Our study demonstrates how a child’s rubber popper, when snapped underwater, creates fascinating cavitation bubble formations. By using high-speed imaging, we explore the fluid mechanics behind the formation and collapse of these cavitation bubbles due to pressure changes. Interestingly, the cavitation bubble forms a toroidal shape rather than a spherical one, with a lifespan shorter than that of a spherical bubble with the same outer radius. This research illuminates the intricate interplay between bubble dynamics within a thin gap and material elasticity.

We show that particle roughness can significantly modify viscous dissipation in the limit of small particle-wall separation, with the corrugation amplitude comparable with the separation. In particular, a lubrication analysis provides the rectilinear and angular velocities of the two-dimensional particle as functions of the instantaneous angular configuration. The time-averaged rectilinear velocity is a geometric quantity, obtained without the need to address any time dynamics, with the result that the particle may either translate while rotating or become “locked” in a specific phase and translate without rotation.

This letter revisits the problem of the existence of a universal Strouhal number in the separated flow past a stationary bluff body. Theoretical considerations grounded in flow kinetics yield a concise mathematical expression. The resultant model anticipates a universal Strouhal number equal to $1/2\pi \approx 0.159$, aligning closely with experimental finding. Furthermore, the model has been expanded to forecast the Strouhal – Reynolds relationship in the supercritical regime, demonstrating a universal nature as it relies solely on critical Strouhal and Reynolds numbers. The circular, square, and triangular cross-section shapes have been used for validation purposes.

We find that the impact of active matter on Lagrangian coherent structures (LCSs) was much more significant compared to localized random noise with similar energy. This is because the perturbation generated by active matter could couple with the background flow and further deform the LCSs. In addition, rotational elliptical regions of the flow were much more susceptible to active matter perturbation than the hyperbolic regions. Lastly, we revealed that the LCSs could be decently altered even at a small number density of active matter.

This study numerically investigates the wake transition of an unconstrained self-propelled flexible flapping plate, which can move freely both longitudinally and laterally. Three distinct wake patterns, including symmetric, deflected, and chaotic, are identified quantitatively. The symmetry breaking will be triggered when the cruising and flapping Reynolds number as well as translational kinetic energy reach critical values, which follow some simple scaling laws. it is also revealed that passive lateral oscillation and bending deformation of the plate are two key mechanisms affecting wake symmetry properties.

Theoretical and numerical exploration of the transported Filtered Density Function (FDF) in the framework of Self-Adaptive Turbulence Eddy Simulation (SATES) was conducted, encompassing fundamental definitions and a model for the scalar mixing timescale. To address the model inconsistency in terms of the scalar mixing timescale between RANS and LES modes, a novel model was proposed. Subsequently, $a$ $posteriori$ testing showcased the merits of this novel approach, highlighting its potential to be employed in SATES-FDF simulations of turbulent reacting flows.

We explore the vortical interactions in a swirling combustion system via the construction of time-varying weighted spatial turbulence networks whose node strength distribution is derived from the Biot-Savart law. We find widespread evidence of scale-free topology in the vortical networks, with the most coherent flow structures acting as the primary network hubs. Crucially, we find that even after the onset of thermoacoustic instability, the scale-free topology can persist continuously in time, contrary to some suggestions from the literature. This discovery could have important implications for the design of flow controllers that rely on destroying the primary hubs of vortical networks.

The stresslet in a dilute suspension of rigid spheres in a viscoelastic (Oldroyd-B) fluid is studied under imposed shear and uniaxial extensional flow. We observe that, due to its hyperbolic nature, the polymer constitutive equation can be directly evaluated along streamlines of the flow. Specifically, evaluating along streamlines on the particle surface, in the limit of the Newtonian flow fields, produces an analytical scaling which we evaluate and test against numerical results. This approach also provides physical insight into the mechanism driving the observed trends.

This work investigates the interaction of turbulent hypersonic flows with a 15° compression corner using direct numerical simulations. Different values of stagnation enthalpy, ranging approximately from 600kJ to 4MJ, are considered. Vibrational excitation of the gas molecules is also included in the model for the higher stagnation enthalpy cases. The analysis presented in the manuscript illustrates the effects of the flow thermodynamic state on quantities that determine the main mechanical and thermal stresses generated on a hypersonic vehicle, namely the shear stress, heat flux, and pressure fluctuations at the wall.

A physically motivated low-dimensional model describes properly the interaction of two vertically-aligned large-scale circulations (LSC) in two-layer turbulent convection, and predicts their preferred flow states of thermal and viscous coupling. The model reveals that flow reversals can be achieved when turbulent fluctuations drive the LSC azimuthal diffusion into a flow state such that the two LSC planes are orthogonal to each other, the strength of the LSC in the high Rayleigh number fluid layer then reduces to zero deterministically. The model provides satisfactory interpretation for the high occurrence frequency of flow reversals observed in two-layer turbulent convection.

In this work, we conducted a systematic experimental study of turbulent two-layer convection in an upright cylinder, as illustrated. From the measured mean temperature and temperature variance profiles, we find a unique twin-boundary-layer structure across the liquid interface with one of the twin boundary layers (BLs) on each side of the interface. The functional form of the measured mean temperature and temperature variance profiles is well described by the equations for a BL attaching to a solid conducting plate, so long as a thermal slip length is introduced to account for the convective heat flux passing through the liquid interface.

We address the long-standing mystery surrounding the rotational bias of convective vortices in the atmosphere (dust devils) and the ocean. We investigate the bias using large-eddy simulations of free convection configured for the ocean, but the idealization of our simulations makes the results more broadly relevant to a wide range of flows. We propose a theory that the addition of many small convective vortices, each of which exhibit a small bias, leads to a much more significant bias for large convective vortices. We apply this new theory to typical convective conditions in the ocean and the terrestrial and Martian atmospheres.

Liquid bridges are present in different four-phase systems, like the one studied here, which is formed by a liquid connecting a horizontal solid substrate with a gas phase while surrounded by another immiscible liquid. We obtain the equilibrium solutions by treating each interface as a simple curve with axial symmetry and constant curvature, and satisfying the boundary conditions given by Neumann’s and Young’s laws. Therefore, the final equilibrium solution is formed by a union of a spherical cap, a catenoid, and portions of onduloids or nodoids. We find that the $\gamma$-angle is the control parameter of the problem, as it defines the existence and shape of the final equilibrium solution.

A bursting bubble’s production of film drops - liquid from the bubble’s cap - is experimentally measured as a function of salinity, temperature, surfactant concentration, and viscosity. Existing theories for the film drainage rate and number of film drops are shown to be robustly consistent with the experimental results, but the lifetime of a bubble across various conditions remains poorly described by existing scalings. The reported relationships describing the importance of various physico-chemical variables on ocean spray emissions are of particular significance to the atmospheric and ocean science community as sea spray aerosols impact radiative transfer and cloud seeding.

Active drops through active stresses alter drop evaporation dynamics, puncture evaporating drops, and induce an inside-out evaporation. For an active drop with vortex defect, activity can extend contractile drop evaporation lifetimes by 50% and can accelerate extensile drop evaporation by 33%. Non-intuitive deposition patterns emerge due to evaporation induced fluid flows where the fluid is dragged towards both outer and newly formed inner contact lines. Potential applications of such active drop evaporation include enhanced drop longevity in biological contexts and customizable thin film deposits.

This study numerically and theoretically investigates the control of viscous fingering using a transverse electric field. Viscous fingering can be delayed in the presence of an electric field due to the electrical force distribution along the fluid interface and completely suppressed when the electric field strength exceeds a certain value. A non-dimensional parameter $\varphi$ is defined based on the force balance, which shows good capability for predicting the transition from an unstable viscous fingering to stable displacement under various fluid properties and flow conditions.

In this study, we evaluate the effect of kinetic helicity on the slope of the vertical spectrum of kinetic energy in stratified turbulence. Our theoretical approach allows us to define energy-dominated, helicity-dominated, and joint dual cascade regimes in turbulent flows at various stratification rates. Some of them are verified with the balloon measurements from the Troposphere and Lower Stratosphere. To summarize, one of the conclusions of this work states that domination of helicity flattens the spectrum while an increase in the stratification makes it steeper.

The self-regenerating nature of coherent structures is a key feature that sustains turbulence in wall-bounded shear flows. Theoretical and numerical works have examined the smallest flow region that can sustain these processes, but experimental studies have been scarce due to the technical difficulty of measuring three-dimensional velocity fields. This study reports experimental results that quantify two basic physical mechanisms: the lift-up effect and the production of wall-normal vorticity. These processes are quantified for the first time by a novel local analysis of the three-dimensional velocity field.

Surface slip, characterized by a slip length, $\lambda$, suppresses flow separation induced by Gaussian-shaped deformations on a flat plate. Bumps generate more intense regions of reversed flow than gaps, requiring larger $\lambda$ to inhibit separation. The study extends to double-bump configurations, where the distance between bumps establishes either a stabilizing effect or nonlinear oscillatory flow. However, increasing $\lambda$ ultimately suppresses this phenomenon and eliminates all pockets of separated flow.

The laminar-turbulent transition still poses a challenging problem to fluid dynamic research. This study shows how differential image thermography can be utilized to capture the spatiotemporal aspects of this phenomenon along the curved surface of an airfoil. Further, an incredible agreement between the transition characteristics and the (1+1)D directed percolation theory is observed for a broad range of experimental parameters, namely angle of attack and inflow velocity.

Magnetorotational instability (MRI) is responsible for angular momentum transport in astrophysical disks. However, its definitive experimental confirmation is still missing. The upcoming DRESDYN-MRI experiments using a liquid sodium Taylor-Couette flow are a new effort to detect MRI. In preparation for this, here we study the dynamics of nonaxisymmetric MRI via simulations for the DRESDYN-MRI device. It is shown that nonaxisymmetric modes are important for large $Re\gtrsim 4\times {10}^4$ and relevant in these experiments with $Re\gtrsim {10}^6$. These modes are, however, of nonmagnetic nature, which grow and form a turbulent boundary layer near the cylinders, while axisymmetric MRI dominates in the bulk flow.

In this investigation, we dive into the phenomenon of drop encapsulation in elongated bubbles travelling through liquid-filled capillary channels in the presence of surface-active material. Our numerical results reveal that complex interactions between surfactant parameters, Marangoni stresses, viscosity, and inertia are responsible for dramatically altering the pinch-off times, along with the number, size, and velocity of the encapsulated drops. We summarize these interactions in three distinct encapsulation morphological regimes, providing a structured overview of the underlying dynamics.

Viscosity and contact line friction are the main channels of energy dissipation in wetting dynamics. Experiments and theoretical models haven’t singled out a universal scaling between the two. We perform molecular dynamics simulations of droplets spreading over hydrophilic surfaces, aiming to disentangle the effect of viscosity on friction. The viscosity of the fluid is tuned by changing the chemical composition of the liquid, mimicking real-world experiments. It is found that contact line friction does not scale linearly with viscosity. The trend is explained by accounting for molecular depletion in the near-wall region.

The base of a jet impinging on an ultrapure water bath is studied experimentally. At the impact point, a train of capillary waves develops along the jet. A striking observation is the existence of an effective nonzero water/water contact angle between the jet and the meniscus. The rationalization of this finite contact angle requires a full description of the shape of the interface. By doing an analytical matching between the meniscus and the jet, we show that the capillary waves can be considered as reflected waves present to ensure pressure continuity. It is finally shown that the value of the apparent contact angle is fixed by energy minimization.

Liquid droplets depinning due to shear flow aerodynamic loading is relevant to a range of engineering applications. Factors influencing depinning include droplet volume and submergence, shear flow velocity profile and acceleration, and surface wettability. A series of experiments are performed to assess the critical depinning velocity of liquid droplets subjected to an accelerating (i) laminar boundary layer, and (ii) slot jet at varying impingement angles. Critical depinning velocity, cast into a nondimensional critical Weber number, is found to be strongly related to a newly introduced volumetric shape factor, which encapsulates droplet shape and related aspects of substrate wettability.

This molecular dynamics study investigates the effect of fluid density, confinement, and surface roughness on velocity slip in nanochannels. The key finding is the linearity between the fluid-wall friction coefficient and the peak density at the wall regardless of the wall curvature: tighter confinements attenuate fluid layering at the boundary, reducing the interfacial friction and promoting slip, while higher densities have the opposite effect. Additionally, smoother surfaces characterized by lower accommodation reduce friction via the Smoluchowski factor. These results shed light on the atomistic mechanisms of slip in dense fluids and highlight the importance of fluid-fluid interactions.

Bioclogging, the clogging of pores with living particles, is a complex process that involves various coupled mechanisms such as hydrodynamics, particle deformability, and polydispersity. This article aims to study a yeast clog’s permeability in a microfluidic device. We can finely measure the clog’s permeability for different hydrodynamic forcing using an original and precise on-chip flow rate meter. It reveals that yeast clog’s permeability decreases when the applied pressure increases, with saturation at high pressure. A semi-empirical model based on a double-porosity structure accurately captures the experimental observations.

The Saffman force is one of the key factors for particle transport and hence the interaction among phases in two-phase wall-bounded turbulence. This numerical work finds that the accumulation of particles near the wall and preferential concentration in low-speed streaks are suppressed by the lift force, leading to destruction of the conditional hairpin vortices and decreasing velocity fluctuations near the wall. However, in the outer layer, the particle-turbulence interaction is increased by the lift force because of higher particle concentration.

The extent to which particles such as larvae, seagrass pollen, and microplastics are dispersed by waves and currents has many ecological impacts. Here, we systematically examine the effect of a comprehensive set of parameters on the dispersion of ellipsoidal particles in a wave-current flow using a numerical computation approach. Our results show that all of the parameters considered have some effect on the particle dispersion, but that the settling-wave timescale ratio has the greatest effect.

We investigate the concentration instabilities that arise in vertical dense particulate flows during hydraulic conveying, where the widely used two-fluid model is employed. Results show that the system exhibits instability across a broad spectrum of controlling parameters. We also obtain a fully nonlinear transient numerical solution for the system by using the finite difference method. It is found that small disturbances transform into saturation waves with finite amplitudes when the nonlinear effect becomes dominant, which corresponds to plug flow or slug flow.

The case is made that the kinetic theory of granular gases provides the long-sought universal framework to predict the flow of realistic particles from dilute to very dense conditions. In so doing, the popular inertial rheology and its nonlocal extension to deal with heterogeneities based on the granular fluidity concept are derived as special limits of the kinetic theory.

This paper investigates the effects of the Mach number on turbophoresis and the preferential accumulation of particles. A particle relaxation time weighting transformation is proposed which collapses the concentration profiles not only for particles with small inertia, but also for turbulence with various compressibility at the studied low Reynolds number and in the inner region. Inertial particles in compressible wall turbulence tend to distribute in high-fluid-density and negative-fluid-dilatation regions. This is explained by the changes in turbulent structures at varying Mach number, which in turn also account for the varying scales of particle streaks.

The present work addresses the effect of non-zero volume fraction in predicting the self-induced velocity of particles in an Euler-Lagrange (EL) simulation. Hundreds of EL simulations of flow over a random distribution of stationary particles, covering a range of Reynolds number and volume fraction, are performed to calculate the undisturbed flow and the self-induced velocities of particles in the presence of neighbors. The most significant finding is that the self-induced correction procedure of an isolated particle can be applied even at finite volume fraction, with a simple volume fraction dependent modification. However, the perturbation induced by neighbors often has a much larger effect.

In this work, we employ a physics-inspired mathematical model based on successive linear eigenmode projections to solve the relaxation dynamics of liquid oscillations in a U-shaped tube and subjected to a phenomenological nonlinear contact line model. Each projection induces a rapid loss of total energy in the motion and contributes to its nonlinear damping. The present approach not only describes well the transient stick-slip dynamics, but it also captures the global stick-slip-to-stick transition and the residual exponentially decaying bulk motion following the arrest of the contact line. This study offers a further contribution to rationalizing the impact of contact angle hysteresis.

We investigate compressible turbulent mixing evolving in spherical implosions with differing initial conditions using high-resolution implicit large eddy simulations. We examine in detail temporal and spatial turbulent transport budgets including density self-correlation, turbulent mass flux, and turbulent kinetic energy. This analysis provides improved understanding of the mixing process initiated by Richtmyer-Meshkov and Rayleigh-Taylor instabilities including quantification of contributions to asymmetries in the mixing layer and numerical dissipation.

This study investigates how microscale physics impacts anomalous particle dispersion in disordered media. To this end, disordered media with random sorption and random flow properties are considered. To quantify anomalous large-scale particle transport, a continuous time random walk model is developed that represents both disorder mechanisms. While random advection and sorption may give rise to similar large-scale transport behaviors, they can be clearly distinguished in their response to uniform injection conditions. These findings highlight the importance of microscale physics for the interpretation and prediction of anomalous dispersion phenomena in disordered media.

Idealized nonuniform body force profiles are used to explain the root cause of turbulence enhancement in various physical flows encountered within such fields as mixed convection, magnetohydrodynamics, and flow control. A recent theory used to explain laminarization is extended to include turbulence enhancement and it is demonstrated that turbulence enhancement can be explained by an increased “apparent Reynolds number”.

This study focuses on the turbulent channel flow over three-dimensional wavy walls. Through temporal-spatial averaging decomposition, the momentum flux and kinetic energy transfers by mean, time-averaged, dispersive, and turbulent motions are revealed. We find a notable correlation between dispersive shear stress and vorticity enhancement. Another finding is that the dispersion-turbulence exchange significantly contributes to turbulent kinetic energy production.

Our understanding of fluid turbulence has traditionally relied on a few canonical laboratory experiments. In this paper, we present a relatively new canonical experiment, the stratified inclined duct, whose density stratification allows for the study of coherent and intermittent states at higher Reynolds numbers than in unstratified flows. Applying a novel data-driven technique to a large experimental database of shadowgraph visualizations, we automatically identify distinct turbulent states and transitions between them, paving the way for the reduced-order modeling of stratified turbulence.

This work studies the effect of confinement on fastly rotating turbulent flows, leading to a dimensional transition in the presence of large-scale friction. Performing a linear stability analysis under very high rotation, the threshold between the two- and three-dimensional regime is determined. We discuss about two instability mechanisms: centrifugal and parametric. The quantification of the instability length scale is found to scale as the square root of the Rossby number. Spatial and temporal co-relation of strain rate tensor with the growth of perturbations is observed for parametric type instability. Finally, these instabilities were also studied with the oscillatory Kolmogorov flow.

In this work, we bridge the gap between data assimilation using Physics-Informed Neural Networks (PINNs) and variational methods (based on a classical discretization of the flow equations), when used to reconstruct mean flow from accurate sparse pointwise mean velocity measurements. Tested on the turbulent periodic hill flow (Reynolds number of 5600), we propose the use of Spalart-Allmaras turbulence model augmented PINNs for turbulent mean flow reconstruction. Importantly, we demonstrate how these turbulence model augmented PINNs can reconstruct mean flow more accurately than the equivalent variational data assimilation, using the same sparse velocity measurements and physics constraints.

A neural-network interpolation (NNI) is proposed to improve the prediction of preferential concentration in particle-laden turbulence. The NNI uses the particle position and velocity on neighboring grid points to estimate the fluid velocity at the particle position. To evaluate the NNI, we simulate a two-dimensional homogeneous isotropic turbulence subjected to high-wavenumber forcing. The NNI recovers the effect of small-scale motion on particle distribution from the low-resolution field, adding high-wavenumber energy to the turbulence field. Consequently, the NNI improves the prediction accuracy of the preferential concentration on coarse grids.

This study reports on the outer-layer self-similarity of the turbulent boundary layer at low to moderate Reynolds number range from the perspective of the turbulent/non-turbulent (T/NT) interface. From the new perspective, the exponential self-similarity of the mean velocity is discovered in the outer region. For further consideration, we propose the energy-superposition effect based on the outer-layer structures to reveal the similarity of the turbulent kinetic energy in the outer layer. The present work reveals that the interface-based perspective provides an alternative scheme to understand the outer layer of the turbulent boundary layer.

Understanding of the influence of waves on a tidal turbine wake in a turbulent channel flow is developed using large eddy simulations with modeling of the air-water interphase deformation. For wavelengths between two- and sixteen-times channel depth, the rate of wake recovery is faster than in the absence of waves with only small differences in turbine mean loading. The smallest wavelengths cause fastest wake recovery, and hence shortest wake length. This is due to enhanced vertical and transverse components of convection and turbulent transport of mean kinetic energy. Advection of helical tip vortices shed by the turbine blades is impacted by vertical and streamwise wave kinematics.

What would be the ultimate statistical invariance for wall turbulence? In 3D flows, whether turbulent fluctuations are bounded or unbounded for a series of quantities when the Reynolds number (Re) tends to infinity, is debated. Here, we focus on 2D wall turbulence and find that fluctuations in wall units such as root mean square of wall pressure, wall shear stress intensity, streamwise velocity fluctuation, etc., all exhibit a distinct Re${}^{1/3}$ scaling law. This scaling indicates an unbounded behavior for 2D flows due to the inverse energy cascade, which sheds light on the asymptotical behavior of 3D flows that fluctuations might be bounded because of the forward energy cascade.

The development of singular structures and the path to turbulence in fluids is a fundamental problem that has puzzled researchers for a long time. In this work, we study numerically the evolution of a large-scale initial condition under the hyperviscous incompressible Navier-Stoke equations. Our observations revealed the emergence of shrinking vortex pancakes, which lead to vortex ribs and, eventually, vortex ropes, as depicted in the figure. Eventually, a fully developed turbulent state was achieved prior to its viscous decay.

This work explores the streamwise inclination angle of attached eddies in both the streamwise velocity and temperature fields using direct numerical simulations of turbulent channel flows. Though it has been well studied in incompressible flows, scarce work exists in compressible flows. A high statistical similarity is found between the velocity and temperature fluctuations. The increasing trend of the inclination angle with Reynolds numbers observed in incompressible flows remains. On the contrary, the current database shows a minor Mach-number effect on it. These conclusions could serve as references for extending the attached eddy model from incompressible to compressible flows.

Vortices are found everywhere around us at every scale. Current models describing vortices are mainly kinematics-based, in which the resulting dynamics are limiting in their nature. Relying on these models, one can predict vortices response for a prescribed initial condition but cannot study their response under generic conditions and external disturbances. That is because current models are a well devised formulation. However, we propose a model from first-variational principles relying on the Principle of Least Action, and not only was it able to predict the vortices response in a generic sense, but also it can be extended to include varying strength vortices or any dynamical constraints.

We introduce a pump-probe laser setup to explore the interaction of light momentum and fluid mechanics in a two-layer liquid system. Creating a nanometric bulge in the upper layer reveals a transient bulge on the liquid-liquid interface propelled by viscous stress towards the higher refractive index liquid. Noninvasive measurements and numerical simulations validate our findings, unraveling the intricate interplay between light momentum theories (Minkowski and Abraham) and fluid mechanics. The transient deformation height serves as a precise indicator of surface tension and viscosity, enabling nanoscale manipulation with potential applications in sensors, actuators, and optical devices.

When a flat plate impacts water at small deadrise angles, its rotational motion is dominated by the asymmetrical air cushioning effect underneath the plate. In the regime of impact velocity and deadrise angle, four typical motion patterns are found: pitching-down, fluctuating-pitching-down, pitching-up-down, and pitching-up. These motions are mainly dictated by two physical processes of asymmetrical air cushioning effect: keel compression and edge compression. In keel compression, the air underneath the keel is compressed and produces a pitching down moment; in the following edge compression stage, the air underneath the edge is heavily compressed and produces a strong pitching up moment.

Gravito-inertial waves propagate in fixed directions and exhibit a critical slope singularity whenever one of these directions is tangent to a surface boundary. The nature of this singularity is analyzed. It is shown how it governs the scaling and the structure of the intense viscous beam that it generates.