On May 29 at noon ET, Michalis Chatzimanolakis (Harvard University) will sit down with the Physical Review Journal Club to discuss their recently published research, “Learning in two dimensions and controlling in three: Generalizable drag reduction strategies for flows past circular cylinders through deep reinforcement learning.” Please register here. Registration is free and a video recording will be provided to all registrants.

We are greatly saddened by the sudden passing on April 14 of Keith Julien, Chair and Professor of Applied Mathematics at the University of Colorado, Boulder, Fellow of the American Physical Society, and a member of the Editorial Board of Physical Review Fluids.

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 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.

Browse outstanding papers by early career researchers who have received the Frenkiel Award in recognition of their significant contributions to fluid dynamics.

Drops impacting supercooled surfaces adhere to them due to contact line pinning and their solidification. However, distinguishing the influence of each phenomenon on post-impact behavior is challenging since even repellent materials exhibit some drop adhesion. In this study, we examine the impact of water and alkane drops on an omniphobic dry ice surface. We show that the solidification extent within the drop, combined with thermal, elastic, and surface tension forces, dictate outcomes like fragmentation, rebound, or no-bounce. Our findings have critical implications for material design in 3D printing, frost-resistant coatings, and safe biological material transport in cold climates.

Liquid infused surfaces (LISs) are a nature-inspired surface technology that demonstrates multiple functionalities under laminar and controlled flow conditions. We study experimentally the behavior of the infused lubricant under submerged conditions and turbulent flow. When exposed to turbulence, the lubricant layer develops into a pattern of droplets, the length of which depends on the balance between shear and contact force. The stability of the droplets prevents complete drainage of the lubricant and increases the robustness of the LIS in the presence of turbulence. We identify a model that predicts the equilibrium length of the droplets and validate it with numerical simulations.

We examine the influence of wind forcing on the inception of breaking in surface gravity waves using an ensemble of high-resolution numerical simulations. We find that there is a critical point in the energetic evolution of the wave in which the convergence of kinetic energy at the wave crest can no longer be offset by conversion to potential energy, resulting in a rapid growth of kinetic energy up to breaking onset. This energetic signature is shown to consistently differentiate between non-breaking and breaking waves under a range of wind forcing speeds.

500 resolved particles, colored by their temperature, are suspended in Rayleigh-Bénard convection at a Rayleigh number of ${10}^7$. The lines are streamlines colored according to the fluid vertical velocity. Near the cell bottom, the fluid circulation pushes the particles from the base of the descending to that of the ascending plume where they accumulate into a dune. The particles that follow are dragged up the dune acquiring a vertical velocity component which promotes their resuspension. The lift force plays no role in this process. Depending on the particle number (from 500 to 3000) up to 20% of the fluid gravitational energy can be transferred to the particles.

Exploring how the shape of red blood cells influences their flow properties, this study uses numerical simulations to analyze changes from healthy bi-concave forms to abnormal spherical shapes associated with disorders like spherocytosis. The research reveals complex, non-monotonic relationships between cell shape and flow rate across varying channel widths, and its impact on blood perfusion.

We present an asymptotic theory for the dynamics of slender chemically propelled loops and knots. It is valid for nonintersecting three-dimensional centerlines, with arbitrary chemical patterning and varying (circular) cross-sectional radius, allowing many slender active loops and knots to be studied. The theory has closed-form solutions in simpler cases, enabling us to derive the swimming speeds of chemically patterned tori, and the pumping strength (stresslet) of uniformly active slender tori. Using numerical solutions, we find the behavior of exotic active particle geometries, such as a bumpy uniformly active torus that spins and a Janus trefoil knot, which rotates as it swims forwards.

The space-time correlations of both wall-shear fluctuations and the streamwise velocity fluctuations carried by wall-attached eddies are investigated in a multiscale manner, by coupling the inner-outer interaction model (IOIM) with the attached eddy hypothesis. The present results demonstrate that the space-time correlations for the wall-shear stress fluctuation are mainly dominated by near-wall small-scale motions, and wall-attached eddies at a given length scale feature distinctly different space-time properties as compared to those of ensembled eddies with multiple length scales, which provides a new perspective for analyzing the decorrelation mechanisms in turbulence theory.

Experiments show the effects on combustion of adding hydrogen to natural gas—a fuel mixture that could reduce carbon emissions from power plants.

Focus story on:
Byeonguk Ahn et al.
Phys. Rev. Fluids 9, 053907 (2024)

Recent advancements in automatic differentiation, which played a pivotal role in deep learning, offer a promising approach to addressing challenges in controlling fluid flow behavior. We demonstrate the power of the method by optimizing the packing of a polydisperse system of periodically arranged circular rods to minimize the pressure drop across the media. We show how the optimum topology of the porous media changes with changing the packing fraction.

Super-currents, tunneling across insulators in Josephson junctions, have a one-to-one classical analog to action-at-a-distance between two interfacial Rossby waves in shear flows. Quantum avoided crossing between eigenstates, described by the Klein-Gordon equation, is obtained as well for the Rossby wave normal modes. Both the quantum and the classical dynamics are formulated as coupled two-state systems and presented on a Bloch sphere, where the Hadamard gate transforms the two normal modes into an intuitive computational basis of two single Rossby waves. Yet, lacking analogs to quantum collapse and entanglement, the Rossby wave system cannot serve as a qubit prototype, even in principle.

To explore the dynamics of annular combustors, we investigate azimuthal thermoacoustic instabilities under a range of hydrogen power fractions and operating conditions. Using time-series analysis and mode detection techniques, we examine the relationship between longitudinal and azimuthal modes, identifying a transition from chaos to high-amplitude periodic states. Our research sheds light on how hydrogen enrichment affects combustor stability and presents the first identification of type-II Pomeau–Manneville intermittency in annular combustors. These findings contribute to knowledge of the modal dynamics within combustors, with implications for the design and operation of future systems.

Wall vortex occurs when a cavitation bubble oscillates far from a single rigid wall (at a dimensionless standoff distance γ${}_r$>1.3). This study finds that a wall vortex in an expanded new regime forms instead of a free vortex at a smaller γ${}_r$ value, when introducing a water surface. Criteria for vortex flow patterns are proposed based on the direction of the bubble centroid migration at the beginning of the second cycle $t_c$ though a theoretical model developed with a Lagrangian formulation. Numerical analysis reveals that the wall vortex flow with the influence of the water surface contributes to a greater wall shear stress and larger area, thus increasing the surface cleaning potential.

Kolmogorov scaling is used to derive a model for the structure function constant associated with index of refraction fluctuations in Rayleigh-Benard turbulence. The model predicts that the normalized structure function constant depends on the heat flux to the four-thirds power, and is independent of the Rayleigh number. The model agrees with the results of numerical simulations, thereby lending support to the assumptions underlying the theory.

The instability behavior of plane Couette flow is notoriously difficult, because it has no classical unstable modes, and this for any high Reynolds number. Here, the plane Couette flow is modified by means of a constant wall transpiration, i.e. simultaneous blowing from below, which has a destabilizing effect, and suction from above, which has a stabilizing effect. These opposing effects led to a changed in an unpredictable way, i.e. a destabilization at a certain point with increasing transpiration rate, the increase in instability then reaches a maximum and then leads to a slow stabilization again as the transpiration rate increases further. The destabilizing effects clearly dominate here.

The effects of density variations on structures developing in an isotropic incompressible turbulence flow are investigated. Statistical analyses are carried out on datasets obtained from direct numerical simulations of forced turbulence. Numerical evidence shows that the introduction of a variable-density field into a turbulent field modifies the coherent structures and the energy spectrum in the inertial range.

The interaction between a vortex and an impacting body is complex due to the interaction of inviscid and viscous mechanisms. We conduct the first three-dimensional direct numerical simulations of this process and vary the relative impact velocity of the cylinder to explore the parameter space and analyze this process in detail. Strong vortices lead to ejection and interaction of secondary vorticity from the cylinder’s boundary layer, while weak vortices lead to approximately inviscid interaction of the cylinder with the primary vortex through deformations.

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).

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 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.