In a two-dimensional model of a Leidenfrost drop levitating above a flat hot substrate, it is found that the lubrication layer of vapor can develop an asymmetry which has the effect of propelling the drop sideways.

The critical Peclet number for an electro-osmotic instability is reduced in inverse proportion to the geometrically increased electric fields due to curvature of the electrode. This electro-osmotic instability, in contrast to the well-known one, appears exclusively at the enriched interface (anode), rather than at the depleted one (cathode).

Bridging the gap between viscous and elastic solutions: An experimental study of the capillary thinning, and subsequent instability and breakup, of biopolymer’s filaments with tuneable elasticity.

The dissipation range of the turbulent energy spectrum is investigated using high-Reynolds-number direct numerical simulations with unprecedented small-scale resolution, uncovering the universal and nonuniversal aspects.

Fluid flows induced by a flagellated bacterial swimmer are often modeled as a simple force dipole, valid in the far field. A refined swimmer representation is presented that makes use of regularized singularities, retaining simplicity while capturing details of the complex flow field near the swimmer. A simple model system is then considered, demonstrating that these nuanced hydrodynamics are significant for bacterial interactions.

Swimming of Spiroplasma is not as kinky as one might suspect. The reversibility of Stokes flow combined with the symmetry of the swimming gait result in straight trajectories.

Volume-conserving periodic flows feature inhalant and exhalant phases that interact nonlinearly to create dynamic flow structures. Numerical and experimental approaches provide time-resolved flow fields to map the Lagrangian histories of inhale and exhale phases. For biologically relevant intermediate Reynolds number, it is shown that fluid exchange is especially sensitive to asymmetries between the inhale and exhale flow structures.

In a study of the evolving structure of two-dimensional liquid foams flowing through an inhomogeneous confining cell, the motion and deformation of their elementary components, the bubbles, are quantified for various confinement ratios. The flow experiments highlight the elastoplastic properties of foams, controlled by their liquid fraction, which are notably responsible for the symmetry breaking of the flow, with multipolar deformation and velocity fields around the localized inhomogeneity.

A study of the effect of inhomogeneous environments on the swimming direction of the microalgae Chlamydomonas reinhardtii in the presence of a light stimulus is presented. A mean deflection of microswimmers is measured that shows an interesting nonlinear dependence on the direction of the guiding light beam with respect to the symmetry axes of the pillar lattice. This is shown both in experiments and numerical simulations. On the basis of these results, an analytical model for microswimmers is suggested, where the pillar lattice is replaced by an anisotropic scattering medium.

Spiral defect chaos appears near the onset of Rayleigh-Bénard convection for a low Prandtl number fluid. In this state, rotating spirals are continuously nucleated and eliminated, yielding a persistent dynamics. Here we derive an equation for the azimuthal flow induced by an effective body force originating from rotating spirals. This result is verified numerically using a two-dimensional generalized Swift-Hohenberg model and the three-dimensional Boussinesq equations. We identify a correlation between the appearance of spiral defect chaos and the balance between mean-flow advection and diffusive dynamics related to roll unwinding.

Numerical simulations are used to investigate the passage of a droplet through a liquid-liquid interface in the tailing configuration until the breakup of the column, leading to the drop encapsulation. Conditions required for the droplet to cross the interface are discussed. Scaling laws of both the length of the entrained column and the volume of the encapsulating film are proposed, primarily dependent on inertial parameters and exhibiting a strong influence of the drop-to-film viscosity ratio.

Superfluid helium droplets impacting on a solid surface behave much differently than any other fluid. After a short period of initial spreading that is similar to classical fluids, superfluid helium drops quickly shrink and disappear as the superfluid drains out through a thin adsorbed layer of helium on the surface. The lifetime and contact angle of these drops is strongly temperature-dependent, and colder drops (with high superfluid fractions) maintain a constant contact angle throughout the contraction. Above $T_{lambda}$, helium spreads slowly like other classical fluids.

The retraction of a viscous liquid sheet is studied using direct numerical simulations and long-wave asymptotic models. In the viscous regime, there exists a self-similar solution for the interface and the velocity profiles of a retracting sheet. This similarity solution reveals that the tip speed decreases as a function of time for a finite liquid sheet in contrast to the steady speed reached in the inertia dominated regime. Direct numerical simulations corroborate these theoretical predictions.

Bubble expansion and collapse in a Newtonian liquid is studied numerically for a range of dimensionless initial distances from a solid boundary. Bubbles farther from the solid develop the well-known microjet during collapse. However, for bubbles very close to the solid, a different jet mechanism leads to the formation of very fast, thin jets. The results are relevant for a better understanding of cavitational erosion.

The effect of surfactants on the tail and film dynamics of elongated gas bubbles propagating through circular capillary tubes is investigated through an extensive three-dimensional numerical study in the context of a hybrid front-tracking/level-set method. A systematic analysis of the impact of a wide range of surfactant properties on the bubble and flow dynamics in the presence of inertial effects is performed. The presence of surfactant suppresses the tail undulations as it accumulates near the bubble tail, and associated Marangoni stresses lead to local rigidification of the interfacial dynamics.

Room temperature ionic liquids (RTILs) are mixtures of large ionic molecules of importance to energy technology applications, such as supercapacitors and high-performance batteries. A new computational model that uses fluctuating hydrodynamics to allow for efficient and accurate investigation of complex nanometer scale structures is presented. This hydrodynamic model is derived to be consistent with the thermodynamic and electrical properties ultimately responsible for the rich phenomena observed in RTILs. Simulation results demonstrate that the model reproduces important physical effects observed in RTIL experiments.

The late stage of the freefall of homogeneous and heterogeneous cones and the induced flow are studied using three-dimensional particle tracking velocimetry and three-dimensional particle image velocimetry. Results showed four distinct patterns modulated by the degree of heterogeneity and specific gravity. They included straight fall, sinusoidal-like trajectories, inclined translations with large rotations, and tumbling followed by an irregular motion.

Primary and secondary instabilities of buoyancy convection in a laterally heated cube with perfectly thermally insulated horizontal boundaries and perfectly thermally conducting spanwise boundaries are studied. It is revealed that en route to unsteadiness, the flow undergoes a steady symmetry-breaking pitchfork bifurcation. With a further increase of the Grashof number, the nonsymmetric flow bifurcates into an oscillatory state via a Hopf bifurcation.

The transition to turbulence from the perspective of second-order structure functions is analyzed. To separate the contributions from laminar and turbulent events, conditional sampling based on the local instantaneous intermittency is applied. New expressions are derived that decompose the structure functions to conditional-averaged quantities and elucidate the process of turbulent spot formation and penetration inside the boundary layer.

Detailed numerical simulations of a thin liquid film overflowing separated microstructures with sharp corners were conducted. The two-phase flows were described by the coupling between the Cahn-Hilliard and the Navier-Stokes equations. Our results indicate a strong influence of the microstructures on the stability of the liquid film.

A ferrofluid and a nonmagnetic fluid flow in a rectangular Hele-Shaw cell subjected to an in-plane magnetic field. A chemical reaction occurs, and the two-fluid interface becomes elastic. The produced patterns range from straight front to sharp tip ones, and they differ from usual Saffman-Taylor fingers.

Introducing flow pulsation into a water-jet cooling system allows for flow-field control for more efficient heat removal. Here, transient thermal mapping is used to quantify how a pulsed water jet in a falling-film geometry can enhance the overall cooling performance. It is found that the influence of the jet-pulsation frequency on the maximum cooling performance can be predicted by heat flux matching at the solid-liquid interface, while the optimum pulsation frequency is dictated by the thermofluid instabilities that occur in the falling film.

Flow of foams is studied in a model porous medium, in a large range of capillary numbers, Ca, and relative gas flow rates, f${}_g$. Pressure measurements show that the effective viscosity is a decreasing power-law function of Ca, with the exponent ranging from −1 to −0.75. Direct observation reveals that the flow is heterogeneous and the fraction of preferential paths increases with both f${}_g$ and Ca. In a straight channel of varying cross section, a bubble train behaves as a shear-thinning yield stress fluid, accounting quantitatively for the effective viscosity in the micromodel.

Gravity-driven flow of a liquid over an isothermal cylinder is unconditionally unstable. The flow of a thermoviscous fluid over a heated or cooled substrate shows interesting stability behavior. The relevant parameters in the model affect the spatiotemporal nature of the instability.

The lateral adhesion force on a wall-bound drop, propagating along a smooth rigid substrate, is measured using the drop adhesion force instrument (DAFI) and compared with the aerodynamic force applied to the drop in a fully developed channel flow. The agreement is rather good for relatively large capillary numbers. For very small capillary numbers the values of the adhesion and aerodynamic forces differ significantly. This unexpected result is explained by the drop propagation process influenced by the complex three-dimensional oscillations of the drop shape, similar to a shuffling gait.

When a thin film consisting of two layers of different liquids is heated from above, some oscillations of its interfaces can appear spontaneously due to the temperature dependence of interfacial tensions (Marangoni convection). These oscillations have a certain natural frequency. We investigate the possibility of changing that frequency by a periodic modulation of heating or cooling of the interface between the liquid layers. We found that in some intervals of the modulation frequency a synchronization occurs: the natural frequency of oscillations is shifted in such a way that it becomes commensurate to the external modulation frequency.

Low-speed maneuvering and station keeping capabilities for small unmanned underwater vehicles (UUVs) are of great interest to the maritime community and flapping fins provide a potential solution for achieving these capabilities. We show how phasing between tandem bio-inspired fins flapping at high stroke amplitudes, i.e. by leading or lagging the rear fin with respect to the front fin modulates its thrust production, leading-edge suction and the overall wake characteristics of the system using load cell thrust measurements, two-dimensional particle image velocimetry measurements and three-dimensional computational fluid dynamics simulations.

Owing to nonlinearity of induced-charge electrophoresis, the relative two-dimensional motion of a pair of conducting cylinders under a uniform DC field comprises—in addition to the symmetric translation along their line of centers (LOC) associated with the separate contributions of the parallel- and perpendicular- external-field components—LOC rotation resulting from their interaction. Consequently, all particles are eventually moving apart in the general direction perpendicular to the field. When LOC is initially oriented sufficiently close to the field, this takes place through transient pairing events.

Oil is injected into a mixture of the same oil and noncolloidal particles inside a Hele-Shaw cell to investigate the connection between miscible fingering and the interfacial structure that develops inside the thin gap. By tuning the channel confinement relative to the particle size, it is demonstrated that shear-induced diffusion of particles can be enhanced and the interfacial shape caused to become more rounded, which results in changes in fingering morphologies. The results of the study suggest a potential use of the wall confinement to control hydrodynamic instabilities uniquely in suspensions.

Particles suspended in a fluid exert feedback forces that can significantly impact the flow by altering turbulent drag. Flow modulation induced by small spherical heavy particles is studied by means of numerical simulations of an Eulerian two-way coupling model. The amplitude of the mean flow and the turbulence intensity are found to be reduced by increasing particle mass loading and fluid friction is enhanced. Surprisingly, these effects are stronger for particles of smaller inertia.

A study sheds light on the behavior of two interacting gas jets in a fluidized bed. Two regimes are identified: bubbles break off from jets in either a synchronous pattern or an alternating asynchronous pattern. Which pattern forms depends on the separation distance between jets and the particle size in the fluidized bed. By reproducing the magnetic resonance imaging results using simulations, the alternating asynchronous pattern is attributed to the dominance of drag forces over dissipative forces on the particles in between the two jets.

Two distinct scenarios of spatial modulation in two-dimensional plane Poiseuille flow have been studied. The first one is based on the identification of a new family of asymmetric Tollmien-Schlichting waves (TSW) breaking the reflectional symmetry about the channel midplane. The second follows the fate of a branch of time-periodic space-modulated waves that exclusively bridge upper-branch TSW-trains of different number of replicas by means of a codimension-2 bifurcation point. These modulated waves may therefore play a relevant role in the strange saddle governing domain-filling turbulent dynamics at high Reynolds numbers.

An investigation of the Taylor–Aris dispersion in an oscillatory axisymmetric squeeze flow, driven periodically by the motion of one of the confining, parallel planes is presented. Using the method of multiple timescale homogenization, the mass-heat balance equation in this flow is reduced to a one-dimensional equation, indicating three effective mechanisms: diffusion, advection, and reaction. The results show that the transport in the oscillatory squeeze flow can be either enhanced or diminished, depending on the interplay of these three effective (homogenized) mechanisms.

Theoretical models are proposed to describe the merging of dual rows of plumes in a quiescent ambient and a single row of plumes in a crosswind. The theory for the dual row case is applied to model the reduced entrainment rate in the case of plumes emitted from back-to-back cooling towers. The theory of plumes in a crosswind predicts satisfactory agreement with the previous water flume measurements in terms of the plume trajectories.

High turbulent dissipation in inhomogeneous flows can lead to a quasihomogeneous regime of Lagrangian statistics at small scales. This is shown in a canopy flow by an analysis of experimental Lagrangian trajectories. Furthermore, the analysis shows that Lagrangian statistics are affected by turbulence-obstacle interaction, leading to a short velocity decorrelation timescale and attenuation of the Kolmogorov constant for the Lagrangian structure function.

The dissipation of turbulent kinetic energy (TKE) in wall-bounded turbulent flows is found to scale with the Kolmogorov wall velocity, not the friction velocity as previously suggested. A new scaling is also developed for the peak value of the TKE. The new scaling is verified against direct numerical simulation data and is justified by dimensional analysis.

We perform mean-flow reconstruction through variational data assimilation using Reynolds-Averaged Navier-Stokes equations closed by the Spalart-Allmaras model in a high Reynolds number Backward-Facing Step configuration. Two correction terms are used: a force term in the momentum equations and a scalar term in the turbulence model. We show that the force term perfectly reconstructs the target data but only for dense measurements, while the scalar solution is slightly less accurate, but independent of the number of measurements. An observability Gramian analysis shows that the scalar term is much less flexible than the force term.

The local energy flux rate toward small scales in isotropic turbulent flows is investigated. The joint probability density function is calculated, with the local filtered strain rate, for a scale in the inertial range. The flux shows good correlation with the strain, in support of the Smagorinsky eddy viscosity model. The implications of the results for subgrid scale models are discussed and new modeling directions are proposed.

Linear damping is widely applied in experiments and fluid models to describe the large-scale dissipation that absorbs upscale energy transfers in two-dimensional turbulence. However, in many systems, it is not justified that linear damping is a good model and it is hard to obtain the damping coefficient from directly measurable quantities. A method based on the measurable third-order structure function is proposed to justify the form of linear damping and determine the damping coefficient.

A simple model of the transverse plume predicts that at a certain freestream speed, the entrainment rate is a maximum and the flame length is a minimum. This presumably corresponds to the most intense combustion in a compact wildfire. The model is in accord with laboratory experiments of a chemically reacting plume.

Two crossed surface waves generate vertical vorticity in a viscous fluid due to hydrodynamic nonlinearity. We find parameters of the induced flow and investigate its excitation and damping dynamics, focussing on the case of large-scale vorticity (in comparison with the wavelength), corresponding to surface waves crossing at a small angle. We also show that a thin insoluble liquid film (possibly covering the fluid surface due to contamination) increases the induced flow intensity not only at the surface, but also in the fluid bulk. We discuss our results in view of recent experimental observations of large-scale vortex flows.

Laboratory experiments are performed in a partly filled and tilted rotating annulus, which is analog to the classical precession experiments. With the presence of a forcing Kelvin mode, two types of instability are observed under a resonant condition: a parametric triadic resonance, in which two free Kelvin modes form a triad with the forced Kelvin mode, and a shear-type instability, which is related to the nonzero mean flow.