We study the role played by upstream-traveling guided waves on the generation of tones by a supersonic, ideally-expanded round jet impinging on a flat plate. We explore a finite-thickness stability model for the prediction of allowable frequency ranges for resonance based on the dynamics of such waves. We show that the frequency ranges predicted by the finite-thickness model are consistent with the vast majority of experimental and numerical data available in the literature, correcting discrepancies observed previously with vortex sheet models. This provides further evidence for the involvement of guided jet modes in the resonance mechanism.

Viscoelastic fluids may exhibit complex flow behaviors, such as nontrivial normal stress differences and time-dependent responses, which can significantly impact the settling dynamics of suspended particles. We investigate particle-wall interactions for the case of a sphere moving close to and normal to a wall in an Oldroyd-B fluid, considering both prescribed velocity and force scenarios; the lubrication approximation and Deborah number perturbation techniques are used.

We present a theoretical model for viscous liquid systems exhibiting Rayleigh-Plateau instability, considering cases with and without a solid fiber. Using the lubrication approach and hydrodynamic interactions at the solid-liquid interface, we derive one-dimensional evolution equations for the breakup of viscous liquid threads and films on fibers. Our model aligns well with experimental results, unifying Goren’s liquid film on a fiber and Rayleigh’s viscous liquid thread findings. It identifies the most unstable mode as proportional to the wavenumber quadratically and reveals the exponential decay of satellite droplet volume with increasing wavenumber.

Cohesive force chain networks during a granular collapse process. Such networks can be identified by means of community detection approaches.

Droplet impacting on rotating surface experiences the tangential shear force from the rotating surface, generating a centrifugal force that either enhances the spreading or destabilizes the expanding lamella. In this study, we experimentally characterize the impacting of droplets with a wide range of viscosity on rotating surfaces with various wettabilities, and theoretically analyze the observed impacting dynamics, including the enhanced spreading and the transition to the destabilization of the expanding lamella. We propose a simplified approach to predict these key parameters, and validate the theoretical prediction by experimental measurement.

Permafrost thaw plays a crucial role in climate change dynamics, yet the intricate small-scale processes driving thawing remain inadequately understood. Here we reveal that the density anomaly of water has the potential to trigger convection in coarse-grained soils above permafrost. By using high-resolution numerical simulations of the primitive equations in porous media we show that convection significantly accelerates the thawing process. Climate models that predominantly account for thawing induced by diffusive processes alone overlook this aspect, which can self-reinforce and lead to enhanced climate-permafrost feedback.

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.

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.