Non-modal linear stability analysis results are presented for hypersonic flow over an elliptic cone with an aspect ratio of two at zero angle of attack, completing earlier modal instability analysis of flow around the same geometry. The theoretical framework to perform transient growth analysis of compressible flows on a generalized two-dimensional frame of reference is developed for the first time and is then applied to solve the initial-value problem governing non-modal linear instability on planes perpendicular to the cone axis, taken at successive streamwise locations along the elliptic cone. Parameter ranges examined here are chosen so as to model flight of the Hypersonic International Flight Research Experimentation 5 (HIFiRE-5) test geometry at altitudes of 21 km and 33 km, corresponding to Mach numbers 7.45 and 8.05 and unit Reynolds numbers $Re' = 1.07\times 10^7$ and $1.89\times 10^6$, respectively. Results obtained indicate that the significance of the non-modal growth for laminar–turbulent transition increases with increasing flight altitude (decreasing Reynolds number). At a given set of flow parameters, transient growth is stronger in the vicinity of the tip of the cone and in azimuthal locations away from both of the minor (centreline) and major (attachment line) axes of the cone. Linear optimal disturbances calculated at conditions of maximal transient growth are found to peak in the crossflow region of the elliptic cone. These structures are elongated along the streamwise spatial direction, while being periodic along the spanwise direction with periodicity lengths of the same order of magnitude as the well-known structures identified as crossflow vortices in both experiments and simulations.

We report on the discovery, using numerical simulations, of a segregation pattern in high-speed granular flows, in which size segregation is primarily driven by two-dimensional granular temperature gradients, rather than by gravity. In contrast to slower flows on gentle slopes, in high-speed flows on steep slopes, large particles no longer accumulate in the upper layers of the flow, but are trapped in the interior. The strong temperature gradients that develop between the interior of the flow and the surrounding dilute periphery appear to govern the segregation mechanism. Interestingly, these new segregated flows run at a much faster speed than similar monodisperse flows. This opens up promising perspectives for transporting granular material with enhanced efficiency. Importantly, we show that the kinetic theory for dense, inclined flows of binary mixtures can provide a relevant theoretical framework to explain the segregation patterns observed in the numerical simulations.

The structure of the steady boundary layer in the airflow over young wind waves is studied in detail using extensive data accumulated in our laboratory as well as in additional laboratory facilities and in field measurements. Following the approach adopted in studies of turbulent flow over solid rough surfaces, the coupling between the spatial evolution of wind waves and wind velocity profiles over the water is analysed. The roughness of the moving water surface under wind is not constant; wind waves that constitute the roughness elements are unsteady, random and three-dimensional. The effective water surface roughness increases with airflow velocity as well as with downstream distance. Nevertheless, the existence of wall similarity as observed in a flow over solid surfaces is demonstrated; a fully rough boundary layer is obtained for young wind waves at diverse operational conditions. This approach enables quantitative study of the coupling between the airflow and local wind wave characteristics.