Direct numerical simulations of a FENE-P fluid in both two- and three-dimensional straight periodic channels find that elasto-inertial turbulence is fundementally two-dimensional. The spurious effect of artificial diffusion of the polymer is demonstrated.

Experimental observations suggest the existence of an extended transitional region, as far as 30–50 diameter from a synthetic jet exit, where the spreading and decay rates are significantly enhanced over a continuous jet.

Numerical simulations of plane-Poiseuille flow find a state of sparse but persistent oblique turbulent bands in a Reynolds number range between about 660 and 800, which is below the anticipated directed percolation transition.

Simulations show that a suspension of deformable incompressible spheres in a Newtonian fluid displays shear-thinning and that this can be understood in terms of a reduction of the effective volume fraction occupied by the suspended spheres due to their deformation.

Two different patterns of erosion are observed in steady flow past a vertical cylinder emerging from a granular bed: a large hole at the base of the cylinder or two side-by-side elongated holes in the wake of the cylinder.

The Zagarola-Smits scaling is derived directly from a detailed analysis of the mass and mean momentum balance in the outer region of the zero-pressure-gradient turbulent boundary layer.

Direct numerical simulations of turbulent channel flows up to $R{e}_{\tau }=8000$ find logarithmic variations both in the mean velocity and streamwise turbulent variance, although these logarithmic regions do not agree with each other.

Multiple vortex-pair morphologies were studied using particle image velocimetry in steady and pulsatile flows through elastic curved vessels. Torsion effects were more profound than local vessel compliance (elasticity) effects. Torsion induces asymmetry in the vortical structures and causes like-signed vortices to merge.

In a numerical investigation of the flight dynamics and aerodynamics of a two-dimensional jellyfish-like ornithopter, three locomotion states are identified with the variation of control parameters. The responses of the ornithopter to physical and numerical perturbations are also examined.

Experiments show that swimming speed has little impact on the forces or wake produced by a fishlike propulsor. Both continuous and intermittent swimming styles are explored. The results indicate that there may be no need to mimic a free-swimming (self-propelled) condition in laboratory studies.

Adding a small amount of rigid particles to a viscoelastic fluid leads to interesting suspension rheology, because the gradients near the particles induce elastic stresses in the fluid phase. We derive the viscosity of a dilute suspension of spheres by perturbation theory for a weakly elastic fluid.

Creeping motion of a long bubble in 2D planar channels and axisymmetric tubes filled by a Bingham fluid is studied numerically. Bingham and capillary number effects on bubble shape, flow field, yield surfaces, and the wall shear stress profile are discussed and applied to human respiratory airways.

The vortex-surface field (VSF) is extended to investigate the vortex dynamics in compressible Taylor-Green flows at a range of Mach numbers. The compressibility effect on the process of vortex reconnection and the interaction of shocklets and vortex surfaces using VSFs is elucidated.

Removing typically-applied approximations (quasi-steady-state electronic levels and local escape factors) noticeably impacts the predicted shock-layer radiative heating to the afterbody region of Earth entry vehicles.

Domains of regular and irregular reflection for underwater oblique shock waves have been identified theoretically. We compare the Tait and Mie-Grüneisen equations of state in their ability to demonstrate physics of shock wave reflection and find implications for designing blast mitigation devices.

Heat transport enhancement and flow topology of turbulent Rayleigh-Bénard convection under geometrical confinement are studied numerically for over two decades of Prandtl numbers, with Ra fixed at ${10}^8$.

Large scale cyclonic vortices, such as tropical cyclones in the Earth atmosphere, exhibit an eye in their center. A new mechanism to understand the formation of such eyes is investigated. Contrary to standard explanations, it is shown that a purely hydrodynamic instability can yield an eye.

Rayleigh-Bénard convection is studied in a liquid metal exposed to a horizontal magnetic field. Declining field strength at a given Ra number enables the development of diverse flow oscillations, which are attributed to two-dimensional variations of the roll structure and emerging 3D flow effects.

The impact velocity affects the maximum spreading diameter of blood droplets. Observations show that it also affects the width of the bulge visible after evaporation on dried blood drip stains. The interpretation of this phenomenon could have a possible forensic application.

We experimentally observe streaming patterns around pulsating bubbles. Under flow, closed recirculation zones appear, that we can predict with a dipolar-like description of the acoustic streaming flow field. This can be extended to model the complex patterns around a large group of bubbles.

Bubble bursting is a ubiquitous phenomenon, impacting the climate by producing sea spray aerosols above the ocean, while also controlling the fineness of the effervescence of sparkling wine. Numerical and laboratory experiments are used to rationalize the velocity of jets formed by bubble bursting.

An integrated experimental and theoretical investigation of the promenade mode, a bound state formed by a pair of droplets walking side by side on the surface of a vibrating fluid bath, highlights the role of bouncing phase adaptation in stabilizing the promenade mode.

Gravity induces parallel growth of finger structures which forms when air displaces a granular-liquid mixture. The alignment direction, which varies between horizontal and vertical, is explained by the interplay between surface tension, yield stresses, and the hydrostatic potential.

In transitional pipe flow, isolated turbulent spots carry substantial pressure and velocity fluctuations as they travel along the pipe. A model is proposed that links the propagation speed of such spots with the exact coherent solutions to their spatial localization rate.

Organized three-dimensional structures emerge in the turbulent wakes of bluff bodies. Numerical simulations are employed to track these dominant patterns, explore their evolution in the different flow compartments, and quantify their alteration once the body is subjected to flow-induced vibrations.

Optimal velocity perturbations arising from the interaction of a turbulent flow with a soft compliant wall. The elastic nature of the wall gives rise to several modes of instability having variable growth depending on the characteristics of the fluid flow as well as the wall properties.

Numerical simulations show that while a short-duration jet ejected into a Blasius boundary layer directly generates hairpin vortices with narrow legs, they themselves do not lead to turbulent transition. Generation, by vortex reconnection, of a hairpin vortex with widespread legs is key in the transition to turbulence.

Evolution of the flow regimes in the wake behind a streamwise rotating sphere was experimentally studied focusing on the Reynolds number versus rotation rate parameter plane. Four different regimes were observed in the wake and were investigated using modal decomposition and frequency analysis.

Above a critical angle, an oblique shock impinging on a Mach 5.92 boundary layer causes the resulting separated flow to become unstable. Direct numerical simulation and global stability analysis reveal the physical mechanism that drives this three-dimensional instability.

Very-low-amplitude, high-frequency forcing is used to manipulate the laminar separation bubble on a NACA 0012 airfoil pitching at a constant rate. Properly tuning the forcing frequency to track the changing Kelvin-Helmholtz instability results in significant delay of the onset of dynamic stall.

The cylindrical Richtmyer-Meshkov (RM) instability is studied in a shock tube. The growth rate of this instability does not saturate in the nonlinear regime as it does in the planar geometry. Numerical and theoretical studies show that a Rayleigh-Taylor instability superimposes on the RM one and that Bell-Plesset effects enhance the growth of the instability.

Local slip boundary conditions at the liquid-gas interface of superhydrophobic grooves with beveled edges and nonuniform depth are investigated. We show that local slip at deep grooves is controlled only by width and bevel angle. This could enable tuning and constraints on large slip in gas areas.

The aerodynamic drag and consequent deformation of a perforated flexible strip are studied with a combination of wind-tunnel experiments and numerical simulations. Increasing the porosity of the strips causes a reduction in the local drag coefficient similar to that seen in rigid perforated plates.

In many applications of synthetic motors objects are moved through environments with component mixtures. Here active colloid propulsion is experimentally probed versus solution viscosity in hydrogen peroxide solutions, and interpreted with existing diffusion reaction models of self-diffusiophoresis.

The much studied von Kármán swirling flow in a small ratio vessel reveals a novel flow structure with a tornadolike rotating core flow in the laminar regime. Although the flow pattern is complicated and differs from von Kármán’s prediction, the friction coefficient scales in accordance.

The effective viscosity of mixtures is of interest for many applications. A model is proposed that relates this effective viscosity to minimal information regarding the microstructure of the mixture. This model is validated by means of direct numerical simulations of flows of these mixtures.

The stability of the boundary-guided motion of a diffusiophoretically self-propelled locomotor along a planar wall to mechanical disturbances and Brownian forces is examined theoretically. The main goal is to understand the conditions under which this passively guided motion is stable.

A long-range, apparently electrostatic, interaction has recently been found for surface forces across ionic liquids. Here we disentangle equilibrium and viscous contributions, confirming an exponentially decaying static force, and elucidating the slip boundary condition for drainage of ionic liquid.

Molecular dynamics simulations of nanochannel flows generally assume streamwise homogeneity applying an external driving force to represent a pressure gradient. A careful evaluation of this hypothesis shows it to be valid only at low pressure differences due to heating and compressibility.

Partially wetting thin films that are connected to plateau borders are described. The lifetimes of such films are found by matching the regimes of early capillary thinning and later collapse due to intermolecular forces.

The cohesion of a wet grain assembly is encountered in many situations, such as in the fabrication of sandcastles, the preparation of couscous, crumbles, or other granulated systems. Two simple experiments on model liquid/bead mixtures are performed to understand the behavior of such systems.

Leapfrogging occurs, for example, in smoke or bubble rings. We model it in two dimensions with two interacting vortex pairs. Varying the vortices initial positions, we observe instabilities and bifurcations of leapfrogging along with other features including walkabout, braiding and chaotic motions.

The ability of a self-propelled swimmer to displace fluid—thus inducing mixing—in terms of the drift volume is assessed. The drift volume is found to be generally large when the Reynolds number is small, while at larger Reynolds numbers, it varies widely depending on the kinematics of swimming.

Direct numerical simulation data of a jet with two passive scalars is compared to explicitly filtered data of the same case with regard to the influence of the turbulent/nonturbulent interface (TNTI) on differential diffusion. Filtering is found to affect differential diffusion differently at the TNTI and in the jet core.

An experimental investigation of the turbulent jet which issues from a slender open-core annular source reveals that the near-field dilution is enhanced by entrainment of fluid induced through the core. Exploring the streamwise development using particle image velocimetry, the jet is delineated into four distinct regions.

An exploration of how finite dimensional systems reach thermodynamical equilibrium shows that in real space, thermalization first occurs in partial subdomains.

In cold clouds, small crystals of size $\sim$ 100 $\mu$m settling in a turbulent fluid need to aggregate through collisions to form large hail or graupel particles. Using fully resolved direct numerical simulations, the role of the two main physical processes responsible for collisions is clarified, namely, turbulence and the difference in settling velocity due to different crystals orientations.

Spanwise repeating boundary-layer trips are tested to investigate the influence of various geometrical parameters of the constituent obstacles on the nature of the transition region downstream of the trips, where the artificially generated turbulent boundary layer evolves towards a canonical state.

A persistent small-scale anisotropy of turbulent passive scalar fields, which is independent of the scalar field initial conditions, is observed in direct numerical simulations of fully developed channel flow, even after the flow has undergone substantial mixing, and is attributed to the mean velocity gradient.

Axisymmetric turbulence shares features with both two- and three-dimensional flows. A spectral analysis shows the presence of a dual energy-helicity cascade, with energy transferred to the large scales and helicity to the small ones.

Inspection of spatial features of secondary flows in a turbulent boundary layer over longitudinal surface roughness shows that secondary flow size is mostly determined by the valley width and the rotational sense of secondary flows is affected by the turbulent transport term in the energy budget.

The structure of high and low shear-stress events in a turbulent boundary layer is investigated by means of simultaneous wide-field particle image velocimetry and wall-shear stress measurements. Contributions of the small and large scales are discussed based on a scale decomposition of the velocity field.

We study the effect of wall boundary conditions on turbulence intensities in the context of wall-modeled large-eddy simulations. It is shown that the use of a slip boundary condition at the wall alleviates the well known turbulence intensity over-/under-prediction in coarse large-eddy simulations.

A new investigation identifies frequencies in minimal channel flow relevant to critical layer dynamics displayed by exact coherent states and shows their predominance over bursting frequencies away from the wall. It is also shown that strong bursts are related to the instability of coherent states.

Passive scalar fine-scale structure is studied numerically for heated grid-element scalar turbulence. Scalar variance decays earlier in space than turbulent kinetic energy (both with dissipation). A $-5/3$ slope in the scalar-variance spectrum is found beyond the Kolmogorov-Obukhov-Corrsin theory.

The added-mass concept can be validly applied to viscous flows, provided that the circulatory-force expression accounts for image vorticity. The relationship between added mass and image vorticity is explored, and the most appropriate physical interpretation of added mass is discussed.

Coherent vortex structures are tracked in simulations of massively-separated and turbulent flows using Lagrangian saddles found as intersections of positive and negative finite-time Lyapunov exponent ridges. This allows vortices to be tracked objectively in time and space in a variety of flows.