The dynamics of flexible filaments entrained in flow are important for understanding many biological and industrial processes. This work describes a data-driven technique to create high-fidelity low-dimensional models of flexible fiber dynamics using machine learning; the technique is applied to sedimentation in a quiescent, viscous Newtonian fluid, using results from detailed simulations as the data set. Over a wide range of fiber flexibilities, the filament shape dynamics can be represented with high accuracy with only four degrees of freedom.

A circular surface wave (CSW) of a low-temperature gallium alloy in the immovable cell with a central bottom electrode and an upper ring electrode exposed to axial magnetic fields is studied experimentally. The mechanism which provides the existence of a stable CSW is suggested. It is determined that, depending on the force parameter and geometrical characteristics of the cell, three modes can occur in the cell: rest, CSW, or axial rotation with a deep funnel on the surface providing the circular contact of the liquid metal with the electrode. A mode map showing the boundaries of the CSW existence domain is plotted on the parameter plane.

During floods caused by a continuous increase in river flow, the water level often rises suddenly, while the recession takes much longer. This behavior is consistent with the subcritical instability highlighted in this article. This instability emerges for uniform, quasi-stationary flows at high Reynolds numbers in a channel. With increasing flow, a sudden jump in water level occurs when a well-defined flow rate is reached. If the flow rate subsequently decreases, the water level drops again suddenly, but at a flow rate well below the previous one.

We report a high throughput strategy to produce gel microspheres. This method is based on the controlled fragmentation of an aqueous jet in air that results in droplets of monomer solution, their entry and collection in an oil bath containing a catalyzer and surfactants, followed by polymerization of the emulsion droplets which thus turn into gel beads. Adjusting the impact area of the stream of droplets at the free surface with an electric field allows to minimize coalescence of droplets as well as mass transport between the droplets and the continuous phase which is correlated to the sedimentation flow features of the dilute emulsion.

Using lattice Boltzmann simulations and an analytical framework based on far-field approximations and method of images, we study the trajectories of microswimmers inside three-dimensional channels of square and rectangular cross-sections. We find that pusher-type microswimmers move helically inside the square tube, weak pullers slide through the center of the channel while strong pullers exhibit a trajectory which is off-center. The trajectories of the neutral swimmers are challenging to generalize due to the sensitivity to the initial conditions. Finally a method of superposition is used to construct three dimensional trajectories, thus explaining the origin of their apparent complexity.

Wave breaking is recognized as one of the most violent air-sea interaction processes, significantly enhancing the transfer of heat, mass, and momentum between the oceans and the atmosphere. In this study, we investigate heat transfer in wind turbulence over breaking waves using direct numerical simulation, with a particular focus on the effects of wave age. Our findings suggest that temperature responds in a more complex way to wave age than velocity does, emphasizing the need to incorporate this phenomenon into air-sea interaction and weather forecasting models.

The presence of platelets near vessel walls is crucial for clot formation to stop bleeding. We examine platelet margination influenced by red blood cell (RBC) aggregation through numerical simulation. Our findings indicate that moderate to strong RBC aggregation enhances platelet margination in microcirculation, thereby improving the capacity to stop bleeding. This mechanism offers a natural counteraction against major bleeding in conditions such as diabetes, where strong RBC aggregation is commonly observed.

In the present study the axisymmetric flow in an aspect ratio R/H=10 cavity is revisited. The base state is shown to lose stability in a supercritical Hopf bifurcation. Branches of periodic and chaotic self-sustained solutions are computed using harmonic balance method and time integration. In addition, edge states separating the steady laminar and chaotic regimes are identified using a bisection algorithm. All the self-sustained solutions found are shown to exist only for a high enough Reynolds number and are therefore disconnected from the experimentally observed circular rolls.

Our study experimentally investigates the morphology and breakup of a droplet descending into an airstream, analyzing child droplet size distributions through high-speed shadowgraphy and in-line holography. We found that varying the droplet’s release height results in different breakup modes — from vibrational to retracting bag-stamen breakup at the same Weber number — each with distinct size distributions. Our theoretical model, which incorporates the effective Weber number, accurately predicts these distributions, underscoring the critical impact of droplet dynamics and aerodynamic interactions on breakup behavior.

When a jet of fluid emerges into an ambient medium of the same density but higher viscosity, the dominant mode of instability transitions from axisymmetric at low viscosity ratio M to a helical mode at high M. It is shown that these helical modes are unstable global modes associated with enhanced mixing and the emergence of a single dominant frequency observed everywhere in the near field. These frequencies align well with predictions of absolute instability from spatiotemporal linear stability theory.

The implicit U-Net enhanced Fourier neural operator (IUFNO) combines the loop structure of implicit FNO (IFNO) with U-Net, leading to enhanced long-term predictive ability in the large-eddy simulations (LES) of turbulent channel flow. It is found that the IUFNO outperforms the traditional dynamic Smagorinsky model (DSM) and the wall-adapted local eddy-viscosity (WALE) model at coarse LES grids. The predictions of both the mean and fluctuating quantities by IUFNO are closer to the filtered direct numerical simulation (fDNS) benchmark compared to the traditional LES models, while the computational cost of IUFNO is much lower.

The perturbation dynamics in a supersonic shear layer are characterized using large-eddy simulations and linear-operator-based input-output analysis. The Kelvin-Helmholtz instability emerges as the primary mechanism for disturbance energy amplification. To identify feasible actuator placement locations, we conduct a state-space restricted input-output analysis, revealing the splitter plate trailing surface as the most receptive region. Furthermore, simulations with applied forcing demonstrate that the coherent structures predicted by linear analysis remain active within a highly nonlinear turbulent flow.

This paper addresses the artificial bottleneck effect in large-eddy simulations (LES), causing erroneous energy spectrum overshoots due to residual stress model inaccuracies. We use Stokes Flow Regularization (SFR) to improve LES models with detailed residual kinetic energy considerations. Adding a nonlinear gradient component to the residual stress closure accurately captures local stress structures, reduces kinetic energy over-predictions, and enhances energy cascade representation. The mixed model produces vortex tube-like structures similar to those in filtered direct numerical simulations (DNS), while the eddy viscosity model yields shear layer-like features.

In this paper, we first examine the wave morphology induced by reactant thermal stratification in hypersonic reactive flows. Three flow regimes are observed: autoignition-driven reaction front, detonation wave, and decoupled shock/reaction front. The results suggest that local autoignition behavior can be significantly influenced by reactivity nonuniformity in the reactive flow; and an undesirable detonation wave can be initiated given a suitable temperature gradient. This also indicates that oblique detonation waves can be generated without mechanical devices such as blunt bodies and wedges.

The varicose dynamics of gravitational liquid sheet flows issuing into a quiescent gaseous ambient is relevant for technological applications such as coating deposition, where varicose perturbations of the curtain shape can arise due to velocity fluctuations coming from the delivering pump placed upstream of the coating die. We investigate this problem using a one-dimensional linear model and two-dimensional nonlinear simulations, finding novel scaling laws for the curtain oscillation frequency and amplitude with the Weber number. Moreover, an interesting numerical break-up phenomenon of the curtain is outlined, driven by the progressive curtain thinning induced by the varicose disturbances.

We uncover and study a physical process that is hidden in the healthy lung. This process can be triggered by the thickening of the thin layer of mucus lining the bronchi walls. At the air-mucus interface, surface tension and curvature can combine to oppose the physiological motion of mucus induced by cilia, causing mucus to stall or, even, move in reverse. Representing mucus as a Bingham fluid, we characterize analytically the fluid dynamics in the thin layer using dimensionless quantities and employ numerical simulations to compute the layer velocity field in airways bifurcations. This work sheds new light on the intricate workings of the respiratory system.

When a drop impacts next to the edge of a solid substrate, it spreads beyond the edge and forms a liquid sheet surrounded by a rim. As the rim decelerates, ligaments may form, then destabilize in droplets. This fragmentation scenario has been extensively investigated in the axisymmetric configuration of centered impacts on small circular targets. In this work, we investigate star-shaped targets. We show that the rim is shaped complementarily to the substrate: it goes farther and yields more droplet ejections in directions corresponding to troughs in the substrate edge profile.

The energy cascade in turbulence is explored through a novel approach in which the vortical structures of a certain scale in the inertial subrange, colored by yellow, is forcibly suppressed. Their disappearance leads to a slight increase in kinetic energy in the larger scale range and a decrease in the smaller scale range. The vortex-tracking analysis reveals that the vortices twice as large as the target, colored by blue, exhibit smaller curvatures and longer lifespans, while no remarkable changes are found for the vortices that are four times larger, colored by red. These findings indicate that the larger vortices are locally bent by the smaller vortices, shortening their lifespans.

The policy requires authors to explain where research data can be found starting Sept. 4.

When determining the authorship list for your next paper, be generous yet disciplined.

APS has selected 156 Outstanding Referees for 2024 who have demonstrated exceptional work in the assessment of manuscripts published in the Physical Review journals. A full list of the Outstanding Referees is available online.

The recipients of the 40th François Naftali Frenkiel Award for Fluid Mechanics are Aliénor Rivière, Daniel J. Ruth, Wouter Mostert, Luc Deike, and Stéphane Perrard for their paper “Capillary driven fragmentation of large gas bubbles in turbulence” which was published in Physical Review Fluids 7, 083602 (2022).

Physical Review Fluids publishes a collection of papers associated with the 2022 Gallery of Fluid Motion. These award winning works were presented at the annual meeting of the APS Division of Fluid Dynamics.

See the 2022 Gallery for the original entries.

The 75th Annual Meeting of the American Physical Society (APS) − Division of Fluid Mechanics was held in Indianapolis, IN from November 20–22, 2022.

The Collection is based on presentations at the 2022 meeting of the APS Division of Fluid Dynamics in Indianapolis, Indiana. Each year the editors of Physical Review Fluids invite the authors of selected presentations made at the Annual meeting of the APS Division of Fluid Dynamics to submit a paper based on their talk to the journal. The selections are made based on the importance and interest of the talk and the submitted papers are peer reviewed. The current set of invited papers is based on presentations made at the 75th Annual meeting of the APS Division of Fluid Dynamics in November 2022. The papers may contain both original research as well as a perspective on the field they cover.

The Editors of Physical Review Fluids highlight the journal’s achievements, its editorial standards, and its special relationship with the APS Division of Fluid Dynamics (DFD).

Beverley McKeon and Eric Lauga describe their vision as new Co-Lead Editors for Physical Review Fluids, which celebrates its fifth anniversary this year.