Recent Articles

Extreme vortex-gust airfoil interactions at Reynolds number 5000

Kai Fukami, Luke Smith, and Kunihiko Taira

Phys. Rev. Fluids 10, 084703 (2025) - Published 12 August, 2025

This study examines extreme vortex gust-airfoil interactions at Reynolds number 5000 using large-eddy simulations and nonlinear machine learning. We show that aerodynamic responses remain primarily two-dimensional up to gust ratios |G| ≤ 3 but transition to three-dimensional dynamics beyond |G| ≥ 4. We further reveal the low-dimensional nature of extreme aerodynamic flows for cases where the interaction dynamics are primarily two-dimensional throughout nonlinear observable-augmented autoencoder compression. These findings provide a foundation for modeling and control of small-scale aircraft operations under highly gusty environments.

Data-driven shape inference in three-dimensional steady-state supersonic flows: Optimizing a discrete loss with JAX-Fluids

Aaron B. Buhendwa, Deniz A. Bezgin, Petr Karnakov, Nikolaus A. Adams, and Petros Koumoutsakos

Phys. Rev. Fluids 10, 084902 (2025) - Published 12 August, 2025

We present a method for the simultaneous inference of flow fields and obstacle shapes from sparse measurements in steady-state compressible flows. Such inverse problems are highly ill-posed and require strong regularization. We address this by combining the Optimizing a Discrete Loss (ODIL) technique with JAX-Fluids. ODIL minimizes the discrete residual of the governing equations, preserving both the accuracy and convergence properties of the underlying numerical methods. The employed conservative finite-volume scheme, including shock-capturing reconstruction and a sharp-interface immersed boundary method, is crucial for effective regularization and therefore accurate flow field inference.

Bistability and charge-density blowup in the onset of drop Quincke rotation

Gunnar G. Peng and Ory Schnitzer

Phys. Rev. Fluids 10, L081701 (2025) - Published 12 August, 2025

The Quincke effect is a striking symmetry-breaking phenomenon in which a particle undergoes spontaneous rotation when subjected to a sufficiently strong electric field. This study, focused on Quincke rotation of a two-dimensional circular (non-deformable) drop, numerically demonstrates the emergence of bistability as the drop viscosity is reduced relative to that of the surrounding fluid — consistent with experimental observations. It is found that capturing this transition entails resolving the formation of charge-density blowup singularities driven by surface convection.

Microswimmer collective dynamics in Brinkman flows

Yasser Almoteri and Enkeleida Lushi

Phys. Rev. Fluids 10, 083102 (2025) - Published 8 August, 2025

Tiny obstacles in a Brinkman fluid can dramatically alter how swimming microorganisms like bacteria or micro-algae coordinate their motion. The environmental resistance presented by the particulate delays and, at high enough levels, completely suppresses the collective instabilities that arise due to hydrodynamic interactions between the swimmers. By contrasting our results with those for homogeneous fluids, we highlight how the physical structure of a habitat can control and disrupt whether microorganisms swim in coordinated groups.

Nonlinear free-decay oscillations of a magnetically levitated air bubble in water produced by coalescence

G. Hunter-Brown, N. Sampara, M. M. Scase, and R. J. A. Hill

Phys. Rev. Fluids 10, 083601 (2025) - Published 8 August, 2025

While theory has studied the large amplitude single-mode shape oscillations of gas bubbles, this is an idealized case. Real-world scenarios typically involve the excitation of many modes. This study probes the nonlinear shape oscillations produced through coalescence, introducing magnetic levitation for the first time to freely suspend air bubbles in water, 5–6 mm in diameter, with negligible distortion. These experiments, along with simulations, reveal that while the bubble’s shape generally agrees well with theory, the coupling of multiple oscillation modes significantly alters its frequency response, highlighting a key aspect of bubble dynamics not captured by single-mode theory.

Viscoelasticity reduces the droplet size in mucosalivary film fragmentation during intense respiratory events

Mogeng Li, Youssef Saade, Stéphane Zaleski, Uddalok Sen, Pallav Kant, and Detlef Lohse

Phys. Rev. Fluids 10, 084001 (2025) - Published 8 August, 2025

We examine the fundamental fluid dynamical mechanisms dictating the generation of bioaerosols in the human trachea during intense respiratory events, such as coughing and sneezing. Using a ‘cough machine’ and numerical simulations, we observe that when subject to intense shear from the airflow, the mucosalivary-mimetic fluid lining forms bag-like structures. These structures rupture through the appearance of retracting holes on the bag surface, generating droplets via the unstable retraction of liquid rims bounding these holes. Viscoelasticity of the mucosalivary-mimetic fluid promotes the formation of larger, thus thinner bags, leading to the production of smaller droplets upon rupture.

Model-based time super-sampling of turbulent flow field sequences

Qihong L. Li-Hu, Patricia García-Caspueñas, Andrea Ianiro, and Stefano Discetti

Phys. Rev. Fluids 10, 084901 (2025) - Published 8 August, 2025

A novel model-based approach for time super-sampling of turbulent flow fields is proposed, based on POD-Galerkin models. Temporal resolution is recovered by integrating in time the dynamical system obtained from projecting the Navier-Stokes equations onto a low-dimensional space derived through Proper Orthogonal Decomposition (POD). This method enables temporally continuous reconstructions between non-time-resolved Particle Image Velocimetry (PIV) snapshots. Our results demonstrate the capability to accurately reconstruct flow dynamics between available measurements.

Ciliary fluid dynamics of swimming, feeding, pumping, and sensing

Toshihiro Omori and Takuji Ishikawa

Phys. Rev. Fluids 10, 080501 (2025) - Published 7 August, 2025

Cilia are ancient cell organelles that generate fluid flow by beating periodically. They play four key roles: swimming, feeding, pumping, and sensing. This study explores how cilia generate flow and perform these functions. Swimming efficiency peaks when the number of cilia scales with body length squared, matching biological scaling. In choanoflagellates, inward flagella enhance feeding, and outward motion aids swimming. In mouse embryos, nodal flow from motile cilia is sensed by immotile cilia to establish left-right body asymmetry. These findings underscore the diverse roles of ciliary flow and the significance of fluid mechanics in biology.

Magnetic control of magnetotactic bacteria swarms

Mihails Birjukovs, Guntars Kitenbergs, Andrejs Cebers, Klaas Bente, and Damien Faivre

Phys. Rev. Fluids 10, 083101 (2025) - Published 7 August, 2025

Collectively controllable active particle swarms are prospective for object manipulation and payload carrying in fluidic microenvironments, but a theoretical description is missing. Observing the motion of magnetotactic bacteria swarms perpendicular to the applied in-plane magnetic field, we present a torque dipole-based “hydrodynamics with spin” model for active particle swarms, and use it to explain this behavior. The motion direction is given by the left-hand rule, and the velocity magnitude is linear in the magnetic field magnitude. The theory is applicable to a wider class of systems, enabling the control of swarms of various types of active particles via different driving fields.

Two-point correlations conditioned on the turbulent/nonturbulent interface in a turbulent temporal jet

J.-P. Mollicone, A. Cimarelli, E. De Angelis, and M. van Reeuwijk

Phys. Rev. Fluids 10, 084602 (2025) - Published 7 August, 2025

This study explores conditional one- and two-point velocity fluctuation correlations relative to the turbulent/non-turbulent interface (TNTI) in a temporal jet. By comparing classical and TNTI-conditioned averages, it reveals new spatial correlations, scaling behaviors and peculiar turbulent production regions which may affect turbulence models. The analysis uncovers distinct correlation peaks tied to the TNTI’s variability that point to large-scale turbulent motions influenced by the interface and that may be missed by conventional one-point or unconditioned analyses.

Alternative scaling for roughness transitions in turbulent flows: The role of the internal boundary layer

Justin P. Cooke, George I. Park, Douglas J. Jerolmack, and Paulo E. Arratia

Phys. Rev. Fluids 10, 084601 (2025) - Published 4 August, 2025

Internal Boundary Layers (IBL) form when turbulent flows encounter sudden changes in surface roughness. The IBL introduces new length- and time-scales to the flow, characterizing the turbulent motions within. We introduce an alternative scaling for velocity profiles within the IBL using IBL-based parameters: the IBL height and edge velocity. Using our numerical simulations and two experimental datasets, we demonstrate the capability of these new scaling parameters for a variety of flow and surface conditions, offering a simpler, more unified way to understand the behavior within this spatially developing turbulent region.

Vortical structures and streak instabilities over a single deep circular dimple within a laminar boundary layer flow

Jianxun Zhu, Cai Tian, Lars Erik Holmedal, and Helge I. Andersson

Phys. Rev. Fluids 10, 084701 (2025) - Published 4 August, 2025

Dimples – localized indentations in otherwise smooth surfaces – have attracted attention in both numerical simulations and experiments due to their practical applications. Direct numerical simulations have been performed to examine the flow in a zero-pressure-gradient boundary layer over a single deep circular dimple. The flow is characterized by the presence of a tornado-like vortex pair within the dimple, followed downstream by a quasiperiodic shedding of hairpin vortices forming a vortex street. Downstream of the dimple, both sinuous and varicose streak instabilities develop, contributing to the generation of hairpin vortices and the periodic meandering of these vortices, respectively.

Flow-induced rotational vibration of a circular cylinder with attached rigid splitter plate in an inverted configuration

Avinash Kumar Pandey and Rajneesh Bhardwaj

Phys. Rev. Fluids 10, 084702 (2025) - Published 4 August, 2025

While splitter plates are typically employed to suppress flow-induced vibration in bluff bodies, this study reveals an opposite outcome. In a new configuration, attaching a splitter plate to the windward side of a circular cylinder leads to amplified rotational vibrations. Numerical simulations at low Reynolds numbers uncover several regimes, including large-amplitude oscillations, chaos, and symmetry breaking — driven by vortex-structure interactions. The findings highlight the role of leading-edge vortices and demonstrate synchronized structural oscillations with enhanced energy transfer between fluid and structure, offering potential applications in energy harvesting.

Arrested development and traveling waves of active suspensions in nematic liquid crystals

Jingyi Li, Laurel Ohm, and Saverio E. Spagnolie

Phys. Rev. Fluids 10, 083301 (2025) - Published 1 August, 2025

A mean-field theory is derived for a dilute suspension of active particles in a nematic liquid crystal. Beyond a critical active Ericksen number or particle concentration, the suspension first comes into alignment, then buckles via a classical bend instability. Rather than entering the fully developed roiling state observed in isotropic fluids, the development is arrested into a steady, flowing state by fluid elasticity. The image shows the fluid’s elastic energy in such an arrested state. If the particles are motile, they can surf along the bent environment of their own creation.

Effect of the spanwise domain size on the flow characteristics behind a circular cylinder at low Reynolds numbers

Daeun Song (송다은), Young-Jin Yoon (윤영진), and Haecheon Choi (최해천)

Phys. Rev. Fluids 10, 074103 (2025) - Published 31 July, 2025

The flow over a circular cylinder at Re = 220 exhibits mode-A instability, characterized by a spanwise wavelength of around 4d, where d is the cylinder diameter. In the present study, the spanwise domain is extended up to 252d to investigate if the flow field indeed exhibits a periodic pattern with such a spanwise wavelength of 4d. The results reveal that, in addition to the parallel shedding of approximately 4d, very long oblique shedding up to 45d and vortex dislocation are observed in the cylinder wake. In contrast, at Re = 300, where the flow exhibits mode-B instability, such oblique shedding or vortex dislocation does not occur even with a long spanwise domain.

Footprint of laminar separation on a wall-bounded wing section at transitional Reynolds numbers

Charles J. Klewicki, Bjoern F. Klose, Gustaaf B. Jacobs, and Geoffrey R. Spedding

Phys. Rev. Fluids 10, 073905 (2025) - Published 30 July, 2025

Laminar separation plays a crucial role in wing aerodynamics during the transition to turbulence and, depending on the flow parameters and conditions, can lead to multiple stable states. In the presence of bounding walls, the flow is inherently three-dimensional, with strong spanwise motions observed within the boundary layer up to the wing midspan. These effects persist even in time-averaged views and have important implications for potential control strategies.

Dynamics of droplet impact onto spheres: From hydrophobic to superhydrophobic surfaces

Chenlin Zhu, Boyu Zhang, Lijuan Qian, and Hang Ding

Phys. Rev. Fluids 10, 073605 (2025) - Published 29 July, 2025

This experimental investigation examines water droplet impacts on hydrophobic or super-hydrophobic surfaces across moderate Weber numbers (3 ≤ We ≤ 120) and size ratios (1.04 ≤ Ω ≤ 2.08). Through high-speed imaging, we observe distinct regime transitions as Ω increases, identifying the maximum spreading angle (θ_max > 90°) as the critical transition criterion. An energy-based theoretical model is developed for predicting regime boundaries and maximum spreading behavior.

Flow-acoustic resonance in deep and inclined cavities

You Wei Ho and Jae Wook Kim

Phys. Rev. Fluids 10, 074603 (2025) - Published 29 July, 2025

In this study, we perform wall-resolved large-eddy simulations to investigate flow-acoustic resonances in deep cavities (D/L = 2.632) at three inclination angles and two Mach numbers. We discover that inclined cavities generate acoustic responses more than 30 dB stronger at a surprisingly low frequency (St=0.276) than the orthogonal cavity. Through modal and resolvent analyses, we identify the distinctive vortex dynamics mechanism at play and reveal the primary factors that contribute to the enhanced aeroacoustic responses in the inclined cavities. Finally, we propose a predictive criterion for the onset of deep cavity resonance associated with the distinctive vortex dynamics identified.

Force-driven flow of a slightly rarefied gas in a square duct

Masanari Hattori and Shigeru Takata

Phys. Rev. Fluids 10, 073402 (2025) - Published 28 July, 2025

A laminar duct flow is considered based on the kinetic theory of gases. An asymptotic analysis for small Knudsen numbers reveals that the thermal stress, which is missing in the Navier-Stokes equation, must be included in the momentum balance in the duct’s cross-sectional directions. This stress arises from the nonuniform temperature field developed by viscous dissipation of the main axial flow. It induces a slow secondary flow in the cross-sectional plane, which, through convection, produces finite effects on the axial velocity and temperature fields.

Plasticity effects in coarsening bubbly yield-stress fluids: From damped growth to arrest

Nicolò Galvani, Sylvie Cohen-Addad, Brice Saint-Michel, and Olivier Pitois

Phys. Rev. Fluids 10, 073604 (2025) - Published 28 July, 2025

The yield stress of a plastic matrix embedding bubbles can give rise to distinct coarsening regimes, depending on the balance between plastic and capillary forces. These include: (i) classical coarsening, with bubble growth similar to that in simple liquids; (ii) damped coarsening, where growth slows progressively; and (iii) complete arrest of coarsening once a critical threshold is exceeded.

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