This study investigates the interactions between flexural-gravity waves and interfacial waves in a two-layer fluid, focusing on wave blocking. Both liquid layers are of finite depth bounded on top by a viscoelastic thin plate. Both liquids are incompressible and inviscid, and their flows are two-dimensional and potential. Linear wave theory and a linear equation of a thin floating viscoelastic plate of constant thickness are used. We analyse the phenomenon of wave blocking and Kelvin–Helmholtz (KH) instability in a two-layer fluid with a discontinuous background mean flow. A quartic dispersion relation for frequency as a function of wavenumber and other parameters of the problem is derived. Two cases of uniform current and layers moving with different velocities are studied. Wave blocking occurs when roots of the dispersion relation coalesce without accounting for plate viscosity, leading to zero group velocity. Our findings indicate that wave blocking can occur for both flexural-gravity and interfacial waves under various frequency and current speed conditions, provided that plate viscosity is absent. The role of different parameters and the flow velocities of the upper and lower layers are investigated in the occurrence of wave blocking and KH instability. The loci of the roots of the dispersion relation involving plate viscosity depict that no root coalescence occurs irrespective of the values of wavenumber and frequency in the presence of plate viscosity. The amplitude ratio of the interfacial wave elevation to that of floating viscoelastic plate deflection exhibits the dead-water phenomenon as a density ratio approaches unity.

Reconstructing Quaternary-timescale environmental change in drylands provides insights into styles and rates of change in response to direct insolation forcing and variations in global temperature, ice volume and sea level. Changes to the relatively inhospitable environments presented by drylands are also central to debates about the migration and adaptability of hominin species. This review outlines approaches used for reconstructing past environments, which use dated sequences of environmental proxies – the properties of physical sediments, chemical precipitates and biological materials. In addition, climate model simulations can explore responses to known climatic forcing factors. Advances in both approaches remain situated within conceptual frameworks about dryland responses to: (i) cyclical changes in the Earth’s orbit around the sun, either mediated via the global cryosphere during glacial–interglacial cycles (~100 ka periodicity during the last ~0.9 Ma), or mediated via the response of the global monsoon with ~23 ka periodicity (precession) and (ii) millennial-scale climatic shifts, which are thought to originate outside drylands, within the North Atlantic.

In this review, three examples are outlined to demonstrate areas of emerging consensus and remaining contradictions: (1) speleothem and tufa growth records that span hundreds of thousands of years (ka), (2) conditions at the last glacial maximum (LGM) and (3) proxies recording millennial-scale events. Precessional-scale forcing of the monsoon is not always observed, with apparent mediating roles from glacial boundary conditions in parts of the northern hemisphere (NH) and from interglacial boundary conditions in parts of the southern hemisphere (SH). The LGM in drylands was initially conceptualised as experiencing pluvial (wet) conditions, which shifted to a glacial aridity paradigm, and is again shifting to a pattern of global heterogeneity, in which some drylands were wetter-than-present and others drier. However, there remain contradictions between environmental proxies and model simulations, and spatial heterogeneity is observed within many drylands. Dryland proxies that record millennial-scale events demonstrate clearly the importance of climatic teleconnections across the globe in influencing dryland hydroclimate. The records of Quaternary dryland change across a range of timescales get used alongside simulations of ecosystem response through time, hominin habitat and even hominin physiology to better understand likely hominin dispersals through dryland regions.

A numerical study is presented on flow-induced vibration of a circular cylinder, under the effect of a downstream stationary cylinder-induced proximity interference. The interference-induced various types of gap-flow regimes and characteristics of vibration and gap-flow rate $Q^*_g$ are presented, by considering various non-dimensional gaps $G^* = 0.1{-}2.5$ and reduced velocities $U^* = 3{-}20$ at a constant Reynolds number $Re = 100$, mass ratio $m^*= 2$ and damping ratio $\zeta = 0.005$. Decreasing $G^*$ or increasing proximity leads to the four gap-flow regimes: bi-directional gap flow at $G^* \geqslant 1.0$, uni-directional non-orthogonal gap flow at $G^* = 1.5{-}1.0$, uni-directional orthogonal gap flow at $G^* \leqslant 0.5$ and uni-directional one-sided gap flow at $G^* \leqslant 0.3$. Further, the respective regimes at larger $U^*$ are associated with proximity-induced modified vortex-induced vibration (PImVIV), proximity-induced galloping (PIG), transitional PImVIV–PIG, and proximity-induced staggered vibration (PISV). Quantitative presentation of maximum gap-flow rate $Q^*_{{g,max}}$, phase $ \phi _g$ (between $Q^*_{g}$ and displacement $y^*$) and phase portraits ($Q^*_{g}$ versus $y^*$) provides clear demarcation between the various gap-flow regimes. Flow mechanisms are presented for the PImVIV, PIG and PISV responses. For the PIG, the mechanism is presented for the first time on generation of galloping instability, asymptotically increasing $A^*$ and existence of optimum gap $G^* = 0.5$ for the maximum amplitude. This work is significant as it provides new insights into the proximity interference-induced gap-flow dynamics between two cylinders, associated flow mechanism for both vibration mitigation and enhancement and promising potential applications for energy harvesting.

This study investigates the transport of particles in turbulent channel flow with friction Reynolds number $Re_\tau = 1000$ by direct numerical simulation. We focus on how large-scale flow structures, namely the $Qs$ structures (Lozano-Durán et al. 2012, J. Fluid Mech., vol. 694, pp. 100–130), affect the wall-normal transport of particles. Despite occupying less than $10\,\%$ of the physical domain, our results highlight the critical role played by $Qs$ structures in the particle transport, namely that the particle number and momentum flux inside the $Qs$ structures are substantially higher than outside. The fraction of particle wall-normal momentum flux inside $Qs$ structures is considerably larger than their volume fraction, suggesting highly efficient transport inside the $Qs$ structures. This prominent role played by $Qs$ structures in the transport of inertial particles is more effective by diminishing the inertia of particles. Notably, the long-distance transport of particles in the wall-normal direction is driven primarily by the continuous effect of $Qs$ structures. In summary, our findings advance the understanding of the effects of $Qs$ structures on particle transport, and demonstrate their significant role in the process.

The electromagnetically driven magnetised spherical Couette flow is studied experimentally, theoretically and numerically in the laminar regime. The working fluid, Galinstan, is contained in the spherical gap between two concentric spheres at rest. The electromagnetic stirring is primarily generated due to the interaction of a direct current, which is injected through two ring-shaped electrodes located at the equatorial zone of each sphere, and a dipolar magnetic field produced by a permanent magnet located inside the inner sphere. The flows were explored experimentally for a Reynolds number ranging from 450 to 2230 and a Hartmann number of 240. Ultrasound Doppler velocimetry and particle image velocimetry were used to characterise the flow. For low Reynolds numbers, given the symmetry of the problem, a one-dimensional analytic solution is obtained in the equatorial plane from the magnetohydrodynamic equations. The analytical solution reproduces the main characteristics of the flow. In addition, a full three-dimensional numerical model is able to reproduce both the analytical solution and the experimental measurements. To the best knowledge of the authors, this is the first time experimental results of the magnetised spherical Couette flow have been reported with electromagnetic forcing using a liquid metal as the working media.

Viscoplastic fluids exhibit yield stress, beyond which they flow viscously, while at lower stress levels they behave as solids. Despite their fundamental biological and medical importance, the hydrodynamics of swimming in viscoplastic environments is still evolving. In this study, we investigate the swimming of an ellipsoidal squirmer and the associated tracer diffusion in a Bingham viscoplastic fluid. The results illustrate that neutral squirmers in viscoplastic fluids experience a reduction in swimming speed and an increase in power dissipation as the Bingham number increases, with swimming efficiency peaking at moderate Bingham numbers. As the aspect ratio of a squirmer increases, ellipsoidal squirmers exhibit significantly higher swimming speeds in viscoplastic fluids. The polar and swirling modes can either enhance or reduce swimming speed, depending on the specific scenarios. These outcomes are closely related to the confinement effects induced by the yield surface surrounding the swimmer, highlighting how both swimmer shape and swimming mode can significantly alter the yield surface and, in turn, modify the swimming hydrodynamics. In addition, this study investigates the influence of viscoplasticity on swimmer-induced diffusion in a dilute suspension. The plasticity enforces the velocity far from the swimmer to be zero, thus breaking the assumptions used in Newtonian fluids. The diffusivity reaches its maximum at intermediate aspect ratios and Bingham numbers, and increases with the magnitude of the squirmer’s dipolarity. These findings are important to understand microscale swimming in viscoplastic environments and the suspension properties.

Hydrodynamic modulation of short ocean surface waves by longer ambient waves significantly influences remote sensing, interpretation of in situ wave measurements and numerical wave forecasting. This paper revisits the wave crest and action conservation laws and derives steady, nonlinear, analytical solutions for the change of short-wave wavenumber, action and gravitational acceleration due to the presence of longer waves. We validate the analytical solutions with numerical solutions of the full crest and action conservation equations. The nonlinear analytical solutions of short-wave wavenumber, amplitude and steepness modulation significantly deviate from the linear analytical solutions of Longuet-Higgins & Stewart (1960 J. Fluid Mech. vol. 8, no. 4, pp. 565–583) and are similar to the nonlinear numerical solutions by Longuet-Higgins (1987 J. Fluid Mech. vol. 177, pp. 293–306) and Zhang & Melville (1990 J. Fluid Mech. vol. 214, pp. 321–346). The short-wave steepness modulation is attributed 5/8 to wavenumber, 1/4 due to wave action and 1/8 due to effective gravity. Examining the homogeneity and stationarity requirements for the conservation of wave action reveals that stationarity is a stronger requirement and is generally not satisfied for very steep long waves. We examine the results of Peureux et al. (2021 J. Geophys. Res.: Oceans vol. 126, no. 1, e2020JC016735) who found through numerical simulations that the short-wave modulation grows unsteadily with each long-wave passage. We show that this unsteady growth only occurs for homogeneous initial conditions as a special case and not generally. The proposed steady solutions are a good approximation of the nonlinear crest-action conservation solutions in long-wave steepness $\lesssim 0.2$. Except for a subset of initial conditions, the solutions to the nonlinearised crest-action conservation equations are mostly steady in the reference frame of the long waves.

Sound entering the ear is known not only to transmit signals to the nerve system, but also to generate vortex-like steady streaming in the cochlea. This streaming has been suggested as the primary vehicle for drug delivery in the inner ear (Sumner, Mestel & Reichenbach, 2021, Sci. Rep., vol. 11, 57). An alternative vehicle by pure diffusion alone has also been suggested by Sadreev et al. (2019, Front. Cell. Neurosci., vol. 13, 161). This paper purports to examine both mechanisms analytically, and compare their relative importance, based on the two-dimensional model of Allen (1977, Acoust. Soc. Am., vol. 61, 110–119). First, we reconstruct the fluid mechanics of the Békséy vortices by an asymptotic theory of multiple scales as a complement to the two-dimensional numerical theory of Edom, Obrist & Kleiser (2014, J. Fluid Mech., vol. 753, 254–278). For discerning the difference between Sumner, Mestel & Reichenbach (2021) and Sadreev et al. (2019), we combine sound-induced streaming and molecular diffusion by modeling the drug as a solute of known diffusivity. It will be shown that due to the high frequency of sound, advection is augmented by the Lagrangian velocity, but molecular diffusion still dominates drug transport in the cochlear duct, unlike Taylor dispersion of pollutant by tides in a shallow river.

Synthetic-aperture radar images and mesoscale models show that wind-farm wakes differ from single-turbine wakes. For instance, wind-farm wakes often narrow and do not disperse over long distances, contrasting the broader and more dissipating wakes of individual turbines. In this work, we aim to better understand the mechanisms that govern wind-farm wake behaviour and recovery. Hence we study the wake properties of a $1.6$ GW wind farm operating in conventionally neutral boundary layers with capping-inversion heights $203$, $319$, $507$ and $1001$ m. In shallow boundary layers, we find strong flow decelerations that reduce the Coriolis force magnitude, leading to an anticlockwise wake deflection in the Northern Hemisphere. In deep boundary layers, the vertical turbulent entrainment of momentum adds clockwise-turning flow from aloft into the wake region, leading to a faster recovery rate and a clockwise wake deflection. To estimate the wake properties, we propose a simple function to fit the velocity magnitude profiles along the spanwise direction. In the vertical direction, the wake spreads up to the capping-inversion height, which significantly limits vertical wake development in shallow-boundary-layer cases. In the horizontal direction and for shallow boundary layers, the wake behaves as two distinct mixing layers located at the lateral wake edges, which expand and turn towards their low-velocity side, causing the wake to narrow along the streamwise direction. A detailed analysis of the momentum budget reveals that in deep boundary layers, the wake is predominantly replenished through turbulent vertical entrainment. Conversely, in shallow boundary layers, wakes are mostly replenished by mean flow advection in the spanwise direction.

Bubble–particle collisions in turbulence are key to the froth flotation process that is widely employed industrially to separate hydrophobic from hydrophilic materials. In our previous study (Chan et al., 2023 J. Fluid Mech. 959, A6), we elucidated the collision mechanisms and critically reviewed the collision models in the no-gravity limit. In reality, gravity may play a role since, ultimately, separation is achieved through buoyancy-induced rising of the bubbles. This effect has been included in several collision models, which have remained without a proper validation thus far due to a scarcity of available data. We therefore conduct direct numerical simulations of bubbles and particles in homogeneous isotropic turbulence with various Stokes, Froude and Reynolds numbers, and particle density ratios using the point-particle approximation. Generally, turbulence enhances the collision rate compared with the pure relative settling case by increasing the collision velocity. Surprisingly, however, for certain parameters the collision rate is lower with turbulence compared with without, independent of the history force. This is due to turbulence-induced bubble–particle spatial segregation, which is most prevalent at weak relative gravity and decreases as gravitational effects become more dominant, and reduced bubble slip velocity in turbulence. The existing bubble–particle collision models only qualitatively capture the trends in our numerical data. To improve on this, we extend the model by Dodin & Elperin (2002 Phys. Fluids 14, 2921–2924) to the bubble–particle case and found excellent quantitative agreement for small Stokes numbers when the history force is negligible and segregation is accounted for.