Physical Review Fluids

Author(s): Vincent Laroche and Alexis K. Kaminski

The response of biogeochemically active scalars to small-scale turbulent mixing is not well characterized. Using direct numerical simulation, we explore the evolution of idealized phytoplankton and nutrient scalars in an unstable stratified shear layer. Extending theory from stratified turbulence literature, we examine irreversible scalar fluxes and show that the type of stratified shear mixing matters (i.e. overturning or scouring), especially when physical and biological timescales align.

[Phys. Rev. Fluids 10, 084801] Published Fri Aug 15, 2025

Author(s): Ron Shnapp

Lagrangian particles in turbulence separate faster backward in time than forward. We provide a kinematic explanation by decomposing separation into speed and velocity-orientation components. Analysis of direct numerical simulations shows that orientation, not speed, drives the asymmetry: when particles approach, their velocities align more strongly toward each other, whereas during separation this alignment is weaker.

[Phys. Rev. Fluids 10, L082601] Published Fri Aug 15, 2025

Author(s): Bérengère Dubrulle, Ariane Barlet, Amaury Barral, Adam Cheminet, Guillaume Costa, Pietro Dragoni, Abhishek Harikrishnan, Adrien Lopez, Kirone Mallick, and Quentin Pikeroen

For a long time, weather forecasting was based on empirical correlations, producing sayings like “Rain before seven, fine by eleven”. The modern weather forecast uses supercomputers and many ground observations. How does it work? Why is our weather app displaying scores for predictions over more than 3 days? Why is it failing sometimes even for shorter periods? Will it improve if we use larger computers and artificial intelligence? The answer to all these questions is now available, thanks to recent progress in mathematics, and involves possible singularities of the inviscid limit of the primitive equations.

[Phys. Rev. Fluids 10, 083801] Published Thu Aug 14, 2025

Author(s): Priyanka Banga, Surya Narayan Maharana, and Manoranjan Mishra

Viscosity-stratified flows are prone to instabilities that influence industrial transport processes such as oil recovery, pipeline lubrication, polymer deposition, and extraction. This study investigates the onset and growth of instabilities in miscible layered channel flows using an initial value problem framework. By capturing the full time-dependent evolution of the base state, the work reveals how transient dynamics affect instability onset and energy amplification, offering new insights beyond traditional quasi-steady analysis.

[Phys. Rev. Fluids 10, 084002] Published Thu Aug 14, 2025

Author(s): Joanne Steiner, Cyprien Morize, Ivan Delbende, Alban Sauret, and Philippe Gondret

When a disk suddenly moves toward or away from a solid wall, the resulting vortex ring behaves in ways that differ from the unbounded case. Experiments and numerical simulations show that the circulation and core radius of the vortices obey new scaling laws that depend not only on disk diameter, stroke length and time but also on the distance to the solid wall.

[Phys. Rev. Fluids 10, 084704] Published Wed Aug 13, 2025

Author(s): Kai Fukami, Luke Smith, and Kunihiko Taira

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.

[Phys. Rev. Fluids 10, 084703] Published Tue Aug 12, 2025

Author(s): Aaron B. Buhendwa, Deniz A. Bezgin, Petr Karnakov, Nikolaus A. Adams, and Petros Koumoutsakos

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.

[Phys. Rev. Fluids 10, 084902] Published Tue Aug 12, 2025

Author(s): Gunnar G. Peng and Ory Schnitzer

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.

[Phys. Rev. Fluids 10, L081701] Published Tue Aug 12, 2025

Author(s): Yasser Almoteri and Enkeleida Lushi

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.

[Phys. Rev. Fluids 10, 083102] Published Fri Aug 08, 2025

Author(s): G. Hunter-Brown, N. Sampara, M. M. Scase, and R. J. A. Hill

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.

[Phys. Rev. Fluids 10, 083601] Published Fri Aug 08, 2025

Author(s): Mogeng Li, Youssef Saade, Stéphane Zaleski, Uddalok Sen, Pallav Kant, and Detlef Lohse

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.

[Phys. Rev. Fluids 10, 084001] Published Fri Aug 08, 2025

Author(s): Qihong L. Li-Hu, Patricia García-Caspueñas, Andrea Ianiro, and Stefano Discetti

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.

[Phys. Rev. Fluids 10, 084901] Published Fri Aug 08, 2025

Author(s): Toshihiro Omori and Takuji Ishikawa

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.

[Phys. Rev. Fluids 10, 080501] Published Thu Aug 07, 2025

Author(s): Mihails Birjukovs, Guntars Kitenbergs, Andrejs Cebers, Klaas Bente, and Damien Faivre

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.

[Phys. Rev. Fluids 10, 083101] Published Thu Aug 07, 2025

Author(s): J.-P. Mollicone, A. Cimarelli, E. De Angelis, and M. van Reeuwijk

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.

[Phys. Rev. Fluids 10, 084602] Published Thu Aug 07, 2025

Author(s): Justin P. Cooke, George I. Park, Douglas J. Jerolmack, and Paulo E. Arratia

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.

[Phys. Rev. Fluids 10, 084601] Published Mon Aug 04, 2025

Author(s): Jianxun Zhu, Cai Tian, Lars Erik Holmedal, and Helge I. Andersson

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.

[Phys. Rev. Fluids 10, 084701] Published Mon Aug 04, 2025

Author(s): Avinash Kumar Pandey and Rajneesh Bhardwaj

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.

[Phys. Rev. Fluids 10, 084702] Published Mon Aug 04, 2025

Author(s): Jingyi Li, Laurel Ohm, and Saverio E. Spagnolie

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.

[Phys. Rev. Fluids 10, 083301] Published Fri Aug 01, 2025

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

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.

[Phys. Rev. Fluids 10, 074103] Published Thu Jul 31, 2025