Substituting a centrifugal field to gravity makes possible the study of the dynamics of rising deformable drops in another immiscible liquid, keeping free from contamination of the interface. In this experiment, impurities do not have time to adsorb during the very short residence time due to the strong acceleration. Experiments are used to validate direct numerical simulation results in liquid-liquid systems, and the aspect ratio of rising ellipsoidal droplets is found to be a growing function of the sole Weber number, with a smaller rate compared to clean bubbles.

Often considered a childhood pastime, soap bubbles emerged as a captivating domain for rigorous scientific inquiry for generations. While blowing soap bubbles is familiar to everyone, the underlying physics of inflating them remains unanswered. In our investigation, we visualize the previously unexplored internal airflow experimentally, revealing a toroidal vortical flow that resembles a bound vortex ring. The air enters the bubble as a round jet, emerging from the nozzle opening and impinges on the expanding concave interior to form this toroidal vortex. We also predict several scaling laws for the inflation rate and dynamics of this confined vortical flow by varying the source pressure.

We unveil a gyre turbulence regime within a two-dimensional wind-driven ocean model, where energy dissipation becomes independent of fluid viscosity. This anomalous dissipation is driven by a vigorous two-dimensional vortex gas overlaying a low energy western-intensified gyre, shedding light on the effect of boundary instabilities in disrupting the inverse energy cascade.

While heat transport by baroclinic turbulence in oceans and planetary atmospheres is well described by a two-layer model, the relative depth of the two layers varies greatly depending on the situation of interest, making it an important parameter governing the transport properties of the system. Focusing on the low-drag turbulent regime, we extend the vortex-gas scaling theory to address the case of arbitrary layer depths.

In this Letter, we explore the influence of colloids at liquid-liquid interfaces on droplet pinch-off dynamics in microfluidic devices. We uncover a significant deviation in droplet formation time compared to pure systems, similarly to surfactant-laden systems. Yet notably, colloids exert minimal impact on droplet size, indicating potential nonlinear effects. The dynamics of neck thinning without colloids agree with the classic pendant drop scaling laws, while particle presence replaces traditional viscous and inertial-viscous regimes with a single power law, suggesting an elastic behavior driven by soft particle interactions.

The “anomalous” peaks in experimentally obtained power spectral density plots in the wake of wind turbines are investigated with time-resolved volumetric measurements. To promote early tip vortex interaction, blades with different angles-of-attack are used on the same rotor. Using an advanced volumetric technique to obtain the velocity field in the wake, the tip vortex interaction is visualized and quantified. The captured tip vortices corroborate the findings from power spectral density plots at different downstream locations that only one vortex is dominant, demonstrating that a difference in initial vortex strength can result in vortical energy being distributed at unexpected frequencies.

Scraping of a thin layer of viscoplastic fluid from a horizontal surface by a translating rigid scraper generates a mound of fluid upstream of the scraper and a residual layer behind it. We compute numerical solutions for the system modeled via viscoplastic shallow-layer theory. The unsteady dynamics of this system exhibit a variety of self-similar regimes, for which we construct solutions explicitly and identify key scalings for the temporal development of the mound. We further report experimental results, which are compared with predictions from the shallow-layer theory, obtaining reasonable agreement once a slip boundary condition is included in the model.

The dynamics of a free object in an active nematic suspension in a circular container are simulated. For ranges of parameters, unstable chaotic wanderings eventually reach either a fixed-point or limit-cycle (shown) behavior. These flows are analyzed, and similar behaviors confirmed to also occur in more complex geometries.

This paper derives the Enskog equation in the context of orthonormal vielbein fields, allowing the use of arbitrary coordinate systems to describe spatial geometry. Additionally, an adapted coordinate system in momentum space is employed, which is connected to physical space via vielbeins. A suitable finite-difference lattice Boltzmann model is developed and validated against a direct simulation Monte Carlo particle-based method for solving the Enskog equation in curvilinear geometries. The test scenarios include cylindrical Couette and Fourier flow between coaxial cylinders, and spherical Fourier flow between concentric spheres.

Wall modes and bulk modes compete in small-aspect-ratio rapidly rotating Rayleigh-Bénard convection. Wall modes remain robust in the presence of bulk convection and contribute substantially to the global heat transport.

We investigate the inertial and noninertial dynamics of three-dimensional elastic capsules flowing through a square channel presenting a sharp corner. The channel Reynolds number Re ranges from 0.01 to 50 and the Capillary number Ca, which measures the ratio of the viscous and elastic stresses, ranges from 0.075 to 0.35. We report trajectory, surface area, velocity and membrane stress in the case of a single capsule, two capsules and a train of ten capsules released upstream of the corner. This study contributes to the elaboration of practical guidelines for controlling capsule breakup and predicting throughput in both inertial and noninertial microfluidic experiments.

Wall vortex occurs when a cavitation bubble oscillates far from a single rigid wall (at a dimensionless standoff distance γ${}_r$>1.3). This study finds that a wall vortex in an expanded new regime forms instead of a free vortex at a smaller γ${}_r$ value, when introducing a water surface. Criteria for vortex flow patterns are proposed based on the direction of the bubble centroid migration at the beginning of the second cycle $t_c$ though a theoretical model developed with a Lagrangian formulation. Numerical analysis reveals that the wall vortex flow with the influence of the water surface contributes to a greater wall shear stress and larger area, thus increasing the surface cleaning potential.

Exploring how the shape of red blood cells influences their flow properties, this study uses numerical simulations to analyze changes from healthy bi-concave forms to abnormal spherical shapes associated with disorders like spherocytosis. The research reveals complex, non-monotonic relationships between cell shape and flow rate across varying channel widths, and its impact on blood perfusion.

Drops impacting supercooled surfaces adhere to them due to contact line pinning and their solidification. However, distinguishing the influence of each phenomenon on post-impact behavior is challenging since even repellent materials exhibit some drop adhesion. In this study, we examine the impact of water and alkane drops on an omniphobic dry ice surface. We show that the solidification extent within the drop, combined with thermal, elastic, and surface tension forces, dictate outcomes like fragmentation, rebound, or no-bounce. Our findings have critical implications for material design in 3D printing, frost-resistant coatings, and safe biological material transport in cold climates.

Dendrite formation resulting from morphological instability in cathodic electrodeposition of a metal and, especially, the role that related fluid flows play, has long been of major interest to physicists. We focus on the physical mechanisms behind: (1) Underlimiting currents: Selection of electrokinetic-reactive length scale, which is the geometric average of the electric double layer width and the reaction-diffusion length defined as the ratio of cation diffusivity to electrode reaction rate; (2) Overlimiting currents: Domination of emerging electroconvective flow, selecting the cathodic diffusion layer width as dominant length scale for morphological instability and emerging dendrites.

Direct numerical simulations examine the separated flow over a wall-mounted bump subjected to harmonic inflow oscillations, resembling the passage of wakes from the preceding stage along the suction side of low-pressure turbine blade conditions. Three scenarios unfold: (i) analog to the steady inflow where the separated-flow laminar-to-turbulent transition is initiated by self-sustained Kelvin-Helmholtz (KH) instability; (ii) intermittent large vortex cluster formation replacing the KH during a part of the inflow period; and (iii) a continuous vortex cluster formation and release that shorten the separated flow length compared to steady inflow, the desirable one for practical applications.

The iconic branching patterns of the viscous fingering instability, where one fluid invades another in a thin gap, are unconventionally visualized in an experiment that allows us to measure the velocity field throughout the fluid as the patterns form and grow. We find that local pressure gradients decay, both in front of and behind the interface, with a characteristic decay length. This elegant visualization allows us to see how the structure seeded at the interface continues to influence flow long after the instability onset. With this technique we capture features of the late-time flow field that have traditionally been hard to access by simulation or experiment.

A direct numerical simulation of turbulent flow over a backward-facing step is performed. To control flow separation, a wall-normal body force in the form of a wave-machine-like traveling wave is applied to the top surface of the step. In this control, the recirculation bubble is periodically released in time and in the streamwise direction. In addition, a pair of longitudinal vortices is generated above the step and in the recirculation bubble and the released separation bubble, which causes three effects that reduce the reattachment length: a decrease in the secondary bubble streamwise length, an enhancement of the negative wall-normal velocity, and an increase in Reynolds shear stress.

The reciprocal coupled interactions between a forebody gimbaled axisymmetric bluff body, free to precess in pitch, yaw, and roll, and the body’s near wake are exploited for prescribing its attitude using fluidic actuation. It was shown that closed-loop control of pitch and yaw can significantly attenuate the inherent model’s baseline oscillations by more than 80%, or the control could be tuned to achieve a rapid large-amplitude response to amplify the natural yaw or pitch oscillations by more than 225% of the baseline motion. Either suppression or amplification of the natural oscillations of the model are associated with induced changes in the symmetry of the primary dynamical wake modes.

We consider flows subject to a steady symmetry-breaking bifurcation and forced by a weak noise acting on a slow timescale. By employing a multiple-scale weakly nonlinear expansion technique, we derive a stochastically forced Stuart-Landau equation for the dominant symmetry-breaking mode. The probability density function of the solution, and of the escape time from one attractor to the other, are then determined by solving the associated Fokker-Planck equation, which is made possible by the extremely low dimensionality of the amplitude equation. The validity of this reduced order model is then tested on the flow past a planar sudden expansion.

The instability behavior of plane Couette flow is notoriously difficult, because it has no classical unstable modes, and this for any high Reynolds number. Here, the plane Couette flow is modified by means of a constant wall transpiration, i.e. simultaneous blowing from below, which has a destabilizing effect, and suction from above, which has a stabilizing effect. These opposing effects led to a changed in an unpredictable way, i.e. a destabilization at a certain point with increasing transpiration rate, the increase in instability then reaches a maximum and then leads to a slow stabilization again as the transpiration rate increases further. The destabilizing effects clearly dominate here.

To explore the dynamics of annular combustors, we investigate azimuthal thermoacoustic instabilities under a range of hydrogen power fractions and operating conditions. Using time-series analysis and mode detection techniques, we examine the relationship between longitudinal and azimuthal modes, identifying a transition from chaos to high-amplitude periodic states. Our research sheds light on how hydrogen enrichment affects combustor stability and presents the first identification of type-II Pomeau–Manneville intermittency in annular combustors. These findings contribute to knowledge of the modal dynamics within combustors, with implications for the design and operation of future systems.

We integrated theoretical analysis and numerical simulations to investigate the turbulence transition through a crossflow instability in the boundary layer of a cooler rotating disk within a rotor-stator cavity, influenced by a temperature gradient. This gradient induces centrifugal buoyancy forces that alter the radial inflection points in the mean flow. These changes lead to premature bifurcation of spiral waves, crucial in the transition process, resulting in an early onset of turbulence in the boundary layer of the rotating disk. Our findings underscore the importance of manipulating boundary layer stability via temperature gradients to control turbulent transitions.

A viscous drop spreads slowly on a solid at a velocity selected by its viscosity. We show here that liquid-infused solids – a class of materials with properties in between a solid and a liquid – can expedite the spreading dynamics due to interfacial slip, which we investigate at short time.

Liquid infused surfaces (LISs) are a nature-inspired surface technology that demonstrates multiple functionalities under laminar and controlled flow conditions. We study experimentally the behavior of the infused lubricant under submerged conditions and turbulent flow. When exposed to turbulence, the lubricant layer develops into a pattern of droplets, the length of which depends on the balance between shear and contact force. The stability of the droplets prevents complete drainage of the lubricant and increases the robustness of the LIS in the presence of turbulence. We identify a model that predicts the equilibrium length of the droplets and validate it with numerical simulations.

In this paper, we investigate the exit dynamics of a sphere launched underneath a liquid bath surface at a prescribed impact velocity. Spheres with radii approximate or smaller than the capillary length are considered. The process can be sequenced into a partial exit stage that forms a coated layer, and a full exit stage with an attached ligament. A bouncing-off regime, a lower pinch-off penetration regime, and an upper pinch-off penetration regime are identified, separating by a penetration Weber number and a switching Weber number. The phase diagram is revealed, where the two critical Weber numbers are functions of the Bond number.

In this paper a formulation of the incompressible Navier-Stokes equations is introduced which allows one to model boundary-layer flows induced by the motion of a deforming surface. Such a formulation may be used to model flows relevant in a wide variety of industries from polymer processing to glass manufacturing. We show that for particular sheet geometries and velocities, similarity solutions may be obtained that account for sheet thinning (or thickening) and roughness patterns observed in extrusion-type processes.

We report numerical simulations for the transverse flow-induced vibration (FIV) of an elastically supported elliptical cylinder-plate assembly in the regime of low Reynolds number. We investigate the combined effect of the aspect ratio of the elliptical cylinder (namely, AR = 0.5-2), the reduced velocity (namely, $U_r$ = 2-30), and the splitter-plate length (namely, LSP/D = 0.75 and 2.5). Among numerous factors that exert impacts on the assembly’s FIV over an unlimited range of reduced velocity, the aspect ratio determines the nature and width of the synchronization branch in the amplitude response.

Recent advancements in automatic differentiation, which played a pivotal role in deep learning, offer a promising approach to addressing challenges in controlling fluid flow behavior. We demonstrate the power of the method by optimizing the packing of a polydisperse system of periodically arranged circular rods to minimize the pressure drop across the media. We show how the optimum topology of the porous media changes with changing the packing fraction.

We present a numerical study of a thin elastic sheet with small extensibility sedimenting in a viscous fluid in free space or near a wall. The interplay between gravity and the elastic response of sheets gives rise to complex deformation and reorientation dynamics. Near a vertical wall, sheets exhibit asymmetric conformations that cause the sheet to drift toward or away from the wall. Near an inclined wall, sheets show qualitatively different dynamics when the wall angle is large: they either deposit on or slide along the wall with a fixed wall-normal distance.

We investigate high-Reynolds-number polydisperse gravity currents propagating along a channel of general cross-section into a linearly stratified ambient fluid. We formulate and solve numerically the shallow water equations and present typical height and velocity profiles of the current and particle mass concentration. Two dimensionless parameters, Stratification (S) and particle buoyancy (Π), are relevant. Increasing S decreases the current velocity propagation, but as Π increases, the current propagates faster. For a specific S, Π dependence, an equilibrium occurs for a significant time and the system behaves like a system without particles propagating into the ambient of constant density.

We present an asymptotic theory for the dynamics of slender chemically propelled loops and knots. It is valid for nonintersecting three-dimensional centerlines, with arbitrary chemical patterning and varying (circular) cross-sectional radius, allowing many slender active loops and knots to be studied. The theory has closed-form solutions in simpler cases, enabling us to derive the swimming speeds of chemically patterned tori, and the pumping strength (stresslet) of uniformly active slender tori. Using numerical solutions, we find the behavior of exotic active particle geometries, such as a bumpy uniformly active torus that spins and a Janus trefoil knot, which rotates as it swims forwards.

500 resolved particles, colored by their temperature, are suspended in Rayleigh-Bénard convection at a Rayleigh number of ${10}^7$. The lines are streamlines colored according to the fluid vertical velocity. Near the cell bottom, the fluid circulation pushes the particles from the base of the descending to that of the ascending plume where they accumulate into a dune. The particles that follow are dragged up the dune acquiring a vertical velocity component which promotes their resuspension. The lift force plays no role in this process. Depending on the particle number (from 500 to 3000) up to 20% of the fluid gravitational energy can be transferred to the particles.

Numerical simulations of wet particles often use the particle roughness as a minimum separation criterion to limit the viscous force. Here we investigate the validity of this through a comparison of experiments of binary wet particle collisions to numerical discrete element method (DEM) and smoothed particle hydrodynamics (SPH) simulations.

This work studies the rheology of dense granular media, exploring the effects of varying particle size, density, friction and shear profiles across different flow regimes. Utilizing the discrete element method (DEM), the research extends current models by integrating volumetric contributions and introducing a new power-law scaling that unifies local and nonlocal rheology data onto a single master curve. This approach bridges the μ(I)-rheology and Kinetic Theory, offering a framework for predicting the behavior of granular flows in various settings, from geophysical flows to industrial processes.

Self-propelled (active) particles in laminar fluid flows are blocked by one-way barriers called “swimming invariant manifolds” (SwIMs). The SwIM theory is a generalization of manifold approaches previously applied to the mixing of passive tracers. We verify the blocking behavior of SwIMs experimentally for algae swimming in a vortex chain flow. The SwIMs also form chutes that transport swimmers between vortices with a predicted inter-vortex flux that is consistent with the experiments, despite noise and nonuniformities in the swimming directions of the microbes.

The turbulent transport of small particles (dust particles, cloud droplets, etc.) can be viewed macroscopically as a problem of passive scalar turbulence with extremely high Schmidt numbers. To investigate the spectrum of a passive scalar carried by such particles, we conducted Lagrangian particle simulations in homogeneous isotropic turbulence. The scalar variance spectrum obeys the -5/3 and -1 power laws in the lower and higher wavenumber ranges, respectively, with a clear transition at normalized wavenumber of about 0.04. The dimensionless constants for each range are also consistent with the estimations obtained from previous laboratory and simulation studies.

An algorithm is developed, rigorously justified, and numerically implemented that is capable of determining the full body force used to generate chaotic, turbulent dynamics in two-dimensional Navier-Stokes fluid dynamics. The primary contribution of this result is that the accurate reconstruction of the force requires only partial observation of the state, i.e. sparse observations of the state are sufficient to recover not only the state itself but the unknown forcing function as well even in the fully developed turbulent setting.

We carry out a numerical study of swept shock wave/turbulent boundary layer interaction (SBLI) in the hypersonic regime, where a crossflow velocity component is added to the incoming flow to mimic three-dimensional interactions with cylindrical symmetry. The spatiotemporal dynamics of wall pressure well conform with the previously introduced formula for swept supersonic interactions, extending the validity of the model to SBLIs in the hypersonic regime.

The effects of density variations on structures developing in an isotropic incompressible turbulence flow are investigated. Statistical analyses are carried out on datasets obtained from direct numerical simulations of forced turbulence. Numerical evidence shows that the introduction of a variable-density field into a turbulent field modifies the coherent structures and the energy spectrum in the inertial range.

Kolmogorov scaling is used to derive a model for the structure function constant associated with index of refraction fluctuations in Rayleigh-Benard turbulence. The model predicts that the normalized structure function constant depends on the heat flux to the four-thirds power, and is independent of the Rayleigh number. The model agrees with the results of numerical simulations, thereby lending support to the assumptions underlying the theory.

The space-time correlations of both wall-shear fluctuations and the streamwise velocity fluctuations carried by wall-attached eddies are investigated in a multiscale manner, by coupling the inner-outer interaction model (IOIM) with the attached eddy hypothesis. The present results demonstrate that the space-time correlations for the wall-shear stress fluctuation are mainly dominated by near-wall small-scale motions, and wall-attached eddies at a given length scale feature distinctly different space-time properties as compared to those of ensembled eddies with multiple length scales, which provides a new perspective for analyzing the decorrelation mechanisms in turbulence theory.

To investigate the existence of geometrically self-similar eddies in fully developed turbulent pipe flow, stereoscopic particle image velocimetry measurements were performed in two parallel cross-sectional planes, for friction Reynolds numbers Re${}_{\tau }$ = 1310, 2430, and 3810. The instantaneous turbulence structures are sorted by width using an azimuthal Fourier decomposition, then azimuthally aligned to create a set of average eddy velocity profiles. The streamwise similarity is investigated using two-point correlations. Over the range of scales examined, the candidate structures establish full three-dimensional geometric self-similarity.

We present a self-similarity analysis of single-point turbulent statistics across different quadrants in turbulent wakes. We show here that within the wake self-similar region, the distribution of the Reynolds shear stress in different quadrants can also attain a state of self-similarity. The length scaling is the same for the Reynolds shear stress and its different quadrant contributions, while there exists a difference in velocity scaling. There exists a strong connection between ejection events and large-scale coherent structures, as well as deceleration extreme events.

This article presents the first ever systematic demonstration of the drag reduction at supersonic speed regime caused by non-rigid surfaces prepared using compliant viscoelastic coating. This work lays the foundation to a new engineering paradigm that fuses engineered surfaces to create positive aerodynamic outcomes at speeds that are relevant to aerial vehicles.

We report a new Mach number invariant function for the mean temperature field in compressible wall turbulence. We demonstrate its validation by comparing it with the invariant functions derived in the previous studies, i.e., the semi-local-type and van-Driest-type scalings, case by case. The newly proposed temperature transformations based on the new scaling show an improvement in channel flows over adiabatic walls and supersonic/hypersonic turbulent boundary layers with cold walls. The effects of the generated high-order terms during derivation are also clarified. These findings may be revealing for the development of the near-wall model in high-speed aerodynamics.

The transportation of oil through pipelines and channels is a highly energy-intensive operation, primarily due to the significant viscosity of the oil and the consequent high friction. Among the various friction reduction methods utilized in this domain, the water-lubricated approach has emerged as particularly promising. Our investigation focuses on assessing the efficacy of this technique through direct numerical simulations of turbulent channel flow. In this setup, we introduce two thin water layers near the walls, which serve to lubricate the flow of oil within the core.

The interaction between a vortex and an impacting body is complex due to the interaction of inviscid and viscous mechanisms. We conduct the first three-dimensional direct numerical simulations of this process and vary the relative impact velocity of the cylinder to explore the parameter space and analyze this process in detail. Strong vortices lead to ejection and interaction of secondary vorticity from the cylinder’s boundary layer, while weak vortices lead to approximately inviscid interaction of the cylinder with the primary vortex through deformations.

We present an experimental and theoretical study of interfacial waves generated in a two-phase swirling flow by a low-frequency (1 - 10 Hz) rotating magnetic field (RMF) oriented parallel to the interface. In contrast to surface waves excited by axial magnetic fields, we find that the first and dominant wave mode resembles a hyperbolic paraboloid. A good agreement of experiment with a linearized model was obtained. These results can have important implications for metallurgical processes and point the way to further research in the dynamics of the swirl flow, particularly by extending into the nonlinear regime.

Super-currents, tunneling across insulators in Josephson junctions, have a one-to-one classical analog to action-at-a-distance between two interfacial Rossby waves in shear flows. Quantum avoided crossing between eigenstates, described by the Klein-Gordon equation, is obtained as well for the Rossby wave normal modes. Both the quantum and the classical dynamics are formulated as coupled two-state systems and presented on a Bloch sphere, where the Hadamard gate transforms the two normal modes into an intuitive computational basis of two single Rossby waves. Yet, lacking analogs to quantum collapse and entanglement, the Rossby wave system cannot serve as a qubit prototype, even in principle.

We examine the influence of wind forcing on the inception of breaking in surface gravity waves using an ensemble of high-resolution numerical simulations. We find that there is a critical point in the energetic evolution of the wave in which the convergence of kinetic energy at the wave crest can no longer be offset by conversion to potential energy, resulting in a rapid growth of kinetic energy up to breaking onset. This energetic signature is shown to consistently differentiate between non-breaking and breaking waves under a range of wind forcing speeds.