Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session H3: Particle-Laden Flows: Particle Interactions |
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Chair: Katherine Prestridge, Los Alamos National Laboratory Room: 102 |
Monday, November 23, 2015 10:35AM - 10:48AM |
H3.00001: Experimental drag histories of shocked spherical particles Katherine Prestridge, Greg Orlicz, Adam Martinez The horizontal shock tube (HST) facility at Los Alamos is used to investigate the drag forces on micrometer-sized particles dispersed in air when they are accelerated by a shock. Eight-frame, high-speed particle tracking velocimetry/accelerometry (PTVA) diagnostics are implemented to measure the trajectory of individual particles with high spatial and temporal resolution, and a shadowgraphy system is used to measure the shock position on each image. We present experiments over a range of Reynolds numbers, Mach numbers, particle sizes, and particle densities that explore the drag forces on solid, spherical, non-deforming particles. Experimental drag coefficients are calculated from eight dynamic measurements of particle position versus time, for Mach 1.3 and Mach 1.2 experiments. Experimental results show drag coefficients significantly larger than those predicted by the standard drag model for solid, spherical particles. These results are consistent with measurements made by Rudinger (1970) and Sommerfeld (1985). We will present experimental results and analysis of unsteady drag as a function of particle Reynolds number, Mach number and Stokes number. [Preview Abstract] |
Monday, November 23, 2015 10:48AM - 11:01AM |
H3.00002: Interactions of non-spherical particles in simple flows Mehdi Niazi, Luca Brandt, Pedro Costa, Wim-Paul Breugem The behavior of particles in a flow affects the global transport and rheological properties of the mixture. In recent years much effort has been therefore devoted to the development of an efficient method for the direct numerical simulation (DNS) of the motion of spherical rigid particles immersed in an incompressible fluid. However, the literature on non-spherical particle suspensions is quite scarce despite the fact that these are more frequent. We develop a numerical algorithm to simulate finite-size spheroid particles in shear flows to gain new understanding of the flow of particle suspensions. In particular, we wish to understand the role of inertia and its effect on the flow behavior. For this purpose, DNS simulations with a direct-forcing immersed boundary method are used, with collision and lubrication models for particle-particle and particle-wall interactions. We will discuss pair interactions, relative motion and rotation, of two sedimenting spheroids and show that the interaction time increases significantly for non-spherical particles. More interestingly, we show that the particles are attracted to each other from larger lateral displacements. This has important implications for collision kernels. [Preview Abstract] |
Monday, November 23, 2015 11:01AM - 11:14AM |
H3.00003: Analytic expressions for first order correction to inviscid unsteady forces due to surrounding particles in a multiphase flow Subramanian Annamalai, S. Balachandar, Yash Mehta The various inviscid and viscous forces experienced by an isolated spherical particle situated in a compressible fluid have been widely studied in literature and are well established. Further, these force expressions are used even in the context of particulate (multiphase) flows with appropriate empirical correction factors that depend on local particle volume fraction. Such approach can capture the mean effect of the neighboring particles, but fails to capture the effect of the precise arrangement of the neighborhood of particles. To capture this inherent dependence of force on local particle arrangement a more accurate evaluation of the drag forces proves necessary. Towards this end, we consider an acoustic wave of a given frequency to impinge on a sphere. Scattering due to this particle (reference) is computed and termed ``scattering coefficients.'' The effect of the reference particle on another particle in its vicinity, is analytically computed via the above mentioned ``scattering coefficients'' and as a function of distance between particles. In this study, we consider only the first-order scattering effect. Moreover, this theory is extended to compressible spheres and used to compute the pressure in the interior of the sphere and to shock interaction over an array of spheres. [Preview Abstract] |
Monday, November 23, 2015 11:14AM - 11:27AM |
H3.00004: Validation of a Hertzian contact model with nonlinear damping Adam Sierakowski Due to limited spatial resolution, most disperse particle simulation methods rely on simplified models for incorporating short-range particle interactions. In this presentation, we introduce a contact model that combines the Hertz elastic restoring force with a nonlinear damping force, requiring only material properties and no tunable parameters. We have implemented the model in a resolved-particle flow solver that implements the Physalis method, which accurately captures hydrodynamic interactions by analytically enforcing the no-slip condition on the particle surface. We summarize the results of a few numerical studies that suggest the validity of the contact model over a range of particle interaction intensities (i.e., collision Stokes numbers) when compared with experimental data. [Preview Abstract] |
Monday, November 23, 2015 11:27AM - 11:40AM |
H3.00005: Simulating immersed particle collisions: the Devil's in the details Edward Biegert, Bernhard Vowinckel, Eckart Meiburg Simulating densely-packed particle-laden flows with any degree of confidence requires accurate modeling of particle-particle collisions. To this end, we investigate a few collision models from the fluids and granular flow communities using sphere-wall collisions, which have been studied by a number of experimental groups. These collisions involve enough complexities---gravity, particle-wall lubrication forces, particle-wall contact stresses, particle-wake interactions---to challenge any collision model. Evaluating the successes and shortcomings of the collision models, we seek improvements in order to obtain more consistent results. We will highlight several implementation details that are crucial for obtaining accurate results. [Preview Abstract] |
Monday, November 23, 2015 11:40AM - 11:53AM |
H3.00006: The collision efficiency of cloud droplets in a non-continuum gas Anubhab Roy, Donald Koch The collision efficiency of bidisperse drops in a non-continuum gas is determined, subject to the coupled driving forces of differential sedimentation and turbulent shear. A major source of uncertainty in predicting precipitation formation comes from the absence of reliable theoretical predictions for the collision efficiency. Since coalescence requires molecular contact between two drops, it is sensitive to the non-continuum gas flows and van der Waals (vdW) attractions occurring between colliding drops. As two drops interact, the disturbances to the velocity and pressure of the gas induced by the particle motion retard their rate of approach. An especially important aspect of the hydrodynamic interactions between drops (radii $a_{1}$ and $a_{2})$ is the lubrication interaction that occurs when the drop separation r is such that $h=r$-$a_{1}$-$a_{2}$\textless \textless 1. At such small separations, the relative velocity $w_{r}$ of the drops along their line-of centers induces a very large O($w_{r}$/$h)$ force. Since the forces driving this relative motion remain finite, $w_{r}$ will vanish as $h\to $0. This leads to a prediction that the collision efficiency would be zero if one considered the interaction of two drops in a continuum gas in the absence of attractive colloidal forces. Therefore, it is clearly essential to include an accurate description of all the relevant near field interactions to accurately predict the true collision efficiency. We will treat the coupled sedimentation and turbulent shear effects governing cloud droplets, treated independently in previous works. We show that it is the non-continuum effects rather than vdW that primarily allows finite collision efficiency for drop sizes $a$\textgreater 5$\mu $m at atmospheric conditions.~ [Preview Abstract] |
Monday, November 23, 2015 11:53AM - 12:06PM |
H3.00007: Is there solid-on-solid contact when spheres collide in a fluid? Narayanan Menon, Sumit Birwa, G. Rajalakshmi, Rama Govindarajan A solid sphere colliding with another sphere or a wall within a fluid reverses its velocity and bounces back when it is launched with a Stokes number above a critical value, St$_{\mathrm{c}} \quad \approx $ 10. Previous experiments showed that St$_{\mathrm{c}}$ is only weakly dependent on the material or roughness of the sphere, but did not have the spatial or temporal resolution to determine whether solid impact occurs in the collision. A calculation [1] in the lubrication approximation shows that it is possible for an elastic sphere to rebound under fluid forces alone, without contact between the solids. We report experiments which exploit electrical contact between a sphere and wall to study the collision with high temporal resolution. We find unambiguously that there is solid-on-solid contact when the sphere rebounds from a collision. Analysis of the time of contact, and the time between consecutive impacts, indicates that even when there is impact, fluid viscosity is the dominant dissipative mechanism. The exception is for very smooth spheres, at stokes numbers just above St$_{\mathrm{c}}$. We present calculations with the incompressible Navier-Stokes equations to assess viscous dissipation and pressure effects in the collision.\\[4pt] [1] R. H. Davis, J.-M. Serayssol, and E. Hinch, Journal of Fluid Mechanics 163, 479 (1986) [Preview Abstract] |
Monday, November 23, 2015 12:06PM - 12:19PM |
H3.00008: Experimental Exploration of Electrostatic Charge on Particle Pair Relative Velocity in Homogeneous and Isotropic Turbulence Adam Hammond, Zhongwang Dou, Anjan Tripathi, Zach Liang, Hui Meng Study of droplet collision and cloud formation should consider the effects of both turbulence and electrostatic charge on particle dynamics. We present the first experimental observation of radial relative velocity (RV) of charged particles in homogeneous and isotropic turbulence (HIT). Charges on particles were generated through triboelectric effect between the inner wall of the chamber and the particles. To measure charge distribution, a particle-laden head-on impinging flow mimicking our HIT chamber conditions was built and holographic particle tracking was applied to quantify particle charges by measuring their displacements in an electric field. Particles were observed to have opposite charges. Next, in our HIT chamber, we measured particle RV by a novel 4-frame particle tracking velocimetry technique with and without charges on particles, wherein charges were neutralized by coating the interior of the HIT chamber with conductive carbon paint. We compared RV under the same turbulence conditions between charged particles and neutral particles and observed that when particles were oppositely charged, their mean inward RV increased at small separation distances. This result is consistent with recent theory and simulations (Lu and Shaw, Physics of Fluids, 2015). [Preview Abstract] |
Monday, November 23, 2015 12:19PM - 12:32PM |
H3.00009: Correcting velocity and volume-fraction calculations in two-way-coupled, particle-laden-flow simulations Peter Ireland, Jesse Capecelatro, Rodney Fox, Olivier Desjardins In many flows, the motion of the carrier phase is altered by the presence of inertial particles. To alleviate the computational demands associated with resolving the boundary layers around these particles, volume-filtering is often applied to the underlying flow field, and model equations are solved for the forces on the particles. These model equations involve terms which depend on the fluid properties at the particle center in the absence of the disturbance induced by the particle (i.e., the `undisturbed fluid properties'). In a two-way-coupled simulation, however, we generally only have access to fluid properties after the particle-induced disturbance (i.e., the `disturbed fluid properties'). Using the disturbed fluid properties in the particle model equations leads to an under-prediction of the drag on the particles and an over-prediction of the particle settling velocity. We introduce analytical corrections to alleviate this issue for low particle Reynolds numbers, allowing us to recover undisturbed fluid properties from the disturbed fluid field, and thereby providing more accurate calculations of the particle velocity and drag. We show comparisons between the results with and without the corrections in both uniform Stokes flows and cluster-induced turbulent flows. [Preview Abstract] |
Monday, November 23, 2015 12:32PM - 12:45PM |
H3.00010: Modeling and Prediction of the Effects of Collisions in a Gas-Solid Turbulent Channel Flow Using Moment Methods Dennis Dunn, Kyle Squires Modeling dispersions of particles in multiphase flows is especially challenging in gas-solid suspensions. Lagrangian methods are suitable for dilute particle mediums, but are not cost effective at denser concentrations and impose additional modeling challenges. A moderately dense particle phase is neither sufficiently dense for a continuum limit assumption (collisional equilibrium) nor sufficiently dilute for a Lagrangian method, and resides in the intermediate regime under consideration in the current work. A quadrature-based moment method (QBMM) is chosen to simulate a particle-laden turbulent channel flow considering inter-particle collision effects. In quadrature-based approaches similarly behaving particles may be grouped together and treated in a stochastic manner within an Eulerian framework. Specifically, the Conditional Quadrature Method of Moments (CQMOM) is implemented to discretize a fully 3-D velocity space and capture particle trajectory crossing (PTC). This has the potential for large computational savings as compared to Lagrangian methods, especially when dense collisions are prominent. The probability density function is discretized with a two-point-quadrature in each dimension -- the minimum requirement to capture PTC and enforce collisions. Predictions of the channel flow demonstrate that the collision treatment leads to the expected effects (e.g., redistribution of kinetic energy) and also offer improved accuracy relative to simpler approaches. [Preview Abstract] |
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