The interaction of a turbulent eddy with a semi-infinite poroelastic edge is examined with respect to the effects of both elasticity and porosity on the efficiency of aerodynamic noise generation. The scattering problem is solved using the Wiener–Hopf technique to identify the scaling dependence of the resulting aerodynamic noise on plate and flow properties, including the dependence on a characteristic flow velocity $U$ . Special attention is paid to the limiting cases of porous-rigid and impermeable–elastic plate conditions. Asymptotic analysis of these special cases reveals parametric limits where the far-field acoustic power scales like ${U}^{6} $ for a porous edge, and a new finite range of ${U}^{7} $ behaviour is found for an elastic edge, to be compared with the well-known ${U}^{5} $ dependence for a rigid impermeable edge. Further numerical results attempt to address how trailing-edge noise may be mitigated by porosity and flexibility and seek to deepen the understanding of how owls hunt in acoustic stealth.

The ability of some species of owl to fly in effective silence is unique among birds and provides a distinct hunting advantage, but it remains a mystery as to exactly what aspects of the owl and its flight are responsible for this dramatic noise reduction. Crucially, this mystery extends to how the flow physics may be leveraged to generate noise-reduction strategies for wider technological application. We review current knowledge of aerodynamic noise from owls, ranging from live owl noise measurements to mathematical modeling and experiments focused on how owls may disrupt the standard routes of noise generation. Specialized adaptations and foraging strategies are not uniform across all owl species: Some species may not have need for silent flight, or their evolutionary adaptations may not be effective for useful noise reduction for certain species. This hypothesis is examined using mathematical models and borne out where possible by noise measurements and morphological observations of owl feathers and wings.

Strategies for trailing-edge noise control have been inspired by the downy canopy that covers the surface of exposed flight feathers of many owl species. Previous wind-tunnel measurements demonstrate that canopies of similar characteristics can reduce pressure fluctuations on the underlying surface by as much as 30 dB and significantly attenuate roughness noise generated by that surface. In the present work, surface treatments are designed to replicate the effects of the canopy in a form suitable for application to an airfoil. These treatments are installed directly upstream of the trailing edge to modify the boundary-layer turbulence before acoustic scattering by the edge. Over 20 variants of these designs have been tested by performing aeroacoustic wind-tunnel measurements on a tripped DU96-W180 airfoil at chord Reynolds numbers of up to 3 million. Compared with the unmodified airfoil, the treatments provided up to 10 dB of broadband attenuation of trailing-edge noise. The effectiveness of the treatment is not highly dependent on a particular geometry, but there appears to be strong potential for optimization. The surface treatments remain effective over an angle-of-attack range that extends over 9 deg from zero lift. Aerodynamic impact of the treatment appears minimal.

The Leidenfrost effect occurs when a drop of liquid (or a sublimating solid) is levitated above a sufficiently hot surface through the action of an insulating vapour layer flowing from its bottom surface. When such a drop is levitated above a surface with parallel, asymmetric sawtooth-shaped ridges it is known to be propelled in a unique direction, or ratcheted, by the interaction of the vapour layer with the surface. Here we exploit this effect to construct a ‘ratchet trap’ for Leidenfrost drops: a surface with concentric circular ridges, each asymmetric in cross-section. A combination of experiment and theory is used to study the dynamics of drops in these traps, whose centre is a stable fixed point. Numerical analysis of the evaporating flows over a ratchet surface suggests new insights into the mechanism of motion rectification that are incorporated into the simplest equations of motion for ratchet-driven motion of a Leidenfrost body; these resemble a central force problem in celestial mechanics with mass loss and drag. A phase-plane analysis of experimental trajectories is used to extract more detailed information about the ratcheting phenomenon. Orbiting drops are found to exhibit substantial deformations; those with large internal angular momentum can even undergo binary fission. Such ratchet traps may thus prove useful in the controlled study of many properties of Leidenfrost drops.

There is a growing desire to operate Uncrewed Air Vehicles (UAVs) in urban environments for parcel delivery, and passenger-carrying air taxis for Advanced Air Mobility (AAM). The turbulent flows and gusts around buildings and other urban infrastructure can affect the steadiness and stability of such air vehicles by generating a highly transient relative flow field. Our aim is to review existing gust models, then consider gust encounters in the vicinity of buildings as experienced by flight trajectories over the roof of a nominally cuboid building in a suburban atmospheric boundary layer. Simplified models of fixed- and rotary-wing aircraft are used to illustrate the changes in lift and thrust experienced by flight around the building. The analysis showed that fixed-wing aircraft experienced a substantial increase in angle of attack over a relatively short period of time (<1 s) as they fly through the shear layer at a representative forward velocity, which can be well above typical stall angles. Due to the slow flight speeds required for landing and take-off, significant control authority of rotor systems is required to ensure safe operation due to the high disturbance effects caused by localized gusts from buildings and protruding structures. Currently there appears to be negligible certification or regulation for AAM systems to ensure safe operations when traversing building flow fields under windy conditions and it is hoped that the insights provided in this paper will assist with future certification and regulation.