Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session L35: Multiphase Flows: Turbulence 
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Chair: Francesco Zonta, Vienna Univ of Technology Room: 202A 
Monday, November 20, 2023 8:00AM  8:13AM 
L35.00001: Characterizing turbulenceinterface interaction in a twophase mixing layer Tanjina Azad, Yue Ling In a mixinglayer induced by two parallel streams of gas and liquid, the velocity difference between the two streams introduces a shear instability at the gasliquid interface. The interaction between the turbulence in the fast gas stream and the interface modulates the shear interfacial instability, including the selection of the most unstable mode and the transition from convective to absolute instabilities. Consequently, the modified shear instability exerts its influence downstream, impacting the formation of longitudinal interfacial waves, the occurrence of transverse RayleighTaylor instability at the wave crest, and the statistical characteristics of multiphase turbulence. In the present study, both linear stability analysis and direct numerical simulation (DNS) are conducted to investigate the effect of inlet gas turbulence intensity. In the linear stability analysis, the OrrSommerfeld equation was solved to analyze the spatiotemporal viscous modes, and the turbulent eddy viscosity model has been used to represent the effect of inlet gas turbulence intensity. In DNS, the volumeoffluid method has been used to capture the sharp gasliquid interface, and the pseudo turbulence at the gas inlet is generated by the digitalfilter approach. As the inlet gas turbulence intensity increases, the effective gas viscosity rises, resulting in a decrease of the Reynolds number and an increase in the gastoliquid viscosity ratio. The effects of these two parameters on interfacial instability have been investigated systematically. The results indicate that modification in Re is more important. We have chosen a benchmark case where the instability is convective in the absence of gas inlet turbulence. When Re decreases, the interfacial instability transitions from convective to absolute regimes. A new weakabsolute regime is identified, for which the absolute mode dominates the perturbations induced at the inlet, but its frequency varies with the inlet perturbation amplitude. In contrast, the frequency of the absolute mode will not be influenced by the inlet perturbations when the interfacial instability is in the absolute regime. 
Monday, November 20, 2023 8:13AM  8:26AM 
L35.00002: Quantifying bubble degassing in entraining freesurface turbulence Declan B Gaylo, Kelli L Hendrickson, Dick K Yue Understanding the volume and size distribution of bubbles due to air entrainment by strong freesurface turbulence is important to a variety of natural and engineering applications. In these freesurface flows, the balance between the creation of bubbles by entrainment and subsequent degassing is critical to the volume, as well as the size distribution, of bubbles. Previous work on strong freesurface turbulence has been limited to measuring the net effect of entrainment and degassing. In this work, we utilize Eulerian label advection bubble tracking (Gaylo et al., J. Compt. Phys., vol. 470, 2022) to separately measure entrainment and degassing. We study the period of active entrainment in direct numerical simulations of the canonical strong freesurface turbulence driven by an underlying shear flow. We find that the volume flux of bubbles due to degassing is similar in magnitude to the volume flux due to entrainment, showing that degassing plays an important role in determining the bubble population during active entrainment. Studying the sizedistribution of degassing bubbles, we highlight the importance of the bubble Froude number, Fr_{b, }characterizing the relationship between bubble buoyancy and the surrounding turbulence. For small Fr_{b }(large bubbles), we observe that the degassing rate scales roughly with bubble radius squared, consistent with the terminal velocity of a creeping bubble in quiescent flow. For large Fr_{b }(small bubbles), we show that the effects of turbulence lead to a weaker dependence of degassing rate on bubble radius. 
Monday, November 20, 2023 8:26AM  8:39AM 
L35.00003: Experimental investigation of viscous effects on the deformation of oil droplets in turbulence Xu Xu, Cecilia EstepaCantero, Rui Ni In recent years, much effort has been devoted to understanding the fundamental mechanism of bubble/droplet breakup in turbulent environment. However, relatively little attention has been put into the dynamics of deformation in turbulence, which contains clues to the interplay between bubble/droplet and eddies of various sizes. In this presentation, we will show experimental investigations of the deformation of oil droplets with various viscosity ratios ranging from 0.02 to 140 in turbulence, while surface tension is kept at a similar magnitude. We conducted 3D simultaneous measurements on both the droplet shape and the surrounding fluid flow, allowing us to study the complex couplings between slip velocity, local turbulence, droplet size, and 3D droplet deformation. These results provide new insights into how droplets/bubbles interact with turbulent eddies of various sizes. 
Monday, November 20, 2023 8:39AM  8:52AM 
L35.00004: Effect of Taylor rolls on twofluid turbulent TaylorCouette flow Naoki Hori, HaoRan Liu, Detlef Lohse, Roberto Verzicco We conducted direct numerical simulations of two immiscible and incompressible fluids in a TaylorCouette configuration. The Taylor number was set at 10^{8}, while we varied the volume fraction and the Weber number to explore the interaction between the interfacial structures and the underlying turbulent flow field. 
Monday, November 20, 2023 8:52AM  9:05AM 
L35.00005: Abstract Withdrawn

Monday, November 20, 2023 9:05AM  9:18AM 
L35.00006: Propagation of capillary waves in twolayer oil–water turbulent flow Francesco Zonta, Georgios Giamagas, Alessio Roccon, Alfredo Soldati We use pseudospectral Direct Numerical Simulation (DNS), coupled with a Phase Field Method (PFM), to investigate the turbulent Poiseuille flow of two immiscible liquid layers inside a channel. The two liquid layers, which have the same thickness (h_{1}=h_{2}=h), are characterized by the same density (ρ_{1}=ρ_{2}=ρ) but different viscosities (μ_{1} ≠ μ_{2}). The full problem is described in terms of three parameters: the shear Reynolds number (Re_{τ}, which quantifies the importance of inertia compared to viscous effects), the Weber number (We, which quantifies surface tension effects compared to inertia) and the viscosity ratio, λ, between the two fluids. In particular, we fix Re_{τ}=300, We=1, and we consider viscosity ratios in the range 0.25≤ λ=μ_{1}/μ_{2} ≤ 1. We focus on the role of turbulence in initially deforming the interface and on the subsequent growth of capillary waves. Compared to a single phase flow at the same shear Reynolds number (Re_{τ}=300), in the twolayers case we observe a strong interaction between the turbulent flow and the deformable liquidliquid interface. A full characterization of the interface deformation, in terms of spatiotemporal specta of wave elevation will be presented and discussed. 
Monday, November 20, 2023 9:18AM  9:31AM 
L35.00007: LES and PANS simulations of turbulent dambreak flow: Potential – kinetic energy interactions and flow features Haneesha Iphineni, Björn Windén, Sharath S Girimaji The dambreak problem is a classical benchmark case for studying fluid dynamics aspects of flood management, coastal protection system and waveenergy conversion. The flow is initiated by a sudden release of water from a reservoir. The potential energy of the water in the reservoir is converted into kinetic energy as the water column collapses under the influence of gravity. The airwater interface deforms as the wavefront propagates downstream. The rushing waters exert force on any structures or obstacles along its path. Turbulence is generated during the potentialkinetic energy conversion initially and from shear at later times. We perform large eddy simulations (LES) and partially averaged Navier Stokes (PANS) simulations to investigate various aspects of the dambreak flow. By analyzing the flow patterns, pressure distribution, and forces acting on the dam and surrounding structures, we gain valuable insights into the underlying physics. This understanding can lead to reducedorder models of practical utility. 
Monday, November 20, 2023 9:31AM  9:44AM 
L35.00008: Abstract Withdrawn

Monday, November 20, 2023 9:44AM  9:57AM 
L35.00009: Assessment of a dualscale approach to LES modeling of propagating interfaces Nihar Rameshbhai Thakkar, Marcus Herrmann Deflagration fronts and phase interfaces undergoing phase change are just two examples of interfaces propagating normal to themselves through a background flow field that can be turbulent. To model these propagating interfaces in turbulent flows using a Large Eddy Simulation (LES) approach, a dual scale model based on the volume of fluid method was recently proposed. The model reconstructs the subfilter interfacial velocity field on a highresolution overset narrow band mesh and uses it to transport a fully resolved realization of the interface. Unclosed interfacial terms on the LES scale can then be closed by explicit filtering of the fully resolved interface geometry. 
Monday, November 20, 2023 9:57AM  10:10AM 
L35.00010: The effect of forcing schemes on the statistical properties of twophase turbulent flows Suhas S Jain, Ahmed Elnahhas One of the simplest forms of a turbulent twophase flow is drops/bubbles suspended within a homogeneous isotropic turbulent carrier phase. While the equilibrium state of the singlephase counterpart is very well understood, that of the twophase system remains unexplored. This is due to a lack of knowledge on the effect of forcing schemes that maintain constant properties of the system, especially for systems with nonunity density ratios between the carrier and dispersed phases. Prior studies with nonunity density ratios report the statistics of the flow as the turbulence decays in time, where the statistics could be susceptible to the initial unphysical state of the system. In this talk, we present a controllerbased forcing scheme capable of forcing one or both phases simultaneously, maintaining a constant individualphase kinetic energy and the surface energy between the two phases. The effect of such a forcing, and its variants, on the statistical properties of each phase, such as the integral kinetic energy, Reynolds number, energy spectra, and bubble/droplet size distributions are explored as the physical properties such as the density ratio and Weber number, amongst others, are varied. 
Monday, November 20, 2023 10:10AM  10:23AM 
L35.00011: Particle turbulent diffusion, bias, and modulation Eric Loth This work reviews recent advances in turbulent diffusion of particles, the bias effects of turbulence on particle velocity, and the effects of particles on turbulence. Based on the Hinze’s theory of particle motion for a theoretical kinetic energy spectrum, turbulent diffusion as well as absolute and relative particle velocity fluctuations can be related to directional particle Stokes numbers at the microscale and integral scale for a broad range of inertias and drift parameters. The influence of turbulence can also lead to biases on the mean particle motion and these include nonlinear drag bias, preferential bias, clustering bias, diffusiophoresis, and turbophoresis. These effects can be considered individually to show their controlling parameters and it is often reasonable to linearly combine their effects (as demonstrated by comparison with experimental and computational results). Lastly, predicting turbulence modulation has generally proved very difficult, but a new lengthscale approach based on turbulent dissipation is found to predict trends for flows that are nearly homogenous and isotropic as well as for pipe flows. 
Monday, November 20, 2023 10:23AM  10:36AM 
L35.00012: Comparing phasespace and phenomenological modeling approaches for Lagrangian particles settling in a turbulent boundary layer Andrew P Grace, David H Richter, Andrew D Bragg Under the right circumstances, particles (such as sand, dust, pollen, or water droplets) settling through the lowest 100 meters of the atmospheric boundary layer can experience a net enhancement in their average settling velocity due to their inertia. Since this enhancement arises from their interactions with the surrounding turbulence it must be modelled at coarse scales. Exact phase space methods can be used to model the physical mechanisms responsible for the enhanced settling, and these individual mechanisms themselves can be estimated or modelled to build a more general parameterization of the enhanced settling of inertial particles. In this presentation, we will describe recent work focused on using DNS data of a turbulent boundary layer coupled to Lagrangian point particles to estimate profiles of a driftdiffusion based parameterization of the fluid velocity sampled by the settling particles, which is key for determining the settling velocity enhancement of particles with low to moderate Stokes number. Our goal is to use these profiles to evaluate the efficacy of phenomenological modelling approaches for the enhanced settling velocity of inertial particles for particles with varying friction Stokes numbers and settling velocity parameters. We show that by increasing the settling velocity parameter at fixed moderate friction Stokes number, the drift component captures the increasing strength of Wang and Maxey's preferential sweeping mechanism, while the diffusion coefficient decreases due to the crossing trajectories mechanism. We then use these profiles to argue that the eddydiffusivitylike closure commonly used in phenomenological models is incomplete, relying on inadequate empirical corrections. We will finish the talk with a brief discussion of opportunities for reconciling exact phase space approaches with simpler phenomenological approaches for use in coarsescale weather models. 
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