Long, flexible fibers in a turbulent channel flow showcase fascinating dynamics, sampling nonlinear fluid velocities along their length. Tumbling and colliding with boundaries, they bounce off like pole vaulters, propelling themselves toward the flow center. This motion depletes fibers near the walls and concentrates them in the bulk, boosting the net fiber flux beyond the initial flux of the fluid. The effect grows stronger with longer, more flexible fibers, highlighting crucial implications for transport phenomena in turbulent flows.

Our experimental research demonstrates that helicity in turbulent flows undergoes a direct spectral transfer from large to small scales. Tomographic particle image velocimetry provides insights into the spatial and spectral segregation of turbulent flows with different helicity signs. We show that helicity generation and decay along the jet dramatically depends on the inflow swirl. Notably, we provide direct experimental evidence of the helicity cascade, discovering that swirls of the same sign can impart turbulent helicity of the opposite sign, challenging conventional assumptions. These findings offer valuable benchmarks for numerical simulations using different turbulent closure methods.

Clouds of fine particles held aloft by turbulent fluctuations are widespread in natural flows. Although these suspensions are known to inhibit the turbulence supporting them, this effect remains unstudied in many of the most basic settings, such as shear-driven flows. We trace part of the laminar-turbulent boundary for particle-laden plane Couette flow and find unexpectedly that turbulence suppression depends non-monotonically on particle settling velocity. On either side of laminar flow lie two very different states: near-homogeneous weakly stratified turbulence and a patchier regime where sediment is intermittently lifted into suspension from a highly suppressed boundary layer.

We develop a fluid–structure interaction model that couples one-torsional-spring-based elastic wing-hinge dynamics with flapping aerodynamics to study the aerodynamics and flight stability of hawkmoth at various flight velocities. Both leading-edge vortex, body vortex, and their interactions are responsible for augmenting the vertical force production, achieving high power efficiency from the elastic storage. We verify that realistic wing-hinge stiffness leads to optimal aerodynamic performance and external disturbance-rejection is highly robust in multiple directions. This study highlights the significance of flexible wing hinges in biomimetic designs for micro-aerial vehicles.

To study the effects of powder injection on hydrogen flame flow field parameters in scramjet, numerical simulation methods are employed. The study focuses on the influence of particle injection expansion angle and particle injection swirl on hydrogen flame flow field parameters. The computational results indicate that injecting powder fuel into the hydrogen flame leads to a decrease in flame flow field temperature.

This study explores, for the first time, the impact of plasticity on inertialess viscoelastic instabilities in strong elongational flows. Through detailed numerical simulations, it reveals how elastoviscoplastic effects induce complex and dynamic flow behaviors, leading to new flow states. Crucially, our findings reveal that plasticity can laminarize and suppress these instabilities, offering new strategies for controlling the instability mechanism.

We consider different boundary conditions for imposing flow of yield stress fluids in porous media. In contrast to Newtonian fluids in porous media, imposing pressure or a given flow profile at the boundary leads to significantly different flow fields. In particular, we show that imposing a flow profile leads to a merging tree structure whose properties are governed by the dynamics of a directed polymer in a random medium.

We perform a comprehensive numerical study on all three modes of thermal convection (forced, free, and mixed) within a system comprising two concentric horizontal cylinders filled with viscoelastic fluids, with the inner cylinder rotating. In forced convection, the flow field remains stable, while in free and mixed convection, an increase in the Weissenberg number leads to a transition from steady to unsteady periodic, quasiperiodic, and finally, an aperiodic and chaotic behavior. This transition arises due to the presence of elastic instability and the subsequent appearance of elastic turbulence in viscoelastic fluids with the increasing Weissenberg number.

Solar radiation is known to drive vertical motion under ice-covered lakes in the late winter. Numerical studies in this context tend to neglect the effects of nonuniform solar radiation on fluid motion in the lake interior. This research provides direct numerical simulations and the subsequent analysis of the resulting gravity-current-like flow, which propagates into an inversely stratified ambient with developing convection in the form of three-dimensional Rayleigh-Taylor instabilities. We find that with the chosen parameters, typical of an ice-covered lake, geometry plays a much larger role in its development and cessation.

Natural convection motifs are identified in a Rayleigh-Bénard cubic cell using a probabilistic clustering method, Latent Dirichlet Allocation (LDA). The spatiotemporal features of the motifs at different Rayleigh numbers provide insight into the dynamics of the large-scale circulation (LSC), which is characterized by intermittent reorientations. A model based on the dominant heat flux motifs is found to predict successfully the average LSC reorientation rate, including in cases where few or even no reorientations are observed.

The phenomenon of supercritical and subcritical transitions from one state to another with the variation of a control parameter is widely observed across a variety of natural as well as artificial systems. This paper investigates those transitions in the rotating Rayleigh-Bénard convection (RRBC) system. However, the complexity of RRBC so far hindered the simplest possible description of these transitions. Here, a very simple description of the phenomenon is presented using a pair of one dimensional reduced order models of the system in the presence of free-slip and no-slip boundary conditions. The results of the models are then validated with that of the direct numerical simulations.

Liquid drops exhibit nonspherical surface shapes with strong deformations upon pinch-off from jets or sheets. The deformed state induces shape oscillations, which are analyzed both by the weakly nonlinear approach and high-order simulations using the extended Discontinuous Galerkin method as two alternative nonlinear theories to investigate the oscillations at moderate to large deformations. The coupling of oscillation modes is found to induce quasiperiodic motion, which is shown by Fourier power spectra of the frequencies. The interconversion of kinetic and surface energies during the oscillations at strong initial deformations is quantified by the numerical simulations.

Hypersonic boundary layers are susceptible to flow instabilities that cause laminar flow to transition to turbulence, significantly increasing aerodynamic drag and wall heating. We focus on how these instabilities are triggered by the environment by applying a control systems theory technique called “input-output analysis” that relies in part upon solving the Navier-Stokes equations in reverse, tracing instabilities back to their origins. In the complex interactions between atmospheric disturbances, shock waves created near the nose cone of a hypersonic vehicle, and boundary layer instabilities, we find two physical processes strongly connected to the bluntness of the nose cone tip.

An experimental study is conducted on a thin symmetric airfoil at stall. Below a critical Reynolds number, the flow exhibits low-frequency oscillations (LFOs) characterized by a broadband peak in the aerodynamic force spectrum. Beyond this threshold, the LFOs are replaced by intermittent random switches between two states of either high or low lift (attached or detached flow). The states are explored randomly in time for a fixed angle of attack, contrary to the classical hysteresis often observed in airfoil flows at stall, where both states are absorbing. We model this using a continuous Markov chain and extreme value theory, a framework that can determine the system bifurcation points.

We study the drainage of a viscous liquid layer on a horizontal cylinder under gravity, focusing on cases where viscous effects dominate inertia. Nonlinear simulations distinguish, as a function of film thickness and Bond number, two regimes where the draining liquid either ruptures or forms a quasistatic curtain. The liquid curtain subsequently destabilizes due to capillary and gravity forces. When surface tension dominates gravity, pearls form around the cylinder, whereas when gravity dominates surface tension, hanging droplets form, as confirmed by a linear stability analysis of the curtain.

In this work we propose a machine learning methodology for flow modeling and control design based on an iterative approach for training neural networks. We demonstrate that the methodology is able to achieve stabilization of complex nonlinear plants, such as an unstable confined flow past a cylinder. We also show that, through linearization of neural network models, we can use the methodology to conduct optimal sensor selection, as well as to perform unstable equilibrium estimation and stability analysis.

When an unstable boundary layer is perturbed by free-stream turbulence, the combination of that perturbation with two-dimensional instability waves creates a state that transitions to turbulence by an intriguing, helical breakdown. Helical breakdown is analyzed as a secondary instability; the three-dimensional structure of the eigenfunction of the secondary instability mode reveals the helical pattern. The streak configuration leading to the formation of the helical mode is different from those leading to sinuous and varicose modes reported for pure bypass transition in the absence of instability waves. The mixed mode precursor is the distinctive cause for the helical mode transition.

Linear stability analysis of the mean flow in turbulent plane channel flow in the large-scale pattern-forming range. Growth rate of the least stable mode as a function of streamwise and spanwise wavenumber $\alpha$ and $\beta$, respectively. Strict linear stability for all parameters is predicted for all parameters, suggesting that the mean flow stability does not explain pattern formation.

Fluid-fluid displacement is often irreversible—exhibiting hysteresis where reversal of the driving force (e.g. external pressure) does not reverse the fluids’ configuration. This irreversibility is linked to energy dissipation, a key to efficient design of engineering operations such as subsurface cleanup or energy storage. Here, we analyze (analytically, numerically, and experimentally) a novel model system that exposes a striking phenomenon: emergence of hysteresis and dissipation in a system made of individually “reversible” (non-hysteretic) entities, due to their spatial interactions mediated by interfacial tension.

We demonstrate control of the free surface profile of a rivulet of a nematic liquid crystal through the electric field-induced local increase of the effective viscosity of the rivulet. This resulting increase in rivulet height is studied experimentally, and is described theoretically in terms of the volume flux and the electric field strength. The localized effective viscosity change of the flow in this study occurs under isothermal conditions with the other physical properties of the liquid kept constant. The reported effect contrasts with temperature-dependent viscosity control techniques, which involve changes to the physical properties of the liquid.

Most of the literature on flow through nanoporous two-dimensional membranes has focused on static membranes, yet various studies have shown the relevance of fluid-structure interactions – particularly dynamic coupling – on flow through nanopores. Herein, we use Molecular Dynamics (MD) simulations to study the effects of rigid in-plane harmonic pore oscillations on water flow through nanoporous graphene. First, we repurpose a used technique as a framework to isolate the physical mechanisms caused by the dynamic pore from the injected heat. We show that dynamic opening/closing of flow routes inside the pore enhances flow by increasing axial velocity and decreasing water density inside the pore.

We study water transport in bi-disperse porous structures inspired by xylem tissue in vascular plants (arrays of microchannels interconnected by a nanoporous layer). With various experiments (high pressure-driven flow, spontaneous imbibition, transpiration-driven flow at negative pressure), we show that transport rates can be tuned by varying the shape of the microchannels. Even with a fixed shape, spontaneous imbibition behaves very differently depending on sample preparation (air-filled vs. evacuated), because of a dramatic change of transport mechanism in the microchannels. We provide analytical (effective medium) approaches and numerical simulations to rationalize these observations.

Our study combined experiments and three-dimensional lattice Boltzmann simulations to investigate the dynamic breakup of spatially asymmetric Janus droplets in microchannels under two different bifurcation orientations. We elucidated three characteristic flow regimes: (i) division into two daughter Janus droplets; (ii) breakup into a single-phase droplet and a smaller Janus droplet; and (iii) non-breakup. Unlike single-phase or double emulsion droplets, the dumbbell-shaped Janus droplets might exhibit oblique flow in the channel. The strong confinement of the main channel on mother droplets and large flow rates are essential to the symmetrical breakup of Janus droplets.

We use immersed boundary methods to simulate finite-size spheres and fibers in turbulent flows across a range of Taylor Reynolds numbers ($12.8<R{e}_{\lambda }<442$) and solid mass fractions ($0\le M\le 1$). Both particle shapes act as a “spectral shortcut” to the flow, with fibers extending this effect further into the dissipative range. Spheres enhance dissipation in two-dimensional sheets, while fibers enhance dissipation in structures with dimension between one and two. However, the particles’ effect on the anomalous dissipation tends to vanish as $R{e}_{\lambda }\to \mathrm{\infty }$. These findings have implications for microplastics in oceans, volcanic ash clouds, and sandstorms.

A numerical study on the inertial migration of particle suspension in a circular pipe with thermal effect is performed by means of the Lattice Boltzmann method coupled with the discrete element method (LBM-DEM). The particle position and heat transfer for single particle as well as particle suspensions are discussed. Then, we extend the work to varied temperature conditions. It is shown that the variation of the circumferential equilibrium position can be well regressed by the Richardson number. A nonmonotonic variation of the radial equilibrium position as well as the Nusselt number is discovered, which is attributed to the particle crowding effect.

Fluctuating force, fluctuating torque simulations accurately predict the particle dynamics in particle-laden turbulent flows.

The ubiquitous coffee ring effect, referring to the accumulation of suspended particles at the contact line of an evaporating sessile droplet, arises due to evaporation-induced capillary flow. At high evaporation rates, particle accumulation is also observed at the air-liquid interface, a phenomenon known as surface capture. While the coffee ring effect is well understood theoretically, the transition to surface capture has received less attention. Here we aim to remedy this using a simple low-dimensional model to interpolate between the pure coffee ring and pure surface capture regimes. This interpolation also provides insight into intermediate behaviors.

Experiments based on hydraulic loading of artificial cemented granular layers by a localized upward water flow revealed the existence of three failure modes: (i) Overall block uplift; (ii) Block rupture by median crack at the inflow zone; (iii) Progressive excavation of a fluidized path along the walls. Despite these distinct scenarios driven by the boundary conditions, the critical flow values at breakpoint are consistent with each other, underlining the local character of the instability, and can be rationalized by extending Archimedes’ number to the present case involving adhesion between grains. Agreement is then established with the purely granular case.

Inertial solid particles suspended in dense turbulent channels modulate turbulence and fluid mass flow rate through two mechanisms: (I) the increase of the suspension’s apparent kinematic viscosity with increasing solid volume fraction and (II) turbulence modulation through the particle feedback force. For particle volume fractions below 3%, the increase in suspensions apparent viscosity accounts for most of the modulation. As the volume fraction increases, the particle feedback force drives additional modulation beyond what the increased viscosity accounts for. Namely, this is a reduction in the bulk fluid velocity, reduction of turbulent fluctuations, and increased coefficient of friction.

We present the first experimental study of the turbulent entrainment in non-Boussinesq steady plumes, focusing on moderate-Reynolds helium releases issued from an axisymmetric source. Our results show that, downstream of the turbulent transition, the vertical variations of the entrainment coefficient are primarily affected by the near-field generation of turbulent kinetic energy and by a rising contribution of buoyancy effects. Both features do not exhibit a clear dependence on local variations of the density ratio.

Multilayered porous media are prevalent in both natural and engineered systems, exerting significant influence on flow and transport processes. This study presents a hybrid analytical-numerical approach to compute and understand the dynamics between scalar properties and media characteristics in a coupled system with a two-dimensional free flow layer and a heterogeneous porous medium under laminar flow conditions. Our research underscores how variations in the permeability field within multilayered porous media profoundly influence and control scalar mixing behavior.

We compare supervised super-resolution convolutional neural networks (CNNs) against generative adversarial networks (GANs)-based architectures in the ability to reconstruct turbulent flow fields. GANs demonstrated superior in-sample performance but faced challenges with out-of-sample flows. Incorporating a partially unsupervised adversarial training step with large eddy simulation inputs and dynamic upsampling selection improved GANs’ out-of-sample robustness, capturing small-scale features and turbulence statistics better than standard supervised CNNs. The study recommends integrating discriminator-based training to enhance super-resolution CNNs’ reconstruction capabilities.

With the goal of performing opposition control of large-scale drag-producing turbulence structures, we present a successful control strategy that attenuates large-scale velocity fluctuations in a turbulent boundary layer. Our control architecture consists of a wall-embedded sensor that feeds information to a real-time controller, which selectively operates a jet actuator. We quantify the performance of this single-input/single-output system with spectral statistics and direct skin-friction measurements. Additionally, we link the changes in skin-friction drag to changes in the statistical integral quantities to gauge the correlation between control output and skin-friction variation.

Passive scalar turbulence in the viscous-convective range is investigated via a self-consistent closure theory. Without relying on any empirical parameter the theory successfully explained the scalar-variance spectrum proportional to the inverse wavenumber $k^{-1}$ from the scalar’s deformation timescale dominated by the Kolmogorov-scale eddy, which agrees with the physical viewpoint of Batchelor (1959). High Schmidt number (Sc) calculations up to Sc=100000 suggest that a clear Batchelor spectrum may appear in $k\eta \gtrsim 1$ for $Sc\gtrsim 10000$ where $\eta$ is the Kolmogorov length.

Most of the literature on flow over cliffs and its interaction with a wind turbine wake deals with wind direction perpendicular to the cliff. As wind direction can be oblique in reality, it is important to understand how the flow over a cliff can change with wind direction and what implications it can have for a wind turbine wake. In this study, we showed that above a certain wind direction, streamwise flow recirculation is replaced by a spanwise one, affecting flow shear and turbulence. In addition, we explored the complex interactions between flow over a cliff and a wind turbine wake, affecting its characteristics such as shape, recovery rate, and turbulence level in a nontrivial manner.

We study turbulent fluctuation-induced transitions between hurricane-like large-scale vortices and unidirectional jets in stochastically forced, viscously damped two-dimensional turbulence within an elongated periodic domain. Using direct numerical simulations of unprecedented duration, lasting up to 10000 viscous time units, we collect detailed statistical data on the lifetimes of these metastable structures and quantify the impact of the domain aspect ratio, the forcing scale, and the Reynolds number. We also uncover irreversible transition paths between jets and vortices, which consist of two stages: a rapid change in structure and a subsequent slow viscous adjustment of kinetic energy.

Have you ever tried spinning your water bottle to empty it more quickly? This experiment, familiar to the general public, has rarely been studied in the scientific literature, which focuses mainly on the non-rotational case. We show that this popular experiment is surprisingly complex. Our study reveals the presence of three flow regimes, which have a direct impact on the efficiency of the draining process.

Hydrofoils with symmetric oscillations can generate asymmetric vortex wakes. This surprising asymmetry has been widely reproduced, but a simple metric to predict its onset has remained elusive. Here, using a combination of vortex modeling and water channel experiments, we show that vortex wake deflection is well-predicted by the “relative dipole angle”. In addition to offering a predictive physics-based metric, our results show that a hydrofoil’s wake can converge much more slowly than previously thought (200+ oscillation cycles), and that the wake’s asymmetry is more than a memory of the hydrofoil’s initial condition - it is an instability inherent to the vortex street.

Vertically propagating internal wave packets incident upon a density staircase can resonantly excite natural modes of the staircase that then re-emit upward and downward propagating internal waves, changing the prediction for energy transmission of incident plane internal waves.