The canonical form of the equations for the free-surface elevation and potential of an irrotational fluid is more than a coincidence. The elevation is a ‘generalized coordinate’ field sufficient to define the system Lagrangian without explicit reference to the motion of the fluid interior. The Lagrangian and the associated field equations are complete and self-contained in the two-dimensional surface co-ordinates, but non-local (integro-differential) in form; the canonical equations derived by Miles are just the Hamiltonian counterparts.

A shallow three-dimensional hump disturbs the two-dimensional incompressible boundary layer developed on an otherwise flat surface. The steady laminar flow is studied by means of a three-dimensional extension of triple-deck theory, so that there is the prospect of separation in the nonlinear motion. As a first step, however, a linearized analysis valid for certain shallow obstacles gives some insight into the flow properties. The most striking features then are the reversal of the secondary vortex motions and the emergence of a ‘corridor’ in the wake of the hump. The corridor stays of constant width downstream and within it the boundary-layer displacement and skin-friction perturbation are much greater than outside. Extending outside the corridor, there is a zone where the surface fluid is accelerated, in contrast with the deceleration near the centre of the corridor. The downstream decay (e.g. of displacement) here is much slower than in two-dimensional flows.

An asymptotic description is given of Newtonian fluid flow in a channel which is suddenly heated or cooled. The viscosity is assumed to be purely a function of temperature. The asymptotic approximation is that the downstream viscosity at the channel wall differs by an order of magnitude from that in the upstream flow. Although we make the drastic assumption that viscous dissipation is negligible, we can analyse flows where the viscosity depends either algebraically or exponentially on the temperature.

This paper presents a similarity solution for plane channel flow of a very viscous fluid, whose viscosity is exponentially dependent upon temperature, when heat generation is very large. A dimensionless formulation of the problem involves two length scales (the depth h and length l, respectively, of the channel), one velocity scale (the mean velocity V of the fluid along the channel), the thermal conductivity k, thermal diffusivity k and viscosity V of the fluid, and the temperature coefficient b of the viscosity. From these, two important dimensionless groups arise, the Graetz number (Gz = Vh2/kl) and the Nahme–Griffith number (G = μ V2b/k). In the case of steady flow with G−1 [Lt ] Gz−1 [Lt ] 1 a thin thermal boundary layer of thickness proportional to Gz−½ arises at each wall with an even thinner shear layer, detached from the wall and embedded in the thermal boundary layer, of thickness proportional to Gz−½(ln G)−1, coinciding with the region of maximum temperature (ln G)/b. The similarity variable is (Pe½y/x½) where Pe is the Péclet number (Vh/k) and y and x are measured away from and along (either) boundary wall. The analogous unsteady uniform flow solution is also given.