Direct numerical simulation is applied to the Pi Chamber experimental facility to understand droplet growth and activation in the context of moist Rayleigh-Bénard turbulence. While many bulk features of the experimental observations are represented well, the Lagrangian approach to droplet microphysics is used to gain insight where experiments cannot. Common assumptions regarding the activation-deactivation cycle and droplet lifetime are shown to be likely erroneous.

Marangoni contraction describes the apparent dewetting behavior of certain volatile mixtures from high-energy surfaces due to Marangoni flows. These flows also lead to strong mixing in the droplets. Here we demonstrate that this process is fully captured in the lubrication approximation if one accounts for Taylor dispersion. We derive the dimensionally reduced form of the advection-diffusion problem of bulk mixtures in thin films, showing that Taylor dispersion is required to establish consistent orders in the long-wave expansion.

Classically, the rotation of ellipsoids in shear Stokes flow is captured by Jeffery’s orbits. Here, we demonstrate that Jeffery’s orbits also describe high-frequency shape-deforming swimmers moving in the plane of a shear flow. In doing so, we support the use of these simple models for capturing shape-changing swimmer dynamics in studies of active matter and highlight the ubiquity of ellipsoid-like dynamics in complex systems.

Many swimmers oscillate rapidly as they move, though the details of this fast motion are often neglected in their study. Here, we explore the long-term effects that these oscillations can have on swimmer motion, identifying intuitive conditions on the rapid oscillations that preclude the accumulation of long-term biases in self-propelling objects. We also highlight how the combination of fast oscillations and external flow can lead to significant and unexpected changes of behavior, which we consider in the context of the classical Jeffery’s orbit.

Feeding of ciliates on suspended food particles is complex and relies typically on coordinated motion in bands of transversal rows of cilia known as membranelles. We explore and model the fluid dynamics of feeding flow and particle retention in upstream collecting ciliates that use a single membranelle band to both generate feeding flow, retain food particles, and transport them to the cell mouth.

Using reinforcement learning, we study the coevolution of pursuing-evasion policies of two microswimmers that can sense each other only through hydrodynamic signals, which provide ambiguous information. We show that both agents find effective ways to overcome the difficulties set by partial information, and we explain the main discovered strategies. The setting here developed may offer a framework to study prey-predator interactions in more complex situations.

In reacting flows, changes in fluid physical properties induced by chemical reactions can alter the flow dynamics. Generally, flows change more significantly with larger changes in properties. However, here we demonstrate that the effect on the flow reverses depending on the degree of change in the properties due to the reaction in a reacting flow with gel production, through experiments involving high-precision rheological measurements and our proposed theory. This occurs because viscoelastic properties of the gel differently affect flow dynamics depending on the gel strength.

A numerical investigation is conducted of Rayleigh-Bénard convection in a rectangular enclosure of aspect ratio 2:1 filled by a class of nonlinear viscoelastic fluids represented by the Phan Thien-Tanner (PTT) constitutive equation. The objective is to provide a detailed interpretation of the physical mechanisms involved in Rayleigh-Bénard convection for a viscoelastic fluid. An energy analysis highlights the different physical mechanisms involved in oscillatory convection. The results show the contribution of the elastic terms in the evolution of the kinetic energy of the structures. At a second critical Rayleigh number the oscillatory convection with two rollers becomes stationary again.

Cylindrical cavitation bubbles, capable of generating extreme strain rates, are experimentally and theoretically studied in shear-thickening suspensions. Starting from the limit between continuous and discontinuous shear thickening regimes, cavitation bubbles deform increasingly with increasing solid volume fraction until bubbles are replaced by cavitation-induced fractures.

Due to its vast range of industrial applications, viscoelastic entry flow behavior and stability in abrupt contraction/expansion geometries have been a subject of interest for quite some time. However, due to the strong dependence of flow instabilities and vortex formation near the constriction entrance in non-Newtonian fluids on the geometry and the fluid rheology, generalizations can not easily be made. In the present study, we examine and compare the flow behavior of several shear-thinning fluids in a new planar contraction/expansion geometry that is symmetric in the flow direction and about the midplane of the depth but asymmetric with respect to the “contraction” direction.

In this paper, particle-resolved inviscid simulations of shock propagation over randomly distributed beds are performed, with the goal of quantifying the force on individual particles. An important observation was that the inviscid drag force on some particles remained consistently positive, while others consistently negative, even long after the passage of the shock. These persistent forces are inviscid quasi-steady contributions which, with the viscous counterpart, will play an essential role in the particle’s long-term dispersion if allowed to move in response to the force.

In cold water (temperatures between water’s freezing point and the temperature of maximum density), near-surface heating (from the sun) generates dense water which in turn induces vertical currents. If there is a near-surface current, the resulting convective instabilities efficiently move momentum from the current to regions lower in the water column. Then, there is an induced momentum flux across the plume boundary leading to a complicated series of three-dimensional interactions resulting in turbulence. How might this process be affected by factors such as water clarity and current speed?

Three-dimesional direct numerical simulations are performed to investigate the effect of the horizontal aspect ratio on magnetoconvective instabilities using plane layer Rayleigh-Bénard geometry in the presence of an external uniform horizontal magnetic field. The onset of oscillatory instability is found to scale with magnetic field strength with two distinct scaling laws. The scaling exponent gradually decreases with the increment in the aspect ratio. Different transition routes to chaos have been found depending on the aspect ratio and the magnetic field strength. We find that transient chaotic flow reversals eventually lead to persistent chaotic flow reversals.

Dynamics of freely suspended elastic sheets in uniaxial extensional flow at zero Reynolds number are examined. Sheets can exhibit a coil-stretch-like transition with increasing flow strength, together with a bistable regime in which both a relatively compact state or a fully stretched state can be found. Sufficiently flexible sheets can wrinkle to form different conformations that can be predicted by a linear stability analysis. Wrinkling strongly modifies the dynamics via hydrodynamic screening of the fluid trapped in the folds of the wrinkled sheet.

Understanding and controlling the motion of liquid droplets is important both from a fundamental point of view and in many applications. In this work, we theoretically analyze the dynamics of a liquid droplet sliding down a ramp under the action of gravity. We show that when the dissipation (which we account for by means of an effective contact line viscosity) has a gradient in the direction orthogonal to gravity, the droplet, while sliding down, drifts laterally in the direction opposite to the dissipation gradient. We validate our analytical results, obtained in the limit of low Bond numbers, by a numerical solution of our model.

Recent studies have shown that enveloped viruses contained in airborne respiratory droplets lose infectability fastest at intermediate ambient relative humidities. Studying the evaporation dynamics of respiratory-like droplets in air reveals that, at high humidity, the salt dissolved in respiratory drops inhibits their evaporation indefinitely while at low humidity the drop evaporates leaving a porous solid residue, inside which virions may survive for long times. We conclude that the optimal relative humidity for minimal infectability should coincide with that at which droplets remain liquid and contain high salt concentrations for long periods of time.

This work studies nonlinear spatiotemporal stability of two-dimensional electroconvection between two flat plates subjected to a through-flow, using numerical simulations and weakly nonlinear analyses. We found that the traveling speeds of the leading and trailing edges of the wave packet in the nonlinear regime are consistent with the linear ones. We derived for the first time the Ginzburg-Landau equation (GLE) using an amplitude expansion method extending earlier work of Pham and Suslov. This GLE can predict the absolute growth rate even when the parameters are away from the linear critical conditions, outperforming the GLE derived using a multiple-scale expansion method.

Ferrofluids have a very wide range of possible values of Prandtl number since they are water-based or oil-based. Recent research has highlighted the important role played by the centrifugal buoyancy in highly viscous fluids. The present paper investigates the impact of the centrifugal buoyancy on the thermomagnetic instability in a ferrofluid confined in a cylindrical annulus with a rotating inner cylinder, with a special interest on the effect of viscosity.

For the compressible Rayleigh-Taylor instability (RTI), density stratification is found to cause different bubble behaviors for different Atwood number (At) values. We propose a modified buoyancy-drag model that interprets the physical mechanism causing these behaviors, and analyze the acceleration behavior of a bubble at high At in detail. This work provides a more comprehensive understanding of the compressibility effects on nonlinear evolution of RTI.

By revisiting the linear dynamics of plane channel flow between compliant walls it is shown that the instability is mainly dominated by perturbations of varicose symmetry. The prevailing modes are of traveling-wave-flutter type and governed by the reduced velocity, a nondimensional control parameter measuring the response of the flexible wall to hydrodynamic loading. Analysis of the energy transfer mechanisms reveals that a stabilizing effect for one class of modes is often accompanied by a destabilizing effect for another class. Thus it seems impossible to significantly delay instability onset by wall compliance.

Resonant triad interactions among internal gravity waves in the presence of a uniform shear flow have been explored using analytical and numerical techniques. The existence of self-resonances among internal waves induced by the background shear flow is shown from the linear theory. For different mode interaction cases, the linear approach alone is insufficient for a broader range of control parameters. Hence, the divergence of the second-order solutions representing the superharmonic wave has been used to predict the resonating modes.

Flow-induced vibration (FIV) of bluff bodies has recently been considered as a potential source for renewable energy harvesting. This study presents a comprehensive experimental investigation of structural damping effects on the rotation-enhanced FIV response of an elastically mounted circular cylinder at moderate Reynolds numbers in a free stream. We find that the FIV of a rotating cylinder is associated with different harmonic fluid forcing components and wake modes than the non-rotating counterpart, despite a similar three-branch response. For implied energy harvesting we demonstrate a 33% increase in the peak power output compared to the non-rotating case in a range of rotation rates.

Kirigami cutting technique turns inextensible sheets into highly stretchable devices via the opening of pores. This work investigates experimentally the deformation of such poro-elastic structures in a water flow. We show that the cut pattern allows tailoring the magnitude of sheet expansion, by providing a simple and robust way to tune mechanical properties. But importantly, stretched kirigami sheets feature a three-dimensional mesostructure that dictates the direction of local fluid forces, notably leading here to asymmetric deformation of symmetric planar sheets.

This work examines the interfacial instability and flow patterns of the liquid cone in capillary co-flow focusing. The flow field of the outermost driving flow is estimated with the potential flow model, and the startup process of the coaxial cone is analyzed. The effect of various parameters on the cone instability is studied. Moreover, the flow field inside the cone is visualized through the particle tracing method and numerical simulation, showing that flow rates have a significant impact on the size of the recirculation flow.

Various surface friction regimes, including a transition from slippery to sticky behavior, are uncovered through multiscale asymptotic analysis of flow over a rough surface.

Fractures typically contain multiscale surface features and can impact the fluid flow within. Direct numerical simulations across a wide range of Reynolds number have been carried out on surfaces with roughness details of different length scales. The overestimation in permeability due to surface approximation is quantified, leading to a unified function combining the effects of fracture aperture and surface roughness features.

Evaporative nanoporous membranes have the potential to unlock the next generation of high-power electronics by providing much needed compact and efficient cooling solutions. Here, we develop a novel modeling framework for this class of flows and, in doing so, demonstrate that conventional fluid mechanics, based on the Navier-Stokes-Fourier paradigm is inaccurate, because the domain sizes are comparable to the mean free path in the gas (i.e. the vapor flow is rarefied). Instead, we show that recently developed higher-order moment methods provide both an accurate and efficient method for computing these evaporative flows and thus providing simulation-for-design capabilities at the nanoscale.

We produce images of water-gas interfaces at a size of tens to hundreds of nanometers. An environmental scanning electron microscope with reduced electron-beam-induced heating is used for nanoscale liquid observation. We found that water completely wets very narrow hydrophobic nanometric grooves when condensing, although it cannot invade the gaps when an external drop contacts the same patterns.

A stochastic lubrication equation (SLE) is solved numerically to investigate fluctuating hydrodynamics of three nanoscale thin-film flows: (1) droplet spreading, where power laws are derived; (2) droplet coalescence, where molecular dynamics results are reproduced by the SLE and it is discovered that thermal fluctuations decelerate the process; and (3) thin-film rupture, where, in the regime considered, disjoining pressure dominates the final stages of rupture.

This work studies experimentally the angular and translational dynamics of elongated cylindrical particles settling in a quiescent fluid at low Reynolds numbers. The measured force and torque are compared with expressions accounting for the effect of fluid inertia, obtained either in the slender-rod limit or in the case of spheroids.

The applicability of large eddy simulation (LES) (Smagorinksy and dynamic Smagorinsky) models in predicting turbulence modulation is investigated. The LES models capture the turbulence modulation/fluctuations accurately at low volume fractions. However, at high volume loading, the LES models underpredict the turbulent kinetic energy production, which results in an underprediction of the particle volume loading where complete turbulence collapse is observed. The study highlights that modeling error in LES models is more significant at high particle volume loading than the filtering error and inaccuracy in feedback force.

The volume of fluid and discrete element model (VOF-DEM) method is employed to calculate the formation and rising of double bubbles generated through two different orifices in a gas-liquid-solid flow system. The deviation and coalescence of bubble queues which occur during the bubble rising process are discussed and analyzed with velocity vector diagrams. By comparing when particles are laid at the bottom or settling, it is found that free settlement of particles can weaken the deviation of the bubble queues. Moreover, finding a suitable distance between two orifices for enhanced particle entrainment under different air in-taking velocities is also studied.

Strong continuous and discontinuous shear thickening (CST and DST) have been shown to be rationalized by the formation of a network of frictional contacts. This continuously evolving network is investigated here using k-core analysis of configurations obtained based on a discrete-particle simulation method. The approach identifies clusters in which each particle has at least k contacts with other particles in the same cluster. We find that the k-core structures display universal behavior as a function of contact number except at the shear thickening transition, where there is a sharp increase in the stress carried by the 3-cores in DST, with this increase largely absent for CST.

Machine learning turbulence closures for non-homogeneous and non-isotropic flows is a challenging task. The novelty of the presented approach is the adoption of a universal constraint associated with the energy preservation of the nonlinear terms, which is valid for any turbulent system. This constraint is embedded in the training process, and in combination with nonlocal representations in space and time, results in significant improvement for the resulted coarse scale models.

Space-Lagrangian random walk models conceptualize advective transport in heterogeneous media in terms of collections of particles undergoing fixed-length steps along flow streamlines. We study the impact of velocity correlation structure on longitudinal dispersion for different point velocity statistics. We find that asymptotic equivalence requires the step length of an uncorrelated continuous time random walk to be higher than the correlation length when velocity correlations decay exponentially with distance. We characterize the conditions for equivalence for broad classes of velocity distributions.

In modeling turbulent mixing at high Reynolds number (Re), the small-scale statistics of a passive scalar are assumed similar to those of the velocity. Research shows this to be invalid at either high Re or high Schmidt number individually. We hypothesize that both inertial-convective and viscous-convective subranges are needed for sufficient scale separation in both velocity and scalar fields, such that small-scale statistics of the scalar approach those of the velocity. We explore this using direct numerical simulations of up to ${14256}^3$ and find that at high Taylor Reynolds number of 633, similarity may be assumed between velocity and scalar statistics for mixing in water, but not in air.

This work connects attached eddies to wall friction by relating momentum transfer normal to the lower-bound of the energetic range with the wall shear stress. Our model predicts successfully the transition of the friction factor after the Blasius regime to the extreme Reynolds numbers range. The results show the same accuracy as the well-known logarithmic laws for both the friction factor and the mean velocity profile.

We analyze the coherent motions of the velocity and passive scalar field in a turbulent pipe flow. Using Extended Proper Orthogonal Decomposition (EPOD), we can identify the velocity modes most correlated with the scalar modes. We then apply the Fukagata-Iwamoto-Kasagi (FIK) identity to identify the contribution of each velocity and scalar mode on the time-average Sherwood number.

A pair of counter-rotating vortices is produced in the wake of typical cargo aircraft fuselages due to their ramp shape. With a fast polymer-ceramic pressure sensitive paint (PC-PSP), we measure the global pressure fluctuations at very low velocities (15 to 75 m/s) in a surrogate cylinder with a slanted edge model. Through spectral proper orthogonal decomposition, we observe a family of convective pressure waves with peak energy at frequencies matching the vortex wandering frequencies in this flow.

Mammals such as rats and seals can sense their surroundings with their whiskers. This work examines the coupled dynamics of a flexible cantilever cylinder in the laminar subcritical Reynolds regime to pinpoint the underlying mechanism of flow-induced vibration in sensory whiskers. We find that when certain conditions are satisfied, the cylinder undergoes sustained oscillations due to the frequency lock-in phenomenon in this Reynolds regime. The knowledge generated through this work helps clarify the role of flow-induced vibrations in sensing via whiskers and has relevance to the development of innovative bio-inspired flow-sensing devices for next-generation robotic vehicles.

Due to advances in materials science, surface-driven actuation is an increasingly promising strategy for altering aerodynamic flows to improve aerodynamic performance. There are questions about how it can be used for aerodynamic flow control. We numerically study the effect of traveling-wave surface morphing actuation on an airfoil at low Reynolds number (Re). For the flow considered, we discuss the interaction between actuation and the vortex formation process and the resulting variation in temporal and time-averaged lift. The relation between surface kinematics and flow features found could be extended to other vortex dominated flows at comparable as well as higher Re.

We numerically study laminar separated flows over forward-swept wings at Reynolds number Re=400. The flows over forward-swept wings generally feature a pair of counter-rotating tip vortices, which shape the wake dynamics significantly. The forward-swept wing also experiences enhanced lift with additional vortex lift under tip-vortex-induced downwash effects. The present results provide a comprehensive understanding of sweep effects on laminar separated flows over finite-aspect-ratio wings and offers coverage of less-explored areas of the low-Re aerodynamic database.

In this work, we quantify and model the unsteady lift response to dynamic flap actuation from wind tunnel experiments at Re = $1.8\times {10}^6$. The quantification relies on two newly proposed metrics that characterize the dynamic lift hysteresis and allow describing its evolution under different pitching conditions. The modeling strategy yields one nonlinear model and a set of linear models relating the unsteady airfoil lift coefficient distribution to the flap deflection angle $\delta$.

In Part II of this series we characterize and model gust effects on the research airfoil, and employ them for control. Critical to gust mitigation is detection of the gust onset and its amplitude before impact, which we measure by an X-wire sensor positioned at two separate locations upstream of the test airfoil. Different closed-loop control strategies are compared, including model-based feedback, hand-tuned proportional-integral (PI) feedback, only feedforward, and combined feedback and feedforward controllers. The experimental results demonstrate the effectiveness of the combined approach in mitigating the lift fluctuations during quasi-random gust encounters at various $\alpha$ and $\delta$.

Despite the rising use of machine learning for flow estimation problems, a standard set of flows from which estimators and predictors can be developed, compared and assessed is currently lacking within the fluid dynamics community. This work presents two challenge flow cases for the advancement of sparse sensor-based flow estimation techniques. Feedforward and long short-term memory neural networks are used in conjunction with the proper orthogonal decomposition to predict velocity fields in the wake of two cylinders in proximity using sparsely-placed pressure sensors.

We present experiment and analysis of the effectiveness of a flexible splitter plate in the suppression of vortex-induced vibration (VIV) of a circular cylinder. The important finding is that it is not necessary to suppress VIV by a full-span flexible splitter plate, while efficient control can be achieved by a finite-span one. The fluid-structure interaction and control mechanism are revealed by analysis of wake evolution and force characteristics.

In a density-stratified fluid, the wakes generated by a submerged body or topography can be placed into a number of distinct flow regimes, depending on the balance of forces represented by the Reynolds number, Re, and Froude number, Fr. Here we propose a dynamic mode decomposition (DMD) based classifier to automatically sort stratified wakes based on their strongest DMD modes. The performance of the classifier in a test range of low {Re, Fr} yields insights into the development of further data-driven methods for the more challenging and fully turbulent wakes expected from bodies and geographical features.