. The particles are illuminated using a laser~light~sheet and their positions are obtained using high speed cameras downstream of the grid. Our results are compared with measurements using a~phase doppler interferometer~in the same flow, and with other recent measurements in box turbulence. [Preview Abstract] |
Monday, November 19, 2007 9:18AM - 9:31AM |
FP.00007: Clustering of randomly advected low-inertia particles using an economical simulation method Jaekyoon Oh, Steven Krueger, Alan Kerstein In the EMPM (Explicit Mixing Parcel Model), turbulent advection of fluid is implemented by rearranging the fluid cells. Each permutation represents an individual turbulent eddy, and is called a ``triplet map.'' This implementation of the triplet map captures flow processes as small as the smallest turbulent eddy (Kolmogorov microscale), but the response of small droplets to turbulence has important features at scales as small as the droplet radius. Namely, droplet motion relative to the fluid at scales less than the Kolmogorov microscale induces droplet clustering that is estimated to increase droplet collision rates significantly. We have developed (Kerstein and Krueger 2006), implemented, and tested a 3D triplet map for droplets that captures this clustering effect. There is excellent agreement between our results and DNS (direct numerical simulation) results obtained by Reade and Collins (2000). We are now implementing a collision detection algorithm into the model so that we can simulate collisions and coalescence between finite-inertia particles. [Preview Abstract] |
Monday, November 19, 2007 9:31AM - 9:44AM |
FP.00008: Particle segregation by Stokes number for small neutrally buoyant spheres in a fluid Shane Ross, Phanindra Tallapragada The segregation of particles by Stokes number is investigated using sensitivity analysis of a simplified version of the Maxey-Riley equations appropriate for small neutrally buoyant spheres. By considering the sensitivity of the final location of a particle with respect to the initial velocity of the particle (relative to the fluid), we can partition the relative velocity space at each position. Considering particles in a periodic, cellular fluid flow, the partition gives the final cell location of a particle. The partition is a function of the separatrices in the underlying fluid flow and the Stokes number. We demonstrate how this partition framework can be used to segregate particles by Stokes number in a fluid. [Preview Abstract] |
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