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Top 20 Most Read Articles
April 2013
The 20 articles with the most full-text downloads during the month, in descending order.
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Jet meandering by a foil pitching in quiescent fluid Phys. Fluids 25, 041701 (2013); http://dx.doi.org/10.1063/1.4800321 (7 pages) Online Publication Date: 4 April 2013
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The flow produced by a rigid symmetric NACA0015 airfoil purely pitching at a fixed location in quiescent fluid (the limiting case of infinite Strouhal number) is studied using visualizations and particle image velocimetry. A weak jet is generated whose inclination changes continually with time. This meandering is observed to be random and independent of the initial conditions, over a wide range of pitching parameters.
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Evaporation-induced saline Rayleigh convection inside a colloidal droplet Phys. Fluids 25, 042001 (2013); http://dx.doi.org/10.1063/1.4797497 (21 pages) Online Publication Date: 1 April 2013
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Inside evaporating two-component sessile droplets, a family of the Rayleigh convection exists, driven by salinity gradient formed by evaporation of solvent and solute. In this work, the characteristic of the flow inside an axisymmetric droplet is investigated. A stretched coordinate system is employed to account for the effect of boundary movement. A scaling analysis shows that the flow velocity is proportional to the (salinity) Rayleigh number (Ras) at the small-Rayleigh-number limit. A numerical analysis for a hemispherical droplet exhibits the flow velocity is proportional to the non-dimensional number Ras1/2, at high Rayleigh numbers. A self-similar condition is established for the concentration field irrespective of the Rayleigh numbers after a moderate time, and the flow field is invariant with time at this stage. The scaling relation for the high Rayleigh numbers is verified experimentally by using aqueous NaCl droplets.
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Multiscale considerations in direct numerical simulations of multiphase flows Phys. Fluids 25, 031302 (2013); http://dx.doi.org/10.1063/1.4793543 (13 pages) Online Publication Date: 19 March 2013
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Direct Numerical Simulations of multiphase flows have progressed rapidly over the last decade and it is now possible to simulate, for example, the motion of hundreds of deformable bubbles in turbulent flows. The availability of results from such simulations should help advance the development of new and improved closure relations and models of the average or large-scale flows. We review recent results for bubbly flow in vertical channels, discuss the difference between upflow and downflow and the effect of the bubble deformability and how the resulting insight allowed us to produce a simple description of the large scale flow, for certain flow conditions. We then discuss the need for the development of numerical methods for more complex situations, such as where the flow creates spontaneous thin films and threads, or where additional physical processes take place at a rate that is very different from the fluid flow. Recent work on capturing localized small-scale processes using embedded analytical models, focusing on the mass transfer from bubbles in liquids with low mass diffusivity, suggests one approach. We conclude by discussing immediate needs for progress on the theoretical framework for describing the large-scale motion of multiphase flows and the need for multiscale methods to capture physical processes taking place at diverse length and time scales.
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Experimental manipulation of wall turbulence: A systems approach Phys. Fluids 25, 031301 (2013); http://dx.doi.org/10.1063/1.4793444 (34 pages) Online Publication Date: 19 March 2013
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We review recent progress, based on the approach introduced by McKeon and Sharma [J. Fluid Mech. 658, 336–382 (2010)10.1017/S002211201000176X], in understanding and controlling wall turbulence. The origins of this analysis partly lie in nonlinear robust control theory, but a differentiating feature is the connection with, and prediction of, state-of-the-art understanding of velocity statistics and coherent structures observed in real, high Reynolds number flows. A key component of this line of work is an experimental demonstration of the excitation of velocity response modes predicted by the theory using non-ideal, but practical, actuation at the wall. Limitations of the approach and promising directions for future development are outlined.
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Hairpin vortex organization in wall turbulence Phys. Fluids 19, 041301 (2007); http://dx.doi.org/10.1063/1.2717527 (16 pages) Online Publication Date: 18 April 2007
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Coherent structures in wall turbulence transport momentum and provide a means of producing turbulent kinetic energy. Above the viscous wall layer, the hairpin vortex paradigm of Theodorsen coupled with the quasistreamwise vortex paradigm have gained considerable support from multidimensional visualization using particle image velocimetry and direct numerical simulation experiments. Hairpins can autogenerate to form packets that populate a significant fraction of the boundary layer, even at very high Reynolds numbers. The dynamics of packet formation and the ramifications of organization of coherent structures (hairpins or packets) into larger-scale structures are discussed. Evidence for a large-scale mechanism in the outer layer suggests that further organization of packets may occur on scales equal to and larger than the boundary layer thickness.
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On the fluctuating wall-shear stress in zero pressure-gradient turbulent boundary layer flows Phys. Fluids 23, 021704 (2011); http://dx.doi.org/10.1063/1.3555191 (4 pages) Online Publication Date: 24 February 2011
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Recent direct numerical simulation (DNS) results relating to the behavior of the fluctuating wall-shear stress τw,rms+ in turbulent boundary layer flows are discussed. This new compilation is motivated by a recent article [
Wu and Moin, “Transitional and turbulent boundary layer with heat transfer,” Phys. Fluids 22, 085105 (2010)
], which indicates a need for clarification of the value of τw,rms+. It is, however, shown here, based on other recent DNS data, that most results, both in boundary layer and channel geometry, yield τw,rms+ ≈ 0.4 plus a small increase with Reynolds number coming from the growing influence of the outer spectral peak. The observed discrepancy in experimental data is mainly attributed to spatial resolution effects, as originally described by
Alfredsson et al. [“The fluctuating wall-shear stress and the velocity field in the viscous sublayer,” Phys. Fluids 31, 1026 (1988)]
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Rebound and jet formation of a fluid-filled sphere Phys. Fluids 24, 122106 (2012); http://dx.doi.org/10.1063/1.4771985 (17 pages) Online Publication Date: 27 December 2012
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This study investigates the impact dynamics of hollow elastic spheres partially filled with fluid. Unlike an empty sphere, the internal fluid mitigates some of the rebound through an impulse driven exchange of energy wherein the fluid forms a jet inside the sphere. Surprisingly, this occurs on the second rebound or when the free surface is initially perturbed. Images gathered through experimentation show that the fluid reacts more quickly to the impact than the sphere, which decouples the two masses (fluid and sphere), imparts energy to the fluid, and removes rebound energy from the sphere. The experimental results are analyzed in terms of acceleration, momentum and an energy method suggesting an optimal fill volume in the neighborhood of 30%. While the characteristics of the fluid (i.e., density, viscosity, etc.) affect the fluid motion (i.e., type and size of jet formation), the rebound characteristics remain similar for a given fluid volume independent of fluid type. Implications of this work are a potential use of similar passive damping systems in sports technology and marine engineering.
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Reynolds number effect on the velocity increment skewness in isotropic turbulence Phys. Fluids 24, 015108 (2012); http://dx.doi.org/10.1063/1.3678338 (21 pages) Online Publication Date: 31 January 2012
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Second and third order longitudinal structure functions and wavenumber spectra of isotropic turbulence are computed using the eddy-damped quasi-normal Markovian model (EDQNM) and compared to results of the multifractal formalism. It is shown that both the multifractal model and EDQNM give power-law corrections to the inertial range scaling of the velocity increment skewness. For the multifractal formalism, this is an intermittency correction that persists at any high Reynolds number. For EDQNM, this correction is a finite Reynolds number effect, and it is shown that very high Reynolds numbers are needed for this correction to become insignificant with respect to intermittency corrections. Furthermore, the two approaches yield realistic behavior of second and third order statistics of the velocity fluctuations in the dissipative and near-dissipative ranges. Similarities and differences are highlighted, in particular, the Reynolds number dependence.
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Multiple scaling in the ultimate regime of thermal convection Phys. Fluids 23, 045108 (2011); http://dx.doi.org/10.1063/1.3582362 (6 pages) Online Publication Date: 26 April 2011
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Very different types of scaling of the Nusselt number Nu with the Rayleigh number Ra have experimentally been found in the very large Ra regime beyond 1011. We understand and interpret these results by extending the unifying theory of thermal convection [
Grossmann and Lohse, Phys. Rev. Lett. 86, 3316 (2001)
] to the very large Ra regime where the kinetic boundary-layer is turbulent. The central idea is that the spatial extension of this turbulent boundary-layer with a logarithmic velocity profile is comparable to the size of the cell. Depending on whether the thermal transport is plume dominated, dominated by the background thermal fluctuations, or whether also the thermal boundary-layer is fully turbulent (leading to a logarithmic temperature profile), we obtain effective scaling laws of about Nu∝Ra0.14, Nu∝Ra0.22, and Nu∝Ra0.38, respectively. Depending on the initial conditions or random fluctuations, one or the other of these states may be realized. Since the theory is for both the heat flux Nu and the velocity amplitude Re, we can also give the scaling of the latter, namely, Re∝Ra0.42, Re∝Ra0.45, and Re∝Ra0.50 in the respective ranges.
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Large-eddy simulation of a very large wind farm in a stable atmospheric boundary layer Phys. Fluids 23, 065101 (2011); http://dx.doi.org/10.1063/1.3589857 (19 pages) Online Publication Date: 3 June 2011
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When deployed as large arrays, wind turbines significantly interact among themselves and with the atmospheric boundary layer. In this study, we integrate a three-dimensional large-eddy simulation with an actuator line technique to examine the characteristics of wind-turbine wakes in an idealized wind farm inside a stable boundary layer (SBL). The wind turbines, with a rotor diameter of 112m and a tower height of 119m, were “immersed” in a well-known SBL case that bears a boundary layer height of approximately 175m. Two typical spacing setups were adopted in this investigation. The super-geostrophic low-level jet near the top of the boundary layer was eliminated owing to the energy extraction and the enhanced mixing of momentum. Non-axisymmetric wind-turbine wakes were observed in response to the non-uniform incoming turbulence, the Coriolis effect, and the rotational effects induced by blade motion. The Coriolis force caused a skewed spatial structure and drove a part of the turbulence energy away from the center of the wake. The SBL height was increased, while the magnitude of the surface momentum flux was reduced by more than 30%, and the magnitude of the surface buoyancy flux was reduced by more than 15%. The wind farm was also found to have a strong effect on vertical turbulent fluxes of momentum and heat, an outcome that highlights the potential impact of wind farms on local meteorology.
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Phys. Fluids 25, 043101 (2013); http://dx.doi.org/10.1063/1.4797499 (7 pages) Online Publication Date: 4 April 2013
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We study experimentally the impact of solid spheres in a viscous liquid at moderate Reynolds numbers (Re ∼ 5–100). We first determine the drag force by following the slowdown dynamics of projectiles. We then focus on the shape of the free surface: such impacts generate cavities, whose original shape is described and modeled.
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Fluid elasticity increases the locomotion of flexible swimmers Phys. Fluids 25, 031701 (2013); http://dx.doi.org/10.1063/1.4795166 (7 pages) Online Publication Date: 13 March 2013
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We conduct experiments with flexible swimmers to address the impact of fluid viscoelasticity on their locomotion. The swimmers are composed of a magnetic head actuated in rotation by a frequency-controlled magnetic field and a flexible tail whose deformation leads to forward propulsion. We consider both viscous Newtonian and glucose-based Boger fluids with similar viscosities. We find that the elasticity of the fluid systematically enhances the locomotion speed of the swimmer and that this enhancement increases with Deborah number. Using particle image velocimetry to visualize the flow field, we find a significant difference in the amount of shear between the rear and leading parts of the swimmer head. We conjecture that viscoelastic normal stresses lead to a net elastic forces in the swimming direction and thus a faster swimming speed.
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Reynolds-number scaling of turbulent channel flow Phys. Fluids 25, 025104 (2013); http://dx.doi.org/10.1063/1.4791606 (13 pages) Online Publication Date: 21 February 2013
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Results of an experimental study of smooth-wall, fully developed, turbulent channel flow are presented. The Reynolds number (Rem) based on the channel height and the bulk mean velocity ranged from 10 000 to 300 000. The present results indicate that the skin-friction coefficient (Cf) closely follows a power law for Rem < 62 000. At higher Reynolds numbers, Cf is best described by a log law. Detailed two-component velocity measurements taken at friction Reynolds numbers of Reτ = 1000–6000 indicate that the mean flow and Reynolds shear stress display little or no Reynolds-number dependence. The streamwise Reynolds normal stress (  +), on the other hand, varies significantly with Reynolds number. The inner peak in + is observed to grow with Reynolds number. Growth in + farther from the wall is documented over the entire range of Reynolds number giving rise to a plateau in the streamwise Reynolds normal stress in the overlap region of the profile for Reτ = 6000. The wall-normal Reynolds normal stress ( +) displays no Reynolds-number dependence near the wall. Some increase in + in the outer layer is noted for Reτ ≤ 4000. The trends in the present Reynolds stress results agree qualitatively with recent experimental results from pipe and boundary layer flows. |
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Some exact solutions for debris and avalanche flows Phys. Fluids 23, 043301 (2011); http://dx.doi.org/10.1063/1.3570532 (16 pages) Online Publication Date: 8 April 2011
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Exact analytical solutions to simplified cases of nonlinear debris avalanche model equations are necessary to calibrate numerical simulations of flow depth and velocity profiles on inclined surfaces. These problem-specific solutions provide important insight into the full behavior of the system. In this paper, we present some new analytical solutions for debris and avalanche flows and then compare these solutions with experimental data to measure their performance and determine their relevance. First, by combining the mass and momentum balance equations with a Bagnold rheology, a new and special kinematic wave equation is constructed in which the flux and the wave celerity are complex nonlinear functions of the pressure gradient and the flow depth itself. The new model can explain the mechanisms of wave advection and distortion, and the quasiasymptotic front bore observed in many natural and laboratory debris and granular flows. Exact time-dependent solutions for debris flow fronts and associated velocity profiles are then constructed. We also present a novel semiexact two-dimensional plane velocity field through the flow depth. Second, starting with the force balance between gravity, the pressure gradient, and Bagnold’s grain-inertia or macroviscous forces, we construct a simple and very special nonlinear ordinary differential equation to model the steady state debris front profile. An empirical pressure gradient enhancement factor is introduced to adequately stretch the flow front and properly model nonhydrostatic pressure in granular and debris avalanches. An exact solution in explicit form is constructed, and is expressed in terms of the Lambert–Euler omega function. Third, we consider rapid flows of frictional granular materials down a channel. The steady state mass and the momentum balance equations are combined together with the Coulomb friction law. The Chebyshev radicals are employed and the exact solutions are developed for the velocity profile and the debris depth. Similarly, Bagnold’s fluids are also used to construct alternative exact solutions. Many interesting and important aspects of all these exact solutions, their applications to real-flow situations, and the influence of model parameters are discussed in detail. These analytical solutions, although simple, compare very well with experimental data of debris flows, granular avalanches, and the wave tips of dam break flows. A new scaling law for Bagnold’s fluids is established to relate the settlement time of debris deposition. It is found analytically that the macroviscous fluid settles (comes to a standstill) considerably faster than the grain-inertia fluid, as manifested by dispersive pressure.
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Origin of line tension for a Lennard-Jones nanodroplet Phys. Fluids 23, 022001 (2011); http://dx.doi.org/10.1063/1.3546008 (11 pages) Online Publication Date: 1 February 2011
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The existence and origin of line tension has remained controversial in literature. To address this issue, we compute the shape of Lennard-Jones nanodrops using molecular dynamics and compare them to density functional theory in the approximation of the sharp kink interface. We show that the deviation from Young’s law is very small and would correspond to a typical line tension length scale (defined as line tension divided by surface tension) similar to the molecular size and decreasing with Young’s angle. We propose an alternative interpretation based on the geometry of the interface at the molecular scale.
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Inertial particle trapping in viscous streaming Phys. Fluids 25, 033602 (2013); http://dx.doi.org/10.1063/1.4795857 (21 pages) Online Publication Date: 28 March 2013
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The motion of an inertial particle in a viscous streaming flow of Reynolds number order 10 is investigated theoretically and numerically. The streaming flow created by a circular cylinder undergoing rectilinear oscillation with small amplitude is obtained by asymptotic expansion from previous work, and the resulting velocity field is used to integrate the Maxey–Riley equation with the Saffman lift for the motion of an inertial spherical particle immersed in this flow. It is found that inertial particles spiral inward and become trapped inside one of the four streaming cells established by the cylinder oscillation, regardless of the particle size, density and flow Reynolds number. It is shown that the Faxén correction terms divert the particles from the fluid particle trajectories, and once diverted, the Saffman lift force is most responsible for effecting the inward motion and trapping. The speed of this trapping increases with increasing particle size, decreasing particle density, and increasing oscillation Reynolds number. The effects of Reynolds number on the streaming cell topology and the boundaries of particle attraction are also explored. It is found that particles initially outside the streaming cell are repelled by the flow rather than trapped.
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Three dimensional flow around a circular cylinder confined in a plane channel Phys. Fluids 23, 064106 (2011); http://dx.doi.org/10.1063/1.3599703 (14 pages) Online Publication Date: 22 June 2011
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This paper presents two- and three-dimensional direct numerical simulations of the flow around a circular cylinder placed symmetrically in a plane channel. Results are presented in the Reynolds number range (based on the cylinder diameter and centerline velocity) of 10 to 390 for a blockage ratio (ratio of the cylinder diameter to the channel height) of 0.2. The aim of this work was to investigate in detail the confinement effect due to the channel’s stationary walls on the force coefficients and the associated Strouhal numbers, as well as on the generated flow regimes. Present results suggest a transition from a 2-D to a 3-D shedding flow regime between Re = 180 and Re = 210. This transition was found to be dominated by mode A and mode B three dimensional instabilities, similar to those observed in the case of an unconfined circular cylinder. This is the first time that the existence of the two modes, and of naturally occurring vortex dislocations, has been confirmed via full 3-D simulations for the case of a confined circular cylinder in a channel. A discontinuity in the variation of the Strouhal number St, and of the base pressure coefficient Cpb, with Re was also observed. This was found to be associated with the onset of mode A instability and the development of vortex dislocations, and parallels what occurs in the unconfined case, but previous studies could not confirm its existence in the confined case. Furthermore, by analyzing the mechanisms affecting the shape and evolution of these instabilities, it is demonstrated that they are significantly affected by the confinement only in the far wake.
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Droplet formation in microfluidic cross-junctions Phys. Fluids 23, 082101 (2011); http://dx.doi.org/10.1063/1.3615643 (12 pages) Online Publication Date: 4 August 2011
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Using a lattice Boltzmann multiphase model, three-dimensional numerical simulations have been performed to understand droplet formation in microfluidic cross-junctions at low capillary numbers. Flow regimes, consequence of interaction between two immiscible fluids, are found to be dependent on the capillary number and flow rates of the continuous and dispersed phases. A regime map is created to describe the transition from droplets formation at a cross-junction (DCJ), downstream of cross-junction to stable parallel flows. The influence of flow rate ratio, capillary number, and channel geometry is then systematically studied in the squeezing-pressure-dominated DCJ regime. The plug length is found to exhibit a linear dependence on the flow rate ratio and obey power-law behavior on the capillary number. The channel geometry plays an important role in droplet breakup process. A scaling model is proposed to predict the plug length in the DCJ regime with the fitting constants depending on the geometrical parameters.
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An analysis of premixed flamelet models for large eddy simulation of turbulent combustion Phys. Fluids 22, 115109 (2010); http://dx.doi.org/10.1063/1.3490043 (24 pages) Online Publication Date: 12 November 2010
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When premixed flamelet models are applied in the context of large eddy simulation, a number of assumptions are implicity made. The validity of these assumptions depends on, for example, the simulated flame’s location within the premixed regime diagram, the accuracy of the presumed subfilter flamelet coordinate distributions, and the extent to which the asymptotic flamelets capture the turbulence-perturbed chemistry. Here, the errors that arise due to these assumptions are considered, analyzed, and compared using a direct numerical simulation of a premixed turbulent flame propagating in the thin reaction zones regime. Flamelet representations of the progress variable source term are formed in an a priori fashion. Level set flamelet methods in particular are considered because, although they offer a number of advantages, they make some of the most stringent flame structure assumptions. Errors due to the level set model are evaluated relative to other flamelet error sources, such as the shape of the presumed probability density function and the influence of the variance model. The results provide guidance on the importance of the individual modeling assumptions, and are used to propose a new modeling strategy in an effort to improve the level set framework.
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Lattice Boltzmann simulations of a single n-butanol drop rising in water Phys. Fluids 25, 042102 (2013); http://dx.doi.org/10.1063/1.4800230 (29 pages) Online Publication Date: 11 April 2013
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The motion of an n-butanol drop in water under the influence of gravity was numerically studied using a diffuse interface free energy lattice Boltzmann method. A pure two-liquid system without mass transfer between the phases was considered. A range of drop diameters of 1.0–4.0 mm covered the flow conditions. Most calculations were carried out in a moving reference frame. This allowed studying of long-term drop behavior in a relatively small computational domain. The capability of the method to capture the drop shape especially in the oscillating regime was demonstrated. For each drop diameter the evolution of the drop velocity in time, the terminal rise velocity and drop's shape were determined. The results were compared to experimental and numerical results and to semi-empirical correlations. The deviation of the simulated terminal velocity from other results is within 5% for smaller drops and up to 20% for large oscillating drops. It was shown that beyond the onset of shape oscillations the binary system converges towards a constant capillary number of 0.056.
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