Turbulent wall-bounded flows of dilute polymer solutions achieve a universal state known as Maximum Drag Reduction (MDR). At MDR, elongated polymers primarily sustain velocity fluctuations, destroy the classical path of turbulent kinetic energy of wall-bounded Newtonian turbulence, and induce a mean linear effective viscosity, whose slope defines a new inner length scale for the. system. Analogously to Newtonian turbulence, the mean velocity shows a universal logarithmic behavior (Virk’s law) in the case of a large separation between the inner and the outer scale of the system.

Colloidal particles at fluid interfaces stabilize drops and bubbles, yet their effect on mass transfer remains ambiguous, with experiments showing either strong hindrance or minimal effect, even at near-complete surface coverage. We resolve this ambiguity by modeling transient diffusion with the Fick-Jacobs equation, revealing that particle layers hinder diffusion only at short times due to reduced cross-sectional area. Our model provides a simple criterion for predicting hindered diffusion and captures prior experimental findings into a regime map, offering a unifying framework for diffusion in particle-stabilized multiphase systems.

A continuum model is developed to analyze the stability of finite-size coherent structures in suspensions of strongly aligned swimmers. For dilute active jets, pullers undergo pearling instabilities while pushers destabilize into helical structures. The long-term nonlinear evolution reveals spreading and interaction of puller clusters and wavelength coarsening of pusher helices. These results are in close agreement with experiments performed with photophobic micro-algae controlled by light and hydrodynamically interacting agents-based numerical simulations.

For systems of interfacially driven microswimmers, breaking symmetries of the particle shape and interfacial actuation can lead to self-organization on multiple length scales. For instance, under certain conditions, there is an absorbing phase transition for discoidal swimmers with non-axisymmetric actuation. The particles initially form immotile ordered clusters, and on larger length scales, the clusters realize a spatial distribution characterized by class I disordered hyperuniformity.

When cerebrospinal fluid (CSF) flow in human brains is disrupted, glymphatic waste clearance is jeopardized, which could lead to neurodegenerative conditions, such as dementia. We explore CSF flow in the tiny spaces that surround brain penetrating arteries by modelling the interaction between blood pressure waves and the arterial wall, and postulate a brain pulsation driver. It is shown that flow is highly dependent on the localized anatomical characteristics and brain pulsations significantly magnify such flows. Clinical implications are discussed.

Turbulent premixed bluff-body flames can host both hydrodynamic and self-excited thermoacoustic modes, yet their coupling remains unclear. By fixing the Reynolds number and equivalence ratio while sweeping only the combustor length, we map the route from desynchronization to two-frequency quasiperiodicity and ultimately to 1:2 mutual synchronization, accompanied by strong pressure and heat-release-rate (HRR) oscillations. Spatiotemporal analyses reveal the recirculation zone as the dominant energy-injection site. We also identify two blow-off pathways with clear, local HRR precursors.

This paper reports highly resolved measurements of the three-dimensional velocity field close to a hot solid surface which is surrounded by a colder fluid. The results provide new insights into the specific structure of the boundary layer flow close to a rough surface and how roughness elements influence the transport of heat between the surface and the fluid. The main finding of our work is that, in the domain of Rayleigh and Prandtl number we investigated, roughness only changes the flow field in a passive manner. Contrary to previous assumptions, it does not introduce additional buoyancy forces that could enhance the local heat transfer.

When a porous layer beneath a fluid layer is heated from below, the region in which convective instability develops depends on the intrinsic properties of the coupled fluid–porous system. The present study proposes periodic heating of the superposed system, bounded by a free surface above and an impermeable surface below, to confine convective motion to a desired region and to regulate its onset without altering the system’s physical properties. The amplitude of modulated heating serves as an external control parameter, governing both the onset and the region of convection in a configuration where a thin fluid layer overlies a porous layer, allowing control without modifying the system.

When a droplet falls through another liquid, it usually oscillates between oblate and prolate shapes, but what if its interface could resist those oscillations? In this study, researchers demonstrate how an in situ–formed viscoelastic “skin” at the droplet surface suppresses oscillations entirely, transforming falling droplets into stable oblate bodies. By coupling high-speed imaging with interfacial rheology, the study reveals that nanoparticle, surfactant assemblies can tune interfacial elasticity and damping in real time, shedding new light on how interfacial viscoelasticity governs droplet dynamics across multiphase flows.

This study examines the parameter region in which a solitary rotating vortex can form in an axial liquid metal flow. For different ratios of azimuthal and axial flow intensities, the flow pattern changes significantly. At high intensity of the axial flow the second vortex is completely suppressed by the first vortex. The rotating vortex has clear-cut boundaries. In the flow regions before and after the vortex generation, the vorticity is close to zero. After switching off electromagnetic forces, the vortex is carried by the axial flow along the channel.

A simple tilt transforms the dynamics of a liquid thread flowing along a wire. At a fixed flow rate of 350 mL/h, the system transitions between Rayleigh–Plateau, convective, and immediate droplet drop-off detachment by varying the inclination angle of the wire. Even within the classical Rayleigh–Plateau regime, both the droplet spacing and velocity change significantly with angle, revealing how geometry alone can tune the instability.

A linear nonmodal analysis is performed to investigate the transient growth of diffusive convection in the absence/presence of a bounded Couette flow. The results first examine the properties of transient growth of the pure diffusive convection and interpret the mechanism to cause this growth. Then, the transient growth of diffusive convection with a Couette flow is investigated. Three nonmodal instability regimes regarding the transient growth of the mixed convection are identified, and two mechanisms of the double diffusion to enhance the lift-up mechanism are revealed.

On smooth surfaces, condensing droplets grow, merge, and eventually slide away. Add narrow grooves, and the same water follows a hidden path: it drains through the surface itself. Using a high-throughput condensation setup, we show that groove spacing governs the transition between droplet shedding and capillary drainage. Below a critical spacing, grooves collect and channel all water before large drops can form, offering new routes for efficient dew harvesting and cooling.

We demonstrate the possibility of experimentally obtaining a steady chaotic flow in a closed box without external mechanical forcing. We study how a weakly conductive viscous fluid moves in this cubic domain when subjected to the Lorentz force created by two pairs of magnets and a small electric current. The flow pattern consists of a large vortex created by the first pair of magnets and a double vortex created by the other pair placed perpendicularly. Although each vortex taken separately has poor mixing properties, the combination of the two creates chaotic advection, leading to effective fluid mixing.

How much can a passively deforming trailing-edge alleviate unsteady aerodynamic loads, and which dimensionless numbers govern the deflection and load alleviation? This paper experimentally investigates the unsteady load alleviation of plunging airfoils with a passively deforming trailing-edge. We show that both the deflection amplitude and unsteady load alleviation scale with the product of two Cauchy numbers, or dimensionless flexibilities, one based on the freestream velocity, and the other on the plunging velocity.

In this work, we use numerical simulations to investigate the physical mechanisms at play along a laminar jet driven by an axisymmetric beam of traveling sound waves (Eckart streaming). In particular, we derive scaling laws capturing both the magnitude and longitudinal distribution of the jet velocity along its axis. These scaling laws are defined on distinct regions of the jet, ranging from regions of high acoustic forcing close to the source to forcing-free regions where the beam is fully attenuated. By highlighting the different flow regimes along the jet, these scaling laws are thus able to inform the design of experimental and industrial setups involving Eckart streaming jets

We study the dynamics of a droplet subjected to a thickness indentation in a microchannel. The droplet either reaches an equilibrium shape or splits depending on geometry. We show that its deformation is self-similar but that scaling laws are not sufficient to describe its dynamics and the possible breakup. We propose a model based on surface energy minimization that reproduces our observations in a microfluidic device. We predict whether the drop splits and model the dynamics of the deformation up to breakup, in agreement with our experiments. With our setup, the droplet breakup can therefore be controlled on-demand in situ with an active indentation of the channel thickness.

Thermo-osmosis is a nanoscale fluid flow along solid surfaces driven by temperature variation. In this paper, a model for thermo-osmosis is proposed within a slip-flow theory for molecular fluids. The key is to combine the generalized slip-flow theory for molecular gases with the effects of fluid–solid interaction potentials. By tuning the potentials, or molecular “affinity,” the theory reproduces the reversal of flow direction observed in molecular simulations: when the fluid–solid interaction is favorable (unfavorable), the flow is directed toward the hot (cold) region. This work provides a starting point toward a universal model of slip phenomena in gases and liquids at the nanoscale.

Discrete simulations of granular flows on smooth inclines reveal that a simple law, based on a Froude-like number—the ratio of slip velocity to the square root of wall pressure—accurately describes local wall friction over various angles and mass loads, in both steady and unsteady regimes. A similar law governs wall packing fraction, providing general boundary conditions for flows between smooth walls. Interestingly, a rich variety of flow patterns emerges. The example below illustrates the temporal evolution of the packing fraction in a cross-section of the flow at an inclination of 65 degrees. The flow exhibits successive condensation and evaporation of a dense core.

There are a variety of shock waves in fast-moving granular flows colliding with obstacles which crucially shape flow resistance and impact dynamics. This study employs a depth-integrated numerical model to reveal four distinct interaction regimes—oblique, attached bow, detached bow, and runover—depending on the Froude number and wedge geometry. The results also identify a previously unreported transitional attached bow shock that bridges classical regimes, which would be helpful for improving understanding of flow-obstacle interactions relevant to geophysical hazards.

In vacuum nanoelectrospray, a stream of electrically charged nanoliter droplets moving at high speed through a rarefied space at sub-atmospheric pressure. A supersonic gas microjet in crossflow can effectively redirect the nanodroplets to control their destination. The fundamental theory predicts the droplet fate to enable applications such as high-resolution inkjet printing, trust vectoring for precise satellite control, and biochemical imaging using desorption electrospray ionization.

Sediment-laden flows are inherently density stratified due to their vertical variation of particulate concentration. Stratification provides an inherent mechanism for flow-scale mixing processes. Here we investigate how this change in mixing mechanics impacts sediment transport using simulations of a thermally stratified turbulent channel flow with passively transported particulates. Flow-scale mixing structures (hairpin vortices) are shown to have a profound impact on sediment transport due to their coincidence with strong concentration gradients. As a result classical diffusive-based Fickian models, which assume small-scale mixing, underpredict the capability of flows to suspend sediment.

T-shaped mixers are workhorses for rapid mixing across scales, yet turbulent regimes remain underexplored experimentally. We scale up T-shaped mixers from sub-millimeters to centimeters, and implement flow measurements using techniques of particle image velocimetry and planar laser-induced fluorescence across laminar to turbulent regimes, validated against direct numerical simulations. We successfully replicate the flow characteristics in low-Reynolds-number regimes from micro-scale devices in literature, and also reveal enhanced turbulent mixing in the outlet channel, offering new insights into mixing dynamics at multiple scales.

A system of self-gravitating bosons can form massive condensates, such as dark matter halos around galaxies. Studying such systems can help constrain the nature of dark matter. Yet, the role of turbulence and vortex dynamics within these structures remains elusive. Using direct numerical simulations of the Gross-Pitaevskii–Poisson equation, we show that halos like structures form through a sequential collapse — from sheets to cylinders to spheres. The resulting tangled vortical state alters energy transfer across scales, revealing a pathway for the emergence of large-scale cosmic structures.

The ordered state of microswimmers can be destroyed by an instability created by their swimming stresses. This leads to chaotic flows that resemble turbulence characterized by the presence of topological defects, a phenomenon known as active turbulence. We show that for pushers, the defect turbulent state transitions to a novel concentration-wave turbulent state reported earlier, where defects coexist along with concentration waves. This state emerges from an instability where fluctuations in the concentration of swimmers play a dominant role. Our study aims to provide a comprehensive understanding of the instabilities and turbulence in weakly inertial suspensions of pushers.

We explore how efficiency, a measure of the energy stored in a flow, governs the transition from smooth, viscous dynamics to singular, inviscid ones. Using fluids on log-lattices within a reversible framework, we reveal self-similar blow-ups and their continuation beyond blow-up through stochastic friction. These post-blow-up states connect non-dissipative and dissipative regimes, offering a dynamical route to construct singular solutions of the Euler equations.

Turbulent flow through a longitudinally-rib-roughened square duct is studied using direct numerical simulation (DNS). To understand the rib effects on the velocity field, DNS of a smooth-wall duct flow is also performed which serves as a baseline case of comparison. The impacts of longitudinal ribs on the flow structures, statistical moments of the velocity field and turbulence kinetic energy budget balance are investigated. It is interesting to observe tertiary vortex structures, which significantly alter the distributions of viscous and turbulent stresses within a longitudinally-rib-roughened square duct.

When flexible filaments are exposed to flow, they naturally reconfigure into streamlined shapes—but how filament inclination alone alters turbulence remains unclear. Using large-eddy simulations of inclined filament canopies, we show that tilting the filaments transforms the flow from a canopy-turbulence regime to one where the canopy is largely sheltered from the outer flow, even yielding net drag reduction. A unified virtual-origin framework explains this transition, linking geometry, turbulence penetration, and drag.

In this study, the skewness in both canonical and actuated zero-pressure-gradient turbulent boundary layers (Re${}_{\theta }$ =1770) is decomposed using the real part of the bispectrum, revealing the triadic interactions contributing to skewness. The bispectra of the canonical TBL are compared with those from a case where plasma actuation introduces large-scale spanwise vortices in the outer layer. Actuation serves to examine how imposed outer-layer structures influence near-wall dynamics. Results indicate that linear inner–outer interactions dominate: in the actuated TBL, outer-layer structures modulate near-wall vortex strength but do not trigger their formation.

Turbulent separation bubbles (TSBs) are known to exhibit broadband low-frequency unsteadiness; however, the origin of this phenomenon remains disputed. This work demonstrates that the low-frequency dynamics of a family of TSBs with varying separation extent arise from a forced response to a stationary global mode, rather than from self-sustained oscillations. The forced response remains robust even when the TSB vanishes in the time average or when linear global instability arises. These findings reconcile previous ambiguities regarding the origin of low-frequency unsteadiness in TSBs and further provide guidance for future flow control strategies.

Swirl switching is the temporal rotation of the plane of symmetry of the cross-sectional vortices about the equatorial plane of a curved pipe and can induce considerable structural vibrations. We investigate the effect of bending angle and inflow conditions by performing high-fidelity direct numerical simulations of spatially developing bent pipe flows and extracting spatially coherent structures through proper orthogonal decomposition. Our results show that upstream turbulence is not the primary cause of swirl switching. Instead, the phenomenon likely arises because of a symmetry-breaking instability of the shear layer originating within the curved section.

We present a novel mathematical framework to compute mode-to-mode energy transfers and fluxes for compressible flows. This formalism captures detailed energy conservation within triads and allows decomposition of transfers into rotational, compressive, and mixed components, providing a clear picture of energy exchange among velocity and internal energy modes. The key image shows the decomposed energy fluxes.

This study examines the effects of freestream turbulence on the wakes of circular and square cylinders, with a particular focus on the streamwise evolution of the wake properties. Four different inflow conditions are created by an active grid and are good approximations of homogeneous, isotropic turbulence, thus enabling us to isolate its effects on the wakes. Contiguous high resolution PIV data is gathered up to 1 m downstream of the cylinders. The results reveal that increasing turbulence intensity promotes early transition to self-similarity of the wake velocity deficit, and the earlier breakdown of coherent structures, leading to a reduced average number of vortices in the wake.

We investigate the influence of Prandtl number (Pr) on turbulent compressible convection by performing extensive numerical simulations in both two- and three-dimensions. We find that the bulk remains adiabatic across all Pr, while the global heat and momentum transport exhibits scalings similar to the incompressible Rayleigh-Bénard convection (RBC). In contrast, the boundary layers exhibit distinct scalings near the top and bottom boundaries, unlike RBC, accounting the effects of compressibility. The key image shows the flow structures at two different Pr’s.

The omission of time as a parameter in the classical triad interaction analysis is shown to produce a much too simplistic picture of turbulence. Including time into the analysis shows that not only the spatial wave overlap contributes to energy exchanges between wavenumbers, but the temporal overlap is equally important. The phase match condition is thus broadened to also include temporal frequencies. This can explain much of so-called nonequilibrium turbulence. Not least fractal grid generated turbulence, which is a prime example of these effects. Part II investigates the effects on triadic analysis of practical signals with finite temporal and spatial domains and resolutions.

The inclusion of time as a parameter omitted in the classical triad interaction analysis was introduced in the companion paper Part I. The present work illustrates the effects of practical sampling on the resulting triad interactions. Practical sampling effects include both temporal and spatial digitization (sampling) as well as finite temporal and spatial domains. These effects are seen to broaden the interaction peaks beyond the classically expected delta-functions and the finite domains contribute to a more complex interaction evolution for domains sufficiently small in comparison to the largest scales in the flow under investigation.

The modulation frequency exerts a significant effect on the evolution of plasma-induced vortices, giving rise to three distinct flow structures: vortex-free evolution, leapfrogging, and coexistence of multiple vortex pairs. Among them, the formation of leapfrogging enhances the entrainment coefficient of the plasma jet, thereby potentially enabling more effective flow control.

This article reports an investigation into the statistical properties of an ensemble of random nonlinear water waves propagating in two dimensions in a large scale wave tank (27m x30m, 35 cm deep). By varying the peak frequency of the wave spectrum, we modify the wave dispersion and observe a transition in the system’s behavior. As the frequency decreases, the dynamics evolve from weak wave turbulence to a soliton gas in the shallow water regime. This transition is striking as these two theoretical frameworks are extremely different on fundamental grounds, with the former supporting an energy cascade while the latter is integrable.

Nonuniform inflow alters the linear noise of an underwater propeller by amplifying the overall sound pressure level (OASPL) and introducing asymmetry into the noise directivity patterns. To uncover the underlying mechanisms we develop an equivalent emission point (EEP) acoustic model, which provides an intuitive framework for investigating the propagation behavior and source distribution of the loading noise. Results show that the amplified blade passing frequency (BPF) tone under nonuniform inflow arises from components associated with different harmonics of the blade force, and interference among these components is the primary cause of asymmetric noise radiation in the near field.

Precessing flows in cylinders are highly effective for mixing a passive scalar, as illustrated here with the thin streaks of fluorescent dye. The stretching and folding of these streaks results from chaotic advection by the flow which becomes resonant at specific cylinder heights. This paper reviews theoretical, experimental, and numerical studies of the resonances and the instabilities of a precessional flow, as well as their implications for efficient mixing.

This work establishes a nonlinear phase-resolved wave simulation framework that assimilates observations through the ensemble Kalman filter - pseudospectral Fourier-Legendre (EnKF-PFL) approach. The key image shows that it consistently suppresses the error growth of the PFL-only model and achieves close agreement with reference wave profiles for both regular and irregular waves. It further demonstrates robust performance under highly nonlinear conditions and strong disturbances, where conventional models deteriorate. A consistent set of optimal assimilation parameters is also identified, enabling a practical and predictive strategy for accurate ocean wave forecasting.

This uncertainty-aware Reduced Order Model (ROM) demonstrates enhanced robustness and generalization across varying dynamical regimes of unsteady flow. It provides systematic and reliable predictions by leveraging a Variational Autoencoder (VAE) to construct a suitable latent manifold, and attention mechanisms in the latent space to capture temporal and parametric dependencies.

We report an experimental technique that fully resolves the three-dimensional angular velocity of magnetic particles, suspended in turbulence and actuated by an oscillating magnetic field, using only single-camera two-dimensional imaging. The particles, smaller than the Taylor microscale of the turbulent flow and less dense than water, are tracked with high accuracy to reveal their magnetically driven rotational dynamics affected by turbulence-induced hydrodynamic torque. This method to measure the rotational dynamics of small particles overcomes a key experimental limitation and the experimental apparatus enables active modulation of turbulence through external magnetic fields.

The Macroscopic Forcing Method (MFM) is a numerical tool for determining closure operators, including turbulent closures, through forced numerical simulations. In this work, we outline and demonstrate the recommended MFM procedure to avoid slow statistical convergence of MFM measurements as well as maintain consistent boundary condition treatment. We apply the method to quantify eddy diffusivity moments in two-dimensional Rayleigh-Taylor instability to improve statistical convergence for analysis.

When immersed in water, a solid coated with a superhydrophobic surface traps an air layer, called a plastron, that shields it from liquid contact. This layer enables underwater respiration in animals and provides drag reduction, anti-corrosion, and antifouling effects. However, plastrons are sensitive to external disturbances and can destabilize. This study demonstrates how plastron acoustic resonance can be used to monitor and measure the trapped air layer volume. Validated against direct observations, the method is portable, noninvasive, and effective for studying plastron stability under realistic conditions and various flow conditions.