The modification of a gravity current past a thin two-dimensional barrier is studied experimentally, focusing on propagation characteristics as well as turbulence and mixing at the gravity-current head near the obstacle. The broader aim is to develop an eddy-diffusivity parametrisation based on local governing variables to represent gravity-current/obstacle interactions in numerical weather prediction models. A gravity current is produced in a rectangular tank by releasing a salt solution via a lock-exchange mechanism into an aqueous ethanol solution with matched refractive index, and it is allowed to interact with the barrier. A combined particle image velocimetry and planar laser-induced fluorescence system is used to obtain instantaneous velocity and density fields. The experiments span two Reynolds numbers and four obstacle heights, with each case replicated ten times for conducting phase-aligned ensemble averaging. Four evolutionary stages of the front are identified: approach, vertical deflection, collapse and reattachment. Particular focus is placed on the vertical deflection and collapse stages (dubbed collision phase), which includes flow (hydraulic) adjustment, flow modulation over the obstacle, instabilities, turbulence and mixing, and relaxation to a gravity current downstream. The time scales for various flow stages were identified. The results demonstrate that the normalised eddy diffusivity changes significantly throughout these stages and with the dimensionless height of the obstacle.

Rogue waves are associated with various ocean processes, both at the coast and in the open ocean. In either zone, inhomogeneities in the wave field caused by shoaling, crossing seas or current interactions disturb the wave statistics, increasing the rogue wave probability and magnitude. Such amplification of the frequency of rogue waves and their intensity, i.e. the maximum normalised height, have been attested to in numerical simulations and laboratory studies, in particular for wave–current interactions. In this study, we investigate the effect of the current intensity and direction on rogue wave probability, by analysing long-term observations from the southern North Sea. We observe that the amplification is similar for opposing and following currents. Despite the sea states being dominantly broadbanded and featuring a large directional spread, the anomalous statistics are of the same order of magnitude as those observed in unidirectional laboratory experiments for stationary currents.

Local eddy viscosity and diffusivity models are widely used to understand and predict turbulent flows. However, the local approximations in space and time are not always valid for actual turbulent flows. Recently, a non-local eddy diffusivity model for turbulent scalar flux was proposed to improve the local model and was validated using direct numerical simulation (DNS) of homogeneous isotropic turbulence with an inhomogeneous mean scalar (Hamba 2022 J. Fluid Mech. 950, A38). The model was modified using the scale-space energy density in preparation for application to inhomogeneous turbulence (Hamba 2023 J. Fluid Mech. 977, A11). In this paper, the model is further improved by incorporating the effects of turbulence anisotropy, inhomogeneity and wall boundaries. The needed inputs from the flow to evaluate the model are the Reynolds stress and the energy dissipation rate. With the improved model, one- and two-dimensional profiles ofthe non-local eddy diffusivity in turbulent channel flow are evaluated and compared with the exact DNS values. The DNS results reveal a contribution to the scalar flux from the mean scalar gradient in a wide upstream region. Additionally, the temporal profile of the non-local eddy diffusivity moves downstream, diffuses anisotropically and is tilted towards the bottom wall. The model reproduces this behaviour of mean flow convection and anisotropic turbulent diffusion well. These results indicate that the non-local eddy diffusivity model is useful for gaining insights into scalar transport in inhomogeneous turbulence.

The goal of the Global Plastics Treaty is to “end plastic pollution and to protect human and environmental health.” Despite overwhelming evidence of the adverse impacts of microplastics on human and environmental health, the inclusion of how microplastics will be addressed within the Global Plastics Treaty remains unclear. Yet, a contaminant as morphologically and chemically diverse as microplastics should be considered a priority for regulation under the Global Plastics Treaty like the 1979 Convention on Long-Range Transboundary Air Pollution (CLRTAP). As the goal of finalizing the Global Plastics Treaty at INC-5 was not realized, a valuable opportunity to push for more inclusion and objectives that include atmospheric microplastics exists. To ensure that atmospheric microplastics are present within the Global Plastics Treaty text, the following are recommended. Ensure that inclusion of terms “air” and/or “atmosphere” and/or “atmospheric microplastics” be present alongside other environmental compartments like terrestrial and aquatic (marine and freshwater) environments. Capping plastic production is critical to reducing subsequent microplastic pollution. Improving product design or developing microplastic capture technologies should be prioritized for plastic products known to contribute to atmospheric microplastic pollution. Empirical targets should be established to mitigate emissions of atmospheric microplastics from “leaky” regions (e.g., urban centres and industrial regions), and targets should also be placed on activities that result in generation and release of secondary microplastics (e.g., dryer emissions and automotive tires). A framework to reduce pollution should be informed by the structure of other relevant successful multilateral air pollution agreements (e.g., the Montreal Protocol and CLRTAP), and goals should be informed by the present science and data obtained from monitoring. A global network of atmospheric monitoring observatories should be established to identify “leaky” regions or sources/activities and monitor temporal changes in microplastic concentrations via long-range processes. Establish specialized working groups of global experts that can develop and harmonize atmospheric microplastic monitoring and analytical methods. Assess effectiveness of mitigation and reduction strategies for atmospheric microplastics in relation to the empirical targets established by the Global Plastics Treaty. Additionally, atmospheric microplastics can be used as a metric to ensure that broader scale changes to pollution reduction strategies implemented by the Global Plastics Treaty are effective.

A ground vortex engendered by the interaction of uniform flow over a plane surface with suction into a cylindrical conduit whose axis is normal to the cross-flow and parallel to the ground plane is investigated in wind tunnel experiments. The formation and evolution of the columnar vortex and its ingestion into the conduit’s inlet are explored using planar/stereo particle image velocimetry over a broad range of formation parameters that include the speeds of the inlet and cross-flows and the cylinder’s elevation above the ground plane with specific emphasis on the role of the surface vorticity layer in the vortex initiation and sustainment. The present investigations show that the appearance of a ground vortex within the inlet face occurs above a threshold boundary of two dimensionless formation parameters, namely the inlet’s momentum flux coefficient and its normalised elevation above the ground surface. Transitory initiations of wall-normal columnar vortices are spawned within a countercurrent shear layer that forms over the ground plane within a streamwise domain on the inlet’s leeward side by the suction flow into the duct. At low suction speeds, these wall-normal vortices are advected downstream with the cross-flow but when their celerity is reversed with increased suction, they are advected towards the cylinder’s inlet, gain circulation and stretch along their centrelines and become ingested into the inlet at a threshold defined by the formation parameters. Finally, the present investigations demonstrated that reduction of the countercurrent shear within the wall vorticity layer by deliberate, partial bypass of the inlet face flow through the periphery of the cylindrical duct can significantly delay the ingestion of the ground vortex to higher level thresholds of the formation parameters.

We have established a novel molecular kinetic model that addresses fundamental challenges in the non-equilibrium transport of nanoscale confined fluids, such as rarefaction and fluid inhomogeneities, which are crucial to a range of scientific and engineering fields. The proposed model explicitly considers fluid–solid molecular interactions in the transport equations, eliminating the reliance on predefined boundary conditions. By consistently accounting for molecular interactions between fluids and solids, the unified model captures both intrinsic and apparent non-hydrodynamic effects, as well as real fluid behaviours. Rigorous comparisons with molecular dynamics simulations demonstrate that the present model accurately predicts unique features of strongly inhomogeneous fluid flows, including fluid adsorption, solvation force, velocity slip and temperature jump. Therefore, this mesoscopic model bridges the gap between molecular-scale dynamics and macroscopic hydrodynamics, enabling a practical simulation tool for nanoscale surface-confined flows. Moreover, it offers valuable insights into the molecular mechanisms underlying anomalous transport phenomena observed in confined flows, such as the disappearance and re-emergence of the Knudsen minimum.

Large-eddy simulations have been conducted to investigate the decay law of homogeneous turbulence influenced by a magnetic field within a cubic domain, employing periodic boundary conditions. The initial integral Reynolds number is approximately 1000, while the initial interaction number $N$ ranges from 0.1–100. The results reveal that the Joule cone angle $\theta$, half of the Joule cone, decays as $\cos \theta \sim t^{-1/2}$ when $N \gg 1$. In the nonlinear stage, small-scale vortices gradually recover and restore three-dimensionality. Moreover, the corresponding critical state at small scales, marking the transition from quasi-two-dimensional structure to the onset of three-dimensionality, has been quantitatively defined. During the linear stage, based on the true magnetic damping number ($\tau _t = \rho / (\sigma {\boldsymbol{B}}^2 \cos ^2 \psi )$, where $\sigma$, $\boldsymbol{B}$ and $\psi$ denote the electrical conductivity, magnetic field and the angle between the wavevector and $\boldsymbol{B}$ in Fourier space, respectively), Moffatt’s decay law, $K \sim t^{-1/2}$, manifests at distinct times and zones in the Fourier space, with $K$ signifying turbulent kinetic energy. In the nonlinear stage, for $N \gg 1$, a $-3$ slope in the energy power spectrum is prominently observed over an extended period. The near-equivalence of the characteristic time scales of inertial and Lorentz forces in the inertial subrange suggests a quasiequilibrium state between energy transfer and Joule dissipation in Fourier space, thereby corroborating the hypothesis proposed by Alemany et al. 1979 Journal de Mecanique 18(2): 277–313. Additionally, it is observed that pressure mediates energy transfer from horizontal kinetic energy ($K_{\parallel }$) to vertical kinetic energy ($K_{\bot }$), accelerating the decay of $K_{\parallel }$. Notably, concurrent inverse and direct energy transfers emerge during the decay process. Our analysis reveals that the ratio $R$ of the maximum inverse to maximum direct energy flux correlates with the dimensionality of the turbulence, following the scaling law $R\sim (\cos \theta )^{-2.2}$.

Around 1000 years ago, Madagascar experienced the collapse of populations of large vertebrates that ultimately resulted in many species going extinct. The factors that led to this collapse appear to have differed regionally, but in some ways, key processes were similar across the island. This review evaluates four hypotheses that have been proposed to explain the loss of large vertebrates on Madagascar: Overkill, aridification, synergy, and subsistence shift. We explore regional differences in the paths to extinction and the significance of a prolonged extinction window across the island. The data suggest that people who arrived early and depended on hunting, fishing, and foraging had little effect on Madagascar’s large endemic vertebrates. Megafaunal decline was triggered initially by aridification in the driest bioclimatic zone, and by the arrival of farmers and herders in the wetter bioclimatic zones. Ultimately, it was the expansion of agropastoralism across both wet and dry regions that drove large endemic vertebrates to extinction everywhere.

Flow over bluff bodies encounters instability at supercritical Reynolds numbers, exhibiting the periodic vortex shedding that leads to structural vibrations and acoustic noise. In this paper, a new aerodynamic shape optimisation strategy based on resolvent analysis is proposed to passively control the vortex shedding over two-dimensional cylinders. Firstly, we show that when the flow satisfies the rank-1 approximation, minimizing the maximal resolvent gain enhances flow stability. Secondly, we formulate the geometry-constrained resolvent-based optimisation problem that can be solved by the nonlinear conjugate gradient algorithm. Compared with conventional stability-based optimisation, the proposed approach is more effective as it avoids the cumbersome eigendecomposition of the high-dimensional Jacobian matrix. The efficacy of the proposed resolvent-based optimisation is validated through improving the stability of the one-dimensional Ginzburg–Landau equation. Thirdly, this approach is applied to suppress the vortex shedding of bluff bodies, initialised by a circular cylinder. Although the optimisation is performed at a subcritical state $Re = 40$, reduced vortex shedding and drag forces can be achieved at supercritical Reynolds numbers, while the critical Reynolds number is extended from $47$ to $60$. Dynamic mode decomposition is then performed to reveal that the optimised system becomes more stable and satisfies the rank-1 approximation. Finally, we demonstrate that the combined effects of the flattened surface and the Coanda effect delay flow separation, keeping the separation point nearly unchanged at supercritical Reynolds numbers (e.g. between 80 and 140) for the optimised geometry. This results in a substantial reduction in the strength of vortex shedding, which in turn leads to decreased drag forces. The optimised shape still achieves drag reduction in turbulent flows at a relatively high Reynolds number.

We derive boundary conditions for two-dimensional parallel and non-parallel flows at the interface of a homogeneous and isotropic porous medium and an overlying fluid layer by solving a macroscopic closure problem based on the asymptotic solution to the generalised transport equations (GTE) in the interfacial region. We obtained jump boundary conditions at the effective sharp surface dividing the homogeneous fluid and porous layers for either the Darcy or the Darcy–Brinkman equations. We discuss the choice of the location of the dividing surface and propose choices which reduce the distance with the GTE solutions. We propose an ad hoc expression of the permeability distribution within the interfacial region which enables us to preserve the invariance of the fluid-side-averaged velocity profile with respect to the radius $r_0$ of the averaging volume. Solutions to the GTE, equipped with the proposed permeability distribution, compare favourably with the averaged solutions to the pore-scale simulations when the interfacial thickness $\delta$ is adjusted to $r_0$. Numerical tests for parallel and non-parallel flows using the obtained jump boundary conditions or the generalised transport equations show quantitative agreement with the GTE solutions, with experiments and pore-scale simulations. The proposed model of mass and momentum transport is predictive, requiring solely information on the bulk porosity and permeability and the location of the solid matrix of the porous medium. Our results suggest that the Brinkman corrections may be avoided if the ratio $a=\delta /\delta _B$ of the thickness $\delta$ of the interfacial region to the Brinkman penetration depth $\delta _B$ is large enough, as the Brinkman sub-layer is entirely contained within the interfacial region in that case. Our formulation has been extended to anisotropic porous media and can be easily dealt with for three-dimensional configurations.

We examined theoretically, experimentally and numerically the origin of the acoustothermal effect using a standing surface acoustic wave-actuated sessile water droplet system. Despite a wealth of experimental studies and a few recent theoretical explorations, a profound understanding of the acoustothermal mechanism remains elusive. This study bridges the existing knowledge gap by pinpointing the fundamental causes of acoustothermal heating. Theory broadly applicable to any acoustofluidic system at arbitrary Reynolds numbers, going beyond the regular perturbation analysis, is presented. Relevant parameters responsible for the phenomenon are identified and an exact closed-form expression delineating the underlining mechanism is presented. We also examined the impact of viscosity on acoustothermal phenomena by modelling temperature profiles in sessile glycerol–water droplets, underscoring its crucial role in modulating the acoustic field and shaping the resulting acoustothermal profile. Furthermore, an analogy between the acoustothermal effect and the electromagnetic heating is drawn, thereby deepening the understanding of the acoustothermal process.