Physical Review Fluids

Author(s): Nghia Le, Casey M. Miller, Julianna S. Detrick, Cameron R. Lodi, Kevin A. Mitchell, and Thomas H. Solomon

Self-propelled (active) particles in laminar fluid flows are blocked by one-way barriers called “swimming invariant manifolds” (SwIMs). The SwIM theory is a generalization of manifold approaches previously applied to the mixing of passive tracers. We verify the blocking behavior of SwIMs experimentally for algae swimming in a vortex chain flow. The SwIMs also form chutes that transport swimmers between vortices with a predicted inter-vortex flux that is consistent with the experiments, despite noise and nonuniformities in the swimming directions of the microbes.

[Phys. Rev. Fluids 9, 054501] Published Fri May 03, 2024

Author(s): Izumi Saito, Takeshi Watanabe, and Toshiyuki Gotoh

The turbulent transport of small particles (dust particles, cloud droplets, etc.) can be viewed macroscopically as a problem of passive scalar turbulence with extremely high Schmidt numbers. To investigate the spectrum of a passive scalar carried by such particles, we conducted Lagrangian particle simulations in homogeneous isotropic turbulence. The scalar variance spectrum obeys the -5/3 and -1 power laws in the lower and higher wavenumber ranges, respectively, with a clear transition at normalized wavenumber of about 0.04. The dimensionless constants for each range are also consistent with the estimations obtained from previous laboratory and simulation studies.

[Phys. Rev. Fluids 9, 054601] Published Fri May 03, 2024

Author(s): Hassan El Itawi, Benjamin Lalanne, Subhadarshinee Sahoo, Emmanuel Cid, Gladys Massiera, Nathalie Le Sauze, and Olivier Masbernat

Substituting a centrifugal field to gravity makes possible the study of the dynamics of rising deformable drops in another immiscible liquid, keeping free from contamination of the interface. In this experiment, impurities do not have time to adsorb during the very short residence time due to the strong acceleration. Experiments are used to validate direct numerical simulation results in liquid-liquid systems, and the aspect ratio of rising ellipsoidal droplets is found to be a growing function of the sole Weber number, with a smaller rate compared to clean bubbles.

[Phys. Rev. Fluids 9, L051601] Published Fri May 03, 2024

Author(s): Lennard Miller, Bruno Deremble, and Antoine Venaille

We unveil a gyre turbulence regime within a two-dimensional wind-driven ocean model, where energy dissipation becomes independent of fluid viscosity. This anomalous dissipation is driven by a vigorous two-dimensional vortex gas overlaying a low energy western-intensified gyre, shedding light on the effect of boundary instabilities in disrupting the inverse energy cascade.

[Phys. Rev. Fluids 9, L051801] Published Fri May 03, 2024

Author(s): Junichi Morita, Ryuichi Kimura, Hiroya Mamori, and Takeshi Miyazaki

A direct numerical simulation of turbulent flow over a backward-facing step is performed. To control flow separation, a wall-normal body force in the form of a wave-machine-like traveling wave is applied to the top surface of the step. In this control, the recirculation bubble is periodically released in time and in the streamwise direction. In addition, a pair of longitudinal vortices is generated above the step and in the recirculation bubble and the released separation bubble, which causes three effects that reduce the reattachment length: a decrease in the secondary bubble streamwise length, an enhancement of the negative wall-normal velocity, and an increase in Reynolds shear stress.

[Phys. Rev. Fluids 9, 053903] Published Thu May 02, 2024

Author(s): Himpu Marbona, Daniel Rodríguez, Alejandro Martínez-Cava, and Eusebio Valero

Direct numerical simulations examine the separated flow over a wall-mounted bump subjected to harmonic inflow oscillations, resembling the passage of wakes from the preceding stage along the suction side of low-pressure turbine blade conditions. Three scenarios unfold: (i) analog to the steady inflow where the separated-flow laminar-to-turbulent transition is initiated by self-sustained Kelvin-Helmholtz (KH) instability; (ii) intermittent large vortex cluster formation replacing the KH during a part of the inflow period; and (iii) a continuous vortex cluster formation and release that shorten the separated flow length compared to steady inflow, the desirable one for practical applications.

[Phys. Rev. Fluids 9, 053901] Published Wed May 01, 2024

Author(s): Savannah D. Gowen, Thomas E. Videbæk, and Sidney R. Nagel

The iconic branching patterns of the viscous fingering instability, where one fluid invades another in a thin gap, are unconventionally visualized in an experiment that allows us to measure the velocity field throughout the fluid as the patterns form and grow. We find that local pressure gradients decay, both in front of and behind the interface, with a characteristic decay length. This elegant visualization allows us to see how the structure seeded at the interface continues to influence flow long after the instability onset. With this technique we capture features of the late-time flow field that have traditionally been hard to access by simulation or experiment.

[Phys. Rev. Fluids 9, 053902] Published Wed May 01, 2024

Author(s): Robben E. Migacz, Morgan Castleberry, and Jesse T. Ault

The diffusiophoretic velocity of a particle depends nonlinearly on solute concentration. We examine the implications of this nonlinearity in a dead-end pore, which is a geometry found both in nature and in microfluidic devices. We define the efficiency of injection and withdrawal processes, then describe particle dynamics with a step-like change in solute concentration at the pore inlet (commonly used in experiments) and with time-dependent boundary solute concentration. We show that particle migration becomes linear with slow transitions in solute concentration and demonstrate changes in particle dynamics with oscillatory solute concentration at the pore inlet.

[Phys. Rev. Fluids 9, 044203] Published Tue Apr 30, 2024

Author(s): SungGyu Chun, Evgeniy Boyko, Ivan C. Christov, and Jie Feng

The flow rate–pressure drop relations for laminar flow of Newtonian fluids in common geometries are well understood. However, a complete understanding of how the interplay between shear-thinning rheology and wall compliance sets the flow rate–pressure drop relation for a deformable configuration is still lacking. Here, we provide detailed quantitative comparisons between theory and experiments for the flow rate–pressure drop relation for Newtonian and shear-thinning fluids in two common deformable configurations: a rectangular channel and an axisymmetric tube. Such a comparison is of fundamental importance since it provides insight into the adequacy of the constitutive model used.

[Phys. Rev. Fluids 9, 043302] Published Thu Apr 25, 2024

Author(s): Neeraj S. Borker, Abraham D. Stroock, and Donald L. Koch

A self-aligning particle (SAP) attains near perfect alignment with the fluid lamellae of a low Reynolds number simple shear flow without application of external torques in contrast with the continuous rotation exhibited by most rigid bodies including thin fibers and disks. We characterize the robustness of the flow alignment of SAPs to secondary perturbations such as flow disturbances, Brownian motion, inter-interparticle interactions, and the presence of a wall using dynamic simulations of ring-shaped SAP geometries. The robust flow alignment of SAPs provides an alternative route to access highly aligned microstructures that are inaccessible to suspensions of traditional particle geometries.

[Phys. Rev. Fluids 9, 043301] Published Wed Apr 24, 2024

Author(s): E. Dormy and H. K. Moffatt

Stokes flow driven by rotation of two parallel cylinders inside a cylinder of large radius R0 is investigated, and the flow in this triply-connected domain is determined numerically, with particular focus on the narrow-gap situation, when the local behavior is well described by lubrication theory. The asymptotic situation for infinite R0 is inferred (i) when the cylinder axes are unconstrained, and (ii) when they are held fixed. Contributions to the far-field are identified: a torquelet and a radial quadrupole in the counter- and co-rotating cases, respectively. In the former case, when the cylinders make contact (zero gap) a contact force acting on the cylinder pair is identified.

[Phys. Rev. Fluids 9, 044102] Published Mon Apr 22, 2024

Author(s): Nan He, Yutong Cui, David Wai Quan Chin, Thierry Darnige, Philippe Claudin, and Benoît Semin

We report experiments on the dissolution of a single particle during its sedimentation in a quiescent aqueous solution in the regime of low Reynolds and high Péclet numbers. We use butyramide, a chemical which does not change the density of water when it dissolves. The particle shrinks at a rate independent of its initial radius, in agreement with the model that we derive assuming Stokes drag and a mass transfer rate given by Levich (1962). This model becomes quantitative when including two correction factors to account for the non-sphericity of the particle and for the inclusions of air bubbles inside the particle.

[Phys. Rev. Fluids 9, 044502] Published Mon Apr 22, 2024

Author(s): Alexis Commereuc, Sandrine Mariot, Emmanuelle Rio, and François Boulogne

The structure of liquid foams follows simple geometric rules formulated by Plateau 150 years ago. On smooth surfaces, the foam liquid channels, also called pseudo Plateau borders, are straight between vertices. We demonstrate experimentally that on rough surfaces and under some conditions that we establish, the bubble footprint exhibits a morphological transition. The footprint can adopt a zigzag shape between vertices. We rationalize the number of zigzag segments by a geometric distribution describing the observations made with the footprint perimeter and the mesh size of the asperities.

[Phys. Rev. Fluids 9, L041601] Published Mon Apr 22, 2024

Author(s): Rebecca Liyanage, Xiaojing Fu, Ronny Pini, and Ruben Juanes

We examine how fluids mix in three-dimensional (3D) porous materials due to differences in density which represent one mechanism of underground carbon dioxide storage. The experiment closely matched the simulation in terms of the patterns and speed of mixing. Interestingly, the experiment reveals columnar plumes self-organizing into a reticular pattern, previously seen only in 3D simulations. Results demonstrate quantitative matching over time in concentration, variance, scalar dissipation rate, and dissolution flux. A new relation between dissipation rate and flux is established, highlighting a 30% higher flux in 3D versus 2D systems, affirming prior estimations.

[Phys. Rev. Fluids 9, 043802] Published Thu Apr 18, 2024

Author(s): G. Corsi, P. G. Ledda, G. Vagnoli, F. Gallaire, and A. De Simone

Seed dispersal strategies exemplify the role of morphology in defining falling paths. Here, macroscopic geometry effects are systematically studied by considering trajectories of annular disks. Via linear stability analysis, we identify the stability boundary for vertical fall, with a non-monotonic behavior of the critical falling velocity with the hole size, before increasing for large holes. Nonlinear simulations confirm linear analyses and suggest strategies for annular seed release at different heights, with the emergence of paths possibly beneficial for controlled positioning or advantageous for covering large lateral distances, depending on the hole size and disk weight.

[Phys. Rev. Fluids 9, 043907] Published Wed Apr 17, 2024

Author(s): Hao Zeng, Feng Wang, and Chao Sun

The impact and freezing process of a water droplet are studied experimentally, incorporating the presence of frost and salt. A distinct transition of the spreading dynamics is observed, altering from the well-known 1/2 inertial scaling law to a 1/10 capillary-viscous scaling law. By considering the effect of impact inertia, partial-wetting behavior, and salinity, the mechanism of this transition is elucidated, and a unified model for predicting the droplet arrested diameter is proposed.

[Phys. Rev. Fluids 9, 044001] Published Tue Apr 16, 2024

Author(s): Diego Donzis and Shilpa Sajeev

Turbulent flows comprise a multitude of scales which make them extremely difficult to analyze and simulate. In this work, we study homogeneous isotropic turbulence using a novel approach which solves the Navier-Sokes equations on a reduced set of modes that are sampled stochastically. The complementary set are solved using trivial dynamics. This method, called Selected Eddy Simulations, is able to capture broad dynamics of turbulence with just 10% of resolved modes, suggesting the turbulence attractor may be smaller or more robust to modeling than previously thought. This result also holds promise for developing alternative low-cost numerical approaches to study turbulent flows.

[Phys. Rev. Fluids 9, 044605] Published Mon Apr 15, 2024

Author(s): Alexander Yu. Gelfgat and Gerrit Maik Horstmann

Electrocapillary flows in a classical experiment of Melcher and Taylor are studied. The computed streamlines qualitatively represent the experimental image. With the increase of electrocapillary forcing, the main circulation localizes near a boundary with a larger electric potential. When a dielectric liquid is replaced by a poorly conducting one, the system becomes non-isothermal owing to the Joule heating, and the flow is driven also by buoyancy and thermocapillary convection. The results show that consideration of the two-phase model is mandatory. The Lippmann equation, connecting electrically induced surface tension with nonuniform surface electric potential, is numerically verified.

[Phys. Rev. Fluids 9, 044101] Published Fri Apr 12, 2024

Author(s): S. Balachandar, C. Peng, and L.-P. Wang

The presence of a dispersed phase substantially modifies small-scale turbulence. Here we present a comprehensive mechanistically based model to predict turbulence modulation, the predictions of which, compared with particle-resolved simulations and experiments, is shown.

[Phys. Rev. Fluids 9, 044304] Published Thu Apr 11, 2024

Author(s): Xiaokang Zhang, Jake Minten, and Bhargav Rallabandi

Acoustic fields are used in numerous applications to induce movement of suspended particles. We develop a rigorous theory that systematically unifies inviscid acoustophoresis with viscous streaming. The theory connects particle motion to a generalized form of the secondary radiation force, which depends on the Stokes layer thickness around the particle, and the contrast of density and compressibility between the particle and the fluid. We identify a reversal of particle motion when inertial and viscous forces are comparable, validated with numerical solutions. This has significant implications for applications involving particle sorting or focusing based on size or material properties.

[Phys. Rev. Fluids 9, 044303] Published Wed Apr 10, 2024