This paper explores the intricate dynamics of interfacial flows, focusing on the dual role of surface tension in shaping and destabilizing interfaces within solidifying polymer melts. It reviews recent developments in the experimental characterization and rationalization of these complex flows, emphasizing key challenges and opportunities. The insights presented aim to inform the strategic harnessing of interfacial effects to advance soft material technologies.

A nematic liquid crystal, a phase of matter composed of rod-like molecules, exhibits a tendency towards uniform molecular alignment. Bodies inserted into such a fluid can disturb this orientational order, resulting in elastic stresses in the bulk fluid, which may be relaxed by body repositioning or deformation. We review classical work on rigid particles and their interactions in nematic liquid crystals, and more recent work on the shapes and interactions of deformable bodies in this complex fluid medium.

Metachronal swimming is a common form of locomotion in which organisms stroke multiple appendages sequentially. It is used by many aquatic organisms, including paramecia, copepods, ctenophores, and krill, and spans the viscous to inertial regimes across seven orders of magnitude of Reynolds numbers (Re). Through analysis of morphological and kinematics data collected from the literature, we find a strong power law relationship between Re and the Swimming Number Sw, which describes the appendage kinematics. This scaling law is maintained for all flow regimes, explains why metachronal swimming is successful at low Re, and may be useful in designing bio-inspired metachronally paddling robots.

Since 1756, Leidenfrost droplets have been extensively investigated for their intriguing physics and promising applications. Here, we identify and study an interesting phenomenon which we term Leidenfrost droplet billiard ball collision, providing an answer to the fate of two identical Leidenfrost droplets after they collide. Fundamental mechanisms governing the collision between multiple Leidenfrost droplets are elucidated, expanding the temperature range of droplet manipulation technologies from room temperature to several hundred degrees Celsius. This work helps guide the development of liquid transport technologies for a plethora of applications, such as selective chemical synthesis.

A sub-millimetric jet impinges on a substrate heated above the boiling point of the liquid. Under specific conditions - determined by the jet radius, substrate temperature, and the impinging angle - a “Leidenfrost jet” can be observed, bouncing off the surface. Surfing on its own vapor layer, this jet rebounds without making any contact with the heated substrate.

Recent experiments and simulations have suggested that flows of strongly shear-thinning liquids can be greatly enhanced by superhydrophobicity, above and beyond the enhancements familiar for Newtonian liquids. Using asymptotics and numerics, we illuminate the singular mechanism underlying such enhancement, considering the prototypical problem of shear-driven flow over a grooved superhydrophobic surface. A key finding is that the singular scaling of the effective slip length at small solid fractions transitions from logarithmic to algebraic as the viscosity at infinite shear is reduced relative to its value at zero shear.

In this work, we provide experimental evidence of negative refraction using a locally resonant metamaterial with hyperbolic dispersion, supported by a theoretical model. By characterizing both elliptical and hyperbolic regimes, our results demonstrate the potential of metamaterials in controlling water waves, opening up new possibilities in wave manipulation technologies.

Flame-wall interaction (FWI) for two flame configurations under different laminar burning velocity to nonreacting wall friction velocity ratios is investigated. The flame orientations with respect to the streamwise component of velocity and wall-normal direction have a significant impact on the variations of wall heat flux, wall shear stress, and wall friction velocity in premixed FWI within turbulent boundary layers. The behavior of nondimensional streamwise velocity and nondimensional temperature using wall units for different laminar burning velocity to nonreacting wall friction velocity ratios show that the standard log-law profiles in wall units are not valid.

Pinch-off of premixed flames typically results from flame extinction induced by aerodynamic strain. However, for a jet diffusion flame, we find the strain effect to be secondary. Instead, pinch-off is initiated by an insufficient fuel supply to the reaction region, as a result of outward fuel diffusion overwhelmed by inward convection. This competition of transport effects inspires the definition of a characteristic Peclet number as the pinch-off criterion.

We explore how adding a reaction to a Rayleigh-Taylor unstable interface affects the way that two short wavelength modes couple to generate longer wavelength modes. Using simulations, we identify five distinct flame growth solution types. Depending on the greatest common divisor of the wavenumbers of the two modes, the flame may stall, develop coherent pulsations, or even become a metastable traveling wave. We also compare our results with two-mode coupling in ablative and classical Rayleigh-Taylor and show that all three systems may follow the same mode coupling dynamics.

We perform CFD simulations to study vortex-induced vibrations of a cylinder in inertial-elastic flow and in the range of Reynolds and Weissenberg numbers where both inertia and elasticity of the flow must be considered. For increased fluid elasticity, we observe significant polymer deformation in the upstream stagnation region resulting in a region of large elastic stress that acts as a wall around the cylinder. The region of flow separation is both widened and extended further downstream. We show that higher harmonic forces which cause increased structural fatigue and are typically observed for two-degrees-of-freedom in a Newtonian flow are suppressed by adding elasticity to the flow.

This study explores non-modal elastic instability in low-inertia viscoelastic channel flow with a smoothed inlet, revealing how reduced perturbations affect flow behavior. The transition from laminar to chaotic flow is captured by universal scaling laws for velocity fluctuations and friction, independent of perturbation intensity. Most notably, the study presents a critical scaling relation between the onset of instability and the intensity of inlet perturbations characteristic of non-modal instability with onset of elastic waves, offering a new understanding of the conditions leading to elastic instability and turbulence in inertialess viscoelastic channel flow.

When liquid droplets encounter high-speed gas flows, they fragment through various modes, with bag-mediated breakup mode observed at lower Weber numbers. This study shows that even minute concentrations of long-chain polymer molecules in a Newtonian solvent can drastically modify its fragmentation characteristics. By investigating the role of liquid elasticity, we uncover how polymeric additives control the fragmentation dynamics of a droplet specifically in the bag breakup regime.

Seasonally ice-covered lakes cool during the Fall season due to a loss of heat through their exposed surface. This heat loss drives convection within the lake, which warms the surface. A balance between the heat lost to the atmosphere and the convective warming creates a dynamic equilibrium between the surface water temperature and the convection below. We identified the three convective regimes in this system and predict the transitions between them. Understanding these transitions is essential for predicting the time of ice formation in lakes.

Convective dynamics in the interiors of rapidly rotating oblate spheroidal planets are generally marked by extremely small Ekman numbers and moderate Prandtl numbers. In the relevant parameter space, at the onset of thermal convection, the flow structure can be represented by the superposition of multiple inertial modes of the same azimuthal wave number. This paper discusses why such thermal convective motions occur and how the degree of flattening of a planet due to rotation would affect the flows. More importantly, the theory discussed in this paper establishes a necessary foundation for future studies of turbulent convection and MHD dynamo actions.

Radiatively driven convection in freshwater lakes occurs when water temperature is below that of maximum density (3.98 ℃); as solar radiation penetrates a lake, the surface becomes warmer, denser, and sinks. We derived the vertical velocity scale and buoyancy flux profile, which are then verified by large eddy simulations. Convection cells in inertial frames grow continuously, whereas the Coriolis effect confines the growth. The duration of radiative forcing is a second factor arresting the growth.

Although the scaling law We${}^{1/2}$ where We is the Weber number for describing the maximum spreading of an impacting drop in the capillary regime has been validated by subsequent works, the We${}^{1/4}$ scaling persists in the literature. We found not only viscous dissipation but also the initial diameter result in the gap. Taking the initial diameter and wettability into account, a generalized scaling for the drop’s maximum spreading is established, showing excellent agreement with data for both low and relatively high We.

We present a physically based derivation of the slender-jet equations when the jet’s surface is covered with surfactant and surface rheological effects are important. We allow for the possibility that the bulk fluid and the interface may be non-Newtonian, e.g. a bulk phase that is viscoelastic and a surface phase rheology that may not follow the Boussinesq-Scriven model. Theoretical predictions deduced from slender-jet theory are compared with computational results from a fully three-dimensional axisymmetric (3DA) algorithm for a breaking surfactant covered Newtonian jet. For such jets, analytical results are shown to be in excellent accord with ones obtained from 3DA simulations.

When droplets impact solid surfaces with different contact angles, different types of jets are produced. The atomized jet is observed in the low-speed and small-contact-angle region. The atomized jet produces droplets of smaller sizes and faster velocities compared to other jet types.

Fluid suction/injection through porous boundaries is a classic strategy for boundary-layer control. However, its utility in channel flows is comparatively ambiguous. Using two canonical configurations, we show that non-modal perturbations well below the linear instability threshold are heavily amplified by weak vertical crossflows. Only very strong (thus costly) crossflows can sufficiently inhibit modal and non-modal instabilities. However, these are shown to be accompanied by declining mass flow rates, deprecating any apparent advantage. Our results challenge previous literature and the suitability of crossflow-based control in internal flows.

The Moody diagram is a well-known engineering plot of friction factor versus flow rate which shows definite relationships over the majority of parameter space. However, in the transitional regime between laminar and turbulent flows where the relationships are deemed indefinite the region is shown hatched. We investigate the transitional regime using a novel method to minimize the influence of initial conditions. This regime is approached from above by reducing the flow speed from a turbulent flow state. We find that flows driven by a constant pressure difference yield a single curve near the maximum of Moody’s hatches, while flows driven by a syringe produce a distinctly different curve.

Liquid jets under interfacial shear effects are critical in various scientific and industrial applications. External shear induces axial elastic tension due to velocity gradients across the jet, significantly impacting the linear instability of viscoelastic jets. This study, using slender-jet approximations, reveals that standard models fail to account for the effects of elastic tension. Higher-order approximation models, including parabolic and averaged-parabolic models, are developed to capture these effects accurately, offering a new stabilization mechanism for viscoelastic jets and providing insights for future research.

In this work, we study the deposition and erosion of solid particles at the microscale level and their direct consequences on the internal structure of porous media with branching structure consisting of axisymmetric channels, undergoing a unidirectional flow. We characterize the evolution of the internal morphology of the porous medium using geometric parameters such as the gradients of initial pore radii and layer thicknesses.

This study explores the evolution characteristics of pressure pulses caused by cavitating flow around a highly-skewed propeller. The fluctuation law of low-frequency pressure around the propeller is mainly determined by the rotational motion of the propeller, which is less affected by cavitation. The occurrence, rupture, shedding, and collapse of a vapor cavity will promote a significant increase in medium and high frequency pressure. Most notably, the fragmentation and shedding of small-scale cavities can cause pressure peaks in the mid-frequency region. Meanwhile, the collapse of microbubbles will lead to pressure pulses fluctuating at a decreased rate of 6 dB per octave.

When multiple intruders move through a granular material, complex grain-mediated interactions can arise between them. In this study, we experimentally investigate the case of two spheres moving side by side in a granular medium, focusing on how their interaction affects the drag force experienced by each sphere. Our results show that the presence of a neighboring sphere reduces the drag on the first sphere, with this reduction becoming more significant as the spheres are closer to each other. Furthermore, the amount of relative drag reduction is observed to increase with the intruders’ depth in the granular medium.

Shearing shape-anisotropic grains in a split-bottom shear cell form a localized shear band with a depression at its center. Our study reveals how particle alignment affects the packing density, stress distribution, macro-friction, viscosity, and shear-thinning behavior with the increase in the aspect ratio of the particles. A scaling law correlates the particle aspect ratio to macroscopic friction and effective viscosity, revealing shear-thinning behavior in the bulk and near the surface.

The evolution and accumulation of bubbles formed by gas release at the reaction interface present a considerable challenge for improving the efficiency of photoelectrochemical water splitting. Bubble oscillation enhances mass transfer and influences flow dynamics within the system. This study successfully regulates the amplitudes of bubble oscillations by adjusting system temperature and operating voltage and analyzes the mechanism and the mass transfer theory of oscillating bubbles, offering a new approach to managing bubble dynamics.

Turbulent drag reduction by spanwise wall oscillations has been understood through reducing skin friction and manipulation of Reynolds stress and stress producing events. This work aims to contextualize spanwise wall oscillations around the optimal frequency in terms of Fourier spectra in streamwise and spanwise directions along with the wall normal distributions of coefficients spectra. Analysis of where drag reduction occurs and potential physical mechanisms is provided by determining scale contributions of the Reynolds stress to the bulk mean velocity via the scale dependent Fukagata-Iwamoto-Kasagi (FIK) identity and vorticity contribution to the gradient of turbulent Reynolds’ stresses.

Substrates with streamwise-preferential permeability are theorized to reduce drag in turbulent boundary layers by relaxing the no-slip condition. This study presents the first experimental validation of this concept in airflow using a 3D-printed structure. Contrary to predictions, results show an increase in drag driven by the inverse wall-normal Forchheimer coefficient, agreeing with recent direct numerical simulation data on acoustic liners. The findings highlight the challenge of modeling porous substrates for drag predictions, and suggest that translating their abstraction into physical realizations relevant for practical applications would result in structures similar to riblets.

We propose a “frozen-phase” experiment to reveal the effect of Fourier phase dynamics on the energy transfer in three-dimensional turbulence. The method employed to freeze the phases can be considered as a designed forcing term in the spectral space that only acts on the evolution equation of Fourier phases. The results from this experiment demonstrate the necessity of phase dynamics in sustaining a standard forward energy cascade but also the possibility of developing an attenuated cascade without phase dynamics. These findings provide novel insights for modeling the nonlinear triadic interaction in realistic fluid flows.

Ned Kahn’s artwork “Kinetic Façade” is composed of thin aluminium plates covering the entire façade of a building. When the wind blows, the panels ripple and form large scale patterns traveling downstream. We perform video analysis and laboratory experiments on a one-dimensional pendulum chain immersed in a turbulent flow. Thanks to our reduced model, we identify the mechanisms that generate the patterns on the facade: the turbulent flow carries pressure fluctuations which actuates each thin plate. Ned Kahn’s original idea, revealing the invisible aspect of the wind, now finds a scientific ground.

Fluid flows with partial relaminarization driven by favorable pressure gradients pose significant predictive challenges for equilibrium wall models in large eddy simulations, resulting in an overprediction of the skin friction coefficient. This work introduces a wall modeling approach that smoothly blends equilibrium wall-model and no-slip/laminar boundary conditions based on local flow conditions identified by sensors of acceleration/relaminarization. The proposed modeling approach is applied to the prediction of the flow over a Gaussian-shaped bump at three Reynolds numbers. The results compare favorably with prior experiments and direct numerical simulations across different flow regimes.

We use numerical simulations to explore how turbulent Plane Couette flow — the flow between two moving plates — responds to modulations in the speed of one plate. We find that the resulting flow behavior, including amplitude and phase response, can largely be predicted using simplified flow models that satisfy linear superposition. The work advances understanding of how turbulent flows react to controlled perturbations, and when do turbulent flows behave in approximately linear fashion.

As wind turbines increase in scale and are deployed in diverse atmospheric conditions, understanding wake dynamics in stratified atmospheric boundary layers becomes critical. This study uses large eddy simulations to analyze the mechanisms that govern wake momentum deficit and added turbulence kinetic energy transport in stable and neutral atmospheric boundary layers. Findings reveal that dynamics are strongly influenced by effects of stratification on the atmospheric inflow, rather than buoyant forcing, and the dependence of wake-added turbulence on stability. These insights can guide new models that better predict wakes across conditions, contributing to more efficient wind power.

Anomalous dissipation in turbulence is a dissipation mechanism of kinetic energy that is independent of fluid viscosity; this mechanism requires sufficiently rough velocity fields at high Reynolds numbers. Our study links this statistical behavior to fundamental vortex stretching mechanisms, exploring both, classical kinematic Burgers vortex models and simulations of three-dimensional box turbulence. We identify intense vortex structures in turbulence as key contributors to anomalous dissipation, offering insights into the underlying dynamics of energy transfer in turbulent flows.

We conduct comprehensive investigations by direct numerical simulations on the wall heat flux (WHF) in the hypersonic boundary layer over the windward side of a lifting body, which is affected by non-canonical effects, such as the wall curvature and pressure gradient. Based on the framework of conditional analysis, the generation mechanism of extreme WHF events is illustrated in two perspectives: identifying the coherent structures and quantitatively characterizing energy transport. We explore quite different features of WHF in the central and side regions of the lifting body.

This work provides key insights into the interaction between grazing turbulence and a Helmholtz resonator. By tuning the resonator’s frequency to align with the scales of energetic pressure fluctuations in turbulent boundary layers, the device amplifies small-scale velocity fluctuations while reducing large-scale turbulence energy. This frequency-specific modulation suggests that Helmholtz resonators could serve as scalable, wall-embedded surfaces for targeted turbulence control, offering new pathways in flow control.

Secondary streamlines (left half) and stagnation pressure isolines (right half) colored by stagnation pressure, in a transverse view of the 6:1 prolate spheroid for a Reynolds number based on length of 10,000 and an angle of attack of 70 degrees. A high degree of correlation is observed between the streamlines and the stagnation pressure isolines, which are able to capture the complexity of the vortical flow. Thus, the current study proposes to use the largest closed isolines of stagnation pressure as a boundary for a vortex.

Flow dynamics of pulse-jetting aquatic animals are often studied with apparatus kept stationary. This paper examines whether the stationary configuration captures the mechanics of the swimming animal. A clapping body, simulating a pulse-jet animal, is tested in freely propelled (dynamic) and forward motion constrained (stationary) conditions. Significant differences are observed in body motion and wake structure: clapping motion is nearly twice as fast in dynamic cases, while mean thrust coefficient and vortex circulation are higher in stationary cases, varying with body depth but which remain constant in dynamic cases. Care is needed in extrapolating data from stationary to swimming bodies.

This work reveals a before unknown pathway to triadic resonance of an internal gravity wave beam. Previously, for a finite-width beam, this resonance was only known to occur after the beam was forced at a sufficient amplitude. We now show how, when turbulent structures (vortex rings and bubbles) interact with and sufficiently disturb the beam, this resonance can occur at lower amplitudes. In some cases this resonance decays and in others it is self sustaining, long after the disturbance from the turbulent structure has decayed. These findings call into question our current description of a internal wave beams pathway to triadic resonance.