Variable-density has important physical effects on turbulence, including driving mixing from small to large scales. Many compressible and variable-density flow applications cannot be simulated directly. Recent experimental measurements have implications for modeling and underresolved simulations.

When a mixture of noncolloidal particles and oil displaces air inside a Hele-Shaw cell, particles accumulate on the interface and cause fingering by generating an unstable viscosity gradient inside the suspension. A review and new data of the fingering onset and evolution are presented.

The image of a turbulent jet reveals complex spatial patterns, which result from the decay of turbulent eddies into smaller and smaller structures. Here we lay out a road map to describe such patterns as a result of singularities which possess both a nontrivial spatial structure, and exhibit instability as they progress toward smaller and smaller scales.

This paper sketches my personal scientific bubble journey, starting with single-bubble sonoluminescence, continuing with sound emission and scattering of bubbles, cavitation, snapping shrimp, impact events, air entrainment, and surface micro- and nanobubbles, and finally arriving at effective force models for bubbles and dispersed bubbly two-phase flow.

Liquid transport in a dry foam, a deformable porous material, in the presence of an applied electric field, is computed in the limits of rigid and mobile interfaces. The evolution of the liquid distribution is diffusive-like and significantly affected by the hydrodynamic boundary conditions.

Since the development of propagating-wave-based remote sensing more than a century ago, signal analysis has been limited to in-band recorded frequencies. However, this limitation is artificial. A new discovery from the realm of sonar shows that remote sensing is possible at out-of-band frequencies.

Flashback is a key operational and safety issue for modern low-emission burners. With direct numerical simulations we study upstream flame movement through the turbulent flow within a channel and address the marked change in flashback behavior when the fuel and oxidant mixture is not homogeneous.

A scenario in which shear thickening results from a transition of interactions between suspended particles from lubricated to frictional (LF) with increasing stress is described, with a presentation of primary results and a perspective on outstanding questions which are raised by the LF mechanism.

The power density in a rocket engine is 50 $GW/m^3$. If flame oscillations lock into the acoustic modes of the chamber, the resultant noise can blow up the engine. This paper explains pedagogically, with example code, how best to use adjoint methods to passively control this thermoacoustic instability.

An investigation of a leaking aortic valve finds that it prompts double-jet filling in the compliant left ventricle of the heart, inducing complex and inefficient blood transport. This gives rise to increased blood residence time and the emergence of attracting material lines, presenting favorable conditions for thrombus formation.

A model problem of low-Mach number lean premixed reacting swirling flows with wall-separation zones in a contracting open chamber is studied. A theoretical feasibility for a technology of swirl-assisted combustion where the reaction zone is supported by a wall-separation zone is established.

An investigation of fluctuations occurring in rotationally symmetric systems draws a link between these fluctuations and light polarization, two-state quantum systems, and quaternion numbers. An ansatz is proposed to study this class of problems and is applied it to azimuthal acoustic instabilities in an annular combustion chamber.

Cross-slot devices are routinely used to study the extension of high molecular weight polymer molecules and DNA in solution. We simulate the extensional flow of Boger fluids in an optimised cross-slot rheometer and investigate various emerging time-dependent instabilities.

This paper presents a new implementation of the Macheret-Fridman dissociation model for the direct simulation Monte Carlo method to model thermal nonequilibrium dissociation reactions. We find good agreement with recent quasi-classical trajectory calculations and recent experimental results.

Large-scale patterns in three-dimensional turbulent Rayleigh-Bénard convection in a large-aspect-ratio cell are probed by the clustering of Lagrangian particle trajectories.

Lagrangian dynamics of turbulent Rayleigh-Bénard convection is studied experimentally with 3D particle tracking velocimetry at various aspect ratios $G$. Insight on the pair dispersion, conditional acceleration probability density functions, and roll dynamics induced by $G$ was found from nearly $3.5\times {10}^7$ events.

Multiple stable states exist for two-dimensional tilted convection with large aspect ratios (Γ≥2). The Nusselt number generally decreases monotonically with increasing tilt angle for large Γ cases with Γ≥8.

Hydrophobic particles are selectively separated from aqueous suspension by a novel oil-in-water binder. The collection efficiency is predicted in this work by a hydrodynamic trajectory analysis, and shown to be greatly enhanced by water permeation through the oil layer.

Longitudinal strips are suggested as a new device that can significantly increase the aerodynamic performance of a low Reynolds number airfoil at post-stall angles of attack. Its mechanism relies on the generation of corner vortices that delay flow separation on the airfoil suction surface.

A stability analysis of the Blasius boundary layer formed by a fluid with a temperature-dependent viscosity is examined. The results inform whether hydrodynamically unstable flows may exist inside chemical vapor deposition reactors (CVD) and how a comprehensive CVD stability model can be developed.

A theoretical study shows that adding disk-shaped particles to the fluid between counter-rotating cylinders reduces the onset-speed for secondary flow.

Thin-film experiments with an interferometric apparatus show an absence of a strong relationship between interfacial elasticity and liquid entrainment in thin foam films, which can affect rational design of aesthetic and tactile properties in foaming consumer products.

Space/time-resolved film-thickness data are presented for liquid films flowing under an inverted planar substrate. Different flow regimes are observed depending on the inclination, liquid properties, and Reynolds number, with waves characterized by pronounced three-dimensionality and rivulet formation.

A surfactant coated drop is drawn through an aperture by a converging flow. Under suitable conditions that have also been investigated in a series of independently led experiments, the drop tip emits a thin tipstreaming thread that later breaks up into near-monodisperse droplets.

Evolution of a shocked inverse-chevron interface is studied experimentally, and the effect of vertex angle is highlighted. A new nonlinear model is constructed and provides a better prediction of interface growth than previous models.

Bubble-collapse-driven penetration of liquid-liquid interfaces exhibits two scaling ranges of penetration depth vs time. Detailed numerical simulations show that size and evolution of generated interface perforations depend on viscosity, shock strength, and single- or multiple-bubble configurations.

The flow of water vapor against temperature gradients naturally occurring in drying of porous media leads to condensation and simultaneous gas-liquid invasion of pore space. This phenomenon is studied with a model porous medium in a microfluidic device and a pore network model of drying and imbibition.

A design for and experimental study of microfluidic nozzles that generate thin liquid sheet jets is presented. A parametric study is conducted by varying nozzle geometry, flow rate, and fluid type, and scaling theories to scale and predict jet sheet thickness, width, and length are proposed.

Intrusion of a particle-laden mixture into a pure fluid is studied experimentally within a duct. Applications are found in the discharge of slurries, mine tailings, pastes, sludge, effluents, and sewage. Exotic sedimentary, transitionary, and mixing regimes are found over various inclination angles.

If flow inertia is finite at the particle scale in a channel, particles align parallel to the flow streamlines, forming stable trainlike structures near the walls at low concentration. A numerical investigation finds that hydrodynamic interactions prohibit train formation at lengths exceeding the channel height.

Suspensions made of either faceted or spherical particles behave in quite different ways. However, experiments show that it is possible to unify their behavior by considering the contact contribution to the viscosity that is evaluated through shear reversal experiments.

An investigation of how large-scale flow is affected by changes in the properties of small-scale flows, the layer thickness, and viscosity is presented.

A nonlocal version of the Gross-Pitaevskii equation is investigated. The role of rotons in vortex reconnection is probed through numerical simulations of both local (standard) and nonlocal models. Observed dynamics are compared with the predictions of the local induction approximation.

Current understanding of turbulence in the vicinity of surface roughness is mostly limited to narrow-scale or regular roughness geometries. A numerical analysis shows how various scales of a multiscale, realistic roughness affect friction, momentum balance, and turbulence production in the roughness sublayer.

A microelectromechanical-systems-based floating element wall-shear-stress sensor ($1\times 1$ mm) is validated in a turbulent boundary layer for its ability to measure characteristics of large-scale motions.

A velocity derivative skewness akin to inertial-scale motions of turbulence is introduced and studied analytically, numerically, and experimentally. As an alternative to a conventional fine-scale parameter, it has an advantage in measurability while retaining a connection to vorticity dynamics.

An exact relation between the Reynolds stress and the resolved stress of filtered data is derived and tested in order to quantify the interference effect between mean and subfilter scales. Besides the well-known resolved and subfilter stresses, two additional terms appear in the decomposition.

The flow topology in the vicinity of critical points in a pipe flow reveals turbulence motions resembling hairpin-like vortical structures. Three-dimensional U separation is found to be closely associated with critical points that could form a simple model for wall turbulence.

An experimental investigation of steady-state thermal counterflow of superfluid ${}^4$He indicates that, in the range of investigated parameters, interactions between flow-probing particles and tangles of quantized vortices are appreciably influenced not only by the tangle geometry but also by the particle inertia.

The recirculation flow bounding a forced separating and reattaching shear layer is modeled. Experimental results emphasize that mass entrainment is a key parameter on which the parameters of the model scale. It is shown that an inexpensive and easily deployable sensor can be used to estimate the backflow.

An analytical model is proposed for the strength and position of the Leading Edge Vortex on a revolving wing at an arbitrary angle of attack. Predictions are compared with experiments and numerical solutions.

Motivated by plasma particles impacting a liquid-metal divertor, an analysis is performed to investigate how a liquid’s free surface is deflected by a moving applied pressure. It is found that the plasma can be swept up and down the liquid in order to spread the heat load, however, this may induce dangerously large deflections.

An experimental study reveals a new statistical regime for 1D propagation of deep-water gravity waves where coherent structures (solitons, extreme events) coexist with random waves. Such a state is predicted theoretically by integrable turbulence but thus far had not been observed in this context.

Climate change in the Arctic Ocean will subject its thermohaline staircases to increased internal wave activity. An exploration of the nonlinear conversion of incident waves to trapped staircase waves identifies wave number and frequency bands where energy transfer may be significant.

Steady free-surface flow over a semi-infinite and finite corrugated bottom is considered. It is demonstrated that by varying the Froude number a wide range of nontrivial solutions can be constructed, including generalized hydraulic falls, table-top solitons, and perturbed solitary waves.