Swirl switching refers to the temporal rotation of the plane of symmetry of the cross-sectional vortices about the equatorial plane of the curved pipe and may induce significant vibrations in piping systems. The origin of this phenomenon is not yet understood, and its relationship with turbulent fluctuations in the upstream straight section is still debated in the literature. To assess the effect of upstream turbulence, direct numerical simulations are performed for spatially developing bent pipe flows under both laminar and turbulent inflow conditions. Additionally, the influence of the bending angle on swirl switching and time-averaged flow characteristics is investigated. Proper orthogonal decomposition (POD) is applied to three-dimensional velocity fields to extract spatially coherent structures responsible for swirl switching. The phenomenon is observed for both inlet velocity profiles and bending angles, although it does not correspond to the dominant mode in the case of laminar inflow conditions. A reduced-order reconstruction using the most energetically relevant antisymmetric POD modes indicates a link between swirl switching and the shear layer within the bend. We therefore infer that swirl switching could arise because of a symmetry-breaking shear layer instability, representing a stage in the bifurcation cascade that leads to turbulence.

We develop a robust numerical method for thin plates falling in inviscid fluid that allows for leading-edge vortex shedding. The inclusion of leading-edge shedding restores physical dynamics to vortex-sheet models of falling bodies, and large-amplitude fluttering and tumbling are observed in inviscid simulations. Leading-edge shedding is achieved by introducing a quadrature rule and smoothing procedure for the Birkhoff-Rott equations. The smoothing error is controlled by a projection procedure similar to fencing. We find a transition point between fluttering and tumbling that is consistent with previous viscous simulations and experiments, and other falling motions such as looping and autorotation are also observed as the plate density increases. The dipole street wakes behind the fluttering plates resemble those in experiments. We consider plates bent into V shapes and study the effects of density and bending angle on the qualitative falling dynamics. At small densities, increasing the bending angle stabilizes the falling motion into fluttering, while at large densities, decreasing the bending angle stabilizes the falling motion into autorotation. In the autorotation regime, the magnitude of angular velocity increases as time cubed before it reaches a terminal angular velocity, and in the fluttering regime, the fluttering frequency scales as the $-1/2$ power of $R_1$, the plate density.

Liquid films coating vertical cylinders can form annular liquid collars which translate downwards under gravity. We investigate the dynamics of a thin viscoplastic liquid film coating the interior or exterior of a vertical cylindrical tube, quantifying how the yield stress modifies both the Rayleigh-Plateau instability leading to collar formation and the translation of collars down the tube. We use thin-film theory to derive an evolution equation for the layer thickness, which we solve numerically to examine the nonlinear dynamics. Instability and collar formation occur when gravity is sufficiently strong to make the fluid yield initially. We use matched asymptotics to derive a model describing the quasisteady translation of a slender liquid collar when the Bond number is small. The structure of the asymptotic solution for a viscoplastic collar shares some features with the Newtonian version, but there are several novel asymptotic regions that emerge at the two ends of the collar. The global force balance, which determines the collar's speed, is modified by a leading-order contribution from viscous drag in the collar when the liquid is viscoplastic. We use the asymptotic model to describe slow changes in collar volume when the film thicknesses ahead of, and behind, the collar are unequal. When the film thickness ahead of the collar is less than a critical value that we determine, viscoplastic collars adjust their volume and reach a steadily translating state. This contrasts with the Newtonian problem, where the only state in which steady translation occurs is unstable to small changes in the film thickness.

We present direct numerical simulations (DNS) of particle deposition in a turbulent channel flow, incorporating a viscoelastic soft-sphere collision model with van der Waals adhesion. Particle-wall contact is governed by an adhesion number that varies with temperature, enabling exploration of a wide range of deposition behaviors. Deposition is strongly heterogeneous, especially for inertial particles, where rolling and sliding enhance nonuniformity. Spanwise radial distribution functions reveal that deposited particles form streaks with characteristic spacing set by near-wall two-point velocity correlations. A clustering metric confirms that high-inertia, low-adhesion particles deposit in elongated, anisotropic patterns due to spanwise particle creep driven by velocity fluctuations. Finally, it is shown that this heterogeneity in deposition leads to localized wall wear rates exceeding 10 times the mean, with the most severe wear associated with particles that are carried to the wall in clusters by sweep events.

Recent numerical experiments of dynamo action relevant to the generation of the geomagnetic field have produced different regime branches identified within bifurcation diagrams. Notable are separate branches in which the resultant magnetic field is either weak or strong. Weak-field solutions can be identified by the prominent role of viscosity on the motion whereas the magnetic field has a leading order effect on the flow in strong-field solutions. For a given Ekman number, $\mathcal{E}$ (measuring the ratio of viscosity to rotational effects), the existence of these branches and bistability between them is reliant on a small enough magnetic Ekman number, ${\mathcal{E}}_{\text{m}}$ (measuring the ratio of magnetic diffusion to rotational effects, so $\mathcal{E}/{\mathcal{E}}_{\text{m}}={\mathcal{P}}_{\text{m}}$, the magnetic Prandtl number). Both branches are known to produce large scale dipolar magnetic fields but do not exhibit an expected scale separation between the flow and magnetic field. In this work, by reducing ${\mathcal{E}}_{\text{m}}$, we identify a variety of dynamo states on the weak-field branch beyond the known dipolar solutions. Specifically, hemispherical and nondipolar dynamos were found, in addition to the usual dipolar solutions. Some solutions exhibit clear scale separation between small-scale flow and large-scale magnetic field, despite the large ratio of viscosity to magnetic diffusion. Numerical solutions in this regime have not been observed before and they offer a first connection with earlier theoretical work based on mean-field theory.

The growth of ice stalagmites obtained by the solidification of impacting droplets on a cooled substrate ($-50{}^{\circ }\mathrm{C}$ to $-140{}^{\circ }\mathrm{C}$) is investigated experimentally. It is shown that for any combination of substrate temperature and drop flow rate, there is a critical height above which unfrozen water accumulates at the stalagmite's tip, drips, and develops into fingers that give a star shape to the stalagmite. Both the vertical growth and the radial growth of the stalagmite are discussed through the Stefan problem and mass scaling arguments, respectively. Finally, a phase diagram that presents the stalagmite aspect ratio in function of the main control parameters is proposed.

A dynamic averaging algorithm combined with a high-order spectral (HOS) model is introduced for wave reconstruction and prediction using radar data from stationary or moving ships. The algorithm enables gradual reconstruction of waves entering the radar's blind zone and improves accuracy by reducing the effects of shadowing modulation, particularly under steep wave conditions. The effects of nonlinearity on wave prediction accuracy are examined across a range of sea states with varying wave steepness and directional spreading angles. Results show that the HOS model, which includes third-order nonlinear effects, outperforms both linear and second-order models. Its advantage is especially clear in steep or narrowly spread wave fields, where simpler models fail to accurately predict the timing and height of large waves, leading to wave prediction errors at the ship's position up to three times higher than those from the HOS model. Additionally, when the ship slows down in head waves or speeds up in following waves, the relative velocity between the waves and the ship decreases. This reduction in relative speed extends both the time required for accurately reconstructing waves in the blind zone and the length of the predictable wave field ahead of the ship.

Drying of fluids undergoing sol-gel transition in porous media, a process crucial for the consolidation of damaged porous structures in cultural heritage, often leads to skin formation at the surface. This phenomenon significantly hinders evaporation, yet its precise impact on drying kinetics remains poorly understood. To uncover the governing mechanisms, we develop a novel pore network model that closely replicates quasi-two-dimensional experimental porous media in a previous paper [Le Dizès Castell et al., J. Phys. Chem. Lett. 15, 628 (2024)], incorporating spatial gradients in pore size distribution, with smaller pores near the evaporation side. We demonstrate that this pore distribution dictates the air invasion path and extends the constant-rate period of drying in Newtonian liquids, reproducing the experimental drying curves for water. We further extend our model to capture the interplay of capillary-driven liquid flows, space-dependent viscosity increases, and localized skin formation. To incorporate skin formation, we implement a viscosity-dependent vapor pressure rule derived from experimental data on evaporation-induced sol-gel transition within a capillary tube. We identify a simple relationship: vapor pressure decreases once the meniscus fluid viscosity exceeds a critical threshold. By accounting for localized skin formation through reduced evaporation pressure at high-viscosity throats, our model successfully captures the slowdown of drying kinetics, achieving remarkable agreement with experimental drying curve.

Ice melting is a complex, dynamic, and multiscale process involving many feedback effects. An essential but poorly understood factor for floating ice is the potential to overturn, as dramatically demonstrated by capsizing icebergs. Here we study the melting-induced reshaping of ice coupled to its free rotation dynamics and find that periods of stable equilibrium are punctuated by spontaneous capsize events. Laboratory experiments on floating ice show that melting produces faceted and angular shapes whose tendency to orient to the waterline is repeatedly upset by rolling over. Similar geometries and motions are reproduced in simulations that drive the shape development via differential melt rate over the surface and which solve for the rotational dynamics subject to shape-dependent forcing from gravity, buoyancy, and hydrodynamics. Melting gradually drives the structure toward postural instability while also priming other orientations that prove stable when reached by abrupt flipping. These findings show how the shape and motions of ice are inextricably linked at fundamental scales, and they reveal mechanisms that may be operative for natural conditions.

The translation and shape deformations of a passive viscous Newtonian droplet immersed in an active nematic liquid crystal under circular confinement are analyzed using a linear stability analysis. We focus on the case of a sharply aligned active nematic in the limit of strong elastic relaxation in two dimensions. Using an active liquid crystal model, we employ the Lorentz reciprocal theorem for Stokes flow to study the growth of interfacial perturbations as a result of both active and elastic stresses. Instabilities are uncovered in both extensile and contractile systems, for which growth rates are calculated and presented in terms of the dimensionless ratios of active, elastic, and capillary stresses, as well as the viscosity ratio between the two fluids. We also extend our theory to analyze the inverse scenario, namely, the stability of an active nematic droplet surrounded by a passive viscous layer. Our results highlight the subtle interplay of capillary, active, elastic, and viscous stresses in governing droplet stability. The instabilities uncovered here may be relevant to a plethora of biological active systems, from the dynamics of passive droplets in bacterial suspensions to the organization of subcellular compartments inside the cell and the cell nucleus.

We present a data- and first-principles-driven method for inferring the shape of a solid obstacle and its flow field in three-dimensional steady-state supersonic flows. The proposed method combines the optimizing a discrete loss (ODIL) technique with the automatically differentiable JAX-Fluids computational fluid dynamics (CFD) solver to study the joint reconstruction of flow fields and obstacle shapes. ODIL minimizes the discrete residual of the governing partial differential equation (PDE) by gradient descent-based algorithms. The ODIL framework inherits the characteristics of the chosen numerical discretization of the underlying PDE, including its consistency and stability. Discrete residuals and their automatic differentiation gradients are computed by the JAX-Fluids solver, which provides nonlinear shock-capturing schemes and level-set-based immersed solid boundaries. We use synthetic data to validate this approach on challenging inverse problems, including the shape inference of a solid obstacle in three-dimensional steady-state supersonic flow. Specifically, we study flows around a cylinder, a sphere, and an ellipse. We investigate two distinct approaches for the obstacle shape representation: (1) parametric shape representation, where the obstacle is described by a small set of parameters (e.g., the radius of the cylinder and the sphere) that are optimized together with the flow field, and (2) free shape representation, where the level-set function is directly optimized at each point of the computational mesh, without relying on predefined shapes. For the former, a thorough comparison with physics-informed neural networks is provided. We show that the nonlinear shock-capturing discretization in combination with the level-set-based interface representation allows for accurate inference of the obstacle shape and its flow field for the ODIL method. The proposed approach opens new avenues for solving complex inverse problems in supersonic aerodynamics.

Particles in a sufficiently strong electric field spontaneously rotate, provided that charge relaxation is slower in the particle than in the suspending fluid. It has long been known that drops also exhibit such “Quincke rotation,” with the electrohydrodynamic flow induced by electrical shear stresses at the interface leading to an increased critical field. However, the hysteretic onset of this instability, observed for sufficiently low-viscosity drops, has so far eluded theoretical understanding—including simulations that have struggled in this regime owing to charge-density steepening driven by surface convection. Here, we conduct a numerical study of the leaky-dielectric model in a simplified two-dimensional setting involving a circular drop, considering arbitrary viscosity ratios and field strengths. As the viscosity of the drop is decreased relative to the suspending fluid, the pitchfork bifurcation marking the onset of drop rotation is found to transition from supercritical to subcritical, giving rise to a field-strength interval of bistability. In this subcritical regime, the critical field is always large enough that, at the bifurcation, the symmetric base-state solution exhibits equatorial charge-density blowup singularities of the type recently described by Peng et al. [Phys. Rev. Fluids 9, 083701 (2024)]. As the rotation speed increases along the initially unstable solution branch from the bifurcation, the singularities gradually shift from the equator and ultimately disperse once the rotational component of the flow is strong enough to eliminate the surface stagnation points.

We examine the fundamental fluid dynamical mechanisms dictating the generation of bioaerosols in the human trachea during intense respiratory events such as coughing and sneezing, with an emphasis on the role played by the mucosalivary fluid viscoelasticity. An experimental investigation of the shear-induced fragmentation of a mucosalivary-mimetic fluid in a confined geometry reveals that viscoelastic liquids undergo atomization in a manner akin to Newtonian liquids—via the formation of baglike structures—which ultimately rupture through the appearance of retracting holes on the bag surface. Droplets are produced via the unstable retraction of liquid rims bounding these holes. However, in comparison to Newtonian liquids, viscoelastic bags inflate to larger sizes—implying thinner sheets and, consequently smaller droplets upon rupture. Numerical simulations support that the smaller droplets can be attributed to the thinner sheets, with a more uniform thickness, for viscoelastic bags prior to rupture. Hence, we highlight the role of the viscoelasticity in determining the thickness of the intermediate baglike structures, which, in turn, govern the droplet size distribution of the expelled aerosol.

We study the dynamics of sail membranes during a model of a tacking maneuver, when the sail angle of attack is reversed in order to sail upwind. In successful tacking the sail flips to the mirror-image shape, while in unsuccessful tacking the sail remains stuck in a metastable state close to the initial shape. We investigate whether the sail flips, and if so, how long it takes and the subsequent dynamics, over a parameter space that describes the sail membrane properties and the kinematics of tacking. We find that the “steady” parameters—stretching rigidity, pretension, and angle of attack—mostly determine whether a membrane flips or not. Flipping is more likely with larger values of the stretching rigidity, pretension, and angle of attack. The dynamical parameters—membrane mass density, concavity of angle-of-attack transition kinematics, and time length of the tacking maneuver—mainly affect how long flipping takes. With large membrane mass the membrane can maintain the momentum from the tacking motion long enough to reach the flipped state, but it may also take longer to converge to a steady shape. Concave-down angle-of-attack profiles and small tack times are generally associated with shorter flip times but a few exceptions exist, because these kinematics can also give a larger acceleration to the fluid-sail system that persists for longer times. We also investigate slack sails and find that they are more difficult to flip.

Many complex wetting behaviors of fibrous materials are rooted in the behaviors of individual droplets attached to pairs of fibers. Here, we study the splitting of a droplet held between the tips of two cylindrical fibers. We discover a sharp transition between two post-rupture states, navigated by changing the angle between the rods, in agreement with our bifurcation analysis. Depinning of the bridge contact line can lead to a much larger asymmetry between the volume of liquid left on each rod. This second scenario enables the near-complete transfer of an aqueous glycerol droplet between two identical vinylpolysiloxane fibers. We leverage this response in a device that uses a ruck to pass a droplet along a train of fibers, a proof-of-concept for the geometric control of droplets on deformable, architected surfaces.

We study experimentally a symmetrical rigid foil performing pitching oscillations around a mean incidence angle (${\alpha }_m$) with respect to an incoming flow in a hydrodynamic channel at a constant velocity where the Reynolds number based on the chord of the foil is ${\mathrm{Re}}_c=\rho U_{\infty }c/\mu =14400$. The problem is inspired by the pumping maneuver used by athletes on the new hydrofoil-based windsurf boards. In windsurfing, the pumping mode is used by athletes to produce unsteady propulsion. Because of the complex kinematics and the sail's position relative to the perceived wind, the sail produces thrust or drag, and lift. In sailing, the wind perceived by the sail is not parallel to the mean chord. Even if the sail is in a steady mode, it can propel the board by adjusting the mean incidence angle. Few studies investigate the impact of ${\alpha }_m$ on the mean values of aerodynamic force. The goal of this study is to quantify the forces on this configuration by varying the pitching kinematics characterized by the Strouhal number (${\text{St}}_A=fA/U_{\infty }$, with $f$ and $A$ the frequency and amplitude of pumping, respectively, and $U_{\infty }$ the wind velocity perceived by the sail) from 0 to 0.27, and the mean incidence angle ${\alpha }_m$, from 0° to ${30}^{\circ }$, of the foil. The force measurements show a high lift production and the delay of the stall angle according to ${\text{St}}_A$ which can be linked to previous studies about the generation of vortices at the trailing edge. A general trend of decrease is observed for the drag force coefficient in pitching compared to the static case. For the highest Strouhal numbers tested, the drag coefficient can become negative (thrust) in a range of ${\alpha }_m$ up to ${15}^{\circ }$ in specific cases. By using a sport-mimetic approach, we transform the measured lift and drag forces into propulsive and drifting forces, which are the decomposition of the aerodynamic force in the board frame. It is the reference frame in sailing to characterize and optimize physical parameters of the boat such as speed and trajectory. We investigate the impact of unsteady propulsion in upwind conditions. Doing so allows us to investigate race strategies because the generation of propulsion resulting from the drive force is linked to a sideways motion caused by the drift force. We also discuss the impact of using the pumping maneuver rather than the steady propulsion and show several behaviors that could help athletes with decision making.

When a slice of beet is placed on a plate with a thin layer of beet juice, one can observe a clear fringe around the beet, where the color is more translucent than the rest of the juice. The hypotheses in literature were inconsistent and limited, which motivated us to revisit this phenomenon. Using a motorized confocal displacement sensor, we measured the temporal evolution of the liquid surface profile across the fringe. Our findings suggest that a suction flow, induced by the capillary rise of the contact line, causes a dimple—a small concave depression—to form on the liquid surface. While surface tension and gravity tends to smooth out the dimple, viscous drag acts against them if the liquid film is sufficiently thin. Our scaling analysis correctly estimates the dependence of dimple lifetime on liquid properties and film thickness. We also capture the dimple formation dynamics by numerically solving the lubrication equation with the Young-Laplace equation. This work provides an interpretation for a common phenomenon.

This study examines the dynamics of roll waves that spontaneously arise in muddy debris flows, where phase separation is not observed. To this end, a set of bulk mass and momentum balance equations is derived from the depth-averaged model proposed by Meng et al. [J. Fluid Mech. 943, A19 (2022)]. The derived equations are shown to be well-posed, with the threshold Froude number for flow instability slightly exceeding the classical $2/3(1-\mathrm{\Gamma })$, which corresponds to dry granular flows on rough planes. A high-resolution shock-capturing scheme is employed to resolve the evolution of debris roll waves, which emerge from initially steady, uniform flows subjected to sinusoidal perturbations. These waves grow under the influence of inertial forces and eventually saturate into steady, traveling waveforms. A traveling-wave solution is subsequently constructed, accurately capturing the steady traveling waves observed in the simulations. This theoretical framework is further applied to natural debris roll waves observed in the Illgraben torrent, Switzerland. Simulated time series of the flow depth and wave speeds, based on the $\mu \left(I\right)$-dependent basal granular friction law, exhibit excellent agreement with field measurements. Numerical results also reveal that large-amplitude waves, which carry significant mass flux, are dynamically unstable and tend to fragment into multiple smaller-amplitude waves. These smaller waves are overtaken and merged by trailing waves with larger amplitudes and faster propagation speeds, a phenomenon consistent with field observations.

Although it is commonly expected that a metal disk placed on the surface of water will sink, our investigation has revealed a surprising phenomenon: a vertical jet directed onto the disk from above can allow it to remain afloat. This result defies intuition, as one would assume that the force of the jet's impact would cause the disk to sink. We have discovered that this phenomenon occurs as a result of water displacement from the top of the disk caused by the impacting jet, operating through a mechanism similar to the hydraulic jump. This displacement increases the effective immersed volume, resulting in an increased buoyant force that balances gravity. In contrast to the classical case, here the jump radius is fixed by the geometric parameters of a disk, a phenomenon we refer to as the boundary jump. To further explore this effect, we have presented a theoretical model based on scaling laws, which provides the conditions required for the disk to float. The prefactor was determined through an independent experiment. Finally, we conducted experiments on the disk's floating and sinking, which showed a good match with the proposed theory.

Singular or weak solutions of the incompressible Euler equations have been hypothesized to acccount for anomalous dissipation at very high Reynolds numbers and, in particular, to explain the d'Alembert paradox of nonvanishing drag. A possible objection to this explanation is the mathematical property called “weak-strong uniqueness,” which requires that any admissable weak solution of the Euler equations must coincide with the smooth Euler solution for the same initial data. As an application of the Josephson-Anderson relation, we sketch a proof of conditional weak-strong uniqueness for the potential Euler solution of d'Alembert within the class of strong inviscid limits. We suggest that the mild conditions required for weak-strong uniqueness are, in fact, physically violated by violent eruption of very thin boundary layers. We discuss observational signatures of these extreme events and explain how the small length scales involved could threaten the validity of a hydrodynamic description.