Observations of ultrasound interacting with coated microbubbles has become an important biomedical tool for diagnostic imaging and therapeutics. This paper presents a perspective which highlights the underlying linear and nonlinear bubble dynamics. Mathematical modeling of contrast microbubbles, including an interfacial rheological model, and model building and improvement using attenuation and scattering experiments are presented. A method of noninvasive organ-level blood pressure monitoring is described. Microbubbles together with low-intensity pulsed ultrasound are also demonstrated to be an effective tool in tissue engineering.

Rotation-induced traumatic brain injury is a serious health concern. We present a novel experimental and theoretical approach to studying fluid-structure interactions between soft matter and its liquid surroundings under rapid rotational impact. Our theoretical model, developed using the Arbitrary Lagrangian-Eulerian method, has been verified and validated against experimental data. This work establishes a strong foundation for future research into rotation-induced brain concussions, where the transient fluid-structure interaction between the cerebrospinal fluid and the soft brain matter plays a critical role in impact transmission and mitigation.

This study introduces a novel approach to shielding minerals from chemical reactions. By flowing microparticles around dissolving calcite, we demonstrate the formation of a protective layer that inhibits the reaction. The process is driven by diffusiophoresis - the motion of colloidal particles induced by solute concentration gradients. This mechanism opens new possibilities for applications such as repairing damaged zones in geological gas storage, addressing groundwater contamination, or advancing medical techniques like bone-crack repair and targeted drug delivery.

Low Reynolds number sphere motion near boundaries has been studied theoretically since the early 1900s because these canonical particles also provide insights into the motion of arbitrarily shaped objects, but theory has not been completely tested experimentally. Our experiments use novel techniques to verify the theory, and we use the data and theory to optimize the method of images for regularized Stokeslets. We find the optimal regularization parameter is related to the average discretization length, which provides a principled method for choosing its value. We also find the edge of the sphere must be kept at least one discretization length from the boundary for accurate results.

This study investigates how wave reflection influences pulsatile power in compliant tubes, providing new insights into cardiovascular biofluid dynamics. Using a reduced-order fluid-structure interaction model, we identify distinct frequency-dependent regimes in pressure-flow dynamics, separated by optimum frequencies that minimize pulsatile workload. Our findings underscore wave reflection as a key mechanism for strategically controlling inflow generator workload, informing the optimization of medical devices involving fluid-filled compliant tubes.

Inside the small intestine, the body absorbs most nutrients from chyme, a Non-Newtonian fluid formed by gastric disintegration of food. Despite its critical role in digestion, the impact of chyme’s rheology remains poorly understood. Using finite-element simulations, we reveal how viscosity governs nutrient uptake in intestinal contraction patterns. Our findings show that shear-thinning enhances absorption without compromising waste clearance, offering new insights into intestinal transport and disease prevention.

Encapsulating active particles within droplets to drive their motion presents exciting possibilities for biomedical applications, such as targeted drug delivery. In realistic biological and environmental settings, droplet interfaces often exhibit complex rheological behaviors that influence propulsion. This work investigates how surface shear and dilatational viscosities affect the motion of an active droplet, where a liquid droplet encloses an active particle modeled as a squirmer. The findings provide insights into the behavior of microswimmer-driven droplets in complex environments, paving the way for their potential biomedical and environmental applications.

The impact of a droplet onto a solid surface is a phenomenon we experience in our daily life, just like when raindrops fall onto windows, or even impact us. Droplet impact onto a solid surface is a process that has been well studied, from the shape the droplets take during flight, their spreading and splashing, to the force they exert on the surface they fall onto. The presence of bubbles may strongly influence the forces that occur during impact. Here, we study this effect numerically, by simulating the short-time interaction between an impacting droplet and a solid surface, mediated by the gas layer between droplet and solid just before collision, in the presence and absence of bubbles.

A water droplet impacting the center of a rotating superhydrophobic surface is experimentally observed with high-speed side-view imaging to examine the bouncing and retraction dynamics. The circumferential velocity in the retracting liquid film generates a centrifugal force which either induces rapid rebound like a spinning-top or leads to drop break up. A model is used to predict the retraction and circumferential velocities. We find that the droplet rebound is weakened by the centrifugal force with a transition between the two regimes around a critical Froude number (Fr=2.4). Good agreement is found between the theoretically predicted parameters and the experimentally measured ones.

We experimentally investigate the magnetophoresis of an oil droplet in a paramagnetic aqueous solution. Before the droplet reaches its equilibrium position - where the Kelvin force balances buoyancy - it undergoes rectilinear oscillation that closely resembles a damped harmonic oscillator. With that, we derive the form of the memory kernel that follows an exponential decay ($\propto e^{-t\prime }$) rather than ($\propto t{\prime }^{-1/2}$) for bubbles and particles. At equilibrium, interfacial tension works to restore the droplet’s spherical shape. Quantifying this deformation by incorporating magnetic pressure into the Young–Laplace equation, we inversely determine the interfacial tension between the two fluid phases.

We investigate numerically the early-to-mid-time deformation of liquid droplets in turbulent airflow with a non-zero mean speed. Ambient turbulence enhances the drag coefficient of the droplet as it flattens. Concurrently, the droplet becomes tilted and increasingly corrugated under strong turbulence intensity. We quantify these phenomena and discuss their possible origins associated with turbulence intermittency.

Finite-sized bubbles in turbulent flows are more than passive tracers—they actively influence flow structures. Using high-speed 3D imaging, we reveal how deformable, finite-sized bubbles alter turbulence, with slip velocity decorrelating over bubble-sized eddy turnover times. This rapid wake reorientation constrains wake development, yet at high Reynolds numbers, these bubbles still amplify local turbulence. Our findings highlight how bubble deformation, size, and orientation shape the surrounding turbulence, offering new insights into bubbly flow dynamics.

We numerically investigate the dynamics of bubble bursting affected by a uniform electric field. Our findings indicate that the electric field modifies both the cavity collapse and jet pinch-off processes by disrupting the typically observed self-similar behavior. This study provides a detailed analysis of how the electric field affects the mechanisms of jet pinch-off, local curvature, wavelength and energy distribution. These insights have significant implications for designing at small scales.

Understanding the physics of wall mode in Rayleigh-Bénard convection under intense magnetic field is critical for heat transfer designs. This study reveals the flow and thermal behaviors of wall mode, uncovering how magnetic intensity, temperature difference, and aspect ratio affect fluid dynamics. The volume of wall modes plays an important role in heat transport, while the hot and cold wall surfaces with no-slip condition triggers reverse flows. These findings provide new insights into the heat transfer design of magnetohydrodynamics.

We present experimental measurements revealing the density and velocity structure of gravity currents flowing over closely spaced roughness elements. We focus on their transient response, extending beyond the steady-state analysis of earlier studies. Our observations demonstrate that while currents flow above the obstacle array, convective exchange at the current’s underside cause dense fluid to drain into the obstacle array, significantly altering the current’s structure and evolution. These findings provide new insights into the fundamental mechanics of gravity currents interacting with complex topography.

Axial symmetry is a common assumption in hypersonic flows over conical geometries, yet many exhibit unsteady, three-dimensional instabilities. Using triple-deck theory, linear stability analysis, and direct simulation Monte Carlo, we examine non-axisymmetric azimuthal eigenmodes in Mach 16 flows. The strongest amplification occurs for azimuthal wavenumber n=1 near the cone tip due to interactions between the conical shock and the viscous shear layer. In double-cone flows, these three-dimensional effects alter the surface properties where the transmitted conical shock hits the wall, challenging axial symmetry assumptions and laminar-to-turbulent transition.

Low-order linear models serve as a foundational tool for flow analysis and control design for unstable flow systems. Conventional modeling approaches impose strict requirements on initial base flow and training process to ensure input-output identifiability. To address these limitations, this study presents a novel open-loop input-output modeling framework based on closed-loop identification principles, which leverages real-time control feedback generated by an adaptive data-driven, model-free controller to dynamically balance external excitation and inherent flow disturbances. This framework demonstrates enhanced modeling effectiveness and operational adaptability.

Two fluid annuli are nested between an inner and outer cylinder. The capillary instability at the fluid interface is controlled by axial motion, driven by a pressure gradient and translation of the inner cylinder. Yet a strong flow can generate other instabilities, often non-axisymmetric, either at the interface due to the viscosity jump, or in critical layers near both boundaries. Completely stable regions in the 6-dimensional parameter space are identified.

Tiny suspended particles are moved ahead of a freezing front, leaving behind pure ice. Yet, if particles are big or freezing occurs rapidly, they can get trapped in ice. Here, we investigate this phenomenon, focusing on the behavior of spherical objects, near the critical entrapment speed. Through experiments and theory we quantify how far particles travel before becoming entrapped. Moreover, we observe that two nearby particles either attract or repel during freezing, depending on their thermal conductivities. Our findings help to understand cluster formation during solidification, which we show to be analogous to clustering of particles at liquid interfaces, known as the Cheerios effect.

Double-mode Faraday waves, excited near overlapping instability tongues, exhibit rich and interesting pattern-forming dynamics. This work experimentally studies the transitions from the five-fold mode to the eight-fold mode, governed by mode interactions and suppressions. With single-point and full spatial measurements, we quantify amplitude evolution and identify distinct supercritical (five-fold) and subcritical (eight-fold) excitations. The analysis reveals that incorporating fifth-order terms into the amplitude equations is essential for accurately capturing the observed transitional behaviors, thereby providing deeper insights into the multi-mode dynamics of Faraday wave systems.

Understanding and optimizing the design of helical microswimmers is crucial for advancing their application in various fields. This study integrates free-form deformation with Bayesian optimization to discover optimal shapes that optimize microswimmer speed and efficiency. Applied to both monoflagellated and biflagellated swimmers, the method revealed novel optimal designs, expanding the known diversity of pushers and pullers. These findings provide insights into bio-inspired locomotion and microscale propulsion

We study the streaming flow induced by periodic forcing on a fluid bounded in a millimeter-sized channel with a wedge obstacle on one of its walls. Using both experimental and numerical approaches across relevant parameter ranges, we observe different regimes of symmetric, transitional, and asymmetric flows for a large range of forcing amplitudes. We show regimes with an asymmetrical pair of streaming vortices under large vibration amplitudes at 280 Hz, and also evidence of slow-oscillations of symmetric vortices in a transitional mode. The radius of curvature of the wedge tip is found to be crucial for the existence range of these regimes.

When a static granular layer on a rough inclined plane is set in motion, the inclination angle at which a steady granular flow is observed is higher than the one observed when a granular flow is brought to rest by reducing progressively the inclination angle. This hysteretic phenomenon at the static-flowing transition in granular media is numerically investigated. This work highlights the physical mechanisms responsible for hysteresis: a combination of inter-particles friction and particles inertia. Their influence on hysteresis are quantified and characterized through the evolution of the granular microstructure describing the contact network.

A simple one-dimensional power law model was proposed based on the assumption that the product of the water permeability and the pressure gradient is proportional to the cube of the water saturation. This model is semi-quantitatively consistent with the experimental results and suggests that the role of small gradients in water saturation in the wet region can not be ignored in determining the transition point from a constant rate period (CRP) to a falling rate period (FRP) in evaporation rates.

Hoppers are widely used in industrial granular processes. Discrete element simulations of the flow in an idealized wedge-shaped hopper, with frictionless walls and radial gravity, are compared to predictions of an exact theory by Savage, based on purely frictional rheology, and an extension of the theory based on a frictional-collisional rheology ($\mu$-I). The agreement is excellent when the finite stress at the exit of the hopper is taken into account. The theory and simulations provide new physical insights into the converging flow, for varying flow rates, inter-particle friction, particle stiffness and particle diameter.

In low-Reynolds-number flows, active swimmers can create non-reciprocal swimming strategies to achieve sustained propulsion. However, by virtue of their geometry and deformability, it is also possible for passive particles to drift or move in directions different from the mean background flow. We study hinge-shaped particles and demonstrate that adding elasticity leads to symmetry breaking and drift in oscillating flows and characterize the influence of deformability on drift.

Heat transfer in porous materials, such as Earth’s soil, often forms stationary convective patterns. Our study reveals that when the upper boundary of a tilted porous layer is only partially conducting, these patterns begin to drift upslope or downslope—an effect not previously explored. Extensive numerical simulations show that the drift speed and direction depend on boundary heat conductivity, which dramatically alters the behavior of patterns: below a threshold conductivity, convection cells spread and repel; above it, they cluster and interact dynamically. Our findings enhance the understanding of heat transfer in real-world fluid systems where boundaries do not conduct heat perfectly.

We numerically investigate the evolution of small-scale synthetic streamwise vortices in low-friction-Reynolds-number turbulent boundary layers. After analyzing statistical structures associated with these near-wall synthetic and naturally occurring streamwise vortices, we observe the similarities regarding their scales and the signature of kinetic energy transport. These similarities indicate that embedded small synthetic streamwise vortices of a spanwise scale comparable to those in canonical turbulent boundary layers are self-contained in the near-wall region and directly interact with the structures in this area toinfluence the associated turbulent transport.

This study examines how underwater turbulence shapes the movement and clustering of floating particles with relevance to environmental processes like micro-plastic dispersion or oil spills. Experiments in water tunnels were conducted, generating controlled turbulence beneath a flat surface. We find that floating particles form clusters matching the size and duration of large underwater swirls, persisting as long as these turbulent structures exist. While the water surface appears calm, hidden three-dimensional turbulence below creates surface flows with two-dimensional-like features, trapping particles in long-lived vortices.

This study investigates the noise generation mechanism of a circular cylinder in a quasisteady transonic condition using direct noise computation. The flow contains complex features such as weak oblique shocks, expansion fans, fluctuating separated shear layers, suppressed vortex shedding, 𝜆 shocks, and quasisteady bow shocks. Near-wake pressure fluctuations are found to be more strongly correlated with far-field pressure fluctuations, whereas wall pressure fluctuations are uncorrelated with far-field pressure fluctuations, suggesting that the sound sources are located in the near wake rather than on the cylinder surface.

In two-dimensional flows, coherent structures often coexist with a turbulent background. We demonstrate that a large-scale dipole can be distinguished from turbulent noise by considering it as an equilibrium solution of the Euler equations.

Rotation and stratification, which are essential in geophysical turbulence, affect the way groups of particles disperse. Unexpectedly, we find numerically, that the dispersion between pairs of particles becomes more intermittent when the stratification increases, the ratio between stratification and rotation being kept fixed. We find, however, that the time-asymmetry between forward and backwards dispersion diminishes with stratification. By studying the dependence of the dispersion between particles on the angle between the initial separation and the vertical, we find evidence that shear layers spontaneously develop in the flow.

The widespread application of data-driven turbulence models is currently limited by challenges in generalization and robustness to inconsistencies between input data of varying fidelity levels. This is especially true for thermal turbulent closures, which inherently depend on momentum statistics provided by low or high-fidelity turbulence momentum models. This work investigates the impact of momentum modeling inconsistencies on a data-driven thermal closure trained with a dataset with multiple fidelities (DNS and RANS).

Simulation of superhydrophobic surfaces (SHS) commonly utilizes a slip length boundary condition, relating slip velocity to wall-normal velocity gradient. In the Stokes flow limit, this effect can be shown to only depend on local velocity gradients at the wall; at finite Reynolds numbers, we find that nonlocal effects emerge. We investigate both scalar and momentum transport using the macroscopic forcing method to construct nonlocal eddy diffusivity, eddy viscosity, and slip kernels for turbulent channel flow over pattern-resolved SHS. The importance of nonlocality in these operators is assessed using the Reynolds-averaged representations for mean scalar and velocity fields.

We experimentally study turbulence in water close to an air-water interface using an experimental setup based on the randomly actuated synthetic jet array forcing. We are able to tune the turbulence intensity by varying the flow of a unique pump, while each jet is controlled by a solenoid valve. Particle image velocimetry measurements are made to characterize the turbulent fluctuations becoming strongly anisotropic when approaching the free surface. The results are compared to the rapid distortion theory and stratified turbulence model.

While the scattering of turbulent flows by an edge radiates sound waves into the far field, the roles of hydrodynamic and acoustic pressures remain unclear. This study examines these components to elucidate their roles and characteristics, focusing on trailing-edge noise through numerical and analytical approaches. Hydrodynamic pressure, characterized by streamwise turbulence, exhibits out-of-phase and incoherent behavior along the wall, forming pseudo-sound sources. In contrast, scattered acoustic pressure, governed by spanwise-coherent structures with weak energy and in-phase waves along the wall, is responsible for far-field sound radiation.

In this paper, we investigate compressibility effects on helicity using structure functions and third-order relations. The helicity scaling law $r^{2/3}$ holds in compressible turbulence, as helicity remains unaffected by compressive components. A new third-order relation introduces pressure and divergence terms, which are crucial in the dissipative range and drive inverse helicity cascades in the inertial range, respectively. Our findings clarify the role of compressibility in interscale helical dynamics, providing insights for its application in compressible turbulence and future theoretical advancements.

In this study, we show that side jets can be produced by introducing strong forcing on V-notched nozzle jets. We demonstrate that the plane along which the side jets are formed can be controlled by varying the forcing frequency, where the side jets can drastically increase the spread rate and enhance mixing.

Compliant membrane oscillating foil turbines (OFTs) can enhance power generation by stabilizing leading-edge vortices (LEVs), a key mechanism for lift. Through experiments in a water flume, we map the performance of compliant OFTs across a broad range of kinematics and isolate the roles of camber and extensibility in LEV stabilization. Membrane extensibility is shown to be critical for dynamic decambering at high angles of attack, delaying stall and improving lift and power, while camber alone suffices at low angles of attack. This study provides new insights into optimizing compliant OFTs for renewable energy applications.

Vortex rings (VRs) are intriguing fluidic structures capable of self-propulsion once created. We experimentally investigate the behavior of a VR as it interacts with perforated plates set at different angles. Remarkably, a single VR splits into two distinct VRs when the included angle (θ ≤ 120°) is small. Moreover, using n-faced perforated surfaces makes it possible to generate multiple VRs from a single vortex. On the other hand, this process can be suppressed by adjusting the θ values. The study explores key phenomena such as the growth of mushroom-shaped structures before interaction, the formation, and interaction of jets during the vortex impingement, and the subsequent reformation.

Wave wakes produced by a finite-size source uniformly moving on an ice plate overlying deep water is studied. The kinematic and amplitude characteristics of source-generated flexural-gravity waves are presented in terms of isophase patterns; the wave patterns are determined by ad hoc defined analogues of Mach and Bond numbers. The Reference Solution Approach is used to describe the distribution of wave amplitudes in the wake accounting for the source size and shape. This approach agrees with the Stationary Phase Method in the far-field zone and reproduces also specific wave dynamics at short and intermediate distances from the source.

Electromagnetically driven large-scale vortices appear in horizontal electrically conducting soap films formed between two concentric cylindrical electrodes with the radii of several centimeters. In nonuniform magnetic fields formed by conventional permanent magnets, instability of the base flow develops if the applied DC voltage exceeds several volts. The number of vortices is controlled by the aspect ratio of the radii of the electrodes.

In oceans, multiple inertia-gravity waves often coexist in a region. We study the stability of two coexisting plane inertia-gravity waves, with the same frequencies and wavevector norms. Specifically, we explore the decay of two primary waves through triadic resonant instabilities in cases where two primary waves have a common secondary wave, and this results in a 5-wave system composed of two different triads. We show that 5-wave systems are the dominant instabilities with higher growth rates than standard triads for latitudes greater than 9 degrees.

Measurements taken from fluid flows are often sparse, noisy, and from a mix of pressure and velocity data. In this paper, we develop a physics-constrained neural network to reconstruct the full flow field from incomplete measurements. We propose a new loss function specifically for reconstructing flows from noisy, sparse measurements. We reconstruct a laminar bluff body wake and a chaotic Kolmogorov flow from sparse measurements and stochastic noise.