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

We show that the apparent relaxation time inferred from the exponential thinning regime in viscoelastic pinch-off is not necessarily a material property, as was assumed so far, but depends on the system size for various polymer solutions and filament thinning techniques. It depends on the plate size in Capillary Breakup Extensional Rheometry (CaBER) with both slow and fast plate separation protocols, and on the nozzle size in Dripping-onto-Substrate (DoS), corroborating recent observations with the dripping technique. It is not due to artifacts such as solvent evaporation or polymer degradation and it cannot always be rationalized by finite extensibility effects.

We address coherent pressure structures in turbulent channel flows through SPOD and resolvent analysis with and without an eddy-viscosity model. The spectral analysis revealed energetic structures in the near-wall region, as well as large-scale and spanwise-coherent structures. Pressure structures are targeted in both SPOD and resolvent analysis by selecting an adequate norm through the quadrature weights and observation operator, respectively. The first SPOD and the leading resolvent modes closely agree and show low-ranking behavior. The analyzed modes comprise quasi-streamwise (for near-wall and large-scale structures) and spanwise vortices with pressure peaking at vortex centers.

Uncharged micron-sized water droplets flying toward each other do not always coalesce due to the cushioning effect of the air between them.For oppositely charged droplets, we discover a regime for which droplets always collide when they move inside the stable manifolds of a saddle point of the relative droplet dynamics. A consequence is that only for small electrical charges does the droplet coalescence rate depend primarily upon the Knudsen number (Kn), the ratio of the mean-free-path of air to the mean droplet radius. For much larger charges, coalescence does not depend upon Kn. Our theory predicts the critical charge at which the transition between the two regimes occurs.

Novel formulations involving a control surface at a distance from the body are developed to compute drift loads on structures composed of an impermeable hull and a perforated surface accurately and efficiently. The developed formulation can not only give all six components of the mean wave drift force and moment, but also determine the drift loads on each individual body of a multi-body system.

A viscous fluid in contact with a solid channel that has a preferential affinity with respect to a second fluid embedded in the channel leads to a spontaneous imbibition process. Due to the increasing friction of the invading phase, the invading fluid scales diffusively in time. What happens when a third liquid is a lubricant coating the channel? We show that when the lubricant viscosity is decreased, dissipation switches from being localized in the bulk of the invading phase, to the lubricant layers. This leads to a new crossover, below which diffusive dynamics are not observed. Our results open up the possibility of using this mechanism in SLIPS and LIS to control capillary flows.

Our experiments demonstrate that pancake-shaped bubbles in thin tilted Hele-Shaw cells rise slower than what is expected by simply correcting the effective gravity. This effect is shown to be more important for larger inclination. A careful study highlights an asymmetry for the lubrication film between the bubble and the top and bottom walls. As the cell inclination increases, this asymmetry also increases. We propose that it induces an additional “friction” due to fluid motion between the surrounding of the bubble and the Poiseuille flow further away.

Ion selective concentration shocks have been shown to develop in electrochemical cells with homogeneous porous media of low surface charge. Here we demonstrate through a set of simulations that heterogeneity in the porous structure can lead to substantial differences in separation performance. Both variation in surface charge and characteristic pore size, result in vortical flow in the depleted area, affecting multiple metrics such as energy consumption, water recovery, and desalination. These effects can also be observed for hierarchical media and may be exploited in future designs of porous materials in electrochemical applications beyond shock electrodialysis

The follower force model is a fundamental model for active filaments, commonly utilized to model microtubule-motor protein complexes and collections of cilia. In this work we perform a thorough analysis of this model, employing techniques from computational dynamical systems, adapted from high Reynolds number fluid dynamics, to map out the bifurcations in the system and classify emergent states. This approach allows us to bridge the gap between 2D and 3D analyses, in particular establishing the initial buckling as a double Hopf bifurcation. Additionally, we identify the existence of a quasiperiodic solution at the second bifurcation, and categorize the dynamics at higher values of forcing.

Inspired by industrial processes in oil and gas well plugging and abandonment (P&A) operations, this study investigates the injection of heavy, thick fluids into lighter fluids. By experimenting and analyzing flow behaviors using dimensionless numbers such as Reynolds number, Froude number, inclination angle, Bingham number, and viscosity ratio, we identified different flow regimes such as stable and unstable slumping, separation, and mixing. These findings offer valuable insights for improving fluid flow analysis in applications like 3D printing and other industrial processes.

We study numerically and analytically the effect of Navier slip on the orientational dynamics and effective shear viscosity of a semi-dilute suspension of two-dimensional particles with either circular or elongated (plate-like) shape, interacting only via hydrodynamic and contact forces. We show that at dilute concentrations slip causes the elongated particles to align in the flow direction, whilst for large concentrations tumbling of the particles occurs due to particle-particle interactions. We show this change in orientational microstructure directly impacts the effective viscosity of the suspension: a minimum in the effective viscosity occurs at a threshold concentration.

Cohesive forces occur at the particle scale and have effects up to the macroscopic scale. Using the canonical configuration of a column of grains collapsing under its own weight in air, this paper reports that a bulk description framework of cohesive effects can account for the macroscopic observations. Experiments are reported with two different cohesion sources, capillary bridges or a polymer coating, for the collapse of rectangular and cylindrical granular columns. The bulk framework is shown to capture the effects of cohesion on the final deposit for both sources of cohesion.

The natural frequency of a fluid overlying on a wavy wall in general reduces. This reduction is observed by a shift in the minimum of the Faraday threshold, i.e., in the parametric acceleration versus frequency plot.

Laboratory experiments of ocean gravity currents show laminar transport supplemented by spontaneous and intermittent cascading of dense water. Statistical analysis reveals self organized criticality of the downward transport. Cascading intrusions are a major contributor of turbulence and vorticity in the ocean interior.

On July 26, Donovan Allum (University of Saskatchewan) will sit down with the Physical Review Journal Club to discuss their recently published research, “Simulations of buoyant flows driven by variations in solar radiation beneath ice cover”.

Spatial variations in ice and snow characteristics imply that radiative forcing in late winter lakes is spatially heterogeneous. In this work, idealized, three-dimensional simulations of buoyancy-driven flows driven with heterogeneous solar radiation intensity were performed. The radiative forcing in fresh water at temperatures below 4◦C initiates an unstable stratification near the surface, leading to Rayleigh-Taylor instabilities. The variations in radiative forcing intensity generates gravity current-like flow along the surface. In the published paper, the authors provide an in-depth analysis of the development and death of the gravity current-like flow and link their results to possible melt rate variations associated with the spatial variability of the radiative forcing.

After the presentation, Allum and co-author Marek Stastna (University of Waterloo) will be available to answer attendee questions in a live Q&A session moderated by Phys. Rev. Fluids Board Member, Pascale Garaud (UC Santa Cruz).

The verification of small-scale isotropy requires three-dimensional information of the flow field, a condition rarely satisfied in experiments. To examine this we develop a framework that considers how the presence of bursts at smaller flow scales generates turbulent kinetic energy differently between the horizontal and vertical directions. This framework can be applied both to flow fields obtained via numerical simulations, and to data from field and laboratory measurements. Moreover, a universal relationship emerges to predict small-scale anisotropy from large-scale flow conditions, thus contributing towards the development of next-generation closure models of wall turbulence.

A statistical tool tests the long-held assumption that small-scale turbulence is isotropic.

Synopsis on:
Subharthi Chowdhuri and Tirtha Banerjee
Phys. Rev. Fluids 9, 074604 (2024)

The dynamics and energy-harvesting performance of piezoelectric plates in oscillatory flows have been studied numerically. The simulations show that when these plates are arranged in an array with certain distance between neighbors, the average energy-harvesting capacity of each individual plate may be increased by as much as 110% within the range of parameters considered. The underlying physical mechanism has been identified as wake energy recovery - a plate in such a formation is able to extract energy from the wakes of its neighbors that will otherwise be dissipated. This finding can be used in the development of environmental-friendly soft-body wave energy harvesters.

In this study, we explore the complex dynamics of a dam break wave comprising a clay-water mixture under turbulent flow conditions as it interacts with a vertical rigid wall. Utilizing advanced three-dimensional Large Eddy Simulations (LES), we investigate the evolution of the dam break over time as a function of fluid rheology. The study aims to provide useful information for the development of risk mitigation strategies and the design of protective structures by examining the influence of clay concentration and initial fluid depth on the wave’s behavior, bed shear stresses, and impact forces.

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.