The swimming velocity, trajectory, and flow streamlines of a 2D undulating sheet are remarkably similar to a 3D filament. However, flow derivatives such as the shear rate are dramatically different. Thus, 2D modelling in shear-thinning fluids can result in misleading viscosity fields, or “Fake $\mu$s”.

Two-dimensional simulations of a discontinuous shear-thickening suspension shows shear-protocol-dependent flow curves under imposed pressure (e.g., negative dynamic compressibility when increasing the pressure), which can be rationalized by the way the frictional contacts evolve.

The onset of cellular motion in Taylor-Couette flow is a pitchfork bifurcation under the assumption of periodicity. New experimental data show that an explanation of the observed bifurcation requires the interaction between neighboring states.

The Marangoni elasticity of a soap film is found by measuring the speed of an oblique shock on a flowing film.

In the fully rough limit, $k_s/L$ fixed, the local skin-friction coefficient $C_f$ and the roughness-normalized boundary-layer thickness $\delta /k_s$ approach universal dependencies on $x/k_s$, while in the long-plate limit, $U_{\mathrm{\infty }}{k}_s/\nu$ fixed, the flow approaches the behavior of a smooth wall.

Large-eddy simulation is used in a lung geometry to study flow in the bronchial tree. After becoming unsteady at a constriction in the oropharynx, the flow turns chaotic, exhibiting fluctuations with broadband spectra even at the most distal simulated airways with Reynolds numbers as low as 300.

Why do most fish swim with a relatively constant tail-beat amplitude of approximately 20% of their body length while their speed is linearly correlated with their tail-beat flapping frequency? Scaling analysis and experiments show that this behavior is rooted in minimizing input power for swimming.

Key theoretical variables of the tube model are examined with nonequilibrium molecular dynamics simulations. Although quantifying corresponding physical properties with these variables appears realistic, it is shown that the evolution equations arising from various tube models are not complete for describing flow process dynamics.

Flow instabilities have long been observed in viscoelastic fluids, even at low Reynolds numbers. Particle tracking methods are used to compare the turbulentlike features of polymeric solutions flowing in straight microchannels with those in curved geometries.

When a viscoelastic, shear-thinning fluid is extruded through a large aspect ratio capillary, it exhibits an inertia free, bulk flow instability resulting in helical undulations of the extrudate. This instability is shown to be supercritical, at odds with that occurring in non-shear-thinning fluids.

Simulations of compressible convection in stratified atmospheres in both two and three dimensions display turbulent heat transport that is surprisingly similar to transport in incompressible, Rayleigh-Bénard convection. Further, this heat transport is insensitive to the Mach number of the flows.

In coalescence, two bubbles touch and merge as the bridge connecting them grows from micro to macro scales. This multiscale free surface flow, involving a hydrodynamic singularity and length scales that differ by many orders, is analyzed by simulation and benchmarked against theory and experiments.

Oblique drop impact onto a deep liquid pool may result in deposition, single-sided splashing, or omnidirectional splashing. Here we experimentally study these different regimes as well as the maximum cavity dimensions, and complement our findings with basic scaling arguments.

Direct numerical simulation is used to study large deformable drops in channel turbulence. Surface tension and drop-to-fluid viscosity ratios on breakup and coalescence are examined. At large Weber values, an increase in drop-to-fluid viscosity decreases breakup rates, very like an increase in surface tension does.

Electrohydrodynamic instabilities of a semiconducting oblate spheroid suspended in weakly conducting fluids are theoretically analyzed, and an optically tunable Quincke rotor is proposed. With increasing light intensity, the spheroid can leave chaos or a limit cycle for a stable spinning (or nonspinning) state.

How rotation affects transition to turbulence of pressure-driven flow in a channel is investigated. Even at extremely low rotation rates, optimal perturbations are asymmetric about the centerline. Subcritical transition features are significant even in regimes of exponentially growing instabilities.

A nonlinear optimization technique which maximizes energy growth of a finite-amplitude disturbance to a known solution is shown to generate flow fields convergent to another “nearby” solution of the Navier-Stokes equations. Several examples are explored in detail.

A bathtub vortex has been found to be induced autonomously by instability of the steady symmetric flow if the bathtub has a rectangular cross section. A new study finds that the vortex axis deviates from the center of the drain hole by further instability and even further instability causes temporal oscillation of the vortex.

Direct numerical simulations and a theoretical model are applied to study the undulations on the surface of elongated bubbles in confined gas-liquid flows. When the Weber number of the flow is above 0.1, several undulation crests appear at the rear meniscus of the bubble because of inertial effects.

The influence of a time-dependent chemical reaction and interfacial shear stress on evolution and stability of a thin liquid film is investigated. An initially unstable film can become stable with time as the reaction rate decreases. The shearing of the film influences stability in a complex manner.

Apparent slip occurs when liquids flow over solid surfaces with trapped gas films or bubbles. If these films are very thin, the amount of slip can be different from conventional gas cushion model predictions because rarefied gas effects need to be taken into account.

The role of inertia in interfacial pattern formation in radial viscous fingering is studied for arbitrary viscosity and density ratios. A surprising effect of inertia is found: At finite Reynolds numbers the tip-splitting regime is replaced by a “dendriticlike” regime with side-branching structures.

Linear instability of compound liquid threads in the presence of surfactant is investigated. The limitation of one-dimensional approximation in previous work is removed so both unstable modes can be captured. The squeezing mode is much more sensitive to surfactant effects than the stretching mode.

The low-Reynolds-number hydrodynamics of slender-ribbon ellipsoids and tori are determined analytically. In doing so, it is shown that the hydrodynamics of arbitrary slender ribbons can be represented by a single line integral, similarly to the hydrodynamics of slender bodies.

The vortex cooling (Ranque) effect is easy to see in experiment, but theories are confusing, leading to questionable applicability of thermodynamics. A minimal hydrodynamic model reproducing the main aspects of the effect is presented and is used to examine a cooling effect with theoretical efficiency of more than 1.

A numerical investigation of laminar duct flows of dilute and semidilute suspensions shows that mean particle concentration depends mainly on the bulk Reynolds number and less on the volume fraction. The solid phase induces a cross-stream secondary motion, absent in single-phase laminar duct flows.

Effect of neutrally buoyant large particles on turbulent plane Couette flow is numerically investigated at $Re$ near transition. Second moments of the flow are relatively unchanged. However, slightly inertial particles modulate the streak dynamics and flow intermittency.

Cavitating flow around a blunt axisymmetric body close to the free surface is examined experimentally and numerically with LES and VOF methods. Unsteady behavior, including air entrainment and shedding of the cloud cavity, is observed. The numerical and experimental results are consistent.

A new experiment shows that clogging in silo discharge depends on the silo width and correlates with some features of the velocity of the grains and the kinetic stress near the orifice. The probability of clogging is maximum when the kinetic stress values in the arching region are more homogeneous.

A turbulent flow created by two counter-rotating disks in a square box is found to be bistable due to a vortex network, each state displaying a stagnation point topology. The turbulent kinetic energy budget shows that local production reaches twice the dissipation rate, leading to strong turbulent fluxes.

An easily extensible, computationally efficient, slow-growth formulation for direct numerical simulation of compressible boundary layers with complex physics is developed. The approach is specifically tailored to produce data for use in Reynolds-averaged Navier-Stokes model evaluation and is demonstrated on both low-Mach and transonic flows.

Novel statistical and dynamical approaches are used to improve the understanding of turbulence in the stable boundary layer. Nonturbulent motions appear to have a specific dynamical signature, highlighted by our indicators. The analysis is performed on data collected over Plaine Morte glacier.

Fluids with generalized temperature-dependent properties are simulated in internally heated turbulent channel flows in order to isolate the relevant physical parameters that govern the scalar statistics and to develop scaling relations for mean temperature.

The active and hibernating turbulence in drag-reducing plane Couette flows is analyzed. The qualitative picture that emerges from the investigation is a cyclic mechanism of energy exchange between the polymers and turbulence that drives the flow through an oscillatory behavior.

How large are the heat fluctuations induced by viscous dissipation in turbulent flow? This question is addressed by varying the fluid and flow properties in numerical simulations of isotropic turbulence.

A synthetic velocity gradient tensor generation algorithm is developed and used to test hypotheses regarding the structure of turbulence. Aspects of homogeneous, isotropic turbulence that exhibit significant differences compared to these synthetic tensors are identified.

A second-order statistical state dynamics model shows that turbulence in Couette flow is maintained by parametric instability of embedded fluctuating streaks, and the statistical mean turbulent state is feedback regulated to enforce statistical neutrality of associated Lyapunov instabilities.

Bandwidths of space-time energy spectra in turbulent flows are shown to depend on both amplitudes and phases of velocity modes. Therefore, phases alone cannot determine spectral bandwidths. A rescaling approach is proposed to reconstruct space-time energy spectra with the cross-spectral method.

Helical vortices are found, e.g., in wind turbine wakes. An accurate description of viscous helical solutions is needed for instability studies and control. New generic quasisteady states are elaborated in the helical framework and characterized based on high-precision direct numerical simulation results.

Prescribing an orbital motion to a glass of wine generates a rotating gravity wave that comes along with a swirling mean flow. The scaling and the spatial structure of this mean flow are analyzed in the weakly nonlinear regime using stroboscopic particle image velocimetry.

In a numerical study, a cylinder is split in half and rapid rotation in each half is modulated harmonically with a small amplitude. Because of finite viscosity and nonlinear flow conditions, wave beams produced from the split, and from corners where endwalls and sidewall meet, form intricate patterns due to wave interference.