Latest papers in fluid mechanics

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

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

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

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

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

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

Author(s): S. Ghosh, P. S. Goswami, and V. Kumaran

Fluctuating force, fluctuating torque simulations accurately predict the particle dynamics in particle-laden turbulent flows.

[Phys. Rev. Fluids 9, 064303] Published Mon Jun 10, 2024

Author(s): Jake Langham and Andrew J. Hogg

Clouds of fine particles held aloft by turbulent fluctuations are widespread in natural flows. Although these suspensions are known to inhibit the turbulence supporting them, this effect remains unstudied in many of the most basic settings, such as shear-driven flows. We trace part of the laminar-turbulent boundary for particle-laden plane Couette flow and find unexpectedly that turbulence suppression depends non-monotonically on particle settling velocity. On either side of laminar flow lie two very different states: near-homogeneous weakly stratified turbulence and a patchier regime where sediment is intermittently lifted into suspension from a highly suppressed boundary layer.

[Phys. Rev. Fluids 9, L062602] Published Mon Jun 10, 2024

Author(s): Danail Obreschkow

Differential equations of the form R̈=−kRγ, with a positive constant k and real parameter γ, are fundamental in describing phenomena such as the spherical gravitational collapse (γ=−2), the implosion of cavitation bubbles (γ=−4), and the orbital decay in binary black holes (γ=−7). While explicit ele…

[Phys. Rev. E 109, 065102] Published Fri Jun 07, 2024

Author(s): Jingwen Fu, Wenwei Liu, Xing Jin, and Yun Huang

A numerical study on the inertial migration of particle suspension in a circular pipe with thermal effect is performed by means of the Lattice Boltzmann method coupled with the discrete element method (LBM-DEM). The particle position and heat transfer for single particle as well as particle suspensions are discussed. Then, we extend the work to varied temperature conditions. It is shown that the variation of the circumferential equilibrium position can be well regressed by the Richardson number. A nonmonotonic variation of the radial equilibrium position as well as the Nusselt number is discovered, which is attributed to the particle crowding effect.

[Phys. Rev. Fluids 9, 064302] Published Fri Jun 07, 2024

Author(s): Taketo Ariki

Passive scalar turbulence in the viscous-convective range is investigated via a self-consistent closure theory. Without relying on any empirical parameter the theory successfully explained the scalar-variance spectrum proportional to the inverse wavenumber k−1 from the scalar’s deformation timescale dominated by the Kolmogorov-scale eddy, which agrees with the physical viewpoint of Batchelor (1959). High Schmidt number (Sc) calculations up to Sc=100000 suggest that a clear Batchelor spectrum may appear in kη≳1 for Sc≳10000 where η is the Kolmogorov length.

[Phys. Rev. Fluids 9, 064603] Published Fri Jun 07, 2024

Author(s): Qiang Zhong and Daniel B. Quinn

Hydrofoils with symmetric oscillations can generate asymmetric vortex wakes. This surprising asymmetry has been widely reproduced, but a simple metric to predict its onset has remained elusive. Here, using a combination of vortex modeling and water channel experiments, we show that vortex wake deflection is well-predicted by the “relative dipole angle”. In addition to offering a predictive physics-based metric, our results show that a hydrofoil’s wake can converge much more slowly than previously thought (200+ oscillation cycles), and that the wake’s asymmetry is more than a memory of the hydrofoil’s initial condition - it is an instability inherent to the vortex street.

[Phys. Rev. Fluids 9, 064702] Published Fri Jun 07, 2024

Author(s): Olivier Vincent, Théo Tassin, Erik J. Huber, and Abraham D. Stroock

We study water transport in bi-disperse porous structures inspired by xylem tissue in vascular plants (arrays of microchannels interconnected by a nanoporous layer). With various experiments (high pressure-driven flow, spontaneous imbibition, transpiration-driven flow at negative pressure), we show that transport rates can be tuned by varying the shape of the microchannels. Even with a fixed shape, spontaneous imbibition behaves very differently depending on sample preparation (air-filled vs. evacuated), because of a dramatic change of transport mechanism in the microchannels. We provide analytical (effective medium) approaches and numerical simulations to rationalize these observations.

[Phys. Rev. Fluids 9, 064202] Published Thu Jun 06, 2024

Author(s): Giulio Dacome, Robin Mörsch, Marios Kotsonis, and Woutijn J. Baars

With the goal of performing opposition control of large-scale drag-producing turbulence structures, we present a successful control strategy that attenuates large-scale velocity fluctuations in a turbulent boundary layer. Our control architecture consists of a wall-embedded sensor that feeds information to a real-time controller, which selectively operates a jet actuator. We quantify the performance of this single-input/single-output system with spectral statistics and direct skin-friction measurements. Additionally, we link the changes in skin-friction drag to changes in the statistical integral quantities to gauge the correlation between control output and skin-friction variation.

[Phys. Rev. Fluids 9, 064602] Published Thu Jun 06, 2024

Author(s): Vivaswat Kumar, Federico Pizzi, George Mamatsashvili, André Giesecke, Frank Stefani, and Adrian J. Barker

We reveal and analyze an efficient magnetic dynamo action due to precession-driven hydrodynamic turbulence in the local model of a precessional flow, focusing on the kinematic stage of this dynamo. The growth rate of the magnetic field monotonically increases with the Poincaré number Po, characteriz…

[Phys. Rev. E 109, 065101] Published Wed Jun 05, 2024

Author(s): Ivan Kharsansky Atallah, Luc Pastur, Romain Monchaux, and Laurent Zimmer

An experimental study is conducted on a thin symmetric airfoil at stall. Below a critical Reynolds number, the flow exhibits low-frequency oscillations (LFOs) characterized by a broadband peak in the aerodynamic force spectrum. Beyond this threshold, the LFOs are replaced by intermittent random switches between two states of either high or low lift (attached or detached flow). The states are explored randomly in time for a fixed angle of attack, contrary to the classical hysteresis often observed in airfoil flows at stall, where both states are absorbing. We model this using a continuous Markov chain and extreme value theory, a framework that can determine the system bifurcation points.

[Phys. Rev. Fluids 9, 063902] Published Wed Jun 05, 2024

Author(s): Rodion Stepanov, Peter Frick, Vladimir Dulin, and Dmitriy Markovich

Our experimental research demonstrates that helicity in turbulent flows undergoes a direct spectral transfer from large to small scales. Tomographic particle image velocimetry provides insights into the spatial and spectral segregation of turbulent flows with different helicity signs. We show that helicity generation and decay along the jet dramatically depends on the inflow swirl. Notably, we provide direct experimental evidence of the helicity cascade, discovering that swirls of the same sign can impart turbulent helicity of the opposite sign, challenging conventional assumptions. These findings offer valuable benchmarks for numerical simulations using different turbulent closure methods.

[Phys. Rev. Fluids 9, L062601] Published Wed Jun 05, 2024

Author(s): Ludovico Nista, Heinz Pitsch, Christoph D. K. Schumann, Mathis Bode, Temistocle Grenga, Jonathan F. MacArt, and Antonio Attili

We compare supervised super-resolution convolutional neural networks (CNNs) against generative adversarial networks (GANs)-based architectures in the ability to reconstruct turbulent flow fields. GANs demonstrated superior in-sample performance but faced challenges with out-of-sample flows. Incorporating a partially unsupervised adversarial training step with large eddy simulation inputs and dynamic upsampling selection improved GANs’ out-of-sample robustness, capturing small-scale features and turbulence statistics better than standard supervised CNNs. The study recommends integrating discriminator-based training to enhance super-resolution CNNs’ reconstruction capabilities.

[Phys. Rev. Fluids 9, 064601] Published Tue Jun 04, 2024

Author(s): A. Caquas, L. R. Pastur, and A. Genty

Have you ever tried spinning your water bottle to empty it more quickly? This experiment, familiar to the general public, has rarely been studied in the scientific literature, which focuses mainly on the non-rotational case. We show that this popular experiment is surprisingly complex. Our study reveals the presence of three flow regimes, which have a direct impact on the efficiency of the draining process.

[Phys. Rev. Fluids 9, 064701] Published Tue Jun 04, 2024

Author(s): V. Dzanic, C. S. From, and E. Sauret

This study explores, for the first time, the impact of plasticity on inertialess viscoelastic instabilities in strong elongational flows. Through detailed numerical simulations, it reveals how elastoviscoplastic effects induce complex and dynamic flow behaviors, leading to new flow states. Crucially, our findings reveal that plasticity can laminarize and suppress these instabilities, offering new strategies for controlling the instability mechanism.

[Phys. Rev. Fluids 9, 063301] Published Mon Jun 03, 2024