New Papers in Fluid Mechanics

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

Dynamics of particle-laden turbulent suspensions: Effect of particle roughness

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

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

Two regimes of dilute turbulent settling suspensions under shear

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

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

From cavitation to astrophysics: Explicit solution of the spherical collapse equation

Physical Review E - Fri, 06/07/2024 - 11:00

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

Numerical investigation on particle inertial migration in circular Poiseuille flow with thermal convection

Physical Review Fluids - Fri, 06/07/2024 - 11:00

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

Statistical theory of passive scalar turbulence within the viscous-convective range

Physical Review Fluids - Fri, 06/07/2024 - 11:00

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

Predicting the slowly converging dynamics of asymmetric vortex wakes

Physical Review Fluids - Fri, 06/07/2024 - 11:00

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

Tunable transport in bidisperse porous materials with vascular structure

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

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

Opposition flow control for reducing skin-friction drag of a turbulent boundary layer

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

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

Dynamo action driven by precessional turbulence

Physical Review E - Wed, 06/05/2024 - 11:00

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

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