Latest papers in fluid mechanics

Author(s): Joachim Falck Brodin, Kevin Pierce, Paula Reis, Per Arne Rikvold, Marcel Moura, Mihailo Jankov, and Knut Jørgen Måløy

Using a custom-built three-dimensional scanner, this study reveals how immiscible fluid interfaces destabilize during invasion through disordered porous media. As the flow rate is changed, the interface transitions from stable sheets to tangled, unstable fingers. A stability criterion incorporating pressure dynamics and relative permeability is introduced. The findings bridge two-dimensional theory with complex three-dimensional reality, advancing our understanding of multiscale interactions between flow, structure, and interface dynamics.

[Phys. Rev. Fluids 10, 064003] Published Wed Jun 11, 2025

Author(s): Dinesh Kumar Kinjangi and Daniel Foti

Two distinct hypotheses, related to upwind and turbine scales, for the formation of wake meandering —the large-scale, periodic oscillating motion of the far wake of a wind turbine —are tested via LES by varying the incoming scales. Wake meandering is observed throughout the range of scales but exhibits variation in turbulence and spectral characteristics. Two wake meandering scales are observed: (1) turbine influenced scale with regular Strouhal number, St ≈ 0.3 and corresponding harmonics, with a similar wake center distribution across all upwind conditions, and (2) upwind produced scale if low upwind scales are present, St < 0.1 with irregular wake center distributions.

[Phys. Rev. Fluids 10, 064611] Published Tue Jun 10, 2025

Author(s): Anna Frishman and Daniel Lecoanet

A slowly stretched fluid bridge is destined to break, leaving behind a satellite droplet. Combining experiments and one-dimensional simulations, the size of this droplet is shown to be highly reproducible. It is demonstrated to follow a simple formula depending only on the normalized volume of the bridge and the Weber number, so by stretching the bridge faster or increasing its volume the droplet size can be dramatically increased. The robustness of the satellite size is revealed to be due to an underlying universal dynamical solution.

[Phys. Rev. Fluids 10, 063604] Published Mon Jun 09, 2025

Author(s): Doyel Pandey and Steffen Hardt

An electrolyte-filled nanochannel can promote significant thermo-osmotic flow in the presence of wall slip. This flow augments the thermoelectric response of the channel especially in situations with thin electric double layers (relative to the channel width). For example, for a slip length of 40 nm, the thermoelectric power can increase by more than a factor of 200 due to convective charge transport.

[Phys. Rev. Fluids 10, 063701] Published Mon Jun 09, 2025

Author(s): Bradley Pascoe, Michael Groom, and Ben Thornber

The compression or expansion of turbulent mixing layers often occurs under anisotropic strain rates, causing the mixing layer to compress or expand at different rates in different directions. Anisotropic strain patterns are common in nozzle flows and implosions yet are rarely addressed in turbulence models. The treatment of the turbulent length scale under bulk compression is investigated for a two-equation Reynolds—Averaged Navier—Stokes model. Comparing to strained implicit large eddy simulations, the mixing layer properties are better predicted when accounting for the alignment of the strain rates and the mixing layer growth direction.

[Phys. Rev. Fluids 10, 064609] Published Mon Jun 09, 2025

Author(s): Gregory Eyink and Hao Quan

Weak Euler solutions have been hypothesized to explain the d’Alembert paradox of non-vanishing drag. A difficulty is the “weak-strong uniqueness” property, which requires that an admissible weak Euler solution must coincide with the smooth Euler solution for the same initial data. Using the Josephson-Anderson relation adapted from superfluids, we show that weak-strong uniqueness for d’Alembert’s solution requires mild conditions. To explain drag we therefore predict that these conditions are violated by violent eruption of very thin boundary layers. We discuss observational signatures and explain how the small length-scales involved could threaten the validity of a hydrodynamic description.

[Phys. Rev. Fluids 10, 064610] Published Mon Jun 09, 2025

Author(s): Anjali Vajigi and Subhadeep Roy

We investigate the dynamics of active paths during a two-phase flow of immiscible fluids under an external pressure drop ΔP. We demonstrate that this dynamics of new path opening plays a crucial role in understanding the nonlinear rheology for the one-dimensional capillary fiber bundle model in the …

[Phys. Rev. E 111, 065102] Published Mon Jun 09, 2025

Author(s): Peyman Rostami, Alexander Erb, Reza Azizmalayeri, Johanna Steinmann, Robert W. Stark, and Günter K. Auernhammer

We investigate the coalescence dynamics of two identical polymer solution drops on a solid substrate. Our primary focus is the influence of the elastocapillary number (Ec), which is defined as the ratio of the sample timescale (i.e., the polymer relaxation timescale) to the experimental viscous timescale of drop merging. The dynamics of the liquid bridge depend non-monotonically on Ec. A combination of surface tension, polymer stress, and viscosity shapes the bridge profile during the process.

[Phys. Rev. Fluids 10, 063603] Published Fri Jun 06, 2025

Author(s): Jiacheng Yang, Xinshu Zhang, and Jinyu Yao

In this study, with the prediction directly from the framework based on the Zakharov equation using kernels in Hamiltonian form, instability regions were obtained in different water depths. Extensive numerical simulations were conducted using a higher-order spectral method to investigate the modulational instability. Numerical simulations confirm that sidebands grow exponentially, suggesting that there is noticeable modulational instability in shallow water at dimensionless water depth 0.8. An evident amplification can also be achieved.

[Phys. Rev. Fluids 10, 064801] Published Fri Jun 06, 2025

Author(s): Luke Neville

The steady Stokes flow around two unequally sized parallel cylinders is solved for exactly using tools from complex analysis and conformal mapping, generalizing the known solutions for a cylinder moving by a plane wall, and two equal cylinders. The resulting flows are highly constrained by the condition that the system be force and torque free, with it impossible to move one cylinder independently of the other.

[Phys. Rev. Fluids 10, 064102] Published Thu Jun 05, 2025

Author(s): Denis Dumont, Francisco M. Rocha, Maxime Nicolas, and Olivier Pouliquen

Discontinuous shear thickening (DST), extensively studied in dense suspensions, can also occur in dry granular materials when interparticle friction depends on contact force. Using discrete-element simulations, this study shows that introducing a non-Coulombian friction law, mimicking the effect of coatings or lubricants on grain interactions, leads to DST even in the absence of a suspending fluid. A mean-field model is developed that links the microscopic friction law to the bulk rheology, capturing the transition from continuous to discontinuous thickening and offering insights for tuning flow properties in dry industrial powders.

[Phys. Rev. Fluids 10, 064302] Published Thu Jun 05, 2025

Author(s): Xuanxuan Fan, Hezhen Fang, Shuze Tang, Xiuyu Wang, Dazhuan Wu, and Shijie Qin

Understanding plastron deformation of hydrophobic surfaces under turbulent flow is crucial for optimizing drag reduction. This study experimentally reveals that longitudinal plastron deformation significantly undermines the drag reduction performance of micro-grooved hydrophobic surfaces. By correlating changes in plastron morphology with skin friction and turbulence characteristics, we demonstrate that reducing groove length effectively suppresses plastron longitudinal deformation and enhances drag reduction. These findings provide valuable insights for the design of hydrophobic surfaces for turbulent drag reduction.

[Phys. Rev. Fluids 10, 064608] Published Thu Jun 05, 2025

Author(s): Jeyapradhap Thirisangu, Anjan Mahapatra, and Karthick Subramani

How do droplets stay suspended against gravity in an acoustic field, especially when they are too large for the rigid particle assumption to hold? This study explores droplet dynamics beyond the Rayleigh limit, revealing how the balance between acoustic, interfacial, and gravitational forces governs suspension. By modeling the droplet as a fluid, we uncover size-dependent switching between nodes and antinodes, non-monotonic trends in critical energy for suspension, and the potential for droplet sorting, offering new insights into complex droplet behavior in standing acoustic waves.

[Phys. Rev. Fluids 10, 063602] Published Wed Jun 04, 2025

Author(s): Daiki Watanabe and Susumu Goto

What kind of flow is sustained in a horizontally rotating cylinder? When the cylinder is completely filled with liquid, the flow tends to solid-body rotation. However, when the cylinder is partially filled, unexpectedly nontrivial vortical structures can arise. This system is both fundamental to science and practical in application, making it of great interest. In this study, we conduct direct numerical simulations to investigate flow structures that emerge in the presence of axial flow and observe a wide variety of flow patterns induced in the system.

[Phys. Rev. Fluids 10, 063902] Published Wed Jun 04, 2025

Author(s): Dae Yeon Kim, Sachit G. Nagella, Kyu Hwan Choi, and Sho C. Takatori

Many-body hydrodynamic interactions play an important role in the dynamics of fluid suspensions. However, there is a dearth of experimental frameworks with which to quantify them. To address this, we develop an optical tweezer-based technique that accurately measures translation-rotation hydrodynamic coupling between trapped colloids with exquisite precision, obtaining a direct reporter of few- to many-body hydrodynamic interactions experimentally. With our technique, we can precisely quantify distant fluid disturbances that are generated by ∼2 pN of hydrodynamic force at 12 particle radii of separation.

[Phys. Rev. Fluids 10, 064301] Published Wed Jun 04, 2025

Author(s): Max W. Knoop, Rahul Deshpande, Ferry F. J. Schrijer, and Bas W. van Oudheusden

Despite extensive efforts, the mechanisms of drag reduction via transverse wall forcing are not fully understood; here, we emphasize the importance of the Stokes strain rate (SSR). Imposed through a streamwise-periodic square-wave type forcing, SSR forcing is found to be strong and impulsive during reversal of forcing direction but nearly zero otherwise. As the wavelength extends beyond optimal conditions, the impulsive SSR-topology promotes the asymmetry between a short turbulence attenuation (high SSR), and the extended recovery phase (near-zero SSR), while the skin-friction is marked by an out-of-phase response. These insights may also prove valuable in passive forcing surrogates.

[Phys. Rev. Fluids 10, 064607] Published Wed Jun 04, 2025

Author(s): Manikandan Balasubramaniyan, Linghan Chen, Wen Ao, Peijin Liu, Yu Guan, and Larry K. B. Li

We produced globally unstable (sinusoidal mode) and stable flames (varicose mode) with equivalence ratios of 0.47 and 0.51, respectively. When these flames were subjected to different forcing frequencies and amplitudes, they exhibited heat release rate (HRR) amplitude suppression and resonance. However, under higher detuning conditions, both flames showed amplitude suppression, with the globally unstable flame exhibiting greater suppression. We also found that the HRR amplitude suppression is not uniform throughout the flame and shows spatial variations.

[Phys. Rev. Fluids 10, 063201] Published Tue Jun 03, 2025

Author(s): Haryl Ngoh and Jonathan Poggie

The unsteady separation motion in three-dimensional swept shock-wave/boundary-layer interactions has been shown to exhibit different characteristics compared to nominally two-dimensional interactions. In this numerical study of a sharp-fin induced swept interaction, we show that the separation unsteadiness was strongly correlated with fluctuations in the incoming boundary-layer flow. Oscillations of the separation shock foot occurred at a lower frequency range compared to that of the separation position. Time-periodic forcing of the incoming boundary layer modulated the unsteady separation motion where the response was significantly influenced by the frequency and spanwise form of the forcing.

[Phys. Rev. Fluids 10, 063901] Published Tue Jun 03, 2025

Author(s): Mohammad Autif Shahdhaar, Atul Srivastava, and Suneet Singh

The problem of the science behind the impact of droplets on a liquid pool is quite relevant as it uncovers one of the the most abundant fluid interactions. A wave is generated at the impact and it traverses along the air-pool interface. This study provides spatiotemporal resolved whole-field topography of interface following the impact of an immiscible silicon oil droplet on water pool. We report different modes of interaction of the oil droplet based on pool height, droplet viscosity and Weber number, revealing their effect on the surface wave characteristics after the impact.

[Phys. Rev. Fluids 10, 064002] Published Tue Jun 03, 2025

Author(s): Prabhash Kumar, Prahallada Jutur, Anubhab Roy, and Mahesh Panchagnula

Stirring slow viscous flows is a problem common to several physiological systems. We demonstrate that Stokesian oscillatory flows can be stirred via chaotic advection driven by sub-periodic variability and blinking Moffat eddies. Through experiments and simulations in a bifurcating T-section, we identify transitions between open streamlines (non-stirred) and regimes where the stretch-and-fold mechanism, central to Lagrangian chaos, is activated and stirring accentuated. Irreversibly stirring Stokesian flows has been achieved for the first time with greater efficiency than reported in the literature.

[Phys. Rev. Fluids 10, 064101] Published Tue Jun 03, 2025