Physical Review Fluids

Author(s): M. Crialesi-Esposito, G. Boffetta, L. Brandt, S. Chibbaro, and S. Musacchio

Droplet formation is an important research topic with applications in geophysics, where fragmentation is key in the exchange between atmosphere and ocean, and in industrial processes, with many products obtained by emulsification. Despite its importance this fundamental physical problem remains a puzzle. In this work we examine small droplet formation in turbulent multiphase flows. By surgically acting on the Navier-Stokes equations we show that small-droplet formation is the main cause of high dissipation events characterizing multiphase turbulence, and not vice versa. We also prove that tiny droplets are formed through the internal capillary dynamics of large droplets during break-up.

[Phys. Rev. Fluids 9, L072301] Published Mon Jul 29, 2024

Author(s): Grant Rydquist and Mahdi Esmaily

Algorithms used to calculate red blood cell damage are often constructed empirically from laminar flows, and the extensibility of these algorithms to turbulent flows is an open question. To examine this question, we investigate the effect of turbulent flows on red blood cell damage by simulating individual red blood cells in turbulent channel flow using a multiscale computational framework. The results from the turbulence simulations are compared to laminar channel flow at equivalent wall shear stress. While the laminar flows generate greater stretch in a time-averaged sense, the turbulent flows produce spikes in deformation associated with short bursts of high-shear conditions.

[Phys. Rev. Fluids 9, 073102] Published Fri Jul 26, 2024

Author(s): Shubhra Sahu, Bapan Mondal, and Somnath Bhattacharyya

In this study we have extended the mean-field based model to analyze the layered structure of the electric double layer and found that oscillations in charge density arise in multivalent counterions. Formation of the counterion condensed layer and overscreening creates a larger accumulation of coions in the electric double layer, leading to an electroosmotic flow reversal and attenuation in ion selectivity of the pore. The present model shows that the counterion size has an impact on the condensed layer and hence, overscreening. We demonstrate that the electroosmotic flow reversal of multivalent electrolytes can be suppressed by mixing with monovalent slats.

[Phys. Rev. Fluids 9, 074201] Published Fri Jul 26, 2024

Author(s): I. Misra and V. Kumaran

The dynamics of a magnetic particle of moment m in an oscillating magnetic field H has a qualitative dependence on the magnetization model. There are sustained oscillations for a particle with a permanent dipole. The particle is static for an induced dipole with no hysteresis. For an induced dipole with hysteresis, there is a transition between oscillations with a magnetic moment oscillating about a particle axis, and a magnetic moment flipping relative to the particle axis.

[Phys. Rev. Fluids 9, 074303] Published Fri Jul 26, 2024

Author(s): Tristan Aurégan, Sylvain Courrech du Pont, and Benjamin Thiria

Flexible structures subjected to varying flow conditions are often more resilient than their rigid equivalents. We exploit this bio-inspired strategy, called reconfiguration, to improve the efficiency of a scale model underwater propeller. Using flexible blades that deform passively, we show experimentally that it is possible to program the deformation of the rotor such that the efficiency of the propeller is improved in off-design conditions. We also show that the deformation of the blades can be predicted using a simple model.

[Phys. Rev. Fluids 9, 074402] Published Fri Jul 26, 2024

Author(s): Arefe Ghazi Nezami and Blair Anne Johnson

Turbulence generated with a random jet array (RJA) can be homogeneous, isotropic, and exhibit negligible mean flow, making it suitable for many applications. However, achieving these characteristics requires a thorough understanding of the turbulence generation process within RJA facilities. In this study, we investigate the evolution of turbulence within an RJA-driven water tank, from its generation to its decay, and distinct features of the flow. Additionally, we propose a relationship for estimating the energy of the generated turbulence in cases with minimal mean flow, and we introduce an efficiency parameter to compare the input energy with the resulting turbulent flow.

[Phys. Rev. Fluids 9, 074610] Published Fri Jul 26, 2024

Author(s): Atul S. Vivek, Ranabir Dey, and Harish N. Dixit

As liquid films on solid substrates approach submicron thickness, they tend to become naturally unstable due to the action of long-range dispersion forces. Interestingly, the presence of surfactants and gravitational drainage alters the stability characteristics of these films. Using linear and nonlinear stability analyses in the lubrication limit, we show that draining films containing surfactants exhibit greater stability compared to stationary films with surfactants, as well as draining films with clean interfaces. Our findings can have potential implications for the stability of a wide variety of thin films, ranging from precorneal tear films to industrial coatings.

[Phys. Rev. Fluids 9, 074004] Published Thu Jul 25, 2024

Author(s): Cat Tuong Nguyen and Martin Oberlack

A large-scale direct numerical simulation of a spatially evolving turbulent round jet flow has been conducted at a Reynolds number of 3500 based on the orifice diameter and the mean bulk velocity at the orifice. A fully turbulent pipe flow profile is used as the inlet condition, and high-quality statistical data is generated by averaging over 200 washouts of a particle. The present data include statistical moments up to the third order and the probability density function of the axial velocity at various distances from the centerline and the orifice, showing remarkable self-similarity. The present data is compared with data of previous numerical and experimental studies.

[Phys. Rev. Fluids 9, 074608] Published Thu Jul 25, 2024

Author(s): Kevin Ley, Olivier Soulard, Jérôme Griffond, Antoine Briard, and Serge Simoëns

How fast can a Rayleigh-Taylor unstable flame accelerate? How rapidly can its width increase? To investigate these issues, a link is established between the level of molecular mixing and the self-similar evolution of a reactive Rayleigh-Taylor turbulent flow.

[Phys. Rev. Fluids 9, 074609] Published Thu Jul 25, 2024

Author(s): Safir Haddad, Samuel Vaux, Kevin Varrall, and Olivier Vauquelin

This paper presents analytical solutions to the theoretical problem proposed by Ellison and Turner. Their equations describe the longitudinal evolution of a miscible gravity current when it reaches a steady state. The solutions here proposed allow the evolution of the velocity, height and density of the current to be calculated solely from the local Richardson number. The latter is obtained using a universal function F that can be tabulated or plotted whatever the release conditions. The solutions are then extended to configurations in which a jump appears, and finally for several entrainment laws available in the literature.

[Phys. Rev. Fluids 9, 074803] Published Thu Jul 25, 2024

Author(s): Xiao-Zheng Zhao, Can Qiu, Sheng-Qi Zhou, Yi-Zhen Li, Heng-Dong Xi, and Ke-Qing Xia

The mean temperature profile is measured in a three-layer water-FC77-mercury system (FC77 is a type of Fluorinert electronic liquid). The mean temperature profile is measured across (a) the mercury-FC77 interface and (b) the FC77-water interface. The three-layer system provides slippery boundary conditions for the middle FC77 layer in an otherwise standard Rayleigh-Bénard convection system. This results in greatly enhanced heat transport across the FC77 layer, both in magnitude and in the scaling exponent with the Rayleigh number Ra.

[Phys. Rev. Fluids 9, 073501] Published Wed Jul 24, 2024

Author(s): Zhenyu Ouyang, Chen Liu, Zhaowu Lin, and Jianzhong Lin

We simulate a spheroidal swimmer through a complex fluid. A spheroidal swimmer model is developed and exerted in a direct-forcing fictitious domain method framework. We find an inertial spheroidal puller with a small swimming intensity swims faster than the counterpart subjected to the Stokes flow regime, a departure from observations of spherical pullers. Within the Giesekus fluid medium, an augmented mobility factor correlates with an increased squirmer velocity, while a larger aspect ratio contributes to neutral squirmer speed enhancement in the presence of fluid inertia. Meanwhile, a slenderer, inertial squirmer with a vigorous swimming intensity expends more energy.

[Phys. Rev. Fluids 9, 073303] Published Tue Jul 23, 2024

Author(s): Yi-Feng Wang, Yi-Bo Wang, Ling-Zhe Zhang, Xin He, Yan-Ru Yang, Xiao-Dong Wang, and Duu-Jong Lee

Impacting nanodroplets on pillared surfaces are investigated with molecular dynamics simulations. The difference between the liquid spreading dynamics on flat and pillared surfaces is shown to arise from the intrusion effect of liquid into pillar gaps. The altered dynamics can be understood by modeling the maximum spreading factor βmax. The scaling laws of βmax on flat surfaces can successfully predict βmax on pillared surfaces when the volume term of the bulk droplet is properly accounted for. For a six parameter group of the initial problem we propose a universal phase space which contains six outcome regimes for impacting nanodroplets.

[Phys. Rev. Fluids 9, 073602] Published Tue Jul 23, 2024

Author(s): Margaux Ceccaldi, Vincent Langlois, Olivier Pitois, Marielle Guéguen, Daniel Grande, and S. Vincent-Bonnieu

The time evolution of the liquid relative permeability of grain packings initially filled with liquid foam is measured to increase until a constant value is eventually observed. We demonstrate that this evolution is directly related to the coarsening of the liquid foam confined in the pore space. Using the measured bubble size evolution combined with the intrinsic liquid permeability of the bulk foam, we have determined values corresponding to the function that describes the foam confinement effects as a function of the bubble-to-grain size ratio.

[Phys. Rev. Fluids 9, 074003] Published Tue Jul 23, 2024

Author(s): Xiaohan Hu, Keshav Vedula, and George Ilhwan Park

Which direction matters more for subgrid-scale (SGS) turbulence? Our analysis reveals that the essence of dynamic SGS models is often condensed in only a few (sometimes, just one) special directions, offering new insights into the success of dynamic large-eddy simulation models.

[Phys. Rev. Fluids 9, 074607] Published Mon Jul 22, 2024

Author(s): A. Gaillard, M. A. Herrada, A. Deblais, J. Eggers, and D. Bonn

We show that the apparent relaxation time inferred from the exponential thinning regime in viscoelastic pinch-off is not necessarily a material property, as was assumed so far, but depends on the system size for various polymer solutions and filament thinning techniques. It depends on the plate size in Capillary Breakup Extensional Rheometry (CaBER) with both slow and fast plate separation protocols, and on the nozzle size in Dripping-onto-Substrate (DoS), corroborating recent observations with the dripping technique. It is not due to artifacts such as solvent evaporation or polymer degradation and it cannot always be rationalized by finite extensibility effects.

[Phys. Rev. Fluids 9, 073302] Published Fri Jul 19, 2024

Author(s): A. Dubey, G. P. Bewley, K. Gustavsson, and B. Mehlig

Uncharged micron-sized water droplets flying toward each other do not always coalesce due to the cushioning effect of the air between them.For oppositely charged droplets, we discover a regime for which droplets always collide when they move inside the stable manifolds of a saddle point of the relative droplet dynamics. A consequence is that only for small electrical charges does the droplet coalescence rate depend primarily upon the Knudsen number (Kn), the ratio of the mean-free-path of air to the mean droplet radius. For much larger charges, coalescence does not depend upon Kn. Our theory predicts the critical charge at which the transition between the two regimes occurs.

[Phys. Rev. Fluids 9, 074302] Published Fri Jul 19, 2024

Author(s): Filipe R. do Amaral and André V. G. Cavalieri

We address coherent pressure structures in turbulent channel flows through SPOD and resolvent analysis with and without an eddy-viscosity model. The spectral analysis revealed energetic structures in the near-wall region, as well as large-scale and spanwise-coherent structures. Pressure structures are targeted in both SPOD and resolvent analysis by selecting an adequate norm through the quadrature weights and observation operator, respectively. The first SPOD and the leading resolvent modes closely agree and show low-ranking behavior. The analyzed modes comprise quasi-streamwise (for near-wall and large-scale structures) and spanwise vortices with pressure peaking at vortex centers.

[Phys. Rev. Fluids 9, 074606] Published Fri Jul 19, 2024

Author(s): Peiwen Cong, Hui Liang, Yingyi Liu, and Bin Teng

Novel formulations involving a control surface at a distance from the body are developed to compute drift loads on structures composed of an impermeable hull and a perforated surface accurately and efficiently. The developed formulation can not only give all six components of the mean wave drift force and moment, but also determine the drift loads on each individual body of a multi-body system.

[Phys. Rev. Fluids 9, 074802] Published Thu Jul 18, 2024

Author(s): Sergi G. Leyva, Ignacio Pagonabarraga, Aurora Hernández-Machado, and Rodrigo Ledesma-Aguilar

A viscous fluid in contact with a solid channel that has a preferential affinity with respect to a second fluid embedded in the channel leads to a spontaneous imbibition process. Due to the increasing friction of the invading phase, the invading fluid scales diffusively in time. What happens when a third liquid is a lubricant coating the channel? We show that when the lubricant viscosity is decreased, dissipation switches from being localized in the bulk of the invading phase, to the lubricant layers. This leads to a new crossover, below which diffusive dynamics are not observed. Our results open up the possibility of using this mechanism in SLIPS and LIS to control capillary flows.

[Phys. Rev. Fluids 9, L072002] Published Thu Jul 18, 2024