A radially symmetric sinusoidal wave structure is imprinted on an impacting circular disk to modulate the way the disk forces the free water surface. The experiments support the argument that the surface elevation around the disk edge prior to impact is an instability of the Kelvin-Helmholtz type, as the free surface resonates when the forcing wavelength on the disk is close to the most unstable wavelength predicted by theory. Besides, our wave-structured disk is also found to promote gradual inertial wetting of the impacting surface to effectively retain the entrapped air pocket (as shown in the figure), which, in turn, mitigates the peak impact force.

We present an extraordinary phenomenon that emerges from seemingly simple ingredients: a volatile droplet deposited on a highly wetting and heat-conducting warm substrate. Contrary to prevailing intuition that the deposited droplet would spread more and evaporate faster, we find that the droplet instead undergoes contraction and in addition it spontaneously and erratically moves, for substrate temperatures well below the boiling point of the liquid. We term this remarkable phenomenon “Autothermotaxis” and show that it originates from the thermal Marangoni flow in the droplet which undergoes an instability. The thermal Marangoni flow is also the reason for the contraction of the droplet.

The collective motion of nonspherical particles displaying preferential alignment can be modeled by extending the kinetic theory of granular gases. A linear dependency on the orientational tensor into the constitutive relations for the stresses, and a balance law for the orientational tensor itself, in which a key role is played by the randomizing effect of collisions, permit to reproduce the discrete simulations of homogeneous shearing flows of cylinders at different aspect ratios.

Suspensions, comprising particles and fluid, play a crucial role across diverse domains, including in nature, such as submarine landslides, biological systems such as blood flows, as well as industrial materials like concrete. The rheology of these materials is a combination of both fluid and solid properties. This study introduces a dimensionless number derived from the ratio between characteristic length scales, offering a unified perspective on the rheology of dense suspensions across a spectrum from conditions where fluid-like effects dominate to regimes influenced by inertial forces. The derivation contained herein is strict, offering significant physical insight into these systems.

In turbulent flows, slender flexible bodies exhibit complex motions such as the swaying of seagrass or the oscillations of a pylon in the wind. Here we characterize the dynamical behavior of a clamped flexible fiber immersed in wall turbulence over a wide range of natural frequencies by DNS. Only two flapping states are possible: one where the fiber oscillates at the characteristic frequency of the largest turbulent eddies and another where the natural structural response dominates. We observe for the first time that in the turbulence dominated regime the fiber always sways at a frequency proportional to the largest scale of the flow, regardless of its structural parameters.

This work investigates the dynamics of a unique, radially expanding vortex ring in a water layer when a miscible, volatile drop of ethanol spreads as a film on the air-water interface. The study unravels the link between the dynamics at the interface and the generation of vorticity in the water layer below it. A novel scaling is proposed for the radius and velocity of such vortex rings, where they are shown to be dependent on time as well as the properties of the drop and the substrate.

The study of the endothelial glycocalyx layer (EGL) has been of great interest in recent years due to its direct relationship with human health. Some works have modeled the EGL under the triphasic mixture theory (TMT), considering an electrically charged porous medium interacting with the electrolyte under a pressure gradient. Our study is based on a model that couples the mechanical and electrical interaction, considering TMT, a non-Newtonian electrolyte, and wavy walls using the domain perturbation method, focusing on the shear stresses in the EGL and on the induced streaming potential that can be used as a biocompatible energy source.

Many biological systems rely on interactions between active processes and passive, deformable structures to properly function. An important example is chromatin in the cell nucleus, where ATP-powered processes, such as transcription or DNA repair, act on the chromatin fiber and influence its motion. Motivated by this system, in this study we develop and analyze a model of a flexible fiber in an active suspension as an analog to a chromatin fiber in an active environment - the nucleoplasm. Interactions between the suspension and the fiber lead to a novel bend instability, and nonlinear simulations identify coherent motions and conformations of the fiber over long timescales.

Our Brownian dynamics model uncovers response mechanisms of microorganisms such as Escherichia coli to shear flows and constrictions, shedding light on anomalous accumulation of active particles in confined channels. Our findings highlight the butterfly-like attractors that trap particles past the constriction and characterize relevant dynamical regimes of active particle accumulation. We also explore the impact of particle size, channel geometry, and fluid velocity on trapping probability and accumulation strength. This intriguing phenomenon could be utilized as geometrical sift for mixture of active particles with different sizes/velocities and lead to innovations in particle manipulation.

The recently discovered elastic center-mode instability in plane-Poiseuille flow is shown to be present even in base flows devoid of centerline symmetry. This is illustrated using the Couette-Poiseuille family of asymmetric velocity profiles, wherein the instability is present when the maximum of the base flow is present within the channel domain. The predicted instability is potentially relevant to viscoelastic flows in the Taylor-Dean, shallow-cavity, and slider-bearing configurations.

The accurate characterization of molecular transport properties is essential for high-fidelity simulations of reactive, hypersonic flows. The correct prediction of energy and mass diffusion in the laminar, high-temperature, multicomponent boundary layer of a hyper-velocity flow has profound implications for the accurate modeling of gas-surface interactions and thermal loads on the aeroshell. In this work, molecular transport is investigated by solely using $ab$ $initio$ potential energy surfaces. Our approach removes the empiricism associated with simplified molecular interactions models used in previous studies and is applicable to arbitrary gas mixtures.

Using a needle-plate electro-atomization experimental device, and applying a single pulse disturbance voltage signal, the response of the Taylor cone to a disturbance signal was explored. At the experimental level, the coupling relationship between polarization charge relaxation time and the oscillation period of the Taylor cone was uncovered, revealing the oscillation mechanism of the Taylor cone under this voltage disturbance.

We develop neural-network active flow controllers through a deep learning PDE augmentation method (DPM). In two-dimensional, incompressible, confined cylinder flow with Re = 100, we compare drag-reduction performance and optimization cost of adjoint-based controllers and deep reinforcement learning (DRL)-based controllers. The DRL-based controller demands 4,229 times the model complexity of the DPM-based one. The DPM-based controller is 4.85 times more effective and 63.2 times less computationally intensive to train than the DRL-based counterpart. In laminar compressible flows, successful extrapolation of the controller to out-of-sample flows demonstrates the robustness of the learning approach.

The convective flow of dielectric liquids with finite electric conductivity subjected to the simultaneous action of an electric field and a destabilizing temperature gradient is investigated. The instability of the system is studied through modal stability analysis, and it is found that the onset of the flow is delayed when an electric field is applied. Numerical simulations are performed to show the flow patterns and heat transfer characteristics. The dual solutions are highlighted in the bifurcation diagram.

The case of a round jet of one liquid emerging into an ambient medium of a different viscosity is relevant to many industrial mixing situations. A linear spatiotemporal analysis of such a configuration shows that for sufficiently high viscosity contrast, the flow is absolutely unstable. Both axisymmetric and helical modes are unstable, with the axisymmetric mode being slightly dominant. The absolute/convective instability transition boundary defined in terms of viscosity ratio and jet Reynolds number is compared to the results of recent experiments and good agreement is found.

This work aims at studying the modal and nonmodal stability of stably stratified turbulent channel flow, assessing the influence of stratification at fixed friction Reynolds number. When increasing the stratification, the energy gain for streamwise-independent perturbations increases by two orders of magnitude, and the spanwise wavenumber for which the energy gain peaks reaches values comparable to those reported in the direct numerical simulations. Moreover, for nonzero values of the streamwise and spanwise wavenumbers, α and β, the energy gain curve has two peaks, one for shorter target times and α>β, leading to a center-channel temperature peak, and another for α<β at larger target times.

Catalytic motors, which self-propel in a liquid due to an inhomogeneous surface reaction, constitute an important illustration of active matter in a non-biological context. Prevailing models of the associated self-diffusiophoretic transport assume a chemical reaction at the boundary of the swimmer. We here address the more realistic scenario where that reaction is balanced by a homogeneous reaction in the bulk. The associated diffusive transport of solute, described by two Damköhler numbers, exhibits a boundary-layer topology which is not encountered in the prevailing models.

Comprehensive numerical investigations of droplet displacement in soluble surfactant driven flows using the front-tracking method are presented. Surfactant transport in the bulk and at interfaces shapes droplet displacement and determines the transition conditions between steady-state sliding and detachment. Detachment is highly dependent on surfactant replenishment at interfaces, especially at receding contact lines where the nonuniform concentration induced Marangoni flow impedes movement. The critical effective capillary number can be used as a criterion to evaluate the ability of the surfactant to detach the droplet, giving a unique logarithmic relationship with detachment time.

When the surface tension of a droplet or a film varies inhomogeneously, a surface-tension gradient occurs, which induces a flow inside the fluid. Thermocapillary flows induced by localized heating have been observed experimentally in millimeter-sized water droplets. In particular, these experiments reveal that when such droplets are heated from below by a localized heat source targeted at the droplet center, a twin vortex pair perpendicular to the substrate is observed. This work aims to obtain some theoretical understanding to explain the onset of such vortices.

This work investigates the linear stability of an ultrathin non-wetting liquid film on a spherical substrate in the limit of negligible inertia. The interplay between destabilizing van der Waals forces, with an exact potential description, and stabilizing surface tension gives rise to a discrete set of unstable temporal modes. Also, the performance of classical lubrication models has been assessed against the complete Stokes equations of motion.

Equations are derived for the evolution of a three-dimensional perturbation of a straight slender elastic fiber at an arbitrary orientation in shear flow at low-Reynolds-number. For the orientation in the plane of the shear flow and the flow gradient, the spectral analysis is performed for in-plane and out-of-plane perturbations. The most unstable eigenfunctions and eigenvalues are analyzed and compared to the previous results for the in-plane perturbations in the shear flow, and for arbitrary perturbations in the compressional flow.

Locomotion is a combination of translation and rotation. With suitable boundary conditions, rotation can induce translation. The induced translations are more intriguing when multiple objects are rotating together. In this paper, we study theoretically the hydrodynamic bound states of two active cylinders rotating inside cylindrical confinement filled with viscous fluid. We find that the active cylinder pair can fall into four distinct non-trivial hydrodynamic bound states depending on their initial positions. Our findings may be helpful in understanding bacterial flagella rotating within a confining geometry.

We have conducted experiments which show that when two sources of different salts dissolving in water overlap, colloidal particles suspended anywhere in the solution move due to an action-at-a-distance force. As soon as the salts from the pair of sources overlap, an electric field appears practically instantaneously throughout the solution. The electric field exerts a force on any charged colloidal particles, causing them to move at speeds of order 0.1 micrometer/s. This new way of moving particles suspended in water could be used, for example, to remove them from solution, thereby purifying the water.

Phoretic particles swim and interact exploiting their dual chemical and hydrodynamic footprint on their environment, resulting in complex collective behavior spontaneously but also in response to external forcing. Here, based on a kinetic model for dilute suspensions, we numerically characterize their response to shear in a confined environment and identify three different regimes depending on the relative magnitude of shear forcing, chemotaxis, self-propulsion and confinement. The particles, in turn, exert microscopic stresses on their surroundings, resulting in a complex rheological response tightly linked to the self-organization regime.

In this work we compare long wavelength fluctuations in nanoconfined simple fluids from molecular dynamics simulations with continuum descriptions. Quasi-two-dimensional descriptions of these confined fluids give rise to attenuation behavior of hydrodynamic fluctuations distinct from the bulk. Interactions with the walls introduce additional dissipation and can be related to wall slip and an anomalous dispersion relation for sound.

Deformable elastic particles can accumulate around a vortex center even if the particle is neutrally buoyant. The angle between the deformed particle and the pathline plays important roles in this accumulation process. In this paper, we propose a simple model to explain this interesting accumulation phenomenon.

The dispersion of heavy inertial particles in a cellular flow made of Taylor-Green vortices is found to display non-ergodicity and sensitive dependence on initial particle location. Even more surprising is the sensitive and non-monotonic dependence on Stokes number. The large time dispersion of particles can be ballistic (red), diffusive (green) or trapped (blue), depending on where they have started in the flow. Diffusive particles show chaotic dynamics. Here the mutually exclusive group of initial particle locations form a non-ergodic set (as in the figure), unlike a turbulent flow, which is known to be ergodic.

A novel pore-corner network extraction method is proposed. To validate our proposed extraction method, a generalized network model is developed to simulate evaporation in a porous medium composed of packed spherical beads. The modeling results are in good agreement with the experimental data, particularly in terms of the variation of liquid distribution over time. Our proposed extraction method not only contributes to disclose the structures of pores and corners in real porous media but also benefits the development of generalized network models that can be employed to understand in detail the multiphase transport in porous media from the pore scale perspective.

This work reveals the mechanism of cavitation-structure interaction by comparing experimental results between a stainless-steel hydrofoil and a flexible hydrofoil, encompassing hydrodynamic loads, cavitation structures, vibrations, and deformations. The hydrodynamic load induces nose-up twisting deformation, which subsequently increases the angle of attack, promoting cavitation inception. Additionally, the dynamics of cloud cavities induced by the shock wave mechanism are clarified based on high-speed images.

We study fluid motions excited by a spatially periodic force in a plane region with periodic boundary conditions; mean drift in both directions across the domain is nonzero. Under weak force, the flow is stationary; Fourier spectra of velocity for advected tracer particles are singular continuous (fractal). At higher amplitudes of the force, the flow pattern periodically oscillates, ensuring the onset of Lagrangian chaos in tracer dynamics. We illustrate the transformation of the power spectrum from the fractal object to the conventional smooth curve (subplots (i-iv)), and describe the accompanying changes in the pattern of autocorrelation for the tracer velocity.

This study elucidates the regime of fluid instabilities that can arise in a stratified shear flow when density and velocity interfaces are not aligned - a common occurrence in various geophysical flows. Through a combination of linear stability analysis and direct numerical simulations, we unveil a hybrid mode characterized by features of both Kelvin-Helmholtz and Holmboe instabilities. By quantifying the crucial role of asymmetry, our findings contribute to a refined understanding of the dynamics and mixing in these stratified shear flows.

This letter “H” is written in turbulent air by tagging molecules in the focus of intense laser beams that cross in space. In the course of 40 microseconds, turbulence deforms and disperses the pattern. It could be used as a way to measure the velocity of the turbulent eddies. However, more importantly, it reveals the intricate interplay between molecular diffusion and turbulent dispersion.

Detection of coherent structures in experiments, when using single-point temporal measurements, poses challenges due to their three-dimensional nature. Past methods, relying on ad hoc thresholds, lacked consistency across studies. To address this, the level-crossing method has been introduced and applied to two datasets of wall-bounded turbulent flows. This method allows the identification of coherent structures in a more objective manner, avoiding the need for arbitrary thresholds. An interesting discovery is that coherent structures influence near-wall turbulence through nonlinear interactions, a phenomenon not discernible through traditional spectral analysis.

Structures in turbulent flow are largely responsible for variations in the forces experienced by the body of interest. In this work, the impact of specific structures in the velocity field on the resulting pressure field are revealed using a data-driven framework. The body of interest is a stalled NACA 0012 airfoil obtained from high-resolution large-scale time-resolved particle image velocimetry.

How local is turbulence? This question has been extensively examined in scale space. In this study, we investigate the locality in physical space by artificially truncating the spatially nonlocal nonlinear interactions and see how the energy cascade is affected by the truncation in three-dimensional turbulence.

An experimental apparatus for studying vortex-induced vibration (VIV) of a flexible pipe was designed and constructed in an ocean basin. A VIV test of a tensioned flexible pipe in oscillatory sheared flow (OSF) was performed based on this rotation rig. One end of the test pipe is fixed, and one end is forced to oscillate harmonically with various amplitudes and periods. The results show that VIV under OSF exhibits amplitude modulation and hysteresis phenomena. A critical Keulegan-Carpenter (KC) number is proposed to describe the occurrence of modulated VIV under OSF. The experimental results provide benchmark data for future VIV prediction research.

Understanding the flow physics near stall at high chord-based Reynolds numbers (> ${10}^6$) remains a challenge for both experimental and numerical approaches. Generally statistical bi-dimensionality of the flow is assumed $a$ $priori$. In the present study, the wall pressure at two chords provides a description of the flow evolution with the angle of incidence. Our results indicate that flow separation at high Reynolds numbers is an inherently local, three-dimensional and unsteady process that occurs in a continuous manner. However, as it can be represented with mainly two proper orthogonal decomposition modes, our results also suggest that a low-order approach may offer a viable modeling route.

In this paper, we experimentally investigate the flow fields and lift transients associated with transverse gust encounters. We highlight the key differences between inviscid and viscous gust encounters and discuss how contrasting shed vorticity distributions lead to dissimilar lift behavior during the gust exit. Our findings contribute to a refined understanding of the utility of inviscid models in the prediction of loads during atmospheric gust encounters.

In this work, we demonstrated how model predictive control (MPC) can effectively manipulate a highly nonlinear foil-and-flap system to follow designated lift trajectories to sub-5% error. The surrogate model is built through a data-driven method merging fluidic and structural information which can be readily deployed to numerous fluid-structure interaction systems. The rapid optimization procedure utilized to generate the control signal can also be used for estimating ambient environmental change

In the Arctic region, with the reduction of ice extent and thickness, a shipping route may become possible. We theoretically derive the wave motions induced by a point source pulsating and advancing at the marginal ice zone. It is found that when a ship navigates along the edge of an ice sheet, the free surface wave pattern has two V-shaped components. The outer V-wave is very similar to the common free surface wave without the ice sheet, while the inner V-wave is mainly due to the reflection of the outer V-wave by the ice sheet.

Density-driven flows climb out of topographic depressions if they are sufficiently energetic. We investigate the inertial dynamics of these unsteady flows theoretically as fluid climbs from a lower to an upper plateau and then simultaneously propagates away from and drains back into the depression. The volume of fluid that escapes the confinement diminishes with a power-law dependence upon time; the draining flow becomes self-similar, and the self-similarity is of the second kind, featuring an exponent which is a function of the frontal Froude number. The volume continues to decrease even when viscous processes are non-negligible and ultimately none of the fluid escapes from the depression.

In this study, we explore the transition from fully developed turbulence to global intermittency in stably stratified channel flow using direct numerical simulations across varying friction Reynolds and shear Richardson numbers. We quantify intermittency by measuring the volume fraction of turbulent patches and find that intermittency can originate from either near-wall or mid-channel regions, depending on the above parameters. The study identifies a critical value of the Nusselt number (approximately 3.0) below which near-wall intermittency consistently occurs. The critical friction Richardson number for intermittency is found to be proportional to the squared Reynolds number.