Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session A28: Particle-laden Flows: General I |
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Chair: Sarma Rani, University of Alabama in Huntsville Room: F149 |
Sunday, November 20, 2016 8:00AM - 8:13AM |
A28.00001: Effects of Deterministic and Stochastic Forcing Schemes on Inertial Particle Statistics in DNS of Isotropic Turbulence Rohit Dhariwal, Kiruthika Sundararajan, Sarma L. Rani In DNS of isotropic turbulence, statistical stationarity is achieved through a forcing scheme that supplies energy to the large scales. These schemes may be broadly classified into deterministic and stochastic forcing schemes. In the deterministic scheme, the turbulent kinetic energy dissipated during a time step is resupplied, whereas in stochastic schemes, forcing is determined based on the evolution of Ornstein-Uhlenbeck processes. Both approaches add the forcing within a band of wavenumbers at the low-wavenumber end of the energy spectrum. The goal of this study is to investigate the effects of the two forcing schemes on the flow, and on the relative motion statistics of inertial particles in forced isotropic turbulence. An important parameter in stochastic forcing is the forcing time scale $T_F$. DNS was performed using both forcing schemes for $T_F = T_E/4,~T_E/2,~T_E,~2T_E,~4T_E$. Here $T_E$ is the large eddy turnover time obtained from the DNS with deterministic forcing. Three Taylor micro-scale Reynolds numbers $Re_\lambda = 76,~131,~195$, and twelve particle Stokes numbers based on the Kolmogorov time-scale, $St_\eta = 0.05$ to $40$ are considered. Detailed analysis of the effects of forcing time scales on both fluid and particle statistics is undertaken. [Preview Abstract] |
Sunday, November 20, 2016 8:13AM - 8:26AM |
A28.00002: Comparison of Stochastic Theory and DNS for the Relative Motion of High-Inertia Particle Pairs in Isotropic Turbulence Sarma Rani, Rohit Dhariwal, Donald Koch In an earlier work, we derived closures in the limit of high Stokes number for the diffusivity tensor in the PDF equation for particle pairs. The diffusivity contained the time integral of the Eulerian two-time correlation of fluid relative velocities seen by pairs that are nearly stationary.The two-time correlation was analytically resolved through the approximation that the temporal change in the fluid relative velocities seen by a pair occurs principally due to the advection of smaller eddies past the pair by large scale eddies. Two diffusivity expressions were obtained based on whether the pair center of mass remained fixed during flow time scales, or moved in response to integral-scale eddies. A quantitative analysis of the stochastic theory is performed through a comparison of the pair statistics obtained using Langevin simulations with those from DNS. Langevin simulations of particle pair dispersion were performed using the diffusivity closures for four particle Stokes numbers based on the Kolmogorov time-scale, $St_\eta = 10,~20,~40,~80$ and at two Taylor micro-scale Reynolds numbers $Re_\lambda = 76,~131$. Statistics such as RDF, PDF, variance and kurtosis of particle-pair relative velocities were computed using both Langevin and DNS runs, and compared. [Preview Abstract] |
Sunday, November 20, 2016 8:26AM - 8:39AM |
A28.00003: Effects of Reynolds Number and Stokes Number on Particle-pair Relative Velocity in Isotropic Turbulence: An Experimental Study Zhongwang Dou, Andrew Bragg, Adam Hammond, Zach Liang, Lance Collins, Hui Meng Effects of Reynolds number ($R_{\lambda } )$ and Stokes number ($St)$ on
particle-pair relative velocity (RV) were studied using four-frame particle
tracking in an enclosed turbulence chamber. Two tests were performed:
varying $R_{\lambda } $ between 246 and 357 at six $St$ values, and varying
$St$ between 0.02 and 4.63 at five $R_{\lambda } $ values. By comparing
experimental and DNS results of mean inward particle-pair RV, $\left\langle
{w_{r}^{-} } \right\rangle $, we observed excellent agreement for all test
conditions across a large range of particle separation distance ($r)$;
however at $r\le 10\eta $ ($\eta $: Kolmogorov length scale), experimental
$\left\langle {w_{r}^{-} } \right\rangle $values were higher than
simulation. At fixed $St$,$\left\langle {w_{r}^{-} } \right\rangle $ was
found to be independent of $R_{\lambda } $ in the observable $St$, $r$, and
$R_{\lambda } $ ranges. At fixed $R_{\lambda } $, $\left\langle {w_{r}^{-} }
\right\rangle $ increased with $St$at small $r$ and decreased with $St$ at
large $r$. We further compared $\left\langle {w_{r}^{-} } \right\rangle $
and variance of RV, $\left\langle {w_{r}^{2} } \right\rangle $, between
experiments, DNS and theoretical predictions by Pan and Padoan (2010). At
$0 |
Sunday, November 20, 2016 8:39AM - 8:52AM |
A28.00004: Effects of elevated line sources on turbulent mixing in channel flow Quoc Nguyen, Dimitrios Papavassiliou Fluids mixing in turbulent flows has been studied extensively, due to the importance of this phenomena in nature and engineering. Convection effects along with motion of three-dimensional coherent structures in turbulent flow disperse a substance more efficiently than molecular diffusion does on its own. We present here, however, a study that explores the conditions under which turbulent mixing does not happen, when different substances are released into the flow field from different vertical locations. The study uses a method which combines Direct Numerical Simulation (DNS) with Lagrangian Scalar Tracking (LST) to simulate a turbulent channel flow and track the motion of passive scalars with different Schmidt numbers (Sc). The particles are released from several instantaneous line sources, ranging from the wall to the center region of the channel. The combined effects of mean velocity difference, molecular diffusion and near-wall coherent structures lead to the observation of different concentrations of particles downstream from the source. We then explore in details the conditions under which particles mixing would not happen. Results from numerical simulation at friction Reynolds number of 300 and 600 will be discussed and for Sc ranging from 0.1 to 2,400. [Preview Abstract] |
Sunday, November 20, 2016 8:52AM - 9:05AM |
A28.00005: Rolling and sliding motion of spheres propagating down inclined planes in still water Yi Hui Tee, Ellen Longmire In modelling the motion of spheres submerged in liquid, gravity, drag, lift, and added mass forces have to be taken into account. For spheres contacting bounding surfaces, friction coefficients due to rolling and sliding increase the complexity of the model. In this study, experiments are conducted to investigate the effects of particle density and diameter on the rolling and sliding motion of spheres. Spherical particles with marked surfaces are released from rest on an inclined glass plate in still water at various inclination angles and allowed to accelerate. A $45^{\circ}$ mirror mounted beneath the plate allows simultaneous capture of both longitudinal and spanwise motions of the sphere. Based on sequences obtained by high speed imaging, the translational and rotational velocities are determined. Particle Reynolds numbers at terminal velocity range from 400 to 2500 corresponding with Galileo numbers of 800 to 2800. By comparing the translational and rotational velocities, the occurrence of sliding motion can be identified. The onset of sliding motion is then determined as a function of inclination angle and Galileo number for multiple particle materials. The experimental results are also compared against the existing models from the literature. [Preview Abstract] |
Sunday, November 20, 2016 9:05AM - 9:18AM |
A28.00006: Shear resuspension and rheology of dense particles at moderate and large Re Esperanza Linares-Guerrero, Melany Hunt, Roberto Zenit We experimentally investigate the resuspension behavior of dense particles subjected to lateral shear in a Couette device for particle Reynolds numbers from 15 to 500. Before the shear is applied, the particles sediment and form a compact bed that sits in the bottom of an annular ring of the apparatus. At sufficiently high shear rate, applied by rotating the inner cylinder, the particles re-suspend and the bed expands reaching a steady state. The mean volume fraction of the bed is determined from the initial bed height, the Archimedes number, and the Stokes number. The measurements are compared with a model that predicts the bed expansion by considering a balance between the rate of settling and a shear induced Fickian flux for moderate to high Reynolds numbers. Good agreement was found between experiments and predictions, considering values of the diffusion coefficient found in the literature. Once the column has resuspended, a measurement of the effective viscosity is performed. We discuss the implications of the sedimenting granular phase on the rheology of the suspension. [Preview Abstract] |
Sunday, November 20, 2016 9:18AM - 9:31AM |
A28.00007: Fingering instability of a suspension film spreading on a spinning disk Mayuresh Kulkarni, Subhadarshinee Sahoo, Pankaj Doshi, Ashish Orpe We have experimentally investigated the spreading of a suspension drop when rotated atop a spinning disk using flow visualization techniques. The suspension is made of $50 \pm 10$ micron glass beads suspended in a low viscosity, partially wetting Newtonian liquid having same density as the glass beads. The suspension drop is placed centrally on a horizontal disc and the disc is then rotated at a desired speed. The spreading behavior is captured using a high speed camera and the acquired images are analysed to find the edges of the spreading film. For all the particle volume fractions ($\phi_{p}$) studied, the suspension drops spread radially until they reach a critical radius, following which the contact line develops instabilities which further grow into fingers. The critical radius for the onset of instability shows an increase with increase in the particle fraction ($\phi_{p}$) before decreasing slightly at the highest value of $\phi_{p}$ studied, while the instability wavelength ($\lambda$) exhibits a non-monotonic dependence. The value of $\lambda$ is close to that for a partially wetting liquid at lower $\phi_{p}$, it decreases with increasing $\phi_{p}$ to a minimum before increasing again at the largest $\phi_{p}$. [Preview Abstract] |
Sunday, November 20, 2016 9:31AM - 9:44AM |
A28.00008: A collision model for simulating dense suspensions 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 will present the collision modeling strategy for our code PARTIES (PARTicle-laden flows via immersed boundarIES), which includes lubrication, normal contact, and tangential contact forces. While our strategy is based on other collision models, we will highlight several improvements we have made and then demonstrate the effectiveness of the collision model in reproducing experimental results for binary particle-wall collisions as well as bulk transport rates for a laminar shear flow over a bed of thousands of spheres. [Preview Abstract] |
Sunday, November 20, 2016 9:44AM - 9:57AM |
A28.00009: A settling-driven instability in two-component, stably stratified fluids Ahmad Alsinan, Eckart Meiburg, Pascale Garaud We analyze the stability of stably stratified fluids whose density depends on two scalar fields, for situations where one of the scalar fields is unstably stratified and involves a settling velocity. Such conditions may be found, for example, in flows involving the transport of sediment and heat or salt. A linear stability analysis for constant-gradient base states demonstrates that the settling velocity generates a phase shift between the perturbation fields of the two scalars, which gives rise to a novel instability mode. This instability mechanism favors the growth of waves that are inclined with respect to the horizontal direction. It is active for all density and diffusivity ratios, including for cases in which the two scalars diffuse at identical rates. If the scalars have different diffusivities, the new instability mechanism competes with the dominant elevator mode of the classical double-diffusive instability. We present linear stability results as a function of the governing dimensionless parameters, including for lateral gradients of the base state density fields that result in predominantly horizontal intrusion instabilities. Highly resolved direct numerical simulation results serve to illustrate the nonlinear competition of the various instabilities for such flows in different parameter regimes. [Preview Abstract] |
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