A laboratory experiment is used to examine the vertical mixing resulting from the breaking of internal waves on a sloping boundary in a continuously stratified fluid. Attention is confined to the case of critical waves when the slope of the group velocity vector of the incident waves is equal to the bottom slope. Along the sloping boundary a turbulent bottom boundary layer forms with a thickness dependent on the incident wave amplitude. The mixing efficiency, defined as the ratio of the increase in potential energy due to mixing to the loss of kinetic energy by the incident waves, is dependent upon the stability of the flow and has an upper bound of approximately 0.20.

By examining the increase in potential energy of the fluid as a result of sustained mixing, we are able to compute the transition value of the dissipation εtr below which no mixing occures. For mixing due to the breaking of critical internal waves on sloping boundaries we find that εtr = (8±2)νN2. From comparisons with experiments with grid-generated turbulence, this suggests that while εtr/νN2 = 0(10) in the available data sets, the specific value of εtr may be mechanism dependent.

Screens used in settling chamber of wind tunnels are known to introduce non-uniformities into the flow which, in turn, may influence the flow in the working section. A thorough investigation was made, using a flow visualization technique, of the development of non-uniformities generated by screens and their influence on the flow in the stagnation region of circular cylinders placed normal to the flow. A theoretical model of the flow downstream of screens is presented that is consistent with the experimental findings.

A pair of curved gauzes with non-uniform porosity was used to generate a strong constant-shear flow with low turbulence intensity. The complex features of this flow around a circular cylinder with its axis normal to the vorticity and mean velocity vectors were studied. Methods such as the use of end plates, inclusion of gaps at the junctions of the cylinder with the end plates, and fluid withdrawal were applied to minimize end effects. Exploratory studies were made to investigate shear effects on the vortex-wake formation region with Reynolds numbers between 800 and 1.4 × 104. Effects of the steepness factor on vortex formation were explained by regarding secondary flow in the base region of the cylinder as negative base bleeding which behaves as a wake interference element.

An attempt is made to derive complete sets of conservation laws for various flows. It is shown that the equations of three-dimensional adiabatic flow cannot be transformed into a complete set of conservation laws. It is demonstrated that potential vorticity is the only material invariant of shallow-water flow. A complete set can be derived for three-dimensional homogeneous flow, if Lagrangian tracers are added.

The development of an incompressible, laminar, pulsatile boundary layer over a semi-infinite flat plate is studied. Although the undisturbed free stream flow is taken to be non-reversing (following previous studies), sufficiently far downstream, the flow in part of the boundary layer must ultimately reverse direction during part of the cycle.

A novel numerical (finite-difference) scheme is described, which builds in the time periodicity of the flow, and also takes into account the direction of the flow in deciding the form of the differencing in the streamwise direction. The effect of variations in numerical grid is investigated, and comparison is made with asymptotic formulae applicable close to and far from the leading edge of the plate, together with linearized (small oscillation) results obtained using the analysis of Ackerberg & Phillips (1972), which is shown to yield remarkably accurate results when compared with the solution for the full problem, even for quite large oscillation amplitudes.

The method of Tanaka (1983) is used to solve the eigenvalue problem determining the form of the first superharmonic instability of periodic Stokes waves. Comparisons are made with other approaches to this problem and a discussion of the advantages of Tanaka's method is given. The accurately resolved eigenfunction solution is then taken as the initial state for commencing the computational time-stepping method of Dold & Peregrine (1985), by which we investigate the full nonlinear development of the growing and decaying modes of this instability. It is observed that all unstable modes develop to breaking in the periodic regime and this result is compared and contrasted with the solitary wave case.

For some years now, much effort has been devoted to the study of thermosolutal convection in the liquid phase during upwards solidification of a binary alloy, which is coupled to the dynamics of the solid-liquid interface. While the theoretical analysis is well developed, there is a need for experimental evidence. Experiments in cylinders have thus been carried out on lead – 30 wt % thallium alloys in order to obtain significant information about the convective patterns in the melt adjacent to the solidification front, from a knowledge of the macroscopic shape of the phase boundary. This shape is determined as a function of the lateral confinement θ (the ratio of the crucible diameter to the unstable wavelength at the threshold for an infinite medium) from a series of contour lines for the solid in the two-phase region of the quenched samples. When θ is small, the pattern always has a central axisymmetric core and an outer annulus which is at first complex or structureless and then presents a mixture of festoons and solid sectors. For θ very close to unity, a hexagon, which is the basic element of a laterally infinite array, dominates the morphology. At higher θ, a hexagon can no longer remain naturally centred and is replaced by two main cells which contact the wall by again making a completely festooned ring. The fluid flow in the liquid just ahead of the solid-liquid interface is then inferred. Analogy with Bénard-Marangoni patterns suggests a qualitative analysis of the convective structures. The present observations are finally compared to previous ones on similar alloys grown in crucibles with a smaller diameter.