This work demonstrates the occurrence of travelling wave flutter instabilities on viscoelastic coatings in supersonic flows, which forms an important milestone towards developing viscoelastic materials for high-speed flow applications. The flow-induced travelling surface waves were generated by elevating the air speed such that the flow dynamic pressure and the material Young’s modulus are of the similar order, their ratio defined as the compliance parameter. Experimental investigations probed the critical compliance parameter corresponding to surface instability onset across different fluid-structural non-dimensional parameters. The results revealed a strong dependence of the critical compliance parameter on the flow Mach number in addition to other flow and structural parameters that were found to influence the instability onset in earlier studies with aqueous flows. A broader parametric sweep of these parameters was performed using linear stability analysis. Consistent qualitative comparisons were obtained between the experiments and stability calculations on the most amplified wave characteristics. Further investigations revealed that the interaction mechanisms are similar to incompressible flows.

Subspecies classification plays a critical role in understanding evolutionary processes and informing conservation strategies. In the western Mediterranean, the European Turtle-dove Streptopelia turtur is traditionally divided into two subspecies: S. t. turtur, which breeds across most of Europe, and S. t. arenicola, which occurs in North Africa and parts of southern Europe. However, the validity of this subspecific distinction remains controversial. Here, we test the validity of this classification by integrating morphometric and mitochondrial DNA (mtDNA) from populations sampled across their breeding ranges, including mainland Europe, UK, North Africa, and the Canary and Balearic Islands (Spain). Principal Component Analysis (PCA) and Linear Discriminant Analysis (LDA) revealed no statistically significant morphometric differences between subspecies, with a strong overlap in key traits such as wing, tail, and tarsus length. Mitochondrial haplotypes formed a star-like network with no geographical structuring or lineage exclusivity. AMOVA and pairwise Fst analyses showed no genetic differentiation between subspecies, and variation was greater within S. t. turtur populations than between previously proposed subspecies. Our findings suggest that the current subspecific classification of European Turtle-dove may not be supported by genetic or morphometric evidence. Instead, the observed homogeneity could be attributed to on-going gene flow facilitated by dispersal during migration or a relatively recent shared evolutionary history. These results have important implications for the conservation management of western Mediterranean breeding populations, particularly in relation to hunting regulations. We advocate for an integrative approach using genomic and ecological data to reassess subspecies boundaries in S. turtur and ensure evidence-based conservation strategies.

We study interactions between anticyclonic and cyclonic coherent barotropic vortices with inertial oscillations across a broad range of Rossby numbers and wave–vortex energy ratios. We examine parameter regimes where the interaction leads to high vertical velocities along with fine-scale dissipative structures and identify these as cases where the flow transitions to turbulence. The analysis reveals that flows that transition to turbulence have Rossby numbers and energy ratios inversely correlated. As a result, low Rossby number flows need a higher wave–vortex energy ratio while higher Rossby number flows can transition to turbulence at lower wave–vortex energy ratios. We also explore energetic interactions between the flow components using baroclinic–barotropic, linear wave–balance and a fast–slow decomposition. The results show that the three decompositions yield similar results at low Rossby numbers while the fast–slow and baroclinic–barotropic decompositions are better suited for high Rossby number flows. In all the cases we find that the slow barotropic vortical flow facilitates the forward cascade and dissipation of the fast baroclinic wave field, while its own dissipation and energy exchange with the wave field remain relatively small.

We derive asymptotic models for the ultimate regimes in horizontal convection (HC) and pure internally heated convection (IHC), in analogy with our recent (2024) extension of the ultimate-regime model for Rayleigh–Bénard convection (RBC). To derive the corresponding models for HC and IHC, we combine turbulent boundary-layer relations with the exact dissipation balances for these two systems. For HC, the resulting scaling relations are consistent with the rigorous transport bound of Siggers et al. (2004 J. Fluid Mech. vol. 517, 55–70). For pure IHC, they are consistent with the exact HC–IHC balance analogy of Wang et al. (2021 Geophys. Res. Lett. vol. 48, e2020GL091198) and with the rigorous bounds on the convective-flux asymmetry in the equal-temperature-plates configuration (Arslan et al. 2021 J. Fluid Mech. vol. 919, A15). The main difference between RBC and HC/IHC is that, in the latter two cases, the global kinetic-energy balance does not contain the additional response factor (dimensionless convective heat flux in HC or inverse bulk temperature in IHC), whereas it does in RBC. As a consequence, for fixed Prandtl number ${\textit{Pr}}$, the ultimate-regime scaling exponent is $1/3$ for both HC and IHC, rather than $1/2$ as in RBC.

This study reports mainly experimental findings on the evolution of confined axisymmetric gravity currents of non-Newtonian fluids within a gap of fixed height, focusing on the combined effects of rheology and surface tension. The inflow rate follows a power-law dependence on time, with the exponent related to the fluid behaviour index. Prior theoretical work, validated against axisymmetric experiments with Newtonian fluids, has shown that (i) surface tension acting primarily at the grounding line can significantly modify the structure of classical self-similar solutions and (ii) the presence of a meniscus at the grounding line can be incorporated into the similarity formulation through a parameter relating its height to the gap thickness, thereby preserving self-similarity. Here, this framework is extended to non-Newtonian fluids, adopting a power-law rheology for analytical convenience, which is appropriate since the tested shear-thinning and shear-thickening fluids behave as power-law fluids over the relevant shear-rate range. The influence of surface tension is confirmed through twenty-one axisymmetric experiments using Newtonian and non-Newtonian fluids. Good agreement between theory and experiments is obtained only when the grounding-line meniscus is included in the analysis. In addition, surface tension is found to play a central role in the hysteresis observed during the initial stages of current propagation, determining whether the current remains confined or transitions to an unconfined state. Finally, we show that the self-similar solution is robust, remaining valid even when the inflow rate deviates from the ideal power-law form, for example, in cases involving a delay in filling the experimental cell.