Self-generation of highly organized waves in a nominally turbulent jet at very low Mach number can arise from its impingement upon the downstream orifice of an axisymmetric cavity, having an impingement length much shorter than the corresponding acoustic wavelength. The oscillation frequencies are compatible with the resonant modes of a long pipe located upstream of the cavity and with jet-instability frequencies based on the column mode (0·3 [siml ] SD [siml ] 0·6), as well as the near-field shear layer mode (0·016 [siml ] Sθ0 [siml ] 0·03). Moreover, the frequency of the organized wave is constant from separation to impingement; consequently vortex pairing does not occur.

Within the cavity, the pressure amplitude associated with the organized wave is directly related to the phase difference between the organized velocity fluctuations at separation and impingement. Maximum pressure amplitude occurs when this phase difference, measured along the cavity (i.e. jet) centre-line, is 2nπ. Streamwise amplitude and phase distributions of the organized wave cannot be explained from purely hydrodynamic considerations; however, they can be effectively modelled by superposing contributions from hydrodynamic and acoustic waves. This aspect has important consequences for externally excited jets as well.

The flow is considered of a Newtonian fluid, of viscosity η and surface tension T, in the narrow gap between a pair of rollers of radii R1 and R2, whose peripheral speeds are constant and equal to U1 and U2 respectively. The objective is to determine the coating thickness h1∞ on the upper roller as a function of the non-dimensional parameters H0/R, ηU/T and U1/U2, where H0 is the minimum gap thickness, U = ½(U1 + U2), and 2R−1 = R1−1 + R2−1.

Using lubrication theory to provide an adequate description of the fluid flow, two mathematical models are derived whose essential difference lies in the specification of the boundary conditions. In the separation model it is assumed that the pressure distribution will terminate at a position which is both a stagnation point and a point of separation, whereas the Reynolds model incorporates the classical Reynolds conditions. In each case, theoretical predictions for the non-dimensional coating thickness, h1∞/H0 as a function of U1/U2 are found to compare well with experiment. However, theory does suggest that the two models are applicable to different and complementary regions of parameter space, and hence together they may form a basis for further investigations into the various features of coating processes.

A nonlinear amplitude equation of second order in time, which governs the temporal evolution of linearly unstable modes in stratified shear flows, is derived. It applies to a class of flows with continuous velocity and density profiles, and two examples of such flows are studied.

One of the flows that is studied is the stratified Couette flow with the buoyancy frequency equal to Qy2, where Q is a constant and y the vertical co-ordinate. The nonlinear amplitude equation is studied for various values of Q.

For the Garcia flow the nonlinear amplitude equation for the long-wave modes is evaluated, and it is compared with the corresponding equation in the Kelvin–Helmholtz flow, which has been found previously.

Profiles of gasdynamic parameters in self-similar blast waves, taking into account the influence of conduction and radiation fluxes due to high temperatures attained at the centre, are determined. In the blast-wave equations these fluxes are expressed in terms of the Fourier law for heat conduction and a differential expression for radiative transport in a semi-grey gas model. Various boundary conditions are considered in order to account for different ways in which blast waves are initiated and driven. Similarity requirements are implemented in the solution by compatible functional forms of gas conductivity and absorptivity, as well as the opacity of the shock front. This formulation yields a two-point boundary-value problem, which is then transformed into an initial-value problem in order to facilitate the integration. As a particular example, a detailed solution for the constant-energy case is obtained, covering the whole range of relative heat-transfer effects expressed in terms of radiative to gasdynamic energy fluxes, from the adiabatic flow field, on one extreme, to the isothermal, on the other.

According to Miles’ theory of wind-wave generation, water waves grow if the curvature of the wind profile at the critical height is negative. As a result, the wind profile changes in time owing to the transfer of energy to the waves. In the quasilinear approximation (where the interaction of the waves with one another is neglected) equations for the coupled air–water system are obtained by means of a multiple-time-scale analysis. In this way the validity of Miles’ calculations is extended, thereby allowing a study of the large-time behaviour.

While the water waves grow owing to the energy transfer from the air flow, the waves in turn modify the flow in such a way that for large times the curvature of the velocity profile vanishes. The amplitude of the waves is then limited because the energy transfer is quenched.

In the high-frequency range the asymptotic wave spectrum is given by a ‘–4’ law in the frequency domain rather than the ‘classical’ ‘–5’ law.

Model experiments have been done on two-dimensional inflated buildings in thick boundary layers that simulate either an onshore wind, or flow over sparsely wooded country. A theory has been developed which replaces the boundary layer by an inviscid flow of uniform vorticity. The replacement flow matches both the velocity and the velocity gradient of the wind at the maximum height of the building. The tension in the membrane is quite well predicted by the theory, but the external pressure is in general too low owing to the presence of separation bubbles at the leading and trailing edges.