Linearized equations are derived for disturbances in an infinite plasma without an imposed magnetic field. It is shown that besides the electrostatic, or longitudinal, waves which are usually considered, there can also exist electromagnetic, or transverse, waves. The two sets of waves are generally coupled, but one can nevertheless classify the waves as either mainly longitudinal or mainly transverse. It turns out that a plasma which is stable to longitudinal waves will be unstable to transverse waves unless the velocity distribution of its particles satisfies some rather stringent conditions. In a practical case these conditions would require the distribution to be isotropic.

In flow between concentric rotating circular cylinders, it was shown by Taylor (1923) that instability may occur in the form of toroidal vortices spaced regularly along the axis. When the vortex motion occurs additional torque is required to keep the cylinders in motion at given speeds. Stuart (1958) used an energy-balance method, in the case when the annular gap is small compared with the radius, to estimate the additional torque and the associated finite amplitude attained by the vortices. He included the effect of distortion of the mean motion, but ignored the generation of harmonics of the fundamental mode and the distortion of the velocity associated with the fundamental mode. It is now known that these are not valid mathematical approximations and a rigorous perturbation expansion is developed here to remedy the deficiency. The analysis is valid for any gap width and any angular speeds of the containing cylinders, but requires the amplification rate of the disturbance to be small.

Numerical results using a digital computer are obtained for the shape and amplitude of the vortices in three cases: (i) when the outer cylinder has twice the radius of the inner one and is kept at rest, (ii) when the gap is small and the cylinders rotate with nearly the same speeds, and (iii) when the gap is small and the outer cylinder is kept at rest. The equilibrium amplitude obtained in the last case is substantially the same as that found by Stuart.

The results for cases (i) and (iii) give close agreement with the experimental values obtained by Taylor (1936) and Donnelly (1958) for the torque required to keep the inner cylinder rotating with constant speed while the outer one is at rest, for a certain range of speeds. In the small-gap problem it is shown that the equilibrium amplitude is almost proportional to 1 − m, where m is the ratio of the angular speeds of the outer and inner cylinders.

The development of large amplitude compression waves in a collision-free plasma is studied by considering the motion of a plane piston into a uniform stationary plasma containing a magnetic field parallel to the plane of the piston. The adiabatic two-fluid equations are solved by finite-difference methods and the form of the waves after a long time is compared with the possible steady-state solutions.

A generalized discontinuous solution of the steady-state equations is found for sufficiently high Mach numbers. At the highest Mach numbers this leads to a constant state at the piston; while at lower speeds a wave train results whose amplitude increases as the speed decreases. In each of these cases the numerical solutions of the time-dependent equations converge rapidly to the steady-state solutions. At still lower speeds, where the solitary-wave solution exists, the situation is less clear.

In the light of accumulated evidence in favour of Miles's mathematical theory of wave generation by wind, the author has thought it desirable to translate the theory into the form of a physical argument, which goes as follows. Travelling water waves in a wind produce, to a first approximation, airflow undulations with pressures least over crests and greatest over troughs. Hence, just above the ‘critical height’ where the airflow component in the direction of propagation equals the wave velocity, air after slowly overtaking a crest is turned back by the higher pressure near the trough, moves down to a lower level and back towards the crest. Similarly, behind crests, an upward movement at the critical height occurs. Quantitatively, the vertical velocity v is such that the ‘vortex force’ − ρων (where ρ is density and ω vorticity) balances the sinusoidal pressure gradient. Furthermore, since in turbulent boundary layers vorticity decreases with height, any downflow produces a local vorticity defect, and upflow a local vorticity excess, and hence the vortex force varies about a negative mean at the critical height (although at other levels, where air moves sinusoidally, with vertical displacement and velocity 90° out of phase, the mean vortex force is zero). The negative total mean force extracts wind energy, and transfers it to the wave, enabling it to grow exponentially. For pressure gradients adequate to initiate substantial energy transfer, the critical height must be fairly small compared with the wavelength, and waves can grow whenever their velocity and direction satisfies this condition, a conclusion supported by measurements (Longuet-Higgins 1962) of the directional spectrum of wind-generated waves.

The analogy between the transport of heat and vorticity when the Prandtl number is unity is used to provide a simple complementary solution of the boundary-layer energy equation for plane flow. The solution is extended to apply to axisymmetric boundary layers by suitably stretching the co-ordinate normal to the wall. Several applications of the solution are discussed.

Some general properties of one-dimensional deflagration waves in a non-conducting inviscid gas at rest are discussed when ionization of the gas takes place across a shock wave which precedes the flame front, and electromagnetic fields are present. The direction of wave propagation, the electric field and magnetic field are taken as a mutually orthogonal triad of vectors. The jump relationships across the gas-ionizing shock wave and magnetogasdynamic combustion wave are investigated and the two Hugoniot curves analysed in detail in the pressure-specific volume plane. The possible types of wave are indicated for arbitrary magnitudes of the upstream electromagnetic field. It is shown that weak gasionizing shock waves cannot exist. For suitably chosen electromagnetic field strenghts the density ratio across the shock wave may be greater than the ordinary gasdynamic limit and, in such cases, the pressure and density ratios are related in an inverse manner, in contrast to the behaviour for ordinary gasdynamic or magnetogasdynamic shock waves. The magnetogasdynamic combustion wave has similar properties to that in ordinary gasdynamics.

The Navier-Stokes equations for a viscous, incompressible fluid are considered for a steady, axisymmetric flow composed of a strong rotation combined with radial sink flow which exhausts axially inside a finite radius. The equations are reduced to two coupled partial differential equations in terms of the stream function and circulation. The equations contain three dimensionless parameters: the radial Reynolds number, a characteristic ratio of mass flow per unit lenght to circulation, and a characteristic ratio of an axial dimension to a radial dimension. The product of these last two dimensionless parameters is used as a new expansion parameter for generating an asymptotic series solution. To zeroth order in this parameter, the solution for the stream function is a linear distribution between two axial boundary values. First-order correction terms are calculated for a specific example.

In discussing these equations the limitations of the exact solutions due to Donaldson & Sullivan (1960) and Long (1961) are noted. These exact solutions are contrasted with the approximate treatment of this type of vortex originated by Einstein & Li (1951) and generalized by Deissler & Perlmutter (1958).

The effect of radiative transfer on the thermal stability of an ionized fluid, in the presence of uniform vertical magnetic field has been examined from hydromagnetic approximations. Two asymptotic cases (i) when the fluid is optically thin and (ii) when it is optically thick, have been examined in detail. The principle of exchange of stabilities and the concept of over-stability have been discussed and the physical conditions under which the former will hold, obtianed. Also the conditions as to which type of instability will arise first are derived and it has been shown that in the presence of large magnetic field over-stability arises earlier than convection under the first approximation.

The boundary layer on a semi-infinite flat plate placed in a two-dimensional, unbounded, steady, constant shear flow of a viscous incompressible fluid is examined on the basis of the constant-pressure assumption. An asymptotic solution is obtained first for large vorticity numbers. Then an approximate solution is found for arbitrary vorticity numbers that gives good agreement with exact calculations for the extreme cases of small and large vorticity numbers. The present calculations are limited to the boundary layer on the top side of the plate only.

Some general stability criteria for non-dissipative swirling flows are derived, and extended to the case of an electrically conducting fluid in the presence of axial magnetic field and current. In particular it is shown that the analogy between a rotating and a stratified fluid holds in this case, and that an important determinant of stability is a ‘Richardson number’ based on the analogue of the density gradient and the shear in the axial flow.