Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session A8: Particles: Gravity and Settling |
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Chair: Kenneth Kiger, University of Maryland Room: 25A |
Sunday, November 18, 2012 8:00AM - 8:13AM |
A8.00001: Settling Regimes of Inertial Particles in Turbulence Garrett Good, Peter Ireland, Ewe Wei Saw, Gregory Bewley, Eberhard Bodenschatz, Zellman Warhaft We present numerical and experimental evidence regarding the (1) enhancement and (2) reduction of particle settling speeds from their Stokes velocities by turbulence. Settling enhancement has long been attributed to particle path biases while various models have been proposed for settling reduction, including vortex trapping, loitering, and most commonly, non-linear drag effects. There is disagreement between experiment and DNS, and this work in particular is motivated by that highlighted in the entrainment study of Ireland \& Collins (2012). Here, we take a new look at particles with both enhanced and reduced settling velocities, and focus on the transition between these two regimes and how it is affected by the large- and small-scale features of the turbulence. The numerical results show the relevant parameter space in unprecedented detail, while the experiments use a new apparatus which creates uniquely adjustable turbulence at its center (generated by thirty-two randomly driven loudspeaker jets) to measure settling speeds in real flows. We address current contention, and aim to paint a more comprehensive picture of particle settling. [Preview Abstract] |
Sunday, November 18, 2012 8:13AM - 8:26AM |
A8.00002: Dynamics of gravity-driven, particle-laden thin-film flows Aliki Mavromoustaki, Andrea Bertozzi Our study focusses on gravity-driven, particle-laden flows which are pertinent to a wide range of industrial and geophysical settings in which transport of suspensions occur. In the case of gravity-driven, single-species, particle-laden flows, there exist three distinct regimes which are dependent on the plane angle of inclination and bulk particle volume fraction: settling of particles to the substrate versus settling to the front of the flow and, an intermediate, unstable, ``well-mixed'' regime. The dynamics is described by a previously derived equilibrium model, using lubrication theory, based on a balance between hindered settling and shear-induced migration; this consists of a coupled system of hyperbolic conservation equations which describe the interface position and the particle volume fraction. We investigate the governing system analytically and numerically; an analysis of the governing equations exhibits rich mathematical structure where we observe the formation of double-shock wave solutions while, as the limit of maximum permissible particle concentration is approached, the numerical solutions are described by singular shocks. Finally, we discuss the physical interpretation of our solutions as applied to the experimental setting. [Preview Abstract] |
Sunday, November 18, 2012 8:26AM - 8:39AM |
A8.00003: Dynamics of particle settling and resuspension in viscous liquid films Andrea Bertozzi, Nebojsa Murisic, Benoit Pausader, Dirk Peschka We develop a dynamic model for suspensions of negatively buoyant particles on an incline. Our model includes settling due to gravity as well as resuspension of particles by shear-induced migration. We consider the case where the particles settle onto the solid substrate and two distinct fronts form, namely a faster liquid and a slower particle front. We show that the resulting transport equations for the liquid and the particles are of hyperbolic type, and study the dilute limit, for which we compute exact solutions. We also carry out systematic laboratory experiments, focusing on the motion of the liquid and the particle fronts. We show that the dynamic model predictions for small to moderate values of the particle volume fraction and the inclination angle of the solid substrate agree well with the experimental data. [Preview Abstract] |
Sunday, November 18, 2012 8:39AM - 8:52AM |
A8.00004: Effect of gravity on the preferential concentration of heavy particles Yongnam Park, Changhoon Lee The effects of gravity on the preferential concentration of heavy particles are investigated in forced isotropic turbulence using direct numerical simulation. Preferential concentration of heavy particles is usually observed for particles with St$\approx $ 1 and the mechanism of preferential concentration is strongly related with the vortical structure of background flows. In this study we found that strong gravity causes a different kind of preferential concentration for high Stokes number particles which is not related with the vortical motion of fluid. We introduce average distance concept between particles for quantitative analysis of preferential concentration, and suggest that the closest distance between particles is a good indicator of preferential concentration. Moreover, we investigate the effect of gravity on the geometric nature of heavy particle's trajectories such as curvature and torsion. PDF of curvature and torsion are determined by the Gaussian distribution of particle velocity, and thus their PDFs are not modified by gravity as long as the particle's velocity maintains the Gaussian characteristics. More detailed analysis will be discussed in the meeting. [Preview Abstract] |
Sunday, November 18, 2012 8:52AM - 9:05AM |
A8.00005: A Voronoi Analysis of Preferential Concentration of Heavy Particles in Active Grid Generated Turbulence Martin Obligado, Alain Cartellier, Mickael Bourgoin Particle laden flows are of relevant interest in many industrial and natural systems. When the carrier flow is turbulent, a striking feature is the phenomenon called preferential concentration: particles denser than the fluid have the tendency to inhomogeneously distribute in space, forming clusters and depleted regions. We present a study on the preferential concentration and clustering in homogeneous and isotropic turbulence based on Voronoi diagrams. We have formerly quantified preferential concentration as a function of the Stokes number (defined as the ratio of the particle viscous relaxation time to dissipation timescale of the flow) in moderate turbulence conditions, up to Reynolds number based on Taylor microscale of the order of $R_\lambda \sim 120$. Using an active grid recently implemented in our windtunnel, we investigate in the present study the effect of Reynolds number on particles clustering in the range $R_\lambda \sim 200-400$. Clustering level is found to be significantly higher than previous measurements at lower Reynolds number. We also present an analysis of the geometry of clusters and voids and investigate the possible connection with stick-sweep mechanisms using direct numerical simulation data of homogeneous isotropic turbulence. [Preview Abstract] |
Sunday, November 18, 2012 9:05AM - 9:18AM |
A8.00006: How gravity and size affect the acceleration statistics of bubbles in turbulence Vivek N. Prakash, Yoshiyuki Tagawa, Enrico Calzavarini, Julian Martinez Mercado, Federico Toschi, Detlef Lohse, Chao Sun We report results from a Lagrangian experimental investigation in the largely unexplored regime of very light (air bubbles in water) and large particles ($D / \eta >> 1$) in turbulence. Using a traversing camera setup and particle tracking, we study the acceleration statistics of $\sim 3 \ mm$ diameter ($D$) bubbles in a water tunnel with nearly homogeneous and isotropic turbulence generated by an active-grid. The experiments reveal that gravity increases the acceleration variance and reduces the intermittency of the PDF in the vertical direction. Moreover, the experimental acceleration PDF shows a substantial reduction in intermittency at growing size ratios, in contrast to neutrally buoyant or heavy particles. All these results are closely matched by numerical simulations of finite-size bubbles with Fax\'en corrections. [Preview Abstract] |
Sunday, November 18, 2012 9:18AM - 9:31AM |
A8.00007: Modeling Gravitational Settling of Inertial Particles in Turbulent Like Flow Sathyanarayana Ayyalasomayajula, Soham Banerjee, Zellman Warhaft Gravitational settling of inertial particles in a turbulent fluid is an important aspect of many natural and engineering flows. The recent experiments of Good et al 2012, {\em J. Fluid Mech}, 694, and earlier experiments reveal the presence of two effects, loitering and fast-tracking that modulate the gravitational settling rates of the inertial particles in a turbulent flow. The equivalent DNS simulations of Ireland \& Collins 2012, \textit{J. Fluid Mech}, 704, in many respects show good agreement with the experiments but the magnitude of loitering and fast-tracking is much smaller. The Vortex model of Ayyalasomayajula et al 2008, \textit{Phys. Fluids}, 095104, was earlier used to study the inertial particle acceleration statistics in turbulent like model flow to explain the reduced inertial particle accelerations. The Vortex model is again used here in this current study to better understand the nature of fast-tracking and loitering effects for a turbulent like fluid flow. We present results where the parametric variation of fluid flow intensity (analogous to changing Re), Stokes numbers and gravity is performed and the relative changes of loitering and fast-tracking effects are studied. We offer possible explanations for the discrepancy between the experiments and DNS also. [Preview Abstract] |
Sunday, November 18, 2012 9:31AM - 9:44AM |
A8.00008: Effect of ambient flow inhomogeneity on shear-induced lift on a sphere at finite Reynolds number Jungwoo Kim In particle-laden flows involving particle transport and dispersion, the prediction capability of hydrodynamic forces on the particle in a non-uniform flow is one of the central issues. However, existing analytical expressions and empirical correlations are mainly based on uniform or other simple linear ambient flows such as uniform shear and uniform vortex. Therefore, the objective of this study is to investigate the effect of flow inhomogeneity on shear-induced lift on a sphere. To do so, we perform direct numerical simulations of a sphere in an inhomogeneous shear. One of the inhomogeneous shear flows considered is the sine profile having the form of $u(x,y)/U_\infty =1+K\sin (2\pi \frac{y/D}{L/D})$. Here, $U_\infty $ is the fluid velocity at the center of the sphere, $D$ the sphere diameter, $L$ the period of the sine profile. Also, the Reynolds number is $\frac{U_\infty D}{\nu }=100$. The present simulations show that the lift forces are decreased with increasing the degree of the flow inhomogeneity (that is, $D/L)$ while the non-dimensional shear rate at the location of sphere center is fixed to be $2\pi \frac{K}{L}=0.1$. Comparing the change in the lift force with respect to surface-averaged vorticity under uniform inlet shear, that under inhomogeneous shear has certain systematic deviations. In the final presentation, more details of the shear-induced lift on a sphere in inhomogeneous shear flows considered would be presented. [Preview Abstract] |
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