New Papers in Fluid Mechanics

Dual residence time for droplets to coalesce with a liquid surface

Physical Review E - Tue, 06/18/2024 - 11:00

Author(s): Ting-Heng Hsieh, Wei-Chi Li, and Tzay-Ming Hong

When droplets approach a liquid surface, they have a tendency to merge in order to minimize surface energy. However, under certain conditions, they can exhibit a phenomenon called coalescence delay, where they remain separate for tens of milliseconds. This duration is known as the residence time or …


[Phys. Rev. E 109, 065109] Published Tue Jun 18, 2024

Analysis of Rayleigh-Bénard convection using latent Dirichlet allocation

Physical Review Fluids - Tue, 06/18/2024 - 11:00

Author(s): B. Podvin, L. Soucasse, and F. Yvon

Natural convection motifs are identified in a Rayleigh-Bénard cubic cell using a probabilistic clustering method, Latent Dirichlet Allocation (LDA). The spatiotemporal features of the motifs at different Rayleigh numbers provide insight into the dynamics of the large-scale circulation (LSC), which is characterized by intermittent reorientations. A model based on the dominant heat flux motifs is found to predict successfully the average LSC reorientation rate, including in cases where few or even no reorientations are observed.


[Phys. Rev. Fluids 9, 063502] Published Tue Jun 18, 2024

Generalized scaling laws for the irrotational motions bordering a turbulent region

Physical Review E - Mon, 06/17/2024 - 11:00

Author(s): Marco Zecchetto, Ricardo P. Xavier, Miguel A. C. Teixeira, and Carlos B. da Silva

In turbulent free shear flows such as jets and wakes, and also in turbulent boundary layers, the turbulent region is bounded by a region of irrotational flow where the magnitude of the potential velocity fluctuations can be very high. This is particularly true close to the turbulent-nonturbulent int…


[Phys. Rev. E 109, 065107] Published Mon Jun 17, 2024

Simulations of buoyant flows driven by variations in solar radiation beneath ice cover

Physical Review Fluids - Mon, 06/17/2024 - 11:00

Author(s): Donovan J. M. Allum and Marek Stastna

Solar radiation is known to drive vertical motion under ice-covered lakes in the late winter. Numerical studies in this context tend to neglect the effects of nonuniform solar radiation on fluid motion in the lake interior. This research provides direct numerical simulations and the subsequent analysis of the resulting gravity-current-like flow, which propagates into an inversely stratified ambient with developing convection in the form of three-dimensional Rayleigh-Taylor instabilities. We find that with the chosen parameters, typical of an ice-covered lake, geometry plays a much larger role in its development and cessation.


[Phys. Rev. Fluids 9, 063501] Published Mon Jun 17, 2024

From weakly to strongly nonlinear viscous drop shape oscillations: An analytical and numerical study

Physical Review Fluids - Mon, 06/17/2024 - 11:00

Author(s): Martin Smuda, Florian Kummer, Martin Oberlack, Dino Zrnić, and Günter Brenn

Liquid drops exhibit nonspherical surface shapes with strong deformations upon pinch-off from jets or sheets. The deformed state induces shape oscillations, which are analyzed both by the weakly nonlinear approach and high-order simulations using the extended Discontinuous Galerkin method as two alternative nonlinear theories to investigate the oscillations at moderate to large deformations. The coupling of oscillation modes is found to induce quasiperiodic motion, which is shown by Fourier power spectra of the frequencies. The interconversion of kinetic and surface energies during the oscillations at strong initial deformations is quantified by the numerical simulations.


[Phys. Rev. Fluids 9, 063601] Published Mon Jun 17, 2024

Linear stability of turbulent channel flow with one-point closure

Physical Review Fluids - Mon, 06/17/2024 - 11:00

Author(s): P. V. Kashyap, Y. Duguet, and O. Dauchot

Linear stability analysis of the mean flow in turbulent plane channel flow in the large-scale pattern-forming range. Growth rate of the least stable mode as a function of streamwise and spanwise wavenumber α and β, respectively. Strict linear stability for all parameters is predicted for all parameters, suggesting that the mean flow stability does not explain pattern formation.


[Phys. Rev. Fluids 9, 063906] Published Mon Jun 17, 2024

Dispersion control in coupled channel-heterogeneous porous media systems

Physical Review Fluids - Mon, 06/17/2024 - 11:00

Author(s): Bowen Ling, Runqing Shan, and Felipe P. J. de Barros

Multilayered porous media are prevalent in both natural and engineered systems, exerting significant influence on flow and transport processes. This study presents a hybrid analytical-numerical approach to compute and understand the dynamics between scalar properties and media characteristics in a coupled system with a two-dimensional free flow layer and a heterogeneous porous medium under laminar flow conditions. Our research underscores how variations in the permeability field within multilayered porous media profoundly influence and control scalar mixing behavior.


[Phys. Rev. Fluids 9, 064502] Published Mon Jun 17, 2024

Transient internal wave excitation of resonant modes in a density staircase

Physical Review Fluids - Mon, 06/17/2024 - 11:00

Author(s): Joel Bracamontes-Ramirez and Bruce R. Sutherland

Vertically propagating internal wave packets incident upon a density staircase can resonantly excite natural modes of the staircase that then re-emit upward and downward propagating internal waves, changing the prediction for energy transmission of incident plane internal waves.


[Phys. Rev. Fluids 9, 064801] Published Mon Jun 17, 2024

Propulsion of a three-sphere microrobot in a porous medium

Physical Review E - Thu, 06/13/2024 - 11:00

Author(s): Chih-Tang Liao, Andrew Lemus, Ali Gürbüz, Alan C. H. Tsang, On Shun Pak, and Abdallah Daddi-Moussa-Ider

Microorganisms and synthetic microswimmers often encounter complex environments consisting of networks of obstacles embedded into viscous fluids. Such settings include biological media, such as mucus with filamentous networks, as well as environmental scenarios, including wet soil and aquifers. A fu…


[Phys. Rev. E 109, 065106] Published Thu Jun 13, 2024

Neural networks in feedback for flow analysis and control

Physical Review Fluids - Wed, 06/12/2024 - 11:00

Author(s): Tarcísio C. Déda, William R. Wolf, and Scott T. M. Dawson

In this work we propose a machine learning methodology for flow modeling and control design based on an iterative approach for training neural networks. We demonstrate that the methodology is able to achieve stabilization of complex nonlinear plants, such as an unstable confined flow past a cylinder. We also show that, through linearization of neural network models, we can use the methodology to conduct optimal sensor selection, as well as to perform unstable equilibrium estimation and stability analysis.


[Phys. Rev. Fluids 9, 063904] Published Wed Jun 12, 2024

Mixed mode transition in boundary layers: Helical instability

Physical Review Fluids - Wed, 06/12/2024 - 11:00

Author(s): Rikhi Bose and Paul A. Durbin

When an unstable boundary layer is perturbed by free-stream turbulence, the combination of that perturbation with two-dimensional instability waves creates a state that transitions to turbulence by an intriguing, helical breakdown. Helical breakdown is analyzed as a secondary instability; the three-dimensional structure of the eigenfunction of the secondary instability mode reveals the helical pattern. The streak configuration leading to the formation of the helical mode is different from those leading to sinuous and varicose modes reported for pure bypass transition in the absence of instability waves. The mixed mode precursor is the distinctive cause for the helical mode transition.


[Phys. Rev. Fluids 9, 063905] Published Wed Jun 12, 2024

Molecular dynamics study of the sonic horizon of microscopic Laval nozzles

Physical Review E - Tue, 06/11/2024 - 11:00

Author(s): Helmut Ortmayer and Robert E. Zillich

A Laval nozzle can accelerate expanding gas above supersonic velocities, while cooling the gas in the process. This work investigates this process for microscopic Laval nozzles by means of nonequilibrium molecular dynamics simulations of stationary flow, using grand-canonical Monte Carlo particle re…


[Phys. Rev. E 109, 065104] Published Tue Jun 11, 2024

Precipitation-induced filament pattern of injected fluid controlled by a structured cell

Physical Review E - Tue, 06/11/2024 - 11:00

Author(s): Shunsuke Tanaka, Kojiro Otoguro, Miyuki Kunihiro, Hiroki Ishikawa, and Yutaka Sumino

Mixing of two fluids can lead to the formation of a precipitate. If one of the fluids is injected into a confined space filled with the other, then a created precipitate disrupts the flow locally and forms complex spatiotemporal patterns. The relevance of controlling these patterns has been highligh…


[Phys. Rev. E 109, 065105] Published Tue Jun 11, 2024

Aerodynamics and stability of hawkmoth forward flight with flexible wing hinge

Physical Review Fluids - Tue, 06/11/2024 - 11:00

Author(s): Yujing Xue, Xuefei Cai, and Hao Liu

We develop a fluid–structure interaction model that couples one-torsional-spring-based elastic wing-hinge dynamics with flapping aerodynamics to study the aerodynamics and flight stability of hawkmoth at various flight velocities. Both leading-edge vortex, body vortex, and their interactions are responsible for augmenting the vertical force production, achieving high power efficiency from the elastic storage. We verify that realistic wing-hinge stiffness leads to optimal aerodynamic performance and external disturbance-rejection is highly robust in multiple directions. This study highlights the significance of flexible wing hinges in biomimetic designs for micro-aerial vehicles.


[Phys. Rev. Fluids 9, 063101] Published Tue Jun 11, 2024

Dynamic breakup of Janus droplet in a bifurcating microchannel

Physical Review Fluids - Tue, 06/11/2024 - 11:00

Author(s): Hao Wang, Shiteng Wang, Yao Mu, Qing Han, and Yi Cheng

Our study combined experiments and three-dimensional lattice Boltzmann simulations to investigate the dynamic breakup of spatially asymmetric Janus droplets in microchannels under two different bifurcation orientations. We elucidated three characteristic flow regimes: (i) division into two daughter Janus droplets; (ii) breakup into a single-phase droplet and a smaller Janus droplet; and (iii) non-breakup. Unlike single-phase or double emulsion droplets, the dumbbell-shaped Janus droplets might exhibit oblique flow in the channel. The strong confinement of the main channel on mother droplets and large flow rates are essential to the symmetrical breakup of Janus droplets.


[Phys. Rev. Fluids 9, 064203] Published Tue Jun 11, 2024

Colloidal deposits from evaporating sessile droplets: Coffee ring versus surface capture

Physical Review Fluids - Tue, 06/11/2024 - 11:00

Author(s): Nathan C. J. Coombs, James E. Sprittles, and Mykyta V. Chubynsky

The ubiquitous coffee ring effect, referring to the accumulation of suspended particles at the contact line of an evaporating sessile droplet, arises due to evaporation-induced capillary flow. At high evaporation rates, particle accumulation is also observed at the air-liquid interface, a phenomenon known as surface capture. While the coffee ring effect is well understood theoretically, the transition to surface capture has received less attention. Here we aim to remedy this using a simple low-dimensional model to interpolate between the pure coffee ring and pure surface capture regimes. This interpolation also provides insight into intermediate behaviors.


[Phys. Rev. Fluids 9, 064304] Published Tue Jun 11, 2024

Helium plumes at moderate Reynolds number

Physical Review Fluids - Tue, 06/11/2024 - 11:00

Author(s): Stefano Lanzini, Massimo Marro, Mathieu Creyssels, and Pietro Salizzoni

We present the first experimental study of the turbulent entrainment in non-Boussinesq steady plumes, focusing on moderate-Reynolds helium releases issued from an axisymmetric source. Our results show that, downstream of the turbulent transition, the vertical variations of the entrainment coefficient are primarily affected by the near-field generation of turbulent kinetic energy and by a rising contribution of buoyancy effects. Both features do not exhibit a clear dependence on local variations of the density ratio.


[Phys. Rev. Fluids 9, 064501] Published Tue Jun 11, 2024

Enhanced transport of flexible fibers by pole vaulting in turbulent wall-bounded flow

Physical Review Fluids - Tue, 06/11/2024 - 11:00

Author(s): Jérémie Bec, Christophe Brouzet, and Christophe Henry

Long, flexible fibers in a turbulent channel flow showcase fascinating dynamics, sampling nonlinear fluid velocities along their length. Tumbling and colliding with boundaries, they bounce off like pole vaulters, propelling themselves toward the flow center. This motion depletes fibers near the walls and concentrates them in the bulk, boosting the net fiber flux beyond the initial flux of the fluid. The effect grows stronger with longer, more flexible fibers, highlighting crucial implications for transport phenomena in turbulent flows.


[Phys. Rev. Fluids 9, L062501] Published Tue Jun 11, 2024

Insights on phase speed and the critical Reynolds number of falling films

Physical Review E - Mon, 06/10/2024 - 11:00

Author(s): Arghya Samanta

We revisit the studies of gravity-driven viscous falling films with and without imposed shear stress to provide new perspectives on phase speed and the critical Reynolds number for surface instability. We use the traditional long-wave expansion technique implemented for investigating the linear stab…


[Phys. Rev. E 109, 065103] Published Mon Jun 10, 2024

Stability of a liquid layer draining around a horizontal cylinder: Interplay of capillary and gravity forces

Physical Review Fluids - Mon, 06/10/2024 - 11:00

Author(s): Shahab Eghbali, Simeon Djambov, and François Gallaire

We study the drainage of a viscous liquid layer on a horizontal cylinder under gravity, focusing on cases where viscous effects dominate inertia. Nonlinear simulations distinguish, as a function of film thickness and Bond number, two regimes where the draining liquid either ruptures or forms a quasistatic curtain. The liquid curtain subsequently destabilizes due to capillary and gravity forces. When surface tension dominates gravity, pearls form around the cylinder, whereas when gravity dominates surface tension, hanging droplets form, as confirmed by a linear stability analysis of the curtain.


[Phys. Rev. Fluids 9, 063903] Published Mon Jun 10, 2024

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