We present a Lagrangian analysis of turbulent blood flow in the human left heart using high-fidelity simulations based on a patient-specific anatomical model. Leveraging a fully coupled fluid-structure-electrophysiology interaction framework, we track the motion of Lagrangian (passive) tracers to investigate the multiscale statistical properties of velocity fluctuations over more than 4 orders of magnitude. Our analysis reveals strong Lagrangian intermittency throughout the left heart, reflecting the complex and unsteady nature of cardiovascular flow. The present work underscores the sensitivity of Lagrangian statistics to physiological parameters and highlights their potential for improving the understanding of pathological flow conditions in cardiovascular systems. Such Lagrangian tool provides a statistical foundation for modeling shear-induced damage in red blood cells (hemolysis), with implications for the evaluation of prosthetic valves and blood-contacting medical devices.

We present an experimental study of sprays produced from microfabricated spray nozzles as a function of nozzle geometry, flow rate, and ambient pressure. Our findings reveal that spray cloud formation is driven by friction with the surrounding air, resulting in a conical structure whose spatial extent scales as the square root of time. We identify a critical pressure threshold below which the characteristic fanning out of spray clouds, attributed to the Kelvin-Helmholtz instability, fails to occur.

Thermally driven flows have been extensively studied using the canonical Oberbeck-Boussinesq (OB) approximation, which treats material properties as constant except for the buoyancy force, where small density variations are fundamental to driving motion. Yet the validity of this approximation in coldwater systems remains largely unexplored. In such environments, density variations associated with a given temperature difference are relatively small due to the nonlinear equation of state. In contrast, variation in dynamic viscosity and thermal conductivity remains moderate, comparable to those in warmer systems. Here we numerically investigate the effects of temperature-dependent material properties in two canonical configurations of ice-bounded waterbodies: horizontal and vertical convection. Our results show that variable material properties remain dynamically important and significantly affect global transport quantities in quasi-steady states. In particular, we identify anomalous dynamics under specific conditions, in which the dominance of cold and warm circulations completely reverses. This anomaly leads to up to $O(10\%)$ of over- or underestimation in global quantities, including the water-to-ice heat flux, which controls systems' energy and ice melt rates. Although the relative importance of the variable material properties depends on domain size and boundary conditions, their persistent influence in both local dynamics and global quantities underscores their non-negligible and, sometimes, pivotal effects in cold aquatic systems. We therefore conclude that the conventional OB approximation may lead to misinterpretation of the dynamics of cryospheric water bodies by excessively simplifying the underlying thermophysical variability.

The injection of an elongated bubble into a microchannel is a well-established strategy for enhancing convective heat transfer, with applications ranging from electronics cooling to miniaturized heat exchangers. So far, a rigorous characterization of the heat removal capabilities of elongated bubbles (also known as Bretherton's or Taylor's bubbles) is still lacking. To address this gap, we investigate the forced convection problem in the thin film formed by an elongated bubble under uniform wall heat flux, examining the competition between advection, diffusion, viscous dissipation, and an imposed heat flux at the channel walls. By means of two-scale asymptotic analysis, we derive a one-dimensional advection-diffusion–heat-transfer equation with shape-dependent effective coefficients. This model generalizes the classical Graetz problem to capillary-driven flows and extends the Aris-Taylor dispersion to the case of an elongated bubble, clarifying the interplay between the imposed heat flux and the recirculating flow patterns at both front and rear menisci. Interestingly, the model recovers the Péclet-squared scaling of the effective diffusion coefficient and can be used to determine the heat transfer coefficient in the film region. In fact, we derive a closed-form scaling law for the Nusselt number, revealing the influence of the problem parameters (i.e., the bubble profile and the axial temperature gradient) and the dimensionless groups (Péclet, Brinkman, and capillary numbers) on forced convection. Grounded in first principles, our analysis contributes to a deeper understanding of capillary-driven forced convection in confined environments.

Gravity-driven geophysical flows, such as landslides and avalanches, pose significant hazards due to their instability and destructive potential. A mechanistic understanding of the obstacle-granular interaction processes and their coupled effects on deposition dynamics is crucial for designing engineered barriers. This study investigates the postimpact evolution of downslope granular flow and its subsequent deposition dynamics on horizontal base within a two-dimensional channel configuration. Through laboratory experiments and particle image velocimetry measurements, we identify a “third jump” phenomenon arising from late-stage lateral particle convergence. Furthermore, we demonstrate the complete elimination of the granular vacuum zone behind the obstacle, a phenomenon driven by subcritical basal flow and the low hysteresis of glass beads, which allows particles to “creep” into the void. Our results reveal a fundamental decoupling between localized obstacle-induced disturbances and global deposition dynamics, which remain primarily governed by the initial mass. We provide robust scaling laws for deposit height, run-out distance, and spreading velocity, advancing the predictive framework for geophysical hazard assessment beyond existing unobstructed flow models.

We develop a nonlinear model for electroosmotic transport through a constricted microchannel with a rigid curved upper wall and a compliant lower wall. The framework couples Helmholtz-Smoluchowski slip under a globally constrained electric field, lubrication hydrodynamics, quasistatic Kirchhoff-Love wall bending, and Derjaguin-Landau-Verwey-Overbeek disjoining pressure. The response is governed by wall stiffness, geometric curvature, surface conduction, and the strengths of the repulsive and attractive intermolecular interactions. Asymptotic analysis and fully coupled Chebyshev-collocation computations reveal three regimes: a stiff-wall regime with negligible deformation, a compliance-limited regime in which localized throat narrowing suppresses throughput, and a small-gap saturation regime in which further thinning becomes progressively slower because of the coupled elastic, hydrodynamic, and disjoining-pressure response. In the strong-constriction limit, geometric focusing amplifies the electric field and produces a sublinear increase of throughput with curvature, whereas in the localized-throat limit the deformation and compliance-induced flux reduction are organized by the effective stiffness $\mathcal{B}{C}^2$. The results provide compact, design-oriented scaling laws for compliant electroosmotic constrictions and clarify how stiffness, curvature, surface conduction, and minimum-gap interactions regulate transport in soft microfluidic, biosensing, drug-delivery, and iontronic systems.

An analytical theory is developed for the deformation of a viscous ferrofluid droplet subject to a uniform magnetic field together with a linear background flow. Under weak flow conditions ($\text{Ca}\ll 1$), we apply the method of domain perturbation to compute ellipsoidal corrections to the droplet shape up to $O\left({\text{Ca}}^2\right)$ in the capillary number. The resulting small-deformation equations show that droplet deformation and breakup depend strongly on both the direction and the strength of the imposed magnetic field as demonstrated in existing computational studies. Steady-state bifurcation analysis is used to determine droplet breakup and the critical capillary number as functions of the flow type and magnetic-field orientation. We then analyze the rheology of a dilute ferrofluid emulsion by extending the classical approach of Batchelor [J. Fluid Mech. 41, 545 (1970)] to compute the emulsion extra stress and the associated rheological material functions (e.g., shear and extensional viscosities). The existence of a nonsymmetric stress tensor, reported by previous computational studies, is demonstrated and the extra torque in the suspension is calculated. Analytical expressions for rheological properties of bulk emulsions are derived up to $O\left(\text{Ca}\right)$.

Taking advantage of the recently developed Linearized Arbitrary Lagrangian-Eulerian framework [Sierra-Ausin et al., Phys. Rev. Fluids 7, 113603 (2022)], we characterize the linear dynamics of an incompressible gas bubble immersed in a biaxial straining flow. We show that the system undergoes a saddle-node bifurcation with strongly different equilibrium shapes when varying the Ohnesorge number, $\text{Oh}$, which compares viscous and capillary effects. Equilibrium shapes are found to be oblate for sufficiently large $\text{Oh}$ while, counterintuitively, they are prolate for low-enough $\text{Oh}$. The bifurcation diagram is found to contain also two sets of disconnected branches that cannot be obtained by continuation starting from a spherical shape. One set corresponds to bubble shapes expected to be unstable, while the second set comprises a wide region exhibiting stable shapes that might be observed experimentally. We then characterize the linear stability of the various branches. In addition to the unstable axisymmetric mode arising at the saddle-node bifurcation, two nonoscillating drift modes are also identified, together with two unstable modes with azimuthal wave number $m=2$ and two oscillating modes with $m=3$ that appear on oblate bubbles in the presence of large inertial effects. Present results lead to conclusions that may be relevant in the context of bubble breakup in turbulence. In particular, they show that bubbles are much more stable in the biaxial flow geometry than in the uniaxial one. This presumably explains why turbulent breakup mostly happens in uniaxial regions, although biaxial regions are significantly more frequent.

Solute transport in porous media is strongly influenced by both hydrodynamic instabilities and physicochemical interactions, such as adsorption. In this study, we investigate the coupled dynamics of a finite solute blob subjected to Langmuir type nonlinear adsorption and viscous fingering (VF) instabilities during miscible displacement. To resolve the complex interplay between adsorption and instability, we solved the governing equations using a high-resolution Fourier pseudospectral method. While linear adsorption leads to symmetric blob spreading, the nonlinear Langmuir regime exhibits highly asymmetric solute profiles characterized by the formation of a rear rarefaction wave and a sharp leading shock layer. Additionally, the net advection of the blob is noticeably reduced in the upstream direction as the Langmuir adsorption parameter $b$ increases. We have found that for the range of adsorption parameters for a given suitable log mobility ratio, the VF instability was suppressed and introduced a new comet shape structure. Furthermore, we examine the impact of adsorption on mixing by tracking the degree of mixing over time. Interestingly, we observe a nonmonotonic dependence of mixing on the Langmuir parameter for a suitable log mobility ratio. The coupled effect of VF at the rear interface and of Langmuir adsorption provides a powerful mechanism to enhance or reduce the mixing and VF depending on the need of the application.

We study the evaporation dynamics of spray lamellae—the paradigm of dense sprays evaporation—injected into a quiescent dry environment from an oscillating source. Lamellae are made of a collection of densely packed microns in size droplets of liquids with different volatilities, initially in equilibrium with their vapor. Building on an earlier study [de Rivas et al., Phys. Rev. Fluids 1, 014201 (2016)], we confirm that the lifetime of an individual droplet embedded in such a spray is much larger than expected from the usual d-squared law describing the fate of a single isolated droplet evaporating in a quiescent environment. By analogy with the way mixing times of scalars are understood from the coupling between stretching and diffusion, we show that the boundary between the spray and the diluting environment is slaved to the dynamics of its saturating vapor concentration field as envisaged in Villermaux et al. [Phys. Rev. Fluids 2, 074501 (2017)], thus explaining the very substantial evaporation delay of the liquid phase in spite of its division into fine droplets. We quantify the evaporation time for the stirring protocol of an undulating lamellae as well as, thanks to unique in situ measurements, the concomitant evolution of the droplets number, and size distribution in the spray.

The collision kernel of droplets in warm clouds is a crucially important quantity for the parametrization of precipitation in weather and climate models. Nevertheless, its accurate representation remains a challenge, specifically in the bottleneck range $15\mu \mathrm{m}<r<40\mu \mathrm{m}$, within which turbulence is believed to substantially contribute to droplet growth. In this work, we address this problem by performing direct numerical simulations of polydisperse inertial particles suspended in three-dimensional turbulence at Reynolds number up to ${\text{Re}}_{\lambda }=418$. Collision statistics are compiled for droplet pairs across the Stokes number range $\text{St}\in [0.02,2]$, yielding comprehensive bidisperse maps of collision kernels, radial relative velocities, and radial distribution functions at contact. Our analysis reveals that polydispersity enhances collisions between light droplets through differential sampling, but attenuates collisions at larger Stokes numbers by rapidly reducing the spatial overlap of droplet clusters. By benchmarking existing models, we show that dominant bidisperse errors arise from overpredicted cross-species clustering. In light of these results, we propose an adapted model for the bidisperse radial distribution function, as well as a novel parametrization for the associated collision kernel, applicable to the smallest droplets in the bottleneck range with small settling velocities. Finally, we study the broadening of the droplet size distribution due to collision-coalescence and demonstrate that droplet growth is markedly accelerated in parcels of large local dissipation rate, supporting the hypothesis that turbulent intermittency may help overcome the bottleneck barrier.

In this paper, numerical simulations are conducted of flow past a circular cylinder confined symmetrically in a channel. The effects of upstream turbulence on wake properties are studied with upstream conditions assumed to be a canonical turbulent channel flow. Simulations have been conducted at blockages of $\beta =0.3$, 0.5, and 0.7, where blockage is defined as the ratio of cylinder diameter to channel height. It is found for the lowest blockage, $\beta =0.3$, turbulence trips the shear layers separating off the cylinder, forming Kelvin-Helmholtz vortices which rapidly roll-up and form Kármán vortices further downstream. This mechanism prevents merging of Kelvin-Helmholtz vortices, evidenced by a lack of a subharmonic peak in spectra of probes placed in the shear layer. For $\beta =0.5$, acceleration of fluid through the gap between the cylinder and channel wall partially relaminarizes the flow, reducing turbulence. Despite this, the surviving turbulence still trips the cylinder shear layer, causing it to break down. Finally, for $\beta =0.7$, a large change in wake dynamics is found to occur. Relaminarization again occurs through the gap, but is insufficient to suppress all turbulence. Hence, tripping the cylinder shear layer and breaking it down before wake bias could occur. This therefore results in the time- and spanwise-averaged flow being symmetric, unlike its laminar upstream counterpart. Spectral analysis also finds a strong subharmonic peak, indicating merging of Kelvin-Helmholtz vortices for $\beta =0.5$ and 0.7.

The low-frequency unsteadiness of turbulent separation bubbles is manifested in a contraction and expansion of the reverse-flow region. This study shows that, in a pressure-gradient-induced turbulent separation bubble developing in a half-diffuser, the dominant long-time behavior follows nonlinear stochastic dynamics. Planar time-resolved particle image velocimetry reveals a spanwise-organized coherent structure with anticorrelated core and sidewall regions. Its temporal evolution is quantitatively described using a Langevin equation, modeling both the deterministic drift and stochastic forcing of the dynamical system. This model suggests that the low-frequency unsteadiness may arise from stochastic forcing, resulting in transitions between two equilibrium states. The methodology and findings documented in the present paper therefore constitute a significant step toward understanding turbulent separation bubbles as probabilistic dynamical systems.

Waves refract when traveling over water of variable depth, and the theory of radiation stress implies consequent forces imparted to the bottom topography. Such forces may be relevant for deformable boundaries and submerged objects that are free to move, but such wave-structure interactions have yet to be experimentally observed nor utilized in applications. Here we demonstrate such effects at the laboratory scale by introducing experimental techniques for studying the movements of submerged structures due to water waves. Asymmetrically shaped bodies are shown to move perpendicularly to plane traveling waves, and visualizations of the distorted wave fields are consistent with a mechanism based on refraction in the shallow layer over the structure. Further, we show how this effect can be exploited to stably position or "tweeze" objects within focused wave fields. Simplified calculations suggest that refractive redirection of momentum flux can explain these outcomes, and the form of the wave field and geometry of the body can be used to control forces. The systems presented here are hydrodynamic analogs to acoustic and optical transport and trapping, suggesting generalizations that could guide further strategies for wave-based motion control.

We demonstrate the feasibility of a closure model for large-scale eddies in the energy-containing range of turbulence. To this end, we construct a data-driven closure model for the largest eddies in turbulence driven by the steady Taylor-Green force in a periodic cube, and thoroughly evaluate its stability and accuracy. A recurrent neural network is employed to autoregressively predict the temporal evolution of low-wave-number components of the Fourier coefficients of vorticity with a prediction interval $\mathrm{\Delta }{t}_{\mathrm{p}}$. The key factors for successful modeling are not only spatial low-pass filtering to construct the closure model, but also preprocessing the training data with a temporal low-pass filter. More concretely, an appropriate cutoff frequency, which is inversely proportional to $\mathrm{\Delta }{t}_{\mathrm{p}}$, is needed for stable and accurate reproduction of the dynamics and statistics of the largest eddies. Without this preprocessing, the model exhibits steady or periodic behavior and fails to reproduce the chaotic dynamics of the largest eddies.

The problem of inertia-gravity waves excited by harmonic flow over topography is often simplified by the traditional approximation, which neglects the horizontal component of the Coriolis force. We consider waves forced by a two-dimensional tidal potential in a finite-depth domain, considering the full (nontraditional) Coriolis force, nonhydrostatic effects, and contribution from higher-order harmonics. We provide formulas for the vertical velocity, form drag, and conversion rate of inertia-gravity waves. Overall, including the nontraditional Coriolis force enhances the conversion rate and torque of inertia-gravity waves and extends the range of latitudes that can support them. Inertia-gravity waves under nontraditional rotation exhibit weaker suppression of the conversion rate in the low-depth limit, and weaker nonacoustic effects in the rapidly rotating limit at low latitudes.

Inertia-gravity waves present in a wide range of fluid dynamical contexts, necessitating a generalized and adaptable theory to accurately describe their behavior. We provide an analytical solution for form drag and velocity, in addition to the energy conversion rate of such waves excited by harmonic flow over a topographic source. Our analysis extends the investigations of Bell et al. [J. Fluid Mech. 67, 705 (1975)] and Llewellyn Smith et al. [J. Phys. Oceanogr. 32, 1554 (2002)] to consider a finite depth ocean without the hydrostatic or acoustic approximations. Eschewing these approximations enables consideration of waves in subinertial contexts (e.g., weak stratification, diurnal tides, or flow at high latitudes). The nonhydrostatic solution supports any waves forced at frequency $\omega$ between the stratification $N$ and Coriolis $f$ frequencies. The nonacoustic solution does not neglect the tidal excursion length and thereby enables superharmonics $n\omega$ ($n>1$) to lie within the aforementioned interval, even when the fundamental frequency does not. The finite depth of the domain is found to suppress inertia-gravity wave power when the distance between reflections of the wave approaches the thickness of the wave beam. This suppression is enhanced in strongly nonacoustic contexts.

We investigate the fragmentation dynamics of acoustically levitated microemulsion droplets subjected to laser irradiation. The influence of irradiation intensity and emulsion composition on bubble dynamics and subsequent breakup characteristics is investigated. Our findings demonstrate an inverse relationship between irradiation intensity and the onset of vapor bubble nucleation, which is consistent with theory. Depending on the composition and irradiation intensity, distinct regimes of bubble growth are revealed. The growth and collapse of these bubbles result in droplet fragmentation via ligament-mediated breakup and sheet breakup. These breakup modes are governed by interfacial instabilities, including Faraday, Rayleigh-Taylor, and Plateau-Rayleigh mechanisms. The bubble growth and collapse events are quantified using a breakup impact parameter ($\beta$), which, in turn, reveals the onset of instability triggering droplet breakup. Consequently, the variations in ligament dynamics [using Weber number (We)] are explained and correlated to the respective bubble growth and collapse events. In particular, our experimental data show that small-sized bubbles (lower value of $\beta$) typically result in the ejection of high-momentum ligaments (larger We). Finally, we report the growth and collapse dynamics of a highly viscous bubble that forms toward the end of the droplet lifetime due to the high concentration of nonvolatile surfactant.

We calculate the effective slip length for a rectangularly grooved periodic surface encapsulated (i.e., fully wetted) by a lubricant fluid and subjected to exterior shear flow parallel to the grooves. Our focus is the limit of a nearly inviscid lubricant, where the ratio $\mu$ of the lubricant viscosity to that of the exterior fluid is small. This limit is singular for an encapsulated surface, indicating a dominant lubricant-flow effect—a stark contrast to superhydrophobic surfaces where the role of the lubricant is typically negligible. In addition to $\mu$, the ratio $\lambda$ of the slip length to the grooving semiperiod depends on three geometric lengths, all normalized by the semiperiod: $b$, the thickness of the lubricant films wetting the groove ridges, and $\varphi$ and $h$, the semiwidth and height of the ridges, respectively. We identify two key limits characterizing the regime $\mu \ll 1$. In the first, with $b$ held fixed, we find $\lambda \sim {\mu }^{-1}\tilde{\lambda }(b,\varphi ,h)$, where the rescaled slip length $\tilde{\lambda }$ is determined by an interior lubricant-flow problem. In the second, with $b/\mu$ held fixed, we find $\lambda \sim \mathrm{\Lambda }(b/\mu ,\varphi )$, where $\mathrm{\Lambda }$ is governed by an exterior flow problem in which the thin lubricant films wetting the ridges are effectively replaced by a Navier-slip condition, while the rest of the interface is shear free. As $b/\mu \to 0$, the Navier-slip condition simplifies to no slip, whereby the exterior problem reduces to that for a superhydrophobic grooved surface—famously solved by Philip using complex variables [Philip, Z. Angew. Math. Phys. 23, 353 (1972)]. Asymptotic and numerical analysis of the exterior problem demonstrates (i) the transition from Philip's solution at $b/\mu \ll 1$ to an algebraic behavior $\mathrm{\Lambda }\sim b/\left(\mu \varphi \right)$ at $b/\mu \gg 1$, matching with the small-$b$ limit of the interior problem, and (ii) for $\varphi \ll 1$, the transition from a logarithmic $O(ln\varphi )$ scaling for $b\ll \mu \varphi$—familiar from the superhydrophobic case, where $\varphi$ constitutes the solid-to-air fraction—to the aforementioned algebraic regime when $\mu \varphi \ll b\ll \varphi$. As $b$ becomes comparable to $\varphi$, the exterior problem loses validity in favor of the interior problem. By analyzing the latter in the distinguished sublimit where $b$ and $\varphi$ are comparably small, we demonstrate how, as $b/\varphi$ is increased, the algebraic growth of $\lambda$ with $b/\mu$ is arrested at order ${\mu }^{-1}/lnb$.

The role of interparticle collisions in turbulent particle-laden channel flow is investigated through a detailed comparison between two-way and four-way coupled point-particle direct numerical simulations. Simulations are performed with particle Stokes number ${\mathrm{St}}^{+}$ ranging from 3 to 300, at a fixed mass loading ${\mathrm{\Phi }}_m=0.6$ and bulk Reynolds number ${\mathrm{Re}}_b=U_{b}h/\nu =5150$. It is found that interparticle collisions significantly modulate the dynamical features of both fluid and particle phases, resulting in a substantial reduction of near-wall particle concentration, enhanced turbulence attenuation, and more pronounced drag reduction especially for high ${\mathrm{St}}^{+}$ cases. These modulating effects mechanistically arise from the following two differences compared with the two-way coupling cases. First, interparticle collisions increase particle velocity fluctuations in the wall-normal direction, which enhances particle dispersion and consequently weakens near-wall particle accumulation. Second, interparticle collisions amplify the slip velocity fluctuations and thereby increase the extra dissipation, which leads to turbulence attenuation and further drag reduction. The above findings suggest that four-way coupling is of fundamental importance for accurate modeling of interphase momentum and energy dynamics in particle-laden turbulent flows.