Spatial regions that do not mix effectively with their surroundings in fully turbulent three-dimensional Rayleigh-Bénard convection are identified by clusters of Lagrangian trajectory segments. By monitoring a locally defined Nusselt number along these trajectories it is quantified that these Lagrangian coherent sets, which are indicated by the tracer clouds in the figures, contribute significantly less to the global heat transport than their spatial complement where thermal plumes rise and fall.

An inviscid and planar mixing-layer analogy has been recently developed to qualitatively identify the transverse jet transition from convectively to absolutely unstable. We have shown that introducing Reynolds number effects on the disturbance behavior as well as evaluating the entire problem in cylindrical instead of Cartesian coordinates leads to significant improvements. This novel viscous and round mixing-layer analogy can accurately identify the transverse jet transition.

A solid plate about to slam onto a water surface makes it presence felt before the actual contact by squeezing out a mediating air cushioning layer. This air cushioning layer has regions of low and high pressures. At the point of high (stagnation) air pressure, the water surface is deflected away from the impactor. While in the low pressure region, a Kelvin-Helmholtz instability initiates the suction of the water surface towards the impactor. Using a new measuring technique we measure such deflections, of the order of 10-300 microns, and explain the mechanisms driving them.

Aquatic animals swim with a wide range of kinematic motions affecting their shed vortex structures and propulsive performance. We explore the mechanistic trade-offs that occur when caudal fin swimmers use continuous or intermittent combined heaving and pitching motions. It is determined that intermittent swimming can improve efficiency for pitch dominated motions whereas heave dominated motions lead to higher efficiencies with continuous swimming. This phenomenon is a consequence of the physical origins of the force production for heave dominated and pitch dominated motions, which is discussed in light of unsteady thin airfoil theory.

We conduct experiments to study magnetic helical swimmers in viscoelastic fluids. The swimming speed is strongly influenced by the tail-to-head size ratio: the speed can be larger, similar, or smaller than the Newtonian one depending on the value of the size ratio. We conjecture that this size asymmetry induces a net viscoelastic force that affects the swimming speed.

Two-dimensional simulations considering detailed chemistry are conducted to investigate the weakly unstable detonation diffracting through an obstacle. Subcritical, critical, and supercritical regimes are identified and their distributions are significantly affected by the obstacle size and shape. In contrast, the mixtures with different nitrogen dilution have little influence on the regime distributions.

We report the first-ever experimental study of thixotropic aging in viscoplastic drop impact. A new dimensionless group is proposed and validated. The results will be useful in predicting splash behavior in a variety of applications from spray coating to fire suppression.

Surface attached micro- and nanobubbles are long-lived gaseous domains which are remarkably difficult to move or destroy. By creating a localized shear flow of sufficient shear rate from jets of cavitation bubbles, we observe that surface attached micro- and nanobubbles form long gaseous tethers, leading to the pinch-off and release of submicroscopic daughter bubbles, i.e. streaming gaseous nuclei. Detailed three-dimensional simulations and theoretical analysis show that the condition for pinch-off is dependent on the capillary number coupled with the Rayleigh-Plateau instability.

Numerical simulations are used to compare the impact of the internal fluid viscosity and the membrane viscosity on an isolated tank-treading red blood cell. Both decrease the tank-treading frequency, with moderate changes in the deformation and inclination of the red blood cell. However, it is shown that tank-treading frequencies from existing experiments are only retrieved if membrane viscosity is accounted for, and with an apparent shear-thinning of the membrane.

Soap bubbles are easy to deform: a child blowing, the wind, or an electric field imposed by a plane capacitor are some possible examples. When an electric field is applied, a bubble elongates along the direction of the field, deforming into a spheroid. For a sufficiently high electric field, bubbles eventually become conical, forming a so-called Taylor cone. In addition to the dependence on the electric field and the soapy solution used, the shape of a bubble is also related to the substrate on which it rests, namely a solid plate or a liquid bath.

Drop splashing on surfaces is important for a wide variety of processes ranging from inkjet printing to pollination by rain and forensic blood pattern analysis. The critical impact velocity beyond which the drop disintegrates is well understood for smooth surfaces, but remained a puzzle for rougher ones that are often encountered in practice. We find that the splashing threshold on rough surfaces is lower, which can be understood as an interplay between the surface roughness and the viscous, inertial, and capillary forces on the drop.

The spin-up effect - entrainment of a ferrofluid by a rotating magnetic field - is still poorly understood despite its 50-year history and many published works. This work shows that the generally accepted theory of spin diffusion can not explain this effect due to the completely insignificant value of the spin viscosity of real ferrofluids. Instead, it is shown that the heat generated in the micro-eddies that arise around the rotating magnetic particles makes the fluid magnetization inhomogeneous, and this is enough to explain the bulk flow in the fluid. The flow rate is proportional to the cubes of the field rotation frequency and the vessel radius.

The orientation vector of a spheroid in a viscous shear flow rotates in closed Jeffrey orbits on a unit sphere. If the spheroid is polarizable and it is subjected to a magnetic field, the particle tends to align along the field. The transition between static and rotating states of a polarizable spheroid in a magnetic field subjected to shear flow is analyzed, and the phase boundaries and nature of the bifurcations are identified.

A tiny electrical voltage can be generated by the collective coupling of the electron spins to the flow vorticity in laminar and turbulent liquid metal flows. Here, we demonstrate a linear scaling law between electrical voltage and pressure drop for laminar flows through capillaries with different cross sections, both, analytically and experimentally.

We report experiments observing the gravitational axisymmetric spreading of a viscous liquid with a fixed volume on inclined geometries that cause converging flows, for example, on a funnel and a bowl. The spreading of the liquid on these geometries is different from that on typical geometries such as an inclined plate: the thickness of the spreading front first decreases in time and then increases. These geometries also induce different new thresholds of fingering instabilities.

We study the stability of a thin membrane (of zero bending rigidity) with a vortex sheet as a nonlinear eigenvalue problem in the parameter space of membrane mass ($R_1$) and pretension ($T_0$). With both ends fixed light membranes become unstable by a divergence instability and heavy membranes lose stability by flutter and divergence for a $T_0$ that increases with $R_1$. With the leading edge fixed and trailing edge free, or both edges free, membrane eigenmodes transition in shape across the stability boundary. We find good quantitative agreement with unsteady time-stepping simulations at small amplitude, but only qualitative similarities with the eventual steady-state large-amplitude motions.

The three-dimensional flow features produced by a single sweeping jet actuator in a flow separation control application is investigated using stereoscopic PIV. By varying the freestream Reynolds number and actuator’s position, noticeable differences in the near-field vortex dynamics, local flow entrainment levels, and geometry of the separation bubble can be observed. Insights into these three-dimensional flow features and its effects on flow separation control are offered which may be useful in further improving the efficiency of these actuators and optimizing their operating conditions.

The physical problem of a shock-accelerated bubble has long been a fascinating subject in the study of the Richtmyer-Meshkov (RM) instability. In the current study, the behavior of a shock-accelerated heavy cylindrical bubble under the nonequilibrium conditions of diatomic and polyatomic gases is investigated numerically in describing the RM instability. In contrast to a monatomic gas, the generation of larger rolled-up vortex chains, different kinds of outward jet formation, and large mixing zone with strong and large expansion are observed in diatomic and polyatomic gases.

Drop impact with a smooth surface can produce a dimple mode of contact due to a combined effect of a capillary wave and a thin film instability.

Water entry studies traditionally employ homogeneous projectiles of varying impactor shape, entry speed, and surface roughness. Surface heterogeneity is yet another means to manipulate splash dynamics. In this experimental study, we systematically investigate the water entry of smooth, free-falling, hemispherically-coated spheres for Froude numbers in the range of 2.8-6.7. Half-hydrophobic, half-hydrophilic spheres released in various orientations produce deep seal cavities, and provoke orientation dependent super-surface splash features and sphere dynamics.

Bimetallic Pt-Au nanorods in the form of microswimmers within an aqueous solution exhibit self-propulsion that is powered by self-electrophoresis. This bimetallic Pt-Au system can be immobilized to generate convective fluid flow, thereby acting as a micropump. Here we use a combined experimental and numerical approach to investigate the key elements, including the self-induced electric field, the proton gradients, and the reaction kinetics, that impact the chemomechanical actuation of the Pt-Au electrocatalytic system. Our findings contribute towards the fundamental understanding of fluid flow powered by an electrocatalytic micropump that applies to mass transport enhancement in systems.

Like the bubble tip frozen into a drinking glass, very sharp tips are formed generically at the end of drops and bubbles in strong flows. We show that the tip curvature is exponentially large in the square of the flow strength, and that the bubble ends are almost conical, but with a slope that increases logarithmically as the tip is approached. This solution of the viscous flow equations is shown to match to the slender bubble shape valid away from the tip, found by G.I. Taylor.

We study the spreading of drops made of yield-stress fluids. In contrast to what is observed in Newtonian fluids, the final contact angle reached by the drop depends on the drop size, on its yield stress, and on the liquid/solid hydrodynamic boundary condition. This highlights the crucial role of dynamic history. We then extend the classical Young’s law to the case of yield stress fluids. In particular, by considering that the final shape of the drop is set by a dynamical arrest, we predict the observed final contact angle.

The molecular rotor 9-(2-Carboxy-2-cyanovinyl)julolidine (CCVJ) exists in two isomers. While the E isomer, which is present in the absence of light, yields a fluorescent behavior, the Z isomer is a photoproduct with no detectable luminescence. This regenerative behavior is utilized for Molecular Tagging Velocimetry (MTV).

In an acoustofluidic system, the co-flow of fluids having a difference in acoustic impedance can result in an undesirable bulk transport (or relocation) of the fluids. In this work, we show how fluid relocation in combination with primary radiation force can be used to effect medium exchange and separation of different-sized particles suspended in the fluid by controlling the flow rates and acoustic energy density.

We perform simulations of suspension of capsules with nucleus in shear flow. Every capsule is modeled as a neo-Hookean hyperelastic membrane enclosing a rigid particle with radius equal to the half capsule radius. The rheology of the suspensions is related to the capsule deformation and orientation when varying the membrane stiffness and capsule volume fraction and compared to the dynamics of capsules without nucleus.

Granular flows exhibit complex, nonlocal, behavior that has been characterized by an introduced ‘granular fluidity’ term. We use numerical simulations to investigate whether granular fluidity is solely defined by the local granular temperature and solid fraction, as was recently proposed. We show that, in simple flows, the definition holds but depends on inter-particle friction parameters. In more complex flows, the proposed definition breaks down and granular fluidity cannot be simply characterized by solid fraction and granular temperature.

In inhomogeneous turbulent flows particles whose size are close to the flow integral length scale do not sample the flow homogeneously. Instead they explore preferentially regions of low velocity fluctuations. We study how this preferential sampling becomes more and more accentuated when the flow fluctuations decrease as the Reynolds number is lowered. In the absence of fluctuations, the large particles are trapped in the vicinity of the islands of the laminar flow, due to a shear-induced lift force, for times several order of magnitude longer than those associated with the flow forcing.

The settling of a pair of particles through a density interface is analyzed for various particle positions and stratifications. The particles decelerate through the interface as surface fluid remains attached to the particles. Heightened transport is found to occur for vertically aligned particles.

Turbulent particle-laden flows have a widespread presence in industrial and natural processes. In this context, when gravity is present a natural question that arises is how the particles will settle. In this work, we show that the Rouse and Reynolds numbers seem to describe the particles settling dynamics under homogeneous isotropic turbulence. Our claim is supported by ours as well as previous experimental datasets available in the literature.

At a density ratio of 2, Galileo numbers of 70 and 160, and volume fractions in the range of 0.01 to 0.2, the pronounced angular velocities of cubes and the resulting orientation- and rotation-induced lift forces significantly promote transverse motions and the likelihood of escaping from clusters. Consequently, cube suspensions are found to be structurally more homogeneous than sphere suspensions, in addition to being more isotropic in terms of their momentum transfer properties.

Adding fibers to a liquid thread increases the viscosity, hence slows the early stretching of the thread, but it also alters the later breakup dynamics and induces a considerable variability in the thread necking shapes.

We use fully-resolved simulations to predict the force and torque acting on a long cylinder or a fiber inclined with respect to the incoming flow. Results obtained in the viscous regime are compared with predictions of the slender-body theory, possibly incorporating finite-inertia corrections. Numerical results are used to build approximate models for the force and torque valid from creeping-flow conditions up to the upper limit of the stationary inertial regime.

When modeling electrolytes, molecular scale features, for example the electric double layer, often play a role in determining meso- and macro-scale dynamics. These features cannot be captured by continuum fluid dynamics based approaches, and direct simulation methods such as molecular dynamics can be computationally expensive. Here we present a mesoscale approach for the simulation of electrolytes which alleviates these issues: the discrete ion stochastic continuum overdamped solvent (DISCOS) method.

The time-efficient method of recurrence CFD (rCFD) uses the pseudo-periodic nature of a flow, extrapolating the passive transport quickly to infinity. In that scope, the recurrence/distance matrix for bubbling and turbulent fluidization regimes are investigated to find the recurrence properties. The results indicate the need for posterior spatial filtering in the turbulent regime which, in turn, reveals recurrent uniform superstructures with a clear fingerprint on the distance/recurrence matrix.

The two-phase flow generated behind a cavitating backward-facing step is studied using the combination of three experimental techniques: wall-pressure measurements, global high-speed imaging with visible light, and high spatial and temporal resolution x-ray imaging. Three zones are identified based on the topology of the vapor fraction maps, that correspond to vaporization, transport, and condensation. Simultaneous pressure and void fraction measurements reveal that extreme events are associated with a change from a shear layer mode to a wake mode, with a temporal signature that is heavily affected by the presence of the vapor phase.

The dynamics of incompressible flows are governed by an interaction between non-normal linear dynamics and a static nonlinearity. We propose a framework for stability analysis that considers the linear dynamics subject to constraints that reflect the fact that the nonlinearity is quadratic and energy conserving. The approach can be used to conduct global, local, and non-modal stability analyses and to uncover dominant nonlinear flow interactions that drive these instabilities.

We develop a semi-analytical solution based on integral transforms (GITT) that can be employed to predict macroscopic transport in channel-matrix shear flows in a computationally efficient manner. We further demonstrate that the newly developed solution enables real-time macroscale concentration estimation in relevant applications.

In homogeneous shear flow, turbulence exhibits anisotropic properties affecting all scales of motion at finite Reynolds numbers. Upon releasing the mean shear, the anisotropy characterizing the velocity field decays over a large eddy turnover time. The decay of the anisotropy of the vorticity field, however involves a range of time-scales, from the short (Kolmogorov) time scale, up to the large eddy turnover time.

Structures of wall turbulence due to the mean shear created by the wall are generated. Numerical tests are performed with the rough-wall-like mean shear imposed in the near-wall region without resolving the surface roughness in the constrained large-eddy simulation. The results indicate that the major effects of roughness on wall turbulence can be well reproduced.

A velocity-temperature coupled control method is proposed for turbulence drag reduction of supersonic boundary layers. Direct numerical simulations indicate that heated wall blowing achieves the best drag reduction, whereas cooled wall blowing leads to a drag increase. The reduction of mean viscous shear stress is mainly responsible for the drag reduction mechanism, though there is a substantial increase in Reynolds stresses. Nevertheless, the control efficiency of heated wall blowing is low due to high energy consumption, but further investigation is needed.

The dependence of turbulence statistics in wall-bounded flows on the friction Reynolds number $R{e}_{\tau }$ is complex. Luchini proposed an empirical law for the mean velocity $U^{+}$ which has a dependence proportional to $1/R{e}_{\tau }$ at fixed $y^{+}$, and agrees well with direct numerical simulation for channel (Poiseuille), Couette, and pipe flow. We test similar laws for the Reynolds stresses and their budgets. The figure shows the wall-normal Reynolds stress $\overline{v^{+}{v}^{+}}$ in channel. The two lower curves are results at two Reynolds numbers, and the upper one the extrapolation to infinite $R{e}_{\tau }$. The symbols show an experiment in pipe at higher $R{e}_{\tau }$.

Small heavy particles can cluster in turbulence by the action of coherent vortices. We propose a general mechanism for particles with any time scale in the inertial range to form clusters. The proposed mechanism successfully describes, in terms of the coarse-grained acceleration field, the spatial distribution of sheetlike clusters of the particles. Our direct numerical simulations of turbulence at the Taylor-length Reynolds number of 740 verify the mechanism.

A new physics-based uncertainty quantification methodology is developed for understanding model form uncertainty. This framework provides insights into the reasons and conditions under which model assumptions are wrong and is used to study model form uncertainty in two-equation Reynolds-Averaged Navier-Stokes turbulence models in wall-bounded flows. Through this analysis, the shortcomings of these models are understood with an emphasis on the role of anisotropy. Error cancellation is shown to largely benefit two-equation turbulence models in wall-bounded flows, leading to models that provide correct predictions but lack physical correctness.

Compressible isotropic spectral transfer and Karman-Howarth-Monin equations equivalently quantify different processes in weakly and moderately compressible direct simulations of decaying hydrodynamic turbulence with moderate Reynolds numbers. The simulation results show that pressure dilatation does not lead to a net exchange between the kinetic and internal energies but that it may lead to a cross-scale energy transfer of the kinetic energy.

A deterministic forcing method and stochastic forcing method are proposed to keep mean turbulence kinetic energy and mean helicity at ideal values. Using the proposed forcing methods, it is shown that characteristics common to energy and helicity dissipation rates, and results of direct numerical simulations, support the joint cascade scenario of energy and helicity.

We show that the task of subgrid scale (SGS) modeling can be split into computing the total SGS energy transfer and determining its distribution among scales of motion. The former can be computed from energy transfers involving only resolved scales, providing a physical constraint on any proposed SGS model. The latter can be prescribed through classical spectral SGS modeling expressions shown in the figure or computed directly from resolved fields, allowing self-contained large eddy simulations.

We simultaneously measure both the spinning and the tumbling components of rotation for long inertial fibers in isotropic turbulence. The spinning rates of these fibers are higher than the tumbling rates, manifesting dynamics analogous to sub-Kolmogorov fibers in turbulent flows. Similar to how sub-Kolmogorov fibers preferentially align with the local vorticity, long fibers preferentially align with the large scale coherent vortex filaments that can be as long as the integral scale of the turbulent flow.

The influence of a local fluctuation in a turbulent boundary layer can propagate in many directions, potentially resulting in correlation patterns that reflect the common origin of the propagating disturbances. This process-based origin of correlation patterns is complementary to the well-known statistical signatures of organized structures. The present study introduces a novel conceptual framework termed ‘cascade analogy’ that systematizes the analysis of process-based correlations. Predicted behaviors are supported by previously reported measurements and by computational modeling involving a newly formulated reduced version of hierarchical parcel swapping (HiPS).

Near-field coherent structures in turbulent jets from round and square fractal orifices are examined with a Fourier-POD (FPOD) technique at two nozzle diameters from the exit. In the round jet, energy is mostly contained at wavenumber $m=0$, associated with Kelvin-Helmholtz vortex rings, while coherent structures in the fractal jet at the fundamental azimuthal $m=4$ capture the most energy. Nevertheless, the radial FPOD profiles are nearly insensitive to orifice geometry, forming a universal distribution with characteristic radial length scaling. Analysis of near-field fluctuations finds that streamwise vorticity and velocity are highly coupled by a lift-up mechanism in both jets.

Bubble-driven plume in stratified crossflow is modeled using Eulerian-Eulerian large-eddy simulation. Various bubble sizes and crossflow velocities are considered, and noticeable differences in the plume characteristics and material transport are observed. Statistical analysis of the simulation results provides insights on how the crossflow affects the exchanges of momentum and mass between the bubble-driven plume and the surrounding water.

Large scale application of synthetic jet actuators requires the implementation of an array of such devices. We investigate the interaction between parallel rectangular twin synthetic jets and an incoming turbulent boundary layer, using stereoscopic PIV measurements. By varying critical parameters, such as the orifice separation distance and the phase difference between the jets, we observed noticeable differences in the vectoring behavior and related shear stresses. These results may be useful in establishing optimum configurations and operating conditions for synthetic jet arrays, in the context of flow control and drag reduction.

Although normal-fluid vorticity tends to induce homogeneous, chaotic, quantum-vortex tangles, normal-fluid strain generates structured superfluid vorticity. In extensional flows, the latter is organized into vortex-sheet structures with hyperboloid geometry and fractal-like configurations.

Simulations of fully localized internal gravity wave packets with their induced mean flow superimposed reveal that waves overturn even for initial amplitudes significantly lower than the critical amplitudes predicted by linear theory, in contrast with previous results of one- and two-dimensional wave packets without rotation.

Using numerical simulations shock wave physics in a constant area channel behind a convergent or divergent channel is investigated. It is found that shock wave propagation in the downstream uniform area region is influenced by the post-shock flow unsteadinesses. A detailed flow description is provided and a quasi-steady model for determining the waves intensity at large times is proposed. Moreover, this study points out the limits of the use of Whitham’s theory in a variable area channel.