Using toroidal co-ordinates, an exact solution is derived for the velocity field induced in two immiscible semi-infinite viscous fluids, possessing a plane interface, by the slow rotation of a concave spherical lens, which is such that the circle of intersection of the composite spherical surfaces lies in the plane of the interface. The expression for the torque acting on the lens is derived, and this is shown to be reducible to an analytic closed form, when the lens degenerates into a spherical bowl and the fluids are identical.

Using toroidal coordinates, an exact solution is derived for the velocity field induced in two immiscible semi-infinite fluids possessing a plane interface, by the slow rotation of an axially symmetric body partly immersed in each fluid. The surface of the body is assumed to be formed from two intersecting spheres, or a sphere and a circular disc, with the circle of intersection of the composite surfaces lying in th interface.

It is shown that when the rotating body possesses reflection symmetry about the plane of the interface of the fluids, the velocity field in either fluid is independent of the viscosities of the fluids. The torque exerted on the body is then proportional to the sum of the viscosities. Analytic closed-form expressions are derived for the torque when the body is either a sphere, a circular disc, or a tangent-sphere dumbbell, and for a hemisphere rotating in an infinite homogeneous fluid. Closed-form results are also given for an immersed sphere, tangent to a free surface. For other geometrical configurations, numerical values of the torque are provided for a variety of body shapes and two-fluid systems of various viscosity ratios.

We shall be concerned with two boundary value problems for the Falkner-Skan Equation

when –β is a small positive number. Our interest is in solutions of (1) which exhibit “reversed flow”; that is, solutions f such that f′(x) < 0 for small positive values of x. The boundary conditions which we wish to consider are

and

In this note we report on some numerical results regarding reverse flow solutions of the Falkner-Skan equation

on the half line 0 < t < ∞, which satisfy the boundary conditions

and

The study of similarity solutions of Prandtl's equations for the steady two dimensional flow of an incompressible fluid past a rigid wall leads to the equation

where the primes denote differentiation with respect to the independent variable t, and λ is a parameter. It was first obtained in 1930 by Falkner and Skan [3]. For its derivation we refer to Schlichting [6] here we merely note that the function f′(t) represents, after suitable normalization, the velocity parallel to the wall.

The properties of the solution of the differential equation governing the evolution of localised line-centred disturbances to a marginally unstable plane parallel flow were described by Hocking and Stewartson (1972). A corresponding study of the properties when the initial disturbance is point-centred is presented here. A localised burst at a finite time can be produced, for certain values of the coefficients which can be determined analytically. When the equation permits solutions with circular symmetry, two kinds of bursting solutions, as well as solutions which remain finite, are possible, but the boundary between bursting and finite solutions could not be determined analytically.