Physical Review Fluids

Author(s): Zhengyang Wei and Chang Liu

Input-output analysis has been widely used to predict the transition to turbulence in wall-bounded shear flows, but it typically does not capture the full nonlinear effects. This work analyzes nonlinear input-output stability of transitional shear flows using the Small-Signal Finite-Gain (SSFG) stability theorem. This SSFG stability can predict permissible forcing amplitudes below which a finite nonlinear input-output gain can be maintained. The nonlinear input-output gain obtained from the SSFG stability theorem is higher than the linear input-output gain. The permissible forcing amplitude identified from the SSFG stability theorem is consistent with that obtained by bisection search.

[Phys. Rev. Fluids 10, 103903] Published Fri Oct 24, 2025

Author(s): Mingyun Xie, Qichao Li, Shengqi Wu, Hong Liu, and Lin Fu

In this study, we simulated liquid jets in supersonic crossflow (LJSC) under various inflow Mach numbers using the diffuse interface method. The results show that as the Mach number increases, the liquid jet exhibits reduced penetration and faster fragmentation, accompanied by different breakup modes. A theoretical model was developed to predict the near-field trajectory, which shows good agreement with experimental and numerical data. The proposed primary breakup model can be incorporated into Lagrangian methods to enhance computational accuracy.

[Phys. Rev. Fluids 10, 104006] Published Fri Oct 24, 2025

Author(s): Rémi Macadré, Frédéric Risso, Olivier Masbernat, and Roel Belt

Direct numerical simulations are used to analyze the flow in an annular plane Couette geometry in the laminar regime. A secondary flow is consistently present due to centrifugal effects associated with rotation, regardless of Reynolds number (Re). By increasing the rotation speed, the flow becomes more confined to the walls, leading to progressively thinner boundary layers. Consequently, the primary flow develops into an S-shaped profile, reminiscent of turbulent regimes. At high Re and large channel aspect ratios, an asymptotic regime is observed, the characteristics of which are discussed. This flow is well suited for studying the rheology of highly concentrated two-phase dispersed flows.

[Phys. Rev. Fluids 10, 104102] Published Fri Oct 24, 2025

Author(s): He Zhao (赵河), Zexu Yuan (苑泽旭), Wenjin Han (韩文晋), and Dengming Wang (王等明)

Granular flows of non-spherical particles exhibit complex dynamics that challenge classical rheological descriptions. Using discrete element method (DEM) simulations, we show that superballs display distinct flow behaviors, including enhanced boundary effects, modified velocity profiles, and increased bulk viscosity, compared to spheres. We develop a particle-shape-dependent constitutive model incorporating dimensionless granular temperature to capture nonlocal effects, validated through continuum simulations. This framework enables accurate prediction of flow behaviors across quasistatic and inertial regimes, advancing the modeling of granular systems with complex particle geometries.

[Phys. Rev. Fluids 10, 104306] Published Fri Oct 24, 2025

Author(s): Long Wang and Guowei He

The space-time decorrelation of a passive scalar advected by turbulent flows is dominated by three physical processes: mean-flow carrying downstream, large-eddy random sweeping, and small-eddy distortion. Each process has been investigated through Taylor’s frozen-flow hypothesis and Kraichnan’s random-sweeping and white-noise models. However, their coupling effects remain unexplored. The present paper proposes the Taylor–Kraichnan model to represent the coupled effects of the three processes and leads to the exact solution of space–time correlation. The scale invariance of the space–time correlation is found and consistent with the elliptical approximation model.

[Phys. Rev. Fluids 10, 104607] Published Fri Oct 24, 2025

Author(s): S. H. Ferguson Briggs, A. J. Mestel, and M. G. Blyth

The flow of two concentric ferrofluid layers of differing viscosity and magnetic susceptibility through an annular pipe is examined. The basic flow is driven by an axial pressure gradient and/or by the axial translation of the inner wall. In the absence of a magnetic field, the interface between the fluids is prone to capillary and shear instabilities. Axial and azimuthal magnetic fields can suppress these instabilities, though strong fields may themselves induce unstable magnetic modes. Stability boundaries are mapped across the parameter space, showing how the interplay of capillary forces, shear, and magnetic stresses determines the stability of the system.

[Phys. Rev. Fluids 10, 103704] Published Thu Oct 23, 2025

Author(s): Johan Carlier and Christophe Collewet

Feedback gains derived from balanced truncation of the respective two-dimensional linearized models for the channel and mixing-layer flows reveal spatial patterns similar to Orr structures – the transient vortical features responsible for energy amplification in shear flows. These gains oppose such structures, providing a physically interpretable mechanism by which optimal control suppresses disturbances and delays transition in these shear flows.

[Phys. Rev. Fluids 10, 103902] Published Thu Oct 23, 2025

Author(s): Prabhash Kumar, Anu V. S. Nath, Mahesh Panchagnula, and Anubhab Roy

How do tiny inertial particles behave in a vortex? We revisit this classical problem by incorporating the often-neglected added mass and history forces. Our analysis uncovers additional fixed points beyond the conventional ones, reshaping the trapping landscape, particularly in near density-matched scenarios where added mass becomes significant. Most strikingly, while the absence of the history force permits trapped, diffusive, or ballistic states depending on parameters, its inclusion releases all particles, eliminating trapping and driving them into inevitable long-time ballistic motion.

[Phys. Rev. Fluids 10, 104304] Published Thu Oct 23, 2025

Author(s): Zilong He and Sungyon Lee

We experimentally apply an oscillatory flow to a densely packed suspension plug inside a cylindrical tube in the limit of low Reynolds and high Peclet numbers. The plug becomes reversible in shape once it gradually expands and dilutes to reach a critical particle concentration, matching the previous experimental observations. Surprisingly, our particle-scale measurements reveal that above a critical strain amplitude, individual particles remain irreversible even after the plug shape has stabilized. We further explore this decoupling between particle-scale and system-scale reversibility with simplified two-dimensional simulations.

[Phys. Rev. Fluids 10, 104305] Published Thu Oct 23, 2025

Author(s): Haoqi Fei, Rui Wang, Byron Guerrero, Feng Wang, and Hui Xu

Extreme wall shear stress (WSS) events are traditionally categorized into two types: extreme positive events and backflow events. By introducing an additional azimuthal constraint, both categories can be further divided into subtypes that display distinct statistical and structural characteristics. This study underscores the crucial role of azimuthal shear stress in the formation and evolution of extreme WSS events, providing a more nuanced understanding and a fresh perspective on near-wall turbulence.

[Phys. Rev. Fluids 10, 104605] Published Thu Oct 23, 2025

Author(s): Jun-Ning Wang, Jin-Han Xie, and Xiaojing Zheng

This study investigates the potential-temperature-related third-order structure function in the atmospheric surface layer (ASL), which reflects the properties of potential-energy transfer in the flow, using theory and data from the Qingtu Lake Observation Array (QLOA). We formulated a Kármán–Howarth–Monin equation for potential energy and, using Townsend’s attached eddy hypothesis, derived that the structure function exhibits a horizontal logarithmic region and proposed a universal expression. The image reveals a distinct logarithmic region and demonstrates the validity of our expression for QLOA data.

[Phys. Rev. Fluids 10, 104606] Published Thu Oct 23, 2025

Author(s): James D. Shemilt, Alice B. Thompson, Alex Horsley, Carl A. Whitfield, and Oliver E. Jensen

The surface-tension-driven instability of a liquid film coating a vertical tube can lead to the formation of liquid collars that drift downwards under gravity. This scenario is relevant to the flow of mucus in lung airways. Using thin-film theory, we investigate the formation and motion of collars when the liquid film is viscoplastic. In the limit of weak gravity relative to capillary effects, we quantify the reduction in steady collar speed due to viscoplasticity, and identify conditions under which viscoplastic collars translate steadily, whilst steady motion does not occur in the Newtonian case.

[Phys. Rev. Fluids 10, 103301] Published Wed Oct 22, 2025

Author(s): Abhijit Kumar Kushwaha, Sankara Arunachalam, Ville Jokinen, Dan Daniel, and Tadd T. Truscott

Seven superimposed images capture the motion of a 42 μL water droplet sliding down an inclined superhydrophobic surface. Upon deposition, the droplet partially wets the surface, with interferometry revealing a heterogeneous distribution of white and black patches characteristic of the Cassie–Baxter state. As the droplet accelerates and reaches higher velocities, it entrains air from the surroundings, forming a thin lubricating air layer beneath it. This air layer thickens progressively with increasing velocity, and once a critical threshold is exceeded, the droplet transitions into a state of aerodynamic levitation.

[Phys. Rev. Fluids 10, 103603] Published Wed Oct 22, 2025

Author(s): Takahiro Kanazawa and Kenta Ishimoto

Crawling animals like snails and slugs move by generating waves along their bodies over thin layers of fluid. Using lubrication theory, we derived a general formula for locomotion speed when the viscosity of the fluid layer varies in space and showed that the model captures two common locomotion patterns: transverse and longitudinal crawling. Furthermore, through multiple-scale perturbation expansions, we analytically demonstrate how position-dependent viscosity can slow locomotion, depending on the crawling gait and direction of motion. The results reveal nonlinear, accumulative mechanical interactions between locomotion and a heterogeneous environment.

[Phys. Rev. Fluids 10, 103102] Published Tue Oct 21, 2025

Author(s): Sidyant Kumar, Sachchida Nand Tripathi, and Sanjay Kumar

Atomization of liquid drops has practical engineering applications such as in combustion, sprays, and others. We experimentally study the shear stripping mode of atomization where a liquid drop interacts with a normal shock wave and deforms under shock-induced flow. The initial deformation and circumferential surface waves on the drop are governed by shear instability (Kelvin-Helmholtz). The wave amplification redistributes liquid and the drop evolves into a bowl. Flow acceleration induces modulations, forming an azimuthal ring with its own rim. Two sub-modes emerge: Ligament mode with sheet shearing at lower velocities, and Cellular mode with localized cells at relatively higher velocities.

[Phys. Rev. Fluids 10, 103602] Published Tue Oct 21, 2025

Author(s): Yuchen Ye (叶宇晨), Yan Yang (杨艳), Bin Jiang (蒋彬), Cheng Li (李程), Minping Wan (万敏平), Yipeng Shi (史一蓬), and Sean Oughton

We investigate how finite Reynolds number affects the width of the Inertial Range (IR) in magnetohydrodynamic (MHD) turbulence. A set of high-resolution incompressible MHD simulation data is employed, with Taylor Reynolds Number Reλ ranging from about 200 to 400. The IR width is quantified using third-order laws. A semi-empirical model for second-order structure functions is established and applied to the MHD von Karman-Howarth equation, determining IR boundaries for given Reynolds numbers. This model allows extrapolation of Reλ beyond the reach of current simulations or experiments. We suggest that establishing a two-decade IR in MHD turbulence requires Reλ to be at least 1,500.

[Phys. Rev. Fluids 10, 103703] Published Tue Oct 21, 2025

Author(s): Max Herzog and Jesse Capecelatro

We present results from direct numerical simulations of turbulent channel flow laden with adhesive (viscoelastic) particles. Particles demonstrate higher adhesion strengths at elevated temperatures, an effect we probe by varying the adhesion number. Using spanwise radial distribution functions, we show that particle heterogeneity near and on the wall is promoted by turbulence. Furthermore, low-adhesion, high-inertia particles demonstrate spanwise creep along the wall, leading to elongated streamwise deposits. Abrasive wear profiles highlight the consequences of heterogeneity, with local wear exceeding ten times the mean.

[Phys. Rev. Fluids 10, 104302] Published Tue Oct 21, 2025

Author(s): Lokendra Mohan Sharma, Harish N. Dixit, and Lakshmana Dora Chandrala

While immiscible liquid jets play vital roles in applications from chemical reactors to environmental flows, understanding their dynamics has been hindered by optical distortions at fluid interfaces. Using simultaneous Particle Image Velocimetry (PIV) and Planar Laser-Induced Fluorescence (PLIF) with refractive index-matched fluids, this study provides comprehensive experimental validation of both inviscid and viscous analytical models for buoyant liquid-in-liquid jets in the laminar regime. The study uncovers two distinct scaling regimes: an inertia-dominated near field and a viscosity-governed far field, with buoyancy and jet Reynolds number controlling dynamics while surface tension and outer-phase viscosity play minor roles.

[Phys. Rev. Fluids 10, 104303] Published Tue Oct 21, 2025

Author(s): Samuel Kok Suen Cheng and Jian Sheng

The reconstruction of the conserved scalar from gradient field like pressure is important in many applications. Most current direct integration algorithms initiate the integration at the domain boundaries, where gradient measurement is often unreliable. This study proposes the Boundary-Independent Shortest Path (BISP) integration method, which initiates at an internal node and grows outwards towards the boundaries, thereby eliminating any dependency on the boundaries. This integration algorithm can be directly applied to pressure gradient fields containing inner voids of arbitrary shapes and sizes without compromising the accuracy.

[Phys. Rev. Fluids 10, 104604] Published Tue Oct 21, 2025

Author(s): F. Zabaleta, B. Bornhoft, S. S. Jain, S. T. Bose, and P. Moin

Accurate heat transfer prediction on rough surfaces is critical for ice accretion prediction and aviation safety. Using high-fidelity simulations of conjugate heat transfer, we resolve heat transport in both the fluid and a low-conductivity solid featuring laser-scanned ice roughness. Contrary to the behavior of isothermal surfaces, the low thermal conductivity of the solid causes roughness crests to become the coolest points, sometimes even drawing heat from the air. This work highlights the necessity of including solid conduction effects in next-generation icing models.

[Phys. Rev. Fluids 10, 104603] Published Mon Oct 20, 2025