The relaxation of comb polymers in semidilute solutions is studied using single-molecule fluorescence microscopy and Brownian dynamics simulations with long-range hydrodynamic interactions (HI). Unexpectedly, comb polymer relaxation is found to depend nonmonotonically on branching density, such that lightly branched chains relax faster than linear chains in semidilute solutions. These results challenge commonly held notions in the field and show that the dynamics of branched polymers in nondilute solutions is governed by a coupling between polymer architecture, long-range HI, and excluded volume interactions.

A theoretical study finds that above a critical Péclet number particles can propel themselves through a spontaneous symmetry breaking instability in which a phoretic flow is driven by the advection of mobile surface enzymes.

An examination of the concentration of a pathogen exhaled while speaking in a poorly ventilated space suggests that the probability of infection is relatively high for a few minutes of contact time at a separation of 1 meter separation distance and double that time at a separation of 2 meters.

Swimming microorganisms self-propel in viscous fluids using traveling wavelike deformations of appendages called flagella. A long-wavelength mathematical model is used to show that the flagellar motion maximizing the propulsive force for a given amount of energy dissipated in the fluid is waves traveling with constant speed (potentially on a curved flagellar centerline), with propulsion always in the direction opposite to the wave.

Peritrichous bacteria swim through rotation of a helical bundle of filaments. Here, physical mechanisms that contribute to its formation in terms of direct and indirect hydrodynamic interactions are categorized. A simple analytical singularity model shows that indirect effects generically dominate at all except the very last stages of bundling. A numerical elastohydrodynamic model allows for a detailed analysis along the entire length of the filaments.

Filament elastohydrodynamics has canonically been plagued by severe numerical stiffness. Building upon recent advances in two dimensions, a framework for rapid three-dimensional elastohydrodynamic simulations is presented. This simple, readily extensible formulation yields gains of several orders of magnitude in terms of computational efficiency over existing approaches, enabling a new generation of studies into slender fluid-structure interactions in three dimensions.

Many aquatic microorganisms attach to solid surfaces while creating feeding flows that bring prey particles to them. A theoretical exploration quantifies the flow structures, clearance rates, and recirculation times in such flows using a simple low-Reynolds-number flow model.

Uncertainty-quantification-driven simulations of an explosive experiment are performed to minimize the flow prediction error in the PBXN-5 detonation products and air flow. The global sensitivity response of JWL model parameters based on simulation transverse shock position is given by main sensitivity index over time. The calibration of explosive products model parameters based on experimental shock position data produces good agreement between simulation and experimental results.

Carbon dioxide is a key species for many fundamental and applied problems. Using state-resolved models gives a deep insight into the physics of vibrationally excited states, but at the same time, it is computationally very expensive. A study examines the routes to simplify the simulation of CO${}_2$ vibrational relaxation by revelation of main vibrational relaxation channels and by using the reduced-order (multitemperature) models

Shock waves are generated when a laser pulse ablates a thin jet of water. The shocks have irregular initial shapes, but they flatten and generate secondary shocks as they propagate inside the jet. The shocks can then travel together over long distances and be transmitted inside the jet nozzle.

The effect of high-frequency vibrations with different orientations on the convective flow in a liquid metal inside a cubic cell heated from the side is investigated using the collocation spectral method. The vibration is in three dimensions with a vibration vector contained in one of the three principal planes of the cavity ($xz$, $xy$, and $yz$ planes). In either weightlessness or gravity conditions, the influence of the vibrations on the flow is found to strongly depend on their direction: each vibration orientation has its own features and affects the flow intensity and pattern differently.

Efficient thermal transport using passive means is highly desirable for many technological applications. A numerical study of Rayleigh-Bénard convection with a single passive conductive barrier over a broad range of barrier geometric parameters is presented, aiming to control the thermal transport and flow organization. It is found that, although the resistance induced by the barrier always reduces the flow strength, the global heat transfer can be markedly enhanced because of the funneling of the ascending hot plumes and descending cold plumes and the modifications of boundary layer properties.

An antibubble, the counterpart of an ordinary bubble, consists of a liquid drop separated from the bulk liquid by a thin film of gas, where understanding of its formation mechanism remains limited. Four criteria are reported for antibubble formation by drop pairs impinging on a free surface based on scaling analysis and experimental characterization. This work not only elucidates the underlying formation physics of antibubbles, but it also provides useful guidelines to control antibubble generation.

Electrohydrodynamic (EHD) nanoprinting is used to demonstrate a novel open-atmosphere microfluidic platform, where femtoliter sessile droplets can be formed sustainably despite high volatility, manipulated, and made mobile in any planar direction on-demand, by a sequence of controlled coalescence events. By studying sessile droplet coalescence kinetics in a partially wetting regime, we found that, even for minute droplet sizes, contact line motion still dominates. Coalescing moving droplets were tasked to perform typical microfluidic operations like collecting, transporting, and merging solid materials on otherwise unpatterned substrates.

The rheology of a dilute ferrofluid droplet suspension under simple shear flow is studied using three-dimensional lattice-Boltzmann simulations and the phase-field model. By changing the external field strength and direction, it is found that the suspension rheologies (viscosity and normal stress differences) can be drastically modified. For example, by imposing an external magnetic field, the specific viscosity becomes 12-620% of that under no external field. This suggests that a ferrofluid droplet would be a practical complex fluid for controlling suspension properties, just by changing the external magnetic field strength and direction.

The sessile drop oscillation with free contact lines (the contact angle remains constant when the contact line moves) is investigated through numerical and theoretical analysis. A parametric study is carried out to characterize the effects of contact angle and Bond number on the frequencies for the first and high-order modes. An inviscid theoretical model is developed to provide an explicit expression for the dominant first mode frequency.

For numerous applications in microfluidic engineering, material science, and drug production, droplets of desired size and distribution are generated through liquid impingement on a perforated substrate. The dynamic process of droplet impact, spread, retraction, penetration, jet formation, and fragmentation into satellite droplets is simulated in a droplet impacting a mesh array configuration using an advanced lattice-Boltzmann method. The Weber number and Ohnesorge number are found to significantly influence the dynamics and outcomes, which can be explained by a qualitative and quantitative analysis based on the force and energy balance.

Permeable porous coatings on a flat solid support significantly impact its electrostatic and electrokinetic properties. To predict and interpret them, a closed-form analytical solution is obtained for electrostatic potential profiles that is valid for porous films of any thickness and volume charge density. The analysis interprets and predicts super properties specific to porous films, from an enhanced ion absorption to a consequent amplification of electro-osmotic flows. The results are relevant for hydrogel coatings, porous carbon and silica, polyelectrolyte brushes, and more.

Surface-acoustic waves (SAWs) induce a time-averaged electroosmotic flow (EOF) in an aqueous electrolyte confined in a narrow channel that may exceed flow actuation via acoustic streaming. For a parallel-plate channel the EOF can be maximized by using two SAWs on both channel walls that have the same frequency but are phase-shifted by 180°. The proposed actuation might be a viable alternative for driving liquid electrolytes through narrow channels, without the need for electric interconnects and electrodes.

The mechanism of poloidal flow suppression in an electrovortex flow is verified in a liquid metal experiment and supported by numerical simulations. The results are shown to be valid for both converging and diverging poloidal flows, as well as for solid and free upper boundaries. It is shown that the threshold value of the external magnetic field, above which the poloidal suppression occurs, is comparable with the magnetic field of Earth that is weak.

The turbulent flow within short canopy gaps with lengths $L/h$ = 0.5, 1, 2, 3, and 4, where $h$ denotes the canopy height, is experimentally investigated using planar particle image velocimetry. A thorough examination of the time-averaged streamwise and wall-normal velocities, shear layer growth, turbulent kinetic energy, Reynolds shear stress, and two-point correlations suggests a classification of the flow into two regimes: a skimming flow regime and a shear layer growth regime. In contrast with the skimming flow regime, the shear layer and enhanced turbulence exhibit deep penetration within the gaps and promote mixing in the shear layer growth regime.

The characteristic heavily branched patterns that result from viscous fingering instability are due to highly nonlinear growth of the fingers at late time. Here, the existence of an onset length scale associated with the transition between linear and nonlinear growth is revealed. Remarkably, by measuring this onset point, features of the patterns that form well into the nonlinear regime can be predicted. Existing theoretical work is not sufficient to understand this length scale, especially for miscible fluids where the fluid structure within the small scale of the gap becomes important.

Small spatial perturbations grow into fingers along the unstable interface of a fluid displacing a more viscous fluid in a porous medium, reducing the efficiency of fluid displacement applications. For analogous flow in a radial Hele-Shaw cell, a combined analytical and experimental study reveals that linear instability of the interface is mildly mitigated by adding to the base injection low-frequency oscillations of small magnitude and, to a lesser extent, high-frequency oscillations of large amplitude. This is due to selective suppression of the growth of large wave numbers in the linear regime. By contrast, adding oscillations of an intermediate frequency significantly destabilizes the interface.

Films of partially wetting liquids falling vertically with side contact lines and in the presence of air are studied by means of three-dimensional direct numerical simulations. Fingers developing at the front contact line are subject to a cascade instability, as fingers on the side grow earlier than fingers close to the center. The dynamics of the cascade is influenced by inertial and film thickness effects, and by initial shape perturbations of the contact line. Varying the contact angle does not impact the cascade development.

Viscosity and surface tension of fluids regulates the collapse dynamics of a large cavity where collapse is initially inertial with the minimum radius $r_m\sim (t_0-t{)}^{1/2}$, where $t_0-t$ is the time remaining for collapse. In low-viscosity fluids, an inertial-capillary transition occurs (exponent changes from 1/2 to 2/3), while it is inertial-viscous (exponent changes from 1/2 to 1) in viscous fluids at a late stage of collapse. The inertial-viscous transition occurs when the local capillary number is greater than 1 and the local Ohnesorge number is greater than 0.1.

Simulations reveal the emergence of complex starlike ferrofluid structures having sharp finger tips. These patterns result from an interplay of surface tension, magnetic, and viscous effects. This work supplements previous studies, which examined the system via linear and weakly nonlinear analyses.

Analytical solutions are presented describing the collective propulsion of a flotilla of Marangoni boats on the free surface of a deep layer of viscous fluid. The motion is driven purely by surface diffusion of an insoluble surfactant ejected onto the free surface from the rear of each boat. This sets up a Marangoni stress causing a flow in the viscous fluid. Explicit formulas are found for the collective propulsion speed of the flotilla and the stream function of the Marangoni-induced flow.

Nonlinear simulations are utilized to investigate the viscous fingering formation in a three-layer radial Hele-Shaw cell. The two interfaces of this system are coupled, and the initial distance between them has a great impact on the interfacial morphologies. This work supplements previous studies, which examined the system via linear and weakly nonlinear analyses.

Theory and simulations show that actuating an intrinsically curved elastic filament in viscous fluids can produce bidirectional propulsion by adjusting the actuation frequency. A reduced-order model is developed to characterize this new behavior. The results could inspire the design of swimming microrobots with better maneuverability and soft material systems responsive to a changing environment.

Periodic surface microtextures inspired by textures of shark denticles can alter the frictional response of a solid wall in flow. Using 3D-printed texture-covered rotors in a bespoke Taylor-Couette cell, the effect of the geometry of the textures and flow dynamics on the frictional torque measurements in Couette flow and Taylor vortex flow regimes are investigated. The changes in the torque measurements show direct dependence on the geometric features of the textures in both flow regimes, while the effect of the Reynolds number is only seen in the Taylor vortex flow.

A variety of biological and artificial porous materials can endure cold weather and freezing damage. For concrete pavements, the observed correlation between pavement damage and deicing salts, and the freeze-thaw damage of cement paste loaded with benzene (which contracts upon freezing), challenge the common wisdom that water expansion upon freezing causes damage. A mechanism of nanofluidic salt trapping is proposed in an attempt to explain these puzzling facts and better understand cryotolerance from a physical chemistry perspective.

Motivated by the problem of microplastics in the ocean, we experimentally investigate settling of plastic rods, disks, and spheres in wavy flows. We find that particle average vertical velocity can both increase and decrease in waves, relative to particle settling velocity in quiescent flow. This variation is a function of flow inertia at the particle length scale, characterized by particle Reynolds number Re${}_p$, and also of particle shape. We find that even though the relative vertical velocities between particles and flow remain constant with Re${}_p$, the particles nonuniformly sample the flow as a function of particle shape and Re${}_p$.

The rupture of a liquid film due to co-flowing turbulent shear gas flows is studied using a volume-of-fluid method. This was done by “sandwiching” the film between two fully developed boundary layers from a turbulent channel simulation. Previous theoretical work has indicated that aerodynamic forces drive its evolution. A classical aerodynamics approach is employed to quantify the role of the inviscid lift and drag forces over the deforming film, which, while suggested by previous authors, has not been systematically studied before.

The low-dimensional stochastic flow simulation model called one-dimensional turbulence (ODT) is extended to capture turbulence modulation in particle-laden stationary homogeneous shear turbulence. This study provides an overall characterization of the potential of ODT to support, on the one hand, direct numerical simulation studies with a computationally cheaper way to capture parameter variations, and on the other hand, the incorporation of particle-induced turbulence modulation into subgrid-scale closures of large-eddy simulations.

The maximum dissipation principle is used to derive the boundary conditions at the fluid-gel interface $\mathrm{\Gamma }$, finding that a shear flow in pure fluid can entrain fluid in the gel.

During the course of a complex flow, many coherent structures may be created and destroyed. Here, tools from object tracking in video analysis are used to dynamically track these transient coherent structures, allowing for coherence timescales to be considered independently of the overall flow timescale and revealing the inner dynamics of the structures themselves.

Giant planets like Jupiter, which undergo strong convection in their gaseous envelopes, can develop a composition gradient (CG) during their formation. During their early evolution, a convection zone (CZ) propagates inwards as the planet cools. How quickly does the CZ move inward? What is the mixing mechanism? Using numerical simulations, we find that the CZ evolution rate depends on the cooling speed and initial CG size (a large CG slows the evolution). Further, we find that turbulent motions near the CZ boundary carry enough energy to lift and mix heavier fluid from below. This “entrainment” is important in CZ growth. We present a simple model that reproduces the numerical results.

When mixing of scalar happens in confined environments that are characterized by the presence of impermeable boundaries, its spatial organization deviates from the one observed in unconfined systems. Here, the impact of confinement on mixing state and rate is investigated. The solution for no-flux boundary conditions is given by the superposition of modes whose leading term $m$ leads to a new timescale characterizing confinement-limited mixing that is controlled by diffusion, ${\lambda }^2/(D{\pi }^2{m}^2)$, which is significantly shorter (one order of magnitude shorter) than the classical diffusive timescale ${\lambda }^2/D$ in an unconfined domain.

The coupling effect of the isotropic wall slip and the spanwise oscillation boundary conditions on the drag reduction and turbulence properties in a turbulent channel flow is investigated using direct numerical simulations. As the slip length increases, the drag reduction gradually changes from the oscillation dominated to the slip dominated. For turbulent dynamics, in terms of amplitude, the wall slip condenses the envelope range of the phase fluctuations caused by the oscillatory wall motion, while in terms of phase, the hysteresis of the turbulent dynamics leads to the lag phase of the phase-averaged skin-friction drag, which is larger than that of the Stokes strain.

In homogeneous isotropic turbulence, the degree of heavy particle clustering is reduced with increasing Taylor Reynolds number ($R_{\lambda }$) for 52 ≤ $R_{\lambda }$ ≤ 139 when the Stokes number (St) is small. Contrarily, the clustering of high-St particles tends to stegthen with increasing $R_{\lambda }$. Quantities invoked for low-St particle clustering include both the strength of and characteristic time of particle trapping by “shear structures” quantified by the second invariant of the velocity gradient tensor. The enhancement of high-St particle clustering results from the increasing large-scale eddies at higher $R_{\lambda }$.

In scale-resolving simulations of spatially evolving turbulence, it is vitally important to specify mutually compatible inflow profiles for the resolved flow field and unresolved turbulence. Here, inflow profiles are developed for the unresolved turbulence by employing the equilibrium boundary layer analysis. Based on the proposed algorithm, unresolved kinetic energy and turbulence frequency or dissipation are compatible with each other and consistent with the inflow resolved velocity field.

Simulations of isotropic and shear turbulence in domains of different aspect ratios are performed. By including shear convection and introducing an inflow/outflow in the cross-stream direction, low-cost statistically stationary simulations are attained, the results of which agree very well with the results from experiments and simulations of shear-dominated turbulent flows like mixing layers, planar jets, and round jets.

To study coherent structures and low-dimensional modes in turbulent shear flows we apply spectral proper orthogonal decomposition (SPOD) to simulation data from the turbulent wake of a disk at a Reynolds number of 50,000. Two principal constituents are found: a vortex shedding (m=1) and a double helix (m=2) mode. The SPOD spectrum illustrates the dominance of the leading eigenvalues. The turbulent kinetic energy and Reynolds stress profiles, when reconstructed at different downstream locations, show that the Reynolds shear stress exhibits strong low-rank behavior. The streamwise evolution of modal constituents reveals a transition from the m=1 mode in the near wake to m=2 in the far wake.

Recent experimental results from the drag wake of a dimpled sphere show that the observed wake growth and decay are inconsistent with a widely accepted, 100-year-old scaling law for these quantities. While previous experiments have found a similar discrepancy just behind the wake source, this work shows that the new law holds over a full decade in scaling of distance. The theory behind the existing wake scaling law is revisited, and a faulty assumption which may account for the discrepancy is identified.

Because of the existence of sidewalls and the Coriolis force, temporally and spatially persistent Ekman motion is induced in rectangular ducts subjected to spanwise rotation. The Ekman layers affected by the duct aspect ratio (AR) interact with the Taylor-Görtler vortices, resulting in a peak shift of the mean streamwise velocity toward the suction or pressure wall. A critical value of AR = 4 is identified for the given Reynolds number of 150 and rotation number of 2.5.

By adopting a proper slip length boundary condition in Large Eddy Simulations of airfoil flows at high Reynolds numbers it can be shown that riblets reduce both friction and form drag. We show that riblets induce small but significant modifications of the pressure distribution which tends towards inviscid with improved pressure recovery in the airfoil aft. A quantitative analysis using a classical matched asymptotic expansion reveals that riblets reduce the boundary layer displacement thickness. Reduced thickening of the equivalent body introduced by the boundary layer reduces form drag leading to the unexpected improvement in pressure gradient flows measured in various experiments.

A scale-by-scale energy budget is investigated near the turbulent–nonturbulent interfacial layer with direct numerical simulations of a local turbulent front evolving without mean shear. The forward interscale energy transfer is caused by the velocity gradient in the interface normal direction. The pressure diffusion removes the energy at small scales within the interfacial layer.

The effects of imposing periodic rotations on the vortex-induced vibrations (VIV) of a one-degree-of-freedom rigid cylinder in crossflow are examined in detail. The dynamic response of the cylinder is in general reduced if compared to the nonrotating case, especially when forcing frequencies were larger than the VIV responding frequency under locked-in conditions.

When a light turbulent wind blows at the surface of a liquid at rest, it first generates random surface deformations of weak amplitude elongated in the wind direction. These structures, named wrinkles, are the superposition of the incoherent wakes originating from the pressure fluctuations traveling in the turbulent boundary layer in the air. An investigation of how the geometry of these wrinkles is modified by a sheared current in the liquid is presented.

The upper ocean is host to a zoo of submesoscale instabilities. An analytical derivation shows how an ocean jet whose vertical relative vorticity is of opposite sign to that of the Coriolis parameter, and exceeds it in magnitude somewhere, is unstable to changes in wind stress. It draws kinetic energy from the lateral shear of the jet, as in classical inertial instability, and its vertical extent is controlled by momentum diffusion, as in Ekman spirals. This Ekman-Inertial instability could explain several features of the subsurface submesoscale dynamical landscape.

Timescales relating to the retention of oil droplets in a two-layer stratification are studied using laboratory experiments. It is shown that retention is primarily driven by the gradual recovery of a droplet from its velocity minimum to the upper-layer terminal velocity. The timescales associated with this process are governed by the Reynolds and Froude numbers, as defined by upper-layer characteristics.

Rotating stratified turbulence is of great importance for geophysical flows and industrial applications. Here, the effects of different stratification on energy transfers across scales and kinetic-potential energy exchange in the inverse cascade range are addressed. Moreover, the issue of scale locality in rotating stratified turbulence is studied.