The dynamics of spherical particles in a fluid flow is governed by the well-accepted Maxey–Riley equation. This equation of motion simply represents Newton’s second law, equating the rate of change of the linear momentum with all forces acting on the particle. One of these forces, the Basset–Boussinesq memory term, however, is notoriously difficult to handle, which prompts most studies to ignore this term despite ample numerical and experimental evidence of its significance. This practice may well change now due to a clever reformulation of the particle equation of motion by Prasath, Vasan & Govindarajan (J. Fluid Mech., vol. 868, 2019, pp. 428–460), who convert the Maxey–Riley equation into a one-dimensional heat equation with non-trivial boundary conditions. Remarkably, this reformulation confirms earlier estimates on particle asymptotics, yields previously unknown analytic solutions and leads to an efficient numerical scheme for more complex flow fields.

The flow of fluid through a pipe has been instrumental in illuminating the subcritical route to turbulence typical of many wall-bounded shear flows. Especially important in this process are the turbulent–laminar fronts that separate the turbulent and laminar flow. Four years ago Michael Graham (Nature, vol. 526, 2015, p. 508) wrote a commentary entitled ‘Turbulence spreads like wildfire’, which is a picturesque but also accurate characterisation of the way turbulence spreads through laminar flow in a straight pipe. In this spirit, the recent article by Rinaldi et al. (J. Fluid Mech., vol. 866, 2019, pp. 487–502) shows that turbulent wildfires are substantially tamed in bent pipes. These authors find that even at modest pipe curvature, the characteristic strong turbulent–laminar fronts of straight pipe flow vanish. As a result, the propagation of turbulent structures is modified and there are hints that the route to turbulence is fundamentally altered.

Whereas the flow of simple single-phase Newtonian fluids tends to become more complex as the characteristic length scale in the problem (and hence the Reynolds number) increases, for complex elastic fluids such as dilute polymer solutions the opposite holds true. Thus small-scale, so-called ‘microfluidic’ flows of complex fluids can exhibit rich dynamics in situations where the ‘equivalent’ flow of Newtonian fluids remains linear and predictable. In the recent study of Qin et al. (J. Fluid Mech., vol. 864, 2019, R2) of the flow of a dilute polymeric fluid past a $50~\unicode[STIX]{x03BC}\text{m}$ cylinder (in a $100\times 60~\unicode[STIX]{x03BC}\text{m}$ channel), a novel 3-D holographic particle velocimetry technique reveals the underlying complexity of the flow, including inherent three-dimensionality and symmetry breaking as well as strong upstream propagation effects via elastic waves.

The fundamental challenge to characterize and quantify thermal transport in the strongly nonlinear regime of Rayleigh–Bénard convection – the buoyancy-driven flow of a horizontal layer of fluid heated from below – has perplexed the fluid dynamics community for decades. Rayleigh proposed controlling the temperature of thermally conducting boundaries in order to study the onset of convection, in which case vertical heat transport gauges the system response. Conflicting experimental results for ostensibly similar set-ups have confounded efforts to discriminate between two competing theories for how boundary layers and interior flows interact to determine transport through the convecting layer asymptotically far beyond onset. In a conceptually new approach, Bouillaut, Lepot, Aumaître and Gallet (J. Fluid Mech., vol. 861, 2019, R5) devised a procedure to radiatively heat a portion of the fluid domain bypassing rigid conductive boundaries and allowing for dissociation of thermal and viscous boundary layers. Their experiments reveal a new level of complexity in the problem suggesting that heat transport scaling predictions of both theories may be realized depending on details of the thermal forcing.

An acoustically forced fluid system is known to generate a time-averaged mean, or streaming, flow that evolves on a slow time scale compared to the acoustic-wave period. Classical acoustic streaming in a homogeneous fluid is typically associated with a one-way coupled system wherein the oscillatory acoustic fields inform the streaming mean flow, without any appreciable feedback. In contrast, Michel & Chini (J. Fluid Mech., vol. 858, 2019, pp. 536–564) investigate acoustic streaming in a stratified fluid and demonstrate that the streaming is sufficiently strong to induce significant rearrangements of the background temperature and density fields, resulting in a strong coupling between the acoustic waves and mean flow. This new class of streaming, referred to as baroclinic acoustic streaming, is shown to result in altered streaming patterns with enhanced heat transport that makes possible a range of new applications.

Fluid mechanics and rheology involve many unsolved challenges related to the transport mechanisms of mass, momentum and energy – especially when it comes to realistic, industrially relevant materials. Very interesting are suspensions or granular fluids with solid, particulate ingredients that feature contact mechanics on the micro-scale, which affect the transport properties on the continuum- or macro-scale. Their unique ability to behave as either fluid, or solid or both, can be quantified by non-Newtonian rheological rules, and results in interesting mechanisms such as super-diffusion, shear thickening, fluid–solid transitions (jamming) or relaxation/creep. Focusing on the steady state flow of a granular fluid, one can attempt to answer a long-standing question: how do realistic material properties such as dissipation, stiffness, friction or cohesion influence the rheology of a granular fluid? In a recent paper Macaulay & Rognon (J. Fluid Mech., vol. 858, 2019, R2) shed new light on the effect cohesion can have on mass transport in sheared, sticky granular fluids. On top of the usual diffusive, stochastic modes of transport, cohesion can create and stabilise clusters of particles into bigger agglomerates that carry particles over large distances – either ballistically in the dilute regime, or by their rotation in the dense regime. Importantly, these clusters must not only be larger than the particles (defining the intermediate, meso-scale), but they must also have a finite lifetime, in order to be able to exchange mass with each other, which can seriously enhance transport in sticky granular fluids by rotection, i.e. a combination of rotation and convection.

The equations describing classical viscous fluid flow are notoriously challenging to solve, even approximately, when the flow is host to one or many immersed bodies. When an immersed body is slender, the smallness of its aspect ratio can sometimes be used as a basis for a ‘slender-body theory’ describing its interaction with the surrounding environment. If the fluid is complex, however, such theories are generally invalid and efforts to understand the dynamics of immersed bodies are almost entirely numerical in nature. In a valiant effort, Hewitt & Balmforth (J. Fluid Mech., vol. 856, 2018, pp. 870–897) have unearthed a theory to describe the motion of slender bodies in a viscoplastic fluid, ‘fluids’ such as mud or toothpaste which can be coaxed to flow, but only with a sufficiently large amount of forcing. Mathematical theories for some tremendously complicated physical systems may now be within reach.

Jet noise prediction is notoriously challenging because only subtle features of the flow turbulence radiate sound. The article by Brès et al. (J. Fluid Mech., vol. 851, 2018, pp. 83–124) shows that a well-constructed modelling procedure for the nozzle turbulence can provide unprecedented sub-dB prediction accuracy with modest-scale large-eddy simulations, as confirmed by detailed comparison with turbulence and sound-field measurements. This both illuminates the essential mechanisms of the flow and facilitates prediction for engineering design.

The atmospheric boundary layer is the level of the atmosphere where all human activities occur. It is a layer characterized by its turbulent flow state, meaning that the velocity, temperature and scalar concentrations fluctuate over scales that range from less than a millimetre to several kilometres. It is those fluctuations that make dispersion of pollutants and transport of heat, momentum as well as scalars such as carbon dioxide or cloud-condensation nuclei efficient. It is also the layer where a ‘hand-shake’ occurs between activities on the land surface and the climate system, primarily due to the action of large energetic swirling motions or eddies. The atmospheric boundary layer experiences dramatic transitions depending on whether the underlying surface is being heated or cooled. The existing paradigm describing the size and energetics of large-scale and very large-scale eddies in turbulent flows has been shaped by decades of experiments and simulations on smooth pipes and channels with no surface heating or cooling. The emerging picture, initiated by A. A. Townsend in 1951, is that large- and very large-scale motions appear to be approximated by a collection of hairpin-shaped vortices whose population density scales inversely with distance from the boundary. How does surface heating, quintessential to the atmospheric boundary layer, alter this canonical picture? What are the implications of such a buoyancy force on the geometry and energy distribution across velocity components in those large eddies? How do these large eddies modulate small eddies near the ground? Answering these questions and tracking their consequences to existing theories used today to describe the flow statistics in the atmospheric boundary layer are addressed in the work of Salesky & Anderson (J. Fluid Mech., vol. 856, 2018, pp. 135–168). The findings are both provocative and surprisingly simple.

Tracer particles are the workhorse of the fluid dynamicist for visualizing flow in transparent liquids. Thus a tracer becomes useless if its signal disappears, which frequently happens in practice, for instance due to bleaching. The opposite occurs in a recent work by Kim & Stone (J. Fluid Mech., vol. 850, 2018, pp. 769–783): the fading signal of a dissolving particle may reveal the local composition in a mixture. Such information is highly valuable in the study of evaporating droplets. In virtually all realistic cases, droplets consist of multiple components, ranging from trace impurities to engineered cocktails, which, for instance, generate a desired deposit pattern for a printing process. Different components typically evaporate at different rates, which causes inhomogeneities in droplet composition. Determining the latter is one of the main challenges in the field.

Stably stratified sheared flows are ubiquitous in geophysical flows from the ocean to the stars, and the route to turbulence in these flows remains an open question. The article by Lefauve et al. (J. Fluid Mech., vol. 848, 2018, pp. 508–544) is an invitation to this journey. With impressive experimental precision mastered by few teams in the world, the nature of the coherent structure that dominates the flow on the verge of turbulent breakdown is revealed and analysed through one- or two-dimensional modern stability analysis of an experimentally obtained base flow. The effect of confinement is surprisingly strong, advocating for leaving the textbook flows, inhomogeneous in only one direction, for the more complex shores of real flows, now accessible to analysis of multidimensional stability problems. The route explored by Lefauve et al. (2018) renews with the long tradition of the supercritical bifurcation scenario, it revisits the linear stability theory with possibility of resonances, critical layers and more to be imagined, since complex base flows are now available to explore both experimentally and analytically.

The flow of liquids through paper is challenging to model due to the complexity and disordered layout of the fibre matrix. The expanding use and capability of microfluidic paper-based analytical devices ($\unicode[STIX]{x1D707}$PADs) and their requirement for precision has increased the need to accurately predict the flow of liquids through paper. Many studies have developed models and revealed some of the physical mechanisms responsible for the flow behaviour, but we still lack a complete understanding, particularly in relation to how the fluid fills the various voids with a wide range of shapes and sizes in the fibre matrix of paper. In the featured article, Chang et al. (J. Fluid Mech., vol. 845, 2018, 36–50) used a combined experimental and theoretical approach to uncover the importance of the liquid filling the intra-fibre pores, showed that this results in deviation from the flow behaviour predicted by the Lucas–Washburn equation and developed a model which accounts for this effect.

Liquid drops deposited on a hot plate can experience a boiling crisis, when the vapour flux is strong enough to ensure the levitation of the drop and the relative insulation of the liquid from the solid. It is often denoted Leidenfrost effect, after the German Johann Gottlob Leidenfrost, who first reported it in 1756. While many studies have encompassed various applied issues associated with this phenomenon, aiming to control and prevent its appearance, Ma & Burton (J. Fluid Mech., vol. 846, 2018, pp. 263–291) focused on the spontaneous appearance of a standing wave at the free surface, together with temporal oscillations, making the drop adopt the shape of a star. Their far-reaching study presents exhaustive results using six different liquids with a range of different volumes and temperatures, in which they systematically extracted the drop dynamics together with the pressure fluctuations in the vapour cushion below.

Models of flowing complex fluids such as polymer solutions often use a conformation tensor that reflects the state of the fluid microstructure. In polymer solutions, this quantity measures the orientation and stretching of the molecules, and reflects the fact that the squared length of a polymer molecule must be positive. By exploiting results from differential geometry and continuum mechanics, Hameduddin et al. (J. Fluid Mech., vol. 842, 2018, pp. 395–427) introduce a new approach for analysing the conformation tensor that respects this positivity constraint. With this approach, they present computational results for turbulent flow of a polymer solution that exhibits turbulent drag reduction, showing that the new measures of polymer stretching afforded by their approach lend insights not available in traditional methods.

Surface tension plays a dominant role in the formation and stability of soap films. It renders them both a quasi-two-dimensional fluid and an elastic membrane at the same time. The techniques for measuring the surface tension of the soap solution may very well apply to the static soap film, but how can the surface tension of a soap film be unintrusively measured, and what value would it assume? The answer, being at the intersection of physical chemistry, non-equilibrium physics and interfacial fluid dynamics, is not amenable to deduction via established methods. In a joint theoretical and experimental study, Sane et al. (J. Fluid Mech., vol. 841, 2018, R2) exploit elasticity theory to glean the answer through a simple, yet elegant framework.

The contraction of the heart muscle combines with the opening and closing of the cardiac valves to generate a complex flow in the heart. Predicting this flow presents a significant challenge for computational models, a challenge that Meschini et al. (J. Fluid Mech., vol. 834, 2018, pp. 271–307) tackle head-on by simulating not only the flow in a modelled left ventricle of the heart, but also the coupled dynamics of the mitral valve. The model is validated against a well-designed companion experiment and the authors then proceed to examine the effects of cardiac and valvular dysfunction, as well as prosthetic valves, on ventricular haemodynamics. The insights provided by this study extend from the functional morphology of the left ventricle to the implications of the choice of valve implant on ventricular function.

Thakkar et al. (J. Fluid Mech., vol. 837, 2018, R1) represents a significant advancement in the ability to computationally model rough wall flows. Direct numerical solution (DNS) of turbulent boundary layer flow over an industrial grit blasted surface at relevant roughness Reynolds numbers, from hydraulically smooth to fully rough regimes, is a path forward to parametrically study a wide range of surface roughness. The methodology described in this paper, coupled with validation experiments, ultimately should lead to improved frictional drag predictions.

Using high-resolution numerical simulation, Short et al. (J. Fluid Mech. vol. 835, 2018, pp. 970–998) study diffraction of a detonation as it traverses a $270^{\circ }$ finite-thickness condensed-phase explosive arc. This geometry admits a steady solution in a frame rotating with angular speed $\unicode[STIX]{x1D714}_{0}$ , which thereby facilitates a detailed analysis of how the loss of energy from the detonation reaction zone due to the diffraction process slows the propagation of the detonation. There exists a region of subsonic flow, between the detonation shock and the curve of sonic flow (labelled the DDZ), which is responsible for setting $\unicode[STIX]{x1D714}_{0}$ . Although the DDZ spans the entire thickness for thin arcs, it is localized to a region near the inside surface as the arc is thickened. Thus the explosive energy release near this inside surface plays a disproportionate role in the diffraction process.

Flowing air and water are persistent sculptors, gradually working stone, clay, sand and ice into landforms and landscapes. The evolution of shape results from a complex fluid–solid coupling that tends to produce stereotyped forms, and this morphology offers important clues to the history of a landscape and its development. Claudin et al. (J. Fluid Mech., vol. 832, 2017, R2) shed light on how we might read the rippled and scalloped patterns written into dissolving or melting solid surfaces by a flowing fluid. By better understanding the genesis of these patterns, we may explain why they appear in different natural settings, such as the walls of mineral caves dissolving in flowing water, ice caves in wind, and melting icebergs.

The bizarre behaviour of a cornstarch suspension (sometimes called oobleck) is well known to all of us who have led public engagement events. At the right solids fraction, it flows smoothly at slow speeds, but can be shattered with a quick spoon movement; if you prepare a large enough sample, you can run across the surface (but if you stand still, you will sink). In rheology circles this phenomenon is known as shear thickening, though the flows described above are not necessarily shear-dominated. In recent years there has been a proliferation of research on the mechanism behind true shear thickening, using both experiments and numerical simulations of shear flows. The understanding of the underlying mechanism is improving markedly. But the paper ‘Microstructure and thickening of dense suspensions under extensional and shear flows’ (Seto, Giusteri & Martinello, J. Fluid Mech., vol. 825, 2017, R3) is the first to consider more general flows. We have, for the first time, simulations of thickening in extensional flows, which are a far better description of oobleck with a runner on top – and can begin to quantify the difference between the idealised shear thickening and the extension thickening that happens in practice.