Earlier measurements of the contribution of four distinct classes of motions, i.e. (u < 0, v > 0), (u > 0, v < 0), (u < 0, v < 0) and (u > 0, v > 0), to the Reynolds stress $-\rho \overline{uv}$ in the wall region of a bounded turbulent shear flow have been extended. These classes were obtained by truncating the u and v signals about zero. Various statistical properties of the truncated streamwise and normal velocity components u and v and of their product uv have been determined in an attempt to characterize quantitatively the motions in this flow. Average values and probability density distributions both of the truncated and untruncated signals have been taken.

A point-vortex representation is used to study numerically the evolution of an initially plane vortex sheet. By introducing a tip vortex to represent the tightly rolled portion of the vortex sheet, the chaotic motion which was a feature of some earlier studies is eliminated and the details of the outer portion of the spiral are calculated. The rate of rolling up is calculated and is shown to be governed by the analytically predicted similarity law of Kaden during the initial stages of the rolling up. The calculations are continued until 99% of the vorticity has been rolled up, at which stage the spiral displays a marked ellipticity.

A mixing layer is formed by bringing two streams of water, moving at different velocities, together in a lucite-walled channel. The Reynolds number, based on the velocity difference and the thickness of the shear layer, varies from about 45, where the shear layer originates, to about 850 at a distance of 50 cm. Dye is injected between the two streams just before they are brought together, marking the vorticity-carrying fluid. Unstable waves grow, and fluid is observed to roll up into discrete two-dimensional vortical structures. These turbulent vortices interact by rolling around each other, and a single vortical structure, with approximately twice the spacing of the former vortices, is formed. This pairing process is observed to occur repeatedly, controlling the growth of the mixing layer. A simple model of the mixing layer contains, as the important elements controlling growth, the degree of non-uniformity in the vortex train and the ‘lumpiness’ of the vorticity field.

The theoretical and experimental evaluation of a laboratory device for creating a controlled shear flow in a continuously stratified liquid is described. The shear is created by the movement of the end barriers of a rectangular channel. If these barriers are impulsively set into uniform shear motion, this motion propagates away in the form of a front of width $O((h/n) (Nt)^{\frac{1}{3}})$ travelling with uniform speed Nh/nπ, where N is the (constant) buoyancy frequency, t is time, n is the vertical modal number and his the channel depth. The shear produced has roughly twice the duration of that attainable using a tilting tube (Thorpe 1968) of similar dimensions. Shear enhancement is possible by introducing a properly designed flow contraction. If N(z) is variable, the shear profile obtained is that appropriate to the lowest internal wave mode of infinite length.

A time-dependent current distribution is abruptly switched on, initiating an unsteady disturbance of a two-phase magnetohydrodynamic configuration. This comprises a magnetically permeated, conducting fluid flow contained by a vacuum magnetic field, from within which the source current radiates. An exact solution to the proposed part-time problem is constructed for the magnetic line distortion of the vacuum field.

If the source current is oscillatory, two progressive, non-dissipative waves are normally encountered, these being superposed upon an infinite discrete set of terms obeying separate rules of decay. Propagation occurs longitudinally, parallel to the flow, but with amplitudes dependent on the transverse variable. The waves advance behind two fronts, the faster of which always travels down-stream. Depending on whether the flow speed exceeds or is exceeded by (or equals) √2 × the quadratic mean of both Alfvkn speeds involved, the slower front proceeds, respectively, downstream or upstream (or disappears, in which case, only one wave exists). Along a characteristic (a front path), appropriate contributions depend solely on the transverse co-ordinate, behaving otherwise like Riemann invariants. Contrary to expectation, the net perturbation is continuous across each characteristic. Various steady modes are ultimately attained after an infinite period. The radiation principle is satisfied.

Both travelling waves are vibrationally sustained, vanishing with the source frequency. In such an event, the infinite series result is summable to a closed form. From this, the general solution, corresponding to an arbitrary space–time source distribution, is deduced. Certain characteristic-associated equivalence laws are then established. An asymptotic approximation is made.

A potential-flow based analytical model is developed for the temporal development of the spectra of ripples and dunes generated on an initially flattened bed by an open-channel flow. The analysis predicts the occurrence of two distinct spectral peaks at small times; the physical origin of each is explained. One of the peaks has spatial frequency equal to that given by the Airy relation for a small amplitude stationary free-surface wave. These results are confirmed by Jain's (1971) experimental data. Jain's data also are used to determine quantitativevalues of the lag distance andHayashi's (1970) inclination factor, α, which appear in the sediment-transport relation used in the analysis. A variance-cascade model is used to derive the ‘minus-three-power law’ that describes the spatial spectra of rippIes and dunes at higher wavenumbers. Finally, the relation between the Froude number and dominant wavelength implied by Jain & Kennedy's (1971) non-dimensionalized spectra is discussed and compared with data presented by Jain (1971) and Nordin (1971).

The sharp interface which exists in unbounded turbulent shear flows was studied using a linear array of twenty hot-wire probes. The probe arrangement was such that the location of the interface could be monitored instantaneously. The particular flow examined was a two-dimensional turbulent wall jet. It was found that the interface is a highly contorted surface which exhibits a significant amount of folding. Quantitative methods for characterizing this behaviour are presented, together with pertinent measurements. In addition, measurements of the mean surface area of the interface, and space-time corrrelations of the width of the turbulence were obtained. The latter were used to find characteristic scales and convection velocities of the interface.

Detailed measurements of pressure distributions, mean velocity profiles and Reynolds stresses were made in the thick axisymmetric turbulent boundary layer near the tail of a body of revolution. The results indicate a number of important differences between the behaviour of a thick and a thin boundary layer. The thick boundary layer is characterized by significant variations in static pressure across it and an abnormally low level of turbulence. The static-pressure variation is associated with a strong interaction between the boundary layer and the potential flow outside it, while the changes in the turbulence structure appear to be a consequence of the transverse surface curvature. In order to predict the behaviour of the flow in the tail region of a body of revolution it is not therefore possible to use conventional thin-boundary-layer calculation procedures.

Mean velocity and mean turbulent field measurements are performed for the case of a three-dimensional turbulent boundary layer on an axially rotated cylinder. The cylinder model consists of two parts: a stationary section followed by a spinning afterbody. Techniques of hot-wire anemometry are employed, which yield complete mean velocity and turbulence measurements in skewed flows. The general behaviour of the three-dimensional boundary layer is first discussed: two asymptotic layers analogous to the two-dimensional wall and defect layers are observed; they are shown to evolve from the equations of mean motion. The hypothesis of scalar eddy viscosity is investigated in the light of these results. Using conventional length scale assumptions together with the Reynolds stress tensor equations, a prediction of curvature effects in the law of the wall region is developed; a result in the present case is a smaller slope of the semi-logarithmic portion of the law of the wall, No assumptions over and above those necessary for plane, two-dimensional flow are required for this analysis. The geometry of the model is such that a rapid change in mean rate of strain occurs along the streamlines. From the history of the components of the $\overline{u_iu_j}$ tensor, it is possible to draw some fundamental conclusions concerning the dynamics of the energy dissipation, diffusion and redistribution processes.