Viscoelastic turbulent flows of particle suspensions are studied using direct numerical simulations. In the presence of a moderate concentration of particles, the beneficial effect of polymers in terms of drag reduction are lost, and an increase in drag is observed for sufficiently dense suspensions.

A water drop impacts a cold surface, spreads and then retracts while simultaneously freezing. Experiments explain the surprising shape of the frozen drop and measure the contact angle dynamics of water on ice, by examining the competition between freezing and retraction.

Considering a non-Brownian particulate suspension undergoing Taylor-Couette flow with only the inner cylinder rotating, we report new patterns such as interpenetrating spiral vortices in which upward and downward helical vortices co-exist, and also Taylor vortices coexisting with either spiral or wavy vortices.

Turbulent flows are strongly irreversible and dissipate energy in a highly nonuniform and intermittent manner. In a Lagrangian view the flow irreversibility manifests as a temporal asymmetry in the kinetic energy fluctuations of tracer particles. Using direct numerical simulations we find a surprising connection between tracer energy fluctuations and their passage through intense Eulerian dissipation zones. We also study a nonintermittent turbulent flow and find Lagrangian irreversibility to be insensitive to extreme events in Eulerian dissipation statistics.

Deformation of vulnerable stenosis under pulsatile flow conditions and flow-induced stress acting on a fibrous cap are revealed to be proportional to the square of flow rate divided by fibrous cap thickness. Angle variation of jet flow at the throat of vulnerable stenosis is recommended as a diagnostic index for predicting the stress on the cap.

A complete theory to assess the longitudinal dispersion of a passive solute injected in an annular porous cavity and subjected to a pulsatile flow is provided. The model serves to ameliorate intrathecal therapies in the cerebrospinal fluid within the subarachnoid cavity, and it highlights the key role of the anatomical fine structures and cerebrospinal fluid pulsations.

The exact geometry of the thin glycocalyx layer that coats the interior of blood vessels is unknown. A mathematical model is developed to predict how differences in geometry affect the hydraulic flow through the glycocalyx, and how the glycocalyx geometry could be inferred through experiments.

A new type of transient shear banding in entangled polymer solutions observed in Taylor-Couette flow is reported. A novel combination of rheo-particle-tracking-velocimetry and rheo-microscopy reveals that the banded structure correlates with spatial variations in shear-enhanced concentration fluctuations. The results suggest that local concentration fluctuations, enhanced by shear, lead to extra dissipation within the fluid, which increases the effective fluid viscosity that can drive large flow nonuniformities when such dissipation varies spatially within the flow.

The Navier-Stokes equations with a no-slip boundary condition can be used to model continuum gas flows. For rarefied flows, however, alternative transport equations and boundary conditions are needed. We perform a matched asymptotic expansion on the Boltzmann equation and formulate transport equations and boundary conditions for slightly rarefied acoustic flows. We demonstrate their utility by studying oscillatory flows generated between two heated plates, thermal creep and rectilinear motion of a sphere, and compare to the Navier-Stokes-Fourier equations with slip boundary conditions.

We study a bubbly flow confined in a Hele-Shaw cell in the presence of a linear stratification through numerical simulations. Under confinement, turbulence is suppressed, and mixing comes primarily from transport in bubble wakes. Simulations are run for a range of void fractions and Froude numbers Fr, which varies the stratification strength. Among other results we find that when the stratification strength is increased, the fraction of total energy lost to buoyancy increases while the cross isopycnal diffusion decreases.

A numerical study of the steady motion and deformation of a vesicle freely suspended inside a circular tube in a pressure-driven flow is presented. A phase diagram of vesicle shapes is drawn and a shape transition line is proposed separating the parachute-shaped region from the bullet-shaped one in the reduced volume versus confinement phase space. High-resolution simulations allow examination of the hydrodynamic interaction between the wall boundary and vesicle surface at conditions of very high confinement. Furthermore, several correlations are presented and their practical implications discussed.

A combined numerical and experimental study reveals the optimal range of fluid properties for inkjet printing. The parameter space of fluid viscosity and surface tension required to produce single drops at a prescribed speed is identified and shows an additional restriction on the Reynolds number in order to jet single droplets.

The features of axisymmetric magnetohydrodynamic duct flows in annular geometry, driven by current injection perpendicular to an externally applied magnetic field, is investigated numerically and analytically. Scaling laws for the velocity in the three regimes previously reported in the literature are derived for a wide range of control parameters and systematically compared to experimental data.

Exponential amplification of weak seed magnetic fields in a turbulent medium eventually slows and saturates due to back reaction by the Lorentz force. This is known as the fluctuation dynamo action. Using numerical simulations of driven turbulence, we show that the magnetic fields achieve a (statistically) more force-free state as they saturate. The spatially intermittent random magnetic field structures are also larger in the saturated stage than in the kinematic stage. The fluctuation dynamo saturation is explained by a decrease in amplification and an increase in magnetic field diffusion.

The breakup of jets of superfluid and normal liquid ${}^4$He is studied between 1.2 K and the liquid-vapor critical point at 5.2 K. Both gas and liquid properties vary widely over this small temperature range, creating a unique parameter space with variations of several orders of magnitude for the Ohnesorge number, Reynolds number, and gas-liquid density ratio. The five breakup regimes seen previously, and transitions between them, are described in detail and shown pictorially. New criteria are proposed for the Rayleigh to 1st wind, 1st wind to sinuous, and sinuous to 2nd wind transitions.

Capillary processes governing fluid-fluid displacements produce leap movements of the interface between invading and displaced fluids, and hysteresis or lag between invasion and withdrawal. We examine the passage of a fluid front through a localized constriction in a Hele-Shaw cell. Our experiments show there are large capillary jumps at one end which cause a microscale hysteresis cycle while at the other end the front is reversibly pinned. With a mass-spring model we reproduce the experimental data without fitting parameters, providing insight beyond phenomenological approaches.

A granular raft may maintain its cohesion at greater than expected external flow. This behavior is also found for two beads at an interface. In both cases, the tilting of the contact line around the particles appears to be the key aspect in order to describe the capillary interactions and account for this enhanced cohesion.

When a particle-laden drop impacts a surface, the spreading and fragmentation of the resulting thin liquid film is modified by the presence of the particles beyond the simple increase in viscosity.

An experimental study shows that polygonal hydraulic jumps, which are formed when a liquid jet impacts a target plate, can exhibit modal hysteresis, yet the normalized jump geometry $A/PH$ is unaffected by the flow history and has a weak dependence on the weir geometry. All experimental data collapses upon scaling with the downstream Weber number, illustrating the role of surface tension in mode selection.

Soft plates immersed in fluids appear in many biological processes, including swimming, flying, and breathing. A model of pressure-driven low-Reynolds-number flow in a narrow slit formed by two elastic plates highlights two competing effects: While the plate bending generally reduces the slit aperture, it also causes the two plates to rotate and move apart thus increasing the gap. This leads to a strongly nonlinear flow-rate versus pressure-drop relationship.

Analytic approaches have been previously applied to compute the weak inertial lift on spheres above a plane wall by using an asymptotic expansion in terms of the Reynolds number. By building on these approaches and combining the method of regularized Stokeslets, a numerical method that computes the weak inertial lift on arbitrarily shaped objects in the presence of a plane wall is developed.

The influence of thermal fluctuations on the dynamics of nanoscale liquid threads is investigated using both molecular dynamics (MD) and a stochastic lubrication equation (SLE) derived from fluctuating hydrodynamics. These methods recover a range of breakup profiles, including the ‘double cone’, and we quantify their occurrence statistics. The SLE is about 5000 times faster than MD and permits access to a broader range of parameters. We use it to probe the final stages of rupture and compare to proposed similarity solutions, showing its usefulness for investigating nanoscale interfacial flows.

A three-dimensional microfluidic device with embedded symmetries enables rich flow controls for complex manipulations of particles. These flow characteristics, especially a strain-free flow with uniform flow velocities, are protected by symmetries. This nonperturbative capability serves as a supplement to current perturbative micromanipulation approaches.

Simulations using a three-dimensional lattice Boltzmann multicomponent model are used to investigate droplet formation in a T-junction microchannel for a shear-thinning continuous phase. The shear-thinning behavior is captured using the Carreau-Yasuda model and is validated with experiments. A greater shear-thinning tendency of the continuous phase fluid is shown to be responsible for an increase in size and a change in shape of droplets from spherical to plug-shaped, typically observed in low capillary number Newtonian flows.

A kinetic-theory-based model of granular flow which predicts the depth-varying segregation velocities of rising and sinking particles in density-bidisperse mixtures is presented. The segregation velocity results from a balance between the buoyant force, due to particle density and concentration, and the interspecies drag force from kinetic theory, which depends on interparticle friction and local flow conditions.

Motions of grains such as in dust storms, sand dunes, snow avalanches, Saturn’s rings, and others are categorized as “granular” flows. We have used the Maxwell-Boltzmann kinetic theory to analyze rapid granular flows with an analogy between the motion of macroscopic grains and the random motion of atoms in molecular gases, with a crucial difference that the grains collide inelastically. The inelasticity is responsible for many peculiar behaviors of granular gases as we demonstrate by considering shock-wave propagation in granular gases with higher-order hydrodynamic equations.

Particle- and droplet-laden flows occur in many natural and industrial processes. To probe their dynamics at high particle concentration we use phosphorescent droplets that can be tagged with a laser. Only the tagged droplets emit light, so that subsets of particles can be selected inside a dense cloud. In our proof of concept experiments we obtain statistical information about the droplet distribution in space, and observe their clustering dynamics in a time period that spans a few turnovers of the smallest eddies in turbulence.

Instability and transition in an elementary porous medium are investigated using global linear stability analysis and numerical simulation. The representative elementary volume porous medium consists of a staggered array of square cylinders. Two unstable modes are captured from the linear stability analysis: a two-dimensional oscillatory mode and a three-dimensional stationary mode. Both the lift-up and converging flow effects are responsible for the instability.

A novel implementation of a boundary integral method is used to compute the flow around models of marine aggregates made of cubic particles. These aggregates have a fractal structure and two formation mechanisms, each with a corresponding fractal dimension. The drag, torque, and straining force on these aggregates is characterized as a function of an appropriate measure of their size.

Pleated membrane filters are used in a wide variety of applications. A model is derived to predict and optimize the performance of a pleated membrane filter (in terms of how pore radius varies in the depth of the filter membrane) as adsorptive fouling (standard blocking) occurs. For the chosen objective function the optimized pore shape is found to be funnel-like. The model may be the first to predict deterioration in particle retention with standard blocking alone.

A spatial filtering strategy is used to investigate inertial particles interacting with certain features of wall turbulent flow, thus isolating large-scale motions (LSMs) and very large-scale motions (VLSMs). In the one-way coupled limit, low-inertia particle transport is dictated both by LSMs and VLSMs, while high-inertia particles are more influenced by VLSMs. In the two-way coupled limit, particle-flow interaction is dictated by interactions with LSMs. However, the effects of VLSMs still cannot be ignored.

The one-dimensional-turbulence method is extended to capture turbulence modulation caused by particles in stationary forced, homogeneous isotropic turbulence. Because of the very low computational costs of the one-dimensional model, it enables, after validation with direct numerical simulations, the study of turbulent parameter ranges that are not otherwise accessible. This study is a first step in the development toward a reliable subgrid-scale model for high-turbulent large-eddy simulations.

Direct numerical simulations show that a tracer introduced into a turbulent homogeneous medium will not be fully mixed. Natural barriers are present, across which the tracer concentration jumps, typically from the smallest to the largest value. An analysis of these mixing characteristics is presented.

Large-scale vortices (upper panel) are broadly observed in turbulent flows subject to system rotation, wall curvature, and buoyancy, and are characterized by peaks in the energy spectra at large wavelengths in Fourier analysis (lower panel). We elucidate the underlying energy transport process which sustains large-scale turbulent motions in a streamwise-rotating channel flow with a systematic investigation of the budget balance of the energy spectra. A comprehensive energy-transport chain is summarized to provide a new perspective on the modeling of turbulent flows with large-scale vortices.

The vibrational mode of internal energy is excited in high-temperature compressible turbulence in many physical problems and applications. Direct numerical simulations are used to investigate the vibrational rate and the dissipation and production of vibrational energy fluctuation in compressible isotropic turbulence with vibrational nonequilibrium. Effects of thermal nonequilibrium and compressibility are taken into account.

In the framework of wave turbulence, it is shown analytically and numerically that rotating hydrodynamics and magnetized plasmas at kinetic scales can be described by the same nonlinear diffusion equation. This result means that laboratory experiments can be useful for better understanding solar wind turbulence and vice versa.

To determine how a fluid carries its energy down to microscopic scales where it is dissipated, a result from the assumption-free theory of the nonperturbative renormalization group was tested, using numerical simulations and data from the ONERA wind tunnel. The findings confirm that the kinetic energy spectrum follows the predicted stretched exponential in the dissipation range .

A discrete exterior calculus investigation of flow past a stationary cylinder of circular, triangular, or square cross section embedded on a spherical or cylindrical surface shows insignificant effect of the embedding surface curvature. The contributing parameters, such as the magnitude of the Gaussian curvature term relative to the magnitude of the viscous term around the cylinder and the effect of the geometrical constraint, are insignificant. Is the dynamics of the flow past bluff bodies universal?

Accurate measurements and axisymmetric numerical simulations of the spin-up of a fluid driven by a rotating disk are carried out. In particular, disk dewetting is taken into account. This constitutes a solid benchmark and paves the way for future numerical investigations on the three-dimensional regime of rotating polygons, for example.