We investigate the effect of the transverse aspect ratio ($B$) of a thermogravitational column (TGC) on the stability of a binary fluid mixture with a negative separation ratio ($\psi$), which generates an adverse vertical density gradient. In extended systems, this configuration is unstable and limits separation. Here we show that confinement fundamentally alters this behaviour and can fully suppress the instability. Combining linear stability analysis of the three-dimensional basic state, three-dimensional simulations and experiments, we identify $B$ as the key control parameter. Linear stability analysis yields a correlation for the critical aspect ratio $B_{\textit{c}}(\psi )$ separating unstable and stable regimes. For $B \lt B_{\textit{c}}$, the system remains in a quiescent state despite the presence of the adverse density stratification. Energy budget analysis shows that confinement enhances transverse diffusive transport, preventing the growth of long-wave instabilities that dominate in extended systems. Direct numerical simulations confirm the predicted transition and reveal the nonlinear evolution towards convection when $B \gt B_{\textit{c}}$. Experiments performed with different mixtures, Tol$|$Ch 0.50$|$0.50, Tol$|$EtOH 0.80$|$0.20 and Tol$|$MeOH 0.8420$|$0.1520, and geometries using two independent $\mu$TGCs ($B$ = 5.88 and $B$ = 1.69) validate the theoretical predictions. These results demonstrate that transverse confinement provides a robust and tunable mechanism for controlling thermogravitational stability.

An experimental investigation is conducted to examine aeroacoustic instabilities in high-speed rectangular dual-impinging jets (DIJs), with a focus on the modal dynamics responsible for tonal noise generation. Comparisons are made with a circular baseline over expanded Mach numbers M = 1.0–1.2, impinging distances h/D = 2–3 and a fixed centre-to-centre spacing of S/D = 3.5. Time-resolved wall-pressure fields on the impingement plate and far-field sound are measured synchronously using fast-response pressure-sensitive paint and microphones. Dominant impingement tones and their associated surface-pressure patterns are identified and linked using cyclostationary spectral proper orthogonal decomposition and coherent output power analysis. Both circular and rectangular DIJs exhibit an annularly symmetric impingement mode whose tonal frequency is broadly consistent with classical single-loop jet–plate feedback scaling. In the rectangular configuration, this mode develops an elliptical footprint and a reduced dual-jet interference region, leading to attenuation of the corresponding tone relative to the circular case. In addition, rectangular DIJs support a flapping antisymmetric plate-pressure mode characterised by alternating pressure-wave orientations and a four-stage evolution over the oscillation cycle. This mode is associated with a prominent tone at Strouhal number St = 0.42, which departs from the integer harmonics of the baseline feedback loop. Finally, a co-resonance framework is employed to interpret why the geometry-enabled tone is strongly amplified under the present dual-jet conditions. Overall, these results demonstrate that transitioning from circular to rectangular nozzles redistributes modal dominance in high-speed DIJs and reorganises the balance between attenuated harmonic peaks and an additional geometry-enabled tone.

Controversy exists regarding whether grain morphology reduces or enhances the drag of a single grain in creeping flows. Further complication occurs when orientation dependence of aspherical grains comes into play. To bridge this gap, this study investigates numerically the drag on fractally rough grains depicted by spherical harmonics. Rough shapes could induce drag reduction, indicated by a lower mean value of drag coefficients $C_{\!D}$ at various rotation angles, compared with that of the corresponding smooth sphere. Moreover, the derived power law between $C_{\!D}$ and the projected area $A_{\!p}$ perpendicular to the flow direction, expressed as $C_{\!D}\propto {A_{\!p}}^{-0.8}$ for spheroidal and triaxial-ellipsoidal grains, remains valid for irregular shapes. Such a rotational dependence helps to explain the paradox where drag enhancement is consistently encountered in settling grain experiments prevalent in geophysics. The macroscopic observations are elucidated by microanalysis on the fluid–grain contact pressure differences relative to the volume-equivalent sphere, revealing that the net drag reduction is mainly rooted in the frictional drag. By gaining a deeper understanding of the drag force on rough grains, this research provides valuable insights into particle–fluid interactions in creeping flows, and holds promising implications for unresolved simulations of fluid–particle systems.

As insects flap their wings, they generate complex wake structures critical to their aerodynamic force production. Specific flow structures such as the leading-edge vortex have been studied for decades; however, a complete understanding of the transient dynamics and energy exchange mechanisms in insect wakes remains elusive. To help bridge this gap, we employ data-driven reduced-order modelling techniques to identify a simple and interpretable model for a hovering hawkmoth’s wake. We begin by using an in-house immersed-boundary-method computational fluid dynamics solver to simulate hovering hawkmoth flight. We then perform dynamic mode decomposition to distil the resulting flow field into a set of time-varying modes. Finally, we employ sparse regression to identify a model capturing the driving modes’ temporal evolution, ranging from quiescent flow to periodic steady state. Notably, the model takes the form of a Stuart–Landau oscillator with higher-order nonlinear terms. The presence of a limit-cycle dynamics suggests a balance between energy input from wing motion and energy lost due to advective energy transfer and viscous dissipation. Using an impulse-based wake survey method, we show that this model provides an accurate estimation (mean absolute error within 3.5 % of body weight) of the hawkmoth’s long-term lift production. These findings highlight the significance of stability and energy transfer in flapping-flight aerodynamics, offering a framework for future studies of biological flight systems. Furthermore, by linking the wake dynamics to simple dynamic equations, this work provides inspiration for the design and control of bio-inspired micro-aerial vehicles.

This study develops wavenumber–frequency spectrum models for wall-pressure fluctuations in turbulent boundary layers on flat plates and cylinders in compressible flow. Through non-dimensionalisation and solution of the momentum and continuity equations, a unified physical framework integrating near-field pressure and far-field acoustic regions is established, extending Lighthill’s acoustic analogy. Starting from the fluctuating pressure governing equation, Mach number effects in the acoustic region are explicitly introduced for the first time, and unified analytical expressions across the full wavenumber range are derived for both geometries. The proposed models achieve high-precision prediction across the entire wavenumber domain. Validation is performed via cylinder wind tunnel experiments at Mach $0.12$–$0.18$ and flat-plate direct numerical simulation (DNS) at Mach $0.1$–$0.5$. Compared with classical models such as Chase II, Smol’yakov and Corcos, the present model shows better agreement with experimental and DNS data, particularly in low-wavenumber and acoustic regions, improving prediction accuracy by more than $10$ dB. Key findings: (i) acoustic amplitude highly correlates with Mach number, while the convective ridge does not; (ii) the acoustic of flat-plate boundary is defined by $k_{1}^{2} + k_{3}^{2} - (k_{0} - Mak_{1})^{2} = 0$, where $k$ is the wavenumber and $Ma$ is the Mach number; (iii) the cylinder model degenerates to the flat-plate form as the curvature radius approaches infinity, with curvature effects confined mainly to the acoustic region and large circumferential wavenumbers. This work provides a physically self-consistent and practical engineering spectral model with significantly enhanced predictive capability under compressible-flow conditions.

Although thousands of prehistoric standing stones have been recorded across Iberia, the lack of associated detailed contextual data allows ongoing debate about their possible functions. Following the chance discovery of a ‘diademated’ stela in 2018, excavations at Las Capellanías (Cañaveral de León, Huelva), in south-west Spain, led to the discovery of two more stelae and facilitated the compilation of vital contextual information. Here, the authors explore the association between these stelae, Bronze and Iron Age burials and an ancient routeway, revealing reuse, temporal persistence, geographical connectivity and cultural hybridisation in the widening networks of prehistoric Iberia.