It has long been thought that birds and fish form into flocks and schools to take advantage of flows. It is now shown that flapping bodies not only move faster when grouped together but that the flows also help to arrange, or order, the members. This suggests that a school can be viewed as a “swimming crystal” of fish organized by flows.

In many contexts in biology, flagella or cilia typically beat in synchrony. An asymptotic analysis of filaments separated by a typical distance that is small compared to their own length is developed in its general form, and it is applied to a heuristic model of beating filaments, thereby elucidating how the so-called “elastohydrodynamic” mechanism of synchronization operates with extended objects.

Experiments and computational lamprey models address the question: How does flexibility affect swimming performance? Can we tune the passive material properties of a swimming body to improve performance?

A thorough combined numerical and experimental investigation of nanosecond dielectric barrier discharge actuation provides a description of the dynamics of the flow actuation process and elucidates the associated flow control mechanisms.

A time analysis of electroconvective flows from 3D simulation data is introduced for the first time.

New helical structures are seen in DNS of both adverse and zero pressure gradients, structures which can evolve into turbulence.

Laboratory and numerical experiments have shown evidence of the helicoidal instability of the hub vortex in wind turbine wakes. A new study predicts the characteristics of the hub vortex instability using a linear stability analysis for different turbine loading conditions, blade aerodynamics, and incoming turbulence

A widely unexplored type of hydrodynamic instability is examined—long-time algebraic growth. Such growth can occur on the threshold of neutral stability, as classified by dispersion relations arising from exponential normal modes. The morphology of responses exhibiting algebraic growth are compared with those of well-known exponential growth.

The two-dimensionalization of the flow produced by a rotating impeller in a rapidly rotating frame is investigated experimentally. It is shown to proceed via the emergence of wakes of inertial waves traveling with the impeller blades as a consequence of a barotropic instability of the large-scale flow.

Morphological changes of liquid drops sliding down an incline are studied in a long-wave model employing numerical path-continuation and time-simulation techniques. Highlights of the found behavior include multistability between different droplet types, pearling-coalescence cycles, and a period-doubling route to chaos.

Heating a superhydrophobic substrate can restore the nonadhesive properties expected for such materials under certain conditions. A set of experiments on several model geometries, and a scaling analysis, help elucidate the dominant mechanisms associated with this Wenzel to Cassie transition for impinging droplets.

A theoretical investigation examines the effect of finite substrate thermal diffusivity on the stability of a thin liquid film flowing downward due to gravity.

Experiments and lattice Boltzmann simulations demonstrate that, in contrast with currently accepted opinions, the Cassie state for sufficiently large droplets condensing onto patterned hydrophobic surfaces, is stable, while the Wenzel state is metastable or unstable.

A micropillar array column is used to study miscible viscous fingering in the presence of a nonmonotonic viscosity profile caused by nonideal mixing properties in a porous medium. Fingering is observed at both the frontal and rear miscible interfaces of a finite-size sample phase, with different spatial extents, however.

Anomalous dispersion in heterogeneous porous media is linked to the medium properties in terms of hydraulic conductivity. Spacetime particle transitions follow a continuous time random walk, which quantifies the dominant role of low-conductivity regions. An explicit map between the conductivity and transition time distributions is derived.

Liquid-infused surfaces are being studied for possible applications in surface protection, functionalization, and drag reduction. Researchers investigate the shear-driven failure of these surfaces under a broad range of ratios of the viscosity of the external fluid to that of the lubricant, and they compare mathematical models and experimental results.

A continuous-time random walk approach is developed that quantifies the evolution of isochrone and equidistant Lagrangian velocities in steady heterogeneous flows. It is based on the Eulerian velocity distribution and predicts particle transport for arbitrary stationary and nonstationary initial conditions.

Molecular simulations of the recovery of gas from single nanopores address the need to understand gas recovery from shale formations. A new study shows that in very narrow pores, gas recovery follows a superdiffusive scaling law rather than the classical diffusive law. The gas diffusion coefficient is also much smaller than that predicted by kinetic theories.

Molecular dynamics simulations are used to investigate fluid structure near walls with different types of roughness. Slip depends on both large-scale changes in slope and atomic-scale corrugations that are modulated by local curvature.

A numerical scheme for studying electrolyte solutions in the presence of thermally induced fluctuations is presented and analyzed. The inclusion of fluctuations consistent with fluctuation-dissipation balance is important for modeling small-scale electrokinetic transport phenomena and instabilities.

The results of MRI measurements of void fraction and velocities of gas and solids in a bubbling fluidized bed are compared favorably with theory and CFD-DEM numerical simulations, and also find a wide distributions of velocities.

The effect of wettability on slow fluid displacement from a Hele-Shaw cell with cylindrical obstacles is studied both in microfluidic experiments and in particle-based simulations. Upon increasing the contact angle of the cell walls, a crossover from cooperative to noncooperative interfacial instabilities is found that explains the sudden change of the observed displacement patterns.

The effect of the Basset history force on the sedimentation of nearly neutrally buoyant inertial particles, exemplified by marine snow, in a three-dimensional turbulent flow is analyzed. The main effect of the history force is an extraordinary slow, power-law-type convergence to an asymptotic settling velocity.

The settling of heavy spherical particles in a column of quiescent fluid is investigated. A coherent set of observations is found for settling particles that explain the observed features: trajectory type, vertical alignment, high density regions, and enhanced settling velocity.

Numerical simulations demonstrate that shearing a cluster of randomly oriented Janus spheres adsorbed at the interface between two immiscible fluids drives their assembly into ordered chains due to the formation of capillary dipoles and the resulting interparticle capillary-induced interactions.

Fully resolved simulations of axisymmetric turbulent jets and plumes are used to study how mixing, dilution, and buoyancy are related to turbulent entrainment.

To understand better the spread of ash from volcanic eruptions penetrating into the stratosphere, laboratory experiments are performed using a recently developed light attenuation technique to measure the deposit of sediments from a particle-bearing plume rising in stratified fluid. The results are used to calibrate an adaptation of a plume model that accounts for particle settling and re-entrainment.

Using particle image velocimetry, the flow in the near field of two different arrays of wall-mounted obstacles is investigated. Two clearly distinct formation mechanisms seem to be generated: wall-driven mechanism, associated with vertical slender cylinders, and a wake-driven mechanism associated with longer adaptation regions.

A detailed control volume analysis of the flow around a wind turbine illustrates the dominant components of the mean kinetic energy budget as well as their corresponding spatial distribution.

The transient growth of optimal disturbances constrained to the near-wall region (y+≤40) demonstrates that the ubiquitously observed near-wall turbulence structures are indeed related to linear optimal disturbance in the region where linear approximation is valid.

Releasing a beach ball just under the surface of the water will result in a large and violent “pop-up” of the ball above the surface. Contrary to intuition, increasing the release depth sometimes results in a lower pop-up height. Experiments show that the pop-up height is directly related to the release depth, but the correlation is not always increasing. Particle image velocimetry reveals that vortical structures shed during ascent result in nonvertical trajectories that alter the pop-up height.