Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session A32: Particle-Laden Flows: General I |
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Chair: Federico Toschi, Technische Universiteit Eindhoven Room: 2020 |
Sunday, November 23, 2014 8:00AM - 8:13AM |
A32.00001: Preferential accumulation and enhanced relative velocity of inertial droplet due to interactions with homogeneous isotropic turbulence Colin Bateson, Alberto Aliseda We present results from wind tunnel experiments on the evolution of small inertial ($d\approx 10-200~\mu m$) water droplets in homogeneous, isotropic, slowly decaying grid turbulence. High-speed imaging and Particle Tracking Velocimetry are used to calculate relative velocity distributions. We analyze the preferential concentration and enhanced relative velocity of droplets resulting from their inertial interactions with the underlying turbulence. The two-dimensional particle velocity, measured from PTV and long-time tracks along a streamwise plane, are conditionally analyzed with respect to the distance to the nearest particle. We focus on the non-normality of the statistics for the particle-particle separation velocity component to dissect the influence of the inertial interaction with the turbulence on the dynamics of the droplets. We observe a negative bias (in the mean and mode) in the separation velocity of particles for short separations, signaling a tendency of particles to collide more frequently than a random agitation by turbulence would predict. The 2-D Radial Distribution Function is also analyzed and compared to previous 1-D results. [Preview Abstract] |
Sunday, November 23, 2014 8:13AM - 8:26AM |
A32.00002: Clustering of vertically constrained passive particles in homogeneous and isotropic turbulence Michel van Hinsberg, Massimo De Pietro, Luca Biferale, Herman Clercx, Federico Toschi We analyze the dynamics of small particles confined within a horizontal fluid slab in a three-dimensional (3D) homogenous isotropic turbulent velocity field. Particles can freely move horizontally as fluid tracers but are vertically confined around a given horizontal plane via a simple linear restoring force. The present model may be considered as the simplest description for the dynamics of small aquatic organisms that, due to swimming, active regulation of their buoyancy or other mechanisms, are capable to maintain themselves in a shallow horizontal layer somewhere below the free surface of oceans or lakes. In the model varying the strength of the restoring force can control the thickness of the fluid slab in which the particles can move. Whenever some confinement is present, particle trajectories deviate from fluid tracers and experience an effectively compressible velocity field. We report a quantification of this effective compressibility as well as a quantification of preferential concentration of tracer particles in terms of the correlation dimension. We found that there exists a particular value of the force constant, corresponding to a mean slab depth approximately equal to a few times the Kolmogorov length scale, that maximizes the clustering of the particles. [Preview Abstract] |
Sunday, November 23, 2014 8:26AM - 8:39AM |
A32.00003: Quantitative prediction of clustering instabilities in gas-solid homogeneous cooling systems Christine Hrenya, Peter Mitrano, Xiaoqi Li, Xiaolong Yin Dynamic particle clusters are widely documented in gas-solid flow systems, including gasification units for coal or biomass, gravity-driven flow over an array of tubes, pneumatic transport lines, etc. Continuum descriptions based on kinetic theory have been known for over a decade to qualitatively predict the presence of such clustering instabilities. The quantitative ability of such continuum descriptions is relatively unexplored, however, and remains unclear given the low-Knudsen assumption upon which the descriptions are based. In particular, the concentration gradient is relatively large across the boundary between the cluster and the surrounding dilute region, which is counter to the small-gradient assumption inherent in the low-Knudsen-number expansion. In this work, we use direct numerical simulations (DNS) of a gas-solid homogeneous cooling system to determine the critical system size needed for the clustering instability to develop. We then compare the results to the same quantity predicted by a continuum description based on kinetic theory. The agreement is quite good over a wide range of parameters. This finding is reminiscent of molecular fluids, namely the ability of the Navier-Stokes equations to predict well outside the expected range of Knudsen numbers. [Preview Abstract] |
Sunday, November 23, 2014 8:39AM - 8:52AM |
A32.00004: Inertial Particle Caustics around a vortex Ravichandran Sivaramakrishnan, Rama Govindarajan Caustics are formed when particles with different velocities end up in the same location in space. Inertial particle caustics are thought to be responsible for the rapid onset of rainfall in vigorously convecting clouds. It is generally also believed that caustics are effective only at large Stokes numbers. We study inertial particles caustics in the canonical flow of a single vortex. We also show what kinds of vortices can have caustics around them. For point vortices, we show the existence of critical Stokes numbers below which there can be no caustics. We find that the value of the critical Stokes number depends on the initial conditions chosen for the particles. The existence of a critical Stokes number translates to the existence of definite regions around vortices where particles have to start in order to form caustics. We use this fact to perform simulations with many vortices in a periodic box. We use the lagrangian tracking technique of Osiptsov to track the densities associated with the particles. These simulations show that the effect of caustics on particle clustering is more complicated than is generally believed. While larger particle inertia leads to a larger number of very ``dense'' particles, we find that smaller inertia can lead to higher average densities. [Preview Abstract] |
Sunday, November 23, 2014 8:52AM - 9:05AM |
A32.00005: Backward transformation of the colored-noise Fokker-Planck equation for shear-induced diffusion processes of non-Brownian particles Laura Lukassen, Martin Oberlack As described in literature, non-Brownian particles in shear flow show a diffusive behavior due to hydrodynamic interactions. This shear-induced diffusion differs from the well-known Brownian diffusion, as there is no separation of time scales. That means that the configuration of non-Brownian particles changes on the same time scale as the hydrodynamic velocity. This fact impedes the derivation of a Fokker-Planck equation describing non-Brownian particles in pure position space. In this context, we derived a new Fokker-Planck approach in coupled position-velocity space to assure the validity of the Markov process assumption which is violated in pure position space formulation (Lukassen, Oberlack, Phys. Rev. E 89, 2014). Here, we present a further validation of our new Fokker-Planck approach that allows us to establish a relation to a modified purely position space Fokker-Planck equation. This backward transformation exhibits additional correction terms when compared to other position space Fokker-Planck equations in that context known from literature. Our extended approach shall enable a better stochastic description of non-Brownian particle flows. [Preview Abstract] |
Sunday, November 23, 2014 9:05AM - 9:18AM |
A32.00006: A Stochastic Model for the Relative Motion of High Stokes Number Particles in Isotropic Turbulence Rohit Dhariwal, Sarma Rani, Donald Koch In the current study, a novel analytical closure for the diffusion current in the PDF equation is presented that is applicable to high-inertia particle pairs with Stokes numbers $St_r \gg 1$. Here $St_r$ is a Stokes number based on the time-scale $\tau_r$ of eddies whose size scales with pair separation $r$. Using this closure, Langevin equations were solved to evolve particle-pair relative velocities and separations in stationary isotropic turbulence. The Langevin equation approach enables the simulation of the full PDF of pair relative motion, instead of only the first few moments of the PDF as is the case in a moments-based approach. Accordingly, PDFs $\Omega(U|r)$ and $\Omega(U_r|r)$ are computed for various separations $r$, where the former is the PDF of relative velocity $U$ and the latter is the PDF of the radial component of relative velocity $U_r$, both conditioned upon the separation $r$. Consistent with the DNS study of Sundaram \& Collins, the Langevin simulations capture the transition of $\Omega(U|r)$ from being Gaussian at integral-scale separations to an exponential PDF at Kolmogorov-scale separations. The radial distribution functions (RDFs) computed from these simulations also show reasonable quantitative agreement with those from the DNS of Fevrier et al. [Preview Abstract] |
Sunday, November 23, 2014 9:18AM - 9:31AM |
A32.00007: Measurements of polystyrene bead trajectories and spatial distributions in a turbulent water flow, square duct using high-speed digital holography Rene van Hout, Boris Rabencov, Javier Arca Near neutrally buoyant, polystyrene beads (583 micrometers) were tracked in a square (50x50mm$^{2}$), closed-loop, turbulent water duct at a bulk flow Reynolds number of 10,602 (friction velocity 0.0208m/s) using single view, inline digital holographic cinematography (at 1 kHz). The volume of interest (50x17.4x17.4mm$^{3}$) was positioned at the bottom part of the channel. The mean bead diameter normalized by inner wall coordinates was $d^{+}=$14.2, with Stokes numbers of 8.5. In-house developed algorithms, fine-tuned to tracking single and overlapping beads were developed. Bead in-focus positions were determined by maximum intensity gradient method. Results showed that in agreement with literature publications, ascending beads lagged the mean streamwise water velocity while descending ones had similar velocities. Average streamwise bead velocities and number densities collapsed onto wall-normal-streamwise and spanwise-streamwise planes, indicated preferential segregation of ascending and descending beads up to a height of 100 wall units. Spanwise ``lane'' separation distances ranged between 150-200 wall units, larger but of the same order as the spanwise extent of coherent near-wall turbulence structures. Duct corners were nearly devoid of beads likely caused by secondary flows. [Preview Abstract] |
Sunday, November 23, 2014 9:31AM - 9:44AM |
A32.00008: Scaling of sediment erosion rates for unsteady, nonequilibrium flows Ken Kiger, Kyle Corfman, Rahul Mulinti Traditional approaches to sediment transport are typically based on assumptions of fully developed, equilibrium conditions. Many flows, however, are dominated by the presence of intermittent, coherent large-scale structures, which may not satisfy such assumptions. The current study examines the erosion rates produced by a strongly forced air jet impinging on a mobile bed of fine glass beads. A parametric study is conducted using a range of mean flow and forcing conditions to elucidate the role of the dominant structure on the transport process. The evolution of the bed surface is compared to single-phase PIV measurements. The results show that the use of the time-averaged stress on the bed cannot be used to effectively predict the location or magnitude of erosion. Instead, it is shown that the erosion rate can be related to the location and magnitude of the periodic stress produced by the vortex interacting with the bed. After an initial transient, the erosion for all cases was observed to proceed at a relatively constant rate. The net removal rate was found to correlate closely to the unsteady stress produced by the periodic structures, and predicted the erosion rate for all of the conditions studied when scaled by the integral of the excess wall stress raised to the power 1.2. [Preview Abstract] |
Sunday, November 23, 2014 9:44AM - 9:57AM |
A32.00009: Reducing thermophoretic deposition in heat exchangers using wavy walled channels Zachary Mills, Alexander Alexeev Using computational simulations, we examined the effect of wavy walled geometries on the fouling of heat exchangers. Our model combines a lattice Boltzmann model for simulating the fluid flow, a finite difference temperature model and a Brownian dynamics model used to model the transport and deposition of aerosol particles. In our previous studies, we investigated how the geometry influences the structure of the flow within the channel. Specifically, we determined the critical pressure gradients at which the flow transitions between different flow regimes for various wave amplitudes and periods. We observed three separate flow regimes including steady flow with and without circulation and unsteady time-periodic flow. We have extended this investigation to examine the effects of these different geometries and flow regimes on heat and mass transport within the channel. In our simulations we investigated particle deposition resulting from convection and thermophoresis. From the results of our investigations we will be able to determine the geometries which reduce the rate of fouling in heat exchangers while increasing heat transport. [Preview Abstract] |
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