Latest papers in fluid mechanics

Author(s): Artur Gesla and Eunok Yim

This study investigates the formation of the helical vortex rope in a Francis hydropower turbine by treating it as an unstable vortex breakdown mode in a simplified laminar flow. The vortex rope emerges through a supercritical Hopf bifurcation from an axisymmetric base flow in the draft tube. Without wall friction, a central recirculation zone develops, revealing subcritical solutions and hysteresis under partial-load conditions. The work describes the cyclic formation and collapse of the recirculation bubble as the helical vortex evolves. As flow approaches nominal load, the steady solution branch undergoes a transcritical bifurcation at finite Reynolds number.

[Phys. Rev. Fluids 11, 094701] Published Thu Sep 03, 2026

Author(s): Pim J. Dekker, Christian Diddens, Marjolein N. van der Linden, and Detlef Lohse

Surface tension is frequently measured with the pendant drop method, while the ambient humidity is usually an afterthought. With carefully calibrated experiments and detailed numerical simulations, we show that evaporative cooling lowers the drop temperature by up to 9.5 K which in turn raises the measured surface tension by more than 1 mN/m. Evaporation-driven and Marangoni-induced flows additionally deform the drop, but only marginally. A passive humidity control removes these artifacts entirely and reveals the shallow minimum in the surface tension of aqueous 1,2-hexanediol mixtures.

[Phys. Rev. Fluids 11, 094901] Published Thu Sep 03, 2026

Author(s): Daniela Dávalos Ruedas, R. E. Moctezuma, and J. Rodrigo Vélez-Cordero

Theoretical expressions used to study the assembly of magnetic particles under the action of external magnetic fields are generalized to incorporate carrier fluids with varying viscosity (shear thinning fluids). The theory is essentially local, and the drag force assigned to each particle considers …

[Phys. Rev. E 114, 035102] Published Wed Sep 02, 2026

Author(s): Nianhua Liu and Serhiy Yarusevych

Lifting surfaces operating at aerodynamically low Reynolds numbers often have laminar separation bubbles (LSBs) whose characteristics can significantly affect performance. We investigate the effect of localized surface roughness, which might form due to manufacturing, local contamination, damage or icing, on LSBs. We find that the localized roughness eliminates downstream laminar separation and modifies the LSB topology and dynamics over a substantial spanwise region extending well beyond the roughness itself. The associated effect on aerodynamic performance is considerably greater than would be expected based solely on the relative spanwise extent of the localized roughness.

[Phys. Rev. Fluids 11, 093901] Published Tue Sep 01, 2026

Author(s): Lin-Ling Li (李临玲), Juan-Cheng Yang (阳倦成), and Ming-Jiu Ni (倪明玖)

Compared with a static liquid film, gas jet impingement on a flowing liquid metal film produces a shallower cavity and a strongly asymmetric profile through upstream liquid accumulation. Counter-current coupling upstream and co-current coupling downstream cause the wave-amplitude response to jet momentum to weaken upstream but strengthen downstream as the liquid Reynolds number increases. Under pulsating jets, an attenuation coefficient correlates wave amplitude with jet momentum and characterizes a pronounced spatial asymmetry in momentum transfer. These findings clarify how gas jets influence the liquid metal film flow and provide a reference for the design of liquid metal divertors.

[Phys. Rev. Fluids 11, 094801] Published Tue Sep 01, 2026

Author(s): Vladimir Rosenhaus and Daniel Schubring

We study wave turbulence in systems with two special properties: a large number of fields (large N) and a nonlinear interaction that is strongly local in momentum space. The first property allows us to find the kinetic equation at all interaction strengths—both weak and strong, at leading order in 1…

[Phys. Rev. E 114, 035101] Published Tue Sep 01, 2026

Author(s): Marek Stastna and Andrew P. Grace

This article presents a simple to implement algorithm for detecting strong cabbeling in stratified fluids with a nonlinear equation of state. The algorithm is used to identify the manner in which the arrested development of the Rayleigh-Taylor instability occurs in the strongly cabbeling regime.

[Phys. Rev. Fluids 11, 084505] Published Mon Aug 31, 2026

Author(s): Sedat Tardu and Benjamin Arrondeau

Large staggered roughness elements in the entrance region of a subcritical channel flow generate intense turbulent eddies that gradually break down as the flow progresses into the downstream smooth channel (SC) of the configuration studied here. This flow combines several sources of complexity: surface roughness, a rough-to-smooth step change and relaminarization. The characteristics of the turbulence decay along the SC centerline agree reasonably well with those of homogeneous isotropic turbulence. Owing to the slow rate of relaminarization, the Nusselt number averaged over large streamwise distances in the SC remains close to that of an equivalent fully developed turbulent channel flow.

[Phys. Rev. Fluids 11, 084611] Published Mon Aug 31, 2026

Author(s): H. A. J. Middleton-Spencer, B. McCanna, D. Proment, and H. M. Price

Vortex reconnections are fundamental to quantum fluids. They are reconnections of quantum vortices defined by a topological change and irreversible energy transfer given by universal scaling laws, leading to quantum turbulence. We generalize this mechanism by studying reconnections of extra-dimensional quantum vortices. We find a new range of vortex interactions depending on their initial orientation – a four-dimensional generalization to the known three-dimensional case; vortex interactions with no reconnections; and a new class reconnection that keeps the same scaling law with no energy transfer, suggesting a form of reversible reconnection not allowed in three-dimensional systems.

[Phys. Rev. Fluids 11, 084701] Published Mon Aug 31, 2026

Author(s): Prasoon Suchandra and Shabnam Raayai-Ardakani

We study flow past V-formations of cylinders, inspired by migratory birds’ flight, examining how formation angle affects the drag of both individuals and the group. Using particle image velocimetry in a water tunnel, we evaluate the impact of changing angles on complex wake-wake/wake-body interactions, quantifying mean and fluctuating quantities, and analyzing vortex dynamics. We find that members with streamwise overlap experience major reductions in the drag force compared to a solo cylinder. Our study establishes a baseline for probing flow past groups of complex bodies (like drones) and demonstrates how optimizing formations can achieve desired performance goals, such as minimum drag.

[Phys. Rev. Fluids 11, 084702] Published Mon Aug 31, 2026

Author(s): Peng Wang, Jiawei Han, Kun Xue, and Yu Guo

We present a numerical study of shock-induced dispersal in dense, flexible fiber curtains using a coupled discrete element method–computational fluid dynamics (DEM–CFD) approach. The fiber curtains exhibit distinct particle dynamics compared to spherical particle curtains. Increasing the fiber aspect ratio and reducing flexibility enhance geometric interlocking and prolong clustering, thereby increasing resistance to shock propagation. Finally, we modify a scaling law using an effective fiber aspect ratio to predict the temporal evolution of curtain expansion, accounting for fiber elongation and significant bending deformation.

[Phys. Rev. Fluids 11, 084302] Published Fri Aug 28, 2026

Author(s): Francisco Monteiro, Tommaso De Maria, Samuel Ahizi, Ramon Abarca, Giuseppe C. A. Caridi, and Miguel A. Mendez

Vertical accelerations can amplify small free-surface disturbances into strongly nonlinear sloshing through parametric resonance, yet the resulting regimes remain poorly characterized in horizontal cylindrical tanks. While the classical Mathieu equation predicts the onset of parametric instability, here, high-speed experiments, combined with data-driven modal analysis and classification, reveal nonlinear responses that develop beyond the instability threshold. The resulting dimensionless regime maps organize stable, longitudinal, breaking, and mixed-mode dynamics across different fill levels, exposing the rich flow phenomenology surrounding the primary parametric-instability region.

[Phys. Rev. Fluids 11, 084804] Published Fri Aug 28, 2026

Author(s): Smiron Varghese, Benjamin Miquel, and Wouter J. T. Bos

We present an analytical investigation of the global helicity budget associated with the salt-fingering instability within the two-dimensional, three-component framework. Our analysis shows that in the region of parameter space corresponding to salt fingering, helicity amplification occurs when the …

[Phys. Rev. E 114, L023101] Published Thu Aug 27, 2026

Author(s): Huifeng Chen (陈慧锋), Yixin Yang (杨揖心), Mingbo Sun (孙明波), Hongbo Wang (汪洪波), Dapeng Xiong (熊大鹏), Changhai Liang (梁昌海), Wenxiao Long (龙文骁), and Wenming Li (李文明)

This paper uses direct numerical simulations to systematically investigate supersonic flows in a bent pipe with a developing turbulent boundary layer and a core flow region. The results reveals that boundary layers experience intricate flow patterns at different azimuthal angles: secondary flows that drives the streaks move from the lower to the upper side; separation triggered by the combined effects of the adverse pressure gradient and flow deceleration on the upper wall; and the increase of the Görtler instability on the lower wall inducing the clustering and uplift of low-momentum fluids, the presence of Görtler-like vortices, and the baroclinic effect of turbulent transport processes.

[Phys. Rev. Fluids 11, 083401] Published Thu Aug 27, 2026

Author(s): Yu Cheng and Tianle Liu

Turbulent transport in large-eddy simulations relies on subgrid-scale (SGS) closures, yet conventional models typically assume that SGS stresses are uniquely determined by the resolved flow. We introduce a conditional diffusion model that learns the conditional distribution of SGS stresses from high-resolution atmospheric boundary layer simulations. By representing conditional variability rather than a single deterministic mapping, the proposed framework accurately reproduces SGS stress statistics, generalizes across unseen stability regimes and grid resolutions, and provides a new probabilistic framework for SGS turbulence modeling.

[Phys. Rev. Fluids 11, 084610] Published Wed Aug 26, 2026

Author(s): Pranav Hegde, Wietze Herreman, Jorge César Brändle de Motta, Romain Canu, Marie-Charlotte Renoult, and Gerrit Maik Horstmann

Metal-pad-roll instabilities limit the safe and efficient operation of aluminum reduction cells, yet laboratory-scale models fall outside the assumptions of most existing theories. We develop an analytical stability theory for small rectangular two-layer cells that captures finite-depth, viscous, and capillary effects, including a parameter-free description of interfacial-wave damping. Validated against direct numerical simulations and experiments, the theory provides quantitative benchmarks for designing and interpreting small-scale MHD experiments and multiphase simulations.

[Phys. Rev. Fluids 11, 084803] Published Wed Aug 26, 2026

Author(s): Johnathan Hoggarth, Daniel M. Harris, John W. M. Bush, and Bauyrzhan K. Primkulov

We introduce a simple droplet generation technique that rapidly stretches a liquid ligament on both quiescent and vibrating baths. By systematically varying the stretching distance and the radius of the cylindrical probe used to form the ligament, we map the parameter space in which the ligament pinches off and collapses into a single droplet. The resulting droplet size follows the volume-conservation scaling R∼a2/3L1/3 , with excellent reproducibility (radius variation below 5%).

[Phys. Rev. Fluids 11, L082001] Published Wed Aug 26, 2026

Author(s): Mehdi Riahi and Mohamed Hayani Choujaa

Previous studies dealing with Floquet stability analysis of non-zero-mean time-modulated Rayleigh-Bénard convection have shown the existence of only harmonic and subharmonic instability modes. Here, we emphasize the existence of quasiperiodic instabilities that have not yet been reported in the lite…

[Phys. Rev. E 114, 025108] Published Mon Aug 24, 2026

Author(s): Chien-Chia Liu and Jui-Yin Lin

Wall roughness plays a central role in determining turbulent friction in pipe flows, yet most predictive frameworks continue to characterize the wall by a single effective roughness scale. Classical datasets—notably Nikuradse's sand-grain experiments and the rough-pipe measurements of Colebrook and …

[Phys. Rev. E 114, 025109] Published Mon Aug 24, 2026

Author(s): Bin Zhang, Yuke Li, Hongna Zhang, Guiren Wang, Rong Liu, Shaowei Wang, and Zijing Ding

Polymer diffusive instability (PDI) provides a unique route to instability in viscoelastic flows at vanishing Reynolds numbers, yet how realistic wall conditions modify its behavior remains unclear. Here, we show that wall slip regulates PDI in viscoelastic Poiseuille flows by altering the dominant instability pathway. Increasing slip weakens the near-wall PDI-1 mechanism and favors a shear-driven PDI-2 regime. Combining linear stability analysis and direct numerical simulations, we reveal the energy-transfer processes underlying this slip-induced transition and provide new insight into controlling polymer-flow instabilities.

[Phys. Rev. Fluids 11, 083905] Published Mon Aug 24, 2026