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.

Drops sliding down an incline exhibit fascinating shapes, which indirectly provide a great deal of information about wetting dynamics. Puthenveettil, Kumar & Hopfinger (J. Fluid Mech., vol. 726, 2013, pp. 26–61) have renewed this subject by considering water and mercury drops sliding at high speed. The results raise puzzling questions: how to take into account inertia at a high-speed contact line, large contact angles, the nature of the dissipation at small scale and sliding versus rolling behaviours?

Inspired by natural interfaces with surprising transport properties, innovative modifications of surfaces have been engineered to reduce drag. The common theme across these new developments is the presence of lubricant patches or layers that decrease the direct contact of viscous liquid with non-slippery solid walls. For laminar flow, the traditional assumption regarding the lubricant layer is a constant shear rate or a steady pressure gradient, implying a net flow rate of the lubricant film. By challenging this assumption, Busse et al. (J. Fluid Mech., vol. 727, 2013, pp. 488–508) rigorously found that the hydrodynamic slip is reduced by the presence of a reversal of lubricant flow close to the wall. The analytical results for velocity field and change in drag provide insight into the optimal design of slippery surfaces with lubricant layers for drag reduction.

The complex dynamics of turbulent flow in the vicinity of a solid surface underlie numerous scientifically important processes, and pose persistently daunting challenges in many engineering applications. Since their discovery decades ago, coherent motions have presented a tantalizing prospective opportunity for constructing descriptions of wall-flow dynamics using only a relatively small number of elements. The veracity and reliability of such representations are, however, ultimately tied to their basis in the Navier–Stokes equations. In this regard, the study by Sharma & McKeon (J. Fluid Mech., vol. 728, 2013, pp. 196–238) constitutes an important contribution, as it not only provides insights regarding the mechanisms underlying wall-flow coherent motion formation and evolution, but does so within a Navier–Stokes framework.

Wall-bounded flows, in their transition from a laminar state to turbulence, pass through a set of particular stages characterized by different physical processes. Among wall-bounded flows, separated flows have a special place because their dynamics can either be noise amplifiers or oscillators. For several years Marxen and co-workers have been studying the evolution of two- and three-dimensional perturbations in the laminar part of a laminar separation bubble. In Marxen et al. (J. Fluid Mech., vol. 728, 2013, p. 58) they study vortex formation and its evolution in laminar–turbulent transition in a forced separation bubble. By the combined use of numerical and experimental methods, different mechanisms of secondary instabilities have been highlighted: elliptic instability of vortex cores and hyperbolic instability responsible for three-dimensionality in the braid region. This work shows, for the first time in laminar separation bubbles, the first nonlinear stages of transition to turbulence of such a flow. However, since this type of flow is very sensitive to various environmental stresses, several scenarios for transition to turbulence remain to be explored.

Multiphase flows in porous media represent fluid dynamics problems of great complexity involving a wide range of physical phenomena. These flows have attracted the attention of an impressive group of renowned researchers and have spawned a number of classic problems in fluid dynamics. These multiphase flows are perhaps best known for their importance in oil recovery from petroleum reservoirs, but they also find application in novel areas such as hydrofracturing for natural gas recovery. In a recent article, Zinchenko & Davis (J. Fluid Mech. 2013, vol. 725, pp. 611–663) present computational simulations that break new ground in the study of emulsions flowing through porous media. These simulations provide sufficient scale to capture the disordered motion and complex break-up patterns of individual droplets while providing sufficient statistical samples for estimating meaningful macroscopic properties of technical interest.

Hydraulic jumps, roll waves or bores in open channel flows are often treated as singularities by hydraulicians while slowly varying shallow water flows are described by continuous solutions of the Saint-Venant equations. Richard & Gavrilyuk (J. Fluid Mech., vol. 725, 2013, pp. 492–521) have enriched this model by introducing an equation for roller vorticity in a very elegant manner. This new model matches several experimental results that have resisted theoretical approaches for decades. This is the case of the roller of a stationary hydraulic jump as well as the oscillatory instability that the jump encounters when the Froude number is increased. The universality of their approach as well as its convincing comparisons with experimental results open the way for significant progress in the modelling of open channel flows.

The understanding of chaotic dynamics in high-dimensional systems that has emerged in the last decade offers a promising dynamical framework to study turbulence. Here turbulence is viewed as a walk through a forest of exact solutions in the infinite-dimensional state space of the governing equations. Recently, Chandler & Kerswell (J. Fluid Mech., vol. 722, 2013, pp. 554–595) carry out the most exhaustive study of this programme undertaken so far in fluid dynamics, a feat that requires every tool in the dynamicist’s toolbox: numerical searches for recurrent flows, computation of their stability, their symmetry classification, and estimating from these solutions statistical averages over the turbulent flow. In the long run this research promises to develop a quantitative, predictive description of moderate-Reynolds-number turbulence, and to use this description to control flows and explain their statistics.

One way that large-scale oceanic internal waves transfer their energy to small-scale mixing is through parametric subharmonic instability (PSI). But there is a disconnect between theory, which assumes the waves are periodic in space and time, and reality, in which waves are transient and localized. The innovative laboratory experiments and analysis techniques of Bourget et al. (J. Fluid Mech., vol. 723, 2013, pp. 1–20) show that theory can be applied to interpret the generation of subharmonic disturbances from a quasi-monochromatic wave beam. Their methodology and results open up new avenues of investigation into PSI through experiments, simulations and observations.

Convection in a closed porous domain is a temporally and spatially complex flow which evolves over long time scales as the driving buoyancy contrasts are eliminated by mixing. In a contribution that combines numerical, experimental and asymptotic approaches, Hewitt, Neufeld & Lister (J. Fluid Mech., vol. 719, 2013, pp. 551–586) demonstrate that the essential dynamics can be captured by simple ‘box’ models, both when the buoyancy supply is imposed at the upper boundary and when the domain contains a moving interface between different fluids. This work provides insights into the dynamics and viability of schemes for the geological sequestration of CO2.

If, in a large expanse of fluid such as air or water, an object that is heavier than the fluid displaced is released from rest, it descends in a manner that can depend in a complex way on its geometry and density (relative to that of the fluid), and on the fluid viscosity, which, as in other fluid contexts, remains important no matter how small this viscosity may be. A major numerical attack on this problem for the case in which the object is a thin circular disc is presented by Auguste, Magnaudet & Fabre (J. Fluid Mech., vol. 719, 2013, pp. 388–405).

Marusic et al. (J. Fluid Mech., vol. 716, 2013, R3) show the first clear evidence of universal logarithmic scaling emerging naturally (and simultaneously) in the mean velocity and the intensity of the streamwise velocity fluctuations about that mean in canonical turbulent flows near walls. These observations represent a significant advance in understanding of the behaviour of wall turbulence at high Reynolds number, but perhaps the most exciting implication of the experimental results lies in the agreement with the predictions of such scaling from a model introduced by Townsend (J. Fluid Mech., vol. 11, 1961, pp. 97–120), commonly termed the attached eddy hypothesis. The elegantly simple, yet powerful, study by Marusic et al. should spark further investigation of the behaviour of all fluctuating velocity components at high Reynolds numbers and the outstanding predictions of the attached eddy hypothesis.

The parameter regime of strong stable density stratification and weak rotation is an important one in geophysical fluid dynamics. These conditions exist at intermediate length scales in the atmosphere and ocean (mesoscale and sub-mesoscale, respectively), and turbulence here links large-scale quasi-geostrophic motions with small-scale dissipation. While major advances in the theory of stratified turbulence have been made over the last few decades, many open questions remain, particularly about the nature of the energy cascade. Recent numerical experiments and analysis by Augier, Chomaz & Billant (J. Fluid Mech., vol. 713, 2012, pp. 86–108) present a remarkably vivid illustration of the nonlinear interactions that drive such turbulence. They consider a columnar vortex dipole, which naturally three-dimensionalizes under the influence of strong stratification. Kelvin–Helmholtz instabilities subsequently transfer energy directly to small scales, where the flow transitions into three-dimensional turbulence. This direct link between large and small scales is quite distinct from the usual picture of a turbulent cascade, in which nonlinear interactions are local in scale. But how important is this mechanism in the atmosphere and ocean?

The recent article by Schoof (J. Fluid Mech., vol. 712, 2012, pp. 552–578) provides a technically demanding solution to the problem of determining ice-stream margin evolution. It is important in opening the way to the future theoretical description of how the ice sheets will melt and sea level will rise as the climate warms. But the sophistication of the mathematics should not operate as a mask to an examination of the credibility of the model.

Rotating, stratified flows are important in a wide variety of both geophysical and engineering applications. Whilst ‘steady state’ flows of this type are generally very simple (in effect, rigid body rotation), the effect of abruptly altering (even a little) the rotation rate can induce significant temporal flow disruptions, made all the more complicated when the fluid is bounded inside a closed finite container, a problem studied both experimentally and theoretically by Foster & Munro (J. Fluid Mech., this issue, vol. 712, 2012, pp. 7–40).

A turbulent boundary layer formed over an external surface is spatially evolving. The flow develops from an initial set of conditions and accordingly the state of the layer at some downstream location owes a debt to its upstream history. Schlatter & Örlü (J. Fluid Mech., this issue, vol. 710, 2012, pp. 5–34) demonstrate succinctly this sensitivity to upstream history, finding that relatively minor modifications to the trip parameters can produce non-canonical development up to surprisingly high Reynolds numbers. This interesting study will serve as a cautionary note to experimentalists and numericists while providing a plausible explanation for some of the disparity noted among previously published results.

In their transition from a laminar state to turbulence, some unstable flows pass through a set of well-defined stages involving different and distinct processes. This is so, in particular, for Kelvin–Helmholtz instability, although details of its transition still retain many mysterious aspects. Billows develop in the primary stage of this stratified shear flow instability, separated by thin braids in which the shear is relatively high. Fluid is statically unstable within the billows and consequently potentially prone to convective instability. Numerical studies by Mashayek & Peltier (J. Fluid Mech., this issue, vol 708, 2012a,b, pp. 5–44 and 45–70) have discovered several new types of secondary instability in the braids and billow cores that may hasten the eventual transition to turbulence. The instabilities are illustrated by the authors in colour figures, remarkable for their beauty and (recalling William Blake’s ‘The Tyger’) ‘fearful symmetry’. But are they helpful in establishing the subsequent turbulence in the natural environment?

The structure of viscous and thermal boundary layers at the heated and cooled plates in turbulent thermally driven flows are of fundamental importance for heat transfer and its dependence on the thermal forcing (the Rayleigh number in non-dimensional form). The paper by Shi, Emran & Schumacher (J. Fluid Mech., this issue, vol. 706, 2012, pp. 5–33) stresses the deviations of the boundary layer vertical profiles from the Prandtl–Blasius–Pohlhausen theory. Recent papers showing very similar results, in contrast, focus more on the similarities.