A wind-turbine wake in a pressurized-air wind tunnel was forced via slow periodic oscillations in the turbine rotation rate. Flow measurements identified traveling waves in the wake, which could be controlled by independently varying the turbine thrust and tip-speed ratio. The results demonstrate the importance of advection for the modeling and control of wind-turbine wakes, even at nominally quasi-steady time scales.

We embark on a 30-day journey following the flow of a high-viscosity liquid through a capillary tube to assess how existing theoretical models, which also account for the dynamic nature of the contact angle, perform in long-term imbibition processes. The journey reveals the persistent and diffusive nature of imbibition processes for liquids with extreme values of viscosity.

Water injection is a promising technology for mitigating pollutant emissions from hydrogen combustion. However, the high reactivity and diffusivity of hydrogen flames make droplet–flame interactions particularly intricate. Using two-dimensional detailed-chemistry simulations, we show that a single droplet locally modifies the flame displacement speed and heat release rate, while altering flame stretch through droplet-induced curvature. We also propose a phenomenological model to predict the local flame displacement and velocity during the droplet-flame interaction.

We show that the interaction of a vortex pair with a localized polymeric fluid layer can generate secondary and tertiary vortices through polymer stress gradients. Numerical simulations reveal that elastic stresses significantly modify vorticity production and energy transfer, leading to flow behaviors not observed in Newtonian fluids. The image shows the vorticity field during the interaction.

The pinch-off of drops and bubbles from a needle are classic examples of hydrodynamic singularities, in which a fluid body splits into two. While even small amounts of polymers strongly delay drop breakup by forming long liquid threads, recent experiments show that bubble pinch-off remains largely unaffected. In this article we demonstrate that polymer stretching cannot compete with the violent inertial collapse governing bubble pinch-off. Only for sufficiently high polymer concentrations and small needle sizes can viscoelastic effects delay breakup, resulting in the formation of air cavities.

We study how viscoplastic filaments retract and break. Unlike Newtonian liquids, viscoplastic fluids can resist motion through a yield stress. We identify two mechanisms that can prevent droplet detachment from its edge depending on the shear-dependence: shear thickening reopens due to generation of vorticity near the neck, whereas strong shear-thinning allows capillary pressure to reopen it due to curvature changes. Remarkably, this latter mechanism also arises in the Newtonian limit as the viscosity approaches zero, demonstrating that an almost inviscid filament need not undergo classical end pinching, contrary to the prevailing picture in the literature.

Using qualitative flow diagnostics and quantitative pressure measurements, this study investigates hypersonic flow over a cone-mounted axisymmetric cavity to assess the influence of Reynolds number, aspect ratio, and excess rear-face height. A distinct mode-shifting behavior from flapping-dominated to Kelvin-Helmholtz-dominated oscillations is identified at the highest aspect ratio, owing to a possible turbulent transition of the shear layer with a change in Reynolds number, which is found to be absent in the two-dimensional cavity configuration. The excess rear-face-height cases also demonstrate the ability to alter the dominant instability mechanism and resonance characteristics.

We characterize the nonlinear pressure drop–flow rate relation of elongated bubble trains in capillary tubes by incorporating thin-film hydrodynamics at low capillary numbers. We show that pore-scale statistical heterogeneity in capillary bundles induces systematic deviations from the single-phase limit, controlled by both the pore-size distribution and flow parameters. We identify a crossover between sublinear regimes with exponents ranging from 2/3 to 1, connecting the Bretherton and Darcy limits and in qualitative agreement with values reported for immiscible two-phase flow in porous media.

We experimentally and numerically investigate nanosecond laser-induced jetting dynamics of a free-falling micro-sized liquid tin droplet. Following laser impact, the droplet rapidly expands and retracts, leading to the formation of a high-speed axial jet. A specific combination of laser-induced pressure and its angular distribution on the droplet surface drives cavity formation, ultimately giving rise to a singular jet. We reveal the underlying mechanism and construct a phase diagram explaining jet emergence as the result of a subtle interplay between droplet curvature during retraction and radial flow.

Droplet mixing is vital in many practical applications, yet the underlying physical mechanisms remain poorly understood. This work uses carefully validated numerical simulations to investigate the mixing dynamics of inkjet printed droplets. Our results show that impact-driven flows contribute only weakly to mixing, while molecular diffusion governs homogenization over a timescale of seconds. We perform studies to understand how droplet size, spacing, volume ratio, and substrate wettability influence mixing, producing methods to estimate mixing times for droplet-based manufacturing processes.

Why do clean spherical bubbles spread laterally even after they leave each other’s wakes? Experiments and reduced-order modeling reveal that bubble chains disperse through two successive mechanisms: wake-induced lift initiates lateral dispersion, while a bubble-induced upward flow drives the second stage. The findings reveal how pairwise wake interactions give rise to collective bubble dynamics.

The sound produced by a detaching bubble is known to arise from its volumetric oscillations, but the factors driving the oscillation amplitude has remained an open question. We perform experiments using two bubble-generation methods, and observe that both the shape and amplitude of the pressure signal depend strongly on the bubble formation mechanisms. Using a large set of experimental data, we identify that the oscillation amplitude is mainly controlled by the initial growth velocity of the bubble. Furthermore, we propose a simple model that captures the onset of the acoustic signal as a consequence of the bubble pinch-off dynamics.

In this work smectic bubbles with millimeter diameters, but only nanometer film thicknesses, are exposed in microgravity experiments to high electric fields to study field effects on film inclusions. The observed motion of islands and droplets in the film plane can be considered as a quasi-two-dimensional analogue of electrospraying. Different mobilities of anions and cations of the ionic dopants make the effect polarity dependent. The image shows the motion of islands (color coded blue to brown) when a high electric field at the anode (brown bar) is switched off at time $t=0$.

Charge transfer between particles is believed to strongly influence the mechanical properties of many suspensions, yet it remains difficult to observe directly. We demonstrate that biopolymer microcapsules exhibit a unique bouncing dynamics under an alternating electric field that is consistent with repeated charge transfer during contact. These findings identify Coulombic interactions as the origin of the attractive forces responsible for the fragile gel behavior and yield stress previously observed in suspensions of these microcapsules.

Electrophoresis is often used in combination with external flows for enhanced particle separation. However, the resulting motion when particles have nonuniform surface charge remains poorly understood. This dynamic problem is solved here using an efficient and exact reduced order model. It reveals the diverse set of trajectories emerging from the coupling between the particle’s rotation and its uneven surface charge with potential applications in medical diagnosis and analytical chemistry.

Low Reynolds number airfoils may stall abruptly due to laminar separation bubble bursting. Periodic boundary layer forcing can promote transition through the excitation of natural instabilities, and thereby delay or prevent stall. In this study, forcing is provided by a plasma actuator and the influence of varying forcing amplitude on the bursting transient is examined using particle-image velocimetry and surface pressure measurements. Increasing the forcing amplitude delays and increases the variance in the bursting start time. However, the dynamics of the bursting process are largely insensitive to forcing amplitude. For higher forcing amplitudes, bursting is entirely prevented.

Shear-thinning is ubiquitous in biological and industrial fluids, yet its influence on the instability of time-periodic shear flows remains poorly understood. Using Floquet stability analysis of the Stokes layer in Carreau fluids, we show that shear-thinning can either suppress or promote instability depending on the characteristic fluid response time. The study further identifies a phase-dependent energy-transfer mechanism governing instability in a time-periodic shear flow, which has not been revealed in the literature, providing a dynamic counterpart to the classical energy-production mechanism in steady shear flows.

We revisit the classical homogeneous shear assumption in cone-and-plate rheometry over a slip boundary condition, and resolve inconsistencies between two slip boundary formulations. Our results show that a fixed slip-length model induces radial shear-rate non-uniformity, whereas a constant wall-stress formulation restores homogeneous shear via a radius-dependent slip length. Extending our work to shear-thinning Carreau fluids, we demonstrate a strong shear-rate dependence of drag reduction and effective slip length. Despite distinct local flow structures, both models yield consistent area-averaged slip from torque measurements, providing a unified framework for slip quantification.

In multiphase turbulent flows, kinetic and interfacial energies usually coexist in a subtle balance, but their mutual conversion often goes unnoticed in statistically steady regimes. Our work introduces a controlled, time-periodic forcing to break this steadiness, thereby revealing the dynamic cycle of energy injection, conversion, and dissipation. By extending the $k-ϵ$ model to include surface energy and nonequilibrium effects, and by linearizing the system, we uncover the intricate time scales that govern the coupling between these processes.

Put a soap bubble in an electric field and it stretches into a taller, smooth dome. Turn the field up and, past a critical point, the top sharpens into a pointed cone that fires off a thin jet. Imaging the bubble from the side, we follow this whole sequence in one experiment and find two things. Bubbles of different sizes deform along the same curve once the field is rescaled by bubble size and surface tension, so one balance between electric and capillary forces sets the shape. And the cone is far sharper than Taylor’s classic value, because it is selected while the film is still rushing toward the jet rather than resting in equilibrium.

Rotating bluff bodies are widely used to control wake instabilities and fluid forces, but the role of wake–body interactions in multi-body configurations remains less understood. Direct simulations of isolated and tandem transversely rotating spheres show that rotation can suppress classical shedding and reorganize the wake into double-threaded vortical structures. However, tandem interactions can overturn this stabilizing effect, sustaining unsteady wakes and strongly modulating drag and lift.

Oscillatory flows in compliant channels are common to a number of soft-hydraulic problems, from physiological transport to microfluidics. Recently, elastoinertial rectification was shown to lead to streaming in these systems. We develop this theory for a two-dimensional (2D) channel with a confined, nearly incompressible elastic layer as its wall and validate the predictions against detailed Lagrangian–Eulerian fluid–structure interaction simulations. Surprisingly, the nearly incompressible 2D layer introduces new physics: resonance-like amplification of streaming at specific Womersley numbers. Our results offer design principles for optimizing flow rectification in soft microsystems.

Controlling the distribution of colloidal particles during drying of a thin porous medium is of importance for various technological applications. The liquid phase distribution during drying in a thin system is strongly affected by two effects, referred to as the edge and breakthrough effects. The impact on the colloid distribution during drying of both effects is studied. Simulations indicate markedly more uniform colloid spatial distributions as the result of both effects compared to predictions based on the standard model ignoring these effects.

Dense non-Brownian suspensions undergo a rheological transition with increasing shear rate, from a Newtonian scaling where stress grows linearly, to a Bagnoldian scaling where it grows quadratically. For suspensions devoid of frictional contacts due to electrostatic repulsive forces keeping the particles apart, our discrete element simulations reveal the shear rate marking the onset of the inertial regime to exhibit a critical behavior as the suspension approaches jamming. Our results show this criticality to be tied to a diverging microstructural length scale, larger than any individual particle, that governs the emergence of inertial effects within the suspension.

High-fidelity simulations have become indispensable for uncovering the physics of turbulent two-phase flows, yet quantitative guidelines for the grid and time-step requirements needed to accurately resolve interface dynamics have been lacking. We derive scaling laws that predict these computational requirements as functions of Reynolds, Weber, and Capillary numbers; identify distinct inertia- and viscous-dominated regimes; and introduce a new dimensionless parameter that unifies their classification. The resulting framework provides practical $a$ $priori$ resolution criteria and computational cost estimates for predictive interface-resolved simulations.

We study collisions of small inertial particles suspended in a background flow. These particles may be inert plankton in the ocean or water droplets in clouds. Previous studies have shown that such collisions or caustics require particles to pass through regions of high strain in the flow. Our work finds that the type of strain is crucial, not just its magnitude. Regions of large compressive strain trigger caustics, whereas particles survive caustics despite encountering large strain if they come from extensional strain. Extending the study of caustics to finitely dense particles, we show that the level of strain required for caustics increases with decreasing particle density.

Ordered vortex lattices, familiar from quantum systems like Bose-Einstein condensates and type II superconductors, have emerged in classical rotating turbulence, but the mechanisms controlling their stability remain elusive. By embedding ideal, defect-free triangular arrays into three-dimensional turbulent flows, we map the region of parameter space where these structures survive and establish their characteristics. Their lifetimes obey a memoryless random process whose mean depends sensitively on the Rossby number and energetic balance, with optimally tuned dissipation yielding long-lived states whose statistics resemble those of critical systems.

The intense fluctuations of the rate of energy dissipation, an essential aspect of intermittency, are of constant interest in turbulence research. Measuring the energy dissipation, however, is extremely challenging: Most of our knowledge comes from hot-wire measurements of the derivative of the streamwise velocity component, which is generally regarded as a surrogate of the energy dissipation. Here we show that the moments of the energy dissipation cannot reduce to those of its surrogate, as a consequence of a fundamental property of the rate of strain tensor in turbulence. However, the predictions based on assuming a simplified strain rate tensor are accurate to within a few percent.

Particle-vortex interactions in quantum turbulence are essential for interpreting the results from particle-laden experiments in superfluid He II thermal counterflow. In this experimental investigation, we extracted and analyzed trapping and de-trapping processes of small particles on quantum vortex lines. Our results show an asymmetry between the two processes, indicating that they are governed by distinct physical mechanisms, and suggest that they exhibit self-similar features across small time lags. These results deepen our understanding of trapping and de-trapping processes of particles, and the dynamics of particle motion in turbulent thermal counterflow under two-fluid interactions.

Using direct numerical simulations of incompressible turbulence at Taylor-scale Reynolds numbers $R_{\lambda }$ up to about 1740, we examine spectra, two-point correlations, and second-order local-average moments of the energy dissipation rate ϵ and enstrophy Ω. Correlations and local-average moments exhibit larger scaling exponents for fluctuating fields than for total fields over nearby but distinct ranges. In both statistics, squared-mean contributions are nonnegligible relative to fluctuating contributions. Thus, total- and fluctuation-field exponents need not coincide over these ranges. Results suggest $R_{\lambda }$ ≈ 1740 remains insufficient to reach the asymptotic regime assumed in intermittency theories.

Data assimilation and symbolic regression are used to formulate an analytical correction to the Spalart-Allmaras model, addressing local deficiencies in its production term. The correction improves separated-flow predictions while preserving the performance of the baseline model for wall-attached flows. Tests on multiple two-dimensional flow cases confirm the applicability of the correction across diverse configurations.

Invariants, such as energy or enstrophy, are central to turbulence theory. Some systems behave in a sub-space between two well-known limits where invariants are known. Here, we demonstrate that invariants can also be defined for these intermediate cases. Knowledge of the invariants enables the development of simple models for the multiscale dynamics of such systems.

The comprehensive statistics of a turbulent flow field can in principle be captured by Hopf’s functional approach; however, the resulting functional equations have remained largely intractable. We here study a simplified passive vector model, whose Hopf equation is solved exactly by an ensemble of Gaussian fields. Based on both theory and simulations, we find that the model displays a fluctuating energy flux from large to small scales. The resulting intermittency at the small scales can be understood as arising from a probabilistic mixture of Gaussian sub-ensembles.

We introduce a sequential approach to augment the shear stress transport-conditioned (SST-CND) model, which was originally trained on 2D flows, in 3D scenarios. 3D field inversion is first performed to obtain the distribution of the augmentation term. Then, features that are nonzero only in 3D scenarios are selected to build the expression using symbolic regression. The resultant SST-CND3D model preserves the original model’s capabilities in 2D flows, while showing significant improvement in 3D complex cases, including a real-world high-lift device.

Acoustic liners are widely used for noise reduction in aero-engines, but the physical mechanisms governing their aerodynamic drag remain insufficiently documented. Based on pressure-drop measurements in a turbulent channel over a wide range of Reynolds numbers, this study identifies distinct drag regimes and reveals two markedly different behaviors within the transitional regime. In particular, large drag increases are shown to be associated with the onset of aeroacoustic resonance at the liner resonance frequency, while the relative importance of liner geometry is captured through a frontal-solidity-based parameter.

Unsteady thin-airfoil theory is a straightforward reduced-order framework for arbitrary airfoil motions and geometries. However, an inconsistency appears when recovering classical unsteady problems: Glauert’s bound-vorticity expansion is not uniformly convergent at the trailing edge in unsteady motion. Revisiting Wagner’s problem through the unsteady Kutta condition, this work derives a recurrence for the bound-vorticity coefficients and builds a uniformly convergent bound vortex-sheet formulation with a discrete-vortex equivalent. With few terms, the method ensures bound-wake continuity, zero trailing-edge loading, and smooth transient predictions for Wagner’s indicial problem

A previously undocumented Reynolds-number-dependent transition is identified in the wake of a rounded-edge slanted afterbody, from a centerline-separated to a novel centerline-attached vortex state. The transition produces a pronounced drag reduction through the collapse of the centerline recirculation region as the upstream boundary layer becomes turbulent. These findings explore experimental evidence linking laminar separation bubble dynamics, shear-layer instability, and wake-state transitions in this canonical bluff-body geometry.

Experimental studies of fluid–structure interactions (FSI) rarely capture dense, simultaneous measurements of both the fluid and solid phases, which limits analysis of the coupled dynamics. We present a physics-informed data assimilation framework that reconstructs both the flow and structure from sparse, off-body particle tracks, i.e., without requiring a constitutive model or direct observations of the structure. The method accurately recovers coupled dynamics in 2D and 3D benchmarks, allowing for quantitative measurements of FSI from limited data.