When a wetting drop is deposited on a fibrous material, such as water on paper or fabric, its spreading is coupled with absorption between and inside the fibers, which often causes swelling and large deformations of the fiber network. Using experimental model systems and simple modeling, we describe the coupled effects of perfect wetting, capillary forces, and liquid absorption/swelling in assemblies of fibers.

Molecular rotors are molecules that form twisted conformations upon photoexcitation, with their fluorescence relaxation time serving as a measure of viscosity. They have been used to estimate the viscosity of biological and other membranes, but yield higher values than more common methods. We show that the rotor’s relaxation time is influenced by a combination of membrane viscosity and interleaflet friction, and rationalize the existing discrepancy among methods.

Newtonian turbulence has been the object of study for 150 years with mechanisms driving transitions to turbulence of paramount interest. Recently discovered elastic turbulence in polymeric fluids occurs even at Re=0, thus is driven by different mechanisms. Coherent structures such as traveling wave, oscillatory, and chaotic solutions have been observed, without an understanding of the transitions between states. Here we provide a characterization of transitions from coherent structures which oscillate undergoing period-doubling bifurcations to chaos. The system we study is sufficiently “simple” that further analysis will provide insight into mechanisms driving elastic turbulence.

The very first moments of liquid coalescence remain elusive, with discrepancies between experiment, theory, and simulations. By combining high-speed imaging, electrical measurements, and numerical simulations in both drop-drop and drop-bath configurations, we reveal the critical role of the surrounding gas and van der Waals forces. Our study shows that merging is initiated by jump-to-contact, followed by the formation of air pockets that shape the early-stage neck dynamics, offering new insights into this fundamental fluid process.

Are extreme events in turbulence universal? While small-scale universality has long been accepted for some low-order statistics like the energy spectra, its validity for intense, intermittent fluctuations remains unclear. Using state-of-the-art direct numerical simulations and laboratory experiments, we analyze extreme velocity gradients across several different turbulent flows, and uncover striking universal behavior not only in scaling laws, but also in their detailed tensorial structure. Our findings support a deeper level of universality than previously thought, with profound implications for theory and modeling.

Undulatory swimming is a widespread locomotion strategy in aquatic animals, yet understanding and applying it to efficient artificial systems remains challenging. Using a biomimetic robotic swimmer and reinforcement learning, we identify an optimal control strategy that maximizes thrust. Our findings are validated through experiments, theoretical modeling, and fluid-structure simulations. They provide key insights into efficient aquatic propulsion and open new perspectives for autonomous underwater vehicles.

Human respiration releases a droplet laden air cloud that can carry pathogens and transmit infectious diseases. Despite extensive research, the characterization of exhaled droplets remains incomplete, with significant variability in droplet size distribution and limited measurement of droplet velocity. This study employs an enhanced version of the Interferometric Laser Imaging for Droplet Sizing (ILIDS) technique to simultaneously measure droplet size and velocity in 23 volunteers during speaking, coughing, and breathing. We also evaluate the impact of protective masks on droplet size and velocity distributions, and the variability observed between and within individual volunteers.

Shrimp propel themselves via metachronal paddling with five pairs of pleopods, generating both tip and edge vortices. In our high-fidelity 3D reconstruction and CFD simulations of the marsh-grass shrimp (Palaemonetes vulgaris), we found that interactions among adjacent pleopods significantly enhance propulsion and reduce energetic costs. Eliminating these inter-appendage interactions raised the cost of transport by 20%. During the recovery stroke, edge vortex merging reduces drag by 273% and power consumption by 137%, chiefly by aligning flow to lower pressure on the pleopods’ anterior surfaces. These findings underscore the importance of edge vortices in improving swimming efficiency.

Fish schooling is believed to provide benefits by allowing individuals to leverage vortices generated by nearby fish, thereby improving their performance. While prior works have characterized horizontal planar formations of fish, our comprehensive analysis of the hydrodynamics in the vertical diamond formation reveals significant interactions among vertically arranged fish. In the densest vertical diamond formation, fin-fin, body-body, wake-body, and wake-fin interactions enhance force generation and propulsive efficiency for each individual in the school. The findings of this study could guide school configurations that enhance the performance of fish-inspired bio-robotic swarms.

Lean premixed hydrogen combustion is a promising technology for reducing NOₓ and CO₂ emissions in gas turbines, but suffers from thermoacoustic instabilities. This study experimentally investigates the influence of the Lewis number on hydrogen and methane premixed flames under thermoacoustic instability in a low-swirl combustor. Simultaneous measurements of pressure fluctuations, OH* chemiluminescence, and PIV reveal that the pressure fluctuation in hydrogen flame is three times larger than that in methane flame, highlighting the significant role of the Lewis number in flame–flow interactions.

Shear-thickening suspensions can be brought to shear jamming: under constant volume flow at finite stress is impossible. If the volume is not fixed and the suspension can dilate, what are the stress levels in the suspension? We show that Darcy flow couples to dilation to give stresses scaling as the square of the size over which dilation occurs. This result is quantitatively modelled using a Reynolds-like dilatancy law and the Wyart-Cates constitutive model, which can e.g. shed light on the stresses observed during impact on shear-thickening suspensions.

We investigate experimentally and numerically the fingering instability caused by the dissolution of CO${}_2$ in water in a heterogeneous Hele-Shaw cell of variable gap width. Heterogeneity accelerates the instability, which presents a larger amplitude and faster growth rate than in homogeneous constant-gap cells. Experimental and numerical data are compared and provide new insight on the interaction between permeability heterogeneity and fingering instabilities.

In the classical Leidenfrost framework, the top evaporation of levitating drops is typically ignored, yet combustion redefines the evaporation dynamics for inflamed Leidenfrost drops. This work shows that inflamed Leidenfrost drops burn much longer than those in sub-Leidenfrost states, with evaporation split into independent bottom and top parts. Bottom evaporation dominates—especially for large drops—while the temperature-independent top rate reflects the intrinsic burn rate at saturation temperature. These findings challenge fire suppression strategies by highlighting the prolonged fuel lifetime in the Leidenfrost state and offer a boundary-free method to measure intrinsic burning rates.

Colloidal particles can be driven to migrate under applied electric fields (electrophoresis) or in response to solution gradients (diffusiophoresis). These phenomena are often discussed and derived differently, due to the physical differences in the forces that drive them. This paper develops a complementary, conceptually unified approach to predict phoretic drift and osmotic slip velocities, which naturally reproduces existing results while providing natural extensions and generalizations, as well as a natural way to understand and predict global characteristics of particle flow fields.

Large-eddy simulations of atmospheric flow through hybrid wind-solar farms were performed, showing that the significant temperature difference between the sea surface and floating photovoltaic arrays can induce secondary flows in the plane perpendicular to the mean flow. These motions lead to larger wind speed in the area between the strips, from which wind turbines will benefit if they are placed in these locations. The simulations indicate that this results in a gain of up to 30% in wind power production in a hybrid wind-solar farm, as compared to an isolated wind farm.

We consider conditions for minimum energy dissipation and advective travel time along path lines for potential flows in heterogeneous porous and fractured media. The work employs some concepts and techniques not often seen in the literature on Darcy (and cubic law) flows, including M. King Hubbert’s energy-based conception and variational methods. We explain a seemingly surprising result concerning how travel time through a series of fracture segments responds to small, local aperture perturbations.

We studied the feedback of extreme vertical velocity drafts on the dynamics of stratified turbulent flows using a coarse-graining approach. This approach allowed for a local-in-scale analysis while preserving spatial detail, which is critical for assessing energy transfer and conversion in large-scale intermittent flows of geophysical interest. We found that vertical drafts may act as a local energy injection mechanism throughout the flow domain, affecting the exchange between kinetic and potential energy.

Accurately modeling boundary-layer transitions is a top research priority in the NASA CFD Vision 2030 Study and can help reduce the environmental impact of aviation. Here we apply stability analysis to subsonic flow over a two-dimensional zero-pressure-gradient boundary layer in the presence of a forward-facing step which has varying height, slope, and corner radii. The computational predictions capture the overall trend from experiments associated with a gradual upstream shift in the transition location as the step height increases. However, the critical step height corresponding to a very large upstream shift in the transition location is not well predicted compared to the measured data.

This numerical study delves into the complex dynamics of transitional hypersonic boundary layers, examining how streaks—elongated flow structures—affect linear instability mechanisms. By exploring the interplay between streak amplitude and boundary layer instabilities, the research uncovers intricate growth mechanisms of the first and second Mack modes, which differ significantly from idealized conditions without streaks. The study proposes a clear classification of growing instabilities and analyses modified growth trends. These insights provide valuable perspectives on potential pathways to turbulence in distorted boundary layers, contributing to advancements in transition prediction.

The suppression of Marangoni-driven instability in a heated viscoelastic thin film was investigated under parametric forcing. It was found that, unlike Newtonian systems which exhibit a single-frequency stabilization threshold, an island of stability bounded by two critical frequencies emerges due to fluid elasticity. Stabilization and destabilization were both shown to result from the memory effects imparted by elasticity. A fundamental shift in the conditions for instability suppression was thereby revealed.

Transition state theory is used to model slip of a simple liquid at a liquid-solid interface. In the linear regime of low shear rate, the model leads to a simple expression for the slip that is in excellent agreement with molecular dynamics simulations for a wide range of system parameters and thermodynamic conditions.

We explain how self-propelling active droplets can squeeze through increasingly narrow microchannels with shape and velocity field adaptations. Using microscopy techniques, we show that these droplet microswimmers transform from a spherical to a ‘stadium’-like shape and eventually into an elongated ‘capsule’-like shape as they traverse extreme confined spaces. During this shape evolution, their hydrodynamic signature changes from a symmetric, quadrupolar to an asymmetric velocity field. Using finite-element based numerical simulations, we explain the flow field evolution by considering the thin lubricating film between the microchannel wall and the active droplet interface.

Theory and simulation are used to demonstrate that the concentration polarization induced electro-osmotic (CPEO) flow is the origin of the experimentally observed nonlinear electrokinetic flow near a charged dielectric corner. The CPEO explains many experimentally observed electrokinetic phenomena that cannot be captured by existing electrokinetic theories. The CPEO can become a versatile technique in the microfluidic toolbox and open many new possibilities to use dielectric structures for fluidic flow control.

This study investigates the systematic connection between the statistics of non-decaying homogeneous isotropic turbulence and droplet fragmentation outcomes, based on the concept of the enstrophy transport equation. Analysis of the interface-resolved direct numerical simulations (DNS) underlines the role of different vorticity generation mechanisms, such as vortex stretching and surface tension, in determining the size of the largest stable droplet during turbulent fragmentation known as the Kolmogorov-Hinze scale. The findings serve as the basis for future theory development under more complex turbulent fragmentation conditions.

Spatially localized states of convection surrounded by quiescent fluid, known as convectons, have been widely studied in natural doubly diffusive convection in the case where thermal and solutal effects are in balance. Here, we investigate localized pattern formation when this balance is broken. We find convectons persist away from the balanced case and show how their emergence is related to different patterned states, including anticonvectons (localized convection rolls attached to the end walls) in thermally dominated regimes and domain-filling patterned states in solutally dominated regimes.

Nonlinear waves over varying topography remain important topics in oceanographic applications. Modeling solitons over a variable seabed is more challenging than modeling waves on a flat surface, where integrable autonomous equations, such as the Korteweg-de Vries equation, are effective. The geometric complexity of the bathymetry leads to non-integrable, non-autonomous nonlinear systems. We map the perturbations caused by periodic bathymetry on Boussinesq solitons into a perturbed KdV equation and compare our analytical solutions and their stability with numerical simulation across different parameters related to nonlinearity, dispersion, and bathymetry.

Understanding how turbulent flows disperse particles is essential both for unraveling fundamental aspects of turbulence and for their environmental and industrial implications. Using experiments and numerical simulations, we show that the local flow geometry, especially around stagnation points, significantly affects particle-pair dispersion. By examining the angle between particle-pair displacement and their relative velocity, we characterize dispersion and show that flow anisotropies alter dispersion behavior beyond classical predictions based on homogeneous isotropic turbulence.

Certain animals have evolved complex strategies to track sources of odors which are advected by turbulent flows. In this paper, we model this search task as a partially observable Markov decision process, which allows us to compute optimal Bayesian search strategies in the sense that they reach the source in minimal average time. We apply this approach to realistic data taken from direct numerical simulation. Focusing on the especially difficult decision of what to do when contact with the odor has been lost, we study the optimal trajectories in this scenario — which strongly resemble known animal behaviors — and try to understand the results by way of a simplified model.

We study an $a$ $posteriori$ data-driven approach—solver-in-the-loop—for subgrid modelling of a shell model of turbulence. Leveraging differentiable physics, this method allows a neural network to interact with the solver during training, exposing it to physically informed inputs. This departs from the traditional $a$ $priori$ training and yields more accurate and stable closures. Our model is able to capture the intermittent and non-Gaussian nature of the subgrid scales, characteristic of high Reynolds number turbulence, and is able to reproduce high-order statistical moments within error bars.

In the context of turbulent flow, equilibrium can be defined as the state in which the rate of energy input matches the rate of energy dissipation. Departures from this balance characterize nonequilibrium conditions, which can be treated, to leading order, as linear perturbations around the equilibrium state.

Being able to accurately predict the flow near walls with preferably coarse resolutions is a key challenge in most practical flow applications. However, the wall-modeled large eddy simulations (WMLES) commonly applied typically rely on ad hoc interventions, such as assuming a mean velocity profile near the wall instead of the instantaneous filtered velocity profile. In the present paper, classical filtering is extended to domains confined by walls using volume-filtering. By rigorous application of volume-filtering, we derive a consistent wall-modeling framework that we refer to as volume-filtered WMLES.

The full-life cycle evolution of the Lamb vector is investigated thoroughly. Numerical validation confirms its asymptotic behavior near no-slip boundaries and formation mechanisms within viscous sublayers of supersonic flat plate flows. Using Stokes-Helmholtz decomposition, the spatiotemporal evolution equation reveals competition between advection, stretching/tilting of generalized Lamb vectors’ transverse components, and viscosity. Defining a weakly nonlinear mid-field wake region fills gaps in its full-life cycle, with this region’s behavior and physics examined.

To address discrepancies in the pressure peak variation law during the impact phase of flat plate ditching, we adopt a CFD method that has undergone detailed verification and validation to simulate flat plate ditching under various conditions. The results confirm that the pressure peak exhibits a “rapid increase-gradual decrease” variation pattern. Further analysis indicates that this phenomenon can be attributed to formation and development of the spray sheet and the resulting alteration of the stagnation streamline direction. The widely employed Wagner and self-similar theories, which fail to consider the spray sheet formation, are not applicable during the pressure peak increase stage.