Latest papers in fluid mechanics

Author(s): Nathaniel Hildebrand, Pedro Paredes, and Meelan M. Choudhari

Accurately modeling boundary-layer transitions is a top research priority in the NASA CFD Vision 2030 Study and can help reduce the environmental impact of aviation. Here we apply stability analysis to subsonic flow over a two-dimensional zero-pressure-gradient boundary layer in the presence of a forward-facing step which has varying height, slope, and corner radii. The computational predictions capture the overall trend from experiments associated with a gradual upstream shift in the transition location as the step height increases. However, the critical step height corresponding to a very large upstream shift in the transition location is not well predicted compared to the measured data.

[Phys. Rev. Fluids 10, 043901] Published Mon Apr 14, 2025

Author(s): Wouter J. T. Bos and Ryo Araki

In the context of turbulent flow, equilibrium can be defined as the state in which the rate of energy input matches the rate of energy dissipation. Departures from this balance characterize nonequilibrium conditions, which can be treated, to leading order, as linear perturbations around the equilibrium state.

[Phys. Rev. Fluids 10, 044603] Published Mon Apr 14, 2025

Author(s): Li Fu, Sardor Israilov, Jesús Sánchez-Rodríguez, Christophe Brouzet, Guillaume Allibert, Christophe Raufaste, and Médéric Argentina

Undulatory swimming is a widespread locomotion strategy in aquatic animals, yet understanding and applying it to efficient artificial systems remains challenging. Using a biomimetic robotic swimmer and reinforcement learning, we identify an optimal control strategy that maximizes thrust. Our findings are validated through experiments, theoretical modeling, and fluid-structure simulations. They provide key insights into efficient aquatic propulsion and open new perspectives for autonomous underwater vehicles.

[Phys. Rev. Fluids 10, 043101] Published Fri Apr 11, 2025

Author(s): J. Tumelty, C. Beaume, and A. M. Rucklidge

Spatially localized states of convection surrounded by quiescent fluid, known as convectons, have been widely studied in natural doubly diffusive convection in the case where thermal and solutal effects are in balance. Here, we investigate localized pattern formation when this balance is broken. We find convectons persist away from the balanced case and show how their emergence is related to different patterned states, including anticonvectons (localized convection rolls attached to the end walls) in thermally dominated regimes and domain-filling patterned states in solutally dominated regimes.

[Phys. Rev. Fluids 10, 044401] Published Thu Apr 10, 2025

Author(s): André Freitas, Kiwon Um, Mathieu Desbrun, Michele Buzzicotti, and Luca Biferale

We study an a posteriori data-driven approach—solver-in-the-loop—for subgrid modelling of a shell model of turbulence. Leveraging differentiable physics, this method allows a neural network to interact with the solver during training, exposing it to physically informed inputs. This departs from the traditional a priori training and yields more accurate and stable closures. Our model is able to capture the intermittent and non-Gaussian nature of the subgrid scales, characteristic of high Reynolds number turbulence, and is able to reproduce high-order statistical moments within error bars.

[Phys. Rev. Fluids 10, 044602] Published Thu Apr 10, 2025

Author(s): Antoine Deblais, Kaili Xie, Peter Lewin-Jones, Dirk Aarts, Miguel A. Herrada, Jens Eggers, James E. Sprittles, and Daniel Bonn

The very first moments of liquid coalescence remain elusive, with discrepancies between experiment, theory, and simulations. By combining high-speed imaging, electrical measurements, and numerical simulations in both drop-drop and drop-bath configurations, we reveal the critical role of the surrounding gas and van der Waals forces. Our study shows that merging is initiated by jump-to-contact, followed by the formation of air pockets that shape the early-stage neck dynamics, offering new insights into this fundamental fluid process.

[Phys. Rev. Fluids 10, L042001] Published Thu Apr 10, 2025

Author(s): Shivakumar Athani, Bloen Metzger, Yoël Forterre, and Romain Mari

Shear-thickening suspensions can be brought to shear jamming: under constant volume flow at finite stress is impossible. If the volume is not fixed and the suspension can dilate, what are the stress levels in the suspension? We show that Darcy flow couples to dilation to give stresses scaling as the square of the size over which dilation occurs. This result is quantitatively modelled using a Reynolds-like dilatancy law and the Wyart-Cates constitutive model, which can e.g. shed light on the stresses observed during impact on shear-thickening suspensions.

[Phys. Rev. Fluids 10, 043301] Published Wed Apr 09, 2025

Author(s): R. A. Heinonen, L. Biferale, A. Celani, and M. Vergassola

Certain animals have evolved complex strategies to track sources of odors which are advected by turbulent flows. In this paper, we model this search task as a partially observable Markov decision process, which allows us to compute optimal Bayesian search strategies in the sense that they reach the source in minimal average time. We apply this approach to realistic data taken from direct numerical simulation. Focusing on the especially difficult decision of what to do when contact with the odor has been lost, we study the optimal trajectories in this scenario — which strongly resemble known animal behaviors — and try to understand the results by way of a simplified model.

[Phys. Rev. Fluids 10, 044601] Published Wed Apr 09, 2025

Author(s): A. W. C. Lau and J. B. Sokoloff

In this paper, we provide a theoretical framework for understanding the friction of water flowing in carbon nanotubes with diameters of the order of a nanometer. Molecular dynamics simulations show that under such circumstances, water forms one-dimensional water wires or hollowed cylindrical periodi…

[Phys. Rev. E 111, 045103] Published Wed Apr 09, 2025

Author(s): Rima Benhammadi, Patrice Meunier, and Juan J. Hidalgo

We investigate experimentally and numerically the fingering instability caused by the dissolution of CO2 in water in a heterogeneous Hele-Shaw cell of variable gap width. Heterogeneity accelerates the instability, which presents a larger amplitude and faster growth rate than in homogeneous constant-gap cells. Experimental and numerical data are compared and provide new insight on the interaction between permeability heterogeneity and fingering instabilities.

[Phys. Rev. Fluids 10, 043501] Published Tue Apr 08, 2025

Author(s): Xujun Fan, Stéphane Dorbolo, Fangye Lin, and Jun Zou

In the classical Leidenfrost framework, the top evaporation of levitating drops is typically ignored, yet combustion redefines the evaporation dynamics for inflamed Leidenfrost drops. This work shows that inflamed Leidenfrost drops burn much longer than those in sub-Leidenfrost states, with evaporation split into independent bottom and top parts. Bottom evaporation dominates—especially for large drops—while the temperature-independent top rate reflects the intrinsic burn rate at saturation temperature. These findings challenge fire suppression strategies by highlighting the prolonged fuel lifetime in the Leidenfrost state and offer a boundary-free method to measure intrinsic burning rates.

[Phys. Rev. Fluids 10, 043601] Published Tue Apr 08, 2025

Author(s): Subhasish Guchhait, Smita S. Sontakke, Shubhadeep Mandal, and Ranabir Dey

We explain how self-propelling active droplets can squeeze through increasingly narrow microchannels with shape and velocity field adaptations. Using microscopy techniques, we show that these droplet microswimmers transform from a spherical to a ‘stadium’-like shape and eventually into an elongated ‘capsule’-like shape as they traverse extreme confined spaces. During this shape evolution, their hydrodynamic signature changes from a symmetric, quadrupolar to an asymmetric velocity field. Using finite-element based numerical simulations, we explain the flow field evolution by considering the thin lubricating film between the microchannel wall and the active droplet interface.

[Phys. Rev. Fluids 10, 044202] Published Tue Apr 08, 2025

Author(s): Haokui Jiang, Jean-Lou Pfister, Daniel Zhengyu Huang, and Shunxiang Cao

We develop a Koopman reduced-order model (ROM) to analyze the instability mechanism and predict the hydrodynamic behavior for the flag flapping in the wake of a cylinder. The Koopman ROM is constructed using a kernel dynamic mode decomposition method and enhanced through a residual dynamical mode de…

[Phys. Rev. E 111, 045101] Published Tue Apr 08, 2025

Author(s): Dongdong Liu, Binjie Tan, Yunzhi Liu, Yi Niu, Jianing Guo, Junzhuo Wu, Jiakai Li, Jiankun Li, and Xiang Luo

The interaction between a droplet and superhydrophobic particles has a duration that significantly deviates from the inertia-capillary timescale. Both the rapid and delayed rebounds of the droplet are observed on the superhydrophobic particles with various particle sizes and impact velocities. Typic…

[Phys. Rev. E 111, 045102] Published Tue Apr 08, 2025

Author(s): Nicolas G. Hadjiconstantinou

Transition state theory is used to model slip of a simple liquid at a liquid-solid interface. In the linear regime of low shear rate, the model leads to a simple expression for the slip that is in excellent agreement with molecular dynamics simulations for a wide range of system parameters and thermodynamic conditions.

[Phys. Rev. Fluids 10, 044201] Published Mon Apr 07, 2025

Author(s): Yunlong Zheng, Qiulin Qu, Peiqing Liu, Tianxiang Hu, Xiaohang Shi, and Peizhe Zhou

To address discrepancies in the pressure peak variation law during the impact phase of flat plate ditching, we adopt a CFD method that has undergone detailed verification and validation to simulate flat plate ditching under various conditions. The results confirm that the pressure peak exhibits a “rapid increase-gradual decrease” variation pattern. Further analysis indicates that this phenomenon can be attributed to formation and development of the spray sheet and the resulting alteration of the stagnation streamline direction. The widely employed Wagner and self-similar theories, which fail to consider the spray sheet formation, are not applicable during the pressure peak increase stage.

[Phys. Rev. Fluids 10, 044801] Published Thu Apr 03, 2025

Author(s): Mostafa Kamal and Perry L. Johnson

[Phys. Rev. Fluids 10, 049901] Published Thu Apr 03, 2025

Author(s): Nan Shi, Amr abdel Fattah, and Todd M. Squires

Colloidal particles can be driven to migrate under applied electric fields (electrophoresis) or in response to solution gradients (diffusiophoresis). These phenomena are often discussed and derived differently, due to the physical differences in the forces that drive them. This paper develops a complementary, conceptually unified approach to predict phoretic drift and osmotic slip velocities, which naturally reproduces existing results while providing natural extensions and generalizations, as well as a natural way to understand and predict global characteristics of particle flow fields.

[Phys. Rev. Fluids 10, 043701] Published Tue Apr 01, 2025

Author(s): Xu Xu, Shiyong Tan, Yinghe Qi, and Rui Ni

Finite-sized bubbles in turbulent flows are more than passive tracers—they actively influence flow structures. Using high-speed 3D imaging, we reveal how deformable, finite-sized bubbles alter turbulence, with slip velocity decorrelating over bubble-sized eddy turnover times. This rapid wake reorientation constrains wake development, yet at high Reynolds numbers, these bubbles still amplify local turbulence. Our findings highlight how bubble deformation, size, and orientation shape the surrounding turbulence, offering new insights into bubbly flow dynamics.

[Phys. Rev. Fluids 10, 033605] Published Mon Mar 31, 2025

Author(s): Chuanqiang Gao (高传强), Xinyu Yang (杨新宇), Kai Ren (任凯), and Weiwei Zhang (张伟伟)

Low-order linear models serve as a foundational tool for flow analysis and control design for unstable flow systems. Conventional modeling approaches impose strict requirements on initial base flow and training process to ensure input-output identifiability. To address these limitations, this study presents a novel open-loop input-output modeling framework based on closed-loop identification principles, which leverages real-time control feedback generated by an adaptive data-driven, model-free controller to dynamically balance external excitation and inherent flow disturbances. This framework demonstrates enhanced modeling effectiveness and operational adaptability.

[Phys. Rev. Fluids 10, 033902] Published Mon Mar 31, 2025