Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session X08: Multiphase Flows: Turbulence (10:45am  11:30am CST)Interactive On Demand

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X08.00001: Global and local statistics in turbulent emulsions Lei Yi, Federico Toschi, Chao Sun Turbulent emulsions are complex systems characterized by a coupling between smallscale droplets and largescale rheology. By using a specifically designed TaylorCouette shear flow system, we can characterize the statistical properties of a turbulent emulsion made of oil droplets dispersed in an ethanolwater solution, at the oil volume fraction up to 40{\%}. We find that the dependence of the droplet size on the Reynolds number of the flow at the volume fraction of 1{\%} can be well described by Hinze's criterion. The droplet sizes are found to have a lognormal distribution, hinting at a fragmentation process in the droplet formation. Additionally, the effective viscosity of the turbulent emulsion increases with the volume fraction of the dispersed oil phase, and decreases with increasing shear strength. We find that the dependence of the effective viscosity on the shear rate can be described by the HerschelBulkley model, with a flow index decreasing with increasing the oil volume fraction. This finding indicates that the degree of shear thinning increases with the volume fraction of the dispersed phase. The current findings have important implications for bridging the knowledge on turbulence and lowReynoldsnumber emulsion flows to turbulent emulsion flows. [Preview Abstract] 

X08.00002: SubHinze scale bubble production in turbulent bubble breakup Ali\'enor Rivi\`ere, Wouter Mostert, St\'ephane Perrard, Luc Deike Through direct numerical simulations we study the dynamic of bubble breakup under isotropic and homogeneous turbulence. We create the turbulent flow by forcing in physical space and inject a bubble of initial radius $d_0$ once a stationary state is reached. We investigate the effect of the Weber number (ratio of turbulent and surface tension forces) on the breakup dynamics and statistics from large ensemble of simulations. We identify three regimes depending on the bubble size: below the critical size, $d_H$, the Hinze scale, bubbles are stable; close to the critical conditions $d_0 \approx d_H$ we observe binary and tertiary breakups, leading to bubbles mostly between $0.5d_H$ and $d_H $, a signature of a production process local in scale. Finally for bubbles much larger than $d_H$ numerous bubbles much smaller than the critical size are produced: typically between $0.1 d_H$ and $0.3d_H$. We show that their formation relates to rapid large deformations and successive breakups: the first breakup in a sequence leaves highly deformed bubbles which will break again, without recovering a spherical shape and creating an array of much smaller bubbles. [Preview Abstract] 

X08.00003: Multiscale characterization of the effect of gas swirl on twofluid coaxial atomization. NathanaĆ«l Machicoane, Rodrigo OsunaOrozco, Peter D. Huck, Alan L. Kastengren, Alberto Aliseda This work aims at developing a better mechanistic understanding of the processes that control the instabilities leading to spray formation in coaxial twofluid atomization. The goal is to characterize the nearfield of the spray, where a dense multiscale turbulent multiphase flow evolves from the nozzle exit as the liquid breakup progresses and the formed inclusions interacts with the turbulent gas jet. We conduct experimental measurements in the optically dense region of the spray created by a canonical twofluid atomizer, using Synchrotron highspeed radiography at the Advanced Photon Source of Argonne National Lab. We retrieve not only the statistics of the liquid core length, as has been done in previous studies of atomization in a narrower range of spray parameters, but also its characteristic timescales, with the goal of providing a better picture of the dynamic processes that control this important spray metric. This work, together with a complementary study using shadowgraphy at overlapping swirl and gastoliquid momentum ratios, yields a complete description of the spray nearfield, over a wide range of parameters that are relevant for a broad scope of applications. [Preview Abstract] 

X08.00004: Breakage, coalescence and droplet size distribution of surfactantladen droplets Giovanni Soligo, Alessio Roccon, Alfredo Soldati The effects of surface tension modifications on the dynamics of surfactantladen droplets in wallbounded turbulence are numerically investigated: direct numerical simulations of the NavierStokes equations are used to compute the flow field, while a phasefield method in a twoorderparameter formulation is adopted to track the interface (firstorder parameter) and the surfactant concentration (secondorder parameter). The surfactant acts on the interface reducing the local surface tension according to its strength and local concentration. To investigate surface tension effects, here we change the reference value of the surface tension (i.e. that of a surfactantfree interface) and the strength of the surfactant. Enhancement of both the coalescence and breakage rate is observed as the average surface tension over the interface is reduced. We also compute the droplet size distribution and find a fair agreement with available analytic scaling laws and experimental measurements in the breakagedominated regime. [Preview Abstract] 

X08.00005: Coupled Population Balance and Large Eddy Simulation model for Polydisperse Droplet Evolution in a Turbulent Round Jet Aditya Aiyer, Charles Meneveau A population balance model coupled with large eddy simulations (LES) is adapted and applied to study the evolution of oil droplets in an axisymmetric turbulent jet including the effects of droplet breakup. We develop a hybrid approach where the inlet condition is prescribed using a one dimensional (1D) parcel model that accounts for the evolution of the dispersed phase along the jet centerline due to the combined effects of advection, radial turbulent transport and droplet breakup. LES results are compared to published experimental data, with good agreement and we examine the statistics of the velocity field and the concentration of the polydisperse oil droplet plumes for two droplet Weber numbers. We find that the centerline decay rate of the concentration for different droplet sizes is modified in the breakup dominated zone. Moreover, the transverse dispersion of larger droplets is suppressed due to trajectory crossing effects. Unlike Reynolds averaged approaches, LES also allows us to quantify size distribution variability due to turbulence. We quantify the radial and axial distributions and the variability of key quantities such as the Sauter mean diameter, total surface area and droplet breakup timescale and explore their sensitivity to the Weber number. [Preview Abstract] 

X08.00006: Rise Velocity of Bubbles in Turbulence Daniel Ruth, Marlone Vernet, Luc Deike Turbulence in the liquid phase impacts the mean rate at which bubbles rise or heavy particles sink through the fluid, with practical relevance in environmental and industrial processes in which bubbles mediate mass transfer between liquid and gaseous phases. With a turbulent water flow generated in the laboratory, we show that air bubbles significantly smaller than the integral length scale of the turbulence experience a reduction in mean rise speed $\langle v_z \rangle$ by an amount approximately equal to the scale of the turbulent fluctuations $u'$ (that is, $\langle v_z \rangle \approx v_\mathrm{q}  u'$, where $v_\mathrm{q}$ is the bubble rise speed in a quiescent environment), when the dimensionless intensity of the turbulence $\beta = u' / v_\mathrm{q}$ is less than 1. Our experiment employs planar particle image velocimetry to characterize the turbulence throughout the measurement volume and a twocamera stereo vision system to track bubbles in three dimensions. We then utilize the homogeneous, isotropic turbulence simulation from the Johns Hopkins Turbulence Database to integrate the MaxeyRiley equation for a pointbubble in a carrier flow, revealing the mechanisms responsible for the turbulence's reduction of the bubbles' rise velocity. [Preview Abstract] 

X08.00007: Phaseconditioned turbulent statistics of an immiscible buoyant jet in the nearfield Xinzhi Xue, Lakshmana Dora, Joseph Katz Simultaneous applications of PIV and PLIF in a refractive index matched facility provide the velocity and phase distribution in the nearfield of an immiscible buoyant oil jet in water. Close to the nozzle, vertical momentum exchange occurs as water is entrained and oil ligaments extend outward. KevinHelmholtz vortices form in the water with their centers located at the tip of oil ligaments. Further downstream, the momentum diffuses in both phases as the oil fragments into compound droplets. The spreading and decay rates of the centerline oil fraction are lower than those of the axial momentum. Comparison to a singlephase jet at the same Re shows that the transition from azimuthal shear layers to selfsimilar profiles of velocity and Reynolds stresses occur earlier in the oil jet. The phaseconditioned turbulence reveals differences in velocity and all Reynolds stress components. The peripheral turbulence in the water is higher near the jet exit, but lower at 67 diameters downstream, the latter owing to the intermittency of entrained water. Shear production dominants the turbulence production in the periphery of the jet fragmentation region. Near the centerline, the TKE production rate, hence the turbulent kinetic energy, is higher in in the water. [Preview Abstract] 

X08.00008: LES of dropletladen isotropic turbulence using artificial neural networks Andreas Freund, Antonino Ferrante Developing accurate SGS models for LES is important for making the simulation of highReynoldsnumber turbulence computationally feasible. We are particularly interested in the LES of decaying homogeneous isotropic turbulence laden with finitesize droplets. Only until recently has the DNS of such flows become possible (Dodd \& Ferrante, \textit{J.\@ Fluid Mech.\@} 806 (2016), 356412), meaning that their LES is relatively unstudied. Part of the challenge of creating LES models for such flows is that their multiphase nature introduces additional closure terms besides just SGS stress. Using DNS data, we analyze these terms a priori to determine which are sufficiently significant to warrant modeling. We then employ artificial neural networks to develop models for these terms and show in our a posteriori analysis that our LES faithfully reproduces the decay of TKE seen in DNS. [Preview Abstract] 
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