Physical Review Fluids

Author(s): Akhshay S. Bhadwal, Joseph R. L. Cousins, Nigel J. Mottram, Stephen K. Wilson, Brian R. Duffy, Ian C. Sage, and Carl V. Brown

We demonstrate control of the free surface profile of a rivulet of a nematic liquid crystal through the electric field-induced local increase of the effective viscosity of the rivulet. This resulting increase in rivulet height is studied experimentally, and is described theoretically in terms of the volume flux and the electric field strength. The localized effective viscosity change of the flow in this study occurs under isothermal conditions with the other physical properties of the liquid kept constant. The reported effect contrasts with temperature-dependent viscosity control techniques, which involve changes to the physical properties of the liquid.

[Phys. Rev. Fluids 9, 064002] Published Mon Jun 24, 2024

Author(s): Abbas Farhat, Pierre Philippe, Li-Hua Luu, Alexis Doghmane, and Pablo Cuéllar

Experiments based on hydraulic loading of artificial cemented granular layers by a localized upward water flow revealed the existence of three failure modes: (i) Overall block uplift; (ii) Block rupture by median crack at the inflow zone; (iii) Progressive excavation of a fluidized path along the walls. Despite these distinct scenarios driven by the boundary conditions, the critical flow values at breakpoint are consistent with each other, underlining the local character of the instability, and can be rationalized by extending Archimedes’ number to the present case involving adhesion between grains. Agreement is then established with the purely granular case.

[Phys. Rev. Fluids 9, 064305] Published Mon Jun 24, 2024

Author(s): Jonathan S. Van Doren and M. Houssem Kasbaoui

Inertial solid particles suspended in dense turbulent channels modulate turbulence and fluid mass flow rate through two mechanisms: (I) the increase of the suspension’s apparent kinematic viscosity with increasing solid volume fraction and (II) turbulence modulation through the particle feedback force. For particle volume fractions below 3%, the increase in suspensions apparent viscosity accounts for most of the modulation. As the volume fraction increases, the particle feedback force drives additional modulation beyond what the increased viscosity accounts for. Namely, this is a reduction in the bulk fluid velocity, reduction of turbulent fluctuations, and increased coefficient of friction.

[Phys. Rev. Fluids 9, 064306] Published Mon Jun 24, 2024

Author(s): Arslan Salim Dar and Fernando Porté-Agel

Most of the literature on flow over cliffs and its interaction with a wind turbine wake deals with wind direction perpendicular to the cliff. As wind direction can be oblique in reality, it is important to understand how the flow over a cliff can change with wind direction and what implications it can have for a wind turbine wake. In this study, we showed that above a certain wind direction, streamwise flow recirculation is replaced by a spanwise one, affecting flow shear and turbulence. In addition, we explored the complex interactions between flow over a cliff and a wind turbine wake, affecting its characteristics such as shape, recovery rate, and turbulence level in a nontrivial manner.

[Phys. Rev. Fluids 9, 064604] Published Mon Jun 24, 2024

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

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

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

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

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

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

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

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

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

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): 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