The rupture of soap films is traditionally described by a law accounting for a balance between momentum and surface tension forces, derived independently by Taylor and Culick in the 1960s. This law is highly relevant to the dynamics of thin liquid films of jets when viscous effects are negligible. However, the minute amounts of surfactant molecules present in soap films play a major role in interfacial rheology, and may result in complex behaviour. Petit et al. (J. Fluid Mech., vol. 774, 2015, R3) challenge standard thin film dynamics via intriguing experiments conducted with highly elastic surfactants. Unexpected structures reminiscent of faults are observed.

The flow inside a circular pipe is known to support localised patches of turbulence travelling downstream. When the diameter of the pipe is slowly increasing, seemingly analogous but motionless patches can congest the flow and lead to disastrous energetic performance. Until now, no theory has predicted quantitatively the flow rates at which such a steady turbulent puff could be expected. The recent large-scale numerical study by Selvam et al. (J. Fluid Mech., vol. 771, 2015, R2) offers the first realistic simulation of this phenomenon, with a detailed analysis of the bifurcation sequence leading from the laminar flow to the localised turbulence regime. Similarities and differences between straight and divergent cases are also discussed.

Orlandi et al. (J. Fluid Mech., vol. 770, 2015, pp. 424–441) present direct numerical simulations over a very wide Reynolds number range for plane Couette and Poiseuille flows. The results reveal new information on the abrupt nature of transition in these flows, and the comparisons between Couette and Poiseuille flows help to provide a clearer picture of Reynolds number trends, especially with regard to inner/outer layer interactions. The stress distributions give strong support to Townsend’s attached eddy hypothesis, particularly for the wall-parallel component where there has been little experimental data available. The results pose some intriguing questions regarding the reconciliation of the present results with data at higher Reynolds numbers in different canonical flows.

Anisotropic particles are suspended in a wide range of industrial, environmental and biological fluid flows. The orientations of these particles are sometimes randomized by turbulence, but often they are brought into preferential alignment by the fluid flow. In a recently published study, Challabotla, Zhao & Andersson (J. Fluid Mech., vol. 766, 2015, R2) performed the first numerical simulations of inertial disks in a turbulent channel flow. They find that disks can be made to preferentially align either parallel or perpendicular to the wall depending on the particle density. Particle shape also affects alignment, particularly for lower density particles, and the alignment of disks is quite different from the alignment of fibres.

The mechanisms by which a wetting, non-saturating liquid bestows macroscopic cohesion and strength to a granular material are usually not accessible to micromechanical investigations for saturations exceeding the pendular regime of isolated menisci, easily studied by discrete element models (DEM). The paper by Delenne et al. (J. Fluid Mech., 2015, vol. 762, R5) exploiting a multiphase lattice Boltzmann approach, pioneers the simulation of the micromorphology and of the mechanical effects on grains of an interstitial liquid, in equilibrium with its vapour, for the whole saturation range. Interestingly, in accordance with some experiments and phenomenological models, the results suggest that the mechanical effect of capillary forces is maximized for some intermediate saturation level (near 40 % in the model), well beyond the pendular range. In general, the proposed simulation technique opens the way to many studies of partially saturated granular assemblies, for different saturation or imbibition processes and histories.

Picano et al. (J. Fluid Mech., vol. 764, 2015, pp. 463–487) have conducted fully resolved numerical simulations of many thousands of spherical particles in a turbulent channel flow with $\mathit{Re}=5600$. Their results give a tantalizing demonstration of the vastness of the vistas that this line of research is about to open.

Extreme behaviour of fluid and material motions needs to be understood for engineering processes and the behaviour of clouds or plumes of pollution. Applications in the natural environment require scaling of turbulence behaviour and models beyond current computational or laboratory understanding. New computational studies of Biferale et al. (J. Fluid Mech., 2014, vol. 757, pp. 550–572) are probing new regimes of scaling of extreme random events in nature produced by turbulent fluctuations trending towards applications in environmental prediction.

The large eddy simulation (LES) technique will soon be 50 years old. Since Deardorff’s first papers in 1970 introducing this approach, major advances in the theory of LES and its computational implementation have been made and widely adopted. However, in terms of validation, LES studies continue to largely focus on the first- and second-order statistics, which in fact are the same tests that Deardorff conducted 45 years ago. Further advances in LES and wider adoption for new flows require advanced and more challenging tests to be developed and documented to serve as benchmarks. The paper by Stevens, Wilczek & Meneveau (J. Fluid Mech., 2014, vol. 757, pp. 888–907) does precisely that. The authors demonstrate the ability of LES to capture the recently established log-law of streamwise velocity variance and the related log-laws for even-order statistics up to order 10, as well as the departure of these statistics from a Gaussian distribution. The paper also provides key insights into the role of grid resolution on the computed turbulence field.

Understanding biolocomotion in fluids has long been a focus of fluid dynamicists. One method to quantify the impact of environmental stresses on locomotion is to systematically change the mechanical properties of the surrounding medium, and measure how that change influences swimming kinematics and energetics. In a recently published investigation, Gagnon et al. (J. Fluid Mech., vol. 758, 2014, R3) employ that approach to investigate the locomotion of the nematode Caenorhabditis elegans in complex fluids. Specifically, they characterize experimentally how the presence of shear-thinning rheology influences the flow around the organism and its swimming ability. Surprisingly, while they measure an important change to the flow structure around the organism, they find no change in its waving motion and the speed at which it is able to swim. While ‘gooeyness’ is a universal feature of natural biological and environmental media, C. elegans seems to find it perfectly bearable.

Cross-streamline migration of soft particles in suspensions is essential for cell and DNA sorting, blood flow, polymer processing and so on. Pioneering work by Poiseuille on blood flow in vivo revealed an erythrocyte-free layer close to blood vessel walls. The formation of this layer is related to a viscous lift force caused by cell deformation that pushes cells towards the centre of blood capillaries. This lift force has in this case a strong impact on blood flow. In contrast, rigid spherical particles migrate from the centre towards the periphery, owing to inertia (the Segré–Silberberg effect). An important open issue is to elucidate the interplay between particle deformation and inertia. By using a capsule suspension model, Krueger, Kaoui & Harting (J. Fluid Mech., 2014, vol. 751, pp. 725–745) discovered that capsule flexibility can suppress the Segré–Silberberg effect and inertia promotes overall flow efficiency thanks to a strong inertial flow focusing effect.

Much of our understanding of the transition to turbulence in flows without a linear instability came with the discovery and characterization of fully three-dimensional solutions to the Navier–Stokes equation. The first examples in plane Couette flow were periodic in both spanwise and streamwise directions, and could explain the transitions in small domains only. The presence of localized turbulent spots in larger domains, the spatiotemporal decoherence on larger scales and the ability to trigger turbulence with pointwise perturbations require solutions that are localized in both directions, like the one presented by Brand & Gibson (J. Fluid Mech., vol. 750, 2014, R3). They describe a steady solution of the Navier–Stokes equations and characterize in unprecedented detail, including an analytic computation of its localization properties. The study opens up new ways to describe localized turbulent patches.

This contribution is a remake of the one published under the same title by Reynolds (Nord. Hydrol., vol. 7, 1976, pp. 161–183). As in that paper, attention is given to the latest developments in the field of river morphodynamics and, in particular, on the formation of bed patterns. Stimulated by the work by Kidanemariam and Uhlmann (J. Fluid Mech., vol. 750, 2014, R2) the opportunity is taken to examine the most recent developments in terms of sediment transport models.

Closed bilayer membranes (vesicles) display a plethora of non-spherical shapes under equilibrium conditions, unlike drops and bubbles which are kept spherical by surface tension. Even more complex behaviour arises under applied flow. Intriguingly, a vesicle can adopt asymmetric shapes even under symmetric forcing such as uniaxial extensional flow. Narasimhan, Spann & Shaqfeh (J. Fluid Mech., vol. 750, 2014, pp. 144–190) explain the mechanism of this peculiar instability and trace its origin to the tension which develops in the area-incompressible membrane in response to the applied stress. The authors also show that this mechanism is relevant to the pearling of tubular vesicles. This study raises many questions, e.g. whether other soft particles with load-dependent tension such as capsules can undergo similar shape transformations.

The dynamics of flowing, concentrated suspensions of non-colloidal particles continues to surprise, despite decades of work and the widespread importance of suspension transport properties to industrial processes and natural phenomena. Blanc, Lemaire & Peters (J. Fluid Mech., 2014, vol. 746, R4) report a striking example. They probed the time-dependent dynamics of concentrated suspensions of rigid and neutrally buoyant spheres by simultaneously measuring the oscillatory rheology and the sedimentation rate of a falling ball. The sedimentation velocity of the ball through the suspension depends strongly on the frequency of oscillation, though the rheology was found to be independent of frequency. The results demonstrate the complexities of suspension flows and highlight opportunities for improving models by exploring suspension dynamics and rheology over a wide range of conditions, beyond steady and unidirectional ones.

Taylor–Couette flow is inevitably associated with the visually appealing toroidal vortices, waves, and spirals that are instigated by linear instability. The linearly stable regimes, however, pose a new challenge: do they undergo transition to turbulence and if so, what is its mechanism? Maretzke et al. (J. Fluid Mech., vol. 742, 2014, pp. 254–290) begin to address this question by determining the transient growth over the entire parameter space. They find that in the quasi-Keplerian regime, the optimal perturbations take the form of Taylor columns and that the maximum energy achieved depends only on the shear.

In their recent publication Crouch et al. (J. Fluid Mech., this issue, vol. 748, 2014, pp. 5–35) use wind tunnel experiments to quantify the large-scale vortical structures that develop as a cyclist progresses through a full rotation of the pedals. The authors identify asymmetries in the trailing vortex wake, which intensify as one leg straightens, as the primary source of drag variation over one pedal cycle. These new data suggest that targeted approaches to mitigate asymmetries in the trailing wake present an intriguing opportunity to reduce drag in cycling strategies and technologies.

Ship wakes are fascinating. They can be observed by the human eye and appear to have a V shape when the ship is advancing at constant speed along a straight trajectory. Under idealized conditions, Kelvin found that the angle between the two branches of the V is ${\sim }39^\circ $. However, in a number of cases, this angle appears to be smaller. This phenomenon has been studied by various authors, and several explanations have been suggested. The most elegant one, which is based on the amplitude of the ship waves rather than their phase, has recently been revisited by Darmon, Benzaquen & Raphaël (J. Fluid Mech., vol. 738, 2014, R3).

The complex patterns observed in experiments on suspensions of swimming cells undergoing bioconvection have fascinated biologists, physicists and mathematicians alike for over a century. Theoretical models developed over the last few decades have shown a strong similarity with Rayleigh–Bénard thermal convection, albeit with a richer dynamical behaviour due to the orientational degrees of freedom of the cells. In a recent paper, Hwang & Pedley (J. Fluid Mech., vol. 738, 2014, pp. 522–562) revisit previous models for bioconvection to investigate the effects of an external shear flow on pattern formation. In addition to casting light on new mechanisms for instability, their study demonstrates a subtle interplay between shear, swimming motions and bioconvection patterns.

Nature shuns homogeneity. In turbulent clouds, industrial reactors and geophysical flows, discrete particles arrange in clusters, posing difficult challenges to theory. A persistent question is whether clusters can be modelled with continuum equations. Recent evidence presented by Mitrano et al. (J. Fluid Mech., vol. 738, 2014, R2) indicates that suitable equations can predict the formation of clusters in granular flows, despite violating the simplifying assumptions upon which they are based.

Sequestration of carbon dioxide by injecting it into the deep subsurface is critical to successful mitigation of climate change by reducing anthropogenic emissions of greenhouse gases into the atmosphere. To achieve this we must understand how CO2 moves in the subsurface. Many interesting fluid mechanics problems emerge. Szulczewski, Hesse & Juanes (J. Fluid Mech., vol. 736, 2013, pp. 287–315) focus on one critical aspect, namely the dissolution of CO2 into the fluid resident in the subsurface and the flow dynamics that ensue. Even for this single problem, an elegant analysis identifies seven behavioural regimes that control the amount and timing of dissolution.