The stability of the interface in a core–annular flow (CAF) of two immiscible Newtonian fluids with contrasting densities has been investigated, emphasising the role of strong circumferential rotation for the first time. The aim of the investigation is to give insight into the physical mechanisms underlying interfacial disruption. We examine the combined effects of gravity, interfacial tension, axial and azimuthal shear stresses, and centrifugal force on interface stability. The Rayleigh–Taylor instability, induced by gravity, appears as a spiral mode with a azimuthal wavenumber of one. As gravitational effects decrease, the most unstable mode number increases sharply before decreasing with increasing rotation. This non-monotonic behaviour is attributed to the interplay between azimuthal shear and centrifugal acceleration. We demonstrate that this velocity ratio fundamentally governs the onset of spiral modes by varying the ratio of the axial velocities of the core and annular fluids. Higher Reynolds numbers in the annular phase promote the emergence of higher-order spiral modes concomitant with amplified azimuthal shear at the interface. In a parametric study of the gap between the core and pipe wall, we identified a suppressive effect of reduced annular thickness on the growth of higher azimuthal wavenumbers. An energy budget analysis further delineated distinct mechanisms underpinning each instability regime and clarified transitions between them. These findings extend our understanding of interfacial stability in swirling CAFs and provide a predictive framework to control spiral-mode selection.

We study the bursting of a bubble on a liquid free surface under critical conditions, i.e. those leading to the minimum (maximum) size (velocity) of the first-emitted jet droplet. We consider the effect of a surfactant remaining in the monolayer during the cavity collapse and jetting (the surfactant is considered as insoluble). Our experiments show that a tiny amount of surfactant considerably increases (decreases) the droplet radius (velocity). The volume of the first-emitted droplet increases by a factor of 20 for a concentration that produces an insignificant reduction in the bubble surface tension. The total liquid volume ejected by the bubble increases with the surfactant concentration. Surfactant accumulates at the bubble base due to the shrinkage of the cavity bottom and surfactant convection. The resulting reduction in surface tension narrows the region of free surface reversal. Despite this effect, the droplet size increases because Marangoni stress widens the jet and slows the liquid jet interface, delaying droplet detachment. More liquid flows into the droplets, increasing the mass and energy transfer to the resulting spray. A significant increase in the droplet size is also observed with a weak surfactant. This indicates that natural water contamination can substantially alter bubble bursting under critical conditions. Our results may explain the size of the particles emitted by bubble bursting in seawater.

This study investigates the influence of wind tunnel ground conditions (stationary/moving) on flow topology and passive scalar dispersion in the wake of the Ahmed body with rear slant angles, $\phi$ = 25$^\circ$ and 40$^\circ$. We implement field measurements of both velocity and scalar concentrations in the wake, for both the ground conditions, within the same experimental set-up, allowing for structural correlation between wake topology and scalar dispersion. Particle image velocimetry measurements reveal the existence of a third spanwise vortex (vortex G) near the stationary wind tunnel ground, due to the floor boundary layer, for both of the Ahmed bodies ($\phi$ = 25$^\circ$, 40$^\circ$). Concentration field measurements performed using quantitative smoke visualisation show higher scalar dispersion in the wake of both Ahmed bodies for the stationary ground condition. Comparing the velocity and concentration fields further identifies vortex G as the primary physical driver for the enhanced vertical dispersion of the scalar, observed in stationary ground conditions. To quantify the dispersion and characterise these effects, we introduce dispersion parameters, such as non-dimensional dispersion ($\mathscr{D}$) and dispersion length scales ($\mathscr{L}_y, \mathscr{L}_z$). These parameters confirm that, while lateral dispersion remains relatively insensitive to wind tunnel ground conditions, the presence of vortex G in stationary ground conditions leads to an overestimation of vertical dispersion by up to $\approx$29 % ($\phi$ = 25$^\circ$) and $\approx$49 % ($\phi$ = 40$^\circ$). This study quantifies the overestimated dispersion, identifies the vortical structures responsible for scalar redistribution, provides physical insight into the wake dispersion phenomenon and highlights the importance of correct wind tunnel ground conditions in the vehicle wake dispersion studies.

Recently, flow-reversal mechanisms in Rayleigh–Bénard (RB) convection and controlling strategies via modifying local temperature boundaries have received increasing attention due to the impact on heat-transfer efficiency and extreme eruption events. We consider an alternative possibility of altering fluid density: an added scalar field that induces double-diffusive convection, implemented by imposing local iso-concentration bands on the horizontal plates. In addition to the Rayleigh number (${\textit{Ra}}$) and Prandtl number (${\textit{Pr}}$), the system is governed by the Lewis number ($Le$), buoyancy ratio ($Br$), normalised bandwidth ($\delta$) and normalised band-centre-to-midline distance ($c$). We examine the influence of $\delta$ and $c$ on flow reversal at ${\textit{Ra}}=5\times 10^7$, ${\textit{Pr}}=2$, $Le =1$ and ${\textit{Br}}=1.5$. Paired bands effectively reduce reversal frequency, with stronger suppression for larger $\delta$; the optimal band position is $c=0.2$. Fourier mode analysis reveals a previously underappreciated role of the $3\times 3$ roll structure in reversal suppression, whose mean energy correlates positively with the single-roll structure. In standard RB convection, turbulence destabilises the symmetric $2\times 2$ roll configuration, causing frequent reversals owing to competition with asymmetric (1, 1) and (3, 3) modes. The concentration bands enhance the (1, 1) and (3, 3) modal energies, especially at the optimal band position, producing a steady mean flow structure comprising a large-scale circulation and four corner rolls. Despite the local boundary modification, scaling laws for the response parameters (Nusselt number (${\textit{Nu}}$) and Reynolds number (${\textit{Re}}$)) remain close to standard RB convection: ${\textit{Nu}}\sim Ra^{1/3}$ and ${\textit{Re}}\sim Ra^{4/9}Pr^{-2/3}$. These findings demonstrate an effective approach to suppress flow reversal and alter heat transfer efficiency.

Shock tube experiments are essential in understanding the environment encountered by hypersonic vehicles. Such experiments provide information used to determine rate constants of chemical, relaxation and radiative processes taking place in non-equilibrium plasmas. These constants are significant drivers of uncertainty in surface heat flux predictions. Recent work has shown that flow non-uniformities in real shock tube experiments can be misinterpreted as a need to alter these parameters; however, no comprehensive model exists to decouple the effects. We show that there is a rigorous method to achieve this by using experimental measurements as boundary conditions and including their effects via reverse time integration. This method improves over previous implementations by rigorously enforcing conservation laws, incorporating two-temperature, non-equilibrium thermochemistry and explicitly modelling both forward- and backward-running sound waves in the shock tube test slug through a method of characteristics formulation. This approach allowed the effect of shock speed variation in highly non-equilibrium tests, specifically those relevant to Titan entry, to be studied for the first time. A validation study showed that properties predicted by the method were found to agree with results from a viscous, two-dimensional axisymmetric Navier–Stokes solver within 1.5 %. When applied to shock tube test cases from the EAST and T6 facilities for simulation of lunar return and Titan entry representative conditions, the method offered improved agreement with experimentally measured oxygen 777 nm and 240–440 nm radiance, respectively, when compared with previous implementations, particularly towards the rear of the test slug where forward-running sound waves from the driver become influential.

This study explores partial synchronisation in turbulent channel flows using sequential variational data assimilation with sparse observations, emphasising the roles of model and observation uncertainties. Unlike previous work that focused on synchronisation using direct numerical simulation, this study considers synchronisation under imperfect models and noisy data. In the first part, a synchronisation map is constructed, revealing invariance with respect to variations in the predictive model, Reynolds number and mesh resolution. Full synchronisation emerges above a critical level of equivalent observation density. At lower observation densities, modal synchronisation is observed, where the energies of dominant modes evolve independently of initial conditions. As data become sparser, the system transitions to a non-synchronisation regime, with assimilated flows exhibiting minimal correlation with the observations. The second part of this study uses the master flow interpolated from down-sampled sparse observations. The delay-coordinate strategy is introduced to enhance the modal synchronisation. Results indicate that the optimal $\sigma$ lies near the threshold between modal synchronisation and non-synchronisation. This demonstrates that the modal synchronisation serves as a critical prerequisite for leveraging historical information in data assimilation to improve the accuracy of turbulence reconstruction. These findings extend the scope of synchronisation theory and provide valuable guidance for advancing data assimilation methodologies.

In compressible gas–particle flows the dispersion of particle clouds driven by a blast is widely observed in extreme natural and engineering scenarios. Whereas prior research has primarily focused on planar shock or blast-driven configurations, this study investigates a gas–particle system combining a finite-source blast with supersonic inflow. Accordingly, the compressible multiphase particle-in-cell method is employed to simulate the flow. The resulting waves including main shock, contact surface and secondary shock are parametrically investigated, where the main shock radius follows an approximate power law to time. Driven primarily by the drag force, a simplified two-stage scaling law for spanwise leading particle dispersion is derived: a time-squared dependence during the blast-dominated stage and growth behaviour ranging from linear to logarithmic in the subsequent flow-impingement stage. Furthermore, four dispersion morphologies are identified: compressed, uniform, eroded and jetting, each explained by specific wave–particle interaction mechanisms. Finally, a phase diagram correlating these morphologies with the inflow Mach number, Stokes number and pressure ratio is constructed. These findings reveal the coupled mechanisms in gas–particle systems driven by a blast and supersonic inflow, providing a predictive basis for impulse effects and particle dispersion.

Energy poverty remains a persistent challenge in Nigeria, where over 40% of the population lacks reliable electricity despite vast renewable energy potential. While SDG 7 frames universal energy access as a justice imperative, renewable energy transitions generate complex social and environmental trade-offs that remain underexamined. This study assesses Nigeria’s renewable energy transition through the lens of energy justice, incorporating distributional, procedural, recognition, and restorative dimensions. Guided by three research questions, it evaluates: (1) the integration of energy justice principles in policy, (2) their implementation in practice, and (3) whether the transition can be considered just overall. Drawing on qualitative expert interviews, findings reveal multidimensional non-economic impacts. Benefits include improved health, enhanced educational access, livelihood opportunities, and environmental gains. However, significant harms persist, including displacement, land-use conflicts, electronic waste, cultural disruption, and gender-based vulnerabilities. While justice principles are often articulated in policy, implementation remains uneven: participation is frequently tokenistic, benefits are short-lived or unevenly distributed, vulnerable groups are insufficiently recognised, and reparative mechanisms are weak or absent. By linking these deficits to the persistence of energy poverty, the study shows that Nigeria’s transition remains incomplete from a justice perspective, underscoring the need for more inclusive and accountable governance frameworks.

Quantifying the contribution of vortex structures to wall forces is essential for identifying the primary sources of forces. The traditional force-element method focuses on the contribution of flow structures to the resultant force. However, the contribution of flow structures to the distributed force cannot be identified. This work proposes a distributed force element method to address this issue. Inspired by the framework of matched asymptotic expansions, the method resolves the surface pressure by matching the fundamental solutions of the outer wave and inner flow regions. The pressure is thus decomposed into contributions from a convective acceleration term, a boundary acceleration term and a boundary vorticity term. The method is implemented by solving the resulting linear system with singular value decomposition. The volume source is further decomposed into direct radiation and boundary scattering components. It is found that compared with the direct radiation component, the boundary scatter component decays fast in the wake. Consequently, the direct radiation component is dominant in the far wake. A finite-domain pressure correction is proposed based on the direct radiation component. The distributed force element method is validated using several benchmark cases: two-dimensional configurations including laminar flow around stationary and oscillating circular cylinders; three-dimensional cases comprising laminar flow past a sphere, subcritical flow past a sphere and laminar flow over an inclined spheroid. The results suggest that the proposed distributed force element method enables the precise quantification of how flow structures in the wake and around the bluff bodies contribute to the surface pressure.

The present study establishes a general theory for fluid-element rotation and intrinsic vorticity decompositions within the framework of vorticity kinematics. We propose two direction-dependent vorticity decompositions (DVDs) based on the analysis of rotation of directed material line and surface elements, with the rigid-rotation and spin modes of vorticity being explicitly defined. Intrinsic coupling relations are then derived for a pair of orthogonal line and surface elements, demonstrating their complementary roles in both kinematics and geometry. Notably, the surface-element-based spin mode is shown to coincide with the relative vorticity in the generalized Caswell formula, thereby providing a faithful representation of surface shear stress in Newtonian fluids. Correspondingly, another two DVDs are constructed based on the geometry of streamlines and streamsurfaces in the field description. Furthermore, within the characteristic algebraic description, in terms of the rotational invariants $(\psi ,\gamma )$ in the real Schur form of the velocity gradient tensor, two invariant vorticity decompositions (IVDs) are formulated. The first IVD with positive spin aligns with the Liutex-shear decomposition, which corresponds to the Klein–Kaden–Betz (KKB) mechanism by which wrapping shear layers form axial vortices. The second IVD is indispensable for understanding unidirectional swirling motion around a point on the rotation-axis-normal plane ${\mathcal{P}}$, corresponding to an anti-KKB mechanism/phenomenon characterized by the negative spin. Importantly, it is proved that the DVD vorticity modes are rigorously bounded by the IVD vorticity modes $(R_{N}^{\pm },s_{N}^{\pm })=(2\psi ^{\pm },\gamma ^{\pm })$ on ${\mathcal{P}}$. Finally, distinctive features and applicability of these kinematic tools are demonstrated with representative examples. The results indicate that a coupled IVD–DVD approach provides a powerful diagnostic tool for unravelling the subtle structures and fundamental physics inherent to complex flow fields.

The influence of hydrodynamic interactions on the schooling behaviour of fish is still poorly understood. This paper numerically investigates the collective motion of two parallel fish that move freely in both the longitudinal and lateral directions, focusing on the effects of wavelength and phase difference on their stable formations, swimming speed and energy efficiency. It is found that two parallel fish can achieve stable formations in both longitudinal and lateral directions, only via the hydrodynamic interactions. Three distinct modes are classified based on the cycle-averaged longitudinal speed, i.e. the steady slow mode, the steady fast mode and the fluctuating fast mode; which mode occurs depends on the wavelength and phase difference. Compared to a single fish, two fish in the steady slow mode swim slower, whereas they swim faster in both the steady fast and fluctuating fast modes, with a maximum speed increase of 12 % observed in the latter mode. Moreover, the fish school exhibits higher propulsive efficiency than a single fish in most cases. Furthermore, the power consumption and propulsive efficiency of each fish in different modes are discussed in detail. Finally, the mechanism behind the stable formations has been analysed. These results may shed some light on understanding the underlying mechanisms of fish schooling behaviour.

Planar particle image velocimetry (PIV) measurements were conducted to investigate turbulent flows through a square duct roughened by transverse rectangular ribs of four blockage ratios (${\textit{Br}}=0.1$, 0.15, 0.2 and 0.25) at a bulk Reynolds number of ${\textit{Re}}_b = 9400$. In contrast to the classical two-dimensional (2-D) rib-roughened boundary-layer flows, the turbulent flow studied here is intrinsically three-dimensional (3-D) and inhomogeneous, complicated by not only the internal shear layers (ISLs) triggered by the rib crests, but also the intense interaction of the four boundary layers developing over duct sidewalls. It is observed that turbulent motions near the rib crest are mainly dominated by the ejection and sweep events. As the blockage ratio increases, the magnitudes of Reynolds stresses near the rib crest increase significantly attributed to enhanced sweep events and large-scale flapping motions. The results of temporal auto-correlations and spatial two-point auto-correlations show that both temporal and spatial integral scales of turbulence structures are dominated by the streamwise velocity fluctuations, which increase as the rib height increases. Based on proper orthogonal decomposition (POD) analyses, it is interesting to observe that the ISL near the rib crest is dominated by both the low- and high-frequency flapping motions characteristic of the first POD mode.