Mitigating energy consumption in fluid transportation is crucial for industrial processes. Our research addresses this by investigating the reduction of friction in liquid foams on Slippery Liquid-Infused Porous Surfaces (SLIPS). These surfaces have asperities filled with oil, significantly reducing friction compared to smooth surfaces. While oil is unsuitable due to its anti-foaming properties, we propose a unique approach using self-SLIPS, introducing the idea that the foam liquid can serve as a lubricant. Our findings demonstrate that these passive surfaces can achieve a 25% reduction in foam friction, promising significant advancements in energy-efficient fluid transport.

This numerical study on two-dimensional (2D) turbulence reveals that transverse structure functions in the inverse energy cascade display anomalous scaling properties, differing from the self-similar behavior of longitudinal ones. Using direct numerical simulations of incompressible Navier-Stokes equations, this study shows a link between the scaling exponents of transverse structure functions and velocity circulation moments. These findings provide new insights into the dynamics of 2D turbulence, with implications for understanding geophysical flows.

Microorganisms often navigate in heterogeneous complex fluids, such as mucus or soil. The heterogeneity affects swimming in surprising ways. In this work we use experiments and mathematical modeling to understand the effect of suspended particles on the efficiency of helical propulsion. Strikingly, we find that suspensions can significantly enhance propulsion.

The hydrodynamic instabilities of propagating interfaces in Hele-Shaw channels or porous media under the influence of an imposed flow and gravity are investigated within the framework of Darcy’s law. The stability analysis pertains to an interface between two fluids with different densities, viscosities, and permeabilities, which can be susceptible to Darrieus-Landau, Saffman-Taylor, and Rayleigh-Taylor instabilities. An explicit dispersion relation is derived.

When particles suspended in a fluid are heated, their localized temperature modifies the fluid’s viscosity in nontrivial ways. Einstein famously showed that adding particles to a fluid increases its viscosity. This work demonstrates that heating these particles can cause the viscosity to increase, decrease, or remain unchanged, depending on the temperature gradient. Moreover, an uneven heat distribution on the particles gives rise to surprising effects — making the fluid’s behavior directionally dependent and inducing what is known as odd viscosity. These findings open new avenues for controlling fluid flow.

Pilot-wave hydrodynamics concerns the dynamics of droplets walking on a vibrating liquid bath, and forms the basis for the field of hydrodynamic quantum analogs. We here investigate a theoretical model that captures both vertical and horizontal drop dynamics. The model provides new rationale for a number of phenomena, including colinear swaying, intermittent walking, and chaotic speed oscillations, all of which are linked to variability in the droplet’s impact phase. Our study also highlights the degeneracy in the droplet’s vertical dynamics, consideration of which is essential for understanding the dynamics of droplets in confined geometries and interacting with standing Faraday waves.

Drops impacting superhydrophobic surfaces have a rim-lamella structure at maximum spreading. The volume ratio of these two components is shown to be Weber-independent and related to a new Weber-independent characteristic time, the transition time. Volume ratios from experiments and simulations at different Ohnesorge numbers are shown.

Coalescence of droplets is connected to fascinating interfacial fluid dynamics that is of great importance in a variety of natural and engineering systems. We here explore the growth dynamics of liquid bridges during droplet coalescence under acoustic levitation. We show that the early-time evolution of the liquid bridge follows a scaling law $d\sim t^5$ in the inertial regime, with different prefactors for horizontal and vertical growth. We also highlight the interplay between acoustic radiation pressure and Laplace pressures. A new dimensionless parameter, the Acoustic-Capillary Dynamics Number, is introduced to enhance our understanding of liquid bridge dynamics in acoustic fields.

This work investigates drop impact dynamics on state-of-the-art lubricant-infused micro/nanotextured surfaces. The results of this study show the presence of an optimal lubricant layer thickness (≈3-5 𝜇m) that maximizes drop breakup and splashing. Moreover, our experiments show that drop splashing can be suppressed by increasing lubricant viscosity. Lastly, the density mismatch between the drop and the lubricant oil has also been shown to amplify the breakup of the radially expanding liquid rim into tiny droplets. The insights gained from this work provide new avenues to suppress and/or amplify drop breakup during high-velocity impact.

We use a continuous data assimilation approach to obtain low-cost stand-alone computational models for fluid flows. A nudging method is used to enforce global flow statistics, yielding a data-driven stochastic model that obtains accurate flow representations at coarse grids at severely reduced computational costs. This is demonstrated for the two-dimensional Navier-Stokes equations and the quasi-geostrophic equations.

Due to the inviscid cross-flow instability caused by the inflection point in radial velocity, both convective and absolute instabilities coexist in the rotating-disk boundary layer within rotor-stator cavities. To investigate the laminar-turbulent transition process through convective instability under specific roughness element excitation, this study employs numerical simulation with body force to simulate roughness elements. The findings reveal that under convective instability, the rotating-disk boundary layer initially exhibits stationary spiral waves mode, followed by the development of traveling spiral waves mode in the radial direction, ultimately transitioning to turbulence.

We conduct experimental and theoretical investigations into the previously unexplored effects of intergranular bonds within cohesive granular skeletons on multiphase flow dynamics. We illuminate that fluid forces can become large enough to surpass bond strength, leading to bond breakage and fracture initiation as capillary numbers increase. A first-ever phase diagram of five distinct fluid-fluid-grain displacement morphologies is established under varying flow and cohesion conditions. Through dimensional analysis, we propose a fracturing number $N_f^{*}$ = 1 as a theoretical threshold to characterize the onset of fluid-induced fracturing in cohesive media.

This study explores how distributed surface roughness influences separation-induced transition on a flat plate, under the influence of freestream turbulence and adverse pressure gradients. Using direct numerical simulations, the interactions between freestream perturbations and vortices generated by roughness elements are investigated. Results reveal that low roughness accelerates instability development without dominating the transition process, while higher roughness causes a more intensified vortex mixing process and directly results in transition to turbulence.

Archetypal falling behaviors of impervious objects are classified into four modes: fluttering, tumbling, steady descent, and chaotic motion. We present in this paper an experimental result of stability induced by porosity and permeability. We discover that by drilling different porosity patterns, we can avoid tumbling and chaotic behavior in plates that, due to their inertia and Reynolds values, should tumble while falling according to the regime diagram of impervious plates. Instead, the majority of the plates flutter and a few even descend steadily.

The intricate dynamics between thin sheets and compressible fluids plays a pivotal role in microelectromechanical systems and microfluidic switches, where precise control of fluid dynamics and structural movement is essential. In this study we present an analytical model that integrates the elasticity of thin sheets with the hydrodynamics of compressible fluids to investigate how material properties and compression influence the system’s vibrational modes.

An experimental investigation of the Richtmyer-Meshkov instability is conducted in spherical geometry where the displacement and the growth of the perturbations at the interface are given and compared to numerical simulations and new theoretical predictions. The results show that the instability amplitude initially grows, stabilizes, and then reduces before the arrival of the reflected shock wave. The theoretical model developed here agrees well with the experiments, although a time shift is observed in the stabilization regime. Furthermore, we show that convergent Rayleigh-Taylor effects are the main stabilizing mechanisms, and that compressibility has a negligible effect.

We focus on how cavity shape, evaporation rate, and thermal Marangoni flow affect particle deposition when the contact line is free to move. A lubrication- theory-based model is developed to derive evolution equations for the droplet height and vertically averaged particle concentration. Our model shows that a pressure maximum appears at the droplet edge as the evaporation rate or thermal Marangoni flows increase, or as the cavity depth decreases. The resulting inward flow causes the contact line to depin, which leads to an increase in the uniformity of particle deposition but also to the occurrence of a pinch point (a sharp local minimum of particle number density) near the droplet edge.

The article presents a mesoscopic hydrodynamic model for a spreading drop of volatile partially wetting liquid on a solid porous substrate. The model describes the coupled dynamics of the the three-phase system in terms of the drop height profile, the vertically averaged saturation profile in the porous layer and the vertically averaged vapor density above the substrate. Our approach is based on the gradient dynamics framework widely used for modeling thin liquid films. After developing the model, we discuss a selection of theoretical and numerical results, e.g., the resulting sorption isotherm or a simulation of coupled spreading, imbibition, and evaporation dynamics.

Droplet dynamics in (di)electrowetting and dewetting on dielectric configuration is elucidated using a lubrication model. Electric field-modified Frumkin-Derjaguin theory through Lippmann’s principle of electrocapillarity is employed. The research uncovers a field strength dependent power-law relationship for the contact line motion that conforms to Tanner’s law for the case of critical field leading to complete wetting. Electrodewetting shows a faster retraction compared to wetting due to reduced viscous dissipation. These findings provide a new theoretical framework for electric field-driven wetting and dewetting, enabling advancements in digital microfluidics.

We simulate the dynamics of a flexible stem submerged under a surface gravity wave. Varying the rigidity of the stem, we explore its motion in the drag-dominated regime with realistic air and water properties. A stiffer stem maintains on average a straight configuration and exhibits streamwise oscillations in phase-opposition with the wave, moving symmetrically with respect to the vertical direction. Conversely, a more compliant stem reconfigures under the influence of the Stokes drift, bending forward and breaking the symmetry, and exhibits oscillations that are more coherent with the surrounding flow field. Resonance is observed at the transition between the two regimes.

This paper introduces an optimization framework for identifying sparse finite-amplitude perturbations that maximize transient growth in nonlinear systems. An iterative direct-adjoint looping algorithm is formulated based on the first-order necessary conditions for optimality. The method is applied to a reduced-order model of sinusoidal shear flow. Our results show that optimal sparse perturbations can achieve comparable energy amplification as the optimal non-sparse solution by triggering many of the same nonlinear modal interactions responsible for driving transient growth. We anticipate the approach will be a useful tool in future investigations into flow stability and control.

Heating a stratified fluid in a valley-shaped container triggers a self-organizing flow. Starting from a quiescent state, any infinitesimal disturbance leads the flow through a transient three-dimensional phase before settling into a two-dimensional steady state. This complex pathway arises from the dominance of viscous dissipation over buoyant production. The orderly transition between 3D and 2D, supported by linear theory, sets our findings apart from transient chaos. While the final pattern is not predicted by linear theory, several stages align well with it. Our example offers new insights into self-organization in fluid systems with geophysical and astrophysical implications.

Stratified flow instabilities play a critical role in particle sedimentation in marine environments, influencing the efficacy of marine carbon dioxide removal strategies. Using direct numerical simulations, we reveal how these instabilities enhance or inhibit settling across flow regimes. Our findings highlight the dynamic interplay between turbulence, stratification, and particle dynamics, providing new insights into optimizing carbon removal techniques.

Chemical reactions significantly affect the solute dynamics in the couple stress fluid flow. Using multiscale homogenization, our findings highlight how combined bulk and wall chemical reactions significantly reduce the solute concentration for all values of the couple stress parameter in flows between parallel plates. Bulk and wall chemical reactions consume the solute across the channel, and at the boundaries, these reactions override the effects of couple stress, ensuring precise control over concentration profiles. Crucially, the couple stress effect remains impactful for all reaction conditions, opening new frontiers in advanced fluid system design.

Neural networks applied to turbulence modeling often do not learn locality or generalize to very high Reynolds number reasonably. Here, a two neural network architecture is introduced that learns the relevant neighborhood needed for sub-filter stress modeling through convolutions and the U-net architecture while generalizing reasonably to Reynolds numbers far larger than the training set on forced homogeneous isotropic turbulence and channel flow.

This work focuses on the impact of meandering on wake statistics in the stratified environment. The range of wake meandering increases in the vertical direction while wake height does not. Meandering does not directly change the velocity scaling, or the wake width and height, but it leads to deviation of the velocity profile from self-similarity, and the development of layered flow structures. We can accurately measure how meandering distorts the self-similar velocity profile and impacts scaling of the width and height, and thus how meandering changes the scaling of the velocity deficit.

Artificial-neural-network-based (ANN-based) subgrid-scale (SGS) models for turbulent channel flow often suffer from instability and poor generalization. Here, we propose an ANN-SGS model based on the strain-rate eigenframe and apply it to large eddy simulations of compressible turbulent channel flow. Our results indicate that the newly proposed model can predict flow statistics more accurately than traditional SGS models, and it also exhibits generalization capability for both Reynolds and Mach numbers.

Motivated by countergradient diffusion phenomenon in the turbulent premixed flame, we discuss the effects of heat release on turbulence. We devised and implemented a direct numerical simulation of the turbulent Rayleigh flow generated by isotropic turbulence flowing into the Rayleigh flow. We show that countergradient diffusion of the turbulent heat flux occurs in the turbulent Rayleigh flow, and we discuss its mechanism based on the transport equations of the turbulent statistical quantities. It is shown that the released heat amplifies the density-internal energy correlation and contributes countergradient diffusion through the pressure gradient effect.

The valley-mountain arrangement excites secondary vortices when interacting with the atmospheric boundary layer. In this paper we develop an analytical model to predict these secondary vortices in the case of thermal stratification. The model was compared to recent direct numerical simulations available in the literature which studied secondary motions in channel flow for a wide range of Reynolds and Richardson numbers. The model showed robust performance for the range of cases considered, showing error less than 5% for the temperature and below 20% for velocity in most cases.

Inspired by recent observations of stable cyclone patterns on the North and the South poles of Jupiter, we present observations of monopolar barotropic cyclones in a model experiment for atmospheric polar flows. We show, both experimentally and with idealized quasi-geostrophic simulations, that starting from an initial vortex, there are two evolution regimes depending on vortex strength relative to the local beta-effect across its surface: a Rossby wave emission dominated regime and a strong vortex regime. In the latter regime, the generated cyclone drifts in a northwest direction. We show that the “beta gyre” induced “beta drift” mechanism locally applies in our experimental polar plane.

This study examines how the unsteady wake from an upstream stationary cylinder impacts the flapping response of an elastic inverted foil. Through high-fidelity simulations, we identify a critical nondimensional bending rigidity above which wake interference effects result in distinct flapping dynamics governed by stiffness and mass ratio. Additionally, we propose a nondimensional parameter that captures the interplay between inertia and elasticity, offering insights into fluid-structure interactions with implications for renewable energy harvesting systems.

Localized vortices play a major role in complex flows. Their interaction with the boundaries of the medium determines the complex structure of many flows. The finite dimensions of the vortex cores lead to nonintegrability for a small number of such vortices, complicating the flow analysis. To understand interactions and motions in limited areas, we use limiting point vortices, which can be described within the framework of Hamiltonian particle mechanics. The considered example of the motion of a point dipole vortex in a channel demonstrates both integrability and the possibility of a complete classification of all possible motions and flows accompanying its motion in the channel.