Dynamic neutron radiography is used to observe the effect of a transverse magnetic field on argon bubbles rising through a thick layer of liquid gallium without interactions with the container walls.

Water flowing down a freezing plane builds a static layer of ice on which it keeps flowing. After a rapid diffusive ice growth, the system reaches a steady state resulting from the balance between the heat flow in the water and in the ice. This gives the ice a surprising linear shape.

Adjoint-based optimization is performed in a Blasius boundary layer flow, leading to the computation of the subcritical transition critical energy threshold and associated minimal seed. Simulation results demonstrate the physical characteristics and key energy growth mechanisms of the perturbations. Phase-space representation of the minimal seed’s evolution leads to the identification of a persistent attracting region.

Rayleigh-Taylor mixing in a porous medium is studied numerically in two and three dimensions. Density plumes are found to grow in a non-self-similar way, with length increasing faster than width. The evolution of the mixing layer is found to be quantitatively different in two and three dimensions.

Since flow regions of high helicity are associated with low dissipation of kinetic energy, helicity can be used to mark the flow structures that are major contributors to turbulent scalar dispersion. Employing a Lagrangian approach to compare the helicity between scalar tracers and fluid particles in the same flow field shows that flow structures that contribute to the transport of low Schmidt number markers are different than those for fluid particles. This is quite important in anisotropic wall turbulence, where the interplay between molecular and turbulent transport is critical.

In a study of the Leidenfrost levitation of a droplet on a hot liquid bath, rather than, classically, on an extremely hot plate, an existential feedback is revealed. The droplet induces a bath flow, whose fickle structure is explored both experimentally and theoretically. In turn, the flow drastically enhances the heat transfer to the droplet by convective means, thus ensuring such an evaporative levitation despite the poorly conducting liquid medium of the bath.

Compressible direct numerical simulation of vortex reconnection uncovers the source and far-field pattern of aeroacoustic noise.

An experimental demonstration is presented that a chirped array of cylinders can be designed to control the spatial distribution of water wave energy and substantially amplify target frequencies at specified locations, over a broad range of frequencies, consistent with linear band-gap theory.

Magnetic resonance velocimetry is used to investigate the steady inhalation, steady exhalation, and oscillatory flow in a 3D-printed realistic airway geometry for physiologically relevant regimes ranging from quiet breathing to respiration under high-frequency ventilation. Additionally, the pendulluft phenomenon is demonstrated experimentally for the first time, using both Eulerian velocity fields and Lagrangian path lines.

Numerical techniques are employed to study the locomotion of a spheroidal squirmer in a complex fluid with a nutrient-dependent viscosity. It is found that the nonuniform viscosity significantly affects the pressure field around the swimmer rather than the velocity field. Results obtained here will be helpful in interpreting experimental observation where a microswimmer significantly affects a fluid’s local rheology.

A study of the kinematics, power and recovery strokes, fluid flows, and efficiency of a reciprocal dumbbell swimmer with finite inertia finds that the swimmer’s average flow field is dominated by the flow during its power stroke, and it switches from pullerlike to pusherlike depending on the (finite) Reynolds number.

Unfolding the connection between kinematics and physiology of natural swimming species is of much value for bioinspired designs of autonomous underwater vehicles. Extensive numerical investigations find which wavelength of the undulating motion of carangiform and anguilliform swimmers is suitable to maximize their swimming performance in terms of thrust production and efficiency under different flow conditions. It is also explained how jet switching in the wake of a swimmer deteriorates the hydrodynamic efficiency of anguilliform and carangiform swimmers.

Chemical reactors often need to generate products quickly, with minimal stirring energy. What flow is best? By studying two-dimensional laminar flows via advection-diffusion simulations and via simulations of moving chemical reaction fronts, analytic predictions of converged front shape and reaction rate are devised. In the regime studied, concentrating kinetic energy in small regions causes the fastest reactions.

Simulations and theory reveal oscillations of droplets when driven by viscoelastic fluids because of their elastic memory effect. The viscoelastic oscillation helps release trapped droplets from hard-to-displace positions.

Experiments demonstrate that viscoelastic-induced particle migration concentrates solids at the core of vortices in particle suspensions. The findings can successfully be applied to the separation of solids suspended in a viscoelastic liquid with small density difference between the phases, high liquid viscosity, and small-sized particles. The proposed separation method is low cost and relevant to many chemical engineering processes.

The cooperation and competition of viscoelastic fluids, subject to pulsatile forcing, with the elastomeric microtubes that confine them is explored. Tuning of system parameters allows for the excitation of different modes, and resonances can be achieved by driving the fluid with the appropriate pulsatile pressure drop. The results are relevant at microscales and potentially useful for tailoring composite microfluidic devices, where one can induce an increase or decrease of the amplitude of the longitudinally averaged flow, relative to the one of tubes made of a single material.

In turbulent Rayleigh-Bénard convection in a cubic cell, a new oscillation is found in the temperature profile shape of the large-scale circulation (LSC). This geometry dependence is explained by a model which assumes that heat conducted to the LSC from thermal boundary layers is proportional to the LSC path length along the boundary layers at the top and bottom plates. In a non-circular cross-section cell, oscillations of the flow orientation around a corner lead to container shape oscillations in the LSC reference frame, and thus a path length oscillation of the LSC.

The GeoFlow experiment on the International Space Station was the first experiment to investigate rotating convection in a radial gravitylike force field. This led to the first observation of the existence of global columnar cells in a microgravity environment. The flow structures are identified by a machine learning algorithm and complemented by numerical simulations.

We investigate conditions for the breakup of a droplet with viscous surface moduli, under the assumption of weak flow and negligible Marangoni forces. The viscous interface is treated as a homogenous fluid obeying the Boussinesq–Scriven constitutive law. We observed that the presence of surface shear viscosity stabilizes the droplet, while that of surface dilational viscosity destabilizes it. The destabilizing effect of surface dilational viscosity appears similar to surfactant convection effects, while the stabilizing impact of surface shear viscosity appears similar to surfactant dilution.

We study the unique morphodynamics of a compound droplet resulting from the interplay between an imposed electric field and an extensional flow. The nonintuitive findings include the interconversion of shape-evolution patterns of the compound droplet system depending on the background flow strength, electric field strength, electrophysical properties, and here unveiled post-breakup dynamics. We also show that including an electric field causes an intricate dependence of extensional viscosity on the electrical properties of the inner droplet - a paradigm not prevalent in pure extensional flow.

The time a single bubble stays on a liquid bath surface depends on deterministic drainage. However, for pure water, stochastic local disturbances also affect bubble death. For binary systems like water/alcohol mixtures (WAM), preferential evaporation of one component is crucial. We examine stochastic and deterministic effects on WAM bubble lifetime statistically using an automated bubble generator. The bubble lifetime probability density and the increase in its average with ethanol concentration depends on stabilizing Marangoni stresses arising from evaporation-induced heterogeneities.

A theoretical study shows that mechanowetting can transport fluid compartments by means of traveling surface waves. Mechanowetting is a propulsion mechanism that exploits capillary forces at fluid interfaces and deforming surface topographies, enabling fluid slugs to be transported at the speed of the surface waves. The effectiveness of the method for microfluidic propulsion is quantified by means of computational fluid dynamics simulations and an intuitive theoretical model.

The effect of ultrasound on the diffusive growth of a single spherical bubble growing on a substrate in supersaturated carbonated water is quantified. It is found that the diffusive growth of surface bubbles can be easily enhanced by 2 orders of magnitude during volumetric resonance. This convective growth is shown to be caused by vigorous acoustic microstreaming arising from the nonspherical bubble oscillations.

The jetting phenomenon from impinging droplets on partially wetting hydrophilic substrates composed of cylindrical micropillars is studied. Impact velocity and fluid viscosity are varied to characterize the jets. It is found that the jetting phenomenon arises for certain ranges of Weber and Ohnesorge numbers. Jet speed, height, and diameter scale linearly with the Weber number. The scaling analysis indicates that the jet is produced by pure inertial focusing of radial flow due to the collapse of an air cavity formed at the center of the drop during the recoiling phase of the impact.

The interaction of surrounding drops is modeled as a temporally fluctuating electric field around a test drop. Because of the field-squared dependence, the drop deformation is larger under a fluctuating electric field than under a steady field. Consequently, drop breakup is observed at mean capillary numbers smaller than the critical capillary number for breakup under a steady field. Moreover, fluctuations drive drop breakup faster than a steady field.

We present the first laboratory-based results on thermohaline vortex decay. Anticyclonic vortices 1m in diameter were generated within a stratified tank on a rotating platform. Besides an m=2 baroclinic instability, if heated, initially convective edge features yield at later times to diffusive convection and layering: the presumed beginnings of a thermohaline staircase.

Gravity currents, a genre of stratified shear flow, are a tremendous source of turbulence and mixing in ocean and atmosphere. Using high-resolution laser diagnostics, large-, intermediate- and small-scale flow features of an evolving current are measured in laboratory experiments. From this, the turbulent fluxes, mixing efficiency, and eddy diffusivity are quantified, which helps in inferring the state of turbulence inside the current. The results add new insights into modeling of stratified shear flows.

Quasinormal scale elimination theory of rotating turbulence offers a new explanation of physics governing atmospheric and oceanic spectra including the well-known Nastrom and Gage spectra observed in the upper troposphere and lower stratosphere. The terrestrial circulations may be characterized as flows with “compactified dimensionality” (spatial dimension between 2 and 3). Such flows may have both inverse and direct cascades in the same inertial range (dual cascade) and their spectral amplitudes may be determined not by energy/enstrophy fluxes, but by the magnitude of the Coriolis parameter.

The flow instability in a precessing spheroid is a fundamental subject of fluid mechanics and is also important in geophysics because Earth is precessing. There are at least three instabilities: elliptical, shearing, and conical shear. Theory predicts the dominance of the latter in a regime that depends on the spin rate and container’s ellipticity. Laboratory experiments that perfectly support the theory are presented.

A discrete framework to calculate eigenvalue sensitivity gradients to geometrical changes is introduced and applied to the compressible Navier-Stokes equations. The novel formulation is used to control global instabilities in compressible turbulent flow scenarios, modeled through Reynolds-averaged Navier-Stokes approaches. The sensitivity gradients are exploited to include local surface deformations that delay or enhance the onset of instabilities and can modify their associated frequency.

To identify the mechanism triggering boundary layer transition on a spherical forebody of an Apollo type re-entry capsule, direct numerical simulations of perturbed flow are analyzed. The perturbations are generated by deterministic distributed surface roughnesses that resemble model surface imperfections that are mounted on a capsule model in corresponding experiments. The receptivity of the capsule boundary layer to the roughness and the subsequent disturbance growth are analyzed.

Experimental studies of a transitional shock-wave–boundary-layer interaction identify robust streaklike flow structures. In order to account for the emergence of these three-dimensional flow features, growth of small initial perturbations around the two-dimensional laminar base flow is examined. The most significant transient amplification originates from the upstream streamwise vortices, which are not related to modal instabilities. Unique to separated high-speed flows, the impinging shock compresses the boundary layer, thereby causing the growth of the streaks.

After breakup and emergence of a three-phase contact line, an aerodynamically driven surface-bound liquid film contracts in a characteristic triangular manner to eventually feed a rivulet. The dynamics in this film contraction zone is described experimentally and analytically where two side rims can be identified, framing a center region of thin-film flow. The side rims show a converging shape, thus defining the overall appearance of the film contraction zone.

Surface oscillations of a liquid jet, like water falling from a faucet, are common. For an orifice with n-fold rotational symmetry, the jet cross section oscillates between two symmetrical shapes. Called axis switching, it is a standing wave superposition of two opposite azimuthal ripples. We show that by adjusting the velocity profile at the orifice just one ripple can form. The jet surface is twisted from the single ripple, although the jet has no axial angular momentum. Simulations show that twisted jets can form with various n-fold rotational symmetries, including the regular square.

The rupture process of stretching liquid bridges is determined by competition between contact line slip velocity and liquid bridge thinning velocity. Poorer surface wettability and larger stretching velocity help increase slip velocity and enable the liquid bridge to slip off surfaces. In addition, the shift moment of minimal radius of stretching liquid bridges is related to surface wettability, regardless of plate stretching velocity. Finally, satellite drop formation is related to the sequential order of appearance of the liquid bridge slipping off the plate and the breakup of the filament.

If a thin liquid film is generated from a source region on a vertical surface, it may flow down the wall as a result of gravity or, if there is a strong upward air current, get carried up by the resulting shear stress. An analysis of the dynamics of such a system both with and without surface tension finds that at low source strengths, the film is carried entirely upward, whereas for stronger source strengths, the bulk of the film falls down while some is still carried upward by the air flow. Surface tension strongly affects the film profiles and the speeds of the fronts.

What happens when a fluid is pumped into a soft microchannel with height as small as the diameter of a human hair? The microchannel inflates under the flow pressure, while the flow within is affected by the resistance of the wall to deformation. We develop a mathematical model and simulate the ensuing fluid–structure interaction. Even after complex oscillations of the channel wall, we show that the final inflated state is stable. Our model highlights the parameter sets that determine the steady state wall deformation and hydrodynamic pressure distribution across a wide range of systems.

Engineered surface textures can manipulate boundary layers affecting fluid drag. We study periodic, infinitely long spanwise grooves on a laminar boundary layer over a plate for 1000 < $R{e}_L$ < 25000. Below a certain width-to-depth aspect ratio (AR), a primary vortex inside each groove causes the freestream to “slip over”, reducing skin friction. Increasing AR poses a tradeoff in drag reduction due to pressure drag from groove vertical walls. Overall, transverse grooves for laminar flow can reduce total drag up to 10% compared to a flat plate, despite increasing the wetted surface area.

In a binary fluid subject to periodic forcing the compressibility of the lower viscosity phase generates resonances in the system dynamics. In a zero-mean flow the flow amplitude could be optimized by driving the system at certain frequencies with possible increases of over an order of magnitude. When compressibility is irrelevant single fluid universal behavior can be recovered. In both situations the results may apply to a variety of microfluidic systems, such as devices with drops, fluids with trapped air bubbles, and mixing devices, since these are often subject to periodic forces.

The magnetization of real materials responds to the applied magnetic field, notably by saturating at a maximum magnitude and being coerced to be closer to the applied field direction. These effects are particularly important for soft magnetic materials, which are often used in magnetic microrobotic swimmers. Previous understanding has been based on models of permanent magnets which neglect these effects, but new work shows that taking them into account leads to an unappreciated physical limit on swimming velocities for magnetically rotated microswimmers.

In electrowetting, an electrostatic potential is applied to a droplet to increase its wettability. As the droplet rapidly spreads to its new equilibrium state, contact line friction is greatly diminished. Molecular dynamics simulations show that this effect is present at molecular scales and is related to how liquid molecules advance the contact line.

A machine-learning technique is used to develop data-driven models for turbulent heat flux prediction in wall jets with co-flow The training data are obtained by performing highly-resolved large-eddy simulations of nine cases covering various flow and geometry conditions. Robust Reynolds-averaged Navier-Stokes based heat-transfer closures are obtained, and a significant error reduction in predicting adiabatic wall effectiveness is achieved using the machine-learnt models.

Examining fluid flow inside a monolithic, three-dimensionally oriented, microfluidic channel is experimentally challenging because it does not permit a laser sheet to be effectively applied; tracking microscopic tracer particles is difficult; and monolithic construction prevents access to any material inside. We overcome these problems with an approach which captures the flow both along the channel axis and in a plane perpendicular to it, yet lets us capture the flow pattern over a wide Reynolds number range. Our observations corroborate results from flow simulations.

The locally dynamic one-equation based subgrid-scale is extended for large-eddy simulation of stably stratified turbulent flows. It is comprehensively assessed by simulating turbulent channel flow for a range of Reynolds and Richardson numbers. It captures all the key flow features, such as suppression of vertical turbulent transport, presence of internal waves and pycnocline, and decrease of the friction coefficient and Nusselt number. The turbulence statistics show good agreement with the reference results thus demonstrating its applicability to stratified flows with variable mean shear.

Axisymmetric turbulence is shown to exhibit a critical transition between two flow states. This transition is triggered by the anisotropy of the forcing of the system.

Turbulent open channel-flow under strongly stable thermal stratification is investigated. It is shown that the dominant effects of strong stable stratification on the characteristics of near-wall turbulence are transient. The budget of turbulent kinetic energy, the budget for the tangential Reynolds stress, and the relevant length scales are discussed. It is shown that near-wall turbulence at quasistationarity is approximately insensitive to the choice of upper thermal boundary condition.

Interscale energy transfers at the turbulent/non-turbulent interface are from small to large scales near the interface’s tangent plane where motions are predominantly stretching, but from large to small scales in other directions where motions are predominantly compressive. This predominance is partly due to extreme compressive motions which can be significantly more likely than extreme stretching motions even where motions are on average stretching.

Particle-laden turbulent flows in von Karman laboratory experiments and in Taylor-Green direct numerical simulations display remarkable similarities, both for tracers as well as for inertial particles. A method to test models for particle dynamics in turbulent flows, exploiting these similarities, is presented. The effect of the mean flow is also considered, showing how it impacts scaling properties of particles’ statistics, and how it affects inertial range behavior.

Low-dimensional models are desirable for turbulent flow problems that are otherwise impractical to solve. A model describing the dynamics of the orientation of convection rolls as diffusion in a potential determined by the shape of the cell is tested. In a cubic Rayleigh-Bénard convection cell, measurements confirm the model prediction of a four-well potential, along with advected oscillation modes centered around potential minima at corners, driven by turbulent fluctuations with a restoring force due to the noncircular shape of the cell cross section.

A data compression methodology for fluid dynamics is introduced. The simulation domain is divided into subdomains with equal size, and the simulation data is compressed by storing it only on the subdomain boundaries. When data is requested by users, a re-simulation of the Navier-Stokes equations within the subdomain is performed using the stored boundary data. The data storage scheme is carefully designed to avoid any errors during the compression-decompression process. A 40:1 lossless compression ratio is easily achieved.

Using fully resolved direct numerical simulations of two-dimensional homogeneous and isotropic turbulence, the Kolmogorov-Sinai entropy and attractor dimension are measured across a wide range of Reynolds numbers. In contrast with the three-dimensional case, these quantities are dependent on the system size and forcing length scale, providing further evidence of nonuniversality in two-dimensional turbulence.

Wall turbulence is characterized by a near-wall cycle of streaks and quasistreamwise vortices apparent as an invariant inner peak in the premultiplied energy spectra. A second, outer peak is known to emerge in this spectral view and become energized with increasing Reynolds number (Re) as well as adverse pressure gradient (APG). An analysis of experimental data sets examines how this outer peak scales with Re and APG and whether their imprint on the near-wall small scales are different.

The transition from forward to inverse energy cascade in turbulent flows in thin layers, varying the functional form of the forcing and the thickness of the layer, is investigated. As the forcing function becomes more three-dimensional, the inverse cascade is suppressed and the critical height $h_c$, where the transition occurs, is decreased.

Drag reduction in turbulent boundary layer flow via swept transversal surface waves is investigated by large-eddy simulations. For partially downstream traveling waves, a decreased friction drag reduction and positively contributing pressure drag are determined while the opposite occurs for partially upstream traveling waves. It is also shown that an amplification of the outer layer turbulence can coexist with a drag-reduced near-wall state.

The global wake dynamics of an Ahmed body are correctly captured by a proper-orthogonal-decomposition (POD)-based low-dimensional model. The POD modes include the quasisteady wake deviation with intermittent switches, both vortex shedding modes and bubble pumping mechanisms.

Density variations modify the way pressure transports information over large distances. As a result, the long-range correlations of the velocity field behave differently in constant- and variable-density flows. This raises the question of the permanence of large eddies in variable-density turbulence. Does this principle also hold in this context? If yes, how is it expressed? An exploration of these issues is presented.

Experiments that explore leading-edge vortex formation for plunging high-aspect-ratio swept and unswept wings are presented. For the unswept wing, the leading-edge vortex sheds and moves inboard, whereas on the swept wing, it moves outboard, leading to significant differences in the wing root bending moment. Axial velocity distribution through the vortex filament revealed jetlike, wakelike, and uniform axial velocity profiles.

A general Hamiltonian description for the trajectories of any number of interacting buoyant vortices in a homogeneous ambient fluid is presented. It constitutes an idealized description of coherent vortex structures in environmental flows where buoyancy forces are relevant for the evolution of such structures. The addition of buoyancy to this simple system results in the earlier appearance of chaos in the motion of buoyant vortex structures.

Turbulent fluid flows such as those in the ocean and the nocturnal atmosphere are strongly affected by Earth’s rotation and a stable density stratification. Using direct numerical simulations of the governing equations, the Lagrangian dispersion of particles in such flows is investigated. The anisotropy of various Lagrangian statistics is quantified, and different flow regimes are identified based on underlying linear and nonlinear physical processes. Comparisons with theory are made when appropriate.

An efficient path for energy dissipation is often looked for in wave turbulence systems. Fluid elements in stratified turbulence are shown to alternate between two invariant sets of solutions: waves and local convection, the latter connected to effective dissipation of energy. The fast evolution between these solutions explains enhancement of extreme events and balance relations in stratified flows.

When a boat is pushed at constant force from deep water to shallow water, the drag changes in such a way that two possible states emerge, corresponding to a slow speed and a fast speed. A study of the dynamical behavior in such a transition, including possible hysteresis routes, with reference to real applications such as rowing sports is presented

Recent studies of surface gravity waves propagating over a sloping bottom have shown that an increase in the probability of extreme waves can be triggered by depth variations in sufficiently shallow waters. A boundary element method is used to show that this increase in probability is greatest when the slope is steepest, i.e., for a step. A harmonic separation technique shows that the second-order terms in wave steepness are responsible for the change in the statistical properties near the depth transition.

In the absence of background rotation, the Eulerian flow induced by two-dimensional internal wave packets is well known to have the structure of long trailing internal waves. We show that rotation makes these waves evanescent and the structure of the flow over the waves in the wave packet qualitatively changes, with implications for the modulational stability of the waves.