We uncover and study a physical process that is hidden in the healthy lung. This process can be triggered by the thickening of the thin layer of mucus lining the bronchi walls. At the air-mucus interface, surface tension and curvature can combine to oppose the physiological motion of mucus induced by cilia, causing mucus to stall or, even, move in reverse. Representing mucus as a Bingham fluid, we characterize analytically the fluid dynamics in the thin layer using dimensionless quantities and employ numerical simulations to compute the layer velocity field in airways bifurcations. This work sheds new light on the intricate workings of the respiratory system.

Permafrost thaw plays a crucial role in climate change dynamics, yet the intricate small-scale processes driving thawing remain inadequately understood. Here we reveal that the density anomaly of water has the potential to trigger convection in coarse-grained soils above permafrost. By using high-resolution numerical simulations of the primitive equations in porous media we show that convection significantly accelerates the thawing process. Climate models that predominantly account for thawing induced by diffusive processes alone overlook this aspect, which can self-reinforce and lead to enhanced climate-permafrost feedback.

Pendant drops spontaneously appear on the underside of wet surfaces through the Rayleigh-Taylor instability. Here, I show that such pendant drops can get stuck on topographic defects, despite having no contact line. Unlike traditional pinning, the trapping force has a gravito-capillary origin: liquid has to move up or down and the interface has to deform for the drop the pass the defect. I model this topography induced force, demonstrate how to harness it to guide pendant drops, and expect it to be relevant to other contact line free systems.

Impulsively generated focused jets play significant roles in advanced manufacturing and biomedical applications, where the viscous effects are crucial in jet dynamics. In this study, we find that mass and momentum transfer along the tangential direction of the free surface contribute to focused jet formation. The viscosity-induced diffusion of the shear flow and vorticity near the free surface reduces the jet speed. These findings offer new perspectives on viscous interface dynamics.

Direct Numerical Simulations (DNS) are among the most powerful tools for studying turbulent flows. Even though the achievable Reynolds numbers are lower than those obtained through experimental means, DNS offers a clear advantage: the entire velocity field is known, allowing for evaluating any desired quantity. One is the indicator function, which is crucial for understanding inner and outer interactions in wall-bounded flows and describing the overlap region between them. We find a clear dependence of this indicator function on the mesh distributions we examined, raising questions about classical mesh and convergence requirements for DNS and achievable accuracy.

The reason for the deviation of measured generalized Townsend-Perry (T-P) constants from the previous Gaussian prediction based on the attached eddy model is found and the universal expression of the generalized T-P constants, regardless of the eddy type, is derived. These findings may contribute further insight into the fundamental statistical characteristics of coherent motions in flow fields and provide good indications for testing the accuracy of numerical simulations in high-Reynolds-number flows.

In biological fluid dynamics, as studied in the lab or in natural environments, it is often the case that organisms swim in a thin fluid layer above a bottom no-slip surface and below an upper stress-free interface. Here we examine in this “Petri dish” geometry the image system for the elementary and composite singularities of Stokes flow: the Stokeslet, rotlet, source, rotlet dipole, source dipole and stresslet. As an application of these results, we reconsider the problem of hydrodynamic bound states of spherical microswimmers in finite-depth chambers.

Carotid artery disease is a significant contributor to mortality in the United States. The role of internal vortical structures in enhancing pro-atherosclerotic wall shear stresses and how they differ between healthy and disease-prone patient cohorts has not been studied. This study revealed an important vortex which forms in the cardiac cycle at a time primarily dictated by bifurcation geometry, whereas its lifespan is determined by flow conditions, such as pressure drop and flow rate. We found that high internal carotid artery mass flow rate and a high favorable pressure gradient maximum occurring near peak systole are strong indicators of a greater pre-disposition towards atherogenesis.

The substantial rise in fluid temperature behind the shock at the head of hypersonic vehicles induces molecular internal energy excitation and dissociation, potentially resulting in thermochemical nonequilibrium flow. The high vibrational energy molecule of oxygen and nitrogen undergo overdistribution and underdistribution states, resulting in increased and decreased dissociation rates in nonequilibrium flow. The latter state dominates and notably increases fluid temperature and wall heat flux, impacting the vehicle’s thermal protection design. This study proposes an improved two-temperature dissociation rate model to more accurately simulate the nonequilibrium flow behind the shock.

In this paper, we first examine the wave morphology induced by reactant thermal stratification in hypersonic reactive flows. Three flow regimes are observed: autoignition-driven reaction front, detonation wave, and decoupled shock/reaction front. The results suggest that local autoignition behavior can be significantly influenced by reactivity nonuniformity in the reactive flow; and an undesirable detonation wave can be initiated given a suitable temperature gradient. This also indicates that oblique detonation waves can be generated without mechanical devices such as blunt bodies and wedges.

This study reports a novel dynamic in shear-thickening suspensions with a free surface under bottom shear, revealing localized, persistent jammed clusters (bumps in the image). We clarify the essential role of shear-thickening properties and the effect of suspension depth, showing that at small depth this heterogeneous state could occur below the minimum density of discontinuous shear-thickening. Moreover, our findings emphasize the critical role of free surfaces in cluster growth. This advances our understanding of the evolution of heterogeneity associated with shear-thickened states.

Using lattice Boltzmann simulations and an analytical framework based on far-field approximations and method of images, we study the trajectories of microswimmers inside three-dimensional channels of square and rectangular cross-sections. We find that pusher-type microswimmers move helically inside the square tube, weak pullers slide through the center of the channel while strong pullers exhibit a trajectory which is off-center. The trajectories of the neutral swimmers are challenging to generalize due to the sensitivity to the initial conditions. Finally a method of superposition is used to construct three dimensional trajectories, thus explaining the origin of their apparent complexity.

Viscoelastic fluids may exhibit complex flow behaviors, such as nontrivial normal stress differences and time-dependent responses, which can significantly impact the settling dynamics of suspended particles. We investigate particle-wall interactions for the case of a sphere moving close to and normal to a wall in an Oldroyd-B fluid, considering both prescribed velocity and force scenarios; the lubrication approximation and Deborah number perturbation techniques are used.

We investigate an ice block melting in a box and reveal that the aspect ratio and ambient temperature play a crucial role in the melting process, leading to substantial variations in melt rates and shape evolution. In general, the shape which melts the slowest is quite distinct from that of a disk, due to the symmetry breaking by buoyancy-driven thermal convection. Predicting their melting rates is crucial for understanding the interplay between melting icebergs with the climate.

Through experiments and scaling analysis, we describe the deformation of an impacting drop at low impact Reynolds number. We focus on deformations driven by impact inertia and limited by viscous dissipation only. The main result is a universal power-law dependence of the nondimensional contact diameter in the maximum deformation state as a function of the Reynolds number, when the latter is smaller than 1. This specific case was not described before in the literature, despite its relevance in earth sciences for example.

When a drop impacts next to the edge of a solid substrate, it spreads beyond the edge and forms a liquid sheet surrounded by a rim. As the rim decelerates, ligaments may form, then destabilize in droplets. This fragmentation scenario has been extensively investigated in the axisymmetric configuration of centered impacts on small circular targets. In this work, we investigate star-shaped targets. We show that the rim is shaped complementarily to the substrate: it goes farther and yields more droplet ejections in directions corresponding to troughs in the substrate edge profile.

The presence of platelets near vessel walls is crucial for clot formation to stop bleeding. We examine platelet margination influenced by red blood cell (RBC) aggregation through numerical simulation. Our findings indicate that moderate to strong RBC aggregation enhances platelet margination in microcirculation, thereby improving the capacity to stop bleeding. This mechanism offers a natural counteraction against major bleeding in conditions such as diabetes, where strong RBC aggregation is commonly observed.

We report a high throughput strategy to produce gel microspheres. This method is based on the controlled fragmentation of an aqueous jet in air that results in droplets of monomer solution, their entry and collection in an oil bath containing a catalyzer and surfactants, followed by polymerization of the emulsion droplets which thus turn into gel beads. Adjusting the impact area of the stream of droplets at the free surface with an electric field allows to minimize coalescence of droplets as well as mass transport between the droplets and the continuous phase which is correlated to the sedimentation flow features of the dilute emulsion.

Droplet impacting on rotating surface experiences the tangential shear force from the rotating surface, generating a centrifugal force that either enhances the spreading or destabilizes the expanding lamella. In this study, we experimentally characterize the impacting of droplets with a wide range of viscosity on rotating surfaces with various wettabilities, and theoretically analyze the observed impacting dynamics, including the enhanced spreading and the transition to the destabilization of the expanding lamella. We propose a simplified approach to predict these key parameters, and validate the theoretical prediction by experimental measurement.

This study investigates the key stages before aerosol formation by bursting bubbles: capillary wave propagation, convergence at the bubble apex, Worthington jet ascent, and droplet release. We focus on overlooked factors: surface-active agents and gravitational effects, quantified by the Bond number. Our results propose a new mechanism explaining capillary wave retardation in surfactants, involving the transition from bi- to uni-directional Marangoni stresses, which counter wave motion. We also elucidate the varying wave velocities with different surfactant properties, challenging the constant velocity observed in clean interfaces.

We illuminate effects of surface-charge convection intrinsic to leaky-dielectric electrohydrodynamics by analyzing the symmetric steady state of a circular drop in an external field at arbitrary electric Reynolds number ${\mathrm{Re}}_E$. In particular, we characterise the development of an equatorial charge-density blowup singularity at moderate ${\mathrm{Re}}_E$ (in the case where charge relaxation is slower in the drop phase than in the suspending phase) and the formation of polar stagnant caps at large ${\mathrm{Re}}_E$ (in the opposite case)

A circular surface wave (CSW) of a low-temperature gallium alloy in the immovable cell with a central bottom electrode and an upper ring electrode exposed to axial magnetic fields is studied experimentally. The mechanism which provides the existence of a stable CSW is suggested. It is determined that, depending on the force parameter and geometrical characteristics of the cell, three modes can occur in the cell: rest, CSW, or axial rotation with a deep funnel on the surface providing the circular contact of the liquid metal with the electrode. A mode map showing the boundaries of the CSW existence domain is plotted on the parameter plane.

Electroconvection (EC) turbulence is an important branch of electrohydrodynamics (EHD). This work applies large-eddy simulation (LES) to EHD turbulence based on the lattice Boltzmann method. The Smagorinsky and WALE models are used for the momentum equation, and the turbulent Schmidt number models the charge transport equation. The results are compared with those computed through different numerical models. Both two-dimensional and three-dimensional numerical tests show that the LES models applied in EHD turbulence simulation perform reliable results at high computational speed, making them suitable for further simulations of EHD turbulence.

Is transient growth critical for boundary-layer stability in fluids at supercritical pressure with their strong property variations across the pseudo-boiling line? This study on non-modal growth and its competition with modal growth under various heat-transfer scenarios reveals new insights into instability mechanisms in non-ideal fluids. We identify optimal growth in streamwise-modulated streaks as the fluid is heated beyond the pseudo-boiling line, with transcritical Mode II responsible. In this scenario, transition below the critical Reynolds number might prevail, mimicking the effect of an adverse pressure gradient in the ideal-gas regime.

When a jet of fluid emerges into an ambient medium of the same density but higher viscosity, the dominant mode of instability transitions from axisymmetric at low viscosity ratio M to a helical mode at high M. It is shown that these helical modes are unstable global modes associated with enhanced mixing and the emergence of a single dominant frequency observed everywhere in the near field. These frequencies align well with predictions of absolute instability from spatiotemporal linear stability theory.

In the present study the axisymmetric flow in an aspect ratio R/H=10 cavity is revisited. The base state is shown to lose stability in a supercritical Hopf bifurcation. Branches of periodic and chaotic self-sustained solutions are computed using harmonic balance method and time integration. In addition, edge states separating the steady laminar and chaotic regimes are identified using a bisection algorithm. All the self-sustained solutions found are shown to exist only for a high enough Reynolds number and are therefore disconnected from the experimentally observed circular rolls.

We study the role played by upstream-traveling guided waves on the generation of tones by a supersonic, ideally-expanded round jet impinging on a flat plate. We explore a finite-thickness stability model for the prediction of allowable frequency ranges for resonance based on the dynamics of such waves. We show that the frequency ranges predicted by the finite-thickness model are consistent with the vast majority of experimental and numerical data available in the literature, correcting discrepancies observed previously with vortex sheet models. This provides further evidence for the involvement of guided jet modes in the resonance mechanism.

Baroclinic instability plays an important role in rotating stratified fluid systems, such as Earth’s atmosphere and ocean, and can be modeled as a pair of constructively interfering edge-waves. We provide a self-consistent explanation of how the edge-wave interaction mechanism is modified in the presence of slopes, extending existing simple but incomplete explanations. We also put forth speculations on using linear instability theory to inform eddy parameterizations, and highlight parity-time symmetry in the governing equations, finding links between shear instabilities, interacting edge-waves, eddy-mean interactions, and concepts from quantum field theory.

The experimental violation of Bell’s Theorem in quantum systems is generally taken as proof of the impossibility of local hidden variable theories, as would provide a rational dynamical underpinning for quantum mechanics. We here present a platform for executing Bell tests in pilot-wave hydrodynamics, a system known to capture many features of the quantum realm. We execute a static test in a bipartite tunneling system, and rationalize the emergent violations in terms of the wave-mediated coupling between the two subsystems.

We examine the linear thermocapillary instability of a two-dimensional surfactant-laden gravity-driven shear-imposed film flowing over a uniformly heated inclined wall. We observe that the surfactant Marangoni number stabilizes, while the thermal Marangoni number destabilizes H-mode, S-mode, P-mode, and shear mode. These opposing impacts establish an analytical relationship between surfactant and thermal Marangoni numbers, for which the critical Reynolds numbers for the H-mode instability of the non-isothermal and isothermal film flows coincide. When comparing results with and without inertia, we predict that inertia stabilizes surfactant mode.

The varicose dynamics of gravitational liquid sheet flows issuing into a quiescent gaseous ambient is relevant for technological applications such as coating deposition, where varicose perturbations of the curtain shape can arise due to velocity fluctuations coming from the delivering pump placed upstream of the coating die. We investigate this problem using a one-dimensional linear model and two-dimensional nonlinear simulations, finding novel scaling laws for the curtain oscillation frequency and amplitude with the Weber number. Moreover, an interesting numerical break-up phenomenon of the curtain is outlined, driven by the progressive curtain thinning induced by the varicose disturbances.

Our study experimentally investigates the morphology and breakup of a droplet descending into an airstream, analyzing child droplet size distributions through high-speed shadowgraphy and in-line holography. We found that varying the droplet’s release height results in different breakup modes — from vibrational to retracting bag-stamen breakup at the same Weber number — each with distinct size distributions. Our theoretical model, which incorporates the effective Weber number, accurately predicts these distributions, underscoring the critical impact of droplet dynamics and aerodynamic interactions on breakup behavior.

We present a theoretical model for viscous liquid systems exhibiting Rayleigh-Plateau instability, considering cases with and without a solid fiber. Using the lubrication approach and hydrodynamic interactions at the solid-liquid interface, we derive one-dimensional evolution equations for the breakup of viscous liquid threads and films on fibers. Our model aligns well with experimental results, unifying Goren’s liquid film on a fiber and Rayleigh’s viscous liquid thread findings. It identifies the most unstable mode as proportional to the wavenumber quadratically and reveals the exponential decay of satellite droplet volume with increasing wavenumber.

Beyond a threshold speed, the receding contact line of a partially wetting liquid on a solid substrate becomes unstable, forming a corner. At these speeds, one expects the inertial effects near the contact line to be significant for many liquids commonly used in industrial processes. To account for it, we provide the self-similar leading-order inertial correction to the well-known Stokes solution for the interface shape and flow field near the moving contact line. Furthermore, inspired by recent experiments, we make quantitative predictions for water and liquid mercury and argue that it is essential to consider inertial contributions when modeling fast-moving contact lines.

The dynamics of flexible filaments entrained in flow are important for understanding many biological and industrial processes. This work describes a data-driven technique to create high-fidelity low-dimensional models of flexible fiber dynamics using machine learning; the technique is applied to sedimentation in a quiescent, viscous Newtonian fluid, using results from detailed simulations as the data set. Over a wide range of fiber flexibilities, the filament shape dynamics can be represented with high accuracy with only four degrees of freedom.

Our study investigates the entrapment of air bubbles beneath micrometer-sized particles upon immersion, employing theoretical, computational, and experimental approaches. We reveal that the wettability of both particles and substrates significantly influences the adhesion forces due to trapped bubbles. Our findings, validated through experiments, highlight that hydrophobic surfaces increase the likelihood of bubble entrapment, thereby enhancing particle detachment forces. This work offers valuable insights for various industries, such as food processing, where optimizing surface coatings could improve cleaning efficiency and promote greener practices.

Cohesive force chain networks during a granular collapse process. Such networks can be identified by means of community detection approaches.

Miscible multiphase flow in porous media is a key phenomenon in various industrial and natural processes, such as hydrogen storage and geological carbon sequestration. We experimentally find that inlet pressures and heterogeneity levels control the miscible displacement of high-viscosity glycerol by low-viscosity water and their mixing due to shear forces, by transitioning from uniform to fingering patterns at the pore scale. We derive a non-dimensional modified Sherwood number that links these microscale patterns to physical properties like velocity distribution, diffusion, and viscosity contrasts; thus linking pore-scale dynamics with macroscale Darcy-scale observations.

Using a Reyolds shear stress model in the boundary layer equations for flow over a flat plate, the streamwise velocity ($U$) is related to the wall-normal velocity ($V$). The empirical power-law $U\sim {\eta }^{1/n}$ for the streamwise velocity is derived theoretically, followed by a power-law for $V$.

The energy cascade in turbulence is explored through a novel approach in which the vortical structures of a certain scale in the inertial subrange, colored by yellow, is forcibly suppressed. Their disappearance leads to a slight increase in kinetic energy in the larger scale range and a decrease in the smaller scale range. The vortex-tracking analysis reveals that the vortices twice as large as the target, colored by blue, exhibit smaller curvatures and longer lifespans, while no remarkable changes are found for the vortices that are four times larger, colored by red. These findings indicate that the larger vortices are locally bent by the smaller vortices, shortening their lifespans.

The perturbation dynamics in a supersonic shear layer are characterized using large-eddy simulations and linear-operator-based input-output analysis. The Kelvin-Helmholtz instability emerges as the primary mechanism for disturbance energy amplification. To identify feasible actuator placement locations, we conduct a state-space restricted input-output analysis, revealing the splitter plate trailing surface as the most receptive region. Furthermore, simulations with applied forcing demonstrate that the coherent structures predicted by linear analysis remain active within a highly nonlinear turbulent flow.

The implicit U-Net enhanced Fourier neural operator (IUFNO) combines the loop structure of implicit FNO (IFNO) with U-Net, leading to enhanced long-term predictive ability in the large-eddy simulations (LES) of turbulent channel flow. It is found that the IUFNO outperforms the traditional dynamic Smagorinsky model (DSM) and the wall-adapted local eddy-viscosity (WALE) model at coarse LES grids. The predictions of both the mean and fluctuating quantities by IUFNO are closer to the filtered direct numerical simulation (fDNS) benchmark compared to the traditional LES models, while the computational cost of IUFNO is much lower.

This paper addresses the artificial bottleneck effect in large-eddy simulations (LES), causing erroneous energy spectrum overshoots due to residual stress model inaccuracies. We use Stokes Flow Regularization (SFR) to improve LES models with detailed residual kinetic energy considerations. Adding a nonlinear gradient component to the residual stress closure accurately captures local stress structures, reduces kinetic energy over-predictions, and enhances energy cascade representation. The mixed model produces vortex tube-like structures similar to those in filtered direct numerical simulations (DNS), while the eddy viscosity model yields shear layer-like features.

Wave breaking is recognized as one of the most violent air-sea interaction processes, significantly enhancing the transfer of heat, mass, and momentum between the oceans and the atmosphere. In this study, we investigate heat transfer in wind turbulence over breaking waves using direct numerical simulation, with a particular focus on the effects of wave age. Our findings suggest that temperature responds in a more complex way to wave age than velocity does, emphasizing the need to incorporate this phenomenon into air-sea interaction and weather forecasting models.

During floods caused by a continuous increase in river flow, the water level often rises suddenly, while the recession takes much longer. This behavior is consistent with the subcritical instability highlighted in this article. This instability emerges for uniform, quasi-stationary flows at high Reynolds numbers in a channel. With increasing flow, a sudden jump in water level occurs when a well-defined flow rate is reached. If the flow rate subsequently decreases, the water level drops again suddenly, but at a flow rate well below the previous one.

The presented methodology fuses computational models with experimental data to enhance the Spalart-Allmaras (SA) turbulence model for Reynolds-averaged Navier-Stokes equations. By leveraging the Ensemble Kalman filtering approach (EnKF), this study refines the SA model’s coefficients, ensuring improved performance on separated flows without any accuracy trade-off on flows already well captured by SA. Validated on different flow conditions, including a backward-facing step and a NASA wall-mounted hump, the recalibrated model demonstrates significant improvements in key metrics.

This study proposes a machine-learning solution to address the log-layer mismatch problem that occurs when the erroneous near-wall flow variables are used as the input for the wall model in a wall-modeled large eddy simulation. Neural networks are employed to correct the errors in the near-wall flow variables before they are used as the input for the wall model. The input and output features of the neural networks are selected based on the physical relations of the turbulent boundary layer for robustness against various Reynolds and Mach number conditions. The proposed neural networks enable the wall model to accurately predict the wall shear stress and the resultant turbulence statistics.

Turbulence is ubiquitous in our world, attributed to diverse mechanisms that may act alone or together. Traditionally, inspired by the reductionist approach, the dominant turbulence-inducing mechanism has been isolated and minor ones neglected. However, are the minor factors truly negligible? Recent research, grounded in conservation of kinetic energy and multicomponent scalar variance fluxes, suggests otherwise. Even weaker mechanisms, such as electric body forces, can significantly influence the cascades of turbulence in the presence of a dominant mechanism like buoyancy-driven turbulence, leading to the emergence of new scaling indices and diverse observations in real-world turbulence.

We investigate the effect of rotation on the two-dimensional boundary layer on a ridge in high Reynolds number flow using numerical simulations to consider both shallow and deep flows as well as flow past a horizontal cylinder, motivated by experimental results by Machicoane et al. In all cases the boundary layer remains attached, even at large Reynolds numbers (Re), provided the Rossby number (Ro) is sufficiently small. At larger Ro, the flow detaches at sufficiently high Re to form a steady recirculating region in the lee of the ridge; at even higher Re no steady flow is found. The figure shows streamlines at Re=256000 for Ro=1.2, 1.5 and 2.