A swimming fish leaves behind an orderly pattern of vortices, but schools of fish are not ordered into lattice arrangements. Our experiments on robotic hydrofoils show how flapping swimmers can benefit from hydrodynamic interactions and stay in a school even without specific positioning relative to neighbors.

The lifetime of antibubbles - droplets separated from a liquid bath by a thin layer of air - has typically been too short for further study or application. We experimentally demonstrate a mechanical method for extending the antibubble lifetime from minutes to hours. The recorded longest lifetime is up to 13 hours. In our method, vertical vibrations stabilize the air film as successive shocks on the antibubble top compensate the upward air drainage. To suppress Faraday waves, the liquid surface is covered with bubbles. A theoretical model is built to obtain the threshold for a long-lived antibubble. In the figure, the black scale bar is 20mm and the blue one 2mm.

We study viscous fingering instability in a soft Hele-Shaw cell, in which one of the walls is replaced by a bounded elastomer. For sufficiently high flow rates in our experiments, fingers can get trapped because the deforming solid contacts the opposite wall, sealing the cell near its rim, although in numerical simulations of axisymmetric bubble expansion at the same control parameters, the interface escapes. For parameter regimes in which narrower fingers grow, we find the opposite behavior: the cell seals in axisymmetric numerics but fingers escape the cell in experiments, suggesting that viscous fingering can either promote or suppress the choking of the cell.

We have built a mean-field kinetic model to study the underlying hydrodynamic instabilities in dilute, activity-balanced pusher-puller binary suspensions that produce near-zero mean extra stress. We have successfully demonstrated that mixing pushers and pullers in such binary systems may lead to much richer behaviors than a simple neutralization process due to the cancellation of stresslets from both species. Also, we find the instability mechanisms to be different from those derived for pure pushers or pullers, where a net extensile extra stress typically indicates unstable dynamics.

Starting from continuum kinetic theory, we derive a mean-field theory and entropy-based closure model for a polar active fluid. Analysis and simulation show the model closely approximates both the transient and nonequilibrium dynamics of the kinetic theory. Taking advantage of the model’s reduced degrees of freedom, we perform large-scale simulations of turbulent active suspensions in two and three dimensions.

Discontinuous shear-thickening (DST) fluids exhibit complex stress relaxation upon flow cessation. We investigate the relaxation behavior of dense cornstarch suspensions via rotational rheometry and find that the relaxation timescales and the residual stress increase with the apparent viscosity measured in the DST state prior to the flow cessation. Within the framework that attributes DST to the lubricated-to-frictional transition of interparticle contact, our results indicate that the lasting effects of the DST state arise from partial retention of frictional force chains after flow cessation.

Double diffusive layers (DDLs) are one of the flow patterns which arise due to the coupled effects of thermal and solutal gradients and find relevance in large-scale applications. We report observations of DDLs using unidirectional solidification of a binary mixture, which naturally introduces thermal and solutal buoyant flows. The convection rolls at the opposite sidewalls, having an opposite sense of rotation, ultimately merge to form the DDLs that expand throughout the breadth of the test chamber. Our research on exploring the mechanisms of DDLs uses a combination of nonintrusive techniques capturing full-field measurements in real-time to obtain insights on their evolution.

Convection in a frame of reference rotating rapidly about an axis perpendicular to the direction of gravity is a model system for convection in the equatorial region of gaseous planets and is realized in recent laboratory experiments. It is found that the transition from rotationally constrained two-dimensional to three-dimensional flows in this system does not occur at some fixed Rossby number. Instead, the Rossby number at the transition depends on the Reynolds number of the flow. The transition occurs when the Taylor columns which constitute the two-dimensional flow become unstable to elliptical instability.

In a wall-bounded shear flow, deformable particles form a crystal-like structure. To examine particle sorting in microfluidic devices we added small capsules (SC) to a system where large capsules (LC) form a crystal structure, and find that the movement of SC changes significantly with size ratio $R_a$. When SC and LC are of comparable size ($R_a\sim 1$), the small capsules are trapped in the crystal structure, whereas when SC are smaller ($R_a\sim 0.6$ or lower) the SC deplete from the crystal layer due to low lift velocity. This suggests that the crystal structure of capsules can be used to trap capsules of comparable sizes, or to exclude capsules of smaller sizes.

Liquid atomization processes are used in many delivery and coating systems involving pure solvents, solutions, and suspensions. Here we develop a theoretical description of droplet sizes and flow rates in aerosols and sprays, using the first principles of conservation of mass and energy, and employing dimensional and scale analyses. Our study explores the overall range of mean droplet diameters between 0.1-100 μm and Ohnesorge numbers between 0.01-100. We find a reasonable agreement between the theory and experiments, and our theoretical framework contributes to understanding of liquid atomization and can be used for comparison between different spray and aerosol production techniques.

In this work, we study the transport of surfactant molecules to the surface of an oscillating microbubble when an acoustic pressure is used as driving force to promote the nonlinear oscillation. We consider a dynamic surface tension as a function of the surfactant concentration and a large diffusive Péclet number, as occurs in several applications. The surfactant concentration equation is solvable by using a similarity transformation which simplifies the problem, whereas the equation for the radius evolution is solved by the 4th order Runge-Kutta method.

Electrohydrodynamic (EHD) jet printing is an additive manufacturing technique in which an electrified jet of functional ink is deposited on a motion-controlled substrate. Through experimental visualization of EHD jets, current measurements and scaling analysis, we show that steady EHD jetting can occur in three regimes: the cone-jet, moderately stretched jet, and thick-jet regimes. These EHD jetting regimes differ with respect to the relative importance of surface convection and bulk conduction currents downstream of the jet.

The downburst outflow that triggers thunderstorms and sandstorms can be regarded as a cooling source gravity current (CSGC), which may be quite different from the lock-release gravity current (LRGC). We propose a unified integral model incorporating a continuous velocity transition across the density interface for both CSGC and LRGC. The integral model for CSGC correlates the front characteristics with the thermodynamic and geometric properties of the cooling source, verified by direct numerical simulations. Results also demonstrate that the influence of the shear layer in the dense fluids tends to vanish in LRGC and could be predominant in CSGC.

We investigate the transient response of gravity currents moving through a relatively sparse array of roughness elements. The work builds on earlier studies which assumed that the currents were in a quasi-steady state. We find that the current decelerates and that the density structure changes from the head and tail structure of a smooth bed gravity current to a wedge shape.

Adding tiny amounts of long polymer chains to a fluid dramatically alters its behavior. In this work, we show that channel flows of such viscoelastic fluids at vanishingly small Reynolds numbers are highly sensitive to weak disturbances, even though they are linearly stable. We thus show experimentally that a weak disturbance elicits strong fluctuations with a continuous spectrum in three different regimes and elastic waves far from the excitation location through a non-modal bifurcation.

In gas phase experiments we explore the effect of asymmetric and helical excitation of the flow about the exit plane of a jet injected perpendicularly into crossflow. Simultaneous acetone planar laser-induced fluorescence imaging and stereo particle image velocimetry quantify the effects of such excitation on jet structure, vorticity, and scalar mixing, while proper orthogonal decomposition of the images enables determination of characteristic dynamical signatures related to important jet features.

We use an asymptotic model to study instabilities of free surface films of nanometric thickness on thermally conductive substrates in two and three spatial dimensions. The film evolution is simulated using efficient GPU-based simulations which, when combined with the developed asymptotic model, allow for fully nonlinear time-dependent simulations in large three-dimensional computational domains. We find that the properties of the thermally conductive substrate - in particular its thickness and rate of heat loss - play a critical role in controlling the film temperature and dynamics. The graphic shows the influence of substrate thickness ($H_s$) on film temperature and breakup.

The stability of a dynamic meniscus depends upon its macroscopic curvature. In particular, the critical velocity for the dynamic wetting transition is affected by the meniscus curvature which, as such, emerges as a thickness selection mechanism for liquid film deposition. These ideas have been demonstrated experimentally by flowing water in tubes at high velocities and rationalized by numerical modeling.

In this study, we experimentally reveal the liquid-like and solid-like behaviors of millimetric liquid marbles when they sediment in oil towards a water layer below. Upon dynamically interacting with the oil-water interface, an aqueous armored droplet is shown to either rupture or remain intact as the oil surrounding the drop pinches off. The pinch-off in the novel gravity-driven regime leads to the double-encapsulation of aqueous solutions, which enables their secure transport in a wet environment. We also present scaling laws that account for the drop’s weight and the oil viscosities, in good agreement with the experiments.

We explore the axisymmetric breakup of electrified jets under wide ranges of the Taylor number (Γ), the Ohnesorge number (𝑂h) and the wavenumber (𝑘) which are relevant to electrosprays operating in the cone-jet mode. We extend prior numerical and experimental results by developing a phase field EHD model to account for the behavior of the jet after the pinch-off. For 𝑂h > 1, a much broader distribution of droplet size is observed which originates from the pinch-off region of the jet. For Γ ≥ 1.5, the primary droplets are always unstable irrespective of the viscosity level.

Based on the conservative phase field model of Lowengrub and Truskinovsky for almost incompressible liquid binary mixtures, we propose an extended scheme for studying immiscible/miscible liquids. Below a critical temperature $T_c$, the liquids are immiscible with separating interfaces. Above $T_c$ interfacial effects vanish and the liquids become perfectly miscible. The free energy density of the system depends not only on the system composition through the phase field $\varphi$ but also on the reduced temperature $r=(T_c-T)/T_c$. The free energy transforms through $T_c$ to permit a two-phase system in the subcritical (immiscible) regime and a mono phase in the supercritical (miscible) regime.

The lifetime $\tau$ of an isothermal and purely diffusively dissolving droplet in a host liquid scales as $\tau \sim R_0^2$ with its initial radius $R_0$. Here we experimentally find and theoretically derive a completely different droplet dissolution behavior, resulting in $\tau \sim R_0^4$. It occurs when the dissolution dynamics is controlled by local heating of the liquid, leading to a modified solubility and a thermal Marangoni flow around the droplet.

A theoretical model is presented to investigate the stability of a thin viscoelastic fluid draining down a uniformly heated slippery inclined plane. The instability is enhanced as the slippery length and viscoelasticity increase. When the wall is heated, the instability is reinforced.

An analytical solution is presented for dipole-flow disturbed by a circular inclusion of conductivity different from the background. The effective conductivity is computed by means of the self-consistent approach and results, in general, in a nonlocal property.

The flow over two three-dimensional bluff bodies in tandem arrangement is widely encountered in both natural environments and engineering applications. Here we perform a detailed numerical investigation on the wake bifurcations behind two axisymmetric bluff bodies in tandem. Our study explores the overall range of Re between 100 - 500 and the separation distance $S/d$=1, 2, 4, and 6. Seven main wake regimes are identified, and detailed wake bifurcation scenarios depending on Re and the separation distance are provided.

A scheme is presented to actively control particulate position in inertial microfluidics. The unilateral slip boundary is applied to regulate the velocity distribution of the flow field in the microchannel, thus the vertical equilibrium position of the particle can be changed with the slip length. Three kinds of typical particles, namely circular, elliptical, and rectangular, are simulated. The results demonstrate that the vertical equilibrium positions can be manipulated within 40% of the channel width. If the boundary slip is adjusted by some technique, such as electrowetting, then real-time control can be achieved.

The treatment of fluid-fluid interfaces in flows becomes important as systems approach the nanoscale. At such scales interfaces have a distinct width and properties such as shear viscosity differ from those of the bulk phases. Using molecular dynamics simulations we show how these effects can be quantified and modeled in the case of nanoscale Marangoni convection. The success of our approach suggests that it can also be used for generalized nanoscale flows involving fluid-fluid interfaces.

Capillary flow through hydrogel-coated capillary tubes was investigated experimentally and theoretically. The results show how the absorption and swelling of hydrogel regulate capillary flow. Water imbibition through porous hydrogel materials can be understood as analogous to flow through a channel that deforms with water absorption. Therefore, this study provides a better understanding of water absorption through porous hydrogel materials, which are widely used for agricultural substrates, hygienic products, and microfluidic devices.

With molecular dynamics simulations (MDS) we explore the effects of the liquid-to-vapor transition on sound waves including observation of bubble dynamics due to Bjerknes forces on a molecular scale. The usefulness of MDS for the phase transition, bubble growth, and bubble-sound interaction is demonstrated without the use of any phenomenological models.

By adopting the direct force formulation in a finitely extensible nonlinear elastic (FENE) model for the feedback force by laden polymers to stationary isotropic turbulence, we clearly identified the energy flow between turbulence and polymers. The stretching motion of turbulence is suppressed by the polymer springs with large Weissenberg number, resulting in suppression of turbulent kinetic energy and energy dissipation.The direct force model seems to be more efficient in providing physical insight than the polymer stress model based on the conformation tensor.

Evaporation – condensation effects in a porous medium containing saline water enhance water transport and explain salt crust faster upward displacement.

We investigate experimentally the behavior of a large particle in turbulent bedload transport when varying the Shields number and the size ratio. For size ratios above unity the large particle segregates upward with two steps: a first stage with slow and intermittent motion and then, a second stage in which the particle accelerates suddenly until the top of the bed. While the kinematics of the second stage is repeatable for a given configuration, it is not the case for the first stage. A transition in the size ratio is also observed: above a size ratio $r=1.7$, the spatial trajectory of the intruder becomes linear with a constant slope, independent of the Shields number and the size ratio.

Motivated by an application to nuclear safety, we studied the effect of an imposed gas over-pressure on the discharge flow of granular media from a cylindrical silo. The size and type of particles and surrounding fluid where discharge occurs were varied, using air and water to test a coolant fluid for nuclear safety. Particle and air flow rates are found to increase during discharge with constant over-pressure conditions. A two-phase continuum model is used with a frictional rheology for particle-particle interactions. This model is solved numerically and we propose a simple quasi-steady analytical model with the air-pressure gradient at the orifice as a driving force in addition to gravity.

We explore the linear stability and subsequent formation of nonlinear waves in a colloidal, gravity-driven falling film flow. The problem of a gravity-driven falling film devoid of any microstructure has been well studied. However, the role of particulate suspensions in the stability and dynamics of such systems is less explored. We perform a linear stability analysis and observe that the presence of colloidal particles stabilizes both the long-wave surface and short-wave shear instability modes. We also derive nonlinear models in the framework of long-wave theory using Benney’s gradient expansion and the Integral Boundary Layer (IBL) approach to study the systems’ wavy dynamics.

In this study we explore the role of Brownian coagulation on bidisperse like-charged spherical particles interacting via non-continuum hydrodynamics interactions. It is well established in the existing literature that van der Waals interactions enhance the coagulation rate. However, the role of near-field attractive electrostatic forces on the pair interactions of like-charged aerosols is relatively less explored. We have found that electrostatic interactions enhance the coagulation rate between like-charged Brownian particles while interacting through non-continuum hydrodynamics.

We use large eddy simulations with the Euler-Lagrange point-particle approach to assess the appropriateness of the well-mixed theory in predicting indoor risk of contagion. A novel ‘statistical overloading’ technique allows us to analyze infinitely many scenarios, with only one data-rich simulation. The figure shows the droplet nuclei (left half of the room; see color map for radius of nuclei ‘r’) and an iso-surface of velocity magnitude (in yellow). The inset (bottom right) shows one of the infinitely many expiratory events that can be analyzed.

Recent studies show that hydrodynamic interactions qualitatively change the collision rate between droplets settling in a steady straining flow. We explain the dependence of the collision rate on the non-dimensional settling speed by analyzing the bifurcations of the underlying dynamical system.

We investigate the mixing properties of an acoustic driven flow in a parallelepipedic cavity with square basis. Relying on nonnormal reflections, an acoustic beam with a square path is obtained, generating a global complex flow in the cavity which enables chaotic advection. For a moderate power of the source, the chaotic region invades the whole cavity, illustrating that acoustic streaming can be used successfully as a nonintrusive tool to mix efficiently.

A flow of an incompressible viscous fluid in a wedge generated by a source at the apex is a rare exact solution of the Navier-Stokes equations. We analyze a diffusing particle advected by this flow and show that in a wedge with absorbing boundaries the survival probability decays algebraically with time. The computation of the decay exponent reduces to finding the ground state energy of the quantum particle in a potential well with a shape determined by the radial flow velocity.

A model for the second order velocity structure function tensor, based on Kolmogorov’s theory and valid for the inertial range, is proposed, incorporating the effect of anisotropy as a linear perturbation to the standard isotropic form. Structure functions measured from direct numerical simulations of channel flow and from experimental measurements in turbulent boundary layers are compared with predicted behavior and reasonable agreement is found.

Surface fouling by marine organisms such as barnacles has a major impact on the shipping sector. We use direct numerical simulations to investigate how the fluid dynamical properties of a surface change as it is gradually covered by barnacle colonies. The roughness effect peaks at intermediate coverage when approximately half of the surface is covered by barnacles. We further investigate how the roughness is perceived by the outer part of the flow using the blanketing layer concept and find a linear relationship between the roughness function and the effective slope of the blanketing layer.

Large-scale flow components are known to affect Lagrangian inertial-range turbulent statistics. Multitime structure functions are shown to be a powerful and robust tool to disentangle these contributions from tracers’ statistics in a turbulent von Kármán experiment, in Taylor-Green direct numerical simulations, and in isotropic and homogeneous turbulence. Multitime statistics also improve Lagrangian intermittency measurements.

We numerically investigate a two-time Lagrangian velocity correlation function for particle pairs in two-dimensional energy inverse-cascade turbulence. We propose a self-similar form of the correlation function via incomplete similarity and numerically verify it and determine the scaling exponents. Our results suggest a possibility not to recover the Richardson-Obukhov law at infinite Reynolds number.

We experimentally investigate the drag-reduced channel flow of an inelastic, shear-thinning rigid polymer solution. Particle image velocimetry and shear-viscosity measurements are used to obtain instantaneous distributions of the shear viscosity within the turbulent domain. We elucidate that drag reduction for rigid polymers is primarily driven by a thin lubricating layer that consists of low-viscosity fluid near the channel wall.

Covert-feathers-inspired passive flow control is beneficial for aerodynamic performance at post-stall angles of attack. However, most studies model covert feathers as a rigidly attached or a freely moving flap on a wing. The performance of a flap mounted via a torsional spring, emblematic to the finite stiffness of bird feathers, has remained unexplored. We simulate strongly coupled fluid-structure interactions of flow past a stationary airfoil with a passively deployable, torsionally mounted flap on the suction surface. We then perform a parametric study and an in-depth analysis of the dominant flow features that affect performance.

The vortex-induced vibration of a circular cylinder near a solid wall exhibits two modes of collision with the wall, namely, “Impact” and “Gracing.” At the minimum position of the cylinder nearer the wall, the “Impact” case corroborates an abrupt change in the velocity gradient with a positive repulsive force magnitude. In contrast, the cylinder embraces the wall and moves away during the “Gracing” case, revealing a linear variation of the velocity gradient and nearly-zero repulsive force. The lift signal shows a sudden discontinuity for the “Impact” situation while rendering a continuous variation for the “Gracing.”

We quantify the mutual interaction effects between two parallel wakes caused by a mismatch in the size of a cylinder pair.

The effect of random waves and steady current on the wake of an elastically mounted cylinder is studied in a series of experiments. The relation between the vortex shedding frequency and waves’ nonlinearities is discussed. A phase-based method is proposed in order to detect the presence of a subharmonic phase lock-in phenomenon.

This work is a first preparatory theoretical study for upcoming magnetorotational instability (MRI) experiments with liquid sodium planned within the DRESDYN project. Therefore, we focus on the characteristics of MRI for typical values of the main parameters of the basic magnetized Taylor-Couette (TC) flow achievable in these experiments. In contrast to previous attempts at finding MRI in the laboratory, our results clearly indicate that standard MRI and its helically modified version can be detected for the first time in the DRESDYN-TC device, including the astrophysically important Keplerian rotation, despite the very small magnetic Prandtl number of liquid sodium.

We study the effect of superhydrophobic boundaries on the propagation of nonlinear acoustic waves. A cylindrical liquid-filled tube with longitudinally grooved boundaries, whose cross-sectional sketch is shown in the figure, is considered. We derive a model containing an interplay between the pressure and the shape of the liquid-gas interface, subject to contact line hysteresis, which is a dominant mechanism for energy dissipation. For example, a sudden increase in the inlet (located at 𝑋 = 0) pressure and no flux boundary conditions in the outlet (𝑋 = 1), results (after an initial transient) in slowly decaying pressure oscillations with an increasing amplitude towards the outlet.