The broad relevance of fluid mechanics to biology has been increasingly appreciated by engineers and biologists alike, leading to continued expansion of research in the field of biological fluid dynamics. A selection of classic and recent work that can guide future research is highlighted.

A dynamical system is described that captures the near-singular character of vortex reconnection, thus indicating how enstrophy in turbulent flow can become infinite in the zero-viscosity limit. Similarities with the near-singular behavior in the problem of cusp-formation at a free surface are identified.

Professor Itai Cohen describes the challenge of studying insect flight and, in the accompanying video recording of his invited lecture from the 71st annual APS DFD meeting, illustrates the mechanisms used by insects to control this behavior and achieve spellbinding acrobatic feats.

The flow conditions of a full-scale wind turbine are reproduced in a laboratory with dynamic similarity, using a pressurized wind tunnel. Aerodynamic scale-effects persist at higher Re than previously believed, and it is shown that the boundary layer state is critical for turbine performance.

A restricted nonlinear representation of the flow as a streamwise constant large-scale interacting with dynamically restricted perturbations faithfully reproduces low order statistics, cross-stream structures and energy transport in turbulent channels, at vastly reduced computational costs.

Colloidal biology is a frontier of exploration in biological cells bridging the operational gap between structural biology (atomistic resolution over nanoseconds) and systems biology (minutes of operation, no spatial resolution). Colloid physics bridges this gap where much of cell machinery operates.

Artistic painting is analyzed with fluid mechanics. We identify hydrodynamic instabilities which are either prevented or induced to create textures of aesthetic value. In watercolor painting the ‘coffee stain’ instability can be induced or prevented by varying pigment type and amount and paper wetness.

Simple models for hairy surfaces interacting with flows are derived, and used to extract optimization and design principles in idealized contexts. Reconfiguration, in which flow couples to the hairs geometric configuration, is investigated along with drainage of thin films through beds of hairs.

Shelled Antarctic pteropods (aquatic snails nicknamed “sea butterflies”) swim with a pair of parapodia (or “wings”) in a high efficiency propulsion regime when the Reynolds number based on the flapping exceeds 35.

Many systems in physics and life sciences are characterised by microscopic flexible fibers interacting through viscous flow. We describe an efficient modeling framework taking into account nonlocal interactions, applied to sedimenting and shear flows with multiple fibers, and flagellar propulsion.

An investigation shows that simple flow inside a droplet can propel it along regular trajectories while simultaneously leading to chaotic motion of tracer particles. The resulting advective mixing can accelerate and even dominate transport by diffusion for biologically plausible Péclet and Batchelor numbers.

A numerical study finds that shear induced diffusivities increase with distance from the channel walls because of increasing mobility, up to an artery-size dependent maximum due to the diminishing local shear rate. At the center they remain finite owing to crossflow sweeps generated from the interaction of near-wall eddies.

We numerically study the thin film evaporation process enabled by nanoporous membranes for electronic device cooling. Results show that the net evaporative mass flux is determined by an interplay between physical effects, quantified by the Knudsen number, porosity, evaporation coefficient, and meniscus shape.

A well-known rheological property of a suspension of two sizes of rigid particles is a reduction in the shear viscosity. A numerical analysis shows that for bimodal capsule suspensions, the extent of the viscosity reduction is amplified by the deformability of the capsules.

We study the motion of a low viscous non-wetting droplet in a Hele-Shaw cell while it is pushed by an external phase at low capillary numbers. The velocity can be strongly affected by a stagnant cap at the rear of the drop and we propose a model that remarkably reproduces the experimental data.

The normal impact force of Rayleigh jets is investigated for three possible impact scenarios: a) steady-state jet, b) wavy jet, and c) droplet train. Owing to momentum conservation, the peak force experienced by the droplet train is over three times greater than that of the steady jet.

The deionization shock in microstructures has been essentially attributed to surface charge. It is demonstrated that deionization shock can also be driven by bulk electroconvection up to millimeter scale, shedding light on a new design for shock electrodialysis for desalination and water purification.

Electrovortex flows occur whenever thin electrodes are put in contact with wider liquid metal regions. An investigation shows that in liquid metal batteries, electrovortex flows can become so intense they can compromise the layered structure of the battery and cause a short circuit.

We derive a new “tube law” for elastic-walled channels, which takes into account the axial stretching that arises because of axial curvature in the base state. We quantify the effect of the new law on oscillatory fluid-structure-interaction modes in the channel and their stability.

We find unique perturbations in rotational flows because of flow instabilities.

In channel flow, the transition from turbulence displays a crossover from laminar-turbulent patterns following a two-dimensional directed-percolation scenario to a regime with localized turbulent bands statistically propagating along a single direction. A simple model accounts for this bifurcation.

Precise mathematical criteria for contact line depinning from sharp corners is derived using a variational formulation and turning point arguments. The application of these results to the analysis of the stability of nonwetting equilibria on superhydrophobic surfaces is also briefly addressed.

A study shows that hydraulic jumps are governed by both gravitational and capillary forces. The jump location results from a competition of supercritical flow with gravity-capillary waves traveling in the upstream direction. Fluid properties, flow conditions, and the state of development scale the respective forces.

An extended lubrication model is proposed by taking into account a larger surface-to-surface distance than that for the Reynolds lubrication theory, and the wall-normal variation of the pressure is related to the longitudinal derivative of the local velocity of the Couette-Poiseuille flow.

Crustaceans use their hairy appendages to sense and track food in flows with intermediate Reynolds number. A reduced order model is developed to predict the flow phase and generate a design principle for engineering flows past hairy surfaces.

We find that an analytical expression of liquid response to oscillatory motion of confining walls reproduces molecular dynamics simulation results.The viscoelastic friction coefficient and hydrodynamic boundary conditions with different wettabilities are unambiguously identified as fitting parameters.

The effect of pulsating channel walls on transport and dispersion of solutes along a channel is investigated. Hands-on analytic expressions to model dispersion in this setting using a versatile method are derived. Different scenarios where dispersion may be enhanced or suppressed are found.

A new flow-based technique for precisely manipulating the orientation and trajectory of anisotropic Brownian particles using path-following control, without the need for optical or electric fields, is presented.

A method is introduced for approximating the fully resolved flow through a monodisperse array of spheres using only the locations of the spheres and the volume-averaged Reynolds number. A pseudoturbulence stress model is proposed for use in a Lagrangian framework.

Development of “tortuous” mean ganglia flux paths during immiscible two-phase flow through a disordered and periodic 2D porous domain is studied. Results reveal that such paths are related to the gradual mobilization of stranded ganglia due to the pore-scale interplay between capillary, gravity, and viscous forces.

A higher-order realizable algebraic Reynolds stress modeling is proposed based on the square root tensor of the Reynolds stress. This model is expected to be useful in numerically stable predictions of turbulent flows with three-dimensional mean velocity, such as an axially rotating pipe flow.

Direct numerical simulations of an incompressible passive vector in turbulence find that high pseudo-enstrophy domains are sheet-like, unlike the tube structure of the high enstrophy domain. The scaling exponents of the structure functions are intermediate between the velocity and passive scalar.

The observability of the flow away from the wall in turbulent channels is studied using noiseless, although potentially incomplete, wall measurements. The reconstructions deteriorate with the distance to the wall, but coherent motions are still observable.

We perform direct numerical simulations of turbulent channel flow with superhydrophobic or liquid infused surfaces on the lower wall. The texture reproduces etched sand-blasted aluminum. Dependence of the amount of drag reduction on interface deformation and pinnacle height distribution is discussed.

Large eddy simulation (LES) of complex geometries often require discretization with high aspect ratio cells which can cause loss of simulation fidelity. We examine the effects of anisotropic resolution in LES and propose a tensor eddy viscosity to directly address resolution anisotropy.

The entrainment phenomena of the quiescent core region in a turbulent pipe flow are examined by characterizing the tall wall-attached structures of the streamwise velocity fluctuations. The quiescent core region is the uniform momentum zone with the highest streamwise velocity magnitude.

The axial and azimuthal mean momentum equation for swirling pipe flows are derived. Increasing the swirl strength increases the extent of the inertial region by pushing the beginning of the inertial region closer to the wall. This is due to the axial viscous forces rather than the azimuthal ones.

A local point of view is adopted on the chaoticity of turbulent flow, using the Lyapunov analysis. Among other findings, it is found that chaotic perturbation growth is a highly localized phenomenon that occurs in regions of high-velocity gradient but not in regions of high turbulence intensity.

An inverse cascade is found in the energy transfer in scale space of channel flow. The conditional average of the velocity field associated with the inverse cascade or negative subgrid-scale production reveals a long streamwise vortex near the wall with a short vortex in the upstream region.

We observe experimentally how two crossed standing surface waves excite a regular lattice of near-surface vortices. The dynamics of formation and decay of the lattice coincides with that predicted by our theoretical model. The mass transport in the vortices is primarily determined by Eulerian flow.

It has long been surmised that the “log” law of the mean-velocity profile (MVP) is closely related to the “$-5/3$ or $-1$” power law of the energy spectrum in wall-bounded turbulence. A refined model for the connection between MVP and the energy spectrum based on eddy’s inclination angle is proposed.

A data-based, linear dynamic estimator is developed to reconstruct the velocity field around an airfoil using measurements from a single sensor point in the wake. The performance of the estimator is very robust to the sensor location and to small changes in Reynolds number.

We derive a multiple-scales solution for the Eulerian mean flow under wavepackets, which is driven by the divergence of Stokes drift and the setdown. This solution is valid for all water depths and is validated by flume experiments and recovers all previous solutions in their respective limits.

Flow past a rotating circular cylinder is investigated experimentally in flowing soap film at ${\mathrm{R}}_{\mathrm{e}}$ of 200 to 250. Vortex shedding suppression is not found. A mode of single signed vortices dependent on surface hydrophobicity, with a rotation sense opposite to that of cylinder rotation, is observed.