Experiments on a submerged sediment layer creep were performed, varying the system angle and a porous flow, far under the avalanching stress criterion. Logarithmic decay rates of the deformation are observed, with the rate increasing with both control parameters.

Here, inertial forces on particles in nonuniform ambient flows, previously missing from the Maxey—Riley formulation of inertial particle dynamics, are identified. These forces involve both local and convective fluid inertia and depend on the curvature of the ambient flow. These new terms provide quantitative corrections and qualitative additions to the Maxey—Riley equation and are relevant when the particle size is appreciable relative to the characteristic length scale of the ambient flow.

Invariant manifolds blocking the motion of active particles in fluid flows are identified and experimentally confirmed for swimming bacteria in a microfluidic cross-channel flow. These manifolds project to one-way barriers for swimmers in physical space. Distinct barriers are predicted for perfect smooth swimmers versus noisy swimmers in general, nonlinear flows.

A study shows that the shell model of turbulence has a hidden scaling symmetry, which involves nonlinear transformations of shell speeds and time. This symmetry is restored statistically in the developed turbulence despite all original scaling symmetries being broken by the intermittency.

Flow features and forces of three-dimensional flapping foils are presented with aspect-ratio (AR) variations. The similitude of rolling and twisting kinematics is maintained across different ARs. An AR correction analogous to Prandtl finite-wing theory is developed, enabling future use of strip theory in the analysis and design of finite flapping foils.

Analytical expressions are obtained for the propulsion and the wake structure of tandem flapping foils with chordwise deflection, explaining the propulsive performance improvement in relation to single foils or rigid-foil counterparts for certain combinations of frequency, spacing, and phase shift. The findings are of interest for the design of small aerial or aquatic vehicles using tandem propulsors.

Colonial physonect siphonophores swim via laterally distributed multijet propulsion. Here, computational fluid dynamics is employed to investigate the underlying fluid mechanics and adaptive values of this unique way of propulsion. It is found that colonial swimming achieves energetic benefits for jetting individuals within the colony because they require significantly lower per-module power than that required by a lone jet module swimming at the same speed.

The collective behavior of active suspensions of microswimmers immersed in a viscous fluid is investigated through numerical studies. It is shown that a bioturbulent state emerges both in suspensions of pushers and of pullers. Which conditions are needed to trigger such phenomenology and what mechanisms underlie such phenomena are discussed. An investigation into whether the difference in puller and pusher dynamics is due to a spontaneous breaking of the hydrodynamics time-reversal symmetry is presented, and the mechanisms underlying such broken symmetry in biological swimmers are examined.

The breakup of jets and cavities may give rise to cascades of satellites (drops or bubbles) of a broad range of sizes. A dynamical mechanism for the formation of such cascades, based on the notion of discrete self-similarity (DSS), is presented. In DSS, structures repeat indefinitely at periodic spacetime windows in similarity variables.

Transient 3-dimensional simulations of viscoelastic fluidlike adhesives access the physics behind the debonding process of pressure sensitive adhesives (PSA). A novel finite element method is developed to simulate viscoelastic flows with multiple free surfaces and contact lines. The rheological properties of the adhesive are correlated with its tackiness and stickiness, targeting the development of materials with improved adhesive energy.

For high-Mach-number turbulent flows, the Kolmogorov length scale can be comparable to the gas-molecule mean-free path, which could introduce noncontinuum molecular-level effects into the turbulent energy cascade. To investigate this issue, compressible Taylor-Green vortex flow is simulated using both noncontinuum molecular gas dynamics and continuum computational fluid dynamics. Although the energy-decay rates are the same, molecular-level fluctuations break the flow symmetries and thereby produce different but statistically similar routes from the initial nonturbulent flow to the long-time turbulent flow.

Optimal, exact coherent solutions have been shown to tightly bound the scaling of heat transport in two-dimensional turbulent Rayleigh-Bénard convection. These optimal solutions are unstable, but it is of interest to detect their signature in turbulent flow fields. Here, a direct link is established between the exact coherent solutions that optimize heat transport and transition to turbulence in two-dimensional Rayleigh-Bénard convection.

Flow phenomena on a roof are investigated by employing direct numerical simulation. A sequence of pitchfork bifurcations of steady plumes on the roof occurs for small Rayleigh numbers, which is analyzed using a topological method. Furthermore, a Hopf bifurcation followed by periodic doubling, quasiperiod bifurcations, and a transition to chaos appear as the Rayleigh number is increased.

Coated microbubbles exhibit a complex surface rheology. When accelerated towards a wall subject to an acoustic disturbance they behave like a deformable solid and assume a prolate rather than oblate shape, as is the case with conventional bubbles, due to the dominance of friction over pressure drag. This results from the balance between viscoelastic stresses that develop on the protective shell and pressure and viscous stresses from the surrounding fluid which generate a progressively more prolate shape as the translational Reynolds number increases. Proper coating design can optimally control particle shape and motion in applications, notably in biomedical ones.

A coated microbubble approaching a solid substrate changes shape from prolate to oblate due to the balance of Bjerknes force and elasto-lubrication pressure. The bubble then undergoes trapped pulsations about an average static state corresponding to the Reissner response of a coated shell compressed by a rigid plate corrected for surface tension. The contact region length is inverse to shell stiffness, with thickness of tens of nanometers due to the balance of viscous shell and liquid stresses. The contact region flow is a Stokes layer generated in response to bubble translational and vibrational motion that are found to be in phase in our simulations, so steady streaming was not captured.

The thinning dynamics and film lifetime between two bubbles depend on the extent to which the interfaces are flattened upon close contact, characterized by the film initial radius and thickness. Approaches that account for both dimple formation and attractive van der Waals forces have not been used to reveal film lifetime dependence on these initial features. Here we address this gap with numerical simulations of the classical thin film equation, formulated so that the film initial radius and thickness are direct input parameters. We find different thinning behavior for large and small radii films and characterize it using known analytical solutions from the literature.

An experimental study focuses on the airborne sound generated during bubble bursting at the surface of a liquid bath. It is found that the acoustic frequency drifts and increases, consistent with a Helmholtz-type resonance of the cavity being more and more opened as the thin film in the air retracts. As an extension, a simple model based on a collection of drifting Helmholtz resonators is proposed, capturing the main features of the fizzing sound of an effervescing beverage.

On the basis of the boundary element method and explicit finite element method, a three-dimensional fully coupled model is developed to investigate the interaction between a bubble and an elastic-plastic thin-walled plate. The model can accurately calculate the bubble loading acting on the plate surface and describe the structural motion coupling with the flow field on two sides of the plate. As a result of the elastic-plastic effects of the thin-walled plate, the bubble displays attractive motion, repulsive motion, or splitting.

The interaction between an oscillating bubble and a free surface is experimentally and computationally investigated. The evolution of the free surface is characterized by measuring the surface area and the volume of the jets that are formed at the free surface.

Electrosprays of highly conducting liquids operated in the cone-jet mode produce charged nanodroplets of controllable size and molecular ions. The study of this electrospraying regime is challenging because of the lack of experimental techniques for probing these nanometric systems. An experimental technique based on time-of-flight and retarding potential analysis is presented for measuring the velocity and potential of the jet at its breakup, and it uses this information to rationalize the physics of both the droplet formation and the emission of ions from cone-jets of highly conducting liquids.

Electrothermohydrodynamic convection between parallel electrodes with unipolar injection is investigated using a two-relaxation-time lattice Boltzmann method. The interactions between the stabilizing buoyancy force and the destabilizing electric force lead to either monotonic instability or overstability, depending on the Rayleigh number and the Taylor number. A two-stage bifurcation is observed for overstability near the threshold Rayleigh number with a significant change in phase and amplitude.

An experimental measure of irreversible mixing efficiency, $R{i}_f^{*}$, for a lock-exchange gravity current is presented. The results show that mixing efficiency increases and is sustained in the presence of strong stratification provided the flow energy is high. Following this, various turbulence regimes are mapped based on turbulent Froude number and turbulent Reynolds number. Additionally, $R{i}_f^{*}$ is shown to have a strong dependence on turbulence regime and molecular effects induced by Prandtl number.

We investigate MHD-Couette–Shercliff flows driven by lateral wall motion with turbulent transitions observed only at supercritical Re. Linearly, the Tollmien–Schlichting (TS) wave instability was isolated at each wall for friction parameter H $\gtrsim$ 300 in MHD-Couette flow, and H $\gtrsim$ 1000 for Shercliff flow. At low H, linear and nonlinear growth led only to saturation to a traveling wave, where the waves at each wall conjoin. For larger H≥10, nonlinearity isolates the TS wave to one wall. At H=10 turbulence is sustained and a distinct inertial subrange formed. For H≥30 the turbulent state relaminarized, with low bulk production due to the flattened base flow.

A new framework analyzes energy flux in turbulent channels under various controls, enabling estimations of drag reduction and net power saving, and also suggesting a perspective of composite control for future exploration.

The problem of the origin of turbulence, and hence, transport of angular momentum, in accretion flows (particularly cold flows) as well as laboratory flows like plane Couette flow, prevails due to their stability under linear perturbation. We attempt to resolve this long-standing issue with linear analysis by considering an extra stochastic force with a nonzero mean (m). We show that these flows become unstable, and the corresponding maximum growth rate (Im(β)max) increases, if the mean is increased, with other parameters fixed. Since accretion flow has a central sink a fluid parcel must take less time to become nonlinear than to cross the local analysis region, consistent with our analysis.

Deposition of colloidal particles in a porous medium is typically considered to be problematic in energy and water applications. Here, experiments demonstrate that colloidal deposition can, surprisingly, promote mobilization of a trapped immiscible fluid from a porous medium without requiring any surface activity. Analysis of the underlying physics provides a way to predict the characteristics of fluid that is mobilized as deposition progresses.

Pattern selection in oscillatory long-wave Marangoni convection in a heated thin layer of liquid with weak heat flux from the free surface is investigated. The research is performed under the assumption that the surface tension of the liquid nonlinearly depends on the temperature. The pattern selection is analyzed for square, rhombic, and hexagonal planforms.

The rebound of large water jets from a superhydrophobic substrate in the low-gravity environment of a drop tower is investigated. A regime map is constructed from drop tower test data. Scaling laws and simulations are used to identify boundaries between jet rebound regimes and to predict landing flow geometry.

Plants and insects use slender conical structures to transport and collect small droplets that are propelled along conical structures by capillary action. It is shown that these droplets can deposit a film with a thickness that depends on the fiber’s radius and the droplet size, highlighting that the coating is affected by finite size effects relevant to film deposition on fibers of any slender geometry. These self-propelled droplets have significant potential to create passively coated structures.

A study of surface transpiration demonstrates its use for reduction of forces required to maintain the relative movement of parallel plates. Competition between reduction of the effective spacing between the plates, elimination of the direct contact between the transpired wall and the bulk of the fluid, and generation of a driving force through the nonlinear streaming leads to a wide range of responses, including resistance reduction. It is argued that transpiration can be used as an alternative propulsion method.

The cell cytoskeleton is a dynamic gel of semi-flexible, inextensible filaments and dynamic cross-linkers, all suspended in a viscous fluid. Here a spectral method is developed to simulate this gel, assuming filament smoothness. Interactions between filaments and the fluid are accounted for with a nonlocal slender body theory, reformulated in terms of the Rotne-Prager-Yamakawa tensor. Inextensibility is treated through a novel weak formulation, with constraint forces obtained using the principle of virtual work, and the filaments updated via tangent vector rotations on the unit sphere. Gel simulations show that nonlocal hydrodynamic forces contribute nontrivially to total suspension stress.

A theory based on a triple-deck approach is developed to rapidly assess the velocity field over a rotating disk with surface roughness. The theory results are validated with numerical simulations and suggest new ways to incorporate roughness in flow calculations.

The applicability of some constitutive equations for the solid stress tensor used in the mixture model to describe the shear-induced migration of neutrally buoyant particles in Newtonian fluids is investigated. It is shown that in moderately dense suspensions, where direct particle contacts and interparticle forces are negligible, migration must be due to the lubrication forces between the particles. Results highlight that the tensor accounting for these forces is part of the stress tensor of the fluid phase, not of the solid phase and, in particular, that it coincides with the part of the particle-presence stress tensor related to the lubrication forces.

Experiments and simulations reveal that rigid fibers suspended at high concentration in a viscous fluid can be aligned in the vorticity direction using a shearing flow. Creating vorticity alignment requires an oscillatory shear over a limited range of strain amplitudes. The suspension must also be confined, as only those particles near the bounding walls align.

The fluid-structure interaction of an elastically restrained inclined tangent ogive-cylindrical body with a blunt elliptical base is investigated numerically. The flow is three-dimensional, compressible, and laminar, and the slender body is allowed to yaw at high incidence. The resulting response exhibits an intricate bifurcation structure that includes bistable periodic (finite amplitude) and nonstationary (small amplitude) limit cycles for moderate angles of attack, and nonstationary (finite amplitude) oscillations for high angles of attack.

Small colloidal floaters are poured at the air-water interface of a tank stirred by many camphor swimmers. The system rapidly reaches a statistically stationary state, resulting in competition between (i) efficient stirring by the disordered motion of the swimmers and (ii) unmixing promoted by the chemical cloud attached to each individual self-propelled disk.

Fluid flow and mass transport in rough channels are ubiquitous phenomena occurring in numerous engineering applications and natural processes. Comprehensive numerical simulations and stochastic upscaling elucidate how the complex interplay between channel roughness, inertia, and diffusion controls solute transport in channel flows. A mechanistic link between the complex interplay and anomalous transport in rough channel flows is successfully established.

Polymer-induced drag reduction in turbulent flow is bounded by a universal upper limit with increasing fluid elasticity. For decades, efforts to understand this maximum drag reduction asymptote have focused on the search for an ultimate flow state whose dynamics is no longer influenced by polymer elasticity. Contrary to common assumption, this study shows that behind the converged mean flow, the underlying dynamics continues to evolve through distinct stages, with no sign of convergence.

A new modal decomposition of fluid flows, labeled cross proper orthogonal decomposition (CPOD), is presented. CPOD modes are ranked by their contributions to the trace of the cross-covariance between flow fluctuations. An application of CPOD is presented for turbulent channel flow, with modes with maximal contribution to the Reynolds shear stress. Low-order reconstructions of the velocity field using CPOD modes lead to fast convergence of the Reynolds stress and, consequently, of the turbulent mean flow.

Low-level jets (LLJs) are the wind maxima frequently observed in a stably stratified atmosphere, and they possess a high wind energy potential. Large-eddy simulations show that in a wind farm, the power production of the first row increases with a decrease in LLJ height; however, downwind of the first row, power production is reduced as a result of the limited wake recovery and turbulent entrainment. In the presence of an LLJ, wind veer due to the Coriolis forces causes an asymmetry in the wind availability for different turbine columns.

Biphase is introduced as a nonlinear alternative to traditional amplitude modulation coefficients for studying the interaction delays between large- and small-scale motions in wall-bounded turbulent flows. The biphase combines energetic and geometric interpretations to provide an integrated diagnostic for the scale interaction problem.

The understanding of the large-scale dynamics of the turbulent jet in counterflow remains limited. By employing proper orthogonal decomposition and spectral proper orthogonal decomposition on large eddy simulation data, new insights are presented. The fundamental mode dynamics are described as varying penetration, precession, and an alternating stretching-contracting motion. Furthermore, intermittency in the temporal evolution of these modes is identified.

Intense resonant interactions between two deep-water gravity waves can lead to the bending and then splitting of crests and troughs. The curvatures of the crests and troughs increase with an increase of the primary wave’s steepness and the resonant wave’s propagation distance. For primary waves with small steepness, the crest and trough lines will eventually split, as long as the propagation distance is sufficiently large. The crest line is split into a longer segment with a larger wave crest and a shorter one with a smaller wave crest.

The existence of a resonant triad interaction among two primary internal waves and a superharmonic wave in a stably stratified uniform shear flow is proved. Under the pump-wave approximation, the resonant triad becomes unstable when the first mode acts as the pump wave. The exact solutions of the amplitude equations reveal that the stability of a triad depends on the mode numbers and initial conditions.