Abstract. Interactions between a finite number of bodies and the surrounding fluid, in a channel for instance, are investigated theoretically. In the planar model here the bodies or modelled grains are thin solid bodies free to move in a nearly parallel formation within a quasi-inviscid fluid. The investigation involves numerical and analytical studies and comparisons. The three main features that appear are a linear instability about a state of uniform motion, a clashing of the bodies (or of a body with a side wall) within a finite scaled time when nonlinear interaction takes effect, and a continuum-limit description of the body-fluid interaction holding for the case of many bodies. §1. Introduction. The study of interactions between moving solid bodies and the surrounding fluid has many natural, industrial and biomedical applications. These and background motivations are considered in §1.1. Previous studies are discussed in §1.2, and §1.3 focuses on the present work.1.1. Applications and motivation. A great many applications arise across nature such as with falling leaves and moving seeds and coffee grains (e.g. [4, 7, 27, 52]), not to forget the motion of frozen ice particles and hailstones as well as sedimentation and fluidization phenomena. Applications also arise in sporting contexts such as running and cycling groups and to some extent in longdistance swimming competitions. The behaviours of various swarms similarly have an interactive fluid-dynamical element to them, while a communication to the authors has pointed out the collisions of ships due to rushing water and suction between them, as occurred for example during the UK-Iceland cod war and the similar danger of suction for barges in the Suez Canal.Three industrial applications are concerned with the falling of lumps of ice into an engine intake in an aerodynamic safety context, the travel of windblown particles of ice along a wing surface again in the aerodynamic safety context, and the falling of rice grains down a chute in a food-sorting context. In addition various disintegration, deposition, oil-well and sequestration modelling applications exist for interactions between solid bodies and fluids.There are also many biomedical applications in principle, for example to travel of solids within vessels of major networks in the human body. Specific applications are to transport of blood clots, embolization procedures in stroke

Near-impact behaviour is investigated for a solid body approaching another solid body with two immiscible incompressible viscous fluids occupying the gap in between. The fluids have viscosity and density ratios which are extreme, the most notable combination being water and air, such that either or both of the bodies are covered by a thin film of water. Air–water interaction and the commonly observed phenomenon of air trapping are of concern in the presence of the two or three thin layers and one or two interfaces. The subcritical regime is of most practical significance here and it leads physically to the effect of inviscid water dynamics coupling with a viscous-dominated air response locally. This physical mechanism induces touchdown (or an approach to touchdown), which is found to occur in the sense that the scaled air-gap thickness shrinks towards zero within a finite scaled time according to analysis performed hand in hand with computation. A global influence on the local touchdown properties is also identified. Comparisons with computations prove favourable. Air trapping is produced between two touchdown positions, at each of which there is a pressure peak; an oblique approach would not affect the finding unless the approach itself is extremely shallow. The mechanism of air–water interaction leading to air trapping is suggested as a quite wide-ranging result.

A theoretical and computational study is presented of the wind flow over the mountain ridges to the south of Hong Kong International Airport given the upwind profiles of 'mean' velocity and temperature. A sensitivity study shows how large variations in wind speed, wind shear and wind direction occur on the approach path when the upwind flow from the southeast is stably stratified above the boundary layer with a significant inversion no more than a few hundred metres above the ridge (height about 450 m). The Froude number is close to unity. The effects of vertical wind shear across the inversion layer reduce the speed-up, and directional wind shear changes the surface flow direction. The observed horizontal length scales along the approach are about equal to the projected widths of the predominant features of the ridges and valleys. For an extreme wind event there was found to be satisfactory agreement between the wind speed measured by a landing aircraft and the predictions of the fast quasi-linear analytically based computer model FLOWSTAR. This suggests that near real time application of the model and near real time monitoring with remote and in situ instruments can be used to predict these extreme flow events and hence give warnings about them. Conclusions are drawn about how such predictive modelling could be developed.

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Ensuring long-term performance from key infrastructure is essential to enable it to serve society and to maintain a sustainable economy. The future-proofing of key infrastructure involves addressing two broad issues: (i) resilience to unexpected or uncontrollable events (e.g., extreme weather events); (ii) adaptability to required changes in structure and/or operations of the infrastructure in the future. Increasingly, infrastructure owners, designers, builders, governments and operators are being required to consider possible future challenges as part of the life cycle planning for assets and systems that make up key infrastructure. A preliminary study is reported here that aimed at exploring the following questions related to infrastructure (systems): what does 'future-proofing' of infrastructural assets mean? Why and when should critical infrastructure be future-proofed? How can infrastructure assets (systems) be prepared for uncertain future events? How can future-proofing considerations be incorporated into infrastructure asset management practices? To seek answers to the above questions, two industrial workshops were conducted that brought together leading practitioners in the UK infrastructure and construction sectors, along with government policymakers. This paper captures lessons learnt from the workshops and proposes a framework for linking future-proofing into asset management considerations. Case studies of Dawlish railway and Heathrow airport are also presented.