Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session D29: Turbulence: ParticleLaden and Multiphase FlowsMultiphase Turbulence

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Chair: Andrew Bragg, Duke University Room: 205 
Sunday, November 19, 2017 2:15PM  2:28PM 
D29.00001: Is the kinetic equation for turbulent gasparticle flows illposed? Michael Reeks, David Swailes, Andrew Bragg We examine recent claims that the kinetic equation for dispersed gasparticle flows has the properties of a backward heat equation and as a consequence, its solutions will in the course of time exhibit finitetime singularities. We show that the analysis leading to this conclusion is fundamentally incorrect because it ignores the coupling between the phase space variables in the kinetic equation and the time and particle inertia dependence of the phase space diffusion tensor. This contributes an extra $+$ve diffusion that always outweighs the contribution from the ve diffusion associated with the dispersion along one of the principal axes of the phase space diffusion tensor. This is confirmed by a numerical evaluation of analytic solutions of these $+$ve and ve contributions to the particle diffusion coefficient along this principal axis. We also examine other erroneous claims and assumptions made in previous studies that demonstrate the apparent superiority of the GLM PDF approach over the kinetic approach. In so doing we give a more balanced appraisal of the benefits of both PDF approaches. [Preview Abstract] 
Sunday, November 19, 2017 2:28PM  2:41PM 
D29.00002: Effects of Homogenous Isotropic Turbulence on the Droplet Size Distribution and Clustering Rachael Hager, Ömer Savas In clouds, the main growth mechanism of droplets with diameters 1050 $\mu m$, known as the sizegap, is collision and coalescence. Atmospheric turbulence is known to increase the droplet growth rate in this range by enhancing the relative velocity between droplets and the formation of droplet clustering, thus increasing the droplet collision rate. The purpose here is to understand further how isotropic, homogeneous turbulence affects the evolution of the droplet size spectrum and the droplet concentration characteristics in the sizegap. Two sets of experiments are conducted in a 40cm Eaton box, at the center of which homogeneous turbulence is generated. Flow images are taken of aluminumoxide particles ranging from 0.55 $\mu m$ in various flow conditions using a continuous wave laser sheet. Particle clustering and flow structures are examined for a range of Stokes numbers, where clustering is quantified using the radial distribution function. Secondly, droplets with an average diameter of $\sim 10 \mu m$ are injected into the turbulence box under various flow conditions. PDA is used to study the development of the droplet size distribution in isotropic, homogeneous turbulence. [Preview Abstract] 
Sunday, November 19, 2017 2:41PM  2:54PM 
D29.00003: The role of collective effects on the enhancement of the settling velocity of inertial particles in turbulence Peter Huck, Colin Bateson, Romain Volk, Alain Cartellier, Mickael Bourgoin, Alberto Aliseda A particleladen homogeneous isotropic turbulence experiment is used to study the role that collective effects (e.g. particleparticle aerodynamic interactions ) have on the settling velocity of inertial particles (Stokes $0.1 < \mathrm{St} < 0.3$). Conditional averaging of the particle vertical velocity on the local concentration identifies three settling regimes: low concentration fasttracking, rapid increase in settling velocity at intermediate concentrations, and saturation at large concentrations. The latter effect, associated with fourway coupling, displays qualitative agreement with simulations in the literature and is a new experimental observation. Fluctuations up to an order of magnitude larger than the background volume fraction are measured using Voronöi analysis which is used in a model developed in the spirit of volumeaveraged multiphase flow methods, that gives a consistent interpretation, and quantitative predictive power, of the three settling regimes measured. [Preview Abstract] 
Sunday, November 19, 2017 2:54PM  3:07PM 
D29.00004: Turbulence modification in a turbulent particle laden channel flow with geometric perturbations in one wall Jesus RamirezPastran, Carlos DuqueDaza Experiments and numerical simulations of particleladen flows have shown that wallroughness level in a turbulent channel flow affects the statistics of both phases. It seems then that induction of geometric perturbations on the channel walls would allow modifying and controlling the particles distribution aiming to enhance processes of heat transfer and particle transport. In this work, Large Eddy Simulations were used to evaluate the effect of two different geometric arrangements (protuberances as well as cavity disturbances) on the particles distribution and the turbulent behavior of a particle laden channel flow. The goal of this work is to evaluate the possibility of controlling the particles distribution and the modification of turbulence by introducing geometric perturbations in one wall. The geometric perturbations are located at the bottom wall of the channel and imposed perpendicularly to the main flow. Measurements of kinetic energy budgets terms, particle concentration profiles, coherent structures, as well as an energetic balance are shown. Results seem to indicate that the geometric perturbations affect considerably the distribution of particles, the turbophoresis effect, as well as the turbulent behavior of the flow. [Preview Abstract] 
Sunday, November 19, 2017 3:07PM  3:20PM 
D29.00005: Deformation dynamics of thin flexible sheets in homogenous isotropic turbulence Vamsi Spandan, Maziyar Jalaal, Roberto Verzicco, Detlef Lohse The dynamics of rigid anisotropic bodies such as ellipsoids and rods immersed in turbulent flows has been studied extensively in the past both for fundamental scientific research and for its relevance in industrial applications. In this work, we move a step further by studying the deformation dynamics of thin flexible sheets immersed in a homogeneous isotropic turbulent environment. We use direct numerical simulations to simulate a coupled system of a fluid which is driven by stochastic forcing at the large scales and a thin flexible sheet, the deformation of which depends on local flow conditions. We analyse the effect of bending stiffness and density ratio on the response of the sheet and find new deformation modes which are absent in onedimensional fibers. We analyse these modes through characterising the distribution of the local curvatures. Furthermore, we provide results on turbulence modulation due to the presence of the sheet. [Preview Abstract] 
Sunday, November 19, 2017 3:20PM  3:33PM 
D29.00006: Atomisation and droplet formation mechanisms in a model twophase mixing layer Stephane Zaleski, Yue Ling, Daniel Fuster, Gretar Tryggvason We study atomization in a turbulent twophase mixing layer inspired by the Grenoble airwater experiments. A planar gas jet of large velocity is emitted on top of a planar liquid jet of smaller velocity. The density ratio and momentum ratios are both set at 20 in the numerical simulation in order to ease the simulation. We use a VolumeOfFluid method with good parallelisation properties, implemented in our code \url{http://parissimulator.sf.net}. Our simulations show two distinct droplet formation mechanisms, one in which thin liquid sheets are punctured to form rapidly expanding holes and the other in which ligaments of irregular shape form and breakup in a manner similar but not identical to jets in RayleighPlateauSavart instabilities. Observed distributions of particle sizes are extracted for a sequence of ever more refined grids, the largest grid containing approximately eight billion points. Although their accuracy is limited at small sizes by the grid resolution and at large size by statistical effects, the distributions overlap in the central region. The observed distributions are much closer to log normal distributions than to gamma distributions as is also the case for experiments. [Preview Abstract] 
Sunday, November 19, 2017 3:33PM  3:46PM 
D29.00007: Impact of interfacial instability on the multiphase turbulence statistics in a twophase mixing layer Yue Ling, Daniel Fuster, Gretar Tryggvason, Stephane Zaleski A gasliquid mixing layer formed between parallel gas and liquid streams is an important fundamental problem in turbulent multiphase flows. The velocity difference between the gas and liquid streams triggers a KelvinHelmholtz instability at the gasliquid interface, which develops into an interfacial wave moving downstream. The development of the interfacial wave perturbs the gas stream vorticity layer and the latter transitions from laminar to turbulent flows. The interfacial instability can be convective or absolute depending on the input parameters. In the present study we investigate a case in the absolute instability regime. As a result, a dominant frequency arises in the gasliquid mixing layer, which is well predicted by viscous instability theory. As the interfacial wave plays a critical role in the transition to turbulence and their development, the temporal evolution of turbulent fluctuations is found to follow a similar frequency. The turbulence statistics, including Reynolds stresses and turbulent kinetic energy (TKE) budget are computed. High mesh resolution is required to yield converged turbulent dissipation. Kolmogorov and Hinze scales are calculated based on the measured dissipation. [Preview Abstract] 
Sunday, November 19, 2017 3:46PM  3:59PM 
D29.00008: Transition to turbulence in particle laden flow Nishchal Agrawal, George Choueiri, Björn Hof The critical Reynolds number for transition to turbulence in particle laden flows, depends not only on the particle size but also on the particle concentration. Although this dependence is known to a certain extent, the mechanism of transition is not well understood. In this study, we experimentally investigate the effect of inertial particles on transition to turbulence in pipe flows. The particles used were spherical, monodispersed and neutrally buoyant. At low particle concentrations transition occur abruptly via localized structures called ‘puffs’, similar to that for a single phase Newtonian flow i.e., a subcritical transition, and the critical Reynolds number above which turbulence can be sustained decreases with particle concentration. At higher particle concentrations however, these localized structure cease to exist and turbulence arises continuously from laminar flow, unlike in the case of a subcritical transition. This suggests a mechanism of transition significantly different from that for a single phase Newtonian flow. We infer that at high concentrations of particles the subcritical transition is replaced by a supercritical transition. [Preview Abstract] 
Sunday, November 19, 2017 3:59PM  4:12PM 
D29.00009: A numerical study of flow around a sedimenting particle in a linearly stratified fluid at small Reynolds numbers Hojun Lee, Changhoon Lee Falling heavy particles are frequently found in the atmosphere or the ocean which is typically thermally stratified. In this study, numerical simulations are conducted for flow around a sedimenting sphere at low Reynolds number embedded in linearly stably stratified or linearly unstably stratified fluid using the decoupled monolithic projection method in order to investigate the effect of the stratification on the characteristics of flow over a sphere. Fluid considered in this study is air and water. The range of Reynolds number considered is $0.01 \leq Re \leq 10$ based on the sedimenting velocity and the sphere diameter, and the Rayleigh numbers are in the range of $10^{5}\leq Ra \leq 1$, where $Ra$ is defined by $Ra=g\beta\gamma d^4/\kappa\nu$ based on the temperature gradient $\gamma$ and the sphere diameter $d$. From simulations we observed a modification in drag and the flow structure due to stratification. The normalized enhanced drag coefficient linearly increases with Ra for very low Ra and increases with 1/3 power of Ra for large Ra. The unstably stratified fluid is found to decrease the drag for limited range of Rayleigh numbers beyond which the flow becomes unstable. [Preview Abstract] 
Sunday, November 19, 2017 4:12PM  4:25PM 
D29.00010: Statistics of the relative velocity of particles in bidisperse turbulent suspensions Akshay Bhatnagar, Kristian Gustavsson, Bernhard Mehlig, Dhrubaditya Mitra We calculate the joint probability distribution function (JPDF) of relative distances ($R$) and velocities (${\bf V}$ with longitudinal component $V_R$) of a pair of {\it bidisperse} heavy inertial particles in homogeneous and isotropic turbulent flows using direct numerical simulations (DNS). A recent paper (J. Meibohm, {\it et. al.} 2017), using statisticalmodel simulations and mathematical analysis of an onedimensional whitenoise model, has shown that the JPDF, $\mathcal{P}(R,V_R)$, for two particles with Stokes numbers, $St_1$ and $St_2$, can be interpreted in terms of $St_M$, the harmonic mean of $St_1$ and $St_2$ and $\theta \equiv \mid St_1St_2 \mid/(St_1+St_2)$. For small $\theta$ there emerges a smallscale cutoff $R_c$ and a smallvelocity cutoff $V_c$ such that for $V_R \ll V_c$ and $R \ll R_c$ the JPDF, $\mathcal{P}(R,V_R)$, is independent of $R$ and $V_R$. Beyond these two smallscale cutoffs the JPDF for the bidisperse case shows the same scaling behavior as the JPDF for {\it monodisperse} particles with $St = St_M$. Our DNS demonstrate that this is true and the scales $R_c$ and $V_c$ are proportional to $\theta$ for small $\theta$. [Preview Abstract] 
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