A perspective is presented on how machine learning methods might advance fluid mechanics. Current limitations are discussed, though the potential impact is deemed high, so long as outcomes are held to the long-standing critical standards that should guide studies of flow physics.

The evaporation of a drop of American whiskey leaves a characteristic web-like pattern that isn’t observed in Scotch whisky and other distillates.

Gravity-driven Rayleigh-Taylor convective dissolution in porous media is controlled by buoyancy and diffusion. We show that the main features of the mixing dynamics of dissolution exhibits a superlinear scaling that is universal.

Alternating current (AC) immersed electrospray exhibits a new microdripping phenomenon whose droplet size is exponentially sensitive to the applied field.

A model is derived by assuming that agitation in dense suspensions is driven by fluctuations of the solid-phase indicator function. The model is shown to accurately predict the velocity variances of both liquid and particles in inertial homogenous fluidized beds.

The homoclinic tangle at the laminar-turbulent boundary explains transient chaos observed in spatially localized turbulence.

Groups of active microtubule bundles spontaneously form wavelike patterns, and the physical mechanism for this hasn’t been well understood. We present a first-principles model that exhibits these waves with only a handful of inputs, and predicts how observations depend on experimental conditions.

Statistical behavior of turbulent kinetic energy (TKE) transport in boundary layer flashback of hydrogen-rich premixed combustion is analyzed for the first time. Flame alters the structure of the turbulent boundary layer, which is evident from the changes in the budget of the TKE transport equation.

Classical modes of one-dimensional detonation are reproduced by using a real chemical kinetics. A bifurcation diagram is formed to identify the bifurcation points of pulsating instability and the detonation limits of pulsating detonations are found respectively at low and high Ar dilutions.

An analytical formula is derived for the drag force on a particle translating in a viscoelastic fluid under the influence of an externally imposed torque. A decrease of the drag force as the rotation rate of the particle increases is predicted in agreement with numerical results from the literature.

Unlike force-dipole swimmers, amoeboid swimmers affect the effective suspension viscosity in a nontrivial manner. Transitions between shear-thickening and shear-thinning behaviors, depending on applied shear rates, are due to the phase-locking of the swimmer dynamics with the shear flow.

Experiments find that millimetric drops of a viscoelastic fluid impacting on a superhydrophobic surface at several meters per second do not rebound above a critical Weissenberg number of 1.

A fluid-dynamic-type system for acoustic phenomena in the slip-flow regime is obtained with an asymptotic analysis of the linearized Boltzmann equation. The system permits a comprehensive understanding of the phenomena. The occurrence of various non-Navier-Stokes effects is clarified.

A numerical analysis reveals that the heat transfer inside a horizontal porous annulus filled with a saturated porous medium depends strongly on the waviness of the cold wall. Particular attention is paid to the influence of the waviness parameters on the thermoconvective instabilities.

We study the onset of convection in two superposed fluid layers that have different properties. Depending on the parameters of the problem, the most unstable mode can take one of three distinct forms: whole layer solutions, localized solutions, and segregated solutions.

Effects of membrane-membrane adhesion on vesicle hydrodynamics are studied theoretically (using lubrication theory) and numerically (using time-adaptive boundary integral simulations). Novel vesicle dynamics are quantified to help design future microfluidic experiments to probe membrane adhesion.

The effect of an off-center stable steady state of a vesicle (a model of red blood cells) in a bounded shear flow is investigated numerically. It is found that this new solution (off-center) strongly alters the rheological properties of dilute as well as concentrated vesicle suspensions.

The nanotrench structure can obviously enhance the stability of bulk nanobubbles and control the formation and positions of surface nanobubbles. The hydrophilic deformed graphene domains prevent interaction between two neighboring surface nanobubbles.

Droplet breakup is classically controlled by the ratio of flow kinetic to surface energy (Weber number). From axisymmetric simulations and simple model-scaling arguments we show how the critical Weber numbers for breakup and bursting/stripping transition depend on the droplet/carrier density ratio.

Electrohydrodynamic convection between parallel electrodes with unipolar injection is investigated numerically. A parameter $Y$, defined as the ratio of the local electric force to the viscous force, characterizes the stability of the flow system.

The standard electrokinetic model (SEM) is modified by considering hydrated ions as charged dielectric spheres so that in the modified model (MEM) the electrolyte solution becomes a non-homogeneous fluid with permittivity and viscosity values that are both functions of the local ionic concentrations.

The buoyancy instability occurring at the interface of a soluble body suddenly immersed in a quiescent solvent is explored. Numerical simulations are used to derive and confirm scaling laws based on a constant solutal Rayleigh number. Dissolution erosion rates are predicted in some geological situations.

The physics behind the instability at the source of oscillations in a single-branch pulsating heat pipe is investigated. Oscillations produced by the resonator increase in amplitude when pressure induced by evaporation-condensation (positive feedback mechanism) overcomes the viscous losses (dissipation).

With the Hall-magnetohydrodynamics model for conducting fluids we study the stability of an impulsively accelerated, perturbed density interface, separating two fluids immersed in a background magnetic field. The presence of the magnetic field is found to suppress the Richtmyer-Meshkov instability.

A model for estimating the onset of parametric instability in a stably stratified two-dimensional confined viscous flow is presented. The model uses a superposition of modal responses to independent Mathieu equations and is calibrated using a small number of numerical Floquet results.

The transition mechanism and unsteady behavior behind a backward-facing step in the supersonic regime is investigated using large eddy simulation. Results show that there are three main frequency scales in the unsteady transition process.

We report a new type of double-diffusive magnetorotational instability in magnetized flow with radially increasing angular velocity (positive shear/Rossby number), which up to now was deemed stable. It can arise in the equatorial part of the solar tachocline, with a key role in magnetic activity.

Global instability leads to the transonic buffet around a 2-D wing profile. We performed linear stability analyses for different swept angles to determine the 3-D buffet origin, finding that a 3-D initially stationary mode (zero swept angle) becomes progressively unsteady leading to the 3-D buffet.

Estimating the structure of a flow field from limited and uncertain measurement data is made challenging by the richness of structures in a complex flow. An investigation of sparse representation in a library of examples shows that this can provide robust, accurate reconstructions.

The fluid flow in an elastic pipe stretched and twisted in the axial and azimuthal directions is analyzed using a self-similar formulation. Many of the flows obtained develop intense and narrow swirling axial jets, that may exhibit flow-induced oscillations.

We combine two known independent phenomena, viscous fingering dynamics in miscible fluids and nonlinear wave fronts, to investigate how these two nonlinear dynamics interact. The difference between cases with/without nonlinear adsorption of solute arise in creation/annihilation of nonlinear waves.

Experiments show that low-Stokes-number particles in a thermocapillary-induced convection within a closed geometry with a free surface form several types of coherent structures.

Viscous fingering on a chemically reactive immiscible interface is simulated by the phase-field method, focusing on temporal effects of produced surfactant which alter local interfacial tension continuously. A reference capillary number is proposed for the injected fluid area at breakthrough time.

Linear stability analysis and nonlinear simulations confirm that the onset of viscous fingering (VF) instability is delayed in the miscible VF model with the absorbing boundary as compared to the reflective one. A threshold log mobility ratio is found for which such delayed onset is maximum.

When a more viscous liquid displaces a less viscous one in a Hele-Shaw cell, a new instability of the interface is observed due to spontaneous convection occurring when there is a phase separation if the two liquids are partially miscible.

We develop a lattice model for liquid droplets on solid surfaces incorporating single-particle and larger-scale collective hydrodynamic motion. Contact angles and surface tensions are derived from microscopic parameters. Droplets joining, spreading, sliding, dewetting, and evaporating are analyzed.

An extensive theoretical and numerical study of gravitational settling of small particles in a stratified fluid was performed. Adopting the integral equation on surface traction, we derived the drag enhancement due to stratification at low Reynolds and Peclet numbers, and confirmed with simulations.

For a sphere moving in a viscous liquid inside a second sphere, it is shown theoretically that chirality (the lack of reflectional symmetry) is necessary to produce a mean torque from oscillatory displacements and rotations, both with zero means.

A study of the migration of a hyperelastic particle suspended in a Newtonian pipe flow for different Reynolds numbers and elasticity is presented. The particle deforms and undergoes a lateral displacement while traveling downstream through the pipe, finally focusing at the pipe centerline.

The tracking of a few large density-mismatched particles is used to efficiently􏰁 and accurately reconstruct the background 􏰀fluid fl􏰀ow. Approximating the particulate phase space and taking the limit of vanishing Stokes number (St), we reconstruct three paradigmatic 􏰀flows.

Pressure-imposed rheometry is used to examine the influence of surface roughness on the rheology of immersed and dry frictional spheres in the dense regime and to characterize the singular behaviors of these systems in the vicinity of the jamming transition.

Experiments show that a suspension confined between two bubbles and flowing in a pipe may become heterogeneous when liquid inertia comes into play. Indeed, inertial migration of the particles along the radial direction modifies the axial flux of particles linked to the Poiseuille flow.

Finite-amplitude traveling wave solutions in the asymptotic suction boundary layer are computed for different wall permeabilities. The permeability has negligible effect on the Reynolds number at which they emerge, but it affects their shape and amplitude and the turbulent flow velocity profile.

The dynamics of a passive scalar plume is experimentally studied in a rough-wall turbulent boundary layer. Besides classical statistics, complex networks are exploited to highlight the temporal structure of vertical turbulent transport series in terms of occurrence and intensity of extreme events.

An experimental study on active-grid-generated turbulence finds that different operating protocols can produce different energy cascades. Also, independently of the protocol, the turbulence dissipation constant can be modeled from the zero crossings of velocity fluctuations.

The weak turbulence of internal gravity waves and the strong turbulence of eddies coexist in stratified turbulence. The wave-number range of the anisotropic weak-wave turbulence is identified based on energy decomposition and characteristic time scales.

An investigation finds that in free-surface plunging jets, shear layer vortices form right below the free surface, then develop and disintegrate faster than vortices in canonical free jets. This leads to shorter potential cores, faster decay of the mean centerline velocity, and enhanced liquid entrainment from the ambient.

Large Eddy Simulation and experiments show that the wake of horizontal cylinders in proximity to a solid boundary is asymmetric. The cylinder’s bottom shear layer transitions to rollers, generating negative pressure, which induces a ground vortex to lift-off and interact with the von-Karman street.

The subgrid-scale (SGS) force and the divergence of SGS heat flux are modeled directly by an artificial neural network for large eddy simulation of compressible turbulence, which is based on the first-order and second-order derivatives of filtered fields as input features.

Energy transfers among the mean, the secondary, and the residual fields are different in RPCF with multiple states. The kinetic energy received by the residual field is mainly from the streamwise component of the secondary flows at R3 state (a) while it is mainly from the mean field at R2 state (b).

Closure model for turbulent-transport Prandtl numbers based on the equilibrium boundary layer analysis of filtered flow fields is developed. The analysis represents an important step forward toward using two-equation closures for scale resolving simulations of practical turbulent flows.

The ability of a device to exploit the energy of a flow to generate thrust is the essence of passive propulsion. Such energy conversion is challenging when a flow is turbulent. Our study sheds light on turbulence-driven propulsion and its consequences for turbulent transport of anisotropic objects.

Variable-density effects at a twice shocked mixing layer between gases are studied through Navier-Stokes adaptive mesh refinement simulations. Development of instability after first shock and chaotic mixing after reshock are discussed for 2D and 3D cases and as a function of the Reynolds number.

The relationship between the energy flow and kinematics of the small-scale is investigated in homogeneous isotropic turbulence. The study employs a high order continuum theory to decouple the rotational motion from the translational motion and investigate the contribution of each field alone.

We generalize Pao’s hypothesis for hydrodynamic turbulence to derive the energy spectrum for two-dimensional Ekman turbulence. The model captures the steepening of the spectrum caused by Ekman friction. The paper also covers a similar model for quasi-static magnetohydrodynamic turbulence.

An investigation is conducted on the flow around three tandem prisms, shedding light on the evolution of the primary vortices to the secondary. The secondary-vortex-street onset corresponds to the local maximum vorticity amplitude associated with the secondary frequency.

Surface wave fields with wave-driven droplets that bounce on a vertically vibrated bath of the same liquid are investigated in slit experiments. Complex wave fields are observed during slit passage, and only minute differences between the wave fields are found in comparable single- and double-slit experiments.

When surface gravity waves propagate in a rotating frame, an “anti-Stokes” mean flow develops and cancels out most of the Stokes drift, resulting in extremely weak Lagrangian transport. This might not be the end of the story, however, as we show that such anti-Stokes flows are in fact unstable.